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.     

Preparation of porous Y2SiO5 ceramics with relatively high compressive strength and ultra-low thermal conductivity by a TBA-based gel-casting method

Porous Y₂SiO₅ ceramics with relative high compressive strength (as high as 24.45 MPa) and ultra-low thermal conductivity (～0.08 W/m K) were successfully fabricated by a tert-butyl alcohol based gel-casting method. The formation mechanism of the 3D interconnected pores and the properties of the green body are discussed. The porosity, pore size, compressive strength and thermal conductivity could be controlled by varying the initial solid loading and the sintering temperature. When regulating the initial solid loading (from 20 to 50 wt%) and sintering temperature (from 1200 to 1500 °C), the porosity can be controlled between 47.74% and 73.93%, and the compressive strength and the thermal conductivity of porous Y₂SiO₅ ceramics varied from 3.34 to 24.45 MPa and from 0.08 to 0.55 W/m K, respectively. It should be noted that the porous Y₂SiO₅ ceramics with 30 wt% solid loading and sintering at 1400 °C had an open porosity of 61.80%, a pore size of 2.24 μm, a low room-temperature thermal conductivity of 0.17 W/m K and a relatively high compressive strength of 13.91 MPa, which make this porous Y₂SiO₅ ceramics suitable for applications in high-temperature thermal insulators.     

Synthesis and characterization of porous Y2SiO5 with low linear shrinkage,high porosity and high strength

The emerging porous Y₂SiO₅ ceramic is regarded as a promising candidate of thermal insulator owing to its very low thermal conductivity. However, recent works on porous Y₂SiO₅ are confronted with severe problems such as large linear shrinkage (18.51–20.8%), low porosity (47.74–62%) and low strength (24.45–16.51 MPa) at high sintering temperatures (1450–1500 °C). In this work, highly porous Y₂SiO₅ ceramic with low shrinkage and excellent high-temperature strength was fabricated by in-situ foam-gelcasting method at 1550 °C. The as-prepared sample has unique multiple pore structures, low linear shrinkages of 6.3–4.5%, controllable high porosities of 60.7–88.4%, high compressive strengths of 38.2–0.90 MPa, and low thermal conductivities of 0.126–0.513 W/(m K) (porosity: 87.1–60.2%). The effects of relative density on relative strength, as well as porosity on thermal conductivity were quantitatively discussed. The present results indicate that porous Y₂SiO₅ is the potential high-temperature thermal insulation material of light weight, low thermal conductivity, and high strength.     

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.     

Effects of sintering temperature on mechanical properties of 3D mullite fiber (ALF FB3) reinforced mullite composites

A new method to weaken the interfacial bonding and increase the strength of 3D mullite fiber reinforced mullite matrix (Mu_f/Mu) composites is proposed and tested in this paper. Firstly, Mu_f/Mu composites were fabricated through sol–gel process with varied sintering temperature. Then, the effects of sintering temperature on mechanical properties of the composites were tested. As sintering temperature was raised from 1000 °C to 1300 °C, the three-point flexural strength of the composites firstly decreased from 66.17 MPa to 41.83 MPa, and then increased to 63.17 MPa. In order to explain the relationship between composite strength and sintering temperature, morphology and structure of the mullite fibers and mullite matrix after the same heat-treatment as in the fabrication conditions of the composites were also investigated. Finally, it is concluded that this strength variation results from the combined effects of matrix densification, interfacial bonding and fiber degradation under different sintering temperatures.     

