Thermoelectric Properties of TiS 2 type materials
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Thermoelectric Properties of TiS2 type materials Edward E. Abbott and Joseph W. Kolis Department of Chemistry, Clemson University, Clemson SC 29634 Nathan D. Lowhorn, William Sams, and Terry M Tritt*, Department of Physics, Clemson University, Clemson SC 29634
Abstract
TiS2 belongs to a family of layered compounds that displays promise as a thermoelectric material. At room temperature the thermopower (α) of TiS2 displays an n-type behavior, with a magnitude of ≈ -200 µV/K. The electrical resistivity (ρ), is on the order of 1 mΩ-cm at room temperature and displays a “metallic–like” behavior with dR/dT > 0 from 300 K to 10 K. Thus, these compounds exhibit relatively large power factors (PF = α2/ ρ) with a PF ~30 µW/K2 cm at T = 300 K, which are comparable to the state-of-the art Bi2Te3 type materials, which have a PF ~40 µW/K2,at T = 330K. These values suggest that further investigations of these systems could be profitable. Thin plate-like crystals of TiS2 are grown by the iodine vapor transport method with planar dimensions of 1 cm and thicknesses of 20 µm or more. In this synthetic approach some dopants can be integrated into the parent compound, effectively providing a route for the tuning of electronic properties. We present here some effects of elemental doping on the electronic properties in these TiS2 based materials.
1. Introduction
For many years bismuth telluride (Bi2Te3) type materials have been the state of the art in thermoelectric cooling for moderate temperature applications.
These materials are layered
materials with itinerant electron behavior, thus it has been the focus of our research to investigate other low dimensional systems with itinerant electron behavior as thermoelectric materials with titanium disulfide (TiS2) as one such material. TiS2 is a layered material in which layers of edge shared TiS6 ocatahedra are held together with weak Van der Waals forces giving it highly
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anisotropic behavior.
The thermopower of TiS2 has been investigated in the past
1-3
, however
recently the claim has been made that the thermopower of TiS2 can be increased beyond that of previous reports up to a value of
-251 µV/K 4.
This translates into a figure of merit
(ZT=α2σT/κ) of 0.16 where α is the Seebeck coefficient, σ the electrical conductivity, and κ is the thermal conductivity. It is well known that TiS2 can be grown by iodine vapor transport and will readily form non-stochiometric phases that are titanium rich, with titanium atoms acting as intercalates
5-8
. It is proposed that tight stochiometric control of these reactions is the basis for
this enhanced thermopower as compared to previously grown materials4. As with the maturation of bismuth telluride as a thermoelectric material it would follow that one could tune the properties of TiS2 further with elemental doping and enhance its viability as a thermoelectric material.
2. Experimental 2.1 Crystal growth
Single crystals of TiS2 were grown using chemical vapor transport with iodine as the primary transport agent. Quart
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