JP4120800B2 - Electrode for discharge lamp - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electrode for a discharge lamp used as a backlight of a liquid crystal display (LCD) such as a television receiver or a personal computer.
[0002]
[Prior art]
A liquid crystal display (LCD) used in office automation equipment such as a personal computer or a word processor incorporates a backlight having a cold cathode fluorescent lamp as a light source for illuminating the LCD.
[0003]
Conventionally, what has been conventionally used as an electrode for a backlight of the cold cathode fluorescent lamp or the like has been made of nickel, which has relatively good sputtering resistance and excellent drawing workability.
[0004]
[Problems to be solved by the invention]
However, in recent years, in order to further increase the lifetime and increase the brightness, there has been a demand for an electrode made of a material having a higher sputtering resistance and a lower work function. Conventional nickel can satisfy this requirement. It is gone. For this reason, an electrode material having a lower sputtering rate than nickel is first demanded. The reason why the sputtering rate is low is that the progress of blacking is slow. Also, the lower work function of the electrode material is preferable because when the work function is low, the cathode fall voltage becomes small, and sputtering on the electrode surface hardly occurs at the same voltage. That is, the inner wall of the tube near the electrode due to sputtering is reduced and a long life is obtained. On the contrary, if the lifetime is the same, there is an advantage that the voltage can be increased to increase the luminance.
[0005]
Tungsten, molybdenum, and the like are known as metals having better sputtering resistance than nickel. However, these metals are refractory metals, which are difficult to form. For example, in order to form an electrode having a complicated shape such as a cup shape, processes such as powder pressing, sintering, forging, rolling and drawing are performed. is required. For this reason, with tungsten, it is difficult to obtain a desired shape having an inner diameter: depth of 1: 3, for example. Further, although molybdenum can be manufactured, there is a problem in that the manufacturing cost is significantly increased because the processing rate cannot be increased and the yield is reduced. In view of such circumstances, the present invention is an electrode for a discharge tube that is superior in sputtering resistance to a conventional discharge lamp electrode and can achieve a long life and high brightness when incorporated in a discharge lamp. An object of the present invention is to provide an electrode that is relatively easy to manufacture.
[0006]
In order to solve the above problems, the present invention has the following configuration. That is, the invention according to the present invention is a discharge lamp electrode of the schematic cup-shaped recess openings are formed at one end, the main component of tungsten and / or molybdenum, nickel, iron, cobalt It is characterized by containing 0.01 to 10% by weight of one or more of the iron group metals.
[0007]
Since the discharge lamp electrode is mainly composed of tungsten and / or molybdenum having excellent sputtering resistance, the obtained electrode has better sputtering resistance than the conventional nickel electrode. Also, pure tungsten and pure molybdenum have high melting points and poor formability, so in order to obtain a high-density electrode with a predetermined shape, it is necessary to perform mechanical plastic processing such as rolling forging, rolling and drawing after sintering. There, impossible manufacture of tungsten, but the manufacturing cost becomes high in molybdenum in the present invention, this nickel as the iron group metals, iron, since the sintering by adding a metal cobalt or the like, improves sinterability, A complicated shape such as a cup-shaped electrode can also be manufactured relatively easily.
[0008]
From the viewpoint of work function, in addition to the above, one or two of the group consisting of lanthanum oxide, yttrium oxide, cerium oxide, strontium oxide, zirconium oxide, hafnium oxide, barium oxide, thorium oxide, lanthanum boride It is preferable to contain 0.1 to 4% of the above by weight ratio.
[0009]
When the electrode of claim 1 or 2 is manufactured by MIM or press molding, it is very difficult to make the wall thickness of the cup electrode about 0.1 mm. The reason why such a thin wall thickness is required is to increase the surface area of the cup inner surface as much as possible in order to increase the glow discharge efficiency and to increase the cup inner diameter as much as possible at a predetermined outer diameter. . When the wall thickness is reduced, in the case of MIM, the wraparound of the resin component containing the metal powder becomes insufficient. In addition, shape retention during degreasing becomes difficult. Even in powder press molding, the movement of the powder cannot catch up and cannot be molded into such a shape. Further, since the inner diameter of the fluorescent tube is very small, the outer diameter tolerance management of the cup is also strict.
