SiO2纳米孔中的毛细管冷凝、冻结和熔融：氮在介孔SBA-15中的吸附等温线和扫描量热法研究
Capillary Condensation, Freezing, and Melting in Silica Nanopores: A Sorption Isotherm and Scanning Calorimetry Study on Nitrogen in Mesoporous SBA-15
摘要：
用体积吸附等温线和扫描量热法研究了氮气在介孔二氧化硅颗粒（SBA-15）粉末中的冷凝、熔化和冻结过程。在Saam和Cole的圆柱形孔中蒸汽冷凝的平均场模型内，液氮吸附等温线很好地描述为双峰孔半径分布。它包括以3.3 nm为中心的典型管状中孔的窄峰和位于1 nm处的微孔的显著更宽的峰特征。在微孔中冷凝的材料以及在中孔中的前两个吸附单层不表现出任何热异常。凝固和熔融转变仅影响超过约100 ℃的孔隙冷凝物。中孔的第二单层。在这里，界面熔化导致比热测量中的单峰。均匀和非均匀的冻结，以及延迟过渡的部分填充的中孔导致的热量冻结异常类似的复杂性和依赖于热历史，如已观察到的氩在SBA-15。在孔隙空间中的结晶的轴向传播是更有效的情况下，氮比以前观察到的氩，我们归因于孔隙固体的晶体纹理的差异。
英文摘要：
Condensation, melting and freezing of nitrogen in a powder of mesoporous silica grains (SBA-15) has been studied by combined volumetric sorption isotherm and scanning calorimetry measurements. Within the mean field model of Saam and Cole for vapor condensation in cylindrical pores a liquid nitrogen sorption isotherm is well described by a bimodal pore radius distribution. It encompasses a narrow peak centered at 3.3 nm, typical of tubular mesopores, and a significantly broader peak characteristic of micropores, located at 1 nm. The material condensed in the micropores as well as the first two adsorbed monolayers in the mesopores do not exhibit any caloric anomaly. The solidification and melting transformation affects only the pore condensate beyond approx. the second monolayer of the mesopores. Here, interfacial melting leads to a single peak in the specific heat measurements. Homogeneous and heterogeneous freezing along with a delayering transition for partial fillings of the mesopores result in a caloric freezing anomaly similarly complex and dependent on the thermal history as has been observed for argon in SBA-15. The axial propagation of the crystallization in pore space is more effective in the case of nitrogen than previously observed for argon, which we attribute to differences in the crystalline textures of the pore solids.
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