Reaction-bonded Mullite Research Articles

Preparation and enhancement of mullite reticulated porous ceramics for porous media combustion

The insufficient strength and thermal shock resistance of mullite reticulated porous ceramics (RPC) limited their application in porous media combustion. To strengthen mullite RPC, the struts with three layers were prepared using the combined approaches of polymeric replica and vacuum infiltration. The effects of residual stress within the struts on thermal shock resistance of mullite RPC were investigated by finite element analysis (FEA). The results showed that the strut was composed of the coating of reaction-bonded mullite, middle layer of mullite skeleton and the triangular filling layer. The triangular voids and surface cracks were eliminated in the formed struts, resulting in a higher strength of mullite RPC than that of hollow strut. In addition, FEA results indicated that the residual compressive stress formed in the coating of strut after mullite RPC was sintered at 1450 °C and 1500 °C, while the residual tensile stress formed at 1550 °C. The larger residual compressive stress within the strut coating was beneficial to enhance the thermal shock resistance of mullite RPC.

Design of three-layered struts in SiC reticulated porous ceramics for porous burner

The three-layered struts within SiC reticulated porous ceramics (RPC) were designed to improve the thermal characteristics and thermal shock resistance of porous burner. The effects of strut structure and its composition on the flame stability limits, flame temperature and thermal damage resistance of porous burner were investigated. The results showed that three-layered struts consisted of continuous mullite coating, middle layer of SiC skeleton and inner layer of reaction-bonded mullite after sintering at 1450 °C. Compared to porous media with single layer, RPC with three-layered struts possessed the combined effects of high thermal conductivity and heat radiation because of its middle layer of SiC skeleton and in-situ formed mullite coating. The three-layered struts in porous media extended the flame stability limits and increased the flame temperature of porous burner. Furthermore, the formation of three-layered struts eliminated strut defects and formed continuous mullite coating, thereby significantly improving the oxidation resistance and thermal shock resistance of RPC, which increased the service life of porous burner.

AlPO 4-coated mullite/alumina fiber reinforced reaction-bonded mullite composites

A precursor for reaction-bonded mullite (RBM) is formulated by premixing Al 2O 3, Si, mullite seeds and mixed-rare-earth-oxides (MREO). An ethanol suspension thereof is stabilized with polyethyleneimine protonated by acetic acid. The solid in the suspension is infiltrated into unidirectional mullite/alumina fiber-preforms by electrophoretic infiltration deposition to produce fiber-reinforced, RBM green bodies. Crack-free composites with ≤25% porosity were achieved after pressureless sintering at 1300 °C. Pre-coating the fibers with AlPO 4 as a weak intervening layer facilitates significant fiber pullout on composite fracture and confers superior damage tolerance. The bend strength is ∼170 MPa at 25 °C ≤ T ≤ 1100 °C. At 1200 °C, the composite fails in shear due to MREO-based, glassy phase formation. However, the AlPO 4 coating acts as a weak layer even after thermal aging at 1300 °C for 100 h.

Use of Mixed‐Rare‐Earth Oxide in the Preparation of Reaction‐Bonded Mullite at ≤1300°C

A process for production of near‐net‐shape mullite‐matrix ceramic composites at ≤1300°C has been achieved by reaction‐bonding Al2O3, silicon, mullite seeds, and eutectics of Al2O3–SiO2–mixed‐rare‐earth oxide. The fusion temperature of the eutectic composition utilized is 1175°C. This liquid phase facilitates silicon oxidation, mullitization, and sintering. Mullite phase develops with low residual Al2O3 when 7.5 wt% mixed‐rare‐earth oxide and 5 wt% mullite seeds are used. The final sinter is >90% of theoretical density, >90% mullite (by quantitative XRD), and suffers 2.2% sintering shrinkage.

Mullitization and densification of Y 2O 3-doped reaction-bonded mullite ceramics with different compositions

Mullite ceramics were fabricated by a reaction-bonding technique from the powder mixtures of Al 2O 3 and Si with an Y 2O 3 addition. The effects of the initial powder composition on the mullitization and densification behaviors were investigated. It has been shown that mullitization and densification can be simultaneously enhanced by a low-viscosity yttrious silicate glass. After a 2 h heating at 1400 °C, all the specimens exhibited a dense microstructure. Moreover, mullite grains were observed to have an elongated morphology, while the yttrious silicate glass was found to crystallize into Y 2Si 2O 7 at the later stages of mullite growth.

