A.S. Gurav

Aerosol Processing of Materials

Recent advances in aerosol generation of materials are reviewed. Gas-to-particle and spray processes (spray pyrolysis) for powder generation and various routes for film generation are discussed from the experimental and theoretical perspectives. The range of materials generated by these routes has increased in recent years to include fullerenes and ceramic superconductors. Many metals and various oxide and nonoxide ceramics have also been added to the list of materials generated by gas-phase routes. Established aerosol routes such as vapor condensation have found widespread applications for generation of nanophase materials. The formation of quantum dots via aerosol approaches has also been demonstrated. The theoretical understanding of gas-to-particle conversion routes has advanced to include the finite rate of particle fusion or sintering occurring after collisions of particles. The modeling of spray pyrolysis systems has provided insight into the control of particle morphology and reactor design. In th...

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.

Generation of nanometer-size fullerene particles via vapor condensation

Abstract Ultrafine (30–40 nm) fullerene particles were generated via vapor condensation in a continuous flow system starting from pure C 60 and mixed fullerene extract (C 60 /C 70 ). Gas-phase particle size distributions were measured using a condensation particle counter and differential mobility analyzer. Particle morphology and crystallinity were studied by SEM and TEM. Particles were partly amorphous at 400°C, and became crystalline C 60 when processed above 500°C. The particles had an average size of 30, 35 and 40 nm and total number concentrations of 2.2×10 6 , 3.5×10 6 , and 5.0×10 6 number of particles/cm 3 , respectively at 500, 525 and 550°C.

Palladium metal and palladium oxide particle production by spray pyrolysis

Abstract Spray pyrolysis was used to produce dense, spherical palladium metal particles at and above 900 °C in air and 800 °C in nitrogen, well below the melting point of palladium (1554 °C). Palladium oxide particles were produced at lower temperatures. At 500 °C the PdO particles were composed of nanocrystalline grains 5 to 15 nm in diameter and had surface areas of 30.2 to 32.8 m2/g. The particles became less porous and less polycrystalline as temperature increased. At 800 °C the PdO particles were polycrystalline with grains of 20 to 50 nm and a surface area of 3.23 m2/g. The Pd particles produced at 900 °C by decomposition of the oxide were single-crystalline and fully-dense. These observations are consistent with the formation of porous but not hollow aggregates of PdO at lower temperatures, which can be densified in the gas phase to form solid particles of Pd above 900 °C.

The ash formation during co-combustion of wood and sludge in industrial fluidized bed boilers

Abstract Ashes from an industrial bubbling fluidized bed (BFB) and a circulating fluidized bed (CFB) boilers burning biomass and paper mill sludges are characterized. Bulk sampling was carried out with a simultaneous low pressure impactor sampling (BLPI) and with on-line monitoring of submicron ash gas phase size distribution. The principal ash forming mechanism was sintering of 1–10 μ m inorganic paper filler particles, into 5–200 μ m ash agglomerates. Despite few spherical particles no ash melting was detected. The fraction of vaporized and condensed ash was below 0.1 mass% in both units as determined with BLPI and with on-line monitoring. According to elemental mass size distributions no significant vaporization of any main element occurred including sodium and potassium. Wall deposit observed on refractory lining in BFB furnace was composed of sintered ash binding large ash and sand particles in the structure. No molten phases were detected to participate in the layer growth. Most quartz sand grains in the bed were covered by a thin layer of small ash particles, below 5 μ m in size.

The Structure of Submicron Ash from Combustion of Pulverized South African and Colombian Coals

The formation of submicron ash particles during the utility-scale pulverized combustion of South African Klein Kopie and Colombian El Dorado coals was studied by measuring the ash particle number and mass size distributions in the size range 0.01–1 pm upstream of the electrostatic precipitator (ESP). Ash morphology, composition and microstructure were studied by high resolution scanning and transmission electron microscopes (SEM and TEM).

Phase evolution and gas-phase particle size distributions during spray pyrolysis of (Bi,Pb)SrCaCuO and Ag(Bi,Pb)SrCaCuO powders

Phase evolution, gas-phase particle size distributions and lead loss were studied during formation of (Bi,Pb)SrCaCuO powders and their composites with silver by spray pyrolysis starting from nitrate solutions. The 10 wt% Ag/90 wt% Bi1.8Pb0.44Sr2Ca2.2Cu3Ox composite powders made at 700°C consisted of 20–60 nm grains of silver and mixed-oxide phases with a fine dispersion of Ag grains within the particles. At 700°C, the primary phases present in (Bi,Pb)SrCaCuO powders were (Bi,Pb)2Sr2CuOx (2201), Ca2PbO4 (plumbate), (Bi,Pb)2Sr2CaCu2Ox(2212), and (Bi,Pb)3Sr2Ca2Cu1Ox(3221). For T≥800°C, the powders were considerably depleted in lead, and the plumbate and 3221 phases were absent. For T = 900°C, a large number of ultrafine particles (<30 nm) were formed, probably from the PbO vapor released from the reactor walls. Using spray pyrolysis, it is easy to control stoichiometry and limit the phase segregation at the nanometer-scale so that homogeneous and phase-pure materials can be obtained rapidly during subsequent processing.

Multicomponent particle formation via gas-phase reactions of single-component precursors

Abstract We are examining new routes to vapor-phase generation of multi-component particles with controlled stoichiometry. Generally, gas phase reaction processes use two or more reactants which must be fed simultaneously into a reactor to form such materials. This method can result in the formation of chemically nonuniform particles having different compositions. We have demonstrated a new approach to producing particles by gas-to-particle conversion using volatile single-source compounds that contain all the elements needed to form the final compound. This technique was used to form both Ge-Mn and ZnS particles. By reacting 1,1-bis-(pentacarbonylmanganio)-3,4-dimethylgermacylclopent-3-ene in both nitrogen (at 600°C) and H 2 -N 2 (at 400, 500, 600, and 700 °C), 30 nm Ge-Mn particles were formed. By reacting Zn(S 2 CNEt 2 ) 2 (TMEDA) in nitrogen at 500, 600, and 700 °C, 30 to 50 nm ZnS particles were produced.

Gas-phase particle size distributions during vapor condensation of fullerenes

Abstract Nanometer-size fullerene particles were generated via vapor condensation in a continuous flow system starting from pure C60 and mixed fullerene extract (C60/C70). Vapor condensation was carried out by cooling a nitrogen gas stream containing fullerene vapors from 400–650°C to room temperature. Particles were partly amorphous at 400°C, and became crystalline C60 when processed above 500°C. The average particle diameter and particle number concentration increased with increasing processing temperature. The particles had average sizes of 30, 35 and 40 nm and total number concentrations of 2.2 × 106, 3.5 × 106, and 5.0 × 106 #/cm3, respectively at 500, 525 and 550°C.

Synthesis of nanophase materials by intraparticle reaction routes

Abstract We have generated 0.1-0.5 μm particles that consist of nanophase PdO and V2O5. The particles were produced by an aerosol decomposition technique in which droplets containing precursors were passed through a hot-wall flow system in which the solvent evaporated and the precursors decomposed within the particles to form the product. By controlling the temperature and residence time, grain-growth in the particle could be minimized to allow formation of nanometer-sized crystallites. Advantages of this approach include the ability to form nanophase complex metal oxides with high purity at high production rates.