CONTAINERLESS SOLIDIFICATION OF REFRACTORY METALS BY ELECTROSTATIC LEVITATION

Abstract

We have been developing an electrostatic levitation (ESL) system to investigate the new process of containerless solidification with significant undercooling for researching new materials or metastable phases. In this report, preliminary experiments for three refractory metals Zr Nb and Mo have been performed by the ESL process using 150 and 250 W Nd:YAG (Naodymium:Yttrium Aluminium Garnet) lasers. Since molten metals are solidified without contact in a high-vacuum condition, a significant undercooled state up to 300–500°C is maintained before recalescence followed by solidification. Dendritic growth of these refractory materials of Nb and Mo is also driven with suppression of heterogeneous nucleation. The ESL-processed Nb showed a superconducting transition at 9.1K, which is higher than as-arc melted Nb at 8.6K. The magnetization curves imply that it contains fewer pinning centers.

Keywords:

• Electrostatic levitation
• Containerless solidification
• Recalescence
• Undercool
• Single crystal
• Sphericity
• Superconductivity
• Dendrite
• Refractory metals
• Niobium
• Molybdenum
• Zirconium
• Microgravity
• Pinning center
• Homogeneous nucleation

ACKNOWLEDGMENTS

Technical assistance by A. Tanji, K. Uematsu, and K. Takashima at Ishikawajima–Harima Heavy Industies Co. Ltd. is appreciated. K. Kuribayashi at the Institute of Space and Astronautical Science is also acknowledged for useful discussions. This work was supported by the Core Research for Evolutional Science and Technology project of Japan Science and Technology Corporation.