Phase development of Y2O3-doped reaction-bonded mullite ceramics

Phase development of Y2O3-doped reaction-bonded mullite ceramics

Accelerated Reaction Bonding of Mullite

A suitable way to process mullite ceramics and mullite‐matrix composites with low dimensional changes (“near‐net‐shape processing”) is reaction bonding, using silicon metal and corundum (α‐Al2O3) as starting materials, be‐cause sintering‐induced shrinkage is compensated by vol‐ume expansion caused by the silicon‐oxidation‐induced vol‐ume expansion. This work describes a new reaction‐bonded mullite (RBM) processing route that proceeds at much lower temperatures (≤1350°C) than in “normal” RBM systems (≥1550°C). Accelerated silicon oxidation and mullite formation are effected by adding low amounts of yttria (Y2O3) or ceria (CeO2) to the green powder mixtures, which causes the formation of transient, metastable, and low‐viscosity Y‐Al‐Si‐O and Ce‐Al‐Si‐O partial melts. After long‐term heat treatments at ≤1350°C, however, the intermediate liquids recrystallize and leave behind RBM ceramics that are homogeneous, substantially glass‐free, and relatively dense. These two RBM materials consist of mullite, α‐Al2O3, and Y2Si2O7 or CeO2, respectively.

Preliminary results on a novel fabrication route for α-Al 2O 3 single crystal monofilament-reinforced reaction-bonded mullite (RBM)

Owing to their excellent properties, continuous-fibre reinforced mullite-matrix composites are good candidates for applications in which oxidation resistance and damage tolerance at high temperatures (> 1000 °C) are required. To avoid fibre damage, near net-shape fabrication techniques of the composite are required. This has been achieved by using the reaction-bonding process which benefits the oxidation of metal powders producing volume expansion, and hence fully or partially compensating for the sintering-induced shrinkage. Starting materials include Al-Si alloy (80:20), Si metal, α-Al 2O 3 and mullite precursor powders. Due to the variety of starting compounds with different reaction and sintering kinetics, composite fabrication becomes a complex process. Differential scanning calorimetry (DSC) measurements, scanning electron microscopy (SEM) observations, and X-ray diffractometry (XRD) data show that effective milling of metal powder leads to a high degree of mullite formation (≈ 84%) at temperatures as low as 1500 °C, although densification of the ceramic compacts remains rather low (≈ 45% of theoretical density). Single crystal α-Al 2O 3 monofilaments were used to reinforce the reaction-bonded mullite (RBM) matrix. Although no intense reaction between the matrix and the fibres was observed at process temperature, strong bonding develops between uncoated fibres and the matrix. In order to produce a weaker fibre-matrix interface, which is necessary for improvement of the damage tolerance, the fibres were coated with ZrO 2 by means of high frequency sputtering. Microstructural observations of the fibre surfaces before and after the reaction-bonding process indicate that thick coatings (> 10 μm) produce very weak bonding, insufficient for matrix-fibre load transfer due to shrinkage of the low density ZrO 2 layers. Thinner layers (1 μm) produce a better interfacial relation with suitable pull-out of fibres.

Secondary operations

Owing to their excellent properties, continuous-fibre reinforced mullite-matrix composites are good candidates for applications in which oxidation resistance and damage tolerance at high temperatures (> 1000 °C) are required. To avoid fibre damage, near net-shape fabrication techniques of the composite are required. This has been achieved by using the reaction-bonding process which benefits the oxidation of metal powders producing volume expansion, and hence fully or partially compensating for the sintering-induced shrinkage. Starting materials include Al-Si alloy (80:20), Si metal, α-Al2O3 and mullite precursor powders. Due to the variety of starting compounds with different reaction and sintering kinetics, composite fabrication becomes a complex process. Differential scanning calorimetry (DSC) measurements, scanning electron microscopy (SEM) observations, and X-ray diffractometry (XRD) data show that effective milling of metal powder leads to a high degree of mullite formation (≈ 84%) at temperatures as low as 1500 °C, although densification of the ceramic compacts remains rather low (≈ 45% of theoretical density).Single crystal α-Al2O3 monofilaments were used to reinforce the reaction-bonded mullite (RBM) matrix. Although no intense reaction between the matrix and the fibres was observed at process temperature, strong bonding develops between uncoated fibres and the matrix. In order to produce a weaker fibre-matrix interface, which is necessary for improvement of the damage tolerance, the fibres were coated with ZrO2 by means of high frequency sputtering. Microstructural observations of the fibre surfaces before and after the reaction-bonding process indicate that thick coatings (> 10 μm) produce very weak bonding, insufficient for matrix-fibre load transfer due to shrinkage of the low density ZrO2 layers. Thinner layers (1 μm) produce a better interfacial relation with suitable pull-out of fibres.