This project investigates structure formation and electrical and thermal transport properties of PbTe-Ag2-äSe alloys. Solid-solutions of PbTe-Ag2-äSe display extremely interesting structural behavior, low electrical resistivity and very low thermal conductivity. Hence, this system has the potential to give rise to a new state-of ?the-art thermoelectric material for power generation applications. In addition to the favorable transport properties exhibited by the system, the resulting alloys are stable and mechanically robust thus allowing excessive manipulation for optimization of the thermoelectric performance. It has been observed that dense solid-solutions of PbTe-Ag2-äSe (1:1 molar ratio) result in a predominantly single phase alloy which crystallizes in the NaCl structure. Stabilization of a predominantly cubic single phase alloy in the presence of such a high level of structural disorder is remarkable. PbTe appears to dictate structure formation from the melt and not Ag2Se - which stabilizes in an orthorhombic structure at low temperatures and cubic at high temperatures. The PbTe-Ag2-äSe alloys are degenerate semiconductors whose electrical transport correlates with Ag concentration. Preliminary measurements on several samples with ä = 0.1 showed p-type behavior and a high-temperature (~ 400 oC) total thermal-conductivity value êT< 0.6 W/m-K. This work will investigate the structure formation of these alloys using x-ray diffraction and microscopy, and study the effects of structure manipulation, i.e. further alloying, nano-structuring and nano-composites, on the thermoelectric properties. This research will provide excellent training for graduate and undergraduate students in material synthesis and characterization.

NON-TECHNICAL SUMMARY:

This proposal will utilize solid-solutions, nano-structuring and band-gap engineering in order to obtain a high efficiency thermoelectric alloy for power generation applications. Development of thermoelectric materials for power generation is an essential component in our scientific effort to achieve energy independence and security. Thermoelectric materials convert wasted heat to electricity, which implies that any heat source, either man-made (automotives) or natural (hot springs) can be utilized for production of electrical energy. The presence of a temperature gradient across the thermoelectric will give rise to a voltage. Thermoelectric materials also offer the mechanical advantage of absence of moving parts, which eliminates the need for lubrication and frequent maintenance. This work will research the structure formation and high temperature thermoelectric performance of PbTe-Ag2-äSe alloys. These alloys have been shown to have very low thermal conductivity and good electrical conductivity. Both are needed in order to obtain an efficient thermoelectric material. Additionally, the high temperature thermoelectric performance will be optimized through the chemical and mechanical processing of these alloys. This research will provide excellent training for graduate and undergraduate students in material synthesis and characterization. One graduate student and two undergraduate students will be working on this project and will be involved in all aspects of this investigation. The PI actively encourages the participation of women and minorities in his research.

