Mechanically Activated Solid-State Radical Polymerization and Cross-Linking via Piezocatalysis.

Piezocatalysis offers a means to transduce mechanical energy into chemical potential, harnessing physical force to drive redox reactions. Working in the solid state, we show here that piezoelectric BaTiO3 nanoparticles can transduce mechanical load into a flux of reactive radical species capable of initiating solid state free radical polymerization. Activation of a BaTiO3 powder by ball milling, striking with a hammer, or repeated compressive loading generates highly reactive hydroxyl radicals (⋅OH), which readily initiate radical chain growth and crosslinking of solid acrylamide, acrylate, methacrylate and styrenic monomers. Control experiments indicate a critical role for chemisorbed water on the BaTiO3 nanoparticle surface, which is oxidized to ⋅OH via mechanoredox catalysis. The force-induced production of radicals by compressing dry piezoelectric materials represents a promising new route to harness mechanical energy for solid state radical synthesis.

Magnetic prestressing for a d32-mode single crystal ultrasonic transducer.

This work describes a 35.9 kHz ultrasonic transducer that incorporates a magnetic arrangement to apply a static-compressive prestress to a d32-mode relaxor ferroelectric single crystal drive-element. The magnetic arrangement produces a 22.5 N static-compressive force, inducing a static compression of ∼630 nm on the drive-element. Operating in air with a continuous-wave 10 V peak drive at ∼35.9 kHz, the measured resonant peak displacement of the transducers head-mass was 127 nm. This is well within the predicted static compression, thus, the drive-element is protected from damaging tensile stress. Under the same drive conditions and at an axial distance of 10 mm from the face of the head-mass, the measured acoustic pressure was ∼12 Pa. Analytical and finite element model predictions and the measured behaviour of a prototype device are presented and show good correlation, demonstrating that magnetic prestressing of the drive-element can be a viable alternative to the traditional bolt-clamp.

Simultaneous Large Optical and Piezoelectric Effects Induced by Domain Reconfiguration Related to Ferroelectric Phase Transitions.

Electrical switching of ferroelectric domains and subsequent domain wall motion promotes strong piezoelectric activity, however, light scatters at refractive index discontinuities such as those found at domain wall boundaries. Thus, simultaneously achieving large piezoelectric effect and high optical transmissivity is generally deemed infeasible. Here, it is demonstrated that the ferroelectric domains in perovskite Pb(In1/2 Nb1/2 )O3 -Pb(Mg1/3 Nb2/3 )O3 -PbTiO3 domain-engineered crystals can be manipulated by electrical field and mechanical stress to reversibly and repeatably, with small hysteresis, transform the opaque polydomain structure into a highly transparent monodomain state. This control of optical properties can be achieved at very low electric fields (less than 1.5 kV cm-1 ) and is accompanied by a large (>10 000 pm V-1 ) piezoelectric coefficient that is superior to linear state-of-the-art materials by a factor of three or more. The coexistence of tunable optical transmissivity and high piezoelectricity paves the way for a new class of photonic devices.

Ultra-large electric field-induced strain in potassium sodium niobate crystals.

Electromechanical coupling in piezoelectric materials allows direct conversion of electrical energy into mechanical energy and vice versa. Here, we demonstrate lead-free (K x Na1-x )NbO3 single crystals with an ultrahigh large-signal piezoelectric coefficient d 33* of 9000 pm V-1, which is superior to the highest value reported in state-of-the-art lead-based single crystals (~2500 pm V-1). The enhanced electromechanical properties in our crystals are realized by an engineered compositional gradient in the as-grown crystal, allowing notable reversible non-180° domain wall motion. Moreover, our crystals exhibit temperature-insensitive strain performance within the temperature range of 25°C to 125°C. The enhanced temperature stability of the response also allows the materials to be used in a wider range of applications that exceed the temperature limits of current lead-based piezoelectric crystals.

Ferroelectric Domain Continuity Over Grain Boundaries for Tetragonal, Orthorhombic, and Rhombohedral Crystal Symmetries.

Domain mobility is understood to have a significant impact on ferroelectric material properties. Grain boundaries are known to inhibit ferroelectric domain wall mobility, thereby limiting the properties of polycrystalline ferroelectrics. Continuous domains across grain boundaries can change the grain boundary impact on domain wall mobility. Such microstructures have been observed since the 1950s, but their impact on properties is not well understood. This paper predicts the likelihood of domain wall continuity over grain boundaries for typical perovskite oxide ferroelectric symmetries of tetragonal, orthorhombic, and rhombohedral. Five two-grain systems are studied for domain continuity over all possible grain boundary space. Results show that rhombohedral and orthorhombic symmetries have a higher probability for domain continuity over grain boundaries as compared with tetragonal symmetry. This observation is due to the greater number of domain wall planes and polarization vectors in these symmetries. Grain boundaries probable for domain continuity can be identified in each misorientation for each symmetry. This knowledge, combined with developments in microstructural measurements and ceramic processing techniques, may be used to control domain continuity at grain boundaries, and hence, influence the final properties of the material.

