Showing papers on "Lanthanum published in 2005"

Effect of Yttrium and Lanthanum on the Final-Stage Sintering Behavior of Ultrahigh-Purity Alumina

Abstract: Final-stage sintering has been investigated in ultrahigh-purity Al2O3 and Al2O3that has been doped individually with 1000 ppm of yttrium and 1000 ppm of lanthanum. In the undoped and doped materials, the dominant densification mechanism is consistent with grain-boundary diffusion. Doping with yttrium and lanthanum decreases the densification rate by a factor of ˜11 and 21, respectively. It is postulated that these large rare-earth cations, which segregate strongly to the grain boundaries in Al2O3, block the diffusion of ions along grain boundaries, leading to reduced grain-boundary diffusivity and decreased densification rate. In addition, doping with yttrium and lanthanum decreases grain growth during sintering. In the undoped Al2O3, surface-diffusion-controlled pore drag governs grain growth; in the doped materials, no grain-growth mechanism could be unambiguously identified. Overall, yttrium and lanthanum decreases the coarsening rate, relative to the densification rate, and, hence, shifted the grain-size-density trajectory to higher density for a given grain size. It is believed that the effect of the additives is linked strongly to their segregation to the Al2O3grain boundaries.

Efficacy, tolerability, and safety of lanthanum carbonate in hyperphosphatemia: a 6-month, randomized, comparative trial versus calcium carbonate.

Abstract: Background/Aims: Hyperphosphatemia is an important clinical consequence of renal failure, and its multiple adverse systemic effects are associated with significantly increased risks of morbidity and mortality in dialysis patients. Existing oral phosphate binders have not permitted control of serum phosphate within currently accepted guidelines. This study compares lanthanum carbonate with calcium carbonate for control of serum phosphate in hemodialysis patients. Methods: In this European multicentre study, 800 patients were randomised to receive either lanthanum or calcium carbonate and the dose titrated over 5 weeks to achieve control of serum phosphate. Serum levels of phosphate, calcium and parathryoid hormone were followed over the following 20 weeks. Results: Around 65% of patients in each group achieved phosphate control, but in the calcium carbonate group this was at the expense of significant hypercalcemia (20.2% of patients vs. 0.4%). Consequently, calcium x phosphate product tended to be better controlled in the lanthanum group. Conclusion: This 6-month study demonstrates that serum phosphate control with lanthanum carbonate (750–3,000 mg/day) is similar to that seen with calcium carbonate (1,500–9,000 mg/day), but with a significantly reduced incidence of hypercalcemia. Lanthanum carbonate is well tolerated and may be more effective in reducing calcium x phosphate product than calcium carbonate.

Structural Characterization of a Lanthanum Bistriflimide Complex, La(N(SO2CF3)2)3(H2O)3, and an Investigation of La, Sm, and Eu Electrochemistry in a Room-Temperature Ionic Liquid, [Me3NnBu][N(SO2CF3)2]

Abstract: The reduction of selected lanthanide cations to the zerovalent state in the room-temperature ionic liquid [Me3NnBu][TFSI] is reported (where TFSI = bistriflimide, [N(SO2CF3)2]-). The lanthanide cations were introduced to the melt as the TFSI hydrate complexes [Ln(TFSI)3(H2O)3] (where Ln = LaIII, SmIII or EuIII). The lanthanum compound [La(TFSI)3(H2O)3] has been crystallographically characterized, revealing the first structurally characterized f-element TFSI complex. The lanthanide in all three complexes was shown to be reducible to the metallic state in [Me3NnBu][TFSI]. For both the Eu and Sm complexes, reduction to the metallic state was achieved via divalent species, and there was an additional observation of the electrodeposition of Eu metal.