[0010]
Thus, it has been found that forming the wall thickness larger than a predetermined dimension and then processing the outer diameter by centerless grinding or the like is effective for reducing the wall thickness and managing the outer diameter. In the case of MIM, a sintered body having a predetermined thickness, for example, twice the wall thickness of 0.1 mm may be manufactured, and for example, centerless grinding may be performed with a # 440 GC grindstone. A thick sintered body may be manufactured and centerless ground with a # 440 GC grindstone.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail with specific examples. First, the main components of the electrode of the present invention are tungsten, molybdenum, and the like. These metals have a lower sputtering rate than nickel, and are excellent in sputtering resistance. This is apparent from, for example, the graph of FIG. 3 (see Mitsuharu Onuma, “Plasma and Film Formation Fundamentals”, published on August 29, 1986, published by Nikkan Kogyo Shimbun, page 111). Therefore, the discharge lamp electrode using this can provide a long life.
[0012]
On the other hand, this type of electrode is preferably formed in a cup shape in order to increase glow discharge efficiency. In this case, it is effective to form a relatively deep cup shape having a length of about 4 mm for a diameter of 1.7 mm, for example. However, with tungsten, it is difficult to deep-draw into a cup shape. Cost is high and yield is poor.
[0013]
Therefore, in the case of the electrode of the present invention, it is preferable to mold the metal powder using an injection molding method or a press molding method. This metal powder injection molding method injects a kneaded product of a thermoplastic organic binder composed of a high-molecular binder and a low-molecular binder in a volume ratio of 5: 1 to 0.7: 1 into a metal powder such as tungsten or molybdenum. This is a molding method. The obtained molded body is degreased (removal of organic substances) by heating in a reducing atmosphere, and then sintered in a reducing atmosphere at 1500 to 2000 ° C.
[0014]
In this case, in order to improve sintering properties, tungsten, a small amount of iron group nickel refractory metal such as molybdenum, iron, to leave contain a metal cobalt or the like. It has been found that when these iron group metals are contained in a trace amount, for example, 0.01 to 1% by weight, the sintering properties of tungsten and molybdenum are remarkably improved. The iron group metals, iron, nickel, there is a cobalt or the like, Experimentally, nickel was the most effective. If the content of the iron group metal is too small, the sintering property is not improved, and it becomes difficult to perform sintering at an economical sintering temperature, for example, 2000 ° C. or less. If the sintering property is poor, a dense product cannot be obtained and the sputtering resistance is lowered. On the other hand, when the content of the iron group metal is too large, the electrical conductivity and thermal conductivity are lowered, and the performance as an electrode is lowered.
[0015]
On the other hand, a so-called polymerized gold containing tungsten and molybdenum containing iron group metal of more than 2% and not more than 10% by weight has good sintering properties. The density can be sufficiently improved. In this alloy, if there is too much iron group metal, a problem arises in the sputtering resistance. In particular, if there is too much nickel, it tends to become brittle. Further, when allowed to contain nickel, sinterability liquid phase appears during sintering is improved, tungsten and nickel forms an intermetallic compound (N i 4 _W), so brittle, No rather multi It is preferable to add an iron group metal to suppress the formation of intermetallic compounds.
[0016]
Furthermore, in order to lower the work function of the electrode, one or more of the group consisting of lanthanum oxide, yttrium oxide, cerium oxide, strontium oxide, zirconium oxide, hafnium oxide, barium oxide, thorium oxide, lanthanum boride It is preferable to contain 0.1 to 4% by weight. When these contents are less than the above range, the work function hardly decreases. On the other hand, if the amount is too large, the sintering property is lowered and the densification necessary for the sputtering resistance cannot be achieved. Therefore, the above range is preferable.
[0017]
Further, it may be required to increase the glow discharge efficiency. In order to increase the glow discharge efficiency, it is effective to increase the surface area of the cup inner surface as much as possible. In order to increase the cup inner diameter as much as possible at the determined outer diameter, the cup wall thickness is required to be reduced to about 0.1 to 0.15 mm. In the case of MIM, the wraparound of the resin component containing the metal powder becomes insufficient. In addition, shape retention during degreasing becomes difficult. Even in the powder press molding, the movement of the powder cannot catch up and cannot be molded into such a shape.
