Ultrasonic Modification of Aluminum Surfaces: Comparison between Thermal and Ultrasonic Effects.

Ultrasound has become an increasingly popular tool in the modification of metal surfaces, imbuing them with various desired characteristics and functionalities. The exact role played by ultrasound in such processes remains largely speculative and thus requires clarification. In this study, aluminum was taken as a model metal to probe the nature of the surface modification, focusing on both chemical and physical changes. Using metal plates as substrates, the formation of a characteristic porous surface structure was ascertained to arise from a purely thermal mechanism, with the ultrasound providing an inhibitory influence when compared with controlled experiments matching the thermal conditions of sonication. No beneficial effect was observed through sonication, with regards to surface texture, porosity, and electrochemistry. However, for metal powders, a pronounced change in the phase composition was observed following ultrasonic exposure, largely attributed to the growth of bayerite from the surface. The immobilization of the powder on a thin epoxy film nullified such effects. This suggests that the changes in phase composition are due to the effect of ultrasound-induced mechanical stirring and high speed particle motion on the dissolution and reprecipitation of the metal oxide and hydrated oxide species. This work is of significant value to researchers both in materials science and in sonochemistry

Growth of Mesoporous Silica Nanoparticles Monitored by Time-Resolved Small-Angle Neutron Scattering

Since the first development of surfactant-templated mesoporous silicas, the underlying mechanisms behind the formation of their structures have been under debate. Here, for the first time, time-resolved small-angle neutron scattering (tr-SANS) is applied to study the complete formation of mesoporous silica nanoparticles. A distinct advantage of this technique is the ability to detect contributions from the whole system, enabling the visualization not only of particle genesis and growth but also the concurrent changes to the coexistent micelle population. In addition, using contrast-matching tr-SANS, it is possible to highlight the individual contributions from the silica and surfactant. An analysis of the data agrees well with the previously proposed “current bun” model describing particle growth: Condensing silica oligomers adsorb to micelles, reducing intermicellar repulsion and resulting in aggregation to form initial particle nuclei. From this point, the growth occurs in a cooperative manner, with condensing silica filling the gaps between further aggregating micelles. The mechanistic results are discussed with respect to different reaction conditions by changing either the concentration of the silica precursor or the temperature. In doing so the importance of in situ techniques is highlighted, in particular, tr-SANS, for mechanism elucidation in the broad field of materials science

Ultrasonic Approach for Formation of Erbium Oxide Nanoparticles with Variable Geometries

Ultrasound (20 kHz, 29 W·cm^–2) is employed to form three types of erbium oxide nanoparticles in the presence of multiwalled carbon nanotubes as a template material in water. The nanoparticles are (i) erbium carboxioxide nanoparticles deposited on the external walls of multiwalled carbon nanotubes and Er₂O₃ in the bulk with (ii) hexagonal and (iii) spherical geometries. Each type of ultrasonically formed nanoparticle reveals Er³⁺ photoluminescence from crystal lattice. The main advantage of the erbium carboxioxide nanoparticles on the carbon nanotubes is the electromagnetic emission in the visible region, which is new and not examined up to the present date. On the other hand, the photoluminescence of hexagonal erbium oxide nanoparticles is long-lived (μs) and enables the higher energy transition (⁴S_3/2–⁴I_15/2), which is not observed for spherical nanoparticles. Our work is unique because it combines for the first time spectroscopy of Er³⁺ electronic transitions in the host crystal lattices of nanoparticles with the geometry established by ultrasound in aqueous solution of carbon nanotubes employed as a template material. The work can be of great interest for “green” chemistry synthesis of photoluminescent nanoparticles in water