Effect of Al2O3 addition on the thermal expansion of sodium alkaline-earth silicate glasses: A molecular dynamics study

Coefficient of thermal expansion (CTE) is an important property to consider when utilizing oxide glasses in thermal treatment processes to avoid thermal damage at the interfaces of the glasses with heterogeneous materials. It is thus important to know the effect of additives on CTE for designing glasses. The use of alumina efficiently improves chemical and mechanical durability of oxide glasses while maintaining the functionality and productivity; however, alumina-doping often induces nonlinear variation of CTE. In this work, we therefore tried to investigate the relationship between CTE and the microstructure of sodium alkaline-earth aluminosilicate glasses using classical molecular dynamics (MD) simulations. To accurately model the glasses, we extended a force-matching potential by optimizing the parameter sets for Ca–O, Mg–O, and Na–O pair interactions using Bayesian optimization. The MD simulations reproduced the nonlinear variation of CTE as a function of alumina content, and detailed structural analyses identified inhomogeneous expansion in the glasses. It was found that the nonuniform CTE change at the nanoscale was related to the formation of an alumina-rich region, in which more fivefold-coordinated aluminum exist, when alumina content exceeded Na₂O content. Accordingly, the microstructural change by alumina-doping was identified as the origin of the nonlinear variation in the CTE of the glasses.     

Molecular dynamics simulation study on the structure and properties of polyimide/silica hybrid materials

A type of polyimide/silica (PI/SiO₂) copolymer model was established through the dehydration of tetraethyl orthosilicate molecules (TEOS) and bonding to a silane coupling agent. The content of SiO₂ was controlled by adjusting the number of molecules which bound to the TEOS. Finally, the silica was formed into a hybrid model (hybrid PI/SiO₂) with a small molecule embedded in the PI. The model was optimized by geometric and molecular dynamics and the changes in the model structure, Young's modulus, shear modulus, and glass-transition temperature (T _g) were analyzed. The results showed that the density and cohesive energy density of the composites could be improved by doping SiO₂ in PI. Young's modulus and shear modulus of PI/SiO₂ hybrid materials were higher than undoped PI. The tensile strength reached 568.15 MPa when the doping content was 9%. Therefore, the structure design and content control of SiO₂ was an effective way to improve the performance of a PI/SiO₂ composite. The variation of T _g and tensile strength of PI/SiO₂ hybrid composites is consistent with that of PI/SiO₂ composite synthesized in real experiment, which will be a convenient method for new material design and performance prediction. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47335.     

Structure and vibrations of cerium in silica glass from molecular dynamics simulations

Molecular dynamics simulations are performed to investigate the effect of cerium on the structural and vibrational properties of silica glass. At low-concentration levels, the cerium ions tend to generate longer bonds with bridging oxygens than nonbridging ones, the proportion of which is associated with the average bond length varied with cerium coordination. Formed in the presence of cerium, the bond angles exhibit strong dependence on the types of Ce-O bonds that bring about different angular distributions. Despite the discrepancy in the structures between Ce³⁺ and Ce⁴⁺, similar characteristics of vibrations are observed for the two states. In comparison with the glass formers, the vibrations of cerium that contribute primarily to the low-frequency region show a less localized behavior, whereas the acoustic-like and optic-like modes separate at a much smaller frequency.     

Correlation between IR peak position and bond parameter of silica glass: Molecular dynamics study on fictive temperature (cooling rate) effect

The fictive temperature of glass is a consequence of its thermal history (cooling rate, primarily) and has a direct effect on physical and chemical properties of the glass. But, it is not easy to measure. The ability to nondestructively and spectroscopically measure it at room temperature would be of great benefit. Although empirical correlations have been established between fictive temperature and selected absorption peaks in the infrared spectra of silica glass, the fundamental understanding for this correlation has not been reported. Here, we use molecular dynamics simulations to show that the blue shift in the Si–O–Si asymmetric stretching peak of pure silica glass, which is known to correlate with a decrease in fictive temperature, can be attributed to a decrease in the average length of the Si–O bond in the silica network, not changes in the density or the Si–O–Si bond angle. The decrease in density at higher fictive temperatures of silica is associated with a decreased population of 5‐ and 6‐membered rings and broadening of the ring‐size distribution, and an increase in the average Si–O–Si bond angle.     

