graphite fibers intercalated with nitric acid and arsenic pentafluoride

Carbon Vol. 23, No. 6, Pp. 731-737. Printed m the U.S.A.

1985

000X-6223185 $3.00+ Gil 0 1985 Pergamon Press Ltd.

ELECTRICAL RESISTIVITY AND MAGNETORESISTANCE OF CARBON/GRAPHITE FIBERS INTERCALATED WITH NITRIC ACID AND ARSENIC PENTAFLUORIDE W. DAVID LEE U.S. Army Belvoir Research and Development Center, Fort Belvoir, VA 22060, U.S.A. and GLENN

Moore School of Electrical

P. DAvIst and F. LINCOLN VomLt

Engineering

(Received

and Science, University PA 19104, U.S.A.

1August 1984; in revisedform

of Pennsylvania,

Philadelphia,

4 April 1985)

Abstract-A

series of high performance, experimental carbon/graphite fibers was intercalated and examined with respect to their metallic conductivity behavior by resistivity and magnetoresistance versus temperature measurements. One fiber was a polyacrylonitrile (PAN&type precursor and three were pitch base precursors. All four types showed substantially similar behavior in the pristine state with respect to room temperature resistivity and the sign and magnitude of the temperature coefficient of resistivity. After intercalation with either nitric acid or nitric acid followed by AsF,, the PAN-based fibers displayed a resistivity versus temperature behavior qualitatively similar to their pristine counterparts but displaced to lower resistivity. On the other hand, the pitch fibers with the same intercalation treatment exhibited metallic behavior (a positive temperature coefficient of electrical resistivity and a small magnetoresistance). These manifestations of metallic behavior are usually indicative of some three dimensional graphite structure in the carbon fibers. Key WordsIntercalation,

graphite,

carbon,

fibers, resistivity,

magnetoresistance.

1. INTRODUCTION

Study of the electrical properties of intercalated carbon/ graphite fibers has excited the interest of researchers for more than a decade, primarily for two reasons: (1) the possibility of making a practical, lightweight high strength electrical conductor from this material[ 1,2], and (2) as a medium for determination of the relationship of the carrier transport to the physical and electronic structure of graphite intercalation compounds. While the electrical resistivity of graphite is highly anisotropic, the crystal orientation is such that the low resistivity Z-axis (p, = 40 x 10e6 n-cm) is disposed longitudinally in the fiber, and the higher resistivity Z-axis (p, = 0.10 n-cm) lies radially, yielding a filament well oriented for electrical conductivity purposes[3]. Furthermore, with any anisotropic conductor, it is necessary, when measuring the resistivity using the four-point probe method, that the aspect ratio of the sample, i.e. length divided by diameter, be at least as large as the square root of the anisotropy ratio[4]. This requirement is easily satisfied with filaments having diameters in the vicinity of 10 p.m[S]. Intercalation of carbon fibers was first reported by Henrinckx et a/.[61 in 1968, who thermally inserted potassium into rayon precursor fibers. Their reported resistivity values, although open to some doubt because of questions concerning diameter, density and surface, were quite low: 1000 @-cm for the pristine fiber and 60 pR-

tPresent address: Intercal Company,

Port Huron, MI 48060,

U.S.A. 731

cm after intercalation. From Ubbelohde’s research[7], culminating in the intercalation of graphite crystals with HNO,, it was clear that intercalation with strong acids to produce acceptor compounds resulted in the highest electrical conductivities. Various authors have subsequently reported intercalation of rayon-based, polyacrylonitrile (PAN)-based, pitch-based, and vapor-grown fibers with many different intercalants and combinations of intercalants[7-161 with some room temperature resistivities as low as 3 @-cm[ 131. The general conclusion drawn from these studies was, regardless of the type of precursor, the greater the degree of graphitization of the starting fiber, the lower the electrical resistivity after intercalation. As the final heat treatment temperature (HIT) of the fiber is increased above 12oo”C, the elastic modulus increases and the resistivity decreases to the range of several hundred @-cm. This relation between graphitization and intercalated resistivity has been cited previously by others[ 14,17-191. A major drawback in quoting absolute values of electrical resistivity of the pristine or intercalated fibers is the difficulty in determination of the cross sectional area and the rather large variations encountered from fiber to fiber. Thus, only by resorting to the tedious task of making a large number of single fiber measurements can one determine average values with a reasonable degree of accuracy. The question of changes in the mechanical properties of the acceptor intercalated fibers has been studied by several different groups using various intercalants[20-

W. D. LEE et al.

132

261. While these studies show that there was no serious deterioration of mechanical properties as a result of fiber intercalation, other work[26,27] has shown significant degrading of the fibers’ mechanical properties. Data to date show that these changes are specific to both the fiber group and the intercalant used.

