Structure of Sodium Aluminosilicate Glasses

A series of sodium aluminosilicate glasses composed of varying ratios ( R ) of Al₂O₃/Na₂O (0.25 R 2.0) has been simulated with the molecular dynamics technique using a tetrahedral form of a three-body interaction potential. Extrema in the activation energies for sodium diffusion and in the diffusion constants for all of the atomic species were observed for glasses with equal concentrations of Al₂O₃ and Na₂O ( R = 1.0). These changes corresponded to the minimum observed experimentally in the activation energy for electrical conductivity and to the maximum observed in the viscosity for glasses with compositions of R = 1.0. The coordination of aluminum remained 4 over the entire compositional range, negating the need to invoke a coordination change of aluminum to explain the changes in the physical properties. The changes to the simulated physical properties as R passed through the equivalence point were attributed to the elimination of nonbridging oxygen, to the introduction of oxygen triclusters, and to changes in the distribution of ring structures within the glass networks.     

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.     

非晶态硅酸盐微观结构的研究进展

总结了目前对非晶态硅酸盐微观结构的认识。综述了硅酸盐的分子动力学模拟技术的最新发展以及Raman光谱研究的最新结果。在简要叙述了其它的实验研究手段的基础上,将Raman光谱与它们进行了简单比较,阐明了Raman光谱在硅酸盐微观结构研究方面的优越性,总结了该研究领域需要进一步研究的内容。     

Thermochemistry of Glasses in the Y2O3-Al2O3-SiO2 System

Enthalpies of drop solution in molten 2PbO-B₂O₃ at 1078 K were measured for glasses along the 2YAlO₃-3SiO₂ and return ½Y₃Al₅O₁₂-3SiO₂ joins. The onset glass transition temperature increases slightly with increasing silica content and Y/Al. Enthalpies of mixing were calculated on the basis of amorphous end members. Strongly negative heats of mixing support the absence of miscibility gaps except possibly for very high silica content, consistent with experimental phase analyses, which indicate much narrower miscibility gaps compared with the phase diagrams calculated on the basis of previous data and the CALPHAD formalism.     

层状硅酸盐多孔材料研究现状(英文)

层状硅酸盐是制备多孔材料的优良原料,本文介绍了层状硅酸盐的结构特点,综述了层状硅酸盐多孔材料的水热合成法、结构重排法等制备方法,探讨了该类多孔材料的应用研究现状,并对其进行了对比分析。在跟踪国内外新近研究成果的基础上,提出了若干研究内容或方向。     

Effects of the nature of the aluminum source on the acidic properties of some mesostructured materials

Mesostructured aluminosilicates have been synthesized using gels prepared by reacting colloidal silica (Ludox AS) with Al(OH)₃, aluminum isopropoxide (Al(iPrO)₃) or NaAlO₂ in the presence of a surfactant. The hydrothermal transformation at 110°C of these gels produced solids with the hexagonal structure typical of MCM-41-type materials. These crystals have been characterized by X-ray diffraction, thermal analysis (TG/DTA), N₂ sorption and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The type, strength and density of acid sites have been studied using microcalorimetry and infrared (IR) spectroscopy.As a general trend, pore volume and average pore size decreased as the gel SiO₂/Al₂O₃ ratio decreased from 32–8, while the pore wall thickness remained in the 1.0–1.5 nm range. Except for one sample, Al was incorporated in tetrahedral coordination inside the pristine crystals. However, as expected, dealumination occurred upon calcination at 600°C/12 h, yielding materials having both tetrahedral and octahedral Al-species. Fourier transform infrared (FTIR) experiments with pyridine have indicated that these mesostructured aluminosilicates contain both Brönsted and Lewis acid sites and that acidity is strongest in samples prepared with NaAlO₂.Microcalorimetry experiments with ammonia as the probe molecule have shown that Al insertion into the mesoporous silicate framework affects acid site strength and distribution in a manner controlled by synthesis condition. Samples prepared with Al(OH)₃ contain a wide distribution of acid site strengths, indicating the absence of preferred locations of Si-O-Al groups within the pore walls. In contrast, distinct populations of acid sites with strengths in the 130–140 kJ mol⁻¹ range or near 150 kJ mol⁻¹, appear in materials prepared with Al(iPrO)₃ or with NaAlO₂, respectively. Al sites may be located at the surface of the pore wall, where they interact directly with the basic probe molecule. They may also be sandwiched between silica layers within the pore wall (3–4 Si layers thick), giving acid sites with a strength comparable with that of smectites. Finally, they may also be present in the pore (or within the pore walls) as extraframework Al(VI)-species. The nature, size, concentration, ease of hydrolysis and condensation of the aluminum precursors during synthesis control aluminum incorporation, distribution and location within the structure and with it the acidity of the resulting mesoporous aluminosilicate.     

Substitutional reaction in Si–O network of molecular dynamics-modeled liquid Na2SiO3: Microscopic and statistical study

The study of the bond breaking and formation processes, that is, the chemical reaction, in the Si–O network structure in liquid alkali silicates at temperatures around or higher than the glass-transition temperature is important for understanding kinetic processes such as the structural relaxation of the network, viscous flow, and diffusion of the network former ions. Herein, novel methods for analyzing the reactions in a molecular-dynamics-modeled liquid Na₂SiO₃ were used to confirm the following results: (a) the substitutional reactions (in which a nonbridging O ion of a Si–O chain or a SiO₄ tetrahedron attacks the Si ion of another chain from backside of a bridging O ion, which acts as the leaving group, and the bridging O leaves the Si ion) primarily occur in the Si–O network of liquid Na₂SiO₃; and (b) The abundance ratio of Q_n species can be quantitatively reproduced by the reaction rate.     

Fabrication of Macroporous Alumina with Tailored Porosity

Macroporous alumina materials were fabricated via colloidal processing using polymer spheres as the template and ceramic particles as the building blocks. The influence of the suspension conditions and volume ratio of the polymer/ceramic particles on the formation of the pore structure has been investigated. The results showed that the suspension conditions have a significant effect on the pore morphology. A well-defined three-dimensional, ordered porous structure with a controllable pore size and porosity could be obtained through the hetero-coagulation, self-assembled processing of the polymer/ceramic particles. The pore size and porosity could be easily tailored by varying the polymer size and the volume ratio of the polymer/ceramic particles.     

一种液相色谱新填充介质--多孔玻璃的特性与应用

本文综述了一种新型载体材料多孔玻璃的结构特点和制备原理,及其作为液相色谱新填充介质的良好特性和应用前景.