Localized Densification during the Stage II–III Transition - Compaction Efficiency at High Pressures

Localized densification and compaction efficiency at high pressures was studied using X-ray computed tomography. Stage III begins with the initiation of a region of uniform average density within the overall high-density zone. No further densification occurs in this region; additional pressure is transmitted into adjacent, less-dense zones and the die wall. This localized increase in wall friction continuously decreases compaction efficiency. Discrete element modeling was used to visualize force transfer. The transition was governed by events that ranged from the microscale to the macroscale. Terapascal levels of pressure were required to produce a uniform compact.     

Compaction and Sintering Behavior of Bimodal Alumina Powder Suspensions by Pressure Filtration

The compaction behavior of fine alumina powders with different particle sizes or bimodal particle-size distributions that are undergoing pressure filtration was investigated. Three alumina powders—average particle sizes of 0.2—0.86 μm—were compacted to a solids fraction of 62—65 vol% from suspensions at pH 3, which was the pH level at which the suspensions showed their lowest viscosity. When the powders of different average sizes were mixed, the suspensions showed better flowability, and the lowest viscosity was obtained when the fraction of fines was ∼30 vol% and pH = 3. The mixed-sized powder suspensions were compacted to higher density than the suspensions of unmixed fine or coarse powders, and the maximum density was obtained for mixed suspensions that had the lowest viscosity, despite the different particle-size ratio. Maximum densities of 72.5% and 75.0% were attained when the size ratios were 2 and 5, respectively. The compacts that were pressure-filtered from mixed suspensions exhibited a single-peaked pore-size distribution and a homogeneous microstructure, whereas the pore-size distributions of dry-pressed compacts were double-peaked. The sintering behavior of the compacts that were pressure-filtrated from bimodal powders exhibited significantly better sinterability and much-less linear shrinkage than the coarser powders and the dry-pressed powder compacts.     

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.     

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.     

Densification of Alumina-Silicon Carbide Powder Composites: II, Microstructural Evolution and Densification

Microstructural evolution adn densification kinetics of Al₂O₃-SiC powder composites were studied using two different SiC powders. Examination of the microstructural evolution of Al₂O₃-fine SiC powder composites showed three well-defined stages of densification: the first was characterized by constant pore size and no grain growth; the second involved rapid pore coarsening and grain growth; the third was characterized by pore shrinkage and slow grain growth. Studies of the densification kinetics of Al₂O₃-coarse SiC powder composites exhibited two stages of densification: in the first stage there were no significant differences in densification rate between pure Al₂O₃ compacts and composites; in the second stage, however, differences in densification behavior between pure Al₂O₃ compacts and composites became pronounced.     

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.     

Effect of Externally Applied Plasticizer on Compaction Behavior of Spray-Dried Powders

The effects of an externally applied plasticizer on compaction behavior and green microstructure quality of spray-dried powders was investigated. The plasticizer was applied to the external surfaces of already spray-dried powders by spraying it on tumbling spray-dried granules. The apparent yield point of the spray-dried powder was reduced when the plasticizer was added. Microstructures of compacts made from these granules (with and without the externally applied plasticizer) were compared at different compaction pressures. Better knitting across granule interfaces and fewer defects were obtained for the granules with the externally applied plasticizer.     

Influence of Compaction and Granulation Processes on the Properties of Soft Magnetic Ceramics

The granulation and compaction processes are investigated in relation to the influence they have on the magnetic permeability and electromagnetic power losses of conventionally prepared polycrystalline manganese-zinc ferrites. It appears that, during granulation by spray-drying, rapid evaporation conditions may be reached at the internal of the under drying droplets, giving rise to the formation of large void defects in the interior of the granulate. Those defects may resist compaction and also be present in the compacted specimens, eventually leading to large intragranular isolated pores and lower densities at the sintered material; the magnetic properties of the material are therefore negatively influenced. Compaction at high pressures (e.g., >500 kg cm⁻²) may compensate for the existence of inflation defects by causing their removal on compaction. The previous phenomenon is absent in granulates made by the roll granulation process that exhibit, in general, a better compaction, sintering, and magnetic behavior when compared to spray-drying granulates at conditions where void formation takes place.     

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.     

Densification characteristics of chromia/alumina castables by particle size distribution

The quality of the refractories applied on integrated gasification combined cycle should be a key factor that affects both the reliability and the economics of gasifier operation. To enhance the workability of chromia/alumina castables, three types of ultrafine alumina powder were added to improve the workability. Densification behavior of such castables in the presence of ultrafine alumina was assessed through the measurement of parameters like flow value, viscosity, bulk density, apparent porosity, and microstructure evaluation by an SEM study. It's proved that the specific surface area and particle size distribution of ultrafine powders in matrix parts greatly influence the densification behavior of these castables.     

