Resonance Frequency Shift Research Articles

NMR Spectroscopy and Multiscale Modeling Shed Light on Ion-Solvent Interactions and Ion Pairing in Aqueous NaF Solutions.

The balance between ion solvation and ion pairing in aqueous solutions modulates chemical and physical processes from catalysis to protein folding. Yet, despite more than a century of investigation, experimental determination of the distribution of ion-solvation and ion-pairing states remains elusive, even for archetypal systems like aqueous alkali halides. Here, we combine nuclear magnetic resonance (NMR) spectroscopy and multiscale modeling to disentangle ion-solvent interactions from ion pairing in aqueous sodium fluoride solutions. We have developed a high-accuracy method to collect experimental NMR resonance frequencies for both ions as functions of temperature and concentration. Comparison of these data with resonance frequencies for nonassociating salts allows us to differentiate the influence of solvation and ion pairing on NMR spectra. These high-quality experimental NMR data are used to validate our modeling framework comprising polarizable force field molecular dynamics (MD) simulations and quantum chemical calculations of NMR resonance frequencies. Our experimental and theoretical resonance frequency shifts agree over a wide range of temperatures and concentrations. Structural analysis reveals how both trends are dominated by interactions with water molecules. For the more sensitive 19F nucleus, the NMR resonance frequency decreases as hydrogen bonds between fluoride and water molecules are reduced in number with increased temperature and molality. Through a detailed analysis of the theoretical NMR resonance frequencies for both ions, we show that NMR spectroscopy can distinguish both contact ion pairs and single-solvent-separated ion pairs from free ions. This quantitative framework can be applied directly to other systems.

Flexible surface plasmon based coupled triple band UHF-microwave sensor for glucose sensing application

Although the correlation between a glucose concentration and its permittivity is somewhat weak to be measured, the glucose concentration is a strong function of the dispersion. In terahertz or microwave frequencies, dispersion can be observed or measured along the interface between an object under test and a metamaterial or surface plasmonic surface, which is basically a metal structure characterized by periodically arrayed holes, grooves, or metal grating. In this work, we have focused on the method for improving the accuracy of a glucose measurement by proposing a new triple-band microwave sensor design and by measuring the resonant frequency shift associated with a glucose concentration at three frequencies simultaneously. A new triple-band glucose sensor of dimension (30 mm x 10 mm) was designed with the main sensing region as compact as 14 mm with two conducting microstrip lines on both ends of the sensor. The sensor design has been realized on a thin flexible substrate of 0.15 mm thickness. The proposed sensor has been designed to measure glucose concentration through the measurement of a resonant frequency shift at 650 MHz, 4.45 GHz, and 10.35 GHz. Overall, the glucose concentration has been found to be correlated positively and linearly with the resonant frequency shift at these frequencies.

A microwave sensing system combination of interdigital structure (IDS)-based microstrip line and RF circuits for extracting complex permittivity of liquid samples

In this manuscript, a microwave sensing system for extracting complex permittivity of binary aqueous solutions based on a modified microstrip line with interdigital structure (IDS) etched is proposed. The passive resonance unit and active circuits constitute the microwave sensing system. The passive resonance unit is evolved from conventional microstrip line with the characteristic impedance of 50Ω, then the IDS is etched on the top surface of passive resonance unit to enhance the confinement of electrical field. The modified passive sensor can produce two resonant modes, i.e., odd-mode 1 and odd-mode 2, the electrical fields of these two modes are both concentrating at IDS, and odd-mode 1 with higher density of electrical field is adopted to retrieve complex permittivity of liquid samples. Except for the passive sensor, the sensing system consists of power divider, low noise amplifier (LNA), isolator, orthogonal hybrid coupler, phase shifter, mixer, and low-pass filter (LPF). The proposed microwave sensing system has two output DC voltages, i.e., channel-I and channel-Q. The mathematical relationship between complex permittivity and resonant frequency shift can be transformed into the relation between complex permittivity and two DC voltages by the proposed microwave sensing system, which is beneficial to discard the use of VNA. The two output voltages of microwave sensing system will be changed according to the injections of liquid samples with different complex permittivity, then the mathematical models can be established by summarizing the rule between complex permittivity and DC voltages. Finally, the established mathematical models can be adopted to predict the binary aqueous solutions with unknown complex permittivity. In measurement, the proposed microwave sensing system has the average sensitivities of about 2.478 mV/εr′ and 1.418 mV/εr′ for channel-I and channel-Q, respectively, which are several times higher than other reported ones. Low-cost, high sensitivity, convenient measurement, and easy fabrication are the merits for the proposed sensing system, and it is a good template in the region of detecting liquid samples.

