Effect Of The Grain Size And Interfaces On Properties Of Bulk Nanosubstances

Properties of bulk nanomaterials depending on the grain size and the state of grain boundaries have been analyzed in reviews.'6,38,39'

At 300 K, the microhardness of bulk nanocrystalline substances is usually several times larger than HV of coarse-grained substances. The growth of HV was observed with decreasing size of n-Fe and n-Ni grains.'40' The microhardness HV of nanocrystalline n-Cu copper (D ~ 16 nm) is ^2.5 times larger than that of copper with grains 5 mm in size. However, as the size of n-Cu grains diminishes from 16 to 8nm, HV decreases by ~25%. The decrease in HV is also observed when n-Pd grains are refined from 13 to 7 nm. The microhardness HV of Ni-P, TiAlNb, TiAl, and NbAl3 nanocrystalline alloys drops as the grain size decreases from 60-100 to 6-10 nm.

In a general case, the microhardness of nanosub-stances grows as the grain size decreases to some Dc value and drops at D < Dc. Mechanical and elastic properties of nanocrystalline metals are determined not only by a small size of grains, but also by the state of interfaces. Therefore contradictory results on the dimension dependence of the microhardness may be due to different structures of interfaces.

Strength properties of nanosubstances are enhanced with decreasing size of grains. The yield stress of nanocrystalline Pd (D = 5-15 nm) and Cu (D = 2550 nm) is 2-3 times higher than the yield stress of coarse-grained metals.'41' The tensile strength of nanocrystalline metals is 1.5-8 times larger than that of coarse-grained metals.'40,41'

At temperatures from 150 to 300 K, the heat capacity Cp of n-Pd (D = 6 nm) and n-Cu (D = 8 nm) is 30-50% and ~10% higher than the heat capacity of coarse-grained bulk Pd and Cu, respectively. In the interval of 0.06 to 10.0 K, the low-temperature heat capacity of compacted nanocrystalline copper n-Cu with grains 6.0 and 8.5 nm in size proved to be 5-10 times larger than the heat capacity of coarse-grained copper. Measurements of the heat capacity of amorphous, nanocrystalline, and coarse-grained selenium Se over the temperature interval from 220 to 500 K'42' revealed a small increase in the heat capacity of bulk nanocrystalline n-Se as compared with coarse-grained Se at T < 375 K. A comparison of the heat capacity of substances in nanocrystalline, amorphous, and coarse-grained states'43' showed that the heat capacity of samples prepared by compaction of nanopowders is largely different from the heat capacity of substances in the coarse-grain state (Table 1). Oppositely, this difference does not exceed 2% for samples prepared by crystallization from the amorphous state. One may think that most of the excess heat capacity of compacted nanomaterials is a result of a large surface area of interfaces, structural distortions, and impurities.

The thermal expansion coefficient a is proportional to the heat capacity. Therefore the coefficient a of bulk nanosubstances should be higher than a of coarsegrained polycrystals. Indeed, the coefficient a of nano-crystalline copper n-Cu with grains 8 nm in size on the average is twice as large as a of coarse-grained copper.'44'

A large surface area of interfaces and a high concentration of defects determine an intensive scattering of charge carriers in nanomaterials. A considerable increase in electroresistivity p of nanocrystalline Cu, Pd, Fe, and Ni and various alloys with decreasing size of grains has been noted by many researchers. For example, at temperatures 0 < T < 275 K, electrore-sistivity of n-Cu (D = 7 nm) is 7 to 20 times larger than p of common coarse-grained copper.

The effect of the nanostate on magnetic properties of paramagnetics is well pronounced, e.g., in palladium (Fig. 14).'8' At 300 K, susceptibilities of nanocrystalline n-Pd and the initial coarse-grained palladium differ by 8%. According to Ref.'8' such a considerable variation of the susceptibility is a result of the presence of intra-grain vacancy complexes in n-Pd, which change the density of electron states at the Fermi level.

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