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.

Claims

1. A sintered polycrystalline ceramic body comprising ZnS in a polymorphic form having a cubic structure and having the transmittance of at least 50% in a wavelength range of 0.7-12 μm for a 6 mm thickness, and a Vickers hardness of ≥180 kg/mm.sup.2.

2. A sintered polycrystalline ceramic body of claim 1, wherein said chalcogenide material is zinc sulfide sphalerite.

3. A sintered polycrystalline ceramic body of claim 1, wherein said ceramic body has an average pore radius of less than 0.10 microns.

4. A sintered polycrystalline ceramic body of claim 1, wherein said ceramic body has an average grain size of less than 8 μm.

5. An infrared window or dome for protecting an infrared sensor, comprising a polycrystalline chalcogenide ceramic body of claim 1.

6. An infrared imaging system comprising at least one infrared sensor and the infrared window or dome of claim 5.

7. An infrared lens for focusing light within the 0.4-14 μm wavelength range, wherein said infrared lens comprises a polycrystalline chalcogenide ceramic body of claim 1.

8. The sintered polycrystalline ceramic body of claim 1, wherein the sintered polycrystalline ceramic body is prepared by: heating a chalcogenide powder; and subjecting the heated powder to uniaxial pressing.

9. The sintered polycrystalline ceramic body of claim 8, wherein less than 50 wt % of the chalcogenide powder has a diameter of 5 μm or less.

10. The sintered polycrystalline ceramic body of claim 9, further comprising the step of subjecting the pressed heated powder to hot isostatic pressing.

11. The sintered polycrystalline ceramic body of claim 8, wherein the chalcogenide powder is heated to about 900-1,000° C. and the heated powder is pressed at a pressure of about 40-60 MPa and a temperature of about 900-1,000° C. for about 0.16-6 hours.

12. The sintered polycrystalline ceramic body of claim 10, wherein the hot isostatic pressing is at a temperature of about 880-1,000° C. under an inert gas pressure of about 180-250 MPa for about 6-100 hours.

13. The sintered polycrystalline ceramic body of claim 1, wherein said body has a transmittance of at least 60% in a wavelength range of 0.7-12 μm for a 6 mm thickness.

14. The sintered polycrystalline ceramic body of claim 1, wherein said body has a transmittance of at least 70% in a wavelength range of 0.7-12 μm for a 6 mm thickness.

15. The sintered polycrystalline ceramic body of claim 1, wherein said ceramic body has an extinction coefficient of ≤0.5 cm.sup.−1 at a wavelength of 1100 nm.

16. The sintered polycrystalline ceramic body of claim 1, wherein said ceramic body has a Vickers hardness of ≥230 kg/mm.sup.2.

17. A sintered polycrystalline ceramic body of claim 1, wherein said ceramic body has an extinction coefficient of ≤0.5 cm.sup.−1 at a wavelength of 1100 nm and a Vickers hardness of ≥230 kg/mm.sup.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Various other features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views, and wherein:

(2) FIG. 1 illustrates hot press die mold for use in the manufacture of polycrystalline ZnS ceramic compositions in accordance with the invention;

(3) FIG. 2 illustrates the fine microstructure achieved by the invention in comparison with the microstructures achieved by standard hot pressing processes;

(4) FIG. 3 is a table showing the Vickers Hardness of examples in accordance with the invention as a function of extinction coefficient at 1100 nm; and

(5) FIG. 4 graphically illustrates a comparison of the transmission spectrum for Example 7 in accordance with the invention and the transmission spectrum for a commercially available FLIR (Forward Looking Infrared) material.

(6) FIG. 1 illustrates a hot press mold fabricated of, for example, fine grained isopressed graphite. The mold comprises mold member 1 fashioned as a solid cylinder, mold members 2 and 4 fashioned as hollow cylinders, mold member 3 fashioned as a hollow cylinder with a slit cut down the long axis, and mold member 5 fashioned as a cylinder. Additionally, a first intermediate disk 8 and a second intermediate disk 11 both fabricated from fine grained isopressed graphite are provided.

(7) Powdered ZnS having an average particle size of 5 μm, in the form of a green compact 10, is positioned between the first intermediate disk 8 and the second intermediate disk 11. The surfaces of the intermediate disks 8 and 11 facing toward the green compact 10 and the inside wall of the hollow cylinder 3 form the surfaces of the mold cavity. These surfaces are covered with a sheet of graphite foil 6, 7, 9 having a thickness of about 0.010 inches.

