JP6232816B2 - Nano-polycrystalline diamond, electron gun including the same, and method for producing nano-polycrystalline diamond - Google Patents

Description

  The present invention relates to a nano-polycrystalline diamond and an electron gun including the same, and more particularly to a nano-polycrystalline diamond doped with a different element and an electron gun including the nano-polycrystalline diamond.

  Since diamond has high hardness, it has various industrial values. However, the production and quality of natural diamond are not stable, and since the artificial production of diamond has become possible in recent years, the latter diamond is exclusively used industrially.

  As a method for artificially producing diamond, a chemical vapor deposition (CVD) method is widely known. The CVD method is a method in which a single crystal diamond is grown on a substrate by spraying a hydrocarbon gas on the substrate in a vacuum and a high temperature environment, and the diamond produced by this method becomes a carbon epitaxial layer. .

  In recent years, a method has been developed in which graphite is formed by a CVD method and the graphite is converted to diamond by direct conversion. The diamond produced by this method is a nano-polycrystalline diamond composed of nano-sized single crystal diamond, and since it can have a higher hardness than single crystal diamond, its wide utilization is expected.

  For example, Patent Document 1 (Japanese Patent Laid-Open No. 2013-28499) discloses nano-polycrystalline diamond to which a group 15 element is added. Since such nano-polycrystalline diamond has more electrons than ordinary diamond, it is expected to be used as an electron emission source of an electron gun, for example.

JP 2013-28499 A

  However, the development of nano-polycrystalline diamond has just begun, and further development is desired in order to be widely used as an electron emission source and to be suitably used.

  The present invention has been made in view of the above situation, and an object of the present invention is to provide a nano-polycrystalline diamond that can be used as an electron emission source and an electron gun including the same.

The present invention includes carbon and a heterogeneous element doped in a crystal structure composed of carbon, and the heterogeneous element is at least one selected from the group consisting of group 16 elements other than oxygen, The atomic concentration of the element is nano-polycrystalline diamond that is 1 × 10 ¹⁴ / cm ³ or more and less than 1 × 10 ²⁰ / cm ³ .

  Moreover, this invention is an electron gun provided with the said nano polycrystalline diamond.

  ADVANTAGE OF THE INVENTION According to this invention, the nano polycrystalline diamond which can be utilized as an electron emission source and an electron gun provided with the same can be provided.

It is a perspective view which shows an example of the state which formed the graphite on the base material. It is a perspective view which shows an example of the state which converted the graphite on a base material directly and produced the nano polycrystalline diamond.

[Description of Embodiment of Present Invention]
First, embodiments of the present invention will be listed and described.

  The present inventors have studied improvement of nano-polycrystalline diamond for the purpose of widely and suitably using nano-polycrystalline diamond as an electron emission source.

Specifically, the doping amount of the group 15 element was variously changed in order to enhance the electron emission function in the nano-polycrystalline diamond. However, even when the doping amount of the group 15 element in the nano-polycrystalline diamond was changed in the range of about 1 × 10 ¹⁴ to 1 × 10 ²⁰ / cm ^3, no significant change was found in the emission current obtained from the electron gun. . In addition, various studies were conducted on this nano-polycrystalline diamond, but no significant change was observed in the emission current obtained from the electron gun.

  Therefore, the present inventors focused on group 16 elements in order to achieve breakthrough. The group 16 element is an element widely recognized in the industry as being unable to be sufficiently doped with diamond. In fact, in the above-described conventional CVD method for producing diamond, it can be processed as an electron emission source. It is difficult to obtain a diamond having a thickness of, that is, a diamond that is not a thin layer and that is uniformly and sufficiently doped with a group 16 element. For this reason, a study of doping a group 16 element at a concentration high enough to function as an electron emission source with respect to diamond is not usually performed.

  Contrary to the above-mentioned common sense, when the present inventors moved to the study using the group 16 element, it was confirmed that the nano-single crystal diamond can be doped with the group 16 element at a very high concentration compared to the conventional case. Furthermore, the inventors have found that a high electron emission function can be exhibited by repeated studies using the nano-single crystal diamond, and thus completed the present invention.

That is, the present invention includes carbon and a heterogeneous element doped in a crystal structure composed of carbon, and the heterogeneous element is at least one selected from the group consisting of group 16 elements other than oxygen. The nano-polycrystalline diamond has an atomic concentration of different elements of 1 × 10 ¹⁴ / cm ³ or more and less than 1 × 10 ²⁰ / cm ³ . The nanopolycrystalline diamond of the present invention has a high electron emission function and can be used as an electron emission source.

  In the nano-polycrystalline diamond, the single crystal constituting the nano-polycrystalline diamond preferably has a particle size of 500 nm or less. Thereby, since nano-polycrystalline diamond can have higher hardness, it can have high durability against impact, and thus can have higher characteristics as an electron emission source.

  In the nanopolycrystalline diamond, the heterogeneous element is preferably sulfur. Thereby, the nano-polycrystalline diamond has further oxidation resistance, and thus can have higher characteristics as an electron emission source.

