Electrical conductivity of LaCoxFe1 ? xO3 ? δ and LaLi0.1CoxFe0.9 ? xO3 ? δ (0 ≤ x ≤ 0.4) oxides

We have studied the effect of Co and Li concentrations on the phase composition and electrical conductivity of LaCo _x Fe_{1 − x} O_{3 − δ} and LaLi_0.1Co _x Fe_{0.9 − x} O_{3 − δ} perovskite-like oxides synthesized in air at 1470 K. Single-phase materials with an orthorhombic crystal structure were obtained in the range 0 ≤ x ≤ 0.3. The composition dependences of conductivity have a minimum at x _c = 0.1 and 0.2, respectively. In the range x > 0.1, the conductivity of LaCo _x Fe_{1 − x} O_{3 − δ} increases with increasing Co concentration for T > 700 K and decreases for T < 600 K. The conductivity of La(Li_0.1Co _x Fe_{0.9 − x} )O_{3 − δ} in the range 0 ≤ x ≤ 0.1 and for x ≥ 0.2 increases with Co concentration throughout the temperature range studied.     

Electrical transport in magnesium aluminate

The conductivity of MgAl₂O₄ has been measured at 1273, 1473 and 1673 K as a function of the partial pressure of oxygen ranging from 10⁵ to 10⁻¹⁴ Pa. The MgAl₂O₄ pellet, sandwiched between two platinum electrodes, was equilibrated with a flowing stream of either Ar + O₂, CO + CO₂ or Ar + H₂ + H₂O mixture of known composition. The gas mixture established a known oxygen partial pressure. All measurements were made at a frequency of 1 kHz. These measurements indicate pressure independent ionic conductivity in the range 1 to 10⁻¹⁴ Pa at 1273 K, 10⁻¹ to 10⁻¹² Pa at 1473 K and 10⁻¹ to 10⁻⁴ Pa at 1673 K. The activation energy for ionic conduction is 1·48 eV, close to that for self-diffusion of Mg²⁺ ion in MgAl₂O₄ calculated from the theoretical relation of Glyde. Using the model, the energy for cation vacancy formation and activation energy for migration are estimated.     

A structural,thermogravimetric and ESR study of the RhO x -TiO2 system

Titanium dioxide samples containing a few per cent of rhodium oxide, heated in air at 1273 K, have been investigated in order to study the formation of solid solutions. The results, monitored by X-ray diffraction, thermogravimetry in a hydrogen stream and ESR spectroscopy, show that rhodium is present both as separate phase (Rh₂O₃) and in solid solution in the rutile structure. The incorporated rhodium is mainly present in the 3 + oxidation state, only a small fraction being present as Rh(II). Rh₂O₃ and the rhodium species in solid solution are reduced by hydrogen to metal in different temperature ranges. By combining thermogravimetric and analytical data, the average oxidation state of rhodium,n, has been evaluated. The variation ofn with the rhodium content has been accounted for on the basis of a strong metal-support interaction developing with the reduction. This interaction affects the metal dispersion.     

Effect of admixtures of oxygen on the oxidation of iron and Fe−Cr alloys in lead melts

We study the process of oxidation of Armco iron and Fe−16Cr and Fe−16Cr−1Al model alloys held in lead melts with different concentrations of oxygen for 1000 h at 650°C. It was discovered that the intensity of oxidation, the structure, and phase composition of oxide layers are determined by the activity of oxygen in the liquid metal. By the methods of layer-by-layer X-ray diffraction analysis and microscopic X-ray diffraction analysis, it was shown that, for low concentrations of oxygen in lead (C ₀≤10⁻⁶ wt.%), a thin (1–5μm) oxide [magnetite (Fe₃O₄] film is formed on the surface of iron. If alloys are held under the same conditions, then we also observe an increase in the concentration of chromium in the subsurface layers. For higher concentrations of oxygen (up to 10⁻⁵ wt.%), a film of magnetite (with inclusions of pure lead) is formed on the surface of unalloyed iron. In alloys, under the layer of magnetite, we detect the formation of oxide layers with the same composition as a solid solution of Fe₃O₄ and FeCr₂O₄ and the structure of spinel. These layers efficiently suppress the process of penetration of lead but do not completely terminate the process of diffusion of oxygen into the bulk of the material, which eventually leads to the internal oxidation of alloy. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 3, pp. 97–101, May–June, 1997.     

