NEW ENGINEERING MATERIALS

Metallic Glasses

• It is the newly developed engineering materials. It share the properties of both metals and alloys.

• Most metals and alloys are crystalline. In contrast, a glass is an amorphous (non crystalline) brittle and transparent solid.

• Thus, metallic glasses are the metal alloys that are amorphous. That is, they do not have a long range atomic order.

• Advantages:

1. homogeneous composition

2. strong and

3. superior corrosion resistance.

• To have this particular property, the metallic glasses are to be made

by cooling a molten metal so rapidly at a rate of 2 x 106°Cs-1. during

this process of solidification, the atoms do not have enough time or

energy to rearrange for crystal nucleation. Thus, the liquid upon

reaching the glass transition temperature Tg solidifies as a metallic

glass. Again, upon heating metallic glasses show a reversible glass-

liquid transition at Tg.

Types of Metallic Glasses

• Metallic glasses are of two types based on their base

material used for the preparation.

1. Metal-metal glasses, Exam Ni-Nb, Mg-Zn and Cu- Zr

2. Metal- Metalloid glasses. Transition metal like Fe, Co,

Ni and metalloid like B, Si, C and P are used.

Preparation

• Various rapid cooling techniques such as• 1) spraying,• 2) spinning and• 3) laser deposition

• In this technique, there is spinning disc made of copper. In order to prepare a metallic glass of a particular type a suitable combination of metal-metal or metal-metalloid alloy in their stoichiometric ratio are taken in a refractory tube having a fine nozzle at its bottom. The nozzle side of the tube is placed just over the spinning disc.

• An induction heater attached to the refractory tube melts the alloy. This melt is kept above its melting point till it gets transformed into a homogeneous mixture. An inert gas such as helium is made to flow through the tube containing the homogeneous mixture. As a results, the melt gets ejected through the nozzle. The ejected melt is cooled at a faster rate with the help of spinning cooled copper disc. The ejection rate can be increased by increasing the pressure of the inert gas. Thus, a glassy alloy ribbon starts getting formed over the spinning disc.

• The thickness of the glassy ribbon may be varied by increasing or decreasing the speed of the spinning disc.

The other techniques

• The other techniques used for producing ribbons of metallic

glasses include.

1. Twin roller system

In this technique a molten alloy is passed though two rollers

rotating in opposite directions.

2. Melt extraction system

In this technique the fast moving roller sweeps off

molten droplet into a strip from a solid rod.

Properties

• The strength of metallic glasses are very high (nearly twice that of stainless steel) lighter in weight.

• They are ductile, malleable, brittle and opaque. The hardness is very high.

• The toughness is very high, i.e., the fracture resistance is very high (more than ceramics).

• They have high elasticity. i.e., the yield strength is very high.

• They have high corrosion resistance.• They do not contain any crystalline defects like point defects,

dislocation, stacking faults etc.

• They are soft magnetic materials. As a result, easy magnetization and demagnetization is possible.

• Magnetically soft metallic glasses have very narrow hysteresis loop as shown in figure. Thus, they have very low hysteresis energy losses.

• They have high electrical resistivity which leads to a low eddy current loss.

Applications

1. Metallic glasses are used as transformer core material in high power transformers.

2. Because of their high electrical resistivity and nearly zero temperature coefficient of resistance, these materials are used in making cryothermometers, magnetoresistance sensors and computer memories.

3. As the magnetic properties of the metallic glasses are not affected by radiation they are used in making containers for nuclear waste disposal.

4. These materials are used in the preparation of magnets for fusion

reactors and magnets for levitated trains etc.

5. Metallic glasses can also be used for making watch cases to replace Ni and other metals which can cause allergic reactions.

6 The excellent corrosion resistance property makes these materials to be ideal for cutting and in making surgical instruments. They can be used as a prosthetic material for implantation in the human body.

7 In future, the usage of metallic glasses in the electronic field can yield, stronger, lighter and more easily moulded castings for personal electronics products.

8 Metallic glasses are used in tap recorder as heads, in manufacturing of springs and standard resistances.

Shape memory alloys

• Shape memory alloys (SMA’s) are metals which exhibit two very unique properties,

1 shape memory effect and

2 pseudo elasticity or super elasticity (SE).

