Polycrystalline chalcogenide ceramic material

Abstract

The invention relates to a polycrystalline IR transparent material produced by sintering chalcogenide powder, e.g., ZnS powder, using hot uniaxial pressing followed by hot isostatic pressing. The microstructure of the material described in this disclosure is much finer than that found in material produced using the state of the art process. By using a powder with a particle size fine enough to improve sintering behavior but coarse enough to prevent a lowering of the wurtzite-sphalerite transition temperature, a highly transparent material with improved strength is created without degrading the optical properties. A high degree of transparency is achieved during hot pressing by applying pressure after the part has reached a desired temperature. This allows some degree of plastic deformation and prevents rapid grain growth which can entrap porosity. The crystallographic twins created during this process further inhibit grain growth during hot isostatic pressing.

1 . A sintered polycrystalline ceramic body comprising a chalcogenide material in a polymorphic form having a cubic structure and having an extinction coefficient of ≦2.0 cm −1  at 1100 nm and a Vickers hardness of ≧180 kg/mm 2 . 
     
     
         2 . A sintered polycrystalline ceramic body according to  claim 1 , wherein said chalcogenide material in a polymorphic form having a cubic structure is zinc sulfide sphalerite. 
     
     
         3 . A sintered polycrystalline ceramic body according to  claim 1 , wherein said ceramic body has an extinction coefficient of 0.05-2.75 cm −1  at a wavelength of 1100 nm. 
     
     
         4 . A sintered polycrystalline ceramic body according to  claim 1 , wherein said ceramic body has an extinction coefficient of ≦2.5 cm −1    
     
     
         5 . A sintered polycrystalline ceramic body according to  claim 1 , wherein said ceramic body has a Vickers hardness of 180-265 kg/mm 2 . 
     
     
         6 . A sintered polycrystalline ceramic body according to  claim 1 , wherein said ceramic body has a Vickers hardness of ≧200 kg/mm 2 . 
     
     
         7 . A sintered polycrystalline ceramic body according to  claim 1 , wherein said ceramic body has a Knoop Indentation Hardness measured at 0.1 N of at least 260 kg/mm 2 . 
     
     
         8 . A sintered polycrystalline ceramic body according to  claim 1 , wherein said ceramic body has an extinction coefficient of 2.0 cm −1  at a wavelength of 1100 nm and a Vickers Hardness of at least 200 kg/mm 2 . 
     
     
         9 . A sintered polycrystalline ceramic body according to  claim 8 , wherein said ceramic body has an extinction coefficient of 1.0 cm −1  at a wavelength of 1100 nm and a Vickers Hardness of at least 220 kg/mm 2 . 
     
     
         10 . A sintered polycrystalline ceramic body according to  claim 1 , wherein said ceramic body has an extinction coefficient of 1.0 cm −1  at a wavelength of 1100 nm, and a Vickers Hardness of at least 240 kg/mm 2 . 
     
     
         11 . A sintered polycrystalline ceramic body according to  claim 10 , wherein said ceramic body has an extinction coefficient of about 0.75 cm −1  at a wavelength of 1100 nm, and a Vickers Hardness of at least 250 kg/mm 2 . 
     
     
         12 . A sintered polycrystalline ceramic body according to  claim 1 , wherein said ceramic body has an average pore radius of less than 0.10 microns. 
     
     
         13 . A sintered polycrystalline ceramic body according to  claim 12 , wherein said ceramic body has an average pore radius of less than 0.05 microns. 
     
     
         14 . A sintered polycrystalline ceramic body according to  claim 1 , wherein said ceramic body has an average grain size of less than 8 μm. 
     
     
         15 . A sintered polycrystalline ceramic body according to  claim 14 , wherein said ceramic body has an average grain size of less than 5 μm. 
     
     
         16 . A process for preparing a polycrystalline chalcogenide ceramic material comprising:
 heating a chalcogenide powder to a temperature of 900-1000° C.,   subjecting the heated powder to uniaxial pressing at a pressure of 40 to 60 MPa and a temperature of 900-1000° C. for 0.16-6 hours, and   subjecting the resultant pressed chalcogenide material to hot isostatic pressing at a temperature of 880-1000° C. under an inert gas pressure of 180-250 MPa for 10 to 100 hours.   
     
     
         17 . A process according to  claim 16 , wherein said chalcogenide powder is ZnS powder. 
     
     
         18 . A process according to  claim 16 , wherein said chalcogenide powder is heated to the sintering temperature of 900-1000° C. at a rate of 1.5 to 12 K/min. 
     
     
         19 . A process according to  claim 18 , wherein the particle size of the chalcogenide powder is within the range of ≧400 nm to 10 μm. 
     
     
         20 . A process according to  claim 16 , wherein, before being sintered, the chalcogenide powder is subjected to a vacuum in order to remove trapped gases and/or contaminants. 
     
     
         21 . A process according to  claim 20 , wherein vacuum is within the range of 10 −4  to 10 −2  torr. 
     
     
         22 . A process according to  claim 16 , wherein, before being sintered, the chalcogenide powder is subjected to one or more temperature burnout steps to eliminate entrapped hydrocarbons that may be adsorbed to the surfaces of the chalcogenide particles. 
     
     
         23 . A process according to  claim 22 , wherein the one or more burnout steps are performed under vacuum at 10 −4  to 10 −2  torr and at a temperature of 50-300° C. 
     
     
         24 . An infrared window or dome for protecting an infrared sensor, comprising a polycrystalline chalcogenide ceramic according to  claim 1 . 
     
     
         25 . An infrared imaging system comprising at least one infrared sensor and an infrared window or dome for protecting said at least one infrared sensor from the external environment, wherein said infrared window or dome is according to  claim 24 . 
     
     
         26 . An infrared lens for focusing light within the 0.4-14 μm wavelength range, said infrared lens comprising a polycrystalline chalcogenide ceramic material according to  claim 1 . 
     
     
         27 . A sintered polycrystalline ceramic body consisting essentially of zinc sulfide sphalerite, wherein said body has an in-line transmittance per 6.3 mm thickness for all wavelengths in the wavelength range from about 0.4 microns to about 14 microns of not less than the transmittance shown by line B in  FIG. 3 .