System Cu-Rh-O: Phase diagram and thermodynamic properties of ternary oxides CuRhO2 and CuRh2O4

An isothermal section of the phase diagram for the system Cu-Rh-O at 1273 K has been established by equilibration of samples representing eighteen different compositions, and phase identification after quenching by optical and scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive analysis of X-rays (EDX). In addition to the binary oxides Cu₂O, CuO, and Rh₂O₃, two ternary oxides CuRhO₂ and CuRh₂O₄ were identified. Both the ternary oxides were in equilibrium with metallic Rh. There was no evidence of the oxide Cu₂Rh₂O₅ reported in the literature. Solid alloys were found to be in equilibrium with Cu₂O. Based on the phase relations, two solid-state cells were designed to measure the Gibbs energies of formation of the two ternary oxides. Yttria-stabilized zirconia was used as the solid electrolyte, and an equimolar mixture of Rh+Rh₂O₃ as the reference electrode. The reference electrode was selected to generate a small electromotive force (emf), and thus minimize polarization of the three-phase electrode. When the driving force for oxygen transport through the solid electrolyte is small, electrochemical flux of oxygen from the high oxygen potential electrode to the low potential electrode is negligible. The measurements were conducted in the temperature range from 900 to 1300 K. The thermodynamic data can be represented by the following equations: {fx741-1} where Δ_f(ox) G ^o is the standard Gibbs energy of formation of the interoxide compounds from their component binary oxides. Based on the thermodynamic information, chemical potential diagrams for the system Cu-Rh-O were developed.     

Alloy oxide equilibria in the Cr-Mn-O system

The phase boundaries of the Cr-Mn-O system have been investigated by alloy-oxide equilibria at 1173 and 1273 K and by isopiestic technique at 1323 K. The oxide phases which coexist in equilibrium with the Cr-Mn alloys are determined by x-ray diffraction studies. The results of the experiments indicate the presence of MnO in equilibrium with Mn-rich alloy whereas MnCr₂O₄ and Cr₂O₃ phases coexist with almost pure Cr. A three-phase equilibrium consisting of MnCr₂O₄ and MnO phases has been detected at the alloy composition X_Mn=0·252 at 1323 K. The composition of the alloy delineates the phase boundaries in the isothermal sections of the system. The results are interpreted by thermodynamic analysis of the Cr-Mn-O system using the data from the isopiestic measurements and those available in the literature.     

The mechanism of early oxidation stages of Fe20Cr5Al-type alloys at 1123 K

Abstract

Fe20Cr5Al-type alumina forming alloys are known as α alumina-forming materials at relatively high temperatures (≥1273 K). This paper reports on the oxidation mechanism of commercial and synthetic Fe20Cr5Al alloys, at a considerably lower temperature, 1123 K. Some of the alloys were implanted with oxygen or yttrium ions prior to the oxidation exposure.

Two-stage-oxidation, secondary ion mass spectrometry, secondary electron microscopy and lowangle X-ray diffraction were used as experimental tools. Results showed that even prolonged exposures are not sufficient to ensure that the scale consists essentially of α-Al₂O₃. Mechanisms involved from the observed scale morphology, microstructure and phase composition are discussed     

Oxidation behavior of a refractory NbCrMo0.5Ta0.5TiZr alloy

Isothermal oxidation behavior of a refractory high-entropy NbCrMo_0.5Ta_0.5TiZr alloy was studied during heating at 1273 K for 100 h in flowing air. Continuous weight gain occurred during oxidation, and the time dependence of the weight gain per unit surface area was described by a parabolic dependence with the time exponent n = 0.6. X-ray diffraction and scanning electron microscopy accompanied by energy-dispersive X-ray spectroscopy showed that the continuous oxide scale was made of complex oxides and only local (on the submicron levels) redistribution of the alloying elements occurred during oxidation. The alloy has a better combination of mechanical properties and oxidation resistance than commercial Nb alloys and earlier reported developmental Nb–Si–Al–Ti and Nb–Si–Mo alloys.     

A study on electrodeposited Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloys

Zn_1−x Fe _x alloys were electrochemically deposited on AISI 4140 steel substrates from sulfate bath. The bath was consisted of 40 g dm⁻³ ZnSO₄·7H₂O, 20–40 g dm⁻³ FeSO₄·7H₂O_, 25 g dm⁻³ Na₃C₆H₅O₇, and 16 g dm⁻³ H₃BO_3. The effect of bath composition on the electrical resistivity, the phase structure, and the corrosion behavior were investigated by the current–voltage measurements versus temperature, the X-ray diffraction (XRD) analysis, the atomic absorption spectrometry analysis, and the polarization measurements, respectively. Iron content was shown to strongly affect the structure, the electrical resistivity and the corrosion stability of Zn–Fe alloys.     

