Dielectric dispersion in ZnF2-Bi2O3-TeO2 glass system

Dielectric constant , loss tan and ac conductivity of ZnF₂-Bi₂O₃-TeO₂ glasses with varying concentrations of Bi₂O₃ (from 5 to 20%) are studied as a function of frequency and temperature over moderately wide ranges. The dielectric breakdown strength for these glasses is also determined in air medium. From the analysis of these studies along with the IR spectra & DTA recordings of these glasses, the structural changes in ZnF₂-Bi₂O₃-TeO₂ glass system with change in the concentration of Bi₂O₃, are discussed.     

Comparison Between γ-Bi2MoO6 and Bi2WO6 Catalysts in the CO Oxidation

Bismuth molybdate (-Bi₂MoO₆) and bismuth tungstate (Bi₂WO₆) catalysts were prepared by solid-state reaction and their catalytic properties evaluated in the CO oxidation reaction. We characterize their structure by X-ray diffraction, scanning electron microscopy, Auger electron spectroscopy, and BET nitrogen absorption. X-ray diffraction analysis shows that both -Bi₂MoO₆ and Bi₂WO₆ are structural analogs (SG P2₁ ab). Auger analysis shows that Bi₂WO₆ catalysts have more bismuth on the surface than -Bi₂MoO₆, although both samples are bismuth deficient as compared to the stoichiometric compound. The results regarding catalytic activity show that Bi₂WO₆ prepared at 1073 K reaches total conversion of CO (100%) at a lower temperature when compared to -Bi₂MoO₆. It indicates that Bi₂WO₆ is a potential candidate to be used as catalyst in the CO to CO₂ oxidation.     

Stable and Metastable Phase Equilibria in the Bi2O3–BiPO4 System

The stable and metastable phase equilibria in the Bi₂O₃–P₂O₅ system were studied in the range 0–50 mol % P₂O₅ by differential thermal analysis and x-ray diffraction. The results were used to construct the corresponding phase diagrams. In the equilibrium state, the system contains one sillenite phase with Bi₂O₃ : P₂O₅ = 12 : 1 and four other bismuth phosphates (2 : 1, 3 : 1, 5 : 1, and 1 : 1). In a metastable state, resulting from solidification of metastable melts, there exist * solid solutions (0–25 mol % P₂O₅) based on the high-temperature form of Bi₂O₃. At lower temperatures, the * phase transforms eutectoidally into the metastable phase, which has the same structural basis as the high-temperature solid solutions. At room temperature, the phase exists in a narrow composition range around 15 mol % P₂O₅. At lower P₂O₅ contents, the * phase decomposes at 868 K by a monotectoid reaction to form a mixture of the metastable and phases. The phases 3Bi₂O₃ · P₂O₅ () and 2Bi₂O₃ · P₂O₅ (), melting incongruently at 1193 and 1223 K, respectively, appear in both the equilibrium and metastable phase diagrams.     

Some magnetic properties of γ-Fe2O3 synthesized from ferrous fumarate half-hydrate

-Fe₂O₃ synthesized from ferrous fumarate half-hydrate was studied by measurements of D.c. electrical conductivity, Seebeck coefficient, initial magnetization and magnetic hysteresis, and by Mössbauer spectroscopy and scanning electron microscopy. The phase transformation observed by electrical conductivity measurements matched well with the phase transformation observed by the variation with temperature of initial magnetization measurements of -Fe₂O₃; this magnetic study also established the single-domain character of -Fe₂O₃. The magnetic hysteresis values of the -Fe₂O₃ synthesized indicated improved values over that of a -Fe₂O₃ sample synthesized by established procedures. The scanning electron micrographs showed that the -Fe₂O₃ particles were acicular in shape and the Mössbauer spectrum showed a well-resolved six-band spectrum. The presence of a hydrogen ferrite phase was also confirmed by the electrical and magnetic measurements.Deceased, 10 October, 1985.     

