Low-loss Microwave Research Articles

Quantum percolation phase transition and magnetoelectric dipole glass in hexagonal ferrites

Hexagonal ferrites do not only have enormous commercial impact ({\pounds}2 billion/year in sales) due to applications that include ultra-high density memories, credit card stripes, magnetic bar codes, small motors and low-loss microwave devices, they also have fascinating magnetic and ferroelectric quantum properties at low temperatures. Here we report the results of tuning the magnetic ordering temperature in PbFe$_{12-x}$Ga$_x$O$_{19}$ to zero by chemical substitution $x$. The phase transition boundary is found to vary as $T_N \sim (1-x/x_c)^{2/3}$ with $x_c$ very close to the calculated spin percolation threshold which we determine by Monte Carlo simulations, indicating that the zero-temperature phase transition is geometrically driven. We find that this produces a form of compositionally-tuned, insulating, ferrimagnetic quantum criticality. Close to the zero temperature phase transition we observe the emergence of an electric-dipole glass induced by magneto-electric coupling. The strong frequency behaviour of the glass freezing temperature $T_m$ has a Vogel-Fulcher dependence with $T_m$ finite, or suppressed below zero in the zero frequency limit, depending on composition $x$. These quantum-mechanical properties, along with the multiplicity of low-lying modes near to the zero-temperature phase transition, are likely to greatly extend applications of hexaferrites into the realm of quantum and cryogenic technologies.

Microwave design and analysis of a micromachined self-heating power sensor based on matching thermocouples

Microwave performance is a basic index of the sensors used at microwave frequencies, but also affects the sensing output. For the purpose of low-loss microwave applications, it is important for different microwave sensors to develop microwave design. This paper presents the microwave design and analysis of a micromachined self-heating microwave power sensor in the GaAs MMIC process, where the microwave power is dissipated and converted into output thermovoltages by two matching thermocouples. A dc-blocking capacitor is connected to the thermocouples in series and used to avoid the output short-circuit. In order to characterize the microwave performance, an S-parameter model of this self-heating power sensor is established. Using the model, the effects of the capacitor and the thermocouples on the reflection loss are investigated under different microwave frequencies. To demonstrate the validity of the microwave model, the microwave performance of the self-heating sensor is simulated using an electromagnetic software. In the simulation, the relationship between the substrate membrane underneath the thermocouples and the reflection loss is analyzed. Measured reflection losses of the self-heating sensor are between −15.5 to −15.9 dB at 8–12 GHz. The measured results show good agreement with the microwave model and simulation, and the source of small deviations is discussed. The proposed microwave design and analysis contributes to achieving low reflection loss for the sensor, with the fact that more power is used to convert into the thermovoltages.

A New Low-loss Niobate-based Microwave Dielectric Ceramic GaNbO4

In this work, the GaNbO4 microwave dielectric ceramics were studied for the first time. Ceramic samples were obtained via solid-state reaction method and could be sintered well in the temperature range of 1000°C–1080°C. Preparation, phase structure, grain morphology and microwave dielectric properties of GaNbO4 were investigated. It showed a novel microstructure that two allotropic GaNbO4 phases with different average grain sizes were coexisted in the GaNbO4 ceramics. The GaNbO4 sintered at 1060°C exhibited the excellent low-loss microwave dielectric properties of er = 15.6, Q×f = 98,800 GHz (at 9.2 GHz), τf = -69.4 ppm/°C.

A novel temperature-stable and low-loss microwave dielectric using Ca0.8Sr0.2TiO3- modified Li2Mg3TiO6 ceramics

Microstructure and microwave dielectric properties of (1-x)Li2Mg3TiO6-xCa0.8Sr0.2TiO3 (x = 0.03, 0.06, 0.08, 0.09, 0.12) ceramics prepared by the conventional solid state route have been investigated. XRD and SEM-EDS analysis confirmed that ceramics contain Li2Mg3TiO6 and Ca0.8Sr0.2TiO3 phases. The microwave properties exhibited a significant dependence on the sintering temperature and Ca0.8Sr0.2TiO3 content. With increasing addition of Ca0.8Sr0.2TiO3, the dielectric constant $$({\varepsilon _r})$$ increased from 14.3 to 17.4 when samples sintered at 1410 °C for 4 h; the quality factor (Q × f) decreased from 133,905 to 101,000 GHz, and the temperature coefficient of resonant frequency ( $${\tau _f}$$ ) significantly increased from −39 to 6.35 ppm/°C when samples sintered at 1370 °C for 4 h. Sample with 0.91Li2Mg3TiO6-0.09Ca0.8Sr0.2TiO3 sintered at 1370 °C for 4 h possess excellent microwave properties with $${\varepsilon _r}$$ of 16.4, a Q × f of 113,000 GHz and $${\tau _f}$$ of −3.65 ppm/°C. It is proposed as low-loss and low-cost dielectric materials for microwave and millimeter wave application.

