14 Search Results

Photothermal atomic force microscopy coupled with infrared spectroscopy (AFM-IR) brings significant value as a spatially resolved surface analysis technique for disordered oxide materials such as glasses, but additional development and fundamental understanding of governing principles is needed to interpret AFM-IR spectra, since the existing theory described for organic materials does not work for materials with high extinction coefficients for infrared (IR) absorption. This paper describes theoretical calculation of a transient temperature profile inside the IR-absorbing material considering IR refraction at the interface as well as IR adsorption and heat transfer inside the sample. Furthermore, this calculation explains the differences inmore » peak positions and amplitudes of AFM-IR spectra from those of specular reflectance and extinction coefficient spectra. It also addresses the information depth of the AFM-IR characterization of bulk materials. AFM-IR applied to silica and silicate glass surfaces has demonstrated novel capability of characterizing subsurface structural changes and surface heterogeneity due to mechanical stresses from physical contacts, as well as chemical alterations manifested in surface layers through aqueous corrosion.« less

Vibrational spectroscopy has been widely used to investigate various structural aspects of the glass network, and there are a plethora of papers reporting subtle but consistent changes in infrared and Raman spectral features of glass upon alterations of glass compositions, thermal histories, mechanical stresses, or surface treatments. However, interpretations of such spectral features are still obscured due to the lack of well-established physical principles accurately describing vibrational modes of the non-crystalline glass network. Due to the non-equilibrium nature of the glass network, three-dimensionally connected without any long-range orders, vibrational spectral features of glass cannot be interpreted using the analogy withmore » those of isolated molecular moieties or crystalline counterparts. Here, this feature article explains why such comparisons are outdated and describes the recent advances made from theoretical calculations of vibrational spectral features of amorphous networks or comparisons of computational results with experimental data. For the interpretation of vibrational spectral features of silica and silicate glasses, the following empirical relationships are suggested: (i) the intensity-weighted peak position of the Si-O-Si stretch mode negatively correlates with the weighted average of the Si-O bond length distribution, and (ii) the broad band of the Si-O-Si bending mode negatively correlates with the Si-O-Si bond angle distribution. Selected examples of vibrational spectroscopic imaging of surface defects are discussed to deliberate the implication of these findings in the structure-property relationship of silica and silicate glass materials. Unanswered questions and continuing research challenges are identified.« less

Abstract Hydrogen bonding interactions play an important role in many chemical and physical processes occurring in bulk liquids and at interfaces. In this study, hydrous species (H ₂ O and Si-OH) on nano-porous alteration layers (gels) formed on a boroaluminosilicate glass called International Simple Glass corroded in aqueous solutions at pH 7 and pH 9, and initially saturated with soluble silicon-containing species were analyzed using linear and non-linear vibrational spectroscopy in combination with molecular dynamics simulations. The simulation results revealed various possible types of hydrogen bonds among these hydrous species in nanoconfinement environments with their populations depending on pore-size distribution.more » The nano-porous gels formed on corroded glass surfaces enhance hydrogen bond strength between hydrous species as revealed by attenuated total reflectance infrared spectroscopy. Sum frequency generation spectroscopy showed some significant differences in hydrogen bonding interactions on alteration layers formed at pH 7 and pH 9. The glass dissolution under the leaching conditions used in this study has been known to be ten times faster at pH 7 in comparison to that at pH 9 due to unknown reasons. The simulation and experimental results obtained in this study indicate that the water mobility in the gel formed at pH 9 could be slower than that in the gel formed at pH 7, and as a result, the leaching rate at pH 9 is slower than that at pH 7.« less

Understanding surface reactions of silicate glass under interfacial shear is vital as it can provide physical insights needed for rational design of more durable glasses. In this work, we performed reactive molecular dynamics (MD) simulations with ReaxFF potentials to study the mechanochemical wear of sodium silicate glass rubbed with amorphous silica in the absence and presence of interfacial water molecules. The effect of water molecules on the shear-induced chemical reaction at the sliding interface was investigated. The dependence of wear on the number of interfacial water molecules in ReaxFF-MD simulations was in reasonable agreement with the experimental data. Confirming this,more » the ReaxFF-MD simulation was used to find further details of atomistic reaction dynamics that cannot be obtained from experimental investigations only. The simulation demonstrated that the severe wear in the dry condition is due to the formation of interfacial Sisubstrate–O–Sicounter_surface bond that convey the interfacial shear stress to the subsurface and the presence of interfacial water reduces the interfacial bridging bond formation. The leachable sodium ions facilitate surface reactions with water-producing hydroxyl groups and their key role in the hydrolysis reaction is discussed.« less

Abstract Understanding composition‐structure‐property relationships of high‐alumina nuclear waste glasses are important for vitrification of nuclear waste at the Hanford Site. Two series of glasses were designed, one with varying Al:Si ratios and the other with (Al + Na):Si ratios based on the international simple glass (ISG, a simplified nuclear waste model glass), with Al ₂ O ₃ ranging from 0 to 23 mol% (0 to 32 wt%). The glasses were synthesized and characterized using electron probe microanalysis, X‐ray photoelectron spectroscopy, small angle X‐ray scattering, high‐temperature oxide melt solution calorimetry, and infrared spectroscopy. Glasses were crystal free, and the lowest Na ₂ O andmore » Al ₂ O ₃ glass formed an immiscible glass phase. Evolution of various properties—glass‐transition temperature, percentage of 4‐coordinated B, enthalpy of glass formation—and infrared spectroscopy results indicate that structural effects differ based on the glass series.« less

