Showing papers by "National Physical Laboratory published in 2002"

MTDATA - thermodynamic and phase equilibrium software from the National Physical Laboratory

Abstract: In the past, the complexity of the chemical and phase equilibria established during many industrial processes has prevented the kind of in-depth understanding of their thermodynamics necessary for successful and efficient process control. Predictive thermochemistry, as embodied within MTDATA, makes such an understanding possible. MTDATA allows equilibria to be calculated for multicomponent systems of practical interest, containing many different types of phase, from critically assessed data for their component binary and ternary subsystems. Very complicated calculations can be undertaken with as many as thirty different components and 500 phases being considered simultaneously. A number of modules are incorporated for critically assessing, manipulating and retrieving the data, making various types of calculation and plotting binary, ternary, multicomponent, and predominance area diagrams. Facilities are also available for users to link the complexities of phase equilibrium calculations within MTDATA to their own software or to third party commercial software packages enabling users to simulate unit operations within an industrial plant or to integrate kinetic effects. Special derivatives of MTDATA are also now being developed to provide the sophisticated capabilities within MTDATA to non-expert users for special applications using carefully designed graphical user interfaces, which are both easy to use, and which provide output specifically designed for the particular industrial need.

The evaluation of key comparison data.

Abstract: Guidelines containing two procedures are proposed for the evaluation of key comparison data. They apply to the simple circulation of a single travelling standard around all the participants. The application of the procedures to a specific set of key comparison data provides a key comparison reference value (KCRV) and the associated uncertainty, the degree of equivalence of the measurement made by each participating national institute and the degrees of equivalence between measurements made by all pairs of participating institutes. Procedure A is based on the use of the weighted mean, together with consistency checks based on classical statistics regarding its applicability. Should the checks fail, action to remedy the situation is suggested. If the remedy is inappropriate, Procedure B can be applied instead. It is based on the use of the median (or any another informed choice) as a more robust estimator in the circumstances.

Low voltage analog circuit design techniques

Abstract: Analog signal processing is fast and can address real world problems. The applications of battery powered analog and mixed mode electronic devices require designing analog circuits to operate at low voltage levels. In this paper, some of the issues facing analog designers in implementing low voltage circuits are discussed, and possible low voltage design techniques are examined. The authors describe briefly almost all low voltage design techniques suitable for analog circuit structures along with their merits and demerits.

Gas and humidity sensors based on iron oxide–polypyrrole nanocomposites

Abstract: Nanocomposites of iron oxide and polypyrrole were prepared by simultaneous gelation and polymerization process. This resulted in the formation of mixed iron oxide phase for lower polypyrrole concentration, stabilizing to a single cubic iron oxide phase at higher polypyrrole concentration. The composites in the pellet form were used for humidity and gas sensing investigations. Their sensitivity to humidity was found to increase with increasing concentration of polypyrrole. Gas sensing was performed for CO2, N2 and CH4 gases at varying pressures. The sensors showed a linear relationship between sensitivity and pressures for all the gases studied. The sensors showed highest sensitivity to CO2 gas.

Ultrathin SiO2 on Si II. Issues in quantification of the oxide thickness

Abstract: An analysis is made of various quantification issues concerning the analysis of ultrathin layers of SiO2 on (100) and (111) polished Si surfaces. For analysis of the oxide thickness, a simple equation is generally used involving two parameters; the attenuation length of photoelectrons in the oxide and the ratio, Ro, of the intensities of the Si 2p peak from bulk thermal SiO2 and from pure Si. An analysis of previously reported measurements of the attenuation length gives an average value of only 6% less than the theoretical value. However, careful measurements of Ro, via two routes, indicate consistently that a value of 0.88 ± 0.03 should be used rather than the calculated value of 0.53 ± 0.05. This difference may arise through systematic uncertainties in the values for the relevant inelastic mean free paths, the silicon dioxide density and the shake-up contributions. Previously reported experimental values of Ro range from 0.67 to 0.87. Uncertainties also arise from intensity variations caused by the crystal structure of the substrate. These are mapped and a position ‘A’ is found where further work is best conducted. For the (100) surface, A is 34° from the surface normal in an azimuth midway at 22.5° between the [010] and [011] azimuths. For the (111) surface at A is 25.5° from the surface normal in the [101] azimuth. Data for much of the present work are for the (100) surface at an angle of emission of 27° at position ‘B’ at 28.5° from the surface normal in the [110] azimuth, which is equivalently good but may degrade for spectrometers with high angular resolution. If the same equation is used for calculating the thickness, position B leads to a calculated thickness that is 4% less than that measured for an average orientation, whereas data acquired for normal emission lead to a value 18% lower, and those measured at A are 2% higher. Measurements of the carbonaceous contamination confirm earlier conclusions that the contamination is better described using data for an average polymer than for glassy carbon. © Crown copyright 2002. Published by John Wiley & Sons, Ltd.

