Zinc sulfide optical elements

Abstract: To increase the optical transmission characteristics of zinc sulfide in the visible and infrared portions of electromagnetic spectrum, materials such as metals and, in particular, transition metals are diffused through the zinc sulfide material over a duration of time sufficient to cause the material to turn substantially water clear and substantially colorless. The diffusible material is applied as a surface layer (14) to a substrate of zinc sulfide (12), and the combination heated in an inert atmosphere for preferably at least 48 hours. The excess surface layer material is then removed, leaving the substrate with the surface layer material diffused throughout.

Description: This invention relates to infrared transparent zinc sulfide bodies having enhanced transparency in the visible, as well as, the infrared spectrums. Infra red optical imaging systems are known that include one or more externally mounted optical elements which isolate the remainder of the imaging system from an external environment. It is also known in the art that certain applications of these optical elements are on dual mode systems which require transparency in both the infrared as well as the visible portions of the electromagnetic spectrum. When the portions of the electromagnetic spectrum include long wavelength infrared energy (LWIR) particularly in the 8-12 micron band, the number of materials which also provide suitable transparency in the visible portion of the spectrum are limited. Materials which have theoretical transparency in the infrared 8-12µ and the visible are generally alkaline-earth halides, wide bandgap semiconductor materials and diamond. One such material, in particular, is zinc sulfide. As deposited or grown fully dense, polycrystalline ZnS is orange in color having very little transmittance in the visible due to scattering effects. Chemically vapor deposited, orange, optical grade zinc sulfide, which is transparent over the range of about 0.5µ to 12µ, becomes, when subjected to a hot isostatic pressing (HIP) treatment, transparent in the visible portion of the electromagnetic spectrum, as well as remaining transparent in the infrared portions of the electromagnetic spectrum. HIP is a treatment at an elevated pressure and an elevated temperature. This treatment, however, has one drawback. The HIPping of the zinc sulfide material causes grain growth in the material. An increase in the grain size of the zinc sulfide provides a concomitant reduction in the mechanical strength properties of the material. That is, although HIPping increases the optical performance of the material, HIPping also reduces the strength of the material. Although such treated zinc sulfide material is suitable for many applications, it would nevertheless be desirable to provide zinc sulfide material which is transparent in both the visible and infrared spectrums while still retaining relatively high mechanical strength properties. It is therefore an object of the invention to provide zinc sulfide bodies having improved optical transmittance characteristics and mechanical characteristics. GB-A-2 156 383 discloses a chemically vapour deposited ZnS-material doped with B, C, or Si. According to one aspect of the present invention, there is provided a method of producing a zinc sulfide body that is transparent at infra red and visible wavelengths, the method comprising the steps of providing a layer of a transition metal or a Group III metal over a surface of a body of chemically vapor deposited zinc sulfide; heating the zinc sulfide body and the layer of metal for a period of time sufficient to cause portions of the metal to diffuse throughout the zinc sulfide body; and removing any undiffused portions of the metal. By diffusing transition metals for a long period of time into the zinc sulfide, improvement in optical characteristics is provided. Since the treatment occurs at an elevated temperature without an elevated isostatic pressure, there is negligible grain growth in the LWIR material. Accordingly, improvement in optical properties is provided without a corresponding weakening in the mechanical properties of the zinc sulfide. Preferably, the metal is selected from the group consisting of cobalt, iron, gallium, manganese, nickel, and vanadium. The step of heating the zinc sulfide body may take a period of time greater than about 48 hours to cause the selected transition metal to diffuse into the zinc sulfide body. Further, by diffusing the selected species for sufficient period of time to provide sufficient penetration throughout the body, the material will become substantially water clear and nearly colorless. According to another aspect of the invention there is provided a body of chemically vapor deposited zinc sulfide characterized by a transition metal or a Group III metal disposed as a dopant in the zinc sulfide, the dopant being such that the body is substantially transparent to infra red and visible wavelengths. Brief Description of the Drawings The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following detailed description of the drawings, in which: FIGs. 1-3 are a series of cross-sectional views showing steps in the fabrication of an optical element in accordance with the present invention; FIG. 4 is a plot of percent transmissivity vs. wavelength for optical elements in accordance with the present invention; and FIG. 5 is a plot of percent transmissivity vs. wavelength in the infrared range of 2.5µ to 14µ for the optical elements above mentioned. Description of the Preferred Embodiments Referring now to FIG. 1, a portion of an optical element (not numbered) is shown to include a substrate 12 here comprised of an optical material which theoretically has transparency in the infrared and visible portions of the electromagnetic spectrum, and is comprised of zinc sulfide. The substrate 12 has shown disposed over a surface thereof a layer 14 comprised of a diffusible material which is a transition metal selected from the group consisting of cobalt (Co), iron (Fe), manganese (Mn), nickel (Ni), chromium (Cr), and vanadium (V) or a Group III metal such as gallium (Ga). The transition metal layer 14 is diffused into the substrate 12, as will be described in conjunction with FIG. 2. Referring now to FIG. 2, portions of layer 14 (FIG. 1) are diffused through substrate 12 (FIG. 1) providing residual transition metal layer 14′ and doped substrate 12′. The diffusion technique, here used, is to heat the substrate and film to an elevated temperature in an inert atmosphere at a typically ambient pressure of argon. A typical heat treatment would be to heat the substrate and layer to a temperature in the range of 600°C to 800°C in flowing argon at the rate of one liter per minute for at least 24 hours preferably 48 hours and up to 120 hours or more depending upon the initial properties of the material, the degree of improvement desired in optical transparency, as well as the thickness of the sample. After the heat treatment, the undiffused or remaining portion 14′ of layer 14 (FIGs. 1 and 2) is removed and the surface previously having said layer is polished leaving behind substrate 12′ having diffused therein as a dopant, one of the transitional metals. Samples of standard grade zinc sulfide obtained from Raytheon Company, Waltham, Massachusetts were provided with a 3000Å thick layer of cobalt. The cobalt was diffused into the material over three different time intervals at a temperature of 700°C. The material had a thickness of approximately 0.1 inches (0.2 cm). Percent transmission characteristics for the material are shown in FIGs. 4-5 with FIG. 4 showing the % transmission over the visible portion of the electromagnetic spectrum and over the near-infrared region for an 8 hour treatment (curve 24a) a 24 hour treatment (curve 24b), and a 48 hour treatment (curve 24c). These plots are also shown for comparison with plots of Multispectral® Zinc Sulfide curve 25a and Raytran® Standard Grade Zinc Sulfide curve 25b both from Raytheon Company, Waltham, Massachusetts. Note at 48 hours the % transmission in the visible becomes higher than standard grade. It is to be noted that the transmittance of the material increases substantially at longer durations of the heat treatment. Thus, longer treatments (i.e. 48 to 120 hours or more) should provide improved characteristics comparable to Multispectral® ZnS but without applicable grain growth. The % transmission of these samples over the 2.5µ to 14µ range are shown in FIG. 5. Again, the longer the treatment the higher the % transmission. Curve 26c was measured after a 48 hour treatment, curve 26b after a 24 hour treatment, and curve 26a after an 8 hour treatment. In particular, it should also be noted that the absorption band characteristic at 6 microns which is characteristic of standard grade chemically vapor deposited zinc sulfide is eliminated using this technique. It should also be noted that for thicker samples of zinc sulfide material two coatings can be provided over opposite surfaces thereof and the diffusion treatment could occur from each one of said surfaces towards the center of the optical element. That is, since the time to diffuse a given depth of a material is related to the square of the thickness to be diffused, providing the material over a pair of surfaces of the element will significantly reduce the time required to diffuse such materials, particularly for thicker optical elements such as generally encountered in domes and optical windows. Such optical elements typically have thicknesses of 0.2 inches up to 0.5 inches and greater. With the treatment described above, chemically vapor deposited ZnS, which normally has a strong yellow-orange tint, is changed in visible appearance to near water clear and colorless having a very slight bluish tinge. The typical grain size of Multispectral® Zinc Sulfide is approximately 350 microns whereas the grain size of the material as described above using cobalt as a dopant is 2-6 microns which is substantially the same as standard grade Raytran® Zinc Sulfide which is 2-6 microns. Having described preferred embodiments of the invention, it will now become apparent to one of skill in the art that other embodiments incorporating their concepts may be used. It is felt, therefore, that these embodiments should not be limited to disclosed embodiments, but rather should be limited only by the scope of the appended claims.

