Calcium intercalated boronated carbon fibre

Description

[0001] The invention relates to a mesophase pitch derived carbon fibre which has been boronated and intercalated with calcium.

[0002] It is well known to spin a mesophase pitch into a fibre, thermoset the pitch fibre by heating it in air, and carbonize the thermoset pitch fibre by heating the thermoset pitch fibre in an inert gaseous environment to an elevated temperature.

[0003] It is preferable to use mesophase pitch rather than isotropic pitch for producing the carbon fibres because the mesophase pitch derived carbon fibre possesses excellent mechanical properties. Furthermore, it is preferable to use a mesophase pitch having a mesophase content of at least about 70% by weight for the process.

[0004] Carbon fibres have found a wide range of commercial uses. In certain uses, it is desirable to use carbon fibres which possess both excellent mechanical properties and good electrical conductivity. The electrical conductivity is usually described in terms of resistivity. Typically, a mesophase pitch derived carbon fibre which has been carbonized to a temperature of about 2500°C has a resistivity of about 7 microohm-metres and a Young's modulus of about 413.6 GPa. The same carbon fibre heat treated to about 3000°C has a resistivity of about 3.3 microohm-metres.

[0005] The cost for obtaining temperatures of 2500°C and particularly 3000°C is very high. Not only is it costly to expend the energy to reach the high temperatures, but the equipment needed to reach such high temperatures is costly and deteriorates rapidly due to the elevated temperatures.

[0006] U.S. Patent No. 3,692,577 describes a method of making a carbon filament comprising the steps of subjecting a carbon-containing filament to heattretmentto graphitize said carbon-containing filaments in a furnace containing an atmosphere of an inert gas and a metal adsorbable by said carbon-containing filament and vapourizable at graphitizing temperatures, said metal being taken from the group consisting of the metals from Groups 1, II, IV and VI of the Periodic Table. The carbon-containing filament starting material is stated to be regenerated cellulose, modacrylic or acrylic, e.g. polyacrylonitrile filaments and includes bundles of fibres and yarns, braids or fabrics or papers made from monofilaments, fibre bundles or yarns. Desirably, the filament is held under tension while being subjected to heating, the degree of tension depending on the particular characteristics which it is desired to achieve in the final product, high modulus and high strength resulting from high tension and low modulus and low strength resulting from low tension.

[0007] Offenlegungsschrift DE-2946414 A1 describes a method of permanently increasing the conductivity of graphite and graphite-like carbon in the form of powder, shaped bodies - including films and fibres - by intercalation characterised in that the graphite is brought into contact with a liquid intercalation mixture at a temperature in the range of from minus 30°C to plus 150°C and at a pressure of from 0.5 to 2 bar with exclusion of air, the liquid intercalation mixture comprising an organic liquid containing dissolved therein phosphorus or arsenic compounds substituted by organic radicals, dissolved or dispersed therein unsubstituted and/or substituted salts of T-metals and dispersed therein metals of Groups I to III of the Periodic System of the Elements as well as lanthanides and actimides or alloys thereof, these elements being intercalated alone or together with constituents of the organic liquid.

[0008] British Patent No. 1295289 describes the formation of graphite yarns by the graphitization of fibres of polymeric materials such as polyacrylonitrile stabilized yarns. The process comprises pretreating a carbonised or stabilised polymeric fibre with elemental boron or a boron compound in an amount sufficient to distribute boron or a boron compound therethrough in an amount sufficient to provide 0.05 to 5.0% by weight of boron and thereafter heating the pretreated fibre in an inert atmosphere to a temperature in the range of 1800―3200°C for a period sufficient to effect graphitisation thereof. The patent states that the presence of the boron during graphitization facilitates bond migration, and demonstrates that the application of tension to rayon source carbon fibres during the process to impart a stretch to the yarn of 41 % or more increased the tensile strength of the samples pretreated with boron to 36 to 85% higher than for untreated samples of the same fibre under otherwise identical conditions.

[0009] Offenlegungsschrift DE-A-1949830 discloses carbon fibres formed by carbonization of organic materials such as cotton, regenerated cellulose and synthetic resinous materials such as polyacrylonitrile fibres, having a content, essentially in the graphitic atomic lattice of the carbon fibre of up to 2.5% by weight, particularly about 0.1% by weight dissolved boron.

