Abstract:
The effect of Ca2+ substitution on the structure, microstructure,
and microwave dielectric properties of Sr–gehlenite
(Sr2Al2SiO7) ceramic has been investigated. The structure and
microstructure of Sr2 xCaxAl2SiO7 ceramics were analyzed
via X-ray diffraction (XRD) as well as scanning and transmission
electron microscopic techniques. While the end-members
(x = 0 and 2) form isostructural compounds, a highly defective,
nonstoichiometric, Ca-rich secondary phase was observed via
bright-field transmission electron microscopy and energy dispersive
X-ray spectroscopy in compositions corresponding to
x = 0.75 and 1.5. The concentration of secondary phase in
x = 0.75 is too low to be detected via XRD or scanning electron
microscopy. Identical selected-area electron-diffraction
patterns of the compounds (x = 0, 1, and 2) confirmed that
they belong to the space group P 421m (no. 113) with tetragonal
crystal symmetry. The porosity-corrected relative permittivity
at microwave frequencies showed a gradual increase with
Ca2+ content; however, Ca2+ substitution made only marginal
changes to the microwave dielectric properties except in the
case of x = 1.5, in which the secondary phase reduced the quality
factor considerably. Thermal conductivity decreased with
increasing Ca2+ content, and the compounds with defective
structures showed the lowest thermal conductivity. All the compounds
exhibited low coefficients of linear thermal expansion,
with values varying in the range 2.3–3.6 ppm/°C.
