Influence of the microstructure on the diffusion barrier performance of Nb-based coatings for cyclotron targets

Abstract

The number of medical procedures involving the use of cyclotron-produced radionuclides is constantly growing year by year. The design and construction of the cyclotron targets appropriate for the production of the radionuclides of interest are the most challenging issues. The cyclotron targets for the medical radionuclide production suffer from two main corrosion problems: the corrosion due to proton-irradiated water and liquid metal embrittlement. The design of the target for radionuclide production limits the ability to select an ideal material that meets all of the following requirements: machinability or ease of construction, high melting temperature, high thermal exchange performance, excellent chemical inertness, etc. The use of thermally and mechanically suitable substrate materials protected by chemically resistant coatings can be a good compromise. These two corrosion problems can be attributed to the mechanism of diffusion by the aggressive particles through the protective coating. In this research, niobium has been chosen as the principal material for the design of thin film protective coatings. The coating microstructure was correlated to specific deposition parameters to provide chemical resistance to both proton-irradiated water corrosion and liquid metal embrittlement. Film densification and amorphization were pursued to achieve niobium-based thin films efficient as diffusion barriers to proton-irradiated water and liquid metal. The most important conclusions were that the performance of thin films as diffusion barriers varied dramatically based on various deposition parameters and deposition technologies. Among the configurations studied, only three are acceptable as anticorrosion coatings: niobium deposited on axis with unbalanced magnetron sputtering, niobium coated at a high sputtering rate and on a water-cooled sample holder, and the niobium-titanium alloy sputtered at a low argon pressure.