(Invited) Nanowire-Based Aerogels for Energy Conversion and Stimuli Responsive Device Fabrication
Authors: Vaddiraju Sreeram 
Published: February 2020

Over the last few decades, nanowires have been demonstrated to offer the ability to control simultaneously, but independently, materials’ thermal, electrical, mechanical and optoelectronic properties. This was primarily achieved through variation in the constituent nanowires’ chemical compositions, morphologies or dimensions. Most of these demonstrations have been made using either single nanowire devices or small-scale nanowire arrays and mats. Translating these successes to achieve widespread deployment of nanowire-based devices requires strategies for the mass production of nanowires and their assembly into any desired form, such as nanowire networks composed of interconnected nanowires that have controlled porosities. More specifically, such nanowire networks are highly desirable in the fabrication of energy conversion devices (e.g., thermoelectrics) and stimuli responsive devices (e.g., thermoresponsive shape memory composites). In the case of thermoelectrics, assembly of nanowires into highly dense pellets is essential, while fabrication of composites requires highly porous interconnected nanowire networks (or nanowire aerogels) with controlled porosities.
Current literature on composites is dominated by carbon nanofiber-based/carbon nanotube-based composites. Thin films of nanowire-based composites, although not as extensively studied as carbon nanofiber-based/carbon nanotube-based composites, have typically been fabricated using either layer-by-layer methods or by casting mixtures of nanowires with polymeric/monomeric liquids. Study of these composites has mainly been limited to simultaneously enhancing the electrical conductivities and visible light transparency for use in flexible electrodes, or enhancing the mechanical strength of host matrices for use as reinforced composites. Efforts aimed at tuning the lengths of the nanofibers/nanotubes and the volume fractions of the nanofibers/nanotubes within the polymer matrices to ensure nanofiber-nanofiber/nanotube-nanotube contact and optimizing both the electrical conductivities and the light transparencies of the resulting composites have been successful. However, efforts aimed at binding polymer matrices to the nanowires using organic molecules for altering the mechanical strengths of the resulting nanowire-based composites has only lead to nominal increases in the mechanical strengths of the composites. This is owed to the method employed in the fabrication of composites, Using the process of employing mixtures of nanowires and monomers/polymers for casting composites thin films is fraught with multiple problems, including the lack of strong nanowire-nanowire bonding within the composites, nanowire bunching (or agglomeration), and the non-uniformity in the distribution of nanowires within the composites.
With the intent of overcoming the above-mentioned problems, we have developed all solid-state processes not only for mass producing nanowires, but also for assembling them into welded networks that have controlled porosities. Firstly, mass producing organic molecule functionalized nanowires was accomplished using chemical vapor deposition. The direct reactions of the component elements/molecules at low/moderate temperatures, without reliance on expensive catalysts/templates was employed for producing nanowires. In-situ exposure of the nanowires to organic molecules immediately following their synthesis was employed to functionalize them. Hot pressing of mixtures of nanowires and sacrificial materials, followed by mere heating to remove the sacrificial materials, was employed to assemble the nanowires into nanowire networks with controlled porosities. Infusing polymers into these nanowire networks allowed for the formation of compositionally- uniform composites for the study of their mechanical properties. Similarly, highly dense doped nanowire assemblies necessary for ascertaining their thermoelectric performances were obtained by pressure-assisted consolidation of mixtures of nanowires and dopant nanoparticles (e.g., copper nanoparticles for doping nanowires with copper).
In this talk, the strategies developed for the mass production of nanowires that involve the direct reaction of component elements will be presented (1). Strategies for mass producing nanowires in a by-product free manner will also be presented and discussed (2). The latter strategy eliminates the need to purify nanowires following their production. In-situ methods useful for functionalizing the nanowires with organic molecules immediately after their synthesis will also be discussed (1,3,4). Implementation of pressure-assisted strategies for the assembly of nanowires into welded nanowire networks will also be discussed. Finally, the electrical and thermal transport behavior of these nanowire assemblies will be discussed (3-6). Mechanical properties of nanowire-based composites and their thermoresponsive behavior will also be discussed in this talk. Illustrations will be presented using Si, Zn3P2, ZnO and Mg2Si nanowire systems as examples.
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