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Advanced Engineering Materials Metallic Glasses

   Posted On :  02.12.2016 11:21 pm

1 Introduction 2. Metallic glasses 3 Shape memory alloys 4 Nanotechnology 5 Synthesis techniques 6 Discuss the properties of nanophase materials 7 Applications of nanophase materials 8 Non-linear materials and bio-materials

ADVANCED ENGINEERING MATERIALS METALLIC GLASSES

 

1 Introduction

2. Metallic glasses

2.1 Methods of preparation

2.2 Preparation of metallic glasses

2.3 Types of metallic glasses

2.4 Properties of metallic glasses

2.5 Applications of metallic glasses

3 Shape memory alloys

3.1 Shape memory alloys

3.2 Types of shape memory alloys

3.3 Characteristics of SMA

3.4 Commercial shape memory alloys

3.5 Advantages of shape memory alloys

3.6 Disadvantages of shape memory alloys

3.7 Applications of shape memory alloys

4 Nanotechnology

4.1 Nano materials

4.2 Comparison of different objects

4.3 Classification of nanomaterials

4.4 Top-down and bottom-up process

5 Synthesis techniques

5.1 Pulsed laser deposition

5.2 Chemical vapor deposition

6 Discuss the properties of nanophase materials

6.1 Physical properties

6.2 Magnetic properties

6.3 Mechanical properties

7 Applications of nanophase materials

 8 Non-linear materials and bio-materials

8.1 Birefringence and Kerr effect

8.2 Non-linear properties and second harmonic generation

8.3 Non linear properties

8.4 Second harmonic generation

8.5 Biomaterials with their properties and applications

8.6 Classification of biomaterials

8.7 Applications

8.8 Ceramic


1 INTRODUCTION

 

New engineering materials such as metallic glasses, shape memory alloys etc. are the advanced materials, which are the integral part of our life. Both scientists and technologists are searching for new materials, which can be used for high technology research as well as applications.

 

In this chapter, we are going to discuss the new engineering materials like metallic glasses, shpe memory alloys, etc., along with their properties and its wide range of applications.

 

2 METALLIC GLASSES

 

The Metallic glasses are materials which have the properties of both metals and glasses.

Metallic glass = Amorphous metal

In general, metallic glasses are strong, ductile, malleable, opaque and brittle. They also have good magnetic properties and high corrosion resistance.

 

2.1METHODS OF PREPARATION Principle

 

The principle used in making metallic glasses is extreme rapid cooling of the molten alloy. The technique is called as rapid quenching.

 

The cooled molten alloys are fed into highly conducting massive rollers at high speeds to give ribbons of metallic glasses.

 

2.1PREPARATION OF METALLIC GLASSES

 

Principle

 

The principle used in making metallic glasses is extreme rapid cooling of the molten metal alloy. This technique is called as rapid quenching.

 

Melt spinning system

 

A melt spinner consists of a copper roller over which a refractory tube with fine nozzle is placed. The refractory tube is provided with induction heater as shown in fig.

 

The metal alloy is melted by induction heating under inert gas atmosphere (helium or argon). The properly super heated molten alloy is ejected through the fine nozzle at the bottom of the refractory tube.

 

The molten alloy falls on the copper roller which is rotated at high speed. Thus, the alloy is suddenly cooled to form metallic glass. In this method a continuous ribbon of metallic glass can be obtained.


 

2.3 TYPES OF METALLIC GLASSES.

 

Metallic glasses are classified into two types:

 

(i)Metal –Metal metallic glasses

They are combination of metals                

          Metals                  Metals

Examples:   Nickel (Ni)  -        Niobium (Nb)

          Magnesium (Mg)  -        Zinc (Zn)

          Copper (Cu)         -        Zirconium (Zr)

 

(ii)             Metal –Metalloid metallic glasses

 

These are combinations of metals and metalloids.

 

Examples:  Metals                            Metalloids

 

Fe, Co, Ni            -        B, Si, C, P

 

2.4 PROPERTIES OF METALLIC GLASSES

 

Structural properties

 

1.     They do not have any crystal defects such as grain boundaries, dislocation etc.

 

2.     Metallic glasses have tetrahedral close packing (TCP).

 

 

Mechanical properties

 

1.     Metallic glasses have extremely high strength, due to the absence of point defects and dislocation.

 

2.     They have high elasticity.

 

3.     They are highly ductile.

 

4.     Metallic glasses are not work-harden but they are work –soften. (work harnening is a process of hardening a material by compressing it).

