Preparation And Properties Of CaAl₁₂O₁₉ And CaAl₁₂O₁₉Composite Materials

The rapid development of high-temperature industry put forward higher requirements for refractory. There is increasing market demand for new high-quality refractory materials and the calcium hexaluminate(CA6) received intensive attention as a new type of refractory. In this paper, the solid state reaction method was utilized to synthesize the CA6 powders; moreover, the CA6 ceramics, calcium hexaluminate/alumina(CA6/A) ceramics, calcium hexaluminate/(magnesium aluminium spinel-alumina) [CA6/(MA-A)] functionally graded materials as well as mullite-CA6 composites were fabricated and the properties were optimized. The preparation parameters of plate and grain-like CA6 grains, and the toughening mechanism of CA6 in the alumina ceramic and MA-A ceramic were discussed in detail.The morphology of CA6 grains shows dependence on the kind of raw material, the granularity of starting alumina material, the synthetic temperature as well as preparation processes. Using Al(OH)3 and Ca CO3 as raw materials, well-developed plate-like CA6 grains can be obtained at 1550 oC for 3 h. The results show that the CA6 grains were plate-like in the one-step preparation of the CA6 ceramic, while they became equiaxed grains in the two-step preparation. The two-step preparation exhibits advantages over the one-step method in aspects of bulk density, flexural strength, hardness, fracture toughness and thermal shock resistance properties.The results showed that the peak intensity of calcium hexaluminate increased gradually with the increase of Ca O addition. This resulted in the decrease of bulk density and hardness of samples. However, the bending strength and fracture toughness increased to maximum firstly and then decreased with the increase of Ca O addition.And when the content of Ca O is 2 wt.%, the maximum of bending strength and fracture toughness is 297 MPa and 4.7 MPa·m1/2, respectively.CA6/(MA-A) functionally graded materials were prepared from industrial α-Al2O3 and Mg O by liquid infiltration, using a saturated solution of calcium acetate as infiltration solution. The CA6/(MA-A) region had a better toughness that could prevent the crack propagation and improve the spalling resistance. Meanwhile, the MA-Aregion could provide structural support, because of the higher Vickers hardness and density.The porous CA6 materials coated with alumina has a typical core-shell structure and morphology of golf shape. Both molding and the alumina powder particle size in the coating layer affect the microstructure and properties of the porous CA6 aggregates.Using Kaolin, industrial α-Al2O3 and alumina-coated porous CA6 aggregates as raw materials, and adding 3wt.% polystyrene ball, 7.5wt.% sawdust as pore-forming agent and 3wt.% silica gel as combining agent, the mullite bonded CA6 lightweight refractory were prepared at 1550 oC. The obtained products have bulk density of 0.71 g/cm3,compressive strength of 2.48 MPa, and thermal conductivity of 0.186 W/m·K.