Synthesis and sintering properties of cerium oxide powders prepared from oxalate precursors

Fine cerium oxide powders obtained by low-temperature decomposition from oxalate and hydrazinate oxalate are compared. Studies of thermal behaviour and disagglomerating capability are given. The preparation of oxide powders by chemical and physical methods is able to give a dispersed suspension, and leads with appropriate recovery to increased green density and final density after sintering.     

Synthesis of La1−xPbxMnO3 colossal magnetoresistive ceramics from co-precipitated oxalate precursors

The precursors of La_1–x Pb _x MnO₃ manganite colossal magnetoresistive ceramics were prepared by co-precipitation process via the oxalate route using oxides as the starting materials. The mathematical model of the solution was developed and the conditions of the complete co-precipitation of all cations existing in the solution as insoluble oxalates were determined. La_1–x Pb _x MnO₃ manganites were synthesized by high-temperature thermal treatment of the precursors. It was demonstrated that the two-stage annealing procedure: 12 hours at 800°C in air and 12 hours at 950°C in flowing O₂ permits to synthesize compositions with the required stoichiometry exhibiting colossal magnetoresistive properties.     

Al-doped single-crystalline SiC nanowires synthesized by pyrolysis of polymer precursors

Al-doped 6H-SiC nanowires are synthesized by catalyst-assisted pyrolysis of polymer precursors. The obtained nanowires were characterized using scanning electron microscopy, X-ray diffraction, transmission electron microscopy and selective area electron diffraction. We demonstrate that doping concentrations can be controlled by tailoring the Al concentrations in the precursors. We also find that Al-doping has a profound effect on the morphology and emission behavior of the SiC nanowires. The current results suggest a simple technique for synthesizing Al-doped SiC nanomaterials in a controlled manner, which are promising for applications in optical and electronic nanodevices.     

Synthesis of perovskite-type compounds by drip pyrolysis

Lanthanum-transition metal mixed oxides, with the general formula La_1–x M _x BO₃, have been prepared, whereinM stands for Ca and Sr, andB stands for one or more transition metals selected from the group consisting of Cr, Mn, Fe, Co and Ni, where 0x0.50. A drip pyrolysis method has been developed to synthesize the mixed oxides at low temperature (550C). As combustion fuel, glucose was added to the aqueous solution of metal salts. The prepared compounds have perovskite structures. This paper examines the possibilities of finding a simple and large scale procedure in order to produce a long list of perovskites necessary for fuel cell materials (air electrode and interconnect) and catalysts for oxidation catalysis. X-ray diffraction (XRD) analysis, scanning electron microscope (SEM) measurements and determination of specific surface area, carbon content and particle size distribution have been carried out on the obtained perovskite powders.     

Synthesis of silicon nitride nanowires by the pyrolysis of perhydropolysilazane

Silicon nitride nanowires synthesized by the pyrolysis of perhydropolysilazane without using any catalysts are reported. After pyrolysis at 1073 K in N2/NH3 atmosphere, the synthetic nanowires are discrete and curly with diameters about tens of nanometers and lengths of hundreds of nanometers. While after post-treatment at 1873 K in N2 atmosphere, the nanowires are continuous and randomly distributed with diameters about tens of nanometers and several microns in length. There are no bulbs or droplets on the tips of the nanowires, and two gas-solid mechanisms are proposed to explain their growth.     

Studies on initial permeability and loss factor in Ni-Zn ferrites synthesized by oxalate precursors

Ni_XZn_1−XFe₂O₄ ferrites with (X = 0.28-0.40 in step of 0.2) have been synthesized by oxalate precursor method and investigated for their, initial permeability and loss factor measurements. Initial permeability has been observed to increase with the increase in Ni²⁺ up to X = 0.32, beyond which it decreases. The variation of initial permeability has been explained by considering the factors such as grain size, saturation magnetization and anisotropy constant. Thermal variation of initial permeability reveals a peak height in μ_i-T curves which tends to increase with increase in Ni²⁺ content. μ_i-T curves also exhibit thermal hysteresis, which reveals the inverse relationship between the difference in heating and cooling curves at which hysteresis falls between Hopkinson peak and T_c with value of initial permeability. Loss factor values are small which is attributed to high density of the samples and processing techniques.     

Mixed metal oxalate hydrazinates as compound precursors to spinel ferrites

Mixed metal oxalate hydrazinates MFe₂(C₂O₄)₃(N₂H₄)₆ where M = Mn, Co, Ni, Zn have been prepared, characterised and investigated as compound precursors to spinel ferrites, MFe₂O₄. These precursors decompose exothermically at very low temperatures (130°–250°C) to yield ferrites of large surface area. These compound precursors exhibit autocatalytic behaviour, ie. once ignited combustion is self sustained. Characterisation of the precursors and the combustion products has been done using chemical analysis, infrared and Mössbauer spectra and X-ray diffraction.     

