ISSN:0001-1541    

DOI:10.1002/aic.690431302

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractNanosized silicon powders were prepared by gas‐phase cluster agglomeration reactions in a low‐pressure silane plasma. The formation and agglomeration of clusters leading to the growth of primary particles of powder were studied by in‐situ techniques including mass spectroscopy and laser light‐scattering experiments. These powders, generally amorphous and crystallized in a reducing atmosphere, were studied in detail by Raman spectroscopy and high‐resolution electron microscopy, which revealed a very rough surface of as‐prepared single powder particles with structures of 1 to 2 nm. Upon 1–h annealing at temperatures between 300 and 600°C, circular contrast features, 1.5 to 2.5 nm in size, are observed in the amorphous particles, which show medium‐range order. A distinct onset of crystallization is observed at 700°C with structures ranging from very small crystalline ordered regions of 2.5–3.5 nm in size to fast‐grown multipletwinned crystallites. The crystallization behavior is influenced by the clusters that form primary particles. Observed sintering behavior cannot be explained by a classical approach; hence, theoretical models need to be adapted

ISSN:0001-1541    

DOI:10.1002/aic.690431303

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractElectrostatic adhesion processes are used in preparing composite aerosol particles with the electrostatic adhesion force acting between electropositively‐charged and electronegatively‐charged particles. This process enables one to disperse the core and subcomponent particles more uniformly and to control the amounts of subcomponent particles adhered to the surface of individual core particles by adjusting the charging voltage. Using the composite particles prepared by the electrostatic adhesion process as a starting powder for sintering, a composite bulk body of the Ba2YCu3O7‐Xoxide superconductor with BaZrO3inclusions is produced. BaZrO3particles with diameters less than 100 nm are uniformly dispersed in the Ba2YCu3O7‐Xmatrix. The composite bulk body shows improvement in critical current density, which is based on the uniform dispersion of fine BaZrO3particles as magnetic flux pinning centers. These advantages are attributed to the uniform dispersion of BaZrO3over the whole starting

ISSN:0001-1541    

DOI:10.1002/aic.690431304

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractNanophase SiC/Si3N4composite powders were synthesized by the carbothermal nitridation of SiO2. These powders have desirable characteristics of high quality with oxygen contents on the order of 1.5 to 2 wt. %, surface area of ∼ 10 m2/g, submicron α‐Si3N4, low metallic impurity levels, and a homogeneous distribution of the nanophase SiC phase. High‐resolution TEM analysis has shown that the content and size of the nanophase SiC can be varied from 0.5 to 50 wt. % and 25 to 500 nm, respectively, through proper control of raw materials and reactor conditions. To determine how the nanophase SiC reinforcement affects the mechanical properties of Si3N4, densified components were fabricated using both pressureless and pressure‐assisted densification methods. TEM analysis revealed that the nanophase SiC particles are distributed both intergranularly and intragranularly throughout the Si3N4, matrix. By controlling the sintering additive package and the sintering conditions, the ratio of inter– to intragranular SiC can be adjusted. Mechanical property measurements at elevated temperatures showed a dramatic improvement in high‐temperature strength and creep resistance over components made with commercially avail

ISSN:0001-1541    

DOI:10.1002/aic.690431305

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

ISSN:0001-1541    

DOI:10.1002/aic.690431306

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractThe morphological evolution of flame‐generated “primary” spherules and inorganic aggregates was studied at low particle volume fractions [O(10−1ppm)] in a welldefined/characterized laminar nonpremixed combustion environment which produces particle heating rates of 104K/s. Pure Al2O3particles synthesized in an Al(CH3)3(TMA‐) seeded atmospheric pressure laminar counterflow diffusion flame “fueled” with CH4/O2/N2were used as the model material/combustion system. Experimental techniques included spatially resolved laser light scattering (LLS) and thermophoretic sampling/transmission electron microscopy. Local aggregate morphology was characterized in terms of spherule (“grain”) size, aggregate size, aggregate shape and fractal structure. Effects of flame temperature and TMA concentrations on particle inception location, sizes and morphology studied systematically were interpreted based on parallel theoretical studies. LLS signals and TEM images show particle/aggregate size and morphology evolution as a result of two competing rate processes. Mean spherule diameters prior to high‐temperature coalescence are explained in terms of the strong size dependence of nanoparticle restructuring kinetics due to surface melting, even at 500 K. Mean fractal aggregate sizes reached only 15–27 spherules near a local temperature of only 1,250 K. Final particulate products were isolated spherical particles resulting from complete “collapse” of the aggregates in an interval of only 24 ms immediately upstream of the maximum gas temperature (2,280 K). Experimental results are compatible with the characteristic times governing each participating “unit” rate process. Some of these methods can be applied in controlling the larger‐scale synthesis of valuable nanopowders and guide rational extensions into the domain of turbulent nonpremixed combustors generating ultrafine particles of tailored composition and m

