Cavity Perturbation Technique Research Articles

A Novel Dielectric Resonator-Based Passive Sensor for Drop-Volume Binary Mixtures Classification

In this study, the authors present a dielectric-resonator-based sensor to characterize liquids. The sensor is based on a dielectric resonator (DR) fed by a slot-coupling mechanism in the ground plane of a microstrip transmission line (TL). The obtained device is a fully passive sensor that a radiofrequency (RF) signal can interrogate. A small hole was drilled on the DR surface to allocate a drop of the liquid under test (LUT); as a consequence, the resonant frequency of the sensor will depend on the electromagnetic characteristics of the LUT. The device is modeled both with an equivalent circuit model and through an electromagnetic modal analysis. The cavity perturbation technique is used to study the impact of the LUT on the resonant frequency and modal distribution; full-wave simulations corroborate the theoretical results. Finally, a prototype of the proposed sensor tuned to work in the 2.45 GHz band has been designed, manufactured, and measured. The device is cost-effective given the small size and practical to use, thanks to the minimal volume of the pool, which calls for quantities of LUT in the order of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.13~\mu l$ </tex-math></inline-formula> . The prototype has been tested with an ethanol-water solution set. The experimental results match well with simulations and show good repeatability. The sensitivity of the prototype resulted in being 718 kHz per percentage of ethanol in water.

Detection of Anti-Counterfeiting Markers through Permittivity Maps Using a Micrometer Scale near Field Scanning Microwave Microscope.

This paper describes the use of microwave technology to identify anti-counterfeiting markers on banknotes. The proposed method is based on a robust near-field scanning microwave microscope specially developed to measure permittivity maps of heterogeneous paper specimens at the micrometer scale. The equipment has a built-in vector network analyzer to measure the reflection response of a near-field coaxial probe, which makes it a standalone and portable device. A new approach employing the information of a displacement laser and the cavity perturbation technique was used to determine the relationship between the dielectric properties of the specimens and the resonance response of the probe, avoiding the use of distance-following techniques. The accuracy of the dielectric measurements was evaluated through a comparative study with other well-established cavity methods, revealing uncertainties lower than 5%, very similar to the accuracy reported by other more sophisticated setups. The device was employed to determine the dielectric map of a watermark on a 20 EUR banknote. In addition, the penetration capabilities of microwave energy allowed for the detection of the watermark when concealed behind dielectric or metallic layers. This work demonstrates the benefits of this microwave technique as a novel method for identifying anti-counterfeiting features, which opens new perspectives with which to develop optically opaque markers only traceable through this microwave technique.

A novel production method of antimony trioxide from stibnite concentrate and the dielectric properties of antimony sulfide with different desulfurizer

Dielectric properties of stibnite concentrate, pure antimony sulfide, three different desulfurizers and their mixtures were assessed using resonant cavity perturbation technique. Dielectric properties of ZnO and CaO are largely independent of temperature, while CuO and Sb2S3 have strong dependency with temperature. For the mixtures, the Sb2S3 + CuO exhibit strong responsiveness to microwave from 25 °C to 400 °C. The CuO could be efficiently heated and reacted with Sb2S3 in microwave field, in agreement with thermodynamic result. Based on the dielectric properties and thermochemical characteristics during the desulfurization process, the phase evolution and reaction mechanism were systematically investigated. Results show that the majority of antimony elements in the concentrate were volatilized in the form of Sb2O3 and the rest were left in the form of Sb. Effects of roasting temperature, desulfurizer dosage, and holding duration on microwave roasting were assessed and roasted sample were subjected to XRD, EDX and SEM analysis. Optimum roasting conditions were identified to be 700 °C, 1.4 times the stoichiometric requirement of CuO additive and holding duration of 1.5 h. Microwave roasting of stibnite concentrate with CuO additive promises to be a better alternative technology for traditional pyrometallurgical antimony roasting.

