Showing papers by "National Physical Laboratory published in 2004"

Hertz-level measurement of the optical clock frequency in a single 88Sr+ ion.

Abstract: The frequency of the 5s 2S1/2–4d 2D5/2 electric quadrupole clock transition in a single, trapped, laser-cooled 88Sr+ ion has been measured by using an optical frequency comb referenced to a cesium fountain primary frequency standard. The frequency of the transition is measured as 444,779,044,095,484.6 (1.5) hertz, with a fractional uncertainty within a factor of 3 of that of the cesium standard. Improvements required to obtain a cesium-limited frequency measurement are described and are expected to lead to a 88Sr+ optical clock with stability and reproducibility exceeding that of the primary cesium standard.

Critical Review of the Current Status of Thickness Measurements for Ultrathin SiO2 on Si, Part V: Results of a CCQM Pilot Study

Abstract: A study was carried out for the measurement of ultrathin SiO on (100) and (111) orientation silicon wafer in the thickness range 1.5-8 nm. XPS, medium-energy ion scattering spectrometry (MEIS), nuclear reaction analysis (NRA), RBS, elastic backscattering spectrometry (EBS), SIMS, ellipsometry, gazing-incidence x-ray reflectometry (GIXRR), neutron reflectometry and transmission electron microscopy (TEM) were used for the measurements. Water and carbonaceous contamination about 1 nm were observed by ellipsometry and adsorbed oxygen mainly from water at thickness of 0.5 nm were seen by MEIS, NRA, RBS and GIXRR. The different uncertainty of the techniques for the scaling constant were also discussed.

The viscosity of aluminium and its alloys--A review of data and models

Abstract: In this paper the available data for the measurement of viscosity of aluminium and its alloys are reviewed. Most measurements are performed with an oscillating vessel technique and the merits of this technique are discussed. The purity of the aluminium affects the measured viscosity values and we recommend a value of between 1.0–1.4 mPa·s for the pure element at the melting point. Although studies of the viscosity of aluminium alloys are limited, the effects of elemental additions to the alloy are similar to those for additions to the base metal. Thus an increase in concentration of Ti, Ni, Cr, Mn, Mg tends to increase the viscosity whereas the viscosity decreases with increasing Zn and Si concentrations. Also purification of an alloy decreases the viscosity. There is a wide variety of models ranging from those based on empiricism to thermodynamic methods. With the present quality of input data it is probably better to use a simple rather than a sophisticated model.

Equations for the Calculation of the Thermo-physical Properties of Stainless Steel

Abstract: Equations have been derived to calculate values of the thermophysical properties of all stainless steels for temperatures between 300 and 1800 K (austenitic 3 series, ferritic-4 series and precipitation-hardened 6-series alloys). Values of the following properties are given in both figures and tables: density (ρ), thermal expansion coefficient (α), heat capacity (Cp), enthalpy (HT−H298), thermal conductivity (λ) and thermal diffusivity (a), electrical resistivity (R), viscosity (η) and surface tension (γ).

Altitude profiles of aerosol BC, derived from aircraft measurements over an inland urban location in India

Abstract: [1] Altitude profiles of aerosol black carbon (BC) in the atmospheric boundary layer and above it were measured for the first time in India, over the urban location Hyderabad, onboard an aircraft during two consecutive days of February 2004. The profiles on both the days were consistent, and showed a rapid decrease in BC concentration within the boundary up to ∼550 m AGL (where convective activity prevailed). Sodar measurements from the nearby location revealed the mean boundary layer height to be ∼600 m during the flight period. The decrease in BC above the boundary layer was quite weak up to ∼2.2 km AGL.

Subhertz-linewidth Nd:YAG laser

Abstract: Light from a Nd:YAG laser at 1064 nm is independently stabilized to two Fabry–Perot etalons situated on separate vibration-isolation platforms. A heterodyne beat measurement shows their relative frequency stability to be at the part-in-1015 level at 5 s and the relative linewidth to be less than 1 Hz.

Quantitative analytical atomic force microscopy: a cantilever reference device for easy and accurate AFM spring-constant calibration

Abstract: Calibration of atomic force microscope (AFM) cantilevers is necessary for the measurement of nanonewton and piconewton forces, which are critical to analytical applications of AFM in the analysis of polymer surfaces, biological structures and organic molecules. We have developed a compact and easy-to-use reference artefact for this calibration by bulk micromachining of silicon, which we call a cantilever microfabricated array of reference springs (C-MARS). Two separate reference cantilever structures, each nominally 3 µm thick, are fabricated from a single crystal silicon membrane. A binary code of surface oxide squares (easily visible in light, electron and atomic force microscopy) makes it easy to locate the position of the AFM tip along the length of the cantilevers. Uncertainty in location is the main source of error when calibrating an AFM using reference cantilevers, especially for those having spring constants greater than around 10 N m −1 . This error is effectively eliminated in our new design. The C-MARS device spans the range of spring constants from 25 N m −1 down to 0.03 N m −1 important in AFM, allowing almost any contact-mode AFM cantilever to be calibrated easily and rapidly.

The Route to Atomic and Quantum Standards

Abstract: Over the past half-century, there has been a shift away from standards based on particular artifacts toward those based on physical effects, the most stable being based on quantum properties of systems. This change was proposed at the end of the 19th century but is still not complete at the start of the 21st. We discuss how this vision has been implemented through recent advances in science and metrology and how these may soon lead to an SI system finally free from artifact standards, with a consistency based on fundamental constants.