[0018]
Therefore, it has been proved effective to form the wall thickness larger than a predetermined dimension and then process the outer diameter by centerless grinding or the like.
[0019]
In the case of MIM, a sintered body having a wall thickness twice as large as a predetermined dimension, for example, 0.1 mm is manufactured, and centerless grinding is performed using, for example, a # 440 GC grindstone. In the case of press molding, a sintered body having a wall thickness 4 times is manufactured, and centerless grinding is performed with a # 440 GC grindstone. The outer dimensional accuracy was ± 0.05 mm for the sintered product after press molding and ± 0.02 mm for the MIM product and ± 0.1 mm for the ground product at an outer diameter of 2 mm. Hereinafter, specific examples actually manufactured will be described.
[0020]
[Example 1]
A solution of nickel nitrate in an amount corresponding to 0.02 wt% nickel dissolved in ethyl alcohol was added to tungsten powder having an average particle size of 1.5 microns, mixed and immersed, and then the alcohol was volatilized. The obtained powder was reduced at 750 ° C. in a hydrogen stream.
[0021]
A thermoplastic organic binder was added to the reduced powder and kneaded to obtain a compound for injection molding. Examples of the organic binder include a high molecular binder having a molecular weight of 10,000 or more and a low molecular binder having a molecular weight of 5,000 or less. Examples of the polymer binder include EVA (ethylene vinyl acetate) -BMA (butyl methacrylate) -styrene copolymer, PP (polypropylene), and the like, and these can be used alone or in combination.
[0022]
Examples of the low molecular binder include polyethylene wax, micro wax, carbana wax, stearic acid, and the like, and these can be used alone or in combination. The ratio of the high molecular binder to the low molecular binder is 5: 1 to 7: 1, preferably 3: 1 to 1: 1.
[0023]
The amount of the binder is preferably 30 to 60% by volume with respect to 100% by volume of the tungsten powder. If it is more than this, deformation during degreasing is promoted, and if it is less, injection moldability is lowered. Particularly preferred is 35 to 55% by volume.
[0024]
The kneading of the powder with the binder added was performed at 120 to 160 ° C. using a pressure kneader. The compound obtained by kneading was injection molded into a mold having a cup-shaped cavity having an outer diameter of 1.95 mm, an inner diameter of 1.56 mm, and a length of 5.2 mm by an injection molding machine.
[0025]
The obtained molded body was gradually heated to 750 ° C. in a hydrogen atmosphere and degreased. After that, it was sintered at 1900 ° C. for 1 hour in a hydrogen atmosphere to obtain a tungsten cup-shaped electrode (see FIG. 2) having an outer diameter of 1.2 mm, an inner diameter of 0.9 mm, and a length of 4 mm.
[0026]
The obtained tungsten electrode was resistance-welded with a Kovar lead wire already connected with a nickel lead wire, and the Kovar portion was glass-wrapped and then assembled into a discharge tube (discharge cold cathode fluorescent lamp). FIG. 1 is a partial cross-sectional view of a discharge cold cathode fluorescent lamp. The fluorescent lamp 1 has a straight tube shape having an outer diameter D of about 2.6 mm, an inner diameter D ′ of 1.6 mm, and a length L of about 200 mm. In terms of material, sealing portions 5 and 5 are provided at both ends of a tubular glass bulb 2 made of borosilicate glass or aluminosilicate glass having a linear expansion coefficient of 5 to 5.5 ppm, and an electrode 4 is provided on the sealing portion. The sealing wire 3 connected to is sealed. A fluorescent film 11 is provided on the inner surface of the glass bulb 2.
[0027]
The electrode 4 has a cup shape in which a hollow portion 4a opened on the opposite electrode side is formed. In this hollow portion, lanthanum oxide, yttrium oxide, cerium oxide, strontium oxide, calcium oxide, which are discharge characteristic improving agents, are formed. , Zirconium oxide, hafnium oxide, barium oxide, thorium oxide and lanthanum boride can be filled with one or more. Thus, the point which can be filled with the discharge characteristic improving agent is an advantage of a cup-shaped electrode.
[0028]
[Example 2]
A tungsten powder having an average particle size of 1.5 microns was mixed with a solution of nickel nitrate in an amount equivalent to 0.2 wt% Ni in ethanol to volatilize the alcohol after immersion. The powder was reduced at 750 ° C. To this powder, a solution of lanthanum nitrate in an amount corresponding to ₂ wt% La ₂ O ₃ dissolved in ethyl alcohol was added and mixed, and the soaked alcohol was volatilized.
