JP2004047365A - Cathode and manufacturing method of the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode containing emitter member like tungsten including thorium oxide (hereinafter referred as "material A"), improving electron emitting property by the reducing power of carbon, capable of improving electron emitting property and reliability, and a manufacturing method of the same. <P>SOLUTION: A chip 4 of material A (negative electrode base body) is brazed to the tip of a tungsten wire filament 3 by W<SB>2</SB>C, and W<SB>2</SB>C powder 5 is applied to the gap formed between the chip 4 of material A and the tungsten wire filament 3. A layer, containing Th released from the chip of material A and carbon released from W<SB>2</SB>C powder 5, is formed on the surface of the chip 4 of material A (electron emitting surface). <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention is capable of operating at a high temperature, and uses a high melting point metal or a cathode for a discharge tube using a high melting point metal containing an emitter material such as tungsten containing thorium oxide (hereinafter referred to as tritan), tungsten containing zirconium oxide, and tungsten containing hafnium oxide. The present invention relates to a hot cathode such as an output cathode and a method for manufacturing the same. More specifically, the present invention relates to a cathode capable of improving electron emission characteristics and achieving high current density and long life, and a method of manufacturing the same.
[0002]
[Prior art]
The tritan cathode uses a tungsten wire or chip containing thorium oxide (ThO ₂ ) of 1 to 2% by weight (hereinafter referred to as “wt%”) as a filament or a cathode pellet. By heating to the same value as in the following, the thorium oxide in the tungsten is reduced by the tungsten to generate thorium atoms, which is a cathode capable of improving electron emission characteristics. In other words, the reduced thorium atoms diffuse in the tungsten and are adsorbed on the cathode surface to form a thorium-tungsten (Th-W) monoatomic layer on the cathode surface. Due to the formation of the monoatomic layer, the work function is lowered, and about 2.7 eV is obtained. As a result, electron emission characteristics of about 1 A / cm ² are obtained at a degree of vacuum of 10 ⁻⁵ Pa and 2000 ° C. b.
[0003]
Here, when the surface of the electron emission surface of the tritan cathode is carbonized and a layer of ditungsten monocarbide (W ₂ C) (hereinafter referred to as a carbonized layer) is formed at a maximum thickness of 50 μm, the reduction of ThO ₂ is reduced by the reducing power of carbon. Performed at lower temperatures. At the same time, because of the columnar crystals of the carbonized layer, fine cracks are generated in a direction perpendicular to the surface, which contributes to an increase in surface area. Therefore, the electron emission characteristics are dramatically improved. That is, the electron emission characteristics can be improved by 10 times or more at the same operating temperature as compared with the case without carbonization, and the operating temperature can be lowered by about 300 ° C. b when the electron emission characteristics are the same.
[0004]
However, the formation of the carbonized layer has a disadvantage that the mechanical strength is extremely reduced in the case of a direct-heat type cathode (filamentary cathode), and it is difficult to form the carbonized layer on a thin line having a diameter of 0.1 mm or less. there were. Further, from 1500 to 1800 ° C., decarburization proceeds, and the electron emission characteristics decrease. In addition, as decarburization progresses, the resistance of the filament decreases as decarburization progresses because the resistance of the carbonized layer (W ₂ C) is higher than that of tungsten, and the heating current increases at the same filament voltage. As a result, the temperature of the filament increases, which causes a reduction in the life of the cathode.
[0005]
Further, when this carbonized layer is formed on the surface of the cathode for a discharge tube, the decarburization becomes remarkable at a high temperature of 1800 ° C. or more, so that the tip temperature always exceeds 3000 ° C. during operation of the discharge tube lamp. In the vicinity of the tip of the cathode, decarburization occurs instantaneously, and the electron emission characteristics deteriorate. For this reason, the lamp luminance becomes unstable, so that carbonization was not suitable for the discharge tube cathode.
[0006]
Further, there are some problems in the method of forming the carbonized layer. That is, the main methods of forming the carbonized layer are: (1) a method in which a vapor of a carbon derivative such as heptane, benzene, or naphthalene is fed in a vacuum and heated to 2150 ° C. or higher; Or heating it to 1200 ° C. or higher by bringing it into contact with a carbon jig in a hydrogen furnace. However, in any of these methods, the thickness of the carbonized layer is extremely controlled. There is a problem that is difficult. In addition, since the composition of the carbonized layer is desirably made of W ₂ C instead of WC for the electron emission characteristics, there is a problem that it is necessary to pay attention to composition control.
