S.M. Bennington

Intermediate range structure and low-energy dynamics of densified vitreous silica

Abstract The structure and low-energy dynamics of densified vitreous silica are studied with high accuracy by neutron scattering over a wide range of momenta and energy space. The structure factors (S(Q)) indicate that the change of the SiO4 tetrahedron (the short range order) by compaction is very small, but the intermediate range structure (IMRS) represented by the first sharp diffraction peak is strongly affected. This mainly attributed to a deformation of the sixfold ring structure. The low-energy dynamics the boson peak, BP also has a strong correlation with the change of IMRS. A reduction of the intensity of the boson peak by densification, however, cannot be explained by simple models such as phonon dumping or an eigenmode of IMRS. A shrinkage of the void space, surrounded by the sixfold ring structure, is related to the suppression of the low-energy dynamics. We consider that the origin of the boson peak can be attributed to a soft mode composed of an additional degree of freedom such as the void space.

Peculiar suppression of the specific heat and boson peak intensity of densified SiO2 glass

Abstract Low-energy dynamics of normal and densified vitreous silica was investigated by neutron scattering combined with the specific heat measurements. A large evolution of the Boson peak was clarified to be a suppression of the density of states at low energy, but not a peak shift as observed by Raman scattering. These results are inconsistent with a scenario based on a phonon scattering by a local density fluctuation, but consistent with soft potential model and a recent model by Nakayama.

Turbostratic graphite nanofibres from electrospun solutions of PAN in dimethylsulphoxide

Homogenous turbostratic graphite nanofibres (TGNFs) have been synthesized by heat treatment of electrospun polyacrylonitrile in dimethylsulphoxide, offering a solution route of low toxicity to manufacture sub-60 nm diameter TGNFs. Characterization techniques including X-ray diffraction, scanning- and transmission electron microscopy have been used to study the graphitization process, and structural morphology of the nanofibres. The TGNFs have an entangled micro-fibril structure with turbostratic ordering of up to 40 graphene layers after heat treatment at 3000 °C.

Heterogeneity of amorphous polymers with various fragility indices as studied in terms of non-Gaussian parameter

Abstract Dynamical heterogeneity of amorphous polymers has been studied in terms of non-Gaussian parameter, A0. This parameter was estimated as a function of temperature, time, and fragility of polymers using the incoherent elastic neutron scattering technique. We found that the heterogeneity of the amorphous polymers increases with decreasing temperature, especially below the glass transition temperature and also that the stronger (or less fragile) polymers are more heterogeneous.

INTERMEDIATE-RANGE STRUCTURE AND LOW-ENERGY DYNAMICS OF DENSIFIED SIO2 GLASS

Abstract The structure and dynamics of densified SiO 2 glasses have been studied by neutron scattering and Raman scattering as a function of the density. The detailed evolution of the intermediate-range structure, which appears as a change in the first and second diffraction peaks, were clearly observed and we found that a structural change has a linear proportionality to the density. The low-energy dynamics, the Boson peak, have also a strong correlation with the change of the intermediate-range structure. In this paper we discuss this correlation from the point of view that the six-fold ring structure has a key role.

An Overall Energy Spectrum of Magnetic Fluctuations in the Superconducting La1.85Sr0.15CuO4

High-energy pulsed neutron inelastic scattering experiments have been performed using single crystals of La 1.85 Sr 0.15 CuO 4 to study magnetic fluctuations in the superconducting phase. A peak centered at the reciprocal point (π,π) is well defined below around 120 meV and rapidly damped beyond this energy. However, the \lq\lqbackground-subtracted” scattering intensities indicate that magnetic signals remain up to around 300 meV. These data combined with the results obtained by triple-axis neutron scattering experiments provide the first overall energy spectrum of the dynamical susceptibility in La 1.85 Sr 0.15 CuO 4 . The obtained spectrum is much wider than that of YBa 2 Cu 3 O 7- d ( d ∼0).

Dispersive excitation in different forms of SiO2

Abstract The dynamical structure factors of vitreous silica (v-SiO2), polycrystalline α-cristobalite and polycrystalline α-quartz are measured by inelastic neutron scattering. Clear dispersive behavior extending up to 55 meV is observed for all samples, irrespective of structural order. Especially, the similarity of dynamical behavior between v-SiO2 and polycrystalline α-cristobalite is found to be extremely remarkable in the range of E⩾7 meV. We consider that the essential difference between a glassy system and a crystalline one should be in the low-energy region below the Boson peak energy. The quantum excitation between a two-level system based on buckling motion may contribute to the anomalous low-energy excitation in glasses.

Ammonia borane–polyethylene oxide composite materials for solid hydrogen storage

Co-electrospinning ammonia borane (AB) and polyethylene oxide (PEO) has created a unique crystal phase that promotes faster hydrogen release from AB below its melting temperature with no incubation time. Integral fibres have been produced containing 75%, 50% and 25% AB by weight. As the PEO content was increased, the onset temperature of dehydrogenation was reduced from 110 °C for pristine AB to 85 °C for the 25% AB fibres. The new phase is characterised by hydrogen bonding between the hydridic hydrogen atoms bonded to the nitrogen atom in AB and the oxygen atom in the PEO backbone. Additionally, the usual foaming of AB during hydrogen release was effectively controlled by the addition of PEO. Some impurities which accompany the hydrogen release – ammonia and diborane – are reduced, however, borazine levels in the gas stream were observed to increase during the loss of the 2nd hydrogen equivalent. Nevertheless, co-electrospun composites of AB and PEO show great promise as a safe, portable and versatile hydrogen storage material.

Inelastic neutron scattering study of free proton dynamics in coal

Inelastic neutron scattering is unique to observe protons in solids. New measurements for a coal sample (sub-bituminous A) for large energy and momentum transfers reveal peaks of intensity due to the vibrations of protons bound to heavy atoms and a ridge of intensity due to recoil of free protons. The dynamics are represented as a gas of free particles (bare protons) in a box with walls made of bound protons. Even for the sample at 2 K free protons are not at rest. Their mean kinetic energy corresponds to the mean energy of the bound protons.

Neutron spectroscopic studies of crystalline electric fields in high-Tc ErBa2Cu3O7 doped with zn and Ni

Abstract The crystalline electric field (CEF) splitting of the ER 3+ 4f-shell in the high- T c superconductor ErBa 2 Cu 3 O 7 doped with Zn and Ni has been investigated by the inelastic neutron scattering tecnique. the observed exhibit a large number of well resolved CEF transitions between 8 and 80 meV, so that we have been able to determine all nine CEF parameters required for the orthorhombic symmetry. The results are used to derive information on the effective charge associated with the superconducting CuO 2 planes.