Needle-like Mullite Research Articles

Design and direct preparation of a novel silicon carbide support for zeolite membrane

The support for the membrane is the basis for the preparation and application of the zeolite membrane. Moreover, its cost and properties directly determine the performance of the zeolite membrane and its industrial applications. A novel porous silicon carbide (SiC) support for zeolite membrane bonded with needle-like mullite (3Al2O3·2SiO2) was prepared using SiC powders as the raw material and kaolin, Al(OH)3, and AlF3 as the needle-like mullite precursor additives via an in situ high-temperature reactions in an air atmosphere. Effects of the support material composition and the sintering temperature on the sintering behavior, pore structure, permeability, and microstructure of the resultant supports were extensively investigated. The needle-like mullite formation consumed silicon oxide (SiO2) and generated a rigid skeleton structure with a good pore structure and bonding phase. Hence, the porous SiC support exhibited high porosity, with relatively large pore size and mechanical strength, which helped improve the support performance. Notably, the porous SiC support sintered at 1480 °C with in situ inter-particle needle-like mullite bonding exhibited excellent filtration and permeability performance. The porous support materials and methods used in this work are suitable for designing and preparing a novel porous SiC support for zeolite membranes.

The Mechanism of Porcelain Toughened by Activated Kaolinite in a Lower Sintering Temperature.

A high sintering temperature is usually required to acquire excellent performance in the ceramic industry, but it results in high fuel consumption and high pollution. To reduce the sintering temperature and to toughen the porcelain, a self-produced sintering additive of citric acid activated kaolinite was added to the raw material; X-ray powder diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), and thermal gravity analysis and differential scanning calorimetry (TG-DSC) were used to characterize the samples, and the toughening mechanism was discussed. The citric acid activated kaolinite obtained high activity and a large specific surface area. After introducing the activated kaolinite, the bending strength of porcelain at 1270 °C increased from 100.08 MPa to 124.04 Mpa, which was 11.45% higher than that of porcelain without activated kaolinite at 1350 °C. The results of XRD revealed that the content of mullite increased and the quartz decreased at 1270 °C, and the well-distributed needle-like mullite was observed in the images of SEM with the addition of citric acid activated kaolinite. The TG-DSC results indicated adding activated kaolinite to porcelain raw materials reduced the formation of mullite to 994.6 °C. The formation of mullite in a lower temperature served as mullite seeds in a green body during firing, and it enhanced the growth of mullite. These contributed to the appropriate phase composition and the excellent microstructure of porcelain. Thus, the distinguished mechanical performance of porcelain was obtained. Moreover, the sintering additive had no adverse effect on the porcelain body as citric acid-activated kaolinite was one of the main components of the porcelain raw material.

Novel preparation of low-cost support for NaA zeolite membrane by utilizing natural clay

The membrane support is the basis for the preparation and application of the zeolite membrane and its cost and properties directly determine the performance of the zeolite membrane as well as its industrial applications. In this paper, the direct synthesis of NaA zeolite membrane on a novel low-cost support via an in situ reaction by utilizing natural clay can enable a significant reduction in manufacturing costs. During the preparation of the support for the NaA zeolite membrane, additives, such as Al2O3, Al(OH)3 and AlF3, were combined with kaolin clay and used as alumina precursors. Using the resulting porous mullite support, the effects of the firing temperature on the permeability and pore structure of the support were investigated extensively. The results of the experiment showed that silica in the natural clay was reacted by the Al2O3 precursor additives, producing a needle-like mullite in situ that served as a rigid skeleton porous structure for the support. Thus, a network porous structure of the mullite support could be formed during sintering process. The prepared network porous structure mullite supports exhibited a high pure water permeance of 3324 L m2 h1 bar1, open porosity of 41.8% and a mean pore size of 1.36 µm which was ideal for the preparation of the NaA zeolite membrane.

Influence of calcined halloysite on technological & mechanical properties of wall tile body

ABSTRACT The effect of adding 0.1, 0.3, 0.6, 1.0 wt.% halloysite clay, which calcined at 600, 800, 1000 and 1200°C, to the standard wall tile slurry was studied. Wet milled halloysite was sieved with different sizes to examine the halloysite grain size effect. The granulated samples were pressed by applying a pressure of 30 MPa using a hydraulic press and made ready for sintering at 1150°C for 30 minutes. Detailed technological, mechanical and microstructural characterization studies were done on sintered samples. Compared to standard wall tile, almost all the halloysite added samples displayed similar technological properties but higher green and fired strength values. The produced sample with adding 0.6% halloysite (at 600°C calcined) by weight showed a higher firing strength with 28.75 MPa value than the minimum requirement (17.75 MPa). Observation of needlelike microstructure in the porcelain body led to establishing a simple relationship between the interlocking effects of thin needlelike mullite grains and increasing strength values. This study has shown that the addition of calcined halloysite may be an alternative to produce a thinner section wall tile product.

