Thermal expansion and thermal conductivity of SmxZr1−xO2−x/2 (0.1 ≤ x ≤ 0.5) ceramics

A study was conducted of the effect of additions of samarium oxide on the thermal expansion and thermal conductivity of zirconium oxide for thermal barrier coatings. Sm_xZr_1?xO_2?x/2 (0.1 ≤ x ≤ 0.5) ceramic powders synthesized with a chemical-coprecipitation and calcination method were sintered at 1873 K for 15 h. Structures of the synthesized powders and sintered ceramics were identified by X-ray diffractometer. The morphologies of ceramic powders were observed by transmission electron microscope. The thermal expansion coefficients and thermal diffusion coefficients of Sm_xZr_1?xO_2?x/2 ceramics were studied with a high-temperature dilatometer and a laser flash diffusivity technique from room temperature to 1673 K. The thermal conductivity was calculated from thermal diffusivity, density and specific heat of bulk ceramics. Sm_0.1Zr_0.9O_1.95 ceramics consists of both monoclinic and tetragonal structures. However, Sm_0.2Zr_0.8O_1.9 and Sm_0.3Zr_0.7O_1.85 ceramics only exhibit a defect fluorite structure. Sm_0.4Zr_0.6O_1.8 and Sm_0.5Zr_0.5O_1.75 ceramics have a pyrochlore-type lattice. With the increase of Sm₂O₃ content, the linear thermal expansion of Sm_xZr_1?xO_2?x/2 ceramics increases except for Sm_0.1Zr_0.9O_1.95. The thermal conductivities of Sm_xZr_1?xO_2?x/2 ceramics ranged from 1.41 at 873 K to 1.86 W m^?1 K^?1 at room temperature in a test temperature range of room temperature to 1673 K, and the results can be explained by phonon scattering mechanism.     

Synthesis and negative thermal expansion properties of solid solutions Yb2−xLaxW3O12 (0 ≤ x ≤ 2)

A new series of rare earth solid solutions Yb_2?xLa_xW₃O₁₂ were successfully synthesized by the solid-state method. Effects of substituted ion lanthanum on the microstructures and thermal expansion properties in the resulting Yb_2?xLa_xW₃O₁₂ ceramics were investigated by X-ray diffraction (XRD), thermogravimetric analyzer (TGA), field emission scanning electron microscope (FESEM) and thermal mechanical analyzer (TMA). Results indicate that the structural phase transition of the Yb_2?xLa_xW₃O₁₂ changes from orthorhombic to monoclinic with increasing substituted content of lanthanum. The pure phases can form in the composition range of 0 ≤ x < 0.5 with orthorhombic structure and 1.5 < x ≤ 2 with monoclinic one. High lanthanum content leads to a low hygroscopicity of Yb_2?xLa_xW₃O₁₂. Negative thermal coefficients of the Yb_2?xLa_xW₃O₁₂ (0 ≤ x ≤ 2) also vary from ?7.78 × 10^?6 K^?1 to 2.06 × 10^?6 K^?1 with increasing substituted content of lanthanum.     

Structure stability and microwave dielectric properties of double perovskite ceramics – Ba2Mg1−xCaxWO6 (0.0 ≤ x ≤ 0.15)

The structure stability of double perovskite ceramics – Ba₂Mg_1?xCa_xWO₆ (0.0 ≤ x ≤ 0.15) has been studied by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and Raman spectrometry in this paper. The microwave dielectric properties of the ceramics were studied with a network analyzer at the frequency of about 8–11 GHz. The results show that small amount of Ca substitution for Mg increases the Mg/CaO bond strength, and hence the stability of the double perovskite. But it cannot completely suppress the decomposition of Ba₂Mg_1?xCa_xWO₆ at high temperature. Although space group Fm?3m is adopted for all compositions, nonrandom distribution of Ca²⁺ and Mg²⁺ on 4b-site within the short range scale is observed due to their large cation size difference. Small level doping of Ca (x ≤ 0.1) increases the dielectric permittivity monotonically, but does not affect the Q × f value greatly. As expected, the substitution of Ca tuned the temperature coefficient of resonant frequency (τ_f value) from negative to positive value. Excellent combined microwave dielectric properties with ?_r = 20.8, Q × f = 120,729 GHz, and τ_f = 0 ppm/°C could be obtained for x = 0.1 composition. However the Q × f value degrades considerably when the sample was stored under ambient conditions for a long time.     

