Electrical Conductivity and Thermodynamic Properties of Some Alkaline Earth-Doped Lanthanum Chromites

Alkaline-earth doped lanthanum chromites are currently the interconnecting materials of choice for solid oxide fuel cells (SOFCs). Since these materials in SOFC operating conditions are under a large oxygen potential gradient and at high temperature (1273 K), a thorough knowledge of their physical and thermochemical properties is very important. In the present study, the alkaline-earth doped lanthanum chromites La_1-xSr_xCrO₃ (x=0–0.3) and La_0.7Ca_0.3CrO₃ were prepared from complex precursors isolated from the La(NO₃)₃–Cr(NO₃)₃–urea system. The oxide powders were characterized by means of X-ray diffraction (XRD). The DC electrical conductivities of the samples were measured in the temperature range of 295–1273 K in air. The thermodynamic properties represented by the relative partial molar free energies, enthalpies, and entropies of oxygen dissolution in the perovskite phase, as well as the partial pressures of oxygen, have been investigated by the solid electrolyte galvanic cells method coupled with the solid-state coulometric titration technique, within the temperature range of 1073–1273 K and in a reducing atmosphere (10^–5 Pa). The variation of the electrical conductivities and thermodynamic properties with changing oxygen stoichiometry is discussed. The study demonstrates new correlations existing between the structural, electrical, and thermodynamic properties in the doped lanthanum chromites.Paper presented at the Sixteenth European Conference on Thermophysical Properties, September 1–4, 2002, London, United Kingdom.     

Sinterability and electrical conductivity of calcium-doped lanthanum chromites

Calcium-doped lanthanum chromites, (La_1–x Ca _x ) (Cr_1– y Ca_y O₃, have been synthesized to investigate effects of calcium doping on sinterability and electrical conductivity. X-ray diffractometric results have revealed that in addition to normal perovskites (La_1– xCa _x CrO₃), chromium-deficient perovskites can exist as a single phase in the composition region 0.1 <x < 0.3, although the deficit of chromium is small. These chromium-deficient perovskites show a good sinterability even in air at 1873 K. Electrical conductivity of these perovskites has been measured as functions of temperature and oxygen potential. It has been found that electrical conductivity of the chromium-deficient perovskites increases almost linearly with total calcium content. The magnitudes of electrical conductivity are comparable to those of strontium-doped lanthanum chromites.     

X-ray Photoelectron Spectra of La1 – xCaxCr1 – yMyO3 (M = Mn, Fe, Ni, Cu) Perovskite Oxides

X-ray photoelectron spectroscopy was used to identify the oxidation states of ions in La_{1 – x} Ca _x Cr_{1 – y} M _y O₃ (M = Mn, Fe, Ni, Cu) perovskite oxides, to determine the surface composition of the samples, and to assess the effect of B-site substitutions on the electronic spectrum of the chromites. The binding energies of the La 4d, Ca 2p, Mn 2p, Ni 2p, Cr 2p, and O 1s core levels were determined, and the effect of the dopant on the shape of the La 4d and O 1s peaks was analyzed.     

Synthesis and Electrical Properties of Solid Solutions Based on Bismuth Vanadates

The phase-formation processes in the Bi₂O₃–V₂O₅ and Bi₂O₃–MO–V₂O₅ (M = Zn, Pb, Cu, Co, Ni) systems were investigated. The associated sequence of phase transformations and the conditions for preparing single-phase Bi₄V₂O₁₁ (Bi₂VO_5.5) and Bi₂V_{1 – x} Me _x O_{5.5 – 3x/2} solid solutions were established. The lattice parameters of the solid solutions were determined, and their conductivity was measured as a function of temperature and composition.     

Synthesis and Properties of (Sr1 – xMx)2Nb2O7 (M = Cu, Ni) Solid Solutions

(Sr_{1 – x} M _x )₂Nb₂O₇ (M = Cu, Ni) layered perovskite solid solutions were synthesized, and their stoichiometry ranges were determined. The electrical properties of the solid solutions were compared with those of Sr₂Nb₂O₇.     

