Lanthanum Modification Research Articles

Structure of lanthanum prepared by quenching from a liquid state

The structure of lanthanum quenched from a liquid state at cooling rates of 2.5 × 105–107 K/s is investigated. It is shown that a mixture of the face-centered cubic β-La phase stable above 583 K and the metastable lanthanum modification with a seven-layer hexagonal lattice consisting of alternating atomic layers ABCABAC… is formed in rapidly cooled foils. A mechanism is proposed for the formation of the metastable polytype, according to which the polytype is formed as a result of saturation of the main β-phase with well-ordered stacking faults.

Effect of Lanthanum Modification on Thermal Stability of .GAMMA.-Al2O3 Layers Fabricated by Suspension-dip Process

The forming and thermal stability of a coat layer was investigated through a simple suspension-dipping procedure of aqueous La-modified γ-Al2O3 suspension. The effect of La on the stabilization of surface area for powder, and the morphology of coat layers, which are heated at elevated temperatures, were examined by X-ray diffraction, nitrogen adsorption and scanning electron microscopy. The La-addition stabilized the metastable phases of Al2O3 at temperatures up to 1200'C, and induced the increase of phase-transformation temperature to α-Al2O3. The surface area of pure Al2O3 layer decreased after heat treatment at 900-1200°C. Several factors for controlling the formation of a coat layer from La-modified γ-Al2O3 suspension were discussed. A fabricated La-modified alumina layer was stable to realize high surface area as well as homogeneous shape without large cracks after heat treatment under heating condition at 1100°C in air.

Effect of La modifier on the electric hysteresis of lead zirconate stannate titanate compounds

In order to obtain an antiferroelectric material of slim double-hysteresis loop and proper switch electric fields, the effect of lanthanum modifier on the electric hysteresis characteristics of lead zirconate stannate titanate antiferroelectric compounds was studied. The electric hysteresis loops and dielectric constants with temperature variations were measured. The experimental results showed that La addition enhanced the antiferroelectrics of Pb(Zr,Sn,Ti)O 3 compounds. The switch electric fields increased and the electric hysteresis width Δ E= E F− E B narrowed with La content increase. Meanwhile, the dielectric constant was dispersed with frequency and the dielectric peak that related to the transition from antiferroelectric to paraelectric phase was broadened. La was an available modifier to tune the electric hysteresis width and switch fields of Pb(Zr,Sn,Ti)O 3 antiferroelectrics. Finally, an antiferroelectrics of slim double-hysteresis loop and suited switch electric fields was tailored.

Investigation of Electrical, Dielectric and Electromechanical Properties of Lanthanum Modified Lead Zirconate Titanate Using Impedance Spectroscopy Technique

Complex impedance spectroscopy (CIS) analysis on polycrystalline lanthanum modified lead zirconate titanate (PLZT) has been carried out to investigate its electrical, dielectric and electromechanical properties. CIS has been shown to be an important and very convenient experimental tool to correlate the electrical and dielectric behaviour of ferroelectric ceamics with their microstructure/structure in terms of the relaxations of defect species. The physical structure of the sample pellet has been visualized as comprising of grains separated by grain boundary in accordance with the impedance spectrum observations. The electrical processes in the sample has been modeled in the form of an electrical equivalent circuit made up of a series combination of two parallel RC circuits attributed to grains and grain boundaries. The real and imaginary parts of dielectric constants have been evaluated from impedance spectrum results and expressed in terms of complex permittivity spectrum (Cole-Cole plot). The dielectric phenomenon in PLZT has been observed to be of polydispersive nature with marked deviation from Debye type relaxation process. This result has also been confirmed by complex modulus analysis. Conductivity dispersion has been observed throughout the frequency range in the present investigation up to 400°C beyond which low frequency dispersion and high frequency plateau was found. It has been attributed to Warburg type diffusion phenomenon arising due to ion migration/charge transport at higher temperature.

