Broad-temperature-span and improved piezoelectric/dielectric properties in potassium sodium niobate-based ceramics through diffusion phase transition

Thermal strain induced large electrocaloric effect of relaxor thin film on LaNiO3/Pt composite electrode with the coexistence of nanoscale antiferroelectric and ferroelectric phases in a broad temperature range

Enhanced piezoelectric properties in BF-BT based lead-free ferroelectric ceramics for high-temperature devices

Piezoelectric functional materials have been extensively studied and employed in numerous devices. With the rapid development of modern industries, such as power plants, aerospace, automotive, renewable energy and material processing industries, the high temperature piezoelectric materials that can work in extreme environments are in great demand. Piezoelectric materials including piezoelectric single crystals, ceramics and films, are at the heart of electromechanical actuation and sensing devices. A variety of applications where piezoelectric actuators and sensors operate at elevated temperatures (T 200℃) would be extremely desired. The actuators need to work efficiently with high strokes, torques, and forces while operating under relatively harsh conditions. These include high-temperature fans and turbines, motors for valves or natural gas industries, kiln automation, and actuators for automotive engines such as fuel injectors and cooling system elements. Yet, the majority of industrial actuator applications are at or below the 250℃ temperature limit. In addition to the increase in operational temperatures of piezoelectric motors and actuators, a future area of interest is high-temperature MEMS research, which can be used for high-temperature valving. On the other hand, the piezoelectric sensors have been widely used for structural health monitoring applications. This is due to their wide bandwidth, versatility, simplicity, high rigidity, high stability, high reproducibility, fast response time, wide operating temperature range, insensitivity to electric and magnetic fields, the capacity for miniaturization and minimal dependence on moving parts and low power consumption, and wide piezoelectric materials and mechanisms selections, which will greatly benefit the sensing applications. In addition to the temperature usage range, the piezoelectric sensors must withstand the harsh environments encountered in space, engine, power plants, and also need to possess high sensitivity, resistivity, reliability, stability and robustness. In order to use the piezoelectric materials for a specific high temperature application, many aspects need to be considered together with piezoelectric properties, such as phase transition, thermal aging, thermal expansion, chemical stability, electrical resistivity, and the stability of properties at elevated temperature. In this paper, ferroelectric materials with high Curie point, including perovskite-type ferroelectrics, bismuth layer structured ferroelectrics, tungsten-bronze structured ferroelectrics, together with non-ferroelectric piezoelectric single crystals, are surveyed. The crystal structure characteristics, high temperature piezoelectric properties, and recent research progress are discussed. A series of high temperature piezoelectric devices and their applications are reviewed, including high temperature piezoelectric detectors, sensors, transducers, actuators, etc. Finally, recent important research topics, the future development of high temperature piezoelectric materials and the potential new applications are summarized.

BaBi2Sb2Ti4O15, a four-fold layered structured ceramic compound is synthesized by the mixed oxide process at sintering temperature 875 °C. The structural and electrical properties of the material measured in a broad temperature (25–500 °C) and frequency (103–106 Hz) range are presented here. From the preliminary X-ray structural analysis, it is confirmed that this compound has an orthorhombic structure. The temperature and frequency-dependent dielectric constant of the material is observed. Modified Curie Weiss’s law explains the extent of diffusivity observed in a phase transition. The presence of grain and grain boundary effects is confirmed from the Nyquist plots. Non-Debye-like relaxation is noticed from complex impedance and modulus analyses. Reduction in grain resistance is noticed with the enhancement of temperature indicating the existence of negative temperature coefficient of resistance (NTCR) properties in the compound. It is concluded from the above results that this compound may have wide applications in electronics as well as high-temperature devices.

As the operational demands on piezoelectric devices grow increasingly stringent, there is an urgent need for materials capable of delivering stable performance at elevated temperatures. BiFeO3 (BF), a lead-free piezoelectric oxide with high-temperature resilience, is characterized by its notably high Curie temperature (Tc ∼ 835 °C), rendering it a promising candidate for high-temperature applications. However, its piezoelectric coefficients remain inadequate to satisfy practical requirements. The 0.7BiFeO3-0.3Ba(1-x) SrxTiO3 system (abbreviated as BF-BSxT) was designed to elucidate the roles of chemical disorder and local structural heterogeneities in the enhancement of functional properties through fine-tuning of the Sr content. The phase structure of the samples was carefully examined by X-ray diffraction. Rietveld refinement of the XRD data revealed that all BF-BSxT ceramics consist of coexisting R and PC phases. Optimized compositional disorder and local heterogeneities led to a moderate enhancement in the piezoelectric coefficient d33 value of 160 pC/N, a high Tc of 495 °C, and a remanent polarization Pr ≈ 22.1 μC/cm2 -were achieved in the BF-BSxT system at x = 0.06. These results indicate that BF-BSxT ceramics hold good potential for use in high-temperature piezoelectric devices.

