JP2012184128A - Method for producing carbon nanocapsule precursor including metal carbide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method in which a slag-like deposit is made so as not to be formed in a first step of a precursor thermal treatment, and simultaneously, an including carbon nanocapsule precursor is contained in a large amount in a formed fullerene soot.SOLUTION: The production method is so configured that in a helium atmosphere introduced in a discharge vessel, a polyphase alternating current having phase differences is applied, respectively, to three or more discharge electrodes arranged two- or three-dimensionally to thereby generate arc discharge; a composite carbon raw material is evaporated using a plasma region formed by the arc discharge; and the evaporated entire amount is made to condense in the plasma region and to be deposited on the inner wall of the discharge vessel, wherein the helium pressure in the discharge vessel is made to be 15 to 90 Torr, whereby an including micro-carbon nanocapsule-like substance including an inclusion substance is deposited on the vessel inner wall as an including carbon nanocapsule precursor.

Description

  The present invention relates to a method for producing a metal carbide-encapsulated carbon nanocapsule precursor using an arc plasma generated by a multiphase AC arc discharge, and in particular, the present invention efficiently uses a metal carbide-encapsulated carbon nanocapsule. Suitable for mass production.

It was discovered in 1993 for the first time in 1993 that lanthanum carbide microcrystals were encapsulated inside a multi-layered polyhedron of carbon (called carbon nanocapsules) having a size of several to several tens of nanometers by the arc discharge method in a rare gas. (See Non-Patent Documents 1 and 2). This material is a carbon rod electrode on the consumable side in the contact arc method (see Non-Patent Document 3) improved from the Kratchmer-Huffman method known as a mass synthesis method of C ₆₀ fullerene. Is synthesized for the first time by changing to a composite carbon rod containing rare earth oxide or the like (hereinafter referred to as “modified Kretschmer-Huffman method”). Called).

An anode that becomes high temperature due to electron impact from the counter cathode due to direct current arc discharge evaporates from the tip and gradually wears out, so it is also called a consumable electrode. The evaporated anode material is cooled by the helium gas in the apparatus, and a part of the evaporated amount is condensed in the gas phase and adheres to the apparatus wall surface as soot (historically called “fullerene soot”). Improved Kretschmer - although the Huffman method most of the fullerene soot is amorphous carbon, therein is usually what the contained substance is encapsulated in fullerenes and endohedral (in fullerene represented by C ₆₀ ), Single-walled carbon nanotubes (generated only when a catalytic metal is contained in the anode), and the like (see Non-Patent Document 4).

  The remainder of the material evaporated from the anode condenses on the cathode surface and grows as a columnar carbonaceous material called “cathode deposit”. The place where the above-mentioned “carbon nanocapsule” can be found is almost limited to the surface and inside of the cathode deposit (see P129 of Non-Patent Document 4). Historically, “carbon nanocapsules” were discovered by chance in 1993 as a by-product attached to the surface of multi-walled carbon nanotubes existing inside the deposit (see Non-Patent Document 5).

  Soon after it was discovered that lanthanum carbide microcrystals were encapsulated in carbon nanocapsules, it became clear that many metal elements were encapsulated in carbon nanocapsules. It has come to be called “capsules” or carbon-encapsulated metal nanoparticles (CEMNs) (P839 of Non-Patent Document 6). Since carbon nanocapsules themselves are called by various names such as “carbon nanopolyhedral particles” and “carbon nanopolyhedron” (see P41 of Non-Patent Document 4), there are also various names for encapsulated carbon nanocapsules. . Therefore, hereinafter, carbon nanocapsules encapsulating these metal elements and the like are collectively referred to as “encapsulated carbon nanocapsules”.

  The elements contained in the carbon nanocapsules are mostly metal elements, the majority of lanthanoid elements, iron group elements (iron, cobalt, nickel), alkaline earth elements (calcium, strontium, etc.), noble metal elements (copper, silver, gold) ), Platinum group elements (ruthenium, rhodium, palladium, osmium, iridium, platinum, etc.), IV and V group transition metal elements (titanium, zirconium, hafnium, vanadium, niobium, tantalum, etc.), VI and VII group transition metal elements ( Chromium, molybdenum, tungsten, manganese, rhenium, etc.), boron, silicon, germanium, antimony, sulfur, selenium, actinoid elements (uranium, thorium, etc.) and the like have been reported (see P26 of Non-Patent Document 4). There is a possibility that the types of these included elements will further increase in the future. There are books and reviews about these encapsulated carbon nanocapsules (see Non-Patent Document 4 or 6).

These elements may be encapsulated as a metal phase of a pure element or a carbide thereof (for example, a carbide phase or a cementite phase) (see P26 of Non-Patent Document 4). In addition, examples in which compounds such as metal sulfide (CaS) (see Patent Document 1) and metal phosphide (Pd ₃ P) (Non-Patent Document 7) are also included are reported.

  From another viewpoint of the encapsulated carbon nanocapsules, the surface is a microcrystal having a diameter of 10 to 100 nm coated with a carbon film, and can be considered as nano-sized composite particles. However, the encapsulated carbon nanocapsule has a major feature in the form of carbon that covers the particles, shows a structure in which the carbon network surface is developed by graphitization, and the network surface is defined as being parallel to the capsule surface. It is supposed to be possible.

