Thermal conductivity and diffusivity

 

Thermal conductivity and diffusivity

Thermal conductivity

What is thermal conductivity?

When a system gets heat, it stores some of that heat energy and transports the rest of the heat energy to other systems. The ability of a system to transport thermal energy is called the thermal conductivity of the system. Basically, this is a system transport property.

Thermal conductivity is represented by k. The unit of thermal conductivity we saw earlier is W / m * K.

Thermal conductivity of solids, liquids, and gases

What governs the conduction of solids, liquids, and gases?

Before analyzing the thermal conductivity of the various phases, let's look at the phenomena that govern thermal conductivity through solids, liquids, and gases.

In solids, heat can be conducted by two mechanisms. One is lattice vibration and the other is the flow of free electrons. The increased lattice vibration facilitates the transport of thermal energy through the medium. The flow of free electrons increases electrical conductivity. It is also useful for the process of diffusion of thermal energy through the medium.

In liquids and gases, heat conduction is caused by two main mechanisms. The first is collisions between atoms, molecules, or ions, and the second is molecular diffusion. As the number of collisions increases, so does the energy exchange between molecules. This helps transport thermal energy through the medium. Molecular diffusion is the medium that is random in the movement of molecules in a medium. The increased random movement of molecules impedes the transport of thermal energy in certain directions.

In non-metals, the lattice vibration effect plays a dominant role. Non-metals generally have high electrical resistance and impede the flow of electrons. Therefore, for non-metals, it is k ~ kl.

The lattice component of thermal conductivity is strongly dependent on the method of molecular placement. For example, wood, which is an amorphous solid (where the molecules are very disorderly arranged), has a relatively low thermal conductivity value and acts as a heat insulator. Next, let's think about diamonds. A highly ordered crystalline solid. Therefore, it has the highest thermal conductivity at room temperature. Beryllium oxide which is -(BeO), that is also a non-metal and has relatively a high thermal conductivity due to its crystallinity.

Metals are excellent electrical and thermal conductors because they have free electrons and lattice vibrations. On the other hand, non-metals do not have free electrons, so they are electrically non-conductive materials. And in general, basically, non-metals which are like wood those are thermally non-conductive materials. However, non-metals such as diamond and beryllium oxide are excellent thermal conductors. As a result, such materials are widely used in the electronics industry. for example. A diamond heat sink is used to cool electronic components.

Pure alloys have high thermal conductivity. Alloys made of two metals with thermal conductivity k1 and k2 are expected to have a conductivity k between k1 and k2. Surprisingly, this is not the case. The thermal conductivity of alloys of two metals is usually much lower. For example, the thermal conductivity of copper and aluminum is 401W / m ° C and 237W / m ° C, respectively.

What factors depend on the thermal conductivity of liquids and gases?

In the gas, the collision of molecules is effect plays a dominant role. Molecular diffusion, which represents the randomness of the medium, plays a small role. Which Increased their molecular collisions increase by the exchange of energy between these molecules. This improves the thermal conductivity of the gas.

 

In liquids, molecules are relatively tightly packed than gases. Therefore, the thermal conductivity of a liquid mainly depends on the molecular diffusion effect, that is, the random movement of molecules. As we saw earlier, the increased random movement of molecules impedes the flow of heat through the liquid.

Comparison of thermal conductivity of solid, liquid, and gas

Changes in thermal conductivity due to temperature and pressure

How does thermal conductivity change with temperature?

In the case of pure metals and alloys, the thermal conductivity depends mainly on the electronic effect. As the temperature rises, both the number of free electrons and the lattice vibration increase. Therefore, the thermal conductivity of metals is expected to increase. However, the increase in lattice vibration impedes the flow of free electrons through the medium. Due to the combined effect of this phenomenon, in most cases, the thermal conductivity of metals and alloys decreases with increasing temperature. There are some exceptions to this rule. For iron, the thermal conductivity initially decreases and then increases slightly with increasing temperature. For platinum, the thermal conductivity increases with increasing temperature.

In gases, molecular collisions increase with increasing temperature. Therefore, the thermal conductivity of the gas increases with increasing temperature.

In liquids, as we have seen before, thermal conductivity depends primarily on the molecular diffusion effect. As the temperature rises, the randomness of molecular movement increases. This impedes the transfer of heat through the liquid. Therefore, the thermal conductivity of a liquid decreases with increasing temperature. However, there is one exception, pure water. In the case of pure water, the thermal conductivity first increases with increasing temperature and then begins to decrease.

 

 

How does thermal conductivity change with pressure?

Most solids and liquids are incompressible in nature, so their thermal conductivity does not change with pressure.

In the case of gas, the kinetic theory of gas is predicted, and experiments have confirmed that the thermal conductivity of gas is proportional to the square root of temp. T, & Inversely proportional to the sq. root it's of molar mass M. The gas is pressure independent over the wide range of pressures that are actually encountered.

Thermal diffusivity

What is thermal diffusivity?

When a system gets heat, it stores some of the heat energy and transports the rest of the heat energy to other systems. As we have seen, the ability of a material to transport thermal energy is called thermal conductivity. The heat storage capacity of a material is called the heat capacity of the material. The heat capacity of a material is expressed in Cp.

Thermal diffusivity represents the rate at which heat diffuses throughout the material. It is defined as

 

Note that:- Thermal conductivity is represented that how much heat is material conducts, and heat capacity Cp represents how much energy a material store per unit volume. Therefore, That the Thermal diffusivity of any material may be thought of as the ratio of heat conducted through any material to the heat stored per unit volume.

How is it related to thermal conductivity?

Materials with high thermal conductivity or low heat capacity have higher thermal diffusivity. The higher the thermal diffusivity, the faster the heat transfer to the medium.