Showing papers by "National Physical Laboratory published in 2014"

Piezoelectric and ferroelectric materials and structures for energy harvesting applications

Abstract: This review provides a detailed overview of the energy harvesting technologies associated with piezoelectric materials along with the closely related sub-classes of pyroelectrics and ferroelectrics. These properties are, in many cases, present in the same material, providing the intriguing prospect of a material that can harvest energy from multiple sources including vibration, thermal fluctuations and light. Piezoelectric materials are initially discussed in the context of harvesting mechanical energy from vibrations using inertial energy harvesting, which relies on the resistance of a mass to acceleration, and kinematic energy harvesting which directly couples the energy harvester to the relative movement of different parts of a source. Issues related to mode of operation, loss mechanisms and using non-linearity to enhance the operating frequency range are described along with the potential materials that could be employed for harvesting vibrations at elevated temperatures. In addition to inorganic piezoelectric materials, compliant piezoelectric materials are also discussed. Piezoelectric energy harvesting devices are complex multi-physics systems requiring advanced methodologies to maximise their performance. The research effort to develop optimisation methods for complex piezoelectric energy harvesters is then reviewed. The use of ferroelectric or multi-ferroic materials to convert light into chemical or electrical energy is then described in applications where the internal electric field can prevent electron–hole recombination or enhance chemical reactions at the ferroelectric surface. Finally, pyroelectric harvesting generates power from temperature fluctuations and this review covers the modes of pyroelectric harvesting such as simple resistive loading and Olsen cycles. Nano-scale pyroelectric systems and novel micro-electro-mechanical-systems designed to increase the operating frequency are discussed.

Frequency ratio of two optical clock transitions in 171Yb+ and constraints on the time variation of fundamental constants.

Abstract: Singly ionized ytterbium, with ultranarrow optical clock transitions at 467 and 436 nm, is a convenient system for the realization of optical atomic clocks and tests of present-day variation of fundamental constants. We present the first direct measurement of the frequency ratio of these two clock transitions, without reference to a cesium primary standard, and using the same single ion of 171Yb+. The absolute frequencies of both transitions are also presented, each with a relative standard uncertainty of 6 × 10 -16. Combining our results with those from other experiments, we report a threefold improvement in the constraint on the time variation of the proton-to-electron mass ratio, mu-dot/mu = 0.2(1.1) × 10-16 yr-1, along with an improved constraint on time variation of the fine structure constant, alpha-dot/alpha = -0.7(2.1) x 10-17 yr-1.

Early warning signals of ecological transitions: methods for spatial patterns.

Abstract: A number of ecosystems can exhibit abrupt shifts between alternative stable states. Because of their important ecological and economic consequences, recent research has focused on devising early warning signals for anticipating such abrupt ecological transitions. In particular, theoretical studies show that changes in spatial characteristics of the system could provide early warnings of approaching transitions. However, the empirical validation of these indicators lag behind their theoretical developments. Here, we summarize a range of currently available spatial early warning signals, suggest potential null models to interpret their trends, and apply them to three simulated spatial data sets of systems undergoing an abrupt transition. In addition to providing a step-by-step methodology for applying these signals to spatial data sets, we propose a statistical toolbox that may be used to help detect approaching transitions in a wide range of spatial data. We hope that our methodology together with the computer codes will stimulate the application and testing of spatial early warning signals on real spatial data.

Effects of crop residue burning on aerosol properties, plume characteristics, and long-range transport over northern India

Abstract: Aerosol emissions from biomass burning are of specific interest over the globe due to their strong radiative impacts and climate implications. The present study examines the impact of paddy crop residue burning over northern India during the postmonsoon (October–November) season of 2012 on modification of aerosol properties, as well as the long-range transport of smoke plumes, altitude characteristics, and affected areas via the synergy of ground-based measurements and satellite observations. During this period, Moderate Resolution Imaging Spectroradiometer (MODIS) images show a thick smoke/hazy aerosol layer below 2–2.5 km in the atmosphere covering nearly the whole Indo-Gangetic Plains (IGP). The air mass trajectories originating from the biomass-burning source region over Punjab at 500 m reveal a potential aerosol transport pathway along the Ganges valley from west to east, resulting in a strong aerosol optical depth (AOD) gradient. Sometimes, depending upon the wind direction and meteorological conditions, the plumes also influence central India, the Arabian Sea, and the Bay of Bengal, thus contributing to Asian pollution outflow. The increased number of fire counts (Terra and Aqua MODIS data) is associated with severe aerosol-laden atmospheres (AOD500 nm > 1.0) over six IGP locations, high values of Angstrom exponent (>1.2), high particulate mass 2.5 (PM2.5) concentrations (>100–150 µgm−3), and enhanced Ozone Monitoring Instrument Aerosol Index gradient (~2.5) and NO2 concentrations (~6 × 1015 mol/cm2), indicating the dominance of smoke aerosols from agricultural crop residue burning. The aerosol size distribution is shifted toward the fine-mode fraction, also exhibiting an increase in the radius of fine aerosols due to coagulation processes in a highly turbid environment. The spectral variation of the single-scattering albedo reveals enhanced dominance of moderately absorbing aerosols, while the aerosol properties, modification, and mixing atmospheric processes differentiate along the IGP sites depending on the distance from the aerosol source, urban influence, and local characteristics.

