A multinuclear NMR study of sodium-borovanadophosphate glasses

xNaVO₃ · yNaPO₃ · (1 − y)NaBO₂ glasses were prepared where x = 0.0, 0.05, 0.5 and 0 ? y ? 1. These glasses have been investigated with multinuclear MAS NMR. ⁵¹V NMR spectra show that two vanadate configurations are possible, assigned as four and five coordinated vanadium. The concentration of the latter group decreases upon addition of sodium-borate. The presence of four and three coordinated boron sites can be deduced from the ¹¹B NMR spectra. The latter boron sites appear only when borate groups are connected to each other. ³¹P NMR spectra of borophosphates and borovanadophosphates show that the ratio of pyro- and metaphosphates is greater in the glasses containing vanadate than in simple borophosphates. These results indicate phosphate to be the most acidic agent and therefore the best chain-terminating group in these glasses.     

Magnetostatic bias in soft/hard bi-phase layered materials based on amorphous ribbons and microwires

Bi-phase soft/hard magnetic systems have been prepared by electroplating CoNi alloy with relative hard magnetic character (10² Oe coercivity) onto ultrasoft (coercivity less than 1 Oe) FeCo-based amorphous alloys prepared by rapid solidification techniques. Experiments have been performed in planar (ribbons) and cylindrical (microwires) configuration. Low-field hysteresis loops after premagnetizing to saturation are ascribed to the magnetization reversal process of the soft inner phase, while harder component remains close to its remanence. Such low-field loops exhibit a shift typical of magnetic bias phenomena which in this case is ascribed to the magnetostatic coupling originated through the stray fields created by uncompensated charges at the ends of the harder phase.     

Ab initio study of NMR spectra of Li2S–SiS2 glass system

We obtained the chemical shielding constants of ²⁹Si in Li₂S–SiS₂ glass system on the basis of molecular orbital calculations. The relative chemical shieldings calculated at the HF/6-31G* level is −16.1 and −23.8 ppm for E(1) and E(0), respectively. These calculations are in good agreement with the previous NMR study: σ(E(2))>σ(E(1))>σ(E(0)). It is found theoretically that incorporation of Li₂S into SiS₂ does not influence the ²⁹Si chemical shift, while incorporation of Li₂O into SiO₂ does to a significant extent. It is also found that the existence of oxygen in Li₂S–SiS₂ glasses increases the chemical shielding of ²⁹Si by about 25 ppm, which is in good agreement with the experimental result.     

Atomic structure of beryllium boroaluminate glasses: A multi-nuclear NMR spectroscopic study

The short-range structure and network speciation have been studied in a series of beryllium boroaluminate glasses using ¹¹B, ²⁷Al and ⁹Be NMR spectroscopy. All glasses are characterized by a coexistence of BeO₄, BO₃, BO₄, AlO₄, AlO₅ and AlO₆ species. The concentrations of BO₃, AlO₅ and AlO₆ species in these glasses are significantly higher and the geometry of the B-O and Al-O coordination polyhedra are unusually disordered compared to those in other alkali and alkaline-earth boroaluminate glasses reported previously in the literature. These results indicate that Be atoms may not play the typical role of a network-modifying cation in these glasses. This structural scenario is consistent with the highest field strength and electronegativity of Be among all alkali and alkaline-earth metals.     

NMR study of μc-Si:H

The behavior of hydrogen in glow-discharge (GD) μc-Si:H has been characterized by ¹H NMR. The ¹H spectra consist of two components with different linewidths. The linewidth (FWHM) of the narrow component is about 0.5 kHz at ν₀ = 90 MHz, being much narrower than has been observed in GD a-Si:H deposited under a conventional low RF-power condition. It has been demonstrated that the 0.5-kHz FWHM component originates from the hydrogens in motional narrowing state, and such fast-moving hydrogens are incorporated both in μc-Si:H and high-power deposited a-Si:H.     

Glasses in the system PSeTl: An NMR study of chemical bonding properties

Glasses in the ternary system PSeTl have been studied using ³¹P, ²⁰³Tl, and ²⁰⁵Tl NMR. Information has been obtained from the composition dependence of the phosphorus linewidth, the thallium chemical shift, the pseudo-exchange interaction, and from NMR and X-ray data of crystallized samples. At Tl/P ratios smaller than 1, Tl appears to be more covalently bonded than in AsSeTl glasses, and the introduction of thallium results in a strengthening of the structure, which may be combined with a conversion of phosphorus from three-to four-coordination. Glasses with Tl/P > 1 show increasing bond covalency and important TlTl interactions as the thallium content increases; upon annealing TlPSe₂ is formed. In this region introduction of thallium tends to weaken the glass structure, in agreement with observed properties.     

