The Machining of Polymers

Machining is a non-linear process and involves the consideration of variables such as inelastic deformation, high temperatures and contact conditions. This thesis focuses on investigating the contact conditions between cutting tools and polymeric materials during orthogonal cutting. A test rig has been developed to allow cuts to be taken from a rectangular workpiece, such that the rake angle (the angle of inclination of the tool surface) can be varied. In this work the rake angle was varied from -20o to 30o, and the cutting was performed at rates of 0.01ms-1 and 0.1ms-1 on PMMA, nylon 4/6 and nylon 6/12. As part of a round robin investigation, tests were also performed on HIPS, ABS and LLDPE. The experimental method developed required the measurement of forces in two directions (the direction of cutting, Fc and transverse to the direction of cutting, Ft). The rig allowed for the careful control of the depth of cut, h. After each cut, the thickness of the off-cut chip hc was also measured. A series of cuts were taken at depths varying between the range of 0.02mm to 0.25mm and the forces were measured. A cutting theory has been applied to the experimental data to determine the fracture toughness Gc, and the yield stress σY of the material. The Coulomb friction μ and an adhesion term, Ga representing sticking at the tool-chip interface, were also deduced. Independent fracture mechanics tests were performed at a range of temperatures and rates on the different polymers. Tensile tests were also performed, to compare standard values to the material parameters determined in cutting. The values of Gc and σY deduced were independent of the rake angle however, μ and Ga were not. The calculated values of Gc were typically within 5% of the standard values however, σY was found to be up to 5 times higher. The existence of work hardening is believed to be the cause of these elevated values. The cutting analysis was also applied to some previously published metal cutting data and produced constant values of Gc

Interconnection Networks for Scalable Quantum Computers

We show that the problem of communication in a quantum computer reduces to constructing reliable quantum channels by distributing high-fidelity EPR pairs. We develop analytical models of the latency, bandwidth, error rate and resource utilization of such channels, and show that 100s of qubits must be distributed to accommodate a single data communication. Next, we show that a grid of teleportation nodes forms a good substrate on which to distribute EPR pairs. We also explore the control requirements for such a network. Finally, we propose a specific routing architecture and simulate the communication patterns of the Quantum Fourier Transform to demonstrate the impact of resource contention.Comment: To appear in International Symposium on Computer Architecture 2006 (ISCA 2006

A system for determining Li-ion cell cooling coefficients

Current battery data sheets focus on battery energy and power density, neglecting thermal performance. This leads to reduced system level efficiency since cells with poor thermal performance require larger, heavier cooling systems to maintain cell temperatures in a suitable range. To address this a new metric, the Cell Cooling Coefficient (CCC), has been developed and it’s use as a tool for appropriate cell selection has been demonstrated. It also allows the pack designer to calculate which cooling direction method is most suitable by comparing CCC values for tab and surface cooling. The metric is the ratio between the heat rejected from the cell and the temperature difference between the hottest and coolest point. It therefore has units WK−1 and allows a pack designer to easily calculate the required amount of cooling power for the cell given a maximum acceptable temperature rise. In this paper we describe a system and method for the accurate determination of the CCC with the aim of facilitating wider adoption of the metric. The system is able to reliably quantify the surface and tab cooling CCC of any pouch cell

Novel Methods for Measuring the Thermal Diffusivity and the Thermal Conductivity of a Lithium-Ion Battery

Thermal conductivity is a fundamental parameter in every battery pack model. It allows for the calculation of internal temperature gradients which affect cell safety and cell degradation. The accuracy of the measurement for thermal conductivity is directly proportional to the accuracy of any thermal calculation. Currently the battery industry uses archaic methods for measuring this property which have errors up to 50 %. This includes the constituent material approach, the Searle’s bar method, laser/Xeon flash and the transient plane source method. In this paper we detail three novel methods for measuring both the thermal conductivity and the thermal diffusivity to within 5.6 %. These have been specifically designed for bodies like lithium-ion batteries which are encased in a thermally conductive material. The novelty in these methods comes from maintaining a symmetrical thermal boundary condition about the middle of the cell. By using symmetric boundary conditions, the thermal pathway around the cell casing can be significantly reduced, leading to improved measurement accuracy. These novel methods represent the future for thermal characterisation of lithium-ion batteries. Continuing to use flawed measurement methods will only diminish the performance of battery packs and slow the rate of decarbonisation in the transport sector

Lithium-ion battery second life:pathways, challenges and outlook

Net zero targets have resulted in a drive to decarbonise the transport sector worldwide through electrification. This has, in turn, led to an exponentially growing battery market and, conversely, increasing attention on how we can reduce the environmental impact of batteries and promote a more efficient circular economy to achieve real net zero. As these batteries reach the end of their first life, challenges arise as to how to collect and process them, in order to maximise their economical use before finally being recycled. Despite the growing body of work around this topic, the decision-making process on which pathways batteries could take is not yet well understood, and clear policies and standards to support implementation of processes and infrastructure are still lacking. Requirements and challenges behind recycling and second life applications are complex and continue being defined in industry and academia. Both pathways rely on cell collection, selection and processing, and are confronted with the complexities of pack disassembly, as well as a diversity of cell chemistries, state-of-health, size, and form factor. There are several opportunities to address these barriers, such as standardisation of battery design and reviewing the criteria for a battery’s end-of-life. These revisions could potentially improve the overall sustainability of batteries, but may require policies to drive such transformation across the industry. The influence of policies in triggering a pattern of behaviour that favours one pathway over another are examined and suggestions are made for policy amendments that could support a second life pipeline, while encouraging the development of an efficient recycling industry. This review explains the different pathways that end-of-life EV batteries could follow, either immediate recycling or service in one of a variety of second life applications, before eventual recycling. The challenges and barriers to each pathway are discussed, taking into account their relative environmental and economic feasibility and competing advantages and disadvantages of each. The review identifies key areas where processes need to be simplified and decision criteria clearly defined, so that optimal pathways can be rapidly determined for each end-of-life battery

Isothermal temperature control for battery testing and battery model parameterization

The hybrid/ electric vehicle (H/EV) market is very dependent on battery models. Battery models inform cell and battery pack design, critical in online battery management systems and can be used as predictive tools to maximise the lifetime of a battery pack. Battery models require parameterization, through experimentation. Temperature affects every aspect of a battery’s operation and must therefore be closely controlled throughout all battery experiments. Today, the private-sector prefers climate chambers for experimental thermal control. However, evidence suggests that climate chambers are unable to adequately control the surface temperature of a battery under test. In this study, laboratory apparatus is introduced that controls the temperature of any exposed surface of a battery through conduction. Pulse discharge tests, temperature step change tests and driving cycle tests are used to compare the performance of this conductive temperature control apparatus (CTCA) against a climate chamber across a range of scenarios. The CTCA outperforms the climate chamber in all tests. In CTCA testing, the rate of heat removal from the cell is increased by two orders of magnitude. The CTCA eliminates error due to cell surface temperature rise, which is inherent to climate chamber testing due to insufficient heat removal rates from a cell under test. The CTCA can reduce the time taken to conduct entropic parameterization of a cell by almost 10 days, a 70% reduction in the presented case. Presently, the H/EV industry’s reliance on climate chambers is impacting the accuracy of all battery models. The industry must move away from the flawed concept of convective cooling during battery parameterization