Showing papers by "National Physical Laboratory published in 2006"

Reference data for the density and viscosity of liquid aluminum and liquid iron

Abstract: The available experimental data for the density and viscosity of liquid aluminum and iron have been critically examined with the intention of establishing a density and a viscosity standard. All experimental data have been categorized into primary and secondary data according to the quality of measurement specified by a series of criteria. The proposed standard reference correlations for the density of the aluminum and iron are characterized by standard deviations of 0.65% and 0.77% at the 95% confidence level, respectively. The overall uncertainty in the absolute values of the density is estimated to be one of ±0.7% for aluminum and 0.8% for iron, which is worse than that of the most optimistic claims but recognizes the unexplained discrepancies between different methods. The standard reference correlations for the viscosity of aluminum and iron are characterized by standard deviations of 13.7% and 5.7% at the 95% confidence level, respectively. The uncertainty in the absolute values of the viscosity of the two metals is thought to be no larger than the scatter between measurements made with different techniques and so can be said to be ±14% in the case of aluminum and ±6% in the case of iron.

Progress in Field Spectroscopy

Abstract: The measurement of spectral data in the field has an important role in remote sensing, and a long history, but instruments and methods to achieve this have serious limitations under all but the most ideal conditions. Problems arise from the instruments themselves, from the environment in which they are used, and from the methodologies that are commonly adopted. The variable most commonly sought from field measurements is spectral reflectance, or more strictly the bidirectional reflectance factor (BRF), but this is dependent to some extent on the instrument used to make the measurement, and the conditions of measurement, notably the sky irradiance distribution. In this paper we argue that field spectral measurements should be recorded in the appropriate SI units, which will normally be the derived units of radiance for the flux reflected from the target and irradiance for the incident energy. Reflectance data remain a convenient way to represent the energy interactions occurring at the surface, and they have value in generic spectral libraries, but ultimately they lack reproducibility unless accompanied by much more detailed metadata than is the norm in most spectral libraries.

Comparison between frequency standards in Europe and the USA at the 10-15 uncertainty level

Abstract: Istituto Elettrotecnico Nazionale Galileo Ferraris (IEN), National Institute of Standards and Technology (NIST), National Physical Laboratory (NPL), Laboratoire National de Metrologie et d'Essais—Observatoire de Paris/Systemes de Reference Temps Espace (OP) and Physikalisch-Technische Bundesanstalt (PTB) operate cold-atom based primary frequency standards which are capable of realizing the SI second with a relative uncertainty of 1 × 10−15 or even below. These institutes performed an intense comparison campaign of selected frequency references maintained in their laboratories during about 25 days in October/November 2004. Active hydrogen maser reference standards served as frequency references for the institutes' fountain frequency standards. Three techniques of frequency (and time) comparisons were employed. Two-way satellite time and frequency transfer (TWSTFT) was performed in an intensified measurement schedule of 12 equally spaced measurements per day. The data of dual-frequency geodetic Global Positioning System (GPS) receivers were processed to yield an ionosphere-free linear combination of the code observations from both GPS frequencies, typically referred to as GPS TAI P3 analysis. Last but not least, the same GPS raw data were separately processed, allowing GPS carrier-phase (GPS CP) based frequency comparisons to be made. These showed the lowest relative frequency instability at short averaging times of all the methods. The instability was at the level of 1 part in 1015 at one-day averaging time using TWSTFT and GPS CP. The GPS TAI P3 analysis is capable of giving a similar quality of data after averaging over two days or longer. All techniques provided the same mean frequency difference between the standards involved within the 1σ measurement uncertainty of a few parts in 1016. The frequency differences between the three fountains of IEN (IEN-CsF1), NPL (NPL-CsF1) and OP (OP-FO2) were evaluated. Differences lower than the 1σ measurement uncertainty were observed between NPL and OP, whereas the IEN fountain deviated by about 2σ from the other two fountains.

The use of a Monte Carlo method for evaluating uncertainty and expanded uncertainty

Abstract: The Guide to the Expression of Uncertainty in Measurement (GUM) is the internationally accepted master document for the evaluation of uncertainty. It contains a procedure that is suitable for many, but not all, uncertainty evaluation problems met in practice. This procedure constitutes an approximation to the general solution of the Markov formula, which infers the probability density function (PDF) for the output quantities (measurands) from the model of the measurement and the PDFs for the input quantities. This paper shows that a Monte Carlo method is an effective and versatile tool for determining the PDF for the measurands. This method provides a consistent Bayesian approach to the evaluation of uncertainty. Although in principle straightforward, some care is required in representing and validating the results obtained using the method. The paper provides guidance on optimizing the approach, identifies some pitfalls and indicates means for validating the results.

