Initial Sintering of Alumina and Hematite

Experimental measurements of the rate of shrinkage of pressed Al₂O₃ compacts, the neck growth between single-crystal Al₂O₃ spheres and plates, and the effect of particle size on neck growth between single-crystal Al₂O₃ spheres and plates are mutually consistent with the bulk diffusion sintering model. Tbe temperature dependence of the rate of shrinkage and neck growth in Al₂O₃ is characterized by an activation energy of 165 kcal. per mole. Apparent diffusion coefficients and temperature dependence calculated from the shrinkage of pressed compacts of Fe₂O₃ agree with measured diffusion coefficients for the diffusion of Fe in Fe₂O₃.     

Initial Sintering Kinetics of Beryllium Oxide

The initial sintering kinetics of beryllium oxide are examined, including the effects of calcining temperature, purity, and water-vapor content of the sintering atmosphere. Sintering occurs by a diffusion mechanism in dry-air atmospheres. Small additions of MgO to BeO increase the sintering rate markedly. The diffusion constant calculated from sintering data for pure BeO is three orders of magnitude lower than the measured diffusion of Be in BeO. Water vapor in the sintering atmosphere slows the shrinkage rate of pure BeO.     

Initial Stages of Sintering of Alumina by Thermo-Optical Measurements

The behavior of nanostructured and submicrometer α-Al₂O₃ powders during the initial stages of field-assisted sintering technique (FAST), conventional, and microwave sintering was investigated using the laser-flash technique for thermo-optical measurements (TOM). An enhanced neck formation due to surface diffusion at very early stages of sintering was found in FAST samples. No significant difference due to heating rate has been found in these various samples.     

Initial Sintering Kinetics of Hyperstoichiometric Uranium Dioxide

The initial sintering kinetics of uranium dioxide powder compacts were determined as a function of temperature and stoichiometry (800°< T <1050°C, 2.03−6 exp (−55,000/ RT ) cm²/s when O/U=2.08. The diffusion coefficient was approximately proportional to the square of the excess O present, with some deviation at higher O/U values. This behavior can be explained by assuming that the U vacancy concentration is a function of the O activity.     

Kinetics of Initial Sintering with Grain Growth

Kinetic equations for initial sintering were obtained by combining the conventional kinetic equation with an empirical expression for grain growth in the initial stage. The equations describe the isothermal shrinkage of ZnO in O₂ at 80 torr and 800° to 950°C. The equation also successfully analyzed the sintering of powder compacts of Al₂O₃ studied by other workers.     

Diffusion Sintering: I, Initial Stage Sintering Models and Their Application to Shrinkage of Powder Compacts

Consideration is given to several geometrical models that contribute to shrinkage. Various shapes of particles, vacancy sinks, and diffusion paths are described as they affect sintering shrinkage. These simplified models are extended to compacts of nonuniform particles so that much of the kinetics of sintering of a substance can be determined by measuring shrinkage rates of powder compacts. A nonideal compact may sinter as though it had once been an ideal compact after a specific amount of shrinkage has occurred. This shrinkage is characteristic of the particular compact and its origin and is independent of temperature.     

Kinetics of Initial Sintering of Vacuum-Reduced Titanium Dioxide

Observations of sphere-to-plate bonding of vacuum-reduced monocrystalline rutile have been carried out over the temperature range 1200° to 1275°C. The rate law governing the interfacial growth between sphere and plate indicates that the predominant mechanism of material transport in this sintering process is volume diffusion. In the light of the reasonably satisfactory agreement between the oxygen self-diffusion coefficients calculated from the present sintering data and those directly observed by Haul and Just utilizing an isotopic exchange technique, it is likely that oxygen ion diffusion is the rate-determining step in the sintering experiments described.     

