A high-pressure discharge lamp with a ceramic discharge vessel (8) compri a tubular current feedthrough (10) of a metal whose thermal coefficient of expansion is smaller than that of the ceramics. Gas-tightness is obtained by an internal support member (16) located within the current feedthrough (10).

REFERENCE TO RELATED APPLICATION 
U.S. Ser. No. 07/912,526, filed Jul. 13, 1992, Bunk, Jungst, Maekawa and 
Werner. 
Reference to related patents, the disclosure of which is hereby 
incorporated by reference: 
U.S. Pat. No. 4,545,799, Rhodes et al 
U.S. Pat. No. 4,011,480, Jacobs et al 
U.S. Pat. No. 4,160,930, Driessen et al 
U.S. Pat. No. 3,531,853, Klomp 
FIELD OF THE INVENTION 
The present invention relates to a high-pressure discharge lamp, and more 
particularly to a high-pressure discharge lamp having a ceramic vessel 
which contains an ionizable filling and has two ends which are each closed 
by a ceramic plug in which is positioned a tubular current feedthrough of 
a metal whose thermal coefficient of expansion is smaller than that of the 
ceramics. 
It relates, for example, to high-pressure sodium lamps, more specifically, 
however, to metal halide lamps with improved color rendition. The use of a 
ceramic discharge vessel permits the operation at the higher temperatures 
required for this. Typical wattage ratings are 100 W to 250 W. The ends of 
the tubular discharge vessel are closed by cylindrical ceramic end plugs 
which have a metallic current feedthrough in an axial opening thereof. 
BACKGROUND 
Customarily, feedthroughs of niobium are used (as described in the German 
Patent Specification DE-PS 1 471 379). These feedthroughs, however, are 
only relatively suitable for lamps having long lives and good color 
rendition, since, especially when lamps have a metal halide fill, the 
niobium tube and the ceramic sealing material used for the seal corrode 
considerably. An improvement is described in the U.S. Pat. No. 4,545,799, 
Rhodes et al. The niobium tube is tightly sealed in the plug without 
ceramic sealing material due to the shrinking process of the "green" 
ceramics during the final sintering. This is readily possible as both 
materials have approximately the same coefficient of thermal expansion 
(8.times.10.sup.-6 K.sup.-1). 
Feedthroughs made from other metals have also been tested. 
A feedthrough is known from the U.S. Pat. No. 3,531,853, Klomp, which has a 
surface of platinum, iron, nickel or cobalt and a core of an alloy that is 
adapted to the ceramics. The feedthrough may have a conical shape and may 
be joined to the plug by the use of a ceramic internal support 
member--with both plug and internal support member also having conical 
shape--by axial pressing at a defined pressure and in a defined gaseous 
atmosphere. 
Discharge lamps are known from the U.S. Pat. Nos. 4,011,480, Jacobs et al, 
and 4,160,930, Driessen et al, in which the tubular current feedthrough 
consists of tungsten, molybdenum or rhenium, with the tube being supported 
in the interior thereof by a ceramic cylinder having straight, axially 
aligned walls. The cylinder may be solid or hollow; in the latter case the 
bore serves as the exhaust plug and is subsequently closed. The seal 
between the feedthrough and the internally and externally abutting ceramic 
parts, which have both been finally sintered earlier at a temperature of 
1850.degree. C., however, is still carried out by means of a ceramic 
sealing material so that the susceptibility to corrosion of these lamps 
has indeed been reduced but does not yet satisfy the requirements for the 
use in lamps having metal halide fills. In spite of great efforts it has 
not been possible hitherto to develop a corrosion resistant ceramic 
sealing material. 
THE INVENTION 
It is an object of the invention to provide a feed through which is 
resistant to changes of temperature and corrosion and which can be used, 
more particularly, with halide containing fills. 
Briefly, a ceramic discharge vessel, which contains an ionizable metal 
halide fill, has two open ends. Each one of the open ends has a current 
feedthrough directly sintered to a plug fitted into the open end. At least 
one of these feedthroughs is tubular and an internal support element is 
located within the tubular feedthrough. 
