Research On Preparing Yttria Coatings By Metalorganic Chemical Vapor Deposition

Yttria has great thermal stability and great chemical stability with uranium as well as its alloys. It is well suitable for carbon barrier coating of graphite crucible for melting uranium alloys. Various techniques have been used for the preparation of Y₂O₃ coatings such as Spread coating, Thermal spraying, Plasma spraying. But, no process is suitable, since the coatings are strictly demanded in thickness, bond strength and completeness. Metallorganic Chemical Vapor Deposition (MOCVD) is an advanced preparative technique. Coatings prepared by the method have many strongpoints such as high quality, good completeness and good controllability. In this thesis, for preparing carbon barrier coating of graphite crucible for melting uranium alloys, the feasibility of preparing Y₂O₃ coatings by MOCVD was investigated. The main research results are as follows:(1) By the investigation of Y(TMHD)3 precursor and exploratory experiments, the suitable MOCVD equipment was developed. It had no vacuum system, and the heating element was separated with its deposition chamber which was made of silica glass, and the precursor was volatilized by the radiation heating of the deposition reactor.(2) The dependence of coating quality on depositing parameters, including the concentration of Y(TMHD)3, deposition temperature and substrate was studied.1) The coatings well combined with the substrate were prepared when the concentration of precursor was larger than its saturated concentration.2) With the increasing of the temperature from 600℃ to 700℃, the defect density reduced and the Y₂O₃ coatings combined better with the substrate, and the crystal grain changed from equiaxed grain to columnar grain.3) The substrate largely influenced the quality of Y₂O₃ coatings. The coatings deposited on SiO₂ substrate had the highest tightness and combined the bestwith the substrate. With the changing of the substrate from SiC^to CVD SiC, the range of the deposition temperature changed from 500-700°C to 600-700 °C.metalorganic chemical vapor deposition;Y₂O₃ coating;Y(TMHD)3;equipment...