Annual Review of Materials Research - Volume 42, 2012

In the world of tomographic imaging, atom probe tomography (APT) occupies the high-spatial-resolution end of the spectrum. It is highly complementary to electron tomography and is applicable to a wide range of materials. The current state of APT is reviewed. Emphasis is placed on applications and data analysis as they apply to many fields of research and development including metals, semiconductors, ceramics, and organic materials. We also provide a brief review of the history and the instrumentation associated with APT and an assessment of the existing challenges in the field.

Electron microscopy of biological matter uses three different imaging modalities: (a) electron crystallography, (b) single-particle analysis, and (c) electron tomography. Ideally, these imaging modalities are applied to frozen-hydrated samples to ensure an optimal preservation of the structures under scrutiny. Cryo-electron microscopy of biological matter has made important advances in the past decades. It has become a research tool that further expands the scope of structural research into unique areas of cell and molecular biology, and it could augment the materials research portfolio in the study of soft and hybrid materials. This review addresses how researchers using transmission electron microscopy can derive structural information at high spatial resolution from fully hydrated specimens, despite their sensitivity to ionizing radiation, despite the adverse conditions of high vacuum for samples that have to be kept in aqueous environments, and despite their low contrast resulting from weakly scattering building blocks.

In this short review, we discuss recent developments in electron tomography with examples across a range of materials science and nanotechnology. Challenges related to extending the resolution of the technique from the nanoscale to the atomic level are addressed, and the different routes proposed to meet those challenges are considered. We illustrate improvements in electron tomography brought about by recent developments in hardware and the advent of aberration-corrected microscopes. We focus also on developments in new reconstruction algorithms designed to enable reliable and accurate reconstructions from very limited projection data. These recent technique developments provide a genuine promise of routine 3D atomic imaging.

This paper reviews progress using X-ray computed tomography to study damage accumulation. Since its introduction, X-ray microtomography has been used to diagnose the presence of damage. In this review, a wide range of damage-accumulation mechanisms are covered including cavitation, fracture, microcracking, fatigue cracking, and stress corrosion cracking. In this regard, the advantages of attenuation and phase contrast imaging are discussed. This review includes both measurements of damage accumulation, taken postmortem, and the incremental monitoring of damage-accumulation processes during life (sometimes termed four-dimensional tomography). In addition to the qualitative diagnostic studies, the quantitative analysis of tomography images to extract key failure parameters is examined.

The experimental measurement of the evolution of interfaces in three dimensions is reviewed, concentrating on the evolution of polycrystalline and solid-liquid systems, including growth and coarsening in dendritic systems and evolution during liquid-phase sintering. Both ex situ destructive techniques and in situ nondestructive techniques are considered. The importance of making three-dimensional measurements that can be quantified and unambiguously compared with theory is discussed, showing that these measurements provide a direct validation of theory and critical initial conditions for simulations.

Aberration correction in the transmission electron microscope has led to a reduction in the depth of focus for imaging. The depth of focus in a state-of-the-art scanning transmission electron microscope system, which is currently just a few nanometers, creates an opportunity to explore the three-dimensional structure of a sample by focusing on specific layers, an approach known as optical sectioning. In this article, we review the performance of optical sectioning in the scanning transmission electron microscope. Limitations in the simple optical sectioning approach are used to motivate discussion of confocal electron microscopy. Three imaging modes in scanning confocal electron microscopy have been investigated both theoretically and experimentally, and are reviewed here. The method of implementing a confocal arrangement in a microscope is discussed, along with its comparative performance with other methods for three-dimensional imaging and analysis. Finally, current and future potential applications are discussed.

The thermomechanical properties of multiphase metals are determined by a combination of the properties of the microstructural phases and the internal architecture formed by them. The latter must be described using three-dimensional techniques in cases where the phases are distributed nonuniformly, have complex morphologies, form interconnected structures, and present contiguity between those structures. Furthermore, all these morphological aspects may change during service exposure. One of these techniques is X-ray tomography, which has experienced an increased interest from materials scientists during the past decade owing to the advances in spatial and time resolution to reveal nondestructively the internal structure of materials. The present review summarizes the main features of this technique in regard to its capabilities to image metal-based engineering materials three dimensionally. Special emphasis is put on the contributions of X-ray tomography to understand the relationships between architecture and thermomechanical properties with specific examples for lightweight metals, cast iron, steel, and metal matrix composites.

This article reviews studies in which X-ray tomography has been used to characterize the cellular microstructure or the deformation mechanisms of highly porous materials. The technique is suitable for imaging these materials with much detail. Such images can also be used to quantify the micro-structure (wall thickness and cell size distribution and tortuosity of the porous network). Finally, the methods available to produce finite element meshes from the three-dimensional images are presented and discussed in light of one example.

This review discusses recent advances in materials engineering to control thermal conductivity. We begin by presenting theories of heat conduction for general material classes, focusing on common approximations and trends. Next, we discuss characterization techniques for measuring thermal conductivity and the underlying transport properties. Advanced materials at the frontiers of thermal transport, such as rattlers, complex unit cells, nanowires, and nanocomposites, are treated in depth using experimental data and theoretical predictions. The review closes by highlighting several promising areas for further development.

Biofouling is a complex, dynamic problem that globally impacts both the economy and environment. Interdisciplinary research in marine biology, polymer science, and engineering has led to the implementation of bio-inspired strategies for the development of the next generation of antifouling marine coatings. Natural fouling defense mechanisms have been mimicked through chemical, physical, and/or stimuli-responsive strategies. This review outlines the detrimental effects associated with biofouling, describes the theoretical basis for antifouling coating design, and highlights prominent advances in bio-inspired antifouling technologies.

