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PROPERTIES 74 with similar measurements on a TiO2 (rutile) sample compacted at 0.32 GPa with a polyvinyl-alcohol sintering aid from commercial powders ball-milled to a mean grain diameter of 1.3 µm. The results of these investigations on TiO2 indicate that the nanophase compacts, which are rather well bonded on compaction even at room temperature, densify rapidly upon sintering above 500°C with only small increases in grain size. This gives hardness values greater than those of sintered coarser-grained commercial TiO2, but at some 600°C lower temperatures and without the need for compacting and sintering additives. Fracture toughness was found to be improved as well. Fracture measurements (Li et al., 1988) by SEM on similar samples also indicate improved mechanical properties for the nanophase TiO2 relative to coarser-grained conventionally prepared material, with a reduction in the temperature for transgranular fracture of 200°C in the nanophase samples. It is expected that more efficient mechanisms for crack-energy dissipation should be generally available in such ultrafine-grained materials through their grain-boundary networks, which represent a significant fraction of their volume. The intent of the use of properly treated ultrafine starting materials in processing is to fabricate uniform and reproducible submicron-sized microstructures in the unfired pieces, which would be retained in the fired microstructures. However, grain growth generally occurs during sintering, especially on sintering to theoretical density. This occurrence is not necessarily a failure. Using this processing approach results in uniform and reproducible microstructures with more favorable properties, obtained at lower temperatures (Joseph Pask, personal communication). Diffusion and Solid-State Reactions The study of diffusion processes in nanophase materials is of interest for a variety of reasons. First, the knowledge of such diffusion processes can contribute to the understanding of the nanocrystalline structure, particularly with respect to the nature of the interfaces. Second, because of the expected high diffusivities in a material with a large volume fraction of interfaces, doping of the interfacial regions or even alloying with a second element by diffusion along the nanophase grain-boundary network is a feasible process that would allow the tailoring or engineering of properties. Furthermore, the high diffusivities combined with the short reaction distances, resulting from the small particle sizes and high density of nucleation sites in the interfaces of nanophase materials, might permit the formation of interfacial metastable or stable phases in the solid state at fairly low temperatures. In addition, nanophase processing of ultrafine powders opens up a variety of possibilities for producing new metastable phases in bulk form by reacting binary mixtures of nanocrystals of different elements at low temperatures. The production of binary mixtures of nanocrystals in large quantities, compaction into the desired shape, and formation of the phase by solid-state reaction would be a straightforward process. The studies of diffusion and solid-state reactions occurring in
PROPERTIES 75 nanophase materials described in the next paragraphs form a basis for the understanding and control of such processes. Since nanophase materials are rather new as a research topic, only a small number of diffusion experiments have been performed until now. Horváth and coworkers (1987) have reported results on self-diffusion in nanocrystalline Cu using a radioactive-tracer technique combined with sputter sectioning. The grain-boundary diffusion coefficient at 353 K in nanocrystalline Cu is 3 orders of magnitude larger than that in a coarse-grained polycrystalline Cu sample (with an assumed grain boundary width of 1 nm) and 16 orders of magnitude larger than that (extrapolated from higher temperatures) in single-crystal Cu. A rather rough estimate, due to the limited temperature range of the measurements, yields 0.64 eV for the activation enthalpy for self-diffusion, which is even smaller than that for normal grain-boundary diffusion (1.06 eV), but very close to that for surface diffusion. Yet, one must be very careful with the interpretation of this value until a detailed characterization of the samples for internal surfaces and possible relaxation processes in the grain boundaries at low temperatures is performed. Recent investigations by H. J. Höfler, H. Hahn, and R. S. Averback (unpublished results, 1988) have confirmed these high diffusivities by measurements of Ag and Bi diffusion in Cu nanocrystals and of Cu and Ag diffusion in Pd nanocrystals obtained by a variety of techniques, including electron microprobe analysis, (Rutherford) He- backscattering (RBS), secondary-ion-mass spectroscopy, and sputtered-neutral-mass spectrometry. Similarly high diffusivities appear to pertain to nanophase ceramics as well, according to recent RBS measurements of Pt diffusion into nanophase TiO2 (Siegel and Eastman, 1989). To help provide a basis for the understanding of reaction kinetics in binary nanophase mixtures involving systems with large negative heats of mixing, and thus the tendency for phase formation, the reaction of a thin Bi film on a Pd nanocrystal was measured using He-backscattering and electron microscopy (H. J. Höfler, H. Hahn, and R. S. Averback, unpublished results, 1988). As shown in Figure 20, the formation of the equilibrium intermetallic compound Pd3Bi was observed by He-backscattering at rather low temperatures, and electron microscopy confirmed this phase formation. It was also found that significant grain growth occurred in the reacted zone but not on the opposite face of the sample, where no Bi was available. This is indicative of a grain-growth process that is caused by chemical driving forces similar to diffusion-induced grain-boundary migration (DIGM), which results in the alloying of a zone over which a grain boundary has moved. Under the same conditions, however, no Pd3Bi phase formation could be observed in evaporated thin-film samples, indicating again the increased diffusivities and reactivities in the nanocrystalline structure. The potential for nanophase materials processing shown by the low-temperature formation of Pd3Bi also offers a new method for the production of bulk metastable or stable compounds at low temperatures by interdiffusion starting from a binary mixture of
PROPERTIES 76 nanocrystals; an example of this method is described in the following for the case of Cu-Er alloys. Figure 20 He-backscattering spectra for a nanocrystalline Pd sample with an 80-nm thick Bi film deposited onto its surface after sputter cleaning: Before annealing (solid curve) and after 24 hours at 395 K (dashed curve). The experimental setup is shown in the inset. The Pd3 Bi phase formation with a fixed composition can be seen clearly at the front edge (H. J. Höfler, H. Hahn, and R. S. Averback, 1988, unpublished results). Cu and Er metal particles were produced simultaneously with the gas-condensation method and deposited onto the surface of a rotating cold finger in order to achieve a better mixture of the two components. The scraped powder was then compacted to about 2 GPa without exposing to air. Both chemical analysis and He-backscattering yielded a composition close to 50 atomic percent Cu. Debye-Scherrer x-ray analysis showed that, besides a small amount of unreacted Cu and Er particles, most of the sample was reacted to the equilibrium compound CuEr with a CsCl-structure. No additional phases could be identified. This experiment shows that the nanophase production method is clean enough to subsequently react particles with one another, even when highly reactive metals like Er are being used. Furthermore, it demonstrates that the intermixing of the particles of two metals in the convective He-gas
