Pore structure of sintered and metallized iron oxide compacts

Pore structures of sintered and metallized iron oxide compacts were investigated by mercury porosimetry. The pore volume of the sintered compacts was found to decrease with increasing sintering temperature and time, owing to the progressive elimination of the smallest pores present in the compacts. Reduction in hydrogen generated a bimodal pore size distribution, the larger pores reflecting the original intergranular compact structure and the second band of smaller pores, the intragranular voids produced by the removal of oxygen. The average pore size of the reduction band was entirely independent of the oxide grain size and sintering temperature, but depended strongly on reduction temperature. Apparent activation energies of 13 and 27 kcal/mole were derived for the pore formation process over the temperature ranges 500° - 800 °C and 800° - 900 °C respectively. Swelling phenomena observed on reduction were attributed mainly to an increase in the intergranular voidage.     

Effect of compaction pressure on the sinterability of submicron powders of tetragonal zirconium dioxide

Conclusions We studied the effect of compaction pressure on the pore structure of the paniculate compacts obtained using two types of agglomerated submicron powders of tetragonal zirconium dioxide, on the structure evolution during the sintering process, and on the strength of the obtained material. It was established that the characteristics of the agglomerates present in the powders have a significant effect on their behavior during compaction and sintering. At a given compaction pressure, the powders having weaker agglomerates densify up to a higher density and give a more uniform distribution of pores in the preform. The low-density compacts obtained using agglomerated powders having a high specific surface area sinter faster and attain high strength levels at a lower temperature; however, the sintered materials obtained from such compacts contain several structural defects in the form of large pores and have a lower strength. The uniformity of the distribution of pore volume with respect to size (or the specific content of the interagglomerate pores) forms the main criterion of the quality of particle packing in the compacts obtained from agglomerated powders. The compacts having a low content of the interagglomerate pores give a defect-free dense and strong material after sintering. The presence of the anion impurities in the original powders leads to a decrease of density during the sintering process after the attainment of a threshold density at which formation of closed porosity occurs. Pressure sintering (HIP) forms an effective method of suppressing the decrease of density.Translated from Ogneupory, No. 2, pp. 5–11, February, 1993.     

Study of the Bimodal Pore Structure of Ceramic Powder Compacts by Mercury Porosimetry

Mercury porosimetry was used to study the pore-size distribution of both green and partially sintered alumina compacts. Experiments showed that using the mercury-penetration volume data to study the pore structure changes during sintering for a compact with a bimodal pore-size distribution could lead to wrong conclusions because the mercury-penetration volume was not linearly proportional to the porosity. Instead, the porosity distribution should be used. When the porosity of large pores was <10%, some large pores within the compact were observed to become isolated or channeled to the compact surface; this caused errors in the porosity distribution measured by mercury porosity.     

Mercury porosimetry of ceramics

Mercury porosimetry has been used in ceramics for the characterization of products and for studies of processing. Specifications including PSD (pore size distribution by mercury porosimetry) are now applied to magnesia refractories used in basic oxygen furnaces and to building brick which may be exposed to frost. Other products cited as examples are the silica fiber tile used on the space shuttle, plasma sprayed coatings, carbon composites and filters.In ceramic processing research, PSD has proved valuable for evaluating the firing of basic refractories, brick and sanitary ware. Pore growth during the early stages of sintering several materials was first identified by PSD. The character of clay agglomerates and the presence of alumina aggregates in compacts have been measured, the latter showing bimodal PSD. The progressive change in PSD while compacting glass spheres outlines the stages of compaction. The most frequent pore diameter in plaster slip casting molds correlates directly with plaster consistency.In dense or vitrified ceramics, errors may occur due to closed pores or pores with narrow openings. However, in ceramic compacts and highly porous ceramics, pores have several openings so PSD is a realistic measure of structure.     

Novel model for the sintering of ceramics with bimodal pore size distributions: Application to the sintering of lime

A mathematical model for the sintering of ceramics with bimodal pore size distributions at intermediate and final stages is developed. It considers the simultaneous effects of coarsening by surface diffusion, and densification by grain boundary diffusion and lattice diffusion. This model involves population balances for the pores in different zones determined by each porosimetry peak, and is able to predict the evolution of pore size distribution function, surface area, and porosity over time. The model is experimentally validated for the sintering of lime and it is reliable in predicting the so called “initial induction period” in sintering, which is due to a decrease in intra‐aggregate porosity offset by an increase inter‐aggregate porosity. In addition, a novel methodology for determination of mechanisms based on the analysis of the pore size distribution function is proposed, and with this, it was demonstrated that lattice diffusion is the controlling mechanism in the CaO sintering. © 2016 American Institute of Chemical Engineers AIChE J, 63: 893–902, 2017     

Processing of porous glass ceramics from highly crystallisable industrial wastes

This study was carried out to gain understanding about the sintering behaviour of highly crystallisable industrial waste derived silicate mixtures under direct heating and rapid cooling conditions. The materials used in this study were plasma vitrified air pollution control waste and rejected pharmaceutical borosilicate glass. Powder compacts sintered under direct heating conditions were highly porous; compacts with particle size <?38?μm reached a maximum density of 2.74 g?cm^??3 at 850°C, whereas compacts with particles of size <?100?and <?250?μm reached maximum densities of 2.69 and 2.72 g?cm^??3 at 875 and 900°C respectively. Further increase in sintering temperature resulted in a rapid decrease in density of the glass ceramics. Image analysis results were used to link the sudden drop in density to the increase in volume of microsized pores formed in the samples during sintering. In particular, compacts made from <?38 μm particles sintered at 950°C resulted in 65 vol.-% porosity with a pore size of ～20?μm. Such materials can be used for sound and thermal insulation purposes.     

Fabrication of high-density magnesia using vacuum compaction molding

High-density magnesia was fabricated using vacuum compaction molding, and effects of forming pressure and sintering temperature on bulk density, apparent porosity, diameter shrinkage ratio, volume shrinkage ratio, pore size distribution, cold compressive strength, and thermal shock resistance of the magnesia samples were investigated. There were two ranges of pore distribution in samples that were formed via conventional compaction molding, and these ranges were about 350–2058 nm and 6037–60527 nm. It was considered that the range of larger pores mainly influenced the densification of magnesia. Using vacuum compaction molding, large size pores were removed, and high-density magnesia (with a density greater than 3.40 g cm^?3) was easily prepared when forming pressure was higher than 200 MPa and sintering temperature was higher than 1600 °C. Magnesia samples prepared via vacuum compaction molding showed better performance compared to that of samples prepared via conventional compaction molding.     

High strength porous alumina by spark plasma sintering

High strength porous alumina was fabricated by spark plasma sintering (SPS) at temperatures between 1000 and 1200 °C with nanocrystalline Al(OH)₃ as the starting powder without any seeds, dopants or inclusions. Decomposition of the Al(OH)₃ produced a series of transitional alumina phases depending on sintering temperature and pressure and finally the stable α-alumina phase was obtained. A network of continuous pores with unimodal pore size distribution was estimated by mercury porosimetry and BET surface area measurements, with the porosity ranging between 20% and 60% based on sintering conditions. Predominance of fine grains and extensive necking between them led to better strength in the sintered samples. The bending strength of the sintered compacts rapidly increased with sintering temperature while retaining reasonable porosity suitable for practical applications. The results clearly indicate that in situ phase formation of α-Al₂O₃ and θ-Al₂O₃ provides strength and porosity, respectively. Phase transformation, pore morphology and microstructure evolution were also studied.     

Evaluating Pore Space in Macroporous Ceramics with Water‐Based Porosimetry

We show that water‐based porosimetry (WBP), a facile, simple, and nondestructive porosimetry technique, accurately evaluates both the pore size distribution and throat size distribution of sacrificially templated macroporous alumina. The pore size distribution and throat size distribution derived from the WBP evaluation in uptake (imbibition) and release (drainage) mode, respectively, were corroborated by mercury porosimetry and X‐ray micro‐computed tomography (μ‐CT). In contrast with mercury porosimetry, the WBP also provided information on the presence of “dead‐end pores” in the macroporous alumina.     

