Microstructure development in Sb2O3-doped ZnO

The microstructure development in Sb₂O₃-doped ZnO was studied at doping levels up to 2.0 mol%. Dopant Sb₂O₃ reacted with ZnO to form inclusion particles,α-Zn₇Sb₂O₁₂, and inhibited the grain growth of ZnO. With increasing doping level of Sb₂O₃, the growth rate of ZnO decreased whereas that of inclusion particles increased. Some inclusion particles were trapped in ZnO grains at low doping levels of Sb₂O₃, but the volume fraction of trapped inclusion particles decreased with increasing doping level. Stereological analysis of the size and number ratios of ZnO grains and inclusion particles indicated that a compatible assumption is needed to evaluate Zener effect in two-phase sintering.     

Effect of Powder Processing and Sintering Conditions on the Microstructural,Thermal, and Mechanical Properties of Reticulated Zinc Oxide Ceramic Foams

Ceramic foams are made of zinc oxide using different amounts of Sb₂O₃ and Bi₂O₃ as sintering aids. The effect of a ball milling processing of the starting powders and the sintering temperature on the microstructure and the properties of the ZnO foams is investigated. The focus is set on the evolution of the secondary phases formed within the microstructure of ZnO. A determining effect is identified in the amount of an Al₂O₃ impurity which is introduced by abrasion of the milling vessels during ball milling. Alumina is partially dissolved in a spinel α–Zn₇Sb₂O₁₂ secondary phase which is stabilized by a reduction of the unit cell volume. Remaining Al₂O₃ is incorporated into zinc oxide under formation of a defect wurtzite phase. The phase evolution is a complex function of the content of sintering aids, the Al₂O₃ impurity level and the sintering temperature. The shrinkage during sintering and the porosity evolution are correlated to the phase composition within the ZnO material. The thermal conductivity and the compressive strength of the foams are determined, normalized with respect to their porosity, and correlated to the microstructure and phase composition of the ZnO strut material.     

Sintering of Sb2O3-doped ZnO

The effect of Sb₂O₃ on the densification of ZnO was studied and compared with that for additives such as ZnSb₂O₆,-Zn₇Sb₂O₁₂, and Sb₂O₄. Addition of up to 2.0 mol% Sb₂O₃ raised the densification temperature of ZnO from 600° C to 1000° C regardless of doping level and the particle size of Sb₂O₃. Addition of other antimony-additives, however, slowed down the densification of ZnO, and the densification temperature increased with increasing amount of additives. Results of TG, XRD and SEM showed that the observed densification characteristics in Sb₂O₃-doped ZnO are explained by the volatile nature of Sb₂O₃. Thus, Sb₂O₃ evaporates during oxidation at about 500° C and condenses on the ZnO particle surfaces as a noncrystalline phase of an unknown composition, which checks the material transport across ZnO particles resulting in the retarded densification.     

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.     

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.     

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.     

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.     

Thermal evolution of ZnO-Bi2O3-Sb2O3 system in the region of interest for varistors

In facing the design of new processing strategies for ZnO based ceramic varistors, a precise control of its microstructural development during sintering is demanded. Addition of dopants to zinc oxide results in the formation of secondary phases that to a large extent determine the macroscopic electrical properties of the ceramic. In a varistor system based on ZnO with small additions of Bi₂O₃ and Sb₂O₃ these three oxides govern the reactions at high temperature that give place to the secondary phases. These reactions become then the head point from which the functional microstructure is configured. In this way the present work deals with the thermal evolution of the ZnO-Bi₂O₃-Sb₂O₃ system in the region of interest for varistors, revealing the existence of two simultaneous reactions paths during sintering these ceramics.     

Phase equilibria,crystal chemistry and polymorphism of Zn7Sb2O12 doped with Cr and Ni

The binary phase diagram for Cr-doped Zn₇Sb₂O₁₂ was determined and compared with that for Ni-doped Zn₇Sb₂O₁₂. The β → α transition temperature, 1225 °C for undoped Zn₇Sb₂O₁₂, decreases extremely rapidly with increasing Cr content; the solubility limit of Cr in β-Zn₇Sb₂O₁₂ is <1% but 37.5% in α-Zn₇Sb₂O₁₂. Bond valence sum calculations for the tetrahedral site, which contains exclusively Zn, show it to be significantly underbonded in undoped α-Zn₇Sb₂O₁₂, but less underbonded with increased Ni²⁺ and especially Cr³⁺ content, thus providing an explanation for the stabilisation of α-Zn₇Sb₂O₁₂ to lower temperatures on doping with Ni and especially Cr. Sub-solidus compatibility relations in the ternary system ZnO–Sb₂O₅–Cr₂O₃ were determined at 1100 °C for compositions containing ≤50% Sb₂O₅.     