Synthesis of submicron carbon spheres by the ultrasonic spray pyrolysis method

Submicron carbon spherical particles were obtained by polycondensation of resorcinol and formaldehyde in a solution and subsequent ultrasonic spray pyrolysis of the prepared sol. Microscopic characterization indicates the regular spherical shape of the obtained particles and sphere diameters in 200-700 nm range. The carbon spheres are amorphous as confirmed by electron diffraction, EELS, XRD and HREM characterization. Activation procedure was performed with H₂O in a nitrogen flow for 15 and 30 min at 800 °C. The activation procedure preserved the initial spherical shapes of the particles while the particle porosity and specific surface area were increased. The amount of surface oxygen functionalities was also increased by activation procedure as indicated by FTIR analysis.     

Synthesis of ceramic oxide powders by microwave plasma pyrolysis

Ceramic powders prepared pyrolytically exhibit homogeneity and, in most cases, small grain sizes. The energy efficiency of electrically heated systems performing the pyrolysis in a stream of carrier gas is poor. Similar considerations concerning energy demand are valid for spray drying of suspensions. This situation can be improved using a microwave plasma as a source for thermal energy. The process described in this paper works with any aqueous solution of salts used as starting material in ceramics. The process was demonstrated by the synthesis of alumina, zirconia, and zirconia-based ceramic powders; where an energy efficiency of more than 80% was found. For the powder synthesis, aqueous solutions of the nitrates were used as starting materials. Through proper selection of conditions for synthesis, it is possible to obtain nanocrystalline powders, as demonstrated by electron microscopy. Because of the extreme conditions associated with plasma during synthesis, it is possible to prepare nonequilibrium structures and solid solutions in systems in which nearly no equilibrium solubility exists.     

Graphene-like film CVD on e-beam exposed SiO2/Si by the pyrolysis of different oxygen-containing precursors

ABSRTACT

We have previously found that pyrolysis of ethanol vapor on charge pre-patterned SiO₂/Si substrate leads to controllable deposition of graphene-like films (GLFs). In this work we aim at the GLFs synthesis from acetone and isopropanol on SiO2/Si exposed to an electron beam with various energies and doses. Electron beam induced charge in 200?nm SiO₂ is determined by the shift of the capacitance-voltage characteristics. Synthesized GLFs are characterized using Raman spectroscopy and atomic force microscopy. It is shown that the thickness of GLF on SiO₂/Si increase together with the preliminary exposure dose and inversely associated with electron energies.     

Synthesis of magnetite nanoparticles by thermal decomposition of ferrous oxalate dihydrate

Two different polymorphs of ferrous oxalate dihydrate were synthesized by precipitation of ferrous ions with oxalic acid: α-Fe(C₂O₄) · 2H₂O with a monoclinic unit cell is obtained after precipitation and ageing at 90 °C, whereas the orthorhombic β-type is formed after precipitation at room temperature. The morphology of the oxalate crystals can be tailored from prismatic crystals of the α-polymorph over star-like aggregates of α/β-mixtures to non-agglomerated crystallites of β-oxalate. Thermal decomposition in air gives hematite at T ≥ 250 °C; if the thermolysis reaction is performed at low oxygen partial pressures (e.g., T = 500 °C and p _O2 = 10^?25 atm) magnetite is obtained. The synthesized magnetite is stoichiometric as signaled by lattice parameters of a ₀ = 8.39 Å. The thermal decomposition of ferrous oxalate is monitored by thermal analysis, XRD, and IR-spectroscopy. The morphology of the oxalate crystals is preserved during thermal decomposition; the oxalates are transformed into spinel particle aggregates of similar size and shape. The crystallite size of the magnetite particles increases with temperature and is 40 or 55 nm, if synthesized from β-oxalate at 500 °C or 700 °C, respectively. The saturation magnetization of the magnetite particles decreases with decreasing particle size. Since the particles are larger than the critical diameter for superparamagnetic behavior they display hysteresis behavior at room temperature.     

pH dependent coprecipitated oxalate precursors - a thermal study of lead titanate

Formation of stoichiometric lead titanyl oxalate from mixed solutions of lead nitrate and potassium titanyl oxalate at different pH values was investigated by thermal analysis. At very low pH of 0.45 only lead oxalate was precipitated. Increase in pH from 0.45 to 2.5 did not yield stoichiometric lead titanyl oxalate. However, at pH 3, the precipitate obtained corresponded to lead titanyl hydroxy oxalate, which on heat treatment at 550°C for 1 h yielded phase pure PbTiO₃. Precipitation at pH 9 showed persistence of oxalate as an impurity. Mixed hydroxides of Pb and Ti with no traces of oxalate impurity occurred at pH 10.5.     

