Elastic properties of MgO nanocrystals and grain boundaries at high pressures by Brillouin scattering

摘要

Intercrystalline materials (mostly grain boundaries) constitute a substantial fraction of the volume in nanomaterials and significantly influence the material's elasticity, as well as other physical and chemical properties. The effective elastic behavior of intercrystalline material is poorly understood at room pressure and unknown at elevated pressures. Here, we measured acoustic wave velocities of nanocrystalline MgO to pressures of 30 GPa by Brillouin scattering at room temperature. We estimate that both the zero-pressure bulk and shear moduli of the intercrystalline material (K0JC and Go,;c) are reduced by at least 50% compared with the crystalline material. We show that this profound reduction in elastic moduli is preserved at high pressures. Sample characterization as a function of pressure by synchrotron x-ray diffraction shows that the average crystallite size stabilizes to 7 (± 1) nm at high pressure. The microstructure of the sample material after compression was analyzed by high-resolution transmission and scanning electron microscopy. The implications of our observations for the interpretation of Brillouin scattering results from polycrystalline materials are discussed.