Mechanochemical synthesis of crystalline compounds in the Bi-V-O system

The phase composition of crystalline mechanochemical synthesis products in the Bi₂O₃-V₂O₅ system is determined by x-ray diffraction. The phases identified are sillenite, clinobisvanite, a fluorite phase, the high-temperature form of the Aurivillius phase -Bi₂VO_5.5-x , and new, metastable tetragonal fluorite-like solid solutions. The same compounds are obtained at other Bi : V ratios using Bi₂O₄ and VO₂ as starting reagents. Structural analysis of the synthesized compounds reveals high vacancy concentrations and substitutional disordering. The room-temperature ESR spectra of the samples show V⁴⁺ signals at g 2 and 4. The phase transitions characteristic of clinobisvanite and -Bi₂VO_{5.5 -x} are not detected by thermal analysis, in line with the structural data. Mechanical activation of high-purity -Bi₂O₃ converts it into the form; in the presence of impurities, the resulting phases have fluorite or slightly distorted fluorite-like structure.Translated from Neorganicheskie Materialy, Vol. 41, No. 2, 2005, pp. 201–209.Original Russian Text Copyright © 2005 by Zyryanov.     

Engineered flux pinning centres in Pb-doped high temperature superconducting “Bi2Sr2CaCu2O8” ceramics

The Pb solubility of Bi₂Sr₂CaCu₂O₈ is determined within the concentration range Bi_2.18-x Pb _x Sr₂CaCu_8+d with 0 x 0.6 and the temperature range from 800 to 900°C. From a consideration of the Pb solubility in Bi₂Sr₂CaCu₂O₈ a simple annealing procedure was developed to precipitate (Pb,Bi)₄(Sr,Ca)₅Cu0_10+d (451 phase) in high-temperature superconducting Pb-rich Bi₂Sr₂CaCu₂O₈ ceramics. The precipitation results in a two-fold increase of the critical current density which is believed to express improved pinning properties of the superconducting ceramics. The microstructures of the samples with increased critical current density were studied with respect to the grain size and volume content of the 451 phase.     

Thermodynamic properties of the superconductorsBaPb 0.7 Bi 0.3 O 3 andBa 0.7 K 0.3 BiO 3 using Eliashberg theory

We present results of a thermodynamic analysis for the superconductors compounds BaPb_0.7Bi_0.3O₃ and Ba_0.7K_0.3BiO₃. The physical quantities are calculated making use of the Eliashberg theory and the electron-phonon spectra ²()F() as calculated by Shirai et al. For the superconductor BaPb_0.7Bi_0.3O₃, several models of the ²()F() were studied looking for a better agreement with experimental data. The best fit is achieved with a simple constant scaling (C = 1.25) of the Shirai's spectra. The functional derivative of the deviation function D(t) with respect to changes in ²()F() is also calculated.     

Preparation of superconducting Bi-Sr-Ca-Cu-O ceramics by the sol-gel method

Superconducting Bi-(Pb)-Sr-Ca-Cu-O ceramics were prepared through the sol-gel method using an aqueous solution of metal acetates containing acetic acid and tartaric acid. The conditions of gelation and conversion of gel to Bi₂Sr₂CaCu₂O_x phase were studied by thermal analysis, infrared absorption spectroscopy and X-ray diffraction technique. It was found that transparent gels were obtained from the solutions containing tartaric acid, with a molar ratio of C₄H₆O₆/Cu of about 0.4. The gel matrix was assumed to be composed of carboxylate anions and metal cations. A gel of molar composition BiSrCaCu = 2223 was decomposed to CaCO₃, CaO, CuO and Bi_1-x Sr_xO_y (x = 0.2 to 0.3) at low temperatures of 270 to 600 ° C and produced Bi₂Sr₂CaCu₂O_x phase at 800 ° C via complex intermediate states. A rapid cooling of the heated product increasedT _c (end) in the gel of molar composition BiSrCaCu = 2223, while a slow cooling increasedT _c (end) in the lead-containing gel of molar composition BiPbSrCaCu = 1.850.351.92.03.1. The latter showedT _c (onset) at 115 K andT _c (end) at 105K.     

Ionic conductivity and crystal structure relationships in Ti/Cu substituted Bi4V2O11

