Influence of Boron on Carbon Fiber Microstructure, Physical Properties, And Oxidation Behavior

7/29/2019 Influence of Boron on Carbon Fiber Microstructure, Physical Properties, And Oxidation Behavior

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Cwhon Vol. 29, No 2. pp 251~269, 1991 00084223:Y I $3 I:1 00Prmtcd m Orcat l3ntsin. Copyright (_ 1991 Pqamon Prcs5 plc

INFLUENCE OF BORON ON CARBON FIBERMICROSTRUCTURE, PHYSICAL PROPERTIES, AND

OXIDATION BEHAVIORL. E. JONES and P. A. THROWERDepartment of Materials Science and Engineering. The Pennsylvania State University,University Park, PA 16802

(Recei ved 13 Febru ar _y 1990: accepted 11 May lY30)Abstract-An investigation was conducted to determine the influence of substitutional boron on carbonfiber microstructure, physical properties. and oxidation behavior in 0.1 MPa UHP Cl?. Mesophase pitchP5.5and PAN T-3OOcarbon fibers were substitutionally doped with boron at concentration levels between4 x 10m5and 0.05 B/C atom ratio. Boron was found to enhance graphitization in these fibers atconcentrations greater than 2 x 10Y B/C. Below this concentration level the fiber microstructure andstrength were unaffected. The presence of boron at all concentration levels inhibited carbon fiberoxidation. Oxidation inhibition at high boron concentrations was attributed to changes in the fibermicrostructure and specific site blockage by an oxide of boron which developed on the fiber surfaceduring gasification. At relatively low boron concentrations, the decrease in the reactivity of the fiberwas related to a change in the fibers electronic structure which, in turn, influenced the chemistry ofthe active sites. Boron was also found to significantly influence the fiber structure. Boron modulatedthe (002) reflection by enhancing the development of two separate microstructures in the same fiber.one more oxidation resistant than the other. The room temperature mechanical properties of thesedoped fibers were relatively unaffected up to 5 x 10 B/C. hut strength and modulus sharply decreasedabove this dopant level.Key Words-Carbon fibers. boron. oxidation. mechanical properties.

1. INTRODUCTION

Carbon-carbon (C-C) composites have found in-creasing applications in both the aerospace and theutiftiy communities because of their light weight andhigh performance. They possess specific strengthsand moduli which are greater than other materialsat elevated temperatures (with the possible excep-tion of beryllium composites). The demand for light-weight, high-performance materials has placed apremium on understanding the fundamental pro-cesses affecting their high-temperature heteroge-neous reactions with the surrounding environmentand developing a means of protecting them fromoxidation.

It is well established that although C-C compositespossess uniquely superior mechanical properties, thehigh-temperature application of carbon fiber rein-forced composites is limited by carbon oxidation.Under oxidizing conditions at temperatures of 770K or higher, carbon readily chemisorbs oxygen andsubsequently desorbs oxides of carbon. This oxida-tion process results in the erosion of the structureand eventually in the degradation of the propertieswhich the material originally possessed. This prob-lem is especially acute for those applications whichrequire materials survivability at high temperaturesand during repeated thermal cycling in environmentscontaining steam, oxygen, CO?, or unburned hydro-carbons. Therefore, one of the critical issues in theapplication of structural carbon materials is the in-

hibition of carbon oxidation. The protection of citr-bon-carbon composites from oxidation has beenstudied by many researchers. Much of this researchis not available in the open literature; however, afew complete discussions have been published, mostnotably by McKee et al.[l.Z] and Strife and Shee-han[3]. Very few of these studies have attempted toaddress the kinetics of carbon-carbon oxidation andthe mechanisms operative during oxidation inhibi-tion. One such laboratory study on the reactivity ofC-C in H10iH2 gas mixtures showed that this com-posites reactivity at 973 K was strongly in~uencedby the carbon fiber used in the fabrication. It wasfound that composites fabricated with PAN T-300fibers were roughly three times more reactive thancomposites fabricated with pitch P55 fibers[4].

The significance of carbon fiber oxidation can alsobe seen on the surface of three-dimensional C-Ccomposites used as nozzles for rockets[ilj. These ma-terials, when exposed to the product of combustionfrom a solid propellant, may reach surface temper-atures approaching 3,300 K and pressures as high as5 MPa. The postfired composite samples exhibitpreferential surface oxidation. Fiber ends exposedon the surface were attacked preferentially by theoxidizing environment resulting in a pockmarkedsurface topography. The cause of this preferentialattack under conditions which were clearly transportlimited were unclear. However, it is generally under-stood that the chemistry and structure of the carbonfiber ends is quite different than those of the sur-

251


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252 L. E. JONES nd P. A. THROWERrounding carbon matrix and the outer carbon fibersurfaces. This suggests that the surface chemistry ofthe C-C composite had an influence on oxidationeven under the extreme conditions typical of a nozzlethroat at the postfired end of a solid propellant rocket,and that the preferential oxidation of carbon fibersas dictated by their orientation in the composite isa significant consideration in the overall understand-ing of the carbon-carbon oxidation process.

The impact of carbon fiber oxidation has becomeincreasingly more important in the application ofcurrent advanced carbon-carbon composites in whichthe matrix is protected through the addition of glassforming inhibitors such as B,O, which melt at rela-tively low temperatures and flow to fill cracks in thematrix, providing a diffusion barrier in the compos-ite. In these materials oxidation also occurs prefer-entially at the fiber ends and/or along the fiber length.The carbon matrix is protected because the diffusionof oxygen into and through the matrix is impededby these in-situ glassy sealants which fill any porositywhether original or developed during oxidation.Currently, little is done to retard the oxidation ofthe fibers, and they tend to hollow out during high-temperature use, leaving behind a sheath of stabi-lized outer fiber skin and matrix material. From theseresults, it can be argued that fiber oxidation is criticalin these structural composites and that achieving anunderstanding of fiber oxidation behavior is one keytoward achieving enhanced overall C-C compositeoxidation resistance.

This study was conducted to address compositerecession in terms of the relationship between oxi-dation behavior, microstructure, and physical prop-erties of the carbon fibers typically used in the fab-rication of carbon-carbon composites. The objectiveof this study was to establish that carbon fiber oxi-dation can be inhibited through the use of substi-tutional dopants and to evaluate the impact of theoxidation inhibitors on the fiber microstructure andthe subsequent mechanical properties.

Boron was the dopant of interest because it had

been previously shown to be a strong inhibitor ofbulk graphite oxidation[5], where it was clearly shownto inhibit the oxidation rate by decreasing the acti-vation energy of the reaction. This paper describesthe influence of substitutionally doped boron on themicrostructure, physical properties, mechanical be-havior, and reactivity of typical pitch and PAN car-bon fibers.

2. EXPERIMENTAL2.1 Materials

Two carbon fibers were examined, a mesophasepitch (P55) fiber and a polyacrylonitrile (PAN T-300) fiber, which were both manufactured by AmocoPerformance Products. The research initially fo-cused on fiber oxidation and on the inhibition of theoxidation by substitutionally doping boron into thecarbon fibers. The two types of fibers examined inthis study had dramatic differences in their respec-tive microstructures and also in their overall reac-tivity. Relevant physical properties for both fibersare listed in Table 1. A brief description of each fiberfollows.

The P55 fiber is a mesophase pitch fiber whichexperienced a maximum heat treatment temperatureof 2,773 K during its fabrication. It has a density of2.04 g/cc and an outer diameter of 10.7 pm. ThePAN T-300 fiber is a polyacrylonitrile fiber whichexperienced a maximum heat treatment temperatureof 1,573 K during fabrication and also had a tensileload applied during fiber drawing and heat treatmentto align the layer plane structure parallel to the fiberaxis. It has an outer diameter of approximately 6pm and a fiber density of 1.74 g/cc. Based on thesedata, the geometric surface area for the fibers werecalculated.to be 0.18 mig and 0.37 mig for the P55and T-300 fibers, respectively.

