Microstructure and properties of lightweight fibrous porous mullite ceramics prepared by vacuum squeeze moulding technique

A novel fibrous porous mullite network with a quasi-layered microstructure was produced by a simple vacuum squeeze moulding technique. The effects of organic binder content, inorganic binder and adsorbent on the microstructure and the room-temperature thermal and mechanical properties of fibrous porous mullite ceramics were systematically investigated. An anisotropy microstructure without agglomeration and layering was achieved. The fibrous porous mullite ceramics reported in this study exhibited low density (0.40 g/cm³), low thermal conductivity (~0.095 W/(m K)), and high compressive strength (~2.1 MPa in the x/y direction). This study reports an optimal processing method for the production of fibrous porous ceramics, which have the potential for use as high-temperature thermal insulation material.     

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.     

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.     

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.     

叔丁醇基凝胶注模成型制备氧化铝多孔陶瓷

以微米级Al2O3粉料为原料,叔丁醇为溶剂,采用凝胶注模成型工艺制备了氧化铝多孔陶瓷,并研究了Al2O3浆料的固相体积分数(分别为8%、10%、13%和15%)对1 500℃保温2 h烧后氧化铝多孔陶瓷的气孔率、气孔孔径分布、耐压强度、热导率和显微结构的影响.结果表明:当Al2O3浆料的固相体积分数从8%增加到15%时,氧化铝多孔陶瓷烧结体的总气孔率从71.2%逐渐降低至61.2%,气孔平均孔径从1.0 μm逐渐减小至0.78 μm,耐压强度从16.0 MPa逐渐增大至45.6 MPa,而热导率从1.03 W·(m·K)-1逐渐增大至1.83W·(m·K)-1.     

Fabrication and properties of mullite fiber matrix porous ceramics by a TBA-based gel-casting process

A bird nest-like structure was designed by using the mullite fiber as the matrix and SiO₂ as the high temperature binder. This special material was successfully prepared by a TBA-based gel-casting process. The randomly arranged fiber laps bonded by SiO₂ binder was the most important structure characteristic of this porous material. The effect of sintering temperature on the properties, i.e. porosity, bulk density, linear shrinkage, compressive strength, thermal conductivity and the microstructure was studied. The composite exhibited significant pseudoductility. The fracture mechanism of this composite under compression was discussed. The results indicated that the sintering temperature ranging from 1500 to 1600 °C was suitable for yielding mullite fiber matrix porous ceramics which had a low thermal conductivity (0.19–0.22 W/m K), a relatively high compressive strength (3–13 MPa) and a high resilience (66–70%) for applications in the thermal insulators and high-temperature elastic seal field.     

Structure and properties of Al2O3-bonded porous fibrous YSZ ceramics fabricated by aqueous gel-casting

To meet requirements for high porosity and high strength, novel aqueous gel-casting process has been successfully developed to fabricate Al₂O₃-bonded porous fibrous YSZ ceramics with ρ-Al₂O₃ and YSZ fibers as raw materials. Microstructure, phase composition, apparent porosity, bulk density, thermal conductivity, and compressive strength of fabricated porous ceramics were investigated, and effects of fiber content on properties were discussed. According to results, bird nest 3D mesh with interlaced YSZ fibers and Al₂O₃ binder was formed, ensuring the ability to obtain high performance, lightweight ceramics. An increase in the number of YSZ fibers led to more complex interlaced arrangement of fibers and denser network structure of porous ceramics at retaining their stability. Furthermore, their apparent porosity and bulk density increased, whereas thermal conductivity and compressive strength decreased with increasing the fiber content. In particular, comparatively high porosity (71.1–72.7%), low thermal conductivity (0.209–0.503 W/mK), and relatively high compressive strength (3.45–4.24 MPa) were obtained for as-prepared porous ceramics, making them promising for applications in filters, thermal insulation materials, and separation membranes.     

