Contactless Microwave Research Articles

Evanescent Field Applicator for Contactless Microwave Breast Diagnostics in Air

Evanescent Field Applicator for Contactless Microwave Breast Diagnostics in Air

Effects of Carasau Dough Composition on the Microwave Dielectric Spectra up to 20 GHz

Carasau bread is a traditional product from Sardinia (IT). This flat bread is experiencing industrial advancement, through automation, and has great market potential. However, there is lack of understanding of how the composition (water content, salt, and yeast concentration) affects the product quality. In this work, a microwave dielectric spectroscopy study is performed to investigate how the composition of Carasau bread doughs influences the spectra of this food product up to 20 GHz. A third-order Cole–Cole model was used for the physical and quantitative understanding of the electromagnetic properties of this food product. Then, it has been studied how salt, yeast, and water variations affected the model parameters. This work could pave the route to the development of non-destructive, contactless microwave sensors for Carasau bread quality assessment.

Microwave Sensing Aqueous Solutions Based on Ring-Assisted Mach–Zehnder Interferometer

In this work, we present a novel tunable and ring-assisted Mach–Zehnder interferometer (RAMZI) to realize the highly sensitive and contactless microwave measurement of liquid samples. The designed RAMZI consists of a split ring (SR) and a feedback line for the destructive interference at the resonant frequency of odd or even mode of the SR after appropriately tuning the signal balance. An equivalent circuit model is built to study and explain its working principle. The achieved quality factor is up to 3.1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times \,\,10^{5}$ </tex-math></inline-formula> with water loaded. The minimum ethanol concentration level is measured to be 0.005 mole fraction. With the empirical relation between the sensor resonance and the sample permittivity, the complex permittivity of a liquid can be extracted.

(Invited) Oxygen Reduction Reaction in Alkaline Media: The Effect of Carbon Support Crystallinity, Conductivity, and Doping

The rapid development of energy consumption combined with growing environmental awareness is conducive to the wide-ranging search for ecological energy sources. In the recent decade, a significant amount of research has focused on the development of anion-exchange membrane (AEM) fuel cells (AEMFCs), as such approach exhibits faster oxygen reduction reaction (ORR) kinetics in the alkaline environment of the cell [1]. However, to become a serious alternative to the current mainstream acidic fuel cells, alkaline systems should overcome current limitations of performance and stability [2]. The design and synthesis of high-performance electrocatalysts facilitating the slow oxygen reduction reaction (ORR) is, therefore, a key issue in the context of the full commercialization of fuel cells. Transition metal oxides with spinel structure constitute an interesting group of potential catalysts that can replace the widely used materials based on Pt [3, 4]. However, due to the low electrical conductivity and the tendency to undesirable aggregation of spinel nanoparticles, it is important to support them on appropriate carbon carriers. Since the nature of the carbon carrier plays a significant role in the ORR activity, we aimed to comprehensively investigate the influence of the electric conductivity of carbon supports, the role of the surface functional oxygen-bearing groups, and doping with heteroatoms on the performance of the bare and cobalt-manganese spinels deposited on the functionalized carbon supports, acting as ORR electrocatalysts in the alkaline media [5].We investigated commercially available Vulcan XC-72, Printex85, MWCNT, amorphous carbon (C-am.), and carbons synthesized by structural nano-replication using the mesoporous molecular sieves MCM-48 (carbon denoted CMK-1) and the spherical silica calcinated at temperature range 650-1050 oC (SPH-650-1050). To investigate the effect of doping on the ORR catalytical activity the MWCNT was modified by nitrogen and sulfur heteroatoms. Additionally, the doped supports were then treated by plasma to introduce surface oxygen groups. On all investigated carbon supports the nanoparticles of manganese-cobalt spinel were dispersed. They were prepared at 150oC by a microwave-assisted hydrothermal route. Comprehensive physicochemical characterization of carbon support was performed using Raman and UV-Vis spectroscopies, XPS, TPD, XRD, TGA techniques, electron microscopy (TEM), and N2 adsorption (BET) (Figure 1). The relative electric conductivity measurements were performed using the contactless microwave technique. The electrochemical properties of the obtained materials were determined by cyclic voltammetry (CV), whereas the electrocatalytic activity of the obtained materials towards ORR was determined using the RDE and RRDE techniques.It was found that the nature of the carbon carrier plays a triple role in the ORR activity, determining dispersion and faceting of the spinel nanoparticles, as well as the extent of the undesired 2e– reduction pathway, controlled by the fraction of an amorphous component. In the case of SPH samples, it was shown that with the increasing carbonization temperature of the carbon support the electric conductivity increase in parallel to the crystallization extent. The electrocatalytic performance of the carbon carriers and the supported cobalt manganese spinels exhibits a volcano-type shape reaching the maximum for carbon materials obtained at 850°C. Such dependence results from two descriptors changing oppositely with the carbonization temperature: an increasing electric conductivity and a decreasing amount of the carbonyl and quinone groups in the sample. The electric conductivity multiplied by the amount of the carboxyl and quinone groups exhibits the same variation with the carbon calcination temperature as the number of electrons exchanged in ORR, confirming that optimal conjunction of these both properties plays a decisive role in this process. The dispersed spinel active phase favors the direct 4e− pathway of ORR and improves the value of the E onset potential distinctly. To the best of our knowledge, the role of electric conductivity and surface oxygen functionalities in the ORR activity over carbon materials and supported spinel electrocatalysts was disentangled for the first time in this study. Acknowledgments: This work was supported by the Polish National Science Centre (NCN) project OPUS-14, No. 2017/27/B/ST5/01004.

