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We review the benefits of using 29 Si and 1 H magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy for probing the local structures of both bulk and surface portions of mesoporous bioactive glasses (MBGs) of the CaO–SiO 2 −(P 2 O 5 ) system. These mesoporous materials exhibit an ordered pore arrangement, and are promising candidates for improved bone and tooth implants. We discuss experimental MAS NMR results from three MBGs displaying different Ca, Si and P contents: the 29 Si NMR spectra were recorded either directly by employing radio-frequency pulses to 29 Si, or by magnetization transfers from neighbouring protons using cross polarization, thereby providing quantitative information about the silicate speciation present in the pore wall and at the MBG surface, respectively. The surface modifications were monitored for the three MBGs during their immersion in a simulated body fluid (SBF) for intervals between 30 min and one week. The results were formulated as a reaction sequence describing the interconversions between the distinct silicate species. We generally observed a depletion of Ca 2+ ions at the MBG surface, and a minor condensation of the silicate-surface network over one week of SBF soaking.

The HfO2-SiO2 system is attracting interest as a possible new dielectric material in semiconductor devices. Knowledge of the location of hafnium within the silica network and the effect hafnium has on the structure will be central to the successful use of this material system in this application. Here, sol-gel techniques have been used to manufacture (HfO2)(x)(SiO2)(1-x) samples (x = 0.1, 0.2 and 0.4, each heat treated at 250, 500 and 750 degrees C) and these have been characterised by magic angle spinning (MAS) NMR (H-1, C-13, O-17, Si-29), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis. 29Si MAS NMR showed that increasing the hafnia content decreases the connectivity of the silicate network, i.e. increases the range of differently connected SiO4 (Q(n)) units with more having increased numbers of non-bridging oxygens (i.e. lower n). FTIR and O-17 MAS NMR showed unequivocally that the x = 0.4 sample phase-separated at higher temperatures, while in the x = 0.1 sample the hafnium was homogeneously mixed into the SiO2 phase without any phase separation. (C) 2007 Elsevier Inc. All rights reserved.

Neutron diffraction with 46Ti and 48Ti stable isotopes and isotope-enriched 17O and 49Ti MAS NMR have been used to characterize the structure of (TiO2)x(SiO2)1-x sol−gel glass as a function of composition (x = 0.08, 0.18, and 0.41) and calcination temperature (T = 250, 500, and 750 °C). The results reveal the first direct observation of two Ti−O distances in a homogeneous (TiO2)0.18(SiO2)0.82 sol−gel derived glass. In the sample heat treated at 250 °C, the Ti occupies a distorted octahedral environment similar to that found in the mineral ramsayite with four Ti−O bond lengths of around 1.89 A and two close to 2.11 A. After heating to 750 °C, two shorter bond distances are observed: a short distance at 1.81 A due to tetrahedrally coordinated Ti and a longer distance of 1.94 A due to a minority species of octahedrally coordinated Ti. The (TiO2)0.08(SiO2)0.92 sample exhibits similar behavior. After heating to 250 °C, two Ti−O distances are observed at 1.84 and 2.10 A consistent with the presence of both tet...

Amorphous (TiO2) x (SiO2)1−x (x = 0.08, 0.18, and 0.41) sol-gel formed materials have been characterised by a combination of X-ray and neutron diffraction, infra-red and 29Si and 17O magic angle spinning NMR spectroscopy. This combination allows new insight into the fundamental structural role titanium additions play in silica, entering the network at x = 0.08 but largely phase separating at x = 0.41. At an intermediate value of x = 0.18 there is complex coordination behaviour with initially some Ti—O—Ti linkages forming and both TiO4 and TiO6 coordinations present. It appears that at 500°C for the x = 0.18 sample all titanium is present in Ti—O—Si linkages but that this situation is unstable on further calcination. The new information presented here is amalgamated with that from our previous EXAFS, XANES and SAXS on the same samples to produce the most complete view to date of the local and mesoscopic structural behaviour of the (TiO2) x (SiO2)1−x system produced by the sol-gel method.

The effects of different heat treatment schemes (i.e. successively or directly heated to particular temperatures) and atmospheres (air or nitrogen) on the solid-state NMR spectra obtained from (TiO2)(0.15)(ZrO2)(0.05)(SiO2)(0.80) sol-gel materials are investigated. A combination of H-1, C-13, O-17 and Si-29 NMR is used. Si-29 MAS NMR indicates that the extent of condensation of the silica-based network strongly depends on the maximum temperature the sample has experienced, but the condensation is largely independent of the details of the heat treatment scheme and atmosphere used. For sol-gel produced silicate-based materials the results show that the equilibrium structure at each temperature is reached rapidly compared to the time (2 h) spent at that temperature. The O-17 NMR results confirm that a nitrogen atmosphere does significantly reduce loss of O-17 from the structure but care must be taken since there could be differential loss of O-17 from the regions having different local structural characteristics. (C) 2003 Elsevier Science (USA). All rights reserved.