Modifying the as-received (AR) fiber microstruc-ture to inhibit oxidation and understanding the roleof these modifications on the inhibition chemistry

Table 1. Physical properties of the as-received and reference carbon fibersSamplesProperties P55 AR(VSB-32) P55 HT(VSB-32) T-300 AR T-300 HT

Type Pitch Pitch PAN PANDiameter, (pm) 10.7 11.0 6.2 5.8Density, (g/cc) 2.04 2.13 1.74 1.88Surface area,h (m/g) 0.66 0.58 0.56 0.59Geometric area,< (mig) 0.18 0.17 0.37 0.37d,,,,. nm 0.3430 0.3400 0.35 0.3434X-ray density, (g/cc) 2.218 2.237 2.173 2.215Total volume, (cc/g)ore 0.040 0.022 0.115 0.081Total porosity, % 8.16 4.69 20.01 15.23Based on density (p) gradient techniques.Surface area determined by BET Kr absorption at 77 K.Calculated geometric area.dX-ray density = (0.33538/d,,) 2.268g/cc.Total pore volume = [1 p - l/x-ray p].Total porosity = 100 x total pore volume.


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Influence of boron on carbon fibers 253were the specific objectives of this work. Fiber mod-ifications were accomplished either by heat treatingor substitutionally doping with boron.

2.1.1 Reference fibers (Xl). Due to the large dif-ference between the heat treatment temperatures ofthe AR PAN (1,573 K) and AR pitch (2,973 K)fibers, reference fibers (HT) were prepared. Sam-ples of each AR fiber were heat treated to 2,773 Kfor 15 minutes under 203 KPa of Ar. The AR fibersheat treated in this manner are designated (HT). Forthe P55 pitch fibers this means that the HT samplehas seen a temperature 200 K above the doping tem-perature.

2.1.2 Fibers doped w it h boron (B). In order tostudy the oxidation of the doped fibers, P.55 fiberswere doped with elemental B so that the B/C atomratio ranged between 4 x 10e5 (40 ppm B) and 5 x10 (5% B). One boron loading of the PAN T-300fiber was performed to give a 2 x 10. B/C atomratio (2,000 ppm B)).2.2 Boron doping processIt has been previously established that the dopingprocess used during the course of this study substi-tutionaIIy positions B atoms in the graphite crystallattice[6-81. Fibers were placed in graphite cruciblesand surrounded with an ultrapure flake graphite (SP-l), which was coated with a boron-containing me-dium prior to packing the fibers. The boat was thensealed, placed in a graphite furnace, and heated at2.773 K for 30 minutes under 203 KPa of Ar (ultra-high purity, 99.99% Ar).

The packing powder, a purified natural graphitepowder. SP-1, was produced by the Union CarbideCorporation. SP-1 had an initial total surface area(BET, Kr. 77 K) of 1.8 m?ig and was guaranteed tohave less than 1 ppm, total impurities. The use of apowdered medium was necessary because of the del-icate nature of the fibers and the desire to dopeuniformly. Powdered SP-1 was either coated with aliquid organoborane for dopings up to 1.000 ppm Bor mixed with glassy B,03 for dopings above thislevel.

2.2.1 Preparation of the SP-1 Packing tedium.SP-1 was mixed with a known concentration of or-ganoborane to achieve the B dopant level desired.The organoborane used throughout the course ofthis work was tributylborate-B(OCH,CHJJHz-CH,),-manufactured by Fluka Chemicals. The pur-ity level for this borane is >98%; it has a molecularweight of 230.16 g, a boiling point of 518 K, and adensity of 0.875 g/cc at 293 K. At room temperatureit is an oily liquid. In order to ensure a uniformdistribution of the borane on the SP-1 surface, it wasmixed with methanol and added to the SP-1 powderto make a slurry. This slurry was then mixed for 1.5hours on a mechanical stirring plate, and the solventwas slowly removed by heating, leaving behind SP-1 powder with a uniform borate surface coating. Itshould be noted that the concentrations of organo-

CAR 29:2-E

borane added to the slurry are higher than the finalB dopant level in the fiber. This loss of B duringdoping was useful when trying to obtain very low(40 to 500 ppm) dopant levels.

In the SP-1 batch used to dope the fibers contain-ing 1,000 ppm B, it was not necessary to use meth-anol. T~butylborate was added directly to the SP- 1.It was mixed and dried slowly on a hot plate (toremove the organic fraction) until the SP-1 was againa dry powder. At this level organoborane addition,a uniform white coating was visible on the powdersurface, indicating the presence of the borate. Toachieve high B concentration levels (~2%). the liq-uid organoborane was substituted with glassy B,O,(99.999% purity, Aldrich Chemical Co., Inc.). Again,a specific concentration of the B,O, was ground,added to the SP-1, and mixed to obtain the 5% load-ings. It should be noted that this material was a solid,and the process of slow drying was obviously notrequired.

2.2.2 Substi tut ional doping-heat tr eatment cycle.After coating the SP-1, it was distributed uniformlyover the cut fiber sections in the graphic crucible,which was manufactured using ultrapure graphite(POCO-AXF 5Q). Once a crucible had been usedto hold fibers doped to a specific concentration level,it was not used for dopings at lower B levels. Thereason for this was simple. The amount of graphitein the crucible and lid far outweighed the relativelysmall amount of fiber and SP-1 packing. Once thecrucible had been saturated with a specific concen-tration of B, there was little change in this concen-tration when heated with the fiber and SP- I.

For the high-temperature heat treatments, the fi-bers were heated at 2,773 K for 30 minutes under203 KPa of Ar (ultrahigh purity, 99.99% Ar) in agraphite tube resistance furnace. The crucible wasplaced in the center zone of the furnace, which wassealed and evacuated to 3.1 KPa. It was then pres-surized to 23 KPa with UHP Ar. This process wasrepeated at least 3 times to ensure a negligible ox-ygen content within the furnace prior to heating.Argon was passed through the furnace at a rate of100 ccimin, while maintaining the Ar pressure inthe furnace at 203 KPa. This condition was main-tained during the entire heat treatment cycle. Anaverage heating rate of 293 Kimin was used toreach 2,773 K, at which the sample remained for30 minutes, followed by a two hour cool-down cy-cle. Temperatures were monitored using a Leedsand Northrup Optical Pyrometer (Model 8636-C)calibrated against the melting points for Al,O,(2,345 K), TiB? (3,173 K), and MO (2.890 K). Toavoid contamination of fibers during doping, an iden-tical yet clean graphite tube was substituted for theoriginal in which the caljbration was conducted.Any deviation in the temperature as a result of thesubstitution was thought to be small and to havelittle effect on the fiber preparation since the pro-cedure for doping was consistent.


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254 L. E. JONESand P. A. THROWER2.3 Fiber characterization

After doping with B, samples of each fiber alongwith the as-received (AR) fibers and the reference(HT) fibers were analyzed to assess fiber crystallitestructure, elemental boron content, surface struc-ture, and microstructural properties. Samples werealso characterized on the basis of their rate of oxi-dation in pure 02, using a thermal gravimetric ana-lyzer (TGA). The use of the TGA also providedsamples for microstructural observations in the SEMafter they were subjected to a low-level burn-off inO?. Due to the significance of the TGA in this study,its experimental procedures are described separatelyin section 2.4.

2.3.1 Element al anal ysi s-emi ssi on spectroscopy .Samples (0.02 mg) of each fiber specimen were sub-mitted for trace elemental analysis. They were groundto less than 100 mesh size and analyzed using a Spec-trametrics Spectraspan III, DC arc plasma emissionspectrometer using standards containing known con-centrations of B ranging from 100 to 5,000 ppm.Boron concentrations are reported as a ppm level oras a B/C atom ratio based upon the emission spec-troscopy results. They are considered semiquanti-tative.

2.3.2 Crystall it e propert i es-x-ray diffraction.Samples (0.05 g) of each fiber were ground in anagate mortar and pestle, sieved through a 100 meshnylon screen, and mixed with 25 wt% pure silicon(NBS SRM 640) (Table 2) as an internal standard.After thorough mixing, a thin film of each samplewas bound to a glass slide using collodion and ex-amined using a Rigaku Geigerflex x-ray diffractom-eter with CuKa radiation. Peaks were obtained byscanning at 2 20imin with a time constant of 1 sec-ond. Slower scan rates were used to locate very weakdiffraction peaks.