Project Report

Thermoelectric materials can play a significant part in energy conservation since they can convert wasted heat to useful electrical energy. For power generation applications, the dimensionless figure of merit ZT (defined as ZT = α2T/κρ where α is the thermopower or Seebeck coefficient, ρ is the electrical resistivity and κ is the total thermal conductivity) should have its maximum value at temperatures at or above 400 oC. The expression for the dimensionless figure-of-merit indicates that high thermoelectric performance can be achieved in materials that have low thermal conductivity, low electrical resistivity and large thermopower. The current challenge is achieving values of ZT ≥2 in bulk materials. For this project, we established the synthesis procedure and subsequently synthesized specific composites and measured their thermoelectric performance. The goal was the development of novel and high-efficiency thermoelectric structures for power generation applications. Major Project Outcomes PbTe/Ag2Se: We found that when Pb, Te, Ag and Se are reacted in a 1:1:x:1 (x=1.9, 2.0, 2.01) molar ratio, a two-phase composite is formed. The composite consists of a phase which crystallizes in the fcc cubic PbSe structure and a phase that crystallizes in the Ag2Te structure. By varying the Ag concentration, we can manipulate which variant of the Ag2Te structure stabilizes at room temperature (monoclinic α-Ag2Te or cubic β-Ag1.9Te) and can consequently manipulate the electrical and thermal transport behavior of the composite and hence the thermoelectric performance. CuxAg3-xSbSeTe2 (Cu-doped AgSbTe2:Ag2Se): This composite consists of a phase which crystallizes in the AgSbTe2 cubic structure and a phase which crystallizes in the monoclinic α-Ag2Te structure. The thermal conductivity of this crystalline alloy is very low and its temperature dependence is strikingly similar to that of amorphous solids. The alloy is optimized for x= 0.2 and its thermoelectric performance is very high. Specifically, the maximum dimensionless coefficient of merit for rapidly cooled samples is ZT ~ 1.5 and, for slow cooled samples ZT ~ 1.75. The latter corresponds to one of the highest dimensionless figure-of-merit measured in bulk samples. Publications This research has produced the following publications in peer-reviewed journals: 1) Significant enhancement of the dimensionless thermoelectric figure of merit of the binary Ag2Te. J. Capps, F. Drymiotis, S. Lindsey, and T. M. Tritt. Philosophical Magazine Letters 90, 9, 677 (2010). 2) Glassy thermal conductivity in the two-phase CuxAg3-xSbSeTe2 alloy and high temperature thermoelectric behavior. F. Drymiotis, T. Drye, D. Rhodes, Q. Zhang, J. C. Lashley, Y. Wang, S. Cawthorne, B. Ma, S. Lindsey, and T. Tritt. Journal of Physics-Condensed Matter 22, 3, 035801 (2010). 3) Structure formation and very low thermal conductivity in Pb:Te:Ag:Se mixtures. F. R. Drymiotis, T. B. Drye, Y. S. Wang, J. He, D. Rhodes, K. Modic, S. Cawthorne, and Q. R. Zhang. Journal of Applied Physics 107, 3, 033519 (2010). 4) The effect of Ag concentration on the structural, electrical and thermal transport behavior of Pb:Te:Ag:Se mixtures and improvement of thermoelectric performance via Cu doping. J. Capps, B. Ma, T. Drye, C. Nucklos, S. Lindsey, D. Rhodes, Q. Zhang, K. Modic, S. Cawthorne, and F. Drymiotis. Journal of Alloys and Compounds 509, 5, 1544 (2011). 5) Excess vibrational modes and high thermoelectric performance of the quenched and slow-cooled two-phase alloy Cu0.2Ag2.8SbSeTe2. F. R. Drymiotis, S. Lindsey, J. Capps, J. C. Lashley, D. Rhodes, Q. R. Zhang, C. Nucklos, and T. B. Drye. Journal of Physics-Condensed Matter 23, 13, 135305 (2011). Societal Impact Thermoelectric materials can harness wasted heat that is produced by man-made sources (e.g. internal combustion engines) and natural sources (e.g. hot springs) and convert it to electrical energy. The harnessed energy can be returned to the system, thus enhancing its efficiency and reducing fuel consumption. A small increase of a few percentage points in efficiency can considerably decrease fuel consumption; an average ZT in the range from 1.5 to 2 would enable substantial waste-heat harvesting, and up to a 10% fuel reduction may be within reach (L.E. Bell, Cooling, heating, generating power, and recovering waste heat with thermoelectric systems Science 321, 1457-61 (2008)). Education Funds from NSF-DMR-SSMC-0905322 were used to support two (2) graduate students and five (5) undergraduate students. All students were trained in the synthesis and characterization of advanced functional materials. The majority of the undergraduate students who worked on this project are currently pursuing their doctorate in experimental condensed matter physics. This research has produced five (5) publications in peer-reviewed journals with both graduate and undergraduate students sharing authorship. Technological Impact The results of this research suggest that formation of composites is a viable path for achieving very-high thermoelectric performance in bulk materials. The materials developed in this research can be used in the fabrication of high-efficiency thermoelectric devices.

Agency
National Science Foundation (NSF)
Institute
Division of Materials Research (DMR)
Application #
0905322
Program Officer
Linda S. Sapochak
Project Start
Project End
Budget Start
2009-08-01
Budget End
2011-07-31
Support Year
Fiscal Year
2009
Total Cost
$200,977
Indirect Cost
Name
Clemson University
Department
Type
DUNS #
City
Clemson
State
SC
Country
United States
Zip Code
29634