The vapor-grown fibers of Endo et a1.[28] provided a new materiaf with potential for a high degree of graphitic order. Samples of these fibers were heat treated at 270028OO’C and then intercalated by Davis et al.[l3] with HNOl or AsF,. On limited samples the HNOj intercalation produced resistivities in the range of lo-15 $Icm and the AsF, intercalations in the range of 3.0-15 @-cm, neglecting any dimensional changes. Recently, several groups[ 16,27,29-351 have reported interesting results on the electrical resistivity and magnetoresistance of highly graphitized fibers in both the pristine and intercalated states. These fibers showed electrical resistivities as low as 6.0 p&-cm[21] at room temperature and in some cases[13,16,21,27,30-351 were air stable at elevated temperatures. Furthermore, many fibers exhibited a positive temperature coefficient of resistance from 80 to 400 K. The research reported here was undertaken to determine the conduction properties of some well graphitized and intercalated carbon fibers. 2.

EXPERIMENTAL

Four groups of experimental graphite fibers were examined in this work. The four groups were labeled N-l through N-4, with N-l being of a PAN precursor and N2 through N-4 being mesophase pitch based. The fibers were specially made for the Naval Research Laboratory to examine the effects on electrical resistivity of a very high HTT. They were batch processed at a final temperature of -3200°C. Since the emphasis was to be on electrical measurements, detailed determinations of mechanical properties were not made by us, although the manufacturer reports the moduli are in the vicinity of 120 X lo6 psi. Nominal thicknesses of the fibers were measured using light microscope techniques and are listed in Table 1. Group N-3 was also examined by scanning electron microscopy (SEM) and gave diameter values to within a few percent of those obtained by the light microscope technique. However, a large number of the fibers were observed to have “half moon” cross sections, which introduces obvious errors in the calculation of cross sectional area. These errors have an important effect on

absolute values of resistivities but no effect on relative changes upon intercalation, on magnetoresistance measurements or on temperature coefficient of resistance. individual fibers were mounted in a four point d.c. measurement configuration across platinum contacts that had been silkscreened and fired onto alumina substrates (Fig. 1). Mounting contacts to the fibers were made using gold paste, Cermalloy 4752, and firing for 5 min at 500°C. The sample was then placed in a vacuum tight reactor (Fig. I). Platinum leads were attached to the platinum silkscreenings on the substrates and connected to a vacuum feedthrough to allow in situ monitoring of voltage and current during intercalation. The reactor vessel was then evacuated to
VACUUM FEEO.THRU

1413

Table I. Fiber sample designations NfU Sample NO.

Union Carbide Savqle No.

Nominal Diameter )Im

N-l

DPANG

5.0

N-2

PGD-1

8.5

N-3

TP 4101

19.9

N-4

TP 4104B

19.0

Fig. 1. Reaction chamber for intercalation of fibers, showing sample arrangement on substrate.

