Some thermodynamic aspects of highT c superconductors

Thermochemical and thermodynamical properties of HTSC phases are reviewed for the Y-Ba-Cu-O system and also presented for the newly calculated Bi-Sr-Cu-O system stressing out stoichiometric and phenomenological viewpoints. Simulated data are listed for (H ₂₉₈ ^o -H _o ^o , phase transformation temperatures, standard entropies, standard enthalpies of formation, heat capacities in crystalline phase, etc. Pseudobinary phase diagrams are treated showing the effect of oxygen partial pressure particularly illustrated on the (Sr, Bi, Ba)-Cu-O system.The work was carried out under the project No. A 2010532 supported by the Grant Agency of Academy of Sciences of the Czech Republic and the grant No. 104/97/0589 financed by the Grant Agency of the Czech Republic.     

纳米二氧化钛材料的相变和声子限制效应(英文)

拉曼光谱用于研究二氧化钛材料的相变和声子限制效应。化学溶液方法制备了TiO2纳米晶材料。其平均粒度为 6 8- 2 7 9nm。最低频率 1 5 2cm- 1 Eg 模随粒度减小出现蓝移和加宽。在声子限制模型下 ,理论上对不同粒度的TiO2 纳米晶 (6 8,1 0 3和 2 7 9nm)的频移和线宽进行计算 ,结果与实验吻合得很好。研究了TiO2 纳米晶锐钛矿 -金红石相变 ,其相变温度为 6 5 0 - 6 90℃ ,比体块TiO2 的相变温度 1 0 0 0℃低 ,表明了相变的尺寸效应     

TPR-study of Substitution Effects on Reducibility and Oxidative Non-stoichiometry of La0.8A'0.2MnO3+δ Perovskites

La_0.8 A'_0.2 MnO_3+δ (A'=Sr, Ba, K, Cs) perovskites were studied by temperature-programmed reduction. The amount of non-stoichiometric oxygen was found to decrease from δ=0.16 for LaMnO_3+δ to δ=0.07 (Sr-), 0.06 (Ba-), 0.07 (K-) and 0.03 (Cs-) substituted manganites. The reducibility at low and mean-temperatures (t<500°C) increased in the sequence La<(La, Sr)≈(La, Ba)<(La, Cs)<(La, K). This revised version was published online in July 2006 with corrections to the Cover Date.     

High-temperature electron-hole transport in PrBaCo2O5+δ

The oxygen content, conductivity and thermopower in the double perovskite-like cobaltite PrBaCo₂O_5+δ are reported in the oxygen partial pressure range 2×10⁻⁶-0.21 atm and temperatures between 650 and 950 °C. The electrical properties are shown to be continuous through the transition from δ>0.5 to δ<0.5. The variations of transport parameters with temperature and oxygen content reveal hole polaron hopping conduction within oxygen non-stoichiometry domain δ<0.5.     

Ln_2MO_4 cathode materials for solid oxide fuel cells

One of the major challenges to develop intermediate temperature solid oxide fuel cells is finding a novel cathode material, which can meet the following requirements: (1) high electronic conductivity; (2) chemical compatibility with the electrolyte; (3) a matched thermal expansion coefficient (TEC); (4) stability in a wide range of oxygen partial pressure; and (5) high catalytic activity for the oxygen reduction reaction (ORR). In this short review, a survey of these requirements for K2NiF4-type material wi...     

Microwave-Assisted Elemental-Direct-Reaction Route to Nanocrystalline Copper Sulfides Cu9S8 and Cu7S4

Non-stoichiometric copper sulfides Cu₉S₈ and Cu₇S₄ nanocrystallites were synthesized by the reaction between elements S and Cu under microwave radiation for several minutes. The products were analyzed by using XRD, TEM, XPS, ICP and PL.     

