Electrochemical aspects of black chromium electrodeposition from 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid

The electrochemical behaviour of trivalent chromium (Cr³⁺) in 1-butyl-3-methylimidazolium tetrafluoroborate [BMIm][BF₄] ionic liquid solutions was studied by cyclic voltammetry and chronoamperometry. The reduction of Cr³⁺ occurs in two steps, Cr³⁺ to Cr²⁺ and Cr²⁺ to Cr⁰, respectively. The first step is quasi-reversible with a diffusion coefficient of Cr³⁺ in solution of 3.13 × 10⁻⁸ cm² s⁻¹ at 303 K and 25.8 × 10⁻⁸ cm² s⁻¹ at 358 K, estimated from cyclic voltammetry data.Black chromium films were electrodeposited on copper, stainless steel and carbon steel substrates at a constant potential of −1.5 V vs Pt quasi-reference electrode. The films consist of aggregates of nanosized particles. The coatings in the as-deposited condition present an amorphous structure but after annealing for 4 h, a nanocrystalline Cr₂O₃ phase is formed, with an average crystallite size of 17 nm.     

A critical review of mathematical models used to determine the density of hydrogen trapping sites in steels and alloys

Different mathematical models based on electrochemical permeation tests have been proposed to determine the density of hydrogen trapping sites (N_T) in metals. However, recent mathematical models that have been used to determine N_T contain mathematical errors or have been based on the work of researchers whose works contain no mention of equations or mathematical terms related to hydrogen trapping, which calls into doubt the validity of the models' applicability. In this work, based on permeation test results for X65 steel, we show that the use of incorrect mathematical models leads to values of N_T that differ by four orders of magnitude.     

Design of a combined heat,hydrogen, and power plant from university campus waste streams

This paper presents analysis of a combined heat, hydrogen, and power (CHHP) plant for waste-to-energy conversion in response to the 2012 Hydrogen Student Design Contest. Our team designed the CHHP plant centered on a molten carbonate fuel cell (Fuel Cell Energy DFC-1500) fueled by syngas derived primarily from an oxygen-fed municipal waste gasifier. Catalytic methanation and supplemental utility natural gas increase the fuel methane content to meet the DFC-1500 fueling requirements for maintaining stack thermal energy balance. Internal reforming converts excess fuel from the fuel cell to an H₂-rich stream, which is purified downstream in a pressure-swing adsorption system. The separated H₂ (1000 kg per day) is compressed for storage to provide fuel for a campus fleet of PEM fuel cell buses. The system provides more than 1.1 MWe for the campus grid with approximately 20% of the fuel cell power used for H₂ compression and running the plant. Heat recovery steam generators provide steam for the methanation reactor and 0.4 MW of thermal energy for district heating or steam turbine-driven chillers. Cost analysis indicates that the system requires incentives for economic viability with current estimated operating costs, but advances to reduce capital expenses of comparable urban waste-driven CHHP systems can make them attractive for future implementation.     

Potential of molten carbonate fuel cells to enhance the performance of CHP plants in sewage treatment facilities

In spite of its slow commercial deployment, fuel cells are amongst the most efficient and environmentally friendly electric power generators. The case of Molten Carbonate Fuel Cells is even more interesting since, in addition to these features that are common to all fuel cells, these systems can be used as active carbon capture devices due to their capability to migrate carbon dioxide from one electrode (cathode) to another (anode). In this context, this work presents the operation of a fuel cell of this type coupled to a combined heat and power plant based on gas reciprocating engines as typically used in wastewater treatment plants. The biogas produced in the water sludge digestion process is burnt in the reciprocating engines, whose exhaust gases are mixed with air and blown into the fuel cell cathode. The carbon dioxide contained in this stream is conveyed in the form of carbonate ions (CO₃⁼) through the electrolyte to the anode where it reacts with the hydrogen fuel, being released as carbon dioxide. The exhaust gases from the anode comprise carbon dioxide, water steam and a small fraction of unspent hydrogen fuel. The combustion of the latter species with pure oxygen followed by a cooling process permits separating a gaseous stream of pure carbon dioxide from a liquid stream of water.     

