Xingqin Liu

Preparation and gas-sensing properties of CuFe2O4 at reduced temperature

Abstract Sol-gel and co-precipitation were used for preparation of copper ferrite. Pure CuFe2O4 was not obtained until 800°C by a sol-gel process, which might be due to the loose contact between copper and iron atoms and the possible low diffusion rate. However, nanocrystalline CuFe2O4 was prepared by a co-precipitation method. The tetragonal CuFe2O4 phase was formed at 90°C. The decomposition of amorphous hydroxides in the dried precipitate continued until 450°C, according to a TG-DTA analysis. Conductance–temperature measurements of the sensor made of CuFe2O4 indirectly indicated that the amount of absorbed oxygen was small, as reflected by the relatively low response. The CuFe2O4 sensors showed a good response to alcohol, which is promising for further development of this type of sensor. The difference in response for various tested gases might be attributed to absorption of reducing gases and reaction between these gases and the absorbed oxygen. It is supposed that the gas-sensing properties of this material might be improved by the addition of catalysts.

Cathode processes and materials for solid oxide fuel cells with proton conductors as electrolytes

This article provides a brief review of the cathode reaction mechanisms in proton-conducting solid oxide fuel cells (H-SOFCs) and a comparative analysis of electrochemical performance and the ionic and electronic transport in cathode materials. The transfer of protons from electrolyte to triple phase boundaries (TPBs) and the diffusion of O−ad from catalytic sites to TPBs are proposed to be the rate-limiting steps for H-SOFCs, of which the latter is more related to the cathode components, microstructure, conducting species and electrical conductivity. The experimental and theoretical analyses also suggest that cathode materials with electron and proton conductivity present smaller polarisation resistances due to the ability of protons to transfer from the electrolyte into the bulk, which extends the reaction areas for protons and oxygen species to the entire gas/cathode surface.

Porous mullite ceramics from national clay produced by gelcasting

Gelcasting process has been successfully employed to fabricate porous mullite ceramics in this work. The specimens based on mullite composition are found with open porosity of 58.5–63.9%, mean pore size of 0.76–1.31 μm, and nitrogen permeability of 526–1240 m3 m−2 bar−1 h−1 by reactively sintering the gelled mixture of kaolinite and aluminum hydroxide at 1300–1600°C. The porosity, mean pore size, pore size distribution and gas permeability can be controlled by adjusting raw material ratios and sintering temperatures. The gas permeability of the specimens is found to be more dependent on the average pore size than on the open porosity. In addition, the gas transportation mechanism in porous mullite ceramics is dominated by laminar flow when the specimens are fired at high temperatures.

Ethanol-sensing characteristics of barium stannate prepared by chemical precipitation

Abstract Perovskite-type oxide BaSnO 3 has been prepared at reduced temperature. The cubic BaSnO 3 phase forms at 400°C when a precipitation method is applied. Thermal analysis indicates that the decomposition temperature of BaSn(OH) 6 is below 600°C. The trace amounts of BaCO 3 in the precipitate are due to the reaction of barium ions with the atmospheric CO 2 in a strong basic solution. Conductance measurement between 200 and 550°C indirectly demonstrates the gas-sensing mechanism of BaSnO 3 might be a surface-controlled process. The gas-sensing properties of BaSnO 3 to ethanol are first reported. The sensor made of the BaSnO 3 prepared by a precipitation method exhibits low responses to LPG, petrol, H 2 and CO but high response and good selectivity to ethanol. The as-prepared BaSnO 3 is a promising ethanol-sensing material.

Preparation of porous ceramics by gelcasting approach

Abstract Gelcasting has been developed as a novel ceramic forming method for complex-shaped advanced materials of near-net-shape. In this work, the gelcasting process has been further developed to fabricate porous single- and multi-component ceramics. Porous multi-component ceramics were obtained from gelcast body of precursors by a reaction-sintering process. Large α-Al 2 O 3 powders with a d 50 value of 21.12 μm and mixed precursors were used to prepare porous α-Al 2 O 3 and La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3− δ ceramics, respectively. The experimental results presented here demonstrate the utility of the gelcasting process for fabricating practically useful porous ceramics.

