CA1173609A

CA1173609A - Calcium intercalated boronated carbon fiber

Info

Publication number: CA1173609A
Application number: CA000404038A
Authority: CA
Inventors: Raymond V. Sara
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1981-06-22
Filing date: 1982-05-28
Publication date: 1984-09-04
Also published as: DE3266177D1; JPS588125A; EP0068752B1; JPS639046B2; EP0068752A1

Abstract

CALCIUM INTERCALATED BORONATED CARBON FIBER

ABSTRACT OF THE INVENTION

A mesophase pitch derived carbon fiber which has been boronated and intercalated with calcium possesses a low resistivity and excellent mechanical properties.

S P E C I F I C A T I O N

Description

7 ~ 6~ ~

~ he inven~ion rela~es to ~ mes~phase pitch deri~ed carbon fiber ~nd pastlcular~y t~ a carbon fiber which h~s been bor~n~ted and lntercalated with calcium.
It iB well known to 6pi~ a mes~phase pitch into a fiber, thermoset the pitch fiber ~y heatin~.~t in air, . ~ and carbo~ize ~che thermoset pitch fiber by heating the thermo-6et pitch fiber in an inert gaseous environment to an elevated temperature. ~-It is preferable ~o use mesophase pi~ch rather than isotropic pitch for producing the carbon fibers because ~he mesophase pitch derived c~rbon fiber possesses excellent mechanical properties. Furthermore, it is preferable to use a mes~phase pitch having a mesophase conten~ of ~t least about 70% by weight or the proce~s.
Carbon fiber~ have found a wide range of commer-cial uses. In certain uses, it is desirable to use carbon fibers which possess both excellent mechanical properties and good electrical conductivity. The electrical conducti-vity is usually described in terms of resi~tivity. Typically, a mesophase pitch derived carbon fiber which has been carbon-ized to a temperature of ~bout 2500C has a resisti~ity of abou~ 7 microohm-meters ~nd ~ Young~s modulus of about 413.6 GPa. The ~ame carbon fiber heat treated ~o about 3000DC has a resistivity of about 37 3 microohm~meter~.
The cost for obtaining temperatures of 2500~C and particularly 3000~C is very higho Not only ls i~ costly to expend the energy ~o reach the high ~empera~ures, but the equipmen~ needed to reach such high temperatures is cos~ly and deteriorates r~pidly due to the elevated temperatures.

2.

i 13033 ~3l73~;~)g The pre~ent invention allows the production of a mesophase pitch derived carbon fiber having ~ resi~tivity ~f less than about 2 microohm-meter with a maximum heat tre~tin~ temperature of from about 2000C to about 23~0C
and preferably about 1 microohm-meter.
The present invention relates to a m~sophase pi~ch _ derived carbon fiber which has been boronated and intercalated with calcium.
The preferred embodiment teaches a calcium ~o boron weight ratio of abou~ 2:1 in the carbon fiber.
In the absence of boron, the calcium does not intercalat~ into the carbon fiber 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 fibers.
For any given amount of boron in a carbon fiber, 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 ~oron acts as an acceptor and the calcium ~cts as an electron donor.
The interaction between the boron and the calcium is such that a maximum re~is~ivity is reached and then the resistivity is reduced ~ntil a minimum is reached for a calcium to boron weig~t ratio of sbout 2:1. Apparently high conductivity is associated with the donor st~te. As the amount of calcium increases so that the ratio is greater than 2:1, the resistiv-ity increases because a multiple phase condition exists.
Generally, if one were to boronate a carbon fiber ln the absence of calcium, the maximum amount o boron which could be introduced intD the carbon fiber is abou~ 1.2% by weight. The presence of the intercalated calcium, howe~er, ~ubstantially ~ncreases the max~mum amount of boron. It is , 13033

