Porosity in Spinel Compacts Using Small-Angle Neutron Scattering

Unfired spinel (MgAl₂O₄) compacts and sintered materials with small hard agglomerates (<5 μm) were studied using small-angle neutron scattering (SANS) techniques. The SANS results were compared with those from mercury porosimetry and gas adsorption. The results from green-state samples are consistent with interconnected "ink-bottle"-type porosity. In the latter stages of densification the average void size is significantly larger than that found in the unfired compact. The presence of the hard agglomerates affects the observed SANS scattering much more in the partially densified samples than in the unfired compacts. It was demonstrated that the use of multiple SANS techniques to study large voids (<0.1 μm) and large pore fractions (45%) is a useful, sensitive, nondestructive diagnostic probe for the evaluation of porosity during sintering.     

Microstructural Characterization of Alumina and Silicon Carbide Slip-Cast Cakes

The effect of solids loading, particle-size distribution, and suspension viscosity on the resultant microstructure of slipcast monolithic ceramics prepared from aqueous suspensions of alumina and silicon carbide was studied. Unimodal alumina suspensions (average particle size = 0.6 μm) were prepared at 35, 37, and 42 vol%. Silicon carbide suspensions (average particle size = 0.7 μm) were produced with different quantities of dispersant at 37 vol%. Similarly, aqueous alumina suspensions of 42 and 50 vol% were produced with a bimodal particle-size distribution. The slip-cast microstructures were characterized by mercury porosimetry and small-angle neutron scattering, which provided pore size (distribution), pore fraction, and pore morphology. Essentially, the combination of these techniques deciphered packing differences obtained in the cake microstructures. For the alumina cakes produced from the 35,37, and 42 vol% suspensions, the individual characterization techniques, mercury intrusion, and the neutron scattering measurements showed that the cake microstructures were similar in pore size and quantity. However, comparison of the techniques and their assumptions showed differences in the pore shape. Mercury porosimetry and neutron scattering showed bimodal porosity for the cake produced from a mixture of 85% 6-μm particles and 15% 0.6-μm particles. Pore volume fraction and pore size increases were correlated with increased viscosity in the silicon carbide suspensions. In addition, the silicon carbide cake microstructures were measured, and homogeneity was evaluated as a function of position in the cast.     

Two-Stage Sintering of Alumina with Submicrometer Grain Size

This work verifies the applicability of two-stage sintering as a means of suppressing the final stage grain growth of submicrometer alumina. The first heating step should be short at a relatively high-temperature (1400°–1450°C) in order to close porosity without significant grain growth. The second step at temperatures around 1150°C facilitates further densification with limited grain growth. Fine-grained alumina with a relative density of 98.8% and a grain size of 0.9 μm was prepared by two-stage sintering. A standard sintering process resulted in ceramics with identical relative density and a grain size of 1.6 μm.     

Measurement of Porosity in Ceramic Coatings by Thermogravimetric Volatilization of Liquids

A simple gravimetric method was developed to determine the open porosity in ceramic coatings. The coating's pore space was filled with a liquid and the weight loss on volatilization of the liquid was measured in a thermogravimetric analyzer. This thermogravimetric volatilization of liquids (TVL) method was used to characterize the porosity in titania coatings, alumina/aluminum phosphate coatings, and free-standing films of alumina. Several liquids were used; ethylene glycol and 1,3-propanediol gave the best results due to their low volatilities at room temperature. The measured porosities of the ceramic coatings ranged from 30% to 80% and the pore sizes (as determined by SEM and mercury porosimetry) ranged from 0.1 to 15 μm. The standard deviation of the TVL measurement was smaller for thicker coatings (e.g., ≥20 μm). Porosities determined by TVL were within typically 5-10% of those determined by mercury intrusion porosimetry on identical samples. Characterization of a series of alumina/aluminum phosphate coatings showed a decrease in porosity consistent with expectations based on density and SEM observations. TVL is nondestructive, can be used for small volumes of sample, and when combined with SEM, provides a good means to characterize coating porosity and pore structure.     

Processing of Lightweight, High-Strength Porcelains Using an Alumina Cement to Replace Feldspars and Clays

New lightweight, high-strength porcelain bodies, using only nonplastic raw materials such as glass microspheres, quartz, and alumina cement, were fabricated and the effect of quartz particle size was investigated. Decreases in the green strength, relative to an increasing content of glass microspheres, were attributed to the decrease in the density and the relative decrease in the volume of alumina cement. The phases in the fired body were glass, α-quartz, cristobalite, anorthite, and a small amount of α-alumina. The large quartz particles (10–32 μm in size) could not be densified to closed porosity, because of underfiring, whereas smaller quartz particles (4–10 μm in size) permitted densification to closed porosity at 1300°C. The high flexural strength when using medium-sized quartz particles (6–20 μm, content of 30 wt%) was attributed to a stronger prestress and higher density that was due to better vitrification.     

