Thermal Stability of Na₂O-FeO-Fe₂O₃-P₂O₅ Glasses

The crystallization tendencies of five series of Na2O-FeO-Fe2O3-P2O5 (NFP) glasses with different O/P (3.0-3.5) and Fe/P (0.13-0.67) ratios were studied. Characteristic temperatures, including the glass transition temperature (Tg) and crystallization temperature (Txh), were obtained using differential thermal analysis (DTA), and liquidus temperatures (TL) were determined by microscopic evaluation of heat-treated samples. The compositional dependence of glass structure and the characteristic temperatures are discussed. The glass stability (GS) against crystallization is described using parameters based on the characteristic temperatures. For the glass series with O/P ratios near the pyrophosphate composition (O/P ~ 3.5), the glass stability goes through a minimum value around Fe/P ~ 0.3 to 0.4, which corresponds to the crystalline phase NaFeP2O7 and Na7Fe3(P2O7)4

Glass Formation from Iron-rich Phosphate Melts

Iron-rich phosphate glasses with nominal Fe/P ratios between 1.0 and 1.6 were prepared by a rollerquenching technique. The critical cooling rates (CCR) for glass formation were estimated by differential thermal analysis and found to be in the range of 103-104 °C/s for the iron-rich melts, compared to 1-10 °C/s for conventional iron phosphate melts with nominal Fe/P ratios near 0.50. The Fe(II)/Fetotal fraction in the quenched glasses increases with melt time and temperature, and ranges between 0.30 and 0.55 for theglasses studied. Raman spectroscopy indicates that the structures of the iron-rich phosphate glasses are based on isolated orthophosphate tetrahedra, similar to what are found in α-FePO 4. © 2010 Elsevier B.V. All rights reserved

Structural similarity on multiple length scales and its relation to devitrification mechanism: a solid-state NMR study of alkali diborate glasses and crystals

The nature of the devitrification process is a fundamental problem in glass science and governs the ultimate stability of glass. It is hypothesized that the devitrification mechanism of a given glass composition is strongly correlated with the extent of structural similarity at the intermediate-range level between the glass and crystal phase to which it transforms. However, relatively little information is available until now to test this hypothesis because of the lack of long-range periodicity in glass and the absence of effective analysis methods. In this work Li2O-2B2O3 (L2B) and Na2O-2B2O3 (N2B) glasses, which respectively exhibit homogeneous (internal) and heterogeneous (surface) devitrification mechanisms, and plus corresponding crystals from same batches were prepared and comprehensively studied by using multiple solid state NMR techniques. Of those, 11B MAS and MQMAS experiments were applied to reveal short-range order information; 7Li and 23Na spin-echo decay and 11B{6Li} and 11B{23Na} rotational echo double resonance (REDOR) experiments were used to determine homonuclear and heteronuclear interactions that can reflect the intermediate-range order. It was found that the local bonding environments of the L2B crystal and glass are quite similar, while there are significant differences in the N2B system. Moreover, the experimental results indicated that the second moment (M2) values of both homo- and heterodipolar interactions measured for the L2B glass are very close to those of its isochemical crystal, in contrast, the M2 values for N2B glass differ significantly from those of its crystal phase. These observations indicate the existence of a strong correlation between structural similarity at both the short and intermediate length scale, and nucleation mechanis

Correlation of network structure with devitrification mechanism in lithium and sodium diborate glasses

The intermediate-range structure of the network former in lithium and sodium diborate was studied using nuclear magnetic resonance spectroscopy. Specifically, 11B{10B} rotational-echo double resonance experiments were employed to determine the distribution of dipole couplings between these isotopes and in this way determine whether the intermediate range order of the borate network was the same in the glasses as in the crystal forms of these compounds. It was found that in the lithium diborate case the networks are in fact similar between glass and crystal, while in sodium diborate they differ substantially. Because lithium diborate shows homogeneous nucleation and growth on the laboratory time scale while sodium diborate does not, it was concluded that structural similarity between glass and crystal of the glass former correlates strongly with nucleation mechanism

Thermal Stability of Glasses from the Fe₄(P₂O₇)₃-Fe(PO₃)₃ System

Iron phosphate glasses with nominal Fe/P compositions between 0.50 and 0.67 were prepared and characterized. The effects of melt conditions and the initial composition on the Fe(II)-content are reported. Characteristic temperatures, including the glass transition temperature and crystallization temperature, were measured by DSC. The stability of melts against crystallization, described by the Angel and Weinberg parameters, generally decreases with increasing O/P and Fe/P ratios. © 2010 Elsevier B.V. All rights reserved

Sintering and rounding kinetics of irregular glass particles

Compacts of irregular glass particles sinter up to five times faster than sphericalparticle compacts of the same composition. This effect has been attributed to the sharp edges of irregular particles. In this article, we propose and test a phenomenological model for the sintering kinetics of jagged glass particles considering their rounding during sintering. We assume that the small radii of curvature of the particle edges increase as the particles round off and control the sintering rate. We tested the model by measuring the sintering shrinkage of spherical and irregular particle compacts of a diopside (MgO·CaO·2SiO2) glass and using literature sintering data for particles of different shapes of a soda‐lime‐silica glass. The sintering rate of irregular‐particle compacts is initially much higher but tends to reach that of their spherical counterparts as they round off. Our model describes the experimental shrinkage of both glasses and explains the shrinkage anisotropy of irregular‐particle compacts in the initial stages of sintering, providing a significant step toward the understanding and description of the sintering kinetics of jagged glass particles.Fil: Reis, Raphael M. C. V.. Universidade Federal Fluminense; BrasilFil: Barbosa, Anne J.. Universidade Federal Do Sao Carlos. Departamento de Engenharia de Materiales; BrasilFil: Ghussn, Luciana. Universidade do Estado de Rio do Janeiro; BrasilFil: Ferreira, Eduardo B.. Universidade de Sao Paulo; BrasilFil: Prado, Miguel Oscar. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comision Nacional de Energia Atomica. Gerencia de Area de Aplicaciones de la Tecnología Nuclear. Gerencia de Investigación Aplicada. Grupo de Materiales Nucleares; ArgentinaFil: Zanotto, Edgar D.. Universidade Federal Do Sao Carlos. Departamento de Engenharia de Materiales; Brasi