[0010] In our copending application EP-A-0068751 we have described and claimed a mesophase pitch derived carbon fibre having a diameter of less than 30 micrometers characterised by having at least 0.1 % by weight boron diffused in the lattice of the fibre.

[0011] The present invention provides a mesophase pitch derived carbon fibre having a diameter of less than 30 micrometers which has been boronated and intercalated with calcium.

[0012] The present invention allows the production of a mesophase pitch derived carbon fibre having a resistivity of less than about 2 microohm-metres, and preferably about 1 microohm-metre, with a maximum heat treating temperature of from about 2000°C to about 2300°C.

[0013] In the preferred embodiment there is a calcium to boron weight ratio of about 2:1 in the carbon fibre.

[0014] In the absence of boron, the calcium does not intercalate into the carbon fibre very well. Even very small amounts of boron enhance the intercalation of the calcium. Generally, 0.1 % by weight boron or even less is sufficient to improve substantially the intercalation of calcium into the carbon fibres.

[0015] For any given amount of boron in a carbon fibre, the resistivity generally increases as the amount of intercalated calcium increases at the low end, below a calcium to boron weight ratio of 2:1. It is believed that the boron acts as an acceptor and the calcium acts as an electronic donor. The interaction between the boron and the calcium is such that a maximum resistivity is reached and then the resistivity is reduced until a minimum is reached for a calcium to boron weight ratio of about 2:1. Apparently high conductivity is associated with the donor state. As the amount of calcium increases so that the ratio is greater than 2: 1, the resistivity increases because a multiple phase condition exists.

[0016] Generally, if one were to boronate a carbon fibre in the absence of calcium, the maximum amount of boron which could be introduced into the carbon fibre is about 1.2% by weight. The presence of the intercalated calcium, however, substantially increases the maximum amount of boron. It is expected that about 10% by weight or more of boron can be introduced into the carbon fibre in the presence of the intercalated calcium. In addition, it is expected that as much as 20% by weight of calcium can be intercalated into the carbon fibre in the presence of the boron.

[0017] Surprisingly, the boron and calcium can be introduced into the carbon fibre without chemically reacting with the carbon fibre so that a single phase is maintained. Heat treatments at elevated temperatures can result in the formation of new phase, calcium borographite.

[0018] It is believed that the presence of the intercalated calcium results in cross-linking between layer planes in the carbon fibre and improved mechanical properties are obtained. Excellent values for tensile strength and Young's modulus are obtained for the calcium intercalated boronated fibres even though relatively low process temperatures are used. For example, a carbon fibre according to the invention which has been produced using a process temperature of about 2000°C possesses mechanical properties comparable to a conventional mesophase pitch derived carbon fibre which has been subjected to a process temperature of 3000°C. In addition, the carbon fibre according to the invention possesses much lower resistivity compared to the conventional carbon fibre.

[0019] Surprisingly, the carbon fibre according to the invention possesses a relatively high interlayer spacing as compared to the typical interlayer spacing of 3.37 Angstroms (337 pm) of a carbon fibre which has been subjected to a heat treatment of about 3000°C. According to the prior art, one would expect a deterioration of mechanical properties for larger values of interlayer spacing for the carbon fibres. The maximum interlayer spacing occurs for a calcium to boron weight ratio of about 2:1 as in the case for the minimum resistivity.

[0020] Generally, about 0.5% by weight boron and about 1% by weight calcium provides a good quality carbon fibre according to the invention.

[0021] The present invention also relates to a method of producing a mesophase pitch derived carbon fibre having a low resistivity and excellent mechanical properties, and comprises the steps of boronating and intercalating with calcium, a carbon fibre having a diameter of less than 30 micrometers and derived from a mesophase pitch having a mesophase content of at least about 70% by weight mesophase.

[0022] The steps for boronating and intercalating can be carried out simultaneously or consecutively, boronating being first.

[0023] The preferred embodiment is to carry out the method to produce a calcium intercalated boronated carbon fibre having a calcium to boron weight ratio of about 2:1.

