Pristine Glass Research Articles

Photoluminescence and Structural Characterization of RE‐Doped Sodium Aluminosilicate Oxyfluoride Glass and Nanocrystalline Glass‐Ceramics as a Function of γ‐Irradiation Dose

Dy3+ and Eu3+‐doped oxyfluoride glass and glass nanocomposites are subjected to low and high dose of γ‐irradiation, i.e., 100 Gy and 1 kGy, respectively. The photoluminescence spectra show changes in electric to magnetic dipole ratios signifying variations in local structural environment. Glass‐ceramics depict the tendency of crystallization or the ordering of structure with increasing γ‐irradiation dose. The results are further supported with X‐ray diffraction and scanning electron microscopy (SEM) measurements. The formation of nanometer size fluoride particles in the pristine glass and glass‐ceramics after γ‐irradiation has been observed from SEM studies. Fourier transform infrared (FTIR) study reveals the restructuring of aluminosilicate network under γ‐irradiation.

Eco-friendly nanocomposites derived from geranium oil and zinc oxide in one step approach

Nanocomposites offer attractive and cost-effective thin layers with superior properties for antimicrobial, drug delivery and microelectronic applications. This work reports single-step plasma-enabled synthesis of polymer/zinc nanocomposite thin films via co-deposition of renewable geranium essential oil-derived polymer and zinc nanoparticles produced by thermal decomposition of zinc acetylacetonate. The chemical composition, surfaces characteristics and antimicrobial performance of the designed nanocomposite were systematically investigated. XPS survey proved the presence of ZnO in the matrix of formed polymers at 10 W and 50 W. SEM images verified that the average size of a ZnO nanoparticle slightly increased with an increase in the power of deposition, from approximately 60 nm at 10 W to approximately 80 nm at 50 W. Confocal scanning laser microscopy images showed that viability of S. aureus and E.coli cells significantly reduced on surfaces of ZnO/polymer composites compared to pristine polymers. SEM observations further demonstrated that bacterial cells incubated on Zn/Ge 10 W and Zn/Ge 50 W had deteriorated cell walls, compared to pristine polymers and glass control. The release of ZnO nanoparticles from the composite thin films was confirmed using ICP measurements, and can be further controlled by coating the film with a thin polymeric layer. These eco-friendly nanocomposite films could be employed as encapsulation coatings to protect relevant surfaces of medical devices from microbial adhesion and colonization.

Effects of 7.5 MeV electron beam irradiation on optical properties of Eu3+-doped zinc sodium bismuth borate glasses

The present study aims to examine the effects of beaming 7.5 MeV electron on europium-doped zinc sodium bismuth borate glasses (prepared through melt quench technique) and the resulting changes in structural and optical properties. The study also examines the structural depolymerization of irradiated glasses, analyzed using recorded Fourier transform infrared spectra. Absorbance spectra along with Tauc’s plot were used to find the optical band gap of pristine and irradiated glasses. The study observes changes in photoluminescence spectra of irradiated Eu3+-doped glasses, monitored by exciting the samples at 394 nm. The study notes a decrease in emission intensity of Eu3+ ions observed from the emission spectra of electron beam treated glass samples. The thermally stimulated luminescence (TL) measurements of irradiated glasses resulted in intense glow peaks with increasing electron beam doses [4–50 kGy]. The TL glow curve exhibited two dissimilar peaks at distinct temperatures of 490 K and 610 K respectively. The obtained trap-depth (0.93 eV) using Chen’s relation and intensity variation of first glow peak at 490 K as a function electron beam doses facilitates ZNBBE-4 glass for dosimetry applications.

