Microwave Permittivity Research Articles

Use of Microwave Dielectric Spectroscopy for the In Actu Assessment of Frustrated Lewis Pair Encounter Complexes.

Frustrated Lewis pairs (FLPs) offer an important and promising paradigm for main group catalysis. Reported here is the use of microwave dielectric spectroscopy for the in actu detection of FLP encounter complexes. This technique focuses on the room-temperature measurement of the loss component of microwave permittivity (ε2) over the bandwidth from 0.5 to 6.8 GHz. The microwave loss measured for a Lewis pair in a toluene host solution is compared with the losses of the individual components when measured separately, and the difference in loss Δε2 is used to characterize the electrostatic interaction between the pair. The Δε2 value shows a direct correlation with an ability for the FLP encounter complex to split hydrogen gas and abstract hydrogen from γ-terpinene and has led to the identification of a novel FLP encounter complex, tris-pentafluorophenyl borane-eucalyptol pairing.

Collaboratively Far-Field and Near-Field Regions for Dual-Modalities Microwave Permittivity Sensor Using T-Shaped Resonator Embedded With IDC

Collaboratively Far-Field and Near-Field Regions for Dual-Modalities Microwave Permittivity Sensor Using T-Shaped Resonator Embedded With IDC

Solid materials microwave dielectric properties: features of methods practical use

Features of methods practical use for material dielectric properties measurement in different (factory and metrological) laboratories and influence of above mentioned features on measurements results are considered. Results are obtained by Obninsk Research and Production Enterprise “Technologiya” A. G. Romashin radiophysics laboratory and by East-Siberian branch Federal State Unitary Enterprise “Russian metrological institute of technical physics and radio engineering” department of radio engineering measurements. Results of microwave permittivity and loss tangent solid materials measurements are analysed. This investigation made it possible to get additional information about features of standard methods practical use for material dielectric properties measurement, which was absent in literature earlier. In laboratories for measurements used the waveguide resonator and the standardized techniques. The factory laboratory applied a measurement method at the fixed frequency with control in a resonance movement of the plunger, and in metrological laboratory a measurement method with the fixed length of the resonator and measurement at resonant frequencies. Results of measurements showed good reproducibility of results for samples that are meeting method requirements. It was revealed reducing reproducibility of measurements of hygroscopic, heterogeneous samples and in the case of nonobservance of diameter method requirements. Obtained results can be applied for reproducibility improvement of dielectric properties measurements in practical use of considered methods.

Nanocarbon/Co3O4/Epoxy Composites for Microwave Shielding and Absorption

The epoxy composite materials (CMs) with a random distribution of nanosized Co3O4 and carbon nanoparticles (graphite nanoplatelets GNP and carbon nanotubes CNT) are fabricated. Complex permittivity, permeability, and shielding properties of composite materials with 30 wt% of Co3O4 and (2–5) wt% of nanocarbon are measured in the frequency range of 1–67 GHz using the transmission–reflection method. A significant increase of microwave permittivity and presence of several peaks on the dependencies of the imaginary part of permittivity on the frequency are found for three‐phase nanocarbon/Co3O4/epoxy CMs. The observed microwave shielding efficiency of three‐phase epoxy composites is enhanced compared to two‐phase nanocarbon–epoxy composites. Such enhancement of shielding efficiency correlates with the increase of DC conductivity of these three‐phase composites. The measured complex permittivity and permeability spectra for nanocarbon/Co3O4/epoxy CMs are used for modeling the reflection loss for studied composites. It is shown that the most preferable for microwave absorption is the composites containing 2 wt% of nanocarbon filler for a sample thickness of 0.5–0.7 mm. The highest reflection loss = 30 dB with an absorption bandwidth of 15 GHz is found for epoxy composite filled with 2 wt% of GNP and 30 wt% of Co3O4 for a sample thickness of 0.5 mm.

