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.     

Electrical characteristics and reheat-treatment effects in a ZnO varistor fabricated by two-stage heat-treatment

Conventional ZnO varistors are generally fabricated by sintering ZnO powder mixed with additives such as Bi₂O₃, Sb₂O₃, Cr₂O₃, Co₂O₃, and MnO₂. To reduce abnormal grain growth and change in electrical characteristics in the conventional ZnO varistors caused by volatilization of Bi₂O₃, the ZnO powder with all additive oxides except Bi₂O₃ was pressed into disc form and sintered. The disc was then painted with metal oxide paste containing Bi₂O₃ and again fired. The ZnO varistor fabricated by this process, i.e. a two-stage heat-treatment process, showed typical non-linearI-V characteristics with higher breakdown voltage exceeding 800 V mm^–1. It was also observed that the non-linearI-V coefficient change rate, , in the ZnO varistor due to reheat-treatment is almost linearly proportional to the sintered density.     

Effects of nondensifying inclusions on the densification and microstructure of zinc oxide matrix composites

The densification and microstructure development of ZnO containing Zn₇Sb₂O₁₂, ZrO₂, and aggregated ZnO were investigated to elucidate the effect of nondensifying inclusions on the sintering of ceramic/ceramic composites. The inclusion retarded the densification, and the degree of retardation was found to depend on the chemical species of inclusion; Zn₇Sb₂O₁₂ had the largest effect, followed by ZrO₂ and then aggregated ZnO last. The experimental results for aggregated ZnO was explained by the theory which predicts the generation of backstresses. The backstresses give a less significant effect on the densification. For Zn₇Sb₂O₁₂ and ZrO₂, the microstructure of the matrix varied with distance from an inclusion particle; much porosity was observed in the region surrounding the inclusion. Circumferential voids, which are responsible for the suppression of densification, form during the initial stage of sintering. Inclusion particles generate an anchoring effect which retards the densification of the matrix immediately surrounding the inclusion particle during the intermediate stage.Supported by the Inamori Foundation.     

Mechanochemical Synthesis and Thermal Behavior of Metastable Mixed Oxides in the CaO–Sb2O3–Bi2O3 System

The phase composition of crystalline mechanochemical synthesis products in the CaO–Sb₂O₃–Bi₂O₃ system was determined. Of the known phases in this system, only three could be prepared mechanochemically: Ca₂Sb₂O₅, CaSb₂O₄, and CaBiO_2.5 (fcc). A new metastable phase, "-Bi₂O₃, with an orthorhombic structure close to that of the high-temperature, fluorite phase -Bi₂O₃, was obtained by mechanical processing at 30°C. A number of new metastable fluorite solid solutions of binary and ternary oxides were obtained as single-phase powders by mechanochemical synthesis. The mechanochemical yield of primary crystalline products was shown to be several times higher than that of secondary products. A broad composition range was revealed in which perovskite and fluorite phases are in mechanochemical equilibrium. The composition dependence of the lattice parameter of the metastable fluorite phase Bi_{2 – x} Sb _x O₃ was found to be the opposite of the one predicted by Vegard's law. Metastable mixed oxides undergo phase transformations during heating (starting at 280°C in the case of the ternary perovskite phase). Bi_{2 – x} Ca _x O_{3 – 0.5x} fluorite solid solutions experience a transformation at 400°C, accompanied by oxygen loss. During heating in air, Sb₂O₃-containing fluorite phases partially stabilize owing to oxidation but, nevertheless, undergo structural transformations above 480°C. The transformation of Sb_{2 – x} Ca _x O_{3 – 0.5x} metastable fluorite solid solutions near 500°C in air is accompanied by the formation of needle-like Sb₂O₃ crystals. A mechanism is proposed for the extremely rapid growth of such crystals: extrusion of the Sb₂O₃ resulting from fluorite decomposition in agglomerates through triple junctions of aggregates and through cracks in the surface layer of agglomerates.     

