Cold Sintering of Nanocrystalline Powders and Nanocomposites

Dense nanocrystalline materials can be synthesized by assembling atom clusters that are obtained by physical or chemical methods or by mechanical attrition. Because it is difficult to produce bulk materials by physical or chemical vapor deposition or by electrochemical deposition methods, consolidation of nanosize powders is the only practical way to fabricate bulk nanocrystalline materials (Ref 52, 53, 54). Until recently consolidation of nanoscale powders has resulted in densities "-85% TD and not >95% TD. Porous nanocrystalline materials find a variety of useful applications as catalysts or extra fine molecular filters/sieves. However, the potential of nanoscale materials in the field of high performance structural and functional applications, as well as the properties of dense nanoscale materials, remain practically unexplored due to inability to achieve full density.

Cold sintering performed immediately after reduction treatment has been successfully used for consolidation of nanocrystalline aluminum, nickel, and iron powders as well as Ni-TiC nanocomposites, and densities >99% have been obtained (Ref 55). It should be emphasized that the effect of surface oxides is especially pronounced for nanoscale powders characterized by an unusually high grain boundary density. These films are formed during the manufacturing of powders, because contamination of nanosize particles cannot be completely prevented at this stage. It is believed that the relatively low densities reported in most publications dealing with consolidation of nanocrystalline metal powders (e.g., 95% TD for untreated nickel powder consolidated at 5 GPa and 900 °C (Ref 55) compared to >99% TD obtained by cold sintering in vacuum (10-3 Torr) at 3 GPa and 300 °C of a similar powder heat treated in a hydrogen flow) are the result of poor adhesion caused by the presence of surface oxides.

High strength and microhardness values were obtained in cold sintered nanocrystalline materials (Ref 55). Thus, i7y = 600 and 1100 MPa were obtained for pure nanocrystalline nickel (65 nm) and iron (30 nm), respectively. This is considerably higher than the strength of these metals cold sintered from 1 /'m size powders: tTy = 200 and 300 MPa for nickel and iron, respectively.

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