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BiFeO3-BaTiO3based ceramics are considered to be the most promising lead-free piezoelectric ceramics due to their large piezoelectric response and high Curie temperature. Since the piezoelectric response of piezoelectric ceramics just appears after poling engineering, in this work, the domain evolution and microscopic piezoresponse were observed in-situusing piezoresponse force microscopy (PFM) and switching spectroscopy piezoresponse force microscopy (SS-PFM), which can effectively study the local switching characteristics of ferroelectric materials especially at the nanoscale. The new domain nucleation preferentially forms at the boundary of the relative polarization region and expands laterally with the increase of bias voltage and temperature. The maximum piezoresponse (Rs), remnant piezoresponse (Rrem), maximum displacement (Dmax) and negative displacement (Dneg) at 45 V and 120 °C reach 122, 69, 127 pm and 75 pm, respectively. Due to the distinct effect of poling engineering in full domain switching, the corresponding d33at 50 kV/cm and 120 °C reaches a maximum of 205 pC/N, which is nearly twice as high as that at room temperature. Studying the evolution of ferroelectric domains in the poling engineering of BiFeO3-BaTiO3ceramics provides an insight into the relationship between domain structure and piezoelectric response, which has implications for other piezoelectric ceramics as well.

It is well-known that the performance of BiFeO3BaTiO3(BF-BT) ceramics is sensitive to composition, calcining and sintering temperature (Tcaland Tsint) due to the formation of Bi25FeO39and/or Bi2Fe4O9impurities and/or the volatilization of Bi2O3. We report remarkably stable electrical properties over the range of −0.03 ≤ x ≤ 0.05 and 930 °C ≤ Tsint≤ 970 °C in 0.7Bi(1+x)FeO3-0.3BaTiO3ceramics prepared by one-step process. This method avoids the thermodynamically unstable region of BiFeO3and prevents the formation of Bi25FeO39and/or Bi2Fe4O9impurities even when the addition of α-Bi2O3raw material is intentionally deficient or rich to make off-stoichiometric BF-BT, thus greatly improving the robustness of compositional and processing. Rhombohedral-pseudocubic phase coexists in all ceramics, and their CR/CPCfraction are 48.0/52.0–50.6/49.4 and 55.9/44.1–56.6/43.4 when xincreases from −0.05 ≤ x ≤ 0 to 0.01 ≤ x ≤ 0.05. The stable electrical properties of d33 = 180–205 pC/N, Pr = 17.9–23.8 μC/cm2, and TC = 485–518 °C are achieved. The maximum d33T/d33RTof BF-BT is twice that of soft PZT, superior to most the-state-of-art lead-free ceramics. Our results provide a synthesis strategy for designing high performance piezoelectric materials with good stability and easy industrialization.

NaNbO3-based ceramics usually show ferroelectric-like P-Eloops at room temperature due to the irreversible transformation of the antiferroelectric orthorhombic phase to ferroelectric orthorhombic phase, which is not conducive to energy storage applications. Our previous work found that incorporating CaHfO3into NaNbO3can stabilize its antiferroelectric phase by reducing the tolerance factor (t), as indicated by the appearance of characteristic double P-Eloops. Furthermore, a small amount of MnO2addition effectively regulate the phase structure and tolerance factor of 0.94NaNbO3-0.06CaHfO3(0.94NN-0.06CH), which can further improve the stability of antiferroelectricity. The XRD and XPS results reveal that the Mn ions preferentially replace A-sites and then B-sites as increasing MnO2. The antiferroelectric orthorhombic phase first increases and then decreases, while the tshows the reversed trend, thus an enhanced antiferroelectricity and the energy storage density Wrecof 1.69 J/cm3at 240 kV/cm are obtained for 0.94NN-0.06CH-0.5%MnO2(in mass fraction). With the increase of Mn content to 1.0 % from 0.5 %, the efficiency increases to 81 % from 45 %, although the energy storage density decreases to 1.31 J/cm3due to both increased tolerance factor and non-polar phase.

