ADVANCED ENGINEERING MATERIALS

INTRODUCTION: “Necessity is the mother of invention” this statement is really worth for recently developed engineering materials.— Advanced manufacturing techniques and precise characterisation facility of material have given a born to lots of new materials.— Major materials are developed by changing its original properties. This type of development leads to generate smart civilisation.— In this chapter, many recently developed materials and theirengineering applications will be discussed like metallicglass,shape memory alloys, energy materials etc.

METALLIC GLASSES—> Metallic Glass — word itself creates confusion as metal iscrystalline and glass is amorphous material.

As shown in fig. 7.1, two structures (a) Crystalline and (b) Non—crystalline are having same atoms but different atomicarrangement in unit area.

If we melt crystalline materials at high temperature and cool them rapidly, it shows similar type of structure.

Metallic glasses are made up of two ccmbinations(i) Metal—metal alloys like Ni-Nb, Cu-Zr, Mg-Zn, Fe-Zr etc.(ii) Metal—metalloids alloys like Pd—Cu—Zr,Pd—Cu—Si, Fe—B,Fe—Ni—B etc.

Preparation Method—> There are many methods for prepration like Drop smasher, melt-spinning, pendent—drop melt extraction and twin rollerquenching.* Out of all these methods melt—spinning method is discussed below.

Fig. : Schematic diagram of melt—spinning method

— As shown in fig. , material is being filled inside a quartz tube.Lower end of tube is covered by heating coil for melting ofmaterial placed inside.Once a proper melting of material is done, inert gas is injectedinside tube with highpressure.

— Now a molten alloy inside a tube is being dropped out on a rotatingcopper based cylinder which rotates with high speed.

Properties

Metallic glasses are having very high tensile strength. They arestronger than steel. Metallic glasses are ductile in nature. As shown in fig. , theyhave good elasticity

compared to various materials.

Fig. : Strength vs elasticity of various materials

They do not show any crystalline defects like blowholes, crakes or dislocations. Electrical resistivity of metallic glasses is higher than alloysdue to this eddy current loss

of these materials is very low. It was observed in 1975 that metallic glass shows superconductingnature. When metallic glass alloys are mixed with magnetic materials then it shows excellent

magnetic property with low coercivityand high electrical resistivity. This is major advantage for powertransformer to reduce heat loss.

Applications

In recent days, metallic glasses are used to fabricate MEMS(Micro Electrical and Mechanical Devices) devices due to theirfunctional properties.

Due to good elasticity, high resistivity and low coercivity, theyare used as core materials in power transformers. They tendto reduce eddy current loss and improve efficiency of transformer.

Due to high strength, low elastic modulus and good springproperty, they are used as magneto résistance sensors andpressure sensors. These sensors are having wide range andexcellent accuracy.

Metallic glass shows rapid response against electromagnetic field.Therefore they are used as magnetic tape recorder and sensors.

Due to their unique properties, following military components are being developed using metallic glasses like Fuses, Composite armour, Lightweight casings for ordnance, MEMS casings and components, Thin walled casings and components for electronics, Casings for night sights and optical devices, Missile components fins, nose cones, gimbals, bodies and Lighter weight fragmentationdevices.

Due to corrosion resistive properties, they are used in surgicalcomponents. Some metallic glasses are non-carcinogenic and they have - excellent strength; therefore

they are used as bone replacementinstead of titanium.

SHAPE MEMORY ALLOYS

The term alloy describes a solid structure that is composed oftwo or more elements. Shape memory alloys (SMAs) are uniquealloys in that they can “remember” an original shape after beingdeformed.

Nitinol is an example of a shape memory alloys. It is a combinationof Nickel and Titanium and it was developed by the NavalOrdnance Lab in 1962.

Principle of formation of Shape Memory Alloys

Shape memory alloy” is a unique class of metal alloy thatcan recover apparent permanent strains when they are heatedabove a certain temperature.

As shown in fig., the shape memory alloys have two stablephases.(i) The high temperature phase, called Austenite (named afterEnglish metallurgist

William Chandler Austen)(ii) The low temperature phase, called martensite (named after German

metallographer Adolf Martens).

Fig.: Austenite and martensite phase

Now at lower temperature, SMAs exist in the Martensite form which is the soft and easily deformable phase. To return toundeformed state, value of temperature is taken down to 10 °C.

At higher temperatures, Austenite phase of SMA occurs. Thisis a stronger phase and having cubical arrangement. Here, wecan give specific shape to shape memory alloy.

Rapid cooling of material causes a change of state from Austeniteto Martensite. In the un deformed state, the molecules are twinned as shown in the fig. In deformation

state, the molecules are no longerin twinned shape. Now, let’s discuss how temperature changes state of materialwithout any mechanical

loading. This can be discussed with thehelp of graph and schematic diagram as shown below

Fig. Temperature induced transformationwithout mechanical pressure

As per schematic diagram shown in fig. in first phase, when temperature of system is reduced (cooling phase —Austenite to Martensite) material transforms from Austenite state toMartensite state without any mechanical loading.

