Dielectric and piezoelectric properties of barium-modified Aurivillius-type Na0.5Bi4.5Ti4O15

In this study, monophasic Ba_x(Na_0.5Bi_0.5)_1−xBi₄Ti₄O₁₅ (x=0.03, 0.06, 0.09 and 0.12) ceramics fabricated from the powders synthesized via the solid-state reaction route exhibited relaxor behavior. X-ray diffraction analysis revealed that the barium-modified Na_0.5Bi_4.5Ti₄O₁₅ ceramics have a pure four-layer Aurivillius phase structure. Dielectric properties and phase transitions were studied and are explained in terms of lattice response of these ceramics. A shift in ferroelectric–paraelectric phase transition (T_c) to lower temperatures and a corresponding increase in permittivity peak with increasing concentration of Ba²⁺ are also observed. The decrease of orthorhombicity in the lattice structure by the larger Ba²⁺ ion incorporation, indicating an approach of a and b parameters, results in lower Curie temperature. The piezoelectric activity of Na_0.5Bi_4.5Ti₄O₁₅ (NBT) ceramics was significantly improved by the modification of barium. The Curie temperature T_c and piezoelectric coefficient d₃₃ for the composition with x=0.12 were found to be 635 °C and 21 pC/N, respectively. The relationship of polarization with lattice response is discussed.     

Na0.5K0.5NbO3-BiFeO3 lead-free piezoelectric ceramics

Lead-free (Na_0.5K_0.5)NbO₃-based piezoelectric ceramics were successfully fabricated by substituting with a small amount of BiFeO₃ (BF). Difficulty in sintering of pure NKN ceramics can be eased by adding a few molar percent of BF, and the crystalline structure is also changed, leading to a morphotropic phase boundary (MPB) between ferroelectric orthorhombic and rhombohedral phases. The MPB exists near the 1-2 mol% BF-substituted NKN compositions, exhibiting enhanced ferroelectric, piezoelectric, and electromechanical properties of P_r=23.3 μC/cm², d₃₃=185 pC/N, and k_p=46%, compared to an ordinarily sintered pure NKN ceramics. The MPB composition has a Curie temperature of ∼370 °C, comparable to that of some commercial PZT materials.     

Structure and electrical properties of Bi0.5(Na, K)0.5TiO3−BiGaO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x − y)Bi_0.5Na_0.5TiO₃−xBi_0.5K_0.5TiO₃− yBiGaO₃ have been fabricated by an ordinary sintering technique, and their structure and electrical properties and depolarization temperature have been studied. The results of X-ray diffraction reveal that Bi_0.5K_0.5TiO₃ and BiGaO₃ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure. An obvious change in microstructure with increasing concentration of Bi_0.5K_0.5TiO₃ and BiGaO₃ was observed. The piezoelectric constant d₃₃ and the electromechanical coupling factor k_p of the ceramics attain maximum values of 165 pC/N and 0.346 at y = 0.01(x = 0.18) and x = 0.21(y = 0.01), respectively. The temperature dependence of dielectric constant indicates an obvious relaxor characteristic with strong frequency dependence of dielectric constant. The depolarization temperature decreased with increasing content of BiGaO₃ and first decreases and then increases with increasing amount of Bi_0.5K_0.5TiO₃.     

Electrical properties of SrBi2(Nb0.5Ta0.5)2O9 ceramics

Aurivillius SrBi₂(Nb_0.5Ta_0.5)₂O₉ (SBNT 50/50) ceramics were prepared using the conventional solid-state reaction method. Scanning electron microscopy was applied to investigate the grain structure. The XRD studies revealed an orthorhombic structure in the SBNT 50/50 with lattice parameters a=5.522 Å, b=5.511 Å and c=25.114 Å. The dielectric properties were determined by impedance spectroscopy measurements. A strong low frequency dielectric dispersion was found to exist in this material. Its occurrence was ascribed to the presence of ionized space charge carriers such as oxygen vacancies. The dielectric relaxation was defined on the basis of an equivalent circuit. The temperature dependence of various electrical properties was determined and discussed. The thermal activation energy for the grain electric conductivity was lower in the high temperature region (T>303.6 °C, E_a−ht=0.47 eV) and higher in the low temperature region (T<303.6 °C, E_a−lt=1.18 eV).     

