U. Birkholz, Investigation of the Intermetallic Compound Bi2Te3 as Well as the Solid Solutions Bi2-xSbxTe3 and Bi2Te3-xSex with Regard to Their Suitability as Material

The aim of this work was the discovery, production, and testing of p- and n-semiconducting thermoelectric mate-rials of maximum performance according to new theoretical points of view. The studies covered the intermetallic compound Bi2Te3 and mixed crystals of Bi2-xSbxTe3 and Bi2Te3-xSex, which were fabricated according to the Bridgman method (“normal freezing”). The well-known lack of good n-type ma-terial could be caused by the inversion of Bi2Te3 into the n-type. Such a suitably doped n-Bi2Te3 exhibited even better thermoelectric properties compared to Bi2Te3-xSex. In the case of the p-type, on the other hand, a substantial improvement can be achieved by the transition from Bi2Te3 to Bi2-xSbxTe3. The optimal composition was Bi0.6Sb1.4Te3, which in combination with the mentioned n-Bi2Te3 achieved the effectiveness z=2.1×10-3 K-1, equiv-alent to a maximum Peltier cooling of 80° C. Various preparations were also manufactured using powder metallurgy, and the changes in the properties were studied. the reduction of the lattice heat conductivity, which, however, is compensated by a decrease of mobility, was remarkable. Further, the reduction of the hole concentration, which in the case of weakly p-type material led to an inversion, was surprising. The practical testing of the thermocouples was carried out by measuring the maximum Peltier cooling in a high vacuum. By dissolving the cold soldering points with ultrasound, their contact resistance could be reduced to such an extent that practically the full calculated critical cooling development could be achieved. In this way, the critical cooling of 80°, calculated from the physical data of the developed materials, was achieved for the first time.