• These alloys are a unique class of materials, which remember their shape even after severe deformation. i.e., when a SMA is ones deformed in the cold shape (martensite) these materials will stay deformed until heated; where, upon heating they will spontaneously return to their original pre-determined hot shape (austenite).

• It is observed that, the structural changes at the atomic level contributes to this unique properties of the materials.

Types of shape memory alloys

There are two types of Shape memory alloys (SMA’s)

1. The one way shape memory alloy

2. The two way shape memory alloy

• The materials which exhibit shape memory effect (i.e., taking their

own shape) only upon heating are said to have one way shape

memory.

• In contrast, some of the materials exhibit shape memory effect both

during heating and cooling. Hence, these materials are said to have

two shape memory

Crystal structure of shape memory alloy• The shape memory alloys are said to have two distinct crystal

structure or phases. The effect of temperature and internal stresses determines the crystal structure or the phase that the SMA will have at particular instant.

• The phase which exists at high temperature is called the austenite phase (microscopically they possess small platelets structure). highly symmetric structure such as the one in cubic system

• The other phase which is found to exists at low temperature is called the martensite phase (microscopically they possess needle like structure). Less Symmetric like monoclinic.

Classification of shape memory alloys

• The shape memory materials could be broadly classified into three types

1. Temperature induced shape memory

2. Stress induced shape memory

3.Ferro-magnetic shape memory materials.

Temperature Induced Transformation• The phase transformation takes place in SMA’s not at a particular

temperature but over a range of temperature. This transformation is called the temperature transformation.

• The temperature induced transformation is characterized by four temperatures, Ms and Mf during cooling and As and Af during heating.

• Ms And Mf indicate the temperature at which the transformation from the parent phase austenite into martensite starts and finishes respectively.

• Similarly As and Af indicate the temperature at which the reverse transformation from martensite to austenite starts and finishes upon heating.

• Thus, it is observed that, the overall transformation has a temperature range of 10-15°C depending on the chemical composition of the alloy. This overall transformation is found to exhibit hysteresis in which, the transformations on heating and cooling do not overlap. This hysteresis H is found to depend on the composition of the alloy system.

Stress Induced Transformation

• The stress induced transformation takes place at a constant temperature. At a temperature above the Af temperature, the martensite phase can be induced by applying stress over the austenite phase.

• When stressed above a certain value the austenite phase undergoes a large elastic deformation. Stressing

beyond the elastic limit will result in permanent plastic strains. On the removal of the stress, the material almost

completely recovers to the parent austenite phase at a much lower value of stress.

Functional Properties • The SMA’s are characterized by two important properties.

1. Shape Memory Effect (SME)

2. Super Elasticity (pseudoelasticity).

1. Shape memory effect

The SME is the phenomena in which a specimen apparently deformed at lower temperature reverts to its undeformed original shape when heated to higher temperature.

This SME is a consequence of a crystallographically reversible martensitic phase transformation occurring in the solid state. Schematically, the crystallographic formation of martensite and reversion to austenite on heating.

• From the figure it is seen that, the high temperature austenitic structure undergoes twinning as the temperature is lowered, this twinned structure having microscopically needle like structure is called martensite. This phase is relatively soft and easily deformed phase of SMA which exists at lower temperatures.

• This phase upon deformation (i.e., on applying external stress) takes on a particular shape called the deformed (detwinned) martensite, and in the process undergoes a large elastic strain. If heated in this condition, the deformed martensite returns to the stable austenite structure and in the process recovers the elastic strain. Thus, the shape memory phenomenon is seen.

• This SME is being implemented in making coffee pots,• thermostats,• vascular stents• hydraulic fitting for air planes.

2. Super elasticity (Pseudoelasticity)Super elasticity (Pseudoelasticity) refers to the ability of SMA

to return to its original shape upon unloading after a substantial deformation. This SE based on stress induced martensitic (SIM) transformation. This SE in SMA’s occurs at a constant temperature when the alloy is completely compose of austenite phase (temperature is grater than Af).