Densification and microstructure development in the reaction sintering process of yttrium iron garnet

Factors affecting the densification and microstructure development in the reaction sintering process (RSP) of yttrium iron garnet were investigated. Three different powder mixtures were used: Fe₂O₃/Y₂O₃, Fe₂O₃/YFeO₃ (1100 ° C calcined), and Fe₂O₃/YFeO₃ (1200 ° C calcined). The conventionally prepared garnet powder was also adopted as a reference material. It was found that the RSP using Fe₂O₃-YFeO₃ systems has a beneficial effect on densification from the dilatation occurring along with the reaction of garnet formation. On the other hand, it has a detrimental effect due to the local contraction induced by the reaction in the Fe₂O₃-Y₂O₃ system. The densification rate and ultimate density achievable are also affected by the YFeO₃ powder adopted in RSP. A high grain-growth rate was obtained for garnet when the 1200 ° C calcined YFeO₃ powder was used. This leads to a high densification rate at low temperature. However, the densification ability deteriorates at temperatures above 1425 ° C due to the trap of pores in the fast-grown grains. Conversely, the grain-growth rate in RSP with 1100 ° C-calcined YFeO₃ was moderate, and although it gives a slower densification rate at low temperature, the ultimate density can be raised to 99% theoretical density at 1450 ° C.     

Effect of additives on densification and deformation of tetragonal zirconia

The effect of additives (Bi₂O₃, Fe₂O₃) on densification and creep rates of tetragonal ZrO₂-Y₂O₃ has been investigated. In Bi₂O₃-doped Y-TZP, a reactive liquid forms at temperatures above 800–900C, which leads to a strong enhancement of densification for concentrations of 1–2 mol % Bi₂O₃. However, during cooling from the processing temperature a strong, undesirable transformation of the tetragonal to the monoclinic phase occurs. The addition of 0.6–1.2 mol % FeO_3/2 promotes densification without destabilizing the tetragonal phase. A concentration of 1.2 mol %, however, induces discontinuous grain growth, while this is not the case for 0.6 mol %. Creep rates of Y-TZP were enhanced by a factor of 4–6 by adding 0.6 mol % FeO_3/2.     

Sintering and compensation effect of donor- and acceptor-codoped 3mol% Y2O3-ZrO2

Addition of 0.15–0.5 mol% acceptor oxide, Al₂O₃, to 3 mol% Y₂O₃-ZrO₂ results in enhanced densification at 1350 °C. The enhancement is accounted for by a liquid phase sintering mechanism. The addition of donor oxide, Ta₂O₅, of 0.15–2.5 mol % at 1300–1600 °C results in the destabilization of tetragonal (t-) phase and the decrease of final density in 3 mol% Y₂O₃-TZP (tetragonal ZrO₂ polycrystals). X-ray diffractometry (XRD) reveals that the Ta₂O₅-added 3 mol% Y₂O₃-ZrO₂ contains monoclinic (m-) ZrO₂ and a second phase of Ta₂Zr₆O₁₇. The decreasing in final density is attributed to the increase of m-ZrO₂ content. Complete destabilization of t-ZrO₂ to m-ZrO₂ in samples added with 2.5 mol% Ta₂O₅ is interpreted by the compensation effect based on donor- and acceptor-codoping defect chemistry.     

Phase equilibrium relations in the binary system Bi2O3-ZnO

Phase equilibria in the binary system Bi₂O₃-ZnO were studied by quenching technique. Heat-treated compositions were subjected to X-ray diffraction for phase identification, and differential thermal analysis, optical and scanning electron microscopy were used to determine the solid-liquid equilibria occurring in this system. The data thus obtained revealed that incorporation of a small amount of ZnO to the high-temperature face-centered cubic lattice of Bi₂O₃ leads to the formation of a body-centered cubic solid solution (-Bi₂O₃), which extends up to a composition of 2.2 mol% ZnO at a temperature near 750°C. On cooling, the -Bi₂O₃ solid solution undergoes a eutectoid transformation at a temperature of 710°C to yield the low-temperature monoclinic polymorph of Bi₂O₃ (-Bi₂O₃) and Bi₃₈ZnO₅₈. The eutectoid occurs at a composition of 1.8 mol% ZnO. The compound Bi₃₈ZnO₅₈ has a crystal structure analogous to the body-centered cubic -Bi₂O₃ solid solution and melts incongruently at a temperature near 753 ± 2°C to yield -Bi₂O₃ and liquid. A binary eutectic occurs between Bi₃₈ZnO₅₈ and ZnO at a composition near 25 ± 1.0 mol% ZnO with a melting temperature of 738 ±2°C. Based on the data obtained in this study, a revised phase diagram of the binary system Bi₂O₃-ZnO is proposed.     

