Emitter, method for manufacturing the emitter, and X-ray tube

Abstract

This invention provides a highly reliable emitter, a method for manufacturing the emitter, and an X-ray tube. [Solution] The device comprises a base having an electron-emitting surface that emits electrons, and a pair of leg portions that apply a voltage to the electron-emitting surface. The base contains an alloy in a region other than the electron-emitting surface, consisting of at least one element selected from the group of elements consisting of carbon, niobium, tantalum, titanium, cobalt, chromium, and nickel.

Description

[Technical Field]

[0001] Embodiments of the present invention relate to an emitter, a method for manufacturing an emitter, and an X-ray tube including the emitter. [Background Art]

[0002] Patent Document 1 discloses an emitter including a base having an electron emission surface that emits electrons, and a pair of leg portions that apply a voltage to the electron emission surface. [Prior Art Documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Laid-Open No. 2014-232629 [Summary of the Invention] [Problem to be Solved by the Invention]

[0004] However, in conventional emitters, the electron emission surface reaches a high temperature, which causes deformation due to thermal stress, and there is a risk of reduced strength, abnormal electron emission characteristics, and the like, thus there has been a problem in reliability.

[0005] The problem to be solved by the embodiments is to provide a highly reliable emitter, a method for manufacturing an emitter, and an X-ray tube. [Means for Solving the Problem]

[0006] The emitter according to the embodiment includes: a base having an electron emission surface that emits electrons; and a pair of leg portions that apply a voltage to the electron emission surface, wherein the base includes, in a region other than the electron emission surface, an alloy made of at least one element selected from the group of elements consisting of carbon, niobium, tantalum, titanium, cobalt, chromium, and nickel.

[0007] A method for manufacturing an emitter according to an embodiment includes: a base having an electron emission surface that emits electrons; A method for manufacturing an emitter comprising a pair of leg portions for applying a voltage to the electron emission surface, wherein the base portion is mainly made of tungsten, and the peripheral portion located on the outer periphery side of the electron emission surface in the unfolded state of the base portion is doped with carbon to form a tungsten carbide alloy, wherein the peripheral portion is brought into contact with graphite powder for 1 hour or more, and then heated at a temperature of 1200°C or higher.

[0008] The X-ray tube according to this embodiment comprises a vacuum enclosure, a cathode provided inside the vacuum enclosure for emitting electrons, and an anode provided inside the vacuum enclosure for generating X-rays when electrons emitted from the cathode collide with it, wherein the cathode is an emitter as described above or an emitter manufactured by the above manufacturing method. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is an exploded view of the emitter according to the first embodiment. [Figure 2] Figure 2 is a side view showing the usage state of the emitter shown in Figure 1. [Figure 3] Figure 3 is a cross-sectional view showing the process of doping the edges with alloying elements. [Figure 4] Figure 4 is a front view showing a schematic configuration of an X-ray tube using an emitter according to the first embodiment. [Modes for carrying out the invention]

[0010] Embodiments of the present invention will be described below with reference to the drawings. Note that the disclosure is merely an example, and modifications that can be easily conceived by those skilled in the art while maintaining the spirit of the invention are naturally included within the scope of the present invention. Furthermore, the drawings may schematically represent the width, thickness, shape, etc., of each part compared to the actual embodiment in order to clarify the explanation; however, these are merely examples and do not limit the interpretation of the present invention. In addition, in this specification and in each drawing, components that perform the same or similar functions as those described above with respect to previously shown drawings are denoted by the same reference numerals, and redundant detailed explanations may be omitted as appropriate.

[0011] The X-ray tube 1 according to the first embodiment will be described with reference to Figure 4. As shown in Figure 4, the X-ray tube 1 comprises a vacuum enclosure 3, a cathode 5 located inside the vacuum enclosure 3 that emits electrons, and an anode 7 located inside the vacuum enclosure 3 that generates X-rays when electrons emitted from the cathode 5 collide with it. This X-ray tube 1 is a rotating anode type X-ray tube, and the anode 7 rotates the anode target 9 around the rotation axis TA by a rotation mechanism 8. The cathode 5 is supported by a cathode support 6 and emits an electron beam (electrons) generated at high voltage toward the anode target 9, and is equipped with an emitter 11.

[0012] Figure 1 shows an unfolded view of the emitter 11. As shown in Figure 1, the emitter 11 comprises a base portion 15 having an electron-emitting surface 13 for emitting electrons, a pair of leg portions 17, 17 for applying a voltage to the electron-emitting surface 13, and a peripheral portion 19 provided on the outside of the electron-emitting surface 13, sandwiching the electron-emitting surface 13. The peripheral portion 19 may be used in its unfolded state as shown in Figure 1, or it may be bent to the opposite side of the electron-emitting surface 13 as shown in Figure 2. Furthermore, the dashed line 27 in Figure 1 indicates the boundary between the electron emission surface 13 and the edge portion 19, and as shown in Figure 2, it also serves as a folding line when the edge portion 19 is bent for use.

