High-pressure discharge lamp with ceramic discharge vessel

To protect a niobium tube (9) forming a current supply lead through a thrh-opening (14) in an end plug (10) in a ceramic discharge vessel against attack by halides or condensed sodium, the through-opening is formed in two portions, of different diameters. The outer portion (16), remote from the discharge vessel, has a diameter which is matched to the diameter of the tube (9), leaving only a capillary between the plug opening and the tube; the second, inner portion (17) is narrower than the first one, that is, outer portion, and surrounds the shaft of the electrode by a sufficient distance to permit expansion of the electrode, typically of tungsten, under operating conditions, without cracking the plug by forced engagement with the wall of the inner portion (17) of the aperture. A sealing glass, melt-sealed into the capillary of the outer portion, seals the niobium tube. Preferably, the niobium tube terminates inwardly in a dome-shaped end (20) seated in a similarly shaped cavity forming the transition between the two diameters of the through-opening, and also providing a capillary space for ingress of molten sealing glass. The inner diameter of the outer portion (16) of the through-opening differs, preferably, by at most about 0.05 mm from the outside diameter of the tube (9), whereas the diameter of the inner portion (17) of the through-opening (14) differs by at least 0.5 mm from the diameter of the electrode shaft.

Reference to related patents, the disclosures of which are hereby 
incorporated by reference: 
U.S. Pat. No. 4,501,799, Driessen et al (to which European Published 
Application 60 582 corresponds); U.S. Pat. No. 4,740,403, Oomen et al (to 
which European Patent 230,080 corresponds); 
Reference to related publication: 
British Patent 1,465,212, Rigden. 
FIELD OF THE INVENTION 
The present invention relates to a high-pressure discharge lamp, and more 
particularly to such a discharge lamp which has a discharge vessel made of 
ceramic material, typically aluminum oxide, and especially to such a lamp 
having improved end plugs for the vessel through which electrical current 
supply elements extend to supply energy, and mechanically support the 
discharge electrodes within the vessel. 
BACKGROUND 
Various types of high-pressure discharge lamps use ceramic discharge 
vessels. These discharge vessels can be used with sodium high-pressure 
lamps, as well as with metal-halide discharge lamps. The color rendition 
and color indices of the emitted light can be improved when using a 
ceramic discharge vessel over that of a glass vessel, since the ceramic 
material permits a higher operating temperature with respect to a vessel 
made of glass, typically of quartz glass. Lamps of this type may have 
power ratings for example of between 100 to 250 W. 
Problems arise in the lead-through arrangements for electrical energy to 
the electrodes in the discharge vessel. Various through-melting 
technologies are known, primarily from the technology developed for sodium 
high-pressure discharge lamps. Tubular lead-throughs made of niobium or 
tantalum are frequently used. These lead-throughs are then melt-sealed by 
a sealing glass in a ceramic end plug which is fitted into openings formed 
in the end of the discharge vessel. An arrangement of this type is 
described, for example, in British Patent 1,465,212, Rigden. 
Niobium as well as tantalum have thermal coefficients of expansion which 
are similar to that of the ceramic used. The thermal coefficient of 
expansion is about 8.times.10.sup.-6 /.degree.K. 
The known sealing arrangements cannot be used, unfortunately, for 
metal-halide lamps intended to have a long lifetime, and designed for good 
color rendition, since the 
metal-halide fill has the tendency to corrode the niobium lead-through as 
well as the sealing glass used to seal the niobium tube through the 
ceramic plug. Sodium high-pressure lamps, likewise, after extended use 
attack the lead-through, and hence the lifetime of the lamp is limited by 
attack of condensed sodium on the lead-through arrangement. 
Various types of sealing glasses have been used, for example calcium 
aluminate glass (see the aforementioned British Patent 1,465,212, Rigden) 
as well as special sealing glasses made of materials which are known by 
themselves, such as mixtures of aluminum and earth alkali oxides. The 
referenced U.S. Pat. Nos. 4,501,799, Driessen et al, and 4,740,403, Oomen 
et al, describe materials which are particularly resistant to attack by 
halides. 
