Synthesis, characterization and optical properties of Zn1−xTixO nanoparticles prepared via a high-energy ball milling technique

Zn_1−xTi_xO (x = 0, 0.01, 0.03 and 0.05) nanoparticles were prepared by high-energy ball milling at 400 rpm. The milled powders were characterized by X-ray diffractometer (XRD) and the results exhibited that Ti-doped ZnO nanoparticles consisted of single phase with hexagonal structure when the mixtures of ZnO and TiO₂ powders were milled for 20 h. The crystallite size reduced as a function of the doping content and milling time from 1 to 10 h then increased after milling for 20 h and when the annealing temperature increased. The strain changed inversely to the crystallite size. A wider band-gap was obtained by increasing the doping content and annealing temperature because of a reduction in defect concentration. Both ZnO- and Ti-doped ZnO nanoparticles caused damage to S. aureus, E. coli, P. mirabilis, S. typhi and P. aeruginosa.     

Effect of La/Ce ratio on the structure and electrochemical characteristics of La0.7−xCexMg0.3Ni2.8Co0.5 (x = 0.1-0.5) hydrogen storage alloys

The effect of La/Ce ratio on the structure and electrochemical characteristics of the La_0.7−xCe_xMg_0.3Ni_2.8Co_0.5 (x = 0.1, 0.2, 0.3, 0.4, 0.5) alloys has been studied systematically. The result of the Rietveld analyses shows that, except for small amount of impurity phases including LaNi and LaNi₂, all these alloys mainly consist of two phases: the La(La, Mg)₂Ni₉ phase with the rhombohedral PuNi₃-type structure and the LaNi₅ phase with the hexagonal CaCu₅-type structure. The abundance of the La(La, Mg)₂Ni₉ phase decreases with increasing cerium content whereas the LaNi₅ phase increases with increasing Ce content, moreover, both the a and cell volumes of the two phases decrease with the increase of Ce content. The maximum discharge capacity decreases from 367.5 mAh g⁻¹ (x = 0.1) to 68.3 mAh g⁻¹ (x = 0.5) but the cycling life gradually improve. As the discharge current density is 1200 mA g⁻¹, the HRD increases from 55.4% (x = 0.1) to 67.5% (x = 0.3) and then decreases to 52.1% (x = 0.5). The cell volume reduction with increasing x is detrimental to hydrogen diffusion D and accordingly decreases the low temperature dischargeability of the La_0.7−xCe_xMg_0.3Ni_2.8Co_0.5 (x = 0.1-0.5) alloy electrodes.     

Dispersion studies of La substitution on dielectric and ferroelectric properties of multiferroic BiFeO3 ceramic

Lanthanum La-substituted multiferroic Bi_1−xLa_xFeO₃ ceramics with x = 0.0, 0.05, 0.10, 0.15, 0.20 and 0.25 have been prepared by solution combustion method. The effect of La substitution for the dispersion studies on dielectric and ferroelectric properties of Bi_1−xLa_xFeO₃ samples have been studied by performing x-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), density, dc resistivity and dielectric measurements as well as characterizing the polarization-field hysteresis loop. The results of prepared samples are compared with those of bismuth ferrite (BiFeO₃). In the measuring frequency of 10 KHz to 1 MHz, the dielectric constants and dielectric losses for samples x = 0.20, 0.25 are almost stable and exhibited lowest dielectric loss close to 0.1. The resistivity of Bi_1−xLa_xFeO₃ samples reaches a maximum value of 10⁹ ohm-cm, which is about three times higher than that for pure BiFeO₃. The results also show that stabilization of crystal structure and nonuniformity in spin cycloid structure by La substitution enhances the resistivity, dielectric and ferroelectric properties. Furthermore, the substitution of rare earth La for Bi helps to eliminate the impurity phase in BiFeO₃ ceramic.     

