Severe Instabilities Research Articles

Chemo-mechanical instabilities in lithium cobalt oxide at higher state-of-charge in Li-Ion batteries

Transition metal oxide cathodes are widely used in commercial Li-ion batteries. However, their practical charge capacity is limited due to severe chemo-mechanical instabilities at higher charge voltage, and state-of-charge condition. Here, in situ stress and strain measurements were synchronized to probe mechanical deformations in the lithium cobalt oxide (LCO) cathode via a multi-beam stress sensor and digital image correlation, respectively. In situ mechanical measurements revealed how Li removal from the electrode structure induces deformations on the LCO composite cathodes during cycling. The structure and morphology of the LCO cathodes were further investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies. Two distinct electrochemical and mechanical behaviors were identified when the LCO was charged up to 4.65 V. The LCO undergoes a compressive stress generation when charged up to 4.2 V and surface fractures on the LCO particles were detected by SEM. LCO cathode experienced significantly large contractions (negative strains) when charged up to 4.65 V, where intergranular crack formation and phase transformation were detected on the LCO particles via SEM and XRD, respectively. Overall, the study bridges complicated structural deformations with in situ analysis of mechanical degradations in LCO cathodes charged at higher voltages. The correlation is vital to understanding instability mechanisms in transition metal oxides at high voltages for alkali metal ion batteries.

Consequence simulation of cyber attacks on key smart grid business cases

The increasing threat of cyber-attacks on modern power systems highlights the need for a comprehensive examination through simulations. This study conducts an in-depth simulation of cyber-attacks on critical smart grid components, including smart meters, substation automation, and battery management systems, to expose and analyze potential disruptions to power system operations. We identify vulnerabilities that can lead to severe grid instabilities, such as voltage variations, system collapses, and inverter failures. Our analysis underscores the complex interactions between cyber threats and grid components, revealing how disruptions extend beyond mere load interruptions to affect the core infrastructure. We advocate for integrating established cybersecurity frameworks like NIST, ISO/IEC 27001, and IEC 62443, essential in fortifying grid stability against these dynamic threats. Our findings highlight the urgent need for continuous adaptation and enforcement of these frameworks to enhance resilience and ensure the reliability of modern power grids against cyber-attacks.

Fluorination-mitigated high-current degradation of amorphous InGaZnO thin-film transistors

As growing applications demand higher driving currents of oxide semiconductor thin-film transistors (TFTs), severe instabilities and even hard breakdown under high-current stress (HCS) become critical challenges. In this work, the triggering voltage of HCS-induced self-heating (SH) degradation is defined in the output characteristics of amorphous indium-gallium-zinc oxide (a-IGZO) TFTs, and used to quantitatively evaluate the thermal generation process of channel donor defects. The fluorinated a-IGZO (a-IGZO:F) was adopted to effectively retard the triggering of the self-heating (SH) effect, and was supposed to originate from the less population of initial deep-state defects and a slower rate of thermal defect transition in a-IGZO:F. The proposed scheme noticeably enhances the high-current applications of oxide TFTs.

Derivative shape estimations with resolved overlapped peaks and reduced noise for time signals encoded by NMR spectroscopy with and without water suppression

Derivative estimation in magnetic resonance spectroscopy (MRS) possesses several attractive features. It has the ability to enhance the inaccessible spectral details when time signals encoded by MRS are analyzed by nonderivative shape estimators. These unfolded subtle spectral features can be diagnostically relevant in differentiating between healthy and diseased tissues. Within the realm of shape estimators, the prerequisite for the success of MRS in the clinic is reliance upon accurate derivative signal processing. However, derivative processing of encoded time signals can be very challenging. The reason is that such spectra may suffer from severe numerical instabilities since even small perturbations (noise) in the input data could produce large errors in the predicted output data. Nevertheless, it is presently demonstrated that this obstacle can be surmounted by an adaptive optimization. The benefit is simultaneously increased resolution and reduced noise in quantitatively interpretable lineshapes. The illustrative spectra are reconstructed from time signals encoded by proton MRS with and without water suppression.

