Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR INTRODUCING
MERCURY INTO ARC DISC~ARGE LAMPS
BACRGROUND OF THE INyENTION
Field of the Invention
This invention relates to a method and
apparatus for dosing liquids. More particularly,
this invention relate~ to a process for introducing
mercury into a porous, hollow ceramic article,
followed by introducing the mercury-containing
article into an arc discharge lamp, such as a
fluorescent lamp, and then discharging the mercury
from the article inside the lamp and to a lamp
containing the mercury and the article.
Backg~Qund of the DisçlQ~
Most electric arc discharge lamps employ
mercury as at least one of th~ ionizable components
required to initiate or sustain th~ arc discharge and
it i~ therefore necessary to introduce the proper
amount of m~rcury into the lamp during the
manufacturing process. The lamp industry is
constantly looking for ways to introduce mercury into
such lamps in a facile, inexpensive and reproducible
manner. In the past, various machines and devices
have been employed to introduce the mercury into the
lamp in a liquid form; as an amalgam; in the
interstice~ of a s~ntered metal or composite, and in
mercury-containing metal or glass capsules which must
be ruptured after lamp manufacture in order to
release the mercury.
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SUMMAR~ OF THE INVENTION
It has now been discovered that a porous,
hollow ceramic article, such as a sphere having
porous walls, can be filled with a liquid such as
mercury, the mercury-containing article then
introduced into a lamp and the mercury discharged out
of the ceramic article inside the lamp. The liquid
or mercury is introduced into the porous, hollow
ceramic article by placing the article in a
subatmospheric environment to reduce the pressure
inside the hollow portion or cavity of the article
and then introducing the desired liquid or mercury
into the reduced pressure cavity through the porous
wall. The mercury-containing ceramic article is then
introduced into a lamp and the mercury discharged out
of the article inside the lamp by reducing the
pressure in the lamp, heating the lamp and/or the
article or a combination of heat and reduced
pres~ure. This invention has been found to be
part$cularly useful in dosing m2rcury into low
pressure arc discharge lamps such as the well known
fluorescent lamps.
BRIEF DES~RIp~lON OF THE DRAWINGS
Figure 1 schematically illustrates a porous,
hollow ceramic microsphere made of alumina useful in
the practice of the invention.
Figure 2(a) is a partial sectional view of a
fluorescent lamp assembly having a porous, hollow
mercury-containing ceramic sphere lodged inside the
mount stem at one end of the lamp prior to sealing
the exhaust tube and Figure 2(b) illustrates the lamp
after sealing the exhaust tube and complsting the
lamp.
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DETAILE3 DESCRIPTION
Referring to Figure 1, a cross section of a
porous, hollow ceramic sphere useful in the practice
of the invention is shown in schematic form. Thus,
hollow sphere 10 is shown as comprising cavity 12
surrounded by outer wall 14. Wall 14 is sufficiently
porous to allow mercury to be introduced into the
interior hollow portion or inner cavity 12 of the
sphere under the proper conditions. Hollow ceramic
spheres having porous walls useful in the practice of
the invention may be obtained from Microcel
Technology, Inc., of Edison, New Jersey. These are
hollow, thin-shell spheres having strength, low
density and extreme resistance to thermal shock and
have been made of more than twenty different ceramics
including alumina, zirconia, mullite and kaolin They
ar~ available in sizes ranging from 1 to 7 mm in
diameter with wall thicknesses of from 12-150
microns. Although these hollow, porous microspheres
are made primarily for low mass kiln furniture,
radiant burners, high-temperature low-bearing
insulation and filters for molten metal, they have
be~n found useful in the practice of the present
invention.
These hollow ceramic spheres have been filled
with mercury by first placing the spheres in a vacuum
chamber and subjecting the spheres to vacuum or
subatmospheric pressure in order to insure the
desired vacuum throughout the hollow pore structure
of the wall and inside the hollow cavity 12 of the
spheres. After the desired vacuum or subatmospheric
pressure has been reached inside the cavity, the
spheres are surrounded by liquid mercury either by
placing them in a pool of l~quid mercury within the
vacuum chamber or introducing liquid mercury into the
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vacuum chamber until it completely surrounds and
covers all the spheres, after which time the ambient
surrounding the sphere-containing pool of mercury is
increased to the desired pressure. Hollow
microspheres made of alumina have been filled with
mercury by placing the spheres in a vacuum cha~ber
and reducing the pressure to a vacuum, introducing
mercury into the chamber to form a pool of liquid
mercury containing the spheres and then increasing
the pressure over the mercury to a value ranging from
atmospheric to, i.e., 200 psi. This causes the
liquid mercury to penetrate through the porous walls
14 of the spheres 10 into the cavity 12. The rate of
penetration of the mercury into the interior hollow
cavity 12 of a sphere is dependent upon the pressure
differential across the porsus wall, the porosity and
the temperature, it being well known and understood
to those skilled in th~ art that increased
temperature, porosity and pres~ure will increase the
rate of penetration of mercury into the hollow sphere
portion 12. On the other hand, if the porosity is
too great mercury loss or leakage during storage or
the lamp manufacturing process will be a problem. If
the porosity is too little, release of the mercury
from the sphere into the lamp may take too lonq.
