On the polymer-based nanocomposites for electrical ...1078217/FULLTEXT01.pdfEngineered polymer-based...

i

On the polymer-based nanocomposites for electrical

switching applications

Venkatesh Doddapaneni

PhD Thesis School of Engineering Sciences

KTH Royal Institute of Technology Stockholm, Sweden, 2017

ii

TRITA-FYS 2017:08

ISSN 0280-316X KTH School of Engineering Sciences

ISRN KTH/FYS/–17:08–SE SE-100 44 Stockholm

ISBN 978-91-7729-288-3 SWEDEN

Akademisk avhandling som med tillstånd av Kungl Tekniska högskolan framlägges till

offentlig granskning för avläggande av teknologie doktorsexamen i fysik fredagen den 24

mars klockan 14:00 i FA31, Roslagstullsbacken 21, Albanova Universitetscentrum,

Stockholm.

© Venkatesh Doddapaneni, Mars 2017

Tryck: Universitetsservice US AB

Frontpage: Photograph of the MCB and its internal components, (a, b, c) recorded images

of the electric arc at different times. (Courtesy of ABB, www.abb.com.)

http://www.abb.com/

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"Keep going even when there are obstacles."

- Oliver Hart, awarded The Sveriges Riksbank Prize in Economic

Sciences in Memory of Alfred Nobel 2016.

TO

My dear princess

My beloved mother & father

v

PREFACE

This thesis is submitted as a compilation of scientific papers for fulfilling the requirements

of the doctoral thesis (PhD) in the physics department at KTH Royal Institute of

Technology. The research has been supported by the Department of Electromagnetic

Engineering at KTH and ABB.

ACKNOWLEDGEMENTS

I would like to thank my supervisor Prof. Muhammet S. Toprak for all the support and

encouragement during this thesis work. He has given very object oriented support, which

led to get this thesis done as per our time bound.

My sincere thanks go to Prof. Hans Edin for being my co-supervisor, Prof. Rajeev

Thottappillil and Prof. Joydeep Dutta for their constructive and continuous support to move

ahead smoothly during my PhD studies.

I am very grateful to my industrial supervisor Dr. Rudolf Gati, ABB corporate

research, Switzerland for sharing his experimental and theoretical insights on the switching

electrical arcs. Such insights helped me to understand experimental results with

nanocomposites leading to appropriate scientific reasoning. I will always remember his

interest towards engineering of present polymeric materials for different electrical

switching applications.

I would like to thank KIC InnoEnergy PhD School for giving an opportunity to learn

entrepreneurial skills towards a startup from PhD studies.

In the first part of my thesis, I worked for fabrication of PNCs at the Nano-

characterization Laboratory in KTH. I would like to thank Dr. Mohsin Saleemi, Dr. Fei Ye

and Dr. Carmen Vogt for sharing their knowledge on the synthesis of nanoparticles and

nanocomposites, helping me to fabricate PNCs quickly. In the second part, fabricated PNCs

are tested their impact on electrical arcs generate in a real circuit breaker system setup at

Electromagnetic Engineering Lab, KTH. I would like to thank my colleagues Dr. Ara

Bissal and Jesper Magnusson for their assistance during the experimental tests with the

electric arcs.

My special thanks go to Dr. Mykailo Gnybida and Dr. Christian Rumpler for allowing

me to work on theoretical models to better understand the outgassing process at Eaton

European Innovation Center, Prague. This period was good to start from basic to advanced

models and detailed knowledge on different high voltage breakers.

Many thanks to all my colleagues from Applied Physics, Electromagnetic Engineering

departments for being so friendly during this period. I must admit that I really enjoyed

being a PhD student at KTH.

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Finally yet importantly, I would like to thank my family members for all the encouragement

and support they have given me.

Venkatesh Doddapaneni Stockholm, March 2017

vii

ABSTRACT

Recent research demonstrated that polymer based nanocomposites (PNCs) have been

engineered in order to improve the arc interruption capability of the circuit breakers.

PNCs are the combination of nano-sized inorganic nanoparticles (NPs) and polymers,

opened up new developments in materials science and engineering applications.

Inorganic NPs are selected based on their physical and chemical properties which

could make multifunctional PNCs in order to interrupt the electrical arcs effectively.

In particular, we presented the PNCs fabricated by using CuO, Fe3O4, ZnO and Au

NPs in a poly (methyl methacrylate) (PMMA) matrix via in-situ polymerization

method, recently developed method to avoid NPs agglomeration, leading to good

spatial distribution in the polymer matrix. Thus, several samples with various wt% of

NPs in PMMA matrix have been fabricated. These PNCs have been characterized in

detail for the morphology of NPs, interaction between NPs and polymer matrix, and

radiative/thermal energy absorption properties. In the next stage, PNCs are tested to

determine their arc interruption performance and impact on the electrical arcs of

current 1.6 kA generated using a specially designed test set-up. When PNCs interact

with the electrical arcs, they generate ablation of chemical species towards core of the

electrical arc, resulting in cooling-down the arc due to strong temperature and pressure

gradient in the arc quenching domain. This thesis demonstrates for the first time that

these engineered PNCs are easily processed, reproducible, and can be used to improve

the arc interruption process in electrical switching applications.

Keywords: Polymer-based nanocomposites, Inorganic nanoparticles, Electrical arcs,

Circuit breakers, Ablation/Outgassing, Arc interruption capability, PMMA, CuO,

Fe3O4, ZnO, Au, Radiative energy, Electric power, Arc temperature

http://scholar.google.se/scholar_url?url=http://www.google.com/patents/US5663213&hl=en&sa=X&scisig=AAGBfm2Nhmxnn9jx88vFgTgOoaoadPmpJw&nossl=1&oi=scholarr&sqi=2&ved=0ahUKEwi0sdvzqJzPAhXGCSwKHVe5DdMQgAMIGygAMAA

viii

SAMMANFATTNING

Ny forskning har visat att polymerbaserade nanokompositer (PNCs) har utformats för

att förbättra strömbrytares förmåga att undvika ljusbågar vid överslag. PNCs är en

kombination av nanostora oorganiska nanopartiklar (NP) och polymerer, som har

öppnat upp för ny utveckling inom materialvetenskap och tekniska tillämpningar.

Oorganiska NP väljs baserat på deras fysikaliska och kemiska egenskaper som kan

hjälpa PNCs att motverka elektriska ljusbågar effektivt. I synnerhet, presenterade vi

PNCs tillverkade genom användning av CuO, Fe3O4, ZnO och Au NP i en poly

(metylmetakrylat) (PMMA)-matris via in situ-polymerisationsmetod, nyligen

utvecklad för att undvika NP-agglomerering, vilket leder till god rumslig fördelning i

polymermatrisen. Därför har flera prover med olika vikt% av NP i PMMA-matris

tillverkats. Dessa PNCs har utvärderats i detalj för NP-morfologi, interaktion mellan

NP och polymermatris, och strålnings- och värmeenergiabsorption. I nästa skede

testas PNCs för att bestämma deras förmåga att undvika ljusbågar och påverkan på de

elektriska ljusbågarna av 1,6 kA strömstyrka, genererade med hjälp av en

specialdesignad test-set-up. När PNCs interagerar med de elektriska ljusbågarna,

genererar de ablation av kemiska ämnen mot kärnan i den elektriska ljusbågen, vilket

resulterar i nedkylning av ljusbågen på grund av starka temperatur- och

tryckgradienter i området. Denna avhandling visar för första gången att dessa

konstruerade PNCs är lätta att framställa, reproducerbara, och kan användas för att

förbättra avbrottsprocessen för ljusbågen i elektriska kopplingstillämpningar.

ix

LIST OF PUBLICATIONS

This PhD thesis is based on the compilation of the following six papers, referred in the text

by using the serial number assigned below. All these publications are appended at the end

of the thesis with permission from the respective journal.

I Doddapaneni, V., Magnusson, J., Bissal, A., Gati, R., Edin, H., & Toprak, M. S. (2015).

Spectroscopic investigations of the ablated species from the polymers exposed to electric

arcs in air. International Conference on Electric Power Equipment – Switching

Technology, 3, 337.

II Doddapaneni, V., Zhao, Y., Ye, F., Gati, R., Edin, H., & Toprak, M. S. (2015).

Improvement of UV radiation absorption by cupric oxide NPs/PMMA

nanocomposites for electrical switching applications. Journal of Powder Metallurgy

and Metal Ceramics, 54(7), 397.

III Doddapaneni, V., Gati, R., & Toprak, M. S. Engineered PMMA-CuO nanocompositesfor improving the electric arc interruption process in electrical switching

applications. Manuscript.

IV Doddapaneni, V., Saleemi, M., Ye, F., Gati, R., & Toprak, M. S. (2016). On the

electrical arc interruption by using PMMA/iron oxide nanocomposites. Materials

Research Express, 3(10), 105043.

V Doddapaneni, V., Saleemi, M., Ye, F., Gati, R., & Toprak, M. S. (2017). Engineered

PMMA-ZnO nanocomposites for improving the electric arc interruption process in

electrical switching applications: Unprecedented experimental insights. Composites

Science and Technology, 141, 113.

VI Doddapaneni, V., Ye, F., Saleemi, M., Gati, R., & Toprak, M. S. (2017). New

experimental insights for controoling the electrival arcs in electrical switching

applications: a comparitive study on PMMA nanocomposites of Au and ZnO.

Under review, Composites Science and Technology.

http://iopscience.iop.org/journal/2053-1591

http://iopscience.iop.org/journal/2053-1591

http://iopscience.iop.org/volume/2053-1591/3

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CONTRIBUTIONS OF THE AUTHOR

Paper I. Literature survey, planning, performing the experiments, characterizations,

results analysis and writing the complete article.

Paper II. Literature review on the composites, planning, performing the experiments,

evaluation of results and writing the complete article.

Paper III. Literature survey, experimental design, performing the experiments, analyzing

the results and writing the manuscript.

Paper IV. Literature survey, fabrication of nanocomposites, experimental design, results

analysis and writing the complete article.

