Absorption Relaxation Research Articles

Extracting relaxational features from measurements of sound speeds for gas sensing

In recent years, gas sensing methods by jointly measuring acoustic relaxation absorption and sound speeds have become a research focus in acoustic fields. However, the existing methods are difficult to be employed in practical gas detection due to complex measurement processes and large measurement errors when measuring the sound relaxational absorption. Furthermore, the acoustic relaxation absorption spectrum's small signal is hard to be measured because of the noise and large classical relaxation in the high-frequency range. In this paper, a gas sensing method is proposed by only measuring the sound speeds at the three frequencies, avoiding the problems of measuring the acoustic absorption spectrum. First, sound speeds are measured at three frequency points; second, the relaxation strength and the relaxation time of the acoustic relaxation features are extracted; finally, the absorption spectral peak is acquired and located for gas detection. The simulation results verify the feasibility of our method, and the detection errors from the measurement errors of sound speeds can be eliminated effectively. Thus, our work provides a high-accurate and simple acoustic method for gas sensing.

Photocarrier relaxation and saturable absorption in defect-rich, direct chemical vapor deposition-grown graphene glass

Direct growth of large-area uniform graphene films on insulating materials could facilitate the applications of graphene in optoelectronic devices. The ultrafast photocarrier relaxation and saturable absorption of direct chemical vapor deposition-grown graphene glasses, with lots of defects, are studied by using femtosecond time-resolved pump–probe and Z-scan techniques at 800 nm. We find that both the relaxation times associated with hot carrier cooling and the hot phonon effect are greatly suppressed in these defect-rich graphene glasses, which further leads to the increase of both saturation intensity and anisotropy in transient optical response for graphene glasses as compared with defect-free graphene. And, both the suppression effect and saturation intensity increase with the thickness of graphene film. The dominance of defect-assisted carrier-acoustic phonon scattering (i.e., supercollision, the collision of a carrier with both an acoustic phonon and defects) in the cooling process of hot carriers is responsible for the suppression of relaxation time.

Measurement of the Acoustic Relaxation Absorption Spectrum of CO2 Using a Distributed Bragg Reflector Fiber Laser

Reconstruction of the acoustic relaxation absorption curve is a powerful approach to ultrasonic gas sensing, but it requires knowledge of a series of ultrasonic absorptions at various frequencies around the effective relaxation frequency. An ultrasonic transducer is the most widely deployed sensor for ultrasonic wave propagation measurement and works only at a fixed frequency or in a specific environment like water, so a large number of ultrasonic transducers operating at various frequencies are required to recover an acoustic absorption curve with a relative large bandwidth, which cannot suit large-scale practical applications. This paper proposes a wideband ultrasonic sensor using a distributed Bragg reflector (DBR) fiber laser for gas concentration detection through acoustic relaxation absorption curve reconstruction. With a relative wide and flat frequency response, the DBR fiber laser sensor measures and restores a full acoustic relaxation absorption spectrum of CO2 using a decompression gas chamber between 0.1 and 1 atm to accommodate the main molecular relaxation processes, and interrogates with a non-equilibrium Mach-Zehnder interferometer (NE-MZI) to gain a sound pressure sensitivity of −45.4 dB. The measurement error of the acoustic relaxation absorption spectrum is less than 1.32%.

Nature of Linear Spectral Properties and Fast Relaxations in the Excited States and Two-Photon Absorption Efficiency of 3-Thiazolyl and 3-Phenyltiazolyl Coumarin Derivatives.

