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Oxygen transport in the Sr{sub 2}Fe{sub 3{minus}x}Co{sub x}O{sub y} system.

Description: The mixed-conducting Sr-Fe-Co oxide has potential use as a gas separation membrane. Its superior oxygen transport reveals the feasibility of using oxide membranes in large-scale oxygen separation. Sr{sub 2}Fe{sub 3{minus}x}Co{sub x}O{sub y} (with x = 0.0, 0.3, 0.6, and 1.0) samples were made by solid state reaction. To understand the oxygen transport mechanism in this system, conductivity and thermogravimetry experiments were conducted at high temperature in various oxygen partial pressure environments. The oxygen diffusion coefficient was determined from the time relaxation transient behavior of the specimen after switching the surrounding atmosphere. Mobility of the charge carrier was derived from relative conductivity and weight changes. X-ray diffraction experiments were carried out on these samples to determine their crystal structures.
Date: January 4, 1999
Creator: Ma, B.
Partner: UNT Libraries Government Documents Department

Thermodynamics and stability of the mixed-conducting Sr-Fe-Co-O system.

Description: Mixed-conducting Sr-Fe-Co oxides have potential applications in dense ceramic membranes for high-purity oxygen separation and/or methane conversion to produce syngas (CO + H{sub 2}), because of their combined high electronic/ionic conductivity and significant oxygen permeability. We studied the crystal structure and microstructure of the system in X-ray diffraction experiments and by using scanning electron microscopy, respectively. Thermogravimetric analysis was conducted on the SrFeCo{sub 0.5}O{sub x} sample in environments of various oxygen partial pressures (pO{sub 2}). Conductivity increased while weight decreased with increasing temperature. Activation energy decreased while conductivity increased with increasing pO{sub 2}. The pO{sub 2}-dependent conducting behavior of the SrFeCo{sub 0.5}O{sub x} system can be understood by considering the trivalent-to-divalent transition of transition-metal ions.
Date: April 28, 1999
Creator: Ma, B.
Partner: UNT Libraries Government Documents Department

Structure and property relationship in the mixed-conducting Sr-Fe-Co-O system.

Description: Mixed-conducting ceramic oxides have potential uses in high-temperature electrochemical applications such as solid oxide fuel cells, advanced batteries, sensors, and oxygen-permeable membranes. The Sr-Fe-Co-O system combines high electronic/ionic conductivity with appreciable oxygen permeability at elevated temperatures. Dense ceramic membranes made of this material can be used to separate high-purity oxygen from air without the need for external electrical circuitry, or to partially oxidize methane to produce syngas. Samples of Sr{sub 2}Fe{sub 3{minus}x}Co{sub x}O{sub y} (with x = 0, 0.6, 1.0, and 1.4) were prepared by solid-state reaction in atmospheres with various oxygen partial pressures (pO{sub 2}) and were characterized by X-ray diffraction, scanning electron microscopy, and electrical conductivity measurements. Phase components of the samples are dependent on cobalt concentration and synthesis pO{sub 2}. Total conductivity increases with increasing temperature and cobalt content in the material. Higher ionic transference numbers have been observed in samples with lower cobalt contents. Current-voltage characteristics determined in a gas-tight cell indicate that a bulk effect, rather than a surface exchange effect, is the main limiting factor for oxygen permeation through membranes made of Sr{sub 2}Fe{sub 2}CoO{sub y}. Oxygen permeability measurements at various temperatures showed that oxygen permeability increases with increasing temperature, as expected. At 900 C, an oxygen permeation flux of 2.5 scc{center_dot}cm{sup {minus}2}{center_dot}min{sup {minus}1} was obtained for a Sr{sub 2}Fe{sub 2}CoO{sub y} disk of 2.9 mm thickness.
Date: May 18, 1998
Creator: Ma, B.
Partner: UNT Libraries Government Documents Department

Effect of Dy additions on microstructure and magnetic properties of Fe-Nd-B magnets

