I’m Gaylon Campbell, a senior research scientist here at METER Group. Today, we’ll be talking about how to measure and understand thermal properties of materials. Now we’ll take a more detailed look at some of the applications of these thermal properties.
Heat and cold are used in cancer treatment. It’s obviously important that only the cancer cells be exposed to lethal temperatures, but the heat or cold is applied at a point, so the temperature distribution around that point has to be determined using heat transfer models, and those models require accurate thermal properties measurements. We’ve been working with medical researchers who do research on these models and methods in order to make those measurements.
Our Mars project is a good example of using thermal properties for soil heat flow and temperature modeling.
We know with good accuracy how much energy is delivered to the Martian surface by the sun, but how much of that goes into heating the atmosphere and how much goes into the regolith? If we know the regolith thermal properties, we can accurately model the amount of heat flowing into it and the temperature reached at each depth. And obviously we could do the same thing for soils on Earth.
We read in the news these days of more than 20% of Ireland’s power going to data centers for AI and other uses. That’s not a trivial amount of power. All of that is supplied through buried cables, and as this current flows through those cables, they heat up. If they heat up too much, they melt. So how do you design the soil or other backfill materials that go around the cables so that you’ll be sure that they continuously conduct away the wasted power? You need to measure the thermal resistivity of the film materials and do the calculations.
These projects run on much larger scales too.
The SuedLink power line in Germany links the north—where a lot of the power is produced— to the south—where a lot of the power is used. Many of the thermal properties measurements are being made on the materials for that project.
And finally, we talked about the fact that thermal conductivity and heat capacity of porous materials like soil, are strongly dependent on water content. We could measure those thermal properties accurately and use that to determine the water content or possibly even the bulk density of the soil and thermal properties measurements have been used for that purpose.
So hopefully I’ve convinced you by now that the value of thermal properties data, so if you wanted to know what the thermal properties of a material were, how would you get them? Well, maybe you would go to the internet or go to a book. And if we wanted to know the thermal conductivity of water or aluminum or something like that, that would be a good choice. But we’ve just been talking about the fact that for soil, the thermal properties vary a lot with water content, temperature and other variables.
So, if you want to measure a soil’s thermal properties, the methods for making those measurements are either steady state or transient. For example, we could measure heat capacity with a calorimeter, where we could measure thermal conductivity by flowing a constant amount of heat through the material, measuring the temperature difference with transient methods. Those are often made with heated needles. We put a heat pulse in, we analyze the temperature response of the needle in the material.
So we said, for measuring thermal conductivity by steady state, we:
- Establish a constant heat flow
- Measure a temperature difference
- Calculate the thermal conductivity
So the heat flux density times the thickness divided by the temperature difference. This method is simple and direct, but it has some drawbacks. Typically, it requires pretty large samples to get one dimensional heat flow to establish steady state heat flow. It has to be done in a laboratory. And, it takes a long time to establish the steady flow. As a result, a sample a day is about all you can run.
The biggest drawback, though, is for wet porous materials, the temperature gradient redistributes the moisture, so that measurements that you might make with this steady state method are pretty useless.
Experimental data
Here are some data from a paper that was published over 70 years ago on the redistribution of water with temperature gradients. Two columns of soil were cooled for the graph here is on the left side, cooled on the left and heated on the right. The soil in the left graph was compacted, while the one on the right was left loose.
Within a few hours, water had started moving toward the cold end of the soil column. In the compact soil, the water content on the hot end got down to about 12% on the cold end, it got up to about 20%. On the loose soil, it got down to about 5% on the hot end, the cold end got up to about 23%.
Now there are a lot of things that you could learn from these experiments, but the important point is that water will move in moist soil anytime there’s a temperature gradient, and so a steady state method is pretty useless for trying to determine the thermal properties.
