Nano Letters 2006 doi: 10.1021/nl061978i |ACS:: Abs.Full.Pdf| CrossRef 
ACS published online 26 September 2006

high strength but high porosity

Most mesoporous materials are artificial and exhibit characteristic spongelike structure resembling as their natural counterparts of sands or bones. Artificial porous materials find many applications. Not only are used as structural materials like filter, template, host, and shock buffer but also as functional materials like catalyst, electrode or sensor with integrated systems.

Recent findings of mesostructural material with high-than-expected strength suggest strong guarders during fatally shocking. The typical samples are the multilayer CNTs’ arrays (1), colloids particle film (2) and porous metals, for example, nanoporous Au reported in October 2006 issue Nano Lett. (3). Within these topics, it should be noted that the embedded meanings of mesostructures are very similar to those of meso/nano-porous. Sometimes they are the same concepts. 

nanoporous gold foam: small labyrinth 2006
Credit: Scidea Art 2006 Source: www.ScideaNews.com

Juergen Biener and his colleagues try to elucidate a size-dependent mechanism for this higher-than-expected strength, named a dramatic increase in yield strength with decreasing sample size. The experiment along with MD simulation indicated that the bristle diameter of the porous skeleton is key parameter for attaining better yield strength.  

The results imply an optimal size regime. In some cases, the strength response showed a dominant contradiction to conventional bulklike plasticity of the macroscopic celluar solids (porous foam) –the strength of foam materials always decreases with increasing porosity—which is developed by Gibson and Ashby (4). However, if decreasing the diameter of the skeleton wires of Au nanofoam to 10 nm, so-called “defect-free” length scale regime, the mechanical response seemed normal again: "(it) should weaken the material instead of further increasing its strength, similar to the reverse Hall-Petch regime predicted for nanocrystalline materials", said the authors.

The intrinsic physics is mainly attributes to the dynamics of the crystal defects. The abnormal mechanical response is related to the defects' arrangement during external load, and the dimensions of crystal defects as well as their networks’ parameter, i.e. the distance between two neighboring dislocations. For most dislocations, the length is about 10 nm.

Considering the present progresses in nanoscience, it is too early to claim a mechanical phase map for meso/nano-porous materials, because we have not known clearly about the dislocation mechanics of nanowires as well as the interesting nanocrystal wires such as zig-zag nanowires with periodic twinning boundaries (5). As to the later, making zig-zag nanowires needs higher energy than that of straight wires, thus it is also a pathway to absorb energy. On the other hand, the knots of the skeleton wires also complicate the foam mechanics with its random or fractal distribution. Some care is needed in connecting the press-induced transformation.

As the endnotes, the Au nanofoams used by the authors were fabricated by free corrosion of the start alloys of Ag_0.75Au_0.25 or Ag_0.70Au_0.30 in 70% HNO₃ for 2~5 days, or by electrochemically dealloying with applied potential of ~ 1 V measured versus a Ag pseudoreference electrode in a 1 M HNO₃ + 0.01 M AgNO₃ electrolyte. The porosity thus the size of the bristle diameter was adjusted by etched time.

* Lin Pu is in the Physics Department of Nanjing University, Nanjing 210093, CHINA.

References 

Citation

Lin PU. High strength but high porosity. Scidea Sketch 1 (1)，ss20060926a1(2007).  

♦ doi: 10.3128/ss20060926a1 | Scidea:: Abs . Full | CrossRef
♦ ISSN: 1992 - 8548