Paper V. Literature survey, fabrication of nanocomposites, experimental design, results


Paper VI. Literature survey, fabrication of nanocomposites, experimental design, results


xi

TABLE OF CONTENTS

Preface..................................................................................................................................... v

Acknowledgements ................................................................................................................. v

Abstract .................................................................................................................................vii

Sammanfattning .................................................................................................................. viii

List of publications ................................................................................................................ ix

Contributions of the author ..................................................................................................... x

List of Abbreviations .......................................................................................................... xiii

1. Introduction ................................................................................................................... 1 1.1 Electrical switching applications .............................................................................. 1

1.1.1 Modern air based circuit breakers and materials limitations ............................ 2

1.2 Engineered nanocomposites ..................................................................................... 5

1.2.1 Potential advantages and challenges ................................................................. 5

1.3 Integration of PNCs in electrical switching applications ......................................... 7

1.4 Scope of the thesis .................................................................................................... 9

2. Electrical switching applications ................................................................................ 11 2.1 The electric arc in air .............................................................................................. 11

2.1.1 Introduction to electric arc .............................................................................. 11

2.1.2 Different types of electrodes ........................................................................... 13

2.1.3 Different types of electrical arcs ..................................................................... 14

2.1.4 Electric arc roots and arc column .................................................................... 15

2.1.5 Dissipation of arc energy ................................................................................ 16

2.2 Arc interruption process ......................................................................................... 16

3. Polymer-based Nanocomposites ................................................................................. 19 3.1 Polymers ................................................................................................................. 19

3.1.1 Why thermoplastic polymer? .......................................................................... 19

3.1.2 PMMA ............................................................................................................ 20

3.2 Nanoparticles .......................................................................................................... 20

3.2.1 Surface modification of NPs ........................................................................... 21

3.3 Fabrication of PNCs ............................................................................................... 22

4. Experimental ................................................................................................................ 25

4.1 Materials and Methods ........................................................................................... 25

4.1.1 Synthesis of NPs ............................................................................................. 25

4.1.2 Fabrication of PNCs ........................................................................................ 27

4.2 Characterization techniques ................................................................................... 27

4.2.1 X-ray Diffraction, XRD .................................................................................. 28

4.2.2 Transmission Electron Microscopy, TEM ...................................................... 28

4.2.3 Fourier Transform Infrared Spectroscopy, FTIR ............................................ 28

4.2.4 UV-Vis Absorption Spectroscopy, UV-Vis .................................................... 29

4.2.5 Thermal Gravimetric Analysis, TGA ............................................................. 29

4.2.6 Differential Scanning Calorimetry, DSC ........................................................ 29

xii

4.2.7 Scanning Electron Microscopy, SEM ............................................................. 29

4.3 Experimental Tests with the electrical arcs ............................................................ 30

4.3.1 Electrical arc discharge circuit ........................................................................ 30

4.3.2 Experimental test set-up .................................................................................. 30

5. Results and discussion ................................................................................................. 33 5.1 Polymers ................................................................................................................. 33

5.2 PNCs with Cupric oxide NPs ................................................................................. 34

5.3 PNCs with Iron oxide NPs ..................................................................................... 37

5.4 PNCs with Zinc oxide QDs .................................................................................... 41

5.5 PNCs with Gold NPs .............................................................................................. 44

6. Conclusions................................................................................................................... 47

7. Future work.................................................................................................................. 49

Appendix A: ......................................................................................................................... 51 A: Drawing of the experimental test set-up .......................................................................... 51

Appendix B: ......................................................................................................................... 53 B: TGA-FTIR spectroscopy – PA66, PMMA ...................................................................... 53

Bibliography ........................................................................................................................ 55

xiii

LIST OF ABBREVIATIONS

PNCs Polymer-based NanoComposites

MCB Miniature Circuit Breakers

PMMA Poly (methyl methacrylate)

NPs Nanoparticles

Fe3O4 Iron oxide

CuO Cupric oxide

ZnO Zinc oxide

Au Gold

TEM Transmission electron microscopy

SEM Scanning electron microscopy

FTIR Fourier transform infrared spectroscopy

TGA Thermogravimetric analysis

DSC Differential scanning calorimetry

UV-Vis Ultraviolet-visible spectroscopy

LTE Local thermodynamic equilibrium

MEA Monoethanolamine

1

1. INTRODUCTION

Engineered polymer-based nanocomposites (PNCs) are attracting great interest due to their

multifunctional properties[1], chosen alternative to the ceramic walls and traditional polymers

such as PMMA, PA66 etc. in power switching devices (such as fuses, circuit breakers and

contactors). All these materials are intended to protect the switching devices from high

energetic fault currents, improve the reliability and efficiency. Basically a power switching

device is a switch, which has been used for protecting the electric grid from over and short

circuit currents by automatic interruption[2-6].

1.1 ELECTRICAL SWITCHING APPLICATIONS

A Fuse - Every one of us have it in our homes. It is used to protect people and electrical

appliances from high fault currents that could lead to electric shock and fire[7].

In general, an electric power system is a set of electrical components used during

generation, transmission and distribution of electric power [8, 9]. Fuse is the basic key

component in the system in order to protect from over-current and short circuit currents [10,

11]. By definition, fuses are designed to open the circuit or creating infinite resistance when

high currents are present due to the presence of unintended disturbances in the electric power

system[12]. According to the IEC standard i.e. IEC 60269-1, fuses are designed with metal

wire, it melts when it receives either heat or rated fault current. It is clearly mentioned in the

standard is that, “a device that by fusing of one or more of its specially designed and

proportioned components opens the circuit in which it is inserted by breaking the current when

this exceeds a given value for a sufficient time. The fuse comprises all the parts that form the

complete device”[13]. One of the main limitations of the fuse is that it has to be replaced after

interruption of a fault current, wouldn’t be able to provide complete protection due to its

limitations like materials used for fuse wire and frequency of unintentional faults generated in

the system. If an arc fault was not quickly extinguished, that can cause great damage such as

electric shock, fire and personal injuries[14].

In order to overcome the limitations of the fuses, advanced version of fuses i.e. Miniature

Circuit Breakers (MCBs) are introduced since early1920’s[15]. MCBs are combined with

thermal and magnetic trips due to over and short-circuit currents [16, 17], more importantly

reusability without need of replacing repeatedly. MCBs are mainly aimed for current limiting,

reducing electric arc generation [17, 18], improved response time compared to standard zero

2

crossing circuit breakers, significantly reducing let-through energy (i2t), damage to the

connected electrical appliances[19], improved tripping characteristics.

Early version of MCBs were described by Thomas Edison is his patent application in

1879. The present version shown in Figure 1.1 is invented and patented in Germany by Hugo

Stotz and Heinrich Schachtner in 1924 [15]. This version increased safer and efficient

electrification of private homes. Today, billions of MCBs are produced, and nearly all homes

are using MCBs to control the flow of electric power, safeguarding people and electrical

appliances from high fault currents. It looks like a traffic police, who controls traffic at an

intersection when there’s a heavy snow fall in winter, the MCBs interrupt the electric

power reaching home when something inside or outside of homes goes wrong.

Figure 1.1. Evolution of different MCBs in time, a) First MCB in 1924, b) improved MCB with K-characteristics for

motors in 1928, c) High-efficiency MCB S201-K4 in 1957, d) MCB S161 in 1961, e) MCB from System pro M compact

S200 in 2012 (Courtesy of ABB, www.abb.com.)[15].

According to the standards IEC/EN 60898-1, IEC/EN 60947-2 and UL 1077, MCB is

defined as “a mechanical switching device, capable of making, carrying and breaking currents

under normal circuit conditions and also making, carrying for a specified time, and

automatically breaking currents under specified abnormal circuit conditions such as those of

short-circuit”.

1.1.1 MODERN AIR BASED CIRCUIT BREAKERS AND MATERIALS LIMITATIONS

Modern MCBs shows an improved reliability and safety of operation in terms of trip-

free mechanism operation compared to the early stages of fuse shown in Figure 1.1[17, 20].

Designed for domestic and commercial applications like data centers, high speed trains, etc.

they work up-to breaking currents of 15 kA according to the requirement[21]. MCBs lifetime

is approximately 30 years but it depends more on the number of operations, overloads

currents, short-circuits, operating environment, etc.

(a)

(b) (c)

(d) (e)

3

In general, overloading currents result when the devices draw more current than expected from

the source. Short circuit currents result when the current takes a low resistance path due to a

defect instead of the device being connected in load. In both cases, MCBs work through either

thermal or magnetic tripping mechanisms[22]. In case of overcurrent’s, bimetallic strip triggers

tripping mechanism due to heat generated from high currents while short circuit currents

inductance coil triggers, the key components are shown in Figure 1.2.

Figure 1.2. The cross section of a low-voltage MCB and its components (Courtesy of ABB, www.abb.com.).

Some known facts about MCBs are[23]:

1. The triggering speed of MCB is 10 ms, which is about 10 times faster than a normal

human eye can blink

2. MCB can disconnect up to 50 kA short circuit current, equivalent to the same amount

of current drawn from 50,000 smart phone batteries

3. When it breaks currents, it can go up to 10 000 oC, at which even rocks can melt

4. The pressure inside of MCB can go from 1 to 10 bar, which is similar to the pressure

levels used in water jet cutter to cut materials such as metals and granites

5. There are more than 4500 MCBs available in market, same as many different types of

mammals on the earth

Basically, when MCB triggers due to highly energetic fault currents, it will take little

electric field to ionize air and form an electric arc with highly intense radiative[24] and thermal

energy between two electrodes[25]. Illustrations of detailed working MCB in time domain are

shown in Figure 1.3[26]. Traditionally, ceramics and polymeric materials have been used to

improve the performance of MCBs. These materials outgas (ablate) due to the strong energy

generated by an electric arc[27, 28]. The outgassed chemical species move towards the core of

the electric arc and draws energy from the electric arc, helping effectively to cool down the

temperature of the arc[29]. Besides that, the outgassed chemical species create more pressure

Fixed / moving electrodes PNC / outgassing

polymers

The inductance coil

Extinguishing chamber

Bimetallic strip

4

on the electric arc, which helps to rise the current density of the electric arc by creating Lorenz

forces, helping to push the arc towards arc quenching blades. Under these conditions, it would

be possible to control and interrupt/extinguish the electric arcs quickly and effectively[30].

Based on the detailed understanding of the MCBs, we have noticed the limitation of

materials while controlling the electric arc in order to improve the efficiency. We have

identified the scientific need of improving the controlled outgassing process of polymeric

materials by using the radiative and thermal energy emitted from the electric arc. The aim of

this thesis work is to design composite wall materials (PNCs) by embedding appropriate

nanoparticles into the polymer matrix in order to absorb more energy from the electric arc and

will be utilized in outgassing process to improve the arc interruption process.

Figure 1.3. The engineering schematic of typical MCB shown with working times (Courtesy of ABB, www.abb.com.)

[26].

