JPH0248648B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a process for pyrolysis of methane for producing graphite fibers of a type suitable as fillers in plastic composite materials.

U.S. Pat. No. 4,391,787 (Tibbetts) discloses a method for producing fine, straight graphite fibers by pyrolysis of natural gas carried out in thin-walled stainless steel tubes surrounded by moist hydrogen gas. The fibers are preferably 5 to 15 microns in diameter and up to several centimeters long and are well compatible with plastic fillers. In this method, fiber growth is due to complex interactions between chromium-bearing steel and methane at high temperatures. European patent application no.
No. 0132909A describes a method for growing fibers on ceramic surfaces, but only after the pyrolysis of natural gas has begun in close proximity to the stainless steel. Stainless steel is relatively expensive and has a limited service life at fiber growth temperatures.
Furthermore, carbonization of stainless steel is unavoidable, which unproductively removes carbon and reduces yield.

Fiber growth is believed to proceed in two successive stages. During the first stage, the pyrolytic carbon interacts with the metal core to form long thin filaments with a diameter of less than 100 nm. European patent application no.
As described in 0132909A, suitable nuclei are obtained from deposits prepared by evaporation of ferric nitrate solutions. Once nucleated, the filament grows rapidly in length. However, for reasons that are not fully understood, the filament then stops lengthening. Additional pyrolytic carbon then thickens the filament into fibers with a diameter of several μm. It is believed that the pyrolysis reaction initiated between stainless steel and natural gas somehow controls the nature and concentration of carbon precursors in the gas stream to favor initial filament formation and subsequent thickening. It will be done. It is therefore an object of the present invention to provide an improved method for growing graphite fibers by pyrolysis of natural gas on ceramic surfaces, which does not require pyrolysis to begin in close proximity to the stainless steel. It's about doing.

More generally, it is an object of the present invention to provide an improved method for growing graphite fibers by pyrolysis of methane gas, in which the methane concentration of the gas is first made particularly conducive to the reactions that lengthen the filaments and nucleate them. The purpose of the present invention is to provide a method for controlling the filament to a relatively high value for thickening the filament to a fiber. Control of methane concentration at each stage increases yield, maximizes fiber length and shortens growth time, although other factors such as temperature, gas flow rate and nuclear properties also affect fiber growth. This improves the overall method.

According to a preferred embodiment of the invention, the growth of graphite fibers on a moderately nucleated ceramic surface by pyrolysis of natural gas controls the methane concentration in the reactant gas at each stage of the fiber growth process. It is improved by doing. The growth surface is pretreated by evaporating an iron nitrate solution and depositing an iron compound. During the initial period in which methane gas is passed over the surface while being heated to a temperature sufficient to decompose the methane, the gas is prepyrolysed, a natural gas diluted with hydrogen such that the methane concentration is between 5 and 15% by volume. Consists of gas. As used here, methane concentration is expressed as the concentration in the gas mixture before heating to the pyrolysis temperature, where pyrolysis reduces the methane concentration by producing other carbon species. It is understood that This gas flow can be initiated either while heating the surface to a predetermined reaction temperature or after that temperature is reached. In either case,
The iron compounds on the growth surface, when exposed to hydrogen-based gases at elevated temperatures, decompose to form fine iron particles that are suitable nuclei. This nucleus reacts with carbon derived from methane to produce long, thin, cylindrical carbon filaments protruding from the surface.

After the filaments reach their ultimate length, the methane concentration is increased to cause the fine filaments to grow radially and form macroscopic fibers. At this stage, the gas phase suitably contains at least 25% by volume of methane and is preferably undiluted natural gas. The concentrated gas flow is maintained for a sufficient time to grow fibers of the desired diameter, which diameter is preferably between 5 and 15 micrometers for fibers intended for use as plastic fillers.

Subsequent thickening steps do not significantly increase the number or length of filaments, thus optimizing the yield of product fiber by maximizing the number and length of filaments during the first step. The reaction of nuclei and pyrolytic carbon to grow filaments is dependent on many reaction parameters including temperature and gas flow rate. However, according to the present invention, it has been found that the filament formation reaction is particularly sensitive to the methane gas concentration of the initial gas. If the methane concentration is too high, the number and average length of filaments decreases, probably because the excess carbon stops the filament formation reaction prematurely. On the other hand, if the methane concentration is too low, carbon will not be fed at a sufficient rate to connect the reaction and achieve maximum length.