Synthesis of TiC via polymeric titanates: the preparation of fibres and thin films

Polymerization of titanium isopropoxide via its transesterification with o-xylene-,-diacetate enables the formation of a polymeric titanate that can be thermally converted into carbon-deficient TiC after inert atmosphere pyrolysis at 800 °C. The physical and rheological properties of this polymeric titanate allow for the production of crystalline TiC fibres and thin films. The synthesis of this polymeric precursor and the structural transformations leading to the formation of TiC are discussed.     

Synthesis of iron-dispersed carbons by pressure pyrolysis of divinylbenzene-vinylferrocene copolymer

Versatile carbons with finely dispersed iron were synthesized by pressure pyrolysis of a copolymer prepared from divinylbenzene and vinylferrocene at temperatures below 680? C and pressures of 125 MPa. The pyrolysis conditions of the copolymer were found to influence the final morphology of carbons to give fibrils, spheres and polyhedra. The resulting carbons contained uniformly fine particles of cementite (Fe₃C) which were less than 30 nm in size, whereas the magnetite was dispersed in the carbon matrix by pressure pyrolysis in the presence of water. Highly dispersed cementite in carbon was found to decompose into metallic iron by further heat treatment above 850? C. Porous spherulitic carbons were also synthesized by heat treatment of magnetite containing carbon spherulites.     

Synthesis of boron-dispersed carbon by pressure pyrolysis of organoborane copolymer

Boron-dispersed carbon was synthesized by pressure pyrolysis of divinylbenzene-tris(allyl)-borane and styrene-tris(allyl)borane at 125 MPa below 650° C. Amorphous boron dispersed in a carbon matrix was oxidized easily to yield boric acid by heat treatment under air at 300° C. The BK image of the product showed that boron was dispersed uniformly in a carbon matrix. Boron-dispersed carbon had the morphology of coalescing spherulite and polyhedra depending upon the concentration of boron in the parent copolymer. The grain size of carbon polyhedra decreased from 2.0m to 0.2m with an increase in the boron concentration from 1.3 to 5.7 wt%. The presence of 0.5 wt% boron in a carbon matrix enhanced the graphitization at 4.0 GPa and 1200° C, decreasing the lattice spacing with an increase in the crystallite size. The crystallite sizes were comparable to each other after heat treatment at 1100° C and 4.0 GPa when the specimen contained boron from 0.5 to 2.5 wt%. The lattice constant (c ₀) and crystallite size (L _c) of boron-dispersed carbon containing 2.5 wt% boron were 677.0 pm and 30 nm, respectively, after heat treatment at 1200° C and 4.0 GPa.     

Synthesis of cementite-dispersed carbons by pressure pyrolysis of organoiron copolymers

Cementite-dispersed carbons were synthesized by pressure pyrolysis of divinylbenzene-vinylferrocene and styrene-vinylferrocene copolymer at temperatures below 600° C and the pressure of 125 MPa. The pyrolysis process of both copolymers was analysed by infrared spectra and magnetization of the pyrolysed substances. The absorption band of iron-carbon bond of divinylbenzene-vinylferrocene copolymer decreased on increasing its pyrolysis temperature from 300 to 450° C and finally disappeared at 500° C. The carbonization of divinylbenzene-vinylferrocene proceeded more rapidly than styrene-vinylferrocene at temperatures between 450 and 500° C. Styrene-vinylferrocene was heat-treated at 250° C for 2 h under 100 MPa affording a paramagnetic product, whereas the paramagnetic character of divinylbenzene-vinylferrocene was revealed after heat-treatment at 380° C. The saturation magnetization of cementite-dispersed carbon synthesized from both kinds of copolymers was comparable when the pressure pyrolysis was carried out at temperatures between 520 to 600° C at 125 MPa. The saturation magnetization of cementite-dispersed carbon formed at 550° C under 125 MPa was correlated linearly with the iron content in carbon. Threedimensional cross-linked divinylbenzene-vinylferrocene copolymer gave the highly dispersed cementite particles less than 50 nm with the coercive force of 950 Oe. On the other hand, the larger particle size of cementite up to 120 nm and the lower coercive force about 400 Oe were obtained in carbon matrix prepared by the pressure pyrolysis of styrene-vinylferrocene copolymer.