JPH03271107A - Article of zinc selenide having improved optical characteristics - Google Patents

Abstract

PURPOSE: To provide articles of yellowish green and transparent zinc selenide having transmittability in the visible and infrared region of the electromagnetic spectrum by subjecting a specimen consisting of the zinc selenide to a pressurization treatment using an inert working fluid.

CONSTITUTION: The specimen of the zinc selenide is put into a heat isothermal compression furnace and after the inside of this furnace is evacuated, the specimen is pressurized by inert gas, such as argon, to heat the specimen and to annihilate the impurities, defects, etc., included in the specimen. As a result, the articles of the yellowish green and transparent zinc selenide having the transmittability in the visible and infrared region of the electromagnetic spectrum are obtd. The resulted articles of the zinc selenide having the improved optical property are adequately used for applications, etc., where a large-quantity spectrum performance is required. The time for subjecting the specimen to the isothermal pressurization treatment is required to be made longer as the concn. of the impurities or defect contained therein is higher or the thickness of the starting specimen is larger.

COPYRIGHT: (C)1991,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION [0001] The present invention relates to zinc selenide articles with improved optical properties. More particularly, the present invention relates to articles of zinc selenide that are substantially transparent in the visible and infrared regions of the electromagnetic spectrum and are yellow-green and transparent.[0002] Zinc Sulfide and Selenium It is used in applications that require the ability to transmit long-wavelength infrared rays, such as zinc oxide missile domes. Zinc sulfide is the primary material for windows in airborne FLIR systems. These compounds are transparent in the infrared region of the electromagnetic spectrum below about 10 micrometers and are among the most chemically and mechanically durable materials available in the dimensions used. , which has persistent transmission power in the visible region of the spectrum. One problem with these compounds is that they do not have adequate transmission power in the visible and near infrared regions of the electromagnetic spectrum. The uses of these compounds could be further developed if their transparency to wavelengths in the visible and near-infrared ranges could be improved. More specifically, it could then be used in applications requiring high spectral performance. The fact that there is a limit to the wavelength in the far infrared region is an essential property of this material, and multiple gold absorption (mu 1 t
i-phononabsorption), whereas the limit in the short wavelength region is determined by some non-essential effects of an unclear nature.

[0003] Hot isostatic compression (HIP) is the simultaneous application of heat and pressure using an inert working fluid. It has been discovered that HIP treatment of zinc sulfide and zinc selenide samples results in improvements beyond the disappearance of pores. This improves transparency at wavelengths shorter than 2 microns. The zinc sulfide sample was also found to have improved transmission properties across its effective spectral band. The limited transparency of zinc sulfide and zinc selenide is due to scattering and absorption mechanisms. At wavelengths below 2 micrometers, the primary mechanism limiting transmission is scattering, not absorption. The HIP process not only reduces the porosity or eliminates the pores, but also reduces or eliminates the inclusion of the second phase because the impurities act to diffuse out. , and in the case of zinc sulfide, non-cubic homomorphism of zinc sulfide (non-cubic polymorphism)
It has been found that scattering is reduced by promoting the conversion of -cubic polymorphs) to their cubic system. Overall, absorption is HI.
Exists due to P law. Possible absorption is reduced by diffusing substances. It has also been found that the atomic ratio of both ZnS and Zn5e can be brought to stoichiometric values by the HIP method.

[0004] The invention further comprises adjusting the chemical potential of the surface of the article;
A method is provided for processing ZnS and Zn5e products by heating and applying isobaric pressure to the products. Adjustment of the chemical potential is preferably carried out by wrapping the product in a foil of inert material and still allowing some exchange of vapor.

[0005] Other objects and advantages of the present invention will become apparent from the following description. Please refer to the attached figure showing the transmission spectra of a sample of ZnS before and after treatment.

[0006] Hot isostatic pressing (HIP), a method of simultaneously heating and pressurizing with an inert working fluid, is used in the metallurgical production of powder metal compacts and moldings; The method improves their fracture strength and fatigue resistance. In the present invention, similar HIP
The equipment is used to process samples of zinc sulfide and zinc selenide. The sample to be processed is placed in a HIP furnace of conventional design.

After the furnace is evacuated, it is pressurized with an inert gas such as argon. Apply heat to stabilize temperature and pressure.