Now at fixed particular pressure, when we increase temperature, material transforms from Martensite to Austenite. (heating phase— Martensite to Austenite)

Entire process occurs in temperature range of 10 °c to 50°c. So, change in phase with critical temperature can be explained asbelow:

As shown in fig., conversion of materials from austenite tomartensite starts at Msand ends at Mf.

1. Ms (Martensite start temperature) :At this temperature, transformation starts from austenite to martensite.

2. Mf (Martensite finish temperature) :At this temperature, transformation ends and material is fully transformed intomartensite.

Conversion of material from martensite to austenite starts at Asand ends at Af.3. As (Austenite start temperature):From this temperature, transformation starts from

martensite to austenite.4. Af (Austenite finish temperature): At this temperature, transformation ends and

material is fully transformed into Austenite.

Fig. Change in shape with time and temperature without mechanical deformation (Austenite to martensite and martensite to austenite)

Shape memory effectShape memory effect is observed in two ways:

(1) One way shape memory effect and(2) Two way shape memory effect. One Way Shape Memory Effect

In this SMA can be deformed below austenite start temperatureholds the shape until heated above transition temperature.

Two Way Shape Memory EffectHere material remembers both shapes and it can be changedwith temperature.

Two of the most common methods which are usedto create two-way memory through the introduction of dislocation arrays are, (1) Cyclic deformation at a temperature below Mfwhich is followed by constrained

heatingin the coldshape at a temperature above Af

(2) Cyclicdeformation between hot and cold shapes at a temperature above Af.

Pseudo elasticity or super elasticityIn pseudo elastic method, external stress is applied on SMA toachieve desired shape.

As shown in fig., temperature of system is kept constant and through mechanical loading and unloading transformationof material is being achieved.

In initial phase, austenite phase is achieved under constant temperature. External mechanical loading is increased onmaterial till it completely transforms from austenite to martensite.

As we release pressure, martensitebegins to transform into austenite. Finally material comes back to its original shape.

Fig. Pseudo elasticity effect

Properties1. It can remember shape and can regain original shape by applyingheat, stress, magnetic

field etc.2. It has good multi—shape properties.3. Many SMA have ultimate mechanical strength so they are usedin many heavy

applications.4. It has precise changes with respect to temperature so it helpsto control system.

Applications1. They are used in antenna and smart phone as actuators.2. They are used to control low pressure pneumatics. They arealso used into car seats to

adjust its location.3. They are used in dental surgery and dental braces.4. Eye glass frames are prepared from SMAs to achieve maximumfold and flexibility.5. They are used as a transducer to control temperature becausethey change shape with

temperature and allow gas to flow orstop.

BIOMATERIALS In broad definition, the materials that provide an intimate contact to living tissue;

organ or implants without any adverse reaction are known as “Biomaterials”. They. areused to repair, replace or for additional implantationin the field of medicine,

biomedical etc. Based on interactions biomaterials are classified into three differentcategories

1. Bio inert materials2. Bioactive materials3 Biodegradable materials

Bio inert MaterialsThese materials have minimum interaction with surroundingtissues. They are majorly used

as implants, surgical tools and fixtures.Stainless steel, titanium and alumina are examples of bio inert materials. They are also

used in wires and screws to curefractures. Bioactive materials

These materials have direct interaction with surrounding oftissues and bones. They are used as bio implants. Sometime theirinteraction is timely modified and adjusted as per the kineticmodifications.Materials like glasses, glass—ceramics, calcium phosphateceramics, composites &Coatings, bond to living tissues etc. areexamples of bioactive materials. They are used for dental filling,bone filling, tissue repairing etc.

Biodegradable Materials These materials are made to replace living tissues or organsinside body. Biodegradable materials start reacting with humantissues and dissolves slowly, at last they replace tissue or organ.Calcium oxide, gypsum, plyactic—polyglycolic acid, polymers andtricalcium carbonate are examples of biodegradable materials.

Based on material, biomaterials are classified in different types. Metals and Alloys

Metals and alloys are most common biomaterials in manyorthopaedic surgery, fracture fixation plates, screws and artificialjoints. They are also used as surgical tools, dental materials etc.

Stainless steel, titanium alloys and cobalt alloys are commonexamples of metal and alloys used as biomaterials.

PolymersPolymers are of various types and they are having good elasticity.They are used in joint replacement, vascular prosthesis, dentalrestoration, and intraocular lens. PMMA, PVC, polypropylene, polyethylene and polyesters areexamples of polymers.

Composite materialsCompare to metal and polymers, composite materials are widelyused in dental applications, joint replacement and bone repair.They have very good strength compare to other biomaterials.Carbon composites are very popular in all composites.

Ceramics Mostly ceramics are used in dental application as dentures,cement and crowns. Ceramics are also used in other biomedicalfield.Alumina and hydroxyapatite are most common ceramics, usedin joint replacement, synthetic bones and prosthetic devices.Ceramics have excellent strength, good bio-compatibility andhigh corrosion resistance.