Intelligent wireless walls for contactless in-home monitoring

Human activity monitoring is an exciting research area to assist independent living among disabled and elderly population. Various techniques have been proposed to recognise human activities, such as exploiting sensors, cameras, wearables, and contactless microwave sensing. Among these, the microwave sensing has recently gained significant attention due to its merit to solve the privacy concerns of cameras and discomfort caused by wearables. However, the existing microwave sensing techniques have a basic disadvantage of requiring controlled and ideal settings for high-accuracy activity detections, which restricts its wide adoptions in non-line-of-sight (Non-LOS) environments. Here, we propose a concept of intelligent wireless walls (IWW) to ensure high-precision activity monitoring in complex environments wherein the conventional microwave sensing is invalid. The IWW is composed of a reconfigurable intelligent surface (RIS) that can perform beam steering and beamforming, and machine learning algorithms that can automatically detect the human activities with high accuracy. Two complex environments are considered: one is a corridor junction scenario with transmitter and receiver in separate corridor sections and the other is a multi-floor scenario wherein the transmitter and receiver are placed on two different floors of a building. In each of the aforementioned environments, three distinct body movements are considered namely, sitting, standing, and walking. Two subjects, one male and one female perform these activities in both environments. It is demonstrated that IWW provide a maximum detection gain of 28% in multi-floor scenario and 25% in corridor junction scenario as compared to traditional microwave sensing without RIS.

Predicting Solar Cell Performance from Terahertz and Microwave Spectroscopy

AbstractMobilities and lifetimes of photogenerated charge carriers are core properties of photovoltaic materials and can both be characterized by contactless terahertz or microwave measurements. Here, the expertise from fifteen laboratories is combined to quantitatively model the current‐voltage characteristics of a solar cell from such measurements. To this end, the impact of measurement conditions, alternate interpretations, and experimental inter‐laboratory variations are discussed using a (Cs,FA,MA)Pb(I,Br)3 halide perovskite thin‐film as a case study. At 1 sun equivalent excitation, neither transport nor recombination is significantly affected by exciton formation or trapping. Terahertz, microwave, and photoluminescence transients for the neat material yield consistent effective lifetimes implying a resistance‐free JV‐curve with a potential power conversion efficiency of 24.6 %. For grainsizes above ≈20 nm, intra‐grain charge transport is characterized by terahertz sum mobilities of ≈32 cm2 V−1 s−1. Drift‐diffusion simulations indicate that these intra‐grain mobilities can slightly reduce the fill factor of perovskite solar cells to 0.82, in accordance with the best‐realized devices in the literature. Beyond perovskites, this work can guide a highly predictive characterization of any emerging semiconductor for photovoltaic or photoelectrochemical energy conversion. A best practice for the interpretation of terahertz and microwave measurements on photovoltaic materials is presented.

