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.     

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.     

Porous fibrous ZrO2-mullite ceramics prepared via tert-butyl alcohol-based gel-casting

Mullite fiber was used to fabricate ZrO₂-mullite based porous ceramic via tert-butyl alcohol (TBA)-based gel-casting process using zirconite and bauxite as raw materials. Phase compositions, microstructure, pore size distribution, linear shrinkage, bulk density, apparent porosity, thermal conductivity, and compressive strength were analyzed to investigate influences of mullite fiber content and added Y₂O₃ on prepared porous ceramics. Results show that bird nest-like three-dimensional fibrous reticular skeleton structure was constructed with mullite fibers that evenly enwrapped rod-like mullite and ZrO₂ grains. Prepared porous fibrous ZrO₂-mullite ceramics had narrow pore size distribution that consisted of mullite and m-ZrO₂. With an increase in mullite fiber content, linear shrinkage and bulk density decreased, apparent porosity increased, and relatively good thermal conductivity was obtained. In addition, added Y₂O₃ reacted with Al₂O₃ and SiO₂ to form Y-Al-Si-O glass phase, which promoted sintering and densification of the ceramic, thus improving its compressive strength.     

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.     

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%).     

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.     

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.     

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.     

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).     

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.     

Low sintering shrinkage porous mullite ceramics with high strength and low thermal conductivity via foam-gelcasting

Excessive sintering shrinkage leads to severe deformation and cracking, affecting the microstructure and properties of porous ceramics. Therefore, reducing sintering shrinkage and achieving near-net-size forming is one of the effective ways to prepare high-performance porous ceramics. Herein, low-shrinkage porous mullite ceramics were prepared by foam-gelcasting using kyanite as raw material and aluminum fluoride (AlF₃) as additive, through volume expansion from phase transition and gas generated from the reaction. The effects of AlF₃ content on the shrinkage, porosity, compressive strength, and thermal conductivity of mullite-based porous ceramics were investigated. The results showed that with the increase of content, the sintering shrinkage decreased, the porosity increased, and mullite whiskers were produced. Porous mullite ceramics with 30 wt% AlF₃ content exhibited a whisker structure with the lowest shrinkage of 3.5%, porosity of 85.2%, compressive strength of 3.06 ± 0.51 MPa, and thermal conductivity of 0.23 W/(m·K) at room temperature. The temperature difference between the front and back sides of the sample reached 710°C under high temperature fire resistance test. The low sintering shrinkage preparation process effectively reduces the subsequent processing cost, which is significant for the preparation of high-performance porous ceramics.     

Porous anorthite ceramics with ultra-low thermal conductivity

Porous anorthite ceramics with an ultra-low thermal conductivity of 0.018 W/m K have been fabricated by hydrous foam-gelcasting process and pressureless sintering method using γ-alumina, calcium carbonate and silica powders as raw materials. Microstructure and phase composition were analyzed by SEM and XRD respectively. Properties such as porosity, pore size distribution and thermal conductivity were measured. High porosity (69–91%) and low thermal conductivity (0.018–0.13 W/m K) were obtained after sintering samples with different catalyst additions at 1300–1450 °C. Porosity, pore size, pore structure and grain size had obvious effect on heat conduction, resulting in the low thermal conductivity. The experimental thermal conductivity data of porous anorthite ceramics were found to be fit well with the computed values derived from a universal model.     

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.     

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.     

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.     

Near-net-size preparation of porous mullite matrix ceramics with in situ formed 3D mullite whisker network

Porous mullite matrix ceramics have excellent thermal and mechanical properties suitable for applications such as in thermal insulation. However, their applications are limited by processing defects from nonuniform sintering shrinkage and the trade-off between high porosity (preferred for low thermal conductivity) and high mechanical strength. Herein, we seek to minimize the sintering shrinkage by near-net-size preparation and improve the strength by in situ formed whisker network structure. Gelcasting forming technology and pressureless sintering were used to prepare porous mullite matrix ceramics using kyanite and α-Al₂O₃ powders as the starting materials and using MoO₃ to promote the growth of mullite whiskers. The results showed that the sintering shrinkage could be compensated by the volume expansion from solid-state reaction during reaction sintering. The in situ formed three-dimensional (3D) whisker network further reduced sintering shrinkage and effectively improved the strength of the ceramics. An ultralow sintering shrinkage of .78% was achieved. The near-net-shape porous mullite matrix ceramics strengthened by 3D whisker network had a high porosity of 63.9%, a high compressive strength of 83.8 MPa and a high flexural strength of 53.5 MPa.     

Effects of AlF3 and MoO3 on properties of Mullite whisker reinforced porous ceramics fabricated from construction waste

Mullite-whisker-reinforced anorthite-mullite-corundum porous ceramics were prepared from construction waste and Al₂O₃ powder by adding AlF₃ and MoO₃ as the additive and crystallization catalyst, respectively. The effects of AlF₃ and MoO₃ content on the properties of mullite whiskers, such as open porosity, mechanical properties, pore size distribution, microstructure and phase structure, were investigated in detail. The results showed that the morphology of the mullite whiskers and properties of the porous ceramics were greatly influenced by the AlF₃ and MoO₃ content. The specimen obtained by co-adding 12 wt% AlF₃ and 3 wt% MoO₃, and sintering at 1350 °C for 1 h, exhibited excellent properties, including an open porosity of 67.4±0.5% and biaxial flexural strength of 24.0±0.8 MPa. The mullite whiskers were uniformly distributed; the whiskers had a diameter of 0.05–0.5 µm, length of 8–10 µm, and aspect ratios (length to diameter ratio) of 20–30 on average.     

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 properties of porous mullite ceramics from calcined carbonaceous kaolin and α-Al2O3

High purity calcined carbonaceous kaolin and α-Al₂O₃ powders were employed to prepare porous mullite ceramics (Sample A) using graphite as pore former with the reaction sintering method. For the purpose of comparison, porous mullite ceramics (Sample B) was also fabricated from the uncalcined carbonaceous clay incorporated with α-Al₂O₃ powders. Mullitization in the two samples was both nearly complete at 1500 °C, despite the fact that calcination of the clay remarkably depressed mullitization and promoted the formation of glass phase. The Sample A sintered at 1500 °C fractured mainly in an intergranular way, while the Sample B mainly underwent transgranular fracture. The experimental results revealed that densification behavior/open porosity of the Sample A was far more sensitive to sintering temperature. The pore size of the Sample A as well as the Sample B sintered at 1500 °C was in a narrower range of 0.3–5 μm.     

Porous mullite ceramics with enhanced compressive strength from fly ash-based ceramic microspheres: Facile synthesis,structure, and performance

Porous mullite ceramics are widely used in heat insulation owing to their high temperature and corrosion resistant properties. Reducing the thermal conductivity by increasing porosity, while ensuring a high compressive strength, is vital for the synthesis of high-strength and lightweight porous mullite ceramics. In this study, ceramic microspheres are initially prepared from pre-treated high-alumina fly ash by spray drying, and then used to successfully prepare porous mullite ceramics with enhanced compressive strength via a simple direct stacking and sintering approach. The influence of sintering temperature and time on the microstructure and properties of porous mullite ceramics was evaluated, and the corresponding formation mechanism was elucidated. Results show that the porous mullite ceramics, calcined at 1550 °C for 3 h, possess a porosity of 47%, compressive strength of 31.4 MPa, and thermal conductivity of 0.775 W/(m?K) (at 25 °C), similar to mullite ceramics prepared from pure raw materials. The uniform pore size distribution and sintered neck between the microspheres contribute to the high compressive strength of mullite ceramics, while maintaining high porosity.