Dielectric and tunable properties of barium strontium titanate ceramics under the stresses

The external stress dependence of dielectric and tunable properties of Ba_xSr_1−xTiO₃ (x = 0.65) ceramics has been investigated. The results reveal that the Curie peaks of samples are suppressed and broadened, the Curie temperature (T_c) is slightly shifted to lower temperature with an increase of external stress. Correspondingly, the tunability decreases from 66.6% to 58.8% in the vicinity of cubic-tetragonal (C-T) phase transition at an applied DC electric field of 10 kV/cm. These results could be useful for the design of devices and practical applications of Ba_xSr_1−xTiO₃ ceramics.     

X-ray diffraction study of Ba0.985Na0.015Ti0.985Nb0.015O3, Ba0.6Na0.4Ti0.6Nb0.4O3 and Ba0.3Na0.7Ti0.3Nb0.7O3 compositions

The Ba_0.985Na_0.015Ti_0.985Nb_0.015O₃, Ba_0.6Na_0.4Ti_0.6Nb_0.4O₃ and Ba_0.3Na_0.7Ti_0.3Nb_0.7O₃ compositions of the (1 − x) BaTiO₃–xNaNbO₃ (BTNNx) system have been studied by X-ray diffraction and by measurements of dielectric properties. The specimens with composition BTNN (x = 0.015, 0.40 and 0.70) have been refined by the JANA program from X-ray powder diffraction data. Ceramic samples with composition (1 − x) BaTiO₃ + xNaNbO₃ (where x = 0.015, 0.40 and 0.70) were prepared by calcinations from appropriate mixture of BaCO₃, TiO₂, Na₂CO₃ and Nb₂O₅. The calcined powder was sintered at temperature range 1200–1400 °C. As the composition x increased from 0.015 (and 0.70), the ferroelectric ceramics (x = 0.015, FE) with tetragonal phase changed to the ferroelectric relaxors (RFE, x = 0.40). RFE ceramics showed a peculiar diffuse phase transition and dielectric relaxation at the low temperature (down to 180 K) due to a frustration between RFE and FE state. These ceramics present the classical ferroelectric character when 0 ≤ x < 0.075 and 0.55 < x ≤ 1 and relaxor character when 0.075 ≤ x ≤ 0.55.     

Elaboration and dielectric study of ferroelectric or relaxor ceramics in the ternary system BaTiO3-NaNbO3-BaSnO3

The present work reports the elaboration and physical investigation of new compounds of the following composition Ba_1−xNa_x(Ti_1−ySn_y)_1−xNb_xO₃ (BTSnNxy). The studied ternary system presents some continuous solid solutions between the next 3 phases: the NaNbO₃ antiferroelectric phase that becomes easily ferroelectric at low rate substitutions, the BaTiO₃ ferroelectric phase and the paraelectric stannate phase BaSnO₃. Two different dielectric behaviors can be observed once some substitutions are made either in A or B sites of an ABO₃ perovskite. These substitutions modify the dielectric properties of the material. The introduction of Sn⁴⁺ and Ti⁴⁺ cations in the B site favors, respectively, a decrease of the transition temperature and an increase in the value of the real dielectric permittivity. The transition temperature should be modulated by varying the rate of cationic substitution. Some relaxor materials can be obtained at a temperature around room temperature.     

Synthesis, structural and microwave dielectric properties of Al2W3−xMoxO12 (x = 0-3) ceramics

Low dielectric ceramics in the Al₂W_3−xMo_xO₁₂ (x = 0-3) system have been prepared through solid state ceramic route. The phase purity of the ceramic compositions has been studied using powder X-ray diffraction (XRD) studies. The microstructure of the sintered ceramics was evaluated by Scanning Electron Microscopy (SEM). The crystal structure of the ceramic compositions as a result of Mo substitution has been studied using Laser Raman spectroscopy. The microwave dielectric properties of the ceramics were studied by Hakki and Coleman post resonator and cavity perturbation techniques. Al₂Mo_xW_3−xO₁₂ (x = 0-3) ceramics exhibited low dielectric constant and relatively high unloaded quality factor. The temperature coefficient of resonant frequency of the compositions is found to be in the range −41 to −72 ppm/°C.     

Effect of LiSbO3 on the phase structure, microstructure and electric properties of Sr0.53Ba0.47Nb2O6 ceramics

LiSbO₃ doped Sr_0.53Ba_0.47Nb₂O₆ ceramics were synthesized by conventional mixed-oxide method. The phase structure, microstructure, dielectric and ferroelectric properties of obtained ceramics were investigated. Pure tungsten bronze structure could be obtained in all ceramics and LiSbO₃ additive could promote densification and reduce the sintering temperature. The dielectric characteristics showed diffuse phase transition phenomena for all samples, which was proved by linear fitting of the modified Curie-Weiss law with γ value varying between 1.65 and 1.92. With increasing LiSbO₃ content, the transition temperature T_c decreased gradually to near room temperature. Normal ferroelectric hysteresis loops could be observed in all compositions, but the remnant polarization (P_r) and coercive field (E_c) all decreased gradually. Besides, the underlying mechanism for variations of the electrical properties caused by LiSbO₃ doping was explained in this work.     

