Electron Hole Conduction Research Articles

La ( Sr ) Ga ( Fe ) O 3 Perovskite Oxide as a New Mixed Ionic-Electronic Conductor for Oxygen Permeating Membrane

Fe-doped exhibits a high conductivity (∼1 S/cm) and a high oxygen permeation rate. In particular, the highest conductivity and the oxygen permeating rate were attained at (LSGF). Although a surface catalyst is required, oxygen permeation rate from air to Ar was as high as 2.5 cm3 std/min cm2 at 1273 K and 0.3 mm membrane thickness. Oxygen permeation rate from air to Ar increased in the following order, as the surface catalyst. Since the gradient becomes larger, the oxygen permeation rate drastically increased by changing from air-Ar to -air condition. The products were only CO and having a molar ratio ratio) of almost two. Electronic hole conduction was only observed in LSGF polarization measurement and the oxide ion conductivity estimated is as high as at 1073 K. At high the main defect in LSGF is and and at intermediate concentration of is balanced with that of The estimated transport number of oxide ion was ca. 0.6, which is in a good agreement with that estimated by the electromotive force in gas concentration cells. © 2002 The Electrochemical Society. All rights reserved.

Calcium zirconate: preparation, properties and application to the solid oxide galvanic cells

Stoichiometric, and doped with a small amount of CaO, calcium zirconate dense samples were prepared. X-ray analysis revealed that the solid solution was formed in the concentration range up to x=0.06 in the { xCaO+(1− x)CaZrO 3} mixture. The activity of CaO in the solid solution was determined by the emf method at the temperatures 1073 and 1273 K. The electrical conductivity was measured using both dc four-probe and ac impedance spectroscopy methods. The maximum conductivity values were observed for the sample with x=0.06. This sample was used for further investigation. The oxygen-ion transference number, estimated by the emf method, appeared to be close to unity. The partial electronic (electron and electron hole) conductivities, deduced from current–potential curves following the polarisation method, were found to be very small with respect to total electrical conductivity. Then, the sample was tested as an electrolyte in solid oxide galvanic cells. In this way, the values of the standard free enthalpy of formation of cobalt and nickel silicates at the temperatures 1073 and 1273 K were determined and compared with those obtained by other authors.

P-Type electronic conduction in CeO2- and LaGaO3-based solid electrolytes

Modifications of the e.m.f. and faradaic efficiency techniques, taking into account electrode polarization in the measuring cells, in combination with the use of electrodes having sufficiently high polarization resistances enable a precise determination of minor electronic contributions to the conductivity of solid electrolytes. These methods were used to determine the p-type conductivity of compositions based on La(Sr)Ga(Mg)O3-δ (LSGM) and Ce(Gd)O2-δ (CGO) at 900–1270 K. The oxygen ion transference numbers of these materials under oxygen/air gradient vary in the range 0.999–0.970, increasing with decreasing temperature. Substitution of 2 % gadolinium in Ce0.80Gd0.20O2-δ with praseodymium was found to increase the electron-hole conduction by 2.5 – 4 times. At temperatures above 700 K, both the partial oxygen ionic and p-type electronic conductivities of LaGaO3-based phases are higher than those in CGO. The electron-hole transport in LSGM tends to increase with the magnesium concentration, while the activation energy is essentially independent of composition. Electronic conduction in CGO and LSGM electrolytes was also found to be influenced by the ceramic microstructure.

Oxygen Permeability and Ionic Conductivity of Perovskite-Related La0.3Sr0.7Fe ( Ga ) O 3 − δ

