1,720,977 research outputs found
Lattice symmetry relaxation as a cause for anisotropic line broadening and peak shift in powder diffraction
Full text linkIn powder diffraction, lattice symmetry relaxation causes a peak to split into several components which are not resolved if the degree of desymmetrization is small (pseudosymmetry). Here the equations which rule peak splitting are elaborated for the six minimal symmetry transitions, showing that the resulting split peaks are generally broader and asymmetric, and suffer an hkl-dependent displacement with respect to the high-symmetry parent peak. These results will be of help in Rietveld refinement of pseudosymmetric structures where an exact interpretation of peak deformation is required
Dalla cava alla brocca: un progetto ludico scientifico sulla cottura della ceramica con mezzi di fortuna (campagna 2003)
No full textSol-gel synthesis of microporous amorphous silica from purely inorganic precursors
No full textAqueous sodium silicate solutions and hydrochloric acid were used to prepare silica gels without any additives. In order to establish the optimum conditions for the preparation of microporous and monodisperse gels, the sol–gel reaction has been studied as a function of pH, concentration of silicate, time, and temperature. From 29Si NMR spectroscopy it was found that polycondensation sets on immediately after mixing the reagents, with a continuous increase of the fraction of 4-connected silicate tetrahedra (Q4) at the expense ofQ1andQ2. Polycondensation continues beyond the gelation point for 1 ≤ pH ≤ 7, but not for 7 ≤ pH ≤ 10. Two mechanisms were also observed for the gelation kinetics which is about second order with respect to [Si] for pH ≤ 7, whereas for pH > 7, the order increases rapidly with pH. Gels formed at pH > 7 had an electrostatic nature and redissolved in water, whereas for pH ≤ 7, gelation was irreversible as expected for a gel where individual colloid particles are linked by chemical (≡Si—O—Si≡) bonds. From nitrogen adsorption, it is found that the pore size depends mainly upon pH, the finest pores being obtained for pH 2–3 (BET surface area 650–700 m2/g, porosity 35%, pore radius 5–11 Å)
Preparation of silica-based microporous inorganic gas separation membranes
No full textFully inorganic silica-based membranes have been prepared using sol-gel procedures starting from sodium silicate solutions. An asymmetric membrane structure is obtained by multiple deposition of colloidal silica sols onto commercial ceramic supports. The microporous structure of the silica which forms the active top layer remains stable up to 873 K and is characterised by a mean pore diameter of 1.6 nm, a narrow pore size distribution and a porosity of 35%. The continuous silica top layer of the membrane was analysed by SEM and checked by gas permeation measurements. The permeabilities of CH4, N2 and CO2 were measured for membranes deposited onto zirconia supports as a function of temperature. The
preliminary experimental results indicate reproducible and relatively high fluxes as well as a transport mechanism which basically follows Knudsen flow with contributions from surface and microporous diffusion
"Dalla cava alla brocca": un progetto ludico-scientifico sulla cottura della ceramica con mezzi di fortuna (campagne di lavoro 1995-2000)
No full text(Introduzione)
Il progetto “Dalla Cava alla Brocca” nacque verso la fine del 1995 da un esiguo gruppo di persone provenienti da ambiti assai diversi, ma interessate a vario titolo ai molteplici aspetti che possono riguardare il mondo della ceramica. In particolare, l’idea che sta alla base del progetto è quella della realizzazione di oggetti in ceramica facendo uso pressoché esclusivo di materiali di fortuna, ripercorrendo cioè tappe e problematiche analoghe a quelle di culture primitive (antiche o odierne)
dedite a questa attività: dalla raccolta delle materie prime, alla foggiatura di oggetti, alla realizzazione di forni per la cottura.
La diversità delle competenze coinvolte ha fatto sí che si instaurasse fin dall’inizio un proficuo scambio di conoscenze tra coloro che, di volta
in volta, hanno partecipato agli incontri informali che hanno dato vita al progetto: gli argomenti di discussione hanno cosí spaziato dagli aspetti prettamente pratici sulla lavorazione dell’argilla fino alle basi scientifiche su impasti, reazioni chimiche e transizioni mineralogiche che avvengono durante la cottura.
La prima esperienza del dicembre 1995 è stata seguita da varie altre, con crescente partecipazione di appassionati e anche di archeologi. Questi ultimi risultavano particolarmente interessati alla riproduzione di forni d’epoca per le loro ricerche relative alla fabbricazione di ceramiche nell’antichità. In questo articolo sono riportate le attività sperimentali svolte nell’ambito delle campagne di lavoro effettuate dal dicembre 1995 fino al luglio 2000
Cryptomelane nanocrystals: Pseudosymmetry causes anisotropic peak broadening in Rietveld refinement
Full text linkCryptomelane (□2-xKx)[Mn8O16] is an octahedral molecular sieve with the hollandite framework, a strongly correlated electron system with tunnel structure. Rietveld refinement using powder X-ray diffraction patterns of a nanocrystalline sample was undertaken to assess both structural and morphological details which might be tuned for specific applications. The distribution of peak widths was highly anisotropic but successful refinement of the tetragonal structure could be achieved assuming both grain shape and microstrain anisotropy. The microstrain hypothesis appeared flawed from high uncertainties and correlations and was in conflict with the negative slope of a Williamson-Hall plot. As an alternative, lattice desymmetrization from tetragonal to monoclinic was considered. Comparison of calculated and observed hkl dependence of line broadening confirmed that the pseudosymmetry hypothesis was more appropriate, improving results for both structure and particle shape. This is the first time that lattice desymmetrization was used to explain anisotropic diffraction line broadening, a major issue in nanomaterials where peaks are intrinsically broad and the new model helps to abate correlations which afflict Rietveld refinement in these systems
Study of partial occupancies and Jahn-Teller distortions in (Na,□)5[MnO2]13 by XRPD Rietveld and electron diffraction dynamical refinements
No full textHydrogen positions and thermal expansion in lizardite-1T from Elba: A low-temperature study using Rietveld refinement of neutron diffraction data
No full textThe structure of lizardite-1T from Monte Fico, Elba, was refined in space group P31m using neutron diffraction data, measured at 8, 150, and 294 K, and full-profile Rietveld refinement techniques. The lattice parameters at 8 K [a = 5.3267(2), c = 7.2539(6) Å], 150 K [a = 5.3260(2), c = 7.2574(6) Å], and 294 K [a = 5.3332(2), c = 7.2718(6) Å] show nonlinear expansion, with nearly all volume change above 150 K. H positions were precisely refined at 8 K. The inner H4 atom deviates from the idealized 0,0,z positions and is disordered over three symmetry-related positions 0.24 Å away from the ternary axis. The outer H3 atom location is consistent with the previous single-crystal X-ray structure refinement. On the basis of the present thermal expansion data and previous compressibility measurements, the following equation of state for lizardite-1T is proposed: VP.T = Vo[1 + 32.8 × 10-6(T - 294) - 15.5 × 10-4(P - 0.001)]. Accordingly, the constant volume condition requires geothermal gradients on the order of 15 °C/km
Characterization of Mn octahedral molecular sieves by electron diffraction and Rietveld refinement
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