Low-frequency Slope Research Articles

Engineering the mass transport properties in potassium-ion intercalated pristine Prussian blue for sustainable potassium-ion batteries

Potassium-ion batteries (PIBs) could be a cheaper and more abundant alternative to lithium-ion batteries, offering faster charging and suitability for large-scale energy storage. Prussian blue (PB) is prepared through a simple wet chemical method, and its intercalation affinity for K+ ions is tested using linear sweep voltammetry (LSV). The diffusion resistance (Rw: ∼116 Ω Hz1/2) and low-frequency region slope (m: ∼0.63) are found high for the third LSV drive (Seg-3), indicating passivation of the PB surface which leads to a double-layer formation. Most of the K+ ions are intercalated into PB until three LSV segments. After that PB surface is passivated and prohibits further LSV-driven K+ ions intercalation. On average, the Seg-3 sample exhibits a 58 % higher lattice strain in comparison to the as-prepared PB. A capacity of 27 mAh g−1 @ 2 mA g−1 in aqueous media is recorded for the Seg-3 sample and a capacity drop of 4 mAh g−1 and Coulombic efficiency of 99.3 % is recorded under 100 cycles. These findings are further comprehensively validated through multi-technique approaches. The observed affinity and single-stage LSV-driven intercalation of K+ ions highlight the perspective of PB as an electrode material for aqueous potassium ion batteries (a-PIBs).

The Rapid Decline in Interaural-Time-Difference Sensitivity for Pure Tones Can Be Explained by Peripheral Filtering.

The interaural time difference (ITD) is a primary horizontal-plane sound localization cue computed in the auditory brainstem. ITDs are accessible in the temporal fine structure of pure tones with a frequency of no higher than about 1400Hz. How listeners' ITD sensitivity transitions from very best sensitivity near 700Hz to impossible to detect within 1 octave currently lacks a fully compelling physiological explanation. Here, it was hypothesized that the rapid decline in ITD sensitivity is dictated not by a central neural limitation but by initial peripheral sound encoding, specifically, the low-frequency (apical) portion of the cochlear excitation pattern produced by a pure tone. ITD sensitivity was measured in 16 normal-hearing listeners as a joint function of frequency (900-1500Hz) and level (10-50dB sensation level). Performance decreased with increasing frequency and decreasing sound level. The slope of performance decline was 90dB/octave, consistent with the low-frequency slope of the cochlear excitation pattern. Fine-structure ITD sensitivity near 1400Hz may be conveyed primarily by "off-frequency" activation of neurons tuned to lower frequencies near 700Hz. Physiologically, this could be realized by having neurons sensitive to fine-structure ITD up to only about 700Hz. A more extreme model would have only a single narrow channel near 700Hz that conveys fine-structure ITDs. Such a model is a major simplification and departure from the classic formulation of the binaural display, which consists of a matrix of neurons tuned to a wide range of relevant frequencies and ITDs.

Shock fatigue damage failure boundary study of BGA solder joints based on shock response spectrum

Multiple shock damage failure of ball grid array (BGA) solder joints is one of the important reasons for reusable spacecraft mission failure. Shock fatigue damage failure boundary of BGA solder joints is proposed based on shock response spectrum (SRS), which can be used to assess the damage failure state. Weibull distribution is employed to characterize the shock fatigue life for a specified SRS. A Bayesian estimation method based on the Bootstrap approach is proposed to estimate Weibull distribution parameters. By using the time-domain load synthesized method based on wavelet and the experimentally validated shock fatigue life model, the shock fatigue life data of BGA solder joints corresponding to different SRS are extended. The variation patterns of the scale and shape parameters with the SRS parameters is analyzed. Then the shock fatigue damage failure boundary based on SRS is determined for a given design life and reliability. The results show that the critical SRS amplitude increase with the rise of the inflection point frequency and the low-frequency slope, respectively. The developed method is expected to provide references for the environment-adaptive design of BGA solder joints in reusable spacecraft.

