National Physical Laboratory

About: National Physical Laboratory is a facility organization based out in London, United Kingdom. It is known for research contribution in the topics: Dielectric & Thin film. The organization has 7615 authors who have published 13327 publications receiving 319381 citations.

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.

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.

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.

Universal equation for argon gas cluster sputtering yields.

Abstract: An analysis is made of the sputtering yields of materials for argon gas cluster ion beams used in SIMS and XPS as a function of the beam energy, E, and the cluster size, n. The analysis is based on the yield data for the elements Si and Au, the inorganic compound SiO2, and the organic materials Irganox 1010, the OLED HTM-1, poly(styrene), poly(carbonate), and poly(methyl methacrylate). The argon primary ions have cluster sizes, n, in the range 100–16 000 and beam energies, E, from 2.5 to 80 keV. It is found that the elemental and compound data expressed as the yields, Y, of atoms sputtered per primary ion may all be described by a simple universal equation: Y/n = (E/An)q/[1 + (E/An)q−1] where the parameters A and q are established by fitting. The sputtering yields of the three organic materials are given as yield volumes expressed in nm3. For these, an extra parameter B is included multiplying the right-hand side of the equation where B is found by fitting to be of the order (0.18 nm)3 to (0.26 nm)3. This...