Relationship between superconductivity and nematicity in FeSe1−xTex (x=0−0.5) films studied by complex conductivity measurements

We measured the complex conductivity, $\sigma$, of FeSe$_{1-x}$Te$_x$ ($x=0-0.5$) films in the superconducting state which show a drastic increase of the superconducting transition temperature, $T_\textrm{c}$, when the nematic order disappears. Since the magnetic penetration depth, $\lambda$ $(>$ 400 nm), of Fe(Se,Te) is longer than the typical thickness of the film ($\sim$100 nm), we combined the coplanar waveguide resonator and cavity perturbation techniques to evaluate both the real and imaginary parts of $\sigma$. Films with a nematic order showed a qualitatively different temperature dependence in penetration depth and quasiparticle scattering time when compared with those without nematic order, suggesting that nematic order influences the superconducting gap structure. Conversely, the proportionality between superfluid density, $n_\textrm{s}$ ($\propto\lambda^{-2}$), and $T_\textrm{c}$ was observed irrespective of the presence or absence of nematic order. This result indicates that the amount of superfluid has a stronger impact on the $T_\textrm{c}$ of Fe(Se,Te) than the presence or absence of nematic order. Combining these results with band dispersions calculated using density functional theory, we propose that the change of the Fermi surface associated with nematicity is the primary factor influencing the change of $T_\textrm{c}$ and the superconducting gap structure in Fe(Se,Te).

Complex conductivity of FeSe1–x Te x (x = 0 – 0.5) films

We measured the complex conductivity, σ, of the FeSe1−x Te x (x = 0 – 0.5) films below T c which show a drastic increase of the superconducting transition temperature, T c, when the nematic order disappears. Since the magnetic penetration depth, λ (> 400 nm) of Fe(Se, Te) is longer than the typical thickness of the film (∼100 nm), we combined the coplanar-waveguide-resonator- and cavity-perturbation techniques to evaluate both the real and imaginary parts of σ. Films with the nematic order showed a qualitatively different behavior of the quasiparticle scattering time compared with those without the nematic order, suggesting that the nematic order influences the superconducting gap structure. On the other hand, the proportionality between the superfluid density, n s/m* (∝ λ−2), and T c was observed irrespective of the presence or absence of the nematic order. This result indicates that the amount of the superfluid has a stronger impact on T c of Fe(Se, Te) than the presence or absence of the nematic order itself.

A method to derive the dielectric loss factor of minerals from microwave heating rate tests

Characterizing the loss factor of basic rock-forming minerals is fundamental in understanding the mineral-microwave interactions and predicting the possible weakening effect of microwave on rock. This paper proposed a novel method for deriving the loss factor of minerals based on the microwave heating rate tests, the electromagnetic numerical simulations and the effective medium theory. The heating characteristics of 8 basic rock-forming minerals at 500 W were investigated using a 2.45 GHz single-mode microwave and the minerals were classified into 3 categories based on the heating rate in °C/min/g. Meanwhile, the electric field strengths in the minerals were obtained using electromagnetic simulations and the loss factors of the minerals were calculated based on the effective medium theory. The loss factors calculated were then compared with those measured by the resonant cavity perturbation technique. This method provides an alternative to get the loss factor of minerals from the microwave heating rate tests.

Employing higher order modes in a broadband SIW sensor for permittivity measurement of medium loss materials

AbstractIn this paper, a novel SIW microwave sensor is designed to accurately determine the broadband complex permittivity of medium loss and dispersive liquids using a number of higher order modes in 11–20 GHz. To achieve a higher accuracy in characterization, the sensor is equipped with some methods such as Photonic Band Gap method, slow-wave via, and a new feedline, which enhances the quality factor for the higher order TE1,0,n modes. The operating principle of this sensor is based on the cavity perturbation technique, in which the resonant properties of the cavity are utilized to extract the dielectric properties of liquid under test. To provide a method to decrease the LUT consumption, a winding microfluidic channel is designed and embedded in the cavity. The channel increases the interaction between the induced electric field and the LUT. The accuracy of different perturbation technique for determination of permittivity is compared with each other.