Generally, unpyrolyzed methane concentrations of 5 to 15 volume percent yield abundant filaments of suitable length to form product fibers. It is believed that for a given temperature, there are certain methane concentrations that are particularly advantageous for filament forming reactions. for example,
It can be seen that at 1050°C, the optimum methane concentration is about 11%. However, the optimum concentration is sensitive to temperature and other reaction parameters. Furthermore, temperature and other reaction parameters may vary, for example, even in different regions of the growth surface. In one aspect of the invention, the methane concentration is varied over a range intended to provide an optimal concentration for multiple reaction conditions, including temperature variations.

Thus, for reaction temperatures within the preferred range between 1000°C and 1100°C, the methane concentration ranges from less than 10% by volume to more than 12% by volume, more preferably from less than 9% to 13% by volume. It can vary up to more than %.

Once nucleated, filament formation is completed within a short time. Continued exposure to a gas stream containing methane in a range favorable to filament formation results in
The filament does not thicken appreciably even after 2 hours. According to the invention, in the second step the methane concentration in the gas stream is increased and the filaments are enlarged radially into fibers. The diameter of the product fibers is related to the duration of this second stage. However, the length does not increase significantly. Generally, concentrations greater than about 25% thicken the fibers at a reasonable rate. Higher methane concentrations are preferred to maximize the rate of carbon precipitation and thereby minimize the time required to achieve the desired diameter. Thus, controlling methane concentration in accordance with the present invention not only maximizes the number and length of precursor filaments and thereby maximizes the yield and length of product fibers;
Reduces the time required to produce fibers of a particular diameter, thereby reducing overall reaction time.

The method of the invention is illustrated by the following examples.

Example 1 Graphite fibers were grown in a cylindrical tubular mullite reactor having a gas inlet at one end and a gas outlet at the opposite end. A coaxial cylindrical furnace was enclosed in the longitudinal midsection of the reaction channel so that the end of the reactor extended beyond the furnace to provide a gas line junction at the end of the reactor. In order to heat the intermediate section to reaction temperature, the furnace had a spiral electrical resistance heating device surrounding the intermediate section. The inner diameter of the reactor was approximately 52 mm. The middle part of the furnace was approximately 70 cm long.

Fibers were grown on an alumina substrate inserted into the middle of the reactor such that the surface of the alumina substrate was exposed to the gas within the reactor. Prior to insertion, the surface of the body was pretreated by applying a 1.5M aqueous iron nitrate solution to the surface of the substrate and drying the surface.

A mixture of natural gas and dry hydrogen was introduced into the reactor through the inlet. Exhaust the gas from the reactor outlet,
The gas flow in the reactor was thereby set somewhat above atmospheric pressure. Designation of methane grade
Airco cylinder natural gas with a methane grade of 1.3 was used and was approximately 96% methane by volume. The flow rate of natural gas through the reactor was set at 8/min.
cm ³ (cc/min). The hydrogen flow rate is approximately 452
Adjusted to cc/min. Thus, the methane concentration in the gas stream was estimated to be approximately 9.2% by volume.

After evacuating the air from the reactor, heat the furnace;
The reactor middle section above and the gas therein were heated at a rate of about 7°C per minute to a temperature of 1130°C and maintained at that temperature. The flow of 9.2% methane gas was maintained for approximately 1 hour. The natural gas flow rate through the reactor was then increased to 380 c.c./min and the hydrogen flow rate was increased to 995 c.c./min.
cc/min to increase the methane concentration to approximately 28% by volume. After about 1 hour, the furnace was closed and the reactor was cooled to room temperature. The gas flow was stopped during cooling, but the reactor was not opened to air to avoid oxidation of the heated fibers. A large number of graphite fibers were observed on the substrate surface. The average fiber length was about 1.5 cm, but many fibers with a length of about 3 cm were observed. The average diameter was approximately 10 μm. The fibers were believed to be comparable to those described in US Pat. No. 4,391,787 and were suitable for use as plastic fillers.