[0029]
The obtained powder was dried and then reduced at 800 ° C. in hydrogen. Thereafter, a thermoplastic organic binder (using a high molecular binder having a molecular weight of 10,000 or more and a low molecular binder having a molecular weight of 5,000 or less) is added to the reduced powder, kneaded, and a compound for injection molding. It was. As the polymer binder, EVA (ethylene vinyl acetate) -BMA (butyl methacrylate) -styrene copolymer and PP (polypropylene) were used in combination.
[0030]
As the low molecular binder, polyethylene wax and stearic acid were used in combination. The ratio of the high molecular binder to the low molecular binder was 2: 1. The amount of the binder was 40% by volume with respect to 100% by volume of the tungsten powder. Kneading was performed at 130 ° C. using a pressure kneader.
[0031]
The compound obtained by kneading was injection molded into a mold having a cup-shaped cavity having an outer diameter of 1.56 mm, an inner diameter of 1.17 mm, and a total length of 5.2 mm by an injection molding machine.
[0032]
The obtained molded body was gradually heated to 750 ° C. in a hydrogen atmosphere and degreased (organic binder was removed). Thereafter, sintering was performed in hydrogen at 1700 ° C. for 1 hour to obtain a tungsten cup-shaped electrode containing 2 wt% lantana having an outer diameter of 1.2 mm, an inner diameter of 0.9 mm, and a total length of 4 mm.
[0033]
The tungsten electrode obtained as described above was resistance-welded with a Kovar lead wire already connected with a nickel lead wire, and the Kovar portion was glass-wound and then assembled into a discharge lamp (discharge cold cathode fluorescent lamp).
[0034]
[Example 3]
A molybdenum powder having an average particle size of 0.8 micron was mixed with a solution of nickel nitrate dissolved in ethanol in an amount corresponding to 0.1 wt% Ni to volatilize the alcohol after immersion. The powder was reduced at 750 ° C. A thermoplastic organic binder was added to this and kneaded to obtain a compound for injection molding. As the binder, a high molecular binder having a molecular weight of 10,000 or more and a low molecular binder having a molecular weight of 5,000 or less were used. As the polymer binder, EVA (ethylene vinyl acetate) -BMA (butyl methacrylate) -styrene copolymer and PP (polypropylene) were used in combination.
[0035]
As the low molecular binder, polyethylene wax and stearic acid were used in combination. The ratio of the high molecular binder to the low molecular binder was 2: 1. The amount of the binder was 40% by volume with respect to 100% by volume of the tungsten powder. The kneading was performed at 140 ° C. using a pressure kneader.
[0036]
The compound obtained by kneading was injection molded into a mold having a cup-shaped cavity having an outer diameter of 1.95 mm, an inner diameter of 1.56 mm, and an overall length of 5.2 mm by an injection molding machine. The obtained molded body was gradually heated to 750 ° C. in a hydrogen atmosphere and degreased. Thereafter, it was sintered in a hydrogen atmosphere at 1700 ° C. for 1 hour to obtain a molybdenum cup-shaped electrode having an outer diameter of 1.2 mm, an inner diameter of 0.9 mm, and a total length of 4 mm.
[0037]
The obtained molybdenum electrode was resistance-welded with a Kovar lead wire already connected with a nickel lead wire, and the Kovar portion was wound with glass, and then assembled into a discharge lamp (discharge cold cathode fluorescent lamp).
[0038]
When the performance of the discharge cold cathode fluorescent lamps obtained in Examples 1 to 3 was examined, the discharge tube life was 1.5 to 3 times that of the conventional product. Moreover, the brightness | luminance at the same lifetime was 1.2 to 2 times compared with the conventional product.
[0039]
[Example 4]
Tungsten powder with an average particle size of 2.2 μm, 5 wt% carbonyl nickel powder with an average particle size of 6 μm, 2 wt% of carbonyl iron powder with an average particle size of 9 μm, and 1 wt% of cobalt powder with an average particle size of 1.6 μm were mixed in an attritor. The mixed powder was spray granulated with 0.8% of PVA (polyvinyl alcohol) and 0.4% of stearic acid, which are organic binders for pressing, to obtain granulated powder for press molding. This was put into a press machine and press-molded in a predetermined mold. The dimensions of the obtained molded body were an outer diameter of 2.3 mm, an inner diameter of 1.8 mm, and a length of 5.4 mm.