The invention is based on a further development of the technology described 
in copending U.S. application Ser. No. 07/912,526, filed Jul. 13, 1992, 
Bunk et al (to which Published European Application 0528 428 A1, based on 
filed application 91 113 912.9 corresponds. This application describes 
thin-walled molybdenum tubes (wall thickness 0.05 0.25 mm) into ceramic 
plugs directly. When these plugs are used in lamps having particularly 
good color rendition, a narrow gap will result between the current 
feedthrough and the plug after about 500 temperature cycles (that is, 
switching-ON and switching-OFF of the lamp, each causing a change in 
temperature). The width of this gap is about 15 .mu.m. This is due to the 
great difference (25%) between the coefficients of thermal expansion of 
molybdenum (6.times.10.sup.-6 K.sup.-1) and ceramics (8.times.10.sup.-6 
K.sup.-1) which makes itself felt due to the change in temperatures. 
The invention uses the shrinking process of a green ceramics also for the 
bond between the plug and the non-adapted feedthrough, and thus avoids the 
use of the corrosion susceptible ceramic sealing material additionally, it 
uses an internal support member in the form of an already finally sintered 
ceramics which is no longer subject to a shrinking process. This internal 
support member and plug preferably consist of the same ceramic material. 
Due to the cooperation of these two measures the life of these lamps is 
considerably extended (by up to a factor four). 
The bond is obtained by first leaving the end plug as a green body in which 
the tubular current feedthrough, which includes the internal support 
member, is positioned. The plug is now finally sintered, and the, 
necessary reliable bond is achieved due to the shrinking of the end plug 
(about 2-20%). The shrinking green body of the end plug presses on the 
tube and presses the latter against the internal support member. The 
temperatures required for this (about 1850.degree. C.) will not nearly be 
attained (about 1100.degree. C.) at the end plug during operation of the 
lamp. 
This way of joining is particularly advantageous with halide containing 
fills, since corrosion susceptible components are completely omitted. 
In the event that the tubular current feedthrough is gas-tightly closed on 
the side facing the discharge, it may be possible to use the known ceramic 
sealing material technology for the joint between the internal support 
member and the tube, because, in this case, no halide will get to the 
ceramic sealing material. It must be taken into consideration that only 
ceramic sealing materials are suitable which have a melting point higher 
than the sintering temperature. It has become evident that metallic 
solders can also be used. The latter have a higher elastic elongation and 
are thus more readily capable of joining bodies having different 
coefficients of thermal expansion. 
In the case of a current feedthrough which is open on the side facing the 
discharge, that is, a feedthrough which is not gas-tightly closed, the 
ceramic sealing material is omitted when joining the internal support 
member to the feedthrough tube. The idea is to provide for the tightness 
on the inside of the current feedthrough by the pressure of the plug on 
the outside thereof. 
In both cases a relatively precise fit of the internal support member is 
necessary (about 15-50 .mu.m if a ceramic sealing material is used, it 
must be introduced by a capillary effect; with direct sintering, the 
precise fit is necessary to obtain reliable tightness even when the 
shrinking is only slight (about 2%). 
In the most simple embodiment the internal support member has the form of a 
solid cylinder or of a cylindrical tube (hollow cylinder). In the latter 
case the central bore serves for exhausting and filling purposes. It can 
later be closed by a ceramic sealing material or the like. 
One embodiment has proved particularly suitable, especially when the 
internal support member, too, is secured within the tube without ceramic 
sealing material or metal solder. In this embodiment, the height of the 
internal support member is smaller than the height of the plug. A typical 
value is a reduction by 30%. During the final sintering of the end plug 
with the feedthrough tube positioned therein, the portions of the 
feedthrough extending beyond the internal support member are still further 
pressed together since, here, the resistance of the internal support 
member does not exist, so that there results a particularly reliable tight 
joint at least at one end of the internal support member and, in addition, 
the internal support member is reliably retained. The central positioning 
of the internal support member with respect to the plug height is 
particularly suitable because, in this case, the securing effect occurs at 
both ends of the internal support member. 
Particularly advantageous is an embodiment in which at least a portion of 
the internal support member tapers into a conical shape. This shape 
considerably facilitates the matching of the parts to be joined 
(plug-tube-internal support member), as differences in diameters are 
automatically compensated for by axial displacement. The initial fit needs 
to be precise only to about 200 .mu.m. In addition, the retention of the 
internal support member in the tube is automatically safeguarded prior to 
the joining thereto. This embodiment is especially well suited for the 
joining technique without ceramic sealing material. 