Frequency and temperature dependent electrical properties of Ni0.7Zn0.3CrxFe2−xO4 (0 ≤ x ≤ 0.5)

Series of the ferrite samples with a chemical formula Ni_0.7Zn_0.3Cr_xFe_2?xO₄ (x = 0.0–0.5) were prepared by a sol–gel auto-combustion method and annealed at 600 °C for 4 h. The prepared samples have the cubic spinel structure with no impurity phase. As the Cr³⁺ content x increases, the unit cell dimensions decrease with an increase in Cr³⁺ content x. The crystallite size is decreases from 37 nm to 21 nm as the Cr³⁺ content increases from x = 0.0 to 0.5. Resistivity increases whereas dielectric constant decreases with an increase in Cr³⁺ content x. Maxima in the dielectric loss tangent versus frequency appear when the frequency of the hopping charge carriers coincides with the frequency of the applied alternating field. Dielectric constant and dielectric loss tangent increases with increase in temperature. Saturation magnetization of sintered samples showed higher values as compared to as-prepared sample. Curie temperature deduced from AC susceptibility data decreases with increasing x.     

Crystal structure and mixed ionic–electronic conductivity of La1−xCaxMnO3 (0 ≤ x ≤ 0.8) produced by mechanosynthesis

The aim of this work was to study the relationship between the crystalline structure, the mixed ionic–electronic conductivity and the calcium content in calcium-doped lanthanum manganites (CLM, La_1?xCa_xMnO₃) synthesized by reactive ball milling. Mechanosynthesis was employed to produce nanocrystalline CLM with varying calcium content (x = 0–0.8 in increments of 0.1). Powders of Mn₂O₃, La₂O₃ and CaO mixed in the stoichiometric ratio were used as raw materials. The mechanosynthesis was carried out using a high-energy shaker mixer/mill. X-ray powder diffraction and Rietveld refinement were used to determine the crystalline structure as a function of calcium content. The four-point probe resistivity test was used to measure the electrical resistivity of the compacted and sintered powders using a DC milli-ohm meter. The results showed that the substitution of the La³⁺ ion by the Ca²⁺ ion during mechanosynthesis only changed the lattice parameters but not the orthorhombic Pnma structure. The mixed ionic–electronic conductivity increased with the Ca²⁺ content. The best conductivity was observed for the composition of La_0.2Ca_0.8MnO₃.     

Phase evolution and microwave dielectric properties of novel LiAl5−xZnxO8−0.5x-based (0 ≤ x ≤ 0.5) ceramics

Novel LiAl_5−xZn_xO_8−0.5x microwave dielectric ceramics were synthesized through a solid-state reaction route. Phase evolution of LiAl_5−xZn_xO_8−0.5x was determined by XRD analysis. The XRD results indicated that the phase compositions had a P4₃32 space group when 0 ≤ x ≤ 0.2 and a spinel structure when 0.3 ≤ x ≤ 0.5. The dielectric constant (ε_r) of this series’ solid solutions decreased with the increase in Zn doping content, which was in good agreement with the Clausius-Mossotti relation. Oxygen vacancy and the decreased degree of order degraded the quality factor (Q × f) of the two structures. The deterioration in quality factor was further verified by impedance spectroscopy. The temperature coefficient of the resonant frequency (τ_f) decreased with the increase in x and was correlated with the unit cell volume. Finally, CaTiO₃ was used as a compensation material to obtain a near-zero τ_f of the LiAl₅O₈ ceramic.     

Enhanced “overall” ionic conductivity in Li1+xAlxTi2−x(PO4)3 (0.3 ≤ x ≤ 0.7) ceramics prepared from sol-gel powders by spark plasma sintering

The Li_1+xAl_xTi_2?x(PO₄)₃ (LATPx) series displays the highest “bulk” reported conductivity, but a much lower “overall” contribution, that changes with the powder preparation and sintering conditions. In this work, the preparation of LATPx ceramics is discussed, by using the sol-gel technique for powders synthesis and mild spark plasma (SPS) for ceramics sintering at 800 °C. An “overall” conductivity ~ 2.10^?3 Ω^?1 cm^?1 was obtained for the x = 0.4 composition, that was the result of a high “bulk” conductivity, an optimized microstructure and almost full density, in absence of micro-cracks, with a small content of secondary phases and clean grain boundaries. Fast-ion ceramics prepared by SPS are good candidates for solid electrolytes in all solid state batteries (ASSB).     

Heterogeneous oxidation of 2-octanol on 5 wt%Pt–1 wt%Bi/Carbon catalyst

The low activity of 5%Pt–1%Bi/Carbon for the oxidation of 2-octanol at atmospheric pressure and 343 K was investigated. Using solvents such as heptane and p-xylene, it was shown that the reaction rates decrease dramatically shortly after the start of the reaction due to poisoning by product adsorption. Hence this work investigates the effect of using different solvent mixtures on the oxidation reaction of 2-octanol with 5%Pt–1%Bi/Carbon. Mixtures with different volumetric ratios of heptane and dioxane were investigated to find the best composition capable of effectively removing the adsorbed amphiphilic ketone. It is apparent that the reaction rate is correlated with the adsorption coefficient of ketone on the catalyst, such that the maximum reaction rate occurs at the lowest ketone adsorption coefficient, corresponding to a concentration of 16–18%v/v dioxane. A model based on a Langmuir–Hinshelwood mechanism has been successfully fitted to the experimental rates.     

