Effects of SiO2 and Cr2O3 on the formation process of ZnO varistors

The effects of SiO₂ and Cr₂O₃ on the formation process of ZnO varistors were investigated. Prior to formation of the Zn_2.33Sb_0.67O₄ spinel phase (Sp-phase), a spinel-like phase forms. However, this phase does not control the varistor microstructure. The Sp-phase and the Bi₂O₃-phase which were formed by the decomposition of the Bi₂(Zn_4/3Sb_2/3)O₆ pyrochlore phase played important parts in the control of the varistor microstructure. That is, the Bi₂O₃ phase produced in the reaction promotes the initial sintering of the varistor and the Sp-phase inhibits the ZnO grain growth. In this reaction, SiO₂ and Cr₂O₃ play a role in decreasing the decomposition temperature of the pyrochlore phase. Decreasing the decomposition temperature below 900° C (where ZnO grain growth begins) leads to the inhibition of ZnO grain growth.     

Microstructural development of ZnO varistor during reactive liquid phase sintering

The microstructural evolution, grain growth and densification for the varistor systems ZnO-Bi₂O₃ (ZB), ZnO-Bi₂O₃-Sb₂O₃ (ZBS), ZnO-Bi₂O₃-Sb₂O₃-MnO-Cr₂O₃-CoO (ZBSCCM) were studied using constant heating rate sintering, scanning electron microscopy (SEM) andin situ phase formation measurement by high temperature X-ray diffraction (HT-XRD). The results showed that the densifying process is controlled by the formation and decomposition of the Zn₂Bi₃Sb₃O₁₄ pyrochlore (PY) phase for the ZBS and ZBSCCM systems. The addition of transition metals (ZBSCCM system) alters the formation and decomposition reaction temperatures of the pyrochlore phase and the morphology of the Zn₇Sb₂O₁₂ spinel phase. Thus, the spinel grains act as inclusions and decrease the ZnO grain growth rate. Spinel grain growth kinetics in the ZBSCCM system showed ann value of 2.6, and SEM and HT-XRD results indicate two grain growth mechanisms based on coalescence and Ostwald ripening.     

On the phases formation in the calcination process of ZnO varistor

The degree of phase formation was investigated by Sb₂O₃ behaviour in the calcination process, and the change of microstructure and breakdown properties due to the phases formation in ZnO varistor. The samples were calcined at several temperatures for 2 h and were sintered at 1150 to 1300 ° C, respectively, for 1 h. After that,V-I characteristics were investigated. Phase analysis by X-ray diffraction and microstructures observation by SEM were done. As a result, spinel phase was not formed and low nonlinear resistance was shown in the samples without Sb₂O₃. In the samples containing Sb₂O₃, it was shown that the pyrochlore and spinel phase are formed at the conventional calcination temperature or even below that temperature. This primary spinel phase and the spinel phase transformed from pyrochlore phase in sintering process inhibit ZnO grain growth, and so nonlinear resistances should be changed. Hence ZnO grain growth in ZnO-based varistor system is strongly dependent on the Sb₂O₃ behaviour in the calcination process.     

Effect of chromium on the phase evolution and microstructure of ZnO doped with bismuth and antimony

Crystalline phase formation and microstructure of ZnO varistors with a basic composition ZnO–Bi₂O₃–Sb₂O₃ were examined. Addition of chromium oxide to this basic varistor resulted in an α-spinel (α-Zn₇Sb₂O₁₂) phase dissolving a significant amount of Cr, while the β-spinel did not. β-spinel transformed to pyrochlore during cooling, whereas α-spinel hardly transformed to pyrochlore irrespective of the cooling conditions. When Sb₂O₃ was completely replaced by Cr₂O₃, ZnCr₂O₄ was formed instead of spinel. α-spinel particles were 1–2 μm in size and intra- as well as intergranular. ZnCr₂O₄ particles, smaller than 1 μm in size, however, were present as aggregates in the bismuth-rich matrix phase at the grain boundaries. This revised version was published online in November 2006 with corrections to the Cover Date.     