Acoustic gas thermometry

Abstract: We review the principles, techniques and results from primary acoustic gas thermometry (AGT). Since the establishment of ITS-90, the International Temperature Scale of 1990, spherical and quasi-spherical cavity resonators have been used to realize primary AGT in the temperature range 7 K to 552 K. Throughout the sub-range 90 K < T < 384 K, at least two laboratories measured (T − T90). (Here T is the thermodynamic temperature and T90 is the temperature on ITS-90.) With a minor exception, the resulting values of (T − T90) are mutually consistent within 3 × 10−6 T. These consistent measurements were obtained using helium and argon as thermometric gases inside cavities that had radii ranging from 40 mm to 90 mm and that had walls made of copper or aluminium or stainless steel. The AGT values of (T − T90) fall on a smooth curve that is outside ±u(T90), the estimated uncertainty of T90. Thus, the AGT results imply that ITS-90 has errors that could be reduced in a future temperature scale. Recently developed techniques imply that low-uncertainty AGT can be realized at temperatures up to 1350 K or higher and also at temperatures in the liquid-helium range.

Fundamentals of Measurement in Terahertz Time-Domain Spectroscopy

Abstract: Terahertz time-domain spectroscopy (THz-TDS) has emerged as a main spectroscopic modality to fill the frequency range between a few hundred gigahertz to a few terahertz. This spectrum has been known as “terahertz gap” owing to limited accessibility by conventional electronic and optical techniques. Over the past two decades, THz-TDS has evolved substantially with enhanced compactness and stability. Since THz-TDS is becoming an industrial standard, the performance and precision of the system are of prime importance. This article provides an overview on terahertz metrology, including parameter estimation, signal processing, measurement characteristics, uncertainties, and calibrations. The overview serves as guidance for metrology and further developments of THz-TDS systems.

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.

MWCNT-conducting polymer composite based ammonia gas sensors: A new approach for complete recovery process

Abstract: Highly sensitive ammonia gas sensors were fabricated using Multi-walled Carbon Nanotubes (MWCNTs) reinforced electrically conducting polymer composites following solution casting method. Two types of conducting polymers like poly(3,4-ethylenedioxythiophene)–polystyrene sulfonic acid (PEDOT:PSS) and polyaniline (PANI) were used and compared for their ammonia gas sensing properties at room temperature (RT). Both the sensors were found to exhibit excellent sensitivity and poor recovery for ammonia gas at room temperature, but as compared to PANI, PEDOT:PSS polymer composite was found to be more sensitive (with sensitivity of ∼16%) with less response time (∼15 min). In addition, thermal behavior of both the composites was investigated in detail, where MWCNT–PEDOT:PSS composite showed significantly better thermal stability than MWCNT–PANI composite. Sensor recovery posed a great problem at room temperature and a new approach is proposed to get complete recovery, exclusively tested in polymer–CNT composite sensor, to lessen the heat dependence and improve the cycling behavior. A trial experiment is conducted in combination of heat and DC electric field to optimize the complete recovery of the MWCNT–PEDOT:PSS composite based sensor where the recovery time was reduced from 48 h to 20 min. It is believed that such stimulation process provides sufficient energy to desorb chemisorbed ammonia from CNT surface completely.

Evidence for interacting two-level systems from the 1/ f noise of a superconducting resonator

Abstract: The quantum noise generated as multiple two-level systems switch state is usually described by the standard tunnelling model. By studying superconducting resonators, Burnett et al. show that this model fails at low temperatures, and propose a new model to accurately describe the noise in quantum circuits.

