Algerian kaolinite used for mullite formation

In the present study, mullite was synthesized through reaction sintering of Algerian kaolinite and high purity alumina. The raw powders were wet ball milled in a planetary ball mill. Powders' morphology and the microstructure of the sintered samples were characterized by means of a scanning electron microscope. An X-ray diffractometer equipped with a heating facility and a differential thermal analyzer were used to follow mullite formation. Cylindrical specimens were produced by uniaxial cold compaction at a pressure of 75 MPa and sintered at different sintering temperatures for different sintering times. The heating rate was 10 °C/min. It was found that Algerian kaolinite was suitable for mullite production through reaction sintering with pure Al₂O₃. Formation of complete mullite occurred at 1550 °C. A relative density of 94% (of the theoretical density) was achieved at a relatively low sintering temperature of 1600 °C and a sintering time of 4 h.     

Effects of different potassium salts on the formation of mullite as the only crystal phase in kaolinite

The phase transition in kaolinite from 1050 °C to 1600 °C without and with different potassium salts (KF, KNO₃ and K₂SO₄) as mineralizers and the changes of the composition and morphologies of mullite formed in kaolinite have been investigated. The adding of enough potassium has been found to inhibit the formation of cristobalite in kaolinite. The fluorine is found to be beneficial to increase the reaction activity of aluminium from metakaolinite and so the potassium fluoride has promoted more pseudotetragonal mullite formed at 1100 °C than the other two mineralizers. The influence of potassium salts on the composition and microstructure of mullite formed from kaolinite with increasing the heating temperature has been scrutinized in detail. The formation condition for the only crystal phase of mullite from kaolinite has been shown.     

Mechanical properties of textured ceramics from muscovite–kaolinite alternate layers

An organized network of mullite anisotropic crystals embedded in a silico-aluminate matrix material is obtained at interfaces of sintered alternate layers of muscovite and kaolinite minerals. The nucleation and growth of mullite anisotropic crystals occur preferentially along the muscovite basal planes through topotactic reaction with the high temperature form of muscovite.Simultaneously to structural transformations, dehydroxylation of muscovite induces an exfoliation process, which is temperature and time dependent. The kinetics of this process was controlled using an appropriate thermal cycle and uniaxial load. During sintering, the control of mullite size mainly depends in temperature and the addition of a small quantity of low-temperature liquid phase also favors the growth of mullite. But liquid induces the weakening of the organization degree of the mullite network. Quantitative texture analysis (QTA) and SEM were used to characterize microstructural characteristics.Flexural strength, Young modulus and fracture toughness are closely related to size and organization degree of the mullite network. In general, mullite length favors the increase of strength and fracture toughness. But a high organization degree of the mullite network favors the occurrence of interconnected crystals and increases mechanical properties.     

Preparation of mullite by the reaction sintering of kaolinite and alumina

In the present study, mullite specimens and mullite/alumina composites are prepared by reaction sintering kaolinite and alumina at a temperature above 1000°C. The phase and microstructural evolution of the specimens and their mechanical properties are investigated. Primary mullite appears at a temperature around 1200°C. The alumina particles are inert to the formation of primary mullite. Alumina starts to react with the silica in glassy phase to form secondary mullite above 1300°C. The formation of secondary mullite decreases the amount of glassy phase. Furthermore, the addition of alumina reduces the size of mullite grains and their aspect ratio. The strength and toughness of the resulting mullite increase with the increase of alumina content; however, the mechanical properties of the mullite and mullite/alumina composites are lower than those of alumina for their relatively low density.     

Effect of topaz on the synthesis of mullite in mixtures of kaolinite and alumina

The synthesis of mullite in mixtures of kaolinite and alumina in the presence of small amounts of topaz is studied. Topaz is shown to activate the synthesis of both the primary (from kaolinite) and secondary mullite (through binding silica produced by thermal degradation of kaolinite), decrease the temperature of synthesis by 100°C, and increase the total yield of mullite. Products of the thermal degradation of topaz — mullite and gaseous chemically active fluoride compounds — are shown to play the role of mineralizing agents.Translated from Novye Ogneupory, No. 8, pp. 49 – 53, August, 2004.For previous communication, see Novye Ogneupory, No. 7, 41 – 46 (2004).     

Conventional- versus microwave-hydrothermal leaching of glass from sintered kaolinite to make porous mullite

A microwave-hydrothermal treatment with 1 and 5N-NaOH solution was used at 150°C and 190°C to dissolve excess silica glass and thus make porous mullite ceramics from a fired New Zealand kaolin honeycomb. The effect of microwave-hydrothermal (M-H) treatment time on the dissolution of the glass was examined and compared to the result of conventional-hydrothermal (C-H) treatment. As expected, the rate of the dissolution of glass was faster with M-H treatment compared to C-H treatment. The dissolution of 40–43% glass was almost complete after M-H treatment in 1N-NaOH for 6 hrs at 150°C, in¹N-NaOH for 3 hrs at 190°C, in⁵N-NaOH for 1.5 hrs at 150°C, and in 5N-NaOH for 35 minutes at 190°C. When the M-H treatment in 5N-NaOH solution at 190°C was more than 45 minutes, nonporous prismatic crystals were formed on the honeycomb and these crystals decreased the specific surface area. There were no changes in the morphology of mullite whiskers or in the structure of porous mullite body after M-H treatments in NaOH solutions compared to that of C-H treatment. The M-H treatment has been shown to be a rapid technique to prepare porous mullite ceramics from fired kaolin honey comb.     

