Thermal Expansion of Solids

Thermal Expansion means an increase in the dimensions of solid on heating. The dimensions increase in the same proportion. 

There are three types of thermal expansion which are- Linear expansion, superficial expansion, and cubical expansion. The expansion in length is known as linear expansion, and the average coefficient of linear expansion is the change in length over a temperature range. Metals, for example, expand more and have high values of coefficient of linear expansion. 

The expansion in area is known as area expansion, and the expansion in volume is called volume expansion. The coefficient of thermal expansion measures the fractional change in size due to a small temperature change. It describes how the size of an object changes with the temperature change. In the case of thermal expansion, water contracts on heating between 0 oC and 4 oC. The volume of a given amount of water decreases as it is cooled from room temperature until its temperature reaches 4 oC, and below 4 oC, the volume increases, and density decreases.

Origin of Thermal Expansion

On the mean separation of atoms at a temperature comes potential energy and thermal expansion originates from the effects of anharmonic terms in the potential energy. With the increase in temperature, the kinetic energy of atoms also starts increasing, resulting in atoms vibrating and moving faster, and thus there comes the vibrational origin of thermal expansion. For a substance with a constant separation of atoms at a temperature, its potential energy must be symmetric concerning the separation.  

What is Thermal Expansion of Solids?

When a solid is heated, its atoms and molecules vibrate faster in their fixed points, pushing each other apart. When heated, the relative increase in the size of solids is small.  There are various examples of thermal expansion of solids that we observe in our day-to-day life, such as Metal hot water heating pipes, metal-framed windows, railway bridges, and thermometers. Thermal expansion of solids has many applications in our life.

Coefficient of Thermal Expansion

The coefficient of thermal expansion is often defined as the fractional increase in length per unit rise in temperature.

How To Measure Coefficient of Thermal Expansion

There are two methods which are used to measure the coefficient of thermal expansion which are as follows:

• Dilatometry
• Interferometry

Applications of Thermal Expansion of Solids:

1. The gap between two consecutive rails: There is expansion and contraction of rail tracks in summer and winter respectively. Therefore gaps are kept for their expansion; otherwise, with the increase in temperature, the railway tracks will expand, and there might be chances of overlap or dislodging from the position, thereby causing accidents.
2. Removing Tight Lids of Glass jar to open the tight lids, it is immersed in hot water for a minute, and then the metal cap expands and becomes loose. The glass has a lower coefficient of thermal expansion than the material of the lid.
3. Hot milk is poured into a thick-walled glass vessel: When the milk is poured into a thick-walled glass vessel at room temperature, the vessel’s inner surface expands while the outer surface remains at room temperature. Thus, no expansion occurs at the outer surface because of uneven expansion, the glass cracks.
4. Design of AirCraft: The expansion in aircraft is 15-25 centimeters during its flight due to an increase in temperature. An increase in temperature is due to heat created by friction within the air.
5. Thick bottles for soft drinks: The walls of a bottle of soft drinks are made very thick to avoid bursting as the bottle contains gas.  

Disadvantages of Thermal Expansion in Solids:

If there are uses and applications of thermal expansion in solids, so there are a few disadvantages of thermal expansion in solids such as:

1. Change in shapes and dimensions of objects. For example, doors.
2. Wall collapsed due to bulging.
3. Cracking of glass tumbler due to heating. 
4. Bursting of metal pipes carrying hot water or streams.

Consequences of Thermal Expansion in Solids:

1. Due to thermal expansion, there is a change in the periodic potential of the lattice involved. 
2. There is a decrease of the band gap of semiconducting materials and a reduction in overlapping strength of adjacent orbital due to expansion of the lattice.
3. Thermal expansion of solids results in a decrease in the electrical conductivity of metals due to electron-phonon interactions.  
4. Thermal expansion of solids also results in poor thermal conductivity of non-metallic solids if the phonon-phonon interaction is predominant. 
5. If there is a mechanical expansion of solids, then there is a chance of generation of crystallographic defects, known as dislocations.

Conclusion 

We can thereby conclude that thermal expansion means an increase in the volume of material as its temperature increases. It is expressed as a fractional change in length or volume per unit temperature change. For the expansion of solid, there is a linear expansion coefficient, while a volume expansion coefficient is for liquid or gas.