The oxygen ionic conductivity of determined by the oxygen permeation, faradaic efficiency, and total conductivity measurements at 1023-1223 K, is essentially independent of oxygen chemical potential and structural changes in the wide oxygen partial pressure range from to 0.21 kPa. At oxygen pressures close to atmospheric air, the ion transference numbers of perovskite-like phases vary from to increasing with gallium content and temperature. Although there is a great difference between ionic and p-type electronic conductivities in oxidizing atmospheres, the electron-hole conduction was demonstrated to affect oxygen permeation and ambipolar conductivity. In oxidizing conditions, the oxygen permeability of membranes increase with increasing The bulk ionic transport process and surface exchange kinetics both influence the permeation through ceramics, whereas the oxygen fluxes through Ga-containing materials are predominantly determined by the bulk ambipolar conduction. Thermal expansion coefficients of in air vary in the range at 300-800 K and increase up to at 800-1170 K. Substitution of iron with gallium was found to suppress both thermal expansion and chemically induced expansion of materials, originating from oxygen nonstoichiometry variations under changing the oxygen partial pressure. © 2002 The Electrochemical Society. All rights reserved.

Electrical conductivity of polycrystalline copper(I) bromide at low temperature (160–300 K)

The electrical conductivity of polycrystalline copper(I) bromide (CuBr) was investigated at low temperature (160–300 K) by four-point d.c. measurements and impedance spectroscopy. A change from predominant ionic conduction above 260 K to electron hole conduction below that temperature was observed. The activation energies in the different domains were examined in relation to defect models.

Electron–hole conduction in Pr-doped Ce(Gd)O 2− δ by faradaic efficiency and emf measurements

Non-negligible electrode polarisation in faradaic efficiency and emf electrochemical cells used for transport number determination results in apparent ion transference numbers which are lower than the true values. However, appropriate modifications of these techniques combined with use of electrodes having a high polarisation resistance, enable a precise determination of even minor electronic contributions to the total conductivity of solid electrolytes. Experimental modification of the faradaic efficiency method, taking into account the electrode polarisation resistance, is proposed and verified using Ce 0.80Gd 0.18Pr 0.02O 2− δ ceramics. Substitution of 2% gadolinium cations in the lattice of Ce 0.80Gd 0.20O 2− δ solid electrolyte with praseodymium was found to increase the p-type electronic conductivity by 2.5–4 times, while the activation energy for the electron–hole transport decreases from 145 to 125 kJ mol −1. The oxygen ion transference numbers of Ce 0.80Gd 0.18Pr 0.02O 2− δ in air vary in the range 0.996–0.970 at 873–1223 K, decreasing with increasing temperature. No significant effect of co-doping with praseodymium on the ionic conductivity, crystal lattice and thermal expansion of ceria solid electrolyte was found.

Oxidative coupling of methane on improved bismuth oxide membrane reactors

AbstractFluorite‐structured yuria‐bismuth oxide membranes with improved chemical stability were prepared by doping samarium in the lattice. The samarium‐doped yuria‐bismuth oxide (BYS) membranes are in fluorite structure, with high oxygen permeability and good catalytic properties for oxidative coupling of methane (OCM). The oxygen partial pressure dependence of oxygen permeation flax shows a shift of the rate‐limiting step from the suface reaction to the hulk electron‐hole conduction as temperature increases from 750 to 950°C. OCM reactions were conducted in a membrane reactor made of the disk‐shaped BYS membrane. Merhane‐ and oxygen‐containing streams are fed into the opposite sides of the BYS membrane to allow OCM reactions to occur on the membrane surface exposed to methane. The membrane reactor gives C2, yields of 4–11% with C2, selectivity up to 74%. The performance of this membrane reactor is closely related to oxygen permeation properties of the BYS membrane. Experimental results show the effectiveness in improving the C2 yield by increasing the membrane surface area to reactor volume ratio. In this disk‐shaped membrane reactor the highest C2, yield achieved is 17%, with a C2 selectivity of about 80%.

