Sintering of Zinc Oxide

Sintering studies have been conducted with single crystal spheres of zinc oxide in air at total pressures of 10⁻³ to 1.0 atm over the range 1050° to 1250°C. Quantitative observations were made on the rate of growth of a neck between the spheres. No change in the distance between the geometrical centers of the spheres was observed. An analysis of the kinetic data shows that sintering was predominantly achieved by distillation through the surrounding gas phase. At 1150° the rate of welding was inversely influenced by the total pressure when the latter was changed from 0.003 to 0.75 atm. No conclusion could be reached on the effect of the oxygen partial pressure at constant total pressure. A brief summary of the various sintering mechanisms of zinc oxide from 700° to 1250°C is presented.     

Sintering of Zinc Oxide Doped with Antimony Oxide and Bismuth Oxide

The phase change, densification, and microstructure development of ZnO doped with both Bi₂O₃ and Sb₂O₃ are studied to better understand the sintering behavior of ZnO varistors. The densification behavior is related to the formation of pyrochlore and liquid phases; the densification is retarded by the former and promoted by the latter. The pyrochlore phase, whose composition is Bi_3/2ZnSb_3/2O₇, appears below 700°C. The formation temperature of the liquid phase depends on the Sb/Bi ratio: about 750°C for Sb/Bi < 1 by the eutectic melting in the system ZnO—Bi₂O₃, and about 1000°C for Sb/Bi > 1 by the reaction of the pyrochlore phase with ZnO. Hence, the densification rate is determined virtually by the Sb/Bi ratio and not by the total amount of additives. The microstructure depends on the sintering temperature. Sintering at 1000°C forms intragrain pyrochlore particles in ZnO grains as well as intergranular layers, but the intragrain particles disappear at 1200°C by the increased amount of liquid phase, which enhances the mobility of the solid second phase.     

Sintering and Characterization of Nanophase Zinc Oxide

Nanocrystalline, single-phase undoped ZnO was sintered to 95%–98% of theoretical density at 650°–700°C, using pressureless isothermal sintering. The density increased very rapidly at 500°–600°C, remained constant with sintering temperature until ∼900°C, and then decreased slightly. The estimated activation energy for densification at 600°–700°C (275 kJ/mol) was comparable to grain-growth activation energies previously reported for microcrystalline ZnO but much greater than the grain-growth activation energy measured in the present work. A bimodal microstructure, consisting of nanocrystalline grains within larger ensembles ("supergrains"), was observed, and both modes grew as the sintering temperature increased. The grain-growth activation energy for the nanocrystalline grains was extremely low, ∼20 kJ/mol. The activation energy for the growth of the supergrains depended strongly on temperature but was ∼54 kJ/mol at >500°C. The important mechanisms probably are rearrangement of the nanoparticle grains, with simultaneous surface and boundary diffusion, and vapor transport above 900°C.     

Grain Growth of Zinc Oxide During the Sintering of Zinc Oxide—Antimony Oxide Ceramics

Grain growth in a high-purity ZnO with systematic additions of Sb₂O₃ from 0.29 to 2.38 wt% was studied for sintering in air from 1106° to 1400°C. The results are discussed and compared with previous studies of pure ZnO and ZnO with Bi₂O₃ additions in terms of the kinetic grain growth expression: Gⁿ – Gⁿ ₀= K ₀ t exp(— Q/RT ). Additions of Sb₂O₃ inhibited the grain growth of ZnO and increased the grain growth exponent ( n -value) to 6 from 3 for pure ZnO and 5 for the ZnO—Bi₂O₃ ceramic. The apparent activation energy for the grain growth of ZnO also increased to about 600 kJ/mol from 220 kJ/mol for pure ZnO and 150 kJ/mol for the ZnO—Bi₂O₃ ceramics. Both the grain growth exponent and the activation energy were independent of the Sb₂O₃ content. Particles of the Zn₇Sb₂O₁₂ spinel were observed on the grain boundaries and at the grain triple point junctions. It was also observed that the Sb₂O₃ additions caused twin formation in each ZnO grain. It is concluded that both the Zn₇Sb₂O₁₂ particles and the twins are responsible for the ZnO grain growth inhibition by Sb₂O₃.     

Effect of Bismuth of Oxide Content on the Sintering of Zinc Oxide

Densification and microstructure de velopment in Bi₂O₃-doped ZnO have been studied with a special emphasis on the effect of the Bi₂O₃ content. A small amount of Bi₂O₃ in ZnO (0.1 mol%) retarded densification, but the addition of Bi₂O₃ to more than 0.5 mol% promoted densification by the formation of a liquid phase above the eutectic temperature (∼740°C). The liquid phase increased grain-boundary mobility, which was responsible for the formation of intragrain pores and the decrease in the sintered density. The increase in the Bi₂O₃ content increased the probability of the formation of skeleton structure, which reduced the grain growth rate and the sintered density.     

