Solid-State SO2 Sensor Using a Sodium-Ionic Conductor and a Metal–Sulfide Electrode

All solid-state sulfur oxides (SO _x ) sensor devices combined with a sodium ionic conductor (Na₅DySi₄O₁₂) disk and metal sulfide-sensing electrodes synthesized via solution routes have been systematically investigated for the detection of SO₂ in the range of 20–200 ppm at 150–400°C. Among the various sulfide-sensing electrodes tested, the metal monosulfide-based electrodes gave good SO₂ sensitivity at 400°C. The Pb_{1− x} Cd _x S ( x =0.1, 0.2)-based solid electrolyte sensor element showed the best sensing characteristics, i.e., the EMF response was almost linear to the logarithm of SO₂ concentration in the range between 40 and 400 ppm, with a 90% response time to 100 ppm SO₂ of about 3–15 min, and also showed high selectivity to SO₂ at 400°C.     

Characterization of the Piezoelectric Properties of Pb0.98Ba0.02(Mg1/3Nb2/3)O3–PbTiO3 Epitaxial Thin Films

Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PMN–PT) (70/30) thin films were deposited by pulsed laser deposition using two growth strategies: adsorption controlled deposition from lead-rich targets (∼25–30 mass%) and lower-temperature deposition ( T _d≤600°C) from targets containing a small amount of excess lead oxide (≤3 mass %). The substrates used were (001) SrRuO₃/LaAlO₃. Typical remanent polarization values ranged between 12 and 14 μC/cm² for these films. The longitudinal piezoelectric coefficient ( d _33,f) was measured using in situ four-circle X-ray diffraction, and the transverse coefficient ( d _31,f or e _31,f) was measured using the wafer flexure method. d _33,f and e _31,f coefficients of ∼300–350 pm/V and ∼−11 C/m² were calculated, respectively. In general, the piezoelectric coefficients and aging rates were strongly asymmetric, suggesting the presence of a polarization bias. The large, extremely stable piezoelectric response that results from poling parallel to the preferred polarization direction is attractive for miniaturized sensors and actuators.     

High Critical Current YBa2Cu3O7−δ Thick Films on Improved Rolling-Assisted Biaxially Textured Substrates (RABiTS™)

YBa₂Cu₃O_7−δ (YBCO) films with thicknesses ranging from 1.0 to 6.4 μm were deposited by pulsed laser deposition on rolling-assisted biaxially textured substrates (RABiTS). The RABiTS were of the configuration CeO₂/YSZ/Y₂O₃/Ni–3 at.% W. As the YBCO film thickness increased, I _c continued to increase and reached ∼300 A/cm width for a 4.3 μm-thick YBCO film. Commonly observed mechanisms for J _c decrease with increasing YBCO film thickness were not observed. Homogeneous microstructures obtained in even the thickest YBCO films, suggest that the I _c/width can still be enhanced considerably.     

Novel Fast Lithium Ion Conduction in Garnet-Type Li5La3M2O12 (M = Nb, Ta)

Lithium metal oxides with the nominal composition Li₅La₃M₂O₁₂ (M = Nb, Ta), possessing a garnetlike structure, have been investigated with regard to their electrical properties. These compounds form a new class of solid-state lithium ion conductors with a different crystal structure compared with all those known so far. The materials are prepared by solid-state reaction and characterized by powder XRD and ac impedance to determine their lithium ionic conductivity. Both the niobium and tantalum members exhibit the same order of magnitude of bulk conductivity (∼10⁻⁶ S/cm at 25°C). The activation energies for ionic conductivity (<300°C) are 0.43 and 0.56 eV for Li₅La₃Nb₂O₁₂ and Li₅La₃Ta₂O₁₂, respectively, which are comparable to those of other solid lithium conductors, such as Lisicon, Li₁₄ZnGe₄O₁₆. Among the investigated materials, the tantalum compound Li₅La₃Ta₂O₁₂ is stable against reaction with molten lithium. Further tailoring of the compositions by appropriate chemical substitutions and improved synthesizing methods, especially with regard to minimizing grain-boundary resistance, are important issues in view of the potential use of the new class of compounds as electrolytes in practical lithium ion batteries.     

