185 research outputs found
Molecular simulation study of the heat capacity of metastable water between 100K and 300K
Molecular simulation study of the heat capacity of metastable water between
100K and 300K Molecular simulations have been used to study the heat capacity
of metastable liquid water at low temperature adsorbed on a smooth surface.
These calculations aim at modelling water properties measured by experiments
performed on water films adsorbed on Vycor nanoporous silica at low
temperature. In particular, the study focuses on the non-monotonous variation
of the heat capacity around between 100 and 300 K
Vibrational analysis of d-PCL(530)/siloxane based hybrids doped with two lithium salts
Published online: 22 May 2013The present study has been focused on environmentally friendly sol-gel derived electrolytes based on a di-urethane cross-linked d-PCL(530)/siloxane network (where d represents di, PCL identifies the poly(ε–caprolactone) biopolymer and 530 is the average molecular weight in g.mol-1) doped with a wide range of concentration of lithium perchlorate (LiClO4) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Fourier Transform Infrared and Raman (FT-IR and FT-Raman, respectively) spectroscopies have been applied to evaluate the extent of ionic association. Characteristic bands of the PCL(530) segments, of the urethane cross-links and of the anions have been examined to gain insight into the cation/biopolymer, cation/anion and cation/cross-link interactions. In both electrolyte systems “free” ions and contact ions have been identified. The addition of salt modifies the hydrogen-bonded array of the host matrix, causing the destruction/formation of the urethane/urethane aggregates.Fundação para a Ciência e a Tecnologia (FCT
Geopolymer/PEG Hybrid Materials Synthesis and Investigation of the Polymer Influence on Microstructure and Mechanical Behavior
Geopolymers are aluminosilicate inorganic polymers, obtained from the alkali activation of powders containing SiO2+Al2O3>80wt%, mainly proposed as environmentally friendly building materials. In this work, metakaolin-based geopolymers have been prepared and a water-soluble polymer, polyethylene glycol (PEG), has been added in different percentages to obtain organic-inorganic hybrid geopolymers. The influence of both the polymer amount and aging time on the structure and the mechanical behavior of the materials were investigated. FTIR spectroscopy allowed us to follow the evolution of the aluminosilicate framework during the geopolymerization process. This analysis revealed that PEG leads to a network which is rich in Al-O-Si bonds and forms H-bonds with the inorganic phase. SEM microscope showed that the two phases are interpenetrated on micrometric scales. Traction and bending tests have been carried out on appropriate samples to investigate the mechanical behavior of the obtained hybrids, showing that both PEG content and aging time affect the material behavior
Étude des mécanismes électrochimiques dans les films minces d'oxyde de tungstène
Les films minces de WO3 élaborés par le procédé sol-gel présentent des propriétés électrochromes. Dans cet article, nous étudions les processus physiques mis en jeu lors du processus d'insertion électrochimique. Deux composés cristallisés, WO3,2H2O et WO3 orthorhombique, sont étudiés comme composés de référence. La voltammétrie cyclique et les mesures d'impédance complexe permettent de préciser les variations des cinétiques de réaction. Des études de diffraction des rayons X permettent de préciser les changements structuraux intervenant dans ces composés
Freezing, melting and dynamics of supercooled water confined in porous glass
International audienceThe freezing, melting and dynamics of supercooled water at different hydration of controlled porous glass (CPG) with mean pore sizes 10 nm, 30 nm, 50 nm and 70 nm are studied using differential scanning calorimetry (DSC) and deuteruim nuclear magnetic resonance (2 H-NMR). For saturated samples, the melting tempertaure follows the Gibbs-Thomson relation despite a clear linear decrease of the melting enthalpy when the transition is shifted due to confinement. For partially filled porous glasses the crystalization and melting temperatures as well as enthalpy are lower than for the saturated samples. 2 H-NMR confirms the existence of a non-crystallizable part of water adsorbed on the surface of pores. At room temperature, spin-lattice relaxation rate (1/T1) is proportional to the inverse of the mean pore size indicating that the relaxation is governed by a surface limited process. At low tempertaure relaxation rate follows the Vogel-Fulcher-Tammann (VFT) relation
Melting of ice in porous glass: why water and solvents confined in small pores do not crystallize?
The melting of ice in porous glass having different distribution of pores
sizes is analyzed in details. One shows that confined water crystallizes
only partially and that an interface layer, between the ice crystallites and
the surface of the pore, remains liquid. Properties of this non crystalline
interface at low temperature is studied by NMR and DSC. Both methods lead to
an interface thickness h of the order of 0.5 nm, this explains why water do not
crystallize when the dimension of confinement is less than a critical length
nm. The variation of the melting enthalpy per gram of total amount
of water with the confinement length is explained taking into account two
effects: a) the presence of this layer of water at the interface and b) the
linear variation of the melting enthalpy with the melting
temperature Tm. From the data of the literature one draws the same conclusions
concerning other solvents in similar porous materials. Also one points out
the important role of the glass temperature Tg in preventing the
crystallization of the liquids confined in small pores and/or between the
crystallites and the surface of the pores
Propriétés photoélectrochimiques des gels d’oxyde de tungstène
Des films d’oxyde de tungstène WO3,xH2O (amorphe et cristallisés, 0 ≤ x ≤ 2) ont été élaborés par le procédé sol-gel. Ils présentent des propriétés électrochromes et photoélectrochimiques. L’étude de la réponse photoélectrochimique permet d’obtenir des informations sur le phénomène de réduction-insertion électrochimique
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