Note: Descriptions are shown in the official language in which they were submitted.
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CERAMIC ARC TUBE
TECHNICAL FIELD
This invention relates to ceramic arc tubes for high intensity
discharge lamps. In particular, this invention relates to
forming hermetic seals in ceramic arc tubes.
BACKGROUND ART
Ceramic arc tubes are used to contain the high temperature arcs
of high intensity discharge (HID) lamps. General examples of
ceramic arc tubes for HID lighting applications are shown in
U.S. Patent Nos. 4,387,067, 9,999,145, and 4,799,601 which are
incorporated herein by reference. Historically, these arc tubes
were long cylindrical tubes, but recent lamp developments and
the use of corrosive metal halide fill materials have dictated
the use of more compact and complex shapes. For example, when a
cylindrical geometry is used with a metal halide fill, the lamp
fill tends to reside at the ends of the arc tube in the corner
between the tube wall and the end button. During long term
operation, these lamps are often characterized by corrosion of
the ceramic in this area. The lamp ultimately fails when the
corrosion breaches the arc tube wall allowing the fill gas to
leak. Modifying the arc tube geometry to have hemispherical ends
reduces the arc tube corrosion by eliminating the cold spot
where the fill condenses. Not unexpectedly, the compact and
complex arc tube shapes are more difficult to manufacture. In
addition, if several parts are used in the arc tube assembly,
greater effort is required to keep the parts aligned and obtain
hermetic seals. Thus, it is also desirable to reduce number of
parts and seals used to construct the arc tube.
Blow molding and gel casting can be used to form unitary arc
tube bodies having the desired internal geometry but both
methods have inherent disadvantages. Blow molded lamps are
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limited by the degree of expansion achievable during blowing and
are characterized by a thinning of the arc tube wall. Gel
casting is limited by the need to use a temporary core to define
the cavity shape. The temporary core becomes entrapped inside
the formed arc tube and must be removed. Typically, this is
achieved by melting the core which is made of a low melting
point material such as a wax. Because the core is destroyed by
this process, gel casting tends to be a more expensive process.
Additionally, the temporary core material can contaminate the
arc tube cavity causing problems with lamp operation.
While it is possible to form a unitary arc tube body by blow
molding or gel casting, current designs further require that two
capillaries which contain and seal the electrodes be joined to
the arc tube by interference sintering. This results in a three
piece arc tube construction having two joints that must be
hermetic after sintering.
SUMMARY OF THE INVENTION
It is an object of the invention to obviate the disadvantages of
the prior art.
It is another object of the invention to provide a ceramic arc
tube having a two piece construction.
It is a further object of the invention to provide a ceramic arc
tube assembly which can be formed by conventional ceramic
molding techniques.
In accordance with one aspect the invention, there is provided a
ceramic arc tube comprising a hollow ceramic body having a wall,
electrodes, male and female sections, and a cavity containing a
fill material; the electrodes extending into the cavity through
the wall and being connectable to an external source of
electrical power; the male section being sealed hermetically to
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the female section by a lap joint; the lap joint having internal
and external positioning interfaces and a sealing interface.
In accordance with another aspect of the invention, there is
provided a ceramic arc tube assembly comprising male and female
arc tube sections; each section being substantially hollow and
having a wall and open and closed ends; the open ends having a
sealing surface and internal and external positioning surfaces,
the internal and external positioning surfaces being located at
opposite ends of the sealing surface; the closed ends having an
opening for receiving an electrode; the male and female sections
when joined at the open ends forming a lap joint and enclosing a
cavity.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig 1. is a cross sectional view of a cylindrical arc tube
assembly having cylindrical sealing surfaces.
Fig. 2 is a cross sectional view of an assembled arc tube having
the configuration shown in Fig. 1.
Fig. 3 is a cross sectional view of a cylindrical arc tube
assembly having frustoconical sealing surfaces.
