Method of enhancing the optical transmissivity of polycrystalline alumina bodies, and article produced by such method

The in-line transmission of a translucent polycrystalline alumina arc tube for a high-pressure sodium discharge lamp is improved by as much as 50% by dipping the "green" tubular compact in an aqueous slurry containing finely-divided alumina particles and, after the slurry-dipped compact has been dried, subjecting it to the usual pre-sintering and sintering operations required to convert the compact into a dense ceramic body. The slurry is preferably prepared from the same slurry which contains the blended alumina powder, magnesia and other additives that comprise the raw-mix slurry which is spray-dried to produce the larger size generally spherical particles that are compressed or extruded to form the green compact.

BACKGROUND OF THE INVENTION 
This invention generally relates to the electric lamp art and has 
particular reference to a method of improving the optical transmissivity 
of polycrystalline alumina arc tubes during manufacture, and to arc tubes 
which are produced by such method. 
High-density polycrystalline alumina is a well known translucent ceramic 
material and a detailed disclosure of the various operations required to 
manufacture it from compressed finely-divided alumina of high purity in a 
form suitable for use as light-transmitting arc tubes for high-intensity 
electric discharge lamps (such as high-pressure sodium vapor lamps) is 
presented in U.S. Pat. No. 3,026,210 issued Mar. 20, 1962 to R. L. Coble. 
In accordance with the teachings of this patent, additions of small but 
effective amounts of magnesia up to 0.5 weight percent are included in the 
raw-mix composition to improve the optical transparency of the sintered 
alumina tubing. Since the amount of useful light produced by a 
high-intensity discharge lamp is inherently controlled by the ability of 
the arc tube to transmit the visible radiations generated by the 
discharge, intensive research has been conducted to discover and develop 
various ways of improving the optical transmission characteristics of arc 
tubes composed of polycrystalline alumina. In accordance with one prior 
art approach to this problem, the sintered tubing is subjected to a flux 
polishing treatment in a bath of molten sodium borate at a temperature in 
the range of from about 762.degree. C. to 857.degree. C. This arc tube 
polishing concept is disclosed in U.S. Pat. Nos. 3,935,495 and 4,079,167 
issued to G. E. Scott, Jr. et al. 
As stated in the aforementioned patents, the flux polishing process reduces 
the high spots on individual exterior alumina crystals without materially 
introducing low spots at the grain boundaries with the result that the 
treated arc tube has a smoother surface and exhibits increased optical 
transmission. However, the fluxing action leaves a glassy coating on the 
arc tubes which must be removed by washing them in a dilute mineral acid 
solution after the tubes have been withdrawn from the molten flux bath and 
allowed to cool to ambient temperature. Thus, while the flux polishing 
process achieves the desired objective of increasing the optical 
transmission characteristics of the arc tubes, it is expensive and time 
consuming and, as such, is not particularly suitable for use in the 
manufacture of high-pressure discharge lamps in mass-production 
quantities. 
U.S. Pat. Nos. 4,150,317 and 4,169,875 to H. M. Laska et al. disclose the 
concept of improving the in-line transmission characteristics of 
polycrystalline alumina arc tubes by using alumina powder that is very 
pure (99.99% pure and devoid of grain-growth promoting impurities such as 
sodium and iron) and also limiting the magnesia content in the raw-mix 
composition to 0.1% by weight or less. The patents indicate that the 
reduction in the magnesia constituent prevents the formation of secondary 
magnesia alumina spinel phase at the grain boundaries in the final 
sintered product. 
It would accordingly be very advantageous from both a cost and 
manufacturing standpoint if an inexpensive method could be provided for 
improving the optical transmission characteristics of polycrystalline 
alumina tubing which would not materially disrupt or delay the normal 
sequence of operations required to form the tubing from powdered raw 
materials. 
