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.

Higher accuracy analysis of instrumented indentation data obtained with pointed indenters

Abstract: We review current practice for describing force–displacement curves from pointed indenters, highlighting the consequences of the simplifications normally adopted. We derive two corrections, the 'variable epsilon factor' and the 'radial displacement correction.' These are especially important for highly elastic materials such as fused silica where the combined corrections can amount to 13% in the contact area, significantly increasing the accuracy of hardness and modulus results. In contrast, the so-called beta factor has minor importance. We compare our analytical results with finite element (FE) calculations and experimental results. Indenter area functions, obtained using the corrections, agree well with independent direct measurements by a traceably calibrated metrological atomic force microscope (AFM). Further formulae are derived to calculate the complete force–displacement curve of conical indenters and the indentation elastic and total energy. These formulae immediately identify a physical material limit above which a cone cannot generate plastic deformation; for a Berkovich indenter this is a hardness-to-modulus ratio of 0.18.

Gum Arabic‐Graft‐Polyaniline: An Electrically Active Redox Biomaterial for Sensor Applications

Abstract: The radical copolymerization of gum Arabic and polyaniline was done using peroxydisulfate as oxidant and initiator. The produced copolymer was electrically active, water‐soluble and has redox property. The redox behavior of the copolymer was studied using cyclic voltammetry and compared with the pure polyaniline. The UV‐vis, FTIR, 1H‐NMR, XRD, TGA and SEM techniques have been used for the characterization of grafted biomaterial. The copolymerization was systematically optimized and it was observed that solubility of the copolymers in water decreased with an increase in % G. The intrinsic viscosity of the copolymer has been estimated to be 10.8 dL/g, which indicates an appreciably high molecular weight for the copolymer. The shelf life and electrical conductivity of the copolymer were monitored to evaluate their potential application in the fabrication of semiconductor sensor devices. Results are discussed and proposed a plausible mechanism for the copolymerization.

Large-area, ensemble molecular electronics: Motivation and challenges

Abstract: We review charge transport across molecular monolayers, which is central to molecular electronics (MoE) using large-area junctions (NmJ). We strive to provide a wide conceptual overview of three main sub-topics. First, a broad introduction places NmJ in perspective to related fields of research, and to single molecule junctions (1mJ), in addition to a brief historical account. As charge transport presents an ultra sensitive probe for the electronic perfection of interfaces, in the second part ways to form both the monolayer and the contacts are described to construct reliable, defect-free interfaces. The last part is dedicated to understanding and analyses of current-voltage (I-V) traces across molecular junctions. Notwithstanding the original motivation of MoE, I-V traces are often not very sensitive to molecular details and then provide a poor probe for chemical information. Instead we focus on how to analyse the net electrical performance of molecular junctions, from a functional device perspective. Finally, we shortly point to creation of a built-in electric field as a key to achieve functionality, including non-linear current-voltage characteristics that originate in the molecules or their contacts to the electrodes.

Study of the interaction between the indentation size effect and Hall–Petch effect with spherical indenters on annealed polycrystalline copper

Abstract: Methods to obtain tensile stress–strain properties of materials from a practically non-destructive indentation test are of great industrial interest. Nanoindentation is a good candidate. However, to do this successfully, indentation size effects must be accounted for. An indentation size effect with spherical indenters has been shown for a range of fcc metals with relatively large grain size (Spary et al 2006 Phil. Mag. 86 5581–93); the increase in yield stress being proportional to the inverse cube root of indenter radius. Here, we investigate these differences further and present results for the indentation size effect with spherical indenters on Cu samples with a range of different grain sizes from 1 µm to single crystal. The important experimental control parameter, of the relative size of the indentation compared with the grain size, is also explored by using indenters of different radii on the different grain sized samples. When the grain size, d, is less than 6 times the radius of the projected contact area, a, a Hall–Petch-like behaviour is observed superimposed on the indentation size effect. For d > 6a the indentation size effect dominates. The two effects may be combined by addition in quadrature. This new parametric function is able to predict the indentation pressure in annealed copper given input values of indenter radius and grain size.

Strategy to improve the reproducibility of colloidal SERS.

Abstract: There is a pressing need to improve the reproducibility of surface enhanced Raman spectroscopy (SERS) measurements, if the technique is to be used routinely for trace analysis. This is particularly true for colloidal SERS, in which data reproducibility is dominated by the final shape and size of metal clusters produced during colloid aggregation. This study presents general guidelines for designing appropriate measurement strategies that can be used to identify and optimise crucial steps in a protocol that leads to better reproducibility of the results. We show that the data reproducibility can be improved by optimising vortexing time during colloid aggregation, which we attribute to the formation of more reproducible metal clusters under conditions of forced convection. The study also investigated the effects of different storage conditions on the data reproducibility of SERS during a 6-month study period. Storage conditions did not significantly influence the SERS reproducibility. However, at the end of 6 months, colloids that were stored (in plastic containers) at room temperature showed a difference in their quality, as mirrored by their different opto-physical properties. This was made apparent through the analysis of UV-vis spectroscopy measurements by principal component analysis.