Annealing effects on the microstructure and magnetic properties of Y3Fe5O12 films deposited on Si/SiO2 substrates by RF magnetron sputtering

Crack-free Yttrium iron garnet (Y3Fe5O12, YIG) thin films were successfully deposited on Si/SiO2 substrates by RF magnetron sputtering, through controlling the annealing temperature and cooling rate during post-annealing process. The annealing condition dependences of crystallinity, microstructures and magnetic properties of YIG films were investigated. With the increase of the annealing temperature, the saturation magnetization of YIG films increases and the coercive field decreases, while the ferromagnetic resonance (FMR) linewidth becomes wider. The films annealed at 750°C with cooling rate of 1°C/min are crack-free and present excellent comprehensive performances, the corresponding coercive field is 32Oe and the FMR linewidth is 57Oe at 9.4GHz. These results indicate that high-quality crack-free YIG films with excellent magnetic properties can be achieved on Si/SiO2 substrates by controlling the annealing process, which makes it more convenient to fabricate low-loss microwave integrated devices.

Li4Mg3Ti2O9: A novel low-loss microwave dielectric ceramic for LTCC applications

Low-loss novel Li4Mg3Ti2O9 dielectric ceramics with rock-salt structure were prepared by a conventional solid-state route. The crystalline structure, chemical bond properties, infrared spectroscopy and microwave dielectric properties of the abovementioned system were initially investigated. It could be concluded from this work that the extrinsic factors such as sintering temperatures and grain sizes significantly affected the dielectric properties of Li4Mg3Ti2O9 at lower sintering temperatures, while the intrinsic factors like bond ionicity and lattice energy played a dominant role when the ceramics were densified at 1450°C. In order to explore the origin of intrinsic characteristics, complex dielectric constants (ε’ and ε’’) were calculated by the infrared spectra, which indicated that the absorptions of phonon oscillation predominantly effected the polarization of the ceramics. The Li4Mg3Ti2O9 ceramics sintered at 1450°C exhibited excellent properties of εr=15.97, Q·f=135,800GHz and τf=−7.06ppm/°C. In addition, certain amounts of lithium fluoride (LiF) were added to lower the sintering temperatures of matrix. The Li4Mg3Ti2O9−3wt% LiF ceramics sintered at 900°C possessed suitable dielectric properties of εr=15.17, Q·f =42,800GHz and τf=−11.30ppm/°C, which made such materials promising for low temperature co-fired ceramic applications (LTCC).

High permittivity and low loss microwave dielectrics suitable for 5G resonators and low temperature co-fired ceramic architecture

Bi2(Li0.5Ta1.5)O7 ceramics possess a εr of 65.1, a Qf of 15 500 GHz and a TCF of −17.5 ppm °C−1. The sintering temperature was lowered to 920 °C by the addition of 2 mol% Bi2O3, which makes them potential candidates for dielectric resonators and LTCC applications.

Wireless power transfer based on dielectric resonators with colossal permittivity

Magnetic resonant wireless power transfer system based on dielectric disk resonators made of colossal permittivity (ε = 1000) and low loss (tan δ = 2.5 × 10–4) microwave ceramic is experimentally investigated. The system operates at the magnetic dipole mode excited in the resonators providing maximal power transfer efficiency of 90% at the frequency 232 MHz. By applying an impedance matching technique, the efficiency of 50% is achieved within the separation between the resonators d = 16 cm (3.8 radii of the resonator). The separation, misalignment and rotation dependencies of wireless power transfer efficiency are experimentally studied.