Silicon was chosen as the reference material be-cause its peak positions are close to those of graphite.Scans of each fiber were made between the angularranges of 24-29, 41-48, and 53-57 and angularpositions corrected using the standard Si peak po-sitions. The interlayer spaceing (d) for each fiberwas calculated using Braggs law, and crystallite sizeswere calculated from the full peak widths at half theirmaximum height (FWHM) after correcting for in-

Table 2. Exact angular peak positions forSi SRM 640 and approximate positions forthe major graphite peaksGraphiteSilicon (ref. no. 23-64)hkl 28 hkl 20

111 28.443 002 26.53220 47.303 100 42.44101 44.64311 56.123 004 54.70422 88.032 006 86.99a = 0.543088 nm, A = 0.15406 nm,T = 298 K.

strumental broadening and when possible, for crys-tallite strain[9].

The average crystallite width, L,, can be measuredfrom the widths of the (100) or (101) reflections.However, in most carbon fibers these peaks are weakand measurement is difficult. An empirical value canbe calculated from the following formula[lO] usingthe measured interlayer spacing, d, but its applica-bility to these fibers is uncertain.

d nm = 0.3354 + 0.059L,

2.3.3 Surface analy sis-Fouri er tr ansform infr aredspectroscopy (FUR). Fourier transform infraredspectra of boron-treated fiber samples were obtainedby adding 60 interferograms at a resolution of 2 cm-rusing a Digilab FTS 60 spectrometer. The instrumentwas internally calibrated with a He-Ne Laser so thatthe frequency scale was accurate to within 0.2 cm-r.The advantage of an interferometer, as opposed toa system of slits and gratings, is that the interferom-eter allows greater optical throughput to the detec-tor. An on-line microcomputer also makes it possibleto curve resolve and to apply spectral methods.

Fiber samples were prepared for FTIR analysis bycombining 25 mg of KBr with a few flakes of fiberand ground for 30 seconds. This pregrinding tech-nique was used on all samples. Additional KBr wasadded after pregrind to bring the total weight up to275 mg, and this mixture was then ground for an-other 15 seconds. Total mixing time in the wiggle-bug (sample preparation cell) and transfer time toand from the pellet pressing apparatus had to bekept to a minimum to avoid excessive adsorption ofHZO. The powdered mixture was then pressed andscanned immediately. Infrared spectra were then usedto provide information on the nature of the fibersurface and the presence of a possible B,O, coating.

2.3.4 M icrostructural observat ions-scanningel ectron mi croscopy (SEM ). A scanning electron mi-croscope equipped with an energy dispersive spec-trometer was used to examine the surfaces of allselected fibers. Samples were coated with an Auconducting layer to minimize edge effects. All fibersamples were examined using accelerating voltagesof either 5 or 10 kV.

2.3.5 Substi tut i onalposit ioning of boron-magneti csusceptibility measurements. Magnetic susceptibility(x) measurements were carried out by the Faradaymethod using a modified Cahn RG electrobalance.Samples of the ultrahigh purity flake graphite (SP-l), used as the packing medium during the dopingprocess, were characterized using diamagnetic sus-ceptibility before and after doping.

Diamagnetic susceptibility measurements provideinformation on the overall electron population of thegraphite. A pure crystalline graphite will have a dia-magnetic susceptibility of -21.5 x 10m6 mu/g alongthe layer plane dimension and -0.5 x 10m6 per-pendicular to the layer plane. The presence of sub-


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Influence of boron on carbon fibers 25sstitutional boron in crystalline graphite will lowerthe Fermi level as a result of borons behavior as anelectron acceptor. Substitutional boron will, in turn,cause the diamagnetic susceptibility of graphite todrop. This is the case only when boron is chemicallybonded within the graphite structure. The diamag-netic susceptibility will not be lowered as a result ofits presence in an interstitial site or as a surface ox-ide. Therefore, diamagnetic susceptibility is a tech-nique with which to evaluate the location of boronin the graphite crystal structure.

2.3.6 Mechanicul properties. Fiber tensile strengthand tensile modulus were determined at room tem-perature using a standard Instron frame with an LVDT(capacitance transducer) to measure displacement.Fibers were mounted to cardboard tabs using jew-elers wax and were strained at 0.005 cmisec. Fiberswere tested as individual filaments with gauge lengthsof 10 cm.2.4 Ki neti c study of f iber oxid ati on-therm algravi metri c anal ysis (TGA)

Fiber oxidation rates were measured and recordedusing a Perkin-Elmer Thermogravimetric Analyzer(TGS-2. Thermal Analysis Data Station). Data wererecorded as the percent initial weight loss per minuteand were converted into percent burn-off versus timeand to oxidation rates over specific time intervalsusing software. Data were normalized to the initialsample weight and to the instantaneous sample weight.In the experiments herein described, the initial sam-ple weight was approximately 2.5 mg. Fiber samplescut to 3 mm lengths were placed inside Pt sampleboats. They were oxidized isothermally in 0.1MPa of dry ultra-high purity. hydrocarbon free O2(> 99.99%, Linde Division of Union Carbide). Iso-thermal oxidations were conducted on each fiber atseveral temperatures between 923 and 1,073 K. Gasflow rates were maintained at 40 cc (STP)/min dur-ing each experimental run.

Prior to the introduction of 02, samples were heatedto the temperature of interest in 0.1 MPa of N2 (ultra-high purity, >99.99%). Once the oxidation temper-ature was established, the sample was allowed tosoak in N1 for approximately five minutes, after

which the reactant gas (0,) was introduced and theexperiment initiated. Rates obtained for the con-sumption of carbon in oxygen are expressed in termsof grams of carbon gasified per gram of remainingmaterial per unit time (mg-carbon/mg,-hour).

3. RESULTS

3.1 Boron concentr at i on in carbon fiber sum plesThe concentration of substitutional boron in the

P55 fibers after heat treatment to 2,773 K increasedwith increased initial loadings of boron in the SP-1packing. The smallest concentration was 4 x 10 B/C (40 ppm B). This sample is listed as P5S 40Bin Table 3. The largest concentration was 0.05 B/C.which is listed as P55 5% B. In the T-300 fiber. onlyone boron doping level of 0.002 B/C was achieved.and this is listed as T-300 2,000 B.

Magnetic susceptibility measurments made on theheat-treated SP-1 powder verified the sustitutionalpositioning of boron as a result of the processing. Itwas found that the total magnetic susceptibility (x)of the boron-doped SP-1 powder changed from anoriginal value of -2.5 x lo- Cl mu/g to -0.25 x10 emu/g. This change in x corresponds to a de-crease in the Fermi level of the graphite, which oc-curs when boron atoms are present at trigonal sitesin the lattice. This influence of boron on the dia-magnetic susceptibility of graphite was first reportedby Soule[6] and Delhaes and Marchand[ 111.

McClure[ 121 independently derived a theoreticalcurve for the influence of boron on the Fermi levelof graphite which was based upon his susceptibilitytheory. The Fermi level of single crystal graphite wasshown to be 0.027 eV. The Fermi level begins tomove into the valance band at a substitutional boronconcentration of 1.4 x 10~ B/C and continues todecrease with increasing boron concentration. Thebehavior is due to the electronic structure of boron.