Effect of bismuth deficiency on structure and electrical properties of (Na0.5Bi0.5)0.93Ba0.07TiO3 ceramics

(Na_0.5Bi_x)_0.93Ba_0.07TiO₃ (x=0.500-0.492) ceramics were prepared by a citrate method, and the structure and electrical properties of the ceramics were investigated with respect to the amount of Bi deficiency. It was detected that the Bi deficiency had a considerable impact on the crystal structure and microstructure. The inspection of both the temperature dependence of the dielectric properties (free permittivity ε₃₃^T/ε₀ and dielectric loss tan δ) and the evolution of the polarization-electrical field (P-E) hysteresis loops with measuring temperature suggests that the Bi deficiency served to increase the depolarization temperature (T_d). The Bi deficiency led to an increase in the coercive field (E_c) and mechanical quality factor (Q_m) together with a decrease in the remanent polarization (P_r) and piezoelectric constants (d₃₃). The variation of the structure and electrical properties with Bi deficiency amount was qualitatively interpreted in terms of the formation of Bi and oxygen vacancies in the Bi-deficient specimens. This research indicates the importance of adequately controlling Bi stoichiometry of (Na_0.5Bi_0.5)_0.93Ba_0.07TiO₃ ceramics in obtaining the desired ferroelectric and piezoelectric properties.     

Phase evolution and microwave dielectric properties of Bi3SbO7 ceramic

The Bi₃SbO₇ ceramic was prepared by the solid state reaction method and its phase evolution at different temperatures was studied. Low temperature phase α-Bi₃SbO₇ was formed at about 890 °C and it started to transform to high temperature phase β-Bi₃SbO₇ at about 960 °C. Microwave dielectric constants of α-Bi₃SbO₇ ceramic and β-Bi₃SbO₇ ceramic were 43.2 and 37.6, Qf value were 2080 and 5080 GHz, respectively. TCF of α-Bi₃SbO₇ ceramic was near zero and TCF of β-Bi₃SbO₇ ceramic was about −120 ppm/°C. The Bi₃SbO₇ ceramic is a promising candidate for low temperature co-fired ceramic (LTCC) technology due to its large dielectric constant, low dielectric loss at microwave region, low sintering temperature and simple composition.     

The influence of different fabrication processes on characteristics of excess Bi2O3-doped 0.95 (Na0.5Bi0.5)TiO3–0.05 BaTiO3 ceramics

In this study, we will develop the influences of the excess x wt% (x=0, 1, 2, and 3) Bi₂O₃-doped and the different fabricating process on the sintering and dielectric characteristics of 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃ ferroelectric ceramics with the aid of SEM and X-ray diffraction patterns, and dielectric–temperature curves. The 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃+x wt% Bi₂O₃ ceramics are fabricated by two different processes. The first process is that (Na_0.5Bi_0.5)TiO₃ composition is calcined at 850 °C and BaTiO₃ composition is calcined at 1100 °C, then the calcined (Na_0.5Bi_0.5)TiO₃ and BaTiO₃ powders are mixed in according to 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃+x wt% Bi₂O₃ compositions. The second process is that the raw materials are mixed in accordance to the 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃+x wt% Bi₂O₃ compositions and then calcining at 900 °C. The sintering process is carried out in air for 2 h from 1120 to 1240 °C. After sintering, the effects of process parameters on the dielectric characteristics will be developed by the dielectric–temperature curves. Dielectric–temperature properties are also investigated at the temperatures of 30–350 °C and at the frequencies of 10 kHz–1 MHz.     

