Abstract
To develop a low-loss, lead-free, non-alkali LTCC substrate material for microwave/microelectronics applications, glass composites in the CaO-Al2O3-SiO2 system with nucleating agents were employed to derive anorthite-based glass-ceramic powders. A systematic study of nucleation and growth kinetics in the anorthite-based glass-ceramic was conducted. The determination of crystallization kinetic parameters for anorthite (CaO-Al2O3-2SiO2) glass containing TiO2 as nucleating agents was performed by non-isothermal process. Thermal characteristics, microstructure and phase transformation during non-isothermal heat-treatment were explored with the aid of DTA, HTXRD, OM, EPMA, SEM, TEM technologies. Under non-isothermal conditions, the crystallization activation energy for anorthite glass nucleated with TiO2 was evaluated as 412kJmol-1. An analytical approach to acquire the Avrami constant was established by introducing appropriate boundary conditions. This technique, considered as three-dimensional bulk crystallization during non-isothermal treatment with various heating rates, provided reasonable precision in determination of Avrami constant. Anorthite crystals showed preferential nucleation at specific sites with rutile TiO2 crystals precipitated from the glassy matrix and anorthite crystallization was governed by heterogeneous volume nucleation, in which the efficient nucleation temperature for this glass-ceramic was 780~820°C. High density and unimodal distribution of nuclei throughout glass powders and the associated mechanism was volume crystallization predominant for anorthite-based glass-ceramic powders. The introduced TiO2 thus played the role of nucleating agents to reducing the crystallization temperature lower than 900ºC for anorthite-based glass-ceramics. A dense and low-loss glass-ceramic with predominant crystal phase of anorthite was successfully produced by using ultra-fine glass powders with sub-micron scale particle size distribution. Fully-densified temperature was as low as 900°C, and sufficient crystallization could be achieved by subsequently raising the firing temperature to 950°C, accompanied by the enhanced dielectric quality factor. The degree of crystallization affected the dielectric property of sintered glass-ceramics, such as the dielectric constant(k), quality factor(Q), coefficient of thermal expansion(CTE), temperature coefficient of frequency(TCF), temperature coefficient of dielectric constant(TCK). After considering the sintering and crystallization mechanisms, the optimization of heat-treatment process for as-fabricated CaO-Al2O3-SiO2 glass-ceramics was presented. In addition, by controlling the crystallinity of anorthite crystal, the 5wt.% TiO2 nucleated anorthite glass-ceramic possessed a preferable dielectric constant of 8 and a rather low dielectric loss of 0.00045 at 10GHz, which are attractive for application in future LTCC substrates.