Magnetic and magnetotransport properties of La1−xSrxMn0.5Сo0.5O3 perovskites

La_1?xSr_xMn_0.5Сo_0.5O₃ (x ≤ 0.75) perovskites have been studied as a function of temperature by neutron powder diffraction (NPD), magnetization and magnetoresistance measurements. The NPD data show that x = 0.15 and 0.5 compounds are stoichiometric, so the Sr²⁺ doping transforms Co²⁺ ions into the Co³⁺ ones, whereas manganese ions remain in the 4+ oxidation state as in the parent ferromagnetic compound ${\mathrm{LaCo}}_{\mathrm{0.5}}^{\mathrm{2}+}{\mathrm{Mn}}_{\mathrm{0.5}}^{\mathrm{4}+}{\mathrm{O}}_{\mathrm{3}}$. The magnetization data show a decrease in the Curie temperature from 215 K for the compound with x = 0 down to 147 K for the compound with x = 0.05. The compounds with x > 0.15 show an increase in T_C up to 260 K (x = 0.75) in spite of a gradual decrease of the spontaneous magnetization. The stoichiometric compound x = 0.5 demonstrates a sharp ferromagnet-paramagnet transition with T_C = 250 K. However, there is no visible coherent magnetic contribution to the NPD patterns. All compounds are semiconductors and exhibit large magnetoresistance gradually increasing with decrease of temperature. The magnetic data have been interpreted assuming that the Co³⁺ ions are in high spin state, however, there is a fraction of cobalt ions in low spin state. It is suggested that the superexchange interaction between Co³⁺ ions in the high spin state and Mn⁴⁺ ions is ferromagnetic and that the ferromagnetism of the compounds with x > 0.5 and high T_C is associated with positive exchange interactions between Co³⁺ being in high spin state and Mn⁴⁺ ions distributed within the short range regions. Based on the NPD results and magnetization data the magnetic phase diagram has been constructed.     

Variation in structures and photoluminescence spectra of BaxEu1−xAl2O4 powders

Barium europium(II) aluminate (Ba_xEu_1?xAl₂O₄) powders were prepared by a solid-state reaction among barium carbonate (BaCO₃), europium oxide (Eu₂O₃), and alumina (Al₂O₃) powders at 1400 °C for 3 h under a mixed gas flow of H₂ and N₂. The powders were characterized by powder X-ray diffraction (XRD), infrared and Raman spectroscopy, and photoluminescence (PL). With increasing Ba²⁺ content in Ba_xEu_1?xAl₂O₄, the structure of Ba_xEu_1?xAl₂O₄ changed from a monoclinic (P2₁) to hexagonal (P6₃) phase. The hexagonal (P6₃22) phase was also observed between the two phases. The XRD pattern of a single Ba_0.6Eu_0.4Al₂O₄ phase, which has not been reported in the literature, was refined by the Rietveld method and its structure was confirmed by selected-area electron diffraction. With increasing x value, the emission peak in the PL spectra of Ba_xEu_1?xAl₂O₄ became weaker (x = 0–0.4) and then more intense (x = 0.6–0.98), and its position showed a blue shift from 520 to 498 nm.     

Structural,magnetic and elastic properties of Mn0.3−xMgxCu0.2Zn0.5Fe3O4 nanoparticles