The pressure and temperature are maintained for a sufficient period of time to substantially eliminate various impurities and defects within the sample. The samples treated included chemical vapor deposition (CVD) zinc sulfide as well as heat-pressed zinc sulfide. A CVD zinc selenide sample was similarly treated. Commonly available samples of zinc sulfide and zinc selenide are colored and translucent. In the case of zinc sulfide, the coloration occurs because the ratio of atoms in the material deviates from the strict stoichiometric ratio. Additionally, defects within the volume of the material cause light to scatter, making the sample semitransparent rather than transparent. The exact nature of all of these various types of defects is not known. The types and relative proportions of color and light scattering defects are determined by the technique used to prepare the material and the processing conditions of the preparation. Due to the presence of scattering defects, its transparency is severely limited at wavelengths below 2 microns. Furthermore, there are several absorption bands at different wavelengths depending on the method of sample preparation. The limit on the long wavelength side of the transmission band is an essential property of the material and depends on the multiple gold absorption phenomenon. For wavelengths between about 2 μm and the long wavelength limit, the transmission power is limited primarily by absorption phenomena associated with impurities. The transparency limitations of these materials in the visible and near-infrared regions are due to a combination of imperfect characteristic absorption and scattering phenomena, but the limitations due to scattering are significantly greater. Although the short-wavelength limit of the transmission band is a completely inherent material property, deviations from stoichiometry, impurities, and other point defects can cause loss of transparency at wavelengths near the short-wavelength limit. be. Hot isostatic pressing (HIP) can reduce these limitations not only by reducing the porosity of the material or eliminating pores, but also by reducing or eliminating many of the defects that contribute to scattering or absorption. . This diffuses into the thighs due to a combination of factors caused by simultaneous heating and pressurization using the HIP method.

These impurities may be constituted by actually present impurities formed by contaminating atoms that are elements other than those forming the ideal compound, or by defects in the crystal lattice such as the absence or interruption of atoms. It may consist of defects inside. In any case, these impurities will diffuse to the surface of the sample at a constant rate that is a function of temperature. Impurity atoms may be present within the sulfide or selenide as separate, distinct phases. The applied heat also serves to reduce or eliminate the inclusion of precipitates in the second phase of the compound being treated. The applied pressure helps to annihilate any such residual pores that may have been present in the sample prior to processing, and eliminates any new pores that may have arisen during processing if this treatment had not been performed. suppresses the generation of In addition, since the compounds used have fairly high vapor pressures at the processing temperatures used, the pressurization serves to limit the evaporation of the compounds. In the case of zinc sulfide, the density is higher than that of the birefringent hexagonal crystal form of the optically isotropic cubic crystal form. It has been found that the pressure of the HIP process facilitates the conversion of non-cubic allomorphs into cubic crystals. Furthermore, the equilibrium concentration of interstitial atoms and crystal lattice defects is reduced by pressurization. Also, the solubility of impurities generally decreases.

[0007] Zinc sulfide samples included chemical vapor deposition (CVD) and hot compression types. The zinc selenide sample was of the CVD type. Zinc selenide produced by the heat compression method is not generally used because its permeation properties are significantly inferior to zinc selenide produced by the CVD method. However, according to the present treatment method, the properties of zinc selenide obtained by the heat compression method are improved to the same extent. The length of processing time depends on the initial quality of the sample. The higher the quality or permeability of the sample, the shorter the time required to improve the permeability to a predetermined level. Zinc sulfide material prepared by heat compression method is CVD
It has been found that compared to zinc sulfide prepared by the method, there is a higher concentration of impurities or defects that affect scattering.

Processing time also depends on the thickness of the starting sample. In order to improve the permeation performance to a certain level, the treatment time for thicker seeds must be increased.

[0008] As mentioned above, it has been discovered that the optical properties of an optical element can be improved by treating a sample with a HIP method. This is due to the combined effect of double blades. The applied heat appears to favor the diffusion of impurities from the core of the sample to the outer surface. The pressure serves to constrain evaporation of the compound and to eliminate pores and prevent their formation. In the case of zinc sulfide, it is believed that pressure converts all existing non-cubic polymorphic crystals into cubic crystals. This fact guides the selection of operating temperatures and pressures. The temperature must be high enough to allow impurities from the sample to diffuse out. The pressure must be sufficient to prevent evaporation and nearly eliminate pores in the sample. The length of processing time is determined by both the thickness of the sample and its optical quality. Samples with low transmittance typically require longer processing times to obtain significant optical clarity. However, the upper limit on processing time is limited because excessive grain growth occurs if processing times are unreasonably long. It has also been found that CVD zinc sulfide undergoes optical improvement to a significantly higher degree than hot and pressed zinc sulfide. This is probably due to the fact that in the case of the heating and pressing method, defects of larger size are produced which are better diffused and do not come out than the CVD method.

[0009] A 6 mm CVD type zinc sulfide sample manufactured by the CVD method was processed using a normal HIP device (7).
According to HIP treatment method, 990℃, 5000psi (
350kg/cm2. After treatment for 3 hours at 1 temperature and pressure, the optical properties of the sample were visibly improved. 30000 psi [2100 kg
/cm2. Significant improvements in optical properties were obtained at I (7) pressures and temperatures of 1000°C. For a 15 mm CVD zinc sulfide sample, a temperature of approximately 1000°C and 30000 psi [2100 kg/cm2] (7
) pressure for about 24 hours gave good results. 7 to determine processing times for different types of samples.
Temperature range from 00℃ to 1050℃ and 5ooo/J. 3hr for thick items, 36hr for thick items.
Processing times in the range of r were appropriate. However, the invention is not limited by the operating parameters disclosed herein. Significantly different combinations of temperature, pressure and treatment time will still improve the optical properties of the treated samples to some extent. Practical working parameters are usually determined by specific application requirements. The temperatures and pressures used to achieve a given improvement will be significantly lower.

