Search

We present the study of structural, morphological, dielectric, transport and ferroelectric properties of (1-x)BFO–xPTO solid solutions, with 0.3 ≤ x ≤ 0.6, prepared via non-conventional synthesis methods. These methods include Sol-gel and Single-step solid state method. Structural analysis revealed presence of mixed phases i.e. monoclinic (CC) and tetragonal (P4mm) phases, for all compositions showing a Morphotropic Phase Boundary. For the compositions with higher concentration of PTO, an increase in tetragonal phase fraction has been observed. Quantitative analysis showed, in general, higher value of c/a (i.e. tetragonality) for all samples as compared to the bulk PTO. The morphological analysis shows small grain size irrespective of synthesis method and composition. The low temperature frequency dependent tangent loss shows dielectric relaxation with small magnitude of dielectric constant indicating absence of extrinsic contributions. High temperature dielectric anomaly is observed around 400–500 K corresponding to magnetic phase transformation of BFO at Neel temperature which suggest the presence of magneto-electric coupling in specific compositions. Sol-gel prepared composite appeared to be more resistive than the Single-step synthesized composite and shows Arrhenius type dependence of high temperature ac conductivity. Ferroelectricity was observed in all ceramic samples which sustained high applied electric field up to 190 kV/cm. Finally, a correlation between polarization, tetragonal phase fraction and c/a ratio, has been drawn and discussed. It is concluded that c/a ratio (i.e. tetragonality) is more important parameter which can be tuned to achieve enhanced ferroelectric response as compared to the tetragonal phase fraction in (1-x)BFO–xPTO solid solutions.

The Half-Heusler (Hf0.5Zr0.5)1-xTixNiSn0.998Sb0.002 (x = 0, 0.1, 0.3, 0.5 and 0.7) compounds were prepared by vacuum induction melting method combined with annealing, mechanical grinding and spark plasma sintering (SPS) process. For the sintered bodies, the phase and the temperature dependent thermoelectric properties were characterized. The pure phases of Half-Heusler (HH) structure were successfully obtained by subsequent SPS at 1273 K for 1 h under vacuum. With Ti substitution the lattice parameter decreased systematically and carrier concentration increased showing n-type semiconducting behavior, and the thermal conductivity decreased due to reduction of lattice thermal conduction. As a result, it was obtained the figure of merit ZT∼0.92 at 837 K at x = 0.3.

Hf-free n-type half-Heusler with a nominal composition of Ti0.5Zr0.5NiSn0.98Sb0.02 has been reported to have a high ZT value of almost 1.2. However, the synthesis process requires a long annealing time to achieve single-phase structure, which contributes to high product costs due to energy and time consumption. Here we introduce a new route to prepare (Ti0.5Zr0.5)1−xNbxNiSn (x = 0, 0.0050, 0.0075, 0.0100, 0.0125, 0.0150, 0.0175 and 0.0200) compounds for high thermoelectric (TE) performance along with shortening time for sample preparation. The samples were prepared by a combination of arc-melting (AM) and melt-spinning (MS) followed by spark plasma sintering process (SPS). The combination of these synthetic methods produced (Ti0.5Zr0.5)1−xNbxNiSn samples with high chemical homogeneity, single-phase structure, and fine grain about 300 nm in size, which are preferred for both charge and phonon transport properties. As a result, a maximum power factor of 44.5 µW cm−1 K−2 at 817 K and a maximum ZT of 1.19 at 874 K were achieved for the sample with x = 0.015, which are comparable to the highest ZT value reported so far for the Hf-free n-type MNiSn (M = Ti, Zr) compounds. The calculated output power density Pd and efficiency η based on a single-leg device showed an excellent performance, which yields the maximum Pd of 16.2 W cm−2 and η of 12.08% at the cold side temperature TC ≈ 305 K and the hot side temperature TH ≈ 875 K for the optimized composition with x = 0.0125. Furthermore, it is noted that the synthetic process here does not require a long-annealing time and it can be easily applied to mass production.