Dielectric and piezoelectric properties of Fe2O3-doped (Na0.5K0.5)0.96Li0.04Nb0.86Ta0.1Sb0.04O3 lead-free ceramics

The effect of a small amount Fe₂O₃ (0.1-2 mol%) doping on the electrical properties of (Na_0.5K_0.5)_0.96Li_0.04Nb_0.86Ta_0.1Sb_0.04O₃ (NKLNTS) ceramics was investigated. It was found that the B-site substitution of Fe³⁺ does not change the crystal structure within the studied doping level and all modified ceramics have a pure tetragonal perovskite structure at room temperature. The addition of Fe₂O₃ can promote the sintering of NKLNTS ceramics, and simultaneously cause the grain growth so that Fe³⁺-doped NKLNTS compositions show degraded densification at higher doping level. Furthermore, the dielectric properties of the NKLNTS ceramics do not show a significant change by Fe₂O₃ doping. However, the addition of Fe₂O₃ was found to have a significant influence on the electric fatigue resistance and the durability against water. The presence of oxygen vacancies caused by the replacement of Fe³⁺ for B-site ions makes the NKLNTS ceramics harder.     

Microwave dielectric properties of high quality factor La(Mg0.5−xCaxSn0.5)O3 ceramics

The microwave dielectric properties of La(Mg_0.5−xCa_xSn_0.5)O₃ ceramics were examined with a view to their exploitation for wireless communications. The La(Mg_0.5−xCa_xSn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The La(Mg_0.5−xCa_xSn_0.5)O₃ ceramics contained Ca₂SnO_4, CaSnO₃, and La₂O₃. The amount of Ca₂SnO₄ increased with increasing sintering temperature. However, the relative amount of CaSnO₃ decreased with increasing sintering temperature. An apparent density of 6.52 g/cm³, a dielectric constant (ε_r) of 20.2, a quality factor (Q×f) of 80,500 GHz, and a temperature coefficient of resonant frequency (τ_f) of −79 ppm/°C were obtained for La(Mg_0.4Ca_0.1Sn_0.5)O₃ ceramics that were sintered at 1500 °C for 4 h.     

Magnetic and dielectric properties of Ni0.8Co0.1Cu0.1Fe2O4+PZT composites

Magnetoelectric composites of Ni_0.8Co_0.1Cu_0.1Fe₂O₄ and Lead Zirconate Titanate (PZT) were prepared by using conventional ceramic method. The measured values of saturation magnetization (M_s) and magnetic moments (μ_B) are in accordance with the volume fraction of ferrite content in the composite. The dielectric constant of the composites decreases with frequency. The plots of dielectric constant () against temperature (T) show a peak at their respective transition temperatures. The ME output was measured by varying dc bias magnetic field. A large ME output signal of 776 mV/cm was observed for 35% ferrite +65% ferroelectric composite. The magnetoelectric (ME) response is found to be dependent on the content of ferrite phase.     

Densification, microstructural evolution, and dielectric properties of CaTiO3-LaGaO3 microwave ceramics

The relations among the densification, microstructural evolution, and microwave dielectric properties of the (1−x)CaTiO₃-xLaGaO₃ ceramics with x=0.34 and 0.36 were investigated in this study. The results indicated that (1−x)CaTiO₃−xLaGaO₃ ceramics can be densified at 1300 °C with at least 97% of the theoretical value. The ceramics reported an orthorhombic perovskite structure, and no other detectable phases were found. Both ε_r and Q×f values can be improved by slowing the cooling rate during sintering. The ε_r and Q×f values of the 0.64CaTiO₃-0.36LaGaO₃ ceramics at cooling rates of >10 °C/min and 0.1 °C/min are 48.1 and 27,500 and 48.7 and 38,000, respectively. The higher densification obtained at a slower cooling rate plays an important role in improving the microwave dielectric properties.     

Local order and structural transitions in the molecule-based layer ferrimagnet (n-C4H9)4N FeFe(C2O4)3

Local structural order and temperature-dependent structural variation have been studied in the molecular-based layer ferrimagnet (n-C₄H₉)₄N Fe^IIFe^III(C₂O₄)₃ by EXAFS and high resolution X-ray powder diffraction. The EXAFS spectra measured at the Fe K-edge are successfully modelled by successive O, C, O and metal shells, showing that even when there is extensive structural disorder due to stacking faults, the local structural order in this class of ferrimagnets is fully retained. In this salt, which shows remarkable negative magnetisation at low temperature (Néel class Q), the EXAFS Debye-Waller factor has a discontinuity at 40 K, corresponding to one found in the magnetisation. At the same temperature there is also a change in the expansion of the lattice as evidenced by the high resolution X-ray powder diffraction.     

