Excellent Microwave Performance Research Articles

Phase purifying and dielectric improvement in Zn2SiO4 ceramics via simply controlling the particle size of SiO2

AbstractThe zinc orthosilicate, Zn2SiO4 ceramics, exhibit great potential as microwave dielectric materials due to their low dielectric constant and ultrahigh quality factor. The solid‐state synthesis of Zn2SiO4 ceramics often results in the presence of a secondary phase of ZnO, however, leading to significantly lower Q×f values. Cold‐isostatic‐pressing (CIP) and nonstoichiometry have been proven effective in purifying the Zn2SiO4 but need elaborate process and composition manipulation. In this work, it was found that the particle size of SiO2 plays a crucial role in purifying the Zn2SiO4 phase. Specially, nano‐sized SiO2 particles (15 and 30 nm) led to the formation of a ZnO secondary phase in synthesized Zn2SiO4 ceramics, while micro‐sized (2 and 10 µm) resulted in pure‐phase Zn2SiO4 with excellent microwave properties. Notably, when prepared with 10 µm SiO2 and sintered at 1 340°C, the resulting Zn2SiO4 exhibited outstanding microwave performances with a Q×f value of 122 570 GHz, a temperature coefficient of frequency τf of −51.9 ppm/°C, and a dielectric constant (εr) of 6.5. These findings highlight the importance of controlling particle size during synthesis to achieve desired material properties such as purity and excellent microwave performances.

Novel Tri-rutile Ni0.5Ti0.5TaO4 Microwave Dielectric Ceramics: Crystal Structure Chemistry, Raman Vibration Mode, and Chemical Bond Characteristic In-Depth Studies

Here, a novel tri-rutile Ni0.5Ti0.5TaO4 ceramic configuration was first reported, and the sintering features, chemical bond theory, and microwave dielectric properties were systematically explored. Pure Ni0.5Ti0.5TaO4 ceramic with a tri-rutile structure was well sintered at 1275 °C with excellent microwave performance: εr ∼33.06, Q × f ∼14,600 GHz, and τf ∼93.95 ppm/°C. Based on the P–V–L theory, the dielectric constant was mainly affected by the formation of Ta–O bonds (∼62%) due to the larger ionic polarization and bond susceptibility. The experimental dielectric constant (33.06) was comparable to the theoretical calculation (P–V–L: 35.11). The lattice energy was also dominated by the Ta–O bond (∼71%) and the intrinsic loss properties. The τf was also affected by the [M1/M2O6] octahedral distortion. In addition, the relative density, grain size, and packing fraction also played a certain role in the microwave dielectric properties. Insights from the Raman spectroscopy revealed that the A1g mode dominated the Raman vibration at 699.44 cm–1. These results were helpful to provide ideas for novel medium dielectric constant materials and contributed to the miniaturization of microwave electronic devices.

Effect of Cu2+ substitution on the crystal structure, bond properties, and microwave performance of ZnTiNb2O8 ceramics

Dense Zn1-xCuxTiNb2O8 (x = 0.000, 0.002, 0.005, 0.007, 0.010, 0.020, 0.030, 0.040, and 0.050) ceramics were synthesized using a traditional solid-state reaction approach. In the past, CuO was employed as a sintering additive to reduce the sintering temperature. In this paper, the influence of Cu2+ substitution on crystal structure, microstructure, bond properties, and microwave performance in ceramics with rutile-like structure was thoroughly investigated for the first time. According to the microstructures investigation, a moderate degree of Cu2+ substitution was shown to be useful for establishing uniform grain size and limiting dielectric loss. Furthermore, Raman spectroscopy and complex chemical bond theory were used to identify the relationship between microwave performance and intrinsic components in order to further investigate the influence mechanism of microwave performance. The results support that the determining factor of εr value is ionic polarizability. The major factors of Q×f value are the total lattice energy and the FWHM of Ag(2) stretch mode in Raman spectra. In addition, the τf value is mainly regulated by the total bond energy. Especially, the Zn1-xCuxTiNb2O8 (x = 0.005) ceramic exhibited excellent microwave performance: εr = 31.55, Q×f = 62,045 GHz, and τf = −48.01 ppm/°C, making it a viable choice for microwave components.

