Y.M. Guo

Point-defect-induced colossal dielectric behavior in GaAs single crystals

We herein reported colossal dielectric constant (CDC) behavior in GaAs single crystals. This behavior appears in the temperature range above room temperature and results from the bulk effect due to polaron relaxation caused by hopping motion of EL2 defects. When temperature rises higher than 420 K, the interfacial contribution due to Maxwell–Wagner relaxation caused by sample/electrode contacts appears. When temperature is higher than 560 K, the CDC behavior is mainly contributed by the interfacial effect. These features are quite different from the CDC behavior found in oxides, and therefore, the CDC behavior in GaAs single crystals is considered as a new type of the CDC family. Our results underscore the role of point-defects in CDC behavior and suggest that defect engineering can be a promising strategy to achieve superior CDC behavior in both oxide and non-oxide materials.

Normal and abnormal dielectric relaxation behavior in KTaO3 ceramics

KTaO3 ceramic samples were prepared via a conventional solid state reaction route. The dielectric properties of KTaO3 were investigated in temperatures from room temperature to 1000 K and the frequency range of 102–106 Hz. The sample exhibits an abnormal dielectric behavior contrary to the traditional thermally activated behavior in the temperature range below 450 K. Our results revealed that the sample was very sensitive to humidity, leading to a metal-insulator transition (MIT) at 473 K. It is the positive temperature coefficient of resistance of the MIT that results in the abnormal dielectric behavior. When the temperature is higher than 500 K, the sample shows two normal dielectric relaxations following thermally activated behavior. The low- and high-temperature relaxations were argued, respectively, to be related to dipolar relaxation and Maxwell–Wagner relaxations due to oxygen vacancies hopping inside grains and then being blocked by grain boundaries.

Performance and durability of a layered proton conducting solid oxide fuel cell fueled by the dry reforming of methane

Catalyst layers derived from La2NiO4, LaNiO3 and Ni/La2O3 precursors were applied to a conventional Ni-based anode in a proton conducting solid oxide fuel cell (H+-SOFC) for the dry reforming of methane with CO2. The phase structures, microstructures and catalytic activities of catalysts from the different precursors were systematically investigated. The cell performance and durability of a H+-SOFC with a catalyst layer (layered H+-SOFC) were examined. The layered H+-SOFC had higher cell performances than the conventional H+-SOFC. However, catalyst deactivation and degradation of the cell performance were observed as carbon deposition occurred on the catalyst layer due to CO disproportionation in exhaust gas at a high partial pressure of CO. The structure of carbon deposited on the catalysts was also investigated.

Performance variability of Ba0.5Sr0.5Co0.8Fe0.2O3−δ cathode on proton-conducting electrolyte SOFCs with Ag and Au current collectors

Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) is one of the most active cathode materials and shows significant hydration effect suggesting possible proton conductivity. In this study, the performance of BSCF cathode on a proton-conducting BaZr0.1Ce0.7Y0.2O3−δ (BZCY) electrolyte with silver and gold current collectors was determined. The electrochemical characteristics of the symmetrical and anode-supported cell with diluted silver electrode, silver current collector or gold current collector on BSCF electrode were compared. The significant result is that, although the diluted silver electrode itself shows poor operation stability, the silver current collector has strong electrocatalytic contribution to the BSCF cathode performance on the proton-conducting electrolyte, leading to higher cell performance than that with the gold current collector.

High-temperature colossal dielectric behavior of BaZrO3 ceramics

BaZrO3 (BZO) powders were synthesized by solution combustion based on the glycine nitrate process. The sintering conditions of the BZO sample were optimized by varying the sintering temperature and dwell time. The dielectric properties of the sintered samples were investigated in the temperature range of 300–1080 K and frequency range of 102 to 106 Hz. The sample sintered at 1973 K for 12 h was found to show the best dielectric properties. Detailed investigations on this sample reveal an incipient ferroelectric behavior in the temperature range below 420 K and a colossal dielectric behavior above 420 K. The colossal dielectric behavior is composed of two thermally activated relaxations. The low-temperature relaxation was argued to be a dipolar relaxation caused by an oxygen vacancy hopping motion inside grains and the high-temperature one was ascribed to a Maxwell–Wagner relaxation due to the oxygen vacancies being blocked by the sample/electrode contacts.