Specific heat of the high-Tc oxide superconductors

We review the specific heat measurements on La₂CuO₄, La_2–xM_xCuO₄ (M = Ca, Sr, and Ba), YBa₂Cu₃O₇, and the Bi-Ca-Sr-Cu-O and Tl-Ca-Ba-Cu-O systems. Tables of properties derived from the data are presented. Results on RBa₂Cu₃O₇ (R=rare earth elements other than Y) are summarized, as are results on YBa₂(Cu_3–xM_x)O₇ (M=Zn, Cr, Fe, or Ni). The difficulties of analyzing the specific heat data, and specifically the separation of the contributions associated with magnetic impurities, are discussed. It is tentatively concluded that the data nearT _c are consistent with BCS theory, although they show evidence of fluctuation effects. It is also concluded that the low-temperature zero-field data on a majority of the high-T _c oxide superconductors provide evidence of anintrinsic term that is proportional toT, a result that is inconsistent with a gap in the electronic density of states.     

Hydrothermal synthesis of HNbWO6/MO series nanocomposites and their photocatalytic properties

MO (M = Ni, Cu and Mn) or M₂O₃ (M = Fe and Cr) were intercalated into the interlayer of HNbWO₆ by hydrothermal reaction on HNbWO₆ with 1 M soluble M(NO₃)₂ or M(NO₃)₃ aqueous solution at 120°C. 相似文献   

Fabrication of YBa2Cu3O7−x bulk superconductor from the powder without reoxygenation

The fabrication of YBa₂Cu₃O_7–x sintered ceramic and YBa₂Cu₃0_7–x melt product from the powder without additional reoxygenation is reported. The YBa₂Cu3O_7–x is fabricated inside a commercial silicon carbide tube. The SiC tube effectively does not react with the YBa₂Cu3O_7–x and acts to prevent oxygen from diffusing out of the YBa₂Cu3O_7–x . The fabrication procedure can, in principle, be used with any tube enclosure which neither reacts with YBa₂Cu3O_7–x nor allows diffusion of oxygen to the outside atmosphere.     

Solid solutions with rutile structure: electronic behaviour and ultraviolet photoelectron spectrometry study

Solid solutions of composition Ti_2.1M_0.3(Sb_1–x Nb _x )_0.6O₆ (M=Co, Ni and Cu) in the range 0 x 1 are obtained by thermal treatment of the appropriate amounts of simple oxides. The crystal structures have been determined by employing Rietveld structure analysis of powder X-ray diffraction data. Electrical conductivity, magnetic and ultraviolet photoelectron spectrometry (UPS) measurements for these phases are also discussed.     

Electrical conductivity and dielectric permittivity of Bi5Nb3 ? 3 x M3 x O15 ? δ (M = Cr, Cu, Ni) solid solutions

Bi₅Nb_{3 − 3x} M_3x O_{15 − δ} (M — Cr, Cu, Ni) (M = Cr, Cu, Ni) layered perovskite-like solid solutions have been prepared by solid-state reactions. Increasing the solute content of the solid solutions gives rise to solute-dependent monoclinic distortion of their tetragonal cell. The electrical conductivity and dielectric permittivity of the Bi₅Nb_{3 − 3x} M_3x O_{15 − δ} solid solutions and bismuth niobate, Bi₅Nb₃O₁₅, have been measured at temperatures from 300 to 750 K and frequencies from 1 to 200 kHz. Original Russian Text ? N.A. Zhuk, I.V. Piir, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 12, pp. 1504–1509.     

Effect of M and M-Ca Substitution on the Structure and Superconductivity of GdBa2Cu3O7?δ [M = Mo, Hf ]

The structural and superconducting properties of (Gd_1–x–y Ca _y M _x )Ba₂Cu₃O _z with M = Mo, Hf are investigated using X-ray diffraction, electrical resistivity, and oxygen content measurements. The effect of increasing the Mo concentration in (Gd_1–x Mo _x )Ba₂Cu₃O _z changes the structure from orthorhombic to tetragonal accompanied by a large increase in resistivity and a fast decrease in T _c at the rate of 1.9 K per at.% of Mo, unlike that of Hf substitution in (Gd_1–x Hf _x )Ba₂Cu₃O _z , which maintains the orthorhombic structure and decreases T _c very slowly at the rate of 0.6 K per atm.% of Hf with nearly no change in resistivity. The suppression of T _c by M = Mo, Hf can be counterbalanced by hole doping by Ca which increases T _c with increasing Ca content showing maximum compensation for Mo. A comparative study of M = Mo, Hf doped samples in (Gd_1–x–y Ca _y M _x )Ba₂Cu₃O _z indicates that the valence of the dopant M = Mo^4+,6+, Hf⁴⁺ and its ionic radius play an important role in controlling the structural and superconducting properties of the systems.     

