Exploiting the structure-property relationship in Bragg stacks : from phononic superlattices to bioinspired hybrids
Date issued
Authors
Editors
Journal Title
Journal ISSN
Volume Title
Publisher
License
Abstract
Structural organization found in many examples from nature provides an inspiring model of engineering to produce smart materials. This approach is based on exploiting the theme of structure-property relationship to design new materials in which the applications stem from their structural aspects. The work presented in this thesis highlights this idea by exploiting diverse properties of specially designed hybrid Bragg stacks built up from different constituent polymer and nanoparticles. This thesis is a compilation of work done on a variety of multilayered systems involving their fabrication and a detailed analysis of their phononic, mechanical and magnetic properties. In particular, one-dimensional phononic crystals are studied in detail to devise ways to mold the flow of elastic energy and obtain a full description of the phononic band diagram which is instrumental in providing an insight into the fundamental concepts of heat management and acousto-optic interactions. In addition, this provides a way to manipulate and control the propagation of elastic waves in periodic materials.
One-dimensional hypersonic phononic structures are fabricated with a high degree of control using a soft matter approach and characterized with the non-destructive technique of spontaneous Brillouin Light Spectroscopy (BLS). Hybrid Bragg stacks composed of alternating layers of poly (methyl methacrylate) (PMMA) and porous silica (p-SiO2) are built up on glass substrate using high-speed spin coating, in contrast to the conventional semi-conductor fabrication techniques. The multilayered stacks exhibit large and well-defined band gaps in the Gigahertz (GHz) region of frequency and show direction dependent elastic wave propagation. The complimenting experimental and theoretical dispersion diagrams are fully explained normal to and along the direction of periodicity in the PMMA/p-SiO2 Bragg stacks. The intensities of the lower and upper phononic branches, the width of the band gap and the phonon frequencies are found to be strongly reliant on the structural parameters of the phononic structures investigated. The elastic modulus and elasto-optic coefficients of the individual layers are also estimated. Oblique incidence significantly alters the phonon propagation and offers a way of engineering the phononic band gap along with an estimation of the shear moduli of the constituents. The phonon dispersion is found to be robust to withstand fabrication related structural imperfections.
A second approach to engineer the band gap in one dimensional hypersonic phononic crystals is the introduction of defects in otherwise perfect superlattices. The easy fabrication of PMMA/p-SiO2 superlattices with superb control makes the task of studying defect-controlled hypersound propagation much simpler. This work includes fabrication and characterization of hybrid superlattices of PMMA and p-SiO2 containing surface and cavity defect layers in isolation or in combination with each other. This is the first observation of surface and cavity modes in soft matter based phononic superlattices and their subsequent interaction. The defects are introduced in the perfect phononic lattice by varying the material, thickness or position of the surface and cavity defect layers. This comprehensive study provides a complete theoretical description of the band diagram based on the Green’s function method in addition to the experimental phonon dispersion. Breaking the high symmetry of the phononic superlattice is found to be a way to manipulate the band gap as well as to study the interaction between different defect modes. Such phononic structures with controlled defects are found to contain an optical stop band in addition to a phononic band gap and can qualify as phoxonic in nature.
The strength and load-bearing properties of many structural materials found in nature provide motivation to fabricate artificial structures with high mechanical properties. Taking inspiration from super strong nacre and the adhesive character of the constituent DOPA (3, 4-dihydroxyphenylalanine) in marine mussels, hybrid multilayers of a polymer rich in catechol groups (DOPA-polymer) and iron oxide nanoparticles (Fe3O4) are fabricated by a spin coating procedure. The combination of alternating hard and soft constituent layers cemented by strong interactions between DOPA and iron oxide nanoparticles ensure that the resulting crosslinked network makes the hybrid hard and robust. Nanoindentation studies show very high values of elastic modulus (in GPa) and hardness and the hybrid multilayers can be used as multifunctional adhesive coatings. In addition, the structural ordering in the hybrid multilayers appears to be an important factor in the Mössbauer measurements when the thin films are studied in external magnetic field.