Extending the investigations on Bi-based perovskite solid solutions for high-temperature piezoelectric ceramics, this paper considers the binary solid-solution system (1−x)Bi(Ni1∕2Ti1∕2)O3–xPbTiO3 [(1−x)BNT–xPT] for 0.39⩽x⩽1.00. High-density polycrystalline ceramics were fabricated using conventional solid-state processing methods. These ceramics are then taken for structural and electrical properties and differential scanning calorimetry measurements. A morphotropic phase boundary (MPB) was found at the composition 0.51BNT–0.49PT, with a corresponding paraelectric-ferroelectric phase transition TC≈400°C. The electrical poled ceramics demonstrated piezoelectric d33 coefficients≈260pC∕N at room temperature for the MPB BNT–PT compositions. Experimental data are also given for the influence of MnO2 doping to the BNT–PT system close to the MPB compositions, noting an improvement in dielectric losses but a reduction in d33≈180pC∕N. Tricritical behavior is also identified in the tetragonal phase field, with a TC enhancement above TC∼495°C found similar to other Bi(Me′Me″)O3–PbTiO3 systems with high MPB transitions.

The thermal conductivity of layered metal chalcogenides such as MT2 (M = Mo, W; T = S, Se) shows a marked decrease after exfoliation and subsequent restacking process. Random stacking of two-dimensional crystalline sheets circumvents thermal conduction pathways along a longitudinal direction, which results in a reduction in thermal conductivity. WS2 and WSe2 compounds retain p-type conducting behavior after exfoliation and restacking with decreased electrical conductivity due to the change in carrier concentration. MoSe2 compound exhibits metallic behavior < 130 o C with a small Seebeck coefficient, which results from metastable 1T-MoSe2 structure of the restacked phase.

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Herein, we report a significant enhancement of the thermoelectric power factor in polycrystalline Ga-doped ZnO. Despite its higher carrier concentration, the Seebeck coefficient of Zn0.985Ga0.015O was larger than that of Zn0.990Ga0.010O benefiting from an enhancement of the density of states (DOS) effective mass. A gradual increase in the compressive stress with Ga substitution gave rise to a higher DOS at the bottom of the conduction band. An enlarged solution limit of Ga in the ZnO matrix due to a lower firing temperature accelerated the chemical compression. A single phase n-type Zn0.985Ga0.015O bulk exhibited a power factor of 12.5 μWcm−1 K−2.

Thermoelectrics, which transports heat for refrigeration or converts heat into electricity directly, is a key technology for renewable energy harvesting and solid-state refrigeration. Despite its importance, the widespread use of thermoelectric devices is constrained because of the low efficiency of thermoelectric bulk alloys. However, boundary engineering has been demonstrated as one of the most effective ways to enhance the thermoelectric performance of conventional thermoelectric materials such as Bi2 Te3 , PbTe, and SiGe alloys because their thermal and electronic transport properties can be manipulated separately by this approach. We review our recent progress on the enhancement of the thermoelectric figure of merit through boundary engineering together with the processing technologies for boundary engineering developed most recently using Bi2 Te3 -based bulk alloys. A brief discussion of the principles and current status of boundary-engineered bulk alloys for the enhancement of the thermoelectric figure of merit is presented. We focus mainly on (1) the reduction of the thermal conductivity by grain boundary engineering and (2) the reduction of thermal conductivity without deterioration of the electrical conductivity by phase boundary engineering. We also discuss the next potential approach using two boundary engineering strategies for a breakthrough in the area of bulk thermoelectric alloys.