(8) The mold is placed completely into a hot press assembly. The assembly is initially evacuated to a pressure 50×10.sup.−3 torr, and then subjected to a burnout cycle to remove adsorbed gasses from the ceramic powder. The powdered sample is heated to 50, 150, 200° C. and held at each temperature until a desired vacuum level is reached (for example, 200° C. and 10×10.sup.−3 torr, respectively). The assembly is then heated, without applying pressure, to a temperature between 900° C. and 1000° C., preferably around 950° C. After reaching the desired temperature, at a rate of 7 tons per minute pressure is applied to mold member 1 until a pressure of between 40 and 60 MPa, preferably around 55 MPa, is obtained. The pressure is then held at this level for a time of, for example, 0.16 to 6 hours, e.g., 2-4 hours. The pressed article can then be subsequently removed from the mold without damage by removing mold member 5 and pressing the contents out into a hollow cavity with a depth equal the sum of the thicknesses of intermediate disks 8 and 11.

(9) The part is the placed in a graphite crucible and hot isostatically pressed under argon at a pressure of 180-230 MPa at a temperature of 900° C.-1000° C., e.g., 950° C., for a period between 6 and 100 hours, e.g. 12 hours.

(10) FIG. 2A shows the microstructure of a ZnS ceramic body produced by a standard hot pressing process using ZnS produced by chemical vapor deposition as the starting material. As shown, the microstructure is very coarse (e.g., average grain size of 25 μm) and the material had a Vickers Hardness of 150 kg/mm.sup.2. FIG. 2B shows the microstructure of a ZnS ceramic body produced in accordance with the inventive process. The microstructure is very fine (e.g., average grain size of 3 μm) and the material had a Vickers Hardness of 200 kg/mm.sup.2.

EXAMPLES

(11) Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

(12) Tables 1 and 2 describe the preparation examples of the ZnS compositions in accordance with the invention, and the properties of the resultant materials.

(13) TABLE-US-00001 TABLE 1 Preparation Examples of ZnS Ceramic Compositions According to the Invention Heat Sinter- Uniaxial Exam- Burn- Rate ing Uniaxial Pressing HIP HIP ple out (° C./ Temp. Pressing Hold Time Temp. Time No. cycle min) (° C.) (kpsi) (hrs) (° C.) (hrs) 1 0 6 950 6.5 4 950 6 2 0 6 950 6.5 4 950 6 3 0 2 950 8 0.16 950 12 4 0 10 950 8 2 950 12 5 0 10 950 8 6 950 12 6 0 10 950 5 6 950 12 7 1 2 900 8 2 950 12 8 1 2 950 8 4 950 12
Examples 1-6 were using a die with a radius of 25 cm, whereas Examples 7-8 were using a die with a radius of 127 cm.

(14) TABLE-US-00002 TABLE 2 Properties of ZnS Ceramic Compositions According to the Invention Extinction Coefficient Grain at 1100 nm Knoop Hardness Size Example (cm.sup.−1) (kg/mm.sup.2) (μm) 1 2.1 190 2.09 2 1.8 196 3.73 3 2.0 212 4.32 4 1.8 230 3.15 5 2.4 227 3.19 6 2.3 216 4.18 7 0.2 257 <3 8 0.5 250 <3 ZnS 0.05-0.2 150-165 20-100 MultiSpectral ® CLEARTRAN ® 0.05-0.2 147 20-100 ZnS FLIR  3.60 210-240 2-8  material.sup.1
ZnS MultiSpectral® is a ZnS material from II-VI Infrared which is made by chemical vapor deposition and is modified by a hot isostatic press (HIP) process. The material exhibits transmission in the 0.4 to 12 micron range.
CLEARTRAN® is a ZnS material from DOW which is made by chemical vapor deposition and is modified by a hot isostatic process. The material exhibits transmission in the 0.35-14 μm range.
.sup.1ZnS from II-VI Infrared produced by chemical vapor deposition (CVD). The material is used in the 8 to 12 micron region.

(15) FIG. 3 is a table showing the Vickers Hardness as a function of extinction coefficient at 1100 nm for Examples 1-7 in accordance with the invention, as well as for a commercial available ZnS ceramic used for FLIR applications and commercial available ZnS ceramic used for multispectral applications.

(16) FIG. 4 illustrates the in-line infrared transmittance for Example 7 in the wavelength range from 1 to 14 microns (Line B) measured using a Perkin-Elmer Lambda 900 spectrophotometer. The sample used had a thickness of 6.3 mm Line A in FIG. 4 shows the in-line infrared transmittance for a typical commercially available FLIR grade ZnS material. Here, the in-line transmittance represents the ratio of intensity of the transmitted portion of incident light to the intensity of the incident light.

(17) The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

(18) From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

(19) The entire disclosure{s} of all applications, patents and publications, cited herein, are incorporated by reference herein.