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.     

Pulse Electric Current Sintering of Al2O3/3 vol% ZrO2 with Constrained Grains and High Strength

The pulse electric current sintering technique (PECS) was demonstrated to be effective in rapid densification of fine-grained Al₂O₃/3Y-ZrO₂ using available commercial powders. The composites attained full densification (>99% of TD) at 1450°C in less than 5 min. The composites sintered at a high heating rate had a fine microstructure. The incorporation of 3 vol% 3Y-ZrO₂ substantially increased the average fracture strength and the toughness of alumina to as high as 827 MPa and 6.1 MPa·m^1/2, respectively. A variation in the heating rate during the PECS process influenced grain size, microstructure, and strength, though there was little or no variation in the fracture toughness.     

Retrograde Densification in Bi2Sr2CaCu2O8 Superconductors

Bi₂Sr₂CaCu₂O₈ was prepared using the mixed oxide-carbonate method and sintered at temperatures ranging from 850° to 911°C. The samples were characterized for density, mechanical strength, phase composition, microstructure, and superconducting transition temperatures. A unique retrograde densification characteristic is demonstrated in the temperature range 850° to 890°C whereby the material first becomes less dense as the sintering temperature is raised, and only in a narrow temperature range from 900° to 905°C does the material densify then with the formation of a liquid phase. The retrograde densification mechanism is shown to be that of the formation of thin platelike crystallites which grow in a randomly oriented fashion, thus pushing the structure apart. This retrograde densification, coupled with a narrow sintering range overlapping the melting temperature, makes this compound a difficult one to process.     

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.     

Effect of Heating Rate on the Dielectric Properties of the PMN-PT Relaxor Ceramics

The effect of heating rate (2 to 30°C/min) during sintering on the perovskite phase content, density, microstructure, and dielectric properties of 0.9PbMg_1/3Nb_2/3O₃–0.1PbTiO₃ relaxor ferroelectric ceramics has been studied. Increasing the heating rate reduced the level of PbO evaporation and resulted in high perovskite content and dense ceramics. With slow heating rate (2–10°C/min), excess MgO segregated as isolated submicrometer particles in the perovskite grains; these precipitates were absent in ceramics sintered with a heating rate of 30°C/min. The maximum dielectric constant was optimized with a heating rate of 10°C/min. The temperature of maximum dielectric constant shifted downward as the heating rate was increased.     

Effect of Heating Rate on the Sintering Behavior and the Piezoelectric Properties of Lead Zirconate Titanate Ceramics

The effect of heating rate on the sintering behavior and the piezoelectric properties of lead zirconate titanate (PZT) ceramics was investigated. Two different types of PZT (pure and doped with Nb₂O₅) were sintered at 1150°C for 2 h with a wide range of heating rate (0.5°–100°C/min). The densification of pure PZT was improved significantly by increasing the heating rate. The improvement was attributed to the suppression of PbO volatilization and grain coarsening during heating. In contrast, the densification behavior of a PZT specimen doped with Nb₂O₅ was not much influenced by the heating rate. These densification behaviors affected the piezoelectric properties of the specimens. The piezoelectric properties of pure PZT were enhanced significantly by increasing the heating rate, while those of doped specimens were improved only moderately.     

Transformation and Densification of Nanocrystalline θ-Alumina during Sinter Forging

The sinter forging behavior of α-Al₂O₃ seeded and unseeded nanocrystalline θ-Al₂O₃ was investigated as a function of temperature, stress, and strain rate. Seeded samples exhibited the highest degree of plastic deformation during the θ- to α-AI₂O₃ phase transformation. As a result, microstructure control, increased densification, and a higher degree of transformation were obtained. A uniform microstructure of 150 nm α-Al₂O₃ grains developed, reaching 57% relative density after sintering 1.5 wt%α-Al₂O₃ seeded samples for 30 min at 1060°C. When sinter forged at 0.25 mm/min to 63 MPa and 1060°C for 30 min large deformations during the phase transformation increased the relative density to 74%. When the stress was increased to 235 MPa (1060°C, 30 min), 99.7% dense α-Al₂O₃ with a grain size of 230 nm was obtained. By increasing the sinter forging temperature to 1150°C, 99.5% relative density was achieved at 190 MPa for 30 min.     

Densification of Nanocrystalline Yttria by Low Temperature Spark Plasma Sintering

We investigated the densification of undoped, nanocrystalline yttria (Y₂O₃) powder by spark plasma sintering (SPS) at sintering temperatures between 650°C and 1050°C at a heating rate of 10°C/min and an applied stress of 83 MPa. In spite of the low sinterability of the undoped Y₂O₃, a remarkable densification of the powder started at about 600°C, and a theoretical density of more than 97% was achieved at a sintering temperature of 850°C with a grain size of about 500 nm. The low temperature SPS is effective for fabricating dense Y₂O₃ polycrystals.     

