Fabrication and characterization of porous cordierite ceramics prepared from fly ash and natural minerals

Low cost, single-phase porous cordierite ceramics are successfully synthesized by in-situ solid-state reactions from fly ash, quartz, and magnesite. The effects of sintering temperature and magnesite content on phase transformation, open porosity, bulk density, mechanical properties, and microstructure are carefully investigated. Factsage analyses are carried out to calculate the isopleth diagrams, and the results agree well with the experimental outcomes. Thermal gravimetric and differential scanning calorimeter analyses (TG-DSC) are performed to characterize the weight loss and transient behaviors of the raw materials. Linear thermal expansion properties are also studied. The α-cordierite phase is the only phase observed in S-3 (magnesite content 25%) sintered at 1300 °C for 2 h, and its linear thermal expansion coefficient (2.71 × 10⁻⁶ K⁻¹) is close to that of typical cordierite. Both the compressive strength (72.64 MPa) and flexural strength (23.92 MPa) of the as-synthesized samples are high with an open porosity of 33.16% and a bulk density of 1.61 g/cm³.     

High gas permeability and directional channels of mullite porous ceramics prepared by pectin-based freeze-drying method

New gel system for preparing mullite porous ceramics by gel-casting freeze-drying was proposed, using pectin as gel source and alumina and silica as raw materials. Directional channels were formed due to sublimation of water during freeze-drying and decomposition of pectin during high temperature sintering to prepare porous mullite ceramic membranes. Effects of solid content on the properties of mullite ceramics in terms of phase composition, microstructure, apparent porosity, bulk density, pore size distribution, compressive strength, thermal conductivity, pressure drop, and gas permeability were investigated. It was found that prepared porous mullite possessed high apparent porosity (56.04%–75.34%), low bulk density (.77–1.37 g/cm³), uniform pore size distribution, relatively high compressive strength (.61–3.03 MPa), low thermal conductivity (.224–.329 W/(m·K)), high gas permeability coefficient (1.11 × 10⁻¹⁰–4.73 × 10⁻¹¹ m²), and gas permeance (2.18 × 10⁻²–9.32 × 10⁻³ mol⋅m⁻²⋅s⁻¹⋅Pa⁻¹). These properties make prepared lightweight mullite ceramic membranes promising for application in high temperature flue gas filtration. Proposed gel system is expected to provide a new route to prepare porous ceramics with high porosity and directional channels.     

Effect of Al2O3 Addition on the Flexural Strength and Light‐Transmission Properties of Bone China

To improve the flexural strength and light‐transmission properties of bone china, the effects of adding different amounts of alumina (0–3%) to bone china bodies were studied and the phase composition and microstructure of different samples were studied by X‐ray diffraction (XRD) and scanning electron microscopy (SEM). In addition, physical properties, such as the bulk density, the thermal expansion coefficient (TEC) and thermal shock resistance, were studied. It was found that adding alumina increased the overall sintering temperature while reducing the sintering temperature range of bone china. Furthermore, addition of 1% Al₂O₃ improved the tree‐point flexural strength from 120 MPa to 150 MPa, the light transmittance (at 2 mm thickness) from 6.7% to 7.5%, the thermal expansion coefficient from 8.4 × 10^?6°C^?1 to 8.1 × 10^?6°C^?1 and the thermal shock resistance from 140°C to 180°C. Higher corundum content results in similar high flexural strength but lower light transmittance.     

Fabrication and characterization of bone china using synthetic bone powder as raw materials

The synthetic bone powder was studied as a raw materials for bone china, completely replacing natural bone ash raw materials. The physical and thermal properties of samples obtained by the two bone powders were tested and comparatively studied. Performance tests included pyroplastic deformation, flexural strength, bulk density, sintering shrinkage, water absorption, transmittance, thermal expansion coefficient and the thermal shock resistance. The phase composition and morphology were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results indicated that using synthetic bone powder could shorten the preparation time, reduce the sintering temperature and result in high-quality bone china. The pyroplastic deformation decreased from 9.14% to 7.92%, the three-point flexural strength increased from 123 MPa to 191 MPa, the light transmittance (at a 2-mm thickness) increased from 6.7% to 11.2%, the thermal expansion coefficient decreased from 8.24×10⁻⁶ °C⁻¹ to 7.69×10⁻⁶ °C⁻¹, and the thermal shock resistance increased from 140 °C to 180 °C. A continuous interface layer without cracks was produced by using the synthetic bone powder.     

