Papers by Keyword: Mullite

Abstract: Efforts to reduce the usage of imported raw materials in the ceramic industry have encouraged usage of local calcite minerals for production of refractory ceramic products. Corundum-mullite ceramics as refractory products were prepared by using clay, alumina and calcite mineral (CaO) via powder pressing method. The effect of CaO addition from 2.9 to 6.5 wt.% (R1, R2 & R3) on the mechanical strengths such as bulk density, apparent porosity, firing shrinkage and compressive strength were systematically studied as well as the reheat shrinkage and crystallinity of mullite ceramic bodies. The results indicated that the sintering process was prevented by the formation of anorthite phase at higher CaO content. The bulk density increased continuously as the CaO content was increased from 2.9 to 6.5 wt.%, while the apparent porosity and water absorption decreased with the increment of CaO content especially at temperature 1400 °C. On the other hand, addition of small amount of CaO have improved compressive strength of the ceramic body due to reduction in porosity and increase of crystalline phase. Addition of CaO content higher than 4.8 wt.%, would result in increment of crystallinity of the body due to formation of anorthite phase. It was observed that the R1 body (2.9 wt.% CaO) has the lowest reheat shrinkage percentage compared to other bodies and has complied with the Japanese Industrial Standard of mullite refractory product which is lower than 0.2% shrinkage percentage at 1300°C in the furnace for 8 hours.

Abstract: Due to material and structural issues, fire-assaying crucibles used for analyzing precious metals in ores have encountered challenges related to poor thermal cycling in Ghana’s sub-Saharan region. This study aimed to enhance the crucibles by analyzing aluminosilicate minerals' multiphase development using X-ray diffraction and understanding the effects of composition determined by X-ray fluorescence on thermal behavior and water absorption observed through optical microscopy. The improved crucible design exhibited enhanced thermal cycling stability and lower permeability to the assay charge. Analysis showed that Fosu Clay (FC) demonstrated promise with a favorable Al₂O₃:SiO₂ ratio and low impurities; mullite was identified as the primary phase formed at high temperatures, with quartz and cristobalite also present. Introducing 6% CSM dopant to FC increased the mullite content while supporting the transformation from quartz to cristobalite. The optimal crucible sample included coarse and fine-doped grog with an FC-clay binder, demonstrating excellent thermal stability, adequate porosity, and water absorption. Adjusting the percentage of doped grog further increased mullite content while reducing silica content; this suggests that locally produced improved crucibles are feasible through sintering commercial clay with mullite doping and precise composition adjustments.

Abstract: Porous ceramic is a heat-resistant porous material with extensive applications, especially for filtering inclusions in aluminum casting industry. To meet local need for these filters, ceramic foam is fabricated by mixing raw materials including kaolin and chamotte as base material, wood sawdust (WSD) as pore-forming agent (PFA), and carboxymethyl cellulose (CMC) with water as binder. WSD content variable: 0%, 5%, 10%, and 15% was used for this research. The mixture was then formed with dry-press method and sintered at 1200°C to form mullite-based local filters. Characterization of samples morphology, composition, and phase were done using SEM-EDS, XRD, and XRF, while its mechanical and thermal properties were characterized by conducting STA, porosity, coefficient thermal expansion (CTE), permanent linear change (PLC), and flexural strength testing. Results showed an increase of WSD addition on the local filter increased the formation number of open-type pores in fiber form on the ceramic microstructure, no significant heat exchanges from decomposition were detected, increased CTE value from 0,0071 to 0,0371%, PLC from 0,0025 to 0,0345%, apparent porosity from 33,29 to 47,95%, and water infiltration from 18,27 to 33,05%, as well as decreased flexural strength from 13,48 to 6,33 MPa and density from 1.82 to 1.43 g/cm³.

Abstract: Research on ceramic filters, needed in the aluminum casting industry because of their ability to filter inclusions, has been done. This study's primary material to make ceramic or local filters is kaolin. The manufacture of local filters in this study uses the dry press method. In this study, potato starch with a composition of 5%, 10%, and 15% acted as a pore-former in the local filter. The local filter burned to a temperature of about 1200°C to obtain the mullite phase. Local filter characterization used SEM, XRD, XRF, and DTA. Several tests were carried out in this study, including Permanent Linear Change (PLC) test, thermal expansion test, flexural strength test, and porosity test. The results obtained in this study show that pores on the local filter are not open, have a prolate shape, and have an average pore size of 10 to 55 m. Burning kaolin up to 1200°C proved successful in obtaining the mullite phase. Another result of this study, the more starch content added to the material, the greater the shrinkage and expansion of the material; the highest shrinkage value on the material is 0.17%, and the highest expansion value of the material is 0,29%, the 15% values owned by potato starch. It is different in the flexural strength value, which has the highest value with 0% potato starch at 14.14 MPa and the lowest value with 15% potato starch at 5.39 MPa.

Abstract: The development and subsequent production of these special kind of grog from raw materials available in the Czech Republic is one of the possibilities how to reduce income costs for the production of refractory materials with specific properties. The experimental work is focused on verifying the possibility of producing grog with increased alumina content from available raw materials. The raw materials are kaolins and claystones. To achieve a higher content of alumina in the grog, waste mullite dust is used. In order to improve the physical and mechanical properties of the grog, modifiers are used. Selected modifiers are expected to affect positively on the resulting density while maintaining the heat properties.

