Note: Descriptions are shown in the official language in which they were submitted.
132503~
-1- PATENT
- IMPROVED GETTER FOR INCANDESCENT LAMPS
BACRGROUND OF THE I~V~NTION
This invention relates to incandescent lamps and
more particularly to the gettering of such lamps.
The operating life of an incandescent lamp is
greatly shortened by the presence of oxygen, carbon
dioxide, and/or water vapor in the lamp atmosphere.
Water vapor is particularly harmful because even trace
amounts "catalyze~ the evaporation of the tungsten
filament ~oil by means of the well known ~water cycle.
In the water cycle, the temperature at the
tungsten coil is thermally æufficient to decompose
water vapor into hydrogen and o~ygen. The resulting
osygen reacts with the tungsten in the coil to form
volatile oxides which migrate to cooler parts of the
lamp and condense. These oside deposits are reduced
by the gaseous hydrogen to yield black metallic
tungsten and reformed water, which causes the cycle to
repeat.
'''.''
132~036
-2- PATENT
The problems introduced by e~cess osygen in
incandescent lamps are likewise well known. For
example, in the tungsten-halogen cycle, o~ygen is the
primary agent of attack on the tungsten filament.
~his attack may result in etching, and dendritic
growth, and usually causes early filament failure.
While an e~tremely small amount of osygen is commonly
accepted as a necessary constituent in the lamp, the
amount which ends up in a finished tungsten-halogen
capsule is generally recognized as being e~tremely
variable and is always considered to be excessive.
The presence of this ~necessary constituent~ has long
been recognized as a major impediment to the
fabrication of longer lived and more consistently
performing tungsten-halogen lamps.
A commonly utilized solution to the osygen problem
in tungsten-halogen lamps is the introduction of one
or more compounds into the lamp which will remove the
excess oxygen and prevent its participation in the
tungsten-halogen cycle. Such compounds are commonly
referred to as oxygen qetters.
Various oxygen getters and/or gettering systems
have been used previously. For e~ample, metallic
getters such as tantalum, zirconium, niobium, copper,
hafnium, titanium, aluminum, or various combinations
thereof, have been employed as o~ygen getters.
Metallic getters may be attached to a portion of the
filament mount within the lamp, e.g., in the form of a
crimpPd piece of metal. These metal getters may also
be incorporated as an alloy in the molybdenum leads
which support the filament within the lamp.
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132 ~a 036
-3- PATENT
U.S. Patent No. 4,305,017 describes the use of the
above-identified metals together with precious metals
such as palladium, platinum and gold as o~ygen getters.
Metal flags, such as those described in the '017
patent, tend to be difficult and espensive to attach
to the internal structure of a tungsten-halogen lamp.
Also, some metallic getters that are used in
incandescent lamps are not applicable for use in
tungsten halogen lamps because they will react with
the halogen and terminate the desired halogen cycle.
Likewise, the fabrication of specialized getter alloys
can also add considerably to the cost of manufacturing
a tungsten-halogen lamp. In addition, in certain lamp
types, it is desirable for the getter to be present
across the entire range of locations within the lamp.
Such positioning is impossible with metallic flag
getters, and/or metal alloy gettering systems, which
are generally limited to specific discrete locations.
Another commonly used osygen gettér for
incandescent lamps is phosphorus. Phosphorus oxides
which are formed by the gettering of o~ygen are
volatile, even at the cold spot temperatures found in
hot operating incandescent lamps, including
tungsten-halogen lamps.
Phosphorus can be deposited in a lamp, for
e~ample, on either the filament mount and~or the coil
itself, e.g., by dipping a suspension of red
phosphorus or P3N5 in a suitable solvent.
Alternatively, phosphorus can be deposited on the
filament by evaporative coating of red phosphorus.
-4- 132~036 PATENT
Phosphorus can also be introduced into
incandescent lamps as phosphine ~as (PH3), which is
thermally decomposed into phosphorus and hydrogen by
the heat of the coil at light-up.
Another o~ygen getter which has been employed in
incandescent lamps is the carbon getter. Carbon
getters may be introduced to the lamp as part of a
hydrogenated hydrocarbon gas or as carbon mono~ide.
However, in addition to deleteriously affecting
filament life in certain lamp types, carbon has failed
to perform as expected as an o~ygen getter.
Yet another o~ygen gettering system is described
in U.S. Patent No. 1,941,825. This patent teaches and
claims the use of various gaseous fluoride compounds
having water-absorbing properties. The list~of
fluoride compounds includes SiF4, BF3, AsF3,
PF3, and salts thereof.
New gettering systems are constantly being
developed. The present invention represents another
such advance in this art.
SUMMARY OF THE INVENTION
A gettering system, such as the gettering system
of the present invention, wherein gettered o~ygen is
bound in a permanent, nonvolatile form, so that the
getter is effective in even the hottest operating
lamps, clearly represents an advance in this art.