A MAS NMR structural study of cadmium phosphate glasses

Cadmium phosphate glasses, of general formula xCdO(1−x)P₂O₅ (0.25?x?0.6), have been prepared by melting in alumina crucibles, with resulting dissolution of up to 6.4 mol% Al₂O₃. The local structure in these glasses has been studied using ³¹P, ²⁷Al and ¹¹³Cd nuclear magnetic resonance. The distribution of [PO₄]Qⁿ species as a function of composition has been shown to follow the simple binary model. The rate of change of the chemical shift of the ³¹P species in the Q² environment depends on the bond order, which in turn reflects the extent of double bonding between phosphorus and oxygen.     

Extended constraints,arches and soft modes in granular materials

A granular material is a network of fixed length edges linking two grains in contact, and flexible hinges if the grains can rotate without sliding on each other. The network is then dynamically unfrustrated: it can deform freely under shear and behaves like a dry fluid. A sufficient condition for non-frustration in 3D is that all circuits of grains in contact are even. Frustration is concentrated on odd circuits, and the ‘odd vorticity’ forms loops in 3D. Thus, some constraints in the network cannot be accounted for by local mean-field analysis. These extended constraints and the means of accounting for them are discussed. A granular packing can be described as a graph, with a connection carried by the edges. If all circuits of grains in contact are even, the connection is pure gauge, independent of the path between grains. When shear stress is applied over a large scale, it can break the frustrating odd circuits by disconnecting selected grains in contact, leaving all other contacts and even circuits unaffected. The granular material deforms freely like dry quicksand, as occurs in silent earthquakes, soil liquefaction, wet sand under foot (dilatancy) and, to some extent, in geological faults.     

High-temperature in situB NMR study of network dynamics in boron-containing glass-forming liquids

In situ high-temperature nuclear magnetic resonance (NMR) spectroscopy can be very useful for probing changes in structure and dynamics in glass-forming liquids, and is a unique method for observing chemical exchange among structural species (e.g. BO₃–BO₄, Qⁿ–Qⁿ⁺¹, and NBO–BO) at the seconds to microseconds time scales. High-temperature ¹¹B MAS NMR line shape measurements were made at about 100 K above the glass transitions on (Na₂O)_0.3(B₂O₃)_0.7 and (Na₂O)_0.2(B₂O₃)_0.21(Al₂O₃)_0.08(SiO₂)_0.51 glass-forming liquids. BO₃ and BO₄ groups are well resolved in ¹¹B MAS NMR spectra at 14.1 T with sample spinning at 5000 Hz. At higher temperatures, partial peak coalescence occurred due to exchange of BO₃ and BO₄. Temperature effects on borate speciation were also determined by varying the fictive temperature (T_f) of glasses, where T_f estimated from differential scanning calorimetry measurements. We combined these complementary data sets to model structural exchange in the liquid state. The time scale of BO₃–BO₄ exchange from NMR data, τ_NMR, appears to be “decoupled” from that of the macroscopic shear relaxation process τ_s derived from the viscosity, however, at higher temperatures, τ_s approaches τ_NMR. The “decoupling” at lower temperature may be related to intermediate-range compositional heterogeneities, and/or fast modifier cation diffusivities which trigger “unsuccessful” network exchange events.     

Silicon-29 cross-polarization/magic-angle-spinning NMR study of inorganic-organic hybrids: homogeneity of sol-gel derived hybrid gels

The structures of sol-gel derived hybrid gels prepared by co-hydrolysis of tetraethoxysilane (TEOS)-organotrialkoxysilane (RTES, RSi(OC₂H₅)₃, R = CH₃, C₅H₁₁, C₈H₁₇ and C₆H₅) mixtures (TEOS : RTES : CH₃CH₂OH : H₂O : HNO₃ = 0.5 : 0.5 : 10 : x : 0.3; x = 2, 5, 10 and 20) were characterized based on signal intensities of ²⁹Si CP/MAS NMR as a function of the contact time. The contact time dependencies of the signals for Q⁴ (Si(OSi)₄) units were successfully simulated by assuming distributed cross relaxation times (T_SiH) and an intrinsic ¹H spin-lattice relaxation time in the rotating frame (); the latter was different from the apparent value estimated by assuming single T_SiH. The distribution ranges of T_SiH for the TEOS-RTES gels broadened as the water content increased, suggesting that the Q⁴ units tended to be separated from the T units and that the local concentration of ¹H spins around the Q⁴ unit tended to decrease.     

Theoretical study on N-succinimidyl oligothiophenes: A novel class of materials for biological applications

We present a comparative theoretical study on a novel class of materials which can be used as biomarkers, namely the oligothiophene-N-succinimidyl esters. We study by ab initio coupled-cluster and first-principles density-functional theory calculations the electronic structure and the optical properties of a model system, namely bithiophene-N-succinimidyl, both isolated and coupled to a –C(O)–NH–(CH₂)₃CH₃ group at one terminal position. The latter is supposed to simulate the molecule after the binding to a biomolecule. We show that, although the electronic and optical properties of the isolated molecule are different from those of bithiophene, the binding to a biomolecule almost restores the well known electronic and optical properties of bithiophene.     

