The influence of codoping of Fe, Co, Ni, Cu as acceptors and F as donor on the dielectric properties of screen-printed Ba 0.6 Sr 0.4 TiO 3 ceramic thickfilms has been investigated. The undoped and codoped Ba 0.6 Sr 0.4 TiO 3 powders were synthesized through a solgel route. The thermal gravimetric analysis revealed the difference of the thermal decomposition behaviour between the undoped and codoped precursors. The ceramic powders were characterized with x-ray diffraction, scanning electron microscopy and BET measurements. Larger crystallite sizes of the codoped powders were observed. The densification behaviour of the powders was recorded by dilatometry and indicated that codoping influences the sintering mechanism. The permittivity, dielectric loss and tunability of the undoped and codoped thick-films were characterized with coplanar waveguide structures up to 30 GHz.

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The influence of Fe acceptor and Fe – F acceptor – donor co-doping on the dielectric response of screen-printed Ba0.6 Sr0.4 TiO3 thick-films on alumina substrates has been investigated. The Ba0.6 Sr0.4 TiO3 powders were synthesized by freeze-drying of sols. Permittivity, dielectric loss, and tunability were investigated at kHz frequencies with a planar metal-insulator-metal capacitor structure, and at GHz frequencies up to 40 GHz using coplanar waveguide structures. Pure acceptor doping by Fe was found to have a distinct influence on permittivity and dielectric loss-factor at kHz-frequencies and at GHz frequencies due to an internal bias field and pairs of associated defects. Co-doping with F significantly suppresses the influence of the internal bias field and defect-associates at low and microwave frequencies. The commutation quality factor at 10 GHz and Eeff = 5.8 V μm-1 was increased by co-doping the thick-films with Fe and F. Such doped Ba0.6 Sr0.4 TiO3 films show a high potential for tunable microwave applications.

Nonlinear dielectrics are promising materials for tunable microwave components. However, dielectric loss, tunability, and temperature dependence of such dielectrics are crucial parameters and have to be optimized for an application as microwave device. In part I of this split publication, the impact of doping and processing on the material performance of Ba 0.6 Sr 0.4 TiO 3 (BST) ceramics and thick films are presented. A phenomenological model developed by Vendik (Vendik and Zubko 2000) was used to characterize the influence of doping on the material performance of BST as tunable microwave dielectrics. It is shown that acceptor (iron, Fe)-donor (fluoride, F) codoping can positively influence the tunability of BST fundamentally. This impact is interpreted by charged defects influenced by doping and/or related defect formations. In addition to doping, it is shown that thermal treatment of powders and thick films as well as the deposition method show a strong impact on dielectric performance of the final thick-films. In the following part II, metallization, component design, and RF-characterization of tunable microwave devices will be discussed.