In accordance with the present invention there is
provided a method of gettering an incandescent lamp.
The method comprises introducinq a fill gas and a
getter comprising a silane compound, or a partially
-5- 132~03~ PATENT
halogenated derivative thereof, into an unsealed lamp
envelope; sealing the lamp envelope; and heating the
sealed envelope, for a sufficient period of time, and
at a temperature sufficient to activate the getter.
As used herein, the tPrms ~a silane compound~
refer to compounds having the following formula, (I):
siaH(2a ~ 2)
I
wherein ~a~ is an integer greater than zero. Specific
e~amples include SiH4 (a = 1); Si2H6 (a = 2);
Si3H8 (a = 3); etc., and mi~tures thereof.
As used herein, the terms ~partially halogenated
derivatives~ refers to those compounds of Formula I,
wherein from 1, and up to (2a ~ 1), of the hydrogens
have been replaced ~y a halogen, i.e., F, Cl, Br,
and/or I. Preferably the replacement is accomplished
by bromine. It must be noted that in the method of
the present invention, one hydrogen atom must always
be present in the ~partialIy halogenated derivatives~
of the compounds of Formula I.
During the getter activation step of the present
method, the getter of Formula I is heated sufficiently
to be ~activated.~ It has been discovered that, for
the compounds of Formula I, and the partially
halogenated derivatives thereof, that the temperature
and time required to ~activate~ the getter must be
below the temperature and time which would otherwise
. .
-6- 13 2 ~ 0 3 ~ PATENT
cause decomposition of the getter prior to its
performing the desired gettering function. In fact,
when temperatures and~or heating times sufficient to
cause premature decomposition of the getter compound
are employed, the getter compositions recited above
fail to function.
During the getter activation step, the Formula I
getter reacts with residual impurities such as oxygen,
water, etc., present in the sealed envelope, forming
by-products including nonvolatile silicon dioside and
hydrogen. When a partially halogenated derivative of
the getter of Formula I is employed as the getter,
such by-products further include halogen compounds,
e.g., Br2, I2, and the like.
Advantageously, in the method of the present
invention, the getter removes any osygen impurity from
the envelop by binding the o~ygen in a stable,
nonvolatile form, e.g., as silicon dio~ide ~SiO2),
which does not decompose (and liberate o~ygen) under
the high operating temperatures of the lamp.
For a better understanding of the present
invention, together with other and further advantages
and capabilities thereof, reference is made to the
figures accompanying this specification, the following
detailed description and the claims appended hereto.
BRIEF DESCRIPTION OF THE FIGURES
Figures 1 and 2 compare lamp test data for lamps
fabricated using a phosphine getter against lamps
fabricated in accordance with the method of the
present invention.
-7- ~32~036 PA~ENT
DETAILED DESCRIPTION
The present invention i8 directed to an improved
method of gettering incandescent lamps.
Incandescent lamps are well known in the lighting
art. Such lamps typically include an hermetically
sealed light pervious envelope such as guartz or hard
glass, containiny a fill gas. Typical fill gases
include a halogen and an inert gas. Such fill gases
may further include hydrogen. The principal function
of the fill gas in incandescent lamps is to retard
evaporation of the coil. In some lamps the fill gas
may perform the additional secondary function of
suppressing the arc. The envelope also includes a
filament wire, such as tungsten wire, which is in
connection with lead-in wires sealed into and
estending internally and e~ternally of the lamp
envelope. 5uch lead-in wires may e~tend from opposite
ends of the envelope (double-ended Iamp) or from the
same end of the envelope (single-ended lamp). Such
lamps may further be enclosed within an outer envelope
or a parabolic reflector and a lens.
The present method removes residual impurities
from the lamp after introducing the fill gas into the
lamp en~elope and sealing the lamp. Such sealing
step, commonly referred to in the art as ~tipping
off,~ is a routine step in the fabrication or
manufacture of incandescent lamps.
The method of the present invention includes
introducing a fill gas and a getter comprising a
compound of Formula 1:
:
Si H
a ~2a ~ 2)
8 1~2S03 PATENT
wherein ~a~ is an integer greater than zero, or a
partially halogenated derivative thereof, into an
unsealed lamp envelope comprising a light pervious
envelope having a filament wire therein, the filament
wire being in electrical connection with lead-in wires
which are sealed into and e~tend internally and
externally from the lamp envelope.
Preferably, the gaseous getter is introduced into
the lamp envelope as a minor component of the fill
gas. Alternatively, the fill gas and gaseous getter
are separately introduced into the lamp.
After the fill gas and getter have been introduced
into the lamp envelope, the lamp is sealed by
conventional lamp sealing techniques.
