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Developments originally targeted toward economical manufacturing of telecommunications products have planted the seeds for new opportunities such as low-cost, large-area electronics based on printing technologies. Organic-based materials systems for printed wiring board (PWB) construction have opened up unique opportunities for materials research in the fabrication of modular electronic systems.
The realization of successful consumer products has been driven by materials developments that expand PWB functionality through embedded passive components, novel MEMS structures (e.g., meso-MEMS, in which the PWB-based structures are at the milliscale instead of the microscale), and microfluidics within the PWB. Furthermore, materials research is opening up a new world of printed electronics technology, where active devices are being realized through the convergence of printing technologies and microelectronics.
We have previously reported the successful p-type doping of CsBi4Te6 which had a high figure of merit at temperatures below 300 K. In this study, several dopants were explored to make n-type CsBi4Te6. A program of measurements was performed to identify the optimum doping concentration for several series of dopants. The highest power factors occurred around 125 K for the 0.5% Sn doped CsBi4Te6 sample which had a power factor of 21.9 μW/cm•K2 and 1.0% Te doped CsBi4Te6 which had a power factor of 21.7 μW/cm•K2.
Based on the versatile combination of PbQ- and Bi2Q3-type (Q = S, Se, Te) fragments, we explored new compounds in the Pb/Bi/Se ternary system. The new class of compounds, Pb5Bi6Se14, Pb5Pb12Se23, and PbBi8Se13 are homologues with different combination of alternating Bi2Se3- and PbSe-type layers. α- and β-Pb6Bi2Se9 were obtained in different synthetic conditions and the former is isostructural to heyrovskyite (Pb6Bi2S9) while the latter is a NaCl-type cubic phase. Pb5Bi6Se14 shows a power factor of 11.2 μW/cm·K2 with electrical conductivity of 657 S/cm and thermopower of -131 μV/K at 271 K. The most significant characteristic of this material is the extremely low thermal conductivity of less than 1.0 W/m·K at room temperature. On the basis of these properties, a preliminary doping study for Pb5Bi6Se14 with Sn, Sb, and SbBr3 as dopants was undertaken and the results are presented in this report.
The synthesis, physicochemical, spectroscopic, and structural characterization of the compound β-K2Bi8Se13 has been previously reported. The results indicated that this material should be investigated further for possible thermoelectric applications. β-K2Bi8Se13 exhibits excellent electrical conductivity values at room temperature while maintaining high Seebeck coefficients. In this work, the optimization of the compound β-K2Bi8Se13 is continued by the introduction of varying concentrations of several different dopants. The value of x in K2Bi8– xSbxSe13 was varied in order to find the composition with minimum thermal conductivity. Where possible, transport measurements were carried out on both single crystal and polycrystalline ingot material. From these data, the trends in the key parameters were identified for optimizing the power factor and figure of merit.
Results on the synthesis and characterization of the solid solutions CsBi4-xSbxTe6, CsBi4Te6-ySey, as well as doping experiments on CsBi4Te6 are reported. We report X-ray structural investigations showing that the Sb or Se atoms in these compounds are not uniformly distributed in the lattice but show preferential occupation of specific crystallographic sites. Thermoelectric properties of selected systems are presented.
We present the structure and thermoelectric properties of the new quaternary selenides K1+xM4–2xBi7+xSe15 (M = Sn, Pb) and K1-xSn5-xBi11+xSe22. The compounds K1+xM4-2xBi7+xSe15 (M= Sn, Pb) crystallize isostructural to A1+xPb4-2xSb7+xSe15 with A = K, Rb, while K1-xSn5-xBi11+xSe22 reveals a new structure type. In both structure types fragments of the Bi2Te3-type and the NaCl-type are connected to a three-dimensional anionic framework with K+ ions filled tunnels. The two structures vary by the size of the NaCl-type rods and are closely related to β-K2Bi8Se13 and K2.5Bi8.5Se14. The thermoelectric properties of K1+xM4-2xBi7+xSe15 (M = Sn, Pb) and K1-xSn5-xBi11+xSe22 were explored on single crystal and ingot samples. These compounds are narrow gap semiconductors and show n-type behavior with moderate Seebeck coefficients. They have very low thermal conductivity due to an extensive disorder of the metal atoms and possible “rattling” K+ ions.
