Processes for the production of high-stiffness ex-cellulose carbon fibers and carbon fibers

Abstract

Carbon fibers (CF) uniquely combine high strength, stiffness, and low density. This combination of properties makes them indispensable for lightweight construction, a key technology on the path to the desired energy transition with greater climate neutrality and resource efficiency. CFs are used particularly in the mobility sector, for example in aerospace and automotive, where their use enables significant weight savings and thus reduces fuel consumption. Furthermore, CFs are also used in other end applications, including wind turbine rotors, hydrogen pressure tanks, sporting goods, and medical devices. Almost all CFs used in these applications are produced from polyacrylonitrile copolymer (PAN)-based precursor fibers, which together account for a market share of 96 to 98%.However, CFs based on PAN precursors (ex-PAN CF) are petroleum-based, cost- and CO2-intensive to produce, and generate large quantities of toxic substances such as hydrogen cyanide during the conversion process. 7 20 wt.%). Therefore, bio-based alternatives based on natural resources such as cellulose or CF produced from it (ex-cellulose CF) are experiencing a renaissance, as they do not have the disadvantages listed for PAN, and in comparison, potentially enable a reduction in CO2 emissions as well as production costs.

Description

[0001] Carbon fibers (CF) uniquely combine high strength, stiffness, and low density. This combination of properties makes them indispensable for lightweight construction, a key technology on the path to the desired energy transition with greater climate neutrality and resource efficiency. CFs are used particularly in the mobility sector, for example in aerospace and automotive, where their use enables significant weight savings and thus reduces fuel consumption. Furthermore, CFs are also used in other end applications, including wind turbine rotors, hydrogen pressure tanks, sporting goods, and medical devices. Almost all CFs used in these applications are produced from polyacrylonitrile copolymer (PAN)-based precursor fibers, which together account for a market share of 96 to 98%.However, carbon sulfates based on PAN precursors (ex-PAN CF) are petroleum-based, costly and CO2-intensive to produce, and generate large quantities of toxic substances such as hydrogen cyanide (~20 wt%) during the conversion process. Therefore, bio-based alternatives based on natural resources such as cellulose, or carbon sulfates derived from it (ex-cellulose CF), are experiencing a renaissance, as they do not have the disadvantages listed for PAN and, compared to PAN, potentially enable a reduction in CO2 emissions and production costs.

[0002] A significant disadvantage of ex-cellulose CFs, however, is their low material yield of approximately 15 wt% and their poor mechanical properties, particularly their stiffness (tensile modulus of elasticity) of a maximum of 130 GPa. In comparison, commercially available CFs based on PAN (ex-PAN CF) exhibit a tensile modulus of elasticity of 230-270 GPa (HT type) or 280-300 GPa (IM type) and a material yield of approximately 45 wt%.

[0003] Approaches to increasing the material yield of ex-cellulose CFs are described in the scientific literature and represent the state of the art. These approaches include the addition of Lewis acids or flame-retardant substances to the cellulosic starting fiber material, such as ammonium tosylate, ammonium dihydrogen phosphate, or phosphoric acid, etc. (Parry, A.; Windle, A. Carbon fibers from cellulosic precursors: A review. J. Mater. Sci. 2012, 47, 4236-4250; Spörl, JM; Beyer, R.; Abels, F.; Cwik, T.; Müller, A.; Hermanutz, F.; Buchmeiser, MR Cellulose-Derived Carbon Fibers with Improved Carbon Yield and Mechanical Properties. Macromol. Mater. Eng. 2017, 302, 1700195.), or of additives such as lignin or carbon black (Bengtsson, A.; Hecht, P.; Sommertune, J.; Ek, M.; Sjöholm, E. Carbon Fibers from Lignin-Cellulose Precursors: Effect of Carbonization Conditions. ACS Sustain. Chem. Eng.2020, 8, 6826-6833; Zhang, Xin; Lu, Yonggen; Xiao, Hao; Peterlik, Herwig (2014): Effect of hot stretching graphitization on the structure and mechanical properties of rayon-based carbon fibers. In: Journal of Materials Science 49 (2), pp. 673-684. DOI: 10.1007 / s10853-013-7748-0), the material yield, starting from 15 wt.%, can be increased to 40 wt.%.

