Resistivity Measurements Investigations of the electrical properties of solids is an important and rapidly growing area of experimentation-particularly in the area of organic, polymer and organometallic compounds. Measurement of the bulk specific resistance,^, is one of the easiest, most straightforward, andmost useful electrical property determinations. Specific resistance is one of the few physical quantities which exhibits values differing by more than 102"for materials whieh are readily available under conditions common to our experience. This large range of eonductivity is basic to the wide use of electricity and electrical devices. Conductive materials as copper exhibit a p of about 10-%om-cm whereasgood insulators as polytetrafluoroethylene, polyethylene exhibit p values about 1017ohm-cm. Specific resistance is calculated from the following equation: p = Rfolt) where R is the resistance in ohms; a, the pellet area in cm2; t, the pellet thickness in cm; and p, the specific resistance in ohm-cm. We have experimented with a number of different equipment assemblies and configurations and A. Voltage regulator such as the Hewten Packard 6516A. 8, Ress. C. W m find that an assembly as described in the figure block with two stainless steel electrodes placed lhrough it and a high impact yields adequate, useable results for p values of plastic (nanconductive)sheet p l a w below. The sample pellet resides between lo2-1015 ohm-em as a function of applied voltage the two electrodes. D, Ammeter such as the RAC WV-51lA Piwarnmeter. and pressure. Briefly then, the sample is ground to a fine powder and added t o a pellet press of known diameter. The pellet assembly is exposed to a pressure of 20,000 psi using a press such as a Carver Laboratory Press (Model B). The pellet is removed and thickness measured. (Pellet configuration must he determined under dynamic conditions if measurements are made a t pressures greater than 5,000 psi.) The pellet is placed between two stainless steel (or other suitable metal) electrodes of the same diameter as the pellet enclosed in a hard-wood casing. (We have experimented with a number of non-conductive casings and find that for ease of operation, dependability and longevity that a hardwood casing is superior or equivalent to other casings utilized. An item of safety should be stressed when obtaining measurements a t high pressures in t h a t t h e electrodes may break from t h e casing causing t h e casing a n d electrodes t o be discarded from t h e press in a dangerous manner. T h e hardwood casing minimizes this danger. The assembly is fitted onto the press and measurements obtained as a function of voltage and pressure. Resistance, R, is best obtained utilizing Ohm's relationship R = voltage/amperage by measuring amperage a t applied voltages. Unless much effort is made to define such items as surface gas adsorption, interparticle resistance and electrode contact resistance, absolute p values are not obtained. Even so this assembly yields values which are useful in describing the level of resistance and potential usefulness of a sample.
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Wright S t a t e University Dayton. Ohio 45431
5 7 6 / Journal of ChemicalEducation
Charles E. Carraher. Jr.