After the mercury ha~ penetrated and filled the
hollow interior cavity 12 of the microspheres in the
mercury pool, the mercury is drained from the pool,
or the spheres removed from the pool, and the
mercury-filled spheres placed in storage under
suitable conditions until they are used in the lamp
making process. Storage of the mercury filled
microspheres is done without the need for any special
precautions or handling. The vapor pressure of
- 35 mercury is sufficiently low at ambient conditions and
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its surface tension sufficiently high to prevent the
mercury from leaking or vaporizing out through the
porous wall of the spheres. The amount of mercury
that a hollow ceramic sphere according to the
invention can contain is, of course, dependent on the
size of the sphere and the cavity contained within.
For example, a 3 mm diamet~r alumina sphere having a
0.1 mm wall thickness will hold about 80 mg of
morcury and a 1.5 mm diameter sphere of the same wall
thickness will hold about 20 mg of mercury.
Turninq to Figure 2(a) there is shown, in
schematic form, a partial section of one end of a
fluorescent lamp construction prior to being sealed.
Thus, fluorescent lamp preassembly 20 comprises glass
envelope 22 having a coating of on~ or more phosphors
- 24 on the interior surface thereof and enclosing
c~vity 26. Filament mount structure 28 is shown at
one end of lamp 20 a~ comprising gla~s mount stem 30
which terminates at one end in flare portion 36 fused
to ~nv-lop~ 22 at 38. Mount stem 30 contains a pair
of electrically conductive lead~ 34 hermetically
sealQd within and extending therethrough to one end
of each o~ which is attached electrode 32 inside the
lamp and the other end extends exterior of the lamp
~or subsequent connection to electrically conductive
pins 54. The interior of electrode mount structure
~8 contains exhaust tube 40 which terminates inside
the mount stem at exhaust hole 42. Prior to final
se~ling the lamp, the assembly 20 is heated to about
300-C, exhausted, filled with argon at a pressure of
about 2 torr and exhaust tube 40 is then tipped-off
by heating to seal the lamp. The low pressure in the
lamp assists in collapsing the exhaust tube and
sealing it at the poin~ where it is heated by a torch
(not shown) to achieve sealing and removal of the
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excess portion of the exhaust tube after sealing.
Heating the lamp assembly prior to exhausting or to
applying a vacuum to the interior of the lamp
envelope aids in outgassing the interior of the
lamp. A mercury-containing ceramic sphere 10 is
inserted into the lamp exhaust tube 40. In one
embodiment the sphere passes through exhaust hole 42
and into interior cavity 26 wherein the mercury is
released into the lamp due to the combination of heat
and reduced pressure in the lamp. In another
embodiment hole 42 is sufficiently small compared to
the diameter of the sphere so as not to permit sphere
10 to pass therethrough into the interior cavity 26
of lamp envelope 22. The lamp assembly 20 is then
exhausted through tube 40 and a suitable inert gas,
such as argon at a pres~ure of about 4 torr, is
fillad into the lamp through exhaust tube 40 and hole
42 either be~ore of after the mercury-containing
sphere 10 i5 inserted into the exhaust tube, after
which tube 40 i~ soaled as shown in Figure 2(b)
containing ceramic sphere 10 enclosed either within
it inside the interior of mount stem 30 or in the
interior cavity 26 of the lamp. It is preferred to
reta~n sphere 10 in the sealed and tipped-off
remainder of the exhaust tub~ 40 within mount stem 30
BO th~t the sphere cannot abrade or scratch the
phosphor coating 24 in the lamp. After the sealed
lamp assembly 20 has cooled, end caps 52 and
electrical connection pins 54 are assembled onto the
lamp structure to produce the completed fluorescent
lamp 50.
A number of 40 watt fluorescent lamps having a
coating of a calcium halophosphate phosphor 24
disposed on the inside surface of lamp envelope 22
were made in four foot lengths, being typical F40T12
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cool white fluorescent lamps common in both the home
and in industry. About ten lamps were made wherein
the mercury-containing ceramic sphere was introduced
into the interior cavity 26 of the lamp through the
exhaust tube 40 and exhaust hole 42. The sphere was
3 mm in diameter, contained 80 mg of mercury and was
introduced into the lamp when the lamp had cooled to
about 200-C, after which 4 torr of argon was
introduced into the lamp and the exhaust tube
tipped-off and sealed. Another batch of ten were
prepared in a similar manner, but using a 1.5 mm
diameter ceramic sphere 10 conta~ning about 20 mg of
mercury which was introduced into the lamp cavity 26
after exhaustion and back filling with 4 torr of
argon. The lamp was at a temperature of 200-C. ~oth
batches of lamps were assembled with end caps and
pins. These lamps wers all energized and started
immediately and found to be operating satisfactorily
with no 108~ in maintenance or performance after a
p-riod of 6000 hours, at which time the lamps were
turned off and the te~t was completed. This thus
demonstrates a reduction to practice of the invention
and the fact that the practice of the invention
produces satisfactory fluore~cent lamps.