< 0.5 ms

At 0.5 ms

At 1 ms

At 1.5 ms

At 2 ms

At 10 ms

(a) (b) (c)

(d) (e) (f)

http://www.abb.com/

5

1.2 ENGINEERED NANOCOMPOSITES

According to the Oxford English Dictionary, The term “nano” is introduced from the Greek

etonym nanos meaning “dwarf” and mathematically it means either “billionth part” or “10-9

”.

The term “Nano” is used to refer the fraction/orders of nanometer, nanoscale, nanosecond, or

even nanomaterial (material with dimensions in the order of < 100 nanometers) and etc. In

1959, the physicist Richard Feynman started exploring about Nanotechnology and it

technological advantages when he was working at Caltech. One of his famous lectures titled as

“There’s Plenty of Room at the Bottom” gave a kick start/boost in nano-science and nano-

technology[31, 32].

PNCs can be defined as a combination of a polymer matrix and nanoparticles of a size

where at least one dimension i.e. length, width, or thickness is below 100 nm. In the last

decade, engineering of PNCs are becoming the innovative and smart solutions due to their

ability to tailor or enhance the specific properties of the polymers/materials such as improving

the thermal conductivity, increasing radiative and thermal energy absorption for different

applications. Some key developments are shown in Figure 1.4.

Figure 1.4. The schematic representation of different applications for PNCs.

1.2.1 POTENTIAL ADVANTAGES AND CHALLENGES

There are four generations of PNCs according to the developments in the field of

composites[33]

1st generation: Since 1940’s; Glass fiber reinforced composites fabricated for airplane

wings, helicopter rotors, nose cones

2nd

generation: Since 1960’s; High performance composites is developed after the

launch of the Soviet Sputnik satellite

3rd

generation: Between 1970s and 1980s; The quest for new markets by synergizing

the properties apart from space applications

4th

generation: Since 1990’s; the era of hybrid materials, nanocomposites for

interdisciplinary applications like Automobiles

PNCs

Electrical switching applications

Electrode materials for Li ion batteries

Aerospace and marine applications

http://authors.library.caltech.edu/5456/1/hrst.mit.edu/hrs/materials/public/composites/Composites_Overview.htm#1st_generation

http://authors.library.caltech.edu/5456/1/hrst.mit.edu/hrs/materials/public/composites/Composites_Overview.htm#2nd_generation

http://authors.library.caltech.edu/5456/1/hrst.mit.edu/hrs/materials/public/composites/Composites_Overview.htm#3rd_generation

http://authors.library.caltech.edu/5456/1/hrst.mit.edu/hrs/materials/public/composites/Composites_Overview.htm#4th_generation

6

According to the above division of generations, now we are in the fourth generation where one

can integrate the great developments in PNCs for different applications such as electrical,

automobile applications. Several studies have been performed to develop and optimize the

PNCs in the areas of photonics[34], energy storage[35, 36], polymer LED, solar energy

harvesting[37], optics (or optical applications)[38, 39], automobile[40], food packaging[41],

cosmetics[42], environment[43], military[44], imaging, drug delivery, bio-sensing[45, 46],

cancer therapy[47].

There are several well established methods for the fabrication of PNCs, among which can be

listed the followings[48]:

1. Solution induced intercalation: by solubilizing polymer in a solvent and adding NPs

2. In-situ polymerization: mixing of NPs with liquid monomer and then polymerization

3. Polymer melt intercalation: NPs are directly dispersed into the molten polymers and the

mixture is quenched

There are also several challenges during the fabrication of PNCs, which can be listed as

follows[38]:

1. Homogeneity of dispersion and distribution of NPs

2. Agglomeration

3. Viscosity increase

When inorganic nanoparticles come to the market for fabricating PNCs, there are obvious

questions to answer: “Why nano”? Why not micro?

Nanoparticles exhibit relatively different chemical and physical properties compared to the

micrometer sized counterparts and bulk materials[49]. When materials scale down from bulk

towards nanoparticles, changes in physical and chemical properties are clearly noticed due to

increase of surface to volume ratio shown in Figure 1.5. Recently, NPs attracted significant

research interest for improving the specific properties of composite/hybrid materials. In our

applications, ultra-low quantities (wt %) of NPs in PMMA matrix exhibited variation and

improvement in broad range UV radiation and thermal energy absorption properties.

According to the merits and challenges for engineering of PNCs, the PNCs in this thesis

are fabricated through in-situ polymerization method by combining the polymer i.e. PMMA

monomer solution and inorganic nanoparticles. This approach has been proven successful for

dispersing NPs in PMMA matrix and is also applicable at an industrial scale.

7

1.3 INTEGRATION OF PNCS IN ELECTRICAL SWITCHING

APPLICATIONS

Material science development in the last decades is broadening the great opportunities to

improve electrical switching applications (e .g. breakers, switchgear, furnaces, etc.)[50, 51].

Therefore, it is possible to design new materials with profoundly improved specific properties,

which could be used to optimize the breakers to become more reliable, environmentally

friendly and possibly cheaper. During these studies, we focused mainly on improving the

radiative and thermal energy absorption of PNC.

Figure 1.5. Specific surface area (SSA) as a function of NPs diameter[49].

Our first experimental tests/research demonstrated that PNC can be rather promising

candidates for improving the reliability of the circuit breakers, by controlling the generated

electric arc. Therefore, this thesis is aimed at developing PNCs, which will control the specific

physical process i.e. outgassing (from our experimental tests shown in Figure 1.6) by

integrating carefully designed nanoparticles (various morphology, size and architecture) into

the polymeric matrices to influence the absorption of broadband radiation and thermal energy

emitted by an electric arc. New materials design, absorption and outgassing are at the interface

8

between materials science and engineering of devices, the understanding of which is critical for

further improvements of power devices specifically switching to use newly developed PNCs.

Figure 1.6. Schematic of an experimental test set-up depicting the interaction between the PNCs and electric arcs

generated between the two contacts (a) and a flowchart of energy transfer between them (b).

Outgassing is a spontaneous evolution of chemical species from the PNC due to intense

radiative and thermal energy emitted from an electrical arc. It is also called as ablation. In

Figure 1.6, schematic part is explained as follows. The energy from the electric arc (red

arrows) impinges into the bulk polymer network through NPs where it causes outgassing of the

polymers. The outgassed vapour is injected towards the arc (thin sky blue arrows). In our

experimental test set-up itself, there is a possibility of radiation and outgassed vapour could be

exhausted radially. The vapor is subjected into the arc core, where they have to be heated up in

order to remove the energy from the electric arc, thus improving the arc interruption process. In

our experimental test set-up, quartz and glass slides are placed next to each other to have a

better impact of an electric arc on the PNC.

9

1.4 SCOPE OF THE THESIS

This thesis has three-fold objectives:

1. The fabrication of novel engineered PNCs with appropriate nanoparticles: The

objective includes identifying appropriate NPs (CuO/Fe3O4/ZnO/Au) based on the

physical and chemical properties. It starts with NPs synthesis and surface modification

by using earlier developed energy and resource effective methods. Most effective

fabrication method will be selected and used for PNCs fabrication by embedding NPs

into the PMMA matrix. The PNCs are intended to improve electric arc interruption

process.

2. Comprehensive structural and physicochemical characterization of PNCs: The

synthesized NPs and fabricated PNCs will be characterized in detail to understand their

thermal and radiative energy absorption process.

3. Investigating the effects of the fabricated PNCs on the electric arcs: This involves

designing a test set-up to generate an electric arc of prospective current 1.6 kA. The

electrical measurements unveil the impact of PNCs on the electric arcs, turn to improve

the electric arc interruption process. In addition, the investigation of the surface

morphology of the PNCs will be performed to correlate the outgassing process.

11

2. ELECTRICAL SWITCHING APPLICATIONS

2.1 THE ELECTRIC ARC IN AIR

In electrical switching applications, a low voltage circuit breaker is an automatic switch, which

has been used since 1920’s for protecting the electric grid from fault currents by automatic

interruption. When they interrupt the current, especially high fault currents on the order of

several kA, an electrical arc is generated. This arc produces extreme conditions with

temperatures up to 20 000 oC and pressure nearly of several 10 bars[2]. In order to interrupt the

electrical arc quickly and protect the devices from such extreme conditions, conventionally,

ceramics have been the material of choice due to their resistance to thermal shock, ability to

withstand higher operating temperatures. Afterwards, polymers are used due to their

mechanical and outgassing properties in addition to their influence on the arc interruption

performance of the circuit breakers. In general, when polymers are placed near the electrical

arcs they ablate (is also called as outgassing) due to the interaction of high radiative and

thermal energy releases from the electrical arcs. The outgassed chemical species help to cool

the electrical arcs to improve the arc interruption process effectively[52].

2.1.1 INTRODUCTION TO ELECTRIC ARC

An electric arc is a plasma discharge produced due to ionization/electrical breakdown of

air in between two electrical contacts of a circuit breaker[53]. Such electric arcs are similar to

lightning strikes, northern lights, discharge lamps, welding, and arc furnace.

Generation of an electric arc mainly depends on temperature of the surrounding medium

like SF6, vacuum, air and applied current. Electric arc is generated under a complex process,

which contains highly dense free electrons and ions and continuous ionization and de-

ionization takes place. The generation process of ions and electrons are due to field and

thermionic emission, thermal ionization, photon absorption and secondary emissions from the

surfaces. The nature of an electric arc generation depends on the above processes of supplying

electrons at the cathode, and then how it is confined in air[3, 54-56].

Typical potential between two contacts in MCBs, the higher potential is at the anode and

negative potential is at the cathode side as shown in Figure 2.1. Generally, air gets a small

conductivity continuously due to few electrons generations from the cosmic rays and natural

radioactivity. Such electrons can ionize air molecules by collision, leading to generate a new

12

electron and a positive ion pair moving towards the electrodes due to the potential applied

between them. The applied electric field will accumulate kinetic energy (KE) to move in

between the electrodes i.e.

KE = mv2/2 = eEx,

where E is the applied electric field and x is the distance travelled. In general, energy (U) of the

electron is in electron-volts (eV), where

eU=eEx

and U is the potential applied between the distance (x) of two electrodes. It is noticed that the

electron needs larger electric field (E) in order to accelerate between the two electrodes.

Furthermore, the presence of magnetic field (increases the current density and) radially

constrains the electrons towards center and the arc becomes less diffusive.

Figure 2.1. Typical potential distribution in an electric arc [57].