 

Electrical properties

 

1.     Electrical resistivity of metallic glasses is high and it does not vary much with temperature.

 

2.     Due to high resistivity, the eddy current loss is very small.

 

3.     The temperature coefficient is zero or negative.

 

Magnetic properties

 

1.     Metallic glasses have both soft and hard magnetic properties.

 

2.     They are magnetically soft due to their maximum permeabilities and thus they can be magnetised and demagnetized very easily.

 

3.     They exhibit high saturation magnetisation.

 

4.     They have less core losses.

 

5.     Most magnetically soft metallic glasses have very narrow hysteresis loop with same crystal composition. This is shown in fig.


Fig. Hysteresis loop of iron based alloy in crystalline and metallic glassy phase.

 

 

 

Chemical properties

 

1.     They are highly resistant to corrosion due to random ordering.

 

2.     They are highly reactive and stable.

 

3.     They can act as a catalyst. The amorphous state is more active than the crystalline state from the catalytic point of view.

 

2.5APPLICATIONS OF METALLIC GLASSES

 

Metallic glasses also called as met glasses have found wide applications in different fields.

 

Structural application

 

1.     They posses high physical and tensile strength. They are superior to common steels and thus they are very useful as reinforcing elements in concrete, plastic and rubber.

 

2.     Strong ribbons of metallic glasses are used for simple filament winding to reinforce pressure vessels and to construct large fly wheels for energy storage.

 

3.     Due to their good strength, high ductility, rollability and good corrosion resistance, they are used to make razor blades and different kinds of springs.

 

 

Electrical and Electronics

 

1.     Since metallic glasses have soft magnetic properties, they are used in tape recorder heads, cores of high-power transformers and magnetic shields.

 

2.     They use of metallic glasses in motors can reduce core loss very much when compared with conventional crystalline magnets.

 

3.     Superconducting metallic glasses are used to produce high magnetic fields and magnetic levitation effect.

 

4.     Since metallic glasses have high electrical resistance, they are used to make accurate standard resistance, computer memories and magneto resistance sensors.

 

Metallic glasses as transformer core material

 

5.     Metallic glasses have excellent magnetic properties. When they are used as transformer core, they give maximum magnetic flux linkage between primary and secondary coils and thus reduce flux leakage losses.

 

In view of their features like small thickness, smaller area, light weight, high resistivity, soft magnetic property and negligible hysteresis and eddy current loss, metallic glasses are considered as suitable core materials in different frequency transformers.

 

Nuclear reactor engineering

 

1.The magnetic properties of metallic glasses are not affected by irradiation and so they are useful in preparing containers for nuclear waste disposal and magnets for fusion reactors.

2.Chromium and phosphorous based (iron chromium, phosphorous-carbon alloys) metallic glasses have high corrosion resistances and so they are used in iner surfaces of reactor vessels, etc.

 

Bio-medical Industries

 

1.     Due to their high resistance to corrosion, metallic glasses are ideal materials for making surgical instruments.

 

2.     They are used as prosthetic materials for implantation in human body.

 

 

2     SHAPE MEMORY ALLOYS

 

3.1 SHAPE MEMORY ALLOYS

 

A group of metallic alloys which shows the ability to return to their original shape or size (i.e.,  alloy appears to have memory) when they are subjected to heating or cooling are called shape memory alloys.

 

Phase of shape memory alloys

 

Martensite and austenite are two solid phases in SMA as shown in fig.


Fig. Phases of SMA

 

Martensite is relatively soft and it is easily deformable phase which exists at low temperature (monoclinic) (fig.)

(i)                Austenite is a phase that occurs at high temperature having a crystal structure and high degree of symmetry (cubic) (fig.).

 

3.2TYPES OF SHAPE MEMORY ALLOYS

 

There are two types of shape memory alloys

 

(i)                One-way shape memory alloy

 

(ii)             Two-way shape memory alloy

 

A material which exhibits shape memory effect only upon heating is known as one-way shape memory. A material which shows a shape memory effect during both heating and cooling is called two-way shape memory.