THE INVENTION 
It is an object to provide a high-pressure discharge lamp which has a 
ceramic discharge vessel and an ionizable fill therein, which has a long 
lifetime, improved color rendition over prior art lamps, higher light 
output, and retention of the light output as well as the color rendition 
over an extended lifetime of the lamp. 
Briefly, the lamp ends are closed off with plugs. At least one of the plugs 
has an aperture which is stepped in two parts. The diameter of the plug 
portion adjacent the outer end, which may be termed the outer portion 
thereof, is dimensioned to match the outside diameter of the lead-through 
tube. The diameter of the portion adjacent the inner end of the plug is 
smaller, and dimensioned to permit free passage of an electrode stem 
therethrough, that is, to permit passage with sufficient clearance so that 
the stem can expand under operating conditions and increased temperature. 
Typically, the stem is made of tungsten, and the tubular connecting leads 
of niobium or tantalum. A sealing glass is used to melt-seal the tube into 
the outer portion of the aperture. 
The lead-through technology well known in sodium high-pressure discharge 
lamps cannot generally be carried over to lamps having ceramic discharge 
vessels with metal-halide fills. A number of additional problems must be 
solved all at once: 
(1) the halides attack the sealing glass; 
(2) the halides attack the lead-through metal tubes; and 
(3) there are only very few metals known which have thermal coefficients of 
expansion matching, roughly at least, that of the surrounding ceramic. 
The metals which are suitable and meet the requirement (3) are, especially, 
niobium and tantalum; however, just those metals are particularly corroded 
by halides or by a sodium condensate, respectively. 
In accordance with the present invention, the lead-through metals niobium 
and tantalum which have the requisite thermal coefficient of expansion are 
effectively protected against attack by the halides, or a sodium 
condensate, respectively, and, further, the sealing glass is likewise so 
protected. 
The protection is obtained, in accordance with the present invention, by 
the specific arrangement in which the niobium or tantalum metal tubes are 
fitted in the plug. In accordance with prior art, the plug has a 
through-bore of constant diameter, so that, even if the tube is not 
carried to the inner end of the opening, the tube is subject to attack by 
the aggressive fill within the bulb. This attack will occur at least at 
the end portion of the tube. In accordance with the present invention, 
this attack is minimized by making the aperture or through-bore in the 
plug in two parts of different diameter, which are stepped and adjoin each 
other with the larger diameter accepting the tube at the outside of the 
plug and the narrow one accepting the stem for the electrode interiorly of 
the vessel, or at the inner side of the plug. The diameter of the aperture 
at the inner side or discharge side of the plug is so selected that it can 
readily accept the electrode shaft which is made, customarily, of a 
material which is particularly high temperature resistant, usually 
tungsten. The thermal coefficient of expansion of tungsten, however, 
deviates substantially from that of the ceramic material of the plug. 
The end portion of the connecting tube, which faces the discharge, is 
better protected by the material of the plug itself as the diameter of the 
opening becomes constricted. This constriction forms a shoulder or 
abutment surface which, automatically, also determines the spacing of the 
electrodes from each other, as the plugs are introduced into the discharge 
vessel. 
