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.     

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.     

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.     

Electrocaloric properties of (1 − x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 ferroelectric ceramics near room temperature

Electrocaloric effects of (1 − x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ [abbreviated as (1 − x)PMN–xPT] ferroelectric ceramics with x being, 0.08, 0.10, and 0.25, respectively, were measured near room temperature, and the origins of the electrocaloric effects of these ceramics were discussed. It was found that these ceramics possess large electrocaloric effect with ΔT being, more than 1 K under an electric field of 1.5 kV mm⁻¹ in a wide temperature range (more than 10 K) near room temperature, and this effect is due to both of the electrocaloric effect resulting from the electric field induced first-order phase transition and the linear electrocaloric effect. It is expected that these ceramics could be used for multi-stage cascade ferroelectric refrigeration near room temperature.     

Phase transition in (1 − x)Bi(Mg1/2Ti1/2)O3-xPbTiO3 ceramics

The phase structure and phase transition of (1 ? x)Bi(Mg_1/2Ti_1/2)O₃-xPbTiO₃ (BMT-PT) ceramics with x = 0.0–0.42 were investigated. It was found that pure perovskite phases were achieved for x ≥ 0.28, while Bi₄Ti₃O₁₂ or Bi₁₂TiO₂₀ phase existed for x ≤ 0.15. The anomaly dielectric peaks were observed around 620 °C for BMT-(0.28–0.38)PT samples, thus phase transition in (1 ? x)Bi(Mg_1/2Ti_1/2)O₃-xPbTiO₃ ceramics was studied using thermal expansion. It was found that dielectric anomalies at ~ 620 °C were resulted from the phase transition of the second phase and defects inside samples.     

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.     

Evidence of morphotropic phase boundary displacement in lead-free (Bi0.5Na0.5)1 − xBaxTiO3 polycrystalline thin films

Lead-free (Bi_0.5Na_0.5)_1 ? xBa_xTiO₃ (BNBT) thin films with compositions at x = 0.055, 0.100, and 0.150 were prepared by chemical solution deposition on Pt/TiO₂/SiO₂/(100)Si substrates. The dielectric behavior of the films was studied, and the ferroelectric-antiferroelectric phase transition observed was used to situate the morphotropic phase boundary (MPB) for compositions with x ~ 0.100 (BNBT-10), a value that differs from that reported for bulk materials (BNBT-5.5). Extrinsic effects derived from the thin-film configuration (e.g., microstrains, residual stresses) may be responsible for the shift of the MPB. Consequently, the dielectric permittivity is significantly improved for this composition, showing the best ferroelectric response obtained up to now for films of the BNBT system (P_r = 13.0 μC/cm², E_c = 70 kV/cm).     

Doping effects of Li–Sb content on the structure and electrical properties of [(Na0.5K0.5)1 − x(Li)x(Sb)x(Nb)1 − xO3] lead-free piezoelectric ceramics

Ceramic samples of [Na_0.5K_0.5]_1 ? x(Li)_x(Sb)_x(Nb)_1 ? xO₃ (NKNLS) (x = 0.04–0.06) were prepared by high temperature solid-state reaction method. X-ray diffraction analysis of the powder samples suggests the formation of a single-phase material with transformation from orthorhombic to tetragonal crystal structure with increase in Sb content. Dielectric studies show a diffuse phase transition about 100 °C and another phase ferroelectric–paraelectric transition at 330 °C. Polarization vs. electric field (P–E) hysteresis studies show maximum remanent polarization (Pr ～ 0.66 C m^?2) for composition x = 0.05. AC conductivity in the compound increases with increase in temperature which may be attributed due to oxygen vacancies and show negative temperature coefficient of resistance (NTCR) effect.     

Processing and electrical properties of (1 − x)[(1 − y)(Pb(Mg1/3Nb2/3)O3)–y(Pb(Yb1/2Nb1/2)O3)]–xPbTiO3 ceramics

Ferroelectric ceramics in the vicinity of morphotropic phase boundary (MPB) with compositions represented as (1 ? x)[(1 ? y)(Pb(Mg_1/3Nb_2/3)O₃)–y(Pb(Yb_1/2Nb_1/2)O₃)]–xPbTiO₃ were prepared by solid state reaction. The addition of PYbN to PMN–PT decreased the sintering temperature from 1200 °C (y = 0.25) to 1000 °C (y = 0.75). The PT content, where the MPB was observed, increased with the PYbN addition. A remanent polarization value of 28.5 µC/cm² and a coercive field value of 11 kV/cm were measured from 0.62[0.25PMN–0.75PYbN]–0.38PT ceramics, which were close to the ones measured from PMN–0.32PT ceramics. In addition, the Curie temperature was found to increase with PYbN additions.     

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.     

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.     

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.     

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.     

Effect of bond valence on microwave dielectric properties of (1 − x)CaTiO3–x(Li0.5La0.5)TiO3 ceramics

The origins of microwave dielectric properties (1 ? x)CaTiO₃–x(Li_0.5La_0.5)TiO₃ (0.2 ≤ x ≤ 0.8) ceramics, prepared by a conventional solid-state reaction method, were investigated based on the theory of bond valence. The XRD and SEM results showed that complete solid solutions with orthorhombic perovskite structure were formed in the whole investigated compositional range. The dielectric constant (?_r), quality factor (Q × f) and temperature coefficient of resonant frequency (τ_f) were closely related to B-site, A-site and the difference between A-site and B-site bond valences of ABO₃ perovskite compounds, respectively. As x value increased from 0.2 to 0.8, the dielectric constant increased from 198.3 to 276.8, the Q × f value decreased from 4340 to 1880 GHz, and the τ_f value varied from +489.7 to ?178 ppm/°C. For practical applications, excellent microwave dielectric properties of ?_r = 245, Q × f = 2750 GHz and τ_f = +0.75 ppm/°C were obtained for 0.4CaTiO₃–0.6(Li_0.5La_0.5)TiO₃ ceramics.     

One-step preparation of dense TiC1 − xNx–Ni3Ti cermet by combustion synthesis

TiC_1 − xN_x-based cermets are attractive for high-speed cutting operations, and are usually produced by powder sintering at high temperatures and for a long time. In this paper, dense TiC_1 − xN_x–Ni₃Ti cermet is directly prepared in one step by combustion synthesis in 2 MPa N₂ atmosphere, from the reactant of (Ti + C + Ni) with a molar ratio of Ti:C:Ni = 1:0.7:0.4. The as-prepared TiC_1 − xN_x–Ni₃Ti cermet shows a homogeneous microstructure, in which equiaxed TiC_1 − xN_x grains of a few microns are bounded by Ni₃Ti grain-boundary phase, and a Vickers hardness of 12.1 ± 0.3 GPa. It is proposed that the deficiency of C in the starting reactant plays a key role in simultaneous densification during combustion synthesis. Compared with the conventional powder sintering approach, combustion synthesis may provide a one-step, ultrafast, and furnace-free way to prepare TiC_1 − xN_x-based cermets with reduced time and energy consumption.     

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.     

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.