[0024] The boronating can be carried out with elemental boron, boron compounds, or a gaseous boron compound. A calcium compound such as for example CaNCN can be used. Oxygen-containing compounds of calcium are less desirable because of the possible detrimental effect of the oxygen on the carbon fibre.

[0025] Boronating up to about 1.2% by weight maintains a single phase in the carbon fibre. Greater amounts of boron tend to produce boron carbide, B₄C.

[0026] In carrying out the present invention, the carbon fibre has a diameter of less than 30 micrometers and preferably about 10 micrometers.

[0027] Illustrative, non-limiting Examples of the practice of the invention are set out below. Numerous other examples can readily be evolved in the light of the guiding principles and teachings contained herein. The examples given herein are intended to illustrate the invention and not in any sense limit the manner in which the invention can be practiced.

[0028] The Examples were carried out using mesophase pitch derived carbon fibres having diameters of about 8 micrometers. The mesophase pitch used to produce the fibres had a mesophase content of about 80% by weight.

[0029] The carbon fibres were produced using conventional methods and were carbonized to about 1700°C. Lower or higher carbonizing temperatures could have been used. The use of carbon fibres made the handling of the fibres simple because of the mechanical properties exhibited by carbon fibres.

[0030] The best mode used in the Examples simultaneously boronated and calcium intercalated the carbon fibres. This does not preclude the advantage of consecutive treatments for commercial operations. The method used is as follows:

Finely ground graphite, so-called graphite flour, was blended with elemental boron powder. The weight percentage of boron was selected to about the desired weight percentage for the carbon fibres. This mixture amounted to about 600 grams and was roll-milled for about 4 hours to mix and grind the graphite and boron thoroughly. The mixture was then calcined in an argon atmosphere at a temperature of about 2500°C for about one hour. Any inert atmosphere would have been satisfactory.

[0031] The boronated graphite flour was blended with CaNCN powder having particles less than about 44 micrometers to form a treatment mixture. The amount of CaNCN is determined by the amount of calcium to be intercalated.

[0032] The weight of the carbon fibres being treated as compared to the amount of the treatment mixture used is very small. As a result, the weight percentage of the boron in the treatment mixture is about the same for the combination of the carbon fibres and the treatment mixture. This simplifies the selection of a predetermined weight percentage of boronating for the carbon fibres.

[0033] The amount of calcium intercalation must be determined experimentally by varying the amount of the calcium compound used and the treatment time.

[0034] It should be recognized that the vapour pressure of the boron is much lower than the calcium. The boronation is a result of atomic diffusion whereas the intercalation of calcium is a result of vapour diffusion.

[0035] For each Example, six carbon fibres were used and each fibre had a length of about 10 cm. Each of the carbon fibres was suspended inside a graphite container using a graphite form. The graphite form maintained the carbon fibre in a preselected position while the treatment mixture was added to the graphite container. The treatment mixture was vibrated around each carbon fibre to obtain a uniform and packed arrangement.

[0036] The six graphite containers were placed in a graphite susceptor and heated inductively to a predetermined maximum temperature for about 15 minutes. The furnace chamber was evacuated to about 5x10-⁵ Torr (6.8x10-⁴ Kg per sq. m) prior to the heat treatment and then purged with argon during the heating cycle. An inert gas other than argon could be used.

[0037] The process could be carried out using BCI₃, boranes or a water-soluble compound of boron such as H₃B0₃. In addition, CaC1₂ could have been used. Of course, a wide range of other compounds for supplying boron and calcium could be realized easily experimentally in accordance with the criteria set forth herein.

Examples 1 to 18

[0038] Examples 1 to 18 were carried out to obtain about 0.5% by weight of boron in the carbon fibres and varing amounts of intercalated calcium. The maximum temperature for the heat treatment was 2050°C.

[0039] Table 1 shows the results of Examples 1 to 18. The amount of the intercalated calcium varied from about 0.5% to about 3.6% by weight. The Young's modulus for each of the carbon fibres was extremely high and the tensile strength was also very good. The resistivity showed a minimum of about 1.8 microohm-metres for about 1 % by weight calcium and the interlayer spacing, Co/2, was about a maximum for that value.