Characterization of Crystalline Complexes between Heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin and Various Alkanes or Alkenes (5 ≤ C ≤ 10)°

Crystallization, at room temperature under normal pressure, of heptakis (2,3,6-tri-O-methyl)--cyclodextrin (TRIMEB) in the presence of various hydrocarbons (n-pentane, n-hexane, cyclohexane, methylcyclohexane, (RS)--pinene, (S)--pinene and (R)--pinene) gave two series of isomorphous crystalline phases: Ua (metastable) and Ub (stable). In this study, the molar guest/host ratio is defined as the variable x. On the one hand, the experiments carried out in solution at high supersaturations led to the Ua phase, with x < 1 for linear alkanes (non-stoichiometric and efflorescent phases) and x = 1 for cyclic guests. On the other hand, by slow evaporation, the Ub phase obtained with cyclic components was stoichiometric (x = 1) and non-efflorescent; whereas the Ub phase with n-pentane and n-hexane as guestsdisplayed efflorescent character and was non-stoichiometric (x approx 0.15). Temperature-resolved X-ray powder diffraction allowed the irreversible transition from Ua to Ub to be observed. Following this thermal process and whatever the nature of the cyclic guest molecule, Ub was non-stoichiometric (x 0.7 for methylcyclohexane); thus, x was significantly lower than that of the mother phase Ua with x = 1. This suggests a destructive-reconstructive solid-solid transition. The crystal structure solved at 120K of Ub obtained from solution, with methylcyclohexane, reveals that the guest molecule is totally buried within the cavity. The methylcyclohexane mean plane, defined by the C(2), C(3), C(5) and C(6) atoms, is 45° away from the pseudo seven-fold axis of the macrocycle. The methylcyclohexane molecule is disordered within the cavity, and its possible conformations were twisted chair and twisted boat.These results differ from the conformations reported by Rontoyianni et al., J. Incl. Phenom. 32, 415–428 (1998) for the structure of the same complex solved at 293K. Molecular simulations of n-alkane (C(5) and C(6)) movements along the a axis showed that the Ub phase structure can easily undergo a partial release of this linear alkane, due to the presence of channels in this structure. Comparison between solid stateconformations observed for the TRIMEB molecule in itscomplexes does not support the notion of `induced fit' inthe inclusion process.     

Effect of oxygen non-stoichiometry on the structural and magnetotransport properties of LaMn0.85Cr0.15O3+δ

Thermogravimetry, X-ray diffraction (XRD), d.c. magnetization, high-temperature susceptibility and electrical resistivity measurements were performed for LaMn_0.85Cr_0.15O_3+δ perovskites with accurate control of the oxygen content (0?δ?0.11). For 0?δ<0.09, three orthorhombic structures (Pnma) are found: for 0?δ<0.045, the O′ phase (b/√2<c<a), for 0.045?δ<0.06, the O″ (b/√2<a<c) and for 0.06?δ<0.09, the O^? (a<b/√2<c). For 0.09?δ?0.11, a rhombohedral symmetry coexists with O^? in a biphasic field. Magnetic measurements revealed the ferromagnetic interactions (FM) character of the Mn³⁺-O-Cr³⁺ interaction, but also the intricate magnetic phase diagram due to the presence of multiple interactions (Mn^3+,4+-O-Mn^3+,4+, Cr³⁺-O-Mn³⁺, etc.). The comparison of the results for LaMn_0.85Cr_0.15O_3+δ with those of LaMn_0.9Cr_0.1O_3+δ allows discuss the role of Cr³⁺ on the structural, magnetic and magnetotransport properties of the LaMn_1−xCr_xO_3+δ perovskites.     

Effects of A-site non-stoichiometry on the structural and electrical properties of 0.96K0.5Na0.5NbO3- 0.04LiSbO3 lead-free piezoelectric ceramics

Effects of A-site non-stoichiometry on the structural and electrical properties of 0.96K0.5+xNa0.5+xNbO3- 0.04LiSbO3 lead-free piezoelectric ceramics were examined for 0 ≤ x ≤ 0.02. The piezoelectric coefficients exhibited a maximum, d33 = 187 pC/N at x = 0.0075, coinciding with the maximum of the grain size and the apparent density at x = 0.0075. The apparent density and the piezoelectric coefficients decreased with increasing x at higher x which was likely due to the crystal geometrical distortion of 0.96K0.5+xNa0.5+xNbO3-0.04LiSbO3. In addition, super-large grains were found and this may be due to liquid phase sintering. Excess (K++Na+) attracted a sum of space charges to keep the charge neutral, resulting in charge leakage during the course of ceramic polarization, influencing the piezoelectric and ferroelectric properties. These findings are of importance for guiding the design of K0.5Na0.5NbO3-based lead-free ceramics with enhanced electrical properties.