[0040]
The obtained molded body was gradually heated to 800 ° C. in a hydrogen atmosphere, degreased and pre-sintered. Thereafter, sintering was performed at 1500 ° C. for 1 hour in a hydrogen atmosphere to obtain a sintered body having an outer diameter of 2.3 mm, an inner diameter of 1.5 mm, and a length of 4.5 mm. The relative density of the obtained sintered body was 98.5%.
[0041]
The obtained sintered body was centerless ground with a # 320 alumina grindstone from an outer diameter of 2.3 mm to an outer diameter of 1.74 mm. A cup electrode having an outer diameter of 1.74 mm, an inner diameter of 1.5 mm and a wall thickness of 0.12 mm was obtained. The outer diameter dimensional accuracy was ± 0.01 mm after grinding, while ± 0.05 mm after sintering. Grinded products are slightly higher in cost, but are suitable for applications that require high efficiency and high performance such as thin tubes.
[0044]
[Example 5 ]
A molybdenum powder having an average particle size of 2.2 microns was mixed with a solution of nickel nitrate dissolved in ethanol in an amount corresponding to 0.02 wt% Ni to volatilize the alcohol after immersion. The powder was reduced at 750 ° C. To this powder, 0.8% of PVA (polyvinyl alcohol) as a binder for press and 0.4% of stearic acid were added and spray granulated to obtain a granulated powder for press molding. This was put into a press machine and press-molded in a predetermined mold. The dimensions of the obtained molded body were an outer diameter of 2.3 mm, an inner diameter of 1.8 mm, and a length of 5.4 mm.
[0045]
The obtained molded body was gradually heated to 800 ° C. in a hydrogen atmosphere, degreased and pre-sintered. Thereafter, sintering was performed at 1800 ° C. for 1 hour in a hydrogen atmosphere to obtain a sintered body having an outer diameter of 2.3 mm, an inner diameter of 1.5 mm, and a length of 4.5 mm. The relative density of the obtained sintered body was 98.5%.
[0046]
The obtained sintered body was centerless ground with a # 320 alumina grindstone from an outer diameter of 2.3 mm to an outer diameter of 1.7 mm. A cup electrode having an outer diameter of 1.7 mm, an inner diameter of 1.5 mm and a wall thickness of 0.1 mm was obtained.
[0047]
[Example 6 ]
Molybdenum powder having an average particle size of 3.2 μm, 5 wt% carbonyl nickel powder having an average particle size of 6 μm, 2 wt% carbonyl iron powder having an average particle size of 9 μm, and 1 wt% cobalt powder having an average particle size of 1.6 μm were mixed in an attritor. A thermoplastic organic binder was added to the mixed powder and kneaded to obtain a compound for injection molding. Examples of the organic binder include a high molecular binder having a molecular weight of 10,000 or more and a low molecular binder having a molecular weight of 5,000 or less. Examples of the polymer binder include EVA (ethylene vinyl acetate) -BMA (butyl methacrylate) -styrene copolymer, PP (polypropylene), and the like, and these can be used alone or in combination.
[0048]
Examples of the low molecular binder include polyethylene wax, micro wax, carbana wax, stearic acid, and the like, and these can be used alone or in combination. The ratio of the high molecular binder to the low molecular binder is 5: 1 to 7: 1, preferably 3: 1 to 1: 1.
[0049]
The amount of the binder is preferably 30 to 60% by volume with respect to 100% by volume of the mixed powder. If it is more than this, deformation during degreasing is promoted, and if it is less, injection moldability is lowered. Particularly preferred is 35 to 55% by volume.
[0050]
The kneading of the powder with the binder added was performed at 120 to 160 ° C. using a pressure kneader. The compound obtained by kneading was injection molded into a mold having a cup-shaped cavity having an outer diameter of 2.47 mm, an inner diameter of 1.95 mm, and a length of 5.85 mm, for example, by an injection molding machine.
