Specific Number Of Cycles Research Articles

Is Transition Metal Dissolution a Major Degradation Process in Commercial Li-Ion Cells? Application of Impedance with Extended Sub-mHz Low-Frequency Range

Transition metal dissolution is a well-known degradation process in Li-ion batteries correlated with capacity fade1. Transition metals from a lithium transition metal oxide cathode can dissolve which causes structural changes in the crystal structure leading to deterioration of the material. Additionally, dissolved transition metals can migrate to the anode and reduce on the surface, impacting the surface activity of the anode composite and increasing its impedance.It is generally known that electrochemical performance of a Li-ion insertion battery electrode is primarily defined by the active material chemistry, further it is strongly impacted by the material and electrode-level engineering (particle size and aggregation, electrode porosity and tortuosity, electrolyte amount and properties, conductive additive and binder choice and their distribution in electrode composite). Furthermore, impedance response of an insertion electrode exhibits a strong dependence upon lithiation level (x). The specific dependence of impedance of an electrode upon x should allow to help in separating the two individual contributions of the two electrodes in a full cathode-anode battery cell. Thus, not surprisingly, Electrochemical Impedance Spectroscopy (EIS) is a common electrochemical technique of choice for degradation studies of Li-ion batteries2. Typically, EIS spectra of batteries are measured in frequency spans from MHz (laboratory cells) or kHz (commercial cells) down to mHz ranges. However, impedance measurements down to lower minimal frequencies (below 1 mHz) are often avoided from the practical reasons due to the inevitably longer measurement times being required.In the present work cylindrical commercial 18650-size high-energy NMC-Graphite(+Si) cells were electrochemically (galvanostatically) cycled at various conditions. In the constant temperature regime (10°C, 23°C, 35°C or 45°C) the tested cells were systematically cycled within various depth of discharge (DoD) ranges to observe the effect of the voltage range on the evolution (decline) of cell capacity. EIS responses were measured at different selected states of charge (SoC), with the aim to help to resolve the individual contributions of cathode and anode. State of health (SoH) of a battery was determined by current integrated capacity measurement and was periodically monitored after specific number of cycles and correlated with the impedance growth. EIS measurements down to conventional 10 mHz as well as to very low frequencies (0.1 mHz) were performed and compared to determine what additional information and insights for developing of a physics-based battery cell model and degradation modes can be obtained by including the extended low-frequency range (down to 0.1 mHz). We systematically investigated evolution (increase) of impedance of commercial Li-ion cells including the extended low-frequency range.After completion of the cycling test, post-mortem analysis of the cell components was performed on selected cells. Cells were disassembled, electrodes carefully removed, properly washed and SEM-EDX and ICP-MS analysis was performed on the cathode and anode samples. The amount of transition metals in both cathode and anode was determined.We thoroughly confirmed that the impedance of the commercial (full cells) in general increases during the decrease of capacity at all the tested conditions. Importantly, EIS spectra measured down to very low frequencies show that in addition to the (expected) increase of the real component (Re(Z)) simultaneously also the imaginary component (Im(Z)) significantly increases. We provide novel physics-based explanation of the underlying phenomena. Electrochemical characterization of the half cells prepared from the harvested electrodes in general showed a decrease in capacity and increase in voltage hysteresis for both cathode and anode – the actual changes showing strong dependence upon conditions during cycling. EDX and ICP-MS analysis revealed that the composition of the cathode active material remained practically unchanged and that the amount of the transition metals (TMs) deposited on the anode did not increase greatly with the cycling – the total amount of detected TMs remaining well below 300 ppm (relative to the amount of TMs in pristine cathode). Gilbert, J. A., Shkrob, I. A. & Abraham, D. P. Transition Metal Dissolution, Ion Migration, Electrocatalytic Reduction and Capacity Loss in Lithium-Ion Full Cells. J. Electrochem. Soc. 164, A389–A399 (2017).Meddings, N. et al. Application of electrochemical impedance spectroscopy to commercial Li-ion cells: A review. J. Power Sources 480, 228742 (2020). Figure 1. a) Typical examples of EIS response of 811-NMC-Graphite(+Si) full cells measured at SoC = 0.5 (3.705 V) down to 1 mHz (dark colors) and down to 0.1 mHz (lighter dolors) for pristine cell (black/grey), cell cycled at 23°C (blue), and cell cycled at 45°C (red). The corresponding SoH values were: 1, 0.39, and 0.39, respectively. b) Comparison of capacity of the half-cells prepared with the cathodes and anodes harvested from the initial commercial cells (panel a). Figure 1