SUMMARY OF THE INVENTION 
The foregoing objectives are achieved in accordance with the present 
invention by compressing or extruding powdered raw materials of 
standard-grade purity (alumina that is 99.99% pure) to form a "green" 
tubular compact of alumina particles in the conventional manner and then 
dipping the compact in an aqueous slurry of finely-divided alumina 
particles that are also free of impurities (99.99% pure). At this stage of 
fabrication, the green compact is quite porous and readily soaks up the 
alumina slurry so that the latter enters the pores and coats the surface 
of the compact. The tubular compact is then dried with the result that the 
slurry-deposited alumina particles are not only trapped in the pores but 
fill the low spots and interstitial surface cavities which microscopic 
examination reveals inherently exist in the surface of the compact. The 
slurry-impregnated-and-coated tubular compact is then subjected to the 
normal pre-sintering and sintering operations which convert the porous 
compact into a high-density polycrystalline alumina ceramic tube--with the 
result that the slurry-deposited alumina particles are fused to and become 
integral parts of the finished tube. The "leveling" effect of the fused 
slurry-deposited alumina particles not only provides the surface of the 
sintered arc tube with a smooth "polished" finish but also increases the 
density of the polycrystalline alumina by filling minute pores and other 
voids in its ceramic structure. 
In accordance with the preferred embodiment, only the exterior surface of 
the tubular green compact is exposed to and coated with the aqueous slurry 
of alumina so that air can escape from the interior surface of the compact 
into the atmosphere during and after the dipping operation. This is 
conveniently accomplished by temporarily capping one end of the tubular 
compact and immersing that end of the compact into the slurry without 
allowing any of the slurry to flow into the open end of the compact.

PREFERRED EMBODIMENT OF THE INVENTION 
While the present invention can be used with advantage in the manufacture 
of polycrystalline alumina articles of various shapes and sizes which 
constitute components for various kinds of devices which require a durable 
light-transmitting ceramic material, it is especially adapted for use in 
conjunction with the production of ceramic arc tubes for high-intensity 
electric discharge lamps and it has accordingly been so illustrated and 
will be so described. 
As shown in FIG. 1, the ceramic polycrystalline alumina body comprises a 
hollow cylindrical tube 10 in its finished form when its intended use is 
that of an envelope for a high-pressure sodium discharge lamp or similar 
light source. Such tubular envelopes typically have a translucent 
"frosted" appearance and, in the case of a 400 watt lamp, have a length 
dimension of 14 inches (35.5 cm.), an outer diameter of 3/8 inch (9.5 mm) 
and a wall thickness of approximately 0.03 inch (0.75 mm). 
As illustrated in FIG. 2, the arc tube 12 for such a discharge lamp is 
formed by sealing the ends of the alumina envelope 10 by end caps 13, 14 
that are fabricated from a suitable metal such as niobium and are 
terminated by tubular segments 15, 16 which are hermeticaly closed by a 
welding or brazing operation in accordance with standard lamp-making 
practice. The end caps can also be made from polycrystalline alumina and, 
in this case, would comprise discs that are fused to lead-in rods of 
niobium or other suitable metal. The envelope 10 contains a pair of 
electrode coils 17, 18 that are supported at opposite ends of the envelope 
by metal rods fastened to the associated end caps. The envelope also 
contains an ionizable medium comprising sodium (or a combination of sodium 
and mercury) and an inert starting gas such as xenon at a pressure of 20 
Torr, for example, which sustains an arc discharge that passes between the 
electrodes 17, 18 when the arc tube 12 is connected to a suitable power 
supply. As is customary in the manufacture of this type of discharge lamp, 
the arc tube 12 and its current-limiting components are secured to a 
suitable support-mount assembly which is sealed into an outer protective 
envelope (not shown) that is fitted with a base member. 
The green compact or body which is treated in accordance with the present 
invention is formed in the usual manner from high-purity (99.99% pure) 
alumina (Al.sub.2 O.sub.3) and a small but critical amount of magnesia 
(MgO) that are combined with one another to provide a raw-mix material 
that is processed into generally spherical particles of such size that 
they can be readily compressed into a body of the desired shape and size. 
The blending of the raw-mix ingredients is achieved by suspending the 
finely-divided particles in distilled water to form an aqueous raw-mix 
slurry which is then spray-dried to form spherical particles of the 
desired larger size. 
As a specific example, 10,000 grams (approximately 59% by weight) of 
powdered Al.sub.2 O.sub.3 of sub-micron size (average particle size in the 
range of around 0.3 micron) is added to about 5,000 cc's of distilled 
water (approximately 30% by weight) together with about 10 grams of 
powdered MgO (about 0.06% by weight), 300 cc's (approximately 1.8% by 
weight) of a suitable wetting or dispersing agent that is organic and 
soluble in water, and a selected amount of a suitable organic binder such 
as polyvinyl alcohol in a 20% aqueous solution. The raw-mix slurry can 
also contain minor but controlled amounts of a suitable organic lubricant 
and a defoaming agent that are both soluble in water. In the aforesaid 
specific example of a raw-mix formulation, approximately 1,000 cc's 
(approximately 6% by weight) of the 20% aqueous solution of polyvinyl 
alcohol was employed along with 400 cc's (about 2.4% by weight) of 
polyethylene glycol or other suitable organic lubricant and up to about 
0.005% by weight of an organic defoaming agent. 
