GB2179200A - Improvements in or relating to discharge lamps - Google Patents

Abstract

A lead wire which comprises at least two parts 14, 15, at least one 15 of these parts being made of HRTD (high recrystallisation temperature doped) molybdenum, is provided for use in an electrode assembly in a fluorescent lamp. The molybdenum contains 200-1200 ppm of silicon, 50-500 ppm of potassium and 0-120 ppm of aluminium. End blackening is a problem with the prior art, but HRTD molybdenum is sufficiently refractory and has a sufficiently high melting point for very little end blackening to occur when the section of an electrode lead wire which supports the electrode 9 is made from it. It also has a higher thermal conductivity than materials previously used and is therefore able to conduct sufficient heat away from the electrode with a reduced size of wire. Manufacturing is made easier because its possible to have a smaller difference in diameter between various parts of the lead wire. <IMAGE>

Description

SPECIFICATION Improvements in or relating to discharge lamps This invention is related to electrical discharge lamps and in particularto lead wires for use in electrode mount assemblies.

Discharge lamps ofthe fluorescent type generally have electrodes which comprise tungsten coils carrying a suitable electron-emissive material, each such coil being supported bytwo metal lead wires to which it is clamped. Part of each lead wire is embedded in a glass flare (the glass being softglass).To obtain a reliable and satisfactory seal, it is desirable forthe part ofthe lead wire embedded in the flare to match the thermal expansion ofthe glass. It is also desirable for the part ofthe lead wire closest to the lamp discharge (i.e. the end supporting the electrode coil) to be of a refractory material which will withstand the heat of the discharge.In ordertoaccommodatethesediffering requirements, it is known to use "composite" or "multi-part" lead wires such as those disclosed, for example, in our European patent No.70095.

End blackening is partly caused bythevaporisation of barium from the coating ofthe electrode coil (which is reduced by providing a shield around the coil), and partly by the evaporation of material from the lead wires, the lead wire material commonly being nickel, iron ora nickel/iron alloy.

It is an object ofthe present invention to overcome or reduce some of the problems associated with the prior art, particularly that of blackening ofthe ends of the lamps.

According to the invention there is provided a lead wire for use in a fluorescent lamp, the lead wire comprising at least two parts, at least one of said parts being made of doped molybdenum having a high recrystallisation temperature, the doping being potas sium and silicon and optionallyaluminium,the potassium, silicon and aluminium being within the ranges: from 200 to 1200 p.p.m.ofsilicon, from 50to 500 p.p.m. of potassium, and from Oto 120 p.p.m. of aluminium.

The molybdenum must be doped with a combination of potassium and silicon and optionally aluminium. Different ways of doping are possible but they are not suitable for our purposes. A suitable material is produced by Metallwerk Plansee and sold as "HT moly".

The lead wire may comprise two or more distinct parts, at least one end part being madeofhigh recrystallisation temperature doped molybdenum, the doping being potassium and silicon and optionally aluminium in the proportions from 200 to 1200 p.p.m. of silicon, from 50to 500 p.p.m. of potassium, and from Oto 120 p.p.m. ofaluminium.

For convenience, in the following description, this particular type of high recrystallisation temperature doped molybdenum will be referred to as "HRTD molybdenum".

Ordinary molybdenum recrystallises between 1 0000C and 1 1 OO"C, but HRTD molybdenum recrystallises at a temperature in excess of 1 600'C, which is what is being referred to as a "high recrystallisation tempernture According to another aspect of the invention there is provided an electrode assembly for a fluorescent lamp, the assembly comprising a glass flare, electrode lead wires extending through and sealed into the flare and supporting an electrode, each lead wire being at least partly made of HRTD molybdenum.

According to a further aspect of the invention there is provided a fluorescent lamp comprising a tubular light transmitting envelope, coil electrodes at opposite ends ofthe envelope and respective electrode support assemblies at said ends ofthe envelope, each assembly comprising a glass flare sealed into the envelope to form an end wall thereof and electrode lead wires extending through and sealed into the flare, each lead wire being at least partly made of HRTD molybdenum.

