Showing papers on "Lanthanum published in 1991"

Fullerenes with metals inside

Abstract: Fullerenes with a single lanthanum atom trapped on the inside of the carbon cage were produced by laser vaporization of a lanthanum oxide/graphite composite rod in a flow of argon gas at 1200 °C. When sublimed with C_(60) and C_(70), they formed an air-stable film containing principally LaC_(60), LaC_(70), LaC_(74), and LaC_(82). When dissolved in toluene and exposed to air, LaC_(82) was found to be uniquely stable. Evidence was also obtained for coalescence reactions between these fullerenes at high temperatures to form larger cages with as many as three lanthanum atoms inside. Indications have also been obtained for the successful production of KC_(60), C_(59)B, and KC_(59)B where the boron has substituted for a carbon in the soccerball cage. The use of the @ symbol is advocated for specifying such complex fullerenes as (K@C_(59)B).

Dielectric Characteristics of (A1/21+·A1/23+)TiO3 Ceramics at Microwave Frequencies

Abstract: We have investigated the dielectric characteristics at microwave frequencies of perovskites with the formula (A1/21+A1/23+)TiO3, (where A1+ represents an alkali metal from the lithium to potassium series and A3+ represents a member of the lanthanide series from lanthanum to lutetium). It was found for the first time that in the case of Li1+ substitution at the A1+ site, the larger the ionic radius of the element which substituted at the A3+ site, the higher the dielectric constant was and the greater the negative shift of the temperature coefficient of resonant frequency was. For the (1-x)(Li1/21+Sm1/23+)TiO3-x(Na1/21+Sm1/23+)TiO3 system at 3 GHz, high dielectric properties of er=81, Q=2050 and τf=+17ppm/ °C were obtained when x=0.4.

Thermal stability and its improvement of the alumina membrane top-layers prepared by sol-gel methods

Abstract: The thermal stability of unsupported alumina membrane top-layers was studied by determining the pore structure (mainly pore size) change of alumina gels, prepared by sol-gel methods, after sintering at different temperatures ranging from 450 to 1200 °C. The average pore size of the pure alumina membranes and PVA-added membranes increased sharply after sintering at temperatures higher than 1000 °C. Addition of 3% lanthanum, either by mixing lanthanum nitrate in the alumina sol or impregnating lanthanum nitrate into calcined alumina gel, followed by a second heat treatment, can considerably stabilize the pore structure of the alumina membrane top-layers. The pore diameter for the lanthanum-doped membranes was stabilized within 25 nm after sintering at 1200 °C for 30 h, about one-sixth of that for the pure alumina membranes after sintering at 1200 °C for 30 h. The substantial increase in the pore size for the pure alumina membranes at the sintering temperature of 1000 to 1200 °C was accompanied by the phase transformation from γ-to α-alumina. The addition of lanthanum can raise this phase transformation temperature by about 200 °C.

Oxidative coupling of methane with peroxide ions over barium-lanthanum-oxygen mixed oxide

Abstract: The oxidative coupling of methane on a mixed oxide of La2O3 and an alkaline earth metal oxide was investigated. The formation and decomposition behaviors of surface oxygen species were studied by measuring the weight change in the oxygen atmosphere with thermogravimetry (TG) and by recording X-ray photoelectron spectroscopy (XPS). An active and selective catalyst for methane coupling was prepared by mixing La2O3 with 15 atom-% BaO (15BLO). The XPS study showed the existence of O22− species on the surface of La2O3 and 15BLO. The barium species were well dispersed in 15BLO and they increased the concentration of the O22− species. The high catalytic ability of 15BLO is ascribed to this dispersed O22− species on the surface. On the other hand, La2O3 with 50 atom-% BaO (50BLO) exhibited little activity in spite of a larger content of O22− than 15BLO, because the dispersion of the O22− species was poor. In the actual methane coupling reaction, the surface O22− would be decomposed into active oxygen species, probably 20−, before interacting with methane. The importance of barium dispersion may relate to this step of O22− splitting.

