Electric lamp

Abstract

 The electric lamp has a quartz glass lamp vessel (1) in the wall (2) of which current lead-through conductors (3) are accommodated which comprise a molybdenum foil (5) to a first end portion (5') of which a molybdenum outer current conductor (6) is connected. A capillary space extends around the outer conductor (6) from an end face of a seal (8), in which space the first end portion (5') projects. The outer current conductor (6) has a coating of platinum and a skin (9) of platinum phosphide, the first end portion (5') has a skin (9') of molybdenum phosphide and the capillary space is closed by a borate glass (10). The skins and the borate glass enable to operate the lamp in a surrounding of a relatively high temperature without affecting the life of the lamp, which would otherwise occur as a result of oxidation.

Description

[0001] The invention relates to an electric lamp provided with a quartz glass lamp vessel which is closed in a vacuumtight manner and through whose wall current supply conductors extend to a electric element arranged in the lamp vessel,
   which current supply conductors each have a molybdenum foil and connected thereto at a first end portion thereof a outer current conductor made of molybdenum which issues from the lamp vessel to the exterior, and at a second end portion thereof a inner current conductor,
   which molybdenum foil is embedded in a seal of the lamp vessel and lies with its first end portion in a capillary space which extends around the outer current conductor up to a end face of the seal,
   at least the first end portion of the molybdenum foil and the outer current conductor having a protective coating against oxidation.

[0002] Such a electric lamp is known from US 3,798,058.

[0003] The known lamp has molybdenum foils ad outer current conductors which are coated with molybdenum phosphide or with molybdenum phosphate. This coating delays oxidation of the foils, but it renders a comparatively low upper limit for the seal temperature necessary in order to have a lamp achieve its calculated life.

[0004] US 3,211,826 discloses an electric lamp in which the capillary space around the outer current conductor is closed with borate glass. Borate glass has a comparatively low softening temperature of 350-450° C.
When the lamp seal is at a comparatively high temperature, at which oxidation of the molybdenum foil may shorten lamp life, borate glass is liquid and closes the capillary space around the outer current conductor, thus preventing penetration of oxygen to the foil. When the lamp cools down after extinguishing, the borate glass solidifies and contracts more strongly than the surrounding quartz glass. As a result, cracks develop in the borate glass through which oxygen can penetrate. Borate glass offers only a small delay of the oxidation in the case of lamps which are switched off each time after a short period of operation.

[0005] A major disadvantage of borate glass, such as lead borate glass, is that it is hygroscopic and thus does not counteract transport of moisture to the molybdenum foil, so oxidation in humid surroundings. Another disadvantage is that borate glass can attack molybdenum. US 3,211,826 accordingly expresses a preference for the use of platinum outer current conductors. These, however, are expensive.

[0006] EP 0,245,734 discloses a electric lamp whose capillary space is sealed off with lead borate glass comprising a few mole% bismuth oxide. This borate glass does limit the attack on molybdenum, but it is hygroscopic and exhibits cracks.

[0007] Antimony borate glass used in lamps according to US 4,493,944 for the purpose described above also has the disadvantages described above.

[0008] It is a object of the invention to provide a electric lamp of the kind described in the opening paragraph which offers a satisfactory protection against oxidation, and which renders possible operation at a comparatively high temperature during a comparatively long period.

[0009] According to the invention, this object is achieved in that the outer current conductor is coated with platinum and has a skin of platinum phosphide, at least the first end portion of the molybdenum foil has a skin of molybdenum phosphide, and the capillary space is closed off with borate glass.

[0010] The coating and the borate glass cooperate in protecting the molybdenum parts. The coating counteracts attacks on the molybdenum by the borate glass. The borate glass provides a diffusion barrier to oxygen at operational temperatures. Moisture which penetrates through the borate glass is kept separated from the molybdenum by the coating. Tropical cabinet tests have shown that, whereas borate glass provides no protection against moisture at all, the coating offers a for practical applications unlimited protection against moisture.

[0011] The electric element in the quarts glass lamp vessel may be a incandescent body in a inert gas which possibly comprises a halogen, or a pair of electrodes in a ionizable gas, for example in rare gas, for example with metal halide, and possibly with mercury. The term "quarts glass" is understood to mean a glass having a SiO₂ content of at least 95% by weight. The electric element may be accommodated in an inner, sealed quartz glass envelope.

[0012] The molybdenum phosphide skin may be obtained, for example, through vapour deposition of this chemical. It is very convenient, however, to form the molybdenum phosphide skin simultaneously with the platinum phosphide skin in that the current supply conductors are made to react with phosphine or with red phosphorus. The reaction may be carried out, for example, in vacuo after the lamp has been completely assembled.

[0013] The electric lamp with the coated current supply conductors may be provided with the borate glass in that, for example, a ring of this glass is provided around the external current conductor and the glass is heated to above its melting point, whereby it is sucked into the capillary space.