Cholesterol biosensor based on cholesterol esterase, cholesterol oxidase and peroxidase immobilized onto conducting polyaniline films

Abstract: Cholesterol oxidase, cholesterol esterase and peroxidase have been co-immobilized onto electrochemically prepared polyaniline films. These polyaniline–enzyme films characterized using spectroscopic techniques, have been used to fabricate a cholesterol biosensor. This polyaniline-based cholesterol biosensor has a response time of about 240 s, an apparent Km value as 75 mg dl−1 and can be used to estimate cholesterol concentration up to 500 mg dl−1. These polyaniline/cholesterol oxidase/cholesterol esterase films have a detection limit of 25 mg dl−1 with sensitivity of 0.042 μA mg dl−1. The enzyme films were found to be thermally stable up to 48 °C and have a shelf-life of about 6 weeks when stored at 4 °C. The values of the activation energy before and after the critical temperature were found to be 191 and 95.5 kJ mol−1, respectively.

Enhancement of crystalline perfection by organic dopants in ZTS, ADP and KHP crystals as investigated by high‐resolution XRD and SEM

Abstract: To reveal the influence of complexing agents on crystalline perfection, tristhiourea zinc(II) sulfate (ZTS), ammonium dihydrogen phosphate (ADP) and potassium hydrogen phthalate (KHP) crystals grown by slow-evaporation solution growth technique using low concentrations (5 x 10 -3 M) of dopants like ethylenediamminetetraacetic acid (EDTA) and 1,10-phenanthroline (phen) were characterized by high-resolution X-ray diffractometry (XRD) and scanning electron microscopy (SEM). High-resolution diffraction curves (DCs) recorded for ZTS and ADP crystals doped with EDTA show that the specimen contains an epilayer, as observed by the additional peak in the DC, whereas undoped specimens do not have such additional peaks. On etching the surface layer, the additional peak due to the epilayer disappears and a very sharp DC is obtained, with full width at half-maximum (FWHM) of less than 10 arcsec, as expected from the plane wave dynamical theory of X-ray diffraction for an ideally perfect crystal. SEM micrographs also confirm the existence of an epilayer in doped specimens. The ZTS specimen has a layer with a rough surface morphology, having randomly oriented needles, whereas the ADP specimen contains a layer with dendric structure. In contrast to ADP and ZTS crystals, the DC of phen-doped KHP shows no additional peak, but it is quite broad (FWHM = 28 arcsec) with a high value of integrated intensity, p (area under the DC). The broadness of the DC and the high value of p indicate the formation of a mosaic layer on the surface of the crystal. However, similar to ADP and ZTS, the DC recorded after etching the surface layer of the KHP specimen shows a very sharp peak with an FWHM of 8 arcsec. An SEM photograph of phen-doped KHP shows deep cracks on the surface, confirming the mosaicity. After removing the surface layer, the SEM pictures reveal a smooth surface. A similar trend is observed with other complexing agents, like oxalic acid, bipy and picolinic acid. However, only typical examples are described in the present article where the effects were observed prominently. The investigations on ZTS, ADP and KHP crystals, employing high-resolution XRD and SEM studies, revealed that some organic dopants added to the solution during the growth lead to the formation of a surface layer, due to complexation of these dopants with the trace metal ion impurities present in the solution, which prevents the entry of impurities, including the solvent, into the crystal, thereby assisting crystal growth with high crystalline perfection. The influence of organic dopants on the second harmonic generation efficiency is also investigated.

Probing systems for dimensional micro- and nano-metrology

Abstract: Quality control in the growing field of microsystems technology (MST) demands much higher resolution and accuracy of the testing equipment than conventional products. This is especially challenging in the field of probing. For tactile (i.e., contacting) measurement systems this is equivalent to a demand for miniaturization (in terms of form deviation and size of the contacting element, moving mass, probing force) and hence leads to a top down approach. Another approach is to qualify existing picturing microscopy techniques for three-dimensional measurements such as scanning probe microscopy and various optical microscopy techniques (bottom up approach) emphasizing the improvement of repeatability, linearity and calibration. The challenges and demands of micro systems technology on probing systems and different practical approaches for satisfying them will be presented with their special characteristics, fields and limits of application.

