Thermoelectric properties of Re6GaxSeyTe15−y (0 ≤ x ≤ 2; 0 ≤ y ≤ 7.5)

Thermoelectric properties of Re₆Ga_xSe_yTe_15?y (0 ≤ x ≤ 2; 0 ≤ y ≤ 7.5) were studied in the temperature range 90–320 K. The measurements revealed p-type semi-conductivity in all samples. Relatively high values of the Seebeck coefficients, α, were obtained in all samples. The electrical resistivities and room temperature Seebeck coefficients increased as selenium concentrations increased, for each value of x. The room temperature Seebeck coefficients and resistivities decreased as gallium content increased, for each value of y. Low carrier concentrations were found at room temperatures, in agreement with large Seebeck coefficient values. Measurements suggested hopping conduction between 150 K and 280 K for all samples. Temperature dependences of the Seebeck coefficient below 150 K were accounted for by phonon drag effect. The power factors for the samples were calculated. Theoretical discussions of dependences of the measured quantities on temperature and composition are given. Usefulness of these materials as thermoelectrics is also discussed.     

Multicolor-emitting Ca3 -x-ySry(PO4)2:xEu2 + (0 ≤ x ≤ 0.075, 0 ≤ y ≤ 2.2) phosphors for light-emitting diodes

In this paper, a series of Ca_3 -x-ySr_y(PO₄)₂:xEu^2 +, (0 ≤ x ≤ 0.075, 0 ≤ y ≤ 2.2) phosphors were prepared by flux assisted solid-state reaction method, and their photoluminescence properties were investigated. The β- to β′-phase transition of Ca_3 -ySr_y(PO₄)₂ for high Sr^2 + content was observed from the XRD patterns, and the corresponding optical bandgaps were obtained experimentally. Various Eu^2 + emission centers were found, which generate tunable emission depending on the Sr^2 + concentration. Broad and intense excitation bands exist in Eu^2 + activated Ca₃(PO₄)₂, and the introduction of Sr^2 + further extends and enhances the excitation bands beyond 350 nm, which is beneficial to the applications on near ultraviolet LEDs. The morphology measurement reveals that the average size of particles with smooth surface is about 11.2 μm, which is suitable for the practical applications. These results indicate that the Ca_3 -x-ySr_y(PO₄)₂:xEu^2 + phosphors could be promising candidates for LEDs.     

Thermo-optical properties of LaMg1 − xNixAl11O19 (0 ≤ x ≤ 1) hexaaluminates for metallic thermal protection system

LaMg_1 ? xNi_xAl₁₁O₁₉ (x = 0, 0.25, 0.5, 0.75, 1) ceramics are fabricated by pressureless-sintering method at 1700 °C for 10 h in air. The microstructure and thermo-optical properties of LaMg_1 ? xNi_xAl₁₁O₁₉ ceramics are investigated by the X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy measurements. The influence of NiO doping on structure and thermo-optical properties of LaMg_1 ? xNi_xAl₁₁O₁₉ ceramics is investigated. The partial substitution of Ni²⁺ for Mg²⁺ results in a significant increase in emissivity at low wavelengths as compared with unmodified LaMgAl₁₁O₁₉. When the Ni²⁺ content increases to x = 0.75 or above, LaMg_1 ? xNi_xAl₁₁O₁₉ ceramics have a high emissivity value above 0.70 at low wavelengths at 500 °C. The measured emissivity of all LaMg_1 ? xNi_xAl₁₁O₁₉ ceramics shows a similar trend in the wavelength range of 6 to 14 μm.     

Continuous BaTi1 − yZryO3 (0 ≤ y ≤ 1) nanocrystals synthesis in supercritical fluids for nanostructured lead-free ferroelectric ceramics

This paper addresses for the first time two of the main challenges in the field of electroceramics. The first one is the development of a continuous, fast and reliable synthesis method for producing sub-20 nm barium titanate zirconate (BaTi_1-yZr_yO₃ with 0 ≤ y ≤ 1–BTZ) nanocrystals over the whole solid solution, while keeping a narrow size distribution. The second one concerns the processing of dense and nanostructured lead free electroceramics highlighting the size effect on the crossover from ferroelectric to relaxor, in the case of grains smaller than 100 nm. This was achieved combining the supercritical fluid technology to produce in continuous and at moderate temperature (400 °C) BTZ nanocrystals, with Spark Plasma Sintering (SPS) to prevent grain growth during the sintering.     

