Effect of Inclusions on Densification: I, Microstructural Development in an Al2O3 Matrix Containing a High Volume Fraction of ZrO2 Inclusions

The microstructural changes produced by large (38 to 53 μ m), single-crystal ZrO₂ inclusions (0, 0.09, 0.30 volume fractions, based on solid volume) within an Al₂O₃ powder matrix were detailed as a function of constrained densification. Composite powder compacts were produced by pressure filtration for conditions where the Al₂O₃ slurry was either flocced or dispersed. For both conditions, the ZrO₂ inclusions constrained densification. Microstructural observations for all composites revealed (1) the presence of cracks with large opening displacements between inclusions and (2) large density variations within the matrix. The cracks were most frequent at high volume fraction of inclusions in composites produced from flocced slurries and apparently originated during specimen preparation. Their large opening displacment was a result of matrix densification. Fewer cracks were observed in composites produced from dispersed slurries. Instead, these microstructures were dominated by large variations in matrix density, viz., dense regions surrounding low-density regions, not consitent with the initial packing density of the matrix powder. The denser regions were formed early in the densification schedule. The lower-density regions eventually developed into regions containing large, elongated voids as the Al₂O₃ matrix grains became larger with heat-treatment time. This pore enlargement process was shown to result from the disappearance of necks between originally sintered grains and appeared similar to the thermodynamic instability observed in thin films and constrained fibers.     

Effect of Inclusions on Densification: II, Numerical Model

Experimental studies conducted in conjunction with a numerical analysis of strain-rate gradients have established the origin of the damage process that occurs upon the densification of powders containing nonsintering inclusions and reinforcements. The underlying phenomenon is the development of localized compressive strains in the porous matrix, both around high-aspect ratio reinforcements and between closely spaced reinforcements. These regions of compression densify first and then support grain growth. This process produces nondeformable networks that constrain the shrinkage of the adjacent, porous matrix. The constraint causes desintering and cracklike void formation in the lowdensity regions. The variables shown to be of importance are the volume fraction and aspect ratio of the reinforcements. The process is shown to be sensitive to the green density, such that a high initial density reduces initial damage and lowers the differential in densification.     

The Wulff Shape of Alumina: II, Experimental Measurements of Pore Shape Evolution Rates

The rate at which a facetted tetragonal cavity of nonequilibrium shape approaches a cubic equilibrium (Wulff) shape via surface diffusion was modeled. The shape relaxation rate of a facetted stretched cylinder was also modeled. For the first geometry, only an approximate solution based on linearizing the mean potential difference between the source and sink facets was obtained. For the stretched cylinder, both an approximate and an exact solution can be obtained; the approximate solution underestimates the evolution rate by a factor of ∼2. To assess the applicability of the models, nonequilibrium shape pores of identical initial geometry (∼20 μm × 20 μm × 0.5 μm) were introduced into (0001), {10[Onemacr]2}, {1120}, and {100} surfaces of sapphire single crystals using microfabrication techniques, ion-beam etching, and hot pressing. The large (∼20 μm × 20 μm) faces of the pore are low-index surfaces whose nature is dictated by the wafer orientation. A series of anneals was performed at 1900°C, and the approach of the pore shape to an equilibrium shape was monitored. The kinetics of shape evolution are highly sensitive to the crystallographic orientation and stability of the low-index surface that dominates the initial pore shape. The measured variations of the pore aspect ratio were compared to those predicted by the kinetic model. The observations suggest that when the initial bounding surface is unstable, shape relaxation may be controlled by diffusion. However, surface-attachment-limited kinetics (SALK) appears to play a major role in determining the pore shape evolution rate in cases where the initial bounding surfaces have orientations that are part of the Wulff shape.     

Effect of Pore Distribution on Microstructure Development: I, Matrix Pores

A model has been developed to describe the effect of the matrix (first-generation) pore distribution on microstructure development in the final stages of sintering. A model of simultaneous densification and grain growth was used to predict the effects of the number of pores per grain and the pore size distribution on microstructure evolution. Increasing the number of pores per grain was predicted to increase the densification rate, the grain growth rate, and the relative densification rate/grain growth rate ratio. Narrowing the pore size distribution was predicted to inhibit grain growth initially and to increase the densification rate indirectly. Overall, the pore distribution was predicted to have a strong influence on microstructure development and sintering kinetics.     

Effect of Surface Impurities on the Microstructure Development during Sintering of Alumina

Microstructural evolution during sintering of alumina powder compacts prepared by cold isostatic pressing (CIP) was monitored. For CIP, rubber molds lubricated with silicone oil were used so that a very small amount of impurity was introduced to the surface of the powder compacts. During sintering at 1600°C, grain growth in the surface region was inhibited up to sintering for 1 h, but subsequently abnormal grain growth occurred. In the inner region, however, the grains grew uniformly without abnormal grain growth. Impurities that initially drag the boundary migration but form liquid at the end are suggested to cause abnormal grain growth.     

