DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0048] An embodiment of catalyst and method of making the catalyst, carbon fiber formed using the catalyst, and electron emitting device, cathode of a secondary battery, body for hydrogen storing, electron source and image forming apparatus using a carbon fiber according to the present invention is described in detail. But, size, material, shape and relative position of units described below do not limit the scope of the invention, if not mentioning specific limitation. The making process described below is also not unique.

[0049] First of all, the catalyst of the present invention is explained.

[0050] The catalyst of the present invention particularly contains Pd used for growth of carbon fiber (promoting growth of carbon fiber), and at least one element of Fe, Co, Ni, Y, Rh, Pt, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er and Lu is added to the catalyst. And, in case of being used to form carbon fiber, the catalyst is preferable and important in the viewpoint that it controls shape such as diameter of the carbon fiber of a particle state.

[0051] Among the elements added in combination with Pd, it is preferred that at least any of Fe, Ni and Co is added, and combination of Pd and Co is more preferable. If the carbon fiber is made using catalytic particles containing Pd and any selected element among Fe, Ni and Co and applied the obtained carbon fiber to an electron emitting device, it is possible to obtain very excellent applied voltage-electron emission current characteristics. At the same time, it is also possible to obtain stable electron emitting characteristic for a long time. In addition, it is also preferred that the added element (Fe, Ni or Co) composes the catalytic particles in an alloy state with Pd to make a plurality of carbon fibers stably with high uniformity.

[0052] At this time, the term “carbon fiber” in the present invention means fiber containing carbon as a main component or carbon in state of fiber. And, “carbon fiber” of the present invention includes carbon nanotube, graphite nanofiber, amorphous carbon fiber and diamond fiber. And, as described later, in the electron emitting device of the present invention, graphite nanofiber is preferably used among the carbon fibers in aspect of its electron emission characteristic.

[0053] The reason that Pd is used as a component in the present invention is as followed.

[0054] Though other catalysts except Pd become oxide by reaction with moisture and oxygen in atmosphere as soon as they are exposed to the atmosphere, Pd catalyst maintains metallic combining state more stably rather than other catalysts.

[0055] Particularly, the metallic catalytic particles in Fe group have danger of explosion due to abrupt chemical reaction when being exposed to the atmosphere, while the metallic Pd catalyst does not have such danger. And, metallic catalysts containing Co, Ni, Fe or the like added to Pd progress oxidation reaction slowly, so enabling to treat the catalyst safely.

[0056] On the other hand, Pd shows a unique behavior related to nature of easily receiving hydrogen in the catalyst. If exposing Pd to deoxidization circumstance of such as hydrogen or organic gas, particles containing hydrogen are combined each other at a relatively low temperature (more than about 450° C.) to be particles having bigger shape than that of an initial state. Owing to such phenomenon, there are caused some problems for Pd particles changing into bigger shape in the facts that growth temperature of carbon fiber is raised and threshold of electron emission is increased.

[0057] To avoid such problems, there is a method of not exposing the catalyst to hydrogen or hydrocarbon until reaching at a temperature required for growth of carbon fiber, but there is found more effective method that prevent deformation to be larger particle by adding at least one of Fe, Co, Ni, Y, Rh, Pt, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er and Lu (preferably Fe, Co or Ni) into Pd particles to be combined each other.

[0058] The above result is remarkable when a ratio of the added element to Pd is more than 5 atm % (atomic percentage). And, if the ratio (atomic percentage) of the added element to Pd exceeds 80 atm %, it tends to require deoxidization process such as active hydrogen addition or gets into a slow growth of the carbon fiber. In addition, if the ratio (atomic ratio) of the added element to Pd exceeds 80 atm %, crystal structure of the formed carbon fiber tends to be extremely changed since the catalyst has characteristics similar to the catalyst having 100% of added element. For that reason, the added element to Pd is preferably less than 80 atm %.

[0059] And, the catalyst of the present invention is preferably in a particle state in aspect of forming carbon fiber.

[0060] Furthermore, though described in detail later, when applying carbon fiber to the electron emitting device, one in which carbon fibers are arranged in high density (called as “film containing a plurality of carbon fibers”) is applied to one electron emitting device. When forming such a film containing a plurality of carbon fibers with high uniformity and stability, it is essential that the element added to Pd contained in the catalytic particles of the present invention is an alloy state.

[0061] The present invention, though described in detail later, is to form a minute nucleus (catalyst particle (with a diameter of 1 to 100 nm)) with the above catalytic materials to thereby grow carbon fiber preferably applied to the electron emitting device through the nucleus (catalyst particle) by pyrolysis of hydrocarbon gas using the thermal CVD manner. At this time, there are used, for example, acetylene, ethylene, methane, propane, propylene and so on for the hydrocarbon gas. In addition, steam of organic agents such as ethanol and acetone are often used.

[0062] FIGS. 7, 8 , 16 A, 16 B and 16 C exemplarily show schematic diagrams of the film containing a plurality of carbon fibers obtained by decomposition of hydrocarbon gas using the catalyst particles of the present invention. In each figure, a shape of carbon fiber seen at an optical microscope level (˜1000 times) are schematically shown to left, a shape at an scanning electron microscope (SEM) level (˜30,000 times) to center, and a shape at a transmission electron microscope (TEM) level (˜1,000,000 times) to right.

[0063] As shown in FIG. 7 , if graphene is in a cylindrical shape, it is called as carbon nanotube (if multi-layered cylindrical shape, it called as multi-wall nanotube), and the threshold required for electron emission is lowest when an end of the tube is opend.

[0064] Graphite nanofiber is schematically shown in FIGS. 8, 16A , 16 B and 16 C. This type of carbon fiber is composed as a multi-layered graphene. More specifically, as shown in the right schematic figure of FIG. 8 , graphite nanofiber designates fiber-state material in which graphenes 23 are laminated to a longitudinal direction (fiber axis direction). Or, as shown in the right schematic figure of FIG. 8 , it designates the fiber-state materials in which the graphenes 23 are arranged not parallel to the fiber axis. Even though the graven 23 is substantially perpendicular to the axis of fiber, it is included in graphite nanofiber in the present invention.