Preparation study for the low thermal expansion spodumene/mullite composites

In this work, spodumene/mullite ceramics with low thermal expansion were successfully prepared from spodumene, quartz, talc, and clay. The effects of spodumene content and sintering temperature on the mechanical properties of spodumene/mullite ceramics were investigated. The formed phases were then detected by X-ray diffraction analysis and the microstructures of the sintered bodies were determined by scanning electron microscopy. The interaction effects of the spodumene content and sintering temperature on the apparent porosity and bulk density were studied by response surface methodology. The results demonstrate that an appropriate sintering temperature and spodumene content can promote densification, improve the mechanical properties, and reduce the coefficient of thermal expansion (CTE) of spodumene/mullite ceramics. At the spodumene content of 40 wt.%, the sintering temperature of 1270°C, and the holding time of 90 min, the bending strength was 60.45 MPa, the CTE was 1.73 × 10^–6/°C (α_{[25–650°C]} < 2 × 10^–6/°C), the bulk density was 2.28 g cm^-3, and the apparent porosity was 0.43%. Therefore, this study was of guiding significance for reducing the production cost of spodumene low thermal expansion ceramics and improving product quality.     

Permeability behavior of silicon carbide-based membrane and performance study for oily wastewater treatment

Porous SiC ceramic is considered as a suitable material for hot gas filtration, microfiltration, and many others industrial applications. However, full utilizations of porous SiC ceramics have been limited by high-processing costs. In this study, mullite-bonded porous SiC ceramics membranes were prepared using commercial SiC powder, alumina, clay, and different sacrificial pore formers. The effect of different pore formers on the microstructure, mechanical strength, porosity and pore size distribution, air, and water permeability of porous SiC ceramics were investigated. The average pore diameter, porosities, and flexural strength of the final ceramics varied in the range 3.7-6.5 µm, 38-50 vol. %, and 28-38 MPa, respectively, depending on the characteristics of pore former. The Darcian (k₁) and non-Darcian (k₂) permeability evaluated from air permeation behavior at room temperature was found to vary from 1.48 × 10⁻¹³ to 4.64 × 10⁻¹³ m² and 1.46 × 10⁻⁸ to 6.51 × 10⁻⁸ m, respectively. All membranes showed high oil rejection rate (89%-93%) from feed wastewater with oil concentration of 1557 mg/L. The membrane with porosity ~48 vol% and mechanical strength 31.5 MPa showed and highest pure water permeability of 13 298 Lm⁻²h⁻¹bar⁻¹.     

Near net shape fabrication of porous cordierite by combination of foam gel-casting and freeze-drying

Near net shape fabrication of porous cordierite was successfully achieved through a combination of foam gel-casting, freeze-drying, and in situ synthesis. Environment friendly gelation was used as gel system, and the gelatin concentration influenced the drying shrinkage vastly. Combined with the volume expansion coming from phase transition and solid reaction during in situ synthesis of cordierite, the total linear shrinkage could be controlled around zero (−1.87% to 0.45%) by adjusting the gelation concentration and solid content in the slurry, meanwhile the prepared porous cordierite ceramics showed both high porosity (85.9%–91.1%) and high compressive strength (0.58–3.37 MPa). The sample with 0.05 g/ml gelatin concentration and 20 vol% solid content possessed excellent performance: total porosity of 89.1%, compressive strength of 1.36 MPa, and specific strength of 4.9 MPa/(g/cm³), showing the potential usage of filter carrier.     