The atomic radius of boron (0.098 nm) is approx-imately the same as that of carbon (O.OYl4 nm). andbecause boron is trivalent with a coplanar orbitalstructure identical to graphite, it enters the graphitelattice substitutionally at the trigonal sites[ 131. Bo-ron is also electron deficient. having only three elec-

Table 3. Boron content of carbon fiber samoles

Sample nameFiber type

Boron concentration. ppmEmissionInitial amount spectroscopyused to Dope SP-I analysis Value usedmedium (ppm) tppm) (B/C atom ratio)P55 ARP55 HTP55 40 BP55 200 BP55 1.000 BP55 5% BT-300 ART-300 HTT-300 2.000 B

NANA1506503,500176,000NANA3.500

10IOU40200I .ooo50.00030302.000

111i4 x lo-2 x IO IO :5 x I()~3 x 10 3 x IO 2 x IO i


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256 L. E. J ONESand P. A. THROWERtrons in its outer shell and, as a result, acts as anelectron acceptor. Because of the substitutional na-ture of boron, the n-electrons between the layerplanes are redistributed and the Fermi level is low-ered due to this change in the number of availablecharge carriers[8,12]. The influence of electronicstructure on the oxidation rate of graphite was firstsuggested by Long and Sykes in 1952[ 141. This is thetheory of electron transfer by an impurity atom, andfrom this theory it can be argued that the activity ofthe carbon surface to chemisorb oxygen and sub-sequently desorb oxygen complexes (i.e., CO) isaltered by the acceptance of electrons into thevalance band of carbon. It is clear that the substi-tutional doping of boron into the graphite structuredoes far more than just serve to redistribute rr-elec-trons in carbon fibers, and the purpose of this paperis to identify these changes in the chemical and phys-ical make-up of the fiber. A discussion of the influ-ence of boron on fiber microstructure, physical andmechanical properties, as well as fiber reactivity fol-lows.3.2 Fiber x-ray crystallite parameters

The fiber crystallite parameters of interlayer spac-ing, d, crystallite size, L,, and crystallite width, L,,are given in Table 4. Several boron doping levelswere achieved in these fibers and, in general, the dspacing decreased with increasing boron content. TheP55 fibers containing less than 200 ppm B, however,did not have a corresponding decrease in their dspacing. The x-ray crystallite parameters of the pitchfibers containing 40 and 200 ppm B remained rela-tively unchanged from those of the AR pitch fibers.

Turnbull, Stagg, and Eeles in 1966 studied thesubstitutional solid solubility of boron in single crys-tal graphite[l5]. They produced a graphite contain-ing as much as 4% boron and found that their ma-terials containing high dopant levels (between 2 and4% boron) had no systematic variations between theinterlayer spacings and boron content. The boronconcentrations which were found in each of thesesamples were a function of the heat treatment tem-perature experienced. Heat treatment of these bo-ronated graphites also resulted in a change of the

Table 4. Carbon fiber x-ray crystall ite parameters (nm)Fiber samples dooz L ~w nl , ~~(m,,P55 AR 0.3430 11P55 HT 0.3400 15 - -P55 40 B 0.3427 11P55 200 B 0.3428 11P55 500 B 0.3422 12P55 1,000 B 0.3364 15 10P55 5% B 0.3356 37 10 550.3406 12T-300 AR 0.3500 -T-300 HT 0.3434 6 9T-300 2,000 B 0.3350 50 - -0.3415 5

lattice parameters. The d spacings began to decreasefrom their original values at temperatures -1,470K, and above 2,100 K began to increase toward thevalue of the interlayer spacing for a pure graphite.This result is due to the presence of boron in sub-stitutional positions initially, which results in a con-traction in the c dimension and an increase in the adimension. As the concentration of boron dopedinto the lattice is increased, the substitutional solu-bility limit was reached and the boron atoms occupyinterstitial sites, which led to the increase in the cdimension. This increase in d produced by boron atinterstitial sites was greater than the contraction pro-duced by substitutional boron atoms.The presenceof interstitial atoms does not influence the a dimen-sion. It was also found that by annealing to 1,700 K,interstitial boron diffuses out of the crystal. Substi-tutional boron atoms remain in their positions up toabout 2,300 K. At this temperature or higher, boronis removed from its substitutional position by self-diffusion, leaving behind vacancies which were ob-served as vacancy loops by transmission electron mi-croscopy (TEM).

Lowell[7] examined the solid solubility of boronin carbon over the temperature range 2,070 to 2,770K and found the substitutional solid solubility limitto be 2.35 atomic % at 2,620 K. At higher temper-atures the solubility did indeed decrease. The latticeparameters were found to be linear functions of theboron content with the c dimension decreasing andthe a dimension increasing as the boron concentra-tion increased.

In the current study, we obtained significantly loweroverall boron concentrations in both carbon fibers.The final boron concentrations achieved in the fiberwere the result of changing the boron concentrationin the packing medium or graphite boat prior to heattreatment. The temperature and duration of the heat-treatment cycle were held constant during the courseof the investigation. The influence of boron on thecrystallite interlayer spacing, d, for both the PANT-300 fiber and the pitch P55 fiber are given in Fig.1. Substitutional boron in these fibers strongly de-creased the interlayer spacings. In the P55 fiber theinterlayer spacing approached that of a naturalgraphite (0.3354 nm) and leveled off at approxi-mately 0.3356 nm at a boron concentration of -2,000ppm. At this same boron concentration (-2,000 ppm),the interlayer spacing in the T-300 fiber was reducedto less than that of natural graphite.

The sharp decrease in the d spacing for the P55fiber occurred at roughly 500 ppm (5 X lo- B/C).At a 500 ppm boron level, some fraction of this fiberbecame more graphitic as a result of this increasedboron content. The decrease in the interlayer spac-ing of the pitch fiber progressed steadily up to aboron concentration of 5%. At the highest achieveddoping levels, the diffraction pattern of the pitchfiber had two distinct sets of graphite diffraction peaksresulting in two values for the d spacing for this fiber.Figure 2 is the modulated (002) diffraction peak for


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Influence of boron on carbon fibers 257

EG 0.338

i=wc2: 0.336

0.334

B /C ATOM RATIOFig. 1. Influence of the substi tutio nal boron concentration on the interlayer spacing of pitch 855 iber0 and PAN T-300 fiber A. At the higher doping levels two interlayer spacings were measured for PM

and T-300 as indicated by 0 and 1, respectively.

the pitch fiber containing 5% boron. It is believedthat the modulation of the diffraction profile for thehighly doped pitch fiber is due to the presence oftwo distinct graphite fractions in the pitch fiber. Itcan be argued that these two distinct diffraction peaksarise from two different structures within the samefiber. The most intense peak results from a diffrac-tion by the more ordered graphite fraction of thePSS fiber, (OOZ),;,while the less intense, more diffusepeak results from the turbostratic fraction of the PS5fiber, (002),.

The T-300 fiber also exhibited a similar behavior.The addition of boron to the T-300 fiber resulted intwo distinct peaks at a doping level of 2,000 ppm B(Fig. 3). This is considerably less than the 5% levelnecessary for such a phenomenon in the P55 fiber.The ci spacing of the most intense T-300 2,000 Bdiffraction peak was 0.3350 nm, less than the inter-layer spacing for a well-ordered pure graphite, 0.3354nm. The second (002) diffraction peak for the T-3002.000 B fiber was very diffuse and corresponded toa d spacing of 0.3415 nm. The smaller d spacing thanthat of pure graphite is supporting evidence for thesubstitutional positioning of boron in the fiber struc-ture.

All crystallite size, L,, measurements reflected thechanges in both the P.55 and T-300 d spacings. Thecrystallite size generally increased with boron con-centration. However, at the very low doping levels,

B/C ratios of 4 x lo- and 2 x lo-. the crystallitesize remained constant.

Low-level dopings with boron, up to 2 x 10ml BiC, had little effect on the overall P55 fiber x-raycrystallite parameters. However, higher concentra-tions of boron in the P55 fiber (1,000 ppm B and5%) altered the fiber structure by improving thecrystallite alignment and size within the fiber. Thesecarbon fibers with higher boron concentrations hadlower oxidation rates because of the presence of bo-ron. Mechanisms for oxidation inhibition are be-lieved to be linked to the enhanced crystallite struc-ture within these fibers.3.3 Irlhibition of arbon fiber oxidation

The presence of substitutional boron inhibited theoxidation of both of P55 and T-300 fibers; the degreeof oxidation inhibition depended on the concentra-tion and location of the boron present in the fiber.Burn-off versus time data for the AR, HT. and bo-ron-doped P55 fiber samples at 973 K are given inFig. 4. The presence of 1,000 ppm B reduced theoxidation rate by a factor of 6 at 973 K in 0.1 MPaultra high purity 02. In general, the reduction in thereactivity of the PAN T-300 fiber was similar to thatobserved for the pitch P55 fiber but somewhat larger.Boron was a strong inhibitor of the PAN fiber-o,reaction. At a temperature of 973 K, the reactionrate decreased by a factor of 31.9 as a result of the


8/19

258 L. E. JONESand P. A. THROWER

NBS Si Standard

I f I I 125 26 27 28 2920

Fig. 2. Modulated (002) diffraction profile for the PS5 fibercontaining 0.05 B/C atom ratio.

addition of 2,000 ppm B (2 x 10s3 B/C) to the ART-300 fiber (Fig. 5). Table 5 lists the reaction ratesof the P55 and T-300 fibers as a function of boronconcentration and temperature at 25% burn-off. Re-action rates are normalized to the weight of sampleremaining. Note that an increase of ZOOK in heatingtemperature (HT versus doped) is approximatelyequal to a doping of 500 ppm.

