Microstructure and electrical properties of ZnO–Bi2O3-based varistor ceramics by different sintering processes

The effect of sintering processes, such as open sintering, sintering inside a closed crucible, and sintering within a powder bed, on the microstructure and V–I characteristics of ZnO–Bi₂O₃-based varistor ceramics was investigated at sintering temperatures in the range 1000–1200 °C. The results from the experiments showed that the microstructure and electrical properties of the samples varied according to the sintering method and temperature. Optimal values for the electrical characteristics of the varistor ceramics by different sintering processes were obtained when the sintering was conducted at 1100 °C. At the same sintering temperature, the different processes affected the properties differently. At 1000 °C, the samples sintered within a powdered bed showed better electrical properties than those subjected to the other two processes, while at 1100 or 1200 °C, the samples sintered in an open crucible exhibited the best electrical properties.     

Effects of Sb2O3 on the microstructure and electrical properties of ZnO–Bi2O3-based varistor ceramics fabricated by two-step solid-state reaction route

In this work, the ZnO–Bi₂O₃–Cr₂O₃–Co₂O₃–MnO₂ varistors doped with different content of Sb₂O₃ were prepared by two-step solid-state reaction route, including a pre-calcining of the mixtures of nanosized ZnO and the other additives at an optimized temperature, followed by a consequent sintering process at different temperatures. Meanwhile, the effects of Sb₂O₃ on the sintering temperature, microstructure and electrical properties of the objective varistors were investigated. It was found the densification temperature went up in a proper range and the content of pyrochlore phase, spinel phase and β-Bi₂O₃ phase increased with the increasing content of Sb₂O₃, while the grain size of ZnO–Bi₂O₃-based varistor reduced. The results demonstrated that at the same sintering temperature, the second particles increased with the increasing amount of Sb₂O₃, which was helpful to control the grain growth, leading to a higher breakdown voltage. However, the decrease of α-Bi₂O₃ phase (melting point of α-Bi₂O₃ phase is 825 °C), which is the main component of the liquid Bi₂O₃ phase in the sample during sintering process, leads to the increase of the sintering temperature of the green pallet. As a result, the ZnO varistor doped with 3.0 mol% Sb₂O₃ sintered at 1000 °C exhibited the highest breakdown voltage of 1863.3 V/mm. By contrast, the ZnO varistor without Sb₂O₃ doping sintered at 900 °C had the optimum nonlinear coefficient of 59.8.     

Effect of a Bi–Cr–O synthetic multi-phase on the microstructure and electrical properties of ZnO–Bi2O3 varistor ceramics

ZnO–Bi₂O₃–MnO₂ (ZBM)-based varistors were fabricated via doping a novel synthetic multi-phase (SMP) additive produced by calcining the mixture of 18Bi₂O₃·Cr₂O₃ at a given temperature. The effects of the SMP on the microstructural and electrical properties of ZBM varistors were investigated. It was found that the SMP dopant was a compound crystalline phases including Bi–Cr–O phases (Bi_7.38Cr_0.62O_12+x and CrBi₁₈O₃₀) and small amounts of Bi₂O₃ rather than a synthesized polycrystal. The Bi–Cr–O phases were not emerged for samples with x=1, indicating that the amount of it is tiny and the small Bi₂O₃ may accelerate ZnO grain growth. With more SMP doping (x>1) in the ZBM ceramics, it acted as a barrier inhibiting grain growth. For samples with x=5, excellent electrical properties were obtained: the nonlinear coefficient α increased up to 50.19 corresponding to the highly barrier height of 2.62 eV; the leakage current I_L reduced to 0.3 μA. The dielectric constant ε_a is proportional to the ratio of the grain size d to the thickness of the depletion layer width t, which explained the ε_a increased at f=1 kHz for the samples with x=1 and 5. The improvement of the electrical properties can be explained by the oxygen absorption mechanism.     

Influence of Cr2O3 on highly nonlinear properties and low leakage current of ZnO–Bi2O3 varistor ceramics

ZnO–Bi₂O₃–Sb₂O₃–Co₂O₃–MnO₂–xCr₂O₃ (ZBSCM–xCr₂O₃, 0≤x≤0.6 mol%) varistors were fabricated through the conventional solid state method, and the effects of Cr₂O₃ on the microstructures and electrical properties were investigated. Results showed that the secondary phases CrBi₁₈O₃₀ and Co₂Cr_0.5Sb_0.5O₄ emerged when x ranges from 0.2 to 0.4. In these compositions, Cr₂O₃ acted as a donor and decreased the electrical properties of ZBSCM. For samples with x=0.5, the secondary phases transformed to MnCr₂O₄ and the electrical properties increased significantly: the nonlinear coefficient α sharply increased up to 80.71 and the barrier height ϕ_b reached 3.88 eV. This indicates that the donor effect of Cr₂O₃ disappeared. In addition, with the increase of Cr₂O₃, the average grain size of ZnO decreased from 7.48 μm to 5.46 μm, which in turn resulted in an increase of breakdown voltage E_1mA from 216.17 V/mm to 362.50 V/mm. Besides, all the samples showed the low value of leakage current of lower than 0.1 μA. This varistor might be a promising candidate for highly effective applications.     

