Activation Energy For Particle Growth Research Articles

Growth Kinetics of Individual Co Particles Ex-solved on SrTi0.75Co0.25O3-δ Polycrystalline Perovskite Thin Films.

A precise control of the size, density, and distribution of metal nanoparticles dispersed on functional oxide supports is critical for promoting catalytic activity and stability in renewable energy and catalysis devices. Here, we measure the growth kinetics of individual Co particles ex-solved on SrTi0.75Co0.25O3-δ polycrystalline thin films under a high vacuum, and at various temperatures and grain sizes using in situ transmission electron microscopy. The ex-solution preferentially occurs at grain boundaries and corners which appear essential for controlling particle density and distribution, and enabling low temperature ex-solution. The particle reaches a saturated size after a few minutes, and the size depends on temperature. Quantitative measurements with a kinetic model determine the rate limiting step, vacancy formation enthalpy, ex-solution enthalpy, and activation energy for particle growth. The ex-solved particles are tightly socketed, preventing interactions among them over 800 °C. Furthermore, we obtain the first direct clarification of the active reaction site for CO oxidation-the Co-oxide interface, agreeing well with density functional theory calculations.

Control of the Size of Silver Nanoparticles and Release of Silver in Heat Treated SiO₂-Ag Composite Powders.

The growth of silver nanoparticles, the activation energy for silver particle growth, and the release of silver species in heat treated -Ag composite powders are investigated. The silver particle growth is controlled by heat treatment for 75 min of the as-synthesized -Ag composite powder at 300–800 °C. During heat treatment the mean size of the Ag particles increases from 10 nm up to 61 nm with increasing temperature, however, the particle size distribution widens and the mean size increases with increasing heat treatment temperature. Based on X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) studies, silver particles are crystalline and in a metallic state after annealing in all -Ag composite powders. The growth of Ag particles is suggested to take place via diffusion and Ostwald ripening. The activation energy for particle growth was determined as 0.14 eV. The dissolution of silver in aqueous solutions from the -Ag composites heat treated, at 300 °C, 600 °C, and 700 °C, was investigated by varying pH and temperature. The dissolution was reduced in all conditions with increasing silver particle size, i.e., when the total surface area of Ag particles is reduced. It is suggested that the dissolution of silver from the composite powders can conveniently be adjusted by controlling the Ag particle size by the heat treatment of the composite powder.

Influence of heat treatment on the particle size of nanobrookite TiO2 thin films produced by sol–gel method

Pure nanobrookite titania (TiO2) thin films were deposited on glass substrates by the spin-coating method using titanium butoxide and acetic acid. The particle sizes of TiO2 films were controlled by heat treatment temperatures. The activation energy for particle growth was calculated as 23.1kJ/mol. The structural and optical properties of the nanobrookite TiO2 thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), ultraviolet–visible absorption spectroscopy (UV–vis), and Fourier transform infrared spectroscopy (FTIR).

Molten Salt Synthesis of Anisotropy BaBi<sub>4</sub>Ti<sub>4</sub>O<sub>15</sub> Powders in K<sub>2</sub>SO<sub>4</sub>-Na<sub>2</sub>SO<sub>4</sub> Flux

Anisotropic BaBi4Ti4O15 powder was synthesized by a molten salt synthesis (MSS) method in K2SO4-Na2SO4flux and the effects of different process parameters such as calcining temperature, and ratio of salt to reactant (R) on the phase formation and morphology of anisotropic BaBi4Ti4O15particles were also investigated. The as-synthesized powder calcined at 850-950°C exhibits a single tetragonal BaBi4Ti4O15phase. The morphology of BaBi4Ti4O15powder could be adjusted by changing the synthesis conditions. The average particle size (APS) of BaBi4Ti4O15powder increased with R changing from 0.8 to 1.0, while it decreased with further increasing of R to 1.2. In addition, the APS increased with increasing calcining temperature and it showed an Arrhenius dependence on the temperature. The corresponding apparent activation energy for particle growth is 31.9kJ/mol for calcining temperature of 850-1000°C.

Molten salt synthesis of Ba(Mg1/3 Nb2/3)O3 powder

The single-phase Ba(Mg1/3 Nb2/3)O3 (BMN) powder was successfully prepared by the KCl molten salt synthesis (MSS) method. The temperature for single-phase BMN powders by MSS was about 400°C lower than that by the solid-phase method. The average particle size (APS) was about 0.91 μm at 900°C and increased with increasing synthesis temperature. Based on the APS, the activation energy for particle growth in the MSS, whose value was 64.1 kJmol-1, was attained. The sinterability of the powder prepared by MSS method was better than that prepared by solid-phase method.

Surface Area Loss of Supported Platinum in Polymer Electrolyte Fuel Cells

Life tests of polymer‐electrolyte fuel cells using supported Pt catalyst in thin film catalyst layers are run for up to 4000 h at maximum power. Particle ripening is readily evident using these types of electrodes in which the high catalyst utilization efficiency apparently subjects the majority of the platinum to conditions that sustain particle growth. X‐ray diffraction analyses indicate that the initial platinum specific surface areas of 100 m2/g Pt eventually stabilize to about 40 to 50 m2/g in the cathode and 60 to 70 m2/g in the anode. Interestingly, this loss in surface area does not affect the apparent catalytic activity of these fuel cell electrodes. A crystallite migration particle growth mechanism is suggested by the shape of the particle size distribution curves. Since the presence of liquids is known to lower the activation energy for particle growth, the particle size difference between the two electrodes may possibly be attributable to the different hydration levels at the anode and the cathode in operating polymer electrolyte fuel cells.

Sintering TiO2 in HCl Atmospheres

TiO2 was sintered in HCl atmospheres to enhance the effects of vapor transport. Little or no densification is observed for temperatures between 1000° and 1300°C. Particle coarsening occurs at temperatures above 1200°C. The apparent activation energy for particle growth is 114 kJ/mol. It is concluded that the primary mass‐transport mechanism is vapor transport while the particle growth rate is limited by grain‐boundary mobility.

Characterization of Sputtered Gold-Tungsten and Gold-Molybdenum Metallizations for Microwave Power Transistors

The stability and hence reliability of sputtered gold-tungsten and gold-molybdenum films has been determined as a function of anneal temperature up to 650°c. Isothermal and isochronal resistivity experiments have yielded an activation energy for particle growth in 3000 A gold films of 0.4 eV below 200oc and 0.7 eV about 200oc. The activation energy for the refractory layer was calculated to be 1.8 eV from particle size measurements. Annealing the films up to 500oc for 24 h resulted in a decrease of internal stress in gold from 2.5 x 109 dyn/cm2 to 0.2 x 109 dyn/cm2. Similar decreases in internal stress were observed for tungsten and molybdenum. The observed results are discussed in terms of microstructural changes.