Showing papers by "National Physical Laboratory published in 1998"

Marangoni effects in welding

Abstract: The problem of it variable weld penetration or casttocast variation in GTA/TIG welding is discussed. It is shown that for normal GTA/TIG welding conditions the HeipleRoper theory is valid, i.e. tha...

XPS: binding energy calibration of electron spectrometers 5—re-evaluation of the reference energies

Abstract: The binding energies of the calibration peaks for x-ray photoelectron spectroscopy—Cu 2p3/2, Ag 3d5/2 and Au 4f7/2—have been reassessed based on the traceable data recorded in 1984 using unmonochromated x-rays and an analyser resolution of 0.3 eV. The changes in those energies, for different x-ray sources and analyser resolutions, have been calculated and the results compared with further data. This includes work with monochromatic Al x-rays recorded at high energy resolution, allowing the binding energies to be referred to a new zero value set at the Fermi edge measured for Ag. A consistent set of data is presented for the calibration and assessment of photoelectron spectrometers with energy resolutions in the range 0.2–1.5 eV, when used with unmonochromated Al or Mg x-rays or monochromated Al x-rays. © 1998 John Wiley & Sons, Ltd.

Some advantages of video conferencing over high-quality audio conferencing

Abstract: There are many commercial systems capable of transmitting a video image of parties in a conversation over a digital network. Typically, these have been used to provide facial images of the participants. Experimental evidence for the advantages of such a capability has been hard to find. This paper describes two experiments that demonstrate significant advantages for video conferencing over audio-only conferencing, in the context of a negotiation task using electronically shared data. In the video condition there was a large, high-quality image of the head and upper torso of the participant(s) at the other end of the link and high-quality sound. For the audio-alone condition the sound was the same but there was no video image. The criteria by which these two communication conditions were compared were not the conventional measures of task outcome. Rather, measures relating to conversational fluency and interpersonal awareness were applied. In each of the two experiments, participants completed the same task with data presented by a shared editor. In Experiment 1, they worked in pairs and in Experiment 2 they worked at quartets with two people at each end of the link. Fluency was assessed from transcripts in terms of length of utterance, overlapping speech and explicit questions. Only the latter measure discriminated between the two communication conditions in both experiments. The other measures showed significant effects in Experiment 2 but not in Experiment 1. Given this pattern of results it is concluded that video can result in more fluent conversation, particularly where there are more than two discussants. However, in the case of dyadic conversation auditory cues to turn taking, etc., would seem to suffice. In both experiments there was a large and significant effect on interpersonal awareness as assessed by ratings of the illusion of presence, and most clearly, awareness of the attentional focus of the remote partner (s). In Experiment 2, the ratings for the remote partners were similar to those for the co-located discussants, demonstrating the effectiveness of the video link with regard to these subjective scales.

Factors Controlling Naphthenic Acid Corrosion

Abstract: A laboratory study was conducted to elucidate the influence of chemical and physical parameters on corrosion of type 1018 carbon steel (CS, UNS G10180)and 5% Cr-0.5% Mo steel in oils conta...

Electrochemical polarisation and corrosion behaviour of cobalt and Co(W,C) alloys in 1 N sulphuric acid

Abstract: The influence of carbon and tungsten additions to cobalt on the electrochemical properties has been studied. Carbon additions up to 0.15 wt% and tungsten additions up to 8 wt%, covering a range of compositions equal to that expected in the binder composition of WC–Co hardmetals, have been investigated using potentiodynamic and potentiostatic polarisation techniques in 1 N sulphuric acid. Pure cobalt does not passivate and remains active with increasing potential. A film forms on Co(W,C) alloys causing the current to become relatively independent of potential but remaining very high. This behaviour is termed here `pseudo-passivation'. The mechanism of film growth is determined. With increasing carbon and tungsten additions, the corrosion current density and critical current density are reduced. The corrosion potential shifts to more positive values with increasing additions. It is shown that carbon and tungsten additions influence the corrosion behaviour by a stabilisation of the fcc phase. Empirical equations are derived which relate quantitatively the volume fraction of the fcc phase to the composition of the alloy, i.e. the carbon and tungsten content in the binary and ternary alloys.

