EP0410511A1

EP0410511A1 - Electric lamp

Info

Publication number: EP0410511A1
Application number: EP90201945A
Authority: EP
Inventors: Tjepke Hendrik Ekkelboom; Karel Martinus Van Der Waarde
Original assignee: Philips Gloeilampenfabrieken NV; Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 1989-07-24
Filing date: 1990-07-18
Publication date: 1991-01-30
Also published as: HU904566D0; JPH0364852A; HUT54433A

Abstract

The electric lamp has a quartz glass lamp vessel (1) with tungsten current supply conductors (3) extending to an electric element (2) arranged in the lamp vessel (1). The current supply conductors (3) have a continuous coating (4) of quartz glass which extends from the interior to the exterior of the lamp vessel (1) and contain rhenium at least at their surfaces. The coating (4) has a very strong adhesion to the metal; the lamp vessel (1) has a great strength.

Description

The invention relates to an electric lamp comprising:

a lamp vessel sealed in a vacuum-tight manner and consisting of glass having an SiO_z content of at least 95% by weight,
an electric element arranged within the lamp vessel,
current supply conductors extending through the wall of the lamp vessel to the electric element,
at least one current supply conductor made of tungsten with a continuous coating of glass having an Si0₂ content of at least 95% by weight, which coating extends from the exterior to the interior of the lamp vessel, while the surface of the glass coating encloses with the coated surface of the current supply conductor at the points at which they meet an angle α of at most 90°.