[0013] The material of the emitter 11 is a high melting point metal with a thickness of about 1 mm or less and low vapor pressure in vacuum, such as tungsten, or an alloy containing tungsten as a main component. In the embodiment, potassium (K)-doped tungsten (K-doped tungsten) with a thickness of about 0.3 mm or less is preferably used. In the present embodiment, the electron emission surface 13 is a substantially rectangular flat surface as a whole.

[0014] The pair of leg portions 17, 17 are opposing short sides of the electron emission surface 13 that is substantially rectangular as a whole, and are formed by bending toward the opposite side of the electron emission surface 13 during use as shown in FIG. 2.

[0015] In the present embodiment, the edge portion 19 is formed, during use, by bending the opposing long side (the sides opposing each other between the leg portions 17, 17) 15a, 15a toward the opposite side of the electron emission surface 13. In this embodiment, the edge portion 19 is bent at an angle of 90 degrees toward the opposite side of the electron emission surface 13.

[0016] Elongated holes 22 are formed in the electron emission surface 13 so as to form a zigzag current path 21. The elongated holes 22 are holes each having a band shape with a rectangular shape in plan view. In FIG. 1, the elongated hole closest to each leg portion 17 is denoted by reference numeral 23. The elongated holes 22 are formed to be long in a direction orthogonal to the opposing long sides 15a, 15a of the base portion 15 having a substantially rectangular shape in plan view, and are alternately formed such that one end 22a of each elongated hole 22 is close to a boundary line between the electron emission surface 13 and the edge portion 19 (hereinafter simply referred to as "boundary line") 27, and the other end 22b is far from the boundary line 27. As a result, a zigzag continuous current path 21 is formed like a single stroke, and the position of an inversion portion 21a where the zigzag current path 21 is inverted on the electron emission surface 13 is located away from the boundary line 27 indicated by a two-dot chain line.

[0017] Next, a current limiting hole 25 for allowing current to flow from the leg portion 17 to the current path 21 and making it difficult for current to flow into the edge portion 19 will be described. The current-limiting hole 25 is formed on an end 23a side close to the boundary line 27 in the elongated hole 23 closest to the leg portion 17 among the plurality of elongated holes 22 (elongated holes for forming current paths), and the passage area on the other end 23b side relatively far from the boundary line 27 is increased. In the present embodiment, there are a plurality of current-limiting holes 25, which are formed as a pair of holes formed close to each other. As described above, in the elongated hole 23 closest to the leg portion 17, by forming the current-limiting hole 25 on the end 23a side close to the boundary line 27 among the longitudinal ends 23a and 23b of the elongated hole 23, the area of the current path on the other end 23b side is made larger than that on the one end 23a side. Accordingly, on the side of the elongated hole 23 closest to the leg portion 17, the current resistance on the other end 23b side becomes smaller than that on the one end 23a side, so that current flows toward the other end 23b side, and current flowing toward the one end 23a side and the edge portion 19 on the one end 23a side is limited. Accordingly, in the present embodiment, in the emitter 11 in the unfolded state shown in FIG. 1, current can flow only in the direction indicated by arrow A1 (or the opposite direction thereof).

[0018] The emitter 11 is subjected to alloying treatment on portions excluding the electron emission surface 13 and the leg portion 17 thereof. It is preferable that the alloy includes an alloy made of at least one element selected from the element group consisting of the respective elements of carbon (C), niobium (Nb), tantalum (Ta), titanium (Ti), cobalt (Co), chromium (Cr), and nickel (Ni). According to at least one element selected from such an element group, a high-strength alloy can be obtained as a reinforcing material. In particular, in the present embodiment, the material of the emitter 11 is tungsten (W), and each of the elements of carbon (C), niobium (Nb), tantalum (Ta), titanium (Ti), cobalt (Co), chromium (Cr), and nickel (Ni) easily forms an alloy with tungsten (W).

[0019] The alloying treatment will be described below. Here, a case where the material of the emitter 11 is tungsten (W) and the element that forms an alloy with the material of the emitter 11 is carbon (C) will be described. In the present embodiment, the edge portion 19 is subjected to alloying treatment. As shown in Figure 3, the edge portion 19 of the emitter 11 in the unfolded state shown in Figure 1 is bent at a right angle to the electron emission surface 13. The leg portion 17 remains in the unfolded state as shown in Figure 1. As shown in Figure 3, carbon (C) powder is placed in the crucible 31, and the amount of powder is adjusted to a depth where only the bent edge portion 19 is buried. The surface of the powder is then leveled, and the edge portion 19 is bent into a stapler-like shape. The bent edge portion 19 is then inserted downwards into the carbon (C) powder and embedded. This selectively causes only the edge portion 19 to come into contact with the carbon (C) and react.