Noninvasive Readout of the Kinetic Inductance of Superconducting Nanostructures.

The energy landscape of multiply connected superconducting structures is ruled by fluxoid quantization due to the implied single-valuedness of the complex wave function. The transitions and interaction between these energy states, each defined by a specific phase winding number, are governed by classical and/or quantum phase slips. Understanding these events requires the ability to probe, noninvasively, the state of the ring. Here, we employ a niobium resonator to examine the superconducting properties of an aluminum loop. By applying a magnetic field, adjusting temperature, and altering the loop's dimensions via focused ion beam milling, we correlate resonance frequency shifts with changes in the loop's kinetic inductance. This parameter is an indicator of the superconducting condensate's state, facilitating the detection of phase slips in nanodevices and providing insights into their dynamics. Our method presents a proof-of-principle spectroscopic technique with promising potential for investigating Cooper pair density in inductively coupled superconducting nanostructures.

Nonlinear resonant bar of approximate Ramberg–Osgood type modulus defect

This study investigates the nonlinear vibration of a material bar with a modulus defect that causes observable hysteresis in the elastic regime. The Ramberg–Osgood equation is used to obtain an approximate model of the quasi-static stress–strain relationship for such a material, from which a phenomenological dynamic hysteresis function of arbitrary order is constructed. The nonlinear hysteresis is described with two parameters. Applying the hysteresis function leads to a nonlinear wave equation which is then solved for the longitudinal vibration of the bar using a perturbation method. The leading-order solution provides the strain-dependent resonance frequency and damping factor which are the two quantities measured in the nonlinear resonance ultrasound spectroscopy (NRUS). It is shown that the resonance frequency shift and damping increment hold, in general, a power-law dependence on the strain amplitude and the conventional linear dependence is a special case for the quadratic hysteresis. The derived formula is applied to existing data from bending and torsional NRUS-type experiments where the conventional linear fit appears to be clearly inadequate. Experimental resonance frequency shift data are fitted to determine the two hysteresis parameters, both of which vary with the changes in the material state. It is shown that the nonlinear damping ratio is a more general measure of the energy dissipation due to the increasing hysteresis and thus can be taken as a damage index. Consequently, this paper proposes a general procedure to analyze NRUS data for a wider range of materials. The present analysis is valid up to the yield strain which is sufficient to cover the strain amplitudes used in NRUS measurements.

THz Metamaterial Sensitivity Enhancement by Reduction of Substrate's Fabry-Pérot Oscillations Using Back Plates as an Optical De-Coupler.

This work presents a new method for the enhancement of sensitivity in Terahertz (THz) spectroscopy on metamaterial (MM) in terms of its resonance frequency shift (ΔF), by attaching the dielectric back plate to the MM's silicon (Si) wafer. The dielectric back plates are designed to minimize the Fresnel reflections at the backside of the substrate, identical to a broadband antireflective (AR) plate tailored for THz. Utilizing broadband AR technology, we demonstrate the concept of decoupling MM resonance from the substrate's Fabry-Pérot (FP) oscillations. This is done by effectively coupling the THz light out of the high-permittivity substrate, resulting in the improved quality factor of the MM resonance and overall plasmonic enhancement on the metasurface. The back plate acts as a surface plasmonic enhancer to the THz MM by increasing the field intensity on the front metasurface, leading to enhancement of dielectric response (MM's ΔF). This makes the MM resonance ultrasensitive to the minor changes of particle size/concentration under test spread on the metasurface, contributing to enhanced resonance ΔF. The plate also makes the Si substrate optically lossless, enabling the full effect of MM resonance shift and increasing the resonance ΔF by 8-fold compared with MM's fabricated on conventional Si substrates. This research is backed-up with system-level CST simulations and experimental THz impedance spectroscopy of the MM. This method and chip structure is CMOS compatible having potential applications for any resonant MM fabricated on a substrate aimed to maximize dielectric sensitivity for biosensing and nanoparticle THz spectroscopy.