 

Examples of shape memory alloys

 

Generally, shape memory alloys are intermetallic compounds having super lattice structures and metallic-ionic-covalent characteristics. Thus, they have the properties of both metals and ceramics.

Ni –Ti alloy (Nitinol)

 

Cu –Al –Ni alloy

 

Cu –Zn –Al alloy

 

Au –Cd alloy

 

Ni –Mn –Ga and Fe based alloys

 

 

3.3CHARACTERISTICS OF SMAS

 

1. Shape memory effect

The change of shape of a material at low temperature by loading and regaining of original shape by heating it, is known as shape memory effect.

 

The shape memory effect occurs in alloys due to the change in their crystalline structure with the change in temperature and stress.

 

While loading, twinned martensite becomes deformed martensite at low temperature.

 

On heating, deformed martensite becomes austenite (shape recovery) and upon cooling it gets transformed to twinned martensite (fig.).

 

2.SMAs exhibit changes in electrical resistance, volume and length during the transformation with temperature.

3.The mechanism involved in SMA is reversible (austenite to martensite and vice versa.)


 

4. Stress and temperature have a great influence on martensite transformation.

 

5. Pseudo elasticity

 

Pseudo –elasticity occurs in shape memory alloys when it is completely in austenite phase (temperature is greater than Af austenite finish temperature).

 

Unlike the shape memory effect, Pseudo-elasticity occurs due to stress induced phase transformation without a change in temperature. The load on the shape memory alloy changes austenite phase into martensite (Fig.).

 

As soon as the loading decreases the martensite begins to transform to austenite.

 

This phenomenon of deformation of a SMA on application of large stress and regaining of shape on removal of the load is known as pseudo elasticity.

 

This pseudo elasticity is also known as super elasticity



6. Hysteresis

 

The temperature range for the martensite to austenite transformation which takes place upon heating is somewhat higher than that for the reverse transformation upon cooling.

 

The difference between the transition temperature upon heating and cooling is called hysteresis. The hysteresis curve for SMAs is shown in fig.

 

The difiference of temperature is found to be 20-30oC,


 

 

3.4 COMMERCIAL SHAPE MEMORY ALLOYS

 

The only two alloy systems that have achieved any level of commercial exploitation are,

 

(i)                Ni-Ti alloys, and

(ii)             Copper base alloys.

 

Properties of the two systems are quite different.

 

1.       Nickel-Titanium Alloys

 

The basis of the Nickel-Titanium alloy is the binary, equi-atomic inter-metallic compound of Ti-Ni. The inter-metallic compound is extraordinary because it has moderate solubility range for excess Nickel or Titanium, as well as most other metallic elements. This solubility allows alloying with many of the elements to modify both the mechanical properties and the transformation properties of the system. Excess Nickel strongly depresses the transformation temperature and increases the yield strength of the austenite. The contaminants such as Oxygen and Carbon shift the transformation temperature and degrade the mechanical properties. Therefore, it is also desirable to minimize the amount of such elements.

 

Properties:

 

(i)                The Ni-Ti alloys have greater shape memory strain upto 8.5% tend to be much more thermally stable.

 

(ii)             They have excellent corrosion resistance and susceptibility, and have much higher ductility.

 

(iii)           Machining by turning or milling is very difficult except with special tools.

 

(iv)           Welding, brazing or soldering the alloys is generally difficult.

 

(v)             The material do respond well to abrasive removal such as grinding, and shearing.

 

(vi)           Punching can be done if thicknesses are kept small.

 

 

3.5 ADVANTAGES OF SHAPE MEMORY ALLOYS

They are simple, compact and high safe.

They have good bio –compatibility.

They have diverse applications and offer clean, silent and spark-free working condition

They have good mechanical properties and are strong corrosion-resistant.

 

 

3.6DISADVANTAGES OF SHAPE MEMORY ALLOYS

 

They have poor fatigue properties.

They are expensive.

They have low energy efficiency.

 

 

3.7 APPLICATIONS OF SHAPE MEMORY ALLOYS

 

1. Microvalve (Actuators)

 

One of the most common applications of SMAs is mocrovalves. Fig. shows a microvalve made of Ni –Ti alloy actuator. Actuator is a microsensor that can trigger the operation of a device. The electrical signal initiates an action.


