Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to an improved method for the
manufacture of translucent, dense, polycrystaline, aluminum
oxide bodies. The invention relates more particularly to an
improved method for the manufacture of such bodies for use in
electric lamps.
In modern lighting technology, the need exists for an
efficient and inexpensive lamp of relatively long life. A
variety of different gaseous lamps such as those filled with
the inert gases neon and xenon, those which employ halogens
such as iodine, and a variety of lamps filled with metal com-
pounds, such as mercury and sodium vapor lamps represents
improvements in this area. Of this group, high pressure sodium
vapor lamps have been found to exhibit most desirable character-
istics. However, attempts to use sodium vapor lamps with quartz
tubes has undesirably resulted in the transformation of quartz
to opaque silicon thereby substantially reducing the efficiency
of the lamp.
In searching for suitable alternative translucent
materials for such lamps, it has been found that the use of a
translucent lamp body formed of a polycrystalline alumina is
beneficial. An improved composit~on of the polycrystalline
alumina lamp body material has been provided by the use of
additives such as magnesia, cobalt and titanium oxide.
Both the diversity of applications and the high light-
ing efficiency of such lamps makes it desirable to provide
improved, economical processes for manufacturing the alumina
body. Although the improved polycrystalline alumina materials
have provided lamp bodies having suitable translucency, the
present day processes for fabricating the bodies is not wholly
satisfactory notwithstanding efforts to improve their manufac-
ture by varying some of the manufacturing conditions.
Alumina tubes of this type were initially formed by
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an isostatic compaction and compressing process. However, this
forming process is relatively complex and does not readily lend
itself to automation. On the other hand, the known powder or
ceramics metallurgical processes such as vacuum compacting and
compressing, extruding, or injection molding are also less than
satisfactory since they result in the production of bodies of
low density. In addition, tube bodies fabricated by these
latter techniques often fail by virtue of their own weight due
to their relatively high plasticity.
Metals or metal alloy powders having a plasticizer
additive which are useful in extrusion or injection molding pro-
cesses are also known. This forming technique employs a rela-
tively low forming pressure of the order of 5 to 10 atmospheres
so that the prepared mass of material must be relatively highly
plasticized. The elevated temperature necessary for the removal
of this plasticizer from the alumina mass and the conditions
employed in this process undesirably result in the creation of
a great number of undesirable micropores in the body.
A hot injection molding process is also known wherein
a metal powder containing a plasticizer is pressed at a pressure
of 2 to 5 atmospheres and at a temperature of 60 to 140C before
firing. However, under these conditions only relatively small
size bodies can be prepared since the removal of the relatively
large amount of plasticizer employed is quite difficult and the
plasticity of the intermediate product can readily lead to
deformation.
These prior methods have not fully satisfied the need
for the manufacture of alumina lamp tubes having the desired
shapes and having relatively thin, translucent, containment walls.
Accordingly, it is an object of this invention to pro-
vide an improved process for manufacturing translucent containment
bodies for high pressure metal vapor lamps.
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Another object of the invention is to provide an
improved process for manufacturing translucent containment
bodies for high pressure metal vapor lamps which avoids one
or more of the disadvantages encountered in the prior manu-
facture of such lamps.
A further object of the invention is to provide an
improved method for manufacturing tubular shaped, translucent,
dense, polycrystalline alumina bodies.
Another object of the invention is to provide an
improved material for the fabrication of tubular shaped, trans-
lucent, dense, polycrystalline alumina bodies.
In accordance with the general aspects of this inven-
tion, a composition comprising finely divided alumina, metal
additives and a relatively small quantity of organic plasti-
cizer compound is continuously cold formed under relatively high
pressure into a tubular shape. The plasticizer is removed by
heating the formed tubular material and a translucent polycrystal-
line body is obtained by placing the suitably shaped body in an
inert gas atmosphere or in a vacuum.
In accordance with more particular aspects of the inven-
tion, a quantity of the material comprises an alumina powder
which is substantially alpha but contains some gamma and which
is at least 90% pure by weight, a metal compound additive and a
suitable organic compound plasticzer. This material is continu-
ously formed by a cold flow process into an elongated tubular
body. The cold flow tube formation process is carried out under
a suitable pressure with a suitable tool which preferably has a
forming surface lined or coated with alumina ceramics. After the
tubes have been formed by this cold flow process, the plasticizer
is removed by immersing the tube in aluminum oxide powder and
heating the tubes under carefully controlled conditions to a
temperature of no greater than about 1400C. The alumina powder
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is then removed from the tube surface and the tubes are heated
in a vacuum or in an inert atmosphere, preferably in a plasma,
at a temperature of greater than about 1800C, for about at least
five minutes.