Cyclic Electrogravimetry Reveals Details of Copper Electrodeposition in the Presence of Additives

Copper electroplating was studied based on cyclic voltammetry combined with a new type of acoustic microgravimetry. The instrument operates similarly to a quartz crystal microbalance with dissipation monitoring (EQCM-D), meaning that it reports shifts in resonance frequency, Δf, and half bandwidth, ΔΓ, on several overtones. Compared to the existing instruments, the time resolution was improved by a factor of 10 down to 10 ms using multifrequency lockin amplification. The frequency noise after accumulation was reduced below 10 mHz ≙ 0.12 ng/cm2 ≙ 0.14 pm (assuming ρ = 8.96 g/cm3) by modulating the electrode potential and averaging of Δf and ΔΓ. Mostly, Δf and ΔΓ are in line with the Sauerbrey prediction (Δf/n ≈const. and ΔΓ/n < -Δf/nwith n the overtone order). Small unsystematic deviations between overtones due to compressional wave effects are seen but no evidence for strong roughness or viscoelasticity. Further, the Faraday efficiency is almost unity, meaning that Δf is close to the expectations derived from the electrical current. The improved sensitivity enables to determine the time derivative of the frequency shift, which is equal to an apparent mass transfer rate that can be compared to the current directly. Again, small deviations are seen for bulk deposition and stripping. These go back to weak roughness effects most pronounced in the initial phase of bulk deposition and, also, to the presence of a Cu(I) intermediate during dissolution. When adding benzotriazole (BTA) or thiourea (TU) as plating additives, the gravimetric and the voltammetric response changes. For BTA, the deviations between mass transfer rate and current vanish indicative of decreased roughness. For TU additional features appear, related to a potential-dependent adsorption of TU and to increased underpotential deposition (UPD). In the presence of TU, the electrode-electrolyte interface displays viscoelasticity, evident by the bandwidth increases during deposition.[1] Caption: Currents, time derivatives of the frequency shift, and bandwidth shift obtained during copper electrodeposition without additives (A), in the presence of 10 µM benzotriazole (B), and in the presence of 10 µM thiourea (C). Reprinted with permission from Reference [1].

Square Wave Electrogravimetry Monitors Transient Submonolayer Adsorption of Redox-Active Molecules with a Precision of Less Than 1% of a Monolayer