Microstructure of Ba1−xLaxTiO3−δ ceramics sintered by Spark Plasma Sintering

Nano-sized Ba_1−xLa_xTiO₃ (0.00 ≤ x ≤ 0.14) powders were prepared by a coprecipitation method and calcined at 850 °C in air. The corresponding ceramics were obtained by Spark Plasma Sintering (SPS) at 1050 °C. These ceramics are oxygen deficient and are marked as Ba_1−xLa_xTiO_3−δ. Both powders and ceramics were characterized by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). The effect of lanthanum concentration on the densification behavior, on the structure and the microstructure of the oxides was investigated. Average grain sizes are comprised between 54 (3) nm and 27 (2) nm for powders, and 330 (11) nm and 36 (1) nm for ceramics according to the La-doping level. Powders crystallize in the cubic (or pseudo-cubic) perovskite phase. The structure of ceramics consists in a mixture of cubic (or pseudo-cubic) and tetragonal perovskite type phases. As the lanthanum content increases, the tetragonality of the samples decreases, as well as the grain size.     

Electrochemical and structural study of Ce0.8Sm0.2−xLaxO1.9 electrolyte materials for SOFC

Ce_0.8Sm_0.2−xLa_xO_1.9 powders, denoted as La_xSDC (for x=0, 0.01, 0.03, 0.05, 0.07 and 0.1), were synthesized via the mechanical milling reaction method. The La³⁺ doping content has a remarkable influence on structural and electrical properties. The phase identification and morphology were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Lattice parameters were calculated by the Rietveld method. It was observed that the lattice parameter values in Ce_0.8Sm_0.2−xLa_xO_1.9 systems obey Vegard's law. The pellets were then sintered at 1500 °C in air for 7 h. The relative densities of these pellets were over 93.7%.The electrical conductivity was studied using two-probe impedance spectroscopy and results showed that the conductivity of Ce_0.8Sm_0.2−xLa_xO_1.9 first increased and then decreased with La dopant content x. Results also showed that Ce_0.8Sm_0.17La_0.03O_1.9 had the highest electrical conductivity, σ₇₀₀ °C equal to 3.8×10⁻² Scm⁻¹ and an activation energy equal to 0.77 eV. It was therefore concluded that co-doping with the appropriate amount of La can further improve the electrical properties of ceria electrolytes.     

Lanthanum-doped Bi4Ti3O12 prepared by the soft chemical method: Rietveld analysis and piezoelectric properties

Bi_4−xLa_xTi₃O₁₂ (BLT) thin films and powders with x ranging from 0 to 0.75 were prepared by the polymeric precursor solution. The effect of lanthanum on the structure of BIT powders was investigated by Rietveld Method. The increase of lanthanum content does not lead to any secondary phases. Orthorhombicity of the bismuth titanate (BIT) crystal lattice decreased with the increase of lanthanum content due the reduction of a/b ratio. The BLT films show piezoelectric coefficients of 45, 19, 16 and 10 pm/V for x = 0, 0.25, 0.50 and 0.75, respectively. The piezoelectric response is strongly reduced by the amount of lanthanum added to the system.     

Chemical reduction of nanocrystalline CeO2

The reduction of commercial and mechanochemically processed CeO₂ powders was studied. Nanostructured CeO₂, with the crystallite size of 21 nm and the lattice distortion of 0.37%, was obtained during 60 min of milling in a high-energetic vibratory mill. X-ray diffraction, scanning electron microscopy and Brunauer-Emmett-Teller method were applied to characterize the milled powders. During the thermal treatment at 1200 and 1400 °C in an argon atmosphere the nonstoichiometric CeO_2−x oxides with the defect fluorite structure were formed. Compositions of CeO_2−x oxides were determined according to its lattice parameter. The results showed that the release of oxygen, as well as the rate of reduction, was more effective in nanocrystalline then in the microcrystalline CeO₂, producing at 1200 °C CeO_1.80 and CeO_1.85 oxides, while at 1400 °C were obtained similarly, CeO_1.77 and CeO_1.78, compositions.     

