Electric Field Assisted Sintering of Metal Oxides

Abstract: Electric field assisted sintering techniques, FAST or FLASH have become the novel research fields for the past few years. One of the primary reasons for the rise is the potential benefits which these techniques have brought. The chief benefit is the reduction in the sintering time and temperature, which is very important in terms of energy savings and low production cost. How the electric field enhances the sintering rate is still not fully understood. However, it is believed that electric field halts grain growth which indirectly increases the sintering rate. Joule heating at grain boundaries is another decisive factor in increasing sintering rate by simultaneously increasing the diffusion at grain boundaries and lowering the driving force for grain growth. In this thesis, electric field influence with respect to microstructure, mechanical properties and sintering temperature required for densification is being studied. The materials studied are; yttria-stabilized zirconia, ceria-stabilized zirconia and titanium oxide. It is found that electric current rather than electric field is the main variable which controls the densification rate. Electric field or voltage only governs the conduction behavior due to the dielectric properties of metal oxides. Densification rate is directly proportional to current density and current intensity. Thus by carefully controlling these parameters the onset of FLASH sintering can be controlled. It is also found that application of electric current at higher densification temperature increases grain growth than normal sintering. Microstructure analysis and sample internal temperature measurements showed that Joule Heating, grain growth retardation and increased ionic mobility are the main causes of increased densification rate.