  In particular, in the case of carbon nanocapsules encapsulating lanthanum carbide, the capsule is closed by introducing 12 five-membered rings per graphite layer, as in the case of fullerene, except that the substance is encapsulated. The encapsulated carbon nanocapsule is considered to be the same as the carbon polyhedron called “carbon nanocapsule” described above (see Non-Patent Document 5). Such a capsule structure is considered to be a feature common to carbon nanocapsules including all metal carbide microcrystals regardless of whether they are carbide or cementite. In this type of carbide-encapsulated carbon nanocapsule, there is a feature that an empty cavity is observed between the encapsulated microcrystal and the graphitized polyhedral polyhedral capsule (Non-Patent Document). 4 or 6).

  However, there are some examples that hardly show such cavities, as represented by the case where an iron group element or a noble metal element is included as a pure metal phase. In this case, even if the covering carbon is graphitized, it is characteristic that it is in close contact with the surface of the internal metal fine particles and no cavity is seen. In the graphite layer, it is known that defects such as transition and grain boundaries are complicatedly observed at the curved portion. Such a capsule is not closed by introducing a five-membered ring as in the case of a capsule confining a carbide phase of a rare earth element, but has a structure in which graphite pieces are stacked in parallel along the surface of the internal fine particles. (See P133 of Non-Patent Document 4). And it is thought that the confinement function and the shielding function as the capsule are remarkably lowered due to these defects.

  By the way, it is known that lanthanum carbide has high reactivity and is unstable, and it reacts immediately in normal air containing moisture and rapidly undergoes hydrolysis. However, it has been observed that lanthanum carbide microcrystals encapsulated in carbon nanocapsules do not change at all and do not hydrolyze when left in humid air for many years. This observation indicates that the carbide crystal particles are not only physically confined inside the nanocapsule cavity, but are also chemically shielded, which completely suppresses contact and reaction with water and oxygen in the air. (See P130 of Non-Patent Document 4 or P846 of Non-Patent Document 6).

  Regarding thermal stability and oxidation characteristics of microcrystals encapsulated in carbon nanocapsules in dry air, there is a research report on lanthanum carbide, which is considered to have the highest chemical reactivity among lanthanoids (Non-Patent Document 8). ). This lanthanum carbide powder is known to be so active that it spontaneously ignites in air at room temperature. According to this report, when the lanthanum carbide-encapsulated carbon nanocapsules collected from the cathode deposit are heated in the air (10 ° C./min) through a thermogravimetric analysis (TGA) analyzer, the temperature reaches approximately 500 ° C. Has been shown to be stable without reacting. It has been reported that when this temperature is exceeded, the combustion reaction of lanthanum carbide that has been suppressed until then occurs abruptly, so that the sample temperature rises rapidly (Non-patent Document 8).

  Considering the high physical stability of graphite network (one sheet is called graphene) and the non-permeability of materials, and the high chemical stability of carbon in air and water, this carbon nanocapsule without defects is extremely Although it is expected to have a high barrier function, the oxidation resistance characteristic up to 500 ° C. of the lanthanum carbide-encapsulated carbon nanocapsule shown above is considered to be a result of the barrier function being fully exhibited. (See P130 of Non-Patent Document 4 or P846 of Non-Patent Document 6).

  Because this temperature of 500 ° C. is consistent with the empirically known air oxidation start temperature of about 500 ° C. in air, the high activity that the encapsulated lanthanum carbide microcrystals will show is at the carbon powder level. This is because it is suppressed (see Non-Patent Document 9). Therefore, it is clear that the barrier function of defect-free carbon nanocapsules is not limited to lanthanum carbide, but can be exhibited in the same way when any other substance or element is included. Even if it is a substance that is unstable to oxygen and humidity in the atmosphere, and can be expected to be stably held in various gases, water, and various liquids.

  Due to such high retention performance and shielding performance, encapsulated carbon nanocapsules free from defects such as disposal of radioactive waste are highly expected for various applications (see Non-Patent Document 10). Encapsulated carbon nanocapsules without defects are encapsulated carbon nanocapsules closed by the introduction of the five-membered ring described above. Encapsulated carbon nanocapsules encapsulating noble metal elements and iron group elements in a pure metal phase are structurally Are also functionally different. And it turns out that such an inclusion | inner_cover carbon nanocapsule is restricted to what includes a carbide | carbonized_material phase as mentioned above. In other words, the inclusion carbon nanocapsules handled in the invention described in this specification are limited to carbide inclusion carbon nanocapsules, and do not handle metal phase inclusion carbon nanocapsules.

  In addition to the above-described improved Kretschmer-Huffman method (see Non-Patent Document 3, Patent Document 2, and P7 and P128 of Non-Patent Document 4), the encapsulated carbon nanocapsule is made of tungsten arc / metal pool method (US Patent No. 5). 472, 749 (1995), P853 of Non-Patent Document 6), plasma torch method (see P854 of Non-Patent Document 6), ion beam sputtering method (see P857 of Non-Patent Document 6), diamond nanoparticle mixed heating method It has been reported that it can be produced by a method such as (Patent Document 3) and a precursor heat treatment method (Patent Document 4).