Effects of in situ synthesized mullite whiskers on compressive strength of mullite fiber brick

The method of in situ synthesis of mullite whiskers by gas-phase deposition and reaction was applied to improve the compressive strength of the mullite fiber brick. During the preparation process, silica sol, Al(NO₃)₃ solution and NH₄F solution were introduced into the fibrous brick in the form of ions or sol through vacuum impregnation and freeze drying, and the silica sol, Al(NO₃)₃ and NH₄F served as the silica sources, aluminum source and catalyst, respectively. Effects of process parameters (concentration of impregnation solutions, holding time, sintering temperature) on compressive strength and elastic modulus of the fibrous brick during the in situ toughening process were analyzed. SEM and XRD analysis results demonstrated that the mullite whiskers were synthesized on the surface of mullite fibers based on the reaction of AlOF and SiF₄. What is more, the whiskers on adjacent fibers intersected with each other and formed many unfixed lap-jointing points, resulting in the increase of compressive strength and elastic modulus. Although the density and thermal conductivity of the sample after the generation of mullite whiskers fabricated with the optimum process were 0.406 g/cm³ and 0.1262 W/(m K), respectively, which were slightly higher than that of the raw fibrous brick (0.375 g/cm³ density and 0.1069 W/(m K) thermal conductivity, respectively), the corresponding compressive strength and elastic modulus of the sample reinforced with the whiskers increased to 1.45 MPa and 42.03 MPa, respectively, which were much higher than that of the raw fibrous brick (0.39 MPa compressive strength and 6.5 MPa elastic modulus).     

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.     

Low-temperature sintering and microstructure control of mullite–Mo composites

Dense mullite–Mo (45 vol%) composites with homogeneous microstructure have been obtained by plasma activated sintering of a mixture of Mo and mullite precursors at a relatively low temperature (1350 °C) and a pressure of 30 MPa. The mullite precursor was synthesized by a sol–gel process followed by a heat-treatment at 1000 °C. The influence of different mullite precursors on the densification behavior and the microstructure of mullite–Mo composites has been studied. The precursor powder heat-treated at 1000 °C with only Si–Al spinel but no mullite phase shows an excellent sintering activity. Following the sintering shrinkage curves, a two-stage sintering process is designed to enhance the composite densification for further reducing the sintering temperature. The study reveals that viscous flow sintering of amorphous SiO₂ at low temperatures effectively enhances the densification. Moreover, microstructure of these composites can be controlled by selecting different precursors and sintering temperatures.     

Densification and mechanical properties of cBN–TiN–TiB2 composites prepared by spark plasma sintering of SiO2-coated cBN powder

cBN–TiN–TiB₂ composites were fabricated by spark plasma sintering at 1773–1973 K using cubic boron nitride (cBN) and SiO₂-coated cBN (cBN(SiO₂)) powders. The effect of SiO₂ coating, cBN content and sintering temperature on the phase composition, densification and mechanical properties of the composites was investigated. SiO₂ coating on cBN powder retarded the phase transformation of cBN in the composites up to 1873 K and facilitated viscous sintering that promoted the densification of the composites. Sintering at 1873 K, without the SiO₂ coating, caused the relative density and Vickers hardness of the composite to linearly decrease from 96.2% to 79.8% and from 25.3 to 4.4 GPa, respectively, whereas the cBN(SiO₂)–TiN–TiB₂ composites maintained high relative density (91.0–96.2%) and Vickers hardness (17.9–21.0 GPa) up to 50 vol% cBN. The cBN(SiO₂)–TiN–TiB₂ composites had high thermal conductivity (60 W m⁻¹ K⁻¹ at room temperature) comparable to the TiN–TiB₂ binary composite.     

Microstructure and property of porous mullites with a whiskers framework obtained by a sol–gel process

Porous mullites with a whiskers framework and high porosities were fabricated by the reaction sintering (1100 to 1600 °C, 1 h, in an airtight container) of an aerogel block shaped by the sol–gel transition of a mullite precursor composed of SiO₂ sol, Al₂O₃ and AlF₃ powders (as reaction catalyst). The effect of heating temperatures on porosity, whisker formation, microstructure feature and compressive strength of the porous mullites was determined by XRD, SEM and compressive test. The results indicate that after heating at temperatures from 1100 to 1600 °C, the porosities of the mullites varied within the range of 84.1–80.2%. The whiskers in the framework well lap-jointed each other to form the large space and became elongated and smooth at high temperatures due to the accelerated vapor–solid reaction rate. A maximum compressive strength of 16.1 MPa was obtained for the whiskers framework heated at 1600 °C; this strength was attributed to the strong bonding among the smooth whiskers.     

