Luminescence and electrical properties of Eu-modified Bi0.5Na0.5TiO3 multifunctional ceramics

The Eu³⁺-modified Bi_0.5Na_0.5TiO₃ (BNT) ceramics have been fabricated by the solid-state reaction method. The impact of Eu³⁺ doping on the structure, photoluminescence, and electrical properties has been studied by XRD, SEM, PL spectra, and LCR meter. X-ray diffraction analysis reveals that the crystal structure of the samples is well matched with the trigonal perovskite, and the optimal temperature of presintering is 880°C. The Eu³⁺-doped BNT ceramics show excellent red fluorescence at 614 nm corresponding to the ⁵D₀→⁷F₂ transition of Eu³⁺ under 466 nm excitation and relatively long fluorescence lifetime. The BNT-0.02Eu ceramic density is up to 5.68 g/cm³ and the relative density is up to 94.6% with sintering temperature 1075°C. The piezoelectric constant (d₃₃) of samples has been significantly improved up to 110 pC/N by Eu³⁺ doping. The BNT-0.03Eu ceramic presintered at 880°C and sintered at 1050°C has good dielectric properties and excellent luminescence properties. Eu³⁺-doped BNT ceramics make it potential applications for novel integrated electro-optical and multifunctional devices.     

Understanding the depolarization temperature in (Bi0.5Na0.5)TiO3-based ferroelectrics

Depolarization temperature, or T_d, in ferroelectric materials limits the high temperature operation regime as the long range dipolar ordering could not be maintained beyond T_d during zero field heating. Therefore, the control of T_d has been always drawing the attention of the ferroelectric/piezoelectric community. Some particular issues include: Which point defect is more responsible for the change of T_d? What is the phase equilibrium as a function of T_d? In this paper, our goal is to elaborate the correlation between T_d and defect chemistry in 85% (Bi_0.5Na_0.5)TiO₃-15% BaTiO₃ (BNT-15BT) ferroelectric ceramics. Various experimental tools are employed and cross referenced, including dielectric measurements, thermal stimulated depolarization current, impedance spectroscopy, scanning transmission electron microscopy and in-situ transmission electron microscopy. Collectively this study generates a more insightful understanding of T_d in BNT-based ferroelectrics.     

Mechanism of densification of calcium carbonate by cold sintering process

In general, carbonates cannot be easily hardened by the conventional ceramic sintering process due to their thermal decomposition during heating. However, when the cold sintering process (CSP) is selected, carbonates can be hardened at lower temperatures. It has been demonstrated that calcium carbonate can be hardened by CSP, but the detailed densification mechanisms of cold sintering at various temperatures have not been fully clarified. In this study, the vaterite phase of calcium carbonate was selected as the starting material. As the cold sintering temperature for calcium carbonate powder increased, the bulk density of the hardened calcium carbonate body increased. The compressive strength was maximized when cold sintered at 80 °C due to the balance between the solubility of calcium carbonate and the reactivity of cold sintering. Almost no crystal phase transformation from vaterite to calcite occurred during cold sintering, and reprecipitation of the vaterite phase though dissolution-precipitation densified the body.     

Ultra-high strain responses in lead-free (Bi0.5Na0.5)TiO3-BaTiO3-NaNbO3 ferroelectric thin films

Lead-free (Bi_0.5Na_0.5)TiO₃ (BNT)-based piezoelectric materials, have a great potential for high-precision actuators’ applications. In this work, the high-quality (0.94-x%)(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃-x%NaNbO₃ (x = 2–10, BNT-6BT-xNN) thin films have been successfully deposited on Pt/TiO₂/SiO₂/Si substrates by sol-gel method. An ultra-high poling strain S_pol value of 1.7% with a unipolar strain S_uni value of 1.47% was reported in the BNT-6BT-6NN thin films. The coexistence of the ferroelectric phase and relaxor state was observed in the compositions of x = 2–8. Furthermore, the BNT-6BT-6NN thin films show more active domain switching compared to other compositions. It is demonstrated that the optimized strain responses in the BNT-6BT-6NN are attributed to a synergistic reaction of active domain switching and reversible electric-field-induced phase transition between the ferroelectric phase and relaxor state. Our systematic study demonstrates that the BNT-6BT-xNN thin films with an improved strain response are promising candidates for the applications of miniaturized actuators.     

