Browsing by Subject "Particle size analysis"

Now showing 1 - 4 of 4

Open Access
Mesoscopic model of nucleation and Ostwald ripening/stepping: Application to the silica polymorph system
(American Inst of Physics, Woodbury, NY, United States, 2000) Ozkan, G.; Ortoleva, P.
Precipitation is modeled using a particle size distribution ~PSD! approach for the single or multiple polymorph system. A chemical kinetic-type model for the construction of the molecular clusters of each polymorph is formulated that accounts for adsorption at a heterogeneous site, nucleation, growth, and Ostwald ripening. When multiple polymorphs are accounted for, Ostwald stepping is also predicted. The challenge of simulating the 23 order of magnitude in cluster size ~monomer, dimer, . . . , 1023-mer! is met by a new formalism that accounts for the macroscopic behavior of large clusters as well as the structure of small ones. The theory is set forth for the surface kinetic controlled growth systems and it involves corrections to the Lifshitz–Slyozov, Wagner ~LSW! equation and preserves the monomer addition kinetics for small clusters. A time independent, scaled PSD behavior is achieved both analytically and numerically, and the average radius grows with Rave}t1/2 law for smooth particles. Applications are presented for the silica system that involves five polymorphs. Effects of the adsorption energetics and the smooth or fractal nature of clusters on the nucleation, ripening, and stepping behavior are analyzed. The Ostwald stepping scenario is found to be highly sensitive to adsorption energetics. Long time scaling behavior of the PSD reveals time exponents greater than those for the classical theory when particles are fractal. Exact scaling solutions for the PSD are compared with numerical results to assess the accuracy and convergence of our numerical technique. © 2000 American Institute of Physics. @S0021-9606~00!70123-1#
Open Access
Real-space condensation in a dilute Bose gas at low temperature
(Natsional'na Akademiya Nauk Ukrainy, 2001) Kulik, I. O.
We show with a direct numerical analysis that a dilute Bose gas in an external potential - which is choosen for simplicity as a radial parabolic well - undergoes at certain temperature Tc a phase transition to a state supporting macroscopic fraction of particles at the origin of the phase space (r = 0, p = 0). Quantization of particle motion in a well wipes out sharp transition but supports a distribution of radial particle density p(r) peacked at r = 0 (a real-space condensate) as well as the phase-space Wigner distribution density W(r, p) peaked at r = 0 and p = 0 below the crossover temperature Tc* of order of Tc. Fixed-particle-number canonical ensemble which is a combination of the fixed-N condensate part and the fixed-μ excitation part is suggested to resolve the difficulty of large fluctuation of the particle number (δN ∼ N) in the Bose-Einstein condensation problem treated within the orthodox grand canonical ensemble formalism.
Open Access
Sorption of Cs+ and Ba2+ on magnesite
(Cambridge University Press, 1998) Shahwan, Talal; Süzer, Şefik; Erten, Hasan N.
Sorption behavior of Cs and Ba ions on magnesite was studied using the radiotracer method complemented by X-ray photoelectron spectroscopy. Cs and Ba were used as radiotracers. The sorption of Cs is seen to be temperature dependent, whereas Ba sorption is not much affected by temperature changes. It was found that Freundlich type isotherms provide an adequate description of the sorption process. The magnitude of the free energy of sorption are seen to be within the 8-16 kj/mol range. It can be remarked that the sorption of Cs and Ba on magnesite is a fast process suggesting an ion exchange type mechanism, mainly taking place at the surface of mineral particles.
Open Access
Synthesis of blue-shifted luminescent colloidal GaN nanocrystals through femtosecond pulsed laser ablation in organic solution
(Springer Netherlands, 2016-05) Demirel, A.; Öztaş T.; Kurşungöz, C.; Yılmaz, İ.; Ortaç, B.
We demonstrate the synthesis of GaN nanocrystals (NCs) with the sizes of less than the doubled exciton Bohr radius leading quantum confinement effects via a single-step technique. The generation of colloidal GaN nanoparticles (NPs) in organic solution through nanosecond (ns) and femtosecond (fs) pulsed laser ablation (PLA) of GaN powder was carried out. Ns PLA in ethanol and polymer matrix resulted in amorphous GaN-NPs with the size distribution of 12.4 ± 7.0 and 6.4 ± 2.3 nm, respectively, whereas fs PLA in ethanol produced colloidal GaN-NCs with spherical shape within 4.2 ± 1.9 nm particle size distribution. XRD and selected area electron diffraction analysis of the product via fs PLA revealed that GaN-NCs are in wurtzite structure. Moreover, X-ray photoelectron spectroscopy measurements also confirm the presence of GaN nanomaterials. The colloidal GaN-NCs solution exhibits strong blue shift in the absorption spectrum compared to that of the GaN-NPs via ns PLA in ethanol. Furthermore, the photoluminescence emission behavior of fs PLA-generated GaN-NCs in the 295–400 nm wavelength range is observed with a peak position located at 305 nm showing a strong blue shift with respect to the bulk GaN.