Crystal Structure Of Fused Cast High Zirconia Block

2019-02-19 16:47:58

 In high zirconia fused cast refractories, the majority of the refractory crystalline structure are composed of baddeleyite crystals (ZrO2). During the cooling process, zirconia transforms successively through cubic, tetragonal and monoclinic form, with tetragonal and monoclinic forms exhibiting anisotropy in thermal expansion along their crystallographic axes. The resulting complex microstructure is made of numerous different crystallographic domains called variants. Finally, the shift from the tetragonal to the monoclinic structure is responsible for a large volume expansion which causes cracks. The manufacturing process must be rigorously controlled, in particular the cooling rate after fusion, in order to limit mechanical stresses induced by important thermal gradients and the martensitic tetragonal–monoclinic transformation of zirconia.


When heated, fused cast high zircnoia block undergoes a volume shrinkage at about 1150℃,due to the crystal transformation from monoclinic crystals to tetragonal crystals in accordance with the crystal transformation specific to baddeleyite crystals. When cooled, it undergoes a volume expansion at about 950℃ due to the crystal transformation from tetragonal crystals to monoclinic crystals. The volume expansion may cause cracking, and in an extreme case, the refractory undergoes a spalling phenomenon due to many cracks.


The transformation of the baddeleyite crystals is absorbed by a matrix glass. SiO2 is essential for forming the matrix glass. However, if the SiO2 content is too high, the corrosion resistance tends to deteriorate. The Al2O3 component plays an important role in adjusting the relation between the temperature and the viscosity of the matrix glass, and reduces the concentration of the ZrO2 component in the matrix glass.


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