Fig. 4 is a cross sectional view of an assembled arc tube having
the configuration shown in Fig. 3.
Fig. 5 is a cross sectional view of another axially symmetric
arc tube having frustoconical sealing surfaces.
Fig. 6 is a cross sectional view of an assembled arc tube having
the configuration shown in Fig. 5.
DESCRIPTION OF THE INVENTION
For a better understanding of the present invention, together
with other and further objects, advantages and capabilities
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thereof, reference is made to the following disclosure and
appended claims taken in conjunction with the above-described
drawings.
The present invention uses a two piece arc tube design requiring
only one hermetic joint. The arc tube is made in two sections
and joined using a lap joint. Because the arc tube body is made
in two sections, each section can be produced by conventional
ceramic forming techniques such as isostatic pressing, injection
molding, slip casting, or gel casting. If gel casting is used, a
temporary core is not needed to form the internal contours of
the arc tube because each section has an accessible open end.
The lap joint is designed so that the two sections may be easily
aligned and fitted together. Overlapping flanges are created on
the open ends of each section. The flanges provide present
surfaces for sealing and positioning the two sections. To
maintain a tight seal, the sections may be designed with a
slight interference fit. The amount of interference fit required
to maintain a tight seal is typically 1 to 8~. Greater degrees
of interference fit could be used but might lead to considerable
distortion of the arc tube shape.
After the parts are formed and assembled, any organic binder in
the green parts is removed. The assembled sections are
presintered to tack bond them together and remove residual
organic material. An alternative method of joining the two
sections is to combine a temperature slightly above the
softening or melting point of the organic binders used in the
fabrication of the sections with a slight pressure to weld the
two sections together prior to binder removal. In this manner,
the degree of interference fit could be reduced or eliminated
thereby reducing distortion of the sintered arc tube. The final
sealing of the two sections is achieved by sintering the arc
tube usually in a hydrogen containing atmosphere. The
temperatures, heating rate, cooling rate and soak period at peak
temperature will vary depending on the ceramic composition.
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Various arc tube and lap joint configurations are shown in Figs.
1-6.
Fig. 1 is a cross sectional view of an arc tube assembly having
substantially hollow, cylindrical male 4 and female 2 sections.
Each section has the same inside and outside diameters. At the
closed end 11 of each section, there is an opening 12 in the
wall 19 for inserting an electrode (not shown). Capillaries 30
are attached at the closed ends 11 to facilitate positioning and
sealing of the electrode in the completed arc tube. At open ends
and 17, the sections have internal 10 and external 8
positioning surfaces and sealing surfaces 3 and 6. These
surfaces conjoin to form the lap joint shown in Fig. 2. More
particularly, flange 5 of male section 4 extends coaxially at
15 the periphery of open end 17. The outside diameter of flange 5
is less than the outside diameter of the male section. Sealing
surface 3 is cylindrical and is formed by the outside diameter
of flange 5. Similarly, flange 7 of female section 2 extends
coaxially at the periphery of open end 15. Flange 7 has an
inside diameter which is greater than the inside diameter of
female section 2 and equal to the outside diameter of flange 5.
Sealing surface 6 is cylindrical and is formed by the inside
diameter of flange 7. The recess formed at the open end 15
accepts flange 5 of the male section when the parts are mated to
form the arc tube. Positioning surfaces 8 and l0 are formed by
the discontinuities in the diameters of each section 2 and 4 at
their open ends. Positioning surfaces 8 and 10 ensure proper
mating of the two sections. Preferably, positioning surfaces 8
and 10 are substantially orthogonal to the central axis of
sections 2 and 4 however other configurations are possible.
In the embodiment shown in Fig. 1, sealing surfaces 3 and 6 are
cylindrical and coaxial with the central axis of the two
sections. However, in other embodiments, one or both of the
sealing surfaces may be slightly tapered. For example, when an
interference fit is desired, the outside diameter of flange 5 is
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made slightly larger than the inside diameter of flange 7 and
sealing surface 3 of male section 9 is tapered inwardly.