Fig. Schematic of microvalves that open and close according to temperature

 

When an electrical current of 50 to 150 mA flows in Ni-Ti actuator, it contracts and lifts the poppet from the orifice and opens the valve.

 

2. Toys and novelties

 

Shape memory alloys are used to make toys and ornamental goods.

 

A butterfly using SMA. Moves its wings in response to pulses of electricity.

 

3. Medical field Blood clot filters

 

(i)                Blood clot filters are SMAs, properly shaped and inserted in veins to stop the passing blood clots.

 

When the SMA is in contact with the clot at a lower temperature, it expands and stops the clot and blood passes through the veins.

 

(ii)             They are used in artificial hearts.

 

(iii)           Orthodontic applications

 

NiTi wire holds the teeth tight with a constant stress irrespective of the strain produced by the teeth movement. It resists permanent deformation even if it is bent. NiTi is non-toxic and non-corrosive with body fluid.

 

(iv)           SMAs (NiTi) are used to make eye glass frames and medical tools. Sun-glasses made from superelastic Ni-Ti frames provide good comfort and durability.

 

4. Antenna wires

 

The flexibility of superelastic Ni –Ti wire makes it ideal for use as retractable antennas.

 

5. Thermostats

 

SMAs are used as thermostat to open and close the valves at required temperature.

 

 

6. Cryofit hydraulic couplings

 

SMAs materials are used as couplings for metal pipes

 

7. Springs, shock absorbers, and valves

 

Due to the excellent elastic property of the SMAs, springs can be made which have varied industrial applications. Some of them are listed here.

 

Engine micro valves

 

Medical stents (Stents are internal inplant supports provided for body organs)

 

Firesafety valves and

 

Aerospace latching mechanisms

 

8. Stepping motors

 

Digital SMA stepping motors are used for robotic control.

 

9. Titanium-aluminium shape memory alloys offer excellent strength with less weight and dominate in the aircraft industry. They are high temperature SMAs, for possible use in aircraft engines and other high temperature environments.

 

 

4      NANOTECHNOLOGY

 

4.1 NANO MATERIALS

 

Nanoparticles are the particles that have three dimensional nanoscale, the particle is between 1 and 100 nm in each spatial dimension. A nanometer is a unit of measure equal to one-billionth of a meter, or three to five atoms across.

 

Nanotechnology is the design, fabrication and use of nanostructured systems, and the growing, assembling of such systems either mechanically, chemically or biologically to form nanoscale architectures, systems and devices.

 

4.2COMPARISON OF DIFFERENT OBJECTS

 

1.       Diameter of sun    -        1,393,000km

2.       Diameter of earth -        1,28,000km

3.       Height of Himalaya mountain -        8,848km

4.       Height of man      -        1.65km

5.       Virus -        20-250nm

6.       Cadmium sulphide nanoparticle       -        1-10nm

 

 

4.3CLASSIFICATION OF NANOMATERIALS

 

1.     Clusters

 

A collection of atoms or reactive molecules up to about 50 units.

 

2.  Colloid

 

A stable liquid phase containing particles in 1 to 1000 nm range.  A colloidal particle is one such 1 to

 

1000 nm sized particle.

 

3.  Nanoparticle

 

A solid particle in the 1 to 100 nm range that could be non-crystalline, an aggregate of crystallites, or a single crystallite.

 

4. Nanocrystal

 

A solid particle that is a single crystal in the nanometer size.

 

5.  Nanostructured or Nanoscale Material

 

Any solid materials has a nanometer dimension.

 

Three dimensions  --- >   Particles

Two dimensions  --- >   Thin films

One dimension  --- >   Thin wire

 

6.  Quantum Dots

 

A particle that exhibits a size quantization effect in at least one dimension.

 

 

4.4 TOP-DOWN AND BOTTOM-UP PROCESSS

1. Top-down Process

 

In this processes, bulk materials are broken into nano sized particles as shown in


In to-down processes, the building of nanostructures starting with small components like atoms and molecules that are removed from a bulk material so as to obtain desired microstructure.