The effect of secondary phases on electrical properties of ZnO-based ceramic films prepared by a sol–gel method

The ZnO-based ceramic films doped with Bi₂O₃, Sb₂O₃, MnO, Co₂O₃ and Cr₂O₃ were prepared for use as film varistors by a sol–gel method. The formation and the changes of the phases in the films doped with different dopants and annealed at different temperatures were investigated via X-ray diffraction analysis. Three secondary phases, i.e., Bi₂O₃, Zn₇Sb₂O₁₂ spinel and ZnCr₂O₄, were detected in the films when the annealing temperature was above 550 °C. The lattice constants of ZnO and Zn₇Sb₂O₁₂ spinel phase changed with dopants and the annealing temperature, indicating that the diffusion of the ions into the crystals of ZnO and spinel phase had taken placed. The redistribution of the ions changed the constituents of the intergranular phases and the relevant defect species in ZnO grains, and affected intensively the electrical properties of the films, which were used as film varistors. The highest nonlinear coefficient () with the lowest leakage current was achieved when the film, which was doped with Bi₂O₃, Sb₂O₃, MnO and Cr₂O₃, was annealed at 750 °C.     

A high-resolution transmission electron microscope study of a zinc oxide varistor

Investigations were made of varistor microstructure, the morphology of Bi₂O₃ at multiple ZnO grain junctions, Bi₂O₃/ZnO grain boundaries and ZnO/ZnO grain boundaries (especially whether Bi₂O₃ is present or not at the ZnO/ZnO grain boundary) by means of high-resolution transmission electron microscopy and X-ray microanalysis in the scanning transmission electron microscope. Bi₂O₃ at multiple ZnO grain junctions consists of small particles of 0.1m in diameter, and they are vitrified to some extent. It is suggested that bismuth ions dissolve into ZnO grains over a 30 nm range from a Bi₂O₃/ZnO grain boundary; however, there is no bismuth at ZnO/ZnO grain boundaries.     

Structure and dielectric properties of Nd substituted Bi1.5MgNb1.5O7 ceramics

New pyrochlore ceramics Bi_(1.5?x)Nd _x MgNb_1.5O₇ with x = 0–0.6 were synthesized by doping Nd into the A site in the Bi_1.5MgNb_1.5O₇ pyrochlore. The Nd substituted Bi_1.5MgNb_1.5O₇ ceramics were prepared utilizing solid-state reaction and investigated by X-ray diffraction, scanning electron microscope and dielectric measurements. The crystal structure was characterized as single cubic pyrochlore phase for x ≤ 0.5 while the second phase appeared at x = 0.6. With increasing Nd content from 0 to 0.5, the lattice parameters decreased slightly and the average grain size pronouncedly reduced from ~7.5 to ~5.0 μm. The Nd incorporation resulted in attractive enhancement of the dielectric properties including the temperature coefficient of dielectric constant that was remarkably optimized with the lowest value of ?16 ppm/°C at x = 0.5. A good combination of low dielectric loss, superior temperature coefficient and high dielectric permittivity can be achieved for the composition of x = 0.5, which seems to be very appealing for the practical use in high-frequency multilayer ceramic capacitors.     

Co-precipitation technique for the preparation of nanocrystalline ferroelectric SrBi2Ta2O9

A simple co-precipitation technique had been successfully applied for the preparation of pure ultrafine single phase SrBi₂Ta₂O₉. Ammonium hydroxide was used to precipitate Sr²⁺, Bi³⁺ and Ta⁵⁺ cations simultaneously. No pyrochlore phase was found while heating powder at 800 °C and pure SrBi₂Ta₂O₉ phase was found to be formed by X-ray diffraction. Particle size and morphology was studied by scanning electron spectroscopy. Ferroelectric hysteresis loop parameters of these samples were also studied.     

