Koichi Nakaso

Effect of reaction temperature on CVD-made TiO2 primary particle diameter

The effect of chemical reaction rate on the generation of titania nanoparticles by chemical vapor deposition using two different precursors was investigated by FTIR, XRD, and microscopy. The size of the primary particle exhibited a minimum with increasing reactor temperature. At lower reaction temperatures, the continuous and gradual formation of titania monomers occurred followed by coagulation and/or surface reaction on the existing particles. In addition, unreacted precursor condensed at the reactor exit. As the reaction temperature increased, the rate of monomer production increased, the dominant characteristics of particle growth were coagulation and sintering. The reactor temperature where the minimum primary particle diameter was produced was different for the two precursors due to differences in chemical reaction rates. Phase composition as well as the primary particle diameter of product titania were affected by the chemical reaction rate. Particle-laden reactor wall enhanced the precursor conversion at low reactor temperatures, where surface reactions compete effectively with gas-phase precursor conversion.

Evaluation of the change in the morphology of gold nanoparticles during sintering

Morphological changes of agglomerates consisting of nanometer primary gold particles were studied experimentally and theoretically. Gold aerosol nanoparticles were produced using the evaporation/condensation method, and the change in agglomerate size by reheating was examined experimentally using a tandem DMA setup. Numerical calculations, based on two extreme mechanisms to reshape agglomerates, i.e., subsequent coalescence of primary particles and subsequent rearrangement of primary particles, were carried out. By comparison with the experimental results, the sintering time and the rate constant of restructuring were obtained. Using these values, the change in particle size for different generation conditions could be calculated. The change in morphology of agglomerates can be explained from the comparison of the experimental results with the theoretical calculations: agglomerates with smaller primary particles will compact mainly by the subsequent coalescence of primary particles, while agglomerates with larger primary particles will compact mainly by a rearrangement of primary particles.

Synthesis of non-agglomerated nanoparticles by an electrospray assisted chemical vapor deposition (ES-CVD) method

Abstract Non-agglomerated spherical silicon, titanium and zirconium oxide nanoparticles were prepared using an electrospray assisted chemical vapor deposition (ES-CVD) process. Metal alkoxides in conjunction with an electrospray method were used to introduce charged precursors into a CVD reactor. The ions are produced during evaporation of the charged droplets, and they probably act as seed nuclei (i.e., ion-induced nucleation) and/or, they are attached to the produced particles. The experimental results were compared with those obtained using a conventional evaporation CVD method. The particles generated using the conventional evaporation method were agglomerated to a considerable extent irregardless of the type of particle. Whereas, at the same conditions, high concentrations of non-agglomerated nanoparticles having diameters in the range of 10– 40 nm were obtained using the ES-CVD method. This appears to be due to the charging effects of the generated particles, that is, the electrostatic dispersion of unipolarly charged particles. The size of the non-agglomerated particles in the ES-CVD method was reduced as the results of the decease in the concentration of precursors introduced by electrospray.

Size distribution change of titania nano-particle agglomerates generated by gas phase reaction, agglomeration, and sintering

In the manufacturing of nanometer-sized material particlulates by aerosol gas-to-particle conversion processes, it is important to analyze how the gas-phase chemical reaction, nucleation, agglomeration, and sintering rates control the size distribution and morphology of particles. In this study, titania particles were produced experimentally by the thermal decomposition of titanium tetraisopropoxide (TTIP) and oxidation of titanium tetrachloride (TiCl 4 ) using a laminar flow aerosol reactor. The effect of reaction temperature on the size and morphology of the generated particles was investigated under various conditions. The size distributions of agglomerates were measured using a DMA/CNC system. The size distributions of primary particles were measured using TEM pictures of the agglomerates sampled by a thermophoretic aerosol sampler. In order to model the growth of both agglomerates and primary particles simultaneously, a two-dimensional discrete-sectional representation of the size distribution was employed, solving the aerosol general dynamic equation for chemical reaction, agglomeration, and sintering. Qualitative agreement between the experimentally observed results and the simulation are satisfactory for the large variations in reactor temperature explored.