The preparation and research of reduced graphene oxide/glass composite fiber

In this article, reduced graphene oxide/glass composite fiber was prepared from mixing graphene oxide and glass powder by ultrasonic dispersion, planetary grinding, high-temperature sintering, and melting wire drawing. The effects of reduced graphene oxide content on the mechanical and electrical properties of the fiber were investigated. Thermal gravimetric analyzer, differential scanning calorimeter, x-ray diffraction, and energy-dispersive x-ray spectroscopy analysis revealed that the graphene oxide was reduced to reduced graphene oxide in the sintering process and the performances of the composite fiber were improved. The tensile strength of reduced graphene oxide/glass composite fiber was 20% higher than the pristine glass fibers by the addition of 0.5 wt% of reduced graphene oxide. Reduced graphene oxide content was positively correlated with composites conductivity, and according to the percolation theory, the percolation threshold of reduced graphene oxide/glass composite fiber was about 0.5 wt%, and the conductivity of the composite fibers was increased by four orders of magnitude compared to the pristine glass fibers when the content of reduced graphene oxide was 0.5 wt%.

Composition effects on mechanical properties of pristine sodium borosilicate glass

AbstractSodium borosilicate (NBS) glass is widely used in the disposal of high level nuclear waste nowadays. In order to understand the mechanism of hardness in NBS glass varies with various compositions, nanoindentation experiment and molecular dynamics simulation were carried out. Both experimental and calculated results suggested that, the hardness of glass increased with the ratio of R ([Na2O]/[B2O3] in molar percentage) and K ([SiO2]/[B2O3] in molar percentage). In the process of hardness increase, the [BO3] units were transformed into [BO4] units, which could lead to the polymerization increase for the glass system, and the transformation was linear to R. The hardness increase was also accompanying with the increase of silica content, which is another cause for polymerization increase. There are many possible factors that may affect the mechanical properties of pristine NBS glass, and polymerization increase should be the main reason for hardness increase, which could be resulted from the two processes above: (a) the transformation of [BO3] into [BO4]; (b) silica content increase.

Experimental investigation of mechanical and machining parameters of hybrid nanoclay glass fiber-reinforced polyester composites

In this work, the effect of nanoclay addition to glass fiber-reinforced polyester composites is studied. The pristine glass fiber-reinforced polyester composites (i.e., staking sequence, kind of fabric used, etc.) and hybrid nanoclay in varying weight fraction (0, 1, 2, 3, 4 and 5 wt%) and glass fiber-reinforced polyester composite is prepared by vacuum-assisted resin infusion technique. Fracture toughness studies are done to analyze the critical stress intensity factor and critical strain energy release rate. The optimum mechanical properties are obtained for hybrid nanoclay glass fiber-reinforced polyester composites with clay content of 3 wt%. This paper also deals with the study of optimizing the cutting parameters (Cutting speed and feed rate) to obtain maximum mechanical properties in hybrid nanoclay glass fiber-reinforced polyester composites during drilling process. It has been found that the torque is unaffected with increase in cutting speed. On contrast with increase in feed and nanoclay weight fraction, the torque increases. The drilled samples are further subjected to mechanical testing. Tensile studies confirms that better mechanical properties are obtained for optimum machining parameter of (0.045 mm/rev, 210 rpm) for 3 wt% nanoclay and glass fiber-reinforced polyester nanocomposites. The hybrid clay and glass fiber-reinforced nanocomposites generally posses better mechanical properties compared with pristine glass fiber-reinforced polyester composites.

Quantitative composition–bioactivity relationships of phosphosilicate glasses: Bearings from the phosphorus content and network polymerization

