Gas-filled Void Research Articles

Observations of Gas Evolution at End-of-Life in Commercial Li-Ion Cells – Analyses and Diagnostics

It is well-known that Lithium-ion cells can have safety issues. A large number of battery fire incidents have been reported the last years within e.g. battery energy storage systems [1], electric and hybrid-electric ships [2], electric vehicles and mobile phones [3]. The causes for these incidents are numerous and can include internal shortage of the Li-ion cell, external shortage of the battery system, over-heating and over-charging. All these causes involve overheating of the battery that can lead to an eventual thermal runaway which again could lead to a full battery fire. We have earlier reported changes in thermal stability of Li-ion cells aged by cycling and shown that specific cycling regimes can decrease the thermal stability of Li-ion cells to a large extent [4]. Another safety hazard is the gas evolution in Li-ion pouch cells during and after cycling at normal operating temperatures. The gas evolution can be so high that the pouch cell will expand significantly and eventually burst due to the increased overpressure building up inside the cell. While several studies focus on gas evolution under extreme thermal runaway conditions, reports on gas evolution under “normal” operating conditions and even storage are rare. This gas evolution is not well described in literature and the gas composition might be different compared to gases commonly released during a thermal event in the Li-ion battery.This study is based on results from a cycle life study performed on a large renowned Li-ion pouch cell with an initial capacity of more than 50 Ah. A total of 39 cells were cycled following a selected test matrix including various temperatures, current rates and State-of-Charge limits. All cells were cycled until they reached a remaining capacity of between 60 to 70 % State-of-Health. Already during cycling, the thickness of the cells increased, for some cells even by up to 50% by the time they reached end of life. In addition to the swelling during cycling, one third of the tested cells started to evolve significant amounts of gas during storage at 5 °C, i.e. after the cycling was finished and the cells had reached the lower remaining capacity. This gas evolution was further analyzed to scrutinize the causes of this gassing.To determine the composition of the gas evolved in the cells, the gas was retrieved by carefully inserting a syringe with a sharp needle through the top of the cell into the gas-filled void. The gas composition was then analyzed by gas chromatography (GC), coupled with three different detectors: mass spectrometry (MS), thermal conductivity (TCD) and flame ionization. The analysis revealed that more than 95 % of the gas within the cell comprised of hydrogen, methane and ethane, with hydrogen being the largest contributor. A small amount of electrolyte solvents (EMC and DMC) was found, which was expected due to the low vapor pressure at low temperatures. CO and CO2 levels were found to be below 1 %.A post-mortem study of selected gassed cells was performed to investigate the causes of the gassing. During cycling a large set of diagnostic data was compiled to investigate links between the cycling conditions and the specific findings from the post-mortem study.

The Effect of Protrusions on the Initiation of Partial Discharges in XLPE High Voltage Cables

This paper is focusing on 3D Finite Elements Analysis (FEA) based modelling of protrusions as defects or imperfections in the XLPE high voltage cable. This study is aiming to examine the impact protrusions have on the initiation of partial discharges. Spherical and ellipsoidal protrusions with different sizes at the conductor screen of the high voltage cable is an essential content of this paper. In addition, a spherical gas-filled void is placed inside and outside the protrusions, and a water tree produced from protrusions is under consideration. The partial discharge influence taking place at the protrusions and the stress enhancement factor is determined for all the variations mentioned to quantify the rise in the inception of partial discharges due to the protrusions.

Nitrous Oxide and Methane Dynamics in Woodchip Bioreactors: Effects of Water Level Fluctuations on Partitioning into Trapped Gas Phases.

Woodchip bioreactors (WBRs) are low-cost, passive systems for nonpoint source nitrogen removal at terrestrial-aquatic interfaces. The greenhouse gases nitrous oxide (N2O) and methane (CH4) can be produced within WBRs, and efforts to reduce N2O and CH4 emissions from WBR systems require improved understanding of the biogeochemical and physical-chemical mechanisms regulating their production, transport, and release. This study evaluates the impact of trapped gas-filled void volumes as sinks of dissolved gases from water and as sources of episodic fluxes when water levels fall. Dissolved gas tracer experiments in a laboratory bioreactor were used to parameterize nonequilibrium advection-dispersion-gas transfer models and quantify trapping of gas-filled voids as a function of antecedent hydrological conditions. Experiments following a water-level rise revealed that up to 24% of the WBR pore volume was occupied by trapped gas phases, which were primarily located in pore spaces inside woodchips. This finding was confirmed with X-ray-computed microtomography. N2O (3.3-10%) and CH4 (4.3-14%) injected into the reactor following a water table rise partitioned into gas-filled voids and were released when water tables fell. In the case of N2O, partitioning into trapped gas phases makes N2O unavailable for enzymatic reduction, potentially enhancing N2O fluxes under fluctuating water levels.