Morphology and formation mechanism of the pyrochlore phase in ZnO varistor materials

The secondary phases in ZnO varistor materials, having Bi₂O₃, Sb₂O₃, MnCO₃ and CoO as additives, were investigated using transmission electron microscopy with an energy dispersive X-ray spectrometer and X-ray diffractometer scans. The information about the morphology of the pyrochlore phase and its formation reaction were obtained. The pyrochlore phase is formed from the reaction between the spinel and Bi₂O₃ phase. Below 1200° C, the spinel phase reacts with the liquid Bi₂O₃ phase present at the multiple grain junction and forms the pyrochlore phase. The pyrochlore phase nucleates at the corners of multiple grain junction adjacent to the spinel grain and grows towards the centre of the grain junction.     

Nonlinear electrical properties of ZnO varistors fast-fired by using millimeter-wave sintering process

Fast firing of Bi₂O₃-based ZnO varistor materials, which includes zero minutes soaking at 1100°C with 120°C/min heating and 145°C/min cooling rate, was made possible using millimeter-wave sintering (mS) technique. The overall sintering time of the process is less than 18 minutes, and the varistor characteristics obtained are = 38, J _L = 5.55 × 10^–6 A/cm² and V _bk = 600 V/mm, whereas the intrinsic parameters of the materials are _b = 2.84 eV, N _d = 1.85 × 10²⁴ m^–3 and N _s = 7.02 × 10¹¹ cm^–2. By contrast, conventional sintering (cS) process needs higher sintering temperature (1200°C), longer soaking time (60 min) and slower ramping rate (30°C/min) to obtain ZnO materials with the same marvelous nonlinear properties as those prepared by mS-process. Moreover, millimeter-wave sintering (24 GHz, mS) process enhances the densification kinetics and grain growth behavior more efficiently than the microwave sintering (2.45 GHz, S) process, resulting in better varistor characteristics for ZnO materials. However, sintering by millimeter-wave for too long period induces overfiring of the samples, which results in a density reversion phenomenon. Such a phenomenon leads to the decrease in surface state (N _s) and the potential barrier height (_b), which are presumed to be the mechanism leading to the degradation of ZnO materials' nonlinear properties.     

Oxygen ion conduction in γ-Bi2O3 doped with Sb2O3

Electrical conduction in bcc-Bi₂O₃ doped with Sb₂O₃ was investigated by measuring electrical conductivity, as a function of temperature and oxygen partial pressure , and ionic transference number. The-Bi₂O₃ doped with 1 to 3 mol% Sb₂O₃ was stable up to 550° C and showed an oxygen ionic conduction in the region of 10⁵ to 10^–9 Pa. As the Sb₂O₃ content increased, ionic conductivity increased up to 2.5 mol % Sb₂O₃ (1.8×10^–3–1cm^–1 at 500° C) and then decreased. However, the activation energy for ionic conduction remained almost unchanged. It was proposed that the-Bi₂O₃ contains a lot of oxygen vacancies and incorporated Sb⁵⁺ ions at tetrahedral sites which affect the concentration of oxygen vacancy effective for conduction.     

