Numerical simulations of sonochemical production and oriented aggregation of BaTiO3 nanocrystals

Numerical simulations of sonochemical production and oriented attachment of BaTiO₃ nanocrystals are performed in aqueous solution with pH 14. It is suggested that most significant effect of ultrasound is the dissolution of Ti-based gel in aqueous solution. It results in the dissolution-precipitation mechanism in the production of BaTiO₃ nanoparticles, while with mechanical stirring without ultrasound it is the in situ mechanism that BaTiO₃ is gradually formed on Ti-based gel. The oriented attachment of spherical BaTiO₃ nanocrystals occurs by van der Waals torque (Casimir torque). Large aggregates of nanocrystals do not attach with each other as the repulsive double layer interaction is stronger for larger aggregates. For smaller spherical nanocrystals, the alignment of the crystal axes is less accurate due to more significant rotational Brownian motion of the nanocrystals.     

Stoichiometry of BaTiO3 nanoparticles

Barium titanate nanoparticles with various nominal Ba/Ti ratios were prepared through direct synthesis from solution (DSS) and further annealed at different temperatures. Their deviation from stoichiometry was studied through XRD analysis, and a large deviation from stoichiometry has been observed. The grain size we studied ranges from 50 nm to 1 μm. For the as-prepared particles, the grain size is about 50 nm, and the maximum excess of Ti is over 15%. For the samples annealed at 800 °C, the grain size is increased to 100 nm, and the maximum excesses of Ba and Ti are 8 and 9%, respectively. The defects formed during synthesis and surface effect of nanoparticles are both estimated for their contributions to the deviation from stoichiometry, and the defects have been found to be mainly responsible for the large deviation observed.     

Prediction of flexoelectricity in BaTiO3 using molecular dynamics simulations

Flexoelectric effect, referring to the strain gradient induced polarization, widely exists in dielectric materials, but its molecular dynamics has not been studied so much so far. In this work, the radial distribution function of BaTiO₃ and the phase transition temperatures have been investigated, and the results show that the core-shell potential model is effective and the structure of BaTiO₃ is stable in a temperature range of 10 K-150 K. Molecular dynamics simulated hysteresis loops of BaTiO₃ show that anisotropy can play an important role in the coercive field. Based on the rational simulation process, the effects of cantilever beam bent angle and fixed length on the polarization are analyzed. It is found that the small bent angle of the curved cantilever beam can give a proportional relationship with a fixed end length and a non-linear relationship is presented when the bent angle is much larger. The prediction of flexoelectric coefficient in BaTiO₃ is 18.5 nC/m. This work provides a computational framework for the study of flexoelectric effect by using molecular dynamics.     

A new effect of ultrasonication on the formation of BaTiO3 nanoparticles

A new effect of ultrasonic irradiation on the formation of BaTiO₃ particles was identified. Ultrasonication caused the aggregation of the original 5–10 nm BaTiO₃ particles in the same crystal axis and accelerated the formation of BaTiO₃ particles significantly. Furthermore, narrow size distribution was obtained for the aggregated particles under ultrasonic irradiation.     

Simple sonochemical synthesis and characterization of HgSe nanoparticles

Mercury selenide (HgSe) nanostructures were synthesized via a sonochemical method based on the reaction between HgCl(2), SeCl(4) and hydrazine hydrate (N(2)H(4)·H(2)O) in water, in presence of various capping agents. The effects of preparation parameters such as: the kind of capping agent and its amount, ultrasonic power, reaction time and temperature were investigated. It was found that morphology, particle size and phase of the products could be greatly affected by these parameters. HgSe nanostructures were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), photoluminescence spectroscopy (PL) and X-ray energy dispersive spectroscopy (EDS).     

Lipase-catalyzed diacylglycerol production under sonochemical irradiation

The present paper describes a protocol for production of diacylglycerol by the partial hydrolysis of soybean oil catalyzed by lipase under ultrasound irradiation. Better yields and shorter reaction times were obtained under sonication as compared to the thermal process.     