Preparation and gas-sensitive properties of LaFe1−yCoyO3 semiconducting materials

Abstract In this paper, the ultrafine powders of the perovskite-type solid solution LaFe 1− y Co y O 3 (0≤ y ≤0.40) were prepared by chemical coprecipitation. The preparation conditions, phase constituents, conductance and gas-sensing properties were investigated. The results demonstrate that LaFe 1− y Co y O 3 is a p-type semiconducting material and it has high sensitivity and good selectivity to C 2 H 5 OH when y =0.20.

A novel route of synthesizing La1-xSrxCoO3 by microwave irradiation

Abstract La 1− x Sr x CoO 3 (LSC, x =0.10–0.50) powders with pure perovskite structure have been synthesized from mixtures of La 2 O 3 , SrCO 3 , and Co 3 O 4 , irradiated in a domestic microwave oven (DMO) with 2.4.5 GHz and 700–800 W output for no more than 25 min. It is found that Co 3 O 4 couples with microwave well and raises its temperature greatly within 3 min, while neither La 2 O 3 nor SrCO 3 absorbs microwave energy at the ordinary temperature. The microwave synthesizing conditions depend on the Sr 2+ replacement amount, and get difficult when x is high. The traditional solid state reaction method is compared to treat the same mixtures. It appears that the pure phases of LSC cannot be obtained until 1100–1200°C for 4 h. The lower temperature and shorter time with microwave irradiation might be ascribed to the activating and facilitating effect of microwave on solid phase diffusion. The LSC powders made by the microwave process were screen-printed onto compact yttrium-doped ceria (YDC) pellets, the common electrolyte for solid oxide fuel cells (SOFC), to observe the interaction between them. SEM photograph shows that LSC film has good sintering and porous properties, and fair compatibility with YDC. Thus, microwave irradiation is proved to be a novel, extremely facile, time-saving and energy-efficient route to the synthesis of LSC powders.

Ammonia-sensing characteristics of Pt-doped CdSnO3 semiconducting ceramic sensor

Abstract The initial experimental results concerning the ammonia-sensing properties of Pt doped mixtures of α- and β-CdSnO 3 are reported. The mixtures of different β α ratios can be obtained by calcining the CdSnO 3 powder precursor prepared by coprecipitation under different experimental conditions. As compared with that of other samples, however, the 1 wt% Pt doped mixture with β α ratio of 2.0 exhibits a very high sensitivity and selectivity to NH 3 gas, moderate resistance and good response characteristic, as well as low operating temperature (240 °C) for this application.

Low cost porous mullite-corundum ceramics by gelcasting

A gelcasting process has been successfully employed to fabricate porous mullite-corundum ceramic composites from natural clay and corundum powders. The specimens based on the mullite composition are found with an open porosity of 45–47.9%, mean pore size of 1.28–2.55 μm, and nitrogen permeability of 965–5038 m3· m−2· bar−1· hr−1by reactively sintering the gelled mixture of kaolinite and α-alumina at 1100–1500°C. The open porosity (ρo), mean pore size (dp), pore size distribution and gas permeability can be controlled by adjusting raw material ratios and sintering temperatures. The gas permeability of the specimens is found to be more dependent on the pore size distribution as well as dpthan on ρo. In addition, the gas transportation mechanism in porous mullite-corundum ceramic composites is dominated by viscous flow.

Preparation and electrochemical characterization of Sr- and Mn-doped LaGaO3 as anode materials for LSGM-based SOFCs

Sr- and Mn-doped lanthanum gallate powders (LSGMn, La0.9Sr0.1Ga1 − xMnxO3 − δ, x = 0.20, 0.35, 0.43) were prepared by glycine-nitrate combustion synthesis. X-ray diffraction patterns indicate the perovskite structure was formed without any second phase after calcining the powders at 1000°C for 4 h. Compacts of powders synthesized under stoichiometric combustion were sintered to densities over 95% of theoretical values. The electrical conductivity of this material in both air and H2 were characterized using AC impedance spectroscopy. It showed that the conductivity in H2 atmosphere is lower than that in air due to p-type electrical conduction in this material, and the electrical conductivity increases remarkably with increasing manganese content. Good chemical stability of La0.9Sr0.1Ga1 − xMnxO3 − δ in H2 atmosphere as well as the relatively high conductivity makes it an appropriate anode material for Sr- and Mg-doped lanthanum gallate (LSGM)-based IT-SOFCs. Preliminary fuel cell performance measurements were performed, showing promising electrochemical properties of such anode materials.