Microwave dielectric properties of (Na0.5Bi0.5)MoO4–BaMoO4 composite ceramics with ultralow sintering temperature

AbstractThe rapidly ever‐growing wireless communication demands development of low loss, ultralow sintering temperature dielectric ceramics with excellent microwave properties. In this work, fabrication and properties of novel microwave dielectric composite ceramics (1 − x)(Na0.5Bi0.5)MoO4–xBaMoO4 (x = 0.3–0.6), in short as ((1 − x)NBM–xBM), synthesized via solid‐state reaction method with an ultralow sintering temperature is reported. X‐ray diffraction analysis confirms the coexistence of both (Na0.5Bi0.5)MoO4 and BaMoO4 scheelite phases. Scanning electron microscopy analysis demonstrated dense microstructures of ceramics sintered at 650°C and 700°C. The dielectric properties of (1 − x)NBM–xBM (x = 0.3–0.6) ceramic are significantly tailored by incorporation of BaMoO4. With the increase in x from 0.3 to 0.6, the (1 − x)(Na0.5Bi0.5)MoO4–xBaMoO4 displays noticeable change in microwave permittivity (varying from 25.4 to 16.6), quality factor values, and temperature coefficient of frequency (range tunable from +37.6 ppm/°C to −14.2 ppm/°C). The microwave dielectric ceramic, 0.5(Na0.5Bi0.5)MoO4–0.5BaMoO4, has optimum dielectric properties (, = 15 164 GHz, τf = −0.2 ppm/°C) at sintering temperature of 650°C. The dielectric resonator antenna designed using (1 − x)(Na0.5Bi0.5)MoO4–xBaMoO4 ceramics demonstrated excellent radiation performance at resonance frequency of 5.905 GHz, gain of ∼5.8 dBi and 140 MHz bandwidth, which validates its microwave applications utilization within the field of ultralow temperature cofired ceramics technology.

Room temperature AC electrical, surface micro texture analysis, microwave permittivity and dielectric loss tangent in ZnO films doped by Cu, Al, and CuAl

The purpose of this work is the study of the correlation between the thickness of ZnO films doped by transition metals and their three-dimensional (3D) micromorphology. The as deposited ZnO films became more irregular surface hence the maximum value of Sq belong to as deposited ZnO films was in a bout of 0.1038 mm. The core roughness height Sk calculated as a difference between two extreme levels (maximal and minimal) of surface core, for ZnO films annealed at 500 °C have maximum value in a bout of 0.7325 mm. Since the surface kurtosis (Sku) of all sample's films were above 7, therefore there were high peaks or valleys on the films surface and for as deposited CAZO films with value of with 8.883. The height distribution histograms shown that as deposited films respect to annealed films have more uniform distributions. The AZO films annealed at 500 °C have maximum value of DC conductivity in about 104.167 S cm−1. The value of log (dielectric loss tan(δ)) in AZO films have maximum value in range 0.05–0.15. As the frequency of incident photons were increased the polarization orientation were increased. Log AC conductivity values of films were shown almost linear behavior with increasing incident photons frequency that indicates conduction hopping is followed by a small polaron mechanism. AC conductivity values of as deposited AZO films in all frequencies have maximum value.

LiGaSiO4: An ultra-low permittivity dielectric material enabling application in patch antenna

A low permittivity microwave dielectric ceramic, Li-containing silicate (LiGaSiO4), was synthesized using the standard solid-state reaction method. The nominal LiGaSiO4 ceramic exhibited a relative permittivity of approximately 4.4. The effects of nonstoichiometric Li on the microstructure, sintering behavior, and microwave dielectric properties of LiGaSiO4 were comprehensively investigated. Excess Li content (6 mol%) significantly improved the relative density (96.1 %) and microwave dielectric properties (εr∼5.34, Q × f ∼55,600 GHz, and τf ∼ -48.6 ppm/°C), while reducing the optimum sintering temperature from 1070 °C to ∼1010 °C. Additionally, the incorporation of 2 wt% LiF further reduced densification temperature (∼940 °C). The regulation of the τf was achieved through the addition of CaTiO3 ceramic, and a patch antenna was designed and fabricated using a modified Li1.06GaSiO4-LiF-CaTiO3 composite ceramic. The resulting patch antenna resonates at 5.87 GHz with an impressive bandwidth of 83.1 MHz, while both simulated and measured results demonstrate excellent agreement.