The investigation of the kinetics of the losses of nonequilibrium current carriers in Cu-=SUB=-2-delta-=/SUB=-NiSnS-=SUB=-4-=/SUB=- (0≤delta≤0.2) quaternary copper compounds

By solid-phase synthesis from elemental Cu, Ni, Sn, and S samples Cu2-deltaNiSnS4 (0≤delta≤0.2) was prepared. The parameters of their crystal lattice have been refined. The lifetimes of photogenerated current carriers in Cu2-deltaNiSnS4 were estimated for the first time by the contactless time-resolved microwave photoconductivity method. This times turned out to be tau~7 ns, which is comparable with the literature data for the times in kesterites CZTS. Herewith, in kinetics of loses of photogenerated cureent carriers is observed predominance of bimolecular recombination processes over capture processes. Keywords: Cu2-deltaNiSnS4, onequilibrium current carriers, lifetimes, kinetics.

Optical, structural and electrical properties of sputtered ultrathin chromium films

Systematically prepared ultrathin chromium films were analyzed for their structural, optical, and electrical properties. The films with thickness from 2.5 nm up to 57 nm were deposited by magnetron sputtering, and the deposition process was optimized to obtain compact yet low-stress films. The films were studied by non-destructive techniques like the variable angle spectroscopic ellipsometry (VASE), the spectrophotometry in a wide spectral range, the X-ray diffraction (XRD), the X-ray reflectometry (XRR) and the contactless microwave resistance measurement. Nanoscale thickness measured by XRR was confirmed then by ellipsometry. The films with thickness higher than 5 nm exhibits only metallic chromium phase (c-Cr) with 110 preferred orientation. The obtained optical constants of ultrathin chromium films are significantly higher compared to the traditionally referenced values, particularly in the infrared range. We analyse reliability of the obtained data and, for the thickest of our films, the optical constants are in good agreement with the recently published values obtained for polycrystalline bulky chromium studied by the reflection electron energy loss spectroscopy (REELS). The thickness-dependent resistivity of the films follows the Mayadas and Shatzk's distribution with a modification necessary for the films thinner than the value of the mean free path of electrons in the ideal chromium crystal. For such thin films the resistivity is a function of film thickness value, while the resistivity of the thicker films is governed by the mean size of the film crystallites.

Development of microwave metamaterial-inspired sensors with multiple-band reactivity for mercury contamination detection

Alternative contactless metamaterial-inspired microwave sensors, composed of a multi-band omnidirectional bowtie antenna and metamaterials, are developed to detect and identify mercury, one of the most widespread toxic heavy metals, in the C-band and S-band, using reflection coefficients. A multi-negative refractive index band metamaterial is specifically designed as a sensing enhancer, where the multi bands can effectively trigger the electromagnetic properties, as well as enhance the differentiation between the mercury samples. The geometry of the metamaterial and its arrangement and orientation are optimized to reach the system highest sensitivity. The proposed sensors were tested on mercury traces of various volumes and weights. Our study has shown that the electromagnetic responses of the mercury in the microwave range are nonlinear, yet predictable. Six distinguishable resonances generated while testing the samples specify the capability of identifying the mercury amount, which can effectively trace mercury contamination in water and seawater.

A broadband contactless gap waveguide microwave switch for X- and Ku-bands applications

This paper presents a novel wideband gap waveguide mechanical switch for low-loss and high-power radar and satellite applications. The proposed design has contactless waveguide structures and therefore the different conducting layers are not required to make physical contact joints. This property can make the structure more flexible and simple to fabricate than hollow waveguides at millimeter-waves and does not require high-precision fabrication or mechanical assembly such as diffusion bonding of laminated thin metal plates. The simulation and measurement results of the proposed switch show that the input reflection coefficient (|S11|) is less than −20 dB and the ON-state insertion loss (|S21|) is less than 0.1 dB, within the switch operational frequency bandwidth from 10.6 to 16 GHz. Also, the measured adjacent ports isolation for the OFF-state is greater than 75 dB over the desired frequency range. The measured and simulated results agree very well.

