Fabrication of low-to-zero shrinkage reaction-bonded mullite composites

doi:10.1016/0955-2219(95)00137-9

Journal of the European Ceramic Society

Volume 16, Issue 2, 1996, Pages 255-260

Dedicated to Professor Heinz Mecking on the occasion of his 65th birthday.

https://doi.org/10.1016/0955-2219(95)00137-9 Get rights and content

Abstract

The technology of reaction bonding Al₂O₃ (RBAO) can be modified by the use of Si-containing additives to yield low-to-zero shrinkage mullite composites. In the present work, SiC particles were added to the Al/Al₂O₃ precursor mixture. During air heat-treatment, first Al oxidizes to Al₂O₃ at 300–900 °C, thereafter SiC converts to SiO₂ (900–1200 °C). Both phases form mullite (3Al₂O₃· SiO₂) at temperatures >1400 °C. Depending on the hold time at 900–1200 °C, the extent of SiC oxidation, hence the ratio of mullite to dispersed SiC can be controlled. Since both oxidation reactions and the mullitization are associated with volume expansions, the sintering shrinkage can either be fully or partially compensated for. The process parameters amount of Al and SiC, green density and degree of SiC oxidation can be utilized to fabricate low-to-zero shrinkage mullite composites.

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Moreover, concurrent bonding of the joining layer with upper and lower parent layers also occurs during sintering. At this final sintering temperature, new and old alumina particles are indistinguishable because all γ-alumina are converted to α-alumina [13–18]. However, the sizes of old alumina particles are relatively bigger after sintering.
Seamless joining of zirconia-toughened alumina (ZTA) ceramics without the application of any external pressure was demonstrated in this work through a reaction-bonded aluminum oxide (RBAO) method. Reliable joining of ZTA ceramics in a green-state was successfully accomplished in ambient air at 1600 °C for 2 h with the use of an Al/3Y–ZrO₂/Al₂O₃ powder mixture as joining interlayer. The joining mechanism was discussed based on the RBAO process where the effective joining principally relies on the conversion of all Al particles to alumina. Thermogravimetry and differential thermal analysis confirmed the oxidation behavior of the milled Al particles. The joining interface exhibited excellent compositional and structural homogeneity without defects in commensurate with parent bodies. The mechanical properties of the joints were nearly equivalent to bare ZTA ceramics. This proposed simple and cost-effective joining technique can be used for the joining of other alumina-based advanced ceramics to realize large and complex-structures.
Pressure-less joining of alumina ceramics by the reaction-bonded aluminum oxide (RBAO) method
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Accordingly, it is imperative to develop another new simple and reliable technique to avoid these disadvantages. The reaction-bonded aluminum oxide (RBAO) method is well-known for its fabrication of low-to-zero shrinkage ceramics [18–23]. RBAO is based on a mechanism whereby aluminum powder is oxidized leading to a mass gain with volumetric expansion which counterbalances the shrinkage upon sintering.
The present work demonstrates a pressure-less and reliable joining technique for alumina ceramics through a reaction-bonded aluminum oxide (RBAO) method. Effective joining relies on the RBAO mechanism, in which Al particles are converted to alumina through oxidation and bond with alumina particles from the parts to be joined upon sintering. Alumina ceramics in a green state were successfully joined with the use of an Al/Al₂O₃ powder mixture as an interlayer. The oxidation behavior of the Al particles was confirmed by thermogravimetry and X-ray diffraction analyses. Joining was performed in ambient air at 1650 °C for 2 h without applying any external pressure. Microstructural observations at the joining interfaces indicated a compact joining. The joining strengths were assessed by determining the biaxial strengths at room temperature, and the joined samples exhibited no fractures at the joining interfaces. Moreover, the joints had a strength of almost 100 % when compared with those of the parent alumina ceramics.
Recycling of coal fly ash for fabrication of elongated mullite rod bonded porous SiC ceramic membrane and its application in filtration
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The pore former burns out with creation of void space. Silica is produced with expansion in volume ∼ 108% from oxidation of SiC [27] and mullite is produced with expansion in volume ∼ 13% from oxidation derived silica and alumina. The sum of these volume effects is nullified by the shrinkage in volume due to sintering, leading to no appreciable change in dimensions or volume for the final ceramics.
Mullite bonded SiC ceramic membranes were synthesized by a facile solid-state reaction process, using SiC, solid waste fly ash as raw materials and MoO₃ as catalyst for growth of mullite at 1000 °C. The effect of MoO₃ catalyst on mullitization reaction and mullite morphology was investigated. Different pore formers were used to enhance the porosity and to observe its effects on the permeability parameters and filtration characteristics. At room temperature Darcian (k₁) and non-Darcian (k₂) in both water and air flow were measured and clean water flux was determined. The porous SiC ceramics with addition of 5 wt.% MoO₃ exhibited a flexural strength of 38.4 MPa at porosity 36.4 vol% and showed 92% oil removal efficiency from oily wastewater. This technique, combining low-cost materials and the co-sintering at low temperature, can serve as a cost-effective method for the production of high-performance porous SiC ceramic membrnaes for filtration application.
