National Physical Laboratory

About: National Physical Laboratory is a facility organization based out in London, United Kingdom. It is known for research contribution in the topics: Dielectric & Thin film. The organization has 7615 authors who have published 13327 publications receiving 319381 citations.

Organic photovoltaic cells – promising indoor light harvesters for self-sustainable electronics

Abstract: Photovoltaic cells are attracting significant interest for harvesting indoor light for low power consumption wireless electronics such as those required for smart homes and offices, and the rapidly-growing Internet of Things. Here, we explore the potential of solution processable, small molecule photovoltaic cells as indoor power sources. By optimizing the solvent vapour annealing (SVA) time for the photovoltaic layer, a balance between its crystallization and phase separation is obtained, resulting in a record power conversion efficiency (PCE) of over 28% under fluorescent lamps of 1000 lux, generating a maximum power density of 78.2 μW cm−2 (>10% PCE under AM1.5G). This high indoor performance surpasses that of silicon based photovoltaic cells, and is similar to that of gallium arsenide photovoltaic cells. Besides, the ratios of the voltage at the maximum power point (MPP) to the open circuit voltage are similar from indoor lighting to one sun conditions, which is unique and allows a less power consuming method to track the MPP for a broad range of light intensities (potentially attractive for wearable photovoltaics). New insight into the effect of SVA on the indoor and one sun performance is provided using advanced optoelectronic characterization techniques, which show that the mobility-lifetime products as a function of charge carrier density can be correlated well with the performance at different light levels. Our results suggest that organic photovoltaic cells could be promising as indoor power sources for self-sustainable electronics.

A Lead-Free and High-Energy Density Ceramic for Energy Storage Applications

Abstract: In this work, we demonstrate a very high-energy density and high-temperature stability capacitor based on SrTiO3-substituted BiFeO3 thin films. An energy density of 18.6 J/cm 3 at 972 kV/cm is reported. The temperature coefficient of capacitance (TCC) was below 11% from room temperature up to 200°C. These results are of practical importance, because it puts forward a promising novel and environmentally friendly, lead-free material, for high-temperature applications in power electronics up to 200°C. Applications include capacitors for low carbon vehicles, renewable energy technologies, integrated circuits, and for the high-temperature aerospace sector.

Instabilities of a buoyancy-driven system

Abstract: A buoyancy-driven system can be unstable due to two different mechanisms—one mechanical and the other involving buoyancy forces. The mechanical instability is of the type normally studied in connexion with the Orr-Sommerfeld equation. The buoyancy-driven instability is rather different and is related to the ‘Coriolis’-driven instability of rotating fluids. In this paper, the stability of a buoyancy-driven system, recently called a ‘buoyancy layer’, is examined for the whole range of Prandtl numbers, s. The buoyancy-driven instability becomes increasingly important as the Prandtl number is increased and so particular interest is attached to the limit in which the Prandtl number tends to infinity. In this limit, the system is neutrally stable to first order, but second-order effects render the flow unstable at a Reynolds number of order σ-½. Consequences of the results for the stability of convection in a vertical slot are examined.

Review of meta-heuristics and generalised evolutionary walk algorithm

Abstract: Meta-heuristic algorithms are often nature-inspired, and they are becoming very powerful in solving global optimisation problems. More than a dozen major meta-heuristic algorithms have been developed over the last three decades, and there exist even more variants and hybrids of meta-heuristics. This paper intends to provide an overview of nature-inspired meta-heuristic algorithms, from a brief history to their applications. We try to analyse the main components of these algorithms and how and why they work. Then, we intend to provide a unified view of meta-heuristics by proposing a generalised evolutionary walk algorithm (GEWA). Finally, we discuss some of the important open questions.

Detection limits in XPS for more than 6000 binary systems using Al and Mg Kα X‐rays

Abstract: A simple approach to estimating the detection limits of XPS for any element in any elemental matrix is presented, using the intensity of the background at the expected position for the photoelectron peak to be detected. The approach has been extended to estimate the detection limit for all elements from lithium to bismuth in a similar range of elemental matrices. Using a number of assumptions, it is possible to obtain a reasonable estimate the background intensity at any electron kinetic energy in the XPS spectrum of an element. Therefore a detection limit for an arbitrary element in that matrix can be estimated. The results show that, although most elements are detectable at about the 1 at.% to 0.1 at.% level, for heavy elements in a light element matrix the detection limit can be better than 0.01 at.%, whereas for light elements in a heavy element matrix, detection limits above 10 at.% are not uncommon. Two charts detailing the detection limits for all combinations of trace and matrix elements from lithium (Z = 3) to bismuth (Z = 83) are provided for Al K and Mg K X-ray sources using a typical hemispherical analyser instrument which provides 106 counts eV for the Ag 3d5/2 peak from pure silver. These detection limits can be scaled to estimate the detection limits for any given instrument and operating conditions if the intensity of the Ag 3d5/2 peak from pure silver is known.