Effects of the scale of precursor mixing on densification behaviors and phase-transformation kinetics of cordierite gels

Abstract

Effects of the scale of precursor mixing on densification behaviors and phase-transformation kinetics of multicomponent aluminosilicate gels were examined using cordierite gels prepared by two different processing routes: a monophasic (single phase) gel having molecular homogeneity, and a triphasic gel in which constitutive precursor phases are uniformly mixed in a nanometer scale (-50 nm). In the case of the monophasic cordierite gel, homogeneous molecular-scale mixing of precursors induces a direction formation of fi -cordierite without a preceding crystallization, and the densification stops at the onset of crystallization to /ii-cordierite at 960 °C. On the other hand, the densification of triphasic cordierite gel was not affected by the crystallization of intermediate spinel phase at 700 °C. The triphasic gel continues to densify until amorphous silica matrix crystallizes to quartz at 1180 °C. The enhanced densification observed in the triphasic gel was attributed to the retarded crystallization of amorphous silica component caused by the increase in diffusion distances.

Crystallization and Densification of Nano‐Size Amorphous Cordierite Powder Prepared by a PVA Solution‐Polymerization Route

“…In the PVA solution process, a homogeneously mixed solution was obtained easily by the completely dissolving colloidal SiO 2 without any adjustment of solution pH, compared with the mixed colloidal processing route. [13][14][15]…”

“…8,12 In the synthesis of cordierite, the formation of -phase reduced the chances of forming other silicate compounds, such as spinel and cristobalite, or the residual cordierite-type amorphous phase. 8,14 Thus, the formation of -phase during the development of crystallization resulted in synthesis of high-purity ␣cordierite. The polycrystalline cordierite derived from PVA solution had a lower thermal expansion coefficient of 2.1 × 10 −6 /°C, which is a closer match with the thermal expansion coefficient of pure cordierite (1.6 × 10 −6 /°C at 25-1000°C), 1 than that of cordierite prepared by the Pechini resin, 3.8 × 10 −6 /°C.…”

“…This was because the amorphous SiO 2 was depleted by crystallization of -cordierite before full viscous-flow sintering occurred. 13,14 In the case of the Pechini resin process, however, densification by viscous flow lasted longer than densification of the powder compact prepared with PVA solution due to the residual cordierite-type glasses. This was the reason for the enhanced densification observed in the Pechini resin process.…”

“…In the recent past, some authors reported successful densification of cordierite by a mixed colloidal processing route without using any sintering aid. [13][14][15] This processing route produces a homogeneous cordierite-type gel by adjusting pH of the colloidal suspension and results in cordierite ceramics with ∼98% theoretical density. The Pechini resin process 16 can also be used as an effective powder preparation method to make dense and pure cordierite ceramics.…”

Conventional and spark-plasma sintering of cordierite powders synthesized by sol–gel methods

“…The sintering behavior of commercially available crystalline cordierite powders, which were sintered at temperatures near the incongruent melting point of cordierite (1460 1C), were investigated in several studies [15,16]. Moreover, some researchers [7,17,18] reported that if the densification of pure and homogeneous cordierite powder compact occurs below the crystallization temperature, it could be possible to obtain pure and dense cordierite ceramics with relatively good properties. Therefore, these authors investigated the sinterability of amorphous powders, which were densified by viscous sintering.…”