Relaxor ferroelectric like giant permittivity in PrCrO3 semiconductor ceramics

The electrical behavior of PrCrO₃ ceramics prepared by citric acid route and sintered at 1200 °C has been characterized by a combination of permittivity measurements, and impedance spectroscopy (IS). The effective permittivity obtained in frequency range 100 Hz to 1 MHz and temperature range 80–300 K, exhibits giant permittivity value of 3 × 10⁴ near room temperature. The response is similar to that observed for relaxor ferroelectrics. IS data analysis revealed the ceramics to be electrically heterogeneous semiconductor with room temperature resistivity <10² Ω m consisting of semiconducting grains with permittivity ?′ ∼ 100 and more resistive grain boundaries with effective permittivity ?′ ∼ 10⁴. We conclude, therefore that grain boundary effect is the primary source for the high effective permittivity in PrCrO₃ ceramics.     

Decrease of dielectric loss in giant dielectric constant CaCu3 Ti4 O12 ceramics by adding CaTiO3

CaCu₃ Ti₄ O₁₂–x CaTiO₃ ceramics (x=0,0.1,0.2,0.3,0.4 and 0.5) was studied by X-ray diffraction, scanning electron microscope and dielectric measurements. It was indicated that some CaTiO₃ entered the boundaries of CaCu₃ Ti₄ O₁₂ grains and/or subgrains. Dielectric measurement showed that the addition of CaTiO₃ lowered the dielectric loss remarkably, especially at low frequencies, while the giant dielectric constant still remained. At room temperature, the dissipation factor of the x=0.5 sample was decreased to 0.02 over the frequency range from 50 to 2000 Hz, and the dielectric constant was kept to be 4000. We explain this phenomenon in terms of internal barrier layer capacitance model by using the impedance spectroscopy analysis.     

Dielectric properties of Y2O3 donor-doped Ba0.8Sr0.2TiO3 ceramics

Ba_0.8Sr_0.2TiO₃ ceramics doped with Y₂O₃ from 0 to 0.10 mol% exhibit normal ferroelectric phase transition, while the ceramics doped with Y₂O₃ from 0.20 to 0.30 mol% show a giant dielectric constant behavior with loss less than 0.15 at 1 kHz from −40 °C to 140 °C, which is suggested due to semiconductive grain and the Maxwell–Wagner effect by structure disordering in grain boundary. The analyses of unipolar charge for the semiconductive grain indicate three kinds of dielectric processes: thermally stimulated process of unipolar hopping, dispersion process of dielectric constant with frequency, and phase transition process accompanied with disappearance of giant dielectric constant in cubic phase. The XPS results confirm that some of the barium ions are in low energy state to form e-Ba²⁺ and to provide hopping sites for electrons. The ceramics doped with Y₂O₃ from 0.50 to 0.75 mol% recover the normal ferroelectricity. The possible mechanics are relevant to binding effect of cation vacancies on electrons.     

Influence of ZnO additions to 0.8(Mg0.95Co0.05)TiO3–0.2Ca0.6La0.8/3TiO3 ceramics on sintering behavior and microwave dielectric properties

The effects of ZnO addition on the microstructures and microwave dielectric properties of 0.8(Mg_0.95Co_0.05)TiO3–0.2Ca_0.6La_0.8/3TiO₃ ceramics were investigated. ZnO was selected as liquid phase sintering aids to lower the sintering temperature of 0.8(Mg_0.95Co_0.05)TiO3–0.2Ca_0.6La_0.8/3TiO₃ ceramics. With ZnO additives, the densification temperature of 0.8(Mg_0.95Co_0.05)TiO₃–0.2Ca_0.6La_0.8/3TiO₃ can be effectively reduced from 1450 to 1200–1325 °C. The crystalline phase exhibited no phase difference at low addition levels (0.25–2 wt.%). It is found that low-level doping of ZnO (0.25–2 wt.%) can significantly improve the density and dielectric properties of 0.8(Mg_0.95Co_0.05)TiO₃–0.2Ca_0.6La_0.8/3TiO₃ ceramics. The quality factors Q × f were strongly dependent upon the amount of additives. Q × f values of 36 000 and 13 000 GHz could be obtained at 1200–1325 °C with 1 and 2 wt.% ZnO additives, respectively. During all additives ranges, the relative dielectric constants were significantly different and ranged from 23.1 to 27.96. The temperature coefficient varies from 14.1–24.3 ppm/°C.     