Detector to detector corrections: a comprehensive experimental study of detector specific correction factors for beam output measurements for small radiotherapy beams.

Abstract: Purpose: The aim of the present study is to provide a comprehensive set of detector specific correction factors for beam output measurements for small beams, for a wide range of real time and passive detectors. The detector specific correction factors determined in this study may be potentially useful as a reference data set for small beam dosimetry measurements. Methods: Dose response of passive and real time detectors was investigated for small field sizes shaped with a micromultileaf collimator ranging from 0.6 × 0.6 cm2 to 4.2 × 4.2 cm2 and the measurements were extended to larger fields of up to 10 × 10 cm2. Measurements were performed at 5 cm depth, in a 6 MV photon beam. Detectors used included alanine, thermoluminescent dosimeters (TLDs), stereotactic diode, electron diode, photon diode, radiophotoluminescent dosimeters (RPLDs), radioluminescence detector based on carbon-doped aluminium oxide (Al2O3:C), organic plastic scintillators, diamond detectors, liquid filled ion chamber, and a range of small volume air filled ionization chambers (volumes ranging from 0.002 cm3 to 0.3 cm3). All detector measurements were corrected for volume averaging effect and compared with dose ratios determined from alanine to derive a detector correction factors that account for beam perturbation related to nonwater equivalence of the detector materials. Results: For the detectors used in this study, volume averaging corrections ranged from unity for the smallest detectors such as the diodes, 1.148 for the 0.14 cm3 air filled ionization chamber and were as high as 1.924 for the 0.3 cm3 ionization chamber. After applying volume averaging corrections, the detector readings were consistent among themselves and with alanine measurements for several small detectors but they differed for larger detectors, in particular for some small ionization chambers with volumes larger than 0.1 cm3. Conclusions: The results demonstrate how important it is for the appropriate corrections to be applied to give consistent and accurate measurements for a range of detectors in small beam geometry. The results further demonstrate that depending on the choice of detectors, there is a potential for large errors when effects such as volume averaging, perturbation and differences in material properties of detectors are not taken into account. As the commissioning of small fields for clinical treatment has to rely on accurate dose measurements, the authors recommend the use of detectors that require relatively little correction, such as unshielded diodes, diamond detectors or microchambers, and solid state detectors such as alanine, TLD, Al2O3:C, or scintillators.

Highly sensitive biofunctionalized mesoporous electrospun TiO(2) nanofiber based interface for biosensing.

Abstract: The surface modified and aligned mesoporous anatase titania nanofiber mats (TiO2-NF) have been fabricated by electrospinning for esterified cholesterol detection by electrochemical technique. The electrospinning and porosity of mesoporous TiO2-NF were controlled by use of polyvinylpyrrolidone (PVP) as a sacrificial carrier polymer in the titanium isopropoxide precursor. The mesoporous TiO2-NF of diameters ranging from 30 to 60 nm were obtained by calcination at 470 °C and partially aligned on a rotating drum collector. The functional groups such as -COOH, -CHO etc. were introduced on TiO2-NF surface via oxygen plasma treatment making the surface hydrophilic. Cholesterol esterase (ChEt) and cholesterol oxidase (ChOx) were covalently immobilized on the plasma treated surface of NF (cTiO2-NF) via N-ethyl-N0-(3-dimethylaminopropyl carbodiimide) and N-hydroxysuccinimide (EDC-NHS) chemistry. The high mesoporosity (∼61%) of the fibrous film allowed enhanced loading of the enzyme molecules in the TiO2-NF mat. The ChEt-ChOx/cTiO2-NF-based bioelectrode was used to detect esterified cholesterol using electrochemical technique. The high aspect ratio, surface area of aligned TiO2-NF showed excellent voltammetric and catalytic response resulting in improved detection limit (0.49 mM). The results of response studies of this biosensor show excellent sensitivity (181.6 μA/mg dL(-1)/cm(2)) and rapid detection (20 s). This proposed strategy of biomolecule detection is thus a promising platform for the development of miniaturized device for biosensing applications.

