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.

Measurement techniques for piezoelectric nanogenerators

Abstract: Electromechanical energy harvesting converts mechanical energy from the environment, such as vibration or human activity, into electrical energy that can be used to power a low power electronic device. Nanostructured piezoelectric energy harvesting devices, often termed nanogenerators, have rapidly increased in measured output over recent years. With these improvements nanogenerators have the potential to compete with more traditional micro- or macroscopic energy harvesting devices based on piezoelectric ceramics such as lead zirconate titanate (PZT), polymers such as polyvinylidene fluoride (PVDF) or electrostatic, electret or electromagnetic kinetic energy harvesters. Power output from a nanogenerator is most commonly measured through open-circuit voltage and/or short-circuit current, where power may be estimated from the product of these values. Here we show that such measures do not provide a complete picture of the output of these devices, and can be misleading when attempting to compare alternative designs. In order to compare the power output from a nanogenerator, techniques must be improved in line with those used for more established technologies. We compare ZnO nanorod/poly(methyl methacrylate) (PMMA) and ZnO nanorod/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) devices, and show that despite an open-circuit voltage nearly three times lower the ZnO/PEDOT:PSS device generates 150 times more power on an optimum load. In addition, it is shown that the peak voltage and current output can be increased by straining the device more rapidly and therefore time-averaged power, or time-integrated measures of output such as total energy or total charge should be calculated. Finally, the internal impedance of the devices is characterised to develop an understanding of their behaviour and shows a much higher internal resistance but lower capacitive impedance for the ZnO/PMMA device. It is hoped that by following more rigorous testing procedures the performance of nanostructured piezoelectric devices can be compared more realistically to other energy harvesting technologies and improvements can be rapidly driven by a more complete understanding of their behaviour.

A binary database for III–V compound semiconductor systems

Abstract: A thermodynamic database for binary III-V semiconductor systems has been compiled. It is based upon individual assessments which have been or will be published separately, but each assessment is based on the SGTE recommendation for the pure elements and uses the same solution model expression. The 15 possible binary systems between the group III elements Al, Ga and In on one hand, and the group V elements As, Sb and P are included.

A $^{13}$ and $^{31}$P Magnetic Double Resonance Study of Organo-Phosphorus Compounds

Abstract: Twenty-two organo-phosphorus compounds of a variety of structural types have been examined by $^1$H-{$^{13}$C} and $^1$H-{$^{31}$P} magnetic double resonance spectroscopy. The signs and magnitudes of the $^{31}$P-H and $^{31}$P-$^{13}$C spin-spin coupling constants are sensitive to the valency of the phosphorus atom, and the nature of the groups attached to it. Parallel behaviour is noted between two types of coupling constant. The $^{31}$P chemical shifts agree with results obtained by conventional $^{31}$P single resonance spectroscopy, and the $^{13}$C chemical shifts depend on the polarizability of the phosphorus atom and its associated groups.

Dicarboxylic acids and water‐soluble organic carbon in aerosols in New Delhi, India, in winter: Characteristics and formation processes

Abstract: [1] Day- and nighttime aerosol samples were collected at an urban site in New Delhi, India, in winter 2006–2007. They were studied for low molecular weight dicarboxylic acids and related compounds, as well as total water-soluble organic carbon (TWSOC). High concentrations of diacids (up to 6.03 μg m−3), TWSOC, and OC were obtained, which are substantially higher than those previously observed at other urban sites in Asia. Daytime TWSOC/OC ratio (37%) was on average higher than that in nighttime (25%). In particular, more water-soluble OC (M-WSOC) to TWSOC ratio in daytime (50%) was twice higher than in nighttime (27%), suggesting that aerosols in New Delhi are photochemically more processed in daytime to result in more water-soluble organic compounds. Oxalic acid (C2) was found as the most abundant dicarboxylic acid, followed by succinic (C4) and malonic (C3) acids. Contributions of C2 to M-WSOC were greater (av. 8%) in nighttime than daytime (av. 3%). Positive correlations of C2 with malic acid (hC4), glyoxylic acid (ωC2), and relative humidity suggest that secondary production of C2 probably in aqueous phase is important in nighttime via the oxidation of both longer-chain diacids and ωC2. C2 also showed a positive correlation with potassium (K+) in nighttime, suggesting that the enhanced C2 concentrations are associated with biomass/biofuel burning. More tight, positive correlation between less water-soluble OC (L-WSOC) and K+ was found in both day- and nighttime, suggesting that L-WSOC, characterized by longer chain and/or higher molecular weight compounds, is significantly influenced by primary emissions from biomass/biofuel burning.

Charge transfer between epitaxial graphene and silicon carbide

Abstract: We analyze doping of graphene grown on SiC in two models which differ by the source of charge transferred to graphene, namely, from SiC surface and from bulk donors. For each of the two models, we find the maximum electron density induced in monolayer and bilayer graphene, which is determined by the difference between the work function for electrons in pristine graphene and donor states on/in SiC, and analyze the responsivity of graphene to the density variation by means of electrostatic gates.