Exploiting the structure-property relationship in Bragg stacks: From phononic superlattices to bioinspired hybrids
- 파로하 리아캇
- 자연과학대학 화학부
- Issue Date
- 서울대학교 대학원
- phononics; Bragg stacks; hypersound propagation; band gap; high-strength multilayers; nanoindentation
- 학위논문 (박사)-- 서울대학교 대학원 : 화학부, 2014. 8. 손병혁.
- 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 Greens 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.