Properties of Engineering Materials: Optical and Thermal

In this article we will discuss about the optical and thermal properties of engineering materials.

Optical Properties of General Engineering Material:

Optical property deals with the response of a material against exposure to electromagnetic radiations, especially to visible light. When light falls on a material, several processes such as reflection, refraction, absorption, scattering etc.

1. Refraction:

When light photons are transmitted through a material, they causes polarization of the electrons in the material and by interacting with the polarized materials, photons lose some of their energy. As a result of this, the speed of light is reduced and the beam of light changes direction.

2. Reflection:

When a beam of photons strikes a material, some of the light is scattered at the interface between that we media even if both are transparent. Reflectivity, R, is a measure of fraction of incident light which is reflected at the interface.

3. Absorption:

When a light beam is striked on a material surface, portion of the incident beam that is not reflected by the material is either absorbed or transmitted through the material. The fraction of beam that is absorbed is related to the thickness of the materials and the manner in which the photons interact with the material’s structure.

4. Rayleigh scattering:

Here photon interacts with the electron orbiting around an atom and is deflected without any change in photon energy. This is more vital for high atomic number atoms and low photon energies. Ex. Blue colour in the sunlight gets scattered more than other colors in the visible spectrum and thus making sky look blue.

a. Tyndall Effect:

Here scattering occur form particles much larger than the wavelength of light Ex. cloud look white

b. Compton Scattering:

In this incident photon knocks out an electron from the atom losing some of its energy during the process.

5. Transmission:

The fraction of beam that is not reflected or absorbed is transmitted through the material. Thus the fraction of light that is transmitted through a transparent material depends on the losses incurred by absorption and reflection. Thus, R + A + T = 1

where R = reflectivity,

A = absorptivity, and

T = transitivity

6. Thermal Emission:

When a material is heated electrons are excited to higher energy levels generally in the outer energy levels where the electrons are less strongly bound to the nucleus. These excited electrons, upon returning back to the ground state, release photons in process termed as thermal emission.

By measuring the intensity of a narrow band of the emitted wavelengths with a pyrometer, material’s temperature can be estimated.

7. Electro-Optic Effect:

The behaviour of a material in which its optical isotropic nature changes to anisotropic nature on application of an electric field. This effect is seen in LiNbO₃, LiTiO₃ etc.

8. Photoelectric Effect:

Phenomenon in which the ejection of electrons from a metal surface takes place, when the metal surface is illuminated by light or any other radiation of suitable frequency (or wavelength). Several devices such as phototube, solar cell, fire alarm etc. work on this effect (principle).

9. Photo Emissivity:

Phenomenon of emission of electrons from a metal cathode, when exposed to light or any other radiations.

10. Brightness:

Power emitted by a source per unit area per unit solid angle.

Photo Conductivity- Phenomenon of increase in conductivity of a semi-conductor due to excess carriers arisen from optical luminescence.

Optical Properties of Non-Metals:

i. These materials may be transparent, translucent, or opaque. Therefore, they exhibit different optical properties such as reflection, refraction, absorption and transmission. The phenomenon of refraction is more dominant in them.

ii. The non-metals which are transparent are generally coloured due to light absorption and remission in the visible region by them. Absorption of light occurs due to: Electronic polarization.

iii. Excitation of electrons from filled valence band to empty state within conduction band, and Wide band gaps in dielectric materials.

iv. The non-metallic transparent materials transmit light due to net energy formed by absorption and reflection processes.

Optical Properties of Metals:

i. In metals, the valence band is partially filled and so there are large number of quasi continuous vacant energy levels available within the valence band. When light is incident on metals the valence electrons absorb all frequencies of visible light and get excited to vacant states inside the valence band (intra-band transitions). This result in the opacity of metals.

ii. The total absorption of light by the metal surface is within a very thin outer layer of less than 0.1 jam. The excited electrons return back to lower energy states thereby causing emission of radiation from the surface of the metal in the form of visible light of the same wavelength. This emitted light which appears as the reflected light is the cause of the lustrous appearance of metals.

iii. In copper, inter-band transitions occur for energies greater than 2.2 eV i.e. the photons of energy greater than 2.2 eV are strongly absorbed. This energy corresponds to wavelength below 5625 Å. This means that the radiation in the blue-violet range is absorbed. This is reason for the reddish-orange colour of copper.

iv. In silver and aluminium, there is no absorption in the full range of visible radiation. So, the re-emission occurs over the entire wavelength range of the visible spectrum due to which the white colour of these metals exist.

v. Gold appears yellow because there is absorption in green portion and reflection in yellow and red region.

