The effects of processing on the phase, structure and properties of thick Gd₂O₃ films deposited by reactive electron-beam physical vapor deposition (EB-PVD)

Gadolinium-based materials are particularly attractive for neutron detection due to gadolinium's high thermal neutron capture cross section. Since the utilization of gadolinium metal is challenging due to its tendency to form a low quality oxide on the surface when exposed to air, Gd₂O₃ is an appealing alternative due to its stability at high temperatures and inoxidizing environments. Since the mean free path of thermal neutrons in Gd is approximately 7.7 μm, a large film thickness is needed to ensure proper detection efficiency. This book chapter will focus on the processing-structure-property relationships for micron thick Gd₂O₃ films deposited by reactive electron beam-physical vapor deposition (EB-PVD). Fundamental studies have been conducted to assess the effects of deposition parameters such as temperature, oxygen flow, and deposition rate on film properties. Significant changes in crystallographic phase, orientation, grain size, density, and surface roughness were observed with variation in deposition parameters. Gd2O3 has three stable polymorphs: cubic, monoclinic, and hexagonal phases. Since EB-PVD is a non-equilibrium process, it is possible to deposit films that contain meta-stable phases. Indeed, depending on the processing conditions, the deposition of single phase cubic, monoclinic, and mixed phase films was demonstrated. The residual stress state in the films was found to be an integral component in the phase stability of the films. The existence of a stress induced phase transition was demonstrated by means of ion beam assisted deposition (IBAD). The effects of deposition conditions on the electrical and optical properties of Gd₂O₃ will also be discussed.