The manufacture of this particularly well suited embodiment can be carried 
out in two ways. The tube itself may already have a conical portion, with 
the angle of inclination of internal support member and tube being the 
same (typically about 10.degree. ). Or, the internal support member alone 
can originally be slightly conical (5.degree.-10.degree. ) either entirely 
or over a portion thereof. In this case, the originally circular 
cylindrical tube is first pressed to a conical shape. This is carried out 
preferably by friction welding by drawing the tube onto the internal 
support member while the tube is continuously rotated. For facilitating 
this technique or for obtaining larger angles, the tube can already be 
preshaped so as to be slightly conical (typically 5.degree. ) and can be 
additionally enlarged during friction welding (to typically 10.degree.). 
This unit is then inserted into the conically preshaped green body of the 
end plug and the end plug is finally sintered. 
With the friction welding, care must be taken that due to the friction the 
tube is brought to a temperature which is above the transition from the 
brittle phase to the ductile phase so that the tube can be elastically 
deformed. The temperature of the transition is particularly low in the 
case of molybdenum (200.degree. C.). For this reason molybdenum is 
preferred over tungsten and rhenium for this technique which provides a 
particularly reliable bond between the internal support member and the 
current feedthrough. In tile other embodiments tungsten and an alloy of 
tungsten and rhenium are suited similarly well as molybdenum. Their 
coefficient of thermal expansion (4.times.1O.sup.-6 K.sup.-1) is even 
smaller than that of molybdenum. To summarise, it may be noted that the 
present invention is applicable to a feedthrough whose coefficient of 
thermal expansion is at least 20% smaller than that of the ceramic formed 
parts. 
The invention provides a high-pressure discharge lamp of long life whose 
tightness is not impaired even by the use of halide containing fills. The 
discharge vessel is customarily tubular, either cylindrical or with an 
outwardly bulging portion at the middle thereof. Frequently it is located 
within a single-ended or double-ended outer envelope. 
The referenced application Ser. No. 07/912,526, filed Jul. 13, 1992, 
describes a feedthrough system which is capable of resisting corrosion and 
changes of temperature and which can be used, more particularly, for lamps 
having a metal-halide containing fill. 
Metals having a low thermal coefficient of expansion (molybdenum, tungsten 
and rhenium) are the metals which have a high corrosion resistance against 
aggressive fills. Their use as a current feedthrough is, therefore, highly 
desirable. However, the problem of providing a gas-tight seal while using 
such feedthroughs has remained unsolved in the past. 
Metals such as niobium and tantalum have thermal coefficients of expansion 
that match those of the ceramic; on the other hand, however, they are 
known for having poor corrosion resistance against aggressive fills and 
they have not yet been available for use as a current feedthrough for 
metal halide lamps. 
At least the portion of the feedthrough which is exposed to the aggressive 
fill in the interior of the discharge vessel is made of a corrosion 
resistant material having a low thermal coefficient of expansion, that is, 
a coefficient of expansion which is at least 20% lower than that of the 
ceramic vessel material. 
A very simple and basic embodiment of the invention uses a continuous 
tubular feedthrough of molybdenum which is tightly sintered directly into 
the ceramic plug without using any ceramic sealing material. 
The feedthrough is bonded directly into the plug only by co-firing. This is 
very surprising insofar as it was hitherto believed that a durable direct 
sintering could only be effected by using materials having approximately 
the same thermal coefficient of expansion as the ceramic, such as is the 
case with niobium. 
It has become evident that a similar method can only be used with 
molybdenum, tungsten or rhenium (thermal coefficient of expansion 
.ltoreq.6.times.10.sup.-6 K.sup.-1) if it is modified accordingly. This 
permits manufacture of a bond which is material-locking, free from cracks 
and fissures, and which can be used with less agressive fills and 
relatively low thermal strain. 
It is advantageous that the tubular current feedthrough has very thin 
thickness, a small diameter, and a toughened surface. It is further 
advantageous that the relation between the inside diameter of the plug, 
facing the feedthrough, and the outside diameter of the feedthrough is 
within certain optimum dimensions. The seal made without any ceramic 
Sealing material is obtained by first leaving the end plug as a green body 
into which the current feedthrough is introduced. In the final sintering 
of the plug which will now take place, the required reliable bond of the 
plug and current feedthrough interface will be achieved due to the 
shrinking process of the end plug in which the shrinking green body of the 
end plug finally is firmly forced onto the current feedthrough. 