STABILIZATION OF NOTCHED ELEMENTS' FRACTURE BY USING A CONTROL ALGORITHM

Crack propagation is a significant mechanism for quasi-brittle materials under applied loading. It can occur very suddenly and causes numerical instabilities and deficiencies in some problems. This behavior manifest itself as non-convergence solutions i.e. the inability to obtain the entire load-displacement curve or jumps in the load displacement curve. In this study, a control technique is implemented to obtain the whole load–displacement curve when crack propagation causes severe numerical instabilities such as snap-back behavior. The performance of the control technique was demonstrated by simulating uniaxial tension test of pre-notched plate, three-point bending test of a notched beam and mixed-mode test of a notched plate. This study shows that the control algorithm is able to produce a stable solution path for these kinds of problems. This method can be easily implemented in available commercial finite element codes without the need for any user defined subroutines.

Integrated fault identification in granite tunnel based on the analysis of structural and mineral characteristics of rock masses: a case study

Tunnels are underground structures that are widely used in geology, mining and other related fields. Given the fact that many severe underground structure instabilities are found to be closely associated with fault systems present nearby, an integrated fault identification method, namely the 3M method, is proposed based on analysing the macrostructure, mineral characteristics and microstructure of rock masses in the tunnel. This method comprises three steps, and each step corresponds to one ‘M’: macrostructure identification, mineral analysis and microstructure identification. First, the macrostructure is investigated to determine whether there is a fault-related feature in the tunnel (e.g. a slickenside). Second, mineral analysis infers the spatial extent of the fault from changes to the types and compositions of minerals. Third, the microstructure is investigated to determine the fault-related deformation mechanism in the tunnel (e.g. deformation twinning). Adopting the proposed method reduces the subjective influence of geological engineers and improves the accuracy of fault identification via traditional geological analysis. The results of this study provide new insight into tunnel excavation and support design.

Experimental and theoretical investigation of ethyl methyl carbonate/air flames: Laminar burning velocity and cellular instability at elevated pressures

Ethyl methyl carbonate (EMC) gained great attention because of its advantages shown in practical applications both as a blend with conventional oil-derived liquid fuels, and as a solvent being used in Li-ion cell electrolyte compositions. Thus, a better knowledge of the combustion characteristics of EMC/air premixed flames is necessary, which has not yet been well-explored. Towards this goal, the laminar flame propagation of EMC/air mixture was investigated using a constant-volume combustion chamber at the initial pressures of 1, 2, 5 and 8 atm, with the unburned temperature of 423 K and the equivalence ratios ranging from 0.7 to 1.5. A recently established chemical kinetic mechanism of EMC by Takahashi et al. (Combust. Flame 2022) was adopted for the theoretical prediction, which accurately reproduced the measured LBVs at 1–5 atm; however, it underpredicted the LBVs for the lean flame condition at 8 atm. According to the flame instability analysis, the increase in pressure leads to the elevated hydrodynamic instability and the decreased critical radius of spherical premixed flame. Meanwhile, the rich flames of EMC/air had the smaller critical Peclet numbers and suffered the more severe thermal-diffusive instabilities.

Noncollinear and Spin-Flip TDDFT in Multicollinear Approach

Time-dependent density functional theory (TDDFT) is one of the most important tools for investigating the excited states of electrons. The TDDFT calculation for spin-conserving excitation, where collinear functionals are sufficient, has obtained great success and has become routine. However, TDDFT for noncollinear and spin-flip excitations, where noncollinear functionals are needed, is less widespread and still a challenge nowadays. This challenge lies in the severe numerical instabilities that root in the second-order derivatives of commonly used noncollinear functionals. To be free from this problem radically, noncollinear functionals with numerical stable derivatives are desired, and our recently developed approach, called the multicollinear approach, provides an option. In this work, the multicollinear approach is implemented in noncollinear and spin-flip TDDFT, and prototypical tests are given.

Are Accelerated and Enhanced Wave Function Methods Accurate to Compute Static Linear and Nonlinear Optical Properties?

