Characterization of acoustic streaming in water and aluminum melt during ultrasonic irradiation

It is well known that ultrasonic cavitation causes a steady flow termed acoustic streaming. In the present study, the velocity of acoustic streaming in water and molten aluminum is measured. The method is based on the measurement of oscillation frequency of Karman vortices around a cylinder immersed into liquid. For the case of acoustic streaming in molten metal, such measurements were performed for the first time. Four types of experiments were conducted in the present study: (1) Particle Image Velocimetry (PIV) measurement in a water bath to measure the acoustic streaming velocity visually, (2) frequency measurement of Karman vortices generated around a cylinder in water, and (3) in aluminum melt, and (4) cavitation intensity measurements in molten aluminum. Based on the measurement results (1) and (2), the Strouhal number for acoustic streaming was determined. Then, using the same Strouhal number and measuring oscillation frequency of Karman vortices in aluminum melt, the acoustic streaming velocity was measured. The velocity of acoustic streaming was found to be independent of amplitude of sonotrode tip oscillation both in water and aluminum melt. This can be explained by the effect of acoustic shielding and liquid density.     

Aluminum–Carbon Interaction at the Aluminum–Graphene and Aluminum–Graphite Interfaces

The interaction of liquid and solid aluminum with the graphene and graphite surfaces is studied using the density functional theory and a molecular dynamics simulation. The Morse potential is parameterized using the results of ab initio calculations in order to describe the interaction between aluminum and carbon atoms. This potential is used to investigate the interaction of a molten aluminum drop with the (0001) graphite surface theoretically. The properties of the free aluminum melt surface and the contact surface formed upon wetting graphite by the molten drop are calculated. The calculation results agree well with the available experimental data.     

Numerical simulation of the 3-dimensional combustion of aluminized heterogeneous propellants

We report the first 3-dimensional simulations of aluminized propellant combustion, accounting for heat conduction in the solid, combustion in the gas-phase, and coupling of these via the irregularly moving propellant surface, one that can not be defined by a single-valued height function. The simulations are used to examine the dynamics of aluminum particles in the near-neighborhood of the surface after detachment, and to provide an estimate of the time to ignition of the particles, and their speed and height above the surface at ignition. In addition, we examine the temperature history of the particles during their rise to the surface, determine whether they melt or not, and in this way test Cohen’s well-known melting criterion. And, we discuss a simple model which provides insights into how aluminum particles floating on a binder melt layer would migrate because of surface tension effects, and calculate an average migration distance that is consistent with previous agglomeration studies.     

Acoustic probing of two-temperature relaxation initiated by action of ultrashort laser pulse

Ultrashort laser pulse transfers metal into a two-temperature warm dense matter state and triggers a chain of hydrodynamic and kinetic processes—melting, expansion, stretching, creation of tensile stress and transition into metastable state. We study the response of aluminum film deposited on a glass substrate to irradiation by a pump laser pulse transmitted through glass. Several films with thicknesses from 350 to 1200 nm have been investigated. The smallest thickness is of the order of the heating depth d _T∼100 nm in Al. The d _T-layer and the free rear side of the film are coupled through pressure waves propagating between them. Therefore, the processes within d _T-layer affects the time dependent displacement Δ x _rear(t) of the rear surface. We compare simulated and experimental dependencies Δ x _rear(t) obtained by the pump–probe technique. It allows us to define a thickness of molten Al layer and explore the two-temperature processes occurring inside the heated layer.     

