GB1589473A

GB1589473A - Fluorescent lamps

Info

Publication number: GB1589473A
Application number: GB963280A
Authority: GB
Original assignee: Thorn EMI Ltd
Current assignee: Thorn EMI Ltd
Priority date: 1977-07-11
Filing date: 1977-07-11
Publication date: 1981-05-13

Description

(54) IMPROVEMENTS IN FLUORESCENT LAMPS (71) We, THORN EMI LIMITED (formerly known as Thorn Electrical Industries Limited), a British Company, of Thorn House, Upper Saint Martin's Lane, London WC2H 9ED, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to electrical discharge lamps and more especially to mount assemblies for fluorescent lamps.

In electrical discharge lamps of the fluorescent type it is usual for the electrodes to consist of tungsten coils bearing electronemissive material, each coil being clamped to and carried between two metal supports or lead-wires embedded in a "pinch" seal in a respective glass flare which is sealed into one end of the lamp tube. In order to obtain a reliable seal it is customary to use composite "Dumet" support elements and lead glass mounts.

In a lamp running in an alternating current circuit each electrode acts as positive and negative electrode alternately. During the positive part of the cycle the electrode, being bombarded by electrons, gets overheated and in the past this has led to evaporation of the support wires, causing blackening of the inner surfaces of the lamp tube. The present invention is particularly concerned with reducing or preventing evaporation of the support wires and of so-called "end blackening", and with facilitating the formation of reliable glassmetal seals in leadwire or mount assemblies.

According to this invention there is provided a fluorescent lamp comprising a tubular light-transmitting envelope having a fluorescent coating, a fill comprising inert gas and mercury, coil electrodes at opposite ends of the envelope and respective electrode support assemblies at either end of the envelope, each assembly comprising a glass flare composed of soda-lime glass and sealed in the envelope to form an end wall thereof, and electrode support wires extending through and sealed in the flare, the portions of the wires in the region of the seal being composed of a metal having a coefficient of thermal expansion matching that of sodalime glass, and being formed separately from and welded to the portions of the wires exposed within the envelope.

Preferably, reduction of blackening is achieved by coating the surface of the exposed portions of the support wires with refractory material, or by forming the exposed portions of the support wires entirely of refractory metal.

The invention gives more flexibility in the choice of materials for at least those parts of the support wires located in the region of the seal, which in turn enables the cheaper soda-lime glass to be employed for the flares.

Thus, for example, nickel-iron alloy, which closely matches the glass in coefficient of thermal expansion can be used. Expensive "Dumet" (Trade Mark) material used in the prior art can be avoided, if desired, while the soda-lime glass flares can be butt-sealed to the ends of the lamp tube instead of the conventional drop-seal, which requires the use of lead glass.

Refractory material which is used to coat the surfaces of support wires preferably has lubricant properties so as to prevent its damaging, by abrasion, machinery used in the assembly of the lamp.

Although soda-lime silicate glass has been generally used for making the glass tubes for fluorescent lamps, the flares have been made from lead glass despite the fact that it is more expensive than soda-lime glass and that differences in coefficient of expansion between lead glass and soda-lime glass frequently cause "neck cracks" where the -flare is sealed to the tube. It was thought that serious problems would arise from mismatch between metal and glass, and that electrolysis between the leadwires and sodalime glass would destroy the seal between the wires and the glass and cause air leaks.

Moreover, it has previously been found that oxide formed on the surface of conventional leadwire materials as they are being sealed into the flare results in a poor motalJglass seal. For this reason "Dumet" wires, which have a surface on which oxide does not readily form during the sealing operation, have been used, while if a portion of a refractory inner support wire is embedded in the glass pinch and welded to Dumet, this often gives rise to "pinch cracks" in the flare due to differences between the coefficient of expansion of the glass and the refractory wire.

Now it has been found that leadwires made from the nickel-iron alloy referred to above advantageously have an adherent oxide layer formed on the surface which is sealed into the glass of the flare.

