A PROCESS FOR PREPARING FINE CARBON FIBERS IN A GASEOUS PHASE REACTION

Abstract: A process of preparing carbon fibers in a floating state by reacting a mixed gas consisted of a gas of organic metal compound or compounds, a carrier gas and desirably a gas of carbon compound or compounds at the elevated tempera­ ture of 600 to 1300°C is disclosed, in which the fine carbon fibers having a diameter of 0.05 to 2 µm, a length of 2 to 3000 µm and an aspect ratio of 2 to 30,000 as well as a high degree of preferred orientation of carbon layers may be produced in a high yield and a high productivity.

Description: This invention relates to a process of preparing fine carbon fibers in a gaseous phase reaction by thermal decomposition of a carbon compound in presence of a metal catalyst. Carbon fiber produced in a gaseous phase is known to have a high degree of preferred orientation of carbon layers, as well as excellent properties of high strength, high modulus and high corrosion resistance. Further, the carbon fiber has a graphite structure of substantially single crystal through heat treatment, thereby to form an ideal interlaminar compound, which is characterized by high conductivity comparable to copper or aluminum and by good performance in, for example, occlusion of hydrogen and thus is potential as a functional material. As for mechanical properties, such carbon fiber has much higher performance than PAN (polyacrylonitrile)-based carbon fibers and pitch-based carbon fibers and thus can be an ideal structural material. Heretofore, the carbon fiber according to a gaseous phase process has been produced by sequential steps of placing in an electric furnace a substrate of alumina or graphite, onto which an ultra-fine particle catalyst of iron, nickel or the like is formed, over which catalyst in turn is introduced a mixed gas consisting of a gas of hydrocarbon, such as benzene, and a carrier gas, such as hydrogen, thereby to decompose the hydrocarbon at the temperature of 1010°C - 1300°C to form and grow the carbon fiber on the substrate. A general procedure for forming the ultra-fine particle catalyst on the substrate is to suspend ultra-fine metal particles of about 100 to 300 A in a volatile liquid, such as alcohol, having low surface tension, and to spray or apply the resulting suspension onto the substrate, which is then dried. Corresponding prior art is disclosed for example in laid-open Japanese application 107329/77 (publication no. 7538/78). The known procedure has several disadvantages as follows: (1) High irregularity of fiber length on account of temeperature differences with resulting uneven fiber length and of ununiform spray of catalyst with resulting coarseness and closeness of the fiber formation, (2) as a gas of organic compound as a carbon source is consumed by reaction, there occurs a difference in a concentration of the organic compound between an inlet and an outlet to the reactor, so that sizes of the resulting fibers may vary in the vicinity of the inlet or the outlet, (3) as the carbon fiber is formed only on the surface of the substrate but the central zone of the reactor is not involved in the reaction, resulting in a poor yield, and (4) independent and separate processes, such as dispersion of the ultra-fine particles onto the substrate, placement of said substrate within a furnace, reduction of oxidized metal particles with hydrogen at the elevated temperature, formation of carbon fibers on the substrate through thermal decomposition of the organic compound, decrease of the furnace temperature, and removal of the carbon fibers, are required, so that continuous production is difficult and thus the productivity is low. Consequently, the production cost becomes too high to compete with already commercialized PAN-based carbon fibers and pitch-based carbon fibers except for special usage, for example, in the functional material. Another process for the production of carbon fibers emerges from the article "Carbon from Carbon Monoxide Disproportionation on Nickel and Iron Catalysts" by H.P. Boehm (Carbon, 1973, Vol. 11, pp 583 - 590): In the presence of Iron-Carbonyl or Nickel-Carbonyl which is decomposed at the reaction temperature of 480°C - 740°C to form a catalyst for the disproportionation reaction of CO helical carbon fibers and/or twisted strands are formed from a mixed gas containing CO and H₂. Maximum rates of the disproportionation of CO were observed around 550 °C. An essential feature of the produced fibers is their formation of a plurality of layers oriented perpendicular with respect to the fiber axis. This results in poor mechanical properties, in particular poor tensile