Evaluation of the thermoelectric potential of the type-I clathrate Ba sub(8)Ni sub(y)Zn sub(x)Ge sub(46-x-y)

A detailed investigation of the high-temperature thermoelectric properties of the ternary clathrate of type-I Ba sub(8)Zn sub(x)Ge sub(46-x) for the range 7.2 less than or equal to x less than or equal to 7.8 is presented. Electrical resistivity, Seebeck coefficient, thermal conductivity and Hall-effect measurements clearly demonstrate that the maximum ZT-value can be expected for the composition Ba sub(8)Zn sub(7.7)Ge sub(38.3), which reaches ZT similar to 0.46 at a temperature of 730 K. Furthermore, the influence on the thermoelectric properties was studied for partial substitution of Zn by one Ni-atom (Ba sub(8)Ni sub(y)Zn sub(x)Ge sub(46-x-y) with y sub(nom) = 1, and 6.5 less than or equal to x less than or equal to 7.2) or by three Ni-atoms per unit cell (Ba sub(8)Ni sub(y), Zn sub(x)Ge sub(46-x-y), with y sub(nom) = 3 and x = 1 to 3). Whereas a small amount of Ni increases the figure of merit ZT to 0.65 at 800 K, a higher Ni-content reduces the thermoelectric performance. Hall-effect measurements reveal that this improvement is essentially due to an increased charge carrier mobility.