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.     

Particle Crowding Analysis of Slip Casting

The particle crowding index and interparticle spacing terms were calculated for seven alumina suspensions. The particle crowding index was used to interpret the casting rate for the tested alumina suspensions; the index was successfully correlated with the casting rate for cakes that produced the same modes of porosity. Unfortunately, this index could not be correlated with the casting rate for the particle system that produced varied porosity as a function of composition. The interparticle spacing term was correlated with viscosity for particle size distributions between 31 and 0.1 μm. For particle size distributions extended to 44 μm, the viscosity could not be correlated with interparticle spacing, because the quantity of fine particles, rather than the particle packing, controlled the viscosity.     

Effect of Hollow Spherical Powder Size Distribution on Porosity and Segmentation Cracks in Thermal Barrier Coatings

The effect of characteristics of hollow spherical (HOSP) powders on porosity and development of segmentation cracks in plasma-sprayed thick thermal barrier coatings (TBCs) was investigated. Three powders with particle size ranges of 20–45, 53–75, and 90–120 μm were selected from a commercial HOSP powder feedstock for spraying the TBCs. The 20–45 μm powder has a higher deposition efficiency and a greater capability of producing segmented coatings than the other larger powders. Diagnostics of in-flight particles show that the average surface temperature and velocity of the particles sprayed from the fine powder is higher by 250°C and 50 m/s compared with those sprayed from the 90 to 120 μm powder, respectively, due to its greater ratio of surface area to mass. The lower porosity of the coating sprayed from the fine powder is mainly attributed to the decreased volume of intersplat gaps and voids.     

Fabrication of Porous Ceramics with Unidirectionally Aligned Continuous Pores

Porous alumina ceramics with unidirectionally aligned continuous pores were fabricated via the slurry coating of fugitive fiber. Cotton thread was coated with ceramic slurry by pulling it through the slurry, and specimens were produced by spooling the coated thread. The obtained porous alumina ceramics had an average pore diameter of 165 μm, 35% open porosity, and a bending strength of 160 MPa. It was suggested that the pore size and the porosity could be adjusted using the diameter of the cotton thread and the solids concentration of the slurry, respectively.     

Sintering of Mixtures of Seeded Boehmite and Ultrafine α-Alumina

α-Alumina was fabricated by dry pressing mixtures of seeded boehmite and fine α-alumina (i.e., 0.2 and 0.3 μm diameter) to reduce the large shrinkage of boehmite-derived α-alumina. The maximum green density was obtained with mixtures containing ∼70%α-alumina for both alumina powders. The ∼15% linear shrinkage and microstructures of these samples were comparable to 100% alumina powder samples. Samples with 0.2 μm alumina sintered to densities >95% at 1300°C whereas 1400°C was needed for samples with 0.3 μm alumina. These results indicate that boehmite can be used as a substitute for relatively expensive ultrafine α-alumina powders.     

Effect of Surface Finish on Structural Ceramic Failure

The relation between fracture strength and surface finish of brittle nonmetallic materials was examined and related to surface-crack theory. Specimens used in an experimental illustration were made of a commercially available 96%-pure alumina with an average grain size of 3.8 μm and a porosity of about 6%. Several groups of specimens, each having a different surface finish, were used in a biaxial ball-and-ring     

Processing of Particle-Stabilized Wet Foams Into Porous Ceramics

Direct foaming of colloidal suspensions is a simple and versatile approach for the fabrication of macroporous ceramic materials. Wet foams produced by this method can be stabilized by long-chain surfactants or by colloidal particles. In this work, we investigate the processing of particle-stabilized wet foams into crack-free macroporous ceramics. The processing steps are discussed with particular emphasis on the consolidation and drying process of wet foams. Macroporous alumina ceramics prepared using different consolidation and drying methods are compared in terms of their final microstructure, porosity, and compressive strength. Consolidation of the wet foam by particle coagulation before drying resulted in porous alumina with a closed-cell structure, a porosity of 86.5%, an average cell size of 35 μm, and a remarkable compressive strength of 16.3 MPa. On the other hand, wet foams consolidated via gelation of the liquid within the foam lamella led to porous structures with interconnected cells in the size range from 100 to 150 μm. The tailored microstructure and high mechanical strength of the macroporous ceramics can be of interest for the manufacture of bio-scaffolds, thermal insulators, impact absorbers, separation membranes, and light weight ceramics.     