Investigations of chemical interaction between Bi-based 2212 and (RE)Ba2Cu3O7 high Tc superconducting materials

Composite materials have been synthesized by mixing 90% (or 95%) YBa₂Cu₃O₇ and 10% (or 5%) Bi₂Sr₂Ca₁Cu₂O₈ by weight, and firing at 900°C to promote grain growth by inducing a liquid phase (Bi₂Sr₂Ca₁Cu₂O₈) in the system. The influence of the amount of liquid phase on the X-ray diffraction data and electrical properties is reported. Energy dispersive X-ray (EDX) analyses are also reported. The YBiBa₂O₆ phase is formed during the heat treatment and introduces additional chemical heterogeneities at the grain boundaries. A previously reported 2212-related superconducting phase, Bi₂(Sr,Ba)₂(Ca,Y)Cu₂O_8+y, could also be formed during the synthesis process, and its effect on the electrical resistance versus temperature measurements is discussed. Attempts to substitute RE ions (Dy³⁺, Er³⁺, Ho³⁺) for Y³⁺ in YBiBa₂O₆ have been successful and are reported in an appendix section. X-ray diffraction data are also reported. EDX analyses have been performed specifically for a typical ErBiBa₂O₆ compound and reveal the presence of a new Er₂Ba₄O₇ phase.     

Effect of powder bed on the microstructure and electrical properties of Bi2O3- and Sb2O3-doped ZnO

The effects of powder bed on the microstructure and electrical properties of Bi₂O₃- and Sb₂O₃-doped ZnO specimens are investigated in the present study. By using a sufficient amount of powder bed, the weight loss of Bi₂O₃ reduces from >95 to <20%. The reduction of weight loss enhances significantly the densification and grain growth of ZnO. Furthermore, the use of powder bed can also reduce the size distribution of ZnO grains. Nevertheless, the presence of Bi₂O₃ residue results in the formation of a pyrochlore phase (Zn₂Bi₃Sb₃O₁₄) during the cooling down stage; its presence is detrimental to the nonlinear characteristics of ZnO–Bi₂O₃–Sb₂O₃ system.     

Perovskite phase formation in the PbMg1/3Nb2/3 O3-PbZrO3-PbTiO3 system by the columbite route

The reaction mechanism of PbMg_1/3Nb_2/3O₃-PbZrO₃-PbTiO₃ (PMN-PZT) perovskite phase prepared by the columbite route has been studied in the temperature range from 600 to 800 °C. The effects of heating and cooling rate during the calcination of 3PbO +MgNb₂O₆+PZT powder mixtures have also been investigated. Nearly pure perovskite phase, 0.9 PMN-0.1 PZTsolid solution with no pyrochlore phase, as determined by X-ray diffraction, could be prepared at 800 °C for 2 H. From DTA/TGA, dilatometry and XRD data the reaction mechanism of PMN-PZT solid solution formation could be divided into three steps: (i) decomposition of columbite (MgNb₂O₆) by reacting with PbO at 350 to 600 °C (ii) the formation of a B-site-deficient pyrochlore phase Pb₂Nb_1.33Mg_0.17O_5.50 at close to 650 °C, and (iii) the formation of perovskite phase PMN-PZT solid solution from the reaction of Pb₂Nb_1.33Mg_0.17O_5.50 pyrochlore phase with MgO and PZT above 650 °C.     

Varistor characteristics of vanadium oxide-doped zinc oxide ceramics modified with bismuth oxide

The effect of Bi₂O₃ on microstructure, electrical properties, dielectric characteristics, and aging behavior vanadium oxide-doped zinc oxide varistor ceramics was systematically investigated. Analysis of the phase indicated that the ceramics modified with Bi₂O₃ consisted of ZnO grain as a main phase and a few secondary phases such as Zn₃(VO₄)₂, ZnV₂O₄, BiVO₄, V₂O₅, and Mn-rich phase. The average grain size increased from 5.6 to 7.2 μm and the sintered density decreased in the range of 5.51–5.37 g/cm³ up to 0.05 mol%, whereas a further addition increased it to 5.40 g/cm³ at 0.25 mol%. The breakdown field decreased from 4,874 to 2,205 V/cm with an increase in the amount of Bi₂O₃. The ceramics added with 0.025 mol% Bi₂O₃ were characterized by a surprisingly high nonlinear coefficient (60) and very low leakage current density (20 μA/cm²). Bi atoms in the bulk acted as a donor to increase the electron concentration with an increase in the amount of Bi₂O₃.     