The resulting raw-mix slurry contained approximately 60% solids and, after 
being thoroughly mixed, was spray dried to remove the water and form 
spherical particles of the blended raw-mix materials. The spray-dried 
powder was then passed through a 60 mesh screen to remove oversize 
particles and agglomerates of powder. The screened material comprised 
generally spherical particles of the raw-mix formulation that ranged from 
about 10 microns to 70 microns in size and had an average particle size of 
approximately 50 microns. A predetermined amount of these raw-mix 
particles was subsequently fed into the mold of a suitable isostatic press 
apparatus and compressed between a metal mandrel and outer mold form of 
urethane or other suitable material. Hydraulic pressure (in the range of 
from about 15,000 to 30,00 psi) was applied to the spherical raw-mix 
particles to form a green compact of the hollow tubular configuration 
shown in FIG. 1. 
In accordance with this invention, the green compact produced by the 
above-described sequence of operations is placed in contact with an 
aqueous slurry that contains finely-divided alumina particles of 
high-purity and sub-micron size so that the slurry covers the entire 
surface of the compact which is to be treated. The green compact is 
inherently quite porous at this stage and the aqueous slurry of alumina 
quickly enters the pores of the compact aided by capillary action. The 
porous green tubular compact accordingly readily soaks up the alumina 
slurry and the latter coats the surface of the compact. While the alumina 
slurry can be applied to the compact by a suitable spraying apparatus or 
other means, it is more convenient to simply dip or immerse the compact in 
a bath of the alumina slurry and this method of application is preferred. 
In the case of a hollow tubular compact of the type described which is 
destined for use as the envelope for an arc tube, it is important that 
only the outer surface of the tubular compact be placed in contact with 
the alumina slurry and that the interior surface be left uncoated and 
exposed to the atmosphere to permit the air in the pores of the compact to 
escape from the compact after the dipping operation. If both the inner and 
outer surfaces of the compact are coated with the slurry, the air in the 
pores of the compact forms bubbles in the coating (after the compact is 
withdrawn from the slurry) which burst and leave crater marks on the 
coated surfaces. This not only mars the finish of the sintered compact but 
drastically reduces its optical transmission characteristics. Hence, even 
if the green compact is of planar rather than tubular configuration, only 
one of its faces or surfaces should be placed in contact with the alumina 
slurry and coated. Insofar as the interior surface of the tubular compact 
in this embodiment is formed by compressing the raw-mix particles against 
a metal mandrel, it inherently has a much smoother "finish" or structure 
than the exterior surface and thus is not in such dire need of a leveling 
treatment. 
In accordance with a preferred embodiment of the invention, the selective 
treating of the tubular green compact is achieved by temporarily capping 
or sealing-off one end of the compact and immersing the compact, capped 
end downward, into a pool or bath of the "treating" slurry of alumina 
particles until the slurry substantially covers the compact but does not 
reach or flow into the open end of the compact. 
After the green compact is dipped or otherwise placed in contact and coated 
with the "treating" slurry of alumina particles, it is dried and subjected 
to the usual pre-sintering and sintering operations customarily employed 
to convert the compact into the finished high-density polycrystalline 
alumina ceramic tube. Pre-sintering is achieved in the usual fashion by 
heat treating the green compact in air (or an oxygen-containing 
atmosphere) at a temperature of about 1000.degree. C. for approximately 
one to ten hours to remove the hydrocarbon residues of the organic binder, 
lubricant, dispersant and defoamer additives. Final sintering is 
accomplished by heat treating the tubular bodies in a hydrogen atmosphere 
at a much higher temperature (in the range of 1700.degree. C. to 
1900.degree. C.) for a period of from two to twenty-four hours. The 
elevated temperature of this final sintering operation densifies the 
pre-sintered body and forms the closely-knit crystalline structure which 
converts it into a hard translucent ceramic polycrystalline article. 
