Reduced Thermal Conductivity by Low-Frequency Optic Phonons that Give Rise to Negative Thermal Expansion: Opportunities
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1044-U07-01
Reduced Thermal Conductivity by Low-Frequency Optic Phonons that Give Rise to Negative Thermal Expansion: Opportunities for Thermoelectrics? Mary Anne White, and Catherine A. (Kennedy) Whitman Department of Chemistry and Institute for Research in Materials, Dalhousie University, Halifax, B3H 4J3, Canada ABSTRACT We have recently found that the negative thermal expansion (NTE) materials, ZrW2O8 and HfMo2O8, show exceptionally low thermal conductivity. We surmise that the mechanism is the efficient coupling of the low-frequency optic phonons that give rise to negative thermal expansion with the heat-carrying acoustic phonons. Although neither ZrW2O8 nor HfMo2O8 has suitable electronic properties for thermoelectric applications, perhaps the principle of reduced thermal conductivity by low-frequency optic phonons in NTE materials can be used to develop more efficient thermoelectric materials.
INTRODUCTION Thermoelectric materials can be used to convert electric power to a temperature gradient (active cooling), or the converse (power generation). The efficiency of a thermoelectric material can be quantified by the thermoelectric figure of merit, ZT:
ZT =
S 2σT
κ
(1)
where S is the Seebeck coefficient, σ is the electrical conductivity and κ is the thermal conductivity. Thermoelectric materials and devices in present commercial use have relatively low efficiencies which, in principle, can be improved by controlling the properties of the materials. For example, by increasing the electrical conductivity of a material, the ZT will proportionally increase; however, an increase in electrical conductivity could also result in an increase in thermal conductivity through heat conduction by the charge carriers, reducing ZT. It is recognized that use of semiconductors with low thermal conductivities is one of the most promising ways to elevate ZT. In particular, disruption of the flow of the heat-carrying acoustic phonons could reduce the thermal conductivity and increase the efficiency of thermoelectric devices. Our early work showed that the low-temperature thermal conductivity of a clathrate hydrate (17H2O.THF) is anomalously low compared to ice [1]. The temperature-dependence of the thermal conductivity of THF clathrate hydrate was glass-like due to the resonance interaction of the heat-carrying acoustic modes with the optic modes associated with guest “rattling”, resulting in a very short phonon mean free path [2].
On the basis that κ can be suppressed by addition of “rattlers” in clathrate and related structures, Slack [3] proposed that a “Phonon Glass/Electron Crystal” could carry current through the framework atoms, while the rattlers reduced the thermal conductivity to that of a “Phonon Glass.” This phenomenon provides a promising basis on which new thermoelectric materials could be designed. Here we present the relationship between negative thermal expansion and thermal conductivity, as a potential new direction for thermoelectric material development. Our recent investigation of the thermal conductivi
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