We achieved a large electrocaloric effect over a broad temperature span in lead-free ceramics utilizing successive and metastable phase transitions.

An appropriate amount of Zn-ions are incorporated into the high Curie temperature bismuth layer-structure ferroelectric material to fabricate Sr0.2Na0.4Pr0.4Bi4Ti4O15:xwt%ZnO; (SNPBT:xZn), with x = 0, 0.10, 0.15, and 0.20 ceramic series to investigate the magnetic, ferroelectric, and energy storage efficiency and piezoelectric properties. Pure SNPBT and SNPBT:xZn ceramics have maintained their structure even after the intercalation of Zn-ions at the lattice sites of SNPBT. The addition of ZnO in SNPBT has improved the multifunctional properties of the material at x = 0.15. At room temperature, SNPBT:0.15Zn has shown a high relative density of 96%, exhibited weak ferromagnetic behavior along with a low saturation magnetization (Ms) of 0.028 emu/g with a low coercive field of 306 Oe, a high remnant polarization (Pr) of 9.04 µC/cm2, a recoverable energy density () of ~0.5 J/cm3, an energy conversion efficiency (η) of ~41%, a high piezoelectric co-efficient (d33) of 21 pC/N, and an impedance of 1.98 × 107 Ω, which are much improved as compared to pure SBT or pure SNPBT ceramics. Dielectric Constant (ɛr) versus temperature plots present the sharp peak for SNPBT:0.15Zn ceramic at a Curie temperature (TC) ~ 605 °C, confirming the strong ferroelectric nature of the ceramic. Moreover, SNPBT:0.15Zn ceramic has shown strong, piezoelectric, thermally stable behavior, which remains at 76% (16 pC/N) of its initial value even after annealing at 500 °C. The achieved results clearly indicate that SNPBT:0.15Zn ceramic is a promising candidate for future wide-temperature pulse power applications and high-temperature piezoelectric devices.

Lead free piezoelectric materials are being intensively investigated in order to substitute lead based ones, commonly used in many different applications. Among the most promising lead-free materials are those with modified NaNbO3, such as (K, Na)NbO3 (KNN) and (Ba, Na)(Ti, Nb)O3 (BTNN) families. From a ceramic processing point of view, high density single phase KNN and BTNN ceramics are very difficult to sinter due to the volatility of the alkaline elements, the narrow sintering temperature range and the anomalous grain growth. In this work, Spark Plasma Sintering (SPS) and high-energy ball milling (HEBM), following heat treatments (calcining and sintering), in oxidative (O2) atmosphere have been used to prepare single phase highly densified KNN (“pure” and Cu2+ or Li1+ doped), with theoretical densities ρth > 97% and BTNN ceramics (ρth ~ 90%), respectively. Using BTTN ceramics with a P4mm perovskite-like structure, we showed that by increasing the NaNbO3 content, the ferroelectric properties change from having a relaxor effect to an almost “normal” ferroelectric character, while the tetragonality and grain size increase and the shear piezoelectric coefficients (k15, g15 and d15) improve. For KNN ceramics, the results reveal that the values for remanent polarization as well as for most of the coercive field are quite similar among all compositions. These facts evidenced that Cu2+ may be incorporated into the A and/or B sites of the perovskite structure, having both hardening and softening effects.

GeTe Thermoelectrics

The supercritical carbon dioxide (sCO2) Brayton cycle has been regarded as a promising technique for future energy conversion systems. sCO2 recompression cycle configuration has been widely studied for nuclear reactors with the temperature difference between the reactor inlet and outlet restricted to 200°C. However, this cycle configuration may be not suitable for high temperature gas-cooled reactors (HTGR) with a temperature difference up to 500°C. Herein, we aim to develop an optimal sCO2 cycle configuration for HTGR with a broad temperature span, which has both high cycle efficiency and sufficient mechanical security guaranteed by energy cascade utilization. We propose a recompression cycle combined with reheat and expansion technologies, and perform a parameter optimization in Fortran platform. First, the thermodynamic models of the system are established. Then, the parametric analysis is conducted about the effects of split ratio, temperature difference of the heat exchangers, inlet parameters of turbines and compressors, and isentropic efficiencies on the cycle performance. The sCO2 cycle tailored for HTGR can achieve a cycle efficiency of 44.98%. Our work not only highlights the design criteria of temperature-match between the HTGR and sCO2 cycle but also guides the numerical assessment of the steady-state performance.