Long-range symmetry breaking in embedded ferroelectrics.

The characteristic functionality of ferroelectric materials is due to the symmetry of their crystalline structure. As such, ferroelectrics lend themselves to design approaches that manipulate this structural symmetry by introducing extrinsic strain. Using in situ dark-field X-ray microscopy to map lattice distortions around deeply embedded domain walls and grain boundaries in BaTiO3, we reveal that symmetry-breaking strain fields extend up to several micrometres from domain walls. As this exceeds the average domain width, no part of the material is elastically relaxed, and symmetry is universally broken. Such extrinsic strains are pivotal in defining the local properties and self-organization of embedded domain walls, and must be accounted for by emerging computational approaches to material design.

A sample cell for in situ electric-field-dependent structural characterization and macroscopic strain measurements.

When studying electro-mechanical materials, observing the structural changes during the actuation process is necessary for gaining a complete picture of the structure-property relationship as certain mechanisms may be meta-stable during actuation. In situ diffraction methods offer a powerful and direct means of quantifying the structural contributions to the macroscopic strain of these materials. Here, a sample cell is demonstrated capable of measuring the structural variations of electro-mechanical materials under applied electric potentials up to 10 kV. The cell is designed for use with X-ray scattering techniques in reflection geometry, while simultaneously collecting macroscopic strain data using a linear displacement sensor. The results show that the macroscopic strain measured using the cell can be directly correlated with the microscopic response of the material obtained from diffraction data. The capabilities of the cell have been successfully demonstrated at the Powder Diffraction beamline of the Australian Synchrotron and the potential implementation of this cell with laboratory X-ray diffraction instrumentation is also discussed.

Heterogeneous grain-scale response in ferroic polycrystals under electric field.

Understanding coupling of ferroic properties over grain boundaries and within clusters of grains in polycrystalline materials is hindered due to a lack of direct experimental methods to probe the behaviour of individual grains in the bulk of a material. Here, a variant of three-dimensional X-ray diffraction (3D-XRD) is used to resolve the non-180° ferroelectric domain switching strain components of 191 grains from the bulk of a polycrystalline electro-ceramic that has undergone an electric-field-induced phase transformation. It is found that while the orientation of a given grain relative to the field direction has a significant influence on the phase and resultant domain texture, there are large deviations from the average behaviour at the grain scale. It is suggested that these deviations arise from local strain and electric field neighbourhoods being highly heterogeneous within the bulk polycrystal. Additionally, the minimisation of electrostatic potentials at the grain boundaries due to interacting ferroelectric domains must also be considered. It is found that the local grain-scale deviations average out over approximately 10-20 grains. These results provide unique insight into the grain-scale interactions of ferroic materials and will be of value for future efforts to comprehensively model these and related materials at that length-scale.

Electric-field-induced local and mesoscale structural changes in polycrystalline dielectrics and ferroelectrics.

The atomic-scale response of dielectrics/ferroelectrics to electric fields is central to their functionality. Here we introduce an in situ characterization method that reveals changes in the local atomic structure in polycrystalline materials under fields. The method employs atomic pair distribution functions (PDFs), determined from X-ray total scattering that depends on orientation relative to the applied field, to probe structural changes over length scales from sub-Ångstrom to several nanometres. The PDF is sensitive to local ionic displacements and their short-range order, a key uniqueness relative to other techniques. The method is applied to representative ferroelectrics, BaTiO3 and Na½Bi½TiO3, and dielectric SrTiO3. For Na½Bi½TiO3, the results reveal an abrupt field-induced monoclinic to rhombohedral phase transition, accompanied by ordering of the local Bi displacements and reorientation of the nanoscale ferroelectric domains. For BaTiO3 and SrTiO3, the local/nanoscale structural changes observed in the PDFs are dominated by piezoelectric lattice strain and ionic polarizability, respectively.

A high-specific-strength and corrosion-resistant magnesium alloy.

Ultra-lightweight alloys with high strength, ductility and corrosion resistance are desirable for applications in the automotive, aerospace, defence, biomedical, sporting and electronic goods sectors. Ductility and corrosion resistance are generally inversely correlated with strength, making it difficult to optimize all three simultaneously. Here we design an ultralow density (1.4 g cm(-3)) Mg-Li-based alloy that is strong, ductile, and more corrosion resistant than Mg-based alloys reported so far. The alloy is Li-rich and a solute nanostructure within a body-centred cubic matrix is achieved by a series of extrusion, heat-treatment and rolling processes. Corrosion resistance from the environment is believed to occur by a uniform lithium carbonate film in which surface coverage is much greater than in traditional hexagonal close-packed Mg-based alloys, explaining the superior corrosion resistance of the alloy.

Electric-field-induced phase transitions in co-doped Pb(Zr1-xTix)O3 at the morphotropic phase boundary.