High-temperature electrical transport in La0.3Sr0.7Fe1 − xGaxO3 − δ (x = 0–0.5)

The electrical properties of perovskite-related oxide La0.3Sr0.7Fe1 - xGaxO3 − δ (x = 0–0.5) are reported within the temperature range 750 to 950 °C and the oxygen partial pressure range between 10−19 and 0.5 atm. The maximum solid solubility of gallium in the iron sublattice of La0.3Sr0.7FeO3 − δ is close to 30%. At oxygen pressures about 10−4 atm, the materials undergo a transition from perovskite to oxygen vacancy ordered structures. The type of vacancy ordering depends on gallium content. Both p- and n-type electronic conductivities decrease when insulating gallium cations having stable oxidation state are incorporated into the iron sublattice of the ferrite. The observed temperature and oxygen pressure dependencies of the p-type electronic conductivity in the ordered phases suggest a transition from the intrinsic regime, when the electron–hole conduction is governed by the band gap, to the extrinsic regime, controlled by the amount of the oxygen excess in the vacancy ordered structure. The ordered strontium–lanthanum ferrite was shown to be a mixed conductor with oxygen ionic conductivity of about 0.1 S cm−1 at 900 °C. Ionic conduction in the vacancy ordered phases decreases with increasing gallium concentration.

Electrical Conductivity of the High‐Temperature Proton Conductor BaZr0.9Y0.1O2.95

The impedance of the cubic perovskite BaZr0.9Y0.1O3‐δ has been systematically investigated in dry and wet atmospheres at high and low oxygen partial pressures. In the grain interior, conductivity contributions from oxygen ions, electron holes, and protons can be identified. Below 300°C, proton conduction dominates and increases linearly with the frozen‐in proton concentration. The proton mobility, with an activation energy of 0.44 ± 0.01 eV is among the highest ever reported for a perovskite‐type oxide proton conductor. For dry oxygen atmos‐pheres, electron hole conduction dominates with an activation energy of ∼0.9 eV. At temperatures <500°C, the grain‐boundary conductivity can be separated and increases upon incorporation of protons. The high electrical conductivity and chemical stability make acceptor‐doped barium zirconate a good choice for application as a high‐temperature proton conductor.

Faradaic efficiency and oxygen permeability of Sr 0.97Ti 0.60Fe 0.40O 3− δ perovskite

Oxygen ionic conduction in the perovskite-type Sr 0.97Ti 0.60Fe 0.40O 3− δ was studied using oxygen permeability, Faradaic efficiency and total electrical conductivity measurements at 973–1223 K. The ion transference numbers of the strontium titanate–ferrite in air vary from 0.005 to 0.08, decreasing with decreasing temperature. The electron–hole conductivity of the oxide is relatively low but exceeds the ionic conductivity. The activation energy for the electronic conductivity is 35±3 kJ/mol at 470–890 K and drops at higher temperatures. Studying the oxygen permeation through dense Sr 0.97Ti 0.60Fe 0.40O 3− δ ceramic membranes as a function of membrane thickness showed that at temperatures above 1170 K the permeation fluxes are limited by both bulk ionic conductivity and surface exchange rates. Depositing of porous layers of the same material or a mixture of platinum and praseodymium oxide onto the membrane feed-side surface leads to a significant increase in the oxygen permeability. Decreasing temperature results in increasing role of the bulk ionic transport in Sr 0.97Ti 0.60Fe 0.40O 3− δ as the permeation-determining factor. The oxygen permeation fluxes at 1073 K are limited predominantly by the oxygen ionic conductivity of the ceramics. The thermal expansion coefficients of the ceramic material in air were calculated from dilatometric data to be 11.7×10 −6 K −1 in the temperature range 300–720 K and 16.6×10 −6 K −1 at 720–1070 K.

