Influence Of La Doping Research Articles

The influence of La doping on the catalytic behavior of Cu/ZrO 2 for methanol synthesis from CO 2 hydrogenation

A series of Cu/ZrO 2 catalysts with various La loadings for methanol synthesis from CO 2 hydrogenation were prepared by a urea–nitrate combustion method. The catalysts were characterized with N 2 adsorption, XRD, reactive N 2O adsorption, XPS, TPR, H 2-TPD and CO 2-TPD techniques, and tested for methanol synthesis from CO 2 hydrogenation. With increasing La loading, the Cu surface area increases first and then decreases, whereas the amount of basic site over catalysts increases continuously. The results of catalytic test reveal that a linear relationship exists between the conversion of CO 2 and the Cu surface area. Moreover, it is found for the first time that the selectivity to methanol is related to the distribution of basic site on the catalyst surface. The presence of La favors the production of methanol, and the optimum catalytic activity is obtained when the amount of La doping is 5% of the total amount of Cu 2+ and Zr 4+.

Influence of La doping on elastic and thermodynamic properties of SrMoO 3

The elastic and thermodynamic properties for Sr 1− x La x MoO 3 ( x = 0.0, 0.05, 0.1, 0.15, and 0.2) with temperature have been investigated, probably for the first time, by using modified rigid ion model (MRIM). The computed results on the elastic constants ( C 11, C 12, and C 44) are the first report on them. Using these elastic constants we have computed other elastic properties such as B, β, G′, G, E, σ, B/ G ratio, Cauchy pressure ( C 12 − C 44) and Lame's parameters ( μ, λ). We have also reported the thermodynamic properties such as ϕ, f, θ D, θ D1, υ 0, υ 1, γ, and α. The values of Young's modulus, shear modulus and compressibility for SrMoO 3 are in good agreement with the available experimental data. The concentration ( x) dependence of θ D in Sr 1− x La x MoO 3 suggests that increased La doping drives the system effectively away from the strong electron–phonon coupling regime. Specific heat is reported in the wide temperature range and compared with the respective experimental data available in the literature. The thermal expansion coefficient of SrMoO 3 is in good agreement with the other theoretical data.

Lanthanum-doped Bi4Ti3O12 ceramics prepared by high-pressure technique

We present an effective way in this paper to increase the density of lanthanum doped bismuth titanate ceramics, Bi4—xLaxTi3O12 (BLT), thereby significantly improving the performance of the BLT ceramics. Dense BLT ceramicses, Bi4—xLaxTi3O12 (x = 0.25, 0.5, 0.75, 1.0), are prepared by using nanocrystalline powders fabricated by a —gel method and high-pressure technique. The microstructures of the BLT ceramicses prepared separately by conventional-pressure and high-pressure techniques are investigated by using x-ray diffraction and transmission electron microscope. The influence of La-doping on the densification of bismuth titanate ceramics is investigated. The experimental results indicate that the phase compositions of all samples with various lanthanum dopings sintered at 900 °C possess layer-structure of Bi4Ti3O12. The green compacts are pressed under 2.5 GPa, 3.0 GPa, 3.5 GPa and 4.0 GPa, separately. It is found that the density of BLT ceramics is significantly increased due to the decreasing of porosity in the green compacts by high-pressure process.

The influence of La-doping on the activity and stability of Cu/ZnO catalyst for the low-temperature water–gas shift reaction

The influence of lanthanum (La) doping on the performance of 37 wt% Cu/ZnO catalysts for the low-temperature water–gas shift (LT-WGS) reaction was investigated. A 2.3 wt% La loading improved catalyst activity compared to the neat Cu/ZnO and Cu/ZnO/Al 2O 3 systems and was accompanied by a lowering of the activation energy. Higher La loadings promoted the adsorption of H 2O at the expense of CO, resulting in a decrease in LT-WGS activity. Additionally, 2.3 wt% La acted to stabilise catalyst activity compared with the neat Cu/ZnO. XPS and H 2-TPR assessment demonstrated a strong interaction between Cu and La components, while data from CO and H 2O-TPD studies favoured the associative WGS mechanism in this instance. Activity and stability findings also suggested metallic Cu was not responsible for LT-WGS activity.

The Influence of La Doping on the Oxidation Mechanism and Stresses in the Thermally Grown Oxide on CMSX-4 with Pt-Aluminide Bond Coat

The thermally grown oxide (TGO) formed on CMSX-4 coated with Pt-aluminide bond coat was studied by isotopic oxidation and photo-luminescence piezo-spectroscopy (PLPS). It was found that La doping in the CMSX-4 substrate suppressed inward oxygen diffusion along TGO grain boundaries which was the dominant TGO growth mechanism without La doping. The different oxidation mechanisms led to distinct TGO stress distributions and different crystallographic orientations of the α-Al2O3 in the TGO. In the sample without La doping, the residual stress was uniform and the α-Al2O3 crystallites were found to be preferentially orientated with their c-axis in perpendicular to the substrate surface, whereas in the sample with La doping, the residual stress distribution was found to be bi-modal with the existence of low stress zones, and the α-Al2O3 crystallites were preferentially orientated with the c-axis in parallel to the substrate surface. The TGO growth stress was estimated to be −0.6 GPa (compressive) for the sample without La doping and 0.2 GPa (tensile) for the La-doped sample. Low stress zones are speculated to correspond to local damage caused by the tensile growth stress at high temperature.