Microstructure and fatigue properties of 17Cr2Ni2MoVNb gear steel after gas carburizing and low-pressure carburizing

In this work, the microstructure and fatigue properties of the 17Cr2Ni2MoVNb gear steel after gas carburizing (GC) and low-pressure carburizing (LPC) were comparatively evaluated. The results showed that fewer carbides and a higher fraction of retained austenite (RA) were found in the carburized layer of the LPC specimens, as a result of the different quenching temperature. Rotating bending fatigue tests revealed that both the LPC and GC specimens had a fatigue limit of 980 MPa. Compared with the GC specimens, the LPC specimens had significantly higher fatigue strengths at a specific number of cycles under high cyclic stresses.

Influence of power cycling ageing on the current and voltage transitions during hard switching of IGBT devices

In this paper, the influence of ageing by PCT (Power Cycling Test) on the hard switching behavior of IGBT (Insulated Gate Bipolar Transistor) devices, especially the current change rate di/dt and voltage change rate dv/dt during the switching process, is investigated with standard DC-PCT and a PCT with superimposed switching losses. 1200 V IGBTs with two different package forms are used for the PCTs to stimulate two common failure modes: discrete devices with the bond wire failures and modules without baseplate to trigger the solder layer degradation. To evaluate the hard switching behavior of the IGBTs, a DPT (double pulse test) is repetitively performed after a specific number of cycles at the same test conditions to determine the influence of power cycling ageing. The experimental results show that the current and voltage transient transitions of IGBT devices during hard switching are independent of the power cycling ageing.

Cyclic stress-strain behavior of polymeric nonwoven structures for the use as artificial leaflet material for transcatheter heart valve prostheses

Abstract Xenogenic leaflet material, bovine and porcine pericardium, is widely used for the fabrication of surgically implanted and transcatheter heart valve prostheses. As a biological material, long term durability of pericardium is limited due to calcification, degeneration and homogeneity. Therefore, polymeric materials represent a promising approach for a next generation of artificial heart valve leaflets with improved durability. Within the current study we analyzed the mechanical performance of polymeric structures based on elastomeric materials. Polymeric cast films were prepared and nonwovens were manufactured in an electrospinning process. Analysis of cyclic stress-strain behavior was performed, using a universal testing machine. The uniaxial cyclic tensile experiments of the elastomeric samples yielded a non-linear elastic response due to viscoelastic behavior with hysteresis. Equilibrium of stress-strain curves was found after a specific number of cycles, for cast films and nonwovens, respectively. In conclusion, preconditioning was found obligatory for the evaluation of the mechanical performance of polymeric materials for the use as artificial leaflet material for heart valve prostheses.

Investigation on the plasticity accumulation of Ti-6Al-4V fretting wear by decoupling the effects of wear and surface profile in finite element modelling

A finite-element-based wear modelling methodology and a computational device for decoupling wear effects is presented in this study. The decoupling of wear effects facilitates the capture of plasticity accumulation on a particular wear-scarring profile after a specific number of cycles. It was determined that significant plasticity accumulation due to plastic shakedown was predicted in a partial-slip case, while a saturation of plastic deformation was predicted in a gross-sliding case. It was also predicted that a significant amount of plasticity does not meaningfully contribute to the stress and strain range observed in the contact region. It was assumed that plasticity accumulation contributes towards wear of the material and feeds the stress changes, which indirectly affects fatigue life.

