WEB FIELD ASSISTED SINTERING OF URANIUM DIOXIDEThursday (24.09.2020) 17:35 - 17:50 P: Processing and Synthesis 1 Part of:
Controlled current rate flash sintering with an alternate electric field promoted fast densification of uranium dioxide resulting in dense pellets (93% theoretical density), with good mechanical integrity, thirty times faster than conventional sintering. A heating rate of 110 ºC s-1 was estimated for a current rate of 32 mA mm-2 min-1, while at 252 mA mm-2 min-1 the heating rate was 728 ºC s-1 and led to cracking of the pellet due to thermal stress. The densification rate and pellet temperature estimate were coupled and the Master Sintering Curve (MSC) method was applied to compare the activation energy of sintering during flash and conventional sintering. Notably, conventional sintering showed an activation energy of 380 kJ mol-1, and controlled current rate flash sintering an activation energy of 108 kJ mol-1, which is remarkably close to the activation energy of spark plasma sintering of uranium dioxide. This similarity triggers the consideration that the electrical effects are more relevant to sintering than the mechanical pressure in field assisted sintering techniques. Another interesting feature of controlled current flash sintering of uranium dioxide is the saturation of the electrical conductivity at 125 S m-1 at current densities higher than 250 mA mm-2, i.e., >1250 ºC. Although contrary to the traditional behavior of flash sintering in other oxides, where the electrical conductivity increases with temperature, the electrical conductivity of uranium dioxide is sensitive to temperature, oxygen partial pressure, and the electrochemical effects of the electric field and electric current. Therefore, flash sintering of uranium dioxide is believed to have different signatures. Nonetheless, it was demonstrated that by tuning the parameters of controlled current rate flash sintering uranium dioxide with a good microstructure can be successfully obtained, drastically reducing the sintering time and temperature.