Lanthanum and nitrogen co-doped SrTiO3 powders as visible light sensitive photocatalyst

Abstract: Lanthanum and nitrogen co-doped SrTiO 3 was prepared by a mechanochemical reaction method using SrTiO 3 , urea and La 2 O 3 as the raw materials Nitrogen-doped SrTiO 3 could be prepared by heating the mixture of SrTiO 3 and La 2 O 3 under flowing NH 3 gas at 600 °C The sample prepared with 02 mol% La 2 O 3, 22 mol% urea and 778 mol% SrTiO 3 which has nearly the same nitrogen and lanthanum doping atomic fractions could be obtained by a mechanochemical method and exhibited high photocatalytic activities Under the irradiation of light with wavelengths larger than 400 and 290 nm, the photocatalytic activity of nitrogen and lanthanum co-doped SrTiO 3 was 26 and two times greater than that of pure SrTiO 3 A new absorption edge, which was formed in the visible light region and higher visible light absorption after co-doping were the reason for the high visible light photocatalytic activity of this substance

Dopant distributions in rare-earth-doped alumina

Abstract: The distribution of yttrium and lanthanum dopants has been mapped in yttrium- and lanthanum-doped polycrystalline aluminas using imaging secondary-ion mass spectrometry (imaging-SIMS). Both dopants segregate to grain boundaries and pore surfaces. On average, yttrium occupies 7.1%–9.0% of the available grain-boundary cation sites, whereas lanthanum occupies only 2.0%–5.2%. In 1000-ppm-yttrium-doped alumina, an abundance of yttrium aluminum garnet precipitates also is observed. Implications of these observations to the creep behavior of alumina are discussed. The similarity in the segregation behavior of yttrium and lanthanum highlights the potential of lanthanum-doped alumina for improved creep properties.

Physicochemical and catalytic properties in methane combustion of La1−xCaxMnO3±y (0 ≤ x ≤ 1; −0.04 ≤ y ≤ 0.24) perovskite-type oxide

Abstract: The objective of this work is to understand the role of physicochemical properties of pure and calcium substituted lanthanum manganite perovskite with a particular attention to the importance of the degree of non-stoichiometry. The La1−xCaxMnO3±y (0 ≤ x ≤ 1; −0.04 ≤ y ≤ 0.24) oxides were prepared by thermal decomposition of a liquid precursor of lanthanum and calcium nitrates and manganese acetate followed by calcination for 24 h at 973 K. XRD analysis showed that all samples were single-phase perovskite-type oxides with distorted structure depending on the substitution level. Presently, we have investigated the catalytic activities in methane combustion of this group of oxides. Bulk and surface physicochemical characteristics were determined and reported in this study. XPS analysis revealed the significant changes in the La3d, 4d; Ca2p; O1s and Mn2p characteristics as well as the excess of concentration with respect to the bulk resulting from the incorporation of Ca in La sublattice. Substitution with calcium affected significantly the surface specific area which ranged in 6–17 m2/g with a maximum value for an optimal composition x = 0.6. The light-off temperature for 50% methane conversion was found to be the lowest for La0.4Ca0.6MnO3.03 composition. Samples with oxygen overstoichiometry showed better catalytic performances (specific and areal activities) than those with oxygen substoichiometry. An optimal fraction, x = 0.6 of Ca enhanced significantly these performances.

Sol–Gel Synthesis of Perovskite-Type Lanthanum Manganite Thin Films and Fine Powders Using Metal Acetylacetonate and Poly(vinyl alcohol)

Abstract: A novel sol–gel process was developed for preparing thin films and fine powders of lanthanum manganite (LaMnO3) with perovskite-type structure. Lanthanum and manganese acetylacetonate powders could be easily dissolved in a mixed solution of methyl alcohol and ethylene glycol (or methyl alcohol and propionic acid) to give a spinnable organic solution as a starting material. Pretreatment of a substrate by poly(vinyl alcohol) (PVA) aqueous solution, a gel former, made it possible to deposit a thin film as well as to lower the sintering temperature.

Lanthanum Chromite-Based Interconnects as Key Materials for SOFC Stack Development

Abstract: The historical investigations on the physicochemical and transport properties of lanthanum chromite-based perovskite oxides are reviewed to evaluate the compatibility as interconnects in solid oxide fuel cells. These materials improve sinterability in air. This has led to investigations on other physicochemical properties of these materials, such as thermal expansion, mechanical strength, and chemical stability. Lanthanum and chromium ion can be substituted by alkaline earths or transition metals, and this translates into a large flexibility in physicochemical properties. However, the formation of oxygen vacancies in a reducing atmosphere can result in an undesirable isothermal expansion or oxygen permeation. The chemical stability of these materials is governed by the fast cation diffusion at the grain boundary of the polycrystals.