An important parameter of the present invention is that the current 
feedthrough is not a solid cylinder but a tube having a sufficiently thin 
wall in order to be able to deform slightly to compensate for the force 
acting on the feedthrough caused by the shrinking of the end plug during 
the final sintering. On the other hand, the current feedthrough tube must 
be sufficiently thick in order to be able to warrant mechanical stability 
and, more particularly, to be able to securely retain-the shaft of the 
electrode. A wall thickness of 0.1 to 0.25 mm has proved especially 
suitable. 
A second important parameter is the diameter of the current feedthrough 
which determines the absolute value of the thermal expansion. The smaller 
the diameter is in actual fact, the smaller are the forces of expansion 
occurring during operation of the lamp. Preferably, the outer diameter is 
smaller than 2.0 mm. On the other hand, for most practical purposes, and 
to be able to carry enough current, a minimum inner diameter of 0.5 mm is 
recommended, although a smaller diameter may be used for certain 
low-wattage lamps. 
A third important parameter is the surface roughness of the feedthrough. 
The direct sealing between the feedthrough and the plug appears to be due 
mainly to a mechanical bond and to a lesser degree to a diffusion bond. 
The larger the contacting areas at the interface of feedthrough and plug, 
the more effectively can be attained the gas-tightness-of the direct 
sealing portion. Preferably, the surface roughness of the feedthrough is 
about 10-50 .mu.m by Ra, which means a center-line average surface 
roughness. 
A roughness of less than 10 .mu.m is not effective to the improvement of 
gas-tightnes. A roughness larger than 50 .mu.m, although suitable for 
producing a discharge vessel body with good gas-tightness, is not 
preferable because it decreases the reliability and mechanical stability 
of the current feedthrough. This toughening can be simply done by means of 
various ways such as sand blasting, chemical etching and machining. 
A fourth important parameter is the selection of the optimum relation 
between the inside diameter of the end plug and the outside diameter 
of-the current feedthrough. Prior to sintering, the end plug is in an 
unsintered or so-called "green" state. Upon sintering, the end plug 
shrinks, with both its outside and inside diameter decreasing. If the 
decrease of the plugs inside diameter during shrinking is much too high, 
cracking of the end plug is caused due to the bounding stress from the 
current feedthrough introduced into the plug's inside hole. If it is too 
low, the bonding force at the interface between the end plug and current 
feedthrough becomes weak and it results in the of gas-tightness of the 
discharge vessel. Preferably, the inside diameter of the end plug--if 
sintered without introducing the current feedthrough--would be 5 to 10% 
less than the unvaried outside diameter of the current feedthrough. 
In carrying out this technological process, the seal is obtained by first 
positioning the current feedthrough into the axial hole of the plug while 
the plug is in the green state. One of the assemblies thus obtained is 
inserted in each end of the tubular vessel in the green state, and the 
said inserted assembly is sintered in hydrogen or in a vacuum atmosphere 
at a temperature of about 1850.degree. C. for 3 hours. The required 
reliable seal at the plug feed through interface is achieved due to the 
shrinking process of the plug in the green state during sintering in which 
the shrinked end plug body finally is firmly bonded onto the current 
feedthrough. 
When tubes are used as a feedthrough which are made exclusively of 
molybdenum, and when the discharge vessels are subjected to very great 
strain, for example, in the case of lamps having excellent color 
rendition, and the temperature of its coldest spot is higher than 
700.degree. C., a gap may form between the current feedthrough and the 
plug after about 500 temperature cycles. (or changes of temperature 
subsequent to the switching on and off of the lamp). The width of such a 
gap is about 3 .mu.m. This gap occurs as a result of the large difference 
between the low thermal coefficient of expansion (6.times.10.sup.-6 
K.sup.-1) of the molybdenum and the high coefficient of expansion Of the 
ceramic (8.times.10.sup.-6 K.sup.-1) which has an effect caused by the 
strain from the temperature changes and it may result in lamp failure. 
This basic technology can be modified. 
A first technical modification is to use a modified plug which consists of 
a composite material having a coefficient of thermal expansion between 
those of the ceramic vessel material and of the tubular metallic 
feedthrough material. The tubular feedthrough, e.g. of molybdenum, is 
gas-tightly sintered directly into the plug of composite material, which 
comprises, for example, alumina and tungsten, without using any ceramic 
sealing material. This co-fired body maintains gas-tightness after more 
than 500 numbers of heat cycles between 20.degree. C. and 900.degree. C. 