Structural studies of BiFeO3 modified BMZ-PT ceramics

Bismuth perovskites have been attracting attention as a family of piezoelectric ceramics in place of the widely used Pb (Zr, Ti)O₃ (PZT) system. The advantages of bismuth perovskites over PZT are environmentally more-friendly materials, a higher mechanical strength and Curie temperature. Most recently BiMgZrO₃-PbTiO₃ has been reported to be high temperature morphotropic phase boundary (MPB) piezoelectric with appreciably good ferroelectric and piezoelectric properties.Bismuth containing crystalline solutions [(BiMgZrO₃)_1−y-(BiFeO₃)_y]_x-(PbTiO₃)_1−x, (BMZ-BF-PT) have been synthesized by high temperature solid-state reaction technique. The crystalline symmetry varied with the composition, indicating good solid-state solubility of BMZ and BF with PT. X-ray diffraction (XRD) reveals that BMZ-BF-PT has a single-phase perovskite structure. The Morphotropic Phase Boundary (MPB) of BMZ-PT system lies in the region x = 0.55 to x = 0.6 which is supported by the transformation from tetragonal to rhombohedral phase. The SEM photographs reveal the uniform distribution of grains in the matrix. Variation of dielectric parameters with frequency (at room temperature) exhibit typical dielectric behavior for all compositions.     

The study on electrical behavior of Gd2O3-WO3 complex ceramics at high temperature

Gd₂O₃-WO₃ complex ceramics are fabricated by the conventional solid-state reaction process. The electrical characteristics and dielectric properties of the samples were measured at various ambient temperatures in a low electric field (E < 150 V/mm). As the temperature increases, the dielectric constant and the loss tangent show an obvious change at about 50 °C and 330 °C. When the temperature is above 200 °C, the samples display stable nonlinear electrical properties characterized by semiconductivity, and the nonlinearity increases along with increasing temperature. XRD analysis reveals that Gd₂W₂O₉ is the main phase and Gd₂O₃ is the secondary phase. Based on the phase transition of tungsten trioxide, these electrical properties of Gd₂O₃-WO₃ complex ceramics can be simply explained.     

Microstructures and dielectric properties of CaTiO3–LaSrAlO4 composite ceramics

The microstructures and dielectric properties of xCaTiO₃/(1−x)LaSrAlO₄ composite ceramics were investigated. CaTiO₃ and LaSrAlO₄ could co-exist when the content of CaTiO₃ was no more than 30 mol%, while LaAlO₃ and CaO phases were observed in the composite ceramics with higher CaTiO₃ content. The dielectric constant of the composite ceramics increased with increasing the additive content, and the dielectric loss generally increased with increasing the additive content because of the presence of phase boundary and the high dielectric loss of the minor phases. The temperature coefficient of dielectric constant decreased with increasing the additive content firstly, and then it increased abnormally when x equals to 40 mol%, finally it decreased with increasing the additive content further. The dielectric properties predicted from serial formula were most close to those obtained from experiment among the classical dielectric mixture rules.     

Microstructure and microwave dielectric properties of (1 − y)Li3NbO4+yLi2TiO3(Li2SnO3) ceramics

