Preparation and properties of mullite-bonded porous fibrous mullite ceramics by an epoxy resin gel-casting process

Porous fibrous mullite ceramics with a narrow range of pore size distribution have been successfully prepared utilizing a near net-shape epoxy resin gel-casting process by using mullite fibers, Al₂O₃ and SiC as raw materials. The effects of sintering temperatures, different amounts of fibers and Y₂O₃ additive on the phase compositions, linear shrinkage, apparent porosity, bulk density, microstructure, compressive strength and thermal conductivity were investigated. The results indicated that mullite-bonded among fibers were formed in the porous fibrous mullite ceramics with a bird nest pore structure. After determining the sintering temperatures and the amount of fibers, the tailored porous fibrous mullite ceramics had a low linear shrinkage (1.36–3.08%), a high apparent porosity (61.1–71.7%), a relatively high compressive strength (4.4–7.6 MPa), a low thermal conductivity (0.378–0.467 W/m K) and a narrow range of pore size distribution (around 5 µm). The excellent properties will enable the porous ceramics as a promising candidate for the applications of hot gas filters, thermal insulation materials at high temperatures.     

Effects of fiber length and solid loading on the properties of lightweight elastic mullite fibrous ceramics

Mullite fibrous ceramics were successfully prepared by a TBA-based gel-casting with mullite fibers as the main matrix. The effects of the fiber length and the gel-casting solid loading on the composite properties and microstructure were investigated. The 3D structure of the composite was constructed by the randomly arranged mullite fibers with the fixed crossing point, and therefore the fiber length was the most important factor influencing the microstructure of the composition. Further analyses indicate that long fibers were more suitable for the fabrication of high porosity composite. Compared with controlling the fiber length, adjusting the gel-casting solid loading was an easy method of tailoring the properties of the composite. The composite fabricated with the low solid loading and long fibers exhibited a high porosity, a low thermal conductivity, and an excellent elastic property, and can be regarded as a potential high-temperature thermal insulator applied in the industrial or aerospace thermal protection system.     

Mineralogical and dielectric properties of mullite and cordierite ceramics produced using wastes

In this work technical ceramics containing industrial inorganic wastes was carried out. Ceramic formulations prepared with clay, magnesium oxide and residues of kaolin and alumina as raw materials, were formed in a disk-shaped specimens using the uniaxial pressing process and sintering at temperatures from 950°C to 1400°C. The mineralogical, physical and dielectric characteristics of the fired samples were investigated. The dielectric properties, the relative dielectric constant (ε_r) and the loss tangent (tan δ) were evaluated at frequencies of 0.1, 1, 10, and 100?kHz at room temperature. Mullite and cordierite were present as major phases at the highest temperatures. Relative dielectric constant values closest to that of mullite (ε_r = ~ 5 to ~ 6) and cordierite (ε_r =?~ 4 to ~ 6) at 1?kHz. On the other hand, the lowest dielectric losses (tan δ ~ 0.06 to ~ 0.04) were observed for the formulations containing the mullite major phase, and tan δ ~ 0.009 to ~ 0.003 for formulations that showed cordierite as main phase. It was verified that an increase in temperature promoted a reduction of porosity, a property that had a direct influence on the dielectric properties of the formulations. The materials obtained from the residues presented low dielectric constants and loss tangents, which make them suitable for use in electrical and electronic systems.     

Theoretical and experimental analyses of thermal properties of porous polycrystalline mullite

This paper examined experimentally and theoretically the thermal diffusibility (α), heat capacity (C_P1) at a constant pressure (1 atm, 101.33 kPa) and thermal conductivity (κ=C_P1α) for the porous mullite ceramics with 0–55% porosity in a wide temperature range from 298 to 1073 K. The change in the κ values with temperature or porosity for the porous mullite was similar to the temperature dependence or porosity dependence of the α values, which were greatly reduced by the air included in the pores. The κ values for the porous mullite were theoretically analyzed with two model structures of pore–dispersed mullite continuous phase system (A model) and mullite–dispersed pore continuous phase system (B model). The measured κ values at 0–23% porosity agreed well with the κ values calculated for model A structure. In the high porosity range from 33% to 55%, the measured κ values deviated from the κ curve calculated for model A structure and approached the κ value curve for model B structure with increasing porosity. The real microstructure of 30–60% porosity is equivalent to the mixed microstructure of model A and model B for the thermal conductivity measurement.     

