Smart Materials &Semiconductors

Varun P 1st sem, M Tech UOM

What is Smart Material?

These are designed materials that have one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as stress, temperature, moisture, pH, electric or magnetic fields.

These are materials are able to sense changes in their environment and then respond to these changes in predetermined manners as found in living organisms

Types of Smart Materials In earlier classification we can see only 4 types,1. Shape Memory alloys2. Piezo Electric Materials(or Ceramics)3. Magnetostrictive materials4. Electro/Magnetorheological fluids

But there are many others which may not fall into above classification.

5. Magnetic SMA6. Ferrofluid7. Photomechanical

materials8. Self-healing materials9. Magnetocaloric10. Thermoelectric11. Smart Polymers12. Chromogenic materials

Shape Memory Alloys Other names: SMA, Smart metal/alloy, memory metal/alloy. First discovered in AuCd alloy. Well known and extensively used from 80 years These materials if deformed, it returns to its original shape. SMA remembers a particular shape at specific temperature(memory

temperature) E.g. Mainly Cu-Al-Ni and Ni-Ti alloys as well as Ferrous based: Fe-Mn-Si and Copper based: Cu-Zn-Al and Cu-Al-Ni Fe and Cu based are commercially available and cheaper But NiTi (or TiNi) based SMAs are most preferred due to their

stability, practicability and superior thermo-mechanical performance application in Aircraft, Automotive, medicine (dentistry, eye glass

frames, orthopaedics, angioplasty)

How SMA Works?

Relatively strong SME occurs due to the change in the crystalline structure of materials.

Two phases are: Martensite: Low temperature phase Relatively weak Austenite: High temperature phase Relatively strong

MARTENSITEDEFORMING MARTENSITE

DEFORMED MARTENSITE

AUSTENSITE

MARTENSITE

Piezoelectric materialsCertain materials accumulate electric charge in response to applied mechanical stress. These materials also exhibit reverse piezo-electric effect, where in material develops mechanical strain on applying electric field.Piezo is derived from Greek word which means to

squeeze or press.

Examples: Natural quartz, topaz, sucrose, tourmaline minerals,

In human body: Dry bone(bcoz of collagen), DNAProteins: silk, dentin, viral proteinsSynthetic ceramics: barium titanate, lead zirconate,

potassium niobate.Synthetic(but lead free): bismuth ferrite, bismuth titanate.

Applications:In watches and clocksElectronics: crystal oscillator, microphone, speakers etcElectrical: peizo electric transformerAutomotive : engine management systemIn AFM and STM tipsFor energy harvesting

Magnetostrictive materialsMagnetostriction is the property exhibited by

ferromagnetic materials, they change their shape or dimensions during the process of magnetization.

(it’s the reason we hear humming sound when we go near electric transformer)

Why? Internally ferromagnetic materials have a structure that is divided into domains, each of which is a region of uniform magnetic polarisation. So, when a magnetic field is applied, the boundaries between the domains will shift and domains rotate both of these cause a change in materials dimensions

The reciprocal effect is shown by these materials, where we see change in its magnetising pattern, for the given magnetic field, when mechanical stress is applied.

This inverse magnetostrictive effect is also known as magnetoelastic effect and Villari effect.

Ex: cobalt(largest magnetostriction at room temp.)Alloys : Terfenol-D, Galfanol; Amorphous alloy: MelglasApplication: SONAR, ultrasonic transducers

Electrorheological fluid(ER)

These are suspensions of extremely fine non conducting but electrically active particles in an electrically insulating fluid

The apparent viscosity of these fluids change reversibly by an order of up to 1 lakh in response to an electric field.

It can change from liquid to gel and back in milliseconds.Appn: fast acting hydraulic valves, clutches, brakes .Ex: mixing corn flour in vegetable oil

Magnetorhelogical fluid

A type of smart fluid in a carrier fluid, when subjected to magnetic field, the fluid greatly increases its apparent viscosity to the point of becoming a viscoelastic solid.

Importantly, the yield stress of the fluid can be controlled very accurately by varying the magnetic field intensity, which gives rise to its many possible control-based applications.