Preparation and Ferroelectric Properties of Lanthanum Modified Sr0.8Bi2.2Ta2O9 Thin Films

La modified Sr0.8Bi2.2Ta2O9 (SLBT) thin films were fabricated on Pt/Ti/SiO2/Si substrates by metalorganic decomposition. All the films were annealed at 800°C using a rapid thermal annealing furnace. X-ray diffraction analysis indicated that single-phase layered perovskite SLBT ferroelectric thin films can be obtained and no secondary phase was found. The crystallization and hysteresis behavior of SLBT films showed a dependence on the La content. An increase of Pr and decrease of Ec with increasing La concentration were observed. The fatigue properties were found to be improved when more La was added to SLBT films. The results were discussed with respect to the effects of La3+ and Bi3+ substitution at perovskite A-site.

Effect of lanthanum promotion on the structural and catalytic properties of nickel-molybdenum/alumina catalysts

Effects of lanthanum (La) modification on the surface properties and hydrodesulfurization (HDS) activity of NiMo/Al 2O 3 catalysts were studied. In the La-modified catalysts, La inhibited the lateral growth of MoS 2 layers to form stacked MoS 2 layers. The La-modified catalysts slightly enhanced the HDS of 4,6-dimethyldibenzothiophene (4,6-DMDBT) via the hydrogenation (HYD) pathway. The La loading influences the surface concentration of active components, and causes a decrease in surface Ni concentration of the sulfided catalysts. A low La loading (0.7 wt.%) increased the HDS reaction rate constant of the NiMoLa/Al 2O 3 catalysts by more than 80%, whereas a further increase in the La loading decreased the HDS activity, due to a decrease in the surface Ni concentration.

Comparison of effect of La-modification on the thermostabilities of alumina and alumina-supported Pd catalysts prepared from different alumina sources

The thermostability of La-free and La-modified aluminas derived from different alumina sources including pseudo-boehmite, aluminum sulfate, aluminum nitrate, and aluminum chloride have been studied by characterizations of BET specific surface area (BET) and X-ray powder diffraction (XRD). Further, the activity and thermal durability of monolith-supported Pd catalysts using above different alumina as washcoats for simultaneously catalytic oxidation of C3H8, C3H6 and CO have been investigated using continuous flowing reaction. The experimental results show that the oxidation activity over the alumina-supported Pd catalysts does not keep in agreement with the surface area of aluminas derived from four aluminum sources. Lanthanum modification improves the activity of all catalysts calcined over 1000°C for simultaneously catalytic oxidation of C3H8, C3H6 and CO. Alumina source plays a role on the activity and thermostability of alumina and alumina-supported Pd catalysts. The experimental results show that pseudo boehmite is the best alumina source used for catalytic oxidation at high temperature over 1000°C.

Direct determination of phosphorus in vegetable oils by electrothermal atomic absorption spectrometry

An approach to the practical implementation of the standard method of the direct determination of phosphorus in vegetable oils by electrothermal atomic absorption spectrometry with automated sample injection was proposed. Analytical advantages of the use of a graphite platform and a lanthanum modifier prepared on the basis of vegetable oil are shown. The developed procedure provides the monitoring of the concentration of phosphorus in vegetable oils of various types within the range from 10 to 790 mg/kg with a satisfactory accuracy and precision.

Pyroelectric Properties of Sol-Gel Derived Lanthanum Modified Lead Titanate Thin Films

Lanthanum-modified lead titanate (PLT) thin films were fabricated on Pt/Ti/SiO2/Si substrate using the sol-gel method and rapid thermal annealing technique. The pyroelectric coefficient, and the infrared responsivity and the detectivity of the films were measured using static and dynamic methods, respectively. The ferroelectric properties were also measured. The pyroelectric coefficient increased from 1.3 to 2.5 nC/cm2· K with increasing La content. However, these low coefficients of PLT films are due to their crystal quality and orientations. The infrared response properties of PLT films depended on their thermal time constants. τ t (thermal time constant) for the PLT films decreased with the La contents increasing. The effects of back surface etching on the responsivity and the detectivity were also discussed.

