Examination of Conductive Properties of ZrO2 Film Formed By Self-Oxidation Method
Authors: Takemura Rio , Sasaki Kazuya , Niwa Eiki , Kondo Masatoshi 
Published: December 2020

By forming an oxide film on the surface of metallic zirconium, ZrO2 works as a corrosion resistant film and prevents the permeation of hydrogen. This material is expected to be utilized as a plasma facing material for fusion reactors, because zirconium and ZrO2 have excellent compatibility at high temperature under the redaction atmosphere. Vapor deposition methods such as electron beam vapor deposition and pulse laser vapor deposition are the main methods of forming oxide films on metallic zirconium surfaces. In this study, the self-oxidation method using water vapor was employed as a film forming method, because a dense film of zirconium oxide can be obtained on the surface of metallic zirconium by the heat treatment at 500 °C in water vapor. Although the self-oxidation method using water vapor is promising, the previous reports have been limited yet. An oxide film is formed on the surface of metal Zr by a self-oxidation method in a water vapor atmosphere, and its conductive properties due to the difference in the thickness of each layer of the film are examined.

The fine structure of the preparing film was observed with a field emission Scanning Electron Microscope (SEM). The film thickness was determined as the average film thickness from the cross-sectional SEM image using image processing software Image Processing and Analysis in JAVA (Image J). The O/Zr ratio in the film was analyzed by line analysis of an Energy Dispersive X-ray analyzer (EDX). The Crystallinity and formed phase of the film were identified by Electron Back Scatter Diffraction (EBSD) analysis and "grazing incidence X-ray diffraction". impedance measurement was performed using SP-150 electrochemical measurement system of Bio-Logic Sciences Instruments. It was decided to separate the conductivity of each layer by measuring the oxide film prepared by adjusting the film thickness.

In the previous studies, it was revealed from the SEM observation of the cross section of the oxide film that the oxide film was not a single layer but a laminated structure whose O/Zr ratio gradually decreases with increasing of the film depth. Therefore, by changing the X-ray incident angle (θ = 1 ~ 12 °) by XRD analysis, we analyzed the change of the formed phase in the thickness direction and found that each phase is not an amorphous phase but a crystalline phase. The sample formed by steam oxidation was subjected to EDS line analysis in the direction perpendicular to the oxide film cross section, and the O/Zr ratio of each layer was measured. It was observed that the atomic ratios of the three kinds of layer, bottom layer, second layer and top layer were observed to be 0.3-0.6, 0.6, and 1.5 or more, respectively. The samples with the various thickness of oxide film were prepared and the specific electrical resistances of each film were measured with the impedance measurements. The electrical conductive properties of each layer were investigated with the fitting of equation whose parameters are the thickness and the specific electrical resistance of each layer.