Influence of solid loading and particle size distribution on the porosity development of green alumina ceramic mouldings

Alumina ceramic mouldings with different solid contents ranging from 55 to 70 vol% and different ratios of coarse/fine powders, i.e. 0.4 μm (fine) and 3 μm (coarse), respectively, were prepared by compression moulding at 75 °C under a compressive stress of 10 MPa. The porous parameters, such as porosity, pore size and pore size distribution, of the green compacts were evaluated after removal of organic vehicles. Experimental evidence showed that the green density, as well as the sintered density, of the moulded alumina increased linearly with increased solid loading to an optimum of 65 vol% and decreased roughly linearly with increased coarse/fine ratio. Further increase in solid loading reduced particle packing efficiency, resulting in lower green and fired densities. No considerable improvement in green and sintered density of the moulded alumina was achieved by adjusting the coarse/fine ratio, which is due to the fact that coarse particles suppress the driving force of densification. The green compacts generally showed a bimodal pore size distribution character which may be the most important factor in dominating the densification of the powder compacts. The peak frequency at larger pore region is approximately 20–35 μm in diameter and at the smaller pore region is ˜50–95 nm in diameter. The larger pores are believed to be due to the presence of internal voids originating from entrapped gas and are probably caused by the removal of organic vehicles.     

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.     

Limitations on the sintering of graded particle systems

Improvement of compact density is commonly achieved by blending coarse and fine particles, but these compacts will not densify without the presence of a significant amount of liquid phase. It was proposed that two step sintering (TSS) could be applied to sinter the fine particle matrix, potentially accommodating the presence of inclusions of large particles. This hypothesis was false. Compacts were prepared with similar green density but with different ratios of coarse, medium, and fine particles and then subjected to TSS. The results indicated that constrained sintering limits densification on both ends of the particle packing spectrum: A fine particle matrix containing large particles fails to densify because the matrix cannot shrink around the inclusion; the densification of fine particle pockets in a skeletal network composed of large particles does not allow sufficient shrinkage in the pockets of small particles.     

Low-temperature sintering of coarse alumina powder compact with sufficient mechanical strength

Coarse alumina powder compacts doped with various amounts of titania and copper oxide were pressurelessly sintered from 900 °C to 1600 °C. Their phase assemblages and microstructural evolution, as well as their properties, were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry/thermogravimetric (DSC/TG) analysis, and three-point bending and wetting test. The role of TiO₂ and CuO during the sintering is discussed in detail. The experimental results show that the liquid phase from the copper oxide appeared at approximately 1200 °C, so the solid-state reaction between alumina and titania took place at a lower temperature. Such solid state-reaction sintering had a strong impact on the grain growth and greatly promoted the densification of the alumina compact. In addition, the liquid phase inhibited the abnormal grain growth and microcracking. As a result, the coarse alumina powder compacts doped with 5 wt% TiO₂–CuO were fully densified and exhibited sufficient flexural strength (342±21 MPa) when sintered at a temperature of 1450 °C for 2 h.     

Effect of Calcination on the Sintering of Gel-Derived, Zirconia-Toughened Alumina

The densification behavior of ZrO₂ (+ 3 mol% Y₂O₃)/85 wt% Al₂O₃ powder compacts, prepared by the hydrolysis of metal chlorides, can be characterized by a transition- and an α-alumina densification stage. The sintering behavior is strongly determined by the densification of the transition alumina aggregates. Intra-aggregate porosity, resulting from calcination at 800°C, partly persists during sintering and alumina phase transformation and negatively influences further macroscopic densification. Calcination at 1200°C, however, densifies the transition alumina aggregates prior to sintering and enables densification to almost full density (96%) within 2 h at 1450°C, thus obtaining a microstructure with an alumina and a zirconia grain size of 1 μm and 0.3–0.4 μm, respectively.     

Effect of Seeding on Phase Development, Densification Behavior, and Microstmcture Evolution in Mullite Fabricated from Microcomposite Particles

An investigation was carried out concerning the effect of mullite seed particles on phase development, densification behavior, and microstructure evolution in powder compacts prepared with silica/alumina microcomposite particles. The incorporation of ∼2 wt% seed particles in the microcomposite powder compacts had relatively little effect on densification, but resulted in significant decreases in the temperature for mullite formation and the grain sizes in mullitized samples. Samples could be sintered to almost full density and subsequently converted to mullite with average grain sizes ≤0.4 μm at temperatures in the range of 1300°-1400°C. The available evidence indicated that mullite formation occurred primarily by nucleaton and growth in the siliceous matrix phase.     

Densification and Grain Growth of Porous Alumina Compacts

Densification and grain growth of porous alumina compacts during various high-temperature processes were investigated. Experimental data were obtained for densification and grain growth of alumina powder during hot pressing. A set of constitutive equations was proposed based on the constitutive equations by Helle et al. ¹ for hydrostatic response and by Rahaman et al. ² for deviatoric response. Theoretical results from the proposed constitutive equations were compared with various experimental data for alumina powder compacts in the literature, including pressureless sintering, sinter forging, and hot pressing. The proposed model well predicts the densification and grain growth of alumina compacts.     