[0004] However, all these solutions have in common that, with regard to mechanical properties, only a tensile modulus of less than 130 GPa is achieved, and therefore an application as a reinforcing fiber is not suitable, since the requirements regarding the tensile modulus of less than 170 GPa are.

[0005] The only known way to produce ex-cellulose CF with a train E-modulus greater than 170 GPa is in Fig. 1 (Procedure A) is shown. Fig.Figure 1 shows a schematic representation of the thermo-mechanical process for producing ex-cellulose CF with a tensile modulus of elasticity of 130 to 600 GPa and p less than 25 µΩm. The process direction is from left to right. While process path A describes the prior art procedure with four thermal process stages, process path B shows an example of a process according to the invention with only three thermal process stages, which is discussed further below.

[0006] According to the prior art and in Fig. The procedure described in section 1 (Process A) is used in the CF manufacturing process according to the Fig.1 (Process A) a fourth process stage, the so-called graphitization or UHT stage (6), is carried out at temperatures of 2000 to 3000 °C (see also: Zhang, Xin; Lu, Yonggen; Xiao, Hao; Peterlik, Herwig (2014): Effect of hot stretching graphitization on the structure and mechanical properties of rayon-based carbon fibers. In: Journal of Materials Science 49 (2), pp. 673-684. DOI: 10.1007 / s10853-013-7748-0 (Morgan, P. Carbon Fibers and Their Composites; Taylor & Francis: Boca Raton, FL, USA, 2005; ISBN 0824709837). In this temperature range, the previously carbonized fiber material becomes plastic, can be stretched, and oriented along the fiber axis. The technical feasibility of this process was already demonstrated by Union in the 1970s. Carbides were shown, with commercially available ex-cellulose CF fiber types such as Thornel 25 and Thornel 50, which exhibited Young's modulus values ​​of 170-375 GPa. However, the fourth orThe additional UHT process stage (6) resulted in extremely high production and investment costs (CAPEX and OPEX), so that the production of ex-cellulose CF produced in this way was completely discontinued for economic reasons in 1978 (Morgan, P. Carbon Fibers and Their Composites; Taylor & Francis: Boca Raton, FL, USA, 2005; ISBN 0824709837).

[0007] However, such a process is extremely complex, as high temperatures are essential. Furthermore, these high temperatures often result in additional material loss in the carbon fibers and enormous wear and tear on the technical processing equipment.

[0008] Based on the known state of the art, the object of the present invention is therefore to provide an efficient and less expensive method with which higher tensile E-modulus values ​​(E > 130 GPa) and lower specific electrical resistances can be achieved in carbon fibers.

[0009] This problem is solved by the features of claim 1. Claim 13 specifies carbon fibers according to the invention. The respective dependent claims represent advantageous further developments.

[0010] The present invention thus relates in a first aspect to a process for the production of carbon fibers, in which cellulosic multifilament yarn is thermally stabilized, after stabilization of a first carbonization stage and after completion of the first carbonization stage is fed to a second carbonization stage, wherein the second carbonization stage is carried out at higher temperatures than in the first carbonization stage, wherein at least one boron-containing substance is applied to and / or incorporated into the cellulosic multifilament yarn before the second carbonization stage is carried out, and in the second carbonization stage the cellulosic multifilament yarn is stretched.

[0011] Based on the invention and the underlying process, ex-cellulose CF with tensile E-modulus values ​​of 130-600 GPa and a specific electrical resistance < 25 µΩm can be produced at process temperatures between 1000 and 2000 °C ( Fig. , process route B). Surprisingly, this avoids the uneconomical process step of UHT treatment (6) at 2000 to 3000 °C and allows ex-cellulose CF to be produced on conventional carbonization plants designed for, e.g., ex-PAN CFs.