Molecular dynamics study of UO2 symmetric tilt grain boundaries around [001] axis

Molecular dynamics and CRG empirical potential are used in this work to study the symmetrical tilt grain boundaries around the [001] axis in UO₂. The analysis of atomic structures obtained by simulation shows excellent agreement with the Read and Schokley model (Read WT, Shockley W. Dislocation Models of Crystal Grain Boundaries. Physical Review. 1950;78:275) predicting the existence of regular dislocations in these grain boundaries. We calculated their energy of formation and cleavage as well as the energy of formation of Schottky defects and incorporation of xenon and krypton atoms in their proximity. This allowed us to determine how these properties evolve for this series of grain boundaries presenting similar geometric characteristics, as a function of the misorientation angle. In addition, the boundary between small and large misorientation grain boundaries has been determined around 20°, close to the value of 15° reported in literature.     

Si改性对NiMo/Al2O3催化剂加氢脱硫性能的影响

以中和法合成的不同SiO₂含量的改性氧化铝为载体,本文制备系列Si改性的NiMo/Al₂O₃催化剂,采用X射线衍射（XRD）、N₂物理吸附（BET）、程序升温脱附（NH₃-TPD）、吡啶吸附红外光谱（Py-IR）、程序升温还原（H₂-TPR）、高分辨透射电镜（HRTEM）和X射线光电子能谱（XPS）等分析手段进行详细表征。表征结果显示,引入Si减弱了活性金属与载体之间的相互作用,改善了催化剂的孔结构与表面酸性分布,提高了活性相分散度和金属硫化度,促使形成更多的II类NiMoS活性相。以二苯并噻吩（DBT）为模型化合物,在固定床加氢装置上考察了系列催化剂的加氢脱硫（HDS）性能,结果表明,引入Si可降低DBT的加氢反应活化能,提高反应速率常数,进而提高催化剂的加氢脱硫活性。对比DBT转化率在50%时的脱硫产物分布表明引入Si可影响催化剂的反应路径选择性,直接脱硫路径（DDS）选择性从83.69%增加至92.89%,证实了催化剂的表征规律。     

Transparent polycrystalline nanoceramics consisting of triclinic Al2SiO5 kyanite and Al2O3 corundum

Transparent polycrystalline nanoceramics consisting of triclinic Al₂SiO₅ kyanite (91.4 vol%) and Al₂O₃ corundum (8.6 vol%) were fabricated at 10 GPa and 1200‐1400°C. These materials were obtained by direct conversion from Al₂O₃‐SiO₂ glasses fabricated using the aerodynamic levitation technique. The material obtained at 10 GPa and 1200°C shows the highest optical transparency with a real in‐line transmission value of 78% at a wavelength of 645 nm and a sample‐thickness of 0.8 mm. This sample shows equigranular texture with an average grain size of 34 ± 13 nm. The optical transparency increases with decreasing mean grain size of the constituent phases. The relationship between real in‐line transmission and grain size is well explained by a grain‐boundary scattering model based on a classical theory.     

Molecular dynamics study on nano‐particles reinforced oxide glass

Energy release rate and fracture toughness of amorphous aluminum nanoparticles reinforced soda‐lime silica glass (SLSG) were measured by performing fracture simulations of a single‐notched specimen via molecular dynamics simulations. The simulation procedure was first applied to conventional oxide glasses and the accuracy was verified with comparing to experimental data. According to the fracture simulations on three models of SLSG/‐Al₂O₃ composite, it was found that the crack propagation in the composites is prevented through following remarkable phenomena; one is that a‐Al₂O₃ nanoparticles increase fracture surface area by disturbing crack propagation. The other is that the deformation of a‐Al₂O₃ nanoparticle dissipates energy through cracking. Moreover, one of the models shows us that the crack cannot propagate if the initial notch is generated inside a‐Al₂O₃ nanoparticle. Such strengthening is partly due to the fact that the strength of the interface between nanoparticle and SLSG matrix is comparable to that of SLSG matrix, implying that their interface does not reduce crack resistance of the oxide glass.     