Coumarin-based fluorescent agents play an important role in the manifold fundamental scientific and technological areas and need to be carefully studied. In this research, linear photophysics, photochemistry, fast vibronic relaxations, and two-photon absorption (2PA) of the coumarin derivatives, methyl 4-[2-(7-methoxy-2-oxo-chromen-3-yl)thiazol-4-yl]butanoate (1) and methyl 4-[4-[2-(7-methoxy-2-oxo-chromen-3-yl)thiazol-4-yl]phenoxy]butanoate (2), were comprehensively analyzed using stationary and time-resolved spectroscopic techniques, along with quantum-chemical calculations. The steady-state one-photon absorption, fluorescence emission, and excitation anisotropy spectra, as well as 3D fluorescence maps of 3-hetarylcoumarins 1 and 2 were obtained at room temperature in solvents of different polarities. The nature of relatively large Stokes shifts (∼4000-6000 cm-1), specific solvatochromic behavior, weak electronic π → π* transitions, and adherence to Kasha's rule were revealed. The photochemical stability of 1 and 2 was explored quantitatively, and values of photodecomposition quantum yields, on the order of ∼10-4, were determined. A femtosecond transient absorption pump-probe technique was used for the investigation of fast vibronic relaxation and excited-state absorption processes in 1 and 2, while the possibility of efficient optical gain was shown for 1 in acetonitrile. The degenerate 2PA spectra of 1 and 2 were measured by an open aperture z-scan method, and the maximum 2PA cross-sections of ∼300 GM were obtained. The electronic nature of the hetaryl coumarins was analyzed by quantum-chemical calculations using DFT/TD-DFT level of theory and was found to be in good agreement with experimental data.

Difluoroborate-based bichromophores: Symmetry relaxation and two-photon absorption

Given potential applications of multiphoton absorbers, in the present work we have studied the symmetry-relaxation effects in one- and two-photon absorption spectra in two bichromophore systems based on difluoroborate core linked by biphenylene or bianthracene moieties. We have employed a palette of experimental methods (synthesis, one- and two-photon spectroscopy, X-ray crystallography) and state-of-the-art computational methods to shed light on how symmetry relaxation, a result of twisting of building blocks, affects one- and two-photon absorption of the two studied fluorescent dyes. Electronic-structure calculations revealed that the planarity of central biphenyl moiety, as well as deviations from planarity up to 30–40 deg., ensure maximum values of two-photon transition strengths. Perpendicular arrangement of phenylene units in biphenylene moiety leads to 20% drop in the two-photon transition strengths. More detailed studies demonstrated that equilibrium structures of both compounds in chloroform solution show very different values of two-photon absorption cross sections at absorption band maxima, i.e. 224 GM for and 134 GM for biphenyle and bianthracene linkers, respectively. The latter value is in good agreement with experimental value obtained using Z-scan method. The difference in two-photon absorption cross section between both compounds can be rationalized based on equilibrium geometry differences, i.e. interplanar angle is 35 deg and 91 deg in the case of biphenylene and bianthracene moiety, respectively. It is thus not beneficial to introduce conformationally locked central linker based on bianthracene moiety.

A 3D multifunctional nitrogen-doped RGO-based aerogel with silver nanowires assisted self-supporting networks for enhanced electromagnetic wave absorption

We designed a promising three-dimensional (3D) nitrogen-doped reduced graphene oxide-based aerogel (NGA) with silver nanowires (Ag NWs) assisted self-supporting networks for enhanced electromagnetic wave (EMW) absorption. 3D AgNWs@NGAs (8–12 mg/mL) were synthesized by a three-step strategy of sonication, reduction, and lyophilization. The results show that introduced sp3 defects and disordered sites in nitrogen doping from a rarely-reported liquid nitrogen source of acetonitrile remarkably improved electronic dipole polarization relaxation and EMW absorption ability. Besides, electronic band structure and density of states of the NGA were investigated by first-principles calculations to indicate that the carrier mobility became smaller during nitrogen doping. The Ag NWs were distributed on an NGA as the 3D network structure of an elastic support and a conductive and compatible unit, which limited the aggregation of NGA flakes, and further strengthened conductive loss. Significantly, the synthesized ultralight AgNWs@NGA exhibits excellent performance at 9.52 GHz, with a strong minimal reflection loss of –79.99 dB, a small thickness of 2.660 mm, a wide effective absorption bandwidth of 3.5 GHz, and an ultra-low filler content of 5 wt%. Furthermore, a large specific surface of 331.5 cm3/g, a good conductivity of approximately 50 S/cm, a high resilience, and an excellent heat insulation capability, guarantee the durable stability in EMW absorption of AgNWs@NGAs to resist distortion and heat attacks.