In accordance with a preferred feature of the invention, the dimension of 
the outer portion of the aperture or opening and of the outside diameter 
of the connecting tube are so selected that the tube fits snugly into the 
opening leaving only a capillary space, into which the melt glass or 
sealing glass can flow upon melt-sealing the tube into the plug. In 
contrast, however, the diameter of the narrowed portion is so selected 
that the stem or shaft of the electrode, and possibly a region of 
increased thickness at the inner end thereof, can just be passed 
therethrough. A particularly small diameter of the inner narrowed portion 
can be obtained by inserting the electrode system in such a manner that 
the electrode stem or shaft is smooth to the outer end, without any 
thickening at all; the thickening is applied only afterwards, that is, 
after the electrode has been passed through the opening in the plug. This 
thickening may be a few windings of material around the end of the 
electrode, or may be a ball, which can be formed, for example, by 
melting-back the electrode tip. This melt-back can be obtained, for 
example, by application of an excess current. Care must be taken that the 
diameter of the narrowed, inner portion is larger than the diameter of the 
electrode shaft, or the inner connection to the electrode, respectively, 
so that no capillary effect in the region of the narrowed portion will be 
obtained. This is necessary since the electrode shaft customarily is made 
of high temperature resistant metal. Since the usual metal used, that is 
tungsten, has a thermal coefficient of expansion which deviates 
substantially from the ceramic material of the plug, glass melt which, in 
case of capillary effect surrounding the electrode shaft or stem, might 
wet the narrowed region. Since the thermal coefficients of expansion do 
not match, however, the high thermal loading of the lamp, particularly 
upon intermittent use, would result in the formation of fissures or cracks 
in the sealing glass and, eventually, also in the plug, leading to leakage 
and, then, failure of the lamp. 
In accordance with a preferred feature of the invention, the diameter of 
the tube is so made that it is fitted to the outer bore portion with less 
than 0.05 mm clearance, in order to provide for the capillary effect. To 
prevent capillary flow of the sealing glass, however, the narrowed or 
inner portion of the aperture should have a larger clearance to the 
electrode of at least 0.3, and preferably at least 0.5 mm. The length of 
the outer or wider portion of the bore, preferably, is about two-thirds of 
the axial height or length of the plug in order to provide for a reliable 
seat of the tube in the bore and a sufficiently long sealing path for the 
tube in the bore and to securely seat the tube deeply within the plug. 
In accordance with a particularly preferred feature of the invention, the 
tube is formed with a dome-shaped end portion, which is roughly 
hemispherical. The electrode shaft or stem is butt-welded on the apex of 
this hemispherical end portion of the tube. The hemispherical end portion 
can be obtained by melting an open end of the tube which collapses to form 
a closed end having the shape of a hemisphere or half-ball. The stepped 
portion of the aperture within the plug is rounded to receive the 
essentially hemispherical or domed end of the tube so that the space 
between the metal of the connecting tube and of the ceramic plug is about 
the same at all facing surfaces. This arrangement has the substantial 
advantage that the capillary which will form around the tube is extended, 
effectively, to the discharge end of the connecting tube, that is, to the 
inner end thereof. 
The arrangement with the dome-shaped end permits complete coverage of the 
tube and the end with sealing glass, and especially effectively protects 
the connecting tube from attack of the aggressive fill. The concave 
rounding at the transition from the wider outer aperture portion to the 
narrowed or constricted inner portion of the bore should well match the 
hemispherically or dome-shaped end of the lead or connecting tube. 
It has been found that specific spacers or locating accessories to maintain 
the capillary gap and the electrode spacing are not necessary. The 
electrode system formed of the niobium tube, the electrode shaft or stem 
and the tip thereof positions itself automatically. The electrode spacing, 
which should be precise for operation of the lamp also likewise is 
maintained automatically. The width of the capillary gap is obtained by 
the natural roughness or unevenness of the materials adjacent each other. 
In accordance with another preferred feature of the invention, a niobium 
tube is used which, before being melt-sealed in the plug, is coated with a 
layer of aluminum oxide Al.sub.2 O.sub.3, which has a thickness of between 
about 0.1 to 0.2 mm. Of course, the diameter of the outer bore or aperture 
in the plug should then be correspondingly enlarged. This ceramic coating 
has two advantages: it additionally protects the niobium tube with respect 
to the fill, since the fill must diffuse through the ceramic tube before 
it can attack the metal. This increases the lifetime of the lamp. 
Additionally, the Al.sub.2 O.sub.3 layer has a porous structure, which 
improves the wetting capability of the sealing glass. 
