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.     

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.     

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.     

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.     

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.     

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.     

The effect of B-site cations on the properties of KTaxNb1−xO3 [1 0 0] surface: A study of density functional theory

Ferroelectric material and piezoelectric properties are different in potassium tantalite niobate (KTa_xNb_1−xO₃) crystal according to differences in x. Here, we show the results of KTa_xNb_1−xO₃ (x = 0.25, 0.5, 0.75) crystal [1 0 0] surface properties calculated by density functional theory (DFT). Using local density approximation (LDA) and generalized gradient approximation (GGA), DFT has been employed to determine the structural, electronic, and optical properties of chemically ordered ferroelectric KTa_xNb_1−xO₃ crystal [1 0 0] surfaces. Based on the research, comparison and analysis of the results show that all kinds of properties of KTa_xNb_1−xO₃ [1 0 0] surfaces are vary with respect to x. It is found, for instance, that the ordering of B-site cations obviously influences the properties of the KTa_xNb_1−xO₃ surface. At the same time, this allows the experimental studies to be supervised effectively.     

Room temperature ferromagnetism in sputtered Zn1 − xCrxO thin films

Cr doped ZnO (Zn_1 ? xCr_xO) thin films with different Cr concentrations (0.4, 1.5, and 8.9 at.%) were deposited on Si substrates using RF magnetron sputtering. Film crystal structure was characterized using X-ray diffraction, and vibrating sample magnetometer measurements were used to investigate their magnetic properties. Unstable ZnO structure is present at low Cr concentrations, while secondary phases appear at higher Cr concentrations. 8.9 at.% Zn_1 ? xCr_xO film exhibits room temperature ferromagnetism and high 325 K Curie temperature, even after 300 °C annealing for 1 h. This result is promising and demonstrates Cr doped ZnO film's potential use in practical applications.     

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.     

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.     

Synthesis and electrochemical performance of Li1+xNi0.5Mn0.3Co0.2O2+δ (0 ≤ x ≤ 0.15) cathode materials for lithium-ion batteries

In this work, layered lithium-excess materials Li_1+xNi_0.5Mn_0.3Co_0.2O_2+δ (x = 0, 0.05, 0.10 and 0.15), of spherical morphology with primary nanoparticles assembled in secondary microspheres, were synthesized by a coprecipitation method. The effects of lithium content on the structure and electrochemical performance of these materials were evaluated by employing X-ray diffraction (XRD), inductive coupled plasma (ICP), scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge tests. It is found that Li_1.10Ni_0.5Mn_0.3Co_0.2O_2+δ, i.e., Li[(Ni_0.5Mn_0.3Co_0.2)_0.95Li_0.05]O₂ showed the best electrochemical performance due to the highly ordered layered structure, reduced cation mixing and the lowest charge transfer resistance. Li_1.10Ni_0.5Mn_0.3Co_0.2O_2+δ delivered a discharge capacity of 145 mA h g^?1 at 125 mA g^?1 in the cut-off voltage of 2.5–4.3 V, and had a capacity retention of 100% after 50 cycles at room temperature.     

Density functional theories study on the properties of Ga1 − xCrxAs

The structural, electronic, magnetic and optical properties of Ga_1 − xCr_xAs (x = 0, 6.25%, 12.5%) have been studied by first-principles calculations based on the HSE hybrid density functional theories. The optical properties, including the complex dielectric function, optical refractive index, extinction coefficient and absorption coefficient are discussed for radiation up to 15 eV. The results predicate that the system of Ga_1 − xCr_xAs exhibits typical half-metallic properties, in which Cr forms deep levels in the forbidden band and reduces the energy gap, increases static dielectric constant and obviously red-shifts the absorption edge. With the increase of the fraction of Cr, the material gradually exhibits noticeable anisotropy in the photon energy range of 0–5 eV. In addition, the p-d hybridization reduces the magnetic moment of Cr from its free space charge value of 3 μ_B and a smaller atomic magnetic moments of As and Ga atoms are generated.     

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.     

Structural transformation and magnetic properties of CaMn1 − xFexO3 − δ (0.0 ≤ x ≤ 0.5)

We have investigated the structural and magnetic properties of CaMn_1 − xFe_xO_3 − δ (0.0 ≤ x ≤ 0.5). Solid state method is used for the synthesis of these samples. Sintering of these compositions at 1300 °C stabilizes higher ionic radii Fe^+ 3 (0.645 Å) at Mn^+ 4 (0.53 Å) site in CaMn_1 − xFe_xO_3 − δ. Structural transformation from orthorhombic to tetragonal to pseudo cubic crystal system and the increase in lattice parameters have been observed with the substitution of Fe at Mn site in CaMn_1 − xFe_xO_3 − δ (0.0 ≤ x ≤ 0.5). The magnetization data show the transformation of G type of antiferromagnetic arrangement of Mn^+ 4 electrons spins in CaMnO₃ into paramagnetic spin type arrangement with the substitution of Fe at Mn site. The compositions x = 0.05, x = 0.1and x = 0.2 show a small kink ~ 100 K in the magnetization data, which resulted due to the competition between antiferromagnetic and paramagnetic states with the Fe substitution.     

Magnetisation studies of phase co-existence in Gd1 − xCaxBaCo2O5.5

Magnetic properties of hole doped, oxygen deficient double perovskite compounds, Gd_1 ? xCa_xBaCo₂O_5.5, have been investigated. Ferromagnetic transition temperatures increase and the anti-ferromagnetic transition temperatures decrease with Ca substitution leading to stabilisation of ferromagnetisim for x ≥ 0.05. A detailed study of the ferromagnetic phase indicates the presence of double hysterisis loops for Ca fractions, 0.05 ≤ x ≤ 0.2 in the 50–200 K temperature range, suggestive of the co-existence of two ferromagnetic phases with different co-ercivities. Based on the magnetisation and transport measurements a phase diagram is proposed for Ca doped GdBaCo₂O_5.5.     

Multiferroic properties of Ba(FexTi1 − x)O3 nanorods

Multiferroic Ba(Fe_xTi_1 ? x)O₃ (BFT) nanorods were prepared by a chemical route using polyvinyl alcohol as surfactant. The presence of PVA in excess is responsible to convert cubic or spherical shaped nanoparticles into rodlike structure. Tetragonal phase and nano dimensions in the form of rods of BFT specimens are identified. These BFT nanorods show improvement in the coexistence of ferroelectricity and ferromagnetism of multiferroic properties than their nanoparticles. The effect of low dimensions of BFT rods to control dielectric constant with low loss up to higher frequency region has been observed. With 1% of Fe-doping BFT shows higher value of spontaneous polarization, saturation magnetization and dielectric constant than with other dopants.