Showing papers on "Mullite published in 2017"

Microwave dielectric properties of mineral sillimanite obtained by conventional and cold sintering process

Abstract: The sillimanite (Al 2 SiO 5 ) mineral has been sintered by conventional ceramic route and by cold sintering methods. The mineral has very poor sinterability and transformed to mullite on sintering above 1525 °C. The dielectric properties of sillimanite mineral (Al 2 SiO 5 ) are investigated at radio and microwave frequency ranges. The mineral sintered at 1525 °C has low e r of 4.71 and tanδ of 0.002 at 1 MHz and at microwave frequency e r = 4.43, Q u × f = 41,800 GHz with τ f = −17 ppm/°C. The sintering aid used for cold sintering Al 2 SiO 5 is sodium chloride (NaCl). The Al 2 SiO 5 NaCl composite was cold sintered at 120 °C. XRD analysis of the composite revealed that there is no additional phase apart from Al 2 SiO 5 and NaCl. The densification of the Al 2 SiO 5 NaCl composite was confirmed by using microstructure analysis. The Al 2 SiO 5 NaCl composite has e r of 5.37 and tanδ of 0.005 at 1 MHz whereas at microwave frequency it has e r = 4.52, Q u × f = 22,350 GHz with τ f = −24 ppm/°C. The cold sintered NaCl has e r = 5.2, Q u × f = 12,000 GHz with τ f = −36 ppm/°C.

Modest Oxygen-Defective Amorphous Manganese-Based Nanoparticle Mullite with Superior Overall Electrocatalytic Performance for Oxygen Reduction Reaction

Abstract: Manganese-based oxides have exhibited high promise as noncoinage alternatives to Pt/C for catalyzing oxygen reduction reaction (ORR) in basic solution and a mix of Mn3+/4+ valence is believed to be vital in achieving optimum ORR performance. Here, it is proposed that, distinct from the most studied perovskites and spinels, Mn-based mullites with equivalent molar ratio of Mn3+ and Mn4+ provide a unique platform to maximize the role of Mn valence in facile ORR kinetics by introducing modest content of oxygen deficiency, which is also beneficial to enhanced catalytic activity. Accordingly, amorphous mullite SmMn2 O5-δ nanoparticles with finely tuned concentration of oxygen vacancies are synthesized via a versatile top-down approach and the modest oxygen-defective sample with an Mn3+ /Mn4+ ratio of 1.78, i.e., Mn valence of 3.36 gives rise to a superior overall ORR activity among the highest reported for the family of Mn-based oxides, comparable to that of Pt/C. Altogether, this study opens up great opportunities for mullite-based catalysts to be a cost-effective alternative to Pt/C in diverse electrochemical energy storage and conversion systems.

Foam-gelcasting preparation of high-strength self-reinforced porous mullite ceramics

Abstract: High-strength self-reinforced porous mullite ceramics were prepared via foam-gelcasting using mullite powder as a main raw material, AlF 3 ·3H 2 O (0–8 wt%) as an additive, Isobam-104 as a dispersing and gelling agent, sodium carboxymethyl cellulose as a foam stabilizing agent, and triethanolamine lauryl sulfate as a foaming agent The effects of AlF 3 ·3H 2 O content on rheological and gelling behaviors of the slurries, and porosity and mechanical properties of self-reinforced porous mullite samples were examined Addition of AlF 3 ·3H 2 O promoted the in-situ formation of elongated mullite in the fired porous samples, which improved considerably their mechanical properties Compressive strength and flexural strength of 670% porous mullite ceramics prepared with addition of 6 wt% AlF 3 ·3H 2 O was as high as 413 and 139 MPa, respectively Its hot modulus rupture (HMOR) increased initially with the testing temperature, and peaked (with a maximum value of 166 MPa) at 800 °C above which it started to decrease with the testing temperature Nevertheless, it was still retained as high as 67 and 28 MPa at 1200 and 1400 °C, respectively

