Showing papers by "National Physical Laboratory published in 1993"

An Updated Edlén Equation for the Refractive Index of Air

Abstract: The Edlen equation for the refractive index of air, published in 1966, is still widely used. Subsequent to its formulation, however, improved data have become available on the density of air and the refractivity of water vapour. The practical temperature scale has also been revised and the carbon dioxide content of normal laboratory air has increased. These effects result in a discrepancy of typically 1 × 10-7. The consequent revision of the equation brings the agreement between calculation and experiment within an experimental uncertainty of ± 3 × 10-8.

The Influence of Structure on the Physico-chemical Properties of Slags.

Abstract: The current knowledge of the structures of silicate slags is summarised and the relationships between measures of the depolymerisation of the melt and various physical properties are examined. It is shown that the optical basicity when corrected for the cations required to charge-balance any AlO45- tetrahedra present, provides a reasonable measure of the depolymerisation of the melt and has the advantage over the (NBO/T) ratio that it compensates for cation effects. It is shown that the depolymerisation of the melt is the primary factor affecting most physical properties, the cations having only a secondary effect. The relationships between structure and the viscosity, electrical and thermal conductivity, diffusion coefficient, density, thermal expansion coefficient, thermodynamic and optical properties of melts are discussed.

XPS reference procedure for the accurate intensity calibration of electron spectrometers— results of a BCR intercomparison co‐sponsored by the VAMAS SCA TWA

Abstract: A calibration procedure has been developed for XPS instrument intensity scales. The calibration is achieved by measuring the spectrum for reference foils of Ag, Au and Cu for which the true spectrum has been measured usng the metrology spectrometer. The ratio of the measured and reference spectra at 1 eV intervals from 200 eV kinetic energy to 1600 eV provides the calibration known as Q(E). The calibrations from all reference foils for a given instrument and setting should be the same and may be averaged to enhance accuracy. An interlaboratory study involving 58 different instruments, covering 25 different models from 9 manufacturers, has been completed with over 1200 individual spectra measured. The results show that the calibrations are excellent but that care is required or systematic errors may arise from instrumental problems such as x-ray anode contamination, sample contamination, contributions from the sample holder and internal scattering in the spectrometer. However, these problems may be readily diagnosed. The Cu data often show carbon contamination and so those data are omitted from the assessments of Q(E). The instruments provide unmonochromated x-rays from Al and Mg anodes as well as monochromated Al sources. The Q(E) curves for the unmonochromated sources always agree with each other whereas that for the monochromated source may agree with those for the unmonochromated sources in some instruments but not in others. The reason for the differences may lie in stray magnetic fields or lens magnification effects. The different reference materials give the same Q(E) on any given instrument for a given set of operating conditions with a typical scatter of 3% but actually ranging from 0.99% in the best case to 19.49% where problems, as noted above, occurred. Changing the slits or pass energy can change the relative intensities at low and high energy by more than a factor of 2. However, even larger differences may occur between instruments of the same model operated under identical settings in different laboratories. It is thus shown that each individual instrument needs a separate and regular calibration and that such a procedure would reduce the present intensity variations of a factor of 7 over the energy range 200–1400 eV to an average scatter of 3% or less. The individual calibrations for Q(E) for all of the 58 instruments are presented and it is shown that the methodology and reference data work effectively and consistently in all known situations where the reference procedure is adequately conducted.

Skeletal Point Stresses in Circumferentially Notched Tension Bars Undergoing Tertiary Creep Modelled with Physically Based Constitutive Equations

Abstract: Constitutive equations are proposed in which the stress level dependence of creep rate is described by a sinh function, and two damage state-parameters are used to model the tertiary softening caused by: (i) grain boundary cavity nucleation and growth, and (ii) the multiplication of mobile dislocations. These constitutive equations are applicable to polycrystalline nickel-base superalloys and are used together with a continuum damage mechanics finite element based solver, DAMAGE XX, to study the behaviour of axisymmetrically notched tension bars and simulate the complex stress states that may be encountered at geometrical stress-raisers in high temperature components. Numerical studies of such bars show that their behaviour can be accurately represented in terms of a ‘skeletal effective stress’ located at a point within the notch throat, and the stress state at this point. It is shown that this conclusion is valid not only for those materials that fail by grain boundary cavitation alone, but also for materials such as superalloys where grain boundary cavitation is accompanied by mobile dislocation multiplication.