Other sources of electrons are photon absorption due to photoelectric effect, when light of

sufficiently high energy/short wavelength (hν) falls on electrode surface and releases a photo-

electron. Besides that, when a photon is absorbed by air molecule, an electron and ion pair is

generated. The photon energy needed to release the photo-electron is called the work function.

In case of thermionic emission, the release of electrons due to a heated electrode, can lead to

electric arc discharges. But, the electrode must be heated to incandescence and having lower

work function leads to efficient emission of electrons.

In our case, copper metal surface is knocked by the primary electron to give secondary

electrons leading to the discharge of higher currents. It is noticed in any electric arc discharge,

all the above mentioned processes are dominating one among the other and often explanations

and theories are in subjects of dispute. Therefore, we attempted to explain our tests with

electrical arcs using quantitative results such as I-V measurements.

(II)

(I)

(III)

13

2.1.2 DIFFERENT TYPES OF ELECTRODES

There are two types of electrodes classified as refractory and non-refractory electrodes[58]. A

refractory electrode has the ability to withstand high amount of heat when it gets heated,

therefore emission of electrons are not so easy. It weakens the thermionic emission process

from cathode. They have high melting points and are also called as hot electrodes (e.g.

Tungsten).

A non-refractory electrode has opposite characteristics to refractory electrode. They have

very little thermionic emission when it is heated and also called as cold electrode (e.g. Copper,

Silver). In this case, thermionic ionization is elevated by secondary ionization. Thermionic

emission at cold cathode is a finite value as shown in Figure 2.2.

Figure 2.2 shows the variation of temperature vs. the ratio of vaporized atom to thermionic

electrons of different electrodes. The thermionic electrons emission varies with respect to the

properties of the hot and cold electrode materials studied for electrical switching applications.

Figure 2.2. Computed ratio of the flux of vaporized atoms to the flux of thermionic electrons as a function of cathode

surface temperature of different electrode materials [59].

14

2.1.3 DIFFERENT TYPES OF ELECTRICAL ARCS

In the electric grid, types of electrical arc have been divided based on the electrical power

switching applications as shown in Figure 2.3. Mainly, low pressure arcs are generated and

utilized at the distribution level and high pressure arcs are utilized at the transmission level

breakers[60].

In furtherance, high-pressure arcs are divided into two categories as axisymmetric and

non-axisymmetric. Axisymmetric arcs are produced due to convective-controlled by the gases

produced by the outgassing polymers or gaseous environment such as SF6 or air. Such arcs can

be observed in different (medium and low voltage) circuit breakers. Non-axisymmetric arcs are

also called as cross-flow arcs and such arcs are controlled under Lorentz forces. One can notice

such arcs in auro-puffer and self-pressurizing interruptors, and combination of rotating-arc and

auto-explansion breaker types are under developing stage.

Figure 2.3. Classification of the electric arc [61].

Electric arc

Low perssure arcs

High pressure arcs (> 1 bar)

Axisymmetric

Wall or Outgassing

stabilized/Free burning

Non-Axisymmetric

Rotating/Helical arc

15

2.1.4 ELECTRIC ARC ROOTS AND ARC COLUMN

An electric arc between the two electrodes can be divided into 3 parts i.e. an arc column (I),

cathode region (II) and anode region (III) as shown in Figure 2.4.

Arc column is sandwiched between anode and cathode in a breaker [62]. Arc column has

been characterized by the temperature of ~ 20, 000 K, and current density of ~ 15 kA/mm2 and

pressure of ~ 10 bar. Under these conditions, the air molecules start dissociating into atoms and

ions in air. The atoms and electrons released due to process of recombination and dissociation

of atoms, ions and electrons. Such arc column is either in stable mode i.e. local thermodynamic

equilibrium (LTE) state, the rate of ionization is equal to rate of recombination or unstable

mode when it reaches to non-LTE state. Arc column reaches high conductivity, because most

of the current conduction is due to the electrons as compared to positive ions. In general

electrons tend to have higher mobility than positive ions. In addition, there will be a space

charge layer accumulating near cathode and anode due to the applied voltage between them.

The shape of the arc column is mainly depending on the supplied energy between the

electrodes and the voltage distribution near the electrodes is shown in Figure 2.4. In case of a

high current, pressure and short arc, a large voltage build-up at the anode side and vice versa.

The voltage distribution is sum of the voltages of the cathode, anode and arc column. Arc

voltage gradient depends on the efficiency of cooling in arc column by the outgassed chemical

species and the ambient atmosphere. In general, the arc voltage gradient of an electric arc

depends on several factors such as arc length, arc current magnitude, the ambient atmosphere

and contact materials.

Figure 2.4. Illustration of several regions of typical electric arc[57].

In our experiments, we have used copper electrodes of hemispherical shape. Copper

electrodes have lower evaporation temperature and induced electron emission due to the

applied electric field. Specific shape of the electrode is chosen to initiate electrons from the

center of curvature of hemispherical tip of the cathode towards anode in axisymmetric way and

to confine between electrodes and PNCs.

(I) (II) (III)

16

2.1.5 DISSIPATION OF ARC ENERGY

An electric arc dissipates the supplied energy in several ways such as broad range optical

radiation, conduction and convection [56, 60, 63]. The radiation is in a broad wavelength

regimes of 250-700 nm due to line and continuum radiation, including bremsstrahlung and

excitation/de-excitation processes[28] utilized for outgassing of surrounding PNCs and other

lower wavelengths are absorbed by air. Energy dissipation at higher wavelengths isn’t a

considerable percentile. In case of conduction, generated joule energy is dissipated[7]. It is a

known fact that, infrared wavelength range contributes towards thermal energy transfer.

Besides that, thermal conductivity is mainly due to translation of positive ions, electrons and

availability of high thermal conductivity species such as H2 and CO2. When H2 and CO2 mix

with the electric arc, thermal conductivity will be increased to reach LTE. Such gases are

possible to transfer the energy in the convective mode.

Few researchers published on the dissipated energy distribution in low voltage electric

arcs, 80% of energy in the form of radiation and rest is thermal energy. There are some other

reports on the dissipated energy where 65% of energy is in the form of radiation and 35% in

the form of thermal energy[64].

2.2 ARC INTERRUPTION PROCESS

Since last decade, circuit breaker’s performance has been improved using arc interruption

by outgassing of polymers in air. Nevertheless, such arc interruption process is very complex

and not fully understood till now.

In electric arc interruption process, PNCs start outgassing due to the energy dissipated by

the electrical arcs. During this process, PNCs could have better radiative and thermal energy

absorption via NPs lead to strong outgassing features. Outgassing process can be due to several

physical mechanisms, photochemical, photothermal and thermal processes or combination of

them[65, 66]. All these mechanisms induce outgassed chemical species helping to improve the

arc interruption process.

In the literature, there are different processes proposed to explain the performance

improvement of breakers using the above mechanisms. First, the arc is convectively cooled by

the flow of outgassed chemical species from the polymers[52]. Secondly, outgassed chemical

species change the thermodynamic and transport properties in the arcing environment, which

help to interrupt the arcs effectively[67]. Third, outgassed chemical species could increase the

current density of arc, induced Lorentz force caused to move the arc quickly to arc

splitters[68]. But, all these processes are not well understood due to the complexity involved in

the outgassing process. Our research is motivated to improve the polymers using nanoparticles,

PNCs due to all the above processes.

17

It is worth to mention about few difficulties involved in understanding the complex process are

splitting the broad range of radiation wavelengths ranges contributing for outgassing process,

mix of electrode material erosion, intense joule heating effects, outgassing material properties,

outgassed gases, 3D nature of the process, lack of reliable material database and of course, cost

incurred for these experiments.

19

3. POLYMER-BASED NANOCOMPOSITES

In particular, the combination of polymer and nanosized inorganic particles into a single

material has opened up a new area in materials science that has extraordinary implications in

the development of multifunctional materials. These are considered as innovative advanced

materials, with promising applications expected in many fields, including optics, electronics,

protective coatings, catalysis, sensors, biology, and others.

Among inorganic-organic

composites materials, PNCs are of particular interest since they inherit the properties of the

bulk polymer such as being easily processable, and suitable for cost efficient high-volume

manufacturing, while at the same time introducing new properties of nanoparticles. Significant

scientific and technological interest has therefore focused on polymer based nanocomposites

over the last two decades. [38, 47, 69, 70]

In the last decade, PNCs unveils more inter-disciplinary applications such as

automobiles[40], packaging[41] and bio-medicals[45, 46]. Recent developments in composite

materials science broaden the opportunities to improve electrical power switching components

(e.g. breakers, transformers, contractors, etc.) with optimum performance. Therefore, today it is

possible to design novel materials with profoundly improved specific properties, which could

be used for improving the reliability and efficiency of electrical switching applications. The

generated knowledge form this thesis will be useful for effective design and implementation of

new “tailor-made” materials for the next generation MCBs.

3.1 POLYMERS

Different polymers have been used in several technological applications such as circuit

breakers, arc furnaces, arc heaters, pulsed plasma thrusters, high-density plasma sources[52,

71]. In the last decade, polymers have been widely used to improve the electrical arc

interruption process in electrical switching applications.

3.1.1 WHY THERMOPLASTIC POLYMER?

Several extensive studies are reported on the polymeric materials, screened for arc-

quenching capabilities in breakers and fuses of high breaking fault currents[29, 52, 71-74]. All

such studies show that cellulosic and polyacetal groups are useful for quenching electrical arcs.

These groups are able to generate cooling gases such as H2 and CO for improving the

20

efficiency of breakers[72]. Polytetrafluoroethylene (PTFE) and quartz are known for being an

almost inactive polymer when exposed to electric arcs, used as a reference material[75]. In our

first experimental studies, it is noticed that improvement is achieved by the outgassing

(ablation) of the polymers due to the highly energetic radiation and thermal energy generated

by the electrical arcs. Such electrical arcs are generated between a 5 mm air gap with

prospective current of 1.4 kA in the designed experimental setup. During the early stage of the

project, two commercial polymers i.e. poly (methyl methacrylate) - PMMA (–C5H8O2–) and

polyamide 66 – PA 66 (–C12H22O2N2–) shown in Figure 3.1 were used to investigate the

outgassed chemical species and the arc interruption process.

Figure 3.1. Chemical structure of PMMA and PA66 [76].

3.1.2 PMMA

Since electrical switching application requires the controlled amount of outgassed cooling

vapour, we have done several tests with the polymers with the atomic compositions of C, H, O

and N. Based on our experimental tests reported[77], we have chosen to use poly(methyl

methacrylate) (PMMA) for fabricating PNCs. The release of cooling gases is highly

endothermic, due to outgassing of polymeric material and more energy released prior to an

electric arc interruption.

