Cermet Composites, Process for Producing Them and Arc Tube Incorporating Them

This invention relates to cermets, and more particularly relates to composite cermets having one or more layers around central core, and also relates to a process for producing them and to a lamp incorporating them.

Development of a new higher-temperature metal halide lamp requires the substitution of a ceramic arc tube, often polycrystalline alumina (PCA), for the usual fused silica arc tube, which cannot withstand these higher tempertures. Closure for the PCA tubes must have thermal expansion characteristics similar to Al.sub.2 O.sub.3, while at the same time providing a method for electrical connection to the lamp electrode. Thus, the closure material would ideally have PCA-like thermal expansion characteristics but metal-like electrical conductivity. A further complicating factor is that the closure material must also be chemically resistant to the metal halide environment. This would eliminate Nb as the closure material even though it could satisfy both the thermal expansion and conductivity requirements, as Nb reacts rapidly with the halogens.

The problem of chemical resistance is circumvented by choosing closure materials which are already acceptable as other lamp components: Al.sub.2 O.sub.3, the arc tube material, and W or Mo, the electrode and feed-through materials. A simple mixture of Al.sub.2 O.sub.3 with a small amount of W or Mo will not suffice, as approximately 20 volume percent (55 weight percent) of W, for example, is required to achieve acceptable conductivity. a ceramic/metal mixture (cermet) of such high metal content (see, e.g., U.S. Pat. No. 4,001,625) will not exhibit ceramic thermal expansion behavior, however, and problems in achieving a leak-proof closure will likely occur. On the other hand, use of lower percentages of metal will yield materials of unacceptably low conductivity; if a continuous metal feed-through which extends through the closure is used, expansion mismatch causes problems with leaks around the feed-through.

A more complex approach is represented by U.S. Pat. Nos. 4,155,757; 4,155,758 and 4,354,964. In the '964 patent, for example, a refractory metal-oxide cermet with an unusually high conductivity is achieved by forming a continuous conductive network of the refractory metal surrounding the oxide granules. This is achieved by coating relatively coarse refractory metal oxide granules with fine metal powder, compacting and sintering the powder compact. As will be appreciated, producing such a cermet requires close control over process parameters, especially particle sizes of the constituents.

It is accordingly an object of the invention to provide refractory metal-aluminum oxide cermets having both relatively high electrical conductivities and low thermal expansion coefficients, which cermets may be used as end closures for alumina arc tubes in metal halide lamps.

It is a further object of the invention to provide composite cermets having a combination of physical, chemical and electrical properties which are unattainable in one or more of the constituent materials, which cermets may be readily produced by compacting and sintering of powder constituents.

In its broadest aspects, the invention covers a composite cermet body of a sintered powder compact, having a core of a first cermet composition, and one or more layers of other cermet compositions surrounding the core. These outer layers are preferably coaxial with the central core, and form co-planar top and bottom surfaces with the central core, which surfaces are normal to the axis.

In a specific embodiment, the composite cermet core and layers each include at least one of the refractory metals tungsten and molybdenum, and aluminum oxide.

The invention also covers a polycrystalline alumina arc tube for a metal halide arc discharge lamp, in which the tube is sealed with at least one closure member of a composite cermet having a central core containing refractory metal in an amount to make it sufficiently electrically conductive to provide operating current to an arc tube electrode, and having at least one outer layer containing alumina in an amount to provide a thermal expansion coefficient which is compatible with that of the arc tube wall.

In another aspect of the invention, a process for producing composite cermet bodies includes: (1) pressing a first cermet powder composition to form a powder compact having a central cavity opening into the top and bottom surfaces of the compact; (2) filling the cavity with a second cermet powder composition; (3) pressing the powder-filled compact to form a green composite; and (4) sintering the composite to form an integral cermet body.

FIG. 1 is a front elevation view, in section, of a composite cermet body of the invention;

FIG. 2 is a front elevation view partly in section, of an alumina arc tube having an end closure of a composite cermet of the type shown in FIG. 1, including embedded metal electrodes; and

FIG. 3 is a block flow diagram illustrating one embodiment of a process for producing a composite cermet of the invention.

FIG. 1 illustrates, in cross-section, one embodiment 10 of the cermet composites of the invention, in which cylindrical core 1 of a first cermet composition, having central axis A, is surrounded by coaxial annular layer 2 of a second cermet composition. Typically, both compositions contain the same constituents, but in different proportions, in order to provide a gradation of one or more physical, chemical or electrical properties from the center to the outer wall 10a of the cermet. As will be appreciated, finer gradations can be achieved by increasing the number of layers surrounding the core, and progressively increasing one or more constituents at the expense of the remaining constituents, from layer to layer. Where the gradations are sufficiently fine and sintering is carried out for a time sufficient to produce substantial diffusion of constituents between layers, it is possible to achieve a continuous gradation of composition and properties from center to outer wall. Such continuous gradation may be desired for the most demanding applications. Further control over the properties of the cermet can be achieved by varying the constituents so that each layer does not necessarily contain the same constituents. For example, in a composite having two layers surrounding a core, the outer layer may contain constituents A and B, the inner layer B and C, and the core C and D.