Dielectric properties and high temperature thermochemical properties of the pyrolusite-pyrite mixture during reduction roasting

Manganese sulfate is widely used in electrolytic manganese production, catalysts, batteries, and other fields. However, the preparation of manganese sulfate from conventional roasting process of pyrolusite with additive of pyrite has disadvantages including long roasting time and low efficiency. Thus, microwave heating is used as an alternative for the preparation of manganese sulfate instead of traditional heating to destroy the dense structure of minerals and to further improve the heating efficiency which is dominating by dielectric properties of the material. Therefore, to evaluate the mechanism of preparation of manganese sulfate with microwave heating, experiments were carried out to explore the dielectric and thermochemical properties of pyrolusite-pyrite mixture at high temperature. The dielectric properties of pyrolusite-pyrite mixtures with different ratios were measured at high temperature using the resonant cavity perturbation technique, which has been widely used to study the dielectric properties of different metallurgical materials. It was found that with the increase of temperature, the microwave absorption properties of pure pyrolusite decreased from 28.173 F/M to 25.931 F/M. The wave-absorbing property of the soft pyrite/pyrite mixture increased to 41.218 F/M, indicating that pyrite can obviously improve the wave-absorbing property of the mixture. In addition, the characteristics of pyrolysis and reduction were studied systematically, and the pyrolysis and reduction process was divided into four stages according to the difference of temperature: 30 ℃-200 ℃, 200 ℃-400 ℃, 400 ℃-600 ℃, 600 ℃-900 ℃, corresponding to the separation of adsorbed water and bound water, MnO2 decomposition, desulfurization redox reaction of FeS2 and desulfurization redox reaction of Fe(1-x) S, respectively. This work highlights the possibility of reducing manganese ore by microwave heating with an additive of pyrite and finds that pyrite can modify the microwave absorption properties of pyrolusite.

Experimental and numerical analysis of the complex permittivity of open-cell ceramic foams

Open-cell ceramic foams are promising materials in the field of microwave heating. They can be manufactured from susceptor materials and can, therefore, be used as selective heating elements. In this study, the complex permittivities of ceramic foam materials, including silicon-infiltrated silicon carbide (SiSiC), pressureless sintered silicon carbide (SSiC), silicate-bonded silicon carbide (SBSiC), and cordierite were determined. The dielectric properties of the foams were determined by the cavity perturbation technique using a TE104 WR340 waveguide resonator at 2.45 GHz. Samples were preheated in a tubular furnace, enabling temperature-dependent permittivity measurements up to 200 °C. The effective dielectric constant and effective loss factor were found to depend on the porosity and material composition of the foam. The SiSiC material had a higher effective dielectric constant than the SSiC and SBSiC ceramics. The effective dielectric constant of the foams showed a trend of gradual increase with increasing temperature. Some selected dielectric mixing relations were then applied to describe the effective permittivity of the foams and compare them with predictions from finite element simulations performed using the CST Studio Suite. The foam morphologies and simple block inclusions were used in the simulations.

Dielectric characterization of vegetable oils during a heating cycle.

The knowledge of the dielectric properties of oils is of great importance for several industrial applications, such as microwave-assisted oil frying for foods and high voltage electric power transmission. In this paper, we present the complex permittivity of vegetable oils (canola, olive, soybean, coconut) at 2.50GHz using the cavity perturbation technique from 28 to 200°C (temperature close to the smoking point of oils). The measurements were taken with a cylindrical cavity operating at the TE111 mode with an unloaded Q of 4950. In addition, free fatty acids, peroxide index and color were measured before and after heating.

Measuring fat mass in body equivalent materials using an RF resonant cavity.

Provide a proof of concept for the potential of using a novel RF resonant cavity device for accurately and repeatedly measuring fat and fat-free masses in phantom infants. Design, construct, and characterize an RF resonant cavity with dimensions compatible to holding an infant. The cavity was characterized using spherical phantoms of 0%fat, 50% fat, and 100% fat to empirically calibrate shifts in resonant frequency. The phantoms were constructed using emulsions of bovine lard, water, and dish soap inside spherical containers which do not interact with the electric field. The calibration phantoms were compared with a phantom of a test sample to assess the ability of the resonant cavity perturbation technique for measuring body composition. Phantoms of distinct %fat (0%, 50%, and 100%) were used to calibrate the resonant cavity for measuring body composition. The calibration phantoms were used to create calibration lines of unique %fat and were compared to a 475-mL sample of unknown %fat as a measure of how accurate the resonant cavity technique is for measuring body composition. A 475mL test sample was used to examine the robustness of the RCP technique. The sample was 25% fat and had a fat mass of ( )g. The measured fat mass from the RCP technique was g, or a 2% difference. The resonant cavity perturbation technique provides an accurate and repeatable measurement of fat mass in spherical phantoms and suggests the technology might be an effective obesity research tool for infants. Future studies will focus on extending the work to more complex anthropomorphic shapes.

