Nanofibers
Experiment has shown that polymer- and metal oxide-based nanofibers are the most promising materials for application in gas sensors. Present chapter gives description of the approaches used for fiber preparation, and analyzes operating characteristics of
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Nanofibers
2.1
Approaches to Nanofiber Preparation
Various methods can be used for preparing polymer nanofibers (see Table 2.1), including drawing, hard and soft template synthesis, phase separation, self-assembly, and electrospinning (Jayaraman et al. 2004; Liu and Zhang 2009; Long et al. 2011). Among these methods, electrospinning seems to be the simplest and most versatile technique capable of generating 1D nanostructures (mainly nanofibers). Electrospinning is the technique which uses a strong electric field to produce polymer nanofibers from polymer solution or polymer melt (see Fig. 2.1). If electrostatic forces overcome the surface tension of a solution, a charged jet is ejected and moves toward a grounded electrode. Generally, the electrospun fibers are deposited on a fixed collector in a 3D nonwoven membrane structure with a wide range of fiber diameter distribution from several nanometers to a few micrometers (see Fig. 2.2). More recently, aligned electrospun fibers were obtained by using a rotating or prepatterned collector (Theron et al. 2001; Kameoka et al. 2003). One of the most important advantages of the electrospinning technique is that it is relatively easy and not expensive to produce large numbers of different kinds of nanofiber (Lu et al. 2009). Other advantages of the electrospinning technique are the ability to control the fiber diameters, the high surface-to-volume ratio, high aspect ratio, and pore size as nonwoven fabrics. Moreover, nanofiber composites can easily be made via the electrospinning technique with the only restriction being that the second phase needs to be soluble or well dispersed in the initial solution. The advantage of the facile formation of 1D composite nanomaterials by electrospinning affords the materials’ multifunctional properties for various applications. Electrospinning has been used to convert a large variety of polymers into nanofibers and may be the only process that has the potential for mass production. To date, it is believed that more than 100 different polymers have been successfully electrospun into nanofibers by this technique. It should be noted that composite-based nanofibers can also be synthesized using electrospinning (Huang et al. 2003b). One can find a detailed description of this method and fibers prepared using this method in reviews prepared by Huang et al. (2003b), Ding et al. (2009), and Lu et al. (2009). During the process of electrospraying, the liquid drop elongates with increasing electric field. When the repulsive force induced by the charge distribution on the surface of the drop is balanced by the surface tension of the liquid, the liquid drop distorts into a conical shape. Once the repulsive force exceeds the surface tension, a jet of liquid ejects from the cone tip. Small droplets form as a result of the varicose breakup of the jet in the case of low-viscosity liquids. If this phenomenon is applied to polymer solutions, a solid fiber is generated instead of breaking up into individual drops for the electrostatic repulsions between
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