In this paper, Mn_0.3?xMg_xCu_0.2Zn_0.5Fe₃O₄ (x?=?0.00, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30) nanoparticles were prepared by the nitrate-citrate technique at low temperature. The structural, microstructural, magnetic and elastic properties of the samples were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy, Transmission electron microscopy, field emission-scanning electron microscopy and vibrating-sample magnetometer at room temperature. Rietveld refinement of the XRD patterns indicated the formation of the single phase cubic spinel structure (space group Fd-3m) without any detectable impurity phase in all the samples that also was confirmed by FTIR studies. The lattice parameter is found to increase non-monotonically with an increase in Mg ion concentration. Also, the bond lengths and bond angles (A and B sites) of the studied ferrites were calculated by the refining of the XRD data. The values of the crystallite size decrease with increasing micro-strain (and conversely) and both of them reach extremum at x?=?0.15. The low remanence and coercivity values confirmed the formation of the superparamagnetic ferrites nanoparticles. The saturation magnetization of the samples gradually grows with Mg substitution and reach extremum at x?=?0.15. Variation of saturation magnetization with Mg content can be mainly attributed to change of cation distribution, and Yafet-Kittel angle occurred between magnetic moments on B-site in the samples. The values of Young's modulus, Debye temperature, bulk modulus, rigidity modulus of the samples were determined by the values of elastic constant and wave velocities obtained from the force constants. The improvement of the elastic properties of sample x?=?0.05 could be explained regarding the smaller values of the lattice parameter (a), the bond length and angle and the smaller crystallite size.     

Influence of LiBO2 addition on the microstructure and lithium-ion conductivity of Li1+xAlxTi2−x(PO4)3(x = 0.3) ceramic electrolyte

Concerning the safety problems of conventional Li-ion batteries with liquid electrolytes, it is crucial to develop reliable solid-state electrolytes with high ionic conductivity. Li_1+xAl_xTi_2?x(PO₄)₃ (LATP, x = 0.3) is regarded as one of the most promising solid electrolytes due to its high ionic conductivity and excellent chemical stability to humidity.Herein, a new strategy is proposed for improving the sintering behavior and enhancing the ionic conductivity of LATP by using LiBO₂ as the sintering aid via liquid phase sintering. The as-prepared sample LATP with homogeneous microstructure and high relative density of 97.1% was successfully synthesized, yielding high total ionic conductivity of 3.5 × 10^?4 S cm^?1 and low activation energy of 0.39 eV at room temperature. It was found that the addition of LiBO₂ could effectively enhance the densification and increase the ionic conductivity of LATP electrolyte, proving an effective way to synthesis LATP ceramics by a simple and reliable route.     

Nb modified structural,magnetic and magnetocaloric properties of double perovskite Ba2FeMo1−xNbxO6

A systematic study focusing on the effect of Niobium (Nb) doping on the structural, magnetic and magnetocaloric properties of Ba₂FeMoO₆ samples is presented here. The samples of interest Ba₂FeMo_1?xNb_xO₆ (0 ≤ x ≤ 0.4) were prepared using the solid state reaction method and were confirmed to possess a cubic structure with Fm-3m space group using the X-ray diffraction analysis and Rietveld refinement. A second order of ferromagnetic phase transition was recorded in both the pure as well as the Nb doped samples using the temperature dependent magnetization and Arrott plots analysis. The pristine Ba₂FeMoO₆ (BFMO) sample indicated a spontaneous magnetization (34.6 emu/g at 100 K) with a relatively sharp magnetic transition at the Curie temperature (T_C) of 315 K as compared to the doped samples. A magnetic entropy change of 0.93 Jkg^?1K^?1 at an applied magnetic field of 2.5 T was measured for the pure BFMO sample. The doped BFMO samples with Mo partially substituted by Nb however, were observed to effectively modify the T_C accompanied by a decrease in magnetization. The results investigated in this work suggest that the magnetic and magnetocaloric properties of the BFMO can be tailored by controlled Nb doping which is of significant importance in order to realize the numerous potential applications of the material in the magnetic refrigeration technology.     

Physico-mechanical and photoluminescence properties of EuxMg2−xSiO4 system sintered under different conditions

This work focuses on sintering of Eu_xMg_2?xSiO₄ (x = 0.0, 0.01, 0.03, 0.05) forsterite ceramics under different conditions and study their microstructures, physico-mechanical and photoluminescence characteristics. The starting materials were waste silica fume, pure MgO and europium oxide (Eu₂O₃). The prepared batches were sintered up to 1500 °C in the presence or absence of carbon in ambient atmosphere. The presence of carbon helped in reduction of Eu³⁺ into Eu²⁺. The effect of sintering conditions and formed phases on the properties of final product were investigated. The results indicated that the specimens sintered without carbon exhibited improved physico-mechanical properties with formation of forsterite and Eu_4.67 (SiO₄)₃O precipitated from the formed liquid phase. While the samples sintered in carbon exhibited lower physic-mechanical properties with formation of forsterite and clinoenstatite. Photoluminescence properties were improved with increasing the amount of added Eu₂O₃ for all sintered samples. The specimens sintered without carbon exhibited life time higher than those sintered in the presence of carbon.     