The Al.sub.2 O.sub.3 layer, thus, indirectly improves the diffusion of the 
sealing glass in the capillary gap, by improving the reliability of 
complete and uniform wetting of the tube by the sealing glass, 
substantially reducing rejects occurring in manufacture. The coating of 
the Al.sub.2 O.sub.3 can be easily effected by flame-spraying processes. 
In accordance with a further preferred improvement, the coating can be 
improved by adding yttrium oxide, Y.sub.2 O.sub.3. For example, first a 
pure Al.sub.2 O.sub.3 layer is applied and, then, an additional layer of 
Y.sub.2 O.sub.3 thereover. Alternatively, a mixture of Al.sub.2 O.sub.3 
/Y.sub.2 O.sub.3, in a relative proportion of 1:1 to 1:3 can be applied, 
again for example by flame-spraying. The tube, precoated with Al.sub.2 
O.sub.3, can be dipped into a suspension of Y.sub.2 O.sub.3. The coated 
tube is then fired at about 2000.degree. C., or, respectively, glazed. The 
result will be a crystalline layer with a glassy proportion. Other metal 
oxides may also be used for coating. 
The tube with the mixed coating, or with the two coatings, respectively, is 
then sintered for some minutes at a temperature of at least 1800.degree. 
C., under vacuum, or in a protective gas. The result will be a homogeneous 
layer which is highly halide-resistant, formed of an Al.sub.2 O.sub.3 
/Y.sub.2 O.sub.3 mixture. The conditions of cooling of this mixed coating 
can determine the characteristics. If the cooling is carried out rapidly, 
the mixed coating will be quite glassy, and will have the effect of a 
high-temperature enamel. If cooling is permitted to be slow, the coating 
will be primarily of fine crystalline nature. 
The electrical connection or connecting lead tubes, so pretreated, are then 
fitted into the outer portion of the opening of the plug. A ring of 
sealing glass is fitted on the tube, and the plug, with the electrode 
system fitted thereon, is heated until the sealing glass melts and runs 
into the capillaries in the region of the outer portion of the aperture. 
The result will be a gas-tight melt seal. 
The sealing glass used can be of any well-known metal oxide mixture. When 
using the plugs in metal-halide lamps, the sealing glass, in contrast to 
sodium high-pressure lamps, must be halide-resistant.

DETAILED DESCRIPTION 
For purposes of illustration, FIG. 1 shows a metal-halide discharge lamp of 
150 W rated power. A cylindrical outer envelope 1, defining a lamp axis, 
and made of quartz glass, is formed with two bases 3, just outside of 
pinch-sealed ends 2. A discharge vessel 4 is axially located within the 
outer envelope 1. It is made of Al.sub.2 O.sub.3 ceramic, bulged out in 
the center 5 to have an essentially barrel shape, and has two cylindrical 
ends 6. Two current supply leads 7 are connected to the bases 3 through 
conductive foils 8, typically molybdenum foils, within the pinch seals 2 
of the outer envelope 1. The current supply leads 7 are welded to tubular 
connecting leads 9, which are fitted, respectively, in ceramic plugs 10, 
also made of Al.sub.2 O.sub.3, and fitted in the ends of the discharge 
vessel 4. 
The tubular connections or leads 9 are made of niobium or tantalum, and 
retain electrodes 11 at their inner ends. The electrodes 11 have an 
electrode shaft 12 of tungsten and a closely wrapped winding 13 fitted on 
the tungsten stem or shaft 12. The discharge vessel retains a fill which, 
besides an inert ignition gas such as argon, includes mercury and 
additives of metal halides. 
The tubular lead-throughs 9 are deep-drawn or extruded. They are seated, 
recessed, in a bore 14 of the end plugs 10. At the discharge end or inner 
end, the tubes 9 have a flat bottom 15 on which the electrode shaft 12 is 
butt-welded. 
