Timothy L. Ward

Aerosol-Assisted Self-Assembly of Mesostructured Spherical Nanoparticles

Particles possessing nanometre-scale pores of well-defined size and connectivity are of interest for catalysis, chromatography and controlled release of drugs, and as fillers with low dielectric constant, pigments and hosts for optically active compounds,. Silica containing ordered mesopores (of nanometre-scale width) can be prepared by templating of surfactant, and block copolymer liquid-crystalline mesophases, and interfacial phenomena have been used to control the macroscopic form of these materials, providing mesoporous particles,, fibres, and films,. A variety of spherical or nearly spherical particles has been reported,,,, but the degree of ordering and the range of the porous mesostructures have been limited. Here we report a rapid, aerosol-based process for synthesizing solid, well-ordered spherical particles with stable pore mesostructures of hexagonal and cubic topology, as well as layered (vesicular) structures. Our method relies on evaporation-induced interfacial self-assembly confined to spherical aerosol droplets. This simple, generalizable process can be modified for the formation of ordered mesostructured thin films.

Monodisperse mesoporous silica microspheres formed by evaporation-induced self-assembly of surfactant templates in aerosoles

The present invention provides for evaporation induced self-assembly (EISA) within microdroplets produced by a vibrating orifice aerosol generator (VOAG) for the production of monodisperse mesoporous silica particles. The process of the present invention exploits the concentration of evaporating droplets to induce the organization of various amphiphilic molecules, effectively partitioning a silica precursor to the hydrophilic regions of the structure. Promotion of silica condensation, followed by removal of the surfactant, provides ordered spherical mesoporous particles.

The double‐ring electrodynamic balance for microparticle characterization

A simple form of the electrodynamic balance, suitable for a wide range of microparticle measurements, is described and analyzed. The ac electrode of the device consists of a pair of parallel rings, and the dc endcaps are either simple disks or they can be eliminated entirely by applying suitable dc bias voltages to the rings. The stability characteristics of the device are determined by extension of well‐established stability theory, and experiments are compared with that theory. The device is particularly well‐suited for detection of radioactive aerosols, for it has significant advantages over the bihyperboloidal device for radioactivity measurement. The detection of radioactivity levels of less than 20 pCi is feasible. Coupled with a Raman spectrometer the balance serves as a stable ‘‘platform’’ for the study of the chemistry of microparticles, and both qualitative and quantitative analysis of microdroplet chemistry are demonstrated for binary droplets of 1‐octadecene and 1‐bromoctadecane.

Solid silver particle production by spray pyrolysis

Abstract Solid, spherical, micron-sized silver metal particles were produced by spray pyrolysis from a silver nitrate solution. The effects of reaction temperature, carrier gas type, solution concentration, and aerosol droplet size on the characteristics of the resultant silver particles were examined. Pure, dense, unagglomerated particles were produced with an ultrasonic generator at and above 600° C using N 2 carrier gas, and at and above 900°C using air as the carrier gas. Solid particle formation at temperatures below the melting point of silver (962°C) was attributed to sufficiently long residence times (3.5–54 s) which allowed aerosol-phase densification of the porous silver particles resulting from reaction of the precursor.

Microstructure and properties of the Y-Ba-Cu-O superconductor with submicron “211” dispersions

Abstract A fine-scale dispersion of Y2BaCuO5 (“211”) particles ( ∼8000 A average diam.) within the YBa2Cu3O7-δ supercondu ctor (“123”) has been achieved by local melt-texture processing of off-stoichiometric Y-Ba-Cu-O prepared by the aerosol decomposition technique. The presence of such fine particles significantly reduces the thickness of the superconductor plate to below ∼ 7000 A. The plate thickness almost linearly with 211 particle size. The observed scaling relationship is most likely caused by the 211 inclusions serving as effective nucleation sites for 123 crystallization as well as restricting the plate coarsening below the peritectic temperature. The submicron-sized 211 particles tend to reduce microcracks and segregation of impurity phases at the plate boundaries, however, their effect on flux pinning appears to be insignificant.

Mesoporous carbon/silica nanocomposite through multi-component assembly

Ordered mesoporous carbon/silica nanocomposites were synthesized through a novel multi-component molecular assembly and show promising potential as corrosion-resisted electrocatalyst supports.

