Fabrication and characterization of alumina-based bilayer dielectric high gradient insulator (DHGI)

Abstract

High voltage devices suffer from electrical breakdowns that hamper their operation and shorten their lifespan. Such devices are often sealed and operate under vacuum to increase optical transmission and reduce the interaction with ambient gas molecules. The main failure mode in such devices is a surface breakdown. By introduction of a periodic structure of metal-dielectric, known as high gradient insulator (HGI), improved surface breakdown threshold is achieved. Nevertheless, local breakdown damage and metal deposition on the dielectric layers degrade the HGI performance. In this work, we demonstrate the fabrication and characterization of a novel alternative concept where we replace the HGI's metallic layers with high dielectric coefficient ceramic layers. The novel structure which is denoted as DHGI (Dielectric HGI), reshapes the electric field lines in a similar manner to common HGI. A prototype DHGI composed of two layers: (i) Al2O3 and (ii) Al2O3 with 8 wt%. addition of TiO2 co-doped with Al and Nb (TANO), was fabricated by spark plasma sintering (SPS). The dielectric constants of pure alumina and TANO are 10 and ∼104, respectively. It is shown that during sintering, unique intermediate layers are formed at the interface between the two original layers. The microstructure and composition of each layer were characterized. The decreasing TANO content in the intermediate layers induces a gradual decrease of the Al2O3 grain size across the interface. The DHGI prototype was found to have superior breakdown performance compared to conventional pure alumina insulator.