[0007]
[Problems to be solved by the invention]
As described above, when a carbonized layer is formed on a tritan cathode, there is a problem that the carbonized layer lowers the mechanical strength of tritan. Further, at 1800 ° C. or higher, there is a problem that the decarburization of the carbonized layer becomes remarkable, and the carbonized layer is reduced, so that the electron emission capability is reduced. Furthermore, there is the thickness of the carbide layer and W _{2 C} is difficult to control the production, a problem that it is impossible to form a stable W _{2 C} layer.
[0008]
The present invention has been made in order to solve such a problem, and in a cathode containing an emitter material such as tritan and further improving the electron emission characteristics by the reducing power of carbon, the electron emission characteristics and reliability are improved. And a method for manufacturing the same.
[0009]
[Means for Solving the Problems]
The cathode according to the present invention includes a cathode substrate made of a refractory metal containing an oxide of at least one metal selected from the group consisting of thorium, zirconium, hafnium, yttrium, scandium, cerium, and lanthanum as an emitter material, and an electron emission surface. In a cathode having tungsten carbide disposed in the vicinity, a surface containing at least the metal released from the emitter material and carbon released from the tungsten carbide is provided on the surface of the electron emission surface of the cathode substrate. Features.
[0010]
Here, the cathode base refers to a part constituting a cathode having an electron emission surface, and is commonly used as a wire-shaped heater, a so-called filament cathode, which is directly connected to the heater or a sintered body provided near the heater or Includes cathode pellets made of porous material. The high melting point metal means tungsten or molybdenum. Further, the tungsten carbide includes not only ditungsten monocarbide W ₂ C but also monotungsten monocarbide WC.
[0011]
With this configuration, the cathode material including the emitter material such as tritium such as tritan, zirconium, hafnium, yttrium, scandium, cerium, and lanthanum can be converted into an emitter by carbon supplied from tungsten carbide provided near the electron emission surface. Since the material is reduced, the work function can be reduced and the electron emission characteristics can be greatly improved. In addition, since a tungsten carbide layer is not formed on the electron emission surface, the mechanical strength of the cathode material is not reduced, and the tungsten carbide does not disappear due to decarburization, so that carbon can be continuously supplied. And stable electron emission characteristics can be obtained. Furthermore, since columnar crystals are not formed on the electron emission surface as in the related art, there is no need to use ditungsten monocarbide (W ₂ C), and there is no limitation on the material composition, and the production is facilitated.
[0012]
The layer is a layer including a monoatomic layer of the metal atom and a monoatomic layer of the carbon atom, or a layer in which an oxygen atom is bonded thereto.
[0013]
The method for manufacturing a cathode according to the present invention includes a cathode substrate made of a refractory metal containing, as an emitter material, an oxide of at least one metal selected from the group consisting of thorium, zirconium, hafnium, yttrium, scandium, cerium, and lanthanum. And a method of manufacturing a cathode comprising a layer containing at least the metal released from the emitter material and carbon released from tungsten carbide disposed near the electron emission surface on the surface of the electron emission surface of the cathode substrate. So,
A step of preparing a cathode base having a tungsten carbide powder or a tungsten carbide film in the vicinity of the electron emission surface of the cathode base;
The cathode substrate is subjected to heat treatment in a vacuum or an inert gas atmosphere, and at least one selected from the group consisting of thorium, zirconium, hafnium, yttrium, scandium, cerium, and lanthanum released from the emitter material on the surface of the electron emission surface. A step of forming a layer containing one kind of metal and carbon released from the tungsten carbide powder or the tungsten carbide film.
[0014]
Another embodiment of the method for producing a cathode according to the present invention is the method for producing a cathode according to claim 3, wherein the step of preparing the cathode substrate is performed using a group consisting of thorium, zirconium, hafnium, yttrium, scandium, cerium, and lanthanum. The present invention is characterized in that a cathode substrate made of a sintered body of a refractory metal in which at least one selected metal oxide and tungsten carbide are dispersed is formed.