Example 2 A tubular mullite reactor and an electric furnace have a reactor inner diameter of 19 mm and a heated middle section with a length of about 30 mm.
It was the same as Example 1 except that it was cm. In order to introduce the alumina substrate into the reactor while preventing air entrainment, an air removal chamber was installed close to the reactor outlet. The reactor was heated to about 1050° C. while passing argon gas through it. After reaching the reaction temperature, the alumina substrate was inserted from the air removal chamber into the hot zone of the reactor. The substrate surface is Example 1
It had been treated in the same manner as above using 0.15M iron nitrate solution. After the substrate was placed near the center of the hot zone of the reactor, the argon gas flow to the reactor was stopped and the dilute methane flow was started. Volumes of high purity methane were used to produce a formulation representative of commercial grade natural gas, but with a controlled composition to enable evaluation of the method of the present invention regardless of changes in commercial natural gas composition. with 1% nitrogen
and 2% ethane. Synthetic natural gas flows into the reactor at a rate of approximately 4.4 cc/min, and approximately 35.6 cc/min.
diluted with hydrogen introduced at a rate of min. Thus, the methane concentration in the gas stream was estimated to be approximately 10.7% by volume. This gas flow was maintained for approximately 20 minutes. That time was considered sufficient to allow filaments to develop and grow to maximum length. Thereafter, hydrogen flow through the reactor was stopped and the synthetic natural gas flow rate was increased to about 40 c.c./min for about 40 minutes. The gas flow through the reactor was then stopped and the reactor was cooled. Examination of the alumina substrate revealed numerous graphite fibers having an average fiber length of about 4 mm and an average diameter of about 10 μm.

Example 3 In this example, the unresolved methane concentration of the gas stream was varied over a suitable range during the filament formation step in order to optimize fiber growth regardless of changes in reaction conditions.

The apparatus and method were substantially similar to Example 2 except for the following differences. The body surface was treated with 0.1M iron nitrate solution. After heating the reactor to about 1050°C, dilute methane gas flow was started. Initially, the gases entering the reactor consisted of 3.6 cc/min of synthetic natural gas and 36.4 cc/min of hydrogen, which corresponded to a methane concentration of about 8.7%. The gas formulation was varied over approximately 1/2 hour by uniformly increasing the natural gas flow rate and decreasing the hydrogen flow rate such that the overall gas flow rate was substantially constant. After 1/2 hour, the gas flow to the reactor was 5.4 cc/min of natural gas.
and hydrogen at 34.6 cc/min, which corresponded to a methane concentration of approximately 13.1% by volume. The natural gas was then passed undiluted through the furnace at a rate of about 40 c.c./min for about 15 minutes. The product fibers were approximately 3-4 μm in diameter.

Example 4 Example 3, except that in the filament formation step, the initial gas consisted of 4 c.c./min of synthetic natural gas and 36 c.c./min of hydrogen, corresponding to a concentration of unpyrolyzed methane of 9.7% by volume. repeated. Gas composition was 4.8cc/min of natural gas and hydrogen over 1/2 hour.
It was uniformly adjusted to 35.2 cc/min. Undiluted natural gas was then flowed through the furnace to produce the fibers.

As illustrated in these examples, the method of the present invention includes a preferred initial step in which methane diluted with hydrogen interacts with an iron nitrate deposit to form elongated fine filaments. It is believed that at elevated temperatures the hydrogen-based gas reduces the iron nitrate deposit and forms fine iron particles. These particles interact with the pyrolytic carbon to grow filaments. Hydrogen is preferred as a diluent to help reduce the iron nitrate deposit. Hydrogen is also the main by-product of methane pyrolysis, so the presence of a large proportion of hydrogen can help control methane pyrolysis, following well-known chemical principles to prevent uncontrolled soot. Can be done.

Once nucleated, the filament becomes longer at a very rapid rate. However, it stops growing after only a short time for reasons that are not fully understood. It is believed that filament formation is substantially complete within a few minutes, although the initial stage is preferably prolonged to ensure maximum opportunity for filament growth.

