Structural Characterization of Filled n-type IrSb3 Skutterudites
- Author(s): Nguyen, Diep Thi Ngoc
- Advisor(s): Kaner, Richard B
- King, Daniel J
- et al.
Thermoelectric (TE) materials have been attractive to several research groups all over the world for their ability of directly converting thermal energy into electrical energy by the Seebeck Effect, and vice versa by the Peltier Effect. Due to their reliability and scalability, TE materials can be incorporated into devices for power generation or for cooling applications. Skutterudites have a complex crystalline structure in which the voids in the structure can be filled with foreign atoms. This doping technique has been shown to improve the TE performance of these "filled" skutterudite materials by disrupting the thermal conductivity without significantly impacting the electrical conductivity. Several studies have been done on filled CoSb3-based skutterudites and high ZT values have been obtained for these materials around 6000C. However, for thermoelectric applications that require higher operating temperatures, the more refractory IrSb3 with a peritectic decomposition at 11360C (1409 K) offers an advantage over CoSb3, which decomposes at 8740C (1147 K). Using the traditional synthesis method of melting, quenching, then annealing stoichiometric amounts of the elements to produce n-filled skutterudites, the obtained filling fraction was found to be significantly lower than the nominal filling fraction due to the high reactivity, or sometimes the high volatility, of the elemental fillers. In this experiment, we explored the effects of pre-reacting the reactive metal filler element with one of the elements in the skutterudite structure and utilized this precursor for the synthesis of the filled skutterudites to compare the relative successes of achieving targeted filling percentages of the voids in the structure. KyIr4Sb12 samples were synthesized using both the pure filler element, as well as the pre-reacted K-Sb compound. The phase purities, lattice parameters and elemental compositions were analyzed for each sample, calculated using X-Ray diffraction (XRD), scanning electron microscopy (SEM) and electron microprobe analysis (EPMA), respectively. Their thermoelectric properties were also measured and calculated at room temperature and as a function of temperature ranging from room temperature to about 7500C (1023 K). The preliminary results obtained suggest that pre-reacting the potassium with antimony results in higher percentages of phase impurities and less efficient void filling in IrSb3 skutterudite. The highest ZT among all potassium-filled samples synthesized was measured to be 0.28 at 5000C (773 K) for K0.24Ir4Sb12. The data suggest that the cumulative thermoelectric properties of potassium filled IrSb3 are not as favorable as those IrSb3-based skutterudites filled with other atoms such as Ba or Eu. However, some of the potassium-filled samples achieved high room temperature carrier concentrations, similar to those seen in barium-filled samples, indicating that the potassium effectively participates in doping the structure. This work has compared and contrasted two methods for incorporating potassium fillers in the void spaces of IrSb3 skutterudite. While the ZT of potassium-filled samples was less than required for practical applications, this work does suggest that potassium may be useful in tuning electrical properties in samples that employ multiple element filling.
The relationship between the lattice parameter and the filling fraction was also studied on barium-filled and europium-filled IrSb3 skutterudite samples. A higher nominal filling fraction acquired a higher EPMA filling fraction and a higher lattice parameter until the filling fraction limit was reached. While barium displayed higher filling efficiency, europium possessed higher filling fraction limit. These preliminary results suggest that the optimization of multiple element filling to produce higher ZT is very obtainable with further studies on different single fillers.