[0065] In addition, the first crossed surface of graphite is called as “graphene” or “graphenesheet”. More specifically, graphite is made by laminating carbon planes, which is formed by the covalent bond of carbon atoms by sp ² hybridization(trigonal hybridization) and arranged as if regular hexagons are laid, with maintaining a distance ideally of 3.354 Å. This carbon plane is called as “graphene” or “gafensheet”.

[0066] In case the above carbon fiber is used as electron emitting device, the threshold of this electron emission is about 1V to 10V/μm, which is preferably as the electron emitting materials.

[0067] And, in case of forming the electron emitting device using carbon fiber, the film containing a plurality of carbon fibers are used for one electron emitting device. However, when such a film containing a plurality of carbon fiber is used, graphite nanofiber is preferably used as carbon fiber. It is because the electron emitting device in which the film containing a plurality of graphite nanofibers is used as an electron emitting film may obtain greater electron emission current density rather than the case of using carbon nanotube.

[0068] In addition, when the electron emitting device is used as, for example, a display and an electron source, it is required to maintain the good electron emission characteristic for a long time. As a result of investigation conducted by the inventors, it has been found that it is possible to maintain the good electron emission characteristic of the electron emitting device using the film containing a plurality of carbon fibers if Raman spectrum analysis of the film gives a specific result.

[0069] FIG. 11 shows characteristics remarkably appeared in the film containing a plurality of carbon fibers in which the film may maintain the good electron emission characteristic for a long time. Specifically, it schematically shows intensity distribution characteristic (Raman Spectra) of Raman scattering (scattered light) detected when irradiating laser with a wavelength of 514.5 nm to the film containing a plurality of carbon fibers of the present invention. In addition, in FIG. 11, a horizontal axis designates deviation extent (called as “Raman Shift”) of frequency of Raman scattering from Rayleigh scattering, while a vertical axis designates intensity of Raman scattering.

[0070] As shown in FIG. 11 , the film containing a plurality of carbon fibers of the present invention shows a clear peak (“first peak”) of Raman scattering intensity when Raman shift is in range of 1355±10kaiser(cm ⁻¹ ), and a clear peak (“second peak”) of Raman scattering intensity when Raman shift is in range of 1580±10kaiser(cm ⁻¹ ). It's believed that the first peak may be corresponded to so-called “D band”, while the second peak may be corresponded to so-called “G band”.

[0071] And, the film containing a plurality of carbon fibers of the present invention, as shown in FIG. 11 , has a relative intensity h2, which is a difference between Raman scattering intensity of the first peak and background intensity (baseline), at least 1.3 times of a relative intensity h1, which is a difference between Raman scattering intensity of the second peak and background intensity. And more preferably, in the film containing a plurality of carbon fibers of the present invention, a relative intensity h2, which is a difference between Raman scattering intensity of the first peak and background intensity (baseline), is 1.3 times or more and 2.5 times or lower of a relative intensity hi, which is a difference between Raman scattering intensity of the second peak and background intensity.

[0072] In addition, in this invention, the background intensity designates one connecting Raman scattering intensity at 1100 cm ⁻¹ and Raman scattering intensity at 1700 cm ⁻¹ in a straight line in an averaged result of an actual measured Raman spectrum graph.

[0073] And, the film containing a plurality of carbon fibers of the present invention satisfies the relation between Full-Width Half-Maximum of the first peak FWHM2 and Full-Width Half-Maximum of the second peak FWHM1 that FWHM2/FWHM1≦1.2.

[0074] And preferably, the film containing a plurality of carbon fibers of the present invention satisfies the relation between Full-Width Half-Maximum of the first peak FWHM2 and Full-Width Half-Maximum of the second peak FWHM1 that FWHM2/FWHM1≦1.2, and has a relative intensity h2, which is a difference between Raman scattering intensity of the first peak and background intensity (baseline), at least 1.3 times of a relative intensity h1, which is a difference between Raman scattering intensity of the second peak and background intensity. And more preferably, the film containing a plurality of carbon fibers of the present invention satisfies the relation between Full-Width Half-Maximum of the first peak FWHM2 and Full-Width Half-Maximum of the second peak FWHM1 that FWHM2/FWHM1≦1.2, and a relative intensity h2, which is a difference between Raman scattering intensity of the first peak and background intensity (baseline), is 1.3 times or more and 2.5 times or lower of a relative intensity h1, which is a difference between Raman scattering intensity of the second peak and background intensity.

[0075] More preferably, the film containing a plurality of carbon fibers of the present invention has a relative intensity h3, which is a difference between a minimum intensity between the first peak and the second peak (or between D band and G band) of Raman scattering intensity and the background intensity, {fraction (1/10)} or lower of a relative intensity h2, which is a difference between a minimum intensity of Raman scattering intensity having the first peak, in addition to each of the above characteristics (the Full-Width Half-Maximum of the first peak and the second peak and/or the intensity ratio between the first peak and the second peak). And, more preferably, the relative intensity h3 is {fraction (1/15)} or lower of the relative intensity h2.

[0076] As described above, by the fact that the minimum intensity h3 of Raman scattering intensity is {fraction (1/10)} or lower, particularly {fraction (1/15)} or lower, of Raman scattering intensity having the first peak in relation of the first peak and the second peak (or D band and G hand), crystallization of the carbon fiber is improved and thus it is possible to maintain the good electron emission characteristic for a long time.

[0077] The electron emitting device using the film containing a plurality of carbon fibers of the present invention satisfying the above conditions has greater initial electron emission current density, as described below, or lower deterioration of the electron emission characteristic with time, and may become an electron emitting device with good life characteristic. Though the reason that the good life characteristic is obtained for a long time is not clear, it is judged in the present invention that the relation with h3 is also one of important factors in addition to the relation between h1 and h2 and the relation between FWHM1 and FWHM2. In addition, the relation between h2 and h3 gives a stability of electron emitting characteristic.

[0078] Now, an example of methods for making the film containing a plurality of carbon fibers is described. But, size, material, shape, relative position, used gas and chemicals of units described below are just one example, but not intended to limit the scope of the present invention.