Description: It is shown that increasing additions of Dy causes the remanence B/sub r/ to decrease linearly. The intrinsic coercivity, iHc, increases sharply for small additions of Dy, but the increase is not proportional for higher Dy contents. The iHc increases almost linearly with the effective anisotropy field of the RE/sub 2/Fe/sub 14/B phase until the Dy content is about 10% of the total rare earth content. Above this concentration, there is strong deviation from linearity. Various types of possible concentration profiles of the substituted rare earth are suggested. It is also argued that preferential segregation of Dy to the interfaces could be beneficial in increasing the nucleation field. Morphologically there is no apparent effect of Dy on the microstructure. However, in the 5 atomic % Dy sample, Dy rich oxides were observed. It is shown through Energy Dispersive Xray Spectroscopy (EDXS) line profiling that Dy partitions preferentially into the RE/sub 2/Fe/sub 14/B phase in all the cases. No segregation of Dy to the interphase interfaces has been detected.
Date: May 1, 1987
Creator: Ramesh, R.; Thomas, G. & Ma, B.M.
Partner: UNT Libraries Government Documents Department

Development of ceramic membranes for conversion of methane into syngas.

Description: The abundantly available natural gas (mostly methane) discovered in remote areas has stimulated considerable research on upgrading this gas to high-value-added clean-burning fuels such as dimethyl ether and alcohols and to pollution-fighting fuel additives. Of the two routes to convert methane to valuable products, direct and indirect, the indirect route involving partial oxidation of methane to syngas (a mixture of CO and H{sub 2}) is preferred. Syngas is used as feedstock to produce a variety of petrochemicals and transportation fuels. A mixed-conducting dense ceramic membrane was developed from Sr-Fe-Co oxide. Extruded and sintered tubes of SrFeCoO{sub 0.5}O{sub x} have been evaluated in a reactor operating at {approx}850 C for conversion of methane into syngas in the presence of a reforming catalyst. Some of the reactor tubes have been run for more than 1000 h, and methane conversion efficiencies of {approx}98% and CO selectivities of >96% were observed.
Date: September 23, 1999
Creator: Balachandran, U. & Ma, B.
Partner: UNT Libraries Government Documents Department

Electronic/ionic conductivity and oxygen diffusion coefficient of Sr-Fe-Co-O system

Description: Oxides in the system Sr-Fe-Co-O exhibit both electronic and ionic conductivities. Recently, Sr-Fe-Co-O system attracted great attention because of the potential to be used for oxygen permeable membranes that can operate without the electrodes or external electrical circuitry. Electronic and ionic conductivities at various temperatures have been measured on two compositions in Sr-Fe-Co-O system named SFC-1 and SFC-2. The electronic transference number is much greater than the ionic transference number in SFC-1 sample, while the electronic and ionic transference numbers are very close in SFC-2 sample. At 800{degrees}C, the electronic conductivity and ionic conductivity are {approx}76 S{center_dot}cm-1 and =4 S-cm-1, respectively, for SFC-1. While, for SFC-2, the electronic and ionic conductivities are =10 S-cm-1 and {approx}7 S-cm-1, respectively. By a local fitting to {sigma}{center_dot}T = A exp(-E{sub {alpha}}/{kappa}{Tau}), we found that the oxide ion activation energies are 0.92 eV and 0.37 eV respectively for SFC-1 and SFC-2 samples. Oxygen diffusion coefficient of SFC-2 is {approx}{times}10{sup {minus}7} cm{sup 2}/sec at 900C.
Date: March 1, 1995
Creator: Ma, B.; Park, J.H.; Balachandran, U. & Segre, C.U.
Partner: UNT Libraries Government Documents Department