Colossal magnetoresistance of La0.67Ca0.33MnO3 doped with Ni

In recent years, there has been much interest in the perovskite compounds R1−x Ax MnO3 (R=La, Pr and A= Pb, Ca, Sr, Ba) with a Mn3+/Mn4+ mixed valence [1–13]. In the doping range 0.2< x < 0.5, these manganites undergo a paramagnetic insulator to ferromagnetic metal phase transition upon cooling, leading to a sharp resistance peak near the Curie temperature TC. Application of an external magnetic field of few teslas suppresses the resistance greatly. Within the double– exchange interaction model the itinerant carriers in the substituted R1−x Ax MnO3 oxide provide the mechanism for ferromagnetic interaction between Mn3+ and Mn4+ ions. The ferromagnetic Curie temperature is related to the strength of the transfer intergral ti j between Mn3+ and Mn4+ ions which control the the electronic (hole) conductivity. The unusual temperature and magnetic field dependent resistance exhibited by these compounds reflects a novel interplay between magnetism and electronic transport. It has been shown recently that the substitution of Mn3+ by Co3+, Ni3+, Cr3+ in R0.5A0.5MnO3 destroys the charge–ordered state and renders the material ferromagnetic and metallic when the average size of A–site cation, 〈rA〉, is larger than 1.17 A [10–12]. In the present work, we report the effects of the Ni-doping on the perovskite compound La0.67Ca0.33MnO3. Samples La0.67Ca0.33Mn1−x Nix O3 (x = 0, 0.05, 0.1 and 0.2) were prepared using solid state reaction method. Mixed powders of La2O3, MnO2, Ni2O3 and CaCO3 were ground for 60 min, then ball-milled for 30 min and pressed into flats. The flats were first heated at 800 ◦C for 24 h and subsequently pulverized. After being milled for 30 min, they were pressed into flats again, then sintered at 1300 ◦C for 100 h, followed by furnace–cooling in the atmosphere. X-ray diffraction analysis by Cu-Kα radiation was used to confirm a single phase with perovskite structure. The thermal– magnetic analysis using a vibrating sample magnetometer (VSM) at a small field of 0.04 T was performed from 77 to 300 K. The resistance were measured by a standard four–terminal method. The magnetization curves and magnetoresistances at 5 K, and the temperature dependence of the resistance at both zero field and H = 6 T from 5 to 300 K were determined using a superconducting quantum interference device (SQUID) magnetometer.

Electron hole conductivity of gadolinia doped ceria

The electron hole conductivity of gadolinia doped ceria (GCO) was estimated from electrochemical permeability measurements, at temperatures in the range of 800 to 1000°C. The transient behaviour of the cell (while recovering from reducing to oxidising conditions) was used to assess the accuracy of the experimental procedure, and some of these results still suggested the presence of leaks in the sealings. However, the temperature dependence of steady state results indicates that electrochemical permeability prevails, and these results were thus used to estimate the hole conductivity. These results showed that the hole conductivity of GCO at 1000°C is about two orders of magnitude higher than reported for yttria stabilized zirconia, and the activation energy (129 kJmol −1) is slightly lower.

Tuneable photoluminescence, raman spectroscopy and electrical properties of GaAs doping superlattices

Optical properties of three GaAs doping superlattices (SLs) were studied by low and high temperature photoluminescence (PL) as well as inelastic light scattering. A new optically active transition has been observed at 1.48 eV and the high temperature behaviour of the main e–(Zn)h band has been studied. Electrical properties were monitored by current-voltage, resistivity and Hall measurements. This showed a strong variation of carrier concentration in the p-layers associated with mixed electron–hole conduction. The influence of α-particle irradiation on the superlattice structures was also investigated.

Thermoelectric power of Gd-doped barium cerate

The thermoelectric power and electrical conductivity of BaCe 0.9Gd 0.1O 3−δ were determined as functions of oxygen and water vapour activities and temperature. Regions of oxide ion, electron hole and protonic conductivity were observed. A theory of thermoelectric power of proton conductors is presented and the possibility of using thermoelectric power measurements to detect protonic conductivity is discussed. The theory cannot at present explain the results of all the measurements.

Protonic conduction in La 2Zr 2O 7-based pyrochlore materials

Electrochemical pumping of oxygen can be used to adjust the oxygen partial pressure without changing the total content of hydrogen containing species (H 2+H 2O), and this is suitable for studying materials with proton conductivity selectively dependent on water vapour partial pressure. This method was used to demonstrate that lanthanum zirconate based materials possess a significant protonic conductivity contribution at temperatures lower than about 1000°C, and to revise the defect chemistry model. Y for Zr substitution enhances the electron hole, oxygen ion and protonic conductivity contributions, as expected for addition of an acceptor species. The effective mobility of oxygen vacancies, and the equilibrium constant for formation of anti-Frenkel defects were evaluated by taking into account the oxygen ion conductivities in Y-containing and undoped lanthanum zirconate.

