DEPOSITION OF CRYSTALS FROM THE PLASMAS OF ZRO2, HFO2, THO2, AND CEO2

Deposition occurs onto a cathode from an oxide plasma generated inside a hollow oxide anode by the mechanism called the electrolysis of plasma. The temperature of the cathode is usually over 2000 °C during deposition. Deposits attain fairly large crystalline sizes (about 0.3 mm) in spite of their high deposition rates. And besides, some unusual crystals are formed. For the deposition from ZrO2, HfO2, and ThO2 plasmas, the deposition rate, growth rate, and the current efficiency increase with the plasma current (0.4-8 A) and with feeding O2 gas into the plasmas. On the other hand, heating a cathode externally or feeding Ar gas into them leads to the reduction of those rates, the current efficiency, and yield. The maximum growth rate from the ThO2 plasma is 8.4 μm/sec. Yields exceed 90% in general. Mass-spectrometric analyses showed that the main depositing ion species is ZrO+ for the deposition from the ZrO2 plasma. Deposits obtained are usually black oxides, whose O/M (M=Zr, Hf, and Th) ratios are 1.95, 1.95, and 1.64, respectively. The deposits from ZrO2, HfO2, and ThO2 plasmas are monoclinic ZrO2 with precipitates of metallic α-Zr, monoclinic HfO2 with a small amount of unknown hexagonal crystals (whose lattice parameters are a=3.17 and c=5.02 Å), and a mixture of fcc ThO2 and Th, respectively. When O2 gas is fed into those plasmas, transparent oxide crystals of stoichiometric composition are formed. The deposits from a CeO2 plasma are the black oxides whose O/Ce ratios are 1.53 on the average. Contrary to the phase diagram of the Ce-O system, they show x-ray structures of fcc crystals whose lattice parameter varies from 5.506 to 5.411 Å.