Mechanism of Alumina-Enhanced Sintering of Fine Zirconia Powder: Influence of Alumina Concentration on the Initial Stage Sintering

The isothermal shrinkage behavior of 2.9 mol% Y₂O₃-doped ZrO₂ powders with 0–1 mass% Al₂O₃ was investigated to clarify the effect of Al₂O₃ concentration on the initial sintering stage. The shrinkage of the powder compact was measured at constant temperatures in the range of 950°–1050°C. The Al₂O₃ addition increased the densification rate with increasing temperature. The values of apparent activation energy ( nQ ) and apparent frequency-factor term (β₀ ⁿ ), where n is the order depending on the diffusion mechanism, were estimated at the initial sintering stage by applying a sintering-rate equation to the isothermal shrinkage data. The diffusion mechanism changed from grain-boundary diffusion (GBD) to volume diffusion (VD) by Al₂O₃ addition and both nQ and β₀ ⁿ increased with increasing Al₂O₃ concentration. The kinetic analysis of the sintering mechanism suggested that the increase of densification rate by Al₂O₃ addition largely depends on the increase of β₀ ⁿ , that is, the increases of n with GBD→VD change and β₀ with an increase in Al₂O₃ content, although the nQ also increases with Al₂O₃ addition. This enhanced sintering mechanism is reasonably interpreted by the segregated dissolution of Al₂O₃ at ZrO₂ grain boundaries.     

Influence of Atmosphere on the Final-Stage Sintering Kinetics of Ultra-High-Purity Alumina

The influence of sintering atmosphere on the final-stage sintering of ultra-high-purity alumina has been investigated. Model final-stage microstructures were tailored via a latex sphere impregnation and burnout technique. Critical experiments have been conducted to quantitatively examine the influence of the oxygen partial pressure on the final-stage sintering kinetics. Samples were sintered at 1850°C in either dry hydrogen ( P o₂∼ 3 × 10⁻¹⁷ atm) or wet hydrogen P o₂∼ 5 × 10⁻¹⁰ to 2 × 10⁻¹¹ atm), and their microstructures were characterized as a function of sintering time, Sintering in dry hydrogen decreased the susceptibility of the final-stage microstructure to pore/boundary breakaway. In the kinetic analysis, the variation in the number of pores per grain, N _g, was taken into account. It was found that in both atmospheres, the densification rate was controlled by grain boundary diffusion, and that sintering in dry hydrogen increased the densification rate by a factor of 2.25. In addition, it was determined that the grain growth rate in both atmospheres was controlled by the rate of surface diffusion of matter around the pores and that sintering in dry hydrogen enhanced the grain growth rate by a factor of 5.6. The overall effect of the dry hydrogen atmosphere was that it enhanced the coarsening rate relative to the densification rate by a factor of 2.5, and consequently shifted the grain size-density trajectory to much lower densities for a given grain size.     

Computer Simulation of Final-Stage Sintering: II, Influence of Initial Pore Size

A two-dimensional Monte Carlo simulation procedure has been used to investigate the effect of the initial pore size on the microstructural evolution and the kinetics of final-stage sintering. The sintering time scales with r ⁴₀/ D_gb and the grain-growth time scales with r ²_O/ D_m . Pores are found to effectively pin the grain boundaries from the beginning of final-stage sintering at a porosity of Φ= 0.09 until Φ= 0.03. For Φ 0.03, the remaining pores do not effectively retard grain-boundary migration and normal grain growth occurs. Small pores were found to be less effective at retarding grain growth than expected on the basis of a simple grain-growth pinning model. The mean pore size was found to be nearly constant throughout the simulations.     

Sintering Alumina: Effect of Atmospheres

Two effects of atmosphere on the sintering of alumina powder compacts were investigated. Changing the oxidizing conditions during early-stage sintering measurements showed that, within the range −70°F dew point oxygen to −70°F hydrogen equilibrated with alumina, there was no observable effect on sintering rate. The effect of trapped atmosphere on closure of pores was determined from apparent density limits obtained at high sintering temperatures and long sintering times. The complete elimination of porosity during sintering of alumina was possible if discontinuous grain growth was controlled and if the ambient atmosphere was hydrogen, oxygen, or vacuum. Porosity could not be completely eliminated when the ambient atmosphere was helium, argon, or nitrogen (or therefore air).     