  The nano-polycrystalline diamond is preferably used for an electron emission source of an electron gun. In this case, high characteristics as an electron emission source can be sufficiently exhibited.

  Moreover, this invention is an electron gun provided with the said nano polycrystalline diamond. Since the electron gun of the present invention includes an electron emission source having high characteristics as an electron emission source, it can exhibit high performance as an electron gun.

[Details of the embodiment of the present invention]
Hereinafter, the nano-polycrystalline diamond according to the present invention and the electron gun including the same will be described in more detail.

<Nanopolycrystalline diamond>
The nano-polycrystalline diamond of the present embodiment includes carbon and a different element doped in a crystal structure composed of carbon, and the different element is selected from the group consisting of group 16 elements other than oxygen 1 The atomic concentration of the different elements is 1 × 10 ¹⁴ / cm ³ or more and less than 1 × 10 ²⁰ / cm ³ .

  In the present specification, “nanopolycrystalline diamond” refers to polycrystalline diamond in which the grain size of single-crystal diamond constituting polycrystalline diamond, that is, the grain size of crystal units constituting polycrystalline diamond is nano-sized. The nano-sized single crystal diamond is a single crystal diamond particle having a particle size of less than 1 μm. The “particle diameter” of the single crystal diamond means the longest diameter (major diameter) of the particle. In this specification, the value measured using a scanning electron microscope (SEM) is “ It is described as “particle size”.

  In addition, in this specification, “a heterogeneous element doped in a crystal structure” means that a heterogeneous element is substituted with a part of carbon in a diamond crystal structure formed by covalently bonding carbons to each other. The state, in other words, the state that exists in a state of being covalently bonded to carbon constituting the crystal structure and is dispersed in the crystal structure at the atomic level. The different elements in such a state are different from the different elements clustered in the crystal structure.

  That is, the “heterogeneous elements clustered in the crystal structure” exists in the crystal structure in a form in which a plurality of atoms that are different elements are aggregated. Thus, for example, if nanopolycrystalline diamond contains heterogeneous elements clustered in its crystal structure, the heterogeneous elements will be non-uniformly present in the crystal structure, reducing the homogeneity of nanopolycrystalline diamond. This causes a large distortion in the crystal structure, and consequently reduces the hardness of the nanopolycrystalline diamond.

  On the other hand, since the “heterogeneous element doped in the crystal structure” is dispersed in the crystal structure at the atomic level as described above, the nano-polycrystalline diamond containing “the heterogeneous element doped in the crystal structure” Is suppressed in homogeneity as compared with nano-polycrystalline diamond containing “heterogeneous elements clustered in the crystal structure”.

  In nano-polycrystalline diamond, whether or not different elements are contained and the content (atomic concentration) can be measured by inductively coupled plasma (ICP) analysis. In addition, when nano-polycrystalline diamond contains different elements, whether or not the different elements are dispersed in the crystal structure at the atomic level is, for example, (1) the presence of a crystalline phase of the different elements in nano-polycrystalline diamond (2) by measuring the atomic concentration distribution of different elements in the nanopolycrystalline diamond, (3) by measuring the presence or absence of conductivity of the nanopolycrystalline diamond, and It can confirm by combining (1)-(3) suitably.

  Regarding the above (1), since the different elements dispersed in the crystal structure at the atomic level do not form a crystal phase different from diamond, the crystal phases of the different elements are not observed. On the other hand, since the different elements present in a cluster form a crystal phase different from diamond, the crystal phase derived from the different elements is observed. The presence or absence of such a crystal phase can be observed by, for example, X-ray diffraction, and can also be visually observed depending on the size of the crystal phase.

  Regarding (2) above, when the different elements are dispersed in the crystal structure at the atomic level, the atomic concentration distribution of the different elements is uniform as compared to the case where the different elements exist in a clustered state. Such an atomic concentration distribution can be measured by, for example, secondary ion mass spectrometry (SIMS). When the difference in atomic concentration of different elements measured at two arbitrary points in the crystal structure is less than or equal to a predetermined value, the atomic concentration distribution of the different elements can be considered to be uniform. It can be considered that it is in a state dispersed in the crystal structure and not in a clustered state.

  Concerning (3) above, the nanopolycrystalline diamond is checked for the presence or absence of a heterogeneous element crystalline phase or a graphite crystalline phase by X-ray diffraction, and the electrical resistivity (Ω · cm) of the nanopolycrystalline diamond is measured. To confirm the conductivity. When no crystal phase is confirmed and the resistance value is equal to or lower than a predetermined value, it can be considered that the different elements are dispersed in the crystal structure at the atomic level. In this specification, the electrical resistivity is a value measured according to JIS C2141.

  Since the nano-polycrystalline diamond of this embodiment has a high electron emission function, it can be used as an electron emission source. Regarding the “electron emission function”, when this nano-polycrystalline diamond is used as an electron emission source of an electron gun, it can be considered that the larger the emission current taken out from the electron gun, the higher the electron emission function. Although the reason why the nano-polycrystalline diamond of the present embodiment has a high electron emission function is not clear, the present inventors infer as follows based on various examination results.