3.2 NANOPARTICLES

Nanoparticles (NPs) are the most important components in the design of PNCs[78] for the

power switching devices as they can significantly influence the absorption of broadband

radiative energy (from ultraviolet to infrared) and thermal energy from electrical arcs,

improving the outgassing performance of the polymer for extinguishing the arc. In particular,

specific functionality of PNCs can be improved by using NPs[79]. For example magnetic NPs-

polymer nanocomposites are investigated for magnetic and optical applications [38, 79]. In our

work, we are in a need of enhancing the absorption by proper NPs design from families of

dielectric ceramics, magnetic materials, metallic materials and their combinations. These

materials will have different types of energy absorption characteristics based on their particle

size, shape and the NP architecture[80]. Therefore different materials in different absorption

PMMA PA66

21

regions, with various morphologies and in various configurations have been chosen for the

investigation[81].

Several morphologies of NPs including spherical, rod-like, core-shell, Janus

nanostructures shown in Figure 3.2 can be fabricated. It is well known that electromagnetic

absorption[82] of the materials is influenced by their size and morphology, especially in the

case of metallic nanostructures when they are anisotropic. For that reason, nanoparticles of the

same family with various size and agglomeration characteristics can be developed and used for

the fabrication of PNCs.

Figure 3.2. Morphology of NPs (a,b) and architectures (c-e): (a) spherical, (b) rod-like, (c,d) core-shell, (e) janus type.

Methodologies of colloidal synthesis can be adapted from more energy and resource

effective methods to scale up industrial scale, including microwave assisted synthesis, and

solvothermal synthesis, considering the environmental impact of the precursor and waste

materials. Spherical NPs from different materials families (metallic, dielectric, magnetic

compositions as well as their combinations) with a different size population and extent of

agglomeration can be fabricated.

In particular, proposed NPs in this work used for improving the specific functionality of PNCs,

are listed below:

1. CuO NPs: Enhanced thermal conductivity and radiation absorption

2. Fe3O4 NPs: Magnetic, higher thermal stability and controlled outgassing of PMMA

3. ZnO QDs: Enhanced UV radiation absorption

4. Au NPs: Higher thermal conductivity, and visible-infrared radiation absorption

(plasmon absorption)

3.2.1 SURFACE MODIFICATION OF NPS

In order to fabricate high-quality PNCs, surface modification of NPs has to be done[83]

with respect to chosen polymeric matrix i.e. PMMA.

Poly (methyl methacrylate) (PMMA) is a common commercial polymer that fits best for the

function of the matrix for PNCs. To facilitate strong interactions and energy transfer between

the fabricated NPs and the polymeric matrix, the NPs surfaces need to be tailored as shown in

(a) (b) (c) (d) (e)

(a) (b) (c) (d) (e)

22

Figure 3.3. Ideally, surface treatment of the NPs should (i) passivate any dangling bonds to

minimize contributions of unwanted surface states for charge traps; (ii) allow the particles to

form stable dispersions in MMA monomer solvents without particle agglomeration; and (iii)

allow for thermal coupling with a polymer matrix[48, 84, 85]. Thiol, amine, and carboxylic

acid groups coordinate strongly to the surface of the NPs surfaces to be prepared during this

work. Thus, a variety of surface modification schemes are considered based on combining

these different head groups that have affinity for the particle surface with tail groups that are

compatible with PMMA. In this way both the dispersability of NPs in the PMMA matrix and

effective energy transfer of PMMA are possible to tailor.

3.3 FABRICATION OF PNCS

In the literature, there are mainly three methods of polymerization methods discussed and

classification is shown in Figure 3.4. These methods were explained by top-down and bottom-

up approach[38, 47, 48, 69, 86].

Figure 3.3. Illustration of PNCs fabrication followed in this work[87].

In-situ Intercalative Polymerization: This method is first used by Toyota Research Group

to produce polyimide based nanocomposites[88]. Actually, this method is initiated by the

polymer dissolved in a solvent and then NPs are added to it[89, 90]. The solvents such as

hexane, chloroform and toluene have been used according to the polymer and surface modified

NPs. Afterwards, the solvent is evaporated when the NPs are adsorbed by the PMMA matrix

completely.

In-situ polymerization: This method is processed by mixing of surface modified NPs in the

liquid monomer by using ultra-sonication. When it is stabilized, the polymerization is started

either by heat or UV radiation. This method has been successfully tested using nylon–

montmorrillonite nanocomposite, and then extended to thermoplastics [38, 86, 91]. The

Surface modifier

Monomer chains

NPs

(a) (b) (c)

23

advantage of this method is the stabilization/dispersion, covalently bonded between NPs and

PMMA matrix[85].

Melt Intercalation: This method is started at the molten state of polymer matrix and then

NPs are mixed into it[92, 93]. This method has been mechanically processed by using injection

molding and extrusion technique[93, 94]. It is very popular in industrial scale and aimed for

fabricating thermoplastic based nanocomposites. This technique is alternatively used with

respect to other two methods explained above.

Figure 3.4. Classification of fabrication methods of PNCs[61].

PNCs Fabrication methods

Top-down approach

Polymer intercalation in a

solution Melt Intercalation

Bottom-up approach

In-situ polymerization

[Hybrid sol-gel solution ]

Organic precursor [PMMA - MMA

solution ]

Inorganic precursor

[Suface modified NPs]

25

4. EXPERIMENTAL

A wide variety of fabrication methods have been used for NPs and PNCs during the first phase

of the thesis. In the second phase, PNCs have been tested with the electrical arcs to study the

arc interruption process. In both phases, different physiochemical properties have been

analyzed using several characterization techniques on the NPs and PNCs. All of these

techniques were performed at state of the art Nano-Characterization center at Electrum

laboratory, KTH. In the second phase, tests for arc interruption process were carried out at the

Department of Electromagnetic Engineering at KTH. In this chapter, different fabrication

methods of NPs and PNCs, characterization techniques and experimental test set-up used for

arc interruption process are described in detail.

4.1 MATERIALS AND METHODS

In the first phase of the thesis, all the raw materials used in the fabrication of PNCs are

commercially available from Sigma-Aldrich. Specific details are given on the synthesis

methods of the NPs are presented below, followed by the fabrication methods of PNCs.

4.1.1 SYNTHESIS OF NPS

Due to the broad range of our studies, we have synthesized CuO, Fe3O4, ZnO and Au

nanoparticles. The procedures followed for their fabrication is described below.

4.1.1.1 CUPRIC OXIDE NPS SYNTHESIS

Copper(II) acetate monohydrate (1.2g) dissolved in aqueous solution of 300 ml. Acetic

acid (1ml) was added to prevent the hydrolysis and calcination of the above solution until

100°C with reflux, followed by the addition of NaOH (800mg) at 100 °C till pH value reached

6-7. The mixture was then cooled down to room temperature and the black precipitated powder

was filtred, washed once with distilled water and three times with acetone before final drying

at room temperature.

Oleic acid (OA) is used for surface modification of cupric oxide powders and the

procedure is explained as follows: Initially, 35 mg of NaOH was dissolved in 10 ml of distilled

water with mechanical stirring for 10 min. Then, OA (0.3M) is added drop wise to the above

26

solution in order to prepare a solution with pH of 11. Afterwards, 1 mg of CuO nanoparticles

were added to 10 ml of the above solution and mixing was continued at 85 oC for 60 min. After

cooled to room temperature, HCl (1M) was added to the above suspension to remove unreacted

oleic acid and change the pH of the suspension from 11 to 6.

The surface modified cupric oxide nanoparticles were centrifuged, filtered and washed

with acetone for several times. The prepared OA modified cupric oxide powders were

dispersed in hexane before using for fabrication of PNCs[91].

4.1.1.2 IRON OXIDE NPS SYNTHESIS

All the chemicals used in this synthesis are procured from Sigma-Aldrich Chemie GmbH

(Germany). Iron oxide (-Fe3O4-) nanoparticles are synthesized using thermal decomposition

method[85]. This is a two-step method; the first step, iron-oleate is produced using previously

reported method. Afterwards, in the second step, thermal decomposition of 5 mL of iron-oleate

complex was performed in 20 mL of dioctyl ether in the presence of oleic acid. This

decomposition method was initiated in a dual necked flask by refluxing the complex at ca. 297 oC. This method obtained the black color powder, separated with the help of an external

magnet. The black colored powder containing iron oxide nanoparticles are washed three times

by centrifugation with ethanol to remove the byproducts, and then re-dispersing in hexane for

getting a homogeneous distribution of nanoparticles and stored at 4 oC for fabricating PNCs.

4.1.1.3 ZINC OXIDE QDS SYNTHESIS

The synthesis of ZnO QDs[95] were initiated by hydrolyzing zinc acetate dihydrate

(Zn(CH3COO)2·2H2O, ZAD, > 98% ACS reagent, Sigma-Aldrich) with Ethanolamine (MEA,

≥99% ReagentPlus, Sigma-Aldrich) as described by Znaidi et al[96]. The synthesis process

was followed in two steps. In the first step, Zn(II) solution (0.01–0.1 M) was prepared by

refluxing ZAD in ethanol (Solveco) at 85 °C for two hours under continuous stirring. In the

second step, MEA was added to these solutions with the ratio i.e. [MEA]/[Zn(II)] = 3 to obtain

ZnO QDs. The resulting color of the ZnO QDs suspensions varied form transparent to bluish

and yellow depending on the particular concentrations of [Zn(II)] and [MEA].

4.1.1.4 GOLD NPS SYNTHESIS

The typical synthesis process for Au NPs is as follows[97]: 15 mL of hydrochloroauric

acid (10 mg/mL) (HAuCl4, > 99.99%, Sigma-Aldrich) was added into 40 mL of chloroform

(CHCl3, ≥99.9%, Sigma-Aldrich) containing 1.1 g of tetraoctylammonium bromide (TOAB,

[CH3(CH2)7]4N(Br), 98%, Sigma-Aldrich). After 30 minutes of continuous stirring, 120 mL of

1-dodecanethiol (NDM, CH3(CH2)11SH, ≥98%, Sigma-Aldrich) was added to the

aforementioned mixture. Then, 192 mg of sodium borohydride (NaBH4, ≥98%, Sigma-

27

Aldrich) was added to this mixture of solutions in aqueous form to obtain a desired size of Au

NPs in chloroform. Afterwards, the mixture containing synthesized Au NPs was flushed by DI

water three times in separation funnel and the chloroform suspension of Au NP was

concentrated using rotary evaporator and then re-dispersed in hexane for fabricating PNCs.