[0010] Prior to raising the temperature and pressure in the HIP apparatus, some of the samples were wrapped in foil of a second material. Although this packaging is not a vacuum-blocking packaging, it limits the exchange of vapor between the sample and the reaction chamber, and also modulates the chemical potential of volatiles in the sample to facilitate processing.
Helps increase sample permeability. This adjustment of the chemical potential of volatile substances on the surface of the sample can also be carried out by other means, such as by using dopants or solids that release vapors into the gas used in the operation. Dew. Various types of materials have been used in the past. Graphite, mild steel, tantalum, copper, and platinum foil have been used. Platinum packaging foil was able to improve the transmission performance of the samples best. This is probably due to its inert nature.

[0011] The accompanying drawings show the transmission spectra of a 6 mm thick CVD zinc sulfide sample. Line 10 is the spectrum of the original sample before treatment, and line 20 is the same sample subjected to HIP treatment at 1000°C and 30000 ps.
This is the spectrum after processing for 3 hours at 2100 kg/cm2.l. The HIP treatment significantly improved the material's transmittance to short wavelengths, and also eliminated the infrared absorption band at 6 micrometers.

Although the absorption bands in zinc sulfide vary depending on the sample preparation method and working conditions, these appear to be significantly improved by HIP treatment. The untreated sample was macroscopically orange-yellow and too sharp to be used for imaging at visible wavelengths. The treated sample is colorless because the stoichiometric ratio of zinc and sulfur has been adjusted to 1:1, and the concentration of light-scattering defects is extremely reduced by the treatment, so it is difficult to remove water. It was so transparent. The HIP process significantly improved the transmission power at wavelengths greater than 2 micrometers. Other ZnS samples were similarly heated at 990°C and 30,000 psi.
C2100kg/cm2) for 24 hours. The sample thickness was 0.4 to 1.5 crn.

[0012] The following table summarizes the absorption coefficient measurements of a 6 mm thick ZnS sample similar to one of the attached figures. These apparent absorption power values are calculated by dividing the proportion of light absorbed by the thickness of the sample, and thus indicate the contribution of the surface to the absorption power.

[0013] Apparent absorption coefficient (cm”) wave of CVD zinc sulfide
Long (Micrometer - 1st place 1st place 1st place - 1st place - current role 11th 2.8 4.09xlO-38,6
xlO'3.8 2.19xlO' 2.1
6xlO-39,272,41xlO' 1.29
xlO"10.6 2.54xlO' 1.
Similarly, CVD zinc selenide with a thickness of 6 mm was heated at 1000°C and 30000 psi.
[2100kg/cyn2. 1 for 3 hours to obtain a spectrum. The untreated sample was visually yellow and indistinct. After treatment, the color was yellow-green and transparent. This color is due to the stoichiometrically correct proportion of zinc selenide. The transmission power in the visible region was significantly improved. The transmittance of a 0.5 micrometer thick sample before treatment was measured using a spectrometer and was 5%, whereas the transmittance of the sample after treatment was 50%. This significant improvement is due to the adjustment of the stoichiometry by this treatment. Measurements of light scattering of the samples before and after treatment have also been obtained. 0.6
He-N to provide a 328 micrometer light source
e-laser was used. Input of scattered light at 90°C [0014] Before treatment 2 x 10-3 After treatment 4.5 x lO' This result indicates that the type of impurity in this material is of a nature that causes significant scattering. This phenomenon indicates that the transmittance in the low wavelength region is reduced, and that this reduction is effectively achieved by the HIP method.

[0015] This completes the description of the present invention. Many modifications may be made by those skilled in the art without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as defined by the appended claims.

[Brief explanation of drawings]

FIG. 1 shows the transmission spectra of a 6 mm thick CVD zinc sulfide sample before and after treatment.

[Explanation of symbols]

10: Transmission spectrum of the initial sample before treatment, 20: Transmission spectrum of the sample after treatment, horizontal axis: wavelength (micrometer), vertical axis: transmittance (%).

【Document name】

drawing

Claims (3)

[Claims] 1. An article of zinc selenide characterized by being transparent and yellow-green in color with substantial transparency in the visible and infrared regions of the electromagnetic spectrum. 2. The article of claim 1, wherein the article has optical transmission properties substantially equal to the inherent optical properties of the zinc selenide over substantially the entire optical transmission range of the article. goods. 3. The article according to claim 1, which has substantially no absorption bands in the visible and infrared regions.