Elaboration of porosity for the alumina particle surfaces/ bimodal PP composite cast films under continuous stretching

Efforts have been focused on developing an interrelationship between machine direction orientation (MDO)/material variables and the porosity of Al₂O₃ particulate-filled orientable PP for various applications. Composite films were prepared by cast-extrusion followed by stretching up to 100% and 200% by the MDO machine. Cavitation could occur during film stretching when the adhesion between dispersed alumina and continuous polypropylene phase failed. Composite films with modified micro-particles and also nano ones could not generate any voids. On the other hand, composite specimens of unmodified micro-alumina particles generated micro-pores in the joint between dispersed particles and polymer during the hot stretching. Beta nucleation in these films, plays an important role in the generated ductility of the specimens. Cavitation effectiveness (no. of voids) or film permeability depended on the size and amount of alumina particles, in addition to the ratio and temperature of stretching. Cavitation particles with a size range of 0.7–3 μm, create cavities of about 3–7 μm. Generation of voids, by drawing environmentally friendly and antimicrobial active alumina particles, can be utilized in the fabrication of sustainable hygiene polypropylene films with desirable separation abilities for gases or small molecules. Furthermore, the polypropylene/alumina composite films have the potential for microwaveable packaging.     

Fabrication of Mullite Ceramics With Ultrahigh Porosity by Gel Freeze Drying

Porous mullite (3Al₂O₃·2SiO₂) ceramics with an open porosity up to 92.9% were fabricated by a gel freeze-drying process. An alumina (Al₂O₃) gel mixed with ultrafine silica (SiO₂) was frozen and sublimation of ice crystals was carried out by drying the frozen body under a low pressure. Porous mullite ceramics were prepared in air at 1400°–1600°C due to the mullitization between Al₂O₃ and SiO₂. A complex and porous microstructure was formed, where large dentritic pores with a pore size of ∼100 μm contained small cellular pores of 1–10 μm on their internal walls. Owing to the complete mullitization, a relatively high-compressive strength of 1.52 MPa was obtained at an open porosity of 88.6%.     

Effect of Unfired Tape Porosity on Surface Film Formation by Dip Coating

Thin ceramic layers have been fabricated by dipping green tapes of alumina, formed by the doctor-blade casting method, into aqueous slurries containing mixtures of alumina and either unstabilized zirconia (MZ–ZrO₂) or mullite. It was observed that the formation of a thin layer on the surface of the tape is governed by both liquid entrainment and slip-casting mechanisms, and was accelerated by increasing the withdrawal rate, immersion time, or volume fraction of solids in the slurry used for dip coating. By modifying these parameters, layers as thin as 2 μm and as thick as 108 μm were easily formed. Layer formation was found to be strongly influenced by the structure of the tape surface. Layers formed on the top surface of the tape were found to be as much as 48% thicker than those formed on the bottom surface. This difference appears to be related to the smaller amount of porosity on the bottom surface of the tape. Evidence suggests that the polymer binder, used for doctor-blade casting, concentrated on the bottom of the tape as evaporation occurred from the top surface. The lower porosity on the bottom reduced the casting rate during dip coating and produced significantly thinner layers, relative to the top surface.     

Influence of a Dispersion of Aluminum Titanate Particles of Controlled Size on the Thermal Shock Resistance of Alumina

The possibility of developing fine-grained (∼0.5–3 μm) and dense (≥0.98ρ_th) alumina (90 vol%)–aluminum titanate (10 vol%) composites with improved thermal shock resistance and maintained strength is investigated. One alumina material and one composite with similar microstructures (porosity and grain-size distribution) were fabricated to investigate the effect of Al₂TiO₅ on thermal shock behavior. The size of the Al₂TiO₅ particles was kept under 2.2 μm to avoid spontaneous microcracking. The mechanical and thermal properties of the materials involved in their response to thermal shock and the results for the evolution of indentation cracks of equal initial crack length with increasing Δ T in samples quenched in glycerine are described. The combination of thermal and mechanical properties—thermal conductivity, thermal expansion coefficient, Young's modulus, and toughness—improve the thermal shock resistance of the alumina–aluminum titanate composite in terms of critical temperature increment (>30%). The suitable structural properties of alumina—hardness and strength—are maintained.     