Subsolidus phase relations of Mg-Ga-Fe-O spinel solid solutions

The phase relations of Mg-Ga-Fe-O spinel solid solutions have been analyzed in a topological approach using composition diagrams. Oxides with the nominal compositions Mg_{1 − x} Ga_{2 − 2x} Fe_3x O_{4 + δ} and Mg_{1 − x} Ga_2x Fe_{2 − x} O_{4 + δ} (with x varied in steps of 0.1) prepared by a pyrohydrolytic process and air-annealed at 1270 K have been studied by X-ray diffraction.     

Use of Purbe diagrams in prediction of optimum conditions of separation and determination of halogens in natural objects using ionometry

Directional oxidation in the solution of X ⁻ ion being determined down to elementary halogen X ₂ and separation of it using gas extraction is promising for the separation of halogenide ions (Cl⁻, Br⁻, I⁻). The coinciding Purbe diagrams for redox systems of halogens X ⁻/X ₂ and most widely used oxidizers, namely, MnO₄⁻/MnO₂, MnO₄⁻/Mn²⁺, PbO₂/Pb²⁺, Cr₂O₇⁻/Cr³⁺, HNO₂/NO, were fragmentarily plotted. The predicted working pH intervals of the medium for directional quantitative oxidation of the given halogenide ion by a particular oxidizer were ascertained graphically using the diagrams. The possible schemes for the independent ionometric determination of Cl⁻, Br⁻ and I⁻ ions were proposed.     

Crystal structure and magnetic properties of high-coercivity Sr1 ? xPrxFe12 ? xZnxO19 solid solutions

Sr_1−x Pr _x Fe_{12 − x} Zn _x O₁₉ ferrites with x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5 have been prepared by solid-state reactions between praseodymium, iron, and zinc oxides and strontium carbonate in air at 1470 K. According to X-ray diffraction results, the samples with x ≤ 0.2 were single-phase and those with 0.3 ≤ x ≤ 0.5 contained, in addition to the magnetoplumbite phase, small amounts of α-Fe₂O₃, ZnFe₂O₄, and PrFeO₃. The mixed-phase samples further fired twice at 1470 K for 4 and 2 h contained no impurity phases at x = 0.3 and contained only α-Fe₂O₃ at x = 0.4 and 0.5. In the composition range 0 ≤ x ≤ 0.3, the a and c cell parameters, unit-cell volume V, and X-ray density ρ_x of the magnetoplumbite phase vary linearly according to the relations a(?) = 5.8869 − 0.0162x, c(?) = 23.027 + 0.449 x, V(?³)= 691.10 + 9.65x, and ρ_x(g/cm³) = 5.102 + 0.230 x. The highest degree of combined heterovalent substitution of Pr³⁺ for Sr²⁺ and Zn²⁺ for Fe³⁺ in the SrFe₁₂O₁₉ ferrite (formation of Sr_1−x Pr _x Fe_{12 − x} Zn _x O₁₉ solid solutions) at 1470 K is x = 0.32−0.36. The saturation magnetization per formula unit (n _s) of the x = 0.1 ferrite exceeds that of SrFe₁₂O₁₉ by 1.7% at 6 K and by 15.2% at 308 K. The 308-K n _s and coercive force (_σ H _c) of the x = 0.2 ferrite exceed those of SrFe₁₂O₁₉ by 7.6 and 8.5%, respectively.     

Microstructure and elevated temperature properties of a refractory TaNbHfZrTi alloy

Compression properties of a refractory multi-component alloy, Ta₂₀Nb₂₀Hf₂₀Zr₂₀Ti₂₀, were determined in the temperature range of 296–1473 K and strain rate range of 10⁻¹–10⁻⁵ s⁻¹. The properties were correlated with the microstructure developed during compression testing. The alloy was produced by vacuum arc melting, and it was hot isostatically pressed (HIPd) and homogenized at 1473 K for 24 h prior to testing. It had a single-phase body-centered cubic structure with the lattice parameter a = 340.4 pm. The grain size was in the range of 100–200 μm. During compression at a strain rate of έ = 10⁻³ s⁻¹, the alloy had the yield strength of 929 MPa at 296 K, 790 MPa at 673 K, 675 MPa at 873 K, 535 MPa at 1073 K, 295 MPa at 1273 K and 92 MPa at 1473 K. Continuous strain hardening and good ductility (ε ≥ 50%) were observed in the temperature range from 296 to 873 K. Deformation at T = 1073 K and έ ≥ 10⁻³ s⁻¹ was accompanied by intergranular cracking and cavitation, which was explained by insufficient dislocation and diffusion mobility to accommodate grain boundary sliding activated at this temperature. The intergranular cracking and cavitation disappeared with an increase in the deformation temperature to 1273 and 1473 K or a decrease in the strain rate to ~10⁻⁵ s⁻¹. At these high temperatures and/or low-strain rates the alloy deformed homogeneously and showed steady-state flow at a nearly constant flow stress. Partial dynamic recrystallization, leading to formation of fine equiaxed grains near grain boundaries, was observed in the specimens deformed at 1073 and 1273 K and completed dynamic recrystallization was observed at 1473 K.     