The stress on the SMA is increased until the austenite phase becomes transformed in to a martensite phase simply due to loading. But, as soon as the loading is decreased the martensite begins to transform back to austenite since, the temperature is still above Af. As a result, it comes back to its original shape.

This effect, which causes the material to be extremely elastic is known as pseudoelasticity or superelasticity. This SE is non linear it is temperature and strain dependent. This SE is a mechanical type of behavior of SMA’s.

• At a temperature above Af, the austenite phase is found to have much higher yield and flow stresses. But, the martensite is easily deformed to several percent strain at quite a low stress.

• This martensite phase is found to come back to the austenite phase upon heating after removing the stress as shown by the dashed line. But, no such shape recovery is found in the austenite phase upon staining and heating, because no phase change occurs. The above two behaviors are called as thermo mechanical behaviors.

• The superelasticity behaviour is applied in making1. eye glass frames, 2. medical tools, 3. cellular phone antennae’s and 4. orthodontic arches.

Applications of shape memory alloys

1. Aircraft and space industry

fine-tuned helicopter blades

in antenna opening

hubble telescope and

in triggering devices.

2. Automobile industry

making spring acuators,

clutch systems,

thermostats,

oil pressure control unit

and high pressure sealing plugs.

3. Medical field

1. as dental arch wires. These wires will make the misaligned teeth gradually to return to their original shape exerting a small and nearly constant force on the misaligned teeth.

2. They are also used as blood clot filter.

3. Nitinol needle wire localizers are used to locate and mark breast tumours so that subsequent surgery can be more exact and less invasive.

4. tweezers to remove foreign objects through small incisions.

5. guide wires for catheters through blood vessels.

6. in designing micro surgical instruments and

7. micro grippers etc.

4. Consumer products• SMA’s are used in making eye glass frames which offers improved

comfort and flexibility and in cellular phone antenna.• Nitinol is used in robotic actuators and micro manipulators to

simulate human muscle motion.• Ni-Ti springs in coffee pots.• toys and ornamental goods.• couplers and fasteners.• fixed safely valves • instantly restrict water flow in shower or sinks.• safely valves that provide emergency shutdown of process control

lines that handle flammable and toxic fluid and gases.

• Advantages of SMA’s

biocompatibility,

diverse fields of application and

good mechanical properties.

• Disadvantages of SMA’s

highly expensive to manufacture and machine it.

Biomaterials

• The biomaterials are defined as “the materials with novel chemical, physical, mechanical or “intelligent” properties produced through processes that mimic biological phenomena.”

Biomedical Compatibility of Ti-Al-Nb Alloys for Implant Applications

• Over the last 30 years, biocompatible Ti alloy have been made use of to replace human bones and teeth as these alloys are strong, lightweight and biocompatible.

• The material of choice used in implant is titanium-vanadium-aluminium (Ti-V-Al) alloys because of their

• excellent biocompatibility and • high specific strength, • corrosion resistance, • low density, • good ductility and elastic modulus.

BIOMATERIALS

Definition • ‘Materials with novel chemical,

mechanical, physical or intelligent properties that mimic biological phenomena.’

• ‘Any substance (other than drugs) or combination of substances synthetic or natural in origin which can be used for any period of time, as a whole or as a part of a system, which treats, augments, or replaces any tissue organ or function of the body.’

Need for Biomaterials

• The need for biomaterials stems from an inability to treat many diseases, injuries and conditions with other therapies or procedures – replacement of body part that has lost

function (total hip, heart)– correct abnormalities (spinal rod)– improve function (pacemaker, stent)– assist in healing (structural, pharmaceutical

effects: sutures, drug release)

• Historically, biomaterials consisted of materials common in the laboratories of physicians, with little consideration of material properties.