Electrical properties and defect structure in the Sr-Fe-Co-O system

Description: The ceramic Sr-Fe-Co-O has potential use as a membrane in gas separation. This material exhibits high conductivity of both electrons and oxygen ions. It allows oxygen to penetrate at high flux rates without other gas components. Electrical properties are essential to understanding the oxygen transport mechanism and defect structure of this material. By using a gas-tight electrochemical cell with flowing air as the reference environment, we were able to achieve an oxygen partial pressure ({sub p}O{sub 2}) as low as 10{sup -16} atm. Total and ionic conductivities of Sr-Fe-Co-O have been studied as a function of {sub p}O{sub 2} at elevated temperature. In air, both total and ionic conductivities increase with temperature, while the ionic transference number is almost independent of temperature, with a value of {approx}0.4. Experimental results show that ionic conductivity decreases with decreasing {sub p}O{sub 2} at high {sub p}O{sub 2} ({ge}10{sup -6} atm). This suggests that interstitial oxygen ions and electron holes are the dominant charge carriers. At 800{degrees}C in air, total conductivity and ionic conductivity are 17 and 7 S/cm, respectively. Defect dynamics in this system can be understood by means of the trivalence-to-divalence transition of Fe ions when {sub p}O{sub 2} is reduced. By using the conductivity results, we estimated oxygen permeation through a ceramic membrane made of this material. The calculated oxygen permeability agrees with the experimental value obtained directly from an operating methane conversion reactor.
Date: November 1, 1995
Creator: Ma, B.; Chao, C.C. & Park, J.H.
Partner: UNT Libraries Government Documents Department

Dense ceramic membranes for methane conversion

Description: This report focuses on a mechanism for oxygen transport through mixed- oxide conductors as used in dense ceramic membrane reactors for the partial oxidation of methane to syngas (CO and H{sub 2}). The in-situ separation of O{sub 2} from air by the membrane reactor saves the costly cryogenic separation step that is required in conventional syngas production. The mixed oxide of choice is SrCo{sub 0.5}FeO{sub x}, which exhibits high oxygen permeability and has been shown in previous studies to possess high stability in both oxidizing and reducing conditions; in addition, it can be readily formed into reactor configurations such as tubes. An understanding of the electrical properties and the defect dynamics in this material is essential and will help us to find the optimal operating conditions for the conversion reactor. In this paper, we discuss the conductivities of the SrFeCo{sub 0.5}O{sub x} system that are dependent on temperature and partial pressure of oxygen. Based on the experimental results, a defect model is proposed to explain the electrical properties of this system. The oxygen permeability of SrFeCo{sub 0.5}O{sub x} is estimated by using conductivity data and is compared with that obtained from methane conversion reaction.
Date: May 1, 1996
Creator: Balachandran, U.; Mieville, R.L.; Ma, B. & Udovich, C.A.
Partner: UNT Libraries Government Documents Department

Use of high-temperature gas-tight electrochemical cells to measure electronic transport and thermodynamics in metal oxides

Description: By using a gas-tight electrochemical cell, the authors can perform high-temperature coulometric titration and measure electronic transport properties to determine the electronic defect structure of metal oxides. This technique reduces the time and expense required for conventional thermogravimetric measurements. The components of the gas-tight coulometric titration cell are an oxygen sensor, Pt/yttria stabilized zirconia (YSZ)/Pt, and an encapsulated metal oxide sample. Based on cell design, both transport and thermodynamic measurements can be performed over a wide range of oxygen partial pressures (pO{sub 2} = 10{sup {minus}35} to 1 atm). This paper describes the high-temperature gas-tight electrochemical cells used to determine electronic defect structures and transport properties for pure and doped-oxide systems, such as YSZ, doped and pure ceria (Ca-CeO{sub 2} and CeO{sub 2}), copper oxides, and copper-oxide-based ceramic superconductors, transition metal oxides, SrFeCo{sub 0.5}O{sub x}, and BaTiO{sub 3}.
Date: October 1, 1997
Creator: Park, J.H.; Ma, B. & Park, E.T.
Partner: UNT Libraries Government Documents Department