NMR and infrared study of cation motion in vitreous and polycrystalline TlPO3

The activation energy for Tl⁺ conduction in TlPO₃ glass is obtained from analysis of temperature-dependent motional narrowing for Tl²⁰⁵ NMR spectra and determinations of the localized far infrared (FIR) vibrational frequency for Tl⁺. Use is made of the phenomenological equation of Hendrickson and Bray to analyze the NMR data, yielding E_a = 1.19 eV; the measured FIR Tl⁺ vibrational frequency of 80 cm^?1 yields E_a = 1.09 eV. No significant ionic conduction is observed in polycrystalline TlPO₃. Differential scanning calorimeter measurements yield a glass transition temperature T_g of 96°C and the onset of crystallization temperature of 132°C. Measurements of the Tl²⁰⁵ chemical shift interaction as a function of frequency indicate that (1) the Tl⁺ sites in both polycrystalline and glassy TlPO₃ are ionic, the sites in the polycrystal being slightly more ionic than in the glass; (2) the chemical shift interaction is anisotropic in the glass and isotropic in the polycrystal; and (3) distributions in the values of the principal components of the chemical shift tensor exist in the glass, corresponding to a variety of TlO bond lengths and bond strengths.     

Network connectivity in aluminoborosilicate glasses: A high-resolution 11B, 27Al and 17O NMR study

O-17 enriched calcium and potassium aluminoborosilicate glasses with compositions similar (or analogous) to commercial E-glass were made and studied by ¹¹B, ²⁷Al and ¹⁷O NMR in order to explore the network speciation and mixing. Fractions of non-bridging oxygens associated with silicon and boron can be obtained simply from ¹⁷O magic angle spinning (MAS) NMR, while bridging oxygen populations result from complete peak assignment of high-resolution ¹⁷O triple quantum magic angle spinning (3QMAS) NMR spectra. Dramatic differences between the two samples in boron and oxygen speciation demonstrate a large effect of the charge of the modifier cation on mixing behavior and on the stabilization of non-bridging oxygens. The observed oxygen speciation is compared with that calculated from two models: random mixing of Si, B, and Al, and mixing with avoidance of linkages between tetrahedral aluminum and tetrahedral boron groups. Mixing of B and Al in the K-containing glass tends to follow the latter, while the network speciation in the Ca-containing glass is closer to the random model. The effects of Al and Si on boron speciation are discussed, and indicate that the maximum fractions of four-coordinated boron observed in a wide range of glass compositions are closely related to tetrahedral B and Al ‘avoidance’. A modified ‘Dell and Bray’ model is proposed which seems to accurately approximate boron speciation in many alkali aluminoborosilicate glasses. The higher field strength modifier cation (Ca²⁺ vs. K⁺) promotes the formation of non-bridging oxygen, trigonal over tetrahedral boron, and at least minor amounts of five-coordinated aluminum.     

A study of crystallization in some Li---Al---Si---O---N system glasses

The crystallization behavior of some Li---Al---Si---O---N system glasses under different conditions was studied. Experimental results show that the addition of nitrogen could decrease the crystallization tendency of glasses. In addition, the atmosphere, chemical composition and different sources of nitride could influence the crystallization of oxynitride glasses.     

The effect of fictive temperature on the structure of E-glass: A high resolution,multinuclear NMR study

The variation of the structure of E-glass with fictive temperature has been examined with high resolution, multinuclear (¹¹B, ²⁷Al, ²⁹Si, ¹⁷O, and ¹⁹F) MAS NMR. The previously observed decrease in 4-coordinated boron with increasing fictive temperature is confirmed and, for the first time, is directly correlated with an increase in non-bridging oxygens. An increase in 5-coordinated Al is also observed, and is linked to an increase in association of F with Al at higher fictive temperature.     

Electrical characteristics of materials in the BaO---Fe2O3---B2O3 system

Structural peculiarities, as for example segregation and crystallization in vitroceramic materials, may have a significant influence on the electrical volume resistance of materials. Interesting characteristics can be detected in the case of the system BaO---Fe₂O₃---B₂O₃. This paper presents results showing that very different conductivity behaviours can be obtained by a suitable modification of the composition of the materials and by heat treatment.     

EPR study in the SiO2 system

The EPR spectra of Fe³⁺ and V⁴⁺ ions, and radiation centres have been studied in α-tridimite. Consideration of the EPR data in all crystalline and vitreous forms of the SiO₂ system is made. It is shown that the crystal field strength and symmetry did not change considerably in different polymorphs of silica, the changes in hyperfine structure constants being significant. The relation between electronic structure of vitreous silica and different crystalline forms is established.