Improvement of PEMFC performance with Nafion/inorganic nanocomposite membrane electrode assembly prepared by ultrasonic coating technique

Membrane electrode assemblies with Nafion/nanosize titanium silicon dioxide (TiSiO₄) composite membranes were manufactured with a novel ultrasonic-spray technique and tested in proton exchange membrane fuel cell (PEMFC). Nafion/TiO₂ and Nafion/SiO₂ nanocomposite membranes were also fabricated by the same technique and their characteristics and performances in PEMFC were compared with Nafion/TiSiO₄ mixed oxide membrane. The composite membranes have been characterized by thermogravimetric analysis, scanning electron microscopy, X-ray diffraction, water uptake, and proton conductivity. The composite membranes gained good thermal resistance with insertion of inorganic oxides. Uniform and homogeneous distribution of inorganic oxides enhanced crystalline character of these membranes. Gas diffusion electrodes (GDE) were fabricated by Ultrasonic Coating Technique. Catalyst loading was 0.4 mg Pt/cm² for both anode and cathode sides. Fuel cell performances of Nafion/TiSiO₄ composite membrane were better than that of other membranes. The power density obtained at 0.5 V at 75 °C was 0.456 W cm⁻², 0.547 W cm⁻², 0.477 W cm⁻² and 0.803 W cm⁻² for Nafion, Nafion/TiO₂, Nafion/SiO₂, and Nafion/TiSiO₄ composite membranes, respectively.     

High-power CMUTs: design and experimental verification

Capacitive micromachined ultrasonic transducers (CMUTs) have great potential to compete with piezoelectric transducers in high-power applications. As the output pressures increase, nonlinearity of CMUT must be reconsidered and optimization is required to reduce harmonic distortions. In this paper, we describe a design approach in which uncollapsed CMUT array elements are sized so as to operate at the maximum radiation impedance and have gap heights such that the generated electrostatic force can sustain a plate displacement with full swing at the given drive amplitude. The proposed design enables high output pressures and low harmonic distortions at the output. An equivalent circuit model of the array is used that accurately simulates the uncollapsed mode of operation. The model facilities the design of CMUT parameters for high-pressure output, without the intensive need for computationally involved FEM tools. The optimized design requires a relatively thick plate compared with a conventional CMUT plate. Thus, we used a silicon wafer as the CMUT plate. The fabrication process involves an anodic bonding process for bonding the silicon plate with the glass substrate. To eliminate the bias voltage, which may cause charging problems, the CMUT array is driven with large continuous wave signals at half of the resonant frequency. The fabricated arrays are tested in an oil tank by applying a 125-V peak 5-cycle burst sinusoidal signal at 1.44 MHz. The applied voltage is increased until the plate is about to touch the bottom electrode to get the maximum peak displacement. The observed pressure is about 1.8 MPa with -28 dBc second harmonic at the surface of the array.     

Synthesis of multi-walled C nanotubes by Fe-Ni (70 wt.%) catalyzed chemical vapor deposition from pre-heated CH4

This study aims to investigate effect of pre-heating of pure CH₄ on the growth of C nanotubes by chemical vapor deposition in the presence of Fe-Ni (70 wt.%) alloy catalyst. It was observed that un-preheated CH₄ yielded no C formation at 1050-1150 K owing to its poor decomposition rate, contrary to the results of thermodynamic analysis. Whereas, pre-heating treatment of CH₄ at 1200 K prior to the growth increased pyrolysis of the hydrocarbon which promoted synthesis of C nanotubes at 1050-1150 K, possibly due to enhanced formations of intermediate gaseous species. TEM study revealed that the morphologies of the products consist of multi-walled C nanotubes.     

Large-Area (over 50 cm × 50 cm) Freestanding Films of Colloidal InP/ZnS Quantum Dots

We propose and demonstrate the fabrication of flexible, freestanding films of InP/ZnS quantum dots (QDs) using fatty acid ligands across very large areas (greater than 50 cm × 50 cm), which have been developed for remote phosphor applications in solid-state lighting. Embedded in a poly(methyl methacrylate) matrix, although the formation of stand-alone films using other QDs commonly capped with trioctylphosphine oxide (TOPO) and oleic acid is not efficient, employing myristic acid as ligand in the synthesis of these QDs, which imparts a strongly hydrophobic character to the thin film, enables film formation and ease of removal even on surprisingly large areas, thereby avoiding the need for ligand exchange. When pumped by a blue LED, these Cd-free QD films allow for high color rendering, warm white light generation with a color rendering index of 89.30 and a correlated color temperature of 2298 K. In the composite film, the temperature-dependent emission kinetics and energy transfer dynamics among different-sized InP/ZnS QDs are investigated and a model is proposed. High levels of energy transfer efficiency (up to 80%) and strong donor lifetime modification (from 18 to 4 ns) are achieved. The suppression of the nonradiative channels is observed when the hybrid film is cooled to cryogenic temperatures. The lifetime changes of the donor and acceptor InP/ZnS QDs in the film as a result of the energy transfer are explained well by our theoretical model based on the exciton-exciton interactions among the dots and are in excellent agreement with the experimental results. The understanding of these excitonic interactions is essential to facilitate improvements in the fabrication of photometrically high quality nanophosphors. The ability to make such large-area, flexible, freestanding Cd-free QD films pave the way for environmentally friendly phosphor applications including flexible, surface-emitting light engines.     