Processing Dependence of Texture, and Critical Properties of YBa2Cu3O7−δ Films on RABiTS Substrates by a Non-Fluorine MOD Method

YBa₂Cu₃O_{7− δ} (YBCO or Y123) films on rolling-assisted biaxially textured substrates (RABiTS) were prepared via a fluorine-free metallorganic deposition (MOD) through spin coating, burnout, and high temperature anneal. The effects of substrate texture and surface energy of the CeO₂ cap layer were investigated. Except for the commonly accepted key factors, such as the textures of substrate and buffer layers, we found some other factors, for example, the deposition temperature of the cap layer, are also critical to the epitaxial growth of Y123 phase. With the CeO₂ cap layer deposited at relative high temperature of 700°C, a critical current density, J _c, over 1 MA/cm² has been demonstrated for the first time on Ni-RABiTS by a fluorine-free MOD method. Whereas for samples with CeO₂ cap layers deposited at a lower temperature of 600°C, even though XRD data showed a better texture on these buffer layers, texture degradations of YBCO grains under the optimized processing conditions were observed and a lower oxygen partial pressure around 40 ppm was necessary for the epitaxial growth of Y123 phase. As a result, J _c fell to 0.45 MA/cm² at 77 K. The observed phenomena points to the change of surface energy and reactivity of the CeO₂ cap layer with respect to the CeO₂ deposition temperature. In this paper, the YBCO phase diagram was also summarized.     

A Review of the Influence of Grain Boundary Geometry on the Electromagnetic Properties of Polycrystalline YBa2Cu3O7−x Films

Shortly after the discovery of high-temperature superconducting (HTS) materials in the late 1980s, it was revealed that grain boundaries in these complex oxides are strong barriers to current flow. This fact has remained one of the most significant challenges to a viable HTS conductor, and necessitated the development of technologies capable of producing biaxially textured substrates in long lengths. Multiple studies have reported that the critical current density ( J _c) across grain boundaries in the perovskite-like superconductor YBa₂Cu₃O_{7− x} (YBCO) falls off exponentially below the intragrain J _c beyond a critical misorientation angle θ_c of only ≈2°–3°. Here we review our recent work demonstrating that certain grain boundary geometries permit significant enhancements of J _c well beyond the conventional J _c(θ) limit, and also that the grain boundary structure in YBCO films is tied closely to the films' deposition technique. Pulsed laser deposition, a physical vapor deposition technique, results in a columnar grain structure and planar grain boundaries that exhibit the typical J _c(θ) dependence. Ex situ growth processes, where the YBCO film is converted from a previously deposited precursor, can result in laminar grain growth with highly meandered grain boundaries. These latter grain boundary structures are directly correlated to greatly improved J _c values over a wide range of applied magnetic fields. Consequently, very high J _c values are possible in polycrystalline HTS wire even when significant misorientations between grains are present.     

Solution of MgO, CaO, and TiO2 in α-AI2O3

Atomistic simulation methods are used to predict the preferred solution mechanisms and their associated energies. Both full- and partial-charge models are discussed. It was found that defect clustering always results in a significant lowering of the solution energies but does not change the preferred solution reaction of any of the oxides. The large size of the Ca²⁺ ion results not only in a higher solution energy (lower solubility) for CaO compared to MgO but also to a different preferred compensation mechanism. The predicted energy for the cosolution of MgO and TiO₂ is dramatically smaller than for either oxide separately. Conversely, whereas solution of TiO₂ aids the solution of CaO, additions of CaO do not support TiO₂, Solution.     

Thermoelectric Properties of Ruddlesden–Popper Phase n-Type Semiconducting Oxides: La-, Nd-, and Nb-Doped Sr3Ti2O7

We report herein on Ruddlesden–Popper-type doped Sr _{n +1}Ti _n O_{3 n +1} ( n =2) as a potential candidate for n -type thermoelectric (TE) oxides. The TE properties of 5 at.% La-, Nd-, and Nb-doped Sr₃Ti₂O₇ polycrystalline ceramics were investigated and the origin of Seebeck coefficient was discussed from the viewpoint of the symmetry of TiO₆ octahedra. A significant reduction in the lattice thermal conductivity was observed by the enhancement of phonon scattering at SrO/(SrTiO₃) _n interfaces originating from the inherent superlattice structure, and the maximum dimensionless figure of merit, ZT ∼0.15, at a 1000 K value was obtained in 5 at.% La-doped Sr₃Ti₂O₇.     

