Electrical resistivity of fluorinated carbon black
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I. INTRODUCTION Fluorinated carbon, CF X , is a nonstoichiometric solid prepared by the high-temperature reaction of elemental fluorine and carbon. It was first prepared by Ruff1 in 1934,' and has since found a number of uses in such diverse applications as a cathode material for lithium batteries,2"4 fuel cell electrodes,5 lubrication,6'7 electrophotographic consumables,8'9 specialty electrical composites,1011 and others. Carbon black has been used much less often as a starting material for CFX than either graphite or coke.12"14 Fluorinated carbon black retains many structural similarities to the starting carbon, making it a useful alternative for some applications where carbon black is not suitable. Depending upon the fluorine content, CFX exhibits a wide range of electrical resistivities, has color ranging from black to white, has a low coefficient of friction, and a low surface free energy. This article reports the electrical resistivity of fluorinated carbon black as a function of fluorine content, pressure, and temperature. II. EXPERIMENTAL Samples were prepared for this study by the direct fluorination of carbon black with elemental fluorine. Temperatures between 200 and >600 °C were used, depending upon the degree of fluorination sought. Materials with* = 0.07,0.25, and 1.2 (10%, 28%, and64% F, respectively) are commercially available from AlliedSignal Inc. as Accufluor® fluorinated carbon. All samples had submicron particle size, and Table I compares selected physical properties of the commercial materials. Fluorine analyses were performed by fusing the CFX with a mixture of potassium and sodium carbonates. The resulting water soluble fluorides were mea890
J. Mater. Res. 3 (5), Sep/Oct 1988
http://journals.cambridge.org
sured with a fluoride ion electrode. Values in Table I were also obtained using standard techniques: density was determined with a Micromeritics Autopycnometer 1320, decomposition temperature was measured using a DuPont 9900 thermogravimetric analysis (TGA) apparatus, and surface area was determined by the single point nitrogen Brunauer, Emmett, and Teller (BET) method using a Quantasorb system. X-ray diffraction data were collected with Cu Ka radiation between 1° and 60° with a step size of 0.1° and 5 s counting time. Except where otherwise noted, all resistances were measured at 59.3 MPa and room temperature. Resistances less than 10s ft cm were measured in a four-probe poly (vinyl chloride) (PVC) piston cell similar to the one described by Streifinger et al.15 The rectangular opening in the PVC was 1X 6 cm, current electrodes were at opposite ends on the long dimension, the voltage probes were separated by 4 cm along the long direction, and the compressed thickness of the sample was typically 0.3 cm. Compression was carried out in a hydraulic press equipped with a manometer. Resistance measurements were performed with a Solartron/Schlumberger 7061 multimeter in the four-terminal mode using a measuring current of 1 mA or less in the range of resistances under consideration. Samples wit
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