Synthesis, characterization and superparamagnetic resonance studies of ZnFe2O4 nanoparticles

Superparamagnetic nanoparticles of zinc ferrite (ZnFe2O4) were produced by a microwave induced combustion synthesis method. XRD, FT-IR, SEM, VSM and ESR were used for the structural, morphological, and magnetic investigation of the product, respectively. Average particle size of the nanoparticles was estimated by the Schérrer equation using the full-width at half maximum (FWHM) of the most intense XRD peak and found as 41 nm. Magnetization measurements have shown that the samples have a blocking temperature of 72 K which indicates a superparamagnetic behavior. Superparamagnetic resonance (SPR) spectra at room temperature show a broad line with a Landé g-factor, g(eff) approximately 2. We used a theoretical formalism based on a distribution of diameters of the nanoparticles following lognormal proposed by Berger et al. The nanoparticles behave as single magnetic domains with random orientations of magnetic moments which are subject to thermal fluctuations. A Landau-Lifshitz line shape function presents adequate results which are in good agreement with the experimental ones. At high temperatures, the SPR line shape is governed by the core anisotropy and the thermal fluctuations. By decreasing the temperature, the magnetic susceptibility of shell spins increases. As a result of this, the surface spins produce an effective field on the core leading to a decrease of resonance field, B(r). Also, the effective anisotropy increases as the shell spins begin to order. So, the results are interpreted by a simple model, in which each single-domain nanoparticle is considered as a core-shell system, with magneto-crystalline anisotropy on the core and surface anisotropy on the shell.     

The Use of Cooling Holes to Decrease the Amount of Thermal Damage on a Plastic Gear Tooth

A new method was developed to decrease the amount of thermal damage on the contact surface of a plastic gear tooth. The main aim of this method is to decrease the amount of thermal damage on the tooth surface by reducing the heat accumulated in the tooth under working conditions. For this reason, air-cooling holes on the plastic gear tooth were drilled in different locations, with the intention of lowering the tooth temperature by transferring the heat away from the tooth through these holes via convection. Using three the different configurations of holes, the air and temperature at the tooth contact zone were measured and compared.     

The effect of adsorption on mass transfer in fluidized bed catalytic reactors

Tracer gas residence time distributions (RTD) in a laboratory scale fluidized bed system have been measured for pulses of three different tracer gases (methane, ethane and propane) at different temperatures in the range 323 to 435 K. The fluidized solid was a commercial zeolite based FCC catalyst (CBZ‐2), and measurements were carried out in a superficial air velocity range of 0.01 to 0.04 m/s. The data were interpreted with two‐phase dense phase dispersion models for adsorptive tracers, available in the literature. In addition, modified models were considered by assuming a stationary dense phase and neglecting axial dispersion in this phase. Mean residence time, μ₁, and the variance of the residence time, σ², of RTD data were calculated for each experimental run. Applying the moment technique in the Laplace domain, the differential equations for all models considered were analytically solved. Mass transfer coefficients obtained from dynamic experiments were compared with the values estimated from the relations available in the literature. It was found that methods considering convective flux alone between the bubble and emulsion phases give closer values to the experimental ones than the methods also including the diffusive flux.     

Influence of Pullulan‐Based Edible Coating on Some Quality Properties of Strawberry During Cold Storage

Pullulan‐based edible coatings were used to extend the shelf life of strawberries (Fragaria ananassa cv. Camarosa) under cold storage conditions in perforated package. Some physicochemical properties of the strawberry fruits such as weight loss, firmness, colour, ascorbic acid, total carotenoids and fungal decay were determined during the storage. The coatings delayed mould formation and decreased weight loss, softening and degradation of ascorbic acid and carotenoids in the fruits. The most marked difference was recorded in the firmness value of the fruits coated with a 10% pullulan‐based emulsion. Coating with a 10% pullulan‐based emulsion successfully extended the shelf life of the strawberries for 5 days longer under cold storage conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

Effects of sulfonated polyether-etherketone (SPEEK) and composite membranes on the proton exchange membrane fuel cell (PEMFC) performance

Sulfonated polyether-etherketone (SPEEK) has a potential for proton exchange fuel cell applications. However, its conductivity and thermohydrolytic stability should be improved. In this study the proton conductivity was improved by addition of an aluminosilicate, zeolite beta. Moreover, thermohydrolytic stability was improved by blending poly-ether-sulfone (PES). Sulfonated polymers were characterized by H-NMR. Composite membranes prepared were characterized by Electrochemical Impedance Spectroscopy (EIS) for their proton conductivity. Degree of sulfonation (DS) values calculated from H-NMR results, and both proton conductivity and thermohydrolytic stability was found to strongly depend on DS. Therefore, DS values were controlled time in the range of 55–75% by controlling the reaction time. Zeolite beta fillers at different SiO₂/Al₂O₃ ratios (20, 30, 40, 50) were synthesized and characterized by XRD, EDX, TGA, and SEM. The proton conductivity of plain SPEEK membrane (DS = 68%) was 0.06 S/cm at 60 °C and the conductivity of the composite membrane containing of zeolite beta filled SPEEK was found to increase to 0.13 S/cm. Among the zeolite Beta/SPEEK composite membranes the best conductivity results were achieved with zeolite beta having a SiO₂/Al₂O₃ ratio of 50 at 10 wt% loading.     

Synthesis and Thermal Energy Storage Properties of Erythritol Tetrastearate and Erythritol Tetrapalmitate

Erythritol tetrapalmitate (ETP) and erythritol tetrastearate (ETS) were synthesized as novel solid‐liquid phase change materials (PCM) by means of the direct esterification reaction of the erythritol with palmitic and stearic acids. The ETP and ETS esters were characterized chemically using FT‐IR and ¹H NMR techniques. The energy storage properties of the esters were determined by DSC analysis. The results indicated that the ETP and ETS esters synthesized as novel solid‐liquid PCMs are promising materials for thermal energy storage applications at large scale such as solar energy storage, building heating or cooling, indoor temperature controlling, and production of smart textile and insulation clothing.