Preparation of self-reinforcement of porous mullite ceramics through in situ synthesis of mullite whisker in flyash body

Self-reinforced porous mullite ceramics were fabricated by a starch consolidation method with flyash, different aluminium sources (Al(OH)₃ and Al₂O₃) and the additive AlF₃ as raw materials. The reinforcement mechanism of needle-like mullite whiskers through in situ synthesis in ceramic body was investigated. The bulk density, apparent porosity and bending strength of the samples were tested. Phase compositions and microstructures of the sintered samples were measured by XRD and SEM, respectively. It showed that AlF₃ as additive was helpful to the formation of mullite whiskers at a low temperature. As the aluminium sources, Al(OH)₃ was more suitable for the preparation of mullite whiskers than Al₂O₃. The in situ synthesized mullite whiskers formed an interlocking structure, which enhanced the mechanical strength of the porous mullite ceramics. Porous mullite ceramics with bending strength of about 100 MPa and apparent porosity of about 55% were made at 1550 °C.     

Influence of ZrO2 and WO3 doping additives on the thermal properties of porous mullite ceramics

Porous mullite-corundum refractory ceramics were produced by a patented slurry slip casting method from compositions based on commercially available α-Al₂O₃ and γ-Al₂O₃, fused SiO₂ and kaolin. Pores were formed as a result of a chemical reaction of aluminium with water. The influence of usage of raw materials and doping additives such as micro-size ZrO₂ and WO₃ on the sintering temperature, formation of crystalline phases, linear thermal expansion, thermal conductivity and thermal shock resistance of mullite-corundum ceramic was studied. The best thermal shock resistance and, simultaneously, lower thermal conductivity was achieved for the samples doped with WO₃. This was due to the influence of micro-sized WO₃ on the change in γ-Al₂O₃ modification to α-Al₂O₃ and on the structure of mullite ceramics.     

Development of cordierite/mullite composites using industrial wastes

Ceramics containing cordierite and mullite as their principal phases are promising for many applications, due to properties such as a low coefficient of thermal expansion, high durability, low dielectric constant, resistance to thermal shock, and refractoriness. The objective of this study was to produce ceramic composites suitable for use as refractory materials. The raw materials were subjected to chemical characterization and the formulations to physical, chemical, and mineralogical characterization. The specimens were formed by pressing, dried at 110°C, and sintered from 1150°C to 1300°C for 2 hours. The following properties were then determined: linear shrinkage, water absorption, apparent porosity, and flexural modulus of rupture. The phases formed, crystal morphology, and coefficient of thermal expansion were evaluated. The cordierite and mullite phases were observed in all formulations at 1250°C and 1300°C. The results obtained from the formulations with a higher content of fine kaolin residue suggest that these formulations have the potential to be used for the manufacture of refractory materials such as furnace rollers and supports.     

Effect of starch addition on microstructure and properties of highly porous alumina ceramics

Porous alumina ceramics with ultra-high porosity were prepared through combining the gel-casting process with the pore-forming agent technique. Porosity and pore size distribution of the sintered bulks were evaluated with and without adding starch, respectively. In particular, the influences of starch addition on the properties, including thermal conductivity and compressive strength were studied. It was found that the incorporation of starch increased the nominal solid loading in the suspension and subsequently promoted the particle packing efficiency. The porosity is raised with increasing starch content from 0 to 30 vol%, which brings the decrease in thermal conductivity, whereas the compressive strength isn't seriously degraded. The further higher starch addition (40 vol%), however, would deteriorate the performance of the alumina porous ceramics. It is believed that the appropriate starch amount (lower than 30 vol%), working as a pore-forming agent, suppresses the driving force of densification without affecting the connections of neighboring grains while excessive starch amount would lead to the collapse of the porous structure.     

