Influence of an electric field on grain growth and sintering in strontium titanate

Within the last five years considerable efforts were done in investigating electric field assisted sintering (flash sintering). However the experiments are hard to control: shrinkage occurs within seconds and the local temperature is undefined due to joule heating. Therefore the present study removes these two parameters by investigating grain growth under electric field in the no-current-case for strontium titanate. The impact of an electric field on grain growth in strontium titanate is investigated between 1350°C and 1550°C for fields of up to 50V/mm. To prevent joule heating by a current flowing through the material insulating Al2O3 plates separate the electrodes from the samples. The seeded polycrystal technique is used, which allows evaluating the grain boundary mobility without an influence of the grain boundary energy. The growth direction of the single crystalline seeds is perpendicular to the electric field; hence electrostatic forces do not influence the growth. Below 1425°C the influence of the electric field is weak. However above 1425°C the field results in an increase of the grain boundary mobility at the negative electrode. The range of this increase is in the order of ~1mm. It is shown that abnormal grain growth can be triggered by the electric field. Based on the experimental findings a model is established based on a shift of charged defects. The enhancement of the grain boundary mobility on the negative electrode is explained by an accumulation of oxygen vacancies. This accumulation induces a reduction of the material. A reduction of strontium titanate by atmosphere also results in an increase of the grain boundary mobility, which accords well with the observed behavior under electric field

Zur Grenzflächenanisotropie von SrTiO3

Die Materialgruppe der Perowskite enthält viele wichtige Funktionskeramiken, welche sich durch herausragende elektrische Eigenschaften auszeichnen. In vielen Anwendungen besteht jedoch ein starker Einfluss der Mikrostruktur und daher auch des Kornwachstums.Zur Modellierung des Kornwachstums ist die Kenntnis der Parameter Korngrenzmobilität und -energie nötig. In dieser Arbeit wurden beide Parameter für das perowskitische Modellsystem SrTiO3 abhängig von Temperatur und Atmosphäre gemessen

Impact of AC and DC Electric Fields on the Microstructure Evolution in Strontium Titanate

Herein, the impact of AC and DC electric fields on microstructure evolution in strontium titanate is investigated. The focus is on nonthermal effects by using current-blocking electrodes. The seeded polycrystal technique allows investigating the impact of a DC electric field on grain growth for different grain-boundary orientations and the impact of the surrounding atmosphere. As in previous studies, faster grain growth is observed at the negative electrode. This effect is stronger for the (100) orientation and in reducing atmosphere. In AC electric field at 1450 °C, a low-enough frequency results in faster grain growth at both electrodes. These findings agree well with previous studies, where an electromigration of oxygen vacancies is found to cause a local reduction at the negative electrode, resulting in less space charge, less cationic segregation, and a higher grain-boundary mobility. At 1500 °C, AC electric fields are found to cause a complete grain growth stagnation at very small grain sizes. This behavior is unexpected; the physical reasons are not clear. Herein, a brief study of sintering in DC electric field reveals slightly faster sintering if a field is applied

Dislocation and grain boundary interaction in oxides: Slip transmission or cracking?

Please click Additional Files below to see the full abstract

Impact of an external electric field on grain growth in oxides: Comparison of flash sintered samples to field assisted grain growth

In the last years ample effort was done to investigate the effect of electric fields on matter. We investigated the effect of an external electric field on the oxide ceramic model system strontium titanate. More precisely, we observed that a non-contacting external electric field has an impact on the defect distribution and the grain growth. Oxygen vacancies are migrating towards the negative electrode yielding a higher oxygen vacancy concentration compared to the positive electrode. As a result, faster grain growth was observed on the negative electrode. Recent thermodynamic defect calculations revealed the mechanism for this relationship [1]: A high oxygen vacancy concentration results in less space charge and, as such, in less segregation of cationic defects. As less segregation requires less diffusion for grain boundary migration, faster grain growth occurs. We extended these findings to flash sintering of doped strontium titanate. TEM imaging and EDS analysis were used to investigate the microstructure and to map the dopant segregation at the grain boundaries. Observing different dopant species (acceptors and donors) gives insight on flash sintering for different defect concentration and types with different segregation properties. In addition, field assisted microstructure evolution experiments with titania (no current, insulating electrodes) allow to apply the gained knowledge to different material systems with different defect chemistry. [1] Work of Jana P Parras and Roger A. de Souz

Ultra-Fast High Temperature Sintering (UHS) of Strontium Titanate

Please click Additional Files below to see the full abstrac

Defect redistribution along grain boundaries in SrTiO3_3 by externally applied electric fields

