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Solid-State NMR Studies of Zeolites and Related Systems
Published in Alexis T. Bell, Alexander Pines, NMR Techniques in Catalysis, 2020
C. A. Fyfe, Κ. T. Mueller, G. T. Kokotailo
The lattice frameworks of several common zeolites can be assembled from a single building unit, as shown in Fig. 1. The truncated octahedron or sodalite cage subunit (Fig. la) has faces of four- and six-membered rings where the vertices are Si or Al T-atoms joined by linking oxygens (not shown). When two of these are joined via their four-membered rings with bridging oxygens, the structure shown in Fig. lb is formed, and linking of two of these units together then gives the structure shown in Fig. lc. This is the basic lattice structure of zeolite A, which is not known in nature but is widely available from synthesis and commonly known in research laboratories as “molecular sieve.” There is a large central cavity accessible from three orthogonal straight channels. It is this pore and channel structure that allows the lattice to control the size of the molecules sorbed and the reaction products, thus establishing control of chemical reactions involving molecules of 10 Ä or smaller. The zeolite A based molecular sieves used in the laboratory are referred to by the size of molecules to which they are accessible. Depending on the size of the cations in the cavities, molecules of different sizes will be adsorbed. For example, if the cation is Na+, then molecules up to 4 Á in diameter can enter. The ions K+ and Ca2+ are present in 3 A and 5 A molecular sieves, respectively.
Epitaxial growth of 3C-SiC (111) on Si via laser CVD carbonization
Published in Journal of Asian Ceramic Societies, 2019
Rong Tu, Zhiying Hu, Qingfang Xu, Lin Li, Meijun Yang, Qizhong Li, Ji Shi, Haiwen Li, Song Zhang, Lianmeng Zhang, Takashi Goto, Hitoshi Ohmori, Marina Kosinova, Bikramjit Basu
The preparation of SiC by carbonization, the appearance of voids is usually difficult to avoid. The reason for the voids is occurring Si defects during the film’s growth due to Si atoms on the surface of the substrate taking part in the reaction. The loss of Si atoms on the surface is compensated by migration of Si atoms from the Si substrate. Because of the Kirkendall effect [20], it is then possible that the Si vacancies coalesce and form a small void beneath the SiC film at the layer/substrate interface. Figure 5(a,b) show the plan-view SEM of thin SiC film grown on a Si(111) substrate at a substrate temperature of 1150°C. The dark triangular and hexagonal features are voids in the substrate. Corresponding structural models of triangular and hexagonal voids are shown in Figure 5(c,d). These void facets are the energetically favorableable {111} planes: the {111} planes are the faces with the lowest surface energy (the free surface energy of Si: γ(111) = 1.23 J m−2) [21]. The hexagonal void shown in Figure 5(d) can be seen as a truncated octahedron obtained by cutting off a solid with eight faces [22]. (In the new solid, the corners are transformed into squares and the triangular faces of the octahedron turn into regular hexagons). Figure 6, the size of the voids increases with increases in the carbonization temperature, whereas the density of the voids decreases (Figure 6). At TC = 1000 °C, while the average void size is small (less than 2.0 μm), the void density is extremely high due to large numbers of small nucleating voids on the surface. At TC = 1200 °C, the average void size increase to about 4.0 μm, while the void density is reduced by an order of 1. In Figure 4, the films deposits show a relatively lower surface roughness because of a combination of relatively lower size and density of the voids.