Improved dielectric properties in pyrochlore type oxides: Ca3Sm3?xBixTi7Nb2O26.5 (x = 1.0, 2.0 or 3.0) by Bi substitution

Improved dielectric properties are observed in pyrochlore type oxides, Ca₃Sm_3−x Bi_xTi₇Nb₂O_26.5 (x = 1.0, 2.0 or 3.0) by Bi substitution. The dielectric constant increased with increasing Bi concentration. The dielectric constant obtained for Ca₃Bi₃Ti₇Nb₂O_26.5 is 110, whereas, without Bi (Ca₃Sm₃Ti₇Nb₂O_26.5) it is 62 at 100 kHz. The Powder X-ray diffraction analysis reveals that cubic pyrochlore type phase is formed for all the compositions. The experimental results further show that the formation of Bi substituted compounds is complete at a lower temperature than the compounds without Bi. Microstructure studies reveal that the grains formed are acicular when Bi is present in large amounts compared to cuboid grains in samples having no Bi.     

Co-precipitation method for the preparation of ferroelectric CaBi<Subscript>4</Subscript>Ti<Subscript>4</Subscript>O<Subscript>15</Subscript>

A simple co-precipitation technique has been successfully applied for the preparation of pure single phase CaBi₄Ti₄O₁₅ (CBT) powders. Ammonium oxalate and ammonium hydroxide were used to precipitate Ca²⁺, Bi³⁺ and Ti⁴⁺ cations simultaneously. No pyrochlore phase was found while heating powder at 600 C and pure CBT phase was found to be formed by X-ray diffraction. Particle size and morphology was studied by transmission electron microscopy (TEM). The room temperature dielectric constant at 1 kHz is 400. The ferroelectric hysteresis loop parameters of these samples were also studied.     

Coprecipitation method for the preparation of nanocrystalline ferroelectric CaBi2Ta2O9

A simple coprecipitation technique had been successfully applied for the preparation of pure ultrafine single-phase CaBi₂Ta₂O₉ (CBT). Ammonium hydroxide and ammonium oxalate were used to precipitate Ca²⁺, Bi³⁺ and Ta⁵⁺ cations simultaneously. No pyrochlore phase was found while heating powder at 800 °C and pure CaBi₂Ta₂O₉ phase was found to be formed by XRD. Particle size and morphology was studied by transmission electron microscopy (TEM). The room temperature dielectric constant at 1 kHz is 100. The ferroelectric hysteresis loop parameters of these samples were also studied.     

Comparative X-ray diffraction and Mössbauer spectroscopy studies of BiFeO3 ceramics prepared by conventional solid-state reaction and mechanical activation

The aim of this work was to prepare BiFeO₃ by modified solid-state sintering and mechanical activation processes and to investigate the structure and hyperfine interactions of the material. X-ray diffraction and Mössbauer spectroscopy were applied as complementary methods. In the case of sintering, BiFeO₃ phase was obtained from the mixture of precursors with 3 and 5 % excess of Bi₂O₃ during heating at 1023 K. Small amounts of impurities such as Bi₂Fe₄O₉ and sillenite were recognized. In the case of mechanical activation, the milling of stoichiometric amounts of Bi₂O₃ and Fe₂O₃ followed by isothermal annealing at 973 K resulted in formation of the mixture of BiFeO₃, Bi₂Fe₄O₉, sillenite and hematite. After separate milling of individual Bi₂O₃ and Fe₂O₃ powders, mixing, further milling and thermal processing, the amount of desired BiFeO₃ pure phase was significantly increased (from 70 to 90 %, as roughly estimated). From Mössbauer spectra, the hyperfine interaction parameters of the desired BiFeO₃ compound, paramagnetic impurities of Bi₂Fe₄O₉ and sillenite were determined. The main conclusion is that the lowest amount of impurities was obtained for BiFeO₃ with 3 % excess of Bi₂O₃, which was sintered at 1023 K. However, in the case of mechanical activation, the pure phase formed at a temperature by 50 K lower as compared to solid-state sintering temperature. X-ray diffraction and Mössbauer spectroscopy revealed that for both sintered and mechanically activated BiFeO₃ compounds, thermal treatment at elevated temperature led to a partial eliminating of the paramagnetic impurities.     