Na0.5Bi0.5TiO3-K0.5Bi0.5TiO3系无铅压电陶瓷的极化工艺研究

主要研究了极化电场,极化时间和极化温度等工艺参数对Na0.5Bi0.5TiO3-K0.5Bi0.5TiO3系无铅压电陶瓷介电和压电性能的影响。结果表明:极化电场和极化温度对压电陶瓷的介电、压电性能影响较大,而极化时间则影响较小。适宜的极化电场是3～3.5kV/mm,极化温度70～80℃,极化时间为10～15min。     

溶胶-凝胶法合成(Na0.5Bi0.5)TiO3微粉

以钛酸四丁酯、硝酸铋、醋酸钠和冰醋酸为原料,利用溶胶-凝胶工艺得到透明凝胶,经干燥后煅烧成(Na0.5Bi0.5)TiO3微粉。通过对溶胶体系水/醇盐的摩尔比、初始pH值及胶凝温度对(Na0.5Bi0.5)TiO3凝胶体系溶胶-凝胶形成过程影响的研究,发现水/醇盐比R在35≤R≤60,pH在2.2~3.5,反应温度在40~60℃时,能够得到透明的溶胶;通过TG-DTA、SEM、X-ray等分析手段对(Na0.5Bi0.5)TiO3粉体进行测试,表明在650℃合成1h可以得到单一钙钛矿(Na0.5Bi0.5)TiO3晶体;采用TEM对(Na0.5Bi0.5)TiO3干凝胶粉体分析其粒径大小约为10nm。     

Preparation and properties of LiF transparent ceramics by cold sintering process

The development of optoelectronic devices depends on the development of optoelectronic materials such as transparent ceramics. LiF transparent ceramics are photoelectronic ceramics with excellent photoelectric properties. Still, the traditional preparation of LiF transparent ceramics generally needs a high temperature or high-pressure environment, and the cost is high. This paper adopts a cold sintering process to prepare high-density LiF transparent ceramics at low temperatures to reduce the preparation conditions. The effects of different cold sintering temperatures on microstructure, density, hardness, visible and near-infrared transmittance, and electrical properties of transparent ceramics were studied. The results show that using LiOH solution as the solvent, the relative density of LiF ceramics can reach up to 99.64% under the sintering condition of 375 °C/470 MPa, and the Vickers hardness is 1.34 GPa. Vickers hardness is 1.34 GPa. The transmittance in the visible and near-infrared regions is 60.45% and 85.31%, respectively. The dielectric constant and dielectric loss of 13 GHz are 4.36 and 1.11 × 10⁻³, respectively.     

Rapid preparation of Bi0.5Na0.5TiO3 ceramics by reactive flash sintering of Bi2O3-NaCO3-TiO2 mixed powders

Lead-free Bi_0.5Na_0.5TiO₃ piezoelectric ceramics were successfully prepared by reactive flash sintering of Bi₂O₃-NaCO₃-TiO₂ mixed powders, where phase transformation and densification occurred simultaneously. The influence of electric field strength, current density and holding time at constant current state on the phase transformation and densification were investigated. The current density had a significant influence on the extent of phase transformation and densification. The holding time had no influence on the phase transformation, but had an important effect on crystallinity of sample. The sintered bulks exhibited the maximum polarization P_m of 16.8 μC/cm², remanent polarization P_r of 9.6 μC/cm², coercive field E_c of 29 kV/cmm, maximum electric-field-induced strain of 0.053 %, and piezoelectric coefficient d₃₃ of 85 pC/N. The reactive flash sintering can prepare the dense and single-phase ceramics from multiphase precursor powders in one step of flash, providing a new way for rapid production of ceramic materials.     