  The inventors of the present invention prepared nanopolycrystalline diamond doped with a group 15 element and nanopolycrystalline diamond doped with a group 16 element, and measured the electrical resistivity (Ω · cm). However, it was confirmed that the electrical resistivity of the nano-polycrystalline diamond doped with a group 16 element was much lower. From this result, in the nanopolycrystalline diamond, the atoms derived from the group 16 element are more likely to emit electrons and move more easily than the atoms derived from the group 15 element, so that the emission current is increased more than three times. It is considered a thing.

Furthermore, the nano-polycrystalline diamond of the present embodiment is produced by utilizing direct conversion, which will be described later. According to this method, 1 × 10 ¹⁴ / cm ³ is included in the carbon crystal structure of the nano-polycrystalline diamond. The group 16 element can be doped at such a high concentration. Since the group 16 element is doped at such a high concentration, the nano-polycrystalline diamond can have a high electron emission function. Further, the fact that the group 16 element has one more electron in the outermost core than the group 15 element is considered to contribute to a high electron emission function.

In the conventional diamond manufacturing methods, it is difficult to dope a group 16 element into the crystal structure of diamond, and furthermore, doping at a high concentration of 1 × 10 ¹⁴ / cm ³ or more has not been realized. And for this reason, it was thought that the size of the atoms was the cause so far. However, in completing the present invention, a graphite doped with a group 16 element was prepared by CVD, and this graphite was directly used. It has been found that group 16 elements can be uniformly doped in nanopolycrystalline diamond produced by conversion. From this result, it is speculated that there may be other reasons why it is difficult to dope the group 16 element by the CVD method.

Moreover, since the nano-polycrystalline diamond of this embodiment can maintain the above-mentioned high electron emission function long and stably, it can have high characteristics as an electron emission source. This characteristic enables stable use as an electron emission source. In particular, the atomic concentration of the group 16 element in the nanopolycrystalline diamond of the present embodiment is preferably more than 1 × 10 ¹⁷ / cm ³ , and in this case, it can have higher characteristics as an electron emission source.

  Furthermore, the nano-polycrystalline diamond of the present embodiment can have a high hardness. By having a high hardness, it is possible to suppress damage to itself, so that its handling becomes simple, and even when used as an electron emission source to which various impacts are applied, the electron emission source itself Breakage can be suppressed, thus having a long service life.

The reason why the nano-polycrystalline diamond of this embodiment has a high hardness is that, besides the fact that the single-crystal diamond constituting the polycrystalline diamond is nano-sized, (1) a different element is doped in the crystal structure of carbon. (2) It is considered that the concentration of the different elements is less than 1 × 10 ²⁰ / cm ³ .

  Regarding the above (1), as described above, the heterogeneous elements in the clustered state are present non-uniformly in the crystal structure, so that the homogeneity of the nano-polycrystalline diamond is lowered and the crystal structure is greatly increased. This results in distortion and consequently reduces the hardness of the nanopolycrystalline diamond. In contrast, a doped heterogeneous element can exist uniformly in the crystal structure compared to a heterogeneous element in a clustered state, so that a decrease in homogeneity of nano-polycrystalline diamond can be suppressed. As a result, the high hardness of the nano-polycrystalline diamond can be maintained.

  For this reason, the nano-polycrystalline diamond of the present embodiment preferably does not contain clustered heterogeneous elements. Thereby, all the different elements present in the nanopolycrystalline diamond are doped, and all can contribute to the electron emission function. In addition, since the crystal structure is not distorted, the high hardness of the nano-polycrystalline diamond can be sufficiently maintained. In addition, the heterogeneous elements in the clustered state not only contribute to the electron emission function, but also have low heat resistance, so that nanopolycrystalline diamonds containing them are more heat resistant than nanopolycrystalline diamonds not containing them. It is thought that the nature is lowered.

Regarding (2) above, if the atomic concentration of the different element becomes too high, the hardness of the nano-polycrystalline diamond may decrease, but the different element exists at a concentration of less than 1 × 10 ²⁰ / cm ^3. Thus, the high hardness of the nano-polycrystalline diamond can be maintained. In the nanopolycrystalline diamond of the present embodiment, the different elements are uniformly dispersed regardless of the content rate, so that it is possible to suppress local variations in characteristics in the nanopolycrystalline diamond.

  Moreover, it is preferable that the particle diameter of the single crystal which comprises the nano polycrystal diamond of this embodiment is 10 nm or more and 500 nm or less. In this case, it is possible to provide a more uniform nano-polycrystalline diamond because the variation in the grain size of the single crystal constituting the nano-polycrystalline diamond can be reduced while maintaining high hardness. The properties as a source can also be homogeneous.

  In addition, the nanopolycrystalline diamond of the present embodiment can be produced by a nanopolycrystalline diamond production method described later, and according to this production method, particles can be formed without interposing a binder between single crystal particles. They can be firmly bonded to each other. Such nano-polycrystalline diamond can have a high hardness as compared with the case where particles are bonded together by a binder. Therefore, it is preferable that the nano-polycrystalline diamond of this embodiment does not contain a binder.

