High-Temperature Young's Modulus of Alumina During Sintering

High-temperature Young's modulus of a partially sintered alumina ceramic has been studied dynamically during the sintering process. Comparative, room-temperature Young's modulus data were obtained for a suite of partially sintered alumina compacts with different porosities. The dynamic Young's modulus of a 1200°C partially sintered material was observed to decrease linearly with temperature, but then above 1200°C it increased sharply as sintering and densification of the alumina became dominant. The evolution of the Young's modulus due purely to sintering exhibited an exponential relationship with porosity in excellent agreement with room-temperature measurements of equivalent porous alumina ceramics.     

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.     

Experimental Assessment of Pore Breakaway During Sintering

Experimental measurements of intragranular pore-size distributions and of the shapes of distorted pores attached to grain boundaries were used to examine the podgrain-boundary separation problem. In situ evaluation of the ratio of the boundary mobility-to-surface diffusivity from pore distortion measurements, coupled with prior analytic expressions for the critical pore breakaway condition, permitted specific intragranular pore sizes generated by separation to be predicted. The predictions lie within the distribution of measured intragranular pore sizes.     

Sintering and Phase Evolution of Electroless-Nickel-Coated Alumina Powder

Alumina-nickel powders have been prepared via electroless-nickel (EN) coating of submicrometer-sized alumina powder. The EN layers contain 5.1 ± 0.4 wt% phosphorus and are very unevenly distributed on the surfaces of the alumina particles. These layers consist of amorphous and microcrystalline phases. At temperatures greaterthan equal to1300°C, the EN layers de-wet from the alumina surfaces to become discrete, round particles in the alumina matrix. The alumina-EN pellets can be sintered to reach ~95% of the theoretical density at 1500°C for 4 h in graphite crucibles. The phases of the sintered samples consist of alumina, nickel, and nickel phosphides. However, in the exterior region of the sample sintered at 1500°C for 4 h, Ni₃Al will also form.     

Kinetic Analysis of Solution-Precipitation During Liquid-Phase Sintering of Alumina

Densification controlled by solution-precipitation during liquid-phase sintering was analyzed for the aluminamagnesium aluminosilicate glass system. As a model system for liquid-phase sintering, narrowly sized alumina powders and up to 20 vol% magnesium aluminosilicate glass samples were isothermally sintered at 1550° to 1650°C. Densification rate increases with increasing liquid content and sintering temperature but decreases with increasing density. For samples with >15% grain growth, the densification rate during the solution-precipitation stage of sintering was proportional to (particle size)⁻² and thus interface reaction-controlled. Activation energies ranged from 270 to 500 kJ/mol over the relative density range of 66% to 96%, respectively. The low activation energy is attributed to densification by particle rearrangement, whereas the higher activation energy is due to densification controlled by interface-reaction-controlled solution-precipitation. Intermediate activation energies are attributed to simultaneous densification by the two mechanisms.     

Structure and Grain Coarsening During the Sintering of Alumina

The pore surface area ( S ^p) and grain-boundary area ( S ^gb) were measured during the sintering of undoped and doped (100 ppm MgO) alumina compacts. Since the presence of the additive affects only S ^gb (raising it at a given value of the density), pinning of the boundaries by solid-solution drag is the only additive function evidenced by the results. The importance of such pinning even at densities as low as 75% of theoretical is linked to the existence of microstructural inhomogeneities.     

Effect of Heating Rate on Phase and Microstructural Evolution During Pressureless Sintering of a Nanostructured Transition Alumina

Deagglomeration of a nanocrystalline transition alumina performed using different techniques was first demonstrated to be active in the achievement of a better powder compaction ability under uniaxial pressing and consequently in the development of a highly dense and homogeneous microstructure during pressureless sintering. A major effect, however, was associated to the heating rate chosen during the densification cycle. In fact, the influence of different heating rates (10°C/min or 1°C/min) on phase and microstructural evolution during sintering was investigated in depth on the above best green bodies. A low-rate thermal cycle leads to a significant reduction of the α-Al₂O₃ crystallization temperature and promotes a more effective particle rearrangement during phase transformation. As a consequence, in the low-rate treated material, it was possible to avoid the development of a vermicular structure as usually expected during the densification of a transition alumina and to yield a more homogenously fired microstructure.     