Phase formation, microstructure and microwave dielectric properties of (1 − y)Li₃NbO₄ + yLi₂TiO₃(Li₂SnO₃) ceramics have been studied in this paper. The structure and microstructure of the compounds were investigated using X-ray powder diffractometer (XRD), scanning electron microscope (SEM), Raman spectrometer. The microwave dielectric properties of the ceramics were studied with a network analyzer at the frequency of about 8–12 GHz. Li₃NbO₄ formed ordered solid solutions with the addition of small amount of Li₂TiO₃ (y ≤ 0.2), whereas no solid solution formed with the addition of small amount of Li₂SnO₃. Small amount of Li₂TiO₃ doping suppressed the appearance of impurity phases caused by lithium evaporation for Li₃NbO₄. The Li₂TiO₃ doped compositions with 0.02 ≤ y ≤ 0.08 demonstrated homogeneous and dense microstructure after sintering at 1150 °C/2 h, in contrast the 0.2 ≤ y ≤ 0.6 specimens exhibited porous and subgrains microstructure after sintering at 1250 °C/2 h. Short range ordering was observed in the 0.2 ≤ y ≤ 0.6 compositions. Mechanical mixture phases of Li₃NbO₄ and Li₂SnO₃ based solid solution (Li₂SnO₃ (ss)) existed in the Li₂SnO₃ added specimens_. The dielectric permittivity increased with increasing Li₂TiO₃ addition, but decreased with the increase of Li₂SnO₃ content. All specimens exhibited negative τ_f value for the Li₂TiO₃ added specimens, although its absolute τ_f value decreased with the increase of Li₂TiO₃ addition. Whereas, the τ_f value changed from negative into positive with the increase of Li₂SnO₃ addition. Optimized combined microwave dielectric properties (?_r = 19.8, Q × f = 91,200 GHz, τ_f = −24 ppm/°C and ?_r = 16, Q × f = 75,300 GHz, τ_f = 3 ppm/°C) could be obtained for the Li₂TiO₃ added (y = 0.6) and Li₂SnO₃ added specimens(y = 0.7), respectively. The microwave dielectric properties of the Li₂SnO₃ end member are ?_r = 13.5, Q × f = 61,600 GHz, τ_f = 29 ppm/°C.     

Stoichiometric Pb(Mg1/3Nb2/3)O3 perovskite ceramics produced by reaction-sintering process

Stoichiometric lead magnesium niobate, Pb(Mg_1/3Nb_2/3)O₃ (PMN), perovskite ceramics produced by reaction-sintering process were investigated. Without calcination, a mixture of PbO, Nb₂O₅, and Mg(NO₃)₂ was pressed and sintered directly. Stoichiometric PMN ceramics of 100% perovskite phase were obtained for 1, 2, and 4 h sintering at 1250 and 1270 °C. PMN ceramics with density 8.09 g/cm³ (99.5% of theoretical density 8.13 g/cm³) and K_max 19,900 under 1 kHz were obtained.     

Effect of Pb(Fe1/2Nb1/2)O3 modification on dielectric and piezoelectric properties of Pb(Mg1/3Nb2/3)O3-PbZr0.52Ti0.48O3 ceramics

10 mol% Pb(Fe_1/2Nb_1/2)O₃ (PFN) modified Pb(Mg_1/3Nb_2/3)O₃-PbZr_0.52Ti_0.48O₃ (PMN-PZT) relaxor ferroelectric ceramics with compositions of (0.9 − x)PMN-0.1PFN-xPZT (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9) were prepared. X-ray diffraction investigations indicated that as-prepared ceramics were of pure perovskite phase and the sample with composition of x = 0.8 was close to morphotropic phase boundary (MPB) between rhombohedral and tetragonal phase. Dielectric properties of the as-prepared ceramics were measured, and the Curie temperature (T_c) increased sharply with increasing PZT content and could be higher than 300 °C around morphotropic phase boundary (MPB) area. At 1 kHz, the sample with composition of x = 0.1 had the largest room temperature dielectric constant ?_r = 3519 and maximum dielectric constant ?_m = 20,475 at T_m, while the sample with composition of x = 0.3 possessed the maximum dielectric relaxor factor of γ = 1.94. The largest d₃₃ = 318 pC/N could be obtained from as-prepared ceramics at x = 0.9. The maximum remnant polarization (P_r = 28.3 μC/cm²) was obtained from as-prepared ceramics at x = 0.4.     