[0012] Carbon fibers (CFs) represent an intermediate product in the value chain, which is further processed into technically sophisticated and high-strength composite materials or components, particularly in lightweight construction. The claimed process leads to ex-cellulose CFs with the special technical characteristic of a high tensile modulus of elasticity (E-modulus) of 130-600 GPa in combination with a very low specific electrical resistance < 25 µΩm. Furthermore, the process is characterized in that, compared to the prior art, the resulting CFs can be produced more cost-effectively and with a better CO2 balance (lower CO2 emissions). The CFs produced according to the claimed process and the resulting components would be characterized by lower weight (lightweight construction) and better electrical and thermal conductivity properties, as well as a better CO2 balance.

[0013] Essential to the process according to the invention is the timing of the introduction or application of the substance containing at least one boron atom (hereinafter also referred to synonymously as: treatment with the substance containing at least one boron atom). This treatment takes place before the second carbonization stage and can be performed once or several times. For example, treatment is possible even before thermal stabilization. This includes, for example, treating a multifilament yarn with a boron-containing substance and then stabilizing it. The introduction or application of the substance containing at least one boron atom can also take place during the upstream production of the fiber, for example, by adding at least one boron-containing substance to a spinning solution for fiber production. It is also possible to add the substance containing at least one boron atom to a precipitation bath used in a spinning process.

[0014] Alternatively or additionally, it is also possible that the treatment, i.e. the insertion or application of the substance containing at least one boron element onto or into the multifilament yarn, can take place during or after thermal stabilization and / or during or after the first thermal treatment stage.

[0015] The process can be carried out continuously by feeding quasi-continuous multifilament yarn through the described stages. However, it is also conceivable to first produce a precursor in a separate process (i.e., a cellulosic multifilament yarn treated with at least one boron-containing substance), which is then thermally stabilized and / or carbonized at a later stage. The application or incorporation of the boron-containing substance and the thermal stabilization or carbonization can thus also be carried out locally at different locations. For example, it is possible to first impregnate the cellulosic multifilament yarn. The thermal stabilization and carbonization can then take place at a different time and location.

[0016] An advantageous embodiment provides that the total boron content of the cellulosic multifilament yarn after application or incorporation is at least 0.01 wt.%, preferably 0.01 wt.% to 5.0 wt.%, preferably 0.1 wt.% to 3.0 wt.%, and particularly preferably 0.3 wt.% to 1.5 wt.%.

[0017] The total boron content refers to the total weight content of elemental boron, based on the cellulosic multifilament yarn. The boron content can be determined, for example, using common analytical methods known to those skilled in the art, such as inductively coupled plasma emission spectroscopy (ICP-OES), etc.

[0018] In particular, the substance containing at least one boron is selected from the group consisting of inorganic and organic boron compounds, preferably selected from the group consisting of borates, boranes, boric esters, borneols, boron oxides, in particular orthoboric acid (H3BO3), metaboric acid (HBO2, 3 modifications), diboron trioxide (B2O3), trimethyl boric esters and mixtures and combinations thereof.

[0019] The incorporation or application of the substance containing at least one boron is preferably carried out by impregnating the cellulosic multifilament yarn with an aqueous solution containing at least one boron-containing substance and / or by incorporating it into the spinning solution.

[0020] In particular, the fiber made of cellulose material is immersed or passed through an aqueous solution containing at least one boron-containing substance, for example and particularly advantageously before thermal stabilization.

[0021] In particular, the cellulosic fiber is pretreated with an aqueous solution of boric acid. The cellulosic yarn can be continuously passed through a suitable bath and then dried.