[0015]
That is, in the vicinity of the electron emission surface, a tungsten carbide material is provided in the form of a fixed powder or a film or dispersed in a cathode base, and is subjected to a heat treatment at, for example, 1200 to 1400 ° C., so that the tungsten carbide becomes Upon decomposition, the carbon atoms diffuse to the electron emitting surface of the cathode substrate. Then, together with the thorium atoms diffused from the cathode substrate made of, for example, tritan, on the electron emission surface of the cathode substrate, a layer in which a monolayer of carbon is bonded to a monolayer of thorium or a monolayer of carbon is formed in a monolayer of thorium. A layer in which oxygen is bonded to the bonded layer is formed. As described above, since the tungsten carbide material is provided near the electron emission surface, the temperature does not rise as high as the electron emission surface, does not disappear by decarburization due to high temperature, and this action continues during operation, For example, in order to obtain 1 A / cm ² in a cathode provided with a tungsten carbide layer on the side opposite to the electron emission surface of a tritan chip, it is necessary to lower the operating temperature by about 100 ° C. compared to a conventional cathode provided with a carbonized layer. As a result, a stable current of 1 A / cm ² was obtained at about 1350 ° C.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, the cathode of the present invention and a method of manufacturing the same will be described with reference to the drawings. The cathode according to the present invention is, as shown in FIG. 1, an example of the configuration of a direct-heating type cathode used for an X-ray tube, a cathode for an electron microscope, or the like, as shown in FIG. The chip (cathode base made of tungsten containing thorium oxide) 4 is brazed by W ₂ C. The eutectic point of tungsten and W ₂ C is 2202 ° C., and brazing is performed at this temperature by performing brazing in a vacuum.
[0017]
Then, a W ₂ C powder 5 is applied and provided in a gap between the tritan chip 4 and the tungsten wire filament 3. In this case, before application, the W ₂ C powder 5 is immersed in a binder (adhesive) in which nitrocellulose is dissolved in butyl acetate in advance, and when this is applied, W ₂ C is adhered well. The binder evaporates when the tungsten wire filament 3 is heated.
[0018]
Then, this cathode is incorporated in a vacuum wall or the like, and the tungsten filament 3 is energized and heated at an operating temperature of 1200 to 1400 ° C., whereby carbon atoms of the W ₂ C powder are diffused and the tritan chip 4 A layer in which a monolayer of carbon is bonded to a monolayer of thorium (hereinafter referred to as a Th + C monolayer) or a layer in which a monolayer of carbon is bonded to a monolayer of thorium on the surface 4a (electron emission surface) of (Hereinafter referred to as a Th + C + O monoatomic layer), and electrons are emitted when a voltage is applied between the anode and the anode. When the operating temperature is set to 1200 to 1400 ° C. to operate the cathode from the beginning, it takes time for the carbon atoms of W ₂ C to diffuse and reach the electron emission surface in the early stage of the operation, and the electron emission is performed. Absent. Therefore, in order to quickly diffuse carbon atoms to the surface, the activation is preferably performed at a temperature higher than the operating temperature and equal to or lower than 1500 ° C. By previously covering the cathode surface with a Th + C monoatomic layer or a Th + C + O monoatomic layer, when the operating temperature is lowered, electron emission is performed smoothly.
[0019]
As shown in FIG. 2, the electron emission surface of the cathode (the surface 4a of the tritan chip 4) is deposited such that the carbon atoms 21 diffused on the surface of the cathode substrate 20 made of tritan form a substantially monoatomic layer. I do. Although not shown, the deposited carbon atoms combine with the monolayer of thorium similarly deposited from the cathode substrate as described above to form a monolayer of Th + C or a monolayer of Th + C + O to form an electron emission surface. Is coated. This Th + C monoatomic layer or Th + C + O monoatomic layer has a thickness of several 、, and at most tens of Å or less, which is different from a conventional tungsten carbide layer formed on a cathode surface to a thickness of 50 μm or less. As shown in FIG. 3, the cathode formed by the conventional manufacturing method has W ₂ C provided as a columnar crystal layer 22 on the surface of a cathode substrate 1 made of tritan, and is apparent from the thickness and the crystal structure. This is different from the present invention.
