Novel CTE Tuning of Ultra-Stable ALLVAR Alloy Struts for Large Space Telescopes

ALLVAR Alloys-30 shrinks when heated and expands when cooled, known as negative thermal expansion (NTE). This opposite effect from most materials allows Alloy-30 to compensate for positive thermal expansion (PTE) materials. We have created a new material for athermalizing optics made from any type of mirror material by joining Alloy-30 to other PTE metals to create a specified thermal expansion coefficient. Currently, achievable coefficients of thermal expansion (CTE) range between -30 ppm/K, Alloy-30s CTE, to +24 ppm/K, Aluminums CTE. This provides a new alternative material to currently used carbon fiber composite metering structures and trusses used in optics. The pm-stability of these new metal structures have already been achieved. If this new technology can be scaled to have ultra-stability at meter length scales and the CTE tuned to within ppb/K, they could be used as metering structures in EUV/Optical/IR large area telescopes. The novel CTE tuning method has the potential to simplify the manufacturing and alignment and offer greater thermal stability of these optic systems. ALLVAR Alloys-30 shrinks when heated and expands when cooled, known as negative thermal expansion (NTE). This opposite effect from most materials allows Alloy-30 to compensate for positive thermal expansion (PTE) materials. We have created a new material for athermalizing optics made from any type of mirror material by joining Alloy-30 to other PTE metals to create a specified thermal expansion coefficient. Currently, achievable coefficients of thermal expansion (CTE) range between -30 ppm/K, Alloy-30’s CTE, to +24 ppm/K, Aluminum’s CTE. This provides a new alternative material to currently used carbon fiber composite metering structures and trusses used in optics. The pm-stability of these new metal structures have already been achieved. If this new technology can be scaled to have ultra-stability at meter length scales and the CTE tuned to within ppb/K, they could be used as metering structures in EUV/Optical/IR large area telescopes. The novel CTE tuning method has the potential to simplify the manufacturing and alignment and offer greater thermal stability of these optic systems. The ultimate goal of this effort is to demonstrate consistent meter length scale low CTE telescope metering struts using ALLVAR Alloys. The key objectives of this Phase II project are to 1) Demonstrate repeatable manufacturing of ALLVAR Alloy athermalized strut segments, 2) Determine A-, B-, and S-basis for relevant material properties, and 3) Determine ALLVAR Alloy 30’s long-term dimensional stability. To achieve Key Objective 1, several strut segments will be designed and machined to have zero thermal expansion. To achieve Key Objective 2, material properties will be tested and analyzed using the Metallic Material Property Database (MMPDS) methodology. To achieve Key Objective 3, the length of samples will be measured over a long timeframe. Deliverables for this project are a report outlining the CTE properties and tunability and a set of zero CTE strut segments delivered to NASA.

New large-scale materials with picometer stability and CTE tunability can potentially improve support structures for optic systems critical to NASA’s Science Mission Directorate including LUVIOR, OST, and HabEx. ALLVAR Alloys are a truly cross-cutting technology that can impact ultra-stable coronograph hardware, support structures for deformable mirrors, telescope steering, and star trackers, and other applications in NASA’s Science Mission, Space Technology, and Aeronautics Mission Directorates. ALLVAR Alloy’s unique negative thermal expansion properties can compensate for thermal focus shift in refractive infrared optics allowing infrared and visible refractive optics manufacturers to reduce the size and weight of their optics. Negative thermal expansion washers and spacers can also enable constant force fasteners that do not loosen under repeated thermal cycles.

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