 

2.  Bottom-up Processes

 

In this processes, nano phase materials are produced by building of atom by atom as shown in.


This processes building larger objects from smaller buildings blocks. Nanotechnology seeks to use atoms and molecules as those building blocks. This is the opposite of the top-down approach. Instead of taking material away to make structures, the bottom-up approach selectively adds atoms to create structures.

 

 

5 SYNTHESIS TECHNIQUES

 

Nano materials are newly developed materials with grain size at the nanometre range (10-9m)

 

i.e., in the order of 1 –100 nm. The particle size in a nano material is in the order of nm.

 

5.1 PULSED LASER DEPOSITION

 

Priniciple

 

The laser pulse of high intensity and energy is used to evaporate carbon from graphite. These evaporated carbon atoms are condensed to from nanotubes.

 

Description

 

The experimental arrangement of pulsed laser4 deposition is shown in fig. A quartz tube which contains a graphite target is kept inside a high temperature muffle furnace.


Fig. Pulsed Laser Deposition CNT

 

This quartz tube is filled with argon gas and it is heated to 1473 K. A water cooled copper collector is fitted at the other end of the tube. The target material graphite contains small amount of nickel and cobalt as a catalyst to nucleate the formation of nanotubes.

 

Working

 

When an intense pulse of laser beam is incident on the target, it evaporates the carbon from the graphite. The evaporated carbon atoms are swept from the higher temperature argon gas to the colder copper collector.

 

When the carbon atoms reach the colder copper collector, they condense into nanotubes.

 

5.2CHEMICAL VAPOUR DEPOSITION

 

The deposition of nano films from gaseous phase by chemical reaction on high temperature is  known as chemical vapour deposition.

 

This method is used to prepare nano-powder.

 

Principle

 

In this technique, initially the material is heated to gaseous state and then it is deposited on a solid surface under vacuum condition to form nano powder by chemical reaction with the substrate.

 

Description and Working

 

The CVD reactor built to perform CVD processes is shown in fig.

 

Chemical vapour deposition (CVD) involves the flow of a gas with diffused reactants (substances to be deposited in the vapour) over a hot substrate surface. The gas that carries the reactants is called the carrier gas.


While the gas flows over the hot solid surface, the heat energy increases chemical reactions of the reactants that form film during and after the reactions.

 

The byproduct of the chemical reactions are then removed. The thin film of desired composition can thus be formed over the surface of the substrate.

 

 

6 PROPERTIES OF NANOPHASE MATERIALS.

 

Properties of Nanophase Particles

 

The mechanical, electrical, chemical, magnetic and structural properties of nanophase materials change with the reduction in the particle size of the material.

 

6.1 PHYSICAL PROPERTIES

 

Variation of physical properties with geometry

 

Starting from the bulk, the first effect of reducing the particle size is to create more surface sites. This in turn changes surface pressure and interparticle spacing.

 

(i)                Interparticle spacing decreases with decrease in grain size for metal clusters.

 

For example in copper, it decrease from 2.52 (cluster size –50A) to 2.23A (Cu dimer) fig.

 

The change in inter particle spacing and large surface to the volume ratio in particles have a combined effect on material properties. Therefore, the nanophase materials have very high strength and super hardness.

 

Because of the cluster of grains, the nano phase materials are mostly free from dislocations and stronger than conventional metals.

 


Fig. Interatomic distance in Cun as a function of grain size.

 

 

 

(ii)             Melting point reduces with decrease in cluster size.

 

The melting point of gold in nano phase (Aun) varies as a function of particle size (fig.)


 

Fig. Melting point of small Aun particles as a function of size

 

The melting point decreases from 1200 K to 800 K when the particle size decreases from 300 A to 20 A.

 

(iii)           Ionisation potential changes with cluster size of the nanograins.

 

The electronic bands in metals become narrower when the size is reduced from bulk which changes the value of ionization potential.

Fig. shows the ionization potential and reactivity of Fen clusters as a function of size. Ionisation potentials are higher at small sizes than that for the bulk and show marked fluctuations as a function of size.


Fig. Ionisation potential and reactivity of Fen clusters as a function of size (iv) The large surface to volume ratio, the variations in geometry and the electronic structure have a strong effect on catalytic properties.