Novel mullite ceramic foams with high porosity and strength using only fly ash hollow spheres as raw material

Abstract: A novel approach to fabricate mullite ceramic foams with high porosity and mechanical strength using fly ash hollow spheres (FAHSs) as the only raw material was reported for the first time, in which FAHSs served as both matrix and pore-forming agent. It was demonstrated that uniform FAHS ceramic foams without cracks could be fabricated by both direct stack sintering method and gel-casting route. The effect of sintering temperature on linear shrinkage, porosity, phase composition and mechanical properties was investigated. With the increase of sintering temperature from 1200 °C to 1350 °C, the fracture mechanism changed from fracturing along FAHSs to fracturing across FAHSs, and the compressive strength of the FAHS ceramic foams increased from 4.4 MPa to 33.4 MPa. The FAHS ceramic foams exhibited high porosity in the range of 73–81% and high mechanical strength, which benefited from the dense assembling of small FAHSs with natural diameter distribution and excellent wall-thickness uniformity.

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

Abstract: 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.

Comparative performance of alkali activated slag/metakaolin cement pastes exposed to high temperatures

Abstract: This paper presents a study on the effect of temperature exposure of binders of blast furnace slag (BFS) and metakaolin (MK) in BFS-MK weight ratios of 100-0, 50-50, and 0-100 activated with sodium silicate of modulus Ms = SiO 2 /Na 2 O = 1 and 5, 10 and 15% Na 2 O. A blended ordinary CPC-30R Portland cement reference was used. Pastes were subjected to exposure up to 1200 °C and the performance was evaluated in terms of compressive strength, residual strength, volumetric shrinkage, physical appearance and microstructural changes at different temperatures. All the binders retained more than 30 MPa after exposure to 800 °C for 4 h; specimens of MK and CPC-30R experienced the highest strength losses of 42 and 56% respectively, while those of 100-0 and 50-50 showed minor losses of ∼20%. After heating at 1200 °C the samples showed microstructural damage and more than 65% of strength losses. XRD indicated that the 100-0 and 50/50 binders are prone to form crystalline phases as akermanite, nepheline and nosean at temperatures greater than 1000 °C, while 0-100 geopolymeric binders preserved mostly an amorphous structure even at 1200 °C with some traces of mullite. The dehydration of C-A-S-H and N-A-S-H altogether with the crystallization of the binder gel induced the formation of highly porous microstructures.

High temperature erosion behavior of plasma sprayed NiCrAlY/WC-Co/cenosphere coating

Abstract: High temperature erosive behavior of plasma sprayed NiCrAlY-25WC-Co/cenosphere coating deposited on MDN 321 steel is investigated in the present work. Coating is characterized using Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). Microhardness, porosity, adhesion strength, fracture toughness and ductility of the coating are quantified. Solid particle erosion test is conducted at 200, 400 and 600 °C with 30 and 90° impact angles using alumina erodent. Optical profilometer is used to evaluate erosion volume loss. Erosion resistance of the coating is observed to be higher than the substrate for the test temperatures chosen and noted to be more prominent at lower impact angle and higher temperature. High temperature stability of mullite, alumina and oxide layer assists in increasing erosion resistance of coating. The eroded coating surface morphology reveals the brittle mode of material removal.

Fabrication and oxidation resistance of mullite/yttrium silicate multilayer coatings on C/SiC composites