Intensification of hydroxyl radical production in sonochemical reactors

The efficacy of sonochemical reactors in chemical processing applications has been well established in the laboratory scale of operation though at a given set of operating parameters and no efforts have been directed in terms of maximizing the free radical production. In the present work, the effect of different operating parameters viz. pH, power dissipation into the system, effect of additives such as air, haloalkanes, titanium dioxide, iron and oxygen on the extent of hydroxyl radical formation in a sonochemical reactor have been investigated using salicylic acid dosimetry. Possible mechanisms for oxidation of salicylic acid in the presence of different additives have also been established. It has been observed that acidic conditions under optimized power dissipation in the presence of iron powder and oxygen result in maximum liberation of hydroxyl radicals as quantified by the kinetic rate constant for production of 2,5- and 2,3-dihydroxybenzoic acid. The study has enabled the optimization of the conditions for maximum efficacy of sonochemical reactors where free radical attack is the controlling mechanism for the chemical processing applications.     

Quinone-enhanced sonochemical production of nitric oxide from s-nitrosoglutathione

Sonolysis at 75 kHz of argon- and air-saturated aqueous solutions at pH 7.4 containing s-nitrosogluthathione (GSNO) enhances the production rate of nitric oxide (NO). The quinones, anthraquinone-2-sulfonate (AQ2S) and anthraquinone-2,7-disulfonate (AQ27S) further enhance the NO production over that produced in quinone-depleted sonicated solutions. In contrast, the hydrophobic quinones juglone (JQ) and 1,4-naphthoquinone (NQ) inhibit ultrasound-induced NO detection as compared to quinone-depleted solutions. Larger sonolytical decomposition of the hydrophobic quinones NQ and JQ, as compared to AQ2S and AQ27S, is detected which correlates with a larger production of pyrolysis-derived carbon-centered radicals. Reaction of those radicals with NO could explain NQ and JQ inhibition. This work suggests that sulfonated quinones could be used to enhance NO release from GSNO in tissues undergoing ultrasound irradiation.     

Numerical simulations for description of UV laser interaction with gold nanoparticles embedded in silica

We have performed simulations of laser energy deposition in an engineered absorbing defect (i.e. metal nanoparticle) and the surrounding fused silica taking into account various mechanisms for the defect-induced absorption of laser energy by SiO₂. Then, to simulate the damage process in its entirety, we have interfaced these calculations of the energy absorption with a 2-D Lagrange–Euler hydrodynamics code, which can simulate crack formation and propagation leading to craters. The validation of numerical simulations requires detailed knowledge of the different parameters involved in the interaction. To concentrate on a simple situation, we have made and tested a thin-film system based on calibrated gold nanoparticles (600-nm diameter) inserted between two silica layers. Some aspects of our simulations are then compared with our experimental results. We find reasonable agreement between the observed and simulated crater sizes.     

Effect of ultrasonic irradiation power on sonochemical synthesis of gold nanoparticles

In this work, optimized size distribution and optical properties in the colloidal synthesis of gold nanoparticles (GNPs) were obtained using a proposed ultrasonic irradiation assisted Turkevich-Frens method. The effect of three nominal ultrasound (20 kHz) irradiation powers: 60, 150, and 210 W have been analyzed as size and shape control parameters. The GNPs colloidal solutions were obtained from chloroauric acid (HAuCl₄) and trisodium citrate (C₆H₅Na₃O₇·2H₂O) under continuous irradiation for 1 h without any additional heat or stirring. The surface plasmon resonance (SPR) was monitored in the UV–Vis spectra every 10 min to found the optimal time for localized SPR wavelength (${\lambda }_{\mathrm{L}\mathrm{S}\mathrm{P}\mathrm{R}}$), and the 210 sample procedure has reduced the ${\lambda }_{\mathrm{L}\mathrm{S}\mathrm{P}\mathrm{R}}$ localization at 20 min, while 150 and 60 samples have showed ${\lambda }_{\mathrm{L}\mathrm{S}\mathrm{P}\mathrm{R}}$ at 60 min. The nucleation and growth of GNPs showed changes in shape and size distribution associated with physical (cavitation, temperature) and chemical (radical generation, pH) conditions in the aqueous solution. The results showed quasi-spherical GNPs as pentakis dodecahedron (${\lambda }_{\mathrm{L}\mathrm{S}\mathrm{P}\mathrm{R}}$ = 560 nm), triakis icosahedron (${\lambda }_{\mathrm{L}\mathrm{S}\mathrm{P}\mathrm{R}}$ = 535 nm), and tetrakis hexahedron (${\lambda }_{\mathrm{L}\mathrm{S}\mathrm{P}\mathrm{R}}$ = 525 nm) in a size range from 12 to 16 nm. Chemical effects of ultrasound irradiation were suggested in the disproportionation process, electrons of AuCl₂⁻ are rapidly exchanged through the gold surface. After AuCl₄⁻ and Cl⁻ were desorbed, a tetrachloroaurate complex was recycled for the two-electron reduction by citrate, aurophilic interaction between complexes AuCl₂⁻, electrons exchange, and gold seeds, the deposition of new gold atoms on the surface promoting the growth of GNPs. These mechanisms are enhanced by the effects of ultrasound, such as cavitation and transmitted energy into the solution. These results show that the plasmonic response from the reported GNPs can be tuned using a simple methodology with minimum infrastructure requirements. Moreover, the production method could be easily scalable to meet industrial manufacturing needs.     