It is possible to apply a hydrogen atmosphere for co-firing of an 
assembled body which consists of a metallic feedthrough, a plug of 
composite material and the ceramic discharge vessel. 
A first important parameter of this technology is to use a 
tubular-feedthrough of molybdenum, tungsten, rhenium or alloys thereof. If 
the feedthrough were a solid, for example, a rod or wire, cracking would 
occur at the direct-bonded portion. It is preferable to use a tube of 
small outside diameter. Preferably, the outer diameter is smaller than 2.0 
mm. The thickness of the tube is not limited especially, however, to 
permit the shrinking force caused under the firing process to prevent 
cracking, the inside diameter of the tube should be at least more than 0.3 
mm. 
A second important parameter is the plug material. It must have a 
coefficient of thermal expansion between those of a metallic current 
feedthrough and the ceramic discharge vessel and a good corrosion 
resistance against any agressive fill component such as metal halides and 
sodium. Furthermore, is more desirable to select such a material whereby 
it is possible to co-fire an assembled body under a hydrogen atmosphere. 
The assembled body consists of a metallic feedthrough, a ceramic vessel 
and a plug formed by such a composite material. 
The plug material consist of two components. Alumina is cite main and 
indispensable first component. The second component comprises one or more 
materials selected from the metals tungsten, molybdenum and rhenium, or 
graphite or ceramics having a low coefficient of thermal expansion such as 
AlN, TiC, Si.sub.3 N.sub.4, SiC, ZrC, TiB.sub.2, and ZrB.sub.2. The ratio 
of the two components is the following: the proportion of the main 
component alumina is 60 to 90% by weight, and the proportion of the second 
component is 10 to 40% by weight. The respective coefficients of thermal 
expansion of these composite materials are about 5.5 to 
6.5.times.1O.sup.-6 K.sup.-1. The reason why alumina has to be an 
indispensable component is not only its excellent corrosion resistance. 
Furthermore, due to a solid diffusional reaction under firing at a 
temperature of about 1800.degree. C., the seam originally located at the 
contacting zone between the plug and the end of the discharge vessel is 
eliminated and thus a quasi one-bodied structure is formed. The proportion 
of alumina should be at least 60% by weight. If this proportion is higher 
than 90% by weight, the composite material does not have a desirable 
coefficient of thermal expansion, and, as a result, the direct-bonded 
portion between the plug and the metallic feedthrough is unable to 
maintain the gas tightness after numbers of heat cycles, which finally 
results in lamp failure. If the proportion the second component, 
especially due to the metal included therein, is too high, it is very 
difficult to sinter the plug and to make a highly densified dispersion of 
composite material which is needed to guarantee the gas-tightness of the 
plug itself. For example, in case of a composite material consisting only 
of alumina and tungsten (or one or more of the above mentioned metals), a 
ratio of alumina: tungsten =70 to 83: 30 to 17 by weight shows the best 
results with respect to gas-tightness. For other second component 
materials, the most favorable proportion is within 10 to 25% weight. This 
applies especially to the ceramic materials or blends of ceramic and 
metallic materials. A preferred example is a plug with 20 % SiC, balance 
Al.sub.2 O.sub.3. 
These composite materials can be manufacture nearly without special 
conditions. Basically the procedure is the following: weighing the desired 
proportion of alumina powder and of the second component; adding some 
auxiliary pressing agents for forming, such as water, alcohol, organic 
binder etc.; mixing them by a ball-mill or kneader; making a granular 
powder suitable for the fabrication process by means of a spray-dryer 
and/or in any other way, and finally shaping a plug provided with an axial 
hole for positioning a current feedthrough therein. One special condition 
must be kept in mind: apart from alumina and SiC, the materials for the 
second component oxidize and decompose comparatively easily. Therefore, it 
is necessary to carefully select both the suitable auxiliary agents for 
forming and optimum conditions such as atmosphere and temperature at the 
pre-firing process, which removes the auxiliary agents which have been 
introduced for forming the green body to a plug shape, and to prevent 
oxidation and/or decomposition of the second component materials. 
Otherwise the result would be an undesired coefficient of thermal 
expansion and/ or the occurrence of cracking in the plug body itself. 