Effect of microporous magnesia aggregates on microstructure and properties of periclase-magnesium aluminate spinel castables

The present study investigated three lightweight periclase-magnesium aluminate spinel castables containing microporous magnesia aggregates with a varying apparent porosity (12.8%, 30.8% and 39.3%). The effect of the apparent porosity of the aggregates on the phase composition, microstructure, fracture behavior and strength of the lightweight castables was investigated by XRD, SEM and three-point bending tests. Large cracks between the aggregates with an apparent porosity of 12.8% and the matrix reduced the strength of the castable. For the aggregates with an apparent porosity of 30.8%, an excellent interlocking interface with the matrix increased the strength considerably, but also reduced the fracture toughness. At the highest level of the apparent porosity of the aggregates of 39.3%, the formation of a small number of microcracks between the aggregates and matrix reduced the strength, while the fracture toughness was only slightly affected. The lightweight castables with the best combination of properties were achieved at an apparent porosity of the aggregates of 30.8% since they had a low bulk density of 2.63 g/cm³ as well as a high compressive and flexural strength of 70.2 MPa and 20.9 MPa, respectively.     

Preparation and performance study of cordierite/mullite composite ceramics for solar thermal energy storage

The employment of solar energy in recent years has reached a remarkable edge. It has become even more popular as the cost of fossil fuel continues to rise. Energy storage system improves an adjustability and marketability of solar thermal and allowing it to produce electricity in demand. This study attempted to prepare cordierite/mullite composite ceramics used as solar thermal storage material from calcined bauxite, talcum, soda feldspar, potassium feldspar, quartz, and mullite. The thermal physical performances were evaluated and characterized by XRD, SEM, EPMA, and EDS. It was found that the optimum sintering temperature was 1280°C for preparing, and the corresponding water adsorption was 11.25%, apparent porosity was 23.59%, bulk density was 2.10 mg·cm^?3, bending strength was 88.52 MPa. The residual bending strength of specimen sintered at 1280°C after thermal shock of 30 times decreased to be 57 MPa that was 36% lower than that before. The thermal conductivity of samples sintered at 1280°C was tested to be 2.20 W·(m·K)^?1 (26°C), and after wrapped a PCM (phase change materials) of K₂SO₄, the thermal storage density was 933 kJ·kg^?1 with the temperature difference (ΔT) ranged in 0‐800°C. The prepared cordierite/mullite composite ceramic was proved to be a promising material for solar thermal energy storage.     

Use of steel slag and quartz sand-tailing for the preparation of an eco-friendly permeable brick

In order to effectively utilize industrial solid wastes (ISW), an eco-friendly permeable brick was successfully prepared by using quartz sand-tailing and steel slag. The influence of sintering temperature on the properties including permeability, apparent porosity, water absorption, bulk density, mechanical strengths, and chemical stability was systematically investigated. The research results show that with increasing sintering temperature, the structure of the permeable brick gradually become dense, which may be due to liquid phase sintering caused by a small amount of glass-liquid phase (Ca-Fe-Si-Al-O glass system). The permeability, apparent porosity, and water absorption decrease gradually, while the bulk density and mechanical strengths increase monotonously. Furthermore, the best sintering schedule for the permeable brick is 1320°C for 1 hour, with the optimized parameters, the permeable brick exhibits excellent properties (permeability, 3.65 × 10⁻² cm/s; compressive strength, 34.1 MPa; bending strength, 5.2 MPa; chemical stability, above 99%), wherein the permeability is far beyond Chinese standard (1 × 10⁻² cm/s). The permeable brick in this work provides a promising way for the recycle of ISW. Even better, it can maximize the introduction of quartz sand-tailing and steel slag (reach 100%) and create huge economic benefits while protecting environment.     

