Thermal diffusivity vs conductivity

In summary, thermal conductivity and thermal diffusivity are both thermal properties of a material, but they measure different aspects of heat transfer. Thermal conductivity is the rate at which heat flows through a material in a steady state, while thermal diffusivity is a measure of how quickly a material can reach a steady state when a heat source is applied. While a material with high conductivity may also have low diffusivity, this is not always the case as diffusivity depends on the density and specific heat of the material as well. Therefore, both parameters are important to consider when designing for heat transfer in both steady and non-steady state conditions.
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amm508
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I'm not sure I fully understand the difference between thermal conductivity and thermal diffusivity as thermal properties of a material and would appreciate if anyone could clarify.

I understand thermal diffusivity is for a transient case and conductivity is for steady state.
This is what I've understood of the two parameters:
conductivity: rate at which heat flows through a material when one side is hot and the other is cold [W/mK]
diffusivity: a measure of how long it takes for a temperature pulse to traverse a certain thickness when a heat source is briefly applied to one side of a material. [m2/s]

If a material has high conductivity, wouldn't it automatically have low diffusivity as well? In this case why is there a need to define both of these?

Also in terms of design, if say a saucepan needs a material with high conductivity to ensure the heat from the stove is transferred through the base of the saucepan and to the food. It would also need a low diffusivity as well so that the material gets heated quickly.
 
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amm508 said:
If a material has high conductivity, wouldn't it automatically have low diffusivity as well? In this case why is there a need to define both of these?

Not necessarily.

thermal diffusivity = (thermal conductivity) / (density times specific heat)

specific heat is defined as energy needed to heat a unit mass of material by one degree, so
(density times specific heat) is the energy to heat a unit volume of material by one degree.

As you said, for steady state heat conduction the heat capacity is irrelevant (because the temperature isn't changing), and the rate of heat flow only depends on the thermal conductivity.

But for a non-steady state, you also need to consider the amount of energy it takes to heat up the material itself. If different materials have the same thermal conductivity, the one with the lowest (density times specific heat) will get to a steady state condition fastest, and that material will have the highest thermal diffusivity.
 
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1. What is the difference between thermal diffusivity and thermal conductivity?

Thermal diffusivity is a measure of how quickly heat can pass through a material, while thermal conductivity is a measure of how well a material can conduct heat. In other words, thermal diffusivity takes into account both the rate of heat transfer and the material's ability to conduct heat.

2. How are thermal diffusivity and thermal conductivity related?

Thermal diffusivity and thermal conductivity are related by the equation thermal diffusivity = thermal conductivity / (density x specific heat capacity). This means that they are inversely proportional to each other, as thermal diffusivity increases, thermal conductivity decreases.

3. What are some examples of materials with high thermal diffusivity and low thermal conductivity?

Some examples of materials with high thermal diffusivity and low thermal conductivity include water, air, and most liquids and gases. These materials have low densities and specific heat capacities, making them good at transferring heat quickly but not as good at conducting heat.

4. How do thermal diffusivity and thermal conductivity affect heat transfer in buildings?

In buildings, materials with high thermal diffusivity and low thermal conductivity, such as insulation, are used to slow down the transfer of heat between the inside and outside of the building. This helps to maintain a comfortable temperature inside the building and reduce energy costs for heating and cooling.

5. How are thermal diffusivity and thermal conductivity measured?

Thermal diffusivity and thermal conductivity can be measured using various methods such as the hot wire method, the laser flash method, and the guarded hot plate method. These methods involve heating a sample of the material and measuring how quickly heat is transferred through it, allowing for the calculation of thermal diffusivity and thermal conductivity.

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