A third important parameter is the surface roughness of the metallic 
feedthrough. It is favorable to use a metallic feedthrough having a 
toughened surface, but this is not as important as the other parameters 
because it is possible to maintain a gas-tightness at the direct-bonded 
region between plug and feedthrough, even if the feedthrough is not 
specially prepared. 
A fourth important parameter is the optimum relation between the 
feedthrough and the plug on the one hand and between the plug and the 
ceramic vessel on the other hand. The conditions which make a ceramic 
discharge vessel have a direct-bonded closure, obtained by only co-firing, 
at one or both of its ends are almost the same as in the basic technology: 
The axial-hole diameter of the plug where a metallic current feedthrough 
is positioned passing through the hole and being directly bonded to it by 
co-firing has to be adjusted so that after shrinking it would be 3 to 10% 
less than the original outer diameter of a metallic feedthrough, if the 
plug Were fired without a metallic feedthrough. A similar condition 
applies to the inner diameter of the end portion of the ceramic discharge 
vessel, in which end portion the plug is inserted and a one-bodied 
structure is created by applying a solid diffusional reaction under 
co-firing. This inner diameter has to be adjusted so that after shrinking 
it would be within a range of 2 to 5% less than the outside diameter of 
the plug if only the vessel were fired. The reason for those conditions is 
the same as that of the basic technology.

DETAILED DESCRIPTION 
FIG. 1 illustrates schematically a metal halide discharge lamp of 150 W 
power rating. It comprises a cylindrical outer envelope 1 of hard glass 
defining a lamp axis, the envelope being pinch-sealed 2 and provided with 
a base 3 at each of its ends. The axially aligned discharge vessel 8 of 
alumina ceramics is outwardly bulging at the middle portion 4 and has 
cylindrical ends 9. It is supported in the outer envelope 1 by means of 
two current supply conductors 6 which are connected to the bases 3 via 
foils 5. The current supply conductors 6 are welded to tubular 
feedthroughs 10 which are each fitted in a plug 11 at the end of the 
discharge vessel. 
The two feedthroughs 10 of molybdenum (or tungsten, possibly alloyed with 
rhenium) each support an electrode 12 at the discharge side thereof. The 
electrode comprises an electrode shank 13 and a coil 14 slipped on the 
shank at the side facing the discharge. The fill of the discharge vessel 
comprises an inert starting gas such as argon, mercury, and additives of 
metal halides. 
FIG. 2 illustrates the sealing region at one end of the discharge vessel 8 
in detail. The discharge vessel 8 has at its two ends 9 a wall thickness 
of 1.2 mm. A cylindrical plug 11 of alumina ceramics is inserted into the 
end 9 of the discharge vessel. Its outer diameter is 3.3 mm, its height 5 
mm. A molybdenum tube 10, which has a length of 12 mm, a wall thickness of 
0.1 mm, a constant diameter of 1.4 mm and which is closed at the end 15 
facing the discharge, is fitted in an axial opening in the plug as a 
feedthrough. The shank 13 is welded onto the end 15. 
The tube 10 extends on both sides beyond the plug 11. A ceramic internal 
support member 16 of alumina is located in the interior of the closed tube 
10 at the height of the plug. The internal support member is a solid 
cylinder whose outer diameter is closely matched (to about 15 .mu.m) to 
the inner diameter of the tube 10. The solid cylinder is joined to the 
tube by an intermediate metal solder layer 17. In contrast, no additional 
joining agent is located between the tube 10 and the plug 11 that is, the 
system of feedthrough 10 and plug 11 is devoid of joining or sealing 
material. The plug 11 is directly sintered onto the tube 10. 