Retrieving the Intrinsic Microwave Permittivity and Permeability of Ni-Zn Ferrites

Mixing rules may be extremely useful for predicting the properties of composite materials and coatings. The paper is devoted to the study of the applicability of the mixing rules to permittivity and permeability and the possibility of retrieving the intrinsic properties of inclusions. Magnetically soft Ni-Zn ferrites are chosen as the object of the study due to their low permittivity and the negligible influence of the skin effect. Due to this, the microwave properties of bulk ferrites may be measured by standard techniques. It is suggested to perform the analysis of the microwave properties of composites filled with Ni-Zn ferrite powder in terms of the normalized inverse susceptibility defined as the volume fraction of inclusions divided by the effective dielectric or magnetic susceptibility of the composite. The measured properties of the bulk ferrite are compared with those obtained by mixing rules from composite materials. The experimental evidence for difference between the mixing rules for permittivity and permeability of a composite, which was previously predicted only theoretically, is obtained. The reason for the difference is considered to be the effect of non-ideal electrical contacts between neighboring inclusions. It is also experimentally shown that the measured permeability of the bulk material may differ from the retrieved one. The measured static permeability is 1400 and the retrieved one is 12. The reason for the discrepancy is the difference between the domain structures and demagnetizing fields of particles and bulk ferrite.

Microwave Properties of Ba-Substituted Pb(Zr0.52Ti0.48)O3 after Chemical Mechanical Polishing

We have studied the effect of chemical-mechanical polishing (CMP) on the ferroelectric, piezoelectric, and microwave dielectric properties of Ba-substituted PZT (BPZT), deposited by pulsed laser deposition. CMP allowed for the reduction of the root mean square surface roughness of 600 nm thick BPZT films from 12.1 ± 0.1 nm to 0.79 ± 0.15 nm. Ammonium peroxide (SC-1) cleaning was effective to remove Si CMP residuals. Measurements of the ferroelectric hysteresis after CMP indicated that the ferroelectric properties of BPZT were only weakly affected by CMP, while the piezoelectric d33 coefficient and the microwave permittivity were reduced slightly by 10%. This can be attributed to the formation of a thin dead layer at the BPZT surface. Moreover, the intrinsic dielectric permittivity at microwave frequencies between 1 and 25 GHz was not influenced by CMP, whereas the dead layer series capacitance decreased by 10%. The results indicate that the CMP process can be used to smoothen the BPZT surface without affecting the film properties strongly.

Microwave permittivity and permeability measurements on lunar simulants at low temperatures

Lunar simulants are used to approximate the behavior of the lunar regolith. This work focuses on materials responses relevant for microwave heating to inform in-situ resource utilization of lunar regolith for landing pads and building infrastructure. Microwave dielectric permittivity and magnetic permeability measurements have been made for the first time on several new highland lunar simulants (NU-LHT-4M, NUW-LHT-5M and CSM-LHT-1G). One mare lunar simulant (JSC-1A) is included in this work. We present these measurements in the low temperature range −185 °C to 250 °C and in the frequency range 50 MHz to 3 GHz. The dominant minerals in the simulants are the minerals bytownite, ortho- and clino-pyroxenes, olivine, and glass. Less complex rock feedstocks for highland simulants, anorthosite and pyroxenite, were also measured separately. The dielectric measurements were performed on samples having different densities. Differences in the dielectric responses were noted, however all simulants were weak microwave absorbers at the low temperatures. This data assists with planning for a hybrid approach to microwave sintering at the lunar south pole.

Design of a Microwave Heating and Permittivity Measurement System Based on Oblique Aperture Ridge Waveguide.

In this paper, an oblique aperture ridge waveguide operating at 2450 MHz is proposed, and, using the ridge waveguide, a permittivity measurement system is constructed which can measure the permittivity of materials during microwave heating. The system calculates the amplitudes of the scattering parameters by using the forward, reflected and transmitted powers of the power meters, and it reconstructs the permittivity of the material by combining the scattering parameters with an artificial neural network. The system is used to measure the complex permittivity of mixed solutions of methanol and ethanol with different ratios at room temperature, and the permittivity of methanol and ethanol with increasing temperature, from room temperature to 50 °C. The measured results are in good agreement with the reference data. The system allows simultaneous measurement of the permittivity with microwave heating and provides real-time, rapid changes in the permittivity during heating, avoiding thermal runaway and providing a reference for applications of microwave energy in the chemical industry.