The strain- and polarization-electric field behavior was characterized at room temperature for Pb0.98Ba0.01(Zr1-x Ti x )0.98Nb0.02O3, 0.40 ⩽ x ⩽ 0.60. The investigated compositions were located in the vicinity of the morphotropic phase boundary, giving insight into the influence of crystal structure on the hysteretic ferroelectric behavior. The remanent strain of particular compositions is shown to be larger than theoretically allowed by ferroelectric switching alone, indicating the presence of additional remanent strain mechanisms. A phenomenological free energy analysis was used to simulate the effect of an applied electric field on the initial equilibrium phase. It is shown that electric-field-induced phase transitions in polycrystalline ferroelectrics can account for the experimental observations. The experimental and simulation results are contrasted to neutron diffraction measurements performed on representative compositions in the virgin and remanent states.

Hydration and radiation effects on the residual stress state of cortical bone.

The change in the biaxial residual stress state of hydroxyapatite crystals and collagen fibrillar structure in sections of bovine cortical bone has been investigated as a function of dehydration and radiation dose using combined small- and wide-angle X-ray scattering. It is shown that dehydration of the bone has a pronounced effect on the residual stress state of the crystalline phase, while the impact of radiation damage alone is less dramatic. In the initial hydrated state, a biaxial compressive stress of approximately -150 MPa along the bone axis exists in the hydroxyapatite crystals. As water evaporates from the bone material, the stress state moves to a tensile state of approximately 100 MPa. The collagen fibrillar structure is initially in a tensile residual stress state when the bone is hydrated and the state increases in magnitude slightly with dehydration. Radiation dose in continually hydrated samples also reduces the initial biaxial compressive stress magnitude in the hydroxyapatite phase; however, the stress remains compressive. Radiation exposure alone does not appear to affect the stress state of the collagen fibrillar structure.

Cooperation of length scales and orientations in the deformation of bovine bone.

Bone has a complex hierarchical structure. Combined wide angle X-ray diffraction and small angle X-ray scattering were used together with in situ tensile testing to investigate the deformation and failure mechanisms of bovine cortical bone at three material levels: (1) the atomic level, corresponding to the mineral crystal phase; (2) the nano level, corresponding to the collagen fibrils; (3) the macroscopic level. It was found that deformation was linear at all three levels up to a strain of 0.2% in the longitudinal tensile direction. At this critical strain a sudden 50% decrease in the fibrillar and mineral strains was observed. This suggests the presence of partial local damage that leads to inhomogeneous strain distributions within the probed gauge volume. This also gives rise to diffraction peak broadening in the mineral phase, as well as strain relaxation at the nanoscale. Above the critical strain the longitudinally oriented strains below the nanoscale remained constant at a reduced level until failure. This suggests that the lateral orientation of the nanostructures toughens the bone, while a higher material level dominated the subsequent deformation process, either by sliding between the lamellar layers or by the growth of microcracks. Analysis of the diffraction data also shows that the bone has compressive residual stress in the crystal phase. A better understanding of the basic mechanics of the hierarchical bone structure could be the basis to enhance research in biomimetics, developing new advanced materials, and to find solutions for orthopedic problems.

Time-resolved characterization of ferroelectrics using high-energy X-ray diffraction.

Diffraction provides an effective means to characterize ferroelectric materials under the application of dynamic and cyclic electric fields. This paper describes a typical timeresolved diffraction setup at a synchrotron facility using high X-ray energies. Such a setup is capable of measuring the structural response of ferroelectric ceramics to electric fields of various frequencies, amplitudes, and waveforms. The use of high energies also allows the response of the sample to be measured at various angles to the applied load. The results of 3 different types of electric loading are presented and discussed: the structural response of ferroelectric ceramics to a single electric field step function, a cyclic electric field of square waveform, and a cyclic electric field of sinusoidal waveform. Each type of electric field loading provides unique information about the material behavior.

U-Scores for Multivariate Data in Sports.

In many sport competitions athletes, teams, or countries are evaluated based on several variables. The strong assumptions underlying traditional 'linear weight' scoring systems (that the relative importance, interactions and linearizing transformations of the variables are known) can often not be justified on theoretical grounds, and empirical 'validation' of weights, interactions and transformations, is problematic when a 'gold standard' is lacking. With µ-scores (u-scores for multivariate data) one can integrate information even if the variables have different scales and unknown interactions or if the events counted are not directly comparable, as long as the variables have an 'orientation'. Using baseball as an example, we discuss how measures based on µ-scores can complement the existing measures for 'performance' (which may depend on the situation) by providing the first multivariate measures for 'ability' (which should be independent of the situation). Recently, µ-scores have been extended to situations where count variables are graded by importance or relevance, such as medals in the Olympics (Wittkowski 2003) or Tour-de-France jerseys (Cherchye and Vermeulen 2006, 2007). Here, we present extensions to 'censored' variables (life-time achievements of active athletes), penalties (counting a win more than two ties) and hierarchically structured variables (Nordic, alpine, outdoor, and indoor Olympic events). The methods presented are not restricted to sports. Other applications of the method include medicine (adverse events), finance (risk analysis), social choice theory (voting), and economy (long-term profit).