The direct sintering of the integral current feedthrough into the plug is 
carried out as follows: 
The present process for producing a discharge vessel 8 with cylindrical 
ends 9, provided with a plug 11 and an integral current feedthrough 10 
which is directly sealed into the axial hole of the plug, comprises 
preparing a current feedthrough provided with an electrode system 12, said 
feedthrough being made from a molybdenum tube of which the inside diameter 
and thickness are 1.0 mm and 0.2 mm respectively. Further, the process 
comprises providing two kinds of mixtures of inorganic powders as a 
starting material, so-called dispersions, composed of alumina and doping 
material such as Y.sub.2 O.sub.3 and/or MgO, one-of said dispersions 
applying for the vessel body and the alumina used for this dispersion 
having a specific surface area ranging from about 5 m.sup.2 /g to about 10 
m.sup.2 /g, said other dispersion applying for the plug body and the 
alumina used for this dispersion having a specific surface area ranging 
from about 3 m.sup.2 /g to about 5 m.sup.2 /g. Said dispersions are formed 
into two kinds of green bodies (vessel- and plug-shaped).The difference in 
linear shrinkage (.DELTA.L/Lo(%)), which is the difference in length 
between the green body and the sintered body, .DELTA.L, divided by the 
length-of the green body, Lo, between said two green bodies is preferably 
about 3 to 5%. For example, said vessel-shaped green body has a linear 
shrinkage of 21 to 24% and said plug-shaped green body has a linear 
shrinkage of 17 to 20%. The bonding portion 9 of the vessel-shaped body 
has an inside diameter of 4.00 mm and the plug-shaped green body has an 
outside diameter of 3.96 mm, a height of 6.0 mm and an axial hole diameter 
of 1.56 mm. The process further comprises prefiring or presintering the 
said bodies in an air atmosphere at a temperature ranging from about 
1000.degree. C. to about 1400.degree. C. to eliminate impurities including 
shaping aids and water, positioning the current feedthrough 10 into the 
axial hole of said preferred plug body, inserting said positioned body 
into a bonding portion in each end of said prefired vessel body, and 
sintering said assembled body in an atmosphere of hydrogen or in vacuum at 
a temperature ranging from about 1750.degree. C. to about 1900.degree. C. 
for 3 to 5 hours producing a sintered discharge vessel body directly 
sealed current feedthrough, said discharge portion of the body having an 
optical translucency which light or radiation in the visible wavelength is 
able to pass through sufficiently, said bonding portions inside diameter 
of the vessel body shrinking more than the outside diameter of the plug 
body, and-also said axial hole diameter of the plug body shrinking more 
than the outside diameter of the current feedthrough, but said bonding 
portion of the vessel and direct sealing portion of the plug 
slightly-deforming about the plug and the current feedthrough as is known 
in the prior art, and resulting in said sintered body having a perfect 
gas-tightness at the interfaces of the vessel to plug bonding portion 31a 
and at the plug to current feedthrough direct sealing portion 32a. 
In preferred embodiment, the example is slightly modified in that a 
cylindrical plug 11 of composite material is used, consisting of alumina 
and tungsten of respectively 80% and 20% by weight. The dimensions are the 
same as already discussed above. The manufacturing process is essentially 
the same as discussed above with the following exceptions. The dispersion 
applying for the plug body is composed of alumina and tungsten, the 
alumina having a specific surface area of about 3 to 5 m.sup.2 /g and the 
tungsten having an average particle size of less than one micron, the 
weight ratio of said alumina/tungsten being 80/20. It has to be pointed 
out that such a composite body cannot be considered as a cermet because it 
does not have the typically small resistance of a cermet, For example 20 
m.OMEGA.. On the contrary, the resistance air the composite body is 
advantageously very high (typically, 10.sup.10 .OMEGA.), so that the 
composite body is nonconducting and hence the undesired back-arcing after 
ignition is avoided. Again, the two dispersions are formed into two kinds 
of green bodies (vessel- and plug-shaped). The difference in linear 
shrinkage and the dimensions also can be the same as discussed above. In 
contrast to the basic example, only the vessel-shaped body is prefired in 
air atmosphere at a temperature of about 1,000.degree. C. to 1,400.degree. 
C. to eliminate impurities including shaping aids and water. On the other 
hand, said plug-shaped body is prefired in air atmosphere at a temperature 
of less than 300.degree. C. to prevent the oxidation of the tungsten 
component and to remove shaping aids and water prior to the real 
presintering in a hydrogen atmosphere at a temperature of 1,200.degree. C. 
to 1400.degree. C. By this real presintering, the axial hole diameter of 
the plug-shaped body shrinks to about 1.45 min. 
The process further comprises, as already discussed, positioning the 
current feedthrough 10 in the axial hole of the said presintered plug 
body, inserting the said positioned body into a bonding portion in each 
end of the prefired vessel body, and sintering the assembled body in an 
atmosphere of hydrogen or in vacuum at a temperature of about 1750.degree. 
