Summer 2012 Intern Project- Bethany Lettiere
INCREASING THE EFFICIENCY OF THE THERMOELECTRIC HALF-HEUSLER TiNiSn THROUGH INCLUSION OF A SECONDARY PHASE
UC Santa Barbara
Mentor: Christine Birkel
Faculty Advisor: Galen Stucky
Department: Chemistry and Biochemistry
As energy consumption increases, the need for renewable energy sources intensifies. Thermoelectric material is one type of renewable energy source which converts between electrical and thermal energy. Previous research shows that half-Heuslers (hH) show promising thermoelectric qualities such as high Seebeck coefficient and electrical conductivity. Unfortunately, they have a high thermal conductivity which might be responsible for a low thermoelectric figure of merit. Therefore, greater energy conversion efficiency could be achieved by reducing the thermal conductivity of these materials. We tested the effect of adding larger percentages of nickel into TiNiSn, resulting in different TiNi1+xSn compounds (0<x<0.15). Adding additional nickel creates a sample that includes the hH phase (TiNiSn) as a matrix and the full-Heusler (fH, TiNi2Sn) phase as small segregations within this matrix. The addition of the metallic fH should decrease the thermal conductivity by helping scatter phonons. We first synthesized the samples using microwave synthesis methods and then compared two densification methods: hot pressing and spark plasma sintering (SPS). X-Ray diffraction results of the as-prepared materials initially show a mixture of different phases. The densified materials, however, consist almost exclusively of hH and fH phases proving the significance of the densification which should be regarded as an additional synthesis step. X-ray diffraction for the SPS samples show a smaller ratio of fH to hH phases when compared to the traditional hot pressing densification technique, creating a product that retains the promising thermoelectric qualities of the hH material.