Carbon fiber-reinforced carbon aerogel composites (C/CAs) for thermal insulators were prepared by copyrolysis of resorcinol-formaldehyde (RF) aerogels reinforced by oxidized polyacrylonitrile (PAN) fiber felts. The RF aerogel composites were obtained by impregnating PAN fiber felts with RF sols, then aging, ethanol exchanging, and drying at ambient pressure. Upon carbonization, the PAN fibers shrink with the RF aerogels, thus reducing the difference of shrinkage rates between the fiber reinforcements and the aerogel matrices, and resulting in C/CAs without any obvious cracks. The three point bend strength of the C/CAs is 7.1 ± 1.7 MPa, and the thermal conductivity is 0.328 W m(-1) K(-1) at 300 °C in air. These composites can be used as high-temperature thermal insulators (in inert atmospheres or vacuum) or supports for phase change materials in thermal protection system.
Cyclophanes of perylene tetracarboxylic diimides (PDIs) with different substituents at the bay positions, namely four phenoxy groups at the 1,7-positions (1), four piperidinyl groups at the 1,7-positions (2), and eight phenoxy groups at the 1,6,7,12-positions (3) of the two PDI rings, have been synthesized by the condensation of perylene dianhydride with amine in a dilute solution. These novel cyclophanes were characterized by (1)H NMR spectroscopy, MALDI-TOF mass spectrometry, electronic absorption spectroscopy, and elemental analysis. The conformational isomers of cyclophanes substituted with four piperidinyl groups at the 1,7-positions (2 a and 2 b) were successfully separated by preparative TLC. The main absorption band of the cyclophanes shifts significantly to the higher energy side in comparison with their monomeric counterparts, which indicates significant pi-pi interaction between the PDI units in the cyclophanes. Nevertheless, both the electronic absorption and fluorescence spectra of the cyclophanes were found to change along with the number and nature of the side groups at the bay positions of the PDI ring. Time-dependent DFT calculations on the conformational isomers 2 a and 2 b reproduce well their experimental electronic absorption spectra. Electrochemical studies reveal that the first oxidation and reduction potentials of the PDI ring in the cyclophanes increase significantly compared with those of the corresponding monomeric counterparts, in line with the change in the energy of the HOMO and LUMO according to the theoretical calculations.
Enhanced spin figure of merit in a Rashba quantum dot ring connected to ferromagnetic leadsWe propose a pure thermoelectric spin generator based on a Rashba quantum dot molecular junction by using the temperature difference instead of the usual voltage bias difference. A magnetic flux penetrating through the device is also considered. The spin Seebeck coefficient S S and the spin figure of merit Z S T of the molecular junction are calculated in terms of the Green's function formalism and the equation of motion (EOM) technique. It is found that a pure spin-up (spin-down) Seebeck coefficient can be generated by the coaction of the magnetic flux and the Rashba spin-orbit (RSO) interaction.
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