When sunlight hits the surface of an object, the object mainly absorbs near-infrared light energy to increase its surface temperature, and near-infrared light energy accounts for 50% of the total energy of sunlight. In summer, when the sun shines on the surface of the object, the surface temperature can reach 70~80℃. At this time, infrared light needs to be reflected to reduce the surface temperature of the object; when the temperature is low in winter, infrared light needs to be transmitted for heat preservation. That is, there is a need for an intelligent temperature control material that can reflect infrared light at high temperatures, but transmit infrared light at low temperatures and transmit visible light at the same time, so as to save energy and protect the environment.
Vanadium dioxide (VO2) is an oxide with phase change function near 68°C. It is conceivable that if the VO2 powder material with phase change function is compounded into the base material, and then mixed with other pigments and fillers, a composite intelligent temperature control coating based on VO2 can be made. After the surface of the object is coated with this kind of paint, when the internal temperature is low, infrared light can enter the interior; when the temperature rises to the critical phase transition temperature, a phase change occurs, and the infrared light transmittance decreases and the internal temperature gradually decreases ; When the temperature drops to a certain temperature, VO2 undergoes a reverse phase change, and the infrared light transmittance increases again, thus realizing intelligent temperature control. It can be seen that the key to preparing intelligent temperature control coatings is to prepare VO2 powder with phase change function.
At 68℃, VO2 rapidly changes from a low-temperature semiconductor, antiferromagnetic, and MoO2-like distorted rutile monoclinic phase to a high-temperature metallic, paramagnetic, and rutile tetragonal phase, and the internal VV covalent bond changes It is a metal bond, presenting a metallic state, the conduction effect of free electrons is sharply enhanced, and the optical properties change significantly. When the temperature is higher than the phase transition point, VO2 is in a metallic state, the visible light region remains transparent, the infrared light region is highly reflective, and the infrared light part of the solar radiation is blocked outdoors, and the transmittance of infrared light is small; When the point changes, VO2 is in a semiconductor state, and the region from visible light to infrared light is moderately transparent, allowing most solar radiation (including visible light and infrared light) to enter the room, with high transmittance, and this 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 problem that everyone cares about. At present, the most direct way to reduce the phase transition temperature is doping.
At present, most of the methods for preparing doped VO2 are unitary doping, that is, only molybdenum or tungsten is doped, and there are few reports on the simultaneous doping of two elements. Doping two elements at the same time can not only reduce the phase transition temperature, but also improve other properties of the powder.