[0022] The application and / or incorporation of at least one boron-containing substance onto or into the cellulosic multifilament yarn can also take place directly after a solution spinning process for the production of the multifilament yarn. In this case, it is possible for the boron-containing substance to be applied and / or incorporated into the still-wet multifilament yarn (the so-called "never-dried" multifilament yarn). Alternatively, it is also possible for the spun multifilament yarn to be dried first, and the boron-containing substance to be applied and / or incorporated into the multifilament yarn after it has been re-moistened, if necessary.

[0023] Similarly, the multifilament yarn can be mixed with the substance containing at least one boron atom during the manufacturing process, i.e., during the solution spinning process. In this case, the substance containing at least one boron atom is added to the spinning solution, and the solution spinning process for the cellulosic multifilament yarn is carried out with this solution.

[0024] Alternatively or additionally, the boron-containing substance can also be incorporated after the stabilization stage or after the first carbonization stage. The only important thing is that a certain proportion of a boron-containing substance is present in or on the fiber before the second carbonization stage.

[0025] Thermal stabilization is carried out, for example, at temperatures of 150 to 400 °C, especially 200 to 300 °C.

[0026] Preferred temperatures for the first carbonation stage are in the range of 300 to 1000 °C, while higher temperatures, for example 1000 to 2000 °C, are maintained in the second carbonation stage.

[0027] During the second carbonization stage, the cellulosic multifilament yarn is preferably stretched, preferably by a factor of 1.01 to 1.5, more preferably by 1.05 to 1.30.

[0028] In the second carbonization stage, the cellulosic multifilament yarn is subjected to a fiber tension of 1.0 cN / tex, preferably at least 2.5 cN / tex, particularly preferably at least 5.0 cN / tex in the fiber direction.

[0029] Stretching can also occur during the first carbonization stage.

[0030] In particular, the cellulosic multifilament yarn is not exposed to temperatures exceeding 2000 °C during the process.

[0031] The process according to the invention thus makes it possible to produce carbon fibers with a high tensile modulus and low specific electrical resistance in a significantly more economical way; exposure of the fibers to temperatures above 2000 °C (and thus the performance of a UHT stage) is therefore not necessary.

[0032] In particular, the process is carried out in such a way that, apart from the process stages stabilization, first carbonization stage and second carbonization stage, no further thermal process stages are carried out.

[0033] The cellulosic multifilament yarn comprises at least 50 wt.% cellulose, preferably at least 90 wt.% cellulose, and more preferably at least 95 wt.% cellulose. The cellulosic multifilament yarn can, for example, be made entirely of cellulose.

[0034] In a particularly preferred embodiment, only the cellulosic multifilament yarn is treated with an aqueous solution of the boron-containing substance, for example boric acid.

[0035] Preferably, the cellulosic multifilament yarn was produced prior to thermal stabilization by solution spinning, in particular using the viscose process, lyocell process, carbamate process, cold alkali process, cuproxide-ammonia (cuoxam) process, or by spinning the cellulosic multifilament yarn from ionic liquids. These processes are described, for example, in the textbook "Fibers - History, Production, Properties, Market", ed. Dieter Veit, Springer Verlag, 2023, ISBN 978-3-031-15308-2 (https: / / doi.org / 10.1007 / 978-3-031-15309-9).

[0036] According to a further aspect, the present invention relates to a cellulose-based carbon fiber produced as described above. The carbon fiber is characterized in particular by a high tensile modulus of elasticity and low specific electrical resistance, a combination previously unknown in the prior art. Only the process according to the invention enables the corresponding properties of the carbon fiber.

[0037] Preferably, the carbon fiber has a tensile modulus of elasticity of at least 130 GPa, preferably 130 to 600 GPa, and more preferably 150 to 500 GPa. The tensile modulus of elasticity can be measured, for example, according to ISO 11566:1996.

[0038] The specific electrical resistance of the carbon fiber according to the invention can, for example, have values ​​of < 25 µΩm, preferably of 1 to 20 µΩm, and more preferably of 4 to 12 µΩm. The specific electrical resistance can be measured, for example, according to ISO 13931:2013.