Such a lamp is known from US 4,171,500-A.
In this known lamp, stringent requirements are imposed on the thickness of the coating. This thickness d must be so small that it complies with the formula D(D+2d)-'>0.7, where D is the diameter of the current supply conductor. The thickness of the coating is therefore allowed to be only at most 21 % of the diameter of the current supply conductor. Especially if this conductor must be thin, for example 0.7 mm or even 0.2 mm, therefore, only an extremely small thickness of the coating is admissible (at most 0.14 mm and 0.04 mm, respectively). In the preferred case mentioned in the said Patent Specification, in which D(D+2d)^-1≧ 0.85, i.e. d s 0.09 D, even a thickness of only 0.06 and 0.02 mm, respectively, is then admissible. This forms a serious drawback for the manufacture of the known lamp in mass production. It should be noted that the range of 0.2 to 0.7 mm is very usual for the thickness of internal current supply conductors welded to a metal foil embedded in the wall of the lamp vessel.
In the known lamp according to the aforementioned US 4,171,500-A, the coating must moreover be enclosed between its ends by a thicker envelope of similar glass. The necessity of this envelope forms a further limitation for the industrial application of the known lamp due to the additional fusion step necessarily ensuing therefrom.
The wall of the lamp vessel is fused with the said envelope in the known lamp, but in such a manner that the envelope has a surface extending parallel to the surface of the current supply conductor both inside and outside the lamp vessel. This results in that the current supply conductor is sealed into glass over a comparatively great length. An associated consequence is that in the lamp vessel around the current supply conductor there is a comparatively large space, which - due to its comparatively low temperature during operation of the lamp - can influence the light production of the lamp.
In substantially all types of electric lamps comprising a lamp vessel of glass having an Si0₂ content of at least 95% by weight, the current supply conductors are passed in a vacuum-tight manner through the wall of the lamp vessel in that the current supply conductors comprise a foil-shaped part of molybdenum, which is embedded in a pinched seal of the lamp vessel. In this construction, the foil-shaped part must be connected to a conductor extending into the interior of the lamp vessel and to a conductor extending from the pinched seal to the exterior, for which purpose welding connections must be made. The ohmic resistance of the foil-shaped part leads not only to electric losses, but also to a detrimental heat generation in the pinched seal. The current supply conductor, moreover, is a slack assembly which can be manipulated only with difficulty during the manufacture of the lamp and which makes it difficult to position accurately in the lamp vessel that portion which is to be located within said lamp vessel. The accuracy of positioning could be improved if the current supply conductor with a foil-shaped part could be held and continuously positioned also within the lamp vessel during the manufacture of a first pinched seal of the lamp vessel. A rigid current supply conductor would then have to be used during the manufacture of a second seal. Another disadvantage of lamps having a pinched seal is that the seal is destroyed at a comparatively low gas pressure of about 80 bar. In spite of these disadvantages, pinched seals are generally used in commercially available lamps. Short-arc discharge lamps constitute exceptions.
In short-arc discharge lamps, a construction is used in which the current supply conductor is sealed into glass having a comparatively high coefficient of expansion, which is connected via glasses having stepwise decreasing coefficients of expansion to the glass of the lamp vessel, which has a very low coefficient of expansion. This so-called "graded seal" obtained with the use of so-called "transition glasses" is expensive and can only be realized manually in most cases. Moreover, the construction occupies a large amount of space.
GB 2,064,216A discloses an electric lamp in which the current supply conductors have a continuous coating of a transition glass having a coefficient of expansion in the range of 11 - 17 x 10^-7K^-1. These glasses contain, besides about 81 - 87% by weight of Si0₂, also a comparatively large quantity of B₂0₃ and Ai203. Since these glasses have a comparatively low softening temperature, an embossed part must be formed on the surface of the pinched seal in which the coated current supply conductors are included in order to avoid that the coating of comparatively low viscosity is removed from the conductor by the quartz glass of the lamp vessel of comparatively high viscosity during the manufacture of the pinched seal. Consequently, the known lamp necessarily has a profiled seal, which may be disadvantageous when mounting the lamp vessel in a lamp cap. Moreover, the comparatively low Si0₂ content of the transition glass may involve the risk of giving way to attack by the gas filling of the lamp, while the maximum permissible temperature of the glass is only about 700°C.