Phase transitions and domain structures of ferroelectric nanoparticles: Phase field model incorporating strong elastic and dielectric inhomogeneity

An efficient numerical algorithm based on a Fourier spectral iterative perturbation method is proposed to accurately compute the electrostatic fields in three-dimensional (3D) microstructures with arbitrary dielectric inhomogeneity and anisotropy. The method can be conveniently implemented in phase field modeling of microstructure evolution in systems with inhomogeneous dielectric constants as well as inhomogeneous polarization and charge distributions. It is employed to determine the temperature–shape (aspect ratio) phase diagram, domain structures, and domain switching of PbTiO₃ nanoparticles using phase field simulations. It is shown that the Curie temperature is enhanced for nanowires and nanorods and reduced for nanodots. The critical sizes below which the ferroelectricity disappears for the nanowire and thin film are estimated to be around 1.4 nm. Vortex domain structures are found in nanorods, nanodots, and nanodisks. Results are in general agreement with existing experimental observations and first principle calculations.     

Investigation of the influence of thermal treatment on the morphologies, dielectric and ferroelectric properties of PZT-based ceramics

Pb[(Zr_1/2Ti_1/2)_0.9(Zn_1/3Nb_2/3)_0.1]O₃ (PZT–10PZN) powder was prepared using the columbite precursor method. The phase development of calcined powder precursors was analyzed by X-ray diffraction. Dielectric and ferroelectric properties of the as-sintered and annealed samples were measured and correlated with the microstructure. The morphological evolution was determined by scanning electron microscopy (SEM). The as-sintered ceramic exhibited weak normal-ferroelectric behavior, with a relatively low dielectric constant maximum measured at 1 kHz (_rmax at 1 kHz) of 13,000. Annealing resulted in a transition to relaxor-ferroelectric-like behavior, a shift in the dielectric maximum temperature from 360 to 350 °C, and a dramatic increase in _rmax at 1 kHz to a maximum value of 35,000 for the longer anneal. Furthermore, after thermal annealing at 900 °C for 1 week the composition shifted close to the MPB with a great reduction in the transition temperature and a broadening of the dielectric constant maximum. A strong enhancement of the remanent polarization (P_r) was also observed.     

Sol-gel derived (Li, Ta, Sb) modified sodium potassium niobate ceramics: Processing and piezoelectric properties

Nano-scaled lead-free perovskite powders (50-100 nm) with a composition of (Na_0.52K_0.4425)(Nb_0.8825Sb_0.08)O₃-0.0375LiTaO₃ (NKNS-3.75LT) have been synthesized via a modified low-cost citrate sol-gel route, in which Nb₂O₅ and Ta₂O₅ were transformed into stable and soluble chelate complexes to replace costly metal alkoxides. The thermal decomposition process and structural transformation behavior of the as-prepared xerogel was investigated by means of thermo-gravitometry and differential scanning calorimetry, X-ray diffractometry and Fourier-transform infrared spectrometry. The results indicated that the ideal crystallization of the NKNS-3.75LT powder with a single perovskite structure occurs at 500-600 °C, significantly lower than that for a conventional mixed-oxide route. Transmission electron microscopy observation showed that the crystalline powder has particle sizes in the range of 30-100 nm and exhibits polyhedron-like morphology, probably resulting in a specific grain growth behavior upon densification. The sol-gel derived NKNS-3.75LT ceramics sintered at 1090 °C exhibit excellent piezoelectric properties of d₃₃ ∼418 pC/N and k_p ∼0.54 for NKN based random ceramics.     

溶胶-凝胶法制备Ni掺杂CaCu3Ti4O12陶瓷的显微组织及电性能（英文）

采用溶胶凝胶法制备掺杂不同含量NiO(CaCu3NixTi4O12+x,x=0, 0.1, 0.2, 0.3)的CCTO陶瓷,通过扫描电镜和 X 射线衍射对其显微组织和相成分进行了分析,并研究了NiO掺杂对CCTO介电和压敏性能的影响。研究结果表明:Ni对CCTO陶瓷的相位成分没有影响,与用传统固相法制得的样品相比,用溶胶-凝胶法制成的样品具有更小的晶粒尺寸。从介电测量结果来看,当 x=0.2 时,样品具有最高的介电常数和最低的介电损耗。当x=0.3时,得到最低的漏电流,最小值为 0.295,同时,具有最高的阀值电压与非线性系数,最大值分别为1326V/mm和3.1。     

