Investigation of the mechanisms of flash sintering in oxide ceramics
Creators: name: Charalambous, Harry
Publisher: name: No Publisher Supplied
Year: 2018

description: Flash Sintering (FS), has been the subject of intense study by the ceramics community in recent years. Discovered by Cologna, et al. in 2010, flash sintering utilizes the non-equilibrium rise in current under applied electric field to densify ceramic green body compacts in seconds. The model materials used in this thesis were ZnO, TiO2, and CeO2 as they are simple binary oxides which avoids the complication of atomic segregation of multi-cation compositions such as 3 and 8 mol% yttria stabilized zirconia (3YSZ and 8YSZ). This work analyzes the proposed mechanisms for the onset of the flash, the cause of the enhanced sintering kinetics during FS, and the temperature approximation methods used as supporting evidence for each theory. A new temperature approximation technique, referred to as EDXRD temperature calibration, utilizes white beam energy dispersive x-ray diffraction (EDXRD) from a synchrotron source to track the lattice expansion during FS compared to the lattice expansion during conventional sintering (CS). The temperature has been measured as the rise in temperature causes a proportional lengthening of the bonds, which increases the unit cell volume. For all three test materials the FS temperature has been shown to be comparable to the CS temperature. To estimate average temperature a modification of the blackbody radiation model is presented, which incorporates non-ideal emissivity (ε&lt;1) and cooling due to thermal conduction.A new procedure to conduct flash sintering experiments was developed using ZnO as a test case. Rather than allowing an uncontrolled rise in current followed by an abrupt limit to constant current, the current was ramped up linearly. Using impedance spectroscopy it was determined that the conductivity of ZnO increased with an increasing rate of the current ramp and the highest conductivity was measured for the sample densified using conventional FS. This effect was attributed to the increasing loss of oxygen at higher current ramp rates due to the higher driving force for current flow.Oxygen reduction of TiO2 was observed in situ using EDXRD, where secondary peaks were formed at higher d-spacing. Consistent with the understanding of ionically bonded ceramics, the creation of oxygen interstitials requires oxidation of metal cations to maintain charge neutrality, which resulted in a lattice expansion with secondary peaks forming at higher d-spacing. Upon further analysis, utilizing Raman Spectroscopy and X ray Photoelectron Spectroscopy, a remnant reduction of TiO2 remained even after the electric field was turned off and the specimen was cooled to room temperature. The greater reduction of the anode in comparison to the cathode corresponded to increased densification and grain growth in the anode region, which suggests that the reduction is accelerated by the decrease in the number of vacancy blocking grain boundaries due to grain growth.  In situ EDXRD indicated a significant difference in the lattice expansion for CeO2 when platinum paste is applied to both ends compared to no platinum paste applied on the faces of the sample adjacent to the electrical contacts. For specimens with platinum paste, a large lattice expansion as well as peak splitting was observed towards the cathode of the pellet facing the negative electrode. This effect disappeared when no platinum paste was applied and a somewhat asymmetrical lattice expansion towards the anode was observed as with the case of TiO2. The lattice expansion in the platinum coated sample did not match with the expected microstructure: grain growth was promoted towards the anode and suppressed towards the cathode. Thus, while the lattice expansion was dominated by the production of oxygen interstitials as with the case of TiO2, this was unrelated to the enhanced sintering and grain growth towards the anode.TEM-EDS analysis indicated a significant difference in stoichiometry between grain boundary and bulk regions at the anode. Significantly higher oxygen content was found at thick grain boundaries (~2nm) at the anode, indicating oxygen ion diffusion along the grain boundaries which were trapped due to the blocking effect of the platinum paste. At the cathode the grain boundaries were clean with negligible thickness and similar atomic composition to the bulk. Inhomogeneity in the lattice expansion and microstructure was not observed when FS is performed using an AC power supply. Microstructural inhomogeneity was observed for all three materials in the direction of the electric field. The inhomogeneity in microstructure agreed with EDXRD profile inhomogeneity indicating an inhomogeneous temperature profile. The data was fitted to a new model incorporating the Peltier effect, which should occur under DC electric field for p-type and n-type semiconductors. As ZnO, TiO2, and CeO2 are all n-type semiconductors the grain growth towards the anode in all cases agrees with temperature inhomogeneity due to the Peltier effect as increased temperature leads to grain growth.  As the microstructural (grain size and porosity) inhomogeneity was dominated by the Peltier effect, a Joule heating phenomenon, it was determined that the sintering behavior is dominated by Joule heating. The nucleation and avalanche of Frenkel defects are not required to explain the sintering as the sample temperature was not below CS temperatures. In addition, as no experimental evidence for the generation of metal vacancy interstitial pairs has been produced to date a simpler explanation should be found. The enhanced sintering and grain growth kinetics can also be explained as a result of rapid heating due to the internal heat generation, which is a known effect in CS.