Evaluation of TiTe 2 as a Diffusion Barrier in the Synthesis of (Bi 2 Te 3 ) 5 (SnTe) 5 Misfit Layer Compound
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0886-F03-12.1
Evaluation of TiTe2 as a Diffusion Barrier in the Synthesis of (Bi2Te3)5(SnTe)5 Misfit Layer Compound Sissi L. Li and David C. Johnson University of Oregon, MSI 1254 University of Oregon, Eugene, Oregon 97403 ABSTRACT The evolution of modulated reactants designed to form (Bi2Te3)5(TiTe2)5(SnTe)5(TiTe2)5 and (Bi2Te3)5(SnTe)5 are compared and contrasted. The modulated reactant designed to form (Bi2Te3)5(SnTe)5 interdiffused at 220°C forming SnBi2Te4 rather than the desired superlattice structure. The second sample was designed to have TiTe2 as a potential diffusion barrier to prevent the formation of SnBi2Te4. This second sample remained layered after annealing at 220°C. SnTe crystals are observed in the high angle diffraction pattern after this annealing, but there is evidence for the beginning of SnBi2Te4 formation. Annealing this sample at 300°C results in the formation of SnBi2Te4. The interdiffusion of Sn and Bi appears to occur before the formation of the desired TiTe2 structure. INTRODUCTION A thermoelectric material can interconvert thermal differential and electrical potential. Such materials provide promising new alternatives to conventional temperature control devices as well as radioactive thermoelectric generators that convert heat due to radioactive decay to electrical power [1]. A thermoelectric material is evaluated using the thermoelectric figure of merit (ZT=S2T/ρκ) where S is the Seebeck coefficient, T is the absolute temperature, ρ is the electrical resistivity and κ is the thermal conductivity. Typical ZTs in bulk doped materials are less than unity. [1] A room temperature ZT of ~2.4 has been reported by Venkatasubramanian in superlattice thin films of p-type Bi2Te3/Sb2Te3. [2] This significant increase in ZT has been attributed to the increased phonon scattering by the superlattice structure. [3-6] We have been exploring the potential use of the structural mismatch in misfit layer compounds as a mechanism to further decrease the thermal conductivity in superlattice structures. Known misfit layered compounds consist of alternating layers of X-M-X dichalcogenide layers with M’-X rock salt layers where M is a group 4, 5 or 6 transition metal, M’ is a rare earth, tin or lead, and X is a chalcogen. Previously we reported that we were unable to form a misfit layered structure containing alternating layers of Bi2Te3 and SnTe due to interdiffusion of the layers to form SnBi2Te4. [7] We also reported that (Bi2Te3)x(TiTe2)z(Sb2Te3)y(TiTe2)z superlattices can be prepared where TiTe2 is an effective interdiffusion barrier between Bi and Sb. [8] In this paper we prepare modulated reactants containing sequential layers of Ti-Te, Sn-Te, Ti-Te and Bi-Te designed to form a (Bi2Te3)5(TiTe2)5(SnTe)5(TiTe2)5 superlattice to determine if TiTe2 is an effective diffusion barrier between Bi2Te3 and SnTe.
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EXPERIMENTAL Using the modulated elemental reactant (MER) synthesis method, reactant elements are thermally deposited in a high vacuum (~10-7 torr) onto a silicon substrate. Alternating l
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