  Moreover, according to the manufacturing method mentioned later, the nano polycrystalline diamond in which the amount of inevitable impurities is sufficiently low can be manufactured. Specifically, in the nano-polycrystalline diamond of the present embodiment, the content of each element that is an inevitable impurity can be set to 0.01% by mass or less. When the content of each element that is an unavoidable impurity is 0.01% by mass or less, slip at the single crystal grain boundary can be suppressed, and the bond between the single crystal grains can be further strengthened. Therefore, the hardness of nano-polycrystalline diamond can be further increased.

  In addition, since impurities (especially hydrogen (H)) strongly captures electrons, it is one of the factors that reduce the electron emission function. Therefore, the low content of each impurity causes such an electron emission function. The factor to reduce can be removed. Therefore, in the nano-polycrystalline diamond of the present embodiment, the content of each element that is an inevitable impurity is preferably 0.01% by mass or less. The inevitable impurities mean elements other than C and the intended heterogeneous elements, and examples include oxygen (O), silicon (Si), and transition metals in addition to the hydrogen.

Moreover, in the nano-polycrystalline diamond of the present embodiment, the heterogeneous element is preferably sulfur (S). In this case, the nano-polycrystalline diamond can have a higher oxidation resistance in addition to a high electron emission function. Generally, diamond has low oxidation resistance and tends to be oxidized at about 600 ° C. in the atmosphere. Since the electron emission source of an electron gun is exposed to a high temperature environment due to heat generation and heating due to electron emission, when diamond is used as an electron emission source of an electron gun, the environmental conditions of the electron emission source must be set to prevent oxidation. It is necessary to maintain a high degree of vacuum (pressure), for example, less than 10 ⁻³ Pa.

On the other hand, nano-polycrystalline diamond containing sulfur as a different element has high oxidation resistance, so that it can be used as an electron emission source in an environment having a low vacuum, for example, 10 ⁻³ Pa or higher. The present inventors have confirmed through various studies that when nano-polycrystalline diamond containing sulfur is used as an electron emission source of an electron gun, it can be used at a low vacuum of 10 −2 Pa. . The reason why the oxidation resistance of nano-polycrystalline diamond containing sulfur is increased is not clear, but a sulfur oxide film is formed on the surface of nano-polycrystalline diamond, and this may function as a protective film.

  In addition, the nano-polycrystalline diamond of the present embodiment has high hardness and high electron emission function, and can maintain the high electron emission function for a long time, so that it can be suitably used as an electron emission source of an electron gun. it can.

<Electron gun>
The electron gun of this embodiment is an electron gun provided with the above-mentioned nano-polycrystalline diamond as an electron emission source. The configuration of the electron gun is not particularly limited as long as it is at least capable of emitting electrons from an electron emission source, and may be a thermal electron emission type or a field emission type. In addition, when nano-polycrystalline diamond is used as an electron emission source, it is necessary to process the shape appropriately. However, nano-polycrystalline diamond doped with a different element has electrical conductivity and can be subjected to electric discharge machining. Therefore, efficient processing and precise processing are possible.

  According to the electron gun of the present embodiment, since the electron emission source has high hardness and high electron emission function, it can have high characteristics as an electron gun, and thus can have high performance. In addition to the above, the electron emission source can maintain a higher electron emission function for a long time, and thus can be used stably as an electron gun for a long time.

<Method for producing nano-polycrystalline diamond>
The nano-polycrystalline diamond of this embodiment can be manufactured as follows, for example. With respect to the following manufacturing method, each step will be described with reference to FIGS.

(Preparation process)
This step is a step of preparing graphite. Thereby, as shown in FIG. 1, graphite 1 containing carbon and a different element doped in a crystal structure composed of carbon is formed on a substrate 2. Make it. The produced graphite 1 is converted into nano-polycrystalline diamond by a conversion step described later, and the single crystal grain size in graphite 1 and the atomic concentration of different elements in graphite 1 are related to nano-polycrystalline diamond. Therefore, the graphite 1 has a single crystal grain size of 10 μm or less, more preferably 5 nm or less, and the atomic concentration of different elements in the graphite is 1 × 10 ¹⁴ / cm ³ or more and 1 × 10 ²⁰ / cm ^3. And more preferably 1 × 10 ¹⁸ / cm ³ or more and 1 × 10 ²⁰ / cm ³ or less. Such graphite 1 can be formed on the base material 2 by using, for example, the following CVD method.

(CVD method)
First, the base material 2 for vapor-phase-growing graphite on the main surface is arrange | positioned in a vacuum chamber. As a material of the base material 2, any metal, inorganic ceramic material, and carbon material may be used as long as they can withstand a temperature of about 1500 ° C. to 3000 ° C. However, at least the main surface of the base material is graphite from the viewpoint of reducing impurities mixed in graphite as a raw material of the nanopolycrystalline diamond and from the viewpoint of producing graphite having a desired crystal structure.

