Papers by Keyword: Xerogel

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Authors: Gudrun Reichenauer
Abstract: Aerogels are porous materials with potential applications in fields ranging from thermal insulation, catalyst support, filters, electrical storage, components in optical devices, mechanical damping all the way to drug release. However, careful reliable characterization is the base for both, understanding of fundamental structure - property relationships as well as a directed development of materials and composites for specific applications. The review therefore addressed severe problem upon aerogel characterization that have been identified in the past and presents reliable non-destructive alternatives and novel methods that can be applied for the characterization of aerogels as well as their gel precursors.
Authors: Justine Fenech, Céline Viazzi, Florence Ansart, Jean Pierre Bonino
Abstract: Thermal Barrier Coatings (TBCs) are used as insulators on hot section components to reduce operating temperatures in aircraft engines and industrial gas turbine. The TBC system consists of two layers: the ceramic top coat traditionally Yttria Stabilized Zirconia (YSZ) with a low conductivity, and the bond coat generally MCrAlY, M=Ni and/or Cr or Co or Pd or Pt modified aluminides. In the industry, two dry-route processes used to deposit TBCs give quite different microstructures of coatings. In one hand, coatings resulted by plasma spraying (PS) present a lamellar microstruture with a low thermal conductivity in the range from 0.7 to 0.9 Wm−1K−1. In the other hand, Electron Beam Physical Vapour Deposition (EBPVD) coatings with columnar microstruture coatings present the best mechanical performances but perpendicular orientation of the columns makes their thermal conductivity twice higher compared to PS coatings. The present study proposes the elaboration of zirconia coatings via the sol-gel route combined with dip-coating process. It is a versatile process able to produce either thin ceramic coatings or thick deposits. The main advantage of this method is to decrease the crystallization temperature, much lower than conventional processes. Moreover, the sol-gel process is a nondirectional deposition technique, which is very different to the physical methods described above. Doped zirconia have been chosen to constitute isolating multilayers coatings. Sol formulation, slurries stability but also dip-coating conditions have been optimized in order to obtain homogeneous layers on nickel based superalloys substrates.
Authors: Jacob Zabicky, Giora Kimmel, Elena Goncharov, N. Hazan
Authors: Nicolás de la Rosa-Fox, Victor Morales-Flórez, Manuel Piñero, Luis Maria Esquivias Fedriani
Abstract: Acoustic cavitation effects in sol-gel liquid processing permits to obtain nanostructured materials, with size-dependent properties. The so-called “hot spots” produce very high temperatures and pressures which act as nanoreactors. Ultrasounds force the dissolution and the reaction stars. The products (alcohol, water and silanol) help to continue the dissolution, being catalyst content, temperature bath and alkyl group length dependent. Popular choices used in the preparation of silica-based gels are tetramethoxysilane (TMOS), Si(OCH3)4, and tetraethoxysilane (TEOS), Si(OC2H5)4. The resultant “sonogels” are denser gels with finer and homogeneous porosity than those of classic ones. They have a high surface/volume ratio and are built by small particles (1 nm radius) and a high cross-linked network with low –OH surface coverage radicals. In this way a cluster model is presented based on randomly-packed spheres in several hierarchical levels that represent the real sonoaerogel. Organic modified silicates (ORMOSIL) were obtained by supercritical drying in ethanol of the corresponding alcogel producing a hybrid organic/inorganic aerogel. The new material takes the advantages of the organic polymers as flexibility, low density, toughness and formability whereas the inorganic part contributes with surface hardness, modulus strength, transparency and high refractive index. The sonocatalytic method has proven to be adequate to prepare silica matrices for fine and uniform dispersion of CdS and PbS quantum dots (QDs), which show exciton quantum confinement. We present results of characterization of these materials, such as nitrogen physisorption, small angle X-ray/neutrons scattering, electron microscopy, uniaxial compression and nanoindentation. Finally these materials find application as biomaterials for tissue engineering and for CO2 sequestration by means the carbonation reaction.