Key components of organic-based electro-optic devices are challenging to design or optimize because they exhibit nonlinear optical responses, which are difficult to model or rationalize. Computational chemistry furnishes the tools to investigate extensive collections of molecules in the quest for target compounds. Among the electronic structure methods that provide static nonlinear optical properties (SNLOPs), density functional approximations (DFAs) are often preferred because of their low cost/accuracy ratio. However, the accuracy of the SNLOPs critically depends on the amount of exact exchange and electron correlation included in the DFA, precluding the reliable calculation of many molecular systems. In this scenario, wave function methods such as MP2, CCSD, and CCSD(T) constitute a reliable alternative to compute SNLOPs. Unfortunately, the computational cost of these methods significantly restricts the size of molecules to study, a limitation that hampers the identification of molecules with significant nonlinear optical responses. This paper analyzes various flavors and alternatives to MP2, CCSD, and CCSD(T) methods that either drastically reduce the computational cost or improve their performance but were scarcely and unsystematically employed to compute SNLOPs. In particular, we have tested RI-MP2, RIJK-MP2, RIJCOSX-MP2 (with GridX2 and GridX4 setups), LMP2, SCS-MP2, SOS-MP2, DLPNO-MP2, LNO-CCSD, LNO-CCSD(T), DLPNO-CCSD, DLPNO-CCSD(T0), and DLPNO-CCSD(T1). Our results indicate that all these methods can be safely employed to calculate the dipole moment and the polarizability with average relative errors below 5% with respect to CCSD(T). On the other hand, the calculation of higher-order properties represents a challenge for LNO and DLPNO methods, which present severe numerical instabilities in computing the single-point field-dependent energies. RI-MP2, RIJK-MP2, or RIJCOSX-MP2 are cost-effective methods to compute first and second hyperpolarizabilities with a marginal average error with respect to canonical MP2 (up to 5% for β and up to 11% for γ). More accurate hyperpolarizabilities can be obtained with DLPNO-CCSD(T1); however, this method cannot be employed to obtain reliable second hyperpolarizabilities. These results open the way to obtain accurate nonlinear optical properties at a computational cost that can compete with current DFAs.

Electric vehicles: a review of their components and technologies

The number of electrical vehicles (EVs) on the road has increased in recent years, including battery-electric vehicles (BEV), hybrid-electric vehicles (HEV), plug-in hybrid-electric vehicles (PHEVs), and fuel-cell electric vehicles (FCEV). This mode of transportation is expected to eventually replace internal combustion engine (ICE) vehicles, based on current trends. Each key EV component integrates several technologies that are either currently in use or have the potential to become prominent in the future. Environmental, power systems, and other industries may be adversely affected by electric vehicles (EVs). With sufficient EV penetration, the current power system could be subjected to severe instabilities; nevertheless, with proper management and coordination, EVs can significantly contribute to the success of the smart grid concept. Moreover, EVs have the potential to significantly cut transportation-related emissions of greenhouse gases. However, there are still considerable barriers that EVs must overcome before they can completely replace ICEs. The purpose of this study is to review all the relevant information available on EV architectures, battery energy sources, charging processes, and control approaches. Its goal is to provide a comprehensive overview of current EV technology.

Resolving Chemical and Spatial Heterogeneities at Complex Electrochemical Interfaces in Li Ion Batteries

The development of cheap, high energy density electrode materials for Li ion batteries (LIBs) is critical to reaching net zero emissions targets and mitigating climate change. To this end, Ni- and Mn-rich transition metal oxide cathodes simultaneously offer high energy densities as well as cost effective solutions for LIBs. Despite the immense potential of these materials, both Ni- and Mn-rich cathodes suffer from severe interfacial instabilities that lead to crystallographic rearrangement of the active material surface, transition metal dissolution, and the formation of a cathode electrolyte interphase (CEI) layer on the composite surface during electrochemical cycling. While changes in the crystallographic structure of these materials are readily detected with diffraction-based methods, probing the chemistry associated with transition metal dissolution and characterizing the disordered, heterogeneous CEI layer have proven to be substantially more challenging. In this talk, I will discuss our efforts to use a combination of solid-state nuclear magnetic resonance (SSNMR) spectroscopy and X-ray photoemission electron microscopy (XPEEM) to provide information on the CEI deposited on composite cathode films with high chemical and spatial resolution (on the order of nanometers). In situ and operando magnetic resonance spectroscopies are used to correlate specific cathode degradation modes with electrolyte oxidation events. The final properties of the resulting CEI architectures are evaluated in the context of battery performance, where poor capacity retention and high interfacial resistance in LIBs is related to specific surface moieties on the cathode composite.

