Mullitization of Diphasic Aluminosilicate Gels

Recent studies have shown that the mullitization of diphasic aluminosilicate matrices comprising transitional alumina and amorphous silica occurs via a nucleation and growth process. Nucleation is preceded by a temperature-dependent incubation period. Following this incubation period, rapid nucleation of mullite occurs, producing about 1.8 × 10¹¹ nuclei/cm³, which remains constant throughout the rest of the transformation. Both incubation and mullite growth are thermally activated processes with apparent activation energies of 987 ± 63 and 1070 ± 200 kJ/mol, respectively. The growth rate of mullite grains under isothermal conditions is time dependent. An interpretation of these results is proposed on the basis of the nucleation and growth concepts of LaMer and Dinegar which supports the concept that the growth rate of mullite grains is controlled by the dissolution of transitional alumina into the amorphous matrix.     

Copper ⇌ Alkali Ion Exchange of Alkali Aluminosillcate Glasses in Copper-Containing Molten Salt: I, Monovalent Copper Salt, CuCl

Cu⁺⇌ R⁺ (R = Li, Na, and K) ion exchange experiments were conducted for 20R₂O·10Al₂O₃·70SiO₂ glasses in molten CuCl at 550°C in air and nitrogen atmospheres. The depth profiles of the copper incorporated into glasses were determined with an electron microprobe X-ray analyzer. The total amount of diffusing copper, M _t, strongly depended on the type of alkali ion in the glass and the ion-exchange atmosphere; i.e., M _t increased with increasing cationic size in the order Li < Na < K and M _t was greater in air than in nitrogen. The Cu ⇌ R⁺ ion exchange kinetics are discussed in detail.     

Effects of Al:Si and (Al + Na):Si ratios on the properties of the international simple glass,part II: Structure

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 glasses were characterized by neutron total scattering and ²⁷Al, ²³Na, ²⁹Si, and ¹¹B 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 Ca²⁺), ^[4]Al (nearly equally split Na⁺ and Ca²⁺), and ^[6]Zr (mostly Ca²⁺). 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.     

Atomic and micro-structure features of nanoporous aluminosilicate glasses from reactive molecular dynamics simulations

Long-term chemical durability of borosilicate glasses that makes them a widely accepted form of nuclear waste disposal is achieved through the formation of a porous aluminosilicate gel layer that provides passivity and limits the transport of water to the reaction front. Detailed understanding of the porous silicate gel layer is thus critical in elucidating the corrosion mechanism of these glasses and to design of new glass composition for waste immobilization and other applications. In this paper, we use the diffuse charge reactive potential to generate porous aluminosilicate glass structures with compositions equivalent to the gel layers formed at the glass-water interface with an aim to understand the processing condition on the microstructure and atomic structure of these systems. We demonstrate the use of the charge scaling techniques is an effective approach to generate these porous structures with controllable pore mophologies. After initial validation of the potentials and calcium aluminosilicate glass structures using neutron diffraction, we created gel structures with compositions similar to well-known model nuclear waste borosilicate glasses. The porosities and the pore size distribution bear a strong correlation to the processing temperature, as well as to the local atomic structure. Thus, by controlling the processing parameters, the generated porous structures can be customized to closely resemble gel structures due to borosilicate glass corrosion. These results provide insights of the micro- and atomic structure features of the porous aluminosilicate glasses and on the optimal procedure to generate porous structures that can be comparable to experimentally observed gel layer structures thus to elaborate on the correlations between the structure and phenomena in glass-water interactions.     

Investigation of Al-O-Al sites in an Na-aluminosilicate glass

This paper reports the presence of Al- O- Al linkages in an aluminosilicate glass where Si/Al = 1 by using 2D¹⁷O triple quantum MAS NMR technique (3Q MASNMR). The experiments were performed at external magnetic fields of 8.4 and 14.4T. Despite¹⁷OMAS NMR spectra of the sample in both fields do not give much information about the different kinds of linkages in the sample, 3Q MAS NMR spectrum shows clear evidence that there are some amounts of Al-O-Al linkages in the sample giving two completely resolved peaks. These two peaks were attributed to the Si-O-Al and Al-O-Al linkages on the basis of their chemical shifts and, quadrupolar coupling constants which are quite sensitive to the local structure.     

