Microwave Permittivity Measurements Research Articles

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.

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.

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.

Electromagnetic interference shielding performance enhancement of stretchable transparent conducting silver nanowire networks with graphene encapsulation

Silver nanowire (AgNW) networks are promising transparent conducting materials for electromagnetic interference (EMI) shielding and diverse optoelectronic devices. However, the poor contact between adjacent AgNWs leads to low electrical conductivity and weak mechanical stability of AgNW networks, which are limiting the practical application of these electronics. Here, we report an efficient strategy to improve the overall performance of AgNW networks, in which the AgNW networks are sandwiched between two layers of graphene films. The graphene films improve the contact of overlapped AgNWs and bridge the discrete AgNWs and thus increase the conductivity of graphene/AgNWs/graphene (GAG) films. Microwave permittivity measurements together with mechanism analyses reveal that the graphene films can enhance the EMI shielding effectiveness of AgNW networks through offering extra conduction loss, multiple dielectric polarization centers and multi-reflection processes. As a result, the GAG film with an average transmittance of 88% exhibits a sheet resistance lower than 15 Ω sq−1 and an EMI shielding effectiveness of 31 dB (in the frequency range of 8.2–12.4 GHz) after repeated stretching and release at a strain of 40%. Such a total performance is superior to that of most of as-reported transparent conductors. The GAG films therefore show application potential in the age of Internet of Things that electromagnetic radiation pollutions are everywhere.

Broadband Dielectric Spectroscopy Detection of Volatile Organic Compounds with ZnO Nanorod Gas Sensors

Historically, metal-oxide semiconductor-based gas sensors, such as SnO2 and ZnO, have been used to detect gases based on chemiresistivity [1]. In this approach, the electrical changes in response to analytes are measured with electrical circuits, that necessarily involve making electrical contacts to the sensing materials. These contacts are imperfect and introduce errors into the measurements. In this paper, we will demonstrate the feasibility of BDS-based metrology in addressing this gap in metrology to avoid these tool-measurand interactions / parasitic errors. Specifically, we will discuss how radio frequency (RF) propagation characteristics can be applied to probe and study charge transfer chemical changes in these metal-oxide VOC sensing materials.For this study, the solid-state gas sensor device under test consisted of hydrothermally synthesized ZnO nanorods grown on a fine grain seed layer of atomic layer deposition (ALD) ZnO on a silicon substrate. The sample was placed on a waveguide situated in a controlled environment to study the redox reactions of the ZnO sensor detecting ethanol target gas under thermal heating. In this configuration, the ZnO gas sensing element couples to the microwave through an evanescent mode. The temperature of the assembly was varied from room temperature to approximately 180 °C, in a variety of gaseous environments, while continuously monitoring the microwave signal going through the waveguide.Analysis of the BDS data (at specific frequencies) gave us electrical information that can be correlated to charge-transfer reactions involving the defects in the ZnO material and the molecules in the gaseous environment. For example, according to the S21 data, the sample became more resistive as temperature increased. This can be attributed to the redox reactions occurring on the ZnO nanorod gas sensor surface. Illustratively, Figure 1 shows time-lapse evolution on microwave signal loss in the experimental setup attributable to the discrete events occurring in the ZnO in response to various environmental conditions.The resistivity (as indicated by the S21) of the ZnO is stable below 120°C. At 120C, the system becomes increasing resistive with time, possibly as adsorbed water molecules on the ZnO rods desorb and to the healing of the ZnO surface defects, in a distinct two-step process [1]. Finally, the system became less lossy (i.e., electrical resistivity decreased) when ethanol target gas (technical grade C2H5OH, EtOH) was injected into the reactor. However, this decrease in resistivity might have been influenced by the decrease in temperature once the EtOH was introduced. From the S-parameters, we calculate a microwave attenuation constant (i.e. the real part of the microwave propagation constant) which quantified energy dissipation into the ZnO material. The attenuation constant data suggest the redox reaction induced changes in the semiconductor rods are reversible within the temperature range in this experiment.To the best of our knowledge, such detailed description of the discrete events that occur in the metal-oxide sensing material cannot be obtained with the traditional electrical DC resistivity measurements. In the future, we may be able to use this technique to discern differences, and distinguish between ZnO surface- and bulk reactions, as well as identify / characterize point defects. References H-J. Kim, and J.-H. Lee, “Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview”, Sensors and Actuators B 192 (2014) 607– 627Heine, F. Girgsdies, A. Trunschke, R. Schlögl, and M. Eichelbaum, “The model oxidation catalyst α-V2O5: insights from contactless in situ microwave permittivity and conductivity measurements”, Applied Physics A, vol. 112, pp. 289–296, 2013 DOI 10.1007/s00339-013-7800-6 Figure 1