JP 2007-84364 A Japanese Patent No. 3682792 JP 2001-39797 A Japanese Patent No. 4020265 Japanese Patent No. 338798 JP 2005-343784 A

R.S.Ruoff, D.C.Lorents, B. Chan, R. Malhotra, S. Subramoney, Science, 259, 346-348 (1993). M. Tomita, Y. Saito, T. Hayashi, Jpn. J. Appl. Phys., 32, L280-282 (1993). RE Haufler, J. Conceicao, LFP Chibante, Y. Chai, NE Byrne, S. Flanagan, MM Haley, SC O'Brien, C. Pan, Z. Xiao, WE Billups, MA Ciufolni, RH Hauge, JL MarGrave, LJ Wilson, RF Curl, RE Smalley, J. Phys. Chem., 94, 8634-8636 (1990). Yahachi Saito, Shunji Bando, “Basics of Carbon Nanotubes”, Corona, 1998. Y. Saito, T. Yoshikawa, M. Inagaki, M. Tomita, T. Hayashi, Chem. Phys. Lett., 204, 277-282 (1993). “Fullerenes, Chemistry, Physics and Technology”, K. M. Kadish and R. S. Ruoff, Editors; Wiley-Interscience; New York (2000). Takeo Oku; “Carbon Raw Material Science and Material Design II”, P11, CPC Study Group (2000). “Bulk Synthesis and Characterization of Carbon Nanocapsules Containing Lanthanum Carbide”, in Fullerenes, Vol. 14, PV Kamat, DM Guldi, F. D'Souza and S. Fukuzumi, Editors, PV 2004-12, P110-117, The Electrochemical Society , Pennington, NJ (2004) Edited by the Carbon Materials Society of Japan, “Latest Carbon Materials Experiment Technology (Physical Properties / Material Evaluation)”, P19 (2003). K. Yamamoto, T. Wakahara, T. Akasaka, Prog. Nucl. Energy, 47, 616-623 (2005). Saito Yahachi, Journal of Plasma and Fusion Research, 75, P902-909 (1999). L.F.P. Chibante, A. Thess, J. M. Alford, M. D. Diener, R. E. Smalley, J. Phys. Chem., 97, 8696-8700 (1993).

  However, among the above-mentioned methods for producing encapsulated carbon nanocapsules, the tungsten arc / metal pool method, plasma torch method, ion beam sputtering method, and diamond nanoparticle mixed heating method are transition metal elements such as Fe, Co, and Ni. This method is applicable only to the production of carbon nanocapsules encapsulating pure metal phases and their alloy phases, and cannot be applied to the production of carbon nanocapsules encapsulating metal carbide.

  The improved Kretschmer-Huffman method has led to the discovery of metal carbide-encapsulated carbon nanocapsules, and the production of cathode deposits based on the DC arc discharge method is the main production method of metal carbide-encapsulated carbon nanocapsules. It has become. However, in this method, since the material once reaches a high temperature and passes through a gas phase state, and encapsulated carbon nanocapsules are formed directly near the arc by the subsequent rapid condensation, the particle size distribution of the encapsulated carbon nanocapsules and There was a problem that it was difficult to control the number of graphene phases.

  Many methods have also been reported for producing nanocarbons using three-phase or higher multi-phase AC arc discharge with respect to DC arc discharge (Patent Document 6, etc.). However, there is no reliable report that the metal-encapsulated fullerene has been successfully produced by these multiphase AC arc discharge methods, and the multiphase AC arc discharge method is not utilized. Furthermore, in the multi-phase alternating current arc discharge method, the anode and the cathode are switched with time and there is no stationary cathode, so that the cathode deposit is basically not generated. Therefore, it is considered unsuitable for producing metal carbide-encapsulated carbon nanocapsules using cathode deposits as in the modified Kretschmer-Huffman method, and no successful examples have been reported so far.

  In order to overcome the above problems in the improved Kretschmer-Huffman method, the precursor heat treatment method described above has been developed. In this precursor heat treatment method, direct current arc discharge is performed using a composite carbon electrode containing an encapsulated substance, and first, fullerene soot containing “encapsulated carbon nanocapsule precursor” is produced, and then this is heat-treated. This is a method for producing the desired encapsulated carbon nanocapsule. In this method, a first step of preparing fullerene soot from a composite carbon electrode containing an encapsulated substance at a helium pressure of 15 to 100 Torr, and the resulting fullerene soot in a vacuum or under an inert gas atmosphere is 500 to 3000. It is divided into the second step of heat treatment at ° C. In this precursor heat treatment method, the maximum heat treatment temperature can be arbitrarily selected in the second step, so that it is possible to adjust the particle size and the number of graphene layers of the produced encapsulated carbon nanocapsules.

  In the first step of the precursor heat treatment method, the composite carbon electrode containing the encapsulated substance is evaporated by DC arc discharge to produce a fullerene soot containing the encapsulated carbon nanocapsule precursor (Patent Document 4). However, since the direct current arc discharge method is adopted in this process, it is inevitable that more than half of the material evaporated from the composite carbon anode including the inclusion material is condensed on the cathode as a slag deposit. The generation of such a deposit is purely a loss of electrode material, leading to a decrease in the yield of fullerene soot. For this reason, the yield of fullerene soot (efficiency in which the substance evaporated from the electrode is converted to fullerene soot: wt%) is considerably lower than 100%, usually about 20 to 40% (less than 10% if bad). Currently. A decrease in the yield of fullerene soot due to deposit formation is a serious problem, and this problem is hereinafter referred to as “sediment formation problem”.