In order to illustrate the concept and application of a concentrically graded composite, it is helpful to describe an exemplary situation in which a particular composite is used to fulfill specific requirements.

FIG. 2 shows a portion of an alumina arc tube 20, suitable for use in a metal halide arc discharge lamp. The tube is typically filled with one or more metal halides, mercury and argon, and has a pair of electrodes, not shown, between which an illuminating arc is sustained during operation. Because of the large difference in thermal expansion coefficients between the ceramic arc tube and the metal leads for the electrodes, it is difficult to achieve with a single closure material a hermetic seal which is reliable both at the arc tube end wall and surrounding the lead wire.

A comprimise is achieved, where a concentrically graded composite cermet is used. We have succeeded in producing such a composite which has a central conductive core of about 10 to 30 volume percent tungsten, remainder Al.sub.2 O.sub.3, surrounded by an annulus of about 5 to 10 volume percent tungsten, remainder Al.sub.2 O.sub.3.

A composite cermet closure element 21 is illustrated in FIG. 2, having core 21a of relatively high thermal expansion coefficient and annulus 21b of relatively low thermal expansion coefficient, surrounding the core. In addition, the composite nature of the cermet enables the core to have sufficient electrical conductivity so that it is unnecessary for the electrode lead wire to pass through the closure element 21. In the embodiment of FIG. 2, separate tungsten lead wires 22 and 23 are secured by embedment into opposing surfaces of the closure element 21, thus providing a conducting path to the electrode, not shown, without the risk of failure of the hermetic seal which would occur for a pass-through lead.

The closure element 21 is hermetically sealed to the end wall of the arc tube 20 by means of sealing glass 24. Such glass must soften at a sufficiently low temperature that it does not attack the alumina tube, and must also be impervious to the fill materials. Many such sealing glass compositions are known. One suitable composition is a lanthanum oxide-based frit having in weight percent: 86.5% La.sub.2 O.sub.3, 10.0% B.sub.2 O.sub.3, 1.5% P.sub.2 O.sub.5, 1.5% Al.sub.2 O.sub.3, 0.5% MgO.

The composite cermets of the invention are conveniently formed by first pressing an annular compact of the outer layer, then filling the cavity of the compact with a different powder composition, and pressing again to form a green composite. FIG. 3 illustrates an especially preferred process in which the green composite is pressed at a higher pressure before sintering to form an integral cermet body. Sintering is preferably carried out in a non-oxidizing atmosphere, in order to prevent oxidation of the metallic constituents. For the structure illustrated in FIG. 2, depressions 25 and 26 are formed in the composite during the second pressing, and then lead wires 22 and 23 are pressed into the compact during the final pressing, and sintered in place during a single sintering step, typically carried out at a temperature between 1500.degree. C. and 1800.degree. C.

An exemplary procedure for preparing a composite cermet according to the invention is as follows:

The raw materials for these samples were Al.sub.2 O.sub.3 (Linde A, Union Carbide, 0.3 .mu.m) and powdered W metal (Fisher purified, 0.5 .mu.m) Two batches were mixed by tumbling for 17 hours; one batch was 20% W/Vol., and the other was 71/2% W/Vol. Homogeneous cermets previously made from these batches showed resistivities of 0.029 .OMEGA.cm and 2.times.10.sup.11 .OMEGA.cm, respectively. Composite samples were made by first uniaxially pressing a short hollow cylinder 1/2" in diameter with a 1/4" central opening at 500 to 1000 psi from the 71/2 percent W/Vol. batch. The central opening was filled with 20 percent W/Vol. material and pressed at the same pressure. The whole composite was then pressed at approximately 15,000 psi. This composite was sintered in vacuum at 1650.degree. C. for 1 hour. Resistivity measurements by a four-point probe technique on three samples prepared as described yielded results of 79, 51, and 17 .OMEGA.cm. These results are for the whole composite; the core itself would show lower values as described above. Resistivity can be altered as desired by using a larger conductive core or one of higher metal content. In addition, more than two layers may be used in a single composite if greater differences in properties are required between inner and outer layers. In the pressing process, the central plug may be pressed at low pressures (<1000 psi) to a size slightly smaller than the cylinder opening. The plug may then be inserted into this opening, and the composite may be pressed at up to 15,000 psi.

The above example cites only one application of the composites disclosed here. Other composite compositions may include layers grading from one oxide to another, from oxide to metal as above, or from one metal to another. Such compositions are capable of yielding gradation in resistivity, chemical compatibility characteristics, or physical properties such as thermal expansion, density, thermal conductivity, etc. Thus, layers may be chosen such that the finished composite performs different functions in different layers, depending upon the requirements of the particular application.