Porous mullite ceramics with low thermal conductivity prepared by foaming and starch consolidation

Porous mullite ceramics were prepared from an industrial grade mullite powder by foaming and starch consolidation. The viscosities of the original suspensions and the foamed ones with solid loading of 62.5 and 67.5 wt% were measured. After the steps of forming and drying, the green bodies were sintered under different temperatures from 1,200 to 1,600 °C for 2 h. The influence of solid loading of suspension and sintering temperature on the porosity and compressive strength was evaluated. The sintered mullite ceramics, with porosity from 86 to 73 vol% and corresponding compressive strength from 1 to 22 MPa, contained a multi-modal microstructure with large spherical pores and small pores on internal walls. Thermal conductivity measurement carried out by the transient plane source technique at room temperature resulted in values as low as 0.09 W/mK. In addition, the relationship between thermal conductivity and porosity was discussed in detail.     

Porous Mullite Ceramics Formed by Direct Consolidation Using Native and Granular Cold‐Water‐Soluble Starches

In this article, the processing and microstructures of porous mullite bodies prepared by modifying the conventional route of the starch consolidation casting method were studied. The proposed route, called the “soluble route”, involves the use of native starches (i.e., potato, cassava, and corn starches) and a synthesized granular cold‐water‐soluble (GCWS) starch. Stable aqueous mullite‐starch suspensions (0.25 starch volume fraction of 40 vol% total solids) were prepared by mixing. The total starch content was a mixture of ungelatinized native starch and GCWS starch with a 1:10 ratio of GCWS starch to total starch. Steady‐state shear flow properties of the suspensions were analyzed by measuring viscosity. The addition of CGWS starch increased the starting suspension viscosity and thus prevented the particle segregation. Porous mullite bodies were obtained by heating (80°C, 2 h) the suspensions in metallic molds and by drying (40°C, 24 h) and sintering (1650°C, 2 h) the green disks after burning out the starch (650°C, 2 h). Green bodies obtained before and after the burning‐out process, and the sintered disks were characterized with density and porosity measurements (Archimedes method) and microstructural analysis by scanning electron microscopy. The phases generated after the sintering process were determined by X‐ray diffraction analysis, and pore size distributions were studied by Hg‐porosimetry. The obtained results showed that the use of the GCWS starch made the shaping of homogeneous mullite bodies without cracks or deformations possible along with the development of controlled porous microstructures.     

Processing of porous yttria-stabilized zirconia tapes: Influence of starch content and sintering temperature

The tape-casting process was used to produce porous yttria-stabilized zirconia (YSZ) substrates with volume fractions of porosity ranging from 28.9 to 53 vol.% by using starch as a fugitive additive. Concentrated aqueous YSZ slips with different amounts of starch and an acrylic latex binder were prepared. The influence of the volume fraction of starch and sintering temperature on the sintering behavior and final microstructure were investigated. The microstructure consisted of large pores created by the starch particles with lengths between 15 and 80 μm and smaller pores in the matrix with lengths between 0.6 and 3.8 μm. The pores in the matrix reduced the sinterability of the YSZ leading to the retention of closed porosity in the sintered tapes. The porosities were above those predicted for each of the starch contents. However, larger deviations from the predicted porosity were found as more starch was added. The open to total porosity ratio in the sintered tapes could be controlled by the volume fraction of added starch as well as by the sintering temperature. As the volume fraction of starch increased from 17.6 to 37.8 vol.% there was a gradual increase in the interconnectivity of the pore structure. The sintering shrinkage of the tapes at a given temperature could be directly related to the YSZ packing density in the matrix.     