Digenite (Cu1.8S) as a potential p-type thermoelectric (TE) material has attracted extensive attention due to its environmental benign, abundant resources and low cost of component elements. In this study, the TE properties of MnxCu1.8S bulk samples prepared by mechanical alloying (MA) combined with spark plasma sintering (SPS) were investigated. Doping Mn would initially substitute Cu and tune the band structure of Cu1.8S with an enlarged band gap Eg. However, if Mn content is beyond the solubility limit of x = 0.01 in Cu1.8S will cause the formation of MnS, which contributes to the formation of Cu-rich phases at 0.02 ≤ x ≤ 0.08. Benefiting from the synergetic scattering effect of point defects (MnCu•, VS••) and MnS, Cu1.96S, Cu1.97S, Cu2S phases, the lowest thermal conductivity κvalue of 0.75 W m−1K−1was obtained at 773 K for Mn0.08Cu1.8S. Along with the decreased κ, the highest figure of merit ZTvalue of 0.92 at 773 K achieved in Mn0.08Cu1.8S bulk sample. A maximum engineering ZTengof 0.3 and its efficiency ηmaxof about 6% were obtained at 323–773 K, which is almost 3 times than that of the pristine Cu1.8S (ηmax = 2.2%). Introducing Mn in Cu1.8S is an effective and convenient strategy to improve TE performance.

BiFeO3BaTiO3(BF–BT) ceramics exhibit higher piezoelectric coefficients (d33), Curie temperatures (TC), and temperature stability than other high-temperature lead-free piezoelectric materials. However, despite their crucial role in piezoelectric devices, the mechanical properties of BF–BT ceramics have been underexplored. A thorough evaluation of the mechanical properties of BF–BT is crucial for developing cost-effective and durable lead-free piezoelectric ceramics. Moreover, the specific causes of the high piezoelectric response and excellent temperature stability of BF–BT ceramics remain unclear owing to the instrumental detection threshold, which limits experimental studies to temperatures above 140 °C and below the degradation temperature of d33. To investigate the intrinsic origins of the high piezoelectricity and temperature stability of BF–xBT ceramics and to enhance their mechanical properties, a two-step sintering process is used to fabricate these ceramics (0.25 ≤ x ≤ 0.40). Owing to improvements in grain refinement and reduced Bi3+volatilization, the BF–0.33 BT ceramic exhibits enhanced overall performance, with a modified small punch strength of 155 MPa, Vickers hardness of 5.2 GPa, a d33of 220 pC/N at room temperature, TCof 466 °C, and d33values exceeding 400 pC/N at 260 °C. Phase-field simulations, which address the limitations of device detection thresholds, reveal that with increasing temperature, the domain structure relaxes, and polarization intensity decreases. This indicates that changes in the high-temperature piezoelectric properties can be attributed to domain structure relaxation and the increase in dielectric constant. Overall, BF–BT ceramics exhibit superior piezoelectric performance, mechanical properties, and temperature stability, making them highly suitable for use in high-temperature and demanding environments.

Room-temperature thermoelectric materials provide promising solutions for energy harvesting from the environment, and deliver a maintenance-free power supply for the internet-of-things (IoTs). The currently available Bi2Te3family discovered in the 1950s, still dominates industrial applications, however, it has serious disadvantages of brittleness and the resource shortage of tellurium (1 × 10−3 ppm in the earth's crust). The novel Mg3Sb2family has received increasing attention as a promising alternative for room-temperature thermoelectric materials. In this review, the development timeline and fabrication strategies of the Mg3Sb2family are depicted. Moreover, an insightful comparison between the crystallinity and band structures of Mg3Sb2and Bi2Te3is drawn. An outlook is presented to discuss challenges and new paradigms in designing room-temperature thermoelectric materials.