Anisotropic kinetic of the kaolinite to mullite reaction sequence in multilayer ceramics

Multilayer ceramics with a composite and organized microstructure were realized from kaolin and alumina fibers to improve strength and fracture toughness. Dilatometry experiments along 3 directions reveal anisotropic shrinkages, which are in correlation with different activation energy for sintering. Mullite growth is strongly anisotropic, inducing the formation of an organized microstructure, where larger mullite crystals are mainly oriented in plane of layer and perpendicular to alumina fibers. Kinetic data from thermal transformations show that the starting reaction mechanism is mullite nucleation, and it is continued by a strongly anisotropic grain growth. It is explained by topotactic transformations at phyllosilicate faces and along alumina arrangements. Mullite growth kinetics is also favored perpendicularly to fiber main dimension by the anisotropy of alumina diffusion coefficient. It shows the limited importance of mullite crystallization in microstructural transformation, but it also shows that controlled mullite growth is central in microstructural arrangement.     

Effect of seeds on the formation of sol-gel mullite

Mullite precursor gels have been prepared from a mixture of particulate boehmite sol and tetraethoxysilane at a pH of 4. The sol has been seeded with submicron size -Al₂O₃, γ-Al₂O₃ and mullite limited to two percent by weight of the mullite phase. These gels have been invesitgated with respect to phase formation, densification and microstructural development. Dilatometric studies on the seeded precursor gels indicate marked shrinkage in their profiles. Of the different seed nuclei, fine mullite particles have shown excellent influence in the early ceramic phase formation as well as densification. Further they induce complete transformation of the precursor gel to the high temperature phase as well as a uniform fine-grained microstructure.     

Sintering and microstructural study of mullite prepared from kaolinite and reactive alumina: Effect of MgO and TiO2

Mullite ceramic was prepared using kaolinite and synthesized alumina (combustion route) by solid-state interaction process. The influence of TiO₂ and MgO additives in phase formation, microstructural evolution, densification, and mechanical strengthening was evaluated in this work. TiO₂ and MgO were used as sintering additives. According to the stoichiometric composition of mullite (3Al₂O₃·2SiO₂), the raw materials, ie kaolinite, synthesized alumina, and different wt% of additives were wet mixed, dried, and uniaxially pressed followed by sintering at different temperature. 1600°C sintered samples from each batch exhibit enhanced properties. The 1 wt% TiO₂ addition shows bulk density up to 2.96 g/cm³ with a maximum strength of 156.3 MPa. The addition of MgO up to 1 wt% favored the growth of mullite by obtaining a density and strength matching with the batch containing 1 wt% TiO₂. These additives have shown a positive effect on mullite phase formation by reducing the temperature for complete mullitization by 100°C. Both additives promote sintering by liquid phase formation. However, the grain growth, compact microstructure, and larger elongated mullite crystals in MgO containing batch enhance its hardness properties.     

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

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.     

Influences of mullite fibers on mechanical and thermal properties of silica-based ceramic cores

Mullite fibers composite silica-based ceramic cores were successfully prepared by injection molding. The effects of mullite fibers on the mechanical and thermal properties of ceramic cores were investigated. The results indicated that the linear shrinkage was significantly decreased and the porosity was gradually increased with the increase of mullite fibers. In addition, the flexural strength for the room temperature and the simulated casting temperature of 1500°C was increased to a maximum value when the content of mullite fibers was about 1 wt.%, and then decreased with the increase of mullite fibers. The mullite fibers of 1 wt.% presented excellent mechanical properties with a linear shrinkage of .65%, a porosity of 6.96%, and a flexural strength of 17 MPa at room temperature and 34.83 MPa at the simulated casting temperature of 1500°C. Besides, the change in microstructure and properties in various contents of mullite fibers were analyzed.     

Effect of MgO in the microstructure formation of zirconia mullite composites from sillimanite and zircon

The effect of MgO additive on the structural, microstructural and hardness properties of zirconia mullite (MUZ) has been discussed. The MgO additive in MUZ not only stabilizes the cubic zirconia phase but also acts as a sintering aid for the formation of cross-linked mullite grains. The electron micrographs of plasma fused MgO–MUZ shows a uniform arrangement of platelet structure of mullite and dendrite structure of zirconia on mullite surface. The micrograph of plasma sintered composites shows a ladder like structure and a complete cross-linked mullite grains whereas the surface morphology of conventionally sintered composites clearly indicates the presence of small and big grains close packed to each other. Appreciable hardness and higher optical band gap have been observed for plasma fused MgO–MUZ composites. A complete dissociation of sillimanite and zircon has been occurred in plasma fused composites for the complete conversion of MUZ whereas the complete dissociation of sillimanite and zircon has not observed in plasma sintered and conventionally sintered composites. These observations have been realized from the X-ray diffraction and Fourier transform infrared studies.     