The parts may be of differing thermal expansion characteristics and may be of su bstantial Iy the same size.

It is known that because the part ofthe lead wire supporting the electrode is closer to the lamp discharge, it should have refractory properties. Refractory materials which have been considered for this purpose include molybdenum, tantalum, titanium, vanadium and niobium. Ofthese materials, tantalum, vanadium and niobium, and other materials such as hafium, rhenium, osmium and rhodium are expensive a nd th erefo re it wou I d be desirable not to usethem.A disadvantage with titanium and rhodium is that they have lower melting points than the other materials mentioned. Molybdenum and zirconium tend to be brittle and neithershould be used where ductility or malleability is a requirement.

The present inventor has found that a particularly useful material for the ends ofthe lead wires supporting the electrodes is HRTD molybdenum. The melting pointofthis material is sufficiently high (261 0'C) that very little end blackening is caused due to its vaporisation when it is used for such inner parts ofthe lead wires.

Another problem associated with the prior art is that when a lead wire comprises, for example, an inner section of a more highly refractory material than the other sections as it is closerto the discharge, a middle section of a material having a coefficient of thermal expansion selected to be compatible with the thermal expansion ofthe glass flare in which it is embedded, and an outer section of any suitable electrically conductive material,the sections may be of different diameters or cross-sections. For example, in such a composite leadwire,the inner part may have a diameter of 0.8mm, the middle part a diameter of 0.4mm and the outer partadiameterof0.Smm.This causes problems in automated processes as the wires tend to tilt when stacked.

An advantage of using HRTD molybdenum is that it has a higherthermal conductivity than some materials previously used (such as nickel) and is able to conduct sufficient heat away from the electrode with a reduced size of wire. Although it has similarthermal conductivity to that of ordinary molybdenum, HRTD molybdenum does not have the disadvantage of its low recrystallisation temperature.

Other advantageous properties of HRTD molybde num are that it is ductile, it can be benteasilyto clamp the electrode ends, and a reduced diameter of wire (e.g. 0.5mm ratherthan 0.8mm) does not have to be flattened to facilitate bending. HRTD molybdenum is also cheaperthan other materials having suitable refractory properties.

In orderthatthe invention may be clearly understood and readily carried into effect, it will be described by way of example, with reference to the accompanying drawings of which: Figure 1 shows a fluorescent lamp which has electrode lead wires in accordance with the invention, Figure 2 shows in detail an electrode assembly for a fluorescent lamp having composite lead wires which are of a two part structure, Figure 3 represents part of an electrode assembly with composite lead wires ofathree part structure, Figure 4 shows part of an electrode assembly with a composite lead wire having a four part structure, Figure 5 shows a composite lead wire with end parts of unequal diameters, and Figure 6 shows a composite lead wire according to the invention with end parts of substantially equal diameter.

Referring firstto Figure 1 ,the fluorescent lamp has a tubular lighttransmitting envelope 1 (coated with fluorescent material), into each end ofwhich there is sealed a glass flare 2. The glass flares are circular in cross-section and each comprises a hollowfrustoconical section 3 which at its smaller end is integral and coaxial with a solid, parallel-sided portion 4 known as a pinch. The larger ends ofthe frusto-conical sections 3 are sealed into the ends ofthe tube 1.

Either or both oftheflares 2 may have an axial bore 5 which extends from the outer end or ends ofthe flare orflaresasatubulation 6,through which the lamp may be exhausted, and mercury and the required gas or gas mixture may be introduced before closing the tubulation 6.

Support or lead wires 7 pass through and are sealed into each glass flare 2. The lead wires extend generally parallel to the axis ofthe flare and projectfrom the inner end ofthe flare. At the inner end of each lead wire is an innersupportclampportion8,andan electrode in the form of a coated coil 9 is held between these clamp portions by bending the lead wires over the ends of the coil,the coil being substantially perpendiculartothe axis ofthe lamp.

