Facile preparation of thermal insulation materials by microwave sintering of ferronickel slag and fly ash cenosphere

A novel approach for preparing thermal insulation materials by microwave sintering of ferronickel slag (FNS) in the presence of fly ash cenosphere (FAC) was proposed and evaluated. The study showed that during microwave radiation, the contact interface between FNS and FAC would preferentially form magnesium iron chromate spinel and magnesium iron aluminate spinel particles as hot spots by absorbing microwave vigorously, promoting decomposition and transformation of the raw materials into the thermal insulation phases, mainly cordierite and enstatite. After sintering at 900 °C by microwave for only 20 min with the addition of 25 wt% FAC, a thermal insulation material with thermal conductivity of 0.41 W/(m·K), bulk density of 1.46 g/cm³, compressive strength of 30.72 MPa, water absorption of 21.07%, and linear shrinkage of 7.06% was obtained. Compared with the conventional sintering method, the temperature was reduced by 300 °C, with the sintering time shortened by 6 times. This study represents a good example for clean and efficient value-added utilization of FNS, FAC and other relavent solid wastes.     

Microstructural and mechanical characterization of fly ash cenosphere/metakaolin-based geopolymeric composites

Novel fly ash cenosphere (FAC)/metakaolin (MK)-based geopolymeric composites were prepared by adding FAC to the MK-based geopolymeric slurry. Microstructure, mechanical property, thermal conductivity, and bulk density of the FAC/MK-based geopolymeric composites were investigated. It was confirmed by the scanning electron microscope (SEM) and transmission electron microscopy (TEM) that the FAC did not dissolve in alkaline condition, but element diffusion took place around the interface between geopolymeric matrix and FAC. The compressive strength, thermal conductivity and bulk density of FAC/MK-based geopolymeric composites decreased monotonically with the increase of the FAC content from 15 vol.% to 40 vol.%, and the minimum values for the 40 vol.% FAC/MK-based geopolymeric composite reached 36.5 MPa, 0.173 W m⁻¹ K⁻¹ and 0.82 g cm⁻³, respectively, in the range of FAC content from 15 vol.% to 40 vol.%. The results showed that the FAC could lower thermal conductivity effectively and bulk density of FAC/MK-based geopolymeric composites at a cost of slight decrease of mechanical properties. The 40 vol.% FAC/MK-based geopolymeric composite was a promising candidate material for intermediate-temperature thermal insulation applications due to its low thermal conductivity and low density.     

Preparation and characterization of porous anorthite ceramics from red mud and fly ash

The porous anorthite ceramics with high porosity, good mechanical strength and low heat conductivity were prepared using red mud and fly ash as raw materials via the pore forming method. The effects of sintering temperature and fly ash on phase evolution, densification, compressive strength, thermal conductivity and microstructure of the ceramic materials were investigated. The results showed that the compressive strength of the porous ceramics had an obvious improvement with the increase in fly ash, and the densification and heat conductivity decreased firstly and then increased. In particular, specimen S2 containing 30 wt% red mud and 40 wt% fly ash sintered at 1150°C had the better performances. It had the water absorption of 18.18%, open porosity of 38.52%, bulk density of 1.29 g/cm³, compressive strength of 42.46 MPa, and heat conductivity of 1.24 W/m·K. X-ray diffraction analysis indicated that mullite, anorthite, α-quartz, and diopside ferrian were the dominant phases in the specimens. Scanning electron microscopy micrographs illustrated that plenty of open pores with strip shape and closed pores with axiolitic shape existed in the specimens. Furthermore, the existence of mullite could prevent crack propagation to enhance the energy of inter-granular fracture. It endowed the porous anorthite ceramics with high porosity, good compressive strength, and low heat conductivity.     

