A SOLID LAMP FILL MATERIAL AND METHOD OF DOSING HID LAMPS
CLAIM OF PRIORITY
This application claims the priority of U.S. Provisional Patent Application S.N.
60/188,004 filed March 9, 2000.
BACKGROUND OF THE INVENTION
The present invention relates generally to dosing lamp fill material in lamps.
More specifically, the present invention relates to dosing small quantities of halogens
in high intensity discharge ("HID") lamps.
HID lamps with a vaporizable lamp fill have found widespread use in lighting
large outdoor and indoor areas such as athletic stadiums, gymnasiums, warehouses,
parking facilities, and the like, because of the relatively high efficiency, compact size,
and low maintenance of HID lamps when compared to other lamp types. HID lamps
have also been developed as point sources. In many applications, it is advantageous to
lamp operation to provide a small amount of a halogen in the arc tube of HID lamps.
In other applications, it may be advantageous to provide a small quantity of one or
more metals in the arc tube of HID lamps.
For example, ultra high pressure mercury lamps operate with mercury pressures
of 100 atmospheres and higher and have been found to be good point sources for
projection and optical systems. One disadvantage of such lamps is a reduced
operating life resulting from the blackening of the walls of the arc tube due to
deposition of tungsten from the lamp electrodes on the arc tube wall. It is known that
small quantities of a halogen dosed into the arc tube of the lamp reduces the
blackening of the wall of the arc tube and thus extends the life of the lamp. Typically,
chlorine, bromine, or iodine is dosed into ultra high pressure mercury lamps, however,
bromine has been favored in most applications. The quantity of halogen dosed in
these lamps is typically less than 0.1 mg and may be less than 0.1 μg. For example,
U.S. Patent No. 5,497,049 to Fischer discloses an ultra high pressure mercury lamp
having a dose of bromine of less than 0.1 μg.
There remains the practical question of how to dose such small quantities of a
halogen into the arc tube of a HID lamp. One known method is to add an appropriate
quantity of halogen gas to the inert fill gas of the lamp. In the example of providing
bromine in an ultra high pressure mercury lamp, the bromine in the form of Br2 may
be added to the argon fill gas. However, it is difficult to control the Br2 concentration
in the fill gas and the Br2 may be absorbed on the surfaces of the gas delivery system
gas or react with system components. Thus precise small quantities of bromine are
difficult to dose into lamps using this method.
Another known method of dosing such small quantities of bromine in a HID
lamp includes adding methylene bromide (CH2Br2) vapor to the argon fill gas of the
lamp as disclosed in U.S. Patent No. 5,109,181 to Fischer et al. However, it is
difficult to control the concentration of the vapor in argon in this method. Further,
hydrogen contamination in the lamp is possible.
Yet another known approach to dosing such small quantities of bromine into a
lamp includes the formation of lamp fill particles formed from mercuric bromide
(HgBr2). However, it is very difficult to fabricate and handle a sphere having
quantities of halide as low as 0.1 μg. Even larger spheres having as much as 0.05 mg
of halide are difficult to dose into lamps because of the small size of the spheres. The
spheres are also difficult to handle and dose because of static electricity.
Thus there remains a need for a method of dosing small quantities of a halogen
in a HID lamp in an easily fabricated and dosed lamp fill material.
Accordingly, it is an object of the present invention to obviate the deficiencies
of the known prior art and to provide a novel lamp fill material.
It is another object of the present invention to provide a novel particle suitable
for introducing small quantities of a halogen into a HID lamp.
It is yet another object of the present invention to obviate the deficiencies of
the known prior art and to provide a novel method of dosing a lamp.
It is still another object of the present invention to provide a novel method of
dosing a HID lamp with small quantities of a halogen in a solid lamp fill particle.
It is a further object of the present invention to provide a method of dosing a
lamp which reduces the introduction of impurities into the lamp.
It is yet a further object of the present invention to provide a novel lamp fill
material for introducing a metal and metal halide into a HID lamp.
It is still a further object of the present invention to provide a novel method of
dosing a HID lamp with small quantities of one or more metals and a metal halide.
These and many other objects and advantages of the present invention will be
readily apparent to one skilled in the art to which the invention pertains from a perusal
of the claims, the appended drawings, and the following detailed description of the
preferred embodiments.
DESCRIPTION OF THE DRAWINGS
Figure 1 is a phase diagram of the bismuth-bismuth bromide system.
DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention finds utility in dosing the desired quantities of a metal
halide and metal in all types and sizes of HID lamps. By way of example only, certain
aspects of the present invention may be easily understood in the embodiment of a
vaporizable lamp fill material and method of dosing small quantities of bromine in
ultra high pressure mercury lamps.
It has been discovered that lamp fill material suitable for delivering quantities
of a halogen as low as 0.1 μg or less may take the form of solid particles formed from
a molten mixture of one or more metals and the halide of one or more metals. The
metal halide component of the particle vaporizes during lamp operation to deliver the
desired quantity of the halogen into the lamp. The metal halide in the particle must be
soluble in the molten metal; however, it is undesirable to form two immiscible liquids
or separate molten metal and solid metal halide phases.
It has been found that high solubility of metal halides in metals occurs in a
limited number of systems. The metal halide may be dissolved in the parent metal of
the metal halide as illustrated in the phase diagram for the bismuth-bismuth bromide
system shown in Figure 1. However, the metal halide may also be dissolved in the
parent metal combined with one or more other metals, or with just one or more other
metals. Some systems may provide mixtures comprising a low weight percent of the
metal halide while other systems are suitable for providing mixtures comprising a low
weight percent of the metal.
