Method of manufacturing an article containing at least one glass part in which a metal part is sealed in

A method of making glass beads for current leading-in wires of lamps by internal and external high-frequency heating of a glass tube which surrounds a metal rod.

The invention relates to a method of manufacturing an article containing at 
least one glass part in which a metal part is sealed in. The invention 
also relates to an apparatus for carrying out such a method and to 
articles, for example lamps, which are manufactured by means of such a 
method. 
The metal part may be rod-shaped, filamentary or disc-shaped; the metal 
part may also be hollow, for example be in the form of a tube. 
For many articles, in particular lamps such as gas discharge lamps and 
incandescent lamps, it is necessary to seal metal parts in a gastight 
manner in a glass part, in particular the wall. Successful performance of 
such a sealing operation depends upon many factors. Some of these factors 
are the values of the coefficients of expansion of the parts to be joined, 
the atmosphere in which sealing-in takes place and in particular the skill 
of the person required to make the seal. 
In the literature many directions are found for making seals which are 
gastight and do not crack during use of the articles. In the methods 
described widely different points are mentioned which are to be heeded in 
making the seals. It was found in practice that a good seal is obtained 
only by a particularly fortunate combination of the various directions. 
Obviously it is necessary to pay attention to all the aforementioned 
points, but in addition requirements are to be satisfied which do not 
concern the seal itself but do concern the use of the article to be 
manufactured. For example, for some lamps special metals, such as tungsten 
and molybdenum, or special wall materials, such as fused silica or hard 
glass, must be used. All these circumstances together result in that from 
the literature no universal method is known which can successfully be used 
under widely different conditions. Hence frequently highly complex seals, 
for example using intermediate glasses, or complicated accurately defined 
consecutive stages of manufacture have been resorted to. 
In many cases manufacture starts from a filamentary or rod-shaped metal 
part to which first a glass bead is applied. This glass bead then is 
joined to the remainder of the article to be manufactured by means of a 
discrete step in which the joint is established by heating. 
The glass bead may be applied to the metal part in various manners, for 
example by heating the metal part and then bringing it into contact with a 
glass rod which is made plastic, for example by means of a burner. It will 
be understood that the term "plastic" herein refers to a physical 
characteristic whereby the material may be deformed without rupture. 
Rotating the metal part enables a bead to be applied, the success of the 
method being highly dependent upon the aforementioned circumstances. 
Another method of applying a bead starts from a glass tube which 
approximately fits around the metal part; by heating, for example by a 
burner, or by electric heating the tube is sealed to the metal part. 
An important disadvantage of the known methods further is that some 
combinations of metal and glass which are desirable with a view to the use 
of the article to be manufactured cannot, or cannot readily, be 
reproducibly realized. 
A large disadvantage attendant on many of the known methods further is that 
during the making of the bead the surface of the metal is oxidized when 
the operation is performed in air. This can be avoided by making the bead 
in a shielding gas; however it will be evident that supplying the 
shielding gas complicates the operation and in some methods can hardly 
furnish success. In this connection we have in mind the aforementioned 
method in which the end of a glass rod is heated and is pressed onto a 
rotating metal part. In this process shielding gas may be supplied through 
a pipe near the bead to be made, but even then oxidation cannot completely 
be avoided with certainty. Consequently the metal adjoining the bead will 
oxidize, which often is highly undesirable for use, in particular in 
lamps. Hence it always has been necessary after the making of the bead to 
remove the oxide film from the metal part by grinding or etching. This 
means, however, that the metal part at the relevant location becomes 
thinner, which afterwards may readily give rise to fracture of this part 
and in addition jeopardizes the gas-tightness, because the metal inside 
the bead, in particular at it ends, may be attacked. If in operation the 
metal part is required to carry an electric current, additional heat will 
be developed at the location of the thinner portion so that the risk of 
cracking is further increased at this very location. 
The method according to the invention of manufacturing an article 
containing at least one glass part in which a metal part is sealed is 
characterized in that the article is manufactured by shaping the glass 
part in the form of a tube which internally has about the same shape and 
cross-section as the metal part, the tube is slipped onto the metal part 
and the resulting assembly is heated by passing it through a 
high-frequency electric field so as to produce a melting zone. In this 
zone additionally a non-short-circuited electrically conducting winding is 
located through which the assembly is passed, the heating procedure being 
effected in a non-oxidizing shielding gas. 
Compared with the known methods a method according to the invention entails 
many advantages. Some of these advantages are given in the following list: 
1. the method can generally be applied to widely different materials; 
2. the method may readily be automated so that success does no longer 
depend upon high skill of the operator; 
3. the seal obtained is gas-tight and remains so during use of the article; 
4. 4. the resulting seal there is a uniform build-up of stresses so that 
cracking is reduced to a minimum; 
5. the cross-sectional area of the metal part is not decreased. 
Although some elements of a method according to the invention are to be 
found in the literature, the above-mentioned and other advantages are 
obtained only by the combination of steps according to the invention.

