Piezoelectric materials, which respond mechanically to applied electric field and vice versa, are essential for electromechanical transducers. Previous theoretical analyses have shown that high piezoelectricity in perovskite oxides is associated with a flat thermodynamic energy landscape connecting two or more ferroelectric phases. Here, guided by phenomenological theories and phase-field simulations, we propose an alternative design strategy to commonly used morphotropic phase boundaries to further flatten the energy landscape, by judiciously introducing local structural heterogeneity to manipulate interfacial energies (that is, extra interaction energies, such as electrostatic and elastic energies associated with the interfaces). To validate this, we synthesize rare-earth-doped Pb(MgNb)O-PbTiO (PMN-PT), as rare-earth dopants tend to change the local structure of Pb-based perovskite ferroelectrics. We achieve ultrahigh piezoelectric coefficients d of up to 1,500 pC N and dielectric permittivity ε/ε above 13,000 in a Sm-doped PMN-PT ceramic with a Curie temperature of 89 °C. Our research provides a new paradigm for designing material properties through engineering local structural heterogeneity, expected to benefit a wide range of functional materials.

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Since the discovery of carbon nanotubes in 1991 by Iijima, there has been great interest in creating long, continuous nanotubes for applications where their properties coupled with extended lengths will enable new technology developments. For example, ultralong nanotubes can be spun into fibres that are more than an order of magnitude stronger than any current structural material, allowing revolutionary advances in lightweight, high-strength applications. Long metallic nanotubes will enable new types of micro-electromechanical systems such as micro-electric motors, and can also act as a nanoconducting cable for wiring micro-electronic devices. Here we report the synthesis of 4-cm-long individual single-wall carbon nanotubes (SWNTs) at a high growth rate of 11 microm s(-1) by catalytic chemical vapour deposition. Our results suggest the possibility of growing SWNTs continuously without any apparent length limitation.

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The grain-size effect on deformation twinning in nanocrystalline copper is studied. It has been reported that deformation twinning in coarse-grained copper occurs only under high strain rate and/or low-temperature conditions. Furthermore, reducing grain sizes has been shown to suppress deformation twinning. Here, we show that twinning becomes a major deformation mechanism in nanocrystalline copper during high-pressure torsion under a very slow strain rate and at room temperature. High-resolution transmission electron microscopy investigation of the twinning morphology suggests that many twins and stacking faults in nanocrystalline copper were formed through partial dislocation emissions from grain boundaries. This mechanism differs from the pole mechanism operating in coarse-grained copper.Nanocrystalline ͑nc͒ materials have been reported to have superior mechanical properties such as high strength, which can coexist with very good ductility. 1-3 These superior mechanical properties are attributed to their unique deformation mechanisms, which are different from those in their coarse-grained ͑CG͒ counterparts. 4 -8 For example, molecular dynamics simulations, which used extremely high strain rates in the order of 10 6 to 10 8 s Ϫ1 , predict that NC Al deforms via partial dislocation emission from grain boundaries, which consequently produces deformation twins. 7 These predictions have recently been verified experimentally in nc Al powder processed by ball milling at liquid nitrogen temperature 9 and in nc Al film produced by physical vapor deposition. 8 These observations are very surprising because deformation twinning has never been observed in CG Al.High strain rate, low temperature, and nanometer grain size are major contributing factors for deformation twinning in the ball-milled Al powder. 9,10 In fact, both high strain rate and low temperature are known to promote deformation twinning. 11,12 For example, CG copper does not deform by twinning 13,14 except at very high strain rate 15,16 and/or low temperature. 17 However, the grain-size effect is not so clear. It has been suggested that both the critical slip stress and twinning stress follow the Hall-Petch ͑HP͒ relationship, with the HP slope for twinning (k T ) significantly larger than that for slip (k S ) for many CG metals and alloys. 18 For copper, the k T is about 0.7 MN/m 3/2 , while k S is about 0.35 MN/m 3/2 . 19 Consequently, dislocation slip rather than deformation twinning is expected to become the preferred deformation mode when the grain is smaller than a certain size. Indeed, Meyers et al. 20 reported that shock compression at 35 GPa produced abundant deformation twins in copper samples with grain sizes of 117 and 315 m, but virtually no twinning in a copper sample with a grain size of 9 m. On the other hand, it has been well known that the HP relationship fails in nc materials. 4,5 These literature observations raise some fundamental questions on the grain-size effect on deformation twinning. Does the trend that smaller grains are harder to ...