Effect of annealing on the microstructure, room-temperature strength, and fracture toughness of Al2O3−ZrO2−TiN ceramics

The microstructure, phase composition, room-temperature flexural strength, and fracture toughness of Al₂O₃−ZrO₂−TiN (AZT) ceramics were studied on specimens annealed in air at 1000, 1200, and 1400°C. The strength of the ceramics decreased with annealing temperature. The degradation in strength was caused by defects formed on or near the surface of the ceramics during oxidation of TiN which started at 600–700°C. The surface defects after annealing are influenced by the formation of rutile (TiO₂) at 1000 and 1200°C, aluminum titanate (Al₂TiO₅), and titanium suboxide Ti₅O₉ at 1400°C as well as by diffusion processes associated with ZrO₂. If the annealing of smooth AZT specimens in air resulted in lower strength, specimens in the form of single-edge notched beam (SENB) exhibited a considerable increase in fracture toughness (K _Ic) with annealing temperature. Such behavior was caused by the formation of an oxide layer which hindered the propagation of the main crack from the notch base. Thermal treatment of the smooth AZT specimens and further edge notching and testing did not result in a change of K _Ic values. The Al₂O₃ and Al₂O₃−ZrO₂ ceramics were also tested for comparison. Translated from Problemy Prochnosti, No. 1, pp. 132–138, January–February, 1999.     

Effect of nonstoichiometry on the structure and microwave dielectric properties of Ba1 ? x(Zn1/2W1/2)O3 ? x and Ba(Zn1/2 ? yW1/2)O3 ? y/2

We have synthesized Ba_{1 − x} (Zn_1/2W_1/2)O_{3 − x} and Ba(Zn_{1/2 − y} W_1/2)O_{3 − y/2} barium tungstates with different deviations from cation stoichiometry (x = 0.01–0.05, y = 0.01–0.05), determined the phase composition of ceramics fabricated from the tungstates, and investigated their electrical properties. Even slight deviations from cation stoichiometry in Ba(Zn_1/2W_1/2)O₃ lead to the formation of the scheelite phase BaWO₄, and its content increases with heat-treatment temperature. Barium or zinc deficiency in the systems studied improves the sintering behavior of Ba(Zn_1/2W_1/2)O₃ and increases the degree of 1: 1 B-site cation ordering, which in turn ensures an increase in microwave quality factor, Q.     

Muon-Spin Rotation Study of the Ternary Noncentrosymmetric Superconductors Li2Pd x Pt3−x B

We investigated the superconducting state of the noncentrosymmetric superconductors Li₂Pd _x Pt_3−x B with superconducting transition temperature T _c=5.16(8) K (x=2.25), 3.56(8) K (x=1.5) and 2.60 K (x=0) by means of muon-spin rotation (μSR) and specific heat experiments. The μSR relaxation rate σ _sc was found to be constant at low temperatures for all the compounds. Data taken at different magnetic fields show that the magnetic penetration depth λ is field-independent for Li₂Pd_2.25Pt_0.75B and Li₂Pt₃B. The electronic contribution to the specific heat measured in Li₂Pd_1.5Pt_1.5B and Li₂Pt₃B increases exponentially at the lowest temperatures. These features suggest that the whole family of Li₂Pd _x Pt_3−x B comprises single-gap s-wave superconductors across the entire doping regime.     

