Physical foundations of the oxide cathodes

A novel explanation of the low values of work function in case of activated (partly deoxidized) polycrystalline oxides of alkali and alkaline earth metals is offered. Use of the metallic plasma model to the conducting oxides leads to the following values (in eV): 1.00, 1.67, 1.50, 1.44, 1.46 and 1.59 for activated Cs₂O, CaO, SrO, BaO, Y₂O₃ and La₂O₃, respectively. The main reason of low work function of the oxide cathodes is very low density of free electrons in the emitting surface layer.     

Laser ceramics: Mechanisms of solid-state reactions

The role of twinning in synthesizing Y₃Al₅O₁₂ from Al₂O₃ and Y₂O₃ powders during milling and compacting is shown by the example of obtaining laser ceramics based on yttrium-aluminum garnet. Simulation of twins in aluminum and yttrium oxides revealed the possibility of forming fragments of the garnet structure near the twin boundary. It is concluded that process of solid-state reactions is mainly associated with twinning, which leads to local variation in the symmetry and/or phase transitions.     

Synthesis and characterization of indium oxide-hematite magnetic ceramic solid solution

Indium oxide-doped hematite xIn₂O_3*(1-x)??-Fe₂O₃ (molar concentration x = 0.1?C0.7) solid solutions were synthesized using mechanochemical activation by ball milling. XRD patterns yield the dependence of lattice parameters and grain size as function of milling time. After 12 h of milling, the completion of In^3?+? substitution of Fe^3?+? in hematite lattice occurs for x = 0.1. For x = 0.3, 0.5 and 0.7, the substitutions between In^3?+? and Fe^3?+? into hematite and respectively, In₂O₃ lattices occur simultaneously. The lattice parameters of ??-Fe₂O₃ (a and c) and In₂O₃ (a) vary with milling time. For x = 0.1, Mössbauer spectra were fitted with one, two, or three sextets versus milling time, corresponding to gradual substitution of In^3?+? for Fe^3?+? in hematite lattice. For x = 0.3, Mössbauer spectra after milling were fitted with three sextets and two quadrupole-split doublets, representing In^3?+? substitution of Fe^3?+? in hematite lattice and Fe^3?+? substitution of In^3?+? in two different sites of In₂O₃ lattice. For x = 0.5 and 0.7, Mössbauer spectra fitting required two sextets and one quadrupole-split doublet, representing coexistence of In^3?+? substitution of Fe^3?+? in hematite lattice and Fe^3?+? substitution of In^3?+? in indium oxide lattice. The recoilless fraction studied versus milling time for each molar concentration exhibited low values, consistent with the occurrence of nanoparticles in the system. SEM/EDS measurements revealed that the mechanochemical activation by ball milling produced xIn₂O_3*(1-x)??-Fe₂O₃ solid solution system with a wide range of particle size distribution, from nanometer to micrometer, but with a uniform distribution of Fe, In, and O elements.     

Pressure‐induced structural transformations in In2‐xYx(MoO4)3 systems

In this work, we report results of high‐pressure Raman experiments (P < 8 GPa) on In_2‐xY_xMo₃O₁₂ for x = 0.0 and 0.5. A crystalline to crystalline structural phase transition and pressure‐induced amorphization (PIA) have been identified. The structural phase transition takes place at 1.5 and 1.0 GPa for In₂(MoO₄)₃ and In_1.5Y_0.5(MoO₄)₃, respectively, resulting in the change of structure from monoclinic P2₁/a to a more denser structure. The PIA started at 5 and 3.4 GPa for In₂Mo₃O₁₂ and In_1.5Y_0.5Mo₃O₁₂, respectively. The amorphization process takes place in two stages in the case of In_1.5Y_0.5Mo₃O₁₂ phase, while for In₂Mo₃O₁₂, it is not complete until the pressure is as high as 7 GPa. Our results also suggest that with increase of ionic size of the A³⁺ ions, the octahedral distortion increases and consequently larger local structural disorder is introduced in the A₂(MoO₄)₃ system, where A is a trivalent ion (In, Y³⁺, Sc³⁺, Fe³⁺, etc.). Copyright © 2015 John Wiley & Sons, Ltd.     

