Electron beam emission structure and field emission device

Abstract

In a cold cathode (1) having a columnar electrode substrate (11) an axial one end of which is disposed in a posture facing an anode (2), the dimension in the radial direction of an anode-side facing surface (12) is 0.1 mm to 1 mm, and an electron emission unit (4) having a carbon nanostructure is formed on the anode-side facing surface (12). The surface of the carbon nanostructure is formed such that a granular part (40) having a shape raised from the surface is distributed in an island shape with respect to the surface, and has an island-like structure. The granular part (40) is formed such that the dimension in the raised direction is 10 μm or more, and the dimension in the radial direction of the maximum diameter part of the granular part is 10 μm or more.

Description

Electron beam emission structures and field emission devices

[0001] The present invention relates to an electron beam emission structure and a field emission device that can be applied to various devices such as X-ray devices, electron tubes, and lighting devices.

[0002] Field emission is a phenomenon in which electrons are emitted into a vacuum atmosphere (e.g., in an insulating vacuum vessel) due to the concentration of an electric field. Utilizing this phenomenon has been studied to construct various field emission devices, such as X-ray devices, electron tubes, and lighting devices.

[0003] One example of field emission is a configuration in which a predetermined voltage can be applied to both an electron source (such as a cold cathode) and an anode (target) that are arranged facing each other at a predetermined distance, and the electrons generated in the electron source by the application of the voltage can be used to emit an electron beam toward the anode. By irradiating (colliding) the anode with such an electron beam, a desired function (for example, radioscopic resolution due to the external emission of X-rays in an X-ray device) can be achieved (see, for example, Patent Documents 1 to 5).

[0004] The electron source may be a cold cathode having an electrode substrate such as a Si substrate or a SUS substrate, with an electron-emitting portion provided on the end face of the electrode substrate facing the anode (hereinafter simply referred to as the anode-side facing surface). The electron-emitting portion may be a carbon nanostructure formed by growing carbon nuclei formed on the anode-side facing surface by, for example, a CVD method, and then three-dimensionally growing and stacking carbon films (graphene, carbon nanotubes, etc.) to form a hollow structure.

[0005] The electron beam needs to be focused on a central axis between the electron source and the anode (hereinafter simply referred to as the central axis) so that it can be irradiated onto the anode. This focusing control can be achieved, for example, by using a focusing electrode or the like to reduce the diameter of the electron beam as it approaches the anode from the electron source, or by reducing the focal spot size (electron spot), which is the area where the electron beam collides with the anode.

[0006] Patent No. 4565089 Patent No. 5264055 Patent No. 4408891 Patent No. 4390847 JP 2022-073816 A

[0007] In recent years, as the demand for smaller beam diameters and focal spot sizes has increased, for example, for electron beams in microfocus X-ray tubes, studies have begun to be conducted on reducing the diameter of the electrode substrate. However, simply reducing the diameter of the electrode substrate reduces the amount of electrons generated, which in turn reduces the tube current, and this can prevent the field emission device from performing its intended function.

[0008] Furthermore, even if a high current density can be obtained with an electron beam from a small-diameter electrode substrate, the electron-emitting portion formed on the electrode substrate is likely to have low durability against the high-current-density electron beam, and as a result, there is a risk that the field emission device will not be able to perform its desired function.

[0009] It is possible to reduce the beam diameter and focal spot size by controlling the focusing of the electron beam using an electromagnetic coil or the like, but providing such an electromagnetic coil would lead to an increase in the size and cost of the field emission device, and the focusing control would also be complex and difficult.

[0010] The present invention has been made in consideration of the above technical problems, and aims to provide a technology that can contribute to making it easier to reduce the beam diameter and focal spot size of an electron beam and to making it easier for a field emission device to perform the desired functions.

[0011] The electron beam emission structure and field emission device according to the present invention can contribute to solving the above-mentioned problems. One aspect of the electron beam emission structure includes a columnar electrode substrate arranged with one end of its axis facing an anode, a cold cathode having an electron emission portion on the end surface of the columnar electrode substrate facing the anode, and a cylindrical focusing electrode arranged coaxially with the axis while being insulated from the cold cathode and surrounding the outer periphery of the columnar electrode substrate. The end surface has a radial dimension of 0.1 mm to 1 mm, the electron emission portion is made of a carbon nanostructure obtained by growing carbon nuclei generated on the end surface, and the surface of the carbon nanostructure is formed with granular portions protruding from the surface and distributed in an island-like pattern relative to the surface, forming an island structure, the granular portions having a dimension in the protruding direction of 10 μm or more and a radial dimension at their maximum diameter of 10 μm or more.

