Thermal evolution of metakaolin geopolymers: Part 1 – Physical evolution

The physical evolution of materials during heating is a critical factor in determining their suitability and performance for applications ranging from construction to refractories and adhesives. The effect of different cations (sodium and potassium) on the physical evolution of geopolymeric materials derived from metakaolin is investigated for a range of specimens with Si/Al ratios between 1.15 and 2.15. It is observed that the effect of potassium is to reduce the thermal shrinkage, while thermal shrinkage increases with increasing Si/Al ratio in the presence of each alkali type. The thermal shrinkage behavior of mixed-alkali specimens is observed to change from a mean of the sodium and potassium specimens at low Si/Al ratio to behave similarly to sodium specimens at high Si/Al ratios. It is clear from this investigation that alkali cations only have a significant effect on thermal shrinkage of geopolymer at low Si/Al ratios (⩽1.65), while both Si/Al ratio and alkali cation have little effect on the extent of thermal shrinkage at Si/Al ⩾ 1.65.     

Heterophase inclusions and dissolved impurities in Ge25Sb10S65 glass

The impurity content and microhomogeneity of Ge₂₅Sb₁₀S₆₅ glass samples, prepared by direct synthesis from elements, were investigated. It was shown that the increase in temperature of synthesis of the glass-forming melt resulted in the increase of the content of impurities of H, Na, Al, Si, K, Ca and transition metals in the prepared glasses. The glasses from the melt, subjected to chemical-distillation purification, were characterized by the low content of gas-forming impurities and the increased content of Al, Si and Cl. The glasses contained heterophase impurity inclusions mainly consisting of SiO₂, and their concentration and size depended on the conditions of glass preparation. The impurity content in the purest glasses was as follows: oxygen – <0.5 ppm wt, carbon – <5 ppm wt, hydrogen – 0.1 ppm wt, Si – <1 ppm wt, transition metals – <0.25 ppm wt, heterophase impurity inclusions with sizes larger than 80 nm – <10² cm³. It was shown that heterophase impurity inclusions behaved as the centers of glass crystallization.     

Ex situ XRD,TEM, IR,Raman and NMR spectroscopy of crystallization of lithium disilicate glass at high pressure

The structure of Li₂O · 2SiO₂ (LS₂) glass was investigated as a function of pressure and temperature up to 6 GPa and 750 °C, respectively, using XRD, TEM, IR, Raman and NMR spectroscopy. Glass densified at 6 GPa has an average Si–O–Si bond angle ∼7° lower than that found in glass processed at 4.5 GPa. At 4.5 GPa, lithium disilicate crystallizes from the glass, while at 6 GPa new high pressure form of lithium metasilicate crystallizes. This new phase, while having lithium metasilicate crystal symmetry, contains at least four different Si sites. NMR results for 6 GPa indicate the presence of Q⁴ species with (Q⁴)Si–O–Si(Q⁴) bond angles of ∼157°. This is the first reported occurrence of Q⁴ species with such large bond angles in alumina free alkali silicate glass. No five- or six-coordinated Si are found.     

A comparison of natural and experimental long-term corrosion of uranium-colored glass

The alteration features of historical U-colored glasses exposed to natural weathering for over 150 years were compared with the experimental alteration of similar glass with ～0.3 wt% of uranium using a long-term (up to 426 days) kinetic laboratory batch leaching test in deionized water. Two types of natural corrosion crusts were identified by a combination of SEM/EDS, HRTEM/SAED, EPMA and XRD: (i) formation of a leached layer (up to ～600 μm thick) depleted in alkalis and enriched in Si with stable concentration of U and Al and (ii) formation of lamellae depleted in alkalis, Si and U and enriched in Al. The presence of newly formed gibbsite (Al(OH)₃) and kaolinite (Al₂Si₂O₅(OH)₄) were confirmed in the second type of corrosion crust by HRTEM. Dissolution of the glass components including uranium was determined during the laboratory leaching test. Several μm thick alkali-depleted alteration zones with stable U content relatively enriched in Si and Al were formed on the glass surface. The PHREEQC-2 modeling also predicted the precipitation of secondary gibbsite and kaolinite in the late stages of the leaching. These phases may form especially when sufficient amounts of Al are available from the environment (e.g., soil). Furthermore, they provide surfaces for sorption and may, in some cases, affect the mobility of U ions released from the glass in dependence on pH and U speciation.     

