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.

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%.

Synthesis of nanocrystalline Ni0.5Zn0.5Fe2O4 ferrite and study of its magnetic behavior at different temperatures

Abstract: Ni0.5Zn0.5Fe2O4 ferrite nanocrystals with average diameter in the range of 1–2 nm have been synthesized by reverse microemulsion. X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM) are used to characterize the structural, morphological and magnetic properties. X-ray analysis showed that the nanocrystals possess cubic spinel structure. The absence of hysteresis, negligible remanence and coercivity at 300 K indicate the superparamagnetic character and single domain in the nanocrystalline Ni0.5Zn0.5Fe2O4 ferrite materials. The nanocrystalline Ni0.5Zn0.5Fe2O4 ferrite were annealed at 600 °C. As a result of heat treatment the average particle size increases from 2 nm to 5 nm and the corresponding magnetization values have increased to 21.69 emu/g at 300 K. However, at low temperature of 100 K, the annealed samples show hysteresis loop which is the characteristic of a superparamagnetic to ferromagnetic transition. In addition, a comparative study of the magnetic properties of Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from reverse microemulsion has been carried out with those obtained from the general chemical co-precipitation route.

Dibromocarbene Functionalization of Boron Nitride Nanosheets: Toward Band Gap Manipulation and Nanocomposite Applications

Abstract: We report the covalent functionalization of exfoliated boron nitride nanosheets (BNNSs) using dibromocarbene (DBC) species. The functionalization of BNNSs is enabled as the nanosheets are utilized as two-dimensional phase-transfer catalysts for the migration of carbenes across the organic–aqueous phase boundary. We postulate that BNNSs stabilize carbenes by forming B-CBr2 ylides and in turn act as the reaction substrate. DBC functionalization of BNNSs results in the formation of B–C and B–N bonds to the sp2-hybridized BNNS lattice via the formation of dibromo-bridged bicyclo BCN systems. The covalent functionalization was characterized using HR-TEM, AFM, EELS, XRD, EDX, ToF-SIMS, TGA, Raman, XPS, FTIR, and UV–vis techniques. Utilization of CBr2 groups as a means by which BNNSs may be integrated and interfaced with solvents, molecular species, and condensed-phase materials was demonstrated by grafting alkyl chains from the functional groups via alkyl/halogen exchange. Alkyl-functionalized BNNSs were integr...