Tape casting and sintering of strontium-doped lanthanum chromite for a planar solid oxide fuel cell bipolar plate

Abstract: Nonagglomerated strontium-doped lanthanum chromite powders were prepared by a modified Pechini resinintermediate process and tape cast to form bipolar plates for a planar solid oxide fuel cell. An air-sintering technique for the strontium-doped lanthanum chromite was developed, which involved placing the green tape between Cr{sub 2}O{sub 3}-fired plates. The sintering process was found to be diffusion controlled, with densification beginning at the surface and proceeding to the interior. A bipolar plate of 2-mm thickness was fired to more than 9.3.5% theoretical density when fired at 1670{degrees}C for 7 h.

Application of lanthanum to pseudo-boehmite and γ-Al2O3

Abstract: Elements such as lanthanum, zirconium, thorium and cerium are known to raise the thermostability of γ-Al2O3. Among these elements lanthanum is the most effective. Three procedures have been used to apply lanthanum homogeneously to the surface of pseudo-boehmite and γ-Al2O3, namely, incipient wetness impregnation, deposition-precipitation from a homogeneous solution, and specific adsorption of a La(EDTA)-complex. The latter procedure results in supports covered homogeneously with lanthanum. The minimum amount of lanthanum required to render the catalyst supports thermostable, depends on the procedure to apply the lanthanum. The specific adsorption procedure offers the possibility to obtain thermostable supports with loadings as low as 0.5 wt % La.

Surface processes of H2 in the initial activationof LaNi5

Abstract: The surface processes of H2 molecules in the initial activation of LaNi5 are demonstrated quantitatively by systematic measurements of the rate of H2 absorption by LaNi5 with various surface conditions prepared in ultrahigh vacuum, low vacuum and air in the H2 pressure range 10−10 mbar-101 bar. The reaction probability of H2 with LaNi5 is reduced from 1 to 10−3 at 298 K when the vacuum condition in the degassing treatment of LaNi5 is changed from 10−10 to 10−5 mbar. The dissociation of H2 molecules is markedly inhibited by the oxides and hydroxides of lanthanum and nickel which are formed during exposure of LaNi5 to low vacuum or air. Increasing amounts of these surface layers reduce the reaction probability to 10−6. However, the kinetic data indicate that the dissociation of H2 molecules can take place on lanthanum oxides. The addition of a small quantity of nickel to lanthanum oxides markedly facilitates H2 dissociation and subsequent hydrogen permeation through the lanthanum oxide layers. The presence of suboxides such as LaOx, La2O3−x or La−NiOx in the surface region seems to be responsible for the ready activation of LaNi5 exposed to low vacuum or air. The activation model proposed in this report does not require the presence of metallic nickel or nickel clusters in the surface region.

Preparation and characterization of zirconium dioxide catalyst supports modified with rare earth elements

Abstract: ZrO2 catalyst supports modified with rare earth elements were prepared by coprecipitation from an aqueous solution of zirconium oxychloride and rare earth chlorides. The crystallization of amorphous hydrous ZrO2 was inhibited by doping with rare earths; the crystallization temperature was elevated as the amount and ionic radius of the rare earth modifiers was increased. Only modification using cerium had no effect on the crystallization process. The behavior of cerium was different from that of other rare earth elements with valency + 3. A metastable cubic phase was formed for ZrO2 modified with 10 mol.% lanthanum, neodymium and samarium by heating at 600°C. X-ray diffraction and Raman data indicated that the metastable phase had large microstrain and short-range ordering similar to tetragonal symmetry. Rare earth modified ZrO2 showed a large surface area and good thermal stability as a catalyst support. The carbon monoxide oxidation activity of iron was enhanced by modification with neodymium of ZrO2 supports. The results suggest the effectiveness of rare earth modified ZrO2 as catalyst supports.

Dispersion deposition of metal — Particle composites and the evaluation of dispersion deposited nickel — Lanthanum nickelate electrocatalyst for hydrogen evolution

Abstract: A method for the preparation of high surface area coatings and its application towards fabrication of prospective composite electrocatalysts for hydrogen evolution is proposed. Exploratory studies with such a composite, (Ni/LaNiO3), as a hydrogen electrode in alkaline solutions indicate that it is a more active electrocatalyst than sintered or electrodeposited nickel. The electrocatalytic properties were found to be a strong function of the deposition parameters as well as of the history of the oxide powder.

