Article pubs.acs.org/JPCC
Ultrasonic Modification of Aluminum Surfaces: Comparison between Thermal and Ultrasonic Effects. Adam Brotchie,* Dimitriya Borisova, Valentina Belova, Helmuth Möhwald, and Dmitry Shchukin Max Planck Institute of Colloids and Interfaces, Am Mühlenberg 1, Golm/Potsdam 14476, Germany S Supporting Information *
ABSTRACT: 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. materials.15−17 This technique been applied not only to bulk metals but also to metal powders. Recently, Skorb et al.,17 have ultrasonically modified various metal powders, reporting them also to exhibit a sponge-like surface morphology with a concomitant change of crystal phase. Cavitation, when applied at the high intensities used in these studies, can lead to significant heating of the solution and the substrate itself, rendering the attribution of certain experimental observations exclusively to cavitation difficult. It is thought that cavitation may cause the porous hydrated metal oxide surface structure to form through removal of the passivating native oxide layer by microjetting, followed by radical-induced oxidation of the bare aluminum. However, it has been known since the 1970s that aluminum surfaces develop such a porous morphology upon immersion in warm water at temperatures between 50 and 100 °C.18,19 Therefore, it becomes expedient to isolate the exact role of ultrasound in this process and to evaluate its efficacy in comparison with existing, simpler techniques. To this end, we have, in the present study, sonicated pure aluminum plates, powder slurries, and immobilized powders on solid substrates and compared the results with those from controlled heating experiments.
1. INTRODUCTION The use of ultrasound in both the synthesis and modification of a range of functional materials is well-established.1−4 It is through the chemical and physical effects of acoustically induced microbubble formation and collapse (acoustic cavitation), creating extreme localized heating (ca. 5000 K) and violent fluid flow effects,5−8 that ultrasound attains its great versatility and applicability in material and environmental science, medicine, and a range of other industries. To date, it has been shown to be an efficacious tool for the degradation of macromolecules and water-borne pollutants and in the synthesis of metal, polymeric and composite nanomaterials with superior functionality compared with materials formed via conventional methods.1,9,10 A particular area where ultrasound has been recently applied is in the formation of porous metal structures, a class of new materials, which, when produced in bulk, are important in acoustic insulation and as lightweight highperformance materials in the aerospace and automobile industries.11,12 When a porous surface layer is formed on a solid metal, this renders the surface liable to forming strongly adhering coatings and enables the development of new composite materials.13,14 Recent studies have exploited high intensity ultrasound in the pretreatment of metal surfaces to produce an exciting range of novel, functionalized materials, for example, surfaces with anticorrosive or superhydrophobic properties and novel metal hybrid © 2012 American Chemical Society
Received: February 19, 2012 Revised: March 11, 2012 Published: March 19, 2012 7952
dx.doi.org/10.1021/jp3016408 | J. Phys. Chem. C 2012, 116, 7952−7956
The Journal of Physical Chemistry C
Article
2. MATERIALS AND METHODS 2.1. Materials. Aluminum sheets (99.999% Al; 1.0 mm thickness) were supplied by Goodfellow (Germany), and aluminum powder (particle size 18.2 MΩ·cm). 2.2. Sonication. Sonication was performed in a glass cell fitted with an external cooling jacket, connected to a cryostat. The ultrasound unit was a 20 kHz, horn-type reactor (model: VIP1000hd with a B2-1.2 booster; Hielscher, Germany) with 3.8 cm2 titanium alloy sonotrode tip. The total power delivered to the solution was determined through the standard calorimetric method to be 168 ± 3 W. For high temperature experiments, the temperature was raised to 65 ± 3 °C and was thereafter kept constant with use of the cryostat. For low temperature experiments, the solution was initially cooled to 5 ± 3 °C and the temperature rise during sonication limited to 37 °C using the cryostat. All samples were introduced to the solution after the initial temperature adjustment. The time of sonication was varied between 5 and 45 min. Metal plates were held 10 mm directly below the horn tip with a Teflon holder, and a water volume of 550 mL was used. For the powder slurries (10 wt %), a smaller water volume of 150 mL was employed to facilitate a more homogeneous system during sonication. (This circumvented the powder from settling on the base of the reactor.) Control heating experiments were conducted in the absence of ultrasound by placing a beaker containing the sample in water in an oil bath, with magnetic stirring in the case of the powder suspensions. 2.3. Sample Preparation. The aluminum sheets were cut into 1.0 × 2.0 cm2 pieces. The surfaces were, unless otherwise specified, polished prior to each experiment. The final samples were washed with water and dried under nitrogen flow. The sonicated powder slurries were partially degassed under vacuum due to excessive gas evolution. The dry powder was obtained after centrifugation (3000 rpm, 15 min) and drying for 24 h at 37 ± 2 °C. The metal powder was also immobilized on glass pieces using a thin layer of epoxy glue and left to cure at room temperature for 24 h. 2.4. Characterization. Scanning electron microscopy (SEM) was performed on a Gemini Leo 1550 microscope, applying an operating voltage of 3 kV and detecting the secondary electrons. Nitrogen sorption was used to obtain the BET surface areas, pore volume, and pore size of the metal powders after baking the dried samples overnight under vacuum at 100 °C. Electrical impedance spectroscopy (EIS) of the treated metal plates was also performed over a period of 14 days in 1 M NaCl solution using a CompactStat electrochemical analyzer (Ivium Technologies) to evaluate the physical properties of the treated surfaces and quantify their corrosion resistance. A conventional three-electrode cell with a reference electrode (Ag/AgCl), a counter electrode platinum plate, and a working electrode (the samples under study) was used in a Faraday cage to perform the measurements. The current response was detected in the frequency range 0.01 Hz to 1 MHz at the open circuit potential. The dependence of the impedance on the applied frequency yields information about the capacitance and resistance of the surface. The deterioration of a barrier layer (e.g., a native oxide layer) is revealed by a decrease in the absolute impedance, |Z|. X-ray photoelectron spectrometry (XPS) (Thermo Scientific K-Alpha) and X-ray diffraction spectrometry (XRD) (Enraf Nonius FR 590) were also used for characterization.