Optical Properties of Semiconductors:

i. Intrinsic semiconductors at low temperatures have a completely filled valence band and an empty conduction band. So no intra-band transitions can occur in semiconductors.

ii. Radiation of low frequencies, i.e. infrared radiation are not absorbed and that’s why semiconductors are transparent to infra-red radiation.

iii. The energy gaps in semiconductors are in the range of 0.5 – 3eV. So inter-band absorption occurs for radiation in this range which corresponds to near infra-red and visible range, this is responsible for the opacity of semiconductors.

Visibility Range of Light Spectrum:

i. Semiconductors infrared region-InSb, Ge, Si, GaAs.

ii. Semiconductors lying in visible region- CdSe (red visibility) GaP (yellow visibility).

iii. Semiconductors lying in ultraviolet region- CdS (green visibility), SiC (blue visibility) ZnS.

Luminescence:

Luminescence is the property by which a material emits the light.

In semiconducting materials, the light is emitted under certain conditions which are as below:

(i) When electron-hole pairs (EHP) are generated, or

(ii) When the carriers fall to their equilibrium state after being excited to higher impurity levels.

Different Types of Luminescence:

1. Photo-Luminescence:

It is the phenomenon of emission of light from a semiconductor on account of recombination of excited electron-hole pair (EHP).

Here one photon is emitted from each photon absorbed. Recombination in semiconductors takes place at varying rates; fast and slow.

Accordingly, the photo-luminescence may be of following two types:

a. Fluorescence:

It is a fast process property of material in which the emission of photon stops in about 10^–8s after the excitation is removed.

Example:

(i) Glass surface coated with tungstates or silicates such as in fluorescent lamps.

(ii) Television screen coated with sulphides, oxides, tungstates etc.

b. Phosphorescence:

Slow process property of material in which the emission of photon continues for a longer duration, lasting for seconds and minutes after removal of excitation.

Materials falling in this category are termed as phosphors, example:

a. ZnS coated with Cu an impurity

b. CdS coated with Ag as impurity

c. KCI coated with Tl as impurity

d. Nal coated with Tl as impurity

Example:

‘Fluorescent lamp’. It is a glass tube filled with a gas, which is generally the mixture of mercury vapours and argon. The inside of the tube has a fluorescent coating. When an electric discharge is induced between electrodes of the tube, the atoms of the gas are excited and emit photons.

2. Electro-Luminescence:

This effect can be created by introducing the electric current into a semiconductor. The electrical current can be used in different ways to generate the photon emission from semiconductors. One such way is ‘injection’.

The name of the process is injection electro-luminescence which is use in making light-emitting diodes (LEDs).

In them the minority carriers are injected by electric current, into the regions of a crystal where they can recombine with majority carriers. It results in emission of recombination radiation.

The effect of electro-luminescence can be found in devices incorporating the phosphor powder (such as of ZnS) in a plastic binder.

This phosphor gives-off the light when an alternating current (a.c.) filed is applied on it. Such device is known as ‘electro-luminescence cell’, which is used as lighting panel.

Destriau effect- The emission of photons in certain phosphors occurs when they are subjected to alternating electric field, was observed for the first time by Destriau. Hence this phenomenon is known as ‘Destriau effect’.

Optical Properties of Insulators:

i. Insulators have completely filled valence band and so like as in semiconductors, no intra-band transitions can occur.

ii. The energy gap in insulators are greater than 5 eV and so no inter-band transition can occur in the visible range of radiation.

iii. Absorption occurs only for the ultraviolent radiation. Insulators are transparent from infra-red up to the ultra-violet radiation.