Microwave-assisted self-reduction of composite briquettes of zinc ferrite and carbonaceous materials

Microwave carbothermic reduction of zinc ferrite was studied based on self-reduction of briquettes composed of zinc ferrite and different reducing agents, including coke, coal and biochar, in a vertical microwave furnace from both thermodynamic and experimental perspectives. The thermodynamic analysis showed that zinc ferrite could be reduced using the carbonaceous materials in a temperature range of 910–1250 °C. The experimental results demonstrated that the reducing agents had distinct microwave absorption capabilities, confirmed by the comparisons of corresponding microwave penetration depths (Dp, in this study Dp,coke> > Dp,coal > Dp,biochar) and reflection losses (RL, in this study RLcoke> > RLcoal > RLbiochar) based on the microwave permittivity and permeability measurements. This finding was in agreement with their different roles in the reduction, which produced reduced briquettes and volatilized matters via a series of stepwise reactions. Compared with conventional reduction, it was also found that the presence of microwave led to a much higher reduction efficiency. The coke exhibited a much lower reduction efficiency than coal and biochar. This observation was more obvious in microwave-assisted reduction where the coal and biochar contributed to high iron metallization degree (97%) and reduction degree (above 99%) after reduction at 1050 °C for 15 min even with low fixed carbon contents. Further analysis of the phase composition and microstructure of the reduced briquettes obtained in microwave-assisted reduction showed that metallic iron particles were formed but aggregated in different morphologies, probably associated with the growth of the metallic particles adjacent to the residual carbonaceous reducing agents. Meanwhile, the analysis of the corresponding volatized matter revealed that it was mainly constituted by highly pure metallic zinc particles which presented a hexagonal structure. The results indicated that in comparison with coke, the other two carbonaceous materials, especially biochar, could serve as more efficient and environmentally friendly reducing agents for treatment of zinc ferrite and zinc-bearing materials by its combined usage with microwave external field.

Microwave Permittivity of Trace sp2 Carbon Impurities in Sub-Micron Diamond Powders.

Microwave dielectric loss tangent measurements are demonstrated as a method for quantifying trace sp2-hybridized carbon impurities in sub-micron diamond powders. Appropriate test samples are prepared by vacuum annealing at temperatures from 600 to 1200 °C to vary the sp2/sp3 carbon ratio through partial surface graphitization. Microwave permittivity measurements are compared with those obtained using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and electron energy loss spectroscopy (EELS). The average particle size remains constant (verified by scanning electron microscopy) to decouple any geometric dielectric effects from the microwave measurements. After annealing, a small increase in sp2 carbon was identified from the XPS C 1s and Auger spectra, the EELS σ* peak in the C 1s spectra, and the D and G bands in Raman spectroscopy, although a quantifiable diamond to G-band peak ratio was unobtainable. Surface hydrogenation was also evidenced in the Raman and XPS O 1s data. Microwave cavity perturbation measurements show that the dielectric loss tangent increases with increasing sp2 bonding, with the most pertinent finding being that these values correlate with other measurements and that trace concentrations of sp2 carbon as small as 5% can be detected.