Efficient Constitutive Model for Continuous Micro-Modeling of Masonry Structures

ABSTRACT Masonry is a composite material often modeled as an equivalent homogenous material. However, the complexity of its micro-structure leads to complex mechanical responses, which are almost impossible to capture accurately with homogenous constitutive models. Micro-modeling can be used in these scenarios, allowing for the explicit modeling of microstructural components, leading to an accurate capturing of their interaction. Its main drawback is the computational cost, which often makes this approach suitable only for the simulation of small specimens. This is especially true due to strain-softening leading to severe instabilities and non-convergence of the solution. The objective of this work is to propose a simple yet effective constitutive plastic-damage model for the microstructural components of masonry. It is based on a damage model previously developed by the authors. For a better representation of the cyclic response of masonry, plasticity is added using a simplified implementation that does not strictly follow the rules of standard elastoplasticity, allowing an explicit computation of the stress tensor from the strain tensor without the need for an iterative loop at the material level. To reduce the numerical issues related to strain-softening and thus improve the stability of the solution, an IM-PLEX integration algorithm is adopted.

Thermal performances of a flat-plate pulsating heat pipe tested with water, aqueous mixtures and surfactants

An experimental study is reported about a copper closed-loop flat plate pulsating heat pipe tested in both horizontal and vertical -bottom heated mode- orientations and at two cold source temperatures (20 °C and 40 °C); several working fluids were tested after having modified of their surface tension compared to pure water, verified through dedicated sessile droplets test bench. After which, tests were performed using pure water, pure ethanol, binary aqueous mixtures (water/ethanol, water/butan-1-ol and water/butan-2-ol, the last two being self-rewetting fluids) and water/surfactant mixtures (Tween® 20 and Tween® 40) as working fluids. The thermal performances of the device tested in horizontal orientation were significantly better for the aqueous mixtures such as water/ethanol and water/butan-2-ol than for the others (with values of thermal resistances down to 63%, and 52%, respectively, lower than those of pure water at 20 °C cooling temperature). Concerning pure fluids, the device presents severe temperature and pressure instabilities. Water with surfactants also presents enormous instabilities, calling into question the interstitial influence of the tested surfactants in the hydraulic slug flow pattern occurring in horizontal inclination. The results in vertical orientation, although often studied in the literature, are less conclusive and more difficult to analyze given that surface tension effects are less influential compared to the gravity effects in the annular or bubbly flow patterns occurring in this operating mode. Finally, zeotropic mixtures with butan-2-ol and ethanol performing very similarly, and considering their identical surface tension, the expected rewetting effect of SRWF is not proven, and may not even explain improved FPPHP operation, which is rather assumed to be largely caused by enhanced wettability or increased (∂P/∂T)sat.

Analysis of self-excited transverse combustion instability in a rectangular model rocket combustor

A methane/oxygen mixture is considered to be an appropriate propellant for many future rocket engines due to its practicality and low cost. To better understand the combustion instability in methane/oxygen-fed rocket engines, the spontaneous transverse combustion instability in a rectangular multi-element combustor (RMC) was analyzed both experimentally and numerically. Severe combustion instabilities occurred in the RMC during repeatable hot-fire tests. The physical mechanisms were systematically investigated through numerical simulations based on the stress-blended eddy simulation and flamelet-generated manifolds method with detailed chemical mechanisms (GRI Mech 3.0). The numerical results for the frequency spectrum and spatial modes agree well with the theoretical analysis and experimental data. The driven regions of the combustion instability were identified on both sides of the combustion chamber through a Rayleigh index analysis. The longitudinal pressure oscillations in the oxidizer post were found to be coupled with the transverse pressure waves in the combustion chamber and led to periodic oscillations of the mass flow rate of propellant. Moreover, the mixing was highly enhanced when the pressure wave interacted with walls of the combustion chamber. Therefore, a sudden release of heat occurred. The pressure oscillations were enhanced by pulsated heat release. A closed-loop system with positive feedback associated with periodic oscillations mass flow rate of the propellant, and sudden heat release, was believed to account for the present combustion instability.

