During the use of industrial furnace and vacuum chambers, the viewport window will be subjected to very high pressure and high working temperature. In order to ensure the safety of experimenters, the viewport window must be firmed, reliable, high-temperature resistant, also has excellent optical properties. Synthetic sapphire is an ideal material as an viewport window.
Sapphire has the advantage of its pressure strength: it can withstand the pressure before rupture. Sapphire has a pressure strength of approximately 2 GPa. In contrast, steel has a pressure strength of 250 MPa (almost 8 times less than sapphire) and gorilla glass (™) has a pressure strength of 900 MPa (less than half of sapphire). Sapphire, meanwhile, has excellent chemical properties and is inert for almost all chemicals, making it suitable for where corrosive materials are present. It has a very low thermal conductivity, 25 W m’(-1) K^(-1), and a very low thermal expansion coefficient of 5.8×10^6/C: no deformation or expansion of thermal conditions at high or high temperatures. Whatever your design, you can make sure it has the same size and tolerances at 100 meters under the sea or 40K in orbit.
We have used these characteristics of strength and scratch-resistant windows in customer applications, including vacuum chambers and high temperature furnaces.
Sapphire window for furnace has excellent transmission in the 300nm to 5500nm range (covering ultraviolet, visible and infrared areas) and peaks at transmission rates of almost 90% at 300 nm to 500 nm wavelengths. Sapphire is a double refractive material, so many of its optical properties will depend on the crystal orientation. On its normal axis, its refractive index ranges from 1.796 at 350nm to 1.761 at 750nm, and even if the temperature changes significantly, it changes very little. Due to its good light transmission and wide wavelength range, we often use sapphire window in infrared lens designs in furnaces when more common glasses are not suitable.
Here is a Experience Calculation formula of the thickness for sapphire viewport window:
Th=√( 1.1 x P x r² x SF/MR)
Th=Thickness of the window(mm)
P = Design use pressure(PSI),
r = Unsupported radius (mm),
SF = Safety factor (4 to 6) (suggested range, may use other factors),
MR = Modulus of rupture (PSI). Sapphire as 65000PSI
For example, Sapphire window with diameter 100 mm and unsupported radius 45 mm used in environment with Pressure differential of 5 atmosphere should have thickness of ~3.5mm (safety factor 5).