The physical and chemical properties of electroless nickel–phosphorus alloys and low reflectance nickel–phosphorus black surfaces

Abstract: Novel insights into the manufacture of nickel–phosphorus black surfaces by chemical etching of electroless-deposited Ni–P alloy has been achieved by examining the influence of pre-etch phosphorus composition and etching method on the resulting morphology, composition and reflectance of the black surface produced. An optimum phosphorus composition and etching regime to produce low reflectance blacks of 0.4% or lower in the visible region is proposed. Cross-sectional analysis of the etched surface has allowed, for the first time, an accurate determination of the scale of the enhanced morphologies produced and the thickness of the oxidised black layer itself. AFM studies have also provided information on the phase structure of the as-deposited Ni–P alloy.

Synthesis of precision AC waveforms using a SINIS Josephson junction array

Abstract: A synthesizer of precision ac waveforms based on a binary superconductor insulator-normal metal-insulator-superconductor (SINIS) Josephson junction array has been developed. The array segments are individually biased using a high-speed bias source which gives a switching time of approximately 250 ns between voltage steps. The accuracy limitations of the system due to limited risetime and bandwidth are discussed. Comparisons between the synthesizer and a planar multijunction thermal converter (PMJTC) indicate that it is possible to synthesize sine waves with known rms value having an accuracy in the range of 1 part in 10/sup 7/.

The Provisional Low Temperature Scale from 0.9 mK to 1 K, PLTS-2000

Abstract: An internationally-accepted ultra-low temperature scale is needed to provide the basis for reliable thermometry in the temperature range in which commercial dilution refrigerators operate, and in experiments investigating the properties of 3He and other condensed matter. Several laboratories have developed 3He melting-pressure scales, but there are substantial differences even between the most recent of them. These amount to about 0.3% of T near 500 mK, rising to about 6% of T at 0.9 mK. In 1996 a collaboration was initiated between low temperature physicists in national laboratories and elsewhere to derive an equation for the melting pressure of 3He which could be accepted for international use from 1 K to 0.9 mK, the Neel temperature of solid 3He. After an open workshop in Leiden in 1998, discussions took place to see if thermodynamic calculation of 3He melting pressures could resolve the differences. In January 2000 the authors (apart from ALR and GS) met at NIST and were able to reach a compromise on the Provisional Low Temperature Scale, PLTS-2000. Its 1-sigma uncertainty is estimated to be 0.3% of T (up to a maximum of 0.5 mK), but this rises to about 2% of T at 0.9 mK. The provisional status recognizes that the PLTS-2000 is a compromise, rather than a true consensus, but it is likely to be some years before it can be replaced by a more accurate scale. The scale was announced at the Quantum Fluids and Solids conference in Minnesota, USA, in June 2000, and was formally adopted by the Comite International des Poids et Mesures in October 2000.

Langmuir–Blodgett films of poly(3-dodecyl thiophene) for application to glucose biosensor

Abstract: Monolayers of poly(3-dodecyl thiophene) (P3DT) have been obtained on indium–tin-oxide (ITO) coated glass plates by dispensing mixed solution of P3DT and stearic acid (SA) prepared in chloroform onto water subphase by a microsyringe. The pressure–area isotherms of these P3DT–SA monolayers were studied as a function of temperature and pH. The monolayer stability onto the water subphase has been experimentally studied at different temperatures, pH and surface pressure. These P3DT–SA monolayers fabricated onto the ITO-coated glass plates were characterized using FTIR and cyclic voltammetry studies. The desired enzyme monolayers were fabricated by dispensing glucose oxidase mixed with P3DT/SA in chloroform and were transferred onto desired ITO-coated glass. An attempt has been made to utilize these P3DT/SA/GOX LB films for fabrication of a glucose biosensor.

