Wednesday, February 5, 2014

KIT demonstrates lightweight ALD coated material that mimics the structure of bone

"High-strength cellular ceramic composites with 3D microarchitecture" Bauer et al., Karlsruhe Nano Micro Facility, Karlsruhe Institute of Technology, D-76131 Karlsruhe, Germany LINK (10.1073/pnas.1315147111, including supporting free avaialble material, videos and pictures)
Uniaxial compression test of a polymeric truss structure (right). Failure due to local buckling of diagonal struts under compressive load is followed by large plastic deformations and fracture. Before the collapse, bending of single struts due to processing-related predeformation (shrinking effects) also is observed. Uniaxial compression test of a polymeric truss structure (left) coated with 10 nm of [ALD] Al2O3. Local buckling of individual vertical compression bars leads to the immediate collapse of the whole structure. Only a little plastic deformation can be observed.

To enhance the strength-to-weight ratio of a material, one may try to either improve the strength or lower the density, or both. The lightest solid materials have a density in the range of 1,000 kg/m3; only cellular materials, such as technical foams, can reach considerably lower values. However, compared with corresponding bulk materials, their specific strength generally is significantly lower. Cellular topologies may be divided into bending- and stretching-dominated ones. Technical foams are structured randomly and behave in a bending-dominated way, which is less weight efficient, with respect to strength, than stretching-dominated behavior, such as in regular braced frameworks. Cancellous bone and other natural cellular solids have an optimized architecture. Their basic material is structured hierarchically and consists of nanometer-size elements, providing a benefit from size effects in the material strength. Designing cellular materials with a specific microarchitecture would allow one to exploit the structural advantages of stretching-dominated constructions as well as size-dependent strengthening effects. In this paper, we demonstrate that such materials may be fabricated. Applying 3D laser lithography, we produced and characterized micro-truss and -shell structures made from alumina–polymer composite. Size-dependent strengthening of alumina shells has been observed, particularly when applied with a characteristic thickness below 100 nm. The presented artificial cellular materials reach compressive strengths up to 280 MPa with densities well below 1,000 kg/m3.