X-ray tube target and method for manufacturing same

Abstract

One embodiment of the present invention provides an X-ray tube target 1 including: an Mo alloy substrate 2, a carbon substrate 3, and a bonding layer 4 disposed between the Mo alloy substrate 2 and the carbon substrate 3. The bonding layer 4 includes: a layer containing an MoNbTiZr alloy; and an Nb-containing layer.

Description

X-ray tube target and method of manufacturing the same

[0001] FIELD OF THE INVENTION An embodiment of the present invention relates to an X-ray tube target and a method for manufacturing the same.

[0002] In order to achieve higher resolution, medical CT scanners often use rotating anodes that can obtain high X-ray output. To achieve high output, the energy of the electron beam irradiated onto the target must be increased. With a rotating anode, the target rotates relative to the electron beam, constantly changing the focus, enabling high-power irradiation while minimizing target degradation. However, targets irradiated with high-energy electron beams in a vacuum tube must have high heat resistance and sufficient thermal volume for continuous operation.

[0003] Mo alloys are used as components that meet the heat resistance requirements described above, but in recent years, in order to improve inspection speed, the diameter of the anode has been increasing so that a wide area can be imaged at once. However, as the diameter increases, the weight of the target increases, placing a greater burden on the rotor shaft. Therefore, it has been investigated to reduce the weight while maintaining the size and thermal volume of the irradiation surface by replacing part of the Mo alloy with graphite, which is both heat-resistant and lightweight.

[0004] When joining the Mo alloy and graphite, it is necessary to use a joining material and joining method that take into consideration heat resistance and mechanical properties sufficient for practical use in X-ray tubes.

[0005] Japanese Patent Application Publication No. 4-228480 Japanese Patent Application Publication No. 2004-355922 Japanese International Publication No. 2010-005001

[0006] The problem to be solved is to provide an X-ray tube target that can improve bonding strength while ensuring practical heat resistance and mechanical properties, and a method for manufacturing the same.

[0007] According to an embodiment, there is provided a target for an X-ray tube, comprising: a Mo alloy substrate, a carbon substrate, and a bonding layer disposed between the Mo alloy substrate and the carbon substrate, the bonding layer including a layer including a MoNbTiZr alloy and a Nb-containing layer.

[0008] According to another embodiment, there is provided a method for manufacturing an X-ray tube target including a Mo alloy substrate, a carbon substrate, and a bonding layer disposed between the Mo alloy substrate and the carbon substrate. The method includes the steps of: obtaining a laminate by disposing a first brazing filler metal layer, a second brazing filler metal layer, and an intermediate layer between the Mo alloy substrate and the carbon substrate; and heating the laminate to a temperature of 1800°C or higher and 2000°C or lower to form a bonding layer between the carbon substrate and the Mo alloy substrate. The first brazing filler metal layer is in contact with the Mo alloy substrate. The melting point of the first brazing filler metal layer is higher than 1600°C and lower than the bonding temperature. The second brazing filler metal layer is in contact with the carbon substrate. The second brazing filler metal layer contains Zr. The intermediate layer is located between the first brazing filler metal layer and the second brazing filler metal layer. The intermediate layer contains Nb.

[0009] 1 is a cross-sectional view of the periphery of a bonding layer obtained when the X-ray tube target according to the embodiment is cut along the stacking direction; FIG. 2 is a cross-sectional view showing measurement conditions when EDS analysis is performed on the periphery of the bonding layer of the X-ray tube target shown in FIG. 1; FIG. 3 is a schematic view showing an example of the relationship between the distance on the scanning axis during EDS analysis and the mass concentration of each element; FIG. 4 is a cross-sectional view obtained when a laminate used in a manufacturing method of an X-ray tube target according to the embodiment is cut along the stacking direction; FIG. 5 is a schematic cross-sectional view of an X-ray tube anode target to which the X-ray tube target according to the embodiment is applied;

[0010] For example, in Patent Documents 1 and 2, bonding is achieved by combining multiple high-melting point metals and their alloys, partially melting them, and then solidifying them. The NbTi alloy used as the brazing filler metal in the X-ray tube target of Patent Document 2 is less expensive and more stably supplied than the V and its alloys used in the composite of Patent Document 1. However, when the NbTi alloy is heated for bonding, a solid-phase reaction with the Mo alloy substrate tends to occur below the melting point of the brazing filler metal. The solid-phase reaction occurs when material transfer occurs at the interface where dissimilar materials are in contact in a solid state. As the solid-phase reaction progresses, Ti contained in the brazing filler metal diffuses preferentially into the Mo alloy substrate. The melting point of the NbTi alloy varies significantly depending on the composition. When Ti diffuses out of the brazing filler metal layer, the melting point rises as Nb flows in from the Mo alloy substrate, such as from Mo or Nb plates. As a result, a high bonding temperature is required to obtain a good void-free bonded body. Therefore, the mechanical strength of the Mo alloy substrate decreases due to the coarsening of the crystal grain size, and the excessive grain growth of the intermetallic compounds makes the bonding layer prone to brittle fracture.In addition, the degree of the solid-state reaction varies in complex ways depending on the heating rate, the holding time at the maximum temperature, and the bonding load, making it very difficult to control the process taking the solid-state reaction into account.

[0011] According to an embodiment of an X-ray tube target and a manufacturing method thereof, the melting point of the brazing filler metal can be lowered without impairing the reactivity, thereby facilitating bonding of a Mo alloy substrate and a carbon substrate. Hereinafter, an embodiment of an X-ray tube target and a manufacturing method thereof will be described. (X-ray tube target of embodiment) The X-ray tube target of the embodiment includes a carbon substrate, a Mo alloy substrate, and a bonding layer disposed between the carbon substrate and the Mo alloy substrate. The bonding layer includes a layer containing a MoNbTiZr alloy and an Nb-containing layer. The X-ray tube target of the embodiment will be described with reference to FIGS. 1 to 3. FIG. 1 is an example of a cross-sectional view of the bonding layer and its surroundings obtained when an X-ray tube target according to an embodiment is cut along the stacking direction. In FIG. 1, the direction in which the substrates and other components constituting the target are stacked (stacking direction) is assumed to be parallel to the z direction. One direction intersecting the stacking direction of the X-ray tube target is assumed to be parallel to the x direction. Furthermore, another direction intersecting the stacking direction of the X-ray tube target is assumed to be parallel to the y direction.

