Effect of Al Substitution on the Thermoelectric Properties of the Type VIII Clathrate Ba_(8)Ga_(16)Sn_(30)

摘要

Single-crystal samples of the type VIII clathrate Ba_(8)Ga_(16-x)Al_(x)Sn_(30) (0 <= x <= 12) were prepared by the Sn-flux method, and the structural and electrical properties were studied from 300 K to 600 K. The lattice parameter increases linearly as x is increased from 0 to 10.5, which is the solubility limit of Al. For all samples, the electrical conductivity sigma decreases monotonically as the temperature is increased. sigma(T velence 300 K) increases from 1.88 X 10~(4) S/m for x velence 0 to 3.03 X 10~(4) S/m for x velence 2, and then gradually decreases to 2.4 X 10~(4) S/m with further increase of x to 8. The increase of sigma for Al-substituted samples is attributed to enhancement of carrier mobility. The Seebeck coefficients of samples with 0 <= x <= 8 are negative with large values, and the absolute values increase from 240 (mu)V/K to 320 (mu)V/K as the temperature increases from 300 K to 600 K. At 300 K, the effective mass m~(*)/m_(0) is in the range from 0.53 to 0.67, and the samples with x velence 6 and x velence 8 have a rather low thermal conductivity of 0.72 W/mK and 0.78 W/mK, respectively. ZT reaches 1.2 at 500 K for x velence 6.