Sintering Kinetics of Undoped and Mn-Doped Zinc Oxide in the Intermediate Stage

The intermediate stage sintering kinetics of undoped ZnO and ZnO doped with 0.1–1.2 mol% Mn were investigated. It was found that the densification in the initial and intermediate stages is affected by surface diffusion in both undoped and Mn-doped ZnO, and that this contribution increases with the increase in the Mn content. The surface diffusion causes an initial pore growth, a more significant grain growth, and an apparent increase in the activation energy for densification at lower densities. Further kinetic analysis showed that the densification process in the intermediate stage is dominated by volume diffusion in all the compositions. The calculated values of the diffusion coefficients are consistent with the ones obtained by the tracer diffusion measurements of Zn in ZnO crystals in other works. The origin of the increase of the surface transport is discussed, together with its effect on the second phase mobility.     

Compositional Effects on the Liquid-Phase Sintering of Praseodymium Oxide-Based Zinc Oxide Varistors

The influence of the formation of eutectic liquid phases during sintering of the ternary varistor system ZnO-Pr₆O₁₁-Co₃O₄ was studied. The temperature at which the samples were observed to be liquid-phase sintered was found to depend on the nominal batch composition. The appearance of the liquid phase had a significant effect on the final microstructure as well as on the electrical properties of the varistors. Small additions of ZrO₂, purposely added or resulting from ball-mill contamination, were found to affect moderately the properties of the liquid phases observed in this system.     

Phase Transformations and Melting Effects During the Sintering of Bismuth-Doped Zinc Oxide Powders

ZnO ceramics used as varistors are prepared with bismuth oxide as an essential additive. Some of its effects during the first step of liquid-phase sintering remain unexplained. Therefore, in this work, we characterize the Bi distribution and identify the Bi-rich phases in spherical Bi-chemically-doped ZnO powders. By a suitable method of differential thermal analysis conducted during sintering, we detect thermal phenomena. Characterization of ceramics quenched during sintering allows us to describe them and to analyze their influence on sintering. We also study the parameters which determine the occurrence of these events during sintering.     

Sintering of Zinc Sulfide

The mechanisms of the sintering of ZnS were determined by measurement of the rate of growth of the necks between polycrystalline spheres. In vacuum (10⁻⁶ mm Hg) at temperatures higher than 600° C the mechanism of sintering is that of volume diffusion with coefficient D_v, = 1.06 × 10⁻² exp (-26,400/RT); below 600°C surface diffusion predominates, with coefficient D₃, = 9.14 × 10_-3 exp (-5700/RT). In Zn vapor (10⁻² mm Hg) between 550° and 650°C the predominating mechanism of sintering is surf ace diffusion, D₃, = 7.06 × 10⁻² exp (-8600/RT). It has been found that in an argon atmosphere the diffusion coefficient is much lower.     

Flash Sintering of Nanocrystalline Zinc Oxide and its Influence on Microstructure and Defect Formation

We report the sintering behavior of nanocrystalline zinc oxide under external AC electric field between 0 and 160 V/cm. In situ acquisition of density by means of laser dilatometry, evaluation of specimen temperature, real‐time measurement of electric field and current help analyze this peculiar behavior. Field strength and blocking electrodes significantly affect densification and microstructure, which was evaluated in the vicinity of the flash event and for the fully sintered material. High current densities flow through the sample at high electric fields, entailing a sudden increment of the temperature estimated to several hundreds of K and an exaggerated grain growth. In contrast, low current density flows through the sample at lower electric fields, which guarantees normal grain growth and highest final density. Macroscopic photoluminescence measurements give insights into the development of the defect structure. Electric fields are expected to enhance defect mobility, explaining the high densification rates observed during the sintering process.     

Sintering of High-Purity Nickel Oxide

The results of an investigation of densification and grain growth during the sintering of high-purity NiO in various atmospheres are presented. The sintering of NiO powders is divided into three stages. Grain growth during the third stage follows the empirical equation D ²= K t, where D is the average grain diameter, K a rate constant, t the sintering time. The activation energy of grain growth was found to be 55 kcal. per mole. The shrinkage and weight loss during the sintering process in vacuum were larger than in argon, air, and oxygen. Grain growth during sintering also differed with the sintering atmosphere. It was concluded that the sintering process is not explained reasonably by the theory of a semiconductor.     

Low-Temperature Sintering of Aluminum Oxide

A fine-sized (∼0.1 μm), agglomerate-free Al₂O₃ dispersion was used to prepare homogeneous green bodies with ∼69% relative density and ∼10-nm median pore radius. Samples could be sintered at 1150°C to a relative density >99.5% and an average grain size of 0.25 μm.     