The Impact of Internal Gravity Waves on the Spectra of Turbulent Fluctuations of Vertical Wind Velocity in the Stable Atmospheric Boundary Layer

The wave turbulence interactions in the stable boundary layer (SBL) of the atmosphere are studied based on data from lidar measurements of the vertical component of wind velocity during the propagation of internal gravity waves (IGWs). It is shown that as an IGW appears, the amplitude of the spectra of turbulent fluctuations of vertical wind velocity nearby the frequency of quasi-harmonic oscillations induced by an IGW increases significantly, sometimes by several orders of magnitude, compared to the spectra in the absence of an IGW. Since IGW energy is transferred to small-scale turbulence, the amplitude of spectra with the Kolmogorov–Obukhov −5/3 power-law frequency dependence in the inertial frequency range increases. The slope of the spectra in the low-frequency range between the frequency of IGW-induced oscillations and the frequency of the lower boundary of the inertial range exceeds the slope, corresponding to the −5/3 power-law dependence. In this frequency range, the spectra obey the power-law dependence on the frequency with the exponent ranging from −4.2 to −1.9. The average value of the exponent −3 is consistent with a low-frequency slope caused by IGWs in turbulent spectra in the lower SBL.

Automated classification of seismic signals recorded on the Åknes rock slope, Western Norway, using a convolutional neural network

Abstract. A convolutional neural network (CNN) was implemented to automatically classify 15 years of seismic signals recorded by an eight-geophone network installed around the back scarp of the Åknes rock slope in Norway. Eight event classes could be identified and are adapted from the typology proposed by Provost et al. (2018), of which five could be directly related to movements on the slope. Almost 60 000 events were classified automatically based on their spectrogram images. The performance of the classifier is estimated to be near 80 %. The statistical analysis of the results shows a strong seasonality of the microseismic activity at Åknes with an annual increase in springtime when snow melts and the temperature oscillates around the freezing point, mainly caused by events within classes of low-frequency slope quakes and tremors. The clear link between annual temperature variations and microseismic activity could be confirmed, supporting thawing and freezing processes as the origins. Other events such as high-frequency and successive slope quakes occur throughout the year and are potentially related to the steady creep of the sliding plane. The huge variability in the annual event number cannot be solely explained by average temperatures or varying detectability of the network. Groundwater recharge processes and their response to precipitation episodes are known to be a major factor of sliding at Åknes, but the relationship with microseismic activity is less obvious and could not be demonstrated.

High-Precision Ion Beam Figuring of X-Ray Plane Mirrors for the Bendable KB Focusing System

Two trapezoidal plane mirrors of 240 mm in length were fabricated by ion beam figuring (IBF) technology for application in a bendable KB focusing system. The correction of surface height and slope errors in different spatial frequency ranges of the mirrors was studied systematically. After one to two iterations of IBF, the figure height errors of the vertical focusing mirror (VFM) and horizontal focusing mirror (HFM) were improved from 32.4 and 65.4 nm to 2.7 and 7.2 nm (RMS), respectively. If the best-fit sphere of the surface profile was subtracted, the residual two-dimensional height errors were only 1.1 and 1.2 nm (RMS). The slope errors in the low spatial frequency range were corrected much faster than the middle frequency ones (f = ∼1 mm−1), which make the low-frequency slope error much smaller. After IBF, the two-dimensional slope errors of the two mirrors calculated with a spatial interval of 1 and 10 mm were reduced to approximately 0.29 and 0.08 μrad, respectively. Full spatial frequency characterization of the VFM before and after IBF showed that the low-frequency figure errors (f < 1 mm−1) were significantly reduced while the middle- and high-frequency morphologies (f > 1–2 mm−1) remain almost the same as before figuring. The fabricated plane mirrors were applied in the hard X-ray micro-focusing beamline in the Shanghai Synchrotron Radiation Facility (SSRF), which realized a focal spot of 2.4 μm × 2.8 μm at 10 keV.

On the comparison of AGN with GRMHD simulations: I. Sgr A*

ABSTRACT We present models of Galactic Centre emission in the vicinity of Sagittarius A* that use parametrizations of the electron temperature or energy density. These models include those inspired by two-temperature general relativistic magnetohydrodynamic (GRMHD) simulations as well as jet-motivated prescriptions generalizing equipartition of particle and magnetic energies. From these models, we calculate spectra and images and classify them according to their distinct observational features. Some models produce morphological and spectral features, e.g. image sizes, the sub-mm bump, and low-frequency spectral slope compatible with observations. Models with spectra consistent with observations produce the most compact images, with the most prominent, asymmetric photon rings. Limb-brightened outflows are also visible in many models. Of all the models we consider, that which represents the current data the best is one in which electrons are relativistically hot when magnetic pressure is larger than the thermal pressure, but cold (i.e. negligibly contributing to the emission) otherwise. This work is part of a series also applying the ‘observing’ simulations methodology to near-horizon regions of supermassive black holes in M87 and 3C 279.