Dielectric relaxation and ionic conductivity studies of [N(CH3)4]2Cu0.5Zn0.5Cl4

WO₃-doped zinc titanate ceramics were prepared by conventional mixed-oxide method combined with a chemical processing. The effects of WO₃ additions on the phase structure and phase transitions of zinc titanate ceramics were investigated by high-temperature X-ray diffractometry (HTXRD) and transmission electron microscopy (TEM). The results showed that the major phase of zinc titanate ceramics transformed from zinc orthotitanate phase to zinc metatitanate phase with the amounts of WO₃ additions increasing. Small WO₃ (<1.00 mass%) addition accelerated the transition of ZnTiO₃ to Zn₂TiO₄, while excessive WO₃ addition restrained the transition. HTXRD showed that WO₃ enhanced the stability of Zn₂Ti₃O₈ and weakened the stability of ZnTiO₃. A precipitate within the Zn₂TiO₄ matrix was observed. Viewed along the orientation of Zn₂TiO₄, the precipitate was found to have a rectangular shape and to be nanometer level in size; its composition was concluded to be Zn₂Ti₃O₈. The dielectric properties of WO₃-doped zinc titanate ceramics were measured at different frequencies. The results showed the decreasing tendency with the increasing measuring frequencies for both the dielectric constants and the loss tangents, and there existed maximum values when the amount of WO₃ was 0.50 mass%.     

Critical properties of nanoscale asymmetric ferroelectric tunnel junctions or capacitors

Critical properties, such as the phase transition temperature, critical thickness and Curie-Weiss-type relation, of nanoscale asymmetric ferroelectric tunnel junctions or capacitors are investigated, considering the effects of size, surfaces, asymmetric interfaces and electrodes on the stability and magnitude of the two spontaneous polarization states. Using the modified thermodynamic model and taking into account contributions of the depolarization field, built-in electric field, interface and surface energies to the thermodynamic potential, explicit expressions of the critical properties are derived. For the asymmetric ferroelectric tunnel junction or capacitor, the results illustrate two important behaviors of vanishing critical thickness for the spontaneous polarization and smearing of the phase transitions, respectively. In addition, other critical properties are discussed as functions of the ambient temperature, misfit strain, surface coefficients, work function steps, dielectric constants and screening lengths of electrodes. Owing to the high-sensitivity of the critical properties to structures of asymmetric interfaces and electrodes, the results also suggest that the critical and other functional properties of nanoscale asymmetric ferroelectric tunnel junctions or capacitors can be completely controlled by adjusting the difference between asymmetric interfaces or electrodes.     

Structure, dielectric and electric properties of (Ba0.68−xSr0.308Bi0.006Na0.006Mgx)(Ti0.99Sn0.01)O3 ceramics

(Ba_0.68−xSr_0.308Bi_0.006Na_0.006Mg_x)(Ti_0.99Sn_0.01)O₃ ceramics were synthesized by solid-state reaction process. The samples (X ≤ 0.010) are a mixture of cubic (major) and rhombohedral (minor) phases. The rhombohedral phase causes a large dielectric loss in low temperature regions and plays an important role in diffuse phase transition of ceramics. While X > 0.010, the rhombohedral phase decreases and gradually disappears. The dielectric loss of ceramics in the low temperature regions decreases, and the samples change from the diffuse phase transition to the phase transition of second order, and then to of first order. In the temperature range of 270-370 °C, intrinsic conduction dominates conductivity of ceramics.     

Application of the FITSC method for characterization of PZT-type ceramics with the diffuse phase transition

In the present work the (Pb_0.84Ba_0.16)(Zr_0.54Ti_0.46)O₃ (PBZT 16/54/46) ceramics has been studied from the point of view of its electrical properties. Dielectric properties of PBZT were measured within the temperature range of the ferroelectric-paraelectric (FE-PE) phase transition region. The method of field-induced thermally stimulated currents (FITSC) was applied and thus the thermal relaxation effects were studied. The observed field dependence of thermally stimulated depolarization currents has revealed the existence of different frequency-dependent relaxation processes in the temperature range between 200 and 400 °C.Our experiment demonstrates that modification of ferroelectric materials with isovalent ions, but bigger radii than the origin atom, significantly affect its properties, particularly the PBZT real part of electrical permittivity shows the phase transition character change from ferroelectric to relaxor and finally, that electrical examination can be effectively used for drawing decisive applications conclusions considering polarization parameters distribution.     

