J. Dadda

Tension - Compression Asymmetry in Co49Ni21Ga30 High-Temperature Shape Memory Alloy Single Crystals

This paper reports on the tension-compression asymmetry of [001]-oriented Co49Ni21Ga30 single crystals at elevated temperatures. Maximum strains of -4.8 % and 8.6 % in compression and tension, respectively, were found. A linear Clausius-Clapeyron relationship was observed for both stress-states where the smaller slope in tension resulted in a significant increase of the phase transformation temperatures with stress, which reached 180 °C under a constant stress level of 150 MPa. In addition, the material demonstrated a large pseudoelastic temperature range of about 300 °C under both stress state conditions. The results in this study unequivocally indicate the potential of these alloys for applications where elevated temperatures and stress levels prevail.

Electronic Properties as a Function of Ag/Sb Ratio in Ag1−yPb18Sb1+zTe20 Compounds

In this study efforts have been made to optimize the electronic properties such as the electrical conductivity and Seebeck coefficient of Ag1−yPb18Sb1+zTe20 (lead-antimony-silver-tellurium, LAST-18) compounds by systematically varying the Ag and Sb compositions with constant Pb/Te ratio. It was found that increasing the content of Sb relative to Ag raised the charge carrier density (n) and thereby the electrical conductivity and power factor. The results indicate that, for deficient Ag, the excess trivalent Sb atoms occupy divalent Pb sites in the unit cell, increasing the value of n in the system. It was established that the Seebeck coefficient decreases with increasing n, indicating a dominant acoustic phonon scattering mechanism in the current alloys. The results demonstrate that the interaction between Ag and Sb atoms plays a major role in determining the electronic properties in the current Ag1−yPb18Sb1+zTe20 compounds.

Improvement of electrical conductivity in Pb0.96-yMn0.04SnyTe alloys for high temperature thermoelectric applications

Lead–tin–telluride is a well-known thermoelectric material in the temperature range 350–750 K. Here, this alloy doped with manganese (Pb0.96−yMn0.04SnyTe) was prepared for different amounts of tin. X-ray diffraction showed a decrease of the lattice constant with increasing tin content, which indicated solid solution formation. Microstructural analysis showed a wide distribution of grain sizes from <1 μm to 10 μm and the presence of a SnTe rich phase. All the transport properties were measured in the range of 300−720 K. The Seebeck coefficient showed that all the samples were p-type indicating holes as dominant carriers in the measurement range. The magnitude increased systematically on reduction of the Sn content due to possible decreasing hole concentration. Electrical conductivity showed the degenerate nature of the samples. Large values of the electrical conductivity could have possibly resulted from a large hole concentration due to a high Sn content and secondly, due to increased mobility by sp–d orbital interaction between the Pb1−ySnyTe sublattice and the Mn2+ ions. High thermal conductivity was observed due to higher electronic contribution, which decreased systematically with decreasing Sn content. The highest zT = 0.82 at 720 K was obtained for the alloy with the lowest Sn content (y = 0.56) due to the optimum doping level.

Correlation between microstructure and thermoelectric properties of AgPb18SbTe20 (LAST-18)

This study reports microstructure investigations at AgPbmSbTe2+m (m = 18) (Lead-Antimony-Silver-Tellurium, LAST-18). Two different length scales were explored. The micrometer scale was evaluated by SEM to analyze volume fraction and number of secondary phases as well as the impact of processing parameters on the homogeneity of bulk samples. Site-specific liftout of TEM lamellae from thermoelectrically characterised samples was made to investigate the structure on the nanometer scale. High-resolution TEM and energy-filtered TEM were performed to reveal shape and size distribution of nanoprecipitates, respectively. An attempt is made to derive a structure-property relationship.

Microstructure analyses and thermoelectric properties of Ag{sub 1-x}Pb{sub 18}Sb{sub 1+y}Te{sub 20}

Abstract This study reports microstructural investigations of long-term annealed Ag 1 − x Pb m Sb 1 + y Te 2 + m (m=18, x=y=0, hereinafter referred to as AgPb 18 SbTe 20 ) (Lead–Antimony–Silver–Tellurium, LAST-18) as well as of Ag 1 − x Pb 18 Sb 1 + y Te 20 , i.e. Ag-deficient and Sb-excess LAST-18 ( x ≠ 0 , y ≠ 0 ) , respectively. Two different length scales are explored. The micrometer scale was evaluated by SEM to analyze the volume fraction and the number of secondary phases as well as the impact of processing parameters on the homogeneity of bulk samples. For AgPb 18 SbTe 20 , site-specific FIB liftout of TEM lamellae from thermoelectrically characterized samples was accomplished to investigate the structure on the nanometer scale. High-resolution TEM and energy-filtered TEM were performed to reveal shape and size distribution of nanoprecipitates, respectively. A hypothesis concerning the structure–property relationship is set out within the frame of a gradient annealing experiment. This study is completed by results dealing with inhomogeneities on the micrometer scale of Ag 1 − x Pb 18 Sb 1 + y Te 20 and its electronic properties.