The particles may be formed by admixing the desired quantity of the halogen in
the form of a metal halide with a molten metal and forming particles from the molten
admixture. The amount of metal halide in the particle is limited by the solubility of
the metal halide in the molten metal. The desired amount of metal in the particle is
determined by the desire to have a particle large enough to facilitate handling and
dosing, yet not too large so as to exceed the amount of metal which is tolerable within
the arc tube of the lamp.
U.S. Patent No. 3,676,534 to Anderson dated July, 1972 and assigned to the
assignee of the present invention, the content of which is hereby incorporated by
reference, discloses a process for forming uniformly sized particles of metal halide
mixtures by forcing a homogeneous melt through an orifice of known diameter at a
known velocity and acoustically or electromechanically breaking the molten jet into
controlled lengths.
An alternative process is described in the Anderson U.S. Patent No. 4,201,739
dated May, 1980 and assigned to the assignee of the present invention, the content of
which is hereby incorporated by reference. In that Anderson patent, particles are
formed by the controlled wetting of an orifice which allows the dripping of molten
metal halide spheres of a larger diameter.
Particles suitable for dosing into the arc tube of a HID lamp are typically
produced as spheres having an average diameter between about 50 and about 3,000
microns, and preferably between about 150 and about 1,200 microns. However, such
particles may be produced in the dripping process described above with a diameter
between about 1600 and about 3000 microns, preferably between about 1750 and
about 2500 microns.
Examples of the metal and metal halide combinations suitable for forming lamp
fill particles include:
A. metals from Group IIB, IIIA, IVA, and VA elements in combination
with a halide of the metal, i.e., M + MX„ where:
M is a metal from the group consisting of Bi, Cd, In, Sn, Tl, and
Pb, and
MX,, is a chloride, bromide, or iodide of the metal M (where n
may be 1, 2, 3, 4, or 5);
B. metals from Group IIB, IIIA, IVA, and VA elements in combination
with a halide of another metal from Group IIB, IIIA, IVA, and VA elements,
i.e., M' + M"^ where:
M' is one or more metals from the group consisting of Bi, Cd, In,
Sn, Tl, Pb, and Hg, and
M"^ is a chloride, bromide, or iodide of one or more metals
from the same group as the metal M' (where n may be 1, 2, 3,
4, or 5).
C. alkali metal in combination with a halide of the alkali metal, i.e.,
M + MX ~ where M is a metal from the group consisting of Na,
K, Rb, and Cs, and
MX is a halide of the metal M;
D. alkaline earth metal in combination with a halide of the alkaline earth
metal, i.e., M + MX„ where :
M is a metal from the group consisting of Ca, Sr, and Ba, and
MX„ is a metal halide of the metal M (where n is typically 2); and
E. rare earth metals in combination with a halide of the rare earth metal,
i.e., M + MX,, where:
M is a metal from the group consisting of La and Ce and possibly
Sc and Y and other lanthanides of atomic numbers 59-71,
and
MX,, is a chloride, bromide, or iodide of the metal M (where n is
typically 3 but occasionally 2).
The most effective particles suitable as a lamp fill material for dosing small
quantities of a halide in a lamp have been found to include a combination of one or
more metals and a halide of one of more metals wherein the vapor pressure of the
metal halide is relatively large, assuring the complete vaporization of the particle at
the operating temperature of the lamp. The vapor pressure is preferably near (or larger
than) the vapor pressure of the particular halide X of mercury, i.e., for a particle
comprising M + MX,,, the vapor pressure of MXj, is preferably near or larger than the
vapor pressure of HgX2.
The particles formed from the alkali metals, alkaline earth metals, and rare
earth metals are less desirable than the others because of the halides of these metals
have relatively low vapor pressures. Further, the reactivity of some of the metals in
these groups may not be desirable for introduction into arc tubes formed from fused
silica or for serving as an inert carrier for a metal halide. Thus the particles formed
from the compositions described in groups A and B above may be the most effective
in delivering small quantities of a halogen into a lamp. However, there may be some
applications for particles formed from groups C, D, and E in ceramic arc tubes or in
other applications where reactivity of the particle components is desired.
In the preferred embodiment of the present invention for delivering a small
quantity of a halogen into an ultra high pressure mercury lamp, the particle is formed
by dissolving bismuth bromide in molten bismuth metal.
Example 1:
A particle is formed by admixing 4 g BiBr3 with 96 g Bi metal, melting the
admixture into a homogeneous melt, and solidifying the melt into a 1.0 mg particles
having a composition of 4 weight percent BiBr3 and 96 weight percent Bi metal. The
particles formed are generally spherical and have a diameter of about 720 μm and a
quantity of about 17 μg of bromine.
Example 2:
A particle is formed by admixing 10 g BiBr3 with 90 g Bi metal, melting the
admixture into a homogeneous melt, and solidifying the melt into 0.2 mg particles
having a composition of 10 weight percent BiBr3 and 90 weight percent Bi metal. The
particles formed are generally spherical and have a diameter of about 350 μm and a
quantity of about 8.6 μg of bromine.
While preferred embodiments of the present invention have been described, it
is to be understood that the embodiments described are illustrative only and the scope
of the invention is to be defined solely by the appended claims when accorded a full
range of equivalence, many variations and modifications naturally occurring to those
of skill in the art from a perusal hereof.