The construction having a foil-shaped part and the construction having a graded seal are used because glasses having an SiO₂ content of at least 95% by weight, such as, for example, quartz glass and "Vycor", i.e. a glass containing 96% by weight of Si0₂, have a linear coefficient of expansion which is considerably smaller (in the range of about 4x10^-7K^-1 to about 12x10^-7K^-1) than that of tungsten (about 45xlo-⁷K-¹). This great difference in coefficient of expansion and the great difference between the softening temperature of the glasses and the operating temperature of the lamps on the one hand and room temperature on the other hand result in that tungsten cannot be included in a vacuum-tight manner in these glasses without special steps being taken.
For several decades attemps have been made to devise special measures by which tungsten could be sealed into glasses, such as quartz glass. The result of these efforts is that commercially available lamps in such glasses still have either a pinched seal with an embedded metal foil or a graded seal.
The construction according to the aforementioned US 4,171,500 is not used either. In spite of the mechanical strength which the construction according to this Patent Specification may have, the disadvantages mentioned with respect to this construction are apparently too serious. It has further been found that it is difficult to manufacture the construction described in a reproducible manner. It has been found that the reproducibility is associated with the extent to which a coating of, for example, quartz glass on the current supply conductors, which adheres to the conductors, can be obtained in a reproducible manner.
US 3,448,320 discloses an electric incandescent lamp having a tungsten current supply conductor of at most 0.1 mm thickness, which is directly sealed into the wall of a quartz glass lamp vessel. It is emphasized that no layer of impurities must be present on the tungsten conductor. The tungsten conductor is brought into a non-oxidezed state and is degased by heating at 1750 to 2200°C in nitrogen or rare gas. However, the lamp described is not commercially available. The maximum thickness of the conductor, furthermore, is too small for practical applications.
US 4,086,075 discloses a method of providing a vitreous coating on metal wires. The method consists in that a metal wire together with a glass tube tightly fitting around it is heated in a high-frequency field in a protective gas, such as nitrogen. The high-frequency field may be produced by a coil connected to a current source. A non-shortcircuited coil is present in the high-frequency field, which coil is heated, as is the metal wire, by the high-frequency field. They both heat the glass tube to its melting point. The coated wire is free from oxides and impurities have not been able to accumulate between the wire and the coating. By this method, according to the said Patent Specification, vitreous coatings can also be provided on wires of thoriated tungsten, which was not possible in prior methods because thorium oxide diffused to the surface of the wire and prevented a gas-tight adhesion of the glass to the wire. If a thoriated tungsten wire acting as an electrode was necessary, therefore, a butt weld had to be formed between the thoriated tungsten wire and a tungsten wire free of thorium oxide and the latter wire had to be provided with a glass coating.
The adhesion of a vitreous coating to a tungsten conductor apparently requires that the coating is provided on a tungsten conductor which is free from adsorbed gases, oxides and other impurities at its surface.
The invention has for its object to provide an electric lamp of the kind mentioned in the opening paragraph which has a very simple construction and can be easily manufactured in a reproducible manner and nevertheless has a great strength.
According to the invention, this object is achieved in that the tungsten current supply conductor contains rhenium at least at its surface.
It has been found that the presence of rhenium in the tungsten wire at the surface of the latter results in a strong adhesion between the glass and the wire and also to a good wetting of the wire by the glass. The latter becomes apparent in an. acute angle a.
The strength of the adhesion between the glass and the tungsten wire was shown inter alia by the following experiments with the coated wires as listed in table 1.
The coated wires were heated to 800° C by current passage under nitrogen. After cooling down to room temperature, the cycle was repeated 1000 times without damage to the coating.
The wires were dipped in liquid nitrogen coming from room-temperature surroundings without damage to the coating.
The coating was completely ground off the wires at one side. The coating remained fully intact everywhere else. The wires were heated to 1900° C in protective gas. After cooling down to room temperature the wires had become bent. The wires were concave at the ground-off side owing to the stronger contraction of the metal compared with the glass. The coating remained intact during this.
The ratio of the thickness d of the glass coating to the diameter D of the coated wire is between 0,42 and 0,5 for the coated wires listed in table 1.