Examples:

a. Perfect diamond crystal

b. Fused quartz

c. Window glass

Non Transparent Insulator:

a. Enamels,

b. Porcelains,

c. Opal glass etc.

iv. Above materials are opaque because the incident radiation gets scattered in all direction by the small particles present in these materials.

v. Due to this, there cannot be perfect transmission. Part of the radiation is diffusely transmitted and part is diffusely reflected. This makes the materials appear opaque.

vi. If the particle size is of the order of the wavelength of visible radiation, there will be maximum scattering.

vii. For some applications, such particles are deliberately introduced in dielectrics to make them opaque.

Optical Absorption in Ionic Crystals:

i. Ionic crystals are insulators. The energy gap in these crystal are in the range of 5-8 eV. The electrons cannot absorb photons in the visible radiation and get excited to the conduction band. So the complete range of visible radiation is transmitted by ionic crystals and they are transparent.

ii. The absorption properties of ionic crystals change drastically if point defects such as lattice vacancy or Schottky defects are present in them. Because of this defect materials are found to be colored.

iii. Another method by which the optical absorption in ionic crystals can be changed is by adding impurities.

Thermal Properties of Engineering Materials:

The responses of solids against the thermal effects are termed as thermal properties of materials. Proper selection of materials for favourable low and high temperature applications requires knowledge of their thermal properties.

For example- Liquid ammonia (NH₃) and liquid oxygen (O₂) require thermal protection at very low (cryogenic temperatures) for storage purpose.

Electric bulb needs to be protected from thermal fatigue at high temperatures.

Information about thermal conductivity, thermal, expansion, melting point and heat dissipation are very essential in the design and operation of power plants (gas and steam).

1. Heat Capacity:

Many engineering solids when exposed to heat experiences an increase in temperature i.e. it absorbs heat energy. This property of a material i.e. material’s ability to absorb heat energy is called its heat capacity, C. It is defined as the energy required to change a material’s temperature by one degree.

Where dQ is the energy required to produce a temperature change equal to dT.

Heat capacity is not an intrinsic property i.e. total heat a material. Hence another parameter called specific heat c, it defined unit mass (J/kg-K, Cal/kg-K)

With increase of internal energy, geometrical changes may occur parallely heat capacity is measured either at constant volume, C_V, or constant external pressure, C_p. The magnitude of C_p is always greater than C_v.

Heat energy absorption of a (solid, liquid or gaseous) material exists in mode of thermal energy vibration of constituent atoms or molecules apart from the other mechanical heat absorption such as electronic contribution. With increase of energy, atoms vibrate at higher frequencies.

However, the vibrations of adjacent atoms are coupled through atomic bonding, which may lead to movement of lattices. This may be represented as waves (phonon) or sound waves, Vibrational contribution of heat capacity is varies with temperature.

C_V equal to zero at 0 K, but increase rapidly with temperature.

2. Thermal Expansion:

After heat absorption, atoms started vibrating and having larger atomic radius, leads to increase in materials dimensions. The phenomenon is called thermal expansion.

Thermal expansion (α) defined as the change in the dimensions length, and is expressed as:

Where T₀ and T_f are the initial and final temperature (in K), l₀ and l_f are the initial and final dimensions of the material and ԑ is the strain, α has units as (°C)^-1. For range of 5 – 25 × 10^-6, for ceramics 0.5 – 15 10^-6.

A volume coefficient of thermal expansion, α_v (= 3α) is used to change with temperature.

An instrument known as dilatometer is used to measure the thermal expansion.

3. Thermal Conductivity:

The ability of a material to transport heat energy from high temperature region to low temperature region is defined as thermal conductivity.

Similar to diffusion coefficient, thermal conductivity is a microstructure sensitive property.

The heat energy transported across Q = kA ΔT/ΔI

Where k is the thermal conductivity material. It has units as W/m.K. Metals in the range 20-400, ceramics 2-50, while polymers have in order of 0.3.

Heat energy in solids in transported by two mechanisms: Lattice vibrations (photons) and electrons.

The amounts of energy transported depends electrons, their mobility i.e. type of material, lattice temperature. The thermal energy associated of their motion.

In ceramic phonon are responsible for thermal conduction. Main reason for experimentally observed low conductivity of ceramics is the level of porosity, as phonons are effectively scattered by imperfections. The scattering of phonons becomes more pronounced with rising temperature.