Phase structure and electrical properties of (Li,Ta)-doped (K,Na)NbO<Subscript>3</Subscript> lead-free piezoceramics in the vicinity of Na/K = 50/50

The phase structure and electrical properties of (K,Na)NbO₃ (KNN)-based lead-free piezoelectric ceramics with nominal composition Li_0.03 (Na _x K_1−x )_0.97Nb_0.8Ta_0.2O₃ (LKNNT, x = 0.50−0.55) were investigated with an emphasis on the influence of Na/K ratio. By XRD and Raman measurements, it was revealed that the phase transition from the co-existence of O ₁ and T ₁ to the co-existence of O ₂ and T ₂ occurs when x = 0.52. At this Na/K ratio, a peak of dielectric constants was obtained, which also corresponds to a morphotropic phase boundary between T ₁ and T ₂ besides that between O ₁ and O ₂ in LKNNT ceramics. All the Li_0.03 (Na _x K_1−x )_0.97Nb_0.8Ta_0.2O₃ ceramics with x = 0.50 − 0.55 show excellent piezoelectric performance, and the piezoelectric coefficient d*₃₃ can reach as high as 335 pm/V. The room-temperature piezoelectric properties in the present LKNNT ceramics are not sensitive to the change of Na/K ratio, indicating that the orthorhombic to tetragonal phase co-existence dominates the enhancement of piezoelectric properties. However, the results on the piezoelectricity measurement at elevated temperature showed possible to further enhance piezoelectric properties by adjusting Na/K ratio in the tetragonal LKNNT ceramics.     

Possibility of Superconductivity of the 2212 Phase in the Bi(Pb)-Sr-Ce-Cu-O System

Synthesis of the Bi-2212 compound in the Bi-Sr-Ce-Cu-O system has been already known. But, there has been no report on superconductivity of the compound yet. We have prepared many Bi-2212 samples partially substituted by Pb for Bi in the Bi-Sr-Ce-Cu-O system. The nominal composition is (Bi_{2− y} Pb _y )Sr₂(Sr_{1− x} Ce _x )Cu₂O _z . Then, we have investigated possibility of superconductivity for the samples. As a result, we find that a sample with nominal composition of x=0.23 and y=0.1, which is of almost the single 2212 phase, shows an anomaly at about 70 K in addition to temperature dependence of the resistivity like a semiconductor. Furthermore, the sample also shows a decrease of magnetic susceptibility starting at about 70 K with decreasing temperature. These experimental results can be considered to result from superconductivity of the 2212 phase in the Bi(Pb)-Sr-Ce-Cu-O system.     

Spinel ferrites of the quaternary system Cu-Ni-Fe-O: Synthesis and characterization

The decomposition of the freeze dried Cu(II)-Ni(II)-Fe(III) formate precursors at 1000°C in air yields complex oxides Cu_xNi_1−xFe₂O_4±δ (0 ≤ x ≤ 1) with a cubic spinel structure. For x < 0.7, single phase spinels are formed at 1000°C. However, for 0.7 ≤ x ≤ 1, Copper oxide (CuO) is identified as a second phase and the formation of a pure spinel phase requires an increase of the iron content in the mixture. For example, Cu_0.81Ni_0.1Fe_2.09O₄ is a single phase at 1000°C/air. Other single spinel phases Cu_0.5+yNi_0.5−y−zFe_2+zO_4±δ, 0 ≤ (y + z) ≤ 0.5, in the phase triangle Cu_0.5Ni_0.5Fe₂O₄–CuFe₂O₄–Cu_0.5Fe_2.5O₄ have been synthesized under special p(O₂)/T—synthesis conditions. The increase of the iron content requires an increase of the reaction temperature and/or a decrease of the p(O₂) in the reaction gas stream. The oxygen exchange between Cu_0.9Fe_2.1O_4.02 and the reducing gaseous phases shows that the non stoichiometry δ of copper ferrite is only about ±0.03. Significant changes in the oxygen content lead to the separation in different phases. The electrical and magnetic properties of copper ferrite samples depend on their chemical composition and preparation conditions.     

Possibility of Superconductivity of the 2212 Phase in the Bi(Pb)-Sr-Ce-Cu-O System

Synthesis of the Bi-2212 compound in the Bi-Sr-Ce-Cu-O system has been already known. But, there has been no report on superconductivity of the compound yet. We have prepared many Bi-2212 samples partially substituted by Pb for Bi in the Bi-Sr-Ce-Cu-O system. The nominal composition is (Bi_2−y Pb _y )Sr₂(Sr_1−x Ce _x )Cu₂O _z . Then, we have investigated possibility of superconductivity for the samples. As a result, we find that a sample with nominal composition ofx=0.23 andy=0.1, which is of almost the single 2212 phase, shows an anomaly at about 70 K in addition to temperature dependence of the resistivity like a semiconductor. Furthermore, the sample also shows a decrease of magnetic susceptibility starting at about 70 K with decreasing temperature. These experimental results can be considered to result from superconductivity of the 2212 phase in the Bi(Pb)-Sr-Ce-Cu-O system.