Improved Dielectric Properties and Grain Boundary Effect of Phenanthrene Under High Pressure

In situ impedance measurements, Raman measurements and theoretical calculations were performed to investigate the electrical transport and vibrational properties of polycrystalline phenanthrene. Two phase transitions were observed in the Raman spectra at 2.3 and 5.9 GPa, while phenanthrene transformed into an amorphous phase above 12.1 GPa. Three discontinuous changes in bulk and grain boundary resistance and relaxation frequency with pressure were attributed to the structural phase transitions. Grain boundaries were found to play a dominant role in the carrier transport process of phenanthrene. The dielectric performance of phenanthrene was effectively improved by pressure. A significant mismatch between Z″ and M″ peaks was observed, which was attributed to the localized electronic conduction in phenanthrene. Theoretical calculations showed that the intramolecular interactions were enhanced under compression. This study offers new insight into the electrical properties as well as grain boundary effect in organic semiconductors at high pressure.

Grain Boundary Evolution of Cellular Nanostructured Sm-Co Permanent Magnets

Grain boundaries are thought to be the primary demagnetization sites of precipitate-hardening 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets with a unique cellular nanostructure, leading to a poor squareness factor as well as a much lower than ideal energy product. In this work, we investigated the grain boundary microstructure evolution of a model magnet Sm25Co46.9Fe19.5Cu5.6Zr3.0 (wt. %) during the aging process. The transmission electron microscopy (TEM) investigations showed that the grain boundary region contains undecomposed 2:17H, partially ordered 2:17R, 1:5H nano-precipitates, and a Smn+1Co5n−1 (n = 2, 1:3R; n = 3, 2:7R; n = 4, 5:19R) phase mixture at the solution-treated state. After short-term aging, further decomposition of 2:17H occurs, characterized by the gradual ordering of 2:17R, the precipitation of the 1:5H phase, and the gradual growth of Smn+1Co5n−1 compounds. Due to the lack of a defect-aggregated cell boundary near the grain boundary, the 1:5H precipitates are constrained between the 2:17R and the Smn+1Co5n−1 nano-sheets. When further aging the magnet, the grain boundary 1:5H precipitates transform into Smn+1Co5n−1 compounds. As the Smn+1Co5n−1 compounds are magnetically softer than the 1:5H precipitates, the grain boundaries then act as the primary demagnetization sites. Our work adds important insights toward the understanding of the grain boundary effect of 2:17-type Sm-Co-Fe-Cu-Zr magnets.