Microstructure and properties of inter-locked mullite framework prepared by the TBA-based gel-casting process

Inter-locked mullite frameworks (IM frameworks) were prepared by the tert butyl alcohol (TBA) -based gel-casting method. The inter-locked structure formed through fluorine-catalyzed gas-phase reaction. Inter-locked topaz structure first formed in the samples and then it transformed into inter-locked mullite structure. TBA's volatilization and gaseous reactants' running off, which acted as two kinds of “pore formers” in the gel-casting process, offered enough space for the mullite grains' anisotropic growth, and led to the porous structure's formation. The volume density, apparent porosity, specific surface area and compression strength of the samples that formed at 1100–1500 °C were found to be 0.53–0.59 g/cm³, 82.1–76.1%, 12.3–4.6 m²/g and 1.6–3.6 MPa, respectively, indicating that the IM frameworks are promising as high-temperature catalyst supports.     

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 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.     

Manufacturing of highly porous calcium phosphate bioceramics via gel-casting using agarose

There is a technological need for highly porous bioceramics to be produced in an environmentally friendly manner. Gel-casting of highly porous HAp-(α-TCP) (CaP) foams using agarose as a gelling agent was investigated. Foaming of gel-cast suspension was performed at the temperature of 60 °C followed by transformation of the foams from a liquid state to a gelled state by cooling them to 15 °C. The sintered (1250 °C, 2 h soaking time) foams were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), N₂ adsorption isoterm and Hg porosimetry. XRD study revealed that additives used in the gel-casting process did not influence the phase composition of the investigated materials. The macroporous microstructure of HAp-(α-TCP) foams was typically composed of approximately spherical pores (cells) interconnected by circular windows. The foams exhibited a broad pore size distribution with cells and windows ranging from 250 to 900 μm, and from 25 to 250 μm, respectively. The mode for spherical pore size was approximately 500 μm while the above value for window was ∼100 μm. Additionally, the small amount of wall microporosity in the range of 0.2–0.9 μm was confirmed by SEM and Hg porosimetry. The obtained porous (P = 90%) HAp-(α-TCP) scaffolds with interesting two types of macropores and a small amount of micropores seem to be a promising bone substitution material.     

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.     

Preparation and characterization of mullite microspheres based porous ceramics with enhancement of in-situ mullite whiskers

High-strength insulating ceramic materials were prepared using lightweight mullite microspheres with dense surfaces and high internal porosity as the main raw material and silica sol as a binder. The effects of AlF₃·3H₂O content on the in situ formation and growth of mullite whiskers were analyzed by X-ray diffraction and scanning electron microscopy. The obtained results showed that mullite whiskers were formed in large quantities at 1200 °C using AlF₃·3H₂O and V₂O₅ as additives; their optimal growth was observed at 4 wt% AlF₃·3H₂O and 1 wt% V₂O₅. The apparent porosity of the produced specimens was 39%; the MOR and CCS of the specimens were 31 and 152 MPa, respectively; the HMOR at 1300 °C was 11.32 MPa; and the thermal conductivity at 900 °C was 0.783 W m⁻¹ K⁻¹. The staggered whisker network structure formed between mullite microspheres not only improved the mechanical properties of the material, but also refined its pore size, reduced the thermal conductivity, and enhanced the thermal insulation properties.     

Low-temperature reaction-sintering of mullite ceramics with an Y2O3 addition

Mullite ceramics were fabricated at relatively low temperatures from powder mixtures of -Al₂O₃ and quartz, with an Y₂O₃ addition. The mullitization process was analyzed by X-ray diffraction. The densification behavior was investigated as a function of the Y₂O₃ content, sintering temperature and holding time as well as mullite seeds. It has been shown that mullitization occurs via a nucleation and growth mechanism within an yttrious aluminosilicate glass, but lattice and grain-boundary diffusion becomes important during the densification process. Moreover, the incorporation of mullite seeds was observed to enhance both mullitization and densification. At 1400°C for 5 h or 1450°C for 2 h, 15 mol% Y₂O₃-doped and 5 mol% mullite-seeded specimens can be sintered to almost full density.     