Effect of Ni doping on thick film SnO2 gas sensor

Abstract: Ni- and/or Al-doped and undoped SnO2 thick film gas sensors were prepared using screen printing technique and tested for their LPG gas sensitivity. Tin oxide powder was prepared using a chemical precipitation technique. The sensitivity, optimum working temperature and response time were investigated in relation to dopants as well as preparation route. The results show that the gas sensitivity is affected not only by the additive but the way it is added into the sensor material. The results indicated a reduction in grain size on Al and Ni doping. The results on resistance, response and recovery time were explained in terms of n–p junction formation between SnO2 and NiO, which increases the depletion barrier height.

Sol-gel-derived super-hydrophilic nickel doped TiO2 film as active photo-catalyst

Abstract: Pure and nickel doped TiO2 thin films on soda glass substrates were prepared by sol–gel dip coating process. The resulting films were annealed at 500 °C for 1 h and characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and UV–vis-near IR techniques. AFM reveals that doping of Ni ions suppresses the grain growth of crystal in the TiO2 film. The contact angle with water of these films was measured by sessile drop method. The undoped films could be entirely wetted by water after 1 h UV illuminations, while 0.5 mol% Ni ion-doped films become entirely wetted after 20 min UV illumination. The photo-catalytic activity was characterized by photo-catalytic degradation of aqueous methyl orange under UV radiation. It is found that, with a suitable amount (2–10 mol%), the Ni dopant increases the photo-catalytic activity of TiO2 films. The mechanism can be attributed to these processes: (1) as the anatase grain sizes decrease with Ni doping and the specific surface areas of doped TiO2 films increase, the charge transfer in TiO2 film is promoted; (2) by enhancing the electron–hole pair separation and inhibiting their recombination, the Ni dopant enhances the charge pair separation efficiency for doped TiO2 films.

Optimisation of interdigitated electrodes for piezoelectric actuators and active fibre composites.

Abstract: The optimisation of the interdigitated electrode (IDE) design for active fibre composites was performed using finite element analysis. The effect of the IDE geometry (electrode width and spacing) and electroceramic substrate thickness on the developed strain for bulk PZT substrates was modelled. The modelling results show that the highest strain is generated when the electrode width equals half the substrate thickness and for thin substrates the electrode finger spacing can be reduced to enable lower driving voltages. Approximately 80% of the maximum d33 strain can be achieved with an electrode separation to substrate thickness ratio greater than 4. The results present simple coherent guidelines for the optimisation of electrode geometry for piezoelectric actuators and active fibre composites.

Evolution of the ‘Guide to the Expression of Uncertainty in Measurement’

Abstract: A number of Joint Committees of the Bureau International des Poids et Mesures and other international organizations carry out particular tasks of common interest. The Joint Committee for Guides in Metrology (JCGM) has amongst its tasks the promotion of the 'Guide to the Expression of Uncertainty in Measurement' (GUM), the preparation of further documents for its broad application, and revision and promotion of the use of the 'International Vocabulary of Basic and General Terms in Metrology'. This paper summarizes the documents relating to the GUM planned by JCGM.

Coulomb blockade anisotropic magnetoresistance effect in a (Ga,Mn)As single-electron transistor.

Abstract: We observe low-field hysteretic magnetoresistance in a $(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ single-electron transistor which can exceed 3 orders of magnitude. The sign and size of the magnetoresistance signal are controlled by the gate voltage. Experimental data are interpreted in terms of electrochemical shifts associated with magnetization rotations. This Coulomb blockade anisotropic magnetoresistance is distinct from previously observed anisotropic magnetoresistance effects as it occurs when the anisotropy in a band structure derived parameter is comparable to an independent scale, the single-electron charging energy. Effective kinetic-exchange model calculations in $(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ show chemical potential anisotropies consistent with experiment and ab initio calculations in transition metal systems suggest that this generic effect persists to high temperatures in metal ferromagnets with strong spin-orbit coupling.

Microwave-accelerated synthesis and characterization of potato starch-g-poly(acrylamide)

Abstract: Using a very low concentration of potassium persulfate as initiator, acrylamide could be efficiently grafted onto potato starch under microwave irradiation and for the grafting O 2 removal from the reaction vessel was not required. Under optimal conditions, grafting and efficiency observed were 160% and 89%, respectively. Grafted starch was characterized by using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). It was observed that the microwave irradiation could significantly accelerate the synthesis of starch-graft-poly(acrylamide), because under identical conditions no grafting was observed in a conventional procedure. Viscosity, shear stability and water/saline solution retention of the microwave-synthesized grafted starch were studied and compared with that of the parent starch.

