Powder selection for hydrothermally processed sol–gel composite barium strontium titanate capacitors

Abstract

Ba 0.7 Sr 0.3 TiO 3 (BST) capacitors for high frequency circuits have been fabricated at 175 8C on epoxy-based copper-clad printed circuit boards, having a glass transition temperature of $200 8 C by hydrothermal processing of dried BST acetatebased sol-gel powder slurries. The film thickness was 2-10 mm. BST powders were prepared respectively as ceramic derived powders processed from oxide mixtures fired to 1100 8C, sol-gel derived powders fired to 750 and 1100 8C, and a low temperature solution process dried at 500 8C. The performance of the capacitor was determined by both the dielectric properties of the powder and the interaction between the crystallizing gel and the powder surface. High relative permittivity is obtained for sol-gel derived powders fired to 750 8C, while high voltage tunability is a characteristic of the ceramic derived powders. Low temperature powders are ineffective. The difference is attributed to the nature of the crystalline interface formed between the surface of the powder particles and the hydrothermally crystallized BST formed from the dried gel within the composite. The optimum molar concentration of the hydrothermal solution was 0.1 M with a Ba content larger than that of the powder. Capacitor composites having a relative permittivity of up to 300 at 20 MHz were demonstrated.