Joachim R. Binder

Beam Steering Transmitarray Using Tunable Frequency Selective Surface With Integrated Ferroelectric Varactors

A tunable frequency selective surface (FSS) with beam steering capability is presented. The FSS is used as a transmitarray with a bandpass characteristic in Ku-band. The periodic sub-wavelength (λ0/25) unit-cells are composed of capacitive and inductive structures creating a bandpass for an incident wave. By patterning the capacitive elements on a screen-printed barium-strontium-titanate (BST) thick-film ceramic, the resonant frequency of the FSS can be tuned. This technology offers a simple and cost effective way for integrating varactors into the FSS and is particularly attractive for microwave circuits with a high varactor density. A prototype is fabricated including two capacitive layers with an overall size of 40 mm × 40 mm. Each layer includes a total of 1600 integrated BST varactors that are fabricated by using a patterning and metallization process. A transmitted wave passing through the FSS will experience a phase shift, which can be tuned by tuning the passband. This allows steering the transmitted wave to a certain direction by applying a phase gradient along the FSS interface. A phase shift range of 360° can be covered by cascading several FSS panels. The prototyped tunable FSS demonstrates the feasibility of the proposed technology and its potential for beam steering.

Low-Cost Phased-Array Antenna Using Compact Tunable Phase Shifters Based on Ferroelectric Ceramics

A low-cost phased-array antenna at 10 GHz is presented for a scan angle of ±50°. The array employs continuously tunable phase shifters based on a screen printed barium-strontium-titanate thick-film ceramic. Due to the use of artificial transmission line topology, the proposed phase-shifter design has a very compact size (3 mm × 2.8 mm) for 342° total phase shift. In the frequency range from 8 to 10 GHz, it exhibits a figure of merit >;52°/dB, which is among the best of phase shifters based on ferroelectric thick films. In a prototyped phased array, the RF circuit consists of a feeding network, phase shifters, and antenna elements, which are integrated into one planar metallization layer. Furthermore, a simple way for routing bias lines for phase shifters is demonstrated using high resistive electrodes. Using screen printed thick films and applying a simplified fabrication process for the RF and bias circuitry can reduce the total expense of phased arrays considerably.

Compact Substrate Integrated Waveguide Tunable Filter Based on Ferroelectric Ceramics

This letter presents the design and the realization of a compact tunable filter integrated in ferroelectric ceramic substrate. The bandpass filter bases on an evanescent-mode dielectric cavity, which is loaded by a pair of tunable mushroom-type complementary split-ring resonators. Tunable impedance matching networks are additionally implemented to improve passband insertion loss across the tuning range. A prototype has been realized in a 12.5 mm × 9.5 mm × 0.8 mm planar module. Measurements confirm a frequency coverage from 2.95 to 3.57 GHz, a 3 dB fractional bandwidth below 5.4%, with an insertion loss between 3.3 dB and 2.6 dB.

Influence of Fe–F-co-doping on the dielectric properties of Ba0.6Sr0.4TiO3 thick-films

Abstract 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.

Barium strontium titanate thick-films: dependency between dielectric performance and their morphology

The paper is focused on the influence of the morphology of barium strontium titanate (BST) thick-films on their effective dielectric properties. Therefore two BST thick-films are made with a minor deviation in sinter temperature, and hence their mean grain size of the films and their sintering necks. By modeling the meso-structure using a Nonlinear 3D Finite-Difference Time-Domain field solver the assumed effect of the morphology on the effective dielectric properties are confirmed. The simulation results show the potential for a further improvement of the dielectric tunability of BST thick-films.

Improving mechanical fatigue resistance by optimizing the nanoporous structure of inkjet-printed Ag electrodes for flexible devices

The development of highly conductive metallic electrodes with long-term reliability is in great demand for real industrialization of flexible electronics, which undergo repeated mechanical deformation during service. In the case of vacuum-deposited metallic electrodes, adequate conductivity is provided, but it degrades gradually during cyclic mechanical deformation. Here, we demonstrate a long-term reliable Ag electrode by inkjet printing. The electrical conductivity and the mechanical reliability during cyclic bending are investigated with respect to the nanoporous microstructure caused by post heat treatment, and are compared to those of evaporated Ag films of the same thickness. It is shown that there is an optimized nanoporous microstructure for inkjet-printed Ag films, which provides a high conductivity and improved reliability. It is argued that the nanoporous microstructure ensures connectivity within the particle network and at the same time reduces plastic deformation and the formation of fatigue damage. This concept provides a new guideline to develop an efficient method for highly conductive and reliable metallic electrodes for flexible electronics.