Orientation-dependent dielectric properties of Ba(Sn0.15Ti0.85)O3 thin films prepared by sol-gel method

The orientation-dependent dielectric properties of barium stannate titanate (Ba(Sn_0.15Ti_0.85)O₃, BTS) thin films grown on (1 0 0) LaAlO₃ single-crystal substrates through sol-gel process were investigated. The nonlinear dielectric properties of the BTS films were measured using an inter-digital capacitor (IDC). The results show that the in-plane dielectric properties of BTS films exhibited a strong sensitivity to orientation. The upward shift of Curie temperature (T_c) of the highly (1 0 0)-oriented BTS thin films is believed to be attributing to a tensile stress along the in-plane direction inside the film. A high tunability of 47.03% was obtained for the highly (1 0 0)-oriented BTS films, which is about three times larger than that of the BTS films with random orientation, measured at a frequency of 1 MHz and an applied electric field of 80 kV/cm. This work clearly reveals the highly promising potential of BTS films for application in tunable microwave devices.     

Structural and electrical properties of high-energy ball-milled NASICON type Li1.3Ti1.7Al0.3(PO4)2.9(VO4)0.1 ceramics

Nano-crystallites of Li_1.3Ti_1.7Al_0.3(PO₄)_2.9(VO₄)_0.1 NASICON type material are prepared by means of solid-state reaction of a stoichiometric mixture after milling it for 22 and 55 h. The milling reduces the average crystallite size of the ceramic to 80 and 60 nm, respectively. Mechanical milling changes structural parameters and the strain induced at the grain-boundaries plays a major role in improving electrical conductivity. An order of magnitude increase in electrical conductivity is observed in the material milled for 55 h compared to the unmilled material, which is also reflected in permittivity loss. Modulus and permittivity representations substantiate the constriction effect of grain-boundaries observed in the complex impedance representation.     

Dielectric and piezoelectric properties of lead-free (Na0.5K0.5)NbO3-SrTiO3 ceramics

In this article, we report successful preparation of dense [(Na_0.5K_0.5)_1−xSr_x](Nb_1−xTi_x)O₃ (x=0.005-0.100) ceramics by ordinary sintering in air. The dependence of phase structure on doping content of SrO and TiO₂ has been determined by the X-ray diffraction technique. It was found that the crystal structure changed from orthorhombic to tetragonal at x≈0.040. Dielectric study revealed that the dielectric relaxor behavior was induced by doping of SrO and TiO₂ into (Na_0.5K_0.5)NbO₃. The samples in the composition range from x=0.005 to 0.020 exhibited excellent electrical properties, piezoelectric constant of electromechanical planar and thickness coupling coefficients of k_p=26.6-32.5% and k_t=39.8-43.8%. The results show that the [(Na_0.5K_0.5)_1−xSr_x](Nb_1−xTi_x)O₃ ceramics are one of the promising lead-free materials for electromechanical transducer applications.     

Structural characterization of dense reduced BaTiO3 and Ba0.95La0.05TiO3 nanoceramics showing colossal dielectric values

BaTiO_3−x and Ba_0.95La_0.05TiO_3−x nanoceramics showing colossal permittivity values have been characterized. While starting powders are of cubic symmetry, X-ray and Neutron Diffraction techniques and Raman Spectroscopy measurements show that the one-step processed ceramics obtained by Spark Plasma Sintering (SPS) contain cubic and tetragonal phases. Rather large oxygen deficiency determined in such ceramics by Electron Micro Probe analysis and Electron Energy Loss Spectroscopy analyzes is explained by the presence of Ti³⁺, as evidenced by X-ray Photoelectron Spectroscopy measurements. Transmission Electron Microscopy and High Resolution Transmission Electron Microscopy show that these ceramics contain 50-300 nm grains, which have single-domains, while grain boundaries are of nanometer scale. Colossal permittivity values measured in our dense nanoceramics are explained by a charge hopping mechanism and an interfacial polarization of a large number of polarons generated after sample reduction in SPS apparatus.     