Dielectric and Piezoelectric Properties of Pb(Mg1/3Nb2/3)O3–PbTiO3–Pb(Mg1/2W1/2)O3 Ceramics

Data are presented on the phase composition, crystal structure, microstructure, and dielectric and piezoelectric properties of (1 – y)[(1 – x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃]–yPb(Mg_1/2W_1/2)O₃ (x = 0.30–0.36; y = 0, 0.05, 0.10) ceramics. It is shown that the use of fine-particle magnesia as a starting reagent ensures the formation of single-phase materials. The ceramics with a rhombohedral structure are found to exhibit relaxor behavior. Increasing the content of the Pb(Mg_1/2W_1/2)O₃ perovskite leads to ordering of the domain structure of poled ceramics and increases their piezoelectric charge coefficient d ₃₁ and the width of their phase transitions.     

Machinable and mechanical properties of sintered Al2O3-Ti3SiC2 composites

Two-phase composites consisting of (1 – x) Al₂O₃ and xTi₃SiC₂ (x = 0–1) were prepared by spark plasma sintering (SPS). Sintered densities larger than 98% of theoretical density were achieved when the specimens were sintered at 1300°C for 5 min (in vacuum, at pressure 30 MPa). When content of Ti₃SiC₂ increased up to 30 wt%, composites were found to be machinable—they could be drilled easily using conventional Fe-Mo-W drills or gravers. The mechanical properties of the (1 – x) Al₂O₃–xTi₃SiC₂ composites were evaluated. The bending strength, Vickers hardness of the specimens had the following ranges: 428 ± 10.2 (x = 0) to 673 ± 15.4 Mpa (x = 1) (bending strength at room temperature); 19.9 (x = 0) to 4.0 GPa (x = 1) (Vickers hardness).     

Fine Structure of α-Al2O3-Based Solid Solutions

The fine structure of solid solutions between isomorphic p-block and 3dtransition-metal oxides was investigated experimentally and theoretically in the technologically important systems -Al₂O₃–M₂O₃(M = Ti, V, Cr, Fe). It was shown that the thermodynamic theory of isomorphic miscibility and conventional approaches to estimating solubility limits fail to explain and, at best, only predict the phase relations in the systems examined. The proposed magnetochemical method offers the possibility of studying in detail the M–M interactions in first-row oxides M₂O₃and (M _x Al_{1 – x} )₂O₃solid solutions, probing their homogeneity, and revealing their salient structural features. The marked difference in the solubilities of isomorphic M₂O₃oxides in -Al₂O₃is interpreted in terms of the magnetic structure of the oxides.     

Incorporation of bismuth into Ba6− x R8+2/3x Ti18O54(R = Nd, Gd)

Microstructural investigations and microanalyses of a series of Ba_6–x R _8+2/3x Ti₁₈O₅₄ ceramics (R = Nd, Gd) revealed that the solid-solubility limit for the isovalent substitution of R ³⁺ by Bi³⁺ depends on the composition of the Ba_6–x R _8+2/3x Ti₁₈O₅₄ phase. For the Nd analogue the solid-solubility limit (y in Ba_6–x (Nd_1–y Bi _y )_8+2/3x Ti₁₈O₅₄) decreases with a decrease in x from y = 0.16 for x = 2.0 to y = 0.10 for x = 0.8. An even lower solid-solubility limit (y = 0.06) was found for the Ba_4.5(Gd_1–y Bi _y )₉Ti₁₈O₅₄ compound (x = 1.5). All substituted Nd compositions exhibit higher permittivities (93–99), lower temperature coefficients of permittivity (11–15 ppm/K) and higher dielectric losses (Q · f = 1300–5500 GHz) than the parent compositions. By exceeding the solid-solubility limit, abrupt changes in the microstructural and dielectric characteristics are induced.     

Hardness and elastic properties of Bi2O3-based glasses

The hardness and elastic properties of 20PbO · xBi₂O₃ · (80 – x)B₂O₃ glasses with x = 20–60 were evaluated through usual Vickers indentation and nanoindentation tests. The glass transition temperature (T _g = 295–421°C), Vickers hardness (H _v = 2.9–4.5 GPa), true hardness (H = 1.5–3.8 GPa) and Young's modulus (E = 24.4–72.6 GPa) decreased monotonously with increasing Bi₂O₃ content. This compositional trend demonstrates that the strength of Bi–O chemical bonds in these glasses is considerably weak compared with B–O bonds and plastic deformations under indentation loading occur easily. The elastic recovery after unloading was about 45% for the glasses with x = 20–50, and the Poisson's ratio was 0.27 for the glass with x = 20. The fracture toughness was evaluated to be 0.37–0.88 MPam^1/2 from the values of H _v and E, and it was proposed that not only weak Bi–O bonds but also boron coordination polyhedra (BO₃ or BO₄) and their arrangements affect on crack formation. From the temperature dependence of Vickers hardness up to the glass transition region, it was suggested that the glasses with high Bi₂O₃ contents belong to the category to fragile glass-forming liquids.     

