c. Silicon nitride: It has two crystal forms: α and β, both of which are hexagonal. Due to the lattice stress in the α-Si3N4 grains, the free energy is higher than the β phase, so the stability is poor, and there is no lattice stress in the β-Si3N4. Filling as a filler is helpful to form a particle network and improve thermal conductivity. It has good mechanical properties, so it is mainly β-Si3N4 in practical production applications

2. Oxide fillers and their applications Oxide fillers mainly include alumina (Al2O3), magnesium oxide (MgO), zinc oxide (ZnO), etc. They have certain thermal conductivity and excellent electrical insulation properties. The oxide filler mainly fills the insulating polymer material in a mixed manner with the nitride, so that the thermal conductivity of the material can be improved, the stable electrical performance can be maintained, and the production cost can be reduced.
a. Alumina: The price of needle-shaped alumina is low, but the filling amount is small. In liquid silica gel, the maximum addition amount of ordinary needle-shaped alumina is generally about 300 parts, so the thermal conductivity of the resulting product is limited. The filling amount of spherical alumina is large, and the maximum addition amount in liquid silica gel reaches 600 to 800 parts. The thermal conductivity of the resulting product is high, and the price is higher, but it is lower than the price of boron nitride and aluminum nitride.
b. Magnesium oxide: low price, easy to absorb moisture in the air, strong viscosity increase, cannot be filled in a large amount, and has poor acid resistance, is easily corroded by acid, and is not suitable for application in acid environment.
c. Zinc oxide: good particle size and uniformity, suitable for production of thermal grease, but its low thermal conductivity is not suitable for production of high thermal conductivity products; light weight, strong viscosity increase, not suitable for potting.
3. Carbide fillers and their applications Carbide fillers are mainly silicon carbide and boron carbide fillers.
a. Silicon carbide: a compound with a strong covalent bond, commonly known as hexagonal α-SiC and cubic β-SiC, similar to diamond structure. Silicon carbide has the characteristics of corrosion resistance, high temperature resistance, high strength, good thermal conductivity, and impact resistance. It also has the advantages of high thermal conductivity, oxidation resistance, and good thermal stability. It is commonly used in packaging materials in the microelectronics industry. However, the carbon and graphite produced during the synthesis of silicon carbide are difficult to remove, resulting in low purity and high electrical conductivity, which limits its application in materials with high insulation performance requirements; and its high density, in silicone rubber Easy precipitation and layering.
The researchers filled the epoxy with silicon carbide as a thermally conductive filler, and found that nano-silicon carbide can promote the curing of epoxy resin, and silicon carbide particles are more likely to form thermal conduction paths or thermal network chains within the resin system, reducing the internal void ratio of epoxy resin and improving The mechanical and thermal conductivity properties of the material are discussed.
b. Boron carbide: a refractory material and super-hard material with high thermal conductivity, but expensive, and not widely used in insulating polymer materials. The researchers filled the natural rubber material with boron carbide as a thermally conductive filler, and found that the addition of boron carbide can improve the thermal diffusion coefficient of natural rubber, and the thermal diffusion coefficient of natural rubber also increased after aging.
4. Application of mixed fillers
The use of different types of fillers in a certain proportion can give full play to the characteristics of a single filler. Due to the hybrid effect, not only can the thermal conductivity be improved, but also the cost can be reduced.
The researchers mixed BN, AlN, and MgO at a ratio of 3:2:5, and then blended them with polyetherketone and polyimide in dimethylformamide. As a result, they found that the molded product has high thermal conductivity.