Relationship between discontinuities, magnetic holes, magnetic decreases, and nonlinear Alfvén waves: Ulysses observations over the solar poles

Abstract: [1] Ulysses magnetic field data are used to study magnetic field microstructure over the solar poles. Magnetic holes (MHs) and magnetic decreases (MDs) are found to be located at the phase-steepened edges of nonlinear Alfven waves. The phase-steepened edges (directional discontinuities) occur in time-coincidence with MHs, one edge of an MD, or throughout the whole MD. These MH- and MD-related discontinuities have both rotational and compressive properties, perhaps explaining why many directional discontinuities detected in interplanetary space have non-MHD properties. The dispersive, dissipative and compressive features of nonlinear Alfven waves may be important for the heating of the solar corona.

FTIR investigations on ion–ion interactions in liquid and gel polymeric electrolytes: LiCF3SO3-PC-PMMA

Abstract: FTIR spectroscopy and ionic conductivity measurements have been employed to study the solvation and conduction mechanism of lithium ions in aprotic liquid electrolytes of LiCF3SO3-PC (lithium trifluromethanesulfonate-propylene carbonate) and gel electrolytes containing poly(methylmethacrylate) (PMMA) additionally. Cation solvation occurs via interactions between Li+ ions with the ring oxygens as well as the carbonyl oxygen of PC molecules over the entire salt concentration range under investigation (0.025–2 M) for both liquid and gel electrolytes. The ionic conductivity decline for concentrations ≥0.8 M LiCF3SO3-PC is attributed to the formation of ion pairs and/or triplets. Presence of (a) free CF3SO3 − ions in 0.025 and 0.05 molar LiCF3SO3-PC solutions, (b) the LiCF3SO3 structure with a monodentate coordinated lithium ion for concentrations ≥0.5 molar LiCF3SO3-PC systems and (c) ion triplets comprising two cations and an anion with a bidentate bridging structure in 2 M LiCF3SO3-PC electrolytes, has been established. Ionic conductivity performance concurs with our infrared results. Gel electrolytes containing upto 15 wt% of PMMA have been found to exhibit liquid like behavior but 2 M LiCF3SO3-PC systems that incorporate 25 wt% of polymer show a distinct Li+–O=C (of PMMA) interaction which is unambiguously determined from the remarkable changes observed for the νsC=O) and νs(SO3)—the symmetric stretching vibrational modes.

Characterisation of a gold foil-based Bonner sphere set and measurements of neutron spectra at a medical accelerator

Abstract: The characteristics of a Bonner sphere set with gold foils as the thermal neutron sensor are described. To illustrate the application of this neutron spectrometer in a pulsed field with an intense photon component, the results of measurements at a hospital electron accelerator are presented.

Results of the Sixth International Comparison of Absolute Gravimeters, ICAG-2001

Abstract: The Sixth International Comparison of Absolute Gravimeters was held from 5 June to 28 August 2001 at the Bureau International des Poids et Mesures (BIPM), Sevres. Seventeen absolute gravimeters were used to make measurements at five sites of the BIPM gravity network. The vertical gravity gradients at the sites and the ties between them were also measured using seventeen relative gravimeters. For the first time the ties were also measured using absolute gravimeters. Various methods of processing the absolute and relative data were tested to calculate the results. The final results of ICAG-2001 are presented. The acceleration due to gravity at a height of 0.90 m is given as (980 925 701.2 ± 5.5) µGal and (980 928 018.8 ± 5.5) µGal for sites A and B, respectively, calculated using a combined adjustment of the absolute and relative data.

Evaluation of the EGSnrc Monte Carlo code for interface dosimetry near high-Z media exposed to kilovolt and 60Co photons