Tailoring the Microwave Dielectric Properties of MgNb2O6 and Mg4Nb2O9 Ceramics

The columbites MgNb₂O₆, MgTa₂O₆, and corundum-type Mg₄Nb₂O₉ ceramics were prepared by the conventional solid-state ceramic route. The structure and microstructure of the sintered samples were investigated by X-ray diffraction and scanning electron microscopic techniques. The microwave dielectric properties of the samples were measured by the resonance method in the frequency range 4–6 GHz. The dielectric properties have been tailored by forming a solid solution between MgNb₂O₆ and MgTa₂O₆ and by the substitution of TiO₂ for Nb₂O₅ in both MgNb₂O₆ and Mg₄Nb₂O₉ ceramics. The Mg(Nb_0.7Ta_1.3)O₆ has ɛ_r=29, Q _u× f =67 800 GHz, and τ_f=0.8 ppm/°C and the MgO–(0.4)Nb₂O₅–(1.5)TiO₂ composition has ɛ_r=34.5, Q _u× f =81 300 GHz, and τ_f=−2 ppm/°C.     

Pb(Zn1/3Ta2/3)O3–PbTiO3 Derived from Mechanical Activation

Nanocrystalline Pb(Zn_1/3Ta_2/3)O₃ (PZTa) of perovskite structure, which cannot be synthesized by either the conventional solid-state reaction of mixed oxides or wet chemistry routes, has been successfully synthesized via a mechanical activation route. The effects of PbTiO₃ (PT) doping in PZTa on the phase formation, thermal stability of the nanocrystalline perovskite phase, and dielectric properties of the resulting PZTa–PT are studied. PZTa doped with a low PT content exhibited a diffuse phase transition, while those with high PT contents demonstrated an expected sharp phase transition at the Curie temperature.     

Thermochemistry of Framework Titanosilicate A2TiSi6O15 (A=K, Rb, Cs)

A new family of framework titanosilicates, A₂TiSi₆O₁₅ (A=K, Rb, Cs) (space group Cc ), has recently been synthesized using the hydrothermal method. This group of phases can potentially be utilized for storage of radioactive elements, particularly ¹³⁷Cs, due to its high stability under electron radiation and chemical leaching. Here, we report the syntheses and structures of two intermediate members in the series: KRbTiSi₆O₁₅ and RbCsTiSi₆O₁₅. Rietveld analysis of powder synchrotron X-ray diffraction data reveals that they adopt the same framework topology as the end-members, with no apparent Rb/K or Rb/Cs ordering. To study energetics of the solid solution series, high-temperature drop-solution calorimetry using molten 2PbO·B₂O₃ as the solvent at 975 K has been performed for the end-members and intermediate phases. As the size of the alkali cation increases, the measured enthalpies of formation from the constituent oxides (     ) and from the elements (     ) become more exothermic, suggesting that this framework structure favors the cation in the sequence Cs⁺, Rb⁺, and K⁺. This trend is consistent with the higher melting temperatures of A₂TiSi₆O₁₅ phases with increase in the alkali cation size.     

Chemical Interactions Promoting the ZrO2 Tetragonal Stabilization in ZrO2–SiO2 Binary Oxides

Metastable tetragonal ZrO₂ phase has been observed in ZrO₂–SiO₂ binary oxides prepared by the sol–gel method. There are many studies concerning the causes of ZrO₂ tetragonal stabilization in binary oxides such as Y₃O₂–ZrO₂, MgO–ZrO₂, or CaO–ZrO₂. In these binary oxides, oxygen vacancies cause changes or defects in the ZrO₂ lattice parameters, which are responsible for tetragonal stabilization. Since oxygen vacancies are not expected in ZrO₂–SiO₂ binary oxides, tetragonal stabilization should just be due to the difficulty of zirconia particles growing in the silica matrix. Furthermore, changes in the tetragonal ZrO₂ crystalline lattice parameters of these binary oxides have recently been reported in a previous paper. The changes of the zirconia crystalline lattice parameters must result from the chemical interactions at the silica–zirconia interface (e.g., formation of Si–O–Zr bonds or Si–O⁻ groups). In this paper, FT-IR and ²⁹Si NMR spectroscopy have been used to elucidate whether the presence of Si–O–Zr or Si–O⁻ is responsible for tetragonal phase stabilization. Moreover, X-ray diffraction, Raman spectroscopy, and transmission electron microscopy have also been used to study the crystalline characteristics of the samples.     

