Direct Surface Modification of Zirconium by Reactive Plasma Processing at Elevated Temperatures

Abstract

This paper investigates a surface modification process for Zr to improve its durability, in order to make use of Zr in a blanket module as an effective neutron multiplier with (n, 2n) reaction. Modified ceramic layers were successfully synthesized on Zr substrates by carburizing, nitriding and oxidizing using reactive plasma processes. The microstructural evolution during the plasma process is governed both by the kinetics of the diffusion and by the kinetics of the ordering to form the reaction products. In the case of carburizing, the growth rate of the carbide layer is restrained by the diffusion of C, which was clarified to exhibit high covalency with the neighboring Zr atoms by a first principles molecular orbital simulation, through the layer. By contrast, the diffusion of O which exhibits high ionicity rather than covalency with the Zr lattice is much faster than the rate of ordering to form the monoclinic ZrO 2 phase. A 20 MeV electron beam pre-irradiation process was also conducted at ambient temperatures as a pre-treatment before the plasma process. The electron beam irradiation can influence the reaction behavior during the plasma process, depending on the process conditions.

Novel Reactive Plasma Processing for Transforming Surfaces of Metals and Intermetallics to Ceramics

“…The geometrical optimization was conducted by a DFT program code. [3] Therefore, the diffusion coefficient of these atoms decreases with an increase in covalency with the proximal Ti atoms. The diffusion kinetics of these interstitial atoms strongly depend on the bond characteristics, i.e., covalency and ionicity, with the lattice of substrate materials.…”

“…2b. When the boundary condition, in which the concentration at the surface changes to c 0 from 0 and is then kept c 0 during the diffusion process due to plasma irradiation, is applied, the solution to Fick's second law is represented by the following equations: [3] c ¼ c 0 {1 2 erfðzÞ}; ð1Þ The net charge is defined as the difference between the effective charge of electrons belonging to an atom and the charge of atomic nucleus.…”