[0079] The film containing a plurality of carbon fibers with the above Raman shift characteristics can be obtained by growing carbon fibers through the below-described catalytic particles of the present invention (particularly, particles comprising Pd and Co alloy or particles comprising Pd and Ni alloy).

[0080] The electron emitting device using particularly graphite nanofiber among carbon fibers made using the catalytic particles of the present invention is not limited to device configuration shown in FIG. 2 described later, and may generate electron emission at a low electric field, and be able to obtain large emitting currents, and be easily manufactured, and have stable electron emission characteristics.

[0081] Graphite nanofiber, differently to carbon nanotube and so on, has fine convex and concave protrusions on a surface (a side of fiber) as shown in FIG. 8 or the like, so thus it is thought that electric field concentration may be easily appeared and the electron will be easily emitted. And, it is also considered that the electron emission would be easier since graphene has an extended shape from a central axis of the fiber toward outside (surface) of the fiber. One carbon nanutube is chemically inert since the side of fiber is basically corresponding to (002)plane of graphite and does not have protrusion like the graphite nanofiber, so it could be thought that electron emission is not easy at the side of fiber. For that reason, graphite nanofiber among carbon fibers is considered preferable for the electron emitting device.

[0082] For example, it is possible to set the film containing a plurality of graphite nanofiber as an emitter and then make an electron emitting device by providing electrode (gate electrode) for controlling electron emission from this emitter. Then, by arranging a light emitting member, which emits light by irradiation of electrons emitted from the graphite nanofiber, on a trajectory of the electrons, a light emitting device such as lamp may be formed. In addition, it is also possible to configure an image display device such as display by arranging a plurality of electron emitting devices using the film containing a plurality of graphite nanofibers as well as preparing anode electrode having a light emitting member such as fluorescent substance. The electron emitting device, light emitting device or image display device using the graphite nanofiber of the present invention may stably emit electrons without maintaining inner in an ultra-high-vacuum state as the conventional electron emitting device does, and since they emits electrons at a lower electric field, very reliable devices may be manufactured very conveniently.

[0083] And, particularly, when arranging a plurality of catalytic particles of the present invention and then applying the electric field intensity of 1×10 ⁵ V/cm between the anode electrode and the film containing a plurality of graphite nanofiber grown from the catalytic particles, a density of 10 ³ number/cm ² can be obtained at electron emission site.

[0084] As an example of the methods of making the catalyst (particularly, particulate catalyst) of the present invention, there is vacuum coevaporation, and specifically it is possible to make the catalyst by using Pd as a first evaporation source of the vacuum evaporation and at least one element selected in the group consisting of Fe, Co, Ni, Y, Rh, Pt, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er and Lu as a second evaporation source of the vacuum evaporation.

[0085] As for the vapor evaporation, there may be used electron beam evaporation, resistance heating evaporation, sputter method and so on, but the sputter method is preferably used. In this case, though it is a unified sputter device, the catalyst of the present invention can be manufactured by putting components, added to Pd target, on each test piece and then sputtering them.

[0086] In addition, there is also a liquid coating method for making the catalyst, in which the catalyst of the present invention is made by coating blended solution of a first liquid containing Pd and a second liquid containing at least one element selected in the group consisting of Fe, Co, Ni, Y, Rh, Pt, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er and Lu on a substrate and then drying or heating it.

[0087] Now, an example of electron emitting device using carbon fiber formed using the catalytic particles of the present invention is described with reference to schematic views shown in FIGS. 1 and 2 . At this time, the catalyst of the present invention is in a particle state for example.

[0088] FIG. 2A is a schematic plane view showing an example of configuration of electron emitting device using the film containing a plurality of carbon fibers of the present invention, while FIG. 2B is a schematic sectional view taken along 2 B- 2 B line of FIG. 2A .

[0089] In FIGS. 2A and 2B , 201 denotes an insulation substrate, 202 denotes a drawer electrode (gate electrode), 203 denotes a cathode electrode, 207 denotes a carbon fibers, which is a material of an emitter, and 205 denotes a conductive material layer, which is made by growth of the carbon fiber 207 through the catalytic particles. This conductive material layer 205 is not essential. Though the cathode electrode 203 is piled over the conductive material layer 205 on which the catalytic particles are arranged, it is also possible to arrange the catalyst of the present invention on a surface of the cathode electrode 203 without the conductive material layer 205 . In other words, it is also possible that the carbon fiber 207 is arranged on the surface of the cathode electrode 203 .

[0090] As for the insulation substrate 201 , an insulation substrate using such as quartz glass of which surface is sufficiently cleaned can be used.

[0091] The gate electrode 202 and the cathode electrode 203 are conductive, and formed by a general vacuum film forming technique such as evaporation and sputtering or a photolithography technique. As a material, carbon, metal, nitride of metal, carbide of metal and so on, which have heat resistance, are preferable.

[0092] The material of emitter (carbon fiber) 207 is carbon fiber such as carbon nanotube, graphite nanofiber or the like, grown using the catalytic particles including Pd and additionally at least one of Fe, Co, Ni, Y, Rh, Pt, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er and Lu. Among the additional components, Fe, Co and Ni are preferably used to obtain effects of the present invention. And, particularly, combination of Co and Pd is preferable. Also, it is required that Pd and the component (at least one of Fe, Co, Ni, Y, Rh, Pt, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er and Lu) combined with Pd are alloyed in aspect of forming the carbon fiber of the present invention having high uniformity and reproduction.

[0093] Hereinafter, an example of making the electron emitting element of the present invention shown in FIG. 2 is described in detail with reference to FIG. 1 .

[0094] (Step 1)

[0095] After cleaning the substrate 101 sufficiently, an electrode layer with, for example, a thickness of 500 nm is formed on the full substrate to form the gate electrode 102 and the cathode (emitter) electrode 103 . Quartz glass, glass partially replaced with K in which impurity contents of such as Na is reduced, a substrate in which SiO ₂ is laminated on a silicon substrate, and a ceramic substrate of such as alumina can be used for the insulation substrate 101 .

