Number Of Non-bridging Oxygens Research Articles

Microstructural dependence on relevant physical–mechanical properties on SiO 2–Na 2O–CaO–P 2O 5 biological glasses

Bioactive glasses of the system SiO 2–Na 2O–CaO–P 2O 5 have been prepared by the normal melting and annealing technique. The elastic moduli, attenuation, Vickers hardness, fracture toughness and fracture surface energy have been obtained using the known method at room temperature. The temperature dependence of elastic moduli and attenuation measurements have been extended over a wide range of temperature from 150 to 500 K. The SiO 2 content dependence of velocities, attenuation, elastic moduli, and other parameters show an interesting observation at 45 wt% of SiO 2 by exhibiting an anomalous behaviour. A linear relation is developed for T g, which explores the influence of Na 2O on SiO 2–Na 2O–CaO–P 2O 5 bioactive glasses. The measured hardness, fracture toughness and fracture surface energy show a linear relation with Young's modulus. It is also interesting to note that the observed results are functions of polymerisation and the number of non-bridging oxygens (NBO) prevailing in the network with change in SiO 2 content. The temperature dependence of velocities, attenuation and elastic moduli show the existence of softening in the glass network structure as temperature increases.

The structure of Na 2O–Al 2O 3–SiO 2 glass: impact on sodium ion exchange in H 2O and D 2O

The kinetics of matrix dissolution and alkali-exchange for a series of sodium aluminosilicate glass compositions was determined at constant temperature and solution pH(D) under conditions of silica-saturation. Steady state release rate for sodium was 10–50 times faster than the rate of matrix dissolution, demonstrating that alkali exchange is an important long-term reaction mechanism that must be considered when modeling systems near saturation with respect to dissolved glass components. Sodium release rates were 30% slower in D 2O compared to rates in H 2O; but matrix dissolution rates were unaffected. These results are consistent with rupture of the OH bond as the rate-limiting reaction in Na +–H + exchange whereas matrix dissolution is controlled by OH − or H 2O catalyzed hydrolysis of SiOSi and SiOAl bonds. Changes in Na exchange rate with increasing Al 2O 3 content could not be reconciled with changes in the number of non-bridging oxygen (NBO) sites in the glass alone. A simple model was used to estimate a structural energy barrier for alkali ion exchange using NaO bond length and co-ordination as measured by Na K-edge X-ray absorption spectroscopy, and binding energy shifts for SiONa sites measured by X-ray photoelectron spectroscopy (XPS). The energy barrier was calculated to increase from 34 kJ mol −1 for Na 2O·2SiO 2 glass to 49 kJ mol −1 for a glass containing 15 mol% Al 2O 3, consistent with stronger bonding of Na on NBO sites and increasing mechanical stiffness of the glass network with increasing Al content. The calculated ion-exchange enthalpies were then used to calculate Na ion-exchange rates as a function of glass composition. Agreement between the calculated and measured Na ion exchange rates was excellent.

Photoluminescence in amorphous TiO 2 -PbO systems

Photoluminescence (PL) at room temperature has been achieved in amorphous thin films and powders of the TiO2–PbO system. They were prepared by the polymeric precursor method with [PbO]/[TiO2] molar ratios ranging from 0.0 to 1.0. The energy position of maximum PL emission and the PL intensity showed dependence on Pb concentration. The Pb addition suggests an increase in the number of non-bridging oxygens (NBO) in the amorphous TiO2 network. These results support the relationship between photoluminescence and structure in TiO2-based amorphous materials.

Three-coordinated boron-11 chemical shifts in borates.

Despite the importance of (11)B nuclear magnetic resonance (NMR) in structural studies of borate glasses, no clear means of correlating NMR parameters with the number of nonbridging oxygens on three-coordinate boron has been demonstrated. In this work, a series of anhydrous, polycrystalline, binary borates has been examined by (11)B magic-angle spinning (MAS) NMR to obtain precise measurements of their three-coordinate boron isotropic chemical shifts. The shifts generally increase with the replacement of bridging oxygens by nonbridging oxygens, ranging from 14.6 ppm in crystalline B(2)O(3) to 22.5 ppm in magnesium orthoborate. The underlying physical basis for this trend is satisfactorily accounted for by considering second neighbor effects using bond valence sums. These data are supportive of a structural model for B(2)O(3) glass in which 72% of the boron atoms are in rings. High-field MAS NMR experiments (B(0) = 18.8 T) indicate that the boron shielding is anisotropic, with greater anisotropy measured for three-coordinate borons possessing one or two nonbridging oxygens, than for those with zero or three nonbridging oxygens.