Novel thermal insulating and lightweight composites from metakaolin geopolymer and polystyrene particles

In this work, new foamed thermal insulation geopolymer composite based on polystyrene particles (PP) and metakaolin was developed. Compressive strength, flexural strength, high temperature resistance and microstructure were evaluated. The experimental results show that compressivestrengthand flexural strength of the thermal insulation geopolymer composite decrease with increasing polystyrene particle content. However, it still exhibits considerable and sufficient strength. The dry density and thermal conductivityalso decrease as polystyrene particle content increases due to the contribution of polystyrene particles with low density. The floatation of the thermal insulation geopolymer composite on water surface indicates the relatively low density and a good quadratic function relationship can be found between thermal conductivity and dry density. Furthermore, the dense interfacial transition zone indicates the high compressive strength and flexural strength of thermal insulation geopolymer composites. The cumulative intrusion volume corresponding to the porosity decreases and the critical pore diametersshift to lower values with addition of polystyrene particles. Geopolymer composites gain strength after exposure around 400 °C, and it suffers dramatic strength loss after 800 °C temperature exposure especially for the 100% polystyrene particles addition specimen.     

Spark plasma sintering and high-temperature strength of B6O–TaB2 ceramics

Tantalum diboride – boron suboxide ceramic composites were densified by spark plasma sintering at 1900 °C. Strength and fracture toughness of these bulk composites at room temperature were 490 MPa and 4 MPa m^1/2, respectively. Flexural strength of B₆O–TaB₂ ceramics increased up to 800 °C and remained unchanged up to 1600 °C. At 1800 °C a rapid decrease in strength down to 300 MPa was observed and was accompanied by change in fracture mechanisms suggestive of decomposition of boron suboxide grains. Fracture toughness of B₆O–TaB₂ composites showed a minimum at 800 °C, suggestive a relaxation of thermal stresses generated from the mismatch in coefficients of thermal expansion.Flexural strength at elevated temperatures for bulk TaB₂ reference sample was also investigated.Results suggest that formation of composite provides additional strengthening/toughening as in all cases flexural strength and fracture toughness of the B₆O–TaB₂ ceramic composite was higher than that reported for B₆O monoliths.     

Microstructure and thermophysical properties of B4C/graphite composites containing substitutional boron

B₄C/graphite composites (BGC) containing substitutional boron were fabricated by pressureless sintering of powder mixtures of petroleum coke, coal tar pitch and B₄C. After sintering at 900 °C and graphitizing at 2200 °C, the microstructure of BGC was characterized by SEM, TEM, XRD, Raman spectroscopy and optical microscopy. XPS measurements revealed the formation of BC₃, and the matrix carbon contained around 6 wt.% substitutional boron. The thermal conductivity of the BGC at room temperature is 52.7 W/m K and the flexural strength is up to 35.1 MPa. The bulk density and electrical resistivity are 1.72 g/cm³ and 13.4 μΩ m, respectively. The correlation between microstructure and properties was investigated. The results showed that the microstructure improvement of the BGC has obvious effect on the thermal conductivity, flexural strength, and electrical resistivity.     

Processing and properties of Nextel™ 720 fiber reinforced alumina-zirconia ceramic matrix composites

The continuous Nextel? 720 fiber-reinforced zirconia/alumina ceramic matrix composites (CMCs) were prepared by slurry infiltration process and precursor infiltration pyrolysis (PIP) process. The introduction of submicron zirconia powders into the aqueous slurry was optimized to offer comprehensively good sintering activity, high thermal resistance and good mechanical properties for the CMCs. Meanwhile, the zirconia and alumina preceramic polymers were used to strengthen the porous ceramic matrix through the PIP process. The final CMC sample achieved a high flexural strength of 200 MPa after one infiltration cycle of alumina preceramic polymer and thermal treatment at 1150 °C for 2 h. The flexural strength retention of the improved CMC sample was 104% and 89% respectively after thermal exposure at 1100 °C and 1200 °C for 24 h.     

Preparation of Porous Mullite Ceramics Using Fly Ash Cenosphere as a Pore‐Forming Agent by Gelcasting Process

Porous mullite ceramics were fabricated from an industrial grade mullite powder by gelcasting process using fly ash cenospheres (FAC) as a pore‐forming agent. The influence of content of FAC and sintering temperature on the density and strength was evaluated. The microstructure showed that FAC can act as a sintering aid and a pore‐forming agent. When the sintering temperature at 1200°C, porous mullite ceramics with a relatively high porosity (48.1–72.2%), low density (0.84–1.64 g/cm³), low thermal conductivity (0.16–0.22 W/m · K), and high compressive strength (6.21–14.70 MPa) have been obtained.     