Magnetization study in LaCr1−xAlxO3 (0 ≤ x ≤ 0.95)

The structural and magnetic properties of the Al-doped LaCrO₃ have been investigated. The samples were produced by combustion method using urea as fuel. X-ray diffraction measurement shows that all samples are formed in a single phase. It is observed that increasing the Al content induces a structural phase transition from orthorhombic to a rhombohedral structure. The magnetic measurements indicate that all samples have an antiferromagnetic order with a strong decrease in the magnetic ordering temperature with increasing Al content. The consistent variation of the magnetic moment confirms that the Al³⁺ ion replaced the Cr³⁺ ion on the B-site of the perovskite.     

Synthesis,characterization, enhanced dielectric and antimicrobial properties of WxCu1−xO nanostructures

Herein, we report the chemical synthesis of W_x Cu_1?xO nanostructures with varying concentration of dopant (x = 0.00, 0.01, 0.03 and 0.05). The as-obtained doped CuO nanostructures have been investigated to evaluate their physio-chemical properties like crystallinity, morphology, optical features and infrared active modes. The dielectric study shows that doping induces a significant increase in real permittivity. In addition, the doped nanostructures also show potential towards inhibition of pathogenic microbes. The antimicrobial activity of prepared nanostructures determined against four different bacterial strains shows that W doped CuO nanostructures possess a strong antimicrobial activity against S. aureus, and K. pneumoniae and an intermediate activity against E. coli and C. albicans. These finding recognize the use of W doped CuO nanostructures in permittivity materials and bacterial disinfection nanomaterials.     

Defect chemistry and dielectric behavior of Sr0.99Ce0.01Ti1−xO3 ceramics with high permittivity

Sr_0.99Ce_0.01Ti_1-xO₃ (SCT, x?=?0, ±?0.0025, ±?0.0050, 0.0075) ceramics were prepared by solid state reaction methods and sintered in air atmosphere at different temperatures, with a soaking time of 2?h. The dielectric properties of all samples presented excellent temperature independence over a broad temperature range from 25 to 330?℃ and frequency independence between 10?kHz and 1?MHz. Sr_0.99Ce_0.01Ti_0.9925O₃ (SCT0.9925) ceramics sintered in air atmosphere exhibited a high permittivity (~5400) and a low dielectric loss (~0.01) measured at room temperature and 1?kHz. XPS and complex impedance spectroscopy analysis confirmed that the high permittivity and low dielectric loss were attributed to the fully ionized oxygen vacancies and giant defect-dipoles in Ti-deficient samples. However, a higher dielectric loss of Ti-rich samples is owing to the destruction of giant defect dipoles, in which highly localized electrons were transformed into hopping electrons.     

Photoluminescent properties of the Ba1−xZnxMoO4 heterostructure obtained by ultrasonic spray pyrolysis

In this study, Ba_1?xZn_xMoO₄ (x = 0, 0.25, 0.5, 0.75 and 1) microspheres were the first time described in the literature the by ultrasonic spray pyrolysis method. Powders were characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM-FEG) and UV–visible and Photoluminescence (PL) spectroscopies. The XRD patterns showed that the samples with x = 0 and x = 1 displayed pure tetragonal and triclinic structures, respectively. On the other hand, samples with x = 0.25, 0.5 and 0.75 revealed the formation of tetragonal/triclinic heterojunction structure. The SEM-FEG images showed that the Ba_1?xZn_xMoO₄ particles present the spherical morphology with the decrease of particle sizes when the value of “x” increase. The gap energies vary between 4.56 eV and 4.17 eV, being influenced by the degree of structural order-disorder. The PL emission spectra of the samples showed a broad band behavior with emission intensity predominant in the orange-red region and of a lower intensity in the blue-green region. The chromaticity coordinates x and y showed that the samples with x ≤ 0.75 present emission in orange, while the samples with x = 1 showed emission in white.     

