C.S. Chen

Oxygen permeation through (Bi2O3)0.74(SrO)0.26–Ag (40% v/o) composite

Abstract The dense dual phase composite membrane made from strontium-stabilized bismuth oxide and silver, (Bi2O3)0.74(SrO)0.26–Ag (40% v/o), was investigated. The composite was found to exhibit very high electrical conductivity at the room temperature, revealing that the silver phase has formed electron-conducting networks in the oxide matrix. The composite shows much improved oxygen permeability compared with the bismuth oxide alone. An oxygen flux of 5×10−8xa0molxa0cm−2xa0s−1 was observed for a 1.00xa0mm thick composite at 700°C with oxygen partial pressures of the feed and permeate side at 0.209, 0.0024xa0atm, respectively. Combination of electrical conductivity and oxygen permeation measurements reveals that oxygen-ion conduction through the oxide phase of the composite is the rate-limiting step for oxygen permeation.

Oxygen permeation through oxygen ion oxide-noble metal dual phase composites

Oxygen permeation behaviour of three composites, yttria-stabilized zirconia-palladium, erbia-stabilized bismuth oxidenoble metal (silver, gold) was studied. Oxygen permeation measurements were performed under controlled oxygen pressure gradients at elevated temperatures. Air was supplied at one side of a dense sintered disk specimen, while helium was fed at the opposite side to sweep away the permeated oxygen. This research has demonstrated that in addition to the presence of percolative metal phase in the oxide matrix, a large ionic conductivity of the oxide phase and a high catalytic activity of the metal phase towards surface oxygen exchange are required for the dual phase composite to possess high oxygen permeability. The bismuth oxide-silver composite fulfils these requirements, hence showing the best oxygen permeability.

Mixed oxygen ionic and electronic conduction in CaFe0.2Ti0.8O3−δ: a combined oxygen permeation and electrical conductivity study

Abstract A combined oxygen permeation and electrical conductivity study on CaFe 0.2 Ti 0.8 O 3− δ was conducted in the temperature range of 1100–850°C, aiming at clarifying the oxygen transport mechanism. The oxide shows appreciated oxygen permeability at elevated temperature. An oxygen permeation flux of 6.37×10 8 molxa0cm −2 xa0s −1 was observed at 1100°C by applying an oxygen partial pressure difference to a 0.84-mm thick disk-shaped sample ( P O 2 ( h )=0.209 and P O 2 ( l )=0.004 atm). The apparent activation energy for oxygen permeation remained almost unchanged (170 kJ/mol) with the variation of the sample thickness in the range of 2.5–0.8 mm. This indicates that the oxygen permeation kinetics is controlled by the transport of charge carriers in the bulk of sample, and can by promoted by further reduction of the thickness. In combination with electrical conductivity measurements, the partial conductivities of oxygen ions and electron holes, σ 1 and σ e , were determined. The value of σ 1 is comparable to that for σ e at the high temperature of 1100°C, but the latter became dominant at reduced temperature. This is due to the much higher activation energy associated with the transport of oxygen ions (179±6 kJ/mol) than that for electron holes (22±2 kJ/mol). It becomes clear that increasing the partial conductivity of oxygen ions in the oxide should lead to the improvement of the oxygen permeability of the oxide.

Conductivity of Bi2O3-based oxide ion conductors with double stabilizers

This paper deals with the preparation and conductivity study of complex oxides in the systems Bi2O3−Y2O3−Nb2O5, Bi2O3−Y2O3−Gd2O3, Bi2O3−Y2O3−Sm2O3 and Bi2O3−Y2O3−Pr2O11/3. X-ray diffraction indicated that a less amount of doubledopant oxides than that, when they were separately used, could stabilize the fcc phase down to room temperature. It was also found that in general a small amount of a third oxide did not change the structure of the binary oxide, but slightly increased its conductivity. As the third oxide further increased, the conductivity decreased and even a second phase appeared which would result in a complex electrical behaviour, depending, on the nature of the correst onding binary systems.

Thickness dependence of oxygen permeation through erbia-stabilized bismuth oxide-silver composites

Oxygen permeation measurements were performed on erbia-stabilized bismuth oxide-silver (40 v/o) composite membranes in the range of thickness of 1.60–0.23 mm and temperature of 850–650 °C. Air was fed at one side of the membranes while permeated oxygen on the other side was swept away with helium. An oxygen flux as high as 1.56 × 10−7 mol/cm2/s was observed for a 0.23 mm thick membrane at 750 °C and Po2(1) = 0.056 arm. The examination of the dependence of permeance on the membrane thickness reveals that (a) the oxygen transport kinetics are controlled by the diffusion of oxygen ions in the bulk of membrane with thickness down to about 1.0 mm; (b) with further decrease in thickness, the oxygen permeation becomes partially limited by surface oxygen exchange. The surface exchange rate has been found to decrease less pronouncedly than the bulk process as temperature decreases. The mass transfer is suggested to be the rate-limiting step for the surface process.

