Mullite Crystals Research Articles

Thermal diffusivity of kaolinite–mullite ceramic matrix composite with silicon nitride nanoparticle filler

Thermal diffusivity of mullite fused ceramics was determined from room temperature up to 500°C at intervals of 100°C. The ceramic material was formed by combining kaolinite as the matrix and silicon nitride as the filler powder. The ceramics were sintered at 1000°C, 1100°C, and 1200°C to study the structural differences. Thermal diffusivity data showed that the addition of silicon nitride affects the composites heat flow and this behavior is noticeable at 1100°C and 1200°C samples. The thermal diffusivity data was plotted on 1000/T graph to demonstrate a linear relation for filler effects of silicon nitride on the mullite matrix. Additions of 25% and 30% silicon nitride sintered at 1200°C exhibited thermal diffusivity values lower than pure mullite crystal at 0.55–0.45mm2/s. These values suggest that heat conduction across the system was interrupted by silicon nitride nanoparticle additions via micropore formations. These samples have potential applications for high temperature thermal insulation.

Effect of cobalt oxide additive on the fired properties of tri-axial ceramic

Fired properties of a tri-axial ceramic system were studied in presence of 1–5wt% cobalt oxide (CoO) additive as a function of firing temperatures in the range of 1100–1300°C. The results showed significant improvement in physical and mechanical properties of the batches in presence of 1% CoO additive compared to that of batches without additive in the firing temperature range of 1100–1200°C. Phase analysis showed that 1% CoO additive improved melting of feldspar and enhanced mullitization. Microstructural analysis confirmed that the morphology of the mullite crystals changed significantly in presence of additive at lower temperature. 1% CoO here acted as a good additive and helped to achieve optimum fired properties at a comparatively lower temperature. Except at 1100°C fired batch, the presence of higher quantum of additive (5%) rather deteriorated the material properties.

Recycling of Coal Fly Ash for the Fabrication of Porous Mullite/Alumina Composites.

Coal fly ash with the addition of Al2O3 was recycled to produce mullite/alumina composites and the camphene-based freeze casting technique was processed to develop a controlled porous structure with improved mechanical strength. Many rod-shaped mullite crystals, formed by the mullitization of coal fly ash in the presence of enough silicate, melt. After sintering at 1300–1500 °C with the initial solid loadings of 30–50 wt.%, interconnected macro-sized pore channels with nearly circular-shaped cross-sections developed along the macroscopic solidification direction of camphene solvent used in freeze casting and a few micron-sized pores formed in the walls of the pore channels. The macro-pore size of the mullite/alumina composites was in the range 20–25 μm, 18–20 μm and 15–17 μm with reverse dependence on the sintering temperature at 30, 40 and 50 wt.% solid loading, respectively. By increasing initial solid loading and the sintering temperature, the sintered porosity was reduced from 79.8% to 31.2%, resulting in an increase in the compressive strength from 8.2 to 80.4 MPa.

Synergistic Effect of Soda-Lime-Silica Glass and Porcelain Scrap on the Vitrification Behavior of Porcelanized Stoneware Tile

Porcelain scrap, a by-product from the table ware industry, was gradually added to a soda-lime-silica scrap-glass-containing traditional porcelanized stoneware tile body replacing sand. The effect of this addition on the vitrification and physico-mechanical properties was studied by measuring the linear shrinkage, bulk density, apparent porosity, and flexural strength of the samples heated in the temperature range 1100–1250°C. The results revealed that the presence of both glass and porcelain scrap had beneficial effects in relation to vitrification at lower temperatures and development of higher flexural strength. The improvement in strength was mainly because of the formation of well-developed needle-shaped mullite crystals and stronger pre-stress caused by the difference in thermal expansion coefficient between the glassy matrix, quartz, and mullite during the cooling process. Circumferential cracking around quartz grains was observed in quartzitic sand-containing samples. XRD studies were also carried out to determine differences in phase evolution.

Phase Composition of Ceramic Brick from the Kremlin in Astrakhan

Investigations have shown that the strength of the ceramic brick in the Astrakhan kremlin is mainly due to mullite. The crystallization of mullite in the experimental specimen shows that the firing temperature of the brick was less than 1000 – 1050 °C.

High-temperature behavior of Al-doped polymer-derived SiAlOC glasses in air environment

Aluminum-containing silicon oxycarbide (SiAlOC) glasses have been obtained through pyrolysis in argon atmosphere at 1000°C of a polymethyl(phenyl)siloxane resin and aluminum tri-sec-butoxide-derived siloxane networks. To investigate their high-temperature behaviors in air environment, these glasses were further annealed at 1000, 1200, 1400, 1600, and 1700°C. It was found that the redistribution reactions between Si–O and Si–C bonds initiated at 1000°C and almost finished at 1200°C; oxidation of the SiAlOC glasses occurred at temperature above 1000°C; and the crystallization of mullite took place at 1400°C due to the inclusion of Al ion into the silica network to form Si–O–Al linkages.