Correlating phase and microstructure development versus dielectric properties in La and Er co-doped Bi4Ti3O12 ferroelectric ceramics

Bi_3.25La_0.75−xEr_xTi₃O₁₂ and Bi_3.25La_0.75Ti_3−xEr_xO_12−δ ceramics were prepared and studied in this work in terms of dopant-induced phase and microstructure development as well as dielectric response. The results show that introduction of Er³⁺ tends to reduce the materials’ sintering temperature and average grain size. Moreover, it was noted that in these systems the substitution site of this dopant is controlled by valence state and ionic radii mismatch effects. In particular, even when a nominal substitution of Ti⁴⁺ is conceived, here it is found that Er³⁺ also incorporates at the (Bi,La)³⁺ sites. These and other interesting concluding remarks from this work, including Er³⁺ tolerance, were possible only after comparing, especially, the X-ray diffraction results and the intrinsic ferroelectric characteristics extracted from the dielectric measurements.     

溶胶-凝胶辅助水热法制备硼掺杂纳米VO2粉体

采用溶胶-凝胶辅助水热法制备了硼掺杂纳米二氧化钒(VO_2)粉体,探究了双氧水(H_2O_2)浓度、水热时间、退火温度、不同硼掺杂剂、硼掺杂量等工艺参数对合成硼掺杂纳米VO_2(M)粉体的影响,并采用X射线衍射仪(XRD)、X射线光电子能谱分析仪(XPS)、电感耦合等离子体光谱仪(ICP)、场发射扫描电镜(SEM)、差示扫描量热仪(DSC)对硼掺杂纳米VO_2粉体的结构、形貌和相变特性进行了表征。结果表明:H2O_2浓度为15%,水热时间为72 h,退火温度为600℃,硼掺杂剂为硼酸、硼掺杂量≤10 at%为较优合成工艺条件;所制备出的硼掺杂纳米VO_2粉体尺寸约为100nm,形貌呈珊瑚状;硼原子成功替代了VO_2(M)晶格中的钒原子,当实际掺杂量为0.6 at%时,相变温度降低7.2℃。     

Phase transition and electrical properties of (1 − x)K0.02Na0.98NbO3-xBaTiO3 ceramics

(1 − x)K_0.02Na_0.98NbO₃-xBaTiO₃ ceramics were prepared by the solid state reaction method, and their electrical properties were investigated. The samples showed crystal structure changing from monoclinic to orthorhombic, and then to tetragonal, with an increase in BaTiO₃ content. The addition of BaTiO₃ markedly enhanced ferroelectric and piezoelectric properties of K_0.02Na_0.98NbO₃ ceramics. Remnant polarization increased and coercive field decreased only in the samples with small amount of BaTiO₃. Piezoelectric properties were improved with the addition of BaTiO₃. The 0.9K_0.02Na_0.98NbO₃-0.1BaTiO₃ ceramics showed maximum piezoelectric constant (d₃₃ = 160 pC/N), which was even comparable with that of (1 − x)K_0.5Na_0.5NbO₃-xBaTiO₃ ceramics. Their good piezoelectric properties, along with a low ferroelectric-ferroelectric transition temperature (T_F-F), made the 0.9K_0.02Na_0.98NbO₃-0.1BaTiO₃ ceramics a potential candidate for lead-free piezoelectric applications.     

Preparation and characterization of barium titanate stannate solid solutions

BaSn_xTi_1−xO₃ (x = 0; 0.05; 0.1; 0.15; 0.2) solid solutions were prepared via conventional solid state reaction and sintered at 1300 °C for 4 h, resulting in dense single phase ceramics with homogeneous microstructures. Tetragonal symmetry for x ≤ 0.1, cubic for x = 0.2 and a superposition of tetragonal and cubic for x = 0.15 compositions were found by X-ray diffraction analysis. The temperature and frequency dependence of the complex dielectric constant and dc tunability were determined. A transformation from normal ferroelectric to relaxor with diffuse phase transition was observed with increasing the Sn concentration. All the investigated compositions show a relative tunability between 0.55 (for x = 0.2) and 0.74 (for x = 0.1), at a field amplitude of E = 20 kV/cm.     

Dielectric properties of spark plasma sintering AlN-W composite ceramics

In order to develop AlN composites suitable for high average power electronic tube,AlN-W materials were prepared by spark plasma sintering.The effects of manufacture parameters on dielectric loss tangent and permittivity constant were investigated,which include powder-mixed method,milling time of high-energy ball milling,starting powder particle size,sintering temperature and holding time and adding amount of the conductive second phase.The results showed that AlN-W materials sintered at the temperature of 1700°C holding for 5 min with 10 vol.% W showed the best dielectric loss tangent larger than 0.81 at the frequency 1 kHz-1 MHz.In addition,magnetic stirring mixed powder and lower sintering temperature led to the better properties because of the higher porosity.The samples sintered from the starting AlN powder with smaller particle size also had the better properties.