HadISDH land surface multi-variable humidity and temperature record for climate monitoring

Abstract: . HadISDH.2.0.0 is the first gridded, multi-variable humidity and temperature in situ observations-only climate-data product that is homogenised and annually updated. It provides physically consistent estimates for specific humidity, vapour pressure, relative humidity, dew point temperature, wet bulb temperature, dew point depression and temperature. It is a monthly mean gridded (5° by 5°) product with uncertainty estimates that account for spatio-temporal sampling, climatology calculation, homogenisation and irreducible random measurement effects. It provides a tool for the long-term monitoring of a variety of humidity-related variables which have different impacts and implications for society. It is also useful for climate model evaluation and reanalyses validation. HadISDH.2.0.0 is shown to be in good agreement both with other estimates and with theoretical understanding. The data set is available from 1973 to the present. The theme common to all variables is of a warming world with more water vapour present in the atmosphere. The largest increases in water vapour are found over the tropics and the Mediterranean. Over the tropics and high northern latitudes the surface air over land is becoming more saturated. However, despite increasing water vapour over the mid-latitudes and Mediterranean, the surface air over land is becoming less saturated. These observed features may be due to atmospheric circulation changes, land–sea warming disparities and reduced water availability or changed land surface properties.

Flat clathrin lattices: stable features of the plasma membrane

Abstract: Endocytosis via clathrin-coated pits is a well-understood process; however, clathrin also assembles into large, flat clathrin lattices (FCLs), which remain poorly described. Quantitative electron, ...

Single-molecule reconstruction of oligonucleotide secondary structure by atomic force microscopy.

Abstract: Based on soft-touch atomic force microscopy, a method is described to reconstruct the secondary structure of single extended biomolecules, without the need for crystallization. The method is tested by accurately reproducing the dimensions of the B-DNA crystal structure. Importantly, intramolecular variations in groove depth of the DNA double helix are resolved, which would be inaccessible for methods that rely on ensemble-averaging.

Advances in gold nanoparticle–liquid crystal composites

Abstract: We present the advancement in the research of the dispersion of gold nanoparticles (GNPs) in thermotropic calamitic liquid crystals. The formation/behavior of surface plasmon resonance (SPR) in GNPs is briefly described. The uniform dispersion of GNPs into liquid crystals along with two important aspects, i.e. tuning of GNP properties by liquid crystal and vice versa, are widely discussed. Overall, the article highlights the advances in the research into GNP-liquid crystal composites in terms of their scientific and technological aspects.

Tip‐enhanced Raman spectroscopy – an interlaboratory reproducibility and comparison study

Abstract: Stiftung Stipendien-Fonds des Verbandes der chemischen Industrie e.V.; German National Academic Foundation; Bruker; National Science Foundation (NSF CAREER) [CHE 0748226]; MOST of China [2011YQ03012406]; NSFC [21021120456]; EMRP

A simple, low-cost CVD route to thin films of BiFeO3 for efficient water photo-oxidation

Abstract: A novel method for preparation of BiFeO3 films via a simple solution-based CVD method is reported using for the first time a single-source heterobimetallic precursor [CpFe(CO)2BiCl2]. BiFeO3 films display ferroelectric and ferromagnetic ordering at room temperature and possess direct band-gaps between 2.0 and 2.2 eV. Photocatalytic testing for water oxidation revealed high activities under UVA (365 nm) and simulated solar irradiation, superior to that exhibited by a commercial standard (Pilkington Activ® TiO2 film) resulting in an apparent quantum yield of ∼24%.

Multi-channel absolute distance measurement system with sub ppm-accuracy and 20 m range using frequency scanning interferometry and gas absorption cells

Abstract: We present an implementation of an absolute distance measurement system which uses frequency scanning interferometry (FSI). The technique, referred to as dynamic FSI, uses two frequency scanning lasers, a gas absorption cell and a reference interferometer to determine the unknown optical path length difference (OPD) of one or many measurement interferometers. The gas absorption cell is the length reference for the measurement system and is traceable to international standards through knowledge of the frequencies of its absorption features. The OPD of the measurement interferometers can vary during the measurement and the variation is measured at the sampling rate of the system (2.77 MHz in the system described here). The system is shown to measure distances from 0.2 m to 20 m with a combined relative uncertainty of 0.41 × 10⁻⁶ at the two sigma level (k = 2). It will be shown that within a scan the change in OPD of the measurement interferometer can be determined to a resolution of 40 nm.

Parameter Sensitivity Analysis of Cylindrical LiFePO4 Battery Performance Using Multi-Physics Modeling

Abstract: A multi-physics model for a cylindrical Li-ion battery has been developed by coupling a model of the thermal distribution in the radial direction to an electrochemical P2D model. The model can predict both terminal voltage and surface temperature, which has the advantage that it can be readily validated by measurement. A sensitivity analysis of up to 30 parameters was carried out using model simulation. A parameter sensitivity matrix was established to describe the parameter sensitivity under different operating conditions and clustering theory used to group the parameters according to their average sensitivity. Finally, a stepwise experiment was designed to validate the results of the sensitivity analysis. It was shown that the stepwise approach to parameter identification results in significantly higher accuracy.