Hermetic seals have been obtained using a 1~ interference fit
(0.004 inches) and a 2° taper on the male flange. Similar
performance was observed when the interference fit cut in half
to 0.002 inches.
The arc tube sections are made preferably from alumina which
provides the necessary degree of translucency after sintering. A
preferred composition is a pure alumina containing about 500 ppm
MgO. After forming and assembling the parts, the assembled
sections are presintered in air~at below 1350°C, and preferably
at 1200°C for 1 hour. Final sintering is performed in a hydrogen
containing atmosphere at a temperature from about 1820°C to
1950°C. More preferably, the arc tube parts are sintered in wet
H2 at 1935°C for 4 hours. In addition to forming hermetic seals,
the sintered arc tubes of the present invention exhibit
excellent dimensional control and reproducibility.
Fig. 2 is a cross sectional view of an assembled arc tube having
the configuration shown in Fig. 1. Male section 9 has been mated
to female section 2 and sintered to form a hermetic seal at lap
joint 25. Lap joint 25 is composed of sealing interface 28 and
positioning interfaces 22 and 24. The sealing and positioning
interfaces in the assembled arc tube are formed at the junction
of the sealing and positioning surfaces shown in Fig. 1. A
hermetic seal is formed at the sealing interface 28 after final
sintering. The general placement of sealing interface 28 is
midway through arc tube wall 14. However, the relative thickness
of flanges 5 and 7 may be adjusted to effect the formation of
the sealing interface at different depths in wall 19. It is
preferred that the width of sealing interface 28 be at least
equal to half the thickness of the wall 19 in the region
immediately adjacent to the lap joint. Hermetic seals may also
be formed at the positioning interfaces 22 and 24. However,
depending on the lap joint configuration, gaps may form at the
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positioning interfaces after sintering. In this embodiment,
these gaps may extend up to halfway through the arc tube wall
but have not been shown to affect lamp performance. The interior
gap can be eliminated by making the male flange 0.001 to 0.005
inches longer than the female flange.
Electrodes 20 are sealed hermetically into capillaries 30 by a
glass frit. The electrodes extend into cavity 32 and are
connectable to an external source of electrical power such as a
ballast. The cavity 32 contains fill 35 and a fill gas (not
shown). The fill 32 may consist~of any number of known arc tube
fill materials and fill gases.
Fig. 3 and 4 are cross sectional views of an arc tube assembly
and resultant arc tube similar to those shown in Figs. 1 and 2.
Flanges 5 and 7 are tapered with respect to central axis 99 to
give sealing surfaces 90 and 43 and sealing interface 42 a
frustoconical shape. The taper angle formed by frustoconical
sealing surfaces 40 and 43 and central axis 49 may varied from 5
to 20 degrees. The limit on the taper angle is the angle beyond
which a good diffusion bond seal can be achieved. If the angle
is too steep, the driving force for bonding using an
interference seal would be reduced to a point where sections 2
and 4 would separate during sealing by sliding along the sealing
interface 42 instead of joining. The frustoconical shape can
provide a wider sealing interface than the cylindrical shape and
tends to reduce the formation and penetration of gaps at
positioning interfaces 22 and 29. This is believed to increase
the strength of the lap joint 47.
Fig. 5 and 6 illustrates an embodiment of the present invention
in which the arc tube shape is not a right cylinder but still
remains axially symmetric about central axis 49. In this
embodiment, closed ends 11 have a rounded, hemispherical
geometry. As previously explained, the rounded contour reduces
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corrosion of the arc tube wall caused by the condensation of
fill 35. Lap joint 47 is the same as the lap joint shown in
Figs. 3 and 4.
While there has been shown and described what are at the present
considered the preferred embodiments of the invention, it will
be obvious to those skilled in the art that various changes and
modifications may be made therein without departing from the
scope of the invention as defined by the-appended claims.
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