The Edmund optics catalog says "Calcium Fluoride is also often found in cryogenically cooled thermal imaging systems". Why is it prefered for this purpose? I might want to put an IR window in a cell that would be filled with liquid nitrogen; this would mean considerable thermal stress. Sapphire is strong, cheap, hard, and undamaged by moisture (you could wipe condensation off of it with no worries) but is sold thinner than other materials, thus increasing the thermal gradient.
For that matter, what sort of O-ring would be usable in a cell that will be filled with LN2? I don't mind some leakage, I just don't want the window or the O-ring to crack.