In our common sense, insulation materials and thermal conductive materials seem to be two completely different things. For example, cotton is insulated and can be made into cotton jackets; Iron is thermally conductive and can be used as a frying pan. The opposite is not possible. However, in reality, we often see another phenomenon, which is that the same material can be used in seemingly opposite scenarios of insulation and thermal conductivity. For example, alumina ceramics can be made into insulation bricks and used in high-temperature kilns; It can also be made into a heat sink and used in electronic products such as LED lights.

In theory, as an insulation material, low thermal conductivity is required; As a heat dissipation material, high thermal conductivity is required. Can the thermal conductivity of the same material change at any time?

To answer this question, we need to look at it from two perspectives.

The first aspect, like the question raised above, is that the thermal conductivity of materials can vary. The most typical example is that the thermal conductivity varies at different temperatures. Taking alumina as an example, as the temperature increases, the thermal conductivity will continuously decrease. The thermal conductivity at 1200 ℃ is only about half of that at 400 ℃. However, the thermal conductivity of aluminum oxide is not small. At room temperature, it is 20-30W/m • K, but it has dropped by more than half, and there is still about 10W/m • K, which is higher than the thermal conductivity of many materials.

Therefore, we need to look at the second and main aspect: alumina can both insulate and conduct heat due to structural changes. That is to say, when alumina ceramics are used as insulation materials and thermal conductive materials respectively, their internal structure is different.

When alumina ceramics are used as thermal insulation materials, their biggest structural feature is porosity and low density. For example, making alumina hollow ball bricks. We all know that the thermal conductivity of air is very low, so the thermal conductivity of alumina hollow ball bricks is also very low. Someone may ask, since the thermal conductivity of the air is very low, why not use air insulation instead? Why bother to incorporate air into alumina materials? This is because although the thermal conductivity of air is very low, it cannot prevent thermal radiation, just like the thermal conductivity of a vacuum is zero, the heat of the sun still transfers to the Earth through the vacuum. Porous alumina not only blocks heat conduction but also thermal radiation, so it can effectively play a role in insulation and heat preservation. For example, there are research reports that a type of alumina microporous ceramic has a density of only 0.6g/cm3, a porosity of 85%, and a thermal conductivity of only about 0.3W/m • K at 1200 ℃.
But when alumina is made into thermal conductive ceramics, the requirements are completely different. The first requirement is to have a high density, the higher the better. High density results in fewer pores, and the ceramic grains are tightly bonded, facilitating heat conduction. The second requirement is high purity, the higher the purity, the higher the thermal conductivity. For example, ceramics with a 99% alumina content have a thermal conductivity of~26W/m • K, while when the alumina content drops to 95%, the thermal conductivity is only~20W/m • K. This is because in ceramics with low alumina content, the composition of glass is relatively high, and the thermal conductivity of glass is relatively low, resulting in a lower overall thermal conductivity of the ceramic. Of course, in practical applications, cost also needs to be considered. Although high-purity alumina ceramics have high thermal conductivity, their prices are also high. Therefore, selecting alumina ceramics based on product requirements should not blindly pursue high purity.

In addition to high purity and dense structure, when aluminum oxide ceramics are used as heat sinks, they often have specific requirements for their external shape. For example, when making LED heat sinks, they often have a finned structure to increase the surface area, facilitate heat dissipation to the air, and thus achieve better heat dissipation effects.