陶瓷材料烧结致密化过程的元胞自动机模拟

为研究陶瓷材料烧结致密化过程,以晶界能和晶界曲率生长驱动力理论为基础,建立了含有气孔的二相晶粒生长的元胞自动机模型,对陶瓷材料烧结致密化过程进行了模拟,并与制备的Al2O3/TiN陶瓷材料进行对比.结果表明,模型可有效地模拟陶瓷材料烧结时晶粒的生长及气孔的湮灭情况,能较好地再现烧结致密化过程,模拟结果与制备的陶瓷材料微观形貌组织十分接近.     

A study of ceramic-lined compound copper pipe produced by SHS–centrifugal casting

In this study ceramic-lined compound copper pipe (CLCCP) was produced by self-propagating high-temperature synthesis (SHS) and centrifugal casting technology. The effects of additives such as SiO₂, CrO₃, Na₂B₄O₇ and ZrO₂ on the densification degree, combining strength and the toughness of the ceramic layer were of prime interest. The results show that the densification degree of the ceramic layer can be improved by adding SiO₂ and CrO₃ in the thermite. This improvement correlated with the reduction of porosity of ceramic layer. The combining strength is increased through the addition of a suitable amount of Na₂B₄O₇. The highest fracture toughness of ceramic layer is obtained at 7 wt.% of ZrO₂ in thermite. The CLCCP shows excellent service properties when it is used in tubular billet crystallizer, including the prevention of cracks and flakes.     

Low temperature cofirable Li2Zn3Ti4O12 microwave dielectric ceramic with Li2O–ZnO–B2O3 glass additive

The effects of Li₂O–ZnO–B₂O₃ (LZB) glass additive on the sintering behavior, phase composition, microstructure and microwave dielectric properties of Li₂Zn₃Ti₄O₁₂ ceramics were investigated. The addition of a small amount of LZB glass can reduce the sintering temperature of Li₂Zn₃Ti₄O₁₂ ceramics from 1,075 to 900 °C without much degradation of the microwave dielectric properties. Only a single-phase Li₂Zn₃Ti₄O₁₂ is formed in Li₂Zn₃Ti₄O₁₂ ceramic with LZB addition. Typically, the 1.5 wt% LZB glass-added Li₂Zn₃Ti₄O₁₂ ceramic sintered at 900 °C for 2 h can reach a high relative density of 97.5 % and exhibits good microwave dielectric properties, i.e., relative dielectric constant (ε _r ) = 19.1, quality factor (Q) = 7083.5 at 9 GHz, and temperature coefficient of resonant frequency (τ _f ) = ? 48.9 ppm/°C. In addition, the ceramic could be co-fired well with an Ag electrode, which is made it as a promising dielectric ceramic for low temperature co-fired ceramics technology application.     

Microstructure and properties of spark plasma sintered AlN ceramics

Spark plasma sintering (SPS) is a newly developed technique that enables poorly sinterable aluminum nitride (AlN) powder to be fully densified. It is addressed that pure AlN sintered by SPS has relatively low thermal conductivity. In this work, SPS of AlN ceramic was carried out with Y₂O₃, Sm₂O₃ and Li₂O as sintering aids. Effects of additives on AlN densification, microstructure and properties were investigated. Addition of sintering aids accelerated the densification, lowered AlN sintering temperature and was advantageous to improve properties of AlN ceramic. Thermal conductivity and strength were found to be greatly improved with the present of Sm₂O₃ as sintering additive, with a thermal conductivity value about 131 Wm⁻¹K⁻¹ and bending strength about 330 MPa for the 2 wt% Sm₂O₃-doped AlN sample SPS at 1,780 °C for 5 min. XRD measurement revealed that additives had no obvious effect on the AlN lattice parameters. Observation by SEM showed that AlN ceramics prepared by SPS method manifested quite homogeneous microstructure. However, AlN grain sizes and shapes, location of secondary phases varied with the additives. The thermal conductivity of AlN ceramics was mainly affected by the additives through their effects on the growth of AlN grain and the location of liquid phases.     

The effect of Sb2O5 additions on the dielectric properties of Ag(Nb0.8Ta0.2)O3 ceramics

The sintering behavior and dielectric properties for perovskite Ag(Nb_0.8Ta_0.2)O₃ ceramic with Sb₂O₅ doping was explored. A small amount of Sb₂O₅ (2.5 wt.%) led to high densification at temperatures < 1060 °C. The dielectric constant increased and the temperature coefficient decreased with increasing concentration of Sb₂O₅, and the dielectric constant reached 673, combined with a low temperature coefficient of 147 ppm/°C, and dielectric loss of 0.0044 (at 1 MHz) for the sample with 3.5 wt.% Sb₂O₅ sintered at 1080 °C.     

