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 1,693 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Ultrafine mixed Fe-Mo oxide particles are prepared by the sol-gel technique using citric acid as a complex agent. The formation process of the ultrafine oxides is studied by using XRD, DTA-TG, FT-IR, TEM, and BET surface area measurement. It is found that the morphology and structure of the oxide particles are significantly dependent on the process parameters such as thermal treatment temperature (T), pH of the starting solution, and the molar ratio of citric acid to metallic ions (L/M). The formation temperature of crystalline Fe(2)(MoO(4))(3) by the sol-gel process is much lower than that by the coprecipitation method. The catalytic properties of ultrafine Fe-Mo oxide particles are tested and compared with those of large oxide particles having the same composition. It is shown that for selective oxidation of toluene to benzaldehyde the ultrafine Fe-Mo oxide particles exhibit higher benzaldehyde selectivity and toluene conversion than the large oxide particles. The unique catalytic properties of ultrafine Fe-Mo oxide particles may be correlated to the higher mobility of lattice oxygen species in the ultrafine oxide particles and their higher BET surface area. Copyright 1999 Academic Press.
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PMID:Preparation, Characterization, and Catalytic Properties of Ultrafine Mixed Fe-Mo Oxide Particles. 1041 72

The electrokinetic behavior and stability of aqueous suspensions containing serpentine minerals of different deposits have been studied. Crystallochemical analyses carried out by X-ray diffraction, IR spectroscopy, TEM, DTA/TG, and XRF revealed that all samples are lizardite-type serpentines with a similar chemical composition. It has been determined that the electrokinetic behavior of lizardite aqueous suspensions is mainly a function of the Mg/Si atomic ratio on surface. So, the low isoelectric point observed in some samples has been linked to values of this ratio lower than that of the ideal lizardite (Mg/Si=1.5). Dissolution studies have shown that the removal of Mg cations from the solid at speeds faster than that theoretically expected (Mg/Si=1.5), i.e., incongruent dissolution, is responsible for the enrichment of Si cations on surface. Therefore, it has been clearly established that the surface charge value developed in the lizardite/aqueous electrical interface is a function of the lizardite surface alteration grade. Copyright 2000 Academic Press.
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PMID:Surface Instability of Serpentine in Aqueous Suspensions. 1108 62

The barium ferrite ultrafine particles were synthesized by coprecipitation in an aqueous solution with polyacrylic acid (PAA) as a protective agent. Thermal analysis by TGA/DTA showed that the precursor could yield barium ferrite after calcination above 700 degrees C for 2 h. By analyses of the XRD and electron diffraction pattern, the formation of pure barium ferrite was confirmed and the appropriate molar ratio of Ba/Fe in aqueous solution was determined to be 1/11. The TEM measurements indicated that the average diameter of the precursor was 4.5 nm, and the diameters of the particles calcined at 700 and 800 degrees C were 23-34 and 49-82 nm, respectively. The magnetic properties characterized by a SQUID magnetometer showed that the barium ferrite ultrafine particles calcined at 700-800 degrees C had a saturation magnetization of 36.9-60.8 emu/g, a remanent magnetization of 19.0-31.0 emu/g, a coercivity of 117.3-221.8 Oe, and a squareness ratio of 0.51. The magnetization was also observed to increase with a decrease of temperature at 5-400 K. These magnetic properties all reflected the nature of ultrafine particles and also were influenced by the morphology and microstructure of final products. Copyright 2001 Academic Press.
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PMID:Synthesis of Barium Ferrite Ultrafine Particles by Coprecipitation in the Presence of Polyacrylic Acid. 1123 38

The strontium ferrite ultrafine particles have been prepared using the microemulsion processing. The mixed hydroxide precursor was obtained via the coprecipitation of Sr(2+) and Fe(3+) in a water-in-oil microemulsion of water/CTAB/n-butanol/isooctane. According to the investigation on the thermochemical properties by TGA/DTA and the phase analysis by XRD, it was shown that the precursor could yield pure strontium ferrite after calcination at 700 degrees C for 5 h while using an appropriate molar ratio of Sr/Fe in microemulsions. From TEM measurement, the diameters of the precursor and calcined particles were 3.8+/-0.7 and 50-100 nm, respectively. The magnetic properties characterized by a SQUID magnetometer showed that the saturation magnetization, remanent magnetization, coercivity, and squareness ratio were 55 emu/g, 28 emu/g, 492 Oe, and 0.51, respectively. The magnetization was also observed to increase with the decrease of temperature at 5-400 K. Compared with those reported earlier, the quite low coercivity implies the potential application of final product in the high-density perpendicular recording media. Copyright 2001 Academic Press.
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PMID:Synthesis of Strontium Ferrite Ultrafine Particles Using Microemulsion Processing. 1125 26