Particle Rearrangement and Pore Space Coarsening During Solid-State Sintering

Coarsening of porosity during sintering has been observed in powder compacts of metallic, ceramic, and amorphous materials. Monitoring and modelling of the growth of individual (closed) pores in the late sintering stages are well established. Porosity is interconnected up to very high densities. Coarsening of the continuous pore space takes place during the initial and intermediate sintering stages. This coarsening is caused by localized transport of atoms or molecules (diffusion or viscous flow) as well as by bulk particle movement (rearrangement). Its quantitative exploration poses problems both experimentally and theoretically. Ways to characterize the geometry of the interconnected pore space and of closed pores are discussed with emphasis on stereological parameters. Recent and classical approaches, experimental findings with 2D model arrangements (as the formation and opening up of particle contacts, pore coarsening, and particle rearrangement) and some advances of computer simulations are discussed together with open questions.     

Grain-Growth Transition During Sintering of Colloidally Prepared Alumina Powder Compacts

When the grain size in partially sintered compacts of alumina was measured as a function of density, we found that the grain-growth behavior fell into two distinct regions. In the region where the porosity remained interconnected, grain growth was negligible; when the continuous pore network collapsed into isolated pores, grains grew rapidly. The transition in grain-growth behavior was observed at approximately 90% of theoretical density. A simple phenomenological method for obtaining the transition in grain growth is suggested. It is based on the idea that an abrupt increase in grain size should be accompanied by a significant decrease in the rate of sintering since the sintering rate changes as the third or fourth power of the grain size. The method consists of fitting the sintering data to an exponential function. The transition then appears as a change in the time constant for the exponential. The low rate of grain growth in the region where the pores are interconnected contradicts earlier work in the literature where significant grain growth during intermediate-stage sintering has been reported. This difference is explained in terms of the homogeneity of packing of our powder compacts, which were prepared by colloidal processing.     

Pore Structure Evolution of Lanthana–Alumina Systems Prepared through Coprecipitation

Pure Al₂O₃ and different compositions of La₂O₃–Al₂O₃ samples have been prepared through coprecipitation. Even after heating at 1300°C, the compositions La₂O₃·11Al₂O₃ and La₂O₃·13Al₂O₃ had higher surface area compared to the pure Al₂O₃ and the La₂O₃·Al₂O₃ composition. Ethanol washing is an effective way for improving the textural stability of pure Al₂O₃ and La₂O₃–Al₂O₃ samples. The effect of steam on the thermal stability of La₂O₃·11Al₂O₃ has also been studied. La₂O₃·11Al₂O₃ sample is found to be stable in steam.     

Thermodynamic Benefit of Abnormal Grain Growth in Pore Elimination During Sintering

Free-energy analyses performed on closed particle-pore arrays show that the presence of an abnormal grain thermodynamically favors the shrinkage of large pores to which is is adjacent. This gives an explanation to the experimentally observed phenomenon of abnormal-grain-growth-promoted densification in barium titanate.     

Anisotropic Microstructural Development During the Constrained Sintering of Dip-Coated Alumina Thin Films

Alumina thin films were manufactured from aqueous slurries by dip coating. Film thickness as function of substrate withdrawal velocity could be correctly modeled by Landau and Levich's theory. Samples were sintered on a rigid substrate at 1350°C for different isothermal times to achieve relative densities from 84% to 97%. Microstructural analysis of polished cross sections revealed a continuous development of pore alignment, as expected by theoretical considerations. With increasing density pores become more anisometric and orientate along the thickness direction. A further preferential orientation of pores was found in the normal plane, apparently due to the coating process. Constraining conditions had less influence on the size and shape of the grains; they tend to become more equiaxed in the constrained plane, presumably due to the biaxial tensile stress state.     

