Rudolf Winter

Determination of the optical band-gap energy of cubic and hexagonal boron nitride using luminescence excitation spectroscopy

Using synchrotron-based luminescence excitation spectroscopy in the energy range 4–20 eV at 8 K, the indirect Γ–X optical band-gap transition in cubic boron nitride is determined as 6.36 ± 0.03 eV, and the quasi-direct band-gap energy of hexagonal boron nitride is determined as 5.96 ± 0.04 eV. The composition and structure of the materials are self-consistently established by optically detected x-ray absorption spectroscopy, and both x-ray diffraction and Raman measurements on the same samples give independent confirmation of their chemical and structural purity: together, the results are therefore considered as providing definitive measurements of the optical band-gap energies of the two materials.

Enhancement of ionic conductivity by the addition of plasticizers in cationic monoconducting polymer electrolytes

Abstract Temperature-dependent 7 Li nuclear magnetic resonance (NMR) spin–lattice relaxation time ( T 1 ) data of a lithium monoconducting polymer electrolyte based on maleic anhydride-styrene copolymers are examined in an effort to understand the mechanism of how plasticizers affect significant changes in the observed ionic conductivity ( σ ). The use of strong polar organic plasticizers, such as propylene carbonate, is believed to induce changes in the charge–carrier separation, which is expected to facilitate the transport of ions in these materials. Cationic monoconducting gel electrolytes consisting of polyelectrolytes with immobilized carboxylic anions, charge compensating mobile Li + ions, and various combinations of poly(ethylene glycol) dimethyl ether, dimethylsulfoxide, propylene carbonate, and BF 3 exhibit ambient σ values from 10 −3 to 10 −6 S cm −1 . For systems not containing BF 3 the T 1 data as a function of inverse temperature and the value of the T 1 at its minimum are found to be independent of the electrolyte composition in spite of the fact that the δ values are observed to increase by 1 to 3 orders of magnitude in these samples. For these samples it is suggested that the Li + ions find comparable sites possibly in the form of contact ion pairs with the carboxylic anions and that they are dynamically coupled to the motion of the polymer host. The observed increase in conductivity upon addition of the plasticizers is assumed to arise from a weakening of the ion–polymer interactions and not from a fundamental change in the ionic conductivity mechanism. Upon addition of BF 3 , however, a shift in the T 1 minimum to lower temperatures and a decrease in the depth of the T 1 minimum are observed. These observations suggest that the Li + ions interact preferentially with –O–B ⊖ F 3 moieties and are consistent with the existence of an alternate conduction path such that the ion motions are decoupled from the local motions of the polymer host.

MAS NMR study of soda-lime–silicate glasses with variable degree of polymerisation

29Si and 23 Na magic angle spinning (MAS) NMR has been used as an atomic probe for structural characterisation of soda-lime-silicate glasses. The glasses studied are based on the composition (CaO) x (Na 2 Si 3 O 7 ), where x is varied from x = 0 to x = 1, (increasing CaO from 0 to 20 mol%) finally matching the Na 2 O fraction. The glass network becomes increasingly depolymerised by increasing CaO, and shows a changing distribution of Q n species. Two Q species have been observed co-existing for compositions, where x = 0, 0.2 and 1, while for compositions, where x = 0.4 and x = 0.8, Q 2 , Q 3 and Q 4 are present. Progressive changes in chemical shifts of Q 2 and Q 3 species, and changes in FWHM of all detected Q species accompany the addition of CaO. The addition of CaO introduces a greater distortion into the structure, and there is evidence to suggest preferential arrangement of Ca in the vicinity of Q 2 species rather than Q 3 .

Molecular dynamics simulation of sodium borosilicate glasses

Abstract Molecular dynamics (MD) calculations of (B2O3)x(Na2Si2O5)(1−x) glasses have been performed using Born–Mayer–Huggins interatomic potentials together with three-body potentials and increasing B2O3 concentrations examined (x=0.3, 0.5 and 0.7). The main features in the total radial distribution functions (RDFs) are directly attributable to those found empirically in B2O3 and Na2Si2O5 glasses. Simulations confirm that there is also a tendency for the borate component of the network to separate from the silicate part and for sodium to associate with the former. As the B2O3 content, x, rises the Na2O/B2O3 ratio, R, falls and the fraction of compensated tetrahedral boron sites shifts in favour of 3-fold co-ordinated borons. Indeed, as x increases, the co-ordination of sodium also increases, reflecting its higher co-ordination in borate than in silicate glasses. Compared to binary borate glasses, these changes in the short range order of (B2O3)x(Na2Si2O5)(1−x) glasses are extended because of the widely ranging SiO2/B2O3 ratio, K.

Following the formation of nanometer-sized clusters by time-resolved SAXS and EXAFS techniques

Time-resolved in situ SAXS and XAS measurements were carried out to monitor the formation of nanoparticles of the sulfides of cadmium and zinc, from solutions containing he corresponding acetate, and thioacetamide under solvothermal conditions. Analysis of the SAXS data shows that particles of ca 5 nm in radius form within the first few minutes of the reaction and then grow uniformly to ca 20 nm over a period of two hours resulting in a highly mono-dispersed particle distribution. EXAFS data of the CdS particles also prepared by solvothermal methods and recorded at 20 K, support the formation of nano-meter sized particles.