Effect of borosilicate glasses on the microwave dielectric properties of ZnO–Nb2O5–Ta2O5 system

(1 ? y)[0.5ZnNb₂O₆–0.5Zn₃Nb₂O₈]–yZnTa₂O₆ with y = 0.91 (ZNT) ceramic have been prepared by conventional solid state ceramic route. The effect of glass additives on the microstructure, densification, and microwave dielectric properties of the ZNT ceramic for low temperature co-fired ceramic applications was investigated. Different weight percentages of quenched glass such as ZnO–B₂O₃–SiO₂, BaO–B₂O₃–SiO₂, LiO–B₂O₃–SiO₂ and MgO–B₂O₃–SiO₂ were added to ZNT powder. The crystal structure of the ceramic–glass composites was studied by X-ray diffraction and microstructure by scanning electron microscopy. The microwave dielectric properties such as relative permittivity (ε_r), quality factor (Q_uxf) and co-efficient of temperature variation of resonant frequency (τ_f) of the ceramics have been measured in the frequency range 4–6 GHz. The 5 wt% ZnO–B₂O₃–SiO₂ added ZNT ceramic sintered at 900 °C showed: ε_r = 28.1, Q_uxf = 32820 GHz (at 4.92 GHz), and τ_f = ?7.7 ppm/^oC respectively.     

SbVO4 doped ZnO–V2O5-based varistor ceramics: microstructure, electrical properties and conductive mechanism

The effects of SbVO₄ addition on the microstructure, electrical properties and characteristics of grain and grain boundary of ZnO–V₂O₅ based varistor ceramics were studied using SEM, E–J measurements and impedance spectroscopy. XRD analysis revealed that all the samples consist of main phase of ZnO and the second phase of BiVO₄ and Zn₇Sb₂O₁₂. The microstructural homogeneity of the ceramic was improved through adding SbVO₄. With an increase of SbVO₄, the average grain sizes decrease from 16.1 to 6.1 μm. The resistivity of grain boundary is approximately constant (~10³ Ω). The ZnO–V₂O₅-based varistor ceramics added with 0.3 mol % SbVO₄ sintered at 940 °C for 4 h exhibited good nonlinear properties of α = 51, J = 13.4 μA/mm² and E _1mA = 416 V/mm.     

Sintering of tin oxide using zinc oxide as a densification aid

The physicochemical electronic characteristics of SnO₂ render it useful in many technical applications, including ceramic varistors, stable electrodes used in electric glass-melting furnaces and electrometallurgy of aluminum, transparent windows and chemical sensors. The use of ZnO as a sintering aid was explored in this study to obtain SnO₂ as a dense ceramic. Compacts were obtained by mechanical mixing of oxides, isostatic pressing at 210 MPa and sintering in situ inside a dilatometer at heating rates of 10°C/min. The grain size and microstructure were investigated by scanning and transmission electron microscopy (SEM/TEM). The phases and chemical composition were analyzed by energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results indicated that ZnO acts as a densification aid for SnO₂, improving its grain growth with additions of up to 2 mol%. ZnO forms a solid solution with SnO₂ up to 1 mol%, above which SnZnO₃ precipitates in the grain boundary, potentially inhibiting shrinkage and grain growth.     

Influence of the concentration of Sb<Subscript>2</Subscript>O<Subscript>3</Subscript> on the electrical properties of SnO<Subscript>2</Subscript> varistors

Varistors are electronic materials with nonohmic behavior. In traditional SnO₂ varistors, CoO acts as a densifying agent, Nb₂O₅ increases the electrical conductivity of SnO₂ grains, and Cr₂O₃ produces a more uniform microstructure and acts as an oxygen retaining agent at the grain boundaries. The present work involved a systematic study of the substitution of Nb₂O₅ for Sb₂O₃ in the composition of a ternary varistor system. The compositions were prepared by conventional wet ceramic processing using deionized water, and the resulting slips were dried by spray-drying. Pellets were produced under a pressure of 330 MPa and sintered at 1,350 °C for 2 h. Similar to the behavior of Nb₂O₅, increasing the concentration of Sb₂O₃ reduced the nonlinear behavior of the ceramic and its breakdown electric field while increasing its leakage current. The samples’ microstructure showed greater porosity, suggesting that higher concentrations of Sb₂O₃ reduce the sintering rate, probably in response to the higher concentration of tin vacancies in the structure.     

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.