Square wave voltammetry on electrolytes containing the reversible redox pairs flavin adenine dinucleotide (FAD) and methyl viologen (MV) was complemented by a new form of acoustic microgravimetry. The analytes FAD and MV have promising applications in bacterial fuel cells and redox flow batteries. The instrument operates similarly to a quartz crystal microbalance with dissipation monitoring (EQCM-D). It reports shifts in resonance frequency and half bandwidth on several overtones. Compared to the existing instruments, the time resolution was improved by a factor of 40 down to 2.5 ms by multifrequency lockin amplification, which is important for the analysis of the transients in electrochemistry. The frequency noise after accumulation was reduced below 10 mHz ≙ 0.12 ng/cm2 ≙ 1.2 pm (assuming ρ = 1 g/cm3) by employing modulation and averaging. Square wave modulation of the electrode potential is superior to linear ramps (cyclic voltammetry) because it reduces the contribution of non-Faraday currents and, also, improves sensitivity. The shifts of frequency,Δf, and half bandwidth, ΔΓ, are in line with the Sauerbrey prediction (Δf/n ≈const. and ΔΓ/n < -Δf/nwith n the overtone order). Small deviations (Figure B and D) presumably are caused by the layer’s softness. The apparent shear modulus, G app, of this layer is in the GPa range. The frequency difference, Δf/n|CT (Figure E), amounts to an apparent mass transfer rate. Plots of Δf/n|CT versus potential sometimes show curves similar to the electric current as observed for e. g. MV. For FAD, the results obtained depended on pH and the apparent mass transfer rates are much different from the current (Figure C and E). This difference is most likely caused by adsorbed molecules that are intermediates of the underlying two-electron process. The explanation is further corroborated by the response time of the QCM signals being much larger than the RC time of double layer recharging as determined with EIS. An interpretation in terms of adsorption/desorption is more plausible than an interpretation in terms of changed viscosity in the diffuse double layer.[1] Caption: Currents and frequency shifts obtained on a solution containing FAD. Df/n|av evidences the preferred adsorption of the intermediate state (FADH).

(Invited) Advances in Wide Bandgap Thermal Resonant Infrared Detectors

Exploring the surface of distant planets, notably Venus, requires robust technologies capable of withstanding extreme environmental conditions while providing valuable scientific insights. This paper delves into the advancements made in designing an instrument tailored for capturing infrared (IR) images in the harsh Venusian environment, where temperatures soar to 500°C. The presented IR detector harnesses the power of a sparse array of resonant-based micromechanical devices, strategically addressing the limitations of existing technologies that can only withstand extreme conditions for limited durations.At the core of this technology lies the utilization of high-temperature-tolerant InAlN/GaN as the primary resonating element. GaN's resilience and high temperature tolerance make it a crucial player in challenging space environments. Its wide bandgap and chemical stability are advantageous for enduring harsh conditions during space missions. In tandem with GaN's intrinsic properties, high-temperature-tolerant meta-absorbers strategically positioned on the surface of each pixel play a pivotal role in the functionality of the IR detector. These meta-absorbers exhibit a remarkable capacity to absorb incoming infrared (IR) radiation, promptly transforming it into heat. The resonant frequency of these devices exhibits a linear shift with temperature variations, and a high-temperature-tolerant read-out electronic adeptly senses this frequency shift, enabling the seamless transfer of imaging data. Extraction of IR source power occurs through the resonance frequency shift in the exposed GaN resonator, compared to a reference resonator. The reference resonator, lacking the IR-absorbing layer, establishes a baseline for minimal resonant shifts.The sparse array configuration strategically reduces imaging elements to optimize power consumption and cost-effectiveness. Inspired by techniques employed in medical imaging instruments, this approach allows the construction of images from a low-power 2D array, striking a balance between imaging capabilities and processing efficiency. This sparse array configuration serves a dual purpose, streamlining signal routing for integrated read-out electronics and simplifying the overall integration and packaging of the IR imaging instrument. Monolithic integration of the read-out electronic with the sensor platform on the same chip overcomes challenges related to electrical and mechanical stability at high temperatures.The resonant effect delivers a remarkable enhancement in the signal-to-noise ratio, surpassing existing thermal detector technologies by a factor of 70×. Furthermore, the strategic implementation of sparse arraying results in approximately a 4× reduction in power consumption, presenting an efficient and resilient solution for imaging in extreme environments. Diverging from traditional IR detectors limited in performance at high temperatures, these detectors rely on the temperature coefficient of elasticity (TCE), offering a wide dynamic range of operational temperatures.Finally, this paper includes recent results measured at 500°C, demonstrating the instrument's capability to operate effectively in Venus's challenging conditions. These advancements pave the way for future planetary exploration missions, promising a deeper understanding of Venus's geological and atmospheric processes.