A Measurement of the Nuclear Magnetic Moment of the Helium-3 Atom in Terms of that of the Proton

Abstract: We have measured the ratio of the NMR spin precession frequencies of optically pumped low pressure helium-3 and of protons in water, using free induction decay. This was done by direct substitution in a flux density of 0,1 T of samples in an accurately spherical 25 mm diameter sample cell, without removing the sample cell from the magnet. The water measurements were corrected to 25°C. The two sets of measurements were each referred to a 10 mm diameter spherical reference water sample which was ratioed out. A new method was devised to measure and correct for the different averaging of the residual inhomogeneities in the magnetic field by the helions and by the protons in the water. Our final result is μh (3He)/μ'p= − 0,761 786 131 3(33). Using the CODATA 1986 recommended value for γp gives γh/2π = 32,434 092 1(97) MHz T-1. Combining the result with a measurement of γh (3He)/γp (H2 gives the shielding factor of H2 as σH2 = 26,350(19) × 10-6, in good agreement with a theoretical value. Combining our result with a measurement of μp/μB yields μh (3He)/μB= − 1,158 671 471(14) × 10-3.

ESR of vanadyl ions in Li2O.BaO.B2O3 glasses

Abstract: Glasses with composition xLi2O·yBaO·(100−x−y)B2Oin3 have been prepared over the range 0 ⩽ × ⩽35 and 0 ⩽ y ⩽ 35. The electron spin resonance spectra of VO2+ of these glasses have been measured at X-band (v ≈ 9.12 GHz) at room temperature (RT ≅ 295 K). Spin Hamiltonian parameters g∥, g⊥, A∥, A⊥, dipolar hyperfine coupling parameter, P, and Fermi contact interaction parameters, K, have been calculated. It is found that with a decrease in the B2O3 content from 85 mol % to 65 mol %, the value of g∥, g⊥, A∥, A⊥, P and K change from 1.936 to 1.940, 1.980 to 1.976, 172.2 to 166.6, 64.7 to 60.9, 117.4 to 116.1 and 0.819 to 0.789, respectively, showing that there is an improvement in the octahedral symmetry of the V4+ site. On the other hand, variations in the Li2O:BaO ratio for constant B2O3 have no effect on the vanadium site. Theoretical optical basicity, Λth, of the glasses has also been calculated and is in the range 0.45-0.53.

Elastic scattering corrections in AES and XPS: I. Two rapid Monte Carlo methods for calculating the depth distribution function

Abstract: We present improvements to the Monte Carlo methods used to calculate emission depth distribution functions (DDFs), emission function decay lengths (EFDLs) and attenuation lengths in quantitative XPS and AES. These reduce the time required to reach any desired level of accuracy by a factor of ∼500, so that even a personal computer can produce accurate results in 2 or 3 min. These rapid results are due to the new technique of averaging over an ensemble of possible inelastic events for each feasible set of elastic scatterings in the Monte Carlo simulation, closely related to methods that have been used successfully in nuclear physics for many years. No new approximations are required. The results are compared with exact analytical solutions for simple special cases; agreement is excellent

Superconductivity in Pr- and Ce-doped Bi2CaSr2Cu2Oy systems.

