The Characterisation and Synthesis of Hafnium diboride for Aerospace Applications

Final Report

The Characterisation and Synthesis of Hafnium diboride for hypersonic flight applications

Kishan Desai

2015 3rd Year Individual Project

I certify that all material in this thesis that is not my own work has been identified and that no material has been included for which a degree has previously been conferred on me.

Signed..............................................................................................................

College of Engineering, Mathematics, and Physical Sciences University of Exeter

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Final Report ECM3101/ECM3102/ECM3149

Title: The Characterisation And Synthesis Of Hafnium Diboride For Aerospace Applications

Word count: 8754 Number of pages: 40

Date of submission: Wednesday, 29 April 2015

Student Name: Kishan Desai Programme: MEng Mechanical Engineering

Student number: 620027268 Candidate number: 004466

Supervisor: Professor Shawoei Zhang

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Acknowledgements

I would like to thank my mentor Professor Shaowei Zhang for providing me with the opportunity to take part in an exciting research project about high temperature ceramics. Thank you for guiding me throughout the project.

Thank you Juntong Huang and Ben Jun Chen for assisting me in the laboratory and for helping me understand many different aspects of this project.

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Abstract In this experiment Hafnium diboride (HfB2) was successfully produced by borothermal

reduction. Molten salt synthesis was the synthesis technique that was used and the

temperatures of the furnaces in each experiment varied between 1100°C-1300°C. Boron

dioxide and magnesium were used as reducing agents and hafnium dioxide was used as the

source for hafnium. The powders were characterised using a scanning electron microscope, a

transmission electron microscope, energy-dispersive x-ray spectroscopy and x-ray diffraction.

The results showed that the optimal sintering conditions was 1300°C with a hold time of 12

hours. At a temperature of 1200°C and a hold time of 6 hours, phase pure HfB2 powder was

created that varied in size between 0.5µm and 2.5µm, with very little HfO2 remaining in the

sample.

Keywords: Molten Salt synthesis, Characterisation, Hafnium Diboride (HfB2), X-ray Diffraction (XRD), Scanning electron microscope (SEM), Transmission electron microscopy (TEM), Ultra-High Temperature Ceramics (UHTC)

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Table of Contents 1. Introduction and background ...................................................................................... 1

1.1. Aim .................................................................................................................................. 1 1.2. Objectives ....................................................................................................................... 2 1.3. Evolution of Project ......................................................................................................... 2

2. Literature review ........................................................................................................ 3 2.1. Material Properties of HfB2 .............................................................................................. 3

2.1.1 Thermal Properties .............................................................................................................. 3 2.1.2 Melting Point ....................................................................................................................... 3 2.1.3 Oxidation Resistance ............................................................................................................ 4 2.1.4 Strength Properties .............................................................................................................. 4 2.1.5 Density ................................................................................................................................. 4 2.1.6 Molecular properties ........................................................................................................... 4

2.2. An Introduction to Molten Salt Synthesis ......................................................................... 5 2.3. Previous Research and Synthesis Techniques ................................................................... 5

2.3.1 Borothermal/carbothermal Reduction ................................................................................ 6 2.3.2 Unconventional method (Sol-‐Gel Synthesis) ....................................................................... 6 2.3.3 Molten salt synthesis ........................................................................................................... 6

3. Methodology and theory ............................................................................................ 6 3.1. Reactions ......................................................................................................................... 6

3.1.1 Borothermal reduction ........................................................................................................ 7 3.1.2 Salt Wash ............................................................................................................................. 7 3.1.3 Acid Rinse ............................................................................................................................. 7 3.1.4 Raw materials ....................................................................................................................... 7

3.2. Research .......................................................................................................................... 7 3.3. Sintering and Molten salt synthesis ................................................................................. 8

3.3.1 An Introduction to Sintering ................................................................................................ 8 3.3.2 Sintering Process and description ........................................................................................ 8 3.3.3 Preparation for Molten Salt synthesis ................................................................................. 8

3.4. X-‐Ray Diffraction ............................................................................................................. 9 3.4.1 Preparation of the samples for XRD ..................................................................................... 9

3.5. SEM ................................................................................................................................ 10 3.6. EDS ................................................................................................................................. 10 3.7. TEM ................................................................................................................................ 10

4. Experimental work .................................................................................................... 10 4.1. Procedure ....................................................................................................................... 10

4.1.1 Preparing each sample ....................................................................................................... 10 4.1.2 Heating the samples .......................................................................................................... 11 4.1.3 Washing the salt ................................................................................................................. 11 4.1.4 Acid Rinse and Dry ............................................................................................................. 12

4.2. Calculating the mass of each reactant in the sample ....................................................... 12 4.2.1 Example calculation ........................................................................................................... 12

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4.2.2 Volatility of reducing agents .............................................................................................. 13

5. Results ...................................................................................................................... 13 5.1. Experiment 1 (Samples 1,2,3) ......................................................................................... 13

5.1.1 Initial Conditions ................................................................................................................ 14 5.1.2 Observations ...................................................................................................................... 14 5.1.3 XRD Results ........................................................................................................................ 14

5.2. Experiment 2 (Samples 4,5,6) ......................................................................................... 15 5.2.1 Initial Conditions ................................................................................................................ 15 5.2.2 XRD Results ........................................................................................................................ 16

5.3. Experiment 3 (Samples 7,8,9) ......................................................................................... 17 5.3.1 Initial Conditions ................................................................................................................ 17 5.3.2 XRD Results ........................................................................................................................ 17

5.4. Experiment 4 (10,11,12) .................................................................................................. 18 5.4.1 Initial Conditions ................................................................................................................ 18 5.4.2 Notable Observations ........................................................................................................ 19 5.4.3 XRD Results ........................................................................................................................ 19

5.5. Experiment 5 (13,14,15,16) ............................................................................................. 20 5.5.1 Initial Conditions ................................................................................................................ 20 5.5.2 XRD Results – Pre Acid Wash ............................................................................................. 21 5.5.3 XRD Results -‐ Post Acid Wash ............................................................................................ 23 5.5.4 Pre –acid conclusion .......................................................................................................... 25 5.5.5 Post acid conclusion ........................................................................................................... 25