Sensors for the nitrogen oxides, NO2, NO and N2O, based on In2O3 and WO3 nanowires

Sensing characteristics of ZnO, In₂O₃ and WO₃ nanowires have been investigated for the three nitrogen oxides, NO₂, NO and N₂O. In₂O₃ nanowires of ∼20 nm diameter prepared by using porous alumina membranes are found to have a sensitivity (defined as the ratio of the sensor resistance in the gas concerned to that in air) of about 60 for 10 ppm of all the three gases at a relatively low temperature of 150 °C. The response and recovery times are around 20 s. The sensitivity of these In₂O₃ nanowires is around 40 for 0.1 ppm of NO₂ and N₂O at 150 °C. WO₃ nanowires of 5–15 nm diameter, prepared by the solvothermal process show a sensitivity of 20–25 for 10 ppm of the three nitrogen oxides at 250 °C. The response and recovery times are 10 s and 60 s, respectively. The sensitivity is around 10 for 0.1 ppm of NO₂ at 250 °C. The sensitivity of In₂O₃ and WO₃ nanowires is not affected by humidity even up to 90% relative humidity. The study also reveals that the sensing mechanism for the three nitrogen oxides have a commonality in that the desorption of oxygen is a crucial step in all the cases. PACS 07.07.Df; 85.35.-p; 82.35.Np     

Chemical and structural properties of ternary post-transition metal oxide thin films: InZnO,InGaO and GaZnO

The chemical states of ternary post-transition metal oxide thin films of InGaO, GaZnO and InZnO were investigated using X-ray photoelectron spectroscopy. Detailed binding energy (BE) analyses revealed certain evolution in chemistry in the ternary oxides compared to the reference binary oxides of In₂O₃, ZnO, or Ga₂O₃. In particular, O 1s BEs were changed with the compositions, which suggests that the charge transfer (CT) between In³⁺/Ga³⁺/Zn²⁺ and O²⁻ ions is significant. Results of extended X-ray absorption fine structure analyses further showed that the first shell coordination (cation–O bond) is roughly maintained even though the ternary oxide films were structurally disordered. This implies that the CT process via O²⁻ ions can influence the charge reconstructions in the ternary oxide systems.     

Effect of thermal treatment on the electrotransport properties of thin-film In<Subscript>2</Subscript>O<Subscript>3</Subscript>, ZnO materials and the multilayer (In<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZnO)<Subscript>83</Subscript> heterostructure

The effect heat treatment has on the electrotransport mechanisms in films of ZnO and In₂O₃, and in a multilayer (In₂O₃/ZnO)₈₃ structure obtained via ion-beam sputtering, is studied. It is shown that there is a mechanism of weak electron localization in the In₂O₃ and (In₂O₃/ZnO)₈₃ samples. The relaxation processes that occur during the heat treatment of In₂O₃ films are found to increase the length of elastic electron scattering, but to reduce this parameter in multilayer heterostructures.     

Chemical and phase compositions of the combustion products of thermit-type mixtures based on chromium, lanthanum, and calcium oxides

The effect of a CaO₂-Al high-caloricity additive on the combustion of a CrO₃-La₂O₃-Cr low caloric mixture is studied. The resultant mixture can burn at an additive content above 20%, with the combustion products having a cast appearance at an additive content above 30%. The additive content and gas pressure produce a significant effect on the phenomenology and regularities of combustion, the process of structure formation, and chemical and phase composition of the autowave synthesis products. The mutual high-temperature dissolution of oxides yields an lanthanum-chromite-based oxide material containing CaO and Al₂O₃, components that have a stabilizing effect and make it possible to modify the physical and chemical properties of lanthanum chromite.     

Conductivity of nanostructured India oxide films containing Co<Subscript>3</Subscript>O<Subscript>4</Subscript> or ZrO<Subscript>2</Subscript>

The effect of additives of cobalt and zirconium oxides on the conductivity of nanostructured composites based on indium oxide is studied. It is shown that addition of up to 20 wt % ZrO₂ to In₂O₃ leads to a sharp decrease in the conductivity of the composite. For the Co₃O₄?In₂O₃ system, the conductivity decreases up to a Co₃O₄ content of 60 wt %, after which it increases. At a Co₃O₄ content in the Co₃O₄?In₂O₃ system of up to 60 wt %, n-type conduction takes place, changing to p-type at 80 to 100 wt % Co₃O₄. Zirconium oxide exhibits practically no n-type conduction, so electric current in the ZrO₂?In₂O₃ system flows through In₂O₃ nanocrystals, i.e., n-type conduction takes place. Possible causes of the observed effects are considered.     