Metallic conductivity behavior of carbon/graphite fibers 2.2 Arsenic pentafluoride intercalation Arsenic pentafluoride (AsF,) was purchased in 100-g lecture bottles from Ozark-Mahoning. The AsF, was distilled trap to trap using various slush baths to remove any impurities (SiF,, HF, etc.). The purified AsF, was introduced into the reactor vessel at pressures of up to 1 atm absolute. End-point resistivity of graphite fibers was unaffected by varying pressure between 200 and 1520 torr. AsF, intercalation was generally complete in < 1 h. As was seen with HNO? intercalation and shown by others[ I 1.141, the better the quality of the pristine fiber (as judged by the resistivity value), the less time needed for intercalation to be completed. 2.3 Two-step (HN0,IAsF.J intercalation Intercalation by HNO, followed by AsF, resulted in lower resistivity values than intercalation by either intercalant alone[ 121. The HNO, intercalated fiber was placed under vacuum at < 1 pm of Hg for 1 h at room temperature. AsF, was then introduced into the reactor using the same process as described previously. End-point resistivities were reached in <5 min and typically < 1 min. Representative values of the room temperature electrical resistivity of the pristine and intercalated fibers are shown in Table 2. 2.4 Low temperature measurements Several fibers were mounted and furnished to one of the authors (W.D.L.) for low temperature measurements of electrical conductivity at Belvoir R&D Center, Ft. Belvoir, VA, U.S.A. After being measured in the pristine state, they were returned to the University of Pennsylvania where they were intercalated with HNO, and returned to Belvoir for further study. The work at Belvoir included all four fiber groups. Only N-3, the most highly conducting, was investigated at the University of PennTable 2. Electrical resistivities of pristine and intercalated fibers (G-cm) E W-l, N-l* W-l3 W-l,

PRISTINE@ 412 303 243 1215

lNrERcNATE0 P 38.9 151.0

INTERCUANT

“9 “4

N-2, N-22 N-23

412 494 673

41

HNO,

N-4,

262 255 303

18.4

HNOl

N-42 N-4) N-31_, N-32-1 N-3,_, N-3, N-3$ N-3, N-34 N--3,-2 N-32-2 N-33-2 N-311-1 N-311-3 N-3-l N-3-Z N-3-7 N-3-8 N-3-9

162 231 19a 255 199 167 159 242 188 221 255 176 231 191 215 215 202

9.4 13.4

Nor%

24 8 a.7

HNO, HN031AsF5 HNO,lAsF5

REONYlNS HNO,

733

Sylvania, where all magnetoresistance measurements were made. An alumina substrate of 5-mm width with platinumsilkscreened and fired contacts was prepared (Fig. 2). Individual fibers were mounted in the manner previously described. The mounted fiber was placed in a 3 X 7mm rectangular glass tubing and a glass to metal seal formed at one end to produce a gas tight feedthrough around the platinum leads. The experiments were performed at Belvoir using the apparatus shown in Fig. 3 and at the University of Pennsylvania using a similar apparatus, both consisting of a liquid helium cryostat with a variable temperature insert, which permitted the temperature to be varied over the range between liquid helium and room temperature. The substrates were attached to the sample block (Fig. 3) using silicone vacuum grease. Indium solder connections were used throughout the cold tip portion. The sample block of oxygenfree high conductivity copper with its associated thermal time constant aided in maintaining uniform sample temperatures, and permitted quasistatic temperature variation. Instrumentation leads were heat sunk to the upper portion of the sample block, and a Lake Shore Cryotronics diode temperature sensor was mounted in the block next to the sample location. Electrical measurements were made as described previously. The current was varied in steps above and below the selected value both as a check for nonohmic behavior and to provide relative calibration points. In a typical experiment the sample was cooled to the lowest desired temperature and allowed to warm slowly to room temperature. Hysteresis was checked on the first sample measured and found to be negligible. The analog outputs of the voltage and temperature monitors were connected to an x-y recorder to produce a continuous record of the voltage as a function of the temperature. Magnetoresistance measurements were made at the University of Pennsylvania using a 1.7-teslamagnet. The magnetoresistance measurements were made at 298, 77, and 5 K. Temperature was stabilized for 30 min prior to the taking of data. Error introduced into the temperature measurement by the magnetic field was always
W. D. LEEet al.

734

PUTIN~

PUTIN

Fig. 2. Ampoule of isolation of intercalated fiber.