  A small amount of multi-walled carbon nanotubes and encapsulated carbon nanocapsules are contained in the deposit, but the ratio is extremely small when viewed from the entire deposit. The main components of the deposit are said to be carbon-based materials such as graphite pieces and amorphous carbon, and those in which macro-particles are formed without inclusion of encapsulated materials (see Fig. 9 of Non-Patent Document 8). 1). In order to increase the yield of fullerene soot generation, it is desirable that this slag deposit is not generated as much as possible (see P905 of Non-Patent Document 11).

  There are roughly two different ways to solve this “sediment generation problem”. It is a path to find out the formation of deposits, search for a way to treat it and convert it to fullerene soot, and a path to prevent the formation of deposits themselves.

  In the production of fullerene soot, after the cathode deposit is produced, the polarity of the electrode is reversed and a direct current arc discharge is performed, and the cathode deposit is scattered and re-converted into fullerene soot. (See paragraph “0014” of Patent Document 5). This operation is called “electrode reversal arc discharge method” and the like, but when this operation is repeated, the amount of slag deposits is finally reduced to about 10%, resulting in the conversion yield to fullerene soot. Is 90% (see P906 of Non-Patent Document 11).

  However, if this method is attempted, the deposit will be very easily damaged when the sludge-like collapsible cathode deposit is removed and attached to the anode side, or when a large current is repeatedly applied to perform discharge work. It is recognized. Therefore, this method has many problems, and it has been found that it is quite difficult to implement.

  Furthermore, in the “electrode reversal arc discharge method”, arc discharge that consumes a large amount of power is repeated many times, and the final production cost of the fullerene cake obtained increases. From the above, as a method for solving the “deposit generation problem”, a method that is as low as possible and easy to operate is desirable.

  As another method for solving the “deposit generation problem”, there is an attempt to improve the process conditions of the discharge so that the evaporant does not condense as slag on the cathode. Since the discovery of the fullerene mass synthesis method in 1990, many process conditions such as current and voltage during DC arc discharge, arc gap distance between electrodes, atmospheric gas type and pressure have been optimized. (See P35 to 36 of Non-Patent Document 4, Non-Patent Document 12). However, despite these optimizations, a considerable degree of deposit formation was unavoidable, and it was found that the yield of converting the composite electrode of the anode to fullerene soot was only about 40% (non-patented). (See Table 1 in Reference 8).

  As described above, as long as the direct-current arc discharge method is employed, there is no means for greatly reducing the cathode deposit at low cost, and the “deposit generation problem” remains difficult to solve.

  On the other hand, in the AC arc discharge method, since the cathode is not fixed to one electrode side, it is expected that the formation of deposits hardly occurs (see P905 of Non-Patent Document 12). For this reason, the AC arc discharge method has been considered promising from the beginning when the "improved Kretschmer-Huffman method" was reported, but it was immediately experimentally confirmed that a deposit similar to the DC arc discharge method was formed. It became clear (Non-Patent Document 3). When the electrode structure is asymmetric even in the two-phase AC arc discharge, deposits concentrate on the electrodes having a large size and a low temperature, and grow in a volcanic shape on the peripheral surface where the electrodes contact each other. In this situation, only one direction of the AC component is used, which is substantially the same as DC arc discharge. For this reason, it has been reported that AC arc discharge cannot be continued for a long time, and it is often necessary to stop discharge and remove deposits (see P8634 of Non-Patent Document 3).

  In order to eliminate the formation and adhesion of this deposit, it has been proposed since 1990 to perform a two-phase AC arc discharge between electrodes that are completely equal in size and structurally symmetrical (see P8634 of Non-Patent Document 3). However, it was not possible to find a report that metal-encapsulated fullerenes or encapsulated carbon nanocapsules were produced by conducting a two-phase or multi-phase AC arc discharge test with this configuration. Therefore, the inventors realized the proposed conditions and conducted a fullerene soot production test using the `` symmetric two-phase alternating current arc discharge method '', and found that a considerable amount of slag deposits were produced on the electrode. The 20-year expectation that no deposits would be generated was disappointing. The deposit of the two-phase alternating current arc discharge method adheres to the side of the consumable surface at the tip of the electrode (see FIG. 3 of Comparative Example 1) and grows with time. Thus, it was found that the “deposition generation problem” cannot be solved even by the “symmetric two-phase AC arc discharge method”.

  As described above, in the process of performing arc discharge using the composite carbon electrode containing the encapsulated substance and converting it to fullerene soot, whether the DC arc discharge method or the two-phase AC arc discharge method is used. Regardless of the above, it was found that during the production of fullerene soot, slag-like deposits were always produced as by-products on the electrode, and the yield of fullerene soot was kept low. In other words, the “sediment generation problem” has remained a difficult problem to date. On the other hand, when a multiphase AC arc discharge method is used, slag deposits are not generated, and therefore, fullerene soot is obtained with a yield of almost 100% from a composite carbon electrode. Nanostructured carbon materials typified by nanotubes are included, but metal-encapsulated fullerenes and metal carbide-encapsulated nanocapsule precursors are not included at all, resulting in a problem that they cannot be used.

