The relationship between crystal structure and modified microwave dielectric properties of Ca3SnSi2-xGexO9 ceramics
Ca 3 SnSi 2- x Ge x O 9 (0 ≤ x ≤ 0.8) and (1– y ) Ca 3 SnSi 1.6 Ge 0.4 O 9 – y CaSnSiO 5 – 2 wt% LiF ( y = 0.4 and 0.5) microwave dielectric ceramics were prepared by traditional solid-state reaction through sintering at 1250°C–1425°C for 5 h and at 875°C for 2 h, respectively. Ge 4+ replaced Si 4+ , and Ca 3 SnSi 2- x Ge x O 9 (0 ≤ x ≤ 0.4) solid solutions were obtained. At 0.1 ≤ x ≤ 0.4, the Ge 4+ substitution for Si 4+ decreased the sintering temperature of Ca 3 SnSi 2- x Ge x O 9 from 1425 to 1300°C, the SnO 6 octahedral distortions, and the average CaO 7 decahedral distortions, which affected the τ f value. The large average decahedral distortions corresponded with nearer-zero τ f values at Ca 3 SnSi 2- x Ge x O 9 (0.1 ≤ x ≤ 0.4) ceramics. The τ f value and sintering temperature of Ca 3 SnSi 2- x Ge x O 9 ( x = 0.4) ceramic were adjusted to near-zero by CaSnSiO 5 and decreased to 875°C upon the addition of 2 wt% LiF. The (1 – y ) Ca 3 SnSi 1.6 Ge 0.4 O 9 – y CaSnSiO 5 – 2 wt% LiF ( y = 0.5) ceramic sintered at 875°C for 2 h exhibited good microwave dielectric properties: ε r = 10.3, Q × f = 14 300 GHz (at 12.2 GHz), and τ f = ‒5.8 ppm/°C.
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