Preparation of Calcium Lanthanum Sulfide Powders via Alkoxide Sulfurization

Abstract: This paper reports that calcium lanthanum sulfide powders are prepared by reacting methoxides of calcium and lanthanum with H[sub 2]S. Alkoxide mixtures of various La/Ca ratios dispersed in methanol were reacted with H[sub 2]S at 25[degrees] to 85[degrees] C and the amorphous gels obtained after the removal of methanol were heat-treated in H[sub 2]S for full sulfurization at various temperatures. Up to 500[degrees] C, cubic LaS[sub 2] was the only crystalline phase present. Calcium lanthanum sulfide phase started to form at 550[degrees] C. In the temperature range of 650[degrees] to 750[degrees] C, single-phase calcium lanthanum sulfide powders were obtained for La/Ca = 2.41 and 2.68.

Lanthanum scandoborate as a new highly efficient active medium of solid-state lasers

Abstract: The properties of lanthanum scandoborate activated with chromium and neodymium ions, representing a new active material, were investigated for the first time. The high activator concentrations (up to 100% of neodymium with hardly any concentration quenching of the luminescence), the large cross section of the lasing transition, and the efficient sensitization in combination with a high optical quality of the new crystal ensured a high efficiency of lasing of the neodymium activator. Several comparative lasing tests showed that lanthanum scandoborate was twice and six times as efficient as yttrium scandium gallium and yttrium aluminum garnets, respectively.

Evidence for a ‘coherence’ gap in Ce3Bi4Pt3

Abstract: We present the results of electronic transport, magnetic susceptibility, and thermodynamic measurements on single crystals of the cubic compound Ce3Bi4Pt3. The susceptibility displays a broak peak centered at Tmax = 80K, which in Ce compounds is usually indicative of mixed valence and moderately heavy-electron behavior. Both the electronic transport properties and the low-temperature specific heat indicate that Ce3Bi4Pt4 is nonmetallic with a transport energy gap of roughly 35 K. We argue that this temperature is close to that scale over which coherence should develop in a periodic metallic system with Tmax = 80K. Resistivities typical of a Kondo-impurity system are realized when the compound is alloyed with moderate amounts of lanthanum. Further, lanthanum substitution increases the low-temperature specific heat to a value consistent with the single-ion energy scale determined from the magnetic susceptibility.

Synthesis and Characterization of Barium Lanthanum Titanates

Abstract: Ternary compounds in the system BaO—TiO2—La2O3 were prepared by the solid-state reaction technique at temperatures between 1300° and 1400°C using precursor oxides as the starting materials. In an alternative processing technique, BaTiO3 was reacted with appropriate proportions of prefabricated lanthanum titanates at 1350°C to obtain the compounds. Two compounds were identified in the TiO2-rich region of the system. The X-ray powder diffraction pattern of a compound with a chemical composition BaLa2Ti3O10 (BaO·La2O3·3TiO2) is indexed on the basis of an orthorhombic unit cell with a= 7.665 × 10−1 nm, b= 28.524 × 10−1 nm, and c= 3.876 × 10−1 nm. The other compound, which has a chemical composition Ba4La8Ti17O50 (BaO·La2O3·4.25TiO2) occurs in a narrow homogeneity range within the system. The X-ray powder diffraction pattern of the compound is indexed on the basis of an orthorhombic unit cell with a= 12.317 × 10−1 nm, b= 22.394 × 10−1 nm, and c= 3.881 × 10−1 nm. Both the compounds are compatible with BaTiO3 and form pseudobinary joins with BaTiO3 in the system BaO—TiO2—La2O3.

Vibrational spectra of high-silica glasses of the system K2OSiO2La2O3

Abstract: Parallel and perpendicular-polarized Raman spectra and KBr transmission IR spectra of glasses of the compositions 10K2O·50SiO2−nLa2O3 (n = 0, 1, 5, 10 mol) and (10−x) K 2 O −50 SiO 2 −( x 3 ) La 2 O 3 (x = 0, 6, 7.5) are presented. Increasing lanthanum concentration in glasses with fixed K:Si produces partially polarized high-frequency bands at 1030, 940 and 860 cm−1, in order of appearance. These are assigned to the symmetric stretching modes of SiO bonds in SiO4 tetrahedra containing 1, 2 and 4 non-bridging oxygens, respectively, whose non-bridging oxygens coordinate with lanthanum. Since no disilicate or metasilicate crystals occur in the La2O3SiO2 binary, lanthanum coordinates with silicate anions in glasses inaccessible to it in binary silicate crystals. Lanthanum forms coordination polyhedra of non-bridging oxygen without the aid of potassium; therefore, the solution chemistry of lanthanum in these glasses is governed by homogeneous equilibria between different silicate anions bonded to lanthanum. Glass-glass phase separation of 10K2O50SiO210La2O3 when heated to 900°C indicates that these homogeneous equilibria produce regions within the glass relatively rich in LaOSi bonds but depleted in SiOSi linkages.