After the lamp has been sealed, the lamp is heated
at a temperature, and for a period of time, sufficient
to activate the getter, so a~ to convert residual
impurities in the lamp into materials which are inert
or nondetrimental to the chemical cycle of the lamp.
One preferred method for using a silane getter of
Formula I is to introduce it as a minor component of
the fill gas. After the lamp has been tipped off,
i.e., sealed, it is subjected to a bake cycle of five
(5) minutes at 350~C. During this bake cycle, the
silane reacts, essentially quantitatively, with any
oxygen in the lamp to form nonvolatile silicon dioxide
and hydrogen. When the lamp is lighted after this
bake cycle, any o~ygen liberated from the coil is
consumed and the residual escess silane is thermally
cracked to form elemental silicon and hydrogen. The
132503~
-9- PATENT
silicon forms a faint brown smo~y deposit in the
bottom of the lamp or capsule ~assuming an inverted
light-up position).
In tungsten-halogen incandescent lamps, silane can
react with halogen in the fill gas to form volatile
halosilanes, which are as effective for gettering
oxygen as silane itself. For e~ample, in a lamp
containing HBr as a fill component, monobromosilane is
formed, assuming silane is introduced into the lamp in
stoichiometric excess.
Other preferred silanes for use in the present
method include disilane (Si2H6), trisilane
(Si3H8) and tetrasilane (Si4Hlo), as well as
their various respective partially halogenated
derivatives.
Appropriate getter activation temperatures are
preferably from about 100C to about 1000C, and most
preferably in the area of from 300C to 500C. Lower
temperatures promote more grad~al release of adsorbed
water and significantly lower gettering reaction
rates. With silane, a bake duration of five minutes
at 350C has been successfully used, as shown below.
In order to facilitate autsmated lamp processing, a
shorter time at higher temperatures is favored; e.g.,
one minute at 500C. The optimum activation time and -
temperature will vary with the specific chemical
getter used, from among those listed herein, and on
the particular lamp construction being made. Some
lamps contain internal structures that heat ~lowly by
external heating and ~ould therefore require a longer
tim~ at a given temperature for optimum gettering
action to occur.
-lo- 1325036 PATENT
In like manner the optimum quantity of gettering
additive will depend on the lamp internal volume, fill
pressure, internal surface area, and the specific
getter used. Depending upon lamp internal volume,
fill pressure, and other variables, effective
gettering action in accordance with the teachings of
this invention can be attained over a concentration
range of from about 0.001 percent to five per~ent
getter additive by volume. At lower levels there may
not be sufficient additive present to react with all
the contaminants present. High escesses will promote
undesired light loss due to the formation of elemental
silicon within the lamp. As lamp surface area
increases, as for esample in an inside frosted or
smoked bulb, more getter will be needed because of the
relatively higher quantity of adsorbed moisture -
present. From the esamples given, those skilled in
the art of lampmaking can quickly arrive at
appropriate ~etter additive levels for specific lamps
of interest.
An entirely separate family of alternative
materials which, it is believed, will perform the
function of this invention, are boron hydride
compounds. Elemental boron and silicon are analogous
and they both form volatile hydrides that are
pyrophoric; that is, they ignite spontaneously upon
contact with air. Materials such as diborane
(B2H6) dihydrotetraborane (B4Hlo), pentaborane
(B5Hg), hesaborane (B6Hlo), as well as halogen
derivatives such as, for esample,
monobromodiboropentahydide (B2H5Br) are espected
-11- 1 3 2 ~ 0 3 ~ PATENT
to fully perform the inventive principle~ taught
herein. In fact, these boron chemicals are e~pected
to perform functionally superior to the silanes based
upon their comparative thermochemistry.
The use of silane compounds and partially
haloqenated derivatives thereof may be preferred,
however, because they are less hazardous to work with,
particularly in a non-laboratory, production oriented
lamp manufacturing plant. The boranes have positive
heats of formation ~in contrast to silane) and are
accordingly unstable chemicals that can violently
decompose, especially in concentrated or pure forms.
In addition to this instability, the boranes are
siqnificantly more tosic than are the silanes. For
example, the threshold limit valves (TLV's) published
in 1984-5 by the American Conference of Governmental
Industrial Hygienists, lists time-weights average
(TWA) exposures of 0.1 for diborane and 5.0 for
silane. These values represent parts per million in -
air ~for a normal 8~hour workday and a 40 hour
workweek, to which nearly all workers may be ~-~
repeatedly exposed, day after day, without adverse
effect.~ The 50-fold difference in TWA's shows the
significantly higher hazard level in working with
boranes. Nevertheless, boranes and haloboranes will
function as highly effective o~ygen getters for
incandescent lamps. It should be emphasized, however, -
that neither silanes nor boranes would remain in the
finished lamps after the light-up step included in the ~-~
manufacturinq process, and that no tosic or other
hazards would e~ist in use or handlinq of the finished
lamps.
