Improved transmittance and ferroelectric properties realized in KNN ceramics via SAN modification

Transparent, Sr(Al_0.5Nb_0.5)O₃‐modified K_0.5Na_0.5NbO₃ (KNN) ceramics were successfully fabricated by a solid‐state pressureless sintering method in this work. The obtained microstructure, transmittance, and electrical properties were characterized in detail. Our results indicated that the modification by Sr(Al_0.5Nb_0.5)O₃ significantly limited the grain growth behavior of KNN, resulting in dense ceramics with submicron grain size (<0.5 μm) and small pore size. Consequently, the ceramic with the 0.96K_0.5Na_0.5NbO₃‐0.04Sr(Al_0.5Nb_0.5)O₃ composition showed superior transmittance and electrical properties: T = 55% in the visible region (0.78 μm), d₃₃ = 105 pC/N, ε_r = 1021, and P_r = 15.1 μC/cm², which were significantly higher than those of pure KNN. Our findings implied that the addition of Sr(Al_0.5Nb_0.5)O₃ could be a good strategy to obtain superior transmittance and electrical properties in KNN and may shed light on other ferroelectric systems.     

Improved ferroelectric,piezoelectric, and dielectric properties in pure KNN translucent ceramics by optimizing the normal sintering method

In this study, it is reported that various properties can be selectively derived in a pure (K_0.5Na_0.5)NbO₃, KNN ceramics through optimizing the sintering temperature by the conventional sintering method. High piezoelectric, ferroelectric, and dielectric properties such as d₃₃ = 127 pC/N, P_r = 31 μC/cm², and ε_r = 767 are obtained at the sintering temperature of 1100 °C. On the contrary, the specimen sintered at 1130 °C does not show high piezoelectric and ferroelectric properties, but it is translucent with a transmittance of 22% and 57% at the wavelength of 800 and 1600 nm respectively and shows a very high dielectric constant ε_r of 881. The origin of the high piezoelectric constant owes to large remanent polarization and dielectric constant, and dense microstructure with uniform distribution of large grains with the conjunction of relatively large crystal anisotropy. On the other hand, dense microstructure with almost no porosity, highly compacted grain boundaries, uniform distribution of grains, and relatively low crystalline anisotropy are responsible for the translucency and large dielectric constant of the ceramic specimens. This study demonstrates that the lead-free KNN ceramic has the potential to show multiple noteworthy properties such as piezoelectric, ferroelectric, dielectric, and transparent properties. This work provides a pure KNN ceramic simultaneously with high piezoelectric and transparent characteristics prepared only by using the conventional sintering method at a moderate sintering temperature for the first time in the literature.     

Synthesis routes for enhanced piezoelectric properties in spark plasma sintered Ta-doped KNN ceramics

Ta substitution in K_0.5Na_0.5NbO₃ lead free piezoelectrics helps to prevent grain growth and has been shown to improve the piezoelectric properties but always leads to hetereogeneous microstructures. Two synthesis routes have been studied to prepare K_0.5Na_0.5Nb_0.8Ta_0.2O₃ (KNNTa) substituted powders. Then highly densified KNNTa ceramics have been obtained by spark plasma sintering (SPS). The use of a synthesized oxide precursor Nb_1.6Ta_0.4O₅ during the ceramic elaboration process clearly shows through accurate Rietveld study a successful Ta substitution with 92% of Amm2 K_0.485(8)Na_0.515(8)Nb_0.819(6)Ta_0.181(6)O₃ phase, confirmed by SEM-EDS analysis and a more homogeneous chemical composition. This leads to enhanced electromechanical coupling coefficients with an improvement of 50% of k_t, 15% for k_p and low electrical losses, compared to the conventional synthesis method with a simple mixing of commercial precursors.     

Tape casting and characterizations of MgO ceramics

We report a high density MgO ceramic substrate produced by the tape casting technology. The tape casting formulation and process produced a uniform tape free of cracking. Y₂O₃ and SiO₂ were used as the sintering aid for the pressureless sintering of the green tape. X-ray diffraction phase identification indicates that MgO is the main phase, while both Y₂O₃ and SiO₂ sintering aids react with MgO to form MgY₄Si₃O₁₃ as the second phase. Liquid phase sintering occurs in the temperature range from 1030°C to ~1500°C, which is confirmed by the simultaneous Thermal Gravitation Analysis/Differential Scanning Calorimeter (TGA/DSC) and the percent linear shrinkage and densification. A 96.5% theoretical density was achieved by presureless sintering at 1650°C for 2 hours, which was further increased to a fully dense structure using hot-isostatic-pressing(HIP) at 1650°C and 207 MPa in argon. Scanning electron microscopy (SEM) and energy dispersive(EDS) spectroscopic analysis on the HIP’ed sample show that MgY₄Si₃O₁₃is located at the MgO grain boundary and the sample has a fully dense structure. The refractive indices and extinction coefficient were measured on the HIP’ed sample along with thermal properties and dielectric properties. Thermal diffusivity and heat capacity were measured to calculate the thermal conductivity.     

