Showing papers by "National Physical Laboratory published in 1997"

Elastic Scattering Corrections in AES and XPS. II. Estimating Attenuation Lengths and Conditions Required for their Valid Use in Overlayer/Substrate Experiments

Abstract: We examine substrate/overlayer experiments and the equations commonly used to quantify overlayer thicknesses. Comparisons with accurate Monte-Carlo simulations show that using attenuation lengths (rather than inelastic mean free paths) eliminates most of the error due to elastic scattering without increasing the complexity of the quantification. We give attenutation lengths for 27 elements, calculated by the criterion that systematic errors in such quantifications should be minimized. These are therefore the best attenuation length values to use in layerwise quantification. We show that, provided these attenuation length values are used, the error in estimation of the thickness of an overlayer due to elastic scattering can be limited to +(5% +1 A) for an emission angle ≤58° from the surface normal, and +(10%a + 1 A) for an emission angle ≤63° from the surface normal. This accuracy is acceptable for most analytical work. Other methods (such as analytical transport theory) are much more complicated, and achieve a high precision that is often unnecessary in view of other uncertainties typically present in these experiments (such as errors due to surface morphology and diffraction effects). The results presented here, using the full theory, show that the analyst's simple straight-line approximation is in fact of adequate accuracy, provided that the correct values of attenuation length are used. Simple semi-empirical equations are presented, which allow the analyst to estimate the attenuation length for electrons of kinetic energy between 50 and 2000 eV, to a standard uncertainty of 6%.

Application of conducting polyaniline as sensor material for ammonia

Abstract: The change in resistance of polyaniline on exposure to aqueous ammonia has been utilized for the study of a prototype chemical sensor. This paper discusses various aspects of polyaniline and its suitability as a chemical sensor, particularly with reference to aqueous ammonia solution.

Modelling of thermal desorption of hydrogen from metals

Abstract: The thermal desorption technique can be used in principle to determine the trapping characteristics of different microstructural trap sites in metals provided there are adequate models to fit to the experimental data. A brief review of models of thermal desorption is presented which indicates that there are limitations in the assumptions made or in the scope of existing models. A more rigorous mathematical model has now been developed which accounts for diffusion, detrapping, and retrapping at one or more type of trap site and which allows for varying trap occupancy. The effect of material and experimental variables on the thermal desorption spectrum has been evaluated and the validity of simple models of desorption assessed. The simpler analytical models, such as the detrapping model of Lee and Lee, in which diffusion is neglected relative to detrapping, do not inspire confidence and are applicable only under very limiting circumstances; for example, in low alloy steels at very low hydrogen contents. It is recommended that thermal desorption measurements be made at progressively decreasing values of initial hydrogen content until the simple analysis yields a consistent value for the trapping parameters. This experimental approach is applicable also to models of thermal desorption which account for diffusion using an effective diffusivity, since trap occupancy is neglected in these. The more rigorous model described herein can be used to determine the binding energy of the traps directly which, together with the density of trap sites, is the most important parameter with respect to hydrogen assisted cracking. The height of the energy barrier to trapping, at constant value of the binding energy, is shown to have only a modest effect on the thermal desorption spectrum compared with the impact of binding energy and of density of trap sites.

Observation of an Electric Octupole Transition in a Single Ion

Abstract: The $^{2}S_{1/2}\ensuremath{-}^{2}F_{7/2}$ electric octupole $(E3)$ transition in ${}^{172}{\mathrm{Yb}}^{+}$ has been detected by observing quantum jumps in a single laser cooled ion, stored in an electrodynamic trap. The transition frequency is 642 116 785.3(0.7) MHz $(1\ensuremath{\sigma})$. Consideration of the transition rate and laser parameters implies a $^{2}F_{7/2}$ level lifetime of 3700 days. This is the first time an atomic $E3$ transition has been driven. This transition has applications as an optical frequency reference.

Investigation of the s-2(1/2)-d-2(5/2) clock transition in a single ytterbium ion

Abstract: The suitability of the 411-nm 2S1/2-2D5/2 transition in Yb+ is assessed as an optical frequency standard. The lifetime of the 2D5/2 level has been measured and found to be 7.2±0.3 ms, which implies a natural linewidth of 22.2±0.9 Hz for the 411-nm transition. In addition to a decay to the ground state the 2D5/2 level can decay into the highly metastable 2F7/2 level where it is trapped. The branching ratio for decay into the 2F7/2 level has been measured and found to be 0.83±0.03. A transition capable of rapidly depopulating the 2F7/2 level and returning the ion to the cooling cycle has been demonstrated. This has enabled a quantum jump measurement of the 411-nm transition in 172Yb+ to be performed, yielding a center frequency of 729 476 869.13(0.42) MHz (1σ).

