What Are Nanostructured Materials?

Nanostructured materials are bulk solids or coatings whose internal architecture is organized at the nanometer scale in a way that imparts macroscopic properties not achievable in their conventionally processed counterparts. The defining feature is not merely that nanoscale constituents are present, but that the nanoscale architecture persists throughout the bulk and directly controls properties such as strength, conductivity, permeability, or optical response. Nanocomposites, nanoporous solids, nanocrystalline metals, thin films, and self-assembled block copolymer domains all fall within this category.

Nanocomposites and Nanocrystalline Materials

Nanocomposites incorporate a nanoscale filler phase dispersed within a matrix of polymer, ceramic, or metal. Even at low filler volume fractions (1 to 5 percent), the enormous interfacial area between matrix and filler fundamentally alters mechanical, barrier, and thermal properties. Clay-reinforced nylon nanocomposites, for example, show large increases in stiffness and gas-barrier performance relative to unfilled nylon, because individual clay platelets with aspect ratios exceeding 100 create tortuous diffusion paths for gas molecules. Carbon nanotube-reinforced polymers translate the exceptional axial stiffness of individual tubes into lightweight structural composites. A systematic review of nanocomposite mechanics is available through Nature Reviews Materials.

Nanocrystalline materials are polycrystalline solids with grain sizes below 100 nm, typically 5 to 50 nm. The Hall-Petch relationship predicts that yield strength increases as grain size decreases, because grain boundaries act as barriers to dislocation motion. Nanocrystalline metals such as nickel and copper can reach yield strengths several times those of their coarse-grained equivalents. However, below roughly 10 nm the relationship inverts: grain boundaries become so numerous that deformation shifts to grain boundary sliding rather than dislocation glide. This inverse Hall-Petch effect and strategies to suppress it are analyzed in papers indexed on IEEE Xplore in the context of nanostructured electrode materials.

Nanoporous Materials and Thin Films

Nanoporous materials contain a continuous network of pores with diameters in the 1 to 100 nm range. Mesoporous silica frameworks with highly ordered cylindrical pores, zeolites with sub-nanometer channels, and anodized alumina templates with tunable pore diameters are the most studied examples. The large internal surface area (up to 1000 m2/g in some mesoporous silicas) makes them outstanding candidates for catalysis, adsorption, and controlled release. Nanoporous metal-organic frameworks (MOFs) combine organic linkers and metal nodes to create porous solids with pore chemistries tunable at the molecular level, enabling selective gas separation and storage. The National Institute of Standards and Technology maintains measurement infrastructure for characterizing pore size distributions and surface areas of such materials.

Thin films with nanometer-scale layer thicknesses are a ubiquitous form of nanostructured material. Alternating multilayers of magnetic metals separated by non-magnetic spacers (a few nanometers thick) exhibit giant magnetoresistance (GMR), where electrical resistance drops substantially in the presence of a magnetic field. GMR discovery enabled hard disk read heads that transformed data storage density. Atomic layer deposition (ALD) produces conformal thin films with thickness controlled to a single atomic layer, enabling high-k gate dielectrics and diffusion barriers for advanced transistors.

Self-Assembled Structures

Block copolymer self-assembly produces periodic lamellar, cylindrical, or spherical nanodomains with periods of 10 to 100 nm throughout a bulk film or coating. The spatial arrangement and domain symmetry are controlled by the block length ratio and annealing conditions. These self-assembled templates can be converted to functional nanostructured materials by selective infiltration of inorganic precursors or by using one domain as a sacrificial template for nanoporous membranes. A detailed treatment of self-assembly thermodynamics and applications appears in ACS Nano via PubMed Central.

Applications

• Structural components: Nanocrystalline aluminum alloys and nanotube-reinforced composites reduce weight in aerospace and automotive structures while maintaining strength.
• Magnetic storage: GMR and tunneling magnetoresistance thin-film stacks are the sensing elements in hard disk and MRAM read heads.
• Catalysis: Nanoporous zeolite and MOF catalysts improve selectivity and conversion efficiency in refinery cracking and chemical synthesis.
• Energy: Nanostructured electrode materials increase the active surface area of fuel cell catalysts and battery electrodes, improving power density and cycle life.
• Barrier coatings: Clay-reinforced polymer nanocomposites reduce oxygen and moisture permeability in food packaging films.
• Drug delivery: Nanoporous silica and MOF carriers load and release therapeutics in response to pH, temperature, or light triggers.