Examples 19 to 40

[0040] Examples 19 to 40 were carried out to obtain about 1.0% by weight of boron in the carbon fibres and varying amounts of intercalated calcium. The maximum temperature for the heat treatment was 2050°C.

[0041] Table 2 shows the results of Examples 19 to 40. By interpolation, it can be seen that as in Examples 1 to 18, a calcium to boron weight ratio of 2:1 results in the lowest resistivity, about 1.1 microohm-metres, and a large value for the interlayer spacing.

Examples 41 to 58

[0042] Examples 41 to 58 were carried out to obtain about 2.0% by weight of boron in the carbon fibres and varying amounts of intercalated calcium. The maximum temperature for the heat treatment was 1600°C.

[0043] Table 3 shows the results of Examples 41 to 58.

[0044] The values of the resistivity are not as good as in Examples 1 to 40. The lowest resistivity is for a calcium to boron weight ratio of about 2:1. The value for the Young's modulus for each carbon fibre is fairly high.

Examples 59 to 75

[0045] Examples 59 to 75, Example 59 being a comparative Example with no calcium, were carried out to obtain about 2.0% by weight of boron in the carbon fibres as in Examples 41 to 58 except that the maximum temperature for the heat treatment was 2050°C.

[0046] Table 4 shows the results of Examples 59 to 75.

[0047] Examples 59 to 75 produced much lower values for resistivity than Examples 41 to 58. The lowest resistivity and highest interlayer spacing can be interpolated to be at a calcium to boron weight ratio of about 2:1. The Young's modulus and tensile strength for each of the carbon fibres is excellent.

Examples 76 to 93

[0048] Examples 76 to 93 were carried out to obtain about 2.0% by weight of boron in the carbon fibres as in Examples 41 to 75 except that the maximum temperature for the heat treatment was about 2300°C.

[0049] Table 5 shows the results of Examples 76 to 93.

[0050] Examples 76 to 93 compare well with Examples 59 to 75.

[0051] While a maximum temperature for the heat treatment can exceed 2300°C, there is a reduction of mechanical properties of the fibres when the maximum temperature exceeds 2500°C.

Examples 94 to 109

[0052] Examples 94 to 109 were carried out to obtain about 5% by weight of boron in the carbon fibres.

[0053] The maximum temperature for the heat treatment was about 2050°C.

[0054] Table 6 shows the results of Examples 94 to 109.

[0055] Examples 94 to 109 do not include the preferred calcium to boron weight ratio but the trend of resistivity versus calcium content shows the characteristic increase in resistivity for a calcium to boron weight ratio less than 2:1. In addition, the interlayer spacing increases from a calcium content of about 3.8% to 8.5% by weight and would be expected to be a maximum at about 10% by weight in accordance with the invention.

Claims

1. A mesophase pitch derived carbon fibre having a diameter of less than 30 micrometers and which has been boronated and intercalated with calcium.

2. A carbon fibre according to claim 1, wherein the calcium to boron weight ratio in the fibre is about 2:1.

3. A carbon fibre according to claim 1 or 2, wherein said fibre contains at least about 0.1% by weight boron.

4. A carbon fibre according to claim 1, 2 or 3, wherein the resistivity of the fibre is about one microohm-metre.

5. A method of producing a mesophase pitch derived carbon fibre comprising the steps of boronating and intercalating with calcium a carbon fibre having a diameter of less than 30 micrometers and derived from a mesophase pitch having a mesophase content of at least about 70% by weight mesophase.

6. A method according to claim 5, wherein the boronating and intercalating steps are carried out simultaneously, or the intercalating step is carried out subsequent to the boronating step.

7. A method according to claim 5 or 6, wherein the boronating and intercalating steps are carried out to produce a calcium to boron weight ratio of about 2:1 in said fibre.

8. A method according to claim 7, wherein the fibre contains at least about 0.1% by weight boron.

9. A method according to any of claims 5 to 8, wherein the boronating step is carried out with elemental boron, BCI₃, a borane or a water-soluble compound of boron.

