Plasma-plasmonics synergy in the Ga-catalyzed growth of Si-nanowires

This paper reports on the growth of Si nanowires (NWs) by SiH₄/H₂ plasmas using the non-noble Ga-nanoparticles (NPs) catalysts. A comparative investigation of conventional Si-NWs vapour–liquid–solid (VLS) growth catalyzed by Au NPs is also reported. We investigate the use of a hydrogen plasma and of a SiH₄/H₂ plasma for removing Ga oxide shell and for enhancing the Si dissolution into the catalyst, respectively. By exploiting the Ga NPs surface plasmon resonance (SPR) sensitivity to their surface chemistry, the SPR characteristic of Ga NPs has been monitored by real time spectroscopic ellipsometry in order to control the hydrogen plasma/Ga NPs interaction and the involved processes (oxide removal and NPs dissolution by volatile gallium hydride). Using in situ laser reflectance interferometry the metal catalyzed Si NWs growth process has been investigated to find the effect of the plasma activation on the growth kinetics. The role of atomic hydrogen in the NWs growth mechanism and, in particular, in the SiH₄ dissolution into the catalysts, is discussed. We show that while Au catalysts because of the re-aggregation of NPs yields NWs that do not correspond to the original size of the Au NPs catalyst, the NWs grown by the Ga catalyst retains the diameter dictated by the size of the Ga NPs. Therefore, the advantage of Ga NPs as catalysts for controlling NWs diameter is demonstrated.     

The development of a feasible method for the tribological characterization of gear teeth surface treatments

In this investigation, an experimental campaign, dedicated to the tribological characterization of surface treatments for gears, is presented. Wear in gears is herein simulated by means of a Ring-on-Ring test machine, where the ring and the cylinder are chosen in such a way that the kinematic and dynamic conditions are as much as possible similar to those presented in the teeth contact, in one of the two extreme points of the action line. The dynamics is simulated by imposing the same contact stress, while the kinematic conditions are applied by assuming in the two systems the same ratio of the specific slide rather than the same sliding speed. The latter choice is justified by the results shown in a preliminary test phase, which have shown the importance of the specific slide for the prediction of wear in gear profiles. The apparatus has shown a fair prediction capacity and has been used to compare the different wear resistances and typologies of gear pairs having different surface treatments. Results are herein presented and briefly discussed.     

Acid salt corrosion in a hydrotreatment plant of a petroleum refinery

The operation of a diesel hydrotreatment plant was stopped after only 18 months of service due to the obstruction of a set of heat exchangers, which are responsible for cooling of the stream that flows out from reactors. In particular, the obstruction occurred mainly in one of them that operated with a low temperature. Chemical analysis of fouling of samples obtained from obstructed tubes identified the presence of NH₄Cl and the absence of NH₄HS, whose presence is expected in these units. The stream that flows out of the reactors has mainly H₂S and NH₃; however, the chloride contents lower than the sulfide contents are sufficient to precipitate NH₄Cl due to the differences between the stability of both salts. The steel used in the equipment of this unit is austenitic stainless steel. Thus the presence of a hygroscopic salt must be eliminated since its hydrolysis generates HCl, and severe restriction to the HCl content in the plant feedstocks must be observed considering that only a very small amount of this compound is expected from the organic chloride.     

Laser‐Induced In‐Fiber Fluid Dynamical Instabilities for Precise and Scalable Fabrication of Spherical Particles

Scalable fabrication of spherical particles at both the micro‐ and nanoscales is of significant importance for applications spanning optical devices, electronics, targeted drug delivery, biodevices, sensors, and cosmetics. However, current top‐down and bottom‐up fabrication methods are unable to provide the full spectrum of uniformly sized, well‐ordered, and high‐quality spheres due to their inherent restrictions. Here, a generic, scalable, and precisely controllable fabrication method is demonstrated for generating spherical particles in a full range of diameters from microscale to nanoscale. This method begins with a macroscopic composite multimaterial solid‐state preform drawn into a fiber that defines precisely the initial conditions for the process. It is then followed by CO₂ laser heating to enable the transformation from a continuous fiber core into a series of homogeneous spheres via Plateau–Rayleigh capillary instability inside the fiber. This physical breakup method applies to a wide range of functional materials with different melting temperatures from 400 to 2400 K and 10 orders of difference in fiber core/cladding viscosity ratio. Furthermore, an ordered array of silicon‐based whispering‐gallery mode resonators with the Q factor as high as 7.1 × 10⁵ is achieved, owing to the process induced ultrasmooth surface and highly crystalline nature.     

Biodegradable antimicrobial films based on poly(lactic acid) matrices and active azo compounds

Using solvent casting and melt compounding methods, we realized antibacterial and antifungal poly(lactic acid)‐based films by introducing different percentages of antimicrobial azo dyes into polymer matrices. Concentration up to 0.01% (w/w) of azo compounds permitted the preparation of antimicrobial and transparent films. The thin films retained the properties of the pure PLA matrices, such as glass transition temperature, flexibility, and amorphous nature. The films exhibited antimicrobial activity and the capability to inhibit biofilms formation of Staphylococcus aureus and Candida albicans. Spectrophotometric investigation of azo compounds release from the polymer matrices confirmed that the materials might have applications in fields where an intrinsic antimicrobial ability of the material is required, such as biomedical tools, biodegradable antibacterial coatings, and films for active packaging. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42357.     