Mullite–β-Spodumene Composites from Aluminosilicates

Sintering studies were conducted using kaolin, metakaolin, zeolite 4A, and various synthetic mixtures of Al₂O₃ and SiO₂ in the presence of Li₂CO₃ and LiCl as fluxing agents. Various compositions of the above were prepared, and conventional sintering studies were conducted at temperatures of 900°–1450°C with soaking periods of 1–3 h. Kaolin, metakaolin, and amorphized kaolin in the presence of Li₂CO₃ showed nucleation centers of β-spodumene as pink specks, whereas synthetic mixtures of Al₂O₃ and SiO₂ failed to behave in the same manner. To determine whether the pink specks formed were color centers or F centers, the samples were subjected to UV, IR, and X-ray irradiation; however, the samples showed no tenebrescence properties. External addition of iron as an impurity in a nonlayered system also resulted in pink speck formation. This observation indicated that impurities present in the natural kaolin were the cause of this phenomenon. Moreover, the LiCl-based samples did not result in pink specks, even though the kaolinitic samples contained iron as an impurity. Therefore, although β-spodumene was formed in aluminosilicates in the presence of Li₂CO₃ and LiCl, the pink variety of β-spodumene (kunzite) formation occurred only in the presence of lithium-rich aluminosilicates and in the presence of iron as an impurity. The phase identification and microstructure were explained based on XRD, DTA, and SEM studies.     

Effect of Al Speciation on the Structure of High‐Al Steels Mold Fluxes Containing Fluoride

To design suitable mold fluxes for the casting of high‐Al steels, the structure of mold fluxes based on CaO–SiO₂, CaO–SiO₂–Al₂O₃, and CaO–Al₂O₃ was examined by Raman spectroscopy and magic‐angle spinning nuclear magnetic resonance. The results showed that Si atoms are replaced by Al atoms as the network formers with the increase in Al₂O₃ in the mold fluxes. This converts the silicate slags (CaO–SiO₂ mold fluxes) into aluminosilicates slags (CaO–SiO₂–Al₂O₃ or CaO–Al₂O₃ mold fluxes). The F^? ions in the mold flux containing Al₂O₃ are classified into three categories, according to function: Bridging F's, Nonbridging F's, and Free‐F's. The Al³⁺ ion holds three distinct coordination environments: ^IVAl, ^VAl, and ^VIAl. The addition of F affects the coordination environment of Al³⁺ to form AlO₃F and AlO₂F₂ that accommodate the network structure of slags. The network structure in the CaO–SiO₂ mold fluxes is mainly connected through Si–O–Si linkage. However, the network structure of the mold fluxes containing elevated content of Al₂O₃ is mainly connected through Si–O–Si, Al–O–Al, Al–O–Si, and Al–F–Al linkages. Hence, the structural characteristics of high‐Al steels mold fluxes must be considered during the designing step of the mold fluxes.     

High toughness transparent glass-ceramics with petalite and β-spodumene solid solution as two major crystal phases

Lithium aluminosilicate glass-ceramics are well known for good transparency, high fracture toughness, low thermal expansion, and good ion exchange ability. In this study, new transparent Li₂O-Al₂O₃-SiO₂ (LAS) glass-ceramics with petalite and β-spodumene solid solution as the major crystalline phases were invented for favorable mechanical properties and potential for application in the hollowware, tableware, container, and plate glass industries. Crystal phases are mainly influenced by the ratio of Al₂O₃ to SiO₂ concentrations. The concentration of SiO₂ required to form specific crystalline phases in the glass-ceramics is higher than that inferred from the ternary phase diagram. Al₂O₃ content is required to be sufficiently high for the formation of crystals, instead of balancing excess amounts of Li₂O in the glass. The average transmittances of 2.0 ± 0.1 mm thickness samples in visible light regions (400–700 nm) can reach more than 80% with crystal sizes of 20–40 nm. Transmittance is significantly decreased for heat treatments around 710°C, due to the high growth rate of β-spodumene solid solution crystals. Vickers hardness, indentation toughness, and crack probabilities of transparent LAS glass-ceramics are significantly improved compared with standard soda lime silicate glass, due to the crack bridging and deflection of crystal grains.     

Incorporation of Al and Na in Hydrothermally Synthesized Tobermorite

Calcium silicate hydrate and its Al‐substituted form synthesized by a hydrothermal process were investigated by X‐ray diffraction, compositional analysis, and magic‐angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, in order to determine the mechanism of Al and Na incorporation in the tobermorite structure with varying molar ratios of Ca/Si and Al/Si. At a high molar ratio of Ca/Si, the silicate chains of tobermorite are ruptured, the degree of polymerization of the silicate chains is lowered, and the high calcium concentration lowers the content of Na₂O in the structure. Solid‐state ²⁹Si and ²⁷Al MAS NMR spectroscopy confirm that all Al atoms were incorporated in the silicate chains of tobermorite. The tetrahedrally coordinated Al (Al(IV)) could either act as the bridging tetrahedron () for the dreierketten chain of tobermorite, or be present in Q³ sites that link two dreierketten chains together. Therefore, the degree of polymerization of the silicate chains of tobermorite is increased at high molar ratio of Al/Si. Furthermore, the greater charge deficit due to the replacement of Si⁴⁺ by Al³⁺ ions is compensated by increased adsorption or binding of Na⁺.