Unravelling the mechanism of lithium insertion into and extraction from trirutile-type LiNiFeF6 cathode material for Li-ion batteries

LiNiFeF6 was used as cathode material in lithium-ion cells and studied by in situ X-ray diffraction (XRD), in operando X-ray absorption spectroscopy (XAS) and 7Li MAS NMR spectroscopy. An optimised electrochemical in situ cell was employed for the structural and electrochemical characterisation of LiNiFeF6 upon galvanostatic cycling. The results for the first time reveal the lithium insertion process into a quaternary lithium transition metal fluoride with a trirutil-type host structure (space group P42/mnm). The in situ diffraction experiments indicate a preservation of the structure type after repeated lithium insertion and extraction. The lithium insertion reaction can be attributed to a phase separation mechanism between Li-poor Li1+x1NiFeF6 and Li-rich Li1+x2NiFeF6 (x1 ≲ 0.16 ≲ x2), where not only the weight fractions, but also the lattice parameters of the reacting phases change. The insertion of Li ions into [001]-channels of the trirutile structure causes an anisotropic lattice expansion along the tetragonal a-axes. An overall increase in the unit cell volume of ~6% and a reduction in the c/a ratio of ~4% are detected during discharge. Changes of atomic coordinates and distances suggest the accommodation of intercalated lithium in the empty six-fold coordinated 4c site. This is confirmed by 7Li MAS NMR spectroscopy showing two Li environments with similar intensities after discharging to 2.0 V. Furthermore, in operando XAS investigations revealed that only Fe3+ cations participate in the electrochemical process via an Fe3+/Fe2+ redox reaction, while Ni2+ cations remain electrochemically inactive.

Beam Steering Phased Array Antenna With Fully Printed Phase Shifters Based on Low-Temperature Sintered BST-Composite Thick Films

This letter presents a novel approach to fully-printed phase shifters for electronically steering the beam of a phased array antenna at S-Band. The phased array consists of four microstrip patch antenna elements and phase shifters, as well as a four-to-one feeding network. As key components in the circuit, the phase shifters are provided with a microstrip loaded-line topology, equipping fully-printed metal-insulator-metal (MIM) varactors with low-temperature sintered BST composite thick films between the top and bottom silver electrodes. Furthermore, a simple biasing concept is implemented for controlling the phase shifter. The fully-printed, low-cost phase shifters achieved a phase shift of 274° and a FoM of 37.3°/dB at 3 GHz. The length of the phase shifters was 0.2λ, where their phase shift versus length resulted in a 14.4°/mm measurement, outperforming all previously reported phase shifters based on fully-printed low-temperature sintered BST thick film. The beam-scanning range of the four-antenna element array was ±30°.

Steerable Dielectric Resonator Phased-Array Antenna Based on Inkjet-Printed Tunable Phase Shifter With BST Metal-Insulator-Metal Varactors

This letter presents a steerable phased-array antenna at C/X-band. The phase shifters used in the design, are implementing inkjet-printed barium strontium titanate (BST) thick-films. This method enables low-voltage tuning and low fabrication cost. The phase shifter is tuned by integrated metal-insulator-metal varactors, whose electrodes are fabricated by photolithography and inkjet-printed dielectric layer. A tunability of 46% at 8 GHz is achieved by applying 50 V across a 1.2- μm-thick BST film. The 11-unit-cells loaded-line tunable phase shifter achieves a figure of merit above 40°/dB from 7 to 8.5 GHz. Dielectric resonator antenna, fabricated from bulk-glass ceramic, has been implemented as radiating element with a stacked architecture for wide bandwidth and high gain. A beam steering of ±30° has been measured with a 1 ×4-element phased array in the anechoic chamber.

Tunable dielectrics for microwave applications

This paper gives a short introduction into tunable dielectrics for microwave applications based on nematic liquid crystal and Barium-Strontium-Titanate (BST) thin- and thick-films. Due to their tuning voltage dependent electromagnetic properties, both are very promising material candidates for tunable passives in the microwave and millimeter-wave regions. For applications, such as frequency-agile communications and sensor systems we illustrate the challenges of optimization BST films based on a multi-scale view for material, processing, device and application design.