Abstract:
Crystals formed of a solid-solution of NiSiF 6  6H 2  O provide very good materials for filtering ultraviolet light and will not deteriorate in temperatures as high as 115° C. They are particularly useful in sensing devices which seek to identify the presence of ultraviolet light in the UV missile warning band.

Description:
FIELD OF INVENTION 
     The invention relates to crystals having thermal and optical properties suitable for use in ultraviolet light filters and sensors. 
     BACKGROUND OF THE INVENTION 
     There are a variety of devices which use ultraviolet (UV) light filters that allow selected wavelengths of light to pass therethrough. Such filters are used in missile approach warning systems which locate and track: sources of ultra-violet energy, enabling the system to distinguish the plume of an incoming missile from other UV sources that pose no threat. The benefit of this system is the ability to estimate missile range and closing velocity to time the ejection of flare-decoys to maximize their effectiveness against infrared-guided missiles. This system can be also used to aim a beam of infrared energy at an IR guided missile to confuse its guidance system. 
     The success and efficiency of the system for helicopters or transport-type aircrafts depends on the UV sensors. Commercially available nickel sulfate hexahydrate. crystals are widely used for these sensors. The biggest: problem for these sensors arises due to low thermal stability of nickel sulfate crystals. The crystals start deteriorating as the temperature starts rising above 60° C. This rise can be due to atmosphere as well as heat generated by working systems in the aircraft. Aircraft parked in tropical and desert areas can experience very high temperatures. In such heat the stability of these systems is very questionable. There is an urgent need for an ultraviolet filter material with higher temperature stability, good crystal growth suitability, and desired filter transmittance and bandwidths. 
     The filter should have high transmittance at the desired wavelength. For missile warning systems, the crystal should be transmissive in the ultraviolet spectral region and have strong absorption at longer wavelengths. Furthermore, the crystal should be able to survive prolonged exposures to temperatures above 85° C. and preferably not be adversely affected by temperatures in the range 100° C. to 115° C. 
     In 1964, H. L. Pryce et al. published their article &#34;Low-Temperature Optical Absorption of Nickel, Flouosilicate Crystals&#34;, Phil. Mag. 10, 477 (1964). The article presents data for the light spectrum which will be transmitted through a NiSiF 6  6H 2  O crystal, but contains no information about temperature stability. That data provides no indication that a crystal made from NiSiF 6  6H 2  O will transmit ultraviolet light in the wavelengths required for a missile warning system. Consequently, the art has not: considered making a filter having a NiSiF 6  6H 2  O crystal which will pass ultraviolet light in the missile warning band, such band being well known to those skilled in the art, and block light at longer wavelengths. However, we have found that NiSiF 6  6H 2  O crystals made in accordance with the methods here disclosed have the desired optical properties. 
     SUMMARY OF THE INVENTION 
     We provide a crystal comprised of hydrated nickel fluosilicate (NiSiF 6  6H 2  O). We have found that NiSiF 6  6H 2  O crystals are stable in temperatures up to 115° C. for indefinite periods. Sufficiently large crystals can be grown that are very useful for UV filters of the type used in missile approach warning systems. Crystals of NiSiF 6  6H 2  O have been successful at withstanding 110° C. to 115° C. for indefinite periods of time under relevant operating conditions and survived at temperatures as high as 130° C. for short periods. The crystal also had the required optical properties including absorption at longer wavelengths and it remained transmissive in the UV band. 
     The crystals can be grown using a solution formed by reacting NiCO 3  in fluorosilicic acid or from a seed crystal in a saturated solution of NiSiF 6  6H 2  O 
     Other objects and advantages of the present invention will become apparent from a description of certain preferred embodiments thereof shown in the accompanying drawings: 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a chart showing thermogravimetric analysis for our NiSiF 6  6H 2  O crystal. 
     FIG. 2 is an ultraviolet light transmission curve for our NiSiF 6  6H 2  O crystal. 
     FIG. 3 is a diagram of a sensor device which uses the crystal. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     We provide a NiSiF 6  6H 2  O crystal useful for ultraviolet sensors and filters. Single crystals of NiSiF 6  6H 2  O were grown by solution growth method. We used two approaches for growing single crystals. 
     A. Crystal Growth from NiCO 3  Fluorosilicic Acid 
     The formation of NiSiF 6  6H 2  O is given as: 
     
