Study On Fine Structure And Mechanical Properties Of Sea Shells

Shells have evolved to own a multifunctional shell through hundreds of million in a mild marine environment.Natural shells are marvelous and hierarchical complex assembled by several kinds of inorganic calcium carbonate and a small amount of organic matter.These natural shells always have remarkable mechanical performance such as high strength and high toughness,thereby burgeoning biomimetic materials based on the structure of natural shell have been there search focus in the fields of materials,chemistry and biology,etc.With the development of society and economic,traditional materials can't meet the needs of some projects,so new materials with multi-function and high performance need to be developed urgently.The morphology,composition and structure of shells not only provide inspiration for designs of biomimetic materials,but also provide vital reference value for the synthesis of the artificial materials.The microstructure and composition of different kinds of shells have a great influence on mechanical properties such as the strength,toughness,and hardness.Thus,selecting high-strength shell species and having a deeply understanding on their structure,composition and fracture mechanism can offer a theoretical basis for the preparation of the biomimetic materials.The mineralization mechanism of shells is analyzed,which provides a craft mentality for the synthesis of artificial gemstone materials at a normal temperature,can realize the benefit effect of low cost and high value.Combining with the vital microstructure of shells,it is of great significance to develop and prepare high-performance biomimetic materials.Based on the above assumptions,the thesis takes shells commonly selected in the South China as the research object to represent and measure the inorganic constituent,organic matter content and microstructure of shells.On the other hand,the flexural strength,compressive strength,fracture toughness property and hardness of selected shells are investigated.Meanwhile,in-depth analysis of the changes of physical and chemical properties of deep-ocean Tridacna gigas after permineralization and put forward the permineralization mechanism of shells.Finally,the biomimetic alumina ceramic materials with mineral bridge structure were prepared by room temperature sintering method,and the effect of mineral bridge structure on enhancing the strength and hardness of the materials was studied.The compositions and microstructures of four kinds of types shells,Cassis cornuta,Peristernia reincarnata,Tridacna squamosa and Tridacna crocea were investigated.It was found that the four kinds of shells are mainly composed of aragonite calcium carbonate and organic matter?1%-2%?.The inorganic phase of Cassis cornuta contains a spot of vaterite calcium carbonate.Shell is a hierarchical structure,and each macroscopic layer contains three levels structure of different dimensions.Each sub-layer is composed of fibrous aragonite calcium carbonate aggregation,which exist a certain range of angle.According to the microstructure of shells,the structure diagrams are constructed.After evaluating the mechanical properties of the four kinds of shells,it is found that Tridacna squamosa owns the highest compressive strength and hardness.The flexural strength of the first-order layer perpendicular to the loading direction is higher than that of the first-order layer parallel to the different loading direction which is the mechanical performance study for samples.With the increase of the numbers of the macroscopic layers,the compressive strength and hardness of shells increased.Based on the results of load-displacement curve and fracture path of sample,it is found that shells have plastic deformation,which greatly improves the strength and toughness of shells.In the meantime,there are many toughening mechanisms and synergistic effects in shells,such as unbroken mineral bridge,Zig-Zag path,mineral bridge,crack deflection,platelet pull-out,etc.By studying the microstructure and mechanical properties of deep-ocean permineralized Tridacna gigas,we find that permineralized Tridacna gigas has dense microstructure,small defect size,and high mechanical properties such as hardness,compressive strength and Young'modulus.When the light transmittance of the permineralized Tridacna gigas is 2.3 times than that of the normal one,the hardness,compressive strength and Young'modulus of the mineralized tridacna are increased by33.20%,23.71%and 3.79%respectively,but the anisotropy of the mechanical properties is reduced.Through the preparation of biomimetic structure ceramics with mineral bridge,it was found that the flexural strength and hardness of ceramic materials could be improved by adjusting the proportion of micron and nanometer alumina particles and increasing Ti O₂ nucleating agent.