 

As an example, the reactivity of small clusters is found to vary by higher orders of magnitude when the cluster size is changed by only a few atoms.

 

6.2 MAGNETIC PROPERTIES

 

Nanoparticles of non-magnetic solids also exhibit totally new type of magnetic properties.

 

(i)                Bulk magnetic moment increases with decrease in co-ordination number

 

The change in magnetic moment on the nearest coordination number is shown in fig.-0


Fig. Change in magnetic moment on the nearest coordination number

 

 

 

As the coordination number decreases, the magnetic moment increases with the atomic value which means that small particles are more magnetic than the bulk material.

 

The magnetic moment of iron (Fe) of nanoparticles is 30% more than that of bulk. At smaller sizes, the clusters become spontaneously magnetic.

 

(ii) The nano-materials shows variation in their magnetic property when they change from bulk state to cluster (nano-particle) state.

 

(iii)           Non-magnetic materials become magnetic when the cluster size reduces to 80 atoms.

 

6.2 MECHANICAL PROPERTIES

 

(i)    In nanophase materials, the elastic strength is low however, its plastic behavior is high.

 

(ii) In some nanophase materials, it is noted that there is decrease in hardness when the grain size is less than 10 nm.

 

However for many nanocrystalline, pure metals (10 nm), the hardness is about 2 to 7 times greater than that of large-grained (>1 μ m) metals.

 

(iii)Higher hardness and mechanical strength (2-7 times) when grain size reduces from 1 μ m to 10 nm.

 

(iv)                       It has very high ductility and superplastic behavior at low temperatures.

 

 

7 APPLICATIONS OF NANOPHASE MATERIALS.

 

1.Materials Technology

We can synthesis harder metals having hardness 5 times higher than normal metals using nanoparticles.

 

Stronger, lighter, wear resistant, tougher and flame retardant polymers are synthesized with nanoparticles as fillers. They are used in replacement of body parts and metals (bio-materials).

 

We can produce unusual colour paints using nanoparticles since nanoparticles exhibit entirely different optical properties.

 

Nanophase materials are used in annoelectronic devices such as nanotransistore, ceramic capacitors for energy storage, noise filters and stabilizers. The special features of these devices include smaller sizes and reduced power losses.

 

ZnO thermistors are used in thermal –protection and current-controlling devices.

 

 

2.  Information Technology

 

Nanoparticles are used for data storage.

 

Quantum electronic devices have started replacing bulk conventional devices.

 

Nano materials are used to produce very tiny permanent magnets of high energy products. Hence, they are used in high-density magnetic recording.

 

Magnetic devices made of Cu-Fe alloy are used in RAM, READ / WRITE heads and sensors.

 

Quantum dots, quantum wells and quantum wires are mainly produced from semiconductor nanomaterials. Hence, they are used in computer storage (memory) devices.

 

3.  Biomedicals

 

Biosensitive nanoparticles are used for tagging of DNA and DNA chips.

 

Controlled drug delivery is possible using nanotechnology. Diffusion of medicine through nanoporous polymer reservoir as per the requirement is very useful in controlling the disease.

 

 

Nanostructured ceramics readily interact with bone cells and bence finds applications as an implant material.

 

4.  Energy storage

 

Since the hydrogen absorbing capability increases with decrese of size of nanoparticles, nanoparticles of Ni, Pd and Pt are useful in hydrogen storage devices.

 

Metal nanoparticles are very useful in fabrication of ionic batteries.

 

5.  Optical devices

 

Nanomaterials      are   used   in   making   effici

 

 

Nanoparticulate zinc oxide is used to manufacture effective Sunscreens.

 

Nanoparticles are used in the coatings for eye glasses to protect from scratch or breakage.

 

 

6.     Transmission lines

 

Nanophase materials are used in the fabrication of signal processing elements such as filters,  delay lines, switches etc.

 

7.     Nanomicro-Electro Mechanical Systems (Nano MEMS) have direct implications on integrated circuits, optical switches, pressure sensors and mass sensors.

 

8.     Molecular Nano-Technology (MNT) is aimed to develop robotic machines, called assemblers on a molecular scale, molecular-size power sources and batteries.