SOLAR CELL Solar cell is a component, which is used to convert sunlight into electricity. It is

made up of semiconductor material in which silicon crystal is most common.silicon crystals are laminated into n—type and p—type layers,stacked on top of each other. Usually it is installed in arraysto get maximum output.

Photovoltaic EffectThe photovoltaic effect is the generation of voltage in amaterialwhen it is exposed to light. It works as belowWhen sunlight radiates to solar panel, photons are absorbedby semiconductor. Photon stimulates election from atom and generateselectrical potential difference. Current starts flowing throughmaterial. And this generates direct current (DC)

Importance of material in solar cell fabricationEfficiency of solar cell depends upon type of material used insolar cell. Highest research work is being done to increaseefficiency of solar cell.Materials used currently for photovoltaic solar cells includecrystaffineefficon as a bulk material and thin films of cadmiumtelluride, copper indium gallium, Gallium arsenide etc.

Bulk Crystalline Silicon MaterialBasic structure of solar cell is prepared by bulk crystafline siliconmaterials. Bulk silicon is seperated into various categoriesaccording to its various crystalline types. These are(i) Monocrystalline Silicon(ii) Poly Crystalline Silicon(iii) Ribbon Silicon

Thin Film Solar cell materials

Thin film materials are sandwiched between two panels of glassto make a solar cell. It reduces amount of material requiredto prepare solar cell.It is observed that efficiency of thin film solar panels is lower comparedto Silicon crystal by 2% to 3%.

ULTRACAPACITORS Ultra-capacitors are also known as double—layer capacitors orsuper capacitors. Ultra capacitors are energy storage devices that storeselectrostatic energy by

polarising an electrolytic solution.In this method, no chemical reaction takes place when energyis being stored. Due to

this life cycle of device is very high. Principle Energy is stored in super capacitors or ultra-capacitorsbypolarising electrolytic

solution. The polarisation of charges isdone via electrode interfaces. Construction This device is made up of electrolyte material, metal platecollectors and porous

electrodes. As shown in fig., there isan interface membrane which separates negative and positiveplates. This membrane is called separator.

WorkingBasically, carbon sheet is used as electrolytic separator. This carbon sheet is placed in such a way that it should have high surface area. Highly porous carbon will store more energy.

Now, as shown in fig., when voltage is applied to positive p1ate, it attracts negative ions from electrolyte. And when negative voltage is applied to plate it attracts positive ions from electrolyte.

So, formation of a layer of ions on both the sides of plate is seen. This phenomenon is called “Double layer” formation.

Super capacitor stores energy through electrostatic charges on opposite surfaces of the layer. Separators will not allow charges to move across electrodes.

Now, amount of energy stored by supercapacitor is very high due to large surface area of carbon electrodes.

Materials

In ultra-capacitor more number of chargesare stored because carbon material is fabricated on large surfacearea of capacitor.Now, nano carbon has large surface to volume ratio and it hashigh stability against temperature and corrosion resistance.

For fabrication of ultracapacitors various forms of carbon materialsare taken which are discussed below(1) Charcol electrode: This is basically a fine powder of carbon material. Electrodesare made up of charcol because of its properties like “Spongymaterial”, low density and low cost.

(2) Carbon aerogels:Carbon aerogels are having large area and continuousnetwork of conductive carbon nanoparticles.

(3) Carbon nanotubes:Single—walled, multi—walled carbon nanotubes are used dueto large surface area. CNT based ultracapacitois havehighest efficiency.

Applications:

Ultracapacitors have long life, rapid charging capacity, low cost,high storage capacity and fast discharging. Due to theseproperties they are used in various applications listed below:

— Computer systems— UPS systems— Power conditioners— Welders— Inverters— Automobile regenerative breaking systems— Power supplies— Cameras—Power generators

FUEL CELLS:

Fuel cells” are electrochemical devices which convert energyof chemical reaction directly into electricity with heat as a byproduct.

. Principle of fuel cell is similar to the principle of primarybatteries except that the fuel and oxidants are stored externally and also continuous supply of fuel and oxidants has to be ensured.

Fuel cell consists of an electrolyte material placed between two electrodes. At anode fuel oxidies through oxident and it emits electron which flow towards cathode via external circuit.

Each cell generates around 0.7—0.8 volt and power of hundreds of watt. For higher output, cells are assembled in series and parallel to provide larger output.

Major fuel cells use hydrogen as negative electrode. This hydrogen is obtained from hydrocarbon fuels. With this platinum is used as catalysts to enhance reaction.

This reaction can be considered as

Advantages:

(1) It is pollution free process.(2) In limitation of conventional fossil fuel, this can be goodalternative.(3) The device and its set up is small in size hence it’s weight isless.

Disadvantages:

(I) It is very costly system as production is very less.(2) It is unfamiliar technology to industry.(3) Still system is at its development stage.