  Therefore, the metal carbide-encapsulated carbon nanocapsules can be produced only by the precursor heat treatment method while controlling the particle size and the number of graphene layers, and this is considered the most promising production method. The first step of this precursor heat treatment method is a step of preparing fullerene soot containing an encapsulated carbon nanocapsule precursor from a composite carbon electrode containing an encapsulated substance. The biggest problem with this first step is the “sediment formation problem”. The rate at which fullerene soot is generated by suppressing the condensation of the material evaporated from the composite carbon electrode as slag deposits containing graphite and the like is determined. There is a need to increase it.

  Accordingly, an object of the present invention is to suppress the formation of slag deposits in the first step of the precursor heat treatment method, and at the same time, produce a fullerene soot to be produced contains a large amount of encapsulated carbon nanocapsule precursors. It is to provide a method. For example, it is to provide a method of achieving a significantly higher yield than the conventional approach of nearly 100% as a step of preparing fullerene soot containing an encapsulated carbon nanocapsule precursor from a composite carbon electrode.

  The inventors of the present invention have continued research on the control of arc discharge for the purpose of solving the above-described problems and have been engaged in research on characteristics evaluation of fullerene soot produced for many years. As a result of these studies, in the multi-phase alternating current arc discharge method in which fullerene soot is obtained with a yield of almost 100% from the composite carbon electrode because no slag deposits are formed, this fullerene soot is added to the aforementioned “encapsulated carbon nanocapsule precursor”. I found a condition to contain a large amount of "body". In the present specification, the term “multi-phase alternating current” is defined and used as not including “two-phase alternating current”. That is, in this specification, “multi-phase alternating current” represents alternating current of three or more phases.

  That is, according to the present invention, an arc discharge is generated by applying an alternating current having a phase difference to three or more discharge electrodes arranged two-dimensionally or three-dimensionally in a helium atmosphere introduced into a discharge vessel. A manufacturing method in which a composite carbon raw material is evaporated using a plasma region formed by a phase alternating current arc discharge, and the entire evaporated amount is condensed in the plasma region and adhered to the inner wall of the container as a fullerene soot. By forming a helium pressure of 15 to 90 Torr, a fine encapsulated carbon nanocapsule material encapsulating the encapsulated material is generated in the soot attached to the inner wall of the container as an encapsulated carbon nanocapsule precursor. It is a manufacturing method of the inclusion carbon nanocapsule precursor. Preferably, the composite carbon raw material is an inclusion substance and carbon contained in the discharge electrode. Further, the composite carbon raw material is a composite, compound, or mixture of an encapsulated substance and carbon introduced from the outside into the plasma region.

  According to the present invention, fullerene soot containing the encapsulated carbon nanocapsule precursor can be produced from the composite carbon raw material in a substantially 100% yield while suppressing the generation of slag deposits.

  According to the method for producing fullerene soot according to the present invention, only fullerene soot containing an encapsulated carbon nanocapsule precursor is selectively produced from a composite carbon electrode containing an encapsulated substance without producing a slag-like deposit. be able to.

It is a schematic sectional drawing of embodiment which concerns on this invention. 4 is a photograph of an electrode after being used in the “symmetric two-phase AC arc discharge method” of Comparative Example 1. 2 is a photograph of an electrode after being used in the “three-phase AC arc discharge method” in Example 1. FIG. It is an electrode photograph immediately after the fullerene soot production test by the six-phase alternating current arc discharge method. It is a high-resolution TEM photograph of the lanthanum fullerene soot produced | generated in the helium pressure 100-110 Torr by the six-phase alternating current arc discharge method. It is a high-resolution TEM photograph of the lanthanum fullerene soot produced | generated in helium pressure 50Torr by the twelve-phase alternating current arc discharge method.

  Hereinafter, embodiments according to the present invention will be described in detail. In addition, since embodiment described below contains a preferable specific example in implementing this invention, various restrictions are made | formed technically, This invention limits this invention especially in the following description. Unless explicitly stated, the present invention is not limited to these forms.

  FIG. 1 shows a schematic sectional view of an embodiment according to the invention. The cylindrical discharge vessel 1 is a vacuum device composed of a highly confidential metal vacuum chamber. On the side surface portion 2 of the discharge vessel 1, rod-shaped discharge electrodes 10 to 15 and discharge electrodes 20 to 25 are installed in two upper and lower stages as described later. Each discharge electrode penetrates the side surface portion 2 and has a tip portion installed in the discharge vessel 1. Further, a pressure gauge 43 is fluidly connected to the discharge vessel 1 in order to measure the helium gas pressure in the discharge vessel 1.

  The discharge electrode can be used as a raw material for a nanostructured carbon material such as carbon fullerene when a carbon electrode is used, but can be used as a raw material for an encapsulated nanocapsule precursor when a composite carbon electrode containing an inclusion substance is used. . Further, when a composite carbon electrode containing a catalyst metal used for producing single-walled carbon nanotubes or double-walled carbon nanotubes is used, it can be used as a raw material for these carbon nanotubes. The composite carbon electrode can be used not only for all discharge electrodes but also for some discharge electrodes. Elements mixed in the composite carbon electrode include most lanthanoid elements, iron group elements (iron, cobalt, nickel), alkaline earth elements (calcium, strontium, etc.), IV and V group transition metal elements (titanium, zirconium, Hafnium, vanadium, niobium, tantalum, etc.), group VI and VII transition metal elements (chromium, molybdenum, tungsten, manganese, rhenium, etc.), boron, silicon, germanium, antimony, sulfur, selenium, actinoid elements (uranium, thorium, etc.) It may be mixed alone or several types may be mixed. In addition, it is possible to use an electrode containing the same or different elements for each electrode, and further use a mixture of several kinds of elements mixed.

