Dielectric capacitors with high energy storage performances are exceedingly desired for the next-generation advanced high/pulsed power devices that demand miniaturization and integration. However, poor energy-storage density (U_rec) and low efficiency (η) resulted from the large remanent polarization (P_r) and low breakdown strength (BDS), have been the major challenge for the application of dielectric capacitors. Herein, a high-entropy strategy with superparaelectric relaxor ferroelectrics (SP-RFE) was adopted to achieve extremely low P_r and high BDS in BaTiO₃ system, simultaneously. Due to the high BDS ~800 kV/cm and low P_r ~0.58 μC/cm², high-entropy SP-RFE (La_0.05Ba_0.18Sr_0.18K_0.115Na_0.115Ca_0.18Bi_0.18)TiO₃ (LBSKNCBT) MLCCs exhibited high U_rec ~6.63 J/cm³ and excellent η ~ 96%. What's more, LBSKNCBT MLCCs with high-entropy and SP-RFE characteristic also possess a good temperature and frequency stability. In a word, this work offers an excellent paradigm for achieving good energy-storage properties of BaTiO₃-based dielectric capacitors to meet the demanding requirements of advanced energy storage applications.

Over the past two decades, (K_0.5Na_0.5)NbO₃ (KNN)-based lead-free piezoelectric ceramics have made significant progress. However, attaining a high electrostrain with remarkable temperature stability and minimal hysteresis under low electric fields has remained a significant challenge. To address this long-standing issue, we have employed a collaborative approach that combines defect engineering, phase engineering, and relaxation engineering. The LKNNS-6BZH ceramic, when sintered at T_sint = 1170 ℃, demonstrates an impressive electrostrain with a $d_{33}^{\ast }$ value of 0.276% and 1379 pm·V^–1 under 20 kV·cm^–1, which is comparable to or even surpasses that of other lead-free and Pb(Zr,Ti)O₃ ceramics. Importantly, the electrostrain performance of this ceramic remains stable up to a temperature of 125 ℃, with the lowest hysteresis observed at 9.73% under 40 kV·cm^–1. These excellent overall performances are attributed to the presence of defect dipoles involving ${\mathrm{V}}_{\mathrm{A}}^{{}^{\prime }}\text{–}{\mathrm{V}}_{\mathrm{O}}^{\text{∙∙}}$ and ${\mathrm{B}}_{\mathrm{N}\mathrm{b}}^{{}^{\prime }}\text{–}{\mathrm{V}}_{\mathrm{O}}^{\text{∙∙}}$, the coexistence of R–O–T multiphase, and the tuning of the trade-off between long-range ordering and local heterogeneity. This work provides a lead-free alternative for piezoelectric actuators and a paradigm for designing piezoelectric materials with outstanding comprehensive performance under low electric fields.

Lots of research on thermoelectric materials (TEs) has focused on improving their thermoelectric (TE) properties to achieve efficient energy conversion. However, the mechanical properties of materials are also the object of concern in practical applications. Nowadays, the field of electronic devices is obviously developing in the direction of flexible electronics, so the research on TEs should also consider the plasticity. Since 2018, it has been discovered that inorganic semiconductor materials have the ability of plastic deformation, giving new possibilities for the development of TEs with plasticity. This paper focuses on the TEs with two-dimensional van der Waals (2D vdW) crystal structures, which have good plasticity but low TE properties. However, these materials have the potential to become excellent materials with TE properties and good plasticity through optimization strategies. In this paper, the latest research progress of 2D vdW TE materials and their applications in electronic devices are reviewed. The plasticity and TE properties of 2D vdW materials with M₂X, MX and MX₂ structure are summarized, and their plasticity mechanisms are discussed. We also introduce the application of high throughput screening in the discovery of novel 2D vdW plastic materials, and outline the future research work of 2D vdW TE materials.

The 0.93(Na_0.5Bi_0.5)_1-xSm_xTiO₃-0.07BaTiO₃ multifunctional ceramics were prepared by solid-phase reaction method. The phase structure, microstructure, electrical and photoluminescent properties were systematically studied. With increasing x, the ceramics undergoes the phase transition from rhombohedral to tetragonal with some rhombohedral distortion, along with a reduced grain size and increased relative density. On the other hand, the Sm³⁺ doping enhances the electric-field driven reversible phase transition and domain size, and reduces the domain walls, thereby contributing to improved piezoelectricity and decreased depolarization temperature (T_d) from 91 ℃ to 40 ℃. Excellent piezoelectric properties of d₃₃ = 213 pC/N and k_p = 29.9% are achieved in the x = 0.010 ceramic. Under excitation (407 nm), the Sm³⁺-doped ceramic exhibits bright reddish-orange fluorescence at 564, 599, 646 nm and 710 nm. A polarization-induced enhancement of photoluminescence is obtained in BNBT-xSm ceramics with an improved relative intensity of emission band at 646 nm. These results indicate that Sm³⁺-doped BNBT ceramics show great potential in electro-optic integration and coupling device applications.