Strain Relaxation and Excitonic Absorption of Atomically-Reconstructed WSe2 Moiré Superlattices

Strain Relaxation and Excitonic Absorption of Atomically-Reconstructed WSe2 Moiré Superlattices

The Role of the Reducible Dopant in Solid Electrolyte–Lithium Metal Interfaces

Garnet solid electrolytes, of the form Li7La3Zr2O12 (LLZO), remain an enticing prospect for solid-state batteries owing to their chemical and electrochemical stability in contact with metallic lithium. Dopants, often employed to stabilize the fast ion conducting cubic garnet phase, typically have no effect on the chemical stability of LLZO in contact with Li metal but have been found recently to impact the properties of the Li/garnet interface. For dopants more “reducible” than Zr (e.g., Nb and Ti), contradictory reports of either raised or reduced Li/garnet interfacial resistances have been attributed to the dopant. Here, we investigate the Li/LLZO interface in W-doped Li7La3Zr2O12 (LLZWO) to determine the influence of a “reducible” dopant on the electrochemical properties of the Li/garnet interface. Single-phase LLZWO is synthesized by a new sol–gel approach and densified by spark plasma sintering. Interrogating the resulting Li/LLZWO interface/interphase by impedance, muon spin relaxation and X-ray absorption spectroscopies uncover the significant impact of surface lithiation on electrochemical performance. Upon initial contact, an interfacial reaction occurs between LLZWO and Li metal, leading to the reduction of surface W6+ centers and an initial reduction of the Li/garnet interfacial resistance. Propagation of this surface reaction, driven by the high mobility of Li+ ions through the grain surfaces, thickens the resistive interphases throughout the material and impedes Li+ ion transport between the grains. The resulting high resistance accumulating in the system impedes cycling at high current densities. These insights shed light on the nature of lithiated interfaces in garnet solid electrolytes containing a reducible dopant where high Li+ ion mobility and the reducible nature of the dopant can significantly affect electrochemical performance.

Metastable states, relaxation and failure mechanisms in condensed matter with defects

The article is concerned with experimental studies of anomalous momentum transfer mechanisms in condensed matter exhibiting multiple defect-induced gaped momentum states. The performed experiments revealed the relationship between the metastable states, the specific critical phenomena in condensed matter with defects, the structural-scaling transitions, and the mechanisms of structural relaxation disclosing viscosity anomalies in solids and liquids due to the strong interactions between the structure constituents. The possibility of these mechanisms under intensive loading including turbulence was substantiated in Naimark (1998), Naimark (1997), Naimark (2004). The established regularities of criticality in the behavior of condensed matter with mesoscopic defects made it possible to study the thermodynamics and kinetics of the nonlinear liquid and solid systems exhibiting the metastable states, and to provide an explanation of the anomalous mechanisms of momentum transfer in systems with "strong interaction." Anomalous momentum transfer mechanisms are associated with the subjection of the dynamics of condensed matter to the established types of collective modes in ensembles of defects (breathers, autosolitons, dissipative blow-up structures) manifesting the character of self-similar solutions. The subordination of dynamics to collective modes is accompanied by a sharp increase in the effective relaxation times and absorption anomalies. The theoretical results are confirmed by the experimental data on the formation of self-similar (quasi-plastic) fronts in liquids and the attainment of viscosity asymptotics due to initiation of fracture waves under shock-wave loading of fused quartz (Naimark (2003), Naimark (2010)).

