Search results for "CERAMICS"
showing 10 items of 1599 documents
Properties of methyl radical trapped in amorphous SiO2 and in natural SiO2-clathrate Melanophlogite
2013
Abstract We report an experimental investigation by electron paramagnetic resonance (EPR) on methyl radical (CH3 ) observed in γ-ray irradiated high-purity amorphous silicon dioxide (a-SiO2) and in a polycrystalline sample of Melanophlogite, a rare natural form of SiO2-clathrate. From the analysis of the EPR spectra we estimate the correlation time of the hindered rotational motion of CH3 molecules at T = 77 K in the two different materials. This physical quantity gives a quantitative measure of the freedom of motion of CH3 molecules trapped in the two solid systems, putting forward relevant information on the properties of the cavities/interstices in which the radicals are confined. In par…
Diffusion and reactions of interstitial oxygen species in amorphous SiO2: A review
2008
This article briefly summarizes the diffusion and reactions of interstitial oxygen species in amorphous SiO 2 (a-SiO 2 ). The most common form of interstitial oxygen species is oxygen molecule (O 2 ), which is sensitively detectable via its characteristic infrared photoluminescence (PL) at 1272 nm. The PL observation of interstitial 0 2 provides key data to verify various processes related to interstitial oxygen species: the dominant role of interstitial O 2 in long-range oxygen transport in a-SiO 2 ; formation of the Frenkel defect pair (Si-Si bond and interstitial oxygen atom, 0°) by dense electronic excitation; efficient photolysis of interstitial O 2 into O° with F 2 laser light (λ= 157…
Phase transition kinetics and reorientational dynamics of the plastic crystal meta-carborane studied by deuteron NMR
2002
Meta-carborane shows an orientationally disordered, orthorhombic phase which transforms into an ordered monoclinic phase near 170 K and into a quasi-isotropic cubic phase near 280 K. From deuteron magnetization measurements both transitions are found to exhibit a thermal hysteresis and slow transformation kinetics. Furthermore, solid-echo NMR spectra and spin-lattice relaxation times are reported. These results indicate that in the orthorhombic phase the meta-carborane molecules perform a strongly anisotropic motion which is characterized by a temperature dependent amplitude.
Rietveld Refinement of Tetragonal V4+–ZrO2 Solid Solutions Obtained from Gels by X-ray Powder Diffraction
2002
Abstract The crystal structure of three tetragonal V x Zr 1− x O 2 solid solutions, with x =0.025, 0.05, and 0.075, prepared by heating dried gel precursors at 450°C in air atmosphere, have been determined by Rietveld refinement on the basis of powder X-ray powder diffractometer data. They contain V 4+ cations surrounded by eight oxygens, four at a distance between 2.079 and 2.093 A and another four at longer distances between 2.369 and 2.348 A. The estimation of the crystal average oxygen position from the X-ray lattice parameter of V x Zr 1− x O 2 conform with the relationship proposed by Howard et al. ( J. Am. Ceram. Soc. 81, 241 (1998)).
Phase analysis and oxygen strorage capacity of ceria-lanthana-based TWC promoters prepared by sol-gel routes
2002
Ceria–lanthana-based promoters of three-way catalysts are synthesized by two different sol–gel routes, involving nitrate precursors. The oxygen uptake ability of these compounds is measured by O2 chemisorption. The specific surface area is determined by N2 adsorption (BET). X-ray diffraction data are analyzed by Rietveld refinement, demonstrating that lanthanum forms solid solution with CeO2; its total amount in ceria depends on the competitive formation of La–Al mixed oxides and on the synthetic method. The O2 uptake ability is essentially determined by the La content in the ceria–lanthana solid solution, while it is independent on the surface area and on the CeO2 particle size. The O2 upt…
Translational and rotational molecular motion in supercooled liquids studied by NMR and forced Rayleigh scattering
1994
It has been shown that translational diffusion coefficients, Dt, in the supercooled van der Waals liquids, orthoterphenyl, phenolphthaleindimethylether, and salol, have a weaker temperature dependence than the shear viscosity, η, at T ≲ 1.2Tg and can be described by Dt ∼ η−χ with χ < 1 whereas Dr ∼ η−1 applies for the mean rotational diffusion coefficients, Dr, down to the glass transition temperature, Tg. This apparent decoupling of translational and rotational motion has been discussed in relation with possible anomalous short time diffusion, spatial heterogeneity, and cooperative molecular motions close to Tg.
Effect of Alkali Ions on the Amorphous to Crystalline Phase Transition of Silica
2001
Abstract The effect of the addition of alkali ions to commercial amorphous silica, generally used as support for heterogeneous catalysts, has been investigated from the point of view of morphological and structural changes. Samples of alkali-doped silica were prepared by impregnation and subsequent calcination at various temperatures. The structural effect of Li, Na, K, and Cs was determined by use of techniques such as wide-angle (WAXS) and small-angle X-ray scattering (SAXS). The WAXS diffractograms, analyzed with the Rietveld method using the GSAS program, allowed qualitative and quantitative identification of the fraction of the different silica polymorphs like quartz, tridymite, and cr…
Profiling of RNA modifications by multiplexed stable isotope labelling
2014
The combination of (15)N/(13)C stable isotope labelling (SIL) and LC-MS/MS revealed a total of 52 modifications in RNA from E. coli and yeast, including 10 previously undescribed modifications. Two modifications, N-ribosylnicotinamide and 2-methylthioadenosine, were newly detected in species hitherto thought not to contain these modifications.
Synthesis and crystal structure of α-NH4(VO2)(HPO4)
1992
Abstract Hydrothermal synthesis starting from β-NH 4 (VO 2 )(HPO 4 ) leads to α-NH 4 (VO 2 )(HPO 4 ): orthorhombic, space group Pb 2 1 a ; Z = 4; a = 6.830(1) A, b = 9.233(2) A, c = 8.817(2) A; R = 0.037 for 3371 reflections. NH + 4 cations connect isolated chains of trans -corner sharing VO 5 square pyramids running along the a -axis. The a -phase transforms irreversibly into the β-form by heating near 200°C. Comparison is made between the two phases.
Metamagnetic behaviour in a new Cu(ii)Re(iv) chain based on the hexachlororhenate(iv) anion
2014
A new chloro-bridged heterobimetallic Cu(ii)Re(iv) chain of formula {Cu(pyim)(Him)2ReCl6}n·MeCN (·MeCN) has been prepared and magnetostructurally characterised. Compound is the first example of the [Re(IV)Cl6](2-) anion acting as a metalloligand towards a paramagnetic metal ion.