5.6. XRD Conclusion .............................................................................................................. 26 5.7. SEM, TEM and EDS .......................................................................................................... 26

5.7.1 SEM and EDS Analysis ........................................................................................................ 26 5.7.2 TEM Analysis ...................................................................................................................... 27

6. Discussion and conclusions ....................................................................................... 29 6.1. Summary ........................................................................................................................ 29 6.2. Optimal Conditions ......................................................................................................... 29

6.2.1 Temperature and Hold time .............................................................................................. 29 6.2.2 Reactants ratio ................................................................................................................... 29

6.3. Further research ............................................................................................................. 30

7. Project management, consideration of sustainability and health and safety ............. 30 7.1. Project Management ...................................................................................................... 30 7.2. Considering the Environment ......................................................................................... 31 7.3. Health and Safety Risk Management .............................................................................. 32

References ....................................................................................................................... 33

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1. Introduction and background Ultra high temperature ceramics have been around since the early 1960’s and have been

identified as a possible material for the leading edges of hypersonic jets and atmospheric re-

entry vehicles. With hypersonic jets potentially travelling at speeds of up to 5 times the speed

of sound (in air), a lot of research needs to be conducted now in order for the vehicles to

operate safely in the future. At the stagnation region over the leading edges of these jets, the

temperature is expected to reach over 2000°C in some cases. In these extreme conditions

there will be heavy oxidisation and there are very few materials, which are able to operate

normally in such high temperatures and yet remain structurally stable, resist oxidisation and

conduct the heat quickly away from the leading edges. [1-9]

Hafnium diboride (HfB2) belongs to this family of materials classified as UHTC and it has

been described as a potential covering material for the leading edges of hypersonic flight and

atmospheric re-entry vehicles. These ceramics such as Hafnium diboride and Zirconium

diboride (ZrB2) are possessed with good thermal and electrical conductivity, good chemical

inertness, high melting points and high strength. As a pure substance it has shown poor

oxidation resistance at high temperatures, but studies have shown that its oxidation resistance

will increase if it is fused with another composite such as SiC or TaSi2. [1-10,12]

Hafnium diboride is one of the most exciting high temperature ceramics and there has already

been a large amount of research conducted on this material in the last 2 decades or so. It has

been synthesised using many different reduction routes, but mostly by borothermal reduction.

It has been produced using several different manufacturing techniques, including spark

plasma sintering and the sol-gel method. However very little research has been conducted on

the molten salt synthesis of HfB2 using boron dioxide as a reducing agent.

1.1. Aim

The aim of this is experiment is to:

1. Synthesize a pure ceramic powder of HfB2

2. Characterise the ceramic powder using various characterisation techniques

This experiment used a synthesis technique called molten salt synthesis. This means that

reaction took place in a pool of molten salt. This particular experiment looked at the

possibilities of high temperature molten salt synthesis using boron dioxide as the reduction

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route. High temperature molten salt synthesis occurs in the temperature range of 1050°C to

1350°C. This is because the salts used in this experiment (NaCl and KCl) both boil at around

1420°C.

This experiment started the synthesis at 1200°C and increased to 1300°C before decreasing

back down 1100°C.

1.2. Objectives

The key objectives of this experiment was to:

1. Locate the optimal temperature and hold time of the furnace for the high temperature

synthesis

2. Investigate the effects of changing the composition of the reducing agents and

reactants

3. Check if increasing the hold time would affect the purity of the finished ceramic

powder

4. Identify the average particle size of the purest sample of HfB2 produced

5. Investigate the effects of the acid wash by doing an XRD analysis on both the pre-

wash and post acid wash for an experiment

Personal Objectives of this experiment was to:

1. Gain a clear understanding of sintering processes’ and in particular, Molten salt

synthesis

2. Understand how to interpret the results of the main types of characterisation

techniques including SEM, TEM, EDS and XRD

3. Become familiar with working in a laboratory,

4. Become aware of laboratory safety as well safety of the environment

1.3. Evolution of Project

Initially the main aim of this project was to produce an extremely pure bulk sample of HfB2

through molten salt synthesis. The sample would be characterized using several techniques

such as XRD, SEM and TEM. Once the optimal conditions of the material had been found,

the next part of the project would be to combine this pure Hafnium diboride with Silicon

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Carbide and test out the mechanical properties of this (such as oxidisation rate, density and

strength).

However the aim of this experiment changed slightly as it was understood that the initial

objectives were a little too ambitious, and the new focus was to try to optimize the reaction

conditions for MSS of phase pure HfB2.

2. Literature review

2.1. Material Properties of HfB2

2.1.1 Thermal Properties

Hafnium diboride and other ultra high temperature ceramics possess extremely good thermal

conductivity. This is advantageous because the leading edges of the hypersonic jet will heat

up due to wind resistance. So a material with good thermal conductivity is required because it

will be able to conduct the heat away from the leading edges to the cooler regions of the

aircraft.

Hafnium diboride has a thermal conductivity of 104WmK-1 and this is significantly higher

than ZrB2 a, which has a thermal conductivity 25-56WmK-1. The thermal conductivity of

HfB2 is higher than most pure metals and similar to the aluminium alloys that are currently

used to build the airframe of commercial jets. It has a relatively low co-efficient of thermal

expansion (6.3x10-6K-1) [1,3-6,15]

2.1.2 Melting Point

During hypersonic flight, the leading edges of the aircraft will experience temperatures of

over 2000°C and therefore a key characteristic of a potential material is a high melting point.

Hafnium diboride’s melting point exceeds this temperature and will start to melt at 3380°C.

This is higher than ZrB2 (which melts at 3200°C) and this is because the bonds, which hold

the atoms together, are extremely strong and require a large amount of energy to break it. [1-

6,15]

a ZrB2 is another UHTC that is being researched as a possible material for the leading edge of hypersonic jets [9]

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2.1.3 Oxidation Resistance

The one problem with hafnium diboride is that at high temperatures (1000°C -1300°C) it will

begin to oxidise very quickly. At this temperature, the hafnium separates from the boron and

reacts with the oxygen in the air to form a thin HfO2 layer on the surface of the HfB2 layer.