Si3N4-ZrO2 composites with small Al2O3 and Y2O3 additions prepared by HIP

Si₃N₄-ZrO₂ composites have been prepared by hot isostatic pressing at 1550 and 1750 °C, using both unstabilized ZrO₂ and ZrO₂ stabilized with 3 mol% Y₂O₃. The composites were formed with a zirconia addition of 0, 5, 10, 15 and 20 wt%, with respect to the silicon nitride, together with 0–4 wt% Al₂O₃ and 0–6 wt% Y₂O₃. Composites prepared at 1550 °C contained substantial amounts of unreacted -Si₃N₄, and full density was achieved only when 1 wt% Al₂O₃ or 4 wt % Y₂O₃ had been added. These materials were generally harder and more brittle than those densified at the higher temperature. When the ZrO₂ starting powder was stabilized by Y₂O₃, fully dense Si₃N₄-ZrO₂ composites could be prepared at 1750 °C even without other oxide additives. Densification at 1750 °C resulted in the highest fracture toughness values. Several groups of materials densified at 1750 °C showed a good combination of Vickers hardness (HV10) and indentation fracture toughness; around 1450 kg mm^–2 and 4.5 MPam^1/2, respectively. Examples of such materials were either Si₃N₄ formed with an addition of 2–6 wt% Y₂O₃ or Si₃N₄-ZrO₂ composites with a simultaneous addition of 2–6 wt%Y₂O₃ and 2–4 wt% Al₂O₃.     

Topochemical Memory Effect in the Formation of Barium Hexaferrite

The topochemical memory effect in Ba(NO₃)₂ : Fe₂O₃ = 1 : 6 reaction mixtures containing -Fe₂O₃ with different particle sizes was studied at low (540–630°C; the main reaction product BaFe₂O₄) and high (900–930°C; the main product, BaFe₁₂O₁₉) reaction temperatures. The results indicate that the particle size of Fe₂O₃ has a significant effect on the BaFe₂O₄ yield in the low-temperature solid-state reaction (<585°C), in contrast to the liquid-assisted reaction (>585°C). In the subsequent high-temperature solid-state reaction (900–930°C), the BaFe₁₂O₁₉ yield depends on the particle size of Fe₂O₃, regardless of the initial reaction temperature. It is shown that, to achieve a high BaFe₁₂O₁₉ yield, the process conditions should be optimized in both the low- and high-temperature stages.     

Optimization of the composition and sintering conditions of high-voltage ZnO varistor ceramics

This paper presents a study aimed at optimizing the composition and sintering conditions of highvoltage ZnO varistor ceramics. We demonstrate that, with allowance for the cost of starting materials, the optimal composition of high-voltage ZnO varistor ceramics is as follows (wt %): ZnO, 90; Bi₂O₃, 2.76; Sb₂O₃, 1.92; Al₂O₃, 3.32; and Co₂O₃, 2. The optimal sintering conditions are isothermal holding at a temperature of 975°C for 2 h. The ceramics thus prepared have V _b = 4.5 kV/mm, α = 50, I _l = 1.1 μA/cm², density ρ = 5.67 g/cm³ (relative density of 96.1%).     

Effect of V2O5 and CuO additives on sintering behavior and microwave dielectric properties of BiNbO4 ceramics

The influences of V₂O₅ and CuO additives on the sintering behavior and microwave dielectric properties of BiNbO₄ ceramics were investigated. The V₂O₅ and CuO additives lowered the sintering temperature of BiNbO₄ ceramics to the range 875 °C–935 °C. All BiNbO₄ compounds with additives had the orthorhombic structure. The dielectric constant _r was not significantly changed, while the unloaded Q value was affected with additives. The Qf value was found to be a function of the sintering temperatures and the amount of additives. It varied from 4500 to 15800 (GHz) and 1000 to 8000 (GHz) with additives V₂O₅ and CuO, respectively. The _f values were increased in positive values with V₂O₅ doped, while decreased in negative values with CuO addition. V₂O₅ and CuO additives effectively improved the densification and dielectric properties of BiNbO₄ ceramics. The correlation between the microstructure and the Qf value was observed with different additives.     

Grain growth of ZnO in binary ZnO-V2O5 ceramics

Grain growth of ZnO during liquid-phase sintering of binary ZnO-V₂O₅ ceramics has been studied for V₂O₅ contents from 0.5 to 4 mol% and sintering from 900°C to 1200°C. The results are discussed and compared with previous studies in terms of the phenomenological kinetic grain growth expression: G ⁿ – G _o ⁿ = K _o t exp(–Q/RT).Addition of V₂O₅ is found to decrease the ZnO grain growth exponent, n, as well as the apparent activation energy, Q. The activation analysis also reveals a change in the rate-controlling mechanism for ZnO grain growth. Following a low-V₂O₅-content (2 mol%) of nearly constant Q values of about 88 kJ/mol, further V₂O₅ additions cause an increase of the Q value to about 115 kJ/mol. Consistent with accepted models of liquid-phase sintering, it is concluded that the rate-controlling mechanism of ZnO grain growth during liquid-phase sintering in the presence of V₂O₅ changes from one of a phase-boundary reaction at low V₂O₅ levels to one of diffusion through the liquid phase at more than 2 mol% V₂O₅ levels.     

