Springback effect and structural features during the drying of silica aerogels tracked by in-situ synchrotron X-ray scattering

The springback effect during ambient pressure drying of aerogels is an interesting structural phenomenon, consisting of a severe shrinkage followed by almost complete re-expansion. The drying of gels causes shrinkage, whereas re-expansion is believed to be linked to repelling forces on the nanoscale. A multi-scale structural characterization of this significant volume change is key in controlling aerogel processing and properties. In this work, hydrophobic, monolithic silica aerogels with high specific surface areas were synthesized by modification with trimethylchlorosilane and ambient pressure drying. Here, we report a multi-method approach focusing on in-situ X-ray scattering to observe alterations of the nanostructured material during the drying of surface-modified and unmodified silica gels. Both show a porous fractal nanostructure, which partially collapses during drying and only recovers in surface-modified samples during the springback effect. Distinct changes of the X-ray scattering data were reproducibly associated with the shrinkage, re-expansion and drying of the gel network. Our findings may contribute to tailor aerogels with specific functionality, as the springback effect has a direct influence on properties (e.g., porosity, pore size distribution), which is directly affected by the degree of re-expansion

How defects in lanthanum iron manganite perovskite structures promote the catalytic reduction of NO by CO

Adjusting the defect level during synthesis of A- and B-site deficient lanthanum iron manganite (LFM) perovskites shows that non-stoichiometry can beneficially influence the catalytic reactivity to N2 in the reduction of NO by CO on noble metal-free LFM-based perovskites. Optimal steering of La deficiency and the associated redox chemistry to reduce the near-surface regions during catalytic operation at low temperatures is the key factor. Surface enrichment by reducible B site cations and a proper design of structural defects resulting from the optimum introduction of La defects exclusively cause in-situ reduction of surface-near regions by CO oxidation, as well as formation of oxygen vacancies for enhanced NO and N2O reactivity. Excess doping with defects causes structural instability and continuous supply of oxygen from the catalyst bulk to the surface at elevated temperatures. Introduction of B site vacancies leads to surface enrichment by non-reducible lanthanum cations, causing suppressed catalyst activity undercutting even stoichiometric LFM

High-temperature structure of Co3O4: Understanding spinel inversion using in situ and ex situ measurements

International audienc

Pbca-Type In2O3: the high-pressure post-corundum phase at room temperature

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of Physical Chemistry C, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://dx.doi.org/10.1021/jp5061599High-pressure powder X-ray diffraction and Raman scattering measurements in cubic bixbyite-type indium oxide (c-In2O3) have been performed at room temperature. On increasing pressure c-In2O3 undergoes a transition to the Rh2O3-II structure but on decreasing pressure Rh2O3-II-type In2O3 undergoes a transition to a previously unknown phase with Pbca space group which is isostructural to Rh2O3-III. On further decrease of pressure, we observed a phase transition to the metastable corundum-type In2O3 near room conditions. Recompression of the metastable corundum-type In2O3 at room temperature leads to a transition to the Rh2O3-III phase, thus showing that the Rh2O3-III phase is the post-corundum phase at room temperature. Our results are supported by theoretical ab initio calculations. Furthermore, they show that the Rh2O3-III phase could be present in other sesquioxides, thus prompting to a revision of the pressure-temperature phase diagrams of sesquioxidesFinancial support by the Spanish MEC under Grant No. MAT2010-21270-C04-01/03/04, MAT2013-46649-C4-1/2/3-P, by MALTA Consolider Ingenio 2010 project (CSD2007-00045) and by Generalitat Valenciana (GVA-ACOMP-2013-012). Red Espanola de Supercomputacion (RES) and ALBA Synchrotron Light Source are also acknowledged. B.G.-D. and J.A.S. acknowledge financial support through the FPI program and Juan de la Cierva fellowship, respectively. We also thank J. L. Jorda for fruitful discussions. A.L.J.P. acknowledges financial support through Brazilian CNPq. A.S. expresses thanks to FEDER Grant UNLV10-3E-1253 for financial support.García-Domene, B.; Sans Tresserras, JÁ.; Gomis, O.; Manjón Herrera, FJ.; Ortiz, HM.; Errandonea, D.; Santamaría Pérez, D.... (2014). Pbca-Type In2O3: the high-pressure post-corundum phase at room temperature. Journal of Physical Chemistry C. 118(35):20545-20552. https://doi.org/10.1021/jp5061599S20545205521183

