Showing papers on "Lanthanum published in 1999"

Electrical and structural characteristics of lanthanum-doped barium titanate ceramics

Abstract: Single phase La-doped BaTiO3 with the formula Ba1−xLaxTi1−x/4O3: 0⩽x⩽0.20 was prepared by solid state reaction of oxide mixtures at 1350 °C, 3 days, in O2. The tetragonal distortion in undoped BaTiO3 decreased with x and samples were cubic for x⩾0.05. Both the tetragonal/cubic and orthorhombic/tetragonal transition temperatures decreased with x, but at different rates, and appeared to coalesce at x∼0.08. The value of the permittivity maximum at the tetragonal/cubic phase transition in ceramic samples increased from ∼10 000 for x=0 at 130 °C to ∼25 000 for x=0.06 at ∼−9 °C. At larger x, the permittivity maximum broadened, showed “relaxor”-type frequency dependent permittivity characteristics and continued to move to lower temperatures. Samples fired in O2 were insulating and showed no signs of donor doping whereas air-fired samples were semiconducting, attributable to oxygen loss.

Solid state 27Al NMR and FTIR study of lanthanum aluminosilicate glasses

Abstract: The structure of lanthanum aluminosilicate (LAS) glasses, containing: (15–25) mol% La2O3, (15–35) mol% Al2O3 and (40–70) mol% SiO2, were studied by 27Al nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). The effects of La2O3 and Al2O3 content on the properties and structure of the LAS glasses were evaluated. Al3+ ion enters into the glass structure mainly in fourfold co-ordination, forming ( AlO 4/2 ) − tetrahedra, and only in small amounts in fivefold and sixfold co-ordination. La3+ acts as network modifier, producing more non-bridging oxygen atoms as its content increases.

Formation and migration of cation defects in the perovskite oxide LaMnO3

Abstract: Atomistic simulation techniques have been employed to investigate the energetics of cation formation and migration in cubic, rhombohedral and orthorhombic LaMnO 3 . The calculations suggest that for rhombohedral and orthorhombic lanthanum manganite, oxidative nonstoichiometry leads to the formation of cation vacancies on both La and Mn sites, though tending towards more La vacancies. The activation energy for lanthanum vacancy migration was found to increase with departure from cubic perovskite symmetry in the order: cubic cubic directions. Calculated migration energies for this path also increased with distortion from the cubic form. The effect of composition on cation migration energies was also examined.

Calcium block of Na+ channels and its effect on closing rate.

Abstract: Calcium ion transiently blocks Na+ channels, and it shortens the time course for closing of their activation gates. We examined the relation between block and closing kinetics by using the Na+ channels natively expressed in GH3 cells, a clonal line of rat pituitary cells. To simplify analysis, inactivation of the Na+ channels was destroyed by including papain in the internal medium. All divalent cations tested, and trivalent La3+, blocked a progressively larger fraction of the channels as their concentration increased, and they accelerated the closing of the Na+ channel activation gate. For calcium, the most extensively studied cation, there is an approximately linear relation between the fraction of the channels that are calcium-blocked and the closing rate. Extrapolation of the data to very low calcium suggests that closing rate is near zero when there is no block. Analysis shows that, almost with certainty, the channels can close when occupied by calcium. The analysis further suggests that the channels close preferentially or exclusively from the calcium-blocked state.

Cation clustering and formation of free oxide ions in sodium and potassium lanthanum silicate glasses: nuclear magnetic resonance and Raman spectroscopic findings

Abstract: Alkali silicate glasses containing lanthanum oxide are useful model systems for understanding the structural role of rare earth cations in optical and other types of materials. We report 29 Si and Raman spectra of sodium and potassium silicate glasses, both with added La 2 O 3 and with La 2 O 3 substituted for Na 2 O or K 2 O on an equal-oxygen basis. In the former series, silicate speciation changes show the formation of more non-bridging oxygens (NBO) as more of the network-modifying La 2 O 3 is added. In the latter series, however, in which the nominal ratio of NBO to Si is constant, silicate speciation changes indicate that the actual ratio decreases significantly as La is substituted for 3 Na or K. The simplest explanation of this finding is that up to several percent of the oxygen in the La-rich glasses is not bonded to any Si, but instead forms `free oxide' ions that are part of La-rich domains. Although the size of these domains remains unconstrained, the lack of evidence for phase separation and continuity of trends in structure with composition suggests that the metastable liquid structure at the glass transition contains substantial intermediate-range heterogeneity.

