Phase Relationship of Mg2Si at High Pressures and High Temperatures and Thermoelectric Properties of Mg9Si5.

Magnesium silicide (Mg2Si) is a promising eco-friendly thermoelectric material, which has been extensively studied in recent times. However, its phase behavior at high pressures and temperatures remains unclear. To this end, in this study, in situ X-ray diffraction analysis was conducted at high pressures ranging from 0 to 11.3 GPa and high temperatures ranging from 296 to 1524 K, followed by quenching. The antifluorite-phase Mg2Si decomposed to Mg9Si5 and Mg at pressures above 3 GPa and temperatures above 970 K. The antifluorite-phase Mg2Si underwent a structural phase transition to yield a high-pressure room-temperature (HPRT) phase at pressures above 10.5 GPa and at room temperature. This HPRT phase also decomposed to Mg9Si5 and Mg when heated at ∼11 GPa. When 5Mg2Si decomposed to Mg9Si5 and Mg, the volume reduced by ∼6%. Mg9Si5 synthesized at high pressures and high temperatures was quenchable under ambient conditions. Thermoelectric property measurements of Mg9Si5 at temperatures ranging from 10 to 390 K revealed that it was a p-type semiconductor having a dimensionless thermoelectric figure of merit (ZT) of 3.4 × 10-4 at 283 K.

The conducting fibrillar networks of a PEDOT:PSS hydrogel and an organogel prepared by the gel-film formation process.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a practical conducting polymer. The gel-film formation process produces a PEDOT:PSS organogel with a structure between a PEDOT:PSS water dispersion and a dried film. We found that this film has a high water-swelling ratio and thickens by a hitherto unreported factor of approximately 6600% as its swells to form a hydrogel. In this study, we investigated the drying behaviour of a hydrogel and an organogel with electrical properties to elucidate the internal structures of the gel responsible for the swelling and shrinkage behaviour with high expansion and contraction ratios. SEM revealed that the gel is composed of a 3D fibrillar network consisting of fibrils that are 4.6 ± 1.6 µm long and 0.63 ± 0.29 µm in diameter. This network plays a pivotal role in the conduction of electricity and swelling behaviour with high expansion ratios. The thickness of the gel decreased to 1/66 of its original value after drying on a substrate, while the total electrical resistance decreased by only 20%. The organogel exhibited the same drying behaviour as the hydrogel, which indicates that the network forms first in the organogel and is maintained in the subsequent swelling and drying processes. The electrical conductivity of the hydrogel increased from 9.0 ± 0.1 to 346.4 ± 1.2 S cm-1 under anisotropic shrinking from 3.1 ± 0.2 mm to 77.4 ± 3.3 µm. The network plays an important role as an enhanced swelling framework by providing effective pathways for the conduction of electricity.