Ionic conductivities and phase transitions of lanthanide rare-earth substituted La2Mo2O9

Abstract: The ion conductivities and phase transitions of lanthanum molybdate (La2Mo2O9) substituted with lanthanide rare-earths are investigated using impedance spectroscopy, dilatometry, and X-ray powder diffraction. Among the substituted La2Mo2O9 of 10 mol% Ce, Nd, Sm, Gd, Dy, Er, Yb, the specimens containing Er, and Dy exhibit depressed α–β phase transformation and high conductivities. Their 700 °C conductivities are approximately five to seven times that of La2Mo2O9, around 0.26 S cm−1, comparable with those of (LaSr)(GaMg)O3 and Gd-substituted CeO2. Among the three compositions of 10 mol% Gd, Dy, Er showing depressed phase transition, Er- and Dy-substituted La2Mo2O9 possess relatively low thermal expansion coefficient 11×10−6 K−1, compared with that of the Gd-substituted La2Mo2O9, 18×10−6 K−1, which is near that of La2Mo2O9. Hence, Dy and Er are valuable dopants in improving the La2Mo2O9 properties. Across the lanthanide series, 10 mol%-substituted La2Mo2O9 demonstrates systematic variations in the conductivity–temperature relation. Hysteresis phenomena in both of conductivity and thermal expansion are also observed in those compositions which display phase transition.

Cerium Oxide-Based Sorbents for Regenerative Hot Reformate Gas Desulfurization

Abstract: The activity and stability of lanthanum- or copper-containing cerium oxide sorbents for hot reformate gas desulfurization were investigated in this work. Reformate gas, derived from the catalytic partial oxidation or autothermal reforming of heavy fuels such as JP-8, is rich in hydrogen and CO and may also contain up to 500 ppmv H2S. Desulfurization at temperatures in the range of 650-800°C is required before using the fuel gas in solid oxide fuel cells. In this work, regenerable cerium oxide-based sorbents were used in desulfurization. The sorbents were prepared with high surface area by the urea coprecipitation/gelation method, followed by slow heating and calcination in air at 650 °C for 4 h. Lanthanum doping (up to 50 at. %) was determined to be effective in moderating the surface area loss of cerium oxide (ceria) in H2S-free reformate gas at 800 °C. On the other hand, severe sintering occurred when copper was used as an additive, even in amounts as low as 10 at. % copper in ceria. However, the copper-containing ceria had the best sulfidation kinetics among the ceria-based sorbents. Sorbents were evaluated in cyclic sulfidation/regeneration tests at 650 and 800 °C, using a simulated reformate gas mixture in sulfidation, and a 3% O2-He gas mixture for regeneration. Using very high space velocities, we determined that sulfidation could be limited to the sorbent surface. The surface sulfur capacity of the sorbents was stable in cyclic sulfidation/regeneration under these conditions.

Magnesium-Lanthanum Mixed Metal Oxide: a Strong Solid Base for the Michael Addition Reaction

Abstract: Michael additions were achieved quantitatively at room temperature with a broad spectrum of acceptors and donors, using a magnesium-lanthanum mixed oxide, a strong solid base. The best solvent for the reaction is dimethylformamide, but good yields were also obtained using toluene as solvent. The results obtained with magnesium-lanthanum are comparable to those reported for super bases. The catalyst can be separated from the reaction mixture and recycled five times without loss of activity with no other treatment.

Lanthanum additions and the toughness of ultra-high strength steels and the determination of appropriate lanthanum additions

Abstract: Studies of commercial heats of AF1410 steel suggest that under appropriate conditions additions of rare-earth elements can significantly enhance fracture toughness. This improvement in toughness is not due to an extremely low inclusion volume fraction but is apparently due to the formation of larger and more widely spaced inclusions. The purpose of this work is to discuss our experience in using rare-earth additions to laboratory scale vacuum induction melted and subsequently vacuum arc remelted heats of ultra-high strength steels to achieve inclusion distributions similar to those observed in commercial heats modified with lanthanum additions. The results indicate that lanthanum additions of 0.015 wt.% to low sulfur steels which have been well deoxidized using carbon-vacuum deoxidation can result in lanthanum rich inclusions which are similar in size, volume fraction and spacing to those obtained in commercially produced heats of ultra-high strength steel to which lanthanum has been added. The heat of steel to which lanthanum additions of 0.015 wt.% were made had significantly higher toughness than did the heat of the same steel in which the sulfur had been gettered as small and closely spaced particles of MnS and which had an inclusion volume fraction similar to that of the heat modified by the addition of 0.015 wt.% lanthanum. This improvement in toughness was attributed to an increase in inclusion spacing. An addition of 0.06 wt.% lanthanum was excessive. Such an addition of lanthanum resulted in a huge volume fraction of large cuboidal inclusions which primarily contain lanthanum and oxygen and which are extremely detrimental to toughness.