Electric field-induced irreversible relaxor to ferroelectric phase transformations in Na0.5Bi0.5TiO3-NaNbO3 ceramics

(1-x)Na_0.5Bi_0.5TiO₃-xNaNbO₃ (x = 0.02, 0.04, 0.06, and 0.08) ceramics were fabricated by solid-state reaction. High-resolution synchrotron x-ray powder diffraction (SXPD) data, coupled with macroscopic electromechanical measurements, reveal the occurrence of an electric field-induced irreversible crystallographic transformation for x = 0.02 and 0.04, from a pseudo-cubic non-ergodic relaxor to a rhombohedral or coexisting rhombohedral-tetragonal long range-ordered ferroelectric phase, respectively. The highest unipolar electrostrain, corresponding to an effective longitudinal piezoelectric strain coefficient of approximately 340 pm V⁻¹, was obtained for x = 0.04; this effect is attributed to enhanced domain switching as a result of the co-existing rhombohedral and tetragonal phases for this composition, which is critical for piezoelectric actuator applications.     

Thermally‐induced phase transformations in Na0.5Bi0.5TiO3–KNbO3 ceramics

The structures and functional properties of Na_0.5Bi_0.5TiO₃–xKNbO₃ (NBT‐xKN) solid solutions, with x in the range from 0.01 to 0.09, were investigated using a combination of high‐resolution synchrotron X‐ray powder diffraction (SXPD) and ferroelectric property measurements. For low KN contents, an irreversible transformation from cubic to rhombohedral phases was observed after the application of a high electric field, indicating that the polar nanoregions (PNRs) in the unpoled state can be transformed into metastable long‐range ordered ferroelectric domains in the poled state. In contrast, the near‐cubic phase of the unpoled ceramics was found to be remarkably stable and was retained on cooling to a temperature of ?175°C. Upon heating, the field‐induced metastable ferroelectric rhombohedral phase transformed back to the nanopolar cubic state at the structural transformation temperature, T_ST, which was determined as approximately 225°C and 125°C for KN contents of 3% and 5% respectively. For the field‐induced rhombohedral phase in the poled specimens, the pseudo‐cubic lattice parameter, a_p, exhibited an anomalous reduction while the inter‐axial angle increased towards a value of 90° on heating, resulting in an overall increase in volume. The observed structural changes were correlated with the results of temperature‐dependent dielectric, ferroelectric and depolarization measurements, enabling the construction of a phase diagram to define the stable regions of the different ferroelectric phases as a function of composition and temperature.     

(Na1-xKx)0.5Bi0.5TiO3系的离子挥发性研究

利用固相法制备了(Na1-xKx)0.5Bi0.5TiO3系压电陶瓷,研究其中Bi3 、Na 、K 离子的挥发对其性能的影响。研究结果表明Bi3 的挥发性对样品的性能影响较大,而Na 、K 离子相对较小。     

Composition,microstructure and electrical properties of K0.5Na0.5NbO3 ceramics fabricated by cold sintering assisted sintering

In this paper, cold sintering was served as a forming method to assist the conventional sintering, which is so-called cold sintering assisted sintering (CSAS) method. Lead-free K_0.5Na_0.5NbO₃ piezoelectric ceramics were prepared by the CSAS method, and the effects of the different procedures on the sintering behaviors and electrical properties of KNN ceramics were studied. Compared with conventional sintering (CS), cold sintering process can induce potassium-rich phase on the KNN particle surface, and remarkably increase both the green and sintering density of KNN ceramics. Meanwhile, the potassium-rich phase would transform to K₄Nb₆O₁₇ second phase on the grain surface, and subsequently suppress the volatilization of potassium element. The sinterability and electrical properties were greatly improved, and KNN piezoelectric ceramics with high performance can be manufactured in a wide sintering temperature range (1055 °C–1145 °C), which proves that CSAS has the potential to be an excellent sintering technique for producing KNN based ceramics.     

