Effects of Codoping with Ga and P on Thermoelectric Properties of Ba8Al16Si30 Clathrate System

We have investigated the effects of Codoping With Ga and P on the thermoelectric properties of the Ba8Al16Si30 clathrate system, attempting to optimize the carrier concentration. The elastic properties, which are important for design of thermoelectric devices, were investigated by ultrasonic testing. Ga/P-codoped specimens with nominal compositions Ba8Al16Ga (x) Si30-2x P (x) (x = 1.0, 1.5, 2.0) were prepared by arc melting and spark plasma sintering and their Seebeck coefficient, electrical conductivity, and thermal conductivity were measured. Analytical studies revealed that the total content of Al and Ga, expressed as atoms per formula unit, increased to 15.65 at nominal x = 2.0, exceeding the maximum content (y = 15.16) of Al for the Ba8Al (y) Si46-y clathrate system. Ultrasonic tests determined the Young's modulus, shear modulus, bulk modulus, and Poisson's ratio to be 102.55 GPa, 40.14 GPa, 76.85 GPa, and 0.2775, respectively, for Ba8Al16Ga (x) Si30-2x P (x) (x = 2.0). The Hall carrier concentration decreased from similar to 1.0 x 10(21) cm(-3) for Ba8Al (y) Si46-y to similar to 6.3 x 10(20) cm(-3) for Ba8Al16Ga (x) Si30-2x P (x) (x = 2.0), suggesting that Ga/P codoping may be useful for tuning the carrier concentration. The value of the Seebeck coefficient at similar to 320 K increased from -46 mu V K-1 for Ba8Al (y) Si46-y to -67 mu V K-1 for Ba8Al16Ga (x) Si30-2x P (x) (x = 2.0). The dimensionless thermoelectric figurea of pound merit ZT at 900 K improved from similar to 0.4 for Ba8Al (y) Si46-y to similar to 0.47 for Ba8Al16Ga (x) Si30-2x P (x) (x = 2.0).