Atmospheric greenhouse effect and ionospheric trends

Abstract: Theoretical calculations predict that CO2 doubling would produce a 50 K decrease in the thermospheric temperature which can result in about 20 km decrease in the F2 peak height (hmF2) and a minor decrease in the F2 layer critical frequency (foF2) [Rishbeth and Roble 1992]. In this paper we analyze ionosonde data for some 31 stations to study the long term trends in hmF2 and foF2. Regression coefficients for hmF2 and foF2 as a function of solar activity, are obtained for each station and departures (anomalies) from expected values derived for both these parameters. An analysis of hmF2 and foF2 anomalies indicates negative trends for some stations and positive trends for others. These varied between +29 to −20 km for hmF2 and +0.7 to −0.6 MHz for foF2 in the 34 year period since 1957, the International Geophysical Year (IGY). We conclude that the present data do not provide a definitive evidence of any global long term trend in the ionosphere.

Structural correlations in GexSe1−x glasses – a neutron diffraction study

Abstract: Neutron diffraction measurement is carried out on GexSe1-x glasses, where 0.1 less than or equal to x less than or equal to 0.4, in a Q interval of 0.55-13.8 Angstrom(-1). The first sharp diffraction peak (FSDP) in the structure factor, S(Q), shows a systematic increase in the intensity and shifts to a lower Q with increasing Ge concentration. The coherence length of FSDP increases with x and becomes maximum for 0.33 less than or equal to x less than or equal to 0.4. The Monte-Carlo method, due to Soper, is used to generate S(Q) and also the pair correlation function, g(r). The generated S(Q) is in agreement with the experimental data for all x. Analysis of the first four peaks in the total correlation function, T(r), shows that the short range order in GeSe2 glass is due to Ge(Se-1/2)(4) tetrahedra, in agreement with earlier reports. Se-rich glasses contain Se-chains which are cross-linked with Ge(Se-1/2)(4) tetrahedra. Ge-2(Se-1/2)(6) molecular units are the basic structural units in Ge-rich, x = 0.4, glass. For x = 0.2, 0.33 and 0.4 there is evidence for some of the tetrahedra being in an edge-shared configuration. The number of edge-shared tetrahedra in these glasses increase with increasing Ge content.

Ion recombination corrections for the NACP parallel-plate chamber in a pulsed electron beam

Abstract: The NACP electron chamber is one of three parallel-plate chambers recommended for use in the UK. Measurements with this chamber type have indicated a problem in determining the recombination correction. This is due to a variation of the ionization current I with polarizing voltage V which deviates from the accepted Boag theory. It is shown that there is a chamber-dependent threshold voltage below which the NACP chamber follows the Boag theory. Above this voltage the chamber should be used with caution, although it is still possible to correct for the dependence of the chamber response on the dose per pulse. The existence of such deviations from theory demonstrates the usefulness of the 1/I against 1/V plot and the limitations of the Boag two-voltage analysis. Values for the initial recombination and the coefficient of general recombination are measured for several NACP chambers. It is shown that from these one can derive a value for the effective plate separation and the collector radius of each chamber. Differences in the behaviour of NACP chambers manufactured by Scanditronix and Dosetek are discussed and the implications of free-electron collection are considered.

Quantitative AES. VIII: analysis of auger electron intensities from elemental data in a digital auger database

Abstract: An analysis of the correlation of theoretical predictions for Auger electron intensities is made with the experimental data of the high-resolution digital Auger database. This analysis covers single-element samples measured for 5 keV and 10 keV beam energies. The spectral data are for wide scans of 20–2500 eV at 1 eV energy resolution and 1 eV energy intervals, as well as narrow scans at 0.25 eV energy resolution and 0.1 eV energy intervals. All spectra have the instrument intensity/energy response function removed so that peak areas are measured in units of electrons emitted per steradian per incident electron. Correlations are made for calculations of the intensities using the cross-section of Casnati et al. applied to the K, L, M and N shells, the backscattering factor of Shimizu and the inelastic mean free paths of Tanuma et al. The intensities, summed over all peaks relating to initial ionizations in shells of a given principal quantum number, correlate with the theoretical predictions with no fitting parameters, with a mean error of a factor of only 1.04 and a scatter factor of 1.3. Expressions are provided to extend the database to other beam energies and angles of incidence. General equations are also provided to extract new sensitivity factors for the average matrix, which lead to matrix factors that effectively reduce to unity for peaks above 175 eV, instead of the traditional matrix factors that cover a range of 0.1–7.2. These sensitivity factors for the average matrix may be used for peak areas, peak heights or differential peak-to-peak intensities. In the latter two cases, spectral broadening may be necessary to obtain accurate results. © 1998 John Wiley & Sons, Ltd.