Synthesis and microstructure of (LaSr)MnO3 and (LaSr)FeO3 ceramics by a reaction-sintering process

(La_xSr_1−x)MnO₃ (LSMO) and (La_xSr_1−x)FeO₃ (LSFO) (x = 0.2–0.4) ceramics prepared by a simple and effective reaction-sintering process were investigated. Without any calcination involved, La₂O₃ and SrCO₃ were mixed with MnO₂ (LSMO) or Fe₂O₃ (LSFO) then pressed and sintered directly. LSMO and LSFO ceramics were obtained after 2 and 4 h sintering at 1350–1400 and 1200–1280 °C, respectively. Grain size decreased as La content increased in LSMO and LSFO ceramics.     

Electrochemical properties of La1−xSrxFeO3 (x = 0.2, 0.4) as negative electrode of Ni-MH batteries

La_(1−x)Sr_xFeO₃ (x = 0.2,0.4) powders were prepared by a stearic acid combustion method, and their phase structure and electrochemical properties were investigated systematically. X-ray diffraction (XRD) analysis shows that La_(1−x)Sr_xFeO₃ perovskite-type oxides consist of single-phase orthorhombic structure (x = 0.2) and rhombohedral one (x = 0.4), respectively. The electrochemical test shows that the reaction at La_(1−x)Sr_xFeO₃ oxide electrodes are reversible. The discharge capacities of La_(1−x)Sr_xFeO₃ oxide electrodes increase as the temperature rises. With the increase of the temperature from 298 K to 333 K, their initial discharge capacity mounts up from 324.4 mA h g⁻¹ to 543.0 mA h g⁻¹ (when x = 0.2) and from 147.0 mA h g⁻¹ to 501.5 mA h g⁻¹ (when x = 0.4) at the current density of 31.25 mA g⁻¹, respectively. After 20 charge-discharge cycles, they still remain perovskite-type structure. Being similar to the relationship between the discharge capacity and the temperature, the electrochemical kinetic analysis indicates that the exchange current density and proton diffusion coefficient of La_(1−x)Sr_xFeO₃ oxide electrodes increase with the increase of the temperature. Compared with La_0.8Sr_0.2FeO₃, La_0.6Sr_0.4FeO₃ electrode is a more promising candidate for electrochemical hydrogen storage because of its higher cycle capacity at various temperatures.     

Structural and magnetic characterization of MnxZn1 − xFe2O4 (x = 0.2; 0.35; 0.65; 0.8; 1.0) ferrites obtained by the citrate precursor method

In this work the microstructure and magnetic properties of Mn-Zn ferrites powders were investigated. Mn_xZn1 _− xFe₂O₄ powders where x = 0.2; 0.35; 0.5; 0.65; 0.8 and 1.0 were obtained by citrate precursor method. Citrate resin precursor was burned on air atmosphere at 400 °C for 3 h. Mn-Zn powders were calcined at 950 °C during 150 min under inert atmospheres: N₂ and rarefied atmosphere. Thermal analysis of precursor resin, phase evolution and microstructure of Mn-Zn ferrites powders were investigated by TG, DTA, XRD and SEM techniques. The powders calcined under rarefied atmosphere show spinel cubic structure and contamination of α-Fe₂O₃, while powders calcined under N₂ presents only the spinel cubic structure. Particle size was observed by SEM ranging from 80 to 150 nm. The magnetic properties were measured employing a vibrating sample magnetometer (VSM). It was observed that the saturation magnetization M_s increased with the increase of Mn content. The M_s of Mn_0.8Zn_0.2Fe₂O₄ calcined on rarefied atmosphere and Mn_0.8Zn_0.2Fe₂O₄ calcined on N₂ was 23.31 emu g⁻¹ and 56.23 emu g⁻¹, respectively.     