In this work we improved the thermoelectric figure of merit ZT in a LaxCoFe3Sb12 (x = 0.80, 0.85, 0.90) skutterudite system by applying a conductivity ratio (electrical to thermal) control method based on the phonon-glass electron-crystal (PGEC) approach. Polycrystalline bulks of LaxCoFe3Sb12 were prepared by melt-spinning and subsequent spark plasma sintering. Based on the thermoelectric transport properties and microstructural analysis, we found that large amount of La-filler (x ≥ 0.9) in the LaxCoFe3Sb12 system is necessary to increase the conductivity ratio required to improve the thermoelectric properties in the LaxCoFe3Sb12 system. Resultantly, the maximum ZT value of 0.87 was obtained at 723 K in the heavily filled La0.9CoFe3Sb12. We discuss our results on the basis of the filling level dependence of the rattling effect and on the formation of the skutterudite single phase. We also confirm that a large amount of La-filling level is necessary to meet the electrical neutrality condition of the p-type LaxCoFe3Sb12 skutterudite system.

We investigate the thermoelectric properties of reduced graphene oxide (RGO) incorporated Bi0.36Sb1.64Te3 composites. A melt spinning process enables RGO to be uniformly distributed with Bi0.36Sb1.64Te3 matrix and the incorporated RGO increases the carrier mobility of Bi0.36Sb1.64Te3 matrix with the enhancement of the power factor by 20%. The grain sizes of the Bi0.36Sb1.64Te3 materials decrease with the RGO amount, leading to the reduction in lattice thermal conductivity by enhancing grain boundary scattering. Owing to the above effects, the thermoelectric figure of merit (ZT) can be enhanced in the measured temperature range, where the optimum ZT value is reached to 1.16 at 393 K, ∼15% higher ZT value than pristine Bi0.36Sb1.64Te3. Consequently, this RGO incorporation method could be widely used to improve thermoelectric performances in other conventional thermoelectric materials.

We report the effects of Cu incorporation on the thermoelectric transport properties in n-type CuxBi2Te2.7Se0.3 (0 ≤ x ≤ 0.08) compounds. The acceptor roles of Cu in the compounds were elucidated by the charge and thermal transport properties. Considerable change in the density of state (DOS) effective mass depending on the Cu content was evidenced by the cross-over of Seebeck coefficients, and the effects of the DOS effective mass on the thermoelectric transport properties were interpreted by temperature-normalized Pisarenko plot. Significant enhancement of thermoelectric figure of merit and the control of optimum operating temperature could be achieved by the Cu incorporation, and their origins were discussed.

We report the enhanced thermoelectric properties in Cu x Bi 0.38 Sb 1.62 Te 3 compounds through simultaneous achievement of improved charge transport and suppressed thermal transport by Cu incorporation. Incorporated Cu acted as an acceptor rather than donor, resulting in the significant improvement of p -type electric conduction. Density of state effective mass was almost linearly proportional to the Cu content in the compounds, and it led to the enhancement of power factor in Cu-doped compounds regardless of the increase in hole concentration. Furthermore, reduction of bipolar and lattice thermal conductivities could also be achieved by the Cu incorporation, consequently leading to the improved thermoelectric properties of the compounds at relatively high temperatures. Effects of Cu on the thermal and charge transport properties are discussed in detail, and the controllability of the thermoelectric performance for optimum operating temperature is also reported.

The effects of eco-friendly (BiNa0.84K0.16)0.48Sr0.04TiO3 (BNKS)-content in 0.65Bi1.05FeO3–0.35BaTiO3 (BFBT) dielectrics were investigated by following simple solid state fabrication route. By the introduction of BNKS in BFBT matrix the average grain size was significantly reduced, with relatively high dense microstructure (relative density > 94%). The BNKS-modified BFBT dielectrics demonstrated thermally-stable er (670–1005, from 30 °C to 500 °C), high Tmax (424 °C–465 °C), colossal er-max (58880–69226), and er-mid (2891–5652 ± 15%) across the broad range of temperature from 244 °C to 500 °C. At the optimum composition (x = 0.10) the temperature-dependent (30 °C–150 °C) substantially high energy-storage density (Wstore ∼ 0.81 J/cm3) and efficiency (η > 60%) in bulk ceramics were observed. The thermally-stable dielectric and energy storage properties suggest that present investigated dielectrics can be promising candidates for high temperature dielectric applications and power electronics.