[0012] 1 includes a Mo alloy substrate 2, a carbon substrate 3, and a bonding layer 4 disposed between the carbon substrate 3 and the Mo alloy substrate 2. The bonding layer 4 includes a first layer 5 containing a MoNbTiZr alloy, a second layer 6 containing a MoNbTiZr alloy, and a Nb-containing layer 7.

[0013] The Mo alloy contained in the Mo alloy substrate 2 is not particularly limited, but may contain, in addition to Mo, at least one element selected from the group consisting of oxygen, carbon, titanium, and zirconium, for example.

[0014] The carbon substrate 3 contributes to weight reduction without impairing the mechanical properties of the target. An example of a material constituting the carbon substrate 3 is isotropic carbon. It is preferable that the isotropic carbon be formed by cold isostatic pressing (CIP). CIP-formed isotropic carbon has high density and excellent mechanical properties. The carbon substrate 3 has multiple grooves 3a on the surface (bonding surface) that contacts the bonding layer 4. Each groove 3a has a V-shape with a depth in the stacking direction (z-direction). The multiple grooves 3a are aligned, for example, in the x-direction. The grooves 3a can prevent the bonding layer 4 from peeling off from the carbon substrate 3 due to thermal expansion and contraction of the carbon substrate 3. The number of grooves 3a may be one. The shape of the grooves 3a is not limited to a V-shape and may be, for example, a U-shape. The carbon substrate 3 may be entirely free of grooves.

[0015] The bonding layer 4 will now be described. The first layer 5 containing the MoNbTiZr alloy is adjacent to the Mo alloy substrate 2. Therefore, the first layer 5 containing the MoNbTiZr alloy is bonded to the surface (bonding surface) of the Mo alloy substrate 2 opposite the electron beam irradiated surface 2a. On the other hand, the second layer 6 containing the MoNbTiZr alloy is adjacent to the carbon substrate 3. The second layer 6 containing the MoNbTiZr alloy is bonded to the surface of the carbon substrate 3 on which the groove 3a is formed. The MoNbTiZr alloy of the first layer 5 and the MoNbTiZr alloy of the second layer 6 may have the same composition, but preferably have different compositions. The Nb-containing layer 7 is located between the first layer 5 and the second layer 6. The Nb-containing layer 7 is sandwiched between the first layer 5 and the second layer 6 and bonded to them. The bonding layer 4 contains metal carbide. The metal carbide has a particulate shape. The first metal carbide particles 8 are present in the first layer 5 , and the second metal carbide particles 9 are present in the second layer 6 .

[0016] The form of the metal carbide is not limited to particulate. The target 1 further includes a metal carbide layer 10. The metal carbide layer 10 is located at the interface between the surface of the carbon substrate 3 where the grooves 3a are formed and the second layer 6.

[0017] The bonding layer 4 will be described in detail below.

[0018] The thickness of the bonding layer 4 varies depending on the type of X-ray tube target 1, the size of the Mo alloy substrate 2 and the carbon substrate 3, etc., but can be, for example, 100 μm or more and 1000 μm or less.

[0019] <First Layer 5> The MoNbTiZr alloy contained in the first layer 5 is preferably a MoNbTiZr solid solution. The MoNbTiZr alloy of the first layer 5 preferably has a smaller Zr content than the sum of the Mo, Nb, and Ti content. To suppress a melting point drop due to a eutectic reaction, it is even more preferable that the Zr content be smaller than the sum of the Mo and Nb content. Furthermore, in the MoNbTiZr alloy of the first layer 5, Mo and Ti are preferably components that constitute the alloy matrix. Furthermore, the MoNbTiZr alloy of the first layer 5 preferably has a smaller Zr content than any of the Mo, Nb, and Ti content.

[0020] The Mo content of the MoNbTiZr alloy of the first layer 5 is preferably greater than the Mo content of the MoNbTiZr alloy of the second layer 6. The Ti content of the MoNbTiZr alloy of the first layer 5 is preferably greater than the Ti content of the MoNbTiZr alloy of the second layer 6. Here, the main component is the component that has the greatest amount (mass %, also referred to as content) of all the components constituting the alloy.

[0021] In the MoNbTiZr alloy contained in the first layer 5, the Zr content is preferably 0.1% by mass or more and 20% by mass or less. Like Ti and Nb, Zr is an element that is highly reactive with dissimilar metals and nonmetals, known as an active metal. Therefore, Zr has good wettability with the Mo alloy substrate. Furthermore, Zr dissolves in the MoNbTi alloy at bonding temperatures of 1600°C or higher and does not produce heterogeneous phases that would hinder good bonding. Furthermore, if the Zr content is below a certain amount, a melting point drop due to a eutectic reaction with Mo does not occur, ensuring heat resistance for use as an X-ray tube target. Therefore, by setting the Zr content to 0.1% by mass or more and 20% by mass or less, the bond strength between the first layer and the Mo alloy substrate can be increased while ensuring heat resistance for use as an X-ray tube target. A more preferred range of the Zr content is 0.2% by mass or more and 2.0% by mass or less.

[0022] The first layer 5 may contain a metal carbide. The metal carbide is harder than the bonding layer or the Mo alloy substrate and is stable up to high temperatures, and is therefore expected to have the effect of improving the mechanical strength of the X-ray tube target at room temperature and high temperatures. An example of the metal carbide is MC (M is one or more elements selected from the group consisting of Mo, Nb, Ti, and Zr). The metal carbide preferably contains TiC. It is more preferable that the main component of the metal carbide is TiC. One type or two or more types of metal carbides may be used.

