Showing papers by "National Physical Laboratory published in 1990"

A Realization of the SI Watt by the NPL Moving-coil Balance

Abstract: Mechanical and electrical power in SI units have been equated by measurements made on a coil part of which is in a strong magnetic field The force due to a current I flowing in the coil, is weighed by opposing it with a mass M subject to the earth's gravitational acceleration g This is combined with a separate measurement in which a voltage V is generated in the coil when it is moved vertically with velocity u through the relationship IV = M g u If the current produces a voltage V across a resistor whose value R is known in SI units, then V = (M g u R)1/2 Hence the voltage V and the current I are known in SI units and can be used to express the value of the NPL working standards in SI units The working standard of voltage has hitherto been maintained in terms of a Josephson effect apparatus by ascribing the value 483 594 GHz/volt (maintained) to the Josephson constant KJ presumed equal to 2e/h The measurements reported here suggest a different value of KJ 483 597,903 ± 0,035 ought to be used, based on the premise that the SI value of the quantum Hall resistance is RK = 25 812,8092 ± 0,0014 Ω If one presumed also that RK = h/e2 exactly, the values of elementary charge e and the Planck constant, h, which may be deduced from these measurements are e = 1,602 176 35 ± 0,000 000 14 × 10-19 C, h = 6,626 068 21 ± 0,000 000 90 × 10-34 J s, which may be compared with the values recommended by the CODATA Task Group on Fundamental Constants which are e = 1,602 177 33 ± 0,000 000 14 × 10-19 C, h = 6,626 075 5 ± 0,000 004 0 × 10-34 J s

Optical fringe subdivision with nanometric accuracy

Abstract: Reliable bidirectional optical fringe counting is normally obtained by using two signals derived from the optical outputs of an interferometer varying sinusoidally with path difference and in phase-quadrature. This paper describes a calibration technique that uses these signals to achieve nanometric uncertainties in path length determinations. It discusses some of the limitations to achieving this uncertainty and describes a simple experimental technique for verifying the accuracy of fringe subdivision.

Polarisation Conversion through the Excitation of Surface Plasmons on a Metallic Grating

Abstract: Surface plasmon-polaritons may be excited by photons at a metal-air boundary if a grating is used to provide the necessary extra momentum. Rotation of the grating so that the grooves are no longer perpendicular to the plane of incidence reduces the coupling efficiency while at the same time producing some s-polarised radiation in the reflected beam. When the grooves are at 45° to the plane of incidence maximum p-s conversion is obtained. Conversion efficiencies of up to 66% have been recorded for silver-coated gratings of appropriate depth. By scanning the angle of incidence and using a crossed polariser on the detector a strong resonance peak is observed. The application of this phenomenon to optical sensing is considered.

Creep deformation of single crystal superalloys—modelling the crystallographic anisotropy

Abstract: A generalised model of creep deformation in cubic single crystals is developed that considers the combined effects of viscous glide on two (or more) slip systems and accounts for tertiary creep by the accumulation of mobile dislocations with plastic strain. The model is applied to analyse a database of creep curves for the nickel-base single crystal superalloy SRR99 with the assumption that creep deformation occurs by glide on both the {111}〈 1 01〉 and {001}〈110〉 systems. A procedure for the calculation of creep curves and associated crystal rotations for arbitrary crystal orientations is described. The model predicts changes in the anisotropy of creep behaviour with stress and temperature that is in general agreement with the limited available experimental data. It also includes the contributions of all possible slip vectors to predict crystal rotations that are consistent with observations.

Quantitative AES and XPS: Determination of the electron spectrometer transmission function and the detector sensitivity energy dependencies for the production of true electron emission spectra in AES and XPS†

Abstract: For the use of published general or theoretical sensitivity factors in quantitative AES and XPS, the energy dependence of both the spectrometer transmission function and the detector sensitivity must be known. Here, we develop simple procedures that allow these dependencies to be determined experimentally. Detailed measurements for a modified VG Scientific ESCALAB II, the metrology spectrometer, operated in both the constant ΔE/E and constant ΔE modes, are presented and compared with theoretical estimates. It is shown that an exceptionally detailed electron-optical calculation, involving proprietary information, would be required to match the accuracy of the experimental procedures developed. Removal of the spectrometer transmission function and the detector sensitivity terms allow the measured spectrum to be converted to the true electron emission spectrum, irrespective of the mode of operation. This provides the first step to the provision of reference samples to calibrate the transmission functions and detector sensitivities of all instruments so that they, in turn, may produce true electron emission spectra. This is vital if: (1) all instruments are to give consistent results; (2) theoretical terms are to be used in quantifying either AES or XPS; (3) reference data banks are to be established for AES or XPS.