Low dielectric loss characteristics of [(Mg1−xZnx)0.95Co0.05]1.02TiO3.02 ceramics at microwave frequencies

Low-loss microwave dielectric materials systems of [(Mg1−xZnx)0.95Co0.05]1.02TiO3.02 ceramics were fabricated using solid-state synthesis. Partial Zn2+ and Co2+ substitutions for Mg2+ of [(Mg1−xZnx)0.95Co0.05]1.02TiO3.02 enhanced the Q × f value and allowed densification sintering at lower temperature than MgTiO3 which must be sintered around 1350–1400 °C. [(Mg0.6Zn0.4)0.95Co0.05]1.02TiO3.02 sintered at 1175 °C has an excellent microwave dielectric properties with dielectric constant of 20.1, a Q × f value of 280,000 GHz, and a τf value of −64.7 ppm/°C. (Mg0.6Zn0.4)0.95Co0.05]1.02TiO3.02 ceramic shows high potential for applications in mobile communications.

Ultrahigh Q values and atmosphere-controlled sintering of Li2(1+x)Mg3ZrO6 microwave dielectric ceramics

Ultrahigh-Q Li2(1+x)Mg3ZrO6 microwave dielectric ceramics were successfully prepared by means of atmosphere-controlled sintering through simultaneously adopting double crucibles and sacrificial powder. This technique played an effective role in suppressing the lithium volatilization and further promoting the formation of the liquid phase, as evidenced by the X-ray diffraction, microstructural observation and the density measurement. Both dense and even microstructure, and the suppression of detrimental secondary phases contributed to low-loss microwave dielectric ceramics with Q×f values of 150,000–300,000GHz. Particularly, desirable microwave dielectric properties of εr=12.8, Q×f=307,319GHz (@9.88GHz), and τf=−35ppm/°C were achieved in the x=0.06 sample as sintered at 1275 °C for 6h.

The Field-Programmable Microwave Substrate

A novel low-loss programmable microwave waveguide is demonstrated. The waveguide is developed for applications in field-programmable microwave circuits with similar levels of programmability as that seen in field-programmable gate arrays. The waveguide is realized with a field-programmable microwave substrate (FPMS). The substrate consists of small unit cells that can be individually reconfigured to have a range of positive dielectric constants or a negative dielectric constant. The substrate is contained in a metal parallel plate structure. Programming a positive material sandwiched between two negative material sidewalls results in a waveguide that behaves in a way that is described by the slab waveguide equations. Miniaturization and increased density of unit cells can result in lower loss and better performance. Two implementations of the FPMS are presented. One implementation is on FR4 using all industry standard processes and components. The other implementation uses low-temperature cofired ceramic and custom designed CMOS chips. The FR4 implementation is used to demonstrate programmable waveguides, amplifiers, and oscillators from 0.9 to 3 GHz. The LTCC implementation is used to demonstrate miniaturization of the FPMS. The proposed FPMS concept promises a new era of programmable microwave circuit design for cognitive radio, the Internet of things, and self-healing/adaptive systems.

Superconducting Pb stripline resonators in parallel magnetic field and their application for microwave spectroscopy

Planar superconducting microwave resonators are key elements in a variety of technical applications and also act as sensitive probes for microwave spectroscopy of various materials of interest in present solid state research. Here superconducting Pb is a suitable material as a basis for microwave stripline resonators. To utilize Pb stripline resonators in a variable magnetic field (e.g. in ESR measurements), the electrodynamics of such resonators in a finite magnetic field has to be fully understood. Therefore we performed microwave transmission measurements (with ample applied power to work in linear response) on superconducting Pb stripline resonators in a variable, parallel magnetic field. We determined surface resistance, penetration depth, as well as real and imaginary parts, and , of the complex conductivity of superconducting Pb as a function of a magnetic field. Here we find features reminiscent of those in temperature-dependent measurements, such as a maximum in (coherence peak). At magnetic fields above the critical field of this type-I superconductor we still find a low-loss microwave response, which we assign to remaining superconductivity in the form of filaments within the Pb. Hysteresis effects are found in the quality factor of resonances once the swept magnetic field has exceeded the critical magnetic field. This is due to normal conducting areas that are pinned and can therefore persist in the superconducting phase. Besides zero-field-cooling we show an alternative way to eliminate these even at . Based on our microwave data, we also determine the critical magnetic field and the critical temperature of Pb in a temperature range between 1.6 K and 6.5 K and magnetic fields up to 140 mT, showing good agreement with BCS predictions. We also study a Sn sample in a Pb resonator to demonstrate the applicability of superconducting Pb stripline resonators in the experimental study of other (super-)conducting materials in a variable magnetic field.

