Anomalous dependencies of the dynamic (pulse) and static tunability [k(U)=C(0)/C(U)] as a function of permittivity (ε) were observed in ferroelectric varactors based on doped paraelectric state (Ba,Sr)TiO3 ceramics. The reduction of the relatively high permittivity value from ε≅810 down to ε≅260 by introducing various proportions of a Mg2TiO4 additive resulted in a 20% increase in tunability. Furthermore, ceramics with this additive have demonstrated dynamic tunability noticeably higher than the static tunability, also unexpected for this type of material.
Significant difference in the capacitance tunability of paraelectric state ferroelectric capacitors under dc and pulse voltages is demonstrated. Along with a fast tuning (τ<10ns) the slow relaxation processes (τ⩾100s) responsible for up to ∼30% variation of the capacitance were observed. The observed effect is a main obstacle for application of paraelectric state ferroelectrics on microwaves.
Abstract. Fast switching (< 10 nsec) measurement results on the recently developed BST(M) (barium strontium titanium oxide composition with magnesium-based additions) ferroelectric materials are presented. These materials can be used as the basis for new advanced technology components suitable for high-gradient accelerators. A ferroelectric ceramic has an electric field-dependent dielectric permittivity that can be altered by applying a bias voltage. Ferroelectric materials offer significant benefits for linear collider applications, in particular, for switching and control elements where a very short response time of <10 nsec is required. The measurement results presented here show that the new BST(M) ceramic exhibits a high tunability factor: a bias field of 40-50 kV/cm reduces the permittivity by a factor of 1.3-1.5. The recently developed technology of gold biasing contact deposition on large diameter (110 cm) thin wall ferroelectric rings allowed ~few nsec switching times in witness sample experiments. The ferroelectric rings can be used at high pulsed power (tens of megawatts) for X-band components as well as at high average power in the range of a few kilowatts for the L-band phaseshifter, under development for optimization of the ILC rf coupling. Accelerator applications include fast active X-band and Ka-band high-power ferroelectric switches, high-power X-band and L-band phase shifters, and tunable dielectric-loaded accelerating structures.
The Ginzburg–Devonshire phenomenological approach is used to define the correlation between temperature and electric field magnitudes required to keep permanent values of permittivity of ferroelectrics in rather broad range of operating temperatures. The existence of a crossover of the electric field dependencies for ε at different temperatures is demonstrated theoretically. It makes it possible to establish the upper limit of electrical field strength necessary to provide the thermal stability of ferroelectric devices. Theoretical results have been confirmed experimentally with (Ba,Sr)TiO3 thin film measurements.
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