In accordance with a feature of the invention, the through-opening or bore 
14 has two parts or portions, each of which are of effectively constant 
diameter essentially throughout their axial lengths (see FIGS. 1-3). The 
first part or portion, remote from the discharge space of the discharge 
vessel 5 and forming the outer part or portion 16, is matched to the 
outside diameter of the niobium tube 9. In contrast, the second or inner 
part or portion 17 is constricted or narrowed with respect to the portion 
16. The two portions 16, 17 adjoin each other in stepped form, the bottom 
15 of the tube engaging the step or abutment formed thereby between the 
two portions 16, 17. 
The diameters of the two portions 16, 17 are specifically selected. The 
diameter of the second portion 17 is selected to be so wide or so great 
that, upon assembly, the electrode shaft 12 including the wrapped winding 
13 can be introduced through the opening 14 in the plug 10. 
The tube 9 is sealed in the first portion 16 by a halide-resistant melt 
glass 18 to be gas-tightly retained and sealed in the plug 10. Known 
materials, for example a mixture of aluminum and alkaline earth oxides, 
are suitable for the melt or sealing glass 18. Particularly suitable 
materials which are halide-resistant are described in the referenced U.S. 
Pat. Nos. 4,501,799, Driessen et al, and 4,740,403, Oomen et al. 
FIGS. 2 and 3 illustrate particularly preferred embodiments. The region of 
the lead-through at one end 6 of the discharge vessel is shown to an 
enlarged scale. The discharge vessel, at the end portion 6, has a wall 
thickness of about 1.2 mm. The cylindrical plug 10 is fitted into the end 
6. Its outer diameter is about 3.3 mm; its axial length or height is 5 mm. 
The axial through-opening or bore 14 has a first outer portion 16 with an 
axially constant diameter of 2 mm, and a second constricted inner portion 
17 with an axially constant diameter of 0.8 mm. The length of the portion 
16 is approximately two-thirds of the height or length of the plug 10; the 
remaining third is taken up by the second portion 17. 
The niobium tube 9, which extends externally beyond the plug 10, has an 
outer diameter of 1.95 mm, so that the gap remaining for the melt glass or 
sealing glass has a width of 0.025 mm. The wall thickness of the niobium 9 
is 0.2 mm. The diameter of the electrode shaft or stem 12, butt-welded to 
the dome-shaped end of tube 9, is 0.5 mm, so that the difference between 
the inner portion 17 of the through-opening 14 and the electrode shaft 12 
is 0.3 mm. 
The transition between the two portions 16 and 17 is concave and rounded, 
as seen at 19 in FIG. 2. The discharge end of the tube likewise is closed 
off by a hemispherical rounded dome 20. The two rounded portions 19 and 20 
are matched together, so that the width of the remaining gap, forming a 
capillary, between the tube and the first portion 16 will be retained 
generally also in the region of the concave rounded portion 19 and the 
dome 20. Thus, the sealing glass 18 will wet not only the gap in the axial 
portion of the first part or portion 16, but also the dome-shaped end of 
the tube 20 up to where the electrode shaft 12 extends from the 
dome-shaped end 20. 
The diameter of the second portion 17 of the aperture 14 is selected to be 
so small that, upon assembly, only the electrode shaft can be inserted 
through the bore 14. This ensures an almost complete wetting of the 
surface of the tube 9, especially around the shaft 12. The essentially 
ball-shaped terminal end 21 on the shaft 12 of the electrode 11 is formed 
only afterwards within the electrode vessel by application of excess 
current. 
The arrangement illustrated in FIG. 2 can be further improved, as shown in 
FIG. 3. In essence, the same general arrangement is used and identical 
elements have been given the same reference numerals. The niobium tube 9, 
in accordance with this embodiment, is coated with a layer of Al.sub.2 
O.sub.3 ceramic 22 which, as described, may have an additive of Y.sub.2 
O.sub.3. The thickness of the coating is about 0.2 mm. Consequently, the 
diameter of the portion 16 of the bore 14 in the plug 10 is increased to 
2.4 mm. The sealing glass, when using this embodiment, has been found to 
wet the niobium tube in a particularly uniform and complete manner. 
Various changes and modifications may be made and any features described 
herein with respect to any one embodiment may be used with any others, 
within the scope of the inventive concept.