Effects of lanthanum modification on the antiferroelectric-ferroelectric stability of high zirconium-content lead zirconate titanate

Lanthanum-modified lead zirconate titanate specimens with between 0 and 9 at.% lanthanum and a Zr/Ti ratio of 95/5, Pb1−xLax(Zr0.95Ti0.05)1−x/4O3 (PLZT 100x/95/5), have been investigated using dielectric spectroscopy, Sawyer–Tower polarization, and dilatometric measurements. La modification was found to suppress the stability of the ferroelectric state, changing the phase stability and phase sequence observed on cooling. PLZT 0/95/5 was found to be ferroelectric from room temperature to the Curie point; however, an antiferroelectric state was found to be stabilized as the long-range ferroelectric state was disrupted by La substitution on the Pb sites. With increasing La content (≳3 at.%), a multicell cubic (MCC) state was found to be stable near the temperature of the dielectric maximum. This MCC phase region was characterized by a dielectric plateau in the Curie temperature range and by the lack of any macroscopic symmetry changes. It is suggested that competition between long-range ferroelectric and short-range antiferroelectric couplings is responsible for the MCC state, changes in the relative phase stability, and subsequent changes in the macroscopic properties with increasing La content.

Temperature-programmed study of the oxidation of palladium/alumina catalysts and their lanthanum modification

Chemical modification with lanthana has previously been found to improve the performance of palladium-based three-way catalysts. In this work, the solid-state oxidation and oxide decomposition behavior of Pd/γ-Al2O3 and Pd/(La2O3/γ-Al2O3) catalysts of varying metal loadings have been assessed using temperature-programmed analysis. For Pd/Al2O3 catalysts with low metal concentrations (⩽= 1 wt.-% Pd), the oxygen consumed during TPO (temperature-programmed oxidation) was significantly in excess of that expected from an O/Pd ratio of one, which suggests the formation of a highly dispersed, oxygen-rich Pd/Al2O3 species. For Pd/Al2O3 catalysts of high metal loading (>=5 wt.-% Pd), there were two oxidation peaks in the TPO spectrum. The one at high temperature, which attenuated with lanthanum addition, is thought to be due to the oxidation of Pd/Al2O3 species of lower dispersion with an O/Pd ratio similar to that of PdO; this species may be an alumina-perturbed state of crystalline PdO. The other peak was assigned to the bulk oxidation of palladium particles. TPO revealed that lanthanum promoted the oxidation of palladium. Lanthanum modification impeded the formation of the highly dispersed, oxygen-rich Pd/Al2O3 species when the palladium concentration was low. Instead, there was TPO evidence for the presence of a new oxidation mode of the metal which was assigned to oxidation of a species comprising a Pd-La2O3 interaction of undetermined character. Oxygen TPD (temperature-programmed desorption) spectra also manifested a new feature upon lanthanum promotion of catalysts of high palladium concentration, which may indicate the presence of a Pd-La2O3 interaction, as well. Hydrothermal aging in steam at 1000° C for 16 h had little effect on the solid-state oxidation processes in lanthanum-free catalysts of low palladium concentration. Metal coalescence appeared to have been modest upon steaming of these catalysts, as indicated by the limited decrease in the oxygen uptake during the formation of the highly dispersed, oxygen-rich Pd/Al2O3 species. Thermal treatment brought about more drastic changes in the TPO profile of catalysts of low palladium concentration when lanthanum was present.

Omission of the ashing stage in the determination of aluminium and lead in waters by graphite furnace atomic absorption spectrometry

Omission of the ashing stage in the determination of aluminium and lead in waters by graphite furnace atomic absorption spectrometry on uncoated tubes initially led to lower sensitivity and poor accuracy. Sensitivity could be restored by increasing the drying time to about 40 s or by increasing the drying temperature. The original effect is thought to be due to interference from water vapour; the time was insufficient to remove water vapour before atomisation occurred. Higher drying temperatures (180–260 °C), leading to rapid expulsion of the water vapour, gave results with no loss in precision. A temperature of 250 °C held for 12 s was chosen for drying. With these revised programmes having no ashing stage, satisfactory recovery was demonstrated for aluminium (98 ± 6%, n= 10) and lead (101 ± 2%, n= 8) in domestic water samples. Cycle times were 54 and 55 s for aluminium and lead, respectively. The method for lead involved the use of a mixture of lanthanum and nitric acid as a modifier, but omission of the ashing stage did not affect the accuracy. In a separate experiment, it was shown that the suppression of sulphate interference by the lanthanum modifier was the same with and without an ashing stage.