Interfacial Reaction between Nickel Oxide and Lanthanum Gallate during Sintering and its Effect on Conductivity

Abstract: The results of interdiffusion and reaction between NiO and strontium- and magnesium-doped lanthanum gallate (LSGM) during sintering were investigated. Electron microprobe analysis showed that the NiO content of LSGM sintered 4 h at 1350°C was slightly >2 wt% within 5 µm of the interface, which is equivalent to 7 mol% nickel on B cation sites. This concentration decreased rapidly to <0.25% at a distance of 15 µm. The interfacial reaction between NiO and LSGM, however, led to the formation of a LaSrGa(Ni)O4-delta-type phase. A separate experiment involving conductivity measurements of nickel-doped LSGM indicated that the oxygenion conductivity did not decrease significantly if the nickel content was <5%-8%. At higher nickel contents, the conductivity increased in air because of hole conduction and markedly decreased under conditions of low oxygen pressure because of the formation of the LaSrGa(Ni)O4-delta-type phase. The nickel ions in the LSGM electrolyte and in the reaction product, LaSrGa(Ni)O4-delta, were stable as Ni2+ at reduced oxygen pressures and, therefore, did not cause electronic shorting.

Electrochemical Studies of LaCl3 and GdCl3 Dissolved in Fused LiCl‐KCl

Abstract: Molten systems are important as reaction media for the winning of strong oxidation or reducing elements such as fluorine, alkaline, and alkaline earth metals and aluminum. Recently, fused salt electrochemistry has been used for the preparation of refractory metals opening the way to the deposition of metals in solid state. The thermodynamic properties of solutions of lanthanum or gadolinium chlorides in mixed molten LiCl-KCl were determined by electromagnetic force (emf) measurements of LaCl{sub 3} or GdCl{sub 3} formation cells. The temperature dependence of the emf between 380 and 590 C was used to calculate the Gibbs energy and the enthalpy of formation of dissolved LaCl{sub 3} or GdCl{sub 3}. The electrode processes of La(III) and Gd(III) reduced on a tungsten electrode were investigated by cyclic voltammetry and chronopotentiometry.

Rare earth permanent magnet and method for making same

Abstract: A rare earth permanent magnet alloy having a composition expressed as RxF?100-(x+y+z+m+n)?ByTzMmDn. In this formula, R is one or more of rare earthy elements, such as neodymium, lanthanum, cerium, dysprosium and/or praseodymium; F is Fe or Fe and up to 20 atomic percent of Co by substitution; B is boron; T is one or more elements selected from the group of Ti, Zr, Cr, Mn, Hf, Nb, V, Mo, W and Ta; M is one or more elements selected from the group of Si, Al, Ge, Ga, Cu, Ag, and Au; and D is one or more elements selected from the group of C, N, P, and O. In this formula, x, y, z, m, n are atomic percentages in the ranges of 3∫x∫15, 4∫y∫22, 0.5∫z∫5, 0.1∫m∫2, and 0.1∫n∫4. Fine amorphous particles of such alloy are made by atomization and/or splat-quenching. Both substantially-spherical, irregular and substantially plate-like particles are simultaneously produced.

Destructive adsorption of carbon tetrachloride on lanthanum and cerium oxides

Abstract: The destructive adsorption of CCl4 on La2O3 and CeO2 in the absence of any oxidant, such as oxygen, has been studied by X-ray photoelectron spectroscopy and insitu Raman spectroscopy as a function of the reaction temperature and the amount of CCl4 injected. La2O3 was much more reactive than CeO2, and CCl4 destruction started at around 300°C with the rapid formation of LaOCl, and the release of CO2 into the gas phase. The complete transformation of LaOCl into LaCl3 was much more difficult to obtain, and required high reaction temperatures and large amounts of CCl4. In the case of CeO2, CCl4 destruction started at around 450°C, and was accompanied by the reduction of Ce(IV) to Ce(III) and the formation of CeOCl as an intermediate product. The complete transformation of CeO2 into CeCl3 was only observed at reaction temperatures near 600°C. These results are compared with those recently reported for alkaline earth metal oxides.

Removal of Arsenic(III) and Arsenic(V) Ions from Aqueous Solutions with Lanthanum(III) Salt and Comparison with Aluminum(III), Calcium(II), and Iron(III) Salts

Abstract: Interactions of arsenic(III) and arsenic(V) ions with a lanthanum salt were studied with the aim of developing a new precipitation method for removal of arsenic from aqueous solutions. Performance was compared to those of aluminum, polyaluminum chloride (PAC), calcium, and iron(III) salts. Arsenic(III) was removed by iron(III) and lanthanum in a narrow pH range with less than 605 removal. Arsenic(V) was removed more efficiently by aluminum, PAC, iron(III), and lanthanum. Lanthanum was most effective, meeting Japanese effluent and drinking water standards by adding three times as much lanthanum as arsenic(V).