12- 1 325~)36 PATENT
The qetters taught herein can, of course, be used
together with other known getters such aæ, for
example, phosphorus, if so desired.
Example I.
By way of a specific e~ample, 52 watt, 84 volt
tungsten halogen capsules were fabricated from 10 mm
o.d. aluminosilicate tubing and had a nominal internal
volume of 1.1 cubic centimeters. The fill gas was
introduced to a pressure of five atmospheres and
comprised 0.1 percent hydrogen bromide, 2.0 percent
nitrogen, and the balance senon. The coils were of `
straight coiled coil design wound from no-sag tungsten
wire having a wire weight of 9.28 mg~200 mm, and wound
so as to produce an efficacy of 16.6 lumens per watt.
The finished capsules were mounted in outer bulbs with
a diode in electrical series with the coil for life
~esting at 120 volts AC. These lamps were similar in
construction to the Sylvania Capsylite A-Line lamp.
One group of capsules used no getter. A second
group contained phosphorus, formed in the capsule
before final evacuation by the thermal cracking (with
electrical heating of the coil) of a one percent
mixture of phosphine in nitrogen at a pressure of
approximately 925 millimeters of mercury. The third
group used no phosphine, but instead included a fill
gas containing 0.083 volume percent silane in addition
to the other yases. - -
The lamps were operated equally in the base up,
base down, and horizontal attitudes on the life rack.
The results are summarized in Table I belowr and in
FIGURE 1.
13 1325036 PATE~T
TABLE I
~S~ No. Lamps Avg. Life Range
None 18 2038 Hours581 3553 Hours
Phosphine 19 2611 Hours1851-3484 Hours -
Silane 15 3143 Hours1519-4041 Hours
In FIGURE 1, curve a illustrates the data for the
control lamps fabricated with a phosphine getter,
while curve b illustrates the data for lamps
fabricated in accordance with the present invention.
In this test, silane showed a life improvement of
54 percent over no getter, and 20 percent over the
commonly used phosphorus getter.
E~ample II.
In a second test, 45 watt, 84 volt tungsten
halogen capsules were fabricated from 12.5 mm o.d.
aluminosilicate tubing and had a nominal internal
volume of 2.0 cubic centimeters. The fill gas was
in~roduced to a pressure of five atmospheres and
comprised 0.1 percent hydrogen bromide, 2.0 percent
nitrogen, and the balance senon. The coils were of
straight coiled coil design wound from no-sag tungsten
wire having a wire weight of 7.47 mg~200 mm, and wound :
so as to produce an efficacy of 16.5 lumens per watt.
The finished capsules were mounted in PAR-38 lamps
with a diode in electrical series with the coil. The
lamps were life tested at 120 ~olts AC.
-14- 1 32 5 03~ PA~ENT
One group of capsules contained phosphorus, formed
in the capsule before final evacuation by the thermal
cracking of a one percent mi~ture of phosphine in
nitrogen at 925 millimeters of mercury pressure by
electrically heating the coil. A second group of
capsules used 0.083 volume percent silane in the fill
gas instead of phosphorus.
The lamps were mounted on the life rack and
divided between 45 degrees base up, horizontal, and 45
degrees base down burning positions. The results are
shown in Table II, below, and in FIGURE 2.
TABLE II
Getter No. Lamps Avg. Life Ranae
Phosphine 21 1616 Hours 827-2333 Hours
Silane 44 2358 Hours 632-4627 Hours
In FIGURE 2, curve c illustrates the lamp test
results for the control lamps including a phosphine
getter. Curve d represents the test data for lamps
fabricated in accordance with the present invention.
The use of silane as the o~ygen getter resulted in
a substantial life improvement of 45 percent.
As is clearly illustrated by the foregoing test
results, the method of the present invention
represents a major advance in the state of the art of
incandescent lamp technology by providing a
significantly extended lamp life thereof.
-15- 1325~3~ PATENT
The use of silane compound getters in incandescent
lamps without the method of the present invention
results in decomposition of the silane compound by the
filament coil upon light-up of the lamp before
gettering the contaminants in the lamp. Also, by not
heating the capsules prior to coil light-up, adsorbed -
water on surfaces internal to the capsule is not
released to be gettered by the additive prior to
destruction of the additive by the coil. Furthermore,
elevated temperatures are known to be needed to
promote the rapid and complete reaction of even the
highly active diborane in an escess of air. (~Fate of
Pollutants in the Air and Water Environments,~ Part 2,
pp. 167-192, 1977, by Edward I. Sowinski and
Irwin H. Suffet.)
The present invention has been described in
detail, including the preferred embodiments thereof.
It will be appreciated that the skilled artisan, upon
consideration of this aisclosure, will be able to make
modifications and/or improvements thereon, without
departing from the spirit of the following claims.
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