In-situ observation of magnetic vortices and their manipulation by external fields in Tb(Co1−xFex)5 ribbons

Thermal metamorphism of primitive meteorites—IX. On the mechanism of trace element loss from Allende heated up to 1400° C

Bismuth ferrite (BiFeO 3 )‐based high‐temperature piezoelectric ceramics have high Curie temperatures and excellent thermal stability; however, their applications are limited by inadequate piezoelectric performance due to large coercive fields and high leakage conduction. This study investigates the impact of MnO 2 doping on the phase transition, defect structure, domain morphology, as well as the dielectric, ferroelectric, and electrical conduction behaviors of 75BiFeO 3 –25BaTiO 3 ceramics sintered under an oxygen atmosphere. A structural transformation from distorted rhombohedral to pseudo‐cubic symmetry was observed with increasing MnO 2 content, accompanied by domain fragmentation. MnO 2 doping at an appropriate level inhibited defects such as and . Regarding electrical properties, the reduction in rhombohedral distortion and enhancement in the pseudo‐cubic phase after MnO 2 doping resulted in decreased remanent polarization and electrostrain, which could potentially hinder piezoelectric response. However, appropriate MnO 2 doping effectively reduced leakage conduction, significantly enhancing the poling efficiency of the ceramics. The maximum d 33 value of 107 pC/N and k p of 0.36 were achieved at an MnO 2 doping content of 0.1 wt.%. Excessive MnO 2 addition may generate defect dipoles, as indicated by increased coercive field ( E c ) and dielectric loss. The observed increase in high‐temperature resistivity with MnO 2 content is primarily attributed to reduced grain size and increased concentration of mobile defects. This study provides valuable insights for further research on the application of 75BiFeO 3 –25BaTiO 3 systems in high‐temperature piezoelectric devices.

Potassium-sodium niobate (Na, K)NbO3 (NKN) powder was synthesized at low temperature via microwave-assisted hydrothermal solovthermal method (MHSM). The resultant powder was characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). XRD results showed that pure (Na, K)NbO3 powder with a single perovskite structure was successfully synthesized when the concentration of mineralization exceeded 1 M. In order to reduce the volatilization of alkaline elements, NKN ceramics derived from 5M-powder were prepared in a microwave furnace. Microstructure, stoichiometry, and electrical properties of the obtained ceramics were investigated. The piezoelectric coefficient (d33), electromechanical coupling coefficient (kp) and remnant polarization (Pr) of the sample sintered at 1050 ℃ show optimal values of 132 pC/N, 38% and 26.3 µC/cm2, respectively. The results indicate that the microwave heating is a promising method for synthesizing and sintering NKN-based ceramics.

In this investigation, an complex perovskite structure Ba(FeTa)0.5O3 (BFT) was chosen as the host material and doped with trivalence La3+ ions and divalence Na+ ions which substitute the Ba2+ ion to form Ba1-xAx(FeTa)0.5O3 (A=La, Na) ceramics by reaction sintering. The pure perovskite phase was found in Ba(FeTa)0.5O3 after 8h sintered at 1350℃.The theoretic density increased with PVA increased, and a maximum theoretic density was at PVA 5wt%. The dielectric constant and dielectric loss tangent of Ba(FeTa)0.5O3 at 100 Hz were 19k, 0.279.In addition, in the complex perovskite oxides Ba1-xAx(FeTa)0.5O3(A=La, Na)system, when increasing the La3+ ion concentration, the dielectric constant was decreased owing to the La3+ would made particle growth difficult and produce porosity increased. The dielectric constant is about 1×104¬ at 30 ℃of Ba0.98La0.02(FeTa)0.5O3 ceramic sintered at 1350℃ for 8h, measured at 100 Hz. The grain boundary-layer capacitance was produced for Cole-Cole plot analysis. In addition, when increasing Na+ ion concentration, the density was increased due to the melting point of Na+ is about 851℃, it would made density became earlier. All of samples were over than theoretic density 90%. The dielectric constant is about 8×105 at 30℃ of Ba0.96Na0.04(FeTa)0.5O3 ceramic sintered at 1300℃ for 8h, measured at 100 Hz. The dielectric relaxor behavior with a broad temperature range was obtained for the BLFT and BNFT ceramics which was contributed by orientation and space charge polarization.