strength. It is an object of the invention to provide a process for preparing carbon fibers in a gaseous phase reaction, which may solve the problems as described hereinabove. In particular a process shall be provided appropriate for the production of highly uniform fibers having good mechanical properties, whereüsin the process shall be of continuous nature in order to improve productivity. These objects are solved by a process according to claim 1. It has been found out that a mixed gas consisting of a gas of organic metal compound or compounds, a carrier gas and a gas of carbon compound or compounds may be reacted at the elevated temperature to form the ultra-fine particle catalyst in a gaseous phase reaction and to produce the carbon fibers continuously and efficiently, instead of forming the ultra-fine particle catalyst of iron or nickel on the substrate as in the conventional process. In accordance with the invention, there is thus provided a continuous process for preparing fine carbon fibers in a gaseous phase reaction, wherein a mixed gas consisting of a gas of organic metal compound or compounds, a carrier gas and a gas of carbon compound or compounds is reacted at temperatures of 1000°C - 1300°C. In accordance with the invention, the fine carbon fibers having a diameter of 0.05 to 2 µm, a length of 2 to 3,000 µm and an aspect ratio to 2 to 30,000 may be readily produced in a high yield and high productivity. Further features of the invention, its natures and various advantages will be more apparent from the following detailed description of the invention given by using the accompanying drawings in which: Fig. 1 is a schematic view of an apparatus for practising the process according to the invention; Fig. 2 is a schmeatic view of another apparatus for practising the modified process accroding to the invention; Fig. 3 is a schematic view of a general apparatus for preparing the carbon fibers in the conventional gaseous method. The organometal compounds and carbonyl compounds which may be used in the invention include all types of such compounds, especially those containing transition-metal. They may be usually classified into the following types: (1) alkyl compounds, (2) aryl compounds, (3) olefin compounds, (4) sandwich compounds, (5) carbonyl compounds, (6) cyanides, (7) isocyanides, (8) acetylides and others. The term "sandwich compounds" is referred to as compounds in which two or more aromatic rings of, for example, cyclopentadiene, indene, benzene and their derivatives are arranged in parallel, between which metal atoms are located. The term "transition metal" as referred herein, is intended to include scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, yttrium, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, tantalum, tungsten, rhenium, iridium and platinum with the Group VIII metals such as iron, cobalt and nickel being preferred due to their effective catalytic function, especially iron having the highest catalytic function. Among others, the organometal compounds consisting only of carbon, hydrogen and metal are most preferred. A great number of sugh organometal compounds may be used in accordance with the invention and representative compounds may be listed as follows: (C₄H₉)₄Ti, (CH₃)₃ Re, CH₂CHCH₂Mn(CO)₅, (CH₃O)₅Ta, (C₂H₅)₂FeBγ.(C₂H₅)FeBγ₂, (C₆H₅)₃PtI, (C₅H₅)₂Cγ, (C₅H₅)₂Ti, (C₅H₅)₂Fe, (C₆H₆)₂Mo, (C₉H₇)₂Fe, (C₅H₅)₂Mn, (C₅H₅)₂V, [C₅H₅Fe(CO)₂]₂, [C₅H₅Fe(CO)₂]CN, Fe(CO)₅, Fe₂(CO)₉, Fe(CO)₂(NO)₂, iron tetracarbonyl, iron , carbonyl halide, iron pentacyanocarbonyl, Ni(CO)₄, Cr(CO)₆, Mo(CO)₆, W(Co)₆, and others. Further, a mixture of these compounds may be used. The carrier gas which is used in the invention may include all kinds of gases which are inert to or not involved in the reaction, such as H₂, N₂, He, Ar, Kr, Ne, CO₂ and the like. These gases may be used in the form of a mixture or may contain 0 - 20 % of any other gas such as H₂S, H₂O, halogen and so on. Sulfurous compound, such as H₂S gas is particularly effective but not limited thereto. Halogen or H₂O may also be contained in the carrier gas. In the preferred embodiment of the invention, the carbon compound is used as a carbon source, comprising all types of organic compounds including organic chain compounds and organic cyclic compounds. Any compound other than the organic compounds may be used in the invention, provided that such compound may serve as a carbon source by its thermal decomposition. As examples of the effective carbon compound in the invention, there may be mentioned the following compounds: alkanes, such as methane and ethane; alkenes, such as ethylene and butadiene; alkynes, such as