Thermal properties of materials

What are the thermal properties of materials, and why is it that materials such as steel are chosen for a particular product design and wood is not considered? Thermal properties are present in every product development, as the most diverse parts will have to cope with requirements such as being subjected to intense heat for a short period of time or, conversely, resisting temperature changes over a long period of time in the open air.

Indeed, when heat is supplied to a solid, liquid or gaseous body, some of its properties change. Thermal characteristics are associated with a material-dependent response and basically all material properties (physical, chemical, mechanical, electrical, magnetic and optical) are temperature-dependent, although there are materials specifically designed to resist extreme heat.

Among the thermal characteristics we find some related to heat transport (thermal conductivity, thermal diffusivity or heat capacity…), phase changes, such as first-order transitions (boiling and melting), or physical properties, which are altered when a body is subjected to a heat source.

In this article we look at some of the most important thermal characteristics of materials in engineering, such as heat capacity, thermal conductivity, thermal expansion, fusibility and weldability.

1. Heat capacity

Heat capacity is a property that indicates the ability of a material to absorb heat and change its temperature, thus measuring the external energy required to increase a unit of temperature (typically 1°C or 1°C).

In mathematical terms, heat capacity (C) is the rate of change of heat (Q) with respect to temperature (T):

C= dQ ÷ dT

In practical terms, heat capacity expresses how much more or less difficult it is for a body to undergo change when exposed to heat. For example, the heat capacity of the water in a swimming pool will be much greater than that of a glass of water, which we can easily heat in a microwave.

This is not to be confused with the concept of specific heat (represented by the lower case c), which refers to the heat capacity per unit mass. Thus, this capacity of a body to “store heat” is the quotient between the heat capacity and the mass of the object. The unit of specific heat in the International System is J/(kg∙K). Thus, it is possible to calculate the amount of heat in joules needed to increase the temperature of one kilo of a given substance by 1 degree.

Product development cannot ignore thermal properties and one of the tests performed on materials to determine their ageing is to subject them to a climatic chamber that mimics the behaviour of a material over a period of 0 to 10 years under defined climatic conditions.

Here you can see a practical example of its use in the accelerated ageing of materials to detect critical failures due to corrosion.

2. Thermal conductivity

Thermal conductivity is the ability of a material to transfer heat. Thermal conductivity is expressed in International System units as W/(m∙K). Metals, which are so capable of being extremely hot or icy, do not have the highest thermal conductivity, but diamond does. It is followed by silver, copper, silicon carbide, graphite, iron or steel.

The reason is that the atoms of metals have free electrons in the outermost layers, which allows them to move easily and transport thermal energy (as with electricity). This is not the case with many plastics, insulating materials or, for example, wood. It is therefore never a good idea to cover radiators with furniture.

Thermal conduction, on the other hand, is the phenomenon that causes heat to be transported from higher to lower temperature regions in a material or between different bodies. In fact, heat always flows from the higher to the lower temperature region.

3. Thermal expansion

Most materials expand when they are heated and contract when they are cold. Thermal expansion of materials represents their expansion when heated. It can be in length, volume or some other metric dimension. It can be measured in different ways such as:

  • Linear expansion: when variation in a single dimension predominates.
  • Cubic expansion: the coefficient of volumetric expansion compares the value of the total volume of a body before and after the change in temperature.
  • Area or surface expansion: when the body increases its dimensions in the same proportion.

4. Fusibility

Meltability is the ease with which a material can melt or fuse. It is clear that some materials, such as metal, glass or plastics, melt easily when heated, but this is not always of interest when selecting the materials for a product.

Knowing this ease or resistance to melting is essential for processes such as welding, where the alloy used for welding is required to have a low melting temperature compared to the materials to be welded. For soft soldering, lead and tin alloys are usually used, while for hard soldering, materials such as silver, copper or zinc are used.

On the other hand, there are refractory materials – which can withstand high temperatures without decomposing – such as aluminium, silicon and magnesium oxides, which are used in smelting furnaces and incinerators.

5. Weldability

This is the ability of one or more materials to bond two of their parts together in a homogeneous and high quality weld, so that they meet the requirements for which they were designed. This can be done by applying heat until the melting temperature is reached or by using an intermediate material for adhesion. Steel, aluminium, nickel, copper or titanium and their alloys are metals that are commonly used for welding.

In this practical example, through material innovation, the Infinitia team solved the challenge of finding the ideal material for sustainable corrosion resistant coatings, which also had high temperature, high humidity, weld compatible and cost effective requirements.

At INFINITIA Industrial Consulting we are experts in materials and we help dozens of companies and organisations to solve specific problems from this approach. Contact us for more information.