[0096] Then, positive type photoresist is used to form resist pattern in a photolithography process. The electrode layer is dry-eched using Ar gas while the patterened photoresist is used as a protective mask. Thus, the gate electrode 102 and the cathode electrode 103 with a gap (between the electrodes 102 and 103 ) 5 μm are formed ( FIG. 1A ). Materials of the gate electrode 102 and the cathode electrode 103 are selected from, for example, carbon, metal, nitride of metal, carbide of metal, metallic boron, semiconductor and metal compound of semiconductor. Materials of the gate electrode 102 and the cathode electrode 103 are preferably carbon, metal, nitride of metal, carbide of metal and so on, which have heat resistance. Thickness of the gate electrode 102 and the cathode electrode 103 may be set in range from several tens of nm to several tens of μm.

[0097] Hereinafter, patterning of film or resist using the photolithography process, film forming, lift-off, etching and so on is called simply as “patterning”.

[0098] (Step 2)

[0099] The resist pattern 104 is formed using negative type photoresist used for lift-off of an upper layer in the photolithography process ( FIG. 1B ).

[0100] Then, in this example, a conductive material layer 105 is formed on the cathode electrode. In case of arranging the conductive material layer 105 , it is preferred to use nitride of at least one of Ti, Zr, Ta and Nb as its material. Particularly, TiN is preferred in aspect of growing the carbon fiber stably.

[0101] And then, the catalyst 106 of the present invention is formed on the conductive material layer 105 by using the above-described vacuum evaporation or liquid coating and so on( FIG. 1C ). In order to arrange the catalyst of the present invention in a particle state, there can be used a method of spin coat of liquid containing ultrafine particles or a method of forming a catalytic layer by sputtering and then making it into particles by heating and aggregation under hydrogen circumstance. In the electron emitting device, the method using aggregation by heating under the hydrogen circumstance with assuring sufficient electric contact between the carbon fiber and the cathode electrode is preferred.

[0102] The catalytic particle of the present invention has a diameter 1 nm or more and 100 nm or lower, and more preferably 10 to 80 nm. In order to make the catalyst having such a diameter, the catalyst is piled to a thickness of 1 nm or more and 100 nm or lower, and then heated under hydrogen circumstance.

[0103] (Step 3)

[0104] By lifting off the conductive material layer 105 and the catalytic particle 106 on the resist through use of separation liquid of the resist patterned in the process 2 , the conductive material layer 105 and the catalytic particle 106 are patterned as desired ( FIG. 1D ).

[0105] (Step 4)

[0106] Then, the substrate on which the gate electrode 102 , the cathode electrode 103 and the catalytic particle 106 are arranged is heat-treated under circumstance including gas having carbon. And, as for the gas having carbon, hydrocarbon gas is preferably used. As for hydrocarbon gas, a gas selected among acetylene, ethylene, benzene and acetone is preferably used. And, it is also preferred that the hydrocarbon gas is in contact with the substrate 1 with being mixed with hydrogen. And, such heating (heat treatment) is conducted at a temperature of 400° C. or higher and 800° C. or lower.

[0107] If observing a surface of the conductive material layer 105 with SEM (a scanning electronic microscope) after such treatment, it would be understood that a plurality of carbon fibers are formed ( FIG. 1E ). In the present invention, the region created by arrangement of a plurality of carbon fibers is called as “a film containing a plurality of carbon fibers”. In the present invention, fiber used for particularly the electron emitting element preferably has a diameter of 5 nm or more and 100 nm or lower, and more preferably 10 or more and 30 nm or lower. Deviating this range may cause decrease of the life and make it impossible to obtain sufficient emission current.

[0108] The electron emitting device using the film containing a plurality of carbon fibers, as constructed above, is explained with reference to FIGS. 3 and 4 .

[0109] The device in which the gate electrode and the cathode electrode are spaced apart as much as a gap of several μm is installed to a vacuum device 408 as shown in FIG. 3 , and a vacuum exhaust device 409 exhausts gas until reaching at about 10 ⁻⁴ Pa. As shown in FIG. 3 , the anode electrode 410 is installed to a height H of several mm from the substrate by using a high voltage power source to apply high voltage Va of several kVs (kilo-volts).

[0110] And, a phosphor 411 on which a conductive film is coated is installed to the anode 410 .

[0111] At the device, device current I _f and electron emission current I _e flowing by applying pulse voltage of about several ten V as driving voltage V _f are measured.

[0112] At this time, it is thought that an equipotential line 412 is formed as shown in FIG. 3 . A point where the electric field is most concentrated is considered to be an inner space of the gap from the anode 410 of an electron emitting material (carbon fiber) designated by 413 .

[0113] It is also thought that electrons are emitted from a plurality of carbon fibers positioned near the electric field concentrated point.

[0114] Electron emission characteristic of this electron emitting device is as shown in FIG. 4 . In other words, I _e (electron emission density) is abruptly increased from threshold voltage (V _th ), and not-shown I _f (current measured between the gate electrode and the cathode) has similar characteristics to I _e , but its value is sufficiently low, compared with I _e .

[0115] In the embodiment of the present invention as described above, the gate electrode 102 ( 202 ) and the cathode 103 ( 203 ) are arranged on a surface of the substrate 101 ( 201 ).

[0116] The gap between the gate electrode 102 ( 202 ) and the cathode 103 ( 203 ) and the driving voltage (voltage applied between the gate electrode 2 and the cathode 3 ) are determined so that the electric field required for electron emission has a value 1 to 50 times of that of the horizontal electric field when comparing the vertical electric field required for electron emission from the carbon fiber with the vertical electrical field required for electron emission.

[0117] At this time, “the horizontal electric field” in the present invention means “an electric field to a direction substantially parallel to a surface of the substrate 101 ( 201 )” or “an electric field to a direction that the gate 102 ( 202 ) and the cathode 103 ( 203 ) are faced”.

[0118] And, “the vertical electric field” mentioned above in the present invention means “an electric field to a direction substantially perpendicular to the surface of the substrate 101 ( 201 )” or “an electric field to a direction that the substrate 101 ( 201 ) and the anode 411 are faced”.