acetylene; aryl hydrocarbons, such as benzene, toluene and styrene; aromatic hydrocarbons containing fused ring, such as indene, naphthalene and phenanthrene; cyclo-paraffinic hydrocarbons, such as cyclopropane and cyclohexane; cycloolefins, such as cyclopentene and cyclohexene; cycloaliphatic hydrocarbons having fused ring, such as steroids; sulfur-containing aliphatic compounds, such as methylthiol, methylethylsulfide and dimethylthioketone; sulfur-containing aromatic compounds, such as phenylthiol and diphenylsulfide; heterocyclic compounds, such as benzothiophene, thiophene and others. A mixture of these compounds may also be used. Further, a material made of petroleum, such as naphtha, gasoline, kerosine and natural gas are inexpensive and thus may be utilized industrially and effectively. In accordance with the invention, the carbon compound is used only as the carbon source, so that any other element than carbon in the compound is not critical. Thus, the carbon compound may contain nitrogen, oxygen, sulfur, fluorine, chlorine, bromine, iodine, phosphorus, arsenic and other elements. Upon practising the process of the invention, the mixed gas consisting of the gas of organometal compound or compounds and/or carbonyl compound or compounds, the gas of carbon compound and the carrier gas is reacted at the elevated temperature of 1000 - 1300 °C. This process is effective industrially in view of the yield and economy. A procedure for preparing the mixed gas is not critical but one example for the process will be described hereinbelow. If the organic metal compound - which expression here and hereinbelow is intended to comprise organometal compound or compounds and/or carbonyl compound or compounds - and the carbon compound are in a gaseous state, then they may be mixed as such. If in the solid or liquid state, each of these compounds is charged separately in each heating vessel, to which the carrier gas is introduced separately to form a mixed gas consisting of the gas of carbon compound and the carrier gas on one hand, and another mixed gas consisting of the gas of organic metal compound and the carrier gas on the other hand. Then, both mixed gases are further mixed to form the final mixed gas containing the above-mentioned three gases, namely the gas of organic metal compound, the gas of carbon compound and the carrier gas. In this case, the vaporized or sublimed compounds must be prevented from being recondensed by heating lines for the gases. Alternatively, a solution of the carbon compound having dissolved the organic metal compound therein may be vaporized to form a mixed gas consisting of the gases of the organic metal compound and the carbon compound having the same composition as the starting solution on the elemental basis. In this alternative procedure, the carrier gas may be incorporated either before or after the mixed gas of the organic metal compound and the carbon compound is formed. This alternative procedure is more preferred in view of an apparatus to be employed and stability of the gas concentration than the procedure in which the organic metal compound and the carbon compound are individually and separately vaporized or sublimed and then mixed together, because in the latter case each of the organic metal compound and the carbon compound is liable to vary in its vaporizing degree depending on its residual quantity. In accordance with the invention, a proportion of the gas of organic metal compound in the total mixed gas should be deterimned in such a way that an imaginary vapor pressure, assuming that the organic metal compound is decomposed in the heating zone and all metal exists in the atom state, is more than 10 times higher than the saturated vapor pressure of said metal. In order to increase the yield, however, the proportion of the gas of organic metal compound in the total mixed gas is preferably in the range of 0.01 to 40% when the organic metal compound molecule contains one metal atom. The optimum concentration (by volume) of the carbon compound in the mixed gas may be varied depending on the mumber of carbon atoms in the compound and on the condition of thermal decomposition, but may be preferably in the range of 0.5-40%. The carrier gas, when used as a single component gas, may be most preferably H₂ gas in view of the yield and productivity, which may be, however, mixed with other gas(es) for further improving the yield. For example, the H₂ gas may be mixed with 1-6% of H₂S gas. Further, gases such as He, Ar, Kr, Ne, N₂, or CO₂, which itself may be used as the carrier gas, may be incorporated in the amount of 3-20% for further increasing the yield. It should be noted herein