OUP accepted manuscript

Gravitational wave (GW) radiations from numerous cosmic stellar-compact-binaries form a stochastic GW background (GWB), which is expected to be detected by ground and space GW detectors in future. Theoretical predictions of this GWB were mostly obtained by assuming either circular binaries and/or a specific channel for binary formation, which may have some uncertainties. In this paper, we estimate the GWB and its spectrum by using simple models for the formation of both stellar mass binary black holes (sBBHs) and binary neutron stars (BNSs). We consider that the dynamically originated sBBHs have relatively larger masses and higher eccentricities compared with those from field binary stars and its possible effect on the GWB spectrum. We find that the GWB spectrum may have a turnover in the low-frequency (Laser Interferometer Space Antenna; LISA) band and may be better described by a broken double power-law than a single power-law with the unique index $2/3$, and the low-frequency slope depends on the significance of the dynamically originated sBBHs with high eccentricities. We further generate mock samples of sBBHs and BNSs, and simulate the mock GWB strain in the time domain. We find that GWB can be detected with signal-to-noise ratio (SNR) $\gtrsim274/255/21$ by LISA/Taiji/TianQin over $5$-years' observation and $\gtrsim3$ by LIGO over $2$-years' observation. Furthermore, we estimate that the number of sBBHs that may be detected by LISA/Taiji/TianQin individually with SNR $\gtrsim8$ is $\sim5$-$221$/$7$-$365$/$3$-$223$ over $5$-years' observation.

Low-Frequency Mean Square Slopes and Dominant Wave Spectral Properties: Toward Tropical Cyclone Remote Sensing

Spectral properties near the dominant wave region influence significantly the surface roughness relevant to ocean remote sensing employing low-frequency microwave sensors. The critical parameters characterizing dominant waves are wind speed and dimensionless spectral peak frequency, which is the inverse wave age. The dimensionless spectral peak frequency can be expressed as an equivalent dimensionless fetch or duration. The connection between dominant waves and surface roughness is the spectral slope. This paper presents a surface wave spectral model designed for low-frequency microwave remote sensing, with special emphasis on tropical cyclone (TC) applications. The key elements of the spectral model are: 1) a general spectral function with coefficients accommodating a variable spectral slope and 2) a parametric function connecting the spectral slope and wind speed, which is established with the mean square slope (MSS) observations obtained inside hurricanes by the global positioning system reflectometry technique. In order to make use of the MSS observations inside hurricanes, parametric models of the spatial distributions of wind speed and dimensionless spectral peak frequency inside TCs are developed. The parametric models are based on the wind and wave similarity relationships derived from analyses of hurricane hunter measurements.

The Coronal Volume of Energetic Particles in Solar Flares as Revealed by Microwave Imaging

Abstract The spectrum of gyrosynchrotron emission from solar flares generally peaks in the microwave range. Its optically thin, high-frequency component, above the spectral peak, is often used for diagnostics of the nonthermal electrons and the magnetic field in the radio source. Under favorable conditions, its low-frequency counterpart brings additional, complementary information about these parameters as well as thermal plasma diagnostics, either through gyrosynchrotron self-absorption, free–free absorption by the thermal plasma, or the suppression of emission through the so-called Razin effect. However, their effect on the low-frequency spectrum is often masked by spatial nonuniformity. To disentangle the various contributions to low-frequency gyrosynchrotron emission, a combination of spectral and imaging data is needed. To this end, we have investigated Owens Valley Solar Array (OVSA) multi-frequency images for 26 solar bursts observed jointly with RHESSI during the first half of 2002. For each, we examined dynamic spectra, time- and frequency-synthesis maps, RHESSI images with overlaid OVSA contours, and a few representative single-frequency snapshot OVSA images. We focus on the frequency dependence of microwave source sizes derived from the OVSA images and their effect on the low-frequency microwave spectral slope. We succeed in categorizing 18 analyzed events into several groups. Four events demonstrate clear evidence of being dominated by gyrosynchrotron self-absorption, with an inferred brightness temperature of ≥108 K. The low-frequency spectra in the remaining events are affected to varying degrees by Razin suppression. We find that many radio sources are rather large at low frequencies, which can have important implications for solar energetic particle production and escape.