Analyzing anode and cathode products obtained by the electrolysis of Y-Ba-Cu-O and Y-Ba-Cu-K-O systems

Energy dispersive X-ray spectroscopy (EDX) and electron diffraction and high-resolution electron microscopy are used to study the composition and structure of anode and cathode deposits formed during the high temperature (950°C) electrolysis of the Y-Ba-Cu-O system and low temperature (450°C) electrolysis of the Y-Ba-Cu-K-O system. It is found for the first time that an oxide with the YBa₂Cu₃O _y structure (123 phase) is synthesized during the high temperature electrolysis of Y_0.02Ba_0.30Cu_0.70O _y and Y_0.02Ba_0.25Cu_0.75O _y melts. The 123 phase is not synthesized during low temperature electrolysis, where melts are formed from Y₂O₃, BaO₂, CuO, and KOH. Two new Pt-containing oxides with hexagonal structure are found in the products of high temperature electrolysis.     

Simultaneous removal of nitrogen oxides and diesel soot particulate in nano-structured electrochemical reactor

Simultaneous decomposition of nitrogen oxides (NO_x) and solid state graphite particles were carried out using a 8 mol% Y₂O₃ doped ZrO₂ (YSZ) based electrochemical reactor with a nano-structured NO_x selective multilayer cathode and an oxidative porous anode. The ceramic electrochemical cell was prepared by screen-printing a Pt and a NiO–YSZ pastes as cathode layers and a 12 CaO7Al₂O₃–Pt paste as an anode layer on the YSZ electrolyte, respectively. Simultaneous decomposition of NO_x and graphite particles was investigated using the cell with coated graphite particles on the surface of the 12 CaO7Al₂O₃–Pt composite anode in 1000 ppm NO_x–He gas flow under applying DC voltage at 475 °C. The coated graphite particles at the anode were removed completely with 80% NO_x decomposition by electrochemical reactions.     

Magnetism due to oxygen vacancies and/or defects in undoped semiconducting and insulating oxide thin films

Room temperature ferromagnetism was observed in HfO₂, TiO₂, and In₂O₃ films grown on yttrium-stabilized zirconia, LaAlO₃, and MgO substrates, respectively. While the magnetic moment is rather modest in the case of In₂O₃ films, it is very large in the other two cases. Thin film form, which might create necessary defects and/or oxygen vacancies, must be the main reason for undoped semiconducting and insulating oxides to become ferromagnetic. From the results, a serious question arises if a transition-metal doping indeed plays any essential role in producing ferromagnetism (FM) in non-magnetic oxides.     

Influence of Matrix Nature on the Structural Characteristics of In<Subscript>2</Subscript>O<Subscript>3</Subscript>–CeO<Subscript>2</Subscript> and SnO<Subscript>2</Subscript>–CeO<Subscript>2</Subscript> Composites Fabricated by the Impregnation Method

The structural characteristics, valence states, and distribution of cerium ions between the components in In₂O₃–CeO₂ and SnO₂–CeO₂ nanocomposites fabricated using the impregnation method were studied. X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDX) were used to show that, during impregnation, cerium ions are not included into In₂O₃ crystals and are disposed only on their surface in the form of nano-sized crystallites or amorphous clusters. On the other side, under the contact of CeO₂ clusters with a surface of SnO₂ matrix crystals, cerium ions penetrate into the surface layer of these crystals. In contrast to an In₂O₃–CeO₂ system, where the addition of CeO₂ does not affect the conduction activation energy, where cerium oxide is added to SnO₂, the observed increase in the resistance of a SnO₂–CeO₂ composite is accompanied by a sufficient increase in activation energy. These data and the XPS spectra confirm the modification of the surface layers of conductive SnO₂ crystals as, a result of the penetration of cerium ions into these layers.     

