Changes of Electrical Resistivity of Graphite Fibers with Nitration

28 Changes of Electrical Resistivity of Graphite Fibers with Nitration

Downloaded by SUFFOLK UNIV on January 18, 2018 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0021.ch028

F. LINCOLN VOGEL and R'SUE POPOWICH Moore School of Electrical Engineering and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Penn. 19174

Pure single crystal graphite is a moderately good conduc tor of electricity having a resistivity of 4 x 10 Ω cm in the cystallographic "a" direction (1). That is approximately twenty times the resistivity of pure copper which is 1.67x10 Ω cm at room temperature (2). The electrical carrier concentra tion in graphite, composed of equal numbers of holes and electrons, is 3 x 10 /cm and the carrier mobility, high compared to any metal, is in excess of 10,000 cm /volt sec at room temperature (3). This high value is interpreted as due to the low scattering offered by the orderly resonant bond structure of the hexagonal graphite rings. By comparison, the free electron concentration of copper is approximately 10 /cm with a mobility of 35 cm2/ volt-sec. The formation of a graphite intercalation compound leads to a decrease in the electrical resistivity as has been shown by many workers in this field. Ubbelohde, for instance, has demonstrated this with a variety of intercalants such as bromine (4), bisulfate (5), nitric acid and a number of others (6), (7). Group I elements from the Periodic Table (see for example. Herold (8) ) have been examined as intercalants in graphite and found to produce donor behaviour whereas the aforementioned acid intercalants, sulfuric and nitric, act as acceptors. The largest change of resistivity in the conversion to an intercalation compound reported in the literature is for a Stage II compound of N O -H O (9) where the resistivity atO°Cis approximately two micro ohm cm. The generally accepted mechanism for increase of conductivity in graphite intercalation compounds is the transfer of charge from the nitrate to the graphite trans forming the latter into a giant cation. In these terms then, the stronger the acid-greater the ionization and the higher the conductivity. This paper describes some work in the initial phase of a program to determine the highest conductivity that can be achieved by using very strong acids as intercalants in graphite. "Graphite" fibers, particularly the high modulus type used to reinforce polymers and metals represent a convenient form -5

-6

19

3

2

23

2

5

3

2

411

Deviney and O'Grady; Petroleum Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

PETROLEUM DERIVED CARBONS


412

in which to conduct e l e c t r i c a l r e s i s t i v i t y experiments. The geo metric form of filaments allows for easy calculation of r e s i s t i v i t y values. Further, the high modulus fibers are f a i r l y well structured graphites having the normals to the crystallographic "C" axis lying p a r a l l e l to the fiber axis. (10) Since the direction of highest conductivity i s normal to the "C" axis this orientation configuration i s fortunate for maximizing con ductivity i n the fiber. Probably the most important advantage of the filaments i s the a b i l i t y to make satisfactory contacts in a way that does not c a l l the effects of high anisotrophy into question. The work reported here w i l l concentrate on commerially obtained Thornel 75 fibers since they were found to be well struc tured and typical of high grade fibers. The experiments described herein were done i n the i n i t i a l part of our program to determine the intercalation character i s t i c s of "graphite" fibers with red fuming n i t r i c acid. Experimental Work Working with individual graphite fibers having diameters somewhat less than 10 μ diam presents the investigator with a number of d i f f i c u l t i e s , especially considering the necessity for maintaining multiple e l e c t r i c a l contacts after treatment with very strong acids. The method devised consisted of mounting and soldering an inch long fiber of known cross sectional area to a substrate and measuring the r e s i s t i v i t y . After acid treat ment the r e s i s t i v i t y of the same fiber sample was determined again thus placing reliance on the r e s i s t i v i t y change rather than the magnitude of the absolute values. Resistivity Measurements: Figure 1 i l l u s t r a t e s the fourpoint mounting of graphite fibers on the holder plate. The plate i t s e l f i s alundum with platinum connecting strips s i l k screened on and then baked. The fiber i s l a i d across the p l a t i num strips, a dab of 80 Au-20Sn solder paste put at each con tact point and a short length of Pt wire i s put over the fiber and solder paste. When the solder i s melted, a secure joint i s made holding the fiber between the platinum s t r i p and platinum wire. The choice of materials was governed mainly by the nec essity for maintaining e l e c t r i c a l contact after repeated ex posures to highly reactive environments. With a fiber so mounted, i t i s possible to make a four point r e s i s t i v i t y measurement conveniently. The outside con tacts are connected to the current source through a Keithley Electrometer to measure the current. The voltage drop i s measured between the two inside contacts with a null voltmeter. For con venience i n calculating r e s i s t i v i t i e s the distance between the voltage drop contacts i s one centimeter. The r e s i s t i v i t y , defined as the resistance of a volume of material having a cross section 1 cm χ 1 cm and 1 cm i n length, i s calculated from:


