The Study Of Mechanism And Control Methods Of Growth Bands And Residual Stress In CVDZnS Polycrystalline

Zinc sulfide belongs to ?B-?A group compound.Chemical vapor deposited zinc sulfide(CVDZnS)has high infrared transmittance,mechanical and optical properties.The multi-spectral CVDZnS obtained by hot isostatic pressing transparent treatment has a wider transmission range(0.35-13 ?m),widely used in mid-infrared and far-infrared guided thermal imaging systems.Chemical vapor deposition(CVD)is currently the mainstream preparation technology for growing zinc sulfide bulk material.The as-deposited CVDZnS polycrystals prepared by chemical vapor deposition exhibit growth bands of color and brightness changes along the growth direction,which seriously affects the uniformity of optical transmission.The residual stress of multi-spectral CVDZnS obtained by hot isostatic pressing transparent treatment is serve,which seriously affects the quality of infrared thermal imaging.This study is focus on the mechanism of growth bands in as-deposited CVDZnS and residual stress in multispectral CVDZnS,and obtain the control methods to improve optical performance.The influence of growth bands in as-deposited CVD ZnS on transmittance analyzed by UV/VIS/IR and FT-IR spectrometers showed that growth bands mainly affected the transmission uniformity of as-deposited CVDZnS in visible and near-infrared wavelength,and the visible transmittance can differ by more than 15%due to growth bands.Thus,growth band is a phenomenon based on differences in transmittance due to inconsistencies in optical scattering and absorption.The reasons that affect the transmission uniformity of as-deposited CVDZnS growth bands have been systematically studied.Various test results showed that the hexagonal phase affects transmission uniformity is negligible,the scattering caused by the microscopic defects in the polycrystal has a certain effect on transmission uniformity,and the zinc-sulfur ratio in growth bands of as-deposited CVDZnS is between 1.01 and 1.07.So there is a significant difference of optical absorption between growth bands,which is the main reason for affecting the transmission uniformity of growth bands.The formation mechanism of growth bands in as-deposited CVDZnS is that the zinc-sulfur ratio changes with time during the chemical vapor deposition process.According to the kinetic principle of chemical vapor deposition,the growth bands in as-deposited CVDZnS was controlled by improving the stability of chemical reactant concentration,that is,by stabilizing zinc evaporation amount and H2S gas flow.The experimental results of suppressing growth bands showed that growth bands can be effectively controlled,and the transmittance and transmission uniformity were significantly improved.The residual stress of as-deposited CVDZnS and multi-spectral CVDZnS was compared.The stress birefringence test results showed that the residual stress in as-deposited CVDZnS was small and the distribution uniformity was fine,while the residual stress of multi-spectral CVDZnS was large and the distribution uniformity was poor.The formation mechanism of CVDZnS polycrystalline residual stress is that there is a temperature gradient during preparation and hot isostatic pressing transparent treatment,so there is thermal stress.The experimental samples for the annealing heat treatment research of multispectral CVDZnS are 475×475×18 mm and 165×120×10 mm two main batches.Compared with directly stacking multi-spectral CVDZnS and applying forced convection,the annealing conditions such as separating multispectral CVDZnS by special structural sample chamber and apply natural convection in a hermetic space can release residual stress more significantly and more uniform.Under the conditions of barometric pressure and annealing temperature of 700?750?,the annealing process of 160 h soaking time combined with cooling rate below 5?7?/h reduced the residual stress value of 475×475×18 mm multi-spectral CVDZnS below 10 nm/cm,and the 100 h soaking time combined with a cooling rate below 13?15?/h can reduced the residual stress value of 165×120×10 mm multi-spectral CVDZnS below 15 nm/cm.The hot isostatic pressing in-situ annealing reduced the residual stress of 165×120×10 mm multi-spectral CVDZnS to about 7 nm/cm,and the residual stress was released more significantly.Compared with the barometric pressure annealing,HIP in-situ annealing shorten the heat treatment time and is more effective in reducing residual stress,which is the preferred annealing method,The optimal annealing process for 165×120×10 mm multi-spectral CVDZnS is two constant temperature stage of 950?1000?/130?150 MPa soaking for 40?60 hours and 560?610?/90?100 MPa soaking for 30?40 hours combined with 10?15?/h cooling rate cooling down to room temperature.