Self-diffusion of large solid clusters in a liquid by molecular dynamics simulation

Development of an electrochemical process for production of nano-copper oxides: Agglomeration kinetics modeling

The main objective of this study was the development of a simple, clean, and industrial applicable electrochemical process for production of high pure nano-copper oxides from mining and industrial resources (e.g., ore, spent, slag and wastewater). To conduct the proposed process, a special set up containing an electrochemical cell in an ultrasonic system (28 kHz and 160 W) was proposed. Accordingly, using this set up and applying appropriate voltage (≈ 5 V) at 25 °C, in the presence of N₂ gas, the simultaneous anode dissolution and nano-copper oxides formation (≈ 24 nm) can be occurred, rapidly (less than 45 min). Then, the effect of N₂ gas and free radicals generated by ultrasonic irradiation was studied. The results showed, in the absence of ultrasonic irradiation and N₂, an increase of electrolyte pH from 6.42 to 10.92, a decrease of electrolyte Eh from 285 mV to -1.14 V, and formation of copper nanoparticles. While, in the presence of ultrasonic and N₂, the CuO nanoparticles were formed due to presence of H₂O₂ generated by interaction of free radicals. Moreover, a novel method for kinetics modeling of nanoparticles agglomeration was proposed according to distributed activation energy model and Arrhenius parameters variation. The results showed that, in the absence of ultrasonic irradiation, the nanoparticle agglomerates were firstly formed (interface controlled mechanism) and then, the diffusion of nanoparticle agglomerates was occurred (diffusion controlled mechanism). Therefore, the control of nanoparticles size and shape may be impossible without surfactant. Also, in the presence of ultrasonic irradiation, the whole of agglomeration process followed interface controlled mechanism. Therefore, using ultrasonic irradiation, the nanoparticles shape and size don't change due to prevention of agglomerates diffusion.

Molecular dynamics simulation of fractal aggregate diffusion

The diffusion of fractal aggregates constructed with the method by Thouy and Jullien [J. Phys. A 27, 2953 (1994)] comprised of N(p) spherical primary particles was studied as a function of the aggregate mass and fractal dimension using molecular dynamics simulations. It is shown that finite-size effects have a strong impact on the apparent value of the diffusion coefficient (D), but these can be corrected by carrying out simulations using different simulation box sizes. Specifically, the diffusion coefficient is inversely proportional to the length of a cubic simulation box, and the constant of proportionality appears to be independent of the aggregate mass and fractal dimension. Using this result, it is possible to compute infinite dilution diffusion coefficients (D(o)) for aggregates of arbitrary size and fractal dimension, and it was found that D(o)∝N(p)(-1/df), as is often assumed by investigators simulating brownian aggregation of fractal aggregates. The ratio of hydrodynamic radius to radius of gyration is computed and shown to be independent of mass for aggregates of fixed fractal dimension, thus enabling an estimate of the diffusion coefficient for a fractal aggregate based on its radius of gyration.