Effect of MnO2 addition on microstructure and electrical properties of ZnO–V2O5-based varistor ceramics

The microstructure and electrical properties of ternary system ZnO–0.5 mol% V₂O₅–MnO₂ ceramics sintered were investigated in accordance with MnO₂ content by sintering at 900 °C. For all samples, the microstructure of the ternary system ZnO–V₂O₅–MnO₂ ceramics consisted of mainly ZnO grain and secondary phase Zn₃(VO₄)₂. The incorporation of MnO₂ to the binary system ZnO–V₂O₅ ceramics was found to restrict the abnormal grain growth of ZnO. The breakdown field in the E–J characteristics increased from 175 to 992 V/cm with the increase of MnO₂ content. The incorporation of MnO₂ improved non-ohmic properties by increasing non-ohmic coefficient. The highest non-ohmic coefficient (27.2) in the ternary system ZnO–0.5 mol% V₂O₅–MnO₂ was obtained for MnO₂ content of 2.0 mol%.     

Electrical impedance spectroscopy and structural characterization of liquid-phase sintered ZnO–V2O5–Nb2O5 varistor ceramics doped with MnO

The influence of the MnO content on the microstructure and the electrical response of liquid-phase sintered ZnO–V₂O₅ varistor ceramics wereanalysed using A.C. impedance spectroscopy on samples prepared via the conventional solid state route.The impedance spectra were analysed with the help of one model equivalent circuit at high frequencies and another at low frequencies, involving both resistor and non-Debye constant phase elements (CPEs). The results indicate a significant contribution of grain boundary resistance to the total resistance and non-ohmic characteristic of the studied materials. The Arrhenius plots show two slopes with a turnover at 150 °C/200 °C for both the higher- and lower-frequency time constants. These behaviours can be related to the decrease of the minor charge carrier density. These activation energies were associated with the adsorption and reaction of O₂ (as well as V) species at the grain boundary interface. Consequently, better varistor performance is achieved for 96.9 mol% ZnO+0.5 mol% V₂O₅+0.10 mol% Nb₂O₅+2.5 mol% MnCO₃ with nonlinear coefficient α=21.6, breakdown field E_1mA=191.47 V/mm, leakage current density J_L=36.46 µA/cm² and activation energies of 0.639 eV and 0.644 eV. X-ray diffraction shows that in addition to the major ZnO phases, Zn₃(VO₄)₂ and ZnV₂O₄, were detected as minor secondary phases. SEM analysis of the morphology shows that the grain growth increases with increases in the MnO doping level.     

The transition from non-ohmic to ohmic characteristics in La2O3 doped ZnO–MgO–TiO2 linear resistance ceramics

La₂O₃ doped ZnO–MgO–TiO₂ based linear resistance ceramics were prepared by the solid phase sintering method. The doping content of La₂O₃ is from 0.0 wt% to 2.5 wt%. The solubility of La₂O₃ in ZnO is less than 0.06 mol% (0.5 wt%), La_0.66TiO_2.993 phases will be formed at grain boundary and change the distribution of spinel phase when La₂O₃ is excessive. For I–V test, undoped sample exhibits typical non-ohmic characteristics, but La-doped samples show excellent ohmic behaviors under low DC and high pulse current (PC). The complex impedance spectrum and the frequency dependent conductivity furtherly demonstrate that La-doped samples possess linear characteristics because there is no grain boundary effect which can affect the electron transmission at grain boundaries. Besides, the decrease of the grain boundary barrier from 0.2135eV for undoped sample to 0.0031eV for 0.5 wt% La doped samples can account for the elimination or reduction of grain boundary effect. In this work, the transition from non-ohmic to ohmic properties by doping La₂O₃ in ZnO–MgO–TiO₂ multiphase ceramics is realized.     

