# Thermal Properties of Materials

Thermal properties of materials refer to the properties of a body or region of space that determine whether or not there will be a net flow of heat into it or out of it from a neighbouring body or region and also in which direction the heat will flow.

If there is no heat flow, the bodies or regions are said to be in thermal equilibrium and at the same temperature.
If there is a flow of heat, the direction of the flow is from the body or region of higher temperature to the body or region of lower temperature.

Definition of Temperature Scale

Temperature is a measure of how hot a body is and a scale has to be needed to accurately determined its temperature.

First, a measurable physical property, X has to be selected that varies continuously with temperature.
Next, two standard temperature, called fixed points are chosen to create a linear scale. X is measure at upper fixed point, namely, boiling point of water and lower fixed  point namely, freezing point of water.
Temperature values are them assigned to upper and lower fixed points. For celsius scale, a temperature value of 100 is given to boiling water and a value of 0 is assigned to the freezing point. To find unknown temperature, Ti, simply measure Xi and plug into the following equation.

This method hinges on the assumption that a physical property varies linearly with temperature.

### Specific Heat Capacity

The Specific heat capacity of a solid or liquid is defined as the heat required to raise unit mass of substance by one degree of temperature.

It is the amount of heat energy required to raise the temperature of a unit mass by one unit. Cp is the specific heat capacity at constant pressure and Cv is specific heat capacity at constant volume.
Cp – Cv = R
where R is the gas constant.

### Specific Latent Heat

Specific latent heat is the energy per unit mass absorbed or evolved when a substance changes its phase.
There are two types of latent heat and those are: (i) latent heat of fusion ( the heat given out when a liquid changes into a solid), and (ii) latent heat of vapourization (the heat absorbed when a liquid turns into a gas).
It may be noted that for a given substance, the liquid is in a higher energy state than the solid and the vapour is in a higher energy state than the liquid.

### Thermal Expansion Coefficient

Thermal expansion coefficient is the percentage of dimensional change one can expect per unit increase in temperature. The coefficient is measure with a silatometer which records the change in a sample length over a range of temperature.

### Thermal Conductivity (l)

Thermal conductivity is a property of materials that expresses the heat flux f(W/m2) that will flow through the material if a certain temperature gradient dT (K/m) exists over the material.
The thermal conductivity is usually expressed in W/m-K and denoted as l. The usual formula that expresses l can be represented as f = l x DT

Fourier’s law of heat conduction represented as
q = -k dT/dX

where q is the heat flux and is measure in energy/(time. area); T, the temperature (K); X, the distance in the x-direction; and k is the constant of proportionality.

### Currently Used Techniques to measure thermal conductivity

There are a number of ways to measure thermal conductivity. In general, the steady-state techniques perform a measurement when the material that is analyzed is in complete equilibrium. The disadvantage is that it takes a long time to reach the required equilibrium.

### Thermal Sensor

In many physical phenomena heat is exchanged. Thermal sensor display some specific positive characteristics and those are given below.
Extreme stability
No moving parts
Large dynamic
Little or no energy consumption
The above mentioned characteristics will improve the reliability of measurement and control systems in which they are used. Below a common type of heat flux sensor, called thermopile, is used to explain some principles of measurement.

Measurements using Thermal Sensor

Various measurement like heat flux, radiation and mass flow rate can be determined using thermal sensors.

#### Heat Flux

Heat flux is induced by a temperature gradient across the sensor. The temperature difference is measured by a thermopile.

The absorber absorbs radiation. The radiation is converted into heat. The heat flux to the heat sink is measured by the thermopile.

#### Mass Flow and Heat Transfer Coefficients

A certain amount of heat is generated by a resistor. The ratio between the heat that flows to the heat sink and the heat that was originally generated is a measure for the mass flow. This ratio can be determined using a thermopile.

### Thermal Properties of Materials

Thermal properties of materials smoothen the calculation of the following properties.

Thermal conductivity – Thermal conductivity is the property of a material to conduct heat. Thermal conductivity can be defined as “the quantity of heat transmitted through a unit thickness of a material – in a direction normal to a surface of unit area – due to a unit temperature gradient under steady state conditions”

Thermal Diffusivity – In a sense, thermal diffusivity is the measure of thermal inertia. In a substance with high thermal diffusivity, heat moves rapidly through it because the substance conducts heat quickly relative to its volumetric heat capacity or ‘thermal bulk’.

### High Temperature Materials

Materials at high temperature serve the needs of those who develop and use materials for high temperature applications. The industrial sectors covered include metal extraction, alloy manufacture, chemical processing, power engineering, engine and furnace industries, Within these sectors, the effect of high temperatures on materials performance, particularly creep, fatifue, strength, oxidation, corrosion and wear processes falls within the remit of the research publication.

### Super-alloys

The term “super-alloy” was first used shortly after the world war II to describe a group of alloys developed for use in turbo superchargers and aircraft turbine engines that required high performance at elevated temperatures.

### Materials for Cryogenic Applications

The structural materials undergo a significant change in their mechanical properties at sub-zero temperatures particularly in their toughness. So, the materials suitable for cryogenic applications are those, which have less significant effect on their properties in that temperature range. Most of the aluminium alloys show good properties in cryogenic conditions.

### Insulating Materials

Less dense materials are better insulators. A good insulator is obviously a poor conductor.

### R-value

The R-value of material is its resistance to heat flow and is an indication of its ability to insulate. It is used as a standard way of telling how good a material will insulate. The higher the R-value, the better the insulation is.