Two alternative routes for starch consolidation of mullite green bodies

The starch consolidation forming method can be used in the manufacture of porous ceramics. In this method, based on swelling and gelatinization properties of starch in aqueous suspension at temperature (55–80 °C), the starch granules perform as both consolidator/binder of the green body and pore former at high-temperature.Commercially available powders of mullite and cassava starch were employed as raw materials. Mullite/starch aqueous suspensions (0.25 starch volume fraction of 40 vol.% total solid loading) were prepared by intensive mechanical mixing and homogenization in a ball mill.Two alternative forming routes of thermogelling mullite/starch aqueous suspensions—the Conventional Route (CR) and the Pre-Gelling Route (PGR)—were studied. With the CR, disks were formed by pouring the mullite/starch suspension at room temperature directly into metallic molds and heating at different temperatures (70 and 80 °C) and times (1 and 2 h). With the PGR, disks were shaped by pouring pre-gelled mullite/starch suspensions at 59 °C into heated molds and heating at the same experimental conditions. Once the consolidation process was finished, samples were removed of the mold and dried. Green bodies shaped by the two processing routes and obtained before (CR_bb and PGR_bb) and after (CR_ab and PGR_ab) burning out the starch, were characterized by bulk density and apparent porosity measurements and microstructural analysis by SEM/EDAX on the external and fracture surfaces. The homogeneity of the distribution of raw materials and pores, and the volume porosity were taken into account to establish the optimum consolidation conditions to be used in the preparation of mullite porous materials with homogeneous microstructures.     

Effect of zirconia tape porosity on fluorapatite surface film formation by dip coating

Thin fluorapatite (FA) layers on porous 3 mol% yttria-partially stabilized zirconia (Y-PSZ) substrates have been fabricated by dipping porous zirconia tapes into aqueous 27.4 vol% fluorapatite slurries. Two porous Y-PSZ tapes with different volume fraction of porosity were developed using an acrylic latex binder: tapes with 31.4 vol% porosity were prepared using 16.6 vol% starch as fugitive additive and those with 12.7 vol% porosity were fabricated without starch. The influence of the porous structure of the tape surfaces, top and bottom, on the casting rate and consequently on the layer thickness formed on each surface was studied. Layers formed on the top and bottom surfaces of the tapes with 12.7 and 31.4 vol% porosity were compared. The formation of a thin layer on the surface of the tape was governed by both liquid entrainment and slip casting mechanisms. The data for the FA layer formation were in good agreement with the slip casting model for immersion times>0. The casting rate at the top surface of both tapes was greater than that at the bottom surface. This difference was attributed to a greater porosity of the top surface with respect to that of the bottom one and was more pronounced for the tapes prepared with starch. Layers formed on the top surface were found to be about 55 and 32% thicker than those formed on the bottom surface for the tapes fabricated with and without starch, respectively. For the tapes prepared with starch, the greater porosity and number of smaller pores in the matrix of the top surface increased the casting rate and produced the thickest dip coated layers     

Theoretical and experimental analyses of Young's modulus and thermal expansion coefficient of the alumina-mullite system

Young's moduli (E) and thermal expansion coefficients (TECs) of the alumina–mullite–pore system (96.4–99.5% relative density) were measured for a wide mullite fraction range from 0 to 100 vol%. Both E and TEC values decreased at high mullite fractions. These properties were theoretically analyzed with four proposed model structures that were constructed by three-phase systems of mullite (or alumina) continuous phase 2–pore dispersed phase 1–alumina (or mullite) dispersed phase 3. The ratios of E(theoretical)/E(experimental) and TEC(theoretical)/TEC(experimental) were very close to unity, depending on the mullite fraction. That is, the measured E and TEC values are closely related to the change in the composite microstructure as a function of mullite fraction.     

Mechanical properties of porous ceramics in compression: On the transition between elastic,brittle, and cellular behavior

This paper deals with the uniaxial compression behavior of porous ceramics within a wide range of porosity, varying from 30 to 75 vol%. The load–displacement curves recorded on porous alumina samples showed a transition between a typical brittle behavior at porosity fractions below 60 vol% and a damageable, cellular-like behavior, at higher porosity fractions. This transition in fracture mode was confirmed by in situ compression tests in an X-ray tomograph. Based on a simple model taking into account the competition between the crack length initiating from spherical pores and the mean distance between pores, the porosity at which the transition took place was estimated. The influence of the pore size also depended on the volume fraction of pores: no size effect was noted at the lowest porosity whereas a statistical effect on the size of the solid walls was observed at higher porosity, with an increase in fracture strength with small pores.     