Sintering,crystallization and properties of 2(Mg1−xMnx)O·2Al2O3·5SiO2 ceramics

Mn-substituted cordierites, 2(Mg_1?xMn_x)O·2Al₂O₃·5SiO₂ (x?=?0–1), were prepared from natural components (talc, clay, alumina) and MnO_2. Sintering behavior, phase transformation, and microstructural features of the samples were investigated using X-ray diffraction (XRD), differential thermal analysis (DTA), dilatometric measurements and scanning electron microscopy (SEM) with energy dispersive analysis (EDS). The results of DTA and XRD analysis indicate that MnO₂ is successively reduced to Mn₂O₃ and MnO in the sintering process. Mn²⁺ ions incorporate into the crystal structure of α-cordierite substituting Mg²⁺ ions in octahedral sites and thus increasing the cordierite unit cell volume. Mn promotes the sintering process: the crystallization temperature, melting point, density and open porosity of Mn-substituted cordierites lowered, whereas the shrinkage and medium pore diameter enlarged with an increase in MnO₂ content in the mixture of raw materials. Surface enrichment with Mn with the formation of manganese oxide crystallites was found for the samples with high substitution degree.     

Crystal structure and enhanced microwave dielectric properties of the Ce2[Zr1−x(Al1/2Ta1/2)x]3(MoO4)9 ceramics at microwave frequency

Dense microwave dielectric ceramics of Ce₂[Zr_1−x(Al_1/2Ta_1/2)_x]₃(MoO₄)₉(CZMAT) (x = 0.02–0.10) were prepared by the conventional solid-state route. The effects of (Al1/2Ta1/2)⁴⁺ on their microstructures, sintering behaviors, and microwave dielectric properties were systematically investigated. On the basis of the X-ray diffraction (XRD) results, all the samples were matched well with Pr₂Zr₃(MoO₄)₉ structures, which belonged to the space group R3¯c. The lattice parameters were obtained using the Rietveld refinement method. The correlations between the chemical bond parameters and microwave dielectric properties were calculated and analyzed by using the Phillips—Van Vechten—Levine (P—V—L) theory. Excellent dielectric properties of Ce₂[Zr_0.94(Al_1/2Ta_1/2)_0.06]₃(MoO₄)₉ with a relative permittivity (ε_r) of 10.46, quality factor (Q × f) of 83,796 GHz, and temperature coefficient of resonant frequency (τ_f) of −11.50 ppm/℃ were achieved at 850 ℃.     

Preparation of α-Fe2O3/Ag core/shell nanocomposites and SERS properties

用籽晶法,以甲醛为还原剂、3-氨丙基三乙氧基硅烷(APS)为改性剂,在Ag［(NH₃)₂］⁺溶液中制备α-Fe₂O₃/Ag核壳结构复合粉体。采用XRD、TEM和EDX对样品进行表征,系统研究了APS改性剂、醇水比等对复合纳米颗粒包覆效果及性能的影响;并用吡啶(Py)为探针,研究了α-Fe₂O₃/Ag核壳纳米颗粒作为拉曼衬底时的拉曼增强性能相似文献   

Ionic conductivity of (As2Se3)1 ? x(AgHal)x (Hal = I, Br) nanocomposites

Nanocomposites based on chalcogenide glasses have been synthesized. A differential thermal analysis of (As₂Se₃)_{1 − x} (AgI) _x and (As₂Se₃)_{1 − x} (AgBr)x (0 ≤ x ≤ 0.5) samples has been performed. The size of nanofragments that undergo elementary structural transformations has been evaluated. The data obtained are in agreement with the evaluated sizes of X-ray coherent scattering regions. The electrical properties of the glasses under consideration have been studied using impedance spectroscopy in the temperature range 293–393 K. It has been demonstrated that the ionic component of the electrical conductivity dominates in glasses with a high content of silver halide.     

