Dissipative time-dependent quantum transport theory: quantum interference and phonon induced decoherence dynamics

A time-dependent inelastic electron transport theory for strong electron-phonon interaction is established via the equations of motion method combined with the small polaron transformation. In this work, the dissipation via electron-phonon coupling is taken into account in the strong coupling regime, which validates the small polaron transformation. The corresponding equations of motion are developed, which are used to study the quantum interference effect and phonon-induced decoherence dynamics in molecular junctions. Numerical studies show clearly quantum interference effect of the transport electrons through two quasi-degenerate states with different coupling to the leads. We also found that the quantum interference can be suppressed by the electron-phonon interaction where the phase coherence is destroyed by phonon scattering. This indicates the importance of electron-phonon interaction in systems with prominent quantum interference effect

A gauge-invariant and current-continuous microscopic ac quantum transport theory

There had been consensus on what the accurate ac quantum transport theory was until some recent works challenged the conventional wisdom. Basing on the non-equilibrium Green's function formalism for time-dependent quantum transport, we derive an expression for the dynamic admittance that satisfies gauge invariance and current continuity, and clarify the key concept in the field. The validity of our now formalism is verified by first-principles calculation of the transient current through a carbon-nanotube-based device under the time-dependent bias voltage. Moreover, the previously well-accepted expression for dynamic admittance is recovered only when the device is a perfect conductor at a specific potential

High fidelity mechanical and loss modelling of an interior permanent magnet traction machine for electrical vehicles

High-performance permanent magnet electrical machines have become the leading machine technology in the electric vehicle market due to their combination of high efficiency and high-power density. However, their design optimisation involves complex and often strongly coupled mechanical, electromagnetic and thermal behaviour. Of the many possible topologies of permanent magnet machines, interior (IPM) machines have become the favoured machine type as they offer advantages in field weakening, a contribution from reluctance torque and the ability to retain the magnets within the rotor core without the need in many cases for a separate containment sleeve. However, the trade-off between electromagnetic and mechanical performance is especially important in IPMs because of the use of thin bridge-sections within the rotor core. This thesis reports on detailed design study into the mechanical and electromagnetic optimisation of an 8-pole, 100kW IPM machine with a base speed of 4,000rpm and an extended speed range up to 12,000rpm and makes extensive use of structural and electromagnetic finite element analysis to identify a preferred design. The other aspect of IPM performance which is investigated in this thesis is the influence of high frequency converter switching on the iron loss in the machine. An analysis methodology is developed and applied to an IPM machine with combines a SIMULINK model with pre-calculated finite element characteristics of the machine to predict detailed localised element-by-element flux density variations in the cores of an IPM machine which includes realistic representation of switching events. The effect of current ripple and the grounding of the star-point is investigated. These high frequency flux density waveforms are then used as the basis for estimating the effect of high frequency current ripple on iron loss. This aspect includes a detailed investigation of the limitations of different analytical and numerical models for solving the diffusion equation with 3D eddy current finite element simulations providing a baseline against which to test various models. This aspect of the research results in a time-stepped finite difference representation of 1D eddy current flow in laminations and is applied at full machine level as post-processing tool. The thesis concludes with some experimental measurements of core loss with switching ripple which demonstrates the value of including lamination level eddy current effects in loss predictions

The manufacture and mechanical properties of a novel negative Poisson’s ratio 3-component composite

This paper was presented at the ICCM 20 Conference - 20th International Conference on Composite Materials in Copenhagen, 19-24 July 2015. Full conference proceedings are available via the link in this recordMaterials with a negative Poisson’s ratio known also as auxetic materials [1] exhibit unusual property of getting thicker when stretched and thinner when compressed. The helical auxetic yarn (HAY) is a recently invented auxetic reinforcing structure for composites [2]. A helical auxetic yarn (HAY) consists of two fibres: a low modulus elastomeric core and a high modulus wrap fibre in a double helix structure. When a tensile load is applied the core of the HAY becomes wider as the wrap straightens out, resulting in a lateral expansion of the core, and therefore a large negative Poisson’ ratio behaviour. The auxetic behaviour of the HAY can be tailored by altering fibre properties, the initial geometry and also the applied strain to comply with specific applications, such as composites [3, 4], blast mitigation, and filtration [5]. This paper introduces a further development to the current HAY by addition of a third component (a sheath). The presence of the sheath is expected to overcome problems such as slippage of the wrap and inconsistency in the initial wrap angle previously encountered during the manufacture of the HAY. The auxetic performance of conventional and novel systems is investigated and Poisson’s ratio data are presented.Engineering and Physical Sciences Research Council (EPSRC

The fabrication and mechanical properties of a novel 3-component auxetic structure for composites

Copyright © 2015 Elsevier. NOTICE: this is the author’s version of a work that was accepted for publication in Composites Science and Technology. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Composites Science and Technology (2015), DOI: 10.1016/j.compscitech.2015.06.012Functional auxetic composite materials can be fabricated from conventional or from auxetic components. The helical auxetic yarn (HAY) is a very recently invented auxetic reinforcing component for composite materials. This paper investigates the Poisson’s ratio behaviour of a further development of the HAY, needed for many practical applications. The 3-component auxetic yarn is based on a stiff wrap fibre (the first component) helically wound around an elastomeric core fibre (the second component) coated by a sheath (the third component). The resultant structure can overcome problems such as slippage of the wrap and changes in wrapping angles previously encountered during the manufacture and utilisation of the two-component HAY. The mechanical performance of conventional and novel systems is investigated; with emphasis on the differences between the engineering and true Poisson’s ratio. The importance of the utilisation of a true tensile modulus and a true Poisson’s ratio is demonstrated. This is the first time reported in the literature that an experimental auxetic effect analysis of HAYs was carried out by comparing true and engineering Poisson’s ratio. We show that depending on the coating thickness of the third component, the 3-component auxetic system can demonstrate auxetic behaviour, and the coating thickness can be employed as a new design parameter to tailor both the Poisson’s ratio and modulus of this novel composite reinforcement for a wide range of applications.Engineering and Physical Science Research Council (EPSRC

Utilizing VQ-VAE for End-to-End Health Indicator Generation in Predicting Rolling Bearing RUL

The prediction of the remaining useful life (RUL) of rolling bearings is a pivotal issue in industrial production. A crucial approach to tackling this issue involves transforming vibration signals into health indicators (HI) to aid model training. This paper presents an end-to-end HI construction method, vector quantised variational autoencoder (VQ-VAE), which addresses the need for dimensionality reduction of latent variables in traditional unsupervised learning methods such as autoencoder. Moreover, concerning the inadequacy of traditional statistical metrics in reflecting curve fluctuations accurately, two novel statistical metrics, mean absolute distance (MAD) and mean variance (MV), are introduced. These metrics accurately depict the fluctuation patterns in the curves, thereby indicating the model's accuracy in discerning similar features. On the PMH2012 dataset, methods employing VQ-VAE for label construction achieved lower values for MAD and MV. Furthermore, the ASTCN prediction model trained with VQ-VAE labels demonstrated commendable performance, attaining the lowest values for MAD and MV.Comment: 17 figure