Rapid and real-time monitoring of bacterial growth against antibiotics in solid growth medium using a contactless planar microwave resonator sensor

Infection diagnosis and antibiotic susceptibility testing (AST) are pertinent clinical microbiology practices that are in dire need of improvement, due to the inadequacy of current standards in early detection of bacterial response to antibiotics and affordability of contemporarily used methods. This paper presents a novel way to conduct AST which hybridizes disk diffusion AST with microwave resonators for rapid, contactless, and non-invasive sensing and monitoring. In this research, the effect of antibiotic (erythromycin) concentrations on test bacterium, Escherichia coli (E. coli) cultured on solid agar medium (MH agar) are monitored through employing a microwave split-ring resonator. A one-port microwave resonator operating at a 1.76 GHz resonant frequency, featuring a 5 mm2 sensitive sensing region, was designed and optimized to perform this. Upon introducing uninhibited growth of the bacteria, the sensor measured 0.005 dB/hr, with a maximum change of 0.07 dB over the course of 15 hours. The amplitude change decreased to negligible values to signify inhibited growth of the bacteria at higher concentrations of antibiotics, such as a change of 0.005 dB in resonant amplitude variation while using 45 µg of antibiotic. Moreover, this sensor demonstrated decisive results of antibiotic susceptibility in under 6 hours and shows great promise to expand automation to the intricate AST workflow in clinical settings, while providing rapid, sensitive, and non-invasive detection capabilities.

Исследование кинетики гибели неравновесных носителей заряда в четверных соединениях меди Cu-=SUB=-2-delta-=/SUB=-NiSnS-=SUB=-4-=/SUB=- (0&amp;le;delta&amp;le;0.2)

By solid-phase synthesis from elemental Cu, Ni, Sn, and S samples Cu2 - δNiSnS4 was prepared . The parameters of their crystal lattice have been refined. The lifetimes of photogenerated current carriers in Cu2 - δNiSnS4 were estimated for the first time by the contactless time-resolved microwave photoconductivity method. This times turned out to be τ ≈ 7 ns, which is comparable with the literature data for the times in kesterites CZTS. Herewith, in kinetics of loses of photogenerated cureent carriers is observed predominance of bimolecular recombination processes over capture processes.

Reply to “Comment on ‘Temperature range of superconducting fluctuations above Tc in YBa2Cu3O7−δ single crystals' ”

In a previous article [M. S. Grbi\ifmmode \acute{c}\else \'{c}\fi{} et al., Phys. Rev. B 83, 144508 (2011)] we reported data on the high-temperature superconductor ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ collected by a contactless microwave absorption technique, which provided evidence for superconducting correlations over a limited temperature range (10--20 K above ${T}_{c}$). The paraconductivity signal was determined by subtraction of zero-magnetic-field data from 16-T data. In the preceding Comment [D. S\'o\~nora et al., preceding Comment, Phys. Rev. B 102, 176501 (2020)] D. S\'o\~nora et al. argue that a 16-T magnetic field is not enough to quench all superconducting fluctuations in the microwave spectral range above ${T}_{c}$. As is obvious from the experimental data presented here, as well as from the data shown in the original paper, such conclusions disagree with our experimental results. Moreover, our initial experimental findings recently received independent robust confirmation from several different experimental techniques.

