Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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 electron-emissive 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 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 glass-metal seals in leadwire or mount assemblies.
; According to one broad aspect, the invention provides
a fluorescent discharge lamp comprising a light-transmitting
envelope, electrodes therein, and support wires connected to
said electrodes passing through walls of said envelope and
sealed therethrough, the improvement comprising coating at
least the surfaces of said wires that are exposed within said
envelope to electron bombardment with boron nitride.
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Trademark
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Preferably, this 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.
~he invention gives more flexibility ~n the choice
- of materials for at least those parts of the support wires
located in the region of the se~, which in turn enables a
cheaper glass to be employed for the flares. Thus soda-
lime glass ca~ be used in conaunction with wires, for example
Or nickel-iron alloy, which closely match the glass in
coerficient of thermal expansion. Expensive "Dumet"
components used in the prior art can be avoided, while the
sod~-lime glass flare~ ca~ be butt-sealed to the ends of the
lamp tube instead of the convention~l drop-seal, which
~5 requires the use of lead glass.
Refractory material which is used to co~t the
~urfaces of support wire~ in accordance with the invention
preferably has lubricant properties so as to prevent its
damaging, by abrasion, machinery used in the assembly of the
lamp.
Advantageousl~, where a portion of a support wire
i8 composed of refractory metal this portion is welded to
the other portion of the wire, which may itself comprise
more than one length of wire welded together and may include
a metal which facilitates the formation of the required
glass-metal seal.
Trademark
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~he use of soda-lime silicate gIass for the flares
in electrode assemblies in fluorescent lamps represents i~
itself a second important aspect of the ~nventio~.
~lthough soda-lime silicate glass has been generally
used for m~k;ng the glass tubes for fluorescent lamps, the
flares have been made from lead glass despite the fact that
it i9 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
i 10 to the tube. It was thought that serious problems would arise
from mismatch between metal and ~laæs, and that electrolysis
between the leadwires and soda-lime glass would destroy the
seal between the wires and the glass and cause air leaks.
Moreoverj it has pre~iously been found that oxide
formed on the surface of conventional leadwire materials as
they are being sealed i~to the flare results in a poor metal/
glass seal. For this reason "Dumet" wires, which have a
~urface on which oxide does not readily form during the seal-
ing operation, have bee~ used for at least that portion of the
leadwire passing through the flare, while if a portion of a
refractor~ inner support wire is embedded in the glass pinch
- and welded to Dumet~ this often gives rise to "pinGh cracks"
in the flare due to differences between the coefficient of
expansion of the glass and the refractory wire.
~ccording to this aspect of the present invention a
mount assembly for a fluorescent lamp comprises a flare
Trademark
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made of soda-lime glass and leadwires of which at least
the portions passing through the glass have a coefficient
of thermal expansion matching that of the glass over most
of the temperature range from room temperature to the
sealing temperature at which the leadwires are sealed into
: the flare.
~he leadwires are preferabiy made from the nic~el-
iron alloy referred to above And advantageou~ly have an
adherent oxide layer formed on the surface which is sealed.
~nto the gla~s of-the flare.
~he present in~ention will now be described, by
way of example, with the aid of the aocompanying drawings
; in which :-
~ig. 1 is a diagra~matic view of a fluorescent
lamp in accordance with one embodiment of the present
invention,
~ig. 2 shows one mount assembly for the lamp of
Fig. 1 on an enlarged scale,
- Fig. 3 shows a fluorescent lamp embodying a second
e~ample of the inve~tion,
Fig. 4 shows one mount assembly for the lamp of
Fig. 3 on an enlarged scale, and
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Figs. 5, 6, 7 and 8 are partial views of
flares bearing four examples of leadwires suitable for
use in connection with the second aspect of the
inve~tion.
5 -- ~he fluorescent lamp shown in ~ig. 1 has a glass
tube 1 into each end of which is sealed a glass flare 2.
The glass flares (see Fi~. 2) are circular i~
section and have a tapered port~on 3 which at its smaller
en~ is integral and coaxial with a parallel sided portion . .
4 where the pinch sePl is formed, ~nd the flares are
~ealed, at the larger end of the tapered portion 3, into
the e~ds of the glass tube 1. .
