Composition dependent thermoelectric properties of sintered Mg2Si1−xGex (x = 0 to 1) initiated from a melt-grown polycrystalline source

Fabrication of Mg₂Si_1−xGe_x (x = 0-1.0) was carried out using a spark plasma sintering technique initiated from melt-grown polycrystalline Mg₂Si_1−xGe_x powder. The thermoelectric properties were evaluated from RT to 873 K. The power factor of Mg₂Si_1−xGe_x with higher Ge content (x = 0.6-1.0) tends to decrease at higher temperatures, and the maximum value of about 2.2 × 10^− 5 Wcm^− 1K^− 2 was observed at 420 K for Mg₂Si and Mg₂Si_0.6Ge_0.4. The coexistence of Si and Ge gave rise to a decrease in the thermal conductivity in the Mg₂Si_1−xGe_x. The values close to 0.02 Wcm^− 1K^− 1 were obtained for Mg₂Si_1−xGe_x (x = 0.4-0.6) over the temperature range from 573 to 773 K, with the minimum value being about 0.018 Wcm^− 1K^− 1 at 773 K for Mg₂Si_0.4Ge_0.6. The maximum dimensionless figure of merit was estimated to be 0.67 at 750 K for samples of Mg₂Si_0.6Ge_0.4.     

Synthesis of tellurium nanotubes by galvanic displacement

Tellurium nanotubes with controlled diameter and wall thickness were synthesized by galvanic displacement of cobalt nanowires and their temperature dependent field effect transistor and magnetoresistance properties were systematically investigated. The nanotube diameter was slightly larger than the sacrificial cobalt nanowire diameter with a wall thickness of range from 15 to 30 nm depending on the diameter of cobalt nanowires. Te nanotubes show p-type semiconducting property with the field effect carrier mobility of approx. 0.01 cm²/V s which is relatively lower than other 1D nanostructure. Low mobility might be attributed to porous morphology with small grain size (<10 nm). Temperature dependent mobility also exhibiting a Conwell-Weisskopf relationship to temperatures below 250 K, indicating that the dominant scattering sites are ionized impurity centers. Unique MR behavior was observed from nanotube with a maximum magnetoresistance ratio of 37% at 260 K.     

Hybrid microwave processing of Ca3Co4O9 thermoelectrics

In this study, hybrid microwave sintering of Ca₃Co₄O₉ thermoelectric materials was implemented for the first time. Thermoelectric properties of samples sintered in different conditions and in conventional electrical furnace, using the same temperatures and dwell times, were assessed and compared. Microwave processing was found to promote densification and grain texturing in Ca₃Co₄O₉ bulk ceramics, leading to a significant increase of the electrical conductivity. Seebeck coefficient and thermal conductivity were essentially unaffected by the microstructural changes. Prolonged exposure to microwave radiation at 800 °C led to partial phase decomposition and consequent formation of Ca₃Co₂O₆ and Co₃O₄ impurities, with minor effect on the charge transport. Still, the significant presence of residual porosity suggests the need for further optimization of powder and microwave processing conditions.     

Phase,microstructural investigation and thermoelectric properties of Ga-doped zinc oxide-based ceramics sintered under an argon atmosphere

Compositions in the (Zn_0.98-yAl_0.02Ga_y)O (y?=?0.00, 0.01, 0.02, 0.03) series were fabricated via the conventional solid-state sintering route under an argon atmosphere, and their phase, microstructure and thermoelectric properties were investigated. Single-phase ceramics were formed for the compositions with y?≤?0.01. However, an unknown secondary phase was developed along with the parent phases when y?≥?0.02 due to an over solubility limit of Ga in Zn. For a particular value of Ga substitution for Zn, the (Zn_0.98Al_0.02)O ceramics had decreased electrical resistivity (ρ) and an increased power factor (PF). In the present study, the highest power factor of 5.518?×?10^?4 W?K^?2 m^?1 and lowest electrical resistivity of ~0.82 mΩcm were obtained for the composition with y?=?0.01, i.e., (Zn_0.97Al_0.02Ga_0.01)O.     

Balancing electron and phonon scatterings while tailoring carrier concentration in SnTe for enhancing thermoelectric performance

Balancing electron and phonon scattering is crucial for enhancing the thermoelectric (TE) performance of materials. Herein, the TE performance of Mg-alloyed SnTe was significantly enhanced by managing lattice defects. Formation of Sn vacancies in Mg-alloyed SnTe was suppressed via Sn-compensation, leading to 23% higher carrier mobility and 29% higher power factor (PF) of Sn_0.94Mg_0.09Te than those of Sn_0.88Bi_0.03Mg_0.09Te with Bi-doping. Transmission electron microscopy analysis confirmed the formation of dense dislocation arrays in Sn_0.88Bi_0.03Mg_0.09Te, resulting in an ultra-low lattice thermal conductivity (κ_lat = 0.34 W m⁻¹K⁻¹) at 823 K. A combination of Sn-compensation and Bi-doping in Sn_0.90Bi_0.03Mg_0.09Te resulted in high PF and low κ_lat, simultaneously, owing to the balanced scatterings of electron and phonon. Furthermore, carrier concentration was optimised, with a high figure of merit (ZT ∼1.3) achieved at 873 K, ∼50% higher than that obtained by applying either Sn-compensation or Bi-doping individually.