High temperature permittivity measurements of selected industrially relevant ores: Review and analysis

The mineral processing and extractive metallurgy industries are under increasing pressure to develop more energy efficient and sustainable technologies. As a result, there is increasing interest in the application of alternative energy sources, such as microwave technology. While bench scale testing has been conducted in this area for over thirty years, scale-up has remained a key challenge. One barrier to implementation has been the lack of understanding of the fundamental interaction of microwaves with ores. Key parameters include the real and the imaginary permittivities, which influence both the heating rate and the microwave penetration depth. Over the last decade, high temperature permittivity measurements have been performed on various ores using the cavity perturbation technique. In this paper, this permittivity data is presented as a function of temperature primarily at 912 MHz. Based on the interpretation of this data, recommendations are made for the sustainable utilization of microwave technology in metallurgical processing.

High-temperature dielectric properties and pyrolysis reduction characteristics of different biomass-pyrolusite mixtures in microwave field

Exploring the dielectric properties of mineral-biomass mixtures is fundamental to the coupled application with biomass pyrolysis and microwave technology to mineral reduction. In this work, the microwave dielectric properties of five pyrolusite-biomass mixtures were measured by resonant cavity perturbation technique and the pyrolysis reduction characteristics were systematically investigated, including poplar, pine, ageratina adenophora, rapeseed shell and walnut shell. Results indicated that the dielectric properties commonalities of five mixtures with temperature represented by increasing firstly, dropping intensely and finally rising slightly, with excellent responsiveness to microwaves; which the change trend was mainly attributed to the crystal transformation of amorphous MnO2 and pyrolusite reduction reactions by biomass pyrolysis. Meanwhile, the heating characteristics successfully matched the dielectric properties of the mixtures, and the pyrolusite reduction process by biomass can be divided into two stages: biomass pyrolysis and pyrolusite reduction. The work highlights the universal feasibility of the novel coupled method for mineral reduction.

Effects of Temperature on Relaxation Time and Electrical Conductivity of Spent Automobile Catalyst at Microwave Frequencies

Dielectric properties are some of the main parameters of materials, determining their interaction with electromagnetic energy during microwave heating. The dielectric properties of spent automobile catalyst were measured by using the resonance cavity perturbation technique from room temperature to 800°C at 915 MHz and 2450 MHz. The effects of temperature and frequency on the dielectric constant, dielectric loss factor, loss tangent, and penetration depth were studied. The results indicate that spent automobile catalyst is weakly polar, making it difficult to transform electromagnetic energy into heat. The relaxation time and conductivity obtained by numerical calculations further explain from the microlevel that spent automobile catalyst cannot dissipate electromagnetic energy through dipole loss or conduction loss under microwave irradiation. This work describes an effective way to explore the dielectric heating mechanism and a basis for understanding the interactions between microwaves and spent automobile catalyst.

Structural, magnetic, microwave permittivity and permeability studies of barium monoferrite (BaFe2O4)

Barium monoferrite (BaFe2O4) was synthesized by citrate combustion method. The structural, magnetic properties as well as permittivity and permeability at microwave frequencies have been investigated and discussed in present paper. Formation of orthorhombic crystal structure was confirmed from the X-ray diffraction pattern. These polycrystalline samples sintered at different temperatures have higher Curie temperature (∼700 K) and coercivity (∼4000 Oe) as compared to the values reported in literature. Next to detailed structural and magnetic properties, also permittivity and permeability at X-band microwave frequencies were measured using cavity perturbation technique. The high and quite comparable values of dielectric constant and permeability make this material suitable for use as a microwave absorbing paint as well as a promising electromagnetic interference (EMI) shielding material.

Microwave field: High temperature dielectric properties and heating characteristics of waste hydrodesulfurization catalysts.