Lightweight multi-walled carbon nanotube buckypaper/glass fiber–epoxy composites for strong electromagnetic interference shielding and efficient microwave absorption

Multi-walled carbon nanotube buckypaper (BP) reinforced glass fiber–epoxy (GF/EP) composites were selected to fabricate electromagnetic interference (EMI) shielding and microwave absorbing materials. Six different composite configurations with 3.0 mm thick have been conceived and tested over the X-band (8.2–12.4 GHz). Flexible and low-density (0.29 g/cm3) BP provided a high specific EMI SE of 76 dB with controlled electrical conductivity. GF/EP/BP111 and GF/EP/BP101 composites possess EMI SE as high as of 50–60 dB, which can be attributed to the number of BP inserted and variation in the wave-transmitting layer of the laminates. Furthermore, the shielding mechanism was discussed and suggested that the absorption was the dominant contribution to EMI SE. GF/EP/BP110 laminate demonstrated suitable EMI performance (~ 20 dB), whereas GF/EP/BP011 composite revealed excellent microwave performance, achieving an effective − 10 dB bandwidth of 3.04 GHz and minimum reflection loss (RL) value of − 21.16 dB at 10.37 GHz. On the basis of these results, GF/EP/BP composites prepared in this work have potential applications as both EMI shielding and microwave absorber materials given their facile preparation and lightweight use.

Ultralow-loss (1-x)CaWO4-xNa2WO4 (x = 0.1, 0.2) microwave dielectric ceramic for LTCC applications

In this work, the (1-x)CaWO4-xNa2WO4 (x = 0.1, 0.2, denoted as 0.9CW-0.1NW and 0.8CW-0.2NW, respectively) ultralow-loss microwave dielectric ceramics were prepared via solid-state reaction method. Using low melting-point Na2WO4 as sintering aid to prepare CaWO4Na2WO4 composite ceramics, the sintering temperature of CaWO4 was successfully reduced while maintaining excellent microwave performance. The optimal microwave dielectric properties have been achieved at 900 °C for 0.9CW-0.1NW ceramic: εr = 9.0, Q × f = 105660 GHz, tanδ = 1.1 × 10−4 and τf = −35.4 ppm/°C at a frequency of 12.0 GHz. For the 0.8CW-0.2NW ceramic, the optimal microwave dielectric properties have been obtained at 740 °C, with εr = 8.5, Q × f = 97014 GHz, tanδ = 1.2 × 10−4 and τf = −37.4 ppm/°C at a frequency of 11.8 GHz. In summary, both composite ceramics exhibit low sintering temperatures, excellent dielectric properties and chemical compatibility with the Ag electrode. The findings of this study provide an effective approach to prepare novel composite ceramics as promising candidates for LTCC applications.

Sintering behavior and microwave performance of CaSiO3 ceramics doped with BaCu(B2O5) for LTCC applications

Abstract CaSiO3 (CS) ceramics exhibit excellent microwave performance as a microwave material, however, its sintering temperature is too high and limits its application in the microwave field. In this work, the densification process and microwave performance of the BaCu(B2O5) (BCB) doped CS ceramics were researched. The results showed that the sintering temperature of CS ceramics can be lowered from 1340 °C to 930 °C by BCB additive. In order to obtain ceramic samples with excellent microwave properties and high density, adding 4 wt% BCB to CS ceramics was considered to be optimal. Under this formulation, BCB-CS ceramics sintered at low sintering temperatures (950 °C for 3 h), possessed outstanding microwave performance of e r = 4.7 , Q × f = 34807.5 G H z , τ f = − 21.5 p p m / ° C . Moreover, CS ceramics doped with BCB had a good chemical compatibility with silver powder, which indicated that BCB-CS ceramic was a prospective candidate for low temperature co-fired ceramic (LTCC) applications.

Device and Compact Circuit-Level Modeling of Graphene Field-Effect Transistors for RF and Microwave Applications

Graphene field-effect transistors (GFETs) are promising candidates for future nano-electronic circuitry with excellent radio frequency (RF) and microwave performance due to the ultra-high carrier mobility, large saturation velocity, and good electrical conductivity of the graphene channel. In this paper, a compact circuit-level model of GFETs is proposed for RF and microwave high-frequency applications. An improved double-gate monolayer short channel GFET with an agate length of 150 nm was first designed and fabricated by self-aligned gate processing, enabled by optimized organic, contamination-free graphene transfer, for realizing high working frequency up to 40 GHz. A compact, precise circuit-level model, including the linear and nonlinear operation of GFETs, is then proposed with model parameters extracted based on the test data from the fabricated GFETs. The proposed model is capable of precisely simulating the ambipolar status of GFETs working up to microwave frequency. The calculated errors of the S-parameters, the reflection coefficient, the gain, (all in decibel), and the I–V data (in decimal) are less than 3.5%, 4.1%, 3.1%, and 2.4%, respectively. By defining as the symbolically defined device module, the GFET model with gate dimensional scalability can be easily embedded in the common commercial simulators for RF and microwave circuit and system level designs.