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.     

Spark Plasma Sintering of Bismuth Titanate Ceramics

Spark plasma sintering (SPS) was used to fabricate bismuth titanate (Bi₄Ti₃O₁₂) ceramics. The densification, microstructure development and dielectric properties were investigated. It was found that the densification process was greatly enhanced during SPS. The sintering temperature was 200°C lower and the microstructure was much finer than that of the pressureless sintered ceramics, and dense compacts with a high density of over 99% were obtained at a wide temperature range of 800°–1100°C. Dielectric property measurement indicated that the volatilization of Bi³⁺ was greatly restrained during SPS, resulting in an unprecedented low dielectric loss for pure Bi₄Ti₃O₁₂ ceramics.     

Sintering of Zinc Oxide Doped with Antimony Oxide and Bismuth Oxide

The phase change, densification, and microstructure development of ZnO doped with both Bi₂O₃ and Sb₂O₃ are studied to better understand the sintering behavior of ZnO varistors. The densification behavior is related to the formation of pyrochlore and liquid phases; the densification is retarded by the former and promoted by the latter. The pyrochlore phase, whose composition is Bi_3/2ZnSb_3/2O₇, appears below 700°C. The formation temperature of the liquid phase depends on the Sb/Bi ratio: about 750°C for Sb/Bi < 1 by the eutectic melting in the system ZnO—Bi₂O₃, and about 1000°C for Sb/Bi > 1 by the reaction of the pyrochlore phase with ZnO. Hence, the densification rate is determined virtually by the Sb/Bi ratio and not by the total amount of additives. The microstructure depends on the sintering temperature. Sintering at 1000°C forms intragrain pyrochlore particles in ZnO grains as well as intergranular layers, but the intragrain particles disappear at 1200°C by the increased amount of liquid phase, which enhances the mobility of the solid second phase.     

Formation of a High-Temperature Liquid Phase During the Sintering of PbFe2/3W1/3O3

A high-temperature liquid phase (rather than a low-temperature liquid phase at 690°C as reported recently) has been demonstrated to form at 860°C on heating and to solidify at 840°C on cooling in PbFe_2/3O₃. This liquid phase not only promotes densification, but also induces the formation of rounded PbFe_2/3W_1/3O₃ grains during sintering at 870°C. Through slow cooling at a rate of 25°C/h after sintering, platelike grains, designated G phase, are found to form in a thin surface layer of specimens. This formation of platelike G phase is considered to be related to the solidification and recrystallization of the liquid phase exuded from the interior. The amount of the G phase on the surfaces decreases with the increase of cooling rates, indicating that fast cooling will lead the liquid phase to be solidified in the bulk of specimens. These results reveal that the microstructure of PbFe_2/3W_1/3O₃ is greatly affected by the high-temperature liquid phase; additionally, the slow cooling treatment seems to be a direct and effective method for removing the residual liquid phase from PbFe_2/3W_1/3O₃.     

Effect of Firing Atmosphere on Sintered and Mechanical Properties of Vanadium-Doped Alumina

Vanadium-doped alumina was sintered at 1650°C in both an oxidizing (O) and a reducing (R) atmosphere and the sintered bodies examined. In the O-sintered body, vanadium was present preferentially between the alumina grains, forming a phase (AlVO₄). In the R-sintered body, on the other hand, most of the vanadium was dissolved in alumina as V³⁺, and a small proportion of the vanadium was present as V⁴⁺ in the grain-boundary region. During O sintering, V₂O₅ doping depressed both densification and grain growth, whereas R sintering had no effect on densification but did depress grain growth. The O-sintered, V-doped body exhibited low flexural strength and hardness, whereas the R-sintered body showed comparatively high hardness.     

Densification of Si3N4 with LiYO2 Additive

This paper deals with the densification and phase transformation during pressureless sintering of Si₃N₄ with LiYO₂ as the sintering additive. The dilatometric shrinkage data show that the first Li₂O- rich liquid forms as low as 1250°C, resulting in a significant reduction of sintering temperature. On sintering at 1500°C the bulk density increases to more than 90% of the theoretical density with only minor phase transformation from α-Si₃N₄ to β-Si₃N₄ taking place. At 1600°C the secondary phase has been completely converted into a glassy phase and total conversion of α-Si₃N₄ to β-Si₃N₄ takes place. The grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. Li₂O evaporates during sintering. Thus, the liquid phase is transient and the final material might have promising mechanical properties as well as promising high-temperature properties despite the low sintering temperature. The results show that the Li₂O−Y₂O₃ system can provide very effective low-temperature sintering additives for silicon nitride.     