[0012] The tip of the granular portion in the protruding direction may be curved to protrude at an obtuse angle toward the protruding direction.

[0013] The tip portion may have a radial dimension of 5 μm or more.

[0014] The cold cathode may be characterized in that the outer periphery of the columnar electrode base is surrounded by a cylindrical electrode cover that is arranged coaxially with the axis between the columnar electrode base and the focusing electrode.

[0015] The electrode cover may also be characterized in that the opening edge surface on the anode side has an inclined shape so that a point on the opening edge surface moves toward the anode side as it approaches the radial outside from the inside of the electrode cover.

[0016] One aspect of a field emission device is characterized by having any one of the electron beam emission structures described above, and may also be characterized by including a voltage control unit that can change the voltage applied between the anode and the focusing electrode.

[0017] As described above, the present invention can contribute to making it easier to reduce the beam diameter and focal spot size of the electron beam, thereby making it easier for the field emission device to perform the desired functions.

[0018] Schematic diagram (cross-sectional view along the central axis) for explaining the electron beam emission structure according to Example 1. Schematic diagram (cross-sectional view along the central axis) for explaining an example of a columnar electrode substrate 11. Electron image of the surface of a carbon nanostructure observed with an electron microscope (electron image of an excerpt from the peripheral edge side of the surface). Electron image (perspective image) of a granular portion 40 of a carbon nanostructure observed with an electron microscope. Observation results of current density characteristics versus applied voltage in a verification example (observation results showing IV characteristics). Schematic diagram (cross-sectional view along the central axis) for explaining the electron beam emission structure according to Example 2. Schematic diagram (cross-sectional view along the central axis) for explaining the electron beam emission structure according to Example 3.

[0019] The electron beam emission structure and field emission device in the embodiments of the present invention are completely different from, for example, a structure in which the diameter of the electrode substrate is simply reduced.

[0020] That is, in this embodiment, a cold cathode has a columnar electrode substrate arranged with one end of its axis facing an anode, and an electron emitter is provided on the anode-facing surface of the columnar electrode substrate. The anode-facing surface of the columnar electrode substrate has a radial dimension of 0.1 mm to 1 mm, and the electron emitter is made of a carbon nanostructure obtained by growing carbon nuclei generated on the anode-facing surface.

[0021] The surface of the carbon nanostructure is formed so that granular portions protruding from the surface are distributed in an island-like pattern on the surface, forming an island structure, and the granular portions are characterized in that the dimension in the protruding direction (hereinafter simply referred to as the protruding direction) is 10 μm or more, and the radial dimension at the maximum diameter portion of the granular portion (for example, base portion 41 in FIG. 4 described later) is 10 μm or more (hereinafter simply referred to as the large grain shape).

[0022] In this way, in an electron emission section made of a carbon nanostructure formed on the anode-side surface with a radial dimension of 0.1 mm to 1 mm, by forming large grain-shaped portions distributed in an island pattern on the surface of the carbon nanostructure to form an island structure, it becomes easier to emit an electron beam with a high current density and also easier to obtain durability against the electron beam with a high current density. As a result, it becomes possible to fully demonstrate the desired functions of the field emission device.

[0023] This embodiment may be configured to have the above-described cold cathode, etc., and may appropriately apply common technical knowledge from various fields (for example, the field of field emission devices, the field of carbon nanostructures, etc.) For example, design modifications may be made as needed by appropriately referring to Patent Documents 1 to 5, etc., and examples thereof include Examples 1 to 3 shown below.

[0024] In the following first to third embodiments, detailed explanations will be omitted as appropriate, for example, by using the same reference numerals for overlapping contents.

[0025] As an example of a conventional field emission device, in the case of an X-ray tube with a focal spot size of the order of μm, an impregnated hot cathode formed by combining an oxide cathode that emits electrons is used, rather than a thermionic electron source formed simply by using a tungsten filament. Such an impregnated hot cathode can be made relatively small and has been considered to be capable of generating a large electron current.