During thermal annealing at 1425 °C nominal electric field strengths of 50 V/mm and 150 V/mm were applied along the grain boundary planes of a near 45° (100) twist grain boundary in SrTiO$_3$. Electron microscopy characterization revealed interface expansions near the positive electrode around 0.8 nm for either field strength. While the interface width decreased to roughly 0.4 nm after annealing at 50 V/mm, the higher field strength caused decomposition of the boundary structure close to the negative electrode. Electron energy-loss and X-ray photoelectron spectroscopies demonstrated an increased degree of oxygen sublattice distortion at the negative electrode side, and enhanced concentrations of Ti$^{3+}$ and Ti$^{2+}$ compared to bulk for both single crystals and bicrystals annealed with an external electric field, respectively. Oxygen migration due to the applied electric field causes the observed alteration of grain boundary structures. At sufficiently high field strength the agglomeration of anion vacancies may lead to the decomposition of the grain boundary

Highly conductive grain boundaries in cold-sintered barium zirconate-based proton conductors

Proton-conducting barium zirconate ceramics have shown large potential for efficient electrochemical conversion and separation processes at intermediate operation temperatures. The high energy efficiency, robustness, and intermediate-temperature operation (500-650 °C) make proton-conducting cells promising candidates for future energy conversion systems. However, the major disadvantages of these materials are the inevitable high-sintering temperatures (>1500 °C), leading to Ba-evaporation and formation of high-resistance grain boundaries, which dominate the electrochemical performance. Here, we introduce a novel processing route for proton-conducting barium zirconates, which, on the one hand, significantly lowers the maximum processing temperature and, on the other hand, overcomes the dominating influence of grain boundaries on total conductivity. The key step of this novel processing route is the cold sintering of the powder using pure water as a sintering aid to consolidate BaZrCeYO (BZCY) at 350 °C. We show that clean grain boundaries with a high acceptor-dopant concentration are preserved thanks to the recovery of the perovskite phase during thermal treatment at 1300 °C. This compensates the interfacial core charge, resulting in highly conductive grain boundaries, which do not limit the total conductivity. Consequently, dense BZCY electrolytes produced by our novel approach outperform the conductivity of conventionally sintered BZCY irrespective of the significantly lower maximum processing temperature and its nanocrystalline microstructure. Our presented approach opens up new possibilities for grain boundary engineering and might facilitate novel co-sintering pathways for barium zirconate-based components.The authors acknowledge Dr Doris Sebold for help with SEM investigations and Dr Yoo Jung Sohn for assistance with HT-XRD measurements. M. K. acknowledges financial support from the DFG under project number MA 1280/69-1. Additionally, D. J. and W. R. thank the DFG for funding within the Emmy Noether program (RH 146/1-1). A. V. expresses gratitude to Dr Ivan Povstugar for his insightful discussions on the quality of APT data and its reconstruction. The authors thank Hitachi High-Technologies for providing access to the HF5000 STEM located at ER-C

Microstructure and conductivity of blacklight‐sintered TiO₂, YSZ, and Li₀.₃₃La₀.₅₇TiO₃

Rapid densification of ceramics has been realized and its merits were demonstrated through multiple approaches out of which UHS and flash sintering attract recent attention. So far, however, scalability remains difficult. A rise in throughput and scalability is enabled by the introduction of blacklight sintering powered by novel light source technology. Intense illumination with photon energy above the bandgap (blacklight) allows high absorption efficiency and, hence, very rapid, contactless heating for all ceramics. While heating the ceramic directly with light without any furnace promises scalability, it simultaneously offers highly accurate process control. For the technology transfer to industry, attainable material quality needs to be assured. Here, we demonstrate the excellent microstructure quality of blacklight‐sintered ceramics observed with ultrahigh voltage electron microscopy revealing an option to tune nanoporosity. Moreover, we confirm that electronic, electron, oxygen, and lithium‐ion conductivities are equal to conventionally sintered ceramics. This gives the prospect of transmitting the merits of rapid densification to the scale of industrial kilns

Room‐temperature dislocation plasticity in SrTiO₃ tuned by defect chemistry

Dislocations have been identified to modify both the functional and mechanical properties of some ceramic materials. Succinct control of dislocation-based plasticity in ceramics will also demand knowledge about dislocation interaction with point defects. Here, we propose an experimental approach to modulate the dislocation-based plasticity in single-crystal SrTiO₃ based on the concept of defect chemistry engineering, for example, by increasing the oxygen vacancy concentration via reduction treatment. With nanoindentation and bulk compression tests, we find that the dislocation-governed plasticity is significantly modified at the nano-/microscale, compared to the bulk scale. The increase in oxygen vacancy concentration after reduction treatment was assessed by impedance spectroscopy and is found to favor dislocation nucleation but impede dislocation motion as rationalized by the nanoindentation pop-in and nanoindentation creep tests