Abstract: The mineral mullite has many outstanding refractory properties. The content of aluminum oxide in the raw material is the basic factor influencing the content of mullite in the final product. Depending on Al₂O₃ content, silica-alumina materials can be divided into fireclay and high-alumina with the Al₂O₃ content of 45 % and more. The study describes the influence of raw material, the influence of homogenization of raw material and influence of firing temperature on the mullite formation in fireclay refractory material. Three kinds of refractory clay with different chemical composition were used as raw material for fireclay grog production. Three kinds of homogenization and three different temperatures were studied as factors influencing mullite formation. Powder X-ray diffraction was used to determine mullite content in material. Quantitative phase analysis was conducted by the Rietveld method. Mullite crystals morphology was observed by scanning electron microscopy.

Abstract: Nowadays commonly used thermal barrier coatings (TBC) are based on yttria stabilized zirconia (YSZ). Addition of mullite phase into the YSZ coating can improve resulting high temperature properties. The contribution focuses on high temperature cyclic oxidation behaviour of two TBC systems with different top coats (TC) deposited by the means of atmospheric plasma spraying. The initial mullite-YSZ powder mixture consisted of 29 vol. % of mullite and 71 vol. % of YSZ. The conventional TBC system consisted of ~ 150 µm thick NiCoCrAlYHfSi bond coat (BC) and ~ 300 µm thick YSZ top coat. The experimental mullite-YSZ (MYSZ) TBC system consisted of ~ 150 µm thick NiCoCrAlYHfSi bond coat, ~ 100 µm thick YSZ interlayer and ~ 200 µm thick mullite-YSZ top coat. The experimental TBC proved higher lifetime, durability and phase stability and also lower grow rate of thermally grown oxide (TGO) compared to conventional TBC. Lifetime, phase stability and changes in the microstructure of TBCs after the furnace cyclic oxidation test were evaluated by the means of scanning electron microscopy equipped with EDX analyzer and X-ray diffraction techniques. Oxidation kinetics of TGO was calculated based on thickness determined utilizing digital image analysis.

Abstract: The article deals with the manufacture of modern structural ceramic materials from clay and loam deposits of the Republic of Sakha (Yakutia). The importance and relevance of the development of the production of building materials from local raw materials is emphasized, since this will certainly affect the effectiveness of the construction complex as a whole. The successful development of the construction complex is capable of not only stimulating growth in all sectors of the economy, but also contributes to solving the most pressing social problems. Today, Yakutia has huge reserves of mineral raw materials for the production of a wide range of building materials and products. Of practical interest are wall materials made from clay soils. Given the features of the region’s raw material base, this work focuses on additional processing of traditional material. Controlling the complex physicochemical and structural-mechanical transformations that occur during heat treatment, a methodology has been developed for creating a composite material that will allow competitive innovative materials with enhanced strength properties to be produced with a reinforcing element with a glassy phase matrix of mullite crystals. The fabricated samples have a wide range of physical and mechanical properties and allow using it as a high-quality structural building ceramics, as well as industrial floor technical tile.

Abstract: Increased alumina content in refractory grog is a product of the grog’s firing and, at the same time, it is a raw material used for the production of dense or insulating, shaped or monolithic refractories. Grog characteristics determine the properties of the final product. The specific characteristics of the grog are therefore optimized to suit the refractory product as close as possible. The basic characteristics determining the nature of the grog include its mineralogical composition, chemical composition, and apparent porosity. The present study focuses on ways of reducing the porosity of a high-alumina grog and thus obtaining a raw material suitable for the production of dense refractory materials. The experiments have shown that it is possible to regulate porosity by using clay with better sinterability and phosphoric acid in the production of grog from claystone with alumina content over 40 %, a minimal Fe₂O₃ content, a minimal content of CaO and alkali oxides.

Abstract: Currently, porous SiC ceramics have been a focus of interesting research in the field of porous materials due to their excellent structural properties, high strength, high hardness, and superb mechanical and chemical stabilities even at high temperatures and hostile atmospheres. Porous SiC ceramics have been considered as suitable candidate materials for catalyst supports [1-2], hot gas or molten metal filters [3], high temperature membrane reactors [4], thermal insulating materials [5], gas sensors [6] etc. Porous SiC ceramics are fabricated by various methods including partial sintering [7], carbothermal reduction [8-9], replication or pyrolysis of polymeric sponge [10-12], reaction bonding [13] etc. In all these methods SiC needs to be sintered which requires a very high temperature due to the strong covalent nature of the Si-C bond, selective sintering additives, expensive atmosphere, costly and delicate instrumentation. Processing of porous SiC ceramics at low temperature using a simple technique thus becomes necessary. Bonding of SiC can be done at low temperatures by use of different oxide and non-oxide secondary phases. They include silica, mullite, cordierite, silicon nitride, etc. Various sintering additives are used for the formation of variety of secondary oxide bond phases for formations for porous SiC [14-19] Choice of mullite as a bond for SiC has many advantages. Mullite possesses a high melting point (T_m= 1850°C) and a low oxygen diffusion coefficient (5.6 x 10^-14 m²/sec at 50°C). It has a matching thermal expansion coefficient with SiC (CTE_mullite= 5.3 ×10^-6/K; CTE_SiC = 4.7 ×10^-6/K at RT-1000 °C) and a high strength that can be retained up to a very high temperature. Different sources of aluminum, such as Al₂O₃, Al, AlN, and Al (OH)₃ powders were used for the formation of mullite bonded porous SiC ceramics (MBSC) [20-21]. However, the mullitization temperature of 1550^o C is still necessary. In this work, mullite bonded porous SiC ceramics were fabricated by an in situ reaction-bonding process; the mixture of clay and CaCO₃ were chosen as sintering additives to lower the mullitization reaction between Al₂O₃ and oxidation-derived SiO_2. The effect amount of alumina, sintering temperature and other sintering aids on material property such as porosity/pore size distribution mechanical and micro structural properties of porous oxide bonded SiC ceramics were studied.