• Early biomaterials :– Gold: Malleable, inert metal; used in dentistry

by Chinese, Aztecs and Romans--dates 2000 years

– Iron, brass: High strength metals; rejoin fractured femur (1775)

– Glass: Hard ceramic; used to replace eye (purely cosmetic)

History

– Wood: Natural composite; high strength to weight; used for limb prostheses and artificial teeth

– Bone: Natural composite; uses: needles, decorative piercings

– Sausage casing: cellulose membrane used for early dialysis (W Kolff)

– Other: Ant pincers. Central American Indians used to suture wounds

Essential factors

• Biofunctionality :– The functions of these

materials mainly concern :• Load transmission• Stress distribution

(bone replacements)• Light transmission

(implanted lenses)• Sound transmission

(cochlear implant)• Control of blood flow

(heart related implants)

– Based on these the following need to be considered while seleting a biomaterial :

• Cost effectiveness• Production rate• Mechanical properties

(strength, toughness, fatigue etc.)

• Physicochemical properties (corrosion, durability, electric and thermal conductivity)

• Biocompatibility : – A material is said to be

biocompatible if it does not undergo a degradation in its properties within the environment of the body and does not cause adverse reactions

– Possible outcomes if this factor is overlooked are :

• Corrosion

• Swelling• Leaching

• Dissolution• Wear

• Modification of chemical properties etc.

Classification

Biomaterials

Naturally-derived Semi-synthetic Synthetic

Metals and Alloys

Composites

Ceramics

Polymers

Natural biomaterials

• These materials are derived from living things itself i.e. from plants and animals, for example – collagen, keratin, cellulose, chitin

• They offer several advantages like preventing risk of toxicity, which is the problem arising in case of synthetic implants.

Synthetic biomaterials

• Metal and alloy biomaterials (biometal)

– Used due to mechanical strength and toughness

– Used for load bearing implants

– Include simple wires, screws, plates, total joint, prostheses

– Common ones :

• stainless steel, cobalt alloys, titanium alloys

– Applications :• Bone plates,

stents, orthodontic wires(Ni-Ti)

• Bone and joint replacement(Co-Cr-Mo)

• Heart valves, Dental implants(Cr-Ni-Cr-Mo)

Artificial knee joints

Artificial wrist joint

Bone plates, to assist in the healing of skeletal fractures,

• Fracture fixation, stents

(stainless steel)

• Antibacterial agents

(silver products)

• Dental restoration

(gold alloys and Hg-Ag-Sn amalgam)

• Polymers :– these have properties most similar to natural

tissues.– Their properties depend on their composition,

structure and arrangement of constituent molecules

– examples :

silicones, polyethylene, polyvinyl chloride, polyurethanes, polylactides

– Over a period of time these degrade, which could be :

• Controlled degradation– Here material is designed to serve this process– They get broken down into smaller fragments and are

later eliminated from the body by normal metabolic processes of the body

• Unintentional degradation– Occur due to reactions like oxidation or hydrolysis– Release chemicals that trigger host immune reactions, of

severe nature

– Applications : • Joint lining

• Wound dressing

• Intraocular lens replacement

• Tendon replacement

• Ceramics (bioceramics):– Out of the wide range of ceramics available,

only a few are biocompatible– These can be :

bioinert, bioactive, biodegradable

– These materials have : 1. Great stiffness2. high resistance to corrosion3. excellent wear resistance4. low density

A titanium hip prosthesis, with a ceramic head and polyethylene acetabular cup

– Applications:1. dental, and bone implants

Artificial teeth, and bones are relatively commonplace

2. Surgical cements are used regularly

3. Joint replacements are commonly coated with bioceramic materials to reduce wear and inflammatory response. Other examples are in pacemakers, kidney dialysis machines, and respirators.

• Composites :– ‘Composite materials are solids which contain two or

more distinct constituent materials or phases, on a scale larger than the atomic. The term “composite” is usually reserved for those materials in which the distinct phases are separated on a scale larger than the atomic

– For example – • reinforced plastics such as fiberglass • natural materials such as bone , wood, dentin• A foam is a composite in which one phase is empty

space.

– Natural biological materials tend to be composites. Natural foams include lung, cancellous bone, cartilage, skin and wood

– Natural composites often exhibit particulate, porous, and fibrous structural features

– Properties : 1. Low density2. High strength

(Properties such as the elastic modulus are significantly altered in comparison with those of a homogeneous material.)