A novel low permittivity microwave dielectric ceramic Sr2Ga2SiO7 for application in patch antenna

AbstractThe dielectric properties of a Ga‐based melilite type ceramic Sr2Ga2SiO7 via theoretical prediction based on far‐infrared spectroscopy and experimental measurement by the Hakki–Coleman method were studied in this work. Dense and single‐phase ceramics were fabricated via solid‐state reaction at 1330°C and exhibited comprehensive microwave dielectric properties (εr ∼ 7.6, Q × f ∼ 23 600 GHz, and τf ∼ −35.2 ppm/°C) at 14.3 GHz. Chemical modifications were proposed to adjust the thermal stability and reduce the densification temperature. By adding 10 mol% CaTiO3, the negative τf can be compensated to a near‐zero value of −3.8 ppm/°C. The densification temperature was reduced to 940°C by adding 3 wt.% LiF. A patch antenna was designed using Sr2Ga2SiO7 ceramic with a high radiation efficiency of 99.1% and a gain of 2.788 dBi at the center frequency of 4.371 GHz. All results indicate that the Sr2Ga2SiO7 ceramic has promising application potential for 5G wireless communication technology.

Microstructural evolution and microwave transmission/absorption transition in polymer-derived SiOC ceramics

Herein, we present the structural evolution of polymer-derived SiOC ceramics with the pyrolysis temperature and the corresponding change in their microwave dielectric properties. The structure of the SiOC ceramics pyrolyzed at a temperature lower than 1200 °C is amorphous, and the corresponding microwave complex permittivity is pretty low; thus, the ceramics exhibit wave transmission properties. The Structural arrangement of free carbon in the SiOC ceramics mainly happens in the temperature range of 1200 °C-1300 °C due to the separation from the Si–O–C network and graphitization, while the structural arrangement of the Si-based matrix mainly occurs in the range of 1300 °C-1400 °C owing to the separation of SiC4 from the Si–O–C network to form nanocrystalline SiC. In pyrolysis temperature range of 1200 °C-1400 °C, the microwave permittivity of SiOC shows negligible change. At a pyrolysis temperature exceeding 1400 °C, the carbothermal reaction of free carbon and the Si–O backbone becomes significant, leading to the formation of crystalline SiC. The as-formed SiC and residual defective carbon improve the polarization loss of SiOC ceramics. In this case, the SiOC ceramics show significantly increased complex permittivity, exhibiting electromagnetic absorption characteristics. These characteristics promote the application of polymer-derived SiOC ceramics to high-temperature electromagnetic absorption materials.

Microwave dielectric properties of tetragonal scheelite‐structured (NaY)1/2MoO4 ceramics

Abstract(NaY)1/2MoO4 was fabricated via the solid‐state reaction method of Na2CO3, Y2O3, and MoO3. Scanning electron microscopy results demonstrated that all the (NaY)1/2MoO4 ceramics could be densified well in the sintering temperature range of 900–960°C. Results of X‐ray diffraction analysis demonstrated that the (NaY)1/2MoO4 ceramics crystallized into tetragonal scheelite structure. Sintering (NaY)1/2MoO4 at 940°C for 2 h optimized the microwave dielectric properties of the ceramics. The microwave permittivity, Q × f, and TCF of the (NaY)1/2MoO4 were 10.9, 29 000 GHz and −40.7 ppm/°C, respectively.

Deposition of Thick SiO2 Coatings to Carbonyl Iron Microparticles for Thermal Stability and Microwave Performance.