4.1.2 FABRICATION OF PNCS

In this work, several PNCs are fabricated with different weight percentages (wt %) of

synthesized NPs in methyl methacrylate (MMA) monomer (99 %, Sigma-Aldrich) by in-situ

polymerization. Different wt% of nanoparticles is added to the glass vials containing MMA

monomer (5 mL) separately and wt % of NPs is detailed in chapters of Paper II, IV, V and VI.

Recrystallized 2 2′-Azobis (2-methylpropionitrile) (AIBN, 98 %, Sigma-Aldrich) was added as

an initiator to final monomer solution and well-dispersed by sonication. Afterwards, the

polymerization is initiated in the glass vials at 75 oC in a temperature controlled oil bath for 12

hours. Polymerization process is continued under mechanical stirring, transparent PNCs were

obtained. The schematic flow of the process is shown in Figure 4.1.

Figure 4.1. General schematic of the experimental process followed in the fabrication of PNCs.

4.2 CHARACTERIZATION TECHNIQUES

Several material characterization techniques have been used for the evaluation of NPs and

PNCs at different processing and testing stages. Crystalline structure of the as-synthesized

nanoparticles were studied by x-ray powder diffraction (XRD); their morphology and size was

investigated by transmission electron microscopy (TEM), while surface functionality has been

evaluated by Fourier-transform infrared spectroscopy (FT-IR); Absorption characteristics of

colloidal NP solutions and PNCs have been studied by UV-Vis spectroscopy, while the thermal

characteristics have been investigated using thermal gravimetric analysis (TGA) and

differential scanning Calorimetry (DSC); decomposition products of PNCs have been

identified using TGA system coupled with FTIR. Morphology of the PNCs after exposure to

electric arcs has been studied by using Scanning Electron Microscopy (SEM). Details of each

technique are presented below.

NPs dispersed in a PMMA monomer

Ultrasonication Add initator in-situ

polymerization PNCs

28

4.2.1 X-RAY DIFFRACTION, XRD

The crystalline structures of the NPs before fabricating PNCs were analyzed by using

PANalytical Empyrian X-ray diffractometer (using the source of Cu-kα line wavelength of

1.5405 Å)[98].

Monochromatic X-rays of wavelength of 1.5405 Å are generated by a cathode ray tube

using high voltage source and then incident on the NPs. When electrons in inner shell of NPs

receive sufficient energy, generates characteristic X-ray radiation and collected by the detector

in broad range of angles from 2o to 60

o. The main physics/principle behind this process is the

Bragg’s Law i.e. nλ = 2d sinθ, when the wavelength of incident X-rays with the sample

produces constructive interference with the diffraction angle. One should be able to find lattice

spacing in a crystalline sample, and its crystal structure by using this principle.

4.2.2 TRANSMISSION ELECTRON MICROSCOPY, TEM

The high-resolution microstructure/morphology of the NPs was analyzed by using

Transmission electron microscopy[99], model JEOL FEG JEM 2100F. Most of our TEM

samples were prepared by diluting the NPs in hexane and then drop-casted homogeneously on

the copper grid, later dried under ambient conditions.

The TEM operates based on the same basic principles as the normal light microscope, but

uses electrons instead of light. Electrons are emitted from the electrode using the high voltage

source of 200 kV and passing through vacuum in the column of the microscope. The

electromagnetic lenses are used to confine the electrons into a very thin beam and then passing

through the copper grid. According to sample in the copper grid, some electrons will be

scattered and some disappear from the source beam. The un-scattered electrons are collected

by the fluorescent screen (or nowadays CCD camera), which reveals the morphology of the

NPs.

4.2.3 FOURIER TRANSFORM INFRARED SPECTROSCOPY, FTIR

The surface functionality of the NPs are investigated by using FTIR spectroscopy[100],

(Perkin Elmer Spectrum-One) in the attenuated total reflectance (ATR) mode, by placing the

NPs on the crystal window.

In principle, FT-IR spectrometer analyze and record the interaction between infrared

radiation (wavenumber ranges~500-4000 cm-1

) and a sample i.e. NPs surface, by measuring

the frequencies and their intensity of radiation absorbed by the NPs surface. The absorption

spectrum evidences the number of absorption bands corresponding to functional groups within

the sample, which is then used for the identification of fingerprint regions of molecules and

crystalline materials.

29

4.2.4 UV-VIS ABSORPTION SPECTROSCOPY, UV-VIS

The broad range radiation absorption spectra of nanoparticles and PNCs were recorded using

Cary 100 Bio UV–Vis spectrometer.

Ultraviolet or visible light is absorbed by different molecules in the sample. The

absorbance is directly proportional to the path length travelled with-in the sample (b), and the

concentration of the sample (C).

According to Beer's Law, the absorptivity of the sample i.e. A = ε b c, where ε is molar

absorptivity of the sample. The spectrometer has dual beam-path, passing through the “sample”

cuvettes and background as the ‘Reference’, thus the influence of the background is eliminated

from the resultant spectrum [101].

4.2.5 THERMAL GRAVIMETRIC ANALYSIS, TGA

Thermal degradation of the PNCs was studied using TGA Q500 system (TA Instruments).

The measurements are performed under synthetic air atmosphere with a heating rate of 10 oC/min from 25 to 600

oC [102].

TGA measures % weight change (loss/evaporation) w.r.to. temperature of the PNCs, while

temperature is increased at a controlled rate. These measurements can be done under different

atmospheric conditions such as nitrogen, hydrogen, air or in vacuum. TGA can be interfaced

with FTIR to identify and measure the vapors generated from the PNCs; this is called evolved

gas analysis (EGA).

4.2.6 DIFFERENTIAL SCANNING CALORIMETRY, DSC

Thermal stability and material properties such as melting point, heat capacities are

measured using DSC (Q2000 system from TA Instruments). The measurements are performed

under synthetic air atmosphere with a heating rate of 10 oC/min from 25 to 600

oC [103].

The advantage of using DSC is to observe the physical transformations, or changes, within

the materials, which are not necessarily accompanied by any weight changes. In case of PNCs,

glass transition temperature variation, exothermic and endothermic peaks are important to

evaluate the PNCs.

4.2.7 SCANNING ELECTRON MICROSCOPY, SEM

The morphology of the NPs and PNCs is imaged using scanning electron microscopy

(SEM)[104] (model Zeiss Ultra 55) equipped with field emission gun (FEG) and energy

30

dispersive x-ray analyser (EDX). Imaging can be done without any post processing of NPs as

mentioned for TEM.

The SEM uses intense beam of electrons to interact with the NPs surface, a wide range of signals from electron-NPs interactions. The generated signals will reveal the

information about the sample such as morphology, and chemical composition. In case of

PNCs, the insulating samples were coated with a thin layer of gold/copper by sputtering

process in order to minimize the charging effects during the interactions with the electron

beam.

4.3 EXPERIMENTAL TESTS WITH THE ELECTRICAL ARCS

During the second phase of the thesis, we have tested the PNCs for understanding the arc

interruption process. The experimental setup comprises of the electrical arc discharge circuit

and the test set-up.

4.3.1 ELECTRICAL ARC DISCHARGE CIRCUIT

The discharge circuit as shown in Figure 4.2 is an RLC circuit consisting of a capacitor (16

mF), an inductor (496 μH) for creating a damped oscillation at a frequency of 50 Hz. In

addition, a thyristor switch is connected in series for switching on and off the current [102].

The capacitor is charged to 450 V for all the discharges in order to generate reproducible

electrical arcs with an equivalent peak current of 1.6 kA, approximately. Electrical

measurements such as arc current and voltages are recorded in order to understand the electric

arc interruption process.

4.3.2 EXPERIMENTAL TEST SET-UP

The experimental test set-up is connected in parallel to the discharged circuit and then, the

electric arcs are generated between the two semi spherically shaped contacts (electrodes)

separated by a distance of 5 mm. A thin copper wire of diameter of 20 µm is used for igniting

Vc

Rc Rstray L

Figure 4.2. The high voltage arc discharge circuit containing the power source and test set-up.

Test

se

t-u

p

http://serc.carleton.edu/research_education/geochemsheets/electroninteractions.html

31

the electrical arc. PNCs and non-ablating quartz slide are placed next to the electrodes in this

set-up in order to test the arc interruption process by the PNCs. Detailed image of the test set-

up is shown in Figure 4.3 and in Appendix A.

Figure 4.3. Schematic illustration of the designed test set-up and enlarged view of the specific details of PNC location

and electrode geometry[95].

300mm

10mm

Thick glass slide

Thin quartz slide

PNCs

5mm

33

5. RESULTS AND DISCUSSION

This section is aimed to discuss summary of all the results obtained and presented in the

appended articles. This part has been divided into four sections (as shown in Figure 5.1) based

on the NPs chosen for different PNCs i.e. CuO NPs, Fe3O4 NPs, ZnO QDs and Au NPs. In

each section, we will discuss the results from physiochemical and arc resistant material

characterization of NPs, PNCs and the impact of PNCs on the electrical properties of electrical

arcs (i.e. current and voltage) generated by using designed test set-up at KTH laboratory is

described in detail.

Figure 5.1. General schematic of the experimental process followed in this thesis.

5.1 POLYMERS

It is found that the two polymers showed a clear influence on the arc voltage and current

due to the monomer chemical structure. Two different techniques namely FTIR spectroscopy

and time resolved TGA connected to FTIR spectroscopy are used to identify the dominant

outgassed species such as H2, CO, CO2 produced during the arc interruption process[77]. 3D

and 2D plots of these results are shown in Appendix B: TGA-FTIR spectroscopy. Surprisingly,

the outgassed species were also observed from modelling and simulation works. Furthermore,

the analysis of morphological changes on the outgassed surface of the PMMA and PA66

showed the differences in the outgassing effect. All these results are reported in Paper I.

Synthesis of NPs Suface modification of

NPs Fabrication of PNCs

Validate PNCs impact on the electrical arcs

34

5.2 PNCS WITH CUPRIC OXIDE NPS

CuO NPs are synthesized using precipitation method[91] as detailed in Chapter 2 and Paper

II. TEM image of the as-synthesized CuO NPs shown in Figure 5.2 displaying near spherical

morphology. The average diameter of the NPs was obtained from TEM micrograph as 6±1.3

nm. The SAED pattern of the as-synthesized CuO NPs shows that NPs are crystalline in nature

and the diffraction rings observed matched with the XRD pattern of CuO (Paper II).