3. RESULTS AND DISCUSSIONS 3.1 Intercalation

Intercalation of the graphite fibers by either HNO, or AsF, goes very quickly when compared with known rates for highly oriented p~o~ytic graphite (HOPG) intercalation[36,37]. The “splits” observed by SEM along the length of the N-3 fibers exposed a large surface area. This enhancement of the surface to volume ratio, along with the irregular surface on the balance of the fiber, results in a large number of exposed c-axis edges where inter&ants may enter the lattice. This, combined with the small (relative to HOPG) crystallite size of 0.01-0.02 pm, reduces the distance that an intercalant atom or molecule must travel to enter the lattice. These two physical qualities of graphite fibers lead to the very short intercalation times[ 111. A correlation can be made between pristine resistivity,

-VARIRBlE

intercalation rate and end-point intercalated resistivity. Fibers with lower pristine resistivity intercalate more quickly and to a lower end-point resistivity than those fibers with a higher pristine resistivity]14,37,38]. A two-step interc~ation has generally resulted in a lower end-point resistivity than intercalation by either HNO, or AsF, alone[l2]. Several successive intercalations and deintercalations of HNOSfollowed by AsF, have shown no enhanced resistivity over the initial two-step species insertion. 3.2 Low temperature studies 3.2.1 Resistivity versus temperature. The temperature dependence for each of the four fiber groups is presented in Figs. 4 and 5. Figure 4 shows the resistivity as a function of temperature for the pristine fibers, while Fig. 5 shows the resistivity for the same fibers after interca-

TEMPERATUREHELUJY CRwlSTAl

Fig. 3. Liquid helium cryostat and measurement schematic, showing sample holder.

Metallic conductivity behavior of carbon/graphitefibers 900 -

. PRISTINE N-I,. *

BLIP -

‘*

700 _

l*.*

is limited by defect scattering according to Mathieson’s rule:

d- 5rm

v PRISTINE N-Z,. d= E.&m

‘.* I

* PRlSTlNE N-3,. 4= se.srm * PNtSllNE N-3,. d = 19.3 ,,m

. .

l*._

PNISIINE N-4<, d=lSem

. .

.

ZOO 6

20

3

735

r

’

60

1) 100

’

4

I

’

140 19@ TEMPERATURE (XI

/ 220

1

4

260

300

Fig. 4. Resistivity as a function of temperature for the pristine fibers.

lation. The resistivities are based on the nominal fiber dimension and have not been corrected for changes in fiber size resulting from intercalation or temperature change. Because of the anisotropic characteristics of graphite compounds, four-point d.c. measurements must be evaiuated with care. The fiber samples in these experiments had LID ratios of ~500; thus, current densities are expected to be relatively uniform despite the high degree of anisotropy in the materials. The resistivity versus temperature for the pristine fibers shows a negative temperature coefficient of resistivity, characteristic of a material whose electronic conductivity

This is due to the direct temperature effect on carrier concentration and the temperature independence of defect scattering. This effect has been reported on numerous occasions with various pristine fibers[20,25,29,30]. Based on the data presented by Bright and Singer[S] on mesophase pitch-based fibers, the N-3 fiber group would have an HTT of -3ooO”C. The actual value is -3200°C. After intercalation the resistivity of N-2 was almost inde~ndent of temperature, while N-l showed a definite negative temperature coefficient. The other two fiber groups, N-3 and N-4, showed a definite positive temperature coefficient. The positive temperature coefficient is indicative of a change in the dominant scattering mechanism governing charged carrier mobility from the defectlimited case of the pristine fibers to the phonon-limited case of the intercalated fibers. The positive coefficient may also be indicative of some three dimensionality and is characteristic of metallic behavior. This positive temperature coefficient of resistivity has been reported on several occasions with various other fiber groups and intercalants[16,20,26,30-331. The resistivity ratios between room tem~rature, iiquid nitrogen and liquid helium temperature yield a measure of the temperature coefficients; these are shown for each fiber before and after intercalation in Table 3. 3.2.2 Magnetoresistance.The magnetoresistance data on the N-3 group of fibers in the pristine and intercalated (HNO,) conditions are given in Figs. 6 and 7. From the relationship

-AP =

/.L”W.

PO 170

l

*

l

l

l

l l

18D

’

.

l l

l

.

l

l

.

.

.

.v

l

150 140

F Y

u -I E = 9 p

i

98

I

v.

94

.

.

.

f

.

.

.

.

,

.