Measurement and advanced data post-processing of proton resonance frequency shift in 7 T MRI to obtain local temperature in a tissue-mimicking phantom

Abstract The radio-frequency (RF) transmit power deposition in tissue during magnetic resonance imaging (MRI) at ultra-high fields, i.e. B 0 ≥ 7 T, is a major challenge for pulse sequences requesting large flip angles. The absorbed RF energy can pose safety risk to patients as it is rising temperature in the tissue. The temperature can be accessed using MRI itself via the proton-resonance frequency (PRF) shift technique, which at low B 0 has been shown a valid MR thermometry method. In this paper, we explore the applicability of the PRF method to the assessment of local temperature in 7 T MRI procedures. To this end, we built a phantom filled with a material presenting electrical conductivity and permittivity close to muscle tissue. Tubes filled with oil were placed nearby the phantom to observe the time dependent B 0 drift. MRI phase images were acquired by gradient-echo (GRE) sequences at time points between spin-echo sequences with large flip angle allowing for a continuous assessment of the temperature during a 114 min RF-heating experiment. All acquired phase images were post-processed with attention to the time dependent instability of B 0, and, in addition, to potential spatial and temporal phase discontinuities, known as wraps. In this paper, we present a strategy to analyze and to unfold these phase wraps for large measurement fields and long acquisition times. It is shown that the PRF shift method is beneficial for the assessment of temperature at 7 T MRI. The temperature maps for axial and coronal planes display a temperature increase of approximately 3.5 °C during the time of the RF-heating experiment. Overall it is shown that B 0-drift correction and, importantly, the spatio-temporal unwrapping are an indispensable part of post-processing.

Stretchable terahertz metasurface based on laser induced graphene for contactless deformation sensing

In this paper, a 1D stretchable terahertz (THz) metasurface based on laser-induced graphene (LIG) for deformation sensing is developed with low cost, good sensitivity and robustness. After the LIG metasurface is fabricated by laser-direct writing on a polyimide film, it is divided into columns and bonded to a medical polyurethane (PU) tape, thus endowed with high elasticity due to the excellent flexibility of PU. Because LIG supports surface plasmon polaritons in the THz band, a plasmonic resonance is introduced in the reflection spectrum and red shifts with lateral stretching. Experimental results show that the device can realize a resonant frequency shift of 336 GHz at a deformation of 0-14 mm, and the linearity reaches 0.988, achieving a sensitivity of 24 GHz/mm and excellent robustness (&gt;1000 repetitions). The LIG-based stretchable THz sensor has great application potential for contactless deformation sensing in aerospace, industrial manufacturing, and related fields.

Effect of Zr, Sm and Gd Doped CoFe2O4 on Structural, Spectral and Magnetic Properties

In this work, cobalt composite nano ferrites with chemical formula CoZr0.05-xRExFe1.95O4 (x = 0 and 0.025 and RE = Sm, Gd) were prepared using sol-gel synthesis method and studied structural, optical and magnetic properties of them. The Rietveld assessment of XRD data validated the emergence of a single-phase cubic spinel configuration for all compounds. SEM has been used to study the surface morphology of the compounds. Energy gap has been estimated and the values are 2 eV, 1.8 eV and 1.6 eV for CoZr0.05Fe2O4, CoZr0.025Sm0.025 Fe1.95O4, and CoZr0.025Gd0.025 Fe1.95O4 compounds respectively. FTIR and Raman spectra confirmed the structure with the appearance of standard modes. The RE replacement in CoFe2O4 ferrites has an intense impact on magnetic properties. With the substitution of Sm3+ and Gd3+ ions in cobalt-zirconium ferrite, there is a reduction in saturation magnetization (55 emu g−1 of CoZr0.05Fe2O4 reduced to 25 emu g−1 for Gd3+ and to 29 emu gm−1 for Sm3+) as the size of crystallites decreases. A decrease in crystallite size correlates with an increase in the number of spin disorder occurrences in rare Earth-substituted cobalt-zirconium ferrite nanoparticles Further, the decrease in coercivity (2440 Oe of CoZr0.05Fe2O4 reduced to 720 Oe for Gd3+ and to 29 Oe for Sm3+ with rare Earth element doping is due to the crystallite size and large lattice distortion showed an enhancement of magnetocrystalline anisotropy field which effects a shift of resonance frequencies from higher frequencies which is good for the memory storage devices.

