Measuring bulk resistivity of a powder

The resistivity SR of a material is based on Ohm s law, which relates the current density J (A/m2) to the applied electric field strength E (V/m) in the forms 

R (Ohms) is the measured resistance of the sample over its length L (in the direction of a the electric field) and A is the current carrying cross-section area. The dimensions of SR are reported as Ohm-meter (Q-m). A material following Equation 3.1 is said to be Ohmic whereas, a material following a non-liner power law such as V In in the current-voltage characteristic is non-Ohmic. Non-Ohmic behavior has been discussed by Lampert and Mark (1970), Lacharme (1978), and Kingery (1976). 

An alternative representation of the volume resistivity SR is its reciprocal or electrical conductivity a= SR 1 For isotropic samples, the material resistivity 

The measurement of bulk resistivity of a powder includes volume and surface conduction mechanisms. It is generally not possible to separate out the two effects so that the effective powder resistivity, either the volume or surface resistivity, for dielectric and insulating particles such as glass depends on such factors as the presence of surface impurities and the relative humidity. For clean metal powders, the volume resistivity will dominate conduction in a bed of particles whereas, the presence of a surface oxide film can dominate conduction via the contact resistance for only lightly compacted powders. 

FIGURE 3-1 Measuring the effective powder resistivity by ASME/ANSI PTC 28, 1973. 

---