Electronic states calculated by the DV-Xα cluster method for lithium ion conductive Li 2S–SiS 2–Li 4SiO 4 oxysulfide glasses

The electronic states of the Li 2S–SiS 2–Li 4SiO 4 oxysulfide glasses were investigated by using the DV-Xα cluster method as a non-empirical molecular orbital calculation. The calculation was carried out for the dimer-framed Li 6 Si 2 O m S n (m+n=7) cluster models expected to be present in the oxysulfide glasses. The bond states around lithium ions changed from covalent to ionic with an increase in the number of non-bridging oxygens. The increase of covalency of the Si–S and Si–O bonds in the frame structure brought about the increase of covalency between the frame and lithium ions. The bond states around lithium ions of the cluster with one bridging oxygen were similar to those of the cluster in the pure sulfide glasses. This similarity seems to be one of the reasons for high ionic conductivities, over 10 −4 S cm −1 , in the oxysulfide glasses with small amounts of lithium ortho-oxosalts.

Effects of composition on the vibrational properties of sodium silicate glasses

The vibrational modes and the calculated Raman spectra of computer generated models of sodium silicate glasses are presented and discussed. The localization of the vibrational modes on non-bridging oxygens (NBOs) increases linearly with increasing NBO/Si in all spectral ranges. Strongest localization is observed in the low-frequency range near the 225 and 320 cm −1 vibrational bands. The shape of the partial Raman spectra of SiO 4 tetrahedra with different number of NBOs (Q n species) is complex and depends both on the glass composition and the connectivity of the Q n species. The estimated Q n scattering cross-sections increase with increasing the number of bridging oxygens.

Crack Growth in the High Crack Velocity Region for Binary Sodium Silicate Glasses.

The crack growth behavior in the high crack velocity region (region III) for binary sodium silicate glasses was investigated by using a DCDC (double cleavage drilled compression) specimen under compressive stress. For 15mol% Na2O glass, despite the anomalous characteristic of the bulk modulus, subcritical crack growth in region III was detected. The slope of stress intensity factor (KI)-crack velocity (v) curve for silicate glass depended on the number of non-bridging oxygen per SiO4 tetrahedron rather than on the alkali content in the glass. This compositional dependence of the slope of KI-v curve for silicate glasses could not be explained in terms of thermally activated crack growth process where the crack-tip shape would vary with the pressure derivative of bulk modulus. The smaller KI-v slope and the higher KIC value of 35mol% Na2O glass may be related to the structural change from a three-dimensional network to a two-dimensional network because of the introduction of many non-bridging oxygens into the silica network.

Calculation of Raman spectra and vibrational properties of silicate glasses: Comparison betweenNa2Si4O9andSiO2glasses

First calculations of the vibrational density of states (VDOS), the character of the vibrational modes, and the polarized Raman spectra of depolymerized alkali silicate glass with 20 mol % ${\mathrm{Na}}_{2}\mathrm{O}$ $({\mathrm{Na}}_{2}{\mathrm{Si}}_{4}{\mathrm{O}}_{9}$ glass) using Kirkwood-type potential and computer generated models with periodic boundary conditions are presented and compared with amorphous ${\mathrm{SiO}}_{2}$ $(a\ensuremath{-}{\mathrm{SiO}}_{2}).$ The effects of depolymerization on the localization, bridging oxygen motion, phase quotient, as well as the newly proposed stretching character and atomic participation ratios of the vibrational modes are quantitatively analyzed. The band edges are shown to correspond to strongly localized modes. All vibrational characteristics have singularities near the band edges. The calculated Raman spectra are, generally, in good agreement with experiment. The parallel (VV) polarized Raman spectra are shown to arise mainly from a bond-stretching scattering mechanism which depends on the derivative of the parallel bond polarizability. On the contrary, the depolarized VH spectra arise from mixed bond-stretching and bond-bending scattering mechanisms. It is shown that the perpendicular polarizability of the silicon-bridging oxygen bonds is an order of magnitude smaller than the derivative of the parallel bond polarizability and that their ratio of about 0.1 is not affected by the presence of short-range disorder and depolymerization. The derivative of the parallel bond polarizability for the silicon-nonbridging oxygen bonds is twice larger than that for the silicon-bridging oxygen bonds. Partial Raman spectra of short-range units with different number of nonbridging oxygens (Q species) in the ${\mathrm{Na}}_{2}{\mathrm{Si}}_{4}{\mathrm{O}}_{9}$ glass are calculated and compared with empirically established rules for determination of Q species from polarized Raman spectra.