Bioactive phosphosilicate glasses integrate with bone/tooth tissues by forming a bone-mineral mimicking surface layer of calcium hydroxyapatite (HA). The HA formation (“in vitro bioactivity”) in a simulated body fluid (SBF) solution is known to depend both on the P content (nP) and silicate network connectivity (N¯BOSi) of the glass, but the precise bioactivity–composition relationships remain poorly understood. We present a comprehensive study that clarifies the dependence of the in vitro bioactivity on the {nP, N¯BOSi} parameters for Na2O–CaO–SiO2–P2O5 glass powders (2.6–6.0 mol% P2O5) exposed to SBF for 24 h, using infrared (IR) and solid-state 31P nuclear magnetic resonance (NMR) spectroscopy in conjunction with measured Ca and P concentrations in the solution. IR-derived relative apatite amounts reveal that an increase in the P content of the pristine glass promotes apatite formation by gradually reducing its dependence on the silicate network polymerization: increasing nP widens the N¯BOSi range that provides a high and nearly constant amount of HA, which scales roughly linearly with nP; these properties assist future design of P-rich bioactive glasses. All glasses provide significant HA formation for increasing N¯BOSi values up to ≈2.6, above which the in vitro bioactivity is lost due to insufficient glass dissolution. We also discuss the complex dependence of the SBF-testing outcome on the mass concentration and composition of the glass powder, as well as on its particle sizes, highlighting critical concerns that may guide developments of improved in vitro bioactivity-testing protocols. A strong dependence of the HA formation on the particle sizes is observed for glass powders with low P2O5 content (2.6 mol%), as opposed to their P-richer counterparts that reveal no pronounced particle-size effects.

Xenon solubility and formation of supercritical xenon precipitates in glasses under non-equilibrium conditions

Estimates of noble gas solubility in glasses and minerals are important to understand the origin of these gases, particularly xenon, in the atmosphere. However, technical difficulties and ambiguities in quantifying the dissolved gases introduce large uncertainties in the solubility estimates. We present here the use of transmission electron microscopy (TEM) with in-situ noble gas ion implantation as a non-equilibrium approach for noble gas solubility estimates. Using a suitable Xe equation of state and Monte-Carlo simulations of TEM images, a clear distinction between Xe filled precipitates and empty voids is made. Furthermore, implantation-induced changes in the solubility are estimated using molecular dynamics simulations. These studies allow us to evaluate the xenon solubility of irradiated and pristine silica glasses and monitor in-situ the diffusion-mediated dynamics between the precipitates and voids — otherwise impossible to capture. On exceeding the solubility limit, supercritical xenon precipitates, stable at least up to 1155 K, are formed. The results highlight the high capacity of silicates to store xenon and, predict higher solubility of radiogenic xenon due to the accompanying radiation damage.

Zirconium local environment in simplified nuclear glasses altered in basic, neutral or acidic conditions: Evidence of a double-layered gel

Borosilicate glasses containing 1 to 8 mol% ZrO2 were leached at pH 9, 7 and 1 until complete alteration. The Zr coordination in the bulk and at the gel surface is quantified using Zr L2- XANES spectra in fluorescence and total electron yield mode, respectively. This provides evidence of a single layer at pH 9 whereas a second alteration layer develops at the gel surface at pH 7 and at pH 1. The surface layer, a witness of the first stages of glass alteration, exhibits larger structural variations than the bulk of the gel. This is related with the alteration conditions, from incipient alteration of pristine glasses to chemical diffusion controlled by the gels formed at a later stage. In all gels investigated, a majority of Zr occurs in [6]Zr sites. However, at neutral and acidic pH, the gels contain a high fraction of 7- and 8-coordinated Zr ([7]Zr and [8]Zr, respectively) sites, and only 6-coordinated ([6]Zr) sites at pH 9. The Zr local environment in the gels, determined by Zr K-edge EXAFS, is similar to that in the pristine glasses, with some additional contribution of Zr second neighbors. Both Zr-site geometry and medium-range structure of the alteration gel bring direct evidence that, under the experimental conditions used, glass dissolution is driven by an in situ hydrolysis/condensation mechanism. This allows the gel structure to mimic that of the pristine glass, including the [6]Zr site distortion or the linkage to the silicate gel framework. The pH has a greater influence on the structure of gels than does their Zr content. The structural evolution around Zr is mainly governed by the pH during alteration, while the [6]Zr site symmetry is controlled by the Zr content of the gel. Certain gels contain more octahedral sites than expected from the concentration of charge compensating cations (Ca, Na), a situation similar to that observed in SiO2ZrO2 xerogels.