The unusual case of plastic deformation and high dislocation densities with the cold sintering of the piezoelectric ceramic K0.5Na0.5NbO3

K_0.5Na_0.5NbO₃ (KNN) can be readily densified using the cold sintering process, but despite observing high relative permittivity, the ferroelectric hysteresis is strongly suppressed along with a major suppression in the all-important piezoelectric properties. In this study, KNN is fabricated using a NaOH+KOH transient flux under a uniaxial pressure of 400 MPa and heating to 300 °C for 2 h to drive densification to 93% theoretical. It is only after a secondary heat treatment that we observe improvements of the ferroelectric hysteresis and piezoelectric properties. From a detailed structural-property-processing study using analytical transmission electron microscopy (TEM), X-ray line broadening and high field dielectric characterization methodologies we conclude that there is an unusual in-situ plastic deformation process that takes place in addition to the densification under the cold sintering process. High densities of dislocations within grains were observed that lead to multiple pinning sites that impact both the intrinsic and extrinsic contributions to the high field dielectric and piezoelectric properties. Annealing significantly reduced the dislocation density in the highly defective crystallites, observed directly from the TEM and from the sharpening of the X-ray diffraction peaks, resulting in piezoelectric and ferroelectric properties that approached those of conventionally sintered KNN.     

Rutile TiO2 microwave dielectric ceramics prepared via cold sintering assisted two step sintering

In this work, a sintering route named cold sintering assisted two step sintering process (CSP-TS) is presented to prepare rutile TiO₂ ceramics with submicron grain sizes. Cold sintering process at 300 °C with tetrabutyl titanate and water as the liquid phase yields a ‘green body’ with a relatively high density of ～80 %, and finally dense (98.5–99.8 %) rutile TiO₂ ceramics with grain sizes of ～600 nm can be obtained in the second sintering process at 950?1000 °C. The microstructural analysis with SEM and TEM indicates that the CSP-TS samples sintered at 950 °C have an obvious phenomenon of recrystallization, accompanying by a decrease of amorphous phases and a formation of clear grain boundaries. Besides, the rutile TiO₂ ceramics prepared by CSP-TS possess excellent microwave dielectric properties with relative permittivity of 92.0–98.4 and Q × f values of 27,800?31,900 GHz. Therefore, it is feasible to utilize CSP-TS to prepare ceramics with small grain sizes at low sintering temperatures.     

High energy storage density realized in Bi0.5Na0.5TiO3-based relaxor ferroelectric ceramics at ultralow sintering temperature

The miniaturization and integration trend of electronic applications requires high energy storage performance, and the development of multilayer ceramic capacitors (MLCC) demands the compatibility between ceramic sintering temperature and co-firing temperature of metal electrodes. Herein, we obtained a high recoverable energy storage density and a low sintering temperature simultaneously in 0.5(Bi_0.5Na_0.5)TiO₃-0.5SrTiO₃-x mol% CuO (0.5BNT-0.5ST-x mol% CuO) via the combination of adding CuO sintering aid and citrate sol-gel synthesis method. The optimum sintering temperature decreases significantly from 1130 °C for x = 0 to 820 °C for x = 2.0. The ceramic of 0.5BNT-0.5ST-1.5 mol% CuO exhibits a large W_rec of 2.20 J/cm³ and η of 72.39% under 230 kV/cm. Furthermore, the same sample also possesses a large C_D of 1740.97 A/cm², an extremely high P_D of 139.28 MW/cm³ and an ultrafast discharge speed of 82 ns. These merits reveal that the ceramic of 0.5BNT-0.5ST-1.5 mol% CuO has great potential in practical MLCC production.     

Suppression of high-temperature dielectric loss by designed thermal annealing treatment in (Bi1/2Na1/2)TiO3 ceramics

Dielectrics for automobile applications generally require high temperature reliability. Bi_1/2Na_1/2TiO₃-based materials are excellent high-temperature dielectric candidates with relatively large temperature-insensitive dielectric responses and a maximum dielectric permittivity temperature as high as 300 °C. However, they suffer from a high dielectric loss that increases exponentially above 200 °C due to ionic conduction from thermally activated oxygen vacancy migration. Here, we demonstrate that the impact of ionic conduction on dielectric loss can be effectively suppressed by introducing a sodium deficiency and thermal annealing An appropriate combination of both treatments elevated the temperature insensitivity of the dielectric loss up to ～ 300 °C. A systematic investigation using impedance spectroscopy correlated with microstructure analysis revealed that the sodium deficiency and thermal annealing affected the mobile oxygen vacancy concentration differently.     

