| Poly-crystalline hot-pressed Zinc sulfide (HP-ZnS) is one of the most important IR permeate materials, which has excellent mechanical and optical property comparing with polycrystalline CVD-ZnS. For this reason, it finds its wide applications in many fields such as air-air, air-land, land-air missile dome, infrared optical windows and some other infrared image system.In this paper, studies cover the technology of vacuum hot pressing, properties of powder and treatment of ZnS powders. The structural and optical properties of HP-ZnS crystals are analyzed with modern technologies of Fourier IR spectrum, XRD, TEM and SEM etc. A series of testing on properties are taken, which include density & green density, thermal coefficient of expansion, Knoop hardness, fracture toughness, fracture strength and IR transmittance. In order to calculate IR transmittance using some parameters of hot-press and evaluate parameter effect on properties of HP-ZnS, the neural network, Back-Propagation Network technique is used. It is shown that sintering temperature is the most important parameter that affects IR transmittance of HP-ZnS. The particle size of ZnS powder is the second one. BP-network is useful to map the relationship among the coefficient.The results demonstrated that the structure of HP-ZnS crystal is FCC beta-ZnS. Peak of diffraction 2 θ is 28.52-28.54°, and no other impurity exists. According to the researches of SEM and TEM, HP-ZnS crystal has grain-like structure, which shows typical {111} twin, and this indicates that twinning is one of the main distortion mechanism during sintering when facing hot press.The factors of sintering temperature, pressure and size of ZnS powders have great effect on IR transmittance of HP-ZnS crystals. In the condition of the same size and pressure, transmittance is in direct proportion to temperature in a range; the relative low sintering temperature can produce crystals with high transmittance in the range of 2-5 μm band; at the same tune, relative high temperature is good for that in the range of 8-10μm band. When the temperature goes up over 1050℃, the transmittance begins to go down. In the condition of the same size and temperature, the transmittance increases along with increasing of pressure. In the condition of the same hot pressing, the finer ZnS powder is,the greater activity of powder surface is. As a result, the relative low temperature and pressure are needed to get same quality HP-ZnS.Comparing of different methods of treatment of ZnS powder are also mentioned in this paper. The powders of ZnS are reduced by H2S and H2, at the same time, they are treated to decrease conductivity by use of reducing content of anion. The result is obtained that the IR transmittance is greatly improved with the minimizing ion of impurity.Optimized parameters of hot press at this research are as following: the temperature of sintering is 950℃, the pressure is 350MPa, the time of sintering is 60min, and the vacuum is 0.1 atm. By using of optimized technology, high quality HP-ZnS crystals are obtained. The average IR transmittance of 6mm-thickness sample in the band of 8μm to 12μm is more than 66.6%, which preponderates over that of foreign products.
|
PDF Full Text Request