The present invention will now be described, by way of example, with the aid of the accompanying drawings in which : Fig. 1 is a diagrammatic view of a fluorescent lamp in accordance with one embodiment of the present invention, Fig. 2 shows one mount assembly for the lamp of Fig. 1 on an enlarged scale, and Figs. 3, 4 and 5 are partial views of flares bearing three examples of leadwires suitable for use in connection with the invention.

The fluorescent lamp shown in Fig. 1 has a glass tube 1 into each end of which is sealed a glass flare 2.

The glass flares (see Fig. 2) are circular in section and have a tapered portion 3 which at its smaller end is integral and coaxial with a parallel sided portion 4 where the pinch seal is formed, and the flares are sealed, at the larger end of the tapered portion 3, into the ends of the glass tube 1.

Either one or both (as shown in Fig. 1) of the flares 2 may have an axial bore 5 which extends from the outer end or ends of the flare or flares as a tubulation 6, through which the lamp may be exhausted and the mercury and the required gas or gas mixture introduced before the bore or bores 5 are closed at their inner end or ends, thereby completely sealing the lamp.

Passing through and sealed into each glass flare 2 is a pair of support wires or leadwires 7 which extend generally parallel to the flare axis and project from the inner end of the flare. At the inner end of each leadwire is an inner support damp portion 8, and an electrode in the form of a coated coil 9 is held between the clamps 8, the coil being substantially perpendicular to the axis of the lamp.

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

The inner support clamps 8 and parts of the leadwires 7 which are subject to electron bombardment when the lamp is in use may have a coating of boron nitride, a refractory material which also possesses lubricant properties. The boron nitride may be applied by any suitable method, most conveniently in the form of a suspension in water or organic solvent applied to the leadwire by such means as spoon dipping, brushing, spraying or drip feeding through a jet.

Each leadwire 7 has a portion 14 extending through the flare 2 and into the space within the lamp, and a portion 15 of refractory metal welded to the inner end of the portion 14. The refractory metal portion 15 may extend any distance along the leadwire 7 from the support damp 8 up to a point in the leadwire 7 immediately adjacent, but not in contact with, the glass of the flare 2. Thus it is ensured that the portion 15 of the leadwire 7 that is subject to electron bombardment when the lamp is in operation is composed of wire formed from refractory metal. The preferred refractory metals for this purpose are high temperature molybdenum, tantalum, titanium, vanadium and niobium.

The remaining portion 14 of the leadwire may be made from a nickel-iron alloy having a coefficient of expansion matched with that of the glass flare 2, which is of sodalime glass and may be butt sealed into the end of the soda-lime glass tube 1 at the so-called "neck" 16. The exhaust tubulation 6 is also formed of soda-lime glass.

It is in the neck region 16 of conventional tubes that occasional cracks develop owing to thermal expansion mismatch between the lead glass flare and the soda-lime glass tube and these are eliminated by the use of sodalime glass for the flare. Chemical reduction of the lead glass during sealing often produces a dark seal in the region 3 and makes quality control inspection difficult, and again this is avoided or reduced by the use of soda-lime glass at this point.

In Figs. 3, 4 and 5 the leadwires 7 consist respectively of two, three and four lengths of wire welded where necessary at points indicated generally by the numeral 17.

Fig. 3 shows a two-part leadwire, welded at 17a, of which the portion 19 passing through the flare is made of nickel-iron alloy and has an adherent oxide coating extending over that portion of the leadwire passing through the pinch region of the flare.

The portion 20 of the leadwire forming the inner support wire may be made of refractory metal, or of any other suitable metal with or without a boron nitride coating.

Fig. 4 shows a three-part leadwire welded at 17b and 17c in which the portion 21 sealed into the pinch section of the flare is made of Dumet, and the portion 22 making up the inner support part of the leadwire is of nickel-iron alloy, with or without a boron nitride coating on the clamp.

