Controlling the microstructure of vapor-deposited pentaerythritol tetranitrate films
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We have demonstrated that the microstructure of thick pentaerythritol tetranitrate (PETN) films can be controlled using physical vapor deposition by varying the film/substrate interface. PETN films were deposited on silicon and fused silica with and without a thin layer of sputtered aluminum to demonstrate the effects of the interface on subsequent film growth. Evolution of surface morphology, average density, and surface roughness as a function of film thickness were characterized using surface profilometry, scanning electron microscopy, and atomic force microscopy. Significant variations in density, pore size, and surface morphology were observed in films deposited on the different substrates. In addition, x-ray diffraction experiments showed that while films deposited on bare fused silica or silicon had only weak texturing, films deposited on a sputtered aluminum layer were highly oriented, with a strong (110) out-of-plane texture.
I. INTRODUCTION
Due to the nonequilibrium nature of physical vapor deposition (PVD) methods, resulting film properties are generally highly dependent on preparation parameters. Understanding the basic mechanisms governing the evolution of microstructure is essential to controlling film properties as the physical structure of a film is often directly related to its performance. Despite the importance of film microstructure, there is only a qualitative or semiquantitative understanding of the various physical structures that can be obtained and their origins. A great deal of phenomenological information is available on the relationships between deposition conditions and microstructure in inorganic films, which can be summarized in structure zone models (SZMs) that describe the evolution of film morphology under various deposition conditions.1–3 Although fewer data exist on the relationships between deposition conditions and morphology in vapordeposited organic films, previous studies have shown that varying parameters such as the substrate material,4–6 temperature,4–12 or deposition rate9,10,12,13 can have significant effects on the resultant microstructure. These studies have all been limited to very thin films (;1 lm or less) and efforts to categorize organic film morphologies produced under specified deposition conditions for a broad range of materials have been limited. However, a number of situations exist in which thicker organic films are required, and it is often advantageous to retain the
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.177
morphological control offered by PVD, such as in the case of explosive materials. Explosive materials have a very high energy density, but they require a certain material-dependent minimum thickness to detonate in a reproducible fashion. Microstructure has been shown to have a significant impact on the properties of explosive materials; differences in density, grain size, or pore size and distribution can lead to large changes in ignition sensitivity, detonation velocity, critical diameter, and other behaviors.14–21
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