Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates generally to a
transmissive body of high density polycrystalline alumina
exhibiting improved total optical transmission as well as
in-line transmission. A tubular form of the optically
improved material when used as the light transmissive
envelope for the high intensity electric discharge lamp
provides higher light output than can generally be obtained
with conventional polycrystalline alumina material. Such
improvement is attributable to uniformity in the size and
shape of the individual alumina grains along with substantial
absence of pores and any secondary phase at the grain
boundaries in said material.
The polycrystalline alumina material made in
accordance with U.S. Patent 3,026,210 - dated March 20,
1962 - Coble, assigned to the assignee of the present
invention, has proven generally useful for the light
transmissive envelope in high intensity electric discharge
lamps. This polycrystalline alumina material is
characterized by relatively uniform large grain size and
can be prepared with a minimum of secondary phase magnesia-
alumina spinel at the grain boundaries in order to provide
optimum in-line transmission. A number of further
modified polycrystalline alumina materials are also known
which are said to exhibit improved in-line transmission
attributable to either an addition of various grain-growth
inhibiting agents in the powdered alumina mixture along
with magnesia or otherwise varying the method of preparation.
For example, a uniform grain structure of reduced size is
said to be achieved in U.S. Patents 3,711,585 - dated
3anuary 16, 1973 - Muta and 3,792,142 dated February 12,
1974 - Kobayashi et al by adding lanthana and/or yttria
along with magnesia to the pure alumina powder to provide
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combined in-line transmission improvement and greater
mechanical strength. A different approach which does not
include any addition of grain-growth inhibiting oxides
other than magnesia to achieve the same kind of improvement
is described in U.S. Patent 3,311,483 - Ciapetta - dated
November 19, 1963 where a small size and uniform grain
structure is said to be obtained by modifying the sintering
conditions. All said variations do not eliminate secondary
phase in the final sintered product, however, which
contributes to reduced in-line transmission by reason of
differences in refractive index between alumina and the
secondary phase material.
It has further long been recognized in U.S. Patent
3,026,177 - dated March 20, 1962 - St. Pierre, assigned to
the present assignee, that residual pores in the final
sintered product must also be suppressed for optimum in-line
transmission. A recent investigation finds that a pore
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volume fraction as small as 10 - 10 can be primarily
~ responsible for light scattering in polycrystalline alumina
;~ 20 material and thereby have more of a detrimental effect than
either grain boundary or secondary phase scattering. The
means utilized to reduce porosity in accordance with this
patented method of preparation features no use of a grain
growth inhibiting additive in the powdered alumina starting
material in combination with a two-stage sintering technique
said to enhance removal of residual trapped pores. Such
necessity for double sintering under special conditions and
the attendant costs involved, however, has not lead to
significant commercialization of said method.
The light output or luminous output of high
intensity electric discharge lamps, especially sodium vapor
lamps, depends upon the optical transmission of the light
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transmissive envelope wherein the arc discharge is
generated. More particularly, the in-line transmission
characteristics of said envelope are especially critical
since passage of the emitted radiation without internal
reflection has important advantages. Internal reflection
of the generated radiation within said lamp envelope can
result in significant absorption of the reflected radiation
by the arc discharge. Passage of the generated radiation
through the lamp envelope walls without reflection also
affects heat flow and temperature distribution for the
lamp in a desirable manner. By minimizing such internal
reflection for improved in-line transmission, it has been
found that the lamp envelope walls run significantly cooler
which can permit lamp redesign to run the arc discharge at
higher temperatures for both greater efficiency and a more
desirable whiter color of lamp emission. Consequently,
there is a continuing need to provide still greater in-line
transmission for polycrystalline alumina material and in a
manner which does not require costly modification of existing
commercial manufacture.
It has been found, surprisingly, that in-line
transmission of polycrystalline alumina material can be
significantly increased to such a degree that the light
output characteristics of high intensity electric discharge
lamps utilizing the improved material reflects this change.
Such improvement results from an increase in total optical
transmission between about 0.~ to 1 percent which is further
accompanied by a two- to three-fold increase in the in-line
transmission of the improved material compared to the
previously manufactured product. Specifically, these
improvements are attributable to utilizing a sintered body
of polycrystalline alumina material consisting essentially
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of relatively uniform size equiaxed grains of alumina
containing no more than about 150 parts per million of
Q~ n ~S~
mMg~es~ but essentially devoid of both secondary phase
and residual pores. The total optical transmission of the
improved material has been found to be at least 93 percent
at a 0.75 mm sample thickness for the entire visible
wavelength range. As a still further improvement, it has
also been found that the flux polishing treatment of said
material taught in U.S. Patent 3,935,495 - dated January
27, 1976 - E. Scott, produces additional increase of in-line
transmission for the flux polished material. The flux
polishing can be further characterized as a reduction of the
high spots on the individual surface alumina grains without
materially etching the grain boundaries. A method of
preparing the improved material that provides an average
grain size of approximately 26 microns diameter and where
substantially all grains have an average diameter in the
range extending from about 20 microns diameter up to about
35 microns diameter is hereinafter described for the
preferred embodiments.