New multifunctional porous Yb2SiO5 ceramics prepared by freeze casting

New porous Yb₂SiO₅ ceramics were prepared by a water-based freeze casting technique using synthesized Yb₂SiO₅ powders. The prepared porous Yb₂SiO₅ ceramics exhibit multiple pore structures, including lamellar channel pores and small pores, in its skeleton. The effects of the solid content and sintering temperature on the pore structure, porosity, dielectric and mechanical properties of the porous Yb₂SiO₅ ceramics were investigated. The sample with 20 vol% solids content prepared at 1550 °C exhibited an ultra-low linear shrinkage (i.e. 4.5%), a high porosity (i.e. 79.1%), a high compressive strength (i.e. 4.9 MPa), a low dielectric constant (i.e. 2.38) and low thermal conductivity (i.e. 0.168 W/(m K)). These results indicate that porous Yb₂SiO₅ ceramics are good candidates for ultra-high temperature broadband radome structures and thermal insulator materials.     

Mechanical and fracture properties of zircon–mullite composites obtained by direct sintering

Refractory materials based on zircon (ZrSiO₄) are applied in high temperature applications (1400–1500 °C). They are demonstrated to have an excellent chemical attack resistance, such as corrosion or degradation due to molten glass or metals. On the other hand mullite (3Al₂O₃2SiO₂) is important both in traditional and advanced ceramics. Although multi-phase ceramic materials were always used, nowadays composite materials have an important industrial and technological development, to enlarge the designing capability of the manufacturer in properties and behaviors. The objective of the present work is to study the influence of the starting composition on the mechanical and fracture properties of zircon–mullite composites obtained by direct sintering of consolidated samples by slip cast of concentrated aqueous suspensions in plaster molds. Zircon–mullite composites using 15–45 wt% mullite were prepared and compared with pure zircon material obtained in the same conditions. Flexural strength (σ_f), dynamic elastic modulus (E), toughness (K_IC) and initiation fracture surface energy (γ_NBT) were evaluated. The results were explained by microstructure and the XRD analysis. The presence of mullite increased the zircon thermal dissociation. The ZrO₂ was a product of this reaction and also influence the mechanical and fracture properties of these materials through several combined mechanisms.Zircon composites prepared with 45 wt% of mullite in the starting powder showed a higher fracture toughness and initiation energy than ceramics derived from pure zircon. Microstructure consisting in mullite as a continuous predominant phase in which zircon and zirconia grains were distributed improved almost all the mechanical and fracture properties.     

Synthesis and characterization of Si/SiC ceramics prepared using cotton fabric

A Si/SiC ceramic was prepared from cotton fabric by the reactive infiltration of liquid silicon into the carbon template. A large density difference between the samples has been observed. This may be due to the variation in the pore size and its distribution within the sample. Scanning electron microscopy with energy dispersive spectroscopy shows the presence of three distinct phases, i.e., SiC, free Si and free carbon. X-ray diffraction pattern also confirms the presence of SiC and Si phases. However, there is no peak corresponding to carbon. So, it is inferred that the carbon exists in amorphous form. Micro-hardness, fracture toughness and bending strength of the ceramics were also studied. The values are lower than commercially available SiC ceramics. This may be due to the highly porous nature of cotton fabric-based SiC, as compared to commercially available SiC.     

Microstructure and mechanical properties of in-situ grown mullite toughened 3Y-TZP zirconia ceramics fabricated by gelcasting

In-situ grown mullite toughened zirconia ceramics (mullite-zirconia ceramics) with excellent mechanical properties for potential applications in dental materials were fabricated by gelcasting combined with pressureless sintering. The effect of sintering temperature on the microstructure and mechanical properties of mullite-zirconia ceramics was investigated. The results indicated that the columnar mullite produced by reaction was evenly distributed in the zirconia matrix and the content and size of that increased with the increase of sintering temperature. Mullite-zirconia ceramics sintered at 1500 °C had the optimum content and size of the columnar mullite phase, generating the excellent mechanical properties (the bend strength of 890.4 MPa, the fracture toughness of 10.2 MPa.m^1/2, the Vickers hardness of 13.2 GPa and the highest densification). On the other hand, zirconia particles were evenly distributed inside the columnar mullite, which improved the mechanical properties of columnar mullite because of pinning effect. All of this clearly confirmed that zirconia grains strengthened columnar mullite, and thus the columnar mullite was more effective in enhancing the zirconia-based ceramics. Simultaneously, the residual alumina after reaction was distributed evenly in the form of particle, which improved the mechanical properties of the sample because of pinning effect. Overall, the synergistic effect of zirconia phase transformation toughening with mullite and alumina secondary toughening improved the mechanical properties of zirconia ceramics.     