During the pre-sintering and sintering operations, the finely-divided 
particles of alumina deposited in the pores and interstitial surface 
cavities or "low spots" on the outer surface of the green compact by the 
slurry-dipping treatment become integral fused parts of the finished 
polycrystalline alumina arc tube or body and not only level the exterior 
surface of the tube and provide it with a "polished" smooth finish but 
also increase the density of the arc tube by filling pores and other 
minute voids that would otherwise remain in its ceramic structure. The 
in-line transmission characteristic of the treated tube is thus greatly 
improved. 
In accordance with a preferred embodiment, the aqueous alumina slurry used 
for the dipping or treating operation has the same basic composition as 
the raw-mix slurry that is spray dried to form the spherical powdered 
material which is used for manufacturing the green compacts. However, the 
solids concentration is preferably diluted (by adding distilled water) and 
adjusted so that it is within a range of approximately 1% to 40% by weight 
solids. The preferred range is from about 10% to 30% by weight solids and 
the optimum range is approximately 15% to 25% solids. The raw-mix slurry, 
in contrast, can contain from about 50% to 70% solids. 
The option of using the raw-mix slurry as the "treating" slurry (after 
proper dilution) provides an important advantage from a cost and 
manufacturing standpoint since it requires the preparation of only one 
slurry formulation. The "treating" slurry thus contains finely-divided 
alumina particles of sub-micron size having an average particle size of 
approximately 0.3 micron. However, alumina particles having an average 
particle size in the range from about 0.1 micron to about 1 micron can 
also be employed, depending upon the size of the pores and surface 
cavities or voids in the green compacts or bodies to be treated. Of 
course, the Al.sub.2 O.sub.3 particles in the slurry must be substantially 
free of impurities to avoid contaminating the compact and sintered arc 
tube. Alumina of 99.99% purity has produced excellent results. 
The effectiveness of the "slurry-treating" operation of the present 
invention in improving the "finish" of sintered polycrystalline alumina 
articles and their ability to transmit visible radiations was confirmed by 
tests conducted by partially dipping "capped" green tubular compacts of 
compressed alumina powder in an aqueous alumina slurry that contained 10% 
solids and was made by diluting the raw-mix slurry. After drying and the 
conventional pre-sintering and sintering operations, the portions of the 
ceramic arc tubes that were treated with the alumina slurry had a "glassy" 
sheen and a smooth surface in contrast to the dull rough surface of the 
untreated portions of the tubes. The treated portions of the tubes also 
exhibited enhanced optical transmission properties which produced 
improvements as high as 50% in the in-line transmission (as measured by a 
Metrologic Neon Laser measuring instrument). 
Such marked improvement in the in-line optical transmissivity 
characteristics of polycrystalline alumina bodies treated in accordance 
with the invention was also confirmed by a series of more carefully 
controlled and comprehensive tests. A total of thirty-two green tubular 
compacts of alumina-magnesia particles were made from the same batch of 
raw-mix slurry material that was spray dried and isostatically compressed 
in the same fashion and then divided into four groups or lots of eight 
pieces or compacts. In the first test, one lot was temporarily capped and 
dipped in a "treating" alumina slurry that was prepared by diluting the 
raw-mix slurry so that it contained 20% solids (and finely-divided alumina 
particles having an average particle size of about 0.3 micron). The other 
group or lot was not dipped in the slurry. The dipped compacts were then 
dried and, together with the control lot, were pre-sintered in an electric 
furnace for four hours in air at a temperature of 1100.degree. C. Both 
lots were then sintered for seven hours at 1800.degree. C. in hydrogen and 
the finished polycrystalline alumina ceramic tubes were measured for total 
transmission and maximum and minimum in-line transmission. 
The remaining two lots of eight compacts each were subjected to a second 
test which was identical to the first test except that the dipped and 
undipped green tubular compacts were pre-sintered in air for ten hours at 
1100.degree. C. The results of these two series of tests are given in 
Tables I and II below: 
TABLE I 
__________________________________________________________________________ 
Test No. 1: Tubes Pre-Sintered for 4 hrs. at 1100.degree. C. 
Lot A (Not Dipped) 
Lot B (Dipped) 
% Total In-line Trans. 
% Total 
In-line Trans. 
Transmission 
Max. 
Min. Transmission 
Max. Min. 