The P55 fiber containing 5% boron is not listedin Table 5 because it never reached 25% burn-off atthe temperature of 973 K. The reactivity of this ma-terial was essentially zero after a 20% burn-off (sevenhours of oxidation). Higher levels of burn-off (55%and 70%) were reached over much shorter periodsof time (5.5 and 3 hours) at reaction temperaturesof 1,023 and 1,073 K, respectively. This stopping ofthe fiber oxidation was also evident for the P55 fibercontaining 1,000 ppm B and the T-300 fiber con-taining 2,000 ppm B. A 100% burn-off was notachieved for these materials over the temperaturerange studied. Details of the oxidation study of the

pitch fibers are presented more completely else-where[ 161.3.4 Preferential oxid ati on of th e carbonf iber mi crost ru ctur e

Pitch and PAN carbon fibers were removed fromthe TGA after they had been oxidized to specificburn-off levels (i.e., 150/c, 50%, and 75%) and SEMmicrographs were taken. During oxidation in 0.1MPa of ultra high purity 0, over the temperaturerange of 923 to 1,023 K, the carbon fiber surfaceswere oxidized preferentially at specific sites. In otherwords, accessible areas on the carbon fiber surfacewhich had a high number of active carbon sites wereselectively attacked by oxygen. It was believed thatthe noncrystalline portions of the carbon fiber con-tained the highest active surface area and for thisreason were preferentially removed during the gas-ification process, revealing the well-ordered crystal-line fraction of the carbon fiber. Obviously, the de-gree to which the crystalline microstructure was

(002 IT1NBS Si Standard

~._.....24 i~__~~__--i26 27 2928

Fig. 3. Modulated (002) diffraction profile for the T-300fiber containing 2 x 10m9B/C atom ratio.


9/19

Influence of boron on carbon fibers100

A.R. cl0

0 o, 0 200 B HT 0 oa cl 0cl 1000 Bm-- 0 0 0 0

00 0 cl o40


10/19

260 L. E. JONESand P. A. THROWERTable 5. Gasification rates normalized to remaining weight at 25% burn-off

Rates (mgimg, hour)Samples 1,023 K 973 K 948 K 923 K 873 K

P55 AR 1.96 1.15 0.54P55 40 B 1.74 0.93 0.34P55 200 B 1.11 0.41 0.19P55 1,000 B 0.82 0.33 0.21P55 HT 1.35 0.43 0.25T-300 AR - 10.20 4.76 2.15T-300 HT 2.92 0.74 0.32T-300 2,000 B 0.83 0.32 0.06

revealed was a function of the level of carbon burn-off experienced by each fiber, as well as the con-centration and orientation of the underlying crys-talline microstructure. The microstructure and, hence,the location of preferential oxidation depended onthe carbon fiber precursor, the maximum heat-treat-ment temperature experienced during the fiber pro-cessing, and the influence of the substitutional boronon the microstructure itself.

These points can be seen in the next series ofmicrographs. Figure 6 is a micrograph of the unox-idized AR P55 fiber. This fiber had a uniform cir-cular outer diameter and a textured inner core. Thisinner core appeared to have a random orientationof its microstructure; yet, as with many carbon fi-bers, ribbing was visible along the length of the outerfiber surface. Upon oxidation the inner core regionwas preferentially removed, leaving behind an outerfiber skin or sheath (Fig. 7).

Substitutional doping of the AR P55 fiber withboron at all concentration levels retarded the pref-erential oxidation of the fiber core region. Oxidationof the P55 1,000 B fibers exposed a ribbon-like orfibrillar arrangement of the graphite crystallites aftera 45% burn-off (Fig. 8). Instead of hollowing outthe fiber core, as was the case with the AR pitchfiber, the fiber ends were tapered; attack by oxygenoccurred at the outer fiber surface. Substitutionalboron has enhanced the graphitization of the fibercore, which resulted in the development of crystalliteribbons parallel to the fiber axis. At higher boronconcentrations (P55 5% B), the fibrillar structurewas extremely rigidized. The contoured ribbons ofcrystallites seen in the P55 1,000 B fiber were nolonger present (Fig. 9).

The unoxidized AR PAN T-300 fiber had an ir-regular outer surface and an isotropic core (Fig. 10).The well-defined skin-core heterogeneity observedin the AR P55 fiber was not evident. Upon oxida-tion, all fiber surfaces, including the outer fiber sur-face, were oxidized (Fig. ll), and there are thereforeno unique microstructural features. The AR T-300fiber, however, which was oxidized to a low level ofburn-off (15%), exhibited a concentric shell (circum-ferential) microstructure (Fig. 12). This structure isnot a true onion skin (multilayer) structure as de-scribed by Donnet[l7]; however, it is a single zone

structure (no transverse heterogeneity), which is nottrue of the pitch fiber in this study. There are actuallyonly two shells apparent in this structure, one whichmakes up the outer fiber surface and one in the innercore. This unique microstructure is observed, afterlow levels of oxidation, at the fiber e.nds, and deepgrooves are visible along the fiber length.

Preferential oxidation of the HT T-300 fiber re-sulted in a honeycomb structure when oxidized to-70% burn-off (Fig. 13). Removal of the more dis-ordered fraction of this fiber during oxidation re-vealed a 3D ordering which occurred during gra-phitization at 2,973 K. This 3D honeycomb crystalliteordering in the HT T-300 fiber is unlike the com-monly accepted ribbon model for carbon fibers pre-viously observed for the oxidized P55 1,000 B fiber.A more appropriate 3D model proposed by Johnsonet al.[18] described a structure in which wrappingand folding of crystallites encloses sharp-edged voidsmuch like the structure in Fig. 13.

This structural model proposed by Johnson et al.[lS]and Johnson and Tyson[l9] accounted for the tur-bostratic disorder which was believed to exist andwas not modelled in the proposed carbon fiber rib-bon structure. The basis for this model was theirstudies of graphitized PAN fibers using TEM in whichthey observed tilt and twist boundaries. Crystallitesin PAN fibers were observed to be separated byintercrystallite boundaries and sharp-edged voids.Average crystallite widths were 6.5 nm, and voidswere roughly 1 nm across. The model depicted anunstable turbostratic form of the crystallites in a car-bon fiber which were believed to be restructuredduring graphitization as a result of a growth in thec dimension and a straightening out (untwisting) andgrowth of the a dimension. It is clear from the dif-fraction data that the c dimension of the PAN fiberdoes grow during graphitization and ordering occurs;however, there is no evidence for the growth of thea dimension. Figure 13 seems to indicate that a con-siderable amount of disorder exists along the a di-mension and that it remains twisted as described bythe turbostratic model.