Study of the magnetite to maghemite transition using microwave permittivity and permeability measurements

The microwave cavity perturbation (MCP) technique is used to identify the transition from magnetite (Fe3O4) to the meta-stable form of maghemite (γ-Fe2O3). In this study Fe3O4 was annealed at temperatures from 60 to 300 °C to vary the oxidation. Subsequent to annealing, the complex permittivity and magnetic permeability of the iron oxide powders were measured. The transition to γ-Fe2O3 was corroborated with x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometry (VSM). XRD, XPS and VSM implied that the starting powder was consistent with Fe3O4 and the powders annealed at more than 200 °C were transitioning to γ-Fe2O3. The MCP measurements gave large differences in both complex permittivity and magnetic permeability of the two phases in the frequency range of 2.5–10.2 GHz. Magnetic permeability decreased with annealing temperature, though magnetic losses showed frequency dependent behaviour. Complex permittivity measurements showed a large decrease in both dielectric constant and losses at all measurement frequencies, as well as a prominent loss peak centred around the phase transition temperatures. We interpret the loss peak as being a consequence of field effects due to an intermediate multi-phase mixture. Additionally, almost no frequency dependence was observed. The reduction in complex permittivity implies that the cations in the lattice provide a significant contribution to polarization at microwave frequencies and the effects of are nominal in comparison. The change in loss can be explained as a combination of the differences in the effective conductivity of the two phases (i.e. Fe3O4 exhibits electron-hopping conduction whereas the presence of vacancies in γ-Fe2O3 nullifies this). This shows that the non-invasive MCP measurements serve as a highly sensitive and versatile method for looking at this phase transition in iron and potentially the effects of oxidation states on the polarization in other iron oxides.

Investigating the Broadband Microwave Absorption of Nanodiamond Impurities

Broadband microwave complex permittivity measurements of nanodiamond powders are presented. Previous studies show that measurements of dielectric loss strongly correlate with the presence of nondiamond surface impurities. In this study, the frequency dependence of these losses is investigated using the microwave cavity perturbation (MCP) and broadband coaxial probe (BCP) methods. This allowed further understanding as to what mechanisms contribute to the microwave absorption (free electron conduction or dielectric loss from the disordered surfaces). A multimode MCP system is used which utilizes ${\rm TM}_{0np}$ modes to provide partial spectral characterization. The MCP results revealed minimal frequency dependence, unlike any static conduction-related mechanism. The BCP measurements corroborate the MCP results with much higher spectral resolution, and further demonstrate that disorder related loss may dominate over free electron conduction from 1–10 GHz. From 0.1–1 GHz, free electron conduction has a greater influence with a characteristic $1/f$ dependence implying that conduction may dominate at lower frequencies. However, the BCP method, while repeatable, lacks in precision compared to the cavity method. Nonetheless, the major conclusion in this paper is that through simple microwave permittivity measurements, nondiamond carbon impurities in nanodiamond powders are measurable most likely because of disorder related losses as opposed to free electron conduction.

Quantitative detection of carbon nanotubes in biological samples by an original method based on microwave permittivity measurements

Due to their nanoscale, morphology, and chemical composition, the tracking and the quantitative analysis of carbon nanotubes (CNTs) in biological samples still represent huge challenges. A new technique for the quantitative and accurate detection of CNTs in various biological samples at different scales (whole organisms to organs) was developed using amphibian larvae exposed to double-walled CNTs (DWCNTs). This technique is based on the dielectric relaxation of ultra-low volume suspensions under a microwave electromagnetic field. CNT concentrations were consequently extracted from complex permittivity measurements at 5GHz, making possible to quantitatively assess the animal exposure to CNTs. Our results indicate a detection threshold of 0.02μg of DWCNTs, which is the lowest achieved in the literature to date.

The model oxidation catalyst α-V2O5: insights from contactless in situ microwave permittivity and conductivity measurements

The in situ microwave cavity perturbation technique was used to study the complex permittivity and conductivity of polycrystalline α-V2O5 in a tubular reactor under reactive high-temperature conditions with a TM110 cavity resonating at 9.2 GHz. The sample was investigated at 400 °C in flowing air and air/n-butane mixtures while simultaneously measuring the total oxidation products CO and CO2 by gas chromatography. The V2O5 powder was identified as an n-type semiconductor and the dynamic microwave conductivity correlated well with the near-infrared (NIR) absorption assigned to V3d1 band gap states. Correlations between catalytic performance, real and imaginary parts of the permittivity, and NIR absorption allowed the differentiation between bulk and surface contributions to the charge transport in reactive atmospheres. The stability of the crystalline bulk phase was proven by in situ powder X-ray diffractometry for all applied testing conditions.