         NiCo.sub.3 +H.sub.2 SiF.sub.6 +5H.sub.2 O→NiSiF.sub.6 6H.sub.2 O+CO.sub.2 
    
     We reacted NiCO 3  in fluorosilicic acid at 45° C. and kept adding NiCO 3  until we observed excess of NiCO 3 . This process was carried out in a water bath. After an hour, the solution was decanted and placed in a container which was maintained at 40° C. We progressively lowered the temperature from 40° C. to 25° C. in 100 hours. The solution and water bath both were stirred to maintain the homogeneity of temperature. After several hours, we observed the nucleation and settling of small crystals which grew up to one centimeter in size. 
     B. Crystal Growth from Water Solution 
     As supplied NiSiF 6  6H 2  O was dissolved in water. A saturated solution was prepared at 45° C. by adding NiSiF 6  6H 2  O in water. We had used 300 ml water for the solvent. This volume can be changed depending on the size of container. After we prepared the saturated solution, we decanted it into a container which was already maintained at 40° C. We used a small pregrown seed crystal for the nucleating. The bath temperature was lowered in three steps; from 40° C. to 35° C. in 100 hours, from 35° C. to 30° C. in 100 hours and from 30° C. to 25° C. in 100 hours. The crystal grew on the seed, to a size which would allow a crystal of greater than three centimeters to be fabricated. Then, we removed the crystal from the solution. 
     Those skilled in the art will recognize that the times and temperatures which are used in both methods could be varied. However, we prefer to use temperatures with plus or minus 0.1° C. of those stated. Higher temperatures enable more starting material to be dissolved, but the water evaporates more rapidly. The dominant concern is to create a solution containing enough material to form a crystal of a desired size. 
     C. Characterization 
     The crystal composition was confirmed by matching the X-ray lines. We did not observe any additional phases in grown crystal. The lattice parameters of the crystal were: 
     
         a=9.317 Angstrom 
    
     
         b=9.629 Angstrom 
    
     and the crystal had hexagonal symmetry. Thermogravimetric analysis was carried out to determine the stability of the crystal. We used a heating rate of 5° per minute in our experiment. The results are shown in FIG. 1. It is clear that the crystal did not start losing water before 120° C. Therefore, the crystal and devices made from the crystal can be expected to perform in temperatures below 120° C. Also, we carried out an independent test by placing the crystal in an oven maintained at 100° C. We did not observe any sign of deterioration up to 48 hours at that temperature. Another crystal was subjected to temperatures in the range of 110° to 115° C. for several hours without degradation. 
     The feasibility of desired transmission was evaluated by measuring the transmission of an NiSiF 6  6H 2  O crystal using a Varian Cary-5 spectrophotometer. The results are shown in FIG. 2 and indicate that the NiSiF 6  6H 2  O crystal is transparent in the ultraviolet range and absorptive at longer wavelengths. Based upon this data it is apparent that this crystal can be used in UV filters and sensors for missile warning systems. We attribute the optical properties to the fact that nickel is present in a cluster with six waters of hydration. It should, therefore, be possible to substitute other flurocompounds such as flurostannate (Sn F 6 ) 2-   for fluorsilicate and still achieve substantially the same optical properties. 
     FIG. 3 is a diagram of a sensing device 1 which utilizes the crystal of the present invention. A source of ultraviolet light 2 emits or reflects a beam of ultraviolet, light 4 to the sensing device. The sensing device has an optical system 6 which directs the light to the crystal 8. The optical system 6 may be a single lens as shown in the figure or a series of lenses together with reflectors and filters. Typically, filters comprised of organic dyes in a polyvinyl alcohol plastic are used in this type of system. The beam of ultraviolet light strikes the NiSiF 6  6H 2  O crystal 8 which permits passage of ultraviolet light rays having the required wavelengths. The transmitted ultraviolet light strikes a sensor 10 which generates a signal indicating the presence of the ultraviolet light. That signal is transmitted through wire 12 to an alarm or display device 14. The size of the crystal 8 will depend upon the particular sensing device. In a missile approach warning system a crystal of approximately one centimeter i:n thickness is used. 
     Although we have described certain present. preferred embodiments of our crystal, methods of making the crystal and devices which utilize that crystal, it should be distinctly understood that our invention is not limited thereto, but may be variously embodied within the scope of the following claims.