28.

voGEL A N D POPOwicH

Graphite

413

Fibers

R

Ρ -

A 1


where:

R Ρ A 1

resistance i n ohms r e s i s t i v i t y i n ohm cm cross section i n cm length i n cm 2

Since the main interest i n this study was i n determining the r e s i s t i v i t y change that could be produced by treating fibers, i t was essential to know the resistance before and after treatment, but the geometric quantities of length and area have to be known exactly only for calculation of absolute values of r e s i s t i v i t y . Because the cross sections of the fibers are very often irregular their areas are most accurately determined by examination at high magnification i n a scanning electron microscope. Because of the inconvenience and expense of this electron microscope measurement, i t could not be used routinely on every sample measured. Rather, the cross section shape was determined for each fiber type and a correction factor applied to the circular area derived from the diameter measured by viewing from the side i n a conventional light microscope. Chemical Treatment: Intercalations of the fibers for both the r e s i s t i v i t y experiments and the weight change experiments were done by immersion. The immersions were accomplished simply by submerging the mounted fibers i n about 20 cc of the acid solution—red fuming n i t r i c acid with a specific gravity of about 1.60—under controlled temperature conditions. This was followed by washing i n water and acetone before measurement. In general, nitration by this method produced v i s i b l e results i n a matter of a few minutes. Procedures: For the r e s i s t i v i t y measurements a single graphite fiber selected for study was mounted on a contact sub strate and soldered i n position. Two small lengths—about 3mm long were broken o f f either end and retained for diameter and cross section measurement. The resistance of the centimeter long center section of the test piece was determined by passing a current i n the 10~ ampere range producing a voltage drop i n the m i l l i v o l t range. Smaller voltage drops were avoided to prevent interference from thermal and electrochemical EMF s. The sampxe i s then treated with the acid for a specified length of time, cleaned and remeasured. The anticipated problem of large dimen sion changes or disintegration with intercalation was not encoun tered under the conditions employed i n a l l of these experiments. For the weight change experiments a bundle of fibers about an inch long weighing about 0.1 gram was loaded into a noble metal boat that had been baked to constant weight. The boats were provided with numerous holes which allowed them to f i l l and drain rapidly when placed i n liquid for either nitration treat5

f


P E T R O L E U M DERIVED

414

CARBONS

ment or rinsing i n water and acetone. The procedure then was to load a boat, weigh to the nearest tenth of a milligram, immerse i n acid, rinse, dry and weigh again. For other than room temper ature determinations the acid container was immersed i n a larger bath maintained at the appropriate temperature. Experimental Results: The changes i n r e s i s t i v i t y that were observed after treat ment of the Thornel 75 fibers with red fuming n i t r i c acid are shown i n Table I.


TABLE I RESISTIVITIES OF THORNEL 75 FIBERS INTERCALATED BY IMMERSION IN HNO ?