Research on Buoy Power Management Technology Based on the Battery Switch Theory

The cycle performance and the capacity fading model of 18650 Li-ion batteries and the possibility of dual-battery switching technology applied in marine buoys were studied. The recycling capacity fading data of 18650 li-ion batteries were gained and the data were fit by Matlab cttool toolbox. A fitting curve more in line with the scatters was gained. The number of cycles was calculated when the battery capacity remaining 80%. Then the normal working time and the working time using battery switching theory of a buoy were calculated under that specific number of cycles. The results show that the 18650 li-ion battery capacity fading model adheres more to the power function model than linear function model. Its correlation coefficient reaches 0.9811 and it shows a high fitting degree. The longevity of the battery pack using battery switching technology is 28.79% longer than the traditional single battery pack.

Experimental Study on the Effect of Strain Cycles on Mechanical Properties of AISI 1022 Steel

Abstract: The effect of cyclic loading on various mechanical properties of AISI 1022 steel was investigated in this study using laboratory‐based experimental method. Groups of specimens were tested in push‐pull strain‐controlled cyclic loading. Some of these specimens were tested to failure in pure fatigue tests. For the remaining specimens, cyclic loading tests were terminated at a specific number of cycles and these specimens were then tested to failure in quasi‐static tension. It was found that the strength increased, while the ductility and toughness decreased because of applications of strain cycles. As reduction in ductility weakens plastic strain resistance, and drop in toughness reduces resistance to fracture of this steel, the quasi‐static mechanical behaviour is expected to change as a consequence of application a certain number of strain cycles. This study therefore suggests that these changes in mechanical properties have to be contemplated in the associated design processes.

Morphological Transitions on Lithium Metal Anodes

Coin cells were prepared using a metallic lithium anode, a cathode, and a 1.2 M (30:70 wt %) electrolyte. The cells were cycled galvanostatically between 1 and 2 V vs at a 2C rate. After a specific number of cycles, the cells were disassembled and the morphology of the lithium anode was characterized using scanning electron microscopy. It was observed that the surface morphology of the lithium metal electrode transitioned from a flat and smooth morphology to a microscopically rugged structure that shows three distinct layers: a top dendritic layer, an intermediate porous layer, and a residual metallic lithium layer. Morphological and electrochemical evidence points to the depletion of the electrolyte and the active metallic lithium that reacted to produce the porous layer as the most likely cause of cell failure under the conditions studied.

Size to density coupling of supported metallic clusters

One of the difficulties in standard growth of metallic nano-clusters on oxide substrates as model catalysts is the strong coupling between clusters size and density. Employing multiple cycles, amorphous solid water-buffer layer assisted growth (ASW-BLAG) procedure, we demonstrate how the size to density coupling can be eliminated under certain conditions. In this study, gold clusters were deposited on a SiO2/Si(100) substrate in UHV, using ASW as a buffer layer assisting aggregation and growth. The clusters were imaged ex situ by tapping mode atomic force microscope (AFM) and high-resolution scanning electron microscope (HR-SEM). In situ Auger electron spectroscopy (AES) measurements have led to independent evaluation of the gold covered area. In order to increase the clusters density we have introduced a multiple BLAG procedure, in which, a BALG cycle is repeated up to 10 times. The cluster density can be increased this way by more than five fold without changing their size. Above a specific number of cycles, however, the cluster density reaches saturation and a gradual increase in clusters size is observed. Larger clusters correlate with lower saturation density following multiple BLAG cycles. This observation is explained in terms of long range cluster-cluster attraction between clusters already on the substrate and those approaching in the next BLAG cycle. This attraction is more pronounced as the clusters become larger. We have shown that at saturation density, inter-cluster distance can not be smaller than 20 nm for clusters 4 nm in diameter or larger. Employing two consecutive BLAG cycles, characterized by different parameters (metal dosage and buffer layer thickness) result in a bi-modal size distribution. Moreover, it is demonstrated that one can prepare this way co-adsorbed bi-metallic film of e.g. Au and Pd clusters, with specific density and size on the same substrate. The ASW-BLAG procedure is thus expected to introduce a new pathway for tailor made, versatile model catalysts.