Aqueous electrochemical insertion of Na into the tungsten oxide hybrid WO3(4,4′-bipyridyl)0.5

Aqueous electrochemical insertion of M⁺ (Na⁺ and H⁺) species into WO₃(4,4′-bipyridyl)_0.5 has been carried out. The chemical states and structure of the resulting product were analysed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). XPS showed the presence of W⁶⁺ as well as the usual reduced W species (W⁵⁺) which is responsible for a change in colour. Moreover, the presence of these intercalates correlates with the evolution of the reduced W species. The bulk structure of the layered hybrid, as determined by powder X-ray diffraction, showed no alteration after electrochemistry, in contrast to the same measurements on tungsten trioxide (WO₃). This however concurs with single-crystal X-ray studies, which show little change in lattice parameters with Na⁺ insertion. Four-probe resistance measurements of the layered hybrid coated film display a drop in resistance after electrochemistry, which can be attributed to the injection of charge-carriers into the conduction band.     

Effects of Na2CO3 flux addition on the structure and photoluminescence properties for Eu-doped YVO4 phosphor

The luminescence properties of (Y_0.9Eu_0.1)VO₄ phosphor with Na₂CO₃ flux prepared using the solid-state reaction were investigated. The XRD patterns show that all of the peaks are attributed to the YVO₄ phase. The best crystallinity was obtained with 2 wt% Na₂CO₃ flux addition. The surface morphology of (Y_0.9Eu_0.1)VO₄ phosphor changed from fluffy to a bar shape structure after Na₂CO₃ flux addition due to the tetragonal crystal system of YVO₄. The calcined powders emit bright red luminescence centered at 618 nm due to the ⁵D₀→⁷F₂ electric dipole transition under an excitation wavelength of 318 nm; its intensity was increased about 15% with 2 wt% Na₂CO₃ flux addition. Red shift behavior was observed for the charge transfer state (CTS) absorption, which was due to the grain size of (Y_0.9Eu_0.1)VO₄ phosphor increasing with increasing flux content. For 2 wt% Na₂CO₃ flux addition, the red emission of the (Y_0.9Eu_0.1)VO₄ phosphor had CIE chromaticity coordinates of (0.66, 0.34), which are very close to the NTSC system standard red chromaticity coordinates of (0.67, 0.33).     

Structural and impedance properties of Ca3Nb2O8 ceramics

Polycrystalline sample of Ca₃Nb₂O₈ was prepared by a high-temperature solid-state reaction technique. X-ray diffraction (XRD) analysis confirms the formation of single-phase compound of hexagonal (rhombohedral) crystal structure at room temperature. Scanning electron micrograph of the material showed uniform grain distribution on the surface of the sample. Detailed studies of dielectric properties of the compound, studied in a wide frequency range (10²-10⁶ Hz) at different temperatures (25-500 °C), exhibit a dielectric anomaly suggesting phase transition of ferroelectric-paraelectric and structural type at 300 °C. Electrical properties of the material were analyzed using a complex impedance technique. The Nyquists plot showed the presence of bulk effect in the material in the studied temperature range. Studies of electrical conductivity over a wide temperature range suggest that the compound has negative temperature coefficient of resistance behavior.     

Dielectric and modulus behavior of LiFe1/2Ni1/2VO4 ceramics

The polycrystalline sample of LiFe_1/2Ni_1/2VO₄ was prepared by a standard solid-state reaction technique and confirmed by X-ray diffractometry. LiFe_1/2Ni_1/2VO₄ has orthorhombic crystal structure whose dielectric and electric modulus properties were studied over a wide frequency range (100 Hz–1 MHz) at different temperatures (296–623 K) using a complex impedance spectroscopy (CIS) technique. The frequency and temperature dependence of dielectric constant (ε_r) and tangent loss (tan δ) of LiFe_1/2Ni_1/2VO₄ are studied. The variation of ε_r as a function frequency at different temperatures exhibits a dispersive behavior at low frequencies. The variation of the ε_r as a function of temperature at different frequencies shows the dielectric anomaly in ε_r at 498 K with maximum value of dielectric constant 274.49 and 96.86 at 100 kHz and 1 MHz, respectively. Modulus analysis was carried out to understand the mechanism of the electrical transport process, which indicates the non-exponential type of conductivity relaxation in the material. The activation energy calculated from electric modulus spectra is 0.38 eV.