Thermodynamic stability of Sb2O4 by a solid oxide electrolyte e.m.f. method

The standard Gibbs energy change for the reaction Sb₂O₃ (orthorhombic) (orthorhombic) was measured using the following galvanic cell: Pt, Sb₂O₃(ortho), Sb₂O₄ (ortho), platinum black |15 Wt % Y₂O₃-stabilized ZrO₂|O₂(air,P _{o 2} = 0.21 atm), Pt over the range 585 to 828K. Reproducible e.m.f. values were obtained only when the otherwise poorly electronic conducting oxide mixture was blended with platinum black. The standard Gibbs energy change forthis reaction was found to be (G_R ^o ± 0.81) (kJ) = –16833 + 0.0S019T/K. This yielded thestandard Gibbsenergy offormation G_R ^o of Sb₂O₄ (ortho) to be as follows: [\gDG₁ ^o(Sb₂O₄, ortho) \+- 1.61] (kJ) \s= \t-854.89 \s+ 0.33614T/K. A third-law treatment yielded a value of \t-880.55 (\+-1.21) kJmol^\t-1 for \gDH _1,298 ^o of Sb₂O₄ (ortho).     

Reactive deposition of Y1Ba2Cu3O7 −x superconductor films by pulsed laser ablation from an unreacted mixture of Y2O3, BaCO3 and CuO

Superconducting thin films of Y₁Ba₂Cu₃ O_{7 −x} have been deposited on (100) Y-ZrO₂ substrates by pulsed excimer laser ablation from anunreacted mixture of Y₂O₃, BaCO₃ and CuO. The films deposited at substrate temperature of 680°C and oxygen partial pressure of 200 mtorr were found to be superconducting with zero resistive transition temperature of 89 K and critical current density of over 3 × 10⁵ A/cm² at 77 K. These results are compared with those obtained by laser ablation from a sintered superconducting pellet.     

Microwave dielectric properties and compatibility with silver of low-fired Ba2Ti3Nb4O18 ceramics with BaCu(B2O5) addition

The effects of BaCu(B₂O₅) (BCB) additions on the sintering temperature and microwave dielectric properties of Ba₂Ti₃Nb₄O₁₈ ceramic have been investigated. The addition of BCB can lower the sintering temperature of Ba₂Ti₃Nb₄O₁₈ ceramic from 1,250 to 900 °C and induce no obvious degradation of the microwave dielectric properties. Typically, the 5 wt% BCB added Ba₂Ti₃Nb₄O₁₈ ceramic sintered at 900 °C for 2 h exhibited good microwave dielectric properties of Q × f = 17,600 GHz, ε _r = 38.2 and τ _f  = 7 ppm/°C. The dielectric ceramic demonstrated stability against the reaction with the Ag electrode, which suggests that the ceramics could be applied in multilayer microwave devices requiring low sintering temperatures.     

Gibbs energy of formation of Cu2 Ln 2O5 (Ln = Yb,Tm, Er,Ho, Dy) and CuGd2O4 compounds by the e.m.f. method

Employing electrochemical cells with the solid zirconia electrolyte Cu₂O, Cu₂ Ln ₂0₅, Ln ₂O₃O^2– air and Cu₂O, CuGd₂O₄, Gd₂O₃O^2– air the Gibbs free energy of formation of the solid Cu₂ Ln ₂O₅ (Ln=Yb,Tm, Er, Ho, Dy) and CuGd₂O₄ phases was determined. The results obtained were used to derive Gibbs free energy change of reactions of formation of these compounds from the respective oxides in the following form G _oxides ⁰ (Cu₂Yb₂O₅)=19 429-22.02TJmol^–1 G _oxides ⁰ (Cu₂Tm₂O₅)=35 275-34.09TJmol^–1 G _oxides ⁰ (Cu₂Er₂O₅)=17 427-19.61TJmol^–1 G _oxides ⁰ (Cu₂Ho₂0₅)=18 165-19.49TJmol^–1 G _oxides ⁰ (Cu₂Dy₂O₅)=16 648-17.58TJmol^–1 G _oxides ⁰ (CuGd₂O₄)=9562-12.29TJmol^–1     

High-temperature mass spectrometric study of vaporization in nano- and microcrystalline Fe2O3-SnO2 and NiO-SnO2 oxide systems

Nano- and microcrystalline Fe₂O₃-SnO₂ and NiO-SnO₂ oxide systems have been studied by Knudsen cell mass spectrometry. The results demonstrate that the vaporization thermodynamics in these systems is independent of the particle size.     

The high-temperature thermal expansion of BiPbSrCaCuO superconductor and the oxide components (Bi2O3, PbO,CaO, CuO)