Studies in the System Li2O–Al2O3–Fe2O3-H2O

Subsolidus equilibrium relations in a portion of the system Li2O-Fe₂O₃-Al₂O3 in the temperature range 500° to 1400°C. have been determined near po₂ = 0.21. Of particular interest in this system is the LiFe₅O₈-LiAl₅O₈ join, which shows complete solid solution above 1180°C. Below this temperature the solid solution exsolves into two spinel phases. At 600°C. approximately 15 mole % of each compound is soluble in the other. The high-temperature solid solution and the low-temperature exsolution dome extend into the ternary system from the 1:5 join. There is no appreciable crystalline solubility of LiFeO₂ or of α-Fe₂O₃ in LiFe₅O₈. An attempt to confirm HFe₅O₈ as the correct formulation of the magnetic ferric oxide "γ-Fe₂O₃" was inconclusive, but in the absence of positive evidence, the retention of γ-Fe₂O₃ is recommended. All the metallic oxides of the Group IV elements increase the temperature of the monotropic conversion of -γ-Fe₂O₃ to α-Fe₂O₃. Silica and thoria have a greater effect on this conversion than does titania or zirconia.     

Energetics of La2O3–HfO2–SiO2 Glasses

A series of La₂O₃–HfO₂–SiO₂ glasses, approximately along the join 0.73SiO₂–0.27( x HfO₂–(1− x )La₂O₃), 0< x <0.3), was prepared using containerless processing techniques (aerodynamic levitation combined with laser heating in oxygen). The enthalpy of formation and enthalpy of vitrification at 25°C were obtained from drop solution calorimetry of these glasses and appropriate crystalline compounds in a molten lead borate (2PbO–B₂O₃) solvent at 702°C. The enthalpy of formation from crystalline oxides was exothermic and became less exothermic with increasing HfO₂ content. Heat contents were measured by transposed temperature drop calorimetry and depended linearly on the HfO₂ content. Differential scanning calorimetry showed that both the onset glass transition and the onset crystallization temperature of these glasses increased with increasing HfO₂ content. Upon slow cooling in air, the glasses crystallized to a mixture of baddeleyite, cristobalite, lanthanum disilicate, and hafnon.     

Vaporization Studies of the La2O3–Ga2O3 System

The vaporization of the samples of the compositions Ga₂O₃+ LaGaO₃, LaGaO₃+ La₄Ga₂O₉, and La₄Ga₂O₉+ La₂O₃ was investigated using Knudsen effusion mass spectrometry in the temperature range 1494–1937 K. The partial pressures of the gaseous species O₂, Ga, GaO, Ga₂O, and LaO were determined over the samples investigated. The equilibrium partial pressures were used for the calculation of the thermodynamic activities of the components at 1700 K. Gibbs energies of formation of LaGaO₃( s ) and La₄Ga₂O₉( s ) at 1700 K from the component oxides were derived from the thermodynamic activities as −46.4 ± 4.7 and −99.2 ± 7.9 kJ·mol⁻¹, respectively. The results were compared with the literature data obtained using other methods.     

Reduced Dielectric Loss of Modified ZnNb2O6 Ceramics by Substituting Nb5+ with Ta5+

The effects of substituting Nb⁵⁺ with Ta⁵⁺ on the microwave dielectric properties of the ZnNb₂O₆ ceramics were investigated in this study. The forming of Zn(Nb_{1− x} Ta _x )₂O₆ ( x =0–0.09) solid solution was confirmed by the measured lattice parameters and the EDX analysis. By increasing x , not only could the Q × f of the Zn(Nb_{1− x} Ta _x )₂O₆ ( x =0–0.09) solid solution be tremendously boosted from 83 600 GHz at x =0 to a maximum 152 000 GHz at x =0.05, the highest ɛ_r∼24.6 could also be achieved simultaneously. It was mainly due to the uniform grain morphology and the highest relative density of the specimen. A fine combination of microwave dielectric properties (ɛ_r∼24.6, Q × f ∼152 000 GHz at 8.83 GHz, τ_f∼–71.1 ppm/°C) was achieved for Zn(Nb_0.95Ta_0.05)₂O₆ solid solution sintered at 1175°C for 2 h.     

