Laser-interferometric dilatometry

Abstract

Highly dimensionally stable materials and structures are particularly needed in optical systems such as ultra precise optical clocks, as well as materials with excellent dimensional stability and light weight properties for space applications such as telescopes, optical benches, and optical resonators. Also, the dimensional stability of mounting technologies of materials with different properties is of high interest for such applications. Glass ceramics and composite materials can be tuned to reach a very low coefficient of thermal expansion (CTE) at different temperatures, enabling best stability in the operating temperature for certain applications. In order to determine the CTE of such highly stable materials, very accurate set-ups are needed. In this thesis, metrology set-ups to measure the CTE of a large variety of material samples are designed, realized and verified, measuring dimensionally stable glass ceramics. The set-ups are able to characterize tube shaped samples at a temperature range of 140 K to 333 K. Due to our unique mirror mount design all kind of materials can be characterized. The optical dilatometer set-ups are based on a heterodyne interferometer with a displacement sensitivity at the sub-nanometer level. This instrument is used to measure the expansion of a sample when applying controlled small amplitude temperature signals. A carbon fiber reinforced polymer (CFRP) sample was characterized where CTE levels of 10 -8 K -1 from 140 K to 250 K were measured and a detailed uncertainty analysis was performed. The verified metrology set-up for tube shaped samples was adapted for CTE measurements of larger structures. Therefore, a large thermal chamber was set up and a 0.5 m CFRP spacer with Zerodur endfittings as a representative joint technology demonstrator for the GRACE Follow-On space mission at 302 K was investigated.