Referring now to FIG. 1, reference numeral 1 denotes a metal rod-shaped 
part made, for example, of tungsten; a glass bead 2 is so secured to this 
metal rod as to produce a gastight joint between the rod and the bead. 
Such an assembly may be used, for example, as a current lead-in for a wide 
variety of lamp types. For this purpose other glass parts, for example a 
lamp bulb, may be secured to the bead 2. 
The apparatus shown in FIG. 2 comprises the following component parts. 
Reference numeral 3 denotes a large-bore tube made, for example, of glass 
or fused silica and surrounded by a high-frequency coil 4. By means of 
this coil 4 a melting zone is produced in the tube 3, i.e. a zone in which 
the material of the bead to be made can be heated to a temperature such as 
to become appropriately plastic. A non-short-circuited or open winding 5 
of helical form is placed in the tube 3. The winding 5 is disposed at the 
location of the melting zone and will be understood to be a coil in which 
the ends are not connected. If required by the nature and the size of the 
materials to be worked the coil 5 can be axially displaced. In the drawing 
it is located at the left-hand end of the high-frequency coil 4. As will 
be explained more fully hereinafter, this is a preferred location for 
making a high-quality bead. 
The bead is made from a glass tube 7 slipped onto a metal rod 6. The glass 
tube 7 fits fairly closely around the rod 6. The rod is clamped in a chuck 
8. The chuck is capable of rotation and of axial displacement. At its 
right-hand end the tube 3 has an opening 9. Through this opening a 
shielding gas can be supplied to flow in the direction indicated by the 
arrows. 
Making the bead is effected as follows. While the gas stream is passed 
through the tube 3 and the high-frequency field is generated by means of 
the coil 4 the rod 6 together with the tube 7 it carries are moved from 
left to right. Preferably the rod 6 together with the tube 7 is also 
rotated, but this is not always necessary to obtain a high-quality bead. 
The rod 6 is heated by the high-frequency field generated by the coil 4. 
The rod transfers heat to the inner surface of the tube 7. At the instant 
at which the assembly of 6 and 7 enters the winding 5 the tube 7 is heated 
externally as well, for the winding 5 is also heated by the high-frequency 
field. The winding 5 must not be short-circuited, because otherwise it 
would be melted by the heat developed. Thus in the melting zone the glass 
tube 7 is heated from the inside and from the outside. Owing to the 
displacement in the direction of length from left to right the right-hand 
part of the tube 7 will first become plastic and adhere to the rod 6. This 
instant is shown in FIG. 3. At this stage the tube is still open at the 
left so that impurities evolved from the rod and/or the glass tube 7 can 
escape to the left. FIG. 4 shows the instant at which more than half of 
the glass tube adheres to the rod 6. Finally FIG. 5 shows the instant at 
which the entire bead is finished. The high-frequency field can then be 
switched off and the rod 6 provided with the bead can be retracted. 
Preferably the gas stream is maintained during retracting also. 
Because the entire operation of making the bead is performed in a shielding 
gas there is substantially no likelihood of oxidation of the metal. Hence 
the finished bead is entirely free from oxide and need not be etched. 
Because, as mentioned hereinbefore, no impurities can collect between the 
rod and the glass part of the bead, the joint between the glass and the 
metal is complete and hence gastight. 
By means of a method according to the invention widely different vitreous 
materials and metals can be joined to one another. However, the choice of 
the non-oxidizing shielding gas must be adapted to the materials used, in 
particular to the glass used. It was found, for example, that when fused 
silica parts are used the shielding gas atmosphere must be neutral. For 
fused silica a suitable shielding gas is, for example, nitrogen. If when 
working fused silica in making a bead a weakly reducing gas is used, for 
example a so-called incombustible mixed gas, i.e. a mixture of 8% of 
hydrogen and 92% of nitrogen, a dark-coloured film is formed between the 
metal part and the fused silica which prevents adherence. When using a 
neutral shielding gas, for example nitrogen, a completely smoooth tightly 
adherent bead is obtained. 
When using normal glasses, either hard glass or soft glass, it generally is 
desirable for the shelding gas to be mildly reducing. In such a case 
incombustible mixed gas can be used to advantage. 
By means of a method according to the invention it is possible inter alia 
to make glass beads as thoriated tungsten filaments. Hitherto this has not 
been possible by any of the known methods, because always the thorium 
diffused to the surface of the tungsten, preventing close gas-tight 
adherence. However, for many gas discharge lamps thoriated tungsten is a 
highly attractive electrode material in view of its emitting properties. 
Hitherto when using such a material the lead-in has had to be made of 
tungsten whereas the portion inside the discharge space was allowed to 
consist of thoriated tungsten, which portion was secured to the tungsten 
lead-in member, for example, by welding. 
By means of a method according to the invention lead-ins of widely 
different sizes were made which when used in lamps did not provide 
difficulty in respect of cracking, breaking off etc. The diameter of the 
current leading-in wire can be chosen from a few tenths of millimeters to 
more than two centimeters. Such a leading-in wire may, if required, be 
formed with one or more channels. Thus they may be cooled, for example, by 
a stream of a liquid or gas. 
FIG. 6 shows a high-pressure xenon arc lamp comprising a hard-glass bulb 20 
accommodating a tungsten rod 21 and a thoriated tungsten rod 22. The 
electrodes 21 and 22 each are made in one piece and are secured in a 
gas-tight manner in the wall 20 by means of beads made in a separate 
operation. The beads are designated by 25 and 26. The glass of the beads 
25 and 26 is the same as that used for the bulb 20. No intermediate 
glasses are required. 
It should be mentioned that the high-frequency coil 4 may extend beyond the 
melting zone in order to preheat a glass and/or metal part. However, 
outside the melting zone heating must not become so intense as to make the 
glass plastic at this location. 
The non-short-circuited electrically conducting winding need not be 
helical, although this shape is attractive, for the pitch and/or the 
diameter of a helical winding may be made different at different locations 
and hence heat generation and heat transfer may be varied. This enables, 
for example beads to be applied by starting from tubes which are 
considerably wider than the cross-sectional area of the metal part.