Research On Ferroelectric Materials For Tunable Microwave Application

Barium strontium titanate （BST） ferroelectric material exhibits a large dielectricconstant change with an applied dc electric field. The dc electric-field dependent dielectricconstant can be employed to develop the phase shifting elements in phased array antennasand tuning elements in devices operating at microwave frequencies. Barium strontiumtitanate can be used for high-gradient accelerators, including high-power microwaveswitches of linear collider, phase shifters, and tuning layers for the dielectric basedaccelerating structures. Along with high tunability, BST has large dielectric permittivity anddielectric loss, so as hard to match the device impedance. Therefore the dielectric constantand dielectric loss of barium strontium titanate must be reduced while maintaining a hightunability. Forming ferroelectric-dielectric composite is an efficient method to decrease thedielectric constant of ferroelectrics to a low value and maintain tunability at a sufficientlyhigh level.Ba_0.5Sr_0.5TiO₃/MgO/Mg₂TiO₄ceramics and BST/MgO/Mg₂SiO₄ceramics were preparedby a solid-state reaction method. Both tunability and the microwave dielectric propertieswere investigated. The effect of Mn doping on the dielectric properties ofBa_0.5Sr_0.5TiO₃/MgO/Mg₂TiO₄and Ba_0.4Sr_0.55TiO₃/MgO/Mg₂SiO₄were also studied. TheBST/MgO/Mg₂TiO₄and BST/MgO/Mg₂SiO₄ceramics consist of ferroelectric BST phaseand dielectric MgO and Mg₂TiO₄or Mg₂SiO₄phases, respectively. It is observed that thedielectric constant of composite ceramic significantly reduced due to the addition of lineardielectrics. Fixed the Ba_0.5Sr_0.5TiO₃content in Ba_0.5Sr_0.5TiO₃/MgO/Mg₂TiO₄ceramics, withthe increase of Mg₂TiO₄content and the decrease of MgO content, the dielectric constantand tunability of Ba_0.5Sr_0.5TiO₃/MgO/Mg₂TiO₄composite ceramics increase and the Q×fvalue decrease. Ba_0.5Sr_0.5TiO₃/MgO/Mg₂TiO₄composites have dielectric constant of123.0-156.5at microwave frequency and tunability of14.4-28.5%at10kHz under3.9kV/mm, indicating that Ba_0.5Sr_0.5TiO₃/MgO/Mg₂TiO₄is a promising material with lowdielectric constant for tunable microwave applications. In the BST/MgO/Mg₂SiO₄seriesceramic, with the increase of Mg₂SiO₄content and the decrease of MgO content, thedielectric constant decrease and the tunability increase. The unusual increase of tunabilitywith the decrease of dielectric constant can be attribute to the redistribution of electric field. The Q×f values of BST/MgO/Mg₂SiO₄composite ceramics are generally higher than that ofBa_0.5Sr_0.5TiO₃/MgO/Mg₂TiO₄ceramics. Mn doped BST/MgO/Mg₂TiO₄and BST/MgO/Mg₂SiO₄ceramics show better dielectric properties. Mn doping has no obvious effect on the dielectricconstant of BST/MgO/Mg₂SiO₄ceramics, but can increase the Q×f value effectively andtunability slightly. Up to0.5mol%Mn doping can increase the Q×f value of BST/MgO/Mg₂TiO₄ceramics obviously and decrease the tunability. With Mn content exceeding0.5mol%, Mndoping can result in the decrease of Q×f value slightly and the increase of tunability.Compared to the BST/MgO ceramics, BST composite ceramics with multiple metaloxide materials can maintain a stable dielectric constant while improving the tunability, andare promising tunable microwave composite materials.