Before fabricating PNCs, surface modified CuO NPs were investigated by FT-IR and the

spectra is shown in Figure 5.3. The surface modified NPs with oleic acid (CuO_OA) shows the

peaks of aliphatic monocarboxylates for asymmetric COO-- stretch frequencies at 1545 cm

-1

and symmetric stretch frequencies at 1375 cm-1

instead of –COOH vibration peak at 1710 cm-1

.

Long alkyl chains (–CH, –CH2, and –CH3) peaks were located in between 2750–3000 cm-1

. So,

all these peaks indicate the possible interactions between a carboxylate group and the metal

centre (Cu) in the NPs.

After the surface modification of NPs using OA, PNCs were fabricated with two different

wt% of CuO NPs. A well-dispersed CuO suspension was dispersed in MMA monomer solution

of 5 mL using sonication. Afterwards, the initiator 2,2-azobisisobutyronitrile (AIBN) is added

to the final solution in order to start the polymerization. The polymerization was carried out at

70 oC for 12 h in order to reach the final bulk transparent PMMA/CuO nanocomposites shown

in Figure 5.4. The method of fabrication of such PNCs is also called as in-situ polymerization.

(111)

(a) (b)

Figure 5.2. High resolution TEM image of (a) CuO NPs and (b) SAED pattern[91].

2 1/nm

35

Figure 5.3. FTIR spectra of as synthesized CuO NPs, OA and Oleate modified CuO (CuO_OA)[91].

UV–Vis absorption analysis has been performed on the as-synthesized CuO NPs dispersed in

water, PMMA, PNC_CuO_1 and PNC_CuO_2 detailed in Paper II. Absorption edges of the

PNCs with different wt% of CuO NPs in UV-Vis absorption spectra evidence the improved UV

radiation absorption in PNCs in comparison to bulk PMMA.

In order to understand the impact of PNCs on the electrical arcs, the designed experimental

test set-up was used. The photograph of the test set-up is shown in Paper I in detail such as

electrode shapes, configuration and how the PNCs are placed in the system. Mass loss of the

PNCs are recorded during the tests with the electric arcs, where PNC_CuO_2 has the highest

values of mass loss due to the highest values of arc voltage. The highest mass loss values also

lead to strongest depositions on PNC_CuO_2 compared to PNC_CuO_1 and PMMA as shown

in Figure 5.5.

The electrical signals shown in Figure 5.6 and the arcing time and arc voltage correlates

with the UV radiation absorbance of the PNCs and PMMA. Fresh samples and electrodes are

used during the electrical tests with the electrical arcs. It is noticed that ignition wire is

evaporated at 1 ms and PNC_CuO_2 exhibits relatively high arc voltage which leads to the

limited current and the arc is extinguished about 0.37ms faster than PNC_CuO_1 and PMMA.

There is no significant difference between the electrical signals for PMMA and

PNC_CuO_1[105].

Figure 5.4. Photograph of the PNCs with two different wt% of cupric oxide NPs[91].

PMMA PNC_CuO_1 PNC_CuO_2

0.0017 wt% 0.024 wt%

1 cm

36

Figure 5.5. Mass losses of the CuO-PMMA PNCs during three consecutive tests[105].

Figure 5.6. Electrical signals measured using CuO-PMMA PNCs and PMMA[105].

PMMA

PNC_CuO_1

PNC_CuO_2

Arc

volt

age

(V

)C

urr

ent

(k

A)

37

PMMA PNC_Fe3O4_1 PNC_ Fe3O4_2

1 cm

0.0017 wt% 0.024 wt%

5.3 PNCS WITH IRON OXIDE NPS

Figure 5.7. TEM image of the surface modified Fe3O4 NPs and enlarged image of a single NP in the inset [85].

Iron oxide, Fe3O4, NPs are synthesized using thermal decomposition method as detailed in the

experimental section and Paper IV. High resolution TEM image of surface modified Fe3O4

NPs is shown in Figure 5.7. It is shown that Fe3O4 NPs display nearly spherical morphology,

with average diameter of 11 ± 1 nm. The high-resolution TEM image reveals that NPs are

single crystalline and complemented by the XRD diffraction spectrum shown in Paper IV.

Figure 5.8. Photograph of the PNCs with two different wt% of iron oxide NPs[85].

It is observed that the Fe3O4 NPs are well separated due to their surface functional groups,

allowing homogenous distribution in the PMMA matrix. Afterwards, in-situ polymerization is

used to fabricate PNCs with different wt% of Fe3O4 NPs as shown in Figure 5.8.

10 nm

5 nm

38

TGA and DSC are used to evaluate the thermal properties such as thermal stability, and

thermal energy absorption characteristics of PMMA and PNCs. It is observed that

PNC_Fe3O4_1 has better thermal stability in comparison to PMMA and PNC_Fe3O4_2 as

shown in Figure 5.9. It is also confirmed that the surface modified Fe3O4 NPs and their

homogeneous distribution in the PNC_ Fe3O4_ l lead to enhance its thermal stability,

leading to controlled outgassing process in arc interruption process.

Figure 5.9.TGA (a) and DSC (b) thermograms of PMMA and Fe3O4-PMMA PNCs[85].

(a)

(b)

Wei

gh

t (%

)

Exo up

39

Figure 5.10. Electrical signals recorded during the tests with Fe3O4-PMMA PNCs frontside and backside[85].

The electric arc interruption process of the PNCs is observed using the recorded electrical

signals shown in Figure 5.10. The weights of the samples were recorded by using micro

balance in order to estimate the ablated mass values during the tests. According to the

recorded electrical signals, voltage signals show two different arcing states such as high

voltage and low voltage state. In case of high voltage state, the electric arc is controlled due

to outgassed chemical species lead to buildup voltage, while in case of low voltage state,

the electric arc is much more stable and impact of outgassed chemical species were not

observed. Furthermore, effect of the PNCs and PMMA on the electrical arc is noticed using

the ablated mass values coupled with total dissipated energy as shown in Figure 5.11. In

case of PNC_Fe3O4_1, observed a difference of 61.7 % in ablated mass values between the

front and back sides. It is surprised to notice that front side of the PNC show high

dissipated energy and quick arc interruption, while the amount of outgassed chemical

species is lower. Besides that, trend of these results implies that with increasing wt % of

NPs in PNCs switch the electric arc from high voltage state to low/ more stable voltage

state. More detailed discussion is followed in Paper IV.

40

Figure 5.11.The total dissipated energy (bar charts) and Average ablated mass (solid and dashed lines) measured

during the electric arc and Fe3O4-PMMA PNCs interaction[85].

PMMA PNC_Fe3O4_1 PNC_Fe3O4_2

41

5.4 PNCS WITH ZINC OXIDE QDS

ZnO QDs are synthesized using thermal decomposition method [31]. TEM micrograph

shown in Figure 5.12 reveals nearly spherical NPs with particle diameter of 3.5±0.6 nm,

detailed in Paper V.

Figure 5.12. TEM micrograph of ZnO QDs and inset high resolution image of a single crystalline ZnO QD[95].

In the process of fabricating PNCs, FT-IR spectrum shows the surface functional groups of

ZnO QDs. It is noticed that MEA helps to avoid agglomeration between the ZnO QDs due

to the functional groups revealed by respective absorption regimes, such as C=O stretching

(1748 cm-1

), -CH, -CH2 and –CH3 stretching (2750-3000, 1382 cm-1

) and -NH (3300 cm-1

),

respectively.

Afterwards, six PNCs are fabricated by using in-situ polymerization process with

different wt% of QDs as shown in Figure 5.13, named based on the increasing NP content

as PNC_ZnO_1, PNC_ZnO_2, PNC_ZnO_3, PNC_ZnO_4, PNC_ZnO_5 and

PNC_ZnO_6.

The optical absorption spectrum of PMMA and PNCs are demonstrated by using UV-

Vis spectroscopy. The absorption edges of PNCs shown in Figure 5.14 are increased

systematically and stepwise from PMMA till PNC_ZnO_5, while decreased for

PNC_ZnO_6. The absorption edge values are recorded for PMMA and all PNCs are in

order of 270 nm, 278 nm, 279 nm, 284 nm, 288 nm, 338 nm and 306 nm respectively. It is

noticed that the inversion in the absorption edge for PNC_6 due to the increased wt % of

QDs over percolation threshold level in PMMA matrix. Besides that, the trend of

fluorescence intensity of the samples shown in Figure 5.13 following the UV absorption

edges revealed from UV-Vis spectrum.

Figure 5.13. Photograph of the fabricated PMMA and the ZnO-PMMA PNCs under low intensity UV lamp with

different wt % of ZnO QDs[95].

5 nm

2 nm

0.01 wt %

0.025 wt % 0.05 wt % 0.075 wt % 0.1 wt % 0.25 wt %

5mm

PMMA PNC_ZnO_1 PNC_ZnO_2 PNC_ZnO_3 PNC_ZnO_4 PNC_ZnO_5 PNC_ZnO_6

Figure 5.14. UV-vis absorption spectra of all the ZnO - PMMA PNCs and PMMA[95].

The electric arc interruption process of PNCs is analyzed using the experimental test set-up

detailed in Paper V. It is understood that the arc interruption process is statistical in its

nature and few conclusions are drawn from the electrical signals shown in Figure 5.15 and

5.16. The electrical signals show peaks at 1 ms due to the ignition wire evaporation. As we

observed in iron oxide based PNCs, the electrical signals clearly depict the two states of the

electrical arc such as high voltage (HV) and low voltage (LV) state. In case of HV state, the

voltage starts to building up in the electrical arc due to the presence of ablated chemical

species. It is noticed that for low wt % of QDs in PMMA got the improved UV radiation

absorption lead to reach high voltage state, a further increase of QDs wt% turn to reduce

UV radiation absorption and the electric arc moved from HV state to LV state. Such

transition is observed during the tests with both front and backside.

Figure 5.15. Electrical signals during the tests with the ZnO-PMMA PNCs backside[95].

42

43

Figure 5.16. Electrical signals during the tests with the ZnO-PMMA PNCs frontside[95].