Effect of Pore Distribution on Microstructure Development: II, First- and Second-Generation Pores

A sintering model has been developed to predict the consequences of independently varying the grain growth rate in alumina during final-stage sintering of a microstructure containing both small (first-generation) and large (inter-agglomerate second-generation) pores. The model shows that although it may be thermodynamically favorable to increase the grain growth rate, the kinetics of densification are such that it almost always pays to inhibit grain growth. This conclusion was verified by experiments on undoped, MgO-doped, and ZrO₂-doped alumina impregnated with model spherical large pores produced by the burnt-out latex sphere method. A new type of ceramic processing map has also been developed to aid in the selection of the optimum processing conditions for the sintering of ceramics containing large pores.     

The Wulff Shape of Alumina: III, Undoped Alumina

Controlled-geometry cavities were introduced into the m{10     0} plane of undoped sapphire substrates using photolithographic methods, and subsequently internalized by diffusion bonding the etched sapphire to an undoped high-purity polycrystalline alumina. Pore-boundary separation during growth of the sapphire seed into the polycrystal entrapped the pores within the single crystal. Pores with an equivalent spherical radius of ≈1 μm reached a quasi-equilibrium shape after prolonged anneals at 1600° and 1800°C. The introduction of mechanically induced surface defects accelerated pore shape equilibration. The Wulff shape of undoped alumina was determined by characterizing the shape and facet structure of these equilibrated internal pores using optical microscopy, scanning electron microscopy, and atomic force microscopy. The observed planes in the Wulff shape of undoped alumina, c(0001), r{     012}, s{1     01}, a{11     0}, and p{11     3} planes, were consistent with those reported by Choi et al .; however, a different energy sequence is inferred. The absence of the m-plane in the Wulff shape is consistent with other experimental studies, but inconsistent with those lattice simulations that predict the m-plane to be one of the lowest energy planes in pure alumina. A comparison of Wulff shapes at 1600° and 1800°C suggests that the surface energy of undoped alumina becomes more isotropic as temperature increases.     

Alumina–Aluminide Alloys (3A) Technology: I, Model Development

The general reaction behavior of the 3A process under the thermal explosion mode of synthesis has been investigated via a continuum model. The continuum model uses mass and energy balances to predict temperature difference ( T _s,avg− T _f) curves, as well as profile curves of the reactant conversions and sample temperature. In particular, the effect of the dimensionless parameters associated with the rate of local heat generation (β, the thermicity factor), the activation energy (γ, the Arrhenius number), the rate of heat redistribution (α, the modified thermal diffusivity), the rate of heat transfer by convection (Bi, the Biot number or convective heat transfer parameter), and the rate of heat transfer by radiation (Ω, the radiative heat transfer parameter) were investigated. Conditions to control the reaction process, which should produce high-density final products, were determined. It was found that the overall maximum temperature may be reduced for high γ, low β, high α, and high Bi and Ω. In terms of processing conditions, this may be obtained by reducing the initial reactant concentrations, optimizing the particle size, using small sample sizes and high compaction pressure, and increasing the heat loss by using a high thermal conductivity inert gas.     

Thermodynamics of Densification: I, Sintering of Simple Particle Arrays, Equilibrium Configurations, Pore Stability, and Shrinkage

Equilibrium configurations for linear and closed arrays (rings and regular polyhedra containing a single pore) of identical particles (cylinders or spheres) were determined by minimizing the array's surface and grain-boundary energies with the assumption that each particle conserves its mass. The change in free energy between the initial and equilibrium configuration increases with dihedral angle (i.e., the equilibrium angle). More significantly, it is shown that pores will shrink to an equilibrium size if the number ( n ) of coordinating particles is greater than a critical value. The critical pore coordination number ( n _c) increases with the dihedral angle. Only pores with n n _c are thermodynamically unstable during sintering. It is also shown that any mass-transport mechanism can lead to pore shrinkage while a connecting path to the pore surface remains open. The effective sintering "stress" (i.e., driving force) increases with the dihedral angle and decreases to zero as the equilibrium configuration is reached. Sintering stresses increase with decreasing coordination number. It is also shown that the shrinkage strain for closed arrays increases with the pore coordination number. Rearrangement phenomena within a powder compact are discussed with regard to resultant sintering forces on nonsymmetrically coordinated particles.     