Facile synthesis and X-ray peak broadening studies of Zn1−xMgxO nanoparticles

Magnesium-doped ZnO nanoparticles (NPs) (Zn_1?xMg_xO-NPs, x = 0.0, 0.01, 0.03, and 0.05) were synthesized by a simple sol–gel method. The compounds were synthesized at calcination temperatures of 650 °C for 2 h. The synthesized Zn_1?xMg_xO-NPs were characterized by X-ray diffraction analysis (XRD) and high-magnification transmission electron microscopy (TEM). The XRD results revealed that the sample product was crystalline with a hexagonal wurtzite phase. The TEM showed Zn_1?xMg_xO-NPs with nearly spherical and hexagonal shapes. The size–strain plot (SSP) method was used to study the individual contributions of crystallite sizes and lattice strain on the peak broadening of the Zn_1?xMg_xO-NPs. Physical parameters such as strain, stress, and energy–density values were calculated more precisely for all reflection peaks of XRD corresponding to the wurtzite hexagonal phase of ZnO in the 20°–100° range from the SSP results. The effect of doping on the optical band-gap was also investigated. The results showed that Mg²⁺ is a good dopant to control some of the ZnO properties, with minimum defects to its structure.     

Effects of additives on the synthesis of TiCxN1−x by a solid-gas mechanically induced self-sustaining reaction

The synthesis of TiC_xN_1?x from Ti/C mixtures in a N₂ atmosphere performed in a high-energy planetary mill was used as example to study the influence of the use of additives in mechanically induced self-sustaining reaction (MSR) processes. In particular, the effect of the addition of TiN, TiC, Si₃N₄ and SiC was analyzed. The self-sustaining reaction was extinguished when additive contents of 50, 40, 40 and 30?wt% for TiN, TiC, Si₃N₄ and SiC, respectively, were employed. These additives cannot be regarded as real inert since they served as an extra solid source for nitrogen and carbon, modifying the final stoichiometry of the TiC_xN_1?x phase. The adiabatic temperature (T_ad) determined for the mixtures with no MSR effect was well above the empirical limit value of 1800?K adopted as criterion for the occurrence of the self-propagating high-temperature synthesis (SHS) process. The ignition time (t_ig) of the MSR process was practically invariant for low additive contents (approximately 50?min) and tended to increase up to maximum values of 85–95?min for the larger additive contents.     

Molten-salt synthesis of Ba5−xSrxNb4O15 solid solutions and their enhanced humidity sensing properties

Layered perovskite-type niobates A₅Nb₄O₁₅ (A = Ba, Sr) are thought to be good candidates for microwave dielectric and water-splitting applications, but traditional solid-state syntheses of these compounds usually require high temperatures and complicated procedures. In this work, Ba_5?xSr_xNb₄O₁₅ (x = 0 ? 5) perovskite solid solutions were obtained using a facile molten salt synthetic method. The crystal structure of Ba_5?xSr_xNb₄O₁₅ solid solutions were characterized by X-ray diffraction (XRD). Crystal structure parameters with different Sr²⁺ concentrations show that the lattice parameters and unit cell volumes of Ba_5?xSr_xNb₄O₁₅ decrease with increasing [Sr²⁺]. The humidity sensing behavior of Ba_5?xSr_xNb₄O₁₅ solid solutions was investigated over a wide relative humidity (RH) range, from 11% to 95%. Ba₂Sr₃Nb₄O₁₅ shows the highest sensitivity among the obtained samples with a humidity hysteresis of ca. 4% RH. The response-recovery times of the Ba₂Sr₃Nb₄O₁₅ sensor are only 2 s and 17 s as the humidity alternates between 11% and 95% RH, respectively, showing excellent potential as a humidity sensing material for practical applications.     