[0023] The metal carbide is preferably in the form of particles. The shape of the metal carbide particles is not particularly limited, but can be, for example, granular, fibrous, or the like. The metal carbide particles can have a particle size or aspect ratio (minor axis to major axis). When the metal carbide particles have a particle size, the maximum particle size is preferably less than 10 μm, and more preferably 5 μm or less. When the metal carbide particles have a minor axis, the maximum minor axis is preferably less than 10 μm, and more preferably 5 μm or less. Because metal carbides are brittle, if the particles grow large or are widely distributed, dispersion strengthening cannot be achieved and the particles themselves can become fracture origins, which may actually reduce reliability. Furthermore, if the amount of metal carbide crystallization increases, the solidification of the brazing filler metal may become non-uniform, leading to reduced wettability. Therefore, an increase in the amount of metal carbide crystallization can cause voids. Therefore, it is desirable to disperse a small amount of fine metal carbide in the first layer, and specifically, it is desirable for the first layer to contain metal carbide particles whose maximum particle size or maximum short diameter is less than 10 μm, and the maximum particle size or maximum short diameter is desirably 0.5 μm or more.

[0024] The metal carbide is obtained, for example, when a portion of the carbon of the carbon substrate dissolved in the second brazing filler metal layer diffuses through the intermediate layer, dissolves again in the first brazing filler metal layer, and then combines with elements contained in the first brazing filler metal layer to crystallize.

[0025] The area ratio of the metal carbide in the first layer is 0.1% or more and 2.0% or less. When the metal carbide particles are finely dispersed, they play a role in dispersion strengthening in the bonding layer, improving the mechanical strength of the bonded body and increasing reliability in input tests (actual machine tests). On the other hand, if the ratio of the metal carbide to the area of ​​the first layer exceeds 2%, the bonding layer becomes significantly embrittled and the mechanical properties deteriorate, so it is desirable that the ratio of the metal carbide to the area of ​​the first layer be 2.0% or less.

[0026] <Second Layer 6> The MoNbTiZr alloy contained in the second layer 6 is preferably a MoNbTiZr solid solution. The MoNbTiZr alloy of the second layer 6 preferably contains Zr as a major component. Here, the major component is the component that has the largest amount (mass %, also referred to as content) among the components constituting the alloy. Furthermore, Ti and Mo are preferably trace diffusible elements in the alloy. Ti and Zr each act as active metals necessary for reaction with nonmetals (e.g., carbon substrates). Furthermore, the MoNbTiZr alloy preferably further contains C. C may exist in the MoNbTiZr alloy as a solid solution or as a metal carbide. The second layer 6 may contain only one of C existing as a solid solution and C existing as a metal carbide, or both.

[0027] It is desirable that the Mo content be less than 10 mass % (excluding 0 mass %) in the MoNbTiZr alloy contained in the second layer 6. By making the Mo content less than 10 mass %, it is possible to suppress the formation of a Zr-Mo eutectic structure, and therefore the bonding layer can have sufficient heat resistance.

[0028] The second layer 6 may contain a metal carbide. Metal carbides are harder than the bonding layer or the Mo alloy substrate and are stable up to high temperatures, and are therefore expected to improve the mechanical strength of the X-ray tube target at room temperature and high temperatures. Examples of metal carbides include MC (where M is one or more elements selected from the group consisting of Mo, Nb, Ti, and Zr). The metal carbide preferably contains ZrC. It is more preferable that ZrC be the main component of the metal carbide. One type of metal carbide or two or more types of metal carbides may be used.

[0029] The metal carbide is preferably in the form of particles. The shape of the metal carbide particles is not particularly limited, but may be, for example, granular, fibrous, or the like.

[0030] The thickness of the metal carbide layer 10 present between the second layer 6 and the surface of the carbon substrate 3 where the grooves 3a are formed is preferably 10 μm or more and 100 μm or less. Examples of metal carbides include MC (M is one or more elements selected from the group consisting of Mo, Nb, Ti, and Zr). The metal carbide preferably contains ZrC. It is more preferable that ZrC is the main component of the metal carbide. The type of metal carbide may be one type or two or more types.

[0031] <Nb-containing layer 7> The Nb-containing layer 7 is formed of, for example, Nb or an Nb alloy. The Nb-containing layer 7 is preferably composed mainly of Nb. Nb not only does not melt at the joining temperature, but also allows the diffusion of elements contained in other brazing filler metals to a certain extent compared to Ta and W. Therefore, the Nb-containing layer 7 contributes to the formation of a joining layer of the target composition by interdiffusion of metals.

[0032] The Nb content of the Nb-containing layer 7 is preferably 80% by mass or more. By making the Nb content 80% by mass or more, the heat resistance of the bonding layer can be ensured. The upper limit of the Nb content is 100% by mass.

[0033] 1 and 2, the bonding layer 4 and each layer constituting the bonding layer 4 have a layer shape with a surface parallel to the xy plane, but the shape is not limited to this, and for example, the surface may be uneven. Also, in Fig. 1 and 2, the boundaries of the components constituting the target (Mo alloy substrate, carbon substrate, bonding layer) are shown, but the boundaries may not be clear.

[0034] <Regarding the structural state of the bonding layer> The structural state of the bonding layer 4 can be confirmed by performing scanning electron microscope (SEM) observation and energy dispersive X-ray spectroscopy (EDS) analysis on a cross section obtained by cutting the target 1 along the stacking direction (z direction). An example of a typical cross-sectional structure is shown in FIG. 2.

[0035] The extent of the bonding layer is determined using an EDS device. The EDS device used is a field emission scanning electron microscope (FE-SEM), such as the JEOL EX-74600U4L2Q attached to the JEOL JSM-7200F, or a device with equivalent functionality. Elemental quantitative point analysis is performed continuously at 20 μm intervals along a single axis perpendicular to the thickness direction of the bonding layer, such as the axis indicated by L in Figure 2. If a V-shaped groove 3a is present on the surface of the carbon substrate 3, as illustrated in Figure 2, the position of the scanning axis L is preferably aligned with the valley of the V-shaped groove 3a. The acceleration voltage during EDS scanning is, for example, 15 kV, and the integration time per measurement point is, for example, 48 s. The elements to be quantified are C, Ti, Zr, Nb, and Mo. The quantitative composition values ​​at each point are calculated using the ZAF correction method. An example of the relationship between the thickness of the bonding layer and the quantitative composition values ​​is shown in Figure 3. 3 shows an example of a graph in which the horizontal axis represents the distance from the interface of the carbon substrate on the scanning axis L, and the vertical axis represents the change in mass concentration of each element. The curves representing the change in mass concentration of each element are denoted by reference numerals 20 to 24. Curve 20 is the mass concentration change curve for C, curve 21 is the mass concentration change curve for Zr, curve 22 is the mass concentration change curve for Nb, curve 23 is the mass concentration change curve for Ti, and curve 24 is the mass concentration change curve for Mo.