Development of mixed-conducting oxides for gas separation

Description: Mixed-conducting oxides have been used in many applications, including fuel cells, gas separation membranes, sensors, and electrocatalysis. The authors are developing a mixed-conducting, dense ceramic membrane for selectively transporting oxygen and hydrogen. Ceramic membranes made of Sr-Fe-Co oxide, which has high combined electronic and oxygen ionic conductions, can be used to selectively transport oxygen during the partial oxidation of methane to synthesis gas (syngas, CO + H{sub 2}). The authors have measured the steady-state oxygen permeability of SrFeCo{sub 0.5}O{sub x} as a function of oxygen-partial-pressure gradient and temperature. At 900{degrees}C, oxygen permeability was {approx}2.5 scc{center_dot}cm{sup {minus}2}{center_dot}min{sup {minus}1} for a 2.9-mm-thick membrane and this value increases as membrane thickness decreases. The authors have fabricated tubular SrFeCo{sub 0.5}O{sub x} membranes and operated them at 900{degrees}C for >1000 h during conversion of methane into syngas. The hydrogen ion (proton) transport properties of yttria-doped BaCeO{sub 3} were investigated by impedance spectroscopy and open-cell voltage measurements. High proton conductivity and a high protonic transference number make yttria-doped BaCeO{sub 3} a potential membrane for hydrogen separation.
Date: August 1, 1997
Creator: Balachandran, U.; Ma, B. & Maiya, P.S.
Partner: UNT Libraries Government Documents Department

Structural and electrical properties of biaxially textured YBa{sub 2}Cu{sub 3}O{sub 7-x} thin films on buffered Ni-based alloy substrates.

Description: Oxide high-T{sub c} superconducting wires and tapes with high critical current density (J{sub c}) are essential in future electrical power applications. Recently, YBa{sub 2}Cu{sub 3}O{sub 7{minus}x} (YBCO) thin films grown on Ni-based alloy tapes have attracted intense interest because of their promise for these applications. In order to achieve high J{sub c}, buffer layers are necessary for fabricating biaxially aligned YBCO thin films. In our studies, yttria-stabilized zirconia (YSZ) layers were deposited on Ni-based alloy substrate by ion-beam assisted deposition, and CeO{sub 2} buffer layers were subsequently deposited on the YSZ layer by pulsed laser deposition (PLD) or electron beam evaporation. In addition, MgO layers were deposited on Ni-based alloy substrates by inclined substrate deposition. Finally, biaxially textured YBCO thin films were deposited on these buffered metallic substrates by PLD under optimized conditions. The orientation and in-plane textures of YBCO and the buffer layers were characterized by X-ray diffraction {Theta}/2{Theta} scan, {phi}-scan, and pole figure analysis. The superconductive transition features were examined by measuring inductive T{sub c} and transport J{sub c}.
Date: December 7, 2000
Creator: Li, M.; Ma, B.; Jee, Y. A.; Fisher, B. L. & Balachandran, U.
Partner: UNT Libraries Government Documents Department

Ion-beam-assisted deposition of magnesium oxide films for coated conductors.

Description: The development of high critical-temperature thin-film superconductors and coated conducting wires is important for electric power applications. To achieve high transport current density, template films are necessary for the successful deposition of biaxially aligned YBa{sub 2}Cu{sub 3}O{sub 7-x} (YBCO) on flexible metal substrates. We grew biaxially aligned magnesium oxide (MgO) template films by ion-beam-assisted deposition with electron-beam evaporation. MgO films of {approx}100 {angstrom} thickness were deposited on Si{sub 3}N{sub 4}-coated Si substrates at a deposition rate of {approx}1.5 {angstrom}/sec with an ion flux of {approx}110 {micro}A/cm{sup 2} bombarding the substrate at a 45{sup o} angle. To study crystalline structure by X-ray diffraction, we deposited an additional layer of MgO. Good in- and out-of-plane alignment was observed, with (111) {phi}-scan full-width half-maximum (FWHM) of 6.2{sup o} and (002) {omega}-scan FWHM of 2.2{sup o}.
Date: May 1, 2002
Creator: Weber, T. P.; Ma, B.; Balachandran, U. & McNallan, M.
Partner: UNT Libraries Government Documents Department

Texture formation and superconducting properties of YBa{sub 2}Cu{sub 3}O{sub x} thin films prepared by solution process on LaAlO{sub 3} single crystals.

