Kinetic and Thermodynamic Properties of Nanocrystalline Materials

Abstract

A new class of materials with ultra small grain size has recently been synthesized by combining the methods of inert gas condensation of metal vapors and in situ powder compaction. These ‘nanocrystalline’ materials, with grain sizes of 5-10 nm, can have over 30% of their atoms lying in the highly disordered interfaces or grain boundaries. Because of their unique atomic structure, nanocrystalline materials often have properties far different from their bulk counterparts. In addition, kinetic processes can be rapidly accelerated due to the short diffusion distances between grains. In this review, we will report on the thermodynamic properties and reaction kinetics of nanocrystalline metals and on such kinetic properties as sintering and grain growth in nanocrystalline ceramics.

Nanostructured Materials

“…Furthermore, it has more recently been demonstrated [29] that sintering the same nanophase material under pressure (1 GPa), or with appropriate dopants such as Y, can further reduce the sintering temperatures, while suppressing grain growth as weil, thus allowing for the unique possibility to sinter nanophase ceramics to full density while retaining their ultrafine grain size. The resulting fracture characteristics [35,37,38] developed for sintered nanophase TiO2 are as good, or in some aspects improved, relative to those for conventional coarser-grained rutile.…”

“…For example, nanophase TiO 2 (rutile) exhibits significant improvements in both sinterability and resulting mechanical properties relative to conventionally synthesized coarser-grained rutile [24,29,35,36]. Nanophase TiO 2 with a 12 nm initial mean grain diameter has been shown [24] to sinter under ambient pressures at up to 600°C lower temperatures than conventional coarser-grained rutile, and without the need for any compacting or sintering aid, such as polyvinyl alcohol, which "s usually required.…”

“…Atomic diffusion in nanophase materials, which can also have a significant bearing on their mechanical properties, such as creep and superplasticity, and electrical properties as weil, has been found to be very rapid compared with conventional materials. Measurements of self-and solute-diffusion [35,[39][40][41][42] in as-consolidated nanophase metals and ceramics indicate that atomic transport is orders of magnitude faster in these materials than in coarser-grained polycrystalline samples. This very rapid diffusion in as-consolidated nanophase materials appears to be intrinsically coupled with the porous nature of the interfaces in these materials, since the diffusion can be suppressed back to conventional values by sintering samples to full density [35].…”

“…Measurements of self-and solute-diffusion [35,[39][40][41][42] in as-consolidated nanophase metals and ceramics indicate that atomic transport is orders of magnitude faster in these materials than in coarser-grained polycrystalline samples. This very rapid diffusion in as-consolidated nanophase materials appears to be intrinsically coupled with the porous nature of the interfaces in these materials, since the diffusion can be suppressed back to conventional values by sintering samples to full density [35]. Nonetheless, there exist considerable possibilities for efficiently doping nanophase materials at relatively low temperatures via the rapid diffusion available along their ubiquitous grain-boundary networks, with only short ' diffusion paths remaining into their grain interiors, to synthesize materials with tailored optical, electrical, or mechanical properties.…”

Reaction rate modeling in noncatalytic gas‐solid systems: Species transport and mechanical stress

“…It has been shown that at larger deviations from stoichiometry, so-called Magnkli phases that are two phase regions exist (Reed, 19701, as well as lower oxide phases such as TiO, Ti,O,, and Ti30, (Averback et al, 1989;Rode and Hlavacek, 1994). An assumption of this work is that a single phase exists.…”