Annual Review of Materials Research - Volume 37, 2007

This review describes the mechanisms responsible for low-temperature degradation (LTD) of zirconia ceramics and its detrimental consequences for biomedical devices. Special emphasis is given to the critical issue of zirconia degradation actually observed for hip prostheses. Experimental methods to accurately measure and predict LTD in a given zirconia ceramic are presented. Different solutions to inhibit LTD or at least reduce its kinetics are reviewed, with the objective of highlighting alternative options for the generation of new zirconia-based biomedical ceramic devices.

Single-molecule micromanipulation techniques traditionally have been developed for biophysical applications, but they are being increasingly employed in materials science applications such as rheology and polymer dynamics. Continuing developments and improvements in single-molecule manipulation techniques afford new opportunities in a broad range of fields. In this review we present an overview of current single-molecule manipulation techniques, with an emphasis on optical and magnetic tweezers, followed by a description of the elastic properties of single biopolymers. We then review the use of micromanipulation techniques to locally probe material properties. To provide some insight into biophysical questions addressed by these techniques, we describe two applications, which further serve to illustrate the power and versatility of single-molecule micromanipulation techniques. We conclude with a brief discussion of emerging applications and techniques.

We present an overview of the technique of spin-polarized scanning tunneling microscopy (Sp-STM) and its application to high-resolution magnetic imaging. In STM, the electron density near the sample surface is imaged. Additionally, Sp-STM allows a mapping of the spin polarization of the electronic density, which is related to the magnetic configuration of the sample. Two primary imaging modes of Sp-STM are currently in use: the spectroscopic mode and the differential magnetic mode. The principles of the two modes are explained in the framework of imaging ferromagnetic nanostructures and antiferromagnetic surfaces. The advantages and drawbacks of the two approaches are discussed, and the strength of Sp-STM to map even complex spin structures on the nanometer scale is illustrated.

Lilliputian techniques for measuring the mechanical response of microscale specimens are being developed to characterize the performance and reliability of microelectromechanical systems (MEMS) and other small-scale entities. The challenges associated with the preparation, handling, and testing of small volumes of material have spawned a variety of techniques; this review focuses on uniaxial testing. Results from these experiments provide valuable insight into size-scale effects on the elastic, brittle, and ductile behavior of micron-sized structures. Fundamental elastic interactions show no size effect; in-plane moduli can be predicted from anisotropic elastic constants if crystallographic texture is properly considered. Intrinsic fracture toughness is also size independent, although the fracture strength of brittle MEMS materials is extremely dependent on flaw size and distribution. By contrast, size effects on the strength of ductile materials suggest that the operation of intrinsic dislocation processes in greatly reduced or confined volumes alters their generation, multiplication, interaction, and motion.

This review presents the historical temporal evolution of an atom-probe tomograph (APT) from its genesis (1973) from field-ion microscope images of individual tungsten atoms (1955). The capabilities of modern APTs employing either electrical or laser pulsing are discussed. The results of the application of APTs to specific materials science problems are presented for research performed at Northwestern University on the following problems: (a) the segregation of Mg at α-Al/Al3Sc heterophase interfaces, (b) phase decomposition in ternary Ni-Al-Cr and quaternary Ni-Al-Cr-Re alloys, and (c) 3-D nanoscale composition mapping of an InAs semiconductor nanowire whose growth was catalyzed by gold. These results demonstrate that it is now possible to obtain highly quantitative information from APT that can be compared with modeling, theory, simulations, and/or first-principles calculations.

Many of the structural elements of importance in materials applications (e.g., thin films, barrier layers, intergranular films in ceramics) are small in volume and amorphous. Although the characterization of the structure of amorphous materials by X-ray and neutron diffraction methods is well established, these techniques are not suitable for studies of nanovolumes of materials because of the relatively small scattering cross sections. This chapter reviews recent developments in electron techniques, and particularly electron diffraction, for overcoming this problem.

Functionality of biological and inorganic systems ranging from nonvolatile computer memories and microelectromechanical systems to electromotor proteins and cellular membranes is ultimately based on the intricate coupling between electrical and mechanical phenomena. In the past decade, piezoresponse force microscopy (PFM) has been established as a powerful tool for nanoscale imaging, spectroscopy, and manipulation of ferroelectric and piezoelectric materials. Here, we give an overview of the fundamental image formation mechanism in PFM and summarize recent theoretical and technological advances. In particular, we show that the signal formation in PFM is complementary to that in the scanning tunneling microscopy (STM) and atomic force microscopy (AFM) techniques, and we discuss the implications. We also consider the prospect of extending PFM beyond ferroelectric characterization for quantitative probing of electromechanical behavior in molecular and biological systems and high-resolution probing of static and dynamic polarization switching processes in low-dimensional ferroelectric materials and heterostructures.