NMR and NQR studies of boron in vitreous and crystalline borates

Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) have both been used to study boron in crystalline borates and glasses containing boron oxide (B 2O 3). Both boron nuclear isotopes, 10B and 11B, have magnetic moments and electrical quadrupole moments. The interactions of the latter perturb the NMR Zeeman energy levels and produce broadening and structure in the NMR spectra. Computer analysis of that structure yields quadrupolar parameters that depend very sensitively on the boron coordination, chemical bonding, and the identity of structural groupings containing the boron atoms. NMR spectra for three- and four-coordinated borons are easily distinguished from each other and the fractions of boron in each coordination can be obtained. Similarly, three-coordinated borons bonded to different numbers of non-bridging oxygens are distinguishable and can be quantified. All boron sites are ‘fingerprinted’ by the quadrupolar parameters that can be used to check the validity of values calculated by quantum mechanical procedures. NQR spectra occur at low frequencies (below 1.4 MHz) which poses experimental problems, but they can yield data that are much more accurate than the NMR-derived results, and resolve responses that overlap or conceal each other in the NMR spectra. Identification in glasses of the type and amount of structural groupings found in crystalline borates provides guidance for compositional variations or heat treatments that produce desired macroscopic properties of the glasses.

Mobile hydrogen ion in glass to enable pH-measurement

A number of mechanisms for glass electrode materials for pH measurement have been proposed to date. We found that a key factor in a glass for pH measurement is that a glass contain mobile protons which appear in the glasses containing a number of non-bridging oxygens and are strongly hydrogen bonded.

Acceleration of photochemical hole burning rate for Sm 2+-doped borate glasses by X-ray irradiation

The mechanism of persistent spectral hole burning (PSHB) for Sm 2+-doped solids is known to be the photoionization of Sm 2+ ions, but the detailed processes of hole burning and hole burning kinetics are not clear at this moment. We have investigated the effect of X-ray irradiation on photochemical hole burning and structural changes of Sm 2+-doped borate glasses. The X-ray irradiation and hole burning measurements were performed at room temperature. As a result, the hole depth and width increased after X-ray exposure, and the hole burning rate was increased for all X-ray irradiated specimens. Local structure around the rare earth ions such as symmetry or the number of non-bridging oxygens was changed by X-ray irradiation. These experimental results were explained by the radiation damaged structure of glasses.

Photochemical Hole Burning and Local Structural Change in Sm2+‐Doped Borate Glasses

Photochemical hole burning (PHB) not only can be applied for data storage systems but also serves as a powerful method for studying the local structure around optical centers. The present work investigated the effects of aluminum, magnesium, and silicon ions on hole burning and the phonon sideband for borate glasses that exhibit PHB at room temperature. Hole burning was measured for the 5D0−7F0 transition of Sm2+ and the phonon sideband spectrum for the 5D0‐7F0 transition of Eu3+. The hole width was closely related to local structural change, especially as it seemed to decrease with decreases in the number of nonbridging oxygens produced around the rare‐earth ions. In the case of sodium aluminoborate glasses, the hole width decreased considerably with increasing alumina content. The ratio Γih/Γh for 85B2O3·10Al2O3·5Na2O·1Sm2O3 glass, then, was 80 at room temperature, the largest value ever reported.