Oxidation of the T-300 2000 B fiber revealed noattack of the outer fiber surface; however, the fiberends were preferentially oxidized (Fig. 14). Exam-ination of the ends of fibers oxidized to a burn-off


11/19

Influence of boron on carbon fibers

Fig. 6. As-received pitch P55 fiber

of 35%~exposed a cross-linked network of crystalhtessimilar to that previously seen in the HT T-300 fiber.Further oxidation of this fiber to burn-offs of 70%or greater yielded droplets of boron oxide on thefiber surface (Fig. 15). This oxide accumulated onthe fiber surface at the specific sites of preferentialoxygen attack (i.e.. where oxidation removed carbonfrom the structure and exposed a substitutional bo-ron atom, which then chemisorbed oxygen and pre-vented further carbon consumption at that site). Thisis the mechanism of specific site blockage and is oneof several mechanisms operative during the inhibi-tion of carbon oxidation by substitutional boron(l61.3.5 The effect of boron on carbon fiber mechanicalproperties

There are actually three mechanisms involved inthe inhibition of the carbon fiber oxidation via sub-stitutionally doped boron. These mechanisms are de-scribed in detail elsewhere[l6] and are summarizedhere for clarity.

Table 6 lists mechanical properties for selected The first mechanism, that of specific site blockage,pitch fiber samples. Thus far, we have shown that is the controlling inhibition mechanism when largeboron strongly influenced graphitization and, hence, loadings of substitutional boron are present. As thethe structural properties of the carbon fibers. From oxidation of the fiber progresses, a barrier layer ofthese mechanical data it is clear that the addition of boron oxide develops on the surface and caps offboron degrades to some extent both the fiber strength specific surface active sites, impeding the diffusionand modulus. The decrease in the strength and mod- of oxygen to the graphite and slowing any furtherulus roughly corresponds to the lowering of the crys- oxidation. This process could be seen in its advancedtallite interlayer spacing (Fig. 1). It is unclear why stages in Fig. 15, but it is an operative mechanismthese properties drop off since boron enhances gra- at any stage in the reaction when a specific activephitization and, thus, aligns crystallite planes along site is attacked and boron is exposed. Figures 16athe fiber axis. A previous study by Mayer. Cooper, and 16b are FT-IR spectra of oxidized AR PSS andand Allen[20] on the influence of boron on PAN P55 1.000 B fibers. respectively. Upon oxidation,

Fig. 7. P55 fiber oxidized to a 70% burn-off in 0.1 MPaUHP OZ.

Fig. 8. P55 fiber containing lo- B/C oxidized to a 45%burn-off in 0.1 MPa UHP O1.

fiber mechanical properties found a significant in-crease in both the Youngs modulus and tensilestrength with the addition of substitutional boron.More work is needed in this area to further clarifythe influence of boron.

4. DISCUSSION4. I Mechanisms of carbon fiber oxidationinhibition by substitutional boron

Fig. Y. PS5 fiber containing 0.05 B/C oxidized to a 50%burn-off in 0.1 MPa UHP 0,.CAR 29: 2-F


12/19

262 L. E. JONES nd P. A. THROWER

Fig. 10. As-received PAN T-300 fiber.

the AR samples exhibited a characteristic carbonylspectrum. The most intense peaks for this carbonylcompound were: strong C==O stretch at 1,718 cm-,medium acid O-H stretching vibrations (bonded)between 2,980 and 2,950 cm-, and O-H bendingbetween 1 110 and 1,030 cm-. These bands coupledwith an intense band near 1,400 cm- are indicativeof carboxylic acid (R-C-OH) groups on the ARpitch fiber surface (Fig. 16a).

As a result of boron addition to the P55 fibers andsubsequent oxidation, the surfacs of these fiberschange. Eventually, a level of carbon fiber oxidationis reached at which the reaction stops. This is dueto boron accumulation on the surface, which quicklyforms an oxide coating which inhibits further oxi-dation. The characteristic FT-IR bands for this coat-ing were: a strong B-OH stretch at 3,228 cm-, astrong B-O stretch at 1,446 cm-, and several weakB-O deformation bands between 1,200 cm- and800 cm- (Fig. 16b). A boron oxide protection layeris formed only at sites subjected to oxidation, andthus this mechanism is termed specific site blockage.

The second mechanism of fiber oxidation inhibi-tion is chemical inhibition via electron transfer. Asdescribed previously, the activity of the carbon ac-tive surface sites is reduced due to the presence ofsubstitutional boron in the internal lattice structure.Boron present substitutionally alters the distributionof T-electrons and, as a result, slows the desorption

Fig. 12. As-received PAN T-300 fiber oxidized to a 15%burn-off in 0.1 MPa UHP OS.

of -CO, which is considered the rate controllingstep of the C-O, reaction. Oxidation inhibition con-trolled solely by electron transfer was observed forfibers doped with 200 ppm boron or less. This chem-ically controlled oxidation inhibition mechanism issupported by a large increase in the activation energyfor the C fiber-o, reaction which was observed forfibers with low boron levels.

Finally, the third oxidation inhibition mechanismis due to the development of the fiber structure ascatalyzed through the addition of boron. Carbon fi-bers become more graphitic as the layer planes be-come more ordered and the crystallite dimensionsincrease. This typically results in a decrease in thetotal number of accessible active surface sites (e.g.,area) and, hence, results in a decrease in the reac-tivity of the fiber. At issue is how and where is boronthe most effective inhibitor of carbon oxidation whenit is chemically bound to (subtitutional doped into)the hexagonal graphite lattice structure.4.2 Carbon fiber structure: The influence ofsubsti tut ional boron

It has previously been established that boron ispreferentially doped into disordered or turbostraticcarbon materials, as opposed to graphites with a highdegree of crystallite orientation[21]. Boron also in-duces graphitization in these fibers. The influence

Fig. 11. As-received PAN T-300 fiber oxidized to a 70%burn-off in 0.1 MPa UHP 0,. Fig. 13. Honeycomb structure of the heat-treated T-300fiber oxidized to a 70% burn-off in 0.1 MPa UHP 0,.


13/19

Influence of boron on carbon fibers 263carbon are removed and distortions are relaxed asa result of heat treating to temperatures in excess of2,473 K.

Fig. 14. Preferential oxidation of the end of a PAN T-300fiber containing 2 x 10 ~B/C after burn-off of 35% in 0.1MPa UHP 0:.

Graphitization is often described as a two-stageprocess in which self-diffusion via interstitial migra-tion is considered the mechanism of the first stage.Self-diffusion results in the ordering of layer planesand an increase in the layer plane stacking dimen-sion, L,.The second stage of graphitization involvesthe lateral growth of crystallites during which a con-comitant growth of L, and L, occurs.The activationenergy, E,, value for B diffusion in carbon is roughlythe same as the E, for self-diffusion, -6.8 eV.

of substitutional doping on carbon fiber graphitiza-tion was shown in Fig. 1. The location and influenceof boron on each of these fiber structures is clearlyunique and a function of the fiber precursor and theprocessing cycle experienced by each fiber. A moredetailed description of the role of boron in dictatingcarbon fiber structure is given here.

4.2.1 Boron as a gruphitization catalyst in carbonfibers. The AR pitch fiber is more graphitic than theAR PAN fiber; yet, as with most carbon materials,both fibers contain small regions in which the carbonatoms are arranged in a graphite layer structure andlarge regions where defects and disorder predomi-nate. Upon heat treatment, some reordering of theselayer planes occurs; the graphite crystallites growand the layer plane stacking order improves. Theend result is a material containing a large amount ofthe stable hexagonal form of graphite. Graphitiza-tion, for this reason, can be said to be the processof developing a three-dimensional structural orderin the carbon material through the progressive re-moval of defects within and between the graphitelayer planes.

The influence of boron on the graphitization pro-cess is dictated by the diffusion constant for boronand not by the diffusion activation energy. Hen-nig[22] found that at temperature between 1.970 and2,760 K the diffusion constant for boron migrationin the a direction (6,320 cmis) is much greater thanthat in the c direction (7.1 cmis). The mobility ofboron within the structure was also found to be rel-atively insensitive to the boron concentration for theB/C range between 10 z and lo-. Diffusion con-stants for carbon migration are considerably lowerthan those for boron. Using a C tracer, the self-diffusion constants were found to be 82.8 cm?!s and1.82 cmis in the II and c directions, respectively[23].These findings substantiate the belief that B and Cdiffuse in the lattice structure in an identical mannerand that the graphitization process is acceleratedthrough a mechanism which is enhanced by an amountcomparable to the increased diffusivity of boron overcarbon atoms in the lattice.

