Phase evolution and near-zero shrinkage in BaAl2Si2O8 low-permittivity microwave dielectric ceramics

• BaAl2Si2O8 low-permittivity microwave dielectric ceramics exhibit a very rare characteristic with near-zero or even negative shrinkage.
• Mechanism of near-zero shrinkage was clarified, which is helpful to prepare large-scale and complex-shape ceramic devices.
• Deionized water as dispersant can improve ɛr and Q × f value of BaAl2Si2O8 ceramics, and induce the phase evolution from hexagonal to monoclinic.

The mechanism of near-zero shrinkage, phase evolution induced by calcining temperature and dispersant, and their effects on the microwave dielectric properties of BaAl2Si2O8 ceramics prepared by the solid-state reaction were investigated. Research showed that deionized water was beneficial to preparing BaAl2Si2O8 ceramics with high density of about 95%, and can induce the phase evolution from hexagonal to monoclinic when the calcining temperature was over 1175 °C. With ethanol, however, the single hexagonal phase always existed in the porous ceramics with near-zero shrinkage, and a porous microstructure originated from the decomposition of residual BaCO3 during sintering. The best microwave dielectric properties can both be obtained in the BaAl2Si2O8 ceramics calcined at 1150 °C and sintered 1475 °C for 3 h using dispersants such as deionized water (ɛr = 6.36, Q × f = 44,800 GHz and τf = −46.9 ppm/°C) and ethanol (ɛr = 4.64, Q × f = 35,580 GHz and τf = −43.5 ppm/°C).

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