The CO<SUB align=right>2 and SO<SUB align=right>2 capture capability of two Greek limestones during repeated thermal cycles

In the present study, two Greek limestones from the Megalopolis and Florina areas were used to capture CO2 and SO2. The regeneration ability and decay of the limestone samples were investigated through sorption-calcination cycles in a thermogravimetric analyser operated at ambient pressure. The effects of the sorption time (3, 8 and 30 min), reaction temperature (750°C and 850°C) and particle size (38-53, 150-250 and 500-800 μm) on the sorbent performance were examined. The two limestones demonstrated similar CO2 capture capability. Florina limestone decayed less after a specific number of cycles. When the particle size of the samples increased and the carbonation temperature was reduced, less CO2 capture was observed for both samples. The presence of SO2 inhibited the sorption ability of both limestones. Florina limestone was found to exhibit a higher sorption ability and lower decay when CO2 and SO2 were present simultaneously.

Revision of Loosened Iliac Screws

The present study compared the biomechanics of 2 revision iliac screws: longer and bigger screws, on human cadaveric pelves. To determine if a bigger screw resists loosening under cyclic loading better than a longer screw in revising loosened iliac screws. Iliac screws have been used in treating spinal deformity, spondylolisthesis and many other spinal diseases. Because of the cancellous bone along the screw trajectory, screw loosening over cyclic loading has been experienced in clinical applications. Two popular revision choices are: a longer screw and a bigger screw. However, their biomechanics has not been characterized. The objective of this study is to determine the rate of loosening of longer or larger revision iliac screws under cyclic loading. Eight pairs of human cadaver pelves were harvested. Each side was randomly assigned for a longer revision screw or a larger revision screw. Because of different bone quality in each specimen, applied load was varied according to the peak insertion torque of the primary iliac screws. The load was applied at an anatomic angle with a frequency of 2 Hz. The motion of screw with respect to the pelvis at the bone entry point was recorded with a motion tracking system. The amount of loosening after a specific number of cycles was determined from the screw motion data. The average maximal insertion torque of bigger revision screws (3.2 Nm) was greater than that of longer revision screws (2.7 Nm) with P = 0.03. The average loosening rate was 0.28 +/- 0.13 (SE) mm/thousand cycles for longer revision screws and 0.06 +/- 0.05 (SE) mm/thousand cycles for bigger revision screws. The difference between these 2 revision screws was significant (P = 0.03). In addition, the bigger revision screws had a lower loosening rate than that of the primary screws (P = 0.03). Iliac screws are susceptible to loosening under cyclic loading due to the cancellous bone structure surrounding the screw body. Experimental data showed that the bigger revision iliac screw resists loosening better than the longer screw and the primary screw. Thus, the bigger revision screw is favored if the patient's anatomy allows such operation.

Strain-Induced Martensitic Phase Transformation by Low-Cycle Fatigue in AISI 316L Stainless Steel

The evolution of the strain-induced α′ martensite during the cyclic deformation of austenitic AISI 316L stainless steel (SS) was investigated. The low-cycle fatigue (LCF) test was conducted at various strain amplitudes in air. The amount of α′ martensite was determined for the fatigue-failed specimens as well as for the specimens interrupted after a specific number of cycles. The volume fraction of α′ martensite increased with increasing strain amplitude and number of fatigue cycles. The cyclic hardening behavior was discussed in terms of the dislocation density and strain-induced α′ martensite transformation.