The thermal expansion of superconducting Bi_1.6Pb_0.4Sr₂Ca₂Cu₃Ox (BiPbSrCaCuO) and its oxide components Bi₂O₃, PbO, CaO and CuO have been studied by high-temperature dilatometric measurements (30–800°C). The thermal expansion coefficient for the BiPbSrCaCuO superconductor in the range 150–830°C is =6.4×10^–6K^–1. The temperature dependences of L/L of pressed Bi₂O₃ reveals sharp changes of length on heating (T ₁=712°C), and on cooling (T ₂=637°C and T ₃=577°C), caused by the phase transition monoclinic-cubic (T ₁) and by reverse transitions via a metastable phase (T ₂ and T ₃). By thermal expansion measurements of melted Bi₂O₃ it is shown that hysteresis in the forward and the reverse phase transitions may be partly caused by grain boundary effect in pressed Bi₂O₃. The thermal expansion of red PbO reveals a sharp decrease in L/L, on heating (T ₁=490°C), related with the phase transition of tetragonal (red, a=0.3962 nm, c=0.5025 nm)-orthorhombic (yellow, a=0.5489 nm, b=0.4756 nm, c=0.5895 nm). The possible causes of irreversibility of the phase transition in PbO are discussed. In the range 50–740°C the coefficient of thermal expansion of pressed Bi₂O₃ (_m=3.6 × 10^–6 and _c=16.6×10^–6K^–1 for monoclinic and cubic Bi₂O₃ respectively), the melted Bi₂O₃ (_m=7.6×10^–6 and _c=11.5×10^–6K^–1), PbO (_t=9.4×10⁶ and _or=3.3×10^–6K^–1 for tetragonal and orthorhombic PbO respectively), CaO (=6.1×10^–6K^–1) and CuO (=4.3×10^–6K^–1) are presented.     

Low-temperature specific heat of YBa2Cu3O7, YBa2Cu3O6, Y2BaCuO5, YBa3Cu2O7, BaCuO2, and CuO

We have measured the low-temperature specific heat (1.3T20 K) and the dc magnetic susceptibility (100T250 K) of eight samples of the high-T _c superconductor Y _x Ba_3–x Cu₃O_7– (x=0.9, 1.0, 1.1) and of two samples of nonsuperconducting YBa₂Cu₃O₆₊. We have also performed specific heat measurements on the possible impurity phases: YBa₃Cu₂O₇, Y₂BaCuO₅, CuO, and BaCuO_2+x . The superconducting samples all have a nonzero, sample-dependent linear term * and an upturn inC/T at very low temperature. We show that this anomalous behavior is at least partly due to the presence of a small amount (1%) of BaCuO_2+x impurity phase in the measured samples. This is evidenced by the correlation between * and the Curie component of the susceptibility, which is proportional to the amount of paramagnetic impurities.     

Structural order in Ba(Zn1/3Ta2/3)O3, Ba(Zn1/3Nb2/3)O3 and Ba(Mg1/3Ta2/3)O3 microwave dielectric ceramics

The process and nature of structural ordering and the factors that influence them have been investigated in the microwave dielectric perovskites, barium zinc tantalate (BZT), barium zinc niobate (BZN), and barium magnesium tantalate (BMT), sintered at various temperatures. The samples were characterized mainly by X-ray powder diffraction and transmission electron microscopy. The results show that short-range 1 : 1 B-site order features strongly in the early stages of ordering in BZT and BZN, but it is extremely rare in BMT, for which most grains commence with 1 : 2 order. As sintering progresses, 1 : 1 order is replaced by 1 : 2 long-range order in BZT and by disorder in BZN. Orientational variants of the ordered domains within grains occur in similar numbers when order is fine-scale, but their distribution is less homogeneous in well-ordered samples. Local inhomogeneities in the degree of order within grains, which will affect dielectric properties, correlate with both residual non-stoichiometry and the presence of dislocations. Incompletely reacted starting materials which may persist to late stages of sintering can also strongly influence order. Anomalously large ordered domains at grain boundaries are attributed to grain-boundary migration accompanied by enhanced diffusion. The results indicate that with starting materials that are well-mixed and homogeneous at the nanoscale, tailoring of physical properties should be possible by controlling the type and degree of order through chemical composition. This revised version was published online in November 2006 with corrections to the Cover Date.     

Preparation and characterization of two new dielectric ceramics Ba4NdTiNb3O15 and Ba3Nd2Ti2Nb2O15

Two new cation-deficient hexagonal perovskites Ba₄NdTiNb₃O₁₅ and Ba₃Nd₂Ti₂Nb₂O₁₅ were prepared in the BaO–Nd₂O₃–TiO₂–Nb₂O₅ system by high temperature solid-state reaction route. The phase and structure of the ceramics were characterized by X-ray diffraction and scanning electron microscope (SEM). The microwave dielectric properties of the ceramics were studied using a network analyzer. Ba₄NdTiNb₃O₁₅ has a dielectric constant of 38.15, a high-quality factors (Q _u ×f >18 700 at 5.4422 GHz), and a small temperature coefficient of resonant frequency ( _f )+12 ppm °C^–1 at room temperature; Ba₃Nd₂Ti₂Nb₂O₁₅ has a higher dielectric constant of 46.83 with high-quality factors Q _u ×f >19 500 at 5.0980 GHz, and _f +28 ppm °C^–1.     

Investigation of the crystal growth of CuO from the CuO-Bi2O3 system

The relationship between properties and structure in high-temperature superconducting copper oxides has stimulated the investigation on the properties of single crystal CuO. The phase diagram of CuO-CuBi₂O₄ has been established by improved thermogravimetric analysis (ITGA), differential thermal analysis (DTA) and X-ray diffraction. The crystallization temperature of CuO was determined by ITGA and the dependence of crystallization temperature on cooling rates showed about 20 °C supercooling in the crystal growth of CuO from the CuO-Bi₂O₃ system. Using the theory of crystal growth under stable state conditions, an initial kinetic analysis of seed crystal dissolution was also performed on the ITGA data. Using the phase diagram and the ITGA information, large crystals have been grown near the surface of the high-temperature solution or the bottom of the crucible by a combination of ITGA and the top-seeding method.