However studies have shown that combining HfB2 with other composites can greatly increase

its oxidation resistance. [1,14,15,17]

2.1.4 Strength Properties

Hafnium diboride possess good strength properties, it has been known to have a bending

strength between 300-500MPa. It is also an extremely hard material, with a Vickers hardness

ranging between 20-28GPa. [15]

Hafnium diborides ability to mix with other composites and increase its strength has also

been proven. For example Sciti et al. showed that by adding as little as 15% (by volume)

TaSi2 to an ultra high temperature ceramic, the flexural strength will be 600MPa even at

elevated temperatures of up to 1500°C. This proves that HfB2, when combined with other

composites, shows good thermal stability. [8]

2.1.5 Density

One of the major negative points of hafnium diboride is the high density of the material.

Hafnium diboride has a density of 10.5g/cm3. This is much higher than other composites and

metal alloys which make up most of todays airframes in jets. For example, Carbon Fibre

Reinforced Plastic has a density of 1.9g/cm3, and most aluminium alloys have densities

ranging from about 2.1g/cm3–3g/cm3. ZrB2 has a much lower density, which is

approximately 6g/cm3. [13,21]

2.1.6 Molecular properties

Hafnium diboride is a ceramic that has an extremely strong covalent bond. This strong

covalent bond makes the molecule structurally stable at high temperatures. It also means that

the molecule will require a large amount of energy to weaken the bond and as a result it has a

high melting point. The atoms are arranged in a crystal lattice and this is the reason why it

has good thermal and electrical conductivity. [15]

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2.2. An Introduction to Molten Salt Synthesis

Molten salt synthesis is a synthesis technique that used in manufacturing, to produce a

material at elevated temperatures, with molten salt used as medium for the reaction. Normally

the powders are placed in a non-reactive crucible (carbon) and heated, in a furnace, to a

temperature above the melting point of the salts used. Salt is used as a catalyst in this process

because when it melts, it forms an ionised liquid that helps speed up the reaction time. [7,27]

In molten salt synthesis, normally the salt content ratio to reactants is large. This allows a

pool to form around the reactants and allow them to dissolve and react accordingly. When the

hold time at an elevated temperature is completed, the furnace will begin to cool at a set rate.

This rate can also determine the mechanical properties of the finished product. The salt

solidifies separately from the other reactants and can be easily washed out with water. In

molten salt synthesis, this salt that is washed out can be reused and this is one of the most

important advantages of the MSS method if HfB2 is to be mass-produced industrially.

The role of the molten salts as said by Kimura is [7]:

• To increase the reaction rate and lower the reaction temperature

• To control particle size

• To control particle shape

Molten salt synthesis usually occurs at temperatures in the range of 800°C to 1300°C. This is

because that is the range of melting and boiling points of the salts used. NaCl Melts at 801°C

and boils at 1465°C. KCl melts at 770°C and boils at 1420°C. [19,20]

2.3. Previous Research and Synthesis Techniques

Hafnium diboride has been synthesized in many different ways but mostly by the following

methods:

• Solid-State reduction

• Spark Plasma sintering

• Boro/Carbo thermal Reduction

• Sol-Gel Synthesis

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2.3.1 Borothermal/carbothermal Reduction

HfB2 has previously been synthesised by both borothermal reduction. This method involves

the use of a boron based (sometimes there can be a carbon based) molecule as a reducing

agent in the reaction.

Phase pure hafnium diboride (HfB2) powder has been produced by borothermal reduction of

hafnium dioxide using amorphous boron at relatively low temperature (1600°C) in vacuum.

The synthesized HfB2 powder had an average particle size of 1.37 µm. [14]

Wei-Ming Guoa, Zhen-Guo Yang and Guo-Jun Zhang produced a sub micrometre powder of

HfB2 using borothermal reduction. They produced the powder using a conventional

borothermal route and the average particle size produced was 2.1 µm. In the same experiment

they tested a new borothermal reduction method and managed to produce a fine powder that

had an average particle size of 0.8µm [6]. In another experiment done by Guoa et al., a

hafnium diboride powder was produced that was 1-1.5µm in size. It was synthesised by

borothermal reduction using HfO2 and amorphous boron in a vacuum at 1600°C. [21]

2.3.2 Unconventional method (Sol-‐Gel Synthesis)

Hafnium diboride (HfB2) powder has been synthesized via a sol–gel-based route by

Venugopal et al. using phenolic resin, hafnium chloride, and boric acid as the source of

carbon, hafnium, and boron, respectively. The temperature in this reaction was much higher

than the temperatures used in molten salt synthesis. This method would not be suitable for the

large-scale production of Hafnium diboride. The costs of the raw materials that are used in

this experiment are much higher than the costs of the reactants in this molten salt synthesis

route. Also some of the raw materials are harmful to the environment. Particles of 1.2µm

were achieved at when the pre cursor was heated at 1600°C for 2hrs. [16]

2.3.3 Molten salt synthesis

Almost no research has been published on this technique of the synthesis of Hafnium

diboride. Toshio Kimura conducted research on the “molten salt synthesis of ceramic

powders”, but hafnium diboride was not used in that experiment. [7]

3. Methodology and theory

3.1. Reactions

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3.1.1 Borothermal reduction

In this project, Hafnium diboride was synthesised using the following reaction process:

𝐻𝑓𝑂! + 𝐵!𝑂! + 5𝑀𝑔 !"#$!!"#

𝐻𝑓𝐵! + 5𝑀𝑔𝑂 (1)

The Mg acts as a reducing agent in the redox reaction. The Mg reduces the Oxidising agent

(HfO2 & B2O3) to become Magnesium oxide (MgO). The reaction takes place in a pool of

molten salt (KCl + NaCl). All the raw materials in this reaction are in a powder form.

3.1.2 Salt Wash

After the reaction takes place in the furnace and it is removed, the salt is washed out by

repeatedly rinsing the material that comes out of the furnace. This process is sped up with the

help of a centrifuge. The procedure is explained in detail in Section 4.

3.1.3 Acid Rinse

After the salt is washed out, the magnesium oxide is still remaining in the sample and so it is

removed using an acid (in this case it was Hydrochloric acid/HCl).