Abstract: High atomic number (Z) heterogeneities in tissue exposed to photons with energies of up to about 1 MeV can cause significant dose perturbations in their immediate vicinity The recently released Monte Carlo (MC) code EGSnrc (Kawrakow 2000a Med Phys 27 485-98) was used to investigate the dose perturbation of high-Z heterogeneities in tissue in kilovolt (kV) and 60Co photon beams Simulations were performed of measurements with a dedicated thin-window parallel-plate ion chamber near a high-Z interface in a 60Co photon beam (Nilsson et al 1992 Med Phys 19 1413-21) Good agreement was obtained between simulations and measurements for a detailed set of experiments in which the thickness of the ion chamber window, the thickness of the air gap between ion chamber and heterogeneity, the depth of the ion chamber in polystyrene and the material of the interface was varied The EGSnrc code offers several improvements in the electron and photon production and transport algorithms over the older EGS4/PRESTA code (Nelson et al 1985 Stanford Linear Accelerator Center Report SLAC-265 Bielajew and Rogers 1987 Nucl Instrum Methods Phys Res B 18 165-81) The influence of the new EGSnrc features was investigated for simulations of a planar slab of a high-Z medium embedded in water and exposed to kV or 60Co photons It was found that using the new electron transport algorithm in EGSnrc, including relativistic spin effects in elastic scattering, significantly affects the calculation of dose distribution near high-Z interfaces The simulations were found to be independent of the maximum fractional electron energy loss per step (ESTEPE), which was often a cause for concern in older EGS4 simulations Concerning the new features of the photon transport algorithm sampling of the photoelectron angular distribution was found to have a significant effect, whereas the effect of binding energies in Compton scatter was found to be negligible A slight dose artefact very close to high-Z interfaces exposed to kilovolt x-rays was discovered when atomic relaxation processes following excitation were omitted

Lens arrays for a three-dimensional imaging system

Abstract: This paper describes optical components developed for a three-dimensional imaging system based on integral photography. A variety of lens arrays with differing specifications were required and, by suitable choices and refinements to manufacturing techniques, lens arrays were developed specifically for various stages of the imaging system. Major achievements include an image transfer screen 300 mm × 300 mm with 125 µm pitch lenses formed in photoresist and high-quality replica arrays with a 100% fill factor and pitch value of 850 µm moulded from diamond-machined masters. Correction: In the previously published version of this paper, there were three occasions where the incorrect unit "nm" was printed instead of "mm". The first occurrence was in the abstract (which has been corrected above), and the other two were on page S19 where the text should read as "300 mm × 300 mm" and "150 mm × 150 mm". This error is reported separately as a Corrigendum in the next issue and the full text files attached below have been corrected.

Experimental observation of the phenomenon of spectral switching for a class of partially coherent light

Abstract: Experimental observations of the phenomenon of spectral switching are made for a class of partially coherent light incident on a circular aperture. The on-axis spectrum of the light in the far-field is different from the spectrum of the light at the aperture. It is shown that, depending on the value of the parameter a/L~(/spl omega/) (a is the radius of the aperture and L~(/spl omega/) is the effective correlation length of the light at the aperture at the central frequency /spl omega//sub 0/ of the source spectrum), the spectral shift shows a gradual change but for a particular value of a/L~(/spl omega/) the spectral shift exhibits a rapid transition and the phenomenon of spectral switching occurs. The generation of many spectral lines, from a single spectral line, in the far-field is also demonstrated.

A review of recent work in sub-nanometre displacement measurement using optical and X-ray interferometry.

Abstract: This paper reviews recent work in the field of displacement measurement using optical and X-ray interferometry at the sub-nanometre level of accuracy. The major sources of uncertainty in optical interferometry are discussed and a selection of recent designs of ultra-precise, optical-interferometer-based, displacement measuring transducers presented. The use of X-ray interferometry and its combination with optical interferometry is discussed.

Broadband calculable dipole reference antennas

Abstract: A broadband calculable standard dipole antenna has been developed, with an uncertainty in the antenna factor (AF) of better than /spl plusmn/0.15 dB at the resonant frequency, f/sub res/, in the frequency range 30 to 500 MHz and /spl plusmn/0.2 dB in the range 600 MHz to 1 GHz. For broadband operation of the dipole resonant at 60 MHz the uncertainty is /spl plusmn/0.2 dB over a range 0.33 f/sub res/ to 1.83 f/sub res/. These uncertainties have been validated by close agreement of the measured insertion loss between dipole antennas above a conducting ground plane, with the loss predicted by analytical and numerical methods. The AF measured by the two-antenna method also agrees with the calculated AF. The technique was applied to reference monopole antennas for which AF was determined to an uncertainty of /spl plusmn/0.2 dB over the frequency range 10-100 MHz. The key achievements are: the construction of a very large and flat ground plane, validation of numerical versus analytical calculations of impedance and effective length of resonant dipoles, excellent agreement between measurements and method-of-moments calculations of the coupling between resonant dipoles, good agreement over a broad bandwidth, careful design of antennas and supports, and precision measurements.