Physical and optical properties of Co2+, Ni2+ doped 20ZnO + xLi2O + (30 − x)K2O + 50B2O3 (5 ≤ x ≤ 25) glasses: Observation of mixed alkali effect

Co²⁺ and Ni²⁺ ions doped 20ZnO + xLi₂O + (30 ? x) K₂O + 50B₂O₃ (5 ≤ x ≤ 25) mol% glasses are prepared using melt quenching technique. Structural changes of the prepared glasses by addition of transition metal oxides, CoO and NiO are investigated by UV–vis–NIR, FT-IR spectroscopy and XRD. The XRD pattern indicates the amorphous nature of prepared glasses. FT-IR measurements of the all glasses revealed that the network structure of the glasses are mainly based on BO₃ and BO₄ units placed in different structural groups in which the BO₃ units being dominant. The optical absorption spectra suggest the site symmetry of Co²⁺ and Ni²⁺ ions in the glasses are near octahedral. Crystal field and inter-electronic repulsion parameters are also evaluated. The optical band gap and Urbach energies exhibited the mixed alkali effect. Various physical parameters such as density, refractive index, optical dielectric constant, polaron radius, electronic polarizability and inter-ionic distance are also determined.     

Crystal structure and thermal expansion of LaCr1−xMgxO3, 0 < x ≤ 0.25

Solid solution LaCr_1?xMg_xO₃, 0 < х ≤ 0.25 was prepared by heating stoichiometric amounts of appropriate oxides in air at 1400 °C, 48 h. At room temperature it crystallizes in orthorhombically distorted GdFeO₃-type structure (a ≈ √2 × a_per; b ≈ √2 × a_pe; c ≈ 2 × a_per, where a_per – perovskite subcell parameter). High-temperature X-ray powder diffraction (HT XRPD) and dilatometry revealed first order phase transition to rhombohedral perovskite phase (R-3c, a ≈ √2 × a_per, c ≈ 2√3 × a_per) at 260–311 °C (OR phase transition). Crystal structures of room-temperature orthorhombic and high-temperature rhombohedral phases for LaCr_0.75Mg_0.25O₃ were refined using HT XRPD data. Temperature of OR phase transition increases gradually with increasing of magnesium content. Low-temperature orthorhombic phase exhibits TEC lower in comparison with high-temperature rhombohedral one (e.g. for LaCr_0.85Mg_0.15O₃ TEC(O) = 8.8 ppm K^?1; TEC(R) = 11.6 ppm K^?1). TEC for rhombohedral phase increases with increasing magnesium content from 10.4 ppm K^?1 for LaCr_0.95Mg_0.05O₃ to 12.1 ppm K^?1 for LaCr_0.75Mg_0.25O₃.     

In vitro biocompatibility and antimicrobial activity of wet chemically prepared Ca10−xAgx(PO4)6(OH)2 (0.0 ≤ x ≤ 0.5) hydroxyapatites

Herein, we report the effect of silver ions on the physical, antimicrobial and cytocompatibility properties of wet chemically synthesized silver doped Ca_10?xAg_x(PO₄)₆(OH)₂ (0.0 ≤ x ≤ 0.5) hydroxyapatites (HAp). Silver ions containing HAp exhibit the comparable density, hardness and enhanced antimicrobial properties, in comparison to parent HAp. The optical absorption measurements confirm the presence of silver ions in the doped compositions, which are responsible for as increased antimicrobial property of doped HAp materials for x > 0.3. The cytotoxicity behavior of the doped HAp was evaluated using mouse fibroblast (L929) cell line. The important result has been that doped HAp (x > 0.3) exhibit statistically (significant) lower cell viability in comparison to undoped HAp. However, no difference in cellular functionality on doped HAp surfaces, in terms of cell adhesion and proliferation could be qualitatively observed in reference to undoped HAp. In order to explain the observed antimicrobial and cell viability properties, the in vitro release of Ag⁺ ions has been quantified using Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) and solubility was measured by weight loss in acetate buffer solution.     

Density functional study on the geometric features and growing pattern of AlnNm clusters (n = 1–4, m = 1–4, n + m ≤ 5)

The bonding features of Al_nN_m clusters (n = 1–4, m = 1–4, n + m ≤ 5) are studied within the framework of the density functional theory following a systematic growing mechanism. Various geometries are found to be more stable than structures reported elsewhere for small Al–N aggregates. In other cases, good agreement exists with results already reported by other authors. It is also found that Al–N clusters tend to grow mainly towards nonlinear bidimensional structures up to the pentamer. The ability to form triple N–N bonds is important for the relative stability of clusters as they grow. The growing patterns could be explained mainly in terms of electrophilic attacks of both Al and N atoms to the corresponding sites in the aggregates of lower size. It is found that atomic charges derived from molecular electrostatic potentials allow to rationalize the electrophilic attacks, being then useful to understand the growing paths followed by small Al–N clusters.     