The filaments formed in the first stage had submicron diameters. However, in successive stages, it thickens into fibers. Thus, each product fiber is derived from a filament even though only a fraction of the total number of filaments is properly oriented and lengthened to resist breakage and mature into fibers. Since the thickening step does not significantly add length or produce additional fibers, it is useful to think of fiber yield as the total length of fiber per growth surface area, which in turn increases the number of precursor filaments. and related to length. It was found that the methane concentration during the filament formation stage critically influences the fiber yield.

The optimum methane concentration depends on the reaction temperature. 1050℃
In this case, the unpyrolyzed methane concentration that yields optimal fiber yield is about 11% by volume. Deviations from the optimum value will reduce the yield but can produce reasonable results. In Example 2, the reaction temperature was held constant throughout the fiber growth process. In Example 2, the reaction was repeated at different reaction temperatures while varying the methane concentration to determine the optimal concentration at each temperature.
At 1000°C, the optimum methane concentration was found to be approximately 12.5% by volume. At 1100°C, the optimum value was about 8% by volume. At 1150°C, the optimum value was approximately 5.5% by volume. At temperatures above 1200°C and below 950°C, less fiber was formed under the conditions of Example 2. Generally, a methane concentration of about 5-15% by volume produces sufficient precursor filaments to form dense, suitably long fibers.

The practice of the present invention is not limited to maintaining a constant unpyrolyzed methane concentration during the filamentation step, but rather is suitable for obtaining optimum fiber yields despite other reaction condition differences or temperature variations. This can be done advantageously by varying the methane concentration over a range. Reaction conditions are varied not only over the reaction time, but also between different zones within the reactor. At 1050°C, as in Example 2, filament growth is approximately 11
It is considered that the volume % is optimized using. However, high yields of fiber are in the preferred range 9-13
Methane concentrations within % by volume can be obtained. Thus, as explained in Example 3, the methane concentration can be gradually increased over a preferred range. The unpyrolyzed methane concentration was initially adjusted to a value below 9% and gradually increased to a value above 13%. Alternatively, as in Example 4, the methane concentration can be suitably varied over a narrower range from below to above the optimum methane concentration. The rate at which the methane concentration is increased is suitably slow to allow filaments to form, and then occurs rapidly once optimum conditions are achieved. Generally, changing the methane concentration over a period of about 10 to 30 minutes is sufficient.

Preferred conditions consist of a temperature of about 1000°C to 1100°C and a methane concentration of about 8 to 13% by volume. Although Example 2 consists of an isothermal reaction, it is believed that the filaments are formed at temperatures as low as about 600° C., although such low temperatures are clearly not sufficient to thicken the filaments. Thus, starting the methane gas flow during warm-up, as in Example 1, can accelerate filament formation, especially since filament lengthening reactions are advantageous at lower temperatures. 10 when the gas reaches the reaction temperature
Fiber growth was observed within the reactor after being heated for ~20 seconds. Optimal growth occurs after the gas is heated for about 15 seconds.

The relatively low methane concentrations used in the initial stage to promote filament formation are insufficient to radially enlarge the filaments at a rate sufficient to form fibers within a practical time. In Example 2, methane gas was heated at 1050°C for about 2 hours.
Continuous exposure to 10.7% by volume does not significantly increase diameter. Thus, the method of the invention includes a second step of increasing the methane concentration in the gas stream. At this increased concentration, pyrolysis produces more carbon species that properly precipitate on the peripheral surface of the filament and thicken the filament into a fiber. Generally, about
It can be seen that methane concentrations greater than 25% by volume thicken at an appreciable rate. However, it is preferred to maximize the methane concentration in the gas phase, for example by using undiluted natural gas, to improve efficiency and maximize radial growth rates. For a given methane concentration in the second stage,
Since the average diameter of the produced fibers is expected to increase linearly as a function of time, the diameter is usually determined by the duration of the second stage. Generally, fibers with a diameter of 5 to 100 .mu.m are suitable as plastic fillers, with fibers having a diameter of 5 to 15 .mu.m being preferred.

The rate at which the filaments thicken into fibers also depends on other reaction parameters including temperature and flow rate.
Generally, the flow rate can be increased in the second stage to increase the rate at which pyrolytic carbon is delivered to the growth surface. Also, while temperatures as low as 600℃ are suitable for forming filaments, temperatures as low as 1200℃
Temperatures around or above promote thickening reactions.