[0020]
The electron emission characteristics of the tritan cathode according to the present invention were examined by changing the cathode temperature and measuring the change in cathode current density. FIG. 4 shows a comparison with the electron emission characteristics of the conventional tritan cathode. FIG. 4 shows a tritane cathode according to the present invention in which a monoatomic layer of carbon is formed on a surface A, and a conventional structure of a tritane cathode having a carbonized layer (W ₂ C layer) on the surface of a tritane cathode having a thickness of about 50 μm and forming columnar crystals. In this figure, the relationship between the cathode current density and the cathode temperature is shown by comparing the sample B formed in Example 1 with the sample C formed of only the tritan having no carbonized layer of the conventional structure. As is clear from FIG. 4, the cathode C made of only tritan has a high operating temperature and a low current density. However, the structure B in which the carbonized layer is provided on the surface thereof has a lowered operating temperature and a markedly lower current density. Has improved. However, while the operating temperature is 1300 ° C. and the current density is about 0.1 A / cm ² , the C + Th (+ O) monoatomic layer which is decomposed and diffused by providing W ₂ C powder on the back surface of the cathode according to the present invention is used. Formed A obtained a current density of about 0.5 A / cm ² at 1300 ° C., and obtained about 5 times the electron emission characteristics as compared to B when a carbonized layer was formed on the electron emitting surface. .
[0021]
As is clear from FIG. 4, in order to obtain a current density of 1 A / cm ² , according to the cathode of the present invention, the operating temperature of the cathode can be reduced by about 100 ° C. as compared with the conventional structure in which the carbonized layer is provided. . However, when the temperature is 1400 ° C. or higher, the evaporation rate is higher than the diffusion rate of carbon atoms C, so that the electron emission decreases and the current density decreases. Therefore, the operating temperature needs to be 1200-1400 ° C. That is, according to the cathode according to the present invention, a large current density can be obtained at a low temperature operation, and since the operation is at a low temperature, the burden on the filament is reduced and the amount of thoria evaporated is reduced, so that the life of the cathode can be extended. it can.
[0022]
FIG. 5 shows that the cathode substrate is not a tritan chip, but the filament itself is made of tritan to form a filament and cathode. In this case, the tritan wire 6 is bent as shown in FIG. Fix with such as. The W ₂ C powder 5 is applied to a peripheral portion other than the electron emission surface 8. Then, in the same manner as in the above example, electrons are emitted by energizing the filament and setting the operating temperature to 1200 to 1400 ° C. Also in this embodiment, if activation is performed at a temperature higher than the operating temperature in advance, electron emission is performed smoothly.
[0023]
Also in this example, forming the monoatomic layer can lower the operating temperature by about 100 ° C. to obtain the same electron emission as compared to forming the carbonized layer on the electron emission surface, and the filament Is reduced and the amount of thoria evaporated is also reduced, so that the cathode has a longer life. Further, since the carbonized layer is not formed, there is no deterioration in mechanical strength of the tungsten wire due to the formation of the carbonized layer, and the present invention can be applied to a thin line having a linear shape of 0.1 mm or less. Further, since there is no decrease in electron emission due to decarburization and the resistance value of the filament does not change, the filament current is stable throughout.
[0024]
FIG. 6 is an explanatory cross-sectional view of a discharge tube cathode showing still another example of the present embodiment. In order to manufacture this cathode, tungsten powder and thorium oxide at a maximum of about 20 wt% are mixed, and pressed into a shape of the cathode base 9 shown in FIG. 6 using a metal mold. Next, the W ₂ C powder 10 is put into the concave portion of the cathode substrate 9 and pressed. When the cathode base 9 is fitted into the projection of the lead 11 and sintered in this state at 2400 ° C. in a hydrogen atmosphere, the cathode base 9 contracts due to sintering and is fixed by tightening the lead.
[0025]
In this case, the cathode substrate may be a hot-rolled tritan rod 12 as shown in FIG. 7 in addition to the porous sintered body pressed by the mold. In the case of the tritan rod 12, as shown in FIG. 7, a brazing material (such as molybdenum-ruthenium) 13 is put between the tritan rod 12 serving as the cathode base and the lead 11, and brazed.