Propuesta metodológica para evaluar progamas de orientación educativa

El presente trabajo plantea un modelo de evaluación de programas de orientación que parte del supuesto de que dicha evaluación no es un mero acto puntual, sino que ha de entenderse como un proceso que abarca todas las fases de diseño y validación del programa y ha de iniciarse en el momento en que se aborda la programación y planificación del mismo. El modelo establece cinco fases para el desarrollo de la evaluación: 1ª- De los supuestos previos o preliminares, en la que se van a fijar los supuestos teóricos-previos en los que se basará dicha evaluación ( destinatarios, receptores de la evaluación, finalidad, propósitos, objetivos de la evaluación, papel del evaluador, etc.) 2ª- Viabilidad de la evaluación, en la cual se pretenden establecer las características de la calidad formal/intrínseca del programa, la adecuación / adaptación al contexto y la aceptación en el mismo. 3ª Evaluabilidad o validación de la evaluación; en esta tercera fase se persigue específicamente responder a la cuestión básica de si el programa reúne las condiciones para poder ser evaluado. 4ª- Evaluación del Proceso, que engloba la evaluación de la implementación y la evaluación de desarrollo. 5ª- Evaluación de la Eficacia, la última fase de este modelo, en que se trata de realizar una evaluación de resultados y efectos del programa

Springback effect and structural features during the drying of silica aerogels tracked by in situ synchrotron X ray scattering

The springback effect during ambient pressure drying of aerogels is an interesting structural phenomenon, consisting of a severe shrinkage followed by almost complete re expansion. The drying of gels causes shrinkage, whereas re expansion is believed to be linked to repelling forces on the nanoscale. A multi scale structural characterization of this significant volume change is key in controlling aerogel processing and properties. In this work, hydrophobic, monolithic silica aerogels with high specific surface areas were synthesized by modification with trimethylchlorosilane and ambient pressure drying. Here, we report a multi method approach focusing on in situ X ray scattering to observe alterations of the nanostructured material during the drying of surface modified and unmodified silica gels. Both show a porous fractal nanostructure, which partially collapses during drying and only recovers in surface modified samples during the springback effect. Distinct changes of the X ray scattering data were reproducibly associated with the shrinkage, re expansion and drying of the gel network. Our findings may contribute to tailor aerogels with specific functionality, as the springback effect has a direct influence on properties e.g., porosity, pore size distribution , which is directly affected by the degree of re expansio

Springback effect of ambient pressure dried silica aerogels nanoscopic effects of silylation revealed by in situ synchrotron X ray scattering

Ambient pressure drying APD allows for synthesizing aerogels without expensive and sophisticated equipment for achieving supercritical conditions. Since APD does not eliminate the capillary stress that is induced by the liquid vapour phase boundary, the shrinkage during drying needs to be prevented or reversed. The re expansion of the silylated silica gels during drying is commonly referred to as the springback effect SBE . The SBE is not only important for producing aerogels via APD, but is also a fascinating phenomenon, since it is accompanied by a significant volume change unusual for rigid ceramics. Synchrotron X ray scattering has proven to be especially effective for the investigation of the volume change of these fractal silica structures on different length scales. In this work, we follow the drying, shrinkage, and partial re expansion of various monolithic samples in situ to explore the occurrence of the SBE. For this purpose, various silylation agents, i.e., hexamethyldisilazane, trimethylchlorosilane, and triethylchlorosilane were used to investigate different shrinkage and re expansion behavior. A scattering model was used to extract additional information of the evolving primary particle size, correlation length, fractal dimension, and other intensity contributions of the silica network and the hexane. While the primary particles pointed towards a relaxation at near molecular size, they were likely not involved in the SBE. However, structures near the size of the correlation length could be essential for the occurrence of this phenomenon. These findings may lead to the origin of this interesting phenomenon, as well as a better understanding of the production of APD aerogel

Evaluation of in-situ shear stiffness of asphalt concrete mixtures

In cooperation with the Ontario Ministry of Transportation and the United States Transportation Research Board, researchers at Carleton University in Ottawa, Canada have developed the In- Situ Shear Stiffness Test (InSiSST) - A new field test facility for measuring shear stiffness of compacted asphalt concrete surface layers. To date, various field-testing programs have been conducted to evaluate the in-situ shear stiffness of asphalt concrete mixes using the newly developed InSiSST. In this paper, a brief overview of the InSiSST facility is presented, together with the results of testing completed in Ottawa, Bancroft, Toronto, and at the Long Term Pavement Performance (LTPP) SPS-9A test site located in Petawawa, Ontario. The Petawawa site (870900) is of particular interest as four of the six test sections consist of SuperPave asphalt mixes, while the remaining two sections were designed using the Marshall method. The results of field testing at these locations has indicated that the InSiSST is both sensitive to differences in asphalt mix shear stiffness and is able to rank mix performance between different mixes in terms of measured permanent deformation. With continued refinement and testing, is expected that InSiSST will become a valuable test facility for the design and quality control of hot mix asphalt concrete pavements