Dingkun Peng

Hydrothermal synthesis of LaFeO3 under carbonate-containing medium

Abstract Perovskite-type LaFeO 3 has been synthesized hydrothermally under Na 2 CO 3 -containing medium, and characterized by X-ray diffraction (XRD), SEM, IR and ICP. The product corresponds to orthorhombic perovskite-type LaFeO 3 , with a cubic habit over a small particle size range of 10–20 μm. The LaFeO 3 formation temperature decreased by about 150°C by introducing carbonate into the hydrothermal system.

Sintering and electrical properties of (CeO2)0.8(Sm2O3)0.1 powders prepared by glycine–nitrate process

Abstract (CeO2)0.8(Sm2O3)0.2 (SDC) powders were prepared using the glycine–nitrate process (GNP) with different glycine/metal ratios. The phase identification, morphology and electrical properties of SDC powders were investigated by XRD, TEM and the AC impedance spectroscopy, respectively. It was found that the ratio of glycine/metal had a great effect on both the morphology and the sinterability of the powders as well as the conductivity of the sintered pellets. When the ratio is around the stoichiometric value (about 1.6), the loose powders possessed a foam-like structure. The bulk density of SDC pellets sintered at 1500 °C could reach 95% only when the ratio of glycine to metal was in the range of 1.3–2.0. The conductivity increased with the ratio of glycine to metal till at the ratio of 1.7, and then decreased. The maximum conductivity of the sintered specimens was 0.082 Scm−1 at 800 °C.

Porous YSZ ceramics by water-based gelcasting

Gelcasting, as a novel method to form ceramic bodies, has been successfully developed to fabricate porous YSZ ceramics with an open porosity of 33.1–50.3%, mean pore size of 0.66–0.98 μm and the nitrogen permeability of 215–438 m3/m2.bar.h. In order to further illustrate the features of this water-based gelcasting process to prepare porous ceramics, the same YSZ powders were blended with the same additives, and then cold pressed and sintered at the same conditions employed for gelcasting process. Compared with the cold pressed samples, the gelcast bodies exhibit higher open porosity, lower closed porosity, relatively larger pore size and thus higher gas permeability. Therefore, the developed gelcasting process is a very effective method to fabricate porous ceramics for filters or supports.

Preparation of yttria stabilized zirconia membranes on porous substrates by a dip-coating process

Abstract Gas-tight yttria stabilized zirconia (YSZ, 8 mol.% Y 2 O 3 ) membranes with expected composition were coated on porous ceramic substrates by a dip-coating process with YSZ sol, which was prepared by ultrasonic dispersion of YSZ powder in ethanol. The YSZ powder was synthesized by co-precipitation from inorganic aqueous solutions, followed by azeotropic distillation and sintering at 600°C for 2 h. The crystalline structure of YSZ powder was characterized by X-ray diffraction. Microstructures of YSZ membranes were analyzed by scanning electron microscopy and nitrogen permeation testing. The investigation showed that defects in the membranes could be gradually removed by repeating the dip-coating procedures. The electrical transport properties of the YSZ membrane were analyzed by AC impedance spectroscopy. The electrical conductivity of the membrane was 0.001 s cm −1 at 900°C and the activation energy of electrical conductivity was found to be 87.5 kJ mol −1 .

Intermediate-temperature SOFCs with thin Ce0.8Y0.2O1.9 films prepared by screen-printing

Thin electrolyte films of yttria-doped ceria (YDC, Ce0.8Y0.2O1.9) were fabricated on green substrates of NiO–YDC by a screen-printing technique. Dense YDC thin layers were subsequently formed on the porous NiO–YDC substrates after the green bi-layers were co-sintered at 1350 °C. The YDC films were about 15 μm as characterized by scanning electron microscope. With Sm0.5Sr0.5CoO2.75 as cathodes, single cells were tested from 450 to 650 °C with humidified (3% H2O) hydrogen as fuel and air as oxidant. Open circuit voltage of 0.97 V was obtained at 450 °C, indicating negligible gas permeation through the YDC thin films. Maximum power density of about 360 mW/cm2 at 650 °C was achieved with current density of about 800 mA/cm2. Electrochemical characterizations of the single cells indicate that it is critical to reduce the electrode polarization by developing new electrode materials as well as fabrication processes for solid oxide fuel cells that are operated at intermediate temperature.