The Effects of Na2O and SiO2 on Liquid Phase Sintering of Bayer Al2O3

To determine how grain‐boundary composition affects the liquid phase sintering of MgO‐free Bayer process aluminas, samples were singly or co‐doped with up to 1029 ppm Na₂O and 603 ppm SiO₂ and heated at 1525°C up to 8 h. Na₂O retards densification of samples from the onset of sintering and up to hold times of 30 min at 1525°C compared to the undoped samples, but similar to the as‐received, MgO‐free Al₂O₃, Na₂O‐doped samples sinter to 98% density with average grain sizes of ~3 μm after 8 h. Increasing SiO₂ concentration significantly retards densification at all hold times up to 8 h. The estimated viscosities (20?400 Pa·s) of the 0.3 to 1.8 nm thick siliceous grain‐boundary films in this study indicate that diffusion greatly depends on the composition of the liquid grain‐boundary phase. For low Na₂O/SiO₂ ratios, densification of Bayer Al₂O₃ at 1525°C is controlled by diffusion of Al³⁺ through the grain‐boundary liquid, whereas for high Na₂O/SiO₂ ratios, densification can be governed by either the interface reaction (i.e., dissolution) of Al₂O₃ or diffusion of Al³⁺. Increasing Na₂O in SiO₂‐doped samples increases diffusion of Al³⁺ and Al₂O₃ solubility in the liquid, and thus densification increases by 1%. Based on these findings, we conclude that Bayer Al₂O₃ densification can be manipulated by adjusting the Na₂O to SiO₂ ratio.     

Powder chemistry effects on the sintering of MgO‐doped specialty Al2O3

In this work, we investigate the effects of powder chemistry on the sintering of MgO‐doped specialty alumina. The stages at which MgO influences densification of Al₂O₃ were identified by comparing dilatometry measurements and the sintering kinetics of MgO‐free and MgO‐doped specialty alumina powders. MgO is observed to reduce the grain boundary thickness during densification using TEM. We show that MgO increases the solubility of SiO₂ in alumina grains near the boundaries using EDS. First‐principles DFT calculations demonstrate that the co‐dissolution of MgO and SiO₂ in alumina is thermodynamically favored over the dissolution of MgO or SiO₂ individually in alumina. This study experimentally demonstrates for the first time that removal of SiO₂ from the grain boundaries is a key process by which MgO enhances the sintering of alumina.     

Synthesis of Porous Ceramics with Complex Pore Structure by Freeze-Dry Processing

Porous ceramics with complex pore structure were synthesized by a freeze-dry process. Freezing-in of a water-based ceramics slurry was done while controlling the growth direction of the ice. Sublimation marks of the ice were generated by drying under reduced pressure. Porous ceramics having a complex pore structure were obtained by sintering the green body: aligned macroscopic open pores contained micropores in their internal walls. The pore structure was substantially affected by the starting slurry concentration and sintering temperature. The pore formation mechanism is discussed in relation to these effects.     

Densification Behaviors of Fine-Alumina and Coarse-Alumina Compacts during Liquid-Phase Sintering with the Addition of Talc

The densification behavior of fine alumina (mean particle size of ∼0.31 μm) and coarse alumina (mean particle size of ∼4.49 μm) during liquid-phase sintering with additions of talc have been studied, as well as the microstructural evolution. Small amounts (0, 5, and 10 wt%) of talc were added to the fine alumina and coarse alumina, which were sintered at various temperatures for 2 h. When 5 wt% of talc was added to the coarse alumina, densification proceeded rapidly above the liquid-formation temperature in alumina–talc compacts, because of the promotion of a rearrangement process of the solid grains by the liquid phase. The addition of 10 wt% of talc greatly accelerated densification by increasing the volume fraction of liquid. On the other hand, in the fine alumina, which has a higher activity and a greater driving force for sintering, appreciable densification started below the liquid-formation temperature, which prevented further densification after liquid formation. Moreover, the densification was suppressed as the talc content increased. The rigid skeleton of solid grains that was formed by densification below the liquid-formation temperature is believed to have suppressed the rearrangement process of the solid grains, and further densification of the compacts was retarded, even after the formation of a liquid phase above the liquid-formation temperature.     

Elimination of large Pores During Gas-Pressure Sintering of β'-Sialon

The effect of N₂-gas pressure on the liquid filling of large pores (20 to 120 μm in diameter) is studied in sintered β'-sialon ( z = 1). The initially sintered sialon with large pores is sintered again and infitrated by a liquid (41A1₂O₃-41Y₂O₃-14Si₃Y₄-4AIN (wt%)) at 17800°C for various times under 0.1-, 0.3-, and 0.5-MPa (1-, 3-, and 5-atm) N₂. The liquid fills large interconnected pores; the size of the pores filled with liquid increases with N₂-gas pressure and time. In some liquid pockets, gas bubbles are formed and subsequently disappear during prolonged sintering treatment. The liquid-filling be havior with sintering pressure and time is explained by gas pressure in the pore and thermal decomposition of the material. The benefit of gas-pressure sintering for the elimination of large pores is assessed.     