Crystal structure and oxide ion conductivity of a series of Ti and Ti-Cu double substituted Bi₄V₂O₁₁ compounds, Bi₂V_(1–x)Ti _x O_(11–x)/2 (0.085 x 0.15), and Bi₂V_0.9Cu_(0.1–x)TixO_5.35+x (0 x 0.1), were investigated using X-ray powder diffraction and ac impedance spectroscopy in the temperature and frequency range of 100–700°C and 10^–2–10⁷ Hz, respectively.Structural phase transitions, and , occur as a function of composition in Ti substituted compounds for which the is evidenced to be stable at room temperature when x exceeds 0.125. For all Ti-Cu double substituted compounds studied, the room temperature phase was identified to be phase.The required amount of Ti for phase stabilization at room temperature was significantly reduced and the conductivity improved when Cu substituted a part of Ti. Therefore, for the Bi₂V_0.9Cu_(0.1–x)Ti _x O_5.35+x (0 x 0.075) compounds the ionic conductivity increased and activation energy decreased with decreasing x. At low temperature, the highest ion conductivity was obtained for Bi₂V_0.9Cu_0.1O_5.35. At high temperature (T>500°C), a different behavior was observed. The total conductivity increased at first with decreasing x values down to x = 0.05 and then decreased. The maximum conductivity was obtained for Bi₂V_0.9Cu_0.05Ti_0.05O_5.4, and the activation energy decreased with decreasing x values, such as what happened at low temperature.     

The surface chemical properties of cobalt-modified γ-Fe2-O3 magnetic particles

The surface chemical properties of cobalt-modified -Fe₂O₃ (Co--Fe₂O₃) magnetic particles were studied by adsorption isotherms, FT-i.r. spectroscopy and microcalorimetry. The monolayer-adsorption amount of water on Co--Fe₂O₃ surfaces is 4.8 molecules per square nanometre, and the surface is considered to be covered with a thin water film in its normal environment. The surface hydrophilicity of Co--Fe₂O₃ is comparable to that of TiO₂. The acidic carboxyl group shows the strongest interaction with Co--Fe₂O₃, and a portion of this group chemically interacts. The basic amino group indicates stronger interaction with Co--Fe₂O₃ than the neutral hydroxyl group. It is concluded that the Co--Fe₂O₃ surface possesses a basic rather than an acidic character. This is interpreted in terms of the inherent nature of the adsorption site of Co--Fe₂O₃. Furthermore, the Co--Fe₂O₃ adsorption site appears to be energetically heterogeneous.     

Hydrazine method of synthesis of γ-Fe2O3 useful in ferrites preparation. Part IV – preparation and characterization of magnesium ferrite, MgFe2O4 from γ-Fe2O3 obtained from hydrazinated iron oxyhydroxides and iron (II) carboxylato-hydrazinates

Electro-magnetic properties and microstructural characterization of MgFe₂O₄ synthesized by a ceramic technique at 1000°C from iron oxides, consisting of mainly -Fe₂O₃ and traces of alpha-Fe₂O₃, prepared from iron ore rejects, are compared with the ferrite obtained from commercial alpha-Fe₂O₃. The sources of -Fe₂O₃ are hydrazinated iron (II) carboxylates and iron oxyhydroxides which autocatalytically decompose giving mainly -Fe₂O₃ of uniform particles of 10–30 nm (by scanning electron microscopy (SEM) studies) having high surface area. The ferrite synthesized from such nanoparticle size -Fe₂O₃ gave a porosity of 25% with grains ranging from 0–3 m. On the other hand, MgFe₂O₄ obtained from commercial alpha-Fe₂O₃ grains (of 1–2 m size) gave particles of 0–6 m with a porosity 42%. Saturation magnetization values 922–1168 G are found for MgFe₂O₄ from -Fe₂O₃ source while the alpha-Fe₂O₃ source gave the lowest value, 609. The Curie temperature, T_c, from magnetic susceptibility, initial permeability and resistivity measurements indicated a highest T_c of 737 K for MgFe₂O₄ from alpha-Fe₂O₃, while lower values are found for the ferrite prepared from -Fe₂O₃.     

The constitution of the Ni-Al-Ru system

The constitution of the Ni-Al-Ru system has been investigated in the range 0 to 50 at% Al. Isothermal sections at 1523 and 1273 K have been determined using microstructural observations, electron probe microanalysis and X-ray diffraction. The phases present were: nickel-based solid solution (); (based on Ni₃Al); solid solutions based on NiAl and RuAl, respectively (designated ₁ and ₂), and ruthenium-based solid solution (Ru). The maximum solubility of Ru in was 5 at%. ₁, and ₂ show extensive range of solubilities, namely up to 20at% Ru in ₁ and up to 25 to 35 at% Ni in ₂. Three-phase equilibrium between , ₂ and (Ru) existed at 1523 and 1273 K. Also at 1523 K, three-phase equilibria existed between , and ₁ and ,₁ and ₂, while at 1273 K, the equilibria were between , ₁,₂ and , , ₂ indicating the occurrence of a reaction +₁, +₂ at a temperature between 1523 and 1273 K. Liquidus features have been deduced from data on as-solidified structures. Lattice parameter data and hardnesses are also reported.     