Estimate of the Activation Energies for Boundary Diffusion from Rate-Controlled Sintering of Pure Alumina, and Alumina Doped with Zirconia or Titania

Sintering experiments at constant heating rates were employed to estimate the activation energy for sintering in alumina and in alumina containing 5 vol% zirconia or 5 vol% titania. Grain growth, which can complicate the analysis of sintering kinetics data, was suppressed by using uniformly and densely packed grain compacts prepared by colloidal processing. Grain-boundary diffusion is believed to have been the dominant sintering mechanism. The activation energies were 440 ± 40 kJ/mol for pure alumina, 585 ± 40 kJ/mol for alumina (titania), and 730 ± 60 kJ/mol for alumina (zirconia). The alumina and alumina (titania) results are in agreement with the values reported in the literature. The possibility that the higher activation energies for doped alumina reflect a stronger bonding at alumina interfaces in the presence of zirconium and titanium is discussed.     

Sintering of PZT Ceramics: II, Effect of PbO Content on Densification Kinetics

The presence of a PbO-rich liquid phase was previously shown to affect the densification of lead zirconate-lead titanate (PZT) ceramics. The present work attempts to quantify the role of this liquid phase. Increasing amounts of PbO-rich liquid phase is shown to increase the rates of densification in the initial and intermediate stages, but lower the final density. This behavior is discussed. In the case of PbO-deficient samples, the amount of the deficiency is shown to affect the densification rate at the final stage.     

Kinetics and Mechanism of a Sintering Process for Macroporous Alumina Ceramics by Extrusion

The paper describes an evaluation of kinetic parameters for sintering from the analysis of stepwise isothermal dilatometry data for the pore-forming process of macroporous alpha-Al₂O₃ ceramics fabricated by extrusion. The experimental results demonstrated that isothermal shrinkage data for the sintering process can fit the following rate equation: d Y /d t = nK ( T ) Y (1 - Y )[(1 - Y )/ Y ]^{1/ n} , where Y is the fraction of volume shrinkage. Kinetic parameters, i.e., the average exponent n and the activation energy E for the pore-forming process, were calculated to be 0.232 and 415.5 kJ/mol, respectively, in the temperature range of 1200-1400°C. The results also show the equivalence between isothermal sintering and stepwise sintering in alumina. In the latter case the temperature is increased in steps over a period of time. The equivalence is derived from understanding how the sintering rate changes with porosity. The sintering mechanism in different temperature ranges is discussed, with reference to microstructure development. The bend strengths of porous ceramics with different porosities are also presented.     

Sintering of CoO: I, Initial Stage

The initial-sintering kinetics of compacts of uniform spheres of CoO were investigated at various temperatures and O₂ partial pressures. Volume-diffusion kinetics were exhibited initially, followed by retardation of sintering and deviation from the sintering model as a result of faceting of the spheres. The calculated volume diffusivities lie between those measured for Co and O in CoO.     

Liquid Phase Sintering of Alumina, II. Penetration of Liquid Phase into Model Microstructures

Alumina preforms containing artificial pores were sintered at 1630°C in air and vacuum. Glass penetration into the alumina preforms was conducted at 1600°C in air. It was found that the trapped gases in alumina preforms sintered in air caused the random and incomplete filling of the smaller and larger artificial pores. In contrast, the pores in the alumina preform sintered in vacuum were completely filled during glass penetration.     