Modelling of marangoni effects in electron beam melting

Abstract: Electron beam melting processes exhibit large thermal gradients in the region where the electron beam intercepts the melt; this leads to variations in the surface energy of the melt close to the be...

XPS: binding energy calibration of electron spectrometers 4—assessment of effects for different x‐ray sources, analyser resolutions, angles of emission and overall uncertainties

Abstract: A detailed analysis is made of the binding energy calibration of X-ray photoelectron spectrometers when using monochromated Al Kα x-rays or unmonochromated Al or Mg Kα x-rays. The binding energies of the peaks for Cu 2p3/2, Ag 3d5/2 and Au 4f7/2, as well as for the Ni Fermi edge, are measured at high resolution using monochromated Al Kα x-rays. The apparent binding energy shifts of the peaks are then calculated for this source, and also for the Al and Mg unmonochromated x-ray sources, using full synthetic Kα x-ray structures, as a function of Gaussian spectrometer energy resolutions in the range 0.2–1.5 eV. For all three x-ray sources, the relative binding energies for the Cu 2p3/2 and Au 4f7/2 peaks are contained within ±0.015 eV but the effects for Ag 3d5/2 are stronger and the containment range must be increased to ±0.026 eV. Further data and calculations are provided for surface core-level shifts and here it is found necessary to restrict emission angles to 56° for all the peak separations to be restricted to the above range of ±0.026 eV. Other instrumental effects may give rise to additional larger or smaller effects. Non-optimized settings for monochromators can show further shifts of up to ±0.2 eV. The uncertainties associated with the above calibration are then analyzed to show how the uncertainty at 95% confidence varies across the binding energy range. Example calculations show that seven repeats of both the Cu 2p3/2 and Au 4f7/2 binding energies may be used to define the peak repeatability and that one or two measurements can then be made for each calibration peak to define the calibration. The precise number of measurements to be used depends on the peak energy repeatability and the required confidence limits for the calibration. In practical situations, however, it is likely that the greatest uncertainty in the binding energy scale arises from the drift in the electronics between calibrations. © 1998 John Wiley & Sons, Ltd.

The Kinetics of Dephosphorization of Carbon-Saturated Iron Using an Oxidizing Slag

Abstract: The kinetics of dephosphorization of carbon-saturated iron by oxidizing slags were studied at 1330 °C. Nine slag compositions were investigated in the systems CaO-Fe2O3-SiO2-CaF2 and CaO-Fe2O3-SiO2-CaCl2. Increasing Fe2O3 up to 50 pct was found to increase the rate and extent of dephosphorization, whereas further increases were found to decrease the rate and extent of dephosphorization. This was explained in terms of two competing effects on the driving force, where increased levels of iron oxide increase the oxygen potential for dephosphorization, hence the driving force, but simultaneously dilute the basic components in the slag, lowering the driving force for dephosphorization. CaF2 and CaCl2 were found to decrease the rate and extent of dephosphorization at levels higher than 12 pct. The rate of dephosphorization was found to be first order with respect to phosphorous in the metal and was controlled by mass transport in the slag. The oxygen potential at the slag/metal interface was controlled by the FeO activity in the slag. When the kinetic results were analyzed to take account of different driving forces, Fe2O3, CaF2 and CaCl2 were all found to increase the mass transfer coefficient of phosphorous in the slag, and a quantitative relationship has been demonstrated between these mass transfer coefficients and the slag viscosity for each system studied.

