Zinc Sulphide (ZnS)

Used for IR windows and lenses in the thermal band (8 to 14 microns) where maximum transmission and lowest absorption is required. Also selected for use where visible alignment is an advantage. Zinc Sulphide is produced by synthesis from zinc vapour and H2S gas, forming as sheets on graphite susceptors. It is microcrystalline in structure, the grain size being controlled to produce maximum strength. Multispectral grade is then Hot isostatically pressed (HIP) to improve the mid IR transmission and produce the visibly clear form. Polycrystal ZnS is available.

Specifications of ZnS
Transmission Range 0.37 to 13.5μm
Refractive Index 2.20084 at 10mμm
Reflection Loss 0.0006 cm-1 at 3.8μm
Restrahlen Peak 30.5μm
dn/dT +38.7 x 10-6/°C at 3.39μm
Density 4.09g/cm3
Melting Point 1827°C (dissociates about 700°C)
Thermal Conductivity 27.2 W/(m K) at 298K
Thermal Expansion 6.5 x 10-6/ °C at 273K
Hardness Knoop 160 with 50g indenter
Specific Heat Capacity 515 J · kg-1 · K-1
Dielectric Constant 8
Young's Modulus (E) 74.5 GPa
Apparent Elastic Limit 68.9 MPa (10,000psi)
Poisson Ratio 0.28