A structural and dielectric characterization of NaxCa1-xAl2-xSi2+xO8 (x=0 and 1) ceramics

Albite and anorthite, the end-members of the plagioclase feldspar structural family (NaxCa1-xAl2-xSi2+xO8), were synthesized under subsolidus conditions using the solid-state reaction technique. The structural investigations revealed that both the plagioclase-phase formation and sintering temperature decreased from the anorthite (CaAl2Si2O8; X=0), which started to form at 1000 degrees C and sintered at 1300 degrees C, to the albite (NaAlSi3O8; x=1), which started to form at 800 degrees C and sintered at 1000 degrees C. Dielectric measurements in the microwave (MW) frequency region revealed that a high temperature (1500 degrees C) and an extended heat-treatment time were needed to obtain low-loss anorthite ceramics (Qxf = 10000-12000 GHz). In contrast, the albite attained Qxf = 11 200GHz at a lower temperature (1025 degrees C). A temperature coefficient of resonant frequency (tau(f)) close to zero (-5 ppm/degrees C) was another advantage of the albite over the anorthite, which exhibited tau(f) = -130 ppm/degrees C. The dielectric measurements also revealed that a slow cooling rate considerably improved the Qxf values of the anorthite and albite. (c) 2005 Elsevier Ltd. All rights reserved.