The lamp according to the invention can be obtained in a simple manner. To this end, at least one current supply conductor coated with glass, for example quartz glass, is sealed into a lamp vessel made of, for example, quartz glass. The coating on the current supply conductor may be obtained, for example, by the application of a dispersion of rhenium or a compound thereof, such as an oxide, a salt, such as, for example, a nitrate, chloride, acetylacetoxate, on a tungsten wire, heating of the wire to above the melting point of the glass, for example to approximately 2200° C, upon which rhenium compounds are decomposed, and fusion with the wire under a protective gas, such as, for example, nitrogen or a rare gas, or in vacuum, of a glass such as, for example, quartz glass, which is applied around the wire in the form of, for example, a tube.
It is also possible to start with a conductor made of tungsten which contains rhenium as an additive, for example a conductor of tungsten containing approximately 1% by weight or more of rhenium. In that case rhenium can be brought to the surface of the conductor in that the conductor is oxidized at raised temperature, for example 600° C or higher, for example approximately 1200° C, for example by exposing the conductor to the air at raised temperature and then heating it in a protective gas, such as a rare gas or nitrogen, or in vacuum, up to a temperature above 1400° C, for example to approximately 1600° C. Tungsten oxide then evaporates and rhenium remains at the surface. The surface has an irregular structure, as was apparent from observations with an electron microscope. The conductor is then provided with a coating of, for example, quartz glass, for example in that the conductor with an envelope consisting of a quartz glass tube is heated in a high-frequency field. This method of application has been described above in connection with US 4,086,075. However, a ring may be used in the high-frequency field instead of a non-shortcircuited coil. Electron-microscopic observations have shown that the glass of the coating follows the irregular surface of the tungsten wire, so that an interlocking configuration of glass and metal is the result. This gives a very good connection between these materials.
No rhenium was found in the glass coating in a Scanning Electron Microscope (SEM) through Energy Dispersive Analysis by X-rays (EDAX). Each cross-section did show a higher rhenium concentration at and just below the surface of the tungsten wire compared with the core of the wire. A W Re 1 % wire of 550 µm diameter becomes approximately 16 u.m thinner by oxidation and evaporation. This means that the wire is enriched by a very thin rhenium layer of approximately 0,08 µm at its surface, i.e. by approximately 170 u.g per cm². Such a quantity of rhenium applied on the surface of a pure tungsten wire of 550 u.m diameter means a rhenium content of the modified wire of 0,1% by weight.
If in this manufacturing method a coating must be provided on a comparatively thin conductor having, for example, a diameter of 0.2 mm, it is recommendable to use a glass tube having a small wall thickness of, for example, 0.1 mm. With the use of a tube having a comparatively large wall thickness, the inner side of this tube would not be heated to a sufficiently high temperature by the comparatively thin conductor by means of irradiation. It may then be recommendable to supply heat to the conductor also by direct current passage or by means of a laser. Alternatively, a thin coating may be provided first in a high-frequency field in an environment of rare gas or nitrogen which can be readily maintained with the use of this heat source, possibly in a slightly reducing environment by the addition of a few to a few tenths of % by volume of hydrogen, or in vacuo, and then a thickened glass portion may locally be formed, for example, by means of a burner on this coating. For this purpose, a glass tube may be slipped around the coating and be fused with the coating, for example, by heating with a flame. A comparatively thick coating or a local thickening of the coating may be of importance for readily processing the coated conductor into a portion of a lamp.
It has been found that it is of minor importance for the stability and the quality of the lamp whether the coating of the conductor is thin. It has been found that comparatively very thick coatings also adhere excellently to the conductor without any substantial tensile stress at the surface. It has further been found that coatings on comparatively very thick conductors are of high quality and very stable.
The angle α between the surface of the glass coating and the coated surface of the current supply conductor is at most 90°, but is generally smaller in the lamp according to the invention since the glass wets the metal well. This is the condition for avoiding tensile stresses at the surface.
Although a tungsten current supply conductor can be used, in which rhenium is applied on the surface of a tungsten wire from a suspension, for example, or by vapour deposition, or by some other method, and is then made to penetrate this wire upon heating thereof in order to impart to the wire a rhenium content at and just below the surface, it is attractive to use a wire made of tungsten-rhenium. This is because a tungsten-rhenium wire has the advantage of being ductile, not brittle, so that the wire and lamps comprising the wire can be easily handled and processed. Rhenium is concentrated in small areas throughout the entire wire, however, in commercially available tungsten-rhenium wires. The preparatory treatment consisting of heating, oxidation, and evaporation of tungsten oxide by heating, which precedes the glass coating treatment when these wires are used, is necessary then in order to obtain an even concentration of rhenium at the surface. Since rhenium oxides have a relatively low melting point, and are all dissociated at a temperature of 1000° C, the rhenium can flow out over the wire during this preparatory treatment, so that an even distribution of rhenium over the surface is obtained after dissociation of rhenium oxides.