Boron accelerates the graphitization process. Thecatalytic behavior of boron on the graphitization pro-cess is controlled by the thermally activated mech-anism of carbon self-diffusion. Structural defects in

The effects of B on the crystallite interlayer spac-ing, d. for both the PAN T-300 fiber and the pitchP55 fiber are given in Fig. 1. It has been argued thatas a result of its atomic size, substitutional boronincreases the a lattice parameter and decreases thec parameter[7]. This size argument alone is not enoughto explain the reduction in the c lattice parameter(d spacing) given in Fig. 1. The presence of substi-tutional boron in these carbon fibers strongly re-duces the value of the interlayer spacing, d,,,,,. Gra-phitization. shown by a reduction in the interlayerspacing, is enhanced by substitutional boron at con-centrations greater than 2 x 10 B/C in the PS.5fiber.

The influence of boron on the crystallite width isnot as dramatic. The crystallite width or diameter,L,,, can be determined using line widths of the dif-fraction profiles of the (100) and (101) planes inTable 6. The influence of substitutional boron onmechanical properties

Fig. IS. Droplets of oxide on the surface of the PAN T-300 fiber containing 2 x lo- B/C after oxidation to a 70%burn-off in 0.1 MPa UHP Oz.

FibersP55P55 200BP55 SOOBP55 5% B

Tensile strength Tensile modulus(ksi) (Msi)275 55182 so180 47134 27


14/19

3800 3000 2000 1000WAVENUMBERS

Fig. 16~1.FT-IR spectrum of the oxidized sm-face of the as-received P55 fiber.

L. E. JONESand P. A. THROWERP55AR I 1 I 1

450

P55 0 I I

-

I3800 3000 2000 IO00

WAVE NUMBERSFig. 16b. FT-IR spectrum of the oxidized surface of the P55 fiber co~ta~~i~g lo- BIG

450


15/19

Influence of boron on carbon fibers XI5graphite. Figure 17 defines these dimensions in agraphite crystallite. The crystallite parameter, L (101)(as measured by the breadth of the (101) peak), isas much an L, dimension as an L, dimension sinceit cuts obliquely through the thickness of the crystal.In this sense, it is a true measurement of the degreeof 3D order of the graphite crystals in these carbonfibers. The L, dimension may indeed grow, yet theL,( 101) dimension will remain the same if the ABAB. . stacking sequence is periodically interrupted bycrystallite twist boundaries.

The L parameter calculated from the line widthof the (101) peak is an indication of the regular de-velopment of three-dimensional order within the fi-ber. in the carbon fibers studied, both L,, andL,(101) are not affected by the increase in the boronconcentration up to a 5% boron doping level (Table4). It can. therefore. be argued that since these val-ues are not growing with respect to the increased~~rn(~unt of subsitutional boron. the graphitizati~)nprocess in carbon fibers is accelerated by an increasein the first stage of graphitization (diffusion via in-terstitial migration to eliminate defects), which is aresult of the increase in the rate of boron diffusioninto the fiber structure.

At a boron concentration of 10 B/C in the P55fiber (PSS 1.000 B), the d spacing dropped to 0.3364

nm (Fig. 1). The n spacing for perfect graphite is0.3354 nm. Pitch fiber P55 1,000 B was not entirelygraphitic as a result of boron doping. The d spacingof the P55 1,000 B decreased by 0.0066 nm from thed spacing of the AR P5.5 fiber. A further increasein the P55 fiber boron concentration, to a level of5% B, decreased the d spacing by an additional 0.008nm, which is only a slight improvement in the crys-tallinity of the material as a result of a 50-fold in-crease in the boron concentration. However. it isfairly certain that not all of the boron in P55 5% ispresent substitutionally.

The d spacing for the T-300 fiber doped with boron(T-300 2.000 B) does fall below the d spacing fatperfect graphite. This concentration level (2 x 10 B/C) is far below the 2.35 atom % soiubiiity limitfor boron in carbon. The graphitization of the PANT-300 fiber is also enhanced by the presence of suh-stitutional boron. This very low d spacing value forthe PAN fibers containing boron supports the pres-ence of boron substitutionally in this fibers struc-ture.

4.2.2 Crystallite length. L,,, in boron-dopeci fi'her.s.Bending of the crystallite layer planes was observedin the P5.5 1,000 B fiber (Fig. 8). In this micrograph,the skeleton of the P55 1,000 B fiber was exposedupon oxidation in 0.1 MPa of UHP 0,. This skeleton

Twist.Boundary

Fig. 17. Crystallite dimensions in an ordered graphite crystal


16/19

266 L. E. JONES nd P. A. THROWERis actually a series of undulating ribbons of crystal-lites oriented parallel to the fiber axis. This ribbonstructure was first described by Watt and Johnsonin 1969f24j and is shown schematically in Fig. 18.The proposed structure was one in which ribbons ofcrystallites were aligned parallel to the fiber axis.The average width of the ribbons was about 10 nm,which was reported as the L, value. The crystallitelength, L,, was only 6-12 nm. These small crystallitelengths are indicative of a turbostratic arrange-ment of crystallites in the a dimension. The irregularcontours of these ribbons created voids which werelong and narrow, or slit-shaped. These slit-shapedvoids had cross-sections no greater than l-2 nm andlengths 20-30 nm. The measured L, (based upon the(100) peak width) for these fibers was only 10 nm.

Increasing the boron content in the P55 fiber to5% rigidized the P55 fiber structures. The layer planeshave a needle-like appearance (Fig. 9) which is instrong contrast to the ribbons of crystallites foundin the P55 1,000 B fiber. There was no bending ofthe layer planes in the P55 5% B fiber, and as aresult, well-defined (100) and (101) reflections areobserved.

The HT T-300 structure as revealed via low-levelgasi~cation was a honeycomb network of crystallites(Fig. 13). A considerable amount. of cross-linkingexisted between the crystallite layer planes. Cross-linking of the PAN fiber structure is a residual fea-ture of the oxidation-stabilization step during pro-cessing. Once the fiber is stabilized, it is unable to

I--.---20H-IFig. 18. Ribbon structure of the graphite lattice in a high-modulus carbon fiber, from Fourdeux et al. [25].

react or fuse with other carbon fibers. Unsaturatedside groups present in the linear molecules of poly-acrylonitrile can cause additional polymerization re-actions. To avoid this, PAN fibers are heated in acontrolled oxidizing atmosphere -500 K under ten-sion for several hours. As a result, several thermallyactivated processes occur which cause a reorgani-zation of the polymer chains and three-dimensionallinking of parallel molecular chains by oxygen bonds.These oxygen cross-links keep the chains straightand oriented parallel to the fiber axis even after therelease of tension following the stabilization process.Cross-linking between chains of PAN prevents trueribbons of crystallites from forming. Instead, a ho-neycomb crystallite backbone enveloping box-likeporosity was produced during heat treatment. Dif-fraction by the (100) and (101) planes of the heat-treated T-300 fibers did occur; however, these re-flections were lost upon the addition of boron.

The influence of substitutional boron on theL,, (100) dimension for the PAN fiber (T-300 2,000B) resulted in the loss of the (100) reflection. It ispossible that because the presence of substitutionalboron has forced this fiber d spacing to be belowthat of graphite, some break-up and shortening ofthe a dimension occurred and tilt bounda~es wereformed between these crystallites. Substitutional bo-ron, which catalyzes the first stage of graphitization,does bring the layer planes closer together and in-creases L,; however, boron also disrupts the L, di-mension and creates an increase in the number oftilt boundaries, which results in the loss of the (100)reflection for the T-300 2,000 B fiber.

4.2.3 ~odulut~o~ of the (006) resection by sub-stitutional boron. Modulation of the (002) diffractionprofile of the P55 fiber occurred at a 5% B dopinglevel (Fig. 2). Modulation of the PAN T-300 (002)diffraction profile occurred at a much lower boronconcentration of 2,000 ppm B (Fig. 3).