Figure 1 is a schematic view of a jacketed high
pressure sodium vapor lamp embodying the improved
polycrystalline alumina material of the present invention;
and
Figure 2 is a sectional view of an electrode
configuration for the lamp depicted in Figure 1.
A high intensity sodium vapor discharge lamp in
which the invention may be embodied as illustrated at 1 in
Figure 1 and comprises an outer vitreous envelope or jacket
2 of elongated ovoid shape. The neck 3 of the jacket is
closed by a re-entrance stem 4 having a press seal 5
through which extends stiff in-lead wires 6 and 7 which are
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connected at their outer ends to the threaded shell 8
and center contact 9 of a conventional screw base. The
inner envelope or arc tube 11 is made with sintered high
desity polycrystalline alumina material of the present
invention to provide increased in-line optical transmission
to a degree more fully explained hereinafter. The ends of
the tube are closed by thimble-like niobium metal end caps
12 and 13 which have been hermetically sealed to the
improved alumina arc tube by means of a glass sealing
composition which is shown exaggerated in thickness at 14
in Figure 2.
Thermionic electrodes 15 are mounted on the ends
of the improved arc tube. As best seen in Figure 2, the
electrode comprises an inner tungsten wire coil 16 which is
wound over tungsten shank 17 crimped or welded in the end
of a niobium tube 18 welded to the end cap. The central
turns of the inner coil 16 are spread apart and the outer
tun~sten wire coil 19 is screwed over the inner coil. A
suitable electron emissive mix may be applied to the electrode
coils by painting or alternatively by dipping the coils in
the emissive mix suspension. The material is retained
primarily in the interstices between the turns of outer and
inner coil and of inner coil and shank.
Lower tube 18 is pierced through at 21 and is used
as an exhaust tube during manufacture of said lamp. After
the gas filling sodium mercury amalgam has been introduced
into the arc tube, exhaust tube 18 is hermetically pinched
off by a cold weld indicated at 22 and serves thereafter as
a reservoir for condensed sodium mercury amalgam. Upper
tube 18' has no opening in the arc tube and is used to
contain a small amount of yttrium metal (not shown) which
serves as a getter; the end of the tube is closed by a
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pinch 23 which forms a hermetic seal. The illustrated
lamp is limited to a base-down operation wherein the longer
exhaust tube 18, which must be the coolest portion of the
arc tube for the amalgam to condense therein, is located
lowermost.
The arc tube is supported within the outer envelope
by means of a mount comprising a single rod 25 which extends
the length of the envelope from in-lead 7 at the stem end
to a dimple 26 at the dome end to which it is anchored by a
resilient clamp 27. End cap 13 of the improved arc tube is
connected to the frame by band 29 while end cap 12 is
connected to in-lead 6 through band 30 and support rod 31.
The inter-envelope space is desirable evacuated in order to
conserve heat; this is done prior to sealing off the outer
jacket. A getter, suitably barium-aluminum alloy powder
pressed into channeled rings 32 is flashed after sealing
in order to insure a high vacuum. A method of manufacturing
this type lamp construction is further disclosed in U.S.
Patent 3,708,710 - dated January 2, 1973 - E. Smyser hence
need not be repeated in connection with the present
invention.
Basically, the present improved polycrystalline
alumina material is prepared in accordance with the general
method taught in the aforementioned Coble patent. Said known
method sinters a pressed compact of finely divided alumina
powder containing from a small but effective amount up to
0.5 weight percent of finely divided magnesia at elevated
temperatures in the range 1700 - 1950C in an environment
selected from the group consisting of vacuum and hydrogen
for a sufficient time period to produce a sintered alumina
gra~n structure which de~irahly~ retains little or
essentially no secondary magnesia alumina spinel phase at
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the grain boundaries. As further disclosed in said reference
patent, the purity of the alumina starting material in said
powdered mixture is maintaining about 99 percent purity
in order to reduce the amount of secondary phase formation
that adversely affects transparency of the final body,
especially the in-line transmission through excessive light
scattering. Along with said general method of sintering
preparation, there is also disclosed in said Coble patent
the general technique for measuring total and in-line optical
transmission of the sintered body of polycrystalline alumina
material which makes it unnecessary to repeat such details
in the present specification.