Ultrasound-induced cytolysis of cancer cells is enhanced in the presence of micron-sized alumina particles

Micron-sized alumina particles have been shown to enhance sonochemical free radical formation in aqueous solutions and simultaneously increase the solution temperature and acoustic (white) noise, effects attributable to enhanced inertial cavitation [T. Tuziuti, J. Phys. Chem. A 109 (2005) 4869–4872]. In the current study, the same ultrasound exposure system was applied to in vitro cancer cells as a model system to determine the effect of alumina particles on the long-term survival of cells and on the major pathways of cell death, i.e., either apoptosis or necrosis. Following 6 h of incubation after ultrasound treatment, it was found that the cells died mainly through necrosis, irrespective of whether the exposure was conducted in the presence of alumina particles or not. Alumina particles were non-toxic to cells alone, but were found to decrease the long-term survivability of cells that survived the initial exposure. This effect depended on the size and concentration of particles. These results correlated well with the effect of alumina particles on the sonochemical oxidation of KI under the same exposure conditions. Spin-trapping with 5,5-dimethyl-pyroline N-oxide (DMPO) and electron spin resonance spectroscopy indicated that the sonochemical formation of OH radicals increased in the presence of alumina particles. The current study is consistent with the well known observation that micron-sized particles enhance the acoustic cavitation process.     

Sonochemical synthesis of porous gold nano- and microparticles in a Rosette cell

We report the synthesis of Au nano- and microparticles that relies on α-D-glucose (C₆H₁₂O₆) as the reducer and stabilizer in a Rosette cell under 20 kHz ultrasound irradiation. The chemical and physical effects of ultrasonic irradiation on the synthesis were investigated. The results showed that an optimum pH is required for the formation of insoluble Au(0) particles. Upon irradiation, low pH yielded Au nanoparticles while high pH resulted in microparticles. The Au surface capping by α-D-glucose hydroxyl and carbonyl groups was confirmed by Fourier transform infrared (FT-IR) spectroscopy. X-ray diffraction (XRD) analysis indicated that the Au particles crystallize within the face-centered-cubic (FCC) cell lattice. Moreover, continuous sonication reduced larger amounts of the Au precursor compared to the intermittent mode. Furthermore, tuning sonication time and mode influences the particle size and porosity as characterized by scanning and transmission electron microscopy. Our results shed a new light into the importance of the experimental and ultrasound parameters in obtaining Au particles of desired features through sonochemistry.     

Wave instability of a molten metal layer formed by intense laser irradiation

Wave structure of a molten metal layer flowing over the walls of a vapor-gas cavern that appears as an intense laser radiation penetrates deep into condensed media is studied theoretically taking into account surface tension, gravitation, thermocapillary effect, and nonuniform evaporation from the free surface of the melt. A long-wavelength evolution equation describing the evolution of nonlinear waves on the free surface of a plane molten layer is derived. The spatially periodic running solution to this equation is obtained, and the main characteristics (amplitude and period) of the nonlinear wave structures are determined.     

Design of Some Oxide/Metal Composite Supports and Catalysts

Textural, mechanical and catalytic properties of porous composite materials Al₂O₃/Al, MeO _x (Me)/Al₂O₃/Al with metal particles homogeneously distributed in the alumina matrix were studied. These materials were prepared by mixing the powdered components with aluminum followed by hydrothermal treatment and calcination. The macroporous structure was shown to be controlled by the size of large (>microns) particles in starting blends. The mesoporous structure is primarily determined by the properties of alumina formed by dehydration of hydroxide produced in turn via aluminum oxidation by water. The mechanical strength of porous cermets is determined by the number and properties of contacts between micron-size components of composites. Improved catalytic performance of composites is ensured by the developed macroporous structure providing enhanced mass transfer inside the cermet granules.     

In situ observation and analysis of ultrasonic capillary effect in molten aluminium