[0036] The interface between the carbon substrate 3 and the bonding layer 4 is defined as the measurement point where carbon first falls below 95% by mass when scanned sequentially from the carbon substrate side. The range of the first layer 5 is defined as the measurement point from the Mo alloy substrate side where Mo first falls below 90% by mass to the measurement point where Nb first falls above 80% by mass. The Zr content in the MoNbTiZr alloy of the first layer 5 is the average value of the Zr content at all measurement points belonging to the first layer 5. However, if any measurement point exceeds 20% by mass or if three consecutive measurement points fall below the detection limit of 0.1% by mass, the Zr content in the MoNbTiZr alloy of the first layer 5 is deemed to be outside the range of 0.1% by mass to 20% by mass.

[0037] The range of the second layer 6 is defined as the measurement point defining the interface between the carbon substrate 3 and the bonding layer 4 when scanning sequentially from the carbon substrate side to the measurement point where Nb first becomes 80 mass% or more. The Mo content in the MoNbTiZr alloy of the second layer 6 is the average value of the Mo content at all measurement points belonging to the second layer 6. However, if there is even one measurement point where the Mo content is 10 mass% or more, the Mo content in the MoNbTiZr alloy of the second layer 6 is not defined as being within a range of less than 10 mass%.

[0038] The Nb content of the Nb-containing layer 7 is the average value of all measurement points from the measurement point defining the interface with the first layer 5 to the measurement point defining the interface with the second layer 6 .

[0039] The composition of the bonding layer 4 is determined from the mass ratio obtained from the mass concentration change curve of each element.

[0040] <Regarding the particle size of metal carbide> Metal carbide particles can have a particle size or aspect ratio (major axis and minor axis). The maximum particle size and maximum minor axis are determined by binarizing a secondary electron image obtained by FE-SEM observation using image processing software. The secondary electron image is a field of view that captures the entire first layer in the thickness direction (z direction in the case of Figure 1). The FE-SEM device used is, for example, a JEOL JSM-7200F or a device with equivalent functionality. The image processing software used is, for example, ImageJ. The image imported into the image processing software is a backscattered electron image taken using an FE-SEM (when using a JEOL JSM-7200F, the accelerating voltage is 5 kV) at a magnification that fits the thickness of the first layer within the field of view. In the backscattered electron image, the metal carbide particles are approximated to an ellipse using ImageJ's Ellipses ellipse fitting, and the maximum minor axis is determined.

[0041] A method for manufacturing an X-ray tube target according to the embodiment will be described with reference to FIG.

[0042] The manufacturing method of the embodiment includes a first step of obtaining a laminate by arranging a first brazing filler metal layer, an intermediate layer, and a second brazing filler metal layer between a carbon substrate and a Mo alloy substrate, and a second step of forming a bonding layer between the carbon substrate and the Mo alloy substrate by heating the laminate to a temperature of 1800° C. or more and 2000° C. or less. Each step will be described below.

[0043] <First Step> An example of the laminate is shown in Figure 4. The laminate 30 has a structure in which a Mo alloy substrate 2, a first brazing filler metal layer 31, an intermediate layer 32, a second brazing filler metal layer 33, and a carbon substrate 3 are laminated in this order along the z direction. The direction parallel to one side of the laminate 30 is defined as the x direction. The direction parallel to the other side intersecting with the one side of the laminate 30 is defined as the y direction. The Mo alloy substrate 2 and the carbon substrate 3 are as described above.

[0044] One surface of the first brazing filler metal layer 31 intersecting the thickness direction (z-direction) is in contact with the Mo alloy substrate 2. The first brazing filler metal layer 31 has a melting point higher than 1600°C and lower than the bonding temperature. The bonding temperature is the temperature applied to the laminate in the second step. Because the melting point of the first brazing filler metal layer 31 is low, melting of the first brazing filler metal layer 31 in the second step progresses, making diffusion of the first brazing filler metal layer 31 more likely to occur. This improves the bonding strength. A more preferable range for the melting point of the first brazing filler metal layer 31 is 1670°C or higher and lower than a temperature 100°C lower than the bonding temperature (T-100, where T°C is the bonding temperature). 1670°C is the theoretical melting point of Ti with a purity of 100% by mass. The first brazing filler metal layer 31 is formed, for example, from Ti. Preferably, the Ti is pure Ti. When pure Ti is used for the first brazing filler metal layer, the melting amount increases in the second step, making metal diffusion more likely to occur. Even if the melting point of the first brazing filler metal layer increases due to metal diffusion, brazing can be performed at a relatively low temperature. Furthermore, when metal diffusion occurs in the second step, resulting in a small amount of Zr solid-solubilizing in the first brazing filler metal layer, the melting point of the first brazing filler metal layer can be further and stably lowered. Furthermore, even if a high-purity Ti region remains due to insufficient metal diffusion, it ultimately forms a high-melting-point metal carbide and stabilizes, ensuring heat resistance and resulting in a highly reliable bonding layer. The purity of pure Ti is preferably 95% by mass or more, with a more preferred range being 99% by mass or more. A Ti alloy may be used instead of Ti for the first brazing filler metal layer 31. Examples of Ti alloys include TiNb alloys. The Ti content of the Ti alloy is preferably 95% by mass or more, with a more preferred range being 99% by mass or more.

[0045] The thickness of the first brazing filler metal layer 31 can be, for example, 0.005 mm or more and 2 mm or less, and the preferred range of the thickness of the first brazing filler metal layer 31 is 0.01 mm or more and 0.2 mm or less.