[0026] A typical electron source is a hot cathode with an electron source diameter of several μm to several tens of μm (e.g., 50 μm) that generates an electron current on the order of amperes. It is also known that an aperture (an aperture corresponding to the electron source diameter) is interposed between the hot cathode and the anode, restricting the electron current while irradiating the anode, thereby functioning as a reflection-type X-ray tube. This reflection-type X-ray tube is considered to have the advantage of being able to produce a larger output than a transmission-type X-ray tube.

[0027] However, the hot cathodes described above are installed in a vacuum atmosphere (for example, in a vacuum vessel), and depending on the degree of vacuum in the vacuum atmosphere, the tungsten filament is easily consumed, making it difficult to extend their lifespan. Furthermore, impregnated hot cathodes tend to be expensive and difficult to manufacture.

[0028] Therefore, in recent years, attention has been drawn to a configuration that employs a field emission type cold cathode using a carbon film, typically a carbon nanotube. One example of a typical X-ray tube employing a cold cathode is one in which a carbon film is formed on the anode-facing surface of an electrode substrate having a radial dimension of about several mm (e.g., 5 mm), to form an electron emission section of a carbon nanostructure, and the tube current is about several mA.

[0029] In the case of the electron emission sections of Patent Documents 1 to 5, granular portions are formed on the surface so as to be distributed in an island-like manner, forming an island structure, but the granular portions have a smaller size and shape (for example, a tapered shape with a radial dimension of several nm to several μm; hereinafter, simply referred to as a small grain shape) compared to Examples 1 to 3 described below.

[0030] If the diameter of the electrode substrate of such an X-ray tube is simply reduced (for example, to about 0.5 mm), it may be possible to reduce the beam diameter and focal spot size of the electron beam, but the area of ​​the anode-facing surface of the electrode substrate (the surface area of ​​the electron-emitting portion) will be reduced, resulting in a decrease in the amount of electrons generated and a low tube current. Furthermore, even if a high current density can be obtained in the electron beam from the small-area anode-facing surface (electron-emitting portion), the electron-emitting portion may not be able to withstand the high current density of the electron beam, and its lifespan may be shortened.

[0031] As a specific example, when the radial dimension of the electrode substrate is reduced from 5 mm to 0.5 mm, the surface area of ​​the electron-emitting portion is reduced to about 1 / 100, and the tube current is also reduced to about 1 / 100. In such a reduced-diameter configuration, in order to achieve the tube current (e.g., 1 mA) before the diameter reduction, it is conceivable to increase the current density of the electron beam by about 100 times, but the electron-emitting portion would not be able to withstand the electron beam with such a high current density.

[0032] In contrast, in the electron beam emission structures of Examples 1 to 3 described below, the radial dimension of the anode-facing surface of the cathode is set relatively small (set within the range of 0.1 mm to 1 mm), while the granular portions forming the island structure on the surface of the electron-emitting portion (carbon nanostructure) formed on the anode-facing surface are made large. This not only reduces the beam diameter and focal spot size of the electron beam, but also makes it easier to emit an electron beam with a high current density and ensures durability against such high-current-density electron beams. As a result, even field emission devices such as microfocus X-ray tubes can easily perform their desired functions.

[0033] <<Example 1>> <Schematic Configuration of Example 1> Fig. 1 is a schematic configuration diagram for explaining an electron beam emission structure according to Example 1. The electron beam emission structure shown in Fig. 1 mainly comprises a cold cathode 1 and an anode 2 arranged opposite to each other, and a focusing electrode 3 arranged to cover the outer periphery of the cold cathode 1. In the case of Fig. 1, a power supply E1 for applying a voltage (tube voltage) between the cold cathode 1 and the anode 2, and a power supply E2 for applying a voltage between the anode 2 and the focusing electrode 3 are also provided.

[0034] The cold cathode 1 has a columnar electrode substrate 11 arranged with one end of its axis facing the anode 2 (i.e., arranged coaxially with respect to the central axis), and an electron emission section 4 made of a carbon nanostructure is provided on the anode-side facing surface 12, which is the end face of the columnar electrode substrate 11 on the anode 2 side.

[0035] The focusing electrode 3 has a cylindrical electrode base 31 that is disposed coaxially with the axis of the columnar electrode base 11 (hereinafter simply referred to as the cold cathode axis), surrounds the outer periphery of the columnar electrode base 11, and is configured to be insulated from the cold cathode 1. In the case of the focusing electrode 3 shown in FIG. 1 , a gap is provided between the focusing electrode 3 and the cold cathode 1, thereby providing insulation from the cold cathode 1, but this is not limitative. For example, a desired insulation state can be achieved by interposing an insulator (not shown) between the cold cathode 1 and the focusing electrode 3.