Comparative Study of Microwave Sintering of Zinc Oxide at 2.45, 30, and 83 GHz

Temperature gradients that develop in ceramic materials during microwave heating are known to be strongly dependent on the applied microwave frequency. To gain a better understanding of this dependence, identical samples of ZnO powder compacts were microwave heated at three distinct widely separated frequencies of 2.45, 30, and 83 GHz and the core and surface temperatures were simultaneously monitored. At 2.45 GHz, the approximately uniform volumetric heating tends to raise the temperature of the sample as a whole, but the interior becomes hotter than the exterior because of heat loss from the surface. At 30 and 83 GHz, this interior to exterior temperature difference was found to be reversed, especially for high heating rates. This reversal resulted from increased energy deposition close to the sample's surface associated with reduced skin depth. A model for solving Maxwell's equations was incorporated into a newly developed two-dimensional (2-D) heat transport simulation code. The numerical simulations are in agreement with the experimental results. Simultaneous application of two or more widely separated frequencies is expected to allow electronic tailoring of the temperature profile during sintering.     

Sintering Behavior of Beryllium Oxide

The sintering behavior of beryllia in a reducing atmosphere was studied between 1500° and 2100°C. Above 1700°C. the firing temperature had only a small effect on the density after heating for 24 hours. Examination of the time dependence of sintering showed that at 1700°C. during the first 3 to 5 hours there was a large increase in the density of the body accompanied by a simultaneous rapid rate of grain growth. Firing for longer times resulted in more moderate increases in both density and grain growth. The grain-growth characteristics of beryllia were unchanged by most oxide additions although compacts of higher density resulted. Oxide additives which formed a liquid phase at the sintering temperature enhanced both the sinterability and grain growth of beryllia.     

Sintering in Active Nickel Oxide

Decomposition of nickel oxalate at 450°C in a stream of oxygen for 0.5 hr produced very reactive nickel oxide. Sintering rates in this material were faster (by factors up to 40) than in unreactive nickel oxide, produced by calcining nickel oxalate at 900°C for 5 hr in air. The Arrhenius activation energies for densification were significantly different in the two systems. Preliminary studies confirmed that normal grain growth, which follows a law of the form G³ = kt , occurs in nickel oxide.     

Sintering of Fine Oxide Powders: II, Sintering Mechanisms

Conventional and new sintering mechanisms have been investigated using fine powders of CeO₂ and Y₂O₃ of excellent sinterability. We have verified the validity of Herrings scaling law for 60%–84% relative density and found that it is consistent with grain-boundary-diffusion control. At lower densities, we have found that pores larger than the critical size, in the sense of Kingery and Francois, can still be sintered readily. This is rationalized by a new sintering mechanism based on particle repacking concurrent with particle coarsening, resulting in a higher packing factor. Very fine, surface-active powders that coarsen rapidly are uniquely capable of taking advantage of this new sintering mechanism, which along with their propensity to homogenization, accounts for their remarkable sinterability even at very low green densities.     

Initial Sintering Kinetics of Beryllium Oxide

The initial sintering kinetics of beryllium oxide are examined, including the effects of calcining temperature, purity, and water-vapor content of the sintering atmosphere. Sintering occurs by a diffusion mechanism in dry-air atmospheres. Small additions of MgO to BeO increase the sintering rate markedly. The diffusion constant calculated from sintering data for pure BeO is three orders of magnitude lower than the measured diffusion of Be in BeO. Water vapor in the sintering atmosphere slows the shrinkage rate of pure BeO.     

Zinc Vanadates in Vanadium Oxide-Doped Zinc Oxide Varistors

Convergent-beam electron diffraction has been used to determine the space groups of β- and γ-Zn₃(VO₄)₂ particles in vanadium oxide-doped zinc oxide varistors. The crystal structure of β-Zn₃(VO₄)₂ has been determined to be monoclinic with space group P 2₁ and lattice parameters of a = 9.80 Å, b = 8.34 Å, c = 10.27 Å, and β= 115.8°, whereas that of γ-Zn₃(VO₄)₂ is monoclinic with space group Cm and a = 10.40 Å, b = 8.59 Å, c = 9.44 Å, and β= 98.8°. Energy-dispersive X-ray microanalysis of these two phases shows significant deviations from their expected stoichiometry. It is apparent that the β-phase is, in fact, the metastable Zn₄V₂O₉ phase, whereas the γ-phase either is a new oxide that consists of zinc, vanadium, and manganese or, more likely, is a zinc vanadate phase with a Zn:V atomic ratio of 1:1 that has the ability to go into solid solution with manganese.