Sintering of Acicular Fe2O3 Powder

The sintering of acicular Fe₂O₃ powder has been studied in comparison with an ordinary equiaxed powder. The acicular powder gave a dense material more than 99 % theoretical even from the relatively low green density. Oriented granular structures were observed in the 1200°C compacts. The observed densification behavior has been attributed to the pore configuration in the green compact.     

Sintering of ZnS with a Small Amount of Ba2ZnS3

The effects of alkali and alkali-earth chloride additives on the sintering of ZnS powder compacts in H₂S have been examined. BaCl₂ was found to promote the sintering of ZnS. BaCl₂ reacts with ZnS to form Ba₂ZnS₃, which acts as a flux for sintering.     

Densification of Cr2O3-ZrO2 Ceramics by Sintering

Cr₂O₃ and ZrO₂ were mixed in various ratios and pressed to form compacts, which were then sintered in carbon powder. Compacts with >30 wt% Cr₂O₃ were sintered to densities >98% of true density at 1500°C. This method of sintering in carbon powder can be used to prepare very dense Cr₂O₃-ZrO₂ ceramics at a relatively low temperature, (∼1500°C) without additives.     

Reaction Sintering of ZnO-AI2O3

The reaction sintering of equimolar mixtures of ZnO and A1₂O₃ powders was investigated as a function of primary processing parameters such as the temperature, heating rate, green density, and particle size. The powder mixtures were prepared by two different methods. In one method, the ZnO and A1₂O₃ powders were ball-milled. In the other method, the ZnO powder was chemically precipitated onto the A1₂O₃ particles dispersed in a solution of zinc chloride. The sintering characteristics of the compacted powders prepared by each method were compared with those for a prereacted, single-phase powder of zinc aluminate, ZnAl₂O₄. The chemical reaction between ZnO and A1₂O₃ occurred prior to densification of the powder compact and was accompanied by fairly large expansion. The mixing procedure had a significant effect on the densification rate during reaction sintering. The densification rate of the compact formed from the ball-milled powder was strongly inhibited compared to that for the single-phase ZnAl₂O₄ powder. However, the densification rate of the compact formed from the chemically precipitated mixture was almost identical to that for the ZnAl₂O₄ powder. The difference in sintering between the ball-milled mixture and the chemically precipitated mixture is interpreted in terms of differences in the microstructural uniformity of the initial powder compacts resulting from the different preparation procedures.     

Preparation of Al2O3–AlN–Ni Composites

A method is proposed to prepare Al₂O₃-AlN-Ni composites. The composites are prepared by sintering Al₂O₃/NiAl powder mixtures at 1600°C in a mixture of nitrogen and carbon monoxide. The presence of NiAl particles raises the green density of Al₂O₃/NiAl powder compacts. During sintering, NiAl reacts with nitrogen to form AlN and Ni inclusions. A volume expansion accompanies the reaction. Because of the high green density and the reaction, the volume shrinkage of the Al₂O₃-AlN-Ni composite decreases with the increase of added NiAl content.     

Microstructure and Densification of Pressureless-Sintered Al2O3/Si3N4-Whisker Composites

Composite densification was studied by performing slip casting and sintering experiments on an Al₂O₃ matrix and Si₃N₄ whisker system. Even though all the slip-cast powder compacts exhibited high green densities (up to 70% of the theoretical) and narrow pore-size distribution (pore radius around 15 to 30 nm), significant differential densification on a microscopic scale was found due to the existence of local whisker agglomeration. The inhomogeneous whisker distribution resulted in a binary mixture of large and small pores in the sintered composites, in which whisker-associated flaws remained stable even after prolonged sintering. The sintered microstructures showed that the spatial distribution as well as the volume fraction of the Si₃N₄ affect composite densification. Inhomogeneous whisker distribution dominated the complete densification of the composites.     

Hot Isostatic Pressing of Si3N4 Powder Compacts and Reaction-Bonded Si3N4

Hot isostatically pressed silicon nitride was produced by densifying Si₃N₄ powder compacts and reaction-bonded Si₃N₄ (RBSN) parts with yttria as a sintering additive. The microstructure was analyzed using scanning electron microscopy, X-ray diffraction, and density measurements. The influence of the microstructure on fracture strength, creep, and oxidation behavior was investigated. It is assumed that the higher amount of oxygen in the Si₃N₄ starting powder compared with the RBSN starting material leads to an increased amount of liquid phase during densification. This results in grain growth and in a larger amount of grain boundary phase in the hot isostatically pressed material. Compared with the hot isostatically pressed RBSN samples therefore, strength decreases whereas the creep rate and the weight gain during oxidation increase.     