[0046] One surface of the second brazing filler metal layer 33 intersecting the thickness direction (z direction) is in contact with the surface of the carbon substrate 3 where the grooves 3a are formed. The second brazing filler metal layer 33 contains Zr. The second brazing filler metal layer 33 is formed, for example, from Zr or a Zr alloy. Zr acts as an active metal when reacting with a non-metal (for example, the carbon substrate). The thickness of the second brazing filler metal layer 33 can be, for example, 0.005 mm or more and 2 mm or less. A preferred range for the thickness of the second brazing filler metal layer 33 is 0.01 mm or more and 0.4 mm or less.

[0047] The intermediate layer 32 acts as a shielding layer that prevents elements contained in the second brazing filler metal layer 33 from being mixed into the Mo alloy substrate 2. If the second brazing filler metal layer 33 contains Zr, there is a possibility that the Zr will diffuse and be mixed into the Mo alloy substrate 2. If Mo and Zr are mixed, the melting point will be significantly lowered due to a eutectic reaction, and the heat resistance required for the usage environment of the X-ray tube target may not be guaranteed. If the second brazing filler metal layer 33 contains Zr, the intermediate layer 32 can prevent Zr from diffusing into the Mo alloy substrate 2.

[0048] The intermediate layer preferably contains Nb. Nb may be contained in the intermediate layer either as a single metal or in the form of an Nb alloy. Examples of Nb alloys include Nb-1% by mass Zr and Nb-28% by mass Ta-11% by mass W-1% by mass Zr. Nb not only does not melt at the joining temperature, but also, compared to Ta and W, allows a certain degree of diffusion of elements contained in the first brazing filler metal layer or the second brazing filler metal layer. Therefore, instead of adjusting the composition of the first brazing filler metal layer or the second brazing filler metal layer to the target composition, a joining layer of the target composition can be formed by element diffusion. As an example, by using Nb for the intermediate layer and heating it to 1800°C or higher and 2000°C or lower, not only do the first brazing filler metal layer and the second brazing filler metal layer melt individually, but the elements contained in the first brazing filler metal layer and the second brazing filler metal layer diffuse mutually within the intermediate layer, forming solid solutions in the first layer and the second layer, respectively.

[0049] When the second brazing filler metal layer contains Zr, using an intermediate layer containing Nb allows Zr to diffuse into the first brazing filler metal layer and dissolve in a small amount to the extent that a eutectic reaction with Mo does not occur. This allows the melting point of the first brazing filler metal layer to be lowered. The lower melting point allows the joining temperature to be lowered, resulting in an X-ray tube target with excellent mechanical properties.

[0050] The intermediate layer preferably has a thickness that shields the first brazing filler metal layer from the second brazing filler metal layer and allows elements contained in the first brazing filler metal layer and the second brazing filler metal layer to diffuse into each other. For example, the ratio of the total thickness of the first brazing filler metal layer and the second brazing filler metal layer to the thickness of the intermediate layer ((thickness of the first brazing filler metal layer + thickness of the second brazing filler metal layer) / thickness of the intermediate layer) is preferably 1 or more and less than 2.

[0051] The thickness of the intermediate layer can be, for example, 0.2 mm or more and 2 mm or less. The thickness of the intermediate layer is, for example, preferably 0.2 mm or more and 0.5 mm or less. If the thickness of the intermediate layer is less than 0.2 mm, there is a risk that heat resistance cannot be ensured.

[0052] <Second Step> The laminate 30 is heated to a temperature of 1800° C. or higher and 2000° C. or lower, thereby forming a bonding layer between the carbon substrate 3 and the Mo alloy substrate 2 .

[0053] By setting the bonding temperature in the range of 1800° C. to 2000° C., bonding can be performed while ensuring heat resistance and mechanical properties sufficient for practical use of the X-ray tube. A more preferable range for the bonding temperature is 1850° C. to 1900° C.

[0054] The bonding can be performed, for example, in a vacuum atmosphere.

[0055] The mechanism by which the bonding layer is formed is presumed to be as follows.

[0056] First, the metal (e.g., Ti) contained in the first brazing filler metal layer 31 and the metal (e.g., Zr) contained in the second brazing filler metal layer 33 melt. The molten metal reacts with C in the carbon substrate 3 to produce metal carbides. The metal carbides are generated in the first brazing filler metal layer 31, the second brazing filler metal layer 33, or at the interface between the second brazing filler metal layer 33 and the carbon substrate 3. Next, the metals constituting the Mo alloy substrate 2, the first brazing filler metal layer 31, the intermediate layer 32, and the second brazing filler metal layer 33 diffuse into each other. Metal interdiffusion occurs not only between adjacent layers, but also between layers sandwiching the intermediate layer 32. For example, Ti contained in the first brazing filler metal layer 31 diffuses through the intermediate layer 32 to the second brazing filler metal layer 33. Zr contained in the second brazing filler metal layer 33 diffuses through the intermediate layer 32 to the first brazing filler metal layer 31. This interdiffusion can lower the melting points of the first brazing filler metal layer 31 and the second brazing filler metal layer 33, thereby increasing the reactivity of the brazing filler metal. Next, the laminate is cooled to cause a solidification reaction, thereby forming a bonding layer 4 including a first layer 5 containing a MoNbTiZr alloy, an Nb-containing layer 7, and a second layer 6 containing a MoNbTiZr alloy, thereby obtaining an X-ray tube target 1. As described above, the second step may involve cooling the laminate after heating.

[0057] When the laminate 30 is heated to a temperature of 1800° C. or more and 2000° C. or less, a pressure of 1 kPa or more and 200 kPa or less may be applied to the laminate 30. This can improve the bonding strength.

[0058] The method of the embodiment may include a step other than the first and second steps. After the second step, polishing may be performed as necessary. Also, the method may include a step of providing a rotation axis on the X-ray tube target.

