Properties of phyllosilicate‐based porous ceramics shaped by conventional tape casting and freeze tape casting

Abstract

Silicate ceramics were shaped using tape casting (TC) and freeze tape casting (FTC) processes from three clays labeled HCR, KORS, and KCR. These clays exhibited mass content of 77% halloysite-10 Å, 29% kaolinite, and 98% kaolinite minerals, respectively. After casting the slurries, the dried tapes were sintered at 1200 • C. The microstructure changes were characterized before and after sintering using scanning electron microscopy. The apparent porosity of TC samples was lower (36-47 vol.%) compared to values obtained with FTC samples (67-79 vol.%). The latter samples exhibited a highly textured porosity, with micron-sized pores aligned perpendicular to the tape surfaces. Upon sintering, the porosity of TC samples tended to decrease conversely to the case of FTC samples. Such behavior seemed related to the simultaneous effect of organic additives and ice templating. Consequently, the FTC samples showed a relatively low mechanical strength of 3-7 MPa and thermal conductivity of .14-.22 W m −1 K −1 . After sintering, the mullite crystallization contributed to strengthen the bulk materials, helping to compensate for the detrimental effect of porosity on the stress to rupture and on thermal conductivity values.

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“…This would mean, in addition to the mullite formed, the residual quartz contained in the sintered materials contributes to the improvement of mechanical strength. The increase in the stage time causes the melting of the residual quartz progressively without causing the germination of mullite crystals which can improve mechanical strength [20] [21] [22]. The evolution of the mullite crystals depends essentially on the sintering temperature and the amount of melting oxides [20] [21] [23], in agreement with the results presented in Figure 6.…”