Preparation of Silicon-Titanium-Carbon-Oxygen Fabric/ Mullite Filler/Polytitanocarbosilane Laminates by Polymer Impregnation and Pyrolysis Method

A polytitanocarbosilane (PTC, 20–50 mass%)–xylene solution was infiltrated into a porous, laminated woven fabric of 21–33 vol% Si-Ti-C-O fibers including 26–46 vol% mullite powder (filler) and decomposed at 1000°C in an argon atmosphere. This polymer impregnation and pyrolysis method (PIP) was repeated eight times to produce a laminated composite of 68%–85% of theoretical density. The effects of the polymer concentration and the fraction of mullite filler on the densification rate and microstructure of the layered composite were studied. The pseudoductility of the densified composite, as measured using four-point flexural testing, was caused by buckling after the elastic deformation and was followed by delamination along the direction of the layered fabric. The strength and the energy of fracture were enhanced by controlling the incorporation of mullite filler in the filament yarn (formation of a narrow pore-size distribution) and densification with a low-viscosity PTC solution. The composite with a higher strength provided a higher energy of fracture. The maximum energy of fracture reached 22 kJ/m² in the composite with 330 MPa of strength in four-point flexure.     

Multifunctional Properties of Alumina Composites Reinforced by a Hybrid Filler

Hybrid microstructure design has been used to fabricate alumina composites reinforced by 5 vol% of multiwalled carbon nanotube (MWNT) together with different (1, 2, 3 vol%) contents of SiC nanoparticles by spark plasma sintering. The mechanical, thermal, and electrical properties of the composites were determined as a function of the SiC volume fraction. The thermal conductivity decreased for 1 and 2 vol% of SiC, while for 3 vol%, it increased. Substantial improvements in the fracture toughness, bending strength, and electrical conductivity were observed and attributed to a synergetic effect of the MWNT and SiC nanoparticles in the hybrid microstructure design.     

Effect of Zirconia Content on the Oxidation Behavior of Silicon Carbide/Zirconia/Mullite Composites

The oxidation of hot-pressed SiC-particle (SiC_p)/zirconia (ZrO₂)/mullite composites with various ZrO₂ contents, exposed in air isothermally at 1000° and 1200°C for up to 500 h, was investigated; an emphasis was placed on the effects of the ZrO₂ content on the oxidation behavior. A clear critical volume fraction of ZrO₂ existed for exposures at either 1000° or 1200°C: the oxidation rate increased dramatically at ZrO₂ contents of >20 vol%. The sharp transition in the oxidation rate due to the variation of ZrO₂ content could be explained by the percolation theory, when applied to the oxygen diffusivity in a randomly distributed two-phase medium. Morphologically, the composites with ZrO₂ contents greater than the critical value showed a large oxidation zone, whereas the composites with ZrO₂ contents less than the critical value revealed a much-thinner oxidation zone. The results also indicated that the formation of zircon (ZrSiO₄) at 1200°C, through the reaction between ZrO₂ and the oxide product, could reduce the oxidation rate of the composite.     

Self-Crack-Healing Behavior of Mullite/SiC Particle/SiC Whisker Multi-Composites and Potential Use for Ceramic Springs

To improve fracture toughness and encourage excellent self-crack-healing ability, mullite/SiC particle/SiC whisker multi-composites and mullite/SiC whisker composites were hot pressed. The crack-healing abilities and mechanical properties of these sintered composites were investigated. Based on the obtained results, the usefulness of the mullite composite as a material for springs was discussed. The part of mullite/15 vol% SiC whisker/SiC 10 vol% particle containing healed cracks retained high reliability over the whole measured temperature range. When the crack-healing ability was endowed by SiC whiskers alone, the parts containing the healed pre-cracks were found to have a heat-resistance limit temperature. Mullite/15 vol% SiC whisker/10 vol% SiC particle multi-composite had the best potential as a material for springs used at high temperatures, because it had an adequate crack-healing ability as well as shear deformation ability almost two times stronger than that of monolithic mullite.     