How Far Is Quasar UV/Optical Variability from a Damped Random Walk at Low Frequency?

Abstract Studies have shown that UV/optical light curves of quasars can be described using the prevalent damped random walk (DRW) model, also known as the Ornstein–Uhlenbeck process. A white noise power spectral density (PSD) is expected at low frequency in this model; however, a direct observational constraint to the low-frequency PSD slope is difficult due to the limited lengths of the light curves available. Meanwhile, quasars show scatter in their DRW parameters that is too large to be attributed to uncertainties in the measurements and dependence on the variation of known physical factors. In this work we present simulations showing that, if the low-frequency PSD deviates from the DRW, the red noise leakage can naturally produce large scatter in the variation parameters measured from simulated light curves. The steeper the low-frequency PSD slope, the larger scatter we expect. Based on observations of SDSS Stripe 82 quasars, we find that the low-frequency PSD slope should be no steeper than −1.3. The actual slope could be flatter, which consequently requires that the quasar variabilities should be influenced by other unknown factors. We speculate that the magnetic field and/or metallicity could be such additional factors.

Microwave Radiometric Complex for Studying the Thermal Structure of the Earth’s Atmosphere

We describe a microwave radiometric complex intended for remote passive monitoring of the atmospheric temperatures from the Earth’s surface. The complex consists of three spectroradiometers operating in a frequency range of 50–60 GHz, which covers the central part of the absorption band of molecular oxygen and its low-frequency slope. The radiometers have different spectral resolutions and allow one to simultaneously study the thermal structure of the surface air, the free troposphere, and the stratosphere. To ensure internal calibration of the intensity of the received atmospheric radiation, a built-in device of the modulator-calibrator type on the basis of the Schottky-barrier GaAs diodes is used. The complex is equipped with an automated system to control the measurement process, calibration, and preliminary data processing. Using the microwave sensing results, we intend to retrieve the atmospheric temperature profiles in an altitude interval of 0.05–55 km on the basis on the Bayesian approach to solving ill-posed inverse problems.

Corrosion behavior of X70 steel in sands at different charging times

AbstractTo investigate the corrosion behavior and mechanisms of X70 steel in sands, polarization curves (PCs) and electrochemical impedance spectroscopy (EIS) were carried out using an electrochemical test set-up. X70 steel represents a common and standard API designation for pipelines with 70 ksi or 485 MPa strength. For acceleration of the process at different charging times, a direct current (DC) with 6 V voltage was applied. The results show that DC accelerates the corrosion process of X70 steel in sands and that the corrosion rate increases with charging time of the DC. Corrosion potential changes the process of positive-negative-positive charging with increasing process time, indicating that pore water, oxygen substances and corrosion ions of sands all affect the corrosion reaction. Based on the experimental results from the equivalent circuit of EIS, the corrosion process can be divided into the following three phases: developing and formatting of corrosion product film, pitting corrosion, and progressing pitting period. The low-frequency inductive slope on the EIS graph appears during the period of corrosion pitting as resulting from the corrosion product film. The film slows down the corrosion process and reduces the corrosion rate temporarily. With the increasing charging time, the corrosion product film may be damaged and the corrosion rate will thus increase significantly. The testing results of energy dispersive spectrometry (EDS) and scanning electron microscopy (SEM) indicate iron hydroxides as the original corrosion products. With increasing charging time, the original corrosion products may be oxidized to form stable and protective film products such as Fe2O3 and Fe3O4. Lastly, during the pitting progressing period, oxidized film products will be damaged and the corrosion rate increases with increasing charging time.

