Nowadays, with the development of technology, as a material with high thermal conductivity, high melting point, high hardness, high wear resistance, oxidation resistance, corrosion resistance, etc. , Paper, laser and other fields have been reused. Especially in the application market such as mobile phone substrates, people's demand for it is growing.
Therefore, in order to better expand the application range of thermally conductive ceramics, it is of great significance to improve its heat transfer performance. But before that, we must first understand what affects the thermal conductivity of ceramics. The following editors summarize the various influencing factors.
Influencing factors and improvement analysis
In ceramics, there are three ways of heat transfer, namely convection, radiation and heat conduction. The thermal conductivity of ceramics is related to its composition, internal structure, density, humidity, heat treatment temperature, pressure and other factors.
1 Composition
The thermal conductivity of traditional ceramics is not good, and the reason for the low thermal conductivity is related to the raw materials. The raw materials of traditional ceramics are mainly three types of natural raw materials with poor thermal conductivity: clay, quartz, and feldspar. Therefore, in order to improve the thermal conductivity of ceramics, components must be doped in. This method can be divided into two types according to the different properties of the doped components: ① adding non-metallic materials to ceramics; ② adding metallic materials.
① Adding non-metallic materials: The thermal conductivity of red tiles is better than ordinary tiles, which is due to the Fe203 and mullite crystalline phase it contains. The thermal conductivity of ceramic tiles can also be increased by adding an appropriate amount of A1203, but if it is added too much, the sintering temperature of the ceramic will be increased. In order to reduce the adverse effects, Yu et al. Used the synergistic effect of graphene and alumina to change the internal structure of the material, thereby obtaining a ceramic material with better thermal conductivity.
② Adding metal materials: The heat transfer performance of metals is better than that of most ceramics. Combining the two can effectively improve the thermal conductivity of ceramics. He Yongxiang and others successfully prepared ceramics and metal Cu to diffuse into each other to form a stable metal. The permeation gradient layer effectively reduces the thermal resistance of the ceramic material.
Comparison of thermal conductivity of some materials
2 Stomata
During the firing process of the green body, the green body will produce a large amount due to the presence of organic or inorganic salts, particulate impurities in the powder (such as iron, unrefined residue particles, etc.) and excessive glass phase. Stomata or bubbles. The appearance of pores will inevitably change the way heat is transferred inside the ceramic material, which will have a significant impact on heat transfer.
High porosity ceramics
In ceramics with a high density and a low temperature environment, heat conduction is the main method of heat transfer. In porous ceramics, heat transfer methods such as convection, radiation, and heat conduction exist. Therefore, when analyzing the thermal conductivity of ceramics, the size of the pores, the distribution of the pores, and the connection method should be considered comprehensively.
For example, as early as in the metallurgical industry, in order to reduce heat loss, the ceramic wall of the furnace is made porous (closed pores), which greatly reduces the thermal conductivity of the ceramic wall and makes it easy to maintain a high temperature inside.
Yuan and others studied the thermal conductivity of porous ceramics and found that the thermal conductivity of porous ceramics with a particle size of about 1 μm is 75% of that of dense ceramics with the same particle size–that is, the larger the volume fraction of pores, The lower the thermal conductivity.
3 Internal defects and microstructure
The influence of internal defects and microstructure on the thermal conductivity of ceramics is mainly determined by the phonon thermal conduction mechanism of the material. All kinds of defects are the centers that cause phonon scattering, so they will reduce the average free path of phonons and thermal conductivity. Internal defects are also the centers of phonon scattering. The more such centers, the more energy loss caused by phonon scattering. Therefore, while seeking effective methods to increase the thermal conductivity of the material, measures such as adding sintering aids and increasing sintering time must be taken to reduce the occurrence of internal defects in the material.
For example: SiC and AlN are more commonly used high thermal conductivity ceramics. In theory, the mixture of the two should have better thermal conductivity. In fact, the experimental results of Zhang et al. Prove that although the SiC ceramics will become denser after adding a certain amount of AlN powder, there are differences between the particle size and interatomic forces and other forces between impurities and main atoms. The internal defects caused by this will reduce the thermal conductivity of SiC ceramics.
4 heat treatment process
In the ceramic manufacturing process, heat treatment is one of the most important processes. This process will affect a series of physicochemical changes of the green body and affect the microstructure and mineral composition of the finished product. In the heat treatment process, different components of ceramics will also change differently.
Taking Lingling clay as an example, when studying the thermal conductivity of this single component and its green body as the main component at different temperatures (700 ~ 1400C), it was found that the green body and Ridge has a low effective thermal conductivity, less than 0.3W / (m · K). However, when higher temperature heat treatment is used, its thermal conductivity will increase rapidly, reaching 3W / (m · K). It can be seen that in a certain temperature range, different temperatures have a great influence on the thermal conductivity of the material under the same treatment process. The thermal conductivity of some ceramic materials as a function of temperature can be seen in the figure below.
To sum up
In summary, if you want to improve the heat transfer performance of ceramics, you must consider multiple variables together, such as improving the purity of ceramic materials, increasing the density of ceramic materials, reducing structural defects, reducing pores, reducing grain boundaries, and reducing glass phases, and properly control them. Particle size, reasonable firing system, etc. In addition, adding non-metallic materials such as graphene and graphene-like materials to help ceramic materials improve thermal conductivity may also be a means worthy of further research.