Nanostructured mesoporous tungsten oxide films with fast kinetics for electrochromic smart windows

Abstract: A potential driven self-assembly of sodium dodecyl sulfate/tungsten oxide aggregates at the electrolyte–electrode interface followed by template extraction and annealing yielded mesoporous thin films of electrochromic tungsten oxide (WO3). Electron microscopy images revealed that the films are characterized by a hitherto unreported hybrid structure comprising nanoparticles and nanorods with a tetragonal crystalline phase of WO3 with the measured lattice parameters: a = 0.53 nm and c = 0.37 nm. In addition to pentagonal voids characteristic of the tetragonal WO3 phase at the lattice scale, open channels and pores of 5–10 nm in diameter lie between the nanoparticles, which cumulatively promote rapid charge transport through the film. This resulted in colouration efficiency (ηmax~90 cm2 C−1 at λ = 900 nm) and switching kinetics (colouration time = 3 s and bleaching time = 2 s for a 50% change in transmittance) higher and faster than previously reported values for mesoporous WO3 films. Repetitive cycling between the clear and blue states has no deleterious effect on the electrochromic performance of the film, which is suggestive of its potential as a cathode in practical electrochromic windows.

X-ray microtomography studies of localised corrosion and transitions to stress corrosion cracking

Abstract: Two forms of high resolution X-ray tomographic experiments (i.e. synchrotron based X-ray microtomography and desktop microfocus computed X-ray tomography) are demonstrated in the present paper to illustrate the wide application of these techniques for qualitative and quantitative studies of localised corrosion and environmentally assisted cracking. Specifically, synchrotron based X-ray tomography was used to investigate the localised corrosion morphology within aluminium specimens when exposed in situ to a chloride environment while microfocus computed X-ray tomography was used to investigate the morphology and quantify the transition from localised corrosion to stress corrosion cracking in steel specimens exposed ex situ to a simulated corrosive condensate environment.

Modelling of nanomechanical nanoindentation measurements using an AFM or nanoindenter for compliant layers on stiffer substrates

Abstract: Finite element analysis (FEA) is used to model the nanoindentation process for a rigid, spherically shaped indenter acting on an elastic two-phase system of an elastic layer that is more compliant than the underlying elastic substrate. A review of current analytical equations to model this process is made and compared to FEA. The FEA results may be expressed analytically by a simple function that describes the reduced modulus value obtained with Oliver and Pharr's method for any modulus value, thickness of layer or radius of the indenter tip. This function is used to investigate B?ckle's rule, that to measure the properties of a layer, the indentation depth should be 10% or less of the total layer thickness. The results show that B?ckle's rule is invalid for layer thicknesses below 5??m and a new rule is developed which depends on the layer thickness, the indenter radius and the ratio of the reduced moduli of the substrate and overlayer. This rule is based on FEA data. We present a guide to the analysis of the maximum depth that may be indented in order to keep the uncertainty in the reduced modulus for the layer to better than 10%.

Electrochromic nanostructured tungsten oxide films by sol-gel: Structure and intercalation properties

Abstract: As-deposited sol-gel derived amorphous tungsten oxide films transform into nanostructured films with an interconnected framework of grains and pores and a dominant triclinic crystalline phase upon annealing at 250 degrees C. Transmission electron microscopy and scanning electron microscopy images clearly reveal the annealing-induced microstructural evolution for the film. Subsequent to lithium intercalation, the film annealed at 250 degrees C shows quasi-reversible structural changes, as ascertained by X-ray diffraction and Fourier transform infrared spectral data. Dynamic transmission modulation for film revealed a high optical modulation of 72% (gimel=650nm) and a coloration efficiency maximum of 132 cm(2) C-1 at 800 nm under a lithium intercalation level of x = 0.20. X-ray photoelectron spectroscopy of the W 4f core levels demonstrated a progressive increase in the W5+ content at the expense of W6+ proportion as the insertion coefficient was raised from 0 to 0.25, with 0.20 as the threshold value above which the W5+ content exceeds the W6+ proportion. A new W4+ state also appears which acts to lower the coloration efficiency for x >= 0.22. The presence of charged oxygen interstitials in the vicinity of electrochemically active tungsten sites is also responsible for the coloration efficiency decline at high ion insertion levels.