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.

Evaluation of tropospheric and stratospheric ozone trends over Western Europe from ground-based FTIR network observations

Abstract: Within the European project UFTIR (Time series of Upper Free Troposphere observations from an European ground-based FTIR network), six ground-based stations in Western Europe, from 79 degrees N to 28 degrees N, all equipped with Fourier Transform infrared (FTIR) instruments and part of the Network for the Detection of Atmospheric Composition Change (NDACC), have joined their efforts to evaluate the trends of several direct and indirect greenhouse gases over the period 1995-2004. The retrievals of CO, CH4, C2H6, N2O, CHClF2, and O-3 have been optimized. Using the optimal estimation method, some vertical information can be obtained in addition to total column amounts. A bootstrap resampling method has been implemented to determine annual partial and total column trends for the target gases. The present work focuses on the ozone results. The retrieved time series of partial and total ozone columns are validated with ground-based correlative data (Brewer, Dobson, UV-Vis, ozonesondes, and Lidar). The observed total column ozone trends are in agreement with previous studies: 1) no total column ozone trend is seen at the lowest latitude station Izana (28 degrees N); 2) slightly positive total column trends are seen at the two mid-latitude stations Zugspitze and Jungfraujoch (47 degrees N), only one of them being significant; 3) the highest latitude stations Harestua (60 degrees N), Kiruna (68 degrees N) and Ny-Alesund (79 degrees N) show significant positive total column trends. Following the vertical information contained in the ozone FTIR retrievals, we provide partial columns trends for the layers: ground-10 km, 10-18 km, 18-27 km, and 27-42 km, which helps to distinguish the contributions from dynamical and chemical changes on the total column ozone trends. We obtain no statistically significant trends in the ground-10 km layer for five out of the six ground-based stations. We find significant positive trends for the lower-most stratosphere at the two mid-latitude stations, and at Ny-Alesund. We find smaller, but significant trends for the 18 27 km layer at Kiruna, Harestua, Jungfraujoch, and Izana. The results for the upper layer are quite contrasted: we find significant positive trends at Kiruna, Harestua, and Jungfraujoch, and significant negative trends at Zugspitze and Izana. These ozone partial columns trends are discussed and compared with previous studies.

Characterisation methods for powder bed fusion processed surface topography

Abstract: Powder bed fusion (PBF) is a popular additive manufacturing (AM) process with wide applications in key industrial sectors, including aerospace, automotive, healthcare, defence However, a deficiency of PBF is its low quality of surface finish A number of PBF process variables and other factors (eg powders, recoater) can influence the surface quality It is of significant importance to measure and characterise PBF surfaces for the benefits of process optimisation, product performance evaluation and also product design A state-of-the-art review is given to summarise the current research work on the characterisation of AM surfaces, particularly PBF surfaces It is recognised that AM processes are different from conventional manufacturing processes and their produced surface topographies are different as well In this paper, the surface characterisation framework is updated to reflect the unique characteristics of PBF processes The surface spatial wavelength components and other process signature features are described and their production mechanisms are elaborated A bespoke surface characterisation procedure is developed based on the updated framework The robust Gaussian regression filter and the morphological filters are proposed to be used for the separation of the waviness component due to their robustness The watershed segmentation is enhanced to extract globules from the residual surface Two AM components produced by electron beam melting (EBM) and selective laser melting (SLM), are measured and characterised by the proposed methodology Both of the two filters are qualified for the extraction of melted tracks The watershed segmentation can enable the extraction of globules The standard surface texture parameters of different surface wavelength components are compared A set of bespoke parameters are intentionally developed to offer a quantitative evaluation of the globules

Study of Landau-Lifshitz-Looyenga's formula for dielectric correlation between powder and bulk

Abstract: Landau-Lifshitz-Looyenga's formula for correlation of bulk and powder dielectric parameters has been investigated at microwave and radio frequencies for a number of materials for wide enough experimental conditions of particle size, shape and packing fraction. It has been found that the formula gives good accuracy (about 3%) for dielectric constant determination for particle size not less than 50 μm. For fine powders (particle size less than 30 μm) the deduced dielectric constant is too low by about 3-8%. Dielectric loss determination from the formula is, however, less accurate, the deduced values being always too low by about 10-20%.

Nanostructured ceramics: a review of their potential

Abstract: The driving forces for the recent development of nanoscale features in conventional ceramic materials for both mechanical and functional reasons are discussed, and the causes of some of the resulting property enhancements are identified. Examples from the technical literature are examined, and the potential for future applications is assessed in terms of the tradeoff between enhanced properties and the added costs and difficulties of processing ultrafine powders.