Abstract :
The effects of replacement of MgO by CaO on
the sintering and crystallization behavior of MgO–Al2O3–
SiO2 system glass-ceramics were investigated. The results
show that with increasing CaO content, the glass transition
temperature firstly increased and then decreased, the
melting temperature was lowered and the crystallization
temperature of the glass-ceramics shifted clearly towards
higher temperatures. With the replacement of MgO by less
than 3 wt.% CaO, the predominant crystalline phase in the
glass-ceramics fired at 900 C was found to be a-cordierite
and the secondary crystalline phase to be l-cordierite.
When the replacement was increased to 10 wt.%, the predominant
crystalline phase was found to be anorthite and
the secondary phase to be a-cordierite. Both thermal
expansion coefficient (TCE) and dielectric constant of
samples increases with the replacement of MgO by CaO.
The dielectric loss of sample with 5 wt.% CaO fired at
900 C has the lowest value of 0.08%. Only the sample
containing 5 wt.% and10 wt.% CaO (abbreviated as sample
C5 and C10) can be fully sintered before 900 C.
Therefore, a dense and low dielectric loss glass-ceramic
with predominant crystal phase of a-cordierite and some
amount of anorthite was achieved by using fine glass
powders (D50 = 3 lm) fired at 875–900 C. The as-sintered
density approaches 98% theoretical density. The
flexural strength of sample C5 firstly increases and then
decreases with sintering temperature, which closely corresponds
to its relative density. The TCE of sample C5
increases with increasing temperature. The dielectric
property of sample C5 sintered at different temperatures
depends on not only its relative density but also its crystalline
phases. The dense and crystallized glass-ceramic C5
exhibits a low sintering temperature ( £ 900 C), a fairly
low dielectric constant (5.2–5.3), a low dielectric loss
( £ 10–3) at 1 MHz, a low TCE (4.0–4.25 · 10–6 K–1),
very close to that of Si (~3.5 · 10–6 K–1), and a higher
flexural strength (‡134 MPa), suggesting that it would be a
promising material in the electronic packaging field.
