1,720,978 research outputs found
Freeze cast porous membrane catalyst for hydrogen production via oxy-reforming
Full text linkIn this work, porous catalytic membrane with tailored microstructure has been developed for the production of hydrogen from the oxy-reforming process. Porous catalytic membranes have been produced by freeze-casting of ceria-based powders using camphene as solvent. The process parameters have been optimized in terms of type and amount of dispersant to obtain homogeneous and stable ceramic suspension suitable for the process, while the freezing temperature was chosen on the basis of the microstructure and porosity of the final samples. The obtained freeze cast sample produced showed very high levels of porosity (≈ 80%) and good gas permeability (1.0 × 10−11 m2). The ceria-based porous structure produced was finally evaluated as catalyst in high temperature oxy-reforming reaction for hydrogen production from methane showing high methane conversions (up to 90%) and hydrogen production. The use of a unique catalytic membrane other than pelleted catalysts allowed the obtaining of smooth thermal profiles without the evidence of endothermic or exothermic peaks
Role of different solvents on the purification of as-synthesized nano-Ce1-xGdxO2-d powders
No full textCeria and rare earth-doped ceria powders have important applications in catalysis, gas sensoring, and electronics. Even if many authors report different methods for the synthesis of nano-sized doped-ceria only few of them give information about the necessary washing processes for the powder purification. The organics adsorbed on the as-synthesized particles surface strongly affect, in fact, the properties of the powder. In this work, CeO2 and Ce1-xGdxO2-d (x = 0.10, 0.20, 0.30) solid solutions were produced by polyol microwave assisted method. The amount of synthesis residues adsorbed on the as-synthesized powders was firstly evaluated. The purification ability of different solvents on the as-synthesized Ce0.80Gd0.20O1.90 was, then, accurately studied in order to obtain a clean powder without the need of any thermal treatments. The study shows that water purification allows to reduce the amount of the residues of synthesis leading to the production of nano-particles with a mono-dispersed distribution of dimensions
A glance at processing-microstructure-property relationships for magnetoelectric particulate PZT-CFO composites
Full text linkIn this work, we investigated the processing-microstructure-property relationships for magnetoelectric (ME) particulate composites consisting of hard ferromagnetic CoFe2O4 (CFO) particles dispersed in a Nb-doped PbZrxTi1-xO3 (PZT) soft ferroelectric matrix. Several preparation steps, namely PZT powder calcination, PZT-CFO mixture milling and composite sintering were tailored and a range of microstructures was obtained. These included open and closed porosities up to full densification, PZT matrices with decreasing grain size across the submicron range down to the nanoscale and well dispersed CFO particles with bimodal size distributions consisting of submicron and micron sized components with varying weights. All samples could be poled under a fixed DC electric field of 4 kV/mm and the dielectric, piezoelectric and elastic coefficients were obtained and are discussed in relation to the microstructure. Remarkably, materials with nanostructured PZT matrices and open porosity showed piezoelectric charge coefficients comparable with fully dense composites with coarsened microstructure and larger voltage coefficients. Besides, the piezoelectric response of dense materials increased with the size of the CFO particles. This suggests a role of the conductive magnetic inclusions in promoting poling. Magnetoelectric coefficients were obtained and are discussed in relation to densification, piezoelectric matrix microstructure and particle size of the magnetic component. The largest magnetoelectric coefficient ff33 of 1.37 mV cm-1 Oe-1 was obtained for submicron sized CFO particles, when closed porosity was reached, even if PZT grain size remained in the nanoscale
Additive Manufacturing of Piezoelectric Niobium-Doped Lead Zirconate Titanate (PZT-N) by Binder Jetting