  Next, the base material 2 disposed in the vacuum chamber is heated at a temperature of about 1500 ° C. to 3000 ° C. As a heating method, a known method can be adopted. For example, a method of installing a heater capable of directly or indirectly heating the base material 2 in a vacuum chamber can be mentioned.

  Next, a hydrocarbon gas and a gas containing a different element are introduced into the vacuum chamber. At this time, the degree of vacuum (pressure) in the vacuum chamber is set to atmospheric pressure or lower. Thereby, hydrocarbon gas and the gas containing a different element can be mixed uniformly in a vacuum chamber.

  As the hydrocarbon gas, ethane, butane, methane or the like can be used, and methane is preferably used from the viewpoint of low molecular weight and easy handling. As the gas containing a different element, it is preferable to use a gas composed of a hydride of a different element, a hydrocarbon gas containing a different element, or a halide gas containing a different element. When a gas composed of a hydride of a different element is used, the gas can be easily decomposed at a high temperature, so that the different element can be efficiently supplied onto the substrate. Further, when a hydrocarbon gas containing a different element is used, the different element already bonded to carbon can be supplied onto the substrate, so that the different element can be more efficiently doped into the graphite. . In addition, when a halide gas containing a different element is used, graphite containing a different element can be synthesized without mixing halogen, which is an element other than the different element.

For example, when S is doped as a different element, it is preferable to use hydrogen sulfide (H ₂ S), dimethyl sulfide (C ₂ H ₆ S), or the like. When Se is doped, hydrogen selenide (H ₂ Se), dimethyl selenide (C ₂ H ₆ Se), etc. are preferably used. When Te is doped, hydrogen telluride (H ₂ Te), dimethyl telluride (C ₂ H ₆ Te), etc. are used. It is preferable to use, and when doping with Po, it is preferable to use polonium hydride (H ₂ Po), polonium dimethyl (C ₂ H ₆ Po), or the like.

  Then, by pyrolyzing the mixed gas at a temperature of 1500 ° C. or higher, graphite containing carbon and a different element doped in the crystal structure composed of carbon on the main surface of the base material, Then, the graphite 1 in which the group 16 element is dispersed in the crystal structure at the atomic level is formed.

  In the above CVD method, in order to make the particle size of the single crystal contained in the graphite 1 10 μm or less, the polycrystalline material composed of single crystal particles having a particle size of 10 μm or less, The sintered body is made of fine carbon particles having a diameter of 10 μm or less. By setting the particle size of the single crystal to 10 μm or less, the particle size of the single crystal in the nanopolycrystalline diamond produced by direct conversion can be suppressed to less than 1 μm. Further, by adjusting the particle size of the single crystal contained in the graphite 1 to 10 nm or more and 700 nm or less, the particle size of the single crystal constituting the nanopolycrystalline diamond can be set to 10 nm or more and 500 nm or less. The structure of graphite 1 may be a structure in which a single crystal is included in a part and the other part is in an amorphous state, an indeterminate form, a carbon nanotube, a fullerene, or a polycrystal composed of a single crystal. . In order to obtain nano-polycrystalline diamond having a more uniform particle size, it is preferable to form graphite 1 having a configuration in which one or more of the above states are randomly arranged or an intricate configuration.

Further, in the above CVD method, in order to make the atomic concentration of the different element in the graphite 1 1 × 10 ¹⁴ / cm ³ or more and less than 1 × 10 ²⁰ / cm ³ , the hydrocarbon gas and each gas containing the different element are mixed. Prepare proportions. Specifically, the atomic concentration of the different element in the graphite 1 can be increased by increasing the mixing ratio of the gas containing the different element to the hydrocarbon gas. Moreover, the atomic concentration of a different element can be adjusted also by changing the kind of gas containing a different element. By setting the atomic concentration of the different element in the graphite 1 to 1 × 10 ¹⁴ / cm ³ or more and less than 1 × 10 ²⁰ / cm ³ , the atomic concentration of the different element in the nanopolycrystalline diamond is 1 × 10 ¹⁴ / cm ³ or more 1 It can be less than × 10 ²⁰ / cm ³ .

In this step, by using the above-described CVD method, graphite 1 containing carbon and a different element doped in a crystal structure composed of carbon on the base material 2, is included in the graphite 1. Graphite 1 having a single crystal grain size of 10 μm or less and an atomic concentration of different elements of 1 × 10 ¹⁴ / cm ³ or more and less than 1 × 10 ²⁰ / cm ³ is formed. In other words, group 16 elements (other than oxygen) are dispersed at the atomic level in the crystal structure at an atomic concentration of 1 × 10 ¹⁴ / cm ³ or more and less than 1 × 10 ²⁰ / cm ³ , and single crystal grains Graphite 1 having a diameter of 10 μm or less is vapor-grown on the substrate 2.