that there may occur somewhat difference in the morphology of the resulting carbon fibers depending on gases to be mixed. In case of a mixture of H₂ with H₂S, the resulting carbon fibers may be relatively rigid and straight, while a mixture of H₂ with He, Ar, Kr, Ne, N₂, CO₂ or the like may provide mostly curved and/or solewhat vermicular fibers. In both cases, however, graphitization treatment above 2800°C may eliminate the difference substantially. In accordance with the invention, the mixed gas may be continuously introduced into a reactor tube and the resulting carbon fibers generated in gaseous phase may be continuously withdrawn from its outlet with a mixture gas between a carrier gas and an unreacted gas. In other words, the resulting fibers are fine and thus fall down at a very slow rate, so that very few of them are retained in the reactor tube. In this case, vertical arrangement of the heater may substantially eliminate the accumulation of the fibers within the reactor tube, and enables the fibers to be continuously withdrawn from the tube. The conventional seeding process using the ultra-fine particles on the substrate can utilize only a surface of the substrate as a reaction zone, whereas the process according to the invention enables the ultra-fine metal particles generated by condensation to float or suspend in the whole space within the reactor tube wherein the reaction may proceed in the possible temperature range, thereby to improve the yield considerably. Further, in accordance with the invention it has been found that the concentration of hydrocarbons may be increased in comparison with the seeding process. Thus, due to the continuous production, the improved yield and the availability of the high yield concentration of the hydrocarbons, the process according to the invention may provide the yield more than 100 times higher than the conventional seeding process. Furthermore, in accordance with the invention, the superior fine carbon fibers may be readily obtained in the high yield and the high productivity, which has a diameter of 0.05 to 2 µm, a length of 2 to 3000 µm and an aspect ratio of 2 to 30,000, as well as a high degree of preferred orientation of carbon layers. The length of the carbon fibers produced by the process according to the invention is proportional to the retention time within the reaction zone. Thus, the longer reaction zone may provide the fibers of more than 3000 µm. On the other hand, growth in the diameter may proceed more rapidly than that in the length at the temperature above about 1100°C, so that the temperature gradient within the furnace or the serial connection of furnaces having different temperature ranges may provide the carbon fibers having the diameter of more than 2 µm. The fine carbon fibers prepared by the process according to the invention have a mumber of merits as a composite material or a functional material, compared with the long and thick fibers. The performance required for the short fibers in the composite material is as follows: (1) high strength and modulus of the fiber itself, (2) high aspect ratio of the fiber, (3) large surface area per unit volume, namely fine from, and (4) low density. The carbon fibers prepared by the process according to the invention may satisfy all the requirements as described hereinabove and thus may be an ideal material superior to the conventional fibers for preparing the composite material. Also in the functional material, the carbon fibers according to the invention may form an interlaminar compound and thus may be an efficient catalyst for the organic reactions, for which purpose the high specific area of the fibers is advantageous. The process for preparing the fine carbon fibers in the gaseous phase reaction according to the invention will now be described in detail hereinbelow with reference to the accompanying drawings. Figures 1 and 2 illustrate schematic views of apparatus as used for preparing the carbon fibers in the gaseous phase reaction in accordance with the invention. Referring to Figure 1, each of a vaporizer 42 for the organic metal compound and another vaporizer 46 for the carbon compound is connected to pipes 40, 44 for introducing the carrier gas. Each vaporizer 42, 46 is provided with a pipe 48 for feeding the mixed gas, which pipe is controlled in temperature for preventing the vapor from recondensing. The feeding pipe 48 is merged in the way with a pipe 49 for introducing a replacing gas or a minor gas and then connected to a reactor tube 50, a temperature of which is controlled by a heater 52. The carbon fibers produced in the reactor 50 may be collected in a fiber-collector 