多尺度下多孔Pyrex玻璃材料内高压纳米氦泡的强韧化机制

Porous ceramics with ultra-low density are of great interest as potential engineering material in various industrial fields such as high temperature environment, et.al. The results of our previous studies have proven that the strength and toughness of porous Pyrex glass could be increased by tailoring the enclosed gas pressure of the pressurized argon micro-bubbles, which were “implant” in the glass by capsule-free hot isostatic pressing and subsequent annealing. And helium atoms could be fully dissolved in borosilicate glasses by two-step capsule-free hot isostatic pressing, in which relative lower gas pressure was applied in the first step and higher gas pressure was applied in the second step. However, their potential applications are limited because of the insufficient mechanical properties of porous glass with pressurized argon micro-bubbles. In the present research, we try to “implant” pressurized helium nano-bubbles in glass materials by annealing the glass with large amount of helium atoms under relative lower HIPing pressure at low temperature. By controlling the diffusion and assemble behavior of helium atoms under helium gas pressure atmosphere, sub-micrometer scale helium bubbles could be formed. The formation mechanism of pressurized helium nano-bubbles is clarified. And the mechanical behavior of porous materials with pressurized helium nano-bubbles is discovered. Combining with the nanometer size effect of helium nano-bubbles, the deformation-failure behavior and microstructure evolution of the pressurized helium nano-bubbles under different loading models in sub-micrometer scale are clarified. The influence of the inherent characteristic of the glass (pore size, pore size distribution, and enclosed gas pressure of helium nano-bubbles) on the mechanical properties of glass bulk is investigated. The inherent correlation between the mechanical behavior of sub-micrometer glass materials and the mechanical properties of glass bulk is described. And the multi-scale strengthening and toughening mechanism of pressurized helium nano-bubbles is revealed. By tailoring the microstructure, the mechanical properties of porous materials could be further improved. It is expected this new method will provide scientific foundations for the preparation of ultra-high specific strength porous materials with ultra-low density.

多孔陶瓷材料在轻质耐热承载等领域可发挥重要作用。前期通过热等静压烧结（HIP）和退火在Pyrex玻璃中形成微米尺度高压氩泡，发现通过调控气孔内压可提高多孔材料的强韧性，同时利用低-高压两步HIP可将数量可控的氦原子完全溶入玻璃中。针对微米孔材料强韧性不足的问题，本课题拟对溶氦玻璃进行降压HIP处理，通过调控高气压下氦原子在玻璃中的扩散和聚集行为，控制气泡尺寸至亚微米范畴，在玻璃中“种植”高密度的高压纳米氦泡。阐明高压纳米氦泡的形成机理，揭示高压纳米孔材料的力学特性。结合纳米尺度效应，阐明高压纳米氦泡在亚微米尺度的变形-失效行为和微观结构演变规律；研究纳米氦泡尺寸及其分布和气孔内压等内在特征对宏观力学性能的影响规律，初步建立从微观到宏观力学行为的跨尺度关联，揭示多尺度下高压纳米氦泡的强韧化机制。进一步掌握微观组织和性能的调控方法，改善材料的力学性能，为超轻高比强多孔材料的创新设计提供新思路。

在块体玻璃材料内“种植”高密度的高压纳米气泡，结合闭气孔内压增强和纳米尺度效应，材料可以获得优异的力学性能。本项目采用“高压固溶（两步热等静压法）-降压释放”惰性气体原子技术在Pyrex玻璃材料内形成微纳米尺度、孔径/内压力可调控的高压气泡。通过在多孔玻璃材料的闭气孔内填充高压气体，实现了材料的高性能化。通过收集高压熔融后的玻璃基体和低压处理后的多孔材料中的微量气体，结合热力学计算分析，精确测量了气孔内压值。材料的力学性能远高于同类型多孔玻璃材料，由于气孔内压提高了孔的承载能力。气孔率相近条件下，随着气孔压力增加，材料的弹性模量、比强度和断裂韧性大幅增加。多孔玻璃的断裂韧性高达0.9 MPa·m1/2，压缩强度达到123.5 MPa，弹性模量最高为8.8GPa。以具有分级结构的木材为模板，通过高温热解获得多孔碳，通过SiO气相与碳模板的原位碳热还原反应制备具有木材精细结构的多孔SiC陶瓷和气凝胶材料。在1800 ºC保温4h的工艺条件下，致密的孔壁由直径为100 ~200 nm、长度为100~500 nm的柱状纳米晶SiC组成。木材多孔SiC具有高孔隙率(86%)，高抗弯强度(σ||=37.8 MPa)和高导热系数(k┴ =4.77 W·m-1·K-1, k||=3.08 W·m-1·K-1)，远远优于其他方法制备得到的多孔SiC陶瓷。当孔隙率为78%时，木材SiC陶瓷的导热系数分别为10.3 W·m-1·K-1（k┴）和7.6 W·m-1·K-1 (k||)，对应的各向异性系数为1.6 (k┴ / k||)。