Preparation of Nd-doped BaCeO3 proton-conducting ceramic and its electrical properties in different atmospheres

Abstract Nd-doped BaCeO 3 was prepared by a conventional ceramic processing technique using a special procedure to reduce calcining and sintering temperatures and to avoid possible contamination. BaCe 0.9 Nd 0.1 O 3 − α single perovskite phase was formed when the mixture powders was calcined at T ≥1000 °C. Ball-milling of the calcined powders could well disperse agglomerates. Sintered at T ≥ 1300 °C, specimens with density ≥ 93% of the theoretical and without open porosity could be obtained. Electrical conductivity was measured in different dry atmospheres of Ar, air and O 2 and in moist air. The results showed that in dry Ar, air and O 2 , the conductivity values at a given temperature were similar, and the activation energies almost identical, possibly because Nd-doped BaCeO 3 demonstrated predominately oxygen ion conduction in these environments. In moist air, proton conduction might predominate, leading to an increase in conductivity and a decrease in activation energy.

Chemical stability study of BaCe 0.9 Nd 0.1 O 3-α high-temperature proton-conducting ceramic

BaCe 0.9 Nd 0.1 O 3-α (BCN) ceramic is known to be an excellent high-temperature proton conductor and is a candidate electrolyte for use in solid oxide fuel cells, hydrogen or steam sensors and steam electrolysers. In this work, the chemical stability of BCN was investigated systematically by combining XRD and DTA–TG techniques to study its processing compatibility and its feasibility in potential applications. It was found that above 1200 °C, BCN reacted with alumina or zirconia, leading to the loss of barium and an excess of cerium. In cold water, both sintered BCN disks and powder samples had very low solubility and did not hydrolyse, but they were soluble in some mineral acids, especially in HCl with the liberation of Cl 2 . In boiling water, BCN pellets dissolved readily with decomposition into CeO 2 and Ba(OH) 2 . In 1 atm CO 2 , BCN decomposed to form CeO 2 and BaCO 3 below 1200 °C during heating, but during cooling it was stable above 1000 °C, possibly because BCN has different crystal structures at low and high temperatures. At 600–1000 °C, BCN showed a slight mass loss when exposed to a reducing atmosphere, and a slight mass gain in an oxidizing atmosphere. XRD results revealed that BCN demonstrated chemical and structural stability in both reducing and oxidizing atmospheres.

Effect of powder preparation on (CeO2)0.8(Sm2O3)0.1 thin film properties by screen-printing

Chemical co-precipitation process and glycine-nitrate process (GNP) were used to synthesize (CeO2)0.8(Sm2O3)0.1(SDC) powders for the preparation of thin dense film by screen-printing. XRD, transmission electron microscope (TEM), scanning electron microscope (SEM) and the AC impedance spectroscopy were used to investigate the phase identification, morphology and electrical properties of SDC powders, respectively. It was found that the powders prepared by both processes possessed similar sinterability and conductivity. To both synthesized powders, the SDC pellets sintered at 1350 °C can reach 96% in relative density and 0.08 S cm−1 in conductivity at 800 °C in air. Thin electrolyte films about 30 μm have been successfully achieved by screen-printing with both powders. While the thin electrolyte film by GNP powders possessed better sinterability and higher conductivity, which was attributed to its high pack density during the green film preparation.

Novel intermediate temperature ceramic fuel cells with doped ceria-based composite electrolytes

Abstract A series of novel composite electrolytes, consisting of doped ceria (DCO) and salts were developed and examined. The salts included chlorides and carbonates. Their oxygen ion and/or proton conductivity at intermediate temperature (IT) range (500–700 °C) can be even 2–15 times higher than DCO. Furthermore, they also show higher ionic transference number than DCO. The long-term stability of fuel cells based on these composite electrolytes was also discussed.

Sintering and electrical properties of coprecipitation prepared Ce0.8Y0.2O1.9 ceramics

Abstract Fine Ce 0.8 Y 0.2 O 1.9 (YDC) powders were prepared by a chemical coprecipitation process. The effect of the calcination temperature on the sinterability and electrical conductivity of the resulting YDC ceramics was investigated. Broadening degrees of the X-ray peaks of YDC powders indicate that the crystallite sizes of the powders significantly increase with the calcination temperature from 500 to 1000°C, which is consistent with the transmission electron microscopy (TEM) results. Scanning electron microscopy (SEM) photographs of the surface microstructures revealed that YDC ceramics from the powders calcined at temperatures 500–750°C become highly dense (over 98% theoretical density) when sintered at 1500°C, while the ceramic calcined at 1000°C reached only 93% relative density. On the other hand, the average grain size of specimens obtained from powders obtained at increasing calcination temperature decreased from 2.5 to 1 μm, correspondingly. These results are attributed to the high sintering activity and grain growth rate at the same sintering condition for the finer powders calcined at lower temperature. The electrical measurement by ac impedance spectroscopy shows that the specimen from the powder calcined at 750°C exhibits the highest electrical conductivity, approximately 4 Sm −1 at 750°C in air.