Optical pulse induced ultrafast antiferrodistortive transition in SrTiO3

The ultrafast dynamics of the antiferrodistortive (AFD) phase transition in perovskite SrTiO3 is monitored via time-domain Brillouin scattering. Using femtosecond optical pulses, we induce a thermally driven tetragonal-to-cubic structural transformation and detect notable changes in the frequency of Brillouin oscillations (BO) induced by propagating acoustic phonons. First, we establish a fingerprint frequency of different regions across the temperature phase diagram of the AFD transition characterized by tetragonal and cubic phases in the low and high temperature sides, respectively. Then, we demonstrate that in a sample nominally kept in tetragonal phase, deposition of sufficient thermal energy induces an instantaneous transformation of the heat-affected region to the cubic phase. Coupling the measured depth-resolved BO frequency with a time and depth-resolved heat diffusion model, we detect a reverse cubic-to-tetragonal phase transformation occurring on a time scale of hundreds of picoseconds. We attribute this ultrafast phase transformation in the perovskite to a structural resemblance between atomic displacements of the R-point soft optic mode of the cubic phase and the tetragonal phase, both characterized by anti-phase rotation of oxygen octahedra. Evidence of such a fast structural transition in perovskites can open up new avenues in the field of information processing and energy storage.Comment: 15 Pages, 4 Figure

Longitudinal Eigenvibration of Multilayer Colloidal Crystals and the Effect of Nanoscale Contact Bridges

Longitudinal contact-based vibrations of colloidal crystals with a controlled layer thickness are studied. These crystals consist of 390 nm diameter polystyrene spheres arranged into close packed, ordered lattices with a thickness of one to twelve layers. Using laser ultrasonics, eigenmodes of the crystals that have out-of-plane motion are excited. The particle-substrate and effective interlayer contact stiffnesses in the colloidal crystals are extracted using a discrete, coupled oscillator model. Extracted stiffnesses are correlated with scanning electron microscope images of the contacts and atomic force microscope characterization of the substrate surface topography after removal of the spheres. Solid bridges of nanometric thickness are found to drastically alter the stiffness of the contacts, and their presence is found to be dependent on the self-assembly process. Measurements of the eigenmode quality factors suggest that energy leakage into the substrate plays a role for low frequency modes but is overcome by disorder- or material-induced losses at higher frequencies. These findings help further the understanding of the contact mechanics, and the effects of disorder in three-dimensional micro- and nano-particulate systems, and open new avenues to engineer new types of micro- and nanostructured materials with wave tailoring functionalities via control of the adhesive contact properties

Validating First-Principles Phonon Lifetimes via Inelastic Neutron Scattering

Phonon lifetimes are a key component of quasiparticle theories of transport, yet first-principles lifetimes are rarely directly compared to inelastic neutron scattering (INS) results. Existing comparisons show discrepancies even at temperatures where perturbation theory is expected to be reliable. In this work, we demonstrate that the reciprocal space voxel ($q$-voxel), which is the finite region in reciprocal space required in INS data analysis, must be explicitly accounted for within theory in order to draw a meaningful comparison. We demonstrate accurate predictions of peak widths of the scattering function when accounting for the $q$-voxel in CaF$_2$ and ThO$_2$. Passing this test implies high fidelity of the phonon interactions and the approximations used to compute the Green's function, serving as critical benchmark of theory, and indicating that other material properties should be accurately predicted; which we demonstrate for thermal conductivity

Laser Ultrasonic Characterization of Contact Dynamics in Single- and Few-Layer Self-Assembled Microscale Granular Crystals