Surface relaxation and photoelectric absorption effects on synchrotron X-ray topographic images of dislocations lying on the basal plane in off-axis 4H-SiC crystals

A more sophisticated simulation model is developed based on the principle of ray-tracing to simulate the grazing-incidence synchrotron X-ray topographic contrast of dislocations lying on the basal plane including basal plane dislocations and deflected threading screw and mixed dislocations in off-axis 4H-SiC crystals. The model incorporates effects of surface relaxation as well as the photoelectric absorption to predict dislocation contrast. Compared to conventional ray-tracing images, surface relaxation effects dominate dislocation contrast for diffraction near the crystal surface. The simulated dislocation contrast gradually weakens with increasing depth of the diffracted beam position within the crystal due to photoelectric absorption. The distinctive features of the net simulated dislocation images obtained by aggregating through the effective penetration depth correlate well with contrast features observed on the experimental topographic images. Depth analysis reveals that in some cases the diffracted X-rays from regions below the dislocation can contribute additional contrast features previously not considered.

Acoustic analysis of gas compositions based on molecular relaxation features

The quantitative detection of gas compositions in mixture has been a challenge in the field of acoustic sensing. In this paper, we propose an acoustic gas sensing method using the molecular relaxation features to identify gas composition and determine composition concentration in excitable gases. The proposed method is based on the fact that the relaxation strength and relaxation time constitute relaxation absorption spectrum which characterizes the acoustic molecular relaxation process, and they can be synthesized only using two-frequency acoustic measurements. Then the applications of the method in the simple gas mixtures CO2-air and CH4-air, and the multiple component mixtures CO2-CH4-air demonstrate the feasibility, and the discussion of the sensing performance verifies the robustness. This work provides a more exact, feasible and practical method for the gas sensing applications.

MOLECULAR MECHANISM OF RELAXATION ABSORPTION OF ULTRASONIC WAVES IN AN AQUEOUS SOLUTION OF CALCIUM ACETATE

The absorption of ultrasonic waves in the frequency range from 6 to 146 MHz in aqueous solutions of sodium acetate, sodium chloride and calcium has been studied. It was found that only in solutions of calcium cations and acetate anion present simultaneously, relaxation absorption of ultrasonic waves is observed. The experimental data obtained indicate that the observed relaxation absorption of ultrasound in the studied frequency range by an aqueous solution of calcium acetate is associated with the interaction of the acetate anion with the calcium cation in the solution.

Concentration measurement of carbon dioxide based on ultrasonic principle

Acoustic detection technology has the advantages of low cost, short detection period, online detection and low power consumption. However, the complex circuits and procedures are needed to measure the acoustic absorption while using the conventional method. In order to solve the problems, a dual-frequency measurement based on echo voltage is proposed. The mechanical structure and the measurement circuits are designed. The acoustic absorption coefficients at two different frequencies are calculated and the acoustic relaxation absorption spectrum is obtained. The relaxation frequency and the acoustic absorption are measured at the concentration of 100.0%, 70.0%, 50.8% and 35.5% of carbon dioxide-nitrogen mixed gases. The relationship between the relaxation absorption and the gas concentration is obtained and is utilized to build a mathematical model. Then the unknown concentration of carbon dioxide is able to be obtained. Finally the experiments are done to verify the feasibility of the measurement.

Physical and thermo-mechanical properties of shape memory polyurethane containing reversible chemical cross-links

Orthodontic chains are one of the main parts of orthodontic braces. In this study, in order to obtain a suitable polymer for this application as well as troubleshoot the main drawbacks such as stress relaxation and water absorption, thermoplastic polyurethane (TPU) elastomers with various compositions were synthesized and characterized by Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) analyses. Mechanical properties of samples were evaluated by tensile, elasticity, and stress relaxation tests. According to the required properties for orthodontic chain application, PU2000-311 which contains reversible crosslinks, was selected as the most favorable sample among other pure samples. Moreover, to reduce the water absorption content of PU2000-311, its nanocomposite containing 1 wt% of silica nanoparticles was prepared via solution casting method. As water content angle and field emission scanning electron microscopy (FESEM) images illustrate, incorporation of 1 wt% of modified silica nanoparticles has increased PU2000-311-1S hydrophobicity. In vitro oral environment study showed crystability of samples has recovered great portion of relaxed force. Stress relaxation study indicated samples are applicable in oral temperature range and temperature changes have assisted recovery of relaxed force and reduced treatment period. Finally, shape memory study showed that optimum samples could recover 100% of their original shape.