  In addition, regarding the graphite prepared in this step, the heterogeneous element is uniformly doped in both the thickness direction and the in-plane direction, that is, the atomic concentration distribution of the heterogeneous element in the graphite is uniform. preferable. Since the heterogeneous element is uniformly doped in the graphite, the distribution of the heterogeneous element in the nano-polycrystalline diamond produced by the conversion process described later can be made uniform.

  In order to make the atomic concentration distribution of different elements uniform, it is preferable to introduce the hydrocarbon gas and the gas containing the different elements into the vacuum chamber at the same time. Thereby, each gas can be easily and uniformly mixed, and the graphite in which the different elements are uniformly doped can be efficiently formed on the substrate. Each gas may be supplied from the direction directly above the main surface of the base material toward the main surface of the base material, and supplied from the oblique direction or the horizontal direction to the base material toward the base material. May be. From the viewpoint of more efficiently and more uniformly doping a different element, it is preferable to supply from the direction directly above the main surface of the base material toward the main surface of the base material. In addition, in order to more efficiently and more uniformly dope a different element, a guide member that guides a hydrocarbon gas and a gas containing the different element onto the main surface of the substrate may be provided in the vacuum chamber.

Further, regarding the graphite prepared in this step, the density is preferably 0.8 g / cm ³ or more and 2.2 g / cm ³ or less, more preferably 1.4 g / cm ³ or more and 2.1 g / cm ^3. It is as follows. When the density of graphite is 0.8 g / cm ³ or more, the change in volume when graphite is directly converted into nano-polycrystalline diamond can be sufficiently reduced in the conversion step described later. The probability that cracks occur in polycrystalline diamond can be suppressed, and changes in the environment in the apparatus can be suppressed. As a result, the manufacturing yield can be improved. In particular, the present inventors have conducted various experiments, and when the density of the prepared graphite is 0.8 g / cm ³ or more and 2.2 g / cm ³ or less, the density of the graphite is outside this range. As compared with the above, it has been confirmed that the probability of cracking in the manufactured nano-polycrystalline diamond can be reduced to ½ or less.

  The density of the graphite can be adjusted by, for example, the temperature (° C.) when the graphite is grown on the main surface of the substrate and the introduction rate (sccm) of each gas. Specifically, the density of graphite can be increased by increasing the temperature and increasing the introduction rate of hydrocarbons.

  Further, regarding the graphite prepared in this step, the content of inevitable impurities is preferably low, and specifically, the content of each element that is an inevitable impurity is preferably 0.01% by mass or less. . This is because the content of inevitable impurities in the graphite is inherited by the produced nanopolycrystalline diamond. Further, by suppressing the concentration of inevitable impurities to a low level, grain growth caused by the presence of inevitable impurities can be suppressed, so that a single crystal having a more uniform size can be contained in graphite. Note that an analysis apparatus used for analysis capable of measuring the content of inevitable impurities in graphite, such as SIMS analysis and ICP analysis, generally has a detection limit of 0.01% by mass, and thus has a content of 0. Elements of 01% by mass or less will not be detected by the analyzer.

  Mixing of inevitable impurities into the graphite can be suppressed by setting the degree of vacuum in the vacuum chamber when the gas is pyrolyzed relatively high. Specifically, the present inventors have found that the content of each element, which is an inevitable impurity, can be controlled to 0.01% by mass or less by maintaining the pressure in the vacuum chamber at 13 kPa or more and 40 kPa or less. doing.

  In the above CVD method, the method of introducing the mixed gas into the vacuum chamber after heating the base material has been described. However, the method of heating the base material after introducing the mixed gas may be used, and simultaneously performed. May be.

(Conversion process)
This step is a step in which the graphite formed in the preparation step is sintered and directly converted into nano-polycrystalline diamond, whereby the nano-polycrystalline diamond 3 is placed on the substrate 2 as shown in FIG. Make it.

  Specifically, first, the graphite 1 on the substrate 2 shown in FIG. 1 is placed in a high-temperature high-pressure apparatus. The high-temperature and high-pressure apparatus may be any apparatus that can place graphite inside the apparatus and can control the inside of the apparatus under the above-described conditions. For example, a vacuum chamber used for CVD is used. it can.

  And this graphite 1 is exposed to 1500 degreeC-2500 degreeC and the high-temperature / high pressure conditions of 7 GPa-30 GPa. As a result, the graphite 1 is instantaneously sintered and converted into nano-polycrystalline diamond 3 as shown in FIG. In this case, the shape of the nano-polycrystalline diamond 3 inherits the shape of the graphite 1 except for a slight volume change. In addition, after removing the base material 2 from the graphite 1, only the graphite 1 may be exposed to high-temperature and high-pressure conditions. In this case as well, the produced nanopolycrystalline diamond basically takes over the shape of the graphite 1. It will be.

  In this step, it is preferable not to use additives such as sintering aids, catalysts, and binders. According to this step, it is possible to produce nano-polycrystalline diamond in which single crystals are firmly bonded without using an additive, and by using no additive, compared to the case where the additive is used. High hardness nano-polycrystalline diamond can be produced.