Effect of CeO2 and Al2O3 contents on Ce‐ZrO2/Al2O3 composites

Effect of CeO₂ and Al₂O₃ contents on phase composition, microstructures, and mechanical properties of Ce–ZrO₂/Al₂O₃ composites was studied. The CeO₂ content in CeO₂–ZrO₂ varied from 7 to 16 mol%, and the Al₂O₃ content in Ce‐ZrO₂/Al₂O₃ composites were 7 and 22 wt%. When CeO₂ content was ≤10 mol%, high Al₂O₃ content contributed to hinder the tetragonal‐to‐monoclinic ZrO₂ phase transformation during cooling and decrease the density of microcracks in the composites. Tetragonal ZrO₂ single‐phase was obtained in the composites with ≥12 mol% CeO₂, regardless of the Al₂O₃ content. Hardness, flexural strength, and toughness were dependent on CeO₂ and Al₂O₃ contents which were related to the microcracks, grain size, and phase transformation. The high flexural strength and toughness of the composites with 7wt% Al₂O₃ could be obtained at an optimum CeO₂ content of 12 mol%, whereas those of the composites with 22 wt% Al₂O₃ could be achieved in the wide CeO₂ content range of 8.5‐12 mol%.     

Effect of Al3+ on the spectroscopic properties of Eu2+- and Tb3+-Doped high silica glass

In this study, Eu²⁺/Al³⁺-, Eu²⁺/Tb³⁺-, and Eu²⁺/Tb³⁺/Al³⁺-doped high silica glasses were prepared, and the effect of Al³⁺ on the spectroscopic properties of Eu²⁺ and Tb³⁺ and the energy transfer of Eu²⁺→Tb³⁺ was investigated. The results revealed that the addition of Al³⁺ increased the luminescent intensity of the Eu²⁺-doped glass by approximately 400 times. An obvious Eu²⁺→Tb³⁺ energy transfer occurred in the Eu²⁺/Tb³⁺-doped high silica glass, whose transfer efficiency can reached 66.9%. However, the efficiency continuously decreased as the Al³⁺ concentration increased, and eventually, it dropped to 7.2%. These phenomena can be explained by the following: Al³⁺ not only disperses Eu²⁺ but also changes the field strength and symmetry of its neighboring environment, thus increasing its emission intensity. Moreover, Al³⁺ destroys the strong dipole–dipole interactions between Eu²⁺ and Tb³⁺ in high silica glass due to its dispersive ability, which inhibits the energy transfer of Eu²⁺→Tb³⁺. This indicates that a small amount of Al³⁺ can significantly modify the neighboring environment of Eu²⁺ and change its light-emitting characteristics, potentially yielding efficient blue and green phosphors excited by near-ultraviolet light for use in white light-emitting diodes.     

Gamma-ray irradiation effect on microstructure and physical performances of porous silica

In this paper, the gamma irradiation effect on the microstructure and physical performances of porous silica, including mechanical, thermal, and optical performances, are systematically investigated by using molecular dynamics and density-functional theory-based methods. The study of bond angle distribution, pair distribution function, coordination number distribution, and average ring size distribution show that, after gamma-ray irradiation, the microstructure of porous silica is obviously modified. The tight packing of SiO₂ tetrahedrons in the porous silica network is broken by gamma-ray irradiation. Defects of three-coordinated Si and non-bridging oxygen are induced by gamma-ray irradiation. Moreover, we find that the defects concentrations rapidly grow as gamma-ray dose increases. The mechanical, thermal, and optical performances of porous silica are all seriously degenerated by gamma-ray irradiation. Our results show that, for mechanical performance, Young's modulus, Bulk modulus, and Shear modulus first decrease and then keep stable as gamma-ray dose increases, but the change of Poisson's ratio is slight. For thermal performance, the thermal conductivity decreases exponentially as gamma-ray dose increases. For optical performance, light absorption coefficients increase exponentially and light transmittance drops as gamma-ray dose increases in the working range (photon energy range around 3.5 eV) of inertial confinement fusion. Present work is expected to be valuable for studying the degradation mechanism of silicate materials under gamma radiation and developing gamma-ray irradiation protection technology.     