A multiple-frequency hydrophone calibration technique

Abstract: A method is described for comparing the sensitivity of two hydrophones over the frequency range 1–15 MHz. This technique forms the basis for the dissemination of national ultrasonic standards in the U.K. over this frequency range. A reference hydrophone is placed in an ultrasonic field and then the device being calibrated is substituted and the two output voltages are compared. This substitution method utilizes a broadband ultrasonic field produced by nonlinear propagation. Thus it is possible to cover the whole frequency range with a single measurement on each hydrophone. The overall uncertainty in the intercomparison of two hydrophones increases from ±4.2% at 1 MHz to ±8.2% at 15 MHz (95% confidence level). The method has been compared with discrete‐frequency substitution, time‐delay spectrometry, and absolute calibrations using the National Physical Laboratory (NPL) primary standard laser interferometer. Various designs and sizes of hydrophones were compared, and agreement was within the combined random uncertainties for all the comparisons.

Polyaniline/polymeric acid composite, a novel conducting rubber

Abstract: Preparation du composite par addition de polyaniline a une solution aqueuse de poly (acide styrenesulfonique) puis sechage. La conductivite electrique du composite augmente lentement avec la proportion en polyaniline jusqu'au seuil de percolation (11,6%) puis augmente brusquement. La conductivite augmente egalement avec le taux d'etirage du composite. Discussion de la structure

Increased heating by diagnostic ultrasound due to nonlinear propagation

Abstract: The heating of tissues by the absorption of ultrasound is an important safety consideration in the use of diagnostic ultrasound. This paper shows that models of ultrasonic heating for this situation need to take account of nonlinear propagation. Measurements were made of the temperature rise in a sample of tissue‐mimicking gel, caused by the application of 3.6‐MHz focused ultrasonic beams for 3 min. The propagation path to the focus was in water, to mimic the situation where the fetus is scanned through the full bladder. The effect of nonlinear propagation was seen by changing the pressure amplitude of the pulse, while altering the pulsing regime to preserve a constant spatial‐peak temporal‐average intensity of 1 W cm−2. When nonlinear distortion was present, an enhancement in the temperature rise was observed, which correlated with the value of the shock parameter. The enhancement ratio was typically up to a factor of 3, and the maximum temperature rise observed was 2 °C. This enhanced heating was seen b...

Thermodynamic and phase diagram data for the CaO-SiO2 system

Abstract: A critical assessment of thermodynamic and phase diagram data for the CaO-SiO 2 system has been carried out using an extended Kapoor—Frohberg cellular model for the liquid phase. The assessment incorporates revised data for all the phases of the pure components expressed as the Gibbs energy relative to the weighted enthalpy of the elements in their standard reference states. The data for the intermediate compounds are expressed in the same form. The complete dataset therefore represents all the thermodynamic properties of all the phases as a function of temperature and composition.

Creep Deformation of Engineering Alloys: Developments from Physical Modelling

Abstract: Differences in the creep behaviour of particle-strengthened engineering alloys relative to simple solid solutions are reviewed and their implications for the mechanisms of high temperature deformation in these materials are considered Constitutive equations describing the shapes of creep curves, based on physical models of the important types of damage are considered These are incorporated in a computer software package, designated CRISPEN, that allows the analysis of creep data, the development of a database of model parameters and the simulation of the strain/time trajectory for arbitrary loading conditions Examples of the application of CRISPEN to a range of alloys and loading conditions are described

An automated cryogenic current comparator resistance ratio bridge

Abstract: The method of measurement of two-terminal cryogenic and four-terminal room-temperature resistors using a DC cryogenic current comparator bridge is well established. A bridge which is based on this technique and is fully automated with a computer controlling both the measurement current and the readings of the bridge imbalance current is described. The bridge is designed to measure integer ratios of 1:1 to 1:25 of four-terminal room-temperature resistors in the range 1 Omega to 10 k Omega . An overall accuracy of 1 part in 10/sup 9/ for differences in the ratio from nominal of up to 100 p.p.m. is desired. Some initial determinations of the resolution have been made for a measurement of 10 Omega :100 Omega . These determinations were carried out without the balance servo, and the output from the null detector was measured with a digital voltmeter connected to the computer. An analysis of a set of 11 reversals gave a standard deviation for a single reversal of 1 part in 10/sup 8/ and therefore a standard error of the mean of 3 parts in 10/sup 9/. >

Comparison of two cryogenic radiometers by determining the absolute spectral responsivity of silicon photodiodes with an uncertainty of 0.02

Abstract: To substantiate the NPL primary standard cryogenic radiometer as an absolute instrument it has been compared with the cryogenic radiometer which was successfully used to determine the Stefan-Boltzmann constant. The comparison was carried out with an accuracy of better than 0.02% by the independent determination of the spectral responsivity of silicon photodiodes with each radiometer. Only a detector comprising a number of silicon photodiodes (a trap device) had the required stability to achieve the desired accuracy. Four trap devices were found to have near unity internal quantum efficiency, being self-consistent to within 0.01%.