A GaN–SiC hybrid material for high-frequency and power electronics

We demonstrate that 3.5% in-plane lattice mismatch between GaN (0001) epitaxial layers and SiC (0001) substrates can be accommodated without triggering extended defects over large areas using a grain-boundary-free AlN nucleation layer (NL). Defect formation in the initial epitaxial growth phase is thus significantly alleviated, confirmed by various characterization techniques. As a result, a high-quality 0.2-μm thin GaN layer can be grown on the AlN NL and directly serve as a channel layer in power devices, like high electron mobility transistors (HEMTs). The channel electrons exhibit a state-of-the-art mobility of >2000 cm2/V-s, in the AlGaN/GaN heterostructures without a conventional thick C- or Fe-doped buffer layer. The highly scaled transistor processed on the heterostructure with a nearly perfect GaN–SiC interface shows excellent DC and microwave performances. A peak RF power density of 5.8 W/mm was obtained at VDSQ = 40 V and a fundamental frequency of 30 GHz. Moreover, an unpassivated 0.2-μm GaN/AlN/SiC stack shows lateral and vertical breakdowns at 1.5 kV. Perfecting the GaN–SiC interface enables a GaN–SiC hybrid material that combines the high-electron-velocity thin GaN with the high-breakdown bulk SiC, which promises further advances in a wide spectrum of high-frequency and power electronics.

Microfluidical Microwave Reactor for Synthesis of Gold Nanoparticles.

Microwave treatment can reduce the time of selected syntheses, for instance of gold nanoparticles (AuNPs), from several hours to a few minutes. We propose a microfluidic structure for enhancing the rate of chemical reactions using microwave energy. This reactor is designed to control microwave energy with much higher accuracy than in standard devices. Thanks to this, the influence of microwave irradiation on the rate of chemical reactions can be investigated. The reactor consists of a transmission line surrounded by ground metallization. In order to deliver microwave energy to the fluid under test efficiently, matching networks are used and optimized by means of numerical methods. The monolithic device is fabricated in the low temperature co-fired ceramics (LTCC) technology. This material exhibits excellent microwave performance and is resistant to many chemical substances as well as high temperatures. Fabrication of the devices is described in detail. Measurements of microwave parameters are performed and differences between simulation and experiment results are discussed. Finally, the usefulness of the proposed device is proved in exemplary synthesis.

Sintering behavior and microwave dielectric properties of LiMVO4 (M = Mg, Zn)

Two V-containing microwave dielectric ceramics, LiMgVO4 and LiZnVO4, with excellent microwave performance were prepared by a solid-state reaction route at relatively low temperatures. Using NH4VO3 to replace V2O5 could effectively reduce the sintering temperature of LiMgVO4 and LiZnVO4 to 720 °C and enhance their microwave dielectric properties. LiMgVO4 ceramics sintered at 720 °C exhibited a permittivity ~9.89, a Q × f value of 30,800 GHz and a temperature coefficient of resonant frequency ~−171 ppm/°C. LiZnVO4 ceramics had a relatively smaller τf value of −114 ppm/°C with a er ~ 7.48, a Q × f value of 27,600 GHz. Further efforts are in progress to search for effective approaches to compensate their large negative τf values to be near zero.

Microfluidical Microwave Reactor for Accelerating Chemical Reactions

Microwave treatment can reduce the time of selected syntheses, for instance of the gold nanoparticles, from several hours to the few minutes. We propose microfluidic structure for enhancing rate of chemical reactions using microwave power. This reactor is designed to control microwave power with much higher accuracy than in standard devices. Thanks to this the influence of microwave irradiation on the rate of chemical reactions can be investigated. The reactor consists of transmission line surrounded by the ground metallization. In order to efficient deliver microwave power to the fluid matching networks are optimized using numerical methods. The monolithic device is fabricated in the Low Temperature Co-fired Ceramics (LTCC) technology. This material exhibits excellent microwave performance, and is resistant to many chemical substances as well as high temperature. Fabrication of the devices is described in details. Measurements of microwave parameters are performed and differences between simulation and experiment results are discussed.