[0036] <Configuration example of cold cathode 1> The columnar electrode substrate 11 of the cold cathode 1 is capable of providing an electron emission section 4 on an anode-side facing surface 12 having a radial dimension of 0.1 mm to 1 mm, and various configurations are applicable as long as electrons are generated in the electron emission section 4 by applying a voltage of a desired magnitude from a power source E1, and the electrons are emitted toward the anode 2 to form an electron beam (not shown).

[0037] One example is a conductive metal material (e.g., an Fe—Cr—Ni alloy) formed into a columnar shape (e.g., a cylindrical shape).Other examples include a substrate (e.g., a silicon substrate or a tungsten substrate) that has good bonding properties with carbon (carbon nanostructures) formed into a columnar shape.

[0038] Furthermore, the electrode substrate is not limited to a simple columnar shape, and may have a tapered shape on the anode-side facing surface 12 side, as in the columnar electrode substrate 11 shown in Fig. 2. This allows the radial dimension of the anode-side facing surface 12 to be set small, while the radial dimension on the side opposite to the anode-side facing surface 12 can be set large, making it easier to maintain a certain level of mechanical strength, for example.

[0039] <Configuration example of anode 2> The anode 2 may be any anode that can be struck by an electron beam emitted from the electron emitter 4 and emit X-rays or the like (not shown) due to the impact of the impacting electron beam, and various configurations are possible. The anode 2 in FIG. 1 has a surface 21 (hereinafter simply referred to as the "irradiated surface") that faces the electron emitter 4 of the anode 2 and is irradiated with the electron beam, and the electron beam is configured to impact the irradiated surface 21. The irradiated surface 21 may be configured to be inclined at a predetermined angle with respect to the emission direction of the electron beam. In this case, the X-rays or the like generated by the electron beam that impacts the irradiated surface 21 are irradiated in a direction bent from the emission direction of the electron beam (for example, to one of the left and right directions in the figure).

[0040] <Configuration example of focusing electrode 3> The cylindrical electrode base 31 of the focusing electrode 3 may be configured to surround the outer periphery of the columnar electrode base 11 so as not to interfere with the field emission by the cold cathode 1, and various configurations are possible.

[0041] One example of such a cylindrical electrode substrate is one made of a conductive metal material similar to the columnar electrode substrate 11 and formed into a cylindrical shape (e.g., a shape having a cylindrical cylindrical portion 30) from the metal material. Alternatively, a substrate such as a Si substrate may be formed into a cylindrical shape, as long as it can focus an electron beam by applying a voltage of a desired magnitude from a power source E2. In the case of the cylindrical electrode substrate 31 shown in FIG. 1 , an annular substrate tapered portion 33 is provided at an end 32 of the cylindrical electrode substrate 31 on the anode 2 side, the end 32 being provided with a tapered diameter that protrudes from the end 32 toward the cold cathode axis.

[0042] <Example of configuration of electron emitter 4> The electron emitter 4 is configured from a carbon nanostructure formed on the anode-side facing surface 12, and various configurations can be applied as long as it is capable of generating electrons on the surface of the electron emitter 4 by applying a voltage of a desired magnitude from a power source E1 and emitting the generated electrons toward the anode 2 (radiator).

[0043] As a specific example, as shown in the figure, a thin film (for example, formed by CVD or the like) is provided on the anode-facing surface 12 of the columnar electrode substrate 11 .

[0044] 1, the surface of the electron emitter 4 is recessed toward the anode-side facing surface 12, forming a concave shape. With an electron emitter 4 having such a concave surface, electrons generated on the surface are more likely to be emitted toward the anode 2 in a direction biased toward the central axis. This contributes to reducing the beam diameter and focal spot size of the electron beam.

[0045] The method for making the surface of the electron-emitting portion 4 concave is not particularly limited, but one example is a method in which the electron-emitting portion 4 is formed on the anode-side facing surface 12 that has been previously formed into a concave shape.

[0046] <Example of carbon nanostructure configuration> The carbon nanostructure of the electron emission section 4 has an island-shaped structure on its surface and is capable of emitting electrons generated by application of a voltage of a desired magnitude from the power source E1 to the anode 2 side (radiator), and various configurations can be applied.

