Intermediate Stage Of Sintering Research Articles

Examination of morphological changes of pore channel network during the intermediate stage of sintering of undoped, MgO-doped and ZrO2-doped alumina compacts by modified statistical theory of sintering

The modified statistical theory of sintering was used to evaluate the microstructural parameters, such as the average pore radius, grain size, and total length per unit volume of pore, of undoped and doped alumina compacts during the intermediate stage of sintering. The present results further justify the validity of the modified statistical theory of sintering in characterizing the morphological changes of pore channel network. The evaluated data of the evolution of these morphological parameters provide deep insight of the understanding of the mechanism of the effects of particle size distributions, MgO and ZrO2 on breakup of pore channel network. The possible roles of MgO and ZrO2 in the intermediate stage of sintering have been discussed.

Effect of compositional variation in sintering behaviour of Al–Zr oxide composites

Sintering behaviour of sequentially varied metal ion concentration in Al x –Zr 100− x ( x = 0, 10, 20, …, 100) based oxide composites were studied using conventional solid state sintering process in the temperature range 1400–1600 °C. Critical experimental parameters like green density, sintering time and temperature were varied to achieve higher sintered product. Systematic studies of densification, structural, microstructural, hardness and shrinkage properties were carried out to understand the real sintering process development mechanism involved in this oxide matrix during initial, intermediate and final stage of sintering. Interesting development in connection with microstructural and phase evolution, pore-grain network in relation to physical densification and hardness behaviour are analysed and it is noted that compositional variation in Al x –Zr 100− x oxide matrix ( x = 0, 10, 20, …, 100) produces structural (monoclinic to tetragonal) phase separation (complete or partial) at different sintering temperatures (1400–1600 °C). Long heating schedule (20 h) at high sintering temperature (1600 °C) promotes highest percentage of monoclinic–tetragonal transformation at x = 30 in Al x –Zr 100− x oxide composites.

Microstructural Development during Sintering of Lithium Fluoride

Measurements are reported of the influences of temperature, green density, and pore network breakup on the densification, grain growth, and pore volume distribution in LiF compacts. As long as most of the pore volume remained open to the compact perimeter, the ratio of the rate of densification to the rate of grain growth was higher than that sometimes reported for copper or typical oxides. Plots of the logarithm of densification rates versus sintered density for LiF are approximately linear during intermediate-stage sintering, like those for some oxides. But the plots for LiF are unlike those of the oxides in that, for LiF, densification rates measured at different temperatures converge near the density at which half the pore volume is isolated from Hg intrusion. Calculations suggest that further densification of the LiF compacts is blocked because air trapped in isolated pores becomes sufficiently compressed to balance the sintering stress.

Low temperature processing of dense samarium-doped CeO 2 ceramics: sintering and grain growth behaviors

Samarium-doped CeO 2 is a leading electrolyte for applications in solid oxide fuel cells (SOFCs), which requires a typical sintering temperature of 1400–1600 °C. By synthesizing reactive powders via carbonate precipitation, fully dense CeO 2 ceramics doped with 0–20 at.% of samarium have been fabricated in this work via pressureless sintering at a significantly lowered temperature of 1000 °C. The resultant ceramics show ultrafine grain sizes of ∼0.15–0.75 μm, depending upon the dopant concentration. Sintering studies indicated that samarium doping retards both densification and grain growth but increases the rate ratios of the two in the intermediate stage of sintering. Subsequent investigations on the grain growth in the fully densified ceramics also showed the suppressing effects of dopant, which tend to saturate at 10 at.% of samarium. The activation energy for grain growth increased from ∼186 to ∼254 kJ/mol by raising the samarium concentration from 5 to 20 at.%.

Pore growth during initial and intermediate stages of sintering in ZrO 2–3 mol% Y 2O 3 compact: a small-angle neutron scattering investigation

Small-angle neutron scattering technique has been employed to investigate the evolution of pore morphology during the initial and the intermediate stages of sintering in ZrO 2–3 mol% Y 2O 3. These results show that although there is a reduction in the porosity due to the elimination of the pores, the average growth in the pore size takes place during the sintering. This trend in pore growth corroborates the results of computer simulation based on the Potts model. The SANS data has been analyzed in the light of the polydisperse spherical pore model.