Charge compensation and electrical characteristics of Ta2O5–doped SnO2–CoO ceramics

We investigated the effect of pentavalent donor dopant Ta₂O₅ on microstructure development, electric and dielectric characteristics of SnO₂–CoO based ceramics. Already low additions of Ta₂O₅ (0.05 mol%) effectively reduce the porosity, improve densification and dielectric permittivity and trigger a 3–fold increase in SnO₂ growth rate. Rietveld analysis shows that the amount of Co₂SnO₄ spinel phase drops with the addition of Ta₂O₅ due to incorporation of Co²⁺ and Ta⁵⁺ into SnO₂ structure. With higher additions, however, Ta₂O₅ segregates to the grain boundaries and hinders SnO₂ grain growth, which in turn improves electrical properties. TEM/EDS analysis shows that above 0.5 mol% of Ta₂O₅ the Co:Ta ratio in SnO₂ grains is constant 1:2, which means that a twice lower amount of Ta⁵⁺ is incorporated in the SnO₂ structure compared to the Nb₂O₅-doped SnO₂–CoO system. Accordingly, the following charge compensation mechanism is proposed: 3 Sn(IV)_S^˟_{n (IV)} ⇋ Co(II)_Sn ̎_(IV) + 2 Ta(V)˙_{Sn (IV)}.     

Microstructure and electrical characteristics of ZnO–B2O3–PbO–V2O5–MnO2 ceramics prepared from ZnO nanopowders

A peculiar kind of ZnO–B₂O₃–PbO–V₂O₅–MnO₂ ceramics was produced from the ZnO nanopowders directly co-doped with the oxides instead of lead zinc borate frit in this investigation. The 8 wt.% (PbO+B₂O₃) co-doped ceramics sintered at 950 °C for 2 h displayed the optimum electrical properties, that is, leakage current density J_L=6.2×10⁻⁶ A/cm², nonlinear coefficient α=22.8 and breakdown voltage V_BK=331 V/mm. The co-doping of 8 wt.% (PbO+B₂O₃) resulted in an increase in nonlinear coefficient and a decrease in leakage current density of the ZnO–V₂O₅ varistors while the sintering temperature showed no evident influence on nonlinear coefficient and leakage current density at the range of 800–950 °C.     

Optical and structural properties of ZnO NPs and ZnO–Bi2O3 nanocomposites

Pure ZnO and ZnO–Bi₂O₃ nanocomposites with 5 wt% and 10 wt% of Bi₂O₃ content were synthesized using the co-precipitation method. Optical properties such as refractive index (n), extinction coefficient (k), bandgap (E_g), and Urbach energies, as well as the band structure, were determined by modeling the experimental transmittance and reflectance UV–Vis spectra. The deduced bandgap and Urbach energies for pure ZnO (3.758 eV) increase with the increase of the doping degree of Bi₂O₃ in ZnO–Bi₂O₃ nanocomposite films. X-ray diffraction and scanning electron microscopy (SEM) was used to study the structural and morphological properties of these nanocomposite films. Pure ZnO and nanocomposites with Bi₂O₃ exhibit crystalline domains with wurtzite hexagonal structures, and as the doping degree of Bi₂O₃ increases, the crystallite size decreases. Based on SEM micrographs, the ZnO nanoparticles (NPs) structure shows the presence of aggregation. Moreover, Bi₂O₃ NPs in the nanocomposite film led to the further aggregation in the form of large rods. The elemental and chemical properties of the nanocomposites were investigated using infrared and energy-dispersive X-ray spectroscopy. The charge transfer process in the studied system is between ZnO and Bi₂O₃ conduction bands. Density-functional theory (DFT) calculations were performed for ZnO, Bi₂O₃, and ZnO-Bi₂O₃ compounds to investigate structural, optical, and electronic properties, being in agreement with the experimental results.     

Ge-added TiO2–Ta2O5–CaCO3 varistor ceramics

The influence of doping with Ge on the nonlinear coefficient α and the breakdown electric field E_B of TiO₂–Ta₂O₅–CaCO₃ varistor ceramics was investigated. In this study, TiO₂–Ta₂O₅–CaCO₃ varistor ceramics added with Ge was successfully prepared using the traditional method of ball milling–molding–sintering. The electrical performance, including the nonlinear coefficient α, the breakdown electric field E_B, and the leakage current J_L, are tested using a varistor direct current parameter instrument. The average barrier height Φ_B of each sample is calculated using the relevant formula. X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and scanning transmission electronic microscopy analyses demonstrated that Ge doping notably changed the microstructure of TiO₂–Ta₂O₅–CaCO₃ ceramics, thereby increasing α and decreasing E_B. When the doping contents of Ta₂O₅ and CaCO₃ were 0.2 and 0.4 mol%, respectively, the optimum doping content of 0.9 mol% Ge exhibited high α (10.2), low E_B (14.1 V mm⁻¹), and high Φ_B (0.95 eV). These results are superior to previous findings. In addition, Ge as sintering aid reduced the sintering temperature caused by the low melting point. The optimal sintering temperature was 1300 °C for the TiO₂–Ta₂O₅–CaCO₃ ceramics doped with Ge.     