Pan-STARRS1 variability of XMM-COSMOS AGN

[Abbreviated] We search for scaling relations between the fundamental AGN parameters and rest-frame UV/optical variability properties for a sample of $\sim$90 X-ray selected AGNs covering a wide redshift range from the XMM-COSMOS survey, with optical light curves in four bands provided by the Pan-STARRS1 (PS1) Medium Deep Field 04 survey. To estimate the variability amplitude we utilize the normalized excess variance ($\sigma_{\mathrm{rms}}^{2}$) and probe variability on rest-frame timescales of several months and years by calculating $\sigma_{\mathrm{rms}}^{2}$ from different parts of our light curves. In addition, we derive the rest-frame optical PSD for our sources using continuous-time autoregressive moving average (CARMA) models. We observe that the excess variance and the PSD amplitude are strongly anti-correlated with wavelength, bolometric luminosity and Eddington ratio. There is no evidence for a dependency of the variability amplitude on black hole mass and redshift. These results suggest that the accretion rate is the fundamental physical quantity determining the rest-frame UV/optical variability amplitude of quasars on timescales of months and years. The optical PSD of all of our sources is consistent with a broken power law showing a characteristic bend at rest-frame timescales ranging between $\sim$100 and $\sim$300 days. The break timescale exhibits no significant correlation with any of the fundamental AGN parameters. The low frequency slope of the PSD is consistent with a value of $-1$ for most of our objects, whereas the high frequency slope is characterized by a broad distribution of values between $\sim-2$ and $\sim-4$. These findings unveil significant deviations from the simple "damped random walk" model, frequently used in previous optical variability studies. We find a weak tendency for AGNs with higher black hole mass having steeper high frequency PSD slopes.

On the Space-Time Correlation of Heat Release Rate in Turbulent Premixed Flames

ABSTRACTThe spectral characteristics of combustion noise is dictated by the temporal correlation of the heat release rate fluctuation. This article investigates the two-point space-time correlation of heat release rate fluctuations by analyzing recent direct numerical simulation data of turbulent premixed flames. The results show that this correlation decays rapidly in both space and time due to the abrupt fluctuation of the heat release rate. The two-point space-time correlation can be modeled by Gaussian-type functions including the streamwise direction with strong convection. A flat low-frequency dependence is observed for the spectrum of the correlation function, which will allow the modeling of the spectral peak and the low-frequency spectral slope of combustion noise when multiplying the term ω2 (ω is the angular frequency). This exploratory work shows the potential of this correlation function for capturing the spectral shape of combustion noise.

Poly(ether ether ketone)-based hierarchical composites for tribological applications

Poly(ether ether ketone) (PEEK)/glass fiber (GF) laminates reinforced with single-walled carbon nanotubes (SWCNTs) wrapped in poly(ether ether sulfone) (PEES) were manufactured by extrusion and hot-compression processes. The mechanical, tribological and rheological behavior of the hybrid composites and reference PEEK/SWCNT nanocomposites was analyzed. Viscoelastic measurements as a function of frequency ω revealed an increase in the complex viscosity, storage and loss moduli with the addition of the SWCNTs. Composites incorporating PEES as a compatibilizer exhibited higher moduli values and a smaller low-frequency slope of the storage modulus versus ω, indicating a more homogenous SWCNT dispersion. Moreover, they showed improved compression modulus and strength, suggesting a more effective matrix-SWCNT load transfer. The SWCNTs slightly lowered the friction coefficient of the composites while strongly decreased their wear rate. The compatibilizer reduced the lubricant effect of the SWCNTs and the heat dissipation during sliding, reflected in a smaller improvement in the tribological properties. SWCNT-reinforced laminates displayed outstanding wear behavior, attributed to their superior stiffness and strength, the lubrication capability of the SWCNTs, combined with a synergistic effect arising from the presence of the two types of fillers. The combination of conventional fibers with SWCNTs is a promising route to enhance the performance of the resulting hierarchical composites for tribological applications.

