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The influence of external stress/strain on the phase transitions and dielectric properties of potassium niobate crystal
摘要： We investigated the effects of one-dimensional shock wave compression on the structural, dielectric, and ferroelectric properties of a potassium niobate ferroelectric crystal using Landau–Ginsburg–Devonshire phenomenological theory. Our results show that the cubic to tetragonal and tetragonal to orthorhombic phase transition temperatures increase with increasing shock compression and the derivative of the phase transition temperatures with respect to stain induced by shock compression was estimated to be 21.9 °C/10?3. Additionally, the orthorhombic to rhombohedral phase transition temperature decreased with increasing shock compression and the derivative of the phase transition temperatures with respect to stain induced by shock compression was estimated to be ?144.3 °C/10?3. This result represents a 15.3% increase in the remnant polarization of the tetragonal KNbO3 crystal when the stain induced by shock compression increased from 0 to 4×10?3 at 400 °C. The dielectric susceptibilities ε33 and ε22 were calculated to decrease by approximately 20% and 40%, respectively, when the stain induced by shock compression increased from 0 to 1.6×10?3 at 350 °C. The increase in the remnant polarization and the decrease in the dielectric properties induced by shock compression provide an effective way to improve the power output capacity of a ferroelectric-pulsed power supply.
关键词： phase transition,lead-free ferroelectric crystal,ferroelectric properties,dielectric properties,shock compression
更新于2025-09-09 09:28:46
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High-Temperature Cycle Durability of Superplastic Al-Zn Eutectoid Solder Joints with Stress Relaxation Characteristics for SiC Power Semiconductor Devices
摘要： We have developed a new high-temperature Al-Zn lead-free soldering process that utilizes superplasticity to realize SiC power devices with high-temperature cycle durability. The joining process consists of an Al-78wt.%Zn preparation being sandwiched by a SiC die and insulation substrate, an interfacial cleaning process at approximately 250-270oC, a heating stage to reach the solid-liquid coexisting temperature of 420-430oC, the ejection of low-melting-temperature β(Zn) from the joining area by press stress, and the transformation to a superplastic composition, i.e., Al-70 wt.% Zn at 270-310oC. Many lamellar phases that enable superplasticity can be formed in this microstructure. This solder joint composition was proven to have a better stress-relaxation effect than eutectic Al-95wt.%Zn, and the composition shows a much higher damping capability at the maximum operating temperature of SiC devices (200oC) than that of other joining candidates. The outstanding temperature cycle durability was verified in temperature cycle tests from -40oC to 300oC for 5000 cycles. This durability is due to the high-stress-relaxation effect from the superplasticity transformation realized by the lamellar structures in the Al-Zn alloy solder.
关键词： lamellar structure,damping capability,power semiconductor,SiC,superplasticity,temperature cycle test,Al-Zn eutectoid solder,lead-free solder,stress relaxation
更新于2025-09-09 09:28:46
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Electromechanical properties of ternary BiFeO3?0.35BaTiO3–BiGaO3 piezoelectric ceramics
摘要： In the present work, composition dependent crystal structure, ferroelectric, piezoelectric, and temperature dependent dielectric properties of the BiGaO3-modified (1–x)(0.65Bi1.05FeO3–0.35BaTiO3) (BFBT35–xBG, where x = 0.00–0.03) lead-free ceramics were systematically investigated by solid-state reaction method, followed by water quenching process. The substitution of BG successfully diffuses into the lattice of the BFBT ceramics, without changing the pseudo-cubic structure of the samples. The scanning electron microscopy (SEM) results revealed that the average grain size was increased with BG-content in BFBT system. The BFBT–xBG ceramics showed a maximum in permittivity (?max) at temperatures (Tmax) above 500 °C in the compositional range of 0.00 ≤ x ≤ 0.03. The electro-strain is measured to be 0.125% (d*33 ~ 250 pm/V) under unipolar fields (5 kV/mm) for BFBT–0.01BG ceramics. The same composition (x = 0.01), large static piezoelectric constant (d33 ~ 165 pC/N) and electromechanical coupling factor (kp ~ 25%) were obtained. The above investigated characterizations suggests that BFBT–BG material is favorable for piezoelectric and high temperature applications.
