The absence of plasma in ``spark plasma sintering''

Spark plasma sintering (SPS) is a remarkable method for synthesizing and consolidating a large variety of both novel and traditional materials. There are a number of mechanisms proposed to account for the enhanced sintering abilities of the SPS process. Of these mechanisms, the one most commonly put forth and the one that draws the most controversy involves the presence of momentary plasma generated between particles. This study employs three separate experimental methods in an attempt to determine the presence or absence of plasma during SPS. The methods employed include in situ atomic emission spectroscopy, direct visual observation, and ultrafast in situ voltage measurements. Using these experimental techniques, no evidence of plasma was found during the SPS process. This result was confirmed using several different powders across a wide spectrum of SPS conditions.