Ultra-thin polycrystalline Si layers on glass prepared by aluminum-induced layer exchange

In this work, we present studies of ultra-thin polycrystalline silicon layers (5–100 nm) prepared by the aluminum-induced layer exchange process. Here, a substrate/Al/oxide/amorphous Si layer stack is annealed at temperatures below the eutectic temperature of the Al/Si system of 577 °C, leading to a layer exchange and the crystallization of the amorphous Si. We have studied the process dynamics and grain growth, as well as structural properties of the obtained polycrystalline Si thin films. Furthermore, we derive a theoretical estimate of the grain density and examine characteristic thermal activation energies of the process. The structural properties have been investigated by Raman spectroscopy. A good crystalline quality down to a layer thickness of 10 nm has been observed.     

Fluorine laser-induced silicon hydride Si–H groups in silica

Formation and destruction of silicon hydride (Si–H) groups in silica by F₂ laser irradiation and their vacuum ultraviolet (VUV) optical absorption was examined by infrared (IR) and VUV spectroscopy. Photoinduced creation of Si–H groups in H₂-impregnated oxygen deficient silica is accompanied by a growth of infrared absorption band at 2250 cm⁻¹ and by a strong increase of VUV transmission at 7.9 eV. Photolysis of Si–H groups by 7.9 eV photons in this glass was not detected when the irradiation was performed at temperature 80 K. However, a slight destruction of Si–H groups under 7.9 eV irradiation was observed at the room temperature. This finding gives a tentative estimate of VUV absorption cross section of Si–H groups at 7.9 eV as 4 × 10⁻²¹ cm², showing that Si–H groups do not strongly contribute to the absorption at the VUV fundamental absorption edge of silica glass.     

Characterization of thermo-optical and mechanical properties of calcium aluminosilicate glasses

In this work several different compositions of CaO:Al₂O₃:SiO₂ were prepared under vacuum atmosphere to study the glass forming ability of this system as a function of the SiO₂ content. Samples containing 25–45 wt% of Al₂O₃, 31–44 wt% of CaO, 14–39 wt% of SiO₂ and 4.1 wt% of MgO were prepared in graphite crucibles, for approximately 2 h at ∼ 1600 °C. The influence of silica content is discussed in terms of the mechanical properties, glass transition temperature, crystallization temperature, transmittance spectrum, refractive index, mass density, specific heat, thermal diffusivity, thermal conductivity and the temperature coefficient of optical path length change. The results reinforce the idea that these glasses are strong materials, having useful working-temperature range, good combination of thermal, mechanical and optical properties that could be exploited in many optical applications, in particular, as glass laser materials.     

Germanosilicate and alkali germanosilicate glass structure: New insights from high-resolution oxygen-17 NMR

We present here triple-quantum, magic-angle spinning (3QMAS) NMR spectra for ¹⁷O in a SiO₂–GeO₂ binary glass, and for two sodium germanosilicate glasses, all with Si/Ge ratios of 1. In the binary germanosilicate, three NMR peaks are partially resolved, and correspond to the three types of bridging oxygens, Si–O–Si, Si–O–Ge, and Ge–O–Ge. Peak areas indicate that the relative abundances of these species are close to those expected for random mixing of the Si and Ge in the network. In a sodium germanosilicate glass with a relatively low Na content (Na₂O ≈ 8 mol%), the spectra demonstrate the formation of significant fractions of both nonbridging oxygens bonded to Si, and of oxygens bonded to Ge in five- or six-coordination. At higher Na content (Na₂O ≈ 31%), most or all Ge is four-coordinated and network modification is dominated by the formation of NBO on Si and on Ge. Models of physical properties of alkali germanosilicates, in which modifier oxides are distributed between the Si and Ge components of the network in proportion to the Si/Ge ratio, are thus supported, as is extensive mixing of Si and Ge.     