Raman spectroscopic studies of BaOGa2O3GeO2 glasses

Raman spectra were collected for barium gallogermanate glasses along two compositional joins to determine relationships among glass structure, chemistry, and physical property variations. As GeO 2 content decreases for both glass series, the mid-frequency envelope shifts to higher frequencies while the high-frequency features increase in intensity and shift to lower frequencies. These trends in the Raman spectra parallel those reported for binary Li, Na and K germanate glasses, and are interpreted in terms of a reduction in average ring size, as well as an average lengthening of TO bonds. The more extreme trends for the most barium-rich glasses indicate partial depolymerization of the tetrahedral network and a concomitant increase in the number of non-bridging oxygens. There is no evidence for octahedral Ge or Ga in any of the barium gallogermanate glasses investigated in this study. The findings determined from the Raman data are consistent with earlier conclusions drawn from Ge and Ga X-ray absorption spectra.

Structural influences on the hydroxyl spectra of barium gallogermanate glasses

The infrared absorption of hydroxyl in three series of barium gallogermanate glasses — 10 mol% Ga 2O 3, 60 mol% GeO 2, and BaO/Ga 2O 3 = 1 — have been measured. Four separate bands associated with the OH stretching vibration dominate the absorption spectra of these glasses between 4000 and 1500 cm −1. Two of these bands are attributed to hydroxyl with vibrational frequencies perturbed by hydrogen bonds between the hydroxyl and adjacent non-bridging oxygens. The other two bands arise from the vibration of ‘free’ hydroxyl ions associated with either germanium or gallium tetrahedra. Hydrogen-bonded hydroxyl absorptions are present in the spectra of all compositions. This observation leads to a modification of the previously proposed structural model for these glasses. In addition, the relative concentration of various types of hydroxyl is correlated to the changes in structure associated with changes in composition. Specifically, the fraction of hydrogen-bonded hydroxyl increases at the expense of ‘free’ hydroxyl when the number of non-bridging oxygens in the glass increases.

The importance of melt composition in controlling trace-element behaviour: an experimental study of Mn and Zn partitioning between forsterite and silicate melts

Partition coefficients for Mn 2+ and Zn 2+ between forsterite and melts in the system Fo-Ab-An at temperatures of 1150–1400°C and 1-bar pressure have been determined experimentally. For a given temperature the values of the Nernst partition coefficients ( D Mn and D Zn) depend on the composition, and hence structure, of the melt phase, however, for a given degree of melt polymerization (expressed as the number of non-bridging oxygens per tetrahedral cation, NBO/T), the temperature dependence of the partition coefficients is less than the error in measurement. In these systems, melt composition is therefore more important than temperature in controlling trace-element behaviour. The exchange partition coefficients K D( Mn Mg ) and K D( Zn Mg ) are also more dependent on NBO/T than on temperature. Previous attempts to extract the ΔS and ΔH for the exchange reaction by regressing ln K D vs. 1 T are not valid as they ignore the changes in K D controlled by melt composition. Our data imply that for this system ΔH is small and ΔS varies with melt composition. In deploymerized melts the Nernst partition coefficients are ∼0.8 for Zn 2+ and ∼0.6 for Mn 2+ whereas in highly polymerized melts, values up to 4.5 for Zn 2+ and 3.0 for Mn 2+ have been found. In these highly polymerized systems small changes in the degree of polymerization are accompanied by large changes in the partition coefficients. This implies that fixed values of mineral/melt partition coefficients are unlikely to be adequate in trace-element modelling studies of highly differentiated igneous rocks.

Structure of Sodium Gallotitanate Glass in Relation to Glass Formation

The structure of 40NaO0.5⋅20GaO1.5⋅40TiO2 glass has been investigated by X-ray radial distribution analysis. Nearly all the Ga3+ and Ti4+ ions are four coordinated, constructing highly cross-linked three-dimensional glass networks, and that the number of non-bridging oxygens is very small. The degree of medium-range order in this glass is much lower than in the Na2O⋅2TiO2 glass studied in a previous paper, possibly due to the almost complete absence of TiO6 octahedra in the former glass. The higher glass-forming ability of the Na2O-Ga2O3-TiO2 ternary system compared with the Na2O-TiO2 binary system is ascribed mainly to the higher fraction of four-coordinated Ti4+ ions in the former system. It is suggested that in oxide systems glass-forming ability increases with increasing fraction of four-coordinated Ti4+ ions.