Phase formation and characterization of [Fe, Mg]NbO4 as a new precursor for the PMN–PFN system

An X-ray diffraction (XRD) and scanning electron microscopy (SEM) study of the phase composition and microstructure characteristics of the Mg_{(1 –x)/3}Nb_{(4 –x)/6}Fe _x/2O₂ (x = 0.5) chemical compound is presented. The samples were prepared by the conventional ceramic method and subjected to different heat treatments. Columbite (MgNb₂O₆) and iron niobium oxide (FeNbO₄, Wolframite) were identified as intermediate compounds in the reaction. A new single phase precursor for the (1 –x)Pb(Mg_1/3Nb_2/3)O₃-xPb(Fe_1/2Nb_1/2)O₃ (PMN–PFN) system identified as [Fe, Mg]NbO₄, was obtained, isostructural with the FeNbO₄ where Fe and Mg ions occupy the same crystal site (space group P1 2/a 1). From the Rietveld refinement method the cell parameters of the monoclinic structure were determined. The microstructure analysis indicates that the particles are irregular in shape and the grain size tends to increase with the calcination temperature.     

Structural Studies of the Superconducting La2.5Y0.5CaBa3(Cu1?xFex)7Oz (0 ≤ x ≤ 0.1) Perovskite System by Neutron Diffraction

We have investigated the effect of Fe substitution on the structural and superconducting properties of La_2.5Y_0.5CaBa₃(Cu_1–x Fe _x )₇O _z system by Rietveld refinement of the neutron diffraction patterns of three samples with x = 0.02 (labelled B₁), x = 0.06 (B₂), and x = 0.10 (B₃) along with X-ray diffraction, resistivity, AC susceptibility, and oxygen-content measurements. Samples B₁, B₂, and B₃ are superconducting with T _c ^R=0 values of 73, 62, and 41 K, respectively. Neutron diffraction studies confirm (i) the formation of a single phase tetragonal structure (space group P4/mmm) for all three samples, (ii) Ca and Y ions substitution at the La site concomitantly displaces La onto Ba sites, and (iii) increasing x from 0.02 to 0.10 increases oxygen content (the amount of oxygen per unit cell), as well as Cu(1)— O(4) and Cu(1)— O(1) bond lengths whereas Cu(2)— O(4) bond length decreases with corresponding decrease in T _c to 41 K due to increasing occupancy of Fe ions at Cu(2) site. The change in bond lengths with oxygen content are essentially the same as those of Fe content (x). Present studies establish a correlation between the bond lengths (Cu(1)— O(1), Cu(1)— O(4), and Cu(2)— O(4)) and the measured T _c values of three samples.     

Preparation of copper-cobalt oxide spinels by thermal decomposition of copper-cobalt basic nitrate mixed crystals

Copper-cobalt oxide spinels with a common formula Cu _x Co_3–x O₄ (0x1.0) have been prepared by thermal decomposition at 350° C in air of single-phase mixed basic nitrate precursors (Cu, Co)₂(OH)₃NO₃. The formation of the oxide phase during the thermal treatment has been associated with a diffusionless mode of transformation, established for some hydroxides and hydroxysalts with a layer-type structure.     

Stability of SrZr0.9Yb0.1O3−α protonic conductor in atmosphere containing nitrogen oxides (NOx)

The stability of the SrZr_0.9Yb_0.1O_3– protonic conductor in an atmosphere containing nitrogen oxides (NO _x ) was investigated. When a fine powder of SrZr_0.9Yb_0.1O_3– with a specific surface area of about 50 m²/g was annealed at 440 °C in He gas containing 8% O₂ and 0.1% NO, the formation of Sr(NO₃)₂ was observed by IR measurement, ion-chromatography analysis and ICP analysis. The formation mechanism of Sr(NO₃)₂ was examined by considering the thermodynamic equilibrium. Based on the results of the thermodynamic calculation, H₂O dissolved into SrZr_0.9Yb_0.1O_3– was estimated to play an important role in the reaction for the formation of Sr(NO₃)₂ between SrZr_0.9Yb_0.1O_3– and NO _x .