Preparation, structure and dielectric properties of Ba4LaMNb3O15 (M = Ti, Sn ) ceramics

Two new cation-deficient hexagonal perovskites Ba₄LaMNb₃O₁₅ (M = Ti, Sn) ceramics were prepared by high temperature solid-state reaction route. The phase and structure of the ceramics were characterized by X-ray diffraction, scanning electron microscopy (SEM). The microwave dielectric properties of the ceramics were studied using a network analyzer. The Ba₄LaTiNb₃O₁₅ has high dielectric constant of 52, high quality factors (Q) 3500 (at 4.472 GHz), and temperature variation of resonant frequency (τ_f) +93 ppm °C⁻¹ at room temperature; Ba₄LaSnNb₃O₁₅ has dielectric constant of 39 with high Q value of 2510 (at 5.924 GHz), and τ_f −29 ppm °C⁻¹.     

Effects of Bi2O3 and Cr2Ti3O9 Co-doping on dielectric properties in BaTiO3-based ceramics

A microwave ceramic with general composition (1-x-y) BaTiO₃ + x Cr₂Ti₃O₉ + y Bi₂O₃ has been prepared by solid state synthesis at 1300-1400 °C. The phase composition, perovskite structural parameters and dielectric properties have been obtained by X-ray diffraction and dielectric measurements as a function of chemical composition and temperature. At low doping levels the formation of BaTiO₃-based solid solution has been found. The precipitation of BaCrO₃ has been detected at x = y = 2.0 mol%. A model of the incorporation of Cr³⁺ and Bi³⁺ ions into BaTiO₃-based crystal lattice has been proposed. Diffused phase transition in the temperature range 100-140 °C have been revealed by dielectric measurements for different ceramic composition. As high dielectric constant as 7311 and as low dielectric loss as 0.02 have been found for the composition of 0.98BaTiO₃-0.01Cr₂Ti₃O₉-0.01Bi₂O₃.     

Synthesis of La(Mg0.5Ti0.5)O3 ceramics for microwave applications

La(Mg_0.5Ti_0.5)O₃ ceramics were prepared by a non-conventional chemical route, which was based on the Pechini method. For the synthesis of La(Mg_0.5Ti_0.5)O₃ powders, special attention was paid to calcination and milling conditions. Powder morphology and composition were evaluated. Fine La(Mg_0.5Ti_0.5)O₃ powders were obtained at lower temperatures than by conventional methods. Sintering under different conditions was also tested. Dense La(Mg_0.5Ti_0.5)O₃ ceramics were obtained at lower temperatures showing a single phase composition and a homogeneous microstructure. Preliminary dielectric characterization at microwave frequencies was also performed.     

Crystallographic and dielectric properties of barium-substituted Pb(Mg1/3Ta2/3)O3 ceramics

Ceramic powders of (Ba,Pb)Pb(Mg_1/3Ta_2/3)O₃ were prepared via a B-site precursor route. Crystal symmetries and lattice parameters were determined. Monophasic perovskite was developed after the two-step reaction process, in which the lattice parameters showed linear changes in the entire composition range. Dielectric responses of the ceramics with compositional and frequency changes were investigated. The results were also compared with the (Ba,Pb)(Zn_1/3Ta_2/3)O₃ data.     

Mechanochemical synthesis and characterization of nanocrystalline 0.4Bi(Zn1/2Ti1/2)O3-0.6PbTiO3 powders

Perovskite 0.4Bi(Zn_1/2Ti_1/2)O₃-0.6PbTiO₃ (BZT-PT) powders were successfully synthesized from precursor oxides using a high-energy planetary ball milling. The phase development of the powders during milling was studied by means of X-ray diffraction and Raman scattering techniques. The microstructure of the powders was characterized using transmission electron microscopy, and the thermal behavior was studied as well. The results reveal that after 15 h of milling the formation of BZT-PT phase can be completed and submicron agglomerates of small crystallite sized ~ 12 nm are present in the powders. However, further prolonging the milling time to 25 h leads to the amorphization of the BZT-PT phase.     