Mesoporous CuO/TixZr1 − xO2 catalysts for low-temperature CO oxidation

Mesoporous CuO/Ti_xZr_1 − xO₂ catalysts were prepared by a surfactant-assisted method, and characterized by N₂ adsorption/desorption, TEM, XPS, in-situ FTIR and H₂-TPR. The catalysts exhibited high specific surface area (S_BET = 241 m²/g) and uniform pore size distribution. XPS and in-situ FTIR displayed that Cu⁺ and Cu²⁺ species coexisted in the catalysts. The CuO/Ti_xZr_1 − xO₂ catalysts presented obviously higher activity in CO oxidation reaction than the CuO/TiO₂ and CuO/ZrO₂ catalysts. Effect of molar ratios of Ti to Zr and calcination temperature on catalytic activity was investigated. The CuO/Ti_0.6Zr_0.4O₂ catalyst calcined at 400 °C exhibited excellent activity with 100% CO conversion at 140 °C.     

Heteroepitaxial ZnO/sapphire (0 0 0 1) structure prepared by sol–gel process

ZnO thin films were grown on sapphire (0 0 0 1) substrates by sol–gel process and their structural and optical properties were characterized in detail. High-quality texture was obtained by using precursor solution of zinc acetate and ethanolamine in 2-methoxyethanol, pyrolyzed at 300 °C, then heated at 500 °C, and finally annealed at 750 °C. Highly c-axis oriented ZnO films were confirmed by X-ray θ–2θ scan. A relatively high transmittance in the visible spectra range and clear absorption edge of the film were observed. Epitaxial relationship between ZnO and sapphire and photoluminescence of the film were examined by using a X-ray pole-figure analysis and He–Cd laser. Near-band-edge emission with a deep-level emission was observed.     

Crystal structure,mixture behavior,and microwave dielectric properties of novel temperature stable (1 − x)MgMoO4 − xTiO2 composite ceramics

Novel temperature stable MgMoO₄–TiO₂ microwave dielectric ceramics were prepared by a solid state reaction process at low temperature (950 °C). As TiO₂ content increases, the relative permittivity increases while the Q × f value decreases, and the variation mechanisms are proposed, respectively. The temperature coefficient of resonant frequency (τ_f) shifts to the positive direction as TiO₂ is added. The mixture mechanisms of τ_f value for two-phase composite materials are supposed. A near-zero τ_f value (3.2 ppm/°C) is obtained when x = 0.3, with ε_r = 9.13 ± 0.03 and Q × f = 11,990 GHz. The 0.7MgMoO₄–0.3TiO₂ composites are considered to be appropriate as a low temperature co-fired ceramic material for microwave wireless communication applications.     

Structure and microwave dielectric characteristics of Sr2[Ti1−x(Al0.5Nb0.5)x]O4 (x ≤ 0.50) ceramics

Sr₂[Ti_1−x(Al_0.5Nb_0.5)_x]O₄ (x = 0, 0.10, 0.25, 0.30, 0.5) ceramics were synthesized by a standard solid-state reaction process. Sr₂[Ti_1−x(Al_0.5Nb_0.5)_x]O₄ solid solutions with tetragonal Ruddlesdon-Popper (R-P) structure in space group I4/mmm were obtained within x ≤ 0.50, and only minor amount (1-2 wt%) of Sr₃Ti₂O₇ secondary phase was detected for the compositions x ≥ 0.25. The temperature coefficient of resonant frequency τ_f of Sr₂[Ti_1−x(Al_0.5Nb_0.5)_x]O₄ ceramics was significantly improved from 132 to 14 ppm/°C correlated with the increase in degree of covalency (%) with increasing x. The dielectric constant ɛ_r decreased linearly with increasing x, while high Qf value was maintained though it decreased firstly. The variation tendency of Qf value was dependent on the trend of packing fraction combined with the microstructure. Good combination of microwave dielectric properties was achieved for x = 0.50: ɛ_r = 25.1, Qf = 77 580 GHz, τ_f = 14 ppm/°C. The present ceramics could be expected as new candidates of ultra-high Q microwave dielectric materials without noble element such as Ta.     