Palladium membranes supported on porous ceramics prepared by chemical vapor deposition

Abstract Palladium films on porous alumina supports were prepared by the chemical vapor deposition (CVD) technique of reducing PdCl2 with hydrogen. The phase composition and morphology of Pd composite membranes were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and scanning tunnelling microscopy (STM). A well-crystallized palladium layer was deposited on porous α-alumina supports. The thickness and grain size of the palladium layer were estimated to be about 4 μm. The palladium grains consisted of columnar crystallites of 20–40 nm in length and 5–8 nm in width. The penetration depth of palladium into the α-alumina support was as large as 20 μm. An opposite-diffusion CVD technique is suggested for preparing a dense, thin palladium membrane with high hydrogen permeability and good mechanical stability.

Preparation and characterization of Pd and Pd-Ni alloy membranes on porous substrates by MOCVD with mixed metal β-diketone precursors

A novel metal-organic chemical vapor deposition (MOCVD) process with a mixed metal {beta}-diketone precursor to successfully prepare Pd and Pd-Ni alloy composite membranes is described. A palladium phase deposit consisting of agglomerates of tiny crystallites with a dimension of around 30 x 10 nm was formed on porous Al{sub 2}O{sub 3} substrates, when using a single Pd(AcAc){sub 2} precursor. With a mixed precursor of Pd(AcAc){sub 2} + Ni(AcAc){sub 2}, uniform and dense Pd-Ni alloy membranes (consisting of primary crystallites with a dimension of 40 x 15 nm) were reproducibly obtained on porous alumina supports. N{sub 2} permeation tests indicated that the Pd-Ni alloy layer was gastight and crack-free. XPS studies revealed that no oxygen nor carbon impurities were incorporated into the bulk membranes. The Pd/Ni ratio on the surface of Pd-Ni alloy layer was very close to that of the mixed precursor, but much less Ni content (<1/150) was detected in the deposit body.

Performance evaluation of SOx (x = 2,3) gas sensors using Ag-β′'-alumina solid electrolyte

Abstract Fully solid state SO x sensors have been investigated with a galvanic cell of Pt, Ag/Ag-β′-alumina/SO 3 , SO 2 , O 2 , Pt using tubular Ag-β′-alumina as solid electrolyte, metal silver as solid reference electrode, and porous Pt as working electrode. The values of electric motive force (EMF) of the sensors are consistent with the theoretically calculated ones at temperatures above 550 °C in the p SO x range of at least 10–50 000 ppm. Applying catalysts such as Pt-net and V 2 O 5 in the gas upstream considerably improves the sensor performance and extend its working temperature down to 450 °C. The reproducible EMF values are obtained in the gas flow velocity of 0.2–0.4 cm/s. The response time of the sensors is 50–100 s as the p SO x changes. The sensors exhibit a good selectivity to SO x against hundred-fold concentrations of CO 2 , NO 2 and H 2 O.

MOCVD synthesis of yttria doped perovskite type SrCeO3 thin films

Abstract Yttria doped SrCeO3 films were deposited on YSZ substrates by MOCVD using a novel solid precursor containing a mixture of β-diketonate complexes of Sr, Ce and Y. The deposited films were polycrystalline consisting of nanoscale crystallizes. Single crystal YSZ substrate was found to be most suitable for preparing yttria doped SrCeO3 films. Using dry and well-mixed precursor was critical for obtaining a single phase yttria doped perovskite SrCeO3 films by the MOCVD method.

Electrical conduction and oxygen transport in SrFeCo0.5Ox oxide membranes

Abstract Electrical and oxygen transport properties of SrFeCo 0.5 O x oxide were investigated at elevated temperatures. The electrica1 conductivity was found in the order of 20–10 S/cm in the temperature range of 1100–600°C. An abnormal region was observed at temperatures around 900°C in which electrical conductivity increased with decreasing temperature, which may be related to the undergoing of phase transformation. An oxygen permeation flux of 159×10 −8 mol/cm 2 ·s was measured in an air/helium gradient at 900°C. The acid treatment on the sintered oxide resulted in a considerable improvement of the oxygen permeability, indicating that the surface oxygen exchange limits the overall oxygen permeation process.