[0059] An example in which the X-ray tube target of the embodiment is applied to an X-ray tube anode target will be described with reference to FIG. 5 . The X-ray tube anode target 40 shown in FIG. 5 can generate X-rays by irradiating an electron beam onto an electron beam irradiation surface. The X-ray tube anode target 40 includes a Mo alloy substrate 41, an electron beam irradiation unit 42, a carbon substrate 43, and a bonding layer 44. The Mo alloy substrate 41 has a substantially truncated cone shape. The Mo alloy substrate 41 is provided with a cylindrical through-hole 41a that is concentric with a rotation center 45. The electron beam irradiation unit 42 has a ring shape. The electron beam irradiation unit 42 is provided on an inclined surface of the Mo alloy substrate 41. The surface of the electron beam irradiation unit 42 is the electron beam irradiation surface. The electron beam irradiation unit 42 is formed, for example, from a ReW alloy.

[0060] The carbon substrate 43 has a cylindrical shape. The carbon substrate 43 is provided with a cylindrical through-hole 43a that is concentric with the rotation center 45. Furthermore, a V-shaped groove (not shown) is provided on the ring-shaped upper surface of the carbon substrate 43 at a position that is concentric with the rotation center 45.

[0061] The bonding layer 44 is disposed between the Mo alloy substrate 41 and the carbon substrate 43 to bond them together. The bonding layer 44 is similar to the bonding layer 4 described with reference to Fig. 1. The bonding layer 44 is provided with a cylindrical through-hole 44a that is concentric with the rotation center 45. The through-hole 44a is connected to each of the through-holes 41a and 43a.

[0062] A rotating shaft (not shown) is inserted into a cylindrical space formed by the through-holes 41a, 44a, and 43a.

[0063] The X-ray tube target of the embodiment can be applied to a CT (computed tomography) device such as a medical CT device.

[0064] According to the X-ray tube target of the embodiment described above, the bonding layer for bonding the Mo alloy substrate and the carbon substrate includes a layer containing a MoNbTiZr alloy and a Nb-containing layer, thereby increasing the bonding area between the Mo alloy substrate and the carbon substrate and improving the bonding strength while realizing the heat resistance and mechanical strength required for an X-ray tube target.

[0065] In addition, a method for manufacturing an X-ray tube target according to an embodiment includes the steps of: obtaining a laminate by disposing a first brazing filler metal layer having a melting point higher than 1600°C but lower than the bonding temperature, an intermediate layer containing Nb, and a second brazing filler metal layer containing Zr between an Mo alloy substrate and a carbon substrate; and heating the laminate to a temperature of 1800°C to 2000°C to form a bonding layer between the carbon substrate and the Mo alloy substrate. This method allows the first brazing filler metal layer and the second brazing filler metal layer to melt at a temperature of 1800°C to 2000°C. This promotes interdiffusion of the metals, thereby further lowering the melting points of the first brazing filler metal layer and the second brazing filler metal layer. As a result, the wettability of the first and second brazing filler metal layers with respect to the Mo alloy substrate and the carbon substrate can be improved, thereby increasing the void-free bonding area and improving the bonding strength. Furthermore, the bonding temperature can be kept within the above range, ensuring the heat resistance and mechanical strength required for an X-ray tube target. Therefore, it is possible to provide a highly reliable target for an X-ray tube that has practical heat resistance and mechanical strength.

[0066] The above-described embodiment will be specifically explained below using examples, but the present invention is not limited to the following examples as long as they do not deviate from the gist of the present invention.

[0067] Example 1 As shown in FIG. 4 , a laminate 30 was obtained by laminating a Mo alloy substrate 2, a first brazing filler metal layer 31, an intermediate layer 32, a second brazing filler metal layer 33, and a carbon substrate 3 in this order. The Mo alloy substrate 2 was made of a Mo alloy material containing 0.5 mass % Ti, 0.08 mass % Zr, and the remainder Mo. The carbon substrate 3 was made of isotropic carbon. A V-shaped cross-sectional groove 3a was formed on the entire joining surface of the carbon substrate 3. A Ti foil with a thickness of 0.1 mm was used for the first brazing filler metal layer 31. A Nb foil with a thickness of 0.3 mm was used for the intermediate layer 32. A Zr foil with a thickness of 0.2 mm was used for the second brazing filler metal layer 33. The ratio of the total thickness of the first brazing filler metal layer and the second brazing filler metal layer to the thickness of the intermediate layer was 1.

[0068] The above laminate was placed in a vacuum atmosphere (1.0 × 10 -2The X-ray tube target according to Example 1 was produced by bonding the materials at a temperature of 1890° C. for 60 minutes under a pressure load of 10 kPa at a pressure of 10 kPa or less.

[0069] Example 2 An X-ray tube target of Example 2 was produced in the same manner as Example 1, except that the bonding temperature was changed to 1860°C.

[0070] Example 3 An X-ray tube target of Example 3 was produced in the same manner as Example 1, except that the bonding time was changed to 15 minutes.

[0071] Comparative Example 1 An X-ray tube target of Comparative Example 1 was produced in the same manner as in Example 1, except that the bonding temperature was changed to 1780°C.

[0072] (Comparative Example 2) A 0.2 mm thick Ti foil was used for the first brazing filler metal layer, a 0.2 mm thick Nb foil was used for the intermediate layer, and a 0.3 mm thick Zr foil was used for the second brazing filler metal layer, and an X-ray tube target of Comparative Example 2 was produced in the same manner as in Example 1. The ratio of the total thickness of the first brazing filler metal layer and the second brazing filler metal layer to the thickness of the intermediate layer was 2.5.

[0073] The soundness of the bonded layers of the examples and comparative examples was evaluated by ultrasonic flaw detection testing, and a void-free bonded area rate of 90% or more was judged to be good, and an area rate of less than 90% was judged to be poor. The results are shown in Table 1.

[0074] The structural state of the bonding layer was evaluated by the above-mentioned method using cross-sectional SEM observation and EDS analysis. The FE-SEM device used for the measurements was a JEOL Ltd. JSM-7200F. The EDS device used was a JEOL Ltd. EX-74600U4L2Q attached to the JEOL Ltd. JSM-7200F. Because a V-shaped groove 3a was present on the surface of the carbon substrate 3, the position of the scanning axis L was aligned as closely as possible to the valley of the V-shaped groove 3a. The acceleration voltage during EDS scanning was 15 kV, and the integration time per measurement point was 48 s.