Sintering of Bimodally Distributed Alumina Powders

The densification of alumina powders prepared to have a bi-modal particle-size distribution with a coarse-to-fine particle-size ratio of ≊ 10 can be predicted to a first approximation if the densification of the fine and coarse powders is known. Deviations from model prediction were attributed to compositional heterogeneity. Microstructural observations of bimodal powders show that only the fine powder undergoes grain growth. Thus, the grain-size distribution becomes more uniform than the initial particle-size distribution.     

Enhancement of reaction-sintering of alumina-excess magnesium aluminate spinel in presence of titania

Alumina-excess magnesium aluminate spinel finds use in different high temperature applications including steel ladles. Alumina-excess spinel was prepared by solid oxide reaction using magnesia (MgO=10?wt%) and calcined alumina (Al₂O₃ = 90?wt%), in the sintering temperature range of 1500–1700?°C. The role of titania on the densification, spinelisation, evolution of microstructure and phase assemblage was investigated in this MgO-Al₂O₃ system. Titania addition increased the rate of densification 20x compared to undoped composition at 1500?°C under dynamic heating condition. However, under static firing, the beneficial effect of titania on densification could only be discerned at lower temperatures. The microstructure of titania doped sintered alumina-excess spinel compacts contain magnesium aluminium titanate phase in the grain boundary of corundum and spinel grains. The beneficial effect of titania on densification is attributed to magnesium aluminium titanate phase (Mg_xAl_2(1-x)Ti_(1+x)O₅) development and also by incorporation of Ti⁴⁺ into the spinel structure.     

Shear Deformation and Densification of Powder Compacts

Kinetics of densification and deformation can be studied simultaneously by measuring the time-dependent axial and radial strains in cylindrical specimens of powder compacts when they are sintered under a superimposed uniaxial load. Such experiments, carried out on powder compacts of magnesia-doped alumina, are reported. Several results have been obtained. The experiments have provided a direct measure of the intrinsic sintering pressure, which we find to lie in the range 0.4 to 0.8 MPa. The deformation rate, or shear rate of the porous body, is found to follow Coble creep behavior except for a stress enhancement factor arising from the reduced effective cross-sectional area from the presence of voids. The rate of densification follows a linear dependence on the total mean pressure and a cubic dependence on the average grain size.     

Microstructural Development during Sintering of Lithium Fluoride

Measurements are reported of the influences of temperature, green density, and pore network breakup on the densification, grain growth, and pore volume distribution in LiF compacts. As long as most of the pore volume remained open to the compact perimeter, the ratio of the rate of densification to the rate of grain growth was higher than that sometimes reported for copper or typical oxides. Plots of the logarithm of densification rates versus sintered density for LiF are approximately linear during intermediate-stage sintering, like those for some oxides. But the plots for LiF are unlike those of the oxides in that, for LiF, densification rates measured at different temperatures converge near the density at which half the pore volume is isolated from Hg intrusion. Calculations suggest that further densification of the LiF compacts is blocked because air trapped in isolated pores becomes sufficiently compressed to balance the sintering stress.     

Processing of Al2O3/SiC nanocomposites—part 2: green body formation and sintering

Pressure filtration was used to form green compacts from aqueous slurries of alumina with 5 vol.% silicon carbide. Green densities of 64%TD were achieved for slurries containing a 50 vol.% solids loading. Lower green densities were obtained for a very fine alumina due to the practical limits on maximum slurry solids loading when using finer powders. The samples were dried in a purposely built humidity cabinet to limit sample cracking. It was found that a higher consolidated layer permeability gave a higher initial drying rate. Near fully dense (99% TD) nanocomposites were produced, via pressureless sintering at 1900 °C. Poor sintered densities were obtained in the case of the fine alumina because of localised sintering of these low green density compacts. The required intra/inter-granular nanocomposite microstructures have been obtained for several different systems, with an average grain size of approximately 5 μm. Abnormal grain growth was noted for samples containing the larger particle size silicon carbide. This shows that a maximum particle size limit exists when selecting the powders for a 5 vol.% nanocomposite.     

团聚氧化镁粉料压块的烧结机理与动力学模型

团聚粉料压块具有由高烧结性粉粒所构成的双层堆积结构。在烧结的第一阶段，一级颗粒快速烧结导致二级颗粒的收缩与重排并增加大气孔。在烧结的第二阶段，一级与二级颗粒的烧结同时进行，但压块的烧结是被一级颗粒的烧结所控制。某些中期烧结模型与试验结果吻合。在烧结的第三阶段中，压块的致密和颗粒长大同时进行，其特点是大量的小晶粒晶界存在，气孔有很高的配位数。提出一个方程式，它与团聚氧化镁试块烧结试验结果相一致。     

Morphological Changes in Process-Related Large Pores of Granular Compacted and Sintered Alumina

Large pore defects clearly develop in Al₂O₃ ceramics during sintering. These large pores originate from voids caused by the incomplete deformation and adhesion of powder particles in collapsed dimples at the centers and boundaries of granules in the green compacts. The coalescence of pores, with limited shrinkage, during densification and grain growth in the late intermediate to final stages of sintering, is considered responsible for the development of the large pores. The mechanism of pore coalescence is explained by thermodynamic arguments, which demonstrate that the largest pores result in a stable system.     

