The nano-semiconductor powder is used for the thermal insulation of nano-semiconductor thermal insulation coatings. The barrier effect of coating film on thermal radiation is the result of absorption and reflection. The proportion of absorption and reflection will be different due to the nature of semiconductor materials. The difference is also related to the particle size of the material. For a single nanoparticle, the larger the particle size, the larger the cross section, and the larger the area of ​​the nanoparticle that can block the sunlight through absorption and reflection. As a whole, the larger the total area (A) of the internal nano-semiconductor powder, the better the theoretical heat insulation effect. A can be expressed by the formula (1).
The latest research progress of nano-semiconductor thermal insulation coatings
Where: A - total intercept area, cm2; m - total mass of nano-powder, g; r - particle size, cm; Ï - density, g / cm3.
It can be seen from equation (1) that the smaller the particle size of the nano-semiconductor, the larger the total occlusion area. However, if the particle size is too small, the change in the electron density of the particles will reduce the shielding function of sunlight, so the prepared The nano-semiconductor particles need to have a suitable particle size. From the above analysis, the heat insulating effect of the nano-semiconductor particles is a result of the interaction between the physical structure of the semiconductor itself and the particle size.
Among many nano-semiconductor particles, nano-indium tin oxide (ITO) and antimony-oxide (ATO) are the first metal oxides used in nano-insulation coatings. The carrier concentration of ITO and ATO films is 1020/cm3. The visible light transmittance is 80%-90%, and the infrared reflectivity is 75%-80%. The carrier concentration increases and the infrared emissivity increases. The optical detection of ITO nano-thermal insulation coating by Huang Xushan et al. The insulation coating with nano ITO particles added to the infrared is mainly concentrated in the band of 1200 ~ 1500nm, the infrared barrier of this area can reach 95%, the insulation test shows that when the content of ITO nanoparticles reaches 1.5% of the coating quality The thermal insulation effect of the coating is the best. The contrast of the thermal insulation of the infrared lamp for 5 minutes shows that the temperature of the glass coated with the nano-insulation coating is about 40 ° C lower than that of the blank glass. Sun Guoliang and others are studying the content of strontium (Sb) on the ATO. When the influence of hot paint is found, there are two kinds of existence state of Sb in the coating, Sb3+ and Sb5+. When the content of Sb is low, Sb5+ is dominant, and the number of carriers is small, the infrared blocking rate is low; with the increase of Sb content , the number of carriers More, the infrared blocking rate increases; when the Sb content reaches 6%, the infrared blocking rate is the highest. After more than 6%, the infrared blocking rate decreases. There are two main reasons for the analysis: First, the crystal lattice of tin dioxide is caused by high content of Sb. Distortion, electron scattering becomes larger, and the mobility of carriers is reduced. Second, due to the increase of Sb, Sb3+ is gradually increased. Sb3+ is like an electron trap, eliminating the electrons generated by Sb5+, causing the carrier concentration to decrease, thus infrared The barrier rate also decreased. In addition, Mei et al. studied the ATO content of ATO/epoxy insulation coatings. The results show that as the ATO content increases, the thermal insulation effect increases, and the optimum content of ATO is the total mass of the coating. 2%.
In recent years, with the continuous development of new materials, people have been looking for new types of nanoparticles to replace the original ITO and ATO. Li and other attempts to add Al-doped nano-ZnO into thermal insulation coatings, ZnO itself is also a semiconductor The material, the concentration of ZnO carriers added with Al increased significantly, and the conductivity and infrared barrier properties also increased. Studies have shown that 0.5% of ZnO (in which the molar fraction of Al is 6%) is added to the thermal insulation coating. The barrier property is the best, reaching more than 80%. Zhao et al. added gallium (Ga) into ZnO to form a GZO film. The results show that when the amount of Ga is 4.9%, the reflectance of the film to infrared light can reach 70. %, the transmittance is less than 1%, and the visible light transmittance is still above 90%. It can be seen that GZO coating is an ideal thermal insulation coating for glass. In addition to ZnO dopants, Chen Zhonghua et al. Zirconium oxide was added to the thermal insulation coating, and the microstructure of the coating film was studied. The results show that the coating film has a good barrier effect on infrared rays, and the nanoparticles closely fill the gaps between the coatings to form a complete air insulation layer. Improve the thermal insulation properties of the coating. Wang The lanthanum-cerium oxide nanopowder was prepared by mixing rare earth element lanthanum with lanthanum oxide, and then yttrium-yttria nano-insulation coating was prepared. The optical detection showed that the reflectivity of the coating in the range of 450-1600 nm exceeded 90%, 1600~ The reflectance in the range of 2200 nm exceeds 80%, which is higher than that of ZnO and ITO films.
The application of nano-semiconductor powder has made a great change in the thermal insulation mechanism of the thermal insulation coating. However, the nano-powder has a large specific surface area and a high surface energy, and the nanoparticles are easily agglomerated through interface interaction, so the nanoparticles are The problem of dispersibility in coatings has always been one of the difficulties in coating research. To promote the marketization of semiconductor nano-insulation coatings, it is necessary to strengthen the research on nanoparticle dispersion systems.
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