RU2791775C1 - Method for producing hollow carbon fibers - Google Patents

Abstract

FIELD: production of hollow carbon fibres (CF).

SUBSTANCE: carbon fibres are produced for manufacturing capillaries, membranes, filters, separators for battery compartments and composite materials used when working in aggressive environments and at elevated temperatures of the working area. In accordance with the claimed method for producing hollow carbon fibres, 0.6-1.2% by weight of ultra-long carbon nanotubes are introduced into isotropic petroleum pitch, heated and formed into carbon fibre, oxidized in air at a temperature of 200-300°C with production on the surface of an infusible crust due to the process of oxidative crosslinking carbonized by heating in an inert atmosphere of nitrogen, argon or helium at a temperature of 2200°C.

EFFECT: development of a simplified and accelerated method for producing hollow hydrocarbons.

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Description

The present invention relates to carbon materials, in particular to the production of hollow carbon fibers (CF) used for the manufacture of capillaries, membranes, filters, separators in battery compartments and composite materials. Such carbon materials, due to their chemical inertness and high thermal stability, can be used when working in aggressive environments and at elevated temperatures of the working zone, including as high-temperature filters.

Currently, there are many ways to obtain and use hollow hydrocarbons. The raw materials for producing hydrocarbons are such precursors as polyacrylonitrile (PAN) fibers, ordinary and liquid crystal mesophase pitch (petroleum isotropic pitch), viscose threads, cellulose (cotton fiber), hydrated cellulose, phenol-formaldehyde resin, etc.

The earliest known is [Hulteen, J. C, Chen, H. X., Chambliss, C. K., and Martin, C. R. (1997). Template synthesis of carbon nanotubule and nanofiber arrays. Nanostruct. mater. 9, 133-136. doi: 10.1016/S0965-9773(97)00036-6] a method for producing a hollow carbon fiber using porous aluminum oxide anode plates. In this method, the anode plates are impregnated with a solution of polyacrylonitrile and then dried, then heated without access to air, carrying out the carbonization of polyacrylonitrile in the pores. During carbonization, the resulting carbon material shrinks towards the pore walls with the formation of an internal cavity. At this stage, anode plates are obtained, the pore walls of which are coated from the inside with carbon. Further in this method, aluminum oxide is dissolved in hot alkali, thus releasing hollow hydrocarbons. The disadvantages of this method include the fact that the length of the fiber is limited by the thickness of the anode plate, as well as the high cost of anode plates as a consumable material and a large amount of aggressive liquid waste.

Known in several variants [Loscertales, I. G., Barrero, A., Marquez, M., Spretz, R., Velarde-Ortiz, R., and Larsen, G. (2004). Electrically forced coaxial nanojets for one-step hollow nanofiber design. J. Am. Chem. soc. 126, 5376-5377. doi: 10.1021/ja049443j; Wang, Y., Li, G., Jin, J., and Yang, S. (2017), Hollow porous carbon nanofibers as novel support for platinum-based oxygen reduction reaction electrocatalysts, Int. J. Hydrog. Energy 42, 5938 -5947. doi: 10.1016/j.ijhydene.2017.02.012] a method for manufacturing hollow hydrocarbons by nanospinning of two different polymers: easily decomposable to form a removable core and carbonized to form an outer shell. , then carbonizing the polymer in the outer shell.The disadvantages of this method include the fact that the use of nanospinning leads to the formation of a nanosized fiber with a high specific surface area, which may be contrary to the application in most areas due to reduced stability.

Known [patent US 9,828,700 B2, IPC D06M 13/262, published 09/24/2015] a method of manufacturing hollow hydrocarbons from a preformed multicomponent polymer fiber containing a polyolefin component and an inner core of a readily decomposable component. In this method, first, the multicomponent fiber is heated to remove the easily degradable component and obtain a hollow polyolefin fiber; then the hollow polyolefin fiber is subjected to sulfonation; then the sulphonated fiber is subjected to high-temperature carbonization and a hollow CF is obtained. The disadvantages of this method include a complex manufacturing procedure using expensive polymer precursors, as well as the low quality of the carbon material forming the finished fiber, since high-quality carbon materials are formed during the carbonization of not olefins, but a limited range of precursor polymers - polyacrylonitrile, viscose, pitch.