 

9.     Underwater nanosensor networks are used to detect the movement of ships in an efficient manner with faster response. They can also detect chemical, biological or radiological materials in cargo containers.

 

 

8 NON-LINEAR MATERIALS AND BIO-MATERIALS

 

8.1 BIREFRINGENCE AND KERR EFFECT.

 

The appearance of double refraction under the influence of an external agent is known as artificial double refraction or induced birefringence.

 

Optical Kerr Effect

 

Anisotropy induced in an isotropic medium under the influence of an electric field is known as Kerr effect.

A sealed glass cell known as Kerr cell filled with a liquid comprising of asymmetric molecules is used to study the Kerr effect.

 

Two plane electrodes are placed in parallel to each other. When a voltage is applied to there electrodes, a uniform electric field is produced in the cell.

 

The Kerr cell is placed between a crossed polarizer system (Fig), When the electric field is applied, the molecules of the liquid tend to align along the field direction.

 

As the molecules are asymmetric, the alignment causes anisotropy and the liquid becomes double refracting. The induced birefringence is proportional to the square of the applied electric field E and      to   the   wavelength   λ   of   incident   light.


Fig. Kerr effect –Birefringence is induced in a liquid subjected to an electric field

 

The change in refractive influx is given by

∆μ= K λE2

Where K is known as the Kerr constant

 

8.2 EXPLAIN NON-LINEAR PROPERTIES AND SECOND HARMONIC GENERATION. Basic Principle of Non Linear Properties

 

We know that a light wave is electromagnetic in nature ie., it consists of electric and magnetic fields. When the light propagates through a material, it changes the properties of the medium, such as the refractive index. It depends on the electric and magnetic fields associated with the light.

 

For example, we could not observe nonlinear effects with the ordinary light beam of low intensity, since the electric and magnetic fields associated with the light beams is very weak.

 

With the invention of laser, it is now possible to have electric fields which are strong enough to observe interesting non linear effects.

 

Thus if electric and magnetic fields are strong enough, the properties of the medium will be affected which in turn will affect the propagation of the light beam.

 

8.3NON LINEAR PROPERTIES

 

Few of the nonlinear phenomena observed are

 

1.     Second harmonic generation

 

2.     Optical mixing

 

3.     Optical phase conjugation

 

4.     Soliton

 

 

8.4SECOND HARMONIC GENERATION

 

In a linear medium, polarization P is directly proportional to the electric field E

P E

 

P = εoχE

 

Whereo    ε-     Permittivity of free space

 

                                  χ - electrical susceptibility

 

In nonlinear medium for higher fields ie., higher intensities of light the non linear effects are observed.



 

In the above equation, 1st term gives rise to dc field across the medium, the second term gives external polarization and is called first or fundamental harmonic polarisability.

 

The third term which oscillates at a frequency 2w is called second harmonic of polarization and other terms are referred as higher harmonic polarization.

 

Both first term (dc field) and third term (second harmonic of polarization) added together is called optical rectification.

 

The second harmonic generation is possible only the crystals lacking inversion symmetry. SHG crystals are quartz, potassium dihydrogen phosphate (KDP), Ammonium dihydrogen phosphate (ADP), Barium titante (BaTiO3) and Lithium lodate (LiIO3)

 

The observation of second harmonic generation by KDP is shown in figure.


Fig. Arrangement for observing second harmonic generation

 

When the fundamental radiation (1.064 m) from Nd: YAG laser is sent through SHG crystal like KDP, conversion takes place to double the frequency. i.e., half the wavelength (0.532 m) takes place.

 

 

8.5 BIOMATERIALS WITH THEIR PROPERTIES AND APPLICATIONS.

 

 

The materials which are used for structural applications in the field of medicine are known as Biomaterials.

 

In the recent years, new biomaterials like nanobiomaterials are emerging up due to the requirements in the medical field for different applications.

 

8.6 CLASSIFICATION OF BIOMATERIALS

 

Based on the applications in the field of medicine, biomaterials are classified as

 

1.     Metals and alloys biomaterials

 

2.     Ceramics biomaterials.

 

3.     Polymer biomaterials.

 

4.     Composite biomaterials

 

Sometimes, a single material mentioned above cannot fulfill the complete requirements imposed for specific applications. In such case, combinations of more than one material are required.