According to the manufacturing method described in detail above, the nano-polycrystalline diamond having the above-described characteristics, that is, including carbon and a different element doped in a crystal structure composed of carbon, the different element being other than oxygen And producing a nano-polycrystalline diamond that is at least one selected from the group consisting of the group 16 elements, wherein the atomic concentration of the different elements is 1 × 10 ¹⁴ / cm ³ or more and less than 1 × 10 ²⁰ / cm ^3. be able to.

  In addition, according to the above manufacturing method, since different elements are uniformly dispersed in the graphite, when the graphite is directly converted to diamond, it effectively suppresses local abnormal growth of diamond crystal grains. Can do. Thereby, the particle diameter of the single crystal constituting the nano-polycrystalline diamond can be made more uniform, and as a result, a homogeneous nano-polycrystalline diamond having the above-described characteristics can be produced. Further, by processing the produced nanopolycrystalline diamond into a desired shape, nanopolycrystalline diamond as an electron emission source can be produced.

<Electron emission characteristics>
In Examples 1 and 2, as described in detail below, graphite was prepared by the CVD method, and the obtained graphite was measured for the single crystal grain size, the density measurement, and the inclusion of different elements by the following method. The rate was measured. Thereafter, the graphite is directly converted to produce nano-polycrystalline diamond, and the obtained nano-polycrystalline diamond is measured for the single crystal particle size, X-ray diffraction spectrum, Knoop hardness by the following method, The electrical resistivity was measured. The obtained nano-polycrystalline diamond was processed by electric discharge machining, and this was used as an electron emission source of an electron gun to measure electron emission characteristics.

(Density measurement)
The density of graphite was measured using the Archimedes method.

(Measurement of grain size of single crystal)
The particle size of each single crystal in the SEM image obtained using an electron microscope was measured.

(Measurement of content of different elements)
The content of each element was measured using an ICP-MS analyzer.

(X-ray diffraction measurement)
An X-ray diffraction spectrum was obtained using an X-ray diffractometer.

(Measure Knoop hardness)
Knoop hardness was measured with a micro Knoop hardness meter at a measurement load of 4.9N.

(Measurement of electrical resistivity)
The electrical resistivity at a temperature of 20 ° C. was measured with a resistivity meter.

(Measurement of emission current)
The manufactured nano-polycrystalline diamond was processed into a cylindrical shape having a diameter of 6 μm and a length of 25 μm, and one end thereof was processed into a tapered shape (needle shape), which was set in an electron beam drawing apparatus as an electron emission source. Then, the apparatus was operated under the conditions of an electron emission source temperature of 800 ° C., an extraction voltage of 5 kV, and an acceleration voltage of 20 kV, and the emission current value was measured.

Example 1
First, a substrate made of single crystal diamond was placed in a vacuum chamber. Next, the substrate in the vacuum chamber was heated at 1900 ° C., the degree of vacuum in the vacuum chamber was 13 kPa, methane gas was supplied into the vacuum chamber at 100 sccm, and hydrogen sulfide gas was supplied at 50 sccm, which was continued for 6 hours. . As a result, sulfur-doped graphite having a thickness of about 2000 μm was formed on the main surface of the substrate.

The formed graphite has a density of 2.0 g / cm ³ , single crystal particle sizes of 100 nm to 10 μm, and an atomic concentration of sulfur of 3 × 10 ¹⁹ / cm ³ (0.06 mass%). there were.

  Next, the graphite on the formed substrate was exposed to a high temperature and high pressure environment of 2200 ° C. and 15 GPa to directly convert the graphite into diamond, thereby producing nano-polycrystalline diamond doped with sulfur.

  The produced nano-polycrystalline diamond had a single crystal particle size of 10 to 100 nm, no crystal phase other than the diamond single crystal was observed in the X-ray diffraction spectrum, and the Knoop hardness was 120 GPa. Further, a substrate having a size of 3 mm × 1 mm (thickness 1 mm) was cut out from the nano-polycrystalline diamond, and when this electrical resistivity was measured, the value was 1 kΩ · cm.

  Further, when the emission current when this was used as an electron emission source was measured, the value of the obtained emission current was 470 μA, and such a high value of current could be steadily taken out. Further, the discharge was repeated 10 times, but the electron emission source was not broken by impact. Further, when the emission current was measured again for 50 hours after 10 discharges, it was found that the emission current was 150 ± 10 μA, the change was small and stable.

(Example 2)
Sulfur-doped graphite was formed by the same method as in Example 1 except that dimethyl sulfide gas was used instead of hydrogen sulfide gas. Regarding the formed graphite, it was confirmed that the density was 2.0 g / cm ³ , the particle size was 100 nm to 10 μm, and the atomic concentration of sulfur was 2 × 10 ²⁰ / cm ³ (0.5 mass%). It was. And by the method similar to Example 1, the graphite on the formed base material was directly converted into diamond to produce nano-polycrystalline diamond doped with sulfur.