Microwave applications of high-temperature superconductors

Abstract: For at least eight of the first ten years of high-temperature superconductivity it has been clear that one of the earliest areas of application and commercialization of these materials is as passive devices for microwave electronics. At the time of writing the first prototype products are entering service in the mobile communications industry and a wide range of other devices are at a relatively advanced stage of development. In the further future one can see a cryogenic integrated technology based on HTS materials and the related functional materials which have emerged within the last ten years.

Chirality in Multi-Dimensional Space With Application To Electromagnetic Characterisation of Multi-Dimensional Chiral and Semi-Chiral Media

Abstract: Novel classes of bianisotropic media, termed semi-chirals, plano-chirals, plano-semi-chirals and axi-chirals, are presented based on a reconsideration of the concept of chirality in multi-dimension...

Oxidation growth stresses in an alumina-forming ferritic steel measured by creep deflection.

Abstract: Deflection tests have been used to estimate the stresses developed in the alumina layer formed during short-term oxidation of a Fe-22Cr-5Al-0.3Y Fecralloy steel at 1000°C. Elastic analysis of the deflecting specimen is inappropriate under these test conditions because of the low creep strength of the alloy. Accordingly, a recent creep analysis has been used in this work using currently determined creep properties of the alloy substrate. The results of the analysis show that for the thin oxides produced (<1 μm), the planar stress within the oxide layer is everywhere compressive. Average values are approximately 850 MPa after 0.5 hr oxidation but reduce to <200 MPa after 6.5 hr. These values are very much less than would be expected under conditions of elastic deformation.

Degradation of navigational accuracy with Global Positioning System during periods of scintillation at equatorial latitudes

Abstract: The effect of ionospheric scintillation on navigational accuracy with the GPS (global positioning system) in the equatorial region is presented. The accuracy of position fixing with the GPS as indicated by the PDOP (position dilution of precision) factor is degraded when the raypath from the satellite shows deep fading. It is understood that navigation, particularly using a moderately sophisticated GPS receiver, in the equatorial zone will be severely affected during maximum sunspot number years.

Electrical properties of metal (indium)/polyaniline Schottky devices

Abstract: Schottky devices were fabricated by thermal evaporation of indium on chemically synthesized polyaniline, poly(o-anisidine), and poly( aniline-co-ortho-anisidine) copolymer. Electrical characterization of each of these devices was carried out using current (I)-voltage (V) and capacitance (C)-voltage (V) measurements. The value of various junction parameters such as rectification ratio, ideality factor, and barrier heights of an In/poly(aniline-co-o-anisidine) Shootky device were found to be 300, 4.41, and.4972 V compared to the values of 60, 5.5, and 0.5101 V obtain for an In/polyaniline device, respectively.

Sudden post-midnight decrease in equatorial F-region electron densities associated with severe magnetic storms

Abstract: . A detailed analysis of the responses of the equatorial ionosphere to a large number of severe magnetic storms shows the rapid and remarkable collapse of F-region ionisation during post-midnight hours; this is at variance with the presently accepted general behaviour of the low-latitude ionosphere during magnetic storms. This paper discusses such responses as seen in the ionosonde data at Kodaikanal (Geomagn. Lat. 0.6 N). It is also observed that during magnetic storm periods the usual increase seen in the h'F at Kodaikanal during sunset hours is considerably suppressed and these periods are also characterised by increased foF2 values. It is suggested that the primary process responsible for these dramatic pre- and post-midnight changes in foF2 during magnetic storms could be due to changes in the magnitude as well as in the direction of usual equatorial electric fields. During the post-midnight periods the change in electric-field direction from westward to eastward for a short period causes an upward E × B plasma drift resulting in increased h'F and decreased electron densities in the equatorial region. In addition, it is also suggested that the enhanced storm-induced meridional winds in the thermosphere, from the poles towards the equator, may also cause the decreases in electron density seen during post-midnight hours by spatially transporting the F-region ionisation southwards away from Kodaikanal. The paper also includes a discussion on the effects of such decreases in ionisation on low-latitude HF communications.

High Integrity Ada

Abstract: This paper describes the approach being taken by an ISO group to produce Guidelines for the use of Ada when developing high integrity applications.