Hence, the thermal conductivity of ceramic materials normally diminishes with increasing temperature. Advanced ceramic materials like AIN, SiC are good thermal conductors, they are also electrical insulators. So, these materials are useful as electronic packaging substrates where heat dissipation is needed.

Thermal conductivity of polymers is even low, compared with ceramic materials. Vibration and movement/rotation of molecular chains transfer heat energy. In these materials thermal conductivity depends on degree of crystallinity; a polymer with highly crystalline and ordered structure will have higher conductivity then amorphous polymer.

Thermal conductivity of metals, alloys, semiconductors and dielectrics are in the decreasing order.

Relation between Coefficient of Thermal Expansion and Melting Point:

The coefficient of linear thermal expansion α and the melting point T_m of the solids are related as:

αT_m = constant = λ

The values of λ for different solids are as follows:

(i) λ = 0.02 for ionic compounds and metals,

(ii) λ = 0.03 for some salts, and

(iii) λ = 0.007 for covalent bonded oxides and glasses.

iv. Thermal Stress:

a. Thermal stresses are caused in a material due to temperature variation (thermal gradient) when thermal expansion is restrained. Example:

b. Welded construction of structures and the pressure vessels,

c. Joints of two railroad rails,

d. Jacketed thick cylinders that are shrink fitted,

e. Bimetallic strips in thermostatic controls,

f. Refractory bricks in metallic furnaces and ovens,

g. Outer skins of rockets and missiles,

h. Components of I.C. engines, and

i. Huge concrete structures such as dams.

Warpage:

The distribution of residual stresses is not always symmetrical within the material. Uneven cooling is a cause of such unbalanced stresses, it happens because when one surface of a material is cooled more rapidly than the other, the rapidly cooled surface generates compression whereas tension is developed on other surface. Such asymmetry produces ‘warpage’ and the material develops convexity towards rapidly cooled surface.

v. Spalling (or Thermal) Cracking:

The residual stresses produced within plastic materials may be relieved partially by warpage, but this is not so in case of non-plastic materials. In them, the dimensional changes cannot relieve the stresses, and the stresses in excess of elastic limit produce thermal cracking. This is called spalling. This is a very common phenomenon in glassware.

vi. Thermal Fatigue and Thermal Shock:

Thermal Fatigue:

Behaviour of a material under repeated heating and cooling is known as thermal fatigue. Due to thermal fatigue, thermal stresses of fluctuating nature are produced in the material which may eventually cause its thermal fatigue failure. The ability of a material to withstand such failure is called thermal fatigue resistance.

Thermal Shock:

A situation in the material, when there is a severe and sudden temperature change, is known as thermal shock. The capability of a material to withstand this effects of such drastic change is called thermal shock resistance.

Thermal shocks may be minimized in structural and machine systems by using the following devices:

1. Bellows,

2. Expansion loops,

3. Corrugated parts, and

4. Flexible joints

High Thermal Shock Resistivity Material:

a. Graphite

b. Cermet

c. Glass-ceramics

d. Lithium-ceramics

e. Fused silica

f. Pyrex glass

Technique of Reflective Insulation:

i. The technique of reflective insulation is found in tankers transporting liquid oxygen, liquid hydrogen and liquid nitrogen.

ii. In this technique, highly reflective surfaces are separated by large width of air space.

iii. Conduction and convection is minimum at about 20 mm width of air.

iv. Aluminium foils on paper, reflective aluminum surfaces separated by glass fibre lamina, and Mylar (aluminized plastic films) under high vacuum are used for this purpose.

v. Air is not suitable for thermal insulation at cryogenic temperatures as it solidifies at 81.3 K. Hence the insulating layers are kept free from air and are sealed.

Note:

High temperature effects are those observed above room temperature and low temperature effects are those observed below of it.

High Temperature Effect:

The Effects of high temperatures on materials is to cause the following effects:

i. Loss of Strength,

ii. Grain in ductility,

iii. Reduced stiffness,

iv. Lower yield strength,

v. Polymorphic transformations

vi. Decrease in hardness etc.