The Advanced Super Invar Alloys with Zero Thermal Expansion for Space Telescopes
Kotaro Ona, Naoki Sakaguchi, Haruyasu Ohno, Shin Utsunomiya
Abstract
Invar alloys with low thermal expansion are often used in the structural and optical components of space telescopes and sensors, which require high accuracy and thermal dimensional stability. The thermal expansion of Invar can be reduced through partial substitution of Ni by Co. This alloy is known as Super Invar. However, the decrease in Ni content increases the martensitic transformation temperature (Ms). Consequently, the conventional Super Invar cannot be used at temperatures below 243K. Further, some high performance detectors, such as infrared sensors and bolometers, require cryogenic environment to avoid thermal noise. We resolve this contradiction of low coefficient of thermal expansion (CTE) and low Ms by optimizing the contents of Ni and Co, purifying the alloys, and improving the thermal treatment. We have developed novel alloys, which exhibit near-zero CTE and low Ms simultaneously. The developed Super Invar, IC-LTX, exhibited CTE of 0±5×10-8/K (273-303K), and it can be used at 173K. Stainless Invar (IC-DX) was also developed for applications at cryogenic temperatures. It exhibited CTE of 0±1×10-7/K (60-100K) and martensitic transformation did not occur even at liquid He temperature (4K). These alloys are being tested as telescope components for Small JASMINE satellite, Thirty Meter Telescope (TMT), and other applications.