Structural and superconducting properties of R 1-x Ca x Ba 2 Cu 3 O 7-δ with 0.50>=x>=0.00

Abstract: We report here a consolidated study of structural and superconducting properties of Ca substituted ${R}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ system, with $R=\mathrm{Er},$ Y, Sm, and Nd, and for $0.50g~xg~0.00.$ Interestingly, with increasing x, Er/Y samples show a behavior significantly different from Sm/Nd. In particular (1) orthorhombic distortion of $R:123$ lattice is found to be little affected for Er/Y samples with increasing x in comparison to Sm/Nd samples, (2) although Ca substitution leads to oxygen depletion for all the four sets of samples, the average Cu valence is, in general, found to remain invariant with x, (3) $\ensuremath{\rho}(T)$ is found to show an upward curvature for Er/Y samples, which is absent for the Sm/Nd samples, and (4) ${T}_{c}(\ensuremath{\rho}=0)$ as a function of x and \ensuremath{\delta} shows a much steeper decrease in Er/Y based samples, than in Sm/Nd based ones. Our results unequivocally point to a different structural order or disorder in Er/Y samples as compared to Sm/Nd based samples. We suggest that different site preferences of oxygen vacancies, predominantly created in ${\mathrm{CuO}}_{2}$ planes (CuO chains) of Er and Y (Sm and Nd) based samples, might be responsible for the observed difference in the behavior. This contention is supported by a host of other considerations and experimental observations.

Verification of the sub-nanometric capability of an NPL differential plane mirror interferometer with a capacitance probe

Abstract: A differential plane mirror interferometer developed at the NPL has been built into a system designed to calibrate and certify displacement transducers. Two electrical signals generated from the optical outputs of the interferometer and a voltage output of the transducer to be calibrated are read by a computer and mathematically analysed in order to calculate the proportionality and linearity of the displacement transducer. In order to realize the highest measurement accuracy from the interferometer the interferogram signals are analysed and corrected by an optimizing routine in order to interpolate the fractioning of the optical fringes accurately. Without computer optimization of the signals a standard deviation of measurement of a few nanometres is typical, whereas after optimization this is reduced to a fraction of a nanometre. The interferometer has been used to calibrate a high-precision capacitance probe and the results of these measurements are shown and discussed.

Planning and implementing user-centred design

Abstract: The tutorial presents a structured approach to user centred design, based on the principles of the International Standard "Human centred design processes for interactive systems" (ISO 13407) and other related standards. A core set of practical methods which support the approach are described. These have been selected by the European Usability Support Centres on the basis of their applicability, maturity, availability, and cost-effectiveness. The tutorial gives an overview of each method, and describes criteria which can be used for selecting appropriate methods. The benefits of demonstrating conformance to ISO 13407 are explained.

Depth Sensing Indentation of Thin Hard Films: A Study of Modulus Measurement Sensitivity to Indentation Parameters

Abstract: The sensitivity of modulus measured by traceably calibrated Depth Sensing Indentation (DSI) is investigated as a function of the indentation parameters: maximum load; loading rate; hold time at maximum load; and single vs. repeated loading. Other issues, including instrument compliance, indenter area function and in-plane anisotropy are also discussed.

Microstructural features and heat flow analysis of atomized and spray-formed Al-Fe-V-Si alloy

Abstract: Microstructural features of rapidly solidified powders and preforms of Al80Fe10V4Si6 alloy produced by spray forming process have been studied. The atomization and spray deposition were carried out using a confined gas atomization process and the microstructural features were characterized using scanning electron microscopy and transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. The microstructure of a wide size range of atomized powders invariably revealed cellular and dendritic morphology. The extent of dendritic region and the dendritic arm spacing were observed to increase with powder particle size. The TEM investigations indicated the presence of ultrafine second-phase particles in the intercellular or interdendritic regions. In contrast, the spray deposits of the alloy showed considerable variation in microstructure and size and dispersion of the second-phase particles at specific distances from the deposit-substrate interface and the exterior regions of the deposit. Nevertheless, considerable homogeneity was observed in the microstructure toward the center of the spray deposit. The formation and distribution of a cubic phase α-Al(Fe,V)Si has been characterized in both atomized powders and spray deposits. A one-dimensional heat flow model has been used to analyze the evolution of microstructure during atomization and also during spray deposition processing of this alloy. The results indicate that thermal history of droplets in the spray on deposition surface and their solidification behavior considerably influence the micro-structural features of the spray deposits.

The calibration of therapy level electron beam ionization chambers in terms of absorbed dose to water

Abstract: During 1998, NPL plans to introduce the world's first absorbed dose calibration service for electron beam radiotherapy. The service will be based on the primary standard graphite calorimeter, and will enable the direct calibration of electron ionization chambers, without reference to air kerma standards. This calibration is a two-step process. Firstly, a set of NACP-designed parallel-plate reference chambers have been calibrated against the calorimeter over the last few years. These chambers are then used to calibrate user chambers by direct comparison in a water phantom under standard conditions. This paper describes the calibration of the reference chambers against the calorimeter and the derivation of absorbed dose to water calibration factors (with an estimated uncertainty in this calibration of % at the 95% confidence level).

