Zinc sulfide is a white to yellowish-white crystal. It exists in two crystalline forms: a (wurtzite) and b (sphalerite). Wurtzite crystalline form is hexagonal; refractive index 2.356; density 3.98 g/cm3; melts at 1,700degC. Stable if dry; sphalerite crystalline form is cubic, forming at 1020degC, d 4.102; changes to alpha form at 1180degC; practically insoluble in water, about 6.5 mg/L; soluble in mineral acids, insoluble in alkalis.
Various phosphorescent zinc sulfide compounds have been developed for many different applications, including cathode ray tubes, X-ray displays and electroluminescent panels. The phosphorescent properties of zinc sulfide enable it to glow for a long time when illuminated with blue or ultraviolet light.
It has been shown that the presence of impurities in the phosphorescent zinc sulfide can change the colour of the molecule, resulting in an orange or red appearance with a wavelength of about 590 nanometers. Its luminous quality also makes it useful for the removal of hydrogen sulfide impurities from natural gas.
Phosphorescent ZnS has a unique set of properties that have made it an excellent choice for high-performance infrared windows. Its combination of long-wavelength infrared (812 mm) transparency, mechanical durability and elevated-temperature performance make it a powerful candidate for use in multispectral windows.
Zinc sulfide has also shown promise in flame retardancy. This is important for a wide variety of applications, from rigid PVC and other plastics to coatings for furniture and appliances. In recent years, improved flame-retardant grades have been developed for zinc sulfide alone or in synergistic systems with other materials, such as antimony trioxide.