A Comparative Analysis Between Customized and Commercial Systems for Complex Permittivity Measurements on Liquid Samples at Microwave Frequencies

In this paper, different customized systems for microwave permittivity measurements on liquid samples, based on reflectometric measurements, are presented and analyzed. Their performance is compared against the one deriving from the most widely adopted commercial measurement setup. The systems are designed with the aim of providing less expensive solutions without compromising measurement accuracy. The purpose of the first proposed solution is to replace the commercial measurement software exploiting a reformulation of the classical theory. Based on this alternative formulation, a “homemade” probe is built by properly modifying an N-type coaxial connector, thus providing a system requiring a lower quantity of liquid under test. Moreover, a different experimental approach which uses time-domain reflectometry (TDR) instrumentation is presented. Such solution is by far the least expensive, as it allows avoiding the use of costly instrumentation (such as a vector network analyzer). In order to metrologically characterize the proposed solutions, a series of repeated measurements is performed on a set of well-referenced liquids. After extracting the Cole–Cole parameters through each of the considered measurement methods, the resulting type A uncertainty is evaluated. Finally, comparison with literature data allows the estimation of measurement bias. The analysis evidences that custom solutions generally exhibit an accuracy comparable to the one of the commercial solution, with a slight degradation of performance for the TDR-based setup, which, however, compensates for this drawback with its appealing low cost.

Ferroelectric phase transition in polycrystalline KTaO3 thin film revealed by terahertz spectroscopy

KTaO3 single crystal is an archetypal incipient ferroelectric in which a long-range ferroelectric order does not establish at low temperatures owing to quantum fluctuations. We report on a strong evidence of the ferroelectric phase transition near 60 K revealed by terahertz spectroscopy and microwave permittivity measurements of a polycrystalline KTaO3 thin film on (0001) sapphire substrate prepared by chemical solution deposition. The soft mode behavior is clearly observed in the terahertz (THz) spectra with a minimum frequency at 60 K. At the same temperature microwave permittivity maximum appears. The THz spectra strongly resemble that of strained epitaxial SrTiO3/DyScO3 films: the ferroelectric soft mode is linearly coupled to a central peak which is silent in the paraelectric phase and it becomes coupled to the polarization below the ferroelectric transition temperature with a progressively increasing bare strength.

A Circular Patch Resonator for the Measurement of Microwave Permittivity of Nematic Liquid Crystal

This paper presents a method of performing microwave characterizations of nematic liquid crystals using a circular patch resonator and finite-element software. This technique, which is based on inferring material properties from measured resonances, was demonstrated with BL006 (a nematic liquid-crystal mixture) over the frequency range of 4.8-8.7 GHz. The resonator comprises a thin liquid-crystal sample between a ground plane and a directly coupled inverted patch. The coupling-line design minimizes resonator loading and reflections from substrate impedance discontinuities, and the liquid crystal is aligned with rubbed surfaces and low-frequency voltages. The permittivity is found with simulations that iteratively narrow the difference between the measured and computed resonances. The computed values for BL006 agree well with those found in the literature. This method benefits from simple resonator construction, straightforward simulations, and accuracy that derives from the finite element's detailed modeling of coupling and geometry of the fabricated device.

Microwave Complex Permeability and Permittivity Measurements of Commercially Available Nano-Ferrites

The dielectric permittivity and magnetic permeability of nano-sized magnetic powders are studied for a wide microwave frequency range from 2 to 40 GHz. These powders are studied for their potential application as soft microwave absorbers in wireless systems as well as for use in the manufacture of small magnetic recording tape, transformers and circulators. A newly developed and precise transmission-reflection based waveguide technique has been used to carry out measurements across the eight frequency bands. These materials have been accurately characterized for the first time across this frequency range. In this paper, we present equations for simultaneous calculation of permittivity and permeability across a wide frequency band using the vector network analyzer. To account for variable sample thickness in the in-waveguide measurements, the algorithm for transmission and reflection calculations has been modified. The results show that the dielectric permittivity depends on the particle size and density of the sample used for measurement.