Bioactive Hydrogel Marbles

Liquid marbles represented a significant advance in the manipulation of fluids as they used particle films to confine liquid drops, creating a robust and durable soft solid. We exploit this technology to engineering a bioactive hydrogel marble (BHM). Specifically, pristine bioactive glass nanoparticles were chemically tuned to produce biocompatible hydrophobic bioactive glass nanoparticles (H-BGNPs) that shielded a gelatin-based bead. The designed BHM shell promoted the growth of a bone-like apatite layer upon immersion in a physiological environment. The fabrication process allowed the efficient incorporation of drugs and cells into the engineered structure. The BHM provided a simultaneously controlled release of distinct encapsulated therapeutic model molecules. Moreover, the BHM sustained cell encapsulation in a 3D environment as demonstrated by an excellent in vitro stability and cytocompatibility. The engineered structures also showed potential to regulate a pre-osteoblastic cell line into osteogenic commitment. Overall, these hierarchical nanostructured and functional marbles revealed a high potential for future applications in bone tissue engineering.

Nanoscale imaging of hydrogen and sodium in alteration layers of corroded glass using ToF‐SIMS: Is an auxiliary sputtering ion beam necessary?

The hydrogen (H)/sodium (Na) interface is of great interest in glass corrosion research. Time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) is one of the few techniques that can provide nanoscale H and Na imaging simultaneously. However, the optimized condition for ToF‐SIMS imaging of H in glass is still unclear. In H depth profiling using ToF‐SIMS, H background control is a key, in which an analysis ion beam and a sputtering ion beam work together in an interlaced mode to minimize it. Therefore, it is of great interest to determine if an auxiliary sputtering ion beam is also necessary to control H background in ToF‐SIMS imaging of H. In this study, H imaging with and without auxiliary sputtering beams (Cs+, O2+, and Arn+) was compared on a corroded international simple glass (ISG). It was surprising that the H/Na interface could be directly imaged using positive ion imaging without any auxiliary sputtering ion beam under a vacuum of 2 to 3 × 10−8 mbar. The H+ background was about 5% atomic percent on the pristine ISG glass, which was significantly lower than the H concentration in the alteration layer (~15%). Moreover, positive ion imaging could show distributions of other interesting species simultaneously, providing more comprehensive information of the glass corrosion. If an auxiliary O2+ sputtering ion beam was used, the H+ background could be reduced but still higher than that in the depth profiling. Besides, this condition could cause significant loss of signal intensities due to strong surface charging.

Adaptation of the GRAAL model of Glass Reactivity to accommodate non-linear diffusivity

The empirical GRAAL (Glass Reactivity with Allowance for the Alteration Layer) model, developed to capture the essential corrosion behavior of nuclear waste glasses, was modified in a simple manner by raising the unitless diffusion term by a power factor p. With values of p > 1.0, the diffusivity is a non-linear, power function of the altered layer thickness. The modified model, GRAALP, was used to fit the long-term alteration data for a series of sodium borosilicate glasses reported by Gin et al. J. Non-Cryst. Solids, 358 (2012) 2559 where glass alteration is dominated by growth of the hydrolysis layer. The optimal value for the parameter p was found to vary significantly and could be correlated with glass composition. A value of p = 3.0 was found to be most suitable for the glasses containing both Ca and Al, while p = 2.6 was more appropriate if Ce was present in the glass. For glasses without Ca, the optimal fit to the data followed the original GRAAL model with p = 1.0. In the absence of Al, glasses containing Ca and/or Zr were not as easily characterized. These findings point to a relationship between the residual alteration rate and glass composition. Elements present in the contacting solution may also play a similar role but for this data set only pure water was used. A power law dependence of diffusivity on local water content is proposed which can partially account for the formation of an altered layer with a relatively sharp interface with the pristine glass. The empirical models do not account for the removal of oxide species due to hydrolysis and the associated influx of water and this, in combination with a diffusivity dependent on local water content, may be required to understand the results of depth profiling studies.