Authors: Jin Hong Li, Hong Wen Ma, Ying Cao
Abstract: The xerogel of mullite precursor with high specific surface area (422m2•g-1) was successfully prepared from the sol of aluminium isopropanol (AIP) and tetraethyl orthosilicate (TEOS) tuned by supercritical CO2 and subsequent supercritical fluid extraction. Nanometer mullite was prepared by calcining the xerogel of the precursor, and the obtained mullite together with the xerogel precursor was characterized by SEM, XRD, N2-adsorption and IR. The results showed that the xerogel precursor exhibit a shape of amorphous structure, with size of no more than 100 nm, and no huge blocks was found, indicating that the supercritical CO2 plays an important role in controlling the process of hydrolyzation, sol-gel transition, in which supercritical CO2 may promote the mixture of Si and Al on atomic scale. SEM indicated that the obtained mullite were mainly 2-3 μm in length and lath in shape with aspect ratios of 6-10, and the laths were interlaced with each other to construct a relative dense mullite aggregation. The mineralogical phases tracing experiment showed that the mullitization process underwent from 1200 °C to 1400 °C as followed by the phase transition from -Al2O3 to -Al2O3, and to -Al2O3, and the temperature of complete phase transformation into mullite was 1400 °C.
Authors: L. Noailles, B. Dunn, D. Larson, J. Starkovich, Hong Peng
Authors: Rafael van Grieken, José Aguado, María José López-Muñoz, Javier Marugán
Abstract: This paper presents a review of the work published by the authors on the synthesis, characterization and evaluation of the photocatalytic activity of TiO2/SiO2 materials. The use of titania-silica mixed oxides photocatalysts is proposed basically as a process improvement to overcome the difficulties of recovering titania from the slurries after the photocatalytic treatment of contaminated waters. To understand the mechanism governing the photocatalytic activity of these materials, several titania-silica photocatalysts have been prepared through a sol-gel method that allows controlling the main variables to obtain materials with different textural properties, degree of titania incorporation, dispersion of the photoactive phase and crystallinity of titanium dioxide. The samples have been characterized in depth, looking for correlations between the main physicochemical properties (TiO2 crystallite size, band gap energy and titania surface area) and the activity shown in the photocatalytic oxidation of cyanide, selected as model pollutant. The results suggest that the photocatalytic activity is strongly influenced by the quality of the titania crystal network, which in turn is improved by the use of a hydrothermal crystallization procedure. Additionally, the evaluation of the fraction of the total surface area corresponding to titania is mandatory for comparing the catalytic activity of different materials in processes in which titanium dioxide is the only phase catalytically active and silica behaves as an inert support.
Authors: Jae Won Kim, Seoung Soo Lee, Sung Kang Hur, Yeon Gil Jung, Chang Yong Jo
Abstract: A modified sol-gel method is proposed for the preparation of silicon carbide nano-particles and nano-whisker. Tetraethoxysilane (TEOS), SiO2 bead, and PMMA as precursor of carbon source were used for preparing a ternary carbonaceous silicon xerogel, and morpholine is employed in the sol-gel process as a catalyst reagent. SiC nano-particles and nano-whisker were obtained from carbothermal reduction and vapor-solid (VS) reaction of the ternary xerogel at 1300°C for 9h in a dynamic argon atmosphere (flow rate; 600 cm3/min), and then purified by removing excess silica, residual carbon, and other impurities. The purified SiC sample was characterized by TG/DTA, XRD, SEM, and HRTEM. The nano-particles and the nano-whisker were synthesized by different growth mechanisms, with an initial nucleation via a VS reaction between SiO vapor and carbon precursor (PMMA) and a partially subsequent growth via a vapor-vapor (VV) reaction between SiO and CO, respectively.
Authors: Bambang Priyono, Akhmad Herman Yuwono, B. Munir, A. Rahman, A. Maulana, H. Abimanyu
Abstract: Dye-sensitized solar cell (DSSC) is one of the very promising alternative renewable energy sources to anticipate the diminishing in the fossil fuel reserves in the next few decades and to make use of the abundance of intensive sunlight energy in tropical countries like Indonesia. TiO2 nanoparticles have been used as the photo electrode in DSSC because of its high surface area and allow the adsorption of a large number of dye molecules. In the present study, TiO2 aerogel have been synthesized via sol-gel process with water to inorganic precursor ratio (Rw) of 2.00, followed with subsequent drying by CO2 supercritical extraction (SCE). As comparison, the TiO2 xerogel was also prepared by conventional drying and annealing. Both types of gels were subjected to conventional and multi-step annealing. The resulting nanoparticles in aerogel and xerogel have a band-gap energy of 3.10 and 3.04 eV, respectively. The open circuit voltage (Voc) measurement reveals that the DSSC fabricated with aerogel provided a higher voltage (21,40 mV) than xerogel (1,10 mV).
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