Structure evolution and microwave dielectric characteristics of Ca[(AlxGa0.5-xNb0.5)0.5Ti0.5]O3 ceramics

The microwave dielectric characteristics of Ca[(AlxGa0.5-xNb0.5)0.5Ti0.5]O3 (x = 0, 0.1, 0.2, 0.4, 0.5) ceramics were investigated together with the structure evolution. Distorted perovskite structure in space group Pbnm with b-b-c+ tilting was determined for the present solid solutions, where the microstructures were remarkably affected by Al-content. Owing to the Al-substitution which was accompanied by the subdued octahedral tilting, the Qf value reached up to 40,000 GHz with increasing x, the dielectric constant (εr) remained at around 50, and the temperature coefficient of resonant frequency (τf) could be adjusted to near zero. Low-loss temperature-stable microwave dielectric ceramics with medium dielectric constant were obtained at x = 0.2–0.5, which exhibited Qf values of 35,900–40,000 GHz, εr of 49.2–53.4, and τf of 0.6–3.5 ppm/°C.

Characterization of low loss microwave dielectric materials Zn0.92Co0.08ZrNb2O8 based on the complex chemical bond theory

Low loss microwave dielectric ceramics Zn0.92Co0.08ZrNb2O8 with wolframite structure were prepared by the solid state method. Sintering characteristics, phase composition and microwave dielectric properties were investigated systematically. X-ray diffraction results showed that all samples exhibited a wolframite structure. Rietveld refinement was used to analyze the crystal structure. Based on the lattice parameters, the bond ionicity, the lattice energy, the bond energy and the coefficient of thermal expansion were calculated to evaluate the structural characteristics. The e r values could be strongly dependent on the bond ionicity of the Nb–O bonds. The Q·j values were main affected by the lattice energy and the bond energy of the Nb–O bonds. As for the τ f values, it could be mainly affected by the coefficient of thermal expansion of Zn/Co–O bonds. At the sintering temperature of 1200 °C, Zn0.92Co0.08ZrNb2O8 ceramics exhibited the excellent microwave properties of e r = 27.10, Q·f = 61,036 GHz (at 8.54 GHz) and τ f = −9.81 ppm/°C. These results demonstrated that Zn0.92Co0.08ZrNb2O8 ceramics could be promising candidate materials for the application of highly selective microwave ceramic resonators and filters.

Characterization and microwave dielectric properties of new low loss Li2MgZrO4 ceramics

Low loss microwave dielectric materials Li2MgZrO4 were synthesized by solid-state route. The relationship among sintering characteristics, phase composition, microstructure and microwave dielectric properties of Li2MgZrO4 ceramics were firstly investigated using X-Ray Diffraction, Scanning Electron Microscopy and Network Analyzer. The formation of secondary phases and evaporation of lithium were observed when sintering temperatures exceeded 1175°C. Microwave dielectric properties of Li2MgZrO4 ceramics were affected by sintering characteristics and second phase. In addition, the bond energy of Li2MgZrO4 ceramics was also reported to analyze intrinsic loss. Typically, the ceramics sintered at 1175°C for 4h exhibited excellent microwave dielectric properties with εr=12.30, Q·f =40,900GHz and τf=−12.31ppm/°C.

A novel low loss microwave dielectric ceramics ZnTi0.6Sn0.4Nb2O8 with wolframite structure

Wolframite-structure ZnTi0.6Sn0.4Nb2O8 was firstly prepared by the conventional solid-state method. X-ray diffraction patterns showed that the samples exhibited wolframite structure instead of ixiolite-structure of ZnTiNb2O8. Microwave dielectric properties of ZnTi0.6Sn0.4Nb2O8 ceramics exhibited a significant dependence on sintering characteristics and crystal structure. The phase transition in ZnTi0.6Sn0.4Nb2O8 ceramics played an important role in reducing dielectric loss. In addition, the bond energy of ceramics was also reported for the first time to analyze intrinsic loss. Typically, the ZnTi0.6Sn0.4Nb2O8 ceramics sintered at 1200°C for 4h exhibited excellent microwave dielectric properties with εr=24.61, Q·f =69,500GHz and τf=−2.67ppm/°C.