Effect of hydroxyl impurity on temperature coefficient of refractive index of synthetic silica glasses

We have studied the temperature coefficient of the refractive index of synthetic silica glasses with various hydroxyl impurities. The refractive index was measured at 15 °C and 35 °C at 1.707–0.238 μm wavelengths. The temperature coefficient of a low-OH group (110 wt. ppm) containing glass increased from 8.0 ± 0.2 × 10^?6/°C (at 1.707 μm) to 14.0 ± 0.2 × 10^?6/°C (at 0.238 μm), although it increased respectively from 7.0 ± 0.2/°C to 12.0 ± 0.2 × 10^?6/°C for a high-OH group (1300 wt. ppm) containing glass. The three-term Sellmeier equation, having two terms with resonance photon energies in the vacuum ultraviolet region and one term in the infrared region, was used to analyze the wavelength dispersion of the refractive index. Increasing temperatures shifted the resonance energy in the second term by ?4.14 ± 0.4 × 10^?4 eV/°C for low-OH (110 wt. ppm) glass and ?2.64 ± 0.4 × 10^?4 eV/°C for high-OH (1300 wt. ppm) glass. The fundamental absorption edge in the vacuum ultraviolet region shifted by ?8.8 ± 0.7 × 10^?4 eV/°C for the low-OH glass and ?6.3 ± 0.7 × 10^?4 eV/°C for the high-OH glass in a region of 25–100 °C. Both high-OH glass shift rates were lower than low-OH glass shift rates. The lower temperature coefficient for the Si–OH-related band probably explains the smaller temperature coefficient for high-OH glass: the absorption band of Si–O–H structure is located at lower energy side close to the fundamental absorption band associated with the Si–O–Si structure.     

Mechanically alloyed multi-component Mo-based amorphous alloys with wide supercooled liquid region

A Mo₄₄Si₂₆Ta₅Zr₅Fe₃Co₁₂Y₅ multi-component amorphous alloy was developed via mechanically alloying (MA). It exhibits a record high glass transition temperature of 1202 K and crystallization temperature of 1324 K, an ultrahigh hardness of 18 GPa, as well as a wide supercooled liquid region (122 K) promising for processing through powder metallurgy routes. Here we present the details of the phase evolution during MA and discuss the effects of alloying elements, starting from the Mo–Co and Mo–Si binary systems, through two series of ternary alloys, eventually reaching the desired properties by selecting additional components. The propensity for glass formation and the high thermal stability were interpreted in terms of the negative heat of mixing of the elements introduced, as well as a uniform coverage spanning a wide range of atomic sizes.     

Effect of minor additions on structure and volatilization loss in simulated nuclear borosilicate glasses

Structural and thermal properties are reported for a range of caesium oxide-containing alkali borosilicate glasses, of the form xCs₂O(100 − x)ZMW (0 < x < 10), where ZMW represents a variety of simulated base-glasses. Glass densities increase and glass transition temperatures decrease with increase in caesium oxide concentration. Mass-loss from the melt is found to depend on composition in the same manner as the fraction of silicon Q³ units, resolved from ²⁹Si MAS NMR, and is related to the presence of danburite medium-range order units, resolved from ¹¹B MAS NMR. Volatilization is shown to occur even in the absence of caesium oxide and the mixed alkali borosilicate composition of the volatile species, evolved from the melt at high temperature, is independent of the starting composition of the glass.     

Cadmium ions adsorption in simulated wastewater using structured alumina–silica nanoparticles

Some results on the removal of cadmium ions from simulated industrial wastewater using sol–gel structured nanoparticles of silica and alumina are presented. Two different core-shell nanoparticles were prepared: Al–Si particles (alumina core surrounded by a silica shell) with a molar composition Al:Si of 1:5, and Si–Al particles (silica core and alumina shell) with a molar composition Si:Al of 1:5. Different amounts of cadmium ions were added and the flocculation process of these particles, induced by the ions adsorption, was followed using dynamic light scattering. The efficiency of the ions adsorption was determined using atomic absorption spectroscopy. The results show that it is possible to reduce the cadmium concentration from 140 ppm to less than 5 ppb using Si–Al particles, meeting some international regulations for this contaminant (New European Directive 98-83). The Al–Si particles were not so efficient because the cadmium was only reduced from 125 ppm to less than 90 ppb. The sol–gel technique allows to synthesize model nanoparticles with different morphologies to be used as a prototypes, allowing to choose the better morphology and scaling this knowledge to an industrial level using commercially available silica and alumina nanoparticles chemically modified on the surface.     