__________________________________________________________________________ 
93.4 9.60 
7.39 92.2 11.03 10.14 
93.0 6.01 
5.33 89.2 8.51 7.60 
93.6 7.30 
6.39 88.4 11.18 10.79 
91.4 6.40 
5.46 92.8 10.28 8.82 
90.6 10.76 
8.55 94.4 10.59 9.83 
89.8 5.27 
4.82 92.0 11.30 10.02 
89.4 7.52 
6.30 91.2 11.31 11.23 
89.0 5.89 
5.22 90.8 10.27 9.05 
Avg. 
91.34 7.34 
6.18 
Avg. 
91.37 10.80 9.68 
(0.03% (47% (56% 
Imprvmt.) 
Imprvmt.) 
Imprvmt.) 
__________________________________________________________________________ 
TABLE II 
__________________________________________________________________________ 
Test No. 2: Tubes Pre-Sintered for 10 hrs. at 1100.degree. C. 
Lot C (Not Dipped) 
Lot D (Dipped) 
% Total In-line Trans. 
% Total 
In-line Trans. 
Transmission 
Max. 
Min. Transmission 
Max. Min. 
__________________________________________________________________________ 
91.8 7.08 
5.72 92.6 9.15 7.87 
92.8 6.52 
5.81 94.0 11.27 10.97 
94.4 6.58 
6.30 94.4 10.69 9.66 
93.2 7.05 
6.02 95.0 10.76 9.67 
91.2 6.21 
5.23 92.4 11.17 10.41 
92.2 6.70 
5.71 91.2 11.23 10.39 
93.2 7.34 
6.17 94.6 11.31 11.30 
94.6 8.31 
6.93 91.0 7.64 7.32 
Avg. 
92.92 6.97 
5.98 
Avg. 
93.15 10.40 9.69 
(0.2% (49% (62% 
Imprvmt.) 
Imprvmt.) 
Imprvmt.) 
__________________________________________________________________________ 
As will be noted from the test data, the average total transmission of the 
arc tubes that were pre-sintered for four hours at 1100.degree. C. (Lots A 
and B, Test No. 1) had a percent total transmission of slightly greater 
than 91% and only a very small improvement (0.03%) was exhibited by the 
arc tubes that were dipped in the aqueous alumina slurry prior to 
pre-sintering. However, the maximum value for in-line transmission of the 
treated tubes showed a marked improvement (47%) and the minimum in-line 
transmission showed an even greater improvement (56%). 
The results obtained with the second test (Table II) again showed that 
while the percent total transmission was only slightly improved (0.2%) 
when the green tubular compacts were dipped in the slurry and pre-sintered 
for ten hours rather than four hours at 1100.degree. C., the in-line 
transmission was greatly improved by the treatment (49% improvement for 
the maximum in-line transmission and a 62% improvement for the minimum 
in-line transmission). 
On the basis of these test data, it is believed that pre-sintering the 
"slurry-dipped" or treated green compacts for longer periods of time (10 
hours or so) than that customarily used will provide optimum improvement 
in the in-line transmissive characteristics of the finished 
polycrystalline alumina arc tubes. However, this parameter is not 
especially critical and will vary depending upon the exact formulation of 
the treating-slurry of finely-divided alumina and the porosity and surface 
roughness of the compacts as well as the average particle size of the 
alumina particles in the slurry. 
The total transmission data in the aforementioned tests were obtained in 
the usual manner by slipping the sintered alumina arc tubes over a light 
source of known intensity and then measuring the amount of transmitted 
light in a photometer sphere. The in-line transmission data was obtained 
with a Metrologic Neon Laser instrument (marketed by Metrologic 
Instruments Inc., Bellmawr, N.J.). 
The in-line transmission characteristics of polycrystalline arc tubes for 
discharge lamps are especially important since improvements in this 
property of the fired ceramic material minimize internal reflection of 
radiations generated by the arc within the tube and thus allow such 
radiations to pass directly through the arc tube wall. This prevents the 
radiations from being absorbed by the arc and also tends to reduce the 
operating temperature of the arc tube walls. 
While the arc tubes made for the aforementioned tests were manufactured by 
isostatically compressing the raw-mix powder into green compacts, the 
slurry-treating process of the present invention can also be employed to 
improve the in-line transmission of arc tubes (and other articles) that 
are formed by extruding the raw-mix material into green compacts using 
suitable metal dies or the like. The pore-filling and surface-leveling 
action of the slurry-deposited Al.sub.2 O.sub.3 particles should improve 
the smoothness of the arc tube surfaces even though the extrusion-forming 
operation inherently provides such arc tubes with a finish that is quite 
glossy and very even.