Effects of Particle Packing Characteristics on Solid-State Sintering

Alumina compacts fabricated with different green densities and different pore size distributions were characterized and the changes of the pore characteristics during solid-state sintering were studied. A critical ratio of pore size to mean particle size for pore shrinkage was determined. Porosity in the compact could be classified into two classes: the first class contains pores smaller than the critical ratio, and the second class contains pores larger than the critical ratio. Pores belonging to a different class of porosity behaved differently during sintering. Pores larger than the critical ratio were not totally eliminated during sintering. The first class of porosity controlled the ultimate sintering shrinkage, and the second class of porosity controlled the final sintered density.     

Pore Growth during Initial-Stage Sintering

A quantitative model for pore growth during initial-stage sintering is proposed. During initial-stage sintering, neck formation leads to surface rounding of the pores, thereby causing a decrease in the surface area of the system. The decrease in surface area, without a concomitant decrease in pore volume, leads to a spurious increase in pore size, as calculated by the gas adsorption technique. Geometrical calculations predict a final pore size that is a function of the initial (green) density of the compact. For the 62% dense compacts of the present study, the model predicts a factor of 1.28 increase in pore size, compared to the factor of 1.27, which is experimentally observed. Interestingly, a common, constant factor of 1.27 can also be observed in pore growth data reported by a number of other researchers.     

Microstructural evolution during sintering in MgO powders precipitated from sea water under induced agglomeration conditions

Green compacts pressed by means of uniaxial compaction with Magnesia (MgO) powders precipitated from sea water and calcined at different temperatures were sintered under H₂ atmosphere at 1700 °C. The calcination, carried out between 900 and 1200 °C had a great influence in the final density and the microstructure. The densification of MgO agglomerated powders seems to be predictably related to grain growth and thus coarsening kinetics. At calcination temperatures higher than 900 °C, the volume of large pores was increased notably suggesting that the inhibited grain growth adversely affected the thermodynamics of pore sintering. Relative densities between 74 and 98% of theoretical density were reached in compacts obtained at different compaction pressures. The microstructural differences were examined by Scanning Electron Microscopy (SEM).     

Structural Rearrangement During the Sintering of MgO

MgO compacts were sintered from 900° to 1395°C in dry argon and in argon with 2.3 kPa of water vapor. The surface area and pore size distributions measured by mercury porosimetry showed large changes during the first minute of sintering, indicating that rearrangement instead of coalescence of small particles is the controlling process in the early stage. Rearrangement was found to be temperature-dependent and to extend through the initial stage of sintering.     

Sintering of Mullite-Containing Materials: II, Effect of Agglomeration

The sintering behavior of mullite powder compacts which contained soft and hard agglomerates was studied, The maximum density achieved depended on the size and packing of the agglomerates. Although the initial % total pore volume was kept constant, the presence of larger pores in the green compact, due to larger agglomerates, resulted in lower final densities after sintering. Densification rates were enhanced by the breakdown of agglomerates by grinding. The particle and agglomerate packing arrangements caused densification substages to occur. A schematic model is presented which agrees well with the observed experimental behavior.     

A generalized analysis for mercury porosimetry

A detailed analysis of mercury-intrusion data has been undertaken without specifying a particular pore shape. An equation, derived from a pore-size distribution function in generalized form, has been used to linearize the compression-corrected mercury-intrusion data of several samples on a log-log plot. Using log-log plots, volume changes due to sample compaction can be straightforwardly identified and then separated from volume changes due to mercury intrusion into pores.Equations have also been derived for calculating surface areas from the slope and intercept parameters of these log-log plots. These surface areas, which are compression-corrected and compaction-corrected, have agreed well with BET surface areas.Methods developed for mercury-intrusion porosimetry, to correct data for compression effects, linearize mercury-intrusion data, and calculate surface areas, have been effectively applied to water-intrusion porosimetry. As the hydrostatic pressure was increased on the water surrounding an untreated sample of macroporous polystyrene, water intruded the pores of this sample with a contact angle near 112°.     