Porosity features and gas permeability analysis of bi-modal porous alumina and mullite for filtration applications

Bimodal porous structures were prepared by combining conventional sacrificial template and partial sintering methods. These porous structures were analysed by comparing pore characteristics and gas permeation properties of alumina/mullite specimens sintered at different temperatures. The pore characteristics were investigated by SEM, mercury porosimetry, and capillary flow porosimetry. A bimodal pore structure was observed. One type of pore was induced by starch, which acted as a sacrificial template. The other pore type was due to partial sintering. The pores produced by starch were between 2 and 10 µm whereas those produced by partial sintering exhibited pore size of 0.1–0.5 µm. The effects of sintering temperature on porosity, gas permeability, and mullite phase formation were studied. The formation of the mullite phase was confirmed by XRD. Compressive strengths of 37.9 MPa and 12.4 MPa with porosities of 65.3% and 70% were achieved in alumina and mullite specimens sintered at 1600 °C.     

Preparation and Mechanical Properties of SiC Whisker and Nanosized Mullite Particulate Reinforced TZP Composites

The influence of the addition of nanometer mullite particulates and SiC whiskers coated with alumina on the mechanical properties of tetragonal zirconia polycrystals (TZP) was studied. With increasing mullite( p ) content the high-temperature flexural strength increased, and a maximum value of 360 MPa at 1000°C was reached at 15 vol% mullite( p . Furthermore, 10 vol% SiC( w ) reinforced 15 vol% mullite/TZP composites improved the high-temperature strength up to 490 MPa at 1000°C, 2.7 times that of pure TZP matrix. This high-temperature strengthening is attributed to load transfer from TZP matrix to SiC( w ) and mullite particulates. Significant whisker pull-out and interface debonding were also observed on the fractured surfaces when SiC( w ) was coated with Al₂O₃ film.     

A High-Strain-Rate Alumina-Based Ceramic Composite

An alumina-based ceramic codispersed with zirconia and mullite was synthesized with a submicron grain size. At high temperatures, steady-state creep with very limited grain growth was achieved for samples with 40 vol% alumina, 30 vol% zirconia, and 30 vol% mullite (AZ30M30). Constant stress compressive creep behavior of AZ30M30 showed a stress exponent of 2 and an activation energy of 880 kJ/mol. A strain rate of 0.01 s⁻¹ was obtained for AZ30M30 at 60 MPa and 1500°C, indicating the potential for high-strain-rate superplasticity.     

Colloidal Processing and Mechanical Properties of Whisker-Reinforced Mullite Matrix Composites

Aqueous colloidal suspensions in the two systems of CVD-processed ultrafine mullite powder (<0.1 μm), -Si₃N₄ whisker and -mullite whisker, were prepared near the isoelectric point of mullite (pH 7.0) to prevent cracking during drying of wet green compacts consolidated by filtration. The freeze-dried porous green compacts were hot-pressed with a carbon die at 1500°C for 1 h at a pressure of 39 MPa in N₂ atmosphere. The relative densities of the mullite matrix composites with whiskers of 0 to 10 vol% were in the range of 95.2% to 99.8%. Increasing the fraction of Si₃N₄ whisker increased the density, flexural strength, and fracture toughness of the hot-pressed composites. On the other hand, addition of the mullite whisker increased the fracture toughness but decreased the density and strength of the composites.     

Preparation of C/SiC Composites by Hot Pressing, Using Different C Fiber Content as Reinforcement

Unidirectional C/SiC composites were successfully prepared by hot pressing at 1850°C under 20 MPa, using different fiber volume fractions (from 28 vol% to 55 vol%) as reinforcement. The densification process of the composites became increasingly difficult with increasing fiber volume fraction, and some small pores were still distributed in the intrabundle regions of the composites. The cracks, resulting from the residual thermal stress in the composites due to the mismatch of the thermal expansion coefficient of the matrix and the fiber, were distributed in the matrix. With the increase of fiber content, the mechanical properties of the composites could be improved and the composites exhibited an obvious noncatastrophic fracture behavior due to a decrease in the thermal residual stress and an increase in the fiber pull outs.     

