Note: Descriptions are shown in the official language in which they were submitted.
2Q28589
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MINIMIZING MERCURY CONDENSATION IN
TWO LAYER FLUORESCENT LAMPS
This invention relates to minimizing mercury
condensation in dual layer fluorescent lamps. More
particularly, this invention relates to minimizing
mercury condensation on the inside surface of the
glass envelope of a fluorescent lamp which camprises a
glass envelope enclosing electrodes and a discharge
sustaining fill of mercury and an ionizable inert gas
and having a first layer comprising a mixture of
particles of a phosphor and alumina disposed on the
inner surface of said envelope and a second layer of
phosphor particles having an average particle size
smaller than that of said phosphor in said first layer
disposed on said first layer, wherein the average size
of said alumina particles in said first layer is less
than that of said phosphor in said first layer, but
not less than about one-half micron, to inhibit said
mercury from depositing onto said inner surface of
said envelope.
Fluorescent lamps having two layers of phosphor
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with one layer superimposed or disposed on the other
are well known and old to those skilled in the art.
Two different layers are generally employed to improve
phosphor maintenance and lumen output, to reduce
overall phosphor cost, or both. In many of the high
color rendering premium types of fluorescent lamps
commercially available today, the first layer of
phosphor will be a relatively inexpensive phosphor
which emits a white light, such as a calcium
halophosphate activated with antimony and manganese,
with the second layer of phosphor which is
superimposed over the first layer containing
substantially mare expensive high color rendering red,
blue and green color-emitting phosphors. U.S. Patents
4,088,923 and 4,806,824 disclose a fluorescent lamp
having a dual layer phosphor coating wherein the base
layer is a conventional calcium halophosphate phosphor
on top of which is superimposed a layer of phosphor
comprising a mixture of three different phosphors
which produce red, blue and green color emission.
U.S. Patent 3,937,998 discloses an example of such a
triphosphor layer comprising a mixture of blue, green
and red color emitting phosphors. Yet other examples
are disclosed in U.S. Patent 4,431,941.
More recently it has been found that if the median
particle size of the phosphor particles in the top or
second coat is smaller than the median particle size
of the phosphor particles in the base or first coat
which is disposed adjacent the inner surface of the
glass envelope, a problem occurs With regard to
mercury passing through the two phosphor layers and
condensing on the inside surface of the lamp glass
envelope in the form of droplets and agglomerations or
blotches of mercury. While this phenomenon has not
resulted in any noticeable color change or lumen loss,
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it has resulted in customer dissatisfaction due to its
appearance which some object to as unsightly.
Consequently, there is a need to overcome this problem.
SUMMARY OF THE INTENTION
The present invention relates to a fluorescent lamp
comprising a glass envelope enclosing electrodes and a
discharge sustaining fill of mercury and an ionizable
inert gas within and having a first layer comprising a
mixture of particles of a phosphor and alumina disposed
to on the inner surface of said envelope and a second
layer of phosphor particles disposed on said first
layer, wherein said phosphor particles in said second
layer have a median particle size smaller than that of
said phosphor particles in said first layer and wherein
the median size of said alumina particles in said first
layer is less than that of said phosphor particles in
said first phosphor, but not less than about 0.25
micron. This has been found to significantly inhibit
the mercury from depositing and condensing onto the
2o inner surface of the lamp envelope, without any adverse
affect on lumen output or maintenance of the
fluorescent lamp. In most applications, the phosphor
composition in the first layer will generally be
different from the phosphor composition in the second
layer. The alumina will preferably be a high purity
alumina (i.e., at least about 99.9 wt. ~ A1203) having a
median particle size ranging from between about 0.5-6
microns, preferably from about 0.5-3 microns and still
more preferably from about 1-2 microns. The amount of
3o alumina will broadly range from about 5-40 wt. $,
preferably from about 10-30 wt. ~ and more preferably
from about 12-20 wt ~ of the combined total of
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alumina and phosphor in the first layer.
$RIEF DESCRTPTTON OF THE DRA~~~r~rr
The Figure illustrates in perspective view a
partially broken away section of a low pressure
mercury discharge fluorescent lamp utilizing a dual
layer phosphor coating in accordance with the present
invention.
Referring to the Figure, lamp 1 comprises an
elongated sealed glass envelope 2 having electrodes 3
at each end. Envelope 2 contains the usual discharge
sustaining filling of mercury, along with an inert,
ionizable gas (not shown). Electrodes 3 are connected
to inlead wires 4 and 5 which extend through a glass
seal 6 in a mount stem 7 to the electrical contacts of
base 8 fixed at both ends of the sealed glass envelope
and containing contact pins 11 and 12 which are
electrically connected to leads 4 and 5. The inert
gas will generally be argon or a mixture of argon and
krypton at a low pressure of about 1-4 torr. The
inert gas acts as a buffer or means for limiting the
arc current. Disposed on the inner wall of envelope 2
is a first or base layer 9 of particles of phosphor
mixed with particles of alumina. A second phosphor
layer 10 is disposed on layer 9. The median particle
size of the phosphor particles in first layer 9 will
generally range from about 7 to 15 microns and
preferably from about 8 to 12 microns.