The effect of a Cr2O3-addition on the phase transformation and catalytic properties of γ-Al2O3 in treatment of lean-burn exhausts

The solid-state thermal behaviour of -Al₂O₃ doped with 10 mol% Cr (oxide) was studied with respect to phase-transition behaviour and the co-ordination of the dopant Cr cations. A series of transformations: -Al₂O₃ -Al₂O₃ -Al₂O₃ -Al₂O₃ was observed for Cr₂O₃-doped alumina samples between 500 °C and 1100 °C. Rapid grain growth occurred at temperatures close to 1100 °C. The electron spin resonance (ESR) spectra for the sample heat-treated at 500 °C corresponded to the resonance of -Cr³⁺ in an amorphous Cr oxide impregnated onto the - and -alumina support. The change of ESR spectrum indicated the existence of Cr³⁺, suggesting the formation of a solid solution with the same structures as -Al₂O₃ and/or -Al₂O₃ at 800–1000 °C. The evaluation of catalytic activities for model exhaust was performed under lean-burn (an excess of oxygen) condition of air/fuel ratio A/F = 18 and space velocity SV = 100 000 h^–1 . The modified Al₂O₃ catalyst heat-treated at 1000 °C in air showed removal conversion of 100% for hydrocarbon (C₃H₆), 92% for CO and 5% for NO at 550 °C. Present results suggest that Cr-modification to Al₂O₃ leads to catalytic improvement with good thermal durability.     

Oxygen ion conduction in γ-Bi2O3 doped with Sb2O3

Electrical conduction in bcc-Bi₂O₃ doped with Sb₂O₃ was investigated by measuring electrical conductivity, as a function of temperature and oxygen partial pressure , and ionic transference number. The-Bi₂O₃ doped with 1 to 3 mol% Sb₂O₃ was stable up to 550° C and showed an oxygen ionic conduction in the region of 10⁵ to 10^–9 Pa. As the Sb₂O₃ content increased, ionic conductivity increased up to 2.5 mol % Sb₂O₃ (1.8×10^–3–1cm^–1 at 500° C) and then decreased. However, the activation energy for ionic conduction remained almost unchanged. It was proposed that the-Bi₂O₃ contains a lot of oxygen vacancies and incorporated Sb⁵⁺ ions at tetrahedral sites which affect the concentration of oxygen vacancy effective for conduction.     

Direct current electrical conductivity of gamma ferric oxide

The study of the direct current electrical conductivity, , of freshly prepared -Fe₂O₃ and that of a sample stored for seven days in static air suggests that -Fe₂O₃ adsorbs oxygen and water from the atmosphere. From infra-red spectra it is deduced that the absorbed water in -Fe₂O₃ is present as the physically adsorbed water and as lattice water. The adsorbed oxygen and physically adsorbed water are removed by heating to 100 C, while the lattice water remains in -Fe₂O₃ even up to 280 C. The removal of lattice water is associated with a decomposition during which some of the hydrogen formed occupies the vacancy sites. This suggested formation of the hydrogen ferrite phase is based on the kink in the log against T ^–1 curve observed at 177 C. This kink is very well resolved for a sample equilibrated at 100 C in normal atmosphere, and the measurements of above 100 C of this sample are done in an N₂ atmosphere. The suggestion that the hydrogen ferrite phase is formed has been substantiated by comparison of the X-ray diffraction patterns of -Fe₂O₃ heated under the different atmospheres. From the log against T ^–1 plot for a sample heated under a nitrogen atmosphere the activation energy is small (< 0.05 eV) up to 215 C, and it is comparatively large (0.95 eV) above 215 C. These results suggest a hopping mechanism for the direct current electrical conductivity of -Fe₂O₃. This suggestion has been substantiated by data of the temperature variation of Seebeck voltage.     