Nano Ferrites Microwave Complex Permeability and Permittivity Measurements by T/R Technique in Waveguide

There is a huge demand to accurately determine the magnetoelectrical properties of particles in the nano-sized regime due to the modern IC technology revolution and biomedical applications. In this paper, we present a microwave waveguide measurement technique for measuring complex permeability and permittivity of expensive nano-sized magnetic powder materials. We used a vector network analyzer to provide a standard TRL calibration for free space inside waveguide measurements. In order to maintain the recommended insertion phase range, a very thin prepared sample was loaded inside the calibrated waveguide. The loaded material's magnetic and dielectric effects were also considered in the cutoff wavelength calculation of the propagation constant of the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> wave from the geometrical dimensions of the waveguides. These provisions make the permeability and permittivity measurements more reliable than those found by commonly used techniques. We used six different compounds of nano-sized ferrite powders (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , CuFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , CuFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> Zn, F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> NiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Zn, BaFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sub> , and SrFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sub> ), in which the average diameter of nano particles is less than 40 nm, for measurement purposes. We measured the complex permeability and permittivity from 3.95 to 5.85 GHz. The results show that the dielectric permittivity of these materials is quite different from that of solid-state materials.

Analysis of broadband microwave conductivity and permittivity measurements of semiconducting materials

We perform broadband phase sensitive measurements of the reflection coefficient from 45MHz up to 20GHz by employing a vector network analyzer with a 2.4mm coaxial sensor which is terminated by the sample under test. While the material parameters (conductivity and permittivity) can be easily extracted from the obtained impedance data if the sample is metallic, no direct solution is possible if the material under investigation is an insulator. Focusing on doped semiconductors with largely varying conductivity, here we present a closed calibration and evaluation procedure for frequencies up to 5GHz, based on the rigorous solution for the electromagnetic field distribution inside the sample combined with the variational principle; basically, no limiting assumptions are necessary. A simple static model based on the electric current distribution proves to yield the same frequency dependence of the complex conductivity up to 1GHz. After a critical discussion, we apply the developed method to the hopping transport in Si:P at temperature down to 1K.

Resonant and Broadband Microwave Permittivity Measurements of Single-walled Carbon Nanotubes

ABSTRACTWe report on microwave measurements of complex permittivity of single-walled carbon nanotubes (SWNTs). The SWNT samples are a mixture of semiconducting and metallic nanotubes suspended using Pluronic (F108) as a stabilizing surfactant agent. Other samples that were characterized include pluronic powder and an oriented carpet of multi-walled nanotubes (MWNT). For broadband measurements, the shielded open-circuited transmission line technique was used. Single frequency (resonant) measurements for liquid-based SWNT samples were carried out by employing two different modes in specially designed microwave dielectric resonators (DR), either with a cylindrical hole in the center of a dielectric disk or with a horizontal gap (split resonator). The first resonator can operate in either TE011 or TM011 modes (3.4 GHz and 6 GHz) and was designed for liquid or powder characterization. For pluronic suspended SWNT, real (epsilon) and imaginary parts of permittivity were found at 3.4 GHz as 3.5 and 0.72, respectively.

Nondestructive Sensing of Physical Properties of Granular Materials by Microwave Permittivity Measurement

The dielectric properties of wheat, corn, and soybeans were determined by measuring the scattering transmission coefficient S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21 </sub> in free space at frequencies between 2 and 13 GHz. For better accuracy, a pair of horn-lens antennas was used and time-domain gating was applied to the main response. Variations of the dielectric properties with frequency and physical properties such as bulk density, moisture content, and temperature were investigated. Both the dielectric constant and the loss factor decreased with frequency and increased linearly with bulk density, moisture content, and temperature. Three different approaches are used to correlate the measured dielectric properties and physical properties. Explicit relationships between the dielectric properties and the different physical properties are given at a midrange frequency of 7.0 GHz as an example. The potential use of these relationships is shown for the development of indirect methods for the nondestructive and instantaneous determination of the physical properties of cereal grain and seed from measurements of their dielectric properties. Different sources of error in attenuation and phase shift measurement are discussed, and their effects on the accuracy for the determination of the relative complex permittivity and physical properties are investigated