Ultra-low cost porous mullite ceramics with excellent dielectric properties and low thermal conductivity fabricated from kaolin for radome applications
2019, Ceramics International
Near-net-shape mullite ceramics with high porosity were prepared from ultra-low cost natural aluminosilicate mineral kaolin as raw material and polystyrene micro-sphere (PS) as pore-forming agent. Microstructure, flexural strength, thermal conductivity and dielectric properties of the ceramics were systematically researched. Results show that the porous mullite ceramics possess fibrous skeleton structure formed by a large quantity of interlocked mullite whiskers, which results in good mechanical properties and low-to-zero sintering shrinkage. Flexural strength of the porous mullite ceramics can be up to 41.01 ± 1.12 MPa, even if the porosity is as high as 62.44%. The dielectric constant and loss tangent of the porous mullite ceramics at room temperature are lower than 2.61 and 5.9 × 10⁻³, respectively. Besides, dielectric constant is very stable with the rising of temperature, and the dielectric loss can be consistently lower than 10⁻² when the temperature is not higher than 800 °C. In addition, thermal conductivity at room temperature is as low as 0.163 W/m/K when the porosity of mullite ceramics is 80.05%. The infiltration of SiO₂ aerogels (SiO₂ AGs) can further decrease the thermal conductivity to 0.075 W/m/K, while has just little effects on the dielectric properties. Excellent mechanical, thermal and dielectric properties show that the porous mullite ceramics have potential applications in radome fields. The porous mullite ceramics prepared from kaolin not only have low cost, but also can achieve near-net-shape.
Preparation of low-shrinkage and high-performance alumina ceramics via incorporation of pre-sintered alumina powder based on Isobam gelcasting
2019, Ceramics International
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Therefore, it is of great significance to prepare low shrinkage ceramics for wider applications. Reaction bonding [7–10] is a conventional method for preparing low-shrinkage ceramics. Low shrinkage alumina ceramics with high flexural strength (>300 MPa) and relative density (>90%) have been obtained by reaction of compacts of attrition-milled Al/Al2O3 powder mixtures [11,12].
Mechanically robust alumina ceramics with low shrinkage were successfully prepared via gelcasting using Isobam as the gelling agent and pre-sintered alumina powder as the raw material for the first time. The influence of amount of pre-sintered alumina powder and powder's pre-sintering temperature on the viscosity of suspension and mechanical properties of low-shrinkage alumina ceramics were systematically investigated. The results showed that low-shrinkage alumina ceramics with flexural strength of 294.5 ± 33.0 MPa, linear shrinkage of 7.79% and relative density of 91.5% can be obtained with the addition of 60 wt% alumina powder pre-sintered at 1600 °C. This convenient and facile approach is promising to fabricate low-shrinkage alumina ceramics with large sizes, high dimensional accuracy and complex shapes.
Low-temperature synthesis of highly porous whisker-structured mullite ceramic from kaolin
2018, Ceramics International
Citation Excerpt :
The reason for the slightly greater bulk density of the sample with boehmite was due to its higher apparent density of the mixed powders which led to a high compact density of the green body. Owing to the more liquid existed during sintering, the linear shrinkage of the sample with boehmite was slightly higher than that of sample without boehmite which even exhibited a slight increase in diameter due to the volume expansion during mullitization [44]. Both samples exhibited a broad pore distribution in the range of 30–300 nm, as shown in Fig. 5.
In this study, the mullitization temperature was drastically reduced to 900 °C through adding boehmite sol to the kaolin/α-Al₂O₃ reaction couple. The low mullitization temperature was probably ascribed to: (1) high solubility of γ-Al₂O₃ in the SiO₂-rich liquid during sintering, (2) adhesive effect of boehmite sol and (3) high reactivity of calcined kaolin. Also, starch particles and polystyrene (PS) micro-spheres were utilized as porogens to prepare high-porosity mullite. Compared to starch particles, the PS micro-spheres were more suitable for fabricating high-porosity mullite. The effects of PS content and sintering temperature on the mullitization reaction and properties of porous mullite were studied. The result demonstrated that single-phase mullite could only be achieved at 1300 °C for samples with porogen. Excessive addition of PS micro-spheres (>32 wt%) would inhibit the porosity increase due to the sample shrinkage during sintering. As the amount of PS increased from 22% to 32%, porosity of the samples increased from 73.29% to 80.10%, compressive strength decreased from 15.55 ± 0.52 to 9.89 ± 0.12 MPa and thermal conductivity decreased from 0.26 to 0.16 W/m/K.