[0020] Furthermore, graphite blocks may be used instead of carbon (C) powder. If graphite blocks are used, the material may be mechanically sandwiched and brought into contact with the material. Also, if tar, pitch (a by-product of coal, petroleum, or coal tar), or hydrocarbon gas is used instead of carbon (C) powder, the area to be coated may be masked.

[0021] With carbon (C) in contact, the temperature of the edge portion 19 is raised to approximately 800°C to 1200°C. The method of raising the temperature is by self-electric heating or by radiant heating from the surroundings. At this temperature range, the effect of carbon (C) diffusing into tungsten (W) is dominant, with carbon (C) diffusing from the surface to the deeper layers. Preferably, the mixture is held in this state for one hour or more, or in this embodiment, several hours (for example, half a day or a day), and after a sufficient amount of time has elapsed to obtain sufficient diffusion depth, the temperature is raised to 1800°C or higher to react the diffused carbon (C) with tungsten (W) to form a tungsten carbide (WC) alloy. In the temperature range of 800°C to 1200°C, the dominant phenomenon is the diffusion and movement of carbon (C) into tungsten (W). Around 1800°C, the reaction between tungsten (W) and carbon (C) begins, and the phenomenon of transformation into tungsten carbide (WC) becomes dominant. By utilizing this property, it is possible to control the diffusion of carbon (C) to an appropriate depth, and once the required diffusion length is obtained, react it with tungsten (W) to fix it as tungsten carbide (WC). In other words, in this embodiment, as shown in Figure 1, carbon (C) is doped into the emitter 11 shown in the unfolded view, with each edge portion 19 facing the boundary line 27 bent, and then heated to form a tungsten carbide alloy (WC) on the edge portion 19. In this embodiment, the concentration of tungsten carbide alloy (WC) is formed in an exponential concentration distribution such that it is higher towards the surface side of the edge portion 19, or higher as it moves away from the electron emission surface 13 at the edge portion 19.

[0022] Generally, tungsten carbide (WC) alloys have a higher Young's modulus and higher resistivity than tungsten (W), so the edge portion 19 selectively increases in rigidity and thus increases in electrical resistance. The various values ​​for tungsten (W) and tungsten carbide (WC) are as follows. Tungsten (W) resistance value: 5.5 × 10 -8 Ωm Resistance of tungsten carbide (WC): 19.2 × 10⁻⁶ -8 Ωm The Young's modulus of tungsten (W) is 3.45 × 10⁻⁶. 11 N / m 2 Young's modulus of tungsten carbide (WC): 5.50 × 10⁻⁶ 11 N / m 2

[0023] The operation and effects of the emitter 11 according to the first embodiment will be described. According to the emitter 11 of the first embodiment, when a voltage is applied between the leg portions 17, 17, the current flowing through the current path 21 causes Joule heating at the electron emission surface 13, and thermionic electrons are emitted from the electron emission surface 13. Since the electron emission surface 13 has a zigzag current path 21 formed by the elongated holes 22, sufficient heat generation can be obtained even with a low current.

[0024] By forming the tungsten carbide alloy (WC) concentration so that it is higher towards the surface of the edge portion 19, or so that it is higher as it moves away from the electron emission surface 13 within the edge portion 19, it is possible to obtain strength with appropriate flexibility without becoming brittle, compared to the case where the entire edge portion 19 is completely reacted.

[0025] Furthermore, even in the unfolded state shown in Figure 1, a current-limiting hole 25 is formed between the current path 21 of the base 15 and the leg portion 17. By widening the area on the other end 23b side of the elongated hole 23 near the leg portion 17 and narrowing the area on the one end 23a side, the resistance to the current flowing through the current path 21 is controlled. Thus, the path of the current flowing through the current path 21 can be restricted with simple processing.

[0026] Generally, the electron emission surface 13 becomes hot due to Joule heating (for example, 2400°C to 2700°C), and may deform due to thermal expansion. In particular, repeated stress generation due to the repeated temperature rise and cooling caused by energization may lead to fatigue failure. In addition, deformation and warping may occur due to insufficient strength, which may change the appropriate distance from the focusing electrode (not shown), and the electrons emitted from the electron emission surface 13 may not focus into the intended shape. In contrast, in the emitter 11 according to this embodiment, the edge portion 19 is made of an alloy consisting of at least one element from the element group composed of carbon (C), niobium (Nb), tantalum (Ta), titanium (Ti), cobalt (Co), chromium (Cr), and nickel (Ni). This increases the strength of the edge portion 19 and the emitter 11 as a whole, and reduces deformation due to thermal stress. In particular, the base portion 15 can maintain its shape, reducing problems such as the X-ray focal spot dimensions becoming non-standard due to deformation due to thermal stress or changes in electron distribution during use. Furthermore, it prevents the emitter 11 from being damaged by vibration or shock due to insufficient strength, or from coming into contact with the focusing electrode and causing abnormal electron emission characteristics. Therefore, according to this embodiment, a highly reliable emitter 11 and X-ray tube 1 can be provided.