[0119] As described above, FIG. 3 is a schematic sectional view showing configuration when the anode 411 is arranged upon the electron emitting device and then the electron emitting device is driven. As shown in FIG. 3 , in the electron emitting device of the present invention, assuming that a distance of the gap between the cathode 203 and the gate electrode 202 is d, a potential difference (voltage between the cathode 203 and the gate electrode 202 ) when driving the electron emitting element is V _f , a distance between surfaces of the anode electrode 411 and the substrate 201 on which the elements are arranged is H, and a potential difference between the anode 411 and the cathode 203 is V _a , it is considered that the electric field during driving (horizontal electric field; E1=V _f /d) is 1 times or more and 50 times or lower of the electric field between the anode and the cathode (vertical electric field; E2=V _a /H). As a result, electrons emitted from the cathode 203 collide with the gate electrode 202 may be decreased. Therefore, it may obtain high efficient electron emitting device with low spreading of the emitted electron beam.

[0120] In addition, as shown in FIGS. 2A, 2B and 3 , in the electron emitting device using the film containing a plurality of carbon fibers of the present embodiment, it is preferred that a plane, which includes a surface of the film containing a plurality of carbon fibers and is substantially parallel to the surface of the substrate 201 , is arranged in a position spaced apart from the substrate surface rather than the plane, which includes a part of the surface of gate electrode 202 and is substantially parallel to the surface of the substrate 202 in order to restrain scattering of electrons on the gate electrode 202 or irradiation of electrons toward the gate electrode.

[0121] In other words, in the electron emitting device (electron emitting apparatus) of this embodiment, the plane, which includes a part of the surface of the film containing a plurality of carbon fibers and is substantially parallel to the surface of substrate 1 , is arranged between the anode 411 and the plane, which includes a part of the surface of the gate electrode 202 and is substantially parallel to the substrate surface.

[0122] And, in the electron emitting device (electron emitting apparatus) of this embodiment, a front end of the carbon fiber is arranged to a height “s” (which is defined as a distance between the plane, which includes a part of the surface of gate electrode 202 and is substantially parallel to the surface of substrate 201 , and the plane, which includes the surface of the film containing a plurality of carbon fibers and is substantially parallel to the surface of substrate 201 ), as shown in FIG. 3 .

[0123] The mentioned height “s” depends on a ratio of the vertical electrical field vs. the horizontal electrical field (the vertical electrical field/the horizontal electrical field), and the height is lowered as the ratio of the vertical electrical field vs. the horizontal electrical field is low, while bigger height is required as the horizontal electric field is increased. Practically, the height “s” is in range of 10 nm or more and 10 μm or lower.

[0124] And, the electron emitting device using the film containing a plurality of carbon fibers of the present invention may adopt various configurations in addition to the configuration shown in FIGS. 2A and 2B .

[0125] For example, a cone-type emitter positioned in opening of the gate electrode of so-called Spindt-type electron emitting device as shown in FIG. 15 may be replaced with the film containing a plurality of carbon fibers. Or, it is also possible to arrange a film 4 containing a plurality of carbon fibers of the present invention on the cathode 3 arranged on the substrate 1 as shown in FIG. 14 , position an anode 62 to be oriented to the substrate 1 , and then apply electric field between the cathode 3 and the anode 62 so that electron may be emitted from the film 4 containing a plurality of carbon fibers of the present invention. Alternately, it is also possible in the configuration of the electron emitting element that a grid electrode for controlling electron emission is additionally positioned between the anode and the film containing a plurality of carbon fibers arranged on the cathode.

[0126] However, in the present invention, it is preferred to have configuration that the gate electrode and the cathode are arranged spaced apart on the substrate 1 so that the film containing a plurality of carbon fibers is positioned on the cathode, as shown in the sectional view of FIG. 2 . By using the configuration of FIGS. 2A and 2B , it is possible to obtain the electron emitting device with high efficiency and lower spreading of the emitted electron beam.

[0127] In addition, though it is shown in FIG. 2 that the gate electrode 202 and the cathode 203 have identical thickness, the cathode may have greater thickness than that of the gate electrode in other configurations. Additionally, such configuration can be modified even if an insulation layer with a suitable thickness is arranged between the cathode and the substrate.

[0128] As for preferable content of Co included in the catalyst, the inventors have conducted investigation for different content of Co. As a result, it has been found that voltage width (amplitude voltage width) and threshold required for obtaining certain current is varied depending on concentration of Co. As a result, in the viewpoint of increased threshold and voltage width required to obtain certain current from carbon fiber, it is preferred to add at least 20% of Co to Pd. And, if a fraction of Co is less than 20 atm %, it is impossible to obtain good electron emission characteristic for a long time in the formed electron emitting device using the film containing a plurality of carbon fibers. In addition, it has been found that a minimum growth temperature of carbon fiber can substantially not obtained with a general substrate if a fraction of Co in the catalyst containing Pd and Co exceeds 80 atm %. It has also been found that, if a fraction of Co exceeds 80 atm %, stability is better but the electron emission characteristic is deteriorated. As a result, a fraction of Co in the catalyst containing Pd and Co is 20 atm % or more and 80 atm % or lower.

[0129] As a specific example, a measurement system is configured composed of parallel plate and so on by electrically connecting and the carbon fibers grown with different content (0%, 23%, and 50%) of Co added to Pd and then positioning them in a vacuum chamber, and then applying high positive voltage to a faced anode. FIG. 9 shows results measuring electron emission amounts emitted from the fiber in one embodiment of the present invention.

[0130] As shown in FIG. 9 , in this example, one having the smallest amplitude voltage width (width of voltage required for obtaining certain current as the electron emission current I _e increases from zero) is Co: 50 atm % for the smaller one. As this amplitude voltage width is smaller, cost for the device required for driving control is decreased. In detail, as a result of investigation with changed added contents, the amplitude voltage width is effectively lowered rather than the case of 100% of Pd when Co is or more 20 atm %. Therefore, the cost of driving device is reduced as the threshold is lower even for an increased threshold. As apparently understood in FIG. 9 , it is observed that the threshold is lowered as the content of Co to Pd is increased.