Low-noise Brillouin random fiber laser with Auto-tracking dynamic fiber grating based on a saturable absorption ring

A low-noise Brillouin random fiber laser (BRFL) with stabilized lasing output and narrow linewidth is proposed and demonstrated experimentally. Stimulated Brillouin scattering and Rayleigh scattering are exploited to work as the gain and random distributed feedback mechanisms, respectively. A simple unpumped Erbium-doped fiber (EDF) loop structure which consists of a section of EDF and an optical coupler is introduced into the BRFL cavity, forming a saturable absorption ring (SAR). When light with proper intensity is incident into the SAR, a dynamic fiber grating (DFG) can be built up to work as a narrow-bandwidth filter with the ability of auto-tracking the center frequency. The SAR can effectively filter out dense random longitudinal modes and suppress multi-mode resonances and frequency jitters by restricting the mode-competition-induced mode hopping within a limited frequency range of ∼ 1 MHz and auto-tracking the center frequency of light in the cavity, simultaneously. Thus, the output of BRFL with SAR is much more stabilized than existing free-running BRFLs. In the experiment, a stable lasing output with an ultra-narrow linewidth (less than2kHz) is obtained from the proposed BRFL. The measured relative intensity noise and frequency noise demonstrate that the noise level of the proposed laser is significantly lower than that of the free-running BRFLs. This proposed technique not only provides a solution to the dilemma of previously reported BRFLs with severe lasing instabilities, but also makes a step towards the development of practical and reliable BRFLs. This high-quality laser source with advantages of high stability, low cost, low threshold power, high coherence, narrow linewidth, and low noise can find useful applications in coherent optical detection, high-resolution spectrometers, microwave photonics, and optical sensing fields.

Supervised dimensionality reduction technology of generalized discriminant component analysis and its kernelization forms

Supervised subspace projection technology is a major method for dimensionality reduction in pattern recognition. At present, most supervised subspace projection algorithms are derived from the multi-dimensional extended version of Fisher linear discriminant analysis (FDA), also known as Multi-dimensional Fisher discriminant analysis (MD-FDA). However, MD-FDA needs to be improved further because the projection vectors in the noise-subspace cannot be sorted and the ill-condition of the within-class scatter matrix may cause severe numerical instabilities. Generalized discriminant component analysis (GDCA), the generalization of MD-FDA, together with its kernelization forms are proposed and correspondingly rigorous mathematical proofs are detailed in this paper. By virtue of 5 validation data sets derived from UCI Machine Learning Repository and our laboratory, the theoretical validity and technical advantages of GDCA as well as its kernelization forms are verified, and the effectiveness of the newly proposed method is demonstrated in comparison with 36 kinds of state-of-the-art dimensionality reduction algorithms.

Hydrodynamic limits and numerical errors of isothermal lattice Boltzmann schemes

With the aim of better understanding the numerical properties of the lattice Boltzmann method (LBM), a general methodology is proposed to derive its hydrodynamic limits in the discrete setting. It relies on a Taylor expansion in the limit of low Knudsen numbers. With a single asymptotic analysis, two kinds of deviations with the Navier-Stokes (NS) equations are explicitly evidenced: consistency errors, inherited from the kinetic description of the LBM, and numerical errors attributed to its space and time discretization. The methodology is applied to the Bhatnagar-Gross-Krook (BGK), the regularized and the multiple relaxation time (MRT) collision models in the isothermal framework. Deviation terms are systematically confronted to linear analyses in order to validate their expressions, interpret them and provide explanations for their numerical properties. The low dissipation of the BGK model is then related to a particular pattern of its error terms in the Taylor expansion. Similarly, dissipation properties of the regularized and MRT models are explained by a phenomenon referred to as hyperviscous degeneracy. The latter consists in an unexpected resurgence of high-order Knudsen effects induced by a large numerical pre-factor. It is at the origin of over-dissipation and severe instabilities in the low-viscosity regime.