3 6~ ~

expected that about 10% by weight or more ~f boron can be introducgd in~o the carbon fiber in the presence of the intercalated c~lcium. In addition, ~t ~6 expected that ~s ~uch as 20% by weight ~f calcium can be intercalated int~
the carbon fiber in ~he presence of the boron._ - .Surprisingly, the boron and calc~um can be int~o-d~ced ln~o the carbon fiber without chemically ~eacting with ~he carbon fiber ~o that a ~ingle phase i~ maintained_ Heat treatments at elevated temperatures can result in the formation of a new phase, calcium borographite ~ t i~ believed that the presence of the intercalated calcium results in cr~ss-~inking between layer planes in the carbon fiber and improved mechanical properties are obtained.
Excellent values for tensile ~trength and Young'c modulus are obtained for the calcium intercalated boronated fibers even though relatively low process temperatures are used.
For example, a carbon fiber according to the invention which has been produced using a proce~s temperature of about 2000~C possesses mechanical properties comparable to a con ventional me~ophase pitch derived carbon fiber which has been sub~ected to ~ process temperature of 3000C. In addition, the carbon fiber according to ~he i~vention po~sesses much low~r re~i~tivity compared to the co7lventional carbon fiber.
Surprisingly, the carbon fiber according ~o ~he i~-vention possesses a relatively high interlayer spacing as compared to ehe typical interlayer ~pacin~ of 3.37 Angstroms of a carbon fiber which has been sub3ected eo a heat treat-ment of about 3000C. According to the prior art, one would expPct a deterioration of mechanical properties for larger 30 values of interlayer spacing for the earbon fibers. The ~36~Y 13033 maximum inter~ayer spacing occurs for a calcium to boron weight ratio of a~out 2:1 as in the case for the minimum resistivity.
Generally, about 0,5% by weight boron and about 1% by weight calcium provides a good quality carbon fiber according to the invention.
The present lnvention also relates to the method of producin~ a mesophase pitch deri~ed carbon fiber having a low resistivity and excellent mechanical properties, and comprises the steps of producing a mesophase pitch derived carbon fiber ~om a mesophase pitch having a mesophase con-tent of at least about 70% by weight mesophase, boronating the fiber, and intercalating the fiber with calcium.
The steps for boronating and intercalating can be carried out simultaneously or consecutively, boronating being first.
The preferred em~odiment is to carry out the method to produce a calcium intercalated boronat d carbon fiber havin~ a calcium ~o boron weight ratio of about 2:1.
l'he boronating can be carried out with elemental boron, boron compounds, or a gaseous boron eompound. A cal-cium compound 6uch as CaNCN ca~ be used. Oxygen containin~
compounds of calcium are le~s desirable because of the poss-ible detrimental effect of the oxygen on the carbon fiber.
Boronating up to about 1,2% by weight maintains a single phase in the carbon fiber. Greater amounts of boron tend to produce boron carbide, B~C.
In carrying ou~ the instant inYen~iOn, the carbon fiber has a diameter of less than 30 microns and preferably a~out 10 microns.
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Further ob~ect~ and sdvantsge6 of the invention ~ill be set forth in the following specification and in p~rt will be obviou~ ther~fr~m without specifically bein~
referred to, the same being reallzed and ~ttained as pointed out in ehe claims here~f, Illustratlve, non~limiting example~ of the practice of the invention are ~et out below. Numerous other examples ean readily be evolved in the light of the guidin~ pr1nciples and teachings c~ntained herein. The examples given hcrein are intended to illustrate the invention and not in any ~ense limit t~e manner in which the invention can be prac-ticed, The examples were carried out using mesophase pitch derived carbon fibers having diameters of about 8 microns. The mesophase pitch used to produce the fibers had a mesophase content of about 80% by weight.
The car~on fiber~ were produced using conventional methods and were carbonized to ~bout 1700C. Lower or higher carbonizing ~emperatures could have been usedO The use of carbon fibers made the handling of the ibers simple because of the mechanical properties exhibited by carbo~ fibers.
The best mode used ~n the examples simultaneously boronated and calcium intercalated the carbon fibers. This does n~t preclude the advantage of c~nsecutive ~reatments for commercial operations. The method used is as follows.
Finely ground gr~.~hi,:~, so-called graphi~e flour, was blended with elemental boron powder. The weight per-cent~ge of boron was selected to be abs~ut the- desired weight percentage for the carbon fibers. This mix~ure amounted to ~bou~ 6~0 grams and was roll-milled or about 4 hour~ to . . .

mix and grind the graphite and bolsn ~horoughly, The mixture was then calcined in an argon atmosphere at a temperature of about 2500C for about one hour. Any inert atmosphere ~uld have been satisfactory.
_ The boronated graphite flour was blended ~ith CaNCN powder having particles less than about 4~ microns to form a treatment mixture. The amount of CaMCN is_ determined by ~he amount of calcium to be intercalated.
The weight of the carbon fibers being treated as compared to the amount of the treatment mixture used is very small. As a result, the weight ~ercentage of the boron in the treatment mixture is about the s~me for the combinati~n of the carbon fibers and ~he treat-ment mixture. This simplifies ~he selection of a pre-determined weight percentage of boronating for the carbon fibers.
The am~unt of calcium lntercalation must be de~ermined experimentally by varying the amount of ~he calcium compound used and the treatment time.
It should be recognized tha~ the vapor pressure of the boron i~ much lower than ~he calcium. The boronation is a result of the atomic diffusion whereas th~ intercalation of calcium is a result of vapor diffusion.