Characterization on low loss dielectric Li2MgTiO4 ceramics based on chemical bond theory at microwave frequency

The low loss microwave dielectric materials Li2MgTiO4 with rock salt structure were prepared by the modified solid state method. Sintering characteristics and microwave dielectric properties were investigated under various sintering temperatures ranging from 1200 °C to 1375 °C. The crystal structure was studied by the X-ray diffraction and the micro-structure was studied by the scanning electron microscopy (SEM) techniques. Rietveld refinement was used to analyze the structure of the crystalline phase. Based on the cell parameters, the bond ionicity, lattice energy, bond energy and coefficient of thermal expansion were calculated to evaluate the structural characteristics. The εr values could be strongly dependent on the bond ionicity of the TiO bond. The Q·ƒ values were affected by the lattice energy of TiO bonds. As for the τf values, it could be mainly affected by the coefficient of thermal expansion of LiO bonds. At the sintering temperature of 1350 °C, Li2MgTiO4 ceramics exhibited the excellent microwave properties of εr = 15.07, Q·ƒ = 97,629 GHz (at 8.2 GHz) and τf = −3.81 ppm/°C. These results demonstrated that Li2MgTiO4 ceramics could be promising candidate materials for the application of highly selective microwave ceramic resonators and filters.

Microwave study of superconducting Sn films above and below percolation

The electronic properties of superconducting Sn films () change significantly when reducing the film thickness down to a few , in particular close to the percolation threshold. The low-energy electrodynamics of such Sn samples can be probed via microwave spectroscopy, e.g. with superconducting stripline resonators. Here we study Sn thin films, deposited via thermal evaporation—ranging in thickness between 38 and —which encompasses the percolation transition. We use superconducting Pb stripline resonators to probe the microwave response of these Sn films in a frequency range between 4 and at temperatures from down to . The measured quality factor of the resonators decreases with rising temperature due to enhanced losses. As a function of the sample thickness we observe three regimes with significantly different properties: samples below percolation, i.e. ensembles of disconnected superconducting islands, exhibit dielectric properties with negligible losses, demonstrating that macroscopic current paths are required for appreciable dynamical conductivity of Sn at GHz frequencies. Thick Sn films, as the other limit, lead to low-loss resonances both above and below Tc of Sn, as expected for bulk conductors. But in an intermediate thickness regime, just above percolation and with labyrinth-like morphology of the Sn, we observe a quite different behavior: the superconducting state has a microwave response similar to the thicker, completely covering films with low microwave losses; but the metallic state of these Sn films is so lossy that resonator operation is suppressed completely.

Sintering characteristics and microwave dielectric properties of low loss CoZrNb2O8 ceramics achieved by reaction sintering process

The low loss microwave dielectric materials CoZrNb2O8 with monoclinic wolframite structure were prepared by the reaction sintering process based on conventional solid-state method. The samples were characterized by means of X-ray diffraction, scanning electronic microscopes and energy dispersive X-ray spectroscopy, and the microwave dielectric properties of these samples were investigated using the network analyzer. The results showed that the dielectric properties are strongly dependent on the composition, densification and microstructure of the specimens. Out of these compositions, CoZrNb2O8 with nearly full density obtained at the lower sintering temperature of 1250 °C with excellent microwave properties of ɛr value of 24.40, Q·ƒ of 43,123 GHz and τf of −46.99 ppm/°C. To evaluate the effects of intrinsic factors on the microwave dielectric properties, the chemical bond properties were calculated based on the chemical bond theory.

A low-firing Ca5Ni4(VO4)6 ceramic with tunable microwave dielectric properties and chemical compatibility with Ag

A Ca5Ni4(VO4)6 low loss microwave dielectric ceramic with A site deficient garnet structure was prepared via the conventional solid state reaction method. Ca5Ni4(VO4)6 sintered at 980°C for 4h to a relative density of 96.2% exhibits favorable microwave dielectric properties such as a permittivity of 10.9, a Q×f value of 96,500GHz, and a τf value of −63.6ppm/°C. Its large negative τf could be compensated by forming a solid solution with LiCa2Mg2V3O12, that led to improved properties with a near-zero τf=−3.7ppm/°C, εr=10.2, Q×f=59,300GHz for 0.8Ca5Ni4(VO4)6−0.2LiCa2Mg2V3O12 after sintering at 955°C for 4h. Further on, 0.8Ca5Ni4(VO4)6−0.2LiCa2Mg2V3O12 proved to be chemically compatible with Ag electrodes, so it might be a possible candidate for LTCC applications.