Effect of micro/nano TiO2 addition on the densification behavior and mechanical properties of multifunctional resistant porcelains

The current study aimed at evaluating the effect of sintering temperature and TiO2 content on the density and mechanical properties of novel multifunctional resistant porcelains. In this work, a new selective mixture of available natural ceramic materials of kaolin (30 wt%), feldspar (45 wt%), and quartz (25 wt%) was used. Moreover, 3, 5, and 8 wt% TiO2 nano and microparticles were added to the mixture. The optimum sintering temperature was found to be 1150 °C, which resulted in the highest bulk density and the lowest apparent porosity of porcelain samples. The phase characterization of the prepared porcelains was carried out using the X-ray diffraction (XRD) analysis and Fourier transform infrared spectroscopy (FTIR). The XRD results of the analysis demonstrated that the samples containing 5 wt% TiO2 had the highest peak intensity of the mullite phase. In addition, the FTIR spectra confirmed the presence of crystalline mullite phase. The SEM micrographs revealed that the samples containing 5 wt% micro/nano TiO2 had the highest densification, containing the needle-like mullite phase.

Characterization of fly ash solid-waste for low-cost insulation refractory bricks

Fly ash from Barsingsar, Bikaner thermal power plant in northern India, is significantly generating solid-waste. In India, coal is of poor quality having high ash content around 30–45 wt%, fly ash created at coal thermal power plants. The present work investigates the possible use of fly ash solid-waste for the research of insulation refractory bricks. Fly ash is considered as a powdered material being collected by dust collectors built-in boilers of thermal power plants with the use of coal as a fuel. The chemical, thermal, crystallographic and morphological analysis of fly ash solid-waste has been determined by XRF, DTA-TG, XRD and SEM, respectively. The XRF results show the fly ash solid-waste is alumina and silica-rich. XRD results also reveal that the material is composed of mullite and quartz crystals. SEM investigation exhibits the morphology of fly ash solid-waste is needle-like mullite and fibrous-like quartz type of structure, which supports the XRD results. It can be concluded from the inquiry that fly ash solid-waste is the potential low-cost raw material for insulation refractory bricks.

Fabrication and characterization of a mullite-foamed ceramic reinforced by in-situ SiC whiskers

In this study, a new mullite-foamed ceramic, reinforced with in-situ SiC whiskers (MCS) and applied as the insulating lining of thermal equipment used in cement production, was investigated. Compared with a conventional mullite-foamed ceramic (MC), the MCS phase composition, microstructure, compressive strength, thermal conductivity and alkali corrosion resistance were investigated by using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and Factsage® software. The results showed that after being fired in granular coke, the SiC whiskers formed into MCS struts and were distributed in the pores between the interconnected needle-like mullite. Although the formation of SiC with higher conduction slightly increased MCS thermal conductivity, it significantly enhanced the compressive strength and alkali corrosion resistance of the foamed ceramic. Compared with the MC, although the MCS had higher bulk density (3.9%), and higher thermal conductivity (9.5% at 800 °C), it was more important that greatly improved compressive strength (by 60%) and better alkali corrosion resistance was achieved.

Fabrication of mullite-corundum foamed ceramics for thermal insulation and effect of micro-pore-foaming agent on their properties

Mullite-corundum foamed ceramics were prepared by direct-foaming method using white clay and industrial alumina as raw materials, and micro-pore-foaming agent (MPFA, the main composition was sodium dodecyl benzene sulfonate) as foaming agent. Effect of the MPFA addition on the phase compositions, microstructures and physical properties of the foamed ceramics were investigated by X-ray diffractometer (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS), etc. The results showed that the specimens fired at 1500 °C mainly consisted of needle-like mullite, corundum, and a small amount of glass phase. The MPFA decreased the viscosities and then increased the volume expansion of foamed slurries. Meanwhile, the adding of the MPFA caused the increases of apparent porosities and average pore sizes, and the decrease of strengths of the fired specimens. The optimized product was a specimen with 1.0 wt% MPFA fired at 1500 °C which had a high porosity of 73.7%, a proper compressive strength of 3.05 MPa, a low thermal conductivity of 0.287 W/(mK) (1000 °C) and a positive reheating linear change. Based on the experimental dates, an emendatory GE model was presented, which could predict accurately the effective thermal conductivity at 1000 °C of the mullite-corundum foamed ceramics with different pore characteristics.

