|M.Sc Student||Younis Saeed|
|Subject||Atomic and Nano-Structure of Aragonitic Mollusk|
|Department||Department of Materials Science and Engineering||Supervisors||Professor Emeritus Emil Zolotoyabko|
|Professor Boaz Pokroy|
|Full Thesis text|
Organisms produce more than 60 minerals that serve to maintain their different functions. During last decades, great deal of attention has been drawn to mollusk shells because of superior mechanical properties of the latter, mainly resistance to fracture. Mollusk shells are natural composite materials comprising 95-99 wt.% of ceramic calcium carbonate crystallites, the rest being an organic substance. The alternating hard ceramic and soft organic layers mold complicated inhomogeneous micro- and nano-structures, which have an instrumental effect on crack confinement and de-localization of deformation fields. It is therefore important to study fine details of these ultra-structures towards better understanding of their effect on the physical characteristics of the shells.
In this thesis, we study the structural inhomogeneity in mollusk shells, focusing on the Murex troscheli gastropod shells and Perna canaliculus bivalve shells. While Murex troscheli shells are mainly composed of a crossed-lamellar structure, i.e. a hierarchical lath-like structure, the major component of Perna canaliculus shells is nacre, in the form of uniaxially oriented pseudo-hexagonal tablets. Data obtained by us using synchrotron X-ray diffraction demonstrate that the lattice parameters of biogenic aragonite extracted from both shells differ considerably from those found in geological samples, the relative distortions reaching up to 0.2%. These distortions were found to disappear after mild annealing at 200 0C, which allowed us to assume that intra-crystalline organics is responsible for this effect. In fact, high-resolution transmission electron microscopy (HRTEM) measurements in Perna canaliculus shells revealed nano-sized organic inclusions within individual nacre tablets. Besides that, by the aid of HRTEM we found ceramic asperities and continuous mineral bridges that penetrate into the inter-crystalline organic layers between adjacent nacre tablets. Local pole figures obtained by means of electron backscatter diffraction (EBSD) in scanning electron microscopy (SEM) show three discrete preferred orientations of these tablets in the plane of the inner shell’s surface. The orientation relationships are dictated by twinning processes. In Murex troscheli shells, previously unreported the 4-th and 5-th hierarchical levels in the crossed-lamellar microstructure were visualized by high-resolution SEM and HRTEM. Most probably, these features are also related to twinning in biogenic aragonite.
Our experimental findings support the concept that mollusk shells are compositionally and functionally graded nano-materials. We believe that deeper understanding of the structure and nanostructure of biogenic crystals will aid in the development of new approaches towards growing organic/inorganic composite materials and controlling their properties on a molecular level.