3. RESULTS AND DISCUSSION 3.1. Metal Plates. Aluminum plates were exposed to warm water (65 °C) for various times in the absence and presence of intense ultrasound. The SEM images of the surfaces are shown in Figure 1A−H. With increasing exposure time, the porous
Figure 1. SEM images of aluminum plates treated at 65 °C in water. Panels A−D were taken from nonsonicated samples after 5, 10, 20, and 30 min, respectively; panels E−H were taken from sonicated samples after 5, 10, 20, and 30 min, respectively. Plot I is the mean hole size, as estimated from the SEM images, as a function of treatment time.
metal structure forms, both with and without ultrasound. The hole size in the porous surface structure was estimated from the SEM images using statistical software, Image J, and is presented in Figure 1I. The critical difference observed between the two methods is that the process is markedly slower and less homogeneous in the presence of the ultrasound. Furthermore, 7953
dx.doi.org/10.1021/jp3016408 | J. Phys. Chem. C 2012, 116, 7952−7956
The Journal of Physical Chemistry C
Article
mass gain, rather than loss, during sonication, probably due to oxidation, and no evidence of extensive pitting at low solution temperatures), the effect of this on the surface itself and the development of the porous structure, especially during the initial stages of sonication, is difficult to evaluate. Moreover, interesting questions remain; for example, why does the porous structure develop at a slower rate and show greater site selectivity in the presence of ultrasound and to what extent does ultrasound effect the depth of the porous layer? 3.2. Metal Powders. It is also well known that ultrasound can influence the properties of metal and metal oxide particles. Chave et al. have recently demonstrated how ultrasound accelerates the dissolution of mesoporous alumina particles and leads to a change in phase composition, with nanofibers and other forms of boehmite being reported.21 This was attributed to heating of the particles at the cavitation bubble interface. Ultrasound has also been known for many years to affect particles in solution through inducing high speed collisions, which result from the generation of extremely turbid fluid flow following cavitation bubble collapse. Inelastic collisions between particles have been found to cause localized heating to ∼3000 K, which is sufficient to melt many metals and has consequently been exploited to produce industrially valuable alloys.22−24 We investigated the effect of ultrasound on aluminum particles by comparing ultrasonic and pure thermal treatments of 10 wt % aluminum slurries. The SEM images obtained for samples prepared under different conditions are presented in Figure 3. The powder was initially nonporous as received, but after suspension in water and centrifugation it developed a porous surface structure, similar to that observed on the plates. With the application of ultrasound at low temperature (below 37 °C), not only is porous morphology observed but also crystallites protruding out from the particle surface are visible. A similar change is also observed in the absence of ultrasound, albeit at a higher temperature of 65 °C, ruling out ultrasound as the only critical factor in this process. However, it is clear that although not essential to induce this phase change, ultrasound has a decisively accelerating effect on this process. At 65 °C, the entire surface of the particles is covered with crystallites following sonication. The faceted nature of the particles results from the growth of bayerite and, to a lesser extent, boehmite, as is evident in the XRD spectra presented in Figure 4. In the spectrum from the as-received particles (control), apart from the aluminum peaks, lower intensity diffractions identified as bayerite are observable. The growth of these crystals is facilitated by sonication and not significantly by heating alone. The porosity and surface area of the treated powders shown in Figure 3 were analyzed through N2 sorption measurements and presented in Table 1. Compared with the control (as-received powder), all treated samples exhibit at least a two orders of magnitude increase in specific surface area. It is interesting to note that the largest surface area is achieved either simply through dispersion and centrifugation at room temperature or through sonication at low temperature (
