Vanadium dioxide VO2 powder technology and research status

The intelligent temperature control coating is a coating that transmits infrared light at low temperatures and reflects infrared light at high temperatures. The research status of VO2 and its doped powder and the intelligent temperature control coating based on VO2 powder are introduced. Proposed its future development trend.
Key words: VO2 powder; phase change performance; intelligent temperature control coating
When the sun shines on the surface of the object, the object mainly absorbs the near-infrared light energy to increase its surface temperature, while the near-infrared light energy accounts for 50% of the total energy of the sunlight. In the summer, when the sun shines on the surface of the object, the surface temperature can reach 70~80 °C. At this time, it is necessary to reflect the infrared light to lower the surface temperature of the object; when the temperature is low in winter, it needs to be insulated by infrared light. That is, an intelligent temperature-control material capable of reflecting infrared light at a high temperature and transmitting infrared light at a low temperature and transmitting visible light at the same time is needed, thereby saving energy and protecting the environment.
There are many reports on solar heat reflective thermal insulation coatings, but only staying on the basis of looking for traditional low-infrared light absorbing inorganic fillers, such as titanium dioxide, zinc oxide, barium sulfate, silicon dioxide, iron black. , iron red, iron yellow, talc, glass beads, etc., rely solely on the properties of polymer film-forming materials and traditional inorganic materials to reflect infrared light. The common disadvantage is that the infrared infrared light is not efficient, and only reflects the infrared light function, but does not transmit infrared light at low temperatures, and does not automatically adjust the temperature.
Vanadium dioxide (VO2) is an oxide having a phase change function near 68 °C. It is conceivable that if a VO2 powder material having a phase change function is compounded into a binder and further mixed with other pigments, a VO2-based composite intelligent temperature-control coating can be prepared. After the surface of the object is coated with such a coating, when the internal temperature is low, the infrared light can enter the interior; when the temperature rises to the critical phase transition temperature, a phase change occurs, at which time the infrared light transmittance decreases and the internal temperature gradually decreases. When the temperature drops to a certain temperature, the VO2 undergoes a reverse phase change, and the infrared light transmittance increases, thereby achieving intelligent temperature control. It can be seen that the key to preparing the intelligent temperature control coating is to prepare the VO2 powder with phase change function.
1. Phase change principle
At 68 °C, VO2 is rapidly transformed from low temperature semiconductor, antiferromagnetic, and distorted rutile structure monoclinic phase of MoO2 configuration to tetragonal phase of high temperature metal, paramagnetic, and rutile structure, internal VV covalent bond As a metal bond, it exhibits a metallic state, and the conduction effect of free electrons is sharply enhanced, and the optical characteristics are significantly changed. When the temperature is higher than the phase transition point, VO2 is in a metallic state, the visible light region remains transparent, and the infrared light region is highly reflective. The infrared light portion of the solar radiation is blocked outdoors, and the transmittance to infrared light is small; At the change point, VO2 is in a semiconductor state and is moderately transparent from the visible to the infrared region, allowing most of the solar radiation (including visible light and infrared light) to enter the room, and the transmittance is large, and the change is reversible.
For practical applications, the phase transition temperature of 68 ° C is still too high, how to reduce the phase transition temperature to room temperature is a concern. The most straightforward way to reduce the phase transition temperature is to do it. Doping can change the phase transition temperature. The basic explanation is that the phase transition of the metal semiconductor is due to the center position of the vanadium atom in the center of the lattice deviating from the void of the oxygen octahedron. When the vanadium is replaced by an ion with a larger atomic radius, there is no excess in the octahedron. The space allows the atom to be off-center to stabilize the metal phase of VO2. Secondly, the excess charge of the atom replaces the band gap of the semiconductor phase, and the electron energy is relatively easy to transition, thus lowering the temperature of the VO2 phase transition temperature. Under the doping sample, the phase transition from the semiconductor phase to the metal phase occurs; again, the V4+-V4+ is combined with the same pole, so that VO2 exhibits semiconductor properties, and the doping ions can reduce the V4+-V4+ homopolar bond, and the semiconductor phase becomes Stable, resulting in a decrease in phase transition temperature.
2. Research Status of VO2 and Its Doped Powder
At present, the research on the preparation of VO2 mainly focuses on the film preparation process, and the specific methods include reactive sputtering, reactive evaporation, chemical vapor deposition, sol-gel method, pulse laser ablation, etc., but these methods exist. The equipment is expensive, the process parameters are complicated to control, the process stability is poor or the deposition rate is low, the film formation area is small, and it is not suitable for mass production. In contrast, the preparation of VO2 powder materials has the advantages of low cost and high production efficiency. It was found that the fine-grained VO2 powder material can significantly reduce the phase transformation stress, and both the resistance mutation and the light transmittance increase. Compared with film materials, VO2 powder materials have a wider application field.
The foreign manufacturing equipment is advanced, and the obtained VO2 powder has stable quality and good particle size distribution. Japan, the United States and other countries have achieved industrialization, and VO2 powder has been used as a solar temperature control material, creating high economic and social benefits. Compared with developed countries, the research field of ultrafine powder in China is very different, and the research on VO2 ultrafine powder has only just begun in recent years.
2.1 Research Status of VO2 Powder
Takel et al. selected a mixed gas of H2 and N2 as a protective atmosphere, and hydrolyzed VOCl3 at 800 ° C to obtain a VO2 ultrafine powder. Kimizuka heated V2O5 to 1227 ° C in a white gold crucible in a CO 2 atmosphere for 3 days to obtain VO 2 powder. Valmalette JC is pyrolyzed [(NH4)2V6]16 at 380 °C in N2 atmosphere to obtain metastable VO (2B), and then thermally decomposed at 450 °C under argon to produce VO (2R). ) Powder. In another patent of Valmalette JC, vanadium oxalate is used as a precursor, and nano-VO2 powder is obtained by thermal decomposition at 450 ° C under the protection of argon. Lawton et al. sprayed a VOSO4 dilute solution with a H2-N2 mixed gas stream at ≥1000 K to obtain a VO2 powder having a particle size of <1 μm, and chemically doping VO2 by pyrolyzing a mixture of VOSO4 and WO2Cl2 or MoO2Cl2. TSang et al. performed a slow chemical reaction with KHVO4 solution and KBH4 solution to prepare a suspension, which was filtered, washed with water, dried, and vacuum-treated to obtain nano-sized VO2 powder. Toshiyuki et al. used VOCl3 gas and used a CO2 laser as a laser source to synthesize <100 nm VO2 powder by laser induced gas phase reaction.
Chinese patent CN1522965A discloses a preparation method of a phase B vanadium dioxide nanorod by a rheological phase reaction combined with a self-assembly process. Chinese patent CN1587065A discloses a treatment method for converting vanadium dioxide nanorods from phase B to phase M with a concentration of 20% to 40% (mass fraction) of H2O2 aqueous solution. Chinese Patent No. CN1935909A discloses a method for preparing a stable VO2 powder suspension having an average particle diameter of 70 nm using (NaPO3)6 as a dispersing agent. Chinese patent CN1986125A discloses a method for preparing a VO2 powder by adding a raw material V2O5 powder into a sufficient amount of an organic solvent, adding it to a reactor, and using the high temperature and high pressure of the reaction vessel.
Yin Dachuan et al. obtained a nano-scale VO2 powder by slowly chemically reacting the K3VO4 solution with the KBH4 solution in combination with vacuum heat treatment. Zheng Chenmou and others successfully synthesized VO2 ultrafine powder by synthesizing the parent (NH4)[5(VO)(6CO3)4(OH)9]·10H2O and combining vacuum heat treatment.
Xu Shiqing, Zhao Kang et al. used concentrated sulfuric acid and V2O5 powder as raw materials, ethanol and ether as detergents, and liquid phase precipitation method to study the pulverization technology of VO2 nanopowder with VOSO4 as the precursor, and the crystal of VO2 powder. Process. Lei Deming, He Shan et al. used V2O5, N2H4·2HCl, NH4HCO3 as the main raw materials to synthesize the (NH4)5[(VO)(6CO3)(4OH)9]·10H2O purple crystal matrix stabilized in air. Combined with vacuum heat treatment, the particle size can be controlled to be controlled by 10~60nm, VO1.950±x~VO2.050±(xx<0.005), which can control the amorphous, quasi-crystalline or crystalline state and uniform particles. , spherical VO2 nano powder. Guo Ning, Xu Cailing and others used VOSO4 hydrolysis method and vacuum heat treatment to prepare blue-black VO2 powder samples. They then made reference to Tsang’s work to produce nanoscale VO2 powder. Pan Mei, Zhong Hongmei and others used V2O5, concentrated hydrochloric acid, guanidine hydrochloride, NH4HCO3, ethanol as raw materials to prepare VO2-x nanopowders with different oxygen contents by precipitation method, and analyzed the lattice of oxygen content on vanadium oxide nanopowders. Effects of characteristics and structural phase transitions. Luo Min, Gao Jiqiang et al. obtained a nanoneedle or nanopowder product by melting V2O5 powder and adding a templating agent, using a self-generated pressure in a hydrothermal kettle, and hydrothermal treatment at 150-180 °C. Lin Hua, Zou Jian et al. obtained the blue parent material VOC2O4·H2O by adding industrial V2O5 powder to the oxalic acid solution in proportion, and then the mother was placed in a tubular vacuum furnace to obtain nano VO2 powder. Xu Canyang, Pang Mingjie and others used carbon black as a reducing agent, protected by nitrogen/argon atmosphere, and reduced V2O5 at high temperature to obtain VO2 powder. For the first time, Yang Dongmei, Peng Mingfu and others used vanadium slag to extract vanadium solution as raw material, and successfully produced VO2 powder.

