US 3618193 A
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NOV- 9, 1971 M Q ANERSQN ETAL 3,618,193
GOLD-COATED REFLECTOR Filed Jan. 8, 1969 mmu- INVENTORS NORMAN C. ANDERSON JAMES T. GASPAR I A BY aw/ww ATTORNEYS nitedfStates Paten 3,bl8,l93 Patented Nov. 9, I971].
US. Cl. 29-195 9 Claims ABSTRACTOF THE DISCLOSURE A gold-coated reflector comprising a metallic substrate, an oxide diffusion barrier overlaying said metallic substrate, a layer of refractory metal deposited on said diffusion barrier, and a layer of gold deposited on said layer of refractory metal.
BACKGROUND OF THE INVENTION This invention relates generally to gold-coated reflectors, and particularly to gold-coated reflectors which are subjected to elevated temperatures such as those mounted adjacent a light source.
The reflective surface of reflectors adapted for use in conjunction with light sources typically consists of a thin coating of aluminum or silver. Where the source is to be productive of light in the near-infrared however a gold coating is frequently employed due to the high reflectivity of gold in this spectral region. Furthermore, where the source is produuctive of light of both infrared and visible wavelength, as is the usual case, the use of a gold-coated reflector reduces the amount of visible light filtration required where a purely invisible light beam is sought. This is due to the very substantial fall-off in the reflectivity of gold at and below light of blue-green wave length.
Heretofore it has been known that a gold coating on most metals deteriorates rapidly when subjected to elevated temperatures such as those in excess of 200 C. This is due to the fact that the metal substrate employed is typically one which, though suited to being worked into structurally sound reflector base, inherently interdilfuses with the gold. This problem has heretofore been solved through the use of oxide diffusion barriers overlaying the substrate to the surface of which a solution of gold having a binder in suspension is painted. However, both the percentage of reflectivity and the spectral character thereof from such gold coatings are less than optimum. These disadvantages arise due to impurities present in the form of the binders and to the fact that painted coatings are inherently less smooth than those formed by evaporative, sputtering or ion-plating techniques. These latter techniques cannot be well employed since the distribution of evaporated binders intermixed with evaporated gold is diflicult to control, and since a layer of evaporated gold without such binders would have poor adherence to the surface of an oxide diffusion barrier at elevated temperature.
Accordingly, it is a principal object of the present invention to provide a gold-coated reflector which will not appreciably degrade at elevated temperatures.
More specifically, it is an object of the invention to provide a reflector having a coating of high purity gold overlaying a metallic substrate which substrate will not interdilfuse with the high purity gold-coating at elevated temperatures.
Another object of the invention is to provide a goldcoated reflector having optimum reflectivity in the near-infrared region of the spectrum.
Yet another object of the present invention is to provide a reflector comprising a coating of high purity gold having good adherence at elevated temperatures.
SUMMARY OF THE INVENTION Briefly described, the present invention is a gold-coated reflector comprising a metallic substrate, an oxide diffusion barrier overlaying said metallic substrate, a layer of refractory metal deposited on said diffusion barrier, and a layer of gold deposited on said layer of refractory metal.
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a profile view of a sealed beam arc lamp portions of which are shown in cross-section to reveal an integral reflector made in accordance with principles of the present invention.
FIG. 2 is a greatly enlarged view in cross-section of a portion of the reflector shown in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now in more detail to the drawing, there is shown in FIG. 1 a sealed beam arc lamp having two spaced electrodes 1 housed within an evacuable envelope which electrodes define an arc gap therebetween. The envelope, which contains an ionizable gas such as xenon under pressure, comprises an optical window 2, a ceramic cylinder 3 and a gold-coated reflector 4 made in accordance with principles of the present invention.
When the lamp is ignited light radiating from the arc gap impinges upon reflector 4. About 60% of the light of blue-green wavelength and below is absorbed by the gold coating. However, over of light of longer wavelength, including that in the near-infrared, is reflected. This reflected light is formed into a beam of but slight divergence which is projected from the lamp through window 2. The window has a filter coating on the outer surface thereof which filters light in the visible portion of the spectrum.
As reflector 4 is located in close proximity with the are it acquires an elevated temperature such as that in the order of 350 C. during lamp operation. During fabrication it is subjected to even higher temperatures such as 500 C. during bake-out where the final seal is made by heliarc Welding, or to some 850 C. where brazing techniques are employed. Failure to have a diffusion barrier between the gold coating of the reflector and the metallic substrate over which it lays will result in degradation of the gold reflective surface due to interdiffusion between the coating and substrate.
Referring now to FIG. 2 reflector 4 is seen to comprise a metallic substrate 6 of a nickel-plated alloy of iron, nickel and cobalt. Such alloys are sold under the trademark Kovar and are well suited for use in structural support members due to their strength, workability, and coeflicient of thermal expansion which approximates that of ceramic cylinder 3. Other suitable metals include copper, nickel and alloys thereof which may further comprise iron and cobalt, as the alloys sold under the trademark Kovar do.
After the inner, concave surface of the metallic substrate has been polished and cleansed as by the use of a glow discharge, an oxide diffusion barrier 7 is deposited on the cleansed surface. This barrier serves to prevent interdiffusion between the Kovar and gold coating. Silicon oxide is the preferred diffusion barrier material although other oxides of silicon or of cerium could be used.
Next a layer 8 of a refractory metal is deposited on the diffusion barrier. Here molybdenum is the preferred metal due to its insolubility with gold. Columbium, tungsten or tantalum could likewise be used owing to their low diffusion coefficients with gold. Finally, a layer of high purity gold 9, having a thickness of between 1000 3 and 10,000 angstroms, is deposited on the layer of refractory metal which provides a substrate to which vacuum evaporated gold will adhere well at elevated temperatures without the need for binders. The deposition of layers 7, 8 and 9 may be made by either vacuum evaporation, ion-plating or sputtering techniques.
It should be understood that the above-described embodiment is merely illustrative of applications of the principles of the invention. Obviously, many modifications may be made in this specific example without departing from the spirit and scope of the invention as set forth in the following claims.
What is claimed is:
1. A gold-coated reflector comprising a metallic substrate, an oxide diffusion barrier overlaying said metallic substrate, a layer of refractory metal deposited on said diffusion barrier, and a layer of gold deposited on said layer of refractory metal.
2. A reflector in accordance with claim 1 wherein said metallic substrate comprises a metal selected from the group consisting of copper, nickel and iron-cobalt.
3. A reflector in accordance with claim 1 wherein said metallic substrate comprises an alloy of nickel, iron and cobalt.
4. A reflector in accordance with claims 3 wherein said Kovar is nickel-plated.
5. A reflector in accordance with claim 1 wherein said oxide diffusion barrier is selected from the group consisting of silicon oxide, silicon dioxide and cerium oxide.
References Cited UNITED STATES PATENTS 2,585,128 2/1952 Howe et al 29195 2,859,368 11/1958 Biggs et al 313-113 2,910,605 10/1959 Hodge 313-113 2,974,249 3/1961 Thouret 313-1 13 3,050,667 8/ 1962. Emeis 317240 3,106,489 10/ 1963 Lepselter 117217 3,274,024 9/ 1966 Hill et a1. 117-200 3,409,809 11/1968 Diehl 317-234 L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner US. Cl. X.R. 240103