Functionally graded nanocomposite materials for catalysis: From hard coatings to energy applications
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Introduction Functionally graded nanocomposite materials (FGNMs) are, to a certain extent, bioinspired materials since they are widely present in nature as soft and hard tissues, such as bone, bamboo, teeth, animal appendixes, and wood. One quality of FGNMs is the plentiful availability of numerous microstructural compositions and shapes similar to those found in nature, making their understanding and exploitation an appealing aspect for modern bioinspired engineering. FGNMs are defined as materials in which the chemical, structural, and physical properties change, in a rational manner, over their entire volume. These changes, as one could expect, lead to several variations on application, response, and behavior, the main attribute of this architecture. In the past few years, researchers have included “nanocomposite” under their names, aiming to emphasize the current technological capabilities of the architecture, or perhaps in order to attract the attention of researchers in other fields.1 Although it is arguable that the nanocomposite term is in fact implicit in the original field name, it seems important to include it, since current technology has allowed overcoming some of the original drawbacks of the architecture. The original term, functionally graded material (FGM) was introduced in the early 1980s in Japan,2,3 and was rapidly followed by a series of large Japanese national grants in order to develop the topic in areas such as
tribology and energy. It is worth mentioning that although the variation of functional response with composition and composite materials have been studied since the space race back in the 1960s, the application and proof-of-concept of a single functional graded material in the 1980s and early 1990s is considered an important milestone in materials engineering. Despite the broad range of functional properties that accompany the structural and chemical changes in materials, the use of FGMs has been mainly focused on the field of protective and hard coatings from the beginning. In their initial stages, FGMs based on carbide materials (such as TiC) were developed as high-temperature protective materials, capable of withstanding large temperature gradients, thermal stresses, and strains. While much understanding has been gathered in the past 40 years, and considerable research has provided a deeper understanding of the engineering, modeling, and preparation aspects of FGMs, little literature can be found using this architecture in other fields. Additionally, research outside of Japan was scarce, with some moderate presence in Europe.4 In the late 1980s and early 1990s, the use of FGMs in the thermoelectric field was investigated. In fact, it is feasible to imagine that a material that can withstand a thermal difference of several hundreds of degrees could exhibit a large thermoelectric voltage due to the thermally excited electrons building at one end of the material. This becomes especially attractive
Emerson Coy, NanoBioMedical Centre, Adam Mickiewicz University, Poznań, Poland; coyeme@