硅溶胶结合Al2O3-SiC-C铁沟浇注料的力学性能研究

为了研究硅溶胶结合Al2O3-SiC-C材料的力学性能,以电熔棕刚玉和碳化硅为主要原料,硅溶胶为结合剂,制备了Al2O3-SiC-C铁沟浇注料,研究了其在110、300、500、700、900、1 100、1 300、1 450℃热处理后的常温物理性能和高温(1 400℃)抗折强度,并借助XRD、SEM等进行物相和显微结构分析。结果表明:随着热处理温度的升高,试样常温强度增加,烧后线变化率增大,体积密度先减小后增大,显气孔率先增大后减小,转折温度在700℃;高温抗折强度超过6 MPa。其原因在于:在中低温下,硅溶胶脱水形成—Si—O—Si—凝胶网络结构,保证了浇注料的中低温强度,700℃时因试样大量脱水而使得显气孔率最大,体积密度最小;在高温下,试样中因形成大量纤维状莫来石而为浇注料提供了较高的常温强度和高温强度。     

Preparation and properties of ferromagnetic glass ceramics and glass fibers based on the composition of SiO2-B2O3-Fe2O3-SrO

The ferromagnetic glass ceramics in the system SiO₂-B₂O₃-Fe₂O₃-SrO were prepared via four different fabrication methods, i.e., fiber-drawing, melt-quenching, natural-cooling, and annealing, without performing any nucleation and crystallization heat treatments. The influences of chemical composition and fabrication method on the spontaneous crystallization of magnetite were investigated by X-ray diffraction, scanning and transmission electron microscopy. The X-ray diffraction patterns show the presence of nanometric magnetite crystals in the glass matrix, and the increasing boron oxide can promote the spontaneous crystallization of magnetite. The estimated size of crystallized magnetite varies between 12 and 50 nm. The magnetic properties of the glass ceramics derived from the four fabrication methods were analyzed using a Vibrating sample magnetometer (VSM), Agilent HP8722ES vector network analyzer and Mössbauer spectra. We find that both the saturation magnetization (M_S) and coercivity (H_jc) depend on the chemical composition and fabrication method. The calorimetric measurements were carried out using Orton Standard Dilatometers.     

Effect of Al3+ on the photoluminescence and radioluminescence properties of Mn2+-Doped high silica glass

Mn²⁺/Al³⁺-, Eu²⁺/Mn²⁺-, and Eu²⁺/Mn²⁺/Al³⁺-doped high silica glasses were prepared to investigate their photoluminescence and radioluminescence properties. Particularly, the effect of Al³⁺ on the energy transfer from Eu²⁺ to Mn²⁺ and the regulation law of Mn²⁺ luminescence properties in Mn²⁺/Al³⁺- and Eu²⁺/Mn²⁺/Al³⁺-doped glasses were investigated. A strategy to improve the radioluminescence intensity of glasses was developed, and the radioluminescence intensity formula was improved. The dispersion caused by Al³⁺ improves the luminous intensity of Mn²⁺ and lowers the efficiency of the Eu²⁺→Mn²⁺ energy transfer in high silica glass. Al³⁺ can significantly broaden the excitation spectrum of Mn²⁺ by affecting the coordination number of Mn²⁺ and changing the field intensity around Mn²⁺ ions. Notably, the developed glass can potentially be applied in laser and LED lights. In addition, nonbridging oxygens (NBOs) are considered to be the main factor leading to the low efficiency of glass radioluminescence. Al³⁺ is able to reduce the relative amount of NBOs by forming [AlO_4/2]^?1 tetrahedra, thereby improving the radioluminescence intensity effectively.     