Computer simulation of grain growth of intermediate—and final-stage sintering and Ostwald ripening of BaTiO 3-based PTCR ceramics

This paper presented a novel modified Q-states Potts Monte-Carlo computer simulation procedure applied to simulate the grain growth of intermediate and final stages of sintering and Ostwald ripening process of BaTiO 3 Positive Temperature Coefficient of Resistance (PTCR) ceramics, and the effects of liquid-phases on the grain growth of BaTiO 3-based PTCR ceramics. The computer simulating models of the grain growth of the sintering process were established by the theory of the grain growth. The simulating results indicated that (1) the liquid-phase would hinder the motion of the grain-boundary, and the ratio of the grain growth was limited so that the size of the grains was smaller than those of the single-phase system; (2) the shrinkage of a BaTiO 3 PTCR ceramics almost ceased at the temperature of 1513 K or so in the two-phase system, and the soaking stage involved the densification and grain growth of ceramics controlled by liquid-phases.

Sintering behaviour and microstructural evolution of agglomerated spherical particles of high-purity barium titanate

Abstract Densification and grain growth of high-purity BaTiO 3 spherical particle compacts have been studied in the temperature range of 25–1300 °C by the CRH method. A rapid densification of the samples between 1000 and about 1230 °C with the formation of a continuous interconnected porosity, and a negligible grain growth took place. Above 1230 °C grains grew rapidly and the interconnected porosity progressively collapsed into isolated pores. Measurements of grain size as a function of density showed that the grain-growth behaviour change took place at about 92% of theoretical density, which coincides with the end of the intermediate-stage sintering. From the present experimental results it can be tentatively stated, for the first time in barium titanate, that a linear correlation between density and grain size in the intermediate-stage of sintering exists. In the final-stage of sintering a bloating or desintering feature was present as consequence of the loss of carbon dioxide from the barium carbonate in the barium titanate environment.

Sintering behaviour of Y 2O 3 powders prepared by the polymer complex solution method

High purity fine Y 2O 3 powder has been prepared by a polymer complex solution method using polyethylene glycol as the complexant of Y 3+ cations. Powder morphology and compacts prepared from three different powder characteristics have been studied. SEM observations were used to know the agglomeration degree of the powders, and the uniformity of powder compacts, and the Hg-porosimetry results to investigate the pore size and microstructure evolution during sintering of the green compacts. The compacts made with the powders calcined at the lower temperatures showed an almost single-peaked pore-size distribution and a more homogeneous microstructure, whereas the pore-size distributions of the compacts made with powders calcined at higher temperatures were double or triple peaked. The sintering behaviour of the low temperature calcined powders exhibited significantly better sinterability and higher densification rate/grain growth rate ratio than the high-temperature calcined powder compacts. Complex rearrangement phenomena relating the pore coordination numbers-grain size-grain morphology appear to occur in the initial and intermediate stages of sintering. In the final stage of sintering, grain growth modified the thermodynamic stability of remaining pores permitting its disappearance although kinetics impeded its complete elimination limiting, thus, the final density to values near theoretical density (≈99.5%).

Cosintering Simulation and Experimentation: Case Study of Nanocrystalline Zirconia

Laminates consisting of two different nanocrystalline zirconia materials (undoped and 3Y‐TZP doped) were sintered, and their densification and curvature were quantified using optical dilatometry. The experimental results agreed well with a continuum‐mechanical description for the cosintering of laminates with diffusion coefficients and activation energies determined using densification of the free‐sintering materials. The comparison covered various relative layer thicknesses and heating rates, but was developed only for the intermediate and late stages of sintering. Sintering stresses and compatibility stresses also were determined from the model.

Solid state sintering and high temperature compression properties of Al-alloy5000/AlN nanocomposites

Nanocomposites of Al-alloy5000 reinforced with 4–30 vol.% ceramic phase (oxides and aluminium nitride particles) were produced by cryomilling and hot pressing. Diffusion mechanisms occurring during the intermediate stage of sintering were analysed by a kinetic study at constant rates of heating. The fine and homogeneous dispersion of the ceramic phase in the Al matrix gives the composites a stable nanostructure and influences the diffusion mechanisms (volume or grain boundary). High temperature compression behaviour of the nanocomposites was then examined. In the chosen conditions of temperature and strain rates (823 K, 10 −3–2.7 s −1), the materials exhibit a high resistance to compression. However, after extrusion of a composite containing a low quantity of ceramic phase, deformation by superplasticity occurs.