  A gas supply port 45 is opened in the upper surface portion 4 of the discharge vessel 1, and helium gas is supplied from the gas supply portion. In the gas supply unit, helium gas is supplied from the supply tank 40 via the supply valve 42. As a result of the tests so far, when the helium pressure is less than 15 Torr, the formation of carbon nanocapsule precursors containing metal carbide is hardly observed, and when the pressure exceeds 90 Torr, the generation becomes unstable, and when the pressure exceeds 100 Torr, the formation of carbon nanotubes is not achieved. It came to be seen. Therefore, it is essential to maintain the pressure of the supplied helium at 15 to 90 Torr during arc discharge.

  A gas exhaust port 46 is opened in the lower surface portion 5 of the discharge vessel 1, and the gas in the discharge vessel 1 is exhausted by an exhaust pump 47.

  Each discharge electrode is connected to an AC power supply unit 30 so that an alternating current having a phase difference is applied to each discharge electrode. In this embodiment, the AC power supply unit 30 has a function of converting commercial three-phase AC into twelve-phase AC.

  The encapsulated substance and carbon vaporized in the generated plasma region grow into an encapsulated nanocapsule precursor as the temperature decreases toward the periphery of the plasma region, and adhere to the entire inner surface of the discharge vessel 1. And the fullerene soot adhering to the inner surface after completion | finish of discharge is collect | recovered, and it heat-processes based on a well-known method, and can obtain a desired inclusion carbon nanocapsule (patent document 4).

  The present invention will be described in detail below based on examples.

In the manufacturing apparatus shown in FIG. 1, a stainless steel vacuum chamber (Fukushin Industrial Co., Ltd.) was used as the discharge vessel 1. First, the inside of the vacuum chamber was evacuated to ¹ × 10 ⁻¹ Torr or less by an exhaust pump, and then helium (He) gas (purity 99.99%) was supplied until the pressure reached 60 Torr.

In the same vacuum chamber, 12 discharge electrodes were set in advance before evacuation. As the discharge electrode, a lanthanum oxide impregnated carbon material (Toyo Tanso KM-09LA) containing 0.9 atom% of lanthanum oxide (La ₂ O ₃ ) formed into a cylindrical shape having a length of 80 mm and a diameter of 9 mm is used as the electrode. Used as. The twelve electrodes were set in the upper electrode mounting portion of FIG. 1, and the lower electrode mounting portion was not attached with an electrode.

  When discharging, first, alternating current (voltage 7 to 8 V, current 90 A) having a phase difference was applied to each discharge electrode in a state where the tips of the respective discharge electrodes were in contact with each other. With the AC voltage applied, the electrode body was simultaneously retracted so as to separate the tips of the electrodes, and multiphase AC arc discharge (in this case, twelve-phase AC arc discharge) was started. The distance between the tips of adjacent discharge electrodes was set at a position where the distance was 2 to 10 mm, and the current was gradually increased to continue multiphase AC arc discharge (voltage 10 to 20 V, current 90 to 100 A). The internal pressure of the apparatus during discharge was maintained at 80 to 90 Torr.

  28 minutes after the start of twelve-phase AC arc discharge, the voltage application from the AC power supply is stopped, and after standing to cool, the vacuum chamber is returned to atmospheric pressure, the lid is opened, and the soot-like substance adhering to the inner surface of the vacuum chamber (Fullerene soot) was recovered. At the end of the discharge, the electrode tip was smooth and no deposits or the like were present. Furthermore, no example of slag deposits falling on the bottom of the vacuum chamber below the electrodes was found. From these observations, it was confirmed that almost all of the substance evaporated from the composite electrode was converted to fullerene soot (see the sixth column of Table 1).

  The recovery rate of fullerene soot is slightly less than 100%, but the shortage is not enough for recovery leakage, and oxygen in the lanthanum oxide contained in the electrode reacts with the electrode carbon to become carbon dioxide and is lost to the gas phase side. It is considered to be a fraction. In the case of direct current arc discharge, the recovery rate of fullerene soot is 20 to 40% as described above, and thus the superiority of the multiphase alternating current arc discharge method is clear. In addition to the results of the twelve-phase arc discharge method, Table 1 shows the results of cathode deposit yield and the results of symmetrical two-phase AC, three-phase AC, and six-phase AC in the case of a typical DC arc discharge for comparison. (See Table 1, columns 2-5).

  From this table, it can be seen that the AC arc discharge method from three-phase alternating current to twelve phases produces almost no electrode deposits, and the yield is substantially zero particularly in the six-phase and twelve phases. (See FIGS. 3 and 4). In other words, it can be seen that in the multi-phase AC arc discharge method of three-phase AC arc discharge or more, the yield of fullerene soot is almost 100%, and a more excellent effect is obtained with six or more phases. This trend is not expected to change even in multi-phase alternating current exceeding twelve phases.