Formation of SiC whiskers/leucite-based ceramic composites from low temperature hardening geopolymer

In this study, SiC whiskers (SCWS) reinforced geopolymer composites (SCWS/KGP) and their ceramic products (SCWS/leucite) were prepared, and effects of SiC whiskers contents on the microstructure and flexural strength of the SCWS/KGP and SCWS/leucite composites were investigated. The results show that the whisker addition has little influence on both phase composition and thermal shrinkage of the KGP composites, but a suitable content of whisker will result in the improved flexural strength, and when the SCWS content is 2 wt%, flexural strength of the SCWS/KGP composite is enhanced by 95% compared with the neat geopolymer. The flexural strength of the composites can be further enhanced significantly after the composites being treated at 1100 °C and 1200 °C and flexural strength of the composite with SCWS content of 2 wt% was 107% and 125% higher than the untreated counterpart, respectively. The increase in flexural strength of the composites should be attributed to the strong leucite formation, whisker debonding and pulling out from matrix during the fracturing process based on the good interfacial bonding state between whisker and leucite matrix.     

Preparation of ultra-light ceramic foams from waste glass and fly ash

Ultra-light ceramic foams were successfully prepared by a green spheres technique, which used waste glass powder and fly ash as the main material. Besides, borax and SiC were introduced as fluxing agent and foaming agent, respectively. The effects of fly ash content, borax content and sintering temperature on the microstructures and properties of ceramic foams were systematically investigated. The optimum composition is 30?wt-% fly ash, 70?wt-% waste glass, 15?wt-% borax and 0.5?wt-% SiC. Ultra-light ceramic foams sintered at 680–780°C possess bulk density of 0.14–0.41?g?cm^?3, porosity of 82.9–94.1%, compressive strength of 0.91–6.37?MPa and thermal conductivity of 0.070–0.121?W?m^?1?K^?1, respectively. This method is convenient, low-cost and environment friendly, which makes it a promising way for recycling solid wastes.     

High-strength AlN ceramics by low-temperature sintering with CaZrO3–Y2O3 co-additives

Aluminum nitride (AlN) ceramics with the concurrent addition of CaZrO₃ and Y₂O₃ were sintered at 1450-1700 °C. The degree of densification, microstructure, flexural strength, and thermal conductivity of the resulting ceramics were evaluated with respect to their composition and sintering temperature. Specimens prepared using both additives could be sintered to almost full density at relatively low temperature (3 h at 1550 °C under nitrogen at ambient pressure); grain growth was suppressed by grain-boundary pinning, and high flexural strength over 630 MPa could be obtained. With two-step sintering process, the morphology of second phase was changed from interconnected structure to isolated structure; this two-step process limited grain growth and increased thermal conductivity. The highest thermal conductivity (156 Wm⁻¹ K⁻¹) was achieved by two-step sintering, and the ceramic showed moderate flexural strength (560 MPa).     

Design and characterization of porous mullite based semi-vitrified ceramics

Three porous ceramic composites were prepared from readily available raw materials (kaolin, bauxite, feldspar and kyanite). The porous ceramic formulations were sintered at different temperatures ranging from 1200 to 1400°C. The fired specimens were characterized by determining their porosity, bulk density, flexural strength, thermochemical stability, microstructure, water and mercury permeability. Apparent porosity and bulk density in the range 15.57 ± 1.56–42.73 ± 2.28?vol% and 2.23 ± 0.31–2.68 ± 0.41?g?cm^?3 respectively were obtained after firing. The flexural strength was in the range of 32.31 ± 2.1–74.88 ± 2.57?MPa and the thermal expansion coefficient of 5–9 × 10^?6 C^?1. The values of water permeability were 745.4, 641.45 and 525.91?L/m² h?kPa respectively for PK3, PK4 and PK5. It was found that at high temperature (1400?°C), kyanite particles enhanced the porosity and thermal stability by reducing glass formation and improving crystallization. The presence of the interconnected pores with size between 0.03 and 4.50?µm, the high total volume of pores together with the high flexural strength and thermal stability make the synthesized porous ceramics suitable for high-pressure filtering applications.     