Effects of polycarbosilane infiltration processes on the microstructure and mechanical properties of 3D-Cf/SiC composites

Three-dimensional braided carbon fiber-reinforced silicon carbide (3D-C_f/SiC) composites were prepared through eight cycles of infiltration of polycarbosilane (PCS)/divinylbenzene (DVB) and subsequent pyrolysis under an inert atmosphere. The effects of infiltration processes on the microstructure and mechanical properties of the C_f/SiC composites were investigated. The results showed that increasing temperature could reduce the viscosity of the PCS/DVB solution, which was propitious to the infiltration processes. The density and flexural strength of 3D-C_f/SiC composites fabricated with vacuum infiltration were 1.794 g cm⁻³ and 557 MPa, respectively. Compared to vacuum infiltration, heating and pressure infiltration could improve the infiltration efficiency so that the composites exhibited higher density and flexural strength, i.e., 1.944 g cm⁻³ and 662 MPa. When tested at 1650 °C and 1800 °C in vacuum, the flexural strength reached 647 MPa and 602 MPa, respectively.     

Effects of particle size in silica sol on the mechanical and thermal properties of SiO2f/SiO2 composites

In this paper, quartz fiber-reinforced silica matrix SiO_2f/SiO₂ composites were prepared by the precursor impregnation-heat treatment method using quartz fiber needle felt as the reinforcement and silica sol as the precursor. The effects of particle size in silica sol (10, 50, and 100 nm) on the density, apparent porosity, mechanical properties, and thermal properties of SiO_2f/SiO₂ composites were investigated. The phase composition and microstructure of the composites were characterized by X-ray diffraction and scanning electron microscopy, respectively. The thermal expansion coefficient and thermal conductivity of composites were measured by a push rod method and the laser method. The results show that the density, apparent porosity, and mechanical strength of the specimens firstly increase and then decrease with the increase in the particle size in silica sol. The sample using silica sol with particle size 50 nm has the optimum overall performances (i.e., the flexural strength of 13.7 MPa and the compressive strength of 59.8 MPa), and shows a ductile fracture behavior. At 300°C–700°C, the average thermal expansion coefficient of the optimal sample is .783 × 10⁻⁶/°C. And the thermal conductivity of the samples increases with the increase in temperature, and it reached the highest value of .810 W/(m·K) at 700°C. The SiO_2f/SiO₂ composites show obvious advantages in the application of load-bearing and thermal insulation integration, and they are expected to meet the demanding requirements of hot-pressing sintering and non-ferrous metallurgy industries.     

Preparation of refractory cordierite using amorphous rice husk silica for thermal insulation purposes

This study aims to investigate the effect of sintering temperatures on the phase formation and physical characteristics of refractory cordierite prepared from rice husk silica, Al₂O₃, and MgO powders. The samples were subjected to sintering temperatures of 1050–1350 °C, and development of structures was characterized using Fourier Transform Infrared (FTIR) spectroscopy, X-ray diffraction (XRD) coupled with Rietveld analysis, scanning electron microscopy (SEM) and dilatometry. The results obtained indicated the significant role of sintering temperatures on phase transformation of spinel and cristobalite into cordierite, in which at sintering temperatures of 1230–1350 °C the cordierite emerges as a dominant phase, while spinel and cristobalite are practically undetected. Formation of cordierite was followed by decrease in density, porosity, and thermal expansion coefficient, while for hardness and bending strength the opposite was true. Thermal expansion coefficient of the sintered sample at 1350 °C is 3.3×10⁻⁶/°C and the XRD analysis demonstrated that the main crystalline phase is cordierite. Based on these characteristics, the samples are considered as insulator, suggesting their potential use in refractory devices.     