Viscous behaviour of Bi2O3–B2O3–ZnO glass composites with ceramic fillers

Abstract

Variation in the viscous flow behaviour, nature and extent of glass fluidity in glass/filler composites are addressed with respect to various factors such as filler type, content, size, density and migration distance. The characterisation of a glass (Bi₂O₃–B₂O₃–ZnO) composite consisting of two different fillers (cordierite and willemite) was determined using hot stage microscopy, a differential scanning calorimeter and a flow button test. The microstructure was analysed using a scanning electron microscope. The apparent viscosity of the glass composites increased on increasing concentration and density of the filler. The variation in the viscosity is due to the diffusion of the glass matrix through channels in the cordierite filler of the composite. Based on the calculated migration distance of the filler in the glass matrix, the present work suggests that the interfacial behaviour and the density of the filler play a significant role in determining the viscous flow of the glass composites.     

Electrical and dielectric characteristics of erbium-added ZnO–V2O5-based varistor ceramics

This paper focuses on that the erbium-added ZnO–V₂O₅-based ceramics are attained at a sintering temperature as low as 875 °C. The effect of Er₂O₃ addition on microstructure, electrical properties, and dielectric characteristics has been investigated. Increasing the amount of Er₂O₃ slightly increased the densities of sintered pellets in the range of 5.52–5.59 g/cm³. The increase in the amount of Er₂O₃ increased the breakdown field from 6991 to 7408 V/cm up to 0.1 mol%, whereas a further addition decreased it. The sample added with 0.1 mol% Er₂O₃ exhibited the highest nonlinear coefficient (α=55) and the sample added with 0.25 mol% Er₂O₃ exhibited the lowest nonlinear coefficient (α=14). The donor concentration increased from 2.92×10¹⁷ to 8.48×10¹⁷ cm⁻³ with an increase in the amount of Er₂O₃.     

The structure and properties of 0.95MgTiO3–0.05CaTiO3 ceramics co-doped with ZnO–ZrO2

The (Mg_0.93Ca_0.05Zn_0.02)(Ti_1?xZr_x)O₃ ceramics were prepared by conventional solid-state route. The dielectric properties and structure of (Mg_0.93Ca_0.05Zn_0.02)(Ti_1?xZr_x)O₃ ceramics were investigated. It has been found that MgTiO₃ and CaTiO₃ are the main phases and a second phase CaZrTi₂O₇ appeared in 95MCT ceramics co-doped with Zn–Zr. With Zn–Zr additive, the sintering temperature of 95MCT ceramics can be reduced to 1300 °C, and adjust the temperature coefficient of dielectric constant. With the increasing of Zr content, dielectric constant ?_r decrease from 22.6 to 19.91 and the temperature coefficient of dielectric constant α_c from 5.93 to 2.52 ppm/°C when x = 0.01, 0.02, 0.03 and 0.04 mol respectively. The 95MCT ceramics with x = 0.02 has a dielectric constant ?_r of 22.02, a dielectric loss of 2.78 × 10^?4 and a temperature coefficient of dielectric constant α_c value of 2.98 ppm/°C.     

Fabrication and properties of pressure-sintered reaction-bonded Si3N4 ceramics with addition of Eu2O3–MgO–Y2O3

Dense pressure-sintered reaction-bonded Si₃N₄ (PSRBSN) ceramics were obtained by a hot-press sintering method. Precursor Si powders were prepared with Eu₂O₃–MgO–Y₂O₃ sintering additive. The addition of Eu₂O₃–MgO–Y₂O₃ was shown to promote full nitridation of the Si powder. The nitrided Si₃N₄ particles had an equiaxial morphology, without whisker formation, after the Si powders doped with Eu₂O₃–MgO–Y₂O₃ were nitrided at 1400 °C for 2 h. After hot pressing, the relative density, Vickers hardness, flexural strength, and fracture toughness of the PSRBSN ceramics, with 5 wt% Eu₂O₃ doping, were 98.3 ± 0.2%, 17.8 ± 0.8 GPa, 697.0 ± 67.0 MPa, and 7.3 ± 0.3 MPa m^1/2, respectively. The thermal conductivity was 73.6 ± 0.2 W m^?1 K^?1, significantly higher than the counterpart without Eu₂O₃ doping, or with ZrO₂ doping by conventional methods.     