[0047] The carbon nanostructures themselves that form the island-like structure can be formed by, for example, appropriately applying a known film formation method, and although there are no particular limitations, one example is that they can be formed by applying a CVD method.

[0048] In a specific example of the CVD method, the columnar electrode substrate 11 is first placed in an atmosphere at a predetermined temperature (for example, in a chamber), and a precursor of a carbon film (for example, a precursor generated by plasma decomposition of hydrogen and methane) is deposited on the anode-side facing surface 12, and carbon nuclei are generated and grown on the anode-side facing surface 12.

[0049] As a result, a hollow carbon nanostructure is formed in which carbon films (graphene, carbon nanotubes, etc.) grow three-dimensionally and overlap one another.

[0050] The surface of this carbon nanostructure is not simply flat, but is formed so that granular portions 40 protruding from the surface are distributed in an island-like pattern, as shown in Figures 3 and 4 (for example, as shown in Figure 3, countless granular portions 40 are densely packed together), forming an island-like structure.

[0051] 3 and 4, the surface side of the granular portion 40 has a shape in which minute fibrous carbon films (for example, graphene that appears as stripes in FIG. 4) protrude. Adjacent granular portions 40 are formed so as to be bonded to each other at their respective base portions 41.

[0052] The granular portion 40 can be formed into a desired shape by appropriately setting the film formation conditions of the applied film formation method (e.g., precursor generation conditions, temperature conditions of the anode-side opposing surface 12, electric field and electric field line density relative to the anode-side opposing surface 12, etc.), and preferably has a large grain shape.

[0053] Examples of large-grain granular portions 40 include those in which the dimension of the granular portions 40 in the protruding direction is 10 μm or more, preferably 20 μm or more, and more preferably 30 μm or more, and the radial dimension of the largest diameter part of the granular portions 40 (for example, the base portion 41 in FIG. 4 ) is 10 μm or more, preferably 20 μm or more, and more preferably 30 μm or more, and examples include those in which the large-grain shape can be confirmed with a stereomicroscope.

[0054] There is no particular upper limit to the dimension of the granular portion 40 in the protruding direction or the radial dimension of the maximum diameter portion, and each can be set to, for example, approximately 50 μm, but it is preferable to set them appropriately to an extent that maintains the island structure on the surface of the carbon nanostructure.

[0055] The shape of the granular portion 40 may be, for example, a cone shape, a frustum shape, a spindle shape, a sphere shape, or the like as shown in FIGS. 3 and 4, but is not limited to these.

[0056] Furthermore, the tip 42 of the granular portion 40 in the protruding direction may be formed to have a curved shape that is convex at an obtuse angle toward the protruding direction. This makes it easier to form the tip 42 so that its radial dimension is large (for example, 5 μm or more, preferably 10 μm or more), which may make it easier to obtain durability against, for example, a high current density electron beam.

[0057] Furthermore, the granular portions 40 formed on the surface of the carbon nanostructure do not all need to be large grain shapes, and some may be other grain shapes (e.g., small grain shapes), but it is preferable to suppress the proportion of other grain shapes (e.g., suppress to 30% or less, preferably 20% or less, more preferably 10% or less) so that the desired function of the intended field emission device can be achieved.

[0058] <Configuration example of field emission device> According to the electron beam emission structure shown in FIG. 1, it is possible to configure various types of field emission devices, one example of which is a vacuum container (not shown) in which both ends of a cylindrical insulator are sealed and a vacuum chamber is formed on the inner periphery of the insulator.

[0059] Specifically, the anode 2 is arranged on one side of the vacuum chamber in the vacuum vessel, and the cold cathode 1 and focusing electrode 3 are arranged on the other side of the vacuum chamber, with appropriate wiring to apply the voltages of the power sources E1 and E2 as desired. Also, a Wehnelt extraction electrode (not shown) or the like may be provided to appropriately control the tube current.

[0060] <Others> The voltage application configuration by the power sources E1 and E2 can be configured in various ways, for example, by providing a voltage control unit (not shown) that can change the voltage as appropriate.

[0061] As an example of setting the voltage control unit, when it is necessary to reduce the focal spot size as in microfocus X-rays, etc., it can be set to increase the focusing power of the electron beam. Also, when it is desired to irradiate the anode 2 with an electron beam of high output (power) while suppressing damage to the anode 2, it can be set to decrease the focusing power of the electron beam.