Molecular dynamics study on the viscosity of glass-forming systems near and below the glass transition temperature

Temperature-dependent viscosity is critical to decipher two profound questions in condensed matter physics, namely the glass transition and the relaxation of amorphous solids. However, direct measurement of viscosity over a large temperature range is extremely difficult. Here, using classical molecular dynamics (MD) simulations, we report a novel method to calculate the equilibrium viscosity of supercooled liquid both above and below the glass transition temperature (T_g) and to estimate the nonequilibrium viscosity of glass down to room temperature. Based on the shoving model, we derived an analytical formula showing that the shear viscosity in logarithmic scale changes linearly with the shear-induced variation in shear modulus or potential energy of the glass-forming system. The shear viscosity as a function of steady-state potential energy of liquid under different shear strain rates can be directly calculated in MD simulations; together with its equilibrium potential energy, one can extrapolate the zero-strain-rate equilibrium viscosity. We verified the proposed model by reliably calculating equilibrium viscosity near T_g of four glass-forming systems (Kob–Andersen system, silica, Cu_45.5Zr_45.5Al₉, and silicon) with different fragilities. Furthermore, our model can estimate the nonequilibrium viscosity of glass below T_g; the upper-bound nonequilibrium viscosity of amorphous silica and silicon at room temperature are calculated to be ~10³² and 10²⁵ Pa·s, respectively.     

Molecular dynamics simulation of the synergistic effect of a compound surfactant on the stability of CO2 oil-based foam

It is of great significance to study the stability of foams in the petroleum industry. Therefore, the stability mechanism of Span 20, the fluorinated surfactant FCO-80 and their mixture FS in a CO₂ oil-based foam system were studied by molecular simulation. The sandwich model of CO₂ oil-based foam was constructed to reveal the stability of the foam system from the microscopic perspective. The result shows that under the synergistic effect of Span 20 and FCO-80, the oil–CO₂ distance of the FS foam system and the coordination number of oil molecules are larger than those of Span 20 and FCO-80 foam system. In FS foam system, the diffusion coefficients of CO₂ molecules are small, and the surface tension is reduced, which can improve the stability of foam. The results can supplement previous experimental results on the stability of oil-based foam.     

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.     

Effect of the addition of silica sol on layered extrusion forming of Al2O3-based cores

Layered extrusion forming of ceramic cores with a nanoceramic suspension as a binder was conducted to explore a novel method to produce complex-shaped ceramic cores. Green bodies were prepared using Al₂O₃ particles as precursor materials and silica sol combined with aqueous polyvinyl alcohol solution as a binder. Increasing the silica sol content increased the viscosity of the slurry, enhanced the green bending strength, and decreased the green linear shrinkage. The green microstructure showed the nanosized silica particles were deposited on the surface of the Al₂O₃ particles and among the pores formed by Al₂O₃ particles irregular packing. In addition, increasing the silica sol content increased the bending strength, however, decreased linear shrinkage and open porosity of the sintered bodies. During sintering, the nanosized silica particles converted to the melting phase and reacted with Al₂O₃ and the microstructure of sintered bodies indicated the existence of sintering neck with silica sol addition.     

Fabrication of Al2O3-coated carbon fiber-reinforced Al-matrix composites

Fine Al₂O₃ coating could be obtained from alumina sols modified by chelator acetylaceton, under exact control of the parameters. Al₂O₃ coating by the sol–gel method on the carbon-fiber (CF) surface was investigated in detail to improve the oxidation resistance of CFs. Further study focused on making the Al₂O₃-coated fiber-reinforced aluminum composite prefabrication. XRD, IR, TG–DTA, and SEM methods were used to analyze the alumina gels, the coated CFs, and the prefabrication. After the coating treatment, the oxidation resistance of the carbon fibers is enhanced, the wetting between the fibers and melting aluminum is greatly improved, and the tensile strength of CF/Al prefabrication is heightened. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 177–183, 1998