We have measured the thermal properties of an n-type hot-pressed Bi1.8Sb0.2Te3 alloy with a carrier concentration of 3.1×1019 cm-3. With the adjustable parameters determined from these experimental data, the dependence of thermal properties on the carrier concentrations is investigated, using the two-band-conduction model with a mixed scattering mechanism and a small fluctuation theory. The maximum dimensionless figures of merit of the alloy in a carrier concentration range of 0.8–3×1019 cm-3 have values over 0.7 at temperatures between 250 K and 300 K and a minimum thermal conductivity of 1 W/Km. These alloys are comparable to single crystals, showing the possibility that they provide higher thermoelectric performance than the single crystals, since their microstructure can be controlled by hot-press processing.
We introduce an annealing method performed with only Bi powders at 673 K. With this method, the carrier concentration is controlled and the conductivity type can be changed by adjusting the annealing time. Most importantly, conductivity-type transition occurs in an annealing time of 48 hours. The thermoelectric properties of the alloys are measured and described as functions of carrier concentration and annealing time. n-Type hot-pressed Bi1.8Sb0.2Te3 alloys that deviated from the optimal figure-of-merit Z condition owing to high carrier concentration can be optimized by this annealing method.
Improved lifetime with enhanced efficiency of an Alq 3 based OLED was realized by the co-evaporation of hexamethylbenzene (HMB). Two different solvated crystalline systems of Alq 3 , Alq 3 (C 7 H 8 ) 1/2 , Alq 3 (C 2 H 5 OH), were synthesized to investigate the effect of optically inactive materials on the emission properties of Alq 3 . Clathrate solvents perturb the excited electronic states of Alq 3 so that the incorporated materials in OLEDs should be chemically and optically inactive to control polymorphs. HMB acts as a carrier blocker for both electrons and holes. However, Alq 3 may be stabilized from Joule heat if the Joule heat is efficiently consumed as the vibrational/rotational energy of HMB. Consequently, the self-quenching and the crystallization of Alq 3 are effectively suppressed by the HMB, resulting in prolonged operating time and enhanced efficiency for Alq 3 -OLEDs. q
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