Analysis of Pristine and Tcnq-Loaded MOF FILMS By Contactless Microwave Dielectric Spectroscopy Technique

Metal-organic frameworks (MOFs) are porous and crystalline Coordination Network Compounds (CNCs), also referred to as porous coordination polymers (PCPs), which consist of inorganic metal (or metal/oxo) clusters connected by organic linkers. Surface Anchored Metal-organic-framework (SURMOF) films consist of quasi-liquid phase epitaxially grown highly ordered crystalline MOF films. For this study we used MOF Cu3(BTC)2 (BTC: benzene tricarboxylate) films, also known as HKUST-1 (Hong Kong University of Science and Technology-1), which were infiltrated with tetracyano-quinodimethane (TCNQ) guest molecules to modulate the charge transport characteristics. HKUST-1 MOF thin films have been grown using a liquid phase epitaxy (LPE) spray method in which the metal-containing solution [1 mmol Cu(OAc)2] and the linker solution [0.1 mmol BTC] are sprayed alternately on modified silicon substrates. Subsequently, the samples were immersed into an ethanoic TCNQ solution.We report on localized charge transfer processes at microwave frequencies of porous metal-organic framework (MOF) HKUST-1 Cu2(BTC)3 samples. As part of this study we will compare measurements done in the HKUST-1 pristine form of the material with open pores and HKUST-1, where the pores have been infiltrated with tetracyano-quinodimethane (TCNQ), resulting in a hybrid structure with enhanced electrical conductivity and a significant measurable effect in the microwave propagation characteristics [1]. We present data and analysis from an evaluation of pristine and loaded HKUST-1 SURMOF films using the contactless broadband microwave dielectric spectroscopy (BDS). The technique enables the accurate operando evaluation of the electrical and magnetic properties of materials without compromising the kinetic conditions of the experiment. The BDS method is uniquely suitable for investigating charge transfer reactions, regardless of the nature of the charge carriers, i.e., electrons and holes [2]. As such, these material state-sensitive parameters could be used to monitor charge transfer changes in the SURMOF and SURMOF-TCNQ hybrid analytes.[1] G. M. Espallargas and E. Coronado, “Magnetic functionalities in MOFs: from the framework to the pore”, Chem. Soc. Rev., 2018,47, 533, DOI: 10.1039/c7cs00653e[2] A. M. Wernbacher et al, Operando Electrical Conductivity and Complex Permittivity Study on Vanadia Oxidation Catalysts, J. Phys. Chem. C 2019, 123, 8005−8017, DOI: 10.1021/acs.jpcc.8b07417

Role of the paramagnetic donor-like defects in the high n-type conductivity of the hydrogenated ZnO microparticles

The magnetic and electronic properties of the hydrogenated highly conductive zinc oxide (ZnO) microparticles were investigated by electron paramagnetic resonance (EPR) and contactless microwave (MW) conductivity techniques in the wide temperature range. The EPR spectra simulation allowed us to resolve four overlapping EPR signals in ZnO microparticles. The Lorentzian EPR line with isotropic g-factor 1.9623(5) was related to the singly ionized oxygen vacancy. Another Lorentzian line with g|| = 1.9581(5), g⊥ = 1.9562(5) was attributed to the zinc interstitial shallow donor center, while EPR signal with g|| = 1.9567(5), g⊥ = 1.9556(5) and Gaussian lineshape was assigned to the hydrogen interstitial shallow effective-mass-like donor. The EPR signal with g|| = 1.9538(5), g⊥ = 1.9556(5) and Lorentzian lineshape was tentatively attributed to the shallow donor center. The charge transport properties in ZnO microparticles have been investigated by the contactless MW conductivity technique at T = 5–296 K. Two conduction mechanisms, including ionization of electrons from the shallow donors to the conduction band and hopping conduction process, have been distinguished. The hopping conduction process follows Mott’s variable-range hopping T−1/4 law at T = 10–100 K. The evaluated values of the average hopping distance (15.86 Å), and hopping energy (1.822 meV at 40 K) enable us to estimate the donor concentration in the investigated ZnO microparticles as ~ 1018 cm−3.

Broadband Microwave Signal Dissipation in Nanostructured Copper Oxide at Air-Film Interface.

Contactless broadband microwave spectroscopy (a.k.a., broadband dielectric spectroscopy (BDS)) enables the accurate operando analysis of the electrical and magnetic properties without compromising the kinetic conditions of the experiment. The BDS method is sensitive to the actual electronic structure of species, and it is most relevant to redox reactions involving charge-transfer. In this paper, using BDS, we have studied and characterized the oxidation of a copper layer in a purposely built prototypical 3-D integrated circuit (3D-IC) during cycled high-temperature storage. We show that the microwave signal loss in these devices is attributable to the energy dissipation through the signal's interactions with the copper oxidation product. The results demonstrate that contactless BDS could be leveraged into an excellent metrology for applications that use metal oxide as sensing elements.