Atomic diffusion in the Fe [0 0 1] ∑ = 5 (3 1 0) and (2 1 0) symmetric tilt grain boundary

Both the formation and diffusion activation energies of single vacancy migrating intra-layer and inter-layer near the Fe [0 0 1] Σ = 5 (3 1 0) and (2 1 0) symmetric tilt grain boundaries have been calculated by using the MAEAM and a MD method. From energy minimization, the vacancy concentration in the second layer is higher than the one in the other layers for both (3 1 0) and (2 1 0) STGBs. By the diffusion activation energies of the vacancies migrating intra-layer and inter-layer, the vacancies located from the first to the eighth layers of (3 1 0) STGB as well as the ones located from the first to the tenth layers of (2 1 0) STGB are favorably migrated to the second layer. Thus there is a vacancy aggregation tendency to the second layer near the grain boundary. For the vacancy migrating intra-layer and inter-layer, the influences of the grain boundary are respectively as far as to the fifth and eighth layers for (3 1 0) STGB as well as to the sixth and tenth layers for (2 1 0) STGB.     

XPS and Mössbauer studies on BaSn1−xNbxO3 (x ≤ 0.100)

A few compositions of perovskite oxide BaSn_1?xNb_xO₃ (with x ≤ 0.10) system, prepared by solid state ceramic method, have been studied employing XPS and Mössbauer spectroscopy techniques. Mössbauer spectra of these compositions in the temperature range of 78–300 K reveal that the oxidation state of Sn is +4. In XPS measurements, compositions with x ≤ 0.010 show no evidence of Nb⁵⁺ signal whereas the compositions with x ≥ 0.050 show clear evidence of Nb⁵⁺ signals indicating some unreacted Nb₂O₅ component in the system. This confirms the earlier report where presence of small amount of unreacted Nb₂O₅ was predicted.     

Thermal expansion of hydroxyapatite between − 100 °C and 50 °C

Hydroxy apatite (HAp) ceramic was synthesized using traditional sintering. Dilatometric and lattice thermal expansion properties of a HAp ceramic were evaluated at temperatures of ? 100–50 °C. In that temperature range, the dilatometric thermal expansion coefficient and the lattice thermal expansion coefficient of the HAp ceramic were, respectively, 10.6 × 10^? 6/°C and 9.9 × 10^? 6/°C. Furthermore, thermal expansion properties of a human tooth were measured. The thermal expansion coefficient of the horizontal direction perpendicular to the growing direction of a tooth was 15.5 × 10^? 6/°C; that of the vertical direction along with the direction of tooth growth was 18.9 × 10^? 6/°C at the temperature range described above.     

Measuring spectral transmission and refractive index of AgCl1−xBrx (0 ⩽ x ⩽ 1) and Ag1−xTlxBr1−xIx (0 ⩽ x ⩽ 0.05) at the wavelength of 10.6 μm

We measured the complex refractive index at the wavelength of 10.6 μm with the help of Fourier transform infrared spectroscopy for polycrystalline plates of the following compositions AgCl_1−xBr_x (0 ⩽ x ⩽ 1) and Ag_1−xTl_xBr_1−xI_x, where x varied from 0 to 0.05. In order to do it we chose a segment of the spectrum, which was recorded with a high resolution (0.5 cm⁻¹) using the HgCdTe detector and which had a set of 10 identical peaks. It is shown that the real part of the refractive index rises along with increasing the substituting component fraction in the solid solution from 1.99 to 2.17 for AgCl_1−xBr_x and from 2.17 to 2.24 within the range of TlI mole fraction up to 0.05 for Ag_1−xTl_xBr_1−xI_x. We considered errors introduced by the spectrometer resolution and the accuracy rating of the micrometer, which was used to measure sample thickness. It is seen in the spectra, recorded for the second system with a lower resolution and using a deuterated and l-alanine doped triglycine sulfate detector, that increasing the thallium monoiodide fraction results in widening the transmission range towards bigger wavelengths. We also plan to use the obtained refractive index values for simulating mid-infrared optical fibers, the polycrystalline structure of which is close to the structure of the plates under investigation.     

