Brazing mixture for forming a corrosion-resistant vacuum-tight seal between a ceramic body and a refractory metal
GOVERNMENT CONTRACT 
     The invention herein described was made under a contract with the U.S. Navy. 
    
    
     This is a division of application Ser. No. 330,376, filed 2-7-73, now U.S. Pat. No. 3,918,922. 
    
    
     DESCRIPTION OF THE PRIOR ART 
     Heretofore, attempts have been made to make vacuum tight, high strength, brazed seals between a high alumina body and an alkali corrosion resistant metal member, as of Cb-1Zr. In this prior art, a high temperature brazing alloy, Cb-33Fe, was placed between the metal member and a bare or tungsten metalized ceramic member to be brazed. The assembly was heated in vacuum to a brazing temperature of approximately 1670°C for melting the brazing alloy and forming a brazed joint between the Cb-1Zr member and the high alumina body. 
     The resultant metal-to-ceramic joints were relatively weak, especially with bare ceramic, thereby making such metal-to-ceramic seals unsuited for high temperature high strength applications as required for use in thermionic energy converters and high power infrared radiation sources, such as pulsed cesium vapor arc lamps. The aforecited Cb-33Fe brazement alloy and seal configuration is disclosed in the final technical report on U.S. Navy contract NObs-90496, titled &#34;Research and Development Program of Thermionic Conversion Of Heat To Electricity,&#34; by General Electric Nuclear Thermionic Power Operation, Pleasanton, California, Volume 2, July, 1968. 
     SUMMARY OF THE PRESENT INVENTION 
     The principal object of the present invention is the provision of an improved method for making metal-to-ceramic seals. 
     In one feature of the present invention, the ceramic body is coated with a brazing mixture which includes a volatile vehicle in which is suspended a powdered mixture comprising by weight a majority of columbium powder and a second metal powder selected from a class consisting of iron and nickel. The coated ceramic body is fired in vacuum with an assembled metal member to a temperature sufficiently high to produce a partial melting of the powdered mixture such that upon cooling a solidified bonding phase component or matrix is formed interstitially of the undissolved columbium particles for bonding the columbium particles to the ceramic body, whereby a high strength bond between the columbium particles and the ceramic body is obtained. 
     In another feature of the present invention, the ceramic body is selected from the class consisting of alumina, pore free translucent alumina, and sapphire. 
     In another feature of the present invention, a metal part which is to be bonded to the ceramic is selected from the class consisting of tantalum, tantalum alloys, molybdenum, columbium, and columbium alloys. 
     Other features and advantages of the present invention will become apparent upon a perusal of the following specification. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A brazing mixture is prepared which comprises a powdered mixture of metal powders suspended in a volatile vehicle. The powdered mixture is formulated by combining appropriate amounts of -325 mesh elemental columbium powder and -325 mesh iron powder or -100 mesh nickel powder. The powders are combined with a majority of columbium powder by weight. More particularly, a preferred embodiment employs approximately 17% iron powder or in the case of nickel, approximately 15% by weight nickel powder. 
     The powders are mixed with a methyl methacrylate based lacquer, such as a lacquer having as a base a material available under the trademark Lucite, to give the desired consistency. The solid and liquid constituents of the brazing mixture are mixed by holding them in a rolling container, either with or without a grinding media, for several hours. This serves to intimately mix the metal powders and to promote their suspension in the liquid media. 
     The brazing mixture is applied to that region of a ceramic body to be sealed. Suitable ceramic bodies include high alumina ceramic, (where &#34;high alumina&#34; is defined to mean 85% or more of Al 2  O 3 ), pore free translucent alumina, and sapphire. The brazing mixture may be applied by any suitable means, such as by needle painting, roller coating, brushing, dipping, or spraying. The coating is preferably applied to a thickness, as of 0.005 to 0.010 inch. 
     The coated ceramic bodies to be brazed are assembled with the metal member, to be joined to the ceramic, in a molybdenum or molybdenum-alumina brazing fixture and allowed to dry for several hours. Tungsten or molybdenum weights are used so that the seal area is under a minimum axial compressive load, as of 0.6 pounds per square inch. 