54, while the carrier gas and the unreacted gases are removed through a pipe 56. Referring to Figure 2, an apparatus 20 for feeding a solution, which serves also to store the carbon compound having dissolved the organic metal compound therein, is connected to a pipe 22 for feeding the mixed solution, which pipe 22 is connected together with a pipe 24 for the carrier gas to a reactor tube 26. The feeding pipe 22 for the mixed solution is controlled in temperature for preventing the solution from vaporizing in the way and inserted into the reactor 26 at its temperature range for allowing the solution leaving the pipe 22 to be vaporized instantly. The reactor tube 26 may be controlled in temperature by a heater 28. The carbon fibers produced in the reactor 26 may be collected in a fiber collector 30, while the carrier gas and the unreacted gases are removed through a pipe 32. Figure 3 shows a general apparatus for preparing the carbon fibers in the well-known seeding process. To a vaporizer 60 for the carbon compound are connected a pipe 62 for feeding the carrier gas and a pipe 64 for withdrawing the mixed gas which latter pipe 64 is merged in the way with a pipe 66 for the replacing or reducing gas and then connected to a reactor tube 68 which is controlled for its temperature by a heater 72. In the reactor tube 68 is placed a substrate 70, on which the ultra-fine metal particles are seeded. The carrier gas and the unreacted gases are removed through a vent pipe 74. The process for preparing the carbon fibers in the gaseous phase reaction according to the invention will now be described by non-limiting examples and with reference to the drawings. Example 1 Figure 1 shows an apparatus as used in this Example, in which (C₅H₅)₃Fe (ferrocene) as the organic metal compound, C₆H₆ (benzene) as the carbon compound and H₂ gas containing 3% H₂S as the carrier gas were used. The reactor tube having an inner diameter of 50 mm and a length of 1500 mm was made of aluminium oxide, while the heater having an effective length of 900 mm and a uniformly heating zone of about 300 mm was employed. At first, (C₅H₅)₂Fe and C₆H₆ were charged in the vaporizer 42 and 46, respectively. Then, N₂ gas was supplied through the pipe 49 into the reactor tube at the flow rate of 500 ml/min., while the temperature of the heater 52 was elevated. After the temperature reached the predetermined level, the H₂ gas was fed through the pipe 49 to the reactor tube at the flow rate of 500 ml/min. for replacing the gas. After 30 minutes of the replacement, the H₂ gas was introduced into the pipes 40, 44 for the carrier gas, into which pipes was then introduced H₂S gas through the pipe 49. The mixed gas immediately before entering the reactor had the composition of H₂:H₂S:(C₅H₅)₂Fe:C₆H₅ = 85.0:2.5:0.7:11.8. Its toal flow rate was 588 ml/min., calculated in the standard condition. The uniformly heating zone had the temperature of 1095°C. The resulitng carbon fibers were collected in a stainless fiber packed bed filter 54 as the fiber collector. The experiment was continued for 30 minutes for measuring a deposited amount of the carbon fibers on the reactor tube and on the stainless filter. 3.5 g of fine carbon fibers was obtained, which contained little carbon black and had a diameter of 0.2 µm and a length of about 30 µm in average. Reference 1 Figure 3 shows an apparatus as used in this experiment, in which the heater 72 and the reactor tube 68 are identical to those in Example 1. The vaporizer 60 was charged with C₆H₆ liquid. The reactor tube at its center was provided with a half-circle substrate which was prepared by splitting an aluminium oxide pipe of ID 45 mm and 300 mm length into half. The substrate had previously been subjected to the following treatment. About 1 g of ultra-fine iron particles having an average particle size of 100Å was suspended in 1000 ml of alcohol and then supernatant liquid was taken to be sprayed on the surface of the substrate and then dried. At first, N₂ gas was fed through the pipe 66 at the flow rate of 500 ml/min. while the temperature of the heater 72 was elevated. After the temperature reached the predetermined level, the pipe 66 was supplied with H₂ gas instead of the N₂ at the same flow rate of 500 ml/min. After 30 minutes of the gas replacement, the pipe 66 was closed and the H₂ gas was fed through the pipe 62 for the carrier gas at the flow rate of 100 ml/min. The gas composition in the withdrawal pipe 64 represents H₂:C₆H₆ = 97.6:2.4, while the total flow rate was 102.5 ml/min. The experiment was continued for 4 hours at the temperature of 1090°C for the first one hour, 1130°C for the next one hour