Metamaterials: the case of glassy carbon microlattices

It is often difficult to improve on a definition provided by everyone’s prima-facie source on science and technology. The Free Encyclopedia describes metamaterials thus:

Metamaterials (from the Greek word "meta-", μετά- meaning "to go beyond") are smart materials engineered to have properties that have not yet been found in nature. They are made from assemblies of multiple elements fashioned from composite materials such as metals or plastics. The materials are usually arranged in repeating patterns, at scales that are smaller than the wavelengths of the phenomena they influence. Metamaterials derive their properties not from the properties of the base materials, but from their newly designed structures. Their precise shape, geometry, size, orientation and arrangement gives them their smart properties capable to manipulate electromagnetic waves: by blocking, absorbing, enhancing, bending waves, to achieve benefits that go beyond what is possible with conventional materials. …. Metamaterial research is interdisciplinary and involves such fields as electrical engineering, electromagnetics, classical optics, solid state physics, microwave and antennae engineering, optoelectronics, material sciences, nanoscience and semiconductor engineering.

I have commented on microlattice structures very recently but the topic takes no rest and rather continues to heat up as German scientists at the Karlsruhe Institute of Technology have now built the smallest human-made truss to date, featuring single strut lengths <1 μm and strut diameters of 200 nm made of no composite but of mere glassy carbon – five times smaller than previously known and comparable metamaterials. This dimension achieves hitherto unprecedented strength-to-density ratios that, at the most preliminary consideration, suggest applications in electrodes, filters, or optical elements. Now, it is fairly common knowledge that light and partially hollow materials such as bone and wood may be found just about everywhere in nature. So proof of concept is old news.  These materials typically combine high-load capacity with low weight and serve as a biomimetic model for man-made mechanical metamaterials with a structure planned and produced to have mechanical or optical properties that unstructured solids cannot match as a matter of principle. Think stealth features that direct light, sound or heat around objects, auxetic materials that react counter-intuitively to pressure and to shear, or nanomaterials featuring high specific strength (force per unit area and density). The metamaterial now created in Karlsruhe by 3D laser lithography is extremely stable and its strength in relation to its specific density of up to 3GPa is surpassed only by diamond.

Diamond itself has in recent times been “improved” by significant advances in materials science: in 2003, graphite and later fullerene was compressed to create what has become known as nanodiamond, hyperdiamond, or aggregated diamond nanorods – the hardest and least compressible material known today (~310 GPa). Aggregated nanorods are between 5-20 nm in diameter and have a length of approximately 1 μm. They also occur in nature up to about 1 mm in size when graphite is compressed by meteoritic impact.

Fullerenes have been identified in outer space and may have been the original conveyors of seeds for life on Earth, hence constitute one of the most stimulating research agendas since their discovery circa 1985. Nanodiamonds present a phenomenal range of potential applications spanning a wide range from drug delivery, surgery, skin care, blood testing, sensors, optical and quantum computing.

But back to the seemingly pedestrian glassy amorphous carbon:  in general, it is non-graphitizing, non-graphitizable, free, reactive carbon, combining glassy and ceramic properties with those of graphite. Its essential properties are low density, low friction, low thermal resistance but high temperature resistance, hardness (7 Mohs, like quartz), exceptional resistance to chemical attack and impermeability to gases and liquids. It is without any crystalline structure although, as with all amorphous solids, some short-range order may be observed. It is created in a micrometer-size structure by computer-aided laser hardening of photoresist. Still, the procedure’s maximum resolution only permits strut lengths of 5-10 μm length and 1 μm diameter. But the entire polymer structure is shrunk 80% in a second step by pyrolysis in a vacuum furnace at 900oC, which results in realignment of chemical bonds, shrinking the structure to one-fifth of its previous size. During that process, all chemical elements except carbon disappear from the resist which remains as pure glassy carbon in a shrunk microlattice truss structure. Load capacity of the truss now very closely approximates its theoretical limit, far surpassing unstructured glassy carbon.

Microstructure materials are often used for insulation or to absorb shock. Open pore materials may be used as nanofilters, but also as carriers of catalytic substances with an extremely large internal surface. Metamaterials also have extraordinary optical characteristics with potential applications in telecommunication. Glassy carbon, a.k.a. vitreous carbon, is a high-tech form of pure carbon with fullerene-related structures combining characteristics of glass-like ceramics with those if graphite.

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