Large-magnitude Pauzhetka caldera-forming eruption in Kamchatka: Astrochronologic age, composition and tephra dispersal

Correlation of individual tephra layers over large areas permits the assessment of eruption magnitude and synchronization of disparate sedimentary archives. The middle Pleistocene Pauzhetka caldera with a diameter of ~30 km is one of the largest in Kamchatka. Distal tephra from the caldera-forming eruption has, however, never been found, hampering precise estimates of the eruption volume and magnitude. In this paper, we report first geochemical identification of distal tephra from the Pauzhetka caldera in the Northwest (NW) Pacific and Okhotsk Sea sediments recovered by ODP 145 cores 881B, 882A and 884B, and IMAGES cores MD01-2415 and MD01-2416. Distal tephras are rhyolites of narrow compositional range allowing their reliable identification among the studied marine cores using major and trace element data. Geochemical correlation of the distal tephra to the proximal strongly welded and altered ignimbrite was performed based on immobile trace elements determined in situ by laser ablation ICP-MS. Based on this case study, we propose that a number of trace elements (U, Th, Nb, Ta, Zr, Hf, Ti, REE, Y and Sc) are immobile during on-shore alteration of welded tuffs and can be used for correlation of pristine glass and altered rock groundmass allowing direct identification of volcanic source of distal tephra. Our new data on the spatial dispersal of the airborne Pauzhetka tephra in the NW Pacific sediments defines its minimum dense rock equivalent (DRE) volume of ~46 km3. Together with the exposed volcanic material around the caldera, the total DRE volume is estimated at 150–170 km3 (3.8–4.4 × 105 Mt) corresponding to the eruption magnitude of 7.60–7.65. Stratigraphic position of the Pauzhetka tephra in the studied cores at transition between marine isotope stage 12 and 11c (Termination V) yields a precise astrochronologic age of 421.2 ± 6.6 ka (weighted mean ± 2σ), which is 27 ka younger than the published average 39Ar/40Ar dates on plagioclase from the proximal ignimbrite. Due to the characteristic composition and precise age, the Pauzhetka tephra may serve as a regional marker for Termination V in the NW Pacific and Okhotsk Sea sediments. A multidisciplinary approach adopted in this study is useful for identification and precise dating of the past explosive eruptions in Kamchatka and other volcanic arcs.

Healing of cracks by green laser irradiation in a nanogold particles glass matrix composite

A transparent composite material consisting of a soda-lime-silicate glass matrix containing up to 0.5 %w gold nanoparticles (10 to 20 nm) was successfully obtained by coating glass grains and a subsequent sintering using a spark plasma sintering (SPS) furnace. Coating was achieved thanks to the reduction of gold(III) chloride trihydrate by sodium borohydride in an ethanol solution. The sintered composites were dense and transparent, but for the largest gold content where the sample exhibits a deep red color. When submitted to laser irradiation (532 nm, up to 100 W/cm2), the composite experiences a fast, localized and much more significant increase of its temperature in comparison with the pristine glass material. A temperature as high as 550 °C was reached, allowing the healing of surface crack.

