Rings and rigidity transitions in network glasses

Three elastic phases of covalent networks, (I) floppy, (II) isostatically rigid and (III) stressed-rigid have now been identified in glasses at specific degrees of cross-linking (or chemical composition) both in theory and experiments. Here we use size-increasing cluster combinatorics and constraint counting algorithms to study analytically possible consequences of self-organization. In the presence of small rings that can be locally I, II or III, we obtain two transitions instead of the previously reported single percolative transition at the mean coordination number $\bar r=2.4$, one from a floppy to an isostatic rigid phase, and a second one from an isostatic to a stressed rigid phase. The width of the intermediate phase $~ \bar r$ and the order of the phase transitions depend on the nature of medium range order (relative ring fractions). We compare the results to the Group IV chalcogenides, such as Ge-Se and Si-Se, for which evidence of an intermediate phase has been obtained, and for which estimates of ring fractions can be made from structures of high T crystalline phases.Comment: 29 pages, revtex, 7 eps figure

Structural studies of inorganic glasses

SIGLEAvailable from British Library Document Supply Centre- DSC:D59490 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Assessing the effect of reducing agents on the selective catalytic reduction of NOx over Ag/Al2O3 catalysts

This is the final version of the article. It first appeared from RSC via http://dx.doi.org/10.1039/C5CY01508AThe selective catalytic reduction (SCR) of NOx in the presence of different reducing agents over Ag/Al2O3 prepared by wet impregnation was investigated by probing catalyst activity and using NMR relaxation time analysis to probe the strength of surface interaction of the various reducing agent species and water. The results reveal that the strength of surface interaction of the reducing agent relative to water, the latter present in engine exhausts as a fuel combustion product and, in addition, produced during the SCR reaction, plays an important role in determining catalyst performance. Reducing agents with weak strength of interaction with the catalyst surface, such as hydrocarbons, show poorer catalytic performance than reducing agents with a higher strength of interaction, such as alcohols. This is attributed to the greater ability of oxygenated species to compete with water in terms of surface interaction with the catalyst surface, hence reducing the inhibiting effect of water molecules blocking catalyst sites. The results support the observations of earlier work in that the light off-temperature and maximum NOx conversion and temperature at which that occurs are sensitive to the reducing agent present during reaction, and the proposal that improved catalyst performance is caused by increased adsorption strength of the reducing agent, relative to water, at the catalyst surface. Importantly, the NMR relaxation time analysis approach to characterising the strength of adsorption more readily describes the trends in catalytic behaviour than does a straightforward consideration of the polarity (i.e., relative permittivity) of the reducing agents studied here. In summary, this paper describes a simple approach to characterising the interaction energy of water and reducing agent so as to aid the selection of reducing agent and catalyst to be used in SCR conversions.We gratefully acknowledge funding for this work from the EPSRC CASTech grant (EP/G012156/1). Carmine D?Agostino would like to acknowledge Wolfson College, Cambridge, for supporting his research activities. The authors would also like to thank Dr Jonathan Mitchell for useful discussions

EXAFS STUDIES OF PHOTOSTRUCTURAL CHANGES IN CHALCOGENIDE GLASSES

EXAFS measurements have been performed on a number of chalcogenide glasses exhibiting both reversible and irreversible photostructural effects. Changes in the local structure induced by the absorption of bandgap light have been observed and these are discussed in the light of current models for the effect

Rapid Distinction of Intracellular and Extracellular Proteins Using NMR Diffusion Measurements

In-cell NMR spectroscopy offers a unique opportunity to begin to investigate the structures, dynamics, and interactions of molecules within their functional environments. An essential aspect of this technique is to define whether observed signals are attributable to intracellular species rather than to components of the extracellular medium. We report here the results of NMR measurements of the diffusion behavior of proteins expressed within bacterial cells, and find that these experiments provide a rapid and nondestructive probe of localization within cells and can be used to determine the size of the confining compartment. We show that diffusion can also be exploited as an editing method to eliminate extracellular species from high-resolution multidimensional spectra, and should be applicable to a wide range of problems. This approach is demonstrated here for a number of protein systems, using both 15N and 13C (methyl-TROSY) based acquisition

The degradation of polyglycolide in water and deuterium oxide. Part II: Nuclear reaction analysis and magnetic resonance imaging of water distribution

Magnetic resonance imaging (MRI) and scanning microbeam nuclear reaction analysis (NRA) were used to monitor changes of water ingress into polyglycolide (PGA) disks with degradation time. MRI detects H2O, whereas NRA is sensitive to D2O. The acid-catalysed hydrolysis of the ester is significantly slower in D2O than H2O because of the kinetic isotope effect. This behaviour was investigated in Part I. In this paper, NRA was used to investigate PGA hydration in buffers made from D2O, and NRA and MRI experiments were performed on samples degraded buffers made from a 50% mixture of D2O and H2O (D2O/H2O 50:50) to allow a comparison between the two techniques. The NRA and MRI results provide direct evidence in support of the four-stage reaction - erosion model reported in previous literature, and show that this model applies to polymer degradation in heavy water and in a buffer made from D2O/H2O 50:50. It is believed that this is the first time that NRA and MRI have been compared for the same hydrating system. © 2002 Elsevier Science Ltd. All rights reserved

The degradation of polyglycolide in water and deuterium oxide. Part II: Nuclear reaction analysis and magnetic resonance imaging of water distribution

Magnetic resonance imaging (MRI) and scanning microbeam nuclear reaction analysis (NRA) were used to monitor changes of water ingress into polyglycolide (PGA) disks with degradation time. MRI detects H2O, whereas NRA is sensitive to D2O. The acid-catalysed hydrolysis of the ester is significantly slower in D2O than H2O because of the kinetic isotope effect. This behaviour was investigated in Part I. In this paper, NRA was used to investigate PGA hydration in buffers made from D2O, and NRA and MRI experiments were performed on samples degraded buffers made from a 50% mixture of D2O and H2O (D2O/H2O 50:50) to allow a comparison between the two techniques. The NRA and MRI results provide direct evidence in support of the four-stage reaction - erosion model reported in previous literature, and show that this model applies to polymer degradation in heavy water and in a buffer made from D2O/H2O 50:50. It is believed that this is the first time that NRA and MRI have been compared for the same hydrating system. © 2002 Elsevier Science Ltd. All rights reserved

Nuclear magnetic resonance measurements of velocity distributions in an ultrasonically vibrated granular bed

This paper was accepted for publication in the journal Philosophical Transactions of The Royal Society A-Mathematical Physical and Engineering Sciences and the definitive published version is available at http://dx.doi.org/10.1098/rsta.2013.0185We report the results of nuclear magnetic resonance (NMR) imaging experiments on granular beds of mustard grains fluidised by vertical vibration at ultrasonic frequencies. The variation of both granular temperature and packing fraction with height was measured within the three-dimensional cell for a range of vibration frequencies, amplitudes and numbers of grains. Small increases in vibration frequency were found – contrary to the predictions of classical ‘hard-sphere’ expressions for the energy flux through a vibrating boundary – to result in dramatic reductions in granular temperature. Numerical simulations of the grain-wall interactions, using experimentally-determined Hertzian contact stiffness coefficients, showed that energy flux drops significantly as the vibration period approaches the grain-wall contact time. The experiments thus demonstrate the need for new models for ‘soft-sphere’ boundary conditions at ultrasonic frequencies