Effect of microsilica content on microstructure and properties of foamed ceramics with needle-like mullite

In this study, five foamed ceramics with struts containing needle-like mullite were prepared by direct-foaming method using white clay, industrial alumina and microsilica powder as raw materials. The effects of microsilica content on the phase compositions, microstructures and properties of foamed ceramics were investigated. The results showed that the adding of microsilica decreased the average pore size and apparent porosity and increased the compressive strength and thermal conductivity of the foamed ceramics by affecting the properties of foamed slurry and reaction sintering process. The foamed ceramics with 10 wt.% microsilica content showed the best properties with high porosity of 75.8%, positive reheating linear change, compressive strength of 1.44MPa and low thermal conductivity of 0.219W/(m?K) (at 350?C).

The excellent catalyst support of Al2O3 fibers with needle-like mullite structure and HMF oxidation into FDCA over CuO/Al2O3 fibers

Al2O3 fibers, especially with unique morphology and structure, are a fascinating class of ceramics that have been widely used as catalyst supports. In this work, smooth, porous and needle-like Al2O3 fibers are successfully prepared by electro-blowing spinning (EBS) method. Effects of calcination temperature, calcination rate and pore forming agent content (Polytetrafluoroethylene, PTFE) on the morphology and structure of Al2O3 fibers are systematically studied. Most importantly, Al2O3 fibers with different morphologies are particular suitable to be used as CuO supports for selective oxidation of 5-hydroxymethyl-2-furfural (HMF) to 2,5-furandicarboxylic acid (FDCA). Benefiting from the unique structure, FDCA synthesis over the needle-like CuO/Al2O3 fibers achieves 98.6% yield, which is much higher than the yields over smooth and porous CuO/Al2O3 fibers. Moreover, the fiber catalysts could be easily recovered and reused without the loss of its activity. It is anticipated that Al2O3 fibers with needle-like mullite will shed some light on the development of profiled ceramic fibers for various applications.

Preparation and characterization of mullite foam ceramics with porous struts from white clay and industrial alumina

In this study, four mullite foam ceramics with porous struts and low thermal conductivities, were prepared by direct-foaming method from white clay and industrial alumina with the addition of sodium carboxymethyl cellulose (CMC) as foam stabilising agent. The effect of the CMC addition on the phase composition, pore characteristics, and strength was analysed through XRD, SEM and EDS, etc. The great part of struts for the foam ceramics was the needle-like mullite, the rest was the minor corundum and quartz. The addition of CMC decreased the apparent porosity of foam ceramics and pore area fractions of their struts, while the phase composition was nearly unaffected. Furthermore, the compressive strengths and the thermal conductivities of the foams ceramics increased. The optimized product contained 0.15 wt% CMC, which combined a high porosity of 81.3%, shown a low thermal conductivity of 0.095 W/(mK) (at 350 °C) and a minor positive reheating linear change.

Preparation of mullite/B4C composites: A comparative study on the effect of heating methods

The present study investigated preparation of the mullite/B4C composites through microwave and spark plasma sintering processes. A high-energy ball mill was used to mix the nono-sized mullite and B4C powders in ethanol media for 10 min. The mixed nano-sized powders were pelletized and sintered by microwave (MW) and spark plasma sintering (SPS) processes at 1500 and 1300 °C, respectively. The XRD results revealed the reactions between mullite and B4C, and also the formation of SiC, Al2O3, Si phases in SPS sintered sample. The identified crystalline phases in the microwave sintered sample were mullite and B4C. Samples sintered in SPS reached almost full density and showed improved mechanical properties (the hardness of 16.29 ± 0.2 GPa, bending strength of 348 ± 21 MPa and fracture toughness of 3.01 ± 0.18 MPa m−1/2) than those sintered in microwave. The microstructural investigations showed the formation of needle-like mullite at some areas of sample sintered in SPS with a uniform distribution of B4C in mullite matrix alongside pore-free microstructure. The microstructure of microwave sintered sample contained a large number of pores.

Influence of Spodumene Added as a Fluxing Agent on the Properties of Alumina Porcelain Insulator

Alumina porcelain electrical insulators were prepared using Li2O-bearing spodumene flux as a partial substitute for feldspar. The effects of spodumene on microstructure, crystalline phases, physical properties, modulus of rupture, dielectric breakdown strength and volume resistivity were investigated. The samples with spodumene content >5 mass-% sintered at 1350°C showed both zero percent water absorption (WA) and apparent porosity (AP). Density was found to decrease with increasing spodumene content from 2.39 g/cm3 (at 5 mass-% spodumene) to 2.37 g/cm3 (10 mass-%) to 2.33 g/cm3 (15 mass-%). This may be due to the irreversible phase transformation of α-spodumene to β-spodumene, which accompanies a change in density of spodumene from α-phase (3.20 g/cm3) to β-phase (2.35 g/cm3). The crystalline phases were characterised by X-ray diffraction and scanning electron microscope. They showed that spodumene favours crystallisation of elongated needle-like mullite and spodumene solid solution that coexist with alumina particles embedded in the glassy matrix of bodies sintered at 1350°C. This resulted in an increase in modulus of rupture and enhanced the electrical properties of the porcelain bodies. The maximum 71 N/mm2 modulus of rupture (MOR), dielectric strength of 21 KV/mm and electrical resistivity equal to 10×1012 Ω·cm were recorded with 10 mass-% spodumene.

