Electrical resistivity of carbon fibers

Koyama T, Endo M. Electrical resistivity of carbon fiber prepared from benzene. Jpn J Appl Phys 1974 13 1175-1176. 

Electrical Resistivity. Like the thermal properties, the electrical resistivity of carbon fibers, measured along the axis, is similar to that of pyrolytic graphite in the ab direction and approximately an order of magnitude higher than metal conductors such as aluminum or copper, as shown in Table 8.11. 

Table 8.11. Electrical Resistivity of Carbon Fibers and Selected Metals...

Figure 9.7. Room-temperature thermal conductivity and electrical resistivity of carbon fibers and selected metals.l ...

Fischbach and Komaki, "Electrical Resistance of Carbon Fibers", Biannual Conference on Carbon, 1979. 

The electrical resistivity of the fibers as a function of carbonizing temperature are shown in Figure 14. The resistivity drops from a value of several hundred thousand micro ohm centimeters at low carbonization temperatures to about 675p ohm cm at 2200 C carbonization temperatures. 

The electrical conductivity of carbon fiber composites is affected by the fiber type, density, and waving pattern. Several authors have demonstrated electrical conductivity in carbon fiber composites and measured resistance for a range of composite types. For example, carbon fiber-epoxy has a resistivity ranging from 5000pf2cm to 20,000 pH cm [104,105.110]. It has also been shown that carbon fibers in matrix composites produce a transverse electrical conduction path [112]. Eddy currents flow along fibers and pass from one fiber to another at the points of fiber contact, as shown in Fig. 8. The longitudinal and transversal resistivity of carbon fiber reinforced epo.xy re,sin for a volume fraction of 50% is 0.009 - cm and 0.5 cm respectively [112]. Further detailed infom-... 

Thus, the 2nd fiber category encompasses carbon and carbonized polymers. The foundation materials of this group are the heat convertible, fiber forming polymers, the most common of which today is PAN (polyacrylonitrile). Others are rayon, PBI, and pitch tar. One of the earliest to report results of PAN pyrolysis was Goodhow, et. al.(5i) in 1975. Later, in 1979, Fischbach and Komaki(5 and Brehmer, et. al.(di) in 1980 reported on the electrical properties of carbon fibers made fi om various polymers and described a dependency of resistivity upon the heat treat temperature (HTT) employed to carbonize the fiber which is now well known. The studies by Swift, et. al(52) in 1985 and more recently reported herein were undertaken to expanded upon this base of knowledge and to initiate studies of the stability of the electri properties of fibers made on commercially viable platforms. These studies have led to a launch point for what is believed to have been the first, relatively large scale, commercial application for partially carbonized PAN fibers(50), as resistive carbon fiber based static eliminator brushes. 

Carbon, Carbides, and Nitrides. Carbon (graphite) is a good thermal and electrical conductor. It is not easily wetted by chemical action, which is an important consideration for corrosion resistance. As an important stmctural material at high temperature, pyrolytic graphite has shown a strength of 280 MPa (40,600 psi). It tends to oxidize at high temperatures, but can be used up to 2760°C for short periods in neutral or reducing conditions. The use of new composite materials made of carbon fibers is expected, especially in the field of aerospace stmcture. When heated under... 

As discussed in Chapter 10, a wide variety of additives is used in the polymer industry. Stabilizers, waxes, and processing aids reduce degradation of the polymer during processing and use. Dyes and pigments provide the many hues that we observe in synthetic fabrics and molded articles, such as household containers and toys. Functional additives, such as glass fibers, carbon black, and metakaolins can improve dimensional stability, modulus, conductivity, or electrical resistivity of the polymer. Fillers can reduce the cost of the final part by replacing expensive resins with inexpensive materials such as wood flour and calcium carbonate. The additives chosen will depend on the properties desired. 

Figure 14 ELECTRICAL RESISTIVITY OF SOLVENT EXTRACTED PITCH BASE CARBON FIBER AS A FUNCTION OF CARBONIZATION TEMPERATURE...

Thermal expansion, electrical resistivity and electrical conductivity are key properties of carbon fibers which strongly depend upon their structure. 

In PAN based fibers, the electrical conductivity is low, is dominated by the microtextural defects, and has a semiconducting character. It increases when the fibers are stretched during the high temperature treatment and when the temperature is raised, i.e., when the microtexture becomes more and more ordered [1j. Interestingly, the variations in electrical resistivity of PAN based HM fibers appear to be a function of the inverse of the carbon layer size and obey a linear relationship ... 

The addition of 1.7% v/v of a pitch based carbon fiber lowered the electrical resistivity of a gypsum plaster (o -CaS04-5H20) and significantly improved the effective EMI shielding [59]. 

Piezoresistivity [66] was observed in cement matrix composites with 2.6-7.4 vol% unidirectional continuous carbon fibers. The dc electrical resistance in the fiber direction increased upon tensile loading in the same direction, such that the effect was mostly reversible when the stress was below that required for the tensile modulus to decrease. The gage factor was up to 60. The resistance increase was due to the degradation of the interface of the fiber and matrix, which was mostly reversible. Above the stress at which the modulus started to decrease, the resistance abruptly increased with stress/strain, due to fiber breakage. The tensile strength and modulus of the composites were 88% and 84%, respectively, of the calculated values based on the rule of mixtures. 

Wang and Chung [186] have observed apparent negative electrical resistance in interfaces between layers of carbon fibers in composite material in a direction perpendicular to the fiber layers. 

A patent by Matthews and Ko [188] describes how the electrical resistivity of a PAN based carbon fiber heat treated at 650°-1050°C increases with time when aged in air and the magnitude of this increase is proportional to the initial resistance— the higher the initial resistance, greater is the increase. BASF established that the stability ean be enhanced by treatment in air for 8 8 h at 260-285°C. 

Mei and Chung [235], using electrical resistance measurement studied the Tg and melting behavior of carbon fiber reinforced thermoplastic composites. 

Carbon fiber has been found to be an effective thermistor [192-194], such as a cement paste reinforced with chopped carbon fiber (about 5 mm long) with silica fume (15 wt% cement). Its electrical resistivity decreased reversibly with increasing temperature (1-45°C), with activation energy of electrical conduction (electron hopping) of 0.4 eV. This value is comparable to semiconductors (typical thermistor materials) and is higher than that of carbon fiber polymer matrix composites. The current-voltage characteristics of carbon fiber reinforced silica fume cement paste were linear up to 8 V at 20°C. 

The expansion of the areas of application for carbon fibers is stimulated by their attractive properties, not found in other materials, such as strength, electrical conductivity, stability on exposure to reactive media, low density, low-to-negative coefficient of thermal expansion, and resistance to shock heating. The most representative applications of carbon fibers and element carbon fibers are as sorption materials, electrostatic discharge materials, catalysts, and reinforcement materials in composites. 

---