GB2125023A

GB2125023A - Optical element especially of zinc sulphide or selenide having improved optical quality

Info

Publication number: GB2125023A
Application number: GB08323505A
Authority: GB
Inventors: Charles B Willingham; James Pappis
Original assignee: Raytheon Co
Current assignee: Raytheon Co
Priority date: 1980-12-29
Filing date: 1983-09-01
Publication date: 1984-02-29
Also published as: FR2610730B1; IT8149921A0; JPH03271107A; GB8323505D0; JPH03271122A; JPS57135723A; GB2090237B; IT1172159B; GB2125023B; CA1181557A; FR2610730A1; DE3150525A1; SE8107840L; FR2497361B1; FR2497361A1; GB2090237A; JPH0469090B2; JPH0451489B2

Abstract

Articles of polycrystalline zinc sulphide and zinc selenide achieve substantially improved optical quality by a treatment of heat and isostatic pressure by means of an inert working fluid, e.g. argon. The treated specimens are transparent and have substantial transmission in the infrared and visible range of the spectrum. Additional improvement in the transmission characteristics is achieved by wrapping the specimens in a foil of an inert material prior to treatment. Temperatures from 700 DEG C to 1050 DEG C and pressures from 34 MPa to 205 MPa may be used over periods from 3 hours upwards.

Description

1 GB2125023A 1

SPECIFICATION

Optical element especially of zinc sulphide or selenide having improved optical quality 5 Zinc sulphide and zinc selenide are used in applications such as missile domes requiring long 5 wavelength infrared transmission capability. Zinc sulphide is a major window material for air borne FLIR systems. These compounds are some of the most chemically and mechanically durable materials which are transparent in the infrared range of the electromagnetic spectrum to approximately 10 micrometers. Elements made thereof are available in useful sizes, and have potential for transmission in the visible range of the spectrum. A problem with these compounds 10 is that they do not have adequate transmission in the visible and nearinfrared range of the electromagnetic spectrum. Additional applications for these compounds could be developed if their transparency at visible and near-infrared wavelengths could be improved. More specifically, they could then be used in applications requiring multi-spectral capability. While their far 15 infrared wavelength limitation is an intrinsic property of the material and is related to multi- 15 phonon absorption, their short wavelength limitation is determined by several incompletely characterized extrinsic effects.

According to the present invention there is provided an article of zinc sulphide or zinc selenide compound having substantial transmission in the visible and infrared range of the electromag 20 netic spectrum. 20 Hot isostatic pressing (HIP) is the simultaneous application of heat and pressure by means of an inert working fluid. It has been discovered that HIP treatment of zinc sulphide and zinc selenide specimens produces improvement beyond the elimination of pores. It substantially improves the transparency at wavelengths shorter than two microns. Specimens of zinc sulphide 25 have also been found to have improved transmission characteristics through their effective 25 spectral band. The limitation in the transparency of zinc sulphide and zinc selenide is due to scattering and absorption mechanism. At wavelengths below two micrometers, it is believed that scatter, not absorption, is the principal mechanism that limits transmission. It is found that HIP treatment reduces scatter, not only by reducing or eliminating porosity, but also by reducing or 30 eliminating second phase inclusions, by allowing out-diffusion of impurities, and, in the case of 30 zinc sulphide, by promoting inversion of zinc sulphide non-cubic polymorphs to their cubic form.

Overall, absorption is reduced by HIP treatment by allowing diffusion of absorbing species that might be present. HIP treatment is also found to produce the stoichiometric ratio of the component atoms for both ZnS and ZnSe.

35 A further improvement can be achieved by controlling the chemical potential on the surface of 35 the article while heating the article and applying isostatic pressure. Preferably the chemical potential control is achieved by wrapping the article in a foil of an inert material while still allowing some vapour exchange.

The invention will be further described by way of example, with reference to the accompany 40 ing drawing, which shows the transmission spectra for a specimen of ZnS before and after 40 treatment.

Hot Isostatic pressing (HIP), the simultaneous application of heat and pressure by means of an inert working fluid, is used in metallurgical fabrication of powder metal compacts and castings to improve fracture strength and fatigue resistance. This invention uses similar HIP equipment to 45 treat specimens of zinc sulphide and zinc selenide. The specimens to be treated are placed in a 45 HIP furnace of conventional design. The furnace is evacuated, and then pressurized with an inert gas, such as argon. Heat is applied and the temperature and pressure allowed to stabilize.