X-ray diffraction microstructure analysis of mullite,quartz and corundum in porcelain insulators

The X-ray diffraction microstructure analysis has been performed on commercial samples of the silica and alumina porcelain insulators obtained at 1300 °C, with the same time of firing. The study was carried out on mullite, corundum and quartz by applying several integral breadth methods (i.e. the Williamson–Hall analysis, the Langford method and the Halder–Wagner approximation) and the Fourier analysis (Warren–Averbach method). The apparent crystallite sizes determined for the mullite are direction-dependent (anisotropic) and within each group of samples, on average, the greatest values are obtained along the direction [0 0 1]. With regard to the microstructure of the corundum and the quartz, there are little differences between the two groups of samples. Considering all samples on average, the crystallite sizes follow the order corundum > mullite > quartz. These microstructural data were related with the mechanical strength and with the chemical and mineralogical composition of the samples. Due to similar conditions of formation of the porcelains studied, the content of corundum seems to be the principal factor influencing their flexural strength, coinciding with small differences of crystalline microstructure.     

Phase formation and crystallization kinetics in cordierite ceramics prepared from kaolinite and magnesia

In this work, Algerian kaolinite, a naturally occurring clay mineral, was used as low-cost precursor for the synthesis of cordierite ceramics. The kaolinite was mixed with synthetic magnesia, and the mixture was ball milled and reaction sintered in the temperature range 900–1350 °C for 2 h. Thermogravimetry (TG), differential thermal analysis (DTA), dilatometry, high temperature x-ray powder diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM) complementary techniques were used to analyze sintering behavior, characterize phase transformations, and investigate crystallization kinetics. Milling the kaolinite and magnesia mixture for 10 h yielded a homogenous powder, decreased the average particle size, and improved the roundness of particles. Different crystalline phases were present in the samples sintered in the temperature range 900–1150 °C, the cordierite phase started to crystallize at 1200 °C, and the formation of highly dense cordierite (99%) was complete at 1250 °C. The activation energy values for cordierite formation calculated using Kissinger, Boswell, and Ozawa methods were found to be equal to 577, 589, and 573 kJ/mol, respectively. The kinetic parameters n and m had values close to 2. Bulk nucleation with a constant number of nuclei was the dominant mechanism in cordierite crystallization, followed by two-dimensional growth controlled by interface reaction.     

Kinetic of mullite formation from a porcelain stoneware body for tiles production

The growth of mullite (3Al₂O₃·2SiO₂) in a porcelain stoneware body for tiles production has been investigated using differential thermal analysis (DTA). The activation energy calculated by both isothermal and non-isothermal treatments is 599 and 622 kJ mol⁻¹, respectively. The growth morphology parameters n and m are both about 1.5 indicating that bulk nucleation is dominant in mullite crystallisation followed by three-dimensional growth of mullite crystals with polyhedron-like morphology controlled by diffusion from a constant number of nuclei. The frequency factor calculated by the isothermal treatment is equal to 8.21 × 10²² s⁻¹.     

Effect of TiO2 on the mullite formation and mechanical properties of alumina porcelain

The effect of adding TiO₂ to standard alumina porcelain on its microstructure and flexural strength was investigated. A series of alumina porcelain bodies containing increasing amounts of TiO₂ were prepared by extruding mixtures of raw materials and TiO₂. Porcelain rods were fired under industrial scheduling in a manufacturing kiln. The overall degree of crystalline and amorphous phase content within the porcelain bodies was quantitatively determined using a Rietveld analysis. Results indicated a higher amount of mullite formation in porcelain bodies containing TiO₂. Examination of the product materials using field emission scanning electron microscopy showed a high density of secondary mullite crystals present in the earlier feldspar grain areas of specimens with TiO₂. Energy dispersive X-ray analysis of secondary mullite crystals revealed that Ti⁴⁺ enters into the secondary mullite structure forming a Ti⁴⁺-mullite solid solution. Assessment of the mechanical properties of the TiO₂ containing bodies indicated that small addition raises the flexural strength of the standard porcelain. The improvement in mechanical properties could be associated with an increase of both specimen density and relative content of types II and III secondary mullites. Both observations may be attributed to a decrease in the viscosity of melted feldspar grains, which in turn favours the nucleation and growth of secondary mullite crystals and thus increases the final density of porcelain bodies.     

氧化钛对氧化铝瓷的莫来石形成和力学性能的影响

研究了添加氧化钛对标准氧化铝瓷的显微结构和抗折强度的影响。通过挤压原料和TiO2的混合料,制备了含有不同量氧化钛的系列氧化铝瓷,在生产窑内按照工业化制度烧成瓷棒。定量分析了瓷件内的晶化程度和非晶相含量。结果表明,含有TiO2的瓷件内形成了大量莫来石。用场发射扫描电镜分析材料,表明含有TiO2的试样在早期长石颗粒区域出现了密度较高的二次莫来石晶体。测量含TiO2瓷件的力学性能,表明加入少量能提高标准瓷件的抗折强度。