[0051]
The obtained molded body was gradually heated to 750 ° C. in a hydrogen atmosphere and degreased. Thereafter, sintering was performed in a hydrogen atmosphere at 1450 ° C. for 1 hour to obtain a molybdenum-nickel-iron-cobalt cup electrode (see FIG. 2) having an outer diameter of 1.9 mm, an inner diameter of 1.5 mm, and a length of 4.5 mm. It was.
[0052]
The obtained sintered body was centerless ground with a # 320 alumina grindstone from an outer diameter of 1.9 mm to an outer diameter of 1.7 mm. A cup electrode having an outer diameter of 1.7 mm, an inner diameter of 1.5 mm and a wall thickness of 0.1 mm was obtained.
[0053]
[Example 7 ]
A molybdenum powder having an average particle size of 1.8 microns was mixed with a solution of nickel nitrate in an amount corresponding to 0.2 wt% Ni in ethanol to volatilize the alcohol after immersion. The powder was reduced at 750 ° C. To this powder, a solution obtained by dissolving lanthanum nitrate in an amount corresponding to ₂ wt% La ₂ O ₃ in ethyl alcohol was added and mixed to volatilize the alcohol after immersion.
[0054]
The obtained powder was dried and then reduced at 800 ° C. in hydrogen. Thereafter, a thermoplastic organic binder (using a high molecular binder having a molecular weight of 10,000 or more and a low molecular binder having a molecular weight of 5,000 or less) is added to the reduced powder, kneaded, and a compound for injection molding. It was. As the polymer binder, EVA (ethylene vinyl acetate) -BMA (butyl methacrylate) -styrene copolymer and PP (polypropylene) were used in combination.
[0055]
As the low molecular binder, polyethylene wax and stearic acid were used in combination. The ratio of the high molecular binder to the low molecular binder was 2: 1. The amount of the binder was 40% by volume with respect to 100% by volume of the tungsten powder. Kneading was performed at 130 ° C. using a pressure kneader.
[0056]
The compound obtained by kneading was injection molded into a mold having a cup-shaped cavity having an outer diameter of 1.56 mm, an inner diameter of 1.17 mm, and a total length of 5.2 mm by an injection molding machine.
[0057]
The obtained molded body was gradually heated to 750 ° C. in a hydrogen atmosphere and degreased (organic binder was removed). Then, it sintered at 1700 degreeC in hydrogen for 1 hour, and obtained the cup-shaped electrode made from molybdenum containing 2 wt% lantana of 1.2 mm in outer diameter, 0.9 mm in inner diameter, and 4 mm in total length.
[0058]
The molybdenum electrode obtained as described above was resistance-welded with a Kovar lead wire already connected with a nickel lead wire, and the Kovar portion was glass-wrapped and then assembled into a discharge lamp (discharge cold cathode fluorescent lamp).
[0059]
【The invention's effect】
As is apparent from the above description, the discharge lamp electrode according to the present invention is made of a refractory metal such as tungsten or molybdenum having excellent sputtering resistance, and a discharge lamp using this is a conventional product. Compared to, it became possible to obtain a long life and high brightness. Furthermore, it is possible to obtain a particularly long life and high luminance by reducing the work function of the electrode by containing a low work function substance. Moreover, by performing peripheral grinding, the wall thickness can be reduced, and a higher glow discharge effect and strict outer diameter control can be achieved.
[Brief description of the drawings]
FIG. 1 is a partial cross-sectional view of a discharge lamp.
FIG. 2 is a front view (a) and a sectional view (b) of an electrode.
FIG. 3 is a graph showing various gas ions and sputtering rate.
[Explanation of symbols]
1 discharge lamp 2 glass bulb 3 sealing wire 4 electrode 8 external lead wire

Claims (2)

A discharge lamp electrode of the schematic cup-shaped recess openings are formed at one end, tungsten and / or molybdenum as a main component, one of iron group metals consisting of nickel, iron, cobalt Or containing at least 0.01 to 10% by weight in a group consisting of lanthanum oxide, yttrium oxide, cerium oxide, strontium oxide, zirconium oxide, hafnium oxide, barium oxide, thorium oxide, lanthanum boride An electrode for a discharge lamp characterized by containing one or more kinds in a weight ratio of 0.1 to 4%. 2. The discharge lamp electrode according to claim 1, wherein an outer peripheral surface is ground.

Images