We present the synthesis and structure of the new chalcogenide compounds, Rb0.5Bi1.83Te3, APb2Bi3Te7 (A = Cs, Rb), K1.25Pb3.50Bi7.25Se15 and A1+xPb4−2xSb7+xSe15 (A = K, Rb). The layered structures of the first two telluride compounds are related to each other in a very interesting fashion. These compounds are n-type metallic conductors and solid solutions with Se and Sb have also been synthesized. K1.25Pb3.50Bi7.25Se15 and its Sb analogs have a complex three-dimensional structure composed of NaCl- and Bi2Te3-type building units. The thermal stability, melting behavior, electrical conductivity and thermopower of these compounds are reported.
We are continuing our synthetic investigations in the ternary A/Bi/Se systems and also expanded our interests into the quaternary A/M/Bi/Se (A = Rb, Cs, Sr and Ba; M = lanthanide or Pb) systems. We have synthesized several new ternary and quaternary bismuth selenides with band gaps <0.6eV such as Cs2Bi7.33Se12, A2Bi8Se13 (A = Rb, Cs), Ba4-xBi6+2/3xSe13, and Ba3±xPb3±xBi6Se15. The synthesis, crystal structures and charge transports properties of these new compounds are presented.
In previous investigations we have introduced a variety of new chalcogenide-based materials with promising properties for thermoelectric applications. The chalcogenide CsBi4Te6 was previously reported to have a high ZT product with a maximum value at 260K. In order to improve this value, a series of doped CsBi4Te6 samples has been synthesized. Current doping studies have been very encouraging, with one sample found to have a maximum power factor of 51.5 μW/cm·K2 at 184 K. This paper reports on material characterization studies through the usual transport measurements to determine optimum doping concentration for various dopants.
The ternary compound, SrBiTe3, was prepared by the molten flux method. It crystallizes in the orthorhombic space group Pmmn with a=4.665(2)Å, b=4.517(2)Å, c=16.129Å, and Z=2. The compound is composed of a NaCl-type [Bi2Te4]2- layer and a flat square (Te2)2- net with equal Te-Te distances of 3.24Å. The Sr2+ ions are stabilized between the layers in monocapped square antiprismatic sites. X-ray and electron diffraction studies reveal a 12-fold superstructure due to subtle distortions within the Te net of the compound. The superstructure was refined in the monoclinic space group C2 with a=27.923(5)Å, b=32.228(6)Å, c=4.5069(9)Å, β=90.000(3)°. The compound is a semiconductor with a band gap value of 0.14 eV. It melts incongruently at 613 °C.
New Bi-based chalcogenide compounds have been prepared using the polychalcogenide flux technique for crystal growth. These materials exhibit characteristics of good thermoelectric materials. Single crystals of the compound CsBi4Te6 have shown conductivity as high as 2440 S/cm with a p-type thermoelectric power of ≈ +110 μV/K at room temperature. A second compound, β-K2Bi8Se13 shows lower conductivity ≈ 240 S/cm, but a larger n-type thermopower ≈ −200 μV/K. Thermal transport measurements have been performed on hot-pressed pellets of these materials and the results show comparable or lower thermal conductivities than Bi2Te3. This improvement may reflect the reduced lattice symmetry of the new chalcogenide thermoelectrics. The thermoelectric figure of merit for CsBi4Te6 reaches ZT ≈ 0.32 at 260 K and for β-K2Bi8Se13 ZT ≈ 0.32 at room temperature, indicating that these compounds are viable candidates for thermoelectric refrigeration applications.
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