Optimum bubble temperature for the sonochemical production of oxidants

Numerical simulations of bubble oscillations in liquid water irradiated by an ultrasonic wave are performed for various acoustic amplitudes and various ambient pressures. In the numerical simulations, effect of non-equilibrium evaporation and condensation of water vapor at the bubble wall and that of chemical reactions of gases and vapor inside a bubble are taken into account. The oxidants such as OH radicals, O radicals, H(2)O(2) molecules, and O(3) molecules are created from water vapor inside a heated bubble when a bubble collapses strongly. They are dispersed into the liquid and solutes are oxidized by the oxidants, which is called sonochemical reactions. The computer simulations have revealed that there exists the optimum bubble temperature, which is about 5500 K, for the production of the oxidants inside an air bubble because at higher bubble temperature the oxidants are strongly consumed inside a bubble by oxidizing nitrogen. Correspondingly, there exists an optimum acoustic amplitude for the production of the oxidants, which is about 2.2 atm when the ultrasonic frequency is 140 kHz and the ambient pressure is 1 atm. For an oxygen bubble, on the other hand, the amount of the oxidants created inside a bubble becomes nearly independent of the bubble temperature at the collapse above about 6000 K because nitrogen is absent.     

Numerical simulation of liquid velocity distribution in a sonochemical reactor

Ultrasonically induced flow is an important phenomenon observed in a sonochemical reactor. It controls the mass transport of sonochemical reaction and enhances the reaction performance. In the present paper, the liquid velocity distribution of ultrasonically induced flow in the sonochemical reactor with a transducer at frequency of 490 kHz has been numerically simulated. From the comparison of simulation results and experimental data, the ultrasonic absorption coefficient in the sonochemical reactor has been evaluated. To simulate the liquid velocity near the liquid surface above the transducer, which is the main sonochemical reaction area, it is necessary to include the acoustic fountain shape into the computational domain. The simulation results indicate that the liquid velocity increases with acoustic power. The variation of liquid height also influences the behavior of liquid velocity distribution and the mean velocity above the transducer centre becomes a maximum when the liquid height is 0.4 m. The liquid velocity decreases with increasing the transducer plate radius at the same ultrasonic power.     

Ultrasound artificially nucleated bubbles and their sonochemical radical production

We describe the ejection of bubbles from air-filled pits micromachined on a silicon surface when exposed to ultrasound at a frequency of approximately 200 kHz. As the pressure amplitude is increased the bubbles ejected from the micropits tend to be larger and they interact in complex ways. With more than one pit, there is a threshold pressure beyond which the bubbles follow a trajectory parallel to the substrate surface and converge at the center point of the pit array. We have determined the size distribution of bubbles ejected from one, two and three pits, for three different pressure amplitudes and correlated them with sonochemical OH radical production. Experimental evidence of shock wave emission from the bubble clusters, deformed bubble shapes and jetting events that might lead to surface erosion are presented. We describe numerical simulations of sonochemical conversion using the empirical bubble size distributions, and compare the calculated values with experimental results.     

Twin-seeded BaTiO3 ceramics

Grain size is important in the barium titanate ceramics used as capacitors and thermistors. Normally this is controlled by chemical additives and firing schedules, but growth twins also lead to increased grain size. Seeding experiments described in this paper demonstrate how the twins can be used to produce large-grained BaTiO₃ ceramics.     