Description: YBa{sub 2}Cu{sub 3}O{sub x} (YBCO) thin films were fabricated by the trifluoroacetate (TFA) process on LaAlO{sub 3} (LAO) single crystal in an argon atmosphere. They focused on lowering the heat treatment temperature by decreasing the oxygen partial pressure to adopt the TFA process to metallic substrates. YBCO phase formation was checked by measuring T{sub c} with the inductive method. In-plane and out-of-plane film textures were evaluated by phi-scan and omega scan, respectively. Raman spectroscopy was used to estimate grain connectivity, in-plane texture, and second-phase formation of the films. Although Raman spectroscopy revealed some evidence of cation disorder, the film prepared at 750 C shows a sharp superconducting transition at 91 K and critical current density of 1.3 MA/cm{sup 2} at 77 K. Optimal heat treatment temperature was 750 C in the argon atmosphere, which is consistent with the thermodynamic estimate that heat treatment temperature decreases as oxygen partial pressure decreases.
Date: January 9, 2001
Creator: Jee, Y.-A.; Ma, B.; Fisher, B. L. & Balachandran, U.
Partner: UNT Libraries Government Documents Department

Texture development of MgO buffer layers grown by inclined substrate deposition.

Description: Biaxially textured magnesium oxide (MgO) films used as template layers for YBa{sub 2}Cu{sub 3}O{sub 7-x} (YBCO)-coated conductors have been grown efficiently and consistently by inclined substrate deposition (ISD). Further improvement in texture and a decrease in surface roughness were obtained by depositing a homoepitaxial MgO layer on the ISD MgO layer at an elevated temperature and flat angle. The texture of the ISD layer was studied as a function of thickness by X-ray diffraction and scanning and transmission electron microscopy. Surface roughness of the ISD and homoepitaxial layers was investigated by atomic force microscopy. Based on the results, the optimal thickness of the ISD layer was determined.
Date: August 16, 2002
Creator: Koritala, R. E.; Ma, B.; Miller, D. J.; Li, M.; Fisher, B. L. & Balachandran, U.
Partner: UNT Libraries Government Documents Department

Defect structure of the mixed-conducting Sr-Fe-Co-O system

Description: Electrical conductivity of the mixed-conducting Sr-Fe-Co-O system was investigated at high temperatures and various oxygen partial pressures (pO2). The system exhibits not only high combined electrical and oxygen ionic conductivities but also structural stability in both oxidizing and reducing environments. Conductivity of SrFeCo{sub 0.5}O{sub x} increases with temperature and pO2, within the experiment pO2 range (1-10{sup -18} atm). p-type conduction was observed, the activity energy of which decreases with pO2. A model of the defect chemistry in the Sr-Fe-Co-O system is proposed. The pO2- dependent conducting behavior can be understood by considering the trivalent-to-divalent transition of the transition metal ions in the system.
Date: November 1, 1996
Creator: Ma, B.; Balachandran, U.; Chao, C.-C. & Park, J.-H.
Partner: UNT Libraries Government Documents Department

