WEB Effect of AC field on sintering of polycrystalline ceriaFriday (25.09.2020) 11:20 - 11:35 P: Processing and Synthesis 1 Part of:
The application of electrical fields during sintering aims to reduce the sintering time and temperature as well as improve the targeted properties has aroused the attentions since decades. However, governing principles of this phenomenon are still not fully resolved. In this work, the material system 10 mol % and 0.1 mol % yttrium doped ceria (10YDC and 01YDC) were chosen. The specimens were sintered under ac electrical fields with the frequency of 50 Hz significantly weaker than those required for the ‘flash regime’. For the first time, the dependence of sintering parameters on the applied electrical fields was investigated with constant sample temperature, which was confirmed by direct measurement inside the sample and by thermo-electric finite element simulations. These conditions enable to exclude the macroscopic Joule heating and to identify accurate mechanisms. The sintering behavior was significantly improved, which points to the existence of other mechanisms beside macroscopic thermal effects: either microscopic Joule heating or a direct effect of the field on the grain boundary diffusion process, which leads to the possible decrease in the activation energy. The latter reason is favored by the fact that the activation energy of two ceria materials with two different doping levels decreases under electrical fields. Activation energy for 10YDC varied from 502 kJ/mol without electrical fields, to 311 kJ/mol under Erms = 28 V/cm, while activation energy for 01YDC varied from 485 kJ/mol without electrical fields, to 439 kJ/mol under Erms = 28 V/cm.
So far all sintering parameters for ceramics have been obtained under compressive tests. As basically all existing sintering models silently assume that the compressive and tensile response is symmetric, it is a very crucial question whether the compressive response can be extrapolated to tensile stress configurations. Therefore, the symmetric behavior under compressive and tensile loading could be experimentally verified for the first time. According to the experimental results, the uniaxial viscosity response of 01YDC was confirmed to be the symmetric between compressive and tensile responses, which means the compressive response can be extrapolated to tensile stress configurations.