2𝐻𝐶𝑙 +𝑀𝑔𝑂 → 𝑀𝑔𝐶𝑙! + 𝐻!𝑂 (2)

3.1.4 Raw materials

The materials used in this experiment each had a purity rating and it is important to note this

because slight changes in the purity rating of any material may severely alter the results.

• Hafnium (IV) Oxide, HfO2, was 98% pure.

• Boron Trioxide, B2O3, was 99% pure.

• Magnesium, Mg, was >99% pure.

3.2. Research

The research for this project was conducted and collected through a variety of different

sources. Most of the research that was presented in the literature review came from online

journals that were written by researchers. Using the Boolean search tool provided by EBSCO

E-Journals and the University of Exeter, I was able to gain a solid understanding of my

project and I was able to identify my objectives. All research was written down on paper, or

in a logbook and later copied to this document.

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3.3. Sintering and Molten salt synthesis

3.3.1 An Introduction to Sintering

The furnace used in this experiment consisted of a long quartz tube surrounded by heating

elements. The reaction (in equation 1) takes place inside a non-reactive crucible at elevated

temperatures. The high temperature and pressure inside the carbon based crucible causes the

elements to fuse and form a ceramic. Inside the furnace there is usually a non-reactive gas

that is pumped into the heating chamber. This ensures that the oxygen from the atmosphere

does not react with the magnesium. In this experiment argon gas was used as a non-reactive

gas.

3.3.2 Sintering Process and description

After all the reactants were measured using an electronic scale. They were crushed into a fine

powder using a mortar & pestle and they were placed into a carbon crucible. The crushing

increases the surface area of the particles and aids the reaction process. The crucibles were

wrapped in carbon paper and carefully placed, in the correct order, inside the quartz tube of

the furnace. The quartz tube was then sealed on both ends of the cylinder and checked for

leaks. At one end of the quartz tube, argon gas is pumped in via a pipe, and at the other end

an escape tube is connected to a pipe that has an open end, which is submersed in a bottle of

water. The furnace is sealed when the bubbles of the argon gas escape using the water route.

The furnace is then ready to be switched on. The hold time, hold temperature and heating

rate are all set using the control panel and the machine is turned on.

Once the temperature reading inside the furnace has cooled down to a safe temperature, it is

acceptable to open the seals on each side of the quartz tube. It is important to remember the

order that the samples are placed in and ensure that no mistakes are made.

3.3.3 Preparation for Molten Salt synthesis

In this experiment both KCl and NaCl were used. The ratio of the composition of salts was

50:50. In MSS as mentioned before, the molten salt content needs to be much larger than the

rest of the samples. The total mass of all the reactants in this experiment was between 4g-4.5g

and the total mass of the salts was 20g. Therefore the salt to reactant ratio was 20:4 or 5:1.

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3.4. X-‐Ray Diffraction

X-Ray diffraction is a characterisation method that can be used to evaluate the chemical

composition of a sample of material.

The advantages of using XRD is that it is a relatively cheap method to use, and more

importantly X-Rays are not absorbed by air so there is no need to create a vacuum. XRD does

not work well with lighter elements, but that is not a problem in this experiment. [23]

An XRD machine works by sending a beam of X-Rays into the layer of the prepared sample

at an angle. There is a detector that is placed on the opposite side, which detects the angle at

which the X-ray beams diffract. The detector measures the angle at which the X-rays diffract,

and the intensities of the waves at these peaks are also recorded. [23]

Diffraction occurs when a wave of the electromagnetic spectrum passes through a slit that is

similar in size to the wavelength of the wave. X-ray wavelengths are similar in length to the

distance between atoms and layers in a crystal structure and so diffraction can occur. The

diagram below shows how the diffraction works. The beam is fired at an angle of incidence,

and the detectors measure the reflected angle as seen in Figure 1 below.

Figure 1 shows the how the x-rays diffract in a crystal structure [28]

Using Bragg’s equation, it is possible to be able to calculate the space (d) between the layers

of atoms in a crystal structure. Bragg’s equation is shown below.

2𝑑 sin𝜃 = 𝑛𝜆 (3)

3.4.1 Preparation of the samples for XRD

After the samples had been washed by water, acid and then dried in an oven, they were

crushed into a fine powder by the mortar & pestle. A small area of the fine powder was

carefully placed on a transparent glass disk and pressed gently to form a thin flat circular

sample. This was placed in the XRD machine for further analysis.

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3.5. SEM

SEM/Scanning electron microscopy uses a beam of electrons to view objects on a very small

scale. It is able to magnify several thousand times more than an optical microscope because it

uses the miniscule de Broglie wavelength of an electron to create an image of the contours of

tiny particles. High-energy electron beams are used to energise the specimen and the signals

that are given off are detected and analysed so that an image can be constructed. [24]

3.6. EDS

EDS or energy-dispersive X-ray spectroscopy is another materials characterisation technique

and this was used in this experiment in conjunction with SEM. The EDS in this experiment

used high-energy focused electrons to excite the electrons within the materials sample. The

electrons from the high-energy beam replace the electrons in the shell of the sample and this

gives of energy in the form of X-Rays. These X-rays are detected using an X-ray detector and

characteristics of the X-rays given off; provide details with what materials are contained

inside the sample.

3.7. TEM

Transmission electron microscopy is a method of characterisation that (like SEM) uses a

beam of electrons rather than light to magnify the image. It is different to SEM because TEM

sends electrons through a specimen and the resultant image is then magnified onto an

imaging device. TEM happens in a vacuum and therefore the sample needs to be prepared

carefully. The preparation for TEM is a very complex procedure because the sample has to be

only a few nanometres thick. TEM can produce a much sharper image than even SEM and in

this experiment it will be used to try and identify the crystal structure. [25]

4. Experimental work

4.1. Procedure

4.1.1 Preparing each sample

1. All of the reactants were weighed using a scale, which was accurate to 1/1000th of a

gram.

2. The material was scooped out of each bottle using disposable micropipettes that had

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been cut at the thick end to form a spoon shape. It is important to avoid contamination

of the base materials by regularly changing the disposable micropipettes when

changing from measuring one material to another.

3. The powdered materials were weighed in a disposable plate. The scale was set to zero

each time after the empty plate is placed on the scale.