Thermodynamic Properties and Phase Diagram for the System MoO2–TiO2

The activity of molybdenum dioxide (MoO₂) in the MoO₂–TiO₂ solid solutions was measured at 1600 K using a solid-state cell incorporating yttria-doped thoria as the electrolyte. For two compositions, the emf was also measured as a function of temperature. The cell was designed such that the emf is directly related to the activity of MoO₂ in the solid solution. The results show monotonic variation of activity with composition, suggesting a complete range of solid solutions between the end members and the occurrence of MoO₂ with a tetragonal structure at 1600 K. A large positive deviation from Raoult's law was found. Excess Gibbs energy of mixing is an asymmetric function of composition and can be represented by the subregular solution model of Hardy as follows
The temperature dependence of the emf for two compositions is reasonably consistent with ideal entropy of mixing. A miscibility gap is indicated at a lower temperature with the critical point characterized by T ^c (K)=1560 and     . Recent studies indicate that MoO₂ undergoes a transition from a monoclinic to tetragonal structure at 1533 K with a transition entropy of 9.91 J·(mol·K)⁻¹. The solid solubility of TiO₂ with rutile structure in MoO₂ with a monoclinic structure is negligible. These features give rise to a eutectoid reaction at 1412 K. The topology of the computed phase diagram differs significantly from that suggested by Pejryd.     

Hot Isostatic Pressing of YBa2Cu3O7-δ and YBa2Cu3O7–δ/Ag Composites

Hot isostatic pressing (HIP) can be used to produce fully dense shapes of high-temperature ceramic superconductors. Densification modeling of monolithic YBa₂Cu₃O_7-δ and the composite YBa₂Cu₃O_7-δ/Ag systems allows an understanding of the HIP process and has led to the development of successful protocols for HIP of these materials. Ag metal is the best encapsulation material found for both systems. HIP of monolithic YBa₂Cu₃O_7-δ requires a slow ramp of pressure in order to prevent decomposition into more basic oxides such as Y₂BaCuO₅ and CuO. HIP of composite YBa₂Cu₃O_7-δ/Ag requires careful powder processing to obtain dense material with a fine dispersion of Ag.     

Nano-Blast Synthesis of Nano-size CeO2–Gd2O3 Powders

CeCl₃·7H₂O and GdCl₃·6H₂O that were dissolved in water were precipitated with urea (NH₂CONH₂) to produce matrix agglomerates for three-component nano-reactors. Mixing hexamethylenetetramine with dilute nitric acid resulted in the formation of well-dispersed nano-particles of cyclotrimetilene trinitramine (C₃H₆N₆O₆) (RDX) in the solvent. Nano-reactors were produced by impregnating the nano-C₃H₆N₆O₆ into the matrix agglomerates of an intermediate complex of cerium and gadolinium compounds. Blast initiation of the C₃H₆N₆O₆ resulted in extremely rapid detonation and gaseous products formation at temperatures of 2000°–5000°C, which were compressed into a volume nearly equal to the initial volume of each RDX nano-particle. Multiple "nano-blasts" occurred in the volume of each nano-reactor. The impact of the blast waves led to fragmentation of the surrounding matter. The evolution of a large volume of gaseous products dissipated the heat of the process and limited temperature increase, thus reducing the possibility of local sintering among the primary particles. The short-term high temperature generated during the blasts enhanced the solid solubility of the metal oxides. Uniform aggregates of 22∼74 nm consisting of 6∼14 nm crystallites of gadolinia in ceria solid solution were synthesized.     

Energetics of Substituted Pollucites Along the CsAlSi2O6–CsTiSi2O6.5 Join: A High-Temperature Calorimetric Study

Enthalpies of drop solution for a suite of substituted pollucites with the compositions CsTi _x Al_{1− x} Si₂O_{6+0.5 x} , 0 ≤ x ≤ 1, which are synthesized using the sol–gel method, have been measured in molten lead borate (2PbO·B₂O₃) at 701°C. As Ti⁴⁺ substitutes for Al³⁺, the enthalpies of drop solution become less endothermic and show exothermic heats of mixing within the composition range from x = 0.3 to 1. This nonideal mixing behavior is consistent with the trend seen in variation of lattice parameters, and we interpret it to be a result of the short-range order associated with the framework cations Al³⁺, Si⁴⁺, and Ti⁴⁺ in the structure. Using enthalpies of drop solution of SiO₂, Al₂O₃, TiO₂, and Cs₂O, standard molar enthalpies of formation of these phases from their constituent oxides and from the elements are derived for the first time.