The fullerene soot structure obtained by the twelve-phase AC arc discharge method at an average helium pressure of 85 Torr was observed and evaluated by a transmission electron microscope (TEM, JEOL, JEM-2000FE, HV = 200 kV). FIG. 6 shows an example of a high-resolution TEM photograph of the generated fullerene soot. A portion with strong contrast is LaC ₂ crystal, and many of them are encapsulated in a polygonal carbon nanocapsule substance. The nanocapsule wall surface and the LaC ₂ particles are in close contact with each other and there are no voids, indicating that these are encapsulated carbon nanocapsule precursors. From this photograph, it was confirmed that LaC ₂ encapsulated carbon nanocapsule precursor was contained in a large amount in this bag. These precursors were characterized in that the size was 5 to 10 nm in diameter, the number of graphene layers was about 2 to 6, the inclusions and the graphene shell were in close contact, and no voids were seen.

  The ignition temperature of the lanthanum carbide-encapsulated carbon nanocapsule precursor contained in this soot is 450 ° C from the TGA measurement, which is considerably higher than that of fullerene soot obtained by the DC arc discharge method (259 ° C). High results were obtained. By the way, the ignition temperature of lanthanum carbide-encapsulating carbon nanocapsules contained in the cathode deposit always accompanying the DC arc discharge method was 500 ° C. or more, but the amount contained was very small, and when the temperature of the cathode deposit was further increased A combustion peak corresponding to graphite appeared, showing a large weight loss of over 40%. The graphite combustion peak appearing on the TGA chart of the cathode deposit and the corresponding large weight reduction indicate that most of the material evaporated from the anode is converted to graphite in the DC arc discharge method. Of course, this kind of graphite combustion peak does not appear at all in the TGA chart of either 煤 (DC arc discharge 煤 and twelve-phase AC arc discharge 煤), and only the combustion peak of the lanthanum carbide encapsulated carbon nanocapsule precursor is observed. It was.

  About the recovered lanthanum fullerene soot by the twelve-phase alternating current arc discharge method, untreated soot (A), soot that was heat-treated under vacuum corresponding to the heat treatment temperature, soot (B) (heat treatment temperature 1300 ° C.),煤 (C) (same temperature 1520 ° C), 煤 (D) (same temperature 1700 ° C), 煤 (E) (same temperature 1860 ° C), 煤 (F) (same temperature 2000 ° C), respectively, Table 2 summarizes the ignition temperatures of the encapsulated lanthanum carbide when the heat-treated soot was subjected to thermogravimetric analysis (TGA) in dry air.

  The increase in the lanthanum carbide ignition temperature accompanying the increase in the heat treatment temperature shown in Table 2 shows the same tendency as that of the example described in Patent Document 4 as a prior patent, and is obtained by the present multiphase AC arc discharge method. This proves that the fullerene soot contained contains an encapsulated carbon nanocapsule precursor. In other words, this increase in the ignition temperature correlates with the number of graphene layers constituting the “encapsulated carbon nanocapsule precursor” or “encapsulated carbon nanocapsule” encapsulating the lanthanum carbide contained in each soot, and the lanthanum carbide crystal This is because it has been reported as an experimental fact that the more the average number of graphene layers covering the surface, the harder it is to ignite and the ignition temperature increases (Patent Document 4). This result is consistent with and consistent with a TEM photograph (FIG. 6) showing that a large amount of lanthanum carbide-encapsulated carbon nanocapsule precursor is contained in the lanthanum fullerene soot obtained by the multiphase AC arc discharge method. It is.

As shown in Table 2, the fullerene soot obtained by the multi-phase alternating current arc discharge method has a LaC ₂ ignition temperature in the contained precursor of 450 ° C. slightly higher than that before the heat treatment, and this value until the heat treatment temperature is close to 1500 ° C. It turns out that does not change much. This is considered to indicate that the fullerene soot obtained by the multiphase AC arc discharge method is generated by a heat treatment corresponding to about 1500 ° C. in a wide arc plasma region.

In the present example, the encapsulated substance included in the encapsulated carbon nanocapsule precursor is an example of lanthanum carbide, but the encapsulated substance in the present invention is not limited to lanthanum carbide, and forms carbide. Regardless of whether or not, any element that can be encapsulated as a carbide in the carbon nanocapsule is applicable.
[Comparative Example 1]

  Below, the fullerene soot production test by the "symmetric two-phase alternating current arc discharge method" performed for the comparison is demonstrated based on an Example.

  In the experiment, the same apparatus and conditions as those used in Example 1 were used. In the same vacuum chamber, 12 discharge electrodes were set in advance before evacuation, but before discharging, only two sets of opposing discharge electrodes were in contact with the two-phase alternating current ( A voltage of 6 V and a current of 120 A) were applied, and the other 10 electrodes were largely retracted from the assumed plasma region without applying a voltage. Two-phase AC arc discharge was started by separating the tips of the two electrodes while the AC voltage was applied. The distance between the tips of the discharge electrodes is set at a position where the distance is about 5 mm and the current is gradually increased, and two-phase AC arc discharge (He pressure 50 to 80 Torr, voltage 9 to 16 V, current 110 to 140 A) is continued for 6 minutes. did.

  After cooling the vacuum chamber, returning the vacuum chamber to atmospheric pressure and observing the interior, it was found that fullerene soot was attached to the chamber wall surface. Further, as shown in the photograph of the discharge electrode (FIG. 2), it was confirmed that large slag deposits were generated and adhered to the side of the discharge surface. Moreover, the fallen slag-like deposit was confirmed by the vacuum chamber bottom part. The yield of these electrode deposits was approximately 30% with respect to the total evaporation.

  Below, the fullerene soot production test by the “six-phase alternating current arc discharge method” conducted for specifying the helium pressure range will be described based on examples.

  In Example 2, the same apparatus and conditions as those used in Example 1 were used. In the same vacuum chamber, six discharge electrodes are set in advance before evacuation, and only six discharge electrodes are in contact with each other before discharge, and six-phase alternating current (voltage 6V, current 120A) is applied to the electrodes. Was applied. In this state, the evacuation was stopped, and the chamber was filled with a predetermined pressure of helium so that the helium pressure during discharge was 10, 20, 30, 50, 80, 100, 300 Torr. Then, the tip of the electrode was separated while the AC voltage was applied, and a six-phase AC arc discharge was started. The distance between the tips of the discharge electrodes is set at a position where the distance is about 5 mm and the current is gradually increased, and six-phase AC arc discharge (voltage 9 to 16 V, current 110 to 140 A) is performed under each pressure, and each fullerene I got a spear. Under the condition of a helium pressure of 10 Torr, the formation of fullerene soot attached to the inner wall of the chamber was hardly observed, and this condition was considered inappropriate for the production of the encapsulated carbon nanocapsule precursor.

  Table 3 summarizes the results of evaluation of fullerene soot produced at each pressure. Whether or not the encapsulated carbon nanocapsule precursor was included was determined by TGA measurement. When TGA analysis of fullerene soot produced at a helium pressure of 20 Torr is performed in dry air, a sharp peak ignited by the encapsulated lanthanum carbide appears around 400 ° C., and the encapsulated carbon nanocapsule precursor is produced under these conditions. Became clear. Similar TGA measurement results were also obtained for fullerene soots produced at helium pressures of 30, 50, and 80 Torr (see Table 3, second row). When TEM observation was performed on the fullerene soot produced at 50 Torr, the presence of the lanthanum carbide encapsulated carbon nanocapsule precursor was confirmed (FIG. 6).

  On the other hand, the fullerene soot produced at a helium pressure of 100-110 Torr was mostly composed of amorphous carbon by TEM observation, and the presence of lanthanum carbide-encapsulating carbon nanocapsule precursors could not be confirmed at all (a typical example in FIG. 5). Show).

  On the other hand, fullerene soot produced at a helium pressure of 300 to 350 Torr was found by TEM observation to contain single-walled carbon nanotubes instead of the lanthanum carbide-encapsulating carbon nanocapsule precursor.

  From the above results, it has been clarified that the lanthanum carbide encapsulated carbon nanocapsule precursor is produced in the range of helium pressure of 15-90 Torr.

  The encapsulated carbon nanocapsule precursor that can be produced by the method of the present invention can be easily converted to an encapsulated carbon nanocapsule by heat treatment. The encapsulated carbon nanocapsules produced thereby have excellent characteristics as described above, and therefore can be expected to be used industrially in various fields. Encapsulated carbon nanocapsules containing various substances include various magnetic recording media, magnetic toners, magnetic inks, magnetic fluids, MRI sensitizers (in the above fields, magnetic materials are used as encapsulated substances), pigments , Paint, rubber / paint / plastic filler, refractory filler, tip of scanning tunnel microscope, electrode for energy storage equipment, lightweight parts and wires for ultra-fine electronic circuits, cosmetics, union prevention function Weatherproof capsules that handle fine particles, elements, metals, compounds, etc. that easily oxidize or ignite in the atmosphere, capsules that hold substances that easily evaporate and sublime in a high-temperature reducing atmosphere, and hold radioactive substances in an underground flooded atmosphere It can be used in the field of capsules and the like.

DESCRIPTION OF SYMBOLS 1 Discharge vessel 2 Side surface part 3 Cooling space part 4 Upper surface part 5 Lower surface part 6 Plasma area | region 7 Catalyst body 10-15 Upper stage discharge electrode 20-25 Lower stage discharge electrode 30 AC power supply part 40 Supply tank 42 Supply valve 43 Pressure gauge 44 Mixer 45 Gas supply port 46 Gas exhaust port 47 Exhaust pump.

Claims (4)

  In the helium atmosphere introduced into the discharge vessel, arc discharge is generated by applying multi-phase alternating current having a phase difference to three or more discharge electrodes arranged two-dimensionally or three-dimensionally, and formed by this arc discharge. A composite carbon raw material is evaporated using the plasma region, and the total amount of the evaporated carbon is condensed in the plasma region to adhere to the inner wall of the discharge vessel as a fullerene soot, wherein the helium pressure in the discharge vessel is increased. Metal carbide-encapsulated carbon, characterized in that a fine encapsulated carbon nanocapsule material encapsulating an encapsulated substance is produced in the soot attached to the inner wall of the device as an encapsulated carbon nanocapsule precursor by setting the pressure to 15 to 90 Torr A method for producing a nanocapsule precursor.   The method for producing a metal carbide-encapsulated carbon nanocapsule precursor according to claim 1, wherein the multiphase alternating current has six or more phases.   3. The method for producing a metal carbide-encapsulated carbon nanocapsule precursor according to claim 1, wherein the composite carbon raw material is an encapsulated substance and carbon contained in the discharge electrode.   3. The metal carbide inclusion according to claim 1, wherein the composite carbon raw material is a composite, a compound, or a mixture of an inclusion substance and carbon to be introduced into the plasma region from the outside. A method for producing a carbon nanocapsule precursor.

Images