One-step method for measuring the effect of humidity on powder resistivity

Relation between powder resistivity and humidity is typically measured in several steps. When a standard resistivity cell is used, this procedure may take weeks. We present a method for measuring powder resistivity as a function of moisture using a one-step process. System consisted of two parallel electrodes immersed in powder. High voltage electrode was a uniform square but the ground electrode consisted of two parts: a narrow current electrode connected to an electrometer and a guard electrode surrounding the current electrode. Relative humidity surrounding the particles was measured at the height of current electrode. Relative humidity of ambient air was rapidly increased from 5 RH% to 100 RH% and both current and humidity within the powder were recorded as diffusion of moisture took place. System was modeled using computer simulations. As an example, resistivities of three different lactose, wheat flour and quartz are presented. Resistivity was found to decrease several orders of magnitude between 10 RH% and 90 RH%. As powder was dried, significant hysteresis was observed. Since a small change in humidity results in very large change in resistivity, system could be used to study surface sensitive phenomena such as surface amorphicity and surface chemistry.