The Effect of Lanthanum on the Scales Developed on Thin Foils of Fe-20Cr-5Al at Very High Temperatures

Abstract: A study has been undertaken of the effects oflanthanum on the oxidation of thin foils of Fe-20Cr-5Alin air at 1150°C The addition of lanthanum causesthe time to breakaway to increase from about 24 hr for Fe-20Cr-5Al to over 400 hr Oxidationof the lanthanum-containing alloy occurs in threestages During the first stage, anα-Al2O3 layer is establishedand thickens with time until the aluminum in the foil is depleted sufficiently for alayer of Cr2O3 to become stableand develop at the scale-alloy interface This continuesto thicken at a relatively slow rate until breakawayoccurs The main emphasis in the present paper has been anexamination and analysis of the scale established on thelanthanum-containing alloy in cross section in theanalytical transmission electron microscope (TEM), after an exposure period that coincides with thesecond stage of oxidation, prior to breakaway The scaleat that time consists of three layers The outer layeris composed of equiaxed Al2O3grains The intermediate and inner layers consist of columnarAl2O3 grains and equiaxedCr2O3 grains, respectivelyNumerous voids are observed in the oxide grainboundaries and at the intermediate-inner layerinterface Lanthanum segregates in the oxide grain boundaries andits concentration increases toward the outermost surfaceof the scales These results are consistent with the“dynamic segregation model” to account for the effects of reactive elements on thegrowth of Al2O3 scales

Ionic/electronic mixed conduction relations in perovskite-type oxides by defect structure

Abstract: Perovskite-type oxides La 1- a A a M 1- b B b O 3- x with A=Sr 2+ , Ln 3+ , Ce 4+ and M=Fe, Co, Ga; B=Co, Fe, Mg were prepared in the concentration range a =0·1 to 1 mol and b =0·1 to 0·5 mol. Additionally, A-substoichiometric compositions were prepared. Preparation conditions for monophase materials and structure types of the perovskite were determined by X-ray investigation. The electrical conductivity as a function of pO 2 in the range 10 5 >pO 2 >10 −14 Pa and temperature (500 to 1000°C) was measured on ceramic shapes by a dc four-point technique in combination with solid electrolyte coulometry. The ionic part of conductivity in mixed conductors was determined by oxygen permeation measurements. The II–III-perovskites Sr(Co,Fe)O 3- x in their stabilized form are excellent mixed conductors (maximum 500 S cm −1 at 400°C) and have up to 2 orders of magnitude higher oxygen ionic conductivity than the preferred III–III-perovskite La(Sr)Mn(Co)O 3- x . The oxygen ionic conductivity of the electrolyte La(Sr)Ga(Mg)O 3- x , was increased by doping with 0·1 mol Co. By applying higher Co or Fe doping concentrations the lanthanum gallate, becomes a mixed conductor.

Stability of calcium substituted lanthanum chromites used as SOFC anodes for methane oxidation

Abstract: La 0·7 Ca 0·32 CrO 3 , La 0·75 Ca 0·15 Sr 0·1 Cr 0·95 Mg 0·05 O 3 , La 0·85 Ca 0·15 CrO 3 and La 0·85 Ca 0·15 Cr 0·9 Mg 0·1 O 3 compounds were studied in reducing atmospheres containing hydrogen, methane and CO/CO 2 . They were found to exhibit liquid phase sintering except for La 0·85 Ca 0·15 CrO 3 . Mg enhanced the densification of stoichiometric LaCrO 3 . Current was found to affect the stability of the heavily doped samples, whereas the reducing atmospheres did not have a detectable impact. Calcium and or strontium and chromium enriched secondary phases were detected on current treated La 0·7 Ca 0·32 CrO 3 and La 0·75 Ca 0·15 Sr 0·1 Cr 0·95 Mg 0·05 O 3 samples. La 0·85 Ca 0·15 Cr 0·9 Mg 0·1 O 3 did not degrade under current, at least for 100 h. A current enhanced demixing could be responsable for the segregation. The solubility limit of calcium in LaCrO 3 is thought to be low at 800°C (around 15%).

C 2 -symmetric bis(oxazolinato) complexes of yttrium and lanthanum†‡

Abstract: Chiral 2,2′-methylene-bis[(4S)-4-tert-butyl-2-oxazoline] reacts with Ln[N(SiHMe2)2]3(thf)2 (Ln = Y, La) via an extended silylamide route to yield the first mono- and bis-(bisoxazolinato) complexes, potentially relevant to enantioselective catalysis.