 

Metals and Alloys

 

Metals and alloys are used as biomaterials due to their excellent electrical and thermal conductivity and mechanical properties.

 

TYPES OF BIOMATERIALS USING METALS AND ALLOYS

 

1.     Cobalt based alloys

2.     Titanium

 

3.     Stainless steel

 

4.     Protosal from cast alloy

 

5.     Conducting metals such as Platinum

 

 

8.7APPLICATIONS

 

The metals and alloys biomaterials are used in implant and orthopedic applications.

 

1.     Stainless steel is the predominant implant alloy. This is mainly due to its ease of fabrication and desirable mechanical properties and corrosion resistant.

 

2.     Proposal from cast alloy of Co –Cr –Mo is used to make stem and used for implant hip endoprosthesis.

 

3.     The advanced version of protosal –10 from Co –Ni –Cr –Mo alloy is widely used in Hip joints, Ankle joints, Knee joints, leg lengthening spaceas.

 

4.     ASTMF –136 (composition of Ti –6A1 –4V, EL1 alloy, forged) due to its high strength / weight ratio, high corrosion resistance and high bio compatibility, this alloy is used in dental applications for making screws, wires and artificial teeth.

 

5.     Ni –Ti shape memory alloy is used in dental arch wires, micro surgical instruments, blood clot filters, guide wires etc.

 

8.8CERAMICS

 

Ceramics are used as biomaterials due to their high mechanical strength and biocompatibility.

 

Types of Bio-Ceramic materials.

 

1.     Tricalcium phosphate

 

2.     Metal oxides such as Al2O3 and SiO2

 

3.     Apatite ceramics

 

4.     Porous ceramics

 

5.     Carbons and Alumina

 

Applications

 

 

1.     Ceramic implants such as Al2O3 and with some SiO2 and alkali metals are used to make femoral head. This is made from powder metallurgical process.

2.     Tricalcium phosphate is used in bone repairs.

 

3.     Orthopedic uses of alumina consists of hip and knee joints, tibical plate, femur shaft, shoulders, radius, vectebra, leg lengthening spaces and ankle joint prosthesis. Porous alumina is also used in teeth roots.

 

4.     Apatite ceramics are new bio active ceramics. They are regarded as synthetic bone, readily allows bone ingrowth, better than currently used alumina Al2O3.

 

5.     Carbon has good biocompatibility with bone and other tissues. It has high strengths and an elastic molecules close to that of bone.

 

6.     Carbon coatings find wide applications in heart valves, blood vessel grafts, percutaneous devices because of exceptional compatibility with soft tissues and blood.

 

7.     Percutaneous carbon devices containing high density electrical connectors have been used for the chronic stimulation of the cochlea for artificial hearing and stimulation of the visual cortex to aid the blind.

 

Bio Polymers

 

Biopolymers are macromolecules (protein, nucleic acids and polysachacides) formed in

 

nature during the growth cycles of all organisms.

 

Biopolymers find variety of applications as biomaterials. The most prominent among them are collagens, muco-polysaccharides –chitin, collagens and its derivatives.

 

Collagnes which are major animal structural proteins are widely used in a variety of forms such as solution, gel, fibers, membranes, sponge and tubing for large number of biomedical applications including drug delivery system, vessels, valves corneal prosthesis, wound dressing, cartilage substitute and dental applications.

 

Biomaterials in Opthamology

 

Biomaterials find important applications in opthalmology. They are used to improve and maintain vision. Eye implants are used to restore functionality of cornea, lens, etc, when they are damaged or diseased.

The biomaterials include viscoelastic solutions intraocular lenses, contact lenses, eye shields, artificial tears, vitreous replacements, correction of corneal curvature.

 

Dental Materials

 

Polymers, composites, ceramic materials and metal alloys are four main groups of materials used for dental applications.

 

A large number of materials are tested for porous dental implants, which include stainless steel,Co –Cr –Mo alloy, PMMA, proplast and Daceon, velour coated metallic implants, porous calcium aluminate single crystal alumina, bioglass, vitreous and pyrolytic carbons.

 

The dental applications include impression materials, dentine base and ceorons, bridges, inlays and repair or cavities, artificial teeth, repair of alveolar bone, support for mandible .


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