Magnetic and thermal expansion properties of chromium-substituted lithium ferrite

The structure, magnetic, and thermal expansion properties of chromium-substituted lithium ferrite have been investigated. The lattice constant (Å) decreases linearly as a (x) = 8.32366 − 0.04338x for Li_0.5Fe_2.5−xCr_xO₄ (x = 0.0–1.0). When increasing Cr content, the initial permeability decreased gradually. The average thermal expansion coefficient of Li_0.5Fe_2.5−xCr_xO₄ (x = 0.0–1.0) varied from 15.34 to 17.77 ppm/°C, with increasing Cr content, the average thermal expansion coefficient decreased. The average thermal expansion coefficient (ppm/°C) in the range of 25–850 °C give the polynomial correlation as follows, TEC (x) = 1 7.775 − 0.216x − 0.723x² − 1.493x³ for Li_0.5Fe_2.5−xCr_xO₄ (x = 0.0–1.0).     

Synthesis and electrical conductivity of La-Sr-X-Mg-O (X = Ti, Zr, Al) perovskite solid solution

Perovskite solid solutions of (La_0.6Sr_0.4)(X_1−yMg_y)O_3−δ (X = Ti, Zr, Al) were prepared by a coprecipitation method using corresponding aqueous solutions and ammonium carbonate solution. The freeze-dried powders were sintered in air at 1000-1500 °C for 1-36 h. Single phase solid solutions were produced in the compositions of (La_0.6Sr_0.4)(Zr_0.6Mg_0.4)O_3−δ and (La_0.6Sr_0.4)(Al_0.9Mg_0.1)O_3−δ where (3 − δ) < 3. For the compositions of X = Ti and Zr for y = 0.1 where (3 − δ) > 3, two phases including perovskite solid solution were produced at 1400-1500 °C. The stability of perovskite solid solution was closely related to the fraction of lattice oxygen atom (3 − δ). A relatively high conductivity was measured for (La_0.6Sr_0.4)(Al_0.9Mg_0.1)O_3−δ (σ = 4.15 × 10⁻⁴ S/cm at 600 °C, activation energy 113.4 kJ/mol). The influence of fraction of oxide ion vacancy on the activation energy was small for δ = 0.1-0.3 of perovskite solid solution.     

Synthesis and negative thermal expansion property of Y2−xLaxW3O12 (0≤x≤2)

Y_2−xLa_xW₃O₁₂ solid solutions were successfully synthesized by the solid state reaction method. The microstructure, hygroscopicity and thermal expansion property of the resulting samples were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM) and thermal mechanical analysis (TMA). Results indicate that the structural phase transition of the Y_2−xLa_xW₃O₁₂ changes from orthorhombic to monoclinic with increasing substituted content of lanthanum. The pure phase can form for 0≤x≤0.4 with orthorhombic structure and for 1.5≤x≤2 with monoclinic one. High lanthanum content leads to a low relative density of Y_2−xLa_xW₃O₁₂ ceramic. Thermal expansion coefficients of the Y_2−xLa_xW₃O₁₂ (0≤x≤2) ceramics also vary from −9.59×10⁻⁶ K⁻¹ to 2.06×10⁻⁶ K⁻¹ with increasing substituted content of lanthanum. The obtained Y_0.25La_1.75W₃O₁₂ ceramic shows almost zero thermal expansion and its average linear thermal expansion coefficient is −0.66×10⁻⁶ K⁻¹ from 103 °C to 700 °C.     

Dielectric properties of Sm, Nd and Fe doped Bi1.5Zn0.92Nb1.5O6.92 pyrochlores

New pyrochlore ceramics have been produced by doping Sm and Nd into the Bi site and Fe into the Nb site in the Bi_1.5Zn_0.92Nb_1.5O_6.92 (BZN) pyrochlore. Doped pyrochlore ceramics were produced by conventional solid state mixing of oxides at different doping levels using the compositions of Bi_1.5−xSm_xZn_0.92Nb_1.5O_6.92, Bi_1.5−xNd_xZn_0.92Nb_1.5O_6.92 and Bi_1.5Zn_0.92Nb_1.5−xFe_xO_6.92−x. The solubility limit of cations was determined as x = 0.13, 0.18 and 0.15 for Sm, Nd and Fe, respectively. While Sm and Nd increased the dielectric constant (?), Fe doping led a decrease in ?. Dielectric constant of Sm and Nd doped BZN increased to 199 at x = 0.13 (Sm) and to 219 at x = 0.18 (Nd). At low Fe dopings (x = 0.05), the dielectric constant of BZN increased to 242 but decreased to 211 at x = 0.15. The dielectric losses were lower for Sm and Nd dopings than Fe but in all cases it was lower than 0.006. The dielectric constant of Sm, Nd and Fe doped BZN ceramics was nearly independent of frequency within the frequency range between 1 kHz and 2 MHz, but decreased considerably with temperature between 20 and 200 °C. Temperature coefficient of Sm doped BZN (−354 ppm/°C) was lower than Nd and Fe doped BZN ceramics at solubility limits (−538 ppm/°C for Nd and −565 ppm/°C for Fe).     