Abrasive wear behaviour of laser clad and flame sprayed-melted NiCrBSi coatings

In this work, the influence of the processing conditions on the microstructure and abrasive wear behaviour of a NiCrBSi hardfacing alloy is analysed. The hardfacing alloy was applied in the form of coatings onto a mild steel substrate (Fe–0.15%C) by different techniques: laser cladding (LC) and flame spraying (FS) combined with surface flame melting (SFM). In both cases, the appropriate selection of the process parameters enabled high-quality, defect-free NiCrBSi coatings to be obtained. The microstructure of the coatings was analysed by scanning electron microscopy (SEM), with attached energy dispersive spectroscopy (EDS) microprobe, and by X-ray diffraction (XRD). Their tribological properties were evaluated by micro-scale ball cratering abrasive wear tests using different abrasives: diamond, SiC and WC. Microstructural characterisation showed that both coatings exhibit similar phases in their microstructure, but the phases present differ in morphology, size distribution and relative proportions from one coating to another. Wear tests showed that in three-body abrasive conditions, despite these microstructural differences, the wear behaviour is comparable for both coatings. Conversely, in two-body wear conditions with diamond particles as the abrasive, it was observed that the specific wear rate of the material is sensitive to microstructural changes. This fact is particularly apparent in LC coatings, in which the zones of the layers with higher proportions of very small hard particles present a lower wear resistance. These results indicate that it is important to have good microstructural control of this material, in order to obtain coatings with an optimized and homogeneous tribological behaviour.     

Short‐scale diversity in a dynamic rain fade environment

This article is concerned with using time‐varying uniformly spaced rainfall data to investigate the concept and values of instantaneous diversity gain. This has been obtained on a minute‐by‐minute basis in order to illustrate the behaviour of a VSAT communication system which switches to the most reliable link every minute. The simulations have been carried out using a whole year of rain data obtained from the recordings of a dense rain gauge network located in Barcelona. After illustrating the concept of instantaneous diversity gain, this paper shows the results obtained for various site separations and 20 GHz. These results have been plotted together with the values given by some long‐term‐based models. The comparison shows that such models are not appropriate for dynamic site diversity. This was found to increase the reliability of a VSAT system, especially under severe rain conditions in which the rainfall rates are spatially arranged in rain cells and the attenuation experienced by the two sites is strongly decorrelated. The correlation coefficient between the attenuation series calculated for several separation distances has also been obtained to illustrate this concept. Finally, the paper gives the attenuation distributions calculated for a VSAT system with and without site diversity and for several site separations. Copyright © 2005 John Wiley & Sons, Ltd.     

Fractal generation of rain fields: synthetic realisation for radio communications systems

The spatio-temporal variation of rain fields is a key input into the development of radio systems which operate at frequencies above 10 GHz in a spectrally efficient fashion. Fractal methods have been proven helpful in the analysis and synthesis of rain fields. A fractal model is presented for the simulation of meteorologically representative rain fields, using an additive iterative process in the logarithmic domain. The resulting simulated rain fields are monofractal fields that have appropriate spectral density exponent, fractal dimension and behaviour that is consistent with radar analyses of convective or stratiform types of events. Justification to use a monofractal method of simulation is presented through the analysis of the moment scaling function for fields of rain rate and log rain rate values recorded by the Chilbolton Advanced Meteorological Radar, based in the south of England. The results of the analysis indicate that log rain fields can be analysed and simulated by using monofractal techniques with sufficient accuracy for the purposes of synthesising rain fields for communication systems design and operation. Cumulative distributions of rain rate exceedance derived from the simulated fields are compared with the measured rain gauge data and curves calculated from the ITU-R rain models, with promising results.