Thesis (Ph.D.)--University of Washington, 2018Materials with designed structural discreteness and local resonances have garnered significant interest in recent years due their ability to manipulate waves in new ways. A number of studies over the past decade have explored mechanical wave phenomena in one such system, composed of ordered, close-packed arrays of elastic particles in contact, referred to as ‘granular crystals’. Proposed applications for such acoustic wave tailoring designer materials include vibration isolation, frequency-selective acoustic wave filtering, and acoustic wave guiding and focusing. Besides serving as a platform to gain new insight into the complex dynamic behavior of granular media, macroscale granular crystals (composed of millimeter- to centimeter-sized units) have also shown potential for use as such a designed composite material. Granular crystals also exhibit additional capabilities over some other types of other types of wave-tailoring designer materials, in that they exhibit a tunable dynamic response in the linear, weakly nonlinear and strongly nonlinear regimes to tailor acoustic waves. While macroscale granular crystals are designed to affect sonic frequency acoustic waves, extending granular crystals to the micro- and nanoscale has the potential to enable granular-based devices that operate at megahertz and gigahertz frequencies. In addition, microscale granular crystals also serve as a platform for exploring and developing a more general class of high, frequency, and micro- to nanostructured designer wave-tailoring materials, wherein large-scale fabrication presents significant challenges. Micro- and nanoscale granular crystals, however, cannot be thought of as simply scaled down versions of their macroscale counterparts since effects such as adhesion between particles, which are negligible at the macroscale, become significant at reduced length scales, and can drastically alter the granular crystal dynamics. This thesis focuses on addressing open questions relating to the contact-based dynamics of self-assembled, single- and few-layer-thick microscale granular crystals using an experimentally-driven approach. Convective colloidal self-assembly tech- niques are used to fabricate mono- and multilayer granular crystals comprised of micron- and sub-micron-sized particles. The vibrational dynamics of the microparticle arrays and the interaction of the contact resonances of the particles with surface, bulk and Lamb waves are studied experimentally using photoacoustic techniques. The applicability of adhesive contact models at the microscale, including the effects of adhesion-induced plasticity, is also discussed. Novel mechanisms to tune the interparticle and particle-substrate contact stiffness via nanoscale solid bridges and local ablation of the contact zone are also explored. This work sheds new light on our understanding of the contact-based dynamics of micro- to nanoscale particles, and particle assemblies, and opens avenues for developing a new class of locally-resonant granular acoustic metamaterials with applications such as signal processing and ultrasonic wave imaging, and offers new insights into the mechanical properties of self-assembled materials

Nanocontact Tailoring via Microlensing Enables Giant Postfabrication Mesoscopic Tuning in a Self‐Assembled Ultrasonic Metamaterial

International audienc

Analyse vibratoire de structures sandwiches stratifiees

SIGLECNRS AR 11469 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Longitudinal eigenvibration of multilayer colloidal crystals and the effect of nanoscale contact bridges

International audienceLongitudinal contact-based vibrations of colloidal crystals with a controlled layer thickness are studied

Transformation of a ceramic precursor to a biomedical (metallic) alloy: Part I – sinterability of Ta2O5 and TiO2 mixed oxides

Mixed Ta2O5 – TiO2 binary system was studied by a combination of differential thermal analysis (DTA), scanning electron microscopy-energy dispersive spectrometry (SEM-EDS), X-ray diffraction (XRD) and in situ high temperature X-ray diffraction (HT-XRD) techniques. Different compositions of the mixed oxide powders were fabricated by ball–milling the powdered compositions, pelletizing the homogenized composite powders, and heating the green pellets in air at different temperatures for fixed time intervals. The sintered pellets were evaluated and characterized with respect to porosity, morphology, and phase distribution. DTA runs of the un-sintered powders indicated the onset temperatures for both exothermic and endothermic changes in the binary system. Significant amount of sintering was observed to take place at temperatures higher than 900 °C. Both room and high temperature X-ray diffraction patterns exhibited consistency in phase formation. A ternary compound (TaTiO4) and a ternary solid solution (Ti0.33Ta0.67O2) were observed to form in both room and high temperatures in addition to the respective binary phases (Ta2O5 and TiO2). A sintering temperature in the range 900–1000 °C was observed to be adequate to achieve the requisite mechanical strength and optimum internal porosity (40–48%) for subsequent electrochemical polarization experiments