Dielectric and microwave properties study of TiFeNbO6 ceramics added Bi2O3

In this paper the ceramic matrix of TiFeNbO₆ (TFNO) was studied. The TFNO phase was calcined at 1,075 °C and used to prepare the samples, of 2, 4, 6, 8 and 10 wt% of the Bi₂O₃ and sintered at 1,125 °C. These samples were characterized by X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electron microscopy and dielectric microwave properties. XRD and RS were used to characterize these samples. The samples presented two new phases. The first phase is the tetragonal rutile structure with a space group of P4₂/mnm, equivalent at parent rutile Ti_0.4Fe_0.3 Nb_0.3O₂ (TFNO) with 040725 ICSD code, and the secondary phase belonging to the pyrochlore system Bi_1.721Fe_1.056Nb_1.134O₇ (BFNO), with a space group of Fd-3mZ (227), in a cubic structure. The dielectric properties have shown significant variation for 10 % Bi₂O₃-added sample, because the formation of the new phase (BFNO) contributes with the reduction of τ _f from 281.12 to 77.45 ppm/°C and with increase in ε _r , from 47.23 to 63.77 and an increase in the dielectric loss (tan δ), from 0.0016 to 0.0068, respectively. Even though, Bi₂O₃ additive deteriorates the dielectric loss of the ceramics, the permittivity has enhanced significantly, which is advantageous for reduction of the air gap between the probe and the DRA antennas that influences on the samples for future application in microwave.     

Effects of Bi2O3 addition on the microstructure and electromagnetic properties of NiCuZn ferrites

Effects of Bi₂O₃ addition on the microstructural development and magnetic properties of NiCuZn ferrites were studied. For low temperature sintering (<900 °C), 0.1–1.0 wt % of Bi₂O₃ was added to the NiCuZn ferrites. The grain size and bulk density gradually increased with the increase in the Bi₂O₃ content. Above 0.5 wt % Bi₂O₃, abnormal grain growth was observed. The specimen with 0.25 wt % Bi₂O₃ showed the highest initial permeability with good quality factors and a uniform microstructure. However, the specimen with greater than 0.5 wt % Bi₂O₃ demonstrated abnormal grain growth with lower initial permeabilities and poor quality factors.     

Studies on microstructure and density of sintered ZnO-based non-linear resistors

Systematic studies have been made of the grain growth and density of sintered ZnO-based systems containing one or more additive oxides of the type Nb₂O₅, Bi₂O₃, Sb₂O₃, CoO, Cr₂O₃, MnO₂, NiO and Al₂O₃. These samples were characterized using such techniques as scanning electron microscopy, X-ray diffraction and electron probe microanalysis. The influence of the nature and amount of additive oxides and sintering temperature is discussed in relation to microstructure, density and the phases present.     

Structure and dielectric properties of sputtered bismuth magnesium niobate thin films

The Bi_1.5MgNb_1.5O₇ (BMN) thin films were prepared on platinum coated sapphire by rf magnetron sputter deposition. Effects of substrate temperature, sputter pressure and O₂/(O₂ + Ar) mixing ratio on phase structures and dielectric properties of thin films were investigated. The results indicated that sufficiently high substrate temperature and low sputter pressure would facilitate the formation of cubic pyrochlore in BMN thin films. Meanwhile, the appropriate O₂/(O₂ + Ar) mixing ratio of sputter atmosphere was required. The deposited Bi_1.5MgNb_1.5O₇ cubic pyrochlore thin films with (222) oriented texture exhibited large tunability of ~ 50% at a maximum applied bias field of 1.5 MV/cm, with low dielectric loss of ~ 0.007. The temperature and frequency dependent dielectric measurements indicated that no noticeable dielectric dispersion was detected in BMN cubic pyrochlore thin films.