92

the magneto~sis~nce coefficient or, defined in the region near B = 0 is taken as a “pseudomobiIity” and is inversely related to the scattering factor. The values of JL, obtained in the low field regime for the pristine fibers are given in Table 4 at room, liquid nitrogen and liquid helium temperatures. The pseudomobilities displayed in this table are su~risingly high, which could indicate that the N-3 group of fibers had a well formed. three-dimen-

90 36

Table 3. Resistivity ratios (liquid nitrogen or liquid helium to room temperature) of pristine and intercalated fibers

/

r 1B

I-

........ ...............

20

SO

0 ....

22

...-.-... .....

__ ._

-Qul QRT

... C.C~---.-

220

ZIP

300

Fig. 5. Resistivity as a function of temperature for the intercalated fibers.

W-l

1.58

N-2

1.53

1.71

1.01

1.02

N-3

1.80

2.40

0.88

0.86

I-4

1.83

2.4s

t 0.84

0.83

1.82

1.07

1.10

W. D. LEE et al.

136 0.051 PRIRIWE N-3,

PRlSllNE

N-3,

Fig. 6. Magnetoresistanceversus magnetic field squared for pristine N-3 fibers.

sional structure. Favorable comparison can be made with Hall mobilities obtained on macroscopic crystalline material and highly oriented pyrolytic graphite[38]. The lack of dependence of magnetoresistance on magnetic field after intercalation with nitric acid and arsenic pentafloride shown in Fig. 7 is entirely in accord with a change to metallic behavior. Others[ 16,381 have reported significant magnetoresistance effects with intercalated benzine vapor-grown fibers. Also, the pseudomobilities of two such studies compare favorably, in that Oshima er a1.[27] gave pseudomobility values of -100 cm?/V-set in this study. They reported pseudomobility values for intercalated fibers and HOPG (~.t,, = 4000 cm?/V-see) but not for the pristine

materials.

6

1

T-7711

I

T=SK

0

z

4

6

8

The onset of oscillatory behavior similar to that reported by Woolf[35] may be observable in the pristine fiber magnetoresistance curves (Fig. 6). However, owing to limitations of the magnet used in this study (maximum field = 1.7 tesla), it was not possible to investigate this behavior further.

Acknowledgement-We are most grateful to R. A. Wachnik, J. E. Fischer and L. Pendrys for their enlightening discussions concerning the experimental apparatus and interpretation of the data. The fibers were kindly supplied by Dr. Joseph Reardon, while on the staff of Naval Research Laboratories. This work was supported by the U.S. Army Belvoir Research and Development Center, Fort Belvoir, VA 22060, U.S.A., undercontract DAAG53-76-0061.

10

12

14

16

18

20

MAGNETIC FIELG Ii Fig. 7. Magnetoresistance versus magnetic field squared for N-3 fibers intercalated with HNO,-AsF,.

Metallic conductivity

behavior of carbon/graphite

Table 4. Magnetoresistance coefficient versus temperature pristine fiber N-3 Sample

No.

Magnetoresistance Coefficient !Jm

N-31

N-32

cm2/volt-set

for

Temperature K

1,000

295

1,740

77

1,250

4

1,450

295

1,820

77

2,930

4

REFERENCES 1. J. S. Murday,

2.

3.

4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

14.

D. D. Dominguez, J. A. Moran, W. D. Lee and R. Eaton, Syn. Met. 9, 397 (1984). J. A. Woollam, H. Rashid, V. Natarajan, B. Banks, D. A. Jaworske, J. R. Gaier, C. C. Hung and A. Yavrouian, ExtendedAbstracts, Symp. I, Fall Meeting, Mat. Res. Sot., p. 202 (1984). I. L. Spain, Chem. and Phys. Carbon, Vol. 16, (Edited by P. L. Walker, Jr., and P. A. Thrower), p. 120. Marcel Dekker, New York (1981). H. C. Montgomery, J. Appl. Phys. 42, 2971 (1971). A. A. Bright and L. S. Singer, Carbon 17, 255 (1979). C. Heminckx, R. Perret and W. Ruland, Nature 220, 63 (1968). A. R. Ubbelohde, Proc. Royal Sot. A304, 25 (1968). F. L. Vogel, Proc. Fourth London Int’l. Conf on Graphite and Carbon, p. 332. Sot. Chem Industry (1974). F. L. Vogel and R’Su Popowich, Symp. Series on Petroleum-Derived Carbons, p. 411. Am. Chem. Sot. (1975). F. L. Vogel, Carbon 14, 176(1976). J. G. Hooley and V. R. Dietz, Carbon 16, 251 (1978). G. P. Davis and F. L. Vogel AGARD Conf. Proc. No. 283, p. D-27 (1980). G. P. Davis, M. Endo and F. L. Vogel, ExtendedAbstracts, 15th Biennial Conf. on Carbon, p. 363. Am. Carbon Sot. (1981). D. Dominguez, R. N. Bolster and J. S. Murday, Extended Abstracts, 15th Biennial Conf. on Carbon, p. 365. Am. Carbon Sot. (1981).