Known [patent US 10,774,447 B2, IPC D01D 5/04, published 08/01/2019] a method for manufacturing hollow hydrocarbons from a preformed multicomponent polymer fiber. In this method, a multicomponent polymer fiber is drawn from two polymer precursors, the first precursor being a solution of the first polymer and a readily degradable component in a first solvent into which a plurality of carbon nanotubes (CNTs) are introduced; the second precursor is a solution of the second polymer in the second solvent. In this case, the easily degradable component is selected in such a way that it is immiscible with the first polymer and non-wetting CNT. During joint stretching, the first precursor forms the inner core of the fiber, and the second precursor forms the outer shell, and the role of CNTs is that they facilitate the orientation of the molecules of the first polymer along the axis of the formed fiber. Further, in this method, oxidation is carried out when heated, during which the easily decomposable component is removed from the fiber core forming an internal cavity, and the first and second polymers change the nature of intermolecular bonds and prepare for carbonization. Further, in this method, carbonization is carried out by heating in an inert atmosphere, during which the first and second polymers pass into carbon phases, thus forming a hollow hydrocarbon having an outer shell (based on the second polymer), an inner layer (based on the first polymer oriented CNT) and an internal cavity. The disadvantages of this method include a complex multi-step fabrication procedure using expensive polymer precursors.

The closest in technical essence (prototype) is a method of obtaining hollow hydrocarbons from cotton fiber [Yunming Li, Yong-Sheng Hu, Maria-Magdalena Titirici, Liquan Chen, Xuejie Huang. Hard Carbon Microtubes Made from Renewable Cotton as High-Performance Anode Material for Sodium-Ion Batteries. Advanced Energy Materials, 2016, https://doi.org/10.1002/aenm.201600659]. According to this method, the cotton fiber is subjected to rapid heating at 1300°C, which simultaneously leads to faster carbonization of the outer layers of the fiber, the collapse of the cellulose component of the fiber in the inner part to form an inner cavity, and the removal of decomposition products through the inner cavity. The disadvantages of this method include the imperfect crystalline structure of the obtained carbon material (the so-called hard carbon) and strong distortion of the fiber shape, which is precisely the result of the rapid decomposition of cellulose - this limits the possible areas of application to the area of anode materials for batteries.

The objective of the claimed technical solution is to develop a simplified and accelerated method for obtaining hollow hydrocarbons suitable for the manufacture of membranes, high-temperature filters and composite materials.

To solve the problem, on the basis of experimental observations, isotropic petroleum pitch and ultra-long carbon nanotubes were used as a precursor. Petroleum isotropic pitch is obtained from heavy tar pyrolysis at Tbp. 240°C, fractional composition of the phases: α ₁ - 0%, α - 26.5%, β - 64.1%, γ - 9.4%. The temperature mode of obtaining fusible isotropic petroleum pitch 360°C at reduced pressure (10-13 mm Hg). From such a pitch, it is easy to form, oxidize and carbonize high quality hydrocarbons as described [O.N. Abramov, D.V. Sidorov, T.L. Apukhtin, V.A. Khramkov. Obtaining pitch carbon fiber based on petroleum raw materials. Chemistry and chemical technology, 2015, volume 58, no. 5, p. 86-89]. Extra-long carbon nanotubes are obtained by CVD at a temperature of 1200° by decomposition of a mixture of ethanol-acetone-thiophene on an iron-containing catalyst [VZ Mordkovich, NV Kazennov, VS Ermolaev, EA Zhukova, AR Karaeva, "Scaled-up process for producing longer carbon nanotubes and carbon cotton by macro-spools", Diamond and Related Materials, 2018, 83, pp 15-20. https://doi.org/10.1016/i.diamond.2018.01.017]. As a result of experimental observations, it was concluded that the introduction of a certain amount of ultra-long CNTs of 0.6-1.2 wt. into fusible isotropic petroleum pitch before forming carbon fiber with heating, it affects the process of carbonization of the formed carbon fiber in an inert atmosphere (nitrogen, argon or helium) and makes it possible to obtain a hollow carbon fiber without applying additional complex manufacturing techniques.

The process of obtaining hollow carbon fiber occurs in three stages:

- at the first stage, ultra-long carbon nanotubes in the amount of 0.6 - 1.2 wt.% are introduced into the melt of the precursor - petroleum isotropic pitch, pressed through the spinneret at a temperature of 200-300 ° C and pitch carbon fiber is formed.

- at the second stage, the formed pitch carbon fiber is oxidized by keeping it in air at a temperature of 200-300°C, while an infusible crust is formed on the surface due to the process of thermal-oxidative crosslinking.

- at the third stage, carbonization of the oxidized hydrocarbon pitch is carried out by gradual heating in an inert medium (nitrogen, argon or helium) at a temperature of 2200°C. Under such conditions, aliphatic side chains are almost completely destroyed, aromatization of cycloaliphatic groups occurs, aromatic molecules condense to form polycyclic structures, the chemical composition of the pitch hydrocarbon changes, and the carbon content at the final stage of carbonization increases to 96 wt%. In the case of obtaining CF from petroleum pitch, carbonization causes the gaseous decomposition products to seep through the infusible crust on the outer wall of the CF, resulting in the formation of a continuous CF of Fig. 1A. In the case of adding ultra-long CNTs to the oil isotropic pitch in the amount of 0.6-1.2% of the mass. extra-long CNTs reinforce the outer wall of the CF and impede the penetration of gaseous decomposition products through it, as a result, gaseous decomposition products push the molten core through the end surface of the fiber, forming a hollow CF figb.