美国高技术陶瓷持续增长

（注：２０００年 ２月,美国的商业知讯公司组织人员对全世界的高技术陶瓷市场进行调研,写出的 ３份报告分别为：高技术陶瓷的机遇－技术、经济和市场分析;高技术陶瓷和纳米陶瓷粉－原料、工艺技术、最新进展、工业结构、市场和国际竞争;９９’高技术陶瓷回顾。以下是对美国高技术陶瓷的市场预测。）１概述 高技术陶瓷的销售额从１９８９年的３５亿美元增加到１９９９年的８１亿美元,平均年增长率为８．７％。据美国商业知讯公司的调查,在高技术陶瓷市场中,电子陶瓷不仅将继续占有最大市场份额,而且是未来增长的领头羊,结构陶瓷紧…     

Scaling behavior of dynamic hysteresis of PMN-PT relaxor ferroelectric ceramics near the morphotropic phase boundary

Due to the feature of domains, ferroelectric materials display hysteresis behavior with respect to the change of the applied electric field. Every ferroelectric material has its own unique hysteresis loop reflecting the information of domain reversal under an electric field. In this work, the scaling behaviors of dynamic hysteresis in relaxor (1-x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (PMN-PT) ceramics with different compositions were studied systemically. Our results showed that the evolution of scaling behavior in PMN-PT ceramics can be divided into three stages, which is independent of the phase structure of the ceramics and the testing electric field frequency. The relationship between hysteresis area <A> and field amplitude E₀ obeys the power law <A>∝ in the low and high E₀ regions, where the reorientations of 180° and non-180° domain are dominant, respectively. However, the dynamic hysteresis area <A> does not follow the power law in the intermediate E₀ regions, which is attributed to the interaction of different domain reversal mechanisms. Furthermore, the hysteresis area <A> decreases gradually with increasing frequency at a certain E₀ and the time-dependent domain reversal process was also discussed.     

Electrical and mechanical losses in ferroelectric ceramics

The magnitude of the electrical and mechanical losses of devices made from ferroelectric ceramics, which are currently widely used in electronic circuits, often determines the applications of these materials. In this article the current understanding of the mechanisms and their dependence on the composition of the ceramic and on the experimental parameters is reviewed. The electrical and mechanical losses are proportional to each other. The proportionality factor is determined by material parameters, such as dielectric constant, spontaneous polarization etc., and is independent of the specific loss mechanism. The losses below the Curie temperature are mainly caused by domain wall movements. Above the Curie temperature the losses drop rapidly to a residual level, caused by lattice - and microstructural effects. At higher temperatures the losses are governed by the electrical conductivity of materials. Of major importance for applications are the domain wall losses. Their dependence on doping, aging, field strength, frequency and temperature is described and discussed. Models are presented that attribute the losses to the damping of a moving domain wall.     

Electrical and mechanical properties of ferroelectric lead zirconate titanate/tungsten oxide ceramics

Ferroelectric PZT/xWO₃ ceramics (when x = 0, 0.5, 1, 3 and 5 vol%) were fabricated from PZT and nano-sized WO₃ powders by a solid-state mixed-oxide method. Phase characterization suggested that the reaction between PZT and WO₃ occurred during the sintering. This reaction seemed more pronounced with increasing the content of WO₃. The maximum density at approximately 97% of the theoretical value was achieved at 1 vol% of WO₃ addition. The grain size was reduced with an addition of WO₃ particles from 7.8 μm for PZT to 1.8 μm for 0.5 vol% WO₃ and 0.8 μm for 1–5 vol% WO₃. Mechanical properties of PZT could be improved with an addition of WO₃ nano-particulates. The addition of 0.5 vol% WO₃ could maintain good electrical properties while increasing WO₃ significantly reduced dielectric and piezoelectric constants of the PZT.     

Electric-field-induced strain in Mn-doped KNbO3 ferroelectric ceramics

Potassium niobate, KNbO₃ (KN), ceramics doped with manganese (Mn) were prepared by a modified conventional ceramic fabrication process to characterize their dielectric, ferroelectric and electrostrain properties. In this study, 0.22% of a large electric-field-induced strain was obtained at 80 kV/cm under unipolar driving for 1.2 wt% MnCO₃-doped KN ceramics. Basically, it is difficult to obtain dense and nondeliquescent KN ceramics by the conventional method because of potassium ions. The key factor in obtaining dense and nondeliquescent KN ceramics is the calcination process control. Thus, the two-step calcination pattern is proposed for this purpose. Dense, nondeliquescent and high-resistivity Mn-doped KN ceramics were obtained, resulting in a large electric-field-induced strain under a high electric field.     

