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PROPERTIES 71 5 Properties NANOPHASE COMPACTS The structure-related properties of nanophase materials are expected to be different from those of normally available single-crystal, polycrystal, or amorphous materials because of their ultrafine grain size and large interfacial volume fraction. Initial investigations indicate that such an expectation is indeed justified. Birringer and coworkers (1986) have tabulated a number of properties measured on nanocrystalline metals and compared them with values for their coarse-grained (or single-crystal) counterparts and similar glassy materials. Some of these comparisons are shown in Table 1 (Chapter 1), in which it can be seen that the change in a variety of materials properties appears to be significantly greater in going from conventional crystalline material to nanocrystalline material than in going from crystalline to glassy solid, changes that are generally less than 10 percent. Very few property measurements have been made on nanophase materials to date, and the full impact of their ultrafine microstructures on their properties will only be elucidated by further research in this new area of materials synthesis. Some examples can be given here, however, of such research on nanophase metals and ceramics, dealing with their magnetic and mechanical properties and with diffusion and solid-state reactions in them, that depend on their ultrafine microstructure. Magnetic Properties. A recent study (Cowen et al., 1987) of the magnetic properties of nanocrystalline Er has been carried out, and the results were compared with measurements on the coarse-grained polycrystalline Er starting material. The
PROPERTIES 72 results are shown in Figure 18, where the reciprocal magnetic susceptibility is plotted against temperature for three samples with different grain sizes. The three normally observed magnetic phase transitions, associated with competing exchange and anisotropic interactions, can be seen for the coarse-grained polycrystalline Er starting material at 19, 52, and 85 K in Figure 18(a). However, for the slowly evaporated nanocrystalline Er, with grain diameters in the lower end of the range 10 to 70 nm, these magnetic phase transitions are no longer observed, and a new low-temperature transition to superparamagnetic or spin glasslike behavior arises. On the other hand, for rapidly evaporated nanocrystalline Er, with grain diameters larger than in Figure 18(b) and in the upper end of the range 10 to 70 nm, the normal magnetic phase transitions reappear, but at different temperatures, while the low- temperature superparamagnetic behavior is retained. The detailed magnetic properties of these nanophase Er samples are thus strongly affected by the nanometer scale of their grains and consequently the large volume fraction of grain boundaries. How these properties depend on the grain size, the fraction and location of atoms in grain boundary sites, and the possible formation of Er-oxygen compounds at these interfaces during synthesis, however, remains to be elucidated. Figure 18 Reciprocal magnetic susceptibility versus temperature for three Er samples from the same starting material but with different grain sizes: (a) coarse-grained polycrystal, (b) slowly evaporated nanocrystal, and (c) rapidly evaporated nanocrystal (Cowen et al., 1987).