The pressure and heat are maintained for a period of time sufficient to substantially eliminate a variety of impurities and defects from the specimens. The specimens treated have included 50 chemical vapour deposition (CVD) zinc sulphide as well as hotpressed zinc sulphide. Specimens 50 of CV1) zinc selenide were also treated. Currently available specimens of zinc sulphide and zinc selenide are coloured and translucent. For zinc sulphide, the colouration results from deviations from a strict stoichiometric ratio of the atoms in the material. The specimens are translucent rather than transparent because light is scattered by defects in the bulk of the material. The 55 exact nature of all the different types of defects is not known. The colour, types and relative 55 amounts of light scattering defects are determined by the technique used to prepare the material and by the processing conditions of the preparation. The scattering defects severely limit the transparency at wavelengths shorter than approximately two micrometers. Additionally, there are some absorption bands at different wavelengths which depend on the method of fabrication of 60 the specimen. The long wavelength limit of the transmission band is an intrinsic property of the 60 material and is due to a multiphonon absorption phenomenon. For wavelengths between approximately 2gm and the long wavelength limit, the transmission is limited principally by impurity-related absorption phenomena. The limitation in transparency in these materials at 1 1 visible and near infrared wavelengths is due to a combination of incompletely characterised 65 absorption and scattering phenomena, but scattering predominates. The short wavelength limit 65 2 GB 2 125 023A 2 of the transmission band is ultimately an intinsic material characteristic, but non-stiochiometry, impurities and other point defects can diminish transparency at wavelengths close to the short wavelength limit. Hot isostatic processing (HIP) treatment reduces these limitations not only by reducing or eliminating the porosity of the material but also by reducing or eliminating many of 5 the defects that contribute to scatter and absorption. This is due to a combination of factors 5 produced by HIP treatment through a simultaneous application of heat and pressure. The applied heat allows substantial out-diffusion of impurities normally present in the material. These impurities may consist of actual impurities formed by contaminating atoms of elements other than those forming the ideal compound, or may consist of defects in the crystal lattice, such as 10 vacancies and interstitial atoms. In any event, these impurities will diffuse out to the surface of 10 the specimen at a rate which is a function of temperature. Impurity atoms may be present within the sulphide or selenide crystals as separate distinct phases. The heat supplied also helps to reduce or eliminate these inclusions of second phase precipitates of the compound being treated. The applied pressure helps to eliminate such residual porosity as may be present in the 15 specimen prior to the treatment, and restrains the formation of new porosity which could 15 otherwise develop during the process. Additionally, the pressure is used to limit the volatilization of the compounds, since the compounds used have appreciable vapour pressure at useful treatment temperatures. In the case of zinc sulphide compounds, the optically isotropic cubic crystalline form has a higher density than the birefrigent hexagonal form. The pressure of the 20 HIP treatment is found to favor inversion of non-cubic polymorphs into cubic crystals. 20 Furthermore, the pressure decreases the equilibrium concentration of interstitial atoms and vacancies of the crystal lattice, and generally decreases the solubility of impurities.

Specimens of zinc sulphide have included both the chemical vapour deposition (CVD) and hot pressed types. Zinc selenide specimens have been of the CV1) type. It is found that hot-pressed 25 zinc selenide specimens have substantially inferior transmission characteristics as comapred to 25 CV1) zinc selenide specimens and thus are not generally available. However, this treatment should improve the characteristics of hot-pressed zinc selenide as well. The duration of the treatment depends on the initial quality of the specimen. The better the quality, i.e. transmission capability, of the specimen, the shorter the treatment time can be to achieve a predetermined level of transmission improvement. It has been found that hot pressed zinc sulphide material has 30 larger concentrations of impurities or defects that affect scattering as compared to zinc sulphide prepared by the CV1) process. The duration of the treatment is also determined by thickness of the starting sample. The greater the thickness the longer the treatment has to last to achieve a predetermined level of transmission improvement.

35 As discussed above, it has been discovered that subjecting specimens to HIP treatment 35 improves the optical characteristic of optical elements. This is due to a combination of factors.

The heat provided seems to favour an out-diffusion of impurities from the core of the specimen to the outside surface. The pressure limits the volatilization of the compound and also helps to eliminate and prevent the formation of porosity. In the case of zinc sulphide, the pressure is also 40 believed to force any non-cubic polymorphs present into their cubic form. This provides a 40 guideline in the selection of operating temperature and pressure. The temperature should be high enough to allow the out-diffusion of impurities from the body of the specimen. The pressure should be high enough to both prevent volatilization, and to substantially eliminate porosity in the specimen. The duration of the treatment is determined by both the thickness of 45 the specimen as well as its initial quality. The less transmitting samples normally require a 45 longer treatment time to achieve a predetermined level of optical transparency. However, an upper limit to the duration of the treatment might be determined by an excessive amount of grain growth that might take place during an unreasonably long treatment. It is also found that the CV1) type zinc sulphide achieves a substantially better amount of optical improvement than 50 the hot-pressed zinc sulphide specimens. This is probably due to the fact that the hot-pressing 50 process tends to produce larger size defects which do not out-diffuse as well with this process.