Towards a unifying mechanistic model for silicate glass corrosion

Borosilicate glasses are currently used for the immobilization of highly radioactive waste and are materials of choice for many biomedical and research industries. They are metastable materials that corrode in aqueous solutions, reflected by the formation of silica-rich surface alteration layers (SAL). Until now, there is no consensus in the scientific community about the reaction and transport mechanism(s) and the rate-limiting steps involved in the formation of SALs. Here we report the results of multi-isotope tracer (2H,18O,10B, 30Si, 44Ca) corrosion experiments that were performed with precorroded and pristine glass monoliths prepared from the six-component international simple glass and a quaternary aluminum borosilicate glass. Results of transmission electron microscopy and nanoscale analyses by secondary ion mass spectrometry reveal a nanometer-sharp interface between the SAL and the glass, where decoupling of isotope tracer occurs, while proton diffusion and ion exchange can be observed within the glass. We propose a unifying mechanistic model that accounts for all critical observations so far made on naturally and experimentally corroded glasses. It is based on an interface-coupled glass dissolution-silica precipitation reaction as the main SAL forming process. However, a diffusion-controlled ion exchange front may evolve in the glass ahead of the dissolution front if SAL formation at the reaction interface significantly slows down due to transport limitations.

Influence of intermediates on the optical and theoretical parameters of xLa2O3-(10-x) Y2O3 -15SrO-15CaO -10B2O3-10Al2O3-40SiO2 (2 ≤ x ≤ 8) glasses

In the present investigation, an attempt has been made to study the effect of La2O3/Y2O3 (X) ratio on the optical properties of the novel glass series xLa2O3-(10-x)Y2O3 -15SrO-15CaO -10B2O3-10Al2O3-40SiO2 (x = 2, 4, 6, 8). The glasses have been prepared via melt quenching route. Fourier transform infrared spectroscopy (FTIR) has been done to obtain the band positions and functional groups for all the pristine as well as heat treated glasses, respectively. UV-visible spectroscopy has been performed to obtain the band gap, Urbach energy and refractive indices for all the pristine as well as heat treated glasses. The parameters like density/molar volume have been calculated with the varying La2O3/Y2O3 (X) ratio. Also, some theoretical parameters like electronegativity of metal ion/oxide ion, interionic distance, ionic concentration, optical basicity and polarizability have been calculated. The parameters like average cross link density, average force stretching constant, bulk modulus and number of network bonds per unit volume have also been calculated. Furthermore, electron paramagnetic resonance (EPR) spectra have also been obtained for all the glass samples to determine the magnetic behavior as well as g values (spin Hamiltonian parameters). The optical basicity of all the glasses lies within the range of 0.621–0.631. All the pristine glasses exhibit band gap energy ranging from 5.80 to 5.89 eV.

Femtosecond laser-induced transformations in ultra-low expansion glass: Microstructure and local density variations by vibrational spectroscopy

We report X-ray diffraction, resonance Raman, and infrared (IR) results on pristine ultra-low expansion (ULE) glass, a binary titanosilicate glass with 5.67 mol. % TiO2. ULE processing by femtosecond (fs) laser radiation leads to nanograting writing and photo-darkening for imaging and data storage. We investigate here the vibrational/structural changes induced by fs laser irradiation of ULE at 515 nm. Optical imaging revealed the formation of micro-cavities, and Raman mapping showed molecular oxygen trapped in such cavities of laser-irradiated ULE glass. While titanium in the pristine glass was found predominantly in tetrahedral Ti4+ sites highly dispersed in the silicate matrix, Raman and IR reflectance spectroscopy on laser-irradiated ULE indicated the formation of Ti3+ sites; Ti3+ octahedral sites are formed in the shells of cavities and aggregate in amorphous Ti2O3-type clusters, while the glass around and below cavities contains Ti3+ tetrahedral sites dispersed in the silicate network. Laser-processed ULE glass was found to also exhibit local restructuring of the silicate matrix. Shifts of the strong IR band at about 1080–1100 cm−1 were translated into changes of the average Si-O-Si bond angle in the laser-transformed areas and found to reflect local density variations; the average local density increases relative to silica glass up to about 8% in the shells of micro-cavities and decreases by about 0.5% in the surrounding material. Chemical processes were proposed to account for photo-darkening and the local structural transformation effect in the probed areas of the fs laser-processed ULE glasses.