Microstructural and high-temperature electrical characterization of La1-xSrxFeO3-δ

Abstract: Strontium-doped lanthanum ferrites (LSF) were studied using scanning electron microscopy (SEM), X-ray diffraction (XRD), 4-point D.C. electrical conductivity and bulk property measurements. The results were compared to those of previous studies as well as selected processing conditions. The investigation focused on effects of sintering temperature, time, atmosphere (air, O2 and N2) and composition of La1−xSr x FeO3−δ (x = 0.2–0.9), on the sintering behavior, microstructural development and electrical conductivity. An oxalate precipitation method was used to prepare lanthanum ferrite powders. Simultaneous thermogravimetric and differential thermal analysis (TGA/DTA) studies found calcination temperatures of 800 and 850^∘C were necessary to form single-phase crystalline powders, as determined by XRD. Specimens were sintered from 1300 to 1400^∘C with dwell times from $$\frac{1}{2}$$ to 2 hrs. Results from SEM/EDS analysis showed the presence of a second phase in the samples fired in air or oxygen. The second phase was not detected by x-ray diffraction due to the small amount of material present. Samples fired in nitrogen had the lowest conductivity while those fired in oxygen had the highest. A composition of x = 0.5 resulted in the highest conductivity, 352 S/cm, at an operating temperature of 550∘C in air. High strontium additions (x = 0.9) lowered the linear shrinkage of LSF.

Synthesis by sol–gel route of oxyapatite powders for dense ceramics : applications as electrolytes for solid oxide fuel cells

Abstract: Solid oxide fuel cells have considerable interest in recent years, because of their high efficiency and environmentally friendly nature. Such systems required oxygen-conducting electrolytes and now the most common electrolyte is yttria stabilized zirconia (YSZ). This compound exhibits high oxide ion conductivity at elevated temperatures (850–1000 °C). However, this high working temperature causes problems in terms of materials selection and lifetime. One solution is to develop new oxide ions conductors exhibiting high oxide ion conductivity at intermediary temperatures (700–800 °C). Recent work has identified Ln10−xSi6O26±z (Ln = rare earths) as a good fast oxide ion conductor. Undoped and doped Ln10−xB6O26±z (B = Si or Ge) oxides are currently prepared by solid-state methods. In that work, we propose a sol–gel process to synthesize powders of La9.33Si6O26 type-silicated apatites. The main advantage is to decrease the crystallization temperature in ,comparison to the conventional methods, allowing the synthesis of reactive powders with nanometric particles size. These oxides are synthesized using silicon alkoxide and lanthanum nitride as precursors. In the litterature, no study refers to the synthesis of mixed oxides with silicon alcoxides. However, there are several studies on sol–gel synthesis of glasses with this precursor. In this study, several processing parameters have been investigated (the hydrolysis ratio, the concentration of metallic precursors in the sol and the role of organic compounds) in order to synthesize pure phases after the decomposition of the sols. Pure powders of La9.33Si6O26 type-silicated apatites are obtained at 800 °C. These powders were used to prepare ceramics. Several processing parameters as morphology of powders (agglomeration, particle sizes) and, heating profiles have been studied on the densification. Dense ceramics (90–95%) have been prepared at temperatures around 1400 °C. The used of sol–gel powders allow the decrease of the sintering temperature of about 200 °C.

Trialkylboron/lanthanide metallocene hydride chemistry: polydentate bridging of (HBEt3)- to lanthanum.

Abstract: The reaction of [(C5Me5)2LaH]x with BEt3 is reported, and the solid-state structures of the lanthanum product (C5Me5)2La[(μ-H)(μ-Et)2BEt], 1, and its THF adduct (C5Me5)2La(THF)[(μ-H)(μ-Et)BEt2], 2, are compared with that of the hydride-bridged “tuckover” complex (C5Me5)2La(μ-H)(μ-η1:η5-CH2C5Me4)La(C5Me5), 3.