ϊhe present invention relates to sealing materials suitable for making heat-resistant, and usually hermetic, seals between components of electrical discharge devices. It has been known for a number of years, as revealed in U.S. Patent No. 3■»588, 75 an German Speci¬ fication HO. 2,307,19 ^* 1, that the current sealing material for high pressure sodiura lamps based on a modified calcium magnesium aluminate composition with barium oxide and a small amount of boric oxide is chemically inadequate in the presence of reactive metal halide vapours at around 800°C. In U.S. Patent No. 3*588,573 a family of high temperature metal halide resistant sealing compounds using alumina and rare-eart oxides with high melting points ranging from 1720°C to 1800°C is dr.sclosed. ϊhe use of such high temperatures for sealing the end of the arc tubes, parti¬ cularly in the case of short tubes, presents considerable technical problems. For instance, it can easily cause volatilisation of the metal halide species in the final lamp processing _* ,

German Specification No. 2,307,191 discloses the use of some sealing compounds in the Al O^-SiOp-MnO system, which are claimed to be metal halide resistant, ^• ϊhe silica in the sealing compound is likely ^" to interact with certain metal halides, such as scandium iodide, commonly υ.-.ed in conventional metal halids lamps with a

OMPI

silica envelope. The reaction between scandium iodide and silica is a factor limiting the colour rendition and efficiency of these lamps. The disclosure by Matshuεita in Japanese Patent Application No. *7-34-066 of an unspecified metal halide resistant sealing material of the l ₂ 0 _:5 -Siθ2-- ^β 2θ -*BeO type is also suspect because of the presence of silica. It is also likely to be objectionable in the lighting industry because of the presence of beryllia, which is a highly toxic material. We. have now found in accordance with this inven¬ tion that compositions comprising a rare earth oxide and b.oric oxide, preferably together with minor amounts of phosphorus pentoxide, aluminium oxide and magnesium oxide are useful as sealing compositions in the construction of seals in ceramic discharge lamps, more especially between sintered ceramic oxides such as alumina- and cermet materials such as those disclosed in our co-pending Application DE-OS 26 55 726, for example alumina-tungsten and alumina-molybdenum cermets _e Seals made with these compositions are, moreover, found to retain their hermetic propertios and integrity without any sign of chemical attack after 100 hours at 900°C in reactive metal halide vapours such as mercuric chloride, mercury iodide, sodium* chloride, tin chloride, scandium iodide, sodium iodide or cesium iodido.

Although the preferred rare earth oxide is liuithanum oxide, other rare earth ^* oxides, such as Sin, Nd, Sc, Y, Yb, Dy or Ce oxides, or mixed rare earth oxides, can also be used. The most successful compositions lie in the - range of proportions by weight: 60 to 95 % rare earth oxide, 5 to 5°% boric oxide, 0 to 5 % phosphorus pentoxide, 0 to 5 % aluminium oxide and 0 to 5 magnesium oxide. Also in accordance with this invention it has

O

further been found that hermetic seals between dense alumina components, cermet components or alumina and cermet components can be prepared by using the composition defined above in vacuum or inert atmospheres between 1100°C and 1650°C.

In par icular, compositions around the eutectic compositions of the rare earth and boric oxides, for - example at 89. ^" 68% by weight lanthanum oxide and 10.32% by weight of boric oxide, have been found to seal trans- lucent alumina arc tubes to alumina-tungsten cermets at 1350°C. The sealing material at the join between the sintered alumina and cermet components consists mainly of two major crystalline phases: in the case of lanthanum oxide ^ . ^-. _ _>^)-, and O^.-B - _z - These crystalline phases precipitated from the melt diiring cooling are quite coarse, typically several hundred microns in diameter.