[0062] The voltage applied by the power supply E2 only needs to be large enough to focus the electron beam by the focusing electrode 3, and compared to the voltage (tube voltage) applied by the power supply E1, it can be set to be sufficiently small (for example, set within a range of 90% to 100% of the tube voltage), making it possible to suppress the amount of electrons. Therefore, in the vacuum atmosphere in which the electron beam emission structure is installed, the degree of vacuum in the vacuum atmosphere is suppressed so as not to deteriorate.

[0063] <Verification Example> An X-ray tube device having the electron beam emission structure of Fig. 1 (hereinafter simply referred to as the example device) was created based on Example 1, and the current density characteristics relative to the applied voltage of the power supply E1 were observed, resulting in the results shown in Fig. 5 (results indicated by the symbol "◯" in Fig. 5). Also, in an X-ray tube device created based on Patent Documents 1 to 5 (hereinafter simply referred to as the comparative example device), the current density characteristics relative to the applied voltage of the power supply E1 were observed in the same manner as in the example device, resulting in the results shown in Fig. 5 (results indicated by the symbol "●" in Fig. 5).

[0064] In the example device, a columnar electrode substrate 11 having a shape as shown in Fig. 2 and a diameter of 0.5 mm on the anode-side facing surface 12 was used, and an electron emitting portion 4 made of carbon nanostructures with an island structure as shown in Fig. 3 and Fig. 4 was formed on the anode-side facing surface 12. In the comparative example device, a columnar electrode substrate 11 having a diameter of 5 mm on the anode-side facing surface 12 was used, and an electron emitting portion 4 made of carbon nanostructures with an island structure made of small granular portions was formed on the anode-side facing surface 12.

[0065] The results shown in FIG. 5 show that in the device of the present invention, a high current density is obtained at a low applied voltage, even though the diameter of the surface facing the anode is small (smaller than that of the device of the comparative example).

[0066] Therefore, in the example device, when the applied voltage of the power supply E1 was set to 180 kV, the current density was 5 mA / mm ２ It was confirmed that the above electron beams can be emitted. Furthermore, it was confirmed that the cold cathode 1 of the example device can perform the desired function even when applied over a wide range of voltages, has sufficient durability even at high voltages (for example, several kV to 300 kV), and can obtain an actual current of 5 mA. When applied at a general voltage range, changes over time can be sufficiently suppressed, making it possible to achieve a long life.

[0067] Therefore, according to the above-described first embodiment, not only can the beam diameter and focal spot size of the electron beam be reduced, but also it becomes easier to emit an electron beam with a high current density, and durability against the electron beam with a high current density can be easily obtained.

[0068] <<Example 2>> <Schematic Configuration of Example 2> Fig. 6 is a schematic diagram for explaining an electron beam emission structure according to Example 2. In the electron beam emission structure shown in Fig. 6, the outer periphery of a columnar electrode base 11 is surrounded by a cylindrical electrode cover 5. The electrode cover 5 is interposed between the columnar electrode base 11 and a cylindrical electrode base 31, and is provided so as to be positioned coaxially with respect to the axis of the cold cathode.

[0069] <Configuration Examples of Electrode Cover 5> The electrode cover 5 may be in any cylindrical shape as long as it can surround the outer periphery of the columnar electrode substrate 11, and various configurations are possible.

[0070] One example of such a material is one that does not cause dielectric breakdown, such as a high-melting-point material such as molybdenum, tungsten, or stainless steel, and is formed into a cylindrical shape (for example, a cylindrical shape).

[0071] 6, a gap is provided between the electrode cover 5 and the focusing electrode 3, thereby providing insulation from the focusing electrode 3, but this is not limitative. For example, a desired insulation state may be achieved by interposing an insulator (not shown) between the electrode cover 5 and the focusing electrode 3.

[0072] Furthermore, the end 51 of the electrode cover 5 on the anode 2 side may be configured to have a reduced diameter protruding from the end 51 toward the cold cathode axis, so as to overlap with the peripheral edge of the electron emission section 4 in the direction of extension of the cold cathode axis.

[0073] Furthermore, the electrode cover 5 may be in contact with or at the same potential as the columnar electrode base 11, but it is preferable to appropriately set the distance between the electron emitter 4 and the electrode cover 5 so that they do not interfere with each other too much. For example, in the case of the end 51 having a reduced diameter as described above, it is possible to set the distance so that the electron emitter 4 is not damaged by being pressed by the end 51.

