186,868 research outputs found
Hot pressing of nanocrystalline TiO2 (anatase) ceramics with controlled microstructure
No full textThe preparation conditions of nanocrystalline phase-pure TiO2 anatase ceramics by hot pressing are described. Density, surface area, pore size distribution and grain size are determined by various techniques, including gas adsorption, mercury porosimetry, transmission electron microscopy (TEM) and X-ray diffraction (XRD). The evolution of the structural parameters is followed as function of temperature and pressure programme. It is shown that the porosity, grain and pore size of the ceramics can be controlled by a suitable choice of experimental conditions. Ceramics with densities higher than 90% of the theoretical limit with a mean grain size of 30 nm can be obtained at temperatures as low as 490 ◦C under 0.45 GPa for 2 h. The experimental results are discussed in view of the sintering theory
Polymer electrolyte membrane development for direct methanol fuel cells powering portable microelectronic devices
No full textSulfonated aromatic polymers as proton-conducting solid electrolytes for fuel cells: A short review
No full textThis review describes main strategies for the development of sulfonated aromatic polymers (SAP) with optimal properties for medium temperature (90-120 °C) polymer electrolyte membrane fuel cell applications. SAP are promising economical polymer electrolytes, but there still exist some challenges about these materials due mainly to excessive swelling in water, poor mechanical strength and low dimensional stability. Here, the state-of-the-art of SAP is reviewed and the main focus will be directed to properties of SAP, including proton conductivity, water uptake, mechanical strength, permeability. Especially three approaches to improve the performances are addressed: the formation of copolymers, the formation of hybrid materials and the polymer reticulation
Building bridges: Crosslinking of sulfonated aromatic polymers-A review
No full textSulfonated aromatic polymers (SAP) are promising inexpensive polymer electrolyte membranes for fuel cells, but there still exist some challenges about this kind of materials including excessive swelling in water, poor mechanical strength, low dimensional stability, especially for highly sulfonated SAP. Crosslinking is widely proposed as an efficient strategy to deal with these challenges. Here, the state-of-the-art of crosslinked SAP is reviewed and some prospects for further development are outlined. The following points are addressed: (i) Crosslinked SAP via covalent bonds, including esterification, addition, Friedel-Crafts reactions and formation of -SO2- bridges; (ii) crosslinked SAP by ionic bonds; (iii) combined covalent and ionic crosslinking
Cation-conducting ionomers made by ion exchange of sulfonated poly-ether-ether-ketone: Hydration, mechanical and thermal properties and ionic conductivity
No full textThe hydration in liquid water of various thermally treated SPEEK membranes was determined and confronted with data in saturated water vapor. Significant differences, which correspond to the so-called Schröder paradox, are observed only in un-crosslinked ionomers with IEC>2. eq/kg. After cation exchange in electrolyte solutions (2 and 0.1. mol/L), the hydration and density of the ionomers was determined and confronted with a physical model. There is no clear correlation with cation properties, such as ionic radius or Robinson-Stokes hydration numbers. The ionic conductivity ranges from 3mS/cm for K, NH4 and Mg-exchanged membranes to 0.6mS/cm for Ca and Zn. Although the latter might be related to ionic cross-linking by the divalent cations, the relatively high conductivity of Mg ions might be useful for electrochemical applications
Design of organic-inorganic hybrid membranes based on sulfonated polyaryl-ether-ketones: correlation between water uptake and electrical conductivity.
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