Growth mechanism and optical property of ZnO nanoparticles synthesized by sonochemical method

ZnO nanoparticles have been synthesized by ultrasonic irradiation of an aqueous-alcoholic/aqueous-alcoholic-ethylenediamine (EDA) solutions of zinc nitrate and sodium hydroxide. ZnO nanoparticles possess hexagonal wurtzite structures and they exhibit special photoluminescence properties with a red-shift of 22 nm in UV emission band. It is found that the ultrasonic irradiation time and the solvents both influence the growth mechanism and optical properties of ZnO nanoparticles. The possible growth mechanism of ZnO nanoparticles formation by sonochemical method has been tried to discuss.     

Numerical simulations for sonochemistry

Numerical simulations for sonochemistry are reviewed including single-bubble sonochemistry, influence of ultrasonic frequency and bubble size, acoustic field, and sonochemical synthesis of nanoparticles. The theoretical model of bubble dynamics including the effect of non-equilibrium chemical reactions inside a bubble has been validated from the study of single-bubble sonochemistry. By the numerical simulations, it has been clarified that there is an optimum bubble temperature for the production of oxidants inside an air bubble such as OH radicals and H₂O₂ because at higher temperature oxidants are strongly consumed inside a bubble by oxidizing nitrogen. Unsolved problems are also discussed.     

A sonochemical method for the selective synthesis of alpha-HgS and beta-HgS nanoparticles

A novel sonochemical method for the selective synthesis of alpha-HgS (cinnabar) and beta-HgS (metacinnabar) nanoparticles in aqueous solutions is reported in this paper. alpha-HgS and beta-HgS nanoparticles have been selectively prepared by choosing sodium thiosulfate and thiourea as the sulfur source respectively. To study the crystalline structure, size, morphology and composition of the products, characterization techniques including X-ray powder diffraction, transmission electron microscopy, selected area electron diffraction, X-ray photoelectron spectroscopy and energy-dispersive X-ray analysis are employed. The optical properties of the nanoparticles are investigated by UV-visible absorption spectroscopic measurements. The direct band gap of the as-prepared alpha-HgS nanoparticles with an average size of 12 nm is calculated to be 2.8 eV according to the absorption spectrum. In the case of the beta-HgS nanoparticles with an average size of 13 nm, a broad absorption peak is observed in the UV-visible absorption spectrum, which can be ascribed to the special surface state of this sample. Probable mechanisms for the sonochemical formation of alpha-HgS and beta-HgS nanoparticles in aqueous solutions are presented. The optimum pH value of the stock solutions and the effect of sonication time on the particle size are also investigated.     

Numerical simulations of two-dimensional QED

We describe the computer simulation of two-dimensional QED on a 64 × 64 Euclidean space-time lattice using the Susskind lattice fermion action. The order parameter for chiral symmetry breaking and the low-lying meson masses are calculated for both the model with two continuum flavours, which arises naturally in this formulation, and the model with one continuum flavour obtained by including a nonsymmetric mass term and setting one fermion mass equal to the cut-off. Results are compared with those obtained using the quenched approximation, and with analytic predictions.     

Numerical simulations of generic singularities

Numerical simulations of the approach to the singularity in vacuum spacetimes are presented here. The spacetimes examined have no symmetries and can be regarded as representing the general behavior of singularities. It is found that the singularity is spacelike and that, as it is approached, the spacetime dynamics becomes local and oscillatory.     

Numerical simulations of the reditron

The reflected-electrons discrimination microwave generator (reditron) is a high-power, narrow-band, and single-mode microwave generator that makes exclusive use of the oscillatory character of the virtual-cathode of a relativistic electron beam. The complex, nonlinear character of the virtual-cathode device necessitates particle-in-cell plasma simulation techniques. Investigations indicate two sources of the radiation: (1) the trapped electrons reflexing between the real and virtual cathodes, and (2) the oscillation of the virtual cathode. In the conventional design, the two mechanisms coexist and interfere with each other destructively, causing degradation of the efficiency of microwave generation. The authors have investigated a configuration with a slotted, thick anode and an external magnetic field, which effectively eliminates the reflexing electrons. Two-dimensional particle-in-cell simulations showed that such a configuration exploits the oscillation of the virtual cathode exclusively, and it generates single-mode, narrow bandwidth, and high-power microwave radiation with a potential efficiency over 10%. It was found that further optimization could be achieved by the use of a density (current) modulated electron beam at appropriate frequencies