Approximate solution for crack interacting with a gas-filled void

Approximate analytical solution for crack interacting with a gas-filled void is developed on the basis of Eshelby equivalent inclusion theory and transformation toughening theory. As validated by detailed finite element analysis the approximate solution has good accuracy for prediction of mode I, mode II and I/II mixed mode crack-tip stress intensity factors under plane strain loading conditions.

The contribution of synchrotron X-ray computed microtomography to understanding volcanic processes

A series of computed microtomography experiments are reported which were performed by using a third-generation synchrotron radiation source on volcanic rocks from various active hazardous volcanoes in Italy and other volcanic areas in the world. The applied technique allowed the internal structure of the investigated material to be accurately imaged at the micrometre scale and three-dimensional views of the investigated samples to be produced as well as three-dimensional quantitative measurements of textural features. The geometry of the vesicle (gas-filled void) network in volcanic products of both basaltic and trachytic compositions were particularly focused on, as vesicle textures are directly linked to the dynamics of volcano degassing. This investigation provided novel insights into modes of gas exsolution, transport and loss in magmas that were not recognized in previous studies using solely conventional two-dimensional imaging techniques. The results of this study are important to understanding the behaviour of volcanoes and can be combined with other geosciences disciplines to forecast their future activity.

The micro void neutron detector

The Gas-filled Micro Void Particle Detector is based on gas-filled micro voids placed in an external electric field. This detector presents common features of solid state and gas filled devices as internal amplification, unlimited size and shape, dense, high efficiency parallax reducing structure. The gas filling in the void and/or the wall of the micro void serves as radiation detector. The working principle was tested on syntactic foam composed of glass micro bubbles embedded in an epoxy matrix.

Diffusive shrinkage of a void within a grian of a stressed polycrystal

A method to predict the shrinkage rate of a void centered at an equiaxial grain under hydrostatic pressure is developed. The rate process is controlled by lattice diffusion under the combined influence of void surface energy, grain boundary interface energy and elastic energy stored in the solid. A dimensionless loading parameter that controls the void evolving is introduced. Below a certain value of this parameter the void will be rounded and is stable. Then the shrinkage behavior of the voids is described by the parameters controlling the lattice diffusion. It is found that only gas-free voids could be totally eliminated through lattice diffusion; for a gas-filled void, the internal pressure produced by the gas firstly retards the rate of the shrinkage and eventually stops the shrinkage process as it becomes high enough.

A generalized model for partial discharge processes based on astochastic process approach

A general framework for the physical description of partial discharge (PD) processes is presented that holds for different types of PD causing defects. A PD process is treated as a stochastic process consisting of short duration discharges (point-like in time) and charge carrier drift/recombination intervals between these discharges. It is determined by few basic physical parameters and, in a stochastic process framework, can be described in a closed form by a master equation. Since usually only the fast discharges can be measured as PD signals, a restricted possibility of observing a PD process results. The link between the stochastic process and observable quantities is derived. A specific type of measurements is reported, the so-called phase-resolved partial discharge (PRPD) patterns. Here the total charge transferred during a discharge and the time or alternating current phase at which the discharge occurs are measured. Thus each discharge event is described by the two quantities, charge and phase angle. The modelling of the observation process is explicitly derived for this case. However, the used method can easily be generalized to other types of PD measurements. The proposed approach yields new possibilities for the interpretation and analysis of PD patterns. Features of PD patterns can be derived analytically from the process parameters. Conversely, quantitative information about the discharge physics can be gained from measured patterns. Some limiting cases of model parameter values leading to typical pattern features are discussed explicitly. Examples are presented that demonstrate the applicability of the model for three different discharge types (internal discharge in a gas-filled void, surface discharge in oil, corona in air).

Effects of Solder Joint Voiding and Seating Plane Stability on Surface Mount Lead Standoff

This study considers the influence of solder joint voiding and seating plane stability on the lead standoffs of surface mount technology (SMT) solder joints. A model is presented for approximating the reaction force generated by a gas-filled void in molten solder which is compressed under an SMT lead. Seating plane stability is defined with a simple model. The lead standoffs of a component with an unstable seating plane are sensitive to small forces. For some SMT components, the reaction force generated by compressed voids in molten solder can tilt or rock a component, resulting in lead standoffs much greater than the measured lead coplanarity.

Volume variation of gas-filled voids in a Newtonian medium

The mechanism and kinetics of spontaneous volume variation of a gas-filled void in a Newtonian continuum are investigated. Kinetic equations are derived, describing the variation of the void radius in the case where the process rate is limited by diffusion of gas into the medium surrounding the void. The dissolution of air bubbles in water is studied experimentally. It is found that the kinetics of bubble dissolution is sensitive to the individual characteristics of the gaseous component in the mixture (partial pressure, solubility, and diffusivity in the matrix). The experimental results are discussed in juxtaposition with the analytical part.