Thermal Reaction of Cristobalite in Nano‐SiO2/α‐Al2O3 Powder Systems for Mullite Synthesis

Nanoscaled cristobalite and α‐Al2O3 powders were used as the starting materials for synthesizing mullite by solid‐state reaction. The thermal reaction of the cristobalite with α‐Al2O3 during the thermal treatment was examined. Cristobalite powder with a D50 value of 430 nm was adopted to mix with α‐Al2O3 powders with a D50 values of 230, 310, and 400 nm in a stoichiometric composition of 3Al2O3∙2SiO2 (71.8 wt% α‐Al2O3 and 28.2 wt% SiO2). Samples for thermal reaction were prepared using uniaxial pressed from the three mixtures that showed various particle number ratios of SiO2/Al2O3 due to the different particle sizes of α‐Al2O3. Examinations were performed by differential thermal analysis, X‐ray diffraction, scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, and transmission electron microscopy techniques. The results showed that cristobalite particles amorphized during the thermal treatment, and then reacted with the α‐Al2O3 particle to form mullite via nucleation and growth. The amorphization temperature can be reduced by using finer‐sized α‐Al2O3 powders, thus leading to a lower temperature for mullite formation. Mullite crystals with a multidomain structure were observed in the α‐Al2O3 particle matrixes. The crystal orientation of the mullite was controlled by the α‐Al2O3 matrix, that is, [001] α‐Al2O3 → [001] mullite. These results indicate that the amorphization of cristobalite may trigger the reaction of SiO2 with α‐Al2O3, initiating the nucleation of mullite. The α‐Al2O3 particles act as the hosts for mullite formation and determine the size of the mullite particles.

Recycling of waste fly ash for production of porous mullite ceramic membrane supports with increased porosity

Abstract Low-cost porous mullite ceramic membrane supports were fabricated from recycling coal fly ash with addition of natural bauxite. V 2 O 5 and AlF 3 were used as additives to cause the growth of mullite crystals with various morphologies via an in situ reaction sintering. Dynamic sintering, microstructure and phase evolution of the membrane supports were characterized in detail and open porosity, pore size, gas permeation and mechanical properties were determined. It showed the membrane support with 3 wt.% V 2 O 5 and 4 wt.% AlF 3 addition exhibits an open porosity of ∼50%, mechanical strength of 69.8 ± 7.2 MPa, an interlocking microstructure composed of anisotropically grown mullite whiskers with an aspect ratio of 18.2 ± 3.6 at 1300 °C. Addition of more V 2 O 5 lowered the secondary mullitization temperature, resulting in more mullite formation at lower temperatures. The fabricated membrane supports feature high porosity without mechanical strength degradation, possible strengthening mechanism of the mullite whiskers was further discussed.

Environment-oriented low-cost porous mullite ceramic membrane supports fabricated from coal gangue and bauxite

Porous mullite ceramic supports for filtration membrane were successfully fabricated via recycling of coal gangue and bauxite at sintering temperatures from 1100 to 1500°C with corn starch as pore-forming agent. The dynamic sintering behaviors, phase evolution, shrinkage, porosity and pore size, gas permeation flux, microstructure and mechanical property were systematically studied. A unique volume-expansion stage was observed at increased temperatures from 1276 to 1481°C caused by a mullitization-crystal-growth process. During this stage, open porosity increases and pore size distributions broaden, which result in a maximum of nitrogen gas flux at 1400°C. The X-ray diffraction results reveal that secondary mullitization took place from 1100°C and the major phase is mullite with a content of ∼84.7wt.% at 1400°C. SEM images show that the as-fabricated mullite supports have a porous microstructure composed of sintered glassy particles embedded with inter-locked mullite crystals, which grew gradually with increasing temperature from rod-like into blocky-like morphologies. To obtain mullite membrane supports with sufficient porosity and acceptable mechanical strength, the relationship between porosity and mechanical strength was investigated, which was fitted using a parabolic equation.

Influence of Phosphate-Substituted Flux on Microstructure and Technological Properties of White Porcelain Stoneware Tiles

Calcium phosphate in the range of 3 to 10 mass-% partially substituted soda feldspar in five recipes in an attempt to produce white porcelain stoneware tiles. The added phosphate had a double effect on the behavior of the ceramic body during firing. The penta-valent phosphorous accommodated into the tetrahedron co-ordination of the glass melt and lowered its viscosity, leading to the early dissolution of the primary mullite and the crystallization of acicular secondary mullite, as well as fine grained nano-anorthite grains. The calcium ion played its role in increasing the degree of whiteness of the body.