Abstract: A tri-layer coating of mullite/Y2Si2O7/(70wt%Y2Si2O7+30wt%Y2SiO5) was prepared on carbon fiber reinforced silicon carbide (C/SiC) composite substrate through dip-coating route for the sake of improving oxidation resistance of C/SiC composites. An Al2O3–SiO2 sol with high solid content was selected as raw material for mullite, and a slurry of Y2O3 powder filled silicone resin was used to synthesize yttrium silicate. The microstructure, phase composition, and oxidation resistance of the coating were investigated. The as-fabricated coating shows high density and favorable bonding to C/SiC substrate. After oxidation at 1400 and 1500 °C for 30 min under static air, the flexural strengths of coated C/SiC composite were both increased by ~30%. The desirable thermal stability and the further densification are responsible for excellent oxidation resistance. With the additional help of compatible thermal expansion coefficients among substrate and sub-layers in coating, the coated composite retained 111.2% of original flexural strength after 12 times of thermal shock in air from 1400 °C to room temperature. The carbothermal reaction at 1600 °C between free carbon in C/SiC substrate and rich SiO2 in mullite resulted in severe frothing and desquamation of coating and obvious degradation in oxidation resistance.

The impact of kaolin dehydroxylation on the porosity and mechanical integrity of kaolin based ceramics using different pore formers

Abstract: Thermally induced lattice defects in kaolin play a key role in the mechanical integrity of kaolin based ceramics during heat treatment due to their phase transformations. The effects of three types of pore formers (sawdust, styrofoam and powdery high density polyethylene) on the porosity and mechanical integrity of kaolin based ceramics have been studied based on their batch formulations. The porosity of the ceramic bodies was experimentally determined, while the structure and chemistry of the materials were elucidated via X-ray diffraction (XRD), scanning electron microscopy (SEM), FTIR, DSC/TG/DTA, and zeta potential. The kaolin-based porous samples sintered at 1150 °C exhibited mullite phase transformation attributed to recrystallization effects. Morphologically, open pores were distributed on the sample surfaces due to larger pathways and available channels of eviction of the pore formers. Compressive strength decreased linearly as apparent porosity increased signifying its correlation. The compressive strength showed to be much more sensitive to defects created by dehydroxylation as micro cracks were observed on the sample with HDPE as pore former which could be as a result of large volume change accompanying phase transformations and sensitivity to the dehydroxylation phase. It is noted that the new pore former (HDPE) used in this study produced ceramic bodies with porosity as high as 67% and hence the least compressive strength.

Investigation of high oxygen reduction reaction catalytic performance on Mn-based mullite SmMn2O5

Abstract: An alternative material SmMn2O5 mullite with regard to Pt/C is proposed to catalyze the oxygen reduction reaction (ORR) by combining density functional theory (DFT) calculations and experimental validations. Theoretical calculations are performed to investigate the bulk phase diagram, as well as the stability and electrocatalytic activity of the ORR under alkaline conditions for SmMn2O5 (001) surfaces, which are passivated by nitrogen atoms to avoid any spurious interference. The adsorptions of relevant ORR species (O*, OH*, OOH* and OO*) tend to compensate the coordination of manganese atoms to form Mn-centered octahedral or pyramidal crystal fields, and the corresponding binding energies fulfill a linear relationship. An oxygen molecule prefers to be reduced to OH−via a four-electron pathway and this prediction is verified by electrochemical measurements on the as-prepared SmMn2O5 catalyst with a nanorod structure. Volcano curves are obtained to describe the trends in theoretical ORR activity as a function of a single parameter, i.e. the oxygen binding energy. An overpotential of 0.43 V is obtained at the O* binding energy around 3.4 eV, which is close to the experimental observation (0.413 V) in this work. SmMn2O5 mullite exhibits favorable ORR activity and superior stability with only ∼5% decay in activity over 20 000 s of chronoamperometric operation in contrast to ∼15% decrease of Pt/C, making it a promising candidate for a cathode catalyst.

Microstructure and properties of mullite-based porous ceramics produced from coal fly ash with added Al 2 O 3

Abstract: Using coal fly ash slurry samples supplemented with different amounts of Al2O3, we fabricated mullite-based porous ceramics via a dipping-polymer-replica approach, which is a popular method suitable for industrial application. The microstructure, phase composition, and compressive strength of the sintered samples were investigated. Mullite was identified in all of the prepared materials by X-ray diffraction analysis. The microstructure and compressive strength were strongly influenced by the content of Al2O3. As the Al/Si mole ratio in the starting materials was increased from 0.84 to 2.40, the amount of amorphous phases in the sintered microstructure decreased and the compressive strength of the sintered samples increased. A further increase in the Al2O3 content resulted in a decrease in the compressive strength of the sintered samples. The mullite-based porous ceramic with an Al/Si molar ratio of 2.40 exhibited the highest compressive strength and the greatest shrinkage among the investigated samples prepared using coal fly ash as the main starting material.