Energy efficiency analysis of an integrated glycerin processor for PEM fuel cells: Comparison with an ethanol-based system

The aim of this work is to analyze energetically the use of glycerin as the primary hydrogen source to operate a proton exchange membrane fuel cell. A glycerin processor system based on its steam reforming is described departing from a previous process model developed for ethanol processing. Since about 10% w/w of glycerin is produced as a byproduct when vegetable oils are converted into biodiesel, and due to the later is increasing its production abruptly, a large glycerin excess is expected to oversaturate the market. The reformed stream contains mainly H₂ but also CO, CO₂, H₂O and CH₄. As CO is a poison for PEM fuel cell type, a stream purification step is previously required. The purification subsystem consists of two water gas shift reactors and a CO preferential oxidation reactor to reduce the CO levels below 10 ppm. The reforming process is governed by endothermic reactions, requiring thus energy to proceed. Depending on the system operation point, the energy requirements can be fulfilled by burning an extra glycerin amount (to be determined), which is the minimal that meets the energy requirements. In addition a self-sufficient operation region can be distinguished. In this context, the water/glycerin molar ratio, the glycerin steam reformer temperature, the system pressure, and the extra glycerin amount to be burned (if necessary) are the main decision variables subject to analysis. Process variables are calculated simultaneously, updating the composite curves at each iteration to obtain the best possible energy integration of the process. The highest net system efficiency value computed is 38.56% based on the lower heating value, and 34.71% based on the higher heating value. These efficiency values correspond to a pressure of 2 atm, a water/glycerin molar ratio of 5, a glycerin steam reformer temperature of 953 K, and an extra glycerin amount burned of 0.27 mol h⁻¹. Based on the main process variables, suitable system operation zones are identified. As in practice, most PEM fuel cells operate at 3 atm, optimal variable values obtained at this condition are also reported. Finally, some results and aspects on the system performance of both glycerin and ethanol processors operated at 3 atm are compared and discussed.     

Emission of trace gases and organic components in smoke particles from a wildfire in a mixed-evergreen forest in Portugal

On May 2009, both the gas and particulate fractions of smoke from a wildfire in Sever do Vouga, central Portugal, were sampled. Total hydrocarbons and carbon oxides (CO₂ and CO) were measured using automatic analysers with flame ionisation and non-dispersive infrared detectors, respectively. Fine (PM_2.5) and coarse (PM_2.5-10) particles from the smoke plume were analysed by a thermal-optical transmission technique to determine the elemental and organic carbon (EC and OC) content. Subsequently, the particle samples were solvent extracted and fractionated by vacuum flash chromatography into different classes of organic compounds. The detailed organic speciation was performed by gas chromatography-mass spectrometry. The CO, CO₂ and total hydrocarbon emission factors (g kg⁻¹ dry fuel) were 170 ± 83, 1485 ± 147, and 9.8 ± 0.90, respectively. It was observed that the particulate matter and OC emissions are significantly enhanced under smouldering fire conditions. The aerosol emissions were dominated by fine particles whose mass was mainly composed of organic constituents, such as degradation products from biopolymers (e.g. levoglucosan from cellulose, methoxyphenols from lignin). The compound classes also included homologous series (n-alkanes, n-alkenes, n-alkanoic acids and n-alkanols), monosaccharide derivatives from cellulose, steroid and terpenoid biomarkers, and polycyclic aromatic hydrocarbons (PAHs). The most abundant PAH was retene. Even carbon number homologs of monoglycerides were identified for the first time as biomarkers in biomass burning aerosols.     

Optimal Design of A CPO-Reformer of Light Hydrocarbons with Honeycomb Catalyst: Effect of Frontal Heat Dispersions on the Temperature Profiles

This paper extends a previous investigation on the thermal behavior of CH₄-CPO reformers with honeycomb catalysts. Modeling and experimental studies on the short contact time catalytic partial oxidation (CPO) of CH₄ to syngas from our and from other groups have shown that Rh-catalysts rapidly deactivate at the very high temperatures, close to 1000 °C, that establish in the inlet zone of the reactor. We have previously shown that a significant reduction of the surface hot-spot temperature can be obtained by properly designing the catalyst: beneficial effects are observed at increasing opening of the honeycomb channels, which decreases the rate of O₂ inter-phase mass transfer, and at increasing catalyst activity, which promotes the rate of the endothermic reactions. In this work, we explore the effect of the reactor configuration, namely the effect of heat dispersion from the glowing front face of the monolith. Three reactor configurations were compared in CH₄-CPO experiments: (i) a configuration with perfect continuity between the front heat shield (FHS) and the catalytic module, which behaved close to an ideal adiabatic reactor, (ii) a configuration where the FHS was separated from the catalytic monolith and (iii) a configuration where the FHS was at large distance from the catalytic module. State of the art experimental tools, including the spatially resolved measurement of temperature and concentration profiles were used to characterize the thermal behavior of the various configurations. Detailed kinetic modeling supported the analysis of data. The results showed that, at the expense of a small loss of thermal efficiency, a very moderate loss of performance in terms of conversion and selectivity, but, remarkably, an important reduction of the surface inlet temperatures were achieved. Preliminary experiments with propane/air mixtures suggest that the adoption of a moderately dispersive reactor can represent a promising solution for the stable operation of catalytic units treating heavier fuels than methane.