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.     

Microstructure and current-voltage characteristics of praseodymium-doped zinc oxide varistors containing MnO2, Sb2O3 and Co3O4

The effects of the oxide additives MnO₂, Sb₂O₃ and Co₃O₄ commonly incorporated in commercial Bi₂O₃-doped ZnO varistors on the microstructure and the current-voltage characteristics of 0.5 mol% Pr₆O₁₁-doped ZnO varistors have been studied. A 1 mol% addition of Co₃O₄ to the ZnO-Pr₆O₁₁ binary system does not produce any additional secondary phases in addition to Pr oxides. In contrast to this, the addition of 1 mol% MnO₂ complicates the simple two-phase microstructure of the 0.5 mol% Pr₆O₁₁-doped material by forming the perovskite-type structure Pr(Mn_{1 – x} Zn _x )O₃. The addition of Sb₂O₃ produces the pyrochlore Pr₃Zn₂Sb₃O₁₄ and spinel Zn₇Sb₂O₁₂ phases. Application of the Fresnel fringe technique and the diffuse dark field technique showed that there was an amorphous Pr-rich layer of thickness <2 nm residing between adjacent ZnO grains in all the samples studied.     

Sintering process of Y2O3 and Al2O3-doped Si3N4

The sintering process of Y₂O₃- and Al₂O₃-doped Si₃N₄ has been investigated by dilatometry and microstructural observations. The densification progressed through three processes. The bulk density increased to 85% theoretical without the formation of -Si₃N₄ in the initial process. The densification once terminated after the second process. The / transformation of Si₃N₄ and the related formation of prismatic grains reduced the densification rate in the second process, although the grain size and the aspect ratio were very small. The final process was the densification of -Si₃N₄, where the fibrous grains grew remarkably. The kinetic order for the densification of -Si₃N₄ indicated a diffusion-rate controlling mechanism with the activation energy of 244 kJ mol^–1 (<1450 ° C) and 193 kJ mol^–1 (>1450 ° C). The influence of heating rate on the grain growth was characterized by a parameter derived from kinetic parameters. The relationships between grain growth and densification behaviour have also been discussed.     

Formation of plate-like lanthanum-β-Aluminate crystal in Ce-TZP matrix

The reaction route and morphology of lanthanum--aluminate (LBA) crystals formed in Ce-TZP matrix were studied by examining the crystal phase changes and the microstructures in relation to the heat-treatment time, heat-treatment temperatures and the particle size of raw Al₂O₃ powders. In the Ce-TZP matrix, the LBA crystal was formed by the reaction between La₂O₃ and Al₂O₃ through the LaAlO₃ phase as the intermediate. La₂Zr₂O₇ forms at 800 °C and remains in the temperature range 800–1500 °C, and LaAlO₃ forms between 1200 and 1400 °C. The LaAlO₃ reacts with Al₂O₃ to form LBA above 1500 °C. The diffusion of La³⁺ through the La₂Zr₂O₇ phase was faster than that of Al³⁺. The morphology of LBA crystals was dependent on the particle size of the starting raw Al₂O₃ particle. When submicrometre size Al₂O₃ (0.4m) particles were used as the starting particles, anisotropic, plate-like LBA crystals, about 10m long, were formed during heat treatments. On the other hand, Al₂O₃ of larger grain sizes (3.6, 10.3m) yield conglomerates of LBA crystals. The size of the conglomerates is similar to that of the raw Al₂O₃ particle. The dependence of the morphology of LBA on the particle size of Al₂O₃ can be attributable to the sintering process of the Ce-TZP matrix, leading to the control of the mechanical properties of Ce-TZP ceramics with LBA crystals.