Densification Energy during Nanoindentation of Silica Glass

Load/unload displacement curves at room temperature (humidity 49%) for silica glass have been measured in the penetration range of 0.5–1.2 μm using a Vickers nanoindentation technique (load/unload speed 50 mN/s). Deformation energies have been estimated for the first time. The universal (dynamic) hardness, H _u, and elastic recovery, E _R, at the penetration depth, h _t, of 1.0 μm are H _u= 4.1 GPa and E _R= 0.7. The following energies for total deformation, U _t, elastic deformation, U _e, and plastic deformation (i.e., densification during loading), U _p, are obtained: U _t=190, U _e=135 and U _p= 55 kJ/mol at h _t= 0.5 μm and U _t= 139, U _e= 96 and U _p= 43 kJ/mol at h _t=1.0 μm. All these deformation energies increase with decreasing penetration depth. It is found that plastic deformation energies of 38–55 kJ/mol for 0.5 < h _t < 1.2 μm are very close to the activation energy (46–54 kJ/mol) for the recovery of densification in silica glass, but are very small compared with the single bond strength (443 kJ/mol for Si—O bond) of SiO₂.     

Preparation of Micrometer- to Sub-micrometer-Sized Nanostructured Silica Particles Using High-Energy Ball Milling

Nanostructured porous silica particles with sizes in the micrometer to sub-micrometer range are of great interest due to their potential applications as catalyst supports and nanocomposite materials. However, if these particles are to be used in industry, a process must be developed to affordably produce them on a large scale. This paper reports on a high-energy ball-milling process that has been used to create micrometer- to sub-micrometer-sized mesoporous silica particles starting from a silica xerogel prepared by a surfactant self-assembly sol–gel process. We have studied various milling conditions such as milling media (zirconia, stainless steel, or steel-centered nylon balls), milling time, and the presence of surfactants during milling and the resulting effect on particle size and pore structure. Results from transmission electron microscopy, scanning electron microscopy, X-ray diffraction, light scattering, and nitrogen adsorption demonstrate the feasibility of producing large quantities of nanostructured particles by this simple milling process.     

Formation of Nanocrystalline Transition-Metal Ferrites inside a Silica Matrix

Nanocrystalline transition-metal ferrites were synthesized inside an amorphous silica matrix by the sol–gel method. The formation of spinel ferrites began above 400°C, giving fine particles of about 10 nm at 800°C. This is associated with a specific role of the silica matrix, which facilitates the diffusion of the reacting cations, enhancing the ferrite formation. Above 1000°C the MnFe₂O₄ and CuFe₂O₄ nanoparticles lost their fine nature. The dried gels and crystalline materials were characterized by X-ray diffraction, thermal, FTIR, and BET analyses as well as by high-resolution scanning transmission electron microscopy.     

Nanostructure and Micromechanical Properties of Silica/Silicon Oxycarbide Porous Composites

The microhardness–nanostructure correlation of a series of silica/silicon oxycarbide porous composites has been investigated, as a function of pyrolysis temperature, T _p. The pyrolyzed products have been studied by means of scanning electron microscopy, mercury porosimetry, chemical analysis, solid-state ²⁹Si-NMR, X-ray diffraction, Raman spectroscopy, and microindentation hardness. Two distinct regimes are found for the microhardness behavior with T _p. In the low-temperature regime (1000°C ≤ T _p < 1300°C), the material response to indentation seems to be dominated by the large amount of pores present in the samples. In this T _p range, low microhardness values, H , are found (<110 MPa). Above T _p= 1300°C, a conspicuous H increase is observed. In this high-temperature regime ( T _p= 1300–1500°C), microhardness values are shown to notably increase with increasing pyrolysis temperature. The H behavior at T _p= 1300–1500°C is discussed in terms of (i) the volume fraction of pores and the average pore size, (ii) the bond density of the oxycarbide network, and (iii) the occurrence of a nanocrystalline SiC phase.     

Processing of Lightweight, High-Strength Porcelains Using an Alumina Cement to Replace Feldspars and Clays

New lightweight, high-strength porcelain bodies, using only nonplastic raw materials such as glass microspheres, quartz, and alumina cement, were fabricated and the effect of quartz particle size was investigated. Decreases in the green strength, relative to an increasing content of glass microspheres, were attributed to the decrease in the density and the relative decrease in the volume of alumina cement. The phases in the fired body were glass, α-quartz, cristobalite, anorthite, and a small amount of α-alumina. The large quartz particles (10–32 μm in size) could not be densified to closed porosity, because of underfiring, whereas smaller quartz particles (4–10 μm in size) permitted densification to closed porosity at 1300°C. The high flexural strength when using medium-sized quartz particles (6–20 μm, content of 30 wt%) was attributed to a stronger prestress and higher density that was due to better vitrification.     

二氧化硅气凝胶及其在保温隔热领域应用进展

二氧化硅气凝胶是目前已知最轻的固体材料,具有热导率低、孔隙率高和比表面积大等优点,被誉为新型超级保温隔热材料。然而,二氧化硅气凝胶自身存在力学性能差和制备成本高的问题,大大限制了其在保温隔热领域大规模推广应用。本文简述了二氧化硅气凝胶合成技术和力学性能增强方法,从制备过程控制、老化条件优化、热处理、纤维复合和高分子聚合物复合等方面分析了其对气凝胶性能和工艺的影响,重点介绍了近年来二氧化硅气凝胶保温隔热材料应用在航空航天、军工领域、工业管道、建筑保温以及新能源汽车等领域的研究进展,总结了其在各领域应用的技术挑战。指出未来需进一步拓展二氧化硅气凝胶的使用温区,利用共前体和化学交联等方法增强高温下的隔热性能,同时解决气凝胶纤维复材“掉粉”和微米级粉体分散不均匀等难题,尤其是新能源汽车等新兴应用领域发展迅猛,未来仍需针对新的应用需求对其合成技术进行设计和优化。     

高硅氧玻璃纤维制品烧结工艺可节能性探讨

为减少高硅氧玻璃纤维后处理过程中消耗的大量热能,对传统烧结工艺进行改良,研究烧结工艺的可节能性。研究发现原烧结过程是在敞开体系里进行,易损失大量热能;在封闭体系进行高硅氧玻璃纤维烧结处理,则可减少能量损失。经综合分析,最终确定新工艺至少可节能约25％。     

凝胶法制备熔石英纳米复合陶瓷工艺及性能的研究

对凝胶法制备熔石英纳米复合陶瓷的工艺进行了探讨,并对不同工艺参数下的材料性能进行了分析和对比,讨论了影响材料性能的主要因素,寻求其最佳工艺参数.研究结果表明,用凝胶法可以使纳米相粒子均匀分散于陶瓷基体中,并能方便快速制备熔石英纳米复合陶瓷,纳米相引入后明显改善了材料的烧结性能,增加了材料的密度和强度.