The coil 9 is surrounded by a floating shield 10 held in place by support 11 which is attached to the glass flare 2.

Since it is desirable that the material of the part of the lead wire which passes through the flare matches the glass ofthe flare in thermal expansion, as was mentioned earlier, whereas the partwhich is nearest the discharge should have refractory properties, the lead wire can be made in parts, which may be of different materials, to form a composite lead wire. In accordancewiththeinvention,thesection 15 (as shown in Figures 1 and 2) is made of HRTD molybdenum forthe reasons described hereinbefore. This part of the lead wire is not in contactwith the glass of the flare. Section 14 can then be made of a material which has a coefficient of expansion such that it matches the glass ofthe flare in thermal expansion.The flare material may be soft glass, for example soda-lime or lead glass, and the construction of the lead wire should be such thatthe section 15 is separated from the glass ofthe flare.

Figure 2 shows the structure of the lead wire 7 in greater detail. It can be seen that section 14 extends through the flare 2 into the spacewithin the lamp and may bejoined to the end ofthe section 15 at point 12.

The portion 15 may extend any distance along the lead wire 7 from the support clamp 8 up to a point in the lead wire immediately adjacent, but not in contact with,theglassoftheflare 2.

Structures having lead wires in more parts may be used and Figures 3 and 4show lead wires of respectivelythree parts 16,17 and 18 and four parts 19,20,21 and 22 welded at the points indicated generally by numeral 12. In each case, the inner part 18 and 22 respectively is of HRTD molybdenum. A three part arrangement such as that shown in Figure 3 is preferred,and the section 16 leading out of theflare can be any suitably cheap electrically conductive material such as copper plated mild steel or copper plated iron.

Examples of suitable materials forthe four part structure are copper plated mild steel forthe outer portion 19, Dumetforthe portion 20 sealed into the flare and a suitable nickel/iron alloy (preferably 48% nickel) forthe portion 21.

Referring to Figure 5, this shows a lead wire 23 as known in the prior art (before it is sealed into a glass flare and appropriately bent). Section 24 has a smaller diameter than section 26 and when the wire is placed on a surface 32, the central axis ofthewire makes angle Ato the surface. An example of such a composite lead wire comprises a section 26 of 0.8morn diameter,forpositioning closest to the electrode, a central section 25 of 0.4mm diameter chosen to have a coefficient of thermal expansion compatible with the glass flare in which it will be embedded and a section 24 of 0.5mm diameter, which may be of copper clad metal. (The diameters given are only example other diameters may be used).If ievel 32 is taken to be the datum level of a stacker mechanism, if lead wire 23 is laid flat for horizontal stacking, it makes angleAwith the datum level and when several such lead wires are laid on top of each other, this tilt error multiplies and the degree oftilt may cause malfunctioning ofthe pick-up mechanism.

Figure 6 shows a lead wire according to the invention, with sections 29,30 and 31. If the diameter of section 31, for positioning closes to the electrode, is reduced (compared with that of section 26 of wire 23) until it is substantially the same as that of section 29, this lead wire can lie horizontally and substantiaily parallel to the base ofthestacker mechanism. The diameter of section 31 may be reduced in this way when HRTD molybdenum is used, because it has higherthermal conductivity than some materials previously used and a length of smaller diameter of HRTD molybdenum can conduct as much heat away from the electrodes as, for example, a similar length of larger diameterofstainless steel or nickel platedsteel.

The present invention is of great value for use with high-loaded lamps(operatingwithcurrents higher than about 0.7A).

