J.P. Boilot

NMR study of the sol/gel polymerization

Kinetics and mechanisms of the hydrolysis-condensation reactions, starting with tetraethyl orthosilicate (TEOS), have been studied by 29 Si NMR. A standard experimental procedure for the sol/gel process was used: acid-catalyzed reactions and different concentration ratios of alkoxide and water. The inorganic polymerization was influenced by the following experimenta) parameters: — The starting alkoxides which may exhibit weakly reactive “Si-OR groups”. — The use of gel precursors with different chain lengths which lead to parasite ester exchange reactions. — The variations of pH which modify hydrolysis and condensation rates. — The H 2 O content where an excess of water was required to fully separate hydrolysis and condensation domains, and to eliminate “OR groups” in the gel.

Dimorphism, phase transitions, and transport properties in LiZr2(PO4)3

The authors report the structure, the phase transitions, and the transport properties of two different phases of LiZr{sub 2}(PO{sub 4}){sub 3} materials prepared by the sol/gel technique. The phase have been characterized by X-ray powder diffractometry, thermal analysis, ac conductivity, and NMR ({sup 31}P and {sup 7}Li). A high-temperature phase, prepared at 1,200{degree}C, shows a NASICON-type structure and a monoclinic {yields} rhombohedral first order transition at about 40{degree}C. This transformation is associated with the sharp appearance of a lithium motion leading to high conducting properties above the phase transition {sigma} = 1.2.10{sup {minus}2} ({Omega} cm){sup {minus}1} at 300{degree}C. A low-temperature phase, prepared at 900{degree}C, exhibits the {beta}-Fe{sub 2}(SO{sub 4}){sub 3}-type structure and a monoclinic {yields} orthorhombic transformation at 300{degree}C. A local lithium motion progressively takes place between RT and 120{degree}C and a conducting state is observed at 300{degree}C ({sigma} =5.10{sup {minus}4} ({Omega} cm){sup {minus}1}). In diphasic samples prepared between 900 and 1200{degree}C, both resistivity and activation energy progressively increase with the low-high temperature phase ratio.

Nonmonotonic field dependence of the zero-field cooled magnetization peak in some systems of magnetic nanoparticles

We have performed magnetic measurements on a diluted system of

Kinetic simulations and mechanisms of the sol-gel polymerization

\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}

Relation structure-fast ion conduction in the NASICON solid solution

nanoparticles

Fast ion transport in LiZr2(PO4)3: Structure and conductivity

(d\ensuremath{\sim}7 \mathrm{nm})

Near-field optical patterning on azo-hybrid sol–gel films

, and on a ferritin sample. In both cases, the zero-field cooled (ZFC) peak presents a nonmonotonic field dependence, as has already been reported in some experiments, and discussed as possible evidence of resonant tunneling. Within simple assumptions, we derive expressions for the magnetization obtained in the usual ZFC, field cooled (FC) and thermoremanent magnetization (TRM) procedures. We point out that the ZFC-peak position is extremely sensitive to the width of the particle-size distribution, and give some numerical estimates of this effect. We propose to combine the FC magnetization with a modified TRM measurement, a procedure which allows a more direct access to the barrier distribution in a field. The typical barrier values that are obtained in this procedure show a monotonic decrease for increasing fields, as expected from the simple effect of anisotropy barrier lowering, in contrast with the ZFC results. From our measurements on

Chemical processing of (Ba, Sr)(Ti, Zr)O3 perovskite ceramics

\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}

Structural investigation of K+ and Tl+ β-aluminas

particles, we show that the width of the effective barrier distribution is slightly increasing with the field, an effect that is sufficient for causing the observed initial increase of the ZFC-peak temperatures.

β alumina, a typical solid electrolyte: Latest developments in fundamental approach and in battery utilization

High resolution 29 Si NMR experiments concerning acid-catalyzed sol-gel polymerization from tetraethyl-orthosilicate have been interpreted by means of simulated kinetic curves. In the first hours, the growth process is controlled by the chemical reactivity. The rate constants of the preponderant reactions (hydrolysis-condensation-reesterification) are influenced by the H + concentration and the nature of oligomers (degree of hydrolysis and degree of condensation of silicon atoms). When the size of the polymers becomes sufficiently large, the kinetics of the growth process is mainly controlled by the diffusion which depends on the mobility of the particles and on the molecular separation.