Reaction Bonding and Mechanical Properties of Mullite/Silicon Carbide Composites

Based on the RBAO technology, low-shrinkage mullite/SiC/ Al₂O₃/ZrO₂ composites were fabricated. A powder mixture of 40 vol% Al, 30 vol% A1₂O₃ and 30 vol% SiC was attrition milled in acetone with TZP balls which introduced a substantial ZrO₂ wear debris into the mixture. The precursor powder was isopressed at 300–900 MPa and heattreated in air by two different cycles resulting in various phase ratios in the final products. During heating, Al oxidizes to Al₂O₃ completely, while SiC oxidizes to SiO₂ only on its surface. Fast densification (at >1300°C) and mullite formation (at 1400°C) prevent further oxidation of the SiC particles. Because of the volume expansion associated with the oxidation of Al (28%), SiC (108%), and the mullitization (4.2%), sintering shrinkage is effectively compensated. The reaction-bonded composites exhibit low linear shrinkages and high strengths: shrinkages of 7.2%, 4.8%, and 3%, and strengths of 610, 580, and 490 MPa, corresponding to compaction pressure of 300, 600, and 900 MPa, respectively, were achieved in samples containing 49–55 vol% mullite. HIPing improved significantly the mechanical properties: a fracture strength of 490 MPa and a toughness of 4.1 MPa^.m^1/2 increased to 890 MPa and 6 MPa^.m^1/2, respectively.     

Near-Net-Shape Processing of Metal-Ceramic Composites by Reactive Metal Penetration

Metal-ceramic composites were made to near-net shape by reacting phase-pure mullite and mullite-glass preforms with molten Al using a reactive metal penetration process. Conversion of the two ceramic preforms to Al₂O₃/Al composites was accompanied by a 0.32% volume expansion and a 1.42% volume shrinkage, respectively. Molar volume and density calculations made assuming a net-shape reaction estimate ∼17 and ∼27 vol% Al to be present in the two composites after reaction. Results from quantitative stereology measurements used to quantify the concentration of metal in the reactively formed composites validate the calculations.     

Using polystyrene spheres to produce thin,porous interlayers in alumina laminates

Porous alumina layers were produced by colloidal processing of alumina with the addition of 15, 30, and 45 vol% polystyrene spheres (PS) as pore formers. Alumina laminates were designed with dense layers alternated with porous interlayers using 45 vol% of PS spheres and sintered at 1350°C. The layers’ thickness ranged from 2 to 15 μm, with a random distribution of pores. The higher volume fraction of pores tends to decrease the alumina average grain size, but does not influence the final size of bulk and surface pores. The obtained values of hardness and Young's modulus for the porous interlayer are ∼30% of the values obtained for the dense layer. Vickers indentations suggested that crack propagation can be opposed by the porous interlayers. However, values of mechanical strength, fracture toughness (K_IC), and work of fracture presented no relevant difference compared to a monolithic reference. R-curves presented a slight increase and K_IC a decrease due to crack propagation through the porous interlayers. Although no macrodeviations of the crack path were observed in the fractured surfaces, microdeviations were detected in the interlayer regions.     

Synthesis of Mullite Whiskers and Their Application in Composites

Mullite whiskers were synthesized by a vapor-solid reaction. Mullite composition xerogels were fired at 900° to 1600°C with AlF₃ in an airtight container. An average whisker increased in length from 7 μm at 1100°C to 10 μm at 1600°C, whereas an average whisker decreased in aspect ratio from 25 to 10 with increased firing temperature. The whiskers elongated to the c -axis and the side planes were the {110}. A clear lattice image corresponding to (110) lattice spacing up to the edges of the whiskers was observed with high-resolution electron microscopy, and no droplet was observed on the tips of the whiskers. The chemical composition of the whiskers synthesized below 1100°C showed an apparent Al₂O₃-rich composition of about 72 mol%. Composites reinforced by 15 vol% of mullite whiskers in the matrix of 75 vol% mullite/25 vol% Y-TZP enhanced the fracture toughness compared with those materials without mullite whiskers.