Study of the mixed-conducting SrFeCo{sub 0.5}O{sub y} system

Description: Mixed-conducting Sr-Fe-Co oxides have potential applications in dense ceramic membranes for high-purity oxygen separation and/or methane conversion to produce syngas (CO + H{sub 2}), because of their combined high electronic/ionic conductivity and significant oxygen permeability. SrFeCo{sub 0.5}O{sub y} has been synthesized by the solid-state reaction method. Conductivities were measured at elevated temperatures in various gas environments and rose with increasing temperature and increasing oxygen partial pressure (pO{sub 2}) in the surrounding environment. Neutron powder diffraction experiments revealed that in a high pO{sub 2} environment the SrFeCo{sub 0.5}O{sub y} material consists of three different phases. The relative concentration of each component phase is dependent on temperature and pO{sub 2} in the surrounding environment. In air, Sr{sub 2}(Fe,Co){sub 3}O{sub y} (236 phase) is the majority phase and consists of >75wt.% of the total, while the perovskite and rocksalt phases account for {approx}20wt.% and <5 wt.%, respectively. However, in a reducing environment, the 236 phase decomposes and converts to perovskite and rocksalt phase at high temperature. In an environment of pO{sub 2} < 10{sup {minus}12.2} atm, the 236 phase is completely converted into perovskite (brownmillerite) and rocksalt phases.
Date: May 18, 2000
Creator: Ma, B.; Victory, N. I.; Balachandran, U.; Mitchell, B. J. & Richardson, J. W., Jr.
Partner: UNT Libraries Government Documents Department

Biaxially aligned template films fabricated by inclined-substrate deposition for YBCO-coated conductor applications.

Description: Inclined substrate deposition (ISD) has the potential for rapid production of high-quality biaxially textured buffer layers, which are important for YBCO-coated conductor applications. We have grown biaxially textured MgO films by ISD at deposition rates of 20-100 {angstrom}/sec. Columnar grains with a roof-tile surface structure were observed in the ISD-MgO films. X-ray pole figure analysis revealed that the (002) planes of the ISD-MgO films are tilted at an angle from the substrate normal. A small {phi}-scan full-width at half maximum (FWHM) of {approx}9{sup o} was observed on MgO films deposited at an inclination angle of 55{sup o}. In-plane texture in the ISD MgO films developed in the first 0.5 {micro}m from the interface, then stabilized with further increases in film thickness. YBCO films deposited by pulsed laser deposition on ISD-MgO buffered Hastelloy C276 substrates were biaxially aligned with the c-axis parallel to the substrate normal. T{sub c} of 91 K with a sharp transition and transport J{sub c} of 5.5 x 10{sup 5} A/cm{sup 2} at 77 K in self-field were measured on a YBCO film that was 0.46-{micro}m thick, 4-mm wide, 10-mm long.
Date: August 12, 2002
Creator: Ma, B.; Li, M.; Koritala, R. E.; Fisher, B. L.; Erck, R. A.; Dorris, S. E. et al.
Partner: UNT Libraries Government Documents Department

Separation of gases with solid electrolyte ionic conductors

Description: The authors have developed a novel method of gas separation based on electrolyte ionic membrane technology. Separation of one gas from another occurs through an ion-conducting membrane by the passage of selected ions. Most systems studied have focused on oxygen ion conduction for the separation of oxygen from air, although protonic and halide-conducting solid materials also exist. As an example of this system, this paper concentrates on a study of a membrane reactor used in the production of syngas (CO + H{sub 2}) from methane. The membrane material is a modified perovskite-type oxide exhibiting mixed (electronic/ionic) conductivity. Mixed-conductivity oxides are promising materials for oxygen-permeating membranes that can operate without electrodes or external electrical circuitry. Extruded tubes of this material have been evaluated in a reactor operating at {approx} 850 C for partial oxidation of methane into syngas in the presence of a reforming catalyst. Separated oxygen on one side of the reactor wall was obtained from air on the other side. Methane conversion efficiencies of > 99% were observed, and some of the reactor tubes have been operated for > 1,000 h. Membrane tubes were fabricated from calcined powders by a plastic extrusion technique. Characterization of the mechanical, physical, and chemical properties of this material confirmed the stability exhibited in the reactor.
Date: November 1, 1996
Creator: Balachandran, U.; Dusek, J.T.; Maiya, P.S.; Mieville, R.L.; Ma, B.; Kleefisch, M.S. et al.
Partner: UNT Libraries Government Documents Department