Atomic-level simulations are providing powerful insights into the properties of ferroelectric materials. In particular, we illustrate the effect of the strong coupling among the dipole moment, strain, and microstructure (both physical and chemical) in thin films, nanostructures, superlattices, and solid solutions of ferroelectrics. We assess the domain of validity of the atomic-level approach relative to other theoretical and computational approaches. We identify the strengths and weaknesses of the current generation of interatomic potentials and suggest strategies for improving their performance. We also identify application areas in which atomic-level simulation promises to play an important role.

A new phenomenon—ferroelectric domain breakdown, or the formation of string-like domains—is observed in various bulk ferroelectric crystals under a high inhomogeneous electric field generated at the tip of a high-voltage atomic force microscope. The domains have a high aspect ratio when their length exceeds their lateral size by two or three orders of magnitude. This chapter reviews some of the recent advances in understanding the nature of this phenomenon. The main driving force for ferroelectric domain breakdown is not the tip's electric field, which is negligibly weak near the apex of the long domain, but rather internal forces generated owing to the minimization of depolarization field energy when the domain elongates. Domain breakdown is considered an extreme manifestation of Coulomb instability, which is caused by repulsion of bound charges near the inverted domain apex. This instability leads to strong pulling of the domain apex into a region where the tip-induced electric field is negligibly small.

Most of the ferroelectric and piezoelectric materials used in transducers and electronics are complex mixed-ion alloys, in which atomic disorder plays an important role in the ferroelectric properties. The extreme case is relaxor ferroelectric solids with diffuse ferroelectric transition and glass-like relaxation behavior. In this review I discuss how advanced characterization techniques, such as neutron scattering, enabled a determination of the local structure and facilitated the understanding of the ferroelectric and piezoelectric properties of these complex systems. In particular, I focus on the role of Pb²⁺ ions in mixed ferroelectrics and the atomistic mechanism of relaxor ferroelectrics.

Between electronics and photonics there exists a frequency gap of approximately two octaves, i.e., the frequency range between 100 GHz and 10 THz, across which there are limited capabilities for signal generation, control, guidance, and processing. Here, we demonstrate that phonon-polaritons in ionic crystals like LiNbO³ or LiTaO³ may be used to bridge this gap. The ability to directly visualize polariton fields through real-space imaging, to generate arbitrary THz waveforms through the use of temporally and/or spatially shaped optical waveforms, and to fabricate integrated functional elements for polariton guidance and control through laser machining yields a THz polaritonics platform that enables advanced signal processing and spectroscopy applications.

Combining atomic force microscopy and ultrasonic methods allows near-field detection of acoustic signals and thereby otherwise inaccessible nanoscale mechanical characterization. The two predominant variations, ultrasonic force microscopy and atomic force acoustic microscopy, are reviewed in detail. Applications of each to ceramics, polymers, metals, biological materials, and even subsurface structures are discussed, with a particular emphasis on image contrast mechanisms, data analysis, and experimental challenges. Finally, recent advances of these concepts into high-speed surface property mapping are presented, demonstrating 100-fold enhancements in full-frame imaging speeds.

Magnetoelectric multiferroics is an old but emerging class of materials that combine coupled electric and magnetic dipole order. In these materials, ferroelectric and magnetic (ferromagnetic or antiferromagnetic) states coexist or compete with each other. The interaction leads to a so-called magnetoelectric effect, which is the induction of magnetization by an electric field or electric polarization by a magnetic field. In the past few years, a new set of magnetoelectric multiferroics such as TbMnO³ and Ni³V2O8 has been discovered. In these magnetoelectric multiferroics, ferroelectric order develops upon a magnetic phase transition into a spiral magnetic ordered phase. In addition, these systems show large magnetoelectric effects accompanied by metamagnetic transitions. Noncollinear spiral magnetism is the key to understanding the magnetoelectric properties in these systems. Here I discuss the magnetoelectric coupling in spiral magnets and review recent advances in the understanding of ferroelectricity and the magnetoelectric effect in these new multiferroics. The studies presented here indicate that spiral magnets are promising candidates for magnetoelectrics showing large magnetoelectric effects at low magnetic fields.

