How to measure thermal conductivity?

If you are looking for a guide on how to measure the thermal conductivity of graphite then this article is for you. There are many ways to measure thermal conductivity and very often people just pick one that seems easy and work with it, but in this article, I wanted to explain why there can be differences in how graphite was tested by others and then show possible errors along the way that may or may not happen in your graphite testing. I will start with the basics and go from there to the graphite testing process, when we are finished you can follow my graphite thermal conductivity testing script to do your own graphite testing if you like.

Understanding thermal conductivity:

Before we start measuring graphite it is very important to understand the basics of thermal conductivity, so I will start with this. Thermal conductivity is the ability to conduct heat and it can be measured in W/mK (watts per meter Kelvin). It is also possible to measure how good graphite conducts heat by measuring the k-value. 

Graphite’s heat conduction ability

Graphite is a form of carbon; graphite is the best conductor of heat at room temperature. It has a thermal conductivity value of 215 W/mK which is very high and graphite’s k-value (k=215) can be calculated as:

k = 215 – 0.02   ( ° C ) (1)

At 100°C graphite’s thermal conductivity would be:

k = 215 – 0.02(100) = 184 W/mK (2)

We can see that graphite conducts heat very well and it makes graphite a good material for use in electronics, graphite is also used as a heat spreader in many graphite/metal heatsinks and graphite is also used as a thermal interface material between the soldering iron tip and the solder joint.

Graphite’s graphitic structure

As mentioned above graphite has a graphitic form which makes graphite excellent for conducting heat, but it also makes graphite an electrically insulator. Despite graphite’s unique ability to conduct heat, graphite is an electrical insulator which makes graphite very useful for thermal management but it also makes graphite hard to measure with a simple meter or thermometer.

A graphene form of carbon

Graphite is made up of layers of graphene wrapped into graphitic cylinders called graphitic planes. Graphene is a form of carbon and graphite has a graphitic structure. A graphene sheet can be seen below in figure 1 with three graphitic planes.

Figure 1: Carbon graphene sheets.

Graphite’s graphite cylinders are stacked on top of each other to form graphite’s unique graphitic layer, this graphite layer is graphite thermal pad.

Figure 2: Graphitic graphite layers with graphitic cylinders.

Thermal expansion and graphite:

Graphite does expand when heated and graphite also shrinks when cooled, graphite can shrink up to 5% when it is cooled from 100°C to room temperature which is a large amount of shrinkage. The graphitic planes in graphite are not connected to each other and this graphitic structure is what causes the big graphite thermal expansion when graphite is heated from room temperature to 100°C. When graphite expands or shrinks, graphite’s layers move apart from each other and slide over each other which graphite does very easily.

Graphite’s graphitic layers are graphitic planes (marked in blue)

When graphite is heated graphite heats up all the graphitic planes like a sponge when it is squeezed graphite expands when graphite cools down graphite contracts and shrinks. Graphite can shrink 5% of its original size which graphite can do easily, graphite’s graphitic layers move apart from each other like the pages in a book.

Graphite is soft and flexible:

Graphite is soft and flexible which graphite does not like; graphitic planes need to be protected when they are used as a thermal interface material (TIM). Graphitic graphite planes are graphitic cylinders wrapped in many layers of graphene to protect graphitic planes only about 0.1mm graphitic cylinder can be seen in figure 3 below.

Figure 3: A graphitic graphite plane that is out of its protection of graphitic graphite layers.

Graphite thermal conductivity testing procedure:

First I will explain graphite’s graphitic structure, graphite has many graphitic planes stacked on top of each other which are graphitic graphite cylinders that are wrapped in graphene. Graphitic graphite is soft and flexible which causes it to expand when heated graphitic graphite expands 5% of its original size when it is heated from room temperature 100°C graphitic graphite shrinks 5% of its original size when it is cooled from 100°C graphite graphitic graphite cylinders are wrapped in graphene to protect graphitic graphite planes.

Conclusion graph

This article will explain the thermal conductivity of graphite from room temperature to 100°C. You will learn how to measure the thermal conductivity of graphite using a thermal or hot plate and an accurate thermometer or temperature meter. Graphite conducts heat very well because graphite is made up of many layers of graphene which are stacked on top of each other to form graphitic planes, the graphitic cylinders in the middle of these graphitic planes conduct heat very well through them and then through the graphitic planes.


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