Fabrication of transparent lead-free KNN glass ceramics by incorporation method

The incorporation method was employed to produce potassium sodium niobate [KNN] (K_0.5Na_0.5NbO₃) glass ceramics from the KNN-SiO₂ system. This incorporation method combines a simple mixed-oxide technique for producing KNN powder and a conventional melt-quenching technique to form the resulting glass. KNN was calcined at 800°C and subsequently mixed with SiO₂ in the KNN:SiO₂ ratio of 75:25 (mol%). The successfully produced optically transparent glass was then subjected to a heat treatment schedule at temperatures ranging from 525°C -575°C for crystallization. All glass ceramics of more than 40% transmittance crystallized into KNN nanocrystals that were rectangular in shape and dispersed well throughout the glass matrix. The crystal size and crystallinity were found to increase with increasing heat treatment temperature, which in turn plays an important role in controlling the properties of the glass ceramics, including physical, optical, and dielectric properties. The transparency of the glass samples decreased with increasing crystal size. The maximum room temperature dielectric constant (ε _r ) was as high as 474 at 10 kHz with an acceptable low loss (tanδ) around 0.02 at 10 kHz.     

Fabrication and ferroelectric/dielectric properties of La-doped PMN-PT ceramics with high optical transmittance

Relaxor ferroelectric 0.75(Pb_1–3x/2La_x)(Mg_1/3Nb_2/3)O₃-0.25(Pb_1–3x/2La_x)TiO₃ (La³⁺:PMN-PT x/75/25, where x=2.8, 3.0, 3.5, and 4.0 mol% of La³⁺) transparent ceramics were fabricated by the combination of oxygen atmosphere pressureless sintering and hot-pressing sintering process. The optical transmittances of above four ceramics are higher than 60% at the wavelength of 500–900 nm. La³⁺:PMN-PT 3.0/75/25 exhibits the highest transparency around 70% at 900 nm which is very close to the theoretical transmittance 71%. Each of the four ceramics exhibits the pure perovskite phases. They show fully dense microstructures and their relative densities are higher than 99.8%. The ferroelectric and dielectric measurements indicate that these four ceramics exhibit relaxation characteristics. With increasing La³⁺ content, (200) peak in XRD patterns shifts to higher angles and the average grain size increases, while the temperature Tε_max corresponding to the maximum ε_r, the remanent polarizations P_r and coercive fields E_c decrease gradually.     

Synthesis and characterization of nanocrystalline SrBi2Nb2O9 ferroelectric ceramics using TEA as the polymeric matrix

The processing conditions, reaction mechanism, fine structure of the powders, microstructure, and dielectric properties of SrBiNb₂O₉ (SBN) were systematically studied. A relative density of >80% was obtained using a two-step sintering process at moderate pressure. XRD showed that a single phase with the layered perovskite structure of SrBi₂Nb₂O₉ (SBN) was formed after calcining at 600 °C. No intermediate phase was found during heat treatment at and above 600 °C. The crystallite size (D) and the effective strain (η) were found to be 38.8 nm and 0.01475, respectively, while the particle size obtained from TEM was laid between 25 and 36 nm. SEM revealed that the average grain size after sintering at 900 °C for 4 h was 0.67 μm. Dielectric constant and corresponding tangent loss were measured in the frequency range from 1 kHz to 1 MHz from which the Curie temperature (T_c) was found to be at 450 °C.     

Giant piezoelectric coefficient of PNN-PZT-based relaxor piezoelectric ceramics by constructing an R-T MPB

xTa-PNN-PZT piezoelectric ceramics with [0.55 Pb(Ni_1/3Nb_2/3)O₃ - 0.45 Pb(Zr_0.3Ti_0.7)O₃ - xwt%Ta₂O₅] + 1 wt% PbO composition (0 ≤ x ≤ 0.7) were prepared by conventional solid-state reaction method. Special attention was paid to the effect of Ta₂O₅ concentration on structural, dielectric, piezoelectric, and ferroelectric properties of these ceramics. In particular, rhombohedral-tetragonal morphotropic phase boundary was observed in PNN-PZT crystal structure near the composition of x = 0.5. At this point, volume fraction of rhombohedral phase was almost equal to that of tetragonal phase. In addition, the ceramic exhibited the optimum values of piezoelectric coefficient, bipolar strain, unipolar strain, and inverse piezoelectric coefficient, i.e., 1090 pC/N, 0.135%, 0.165%, and 1493 pm/V, respectively. Moreover, according to in-situ X-ray diffraction, piezoelectric force microscopy, and in-situ strain measurement results, phase structure of 0.5Ta-PNN-PZT revealed relatively stable piezoelectric behavior at 60 °C.     