The assembly of large crystalline phases, although not obviously detrimental to the construction of hermetic seals, lowers the strength of the seal and is liable to initiate cracks during thermal cycling. We have found that "he addition of a small amount of phosphorus pent¬ oxide, aluminium oxide and magnesium oxide, through the formation of aluminium phosphate and magnesium phosphate, reduces the size of the precipitated phases by at least an order of magnitude, typiccύLly to the order of to 5 microns in diameter. The morphology of the precipitated phases is also drastically altered from large octahedral crystalline phases to needle-like platelets. Although the exact mechanisms responsible for the microstructural changes are not well understood, the included additive such as phosphorus pentoxide, aluminium oxide and magnesium oxide, preferably up to a total of about 3 by weight, drastically increase the rate of crystal nucleation during solidification- and subsequently increase the number of crystalline phase per unit volume. Thus a preferred

composition is about 88.50% by weight lanthanum oxide, 10.50% by weight boric oxide, 0.5% by weight aluminium oxide, 0.5% by weight magnesium oxide and 0.5% by weight phosphorus pentoxide. A preferred method of preparing the seal ng compound of this invention consists of mixing the appro¬ priate amount of rare earth oxide, obtained through a soluble salt such as the nitrate, sulphate or oxalate, with boric oxide. The additives can _. also be added as oxides or through a soluble salt or their respective ^' phosphates. The mixture is then fused at 200°C for 2 hours to homogenize the materials, calcined at up to 1200°C for 7 hours, in air or inert atmosphere, crushed and sieved through a 250 micron aperture mesh. The fusion temperature- and fusing time are not particularly critical as this technique simply helps to homogenize all the constituents. However, a preferred calcining temperature in air or inert atmosphere is 900°C for the production of fine mixed oxide powders with good flow, pressing and ejection characteristics, thus permitting the formation of elements such as discs, thin rings or washers. The fusion and calcination must be carried out in high purity alumina or platinum crucibles to avoid picking up undesirable impurities which could adversely affect the sealing behaviour.

The frit can be applied in the form of a slurry, using an organic liquid such as methyl or ethyl alcohol; the frit slurry or a pre ormed ring or disc can then be prefired or premelted on the ceramic or cermet component prior to the final sealing operation. Piemelted frit on the ceramic components offers additional advantages as it removes trapped air, moisture and other residual volatile species which could interfere with the final ceramic lamp processing. The lanthanum oxide compositions set . orth in the

OM

^•

Table possess excellent wettability. They represent preferred percentage ranges of the individual constituents, but not the limits of useful compositions.

. TABLE 1

Sealing the. components between 1325°0 and 1500°C with a tantalum heating element or by radio frequency heating permits the formation of a good fillet between the alumina components and the cermet 'components or between the alumina to cermet components without producing unnecessary flow of the sealing materials, for instance, along the length of the alumina arc tube. The alumina arc tube, may be a sintered alumina or artificial sapphire tube..

The heating rate should preferably not exceed 700°0 per minute to avoid entrapment of air in the melted sealing materials. A suitable heating rate is 4-00 C per minute as this minimises the formation of air bubbles. Holding the temperature for 2 minutes, moreover, helps the sealing materials to vet the alumina arc tube and the cermet components. The sealing materials are suitable for joining components irrespective of whether the surfaces are machined, polished or in the as-sintered condition.

Another important; factor in the construction of a hermetic seal between alumina and cermet materials is the rate of cooling of the melt, a preferred cooling rate being 4-0°C per minute for minutes after melting and holding the melt for 2 minutes, followed by a cooling .

_ O PI

rate not exceeding 80 C per minute for another 15 minutes . The cooling rate allows the additives to act synergetically in the production of small interlocking crystalline phases, which confers improved strength on the sea] , thus enabling it to withstand thermal cycling as is necessary for lamp operation.

It has further been found that by increasing the amount of boric oxide in the La _? 0 .B ₂ 0 system beyond near eutectic proportions, hermetic seals can be effected at temperatures as low as 1100°C. The upper limit of sealing temperaturesis high as l600 C. Table II below shows the compositions and the minimum temperatures at which hermetic seals have been obtained between alumina envelopes and cermet caps.

