When 10:30 AM - 12:30 PM Sep 28, 2017
Where 1200 EECS
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Thermoelectric Behavior of Low Thermal Conductivity Cu- Based and IV-V Chalcogenide Materials


Alan Olvera
Thesis Defense

In the face of an ever-changing global environment, energy-related issues have become a central feature in the day-to-day conversations of the general public. One energy conversion technology that has recently witnessed major advancements is the field of thermoelectrics (TE), where progress in material optimization has resulted in the highest efficiency thermoelectric materials to date. These include superionic copper chalcogenides and IV-VI selenide compounds, such as Cu2Se and PbSe. However, issues related to chemical stability and cost have limited they widespread deployment in TE devices. Hence, this work focuses on the synthesis and characterization of innovative earth abundant and non-toxic Cu-based and IV-V materials for sustainable thermoelectric applications.


The performance parameters of copper chalcogenides, whether photovoltaic or thermoelectric applications, are heavily dependent on electronic defects and impurity phases arising from the large degree of tolerance of these structures to off-stoichiometry. Using traditional synthetic methods, these issues can be very difficult to control. In the first part of this thesis, we develop a simple strategy for the synthesis of CuInSe2 with improved control over the chemical composition. We demonstrate the stepwise direct conversion of the structural template CuSe2 into (1-x)CuSe2/xCuInSe2 nanocomposites without the formation of undesirable binary phases.

The novel synthesis approach was then applied in the second part of this thesis focusing on the electrochemical stabilization and the optimization of the thermoelectric performance of the known TE material, Cu2Se through incorporation of CuInSe2 nanoinclusions. Here we demonstrate improved chemical stability and a record high ZTave of ~ 1.5 over a broad temperature range (300K - 850 K) in Cu2Se/CuInSe2 nanocomposites with ZT values ranging from 0.6 at 325 K to an unprecedented ZT of 2.6 at 850 K for the sample with 1 mol% In. These findings demonstrate new directions for the optimization of Cu2Se- based materials, paving the way for their large-scale utilization in thermoelectric generators.


Lastly, in an effort to find a compatible material for the p-type Cu2Se at high temperatures, a series of materials with the formula Cu4-xAgxSe2 were synthesized. The most important finding is the high temperature n-type behavior of CuAgSe (x = 2), which is the first reported instance of CuAgSe as an n-type superionic material. It is proposed that off- stoichiometry leads to p-type behavior in other studies of CuAgSe materials.

DISSERTATION COMMITTEE


Chair:
Assoc. Prof. Ferdinand Poudeu, MSE
Members:
Asst. Prof. John Heron, MSE
Asst. Prof. Emmanouil Kioupakis, MSE
Cognate:
Prof. Bart Bartlett, Chemistry