关键词： Lead-free,Dielectric,Piezoelectric,Ferroelectric,BiFeO3–BaTiO3
更新于2025-09-09 09:28:46
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Composition-dependent xBa(Zr0.2Ti0.8)O3-(1-x)(Ba0.7Ca0.3)TiO3 bulk ceramics for high energy storage applications
摘要： This work reports the composition dependent microstructure, dielectric, ferroelectric and energy storage properties, and the phase transitions sequence of lead free xBa(Zr0.2Ti0.8)O3-(1-x)(Ba0.7Ca0.3)TiO3 [xBZT-(1-x)BCT] ceramics, with x = 0.4, 0.5 and 0.6, prepared by solid state reaction method. The XRD and Raman scattering results confirm the coexistence of rhombohedral and tetragonal phases at room temperature (RT). The temperature dependence of Raman scattering spectra, dielectric permittivity and polarization points a first phase transition from ferroelectric rhombohedral phase to ferroelectric tetragonal phase at a temperature (TR-T) of 40 0C and a second phase transition from ferroelectric tetragonal phase - paraelectric pseudocubic phase at a temperature (TT-C) of 110 0C. The dielectric analysis suggests that the phase transition at TT-C is of diffusive type and the BZT-BCT ceramics are a relaxor type ferroelectric materials. The composition induced variation in the temperature dependence of dielectric losses was correlated with full width half maxima (FWHM) of A1, E(LO) Raman mode. The saturation polarization (Ps) ≈ 8.3 μC/cm2 and coercive fields ≈ 2.9 kV/cm were found to be optimum at composition x = 0.6 and is attributed to grain size effect. It is also shown that BZT-BCT ceramics exhibit a fatigue free response up to 105 cycles. The effect of a.c. electric field amplitude and temperature on energy storage density and storage efficiency is also discussed. The presence of high TT-C (110 0C), a high dielectric constant (εr ≈ 12285) with low dielectric loss (0.03), good polarization (Ps) ≈ 8.3 μC/cm2) and large recoverable energy density (W = 121 mJ/cm3) with an energy storage efficiency (η) of 70 % at an electric field of 25 kV/cm in 0.6BZT-0.4BCT ceramics make them suitable candidates for energy storage capacitor applications.
关键词： spontaneous polarization,Raman spectroscopy,dielectric constant,lead free ceramics,Energy storage capacitors,fatigue
更新于2025-09-09 09:28:46
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Influence of the Iodide to Bromide Ratio on Crystallographic and Optoelectronic Properties of Rubidium Antimony Halide Perovskites
摘要： Rubidium antimony halides are a promising low toxic alternative to organo-lead halide perovskites as photovoltaic material. In this contribution we systematically investigate the influence of varying the bromide to iodide ratio on the structural, optical and photovoltaic properties of Rb3Sb2Br9-xIx (x = 0 – 9). Single crystal data reveal that all compounds crystallize in a 2D-layered monoclinic crystal structure. Sequential substitution of iodide with the smaller bromide does not change the crystal system, however, increasing the bromide content results in a shrinkage of the unit cell as well as in a blue shift of the absorption onset, increasing the band gap from 2.02 eV to 2.46 eV. Whereas the photovoltaic properties of bromide rich compounds are limited due to a preferential orientation of the layered structure parallel to the substrate, which is detrimental to charge transport, solar cells with Rb3Sb2I9 as absorber material display power conversion efficiencies of 1.37%. Moreover, the devices exhibit low hysteresis properties and are stable for more than 150 days stored under inert atmosphere.
关键词： single crystal,thin film,antimony perovskites,photoluminescence,lead-free perovskite,solar cells,structural characterization
更新于2025-09-09 09:28:46
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High-performance lead-free two-dimensional perovskite photo transistors assisted by ferroelectric dielectrics
摘要： The purpose of this research was to understand the effect of a built-in ferroelectric field on the performance of two-dimensional (2D) lead-free perovskite material-based phototransistor applications. In this work, ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) and two-dimensional (2D) lead-free perovskite ((C6H5C2H4NH3)2SnI4) were utilized as a dielectric layer and a channel layer, respectively. We observed that large hysteresis was successfully eliminated in the transfer curve, as well as the subthreshold swing being significantly reduced by one order of magnitude after P(VDF-TrFE) was introduced as a dielectric layer. Under polarization ‘‘up’’ and ‘‘down’’ states, the device achieved a high photo-switching on/off ratio (4100) and a short photoresponse time (50 ms), respectively. In addition to that, the device also demonstrated a high responsivity of 14.57 A W?1 and a high detectivity of 1.74 × 1012 Jones under the polarization ‘‘up’’ state with an illumination intensity of 21 mW cm?2. In addition, low temperature solution-processed P(VDF-TrFE) and (C6H5C2H4NH3)2SnI4 (except for the contacts of Au electrodes) in this work are considered to be suitable and compatible for flexible-based phototransistor applications in the future.