High-Strength Porous Alumina Ceramics by the Pulse Electric Current Sintering Technique

High-strength porous alumina has been fabricated with a microstructure control using the pulse electric current sintering (PECS) technique. During sintering the discharge, which is assumed to take place in the voids between the particles, is thought to promote the bridging of particles by neck growth in the initial stages of sintering, leaving high porosity. The effect of dopants (MgO, 200 ppm; TiO₂, 1000 ppm) and of secondary inclusions (3 vol% 3Y-TZP) on the constrained densification and the improvement in the mechanical behavior of porous alumina ceramics has been reported. The porosity of the fabricated porous alumina was controllable between 30% and 50% depending on the sintering temperature. The flexural strength of alumina having 30% and 42% porosity showed impressive values of 250 and 177 MPa, respectively. The dominance of the preferential neck growth of grains over densification significantly improved the mechanical properties of porous alumina, besides leaving high porosity.     

Microcavity Formation in Alumina Using Ti Templates I: Formation Conditions

It has been found that fully enclosed microcavities can be engineered within Al₂O₃ ceramics using 125 μm diameter Ti wires as templates. A high-energy milling pretreatment of the alumina causes diffusion of the Ti into the surrounding alumina leaving all of the Kirkendall porosity consolidated into a central cavity. Control experiments using unmilled alumina confirm the necessity of the milling procedure and experiments with different milling media have excluded incidental doping by milling induced contamination as a primary driver of cavity formation. The internal microcavities produced here may lead to new applications in small scale instrumentation and implantable therapeutic devices.     

Preparation Method of Submicrometer-Sized α-Alumina by Surface Modification of γ-Alumina with Alumina Sol

A method is introduced to prepare almost-spherical submicrometer-sized α-alumina via surface modification of γ-alumina with an alumina sol. Milled γ-alumina, in the presence of 3 wt% of α-alumina with a median particle size ( d ₅₀) of 0.32 μm (AKP-30), produced irregularly shaped α-alumina with d ₅₀∼0.3 μm after heat treatment at 1100°C for 1 h. γ-alumina that had been surface-modified by milling in the presence of 3 wt% of the alumina sol resulted in almost-monosized, spherical α-alumina ∼0.3 μm in size after heat treatment at 1100°C for 1 h. Furthermore, almost-spherical α-alumina 0.1—0.2 μm in size was obtained by milling γ-alumina with 3 wt% of AKP-30 alumina in the presence of 3 wt% of the alumina sol, followed by heat treatment at 1100°C for 1 h. The alumina sol that has been introduced in this work seems to act as a dispersant, in addition to helping to form a spherical shape.     

Formation and Densification Behavior of Magnesium Aluminate Spinel: The Influence of CaO and Moisture in the Precursors

Different grades of stoichiometric and non-stoichiometric dense magnesium aluminate spinel (MgAl₂O₄) grains were prepared by a conventional double-stage firing process using two types of alumina and four types of magnesia raw materials. The MgAl₂O₄ spinel formation was found to be highly influenced by CaO and moisture present in the precursor oxides as confirmed by thermogravimetry (TG), differential thermal analysis (DTA), and X-ray diffraction (XRD) techniques. The Fourier transform-infrared spectroscopy (FTIR) study of the precursor oxides revealed the presence of moisture. Influence of alumina and magnesia composition on the densification behavior of MgAl₂O₄ spinels was assessed by characterizing bulk density (BD), apparent porosity (AP), water absorption (WA) capacity, and the microstructures of the stoichiometric, the magnesia-rich, and the alumina-rich spinels sintered at 1650°C for 1 h. Sintering studies indicate that to obtain dense stoichiometric spinel grains with >3.35 g/mL BD, <2.0% AP, and <0.5% WA, the spinel powder should possess a median particle size of <2 μm, CaO content of >0.9%, compact (green) density of >1.95 g/mL, and spinel content of >90%. Among various spinels synthesized, the magnesia-rich spinels exhibited superior properties in terms of high BD, low percentage of AP, and low WA capacity, whereas alumina-rich spinels showed inferior properties. Stoichiometric spinels exhibited an average grain size of 10 μm whereas alumina-rich spinels with 90% alumina had an average grain size of 20 μm. The increase in holding time at higher temperatures enhanced the sintering properties of the spinels, particularly the magnesia-rich spinels. Further, raw mixtures having >0.9% CaO exhibited better sintered properties as compared with others.