Evolution of Average Microstructural Properties in the Final Stage Sintering of Alumina

Models of simultaneous coarsening and densification in final stage sintering commonly assume that the coarsening process results in microstructures that evolve self-similarly from a fixed microstructural geometry, differing only in scale. This assumption is experimentally tested for alumina in the solid volume fraction range of 0.97–1 using nondimensional microstructural parameters. The results clearly show that such models based on assumed geometries often underestimate the pore size relative to the grain size. The largest differences between the model and the experiments occur for lower firing temperatures and higher doping levels. It is concluded that the coarsening reflected in the effect of temperature and dopant level is not a self-similar process from a common microstructural geometry.     

Liquid Phase Sintering of Alumina, I. Microstructure Evolution and Densification

The microstructure evolution and densification of alumina containing 10 vol% calcium aluminosilicate glass and 0.5 wt% magnesium oxide sintered at 1600°C were quantified by measuring the evolution of pore-size distribution, the redistribution of liquid phase, and the fraction of closed and open pores. The densification stopped at a limiting relative density during the final stage of sintering, and the small and large pores were filled simultaneously by glass during sintering. In addition, the results indicate that the pressure build-up of the trapped gases in pores causes a significantly negative contribution to the driving force, and consequently the observed reduction in densification during the final stage of liquid phase sintering.     

Evolution of Defects During Sintering: Discrete Element Simulations

We use discrete element method (DEM) simulations to study the evolution of defects during sintering. In DEM, the particulate nature of the sintering powder is taken explicitly into account because each particle is modeled as a discrete entity interacting with its neighbors. This allows to treat naturally the gain or the loss of contacts between particles, and to explicitly take particle rearrangement into account. These effects are particularly important when looking for the nucleation, growth or healing of local heterogeneities such as defects. We first study the evolution of a crack (generated, e.g., during ejection or drying processes) when no geometrical constraint is imposed. We then investigate how constrained sintering between two parallel planes may lead to crack initiation and growth. We show that the extent of interparticle rearrangement plays a major role in the evolution of the crack under such conditions. The main conclusion of these simulations is that some geometrical constraint is necessary for a defect to grow into a crack and that the presence of an initial defect is not a necessary condition to initiate cracks.     

Effects of Magnesium Oxide on Grain-Boundary Segregation of Calcium During Sintering of Alumina

The advantage of certain amounts of MgO addition in alumina sintering has been realized, and it is common practice. In an attempt to understand the role of MgO in the presence of CaO in commercial-grade alumina, grain-boundary segregation of Ca was investigated by scanning Auger electron microprobe (SAM) using an ultrapure alumina after controlled doping of CaO and/or MgO. The commercial-grade alumina, which usually contains a small amount of CaO, is difficult to sinter to high density. The pure alumina composition (<99.999%) gives "clean" boundaries when it is sintered under "clean" conditions. As the powder was doped with 100 ppm of CaO and sintered at 1300° to 1500°C, all of the grain boundaries were enriched by Ca as observed by others. However, it was also discovered that some of the grain boundaries are enriched by an exceptionally high concentration of Ca. Such a large variation of Ca contents depending on the grain-boundary facets disappeared when samples were codoped with small amounts of MgO. The results suggest that MgO plays a beneficial role in controlling the anisotropic segregation of Ca to various interfaces including grain boundaries and pore surfaces during sintering of alumina. MgO thus enhances chemical homogeneity of commercial-grade alumina.     

Evolution of Mechanical Properties of Porous Alumina during Free Sintering and Hot Pressing

This comparative study addresses the influence of microstructural evolution on mechanical properties of porous alumina sintered with and without the application of uniaxial pressure. A complete set of data on Young's modulus, long-crack fracture toughness, and fracture strength for two alumina powders as a function of density was obtained. The evolution of fracture strength with increased density was modeled using a porosity-dependent crack-tip fracture toughness, linked to the contact area of grains and a porosity-dependent size of the largest defect. The defect shape factor was found independent of porosity. A uniaxial pressure of 13 MPa during densification had negligible effect on the relation of strength to porosity.