Microcavity Formation in Alumina Using Ti Templates II: Mechanism and Kinetics

In part I of this work, it was found that titanium (Ti) wire encapsulated within mechanically milled alumina powder and sintered at 1350°C forms potentially useful microcavities due to the consolidation of Kirkendall porosity. Here a series of samples sintered at 1350°C in the range 0–24 h has shown the remarkable way in which these cavities form. The cavity has already started in samples quenched from the top of the heating ramp (0 min at 1350°C). It is surrounded by a diffusion zone ∼300 μm in diameter, which does not change size throughout the firing process although the contents change markedly. The diffusion zone microstructure is initially complex with phase sequence TiO₂/Al₂O₃/TiO₂+Al₂O₃/Al₂TiO₅. Microstructure evolution may be summarized as outward growth of the cavity accompanied by inward growth of the Al₂TiO₅ resulting in a ∼190-μm-diameter cavity surrounded by a 50-μm-thick layer of Al₂TiO₅. The formation of the cavity and surrounding microstructure is discussed although some features, such as the nucleation of Al₂TiO₅ in the part of the diffusion zone furthest from the Ti source and the ring of Al₂O₃, which persists in between Ti-rich parts of the diffusion zone are still poorly understood.     

Relating Grain Boundary Complexion to Grain Boundary Kinetics II: Silica-Doped Alumina

This second paper in a series describes the relationship between grain growth kinetics and grain boundary complexions in silica-doped alumina. Dense high-purity silica-doped alumina samples were annealed for various times in the temperature range of 1300° and 1900°C and their grain growth behavior was quantified. Four different grain boundary complexions were observed in silica-doped alumina, all of which enhanced the kinetics relative to the intrinsic undoped alumina. These complexions included a thick crystallized film that was likely amorphous at high temperatures, a thin intergranular film, multilayer adsorption, and a type of boundary that showed no observable film by high-resolution transmission electron microscopy. A generational change in the population of grains occurred at 1500°C where all of the abnormal grains impinged and reestablished a new normal distribution. At higher temperatures a new set of abnormal grains containing different complexions formed in the microstructure. The activation energy of the normal and abnormal grains was approximately the same. The effects of varying dopant concentration were analyzed. The results for silica-doped alumina are compared with previous results for calcia-doped alumina in order to draw some generalized conclusions about the effect of complexions on grain growth.     

Sintering Kinetics at Isothermal Shrinkage: Effect of Specific Surface Area on the Initial Sintering Stage of Fine Zirconia Powder

The isothermal shrinkage behaviors of fine zirconia powders (containing 2.8–2.9 mol% Y₂O₃) with specific surface areas of about 6 and 16 m²/g were investigated to clarify the effect of specific surface area on the initial sintering stage. The shrinkage of powder compact was measured under constant temperatures in the range of 1000°–1100°C. The increase in specific surface area enhanced the densification rate with increasing temperature. The values of activation energy ( Q ) and frequency-factor term (β₀) of diffusion at initial sintering were estimated by applying the sintering-rate equation to the isothermal shrinkage data. The Q of diffusion changes little but the β₀ increases with the increase in specific surface area. It is therefore concluded that the increase in the specific surface area of fine zirconia powder enhances the shrinkage rate because of an increase in the β₀ at the initial stage of sintering.     

Spark Plasma Sintering of Alumina

A systematic study of various spark plasma sintering (SPS) parameters, namely temperature, holding time, heating rate, pressure, and pulse sequence, was conducted to investigate their effect on the densification, grain-growth kinetics, hardness, and fracture toughness of a commercially available submicrometer-sized Al₂O₃ powder. The obtained experimental data clearly show that the SPS process enhances both densification and grain growth. Thus, Al₂O₃ could be fully densified at a much lower temperature (1150°C), within a much shorter time (minutes), than in more conventional sintering processes. It is suggested that the densification is enhanced in the initial part of the sintering cycle by a local spark-discharge process in the vicinity of contacting particles, and that both grain-boundary diffusion and grain-boundary migration are enhanced by the electrical field originating from the pulsed direct current used for heating the sample. Both the diffusion and the migration that promote the grain growth were found to be strongly dependent on temperature, implying that it is possible to retain the original fine-grained structure in fully densified bodies by avoiding a too high sintering temperature. Hardness values in the range 21–22 GPa and fracture toughness values of 3.5 ± 0.5 MPa·m^1/2 were found for the compacts containing submicrometer-sized Al₂O₃ grains.