The use of bagasse in the preparation of fireclay insulating bricks

Acidic insulating refractory bricks were prepared by mixing kaolin and grog with polystyrene (PS) beads up to 1.5% and bagasse up to 5% (by weight). This latter is a fibrous waste residue from the sugar cane industry. The bricks were dried and fired at 1250°C in a cycle lasting for 16 hours. The bricks were tested for water absorption, porosity, bulk density, cold crushing strength and thermal conductivity at three different temperatures (400, 600, and 800°C). It was concluded that bricks containing 1% PS and 3% bagasse abided by ASTM C155-97 for C-30 type insulating refractories with a bulk density of 1.01 g/cm³, a cold crushing strength of 4.08 MPa, and a thermal conductivity of 0.37 W/K/m at 800°C.     

Firing behavior of argillites from northern Tunisia as raw materials for ceramic applications

This study reports the firing properties of clayey materials from northern Tunisia to evaluate their possible use as raw material in ceramic. Physical, chemical, and mineralogical characterization and thermal behavior were carried out by inductively coupled plasma atomic emission, X-ray diffraction, Fourier transform infrared spectroscopy, differential thermal analysis, particle size distribution, and Atterberg limits tests. Firing properties were evaluated by color, firing shrinkage, water absorption, bulk density, apparent porosity, and flexural strength. Studied clayey materials are made up mainly by kaolinite and illite and are rich in iron. The main transformations after thermal analysis were identified from 500°C to 1000°C subsequent to the dehydroxylation of clay minerals, calcite decomposition, and the recrystallization process. Fired samples up to 1100°C showed better physical and mechanical properties related with a great densification resulting in a significant increase in linear shrinkage, bulk density, and flexural strength and a decrease in apparent porosity and water absorption up to 1100°C. This behavior is due to a crystalline and liquid phases formed at low firing temperature associated with a high content of fluxing agents. The fired ceramic materials exhibited low water absorption up to 2.26% and high flexural strength up to 32.6 MPa, which makes their potential use for some earthenware and stoneware products.     

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.     

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 and characteristics of porous anorthite ceramics with high porosity and high-temperature strength

High-temperature properties including compressive strength, thermal shock behavior, and thermal conductivity of porous anorthite ceramics with high specific strength were tested and analyzed. The results showed that the prepared materials merit high-temperature compressive strength, thermal stability, and conductivity. With the appropriate fabrication parameters, even though containing 0.33 g/cm³ bulk density and 88.2% porosity, its compressive strength could reach 2.03 MPa at 1000°C, 147% of that at room temperature; the residual strength ratio kept as 114.7% after a thermal shock at 1200°C. The X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) showed that anorthite grains refinement and intergranular voids filling by liquid phase were main factors for the high strength. From room temperature to 1200°C, its thermal conductivity only varied from 0.085 to 0.258 W·(m·K)⁻¹. High porosity, a large number of nanoregions in anorthite grains and amorphous phase in grain boundary were main reasons for low thermal conductivity.     

Preparation and characterization of mullite whisker reinforced ceramics made from coal fly ash

Ceramics with mullite whiskers were prepared from coal fly ash and Al₂O₃ raw materials, with AlF₃ used as an additive. The phase structures and microstructures of the ceramics were identified via X-ray diffraction and scanning electron microscopy, respectively. The results show that pickling of coal fly ash is an effective method for enhancing the flexural strength of ceramics. Sintering temperature and AlF₃ addition were also key factors influencing the creation of ideal ceramics. The ceramic made from pickled coal fly ash, 6?wt% AlF₃, and sintered at 1200?°C, exhibited the highest flexural strength of 59.1?MPa, and had a bulk density of 1.32?g/cm³ and porosity of 26.8%. The results show that ceramic materials made under these conditions are ideal candidates for manufacturing ceramic proppants for the exploitation of unconventional oil and gas resources.     

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.     

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.