Abstract: We report here a systematic study of Pr and Ce doping (at the Ca site) in ${\mathrm{Bi}}_{2}$${\mathrm{CaSr}}_{2}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{\mathit{y}}$. For both Pr and Ce substitutions ${\mathit{T}}_{\mathit{c}}$ decreases with the dopant concentration. Results are interpreted in terms of the hole filling due to aliovalent substitution in the system. At a higher doping level a metal-insulator transition, is observed. Near the transition the localization length (${\mathrm{\ensuremath{\alpha}}}^{\mathrm{\ensuremath{-}}1}$) is calculated and the possibility of having local pairs in the insulating phase is invoked. Around the transition region a dimensional changeover from two to three dimensions is observed in the variable-range hopping (VRH) regime and its consequence on superconductivity is discussed. The lattice parameters have been estimated through x-ray diffractometry and the possibility of Pr and Ce existing in more than the 3+ oxidation state in the system is inferred. Results are also compared with the existing data on other rare-earth dopings (at the Ca site) in the same system.

Experimental validation of predicted temperature rises in tissue-mimicking materials.

Abstract: Increasingly, it is recognised that diagnostic ultrasound is capable of causing temperature rises sufficient to damage tissue. Predictions of this heating are often based on simplified models of both the in vivo conditions and the relevant physical effects. Few measurements have been made to verify these predictions, however, particularly for the tightly focused beams often employed in diagnosis. Furthermore, non-linear effects in both the acoustic field and the surrounding medium have largely been ignored in calculations. To provide an alternative way to estimate the heating, NPL has developed a measurement system to determine directly the temperature rise in tissue-mimicking materials. If necessary, the measurement results can be processed to model the effects of blood perfusion. The temperature is measured using thin-film thermocouples, which have essentially no interaction with the ultrasound or thermal fields. Measurements were made on transducers operating in the frequency range 2-10 MHz and with focal beam-widths from 1 to 3 mm. The measurements agree with theoretical predictions that use either the measured beam-profile or a simplified (Gaussian) model. The results are also compared with the standard soft-tissue models developed by the National Council on Radiation Protection and Measurements (NCRP) and jointly between the American Institute of Ultrasound in Medicine and the National Electrical Manufacturers Association (AIUM/NEMA). On average, the predictions of the NCRP formula are 15% higher than the measurements, confirming its validity as a worst-case model. The predictions based on the AIUM/NEMA formula, however, are typically 30% lower than the measured values.

Observation of the 5s (2)S(1/2)-4d (2)D(5/2) transition in a single laser-cooled trapped Sr(+) ion by using an all-solid-state system of lasers.

Abstract: The first application, to our knowledge, of an all-solid-state system of lasers to the study of a single cooled trapped Sr+ ion is described. Quantum jumps have been observed by driving the 674-nm 5s2S1/2−4d2D5/2 transition, and preliminary observations of the line shape are reported. An upper limit for the temperature of a single ion, derived from the 674-nm linewidth, was 200 mK. If non-Doppler sources of broadening such as unresolved Zeeman structure dominate, then the temperature limit would be even lower.

The effect of variations in the refractive index of industrial air upon the uncertainty of precision length measurement.

Abstract: The extent and causes of air refractivity differences in industrial environments have been assessed at the level of less than 1 part in 107 by the PTB and the NPL using a refractometer instrumentation package capable of both direct measurement and calculation of the refractive index of the atmosphere using an interference refractometer and atmospheric sensor measurements. The results of these measured and calculated values are discussed, together with the comparison of a number of industrial air refractivity measurement sensors.

Predictive thermochemistry and phase equilibria of slags

Abstract: It is well understood that the efficient recovery of values by pyrometallurgical processing of ores requires control of the slag chemistry. In an effort to improve the understanding of slags, a thermodynamic database on subsystems of the CaO-MgO-Fe-O-Al2O3-SiO2 system has been generated through critical assessment of the literature. Data for connecting systems of specific industrial interest are being added. The data can be combined using well-established thermodynamic principles to make calculations on the multicomponent systems of practical interest. Following a description of the calculations, this article illustrates specific applications of thermodynamic modeling to the extraction of copper, nickel, and precious metals; zinc extraction; purification of pig iron; meltdown in nuclear reactors; hot corrosion; and pollution control.

Reassessment of energy transfers in the quasielastic scattering of 250-3000 eV electrons at surfaces.