Oxygen-permeable ceramic membranes for gas separation

Description: Mixed-conducting oxides have a wide range of applications, including fuel cells, gas separation systems, sensors, and electrocatalytic equipment. Dense ceramic membranes made of mixed-conducting oxides are particularly attractive for gas separation and methane conversion processes. Membranes made of Sr-Fe-Co oxide, which exhibits high combined electronic and oxygen ionic conductivities, can be used to selectively transport oxygen during the partial oxidation of methane to synthesis gas (syngas, i.e., CO + H{sub 2}). The authors have fabricated tubular Sr{sub 2}Fe{sub 2}CoO{sub 6+{delta}} membranes and tested them (some for more than 1,000 h) in a methane conversion reactor that was operating at 850--950 C. An oxygen permeation flux of {approx} 10 scc/cm{sup 2} {center_dot} min was obtained at 900 C in a tubular membrane with a wall thickness of 0.75 mm. Using a gas-tight electrochemical cell, the authors have also measured the steady-state oxygen permeability of flat Sr{sub 2}Fe{sub 2}CoO{sub 6+{delta}} membranes as a function of temperature and oxygen partial pressure(pO{sub 2}). Steady-state oxygen permeability increases with increasing temperature and with the difference in pO{sub 2} on the two sides of the membrane. At 900 C, an oxygen permeability of {approx} 2.5 scc/cm{sup 2} {center_dot} min was obtained in a 2.9-mm-thick membrane. This value agrees with that obtained in methane conversion reactor experiments. Current-voltage (I-V) characteristics determined in the gas-tight cell indicate that bulk effect, rather than surface exchange effect, is the main limiting factor for oxygen permeation of {approx} 1-mm-thick Sr{sub 2}Fe{sub 2}CoO{sub 6+{delta}} membranes at elevated temperatures (> 650 C).
Date: February 1, 1998
Creator: Balachandran, U.; Ma, B.; Maiya, P.S.; Dusek, J.T.; Mieville, R.L. & Picciolo, J.J.
Partner: UNT Libraries Government Documents Department

Development of mixed-conducting ceramic membranes for converting methane to syngas

Description: The abundantly available natural gas (mostly methane) discovered in remote areas has stimulated considerable research on upgrading this gas to high-value-added clean-burning fuels such as dimethyl ether and alcohols and to pollution-fighting additives. Of the two routes to convert methane to valuable products direct and indirect, the direct route involving partial oxidation of methane to syngas (CO + H{sub 2}) by air is preferred. Syngas is the key intermediate product used to form a variety of petrochemicals and transportation fuels. This paper is concerned with the selective transport of oxygen from air for converting methane to syngas by means of a mixed-conducting ceramic oxide membrane prepared from Sr-Fe-Co-O oxide. While both perovskite and nonperovskite type Sr-Fe-Co-O oxides permeate large amounts of oxygen when the membrane tube is subjected to oxygen pressure gradients, the work shows that the nonperovskite SrFeCo{sub 0.5}O{sub x} exhibits remarkable stability during oxygen permeation. More particularly, extruded and sintered tubes from SrFeCo{sub 0.5}O{sub x} have been evaluated in a reactor operating at {approx} 850 C for conversion of methane into syngas in the presence of a reforming catalyst. Methane conversion efficiencies of {approx} 99% were observed. In addition, oxygen permeability of SrFeCo{sub 0.5}O{sub x} was measured as a function of oxygen partial pressure gradient and temperature in a gas-tight electrochemical cell. Oxygen permeability has also been calculated from conductivity data and the results are compared and discussed.
Date: April 1, 1997
Creator: Balachandran, U.; Maiya, P.S.; Ma, B.; Dusek, J.T.; Mieville, R.L. & Picciolo, J.J.
Partner: UNT Libraries Government Documents Department