Mechanics and dynamics of the strain-induced M1-M2 structural phase transition in individual VO₂ nanowires

The elastic properties and structural phase transitions of individual VO(2) nanowires were studied using an in situ push-to-pull microelectromechanical device to realize quantitative tensile analysis in a transmission electron microscope and a synchrotron X-ray microdiffraction beamline. A plateau was detected in the stress-strain curve, signifying superelasticity of the nanowire arising from the M1-M2 structural phase transition. The transition was induced and controlled by uniaxial tension. The transition dynamics were characterized by a one-dimensionally aligned domain structure with pinning and depinning of the domain walls along the nanowire. From the stress-strain dependence the Young's moduli of the VO(2) M1 and M2 phases were estimated to be 128 ± 10 and 156 ± 10 GPa, respectively. Single pinning and depinning events of M1-M2 domain wall were observed in the superelastic regime, allowing for evaluation of the domain wall pinning potential energy. This study demonstrates a new way to investigate nanoscale mechanics and dynamics of structural phase transitions in general.

In situ nanomechanical testing in focused ion beam and scanning electron microscopes

The recent interest in size-dependent deformation of micro- and nanoscale materials has paralleled both technological miniaturization and advancements in imaging and small-scale mechanical testing methods. Here we describe a quantitative in situ nanomechanical testing approach adapted to a dual-beam focused ion beam and scanning electron microscope. A transducer based on a three-plate capacitor system is used for high-fidelity force and displacement measurements. Specimen manipulation, transfer, and alignment are performed using a manipulator, independently controlled positioners, and the focused ion beam. Gripping of specimens is achieved using electron-beam assisted Pt-organic deposition. Local strain measurements are obtained using digital image correlation of electron images taken during testing. Examples showing results for tensile testing of single-crystalline metallic nanowires and compression of nanoporous Au pillars will be presented in the context of size effects on mechanical behavior and highlight some of the challenges of conducting nanomechanical testing in vacuum environments.

Ultrahigh stress and strain in hierarchically structured hollow nanoparticles

Nanocrystalline materials offer very high strength but are typically limited in their strain to failure, and efforts to improve deformability in these materials are usually found to be at the expense of strength. Using a combination of quantitative in situ compression in a transmission electron microscope and finite-element analysis, we show that the mechanical properties of nanoparticles can be directly measured and interpreted on an individual basis. We find that nanocrystalline CdS synthesized into a spherical shell geometry is capable of withstanding extreme stresses (approaching the ideal shear strength of CdS). This unusual strength enables the spherical shells to exhibit considerable deformation to failure (up to 20% of the sphere's diameter). By taking into account the structural hierarchy intrinsic to novel nanocrystalline materials such as this, we show it is possible to achieve and characterize the ultrahigh stresses and strains that exist within a single nanoparticle during deformation.

Evaluation of a new modulus mapping technique to investigate microstructural features of human teeth

Teeth contain several calcified tissues with junctions that provide interfaces between dissimilar tissues. These junctions have been difficult to characterize because of their small size. In this work a new technique using a combination of atomic force microscopy (AFM) and a force-displacement transducer was used to simultaneously study the surface topography and map mechanical properties of the junctions and adjacent hard tissues. Prepared specimens from human third molars were scanned by an AFM piezo-tube in contact mode. To measure the dynamic viscoelastic properties of the material a small sinusoidal force was superimposed on the contact force and the resulting displacement amplitude and the phase shift between the force and amplitude were measured. This force modulation technique was used to map the local variation of nanomechanical properties of intertubular dentin, peritubular dentin, enamel, dentin-enamel junction (DEJ) and peritubular-intertubular dentin junction (PIJ). This new technique allowed us to measure the widths of these junctions in addition to local variation in dentin and enamel without causing plastic deformation to the material and with 2 orders of magnitude increase in spatial resolution compared with previous studies that used discrete nanoindentation techniques. Due to the ability to analyze the sample line-by-line, the distribution functions associated with the width of the DEJ and PIJ were conveniently obtained for specific intratooth locations. The data suggested, for three third molar specimens, a DEJ width of 2-3 microm with full-width half-maximum (FWHM) of 0.7 microm and PIJ width of 0.5-1.0 microm with 0.3 microm FWHM. The intertubular dentin storage modulus variation was between 17 and 23 GPa with a mean value of 21 GPa. The range of storage modulus for enamel near the DEJ was between 51 and 74 GPa with a mean value of 63 GPa.