Structure-property relationships in thin films and membranes

The molecular-scale species distributions and intermediate-scale structure of silicate sols influence the microstructures of the corresponding thin films prepared by dip-coating. Using multi-step hydrolysis procedures, we find that, depending on the sequence and timing of the successive steps, the species distributions (determined by 29Si NMR) and intermediate scale structure (determined by SAXS) can change remarkably for sols prepared with the same nominal composition. During film formation, these kinetic effects cause differences in the efficiency of packing of the silicate species, leading to thin film structures with different porosities.

Characteristics of Bi-Pb-Sr-Ca-Cu-O powders produced by aerosol decomposition and their rapid conversion to the high-Tc phase

Abstract Bi-Pb-Sr-Ca-Cu-O powders were produced by aerosol decomposition of nitrate solutions. The effects of reactor temperature and residence time on particle morphology and evaporative Pb loss from particles were demonstrated, and conditions necessary to control Pb loss established. Pb loss was roughly proportional to residence time, and minimal loss occurred with short residence times (3s) and T≤800°C. Particles produced at 700°C typically contained significant porosity, while those produced at T≥800°C were solid. Mixtures of the Bi2Sr2CuOy (2201) and Bi2Sr2CaCu2Oy (2212) phases were produced at 700–900°C in nitrogen and air. However, after hearing in air for 16 h at 850°C, pellets of powder produced at 700°C with nominal composition Pb0.44Bi1.8Sr2Ca2.2Cu3Oy converted to approximately 79 vol.% of the Bi2Sr2Ca2Cu3Oy (2223) phase and displayed a Tc (onset) of 110 K. Rapid conversion to 2223 was promoted by powder synthesis conditions, leading to controlled Pb loss and a homogeneous fine-grained dispersion of mixed-oxide precursor phases within particles.

Synthesis of YBa2Cu3O7−γ and YBa2Cu4O8 by aerosol decomposition

Abstract The synthesis of submicron YBa 2 Cu 3 O 7−γ (1-2-3) and composite 1-2-3/CuO powders using aerosol decomposition of nitrate solutions and the behavior of those powders during subsequent processing were investigated. Transmission Electron Microscopy (TEM) revealed that 1-2-3 and 1-2-3/CuO particles produced at 800°C (residence time of 15 s) were polycrystalline and hollow, while solid particles were produced at temperatures greater than 900°C. Solvent evaporation led to a hollow particle morphology which was retained unless the reactor temperature exceeded 900°C, where densification was rapid enough to result in collapse of the hollow particles within the reactor residence time. Single-phase 1-2-3 powder with a critical temperature ( T c ) of 92K was produced using reactor temperatures of 900–1000°C. Dry-pressed 1-2-3 pellets densified rapidly at 800–900°C to 89% of theoretical density, which is considerably below the temperatures required for coarser powders produced by other techniques. 1-2-3/CuO particles produced at 800°C consisted of 25–50 nm crystallites of 1-2-3 and CuO, and pellets formed from that powder could be converted to single-phase YBa 2 Cu 4 O 8 (1-2-4) by heating in atmospheric-pressure oxygen at 750°C for 24 h, followed by 800°C for 24 h. Larger grained 1-2-3/CuO powder (100–250 nm) produced at 900 and 1000°C showed no conversion to 1-2-4 with the same treatment, demonstrating the critical importance of the small grain sizes obtained by aerosol decomposition for rapid conversion of 1-2-3/CuO to 1-2-4.

Photochemical polymerization of acrylamide aerosol particles

The kinetics of photochemically initiated polymerization of acrylamide and monomer-glycerol solution microdroplets has been explored by following the polymerization reaction by means of a fluorescent dye tracer, auramine-O. Polymerization rate constants have been estimated for the solutions by measuring the decrease in evaporation rate caused by polymerization, using a mathematical model of the simultaneous evaporation/polymerization process. Single charged droplets with initial diameters of order 50 μm were suspended in the paths of two laser beams by ac and dc electrical fields in a combination electrodynamic balance and light-scattering spectrometer. One laser beam was used to size the microsphere by elastic scattering measurements, and the second beam provided the excitation energy for fluorescence. Polymerization rate data and evaporation rate data and are presented for solid acrylamide microspheres and for solution microparticles for various system parameters.