and 1180°C for the last two hours. After the heater was cooled, the substrate was removed to find that the carbon fibers of 7 µm diameter and about 3 cm length were formed on the surface of the substrate. Then, the carbon fibers were scraped off from the substrate and measured for the weight to find 0.32 g. This experiment was chosen from more than 100 experiments due to its highest yield and productivity. Notwithstanding, it will be appreciated that the result of Example 1 is 88 times higher in its productivity than this experiment. This figure of 88 times represents the comparison upon growth phase of the carbon fibers. Thus, on the premise that the procedure of Example 1 ensures the continuous production whereas the reference (the well-known process) does not allow the continuous operation, the productivity may be readily increased over the figure of 88 times up to more than 100 times. Examples 2 to 11 These examples were carried out using the same apparatus as in Example 1 to obtain the results as shown in Table 1 below. As shown in Examples 1 to 11, it is preferred to use the mixed gas consisting of predominant H₂ gas as the carrier gas. When sulfurous compound is used as the organic compound, the use of the H₂ gas as the carrier gas may provide the high yield of the carbon fibers. Example 12 An apparatus as used in this Example is shown in Figure 2. The apparatus 20 for feeding the solution, which is also served as a storage, was charged with a solution having dissolved therein one mole of (C₅H₅)₂Fe per Kg of C₆H₆. Then, the C₆H₆(C₅H₅)₂Fe solution was continuously fed at the flow rate of 0.1 g/min. by constant flow rate pump to the aluminum reactor tube of 50 mm ID heated to 1090°C and was vaporized instantly in the reactor. On the other hand, H₂ gas was fed through the pipe 24 for the carrier gas to the reactor tube at the flow rate of 670 ml/min. The experiment was continued for one hour to determine the yield of the carbon fibers collected in the reactor tube and the stainless fiber filter 30. Further, similar experiments were repeated ten times to evaluate stability of the productivity. The result is shown in Tables 2 and 3 below. Reference 2 The same apparatus as used in Example 1 was employed and the condition was established to obtain the same gas composition as in Example 12. Thus, one our experiments were repeated ten times to obtain the results as shown in Tables 2 and 3. Statistical analysis was carried out for the experimental data as shown in Table 2 to obtain the result which is shown in Table 3 below. It will be appreciated from the data in Table 3 that variance analysis (on one side) with 5% significant level shows clear significant difference in variance between Example 12 and Reference 2. As a result, it may be concluded with 5% significant level that the yield in Example 12 is more stable.

Claims: 1. A process for preparing fine carbon fibers in a gaseous phase reaction by thermal decomposition of a carbon compound in presence of a metal catalyst, characterized in that a gas mixture, consisting of a gas of organometailic compound or compounds and/or carbonyl compound or compounds, a gas of carbon compound and a carrier gas, is subjected to elevated temperatures of 1,000° - 1,300° C, said organometailic compound or compounds and/or carbonyl compound or compounds at said elevated temperatures decomposing to form a metal catalyst which induces growth of carbon fibers in a floating state, and continuously recovering the carbon fibers. 2. A process according to claim 1, wherein the organometailic compound or compounds and/or carbonyl compound or compounds contains Fe, Co and/or Ni. 3. A process according to claim 1, wherein the organometailic compound or compounds and/or carbonyl compound or compounds consists only of carbon, hydrogen and iron. 4. A process according to claim 1, wherein the carrier gas is a mixed gas containing more than 60% of H₂ gas. 5. A process according to claim 1, wherein the carrier gas is H₂ gas containing 1 to 5 % of H₂S. 6. A process according to claim 1, wherein the carrier gas is H₂ gas containing 3 to 20 % of other gas selected from He, Ar, Kr, Ne, N₂ and CO₂. 7. A process according to claim 1, wherein a solution of the carbon compound having dissolved the organometailic compound or compounds and/or carbonyl compound or compounds therein is vaporized to form a mixed gas of the carbon compound with the organometailic compound or compounds and/or carbonyl compound having the same composition as said solution on the elemental basis.

Inventors: ARAKAWA, KOHEI
Applicants: NIKKISO CO., LTD.
Application year: 1984
Main classification: D01F9
IPC codes: D01F9