fibers

737

15. H. A. Goldberg and I. L. Kalnin, Syn. Met. 3, 169 ( 1981). 16. M. Endo, T. Koyama and M. Inagaki, Syn Met. 3, 177 (1981). 17. D. Robson, F. Y. 1. Assabghy and D. J. E. Ingram, J. Phys. D5, 169 (1972). 18. A. A. Bright and L. S. Singer, Carbon 17, 59 (1979). 19. M. Endo, Y. Hishayama and T. Koyama, J. Phys. D15, 353 (1982). 20. C. Henrinckx, R. Perret and W. Ruland, Carbon 10, 71 I (1972). 21. G. L. Hart and G. Pritchard, Nature Phys. Sci., 246, 78 (1973). 22. S. B. Warner, L. H. Peebles and D. R. Uhlmann, Plasticization of Carbon Fibers, Int’l. Conf. on Carbon Fibers, Their Place in Modern Tech., London (I 974). 23. V. R. Dietz and W. H. Vaughn, Proc. 29th Conf. on Reinforced Plastics/Composites Inst., uaner 21 -A C1974) 24. I. L. Kalnin, Extended Abstracts.’ lith Biennial Conf. on Carbon, p. 65. Am. Carbon Sot. (1975). 25. P. Kwizera, M. S. Dresselhaus, J. S. Perkins and C. R. Desper, Extended Abstracts, 15th Biennial Conf. on Carbon, p. 102. Am. Carbon Sot. (1981). 26. D. D. Dominguez, J. L. Lakshmanan, E. E Barbano and J. S. Murday, Proc. MRS Symp., Vol. 20, p. 63. (1982). 27. H. Oshima, J. A. Woollam and A. Yavrouian, J. Appl. Phys. 53, 9220 (1982). 28. M. Endo, T. Koyama and Y. Hishiyama, Japan. J. Appl. Phys. 15, 2873 (1976). 29 I. L. Kalnin and H. A. Goldberg, Syn. Met. 3, 159 (198 1). 30 K. Oshima, J. Ameil, P. Delhaes, J. F. Mareche, D. Guerard and M. Endo, Extended Abstracts, 16th Biennial Conf. on Carbon, p. 102. Am. Carbon Sot. (1981). 31. D. D. Dominguez and I. S. Murday, Extended Abstracts, 16th Biennial Conf. on Carbon, p. 276. Am. Carbon Sot. (1983). 32. M. Endo, T. C. Chieu, G. Timp and M. S. Dresselhaus, Syn. Met. 8, 251 (1983). 33. C. Manini, J. F. Mareche and E. McRae, Syn. Met. 8, 261 (1983). 34. J. R. Gaier, NASA Tech. Memo. #86859, Lewis Research Center, Cleveland, OH (1984). 35. L. D. Woolf, Extended Abstracts. Symp. I, Fall Meeting, Mat. Res. Sot., p. 180. (1984). 36. R. Schlogl, Extended Abstracts. 14th Biennial Conf. on Carbon, p. 278. Am. Carbon Sot. (1979). 37. W. C. Forsman, F. L. Vogel, D. E. Carl and J. Hoffman, Carbon 16, 269 (1978). 38. E. R. Falardeau, L. R. Hanlon and T. E. Thompson, Inorg. Chem. 17, 301 (1978).