Initial Coarsening and Microstructural Evolution of Fast-Fired and MgO-Doped Al2O3

The effect of an initial coarsening step (50-200 h at 800°C) on the subsequent densification and microstructural evolution of high–quality compacts of undoped and MgO–doped Al₂O₃ has been investigated during fast–firing (5 min at 1750°C) and during constant–heating–rate sintering (4°C/min to 1450°C). In constant–heating–rate sintering of both the undoped and MgO–doped Al₂O₃, a refinement of the microstructure has been achieved for the compact subjected to the coarsening step. A combination of the coarsening step and MgO doping produces the most significant refinement of the microstructure. In fast–firing of the MgO–doped Al₂O₃, the coarsening step produces a measurable increase in the density and a small refinement of the grain size, when compared with similar compacts fast–fired conventionally (i.e., without the coarsening step). This result indicates that the accepted view of the deleterious role of coarsening in the sintering of real powder compacts must be reexamined. Although extensive coarsening after the onset of densification must be reduced for the achievement of high density, limited coarsening prior to densification is beneficial for subsequent sintering.     

Sintering of Nanophase γ-Al2O3 Powder

The sintering of ultrafine γ-Al₂O₃ powder (particle size ∼10–20 nm) prepared by an inert gas condensation technique was investigated in air at a constant heating rate of 10°C/min. Qualitatively, the kinetics followed those of transition aluminas prepared by other methods. Measurable shrinkage commenced at ∼ 1000°C and showed a region of rapid sintering between ∼1125° and 1175°C followed by a transition to a much reduced sintering rate at higher temperatures. Starting from an initial density of ∼0.60 relative to the theoretical value, the powder compact reached a relative density of 0.82 after sintering to 1350°C. Compared to compacts prepared from the as-received powder, dispersion of the powder in water prior to compaction produced a drastic change in the microstructural evolution and a significant reduction in the densification rate during sintering. The incorporation of a step involving the rapid heating of the loose powder to ∼1300°C prior to compaction (which resulted in the transformation to α-Al₂O₃) provided a method for significantly increasing the density during sintering.     

Preparation and Properties of Porous α-Al2O3 Membrane Supports

Strong and permeable macro-porous α-Al₂O₃ membrane supports are made by colloidal filtration of 20 vol% dispersions of α-Al₂O₃ with an average particle size of 600 nm. Intact compacts with very good surface quality were obtained at an optimum pH of 9.5 and dosage of 0.2 wt% ammonium aurintricarboxylate (Aluminon), based on dry alumina. The colloidal stability of the aluminon-stabilized slurries is confirmed by ξ potential measurements. Slight sintering of dense-packed α-Al₂O₃ compacts was found to result in >67% packing density and a bimodal pore-size distribution as derived from shrinkage behavior and gas adsorption studies. Non-stationary single gas permeation measurements showed improved gas permeability, compared with α-Al₂O₃ compacts prepared using powder with a smaller particle size (300 nm). The strength of the disk-shaped alumina compacts within the porosity range of 30%–20% increased from 100 to 300 MPa with a standard deviation of 20 and 50 MPa, respectively.     

Effects of Inclusions on the Sintering Behavior of YBa2Cu3O6+x

The sintering behavior of two types of heterogeneous compacts of YBa₂Cu₃O_{6+ x} was studied: (1) Soft agglomerates present in the starting powder were used to study the effect of rapidly densifying inclusions on the overall sample densification. In this case, the induced stresses caused severe cracklike damage in the sintered microstructure. (2) On the other hand, when nondensifying inclusions (same composition) were incorporated into the starting powder, no sintering damage was observed. Further, there was no retardation of densification or coarsening due to the presence of these dense inclusions, over a wide range of inclusion size. Several possibilities for this behavior are discussed, based on the distribution of stresses induced by differential sintering rates.     

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.     

Sintering and Crystallization of CaO–Al2O3–ZrO2–SiO2 Glasses Containing Different Amount of Al2O3

In this work several complementary techniques have been employed to carefully characterize the sintering and crystallization behavior of CaO–Al₂O₃–ZrO₂–SiO₂ glass powder compacts after different heat treatments. The research started from a new base glass 33.69 CaO–1.00 Al₂O₃–7.68 ZrO₂–55.43SiO₂ (mol%) to which 5 and 10 mol% Al₂O₃ were added. The glasses with higher amounts of alumina sintered at higher temperatures (953°C [lower amount] vs. 987°C [higher amount]). A combination of the linear shrinkage and viscosity data allowed to easily find the viscosity values corresponding to the beginning and the end of the sintering process. Anorthite and wollastonite crystals formed in the sintered samples, especially at lower temperatures. At higher temperatures, a new crystalline phase containing ZrO₂ (2CaO·4SiO₂·ZrO₂) appeared in all studied specimens.     

Processing and Sintering of Ultrafine MgO-ZrO2 and (MgO, Y2O3)-ZrO2 Powders

Chemical coprecipitation was used to produce ultrafine and easily sinterable MgO-stabilized and (MgO, Y₂O₃) stabilized ZrO₂ powders. The sintering behavior is very sensitive to post-precipitation washing because "hard" agglomerates form when the precipitated gels are washed with water, whereas "soft" agglomerates form when they are washed with ethanol. The soft agglomerates pack uniformly, resulting in homogeneous shrinkage of powder compacts to near-theoretical density. The hard agglomerates result in compacts which have regions of localized densification and a signifiint fraction of residual porosity.     