Exploiting Nonlinearities of Electrostatic MEMS Resonators for Tunable Low Pressure Sensing

In this paper, we demonstrate the potential functionalization of inherent nonlinearities associated with electrostatic MEMS resonators for low air pressure applications, with operating ranges as low as 0.65-295 mbar. The proposed pressure sensor is made of a polysilicon microplate subject to electrostatic actuation via an underneath electrode and attached to two cantilever microbeams. The pressure sensing mechanism relies on the motion-induced current method, a transduction mechanism that converts the dynamic alteration of the microplate due to pressure variations into an electrical signal detected via a lock-in amplifier. The experimental study showed evidence of the possible tunability of the pressure sensor via the deployment of different detection mechanisms to achieve superior performance in terms of sensitivity and low pressure operating range. These mechanisms rely on nonlinear dynamic features that result from the strong coupling between the microplate and the surrounding fluid along with the electrostatic actuation. These include dynamic pull-in instability, bifurcation and softening behavior associated with the elastic effect of the squeeze-film damping. The experimental results revealed that tracking the bifurcation frequency allows for the detection of low pressure levels, in the range of a few millibars. This capability is not provided by the resonance frequency shift method, which, while offering higher sensitivity, does not achieve the same low-pressure detection.

Label-free detection of inclusion body formation in E. coli with application of terahertz

Extensive production of recombinant protein is essential for use in various fields. Bacterial cells, especially the Escherichia coli (E. coli) expression system, are widely used for the production of recombinant proteins due to their advantages in time and cost. However, the formation of inclusion bodies (IBs) caused by the production of recombinant protein is a significant issue. Therefore, a precise tool for the detection of IBs is in high demand. In this study, we demonstrated a label-free and rapid sensing platform for the detection of IBs formation in E. coli using terahertz-time domain spectroscopy (THz-TDS) system and terahertz metamaterials. The detection was based on the local polarity difference between E. coli samples with induced (positive) and non-induced (negative) protein overexpression. The formation of IBs in positive samples resulted in a decrease in local polarity compared to negative samples. This local polarity difference, strongly related to intermolecular interactions, influenced THz absorption as the THz regime involves weak intermolecular vibrational modes. Experimental results showed that the THz transmittance (ΔT) and resonance frequency shift (Δf) were significantly different between positive and negative samples, confirming the effectiveness of our method. Specifically, positive samples exhibited a ΔT value of 26.57% and a Δf value of 0.12 THz, while negative samples showed a ΔT value of 31.83% and a Δf value of 0.14 THz. These results highlight the capability of our platform to detect IBs rapidly and accurately.

Theoretical analysis of InAs based Bi-tunable narrow band terahertz perfect absorber for thermal sensing application

In this paper, a bi-tunable metamaterial absorber comprising a subwavelength resonator of semiconducting material InAs and a metallic plane adhered to a dielectric layer has been proposed in the terahertz regime. Absorption of about 99.8 % is achieved at 4.446 THz with the application of magnetic field B = 0.4 T and a high tunability rate of 0.4 THz/T in the central resonance frequency due to the presence of a magnetostatically tunable H-shaped InAs resonator and polyimide dielectric layer. The same structure supports dual control over the resonance by replacing polyimide dielectric layer with InSb, as InSb possesses temperature- and magnetic field-dependent dielectric properties. The replacement of polyimide dielectric layer with InSb provides near unity absorption of 99.99 % at B = 0.4 T but when the effect of temperature on the absorption is taken, it provides a high absorptivity of 99.99 % at T = 285 K with a blue shift in the maximum resonance frequency, providing tunability of 0.016 THz/K on increasing the temperature from 280 K to 295 K. Thus, the proposed absorber not only provides dual control over the resonance spectrum but also progresses towards more practical applications in the sensing and detection of temperature variance.

