Journal ArticleDOI
Structure and properties of mullite—A review
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TLDR
Mullite has achieved outstanding importance as a material for both traditional and advanced ceramics because of its favourable thermal and mechanical properties as discussed by the authors. But it is not a suitable material for many applications.Abstract:
Mullite has achieved outstanding importance as a material for both traditional and advanced ceramics because of its favourable thermal and mechanical properties. Mullite displays various Al to Si ratios referring to the solid solution Al 4+2 x Si 2−2 x O 10− x , with x ranging between about 0.2 and 0.9 (about 55 to 90 mol% Al 2 O 3 ). Depending on the synthesis temperature and atmosphere mullite is able to incorporate a number of transition metal cations and other foreign atoms. The crystal structure of mullite is closely related to that of sillimanite, which is characterized by chains of edge-connected AlO 6 octahedra running parallel to the crystallographic c -axis. These very stiff chains are cross-linked by tetrahedral chains consisting of (Al,Si)O 4 tetrahedra. In more detail: Parallel to a the tetrahedra are linked to the relatively short more stiff Al–O(A, B) bonds, whereas parallel b they are linked parallel to the relatively long more compliant Al–O(D) bonds. In mullite some of the oxygen atoms bridging the tetrahedra are removed for charge compensation. This gives rise to the formation of oxygen vacancies and of T 3 O groups (so-called tetrahedral triclusters). The anisotropy of the bonding system of mullite has a major influence on the anisotropy of its physical properties. For example: • the highest longitudinal elastic stiffness is observed parallel c , but lower ones parallel a and especially parallel b , • the maximum of the thermal conductivity occurs parallel c , but maller ones parallel a and especially parallel b , • large thermal expansion especially parallel b , • fastest crystal growth and highest corrosion parallel c . Heat capacity and thermal expansion measurements of mullite display reversible anomalies in the temperature range between about 1000 and 1200 °C. It is believed that tetrahedral cations, bridging O atoms, and O vacancies undergo dynamical site exchange processes at high temperatures. At lower temperatures the dynamic disorder may transform to a static one. Diffraction experiments revealed that also partially ordered states may exist.read more
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Journal ArticleDOI
Mechanical Behavior of Air Plasma-Sprayed YSZ Functionally Graded Mullite Coatings Investigated via Instrumented Indentation
C. V. Cojocaru,Y. Wang,Christian Moreau,Rogerio S. Lima,J. Mesquita-Guimarães,E. Garcia,Pilar Miranzo,María Isabel Osendi +7 more
TL;DR: In this article, two different types of mullite powder (spray-dried and freeze-granulated) and a mullite-YSZ 75/25 vol.% mixture spraydried powder were employed to investigate the role of powder characteristics on the mechanical behavior of air plasma-sprayed mullite bond coats deposited on SiC substrates.
Journal ArticleDOI
Fabrication and characterization of affordable hydrophobic ceramic hollow fibre membrane for contacting processes
M. Abdulhameed,Mohd Hafiz Dzarfan Othman,Haider Nadhom Azziz Al Joda,Ahmad Fauzi Ismail,Takeshi Matsuura,Zawati Harun,Mukhlis A. Rahman,Mohd Hafiz Puteh,Juhana Jaafar +8 more
TL;DR: In this article, the authors studied the changes in the properties of the hollow fibre membranes (gas permeation, mechanical strength, pore size, porosity, tortuosity, morphology, and contact angle) by the addition of alumina (Al2O3) to the pure kaolin with mono or multiparticle sizes.
Journal ArticleDOI
Recycling of Coal Fly Ash for the Fabrication of Porous Mullite/Alumina Composites.
TL;DR: 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.
Journal ArticleDOI
Template free synthesis of highly ordered mullite nanowhiskers with exceptional photoluminescence
TL;DR: An efficient method to produce highly ordered mullite nanowhiskers using B 2 O 3 -doped molten salt synthesis was reported in this article, where the morphology and optical properties were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), TEM, high-resolution transmission electron microscope (HRTEM) and photoluminescence (PL) spectroscopy.
Journal ArticleDOI
Mullite crystallization kinetics of lanthanum doped sol–gel derived precursors
TL;DR: In this paper, the phase separation in two microphases occurs in the gels, the first microphase was alumina-rich and the second silica-and lanthanum-rich.
References
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Book
Petrogenesis of metamorphic rocks
TL;DR: In this paper, the four divisions of metamorphic grade are defined: very low grade, medium grade, high grade and low grade metamorphism, and the change from low grade to medium grade to high grade.