High emission power and Q factor in spin torque vortex oscillator consisting of FeB free layer

Microwave oscillation properties of spin torque vortex oscillators (STVOs) consisting of an FeB vortex free layer were investigated. Because of a high MR ratio and large DC current, a high emission power up to 3.6 µW was attained in the STVO with a thin FeB free layer of 3 nm. In STOs with a thicker FeB layer, e.g., 10 nm thick, we obtained a large Q factor greater than 6400 while maintaining a large integrated emission power of 1.4 µW. Such excellent microwave performance is a breakthrough for the mutual phase locking of STVOs by electrical coupling.

An X‐band Wilkinson power divider and comparison with its miniaturization based on GaAs MMIC process

ABSTRACTThis article presents an X‐band Wilkinson power divider which is completely compatible with GaAs monolithic microwave integrated circuit (MMIC). The novelty is that, GaAs is chosen for the realization for substrate because of its excellent microwave performance, and Au is used to fabricate transmission lines and air bridges due to its good electric properties. The measured return loss of port 1 is about −32 dB at the center frequency of 9 GHz and less than −16 dB over X‐band. The insertion loss of port 2 and 3 is close to 3.3 dB at 8–12 GHz. The measured bandwidth for which the input return loss is less than −20 dB is about 3.2 GHz. For the miniaturized power divider, the measured return loss of port 1 is about −23 dB at the center frequency of 11 GHz and less than −13 dB over the band of 8–12 GHz. The measured insertion loss of port 2 and 3 is close to 3.5 dB over the X‐band. The measured bandwidth of the miniaturized divider is less than 1.6 GHz. Comparison between the Wilkinson power divider and the miniaturized one shows that the shorter the quarter‐wave transmission line is, the larger the impedance is. Meanwhile, the chip area reduction also leads to narrowed bandwidth of the power divider. The experiments demonstrate that the MMIC chip area of the power divider has been reduced by nearly 50% by capacitive loading, whereas the bandwidth is also narrowed by 50%. Therefore, the Wilkinson power divider applies to the situation if wide bandwidth is needed, while the miniaturized one is a better solution for small chip area. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:700–705, 2014

Comprehensive study of InGaP/InGaAs/GaAs dual channel pseudomorphic high electron mobility transistors

In this work, the device characteristics of InGaP/InGaAs/GaAs dual channel pseudomorphic high electron mobility transistors (DCPHEMTs) with triple δ-doped sheets are comprehensively and systematically investigated. The triple δ-doped sheets densities are found to be crucial for improving device performance. Based on a two-dimensional simulator of Atlas, the detailed calculations and studies including energy band diagrams, carrier distributions, and DC and microwave performances are reported. Due to the use of InGaAs DC structure and InGaP Schottky and buffer layers, good pinch-off and saturation characteristics, higher current drivability, larger and linear transconductance, and excellent microwave performance are obtained. For comparison, a practical DCPHEMT with good device performance is fabricated. Generally, good agreements between experimental results and theoretical simulations are found. The DCPHEMT with proper designed triple δ-doped sheet densities is promising the promise for high-performance digital and microwave device applications.

Drift of Electrical Characteristics over Time for Fluid MEMS Capacitors

A variable capacitor with high capacitance ratio (CR) is strongly expected as one of the key devices to realize controllable/ reconfigurable filters for multi-function, multi-band wireless systems. The authors have developed a fluid MEMS capacitor technology as one of the promising candidates for achieving high CR and have reported the excellent microwave performance. In order to verify such technology from a practical viewpoint, the change in the capacitor performance over time was studied, and found out the method to suppress the drift phenomenon.

Wafer Level Integration of MMIC and Microwave IPD with Metal/BCB Multilayer Interconnection Based on Low Resistance Silicon