Characterization of Mg1−xNixAl2O4 solid solutions prepared by combustion synthesis

Mg_1−xNi_xAl₂O₄ (x = 0, 0.25, 0.5, 0.75 and 1) solid solutions have been prepared by combustion synthesis. After annealing the combustion synthesized powders at 1000 °C for 3 h single-phase Mg_1−xNi_xAl₂O₄ was obtained over the entire range of compositions. The lattice parameter of Mg_1−xNi_xAl₂O₄ gradually increased from 8.049 Å (NiAl₂O₄) to 8.085 Å (MgAl₂O₄), which certified the formation of the spinel solid solutions. All samples prepared by combustion synthesis had blue color shades, denoting the inclusion of Ni²⁺ in the spinel structure in octahedral and tetrahedral configuration. The crystallite size of Mg_1−xNi_xAl₂O₄ was in the range of 35-39 nm and the specific surface area varied between 5.8 and 7.0 m²/g.     

Effects of manganese substitution at the B-site of lanthanum-rich strontium titanate anodes on fuel cell performance and catalytic activity

Sr_0.4La_0.6Ti_1−xMn_xO_3−δ with rhombohedral structure has been investigated in terms of their electrochemical performance, redox stability, and electro-catalytic properties for solid oxide fuel cell anodes. The performance of Sr_0.4La_0.6Ti_1−xMn_xO_3−δ anodes for solid oxide fuel cells strongly depends on the Mn substitution at the B-site of the perovskites. Electrical conductivity of Sr_0.4La_0.6Ti_1−xMn_xO_3−δ increases with increasing Mn content. X-ray photoelectron spectroscopy analysis reveals that the amount of Mn³⁺ and Ti³⁺, which is an electronic charge carrier, increases with Mn doping. The reduced anode powders with high Mn/Ti ratio show oxygen storage capability and a low carbon deposition rate. Linear thermal expansion coefficients of Sr_0.4La_0.6Ti_1−xMn_xO_3−δ anodes range from 9.46×10⁻⁶ K⁻¹ to 11.3×10⁻⁶ K⁻¹. The maximum power densities of the single cell with the Sr_0.4La_0.6Ti_0.2Mn_0.8O_3−δ anode in humidified H₂ and CH₄ at 800 °C are 0.29 W cm⁻² and 0.24 W cm⁻², respectively.     

Magnetic and magnetoresistive properties of La0.65−xEuxSr0.35MnO3

Eu-doped perovskites La_0.65−xEu_xSr_0.35MnO₃ (0.05 ≤ x ≤ 0.30) were synthesized by sol–gel method using citric acid and characterized by X-ray diffraction, magnetization, resistivity and magnetoresistance (MR) experiments. All samples had a single hexagonal perovskite structure. As x increased from 0.05 to 0.30, the Curie temperature T_C for the samples decreased from 352 to 242 K. It was found that two transition points appeared when the resistivity changed with increasing temperature, and upon an application of a magnetic field of 20 kOe the maximum magnetoresistivity of 18% for the La_0.65−xEu_xSr_0.35MnO₃ with x = 0.20 was obtained at room temperature 300 K. The mechanism of the transitions for the samples was explored.     