Phase and microstructural development in alumina sol–gel coatings on CoCr alloy

Phase transformation of -Al₂O₃ to -Al₂O₃ in alumina sol gel coatings on biomedical CoCr alloy was studied as function of heat treatment temperature and time. Transformation in unseeded coatings was significant only above 1200 °C. Addition of -Al₂O₃ seed particles having an average size of approximately 40 nm lowered the phase transformation temperature to around 800 °C. These particles were considered to act as heterogeneous nucleation sites for epitaxial growth of the -Al₂O₃ phase. The kinetics and activation energy (420 kJ/mol) for the phase transformation in the seeded coatings were similar to those reported for seeded monolithic alumina gels indicating that the transformation mechanism is the same in the two material configurations. Avrami growth parameters indicated that the mechanism was diffusion controlled and invariant over the temperature range studied but that growth was possibly constrained by the finite size of the seed particles and/or coating thickness. The phase transformation occurred by the growth of -Al₂O₃ grains at the expense of the precursor fine-grained -Al₂O₃ matrix and near-complete transformation coincided with physical impingement of the growing grains. The grain size at impingement was 100 nm which agreed well with that predicted from the theoretical linear spacing of seed particles in the initial sol.     

Effect of zinc on the magnetic properties of acicular cobalt-modified γ-Fe2O3 particles

Acicular -FeOOH particles with a particle length of about 0.35 m and an axial ratio of about 7 were synthesized by the coprecipitation method using the reaction of FeCl₂-NaOH. The (Co, Zn)-modified -F₂O₃ particles were produced by absorbing Co²⁺ and Zn²⁺ ions on the surfaces of -FeOOH particles followed by dehydration, reduction and oxidation. The saturation magnetization and thermal stability of the coercivity of (Co, Zn)--Fe₂O₃ particles were all higher than those of Co--F₂O₃ particles. For the same (Co+Zn) content, the saturation magnetization of (Co, Zn)--Fe₂O₃ particles increased with increasing zinc content but the coercivity decreased.     

In situ conversion electron Mössbauer spectrometry (CEMS) studies on α- and γ-Fe2O3 gas sensors

Clarification of the sensing mechanism of-Fe₂O₃ and -Fe₂O₃ gas sensors was attempted. The information on the surface states of iron oxides at working temperature (400° C for-Fe₂O₃ and 250° C for-Fe₂O₃) was obtainedin situ by applying thein situ CEMS. The-Fe₂O₃ gas sensor, prepared by the precipitation of Fe(OH)₃ from a solution of iron (III) sulphate and tin (IV) chloride, was composed of fine particles (–15 nm) and was superior in sensitivity to other-Fe₂O₃. The gas sensitivity was found to depend on the amounts of remaining sulphate ion, the microstructure and a small amount of iron (II) species generated through the reduction of-Fe₂O₃. The sensing mechanism of-Fe₂O₃ gas sensor was confirmed to be due to the reduction of-Fe₂O₃ to the low resistive Fe_3-x O₄ by combustible gas and to depend on the crystal structure.     

Directionally solidified Bi2O3-6Bi2O3 · SiO2 eutectic

The phase diagrams of the systems Bi₂O₃-6Bi₂O₃ · SiO₂ and (Bi₂O₃)_0.85 (Nb₂O₅)_0.15-6Bi₂O₃ · SiO₂ were determined by a quenching method. Melts of the eutectic and neighbouring compositions in these systems were unidirectionally solidified at various rates of 0.5 to 10 mm h^–1 under a temperature gradient of 100° C cm^–1. At any solidification rate examined, the ingot was of irregular structure in the former system and was of well-aligned regular structure in the latter system at the eutectic composition and those deviated from that by 10 mol%. In the latter ingots, fibrous-(Bi₂O₃)_0.85 (Nb₂O₅)_0.15 crystals were embedded in parallel and with a uniform spacing in a matrix of-6Bi₂O₃ · SiO₂ crystal. The diameter,d, of the fibrous crystals and the interfibre spacing decreased from 3.8 to 0.7 m and 10 to 1.9 m, respectively, with increasing solidification rate,R, in the range 0.5 to 10 mm h^–1, following the relationsd= 2.5R ^–1/2 and=6.9R ^–1/2. The volume fraction of the fibrous crystals increased from 0.14 to 0.23 with an increase in the (Bi₂O₃)_0.85 (Nb₂O₅)_0.15 content of the melt from 46 to 66 mol%. The elongation direction 110 of the fibrous crystal was parallel to the 111 direction of the matrix crystal. The difference in the microstructure of the ingots between the systems is discussed in terms of the solidification process.