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Non Classical Crystallization: From Sea Urchin Spines to Fracture Resistant Cement
Helmut Coelfen
University of Konstanz, Dpt of Physical Chemistry
Fri, Jun. 28th 2013, 11:00
NIMBE Bât.125, p.157, CEA-Saclay

Biominerals teach excellent lessons about advanced materials design. Their structural design is optimized for the specific materials purpose and often, the beneficial properties are generated on several hierarchy levels. Consequently, Biominerals are an intense subject of research to reveal the design principles. This led to the discovery of Non Classical Crystallization.
This presentation begins with an introduction into Biomineralization as a rich source of inspiration for materials scientists. An attempt towards combination of several advantageous biomineral properties, namely the fracture resistance of Nacre, the wear resistance of chiton teeth and the magnetic properties of magnetotactic bacteria will be reported.

Then, the concept of Non Classical Crystallization as well as Mesocrystals will be introduced. Sea urchin spines (see image below) are one example of a so called mesocrystal, a superstructure of crystallographically aligned nanocrystals. Since mesocrystals are an attractive species combining properties of nano-­ and microcrystals, which are formed along non classical crystallization pathways, their formation mechanisms are of interest. Up to now, 6 different formation mechanisms are currently discussed for mesocrystals. Several of these will be presented showing new ways to structure nanocrystals into a mutually oriented structure.

Especially interesting is the adaptation of the mesocrystal structure to the most applied material of humans – cement and concrete. Cement and concrete mechanical properties are based on their glue – calcium silicate hydrates (C-­‐S-­‐H) of pH dependent stoichiometry. Up to now, high compression strength but poor tensile strength are responsible for the lack of the very desired elasticity of cement. It will be shown how specific polymeric additives can be found to address C-­‐S-­‐H in the complex cement mixture leading to colloidally stable C-­‐S-­‐H which can be destabilized by simple means like pH increase as well as supersaturation increase. The result is mesocrystalline C-­‐S-­‐H with very remarkable properties originating from its mesocrystal structure in similarity to the sea urchin spine.    

Microstructure of a sea urchin spine (Scanning electron micrograph) – a biological mesocrystal. Although the spine has the properties of a single crystal, its morphology exhibits no crystal faces which are typical for single crystals. (J. Seto et al., Structure-­‐property relationships of a biological mesocrystal in the adult sea urchin spine. Proceedings of the National Academy of Sciences 109, 3699 (2012).)


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#26 - Last update : 10/10/2012

 

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