High-quality silicon/insulator heteroepitaxial structures formed by molecular beam epitaxy using Al2O3 and Si

Heteroepitaxial growth of γ-Al₂O₃ films on a Si substrate and the growth of Si films on the γ-Al₂O₃/Si structures by molecular beam epitaxy have been investigated. It has been found from AFM and RHEED observations that, γ-Al₂O₃ films with an atomically smooth surface with an RMS values of ∼3 Å and high crystalline quality can be grown on Si (1 1 1) substrates at substrate temperatures of 650–750°C. Al₂O₃ films grown at higher temperatures above 800°C, did not show good surface morphology due to etching of a Si surface by N₂O gas in the initial growth stage. It has also been found that it is possible to grow high-quality Si layers by the predeposition of Al layer followed by thermal treatment prior to the Si molecular beam epitaxy. Cross-sectional TEM observations have shown that the epitaxial Si had significantly improved crystalline quality and surface morphology when the Al predeposition layer thickness was 10 Å and the thermal treatment temperature was 900°C. The resulting improved crystalline quality of Si films grown on Al₂O₃ is believed to be due to the Al₂O₃ surface modification.     

Effect of germanium on the microstructure and the magnetic properties of Fe–B–Si amorphous alloys

In this work, we present a systematic study on the crystallization kinetics and the magnetic properties of melt-spun Fe₈₀B₁₀Si_10 ? xGe_x (x = 0.0 ? 10.0) amorphous alloys. The activation energy for crystallization, determined by differential scanning calorimetry, displayed a strong dependence on the Ge content, reflecting a deleterious effect on the alloys' thermal stability and their glass forming ability with increasing Ge concentration. On the other hand, the alloys exhibited excellent soft magnetic properties, i.e., high saturation magnetization values (around 1.60 T), alongside Curie temperatures of up to 600 K. Complementary, for increasing Ge substitution, the ferromagnetic resonance spectra showed a microstructural evolution comprising at least two different magnetic phases corresponding to a majority amorphous matrix and to Fe(Si, Ge) nanocrystallites for x ≥ 7.5.     

Monte Carlo simulations of the dissolution of borosilicate glasses in near-equilibrium conditions

Monte Carlo simulations were performed to investigate the mechanisms of glass dissolution as equilibrium conditions are approached in both static and flow-through conditions. The glasses studied are borosilicate glasses in the compositional range (80 ? x)% SiO₂ (10 + x / 2)% B₂O₃ (10 + x / 2)% Na₂O, where 5 < x < 30%. In static conditions, dissolution/condensation reactions lead to the formation, for all compositions studied, of a blocking layer composed of polymerized Si sites with principally 4 connections to nearest Si sites. This layer forms atop the altered glass layer and shows similar composition and density for all glass compositions considered. In flow-through conditions, three main dissolution regimes are observed: at high flow rates, the dissolving glass exhibits a thin alteration layer and congruent dissolution; at low flow rates, a blocking layer is formed as in static conditions but the simulations show that water can occasionally break through the blocking layer causing the corrosion process to resume; and, at intermediate flow rates, the glasses dissolve incongruently with an increasingly deepening altered layer. The simulation results suggest that, in geological disposal environments, small perturbations or slow flows could be enough to prevent the formation of a permanent blocking layer. Finally, a comparison between predictions of the linear rate law and the Monte Carlo simulation results indicates that, in flow-through conditions, the linear rate law is applicable at high flow rates and deviations from the linear rate law occur under low flow rates (e.g., at near-saturated conditions with respect to amorphous silica). This effect is associated with the complex dynamics of Si dissolution/condensation processes at the glass–water interface.     

Aluminum induced formation of SiGe alloy in Ge/Si/Al thin film structure

In this work, a study of aluminum induced crystallization (AIC) of thin film germanium/silicon/aluminum (Ge/Si/Al) structure on oxidized silicon is presented. The Ge/Si/Al trilayer structure was prepared in three consecutive thin film deposition processes. The AIC was performed in nitrogen at 500 °C within time duration between 1 and 9 h. The progress of crystallization was monitored by optical microscopy, Raman spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDS). It was found that in Ge/Si/Al structure the AIC can lead to formation of SiGe alloy at temperature of 500 °C. This presents an alternative low-temperature formation method of SiGe which is suitable for integration with the conventional Si technology in electronic device fabrication.     