Qualitative photoluminescence study of defect activation in telecommunication fibers and Bragg gratings in hydrogen-loaded fibers

Despite the relevance of glass fibers and integrated optical circuits for an increasing number of cutting-edge applications ranging from telecommunication to sensing and quantum photonics, the knowledge about their structural and chemical properties is still in its infancy. Optical spectroscopy techniques are challenged due to the intrinsically low cross-sections for inelastic processes. Our approach is to detect these properties along the core, extending the interaction to the fiber length. We report on in-situ temperature-dependent photoluminescence (PL) measurements in transmission geometry of a) pristine optical glass fibers (standard commercial telecom grade and different types of photosensitive fibers) and b) type I fiber Bragg grating (FBG) in hydrogen-loaded fibers of the same type. A laser with 473 nm wavelength and TEM 00 mode is coupled to an optical spectrometer through different fibers. The fibers are thermally cycled between room temperature and 950° Celsius. As a first observation, we detect a clearly visible red emission from the uncoated fibers at the location of the fiber Bragg grating. Fitting the luminescence spectra with a single Gaussian and monitoring the intensity as a function of temperature reveals an irreversible, thermally activated degradation of the luminescence associated to the fiber Bragg gratings. A closer inspection of pristine glass fibers without FBG revealed a faint, yet thermally stable luminescence with similar spectral characteristics. Analyzing qualitative data for two consecutive heating cycles confirmed two distinct activation energies. This may be due to several reasons such as different defects at the basis of this emission or different structural or chemical environments for the same defect. Further experiment will be carried out in the future to investigate the main reason of two distinct activation energies.

Impact of alkali on the passivation of silicate glass

Amorphous silica-rich surface layers, also called gels, can passivate silicate glass and minerals depending on environmental conditions. However, several uncertainties remain on the mechanisms controlling the formation of these layers. In this paper, the influence of exogenous ions supplied by solutions is studied, both on the formation and on the properties of the gel formed on international simple glass (ISG). ISG was altered at 90 °C, pH90 °C 7, in silica-saturated solutions containing various alkaline cations separately (Li+, Na+, K+, and Cs+). The alteration kinetics observed with Li and Na in the solution is similar to that observed with no ions, while K and Cs in the solution tend to decrease glass alteration. Furthermore, for K or Cs ions, the kinetics decreases as the ionic strength of the solution increases. The passivation layer formed in these solutions shows a selectivity toward cations following the series K > Cs > Na >> Li. These alkalis replace Ca from pristine glass in the altered structures, leading to differences in [AlO4]− units charge compensation. Importantly, exchange between Ca and alkali also affects the total quantity of water inside each gel and this effect is well correlated with the observed drop in glass alteration.

Thermally induced amorphous to amorphous transition in hot-compressed silica glass.

In situ Raman and Brillouin light scattering techniques were used to study thermally induced high-density amorphous (HDA) to low-density amorphous (LDA) transition in silica glass densified in hot compression (up to 8 GPa at 1100 °C). Hot-compressed silica samples are shown to retain structural and mechanical stability through 600 °C or greater, with reduced sensitivity in elastic response to temperature as compared with pristine silica glass. Given sufficient thermal energy to overcome the energy barrier, the compacted structure of the HDA silica reverts back to the LDA state. The onset temperature for the HDA to LDA transition depends on the degree of densification during hot compression, commencing at lower temperatures for samples with higher density, but all finishing within a temperature range of 250-300 °C. Our studies show that the HDA to LDA transition at high temperatures in hot-compressed samples is different from the gradual changes starting from room temperature in cold-compressed silica glass, indicating greater structural homogeneity achieved by hot compression. Furthermore, the structure and properties of hot-compressed silica glass change continuously during the thermally induced HDA to LDA transition, in contrast to the abrupt and first-order-like polyamorphic transitions in amorphous ice. Different HDA to LDA transition mechanisms in amorphous silica and amorphous ice are explained by their different energy landscapes.