文摘
We have investigated the effects of Codoping With Ga and P on the thermoelectric properties of the Ba<sub>8sub>Al<sub>16sub>Si<sub>30sub> 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 Ba<sub>8sub>Al<sub>16sub>Ga<sub>xsub>Si<sub>30−2xsub>P<sub>xsub> (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 Ba<sub>8sub>Al<sub>ysub>Si<sub>46−ss="EmphasisTypeItalic ">ysub> 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 Ba<sub>8sub>Al<sub>16sub>Ga<sub>xsub>Si<sub>30−2xsub>P<sub>xsub> (x = 2.0). The Hall carrier concentration decreased from ∼1.0 × 1021 cm−3 for Ba<sub>8sub>Al<sub>ysub>Si<sub>46−ss="EmphasisTypeItalic ">ysub> to ∼6.3 × 1020 cm−3 for Ba<sub>8sub>Al<sub>16sub>Ga<sub>xsub>Si<sub>30−2xsub>P<sub>xsub> (x = 2.0), suggesting that Ga/P codoping may be useful for tuning the carrier concentration. The value of the Seebeck coefficient at ∼320 K increased from −46 μV K−1 for Ba<sub>8sub>Al<sub>ysub>Si<sub>46−ss="EmphasisTypeItalic ">ysub> to −67 μV K−1 for Ba<sub>8sub>Al<sub>16sub>Ga<sub>xsub>Si<sub>30−2xsub>P<sub>xsub> (x = 2.0). The dimensionless thermoelectric figure␣of merit ZT at 900 K improved from ∼0.4 for Ba<sub>8sub>Al<sub>ysub>Si<sub>46−ss="EmphasisTypeItalic ">ysub> to ∼0.47 for Ba<sub>8sub>Al<sub>16sub>Ga<sub>xsub>Si<sub>30−2xsub>P<sub>xsub> (x = 2.0).