Tannin-on-protein adlayers: Secondary adsorption quantified by quartz crystal microbalance with dissipation monitoring (QCM-D)

The quartz crystal microbalance with dissipation monitoring (QCM-D) technique allows to detect adsorption and mechanical characteristics of protein adsorption layers at solid surfaces, including subsequent secondary adsorption of additional species interacting with the proteins. Here, we study the secondary adsorption of two different classes of tannins - hydrolysable vs. condensed - onto a β-lactoglobulin base layer using QCM-D in a protein/tannin adlayer configuration. Sequential flow experiments of protein solutions, tannin solutions and neat solvent or buffer are used to distinguish reversible from desorption-resistant layer formation, evaluate the viscoelastic characteristics the layers, and map protein/tannin interactions in view of their structural properties and origin. We find that for the majority of tannins, secondary layer structural changes (detected via a change in the dissipation factor) occur before significant mass adsorption (detected via the resonance frequency shift). In contrast, simultaneous onset of structural change and adsorbed mass within an observation window of 100 ms was only found for certain hydrolysable tannins. We use these data to build adsorption maps and group the tannins according to their adlayer properties, providing a physical basis for an improved understanding of the organoleptic perception of tannins and polyphenols, in particular with respect to attributes such as ‘astringency’ and ‘lingering’. Moreover, these maps provide insight into the capacity of different tannins to modify the mechanical properties of protein adsorption layers.

Diagnosis and assessment of cyclic freeze–thaw damage in tunnel lining concrete using piezoelectric-based electromechanical impedance technique

Cold regional tunnel linings extensively suffer from severe cyclic freeze–thaw damage, which adversely influences their stability, durability and in-service safety. Consequently, the diagnosis and assessment of cyclic freeze–thaw damage in concrete linings have garnered worldwide attention. This study presents a pioneering effort in using the PZT-based electromechanical impedance (EMI) technique to monitor the evolving damage within lining concrete concurrently exposed to bending loads and freeze–thaw cycles. First of all, ultrasonic measurements and four-point bending tests were performed on cyclically freeze-thawed concrete specimens under two different bending loads. Both the degrading flexural strength and the diminishing ultrasonic wave velocity were measured. Particularly, the damage evolution was characterized by the flexural strength degradation ratio, which exponentially increased by 44.81% and 64.28% under two bending scenarios, respectively. Moreover, conductance signatures driven by two vibration modes (d31 and d33 modes) of embedded piezoelectric transducers were recorded, and both localized and overall variations in the signals were analyzed. It is concluded that both the resonant frequency shift and the statistic metric RMSD within the d31-dominated frequency range could accurately indicate the cyclic freeze–thaw damage of lining concrete. Their linear correlation coefficients with the damage variable reached 0.957 and 0.935, respectively. This research establishes a quantitative framework for monitoring the progressive damage of tunnel lining subjected to freeze–thaw cycling using the EMI technique and lays the groundwork for a broader range of future applications in structure damage diagnosis and assessment.

Accurate thickness evaluation of thermal barrier coatings using microwave resonator sensor

Thermal barrier coatings (TBC) are frequently employed in aircraft engines and turbine blades for protection against high temperatures. Constant exposure to in-service stresses, however, leads to a variety of anomalies in TBC that could prove to be potentially catastrophic for the assemblies they protect. This raises the need for inspection using non-destructive techniques (NDT) to ensure the reliability and structural integrity of the coated assemblies. Thickness evaluation of the topcoat of the TBC is paramount to determining structural integrity of the TBC. Microwaves are particularly suited for such applications since they readily penetrate low loss TBC. However, the performance of previous microwave-based TBC thickness evaluation techniques was limited in accuracy to 15 μm. In this paper, accurate thickness evaluation of thermal barrier coating (TBC) is performed using a microwave resonator sensor operating at a relatively low frequency (<1 GHz). TBC thicknesses as low as 100 μm are evaluated based on shift in resonant frequency of the probe. Overall, practically relevant thicknesses ranging from 100 μm to 600 μm are evaluated herein and multiple trials are performed to ascertain repeatability and assess uncertainty in measurements. For a 250 μm thick TBC sheet, a mean error of 0.9 μm was accomplished which translates to 0.36 % error with a standard deviation of 1.5 μm. Compared to other microwave sensors reported in the literature, a better estimation accuracy is demonstrated in this work, in particular for thicknesses less than <250 μm.