A new high-density wafer-level integration of a GaAs based monolithic microwave integrated circuit (MMIC) chip and a microwave integrated passive device (IPD) is presented. This integration technology, an important and IC-compatible option for system-in-package (SiP), utilizes bulk Si fabrication and film deposition based multichip module (MCM-D) process. MMIC is entirely embedded into the silicon wafer while IPDs are integrated on the dielectric layers simultaneously with the metal/BCB multilayer interconnection. Key fabrication processes and crucial technologies are described in detail. Normal silicon wafer is selected as substrate because of its mature processing technology, low cost, good thermal dissipation as well as its thermal expansion matching with GaAs. To obtain excellent microwave performances and good planarization, thick photosensitive BCB of 25um/layer is adopted as dielectric and thus the use of tapered via that is hollow inside or filled by BCB is a cost-effective way to accomplish inter-layer connection instead of Au bump bonding or column used in dry-etch BCB process. Further promotions on microwave performances are achieved by the shielding effect through ground layer coverage on silicon surface and the application of microstrip lines. Several experiment results such as dc inter-layer connection resistance and thermal resistance measurements are complemented to investigate the characteristic of the whole package. The Microwave properties of the integration sample are measured by transmission performance test from 15GHz to 30GHz. The measurement results are analyzed and discussed comparing with the theoretical or simulation results.

Study on microwave properties of low temperature fired NiZnCu gyromagnetic ferrite

This paper describes the microwave properties of low temperature fired Ni0.6CuxZn(0.4−x)Fe1.96+δO4 gyromagnetic ferrite material synthesized by conventional oxide process with Bi2O3 additives. It is found that 60–63 mol % Ni and 5 mol % Cu contents have the advantage to keep the material with both low-fired characteristic and high saturated magnetization with 3 wt % Bi2O3 additives. Considering the conditions of milling, calcination, and sintering process, the low temperature fired NiCuZn gyromagnetic material was prepared with excellent microwave performances. The saturated magnetization Ms of the sample is 337 kA/m, the ferromagnetic resonance linewidth ΔH and dielectric loss tgδε less than 16 kA/m and 5.7×10−4, respectively, which has the potential applications for low temperature cofired ceramics technology to develop microwave chip circulator/isolator and passive integrated substrate.

Effect of Fe3+/Ba2+ mole ratio on the phase formation and microwave properties of BaFe12O19 prepared by citrate–EDTA complexing method

Abstract Barium hexaferrites (BaFe12O19) with various Fe3+/Ba2+ mole ratio have been prepared by citrate–EDTA complexing method using citrate and EDTA as complexing chelating agents. DSC analysis, XRD patterns, EDS compositional testing and SEM investigations all display that Fe3+/Ba2+ mole ratio is crucial in phase formation and crystal grain growth. The electromagnetic parameters of ɛ′, ɛ″, μ′, and μ″ were determined by network analyzer (Agilent 8722ET) at the frequency range of 2–18 GHz by a reflection/transmission technique. The sample with a Fe3+/Ba2+ mole ratio of 11.5 has the lowest permittivity (ɛ′ = 1–3, ɛ″ ≈ 0) and highest permeability (μ′ = 1.5–3.5, μ″ = 0.5–2.5), exhibiting excellent microwave performances.

The effect of pH values on the phase formation and properties of BaFe 12O 19 prepared by citrate–EDTA complexing method

Barium hexaferrites (BaFe 12O 19) have been prepared by citrate–EDTA complexing method using citrate and EDTA as complexing chelating agents with various pH values. Complexation stability constant analysis, XRD patterns and SEM investigations all display that the chelation between metallic ions and ligand may reach an excellent degree in the condition of pH 6. The precursor with more uniformly distributed Fe 3+ and Ba 2+ can convert to hexagonal barium ferrite more easily and completely. A network analyzer (Agilent 8722ET) was employed to determine the electromagnetic parameters of ɛ′, ɛ″, μ′, and μ″ at the frequency range of 2–18 GHz by a reflection/transmission technique. The sample of pH 6 has lowest permittivity ( ɛ′ = 1–3, ɛ″ ≈ 0) and highest permeability ( μ′ = 1.5–3.5, μ″ = 0.5–2.5), exhibiting excellent microwave performances.

In0.53Ga0.47As∕InAs0.3P0.7 composite channel high electron mobility transistors

An In0.53Ga0.47As/InAs0.3P0.7 composite channel high electron mobility transistor (HEMT) structure was grown by molecular beam epitaxy. Room-temperature Hall measurement showed that the device wafer had an electron mobility of 7300 cm2/V s and a sheet electron density of 3×1012 cm−2. The fabricated HEMT devices with a gate length of 0.25 µm exhibited excellent DC and microwave performance with a peak extrinsic transconductance of 888.3 mS/mm, a cutoff frequency (fT) of 115 GHz, and a maximum frequency of oscillation of 137 GHz. This is believed to be the first report of InGaAs/InAsP composite channel HEMTs. The fT is the highest ever reported for any composite channel HEMTs with the same gate length.