4.1.2 Heating the samples

1. Place the sample in a carbon crucible and place in the centre of the furnace.

2. Seal the furnace and pump in argon gas to stop the Mg from reacting with the oxygen

in the air.

3. Select the maximum temperature in the furnace and the required hold time.

4. Set the temperature to increase slowly at a rate of 5°C per minute until it reaches the

predetermined maximum temperature.

5. Once cooled down, use a rod to push out the carbon crucibles from the other side.

4.1.3 Washing the salt

1. The by-product of the heated sample contains many impurities. One is of these is the

salt and the other is Magnesium Oxide. To remove the salt, remove all the powdered

material from the carbon crucible and place it in a beaker.

2. Add 100ml of water in the beaker and shake well.

3. The Hafnium Diboride and Magnesium Oxide are insoluble in water so they will not

dissolve in the water and be washed away.

4. Place the beaker in an ultrasonic tub of liquid, which sends a high frequency through

the beakers and helps to separate the solution from the insoluble particles. Switch the

machine on and leave to mix for 30-50 minutes.

5. Take the beaker out of the ultra sonic and leave it to rest for several hours. The

particles that cannot dissolve in the water will settle at the bottom of the beaker.

6. Pour out as much water as possible and empty all the remaining contents into a 50ml

centrifugal bottle.

7. Fill the centrifugal bottle and place it in the centrifuge. Turn on the machine for 9

12

minutes at 6000 rpm.

8. Pour out the water, and repeat step 7 a minimum of 3 times.

4.1.4 Acid Rinse and Dry

1. Pour the contents of the salt wash into a 250ml bottle.

2. Pour about 200ml of Hydrochloric acid into the 250ml bottle. Close the lid and shake.

3. Place the bottle in the ultrasonic for 60 minutes.

4. Unscrew the cap slightly (to ensure there is no pressure build up from the reactions)

and leave to settle overnight

5. Repeat the salt wash steps to remove the acid and reduce the PH levels.

6. After the final salt wash, place the samples in an oven at 80°C until dry

4.2. Calculating the mass of each reactant in the sample

The mass of each reactant remained the same throughout the experiment (except experiment

5) but the initial calculation was based on the number of moles required of each reactant.

The total mass of the three reactants HfO2, B2O3 and Mg were supposed to add up to 4g in

this experiment. The ratio of moles required for these reactants were 1:1:5 (From equation 1).

Based on this information, the amount of mass of each reactant can be calculated. The first

step is to calculate the mass of 1 mole of each substance, and this is:

• 210.49g for HfO2

• 69.59g for B2O3

• 24.31g for Mg

But Mg needs 5 moles for every 1 mole of the other reactants. So Mg = 121.55g (5 Moles).

4.2.1 Example calculation

To calculate the mass of each reactant required for each sample, the following formula was

used:

𝑟.𝑁𝑅𝑁×100

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Where N is the number of moles associated with each reactant and R is the mass of 1 mole of

a single reactant in grams.

For example the total mass of Mg required was calculated as shown below.

(24.31×5)210.49×1 + 69.59×1 + (24.31×5) × 100

This gives 30.3%. So Mg will be 1.21g (30.3% of 4g).

Using this formula the other two reactants were calculated and shown below.

HfO2 = 2.09g and B2O3 = 0.693g

4.2.2 Volati l ity of reducing agents

When the reactants are reacting inside the furnace, in the pool of liquid salt, some of the

magnesium and some of the boron trioxide will be vaporised and lost. This is because both

Mg and B2O3 are highly volatile [17,18]. To test this hypothesis, extra Mg and B2O3 were

added in each batch. In each experiment (except experiment 5) there were 3 samples used per

batch, the table below shows how much of each reactant was used in each experiment.

Sample A corresponds to the first sample in each experiment. For example in experiment 2,

sample A corresponds to sample 6, and sample B corresponds to sample 7.

Sample HfO2 B2O3 Mg

A 2.09g 0.693g 1.21g

B 2.09g 0.9g (+30%) 1.21g

C 2.09g 0.9g (+30%) 1.45g (+20%)

The salt content remained constant throughout the experiment and a combination of both

NaCl and KCl was used. For each experiment, 10g of NaCl and 10g of KCl was used.

5. Results

5.1. Experiment 1 (Samples 1,2,3)

14

5.1.1 Init ial Conditions

The temperature of the furnace was set at 1200°C and the only variable between the 3

samples was the composition of the reducing agents. The furnace heated up from room

temperature to 1200°C at a rate of 5°C /min. The hold time was set to 6 hours, and once this

was done, the furnace was allowed to cool naturally.

Sample

No. HfO2 B2O3 Mg NaCl KCl Temperature Hold Time

1 2.0921 0.6943 1.2125 10.021 10.035

1200°C 6 hr. 2 2.0913 0.905 1.213 10.017 10.031

3 2.0943 0.904 1.452 10.013 10.033

5.1.2 Observations

In the first experiment there was a slight problem with one of the samples. When I removed

sample number 2 from the oven, it was cracked. Some of the material had been lost but there

was enough of the sample left inside the crucible to characterise.

5.1.3 XRD Results

In each experiment, the sample that showed the most promising results has the largest image.

Figure 2 (left) shows the XRD results of sample 1. (Right) shows the XRD results of sample 2.

Position [∞2Theta] (Copper (Cu))

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PdB1


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15

Figure 3 Shows the XRD results of sample 3.

The XRD Results show that the third sample produced the most pure HfB2 out of the 3

samples tested. It had the largest amount of pure HfB2 and the least amount of HfO2.

In conclusion, at a temperature of 1200°C and a hold time of 6 hours, the best sample

contained 20% extra magnesium and 30% extra boron dioxide.



In the previous experiment, the carbon crucible that contained sample 2 had cracked. It was

caused because one of the samples got stuck inside the quartz tube, and the rod applied too

much force, which cause it to break. To counter this problem, the carbon crucibles were

wrapped in carbon paper. This would ensure that the force from the rod would be distributed

evenly over the carbon paper.

The temperature of the furnace was set at 1300°C and the only variable between the samples

was the composition of the reducing agents. The furnace increased its temperature from room

temperature to 1300°C at a rate of 5°C /min. The hold time was set to 6 hours, and once this

was achieved, the furnace was cooled naturally.