The alumina mixed with the phosphor particles in
the first layer will have a median particle size
broadly ranging from about 0.5-6 microns, preferably
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0.5 to 3 microns and still more preferably from about 1
to 2 microns. As set forth above, this alumina will
preferably be a high purity alumina having an A1203
content of at least about 99.9 wt. $. It is also
preferred that the alumina be relatively free of alkali
metals, iron and silicon (i.e., <20 ppm Na, <50 ppm K,
<10 ppm Fe and <50 ppm Si). In a particularly
preferred embodiment the alumina will have a purity of
at least about 99.99 wt. ~ A1203.
to By way of an illustrative, but non-limiting
example, the phosphor particles in the first layer will
comprise a calcium halophosphate phosphor, such as a
type disclosed in U.S. Patents 3,109,819 and 4,806,824.
Similarly, the ser_ond phosphor layer 10 will comprise a
triphosphor or mixture of three different red, green
and blue color-emitting phosphors. An illustrative,
but non-limiting example is a mixture of a red-emitting
yttrium oxide activated by trivalent europium, a green-
emitting cerium magnesium aluminate activated by
2o trivalent terbium and a blue-emitting barium magnesium
aluminate activated by divalent europium as is
disclosed in U.S. Patent 4,088,923. Alternatively a
green color emitting terbium activated lanthanum cerium
orthophosphate may be employed as disclosed in U.S.
z5 Patent 4,423,349. A europium activated strontium
chloroapatite phosphor can also be substituted for the
europium-activated barium magnesium aluminate phosphor
to serve as the blue color emitting phosphor companent
in the triphosphor blend, as is known to those skilled
3o in the art.
The above examples are illustrative and are nat
meant to limit the invention to a triphosphor topcoat
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or to any of the particular phosphors in the example.
A number of fluorescent lamps of the type
illustrated in the Figure were made having a 40T12
construction. That is, they were 40 watt lamps of a 4
foot length having an envelope diameter of
twelve-eighths of an inch containing mercury and
argon. All of these lamps were first coated on the
inside of the lamp envelope with a base coat
consisting of a mixture of a calcium halophosphate
phosphor and from 0-20 wt. % of a high purity ceramic
grade of alumina containing 99.99 wt. % alpha
A1203 which was obtained from the Baikalox
International corporation in Charlotte, N.C. The
topcoat was a triphosphor or mixture of three
phosphors which were a yttrium-europium oxide
(red-emitting), a lanthanum orthophosphate
(green-emitting) and a europium-activated strontium
chloroapatite (blue-emitting) in an amount of 55, 35
and 10 wt. % of the total, respectively. The median
particle size in the triphosphor layer ranged from
about 4-6 microns. The base coat or layer was a
mixture of the high purity alumina and an antimony and
manganese-activated calcium halophosphate of a type
disclosed in U.S. Patents 3,109,819 and 4,806,824,
The base coat contained from 0-20 wt. % of the alumina
based on the combined weight of phosphor and alumina.
The median particle size of the calcium halophosphate
phosphor was about 10-12 microns. Each lamp had from
5.5-6 grams of the calcium halophosphate in the first
or base layer and 1.5-2 grams of triphosphor blend in
the second layer.
The results are set forth in the table below.
Each test group contained at least six lamps. The
mercury laydown ratings were qualitative visual
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ratings. Good means that there was no visual
appearance of mercury condensed on the inner surface
of the lamp envelope. A poor rating means that there
were large droplets or agglomerations of mercury
(i.e., about 100 microns) spread out over about
one-third the length of the lamp.
Wt.% A1203
A1203 Particle Hg
Test in Base Sixe, Lamp Lumens Laydown
0
Group Coat Microns 0 hr 100 hr 750 hr Rating
A 0 - 3474 3413 3238 Poor
B 20 0.5 3594 3353 3163 Good
0 hr 100 hr 500 hr
C 0 - 3627 3409 3312 Poor
10 1.5 3591 3332 3201 Medium
20 1.5 3516 3305 3194 Good
F 10 0.25 3607 3418 3257 Good
G 20 0.25 3596 3364 3229 Good
H 0 - 3331 _ _ _ -Poor
- _
10 1.5 - 3337 Good
5 1.5 3352 Medium