Modulation was noticed as a slight shoulder onthe (002) profile of the P55 fiber containing 1,000ppm B. It is believed that in these modulated dif-fraction profiles, the most intense peak results fromdiffraction by the more graphitic fraction of the fiber,while the less intense and broader peak results fromdiffraction by the more disordered (turbostratic)fraction of the same fiber. The crystallite size, L,,of the graphitic fraction of P55 5% B is 40 nm. TheL, value for the turbostratic fraction of the same fiberis 12 nm.

An explanation for these different structures iscentered around the presence of substitutional bo-ron, where influence on the structure, oxidation re-sistance, and physical properties of the fiber cannotbe associated with a homogeneous distribution ofboron throughout the structure. This variation inboron concentration is, as evidenced by SIMS anai-ysis of boron-doped pyrolytic graphite[21], due to alarger boron uptake by that portion of the fibersstructure which was originally amorphous or tur-bostratic as opposed to crystalline (graphitic). Be-


17/19

Influence of boron on carbon fibersTable 7. Boron contents of the progressively oriented pyrolytic graphites analyzedusing SIMS

267

Sample d spacing.nm Boron concentration. ppmafter doping Values used(B/C) atom ratioLOPG 0.3423HTPG 0.3400HPPG 0.3376

cause of this, its structure and properties are signif-icantly more enhanced.

To verify that boron is preferentially doped intodisordered material, three pyrolytic graphites of dif-ferent perfections were doped simultaneously with1,500 ppm boron under conditions identical to thoseused to dope the carbon fibers. Each of these graph-ites was evaluated for crystallite parameters and up-take of elemental boron. The least oriented of thepyrolytic graphite samples (LOPG) was an as-de-posited sample. Next was a slightly more orientedsample, a pyrolytic graphite which was deposited andsubsequently heat treated to 2,973 K (HTPG). Thethird sample was a hot-pressed highly oriented py-rolytic graphite (HOPG) which possessed the highestdegree of crystallite development and preferred ori-entation of the samples investigated. Table 7 liststhese samples, their intertayer spacings, and theirboron contents as measured using SIMS.

As shown, the amount of boron doped into thesesamples decreased with increasing degree of crys-tallite orientation (graphitization). The most disor-dered sample, LOPG. had double the boron con-centration found in the most graphitic sampte, HOPG,even though the initial boron concentrations used todope these materials were identical. No boron con-centration gradient through the specimens was ob-served.

As described, the microstructure of each of theas-received carbon fibers is inhomogeneous. Specificfractions of the AR fiber are quite graphitic, whereasother fractions of the very same fiber are turbo-stratic. An example of this type of structural inho-mogeneity is the AR pitch fiber structure, which hasa graphite outer sheathing and an internal amor-phous core. Upon oxidation the core region pref-erentially oxidizes. leaving behind a hollow graphiteskin.

Because the internal core of this pitch fiber has aconsiderably more random structure than the basalgraphite outer fiber surface. or skin, it takes up sub-stantially more elemental boron than the outer sur-face region. The take-up of substitutional boron en-hances graphitization and the development of theskeletal or inherent substructure of the fiber. Thegraphite fiber outer skin also takes up substitutionalboron, but because of its already ordered and well-oriented microstructure, it takes up a lower concen-tration of boron than the disordered core region.Boron substitutionally doped into both these struc-tural regions catalyzes graphitization and enhances

the crystallite structure within the core region pref-erentially over the fiber outer sheathing. The degreeto which the crystal structure is enhanced in bothregions is a function of the substitutional boron con-centration and the original degree of microstructuralorder. Hence, we observe the modulated diffractionprofiles of these two graphite fractions within thesame fiber.

Upon oxidation, the more graphitic region of thefiber, designated (002),, on the modulated diffractionpeak, is attacked preferentially over the less orderedgraphite region, designated (002)r. Figures 19 and20 are the (002) diffraction profiles for the P55 1.000

1 I I I I24 25 26 27 2828

Fig. 19, The (002) diffraction profile for the oxidized P5Sfiber containing 10 B/C. The graphite fraction of thisfiber is preferentially oxidized; the only diffraction peakwhich remains is that of the turbostatic fraction.


18/19

268 L.E. JONES ndP A. THROWER

1 I I25 26 27

29Fig. 20. The (002) diffraction profile for the oxidized PANT-300 fiber containing 2 x IO- B/C; only the turbostaticfraction of this fiber remains after oxidation.

B and T-300 2,000 B fibers, respectively, after theywere oxidized to a 30% burn-off. Only the (002),profile remains after oxidation for both fibers. Themost graphitic fraction of the fiber, {002),, is pref-erentially removed. The fraction of the fiber yieldingthe (002), reflection contains significantly more sub-stitutional boron after doping and, hence, is moreeasily protected than the graphite fiber fraction whenexposed to oxygen at high temperatures. Upon ex-posure to oxygen, the boron which is located at spe-cific surface active sites becomes an oxide, thus form-ing a protective barrier preventing further oxidationof the carbon substructure.

Micrographs of the boron-doped pitch P55 fiber(Fig. 8) exhibit ribbons of crystallites running thelength of the fiber. There is no longer an outer fiberskin in the oxidized specimen, even though it wasoriginally clear that the fiber skin was considerablymore graphitic than the pitch fiber core in the ARfiber. The pitch fiber skin is preferentially oxidizedin the boron-doped samples because it contains lessboron than does the core region. Oxidation of thefiber core is no longer evident. The ribbons of fibercrystallites developed in the region which was orig-inally the random or turbostratic fiber core are theinherent fiber substructure, which is protected fromoxidation as a result of the higher doped boron con-centration and the subsequent formation of a boronoxide protective layer.

The same argument holds for the PAN T-300 fiber,although this AR fiber contains a significantly lowerconcentration of ordered structure originally. The(002)r reflection given in Fig. 20 is also produced by

the inherent 3D cross-linked substructure of the fiberwhich develops during boron doping. This 3D sub-structure is also protected during oxidation by theformation of an oxide barrier layer.

5. CONCLUSIONThe AR P55 fiber contains a strong skin core het-

erogeneity in which the outer fiber skin is consid-erably more graphitic than the fiber core. Upon ox-idation, the fiber turbostratic core region oxidizespreferentially. This oxidation behavior reverses itselfupon the addition of substitutional boron. The rea-son for this change in behavior is a result of thelocation and concentration of boron in the fiber.Boron is preferentially doped into the disorderedP55 fiber core. The concentration of boron substi-tuted in the fiber is proportional to the number ofdefects which exist in specific regions or zones of thefiber structure. As a result, there is a higher con-centration of boron in the turbostratic core regionthan in the graphitic fiber skin. The addition of boronresults in the ordering of both of these structuralregions in the fiber; however, the oxidation protec-tion of the fiber is not provided by an increase incrystallite order but by the mechanism of specificsite blockage. The fiber core contains more boronthan the fiber outer skin and can, therefore, forman oxide layer more readily. It is also believed thatthe order in the fiber core region does not approachthat of the fiber skin. All carbon fibers maintain theiroriginal structural nature (orientation) as dictatedby their precursor and processing histories; only thecrystallite order is enhanced through heat treatingand/or boron doping. The same processes are op-erative in the PAN T-300 fiber; however, there isnot as clear a structural gradient in the PAN fiber.The 3D cross-linking structure observed throughoutthe oxidized HT PAN T-300 is an inherent part ofthe original fiber structure. The degree of crystallineorder of the 3D cross-linked microstructure is en-hanced through heat treatment. This 3D cross-linkedmicrostructure may itself act as a reinforcement inthe PAN fiber. Since it is not present as a gradientmicrostructural feature, this 3D network in the fibermay result in the enhanced mechanical behavior shownby Mayer, Cooper, and Allen[20].

A gradient microstructure exists in the P55 fiberand is observed as the skin/core regions previouslydescribed. These regions are further delineatedthrough heat treatments and boron doping. Thisnonuniformity in fiber structure gives rise to stressconcentration at the interface between the fiber skinand core, which results in the observed decrease infiber mechanical properties as a result of boron dop-ing.

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Influence of Boron on Carbon Fiber Microstructure, Physical Properties, And Oxidation Behavior

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