Formation, Packing, and Sintering of Monodisperse TiO2 Powders

Monodisperse TiO₂ powders were synthesized by the controlled hydrolysis of dilute alcoholic solutions of titanium alkoxides. The state of powder aggregation in dispersions and powder packing in the green bodies strongly affected the sintering behavior. The sintering of uniformly packed powder compacts resulted in finegrained microstructures with >99% of theoretical density nt temperatures much lower than those required to sinter conventional TiO₂ powders.     

Sintering and Crystallization of a Glass Powder in the Li2O–ZrO2–SiO2 System

Sintering and crystallization of a 23.12 mol% Li₂O, 11.10 mol% ZrO₂, 65.78 mol% SiO₂ glass powder was investigated. By means of thermal shrinkage measurements, sintering was found to start at about 650°C and completed in a very short temperature interval (Δ T similar/congruent 100°C) in less than 30 min. Crystallization took place just after completion of sintering and was almost complete at about 900°C in 20 min. Secondary porosity prevailed over the primary porosity during the crystallization stage. The glass powder compacts first crystallized into lithium metasilicate (Li₂SiO₃), which transformed into lithium disilicate (Li₂Si₂O₅), zircon (ZrSiO₄), and tridymite (SiO₂) after the crystallization process was essentially complete. The microstructure was characterized by fine crystals uniformly distributed and arbitrarily oriented throughout the residual glass phase.     

Influence of Oxygen Activity on the Sintering of MgCr2O4

The sintering behavior of MgCr₂O₄ powder compacts was investigated as a function of temperature, time, and oxygen activity. The results show that MgCr₂O₄ cannot be densified to >70% of theoretical density at temperatures up to 1700°C if the oxygen activity exceeds 10⁻⁶ atm. The oxygen activity must be decreased to <10⁻¹⁰ atm before densities exceeding 90% of theoretical can be achieved. Weight loss and X-ray data indicated that maximum density occurred at an oxygen activity just above that where MgCr₂O₄ becomes unstable.     

Effect of Seeding Sr2KNb5O15 on the Phase Formation and Microstructural Development in Reactive Sintering of Sr2NaNb5O15 Ceramics

The phase formation, densification behavior, and microstructure development of Sr₂NaNb₅O₁₅ (SNN) ceramics in both 10 wt% acicular Sr₂KNb₅O₁₅ (SKN) seed-containing and unseeded systems were investigated in this work. SNN ceramics were reactively sintered from SrNb₂O₆ and NaNbO₃ powders. The results show that the acicular SKN seeds not only accelerate SNN phase formation but also promote the densification at lower temperature. In reactive sintering, the acicular SKN seeds prepared by the molten salt synthesis method can give rise to the formation of a liquid phase and provide the structural framework for the grain growth of ceramics, leading to the formation of large anisotropic grains (>80 μm) in ceramics sintered at 1340°C. However, there are no such large anisotropic grains obtained in the SKN-free system. Observation of the large anisotropic grain growth is explained by the liquid-phase-assisted growth mechanism. For comparison, the microstructure evolution in the system with 10 wt% SKN seed, which was prepared by the conventional mixed-oxide method and without acicular morphology, was also investigated to further support the new growth mechanism.     

Effect of Rigid Inclusions on the Densification and Constitutive Parameters of Liquid-Phase-Sintered YBa2Cu3O6+x Powder Compacts

The presence of rigid inclusions in a powder compact leads to a reduction in the densification rate of the compact and may also lead to processing defects. In this paper, the densification rate and the constitutive parameters of both homogeneous YBa₂Cu₃O_{6+ x} and composite powder compacts (YBa₂Cu₃O_{6+ x} powder with 10 vol% dense inclusions of YBa₂Cu₃O_{6+ x} ) are reported. A small amount of liquid phase, which formed during sintering, was present in the samples. However, even with the presence of a liquid phase, the addition of inclusions still reduces the densification rate of the composite and increases its viscosity. The results have been compared with a published analysis of the problem using measured values of the constitutive parameters. Both the viscosity and viscous Poisson's ratio of the porous body have been measured.     

Thermal Grooving of MgO and Al2O3

High-purity polycrystalline MgO and Al₂O₃ were thermally grooved at 1500° and 1600°C. Accurate techniques were developed for following the growth of a single groove. For high-purity samples growth kinetics were essentially similar to those reported in the literature but were determined to be controlled by volume diffusion. Specimens for thermal grooving were prepared from Al₂O₃ to which transition metal oxides (Fe₂O₃₉, MnO, and TiO₂), which are known to accelerate shrinkage and sintering of Al₂O₃ powder compacts, had been added; the rate of groove growth was increased remarkably by minor amounts of these additives. Control of partial pressure indicated that Fe²⁺ and Ti⁴⁺ are the species active in promoting groove growth. Substantial evidence was found for volume diffusion as the mechanism controlling groove formation.