Fig. 5 shows a four-part leadwire welded at 17d, 17e and 17j, of which the portion 23 between the welds 17d and 17e and sealed into thhe pinch portion of the flare is made from Dumet while the portion 24 between the welds 17e and 17f is made from nickeliron alloy and connects the length of Dumet with the inner support wire 25 of refractory metal or other suitable metal, with or without a boron nitride coating on the clamp.

It is necessary to include the nickel-iron portion 24 because, unlike refractory metal, it has a coefficient of expansion which matches that of the soda-lime glass of the flare sufficiently well to form a seal without an undue risk of the occurrence of pinch cracks.

Nickel-iron alloys, for example as sold under the trade marks NILO 475, 48 and 51, may be prepared for sealing by pickling in dilute hydrofluoric or hydrochloric acid and nitric acid, followed by rinsing. The metal should then be decarbonized in a wet hydrogen atmosphere at 900--11000 C for about one hour and oxidized immediately before sealing into the glass. The wires may be oxidized by heating to 600-10500C in a sulphur-free atmosphere, the time and temperature being chosen to form an oxide film sufficiently thick to have the appearance of a brownish-grey discoloration after sealing. In our cognate Applications Nos.

30007/76 and 420004/76 (Serial No.

1,589,472), from which this application is divided, we claim a fluorescent lamp having electrodes supported by wires sealed into the lamp envelope, in which the portions of the support wires exposed to electron bombardment within the lamp during operation have their surfaces coated with boron nitride.

WHAT WE CLAIM IS:- 1. A fluorescent lamp comprising a tubular light-transmitting envelope having a fluorescent coating, a fill comprising inert gas and mercury, coil electrodes at opposite ends of the envelopes and respective electrode support assemblies at either end of the envelope, each assembly comprising a glass flare composed of soda-lime glass and sealed in the envelope to form an end wall thereof, and electrode support wires extending through and sealed in the flare, the portions of the wires in the region of the seal being composed of a metal having a coefficient of thermal expansion matching that of soda-lime glass, and being formed separately from and welded to the portions of the wires exposed within the envelope.

2. A lamp according to claim 1 in which the portions of the wires exposed to electron bombardment during operation of the lamp have a refractory surface resistant to the electron bombardment.

3. A lamp according to claim 2 in which the said exposed portions of the wires are composed of refractory metal.

4. A lamp according to claim 2 or 3 in which the portion of the wires in the region of the seal are composed of nickeliron alloy.

5. A fluorescent lamp according to claim 1 substantially as described with reference to Figs. 1 and 2 of the drawings.

6. A fluorescent lamp according to claim 1 substantially as described with reference to Figs. 3 to 6 of the drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. between the welds 17d and 17e and sealed into thhe pinch portion of the flare is made from Dumet while the portion 24 between the welds 17e and 17f is made from nickeliron alloy and connects the length of Dumet with the inner support wire 25 of refractory metal or other suitable metal, with or without a boron nitride coating on the clamp. It is necessary to include the nickel-iron portion 24 because, unlike refractory metal, it has a coefficient of expansion which matches that of the soda-lime glass of the flare sufficiently well to form a seal without an undue risk of the occurrence of pinch cracks. Nickel-iron alloys, for example as sold under the trade marks NILO 475, 48 and 51, may be prepared for sealing by pickling in dilute hydrofluoric or hydrochloric acid and nitric acid, followed by rinsing. The metal should then be decarbonized in a wet hydrogen atmosphere at 900--11000 C for about one hour and oxidized immediately before sealing into the glass. The wires may be oxidized by heating to 600-10500C in a sulphur-free atmosphere, the time and temperature being chosen to form an oxide film sufficiently thick to have the appearance of a brownish-grey discoloration after sealing. In our cognate Applications Nos. 30007/76 and 420004/76 (Serial No.