Electric lamp and discharge devices – With gas or vapor – Electrode composition
Reexamination Certificate
2002-11-25
2004-10-26
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
With gas or vapor
Electrode composition
C313S637000, C313S638000, C313S639000, C313S631000, C313S623000, C313S625000, C313S641000, C313S642000, C313S571000, C313S572000
Reexamination Certificate
active
06809478
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a structure of a metal halide lamp for an automobile headlight.
BACKGROUND ART
In recent years, as a lamp for an automobile headlight, a new small metal halide lamp that can be substituted for a conventional tungsten halogen lamp has been developed and commercially expanded. This metal halide lamp has the advantage that it can achieve a luminous flux three times as high as that of the conventional tungsten halogen lamp while the 35 W lamp power of the metal halide lamp is smaller than the 55 W lamp power of the conventional tungsten halogen lamp. On this account, the spread of this metal halide lamp has been promoted as a next-generation lamp capable of achieving improved brightness as well as energy saving and allowing still safer drive of an automobile, particularly at night.
FIG. 9
shows a structure of an arc tube of such a metal halide lamp for an automobile headlight. An arc tube
19
of the lamp has the structure as follows. An envelope
20
of the arc tube is made of quartz, and electrodes
21
and
22
made of a pair of tungsten bars are provided at both ends of the arc tube. Molybdenum foils
25
and
26
are sealed hermetically in sealing end parts
23
and
24
of the envelope
20
, and a rear end of the tungsten electrode
21
is welded and connected to one end of the molybdenum foil
25
and a rear end of the tungsten electrode
22
is welded and connected to one end of the molybdenum foil
26
. External leads
27
and
28
are welded and connected to the other ends of the molybdenum foils
25
and
26
, respectively. Inside the arc tube, 0.01 to 1.0 mg of a mixture of scandium iodide and sodium iodide (NaI+ScI
3
) is sealed as a main component of a luminescent material
29
together with 0.1 to 1.0 mg of mercury and 0.1 to 1.5 MPa of xenon as buffer gases
30
. Typically, the size of the arc tube
19
is such that the distance Le between the electrodes is 4.2 mm, the inner diameter &phgr;i of the arc tube is 2.8 mm, and the inner volume of the arc tube is 30 mm
3
at maximum. It is to be noted here that other metal halide materials such as ThI
4
, LiI, TlI, and the like also may be sealed in the arc tube.
The lighting operation of the above-mentioned metal halide lamp for an automobile headlight is different from that of a normal metal halide lamp for general lighting. Specifically, during the lighting operation of the above-mentioned metal halide lamp, flickering occurs repeatedly, including the flickering at the time of a so-called “instantaneous restart”. Besides, in order to obtain the required luminous flux immediately after the lamp is turned on, the lamp immediately after being turned on is subjected to a lamp current of 2.6 A, which is about seven times as high as the lamp current of 0.4 A during the steady-state lighting. As described above, the metal halide lamp for an automobile headlight is operated according to a unique, relatively demanding lighting system.
At the beginning of the development of the conventional metal halide lamp shown in
FIG. 9
, a first problem was found that the quartz present in the sealing end parts
23
and
24
of the envelope
20
of the arc tube
19
, especially around portions of the tungsten electrodes
21
and
22
sealed therein (hereinafter, such portions are referred to as “sealed portions”), may have cracks and/or may be damaged within a relatively short time of 500 hours or less, resulting in a short lifetime of the lamp. It can be said that this problem is related to the unique lighting system as described above.
Since then, various studies have been made to solve the above-mentioned problem, and means for solving the problem are disclosed, for example, in JP 7(1995)-282719 A, JP 7(1995)-21981 A, JP 10(1998)-223175 A, JP 10(1998)-269941 A, etc.
As a means for preventing the occurrence of cracks and/or damage in the quartz present around the sealed portions of the electrodes, electrodes
21
and
22
made of a so-called “thoriated tungsten material” that contains thorium oxide (ThO
2
) as an additive particularly in an amount of 1 to 2 wt % has been employed. By using such electrodes, the adhesive strength between the electrodes and the quartz can be increased. Further, as shown in an enlarged view of
FIG. 10
illustrating the sealing end part
23
of the envelope, a quartz coating
31
, which is not mechanically connected to the quartz present in the sealing end part
23
, is formed on the periphery of the tungsten electrode
21
(The same occurs on the periphery of the tungsten electrode
22
).
As a result, the quartz present in the sealing end part
23
is no longer subject to stress distortion due to the difference in thermal expansion between the tungsten electrode
21
and the quartz at the time of the flickering of the arc tube
19
. Cracks and/or damage occurring in the quartz present in the sealing end part
23
thus can be prevented. Since this means is extremely effective in preventing the occurrence of cracks and/or damage in the quartz, it has been a major technique generally applied to the conventional lamp shown in FIG.
9
.
Another effective means is winding a tungsten coil
32
on the periphery of the sealed portion of the tungsten electrode
21
of the arc tube
19
as shown in FIG.
11
. In contrast to the case where the above-mentioned means using the thoriated tungsten electrodes is employed, the adhesion between the tungsten electrode
21
in the sealing end part
23
of the envelope and the quartz on the periphery of the electrode
21
is made very weak by using the tungsten coil
32
. However, in this case, the quartz on the periphery of the electrode still is subjected to less stress distortion at the time of the flickering of the arc tube, thereby allowing cracks and/or damage occurring in the quartz to be prevented. It is to be noted that similar means have been applied to a conventional tungsten halogen lamp.
A second problem found in the development of the conventional metal halide lamp shown in
FIG. 9
is that the lumen maintenance factor of the arc tube
19
decreases with the passage of time after the lamp is turned on. As described above, the arc tube
19
having a small volume is operated at a lamp power of 35 W in the steady state. In addition, the arc tube
19
is operated according to a demanding lighting system in which instantaneous restart, high-current operation immediately after the lamp is turned on, etc. are required. Thus, the operating temperature of the envelope
20
made of quartz rises particularly remarkably to reach 1000° C. or more, and the tungsten electrodes
21
and
22
are evaporated and/or worn away considerably, which cause devitrification and blackening of the envelope
20
. Therefore, the decrease in lumen maintenance factor of the lamp due to the devitrification and blackening of the envelope
20
cannot be avoided.
As disclosed in JP 7(1995)-21981 A, for example, the thoriated tungsten electrodes
21
and
22
containing ThO
2
as a means for solving the first problem has been regarded as effective also in solving the second problem and employed as a means for solving the second problem. Conventionally, there has been a widely accepted theory as follows in the art. That is, in a high-pressure discharge lamp employing the thoriated tungsten electrodes, a monatomic layer of Th is formed during the lamp operation to decrease the work function at front ends of the tungsten electrodes and thus decrease the operating temperature thereof considerably, thereby suppressing the blackening of the arc tube caused mainly by the tungsten evaporated from the tungsten electrodes. In fact, in a conventional NaI—ScI
3
-based metal halide lamp for general lighting, thoriated tungsten electrodes generally have been used to suppress a decrease in the lumen maintenance factor caused by the blackening of the arc tube during the lifetime of the lamp.
As a specific means for solving the second problem, a manufacturing process has been developed and introduced into service that provides a high-purity lamp by sufficiently
Kiryu Hideaki
Yoshida Masato
Hodges Matt
Patel Nimeshkumar D.
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