Thick dielectric SiO2 shells on the surface of iron particles enhance the thermal and electrodynamic parameters of the iron. A technique to deposit thick, 500-nm, SiO2 shell to the surface of carbonyl iron (CI) particles was developed. The method consists of repeated deposition of SiO2 particles with air drying between iterations. This method allows to obtain thick dielectric shells up to 475 nm on individual CI particles. The paper shows that a thick SiO2 protective layer reduces the permittivity of the 'Fe-SiO2-paraffin' composite in accordance with the Maxwell Garnett medium theory. The protective shell increases the thermal stability of iron, when heated in air, by shifting the transition temperature to the higher oxide. The particle size, the thickness of the SiO2 shells, and the elemental analysis of the samples were studied using a scanning electron microscope. A coaxial waveguide and the Nicholson-Ross technique were used to measure microwave permeability and permittivity of the samples. A vibrating-sample magnetometer (VSM) was used to measure the magnetostatic data. A synchronous thermal analysis was applied to measure the thermal stability of the coated iron particles. The developed samples can be applied for electromagnetic compatibility problems, as well as the active material for various types of sensors.

Photoinduced Microwave Permittivity of Semiconductors: Exciton Mechanism

Photoinduced Microwave Permittivity of Semiconductors: Exciton Mechanism

Photoinduced Microwave Permittivity of Semiconductors: Exciton Mechanism

Significant differences observed in the behavior of photoinduced permittivity ε of semiconductors in the gigahertz (GHz) and terahertz (THz) ranges are explained within the framework of the exciton mechanism by the different position of these ranges relative to the frequencies of exciton interlevel transitions. The measurements in the GHz range of the photoinduced changes of quantities Imε(P_λ) and Reε(P_λ) of CdS, CdSe and Si samples in a waveguide resonator (f = 4.7 GHz) and transmittance T of Si samples in free space (f = 8–36 GHz ) under fiber-optic irradiation (P_λ = 0–370 mW and λ = 0.97 μm) that exhibit non-Drude response prove the theoretical conclusions: an increase in Reε^(GHz)(P_λ) with increasing P_λ and an increase in transmittance T with decreasing frequency f at fixed power P_λ.

Single Whispering-Gallery-Mode Resonator With Microfluidic Chip as a Basis for Multifrequency Microwave Permittivity Measurement of Liquids

The accurate measurement of complex liquid permittivity in a frequency range provides important information on liquid properties in comparison to single frequency permittivity investigations. The multifrequency microwave characterization technique based on a single quartz whispering-gallery-mode (WGM) resonator with a microfluidic chip are proposed, developed, and demonstrated. This technique allows the complex permittivity of small volumes of liquid to be measured at six resonant frequencies in the 30-40 GHz frequency range. The calibration is performed by simulating the measurement cell and by plotting the calibration nomogram charts for all six investigated frequencies. The novel approach is applied in studies of the complex permittivity of the L-lysine in water solutions. The results open up possibilities to investigate the complex permittivity of biological liquids at several frequencies and to further develop the microwave dielectrometry of small liquid volumes in a certain frequency range using the quasi-optical nature of a single high-Q WGM resonator.

Microwave complex permittivity and anisotropy of conifer wood chips vs moisture content: experiments and modeling

The complex microwave permittivity—including anisotropy- of wood chips of softwood has been measured for different moisture contents in the band 0.75 to 2.5 GHz using an ultra-wide band radio transmission technique. The real and imaginary parts increase monotonically with moisture content. The wood chips are oriented by gravity, which gives anisotropic permittivity. The anisotropy ratio of the real part increases from 1.1 to 1.6 with moisture content from 0 to 120%. The anisotropy ratio of the imaginary part is around 2.5 at all moisture contents. Effective medium models were used to model the permittivity. The Bruggeman, and two versions of the Maxwell Garnett model gave good results at low moisture content (below the fiber saturation point). Above the fiber saturation point only the Bruggeman model gave results in agreement with experiments. The difference in model performance suggests that the free water does not follow the wood chips geometry.

Experimental and Numerical Investigation of Microwave Irradiation-Induced Thermal Characteristics and Permittivity Evolution of Coal

Experimental and Numerical Investigation of Microwave Irradiation-Induced Thermal Characteristics and Permittivity Evolution of Coal