These and other objects and features of the invention
will become apparent with reference to the following specifica-
tion and to the drawings wherein:
Figure 1 is a side view, partly in section of a cylin-
drically shaped tubular body constructed in accordance with
features of this invention;
Figures 2a-2f (incl.) are side views of alternatively
shaped tubular bodies constructed in accordance with features of
this invention;
Figure 3 is a view of an apparatus for forming slugs
from a body material which are employed with a body-forming
apparatus in accordance with features of this invention;
Figure 4 is a view of a cold flow forming apparatus
for forming tubular bodies in accordance with the present inven-
tion; and,
Figure 5 is a view of an alternative cold flow forming
apparatus for forming tubular bodies in accordance with the
present invention.
A material, in accordance with the present invention
from which translucent, containment bodies for high pressure
metal vapor lamps is fabricated, comprises at least about 90%
by weight pure and preferably better than about 99.5% by weight
pure aluminum oxide powder, a maximum of about S~ gan~a aluminum
oxide, less than about 1% of a plasticizer preferably formed of
an organic compound and one or more metals. The metals can
comprise magnesium, cobalt, iron, calcium, nickel, molybdenum,
and tungsten. These metals have been found to be useful in pro-
viding a desired crystal structure and grain boundaries in the
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formed body material. It is desirable, however, that the mater-
ial for forming the polycrystalline tubes of high density be
substantially free of materials such as copper, manganese, lead,
tin and zinc since these materials have been found to impair the
required mechanical properties of the tubes.
Suitable plasticizers for use with aluminum oxide
compounds in cold flow processes comprise a mixture of paraffin
and the fatty acids, wax-like substances such as beeswax and
other organic materials having similar properties. These plasti-
cizer materials should be thermoplastic and melt at temperatures
above about 50C. Upon heating, they should leave or egress from
the body without forming gases such as by a relatively slow
sweating of the plasticizer from the body.
A preferred material in accordance with an embodiment
of this invention is prepared by thoroughly mixing 1,000 grams
of substantially alpha aluminium oxide, which has been treated
with 3% by weight magnesium nitrate, and with a solution of 5
grams of oleic acid, 3 grams of paraffin in 100 grams of benzene.
The mass of material thus obtained is dried, compressed
into cylinders or plugs, and charged to a hydraulic press for
forming elongated tubular-shaped bodies by cold flow.
In Figure 3, the material 10 is initially positioned
within a bore 12 of a body 14. A cylindrically shaped press or
piston 16 is adapted for extending into the bore 12 and for
compressing the material 10 into a cylinder or plug 17. In a
typical example, the mass of material 10 is compressed under a
pressure of about 50 kilograms per square centimeter into a
cylindrical plug 17 having a diameter of about 20 millimeters
and a height of about 20 millimeters.
As illustrated in Figure 4, the cylindrically shaped
plug 17 is then positioned in a bore 20 of a hydraulic press 22.
The cylindrically shaped bore 20 terminates in a centrally
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located aperture 24. The head of a mandrel 26 extends slightly
into this bore. The aperture 24 and the solid mandrel 26
together form an angular ring-shaped aperture having, for
example, an outside diameter of about 10 millimeters and an
inside diameter of about 8 millimeters. This aperture provides
for the formation through cold flow of a cylindrically shaped
tubular body 27 (Figure 1) having a wall thickness of about 1
millimeter when a piston Z8 establishes a suitable continuous
pressure. It is preferable that the tool surfaces and mandrel
surfaces with which the material makes contact be lined or covered
with alumina in order to obtain resistance to wear of the tool
and in order to avoid contamination of the product. The applica-
tion of a continuous pressure by the piston 28 to the plug 17
results in the continous cold flow of the material through the
annular aperture and the formation of the tubular shaped body
27 about the mandrel 26.
In order to fabricate a tube of predetermined length
such as a tube of about 100 millimeters in length, there is
assumed a 25% shrinkage in later stage processing. Accordingly,
a longer tube length, such as 135 millimeter tube is drawn from
the press of Figure 4. In this case the mandrel 26 preferably
has a length for example, of about 140 millimeters. As the
tubular shaped aluminum mass flows from the mandrel, the tube is
cut with a tool to the length of about 135 millimeters. This
cut is made near the end of the mandrel and the tube, by virtue
of its own weight, separates from the mandrel.