Examples of embodiments of the present invention which combinethe use of HRTD molybdenum and reduced diameter of the inner section to provide a composite lead wire include the following: (Referring to Figure 6) SECTION DIAMETER MATERML 29 0.4mm-0.6mm Copper plated mild steel 30 0.4mm-0.6mm Dumet 31 0.4mm-0.6mm HRTD Molybdenum A preferred embodiment has the following composition: SECTION DIAMETER MATERIAL 29 0.5mm Copper plated mild steel 30 0.4mm Dumet 31 0.5mm HRTD Molybdenum The invention does not apply only to cases when the thickest sections of the wire are ofsubstantially equal diamter. lfthey are of slightly differing diameters, the degree of tilt when stacking may be sufficiently reduced to be acceptable. Use of HRTD molybdenum according to the invention allows the diameter ofthe part ofthe wire to be used to clamp the electrodeto be reduced sufficiently to make the angle of tilt tolerable.

This therefore makes manufacturing easier.

Claims (20)

1. A lead wire for use in a fluorescent lamp, the lead wire comprising at leasttwo parts, at least one of said parts being made of doped molybdenum having a high recrystallisation temperature, the doping being potassium and silicon and optionally aluminium, the potassium, silicon and aluminium being within the ranges: from 200 to 1200 p.p.m. of silicon, from 50 to 500 p.p.m. of potassium, and from Oto 120 p.p.m. of aluminium.

2. A lead wire according to Claim 1 in which the doping is potassium and silicon only.

3. A lead wire according to Claim 1 orClaim2,the lead wire being of substantially uniform cross section.

4. A lead wire according to any one of Claims 1 to 3 in which the parts are of differing thermal expansion characteristics.

5. A lead wire according to any one of Claims 1 to 4,the parts being welded together, two of said parts being end parts and each part being of a material suitable to its purpose, one of said end parts being of HRTD molybdenum.

6. A lead wire according to any one ofthe preceding claims, the lead wire comprising three parts, a first part, a second part and a third part, the first part being of HRTD molybdenum and the second part connecting the first part with the third part.

7. A lead wire according to Claim 6, the second part being of Dumet.

8. A lead wire according to Claim 6 or Claim 7,the third part being of copper plated mild steel.

9. A lead wire according to Claim 6 or Claim 7, the third part being of copper plated iron.

10. A lead wire according to any one of Claims 1 to 5, the lead wire comprising four parts, a first part, a second part, a third part and a fourth part, the second part connecting the first part with the third part and the third partconnectingthe second part with thefourth part, the first part being of HRTD molybdenum, the second part being of nickel iron alloy, the third part being ofDumetandthefourth part being of copper plated mild steel.

11. A lead wire according to Claim 10, the nickel iron alloy being 48% nickel.

12. A lead wire substantially as herein described and claimed with reference to the accompanying drawings.

13. A electrode assembly for a fluorescent lamp, the assembly comprising a glass flare, electrode lead wires extending through and sealed intotheflare and supporting an electrode, each lead wire being at least partly made of HRTD molybdenum.

14. An electrode assembly according to Claim 13 in which each lead wire is as claimed in any one of Claims 1 to 12.

15. An electrode assembly according to Claim 13 is which each lead wire is as claimed in any one of Claims 6to 9, the first part supporting the electrode and the second part passing through the glass flare.

16. An electrode assembly according to Claim 13 in which each lead wire is as claimed in Claim 10 or Claim 1 the first part supporting the electrode, and the third part sealed into the glass flare.

17. An electrode assembly substantially as herein described and claimed with reference to the acco mpanying drawings.

18. Afluorescent lamp comprising a tubular light transmitting envelope, coil electrodes at opposite ends of the envelope and respective electrode support assemblies at said ends ofthe envelope, each assembly comprising a glass flare sealed into the envelopetoforman end wall thereof and electrode lead wires extending through and sealed intotheflare, each lead wire being at least partly made of HRTD molybdenum.

19. Afluorescent lamp according to Claim 18 in which each lead wire is as claimed in any one of Claims 1 to 12.

20. Afluorescent lamp substantially as herein described and claimed with reference to the accompanying drawings.