Fast re-oxidation kinetics and conduction pathway in Spark Plasma Sintered ferroelectric ceramics

Abstract

The re-oxidation kinetics of BaTiO3 ceramics sintered by Spark Plasma Sintering (SPS) was investigated using in-situ impedance spectroscopy. Thanks to the flexibility of the SPS process, the grain size of the dense ceramics was tuned from 0.5m to 10m. The reoxidation kinetics are found to be very fast regardless of the grain size and a full re-oxidation of the ceramics are achieved after 20 hours of exposure to an ambient environment at only 600°C. The residual density of charge carriers is reduced when using finer starting powders.SPS ceramics made with micrometer size grains demonstrate a residual charge-carrier density that is one tenth that of ceramics made from 10µm particles. Grain-boundary conduction is dominant through fine-grain SPS ceramics. This latter feature is similar to BaTiO3 sintered using the conventional route with 10m size grain. Finally, the critical grain size for optimal dielectric permittivity is found to shift from 0.7m in standard ceramics to 1.5m in SPS ceramics.

Study of the Physical Properties and Electrocaloric Effect in the BaTiO3 Nano- and Microceramics

“…In other words, the high-density ( over 98% of the theoretical value) nanoceramics con tains a fewer amount of the resistive components as compared with the microcerarnics and thereby have a lower free charge density at the grain boundaries. In such nanoceramics, a decrease in the grain size does not change the type of carriers, associated mainly with oxygen vacancies, but, during reoxidation, leads to a more significant decrease in the density of such carri ers, which affects the transport conductivity mecha nism controlled mainly by the grain boundaries [27]. Figure 4 shows the measured data on the thermal expansion of the BaTiO 3 micro-and nanoceramic samples.…”

Исследования физических свойств и электрокалорического эффекта в нано- и микрокерамике BaTiO-=SUB=-3-=/SUB=-

“…Другими словами, нанокерамика высокой плотности (> 98% от теоретической) содержит в себе меньшее количество резистивной компоненты по сравнению с микрокерамикой и тем самым имеет меньшую плотность свободных зарядов на границах зерен. В такой нанокерамике уменьшение размера зерна не меняет тип носителей заряда, связанных в основном с кислородными вакансиями, но при повторном окислении приводит к более значительному уменьшению плотности таких носителей, что сказывается в значительной мере на транспортном механизме проводимости, регулируемом, в основном, границами зерен [27].…”

Specific core-shell approaches and related properties in nanostructured ferroelectric ceramics

“…A post-sintering annealing under air in the temperature range 700 -1000 °C is thus mandatory to minimize oxygen vacancies associated with the reduction of titanium Ti 4+ into Ti 3+ and to recover insulating properties (Ti 3+ into Ti 4+ ). The defect chemistry at the grain boundaries is then hardly fully mastered, in particular in nanostructured ceramics [10,26]. The combination of low temperature and inert atmosphere enabled to obtain BT@SiO2 ceramics exhibiting high stability of permittivity over a wide temperature range and low dielectric losses at low frequency (0.5% at 10 kHz).…”

“…The optimized sintering conditions are reported in Table 1. They were selected considering (i) the minimization of grain growth and thus the preservation of the core shell design, (ii) the limitation of the reduction of Ti 4+ into Ti 3+ associated with the creation of oxygen vacancies under vacuum at high temperature [26,39,40] and (iii) the control of interdiffusion at the interface between the ferroelectric core and Al2O3 to prevent the formation of barium aluminate (BAO = BaAl2O4) interphase. Further, it is well-know from Finite Element simulations of the SPS process, particularly when insulating materials are sintered, that the current distribution in the tool may induce temperature gradients, overheating of the punches and of the graphite sheet lining the inner wall of the mold [56,57].…”

“…Whatever the composition of the core and the morphology of the shell, SPS moderate conditions allowed to obtain high relative density ceramics (> 90%). It has to be noted that the alumina coating hinders the densification since for ceramics made of uncoated particles (BT and BST), relative density higher than 95% are obtained for similar sintering conditions [26].…”