Effect Of Second Phase And Solution On The Thermoelectric Properties Of YbAl₃

Thermoelectric materials (TEs) are new types of functional materials, which canrealize the direct conversion between electricity and heat. It can be predicted that TEswill play an important role in global sustainable energy solution in the future. YbAl3compound has the highest power factor (34000μW m-1K-2) at low temperatures, whileits simple lattice structure (AuCu3) cannot effectively scatter phonons. The thermalconductivity of YbAl3may be as high as20W m-1K-1, and makes the ZT value be only0.18. Through optimizing preparation processes and studying composite system andsolution system, this study tried to obtaine better properties of YbAl3compound.The procedure of high frequency induction melting-pressing into pellets andannealing-ball milling and spark plasma sintering was employed to prepare pure YbAl3compound. The starting materials were mixed in quartz tube according to thecompositions of the alloys. The alloys were prepared by induction melting under avacuum of810-3Pa. The prepared samples were hand-milled into powder in agatemortar and then pressed into pellets. The pellets were slowly heated to923K andmaintained there for168h to allow the phase transformation of YbAl2to YbAl3to occur.The heat-treated samples were hand-milled into powder in agate mortar and then ballmilled for12h in a planetary ball mill. The milled powders were consolidated by sparkplasma sintering at973K for5min under an axial pressure of50MPa and vacuum of1.010-2Pa to obtain high density samples. The relative density of the samples with goodmechanical properties was larger than97%.Introducing Sb into YbAl3lattice can form (YbAl3)1-x(Yb5Sb3)xcomposite system.Set x=0.01,0.05,0.10,0.20and adopt the above process to acquire the samples forproperty measurements. X-ray diffraction indicated that the composite system wassuccessfully formed and Sb did not enter the lattice of YbAl3. The second phase wasmicrometer-sized and the Rietveld refinement results showed the ratio of each phase. Asthe amount of second phase increased, the resistivity of the sample increased and thethermal conductivity decreased. However, grain boundaries and phase boundariesplayed an important role in optimizing Seebeck coefficient, making the ZT value of(YbAl3)0.95(Yb5Sb3)0.05reach0.23which was28%higher than that of pure YbAl3.Sn has a large solid solubility in YbAl3. Lattice distortion, large residual strain andatomic disorder could be formed in the YbAl3-xSnxsolution system. Introduction of Snmay also have a modulation effect on the electronic structure of YbAl3compound.X-ray diffraction of YbAl3-xSnx(x=0.05,0.10,0.20,0.30,0.50) indicated the change oflattice constant and the existence of residual strain. Sn ions successfully changed the carrier concentration and mobility, leading just to a little degradation of resistivity.Nevertheless, the lattice distortion, residual strain and atomic disorder enhanced theSeebeck coefficient while lowering the thermal conductivity of the samples. As aconsequence, YbAl2.9Sn0.1possessed the best ZT value of0.27, which was50%higherthan that of pure YbAl3.