Deterioration of a Classical Final-Stage Microstructure: A Study in Alumina

Classical final-stage microstructures have been obtained in an ultra-high-purity alumina by a latex sphere impregnation and burnout technique. The isolated equilibrium-shaped pores produced by the latex spheres deteriorated during long-term, high-temperature annealing under reducing atmospheres to produce a cracklike interconnected pore network. Several mechanisms of degradation are considered, including (i) the influence of entrapped gases, (ii) the effect of stresses produced during differential sintering, and (iii) the kinetics of competing mass transport mechanisms. Results suggest that the degradation is an inherent microstructural instability promoted by enhancing the ratio of coarsening rate/densification rate. Furthermore, it is shown that magnesia additions inhibit the process of microstructural deterioration.     

Evolution of the Pore Size Distribution in Final-Stage Sintering of Alumina Measured by Small-Angle X-ray Scattering

Small-angle X-ray scattering was used to follow the evolution of the pore size distribution during final-stage sintering of alumina and of alumina doped with 0.25 wt% magnesia. The volume-weighted (Guinier) results indicate that the effective size of the largest pores increases as the body goes from 97% to more than 99% dense. The surface-area-weighted (Porod) results show that the median size of the smallest pores decreases slightly over the same density range. Taken together, these data indicate that the pore size distribution becomes broader as final-stage densification proceeds. This was confirmed by a maximum entropy analysis, which was used to derive pore size distributions directly from the data. Finally, the evolution of the pore size distributions in alumina, with and without sintering aid, were compared.     

Densification behaviors of Al2O3 ceramics during flash sintering

The densification behaviors of MgO-doped-Al₂O₃ ceramics in the flashing stage and the steady stage were investigated using the classic kinetic model. The results show that the most densification of MgO-doped Al₂O₃ was completed during the flashing stage. The densification mechanism transferred from particle rearrangement resulted from Columbic force among particles under the effect of electrical field in the flashing stage to the lattice diffusion in the steady stage. Therefore, the densification rate in the steady stage dramatically decreased. Additionally, the estimated densification activation energy in the steady stage of flash sintering is 396 kJ/mol, much lower than the activation densification of lattice diffusion measured from conventional sintering, likely due to the effect of electric field/current-induced point defects on the diffusion.     

CaTiO3添加剂对氧化铝陶瓷烧结、显微结构及微波介电性能的影响

CaTiO3添加剂通过湿法球磨与Al2O3粉料混合,并通过无压烧结制备了氧化铝陶瓷,研究了CaTiO3添加剂对氧化铝陶瓷烧结性能、相组成、显微结构和微波介电性能的影响.CaTiO3可以使Al2O3的烧结温度降低至1450℃,但在该温度下烧结的样品由于CaTiO3的加入会产生CaAl12O19第二相.样品中存在大、小两种晶粒,根据EDS能谱分析,大晶粒主要是CaAl12O19,而小晶粒为Al2O3和CaAl12O19的混合相.添加CaTiO3有利于Al2O3陶瓷介电常数的提高,1450℃下掺杂2.5wt％ CaTiO3的氧化铝陶瓷具有较好的烧结性能和微波介电性能,相对密度可达到97.74％,εr～10.86,Q×f～ 8061 GHz.     

Segregation of Magnesium to the Internal Surface of Residual Pores in Translucent Polycrystalline Alumina

For the first time direct evidence for Mg segregation to the surface of pores within translucent polycrystalline alumina grains has been found using energy dispersive X-ray spectroscopy (EDXS) and convergent beam electron diffraction (CBED) in an analytical electron microscope (AEM) on a submicrometer scale. This supports the model that MgO dopant increases the surface diffusivity which, in turn, increases the pore mobility. The MgO dopant's role in retarding grain growth in combination with the enhanced pore mobility allows the achievement of nearly full density and translucency in alumina.     

基于注射成型并三步烧结法制备透明氧化铝陶瓷

本文采用注射成型制备氧化铝坯体,并用三步烧结法获得了完全致密的透明氧化陶瓷.研究了直线透光率与烧结方法,微观结构的关系.结果表明基于注射成型并采用三步烧结方法制备的氧化铝陶瓷没有晶粒异常长大,总透光率和直线透光率分别达到了71％和50.8％.用三步烧结法制备的氧化铝陶瓷直线透光率比一步法制备的氧化铝陶瓷要高25.8％.