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Journal of the European Ceramic Society

Abstract

References (15)

Fracture sources and processing improvements of SiC-whiskerreinforced mullite (ZrO₂) composites

J. Eur. Ceram. Soc.

Reaction bonding of aluminum oxide (RBAO) composites: Processing, reaction mechanisms, and properties

J. Eur. Ceram. Soc.

Processing and microstructure development in Al₂O₃-SiC ‘nanocomposites’

J. Eur. Ceram. Soc.

Sintering and mechanical properties of stoichiometric mullite

J. Am. Ceram. Soc.

Mechanical properties of sintered in situ-reacted mullite/ZrO₂ composites

J. Am. Ceram. Soc.

Development of SiC-whisker-reinforced ceramics

Am. Ceram. Soc. Bull.

Cited by (39)

Seamless joining of ZTA ceramics without the application of pressure

Pressure-less joining of alumina ceramics by the reaction-bonded aluminum oxide (RBAO) method

Recycling of coal fly ash for fabrication of elongated mullite rod bonded porous SiC ceramic membrane and its application in filtration

Ultra-low cost porous mullite ceramics with excellent dielectric properties and low thermal conductivity fabricated from kaolin for radome applications

Preparation of low-shrinkage and high-performance alumina ceramics via incorporation of pre-sintered alumina powder based on Isobam gelcasting

Low-temperature synthesis of highly porous whisker-structured mullite ceramic from kaolin

Fabrication of low-to-zero shrinkage reaction-bonded mullite composites

Abstract

J. Eur. Ceram. Soc.

J. Eur. Ceram. Soc.

J. Eur. Ceram. Soc.

Sintering and mechanical properties of stoichiometric mullite

J. Am. Ceram. Soc.

Mechanical properties of sintered in situ-reacted mullite/ZrO2 composites

J. Am. Ceram. Soc.

Development of SiC-whisker-reinforced ceramics

Am. Ceram. Soc. Bull.

Mechanical properties of sintered in situ-reacted mullite/ZrO₂ composites