In the embodiment described, the ceramic surface was first treated with an iron nitrate solution. The iron nitrate deposit decomposes at moderately elevated temperatures into a mixture of iron oxides, and the iron oxides are then suitable for further decomposition at elevated temperatures and in the presence of a hydrogen-based reducing gas to form filaments. Forms a metal core.
However, nuclei obtained from other sources can be substituted. For example, fibers have been grown using nuclei obtained from submicron magnetite particles.
Iron particles obtained from pyrolysis of iron carbonyl also produce fibers. Iron particles with diameters smaller than 0.1 μm are commercially available and suitable for cores.

Claims (1)

[Claims] 1. A methane pyrolysis method for growing graphite fibers on a moderately nucleated ceramic surface, in which a hydrogen-based gas containing 5 to 15% by volume of methane is decomposed onto the surface. A gas containing at least 25% by volume of methane is then passed over the surface while heating it to a temperature sufficient to form fine carbon filaments protruding from the surface, and then a gas containing 25% or more methane is passed over the surface to decompose the methane and thicken the filaments into fibers. A method comprising: flowing over said surface while heating it to a temperature sufficient to cause 2. A method in which fibers are grown on the surface of a ceramic substrate having a suitable iron-based core, wherein said hydrogen-based gas is a hydrogen gas stream containing unpyrolyzed methane at a concentration of 5 to 15% by volume; flow to 600℃
and 1200°C to decompose the methane in the gas stream and grow the fine elongated carbon filaments, and heating the gas containing more than 25% by volume to a temperature between 950°C and 1200°C. A method for pyrolyzing methane for producing graphite fibers according to claim 1, which comprises heating to a temperature. 3. A method in which fibers are grown on the surface of a ceramic substrate having iron oxide deposits that are reducible to form a suitable iron-based core, wherein said hydrogen-based gas is grown by diluting natural gas with hydrogen. The hydrogen-based gas formed at 950℃
heating to a temperature between 1200°C to decompose the methane in the gas stream and grow said fine elongated carbon filaments, and the gas containing natural gas having an unpyrolyzed methane concentration of 25% by volume or more; Graphite according to claim 1, characterized in that the gas is heated at a temperature within the range of 950°C to 1200°C for a time sufficient to thicken the filament into fibers. Methane pyrolysis method for producing fibers. 4 A method in which fibers are grown on the surface of a ceramic substrate having iron oxide deposits of the type formed by the evaporation of a ferric nitrate solution, wherein said hydrogen-based gas is initially diluted with hydrogen to reduce the methane concentration. methane from a natural gas source formed into a gas stream having 8 to 13% by volume, heating the gas stream and the ceramic surface to a temperature between 1000°C and 1100°C, and not less than 25% by volume of said the gas containing methane is a substantially undiluted natural gas stream, the gas stream radially growing filaments into fibers having a diameter of 5 to 15 μm at a temperature in the range of 1000°C to 1100°C; A method of pyrolysis of methane for producing graphite fibers as claimed in claim 1, characterized in that the filament is passed in contact with a supporting ceramic surface for a sufficient period of time to cause the filament to pyrolyze. 5. A primary hydrogen gas stream having a unpyrolyzed methane concentration lower than a predetermined optimal filament forming concentration is first passed over said surface, and said gas stream has a unpyrolyzed methane concentration that causes said filament to form at said decomposition temperature. gradually increasing the unpyrolyzed methane concentration of the gas stream to above 25% by volume to thicken the filaments into fibers, and then increasing the unpyrolyzed methane concentration of the gas stream to above 25% by volume to thicken the filaments into fibers. A method for pyrolyzing methane for producing graphite fibers according to any one of claims 1 to 4. 6. The initial gas stream contains a source of natural gas mixed with hydrogen such that it has an unpyrolyzed methane concentration of 9% by volume or less, and the methane concentration in the gas stream is increased over a period of between 10 and 30 minutes. Gradually increasing the unpyrolyzed concentration to 12% by volume or more and then applying the substantially undiluted natural gas stream for a period of time sufficient to radially grow the filament into fibers having a diameter between 5 and 15 μm. The methane pyrolysis method for producing graphite fibers according to claim 5, characterized in that the methane is passed through while being in contact with the filaments on the surface.