The cordierite ($2MgO2Al_2O_35SiO_2$)-based glass-ceramic system is a potential candidate for an alternative substrate material for high-speed circuits due to its low-dielectric constant, low temperature sintering capability and a thermal expansion coefficient matching with that of Si chips.
To improve the mechanical properties with the low dielectric constant, cordierite-mullite composite material has been studied. Previous works have concentrated on the densification of cordierite-mullite ceramics at temperatures above 1465℃, where the cordierite melts down to the liquid phase. In order to fabricate a multilayered structure cofired with highly conductive electrodes such as Au, Cu and Ag-Pd, the sintering temperature should be reduced to below 1000℃.
It was suggested that cordierite-mullite composite could be fabricated at temperatures below 1000℃ using glass precursor powder rather than colloidal precursor, since glass is a very homogeneous system on an atomic scale. Many studies have reported on the crystallization mechanism of cordierite glass or cordierite glass with excess $SiO_2$ and MgO, which is close to the ternary eutectic point, but limited study has been carried out on the cordierite-mullite (cordierite with excess $Al_2O_3$ and $SiO_2$ system, since it is not the stable glass-forming region.
The effect of the glass phase separation on the nucleation behavior has been associated with many factors, such as the formation of an amorphous phase with relatively high mobility, phase boundary as a nucleating site for the first crystalline phase and reduction of the energy barrier due to the compositional change. Fine-grained homogeneous composite glass ceramics can be produced by controlled phase separation in the range of 10-50Å.
The present study is concerned with the nucleation and crystallization behavior of cordierite-based glass with excess $SiO_2$ and $Al_2O_3$ (CM glass). Transmission electron microscopical studies on the microstructural evolution have been emphasized. Glass-in-glass phase separation and its effects on the nucleation and crystallization behavior of CM glass were examined. Crystallization behavior of cordierite-based glass with excess $SiO_2$ and $Al_2O_3$ (CM glass) was studied at the initial crystallization stage. Two kinds of glass-in-glass phase separation occurred during the heat treatment of glass powder. The secondary glass-in-glass phase separation, which consists of an $SiO_2$-rich phase and an $Al_2O_3$-rich phase significantly affects the crystallization of the glass and the resultant microstructures of the crystallized glass. Mullite nucleated first in the $Al_2O_3$-rich amorphous phase which is no longer stable glass-forming region. Metastable m-cordierite crystallized in the SiO2-rich amorphous phase and grew rapidly. A fine-grained cordierite-mullite composite with mullite as the grain boundary phase could be produced.