Interactions between mullite saggar refractories and Li-ion battery cathode materials during calcination

Abstract: Used mullite-based saggars from a Li(NixCoyMnz)O2 (LNCM) materials-making plant were collected to characterize the degradation occurring during application. Based on the post-mortem analysis, laboratory scale experiments were designed using mixtures of mullite powders and commercial Li-ion battery cathode precursors. The mixtures were calcined at temperatures ranging from 800 °C to 1100 °C, and characterized with X-ray diffraction and scanning electron microscopy. The influence of temperature on the interactions between mullite and LNCM materials was determined. The combination of the post-mortem study and the laboratory scale tests allowed a more comprehensive understanding of the evolution of the saggar microstructure during degradation.

Fabrication and oxidation behavior of Al4SiC4 powders

Abstract: Al4SiC4 powders with high purity were synthesized by heating the powder mixture of aluminum (Al), silicon (Si), and carbon (C) at 1800°C in argon. The microstructure is characterized as platelike single grain. Both the nonisothermal and isothermal oxidation behavior of Al4SiC4 was investigated at 800°C-1500°C in air by means of thermogravimetry method. It is demonstrated that Al4SiC4 powder possesses good oxidation resistance up to 1200°C and is almost completely oxidized at 1400°C. At 800°C-1100°C, the oxide scales consist of an Al2O3 outer layer and a transition layer. Al4SiC4 remains the main phase. At 1200°C, some spallation resulting from the increment of Al2O3 and the mismatch of thermal expansion coefficient between different product layers can be observed. Above 1300°C, the oxide layer is composed of two part, i.e., large-scale Al2O3 crystals (outer layer) and mullite with less amount of SiO2 (inner layer). The oxidation behavior changes due to the different oxide products. For the reaction kinetics, a new kind of real physical picture model is adopted and obtains a good agreement with the experimental data. The apparent activation energy is calculated to be 176.9 kJ/mol (800°C-1100°C) and 267.1 kJ/mol (1300°C-1400°C).

Chemical cleaning of mullite ceramic microfiltration membranes which are fouled during oily wastewater treatment

Abstract: In this study, chemical cleaning of home-made mullite ceramic membranes which are fouled during oily wastewater treatment was investigated. The high performance and low cost mullite membranes were prepared by local low cost kaolin clay using extrusion method and the characteristic of the fabricated membranes was studied by different methods such as SEM, XRD, mean pore size and porosity analysis. Four types of chemical cleaning agents were selected for chemical in place cleaning of the fouled mullite membrane: Acid (sulfuric acid (H2SO4)), surfactant (sodium dodecyl sulfate (SDS)), chelating agent (ethylene diamine tetra acetic acid (EDTA)) and alkaline (sodium hydroxide (NaOH)). The fouled membranes were cleaned with single, binary and ternary solution of these chemical agents with the concentrations of 5 mM and 10 mM under the best operating conditions. After first cleaning step, membranes cleaned with vinegar and sodium bicarbonate solution as a novel chemical cleaning agent and named as second cleaning step. Results showed that by using single component chemical agent, EDTA and SDS with concentration of 5 and 10 mM were the best cleaning agents which have flux recovery about 31.265% and 57.778% respectively after two steps of cleaning. Binary solution of SDS + EDTA with the concentration of 5 mM was the best cleaning agent among binary and ternary cleaning solution agents which led to 41.802% and 65.163% flux recovery in the first and second cleaning steps of chemical clanging process respectively.