Growth and characterization of GaN and AlN films on (1 1 1) and (0 0 1) Si substrates

GaN and AlN films were grown on (1 1 1) and (0 0 1) Si substrates by separate admittances of trimethylgallium (or trimethylaluminum) and ammonia (NH₃) at 1000°C. A high temperature (HT) or low temperature (LT) grown AlN thin layer was employed as the buffer layer between HT GaN (or HT AlN) film and Si substrate. Experimental results show that HT AlN and HT GaN films grown on the HT AlN-coated Si substrates exhibit better crystalline quality than those deposited on the LT AlN-coated Si substrates. Transmission electron microscopy (TEM) of the HT GaN/HT AlN buffer layer/(1 1 1)Si samples shows a particular orientation relationship between the (0 0 0 1) planes of GaN film and the (1 1 1) planes of Si substrate. High quality HT GaN films were achieved on (1 1 1) Si substrates using a 200 Å thick HT AlN buffer layer. Room temperature photoluminescence spectra of the high quality HT GaN films show strong near band edge luminescence at 3.41 eV with an emission linewidth of ∼110 meV and weak yellow luminescence.     

Solubility of tin dioxide in soda-lime silicate melts

In the present work, the solubility of tin dioxide is assessed as a function of time, temperature and basicity in simple ternary glasses: NC3S, NC4S, NC5S and NC6S (N: Na₂O, C: CaO, S: SiO₂). An increase of silica contents in the glass composition leads indeed to a decrease of the glass basicity. First, a kinetic study of the dissolution has been performed. Consequently, the solubility limits of tin dioxide have been determined after 2 h of heat treatment: this duration is long enough to reach the dissolution equilibrium, and short enough to limit the sodium oxide losses in the melt at high temperatures. Nevertheless the specific case of the most acid glass has been underlined, as its higher viscosity implies longer heating times. At equilibrium state, SnO₂ solubility depends on the temperature (Arrhenius law) and on the glass basicity. In the 1200 °C–1400 °C temperature range, in these soda–lime glasses, the solubility of tin dioxide is between 1.3 and 2.1 at.% Sn and the temperature dependence of solubility exhibits a single mechanism of dissolution. Furthermore, the basicity dependence of the solubilization process is also discussed, and the presence of another oxidation state of tin (Sn^II) is thus proposed.     

The effect of TiO2 on phase separation and crystallization of glass-ceramics in CaO–MgO–Al2O3–SiO2–Na2O system

The experiments were carried out on studying the effect of phase separation on nucleation and crystallization in the glass based on the system of CaO–MgO–Al₂O₃–SiO₂–Na₂O. In the experiments, TiO₂ was chosen as nucleating agent. Three batches of 5, 8 and 10 wt% TiO₂ substitution were investigated by the techniques of DSC, XRD, FTIR and FESEM equipped with EDS. XRD and FTIR analysis indicated that the super cooled glasses were all amorphous, the heat treatment leading to nucleation would cause a disruption of silica network which followed phase separation. The phase separation followed the generation of crystal seeds Mg(Ti, Al)₂O₆. FESEM observation and EDS analysis revealed that the more TiO₂ content of glass, the more droplet separated phase and crystal seeds after nucleation heat treatment. The main crystal phase is clinopyroxene, Ca(Ti, Mg, Al)(Al, Si)O₆, of crystallized glass.     

Effects of hydration,annealing, and melting on heat transport properties of fused quartz and fused silica from laser-flash analysis

Thermal diffusivity (D) at high temperature (T) was measured from 15 samples of commercial SiO₂ glasses (types I, II, and III with varying hydroxyl contents) using laser-flash analysis (LFA) to isolate vibrational transport, in order to determine effects of impurities, annealing, and melting. As T increases, D_glass decreases, approaching a constant (~ 0.69 mm²s^? 1) above ~ 700 K. From ~ 1000 K to the glass transition, the slope of D is small but variable. Increases of D with T of up to 6% correlate with either low water and/or low fictive temperature and are attributed to removal of strain and defects during annealing. Upon crossing the glass transition, D substantially decreases to 0.46 mm²s^? 1 for the anhydrous melt. Hydration decreases D_glass, makes the glass transition occur over wider temperature intervals and at lower T, and promotes nucleation of cristobalite from supercooled melt. Due to the importance of thermal history, a spread in D of about 5% occurs for any given chemical type. Combining prior steady-state, cryogenic data with our average results on type I glass provides thermal conductivity (k_lat = ρC_PD) for type I: k_lat increases from ~ 0 K, becoming nearly constant above 1500 K, and drops by ~ 30% at T_g. We find that D^? 1(T) correlates with thermal expansivity times temperature from ~ 0 K to melting due to both properties arising from anharmonicity.