A six millimeter specimen of CV1) zinc sulphide was processed in three hours by the application of 990C and 5,000 psi (34 MPa) and resulted in visible improvement of the optical characteristics of the specimen. A pressure of 30,000 psi (205 mPa) and a temperature of 55 1 000C was used for a hot-pressed specimen of zinc sulphide and a CV1) zinc selenide 55 specimen, again resulting in substantial optical improvement. A 15 millimeter specimen of CV1) zinc sulphide, using a temperature of approximately 1 000C and pressure of 30,000 psi 205 MPa) as above, was successfully treated in approximately twenty-four hours. Temperatures in the range of 70WC to 1050C and pressures in the range of 5,000 to 30,000 psi (34 to 205 MPa) have been used to date on different types of specimens. The times range from three hours 60 for the smaller thickness mentioned to thirty-six hours for larger sample thicknesses. However, the invention is not limited to these operating parameters. Substantially different combinations of temperature, pressure and duration of the treatment will produce improvements of the optical quality of the treated specimens to some degree. The actual operating parameters are normally 65 dictated by the requirements of specific applications. Substantially lower temperatures and 65 GB 2 125 023A 3 pressures might be used to produce a predetermined amount of improvement.

Some specimens were first wrapped in a foil prior to the application of heat and pressure in the HIP apparatus. The wrappings are not vacuum tight but they serve to limit the vapur exchange between the specimens and the reaction chamber and also serve to control the 5 chemical potential of the volatile species in the specimens in order to enhance the treatment. 5 This control of the chemical potential of the volatile species on the surface of the specimens could be achieved by other means, such as use of dopants in the working gas or solids that will give off vapour species. Different types of material have been used. Graphite, mild steel, tantalum, copper and platinum foils have been used. The platinum wrapping foil results in the 10 best improvement of transmission characteristics for the samples. This is probably due to its 10 inert nature.

Referring now to the drawing, there is shown the transmission spectrum of a six millimeter thick CVD zinc sulphide specimen. Line 10 is for the original specimen prior to treatment and line 20 is for the same specimen after a H I P treatment for three hours at 1 000C and 30,000 15 psi (205 MPa). The HIP treatment has substantially improved short wavelength transmittance of15 the material and has also eliminated the infrared absorption band at six micrometers. Absorption bands in zinc sulphide depend on the manufacturing method and operating conditions but these are expected to be substantially improved by the HIP treatment. Visually, the untreated specimen is yellow-orange and hazy to the extent that it cannot be used for imaging at visible 20 wavelengths. The treated material is colourless because treatment has adjusted the stiochiometry 20 to the correct one-to-one zinc to sulphur ratio and is water clear because the treatment has very substantially reduced the concentration of light scattering defects. HIP treatment substantially improves the transmissivity at wavelengths greater than 2 micrometers. Other specimens of ZnS were similarly treated at 30,000 psi (205 MPa) at 990C for twenty-four hours. Specimens ranged in thickness from 0.4 to 1.5 centimeters. 25 The following table summarizes absorption coefficient measures for a 6 millimeter thick ZnS specimen. These apparent absorptance values were calculated by dividing the fraction of absorbed light by the thickness of the specimen and thus includes surface contribution to the absorption.

30 30 Apparent Absorption Coefficient of CVD ZnS (CM- 1) Wavelength After (Micrometer) Untreated Treatment 35 35 2.8 4.09 X 10-3 8.6 X 10-4 3.8 2.19 X 10-2 2.16 X 10-3 9.27 8.41 X 10-2 1.29 X 10-2 10.6 2.54 X 10-1 1.92 X 10-1 40 40 A six millimeter thick spectrum of CVD zinc selenide was also treated for three hours at 1 OOO'C and 30,000 psi (205 MPa). Visually, the untreated specimen is yellow in colour and hazy. After treatment, the colour is yellow-green and transparent. This colour is due to the correct stoichiometry for zinc selenide. The transparency in the visible range is substantially 45 improved. Using a spectrometer, the transmission of the specimen at 0. 5 micrometer was 45 measured before treatment and was found to be 5%, while after treatment transmission was found to be 50%. This substantial improvement is due mainly to the adjustment to the stoichiometric ratio that the treatment provides. A measure was also obtained of the scattering of light of the specimen before and after treatment. A He-Ne laser was used to provide a source 50 of light at.6238 micrometers. The fraction of light scattered at 90 to the incident laser beam 50 was measured in (Steradian) -I as follows:

Prior to treatment 2 X 10-3 After treatment 4.5 X 10-4 This indicates that the types of impurities in this material give rise substantially to scatter, the 55 phenomenon that is responsible for reduced transmission at low wavelength and the one that 55 HIP treatment is believed to reduce effectively.

Matter described herein is also described and claimed in patent application no. 81 38005

Claims

60 1. An article of zinc sulphide or zinc selenide compound having substantial transmission in 60 the visible and infrared range of the electromagnetic spectrum.
2. An article according to claim 1, further having a substantially stoichiometric ratio of the component atoms of the compound.

4 GB2125023A 4 Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd.-I 984.

Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained- I