Millimeter-wave materials processing in Japan by high-power gyrotron

This paper summarizes recent research activities on materials processing using millimeter-waves in Japan with emphasis on the work performed at the Joining and Welding Research Institute (JWRI), Osaka University. Extensive research results conducted at the JWRI on ceramics sintering and modification of thin film properties with millimeter-wave radiation from a 28-GHz gyrotron are described. In the sintering of pure Al₂O₃, it was found that at a similar density and grain size a big difference in the bending strength appeared in the sample after millimeter-wave sintering as compared with that after conventional sintering. This result was attributed to come from the difference in the grain boundary structures due to the unique millimeter-wave effect. We have also obtained highly dense sintered Si₃N₄ and Aluminum Nitride (AlN) by the millimeter-wave method with new sintering aids containing Yb₂O₃ at a temperature lower by about 200°C-400°C than that by the conventional method. It was verified that selective heating of Yb₂O₃ by millimeter-waves around the grain boundary promoted a densification process in its liquid phase. A high thermal conductivity of 210 W/m·°C could be obtained in AlN samples sintered at 1700°C for 180 min in N₂+ 3% H₂ gas environment. In the research of thin-film modification the millimeter-wave irradiation to SrTiO₃ films prepared by mirror-confinement-type electron cyclotron resonance (MCECR) plasma sputtering method could drastically improve their crystallinity and electrical properties at remarkably lower temperatures than by the conventional thermal methods. The crystallization temperature of amorphous films on Si substrates were 300°C and the dielectric constant reached to about 260.