[0039] Likewise, the carbon fiber obtained is characterized by a total boron content (based on elemental boron) of 0.01 wt.% to 5.0 wt.%, preferably 0.1 wt.% to 3.0 wt.%, particularly preferably 0.3 wt.% to 1.5 wt.%.

[0040] The present invention will be explained in more detail with reference to the following embodiments, without limiting the invention to the specific parameters shown.

[0041] The present invention relates to a thermo-mechanical process at temperatures of max. 1000 to 2000 °C for the continuous production of ex-cellulose CF with a tensile modulus (E) between 130 GPa and 600 GPa and a specific electrical resistance (p) of less than 25 µΩm based on cellulosic endless multifilament yarns having a boron concentration of at least 0.01 wt.% ( Fig., Process B). The thermo-mechanical process is characterized by the fact that in the HT stage (5) in the temperature range of 1000-2000 °C, the fiber is stretched in the fiber direction, e.g., by fiber transport devices (2), thereby inducing a mechanical force or fiber tension. This causes a surprisingly graphitically ordered carbon structure to form in the presence of boron at comparatively low temperatures. This structure is oriented parallel to the fiber axis and thus produces very advantageous properties in the fiber direction. The graphitically ordered carbon structures can be unambiguously detected by X-ray diffraction methods. The higher the distortion ratio (stretching of the fiber) or the fiber tension induced by it, the higher the properties of the resulting ex-cellulose CF with regard to the tensile modulus of elasticity and the specific electrical conductivity.Such an effect is not disclosed in the scientific literature or in the prior art. For a positive effect to occur, a boron content of at least 0.01 wt% must ideally be present in or on the processing cellulosic starting fiber or precursor (1), or a boron content of at least 0.01 wt% must be contained in the intermediate fibers, e.g., between stage (3) and (4), between stage (4) and (5), or in the final C-fiber (7).

[0042] As examples 1 to 12 demonstrate, the surprising effect of the disproportionately strong increase in the ex-cellulose CF properties (tensile modulus and specific electrical resistance p) occurs when a) boron is contained in the fiber material, e.g., in a concentration of 0.3 wt% or 1 wt%, and additionally b) in the temperature range between 1000 and 2000 °C, the fiber material is stretched, thereby inducing a high fiber tension. Examples

[0043] In the following examples, an endless cellulosic multifilament yarn consisting of 1000 individual filaments (1k) was spun using the viscose process (solution spinning process) and impregnated with a boron-containing substance (boric acid) by immersing the filaments in an aqueous boric acid solution, dried, and wound onto the spool (1). The boron content, based on the yarn material, was 0.3 wt% (Table 2) or 1.0 wt% (Table 3). This fiber material was then used for CF production according to process (B). The precursor was unwound from the spool (1) and continuously transported first through the stabilization oven (3) at 200 to 300 °C using a fiber transport device (2), then through the LT carbonization oven (4) at temperatures of 300 to 1000 °C and finally through the HT carbonization oven (5) at temperatures of 1000 to 2000 °C.In the HT carbonization furnace, different distortions (v4 / v3) were set by adjusting the speed ratio of the fiber transport unit positioned at the furnace inlet (v3) and outlet (v4), resulting in varying fiber tensions (Table 1-3). As a result, CF (7) were obtained whose tensile modulus (E-modulus) increased significantly with increasing distortion ratio / fiber tension σ, and whose specific electrical resistance ρ decreased significantly.