Microstructural and thermal properties of plasma sprayed mullite coatings

Thick mullite (3Al₂O₃–2SiO₂) coatings were fabricated by atmospheric plasma spraying (APS) in a mixture of crystalline and amorphous phases, as confirmed by X-ray diffraction (XRD) analysis. The coatings were isothermally heat treated in order to study recrystallization mechanism of the glassy phase. The morphology and the microstructure of both mullite feedstock and coatings were investigated by using scansion electron microscopy (SEM). The porosity of as-sprayed coating was in the range between 2 and 3% and substantially remained unchanged after thermal treatment. The thermal expansion of as-sprayed and annealed coatings was measured during heating up to the temperature of crystallization and the corresponding high-temperature extent of shrinkage was calculated. The differential scanning calorimetry (DSC) curves at different heating rates showed a sharp exothermic peak between 1243 and 1253 K, suggesting a rapid recrystallization of the amorphous phase. Finally, the heat capacity of recrystallized mullite coating was measured by DSC experiments. It was approximately 1.02 × 10³ J/kg K at 373 K and increased with increasing test temperature.     

Effect of mullite phase formed in situ on pore structure and properties of high-purity mullite fibrous ceramics

Porous mullite ceramics are potential advanced thermal insulating materials. Pore structure and purity are the main factors that affect properties of these ceramics. In this study, high performance porous mullite ceramics were prepared via aqueous gel-casting using mullite fibers and kaolin as the raw materials and ρ-Al₂O₃ as the gelling agent. Effects of addition of mullite fibers on the pore structure and properties were examined. The results indicated that mullite phase in situ formed by kaolin, and ρ-Al₂O₃ ensured the purity of mullite samples and mullite fibers bonded together to form a nest-like structure, greatly improving the properties of ceramic samples. In particular, the apparent porosity of mullite samples reached 73.6%. In the presence of 75% of mullite fibers, the thermal conductivity was only 0.289 W/m K at room temperature. Moreover, the mullite samples possessed relatively high cold compressive strength in the range of 4.9–9.6 MPa. Therefore, porous mullite ceramics prepared via aqueous gel-casting could be used for wide applications in thermal insulation materials, attributing to the excellent properties such as high cold compressive strength and low thermal conductivity.     

Recycling of waste red mud for fabrication of SiC/mullite composite porous ceramics

SiC/mullite composite porous ceramics were fabricated from recycled solid red mud (RM) waste. The porous ceramics were formed using a graphite pore forming agent, RM, Al(OH)₃ and SiC in the presence of catalysts. The influence of firing temperature and the pore-forming agent content on the mechanical performance, porosity and the microstructure of the porous SiC ceramics were investigated. Optimal preparation condition were determined by some testing. The results indicated that the flexural strength of specimens increased as a function of firing temperature and a reduction in graphite content, which concomitantly decreased porosity. The ceramic prepared under optimal conditions having 15?wt% graphite and sintered at 1350?°C, demonstrated excellent performance. Under optimal preparation conditions the flexural strength and porosity of the ceramic were 49.4?MPa and 31.4%, respectively. Scanning electron microscopy observation result showed that rod-shape mullite grains endowed the samples with high flexural strength and porosity. X-ray diffraction analysis indicated that the main crystallization phases of the porous ceramics were 6H-SiC, mullite, cristobalite and alumina. This work demonstrates that RM can be sucessfully reused as a new raw material for SiC/mullite composite porous ceramics.     