Influence of La doping on structural and dielectric properties of SrBi2Nb2O9 ceramics

Lanthanum doped SrBi2Nb2O9 ceramics with the chemical formula SrBi2−xLaxNb2O9 (SBLN) (x=0–0.5) have been prepared through conventional solid state route. X-ray diffraction reveals the shrinkage of unit cell of strontium bismuth niobate with incorporation of La3+ dopant, having no lone pair electrons. Shifting of Raman phonon modes indicates the reduced rattling space of NbO6 octahedra with increase in La doping concentration. Further, the softening of lowest frequency phonon mode with increasing x in SBLN shows the transition from ferroelectric to paraelectric at room temperature. The dielectric properties for all the compositions are studied as a function of temperature (25 to 500 °C) over the frequency range of 10 kHz–1 MHz. With increase in lanthanum doping concentration the phase transition becomes diffused and transition temperature gets shifted toward lower temperature. A phase transition from normal ferroelectric to paraelectric has been observed via relaxor-type ferroelectrics with increase in x. The frequency dependence of transition temperature was studied in terms of Vogel–Fulcher relation for SBLN (x=0.4).

Influence of La doping on the properties of molybdenum perovskite Sr1–xLaxMoO3 (0 ≤ x ≤ 0.2)

AbstractThe effects of La‐doping on the structural, magnetic, electrical transport and specific‐heat properties in 4d perovskites Sr1–x Lax MoO3 (0 ≤ x ≤ 0.2) have been investigated. The substitution of Sr ions by La ions does not change the space group of the samples, but increases the lattice parameter a (Å). The resistivity ρ and magnetic susceptibility χ decrease monotonously with the increase of x , while the electronic specific heat coefficient γ e increases. The resistivity of all samples shows a T 2 dependence in the low‐temperature region of 2 K < T < 125 K and a T dependence in the high‐temperature range of 130 K < T < 350 K, related to the electron–electron (e–e) and electron–phonon (e–ph) scattering, respectively. The specific‐heat data agrees with the classical Dulong–Petit phonon specific heat, C cl = 3k BrN A = 124.7 J/mol K at high temperatures and C p(T )/T = γ e + β pT 2 at low temperatures. These behaviors can be explained according to the decrease of the density of states (DOS) at the Fermi energy level (E F), N (E F). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of La doping in multiferroic properties of BiFeO3 thin films

The influence of La doping on the crystal structure and multiferroic properties of BiFeO3 (BFO) thin films was investigated. BFO-based films of pure perovskite phase were deposited on BaPbO3∕Pt∕TiOx∕SiO2∕Si substrates by rf-magnetron sputtering. The increased dielectric constant and remanent polarization of the La-doped BFO films were due to the increased lattice parameters and an improved crystallinity. The polarization switching and the fatigue behavior of the BFO films were significantly enhanced by the La doping. The in-plane magnetization-field curves revealed that the room-temperature saturated magnetization of the BFO films increased with La doping.

Raman and X-ray photoelectron scattering study of lanthanum-doped strontium bismuth titanate

The ferroelectric ceramics of lanthanum-doped SrBi 4Ti 4O 15 (SrBi 4 − x La x Ti 4O 15:SBLT- x, x = 0.00, 0.05, 0.10, 0.25, 0.50, 0.75 and 1.00) have been synthesized. Raman and XPS spectrum measurements have been carried out to analyze the La substitution site and the influence of La-doping on the crystal structure. The results suggest that La 3+ ions prefer to occupy the A site in the case of x ⩽ 0.10, and at higher content they tend to be incorporated into Bi 2O 2 layers. At La content greater than 0.10, the TiO 6 octahedra linking to the Bi 2O 2 layers shrink and the structure distortion relaxes more drastically. The concentration of oxygen vacancy decreases with La-doping, and then saturates when La content is higher than 0.10. The La-doping content has a lower critical value of about 0.10 in comparison with La-doped Bi 4Ti 3O 12, which may be attributed to the stable Sr 2+ ions at A sites of SrBi 4Ti 4O 15.

Influence of La doping on the properties of SrBa hexagonal ferrites

A series of SrBa ferrites with composition (Sr 0.75Ba 0.25) 1− x (La 2O 3) x/2 ·5.3Fe 2O 3 has been prepared by the conventional ceramic technology. The size of the crystallites corresponds to that of a single magnetic domain. Doping of La not only enhances the coercivity of these materials but also improves the remanence. The reasons for improving the hard magnetic properties of the investigated ferrites are discussed.

Influence of La-doping of YBa/sub 2/Cu/sub 3/O/sub 7/ on transport properties of interface-engineered ramp-edge junctions

We have investigated the influence of La-doping of the YBa/sub 2-x/La/sub x/Cu/sub 3/O/sub 7/ (YBCO) thin film electrodes on dc and ac Josephson properties of ramp-edge junctions with interface-engineered barriers. Non-doped optimized junctions exhibit critical current densities, j/sub c/, of 5/spl middot/10/sup 4/ A/cm/sup 2/ and normal sheet resistances, R/sub N//spl middot/A, of 3/spl middot/10/sup -9/ Ohm/spl middot/cm/sup 2/ at 77 K. The transport properties of junctions with x=0.03 La-doping are quite similar to those of non-doped junctions. La-doping of x=0.05 and x=0.07 leads to an increase of R/sub N//spl middot/A and decrease of j/sub c/ by about one order of magnitude. Devices with j/sub c/<10/sup 4/ A/cm/sup 2/, which are operating in the short junction limit exhibit current-voltage characteristics without any excess current and 100% modulation of critical current in external magnetic field. The current voltage characteristics exhibit well defined Shapiro steps. The dependence of current-step height on microwave current can be described in the resistively-shunted junction model. The strong changes in the temperature dependencies of j/sub c/ and R/sub N//spl middot/A suggest different electrical transport properties for junctions fabricated with non-doped and La-doped YBCO. The temperature dependence of the critical current and the normal resistance allows us to draw conclusions to the transport properties of our junctions.