A New Observation on the Phase Transformation of TiO2 Nanoparticles Produced by a CVD Method

Titania (TiO2) nanoparticles with primary diameters of less than 30 nm were produced by the thermal decomposition of TTIP and by the oxidation of TiCl4 in a cylindrical furnace reactor at 1200°C. Particle size, crystalline phase, and phase transformation were investigated as a function of precursor concentration and total flow rate by TEM, a DMA/CNC system, XRD, and TG-DTA. The results show that both particle size and number concentration were increased with increasing precursor concentration, and that the primary size could be controlled by changing the operating conditions. An anatase-to-rutile phase transformation occurred at TTIP concentrations above 7.68 × 10−6 mol/l and this was enhanced with increasing precursor concentration. It is noteworthy that the transformation is independent of grain size but appears to be related to the presence of carbon impurities in the nanoparticles.

Structure of circulation flows in polymer solution droplets receding on flat surfaces

In a previous report where internal flows were experimentally visualized in polymer solution droplets receding on a lyophobic surface [Kaneda et al., Langmuir 2008, 24, 9102-9109], the direction of the circulation flow was found to depend on solvent and solute concentration. To identify the reason for this finding, the internal flow in the droplet is investigated numerically. A mathematical model predicts that double circulation flows initiate after a single flow develops at high Marangoni numbers, while only a single circulation flow develops at low Marangoni numbers. The dependencies of the calculated velocities on the solvent and the initial solute concentration agree qualitatively with experiment. It is concluded that the difference of the flow directions that were investigated experimentally is due to such a change in the flow structures. The effects of the contact angle and dimensions on transport phenomena in a droplet are also discussed.

Optimized combustion of biomass volatiles by varying O2 and CO2 levels: A numerical simulation using a highly detailed soot formation reaction mechanism

To increase syngas production and minimize soot, polycyclic aromatic hydrocarbon (PAH), and CO(2) emissions resulting from biomass combustion, the evolution of biomass volatiles during O(2)/CO(2) gasification was simulated. A highly detailed soot formation reaction mechanism flowing through the reactor, involving 276 species, 2158 conventional gas phase reactions and 1635 surface phase reactions, was modeled as a plug flow reactor (PFR). The reaction temperature and pressure were varied in the range 1073-1873K and 0.1-2MPa. The effect of temperature on product concentration was more emphasized than that of pressure. The effect of O(2)/CO(2) input on product concentration was investigated. O(2) concentration was important in reducing PAHs at low temperature. Below 1473K, an increase in the O(2) concentration decreased PAH and soot production. However, if the target of CO(2) concentration was higher than 0.22 in mass fraction terms, temperatures above 1473K reduced PAHs and increased CO.

Film formation by motion control of ionized precursors in electric field

A chemical vapor deposition (CVD) method, called ionization CVD, in which ionized source molecules were deposited on a substrate by Coulombic force, was developed to control gas-phase reaction and film morphology. This method was applied to the tetraethylorthosilicate (TEOS)/ozone-atmospheric pressure chemical vapor deposition process by using the surface corona discharge. TEOS/O3 films deposited on SiN and SiO2 films by this CVD method showed good properties for the flow shape, the gap filling and the surface morphology. In Fourier-transform infrared spectra of gas-phase intermediates collected in the vapor condenser, the intensity of the absorption peak at 600 cm−1 was different between ionized intermediates and nonionized intermediates.

A reduced mechanism for primary reactions of coal volatiles in a plug flow reactor

In the present paper, the authors study the primary reactions of coal volatiles and a detailed mechanism has been made for three different environments: thermal decomposition (pyrolysis), partial oxidation (O2) and O2/CO2 gasification in a plug flow reactor to analyze the combustion component. The computed results have similar trend for three different environments with the experimental data. A systematically reduced mechanism for O2/CO2 gasification has also been derived by examination of Rate of Production (ROP) analysis from the detailed mechanism (255 species and 1095 reactions). The reduced mechanism shows similar result and has been validated by comparing the calculated concentrations of H2, CH4, H2O, CO, CO2 and polycyclic aromatic hydrocarbon (PAH) with those of the detailed mechanism in a wide range of operating conditions. The authors also predicted the concentration profiles of H2, CO, CO2 and PAH at high temperature and high pressure.

The feasibility of desorption on Zeolite-water pair using dry gas

The increase in temperature, reduction in partial pressure, reduction in concentration, purging with an inert fluid, and displacement with a more strongly adsorbing species are the basic things that occur in the practical method of desorption. In this study, dry gas at constant temperature and pressure was employed as the aid to reduce the partial pressure in the water desorption on the zeolite 13X. The objective of this study is to confirm the feasibility of desorption using dry gas experimentally and numerically. The implication of heat and mass transfers were numerically investigated to find the most influential. The results of numerical simulation agree with the experimental ones for the distribution of local temperature and average water adsorbed in the packed bed.