Vacuum-deposited nanocrystalline polyaniline thin film sensors for detection of carbon monoxide

Abstract: Gas sensors based on metal oxide semiconductors are widely used for detection of toxic gases like CO. These sensors suffer from the draw back of limited lifetime due to their operation at high temperatures. On the other hand it is often difficult to achieve high sensitivity, selectivity, and specificity in presence of a mixture of gases. To overcome this, vacuum-deposited nanocrystalline polyaniline thin films have been found suitable for detection of carbon monoxide. The sensors using these films operate at room temperature. A high sensitivity, selectivity, specificity, and a fast response time have been achieved. High sensitivity, selectivity, and specificity are obtained by doping the polyaniline powder during synthesis. These thin film sensors are highly stable, reliable, and their gas detection limit is low.

Application of high temperature DSC technique to nickel based superalloys

Abstract: Heat capacity is a critical input parameter in mathematical models of solidification and casting. It appears in its own right, and is required for the extraction of thermal conductivity from thermal diffusivity data. It also provides a measure of the latent heat associated with changes of phase, and the precipitation or dissolution of precipitates. Calorimetry is a well-established technique and is used to solve a wide range of materials problems such as studying precipitation or phase changes in alloy systems, and the kinetics of these phase transformations. The quality and range of data that can be obtained will be demonstrated by applying it to nickel base superalloys. This paper discusses important aspects of the measurement including control of the atmosphere, suitable reference materials for temperature and enthalpy calibration, size of the sample and the rate of heating and cooling. The optimised DSC technique was used to investigate the properties of two nickel based superalloys (CMSX4 and IN738LC), comparing the results with previous values and determining sources of error in the method.

Size-dependent conductivity-type inversion in Cu 2 O nanoparticles

Abstract: X-ray photoemission spectroscopy and optical absorption studies have been carried out on ${\mathrm{Cu}}_{2}\mathrm{O}$ nanoparticles prepared using the activated reactive evaporation technique. A strong shift of the valence band edge (from 0.7 to 1.8 eV) has been observed on reduction of the nanoparticle size from 20 nm to 4 nm. The size-dependent modifications in the energy level diagram and ${\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}\ensuremath{-}\mathrm{C}\mathrm{u}}_{2}\mathrm{O}$ nanoparticle Schottky junction characteristics confirm the observation of the inversion of conductivity type from $p$-type bulk conductivity to $n$-type conductivity in ${\mathrm{Cu}}_{2}\mathrm{O}$ nanoparticles at lower dimensions. This study can initiate a further methodology for tailoring the semiconductor properties of a material by controlling the nanoparticle size.

A small-body portable graphite calorimeter for dosimetry in low-energy clinical proton beams

Abstract: Calorimetry has been recommended and performed in proton beams for some time, but never has graphite calorimetry been used as a reference dosimeter in clinical proton beams. Furthermore, only a few calorimetry measurements have been reported in ocular proton beams. In this paper we describe the construction and performance of a small-body portable graphite calorimeter for clinical low-energy proton beams. Perturbation correction factors for the gap effect, volume averaging effect, heat transfer phenomena and impurity effect are calculated and applied in a comparison with ionization chamber dosimetry following IAEA TRS-398. The ratio of absorbed dose to water obtained from the calorimeter measurements and from the ionization measurements varied between 0.983 and 1.019, depending on the beam type and the ionization chamber calibration modality. Standard uncertainties on these values varied between 1.9% and 2.5% including a substantial contribution from the kQ values in IAEA TRS-398. The (Wair/e)p values inferred from these measurements varied between 33.6 J C(-1) and 34.9 J C(-1) with similar standard uncertainties. A number of improvements for the small-body portable graphite calorimeter and the experimental set-up are suggested for potential reduction of the uncertainties.

Measurement of the electric quadrupole moment of the 4d2D5/2 level in 88Sr+.

Abstract: The quadrupole moment of the $4d\text{ }^{2}D_{5/2}$ level in $^{88}\mathrm{S}\mathrm{r}^{+}$ has been measured to be $2.6(3)e{a}_{0}^{2}$, where ${a}_{0}$ is the Bohr radius and $e$ the elementary charge. A single laser-cooled strontium ion was confined in an end cap trap with a variable dc quadrupole potential, and measurements were made on the $5s\text{ }^{2}S_{1/2}$--$4d\text{ }^{2}D_{5/2}$ transition at 674 nm using a femtosecond optical frequency comb. This work shows that measurements of the unperturbed $^{88}\mathrm{S}\mathrm{r}^{+}$ transition frequency with sub-Hz uncertainty are possible and is important in understanding the reproducibility of ion trap optical frequency standards.

The simulation and measurement of the response of Josephson junctions to optoelectronically generated short pulses

Abstract: A new method of driving series arrays of non-hysteretic Josephson junctions using optoelectronically generated pulses to synthesize a variable output voltage is described. Electrical pulses from a photodiode are coupled to the array, and the mean output voltage after filtering is proportional to the pulse repetition frequency with fundamental accuracy. Numerical simulations of the behaviour of the Josephson junctions have been carried out, and these results have been verified by experiment. This technique can be easily extended by using two photodiodes with opposite bias polarities in order to synthesize bipolar alternating or direct voltages.