ISSN:0001-1541    

DOI:10.1002/aic.690431307

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractA process for synthesis of ultrafine ceramic particles using a high‐temperature flame is described. Thermodynamic calculations for the Si‐C‐O‐H system were performed using a computer program for calculation of complex chemical equilibrium compositions. Possible synthesis of ultrafine SiC partcles using Si, SiO, and SiO2powders (1.0–3.0 μm) as Si sources was investigated using an electric furnace (5 kW) at 2,018 K. Particle products were characterized mainly by X‐ray diffraction analysis, field emission scanning electron microscopy, and thermogravimetry‐differential thermal analysis. Ultrafine particles (18–29 nm) of cubic SiC (β‐SiC) could be produced through Si + CH4, SiO + CH4and SiO2+ CH4reaction systems, respectively. The effect of hydrogen addition on the synthesis of SiC was also investigated. Ultrafine particles of 96.0% SiC selectivity were obtained when H2was added to the SiO + CH4reaction system, and the CH4/SiO molar ratio i

ISSN:0001-1541    

DOI:10.1002/aic.690431308

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractSilica and stannic (tin) oxide powders were synthesized by oxidation of their respective chlorides in single and double (inverse) diffusion flame reactors. The effect of reac‐tant gas mixing on the characteristics of these powders (size and morphology) was investigated by altering the position of the fuel (CH4) and oxidant (air or O2) streams in the burner. Reactant gas mixing plays a key role in controlling particle size since it affects the temperature history, residence time, and initial particle concentration in the flame, thus, yielding a simple technique for particle‐size control over a wide size range in flame reactors. The different material properties (such as sintering) of silica and stannic oxide result in particles of different size and morphology, although they were made at nearly identical flame conditions. Moreover, the oxidant composition affects significantly the properties of silica partic

ISSN:0001-1541    

DOI:10.1002/aic.690431309

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractFine particles of dense, high‐purity, crystalline BaTiO3were produced by flame‐spray pyrolysis. A 0.5‐M (Ba:Ti = 1:1) solution of barium acetate, titanium lactate, and water was aerosolized using an ultrasonic generator, and the droplets were delivered into the core of an annular diffusion flame (H2/air) reactor. For all investigated temperatures [∼1,000–∼2,000°C adiabatic], the generated powders were chemically pure, crystalline (primarily tetragonal phase with hexagonal and cubic polymorphs), and unagglomerated. At a low‐flame temperature (∼1,000°C ad.), the particles produced were hollow and irregularly shaped. Particles produced at higher flame temperatures (>∼1,500°C ad.) were dense and homogeneous. Particles showed a transition from a nonspherical porous morphology to a spherical dense morphology with increasing temperature. By increasing residence time, the temperature at which particles became spherical and dense was reduced. Flame‐spray pyrolysis provides a useful method for forming dense particles of high melting point materials by aeros

ISSN:0001-1541    

DOI:10.1002/aic.690431310

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractExperimental investigations of high‐temperature kinetics of Si‐precursor molecules relevant to CVD and ceramic processing are described. Reaction systems using SiH4, Si2H6, and SiCl4highly diluted in argon were studied in a shock tube, a high‐temperature wave reactor, by monitoring in situ the concentrations of atomic or radical reactants Si, H, Cl, SiH, and SiH2. Because of the very high dilution, the measured properties are sensitive to a limited number of elementary reactions, allowing a relatively direct determination of the respective rate coefficients. Both thermal pyrolysis and laser flash photolysis methods were used to expand the investigated temperature range. An overview of the bimolecular Si‐atom reactions i

ISSN:0001-1541    

DOI:10.1002/aic.690431311

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY