Varistor and dielectric properties of SiO2 doped SnO2-Zn2SnO4 ceramic composites

In this work, SiO₂ doped SnO₂-Zn₂SnO₄ ceramic composites with excellent varistor and dielectric properties were prepared through traditional ceramic processing. The obtained nonlinear coefficient α was as high as 9.6, and the breakdown electrical field EB and leakage current density JL was as low as 5.9 V/mm and 62 μA/cm², respectively. At a low frequency of 40 Hz, the relative permittivity εr measured at room temperature was higher than 2.5×10⁴. The nonlinear decrease of the semicircle diameter in the complex impedance spectra with increasing DC bias voltage indicates that the grain boundary effect is an important origin of the varistor and giant permittivity properties. With an increase of temperature, the relaxor peak of the imaginary part M″ of the complex electric modulus shifted to high frequency and the activation energy E_a obtained from the M″ spectrum was about 0.31 eV, much lower than the grain boundary barrier height ϕ_b. The results suggest that other mechanisms may also be responsible for the giant permittivity property besides grain boundary barriers.     

The effect of Sm2O3 on the microstructure and electrical properties of SiO2-doped SnO2-Zn2SnO4 ceramic varistors

In this work, Sm₂O₃- and SiO₂-codoped SnO₂-Zn₂SnO₄ ceramic varistors were prepared through traditional ceramic processing, and the effect of Sm₂O₃ on the resulting microstructure and electrical properties was investigated. The results demonstrated that the ceramics were composed mainly of SnO₂ and Zn₂SnO₄, and Sm was distributed homogeneously in the grains and along the grain boundaries. With 0.2 mol% Sm₂O₃ doping, the grain growth was obviously promoted. Further increases in Sm₂O₃ to 0.4 mol% resulted in trace amount of SiO₂ and segregations containing elemental Sm via X-ray diffraction patterns and microstructure photos, respectively. In the sample doped with 0.3 mol% Sm₂O₃, optimal electrical characteristics of α=9.4, E_B=10 V/mm, J_L=46 μA/cm² and ε′=1.2×10⁴ were obtained. Simultaneously, the sample doped with 0.3 mol% Sm₂O₃ had the lowest conductance activation energy of 0.16 eV at temperatures lower than 110 °C. This good performance indicates that Sm₂O₃- and SiO₂-codoped SnO₂-Zn₂SnO₄ composite ceramics are viable candidate for the manufacture of capacitor-varistor functional devices.     

SnO2 modified PNN-PZT ceramics with ultra-high piezoelectric and dielectric properties

In this work, a two-step solid-state reaction method is used to prepare the 0.55 Pb(Ni_1/3Nb_2/3)O₃-0.135PbZrO₃-0.315PbTiO₃/xSnO₂ (PNN-PZT/xSnO₂) ceramics. The influences of SnO₂ on the crystalline structure, electromechanical properties, and temperature stability of PNN-PZT ceramics were studied in detail. The results demonstrate that the Sn⁴⁺ ions are successfully introduced into the PNN-PZT crystalline lattice and substitute B-site Ni²⁺ and Zr⁴⁺. The x = 0.0025 ceramic with the coexistence of rhombohedral, tetragonal, and pseudocubic phases exhibits the optimized comprehensive properties: the quasi-static piezoelectric constant d₃₃, large-signal d₃₃*, electromechanical coupling coefficients k_p and k_t, free dielectric constant ε_r, and mechanical quality factor Q_m are 1123 pC/N, 1250 p.m./V, 0.63, 0.54, 9529, and 57, respectively. Meanwhile, the Curie temperature for this composition is 103 °C, almost maintaining the same level as the PNN-PZT matrix. After annealing at 75 °C, the retained d₃₃ of x = 0.0025 ceramic is as high as 975 pC/N, superior to the PNN-PZT matrix (retained d₃₃ ≈ 873 pC/N). Our results provide a promising piezoelectric material for board bandwidth, high sensitivity, and miniaturized medical ultrasonic transducers applications.     

Effect of magnesium titanate content on microstructures,mechanical performances and dielectric properties of Si3N4-based composite ceramics

Silicon nitride-based composite ceramics with different contents of magnesium titanate have been fabricated via gas pressure sintering method. The phase compositions, microstructure, mechanical performances and dielectric properties of the composite ceramics were investigated. The density of the Si₃N₄-based composite ceramics firstly increased with additive of magnesium titanate powder up to 5 wt% and then gently decreased, and the mechanical properties firstly increased and then declined. Besides, the dielectric constant and dielectric loss increased with the increase of magnesium titanate contents. For the Si₃N₄-based composite ceramics with 5 wt% magnesium titanate powders, the flexural strength, elastic modulus, dielectric constant and dielectric loss reached 451 MPa, 274 GPa, 7.65, 0.0056, respectively. These results suggested that the magnesium titanate was beneficial for the improvement of mechanical performances and dielectric constant of Si₃N₄-based composite ceramics.     

Structural,dielectric and ferromagnetic behavior of (Zn,Co) co-doped SnO2 nanoparticles

Nanoparticles of basic composition Sn_0.94Zn_0.05Co_0.01O₂, Sn_0.92Zn_0.05Co_0.03O₂ and Sn_0.90Zn_0.05Co_0.05O₂ were synthesized by chemical precipitation method. The incorporation of Co and Zn in SnO₂ lattice introduced significant changes in the physical properties of all the three nanocrystals. The average particle size estimated from TEM data decreased from 15.71 to 6.41  nm with enhancement in concentration of oxygen vacancies as Co content is increased from 1 to 5 wt%. Increasing Co content enhanced the Sn:O atomic ratio as a result concentration of oxygen vacancies increased. The dielectric study revealed strong doping dependence. The dielectric parameters (ε′, tanδ and σ_ac) increased with increasing Co content and attained maximum values for 5% (Zn, Co) co-doped SnO₂ nanoparticles. The dielectric loss (ε′′) exhibited dispersion behavior and the Debye’s relaxation peaks observed in dielectric loss factor (tanδ), whose intensities increased with increasing Co content. The variation of dielectric properties and ac conductivity revealed that the dispersion is due to Maxwell-Wagner interfacial polarization and hopping of charge carriers between Sn⁺²/Sn⁺³ and Co⁺²/Co⁺³. The large dielectric constant of all samples made them interesting materials for device application. Magnetization measurements (M (H) loops) revealed enhancement in saturation magnetization with doping which is due to the formation of large amount of induced defects and oxygen vacancies in the samples. The present study clearly reveals doping dependent properties and the oxygen vacancies induced ferromagnetism in Zn, Co co-doped SnO₂ nanoparticles having applications in ultra-high dielectric materials, high frequency devices and spintronics.     

Microwave dielectric properties of Co2P2O7 ceramics

Co₂P₂O₇ ceramics were prepared through the traditional solid-state sintering technique. The phase composition, grain size distribution, and densification were researched via X-ray diffraction and scanning electron microscope. The influence of pores on permittivity was described by various models. The dielectric loss was found highly dependent on porosity. Moreover, the low ε_r (<10) values of Co₂P₂O₇ ceramics were explained by the covalent feature of P–O bonds. Raman spectroscopy was used for exploring the relationship between polar phonon modes and dielectric properties in terms of intrinsic factors. The optimum dielectric properties (ε_r = 6.76, Qf = 36,400 GHz and τ_f = ?23.9 ppm/°C) were obtained at 1160 °C for 4 h.     

Microstructure and electrical properties of rare earth doped ZnO-based varistor ceramics

ZnO-based varistor ceramics doped with Nd₂O₃ and Y₂O₃ have been prepared by the conventional ceramics method. The phase composition, microstructure and electrical properties of the ceramics have been investigated by XRD, SEM and a V–I source/measure unit. The XRD and EDS analyses show the presence of ZnO, Bi₂O₃, Zn₇Sb₂O₁₂, Y₂O₃, Nd-rich phase and Y-containing Bi-rich phase. The electrical properties analyzed show that the nonlinear coefficient of the varistor ceramics is in the range of 4.4–70.2, the threshold voltage is in the range of 247.1–1288.8 V/mm, and the leakage current is in the range of 1.51–214.6 μA/cm². The 0.25 mol% Nd₂O₃ added varistor ceramics with 0.10 mol%Y₂O₃ sintered at 1050 °C exhibits excellent electrical properties with the high threshold voltage of 556.4 V/mm, the nonlinear coefficient of 61 and the leakage current of 1.55 μA/cm². The results illustrate that doping Nd₂O₃ and Y₂O₃ in ZnO-based varistor ceramics may be a very promising route for the production of the higher threshold voltage and the nonlinear coefficient of ZnO-based varistor ceramics.     

Microstructure and dielectric properties of Ag-BaTiO3 composite ceramics

Di-phase composite ceramics based on BaTiO₃ with 5?vol% of Ag filler have been prepared by sintering the mixture of powders at temperatures above the silver melting point (1000?°C–1300?°C/2?h). As predicted by finite element calculations, the addition of metallic particles should produce a field concentration in some regions of the BaTiO₃ matrix and therefore, an enhanced dielectric response with respect to pure BaTiO₃. The role of oxygen vacancies on the dielectric relaxation mechanisms of Ag-BaTiO₃ composites has been investigated. The sintering temperature of 1200?°C provided optimized ceramics with excellent dielectric properties, i.e. with low losses (tanδ?<?3%) and room temperature permittivity measured at 50?kHz exceeding 6500 (and above 13,000 at the Curie temperature), as result of a good densification (94% relative density) and a synergy effect of the metallic particles inclusions and ceramic grain size in the range of ≈1?μm, where BaTiO₃ has a well-known maximum of its permittivity.     

Low-temperature sintering and microwave dielectric properties of Ca5Co4(VO4)6 ceramics

A novel low temperature firing high Q microwave dielectric ceramic Ca₅Co₄(VO₄)₆ was prepared by the conventional solid-state reaction method. The phase purity, microstructure, and microwave dielectric properties were investigated. The Ca₅Co₄(VO₄)₆ ceramic sintered at 875 °C exhibited excellent microwave dielectric properties: Qxf = 95,200 GHz (at 10.6 GHz), τ_f = −63 ppm/°C, ɛ_r = 10.1, and its ɛ_r corrected for porosity was calculated as 11.1.     

Microwave dielectric properties of multi-ions Ba(Zn,Ta)O3-based perovskite ceramics

In this study, Ba(Zn_1/3Ta_2/3)O₃-based complex perovskite compounds, including Ba(Zn_1/3Ta_2/3)O₃, Ba(Zn_1/3Ta_1/3Nb_1/3)O₃, Ba(Zn_1/6Co_1/6Ta_2/9Nb_2/9Sb_2/9)O₃, and Ba_1/2Sr_1/2(Zn_1/6Co_1/6Ta_2/9Nb_2/9Sb_2/9)O₃, were prepared and characterized. There was no second phase formation shown in the XRD patterns. Though it has been suggested that substitutions of multiple ions over A-site or B-site of the Ba(Zn_1/3Ta_2/3)O₃ ceramics may not be beneficial to their microwave dielectric properties, the Ba(Zn_1/6Co_1/6Ta_2/9Nb_2/9Sb_2/9)O₃ and Ba_1/2Sr_1/2(Zn_1/6Co_1/6Ta_2/9Nb_2/9Sb_2/9)O₃ ceramics in this study were found to perform in a fairly acceptable manner. The Ba(Zn_1/6Co_1/6Ta_2/9Nb_2/9Sb_2/9)O₃ ceramic (sintered at 1575 °C for 6 h) and the Ba_1/2Sr_1/2(Zn_1/6Co_1/6Ta_2/9Nb_2/9Sb_2/9)O₃ ceramic (sintered at 1550 °C for 6 h) reported the following characteristics after annealing at 1400 °C for 10 h: 24.9 and 27.0 for dielectric constants (?_r), 83,000 and 32,100 GHz for quality factors (Q × f) values and −12.8 and −22.6 ppm/°C for temperature coefficients of resonance frequency (τ_f).     

Microstructure and dielectric properties of Dy/Mn doped BaTiO3 ceramics

Dy/Mn doped BaTiO₃ with different Dy₂O₃ contents, ranging from 0.1 to 5.0 at% Dy, were investigated regarding their microstructural and dielectric characteristics. The content of 0.05 at% Mn was constant in all the investigated samples. The samples were prepared by the conventional solid state reaction and sintered at 1290°, and 1350 °C in air atmosphere for 2 h. The low doped samples (0.1 and 0.5 at% Dy) exhibit mainly fairly uniform and homogeneous microstructure with average grain sizes ranged from 0.3 μm to 3.0 μm. At 1350 °C, the appearance of secondary, abnormal, grains in the fine grain matrix and core–shell structure were observed in highly doped Dy/BaTiO₃. Dielectric measurements were carried out as a function of temperature up to 180 °C. The low doped samples sintered at 1350 °C, display the high value of dielectric permittivity at room temperature, 5600 for 0.1Dy/BaTiO₃. A nearly flat permittivity–temperature response was obtained in specimens with 2.0 and 5.0 at% additive content. Using a Curie–Weiss and modified Curie–Weiss low, the Curie constant (C), Curie like constant (C′), Curie temperature (T_C) and a critical exponent (γ) were calculated. The obtained values of γ pointed out the diffuse phase transformation in highly doped BaTiO₃ samples.     

Structural ordering and dielectric properties of Ba3CaNb2O9-based microwave ceramics

Ba₃CaNb₂O₉ is a 1:2 ordered perovskite which presents a trigonal cell within the D_3d³ space group. Dense ceramics of Ba₃CaNb₂O₉ were prepared by the solid-state reaction route, and their microwave dielectric features were evaluated as a function of the sintering time. From Raman spectroscopy, by using group-theory calculations, we were able to recognize the coexistence of the 1:1 and 1:2 ordering types in all samples, in which increasing the sintering time tends to reduce the 1:1 domain, leading to an enhancement of the unloaded quality factor. We concluded that this domain acts as a lattice vibration damping, consequently raising the dielectric loss at microwave frequencies. The best microwave dielectric parameters were determined in ceramics sintered at 1500 °C for 32h: ε′ ~ 43; Q_u×f_r = 15,752 GHz; τ_f ~ 278 ppm °C⁻¹.     

Microstructure and electrical properties of Sc2O3-doped ZnO-Bi2O3-based varistor ceramics

The microstructure and electrical properties of ZnO-Bi₂O₃-based varistor ceramics doped with different Sc₂O₃ content sintered at 1100 °C were investigated. The results showed that the nonlinear coefficient of the varistor ceramics with Sc₂O₃ were in the range of 18-54, the threshold voltage in the range of 250-332 V/mm, the leakage current in the range of 0.1-23.0 μA, with addition of 0-1.00 mol% Sc₂O₃. The ZnO-Bi₂O₃-based varistor ceramics doped with Sc₂O₃ content of 0.12 mol% exhibited the highest nonlinearity, in which the nonlinear coefficient is 54, the threshold voltage and the leakage current is 278 V/mm and 2.9 μA, respectively. The results confirmed that doping with Sc₂O₃ was a very promising route for the production of the higher nonlinear coefficient of ZnO-Bi₂O₃-based varistor ceramics, and determining the proper amounts of addition of Sc₂O₃ was of great importance.     

High-performance 0.9Al2O3-0.1TiO2 microwave dielectric ceramics prepared by digital light processing

In this work, the 0.9Al₂O₃-0.1TiO₂ ceramic sample with good microwave dielectric properties and complex structures can be well fabricated by digital light processing (DLP). A relationship between dispersant content and rheological behavior of 0.9Al₂O₃-0.1TiO₂ slurry was explored. When dispersant content was 3.0 wt%, 0.9Al₂O₃-0.1TiO₂ slurry with high solid loading (50 vol%) and low viscosity (2.9 Pa s) could be obtained. 0.9Al₂O₃-0.1TiO₂ ceramic parts with high accuracy were fabricated successfully by adding 3.0 wt% photoinitiator under 600 mJ/cm² exposure energy. With the increase of sintering temperature from 1400 °C to 1600 °C, relative density, dielectric constant (ε_r), and quality factor (Q × f) of 0.9Al₂O₃-0.1TiO₂ ceramic sample increased first and then decreased, and all reached the maximum value at 1550 °C due to the uniformity and densification of microstructures. The temperature coefficient of resonant frequency (τ_f) value showed an almost monotonous increase, changing from negative to positive, and near-zero τ_f value at 1550 °C. In addition, 0.9Al₂O₃-0.1TiO₂ ceramic samples sintered at 1550 °C fabricated by DLP method presented much better microwave dielectric properties: ε_r = 11.30 ± 0.02, Q × f = 35,345 ± 143 GHz (@～12 GHz), τ_f = 2.16 ± 0.21 ppm/°C than that of by dry pressing method: ε_r = 11.16 ± 0.11, Q × f = 30,195 ± 257 GHz (@～12 GHz), τ_f = 4.45 ± 0.13 ppm/°C, especially the Q × f value achieved a 17% increase. Accordingly, DLP technique, which has advantages of producing relatively high properties and complex geometry of microwave dielectric ceramics as well as without extra high-cost mold, greatly satisfies application requirements.     

Microstructure and dielectric properties of perovskite-structured high-entropy ceramics of Pb(Zr0.25Ti0.25Sn0.25Hf0.25)O3

A dielectric high-entropy ceramic with a composition of Pb(Zr_0.25Ti_0.25Sn_0.25Hf_0.25)O₃ was designed through B-site doping, and then prepared by solid phase reaction method combined with conventional sintering in air for 3 h at 1200 ^°C, 1250 ^°C and 1300 ^°C, respectively. All the high-entropy ceramics of Pb(Zr_0.25Ti_0.25Sn_0.25Hf_0.25)O₃ possess a perovskite structure with uniform elemental distribution and their average grain size falls within the range of 3.19–5.5 μm. For the sample sintered at 1250 ^°C, the dielectric loss is less than 0.07 in the testing frequency of 1 kHz∼1 MHz in 30–350 ^°C, and the dielectric constant reaches a peak of 14356 at about 270 ^°C at 1 kHz. At room temperature, the remnant polarization P_r reaches 28.8 μC/cm². The results demonstrate that the high-entropy ceramic of Pb(Zr_0.25Ti_0.25Sn_0.25Hf_0.25)O₃ has great potentials in the dielectric and ferroelectric field.     

Improved sintering behavior and microwave dielectric properties of SnO2-based ceramics and their LTCC materials with ultrawide temperature stability

Microwave dielectric ceramics with simple composition, a low permittivity (ε_r), high quality factor (Q × f) and temperature stability, specifically in the ultrawide temperature range, are vital for millimetre-wave communication. Hence, in this study, the improvements in sintering behavior and microwave dielectric properties of the SnO₂ ceramic with a porous microstructure were investigated. The relative density of the Sn_1-xTi_xO₂ ceramic (65.1%) was improved to 98.8%, and the optimal sintering temperature of Sn_1-xTi_xO₂ ceramics reduced from 1525 °C to 1325 °C when Sn⁴⁺ was substituted with Ti⁴⁺. Furthermore, the ε_r of Sn_1-xTi_xO₂ (0 ≤ x ≤ 1.0) ceramics increased gradually with the rise in x, which can be ascribed to the increase in ionic polarisability and rattling effects of (Sn_1-xTi_x)⁴⁺. The intrinsic dielectric loss was mainly controlled by r_c (Sn/Ti–O), and the negative τ_f of the SnO₂ ceramic was optimised to near zero (x = 0.1) by the Ti⁴⁺ substitution for Sn⁴⁺. This study also explored the ideal microwave dielectric properties (ε_r = 13.7, Q × f = 40,700 GHz at 9.9 GHz, and τ_f = ?7.2 ppm/°C) of the Sn_0.9Ti_0.1O₂ ceramic. Its optimal sintering temperature was decreased to 950 °C when the sintering aids (ZnO–B₂O₃ glass and LiF) were introduced. The Sn_0.9Ti_0.1O₂-5 wt% LiF ceramic also exhibited excellent microwave dielectric properties (ε_r = 12.8, Q × f = 23,000 GHz at 10.5 GHz, and τ_f = ?17.1 ppm/°C). At the ultrawide temperature range (?150 °C to +125 °C), the τ_ε of the Sn_0.9Ti_0.1O₂-5 wt% LiF ceramic was +13.3 ppm/°C, indicating excellent temperature stability. The good chemical compatibility of the Sn_0.9Ti_0.1O₂-5 wt% LiF ceramic and the Ag electrode demonstrates their potential application for millimetre-wave communication.     

Phase transition,microstructure, and dielectric properties of Li/Ta/Sb co-doped (K,Na)NbO3 lead-free ceramics

Li/Ta/Sb co-doped lead-free (K_0.4425Na_0.52Li_0.0375)(Nb_0.93−xTa_xSb_0.07)O₃ (abbreviated KNLNST_x) piezoelectric ceramics, with Ta-doping ratio of x ranging from 0.0275 to 0.0675, were synthesized using the conventional solid-state reaction method at the sintering temperature of 1130 °C. The effects of Ta content on the microstructure, dielectric properties, and phase transition behavior of the prepared ceramics were systematically investigated. The X-ray diffraction results show that all KNLNST_x ceramics formed a secondary phase, which is assigned to the tetragonal tungsten-bronze type (TTB) structure phase, and showed a phase transition from an orthorhombic symmetry to a tetragonal symmetry across a composition region of 0.0375<x<0.0475. The grain shape and size that correspond to the phase structure transformations can be clearly observed in the scanning electron microscopy images. As x increased to 0.0475, the KNLNST_0.0475 ceramics changed from orthorhombic to tetragonal structure and showed excellent piezoelectric properties of d₃₃=313 pC/N, k_p=47%, and ε_r=1825. By contrast, samples of x=0.0375 with orthorhombic symmetry exhibited poor piezoelectric properties, with d₃₃=200 pC/N and ε_r=1015. These results indicate that phase structure is vital in the piezoelectric properties of KNN lead-free ceramics.     

Effects of cerium doping on the microstructure,mechanical properties,thermal conductivity,and dielectric properties of ZrP2O7 ceramics

A two-step method, combined with cold isostatic pressing, was used to prepare CeO₂-doped ZrP₂O₇ ceramics, and their microstructure, mechanical properties, thermal conductivities, and dielectric properties were determined. It was found that CeO₂ doping could increase the Zr–P and P–O bond lengths, which in turn decreased the thermal conductivity of the ZrP₂O₇ matrix. Doping with 12 wt% CeO₂ simultaneously reduced the sintering temperature and improved the mechanical properties of the ZrP₂O₇ ceramics, while retaining its low thermal conductivity and good dielectric properties. The maximum cold modulus of rupture of a sample at 1250 °C was 75.91 MPa, which met most conditions for use at room temperature. A COMSOL model was used to predict the thermal conductivity, based on the microstructure, with a relatively high degree of accuracy. The thermal conductivity of the CeO₂-doped samples was lower than 1.083 W/(m·K). The dielectric constant was in the range of 5.93–6.52 at 20–40 GHz, and the dielectric loss was less than 4 × 10^?3. The ZrP₂O₇-doped ceramics have potential for application in millimetre wave technology, satellite communication, and vehicle radar fields, because they can meet the high thermal insulation requirements for these applications.