SrZrO3-based proton conducting materials with La additions

The behaviour of La-containing strontium zirconate based ceramics is revised. La additions contribute to improve the sinterability of these materials, but transport properties are also affected. Sr1−xLaxZrO3+δ show some unusual changes in transport properties when samples are exposed to H2-containing atmospheres and then reoxidized. Nevertheless, the transport properties of (Sr,La)(Zr,Y)O3−δ, are still mainly dependent on Y additions, at least for cases when the fraction of Y exceeds the fraction of La. In addition, lanthanum causes significant changes in lattice parameters. A combination of conductivity measurements in different atmospheres, ion blocking experiments, and emf measurements has been used to demonstrate that electron hole conductivity increases with temperature, and the protonic contribution drops. The transport properties at temperatures lower than about 800 °C might be affected by structural changes.

Comparison of NFIM and FIM of polymer layers with tetracyanoethylene and benzene as image gases

Field ion microscopy (FIM) with positive ions and negative ions (NFIM) is compared for polymer layers of two structures grown by field induced anionic polymerization of tetracyanoethylene (TCNE) on a metal tip. The positive and negative ion images of the polymer layer mainly consisting of linear polymer chains are complementary, i.e. positive ion emission is preferentially observed in dark areas of the corresponding negative ion images. For the layer with a crosslinked polymer structure the bright rings of negative ion images are not observed in positive ion images. The dissimilarities between NFIM with TCNE and FIM with benzene and TCNE are attributed to differences in the excess electron and electron hole conductivities of the polymer layers exposed to the image gases.

Membrane reactor for oxidative coupling of CH4with an oxide ion–electron hole mixed conductor

A membrane reactor for the oxidative coupling of CH4 has been constructed with an oxide ion–electron hole mixed conductor, BaCe0.8Gd0.2O3–α. 10% CH4 diluted with Ar and an O2–Ar mixture at a given ratio were fed into opposite sides of the membrane at 1173 K. The formation rate of C2–hydrocarbons (ethane and especially ethene) in the CH4 compartment increased with Po2 in the O2–Ar compartment. This enhancement was due to electrochemical oxygen permeation, causing the conductor to short-circuit itself, resulting form the mixed conduction. For comparison, 10% CH4 and a small amount of O2 were co-fed into one side of the membrane. The membrane operation gave a C2 selectivity two times that of the co-feed operation for all CH4 conversions. From a catalytic study using BaCe1–xGdxO3 –α powders as catalyst, it was found that increasing both the oxide-ion and electron-hole conductivities enhanced the formation of C2 hydrocarbons, but reduced that of CO2. On the basis of these results, a mechanism for the oxidative coupling of CH4 of the mixed-conductor membrane was proposed.

Y 2O 3-doped ZrO 2 membranes for solar electrothermal and solarthermal separations — II. Electron hole conductivity of yttria-stabilized zirconia

In previous work, we examined the suitability of yttria-stabilized zirconia (YSZ) for high- temperature oxygen transport. In the present work, we measured the specific electron-hole conductivity of a 4.5 mole% YSZ tube in the temperature range 1400–1900K using a semipermeability technique. The electron-hole conductivity compares well with values previously reported. Our activation energy, 1.60 eV, gives a temperature dependence similar to the activation energies reported by others, 1.36–1.67 eV.

Role of semiconductor phenomena in boronizing

1. The catalytic effect of metal oxides on the process of boronizing has to do with the semiconductor properties of these oxides. The electron-hole conduction of metal oxides improves the electron interchange between reagents, accelerating oxidation of boton carbide and the boronizing process. 2. Acceleration of saturation with boron based on the oxygen activation ions with lower valency is caused predominantly by oxides that are p-type semiconductors in which the disordered structure and the proportion of electron-hole conduction are determined by the partial pressure of oxygen in the system. 3. The application of the tenets of the electron theory of heterogeneous catalysis of chemical processes to semiconductors together with thermodynamic analysis of redox reactions is an effective direction of searching for optimal activators for currentless boronizing and possibly also for other methods of TCT.