Fabrication and characterization of anorthite–mullite–corundum porous ceramics from construction waste

In this work, anorthite–mullite–corundum porous ceramics were prepared from construction waste and Al₂O₃ powders by adding AlF₃ and MoO₃ as mineralizer and crystallization catalyst, respectively. The effects of the sintering temperature and time on open porosity, mechanical properties, pore size distribution, microstructure, and phase composition were characterized in detail. The results showed that the formation of the mullite whiskers and the properties of the anorthite–mullite–corundum porous ceramics depended more on the sintering temperature than the holding time. By co-adding 12 wt% AlF₃ and 4 wt% MoO₃, mullite whiskers were successfully obtained at sintering temperatures upon 1350 °C for 1 h. Furthermore, the resultant specimens exhibited excellent properties, including open porosity of 66.1±0.7%, biaxial flexural strength of 23.8±0.9 MPa, and average pore size of 1.32 µm (the corresponding cumulative volume percent was 37.29%).     

Investigating the effect of SPRM on mechanical strength and thermal conductivity of highly porous alumina ceramics

For recycling waste refractory materials in metallurgical industry, porous alumina ceramics were prepared via pore forming agent method from α-Al₂O₃ powder and slide plate renewable material. Effects of slide plate renewable material (SPRM) on densification, mechanical strength, thermal conductivity, phase composition and microstructure of the porous alumina ceramics were investigated. The results showed that SPRM effectively affected physical and thermal properties of the porous ceramics. With the increase of SPRM, apparent porosity of the ceramic materials firstly increased and then decreased, which brought an opposite change for the bulk density and thermal conductivity values, whereas the bending strength didn’t decrease obviously. The optimum sample A2 with 50 wt% SPRM introducing sintered at 1500 °C obtained the best properties. The water absorption, apparent porosity, bulk density, bending strength and thermal conductivity of the sample were 31.7%, 62.8%, 1.71 g/cm³, 47.1 ± 3.7 MPa and 1.73 W/m K, respectively. XRD analysis indicated that a small quantity of silicon carbide and graphite in SPRM have been oxidized to SiO₂ during the firing process, resulting in rising the porous microstructures. SEM micrographs illustrated that rod-like mullite grains combined with plate-like corundum grains to endow the samples with high bending strength. This study was intended to confirm the preparation of porous alumina ceramics with high porosity, good mechanical properties and low thermal conductivity by using SPRM as pore forming additive.     

Foam-gelcasting preparation,microstructure and thermal insulation performance of porous diatomite ceramics with hierarchical pore structures

Hierarchically pore-structured porous diatomite ceramics containing 82.9∼84.5% porosity were successfully prepared for the first time via foam-gelcasting using diatomite powder as the main raw material. Sizes of mesopores derived from the raw material and macropores formed mainly from foaming were 0.02∼0.1 μm and 109.7∼130.5 μm, respectively. The effect of sintering temperature, additive content and solid loading of slurry on pore size and distribution, and mechanical and thermal properties of as-prepared porous ceramics were investigated. Compressive strength of as-prepared porous ceramics increased with sintering temperature, and the one containing 82.9% porosity showed the highest compressive strength of 2.1 ± 0.14 MPa. In addition, the one containing 84.5% porosity and having compressive strength of 1.1 ± 0.07 MPa showed the lowest thermal conductivity of 0.097 ± 0.001 W/(m·K) at a test temperature of 200 ̊C, suggesting that as-prepared porous ceramics could be potentially used as good thermal insulation materials.     