Process for obtaining at least one tertiary olefin by decomposition of the corresponding ether

Abstract: A process for obtaining a tertiary olefin, e.g. isobutylene in a very pure state by decomposing the corresponding ether, e.g. methyl tert. butyl ether, in the presence of a catalyst constituted by silica, modified by the addition of at least one element or compound of an element preferably chosen from the group constituted by rubidium, cesium, magnesium, calcium, strontium, barium, gallium, lanthanum, cerium, praseodymium, neodymium and uranium and optionally by the addition of at least one element or compound of an element chosen from the group constituted by aluminium, titanium and zirconium.

Stimulated emission spectra of powders of double sodium and lanthanum tetramolybdate

Abstract: A study was made of the stimulated emission spectra of polycrystalline samples of Na5NdxLa1–x (MoO4)4 with diiferent neodymium concentrations. The stimulated emission wavelength of a mixture of powders with different concentrations of neodymium and slightly different spectra depended on the pump wavelength, indicating a mutual influence of the powder grains in the process of generation of stimulated radiation.

Exhaust gas purifying catalyst excellent in heat resistance and its preparation

Abstract: PURPOSE:To develop a catalyst showing excellent purifying capacity to the exhaust gas discharged from the internal combustion engine of a car even at low temp. and even after high temp. endurance. CONSTITUTION:An exhaust gas purifying catalyst is constituted by supporting at least one kind of a platinum group element, activated alumina, cerium oxide, zirconium oxide stabilized with cerium oxide and oxide of one or more kinds of a metal selected from lanthanum, yttrium, neodymium and an element selected from the Groups IIA, IIIB and an arbitrary zirconium compound on a support having a monolithic structure. In the case of the evaluation on a purifying rate at low temp. after high temp. endurance, this catalyst can enhance a prifying rate against all of prescribing substances (CO, HC, NOx) as compared with a conventional catalyst.

Magnetic materials and process for producing the same

Abstract: This invention relates to a process for producing a rare earth-containing powder comprising crushing a rare earth-containing alloy in water, drying the crushed alloy material at a temperature below the phase transformation temperature of the material, and treating the crushed alloy material with a passivating gas at a temperature from the ambient temperature to a temperature below the phase transformation temperature of the material. Rare earth-containing alloys suitable for use in producing magnets utilizing the powder metallurgy technique, such as Nd-Fe-B and Sm-Co alloys, can be used. The passivating gas can be nitrogen, carbon dioxide or a combination of nitrogen and carbon dioxide. If nitrogen is used as the passivating gas, the resultant powder has a nitrogen surface concentration of from about 0.4 to about 26.8 atomic percent. Moreover, if carbon dioxide is used as the passivating gas, the resultant powder has a carbon surface concentration of from about 0.02 to about 15 atomic percent. The present invention further relates to the production of a permanent magnet comprising the above steps for producing the rare earth-containing powder, and then compacting the powder, sintering the compacted material at a temperature of from about 900° C. to about 1200° C., and heat treating the sintered material at a temperature of from about 200° C. to about 1050° C. An improved permanent magnet in accordance with the present invention includes the type of permanent magnet comprised of, in atomic percent of the overall composition, from about 12% to about 24% of at least one rare earth element selected from the group consisting of neodymium, praseodymium, lanthanum, cerium, terbium, dysprosium, holmium, erbium, europium, samarium, gadolinium, promethium, thulium, ytterbium, lutetium, yttrium, and scandium, from about 2% to about 28% boron and at least 52% iron, wherein the improvement comprises a nitrogen surface concentration of from about 0.4 to about 26.8 atomic percent. The improved permanent magnet can also have a carbon surface concentration of from about 0.02 to about 15 atomic percent if carbon dioxide is used as a passivating gas.