Comparative investigation of structure and dielectric properties of Pb(Mg1/3Nb2/3)O3-PbTiO3 (65/35) and 10% PbZrO3-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 (65/35) ceramics prepared by a modified precursor method

Relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (65/35) and 10% PbZrO₃-doped Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (65/35) ceramics were both prepared by a modified precursor method, which was based on the high-temperature synthesis of an oxide precursor that contained all the B-site cations for the consideration of B-site homogeneity. The dielectric properties of Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (65/35) ceramic was more of normal ferroelectric behavior, but the high dielectric constant (?_m = 34,200 at 1 kHz) and piezoelectric constant (d₃₃ = 709 pC/N) were observed for this composition close to the morphotropic phase boundary. Comparatively, introduction of 10% PbZrO₃ into Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (65/35) ceramics enhanced the diffuse phase transition as well as the rhombohedral to tetragonal phase transition temperature, while it also kept the high dielectric constant (?_m = 29,600 at 1 kHz) and piezoelectric constant (d₃₃ = 511 pC/N).     

Abnormal high dielectric constant in SmFeO3 semiconductor ceramics

The dielectric properties and impedance spectroscopy (IS) of perovskite SmFeO₃ ceramics were studied in the frequency range of 100 Hz-1 MHz in the temperature range from 80 K to 300 K. These materials exhibited colossal dielectric constant of ∼10⁴ at room temperature. The response is similar to that observed for relaxor-ferroelectrics. IS data analysis indicates the ceramic to be electrically heterogeneous semiconductor consisting of semiconducting grains with dielectric constant 20 and more resistive grain boundaries. We conclude, therefore that grain boundary effect is the primary source for the high effective dielectric constant in SmFeO₃ ceramics.     

Characterization and dielectric behavior of CuO-doped ZnTa2O6 ceramics at microwave frequency

The effects of CuO addition on the microstructures and microwave dielectric properties of ZnTa₂O₆ ceramics were investigated. CuO was selected as a liquid-phase sintering aid to lower the sintering temperature of ZnTa₂O₆ ceramics. With CuO addition, the sintering temperature of ZnTa₂O₆ can be effectively reduced from 1350 to 1230 °C. The crystalline phase exhibited no phase difference and no second phase was detected at low addition levels (0.25-1 wt.%). The quality factors Q × f were strongly dependent upon the CuO concentration. A Q × f value of 65,500 GHz was obtained for specimen with 0.25 wt.% CuO addition at 1230 °C. For all levels of CuO concentration, the relative dielectric constants were not significantly different and ranged from 34.2 to 35.7. Tunable temperature coefficient of resonant frequency (τ_f) can be adjusted to zero by appropriately turning the CuO content.     

Effect of CuO on the sintering and cryogenic microwave characteristics of (Zr0.8Sn0.2)TiO4 ceramics

The effect of CuO on the sintering temperature, microstructure and microwave dielectric properties of (Zr_0.8Sn_0.2)TiO₄ (ZST) modified with 1 wt% of ZnO has been investigated. Microwave dielectric properties of ZST ceramics are measured from cryogenic to room temperatures (15–290 K). Crystallite sizes of sintered ZST ceramics as derived from XRD are in the 30–50 nm range. The addition of CuO effectively reduced the sintering temperature to 1300 °C, possibly due to liquid-phase effects. Addition of CuO did not cause any secondary phases up to 1.5 wt% of CuO. The dielectric constant (ε_r) and temperature coefficient of resonant frequency (τ_f) of ZST ceramics do not significantly vary with temperature, whereas the unloaded quality factor (Q_u) changes noticeably. It is found that the Q_u factor of the sample without CuO decreased with increase in temperature, whereas the samples with addition of CuO up to 1.0 wt% showed less dependence on temperature. The Q_u factor of CuO-free ZST is 15,000 and that of ZST with 0.5 wt% of CuO is 11,800 at 15 K. The Q_u factor while measured at room temperature ranged between 2900 and 7000. Efforts were made to understand whether the increase in Q_u factor at both cryogenic and room temperatures is the result of intrinsic or extrinsic factors.