Who are the top five hot ceramics with high thermal conductivity?

At present, the hot thermal conductive ceramic materials mainly include beryllium oxide, aluminum nitride, silicon carbide, silicon nitride, alumina and polycrystalline diamond (PCD) ceramics.  Al2O3 ceramics have low price, high strength, stable chemical performance, good thermal stability and strong insulation, which is one of the most thoroughly studied and widely used ceramic materials in the industry.  But compared with other ceramic materials, the thermal conductivity of Al2O3 ceramics is relatively low, and researchers at home and abroad have also used a variety of sintering methods and different combustion AIDS, but can not further improve the thermal conductivity of Al2O3 ceramics, therefore, Al2O3 is generally not classified as high thermal conductivity ceramics.  

AlN Ceramics  

The thermal conductivity of AlN can reach 320W/(m·K) theoretically, but the thermal conductivity of the actual product is less than 200W/(m·K) due to the scattering of phonons caused by the formation of aluminum vacancy due to the AlN defect.  AlN mainly relies on phonons for heat transfer. In the process of heat transfer, defects, grain boundaries, pores, electrons and phonons themselves all produce phonon scattering, thus affecting the thermal conductivity of AlN substrate.  

Therefore, some sintering AIDS should be added to promote sintering.  The addition of appropriate combustion AIDS can react with oxygen in the lattice to generate the second phase, purify the AlN lattice and improve the thermal conductivity.  Common AlN ceramic combustion AIDS are: Y2O3, Sm2O, CaCO3, CaF2, YF3 and so on.  The method of sintering AlN ceramics with high thermal conductivity has been widely used in production, and AlN ceramics are also widely used in electronic packaging materials and large-scale heat dissipation materials.  However, the production cost of AlN ceramics is high due to the long sintering time, high sintering temperature, high quality AlN powder and other reasons. In addition, the characteristics of AlN, such as easy moisture absorption and oxidation, restrict its large-scale promotion and application.  

Si3N4 Ceramics  

The atomic bond strength, average atomic mass and crystal anharmonic vibration of Silicon nitride (Si3N4) are similar to those of SiC, which provides theoretical basis for high thermal conductivity materials.  Haggerty et al. calculated that the theoretical thermal conductivity of si3N4 crystal at room temperature was 200-320W /mK. However, as the structure of Si3N4 is more complex than that of AlN and scatters phonons greatly, the thermal conductivity of sintered SI3N4 ceramics in the present study is much lower than that of si3N4 single crystal.  

WatariK, HiraoK, Hirosaki, Okamato et al prepared silicon nitride ceramics with high thermal conductivity in the range of 100~162W/mK by various measures.  Silicon nitride has many excellent properties, such as high toughness, strong thermal impact resistance, good insulation, corrosion resistance and non-toxicity, both at high temperature and at normal temperature, which makes it more and more attention of researchers at home and abroad.  

PCD Ceramic  

Among all known materials at present, diamond has the strongest heat transfer ability. The theoretical value of thermal conductivity of its single crystal at room temperature is 1642W/ Mk, and the measured value is 2000W/ mk.  However, large single crystal diamond is difficult to prepare and extremely expensive, so it is not feasible to be used as electronic packaging material from both technical and economic perspectives.  In the sintering process of polycrystalline diamond, it is often necessary to add sintering AIDS to promote the bonding between diamond powder, and high thermal conductivity PCD ceramics can also be obtained.  However, in the process of high temperature sintering, the sintering AIDS will catalyze the carbonization of diamond powder, which will make polycrystalline diamond no longer insulated, so that the polycrystalline diamond ceramics (PCD) with sintering AIDS is not suitable for electronic packaging.  Small single crystals of diamond are often used as reinforcement materials to improve thermal conductivity of ceramics, which plays a role in improving the thermal conductivity of ceramics.  It is found that the thermal conductivity of AlN ceramics can be greatly improved by adding proper amount of nano-diamond when sintering AlN ceramics.  

BeO Ceramic  

In 1971, Slack and Austerman tested the thermal conductivity of BeO ceramics and BeO large single crystal, and calculated that the maximum thermal conductivity of BeO large single crystal reached 370W/mK.  At present, the thermal conductivity of BeO ceramics prepared can reach 280W/mK, 10 times that of Al2O3 ceramics, but BeO is highly toxic, human inhalation will lead to acute pneumonia, long-term inhalation is extremely serious harm to human health.  

SiC Ceramic  

At present, silicon carbide (SiC) is an active thermal conductive ceramic material at home and abroad.  The theoretical thermal conductivity of SiC is very high, reaching 270W/mK.  However, due to the low ratio of surface energy to interface energy, that is, the grain boundary energy is high, it is difficult to sintering high purity and density SiC ceramics by conventional methods.  When conventional sintering method is used, sintering aid must be added and sintering temperature must reach more than 2050℃, this sintering condition will cause SiC grain growth, greatly reduce the mechanical properties of SiC ceramics.  

With the continuous development of science and technology, silicon carbide ceramics are increasingly widely used in the semiconductor field, such as grinding disk, fixture, workbench, etc.  High thermal conductivity is the key index of its application in semiconductor manufacturing equipment components, so it is very important to strengthen the research of high thermal conductivity silicon carbide ceramics.  The main methods to improve the thermal conductivity of sic ceramics are to reduce the lattice oxygen content, improve the density, and rationally control the distribution of the second phase in the lattice.

How to improve the thermal conductivity of ceramic materials?

In order to improve the thermal conductivity of ceramic materials, it is necessary to improve the purity of ceramic materials, try not to add or add additives, but in order to improve the density of materials and control the grain size, adding a certain amount of admixture is necessary.  Also can be doped with some of the high thermal conductivity of non-metal (Fe2O3), metal (such as Cu, etc.) and in the surface of the ceramic load specific organic matter to form high thermal conductivity composite materials.  

When the particle size of raw material decreases to nanometer level, the thermal conductivity of ceramic material decreases, and the thermal conductivity can be significantly increased by properly controlling the particle size.  

Raise the density of ceramic material, reduce stoma and glass phase, make its approach theoretical density as far as possible, also can raise the coefficient of thermal conductivity of ceramic material.  The influence of pores on the thermal conductivity of ceramic materials is more complex, and there are many internal heat transfer modes according to different situations. The thermal conductivity of interconnected pores is higher than that of closed ones.  The higher the sealed porosity, the lower the thermal conductivity.  

The influence of internal defects and microstructure on ceramic thermal conductivity is mainly determined by phonon thermal conductivity mechanism.  All kinds of defects are the centers of phonon scattering, and all these defects reduce the mean free path and thermal conductivity of phonons.  The addition of other components may change the internal structure and affect the properties of ceramics during firing.  In some cases, measures are chosen to modify the internal structure of the ceramic to meet specific functional needs.  

The influence of heat treatment process on ceramics is very significant, the temperature, the length of time, the speed of temperature rise and cooling, the highest firing temperature and the time of heat preservation will affect the thermal conductivity of ceramic materials.