Surface observation of pulsed electric current sintered alumina balls

Abstract

The Pulsed Electric Current Sintering (PECS) method uses electric energy for sintering, however, the phenomenon behind this sintering process is ambiguous. The sintering of green bodies, especially insulating powders, requires Joule heat as well as other phenomena. Insulating materials can be sintered more quickly by the PECS method than by other similar processes such as the hot pressing method. We attempted to clarify why PECS facilitates sintering in a shorter period, using alumina balls as a model starting material. After sintering, the surfaces of the alumina balls were observed by scanning electron microscopy (SEM) and analyzed by electron probe micro analysis (EPMA) in an attempt to clarify the ambiguous phenomena which occurs during PECS. Crystallized alumina of excess aluminum and liquid-state bridging material were found on some parts of the surfaces of the balls, and whisker-like crystalline materials were also found on other parts of the surfaces. If the surface of alumina is exposed to an oxygen partial pressure of 5 × 10-12 Pa at an estimated temperature of above 1880°C, which was about 380 K higher than the die surface temperature, alumina can be reduced. Based on this hypothesis, it is concluded that part of the surface of the alumina balls was heated intensively during the PECS process. The high temperature at the surface pro-motes sintering.