An economic and environment friendly way of recycling boron carbide waste to prepare B4C/Al composite ceramic

Recently, with the rapid growth of sapphire wafer applications, boron carbide as abrasive, has shown an increasing demand. Great amounts of boron carbide waste (low purity and small grain size with D₅₀ ≈ 1 μm) are therefore produced during the production of boron carbide abrasives and barely recovered and utilized. This paper is aimed at developing an economic and environment friendly process to recycle the boron carbide waste through adding a certain amount of Al powder to prepare B₄C/Al composite ceramic. Prior to the sintering process, samples were firstly mixed with different Al powder and then pelleted and dehydrated. The effects of the pelletizing factors on performances of the pellets and the ceramics were optimized as binder hydroxypropyl methylcellulose addition 0.4%, pelleting pressure 30 MPa, Al addition 9 wt%, sintering time 90 minutes. Under these conditions, the apparent porosity, bulk density, compressive strength and flexural strength of the sintered B₄C/Al are 19.08%, 1.84 g/cm³, 246.88 MPa and 71.10 MPa respectively. Al addition can not only attribute to the low-temperature liquid sintering and densification of the product, but also generation of some stable phases including AlB₁₂, AlB₁₂C₂ and Al₃BC, which in turn increase the performance of the ceramic composite.     

SiC junction enhanced carbon nanofiber aerogels with extremely high specific strength and ultra-low thermal conductivity

C/SiC aerogels with both ultra-low thermal conductivity and extremely high strength were fabricated by freeze casting. SiC junctions originated from pyrolysis of polycarbosilane (PCS) were formed between carbon nanofibers (C_f) to enhance the strength of aerogels. The effects of PCS content and total solid content on the phase composition, pore structure, thermal conductivity and compressive property were studied. The fabricated aerogels possess hierarchical pore structure. In the micro-scale, it contains circular pores with size of about 15 µm, while it is mesoporous and macroporous in the nano-scale. Both thermal conductivity and compressive strength increase with the increase in PCS content. Through tailoring PCS content and total solid content, C_f/SiC aerogels with porosity of 99.5%, thermal conductivity of 33 mW·m⁻¹·K⁻¹ and compressive strength of 7.14 MPa can be obtained. The specific strength of the fabricated C_f/SiC aerogels is up to 467.6 MPa/(g/cm³), which is the highest value for ceramic aerogels.     

The microstructure,coefficient of thermal expansion and flexural strength of cordierite ceramics prepared from alumina with different particle sizes

Cordierite ceramics were produced from alumina with 5 and 0.65 μm particle sizes or AlOOH and talc, clays and feldspar, to determine the influence of the alumina particle size on the microstructure, coefficient of thermal expansion (CTE) and flexural strength (FS) of the ceramics. After sintering at 1300 °C the ceramics made from 5-μm-sized alumina consisted of cordierite, glass, quartz, mullite and alumina, and had the highest density, FS and CTE. The alumina grains act as inclusions, from which the trajectories of the cracks were deflected or terminated, which increases the FS and CTE. The ceramics from sub-micrometre-sized alumina or AlOOH contained a negligable amount and no alumina, respectively, together with other phases. This is reflected in the low CTE and FS. The cordierite ceramic with the lowest CTE of ∼2.0 × 10⁻⁶ K⁻¹ and a high FS of 100 MPa was prepared from the 0.65-μm-sized alumina particles.     

Preparation of calcium hexaluminate porous ceramics by gel-casting method with polymethyl methacrylate as pore-forming agent

Calcium hexaluminate (CA₆) porous ceramics were prepared by gel-casting method, with α-Al₂O₃ and CaCO₃ as raw materials and polymethyl methacrylate (PMMA) microspheres as pore-forming agent. The effects of the amount of pore-forming agent PMMA microspheres on the phase composition, bulk density, apparent porosity, flexural strength, microstructure, thermal shock stability and thermal conductivity of CA₆ porous ceramics were systematically studied. The pores of CA₆ porous ceramics are mainly formed by the burning loss of PMMA microspheres and the decomposition of organic matter. Adding an appropriate amount of PMMA microspheres as pore-forming agent has a positive effect on the thermal shock stability of CA₆ porous ceramics. When the amount of pore-forming agent is 15 wt%, the volume density of CA₆ porous ceramics is 1.33 g/cm³, the porosity is 63%, the flexural strength is 13.9 MPa, the thermal shock times can reach 9 times, and the thermal conductivity is 0.293 W/(m·K), which can meet the application in refractory, ceramics or high temperature cement industries.     

