Polycrystalline membrane growth

Very little is known about the influence of grain growth, or crystallization if the membrane is composed of an amorphous alloy, on membrane durability. The as-fabricated permselective metal membrane will be polycrystalline or amorphous, depending on the alloy composition and fabrication method. Amorphous, or metallic glass, structures are far less common than are polycrystalline structures. Both amorphous and polycrystalline structures are quasi-stable, meaning that structures are kinetically stabilized and slow to rearrange to the thermodynamically favored structure. In both cases, this would be a single crystal of the metal. 

Polycrystalline zeolite membranes consist of inter-grown zeolite crystals with no apparent cracks or pinholes (Fig. lA). These films are composed of only zeolite (i.e., there are no non-zeolite components such as amorphous silica or polymer). They are normally supported on a substrate although free-standing films have also been synthesized. Membranes can be prepared on different substrates such as silicon wafer, quartz, porous alumina, carbon, glass, stainless steel (SS), gold, etc. Polycrystalline films are primarily prepared by hydrothermal synthesis methods including in situ crystallization, seeded growth,and vapor transport, " and have potential use in all of the applications discussed in this entry. 

The term polysilicon arises from the structure of these silicon layers, which are essentially polycrystalline in contrast to single-crystalline silicon substrates, due to the growth of these films on amorphous starting layers (in silicon surface micromachining, a silicon oxide layer is usually used as both a seed layer and a sacrificial layer). Polysilicon is widely used in sensor technology it can be used as part of a membrane layer, as an electrical connector, or as a part of a thermopile structure. In this contribution, we focus on its most important function - as the functional layer in surface-micromachined structures. In surface micromachining basically two approaches for producing polysilicon films are used ... 

Fig. 4.14 a, b. Epitaxial deposition of minerals within vesicles is limited by the spherical geometry of the compartments a aqueous ions can be transported across the vesicle membrane and precipitated on an atomic net located at the inner membrane surface b epitaxial growth from the membrane sites results in incoherent development of the mineral nuclei and the formation of a polycrystalline material... 

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