[0131] On the other hand, a maximum content of Co to Pd is investigated in another aspect. In order to dissolve hydrocarbon gas using the catalyst, an oxidized film existing on a surface of the catalyst should be removed. Though the oxidized film is removed by exposing the catalyst to high temperature by using such as hydrogen, this removing temperature becomes one of factors determining the lowest limit of growth. After investigating the lowest limit of growth, the inventors have found that the lowest limit is about 400° C. at Pd: 100%, but about 410° C. at Co: 23 atm %, about 500° C. at Co: 50 atm % and about 600° C. at Co: 100%. As a result of investigation with more modified content of Co in more detail, it has been found that the minimum growth temperature is abruptly increased when a fraction of Co exceeds 80 atm %, which is substantially equal to the case of Co: 100%.

[0132] As easily understood from the results, when using the carbon fiber grown from the catalytic particles using materials including Pd and Co (more specifically, alloy of Pd and Co), a preferable concentration of Co is 20 to 80 atm %. Such a preferable range of concentration is equal for Fe and Ni, which are materials to be added to Pd.

[0133] Now, the image forming apparatus obtained by arranging a plurality of electron emitting elements to which the present invention is applicable is described with reference to FIGS. 5 and 6 . In FIG. 5, 601 denotes an electron source substrate, 602 denotes X-directional wire, 603 denotes Y-directional wire, 604 denotes the electron emitting device of the present invention and 605 denotes a wire connection.

[0134] In FIG. 5, m number of X-directional wires are composed of DX ₁ , DX ₂ , . . . , DX _m . Material, film thickness and width are suitably designed. The Y-directional wires 603 includes n number of wires DY ₁ , DY ₂ , . . . , DY _n , identical to the X-directional wire 602 . An insulation interlayer, not shown, is arranged between m number of the X-directional wires 602 and n number of the Y-directional wires 603 to separate them electrically (m and n are all positive integer).

[0135] The X-directional wire 602 and the Y-directional wire 603 are all drawn out as external terminals, respectively.

[0136] A pair of electrodes (not shown) composing the electron emitting device 604 of the present invention is electrically connected through the wire connections 605 to m number of the X-directional wires 602 and n number of the Y-directional wires 603 .

[0137] A scanning signal applying means, not shown, for applying scanning signals is connected to the X-directional wire 602 in order to select a row of the electron emitting device 604 of the present invention arranged to X direction. On the other hand, a modulated signal generating means, not shown, for modulating each column of the electron emitting device 604 of the present invention, depending on an input signal, arranged to Y direction is electrically connected so as to select individual device to be independently operated.

[0138] Such an image forming apparatus composed using the electron source in a simple matrix arrangement is described with reference to FIG. 6 . FIG. 6 shows a display panel of the image forming apparatus.

[0139] In FIG. 6, 701 denotes an electron source substrate in which a plurality of electron emitting devices are arranged, 703 denotes a rear plate to which the electron source substrate 701 is fixed, and 710 denotes a face plate in which a fluorescent film 708 and a metal back 707 are formed on inside of a glass substrate 709 . 704 is a support frame, which is connected to the rear plate 703 and the face plate 710 . 711 denotes an envelope, which is sealed. 706 denotes the electron emitting device of the present invention. 702 and 705 denote X-directional wire and Y-directional wire connected to the electron emitting device of the present invention.

[0140] The envelope 711 includes the face plate 710 , the support frame 704 and the rear plate 703 , as described above. On the other hand, the envelope 711 has strength sufficient against atmosphere pressure by installing supports (not shown) called as spacer to the face plate 710 and the rear plate 703 .

[0141] The configuration of the image forming apparatus described above is just an example of image forming apparatus applicable to the present invention, and can be modulated depending on technical aspects of the present invention. As for an input signal, TV signal (for example, high qualified TV including MUSE manner) as well as NTSC manner, PAL, SECAM manner and so on can by adopted.

[0142] In addition, the carbon fiber obtained in the present invention can be preferably applied to body for storing a material (ex. hydrogen), cathode material of batteries and complex materials as well as the electron emitting device. Particularly, in case the carbon fiber of the present invention is graphite nanofiber, since graphene having excellent crystallization to an axial direction of fiber is accumulated, more superior hydrogen occlusion(in other words “adsorption” or “storage”) is come out, and better characteristics can be obtained as a cathode material of batteries. Among batteries, the graphite nanofiber of the present invention is preferably applied to cathode of a secondary battery(rechargeable battery), and in particular the graphite nanofiber of the present invention having excellent crystal structure is preferably applied to cathode of a lithium ion secondary battery. Since the graphite nanofiber of the present invention has excellent crystal structure, this may form stable and large charging/discharging capacity. And when the graphite nanofibers formed using the caterytic particle mainly containing at least one of a Fe-Pd alloy, a Ni-Pd alloy and the Co-Pd alloy are applied to a body for storing hydrogen or secondary battery, the especially excellent characteristic can be shown.

EXAMPLES

[0143] Hereinafter, examples of the present invention are described in detail.

Example 1

[0144] In the first Example, Pd and Co are added as catalyzing particles by the common sputtering manner.

[0145] Now, a method of making electron emitting device of the present example is described in detail with reference to FIGS. 1A to 1 E.

[0146] (Step 1)

[0147] After cleaning the substrate 101 sufficiently with the quartz substrate, Ti with thickness of 5 nm and Pt with thickness of 100 nm, not shown, are continuously evaporated on overall substrate by sputtering at an initial stage in order to form the gate electrode 102 and the cathode (emitter) electrode 103 .

[0148] Then, the resist pattern is formed using a not-shown positive type photoresist in the photolithography process.

[0149] Then, the patterned photoresist as mask is dry-etched by using Ar gas on the Pt layer and Ti layer, and the gate electrode 102 and the cathode 103 having an electrode gap (width of interval) of 5 μm is patterned ( FIG. 1A ).

[0150] (Step 2)

[0151] The resist pattern 104 is formed using negative type photoresist used for afterward lift-off of an upper layer in the photolithography process ( FIG. 1B ).

[0152] Then, TiN layer is formed as a conductive material layer on which carbon fiber 107 will be grown with the catalytic particles 106 .