Structure stabilization and dielectric properties of pzn ferroelectric ceramics

Structure stabilization and dielectric properties of Pb (Zn_1/3 Nb_2/3)O₃ -PbTiO₃ -BaTio₃ (PZN-PT-BT) ferroelectric ceramics have been carefully studied. Around morphotropic phase boundary (MPB) of PZN-PT system, heterovalant substitution of K⁺ for Pb²⁺ stabilizes the pesrovskite Structure, while substitution of La³⁺ for Pb²⁺ goes toward the other end. Nonstoichiometry addition of excess ZnO or BaO can also suppress the formation of pyrochlore phase.     

Intergrain boundaries of two types of ferroelectric ceramics

Translated from Steklo i Keramika, No. 8, pp. 20–21, August, 1989.     

Relaxor ferroelectric behavior of La substituted BPZT ceramics

Barium titanate (BT) based compounds have been of great importance in the fabrication of multilayer ceramic capacitors. Several substitutions are used to tailor its structural, electrical and ferroelectric properties. Substituent like zirconium (Zr) for titanium results in decrease in non-centrosymmetric tetragonal to centrosymmetric cubic phase transition temperature (Curie temperature). Whereas substituent like lead (Pb) for barium results in the increase in the Curie temperature. Here we are reporting effect of lanthanum (La) substitution on the properties of Zr and Pb co-substituted BT. The system Ba_0.80−xLa_xPb_0.20Ti_0.90Zr_0.10O₃ with x = 0 and 0.01 was selected for study of structural and dielectric properties. Samples were prepared using solid state ceramic route. X-ray diffraction studies (XRD) were used to confirm single phase structure. Dielectric properties were studied as a function of frequency and temperature. Composition with x = 0.01 was found to show relaxor behavior.     

Electrocaloric properties of high dielectric constant ferroelectric ceramics

In the last two decades, the electrocaloric (EC) effect which is associated to the temperature (θ) dependence of the macroscopic polarization P(E, θ) under electric field E has been spasmodically studied in ferroelectric materials in order to find an alternative to the classical refrigeratory devices using freon. Basically, large electrocaloric temperature variation ΔT originates from electric field-induced phase transition at the Curie temperature, but temperature changes of the sample are difficult to measure and depend on the experimental conditions. In this paper, the electrocaloric effect has been quantified directly and precisely by measuring the thermal energy exchanged under isothermal conditions using a modified Differential Scanning Calorimetry (DSC) apparatus. The DSC technique allowed to compare the EC properties of high-dielectric-constant (ɛ) ceramics in the vicinity of ferroelectric–paraelectric phase transition. The measurements were also simulated starting from polarization versus electric field hysteresis loops for different temperatures. It is shown excellent agreement between simulations and direct DSC measurements, except in a limited temperature range where the hysteresis of the polarization versus temperature is high.     

Enhanced photovoltaic performance via polarization in Bi/Co co-doped KN-based ferroelectric ceramics

A bandgap-tunable KNbO₃ ferroelectric ceramic was prepared by introducing Bi/Co ion. The existence of mixed valence states of Co²⁺/Co³⁺ in the system induced the bandgap reduction and its tunable behavior. KNbO₃–BiCoO₃ solid solutions showed a typical orthogonal perovskite structure and maintained good ferroelectricity (P_s = 15.13 µC/cm²) and high-field polarization ability. The devices based on the .98KNbO₃–.02BiCoO₃ sample exhibited an improved short-circuit photocurrent density (J_sc) of 19.2 nA/cm² under simulated solar radiation, and this was further enhanced to 79.8 nA/cm² after a 60-kV/cm polarizing. The structural analysis of the samples after polarization reveals the effect of ferroelectric polarization on photovoltaic performance. This work provides new insights into the effects of ferroelectric polarization on photovoltaic performance.     

Electrical characterizations of PZT ceramics in large frequency and temperature ranges

Electrical properties of PbZr_0.75Ti_0.25O₃ ceramics have been characterized. The measurements have been made in frequency ranging from 20 to 2.10⁹ Hz and between 20 and 730 °C for low and medium frequencies. Typically at room temperature, the dielectric constant ′ is higher than 500 at 1 MHz whereas the loss tangent is close to 0.01. From ′(T) measurements, the Curie temperature of our sample has been determined at 320 °C. In the paraelectric state, ′(T) follows the empirical Curie–Weiss law near the phase transition which is of second order type. The increase of ′ observed at high temperatures and low frequencies in the paraelectric state are explained: this abnormal behavior is due to the migation of oxygen ions towards the electrodes, creating an additional non-ferroelectric interface which generates a Maxwell–Wagner effect.