P-n codoping induced enhancement of ferromagnetism in Mn-doped In2O3: A first-principles study

We have performed first-principles density functional theory calculation in order to investigate the feasibility of “p-n codoping method” in improving magnetic property of In₂O₃ based diluted magnetic semiconductors. We find that the ferromagnetic state is favored in Mn-doped In₂O₃, and Sn doping can increase magnetic moment in Mn-doped In₂O₃. These findings are in line with our earlier experimental observation. Along with previous works, we now have enough evidences to support that p-n codoping is a valid method to improve magnetism of oxides based diluted magnetic semiconductors.     

Metal oxide additives for the sintering of silicon carbide: Reactivity and densification

The effectiveness of sintering additives for β-SiC was examined based on thermodynamic calculations and experimental observations under hot pressing conditions of 1700–1800 °C. Various types of oxides, such as Al₂O₃, Fe₂O₃, MgO, TiO₂, WO₃ and Y₂O₃, were examined theoretically by considering the Gibbs formation free energy and vapor pressure. According to experimental observations expanded to their binary and ternary systems after hot pressing at 1750 °C and 20 MPa, Al₂O₃, MgO, Y₂O₃ and their mixed systems were found to be the only effective sintering additives that do not react with β-SiC at high temperatures. On the other hand, Fe₂O₃, TiO₂, WO₃ and their combinations with other oxides were not effective because of the reaction with β-SiC by forming the corresponding metal carbides and/or silicides, as predicted by thermodynamic simulations. Moreover, the experimental results for the additional possible components were also included.     

Characterization of iron(III) oxide films used for photoelectrode by means of cems

Thin iron oxides deposited on semi-conductive glass by a spray pyrolysis technique were analysed by Conversion Electron Mössbauer Spectrometry (CEMS). Iron oxide deposited on SnO₂ coated glass was composed of a large grained particles of crystalline α?Fe₂O₃, which showed sextet. The doublet and sextet appeared in CEM spectra of iron oxides deposited on In₂O₃ and WO₃ coated glasses. The sextet was due to α?Fe₂O₃ and the doublet was attributted to the superparamagnetic microcrystalline α?Fe₂O₃ (≈15nm) rather than to spinel compounds of iron. The iron oxide deposited on ZnO coated glass gave a doublet in CEM spectra. It was supporsed to be due to very fine particle of α?Fe₂O₃ (<100nm). It was found that iron oxide films obtained by spray pyrolysis were dependent on the kinds and the temperature of the semi-conductive materials coated on glass.     

Relaxation, self-trapping, and decay of electron excitations in wide-gap oxides

Emission spectra ofMgO, Al ₂O₃, Y₂O₃, andSc ₂O₃ crystals, excited at 8 K by photons with energies 5–40 eV, are measured. The luminescence of free excitons andGe ²⁺ centers forMgO, the self-trapped exciton emission (STEE) forAl ₂O₃ andY ₂O₃, and the tunnel self-luminescence forScO ₃ are studied. A distinction between hole polarons in oxides and self-trapped holes in halides leads to a sharp difference in the mechanisms of STEE creation on electronhole recombination for these two classes of solids. The electron-hole, hole-electron, and exciton mechanisms of excitation multiplication are identified. The high radioresistance of wide-gap oxides and the behavior of oxygen interstitials inMgO crystals are discussed. Institute of Physics of Estonia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 5–16, March, 2000.     

Effect of excess Y<Subscript>2</Subscript>O<Subscript>3</Subscript> addition on the physical properties of melt processed YBCO bulks

The YBCO superconductors with Y₂O₃ addition were prepared by a modified melt textured process and the effects of excess Y₂O₃ addition on the physical properties of melt textured YBCO have been investigated. It is found that the melt temperature of YBCO samples decreases drastically with the increasing Y₂O₃, and the maximum levitation forces are drastically different for samples with different Y₂O₃ addition. It is also found that the optimal Y₂O₃ addition to YBCO is about 10wt%. Considering the microstructure and the starting composition, the results are well discussed and interpreted.     

Control synthesis of octahedral In2O3 crystals,belts, and nanowires

Octahedral In₂O₃ crystals were synthesized by evaporation of a mixture of In₂O₃ and graphite in a horizontal double-tube system. By adjusting the experimental conditions, In₂O₃ nanowires and nanobelts were also obtained. The microstructures of the resultant In₂O₃ materials were characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), X-ray diffraction. In addition, the growth mechanism of the octahedral In₂O₃ crystals was discussed in detail.