The exploration of the dielectric properties of waste hydrodesulfurization catalysts has important guiding significance for the development of microwave heat treatment of waste hydrodesulfurization catalysts for the recovery of valuable metals. The resonant cavity perturbation technique was used to measure the dielectric properties of waste catalyst and the mixture of waste catalyst and Na2CO3 during roasting from room temperature to 700 °C at 2450 MHz. The heating properties of the waste catalyst and mixture of waste catalyst and Na2CO3 were determined in the microwave field. The results show that the waste catalyst and the mixture of waste catalyst and Na2CO3 exhibit strong microwave response capability, and the dielectric constant, dielectric loss factor, and dielectric loss tangent increase with increasing temperature; from 20 to 300 °C, the waste catalyst and the mixture of waste catalyst and Na2CO3 heated at a slower rate, while the material heated rapidly from 300 to 700 °C. In addition, the mechanism of microwave action has been proposed based on the study of dielectric properties and heating properties in the microwave field.

A High-Sensitivity Microfluidic Sensor Based on a Substrate Integrated Waveguide Re-Entrant Cavity for Complex Permittivity Measurement of Liquids.

In this study, a novel non-invasive and contactless microwave sensor using a square substrate integrated waveguide (SIW) re-entrant cavity is proposed for complex permittivity measurement of chemical solutions. The working principle of this sensor is based on cavity perturbation technique, in which the resonant properties of cavity are utilized as signatures to extract the dielectric information of liquid under test (LUT). A winding microfluidic channel is designed and embedded in the gap region of the cavity to obtain a strong interaction between the induced electric field and LUT, thus achieving a high sensitivity. Also, a mathematical predictive model which quantitatively associates the resonant properties of the sensor with the dielectric constant of LUT is developed through numerical analysis. Using this predictive model, quick and accurate extraction of the complex permittivity of LUT can be easily realized. The performance of this sensor is then experimentally validated by four pure chemicals (hexane, ethyl acetate, DMSO and water) together with a set of acetone/water mixtures in various concentrations. Experimental results demonstrate that the designed sensor is capable of characterizing the complex permittivities of various liquids with an accuracy of higher than 96.76% (compared with the theoretical values obtained by Debye relaxation equations), and it is also available for quantifying the concentration ratio of a given binary mixture.

Electronic Properties of Air‐Sensitive Nanomaterials Probed with Microwave Impedance Measurements

Characterization of electronic properties of novel materials is of great importance for exploratory materials development and also for the discovery of new correlated phases. As several novel compounds are available in powder form only, contactless methods, which also work on air‐sensitive samples, are highly desired. We present that the microwave cavity perturbation technique is a versatile tool to study conductivity in such systems. The examples include studies on semiconducting–metallic crossover in carbon nanotubes upon alkali doping, study of vortex motion in the K3C60 superconductor, and the characterization of various alkali atom doped phases of black phosphorus.

A reconfigurable high Q epsilon near zero tunnel for material characterization

A new type of reconfigurable sensing cavity is designed for measurement of complex dielectric permittivity of materials. This circuit is based on an Epsilon Near Zero (ENZ) tunnel integrated on a substrate, and it uses liquid metal switches to reconfigure two tunneling frequencies (2.4 GHz, and 3.5 GHz). As opposed to conventional narrow channel ENZ structures, our tunnel offers high unloaded quality factor by increasing its volume and controlling its external coupling. Using the cavity perturbation technique, the permittivities of 5 different liquids were measured giving very close results to those reported in the literature.

Wide-Range Permittivity Measurement With a Parametric-Dependent Cavity

This paper reports an approach that can determine a very wide-range permittivity using a parametric-dependent cavity based on the field-enhancement method. The method is superior in accuracy as compared with the traditional cavity perturbation technique, especially at high and ultra-high dielectrics. Three regions of interest will be shown. At the low dielectric region ranging from 1 to 15, the field-enhancement method agrees well with the perturbation. At the intermediate or transition region from 15 to 125, the resonant frequency gradually changes from cavity dependence to sample dependence. Beyond 125, the resonant frequency depends mainly on the properties of the sample, not the geometry of the cavity. In addition to the resonant frequencies, the quality factors allow us to determine the imaginary part of the permittivity for low-loss materials. The field-enhancement approach significantly improves the sensitivity and resolution of the measurement. Two representative samples were tested. The results show that the parametric-dependent cavity is intriguing in physics and effective in characterizing materials’ complex permittivity.