10. A method according to any of claims 5 to 9, wherein the intercalating step is carried out using CaNCN or CaCl₂.

Ansprüche

1. Von einem Mesophasen-Pech abgeleitete Kohlenstoffaser mit einem Durchmesser von weniger als 30 um, die bor und eingeschlossenes Calcium enthält.

2. Kohlenstoffaser nach Anspruch 1, in der das Gewichtsverhältnis zwischen Calcium und Bor in der Faser etwa 2:1 beträgt.

3. Kohlenstoffaser nach Anspruch 1 oder 2, in der die Faser mindestens etwa 0,1 Gew.% Bor enthält.

4. Kohlenstoffaser nach einem der Ansprüche 1 bis 3, in der der spezifische Widerstand der Faser etwa 1 Mikroohm-Meter beträgt.

5. Verfahren zur Herstellung einer von einem Mesophasen-Pech abgeleiteten Kohlenstoffaser, das die Schritte des Borzusatzes und des Einfügens von Calcium in eine Kohlenstoffaser mit einem Durchmesser von weniger also 30 pm umfasst, die von einem Mesophasen-Pech mit einem Mesophasen-Gehalt von mindestens etwa 70 Gew.% abgeleitet ist.

6. Verfahren nach Anspruch 5, bei dem die Schritte des Borzusatzes und des Einfügens von Calcium gleichzeitig ausgeführt werden oder bei dem der Schritt des Einfügens von Calcium im Anschluss an den Schritt des Borzusatzes durchgeführt wird.

7. Verfahren nach Anspruch 5 oder 6, bei dem die Schritte des Borzusatzes und des Einfügens von Calcium zur Einstellung eines Gewichtsverhältnisses von Calcium zu Bor in der Faser von etwa 2:1 in der Faser durchgeführt werden.

8. Verfahren nach Anspruch 7, bei dem die Faser mindestens etwa 0,1 Gew.% Bor enthält.

9. Verfahren nach einem der Ansprüche 5 bis 8, bei dem der Schritt des Borzusatzes mit elementarem Bor, BCI₃, einem Boran oder einer wasserlöslichen Borverbindung durchgeführt wird.

10. Verfahren nach einem der Ansprüche 5 bis 9, bei dem der Schritt des Einfügens von Calcium unter Verwendung von CaNCN oder CaCI₂ durchgeführt wird.

Revendications

1. Fibre de carbone dérivée de brai en mésophase, ayant un diamètre de moins de 30 micromètres et qui a reçu du bore et du calcium en intercalation.

2. Fibre de carbone selon la revendication 1, dans laquelle le rapport du poids de calcium au poids de bore dans la fibre est d'environ 2:1.

3. Fibre de carbone selon la revendication 1 ou 2, dans laquelle ladite fibre contient au moins environ 0,1% en poids de bore.

4. Fibre de carbone selon la revendication 1, ou 3, dans laquelle la résistivité de la fibre est d'environ 1 microohm-mètre.

5. Procédé de production d'une fibre de carbone dérivée de brai en mésophase, comprenant les étapes qui consistent à ajouter du bore et à intercaler du calcium à une fibre de carbone ayant un diamètre de moins de 30 micromètres et dérivée d'un brai en mésophase ayant une teneur en mésophase d'au moins environ 70% en poids de mésophase.

6. Procédé selon la revendication 5, dans lequel les étapes d'apport de bore et d'intercalation sont effectuées simultanément, ou l'étape d'intercalation est effectuée après l'étape d'apport de bore.

7. Procédé selon la revendication 5 ou 6, dans lequel les étapes d'apport de bore et d'intercalation sont effectuées de façon à donner un rapport de poids de calcium au poids de bore d'environ 2:1 dans ladite fibre.

8. Procédé selon la revendication 7, dans lequel la fibre contient au moins environ 0,1% en poids de bore.

9. Procédé selon l'une quelconque des revendications 5 à 8, dans lequel l'étape d'apport de bore est effectuée avec du bore élémentaire, du BCI₃, un borane ou un composé hydrosoluble de bore.

10. Procédé selon l'une quelconque des revendications 5 à 9, dans lequel l'étape d'intercalation est effectuée à l'aide de CaNCN ou de CaCl₂.