The electric element of the lamp according to the invention may be a pair of electrodes, possibly surrounded by an inner envelope. The pair of electrodes may be constituted by the inner free ends of the current supply conductors. The inner free ends may have, for example, a thickened portion or a wrapped part or an electrode head may be fixed thereto. The electric element may alternatively be an incandescent body, for example a filament in a halogen-containing gas mixture.
The current supply conductors generally have a thickness in the range of 0.2 to 0.7 mm, but smaller thicknesses of, for example, 0.17 mm, for example with discharge lamps of low power, for example about 35 W, or larger thicknesses, for example 2 mm, for example with short-arc discharge lamps, may be used. In general, current supply conductors will have a thickness in the range of 0.4 to 0.7 mm.
The construction of the lamp according to the invention with coated tungsten current supply conductors is of particular importance for small discharge lamps because of the possibility of accurately positioning the electrodes due to the fact that the current supply conductors are rigid as compared with current supply conductors with a foil-shaped part.
The construction with coated tungsten current supply conductors is also of particular importance for small incandescent lamps, in which the lamp vessel has a very small diameter and the lamp vessel must be capable of withstanding a high filling pressure and hence a very high operating pressure, and in which the filament is centered comparatively accurately in the narrow lamp vessel. Such an incandescent lamp has a tubular lamp vessel having an inner diameter in the range of about 2 to 6 mm, and a gas filling having a pressure at room temperature in the range of 8 to 60 bar, the gas filling mainly consisting of a gas chosen from xenon, krypton and xenon/krypton mixtures possibly containing 2.10-^8- 12.10-⁷mole Hal/cm³, where Hal is chosen from Br, CI and Br/Cl mixtures. The incandescent body has a colour temperature of at least 3300 K during operation at nominal voltage.
Due to the comparatively high pressure in the lamp and to the comparatively narrow lamp vessel, the filament can be operated at a comparatively high colour temperature, while the lamp nevertheless has a comparatively long life. The incandescent lamp is particularly suitable for use in optical systems.
Current supply conductors having a diameter of 0.55 mm were provided by each of the following methods with a coating of glass having an Si0₂ content of at least 95% by weight in order to render them suitable for sealing into a lamp vessel consisting of such a glass. Very satisfactorily adhering coatings were then obtained, which satisfy stringent requirements.
A wire of tungsten containing 3% by weight of rhenium uniformly distributed therein was heated at 1200°C and exposed to air. The tungsten oxide then formed was subsequently evaporated at about 1600° C in an inert environment. After a tungsten skin had been removed in this manner, rhenium was left at the surface. A tube of quartz glass having a length of 15 mm and a wall thickness of 0.275 mm was fused with the wire.
A wire of tungsten doped with 0.01% by weight of K, Al, Si in all, a usual dopant to control the crystal growth in tungsten wire, was immersed in a suspension of 10 mg of rhenium oxide in 0.5 ml of water. The wire was provided with a coating of quartz glass in a corresponding manner.
The use in lamps of tungsten having a rhenium content as an incandescent body or as a support for an incandescent body is known from US 3,236,699-A. Rhenium gives a high degree of ductility to tungsten. Tungsten has a very high ductility at a rhenium content of 3% by weight, but ductility is also high in the range from 1 % to 10%.
Since rhenium is a relatively expensive metal, it is advisable to use material having a relatively low rhenium content, for example up to 3% by weight, for wires made of tungsten-rhenium to be used as current supply conductors in the lamp according to the invention. Tungsten with 3% rhenium by weight as a commercial product is especially suitable for this. The invention, however, also relates to lamps having current supply conductors with a relatively high rhenium content of 20 or 26% by weight.
The glass of the glass coating adjoining the surface of the current supply conductors, and possibly also the current supply conductor itself, may contain an element from the group consisting of thorium, hafnium, chromium, aluminium, titanium, tantalum, magnesium, calcium, strontium, barium, zirconium, lanthanum, scandium, lanthanides, niobium, boron, and yttrium, as described in the non-prepublished European Patent Application 89 200 389 (PHN 12.757). Such an element, however, does not have an additional effect.
An embodiment of the lamp according to the invention is shown in the drawing.
In the drawing:

Fig. 1 is a side elevation on a strongly enlarged scale of an incandescent lamp with a diagrammatically indicated incandescent body,
Fig. 2 is a side elevation of a discharge lamp,
Fig. 3 is a side elevation of an incandescent lamp with a diagrammatically indicated incandescent body,
Fig. 4 is a cross-section taken on IV-IV in Fig. 1.

In Fig. 1, the electric incandescent lamp has a lamp vessel 1 sealed in a vacuum-tight manner and consisting of glass having an Si0₂ content of at least 95% by weight. A tungsten incandescent body 2 is arranged in the lamp vessel 1 as an electric element. Current supply conductors 3 made of tungsten extend opposite to each other through the wall of the lamp vessel 1 to the incandescent body 2. Respective circumferential coatings 4 of glass having an Si0₂ content of at least 95% by weight are disposed on the current supply conductors 3. The coating 4 extends from the exterior of the lamp vessel 1 to the interior of the lamp vessel. The surface 6 of the coating 4 and the coated surface of the current supply conductors 3 enclose at the area at which they meet an angle a of at most 90°. The tungsten current supply conductors 3 contain rhenium at least at their surfaces.
In the lamp shown, the lamp vessel 1 and the coatings 4 consisted of quartz glass.
The current supply conductors 3 consist of tungsten containing 3% by weight of rhenium and have a diameter of 0.55 mm. They are provided with a coating 4 having a thickness of 0.275 mm. The current supply conductors 3 are screwed into end turns of the incandescent body 2, which has an outer diameter of 1 mm. The lamp vessel 1 has an inner diameter of 3 mm and is filled with 55 bar xenon, to which 7 mbar CH₂Br₂ is added, i.e. 2.24 x 10-⁷mole Br/cm³. Upon heating at 800°C in a furnace, the gas pressure increases to about 200 bar, which corresponds to the operating pressure of the lamp. The lamp consumes a power of 55.6 W at 12.1 V and has a colour temperature of 3360 K. The lamp may be used, for example, in motor vehicle headlights.
In Fig. 2, parts corresponding to parts of Fig. 1 have reference numerals which are 10 higher.
The current supply conductors 13, consist of tungsten containing 1% by weight of rhenium and have a diameter of 0.25 mm with a fused tip 12 in the lamp vessel 11. The fused tips 12 as a pair of electrodes constitute an electric element. The coatings 14 have a thickness of 0.125 mm, while a quartz glass ring 17 is provided thereon and fused thereto. The lamp vessel 11 has an inner length of 7.8 mm and an inner diameter of 2.7 mm. The lamp vessel 11 is filled with 6 bar xenon (at 300 K), 0.6 mg of mercury and 0.4 mg of an Nal/Scl₃/TII/Thl₄ mixture. The lamp consumes a power of 35 W at a voltage of 85 V and may be used, for example, as a light source in a a motor vehicle headlight.
In Fig. 3, parts corresponding to parts of Fig. 1 have reference numerals which are 20 higher. The lamp is a 225 V 1000 W floodlight lamp having a colour temperature of 3100 K. The current supply conductors are made of tungsten containing 3% by weight of rhenium and have a thickness of 0.8 mm. They are coated with quartz glass having a thickness of 0.5 mm.The quartz glass lamp vessel 21 is filled with 2.5 bar argon containing 0.3% by volume of CH₂Br₂.
The cross-sections of Figs. 4a, 4b, 4c represent magnifications by a factor 10, 100, and 10.000, respectively. The irregular shape of the surface of the current supply conductor 3 is very clearly visible. It is also visible that the quartz glass 4 follows the surface of the said conductor with great accuracy, so that an interlocking configuration is formed.
Fig. 4b also shows a rhenium line scan. The rhenium concentration in the material is plotted on the abscissa in random units. It is visible that no measurable quantity of rhenium has been diffused into the quartz glass. There is a rhenium concentration at and just below the surface of the current supply conductor. The failure to measure the rhenium at a distance from the surface is caused by the fact that no rhenium agglomerate was present in the cross-section measured.

Claims

1. An electric lamp comprising:

a lamp vessel sealed in a vacuum-tight manner and consisting of glass having an Si0₂ content of at least 95% by weight,

an electric element arranged within the lamp vessel,

current supply conductors extending through the wall of the lamp vessel to the electric element,

at least one current supply conductor made of tungsten with a continuous coating of glass having an Si0₂ content of at least 95% by weight, which coating extends from the exterior to the interior of the lamp vessel, while the surface of the glass coating encloses with the coated surface of the current supply conductor at the points at which they meet an angle a of at most 90°, characterized in that the tungsten current supply conductor contains rhenium at least at its surface.

2. An electric lamp as claimed in Claim 1, characterized in that rhenium is present distributed throughout the entire current supply conductor.

3. An electric lamp as claimed in Claim 1 or 2, characterized in that the electric element is an incandescent body.

4. An electric lamp as claimed in Claim 3, characterized in that the lamp vessel has an interior diameter in the range from 2 to 6 mm and a filling pressure in the range from 8 to 60 bar.