[0026]
Thereafter, a cathode having a structure as shown in FIG. 6 or 7 is incorporated in the discharge lamp. The discharge tube lamp applies a high-voltage pulse as a discharge trigger in an inert gas such as xenon gas to cause glow discharge, and immediately shifts to arc discharge. This transition is determined by the plasma density derived from the atmospheric gas and the strength of the electric field applied to the cathode, and the discharge lamp is designed to automatically transition. During the arc discharge, the temperature of the cathode gradually increases due to the discharge. Normally, in a discharge tube using tritan, this temperature rise causes ThO ₂ to be reduced to Th, and Th diffuses to the surface to form a monoatomic layer and emit electrons. In the case of tritan, since the temperature rise at the tip reaches 3000 ° C. or more, Th diffusion near the tip is actively performed. However, in other parts, the temperature is reduced, so that the Th diffusion is reduced. Therefore, when the discharge operation is performed for a long time, the Th near the tip is depleted, and as the electron emission ability decreases, the discharge voltage increases, the tip temperature also increases, and finally the tip melts and deforms. Decreases.
[0027]
In the case where a carbonized layer is formed on the surface of the discharge tube cathode, the reduction of ThO ₂ is performed at a lower temperature and the diffusion of Th atoms is performed more actively than in the case where the carbonized layer is not formed. However, since the tip temperature is as high as 3000 ° C. or more, decarburization is performed in a very short time, and the electron emission capability is reduced as the carbonized layer is reduced. On the other hand, in the present invention, the temperature at the tip is 3000 ° C. or higher, but if tungsten carbide is arranged in a portion where the temperature becomes 1200 to 1400 ° C., carbon atoms are always supplied to the surface from the tungsten carbide layer. During the discharging operation, the Th + C monoatomic layer or the Th + C + O monoatomic layer is maintained, and the electron emission capability is also maintained. Therefore, the lamp brightness is stabilized without melting deformation of the cathode tip.
[0028]
In the case of this discharge tube cathode as well, as in the example shown in FIG. 1, it takes a long time for carbon atoms to diffuse from the tungsten carbide layer to the surface of the cathode substrate and to be stably supplied from the tungsten carbide layer. It is necessary to activate for several hours under the operating conditions of the discharge lamp. In the case of a discharge lamp, it is difficult to control the cathode temperature, so activation is performed under normal operating conditions. However, even in a discharge lamp using tritan or an impregnated cathode which is currently generally distributed, an activation step is necessary in the lamp manufacturing process, and thus the activation step in the present invention is the same. There is no problem in.
[0029]
FIG. 8 is a view showing another embodiment of the cathode according to the present invention. In this example, W ₂ C14 was directly dispersed in the porous sintered body cathode substrate 9 instead of providing a tungsten carbide layer. Things. In this case, a tungsten powder, a maximum of 20 wt% of thorium oxide powder, and a maximum of 30 wt% of W ₂ C powder 14 are mixed, and as shown in FIG. The cathode base 9 and the lead 11 are fixed by being fitted into the lead 11 and sintered. The subsequent operation is the same as in the example shown in FIG. Here, the mixing ratio of the thorium oxide powder and the W ₂ C powder is such that if the amount of thorium oxide is too large, the conductivity becomes poor, and if the amounts of thorium oxide and W ₂ C are too large, sintering becomes difficult. Is set.
[0030]
As shown in FIG. 9, also in this embodiment, instead of sintering, one in which W ₂ C 14 is dispersed in the tritan rod 12 in advance may be used. The tritan rod 12 and the lead 11 are joined by the same brazing material 13 as in the example of FIG.
[0031]
Although in each of the above examples, tritan was described, the present invention uses, instead of tritan, tungsten containing zirconium oxide, tungsten containing hafnium oxide, tungsten containing yttrium oxide, tungsten containing scandium oxide, tungsten containing cerium oxide, tungsten containing lanthanum oxide, and tungsten containing lanthanum oxide. Etc. can also be used. Further, a combination of these may be used. Further, other high melting point metals such as molybdenum can be used instead of tungsten.
[0032]
Further, in each of the above examples, the direct heating type cathode and the discharge tube cathode used for an X-ray tube, an electron microscope, and the like have been described. However, the present invention is not limited thereto, and the cathode of the present invention may be similarly used for an electron tube cathode, a cathode ray tube cathode, and the like. Can be used. Further, in each of the above-mentioned examples, ditungsten monocarbide W ₂ C is used as tungsten carbide, but monotungsten monocarbide WC may be used.