Abstract High‐alumina containing high‐level waste (HLW) will be vitrified at the Waste Treatment Plant at the Hanford Site. The resulting glasses, high in alumina, will have distinct composition‐structure‐property (C‐S‐P) relationships compared to previously studied HLW glasses. These C‐S‐P relationships determine the processability and product durability of glasses and therefore must be understood. The main purpose of this study is to understand the detailed structural changes caused by Al:Si and (Al + Na):Si substitutions in a simplified nuclear waste model glass (ISG, international simple glass) by combining experimental structural characterizations and molecular dynamics (MD) simulations. The structures of these two series of glassesmore » were characterized by neutron total scattering and ²⁷ Al, ²³ Na, ²⁹ Si, and ¹¹ B solid‐state nuclear magnetic resonance (NMR) spectroscopy. Additionally, MD simulations were used to generate atomistic structural models of the borosilicate glasses and simulation results were validated by the experimental structural data. Short‐range (eg, bond distance, coordination number, etc) and medium‐range (eg, oxygen speciation, network connectivity, polyhedral linkages) structural features of the borosilicate glasses were systematically investigated as a function of the degree of substitution. The results show that bond distance and coordination number of the cation‐oxygen pairs are relatively insensitive to Al:Si and (Al + Na):Si substitutions with the exception of the B‐O pair. Additionally, the Al:Si substitution results in an increase in tri‐bridging oxygen species, whereas (Al + Na):Si substitution creates nonbridging oxygen species. Charge compensator preferences were found for Si‐[NBO] (Na ⁺ ), ^[3] B‐[NBO] (Na ⁺ ), ^[4] B (mostly Ca ²⁺ ), ^[4] Al (nearly equally split Na ⁺ and Ca ²⁺ ), and ^[6] Zr (mostly Ca ²⁺ ). The network former‐BO‐network former linkages preferences were also tabulated; Si‐O‐Al and Al‐O‐Al were preferred at the expense of lower Si‐O‐ ^[3] B and ^[3] B‐O‐ ^[3] B linkages. These results provide insights on the structural origins of property changes such as glass‐transition temperature caused by the substitutions, providing a basis for future improvements of theoretical and computer simulation models.« less

Infrared (IR) and Raman spectroscopic features of silicate glasses are usually interpreted based on the analogy with those of smaller molecules, molecular clusters, or crystalline counterparts; this study tests the accuracy and validity of these widely cited peak assignment schemes by comparing vibrational spectral features with bond parameters of the glass network created by molecular dynamics (MD) simulations. A series of sodium silicate glasses with compositions of [Na₂O]_x[Al₂O₃]₂[SiO₂]_98-x with x = 7, 12, 17, and 22 were synthesized and analyzed with IR and Raman. A silica glass substrate and a crystalline quartz were also analyzed for comparison. Glass structures with the same compositions weremore » generated with MD simulations using three types of potentials: fixed partial charge pairwise (Teter), partial diffuse charge potential (MGFF), and bond order-based charge transfer potential (ReaxFF). The comparison of simulated and experimental IR spectra showed that, among these three potentials tested, ReaxFF reproduces the concentration dependence of spectral features closest to the experimentally observed trend. Hence, the bond length and angle distributions as well as Si–Qⁿ species and ring size distributions of silica and sodium silicate glasses were obtained from ReaxFF-MD simulations and further compared with the peak assignment or deconvolution schemes—which have been widely used since 1970s and 1980s—(a) correlation between the IR peak position in the Si–O stretch region (1050-1120 cm^-1) and the Si–O–Si bond angle; (b) deconvolution of the Raman bands in the Si–O stretch region with the Qⁿ speciation; and (c) assignment of the Raman bands in the 420-600 cm^-1 region to the bending modes of (SiO)_n rings with different sizes (typically, n = 3-6). The comparisons showed that none of these widely used methods is congruent with the bond parameters or structures of silicate glass networks produced via ReaxFF-MD simulations. This result invokes that the adequacy of these spectral interpretation methods must be questioned. Alternative interpretations are proposed, which are to be tested independently in future studies.« less

No abstract provided.

This study explores the corrosion interactions between model nuclear waste glass materials and corrosion resistant alloys, under accelerated conditions that simulate the near field of a nuclear waste repository. The interactions between the corrosion of stainless steel (SS) 316, alloy G30, or alloy 625, and international simple glass or soda-lime silica glass are systematically studied. The dissimilar materials were exposed in close proximity to each other in different electrolytes at 90 °C. After exposure, the glass surface exposed near metals showed different regimes of corrosion, with distinct surface morphologies and chemistries that were likely affected by the local environment createdmore » by the localized corrosion of metals. Surface and solution analyses showed that the corrosion rate of glass was enhanced by the presence of metals. Infrared spectroscopy data suggested the local build-up of stresses in the contact area of glass, which may lead to the mechanical instability of the glass alteration layer. On the other hand, the effect of glass on metal corrosion is strongly dependent on the leaching solution. In electrolytes containing abundant aggressive anions such as Cl^–, glass seems to suppress the localized corrosion of SS by the precipitation of a Si-rich surface film that protects the SS substrate from solutions. However, in less aggressive electrolytes, the corrosion rate of SS was increased by the presence of glass corrosion products. Overall, our study showed that the hidden and localized damage on glass in contact with metals may enhance the release rate of glass components compared to typical uniform glass corrosion.« less