Ionic conductivity of directionally solidified zirconia–mullite eutectics

Abstract The properties of directionally solidified eutectic (DSE) zirconia–mullite composites are presented. These materials combine two oxide ion conductors in eutectic microstructures stable over broad temperature (570–1415 °C) and pO2 (10− 20 to 105 Pa) ranges, which are pertinent for their application as high temperature (> 1200 °C) solid electrolytes, e.g. in Nernstian sensors. X-ray diffraction and scanning electron microscopy/energy dispersive spectroscopy results reveal a composite structure comprising eutectic crystals of mullite and zirconia, and an intergranular amorphous phase rich in Al–Y–Si–O. The amorphous phase is crystallised upon annealing at 1400 °C, and the resulting composites have a composition close to the nominal eutectic consisting of 79 vol.% mullite and 21 vol.% zirconia. The electrical conductivity of these materials is rationalized in terms of percolation, fraction and properties of each phase, attaining values in excess of 0.01 S/cm at 1370 °C and displaying Arrhenius behaviour with activation energy of 70 kJ/mol. The broad electrolytic domain of these solid electrolytes is demonstrated by conductivity measurements carried out from 600 °C to 1370 °C in O2, air, Ar and 10%H2 + 90%N2 atmospheres.

The non-isothermal kinetics of mullite formation in boehmite–zircon mixtures

In this work, we studied the kinetics of mullite formation in different composites under non-isothermal conditions using DTA. Different composites based of mullite, alumina, zircon and zirconia were prepared by reaction sintering of boehmite (as alumina source) and zircon. Several mixtures were used while varying the percentage of the boehmite from 30 to 70 mass% with a step of 10. Five compositions marked as B30, B40, B50, B60 and B70 corresponding to boehmite–zircon ratios (mass%) of 30/70, 40/60, 50/50, 60/40 and 70/30 were fabricated and studied. The DTA conducted at heating rates of 10, 20 and 30 K min−1 showed an endothermic peak in all composites at about 1,603 K associated with mullite formation. The activation energies measured from non-isothermal treatments for five compositions (30, 40, 50, 60 and 70 mass% of boehmite) were 1,029, 1,085, 1,262, 1,508 and 1,321 kJ mol−1, respectively. The n values (Avrami parameter) for all compositions are larger than 2.5, the mullite crystallization of these composites is followed by three-dimensional growth.

The Preparation of Porous Andalusite Refractory by Foaming Method

Andalusite can be used to produce porous refractory with the method of foaming and its properties are very good. The bulk density can be controlled by the quantity of foam easily. The compressive strength can reach almost 9 Mpa (1.0g/cm3). The expansion transmit by pores, the remianing andalusite and the transverse growing of mullite crystal are all good for the stabilization of sample during heating. The agglomeration of foam is bad for foaming slurry while stiring, so one way to well-distribute the pores is enhancing the strength of foams. Andalusite can prolong the lifetime of refractory.

Characteristics of phase formation in Angren deposit clays melted with a solar furnace

The effect different methods of thermoprocessing have on mullite formation in different clay materials is examined. It is shown that a concentrated radiant flux lowers the temperature of mullite crystallization by 100–150°C, while correlations between features of mullite formation and different admixtures go unchanged.

Mechanical and oxidation resistance properties of 3D carbon fiber-reinforced mullite matrix composites prepared by sol–gel process

The 3D carbon fiber-reinforced mullite matrix (3D-Cf/mullite) composites were fabricated via sol–gel process, using mullite sols with high solid content as precursor. The mullitizaiton process of mullite sols was investigated with XRD and TEM. The results showed that the sol was transformed into orthorhombic mullite at 1300°C with a trace of amorphous SiO2 between crystals of mullite. The flexural strength, fracture toughness and interlaminar shear strength of 3D-Cf/mullite composites were 277.6MPa, 12.3MPam1/2 and 36.3MPa, respectively. The 3D-Cf/mullite composites exhibited a significant tough fracture behavior. The oxidation resistance of 3D-Cf/mullite composites was examined. It can be found that the strength retentions of 3D-Cf/mullite composites were 62.9%, 84.8% and 13.6% after oxidation at 1300°C, 1500°C and 1700°C, respectively. The strength transformation was interpreted with the microstructure analysis of oxidized 3D-Cf/mullite composites at different temperatures.