Influence of vanadium substitution on sintering behaviour of Pb 3(VO 4) 2(1− x) (PO 4) 2 x ceramics

Intermediate-stage sintering has been investigated in lead orthophosphovanadates Pb 3(VO 4) 2(1− x) (PO 4) 2 x . It was found that rich-vanadium compounds such as Pb 3(VO 4) 2 and Pb 3(VO 4) 1.6(PO 4) 0.4 densify rapidly with important grain growth. For these compounds grain growth is controlled by grain boundaries and densification occurs by a mixed mechanism with lattice and grain boundary diffusion. For Pb 3(PO 4) 2, sintering mechanism supports a model of grain-boundary-controlled densification and grain growth is a surface diffusion-controlled pore drag mechanism. Moreover, the presence of phosphorus in compounds' formulae, tends to decrease the grain-boundary mobility, preventing pore-boundary separation. The kinetics analysis highlights the importance of vanadium substitution in modifying the diffusion coefficient of rate-limiting species.

Deterioration in the Final‐Stage Sintering of Magnesium Aluminate Spinel

Sintering of magnesium aluminate spinel of the MgO‐excess, stoichiometric, and Al2O3‐excess compositions has been investigated under vacuum and in air for the effect of low oxygen partial pressures. Densification enhancement of the surface layer is due to MgO evaporation which generates oxygen vacancies in the host crystal lattice. Regions of different grain sizes are observed from samples sintered under both conditions. Microstructural features of pairwise breakup of particle chains representing differential sintering are characteristic of the less‐densified sample interior. The densification improved initially and yet was retarded in the intermediate sintering stage when the density exceeded 75% with vacuum‐sintering owing to MgO evaporation.

Magnetic Sintering of Ferromagnetic Metal Powder Compacts

Magnetic sintering of ferromagnetic metal powder compacts has been carried out aiming at precise control of microstructures in polycrystalline materials. Powder compacts of carbonyl iron and cobalt were subjected to magnetic sintering in a direct-current magnetic field ranging up to 1.2 MA/m at temperatures in both the ferromagnetic and the paramagnetic regions. We have found that a magnetic field can enhance the densification and grain growth of iron powder compacts in the ferromagnetic state and in the paramagnetic one. The higher became the magnetic field strength, the more the densification during sintering proceeded. The observed magnetic field effects seemed to he most significant in the intermediate stage of sintering. The observations suggested that a magnetic field was responsible for an increase in the driving force for grain boundary migration that would play an important role in the densification during sintering. In contrast to iron powder compacts the densification of cobalt powder compacts were suppressed by a magnetic lield.

Role of zirconia addition in pore development and grain growth in alumina compacts

The role of 12.4 vol% ZrO2 addition in the microstructure evolution of alumina compacts during the intermediate and final stages of sintering was investigated by means of small-angle neutron scattering measurements and stereological analysis. Both the pore-size evolution results and the grain-growth data indicate a narrowly defined onset density for the transition to the final sintering stage. The presence of ZrO2 as a second phase apparently maintains the stability of the intermediate sintering stage out to significantly higher density than in single-phase alumina and plays an important role in inhibiting grain growth and in preventing pore–grain boundary separation. The influence of the ZrO2 second phase on pore evolution, grain growth, and sinterability of the alumina–zirconia composite is discussed and compared to the behavior of single-phase alumina. The samples were prepared from commercially available powders, with naturally occurring porosity distributions, rather than from artifact(model) pore compacts prepared from nominally pure research-grade materials. The goal was to gain an improved understanding of microstructure development in real materials.

Preparation of YBa2Cu3O7−x superconducting films: influence of the chemical composition on the sintering

Abstract The influence of the cationic ratio Y:Ba:Cu on the kinetics of densification of the YBa 2 Cu 3 O 7− x (YBCO) superconductor ceramic was studied. The powders were prepared from organic precursors and the nominal compositions were: (A) stoichiometric composition YBa 2 Cu 3 O 7− x , (B) composition on the tie line joining YBa 2 Cu 3 O 7− x –BaCuO 2 phases, and (C) composition in the triangle YBa 2 Cu 3 O 7− x –Y 2 BaCuO 5 –CuO. The phases present in these powders heated to 890°C were detected by X-ray diffraction (XRD), while the ratio Y:Ba:Cu was measured by neutronic activation analysis (NAA). The reactions between 900°C and 1070°C under O 2 atmosphere were determined by differential thermal analysis (DTA) at a heating rate of 10°C/min. Compacts of different powders were obtained by uniaxial pressing and were then studied by dilatometric experiments under similar conditions as established in DTA measurements. The dilatometric curves were applied to different sintering models to obtain the validity ranges and activation energies corresponding to the different densification mechanisms that operate in the initial and intermediate stages of sintering.