7.

~ ~ 7 3 ~ g For each example, six carbon fibers were used and each fiber ~ad a length ~f about 10 cm. Each of the carbon fibeIs was suspen~ed inside a ~raphite container using a Draphite formO '~he graphi~e form maintained the carbon fiber in a preselected position while the tre~*ment mixture - was added.to the graphite container. The treatment mixture was vibrated around each carbon fiber ~o obtain a uniform and packed arran~ement. ~_ 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 5 x 10-~ Torr prior to the heat treatment and then purged with ar~on during the heating cycle. An inert gas other than argon could be used.
The process could be carried out using BC13, boranes or water sol~ble salts such as H3B03. In addition, CaC12 could have b~en 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:
Examples 1 to 18 were carried out to obtain about 0.5Y~ by weight of boron in the carbon fibers and varying amounts of intercalated calcium. The maximum temperature for the heat treatment was 2050~C, Table 1 shows the results of the Examples 1 to 18.
The amount of the intercal2ted calcium vari~d from about 0.5% to about 3.6% by wPight. The Young's m~ulus for each of the carbon ibers was extremely high and the tensile strength was also very good. The resistivity showed a ~L~L73~

m~nimum of ~bou 1.8 ~c~Dhm~ t~rs iEor about 1/D 'by w~ight calcium. The lnte~layer ~pacirlg~ C~/2 ~as about a maximum for ^chat v81Ue .

0.5V/~ B~ron - ~oung~
Ca ~n Fiber ~esisti~ity Tens~leModulus ~c~/2 Example % t''L- ~G Pa G Pa R _ Do S 2 ~ 92 ~ 28 44E~-~ 3 ~ 4176 2 0~8 3~8 1~80 5513~4217 3 1 ~ O 1 ~ 81~ 33 4~33 ~ 4224

4 0~5~ 3~5 1~90 5453~4091 ~i 0~6 2~7l~E30 5933~4158 6 0~7 3~6 1~88 ~i583~4174 7 0~7 4~3 1~69 64E~3~4219 B 0. 6 4~ 714 S6 4E~93 ~ 4229 9 0. 8 2 . 91 . S8 5~63 . 4248 0 . 9 1. ~ 8 614~ . 41g8 11 0~ 9 1 ~ 81~ 58 7Z43 ~ 4133 12 0 ~ 9 2 ~ O1 ~ 43 6~413 ~ 4147 13 1 ~ 2 1 ~ 5~L ~I 32 6343 ~ 4205 14 2 . 3 2 . 11 . 84 7383 . 4174 2 . 0 2 ~ 31 . 48 ~843 . 4141 1~ ~ 2 . 6 1. ~i . 44 6623 . 4062 17 ~ 1 . 41 . 25 6~-3 . 4~82 18 3~ 6 1 . 80 . 79 ~0 3 O 403~

Examp le ~ 19 t~ 40 E~amples 19 to 40 ~ere carried out to obtain about 1.0% by weight of boron ~n ~he carbon fibers and Yaryin~ amounts of lntercalated calcium. The_maximum temperature for the heat ~reatment was-2050~C
Table 2 shows the results of the Examples 19 to 40. By interpolation, it can be ~en that as in Examples 1 to ~8, a calcium to boron weight ratio of 2:1 results in the lowest resistivity, about 1.1 micro-ohm-meters, and a large value for the interlayer spacing.

10.

~ 0 ~ 13Q33 TABL~ 2 lV/o Boron Y~ung's Ca in Fiber Resisti~ity Tensile M~dulus Col,2 Exampl.e ~ L- ~ G Pa _ G Pa _ A
19 1.5 4.8 1.89 6413.43~1 0.4 4.3 2.07 4763.4120 _ 21 0.5 2.3 1.9~ 77g.3.3833 22 1.3 4.3 2.53 7863.4348 23 1.~ 3.3 1.85 6923.~265 24 1.5 2.8 1.~3 7453.4638 1.6 3.4 ~.92 66g3.4564 26 1,~ 5.0 1.96 7173.4534 27 I,8 4.4 Z.12 6893.4610 28 1.6 2.3 2.1~ 7583.4540 29 1.8 3.0 1.52 ?173.4571 2.2 1.4 1.33 6273.45~9 31 ~.9 1.7 0.8~ 4483.4488 32 1.9 1.1 1.54 5863.4520 33 3.2 2.0 . 0.58 34~3.4549 34 2,5 1.5 1.15 55~3.4461 4.7 2.3 0.41 3583.4288 36 4.3 2.~ ~.39 33~3.~3~8 37 6.2 2.~ 0.50 2903.43~4 38 5.4 2O0 0.50 3523.44~2 39 6.5 1.7 0.56 4623.4486 8.9 2.2 0.70 5523.4392 36~g Examples 41 to 58:
Examples 41 to 58 were carried out to obtain about 2.0~/~ by weight of b~r~n in the carbon fibers and varyin~ amounts of intercalated c~lcium. The maximum ~empera~ure for the heat treatment was 1600C....
- ,Table 3 shows the r~sults of Examples 41 to 58;
The ~alues of the resistivity are not as good as the Examples 1 to 40. The lowest resistivity is for ~
calci~m to boron weight ratio of abGut 2:1. The value for the Young's modulus for each carbon iber is fairly higho 1~ .