Modular implementation framework of partitioned path-following strategies: Formulation, algorithms and application to the finite element software Cast3M

Damage, cracking, and strain localization mechanisms often lead to unstable structural responses characterized by snap-backs (i.e., force and displacement decrease simultaneously). Standard nonlinear Newton-based solution algorithms with displacement/force control cannot capture the equilibrium curve in its entirety. This can be overcome using path-following formulations. A general (i.e., valid for any finite element code) implementation framework can be designed and applied to the Cast3M software by collecting the essential concepts and the formalism of partitioned path-following arc-length algorithms. Thanks to these developments, Cast3M is now capable of processing path-following equations without any major modifications. Three path-following constraints were selected to demonstrate the applicability of this framework: a first one on the combination of the displacement increment at a given set of nodes, a second one on the maximum strain increment over the computational domain, and a third one on the maximum elastic predictor of the damage/plastic criterion function over the computational domain. Two- and three-dimensional strain localization simulations show that the proposed framework behaves in a stable and convergent manner, even when multiple severe snap-back instabilities are present. Users of Cast3M shall find the proposed study helpful in that it allows them to focus on developing new path-following equations for the software. Cast3M is developed by the French Alternative Energies and Atomic Energy Commission (CEA) and freely available for research purposes. The developments discussed in this paper have been made available to the user/developer community along with Cast3M 2021 (release date: June 2021).

Thermo-mechanical behaviour of the MYRRHA core support structure

This work is part of the research on the design of MYRRHA primary system. MYRRHA, a multipurpose research facility, being developed at SCK-CEN, is based on the Accelerator Driven System (ADS) concept. This research facility intends to show the applicability of the ADS principle, and aims to demonstrate, among other objectives, the feasibility of efficient transmutation of high-level nuclear waste. The present study investigates, mainly, the thermo-mechanical behaviour of the core support bundle, fuel assemblies and core devices, in particular, the deformation in terms of the displacement and gaps distribution. To reach this goal, which is arduous, an efficient modeling methodology based on the finite element method has been devised. An effective solution strategy has been formulated to carry out a highly challenging analysis due to severe numerical issues and instabilities, making the solutions particularly difficult to reach for some cases. Analysis cases are differentiated by some friction coefficient values; and the presence or not of a spring pre-compression force. Despite serious convergence issues, converged solutions have been achieved for all the cases studied. From the results analysis, a remarkable feature observed is the stability of the solutions obtained. And, on the whole, the value of the friction coefficient used does not change significantly the displacement, the gaps patterns, and the stresses. Similarly, account of the pre-compression force does not affect much the global behaviour. The kinematic quantities found should be useful for some preliminary reactivity predictions of the reactor.

Numerical investigations on turbulent jet ignition with gasoline as an auxiliary fuel in rapid compression machines

ABSTRACT Lean burning as one of the low-temperature combustion strategies has the potential to reduce pollution emissions and improve fuel efficiency. However, highly diluted mixtures cause internal combustion engines to suffer from severe combustion instabilities. Turbulent jet ignition (TJI) technology is considered an effective way to solve these issues, but most previous work mainly considers the passive TJI systems fueled by gaseous fuel. In this work, an active TJI system with gasoline as an auxiliary fuel was investigated using three-dimensional computational fluid dynamics (CFD) simulations in rapid compression machines (RCM). High-pressure low-temperature operating conditions were established to mimic the combustion situation of supercharged spark-ignition engines. Different injection parameters for pre-chamber were considered, including injection timing, injection angle, and injection pulse width, and the influence of injection parameters on the evolution of turbulent jet flame in the main chamber with lean mixtures was investigated. The results show that the hedging effect between reverse squeezed flow and high-speed fuel spray during piston compression promotes gasoline spray atomization, evaporation, and mixing in the pre-chamber. The initial lambda of the pre-chamber plays a significant role in the jet exit velocity, while the initial lambda of the main chamber exhibits a greater impact on the jet propagation distance. Meanwhile, there are generally three stages for the entire combustion in the main chamber, including jet penetration, jet ignition, and spontaneous combustion. The present work can provide useful insights into the optimization and design of active TJI systems in realistic engines.