[0014] Borate glasses generally have a comparatively low melting point of 350-450° C because SiO₂ is absent or is present in a quantity up to a few % by weight only. They consist mainly of B₂O₃ and PbO and/or Sb₂O₃ and may possibly comprise up to a few % by weight of other oxides or of fluorides, such as those of alkali or alkaline earth metals, and Bi₂O₃.

[0015] Embodiments of the electric lamp according to the invention are shown in the drawing, in which

Fig. 1 shows a first embodiment in side elevation;

Fig. 2 shows a detail of Fig. 1 on an enlarged scale; and

Fig. 3 shows a second embodiment in axial cross-section.

[0016] In Fig. 1, the electric lamp has a quartz glass lamp vessel 1 which is closed in a vacuumtight manner and through whose wall 2 current supply conductors 3 extend to an electric element 4 arranged in the lamp vessel. The electric element in the Figure is a tungsten incandescent body in a gas comprising halogen.
The current supply conductors 3 each have a molybdenum foil 5 and connected thereto at a first end portion 5' thereof an outer current conductor 6 of molybdenum which issues from the lamp vessel to the exterior, and at a second end portion 5'' thereof a inner current conductor 7, for example made of tungsten or molybdenum, which is connected to the incandescent body.
The molybdenum foil 5 is embedded in a seal 8 of the lamp vessel and lies with its first end portion 5' in a capillary space which extends around the outer current conductor up to a end face 8' of the seal.
At least the first end portion 5' of the molybdenum foil 5 and the outer current conductor have a protective coating against oxidation.

[0017] The outer current conductor 6 is coated with platinum and has a skin 9 of platinum phosphide. At least the first end portion 5' of the molybdenum foil 5 (see Fig. 2) has a skin 9' of molybdenum phosphide. The capillary space is closed off with borate glass 10, for example glass having a composition as listed in Table 1, for example composition 1.

Table 1

	PbO	B₂O₃	ZnO	Al₂O₃	CoO	Sb₂O₃	Bi₂O₃	SiO₂	BaO
1*	90	10
2*	77,2	13,3	2,0	5,5	0,5			1,5
3*	80,0	16,0	4,0
4*	79,3	15,8	4,0		0,9
5*	5,0	30,0				65,0
6#	rest	25-40					3-10		0-15

* composition in % by weight

# composition in mole%

[0018] The lamp shown is a infrared radiator suitable for use as a heat source in an oven. The lamp vessel 1 is tubular ad has a seal 8, 18 at either end. The seal 18 has already been provided with a lamp cap 19 from which a cable 20 emerges so that the lamp can be connected at a distance to terminals of a supply source. The outer current conductor 6 is constructed as a twin conductor for obtaining a mechanically strong coupling to the cable and a comparatively low current density therein.

[0019] The lamp shown has a long life also at a high ambient temperature of, for example, 600° C.

[0020] The molybdenum foil 5 and the outer current conductor 6 may be provided with their coatings separately and subsequently be connected to one another. It is also possible first to weld them together and then to provide them with their coatings. A modification of this is that the lamp is first fully assembled mechanically and the foil and the conductor are subsequently provided with their coatings, for example phosphide skins, through the capillary space. Obviously, only those portions can be coated then which lie in the capillary space or outside the lamp vessel. However, it is these very portions which can come into contact with oxygen and/or moisture ad can be subject to corrosion. In Figs. 1 and 2, the outer current conductor ad the foil were provided with their phosphide skins before being embedded in the quartz glass.

[0021] In Fig. 3, parts corresponding to parts in the preceding Figures have reference numerals which are 20 higher than in those Figures.

[0022] The lamp vessel 21 has a pinch seal 23 through which two current supply conductors 23 are passed. The first end portions 25' are coated with molybdenum phosphide. The outer current conductors 26 have a coating of platinum with a platinum phosphide skin thereon. The capillary spaces around the current conductors 26 are closed off with borate glass 30.

[0023] The lamp vessel 21 is fixed in a reflector body 42 with cement 41, the reflector body having a reflective coating 43 at its inner surface and being closed off with a plate 44. The lamp/reflector unit shown may be used for creating accent lighting.

Claims

1. An electric lamp provided with a quartz glass lamp vessel (1) which is closed in a vacuumtight manner and through whose wall (2) current supply conductors (3) extend to an electric element (4) arranged in the lamp vessel,
   which current supply conductors (3) each have a molybdenum foil (5) and connected thereto at a first end portion (5') thereof a outer current conductor (6) made of molybdenum which issues from the lamp vessel to the exterior, and at a second end portion (5'') thereof an inner current conductor (7),
   which molybdenum foil (5) is embedded in a seal (8) of the lamp vessel and lies with its first end portion (5') in a capillary space which extends around the outer current conductor up to a end face (8') of the seal,
   at least the first end portion (5') of the molybdenum foil (5) ad the outer current conductor having a protective coating against oxidation,
   characterized in that the outer current conductor (6) is coated with platinum and has a skin (9) of platinum phosphide, at least the first end portion (5') of the molybdenum foil (5) has a skin (9') of molybdenum phosphide, ad the capillary space is closed off with borate glass (10).

Drawing