Structural,dielectric, piezoelectric,ferroelectric and electro-caloric properties of Ba1−xCaxTi0.975(Nb0.5Yb0.5)0.025O3 lead-free ceramics

Polycrystalline samples of Ba_1?xCa_xTi_0.975(Nb_0.5Yb_0.5)_0.025O₃ (where x = 0.15, 0.2 and 0.3, abbreviated as BCTYN) were prepared by the conventional solid state reaction method. The effect of calcium (Ca) substitution in BaTi_0.975(Nb_0.5Yb_0.5)_0.025O₃ (abbreviated as BTYN25) on the structural, dielectric, piezoelectric and ferroelectric properties and electro-caloric effects (ECE) was investigated. X-ray diffraction (XRD) results at room temperature showed that the BCTYN samples in the composition x < 0.3 exhibited a pure tetragonal perovskite structure. Dielectric measurements showed a classical ferroelectric behavior for all samples. With the increase of the Ca content, the Curie temperature (T_C) was still maintained with a small shift towards low temperature. The evolution of the Raman spectra was studied as a function of compositions and temperatures. The Raman bands confirmed the structure and the phase transition of the BCTYN ceramics. By adding Ca, the piezoelectric properties and the remanent polarization (P_r) are relatively maintained for the compositions x = 0.15 and x = 0.2. A piezoelectric coefficient of d₃₃ = 130 pC/N and a planar electromechanical coupling factor of k_p = 28% were obtained for these compositions. Two different methods were used to calculate the electro-caloric coefficients of the BCTYN ceramics. The incorporation of Ca was found to enhance the electro-caloric strength (ξ = ΔT/ΔE) within a broad temperature range with a best value of ξ = 0.2?Kmm/kV for x = 0.2.     

High-temperature dielectric properties and microwave absorption abilities of Bi1−xMgxFeO3 nanoparticles

BiFeO₃ (BFO) multiferroic nanoparticles have attracted increasing attention owing to the coexistence of ferroelectric and ferromagnetic properties. In this work, Bi_1−xMg_xFeO₃ (x = 0.05, 0.1, 0.15) multiferroic nanoparticles were synthesized by the sol-gel method. The electromagnetic properties and microwave absorption performance in the temperature range of 323–723 K at X-band were investigated. The qualified bandwidth (absorption intensity < −10 dB) of the Mg-doped BFO materials covers the whole X-band at 673 K, suggesting promising candidates as high-temperature electromagnetic absorbers.     

Reaction path in formation of Ti1−xWxC solid solution by combustion synthesis

Ti_1?xW_xC solid solution powders have been prepared by combustion synthesis in two reaction modes of self-propagating high-temperature synthesis (SHS) and thermal explosion (TE). The reaction mechanism in combustion synthesis is investigated and the effects of processing parameters are discussed. Two reaction paths are proposed for the formation of the Ti_1?xW_xC solid solution. In path 1, Ti_1?xW_xC crystals are directly precipitated from Ti-W-C melt, and in path 2 the Ti_1?xW_xC phase is formed by solid-state reaction between TiC and WC. The availability of the two reaction paths depends on the x value and reaction temperature. For smaller x values and higher temperatures, the reaction path 1 dominates and almost full conversion is achieved. For larger x values and lower temperatures, however, path 2 becomes dominant. In path 2, the rate-limiting step is the formation of WC by slow diffusion-controlled solid-state reaction between W and C, which is unable to be completed during the short reaction period and results in the presence of W₂C and unreacted W in the products. The reaction between W and C cannot be improved by the addition of excessive C, but can be promoted by using carbon black instead of graphite powder as C source or carrying out the synthesis in a high-pressure N₂ atmosphere instead of vacuum. In the fast combustion reaction with high heating/cooling rates, a non-equilibrium state is likely to be created because the mass transfer and energy exchange are limited. The non-equilibrium condition has a strong influence on the reaction kinetics, where the decomposition of W₂C is avoided, a localized compositional heterogeneity is caused, and ultrafine grains on a submicron scale are obtained in the products.