Boron nitride is one of the most valued materials in nitride. It has six crystal forms. The most common being of Boron Nitride are cubic boron nitride (c-BN) and hexagonal boron nitride (h-BN), of which c- BN is similar to diamond and is mainly used to make cutting tools. Its thermal conductivity is strong, but the cost is high. H-BN is a white powder with a layered structure and lattice parameters similar to graphite, also known as white graphite. Although the thermal conductivity is lower than that of aluminum nitride, it has unique advantages as a thermally conductive filler because it does not hydrolyze, and its density is only 2.2g/cm³, not easy to settle in the slurry. 

In addition, h-BN has an insulation voltage of 30~40kv/mm, which can maintain high insulation properties even when used in air at 1000°C, and has a breakdown voltage of 3 kv/mm (twice that of alumina). So it is often used with alumina, aluminum nitride, or silicon nitride to be added to the resin to make composite materials used for insulation and heat conduction.

However, to give full play to the performance of h-BN, there are mainly two aspects that need to be paid attention to:

The first aspect is the purity. The properties of h-BN particles are greatly affected by the purity. Therefore, the preparation of high-purity h-BN particles is the premise to ensure their wide application, especially the content of free boron - h-BN powder. There are often unstable residues of boron-containing compounds. The boron-containing compounds are bound to h-BN powder by hydrogen bonds. Because the hydrogen bonds are easily destroyed, the boron ions are freed and combined with the hydroxide ions of water molecules to form H3BO3 (boric acid). ), resulting in product contamination. Therefore, the lower the content of free boron, the better, and the lower the free boron means, the more stable the h-BN powder is. At present, the factors that directly affect the purity include firing conditions and post-processing of the product.

Secondly, since the shape and aspect ratio of the filler will affect the thermal conductivity of the composite material (commonly there are fibrous, flake, spherical, etc.), the choice of the shape of the thermally conductive filler is also critical. The h-BN fillers currently used are mostly spherical and flake-shaped. Among them, spherical fillers can bring a higher filling amount. In theory, the higher the filling rate, the better the overall thermal conductivity of the material. Therefore, h-BN spherical fillers are rubber-plastic composites. It is a popular candidate for thermal conductive fillers; at the same time, flaky fillers also have their own advantages. It has a high specific surface area, so it is beneficial to form a phonon thermal conduction channel and improve the thermal conductivity of the system. It is also a popular choice in the field of thermal conductivity.

Is it spherical or flake? It doesn't matter if you have difficulty choosing, because both can be used together. When lamellar h-BN is used alone, due to its irregular shape, the thermal conductivity in the direction perpendicular to the crystal plane will be much smaller than the thermal conductivity in the direction parallel to the crystal plane, resulting in that part of the lamellar orientation is vertical when boron nitride is highly filled into the polymer. In the ideal thermal conductivity direction, and the filling rate is also low, the thermal conductivity of h-BN cannot be fully utilized.

Although the use of spherical h-BN alone can bring a higher filling amount, it does not mean that the greater the addition amount, the better the thermal conductivity of the composite material. According to relevant research, spherical boron nitride is used as a filler, and its filling amount affects the thermal conductivity and thermal resistance of silicone resin. With the addition of boron nitride, the silicone resin first reaches a thermal conductivity peak, and the filling amount is the best at this time. With the gradual addition of the filling amount, the thermal resistance of the composite material increases, and the thermal conductivity of the silicone resin decreases.

In summary, h-BN is one of the ideal fillers for polymer-based thermally conductive composites due to its good thermal conductivity and insulating properties. At present, it seems that the exfoliation of h-BN sheets, the preparation of spherical particles and the orientation and arrangement in the matrix all play a decisive role in the thermal conductivity of the composites. In addition, the research on the surface treatment process of BN particles is also very important and should not be ignored.