Alp Sehirlioglu

Thermoelectric properties of CoxNi4-xSb12-ySnyTernary Skutterudites

Jon A. Mackey Mechanical Engineering, University of Akron, Akron, Ohio, 44325 Frederick W. Dynys NASA Glenn Research Center, Cleveland, Ohio, 44135 and Alp Sehirlioglu Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio, 44106 Thermoelectric materials based on the skutterudite crystal structure have demonstrated enhanced performance (ZT>1), along with good thermal stability and favorable mechanical properties. Binary skutterudites, with single and multiple fillers, have been intensively studied in recent years. Compared to binary skutterudites, the ternary systems have received less attention, e.g. Ni4Sb8Sn4. Ternary skutterudites are isoelectronic variants of binary skutterudites; cation substitutions appear to be isostructural to their binary analogues. In general, ternary skutterudites exhibit lower thermal conductivity. Ternary systems of Ni4Bi8Ge4, Ni4Sb8Ge4, and Ni4Sb8Sn4 were investigated using combined solidification and sintering steps. Skutterudite formation was not achieved in the Ni4Bi8Ge4 and Ni4Sb8Ge4 systems; skutterudite formation occurred in Ni4Sb8Sn4 system. P-type material was achieved by Co substitution for Ni. Thermoelectric properties were measured from 298 K to 673 K for Ni4Sb8Sn4, Ni4Sb7Sn5 and Co2Ni2Sb7Sn5. N-type Ni4Sb8Sn4 exhibit the highest figure of merit of 0.1 at 523 K.

Effects of excess lead-oxide and bismuth-oxide content on the electrical properties of high-temperature bismuth scandium lead titanate ceramics

Aeronautic and aerospace applications require piezoelectric materials that can operate at high temperatures. The air-breathing aeronautic engines can use piezoelectric actuators for active combustion control for fuel modulation to mitigate thermo-acoustic instabilities and/or for gas flow control to improve efficiency. The principal challenge for the insertion of piezoelectric materials is their limitation for upper use temperature and this limitation is due to low Curie temperature and increasing conductivity. We investigated processing, microstructure and property relationship of (1-x)BiScO3-(x)PbTiO3 (BS-PT) composition as a promising high temperature piezoelectric. The effect of excess Pb and Bi and their partitioning in grain boundaries were studied using impedance spectroscopy, ferroelectric, and piezoelectric measurement techniques. Excess Pb addition increased the grain boundary conduction and the grain boundary area (average grain size was 24.8¿m, and 1.3¿m for compositions with 0at.% and 5at.% excess Pb, respectively) resulting in ceramics with higher losses (tan ¿= 0.9 and 1.7 for 0at.% and 5at.% excess Pb at 350 °C and at 10kHz) that were not resistive enough to pole. Excess Bi addition increased the resistivity (¿grain = 4.1×1010 ¿.cm and 19.6 × 1010 ¿.cm for compositions with 0at.% and 5at.% excess Bi, respectively), improved poling, and increased the piezoelectric coefficient from 137 to 197 pC/N for 5at.% excess Bi addition. In addition, loss tangent decreased more than one order of magnitude at elevated temperatures (<300 °C). For all compositions the activation energy of the conducting species was similar (¿ 0.35¿0.40 eV).