Negative thermal expansion ALLVAR alloys for telescopes

Abstract

Most materials naturally expand when heated and contract when cooled; this is known as thermal expansion and is typically characterized by a Coefficient of Thermal Expansion (CTE). Competition between different materials, each with their own CTE, can push optic systems out of focus when the system’s temperature changes. The use of low CTE materials like Invar, carbon fiber composites, and silicon carbide help reduce these temperature effects. Unfortunately, they each have drawbacks such as the high density and low corrosion resistance of Invar, the polymer outgassing of carbon fiber composites, and the low fracture toughness of silicon carbide. In comparison, ALLVAR alloys shrink when heated, known as a negative thermal expansion (NTE), and have low density, high corrosion resistance, high ductility, and do not outgas. This NTE behavior, down to -16 x 10-6 °C-1 at room temperature, offers a new way to athermalize optic systems by combining NTE alloys with positive CTE materials. The NTE alloy can compensate for a positive CTE material to achieve a desired CTE. Here we evaluate ALLVAR alloys as a potential material for optics applications.