Effect of sintering temperature on the characteristics of carbons based on mesocarbon microbeads

Abstract: A study was conducted to determine the effect of sintering temperature, ranging between 350 and 2700°C, on the characteristics of carbons made from mesocarbon microbeads (MCMB) of size 4–16 μm, formed in a coal tar pitch by heat treatment at 420 °C for 2.5 h, and separated by solvent extraction using a tar oil. It was found that the MCMB-based carbon plates heat treated to 1000 and 2700°C possess, respectively, an apparent density of 1.64 and 1.78 g cm-3, a weight loss of 11.5% and 15.3%, a volume shrinkage of 32.4% and 45.0%, an open porosity of 7.6% and 14.0%, a bending strength of 72 and 50 MPa, and an atomic C/H ratio of 30 and 417. The 2700°C heat-treated plates revealed homogeneity and fine isotropic structure.

Immobilization and Characterization of Lactate Dehydrogenase on TEOS Derived Sol-Gel Films

Abstract: Physical adsorption and physical entrapment techniques have been utilized for the immobilization of lactate dehydrogenase (LDH) on tetraethylorthosilicate (TEOS) derived sol-gel films. The enzyme (LDH) activity has been assayed as a function of time, temperature, pH and pyruvate concentration. The results of photometric measurements used for monitoring the reaction yield a response time of about 1 min, linearity over a concentration range of 0–1.5 × 10-3 M and detection limit of 5 × 10-5 M. The TEOS sol-gel films containing LDH have been found to be stable for about 30 days at temperatures 4 to 10°C.

Methods for Measuring Usability

Abstract: The tutorial explains the benefits of measuring usability as part of a user-centred design process, and introduces the participants to the methodology for usability measurement developed by the collaborative European ESPRIT MUSiC (Measurement of Usability in Context) project. The tutorial includes demonstration of the use of MUSiC tools, and class exercises to apply the methods to case studies.

A simple resistance network for calibrating resistance bridges

Abstract: This paper describes a simple, passive, low-cost resistance network, closely related to Hamon build-up resistors, that enables the calibration of dc and low-frequency ac resistance and conductance bridges. The network is configured so that the four component resistors can be connected to realize 35 distinct four-terminal resistances, all interrelated by the usual formulas for the series and parallel connections of resistors. Theoretical analysis and experimental results show that with due care in the design, the network can be readily constructed to achieve an accuracy of better than 1 /spl mu//spl Omega/ for resistances of the order of 100 /spl Omega/ (1:10/sup 8/) for angular frequencies from dc to 10/sup 4/ rad/s.

Elastic Scattering Corrections in AES and XPS. III. Behaviour of Electron Transport Mean Free Path in Solids for Kinetic Energies in the Range 100 eV<E<400 eV†

Abstract: Quantitative Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) depend on an accurate knowledge of the correct depth scale of emission of the signal electrons. This depends on both inelastic and elastic scattering processes occurring in the specimen under analysis. A previous paper showed that the depth scale in AES and XPS is significantly influenced by unexpected structure in the transport mean free path for electrons in high-atomic-number elements over the approximate energy range 100-400 eV. This behaviour is implicit in Mott cross-sections, which are known to be reliable and have been used in low-energy electron diffraction (for example) for decades. This paper examines the features of electron-atom elastic scattering over the range of energy and atomic number important in XPS and AES analysis. In particular, the third and fourth partial waves, together with Levinson's theorem are shown to give rise to the structure in the transport mean free path. This is a transport cross-section analogue of the Ramsauer-Townsend effect exhibited at low energies in total elastic scattering cross-sections.

Fracture and spallation of oxides

Abstract: Mechanical damage to thin, protective oxide layers arises principally in service from differential strains produced by temperature changes. For typical alumina- or chromia-forming alloys, in-plane tensile stresses are developed within the oxide layer during upward temperature transients and compressive stresses are produced during cooling. This paper reviews the fracture of oxide scales due to these imposed stresses, under both tensile and compressive loading conditions. Imposed strain rates can vary over many orders of magnitude and, since these are applied at high temperatures, the possibility exists that stress relaxation processes (generically termed ‘creep’) will reduce the development of high stresses within the oxide or at the oxide/metal interface and, thus, have a beneficial influence on the processes of scale failure. This aspect is considered in detail both by reviewing published data and by presenting new numerical results on the influence of metal creep strength on the growth characteristics ...