Structure Of Borate Glasses By Raman Spectroscopy

Raman spectra of the alkali borate glasses are presented with emphasis on the implications of detailed frequency shifts and line shapes. The results are interpreted on a rather complicated three-level hierarchy of glass structures which include boron-oxygen polyhedra, topologically closed rings of polyhedra, and larger scale interconnected clusters of alkali-rich and alkali-poor structures. Both the model and the argument are deliberately vague. The borate glasses appear to be highly disordered materials with a size scale such that no presently available tool gives a really clear view of the structural detail.

Boron Oxide plays a key role in numerous glasses of high technological importance, yet its role in glass structure is far from clear. Indeed, in recent years there have been serious chal lenges to previous structure concepts for both crystalline and glassy borates. These challenges were sufficient to warrant a re examination of the structure of borate glasses using the most pow erful tools currently available. To provide a suitable forum for this undertaking, a four-day conference on "Boron in Glass and Glass Ceramics" was convened at Alfred University, June 3-8, 1977 to review the best scientific thinking on structure and to debate conflicting views and discuss properties and applications of borate glasses. This conference was also the first in a New University series on Glass Science to be rotated among Alfred University, The Pensyl vania State University, Rensselaer Polytechnic Institute, and the University of Missouri-Rolla. The present volume represents the proceedings of the first conference in this series. The volume begins with a review of the remarkable contribution of Jan Krogh-Moe to the understanding of the structure of Borate glasses. This review, authored by Professor N. J. Kreidl, concludes by dedicating the proceedings of this conference as a Krogh-Moe Fest schrift. The volume continues with a historical review by D. L. Griscom, originally prepared for circulation to the contributors prior to the conference. An Epilogue to the opening chapter brings the survey up-to-date in light of the conference papers.

Inorganic glasses are successfully used in the biomedical field, in particular degradable glasses have found applications in tissue engineering, bone regeneration and tooth remineralisation. Silicate glasses are the most commonly used ones but phosphate and borate glasses are attracting more and more interest owing to their special properties, differing from those of silicate bio-glasses. Phosphate and borate glasses thus open up potential routes for new therapeutic applications. This book focuses on these emerging materials. Bridging the phosphate and borate glasses communities, this book provides a fundamental treatment of atomic structure and physicochemical properties before highlighting their current and potential future applications. Phosphate and borate glasses not only feature a broader range of glass formation than silicate glasses. Their ability to completely dissolve in water with the solubility varying over orders of magnitude with compositional changes, makes them exciting materials for delivering therapeutic agents into the human body. Biomaterials scientists working in glasses, hard tissue engineering and regenerative medicine will find this a must-have book to own, alongside their more traditional silicate glass tomes.