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Sample No.

HfO2 B2O3 Mg NaCl KCl Temperature Hold Time

4 2.093 0.6931 1.2133 10.0143 10.0018

1300°C 6 hr 5 2.0935 0.9017 1.235 10.0013 10.0101

6 2.0955 0.9011 1.454 10.0012 10.0167

5.2.2 XRD Results

Figure 4 (left) shows the XRD results for sample 4. (right) shows the XRD results for sample 5.

Figure 5 Shows the XRD results for sample 6.


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Kotoite

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17

Once again the third sample (which contained 20% extra Mg and 30% extra B2O3) in the batch produced the best results. The highest peak was at 42.13° and the height of the peak was 55901 counts.



The aim of Experiment 3 was to investigate the effect of increasing the hold time. The

previous experiments results, where the sample was heated to 1300°C and held for 6 hours

produced a sample of HfB2, which was less pure than the first experiment of 1200°C for 6hr.

So for this next experiment the temperature remained the same, but the hold time was

increased to 12hrs. The aim of this experiment was to check if increasing the hold time would

increase the purity of HfB2.

Sample No.


7 2.0953 0.694 1.2138 10.0182 10.0272

1300°C 12 hr 8 2.0975 0.906 1.245 10.0063 10.0233

9 2.0945 0.9042 1.4537 10.0202 10.0212

5.3.2 XRD Results

Figure 6 (left) shows the XRD results for sample 7. (right) shows the XRD results for sample 8.


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7#


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8#

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HfO2

HfB2

HfO2

18

Figure 7 Shows the XRD results for sample 9

For this experiment, the sample with the highest purity was sample 9. This sample had 20%

extra MgO and 30% extra Boron dioxide. There was very little Hafnium dioxide left over in

sample 9, but there was a small amount of MgO found. This was probably because the acid

wash was not thourough enough. The Hafnium dioxide was still not fully erradicated and so

for the next experiment the temperature was lowered. In conclusion, the results showed that

increasing the hold time did infact increase the purity of HfB2.

5.4. Experiment 4 (10,11,12)


In the previous experiment, the sample produced was still not pure even though the hold time

was increased and there was a possibility that this was occurred because the temperature was

set too high in the furnace. 1300°C is extremely close to the boiling point of both salts used

throughout this experiment (both have a b.p. of around 1400°C), so there is a possibility that

some of the liquid salt was evaporating and escaping from the carbon crucible. So for this

next experiment, the temperature of the furnace was reduced to 1100°C and the hold time set

back to 6 hours as in experiment 1.


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Sample No.


10 2.0933 0.6935 1.2127 10.0051 10.0036

1100°C 6 hr 11 2.0942 0.9042 1.2123 10.0014 10.004

12 2.094 0.9012 1.4531 10.0049 10.0013

5.4.2 Notable Observations

During the acid wash, the reaction of the metal oxide and the acid creates bubbles and if the

bottle is sealed shut; there will be a build of pressure inside the bottle. In this experiment,

sample number 12’s bottle was shut too tightly and left overnight to react and there was a

large build up of pressure in the bottle. The next morning, the bottle had exploded and the

entire sample could not be recovered. No one was harmed during the process and the senior

PhD students in the laboratory cleaned the acid.

5.4.3 XRD Results

Figure 8 Shows the XRD results of sample 10


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HfO2

HfB2

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Figure 9 Shows the XRD results of sample 10

In this experiment, the hafnium diboride powder was successfully produced however the

purity of the sample used in the XRD was low. There was a large amount of hafnium dioxide

found in the sample and this was probably occurring because at this lower temperature the

ceramic powder did not have enough energy to fuse efficiently. In conclusion, this

temperature and hold time produced the worst results so far. A large amount of hafnium

dioxide was found in the finished product.

5.5. Experiment 5 (13,14,15,16)


In this experiment the objective was slightly different to the previous ones. The aim of this

experiment was to investigate the effects of changing the mass of one of the reactants. In this

case 4 samples were prepared (13-16) and in sample 13 the normal amount of B2O3 was

added. In the samples after that, (i.e. samples14, 15,16) the B2O3 was altered with 15%, 30%

and 45% of extra B2O3 added. The Mg content remained constant in all the samples at 1.45g.

The temperature of 1100°C and hold time of 6 hours was chosen again because sample 12

was destroyed in the previous experiment. So for this experiment, sample 15 has the exact

same initial conditions of sample 12 from the previous experiment and could be used as a

comparison.


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Sample No.


13 2.0951 0.6938 1.4537 10.0185 10.00235

1100°C 6 hr 14 2.0905 0.7963 1.4537 10.003 10.005

15 2.0977 0.904 1.4532 10.0036 10.0048

16 2.094 1 1.4547 10.0034 10.001

5.5.2 XRD Results – Pre Acid Wash

Figure 10 Shows the pre-acid wash XRD results of sample 13


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MgO

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q14#


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KCl

MgO

Ag 3SI

HfB2

HfO2

MgO

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5.5.3 XRD Results -‐ Post Acid Wash

Figure 14 Shows the post-acid wash XRD results of sample 13


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5.5.4 Pre –acid conclusion

The XRD results of the pre acid wash show that each sample had excessive amounts of

magnesium oxide. This was expected (as seen in equation 1) and this is the reason why

Hydrochloric acid will be required in the manufacturing process of HfB2. The XRD machine

detected a small amount of Silver sulphide iodide in sample 15 but this is probably an error in

the peak readings, or it is just a contamination and it can be ignored.

5.5.5 Post acid conclusion

After the acid wash, all the samples had the magnesium oxide completely removed from the

XRD results, showing that the acid wash is effective. However sample 13,15 and 16 all

contained a small amount of Magnesium borate (Mg3 (BO3) 2). Sample 14, which had 15%

extra B2O3, contained only HfB2 and HfO2. This does not however mean that it was the most

pure out of all of the samples. Sample 16 had the highest peaks of HfB2 and as explained by

Zhou Jian and Wang Hejing in their experiment ‘The Physical Meanings of 5 Basic

Parameters for an X-Ray Diffraction Peak and their Applications’, the peak intensity

determines the amount of phase in the mixture. [11] In conclusion, the results showed that

this temperature of 1100°C was less effective at producing pure hafnium diboride than the

higher temperatures of 1200°C and 1300°C. The results also showed that adding 45% extra

B2O3 increased the purity of HfB2.