An in situ synchrotron radiographic study of a molten Al–10 wt% Cu alloy under the influence of an external ultrasonic field was carried out using the Diamond-Manchester Branchline pink X-ray imaging at the Diamond Light Source in UK. A bespoke test rig was used, consisting of an acoustic transducer with a titanium sonotrode coupled with a PID-controlled resistance furnace. An ultrasonic frequency of 30 kHz, with a peak to peak amplitude at 140 microns, was used, producing a pressure output of 16.9 MPa at the radiation surface of the 1-mm diameter sonotrode.This allowed quantification of not only the cavitation bubble formation and collapse, but there was also evidence of the previously hypothesised ultrasonic capillary effect (UCE), providing the first direct observations of this phenomenon in a molten metallic alloy. This was achieved by quantifying the re-filling of a pre-existing groove in the shape of a tube (which acted as a micro-capillary channel) formed by the oxide envelope of the liquid sample. Analytical solutions of the flow suggest that the filling process, which took place in very small timescales, was related to micro-jetting from the collapsing cavitation bubbles. In addition, a secondary mechanism of liquid penetration through the groove, which is related with the density distribution of the oxides inside the groove, and practically to the filtration of aluminium melt from oxides, was revealed. The observation of the almost instantaneous re-filling of a micro-capillary channel with the metallic melt supports the hypothesised sono-capillary effect in technologically important liquids other than water, like metallic alloys with substantially higher surface tension and density.     

Surface hydrophobic modification of ultra-fine aluminum silicate by mechanically grinding and heating

Highly hydrophobic ultra-fine aluminum silicate can be obtained by mechanically grinding and heating both aluminum silicate particles and octadecoic acid. The results showed that the organic modifier was adsorbed on the surface of aluminum silicate particles where a little new organic group was generated, and the hydrophobicity of ultra-fine aluminum silicate powder was enhanced. Grafting reaction was mainly carried out in heating process. Due to both grinding and heating, the modifier spread on the surface of particle completely and bonded with Si to form Si-O-C, and with Al to form acyl alumina in bidentate coordination. As a result, a highly hydrophobic layer was formed on the powder surface. However, it was unsuitable for heating temperature to exceed 180 °C, or else the layer would be destroyed. All the above points represent a useful effort for surface-grafting modification of powder particles. In this study, a novel method as a reference was provided on the surface-grafting modification of powder particles.     

Ultrasonic inactivation of Bacillus alpha-amylase. I. effect of gas content and emitting face of probe

Sonoinactivation of alpha-amylase from Bacillus amyloliquefacience was studied at a constant frequency of 30 kHz. The effect of sonotrode emitting face and gas content of medium on the efficiency of enzyme inactivation were investigated at different time-temperature combinations and generation of OH free radicals was also monitored. The results showed that ultrasound effectively inactivated alpha-amylase with a minimum overall inactivation rate at 50 degrees C. The tip diameter of the sonotrode and the gas content of the medium both significantly affected the rate of enzyme inactivation. The increase of tip diameter increased the effect of ultrasound on the enzyme, while the removal of dissolved gas adversely influenced the cavitational events and reduced the rate of enzyme inactivation. Calculation of the kinetic and activation parameters revealed that ultrasound decreased the activation energy, E(a), activation enthalpy, DeltaH(#), and the activation Gibbs free energy, DeltaG(#), and strongly reduced the activation entropy, DeltaS(#), down to negative values. This huge reduction in activation entropy was attributed to the different mechanisms of inactivation induced by heat and ultrasound. It is proved in this study that ultrasonically generated OH free radicals and shear forces, which arise from pulsation- or collapse of bubbles, both can destabilize the enzyme, although their contribution to the overall inactivation varies depending on the temperature and the tip diameter of the sonotrode.     

Fabrication of amine-functionalized magnetite nanoparticles for water treatment processes