Effect of Simultaneous Addition of BiFeO3 and Ba(Cu0.5W0.5)O3 on Lowering of Sintering Temperature of Pb(Zr,Ti)O3 Ceramics

Sintering of 0.5-wt%-MnO₂-added Pb(Zr_0.53Ti_0.47)O₃ ceramics progresses at 935°C for 50 min by the addition of complex oxides of perovskite-type crystal structure, BiFeO₃ and Ba(Cu_0.5W_0.5)O₃. In order to elucidate the low-temperature sintering mechanism of Pb(Zr,Ti)O₃ ceramics, the shrinkage and the evolution of the microstructure of a compacted body during heating were studied. It has been shown that the densification process was separated into the following three stages: the rearrangement of grains, the grain boundary diffusion of atoms, and then grain growth. Also, microstructural and elemental analyses of the ceramics revealed the existence of an amorphous phase at the grain boundaries predominantly composed of lead and copper oxides. Consequently, this process can be facilitated by the occurrence of a transient liquid phase corresponding to the above amorphous phase.     

A Novel Processing Route to Develop a Dense Nanocrystalline Alumina Matrix (<100 nm) Nanocomposite Material

A dense 3-mol%-yttria-stabilized tetragonal zirconia polycrystalline (3Y-TZP) toughening alumina matrix nanocomposite with a nanocrystalline (<100 nm) matrix grain size has been successfully developed by a novel processing method. A combination of very rapid sintering at a heating rate of 500°C/min and at a sintering temperature as low as 1100°C for 3 min by the spark-plasma-sintering technique and mechanical milling of the starting γ-Al₂O₃ nanopowder via a high-energy ball-milling process can result in a fully dense nanocrystalline alumina matrix ceramic nanocomposite. The grain sizes for the matrix and the toughening phase were 96 and 265 nm, respectively. A great increase in toughness almost 3 times that for pure nanocrystalline alumina has been achieved in the dense nanocomposite. Ferroelastic domain switching without undergoing phase transformation in nanocrystalline t -ZrO₂ is likely as a mechanism for enhanced toughness.     

Sintering of LaFeO3 Ceramics

The sintering of LaFeO₃ has been studied in the temperature interval 1100–1600°C in air. The effect of cation nonstoichiometry on densification, microstructure, and phase composition is emphasized. La₂O₃ was observed to inhibit both sintering and grain growth. In Fe-excess materials, exaggerated grain growth occurred, particularly above 1430°C, where a liquid phase was formed. Postsintering swelling was observed in Fe-excess materials above 1430°C. The swelling mechanism is related to phase equilibria, which are reductive in nature and lead to the evolution of oxygen gas. The density in La-excess materials remains high up to 1600°C, but the ceramics might disintegrate in air.     

Densification and Microstructural Development of SrTiO3 Sintered with V2O5

Precipitation of TiO₂ occurs during the sintering of SrTiO₃ with V₂O₅ added as a liquid-phase sintering agent. Satisfactory densification can be obtained at 1250°C when using a high content of V₂O₅ during sintering. However, a microstructure of fine grains and large pores results along with the precipitation of TiO₂. The precipitation of TiO₂ can be repressed by the addition of excess SrO. A well-sintered microstructure with superior densification can thus be obtained at 125O°C from specimens sintered with a low content of V₂O₅ and an appropriate amount of excess SrO.     

Pressureless Sintering of Boron Carbide

B₄C powder compacts were sintered using a graphite dilatometer in flowing He under constant heating rates. Densification started at 1800°C. The rate of densification increased rapidly in the range 1870°–2010°C, which was attributed to direct B₄C–B₄C contact between particles permitted via volatilization of B₂O₃ particle coatings. Limited particle coarsening, attributed to the presence or evolution of the oxide coatings, occurred in the range 1870°–1950°C. In the temperature range 2010°–2140°C, densification continued at a slower rate while particles simultaneously coarsened by evaporation–condensation of B₄C. Above 2140°C, rapid densification ensued, which was interpreted to be the result of the formation of a eutectic grain boundary liquid, or activated sintering facilitated by nonstoichiometric volatilization of B₄C, leaving carbon behind. Rapid heating through temperature ranges in which coarsening occurred fostered increased densities. Carbon doping (3 wt%) in the form of phenolic resin resulted in more dense sintered compacts. Carbon reacted with B₂O₃ to form B₄C and CO gas, thereby extracting the B₂O₃ coatings, permitting sintering to start at ∼1350°C.     

Hot-Pressing of β-SiC Powder with Al-B-C Additives

The sintering behavior of β-SiC powders with additions of Al, B, and C was studied at 1600° to 1800°C with applied pressures of 20 to 60 MPa. Ceramics with densities of ∼3.08 g/cm³ were obtained by hot-pressing at 1650°C and 50 MPa. The bending strength did not degrade up to 1200°C. A large amount of a second phase, which was apparently Al₈B₄C₇, was observed as streaks in the microstructure. It is suggested that a liquid phase, which coexisted with this compound, enhanced densification.