TABLE II

Apart from the addition of small amounts of phosphorus pentoxide, alumina and magnesia, other rare-eaι*th oxides, for example those of yttrium, ytterbium, samarium, dysprosium and cerium, can be additionally incorporated in the La 0-..B 0 system to enhance the properties of the seals. The total amount of these minor oxides should preferably not exceed 5% by weight. It is desirable, but not essential, o inclu these minor additions to effect hermetic seals. Examples of ^' compositions with such additions which have been successfull used for obtaining hermetic seals are shown in the followin Table III.*

TABLE III

The most successful compositions of this kind fall in the range of proportions by weight: 60 to 9 lanthanum oxide, 5 to k % boric oxide, 0 to 5% phosphorus pentoxide, 0 to 5% aluminium oxide, 0 to 5% magnesium oxide, 0 to 5% other rare earth oxide, such ^' as dysprosium oxide, cerium oxide, ytterbium oxide or samarium oxide.

As already mentioned, sealing compositions can be based on rare earth oxides other than lanthanum oxide, such as samarium oxide and neodymium oxide. These sealing compounds in, for example, the Sπi _p O .B _p O and Nd _p 0,.B„0-^ systems are similar to those in the La 0,.B _o 0, system. Table IV below shows some useful sealing compositions of this kind.

TABLE IV

OMPI

Seals described herein as hermetic will usually be impervious to helium. . Such seals are achieved by the preferred compositions and methods here described ^b ut it will- be appreciated that such a degree of hermeticit may not always be required

In the accompanying drawings:

Fig. 1 is a diagram illustrating a typical sealing sequence; Fig. 2 shows one example of a lamp seal construc- ted with the help cf the materials of this

- inventio ; and Pig. 3 shows a further example of a constructed seal. In Pig. 1, in which temperature is plotted against time, one example is given of a heating and cooling sequenc suitable for sealing ^' an alumina component to a cermet component. After initial heating to 1400°C in the region A, the seal is held at this temperature (region B) and subsequently allowed to cool slowly. The first stage of cooling C is more gradual than the second stage D.

Constructed seals are shown by way of example only in Pigs*. 2 and 3 _* These seals have withstood metal halide vapours such as mercuric chloride, mercuric iodide, sodium chloride, tin chloride, scandium iodide, sodium iodide or cesium iodide at 900°C for at least 100 hours without any visible sign of chemical reaction. The seals remain hermetic after exposure to these active metal halides used in a variety of metal halide lamps. In the lamp of. Pig. 2, a cermet can 11 carrying the electrode 15 g placed on a frit ring 12 composed cf the sealing material of this invention at the end of an alumina arc tube 13 with a monolithic alumina plug 14. Pig. 3 shows a completely sealed unit ready for incorporation into a ceramic discharge lamp. The reference numerals have the

same significance as in Pig. 2.

The sealing materials of this invention can be used in a variety of ways for the construction of ceramic discharge lamps containing sodium and/or metal halide vapours in alumina arc tubes. Por instance, they can be used for sealing hermetically alumina and niobium components in the construction of high pressure sodium lamps. Another application of the sealing materials described includes the formatio 'of protective metal halide coatings on cermet materials and on a range of refractory metals such as niobium, tungsten, molybdenum, tantalum for the construction of ceramic metal halide discharge ^' lamps containing sodium vapours and/or metal halide vapours. The sealing materials of this invention can be used to join sintered alumina or artificial single crystal sapphire components, cermet components or alumina to cermet components of any convenient geometry for the construction of ceramic discharge lamps. Such lamps may show improved performance as regards efficiency, colour rendition and higher resistance to metal halide attack at elevated temperature than conventional metal .halide lamps ^' using silica envelopes.

^' BUREΛT '