The use of ultra-long CNTs is due to the fact that the use of other types of (short) CNTs does not lead to the formation of a gas-tight reinforced infusible crust on the CF surface after oxidation.

The used concentration range of ultra-long CNTs in molten petroleum isotropic pitch of 0.6-1.2% wt., due to the fact that when using concentrations of less than 0.6% wt. no gas-tight reinforced infusible crust is formed on the surface of the hydrocarbon after oxidation; and when using concentrations above 1.2% of the mass. it is not possible to carry out the molding of the CF due to the too high viscosity of the mass obtained with the introduction of CNTs.

Thus, the cumulative set of optimal parameter values established experimentally for obtaining hollow SWs makes it possible to achieve the set goal.

Figures 1-2 show a diagram and photographs explaining the invention:

figure 1 shows a diagram showing the transformation of the formed carbon fiber during oxidation and carbonization: A - carbon fiber formed from fusible petroleum pitch; B - carbon fiber formed from a mixture of fusible petroleum pitch and 1% wt., ultra-long CNTs;

figure 2 shows electron micrographs of hydrocarbons obtained on the basis of fusible petroleum pitch: A - from fusible petroleum pitch, B - from a mixture of fusible petroleum pitch and 0.1% wt., ultra-long CNTs, C - from a mixture of fusible petroleum pitch and 1 % wt., extra long CNTs.

Examples of the implementation of the method.

Example 1. The method of obtaining a hollow U B was carried out as follows:

1. Precursor preparation - isotropic petroleum pitch. To obtain hydrocarbons, an isotropic petroleum pitch obtained from a heavy pyrolysis resin is used, the boiling point of the pitch was 240 ° C, fiber-forming properties were provided by a fractional composition with a phase content: α ₁ - 0%, α - 26.5%, β - 64.1 %, γ - 9.4%. The temperature mode for obtaining fusible isotropic petroleum pitch - 360°C at reduced pressure (10-13 mm Hg).

2. Introduction to the fusible isotropic petroleum pitch of CNTs until a concentration of 1% of the mass is reached.

The operation was carried out in 3 stages:

1) a suspension of ultra-long CNTs in toluene (Os.ch.) is mixed with fusible isotropic petroleum pitch in an inert atmosphere, toluene is distilled off (150°C) and the stirrer is stopped due to the solidification of the mixture;

2) heat the mixture of fusible petroleum isotropic pitch and ultra-long CNTs to 280 C and stir for 2 hours;

3) quickly cooled with constant stirring until the carbon mass solidifies and maintained at reduced pressure (~ 10 mm Hg).

3. Pitch carbon fiber molding

The formation of pitch carbon fiber is carried out in a Fourne KS 42 reciprocating machine with a die channel diameter of 250 μm. The feed rate of raw materials (capacity of the piston of the molding machine) is 0.3-1.0 cm ³ /min. The pressure in the chamber is within 0-5 atm.

4. Oxidation (oxidative stabilization) of pitch carbon fiber

Oxidation of pitch carbon fiber is carried out in an NK 6.6.6/5I4 furnace, the dimensions of the working space are 600×600×600. The oxidizing medium is air. Oxidation temperature - 250°C, access to the regime - 67 hours (for hydrocarbons with 1.0 wt.% CNT - 43 hours), time on the regime - 1 hour.

5. Carbonation

The carbonization of pitch carbon fiber is carried out at a temperature of 2200°C, the output to the mode is 8 hours. As a result, a hollow hydrocarbon is formed.

Examples 2-5. The method of obtaining a hollow hydrocarbon is carried out similarly to Example 1, but the content of ultra-long CNTs in petroleum pitch is changed within 0÷1.5% of the mass. The change in the content of ultra-long CNTs in the pitch led to a change in the technical result, as shown in table 1.

Thus, the proposed method is simplified and accelerated in comparison with analogues and allows you to create hollow carbon fibers. EFFECT: hollow carbon fibers with walls reinforced with ultra-long carbon nanotubes.

From the above examples, it can be seen that the parameters claimed in the invention are justified and make it possible to obtain a material that is a hollow carbon fiber.

Claims (2)

1. A method for producing hollow carbon fibers, including the process of heating, oxidation, followed by carbonization of carbon fiber and the formation of a cavity, characterized in that ultra-long carbon nanotubes are introduced into the carbon fiber precursor in an amount of 0.6-1.2 wt.%, from the resulting molten masses perform hot forming of carbon fiber, oxidize carbon fiber in air at a temperature of 200-300°C to obtain a gas-tight infusible crust on the surface due to the process of thermal-oxidative crosslinking and carbonize by heating in an inert atmosphere of nitrogen, argon or helium at a temperature of 2200°C. 2. The method according to claim 1, characterized in that petroleum isotropic pitch is used as a carbon fiber precursor.

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