Enhanced thermal conductivity of low-temperature sintered borosilicate glass–AlN composites with β-Si3N4 whiskers

The effects of β-Si₃N₄ whiskers on the thermal conductivity of low-temperature sintered borosilicate glass–AlN composites were systematically investigated. The thermal conductivity of borosilicate glass–AlN ceramic composite was increased from 11.9 to 18.8 W/m K by incorporating 14 vol% β-Si₃N₄ whiskers, and high flexural strength up to 226 MPa were achieved along with low relative dielectric constant of 6.5 and dielectric loss of 0.16% at 1 MHz. Microstructure characterization and percolation model analysis indicated that thermal percolation network formation in the ceramic composites led to the high thermal conductivity. The crystallization of the borosilicate microcrystal glass also contributed to the enhancement of thermal conductivity. Such ceramic composites with low sintering temperature and high thermal conductivity might be a promising material for electronic packaging applications.     

Improvement of the mechanical properties of SiC reticulated porous ceramics with optimized three-layered struts for porous media combustion

Silicon carbide reticulated porous ceramics (SiC RPCs) with three-layered struts were fabricated by polymer replica method, followed by infiltrating alumina slurries containing silicon (slurry-Si) and andalusite (slurry-An), respectively. The effects of composition of infiltration slurries on the strut structure, mechanical properties and thermal shock resistance of SiC RPCs were investigated. The results showed that the SiC RPCs infiltrated with slurry-Si and slurry-An exhibited better mechanical properties and thermal shock resistance in comparison with those of alumina slurry infiltration, even obtained the considerable strength at 1300 °C. In slurry-Si, silicon was oxidized into SiO₂ in the temperature range from 1300 °C to 1400 °C and it reacted with Al₂O₃ into mullite phase at 1450 °C. Meantime, the addition of silicon in slurry-Si could reduce SiC oxidation of SiC RPCs during firing process in contrast with alumina slurry. With regard to slurry-An, andalusite started to transform into mullite phase at 1300 °C and the secondary mullitization occurred at 1450 °C. The enhanced mechanical properties and thermal shock resistance of SiC RPCs infiltrated alumina slurries containing silicon and andalusite were attributed to the optimized microstructure and the triangular zone (inner layer of strut) with mullite bonded corundum via reaction sintering. In addition, the generation of residual compressive stress together with better interlocked needle-like mullite led to the crack-deflection in SiC skeleton, thus improving the thermal shock resistance of obtained SiC RPCs.     

Porous acicular mullite ceramics fabricated with in situ formed soot oxidation catalyst obtained from waste MoSi2

Porous acicular mullite (3Al₂O₃·2SiO₂) ceramics containing Cu₃Mo₂O₉ as a soot oxidation catalyst was fabricated by a novel approach using commercial powders of Al₂O₃ and CuO, and powder obtained by controlled oxidation of ground waste MoSi₂. The obtained material consisted of elongated mullite grains which are known to be effective in carbon soot removal from diesel engine exhaust. The presence of in situ created Cu₃Mo₂O₉ was found to catalyze the carbon burnout which is an extremely important feature when it comes to filter regeneration, i.e., the captured soot removal. The carbon burnout temperature in the sample containing 12 wt% CuO was by 90 °C lower than that in the sample without CuO. Effect of sintering temperature as well as the effect of amount of CuO additive on mullite properties were studied. It was found that the increase in amount of CuO in samples sintered at 1300 °C decreased porosity and increased compressive strength of the porous mullite ceramics. The addition of 12 wt% CuO increased the strength of the porous mullite ceramics up to 70 MPa, whereas the porosity was reduced from 62% in the mullite without CuO to 44% in the mullite ceramics containing 12 wt% CuO. Although affected by the amount of CuO, the microstructure still consisted of elongated mullite grains.     

Fabrication and properties of porous anorthite/mullite ceramics with both high porosity and high strength

Porous anorthite/mullite ceramics with both high porosity and high strength have been successfully fabricated by foam-gelcasting and pressureless sintering technology, using α-Al₂O₃, SiO₂, and CaCO₃ as starting materials and MnO₂ as sintering aids. The porous mullite ceramics prepared in this study had 83.3% porosity and 0.3 W/m·K thermal conductivity, exhibited compressive strength value as high as 6.1 MPa. The samples fabricated with mullite content of 30 mol% possessed 79.4% porosity and 5.9 MPa compressive strength showed thermal conductivity as low as 0.19 W/m·K. With the addition of MnO₂, the properties of the prepared materials varied slightly when mullite content changed in a large scale. The results showed that the addition of MnO₂ promoted the reaction, affected sintering and grain growth, and contributed to high strength and low-thermal conductivity.     