FATIGUE LIFE CHARACTERISTICS OF WATERWORKS PIPE WELDS

The fatigue characteristic of a material or a structure is derived from fatigue tests of standard specimens. However, many test results of standard specimens are very different from those of real structures or components because of geometrical effect, surface condition and so on. In this study, fatigue tests with specimens and pipes were carried out to evaluate the fatigue characteristic of a real waterworks pipe. Standard fatigue specimens and non-standard specimens were extracted from a steel pipe used in waterworks system. Also, fatigue tests of real pipes used in water service were carried out. This result was compared with that of standard specimens and non-standard specimens. To evaluate pipe's fatigue characteristics based on life distribution, the statistical analysis method was introduced. Probability density functions of the specimen based on the normal distribution function were obtained from fatigue tests at particular stress levels. These functions were then transformed to probability density functions based on a specific number of cycles to failure. This procedure was also adapted to the pipe's test results. From these results, the fatigue characteristic of waterworks pipe was evaluated.

Creep-fatigue interactions in glass fibre/polyester composites

Glass-reinforced plastics (GRP) are employed in the cooling water (CW) systems of coastal power stations, where they are exposed to an aggressive environment, sea water at temperatures up to 40°C, and to a complex loading pattern. This latter comprises a background static load, on which are superimposed sinusoidal variations and periodic unloads. The aim of this project was to ascertain the extent of any interaction between static and dynamic loading. Also examined were the effects of environment and of cyclic loading at low frequencies upon fatigue life. The material was modelled on the CEGB-preferred layup of mixed chopped-strand mat (csm) and woven-roving (wr) in polyester. Two loading regimes were used to model aspects of the CW system service conditions. Both used a frequency of 1 cycle per 2 min (0·0083 Hz). Periodic unloads were represented by a ‘rectangular’ load-time variation (R1), with a ratio of time under load to time off-load of 20:1. Tidal pressure variations were represented by a sinusoidal régime with load amplitude equal to 20% of mean load (S1). When the aggregate time at maximum load during the rectangular-wave test R1 was compared to life under static loading at the same level, intermittent unloading seemed to make no difference. This observation was contrary to some published work. Since the different behaviour might arise from the use of different loading frequencies, the effect of frequency of unloading was investigated over a range from 0·0005 Hz to 0·25 Hz. At the higher frequencies, failure occurs after a specific number of cycles, irrespective of the time under load; at low frequencies a time-at-load criterion is satisfied first. A ‘boundary frequency’ must exist between these régimes, and it is calculated to lie between 0·12 and 1·2 cycles/min (cpm). The frequency of R1 (0·5 cpm) lies within this range. The interaction between sinusoidal and static loading was investigated with two ratios of stress amplitude to mean stress; with frequencies of 1·0, 0·25 and 0·0083 Hz; in air and in water at 40°C. Results were assembled in the form of a partial Haigh diagram. From this it was possible to deduce: (a) that in tension, the effect of combined static and cyclic loading conforms to the expected pattern, (b) that the fatigue design limits of the material in water are less than in air for tests at the same frequency, as was expected, and (c) that a further reduction in limits is required in wet environments at low frequencies, relative to those at 1 Hz. Implications for BS4994, the principal GRP design code, are discussed.

Dynamic Compression Test for Adhesion of Rubber to Cord Fabric

Abstract An improved laboratory method has been developed for the purpose of evaluating rubber-cord adhesions. The test-piece consists of a single cord surrounded by a thin cylindrical layer of casing compound which, in turn, is enclosed and protected by a thicker layer of rubber of standard composition. Such test-pieces are subjected to cyclic compression and bending, under controlled conditions of cord tension and ambient temperature, in a specially designed machine. The apparatus simulates the primary stresses and strains which occur in the crown and shoulder regions of a giant tire running under load, and it has been established that the cord bond in the test-pieces breaks down in a manner analogous to that in an overloaded tire. The degree of bond deterioration is determined by comparing the load required to pull the cord axially through that part of the rubber which has been subjected to compression after a specific number of cycles with that required to do so in a similar untreated test-piece. Data show the rate at which the rubber-cord bond deteriorates with various degrees of compression and temperature, and examples are given of the use of the method for evaluating bonding agents for cotton and rayon cords. A general comparison is made between the Henley test and other adhesion tests known to be in use.