Germanium nanowires, ranging from 10 to 150 nm in diameter, were grown several micrometers in length in cyclohexane heated and pressurized above its critical point. Alkanethiol-protected gold nanocrystals, either 2.5 or 6.5 nm in diameter, were used to seed wire formation. Growth proceeded through a solution-liquid-solid mechanism at growth temperatures ranging from 300 to 450 degrees C. At temperatures exceeding 500 degrees C, large Ge particulates formed due to unfavorable growth kinetics. Temperature, the nature of the precursor, precursor concentration, and the Au:Ge ratio were determining factors in nanowire morphology. The Ge nanowires were characterized using a range of techniques, including XPS, XRD, high-resolution TEM and SEM, nanometer-scale EDS mapping, and DTA.
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PMID:Nucleation and growth of germanium nanowires seeded by organic monolayer-coated gold nanocrystals. 1184 11

Structural evolution upon transformation of sol to gel, and gel to final ceramic during the synthesis of hydroxyapatite is investigated using Fourier transform infrared (FTIR) analysis, X-ray diffraction (XRD), thermal behavior (DTA and TGA), and electron microscopy examination (SEM/TEM). The sol was first thermally aged at 45 C for various time periods up to 120 min. The colloidal sol, which may have an oligomeric structure, was relatively stable against coagulation. Upon drying, the sol particles consolidated into dry gel through van der Waals attraction, and showed X-ray amorphous phosphate structure. The solid gels showed a particulate microstructure, composed of primary particles of about 8-10 nm in diameter. The amorphous gel transformed into crystalline apatite at temperatures > 300 C. The calcined gels showed a nano-scale microstructure, with grains of 20-50 nm in diameter. Through an appropriate heat treatment between 300 and 400d degrees C. the apatite prepared using current process exhibits a nano-scale, low-crystallinity, carbonated apatitic structure, which closely resembles that of human bone apatite.
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PMID:Structural evolution of sol-gel-derived hydroxyapatite. 1192 71

The extraction behavior and phase behavior of TOPO (or TRPO)-kerosene/HCl-ZrOCl(2) systems have been investigated. The middle phase with bicontinuous structure is first used as both the Zr(IV) source and the template to prepare mesoporous material just by precipitation with aqua ammonia under low temperature. By this method, crystal mesoporous ZrO(2) powders with average d-values of 6.3 and 7.48 nm, respectively, have been obtained. TG-DTA, TEM, and XRD are adopted for the characterization of the synthesized sample.
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PMID:Preparation of mesoporous ZrO2 with the middle phase formed in a trioctyl (or alkyl) phosphinic oxide-kerosene/HCl-ZrOCl2 extraction system. 1292 70

Tungsten-promoted zirconia (WO(3)/ZrO(2)) nanoparticles were synthesized in reverse micelles of water/sodium bis(2-ethylexyl) sulfosuccinate/isooctane, which were compared with WO(3)/ZrO(2) prepared by incipient wetness impregnation and sol-gel method. The control of particle size was achieved by varying the process variables, such as water-to-surfactant molar ratio and reagent concentration. Their sizes, appearances, crystal structures, and thermochemical behavior were characterized by UV-vis, TEM, XRD and TG-DTA. NH(3)-TPD and Hammett indicator measurement were also employed to study the acidity of samples. The results revealed that samples prepared in reverse micelles had higher percentages of T-ZrO(2) and greater acid strength than those of other samples. The alkylation of butene with isobutane on WO(3)/ZrO(2) was carried out in a fixed-bed reactor. Samples synthesized in reverse micelles had better reaction performance than others. A parallel relationship could be drawn between the catalytic activity and the acid amounts as well as the acid strength of the catalysis.
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PMID:Controllable synthesis, characterization and catalytic properties of WO(3)/ZrO(2) mixed oxides nanoparticles. 1295 87

Automobile catalytic converters emit nanocrystalline platinum attached to alumina particles. For investigations about the bioavailability of Pt from these particles a model substance with approx. 5% Pt on alumina has been prepared and characterized by physical methods (ESCA, XRD, TEM, DTA, TG). Measuring the platinum solubility of these samples in different solvents revealed high amounts that can be explained assuming a corrosion process. The portion of soluble platinum is dependent on the particle size distribution. For a comparative study platinum black has been used. In general the platinum determination has been carried out by electrothermal atomic absorption spectrometry (ET-AAS). The comparison of ET-AAS results with determinations by adsorptive voltammetry (formazone method) allowed to distinguish between elemental and ionic platinum; in solution samples only ionic platinum has been present. UV spectra of extracts have been used for the semi-quantitative platinum speciation in solutions.
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PMID:Platinum solubility of a substance designed as a model for emissions of automobile catalytic converters. 1506 84

Nano-crystalline Co3O4 and CoO powders have been prepared by a spray pyrolysis approach. The effects of the reaction temperature and initial salts on the crystallinity and phase composition have been studied. Based on the TEM and XRD results, the crystal sizes were in the range of 1-10 nm. SEM and TEM observations also reveal that the nano-powders easily create micron-scale spherical agglomerates. The Co3O4 powders obtained by spraying nitrate solution at 500 degrees C show high specific surface area, which according to the BET method is 82.37 m2/g. The time/temperature phase diagram of cobalt oxides developed from XRD and DTA/TGA analyses shows the existence of a CoO phase at low and high temperature ranges when some specific preparation conditions are applied.
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PMID:In-situ fabrication of nanostructured cobalt oxide powders by spray pyrolysis technique. 1557 Sep 73


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