Many properties of functional materials are quite different at the nanoscale, because at this length scale, the surface/interface energy becomes comparable to the bulk energy. Thus, the nature of various phase transitions at the nanoscale (e.g., structural, electronic, magnetic, metal-insulator) is altered. In addition, in functional materials with many coupled order parameters, quantum effects can dominate the response. We use the term nanoscale with three different context-specific connotations: it could refer to a cluster of atoms or molecules, a confined geometry as in a nanoscale grain or a superlattice, and a nanoscale region in the bulk. This field is still in its infancy, and much needs to be learned in terms of nucleation and thermodynamics at this scale. Materials of interest that we consider in this issue include, but are not limited to, ferroics (ferroelectrics, ferromagnets, ferroelastics), multiferroics (magnetoelectrics, ferrotoroidics), and complex functional materials such as those that exhibit colossal magnetoresistance and high-temperature superconductivity, including the recently discovered iron pnictide superconductors. Superconductors provide a fertile ground for quantum phase transitions.

The physical phenomena and engineering applications of martensitic phase transformations are the focus of intense ongoing research. The martensitic phase transformation and functional properties such as the shape-memory effect and superelasticity are strongly affected by the crystal size at the nanoscale. The current state of research on the impact of crystal size on the phase stability of the martensite is reviewed summarizing experimental results of various nanostructured martensitic materials and discussing the corresponding theoretical approaches. The review outlines the effects of crystal size on the complex morphology of the martensite, leading to interface structures not encountered in coarse-grained bulk materials. The unique shape-memory properties of martensitic materials can persist even at the nanoscale. Nanocrystalline martensitic materials can be processed to obtain tailored functional properties in combination with enhanced strength. Structural changes of metallic nanowires are similar to those arising by martensitic phase transformations and also can lead to shape-memory effects, as predicted by atomistic simulations.

The physical properties that make “functional” materials worthy of their moniker frequently arise because of a phase transition that establishes a new kind of order as the material is cooled from a parent state. Such ordered states include ferroelectrics, ferromagnets, and structurally ordered martensites; because these states all break an orientational symmetry, and it is rare that one can produce the conditions for single domain crystallinity, the observed configuration is generally heterogeneous. However, the conditions under which domain structures form are highly constrained, especially by elastic interactions within a solid; consequently, the observed structures are far from fully random, even if disorder is present. Often the structure of the heterogeneity is important to the function, as in shape-memory alloys. Increasingly, we are surprised to discover new phases inside solids that are themselves a heterogeneous modulation of their parents.

Over decades of effort, investigations of the intrinsic phase transition behavior of nanoscale ferroelectric structures have been greatly complicated by materials processing variations and by the common and uncontrolled occurrence of spacecharge, which interacts directly with the polarization and can obscure fundamental behavior. These challenges have largely been overcome, and great progress in understanding the details of this class of phase transitions has been made, largely based on advances in the growth of high-quality, epitaxial ferroelectric films and in the theory and simulation of ferroelectricity. Here we will discuss recent progress in understanding the ferroelectric phase transition in a particular class of model systems: nanoscale perovskite thin-film heterostructures. The outlook for ferroelectric technology based on these results is promising, and extensions to laterally confined nanostructures will be described.

For decades, a kind of nanoscale microstructure, known as the premartensitic “tweed structure” or “mottled structure,” has been widely observed in various martensitic or ferroelastic materials prior to their martensitic transformation, but its origin has remained obscure. Recently, a similar nanoscale microstructure also has been reported in highly doped ferroelastic systems, but it does not change into martensite; instead, it undergoes a nanoscale freezing transition—“strain glass” transition—and is frozen into a nanodomained strain glass state. This article provides a concise review of the recent experimental and modeling/simulation effort that is leading to a unified understanding of both premartensitic tweed and strain glass. The discussion shows that the premartensitic tweed or strain glass is characterized by nano-sized quasistatic ferroelastic domains caused by the existence of random point defects or dopants in ferroelastic systems. The mechanisms behind the point-defect-induced nanostructures and glass phenomena will be reviewed, and their significance in ferroic functional materials will be discussed.

Shape-memory polymers (SMPs) offer a number of potential technical advantages that surpass other shape-memory materials such as shape-memory metallic alloys and shape-memory ceramics. The advantages include high recoverable strain (up to 400%), low density, ease of processing and the ability to tailor the recovery temperature, programmable and controllable recovery behavior, and more importantly, low cost. This article presents the state-of-the-art regarding SMPs. First, the architecture, type, and main properties of the traditional and recently developed SMPs are introduced. Second, structural and multifunctional SMP composites are summarized and discussed. These composites greatly enhance the performance of the SMPs and widen their potential applications. Finally, current applications of SMP materials in aerospace engineering, textiles, automobiles, and medicine are presented.