  The produced nano-polycrystalline diamond had a single crystal particle size of 10 to 100 nm, no crystal phase other than the diamond single crystal was observed in the X-ray diffraction spectrum, and the Knoop hardness was 120 GPa. Further, a substrate having a size of 3 mm × 1 mm (thickness 1 mm) was cut out from the nanopolycrystalline diamond, and when this electrical resistivity was measured, the value was 10 mΩ · cm.

  Further, when the emission current when this was used as an electron emission source was measured, the value of the obtained emission current was 850 μA, and such a high value of current could be steadily taken out. Further, the discharge was repeated 10 times, but the electron emission source was not broken by impact. Further, when the emission current was measured again for 50 hours after 10 discharges, it was found that the emission current was 850 ± 10 μA, the change was small and stable.

(Comparative Example 1)
A graphite powder having a particle size of 2 μm or less and a sulfur powder were mixed, and the mixture was fired at 2000 ° C. to produce solid carbon in which sulfur was dissolved. The atomic concentration of sulfur in the graphite was 3 × 10 ²⁰ / cm ³ (0.5% by mass). Polycrystalline diamond containing sulfur was produced by exposing the graphite to a high temperature and high pressure environment of 2200 ° C. and 15 GPa.

  With respect to the manufactured polycrystalline diamond, it was confirmed by SEM observation that the particle diameters of single crystals were 100 μm to 500 μm, respectively, and the particle diameters varied greatly. Further, it was confirmed by visual observation that this polycrystalline diamond had an opaque part and a transparent part. In the X-ray diffraction spectrum, the opaque part was a crystalline phase due to sulfur, and the transparent part was It was confirmed that the crystal phase was due to carbon. Moreover, the Knoop hardness of the transparent part was 100 GPa, and the Knoop hardness of the opaque part was 60 GPa.

  Further, a substrate having a size of 3 mm × 1 mm (thickness 1 mm) was cut out from the nanopolycrystalline diamond, and when this electrical resistivity was measured, the value was 800 kΩ · cm. Note that the emission current was tried to be measured using this as an electron emission source, but the current could not be passed to the extent that it could be measured.

<Oxidation resistance>
Nano-polycrystalline diamond not doped with different elements and nano-polycrystalline diamond doped with sulfur were produced, and their oxidation resistance was observed.

(Example 3)
With respect to the nano-polycrystalline diamond doped with sulfur, a nano-polycrystalline diamond doped with sulfur was produced in the same manner as in Example 2. Then, two substrates each having a size of 3 mm × 1 mm (thickness 1 mm) were cut out from the nanopolycrystalline diamond, and each was heated at 600 ° C. and 1000 ° C. for 1 hour in an atmospheric pressure environment.

  After the substrate was heated at 600 ° C. for 1 hour under an atmospheric pressure environment and its mass was measured, no change in mass was observed before and after heating. Moreover, when the mass was measured after heating a board | substrate at 1000 degreeC for 1 hour in atmospheric pressure environment, the change of mass was not seen before and after a heating. From this result, it was confirmed that the nano-polycrystalline diamond doped with sulfur has high oxidation resistance.

(Comparative Example 2)
A nanopolycrystalline diamond not doped with a different element was produced in the same manner as in Example 2 except that dimethyl sulfide gas was not introduced. Then, two substrates each having a size of 3 mm × 1 mm (thickness 1 mm) were cut out from the nanopolycrystalline diamond, and each was heated at 600 ° C. and 1000 ° C. for 1 hour in an atmospheric pressure environment.

  After the substrate was heated at 600 ° C. for 1 hour under an atmospheric pressure environment and its mass was measured, the mass was reduced by 10% before and after heating. Further, when the substrate was heated at 1000 ° C. for 1 hour under an atmospheric pressure environment, the substrate disappeared with combustion.

  It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

1 Graphite 2 Base material 3 Nano-polycrystalline diamond

Claims (6)

Carbon and a heterogeneous element doped in the crystal structure composed of the carbon,
The heterogeneous element exists in a state of being covalently bonded to the carbon,
The heterogeneous element is at least one selected from the group consisting of group 16 elements other than oxygen,
Atomic concentration of the different element state, and are 3 × ¹⁰ 19 / cm ³ to 2 × ¹⁰ 20 / cm ³ or less,
Nano-polycrystalline diamond that does not contain clustered heterogeneous elements .   2. The nanopolycrystalline diamond according to claim 1, wherein a particle diameter of a single crystal constituting the nanopolycrystalline diamond is 500 nm or less.   The nano-polycrystalline diamond according to claim 1, wherein the heterogeneous element is sulfur. The nano-polycrystalline diamond according to any one of claims 1 to 3 , which is used as an electron emission source of an electron gun. An electron gun comprising the nanocrystalline diamond according to any one of claims 1 to 4 . A method for producing nano-polycrystalline diamond according to any one of claims 1 to 4 ,
Preparing a graphite containing carbon and a heterogeneous element doped in the crystal structure composed of the carbon;
Exposing the graphite to high temperature and high pressure conditions of 1500 ° C. to 2500 ° C. and 7 GPa to 30 GPa,
A method for producing nano-polycrystalline diamond.

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