Pore growth during the initial stages of sintering ceramics

Mercury porosimetry was used to measure changes in pore size distribution during initial stage sintering of compacts of submicron size particles of several oxides. Pore growth was observed in MgO and Fe₂O₃, and in Al₂O₃ under certain conditions. Pores can grow by these mechanisms: surface diffusion, particle size distribution effects, particle coalescence, phase transformation, and evaporation/condensation. Surface diffusion may be the mechanism in the case of an alpha alumina. Phase transformation was shown to be the cause when sintering gamma alumina. In the case of magnesia and ferric oxide, particle coalescence appears to be operating. Since pore growth competes with densification for the use of surface energy, it is an important sintering process.     

Cold compaction molding and sintering of polystyrene

It is the objective of this paper to demonstrate the applicability of cold compaction molding followed by a sintering treatment to the processing of polystyrene powders. The influence of pressure, compaction speed, and peak pressure dwell time on the green (as compacted) density and the green tensile strength, as well as the effect of sintering on the tensile strength and dimensional change, were evaluated. The resulting data indicate that room temperature compaction alone is insufficient to provide adequate tensile strength for the compacts. Sintering the green compacts at temperatures of 150 to 173°C markedly improves the tensile strength while simultaneously causing a thickness change in the compacts. This thickness change results from gas evolution, pore shrinkage, and viscoelastic recovery of the residual stresses induced by pressure. For compacts of 0.225 in. thickness, an optimum sintering treatment of 173°C for 30 mins is recommended to provide a tensile strength of 4,000 psi and a thickness change of less than + 7 percent. Coining (repressing) the green compacts does not appreciably affect the sintered strength. However, a finer particle size improves the sintered properties. A review of the literature on the flow behavior of polystyrene suggests that a non-Newtonian viscous flow mechanism is followed by a Newtonian one as sintering progresses.     

Initial Sintering of ZnO

ZnO compacts were sintered in air from 450° to 700° C. By mercury porosimetry, pore size distributions, surface areas, densities, and porosities were determined. No shrinkage occurred below 550°C and, from a surface loss model, the mechanism was concluded to be surface diffusion. Activation energies are compared with those reported by other authors .     

Liquid Redistribution during Liquid-Phase Sintering

A simple two-dimensional packing model, consisting of arrays of circular particles, was used to calculate the free energy changes associated with the filling of pores of different coordinations with liquid. The calculations were used to determine the equilibrium distributions of a liquid in different packing arrangements of particles. The effect of both the volume fraction of liquid and shrinkage on liquid distribution was examined. It was found that, when liquid redistribution can easily take place, the volume fraction of liquid phase, the pore size distribution of a powder compact, and the amount of densification that has occurred all influence the homogeneity of the distribution of the liquid phase. In addition, the model predicts that, as shrinkage occurs or as the volume fraction of liquid phase increases, the pores will try to fill sequentially in order of increasing size. A consequence of this sequential filling of the pores is that the radius of curvature of the liquid meniscus, and hence the driving force for liquid-phase sintering, changes systematically as shrinkage occurs. The modeling suggests that the driving force for sintering changes in a way that depends on the initial overall pore size distribution of the particle arrangement and the way the pore size distribution changes during sintering.     

Influence of green state processes on the sintering behaviour and the subsequent optical properties of spark plasma sintered alumina

The present investigation gives a quantitative correlation between different green microstructures, and their sintering behaviour during spark plasma sintering. The green microstructures were elaborated via various green shaping processes such as direct casting and direct coagulation casting compared to uniaxial compaction of the as-received sub-micron grained corundum powder. Narrowing pore size distribution and reducing pore size (≈40 nm) in the green compact could favour cold densification during initial uniaxial pressing by grain sliding and rearrangement. This is attributed to the soft homogeneous touching network in direct-cast green samples. Consequently, grain growth was impeded and the onset of shrinkage was delayed. Moreover, the small pores and the narrow pore size distribution in the homogeneous green bodies led to higher final densities, with better optical properties compared to the less homogeneous green samples.