Lithium fluoride material properties as applied on the NIRCam instrument

Single crystal Lithium Fluoride (LiF) has been base-lined as one of the optical materials for the Near Infra-Red Camera (NIRCam) on the James Webb Space Telescope (JWST). Optically, this material is outstanding for use in the near IR. Unfortunately, it has poor mechanical properties that make it very difficult for use in any appreciable size on cryogenic space based instruments. In addition to a dL/L from 300K to 30K of ~-0.48% and room temperature CTE of ~37ppm/K, LiF deforms plastically under relatively small stresses. This paper will discuss the heritage of LiF in space-based systems and summarize the mechanical and thermal material data for LiF that is available in the literature. New data will be presented relative to a design limit load for the material so that designers can use this material for space flight applications. Additional new data relative to the cryogenic index of refraction of the material over the near infrared is also provided.