Charge transport and structural morphology of HCl-doped polyaniline

Abstract: The d.c. and a.c. (100 Hz–1 MHz) conductivities of HCl-doped polyaniline have been measured in the temperature range 77–300 K. At 77 K, the a.c. conductivity data, σ(ω), can be described by the relation σ(ω)=Aωs, where the parameter s lies close to unity and decreases with increase in the doping level. The ratio of measured a.c. to d.c. conductivity shows dispersion at 77 K, which decreases with increase in the doping level. This decrease is found to be sharp around pH∼3.0. In the temperature range 77–150 K, the observed d.c. conductivity data can be described by Mott's three dimensional variable range hopping (VRH) model. Scanning electron microscopy studies reveal a sharp change in structural morphology of HCl-doped polyaniline at a pH∼3.0. A strikingly remarkable structural morphology has been observed in the formc of a channel at this pH value. This change is accompanied by a rapid increase in d.c. conductivity, dielectric constant, along with sharp changes in structural morphology, which indicates the existence of a doping-induced structural conductivity correlation in this system. © 1998 Chapman & Hall

Quantitative AES VII. The ionization cross‐section in AES

Abstract: Inner shell ionization cross-sections are analyzed for use with Auger electron spectroscopy (AES). Correlations of the cross-sections of Gryzinski, Casnati et al., Jakoby et al. and Drawin are made with experimental data from the digital Auger database. Jakoby et al.'s cross-section is realistic for K shell ionizations but can become negative for other shells unless modified, so is of limited use for AES. Results for the ratios of experimental and calculated intensities for electron beam energies of 5 keV and 10 keV, for overpotentials in the range 2–60, are used to rank the effectiveness of the remaining three cross-sections. The experimental ratios show means and scatter standard deviations of 1.01±0.12, 0.99±0.05 and 1.04±0.09 when compared with the predictions for the cross-sections of Gryzinski, Casnati et al. and Drawin, respectively. Casnati et al.'s cross-section provides the best correlation, with the goodness of fit being similar for ionizations in the K, L, M and N shells. Casnati et al.'s cross-section is therefore recommended for future use in AES. The use of Gryzinski's or of Drawin's cross-sections, by comparison, can lead to errors as high as 50% at the low value of 1.5 for the overpotential (i.e. for high-energy Auger electrons). © Crown copyright 1998. Reproduced with the permission of the Controller of Her Majesty's Stationery Office.

Superconductivity in the Mg-doped CuBa 2 Ca 3 Cu 4 O 12-y system

Abstract: Synthesis of ${\mathrm{CuBa}}_{2}{\mathrm{Ca}}_{3}{\mathrm{Cu}}_{4}{\mathrm{O}}_{12\ensuremath{-}y}$ (Cu-1234) with Mg doping at the Ca sites, hitherto unexplored, is attempted and reported here. Different samples with 5%, 10%, 20%, and 33% Mg (in nominal stoichiometry) have been prepared in gold capsules under 3.5 GPa pressure and at about 1000 \ifmmode^\circ\else\textdegree\fi{}C temperature using a cubic anvil-type high-pressure machine. The samples were characterized through x-ray diffractometry, resistivity, dc susceptibility, and Hall effect measurements. The samples were then aligned along their c axis under 10 T field at room temperature. Magnetization (both in $H\ensuremath{\parallel}\mathrm{ab}$ and $H\ensuremath{\parallel}c$) was measured between 120 and 5 K in different field values up to 5 T. The superconducting anisotropy (defined as ${\ensuremath{\xi}}_{\mathrm{ab}}/{\ensuremath{\xi}}_{c}{=H}_{c2}^{\mathrm{ab}}{/H}_{c2}^{c}$), estimated in the reversible magnetization regime, comes out to be about 1.4, and is lower than that of the pristine sample. Improvement in the anisotropy factor obtained has been discussed in light of the observed changes in the lattice parameters and the hole concentration, emanating from Mg doping in the Cu-site vacancies in charge reservoir blocks of the 1234 structure.