Analyses of Young's modulus and thermal expansion coefficient of sintered porous alumina compacts

This paper reports the derivation of Young's modulus (E) and thermal expansion coefficient (TEC, β) for a sintered porous structure with open pores. The theoretical E is affected by the number of grains and the grain boundary area in sintered ceramics. The measured E–porosity relationship for porous alumina compacts were compared with the theoretical E values that were derived for the present open-pore structure and also for the dispersed (closed)-pore structure treated previously. With decreasing porosity (50% → 10%), the scattered E values showed a gradually increasing tendency, which were located between two theoretical curves for the open-pore structure. The sudden increase of E values in the porosity range from 10% to 0% was well explained by the theoretical dependence of E on porosity for the open- or closed-pore structure. The β values for the porous alumina structures were independent of porosity and close to the β values reported for fully dense alumina compacts. This result was in accordance with the theoretical β–porosity relationships for the open-pore and closed-pore structures.     

Fabrication and microstructure of porous SiC ceramics with Al2O3 and CeO2 as sintering additives

In the present study, porous silicon carbide ceramics were prepared via spark plasma sintering at relatively low temperatures using Al₂O₃ and CeO₂ as sintering additives. Sacrificial template was selected as the pore forming mechanism, and gelcasting was used to fix the slurry in a short time. The evolution process of the microstructures during different steps was observed by SEM. The influence of the sintering temperature and sintering additives on the shrinkage and porosity of the samples was studied. The microstructures of different samples were characterized, and the mechanical properties were also evaluated.     

Fabrication and properties of three-directional orthogonal aluminosilicate fiber fabric-reinforced mullite composite

In this study, a three-directional orthogonal aluminosilicate fiber fabric-reinforced mullite composite with excellent properties was prepared by the precursor impregnation and pyrolysis (PIP) method. The influence of the number of PIP cycles on the microstructure and mechanical properties of the composite was studied. The density and mechanical properties of the composite enhanced with increasing number of PIP cycles. After the 6th PIP cycle, the density of the composite increased by only 0.8%, and the flexural strength and fracture toughness of the composite reached 130.1 MPa and 4.91 MPa m^1/2, respectively. Moreover, the thermal shock resistance of the composite was investigated by the high-low temperature (from 1300 °C to room temperature) thermal treatment and water quenching method. The strength retention rate of the composite after 10 high-low temperature thermal cycles was 90.1%; the retention rate dropped to 79.5% after 15 thermal cycles. The composite with the critical thermal shock temperature difference 843.9 °C displayed excellent thermal shock resistance.     

Microstructure control and mechanical properties of porous silicon nitride ceramics

Porous silicon nitride ceramics with a fibrous interlocking microstructure were synthesized by carbothermal nitridation of silicon dioxide. The influences of different starting powders on microstructure and mechanical properties of the samples were studied. The results showed that the microstructure and mechanical properties of porous silicon nitride ceramics depended mostly on the size of starting powders. The formation of single-phase β-Si₃N₄ and the microstructure of the samples were demonstrated by XRD and SEM, respectively. The resultant porous Si₃N₄ ceramics with a porosity of 71% showed a relative higher flexural strength of 24 MPa.     

Preparation and properties of porous BaTiO3 nanostructured ceramics produced from cuboidal nanocrystals

The preparation and properties of BaTiO₃ nanostructured ceramics with porosity level in the range of percolation limit (33% and 37% porosity) produced by partial sintering of cubic nanoparticles are presented. Hydrothermally synthesized cuboid-like particles were produced by using Field-Assisted Sintering Technique facility in which temperature and pressure were selected to ensure the consolidation of mechanically stable porous nanoceramics, while preserving as much as possible the starting grain shape. Nanosized grains in the range of (10–40) nm and multiscale porosity ranging from a few nm to hundreds of nm were observed in the sintered ceramics. The dielectric constant of porous nanoceramics assumes low values of ~280–320 and shows a flat thermal response typical to nanostructured ceramics, without a net ferroelectric-paraelectric peak, followed by a Curie-Weiss dependence in the paraelectric state, with negative Curie Weiss temperatures and lowered Curie constant, as result of porosity and ultrafine grain size. A strong conductivity relaxation around room temperature related to air-ceramic interface phenomena indicated a possible sensitivity of these ceramics for gas sensing. Preliminary qualitative tests with saturated acetone vapours have shown a good response of both resistive and reactive components of such porous BaTiO₃ nanoceramics and possible gas sensing interface-related mechanisms were discussed.