Either one or both (as shown i~ ~ig. 1) of the :
; flares 2 may ha~e an aY~al bore 5 which eztends ~rom 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
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- i8 a pair of-support ~res or leadwires 7 which extend
generally parallel to the flare axis and project from the
inner end of the flare. ~t the i~ner 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.
~he 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.
~he inner support clamp8 8 and parts of the lead-
wires 7 which are subaect to electron bombardment when the
lamp is in use ha~e a coati~g 12 (Fig. 2) of boron nitride,
a refractory material which also possesses lubricant
properties. ~he boron nitride may be applied b~ any
suitable method, most co~veniently in the form of a suspension
i~ water or org~n;c solvent applied`to the leadwire by such
means as spoon dipping, brushing, spraying or drip feeding
through a Jet.
~ccording to a second e~ample of the present
invention, a fluoresce~t lamp is sho~n in Fig. 3 having a
glass tube 1 and glass flares 2 (Fig. 4), similar to those
in Figs. 1 and 2, and into which are sealed support wires
or leadwires 7.
Each le~dwire 7 has a portion 14 extending through
the ~lare 2 and into the space within the lamp, and a
portion 15 of refractory metal welded to the i~ner end of
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the portion 14. The refractory metal portion 15 may
extend any aistance along the leadwire 7 from the support
clamp 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 Or the leadwire
; 7 that i9 subject to electron bombardment when the lamp i~
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
can be of soda-lime glass and may be butt se~1ed into the
end of the tube 1.
Fluorescent lamps as shown in Figs. 1 and 2 which
embod~ the second important aspect of the invention have a
soda-lime glass tube 1 into the end of which a soda-lime
glass flare 2 is butt-sealed at the so-called "neck" 16.
~he exhaust tubulation 6 is also formed of soda-lime glass.
It is in the neck region 16 of conventional tube~
that occasional cracks develop owi~g to thermal expacsion
mismatch bet~een the lead glass flare and the soda-lime
glass tube and these are eliminated by the use of soda-lime
glass for the flare. Chemical reduction of the lead glass
during sealing often produces a dark seal in the regio~ 3
and makes quality control inspection difficult, and again
this is a~oided or reduced by the use of soda-lime glass
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at this point.
In ~igs. 5, 6, 7 and 8 the leadwires 7 consist
; respectively of one, two, three and four lengths of wire
welded where necessary at points indicated generally by the
numeral 17.
~he leadwire shown in ~ig. 5 is of nickel-iron
alloy having a coefficient of expansion closely matching
that of the soda-lime glass of the flare, so as to eliminate
or reduce the possibility of pinch cracks forming in the
portion 4 of the flare, and the length 18 of the leadwire has
a uniform adherent surface coating of oxide, formed during
~abrication of the electrode mount, which partially dissolves -
in the glass when the leadwires are sealed into the flare and
thereby improves the seal. Sufficient oxide should be
present to pre~ent complete solution in the glass, as this
may give a weaker seal. The clamp portion 8 may have a re-
fractory coating of boron nitride as already described above.
Fig. 6 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.
~ig. 7 shows a three-part leadwire welded at 1~ and
17c in which the portion 21 sealed into the pinch section of
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~he 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 coati~g on the
clamp.
~i~. 8 shows a four-part leadwire welded at 17d,
17e and 17f, of which ths portion 23 betwee~ the welds 17d
and 17e and sealed into the pinch portion of the flare is
made from Dumet while the portion 24 between the welds 17e
and 17f is made from nickel-iron alloy and connec~ 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 ~
~ickel-iron portion 24 because, unlike refractory met~1, it
has a coefficient of ~expansion which matche~ that of the
soda-lime glass of the flare sufficiently well to form a
seal without an u~due risk of the occurrence of pinch cracks.
~ickel-iron alloy~, for example as sold under the
trade- marks NIL0 475, 48 and 51, may be prepared for sealing
by pickling in dilute hydrofluoric or hydrochloric acid and
nitric acid, followed by rinsing. he metal should then be
decarbonized in a wet hydrogen atmosphere at 900-1100 C for
about one hour and oxidized immediately before sealing into
the glass. he wires may be oxidized by heating to 600-1050C
in a sulphur-rree atmosphere, the time and temperature being
chosen to form ah oxide film sufficiently thic~ to have the
appearance of a brownish_grey disooloration after sealing.
Trademark
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