The Theory of Magnetic Tunnel Junctions

Within this work an investigation into the tunnelling magnetoresistance (TMR) will be presented. A base numerical model is developed to describe the tunnelling through a magnetic tunnel junction (MTJ) so that a simple analytic model can be compared. These models have been extended to the crystalline barrier MTJs. This numerical model was based upon an enhanced Wentzel-Kramers-Brillouin (EWKB) method to describe the tunnelling current density. By correctly considering realistic MTJ parameters, the key result was found to be the correct handling of the effective masses in of the three MTJ layers. The extracted barrier-heights of 3.5-4eV is much higher than found previously and closer to the half band-gap result expected. It is then clear that the correct treatment of the parameters produces a far more realistic result. The key parameter which can be extracted from the I-V characteristics is the product b m*d V , where m* is the effective mass of the barrier, d is the effective barrier thickness and Vb is the effective barrier height. The analytic solution is a transparent model in which the key material parameters are visible and simple enough to be applied by experimental researchers to MTJs. The accurate modelling of both the prefactor and exponent are crucial to estimating the TMR. A simplified analytic result was produced that is in good agreement with numerical and experimental results. The numerical and analytic model are then extended to describe the TMR through a crystalline Fe(001)/MgO(001)/Fe(001) trilayer system. The calculation is based on the free-electron-like numerical solution providing a functional dependence of the TMR. The results were found to be in excellent agreement with the ab initio models and experiment. Furthermore a simplified analytic expression shows the TMR is dependent on the band-widths of the tunnelling electron states, the coupling and the thickness of the barrier. These models will be of great benefit to both experimental and theoretical researchers

An update of the UK’s Design Summer Years Probabilistic Design Summer Years for Enhanced Overheating Risk Analysis in Building Design

Overheating is increasingly becoming a key issue for building design across the world. In the UK better building fabric performance and warmer weather can increase the risk of overheating events in badly designed buildings. The impacts of these overheating events could be reduced by adapting building designs at an early design stage using building thermal models using appropriate weather data such as a design summer year. In this work a method to determine probabilistic Design Summer Years will be presented. These years take into account the return periods of actual events, are presented within a probabilistic framework and are therefore include a description of the severity of the year at each location.Engineering and Physical Sciences Research Council (EPSRC)CIBS

An update of the UK’s test reference year: The implications of a revised climate on building design

Copyright © 2015 SAGE PublicationsAverage weather years have been used around the world for testing buildings to ascertain their likely energy use using thermal modelling software. In the UK, the Test Reference Years which are in current use were released in 2006 but generally consisted of data from 1983 to 2004. In this work, revised test reference years will be proposed which are based on a new climatic period from 1984 to 2013. The differences between the two years will be highlighted and the implications for building design will be discussed.Engineering and Physical Sciences Research Council (EPSRC)CIBS

Generating near-extreme Summer Reference Years (SRY) for building performance simulation

Copyright © 2015 by SAGE PublicationsAt present there is no universally accepted method for deriving near-extreme summer weather data for building performance simulation. Existing datasets such as the Design Summer Years (DSY) used in the United Kingdom (UK) to estimate summer discomfort in naturally ventilated and free running buildings have been criticised for being inconsistent with the corresponding Test Reference Years (TRY). This paper proposes a method for generating Summer Reference Years (SRY) by adjusting the TRY of a given site with meteorological data in order to represent near-extreme conditions. It takes as the starting point that the TRY is robust, being determined on a monthly basis from the most typical months. Initial simulations for the 14 UK TRY locations show promising results for determining building overheating with the SRY

Lumped parameter models for building thermal modelling: an analytic approach to simplifying complex multi-layered constructions

PublishedJournal ArticleThere are many sophisticated building simulators capable of accurately modelling the thermal performance of buildings. Lumped Parameter Models (LPMs) are an alternative which, due to their shorter computational time, can be used where many runs are needed, for example when completing computer-based optimisation. In this paper, a new, more accurate, analytic method is presented for creating the parameters of a second order LPM, consisting of three resistors and two capacitors, that can be used to represent multi-layered constructions. The method to create this LPM is more intuitive than the alternatives in the literature and has been named the Dominant Layer Model. This new method does not require complex numerical operations, but is obtained using a simple analysis of the relative influence of the different layers within a construction on its overall dynamic behaviour. The method has been used to compare the dynamic response of four different typical constructions of varying thickness and materials as well as two more complex constructions as a proof of concept. When compared with a model that truthfully represents all layers in the construction, the new method is largely accurate and outperforms the only other model in the literature obtained with an analytical method. © 2013 Elsevier B.V