BiFeO₃–BaTiO₃ (BF–BT) based piezoelectric ceramics are a kind of high-temperature lead-free piezoelectric ceramics with great development prospects due to their high Curie temperature (T_C) and excellent electrical properties. However, large leakage current limits their performance improvement and practical applications. In this work, direct current (DC) test, alternating current (AC) impedance, and Hall tests were used to investigate conduction mechanisms of 0.75BiFeO₃–0.25BaTiO₃ ceramics over a wide temperature range. In the range of room temperature (RT)−150 ℃, ohmic conduction plays a predominant effect, and the main carriers are p-type holes with the activation energy (E_a) of 0.51 eV. When T > 200 ℃, the E_a value calculated from the AC impedance and Hall data is 1.03 eV with oxygen vacancies as a cause of high conductivity. The diffusion behavior of thermally activated oxygen vacancies is affected by crystal symmetry, oxygen vacancy concentration, and distribution, dominating internal conduction mechanism. Deciphering the conduction mechanisms over the three temperature ranges would pave the way for further improving the insulation and electrical properties of BiFeO₃–BaTiO₃ ceramics.

BiFeO₃-BaTiO₃ based 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-situ using 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 (R_s), remnant piezoresponse (R_rem), maximum displacement (D_max) and negative displacement (D_neg) at 45 V and 120 ℃ reach 122, 69, 127 pm and 75 pm, respectively. Due to the distinct effect of poling engineering in full domain switching, the corresponding d₃₃ at 50 kV/cm and 120 ℃ 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-BaTiO₃ ceramics provides an insight into the relationship between domain structure and piezoelectric response, which has implications for other piezoelectric ceramics as well.

Morphotropic phase boundary (MPB) plays a key role in tuning piezoelectric responses of ferroelectric ceramics. Here, Bi_0·5Na_0·5TiO₃ modified BiFeO₃–BaTiO₃ ternary solid solutions of 0.7BiFeO₃-(0.3-x)BaTiO₃-xBi_0.5Na_0·5TiO₃ (referred to as BF-BT-xBNT, 0.00 ≤ x ≤ 0.04) were prepared for lead-free piezoelectrics. All the ceramics exhibit an MPB with coexisting rhombohedral (R) and tetragonal (T) phases, and the R/T phase ratio decreases upon increasing x. The increment of BNT promotes the grain growth, lowers the leakage current and Curie temperature (T_C), and gradually drives the ferroelectric to relaxor transition. Because of the MPB with appropriate R/T phase ratio, increased grain size and density, and decreased leakage current, the well-balanced performance between d₃₃ = 206 pC/N and T_C = 488 ℃ is obtained in x = 0.01 case. In addition, the further enhanced in-situ d₃₃ = 286–347 pC/N is obtained in BF-BT-xBNT ceramics along with the improved depolarization temperature T_d from 280 to 312 ℃, showing a potential application for lead-free piezoceramics at high temperature.

It is well-known that the performance of BiFeO₃BaTiO₃ (BF-BT) ceramics is sensitive to composition, calcining and sintering temperature (T_cal and T_sint) due to the formation of Bi₂₅FeO₃₉ and/or Bi₂Fe₄O₉ impurities and/or the volatilization of Bi₂O₃. We report remarkably stable electrical properties over the range of −0.03 ≤ x ≤ 0.05 and 930 ℃ ≤ T_sint ≤ 970 ℃ in 0.7Bi_(1+x)FeO₃-0.3BaTiO₃ ceramics prepared by one-step process. This method avoids the thermodynamically unstable region of BiFeO₃ and prevents the formation of Bi₂₅FeO₃₉ and/or Bi₂Fe₄O₉ impurities even when the addition of α-Bi₂O₃ raw 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 C_R/C_PC fraction are 48.0/52.0–50.6/49.4 and 55.9/44.1–56.6/43.4 when x increases from −0.05 ≤ x ≤ 0 to 0.01 ≤ x ≤ 0.05. The stable electrical properties of d₃₃ = 180–205 pC/N, P_r = 17.9–23.8 μC/cm², and T_C = 485–518 ℃ are achieved. The maximum d_33T/d_33RT of 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.

Si₃N₄–SiC ceramics were prepared via reaction sintering with SiC, silicon powder and SiO₂ as main raw materials, and the ceramics were oxidized at a high temperature. The structure and properties of ceramics before and after high-temperature oxidation were investigated. The results show that the main crystalline phases of the samples before high-temperature oxidation are SiC, α-Si₃N₄/β-Si₃N₄ and a small amount of silicon powder, Si₂N₂O and triclinic-SiO₂, while the silicon powder disappears after high-temperature oxidation and forms tetragonal-SiO₂. The mass fraction of β-Si₃N₄ remains constant, and the mass fraction of α-Si₃N₄ decreases from 11.3% before high-temperature oxidation to 6.8% after high-temperature oxidation, while the mass fraction of Si₂N₂O and SiO₂ increases, indicating that α-Si₃N₄ can be decomposed into Si₂N₂O and SiO₂ more easily than β-Si₃N₄. After high-temperature oxidation, the flexural strength of samples decreases from (68.55±6.36) MPa to (49.80±4.96) MPa mainly due to the decomposition of α-Si₃N₄ into Si₂N₂O and SiO₂.

NaNbO₃-based ceramics usually show ferroelectric-like P-E loops 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 CaHfO₃ into NaNbO₃ can stabilize its antiferroelectric phase by reducing the tolerance factor (t), as indicated by the appearance of characteristic double P-E loops. Furthermore, a small amount of MnO₂ addition 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 t shows the reversed trend, thus an enhanced antiferroelectricity and the energy storage density W_rec of 1.69 J/cm³ at 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/cm³ due to both increased tolerance factor and non-polar phase.