Microstructure and thermoelectric properties of y x Al y B14 samples fabricated through the spark plasma sintering

Abstract

Excellent control in p- and n-type transport characteristics was previously obtained for the thermoelectric Y x Al y B14 compounds through Al flux method. In this study, new attempts were made to reduce their grain sizes to obtain dense samples and to possibly lower the thermal conductivity. Introducing the reduction of grain sizes into Y x Al y B14 samples was attempted by two methods; one was through mechanical grinding, and the other was by synthesizing Y x Al y B14 via Y0.56B14 (denoted as "vYB-YAlB14"). Mechanical grinding using ball milling with Si3N4 pots and balls was found not to be an efficient way to decrease the grain size because of contamination of Si3N4. In contrast, vYB-YAlB14 samples were successfully synthesized. Through the synthesis of Y0.56B14, the boron network structure was first formed. Afterward, Y x Al y B14 was obtained by adding Al in the boron network structure through a heat treatment. Due to shorter heating time at lower temperature, the grain sizes were discovered to be smaller than that of Al flux method. The decrease of grain size was found to be beneficial for the densification of Y x Al y B14 and the decrease of its thermal conductivity.