Microstructure of seal formed between a Nb wire and an Al2O3 capillary melted by YAG laser radiation

Cracks occasionally develop inside the glass frit, the sealing material used in ceramic metal-halide lamps that employ polycrystalline alumina as the arc-tube material, although the rate at which these cracks develop is low. Therefore, we proposed a new method to stop their development. Our method did not make use of a glass frit and involved the formation of a seal between the Nb wire and the Al2O3 capillary melted by the radiant energy from a YAG laser. However, cracks or voids developed inside the melted Al2O3 capillary. Therefore, in order to control the Al2O3 microstructure inside the melted Al2O3 capillary and thus to obtain a better seal, grain growth was suppressed by combining the processes of annealing after melting and grain-boundary deposition of Ta. The Al2O3 microstructure could be controlled by these factors. Thus, optimized conditions for forming the seal were achieved.