[0044] Table 1: Process parameters distortion ratio ε and yarn tension σ in HT carbonization (1000 to 2000 °C) and resulting CF properties tensile E-modulus E and specific electrical resistance ρ based on a cellulosic 1k multifilament yarn (precursor) without boron content produced according to the viscose solution spinning process (comparative examples). Example v4 / v3 ε [%] σ [cN / tex] E [GPa] ρ [µΩm] 1 1.000 0.0 0 132 22.0 2 1.050 5.0 1.0 130 23.0 3 1.075 7.5 2.5 135 21.5 4 1.100 10.0 3.9 140 20.2 5 1.125 12.5 5.2 142 20.0

[0045] Table 2: Process parameters distortion ratio ε and yarn tension σ in HT carbonization (1000-2000 °C) and resulting CF properties tensile E-modulus E and specific electrical resistance ρ based on a boron-containing (0.3 wt.% boron) cellulosic 1k multifilament yarn (precursor) produced by the viscose solution spinning process. Example v4 / v3 ε [%] σ [cN / tex] E [GPa] ρ [µΩm] 6 (Comparison) 1.000 0.0 0 131 21.5 7 1.050 5.0 1.1 148 11.2 8 1.100 10.0 2.7 179 9.0 9 1.125 12.5 3.8 209 7.5 10 1.150 15.0 5.4 240 7.0 11 1.175 17.5 6.6 265 6.3

[0046] Table 3: Process parameters distortion ratio ε and yarn tension σ in HT carbonization (1000 to 2000 °C) and resulting CF properties tensile E-modulus E and specific electrical resistance ρ based on a boron-containing (1.0 wt.% boron) cellulosic 1k multifilament yarn (precursor) produced by the viscose solution spinning process. Example v4 / v3 ε [%] σ [cN / tex] E [GPa] ρ [µΩm] 12 (Comparison) 1.000 0.0 0 130 20.5 13 1.050 5.0 1.0 151 10.1 14 1.100 10.0 2.5 185 8.0 15 1.125 12.5 3.9 220 6.4 16 1.150 15.0 5.2 260 6.0 17 1.175 17.5 6.5 295 5.25 18 1.200 20.0 7.6 330 5.0 19 1.250 25.0 9.5 420 4.8 20 1.300 30.0 11.0 500 4.5 Legend: Procedure A - State of the art B Procedure B - claimed advantageous procedure 1 Starting fiber material, cellulosic multifilament yarn 2 Fiber transport unit Process stage 1 Stabilization (200-300 °C) 3 Process stage 2 LT Carbonization (300-1000 °C) 4 Process stage 3 HT Carbonization (1000-2000 °C) 5 Process stage 4 UHT carbonation (2000-3000 °C) 6 Carbonized multifilament yarn (carbon fiber CF) QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited non-patent literature

[0000] Parry, A.; Windle, A. Carbon fibers from cellulosic precursors: A review. J. Mater. Sci. 2012, 47, 4236-4250

[0003] Spörl, J.M.; Beyer, R.; Abels, F.; Cwik, T.; Müller, A.; Hermanutz, F.; Buchmeiser, M.R. Cellulose-Derived Carbon Fibers with Improved Carbon Yield and Mechanical Properties. Macromol. Mater. Eng. 2017, 302, 1700195

[0003] Bengtsson, A.; Hecht, P.; Sommertune, J.; Ek, M.; Sedin, M.; Sjöholm, E. Carbon Fibers from Lignin-Cellulose Precursors: Effect of Carbonization Conditions. ACS Sustain. Chem. Eng. 2020, 8, 6826-6833

[0003] Zhang, Xin; Lu, Yonggen; Xiao, Hao; Peterlik, Herwig (2014): Effect of hot stretching graphitization on the structure and mechanical properties of rayon-based carbon fibers. In: Journal of Materials Science 49 (2), S. 673-684. DOI: 10.1007 / s10853-013-7748-0 [0003, 0006] Morgan, P. Carbon Fibers and Their Composites; Taylor & Francis: Boca Raton, FL, USA, 2005; ISBN 0824709837

[0006] Fibers - History, Production, Properties, Market“, Ed. Dieter Veit, Springer Verlag, 2023, ISBN 978-3-031-15308-2 (https: / / doi.org / 10.1007 / 978-3-031-15309-9

[0035] ISO 11566:1996

[0037]