Two-step sintering of 0.93Bi0.5Na0.5TiO3-0.07BaTiO3 lead-free piezoelectric material

Two-step sintering (TSS) as an efficient sintering method for obtaining dense microstructure while preventing excess grain growth was used for sintering 0.93Bi_0.5Na_0.5TiO₃-0.07BaTiO₃ composition which is located near the morphotropic phase boundary of this binary system. In order to compare the obtained microstructure and piezoelectric properties, conventional single step sintering (SSS) was also examined. Microstructure evolution during sintering at different temperatures was investigated to find the optimum sintering temperature. Ferroelectric hysteresis loop as well as unipolar strain behavior of optimally sintered ceramics was studied. According to density measurement and microstructure studies of the prepared ceramics, TSS resulted in finer and more dense and uniform microstructure compared to SSS method. As a result remnant polarization of TSSed ceramic was increased by 35% and its coercive field was decreased by 16%. The inverse piezoelectric coefficient of the SSSed and TSSed was obtained 220 and 300 p.m./V, respectively. These values are high enough for practical applications such as actuators. The obtained results clearly showed that TSS is capable of sintering 0.93Bi_0.5Na_0.5TiO₃-0.07BaTiO₃ at temperatures lower than which is required for SSS method. Therefore the composition stoichiometry is maintained after sintering and denser microstructure without abnormal grain growth is obtained which is responsible for improved electrical properties of the piezoceramics.     

A位元素非化学计量Bi0.5(Na,K)0.5TiO3-BiFeO3压电陶瓷

采用传统陶瓷制备方法,制备了A位元素非化学计量无铅压电陶瓷0.79(Bi0.5Na0.5)tTiO3–0.18(Bi0.5K0.5)tTiO3–0.03BitFeO3(t=0.95～1.05)。研究了A位元素非化学计量对该体系陶瓷微观结构、压电性能的影响,同时通过测量不同外加应力下压电应变常数(d33),研究了影响d33和径向机电耦合系数(kp)的不同机理。结果表明:A位元素缺乏较多时,析出第二相。kp随A位元素过量与不足的增加而减少,d33随A位元素过量的增加基本不变,随A位元素不足的增加而减少。采用极化相位角(θmax)衡量陶瓷极化程度,发现kp随θmax增加而增加,d33随θmax增加变化不明显。d33在低于临界应力时基本不变,当应力高于临界应力后,随应力增加而下降。压电陶瓷中的应力场对畴壁运动与弹性偶极子的作用是影响d33的作用机理之一。     

Grain size effect on microwave dielectric properties of Na2WO4 ceramics prepared by cold sintering process

In this work, cold sintering was adopted to prepare Na₂WO₄ ceramics with different grain sizes ranging from 0.632 μm to 17.825 μm. Their microstructures, complex impedance, and microwave dielectric properties were studied in-depth. It was found that samples with relative densities higher than 92% can be successfully synthesized by cold sintering process at a low temperature of 240 °C. However, their electrical properties have strong dependence on the grain size. Specifically, the resistance of grain boundaries decreases dramatically with the increase of grain sizes, while the quality factor has a positive correlation with the grain sizes of Na₂WO₄ ceramics. Excellent microwave dielectric properties, including permittivity = 5.80, Q × f = 22,000 GHz, and TCF = −70 ppm/°C, are obtained for Na₂WO₄ ceramics with a grain size of 4.477 μm prepared by cold sintering process.     

准同型相界附近(Na1-xKx)0.5Bi0.5TiO3的制备及性能研究

利用固相法制备了（Na1-xKx）0.5Bi0.5TiO3系压电陶瓷，XRD分析表明所得陶瓷样品为纯的钙钛矿结构，其准同型相界在x=0．18～0．22之间；电子探针显微分析显示所做陶瓷样品晶粒发育良好，具有规则的外形和明显的晶界；实验所得陶瓷样品损耗tanδ最大不超过5％，最好的压电常数d33=153 pC／N，平面机电耦合系数kp=0．299，它们分别出现在x=0．22和0．20处。