C. to 1900.degree.C. for 3 to 5 hours. The resulting gas-tightness of the 
bonding portion 31a and sealing portion 32a is especially good. 
In another embodiment which is shown schematically in FIG. 3, the plug 11 
is also sintered onto the tube 18. The tube 18 is gas-tightly closed on 
the side facing the discharge in that the electrode shank 13 is welded 
into the open end of the tube 18. The internal support member 19, whose 
height is approximately the same as the height of the plug, is tightly 
fitted into the tube 18--the tolerance being about 50 .mu.m--and thus 
forms an opposition during the shrinking process of the plug 11 which 
ensures a strong, gas-tight contact between tube 18 and internal support 
member 19. 
In order to facilitate the positioning of the internal support member in 
the feedthrough, a stop for the internal support member may be used. This 
can be, in the simplest case, an annular spring element of refractory 
material which is placed in the cylindrical tube. As shown in FIG. 3, an 
extension 25 of the internal support member serving as a spacing member 
which rests on the shank 13 of the electrode is particularly suitable. 
In a modified version of this embodiment (FIG. 4) the tightness is further 
improved in that the internal support member 20 is formed as a hollow 
cylinder and has a height of 3.5 mm which is shorter than the height of 
the plug 11, with the hollow cylinder being located at about the middle 
with respect to the height of the plug. During the shrinking process of 
the plug, inwardly extending bulges 21 are formed in the tube 18, which 
bulges extend from the edges 22 of the internal support member to the 
height of the front faces 23 of the plug. The reason for this is that 
there is no resistance of the internal support member during the shrinking 
of the plug ceramics in these regions. The bulges are shown at an enlarged 
scale since, in reality, they can hardly be seen with the naked eye. The 
seat of the plug and the tightness of the feedthrough 18 both on its 
outside and on its inside are thus additionally improved. 
In this version the hollow cylinder 20 can be used as an exhaust chuck if 
the tube 18 is formed with an opening 18'. After evacuation and filling, 
the hollow cylinder 20 is closed by a suitable ceramic sealing material 24 
in well known manner. 
A further possibility which can be used more specifically with an internal 
support member whose length is reduced with respect to the length of the 
plug is shown in FIGS. 5 and 6. The stop is formed by a conical central 
portion 26, respectively 27, of the tube 28, respectively 29, at which a 
corresponding conical end portion 30, respectively 31, of the internal 
support member 32, respectively 33, abuts lit does not matter whether the 
conical portion is located on the side facing the discharge (FIG. 5) or on 
the side facing away from the discharge (FIG. 6) of the feedthrough. In 
both cases, the plug 11 is also provided with the respective inclined 
portions 34, 35. With these partly conical variants, the internal support 
member 33 may be offset with respect to the plug towards the side remote 
from the discharge, or may even project beyond the front face of the plug. 
The securing of the internal support member can be carried out in 
accordance with both the techniques shown heretofore (FIGS. 2, 
respectively 3). 
Embodiments having particular advantages are shown in the FIGS. 7 to 9. An 
entirely conical internal support member is inserted in the conical 
central portions 27 of the tube 29, offset towards the side remote from 
the discharge. 
The internal support member can again be solid (FIG. 7) as a truncated cone 
36, or tubular with conical inner walls (36' in FIG. 8) or also with 
straight inner walls (36" in FIG. 9). By such an arrangement, the 
advantages of a stop may be ideally combined with the reduced requirements 
for the tolerances to be observed. 
The embodiment of FIG. 9 satisfies the extremely high requirements relating 
to tightness and, thus, long life. It corresponds substantially to the 
examples of FIGS. 7 and 8; however, here, a particularly reliable, joint 
between molybdenum tube 29 and conical internal support member 36" has 
been effected by friction welding. During this process, a joining layer 37 
having a thickness of but a few atom layers (shown exaggeratedly thick in 
FIG. 9 for the purpose of illustration) is formed between molybdenum tube 
and internal support member. The angle of inclination of the cone is here 
smaller than 10.degree., in order to keep the mechanical deformation of 
the originally straight molybdenum tube 29 as slight as possible. The 
inclined portions 3S of the plug have the same inclination. The end 
portion 38 of the tube with the enlarged diameter begins, in accordance 
with the method of manufacture, immediately at the base end 39 of the 
internal support member. 
The technique of the friction welding may also be used with the partly 
conical embodiments.