[0153] In addition, the catalytic particles 106 of the present invention are all formed by common sputtering manner. In this case, some Co thin peaces are putted on a Pd sputtering target. The catalytic particles 106 include Pd and Co of 33 atm % in a ratio (atomic ratio) to Pd ( FIG. 1C ).

[0154] (Step 3)

[0155] The resist and the conductive material layer 105 and the catalytic particle 106 on the resist are lifted off by using separation liquid of the resist patterned in the process 2, and the conductive material layer 105 and the catalytic particle 106 are patterned as desired ( FIG. 1D ).

[0156] (Step 4)

[0157] Then, they are heated in ethylene gas flow. After investigation with a scanning electronic microscope, it is found that a plurality of carbon fibers are formed. ( FIG. 1E ) Moreover, the carbon fibers were graphite nanofibers.

[0158] The electron emitting device made as above are mounted in a vacuum device as shown in FIG. 3 , and gas is exhausted by the vacuum exhausting device 409 until reaching to 2×10 ⁵ Pa. Positive voltage V _a =10 kV is applied to the anode 411 H=2 mm spaced apart from the device as shown in FIG. 3 . At this time, device current I _f and electron emission current I _e flowing by applying pulse voltage having driving voltage V _f are measured at the device.

[0159] I _f and I _e characteristics of the device are shown in FIG. 4 . In other words, I _e is abruptly increased at about half of the applied voltage, and about 1 ΞA of the electron emission current I _e is measured when V _f is 15V. On the other hand, though I _f has similar characteristics to I _e , but its value is low more than one order, compared with I _e .

Example 2

[0160] In the second embodiment, Pd and Fe are added as catalytic particles by common co-evaporation method.

[0161] In the present embodiment, after making the electron emitting device in a similar manner to the first embodiment, except that the second process is conducted as below, I _f and I _e are measured.

[0162] (Step 2)

[0163] The resist pattern 104 is formed using negative type photoresist used for lift-off of an upper layer in the photolithography process ( FIG. 1B ).

[0164] Then, TiN layer is formed as a conductive material layer 105 on which the carbon fiber 107 would be grown with the medium of the catalytic particles 106 .

[0165] And, the catalytic particles 106 of the present invention are formed by a common electron beam (two materials simultaneously) evaporation manner as follows. Then, the catalytic particles are deposited on TiN layer by using Pd and Fe as vacuum evaporation sources. As a result, the catalytic particles 106 containing Fe component of 20 atm % to Pd is formed in an island shape ( FIG. 1C ).

[0166] I _f and I _e characteristics of the device are identical to those of FIG. 4 . In other words, I _e is abruptly increased at about half of the applied viltage, and about 1 μA of the electron emission current I _e is measured when V _f is 15V. On the other hand, though I _f has similar characteristics to I _e , but its value is low more than one order, compared with I _e . Moreover, the carbon fiber formed in this example was a graphite nano fiber.

Example 3

[0167] In the third embodiment, Pd and Ni are added as catalytic particles by liquid coating.

[0168] In the present embodiment, after making the electron emitting element in a similar manner to the first embodiment, except that the second process is conducted as below, I _f and I _e are measured.

[0169] (Step 2)

[0170] The resist pattern 104 is formed using negative type photoresist used for lift-off of an upper layer in the photolithography process ( FIG. 1B ).

[0171] Then, TiN layer is formed as a conductive material layer 105 on which the carbon fiber 107 would be grown with the medium of the catalytic particles 106 .

[0172] And, the catalytic particles 106 of the present invention are formed by liquid coating as follows. By using mixed solution of acetic acid complex of Pd and Ni, the mixed liquid is spinner-coated. After coating, it is heated at the atmosphere. As a result, the catalytic particles 106 containing Ni component of 25 atm % to Pd is formed in an island shape ( FIG. 1C ).

[0173] I _f and I _e characteristics of the element are identical to those of FIG. 4 . In other words, I _e is abruptly increased at about half of the applied viltage, and about 1 μm of the electron emission current I _e is measured when V _f is 15V. On the other hand, though I _f has similar characteristics to I _e , but its value is low more than one order, compared with I _e . Moreover, the carbon fiber formed in this example was a graphite nano fiber.

Example 4

[0174] In this embodiment, the electron emitting device shown in FIGS. 1 and 2 is used, like the first embodiment. And, in this embodiment, the cathode 203 ( 103 ) is coated with TiN (titan nitride) with thickness of 300 nm by using the ion beam sputtering manner. The conductive material layer 105 used in the first embodiment is not used.

[0175] And, a catalytic layer made of Pd-Co alloy is evaporated on the cathode as much as amount to be an island shape by the sputtering manner using Ar (argon) gas. As a result of analysis of the evaporated catalytic layer, it is found that 50 atm % of Co is included in Pd. After that, the catalytic particles 106 with particle diameter of about 50 nm are obtained though heating under hydrogen circumstance.

[0176] Then, in this embodiment, the substrate is heated at 500° C. for 10 minutes among gas flow after mixing 1% of hydrogen diluted by nitrogen and 0.1% of acetylene diluted by nitrogen in 1:1 ratio at atmosphere pressure (about 1×10 ⁵ Pa).

[0177] As a result of observing that with the scanning electronic microscope, it is found that a plurality of carbon fibers, curved and elongated in a fiber shape with diameter of about 50 nm, are formed in a region where Pd-Co particles are formed. At this time, the layer composed of a plurality of carbon fibers has thickness of about 5 μm. In addition, as a result of observing this material with the transmission electronic microscope, the image as shown in FIG. 8 is obtained. In FIG. 8, a shape of carbon fiber seen at an optical microscope level (˜1000 times) are schematically shown to left, a shape at an scanning electron microscope (SEM) level (˜30,000 times) to center, and a shape at a transmission electron microscope (TEM) level (˜1,000,000 times) to right.

[0178] And, an interlayer spacing d(002) is inferred to be 0.35 to 0.37 nm. In addition, in the center of fiber axis, there exist an element in which the graphene layer is omitted and an element which is not distinct (it is certainly filled with carbon in amorphous shape).

