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Search query: silica aerogels
14 articles match your search "silica aerogels"Results 1 to 10, sorted by relevance:
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This research uses contrast-matched small-angle neutron scattering to investigate the adsorption behaviour of deuterated methane in a silica aerogel in the pressure range from 0 to 1000 bar. The results reveal a classical reversible two-phase adsorption in the 2.5–50 nm pore size region and no evidence of condensation in the sub-nanometre pores.
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Silica aerogels were doped with cobalt, reduced, and studied with ultra-small-angle X-ray scattering. A scattering pattern rich in structure extends over nearly four orders of magnitude in q space, covering real space from 5 to 3000 Å, exposing micropores, mesopores and macropores for all aerogels.
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Supercritical carbon dioxide within a porous silica aerogel behaves as a two-phase system formed by a film next to the silica walls and a remaining fluid phase. Small-angle neutron scattering allows one to determine the structural parameters of the involved phases.
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Errors in the paper by Ciccariello, Melnichenko & He [J. Appl. Cryst. (2011), 44, 43-51
] are corrected.
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Structural changes during isothermal sintering were studied for two base-catalyzed SiO2 aerogels with initial densities ρ1 = 122 and 256 kg m−3 by means of small-angle X-ray scattering (SAXS). The relevant structural parameters are the specific surface area, which is determined by the size of the compact primary particles, the mean diameter of the secondary particles, which represent the building blocks of the gel network, and finally the size of the macropores within the tenuous tertiary structure. The major finding of our investigations is the observation of two sintering processes with different time scales: a fast reduction of the size of the macropores and a slow increase in the diameter of the secondary particles as well as a relatively small decrease in the specific surface area. For the two samples investigated the changes of these parameters can be unequivocally correlated with the achieved density upon sintering – independent of the initial density.
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Structural properties of organic aerogels were studied by ultra-small-angle X-ray scattering (USAXS) at the Synchrotron Radiation Laboratory HASYLAB at DESY, Hamburg, Germany. The organic aerogels were synthesized from the base-catalyzed sol-gel polymerization of resorcinol with formaldehyde (RF) followed by a supercritical drying process. RF aerogels are low-density materials with a solid matrix composed of interconnected colloidal-like particles. Scattering experiments were carried out using a crystal camera optimized for ultra-small-angle X-ray scattering with synchrotron radiation. The measured SAXS profiles revealed a scattering power depending upon synthetic conditions of the gels. RF aerogels were found to be homogeneous at length scales greater than 20 nm. From Guinier plots, radii of gyration Rg of 3-20 nm were computed. Rg appears to be a measure of the pore (cell) size. Although fractal silica aerogels show similar characteristics, fractal behavior of the organic aerogels is uncertain. These materials are best described as random aggregates of smooth colloidal like particles with open-cell porosity.
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A three-dimensional real-space model has been created for complex hierarchical materials by matching observed and simulated small-angle X-ray scattering patterns. The simulation is performed by arranging the positions of small primary particles and constructing an aggregate structure in a finite-sized cell.
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Base-catalyzed silica aerogels are composed of particles with a mean size of about 5 nm, which form a chain-like porous network. Up to now it has been assumed that supercritical drying (SCD) of highly porous gels in autoclaves leaves the structure nearly unchanged. Small-angle X-ray scattering (SAXS) measurements provide evidence that this is only true for the low-temperature CO2 drying process with a critical temperature Tc = 304.2 K and a critical pressure pc = 73.9 × 105 Pa. In the high-temperature methanol process (Tc = 512.5 K and pc = 80.9 × 105 Pa) with remaining water and catalyst, however, structural changes are introduced. The SAXS measurements can be explained by a narrowing of the particle-size distribution during the heating period of the autoclave process. In a double-logarithmic representation of the scattered intensity I versus the scattering vector q, intermediate slopes smaller than -4 in the Porod region as well as oscillations in an Iq4 vs q plot appear. On the contrary, SCD hardly affects the particle structure of acid-catalyzed gels and, if the basic solvent is exchanged for pure methanol, of base-catalyzed gels.
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Structure development in aerogel-processed nanocrystalline alkaline earth oxides as revealed by SANS
Small-angle neutron scattering (SANS) experiments were conducted on a series of nanocrystalline alkaline earth oxide powders and their precursors in order to characterize structural development in this system. Results show the effect of cation size on the characteristic structural features of each material and the changes that occur during supercritical versus subcritical drying and subsequent calcination.