When 10:30 AM - 11:30 AM Oct 07, 2016
Where 1571 G.G. Brown
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Less is More : New Innovations and Applications for Materials made by Dealloying

Jonah Erlebacher
Department of Materials Science and Engineering, Johns Hopkins University

Dealloying is a materials processing method whereby one component is dissolved out of a multi-component alloy under conditions where the remaining components reorganize into a tortured yet beautiful porous structure. A great thing about dealloying is that the porosity is tunable - by suitable choice of starting materials and processing conditions, almost any pore size can be fabricated, from nanometers to microns, out of almost anything. Most study in this area has focused upon electrochemical dealloying, where dissolution is mediated by an electrolyte, and upon porous precious metals and alloys. Technology drivers in this area have focused primarily in catalysis applications, such as the electrocatalysis found in energy systems, and we will touch on how our dealloyed, nanoporous Ni-Pt is one of the most active catalysts for the fuel cell oxygen reduction reaction at the cathode (currently the rate-limiting reaction in the adoption of fuel cells into the automotive sector).

Mostly, however, we will focus here on the general kinetics of the dealloying process and how the complex choreography of atomic dissolution and interface diffusion drive the pattern forming instability that leads to nanoporosity. Recently, we have extended this fundamental dealloying knowledge to predict and then discover a new class of nanoporous metals - nanoporous refractories such as Ta, Nb, Mo, W, etc. - that cannot be made using electrochemistry. In our case, we alloy these refractory elements with titanium, and then dip our base alloy material into molten copper alloys at temperatures over 1000° C to dissolve out the Ti. This process, "liquid metal dealloying", or, LMD, is indeed as fun as it sounds, and during the process the refractory component reorganizes into a porous structure. Furthermore, once porosity forms in the molten metal bath, you can just simply cool the system down to make a dense nanocomposite. These composites have remarkable mechanical properties such as good ductility and high strength that increases with decreasing feature size and we envision applications for these new materials in extreme environments where a significant fraction of internal interfaces is a critical materials design parameter.

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