关键词： lead-free,phototransistor,two-dimensional perovskite,ferroelectric dielectrics
更新于2025-09-09 09:28:46
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Phase structure analysis and pyroelectric energy harvesting performance of Ba(Hf <sub/><i>x</i> </sub> Ti <sub/> 1 <i>-x</i> </sub> )O <sub/>3</sub> ceramics
摘要： Ba(Ti1-xHfx)O3 ceramics were synthesized by a solid-state reaction process. The evolution of the phase structure was identified by XRD spectrum, dielectric spectroscopy and temperature-dependent Raman spectroscopy for the Ba(Ti1-xHfx)O3 ceramics. In addition, pyroelectric energy harvesting properties based on the Olsen cycle were investigated for the first time. A maximum pyroelectric energy harvesting density value of ND=491.30 kJ/m3 (ΔT=120 °C, EH=50 kV/cm) was achieved in the Ba(Hf0.05Ti0.95)O3 ceramic. Compared with those of BT, the values of ND more than doubled in the temperature range from ΔT=60 °C to ΔT=100 °C in the Ba(Hf0.05Ti0.95)O3 ceramic and even increased 3.2 times at ΔT=80 °C near the Curie temperature (TC) of the Ba(Hf0.05Ti0.95)O3 sample. In addition, a larger pyroelectric energy harvesting density value of ND=367.10 kJ/m3 (ΔT=120 °C, EH=50 kV/cm) was acquired in the Ba(Hf0.12Ti0.88)O3 ceramic. Values of ND-BHT5/ND-BT and ND-BHT12/ND-BT were analyzed in the Ba(Ti1-xHfx)O3 ceramics. The optimal pyroelectric properties can be obtained in the vicinity of the ferroelectric to paraelectric phase-transition region.
关键词： Phase transition,Lead-free ceramics,Raman spectra,Ferroelectric materials,Energy harvesting
更新于2025-09-09 09:28:46
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Orientation dependence of electric field induced phase transitions in lead-free (Na <sub/>0.5</sub> Bi <sub/>0.5</sub> )TiO <sub/>3</sub> -based single crystals
摘要： The orientation dependence of the electric field induced strain and phase transitions in 0.92(Na0.5Bi0.5)TiO3-0.06BaTiO3-0.02(K0.5Na0.5)NbO3 (NBT-6BT-2KNN) single crystals has been investigated. The evolution of Raman spectra with electric field reveals that a tetragonal ferroelectric phase is initially induced at E=14kV/cm and completed above E=25kV/cm for [001] oriented single crystals. When the electric field is applied along [111] direction, a partial phase transition from pseudocubic to rhombohedral structure is triggered at E=19kV/cm, which is higher than that for inducing tetragonal ferroelectric phase along [001] direction. Both field-induced phase structures and stability of NBT-6BT-2KNN single crystal are strongly associated with the crystallographic orientations. These results provide a better understanding to the field-induced macroscopic strain in lead-free NBT-based ferroelectrics.
关键词： X-ray diffraction,lead-free ferroelectrics,electric field induced phase transitions,Raman spectroscopy,NBT-based single crystals
更新于2025-09-09 09:28:46
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Influence of compositional ratio K/Na on structure and piezoelectric properties in [(Na1?xKx)0.5Bi0.5]Ti0.985Ta0.015O3 ceramics
摘要： [(Na1-xKx)0.5Bi0.5]Ti0.985Ta0.015O3 (abbreviated as Ta-NK100x) lead-free ceramics with good piezoelectric properties were prepared using a solid-state reaction method. The structure and electrical properties of Ta-NK100x had been systemically investigated. The highest bipolar strain of 0.458% and the unipolar strain 0.448% are achieved at x = 0.18 at 60 kV/cm. Meanwhile, the corresponding normalized strain d*33 reaches 747 pm/V. In addition, the unipolar strain of the poled Ta-NK18 increases to 0.537%, and corresponding d*33 increases slightly to 894.5 pm/V at 60 kV/cm. The electric-field-induced phase transition between ferroelectric and relaxor is found to play a dominant role in the origin of the large strain. Moreover, the strain behavior remains stable within 105 switching cycles which indicating the prepared ceramics are promising candidates for actuators and stress sensors.
关键词： ferroelectric,relaxor,solid-state reaction,piezoelectric properties,lead-free ceramics
更新于2025-09-04 15:30:14
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Unique ion diffusion properties in lead-free halide double perovskites: A first-principles study
摘要： Pb-free halide double perovskites (HDPs) are proposed as potential candidates for various optoelectronic applications to replace the mainstream hybrid organic-inorganic halide perovskites, e.g., CH3NH3PbI3. While it is known that ion di?usion is a critical problem to a?ect the structural and electronic stability of CH3NH3PbI3, the mechanism of ion di?usions in HDPs is still unclear and highly desired to be revealed. In this study, taking Cs2AgInX6 (X = Cl, Br) HDPs as prototypes, for the ?rst time we suggest that the fast ion di?usion of the dominant defects may play an important role in the performance stability of HDPs. Importantly, we ?nd that the Agi+ di?usion in a multi-ion concerted fashion has a much faster di?usion rate, compared to the VAg? and VX? di?usion in a single-ion fashion. It is revealed that HDPs exhibit quite di?erent di?usion properties from CH3NH3PbI3. Furthermore, we demonstrate that the di?usion rate of Agi+ in HDPs can be e?ectively suppressed by applying an epitaxial strain, which opens a promising way to enhance the performance stability of perovskite materials for various device applications.
关键词： Lead-free halide double perovskites,Ion di?usion,Performance stability,ab initio calculations,Solar cells
更新于2025-09-04 15:30:14