Taking ceramic materials as an example, the thermal conductivity is dependent on their chemical composition, structure, density, sintering process, etc. Increasing the thermal conductivity of ceramic materials within a certain range will improve their capacity for heat conduction, heat convection, and heat radiation. This will also prepare ceramic materials for strong heat energy absorption, high storage, and strong heat dissipation, which will further expand the application of ceramic materials.
Factors Affecting Thermal Conductivity of Ceramic Materials
Chemical Composition
Ordinary ceramics have a thermal conductivity of (0.02-1.5) W/(mK). Low thermal conductivity ceramic materials fall far short of the demands of energy conservation, high efficiency, and special performance in practical applications. In the application field of electronic packaging materials, AlN-based special ceramics with a thermal conductivity of 210W/(m.k), which is 5-8 times that of alumina ceramics, and the ability to withstand high temperatures above 2200°C are prepared through aluminum nitride ceramic composite powder. It is a perfect heat-conducting ceramic material for a new generation of large-scale integrated circuits and power electronic products due to its high thermal conductivity, stable mechanical properties, outstanding electrical insulation properties, extremely high insulation resistance, and low dielectric loss.
Raw Material Powder Particle Size
The thermal conductivity and mechanical properties of ceramic materials are significantly influenced by the particle size, purity, and phase of the raw material powder. The delay in the densification of the ceramic will be more noticeable when the particle size is larger. Unavoidably, pores will develop inside the ceramic during preparation. In general, the more pores a material has, the more thermal conductivity it has.
The thermal conductivity of ceramic powder will decrease as particle size approaches the nanometer range. The properties of the material itself are altered by the surface effect and small size effect of nanoparticles. To prevent abnormal growth of the grains during the sintering process and ensure that the ceramic body's grain distribution is consistent, the grain size is decreased, the distribution is narrowed, the separation area between the grain boundary and the pores is decreased, and the sintering temperature is lowered. As a result, when the powder's particle size is small, the ceramic's thermal conductivity will decrease as the particle size decreases. The performance of ceramic materials' heat transfer can be enhanced during actual production by carefully regulating the particle size of raw material powder.
Pore
Under the same porosity conditions, the larger the pore size, the greater the thermal conductivity. The thermal conductivity of the connected pores is greater than that of the closed pores. The thermal conductivity decreases as closed porosity increases. The cause of this phenomenon is that the gas convection in the pore and the radiation heat transfer between the pore walls increase with pore size. This is the opposite of how density of the material affects thermal conductivity. Even so, two methods can be taken concurrently to increase the material's thermal conductivity after taking the effects of the two methods into account. The study on porous ceramics also came to the conclusion that the convective heat transfer inside the material can be disregarded when the pore size is less than 4 um.
Additionally, porous ceramics enable the transfer of heat through convection, radiation, and heat conduction. As a result, it is important to take into account all aspects of pore size, distribution, and connection mode when examining the thermal conductivity of ceramics.
Organizational Structure
Ceramic materials' thermal conductivity is significantly influenced by their internal structure. The phonon thermal conduction mechanism of the material is primarily responsible for the effect of the internal structure on the thermal conductivity of ceramics. Some materials that can improve the material's thermal conductivity can be added to it in order to increase the material's thermal conductivity. The addition of these materials may result in the appearance of internal flaws in the ceramics because they may undergo a number of physical and chemical reactions with the original substances during the ceramic preparation process, which will affect the internal structure of the ceramics. Therefore, it is necessary to discuss the structural changes that occur in ceramics when other phases are added. The filler's thermal conductivity and the internal structure it develops during the ceramic preparation process play a significant role in the thermal conductivity of ceramics.
In porous ceramics, for instance, impurities distributed along the pore surface, such as impurity atoms, lattice defects, etc., migrate from the hot end of the pore to the cold end as heat is released within the ceramic and precipitate on the pore surface. It will have a great influence on the ceramic's ability to absorb heat, which will have a further effect on its ability to transfer heat.
Sintering Process
One of the most crucial steps in the production of ceramics is sintering. The green body will undergo a number of physical and chemical changes as a result of this process, which will also have an impact on the finished product's microstructure and mineral composition. Different ceramic components will go through different changes during the sintering process. The thermal conductivity of ceramic materials will be affected by the temperature, the length of time, the heating and cooling speed, the maximum firing temperature, and the holding time during the sintering process.
Methods for Improving Thermal Conductivity of Ceramic Materials
Component doping, regulating the size of raw material powder, and improving the internal structure of ceramic materials are the main techniques used to increase the thermal conductivity of ceramic materials. In practice, the increase of ceramics' thermal conductivity is due to the interplay of many variables.
Component Doping
It is necessary to increase the purity of ceramic materials and add as little or no admixture as possible in order to improve the thermal conductivity of ceramic materials. However, a certain amount of admixture must be added in order to increase the material's density and regulate the grain size. A high thermal conductivity composite material can also be created by loading specific organic substances on the surface of the ceramic and doping some non-metals (Al2O3, Fe2O3, etc.) and metals (such as Cu, etc.) with high thermal conductivity in the proper amounts.
Control Particle Size
The thermal conductivity of the ceramic material decreases as raw material particle size approaches the nanometer range. The thermal conductivity can be significantly increased with proper particle size control. Another way to improve ceramic materials' thermal conductivity is to make them denser, with fewer pores and glass phases, and as close as possible to their theoretical density.
Improve the Internal Structure
The effect of the internal structure of ceramic materials on their thermal conductivity is more complex, and there are multiple methods of internal heat transfer depending on the circumstances. The thermal conductivity of the material will be greatly influenced by the way that pores connect, the size of the raw material particles, and the presence of internal flaws like microcracks.
The centers that cause phonon scattering can be any kind of defect, and these defects will result in a decrease in both the phonon mean free path and the thermal conductivity of the material. In the case of silicon nitride ceramics, the mechanism of heat transfer is phonon heat transfer. The mean free path of phonons is greater and the thermal conductivity is higher when the lattice is complete and defect-free, whereas the oxygen in the lattice is frequently accompanied by structural defects like vacancies and dislocations, which significantly lower the mean free path of phonons, resulting in a decrease in thermal conductivity. Therefore, the key to increasing silicon nitride's thermal conductivity is to lower the amount of oxygen in the lattice.