Synthesis and characterization of pure single phase Ni-Zn ferrite nanopowders by oxalate based precursor method

Series of single-phase Ni_1 − xZn_xFe₂O₄ (x = 0.20, 0.35, 0.50 and 0.60) nanopowders with average particle size of ∼ 35 nm have been synthesized by using oxalate based precursor method. Precursor powders were synthesized by reacting aqueous solutions of metal nitrates and oxalic acid by using different total metal ions: oxalic acid molar ratios and then evaporating them to dryness. Pure, single-phase Ni-Zn ferrite nanopowder was formed by calcining the precursor with total metal ion: oxalic acid ratio of 1:0.125 at a temperature of 850 °C. The synthesized nanopowders were characterized by using X-ray diffraction, Thermo-gravimetric and Differential Scanning Calorimetric analysis, Transmission Electron Microscope and Scanning Electron Microscope. Room temperature DC resistivity of the nanopowders was measured with respect to temperature by the two-probe method and was of the order of ∼ 10⁷ Ωcm. Room temperature saturation magnetization was measured by using Vibrating Sample Magnetometer and it varied between 34-49 emu/g depending on the composition. This aqueous solution based method provides a simple and cost-effective route to synthesize single phase, Ni-Zn ferrite nanopowders.     

Structure and thermal properties of La0.6Sr0.4Co0.2Fe0.8O3−δ-SDC carbonate composite cathodes for intermediate- to low-temperature solid oxide fuel cells

The structure and thermal properties of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ-SDC carbonate (LSCF-SDC carbonate) composite cathodes were investigated with respect to the calcination temperatures and the weight content of the samarium-doped ceria (SDC) carbonate electrolyte. The composite cathode powder has been prepared from La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ and SDC carbonate powders using the high-energy ball milling technique in air at room temperature. Different powder mixtures at 30 wt%, 40 wt% and 50 wt% of SDC carbonate were calcined at 750-900 °C. The findings indicated that the structure and thermal properties of the composite cathodes were responsive to the calcination temperature and the content of SDC carbonate. The absence of any new phases as confirmed via XRD analysis demonstrated the excellent compatibility between the cathode and electrolyte materials. The particle size of the composite cathode powder was ∼0.3-0.9 μm having a surface area of 4-15 m² g⁻¹. SEM investigation revealed the presence of large particles in the resultant powders resulting from the increased calcination temperature. The composite cathode containing 50 wt% SDC carbonate was found to exhibit the best thermal expansion compatibility with the electrolyte.     

Synthesis and transport properties in La2−xAxMo2O9−δ (A = Ca, Sr, Ba, K) series

The La_2−xA_xMo₂O_9−δ (A = Ca²⁺, Sr²⁺, Ba²⁺ and K⁺) series has been synthesised as nanocrystalline materials via a modification of the freeze-drying method. The resulting materials have been characterised by X-ray diffraction (XRD), thermal analysis (TG/DTA, DSC), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The high-temperature β-polymorph is stabilised for dopant content x > 0.01. The nanocrystalline powders were used to obtain dense ceramic materials with optimised microstructure and relative density >95%. The overall conductivity determined by impedance spectroscopy depends on both the ionic radius and dopant content. The conductivity decreases slightly as the dopant content increases in addition a maximum conductivity value was found for Sr²⁺ substitution, which show an ionic radii slightly higher than La³⁺ (e.g. 0.08 S cm⁻¹ for La₂Mo₂O₉ and 0.06 S cm⁻¹ for La_1.9Sr_0.1Mo₂O_9−δ at 973 K). The creation of extrinsic vacancies upon substitution results in a wider stability range under reducing conditions and prevents amorphisation, although the stability is not enhanced significantly when compared to samples with higher tungsten content. These materials present high thermal expansion coefficients in the range of (13-16) × 10⁻⁶ K⁻¹ between room temperature and 753 K and (18-20) × 10⁻⁶ K⁻¹ above 823 K. The ionic transport numbers determined by a modified emf method remain above 0.98 under an oxygen partial pressure gradient of O₂/air and decreases substantially under wet 5% H₂-Ar/air when approaching to the degradation temperature above 973 K due to an increase of the electronic contribution to the overall conductivity.