Subsequent to separation of the tubular shaped body
27 from the mandrel, the tubes thus obtained by cold flow are
immersed or embedded in aluminum oxide powder which has been
heated to a temperature of between about 1200C and about 1400C
and preferably about 1250C. The tubular body is thus heated in
the aluminum oxide to a temperature of about 1250C. The rate
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of heating is important in order to remove the organic plastici-
zer compound from the interior of the shaped body without causing
deformation. Therefore, in the temperature range in which the
organic material melts and sweats out, it is desired that the
temperature be increased slowly in a controlled manner at a rate
of about 5-10C per hour. The aluminum oxide powder provides
mechanical support for the shaped bodies during the heating pro-
cess in the temperature range between the melting point of the
plasticizers and the temperature at which complete sweating out
is accomplished. In addition, the aluminum powder absorbs the
plasticizer which sweats out from the body and the evaporation
and thermal decomposition of the plasticizer occurs at higher
temperatures after the plasticizer is completely taken up by
the aluminum powders.
Upon removal of the aluminum oxide powder from the
surface of the preheated tubular bodies, the bodies are fired
in a vacuum, a neutral atmosphere, in a reducing gaseous atmos-
phere, or preferably in a plasma. The tubes are preferably
placed on smooth mandrels made out of dense polycrystalline or
sapphirine type alumina and are positioned, for example, in a
hydrogen gas atmosphere or preferably in a hydrogen plasma and
are fired at a temperature greater than about 1800C and pre-
ferably at a temperature of about 1980C for more than about 5
minutes and preferably for about 10 minutes.
The diameter of the mandrel which supports the pre-
viously described tubular bodies during the sintering process
is preferably 5.9 millimeters. This diameter is .1 millimeters
smaller than the final 6 millimeter inner diameter of the tube
which now has an inner diameter which is reduced in dimension
during the processing by 2 millimeters from an initial 8 milli-
meter size. During this reduction in size of the tube, the tube
should be orientated in a perpendicular position. Accordingly,
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the mandrels are placed within the perpendicularly oriented
tubes or the tubes are placed onto perpendicularly fixed mandrels.
In order to provide a uniform temperature distribution during
this sintering, a molybdenum tube sheath is positioned about the
tubes to be sintered. The molybdenum tube has an inner diameter
of at least 15 millimeters and a wall thickness not greater than
2 millimeters. It has been found thatpolycrystalline aluminum
tubes which are produced by this process when used in a high
pressure sodium lamp yield a light efficiency of 90-100 lumens
per watt.
The cold flow technology described herein may be
employed for the production of tubes having shapes differing
from cylindrical shapes. A variety of shapes, which may be
desirable in certain lamp constructions, can be provided by
shaping of the tube subsequent to the cold flow step. For
example, such tubes can be coiled upon warming into a coil or
by suitable techniques, for example, by warming and blowing
into a mold, they can be transformed into an oblong shape such
as the tubes 29, 41, 42, 43, 44, 45, illustrated in Figure 2.
This shape at times provides a more suitable geometry for the
fabrication of high pressure sodium vapor tubes. In addition,
the cold flow technology described herein can be used not only
for obtaining alumina bodies but also for manufacturing bodies
made of metal powder or glass powders or for the manufacture
of glass wafers which are employed in the electronics arts.
Figure 5 illustrates an alternative cold flow tubular
forming apparatus for forming tubular shaped bodies in accordance
with this invention. The plug 17 is charged to the bore 30 of
a cylinder 32. The cylinder 32 includes a funnel-shaped channel
34 which communicates with a second cylinder 36 within which a
mandrel 38 is positioned. As a piston 40 exerts a continuous
pressure on the plug 17, the plug is forced to cold flow through
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the channel 34 to the space surrounding the mandrel 38 in the
cylinder 36 to provide the tubular shaped body 27.
There has thus been described an improved process and
material for fabricatin~ relatively dense, i.e. substantially
nonporous, polycrystalline, translucent, tubular shaped bodies
partieularly for use in high pressure metal vapor electric lamps.
The process is particularly advantageous in that it can be
readily automated thereby improving the efficiency and reducing
the cost of production of these bodies. In addition, the des-
cribed process provides for altering the initial, cylindrical,tubular shaped form in order to satisfy the particular need or
the use. The material deseribed is particularly advantageous in
that it provides a body with a desired structural and optical
eharacteristics while faeilitating the carrying out of the pro-
cess described.
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