Mechanical Properties of Double Network Poly (Acrylic Acid) Based Hydrogels for Potential Use as a Biomaterial.

Load-bearing applications of hydrogels include soft robots, tissue engineering, and stretchable electronics. This paper presents an extensive study of double network poly (acrylic acid) based hydrogel on stress relaxation, compression fatigue, shear stress, and shock absorption properties as a potential load-bearing soft tissue replacement biomaterial. Double network poly (acrylic acid) hydrogel was selected due to simple processing and availability. The optimized formulation of poly (acrylic acid) hydrogel was used for samples preparation. The compression modulus varied with hydrogel formulation, crosshead speed and swelled amount of the hydrogel. Stress relaxation and shock absorption properties of hydrogel were compared with polyurethane gel used in soft insoles (Shore 5A). Developed hydrogel displayed good fatigue properties up to 10,000 loading cycle at maximum stress of 390±30 kPa and at 84±4% strain. Further, maximum average shear stress and shear modulus of 80 kPa and 140 kPa respectively were observed at 84% strain before fracture.

Elongated magnetic nanoparticles with high-aspect ratio: a nuclear relaxation and specific absorption rate investigation.

Medical application of nanotechnology implies the development of nanomaterials capable of being functional in different biological environments. In this sense, elongated nanoparticles (e-MNPs) with high-aspect ratio have demonstrated more effective particle cellular internalization, which is favoured by the increased surface area. This paper makes use of an environmentally friendly hydrothermal method to produce magnetic iron oxide e-MNPs, starting from goethite precursors. At high temperatures (Td) goethite transforms into hematite, which subsequently reduces to magnetite when exposed to a hydrogen atmosphere for a certain time. It is shown that by adjusting Td it is possible to obtain Fe3O4 e-MNPs with partially controlled specific surface area and magnetic properties, attributed to different porosity of the samples. The particles' efficiencies for diagnostic and therapeutic purposes (in magnetic resonance imaging and magnetic fluid hyperthermia, respectively) are very good in terms of clinical standards, some samples showing transversal proton nuclear relaxivity r2 (B0 = 1.33 T) = 340 s-1 mM-1 and specific absorption rate SAR > 370 W g-1 at high field amplitudes (B0 = 55 mT). Direct correlations between the SAR, relaxivity, magnetic properties and porosity of the samples are found, and the physico-chemical processes underneath these correlations are investigated. Our results open the possibility of using very efficient high-aspect ratio elongated nanoparticles with optimized chemico-physical properties for biomedical applications.

Dielectric relaxation and microwave absorption properties of aurivillius-type multiferroic ceramics

Aurivillius-type multiferroic oxides are increasingly being studied for potential application in multifunctional areas because of their feasible platforms incubating photonic, electric, magnetic, etc. properties. Here, we report the original observations of the dielectric relaxation, electromagnetic properties and high-performance microwave absorption of the single-phase Aurivillius-type ceramics. Two thermally activated relaxation processes were determined by the frequency-dependent behaviour of the dielectric loss peak at low and high temperatures. The complex permittivity and permeability were measured at frequencies 2–18 GHz and the microwave attenuation performances were calculated from the experimentally determined electromagnetic parameters. Significantly, the damped dielectric and magnetic resonances were clearly observed at 12–14 GHz and the excellent microwave absorption properties were presented for the first time and are sensitive to the sample thickness. The minimum reflection loss (RL) surpasses − 30 dB at different thicknesses. Especially, the minimum RL reaches − 70.1 dB at 13.2 GHz and the absorption bandwidth (RL≤ −20 dB) is close to 12 GHz (5.7–17.7 GHz). This work opens up a new pathway to develop Aurivillius-type multiferroic oxides in electromagnetic wave attenuation materials.

