We conducted in situ three-point bending experiments on beams with roughly square cross-sections, which we fabricated from the frustule of Coscinodiscus sp. We observe failure by brittle fracture at an average stress of 1.1 GPa. Analysis of crack propagation and shell morphology reveals a differentiation in the function of the frustule layers with the basal layer pores, which deflect crack propagation. We calculated the relative density of the frustule to be ∼30% and show that at this density the frustule has the highest strength-to-density ratio of 1,702 kN·m/kg, a significant departure from all reported biologic materials. We also performed nanoindentation on both the single basal layer of the frustule as well as the girdle band and show that these components display similar mechanical properties that also agree well with bending tests. Transmission electron microscopy analysis reveals that the frustule is made almost entirely of amorphous silica with a nanocrystalline proximal layer. No flaws are observed within the frustule material down to 2 nm. Finite element simulations of the threepoint bending experiments show that the basal layer carries most of the applied load whereas stresses within the cribrum and areolae layer are an order of magnitude lower. These results demonstrate the natural development of architecture in live organisms to simultaneously achieve light weight, strength, and exceptional structural integrity and may provide insight into evolutionary design.iatoms are single-cell algae that form a hard cell wall made of a silica/organic composite. The ability to produce a functional biosilica shell presents several natural precedents that fascinate and inspire scientists and engineers. One fascinating aspect of such silica glass shells is their intricate, varied, and detailed architecture. Diatoms are generally classified based on the symmetry of their shells: Centric diatoms display radial symmetry whereas pennate diatoms have bilateral symmetry. Fig. 1A shows a schematic of a typical centric diatom and reveals that the shells are composed of two halves, called frustules, that fit together in a Petri-dish configuration. The frustules are attached to each other around the perimeter of the shell by one or several girdle bands. The frustules are usually porous with pore size and density varying between species. The frustule shell can also be composed of multiple layers with a cellular structure within the shell.The proposed evolutionary functions for these intricate shell designs include nutrient acquisition, control of diatom sinking rate, control of turbulent flow around the cell, and protection from grazing and viral attack (1). Evidence in favor of a protective function is that the degree of shell silification depends on the environment, with greater amounts of silica found in shells grown in a predatory environment (2). As a deterrent to predation, the frustule makes use of an inherently brittle glass as a structurally protective material while balancing other evolutionary pressures. A denser ...
This work examines Cu(In,Ga)Se2 thin films fabricated by (1) selenization of pre-sputtered Cu-In-Ga and (2) co-evaporation of each constituent. The efficiency disparity between films deposited via these two methods is linked to differences in morphology and microstructure. Atomic force microscopy and scanning electron microscopy show that selenized films have rougher surfaces and poor adhesion to molybdenum back contact. Transmission electron microscopy and electron energy loss spectroscopy revealed multiple voids near the Mo layer in selenized films and a depletion of Na and Se around the voids. Residual stresses in co-evaporated films were found to be ∼1.23 GPa using wafer curvature measurements. Uniaxial compression experiments on 500 nm-diameter nanopillars carved out from co-evaporated films revealed the elastic modulus of 70.4 ± 6.5 GPa. Hertzian contact model applied to nanoindentation data on selenized films revealed the indentation modulus of 68.9 ± 12.4 GPa, which is in agreement with previous reports. This equivalence of the elastic moduli suggests that microstructural differences manifest themselves after the yield point. Typical plastic behavior with two distinct failure modes is observed in the extracted stress-strain results, with the yield strength of 640.9 ± 13.7 MPa for pillars that failed by shearing and 1100.8 ± 77.8 MPa for pillars that failed by shattering.
The authors create life‐sized synthetic replicas of marine diatom coscinodiscus sp frustules out of cyclohexyl polyhedral oligomeric silsesquioxanes (POSS). The authors demonstrate that these synthetic structures have biosilica‐like amorphous atomic‐level microstructure and mechanical attributes similar to those of a natural diatom. In situ beam bending and fracture experiments on micron‐sized excised sections of natural and synthetic diatoms reveal similarities in their mechanical properties: a Young's modulus of GPa and a fracture toughness of 0.78 ± 0.10 MPa m−1/2 for the synthetic materials; those of natural diatoms are GPa and MPa m−1/2, respectively. In situ single edge notched beam (SENB) bending fracture experiments reveal that fracture behavior of the natural and synthetic specimens is virtually indistinguishable and is characterized by the same brittle failure and crack‐arresting behavior enabled by the double‐wall geometry. Their fracture toughness is comparable to that of fully dense silica, which suggests that the natural diatoms’ frustule maintains its mechanical resilience even at <50% of the weight attained through multi‐scale architecture. The demonstrated ability to fabricate a synthetic hard biomaterial that is virtually indistinguishable from its natural counterpart while capturing its complex architecture, microstructure, and mechanical properties provides a powerful platform for investigating the specific role of each geometrical feature at every relevant length scale in the often sophisticated, multi‐scale hierarchical construct of hard biomaterials, and provides a robust pathway for property optimization.
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