Hypersonic Phononic Composite Materials

G. Fytas
University of Crete, GR

Keywords: phononic, composite materials

Summary:

Phononic crystals, the acoustic equivalents of the photonic crystals, are controlled by a larger number of material parameters. The study of hypersonic phononic crystals (hPnC) imposes substantial demand on fabrication and characterization techniques. Colloid and polymer science offer methods to create novel materials that possess periodic variations of density and elastic properties at mesoscopic length scales commensurate with the wave length of hypersonic phonons and hence photons of the visible light. Polymer-and colloid-based phononics is an emerging new field at the interface of soft materials science and condensed matter physics with rich perspectives ahead. Here, examples from fabricated structures will be highlighted. Depending on the components of the nanostructured composite materials, the resolved vibration eigenmodes of the individual particles sensitively depend on the particle architecture and their thermo-mechanical properties. (i) In periodic structures of polymer based colloids, the dispersion ω(k) has revealed hypersonic phononic band gaps of different nature: Bragg gap for propagation near the edge of the first Brillouin zone due to destructive interference and hybridization gap5 due to the interaction of particle eigenmodes with the effective medium acoustic branch. The shape and topology of the colloidal particles drastically change the phonon propagation as demonstrated for spheroids and polymer brush coated spheres. Since the elucidation of all important parameters towards the general design of optimal phononic structures remains complicated due to the vector nature of elastic wave propagation, 1D-hPnC fabricated constitute appropriate model systems for fundamental studies. (ii) Under normal and oblique incidence the direction -dependent longitudinal and shear moduli are obtained at nanoscale, while the incorporation of defects (cavity and surface layers) holds a wealth of opportunities to engineer ω(k). Since hPnC can simultaneously exhibit phononic and photonic band gaps in the visible spectral region, and phonons are the main heat carriers in dielectrics, many technological applications are feasible. (iii) Elastic wave propagation through hierarchically nanostructured matter can involve unprecedented mechanisms as observed in the dispersion diagram of the spider dragline silk.