Limitations and trends
VO2 ultrafine powders have been prepared in a variety of ways and have made some progress. Despite this, the preparation technology of VO2 ultrafine powder is still slow compared with the application. At present, the preparation of VO2 powder has the disadvantages of low yield, high cost, and insufficient research on the characteristics of ultrafine powder. The industrialization of VO2 ultrafine powder has not been reported in the literature at home and abroad.
In summary, it can be seen that: (1) At present, more one-doping is used, and there are few reports on binary doping. Although the one-doping reduces the phase transition temperature of the VO2 powder, it also reduces other aspects of performance. In order to prepare a more excellent VO2 powder, a binary doping method should be considered. It has been reported that the SiO2 film is coated on the VO2 film, which can greatly improve the visible light transmittance, and the feasibility of coating SiO2 on the VO2 powder can be considered. (2) In order to improve the stability of VO2, consideration should be given to the study of improving the stability of VO2 by adding a certain component. (3) At present, there are more reports on the preparation of VO2 powder, while film materials based on VO2 powder materials are reported less, and in the only report using VO2 powder to prepare film materials, simply VO2 powder is simply reported. The bulk material is mixed with the organic medium and no other pigment filler is added or added. There are few reports on colored intelligent temperature-controlled coatings, and the application of VO2 powders in colored coatings should be considered.
Therefore, future research should focus on finding a variety of high-purity, uniform, stable, and excellent VO2 and its doped ultrafine powder preparation methods, and industrialization, and on this basis, preparation and application Promising VO2 powder based intelligent temperature control coating.

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