Effect of Dynamic Compaction on the Retention of Tetragonal Zirconia and Mechanical Properties of Alumina/Zirconia Composites

Dynamic consolidation techniques were employed to investigate the retention of tetragonal zirconia and degree of consolidation in alumina/zirconia powder compacts. Heating the specimens prior to explosive shock compaction increased the tetragonal-phase retention significantly. Low shock pressures yielded no macrocracking, although final densities were low (60% to 70% of the theoretical density). Heat treatment following dynamic consolidation enhanced the retention of the tetragonal zirconia polymorph regardless of the shock pressure employed. Compact densities were increased to over 90% of theoretical at relatively low sintering temperatures (1300°C). Hardness, toughness, and Young's modulus of the compacts were comparable to those achieved in composites that were synthesized using more conventional techniques. Dynamic compaction offers an alternative method for the fabrication of zirconia-toughened alumina ceramics.     

Fracture Toughness of Spray-Dried Powder Compacts

The strengths and fracture toughness values were measured for alumina powder compacts containing two different binder systems. Diametral compression was used to measure both the tensile strength and the fracture toughness (through-thickness notch). This methodology was very useful in linking processing parameters, such as binder choice and compaction stress, to the quality of the green bodies. Observations of the compact structure before and after fracture showed that the binders segregated to the region between the spray-dried granules. The presence of the excess binder in this region was linked to both the failure mode and the creation of secondary cracks.     

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.     

Adhesion Effects of Intermediate Layers on the Densification of Ceramic Powders

In the ceramic technology the first step to produce sintered bodies is the manufacturing of powders which then are densified. The adhesion mechanisms between the single particles and the agglomerates produced from them determine the densification process. Starting from theoretical considerations adhesion mechanisms, such as solid bridge formation, adhesive bonding and glide-promoting effects, are discussed in principle. Subsequently, the effects of surface-active substances on the densification behaviour of clay-ceramics and oxide-ceramic bodies are discussed. Further, the evaluation of the action of additives to the powder mixtures on the microstructure of the compacts, such as porsity and texture, leads to a compaction equation which describes the transition from the powder pile to a densified green body.     

Evolution of the Grain Size Distribution during the Sintering of Alumina at 1350°C

The objectives of this study are to provide some rare unfolded grain size distribution data for the sintering of alumina and to test for time invariance of the normalized grain size distribution as required by normal grain growth. The results show that ln ς doubled during the sintering times studied. The changes in the grain size distribution may be due to an increase in the number of relatively large grains combined with a reduction in the number of grain annihilation events compared with that required for time invariance of the normalized grain size distribution.     

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.     

Conversion of Polycrystalline Alumina to Single-Crystal Sapphire by Localized Codoping with Silica

The effect of a localized SiO₂ codoping on the conversion of polycrystalline, MgO-doped Al₂O₃ tubes to single-crystal sapphire was investigated. Codoping with SiO₂ before sintering intentionally triggered abnormal grain growth, which resulted in the full conversion of tube surfaces to single crystal without adversely affecting densification to a almost pore-free, translucent state. The degree of surface conversion was strongly dependent on experimental variables, which included furnace temperature and codoping amount. Surface-converted tubes had excellent physical properties, which included good thermal cycling resistance and optical properties superior to unconverted, polycrystalline Al₂O₃.     

Mechanochemical Synthesis of Lanthanum Aluminate by Grinding Lanthanum Oxide with Transition Alumina

Grinding lanthanum oxide (La₂O₃) with Al₂O₃ was conducted to investigate their mechanochemical reactions to form lanthanum aluminate (LaAlO₃) powder using a planetary ball mill. Grinding for 120 min allowed us to obtain single-phase LaAlO₃ with a large surface area when transition alumina was used, whereas no formation of LaAlO₃ was achieved when α-Al₂O₃ was used. The mechanochemical process can be applied to synthesize other rare-earth (RE) aluminates (REAlO₃) from mixtures of a rare-earth oxide and transition alumina.     

Densification of Alumina-Silicon Carbide Powder Composites: I, Effects of a Polymer Coating on Silicon Carbide Particles

One possible approach to improving the densification of powder composites containing a major crystalline phase which densifies (e.g., Al₂O₃) and a difficult-to-sinter phase (e.g., SiC) is to accommodate the matrix volume shrinkage with a "disappearing" polymer coating. A polymer coating prevents contact between the nonsinterable particles and the surrounding matrix. The coating can be burned off before sintering, allowing the matrix phase to "shrink-fit" around the nonsinterable particles during sintering. The effects of a polymer coating on the densification of a two-phase particle system were tested using SiC powder dispersed in an Al₂O₃ matrix. The composites processed with a polymer coating showed more densification during equivalent firing cycles than did those processed without a polymer coating. Densification during sintering was approximately proportional to the amount of polymer adsorbed on SiC, suggesting that the Al₂O₃ matrix did shrink-fit into the gaps between the SiC particles and the surrounding Al₂O₃ matrix. Differences in the pore-size distributions of polymercoated green compacts and uncoated compacts indicated a perturbation of the green microstructure by the gaps. The estimated average thickness of the gap is approximately 20 nm, ∼8% of the average radius of the SiC powder used in this study.