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5.6. XRD Conclusion

The results of the XRD graphs showed that within each experiment, the sample with the most

amounts of B2O3 and Mg produced the purest HfB2. Based on the results of XRD graphs

only, Sample 9 produced the best results, it had 3 large peaks of HfB2 with the maximum

peak at 42.13° and a peak intensity height of 60689 counts. Sample 3 showed the second best

results, its maximum peak was at 42.13° and the intensity was 49611 counts. Sample 6 had

higher peaks of HfB2 than sample 3 but it contained much more hafnium dioxide so it was

deemed less pure.

5.7. SEM, TEM and EDS

The next step in this is experiment is to determine the particle size of the ceramic powder.

The way to do this would be using SEM and TEM. SEM and TEM cannot be done on all the

samples because that would be a waste of money and time. Instead the results of the XRD

graphs were used to determine which samples were the best in each group.

5.7.1 SEM and EDS Analysis

The SEM results confirmed that HfB2 was created and that it was indeed a fine powder.

Sample 3 of experiment 1 can be seen below.

Figure 18 Shows the SEM image of sample 3 (1200°C/6hrs) [A] magnified 2000x [B] magnified 8000x.

The results show that Hafnium diboride was successfully produced and image B shows that

the particles of the powder were very fine, with most of the particles of powder sized in the

range of 0.5µm - 2.5µm. Image A had an EDS analysis completed on this section and the

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results can be found below. The EDS confirms that the white cloudy substance in the image

consists of mostly hafnium, boron and oxygen. The results can be seen in table 1 below.

Spectrum In stats. B O Hf Au Total

Spectrum 1 Yes 4.91 4.81 73.01 17.27 100.00

Spectrum 2 Yes 5.32 5.31 76.57 12.80 100.00

Mean 5.12 5.06 74.79 15.04 100.00

Std. deviation 0.29 0.35 2.52 3.16

Max. 5.32 5.31 76.57 17.27

Min. 4.91 4.81 73.01 12.80

Table 1 Shows the EDS Results from the scan of image A in figure 18

As expected there is both hafnium and boron detected in this sample. There is a little oxygen, which could come from the hafnium dioxide. There was an unusual amount of Gold found in both spectrums that were analysed. This gold is an impurity that has crept in during some stage in the manufacturing process.

5.7.2 TEM Analysis

A scan was completed using TEM and this was done on sample 3 (1200°C/6hrs) and the results can be seen below.

Figure 19 shows where the SEM image was scanned and tested by EDS

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Figure 19 shows the TEM scan of sample 3 (A) magnified 125000x. (B) magnified 251000x

Figure 20 Shows a magnified TEM scan of sample 3 magnified 1250000x

From figure 19 and 20, it can be seen that the particles are extremely fine (10-9m). On average, the particle size of the powder is between 40-70 nm. The darker regions correspond to more layers in the crystal structure and similarly the lighter regions correspond to fewer layers in the crystal structure.

This result is unusual and many thousand times small than the results of the SEM scan. In the literature review section, there was no other synthesis technique that produced a powder this fine. In several different experiments, the powders of HfB2 produced were always measured in micrometres and not nanometres.

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6. Discussion and conclusions

6.1. Summary

A fine power of hafnium diboride has been successfully produced using high temperature

molten salt synthesis. The fine ceramic powders purity depended on the initial conditions of

the experiment.

From the list of objectives described in the introduction a summary of the report can be

concluded:

1. The optimal sintering conditions for this synthesis was located and it was at 1300°C

for 12 hours

2. Changing the composition of the reactants had an effect on the purity rating of the

hafnium diboride. It was shown that a 20% increase in Mg content, and a 30%

increase in B2O3 increased the purity of HfB2.

3. At 1300°C, increasing the hold time from 6 hours to 12 hours reduced the amount of

unwanted hafnium dioxide left in the finished sample.

4. The powders particle size for HfB2, as shown by the SEM image of sample 3, ranged

between 0.5µm - 2.5µm.

5. Hydrochloric acid was an effective in the removal of magnesium oxide.

6.2. Optimal Conditions

6.2.1 Temperature and Hold t ime

The optimal temperature for molten salt synthesis in this experiment was 1300°C with a hold time of 12 hours. This was based on the results of the XRD, however an SEM and TEM scan still needs to be completed on this sample in order to fully confirm this. In this experiment the lowest temperature of the furnace (1100°C) produced the least pure HfB2. The results showed that increasing the temperature and hold time increases the purity of HfB2.

6.2.2 Reactants ratio

The reactant ratio of 1:1:5 between HfO2: B2O3: Mg was correct as suggested in the literature. However increasing the B2O3 and Mg slightly content did prove to be effective. 20% extra Mg produced a more pure ceramic powder than just the normal amount of Mg. A 30% increase in B2O3 also increased the purity of hafnium diboride for all temperatures in the experiment. These extra additions of the reactants were necessary due to their high volatility as discussed in the literature review, and this was expected.

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6.3. Further research

In this experiment there is still plenty of work to be completed. A test should be conducted to try and find the optimal amount of Mg required. This test will be similar to experiment 5 which altered the content of boron trioxide between each sample.

There should have also been a TEM and SEM scan of sample 9 (1300°C/12hrs) that was created in this experiment, because the XRD graphs showed that it produced the most phase pure hafnium diboride. It was not completed due to the time constraints of this project.

A much more detailed test should be conducted to try and locate the optimal hold time.

A similar test can also be done using temperatures between 800°C and 1100°C.

7. Project management, consideration of sustainability and health and safety

7.1. Project Management

This project lasted a full academic year and it would have been quite easy to manage my time

ineffectively. To prepare for this, I used the project management knowledge I gained from the

second year module “Management and Management Science (ECM2102)”, and created a

Gantt chart on Microsoft Project.