Dibromocarbene Functionalization of Boron Nitride Nanosheets: Toward Band Gap Manipulation and Nanocomposite Applications

Abstract: We report the covalent functionalization of exfoliated boron nitride nanosheets (BNNSs) using dibromocarbene (DBC) species. The functionalization of BNNSs is enabled as the nanosheets are utilized as two-dimensional phase-transfer catalysts for the migration of carbenes across the organic–aqueous phase boundary. We postulate that BNNSs stabilize carbenes by forming B-CBr2 ylides and in turn act as the reaction substrate. DBC functionalization of BNNSs results in the formation of B–C and B–N bonds to the sp2-hybridized BNNS lattice via the formation of dibromo-bridged bicyclo BCN systems. The covalent functionalization was characterized using HR-TEM, AFM, EELS, XRD, EDX, ToF-SIMS, TGA, Raman, XPS, FTIR, and UV–vis techniques. Utilization of CBr2 groups as a means by which BNNSs may be integrated and interfaced with solvents, molecular species, and condensed-phase materials was demonstrated by grafting alkyl chains from the functional groups via alkyl/halogen exchange. Alkyl-functionalized BNNSs were integr...

Agreement between two 88 Sr + optical clocks to 4 parts in 10 17

Abstract: The frequencies of two nominally identical ${}^{88}{\mathrm{Sr}}^{+}$ trapped single ion optical clocks, based on the 674 nm 5$s$ ${}^{2}$${S}_{1/2}$--4$d$ ${}^{2}$${D}_{5/2}$ electric quadrupole clock transition, have been compared over a period of nine months. The frequencies of the two clocks were found to agree within a total uncertainty of 4 \ifmmode\times\else\texttimes\fi{} 10${}^{\ensuremath{-}17}$, demonstrating that the individual ${}^{88}{\mathrm{Sr}}^{+}$ optical clocks are reproducible at the 3 \ifmmode\times\else\texttimes\fi{} 10${}^{\ensuremath{-}17}$ level. The absolute frequency of the clock transition was measured to be $f$ = 444 779 044 095 486.71(24) Hz using an optical frequency comb referenced to a cesium fountain primary frequency standard. The standard uncertainty of 0.24 Hz (5.3 \ifmmode\times\else\texttimes\fi{} 10${}^{\ensuremath{-}16}$ of the optical frequency) is dominated by measurement statistics and cesium fountain systematics and is around four times lower than previously published.

Microwave open-ended coaxial dielectric probe: interpretation of the sensing volume re-visited

Abstract: Tissue dielectric properties are specific to physiological changes and consequently have been pursued as imaging biomarkers of cancer and other pathological disorders. However, a recent study (Phys Med Biol 52:2637–2656, 2007; Phys Med Biol 52:6093–6115, 2007), which utilized open-ended dielectric probing techniques and a previously established sensing volume, reported that the dielectric property contrast may only be 10% or less between breast cancer and normal fibroglandular tissue whereas earlier data suggested ratios of 4:1 and higher may exist. Questions about the sensing volume of this probe relative to the amount of tissue interrogated raise the distinct possibility that the conclusions drawn from that study may have been over interpreted. We performed open-ended dielectric probe measurements in two-layer compositions consisting of a background liquid and a planar piece of Teflon that was translated to predetermined distances away from the probe tip to assess the degree to which the probe produced property estimates representative of the compositional averages of the dielectric properties of the two materials resident within a small sensing volume around the tip of the probe. When Teflon was in contact with the probe, the measured properties were essentially those of pure Teflon whereas the properties were nearly identical to those of the intervening liquid when the Teflon was located more than 2 mm from the probe tip. However, when the Teflon was moved closer to the probe tip, the dielectric property measurements were not linearly related to the compositional fraction of the two materials, but reflected nearly 50% of those of the intervening liquid at separation distances as small as 0.2 mm, and approximately 90% of the liquid when the Teflon was located 0.5 mm from the probe tip. These results suggest that the measurement methods reported in the most recent breast tissue dielectric property study are not likely to return the compositional averages of the breast tissue specimens evaluated, and thus, the conclusions reached about the expected dielectric property contrast in breast cancer from this specimen study may not be correct.