Estimation of the urban heat island for UK climate change projections

Copyright © 2010 by SAGE PublicationsCities are known to exert a significant influence on their local climate, and are generally warmer than their surroundings. However, climate models generally do not include a representation of urban areas, and so climate projections from models are likely to underestimate temperatures in urban areas. A simple methodology has been developed to calculate the urban heat island (UHI) from a set of gridded temperature data; the UHI may then be added to climate model projections and weather data files. This methodology allows the UHI to be calculated on a monthly basis and downscaled to hourly for addition to weather generator data. The UHI intensities produced are found to be consistent with observed data.Practical application: There is overwhelming consensus amongst the scientific community that the Earth’s climate is warming. In addition to the effects of climate change the urban heat island (UHI) effect can increase air temperatures significantly in urban areas above those of the rural areas around them. The proposed methodology for calculating the UHI from a set of gridded temperature data allows the UHI to be added to climate model projections such as UKCP09 or HadRM3 and weather data files. The methodology also allows for the temporal downscaling of the UHI from monthly values to hourly data for use in building thermal simulation software

Navigating data governance associated with real-world data for public benefit: an overview in the UK and future considerations

Real-world data encompass data primarily captured for the provision or operation of services, for example, electronic health records for direct care purposes, but which may have secondary uses for informing research or commissioning. Public benefit is potentially forfeited by the underutilisation of real-world data for secondary uses, in part due to risk aversion when faced with the prospect of navigating necessary and important data governance processes. Such processes can be perceived as complex, daunting, time-consuming and exposing organisations to risk. By providing an overview description and discussion around the role of six key legal and information governance frameworks and their role regarding responsible data access, linkage and sharing, our intention is to make data governance a less daunting prospect and reduce the perception that it is a barrier to secondary uses, thus enabling public benefit

Modelling the impact of local reactive school closures on critical care provision during an influenza pandemic

Despite the fact that the 2009 H1N1 pandemic influenza strain was less severe than had been feared, both seasonal epidemics of influenza-like-illness and future influenza pandemics have the potential to place a serious burden on health services. The closure of schools has been postulated as a means of reducing transmission between children and hence reducing the number of cases at the peak of an epidemic; this is supported by the marked reduction in cases during school holidays observed across the world during the 2009 pandemic. However, a national policy of long-duration school closures could have severe economic costs. Reactive short-duration closure of schools in regions where health services are close to capacity offers a potential compromise, but it is unclear over what spatial scale and time frame closures would need to be made to be effective. Here, using detailed geographical information for England, we assess how localized school closures could alleviate the burden on hospital intensive care units (ICUs) that are reaching capacity. We show that, for a range of epidemiologically plausible assumptions, considerable local coordination of school closures is needed to achieve a substantial reduction in the number of hospitals where capacity is exceeded at the peak of the epidemic. The heterogeneity in demand per hospital ICU bed means that even widespread school closures are unlikely to have an impact on whether demand will exceed capacity for many hospitals. These results support the UK decision not to use localized school closures as a control mechanism, but have far wider international public-health implications. The spatial heterogeneities in both population density and hospital capacity that give rise to our results exist in many developed countries, while our model assumptions are sufficiently general to cover a wide range of pathogens. This leads us to believe that when a pandemic has severe implications for ICU capacity, only widespread school closures (with their associated costs and organizational challenges) are sufficient to mitigate the burden on the worst-affected hospitals

An Analytical Approach to the Impact of Heat Waves on Buildings and Their Occupants

This is the author accepted manuscript. The final version is available from ASHRAE via the link in this record.Extreme weather events are expected to become more intense and more frequent, and lead to increases in heat related mortality. Unfortunately, there is no set of agreed heat wave time series to test new buildings or complete building stocks against. In this work, we attempt to solve this by finding analytical relationships between heat waves and increases in internal temperature for 25 monitored dwellings. The result is a methodology that allows the forecasting of the effect of extreme events on buildings with almost no computational effort. Extrapolating the results to the whole UK domestic stock indicates that, for example, a heat wave of three days with a maximum amplitude of 5 kelvin above the situation prior to the heat wave will result on an increment in internal temperature of 1.5 kelvin or more for 43.3% of dwellings and a rise of 2 kelvin or more in 3.5% of the dwellings by the third day of the heat wave