According to my proposed Gantt chart, I was required to complete the synthesis of 1

experiment within 1 week of January 12th. However, the synthesis of the first experiment

took 2 full weeks and I was already behind schedule for the second experiment. To counter

this, a few hours was spent just trying to optimise the synthesis process and managing my

other work around this project. The initial Gantt chart that was proposed at the beginning of

the experiment can be seen below:

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Figure 21 Shows the initial Gantt chart that was used to plan the project.

For my research, my experimental results and my meetings with supervisor/PhD students, I

used a hardcover logbook. The logbook contained most of the important information that was

used in this experiment and helped me keep a record of the results of each experiment as I

completed it.

Other management concerns included data protection. In other words what would happen if a

virus or a computer malfunction suddenly corrupted and destroyed the file. To counter this,

the word document for this project was saved on an external hard drive, Microsoft one drive,

drop box and saved on the desktop of a laptop. These files were updated daily.

7.2. Considering the Environment

One of the main reasons why molten salt synthesis is used in the experiment is because the

salt can be recovered and recycled. The water that is thrown out during the washing process

can be easily distilled, and the salt can be collected. The water that is recovered at the

distillation process can be re-used to wash the next batch of samples, as it will be pure.

The hydrochloric acid in this experiment reacts with the magnesium oxide to form

magnesium chloride and water. Magnesium chloride (MgCl2) is a salt typically known as an

ionic halide [26]. It is extremely soluble in water. The magnesium chloride that is the by-

product of this reaction can be easily separated from the water using distillation and it can

either be sold, or it can be electrolysed to give a pure magnesium metal as shown below:

𝑀𝑔𝐶𝑙!(!) → 𝑀𝑔(!) + 𝐶𝑙!(!)

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Since Magnesium is used as a reactant it is extremely advantageous that it can be recycled.

This is also extremely important when considering the global environmental impact.

Chlorine gas is used extensively in industry, during the manufacturing process of certain

plastics. So it will also not be wasted and could also potentially be sold to another industry.

If further research is conducted, and this manufacturing process is optimised, then molten salt

synthesis would be an extremely good candidate as the chosen method that manufacturers

will use in industry because a large amount of the raw materials can be recycled.

7.3. Health and Safety Risk Management

Health and safety was taken very seriously in this project. Most of the experimental work took place inside a laboratory where there were several high temperature furnaces, high-voltage machines, and plenty of harmful chemicals stored inside this room. Other than that, there were also many experiments being conducted by other students, which involved the use of strong acid and other harmful chemicals. In order to prepare for all risks in this environment, a risk assessment form was completed and it is shown below in Table 2.

ID Risk item Effect Cause

Like

lihoo

d

Seve

rity

Impo

rtan

ce

Action to minimise risk

1 Use of HCl to

remove the MgO from the HfB2

Chemical burns,

Eye Injuries

Spillage 3 8 24

Wearing the appropriate safety

equipment (Personal Protective

equipment PPE)

2 Taking the carbon crucible out of the

furnace

Burns

Pressurised shut, can

explode when de pressurising

2 9 18

Wearing the appropriate safety

equipment (Personal Protective

equipment PPE)

3 Cables, Obstacles Spilling chemicals Tripping 3 4 12

Staying alert, clearing the

workspace before usage

4 Electrocution Electric shock - Death

Ultrasonic, Furnace, XRD 1 10 10

Wearing rubber soled shoes, taking extra precautions

Table 2

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[2] Zhang, G., Guo, W., Ni, D. and Kan, Y. (2009). Ultrahigh temperature ceramics (UHTCs) based on ZrB 2 and HfB 2 systems: Powder synthesis, densification and mechanical properties. J. Phys.: Conf. Ser., 176, p.012041.

[3] Brown-Shaklee, H., Fahrenholtz, W. and Hilmas, G. (2010). Densification Behavior and Microstructure Evolution of Hot-Pressed HfB2. Journal of the American Ceramic Society, 94(1), pp.49-58.

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[6] Guo, W., Yang, Z. and Zhang, G. (2012). Synthesis of submicrometer HfB2 powder and its densification. Materials Letters, 83, pp.52-55.

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[9] Zhang, L., Pejaković, D., Marschall, J. and Gasch, M. (2011). Thermal and Electrical Transport Properties of Spark Plasma-Sintered HfB2 and ZrB2 Ceramics. Journal of the American Ceramic Society, 94(8), pp.2562-2570.

[10] Perkins, C. (2000). Characterization of Hafnium Diboride, HfB2 (0001), by XPS. Surf. Sci. Spectra, 7(4), p.316.

[11] Jian, Z. and Hejing, W. (2003). The physical meanings of 5 basic parameters for an X-ray diffraction peak and their application. Chinese Journal of Geochemistry, 22(1), pp.38-44.

[12] Zhang, S., Hilmas, G. and Fahrenholtz, W. (2007). Pressureless Sintering of ZrB2-SiC Ceramics. Journal of the American Ceramic Society, 91(1), pp.26-32.

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[15] Zapata-Solvas, E., Jayaseelan, D., Lin, H., Brown, P. and Lee, W. (2013). Mechanical properties of ZrB2- and HfB2-based ultra-high temperature ceramics fabricated by spark plasma sintering. Journal of the European Ceramic Society, 33(7), pp.1373-1386.

[16] Venugopal, S., Boakye, E., Paul, A., Keller, K., Mogilevsky, P., Vaidhyanathan, B., Binner, J., Katz, A. and Brown, P. (2013). Sol-Gel Synthesis and Formation Mechanism of Ultrahigh Temperature Ceramic: HfB 2. Journal of the American Ceramic Society, 97(1), pp.92-99.

[17] Shugart, K., Liu, S., Craven, F. and Opila, E. (2014). Determination of Retained B 2 O 3 Content in ZrB 2 -30 vol% SiC Oxide Scales. Journal of the American Ceramic Society, 98(1), pp.287-295.

[18] Kim, J., Maeda, M., Zhao, Y., Shi, D., Dou, S., Choi, S. and Kiyoshi, T. (2008). In situ processed MgB2 conductors: Core densification due to mechanical deformation. Physica C: Superconductivity, 468(15-20), pp.1813-1816.

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The Characterisation and Synthesis of Hafnium diboride for Aerospace Applications

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Transcript of The Characterisation and Synthesis of Hafnium diboride for Aerospace Applications