The total dissipated energy coupled to the ablation rate of the electric arc as shown in

Figure 5.17 in order to understand the electric arc interruption process of the PNCs and

PMMA. It is shown that pure PMMA outgassed low amount of chemical species towards

the electrical arc, revealing a number of fast varying voltage spikes in the electrical signals

shown in Figure 5.15 and 5.16. In case of frontside and backside tests, PNCs up to PNC_5

show higher dissipated energy and better arc interruption process due to lower ablation

rates. The variation of ablation rates varies in a non-monotonous way and correlated to the

UV absorption edge w.r.to wt% of ZnO QDs in PMMA matrix. It is also observed that

variation of ablation rates is same for both sides of PNCs and PMMA. Detailed information

is provided in Paper V.

Figure 5.17. Total dissipated energy of the electric arc (bar plot) and Ablation rate (solid and dashed lines) of the

ZnO QDs –PMMA PNCs [95].

PMMA PNC_ZnO_1 PNC_ZnO_2 PNC_ZnO_3 PNC_ZnO_4 PNC_ZnO_5 PNC_ZnO_6

44

5 nm 2 nm

(a) (b)

5.5 PNCS WITH GOLD NPS

Figure 5.18. TEM images of the Au NPs at low (a) and high (b) magnification[105].

Au NPs are chosen to improve the thermal conductivity of PNCs and to see the arc

interruption process. In general, these PNCs are aimed to withdraw more thermal energy

from the electrical arcs, leading to improve the outgassing nature of PNCs using Au NPs.

Morphology of synthesized Au NPs is studied by using the TEM image shown in

Figure 5.18. NPs are spherical, crystalline and have an average diameter of about 2.75 ± 0.4

nm. Homogeneous distribution of NPs in the PMMA matrix is critical for fabrication of

PNCs, surface modification of NPs is necessary to have better interfacial interaction with

the polymeric monomer chains. The surface modified Au NPs are analyzed using FT-IR

spectroscopy, several functional groups such as methyl and carboxyl groups are exhibited

on the Au NPs. More detailed explanation about FTIR spectrum is presented in Paper VI.

PNCs using Au NPs are fabricated using in-situ polymerization process. The PNCs are

named with respect to increased wt% of the Au NPs as PNC_Au_1, PNC_Au_2,

PNC_Au_3, PNC_Au_4, PNC_Au_5 and PNC_Au_6, as shown in Figure 5.19. Besides

that, PMMA is fabricated without NPs for the reference sample by the same polymerization

process.

Thermogravimetric analysis of the PNCs is done to understand the

degradation/decomposition process w.r.to wt% of NPs. The TGA thermograms reported in

the Paper VI and discussed about thermal stability and extent of outgassing of the PNCs.

Figure 5.19. Photograph of the PNCs with different wt% of Au NPs[105].

5 mm

PMMA PNC_Au_1 PNC_Au_2 PNC_Au_3 PNC_Au_4 PNC_Au_5 PNC_Au_6

45

The electrical signals presented in Figure 5.20 show only one state of the electric arc using

PNCs i.e. high-voltage state and no other state is observed. Nevertheless, it is noticed that

PNCs using ZnO QDs and Fe3O4 NPs observed the two states i.e. high-voltage and low-

voltage state. It could be due to wt% of Au NPs and high molecular weight of Au NPs

compared to ZnO QDs in the PNCs. More details are reported in Paper VI.

Figure 5.20. Electrical signals recorded during the tests with the Au-PMMA PNCs frontside (a); backside (b) [105].

(a)

(b)(b)

PMMA

PNC_Au_1

PNC_Au_2

PNC_Au_3

PNC_Au_4

PNC_Au_5

PNC_Au_6

vdo

Stamp

vdo

Rectangle

46

The electric arc interruption process of the PNCs using Au NPs and PMMA is explained

using total dissipated energy and ablated mass values, shown in Figure 5.21. PMMA

outgassed lower amount of chemical species and evidenced by fast varying voltage spikes.

In case of frontside and backside tests, all PNCs with ultra-low wt% of Au NPs showed

higher dissipated energy turned to improve arc interruption process. The ablated mass

values followed similar trend on the both sides of PNCs except for PNC_Au_3. The

comparison between Au and ZnO based PNCs, detailed information is reported in Paper

VI.

Figure 5.21. Total dissipated energy of the electric arc (bar plot) and Ablated mass (solid and dashed lines) of the

PNCs made of Au NPs[105].

PMMA PNC_Au_1 PNC_Au_2 PNC_Au_3 PNC_Au_4 PNC_Au_5 PNC_Au_6

47

6. Conclusions

In conclusion, few series of bulk homogeneous and transparent PNCs are successfully

fabricated with different low wt % NPs loadings by using an in-situ polymerization process.

Different loading of wt% of NPs in PMMA matrix is chosen based on their chemical and

physical properties. Our preliminary experimental tests were started with CuO NPs, then

followed towards Fe3O4 NPs, ZnO QDs and Au NPs.

The PNCs are fabricated, characterized in detail for the morphology of NPs, interaction

between NPs and polymer matrix monomer chains. Several physiochemical

characterizations were performed on the PNCs and NPs have been performed to assess their

radiative absorption and thermal properties. Such material properties are essential to

understand the electric arc interruption process.

The electric arc interruption process of the PNCs is observed using the designed test

set-up, electrical signals, ablation rate are recorded. Based on the observations with all the

PNCs, the electrical signals showed two states of electrical arc such as high voltage and low

voltage state. In the high voltage state, the voltage of the electric arc starts to build-up due

to the presence of outgassed chemical species. According to our understanding, the

outgassed chemical species move towards core of the electric arc causes a drop-down the

temperature; turn to reduction of the electrical conductivity and rise in arc voltage. This

analogy is characterized by current limited/controlled electric arc, less back-ignition and

spikes in voltage signals. In low voltage state, the outgassed chemical species don’t lead to

build-up the arc voltage and vice versa. For example, as the wt% of QDs is increased in the

PMMA matrix (shown in Paper V), electric arc undergoes a smooth transition from a high

voltage to a low voltage state.

In addition to that, the ablated mass values of PNCs vary in a non-monotonous way

and are correlated to the trend of UV absorption edges, thermal degradation properties.

Furthermore, the total dissipated energy i.e. ∫ 𝐈(𝐭)𝐕(𝐭)𝐝𝐭 values and ablation mass values

are coupled to see the trend of tests with PNCs frontside and backside. Electron microscope

images of the outgassed signature presented for all the PNCs, the variation in blister size

and erosion density observed w.r.to different wt% of NPs addition in the PMMA matrix.

Specific conclusions on different PNC compositions achieved in various articles are

summarized below:

48

Paper I was designed to understand basic polymer-electric arc interactions using two

polymers of PMMA and PA66, which demonstrates the effect on the electric arcs of current

1.6kA using electrical measurements such as the arc voltage and current. We have

understood the effect of chemical composition of the monomer on the electrical arcs.

Furthermore, surface morphology of the outgassed polymers is imaged and interpreted,

revealing differences of outgassing process.

Paper II demonstrated a successful route to the synthesis of CuO NPs and fabrication of

CuO-PMMA PNCs. The range of UV absorption of the PMMA has been broadened due to

the presence of CuO NPs. Paper III demonstrated electric arc interruption measurements,

and correlation of recorded electrical signals of arc with UV radiation absorption of PNCs.

Paper IV was designed for investigating the interactions of magnetic nanoparticle

inclusions with the arc. A highly thermally stable Fe3O4–PMMA PNCs were fabricated

and observed their impact on the electric arc. The result showed a significant influence of

ultra-low wt% of Fe3O4 NPs on the state of electric arc interruption process.

Paper V focused on strong UV absorbing PNCs fabricated by the inclusion of ZnO QDs in

PMMA matrix. The UV radiation absorption edge of PNCs showed red-shift by increasing

content of ZnO QDs. The experimental findings from the tests with both sides of PNCs

show two states of arc interruption process, which are reliable and reproducible.

Paper VI focused on enhanced radiation and thermal energy absorption by introducing a

metallic component in PMMA matrix. Therefore, Au-PMMA PNCs were fabricated to

enhance thermal energy transfer and visible radiation absorption of PNCs. We have

observed important experimental insights during the electric arc interruption process using

Au-PMMA PNCs. Furthermore, a critical comparison of PNCs containing ZnO QDs and

Au NPs revealed a superior electric arc interruption process with enhanced radiative

transfer in case of ZnO QDs-PMMA PNCs.

These experimental insights gained from our industrial collaborators suggested that

these PNCs can be used to replace the traditional polymers in the circuit breakers,

which will reduce the arc interruption time and improve the efficiency and safety of

the high power electrical switching applications.

These are the first experimental investigations with PNCs used for interrupting the

electrical arcs generated in breakers. The electric arc interruption process of the

nanocomposites is investigated using the specially designed test set-up with electrical

measurements. In particular, a clear influence of the nanoparticles loading on the state

of the electric arc interruption in breakers. In conclusion, the reported experimental

insights and results showed that the nanocomposites with tunable nanoparticle

loadings are useful for low power electrical switching applications.

49

7. FUTURE WORK

The purpose of this thesis work was to engineer present polymeric materials by using

developments in material science, in particular towards polymer based nano-composites

(PNCs). The developed PNCs should be able to replace the present insulating, ceramic

materials in power switching devices (such as circuit breakers, contactors, fuses) in order to

protect from the electrical arcs and improve the efficiency and reliability.

This work can be further developed in number ways and possible ideas are mentioned

below:

1. According to current advancements in computational power, developing a

theoretical model to broaden the knowledge on PNCs and electric arc interactions is

necessary to improve predictive power of arc interruption process

2. Theoretical and experimental studies on developing PNCs properties such as UV

radiative absorption and thermal properties

3. Engineering of insulating materials using newly developed core-shell NPs and other

morphology of NPs and their impact on the electric arc interruption process

4. Statistical analysis of electric arc interruption process using PNCs can be studied

5. Perform optical emission spectroscopy and high speed imaging studies to better

understand PNCs and electric arc interactions

6. Integrate mass spectroscopic studies to analyze and quantify outgassed chemical

species during electric arc and PNCs interaction

51

APPENDIX A:

A: DRAWING OF THE EXPERIMENTAL TEST SET-UP

Figure A.1. Drawing of an experimental test set-up connected to arc discharge circuit to generate the electrical arcs

between the two contacts.

53

APPENDIX B:

B: TGA-FTIR SPECTROSCOPY – PA66, PMMA

Figure B.1. TGA-FTIR spectrum of PA66 sample in 2D (a) and temperature resolved 3D (b).

(a)

(b)

54

9

Figure B.2. TGA-FTIR spectrum of PMMA sample in 2D (a) and temperature resolved 3D (b).

(a)

(b)

55

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