Sintering characteristics of a porous Ni/Ni 3Al anode for molten carbonate fuel cells

Studies of sintering behaviour and microstructural pore evolution during sintering of pure Ni particle — and Ni/(4–10 wt.%)Ni 3Al intermetallics — green sheets for the molten carbonate fuel cell (MCFC) anode are conducted. The pore structure in the Ni/Ni 3Al anode can be maintained as an open pore network by restraining the initial stage of sintering. Ni 3Al intermetallics inserted on nickel grain boundaries act as a barrier, which inhibits nickel grain boundary diffusion and controls both densification and nickel grain growth. Pores in the pure nickel anode become smaller than those in the Ni/Ni 3Al anode by an intermediate stage of sintering which results in densification and nickel grain growth.

Relations Between Coarsening and Densification and Mass Transport Path in Solid-state Sintering of Ceramics: Model Analysis

The correlations between coarsening (including grain and pore growth) and densification, and the effects of mass transport on particle coarsening and densification were discussed based on the simple particle array models and for the real particle compacts. Grain boundary motion could cause particle coarsening only under a certain particle size distribution but not densification; mass transport is reasoned to contribute to both grain growth (particle coarsening) and shrinkage for one-dimensional particle arrays. Under a certain limitation for the change of the particle size aspect ratio during sintering, very limited effects of grain grown by itself on the shrinkage of particle a rrays throughreinitiating the sintering could be found. For a real powder compact system, mass transport between the particles, which surround a pore, contributes to the particle coarsening and densification when the pore is thermodynamically unstable and only to particle coarsening when the pore is thermodynamically stable. The mass transport mechanism for both particle coarsening and densification would be the same, which cannot exclude, at least on thermodynamics, the contribution from surface diffusion in the intermediate stage of sintering.

Thermodynamics and Densification Kinetics in Solid-state Sintering of Ceramics

Based on the stability analysis of the closed pores in two dimensions which was determined mathematically with their particle coordination number and dihedral angle, the stability of those in three dimensions was determined with a spherical pore model. The model is set up by first excluding the effect of interface tension, so the pore was supposed to be spherical, and then the tensile stress arisen from the interface tension was supposed to act on this hypothesized spherical pore. On the basis of the spherical pore model, microstructure models based on pores, not grains, for the real powder compacts were first established. Densification kinetics were then determined from the models by the densification rate equations, which were derived by relating density to pore size to grain size ratio, for the intermediate and final stages of sintering. The criteria for pore shrinkage were discussed quantitatively. The derived equations can be used to simulate the relation between densification rate and density during heating with a constant rate and for the explanation of the effects of pore size distribution, agglomerates, and green density on sintering.

Thermally activated step for densification and creep of high-porosity MgO compacts in the intermediate sintering stage

High porosity MgO compacts were sintered in the temperature range 1283–1350 K, under the action of weak uniaxial loads of 3·3 ± 0·4 kPa. Isothermal densification and creep rates were found to be significantly different exponentially decreasing functions of density. The pore size and the pore distribution of samples heated at two different temperatures of 1283 K and 1333 K have turned out to be the same when the relative densities are equal. A common activated step with an average apparent activation enthalpy equal to 480 ± 40 kJ per mole has been experimentally established for both densification and weak load creep. The obtained results are consistent with the model used to explain the nature of the creep phenomena in high porosity MgO compacts when weak uniaxial loads are applied. These results also suggest that both pore growth and grain growth processes obey the same thermal activated mechanism. If this is true, as data of Wong and Pask suggest. The rate-limiting step for intermediate stage grain growth might be different from that applying in the final stage of sintering.

Effects of sintering on properties of alumina microfiltration membranes

The transformation of membrane channels during the sintering process conforms to the Rhines' topological decay model of intermediate sintering stage. The pore size of the membranes enlarges with the increase of sintering temperature. The pore size increment caused by the increase of sintering temperature is more obvious for thin membranes than for thick membranes. With the increase of sintering temperature, the water permeance of membranes increases at first and then decreases after a turning point of sintering temperature.