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

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 Y_0.56B₁₄ (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 Y_0.56B₁₄, 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.