TITLE: BATTERY CASE FEEDTHROUGH
INVENTOR: HISASHI TSUKAMOTO
FIELD OF THE INVENTION This invention relates to a battery case feedthrough and more particularly to a method and apparatus for providing a hermetically sealed electrical feedthrough for use with a metal battery case of low melt point material.
BACKGROUND OF THE INVENTION Battery cases typically require a conductive pin which feeds into the case through a dielectric material which insulates the pin from the conductive case wall. It is generally critical that the feedthrough be hermetically sealed. Techniques are known for forming a hermetic seal between a metal pin of high melt point material and a ceramic cylinder surrounding the pin. Moreover, an annular metal collar of high melt point material can be hermetically sealed around the ceramic cylinder.
It is often desirable to be able to form a battery case of light weight low cost material, such as aluminum. Unfortunately, aluminum melts at a temperature lower than the temperatures normally required to fabricate and install the aforedescribed ceramic-metal feedthrough structure in the case. For example, the fabrication of the aforedescribed structure typically requires the application of a temperature of about 800° C or more to form a hermetic seal between two high melt point metals and/or between a high melt point metal and a ceramic cylinder. Such an elevated temperature could destroy a case formed of aluminum which melts just above 600° C. SUMMARY OF THE INVENTION The present invention is directed to an improved case/feedthrough construction enabling the use of a low melt point case material, e.g., aluminum, in conjunction with a ceramic-metal feedthrough subassembly which requires the application of a high temperature to form a hermetic seal.
In accordance with the invention, a case cover comprised of a layer of high melt point material and a layer of low melt point material is used to respectively seal to a high melt point collar on the feedthrough subassembly and a low melt point case. More particularly, in accordance with a preferred embodiment, a case cover is formed of clad metal comprised of a layer of high melt point material, e.g., stainless steel or titanium, and a layer of low melt point material, e.g., aluminum. A feedthrough hole is formed through the cover. The feedthrough subassembly annular collar is aligned with the hole and hermetically sealed to the clad metal high melt point layer using a high temperature welding process. The clad metal low melt point layer, e.g., aluminum, can be laser welded to the aluminum case at a lower temperature to form a hermetic seal.
BRIEF DESCRIPTION OF THE FIGURES Figure 1 is an exploded sectional view of a feedthrough subassembly, a battery case, and a case cover in accordance with the invention formed of clad metal; and
Figure 2 is a sectional view similar to Figure 1 but showing the feedthrough subassembly, case, and case cover assembled together.
DETAILED DESCRIPTION The following text describes the preferred mode presently contemplated for carrying out the invention and is not intended to describe all possible modifications and variations consistent with the spirit and purpose of the invention. The scope of the invention should be determined with reference to the claims.
Figure 1 depicts a feedthrough subassembly 10 comprised of a feedthrough metal pin 1 penetrating a cylindrical ceramic glass core 2. A hermetic seal can be formed between the ceramic and metal by proper application of processes and materials.
Ceramic to stainless and ceramic to titanium are particular examples of materials that can be bonded to form hermetic seals. Braze bonding, for example, aluminum oxide and zirconium oxide ceramic to metals, including titanium, stainless steel, molybdenum, tantalum, and cobalt-chromium alloys, can be done using a braze, comprising 30% nickel and 70% titanium (U.S.
Provisional Patent Application, Serial No. ; U.S. Patent Application,
Serial No. ). Another example of a braze bond includes the preferred method for joining zirconium oxide containing 3% yttrium to preferably a metal alloy, namely titanium and niobium (55% Ti and 45% Nb), using the nickel-titanium braze (50% Ni and 50% Ti) (U.S. Provisional Patent Application,
Serial No. ; U.S. Patent Application, Serial
No. ).
Thus, by selecting a pin 1 of stainless steel or titanium or one of the other aforementioned metals and a ceramic core 2 of aluminum oxide or zirconium oxide, the pin 1 and core 2 can be hermetically bonded utilizing a high temperature brazing or baking process. Similarly, an annular collar 3 of stainless steel or titanium or one of the other aforementioned metals can be hermetically sealed around the core 2. In accordance with the invention, a clad metal case cover 20 is provided to mount the feedthrough subassembly 10 relative to a case 30 formed of a light weight low cost metal, e.g., aluminum. The case cover 30 is formed of a first layer 4 of a high melt point material, e.g., stainless steel or titanium and a second layer 5 of a low melt point material, e.g., aluminum. There are different methods for forming clad metal, such as a plate of stainless steel clad with aluminum. In one method (U.S. Patent No. 4213558, Hirobe, et al.), sheet metal cladding is fed onto one or both faces of the hot solidified continuously cast strip being withdrawn from the casting means and the assembly is then passed to a hot roll bonding system for cladding. Another method (U.S. Patent No. 4966748, Miyasaka et al.) produces a clad metal by forming a layer of the dissimilar metal powder on the surface of a base metal by cold fixing the powder to the surface under pressure, denting only the surface and a subsurface area of the layer of the dissimilar metal powder by melting and immediately solidifying in a vacuum, compressing the layer of the dissimilar metal powder together with the base metal at a temperature not higher than the solidus-line temperature of the two dissimilar metals under a pressure of not lower than kgf/cm2 using a hot isostatic press, and hot working the layer of the dissimilar metal power together with the base metal.
The case cover 20 defines a feedthrough mounting hole 6 of smaller diameter through layer 4 and larger diameter through layer 5. The feedthrough subassembly 10 is aligned with the hole 6 to place the annular collar 3 contiguous with the upper surface of case cover layer 4. Inasmuch as these contiguous materials are both formed of high melt point materials, e.g., stainless steel to titanium, they can be welded together using a high temperature process to form a hermetic seal. This high temperature is in excess of the melting point of the low melt point case material, e.g., aluminum. The case cover lower layer 5 of aluminum can then be welded to the aluminum case 7 using a lower temperature to form a hermetic seal.
If the clad metal were not used, the high temperature required to hermetically bond the feedthrough pin subassembly to the case would require use of a high melt point case cover. However, in trying to bond the high melt point cover to the aluminum case, the welding, or brazing, temperature would melt the aluminum. Exemplary melting point temperatures of the various metals are aluminum 660.37° C, stainless steel-303, 1427°C, and titanium 1660 +/- 10° C. In Figure 2, Low temperature hermetic welds are shown at (11 ) and (12). High temperature welds are shown at (13) and (14). While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.