Microwave transient reflection in annealed SnS thin films

Tin sulphide (SnS) thin films have been grown by Chemical Bath Deposition (CBD) and annealed up to 600 °C. Non-intentionally doped films show p-type conduction which makes them an ideal partner material for p-n type photovoltaic devices. Most of the films were modestly Ga-doped, but addition of other elements like Fe, Cu, and In was also investigated. We studied essential optoelectronic material parameters like band gap, Urbach tail parameter, and optical absorption using transmission measurements. We found marked differences in the composition of the film surface and of the bulk region. Such complementary results were possible through X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering analysis (RBS), respectively. A tentative model of surface modification and diffusion barrier formation based on thermodynamic arguments is given. Emphasis is put on the study of kinetics of photoinduced carriers after high-power laser pulses by contactless Microwave Transient Reflection (MWTR) analysis. The MWTR signal intensity is increasing strongly upon thermal annealing from 200 to 600 °C, reflecting the strongly improved photoactivity.

Impact of Moisture on Mobility in Methylammonium Lead Iodide and Formamidinium Lead Iodide.

The greatest remaining barrier to the commercialization of perovskite solar cells is their instability to ambient environmental conditions. While most studies of the electronic stability of perovskites employ finished devices, we here exploit the contactless characterization technique time-resolved microwave conductivity to probe electronic properties in the absence of encapsulation and interface effects. By tracking the mobility of charge carriers in two archetypal perovskite compounds, methylammonium lead iodide (MAPbI3) and formamidinium lead iodide (FAPbI3) under various conditions, we are able to make decisive statements about the role of water in the electronic performance of perovskites. Overall, we observe a strong negative correlation between hydration and mobility in MAPbI3, but not in FAPbI3. We anticipate that the data presented herein will serve as a valuable resource in future stability studies in perovskite solar cells and, ultimately, lead to more stable devices.

A Low-Cost Multiple Complementary Split-Ring Resonator-Based Microwave Sensor for Contactless Dielectric Characterization of Liquids

We propose a low-cost, easy-to-fabricate, contactless microwave sensor for dielectric characterization of liquids. The design of the proposed sensor is based on a multiple complementary split-ring resonator (MCSRR) fabricated on a low-cost FR-4 substrate. A glass capillary tube having an inner diameter of $0.008\lambda _{{0}}$ is inserted in the high electric field region of the MCSRR to carry the liquid under test. The sensor is designed to operate at a resonant frequency of 2.45GHz for an empty tube and shifted resonant peaks are utilized for the dielectric characterization of different liquids. The maximum observed shifts in resonant frequency and Q factor are up to 400MHz and 31, respectively. The numerically established relations are experimentally verified through fabricated sensor for various binary mixtures of water and ethanol. The percentage errors between the calculated and reference permittivity of different samples are noticed to be less than 5%. The proposed device promises to be a cost-effective and convenient solution for accurate dielectric characterization of liquids and their binary aqueous solutions.

Contactless Resonant Cavity Dielectric Spectroscopic Studies of Cellulosic Paper Aging

The current analytical techniques for characterizing printing and graphic arts substrates, particularly those used to date and authenticate provenance, are destructive. This limits the amount of data that can be captured from an individual sample. For samples being evaluated in forensic and archeological investigations, any loss or degradation of the materials is undesirable. Furthermore, it is difficult to produce statistically relevant data for such analytes. We have shown elsewhere that a contactless microwave resonant cavity dielectric spectroscopy technique can discriminate between paper samples made from different plant fiber species based on their lignin content. In this publication, we demonstrate the utility of the contactless resonant cavity dielectric spectroscopy (RCDS) technique in the characterization of naturally and artificially aged paper samples. Based on our experimental results, we suggest that the technique could be used in forensic and archeological investigations of unique paper products.