Influence of Sintering Temperature on the Microstructure and Property of Low-cost Ceramic Proppants Prepared by Adding Purple Sands

High-strength corundum-mullite based ceramic proppants have been fabricated from 2nd bauxite (62.3wt%Al2O3) and purple sands, which were used as main raw materials by solid sintering method, combined with the additive of feldspar and pyrolusite powder. The phase analysis was studied by X-ray diffraction (XRD) and the morphology of specimen was observed by the scanning electron microscopy (SEM). The result showed that the additive of feldspar was beneficial to the formation of internal liquid phase of sample with the sintering temperature increasing, which reduced the pores and promoted the densification of sample. Meanwhile the proper liquid phase resulted in the growth of mullite and promoted the close integration of mullite and corundum, forming the network structure of needle-like mullite. Ultimately the proppants exhibited the optimal performance at 1450 °C with bulk density of 1.59 g/cm3, apparent density of 3.11 g/cm3 and the lowest breakage ratio of 3.76% under 52 MPa pressure.

Co-sintered synthesis of needle-like mullite porous membrane materials

Co-sintered synthesis of needle-like mullite porous membrane materials

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.

Low-cost porous mullite ceramic membrane supports fabricated from kyanite by casting and reaction sintering

Porous mullite supports are firstly fabricated by casting and reaction sintering based on kyanite with Al(OH)3 as porogenic agent. The effects of composition and sintering temperature on phase evolution, microstructure, apparent porosity, pore size distribution, linear shrinkage, gas permeation flux and mechanical property of supports are systematically investigated. Results show that the mullitization of kyanite generates needle-like mullite crystals, which form skeleton structures and improve the apparent porosity and strength of supports. Al(OH)3 addition not only promotes the formation of needle-like mullite but also enhances the apparent porosity of supports. Temperature promotes the development of mullite, from 1450 to 1500°C, the amount and size of needle-like mullite crystals increase, ≥1500°C, they reveal columnar morphology. The support prepared with kyanite+40wt%Al(OH)3 sintered at 1500–1550°C exhibits high apparent porosity, good gas permeation flux, excellent mechanical performance and interlocked network structure composed of well development needle-like mullite.

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 1.3µm and a penetrating porosity of 35.9%.

The properties of Southern Thailand clay-based porous ceramics fabricated from different pore size templates

Open cell porous ceramics were produced by low-cost polymeric sponge method using natural clays (Surat Thani clay and Ranong clay) from Southern Thailand and alumina powder as raw materials. Open-cell polyurethane sponges with different pore sizes were used as templates for ceramic slurries. Batch formulations of the samples were formed into green bodies and fired at 1150 and 1200°C at controlled firing rates. The properties studied include solid content, linear shrinkage, porosity, average pore size, pore size distribution, phase composition, phase morphology and compressive strength. This study found that the composition and pore size of sponge template strongly affected the physical properties and mechanical strength of the synthesized porous ceramics. It was found that porous ceramics could be produced from all ceramic compositions without internal cracking and distortion of the bulk structures. Surat Thani clay acted as a source of ball clay giving a better densification and shrinkage after sintering whereas Ranong clay, which is kaolinitic clay gave a better strength due to the presence of mullite as a major phase after firing at 1200°C. The interlocking structure of the needle-like mullite phase could add to the evidence for higher compressive strength in the compositions containing higher relative ratio of Ranong clay to Surat Thani clay.

Effect of Different Sintering Temperature on the Performance of Mullite-Corundum Refractory Materials

Al (OH)3, Suzhou kaolin, AlF3 and V2O5 were premixed and pelleted to form the precursor for fabricating the mullite whisker, and then the precursor was added into the calcined bauxite and Suzhou kaolin mixture according to a certain mass percent. The mullite-corundum refractory materials with well-dispersed needle-like mullite formed in-situ were prepared. Through studying the effect of different sintering temperatures on the performances of the as-fabricated mullite-corundum refractory materials, it was concluded that the appropriate sintering temperature was 1450 °C. X-ray diffraction (XRD), scanning electron microscopy (SEM), water absorption, bending strength, coefficient of thermal expansion and the first thermal stress fracture factor were used to characterize and evaluate the materials. The results show that the sintering character and thermal expansion coefficient of the refractory materials increase with the rising sintering temperature. The bending strength of the refractory materials sintered at 1500 °C presented the maximum value and the first thermal stress fracture factor appeared the highest value at 1450 °C.