Texture development in Bi 2 Te 3 during hot forging
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Texture development in Bi2 Te3 during hot forging E. J. Gonzalez, J. E. Blendell, J. P. Cline, and J. J. Ritter Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
P. Maruthamuthu Department of Energy, University of Madras, Madras 600 025, India
E. H. Nelson Sensors and Electron Devices Directorate, Army Research Laboratory, Department of the Army, Fort Belvoir, Virginia 22060
S. B. Horn Infrared Technology Branch, Night Vision and Electronic Sensors Directorate, Department of the Army, Fort Belvoir, Virginia 22060 (Received 5 August 1996; accepted 3 June 1997)
The development of crystallographic texture in hot-forged polycrystalline Bi2 Te3 samples was studied. Texture was evaluated with the use of the March–Dollase model in conjunction with a Rietveld analysis of x-ray diffraction data. It was determined that during forging a strong (0001) texture develops along the loading axis. The magnitude of the (0001) texture increases systematically with the amount of height reduction during hot-forging. The correlation between the observed deformation and the March–Dollase texture model suggests that grain rotation is the primary mechanism for texture development in Bi2 Te3 .
I. INTRODUCTION
Bi2 Te3 and its alloys, such as Bi2 Te2.85 Se0.15 , Bi22x Inx Te3 , and Bix Sb12x Te3 , have high thermoelectric efficiencies, making them the materials of choice for thermoelectric heating and cooling applications. Bi2 Te3 based cooling devices can replace all the mechanical parts and refrigerants in conventional refrigeration systems. By passing a dc current through a Bi2 Te3 thermoelement consisting of p-type and n-type doped elements, heat can be absorbed at one junction and released at the other junction. Reversing the current flow will reverse the direction of heat flow at each junction. The high thermoelectric efficiency of Bi2 Te3 is due to its high Seebeck coefficient and high electrical conductivity to thermal conductivity ratio. The thermoelectric figure of merit, Z, combines these properties to characterize the thermoelectric efficiency by a single number, Z
S2s , k
(1)
where S is the Seebeck coefficient, s is the electrical conductivity, and k is the thermal conductivity. High values of Z, (2 to 3) 1023 K21 , are characteristic of efficient thermoelectric materials. For instance, the best room temperature thermoelectric materials are Bi–Te–Se–Sb alloys, and one of the highest reported room temperature figure of merit for this alloy is 3.4 3 1023 K21 .1 766
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Bi2 Te3 has a rhombohedral crystal structure, with space group (R3m).2 The structure is usually indexed using Miller–Bravais indices based on a hexagonal system, since the dense packed planes, (111) in rhombohedral, are the basal planes, (0001), in the hexagonal system. The cya ratio of Bi2 Te3 is 6.9, and the atoms stack in layers of Bi and Te along the c-axis. Two types of Te sites exist within the structure; one site is between two Bi lay
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