Abstract: Measurements are reported of the quasielastic scattering energy losses of electrons of 250\char21{}3000 eV from clean Cu, Ag, and Au in order to verify the large loss values recently observed by Erickson and Powell. Large loss values, if they were valid, would lead to a range of interesting analytical possibilities. However, here it is shown that the losses are small and are in agreement with the single-particle scattering model of Boersch et al. A null method is used which allows the energy of the unscattered beam to be referenced at each energy so that the energy losses may be determined directly with an accuracy of 15 meV. It is concluded that the measurements in the study of Erickson and Powell may have included a contribution from the cathode supply circuit that increased as the energy increased. The very low values of the losses observed here are predicted to be significant in surface analysis only for scattering from elements with atomic numbers less than 20.

Development of Standard Measurement Methods for Essential Properties of Ultrasound Therapy Equipment

Abstract: In a European collaborative project, partly funded by the EC Community Bureau of Reference (BCR), reliable methods of measurement for characterising the output and performance of ultrasound physiotherapy equipment have been developed. Experimental investigations using miniature hydrophones to scan the distribution of pressure in therapeutic fields have been undertaken in combination with theoretical simulations of the sound fields. Important parameters such as Beam Cross Sectional Area (BCSA), Effective Radiating Area (ERA) and Beam Nonuniformity Ratio (BNR) (characterising "Hot-spots": potentially harmful to patients) have been redefined, and these new definitions have been incorporated in a revision of IEC 150:1963. The reproducibility and accuracy of measurements of ERA based on these procedures are presented in detail for a variety of therapy fields. Furthermore, it is shown that the value of the BNR for any treatment head should not exceed 8. Values of effective intensity derived using the new procedures are shown to be significantly higher than those obtained using FDA (USA) definitions, a conclusion in agreement with the theoretical expectations. Measurements on four treatment heads were used to validate the procedures of the proposed revised standard. Values of ERA derived by the two laboratories were in agreement to within 2.5%.

Effect of High Pressure on the Electrical Conductivity of TlInX2 (X = Se, Te) Layered Semiconductors

Abstract: The dc electrical conductivity of TlInX2 (X = Se, Te) single crystals, parallel and perpendicular to the (001) c-axis is studied under high quasi-hydrostatic pressure up to 7.0 GPa, at room temperature. Conductivity measurements parallel to the c-axis are carried out at high pressures and down to liquid nitrogen temperatures. These materials show continuous metallization under pressure. Both compounds have almost the same pressure coefficient of the electrical activation energy parallel to the c-axis, d(ΔE∥)/dP = −2.9 × 10−10 eV/Pa, which results from the narrowing of the band gap under pressure. The results are discussed in the light of the band structure of these compounds.

Generalised regression problems in metrology

Abstract: This paper is concerned with generalised regression models in metrology In experiments where much is known about the nature of the error in the measurement data, it is possible to build comprehensive mathematical models which lead to better estimates of the required parameter values We indicate how efficient optimisation algorithms can be developed which exploit the structure of the corresponding regression problems and discuss applications in generalised distance regression and pressure metrology

Effect of lead substitution in bismuth-based cuprate systems containing both low- T c and high- T c phases

Abstract: We report the results of resistivity, ac-susceptibility, and specific-heat measurements carried out on samples of ${\mathrm{Bi}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pb}}_{\mathit{x}}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10+\mathit{x}}$ (0\ensuremath{\le}x\ensuremath{\le}0.7) that were characterized by x-ray diffraction and high-resolution scanning tunneling microscopy. The zero resistance temperature, ${\mathit{T}}_{\mathit{c}}$ (R=0), shows a peak at x=0.3 and the susceptibility data indicate that although a high-${\mathit{T}}_{\mathit{c}}$ 2:2:2:3 phase is present at 110 K, there is a second transition at 90 K that seems to indicate the existence of an additional low-${\mathit{T}}_{\mathit{c}}$ 2:1:2:2 phase. The specific-heat data unambiguously indicate a clear and distinct anomaly around ${\mathit{T}}_{\mathit{c}}$ for samples with x=0.3--0.4 and smearing of the anomaly for xg0.5. Based on the results obtained from these combinations of experiments, the role of lead substitution in Bi samples, particularly in the stabilization of the 2:2:2:3 phase has been discussed.