Assessment of two clayey materials from northwest Sardinia (Alghero district, Italy) with a view to their extraction and use in traditional brick production

Two clayey raw materials collected in the Porto Ferro and Lago Baratz areas of Alghero in northwest Sardinia (Italy) were used to prepare handmade brick samples that were fired under oxidizing conditions at temperatures ranging between 750 and 1000°C to evaluate their possible use in the brickmaking industry. Both raw materials are rich in quartz and phyllosilicates but only the sample from Porto Ferro contains calcite. Granulometrically, the sample from Porto Ferro is a silty sand and shows higher plasticity than the clay from Lago Baratz which is classified as a sand. After firing, the samples acquire a red-orange colour and undergo significant mineralogical and textural changes. Phyllosilicate depletion is accompanied by the crystallisation of mullite and, only in Porto Ferro samples, the breakdown of calcite is followed by the formation of gehlenite, wollastonite and anorthite. Optical and electronic microscopic observations revealed that the temper grains are larger and more abundant in the bricks from Lago Baratz than in those from Porto Ferro. As firing temperature increased, pores became ellipsoidal in shape and the matrix became vitrified. The samples from Lago Baratz also show fissures at grain boundaries, which indicate the formation of an extensive network of pores and make these bricks very brittle when fired at low temperatures (750 and 800°C), so much so that hydric tests could not be performed on them. Bricks produced with the clayey material from Porto Ferro achieved the best results in terms of hydric behaviour, with a reduction in the water absorption capacity, and in terms of compactness, with an increase in ultrasound wave velocity. However, both Porto Ferro and Lago Baratz bricks behaved in similar ways when submitted to the salt crystallisation test, especially at 1000°C when a sufficient degree of vitrification was reached to ensure high quality samples.

Time-resolved powder neutron diffraction study of the phase transformation sequence of kaolinite to mullite

Abstract Mullite formation from kaolinite was studied by means of high-temperature in situ powder neutron diffraction by heating from room temperature up to 1370 °C. Neutron diffractometry under this non-isothermal conditions is suitable for studying high-temperature reaction kinetics and to identify short-lived species which otherwise might escape detection. Data collected from dynamic techniques (neutron diffraction, DTA, TGA and constant-heating rate sintering) were consistent with data gathered in static mode (conventional X-ray diffraction and TEM). The full process occurs in successive stages: (a) kaolinite dehydroxylation yielding metakaolinite in the ∼400–650 °C temperature range, (b) nucleation of mullite in the temperature range ∼980–992 to ∼1121 °C (primary mullite) side by side with a crystalline cubic phase (Si-Al spinel) detected in the ∼983–1030 °C temperature interval; (c) growth of mullite crystals from ∼1136 °C, (d) high (or β) cristobalite crystallization at T > ∼1200 °C and (e) secondary mullite crystallization at T > ∼1300 °C. The calculated activation energy for the kaolinite dehydration was 115 kJ/mol; for the mullite nucleation was 278 kJ/mol and for the growth of mullite process was 87 kJ/mol; finally for cristobalite nucleation the calculated apparent activation energy was 481 kJ/mol.

Complex growth structures of mullite after electrochemically induced nucleation

Glasses with the mol% composition 52.6 SiO2·18.7 Al2O3·14.3 MgO·7.7 TiO2·4.7 B2O3·2.0 CaO were crystallized using electrochemically induced nucleation. During this process, a voltage is applied between a platinum wire and a platinum crucible containing the melt heated to 1330 °C. At the platinum wire, which acts as the cathode, Ti4+ is reduced to Ti3+, leading to a local nucleation of mullite crystals which grow in the glass melt. The square needles of 1–50 μm diameter exhibit complex growth structures including very fine tips surrounded by halos and various stages of needle fragmentation. They are surrounded by an Al/Si-diffusion barrier of up to 7 μm thickness and a dark frame where SiO2 is enriched. It is proposed that the stepwise diameter increase of the needles is mainly initiated by dendritic growth localized to tips formed at the edges of the square needles. The presented results provide visual examples on how crystals manipulate their environment during growth which in turn affects the growth of the crystals.

Influence of Ethylene Glycol on the Mullite Crystallization Processes Analyzed by Rietveld Refinement

Resumen en: Mullite is an excellent structural material due to its high temperature stability, high electrical insulation capabilities and creep resistance. This mat...

Sol-Gel Synthesis of Mullite Starting from Different Inorganic Precursors

Using silicotetraetilortosilicate (TEOS) mixed with aluminum tri-sec-butoxide (TSBAI) or aluminum cloaures mullite ceramics were created by the sol-gel method. The quantities used of each substance were those that led to obtain stoichiometric mullite (3Al2O3·2SiO2). The experimental methodology used for obtaining mullite consisted in: sol-gel synthesis of precursor materials, isothermal treatment of those materials, and characterization of resulting materials. In order to determine the advance of reactions during mullite formation, isothermal treatments between 300°C and 1600°C were performed, keeping the samples at each temperature during 4 h. From XRD results, it may be said that precursor powders originally amorphous start to crystallize in Al2O3 and SiO2 at 1200°C, and the mullite formation starts at 1200°C, with being completed at 1600°C. The use of TSBAI favors the formation of mullite crystals at lower temperature. From SEM observations a microstructure that presents primary mullite with randomly oriented grains of secondary mullite with acicular shapes and sizes that range between 1.25 and 1.50 μm long may be determined.