Claims: A method of producing a zinc sulfide body that is transparent at infra red and visible wavelengths, the method comprising the steps of: providing a layer of a transition metal or a Group III metal over a surface of a body of chemically vapor deposited zinc sulfide; heating the zinc sulfide body and the layer of metal for a period of time sufficient to cause portions of the metal to diffuse through the zinc sulfide body; and removing any undiffused portions of the metal. A method according to claim 1, characterised in that the said period of time is greater than 48 hours for a body that is 2.54 mm (0.1 in.) thick. A method according to claim 2, characterised in that the metal is selected from the group consisting of Co, Fe, Ga, Mn, Ni, Cr, and V. A method according to claim 1 or 2, characterised in that said diffusing occurs at a temperature of about 700°C. A method according to claim 1, characterised in that the metal is cobalt. A body of chemically vapor deposited zinc sulfide characterized by a transition metal or a Group III metal disposed as a dopant in the zinc sulfide, the doping being such that the body is substantially transparent to infrared and visible wavelengths. A body according to claim 6, characterised in that the metal is selected from the group consisting Co, Fe, Ga, Mn, Ni, Cr, and V. A body according to claim 7, characterised in that the metal is selected from the group consisting of Co, Ga, and Fe. A body according to claim 7, characterized in that the metal is Co. A body according to claim 6, characterised in that the dopant is cobalt, and in that the body has a grain size substantially less than 350 microns, is substantially water clear and nearly colorless with a very slight bluish tinge. A body according to claim 10, characterised in that the grain size is in the range of 2 to 6 microns. A body according to claim 11, characterised in that the body has a percent transmission of at least about 30% for wavelengths in the range of 600 nanometers to 1200 nanometers for a sample of the body having a thickness of 2.54 cm.

Inventors: TUSTISON RANDAL W, WAHL JOSEPH MATTHEW, WONG THOMAS Y, TUSTISON, RANDAL W., WAHL, JOSEPH MATTHEW, WONG, THOMAS Y.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1991
Main classification: G02B1
IPC codes: G02B13, G02B1, C01G1, C04B35, C01G45, C01G51, C01G15, C01G53, C01G49, C01G37