Appn: shock absorbers, clutches, brakes etc.Recent researches are in developing body armour and

human prosthetic legs

Magnetic shape memory alloy

Ferro magnetic material which exhibit large strain under the influence of an applied magnetic field.

Compared to SMA magnetic SMA can actuate at higher frequencies.

However magnetic SMA are stiff brittle and recommended for low temperature application.

Ferro fluidA liquid that becomes strongly magnetised in presence

of a magnetic field. Invented in NASA in 1963Has three basic components: a) sub-microscopic,

magnetically permeable particles of iron oxide, usually magnetite, b) a surfactant or dispersant coating and c) a carrier fluid.  

Application: in fuel system and engine cooling system of space craft where there is no movement by gravitation.

Other appn: in manufacturing of hard disks, targeted drug delivery, in loud speakers, as sealant to maintain vacuum.

Surfactants :are soap-like materials that work to coat the particles and keep them from being attracted to each other.Carrier fluid: may be water or oil

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Photomechanical materials

Change in shape of material when exposed to light.

Recorded in 1880 itself by Alexander graham bell.Research efforts are on to induce motion in

miniature robots

Self healing materialsHave intrinsic ability to repair damage due to normal

usage, then expanding the materials life-time. It’s ability to repair itself is structurally incorporatedWhat’s the need? Cracks and other types of damage in a

microscopic level has been shown to change thermal, electrical, and acoustical properties and eventually their propagation leads to full failure of the material or an object made of it.

All this class of materials are not autonomously self-healing, some may require human intervention in one or other form for e.g. light, temperature change to initiate the healing process.

Self healing polymers and elastomers: dicyclopentadiene, thiol based polymers, furan mele-imides

Self healing composites, coating, ceramics are coming into use.

Examples for self healing materials, all above are polymer based

Magnetocaloric MaterialsThese are the materials or compounds that undergo

reversible change in temperature upon exposure to magnetic field.

First observed in nickel but cooling effect was less.Magnetic refrigeration: A cooling technology based on the

magneto caloric effect. Extremely low temperature as well as the ranges used in common refrigerator.

Compared to traditional gas refrigerator safer, quieter, more compact, has higher cooling efficiency and environmentally friendly.

Ex. Element gadolinium and some of its alloys

Thermoelectric materials These materials convert temperature differences into electricity and

vice versa. When temperature differences creates electric potential – Seedbeck

effect. When voltage difference converted to temperature-Peltier effect Most materials exhibit thermoelectric effect, but it is too small. Bismuth telluride, gives significant changes, used in refrigeration and

power generation Importance: to generate electricity from waste heat There are many efforts to create such materials for example by

creating point defects, and vacancies in crystal structure. Other ex. Bi2Se3, Lead telluride

Smart Polymers

It can be classified mainly into 4 types1. Smart inorganic polymers2. pH sensitive polymers3. Temperature responsive polymers4. Shape Memory Polymer (Polycaprolactone)5. Electroactive polymers

pH sensitive polymersPolymers that respond to the changes in the pH of the

surrounding medium by varying their dimensions.They may either increase in size or collapse depending on

pHAcidic group(-COOH) :Swell in basic pH. Ex. Polyacrylic acidBasic group: (-NH2):Swell in acidic pH. Ex. ChitoranApplication: Extensively in controlled drug delivery,

biomimetics, responsive plasmonics

Temperature responsive polymers

Polymers that exhibit drastic and discontinuous changes of their physical properties with temperature.

Research is mainly focussed on polymers that show thermoresponsivity in aqueous solution

Promising areas are tissue engineering, liquid chromatography, drug delivery and bio separation

Polycaprolactone It’s a biodegradable polymer.Can be moulded by immersion in hot water since it has

low melting point of 60 degree centigrade.Has lot of biomedical application and approved by FDA

for implantable devices, for suture, as scaffold for tissue repair

As drug encapsulation for controlled release and targeted delivery.For prototyping, 3D printing, as an ingredient in Shape

memory polymer viz. PCL based Polyurethanes, epoxised soybean oil and PCL

Rapid prototypes of the micro-vascular shape-memory polymer devices (a) Micro-claw with inner vasculatures of different sections. (b) Active spring with inner vasculature. (c) Example of geometry obtained after the training process of the shape-memory effect.