Laser cooling of trapped Yb

Abstract: 172Yb+ ions in an rf trap have been laser cooled for the first time by driving the 2 S 1/2–2 P 1/2 transition at 369.5 nm. It was necessary to irradiate the ions with 2.438 μm infra-red radiation to depopulate the metastable 2 D 3/2 state. An upper limit on ion energies was determined by observing the size of the trapped cloud and corresponds to a temperature below 2 K. Cooled ion lineshapes were compared with simulations and coherence nulls were observed in the infra-red frequency scans.

Simple experimental arrangement for observing spectral shifts due to source correlation.

Abstract: A filter-lens combination in conjunction with partially coherent light is used to produce a secondary source that violates a certain scaling law. The spectral shifts observed have been found to be dependent on the complex degree of spectral coherence, filter peak transmission wavelength, and half-band-width of the filter.

Singlet axial-vector current and the "proton-spin" question.

Abstract: As a preliminary we consider the phenomenology of the European Muon Collaboration experimental result without assuming SU(3) invariance. The most reliable conclusion is that the singlet axial-vector current matrix element is rather small. We calculate this matrix element in a variety of chiral-soliton models in which the axial anomaly equation is satisfied. There are a number of subtleties which we explore. It turns out that reasonable chiral models including only pseudoscalar fields give a zero matrix element. Taking "short-distance" effects consistently into account via the inclusion of vector mesons or explicit quarks does give a small nonzero result. The situation is closely analogous to the prediction of a nonzero result for the nonelectromagnetic part of the neutron-proton mass difference in these chiral models.

Intercomparison of remote measurements of stratospheric NO and NO2

Abstract: During the 1982 and 1983 Balloon Intercomparison Campaigns, the vertical profile of stratospheric NO2 was measured remotely by nine instruments and that of NO by two. Total overhead columns were measured by two more instruments. Between 30 and 35 km, where measurements overlapped, agreement between NO profiles was within + or - 30 percent, which is better than the accuracies claimed by the experimenters. Between 35 and 40 km, there was similarly good agreement between NO2 profiles, but below 30 km, differences of greater than a factor of 3 were found. In the second campaign, NO2 values from most instruments agreed within their quoted errors, except that the Oxford radiometer gave much lower values; the first campaign and the column measurements show a more uniform spread of results.

Application of MTDATA to the Modeling of Multicomponent Equilibria

Abstract: MTDATA embodies the principle that equilibria in multicomponent, multiphase systems can be calculated from a knowledge of the thermodynamic data for the subsystems. A number of modules are incorporated for manipulating and retrieving the data, making various types of calculation and plotting binary, ternary, multicomponent, and predominance area diagrams. The principles and operation of MTDATA are illustrated by reference to systems of practical importance.

Quantitative Solderability Measurement of Electronic Components: Part 1: The Wetting Balance

Abstract: The subject of the quantitative measurement of solderability of electronic components is introduced. The wetting balance in various configurations and modes of operation is being used as the focal point to establish a quantitative measurement capability for solderability of conventional leaded components, surface mounting components and printed circuit interconnections. The principles of operation of the wetting balance and the factors that influence the measurement are discussed. This paper is the first of a series that will cover the development of traceable reference standards for wetting balance calibration, the influence of instrumental design on the measurement, the standardisation of the measurement procedures, the choice and evaluation of a solderability index for the dynamic measurement, and the traceability of the measurement to international standards.

Comparison of three numerical treatments for the open-ended coaxial line sensor

Abstract: The open-ended coaxial line sensor is commonly used for non-invasive microwave dielectric measurements. A comparison is made between three mathematical treatments that allow the reflection coefficient of the sensor to be derived from the complex permittivity of the material with which it is in contact. It is shown that the discrepancies between values computed by the three methods are much smaller than the predicted measurement uncertainties. Mappings of permittivity contours onto the complex reflection coefficient plane are shown.

Superconductivity in Ni substituted Bi2Ca1Sr2Cu2−xNixOy

Abstract: The effect of partial substitution of Ni in place of Cu in Bi 2 Ca 1 Sr 2 Ni x Cu 2− x O y has been systematically investigated for 0⩽ x ⩽0.2. The XRD of the samples confirms them to be of single phase. The superconducting critical temperature T c monotonically decreases with Ni concentration; the rate of T c decrease is around 2.5 K/% of the nominal substitution. All the samples have been subjected to XPS studies. The results show that Ni is incorporated in the lattice in the divalent state with Ni-O bonds substituting for the Cu-O bonds while keeping Bi-O, Sr-O and Ca-O bonds intact. Change in the Ni content does not seem to affect the valence state of Cu. The question of T c decrease with Ni substitution is examined.