Self-cleaning surfaces have drawn a lot of interest for both fundamental research and practical applications. This review focuses on the recent progress in mechanism, preparation, and application of self-cleaning surfaces. To date, self-cleaning has been demonstrated by the following four conceptual approaches: (a) TiO2-based superhydrophilic self-cleaning, (b) lotus effect self-cleaning (superhydrophobicity with a small sliding angle), (c) gecko setae–inspired self-cleaning, and (d) underwater organisms–inspired antifouling self-cleaning. Although a number of self-cleaning products have been commercialized, the remaining challenges and future outlook of self-cleaning surfaces are also briefly addressed. Through evolution, nature, which has long been a source of inspiration for scientists and engineers, has arrived at what is optimal. We hope this review will stimulate interdisciplinary collaboration among material science, chemistry, biology, physics, nanoscience, engineering, etc., which is essential for the rational design and reproducible construction of bio-inspired multifunctional self-cleaning surfaces in practical applications.

Large elastic nonlinearities can lead to elastic hysteresis behavior, which defines ferroelasticity in analogy to its sister ferroics: ferromagnetism and ferroelectricity. Ferroelasticity is the most common nonlinear effect in natural materials and plays a major role in the mineralogical behavior of the Earth's crust and mantle. It produces interfacial twin walls that act as sinks and sources for defects and that show localized effects such as superconducting twin boundaries and ferroelectricity, even when such effects do not exist in the bulk. The movement of twin walls under elastic forcing is creep-like, with some superimposed jerks due to pinning and unpinning by defects and jamming by other twin boundaries. This review applies Landau theory and discusses some aspects of the emerging field of domain boundary engineering.

The influence of increasing strain rate on the mechanical behavior and deformation substructures in metals and alloys that deform predominately by slip is very similar to that seen following quasi-static deformation at increasingly lower temperatures or due to a decrease in stacking-fault energy (γsf). Deformation at higher rates (a) produces more uniform dislocation distributions for the same amount of strain, (b) hinders the formation of discrete dislocation cells, (c) decreases cell size, and (d) increases misorientation, with more dislocations trapped within cell interiors. The suppression of thermally activated dislocation processes in this regime can lead to stresses high enough to activate and grow deformation twins even in high-stacking-fault-energy, face-centered-cubic metals. In this review, examples of the high-strain-rate mechanical behavior and the deformation substructure evolution observed in a range of materials following high and shock-loading strain rates are presented and compared with those seen following quasi-static-loading deformation paths.

In the past 20 years, there has been a surge in research on the magnetocaloric response of materials, due mainly to the possibility of applying this effect for magnetic refrigeration close to room temperature. This review is devoted to the main families of materials suitable for this application and to the procedures proposed to predict their response. Apart from the possible technological applications, we also discuss the use of magnetocaloric characterization to gain fundamental insight into the nature of the underlying phase transition.

This review describes recent developments in stimuli-responsive biointerfaces based on surface-tethered organic molecules, polymer chains, and polymer networks. The existing systems are classified according to the length scale of transformations occurring in the stimuli-responsive material and interactions of the material with the biological environment. In particular, two types of biointerfaces are considered: those whose interactions with proteins and cells can be switched reversibly or one time due to stimuli-triggered changes in molecular conformations or chemical bonds in functional molecules, and those that undergo reversible stimuli-triggered reconstruction at mesoscale due to stimuli-responsive phase behavior. Specific examples of stimuli-responsive surfaces from the recent literature are supplemented with discussion of potential biomedical applications.

Superconductors offer major advantages for the electric power grid, including high current and power capacity, high efficiency arising from the lossless current flow, and a unique current-limiting functionality arising from a superconductor-to-resistive transition. These advantages can be brought to bear on equipment such as underground power cables, fault current limiters, rotating machinery, transformers, and energy storage. The first round of significant commercial-scale superconductor power-equipment demonstrations, carried out during the past decade, relied on a first-generation high-temperature superconductor (HTS) wire. However, during the past few years, with the recent commercial availability of high-performance second-generation HTS wires, power-equipment demonstrations have increasingly been carried out with these new wires, which bring important advantages. The foundation is being laid for commercial expansion of this important technology into the power grid.

Solid films are usually metastable or unstable in the as-deposited state, and they will dewet or agglomerate to form islands when heated to sufficiently high temperatures. This process is driven by surface energy minimization and can occur via surface diffusion well below a film's melting temperature, especially when the film is very thin. Dewetting during processing of films for use in micro- and nanosystems is often undesirable, and means of avoiding dewetting are important in this context. However, dewetting can also be useful in making arrays of nanoscale particles for electronic and photonic devices and for catalyzing growth of nanotubes and nanowires. Templating of dewetting using patterned surface topography or prepatterning of films can be used to create ordered arrays of particles and complex patterns of partially dewetted structures. Studies of dewetting can also provide fundamental new insight into the effects of surface energy anisotropy and facets on shape evolution.

We provide a succinct account of surface-bound gradient structures created from soft materials or those facilitating the study of soft materials behavior. We commence by classifying the chief attributes of gradient structures and offer a few examples of fabrication methods employed to generate such structures. We then illustrate the versatility of gradient assemblies in functioning as recording media for monitoring a physico-chemical process, facilitating fast screening of physico-chemical phenomena, and playing an important part in the design and fabrication of surface-anchored molecular and macromolecular engines for the directed transport of soft materials.