[0074] According to the second embodiment described above, in addition to the same effects as those of the first embodiment, the following can be said. That is, the outer periphery of the columnar electrode substrate 11 (for example, the peripheral edge of the electron-emitting portion 4) may contain electrically unstable elements (for example, elements that may cause electric field concentration or abnormal discharge), but by surrounding the outer periphery with the electrode cover 5, it is possible to suppress such unstable elements. Specifically, the electric field may be more easily relaxed at the peripheral edge of the electron-emitting portion 4.

[0075] <<Example 3>> <Schematic Configuration of Example 3> Fig. 7 is a schematic configuration diagram for explaining an electron beam emission structure according to Example 3. In the electron beam emission structure shown in Fig. 7, an opening edge surface 52 on the anode 2 side of the electrode cover 5 does not simply have a shape extending in the radial direction of the electrode cover 5 (for example, a flat shape as shown in Fig. 6), but has a shape extending in a direction intersecting the cold cathode axis at an oblique angle (hereinafter simply referred to as an oblique shape).

[0076] Specifically, the opening edge surface 52 has an inclined shape such that the point on the opening edge surface 52 moves toward the anode 2 as it approaches the outside from the inside in the radial direction of the electrode cover 5 .

[0077] This inclined shape can be in various forms as long as it can suppress, for example, electric field concentration that may occur around the electron-emitting portion 4. In the case of Fig. 7, both the concave surface of the electron-emitting portion 4 and the opening edge surface 52 extend continuously between them.

[0078] According to the above-described Example 3, in addition to the same effects as those of Examples 1 and 2, the following can be said: That is, the electrically unstable elements described in Example 2 can be more easily suppressed, and specifically, the electric field can be more easily relaxed at the periphery of the electron-emitting portion 4.

[0079] Although the present invention has been described in detail above only with respect to the specific examples, it will be apparent to those skilled in the art that various modifications are possible within the scope of the technical concept of the present invention, and it is natural that such modifications fall within the scope of the claims.

[0080] For example, Examples 1 to 3 may be combined as appropriate. A specific example is applying the columnar electrode substrate 11 shown in Fig. 2 to the electron beam emission structure shown in Fig. 6. In addition, the contents disclosed in Patent Documents 1 to 5 may be applied as appropriate to modify the design, and the same effects as those of Examples 1 to 3 can be achieved.

[0081] DESCRIPTION OF SYMBOLS 1 cold cathode, 11 columnar electrode base, 12 anode-side facing surface, 2 anode, 3 focusing electrode, 4 electron-emitting portion, 40 granular portion, 41 base portion, 42 tip portion, 5 electrode cover, 52 opening edge surface

Claims

1. An electron beam emission structure comprising: a cold cathode having a columnar electrode base with one end of its axis facing an anode, and an electron emission portion provided on the end face of the columnar electrode base facing the anode; and a cylindrical focusing electrode arranged coaxially about the axis while being insulated from the cold cathode and surrounding the outer periphery of the columnar electrode base, wherein the radial dimension of the end face is 0.1 mm to 1 mm, the electron emission portion comprises a carbon nanostructure obtained by growing carbon nuclei formed on the end face, the surface of the carbon nanostructure is formed so that granular portions protruding from the surface are distributed in an island-like shape on the surface, forming an island structure, and the dimension of the granular portions in the protruding direction is 10 μm or more, and the radial dimension of the maximum diameter part of the granular portions is 10 μm or more.

2. An electron beam emission structure as claimed in claim 1, characterized in that the tip of said granular portion in the protruding direction has a curved shape that is convex at an obtuse angle in the protruding direction.

3. An electron beam emission structure as claimed in claim 1 or 2, characterized in that the radial dimension at said tip is 5 μm or more.

4. An electron beam emission structure as described in claim 1 or 2, characterized in that the cold cathode is surrounded on the outer periphery of the columnar electrode base by a cylindrical electrode cover arranged coaxially with respect to the axis between the columnar electrode base and the focusing electrode.

5. An electron beam emission structure as described in claim 4, characterized in that the opening edge surface on the anode side of the electrode cover is inclined so that a point on the opening edge surface moves toward the anode side as it approaches the radial outside from the inside of the electrode cover.

6. A field emission device comprising the electron beam emission structure according to claim 1 or 2.

7. A field emission device according to claim 6, further comprising a voltage control section capable of changing the voltage applied between said anode and said focusing electrode.

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