Approaches for preparing 〈1 1 1〉-textured Bi0.5Na0.5TiO3-based ceramics by hetero-templated grain growth

The preparation of 〈1 1 1〉-textured Bi_0.5Na_0.5TiO₃ (BNT) ceramics was attempted by the templated grain growth method. This method uses anisometric grains as the template for texture development. In this work, plate-like BaTiO₃ grains with the 〈1 1 1〉 direction perpendicular to the plate faces were used. A mixture of the plate-like BaTiO₃ grains and equiaxed BNT grains was consolidated by tape casting to prepare green compacts in which the plate-like BaTiO₃ grains were dispersed in the matrix of the BNT grains and were aligned with their plate faces parallel to the casting direction. The textured BNT ceramics could not be obtained by the sintering of the green compacts, because of the disappearance of the plate-like BaTiO₃ grains by dissolution in the BNT grains. Several approaches were examined to obtain 〈1 1 1〉-textured ceramics. They included the use of plate-like (Sr, Ba)TiO₃ grains as the template, the use of equiaxed grains of BNT–BaTiO₃ and BNT–Bi_0.5K_0.5TiO₃ solid solutions as the matrix, and the addition of excess Na₂CO₃ and Bi₂O₃ to form liquid phases. The only approach successful in obtaining the textured ceramics was the one involving the addition of excess Na₂CO₃. The excess Na₂CO₃ formed a liquid phase and promoted the formation of a shell of BNT grains around a plate-like BaTiO₃ grain, prior to the dissolution of the BaTiO₃ grain. The BNT shell had the same crystallographic orientation as the BaTiO₃ grain and acted as the template for texture development.     

Uncoordinated inorganic salt in 1D chain structure: formation of a novel supermolecule [HgBr2(ptz)]2 · HgBr2 (ptz=phenothiazine)

A novel supermolecule [HgBr₂(ptz)]₂ · HgBr₂ (ptz=phenothiazine) with uncoordinated inorganic salt HgBr₂ presented in a 1D chain was prepared. The bulky ligand phenothiazine has unusual coordination mode with large steric inhibition perpendicular to the chain direction. The uncoordinated HgBr₂ molecule was stabilized by multiple weak Hg⋯Br interactions and the whole structure was also stabilized by strong π–π interactions and N–H⋯Br hydrogen bonds to form 2D network.     

Thermal dehydration of Y(TFA)3(H2O)3: Synthesis and molecular structures of [Y(μ,η1:η1-TFA)3(THF)(H2O)]1∞ · THF and [Y4(μ3-OH)4(μ,η1:η1-TFA)6(η1-TFA)(η2-TFA)(THF)3(DMSO)(H2O)] · 6THF (TFA = trifluoroacetate)

In order to obtain a better anhydrous precursor for various applications in materials science and catalysis, thermal dehydration reactions of Y(TFA)₃(H₂O)₃ (TFA = trifluoroacetate) (A) were investigated. Thermal treatment of A at different temperatures under vacuum (5 × 10^?2 mm) for several hours failed to give totally anhydrous yttrium trifluoroacetate (as indicated by IR). Two different complexes, a partially dehydrated [Y(μ,η¹:η¹-TFA)₃(THF)(H₂O)]_1∞·THF (1) and a partially hydrolyzed [Y₄(μ₃-OH)₄(μ,η¹:η¹-TFA)₆(η¹-TFA)(η²-TFA)(THF)₃(DMSO)(H₂O)] · 6THF (2), were obtained with good and moderate yield, respectively, by crystallization of two different thermally treated batches of A from THF (or THF + DMSO) at room temperature. More efficient dehydration of A could be achieved at 200 °C in a furnace, the obtained anhydrous yttrium tris-trifluoroacetate giving Y(TFA)₃(THF)₂ (3) on crystallization from THF. All the products were characterized by elemental analyses, FT-IR and ¹H NMR spectroscopy as well as thermo-gravimetric analysis. In addition, single crystal X-ray structures are reported for 1 and 2, which show either a terminal (η¹ and η²) or bridging (μ,η¹:η¹) bonding behavior of the TFA ligand.     

Composition-independent Curie point (Tc) in ferroelectric (1 − x)PbTiO3–xBi(Li1/2Nb1/2)O3 solid solution

A new ferroelectric solid solution (1 − x)PbTiO₃–xBi(Li_1/2Nb_1/2)O₃ has been explored to develop high-temperature piezoelectric material. An interesting observation has been found regarding its Curie point (T_C) and tetragonal lattice strain (c/a − 1). With increasing composition (x), the Curie point (T_C) decreases up to x = 0.10 and thereafter remains constant. In concurrence with the T_C, the tetragonal lattice strain (c/a − 1) follows a similar trend. Neutron powder diffraction analysis suggests this anomalous behavior is due to the robust off-centering characteristic of the Bi⁺³ ion 6S² lone pair effect at the A-site compared to ions at B-site.