The dynamics of driven domain walls (DWs) is studied in disordered uniaxial ferroic materials such as periodically poled ferroelectric KTiOPO4; quantum ferroelectric SrTi¹⁸O3; relaxor ferroelectric Sr0.61Ba0.39Nb2O6 (SBN); ultrathin ferromagnetic multilayers Pt/Co/Pt; and discontinuous magnetic metal-insulator multilayers [Co80Fe20/Al2O3]10 (DMIM) with ac susceptibility, , and domain imaging. Sideways creep and slide motion, well known from dc excitation, is mapped onto characteristic dynamic modes in the ac spectra or, more significantly, in Cole-Cole presentations, . Creep and slide are complemented by switching at high enough field amplitudes and by DW segmental relaxation at high enough frequencies. The dynamic relaxation-to-creep transition is well understood within scaling theory, whereas the crossover between the modes creep, slide, and switching is numerically modeled on the basis of the Edwards-Wilkinson equation of DW motion. Clear signatures of all dynamical modes are even found at fractal DWs in dipolarly controlled DMIMs and in polar nanoregions of random-field-controlled SBN above Tc.

Domains and domain walls are a fundamental property of interest in ferroelectrics, magnetism, ferroelastics, superconductors, and multiferroic materials. Unlike magnetic Bloch walls, ideal ferroelectric domain walls are well accepted to be only one to two lattice units wide, over which polarization and strain change across the wall. However, walls in real ferroelectrics appear to show unexpected property variations in the vicinity of domain walls that can extend over micrometer length scales. This chapter specifically reviews the local electrical, elastic, optical, and structural properties of antiparallel domain walls in the trigonal ferroelectrics lithium niobate and lithium tantalate. It is shown that extrinsic point defects and their clustering play a key role in the observed local wall structure and influence macroscale properties by orders of magnitude. The review also raises broader and yet unexplored fundamental questions regarding intrinsic widths, defect–domain wall interactions, and static versus dynamic wall structure.

This chapter reviews experimental results that allow one to interpret the essential features of fracture in ferroelectric ceramics under electric and mechanical load. First, crack growth measurements on unpoled and poled ferroelectric ceramics are reviewed, and numerous experiments that demonstrate the existence and relevance of a domain-switching zone are presented. Thereafter the review concentrates on results of fracture experiments with applied electric fields and addresses the controversial outcomes of such experiments. Theoretical fracture mechanical concepts are then introduced. One part of the discussion focuses on electrical boundary conditions used at the crack surface because they decisively determine the predicted energy release rate. The other part of the discussion, which concerns theoretical concepts, discusses the predicted switching zones around cracks and their influence on the stress intensity factor and energy release rate. Finally, this chapter attempts to mirror the tremendous theoretical framework against the existing experimental results and critically review the relevance of the developed fracture mechanics concepts of piezoelectric ceramics.

Impressive progress has been made in the processing and exploration of new material on an atomic scale (nanomaterials). However, the characterization of such materials by the usual transmission electron microscopy (TEM) techniques suffers from the drawback that the phase of the object-modulated electron wave is virtually lost in the recorded intensity images. Electron holography has opened possibilities for analyzing both the amplitude and phase of the electron wave, hence giving access to the object information encoded in the phase. Examples include intrinsic electric and magnetic fields, e.g. in ferroelectrics or ferromagnetics, which substantially determine the object properties and therefore are indispensable for a complete understanding of structure-properties relations.

Predictions and measurements of the effect of biaxial strain on the properties of epitaxial ferroelectric thin films and superlattices are reviewed. Results for single-layer ferroelectric films of biaxially strained SrTiO3, BaTiO3, and PbTiO3 as well as PbTiO3/SrTiO3 and BaTiO3/SrTiO3 superlattices are described. Theoretical approaches, including first principles, thermodynamic analysis, and phase-field models, are applied to these biaxially strained materials, the assumptions and limitations of each technique are explained, and the predictions are compared. Measurements of the effect of biaxial strain on the paraelectric-to-ferroelectric transition temperature (TC) are shown, demonstrating the ability of percent-level strains to shift TC by hundreds of degrees in agreement with the predictions that predated such experiments. Along the way, important experimental techniques for characterizing the properties of strained ferroelectric thin films and superlattices, as well as appropriate substrates on which to grow them, are mentioned.

The characterization of microstructures in three dimensions is reviewed, with an emphasis on the use of automated electron back-scatter diffraction techniques. Both statistical reconstruction of polycrystalline structures from multiple cross sections and reconstruction from parallel, serial sections are discussed. In addition, statistical reconstruction of second-phase particle microstructures from multiple cross sections is reviewed.