[0075] Measurement of the structural state of the bonding layer revealed that the bonding layers of Examples 1 to 3 included a first layer adjacent to the Mo alloy substrate and containing a MoNbTiZr alloy, a second layer adjacent to the carbon substrate and containing a MoNbTiZr alloy, and a Nb-containing layer located between the first and second layers. The MoNbTiZr alloy of the first layer was a MoNbTiZr solid solution having a composition in which the Zr content was lower than the sum of the Mo, Nb, and Ti content ratios, and the Zr content was lower than the sum of the Mo and Nb content ratios, and the Zr content was lower than any of the Mo, Nb, and Ti content ratios. The MoNbTiZr alloy of the second layer was a MoNbTiZr solid solution having a composition in which Zr was the main component. The Mo content ratio of the MoNbTiZr alloy of the first layer was higher than the Mo content ratio in the MoNbTiZr alloy of the second layer. Furthermore, the Ti content in the MoNbTiZr alloy of the first layer was greater than the Ti content in the MoNbTiZr alloy of the second layer. The Zr content in the first layer, the Mo content in the second layer, and the Nb content in the Nb-containing layer were as shown in Table 1. The bonding layer contained TiC particles and ZrC particles as metal carbide particles. A metal carbide layer was formed between the second layer and the groove-formed surface of the carbon substrate. The metal carbide layer contained ZrC. The thickness of the metal carbide layer was 60 to 80 μm.

[0076] On the other hand, in the bonding layer of Comparative Example 1, the alloy composition of the first layer adjacent to the Mo alloy substrate was MoNbTi alloy, and the alloy composition of the second layer adjacent to the carbon substrate was NbZr.

[0077] In the bonding layer of Comparative Example 2, the alloy composition of the first layer adjacent to the Mo alloy substrate was a MoNbTiZr alloy, and the alloy composition of the second layer adjacent to the carbon substrate was MoNbTiZr. The MoNbTiZr alloy of the first layer was a MoNbTiZr solid solution having a composition in which the amount of Zr was greater than the sum of the amounts of the three components Mo, Nb, and Ti, and the amount of Zr was greater than the sum of the amounts of Mo and Nb. The MoNbTiZr alloy of the second layer was a MoNbTiZr solid solution having a composition in which Zr was the main component. In the formation of the bonding layer of Comparative Example 2, the ratio of the total thickness of the first brazing filler metal layer and the second brazing filler metal layer to the thickness of the intermediate layer was 2.5, so the Zr of the second brazing filler metal layer dissolved the Nb of the intermediate layer, causing the intermediate layer to be eaten away (the intermediate layer became sparse). As a result, Zr in the second brazing filler metal layer diffused through the intermediate layer into the Mo alloy substrate, causing the Mo-Zr eutectic reaction to proceed, resulting in poor bonding.

[0078] The bonding layers of Comparative Examples 1 and 2 included a Nb-containing layer located between the first and second layers. The Nb contents of the Nb-containing layer were 96% by mass and 84% by mass, respectively. The bonding layers of Comparative Examples 1 and 2 also included TiC particles and ZrC particles as metal carbide particles.

[0079] The area ratio and maximum grain size of the metal carbide in the first layer were determined from secondary electron images obtained by SEM observation. The SEM used for the measurement was the same as that used to measure the structural state of the bonding layer. Secondary electron images were obtained in a field of view (acceleration voltage 5 kV) that captured the entire first layer in the thickness direction, and the area ratio and maximum grain size of the metal carbide were obtained by binarization processing using ImageJ as image processing software. The measurement results are shown in Table 1.

[0080]

[0081] As shown in Examples 1 to 3 in Table 1, the bonding rates of the joined bodies of Examples 1 to 3 exceeded 90%, resulting in favorable results. In the joined bodies of Examples 1 to 3, the Zr content of the first layer was 0.1 mass% or more and 20 mass% or less. Furthermore, in Examples 1 to 3, the MoNbTiZr alloy of the first layer had a composition in which the Zr content was less than the sum of the content ratios of the three components Mo, Nb, and Ti, and less than the sum of the content ratios of Mo and Nb. The MoNbTiZr alloy of the second layer had a composition in which Zr was the main component. On the other hand, as shown in Comparative Examples 1 and 2, the bonding state was less than 90%, and a joined body suitable for use as an X-ray tube target could not be obtained. In Comparative Examples 1 and 2, the Zr content of the first layer was outside the above range. Furthermore, the joined body of Comparative Example 1 did not contain a MoNbTiZr alloy. On the other hand, although the joined body of Comparative Example 2 contained a MoNbTiZr alloy, the composition of the MoNbTiZr alloy was different from that of the Examples.

[0082] According to at least one of these embodiments or examples of the X-ray tube target, the bonding layer bonding the Mo alloy substrate and the carbon substrate includes a layer containing a MoNbTiZr alloy and a Nb-containing layer, thereby increasing the bonding area between the Mo alloy substrate and the carbon substrate and improving the bonding strength while realizing the heat resistance and mechanical strength required for an X-ray tube target.

[0083] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as defined in the claims.

[0084] The following describes the invention in terms of embodiments.

[0085] (1) A target for an X-ray tube, comprising: a Mo alloy substrate; a carbon substrate; and a bonding layer disposed between the Mo alloy substrate and the carbon substrate, wherein the bonding layer comprises: a layer containing a MoNbTiZr alloy; and a Nb-containing layer.

[0086] (2) The X-ray tube target according to (1), wherein the layer containing the MoNbTiZr alloy includes a first layer containing the MoNbTiZr alloy and a second layer containing the MoNbTiZr alloy, and the composition of the MoNbTiZr alloy in the first layer is different from the composition of the MoNbTiZr alloy in the second layer.

[0087] (3) The target for an X-ray tube according to (2), wherein the first layer is adjacent to the Mo alloy substrate, the second layer is adjacent to the carbon substrate, and the Nb-containing layer is disposed between the first layer and the second layer.