Preparation and thermal shock resistance of corundum-mullite composite ceramics from andalusite

Corundum-mullite composite ceramics have high hardness, small plastic deformation and other excellent performances at high temperature. Corundum-mullite composite ceramics were fabricated from andalusite and α-Al₂O₃ by in-situ synthesis technology. Effects of mullite/corundum ratio and sintering temperatures on the water absorption, apparent porosity, bulk density, bending strength, thermal shock resistance, phase composition and microstructure of the sample were investigated. Results indicated that the in-situ synthesized mullite from andalusite combined with corundum satisfactorily, which significantly improved the thermal shock resistance as no crack formed after 30 cycles of thermal shock (1100 °C-room temperature, air cooling). Formula A4 (andalusite: 37.31 wt%, α-Al₂O₃: 62.69 wt%, TiO₂ in addition: 1 wt%, mullite: corundum=6:4 in wt%) achieved the optimum properties when sintering at 1650 °C, which were listed as follows: water absorption of 0.15%, apparent porosity of 0.42%, and bulk density of 3.21 g⋅cm⁻³, bending strength of 117.32 MPa. The phase composition of the sintered samples before and after thermal shock tests were mullite and corundum constantly. The fracture modes of the crystals were transgranular and intergranular fractures, which could endow the samples with high thermal shock resistance.     

Design and preparation of high permeability porous mullite support for membranes by in-situ reaction

The natural mineral kaolin combined with alumina additives Al(OH)₃,α-Al₂O₃ and AlF₃ was used to prepare porous mullite ceramic membrane supports using an in-situ reaction. The effects of composition and sintering temperature on the sintering behavior, pore structure, permeability and microstructure of the resulting porous mullite supports were extensively investigated. The experimental results showed that excess SiO₂ in kaolin can be consumed by adding alumina precursors, which resulted in a stiff skeleton of interlinked needle-like mullite crystals in-situ during the sintering. The needle-like mullite crystals touched each other and formed a short network, which acted as a porous skeletal network structure. This network resulted in a highly permeable porous structure. The resulting support is suitable for the preparation of asymmetric ceramic membranes. The densification and pore structure of the support can be effectively adjusted by control of the quantity of alumina precursors in the composition and the sintering temperature. Sintering the subject mullite compositions at 1500 °C for two hours resulted in support structures with an average porosity of 45.9%, an average pore size of 1.3 µm and a penetrating porosity of 35.9%.     

Preparation and properties of in-situ mullite whiskers reinforced aluminum chromium phosphate wave-transparent ceramics

In-situ mullite whisker reinforced aluminum chromium phosphate wave-transparent ceramics were designed and prepared. The phase transformation, microstructure, mechanical and electrical properties of the ceramics were investigated, and the mechanisms of in-situ growth and toughening were discussed. Results indicated that the in-situ growth of mullite whisker significantly improved the mechanical properties of the matrix, especially the high temperature flexural strength. The room temperature flexural strength, 1000 °C flexural strength and fracture toughness of the ceramics were 135.60 MPa, 121.71 MPa and 4.52 MPa m^1/2. After sintering at 1500 °C, the optimum properties of ε^'_r, tanδ and microwave transmittance at region 8–12 GHz were <3.6, <0.03 and>80%, respectively. The sinterability of ACP matrix was improved by the in-situ process of high mullization above 1450 °C. Using ACP binder as the raw material can avoid the phase transformation from B-AlPO₄ to T-AlPO₄. The synthesized mullite whiskers played a role in toughening by whiskers fracture, crack deflection and whisker pulling out.