Heat-capacity comparison among the nanocrystalline, amorphous, and coarse-grained polycrystalline states in element selenium.

Abstract

Nanocrystalline element selenium ~nc-Se! samples were synthesized by completely crystallizing a meltquenched amorphous selenium ~a-Se!. Heat capacities ( Cp) of the nc-Se ~with a mean crystallite size of 10 nm!, a fully relaxed a-Se, and a coarse-grained polycrystalline Se were experimentally determined in a temperature range of 225‐500 K. Measurement results show that there is no pronounced difference between the Cp values of the nc-Se and the a-Se below 250 K. Heat capacities of the nc-Se are found to be only about 2% higher than those of the coarse-grained polycrystalline selenium. The Cp enhancement for the nc-Se, which is much lower than those reported in other nanocrystalline elements, is discussed in terms of the grain-boundary configurations in the nanostructures. @S0163-1829~96!02030-9# Heat capacity of a material is directly related to its atomic structure, or its vibrational and configurational entropy which is significantly affected by the nearest-neighbor configurations. Nanocrystalline ~nc! materials are structurally characterized by the ultrafine crystalline grains, and a large fraction of atoms located in the metastable grain boundaries in which the nearest-neighbor configurations are much different from those in the crystallites. Or in other words, the grain-boundary possesses an excess volume with respect to the perfect crystal lattice. Therefore, heat capacities of the nc materials are expected to be higher than those of the corresponding coarse-grained polycrystalline counterparts. Experimental measurements of heat capacities at constant pressure, Cp in several nc materials synthesized by means of different methods indicated that C p values are frequently higher than those for the coarse-grained polycrystalline counterparts, and are even higher than those for the amorphous states as well. 1‐3 For example, the Cp values of a nc-Pd synthesized by consolidation of ultrafine metal powders were reported to be as much as 60% higher than those for the coarse-grained polycrystalline Pd. 1 The heat-capacity values of a ball-milled nc-Ru sample were found to be about 20% higher than those of its coarse-grained polycrystalline counterparts. 2 Nevertheless, these results should be accepted cautiously because the measured Cp values might be influenced by many factors. Porosity and possible gaseous contamination @e.g., the sample has a density of only 80% and a total impurity content of about 1 at. % in the case of the nc-Pd ~Ref. 1!# in the ultrafine-powder-consolidated nc materials may contribute to the enhanced Cp . While in the ballmilled nc specimens, contamination from the milling media and the subsequent consolidation conditions might alter the heat-capacity results. Up to now, solid experimental results of heat-capacity comparison among the amorphous, the coarse-grained polycrystalline, and the nc states are still lacking.