M 87 at metre wavelengths: the LOFAR picture

M87 is a giant elliptical galaxy located in the centre of the Virgo cluster, which harbours a supermassive black hole of mass 6.4x10^9 M_sun, whose activity is responsible for the extended (80 kpc) radio lobes that surround the galaxy. The energy generated by matter falling onto the central black hole is ejected and transferred to the intra-cluster medium via a relativistic jet and morphologically complex systems of buoyant bubbles, which rise towards the edges of the extended halo. Here we present the first observations made with the new Low-Frequency Array (LOFAR) of M87 at frequencies down to 20 MHz. Images of M87 were produced at low radio frequencies never explored before at these high spatial resolution and dynamic range. To disentangle different synchrotron models and place constraints on source magnetic field, age and energetics, we also performed a detailed spectral analysis of M87 extended radio-halo using these observations together with archival data. We do not find any sign of new extended emissions; on the contrary the source appears well confined by the high pressure of the intra-cluster medium. A continuous injection of relativistic electrons is the model that best fits our data, and provides a scenario in which the lobes are still supplied by fresh relativistic particles from the active galactic nuclei. We suggest that the discrepancy between the low-frequency radio-spectral slope in the core and in the halo implies a strong adiabatic expansion of the plasma as soon as it leaves the core area. The extended halo has an equipartition magnetic field strength of ~10 uG, which increases to ~13 uG in the zones where the particle flows are more active. The continuous injection model for synchrotron ageing provides an age for the halo of ~40 Myr, which in turn provides a jet kinetic power of 6-10x10^44 erg/s.

Characterization of the perceived sound of trauma-induced tinnitus in gerbils

Tinnitus often develops following inner ear pathologies, like acoustic trauma. Therefore, an acoustic trauma model of tinnitus in gerbils was established using a modulated acoustic startle response. Cochlear trauma evoked by exposure to narrow-band noise at 10 kHz was assessed by auditory brainstem responses (ABR) and distortion product otoacoustic emissions (DPOAE). Threshold shift amounted to about 25 dB at frequencies > 10 kHz. Induction of a phantom-noise perception was documented by an acoustic startle response paradigm. A reduction of the gap-prepulse inhibition of acoustic startle (GPIAS) was taken as evidence for tinnitus at the behavioral level. Three to five weeks after trauma the ABR and DPOAE thresholds were back to normal. At that time, a reduction of GPIAS in the frequency range 16-20 kHz indicated a phantom noise perception. Seven weeks post trauma the tinnitus-affected frequency range became narrow and shifted to the center-trauma frequency at 10 kHz. Taken together, by investigating frequency-dependent effects in detail, this study in gerbils found trauma-evoked tinnitus developing in the frequency range bordering the low frequency slope of the induced noise trauma. This supports the theory of lateral inhibition as the physiological basis of tinnitus.

Clinical evaluation of cochlear implant sound coding taking into account conjectural masking functions, MP3000™

Efficacy of the SPEAK and ACE coding strategies was compared with that of a new strategy, MP3000™, by 37 European implant centers including 221 subjects. The SPEAK and ACE strategies are based on selection of 8–10 spectral components with the highest levels, while MP3000 is based on the selection of only 4–6 components, with the highest levels relative to an estimate of the spread of masking. The pulse rate per component was fixed. No significant difference was found for the speech scores and for coding preference between the SPEAK/ACE and MP3000 strategies. Battery life was 24% longer for the MP3000 strategy. With MP3000 the best results were found for a selection of six components. In addition, the best results were found for a masking function with a low-frequency slope of 50 dB/Bark and a high-frequency slope of 37 dB/Bark (50/37) as compared to the other combinations examined of 40/30 and 20/15 dB/Bark. The best results found for the steepest slopes do not seem to agree with current estimates of the spread of masking in electrical stimulation. Future research might reveal if performance with respect to SPEAK/ACE can be enhanced by increasing the number of channels in MP3000 beyond 4–6 and it should shed more light on the optimum steepness of the slopes of the masking functions applied in MP3000.

Tempered relaxation with clustering patterns

This work is motivated by the relaxation data for materials which exhibit a change of the relationship between the fractional power-law exponents when different relaxation peaks in their dielectric susceptibility are observed. Within the proposed framework we derive a frequency-domain relaxation function fitting the whole range of the two-power-law dielectric spectroscopy data with independent low- and high-frequency fractional exponents γ and −α, respectively. We show that this effect results from a contribution of different processes. For high frequencies it is determined by random stops and movement of relaxing components, and the low-frequency slope is caused by clustering in their temporal changes.