Vasculature:arrangement of blood vessels in the body, or within an organ

Electroactive polymers(EAPs)

Polymers that exhibit change in size or shape when stimulated by electric field.

As an alternative to ceramic piezo electric materialsThese produce large strains upto 300% which is

much more than ceramics.Types: Dielectric EAPs, ferroelectric polymers,

electrostrictive polymers,liquid crystal polymers, electrorheologic polymeric fluid etc.

Chromogenic Materials

Materials that change colour due to external stimuli and often reversible.

Types:1. 1.Electrochromic: changes colour on application of

voltage. E.g. LCD2. 2.Thermochromic:change colour depending on

temperature3. 3.Photochromic: change colour in response to light e.g.

sunglasses4. 4.Halochromic: change colour due to change in pH ex. Litmus mixture

Photochromic lens Colour change whenVoltage applied

Electrochromic

Thermochromic

Thermochromic baby spoon

Semiconductors

Why semiconductors? SEMICONDUCTORS: They are here, there, and everywhere Computers, tablet pc, Silicon (Si) MOSFETs, ICs, CMOS laptops, anything “intelligent” Cell phones Si ICs, GaAs FETs, BJTs CD players AlGaAs and InGaP laser diodes, Si photodiodes TV remotes, mobile terminals Light emitting diodes (LEDs) Satellite dishes InGaAs MMICs (Monolithic Microwave

ICs) Fiber networks InGaAsP laser diodes, pin photodiodes Traffic signals, car GaN LEDs (green, blue) taillights InGaAsP LEDs (red, amber) Air bags Si MEMs, Si ICs and, they are important, especially to Elec.Eng.& Computer

Sciences

Introduction Semiconductors are materials whose conductivity,

which is measured by no. of charge carriers (holes & electrons) available to conduct electricity; lies between Conductors and Insulators.

Ex : Silicon and Germanium

Difference in conductivity

Semiconductor MaterialsElemental semiconductors – Si and Ge (column IV of

periodic table) –compose of single species of atomsCompound semiconductors – combinations of atoms of

column III and column V GaAs, InP; and some atoms from column II and VI CdTe. (combination of two atoms results in binary compounds).

There are also three-element (ternary) compounds (GaAsP).

And four-elements (quaternary) compounds such as InGaAsP.

Semiconductor MaterialsGE was widely used in the early days of

semiconductor development for transistors and diodes.

Si is now used for the majority of transistors and integrated circuits.

Compound semiconductors are widely used in high-speed devices and devices requiring the emission or absorption of light.

Intrinsic Semiconductors: pure silicon and germanium are called intrinsic semiconductors.WKT in silicon 4 valence electrons are covalently bonded with neighbouring atom.At sufficiently low temp., at sufficiently low temp all bonds are intact and no electron is available for conductivity.At room temp., some of the bonds are broken by thermal ionisation and electrons are freed

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Two dimensional representation of the silicon crystal, observe covalentBonds are formed by sharing of the valence electrons.At 0 K all bonds are intact and no free electrons are available for current conduction

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It can be calculated by an equation, we get n=1.5X10^10 carrier per cm3

But silicon crystals has 5X10^23 atoms per cm3. Thus at room temp. 1 of every billion atom is ionized. Which is very

low, and not of much practical use. These are termed as intrinsic semiconductors

Doped Semiconductors are also called extrinsic semiconductors. Here carriers of one type(hole: p type or electrons: n type)

predominate For silicon introducing impurity atom of pentavalent phosphorous

(donor)element results in n type And by introducing trivalent impurity boron(acceptor), p type is

created A piece of n-type or p-type silicon is electrically neutral or not?

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Its neutral, the majority free carriers are neutralised by bound charges associated with impurity atom.

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Bibliography

1. www.wikipedia.org2. www.iom3.org Institute of Materials Minerals and Mining,

London3. W D Callister, D G Rethwisch, Materials Science and

Engg.4. A S Sedra, K C Smith, Microelectronic Circuits5. www.czferro.com

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Semiconductor materials

The electronic and optical properties of semiconductors are strongly affected by impurities, which may be added in precisely controlled amounts (e.g. an impurity concentration of one part per million can change a sample of Si from a poor conductor to a good conductor of electric current). This process called doping