Rapid Rate Sintering of Nanocrystalline ZrO2−3 mol% Y2O3

Conventional ramp-and-hold sintering with a wide range of heating rates was conducted on submicrometer and nanocrystalline ZrO₂–3 mol% Y₂O₃ powder compacts. Although rapid heating rates have been reported to produce high density/fine grain size products for many submicrometer and smaller starting powders, the application of this technique to ZrO₂–3 mol% Y₂O₃ produced mixed results. In the case of submicrometer ZrO₂–3 mol% Y₂O₃, neither densification nor grain growth was affected by the heating rate used. In the case of nanocrystalline ZrO₂–3 mol% Y₂O₃, fast heating rates severely retarded densiflcation and had a minimal effect on grain growth. The large adverse effect of fast heating rates on the densification of the nanocrystalline powder was traced to a thermal gradient/differential densification effect. Microstructural evidence suggests that the rate of densification greatly exceeded the rate of heat transfer in this material; consequently, the sample interior was not able to densify before being geometrically constrained by a fully dense shell which formed at the sample exterior. This finding implies that rapid rate sintering will meet severe practical constraints in the manufacture of bulk nanocrystalline ZrO₂–3 mol% Y₂O₃ specimens.     

Reactive Hydrothermal Liquid‐Phase Densification (rHLPD) of Ceramics – A Study of the BaTiO3[TiO2] Composite System

A densification process called reactive hydrothermal liquid‐phase densification (rHLPD), based on principles of hydrothermal reaction, infiltration, reactive crystallization, and liquid‐phase sintering, is presented. rHLPD can be used to form monolithic ceramic components at low temperatures. The densification of barium titanate–titania composite monoliths was studied to demonstrate proof of concept for this densification model. Permeable, green titania (anatase) compacts were infiltrated with aqueous barium hydroxide solutions and reacted under hydrothermal conditions in the temperature range 90°C–240°C. The effects of reaction time and temperature on the conversion of titania (anatase) into barium titanate were studied. Utilizing a 72 h reaction at 240°C between l.0 M Ba(OH)₂, an anatase (TiO₂) powder compact, and a corresponding Ba/Ti ratio of 1.5, it was possible to crystallize a composite 95 wt% (88 mol%) BaTiO₃ and 5 wt% (12 mol%) TiO₂. The composite had a relative density of ~90% with a compressive strength of 172 ± 21 MPa and a flexural strength of 49 ± 4 MPa.     

Effect of Yttria and Yttrium-Aluminum Garnet on Densification and Grain Growth of Alumina At 1200°–1300°C

Densification and grain growth of alumina were studied with yttria or yttrium-aluminum garnet (YAG) additives at the relatively low temperatures of 1200°–1300°C. Yttria doping was found to inhibit densification and grain growth of alumina at 1200°C and, depending on dopant level, had a lesser effect at 1300°C. At 1200°C, yttria inhibits densification more than it hinders grain growth. The rate of grain growth increases faster with temperature than the rate of densification. Alumina-YAG particulate composites were difficult to sinter, yielding relative densities of only 65% and 72% after 100 h at 1200° and 1300°C, respectively. Pure YAG compacts exhibited essentially no densification for times up to 100 h at 1300°C.     

Densification and microstructure evolution during sintering of silicon under controlled water vapor pressure

Sintering of fine silicon powder was studied under controlled water vapor pressures using the Temperature–Pressure–Sintering Diagram approach. The water vapor pressure surrounding the sample was deduced from thermogravimetric analysis and related to the water content of the incoming gas flux with a simple mass transfer model. The thickness of the silica layer covering silicon particles was then monitored by the water vapor pressure and the microstructure evolution and densification during sintering could be controlled. Stabilizing the silica layer indeed inhibits grain coarsening and allows better densification of the compacts under humidified atmosphere as compared to dry atmosphere.     

Preparation of SiO2 Glass from Model Powder Compacts: III, Enhanced Densification by Sol Infiltration

Highly ordered powder compacts were formed using spherical, nearly monosized, amorphous SiO₂ particles. Some of these compacts were subsequently infiltrated with a "silica'sol. After drying, the powder compact interstices contained small, porous xerogel particles, thereby increasing the green relative density of the compact. Sintering studies showed a significant enhancement in densification for the sol-infiltrated samples.     

Initial Sintering with Constant Rates of Heating

Initial sintering of several materials was studied by measuring powder compact densification at constant rates of heating (CRH). The CRH technique was extremely sensitive to the particle size distribution and other characteristics of the compacts. Although the CRH method circumvents several problems encountered in isothermal studies, it cannot be used to identify the mechanism of diffusion. Using the method on carefully prepared alumina powder compacts and assuming a grain-boundary diffusion mechanism, an activation energy of 115±10 kcal/mol was obtained. Zirconia (yttria-stabilized) and titania also exhibited a single densification mechanism with diffusion coefficients which correlate well with values obtained by isothermal measurements.