[0033]
【The invention's effect】
As described above, according to the present invention, since carbon atoms are supplied to the cathode surface to form a monolayer of Th + C or Th + C + O, the electron emission ability is improved. That is, the amount of electron emission is increased about five times as compared with the conventional cathode having a carbonized layer, and the operating temperature can be lowered by about 100 ° C. to obtain the same electron emission. Therefore, the life of the filament and the cathode can be improved.
[0034]
Furthermore, by not forming a carbonized layer, the filament current increases due to a decrease in mechanical strength due to the formation of the carbonized layer, a decrease in electron emission due to decarburization, and a change in resistance due to decarburization, and the cathode temperature increases due to an increase in filament temperature. There are advantages in that problems such as shortening of service life can be solved and the production is very simple because there is no need to control the thickness and composition of the carbonized layer.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of an example in which a tritan tip (cathode base) is welded to the tip of a tungsten wire filament in a direct-heating type cathode according to an embodiment of the present invention.
FIG. 2 is an explanatory cross-sectional view when a monoatomic layer according to the present invention is formed on the surface of an electron emitting surface.
FIG. 3 is an explanatory cross-sectional view when a conventional carbonized layer is formed on the surface of an electron emission surface.
FIG. 4 shows the temperature of a tritan cathode A having a Th + C monoatomic layer according to the present invention, a conventional tritan cathode B having a carbonized layer formed on the electron emission surface, and a conventional tritan cathode C having no carbonized layer. FIG. 3 is a diagram showing a comparison of the amount of electron emission.
FIG. 5 is an explanatory diagram of an example in which a tritan filament is used as a cathode substrate in the direct-heat cathode according to one embodiment of the present invention.
FIG. 6 is an explanatory sectional view of an example applied to a discharge tube cathode using a sintered body according to another embodiment of the present invention.
FIG. 7 is a sectional explanatory view of an example applied to a discharge tube cathode using a tritan rod according to another embodiment of the present invention.
FIG. 8 is a sectional explanatory view of an example in which W ₂ C is dispersed in a porous sintered body in a discharge tube cathode according to still another embodiment of the present invention.
FIG. 9 is an explanatory cross-sectional view of a discharge tube cathode according to still another embodiment of the present invention, in which W ₂ C is dispersed in a tritan rod.
[Explanation of symbols]
Reference Signs List 3 Tungsten wire filament 4 Tritan tip 4a Electron emission surface 5 W ₂ C powder 6 Tritan rod 8 Electron emission surface 9 Cathode substrate 10 W ₂ C layer 11 Lead 12 Tritan rod

Claims (4)

A cathode substrate made of a refractory metal containing, as an emitter material, an oxide of at least one metal selected from the group consisting of thorium, zirconium, hafnium, yttrium, scandium, cerium, and lanthanum; A cathode comprising tungsten, comprising: a layer containing at least the metal released from the emitter material and carbon released from the tungsten carbide, on a surface of the electron emission surface of the cathode base. 2. The cathode according to claim 1, wherein the layer is a layer including a monoatomic layer of the metal atoms and a monoatomic layer of the carbon atoms, or a layer in which oxygen atoms are bonded to these. A cathode substrate made of a refractory metal containing, as an emitter material, at least one oxide of a metal selected from the group consisting of thorium, zirconium, hafnium, yttrium, scandium, cerium, and lanthanum; A method for manufacturing a cathode, comprising a layer containing at least a metal released from the emitter material and carbon released from tungsten carbide disposed near the electron emission surface,
A step of preparing a cathode base having a tungsten carbide powder or a tungsten carbide film in the vicinity of the electron emission surface of the cathode base;
The cathode substrate is subjected to heat treatment in a vacuum or an inert gas atmosphere, and at least one selected from the group consisting of thorium, zirconium, hafnium, yttrium, scandium, cerium, and lanthanum released from the emitter material on the surface of the electron emission surface. Forming a layer containing one kind of metal and carbon liberated from the tungsten carbide powder or the tungsten carbide film. 4. The method for producing a cathode according to claim 3, wherein the step of preparing the cathode base comprises: an oxide of at least one metal selected from the group consisting of thorium, zirconium, hafnium, yttrium, scandium, cerium, and lanthanum; and tungsten carbide. Forming a cathode substrate made of a sintered body of a high-melting-point metal in which is dispersed.

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