Claims

[1] Method for producing carbon fibers in cellulosic multifilament yarn thermally stabilized, after successful stabilization of a first carbonization stage and after completion of the first carbonization stage, it is fed into a second carbonation stage, the second carbonation stage being carried out at higher temperatures than in the first carbonation stage, characterized by , that prior to carrying out the second carbonization stage, at least one boron-containing substance is applied to and / or incorporated into the cellulosic multifilament yarn, and In the second carbonization stage, the cellulosic multifilament yarn is stretched. [2] Method according to claim 1, characterized bythat the total boron content of the cellulosic multifilament yarn after application or incorporation is at least 0.01 wt.%, preferably 0.01 wt.% to 5.0 wt.%, preferably 0.1 wt.% to 3.0 wt.%, particularly preferably 0.3 wt.% to 1.5 wt.%. [3] Method according to any one of the preceding claims, characterized by , that the at least one boron-containing substance is selected from the group consisting of inorganic and organic boron compounds, preferably selected from the group consisting of borates, boranes, boric esters, borneols, boron oxides, in particular orthoboric acid (H3BO3), metaboric acid (HBO2, 3 modifications), diboron trioxide (B2O3), trimethyl borates and mixtures and combinations thereof. [4] Method according to any one of the preceding claims, characterized by, that the application and / or incorporation of the substance containing at least one boron is carried out by impregnating the cellulosic multifilament yarn with an aqueous solution containing the substance containing at least one boron and / or by incorporating it into the spinning solution. [5] Method according to any one of the preceding claims, characterized by that the application and / or incorporation of at least one boron-containing substance onto or into the cellulosic multifilament yarn takes place directly after a solution spinning process for the production of the multifilament yarn and after washing, if necessary. [6] Method according to any one of the preceding claims, characterized by that the application and / or incorporation of the substance containing at least one boron is carried out by adding the substance containing at least one boron to a spinning solution, which is spun into cellulosic multifilament yarn in a solution spinning process. [7] Method according to any one of the preceding claims, characterized by that the stabilization is carried out at temperatures of 150 to 400 °C. [8] Method according to any one of the preceding claims, characterized by , that the first carbonation stage at temperatures of 300 to 1000 °C and / or the second carbonization stage takes place at temperatures of 1000 to 2000 °C is carried out. [9] Method according to any one of the preceding claims, characterized by , that in the second carbonization stage the cellulosic multifilament yarn is stretched, preferably by a factor of 1.01 to 1.5, more preferably by 1.05 to 1.

30. [10] Method according to any one of the preceding claims, characterized by, that in the second carbonization stage the cellulosic multifilament yarn is subjected to a fiber tension of 1.0 cN / tex, preferably at least 2.5 cN / tex, particularly preferably at least 5.0 cN / tex in the fiber direction. [11] Method according to any one of the preceding claims, characterized by that the cellulosic multifilament yarn is not exposed to temperatures exceeding 2000 °C during the process. [12] Method according to any one of the preceding claims, characterized by , that apart from the process stages stabilization, first carbonation stage and second carbonation stage, no further thermal process stages are carried out. [13] Method according to any one of the preceding claims, characterized by that the cellulosic multifilament yarn contains at least 50 wt.% cellulose, preferably at least 90 wt.% cellulose, further preferably at least 95 wt.% cellulose. [14] Carbon fiber produced by a method according to any of the preceding claims. [15] Carbon fiber according to the preceding claim, characterized by a tensile modulus of elasticity, measured according to ISO 11566:1996, of at least 130 GPa, preferably 130 to 600 GPa, more preferably 150 to 450 GPa. [16] Carbon fiber according to one of the two preceding claims, characterized by a specific electrical resistance of < 25 µΩm, preferably of 1 to 20 µΩm, more preferably of 4 to 12 µΩm, measured according to ISO 13931:2013. [17] Carbon fiber according to any one of claims 14 to 16, characterized by a total boron content of 0.01 wt.% to 5.0 wt.%, preferably 0.2 wt.% to 2.0 wt.%, particularly preferably 0.3 wt.% to 1.5 wt.%.

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