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2% Boron_ Young's Ca in Fiber Resisti~i~y Tensile Modulus C~2 Example % ~L-V~ G Pa G Pa _ A
41 ~.2 7.5 2.62 400 3.4202 42 0.2 7~6 2.62 365 3 D 4242 ~ 43 ' ~,3 7.7 2.48 338 3.4324 44 0.7 7.3 2.59 393 3.4283 1.2 6.8 . 2.29 407 3.4179 46 1.8 5.8 1.98 ~20 3.4209 47 2.3 7.1 1.86 ~27 3.4238 48 2.6 5.6 2.03 427 3.4383 49 2.6 4.0 2.38 414 3.436 3.3 4,2 1.97 400 3.4291 51 4cO 3.~ 2.~5 427 3.4483 52 S.l 3.3 1.96 434 3.4491 53 501 3.8 1.27 400 3.4444 54 6.4 4.0 1.32 448 3.4559 6.8 ~.2 1.~3 455 3.4326 56 8.0 4.7 1.13 420 3.4~86 57 8.5 3.5 1.16 510 3.4381 58 . 12.5 4.2 1.23 786 3.433 . 13q33 :~L73tiC~

Examples 59 to 7~:
~ xamples 59 to 75 were carried out to obtain about 2.0% by weight of boron in the carbon fibers as in ~he Examples 41 to 58 exeept that the maximu~ temperature fvr the heat treatment was 2050~C.
Table 4 shows the results of the Exa~ples 59 to 75.
The Examples 5~ to 75 produced much lower ~alues for resistivity than the Examples 41 to 58. The lowest resistivity and highest interlayer spacin~ can be inter-polated to be at a calcium to boron weight ratio of about 2:1. The Young's modulus and ~ensile strength for each of the carbon fibers is excellent.

1~.
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, ~3033 3 ~73~

TABLE_4 2'70 Boron Young ' s Ca in Fib er Re s i s t ivi ty Ten s i l e M~ du l us Co ~ 2 Example % ~ ~ G Pa _G Pa _A
59 O 2. ~ ~.25 6~9 3.381 0.7 2.5 1.6D - 593 3.4003 - 61 3. ~ 2.9 1.31 689 3, s3go 62 0.4 2.8 2.06 641 3.3964 6~ 0.6 2.9 2 12 620 ~-3,4950 lD 64 0.9 2.6 2.07 738 3.4302 ~ . ~ 2.6 1,68 662 3.4489 66 2.9 2.8 1.60 5513 . 4717 67 3.1 2.6 2.11 586 3.4957 6~ 3.2 3.4 1.37 ~27 3.5077 69 3.5 2.5 1.73 ~79 3.5136 3.6 2.0 ~ .48 579 3.5222 71 4.8 1.5 0. ~9 510 3.5293 72 4.5 1. ~ 1 O 25 ~76 3,5349 73 5.1 1.5 1.52 565 3.5027 74 5.1 ~ .5 1.80 634 3. ~930 6. ~ 1.8 0.97 551 3.4886 ~ ~ 3 6 Examples ?6 to 93 Examples 76 to 93 were c~rr$ed out t~ obtain about 2,0X by weight of boron $n the carbon fibers RS in ~he Example~ 41 to 75 except that the maximum temperature or the heat treatmen~ wa~ about 2300~C.
Table S ~h~ws the result6 of the Examples 76 to 93.
The Examples 76 to 93 compare well with ~he Examples 59 to 75.