The thesis begins with a comprehensive review of the structure and properties of borate glasses. This is followed by a predominantly qualitative assessment of highly modified lithium and strontium borates containing yttrium and rare-earth oxide additions that have been prepared by the traditional melt-quenching technique. To the author's knowledge, reports on the ternary glasses studied here are not available in the literature. The feasibility of glass formation for these new compositions is discussed and the structures of the resulting materials have been studied, primarily, with vibrational spectroscopy as well as selectively with differential scanning calorimetry, X-Ray diffraction, and time-resolved fluorescence spectroscopy. Raman and Infrared spectroscopies suggest the glasses formed in the vicinity of the orthoborate stoichiometry are structurally similar to the high temperature phase of the related yttrium or rare-earth orthoborate crystals, regardless of whether the glass transition temperature lies above or below the corresponding phase transformation temperature. A relationship between the crystal phase transformation temperature and the glass transition temperature has been observed through the partial devitrification of borate melts containing relatively high yttrium content. That is, for glasses where the glass transition temperature is above the phase transformation temperature, only short-range order structural units akin to high temperature phase will be present in both the glass and any crystallization products. For glasses where the glass transition temperature is below the phase transformation temperature, the bulk glass is also made up of the short-range order structural units found in the high temperature phase; however, low temperature phase crystallites are detected with XRD. These crystallites, comprised only of tetrahedral orthoborate units arranged n B3O9^9- rings, are seen also as well-defined structures approximately 30 microns in diameter using confocal Raman microscopy. The addition of oxides with the high field strength trivalent yttrium ion to strontium borate glasses was found to depolymerize the borate network into ionic species while simultaneously increasing the glass transition temperature. In this series, the increase of the cation motion band frequency from 180 cm^-1 to 323 cm^-1 indicates the trivalent yttrium ions form stronger bonds with network oxygen than the divalent strontium ions. This correlates with the onset of the glass transition temperature increasing non-linearly from 630.7 °C to 652.2 °C for glasses containing 5 mol% and 25 mol% Y2O3, respectively. Lithium yttrium/rare-earth orthoborate glasses were found to consist solely of isolated trigonal borate units and to be insensitive to the use of either yttrium or other rare-earth elements, all of which have a well-known tetrahedral orthoborate crystalline phase. The absence of isomerization or disproportionation at the orthoborate stoichiometry implies the driving mechanism for glass formation in these glasses can be viewed as having a physical rather than chemical origin. This is to say, vitrification is dependent on the freezing-in of highly distorted, isolated trigonal borate structures - which are favored at high temperatures and in the melt compared to their tetrahedral counterpart. Several of the lithium yttrium/rare-earth orthoborate glasses were doped with Tb^3+ and the measured lifetime of the 543 nm emission using 375 nm excitation was ~2.2 ms. Measured lifetime decays, in conjunction with the relevant literature, suggest that even if trivalent rare-earth cations are present in small quantities, in a partially or fully depolymerized borate glass, the Tb^3+ ions will seek out local sites whose short range order corresponds to that of the high temperature crystalline phase. Indeed, this observation is in excellent agreement with the qualitative Raman interpretation of bulk strontium yttrium borate glasses where the introduction of the small quantities of Y2O3 is seen to induce the formation of trigonal orthoborate units below the pyroborate stoichiometry. Overall, the results obtained from the Tb^3+ doped orthoborate glasses here permit interpretation of previously unexplained published results. This is framed in the context of experimental and computational results pertaining to alkali and alkaline-earth borates that show that metal cations with high field strength will determine their local environment to a greater degree than metal cations present whose field strength is lower.

This works studies the effects of compositional and temperature variations on the structure and properties of aluminoborosilicate glasses. Two groups of aluminoborosilicate glasses, one that has lower boron content and another that has higher boron content, have been studied. The structural changes were mainly observed with high-field B-11, Al-27 and Na-23 magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. In these glasses, boron is either three-coordinate (BO3) or four-coordinate (BO4); aluminum exists predominately as four-coordinate species, but there is a small amount of five-coordinate aluminum ([5]Al). The compositional study focused on the effect of the cation field strength of the network modifiers on the glass structure by varying the ratio of the two network modifiers, CaO and Na2O. Increasing the ratio of CaO to Na2O dramatically lowers the fraction of four-coordinated boron (N4), increases [5]Al, and increases the fraction of non-bridging oxygens (NBO), which was calculated based on the boron and aluminum structural information. However, variations in these fractions are not linear with respect to the average cation field strength. Na-23 spectra reveal that the ratio of bridging to non-bridging oxygens in the coordination shell of Na+ increases with an increasing ratio of CaO to Na2O in Ca-rich glasses. These changes can be understood by the tendency of higher field strength modifier cations to facilitate the concentration of negative charges on NBO in their local coordination environment, systematically converting BO4 to BO3. The effect of temperature on the structure was studied by two ways: cooling the glass-forming melts at different rates to sample the glass structure at different fictive temperature, and using high-temperature in situ NMR. The abundances of BO3 and NBO increase with increasing fictive temperature, suggesting that the reaction BO4 [logical equivalence] BO3 + NBO shifts to the right with increasing temperature. The observed temperature dependence of the abundance of BO4 species allows us to estimate the enthalpy of reaction, [Delta]H, which is closely related to the amount of NBO in the glass. In situ high-T B-11 MAS NMR was used to observe chemical exchange between BO3 and BO4 species over the timescale of microseconds to seconds. The timescale of BO3/BO4 exchange from NMR data, [lowercase Tau](NMR), appears to be "decoupled" from that of the macroscopic shear relaxation process, [lowercase Tau](s), derived from the viscosity data; however, at higher temperatures, [lowercase Tau](s) approaches [lowercase Tau](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.