[0088] (4) The target for an X-ray tube according to (2), wherein the MoNbTiZr alloy of the first layer is a MoNbTiZr solid solution, and the Zr content in the MoNbTiZr alloy is 0.1 mass % or more and 20 mass % or less.

[0089] (5) The target for an X-ray tube according to (2), wherein the MoNbTiZr alloy of the second layer is a MoNbTiZr solid solution, and the Mo content in the MoNbTiZr alloy is less than 10 mass %.

[0090] (6) The target for an X-ray tube according to (2), wherein the first layer contains metal carbide particles having a maximum grain size or a maximum minor axis of less than 10 μm.

[0091] (7) The target for an X-ray tube according to any one of (1) to (6), wherein the layer containing the MoNbTiZr alloy includes a first layer containing the MoNbTiZr alloy and a second layer containing the MoNbTiZr alloy, wherein the MoNbTiZr alloy of the first layer has a smaller amount of Zr than the sum of the amounts of Mo and Nb, and the MoNbTiZr alloy of the second layer is mainly composed of Zr.

[0092] (8) The target for an X-ray tube according to any one of (1) to (7), wherein the bonding layer contains a metal carbide.

[0093] (9) The target for an X-ray tube according to any one of (1) to (8), wherein the Nb content of the Nb-containing layer is 80 mass % or more.

[0094] (10) A method for manufacturing a target for an X-ray tube, the method including: a Mo alloy base material, a carbon base material, and a bonding layer disposed between the Mo alloy base material and the carbon base material, the method comprising: a step of disposing, between the Mo alloy base material and the carbon base material, a first brazing filler metal layer in contact with the Mo alloy base material and having a melting point higher than 1600°C but lower than a bonding temperature, a second brazing filler metal layer in contact with the carbon base material and containing Zr, and an intermediate layer disposed between the first brazing filler metal layer and the second brazing filler metal layer and containing Nb, to obtain a laminate; and a step of heating the laminate to a temperature of 1800°C or higher and 2000°C or lower, to form the bonding layer between the carbon base material and the Mo alloy base material.

[0095] (11) The method for manufacturing a target for an X-ray tube according to (10), wherein the step of forming the bonding layer is performed while applying a pressure of 1 kPa or more and 200 kPa or less to the laminate.

[0096] (12) The method for manufacturing an X-ray tube target according to (10) or (11), wherein the first brazing filler metal layer is made of Ti.

[0097] DESCRIPTION OF SYMBOLS 1...X-ray tube target, 2...Mo alloy substrate, 3...carbon substrate, 3a...groove, 4...bonding layer, 5...first layer, 6...second layer, 7...Nb-containing layer, 8...first metal carbide particle, 9...second metal carbide particle, 10...metal carbide layer, 30...laminated body, 31...first brazing filler metal layer, 32...intermediate layer, 33...second brazing filler metal layer, 40...X-ray tube anode target, 41...Mo alloy substrate, 41a...through hole, 42...electron beam irradiation portion, 43...carbon substrate, 43a...through hole, 44...bonding layer, 44a...through hole, 45...rotation center.

Claims

1. An X-ray tube target comprising: a Mo alloy substrate; a carbon substrate; and a bonding layer disposed between the Mo alloy substrate and the carbon substrate, wherein the bonding layer comprises: a layer containing a MoNbTiZr alloy; and a Nb-containing layer.

2. The X-ray tube target according to claim 1, wherein the layer containing a MoNbTiZr alloy includes a first layer containing a MoNbTiZr alloy and a second layer containing a MoNbTiZr alloy, and the composition of the MoNbTiZr alloy in the first layer is different from the composition of the MoNbTiZr alloy in the second layer.

3. The target for an x-ray tube as recited in claim 2, wherein the first layer is adjacent to the Mo alloy substrate, the second layer is adjacent to the carbon substrate, and the Nb-containing layer is disposed between the first layer and the second layer.

4. The X-ray tube target according to claim 2, wherein the MoNbTiZr alloy of the first layer is a MoNbTiZr solid solution, and the Zr content in the MoNbTiZr alloy is 0.1 mass % or more and 20 mass % or less.

5. The target for an X-ray tube according to claim 2, wherein the MoNbTiZr alloy of the second layer is a MoNbTiZr solid solution, and the Mo content in the MoNbTiZr alloy is less than 10 mass %.

6. The target for an X-ray tube according to claim 2, wherein the first layer contains metal carbide particles having a maximum grain size or maximum shortest diameter of less than 10 μm.

7. The X-ray tube target according to claim 1, wherein the layer containing a MoNbTiZr alloy includes a first layer containing a MoNbTiZr alloy and a second layer containing a MoNbTiZr alloy, the MoNbTiZr alloy of the first layer having a Zr content lower than the sum of the Mo and Nb content, and the MoNbTiZr alloy of the second layer having Zr as a main component.

8. The target for an x-ray tube as defined in claim 1, wherein said bonding layer comprises a metal carbide.

9. The target for an X-ray tube according to claim 1, wherein the Nb content of the Nb-containing layer is 80 mass % or more.

10. A method for manufacturing a target for an X-ray tube, comprising: a Mo alloy substrate, a carbon substrate, and a bonding layer disposed between the Mo alloy substrate and the carbon substrate, the method comprising: disposing, between the Mo alloy substrate and the carbon substrate, a first brazing filler metal layer in contact with the Mo alloy substrate and having a melting point higher than 1600°C but lower than the bonding temperature; a second brazing filler metal layer in contact with the carbon substrate and containing Zr; and an intermediate layer disposed between the first brazing filler metal layer and the second brazing filler metal layer and containing Nb, to obtain a laminate; and heating the laminate to a temperature of 1800°C or higher and 2000°C or lower, to form the bonding layer between the carbon substrate and the Mo alloy substrate.

11. The method for manufacturing a target for an X-ray tube according to claim 10, wherein the step of forming the bonding layer is carried out while applying a pressure of 1 kPa or more and 200 kPa or less to the laminate.

12. The method for manufacturing an X-ray tube target according to claim 10, wherein said first brazing material layer is made of Ti.

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