. TABLE 5 2% Boron Young's Ca in ~iber Resisti~7i~y Tensile M~dulus. Co~2 , Example _ % _ ~f~_nn_ _ Pa G Pa A
76 1 D 9 2.3 1.82 551 3.4385 77 2.5 2.5 1.1~ _~10 3.4585 78 1.1 2.3 0.~6 420 3.3896 79 1.1 2.6 1.70 572 3.4410 1.4 2;4 1.63 558 3.4339 ~1 1.5 2.5 1.~9 724 3.44~2 82 1. S 2 . 3 2. 34 538 3 . 44D5 IB3 1 . 4 2 sl 3 ~ . ~9 ~2~ 3 . 4312 84 2~5 2.3 2.37 696 3.4~81 85, 2.5 ~.~ 2.30 682 3.4~71 867 2.~ 2.3 ~.30 724 3.4667 87 2.4 2O2 2~54 731 3.4752 8~ 2.~ 2.6 1.93 662 3.4913 8g 5.1 1~2 1.90 772 3.507~
~0 6.1 1.~ 1.91 -. ~89 3.~9g2 ` ~1 5.7 1~2 1.9~ 890 ~,5232 ~2 7.0 ~.2 1.~ ~58 3.4~54 93 ~ l.S 1.14 517 3.5159 1~ ~

~ ~7 3 ~ ~

While a maximum temperature for the heat treat-ment can exceed 2300C, there is a reduction of mechanical properties of the fibess when ~he maximum tempera~ure exceeds 2500~C.
Examples 94 to 109:
Ex~mples 94 to 109 were carried out to obtain about 5% by weight of boron in the carbon fibers. The maxi-mum temperature for the heat trea~ment was abou~ 2050C.
Table 6 shows the results of the Examples 94 to 109.
The'Examples 94 to ios do not include the prefer- -red calcium to boron wei~ht ratio but the trend of resisti-vity versus calcium content shows the characteristic increase in resis~ivity for a calcium to boron weight ratio less than 2:1. In addition, the interlayer spacing inCreasPs from a calcium content of about 3.8% to 8.5% by wei~ht ~nd would be expected to be a maximum at about 10% by weight in accordance with the inYentiOn.

TAB~E 6

5% Boron Young's Ca in Fiber Resistivity Tensile Modulus Co~2 Example JO ~ n~ G PaG Pa A
94 ~.6 2.~ 1.43531 3.3928 2.0 ~ .6 1.70462 3.4435 96 3,2 2.6 1.27- 446 3.5160 ~ g7 . 2.8 2.'6 1.58572 3.4830 98 3,~ 2.8 1.40531 3.4822 ~9 4.3 2.8 ~ .61 503 -~ 3.5089 100 2;5 2.9 2 20~89 3.5134 101 3.2 3.0 1,57~00 3.5134 102 3.9 3- 3 2.21~58 3.5473 103 4.5 3.3 1. ~6 579 3.5306 104 408 3O4 0.88517 3.5367 105 _ 607 3.0 0.37317 3.5316 106 ~ 7 3. ~ 0.34290 3.5614 107 8.0 3.6 0.29241 3,57~1 108 8.0 3O4 ~79324 3.583 109 8.5 6.0 0.33186 3.6007 I wish it ~o be understood that I do no~ desire to be limited to the exact details of construction shot~
and described, for obvious modification~ will vccur to a pe~son skilled in the ~rt.
Having ~hus described the inven~ion, what I claim as new and desire to be secured by Letters Patent, is as follow.s:

18.

Claims

WHAT IS CLAIMED IS:

1. A mesophase pitch derived carbon fiber which has been boronated and intercalated with calcium.

2. The carbon fiber of claim 1, wherein the calcium to boron weight ratio in said fiber is about 2:1.

3. The carbon fiber of claim 1, wherein said fiber contains at least about 0.1% by weight boron.

4. The carbon fiber of claim 1, wherein the re-sistivity of said fiber is about one microohm-meter.

S. The carbon fiber of claim l, wherein said fiber contains up to about 10% by weight boron and up to about 20% by weight calcium.

6. A method of producing a mesophase pitch de-rived carbon fiber having a low resistivity and excellent mechanical properties, comprising the steps of producing a mesophase pitch derived carbon fiber from a mesophase pitch having a mesophase content of to least about 70% by weight mesophase, boronating said fiber, and intercalating said fiber with calcium.

7. The method of claim 6, wherein said boronating and intercalating are carried out simultaneously.

8. The method of claim 6, wherein said inter-calating step is carried out subsequent to the boronating step.

19.

9. The method of claim 6, wherein said boronating and intercalating steps are carried out to pro-duce a calcium to boron weight ratio of about 2:1 in said fiber.

10. The method of claim 9, wherein said fiber contains at least about 0.1% by weight boron.

11. The method of claim 6, wherein said boronat-ing step is carried out with elemental boron, BCl3, boranes or water soluble compounds of boron.

12. The method of claim 6, wherein said inter-calating step is carried out using CaNCN or CaCl2.

20.