Improved Near-Infrared Detectors

Abstract: The paper describes the properties of a new large area InGaAs photodiode with near unity internal quantum efficiency. Measurements of linearity, spatial uniformity and dark current of the photodiodes are presented and compared with measurements on germanium photodiodes. The paper also describes the performance of a detector constructed from an arrangement of three of these photodiodes and shows how it has a calculable responsivity from 980 nm to 1 640 nm. It also describes how such a detector can be used as a sensor for a fibre-optic power meter with an uncertainty of around 0,1%.

A proposed design for a polarization-insensitive optical interferometer system with subnanometric capability

Abstract: A bidirectional, fringe-counting, two-beam interferometer system is described that can be used for precision length measurement with any laser source that is adequately coherent for the application. The two electrical phase-quadrature signals required for bidirectional electronic counting are obtained from photodetectors at the two interferometer outputs. A thin metal film design for the beam-splitter coating introduces the required phase difference of nearly 90° for both the perpendicular and parallel polarization components. Electronic alignment and control are performed by a system used to continuously monitor the signals produced from the outputs and automatically process these signals to achieve the optimum performance from the interferometer. Interferometers using the technique require a limited amount of optical alignment and all of the electronic adjustments are automatically controlled by the electronic system. These instruments are economical to manufacture and easy to apply in practice, readily achieving subnanometric accuracy in the measurement of changes in optical path.

Scattering in electron spectrometers, diagnosis and avoidance. I. Concentric hemispherical analysers

Abstract: The unwanted scattering of electrons in electron spectrometers leads to a loss of sensitivity, distortion of the spectral background and problems in calibrating spectrometer intensity/energy transmission functions. Here we develop three methods to analyse the extent of the contribution and how to reduce it to an insignificant level. The three diagnostic methods involve either the measurement of the region of the M4,5VV Auger electron peaks from Ag in AES, or the reflection of an electron beam from a biased sample, or the 3p1/2 photoelectron peak from Ag irradiated with Mg Kα x-rays. In this work analyses are made for concentric hemispherical analysers. In Part II, cylindrical mirror analysers are studied. It appears that in our spectrometers, a modified VG scientific Escalab II with either the original analyser or a 210 analyser, the major scattering contributions arise from high-energy electrons that strike the outer hemisphere giving rise to a spectrum of secondary electrons. These cause the scattered electron contribution to vary approximately as E, where Ep is the pass energy, to increae approximately in proportion to the spectrometer input slit area for AES studies, to fall approximately inversely with an increase in the lens magnification from 1 to 3 and, in the constant ΔE mode, to increase with the analysed electron energy. Thus, for each lens magnification, there exists a range of slits and pass energies for which the scattering may be maintained at <1%. In the constant ΔE mode, pass energies of ≥50 eV are generally safe, whereas in the constant ΔE/E mode, retardation ratios of ≤4 are required.

The standard ridged uniaxial testpiece: Computed accuracy of creep strain:

Abstract: The creep rupture behaviour and the accuracy of the measured strains for the uniaxial tensile testpiece with extensomenter ridges have been studied by use of the Continuum Damage Mechanics (CDM)-based creep finite element solver, DAMAGE XX. Multi-axial creep constitutive equations with multi-damage state variables were used in the computer modelling process. The procedure models the development of failed regions of the testpiece due to the stress concentration in the transition regions from the parallel portion to the extensometer ridges. The accuracy of the creep strain measurement for the testpiece was investigated with the variation of time, gauge length, and stress levels. The computer modelling results show that although the error of the creep strain measurement is a fuction of time, it is dependent upon the stress sensitivity of the material law. For power-law material the maximum error is invariant of the normalized stress, but dependent upon the value of n; and for the sinh-based material ...