     As an alternative to applying the brazing mixture directly to the ceramic body, a first metalizing layer may be applied and sinter-fired onto the ceramic body and the brazing mixture subsequently applied to the metalized layer. Suitable metalizing metal layers include molybdenum and tungsten. Such underlaying metalized layers may be applied to the ceramic body as by evaporation in vacuum or by sputtering in a glow discharge at relatively low pressures, as of 10 -   3  torr. More commmonly such layers can be applied in the form of metalizing paints and subsequently sinter-fired at high temperature to achieve a dense adherent coating. 
     The ceramic, metalized or bare, and metal assembly is fired in vacuum to a brazing temperature sufficiently high to produce only a partial melting of the powdered mixture, such that upon cooling a solidified bonding phase component or matrix is formed interstitially of the undissolved columbium particles for bonding the undissolved columbium to the ceramic body. The undissolved columbium particles form a ductile metal bridge between the ceramic body and the metal part. The lacquer vehicle volatilizes at low temperature and is pumped by the vacuum system. A suitable brazing temperature range is between 1500°C and 1675°C. 
     The resultant metal-to-ceramic seals are vacuum tight, have relatively strong tensile strength, as of 20,000 psi, and are corrosion resistant to alkali metal. The joints may be repetitively cycled to relatively high temperatures, as of 1,000°C or higher, in continuous exposure to alkali metal liquid or vapor. 
     Formation of the final brazement configuration from an initial Cb-17Fe mixture probably occurs as follows: During heatup, the first liquid appears at 1370°C corresponding to the CbFe 2  -Fe (solid solution) eutectic. As temperature is increased to 1600°C, additional columbium dissolves in the melt until the melt composition corresponds to the Cb 3  Fe 2  -CbFe 2  eutectic point. A considerable amount of undissolved particulate columbium remains, and at this point, a kind of pseudo-equilibrium is reached because further dissolution of columbium tends to freeze the melt phase. 
     Quantitative electron probe microanalysis of the particulate and continuous regions in Cb-17Fe brazements confirms the above description. Compositions of the respective regions were found to be Cb-8Fe and Cb-34Fe by weight. Evidently, the undissolved columbium particles are saturated with iron by solid state diffusion during the brazing process. 
     The Cb-17Fe brazement can be thought to consist of columbium-rich particles that form a bridge-like network between the Cb-1Zr and pore free translucent alumina (Lucalox) members, and a hard, continuous cementing or bonding phase that provides the essential bonding. The columbium rich particulate phase probably matches the Lucalox ceramic member quite well in thermal expansion behavior owing to the positive influence of the undissolved iron. 
     The cementing or bonding phase, actually a two phase Cb 3  Fe 2  -CbFe 2  eutectic micromixture, is expected to have a somewhat higher thermal expansion coefficient than the alumina bodies. Ordinarily, a mismatch of this sort, coupled with the high elastic moduli of the ceramic and cementing phase, would impart high shearing stress to the ceramic-brazement interface. The presence of the large volume fraction of columbium-rich particulate phase in the brazement conceivably mechanically constrains the cementing phase to follow an average thermal expansion (or contraction) path determined by the weighted contributions of the phases present (with the weightings being based primarily on respective volume fractions and elastic moduli). Thus, the average thermal expansion behavior of the brazement composite probably is not very much different from that of the Lucalox and, consequently, interfacial stresses are probably low. 
     In the case where nickel powder is mixed with columbium powder, a particularly suitable mixture is Cb-15Ni. When such mixtures are brazed at temperatures above 1500°C, bond strengths are obtained which are comparable to those obtained with the aforedescribed Cb-17Fe brazements. The resultant brazement is quite similar in structure to the Cb-Fe structure. More particularly, a particulate phase of undissolved columbium is obtained with a continuous or cementing phase of CbNi interstitially disposed of the undissolved columbium particles and forming a thin bonding layer between the particulate columbium and the underlaying ceramic body.