A mechanics and electromagnetic scaling law for highly stretchable radio frequency electronics

Many classes of flexible and stretchable bio-integrated electronic systems rely on mechanically sensitive electromagnetic components, such as various forms of antennas for wireless communication and for harvesting energy through coupling with external power sources. This efficient wireless functionality can be important for body area network technologies and can enable operation without the weight and bulky size of batteries for power supply. Recently, antenna designs have received increased attention because their mechanical and electromagnetic properties significantly influence the wireless performance of bio-integrated electronics, particularly under excessive mechanical loads. These mechanical factors are critical for skin-integrated electronics during human motion, as complex skin deformations can damage the conductive traces of antennas, such as those used for near-field communication (NFC), leading to yield or fracture and affecting their electromagnetic stability. Serpentine interconnects have been proposed as a geometric alternative to in-plane circular or rectangular spiral antenna designs to improve the elastic stretchability of the metallic traces in NFC antennas and prevent mechanical fractures. Despite the use of serpentine interconnects within the physiologically relevant strain range for skin (<20 %), the electromagnetic stability of the antennas decreases. This instability, reflected by shifts in resonance frequency and scattering parameters due to inductance changes, reduces the antennas' wireless power transfer efficiency and readout range. Therefore, maintaining the electromagnetic stability of antennas, specifically NFC antennas, under various mechanical deformations has become a critical challenge in practical wireless skin-integrated applications, such as sensing and physiological monitoring. Here, we establish a new mechanics and electromagnetic scaling law that quantifies the inductance changes under strain in a rectangular-loop serpentine structure typically used for NFC wireless communication in stretchable electronics. We present a systematic analysis of the antenna's geometric parameters, material properties of the antenna and substrate, and the applied strain on the inductance change. Our findings demonstrate that the relative change of inductance is solely influenced by the serpentine structure's width-radius ratio, arc angle, aspect ratio of the NFC antennas, and the applied strain. Additionally, under physiological strain conditions for the skin, the relative change of inductance can be minimized to preserve the NFC antenna's performance and prevent mechanical fracture and electromagnetic stability loss.

Design, Fabrication, and Dynamic Analysis of a MEMS Ring Resonator Supported by Twin Circular Curve Beams.

In this paper, we present a compressive study on the design and development of a MEMS ring resonator and its dynamic behavior under electrostatic force when supported by twin circular curve beams. Finite element analysis (FEA)-based modeling techniques are used to simulate and refine the resonator geometry and transduction. In proper FEA or analytical modeling, the explicit description and accurate values of the effective mass and stiffness of the resonator structure are needed. Therefore, here we outlined an analytical model approach to calculate those values using the first principles of kinetic and potential energy analyses. The natural frequencies of the structure were then calculated using those parameters and compared with those that were simulated using the FEA tool ANSYS. Dynamic analysis was performed to calculate the pull-in voltage, shift of resonance frequency, and harmonic analyses of the ring to understand how the ring resonator is affected by the applied voltage. Additional analysis was performed for different orientations of silicon and assessing the frequency response and frequency shifts. The prototype was fabricated using the standard silicon-on-insulator (SOI)-based MEMS fabrication process and the experimental results for resonances showed good agreement with the developed model approach. The model approach presented in this paper can be used to provide valuable insights for the optimization of MEMS resonators for various operating conditions.