Li conductivity of nanocrystalline Li4Ti5O12 prepared by a sol-gel method and high-energy ball milling

Spinel-type structured Li4+xTi5O12 (0 6 x 6 3 ) is actually one of the most promising anode materials for Li ion batteries. In its nanostructured form it is already used in some commercially available Li ion batteries. As was recently shown by our group (Wilkening et al., Phys. Chem. Chem. Phys. 9 (2007) 1239), Li diffusivity in microcrystalline Li4+xTi5O12 with x = 0 is rather slow. In the present contribution the Li conductivity in nanocrystalline samples of the electronic insulator Li4Ti5O12 prepared by different routes is investigated using impedance spectroscopy. The mean crystallite size of the samples is about 20 nm. The ionic conductivity of nanocrystalline Li4Ti5O12 obtained by mechanical treatment is higher by about two orders of magnitude compared to that found for a material which was prepared following a sol-gel method. The latter resembles the behaviour of the microcrystalline sample with an average particle size in the μm range rather than that of a nanocrystalline ball milled one with a mean crystallite size of about than 20 nm. The larger conductivity of the ball milled sample is ascribed to a much higher defect density generated when the particle size is reduced mechanically.

WAXS and NMR studies of intermediate and short range order in K2O-SiO2 glasses

Abstract Wide-angle X-ray scattering and 29 Si NMR have been employed to investigate the medium-range structure of xK2O–(1−x)SiO2 glasses, with x varying in the limits 5% 1.4 A −1 A −1 range. The peaks broaden below x=20%, and at the lowest K2O fraction, a bimodal line shape is found. This broadening is interpreted in terms of phase separation at low K2O fraction. The NMR spectra consist of several (usually three) Gaussian components assigned to the different Q species (SiO4 tetrahedra with different connectivity) present. All three components are uniformly deshielded as K2O is incorporated into the structure. The fraction of non-bridging oxygens (NBOs) derived from the distribution of Q species matches the value obtained from the overall composition, except for the x=17.41% sample, again indicating phase-separation at x

In situ SAXS studies of the morphological changes of an alumina–zirconia–silicate ceramic during its formation

Small-angle X-ray scattering is used at two energies, one either side of the zirconium K-edge, to probe the in situ formation of an alumina–zirconia–silicate ceramic. The use of energies either side of the edge allows the decomposition of information regarding the scattering from the zirconia particles from that of the glass matrix. Porod slope data show how the nanoparticles progress from being relatively isolated particles to becoming agglomerates as the pore network in the glass collapses. The shape of the agglomerates resembles the pore network of the glass at low temperature. The Guinier radii of the particles show the growth of the agglomerates past the Littleton softening point, whilst still resolving the primary particles.

NMR relaxation and line shape study on Li+ diffusion in nanocrystalline layer-structured LixTiS2

Temperature and frequency dependent 7Li spin-lattice relaxation rate measurements on the layer-structured two-dimensional ion conductor LixTiS2 in different order states were carried out in the laboratory frame (T1−1) and in the rotating frame (T1e−1). The activation energies for individual ion hopping, as obtained from these measurements, are about 0.19eV for the polycrystalline, 0.16eV for the nanocrystalline, and 0.07&V for the amorphous material. The frequency dependence of T1−1 is sublinear for both disordered modifications. The NMR central transition lines of the nanocrystalline material decompose into a narrow and a broad component in the course of motional narrowing. The relative intensity of the narrow component corresponding to the fraction of highly mobile Li ions increases gradually with temperature, reaching a limiting value of 50% at high temperatures. Hence, we conclude that the interfacial regions are not structurally homogeneous and comprise about half of the atoms of the sample. Contrary to three-dimensional nanocrystals, diffusion in the two-dimensional nanocrystalline material takes place on the grain surfaces rather than within an amorphous interface medium.

Interfacial structure of annealed alumina - zirconia - silicate nanoceramics

Abstract An alumina – zirconia nanocomposite has been produced using the chloride sol – gel method and embedded into a silicate matrix by dispersing the nanocomposite into a powdered silica glass and subsequent annealing. The resultant nanoceramic was subjected to 27Al magic angle spinning (MAS) NMR, small angle X-ray scattering (SAXS), and X-ray photoelectron spectroscopy (XPS), leading to a core-shell type model of the interfacial region. Initially the particles are agglomerated with the shell containing mainly atoms of octahedral coordination and the core aluminium atoms of tetrahedral coordination. Upon annealing the agglomerates break up, causing a change in the coordination of the aluminium atoms. As the atoms diffuse into the matrix, the ones that were initially in the shell change to be tetrahedrally coordinated, and therefore increase the overall population of tetrahedrally coordinated aluminium atoms within the interface.