Preparation of mullite ceramic microfilter membranes using Response surface methodology based on central composite design

In this paper effect of free silica removal from mullite microfilter membranes using different sodium hydroxide (NaOH) concentrations at different temperatures and for different removal times was studied. The prepared membranes were subjected to XRD, SEM, porosity analysis, and mechanical strength measurement. Response surface methodology (RSM) based on central composite design (CCD) was used to design the experiments and analyze three operating parameters including; NaOH solution concentration, NaOH solution temperature and removal time. The optimum porosity of 49.4 was obtained with NaOH solution concentration of 35 wt% at temperature 75 °C and removal time equal to 8 h.Water flux and mechanical strength as important characteristics were measured for all the membranes. For the membrane with the optimum porosity, water flux, mechanical strength, and free silica removal percentage were 61.7 kg/m² h, 21.6 MPa, and 28.2%, respectively. The maximum rejection percentage was 97.2% and emulsion flux for this state was 15.6 kg/m² h at temperature 25 °C and cross flow velocity of 1.5 m/s.     

Preparation of novel reticulated prickly porous ceramics with mullite whiskers

Porous materials are widely used in heat exchangers, sewage treatment, electromagnetic shielding, thermal insulation, gas adsorption, photocatalytic due to their high specific surface area. The specific surface area of materials plays a pivotal role in them. It can be enhanced by increasing the porosity of the material, but the cost of this improvement is reducing the strength of the material. In order to improve performance, it is necessary to increase its surface area without reducing the strength of the material. In this work, mullite porous ceramics with mullite whisker on the inside and outside surfaces structures, which known as prickly porous ceramics(PPCs). They were fabricated using polyurethane foam coated with slurries as the pore-forming agents, sintered after secondary impregnation with silica sol and ammonium fluoroaluminate. The the sintering temperature as well as slurry composition of secondary impregnation were tuned to tailor the strength and surface structures of the PPCs. In addition, the potential of PPCs as high-temperature catalyst supports was demonstrated. Overall, the PPCs demonstrated large surface areas and high mechanical strength. This study paved the way for the fabrication of high-performance porous ceramics.     

Joining of mullite ceramics with yttrium aluminosilicate glass interlayers

Pellets of yttrium aluminosilicate glass (Y₂O₃:Al₂O₃:SiO₂ = 30:20:50 mol%) powder were used as the filler interlayers (0.4 mm thick) to join two mullite substrates. The glass interlayer partially melted at joining temperature to bond the substrates and then crystallized during cooling to have better bonding strength. The results showed that joining could be performed at 1390–1420 °C for 1–5 h with applied pressure of 0.02 MPa. After joining, the thickness of glass layers varied between 250 μm and 80 μm, depending upon the temperatures. The glass interlayer crystallized into cristobalite, mullite and Y₂Si₂O₇. When joining mullite/3 mol%yttria–zirconia substrates using the same glass pellet, a layer of zircon/mullite was formed at the interface, indicating that reaction occurred between glass and substrates. The formation of zircon usually accompanied with cracks in the substrates. These cracks deteriorated the strength. The achievable three-point bending strengths were 139 MPa for joined mullite and 76 MPa for joined mullite/3 mol%yttria–zirconia.     

Mullite-based cellular ceramics obtained by a combination of direct foaming and reaction bonding

Mullite-based cellular ceramics were prepared via the polymer precursor route using poly(silsesquioxane) in combination with particulate alumina or alumina/aluminum mixtures. The multi-functional preceramic polymer was used as pore-forming agent by employing a self-foaming process during the polymer cross-linking step, as well as a precursor for reactive silica, one of the reagents in mullite formation. The size of filler particulates was found to strongly affect foaming of the polymer/filler mixtures, with coarser particles facilitating an improved foaming performance. Thermal conversion in air at 1600 °C resulted in the formation of cellular ceramics with high mullite contents. The partial substitution of alumina with aluminum in the initial mixtures resulted in improved mechanical properties at comparable porosities, resulting in compressive strengths of 0.3 MPa at total porosities of 93%. A correlation between thermal analysis data and crystalline phase development during the thermal treatment allowed for the clarification of processes taking place during heat treatment, yielding information for a future process optimization approaches.