No full textBinder jetting is an emerging indirect additive manufacturing technique for ceramic materials, which could be employed to produce application-oriented designed components impossible to achieve with traditional processes and featuring enhanced performances. In our study, niobium-doped lead zirconate titanate (PZT-N) powder, usually processed through the standard press-and-sinter route, was employed as the raw material. First, the powder was characterized in terms of granulometry and flowability to assess its suitability for the printing process. Then, shaping by binder jetting was studied, and the effect of three levels of binder saturation (75–90–105%) on the green bodies was assessed. Finally, the microstructure of the sintered samples was studied using SEM, to investigate the effect of thermal treatments on the grain size distribution and residual porosity (~40%). The piezoelectric properties were measured and compared to those of conventionally processed material. The piezoelectric charge and voltage constants (d33 and g33) were evaluated to determine the possible use of printed parts as porous piezoelectric components to be exploited in hydrophones in the direct mode
Influence of chemically synthesized powder addition on K0.5Na0.5NbO3 ceramic's properties
Full text linkA new strategy to produce lead-free K0.5Na0.5NbO3 (KNN) piezoceramics with reliable and improved piezoelectric performance is presented for the first time. KNN powders were synthesized using two distinct synthesis routes: a mechanochemical activation-assisted solid-state route (KNNSSR) and a sol-gel modified Pechini method (KNNchem). KNNchem powders were mixed with KNNSSR at different weight ratios (0, 3, 5, 10 and 20 wt%), and the mixtures were conventionally consolidated and sintered at 1130 degrees C for 2 h. It was found that KNNchem powders influence crystal phase, microstructure and piezoelectric properties of the sintered pellets. Gradually increasing KNNchem content promotes the conversion of the undesired phase present in KNNSSR into the stoichiometric one. It is also proved that the addition of KNNchem between 5 and 10 wt% improves piezoelectric properties, eventually leading to a d(33) piezoelectric charge constant value of 113-115 pC/N. These values are among the highest reported for undoped KNN ceramics obtained by conventional sintering
Fabrication and electrochemical modelling of 8YSZ and GDC10 freeze tape cast scaffolds for solid oxide cells (SOCs)
No full textThe morphology of electrodes in Solid Oxide Cells (SOCs) has a great impact on their mechanical stability during operation as well as transport properties and kinetics, which in turn affect electrode and cell performance. This study proposes a new experimental procedure based on the freeze tape casting technique for the manufacturing of graded porous electrodes for SOCs. The use of water-based freeze tape casting has enabled the effective fabrication of hierarchical porous ionic backbones featuring the typical porosity of functional and supporting electrodes in a single tape. The porous samples are morphologically characterized and subsequently, for the first time according to the authors knowledge, a Computational Fluid Dynamic (CFD) model has been developed to compare the gas transport properties of conventional spongy-like and graded porous electrodes of planar SOCs. The results presented strongly suggest that hierarchical porous electrodes enable higher performance by decreasing the voltage concentration losses
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Study of Reliability of SOFC Cathode Impedances at Applied Overpotentials
No full textIn case of measuring separately the behavior of SOFC electrodes the concern of the reliability of impedance results was highlighted by many authors [1-3], especially when overpotentials are applied. A flat thin electrolyte-supported cell in a three-electrode configuration is often investigated, where the RE is deposited in a ring-shape around the WE, being a very convenient geometry from an experimental point of view. The problem arises when the RE is placed in an electrolyte region, which is not equipotential and inaccuracy becomes particularly pronounced when WE and CE are not aligned or have different area specific resistances.
In this work LSM cathodes were investigated on a YSZ electrolyte-supported cell, using an electrolyte 3 mm thick.
Two types of cells were prepared: the first (CellA) according to the geometric requirements suggested in literature [4]: little WE (diameter 3 mm) aligned to the CE and with equal Rp and time constant; RE co-planar around the WE and placed at a distance greater than three- electrolyte thickness from the WE; the second one (CellB) equal to CellA but with a larger WE (diameter 8 mm). Impedance measurements were carried out both in two- and three-electrode configuration, at OCV and under applied overpotentials (Fig. 1).
A preliminary comparison between the results extracted from CellB both at two- and three- electrodes confirms that a thick electrolyte allows extracting suitable three-electrode impedance results in case of OCV conditions. On the other side, when an overpotential is applied, the impedance spectra extracted from CellA and CellB are comparable only at little overpotentials (η < 0.2 V)
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