As regards the present departure from the prior
art teachings of Coble, however, a particularly reactive
alumina starting material has been selected which can be
characterized by an even lower level of impurities and more
uniform particle size distribution than heretofore used.
The present starting material is 99.99 percent pure alumina
which is substantially devoid of any grain-growth promoting
impurities such as sodium and iron. The present starting
alumina material is further characterized by a relatively
uniform particle size of submicron diameter which excludes
any agglomerates in size exceeding about 10 microns diameter
and further demonstrates a relatively uniform surface area
of approximately 8-9 square meters per gram. Additionally,
the magnesia content in the starting alumina powder mixture
is also maintained at 0.1% by weight or less to insure no
secondary magnesia-alumina spinel phase in the final sintered
product. In the preferred method of preparation, a
conventional binder and lubricant is incorporated in said
powder mixture to permit extrusion of a pressed compact
in the form of tubing and which further requires a
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presintering step as disclosed in the aforementioned Coble
reference in order to remove these further additives prior
to sintering. Accordingly, such presintering is carried
out in an oxygen containing atmosphere at a temperature
from about 950C to about 1200C before the final sintering
operation. The pressed compact has a green density of at
least 30 percent of the theoretical density for a single
crystal of alumina and about 15 percent shrinkage of said
compact occurs during the aforementioned presintering step.
A detailed example for the preferred method of preparation
is given to more fully describe practice of the present
invention.
Approximately 8.75 kilograms of high purity alumina
powder having the particle size distribution and impurities
level above set forth was dry milled for approximately 70
minutes in a vibratory mill after admixture with
approximately 8.75 grams of finely divided magnesia. Said
admixture was then blended with about 220 grams of a liquid
binder suspension (3 percent by weight organic binder) and
425 grams of a stearate lubricant to prepare a blended
admixture suitable for extruding the material in tubular
form. The blended admixture was then compacted in an
extruder in conventional fashion at a total force in the
range 20-35 tons to form the desired shape of pressed compact.
The green density of the tubing prepared in this manner
reached at least 30-35 percent of the theoretical density
for a single crystal of alumina. The green tubing was next
presintered in a conventional resistance element furnace
utilizing an air atmosphere and heated to about 950-1000C.
The presintering schedule being employed held the material
at said elevated temperatures for approximately 4-6 hours
and was further accompanied by slow rates of heating and
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cooling to avoid adverse temperature effects. The final
sintering schedule consisted of firing the presintered
material at approximately 1900C for about 4 hours in an
electric furnace provided with a hydrogen atmosphere.
The optical transmission characteristics of the
final sintered product prepared in the foregoing manner were
measured employing the same general measurement technique
described in the aforementioned Coble patent. Unpolished
tubing samples having an approximate 0.75 mm wall thickness
were measured. These transmission values were also
compared with a sintered polycrystalline alumina product
obtained by the same method of preparation above described
but utilizing conventional alumina powder in the starting
admixture. The conventional sintered product exhibited a
total transmission over the visible wavelength spectral
region of 92.3 percent as compared with 93.5 percent for
the sintered product of the present invention. Correspond-
ingly, the present sintered product exhibited in-line
transmission values of 43 and 67 (in arbitrary units) as
compared with in-line transmission values of 23.6 and 36.5
for the conventional product over a wavelength region
extending from 0.2 micron wavelength to 4.0 microns wavelength.
Further visual comparison of the above respective
sintered products was conducted to provide better under-
standing of the surprising optical transmission improvements
exhibited by the present product. While both sintered
materials were found to have an average grain size of
approximately 26 microns diameter and a relatively uniform
equiaxed grain size distribution extending from about 20
microns diameter up to about 35 microns diameter, there was
found a notable difference between said materials as regards
the nature of optical discontinuities existing at the grain
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boundaries. The present material was found to be
substantially devoid of both pores and secondary phase
inclusions at the grain boundaries whereas the conventional
product contained a far greater number of residual pores.
From this difference combined with a further observation
that the conventional product did not contain any large
concentration of secondary phase inclusions at the grain
boundaries, it can be concluded that residual porosity is
the primary cause of lower in-line transmission for
sintered polycrystalline alumina having no more than about
150 ppm residual ~ content in the sintered product.
It can further be concluded from said observations that a
porosity volume fraction as little as .01-.001 can be
responsible for substantially lower in-line transmission
so that reducing the residual porosity to a 10 4 volume
fraction level should provide in-line transmission comparable
to that obtained with a single crystal of alumina.
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