The anatase phase of titania (TiO₂) nano-photocatalysts was prepared using a modified sol gel process and thereafter embedded on carbon-covered alumina supports. The carbon-covered alumina (CCA) supports were prepared via the adsorption of toluene 2,4-diisocyanate (TDI) on the surface of the alumina. TDI was used as the carbon source for the first time for the carbon-covered alumina support system. The adsorption of TDI on alumina is irreversible; hence, the resulting organic moiety can undergo pyrolysis at high temperatures resulting in the formation of a carbon coating on the surface of the alumina. The TiO₂ catalysts were impregnated on the CCA supports. X-ray diffraction analysis indicated that the carbon deposited on the alumina was not crystalline and also showed the successful impregnation of TiO₂ on the CCA supports. In the Raman spectra, it could be deduced that the carbon was rather a conjugated olefinic or polycyclic hydrocarbons which can be considered as molecular units of a graphitic plane. The Raman analysis of the catalysed CCAs showed the presence of both the anatase titania and D and G band associated with the carbon of the CCAs. The scanning electron microscope micrographs indicated that the alumina was coated by a carbon layer and the energy dispersive X-ray spectra showed the presence of Al, O and C in the CCA samples, with the addition of Ti for the catalyst impregnated supports. The Brunauer Emmet and Teller surface area analysis showed that the incorporating of carbon on the alumina surface resulted in an increase in surface area, while the impregnation with TiO₂ resulted in a further increase in surface area. However, a decrease in the pore volume and diameter was observed. The photocatalytic activity of the nanocatalysts was studied for the degradation of Rhodamine B dye. The CCA-TiO₂ nanocatalysts were found to be more photocatalytically active under both visible and UV light irradiation compared to the free TIO₂ nanocatalysts.     

氧化锆陶瓷板激光切割熔化物颗粒形态研究

本文设计了一套加工装置,对氧化锆陶瓷板激光切割的熔化物颗粒进行收集,并采用Imagine-Pro Pluse(IPP)图像处理软件对熔化物颗粒的形态(数量、形状、平均直径、标准差及其分布情况)进行研究。通过气熔比控制方法,对板厚分别为0.8 mm、1 mm、1.5 mm、3 mm的氧化锆陶瓷板进行激光切割实验。实验结果表明:不同板厚参数下,球形熔化物颗粒所占百分比范围从99.21%降到89.81%,圆饼形从0.79%升至7.44%,哑铃形从0升至2.75%。随着板厚的增加,圆饼形和哑铃形颗粒所占百分比增大,球形颗粒所占百分比降低,球形颗粒平均直径和标准差随之增大,切面粗糙度由2.287μm增加到5.946μm。建立了熔化物去除几何模型,阐述了熔化物颗粒与切割质量的关系,球形颗粒所占的百分比越大,平均直径和标准差越小,切割质量越好,最终获得质量较高切割样件。     

Ultrasound-assisted anodization of aluminum in oxalic acid

Porous anodic alumina is an important nanoscale template for fabrication of various nanostructures. We report a new ultrasound-assisted anodization process in oxalic acid. Under the continuous irradiation of ultrasound, the one-step-anodized sample has a smooth and clean surface, and two-step-anodization brings ordered porous anodic alumina with higher growth rate of 52 μm/h. The ultrasound applied during the anodization can clean the surface and enhance the nanopore growth since it can accelerate the oxide dissolving on the electrolyte/oxide interface. The ultrasound-assisted anodization may be utilized for other anodizations.     

Use of ultrasound irradiation during acid etching of the 2024 aluminum alloy: Effect on corrosion resistance after anodization

While aluminum alloys are widely used in industrial applications, their protection by anodization as surface treatment always requires a preparation step by alkaline or acid etching. In this paper, use of ultrasound during the acid etching step on the 2024 aluminum alloy was investigated. Etching rate, calculated as of weight loss, was measured under ultrasound irradiation, and compared to silent conditions. The etched surface was characterized by Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/EDS) and X-Ray Diffraction (XRD). Surface treatment was performed up to the final anodization step samples, and their final properties were evaluated as a function of various pre-treatments, including acid etching under ultrasound. The main evaluation concerned anticorrosion properties through electrochemical tests: polarization measurements and electrochemical impedance spectroscopy (EIS) in NaCl solution. Finally, use of ultrasound irradiation during acid preparation induced a beneficial effect on the corrosion performance of the anodic layer.     