Viscosity-temperature relationships at 1 atm in the system nepheline-diopside

The viscosities of melts at 1 atm in the system nepheline-diopside have been measured using rotational and beam-bending techniques. At high temperature, melt viscosities decrease with increasing diopside component. This decrease in melt viscosity can be attributed to a reduction in the size of the flow units resulting from an increase in the number of nonbridging oxygens per tetrahedron. The low-temperature viscosity minimum at intermediate compositions develops as a consequence of the different viscosity-temperature profiles of nepheline and diopside melts.

Origin of inhomogeneous linewidth of Eu3+ fluorescence in several oxide glasses

The local structure around Eu3+ ions in several oxide glasses (silicate, borate, germanate, and aluminosilicate) was investigated using both Mössbauer spectroscopy and fluorescence measurement including the fluorescence line narrowing technique. The origin of inhomogeneous broadening of fluorescence spectra is discussed. Except for silicate glasses, the isomer shift of 151Eu varied with the population of oxygen species with different microscopic optical basicity. For silicate glasses, it was found that the isomer shift and Stark splitting of 7F1 level showed no compositional dependence. It was assumed that regardless of alkali content, Eu3+ ions in silicate glasses attract a certain number of nonbridging oxygens (NBOs), which is the only species with a negative charge, when incorporated stably into a glass matrix. Further, it was generally found that larger amounts of alkali ions or smaller amounts of macroanions, such as BO4■, AlO4■, and GeO62■, bring about smaller values of inhomogeneous linewidth, ΔνIH. It was concluded that the compositional dependence of ΔνIH in oxide glasses is successfully explained from the viewpoint of the NBO-donating ability of the glass matrix, which is related to the NBO concentration and the flexibility of the glass matrix.

Lead‐Ion Stability in Soda–Lime Lead Silicate Glasses

Sodium‐calcium‐lead silicate glass compositions were prepared over a wide compositional range by conventional glass‐melting methods. The acid chemical stability of the glass structure was studied by corroding samples of glass in 4% acetic acid solution at 22°C for periods up to 24 h. Lead corrosion stability was evaluated by measuring lead concentrations in the corrosion solution. At short times, parabolic time dependence was observed and the parabolic time coefficients were regressed against composition, yielding a simple additive relationship. A similar model was fitted to 24‐h release data, which showed compositional effects similar to the 2‐h data. Of the oxides studied, sodium was the most detrimental to durability, and the coefficients of the oxides decreased in the series: Na2O, PbO, CaO, SiO2. The effects of the oxides could be partially explained by the number of nonbridging oxygens expected in the glass structure, and the residual effect was attributed to cation characteristics. Small phosphate additions to the glass greatly increased the lead‐ion stability under nearly all experimental conditions examined.

Structure and energetics in mixed-alkali-metal silicate glasses from molecular dynamics

A crystalline orientated interatomic-potential model, including the directionality of Si—O bonds, has been applied in molecular-dynamics simulations of the atomic-scale structures of x[yNa2O·(1 –y)K2O].(1 –x)SiO2 glasses. The structures of the mixed-alkali-metal silicate glasses were investigated both as a function of the total alkali-metal content and as a function of the ratio of concentrations of different types of alkali-metal ions. The interatomic distances in the simulated glasses are in good agreement with experimental results. Bond angles are also reasonable. The numbers of non-bridging oxygens are consistent with theoretical calculations. Alkali-metal ions and non-bridging oxygens form clusters or alkali-metal-rich regions on a nanoscale. The alkali-metal-rich regions are interconnected in glasses with alkali-metal oxide. Although different types of alkali-metal ions are rather randomly distributed with respect to one another within the alkali-metal-rich regions, on an atomic level a few local pairs of alkali-metal ions of the minority type can be found. In the mixed-alkali-metal glasses, sodium ions have a lower average site potential than in the single-alkali-metal glass, which implies a more stable local state for the sodium ions. On the other hand, the average site potential of potassium ions is slightly increased in the mixed-alkali-metal glasses. Our simulations reveal that in the mixed-alkali-metal silicate glasses the average site potentials of alkali-metal ions show non-linearity as a function of composition, hence providing a structural basis for explanations of the mixed-alkali-metal effect.