Acoustic rotational relaxation of hydrogen around normal temperture

Hydrogen is an important energy carrier, and it is widely used due to its extraordinary advantages, such as high heat, clean fuel, being large-scale and renewable. The detection of hydrogen is essential in practical application. Therefore, many researches have focused on monitoring the hydrogen concentration over the past years. Acoustic relaxation theory based on molecular relaxation process is a very promising method of detecting hydrogen gas. However, the existing acoustic relaxation models for gas detection are developed from the vibrational relaxation of gas molecules, and thus they are not applicable for hydrogen and its mixture. In this paper, we present a model for the rotational relaxation process of hydrogen. Firstly, the molecular relaxation process of hydrogen is different from those of other gases due to its large spacing of rotational energy-level and special molecular physical structure. Acoustic relaxation process of hydrogen is mostly determined by the molecular rotational relaxation. Hydrogen molecule is made up of one quarter of para-hydrogen and three quarters of ortho-hydrogen at normal temperature. There is three-rotational-level model for hydrogen rotational relaxation, such as rotational level in states with J=0, 2, 4 (J is rotational quantum-number) for para-hydrogen and J=1, 3, 5 for ortho-hydrogen. Secondly, we introduce effective specific heat into one-mode rotational relaxation at constant pressure, and then extend it to multi-mode rotational relaxation. Upon periodic perturbation of acoustic waves, the temperature and the number of molecules in each rotational level change periodically in the relaxation process. On the basis, we obtain the relaxation equations in a matrix form and calculate effective specific heat at constant pressure for rotational relaxation process. With the relationship between the complex wave number and the effective thermodynamics acoustic speed, we calculate the frequency-dependent acoustic speed and relaxation absorption, and then discuss the difference between the rotational relaxation and the vibrational relaxation. Thirdly, we compare the predicted acoustic speed and absorption spectrum with their corresponding experimental data and investigate the influences of rotational characteristics on absorption spectra in hydrogen and its mixtures. The simulation results show that acoustic speed and relaxation absorption curves calculated by the proposed model are in good agreement with their corresponding experimental data. The model is not only applicable to pure hydrogen gas but also can be used to obtain the acoustic relaxation spectra of gas mixtures with multiple vibrational modes. This model provides a theoretical foundation for the acoustic detecting of hydrogen gas mixtures.

A simple measurement method of molecular relaxation in a gas by reconstructing acoustic velocity dispersion

Recently, a decomposition method of acoustic relaxation absorption spectra was used to capture the entire molecular multimode relaxation process of gas. In this method, the acoustic attenuation and phase velocity were measured jointly based on the relaxation absorption spectra. However, fast and accurate measurements of the acoustic attenuation remain challenging. In this paper, we present a method of capturing the molecular relaxation process by only measuring acoustic velocity, without the necessity of obtaining acoustic absorption. The method is based on the fact that the frequency-dependent velocity dispersion of a multi-relaxation process in a gas is the serial connection of the dispersions of interior single-relaxation processes. Thus, one can capture the relaxation times and relaxation strengths of N decomposed single-relaxation dispersions to reconstruct the entire multi-relaxation dispersion using the measurements of acoustic velocity at 2N + 1 frequencies. The reconstructed dispersion spectra are in good agreement with experimental data for various gases and mixtures. The simulations also demonstrate the robustness of our reconstructive method.

Predicting acoustic relaxation absorption in gas mixtures for extraction of composition relaxation contributions.

The existing molecular relaxation models based on both parallel relaxation theory and series relaxation theory cannot extract the contributions of gas compositions to acoustic relaxation absorption in mixtures. In this paper, we propose an analytical model to predict acoustic relaxation absorption and clarify composition relaxation contributions based on the rate-determining energy transfer processes in molecular relaxation in excitable gases. By combining parallel and series relaxation theory, the proposed model suggests that the vibration-translation process of the lowest vibrational mode in each composition provides the primary deexcitation path of the relaxation energy, and the rate-determining vibration-vibration processes between the lowest mode and others dominate the coupling energy transfer between different modes. Thus, each gas composition contributes directly one single relaxation process to the molecular relaxation in mixture, which can be illustrated by the decomposed acoustic relaxation absorption spectrum of the single relaxation process. The proposed model is validated by simulation results in good agreement with experimental data such as N2, O2, CO2, CH4 and their mixtures.