GB1589472A - Fluorescent lamps - Google Patents

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 fluoresent 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 over-heated 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 inven tion 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 glass-metal seals in leadwire or mount assemblies.

According to this invention a fluorescent lamp has 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.

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 a cheaper glass to be employed for the flares. Thus soda-lime glass can be used in conjunction with wires, for example of nickel-iron alloy, which closely match the glass in coefficient of thermal expansion. Expensive "Dumet" (Trade Mark) components used in the prior art can be avoided, 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.

The use of soda-lime silicate glass for the flares in electrode assemblies in fluorescent lamps represents in itself a preferred feature of the invention.

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 soda-lime 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 metal/glass seal. For this reason "Dumet" wires, which have a surface on which oxide does not readily form during the sealing operation, have been used for at least that portion of the leadwire passing through the flare, 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.

The leadwires are preferably made from the nickel-iron alloy referred to above and 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 des cribed, 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, and Fig. 2 shows one mount assembly for the lamp of Fig. I on an enlarged scale.

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 flarcs 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 clamp 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 attachcd 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 have a coating 12 (Fig. 2) of boron nitride, a refractory material which also possesses lubricant properties. The boron nitride may be applied by any suitable method, most con venicntly 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.

Fluorescent lamps as shown in Figs. 1 and 2 may have a soda-lime glass tube 1 into the end of which a soda-lime glass flare 2 is buttsealed 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.

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-i 1000C for about one hour and oxidized immediately before sealing into the glass. The wires may be oxidized by heating to 600--1050"C 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 Applications Nos. 8009632 and 8009633 (cognate) (Serial No. 1,589,473), divided herefrom, we claim 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 elecrode 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.

WHAT WE CLAIM IS: 1. 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.

2. A lamp according to claim 1, in which the support wires have a coefficient of thermal expansion matching that of the glass in which they are sealed.

3. A lamp according to claim 2, in which the support wires are formed of nickel-iron alloy and the glass seal is formed of soda-lime glass.

4. A fluorescent lamp according to claim 1 substantially as described with reference to the accompanying drawings.

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

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. cribed, 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, and Fig. 2 shows one mount assembly for the lamp of Fig. I on an enlarged scale. 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 flarcs 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 clamp 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 attachcd 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 have a coating 12 (Fig. 2) of boron nitride, a refractory material which also possesses lubricant properties. The boron nitride may be applied by any suitable method, most con venicntly 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. Fluorescent lamps as shown in Figs. 1 and 2 may have a soda-lime glass tube 1 into the end of which a soda-lime glass flare 2 is buttsealed 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. 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-i 1000C for about one hour and oxidized immediately before sealing into the glass. The wires may be oxidized by heating to 600--1050"C 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 Applications Nos. 8009632 and 8009633 (cognate) (Serial No. 1,589,473), divided herefrom, we claim 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 elecrode 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. WHAT WE CLAIM IS:

1. 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.

2. A lamp according to claim 1, in which the support wires have a coefficient of thermal expansion matching that of the glass in which they are sealed.

3. A lamp according to claim 2, in which the support wires are formed of nickel-iron alloy and the glass seal is formed of soda-lime glass.

4. A fluorescent lamp according to claim 1 substantially as described with reference to the accompanying drawings.