TEMPERATURE °C

RESISTIVITY INITIAL

flcm FINAL 4

80

8.5 χ 10-4

1.2

57

8.0 χ 10-4

6.0 χ 10~5

12

27

6.1 χ 10-4

8.1 χ 10"5

8

χ

10~

RESISTIVITY RATIO 7

After intercalation these fiber r e s i s t i v i t i e s are changed by about an order of magnitude and there appears to be a pronounced effect of temperature of treatment on the change i n r e s i s t i v i t y . The weight change experiments shed further light on the what i s occurring during intercalation with red fuming n i t r i c acid. Figure 2 i s a plot of the uptake of weight during this acid treatment. The kinetics are characterized by an i n i t i a l increase which i s sensitive to temperature followed by leveling off which i s roughly the same for the three temperatures, 2°, 22° and 57oc. These i n i t i a l rates of weight increase are shown i n Table II and plotted against the reciprocal of the absolute temperature of the reaction i n Figure 3. The calculated activ ation energy of 20,000 cal/°C mol i s indicative of the rate limiting step of the formation reaction - possibly the step involving indiffusion of the acid molecules. TABLE II RESISTIVITIES OF THORNEL 75 FIBERS INTERCALATED BY IMMERSION IN HNO? TEMPERATURE RATE °C % PER MINUTE 57 42 22 2

88 50 1.8 0.3


AND

POPOwicH

Graphite Fibers


VOGEL

40

50

TIME IN MINUTES

Figure 2.

Intercalation of graphite fibers with red fuming nitric acid


415


PETROLEUM

DERIVED

Figure 3. Intercalation of graphite fibers with red fuming nitric acid


CARBONS

28.

VOGEL

AND

popowicH

Graphite Fibers

417


Discussion and Conclusions It seems clear from the data presented here that "graphite" fibers can be readily intercalated with red fuming n i t r i c acid and that the r e s i s t i v i t y changes that take place are comparable with those experienced i n more perfect graphitic materials such as the stress annealed pyrolytic graphite crystals. The variations of conductivity that occur with variation of treatment temperature can be ascribed to concentration of intercalant changes. The weight change experiments agree with the r e s i s t i v i t y experiments i n a qualitative sense but the temperatures of maximization are not i n agreement. Lacking i n our understanding here i s a knowledge of the exact species that resides i n the l a t t i c e i n t e r s t i t i a l l y after intercalation i s complete. The p o s s i b i l i t i e s are numerous including a l l of the oxides of nitrogen, n i t r i c acid, water or any combination of these. Caref u l and exact determination of these species must be done before an adquate structure of graphite nitrate can be deduced.

Literature Cited (1) . I.L. Spain, A.R. Ubbelohde and D. A. Young in Proc. 2nd Int. Carbon and Graphite, (1967). (2) . Metals Handbook, 8th Edition, Volume 1, Properties and Selection of Metals, Taylor Lyman Ed. (3) . I.L. Spain, Chemistry and Physics of Carbon, ed. by P.L. Walker and P.A. Thrower (Marcel Dekker, Inc. New York 1973) Vol. 8, p. 1 (4) . L.C.F. Blackman, J.F. Mathews and A. R. Ubbelohde, Proc. Roy Soc A258, (1960) (5) . A. R. Ubbelohde, Proc. Roy Soc. A321,445 (1971) (6) . A. R. Ubbelohde, Proc. Roy Soc. A304, 25 (1968) (7) . B. Bach and A. R. Ubbelohde, Proc. Roy Soc. A325, 437 (1971) (8) . Herold, A. Bull, Soc. Chem., France p. 999 (1955). (9) . Fuzellier, H. Doctoral Thesis Presented to University of Nancy, France (1974). (10) .Fourdeaux A, Perret,R and Ruland, W. Conf. on Carbon Fibers Plastics Institute, London (1971) (11) . G. M. Jenkins, K. Kawamura and L.L. Baw, Proc. Roy Soc. A327, 501 (1972).


Changes of Electrical Resistivity of Graphite Fibers with Nitration

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