Measurement of Inherent Material Density of Nanoparticle Agglomerates

We describe a new technique to measure the size dependent inherent material density of chain agglomerate particles. Measurements were carried out for diesel soot and aluminum/alumina agglomerate particles in the nanometer size range. Transmission electron microscopy was used to measure the volumes of agglomerate particles that were preselected by mass using an aerosol particle mass analyzer. We found that the density of diesel exhaust particles increased from 1.27 to 1.78g/cm³ as particle mobility size increased from 50 to 220nm. When particles are preheated to remove volatile components, the density was 1.77±0.07g/cm³, independent of particle size. The densities measured after heating correspond to the inherent material density of diesel soot. Measurements with aluminum nanoparticles were made downstream of a furnace where aluminum (Al) was converted to alumina (Al₂O₃). From measurements of inherent material density we were able to infer the extent of reaction, which varied with furnace temperature.     

Elemental surface enrichment in flame synthesis: A mechanism associated with particle melting-solidification

Elemental surface enrichment is important for functionalities of flame-synthesized particle materials, but its mechanism is poorly understood. In this paper, a mechanism associated with particle melting-solidification is proposed based on an experimental study. Y₂O₃:Eu particles were generated by flame assisted spray pyrolysis (FASP), using H₂/O₂ flames or H₂/air flames. The particles were analyzed by transmission electron microscopy (TEM), X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICP-MS), and X-ray photoelectron spectroscopy (XPS). H₂/O₂ flames resulted in particles with Eu surface enrichment, i.e. the surface Eu concentration was several times higher than the overall Eu concentration; there was no elemental surface enrichment in particles from H₂/air flames. The Eu surface enrichment in H₂/O₂ flames was attributed to elemental partitioning during solidification of molten Y₂O₃:Eu particles; in H₂/air flames the particles did not melt and hence there was no elemental surface enrichment. The findings of this study suggest that elemental surface enrichment may be a common phenomenon for binary metal oxide particles that experience melting-solidification. Such particles should be examined for elemental surface enrichment, both for understanding their functionality and for their potential biological effects.     

Graphene oxide templated alumina nanosheet for the removal of As(V)

The current paper demonstrates a simple preparative method of alumina nanosheet through controlled hydrolysis and condensation on the surface of graphene oxide (GO). Taking into account the moisture sensitivity of the alumina precursor, its hydrolysis was carried out after introduction of interaction between the functional groups of GO and the alumina precursor in a non-aqueous solvent. On pyrolysis of the aluminum oxide-GO composite, a free-standing alumina nanosheet was obtained, which was further confirmed by XRD, TEM, STEM-EDX, FE-SEM, TGA, and N₂ physisorption. Due to a favorable nanosheet structure, the alumina nanosheets showed a better performance in the removal of As (V) when compared to alumina prepared without GO template, even though the latter had a larger surface area than the alumina nanosheet.     

Simulation of ultrasound influence on melt convection for the growth of GaxIn1−xSb and Si single crystals by the Czochralski method

The flow simulation for Ga_xIn_1−xSb and Si melts was conducted for quasi-steady conditions. The maximum velocity was under the solid–liquid interface near periphery of the crystals. An introduction of ultrasound into the liquid formed a standing wave channel under the solid–liquid interface, which acted on melt particles. The calculations of convective and ultrasonic forces acting on the particles in the melt showed that the ultrasonic force is much higher than the convective force.     

Surface properties of mesoporous catalytic supports

The influence of the processing parameters on the properties of zirconia, alumina and silica MCM-41 has been investigated. Digestion of the precursors leads to zirconia, alumina and silica MCM-41 with higher surface area and better thermal stability. The effects of digestion are attributed to increased dehydroxylation, strengthening of the network between primary particles, and to a decrease in the number of surface defects. In the case of zirconia and alumina, digestion also increased the surface acidity. This may be due to the smaller crystallites which expose low-coordinated sites at the surface. Al-MCM-41 prepared by post-synthesis grafting of aluminum is compared with MCM-41 prepared by direct incorporation of aluminum during synthesis. The surface Si/Al ratio was determined using XPS and correlated to the catalytic activity of the Al-MCM-41 for the synthesis of jasminaldehyde.