[0179] As a result of measuring electron emission characteristic (voltage-current characteristic) of this device, the characteristics shown in FIG. 4 are obtained and the threshold electric field required for electron emission is 3V/μm.

Example 5

[0180] In this embodiment, the electron emitting device shown in FIGS. 1 and 2 is used, like the first embodiment. And, in this embodiment, the cathode 203 ( 103 ) is coated with Cr (chrome) with thickness of 300 nm by using the ion beam sputtering manner.

[0181] And, in this embodiment, titan (Ti) is sputter-evaporated as a conductive material layer 105 ( 205 ) to have thickness of 5 nm. And, Pd (Co 25 atm %) to be the catalytic layer is sputter-evaporated to have an island shape with thickness less than 5 nm. After that, the catalytic particles 106 are formed by heat treatment under hydrogen circumstance.

[0182] And, in this embodiment, after putting the substrate into a furnace and then exhausting gas in the furnace sufficiently, gas in which 1 vol % of hydrogen is diluted by nitrogen and 1% of ethylene diluted by nitrogen are put into the furnace in approximately 1:1 ratio up to pressure of 400 Pa.

[0183] Then, in this embodiment, as a result of heating the furnace up to 600° C. and then maintaining the temperature for 30 minutes, carbon fibers having a film shape are grown on the cathode. As a result of observing this carbon fibers with the transmission electronic microscope, the configuration shown in FIG. 16 is checked.

[0184] As a result of measuring electron emission characteristic (voltage-current characteristic) of this device, the characteristics shown in FIG. 4 are obtained and the threshold electric field required for electron emission is 5V/μm.

Example 6

[0185] After sufficiently cleaning the quartz substrate 1 , TiN is coated to have thickness of 300 nm as the cathode by using the ion beam sputtering manner.

[0186] Then, the layer in which catalytic particles composed of Pd-Co alloy are dispersed and arranged by the sputtering manner using Ar gas are evaporated on the cathode with size of 1 mm ² . After that, heat treatment under hydrogen circumstance forms a plurality of catalytic particles composed of Pd-Co alloy with particle diameter of about 30 nm.

[0187] The formed catalytic particles are composed of alloy in which 50 atm % of Co is contained in Pd, as a result of analysis.

[0188] Then, the substrate is heat-treated at 600° C. for 10 minutes at about atmosphere pressure (about 1×10 ⁵ Pa) through flow of gas(gas in which 1 vol % of hydrogen is mixed with nitrogen and gas in which 1 vol % of ethylene is mixed with nitrogen are mixed in 1:1 ratio).

[0189] As a result of observing a surface of the cathode with the scanning electronic microscope, it is found that a plurality of graphite nanofibers 107 ( 207 ), curved with diameter of 30˜50 nm and grown in a fiber shape, are formed on the cathode. At this time, the layer composed of a plurality of graphite nanofibers has thickness of about 5 μm.

[0190] And, an interlayer spacing d(002) is measured to be 0.35 nm.

[0191] In addition, in the center of the fiber, there exist an element in which the graphene layer is omitted and an element which is not distinct.

[0192] As a result of measuring Raman spectrum characteristic by irradiating laser rays with wavelength of 514.5 nm on the graphite nanofibers, the result is as shown in FIG. 10 .

[0193] From this result, it is found that FWHM 1 is 68kaiser(cm ⁻¹ ), FWHM 2 is 55kaiser(cm ⁻¹ ) and h2/h1 is 2.2.

[0194] On the other hand, when being replaced with the catalyst containing 100% of Pd without adding Co and the catalyst containing 23 atm % of Co added to Pd, the film containing a plurality of carbon fibers is formed using each catalytic particle, identically to this embodiment.

[0195] And, the electron emission characteristics of the film formed using the catalyst containing 50 atm % of Co added to Pd prepared in this embodiment, the film formed using the catalyst containing 100% of Pd and the film formed using the catalyst containing 23 atm % of Co added to Pd are compared. Their comparison results are schematically shown in FIG. 12 .

[0196] FIG. 12A shows results of Raman spectrum measurement obtained by irradiating laser with wavelength of 514.5 nm on the carbon fiber formed using the catalyst containing 100% of Pd without adding Co.

[0197] Likewise, FIG. 12B shows results of Raman spectrum measurement obtained by irradiating laser with wavelength of 514.5 nm on the carbon fiber formed using the catalyst containing 23 atm % of Co added to Pd.

[0198] And, FIG. 12C shows results of Raman spectrum measurement obtained by irradiating laser with wavelength of 514.5 nm on the graphite nanofiber formed using the catalyst containing 50 atm % of Co added to Pd, prepared in this embodiment.

[0199] As shown in FIG. 14, a substrate 1 on which a film 4 containing a plurality of carbon fibers arranged on the cathode 3 prepared in this embodiment is positioned in a vacuum chamber 9 . In FIG. 14, 1 denotes the substrate, 3 denotes the cathode, 4 denotes the film containing a plurality of carbon fibers, 61 denotes an anode substrate, 62 denotes a transparent anode using ITO, 5 denotes an insulating spacer, 6 denotes an ammeter, 7 denotes a high voltage power source, 8 denotes a vacuum pump and 9 denotes the vacuum chamber. A glass substrate 61 to which a transparent electrode is attached through the insulating spacer 5 is installed. The high voltage power source 7 and the ammeter 6 are respectively connected thereto, and inner pressure of the vacuum chamber 9 is set to 1×10 ⁹ Pa of vacuum level by using the vacuum pump 8 .

[0200] And, high voltage Va is applied to the anode 62 , and then electron emission amount emitted from the each film containing carbon fibers formed with different content of Co to Pd is measured by the ammeter 6 . And at the same time, time dependence (life characteristic) of electron emission characteristics of the each film containing carbon fibers is measured.

[0201] At this time, it is assumed that three kinds of films corresponding to FIGS. 12A to 12 C have same thickness.

[0202] In addition, in the present invention, current density when electron emission current does not increase in exponential relation though voltage increases in view of V-I characteristic when voltage of the anode is increased slowly is defined as a maximum current density of the each film.

[0203] And, in the first place, the maximum current density is measured for the each film, and set to as an initial value of the maximum current density. And, c