Electronic structure of Cu100−xZrx (x = 40, 50, 60) metallic glasses

The electronic structure of Cu_100−xZr_x (x = 40, 50, 60) metallic glasses was investigated by ultraviolet photoelectron spectroscopy and X-ray photoemission spectroscopy, the valence band spectra of these alloys were analyzed by a large shift of the Cu d-band peaks to higher binding energies upon increasing Cu content. Photoemission experiments and first-principles calculations prove that the values of density of states at Fermi level of Cu_100−xZr_x metallic glasses are mainly dominated by Zr rather than Cu. This work will enlighten further research on understanding the inheritance of metallic glasses and designing new metallic glasses with unique properties.     

Microwave-assisted synthesis and characterization of Ti1 − xVxO2 (x = 0.0–0.10) nanopowders

Ti_1 ? xV_xO₂ (x = 0.0–0.10) nanopowders were successfully synthesized by a microwave-assisted sol–gel technique and their crystal structure and electronic structure were investigated. The products were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman and UV–Vis spectroscopy. The results revealed that TiO₂ powders maintained the anatase phase for calcination temperature below 600 °C, but gradually changed to the rutile phase above 800 °C. The formation of the rutile phase was completed at 1000 °C. For Ti_1 ? xV_xO₂ (x = 0.05) powders, the phase transformation appeared at 600 °C. The absorption edge of Ti_1 ? xV_xO₂ (x > 0) powders broadened to the visible region with increasing V concentration and a strong visible light absorption was obtained with 10% V doping. V doping and subsequent coexistence of both anatase and rutile phases in our Ti_1 ? xV_xO₂ nanoparticles are considered to be responsible for the enhanced absorption of visible light up to 800 nm.     

Deformation-induced α2 → γ phase transformation in TiAl alloys

Deformation-induced α₂ → γ phase transformation in high Nb containing TiAl alloys was investigated using high-resolution transmission electron microscopy (HREM) and energy dispersive X-ray spectroscopy (EDS). The dislocations appearing at the tip of deformation-induced γ plate (DI-γ) and the stacking sequence change of the α₂ matrix were two key evidences for determining the occurrence of the deformation-induced α₂ → γ phase transformation. Compositional analysis revealed that the product phase of the room-temperature transformation was not standard γ phase; on the contrary, the product phase of the high-temperature transformation was standard γ phase.     

Solidification microstructures of the undercooled Co–24 at%Sn eutectic alloy containing 0.5 at%Mn

Solidification samples of undercooled Co–24 at%Sn eutectic alloy containing a small amount of Mn (<1.0 at%) were prepared by the glass fluxing technique. The surface and internal solidification microstructures of the samples were observed by a scanning electron microscope (SEM) and an optical microscope (OM), respectively. The experiment results revealed that the addition of 0.5 at%Mn remarkably changed the solidification behaviors of the undercooled Co–24 at%Sn eutectic alloy. The addition of 0.5 at%Mn influenced the morphological selection of eutectic growth interface by increasing the interface energy anisotropy during the solidification of the undercooled Co–24 at%Sn eutectic melt. As undercooling increases, the coupled eutectic growth interface morphology successively experienced dendritic pattern, factual seaweed pattern and compact seaweed pattern. Besides, the addition of 0.5 at%Mn decreased the critical undercooling for the formation of anomalous eutectic by introducing a new formation mechanism of anomalous eutectic, i.e. divorce eutectic mechanism.     

Microstructural simulation in spinodally-decomposed Cu–70 at.%Ni and Cu–46 at.%Ni–4 at.%Fe alloys

The microstructure simulation of spinodal decomposition was carried out in the isothermally-aged Cu–70at.%Ni and Cu–46at.%Ni–4at.%Fe alloys using the phase field method. The numerical simulation was based on the solution of the Cahn–Hilliard nonlinear partial differential equation by the explicit finite difference method. A slow growth kinetics of phase decomposition was observed to occur in the aged Cu–Ni alloy. The morphology of decomposed phases consisted of an irregular shape with no preferential alignment in any crystallographic direction at the early stages of aging in this alloy. The growth kinetics rate of phase decomposition in the aged Cu–46 at.%Ni–4 at.%Fe alloy was appreciably faster than that in the aged Cu–70 at.%Ni alloy. In the case of the aged Cu–46 at.%Ni–4 at.%Fe alloy, an irregular shape of the decomposed phases was also observed at the early stages of aging. A further aging caused the change of initial morphology to a cuboid and/or plate shape of the decomposed Ni-rich phase aligned in the elastically-softest crystallographic direction <100> of Cu-rich matrix.