Light‐weight thermal insulating fly ash cenosphere ceramics

Light weight fly ash cenosphere (FAC) ceramic composites were developed by simple slip casting method. Thermal properties, Bulk density, Microstructure, flexural strength, and phase analysis of the FAC ceramic composites were measured. The results proved that the FAC have ability to lower bulk density and thermal conductivity effectively. The lowest thermal conductivity achieved for FAC ceramic composites (0.27 W/m.K) was further reduced 0.21 W/m.K by adding combustible additives ie activated charcoal and corn starch. The flexural strength, bulk density and thermal conductivity of FAC ceramic composites reduced consistently with an increase in FAC content. The maximum flexural strength of 13.45 MPa was achieved with 50% FAC and the minimum flexural strength of 4.07 MPa was obtained with 80% FAC. The open porosity increased from 35.51% to 43.76% and 38.19% with an addition of 15% activated charcoal and corn starch, respectively, when compared to no additives. The bulk density of 699, 619, and 675 kg/m³ was achieved with 80% FAC, 80% FAC with the addition of 15% activated charcoal and corn starch, respectively. The 80% FAC ceramic composite shows low thermal expansion coefficient 6.54 × 10^?6/°C at the temperature of 50°C then it varies between 3.7 and 5 × 10^?6/°C in the temperature range above 100°C. These results prove that the developed light weight FAC ceramic are excellent low‐cost thermal insulating materials.     

Preparation of porous cordierite ceramic by gel-casting method

Porous cordierite ceramics were synthesised by gel-casting method, using talcum powder, kaolin and alumina as raw materials. Organic monomers and cross-linker were used as additives. The phase composition and microstructure were investigated by X-ray diffraction and scanning electron microscope. The open porosity, compressive strength and thermal expansion coefficient were tested by the Archimedes method, universal testing machine and thermal expansion instrument, respectively. The results indicate that sintering temperature and holding time have a great influence on the cordierite properties. We obtain the good performance of porous cordierite ceramic sintering at 1350°C for 3?h. The cordierite phase content in the sample is higher and the crystallinity is better. At this point, the porosity is 58.53%, the compressive strength is 22.44?MPa and thermal expansion coefficient reaches 1.69?×?10^?6?°C^?1.     

Thermo-mechanical and high-temperature dielectric properties of cordierite-mullite-alumina ceramics

Heterogeneous ceramics made of cordierite (55–56 wt%), mullite (22–33 wt%) and alumina (23–11 wt%) were prepared by sintering non-standard raw materials containing corundum, talc, α-quartz, K-feldspar, kaolinite and mullite with small amounts of calcite, cristobalite and glass phases. The green specimens prepared by PVA assisted dry-pressing were sintered within the temperature range of 950–1500 °C for different dwelling times (2–8 h). The effects of sintering schedule on crystalline phase assemblage and thermomechanical properties were investigated. The sintered ceramics exhibited low coefficients of thermal expansion (CTE) (3.2–4.2×10⁻⁶ °C⁻¹), high flexural strength (90−120 MPa and high Young modulus (100 GPa). The specimens sintered at 1250 °C exhibited the best thermal shock resistance (∆T~350 °C). The thermal expansion coefficients and thermal shock resistance were studied using Schapery model, the modelling results implying the occurrence of non-negligible mechanical interactions between the phases in bulk. The dielectric properties characterized from room to high temperature (RT– HT, up to 600 °C) revealed: (i) noticeable effects of sintering schedule on dielectric constant (5–10) and dielectric loss factor (~0.02–0.04); (ii) stable dielectric properties until the failure of the electrode material. The thermomechanical properties coupled with desirable dielectric properties make the materials suitable for high density integrated circuitry or high temperature low-dielectric materials engineering.