[0027] The electron emission surface 13 is provided with current-limiting holes 25 to control the resistance through which current flows. This simple configuration allows for control of the thermal and electrical resistance between the electron emission surface 13 and the edge portion 19. This suppresses heat from escaping from the electron emission surface 13 to the edge portion 19, and prevents an increase in the current required to raise the temperature to the necessary level. As a result, it prevents wasted power consumption in the edge portion 19, which does not contribute to electron emission.

[0028] Furthermore, since an alloy is formed at the edge portion 19 to increase its electrical resistance, not much current flows through it, and heat is not easily conducted from the electron emission surface 13, so the temperature of the edge portion 19 does not rise easily. As a result, the amount of thermal deformation can be reduced, suppressing the arch-shaped deformation of the entire emitter 11 due to thermal expansion, and suppressing changes in the distribution of electron flow focused on the anode target 9 due to changes in the distance from the focusing electrode.

[0029] Furthermore, the alloy concentration at the edge portion 19 forms a concentration distribution in a predetermined direction, and the entire material is not alloyed uniformly, thus preventing brittleness and maintaining a predetermined flexibility (Young's modulus).

[0030] The embodiment described above is presented as an example and is not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications are permitted without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents.

[0031] For example, in the first embodiment, the shape of the current-restricting hole 25 is not limited and may be a round hole, or a square or triangular hole. The current path 21 is not limited to the elongated holes 22; it may also be a slit formed to cut the alternately opposing long sides 15a, 15a. The elements forming the alloy, as with carbon (C), may be onib (Nb), tantalum (Ta), titanium (Ti), cobalt (Co), chromium (Cr), and nickel (Ni), either in powder form, in block form, or selectively doped at the edges 19 by processes such as vacuum deposition or ion plating, using masking, and then diffused from the surface.

[0032] The X-ray tube 1 is not limited to a rotating anode type X-ray tube; it may also be a fixed anode type X-ray tube. Furthermore, the emitter 11 is not limited to an X-ray tube; it may be an emitter used in other electronic devices, as long as it serves as an electron emission source. [Explanation of Symbols]

[0033] 1...X-ray tube, 11...Emitter, 13...Electron emission surface, 15...Base, 17...Leg section, 19...Edge section, 21...Current path, 22...Slotted hole, 25...Energy limiting hole.

Claims

1. A base having an electron-emitting surface that emits electrons, The electron emission surface comprises a pair of leg portions for applying a voltage, The base includes an emitter in a region other than the electron emission surface, comprising an alloy of at least one element selected from the group of elements consisting of carbon, niobium, tantalum, titanium, cobalt, chromium, and nickel.

2. The emitter according to claim 1, wherein the electron emission surface has holes or slits that define current paths, and the alloy is formed on the peripheral edge located on the outer periphery side of the current path when the base is unfolded.

3. The emitter according to claim 2, wherein the base is mainly made of tungsten, the alloy is an alloy with tungsten, and the concentration distribution of the alloy is in at least one direction from the surface of the edge portion in the thickness direction and in the direction away from the electron emission surface.

4. A base having an electron-emitting surface that emits electrons, The electron emission surface comprises a pair of leg portions for applying a voltage, The method for manufacturing an emitter wherein the base is made primarily of tungsten, and the peripheral portion located on the outer periphery side of the electron emission surface in the unfolded state of the base is doped with carbon to form a tungsten carbide alloy, A method for manufacturing an emitter, comprising contacting the aforementioned edge portion with graphite powder at a temperature of 800°C to 1200°C for one hour or more, and then heating it at a temperature of 1800°C or higher.

5. An X-ray tube comprising a vacuum enclosure, a cathode provided inside the vacuum enclosure for emitting electrons, and an anode provided inside the vacuum enclosure for generating X-rays upon collision with electrons emitted from the cathode, wherein the cathode has an emitter as described in any one of claims 1 to 3.

6. An X-ray tube comprising a vacuum enclosure, a cathode provided inside the vacuum enclosure for emitting electrons, and an anode provided inside the vacuum enclosure for generating X-rays upon collision with electrons emitted from the cathode, wherein the cathode has an emitter manufactured by the manufacturing method of claim 4.

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