Obtaining ultimate functionalities in nanocomposites: Design, control, and fabrication
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Introduction Composites are a class of materials that combine two or more constituents into a form suitable for technological applications. Although each constituent retains its chemical and structural identity, the composite might display macroscopic multifunctionalities, superior to those of its parent constituents, or completely new functionalities. A classic example is fiberglass-reinforced plastics (FRPs), developed during the 1940s.1 The combination of glass fibers with a thermoset resin matrix launched a commodity industry that currently provides products in a diversity of market areas. Identifying the right combination of two or more known substances especially on the nanoscale has become an effective method for designing and developing entirely new materials with unique and desired properties not obtainable in existing materials. The search for environmentally friendly and application-specific smart materials using nanostructured composites, or nanocomposites, is a much-pursued effort. Nanocomposites, in which at least one of the phases has a dimension in the range of a few angstroms to tens of nanometers, have been recognized as one of the most appropriate materials systems for producing multifunctional properties. Indeed, they represent an engineering solution to synergistically integrate the properties of different materials in a single platform. Examples include polymer-based, metal-based, and oxide-based nanocomposites, as well as organic–inorganic hybrid materials.2
In nanocomposites, emergent behavior can be achieved by interfacing different materials at the nano- or mesoscales. In other words, the properties of a composite can be greatly enhanced in comparison with those of individual constituents. Even more appealing, novel and emergent properties that are not exhibited by any of the constituents in the composite can be produced through synergistic coupling interactions. The synergistic integration of known materials to form composites provides new opportunities and strategies for fabricating smart materials with desired properties for targeted applications.
Microstructures The macroscopic properties of nanocomposites depend strongly on the properties of their constituents and their microstructures. The detailed spatial arrangements of these constituents also play an important role in determining the ultimate functionalities of the nanocomposites. Although the properties of the constituents are often assumed to be known, the microstructures and boundary conditions between the two phases can largely control the properties of the resulting nanocomposites. Optimizing microstructures and interfaces is especially important in achieving desired device performance for specific applications (see the article in this issue by Hu et al.).3 Figure 1 shows the most commonly studied architectures of nanocomposites. The connectivity of each phase in the composite
Ce-Wen Nan, State Key Lab of New Ceramics and Fine Processing, and School of Materials Science and Engineering, Tsinghua University, China; cwnan@tsingh
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