– Applications :• Dentistry – dental filling composites(dental cement),

restorative material• Prosthetic limb• bone cement• orthopedic implants with porous surfaces

synthetic cellular solids (a) open-cell polyurethane,(b) closed-cell polyethylene, (c) foamed nickel, (d) foamed copper, (e) foamed zirconia, (f) foamed mullite, (g) foamed glass, (h) polyester foam with both open and closed cells.

natural cellular solids: (a) cork, (b) balsa wood, (c) sponge, (d) cancellous bone, (e) coral, (f) cuttlefish bone, (g) iris leaf, (h) plant stalk.

PHOTO CONDUCTIVITY

1.24

( )

cg

cg

hc

E

mE eV

λ =

λ = µ

Photo voltaic cell Popularly known as Solar cell

Solar cellworking

Parameters

1.Open Circuit Voltage2.Short Circuit Current3.Fill Factor4.Conversion Efficiency

Isc

The short-circuit current is the current through the solar cell when the voltage across the solar cell is zero (i.e., when the solar cell is short circuited).

The short-circuit current is the largest current which may be drawn from the solar cell.

/0 ( 1)qV kT

total LI I e I= − −

At V=0 Itotal = -IL= Isc

I

Vm

Im

Voc

PmX

V

65

Voc

The open-circuit voltage, Voc, is the maximum voltage available from a solar cell, and this occurs at zero current.

IscI

Vm

Im

Pm

X

Voc

/0 ( 1)qV kT

total LI I e I= − −

by setting Itotal = 00

ln( 1)Loc

IkTV

q I= +

The open-circuit voltage corresponds to the amount of forward bias on the solar cell junction due to illumination.

Pm

IscI

Vm

Im

Pm

X

Voc

Power

Power out of a solar cell increases with voltage, reaches a maximum (Pm) and then decreases again.

Pm = Im x Vm

Remember we get DC power from a solar cell

FF

Isc

I

Vm

Im

Voc

Ideal diode curve

Pm

The FF is defined as the ratio of the maximum power from the actual solar cell to the maximum power from a ideal solar cellGraphically, the FF is a measure of the "squareness" of the solar cell

m m

oc sc

V IMax power from real cellFF

Max power from ideal cell V I= =

η

. m m

in

V IMax Cell Power

Incident light Intensity Pη = =

Efficiency is defined as the ratio of energy output from the solar cell to input energy from the sun.

The efficiency is the most commonly used parameter to compare the performance of one solar cell to another. Efficiency of a cell also depends on the solar spectrum, intensity of sunlight and the temperature of the solar cell.

IscI

Vm

Im

Pm

X

Voc

Power

oc sc

in

V I FF

Pη =

10/22/14 © IIT Bombay, C.S. Solanki

Solar Photovoltaic Technologies

69

Efficiency: η

. m m

in

V IMax Cell Power

Incident light Intensity Pη = =

Efficiency is defined as the ratio of energy output from the solar cell to input energy from the sun.

The efficiency is the most commonly used parameter to compare the performance of one solar cell to another.

IscI

Vm

Im

Pm

X

Voc

Power

Efficiency of a cell also depends on the solar spectrum, intensity of sunlight and the temperature of the solar cell.

oc sc

in

V I FF

Pη =

Applications

Fuel cell

A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent

Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used. Fuel cells are different from batteries in that they require a constant source of fuel and oxygen/air to sustain the chemical reaction; however, fuel cells can produce electricity continually for as long as these inputs are supplied.

The most important design features in a fuel cell are :The electrolyte substance. The electrolyte substance usually defines the type of fuel cell.The fuel that is used. The most common fuel is hydrogen.The anode catalyst breaks down the fuel into electrons and ions. The anode catalyst is usually made up of very fine platinum powder.The cathode catalyst turns the ions into the waste chemicals like water or carbon dioxide. The cathode catalyst is often made up of nickel but it can also be a nanomaterial-based catalyst.

A typical fuel cell produces a voltage from 0.6 V to 0.7 V at full rated load. Voltage decreases as current increases, due to several factors:Activation lossOhmic loss (voltage drop due to resistance of the cell components and interconnections)Mass transport loss (depletion of reactants at catalyst sites under high loads, causing rapid loss of voltage).