Influence of Ge and GeO2 on the microstructure and varistor properties of TiO2–Ta2O5–CaCO3 ceramics

This study investigated the effect of elemental crystal Ge or/and GeO₂ doping on the microstructure and varistor properties of TiO₂–Ta₂O₅–CaCO₃ varistor ceramics, which were prepared via the traditional ball milling–molding–sintering process. X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy, scanning electron microscopy-energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy demonstrated that co-doping with Ge and GeO₂ changed the microstructure of TiO₂–Ta₂O₅–CaCO₃ ceramics, thereby increasing the nonlinear coefficient and decreasing the breakdown voltage. The optimum doping concentrations of Ta₂O₅, CaCO₃, Ge, and GeO₂ exhibited the highest nonlinear coefficient (α=14.6), a lower breakdown voltage (E_B=18.7 V mm⁻¹), the least leakage current (J_L=10.5 μA cm⁻²), and the highest grain boundary barrier (Φ_B=1.05 eV). In addition, Ge and GeO₂ function as sintering aids, which reduce the sintering temperature because of their low melting points.     

Dielectric behaviors of Nb2O5–Co2O3 doped BaTiO3–Bi(Mg1/2Ti1/2)O3 ceramics

Nb₂O₅ and Nb–Co doped 0.85BaTiO₃–0.15Bi(Mg_1/2Ti_1/2)O₃ (0.85BT–0.15BMT) ceramics were investigated. From XRD patterns, undesired phase was observed when the (Nb₂O₅/Nb-Co) doping levels exceed 3 wt.%/2 wt.%, giving rise to the deteriorate dielectric constant. The 0.85BT–0.15BMT ceramics doped with 2 wt.%Nb₂O₅ was found to possess a moderate dielectric constant (? ～ 1000) and low dielectric loss (tan δ = 0.9%) at room temperature and 1 kHz, showing flat dielectric behavior over the temperature range from ?55 to 155 °C. It was found that the formation of core–shell structure in the BT based ceramics is controlled by the doping sequence of Nb- and Bi-oxides.     

Effect of ZnO on the microstructure and electrical properties of WO3–Bi4Ti3O12 ceramics

The aim of the present work is to explore the possibility of incorporate a small amount of ZnO to improve the microstructure control of W-doped BIT-based materials. Two different processing routes have been used according to previous results reported for other materials: reaction and sintering in one single step and a previous calcination step. The sintering behaviour of the samples, the obtained crystalline phases and the microstructure analysis indicate that the reaction between ZnO and Bi₂O₃ plays a critical role during sintering. Both Bi₂Ti₂O₇ and Zn₂TiO₄ secondary phases are stabilized when adding ZnO. Actually, when WO₃ and ZnO are incorporated simultaneously to BIT materials, they interact stabilizing the Bi₂Ti₂O₇ phase and avoiding the incorporation of W⁶⁺ into the BIT lattice. As a consequence, the electrical conductivity of the samples with ZnO is two orders of magnitude higher than that of the samples doped only with WO₃, suggesting that WO₃ does not form a solid solution with BIT. The curve dielectric constant vs temperature also reveals the role played by the Bi₂Ti₂O₇ phase.     

Effect of adding Y2O3 on structural and mechanical properties of Al2O3–ZrO2 ceramics

The effects of adding 1–8 wt% Y₂O₃ on phase formation and fracture toughness of Al₂O₃–xZrO₂–Y₂O₃(AZY) ceramics were studied. Phase formations of the samples were characterized by the X-ray diffraction (XRD) technique. It was found that the major phase was rhombohedral-Al₂O₃, while the minor phase consisted of the monoclinic-ZrO₂, tetragonal-ZrO₂ and monoclinic-Y₂O₃. It was found that Y₂O₃ contents did not clearly influence grain shape of AZY ceramics. The results obtained from the microhardness test could be used to evaluate the fracture toughness. It was found that the smaller grains had high fracture toughness. The maximum fracture toughness of 4.827 MPa m^1/2 was obtained from 4 wt% Y₂O₃. Refinement of lattice parameters using Rietveld analysis revealed the quantitative phases of AZY ceramics. This shows that under adding Y₂O₃ conditions the proportion of tetragonal-ZrO₂ phase plays an important role for the mechanical properties of AZY ceramics.