986 research outputs found
All-optical hyperpolarization of electron and nuclear spins in diamond
Low thermal polarization of nuclear spins is a primary sensitivity limitation
for nuclear magnetic resonance. Here we demonstrate optically pumped
(microwave-free) nuclear spin polarization of and
in -doped diamond.
polarization enhancements up to above thermal equilibrium are observed
in the paramagnetic system . Nuclear spin polarization is
shown to diffuse to bulk with NMR enhancements of at
room temperature and at , enabling a route to
microwave-free high-sensitivity NMR study of biological samples in ambient
conditions.Comment: 5 pages, 5 figure
Residual-based adaptivity for two-phase flow simulation in porous media using Physics-informed Neural Networks
This paper aims to provide a machine learning framework to simulate two-phase
flow in porous media. The proposed algorithm is based on Physics-informed
neural networks (PINN). A novel residual-based adaptive PINN is developed and
compared with the residual-based adaptive refinement (RAR) method and with PINN
with fixed collocation points. The proposed algorithm is expected to have great
potential to be applied to different fields where adaptivity is needed. In this
paper, we focus on the two-phase flow in porous media problem. We provide two
numerical examples to show the effectiveness of the new algorithm. It is found
that adaptivity is essential to capture moving flow fronts. We show how the
results obtained through this approach are more accurate than using RAR method
or PINN with fixed collocation points, while having a comparable computational
cost
Sensitivity analysis using Physics-informed neural networks
The paper's goal is to provide a simple unified approach to perform
sensitivity analysis using Physics-informed neural networks (PINN). The main
idea lies in adding a new term in the loss function that regularizes the
solution in a small neighborhood near the nominal value of the parameter of
interest. The added term represents the derivative of the loss function with
respect to the parameter of interest. The result of this modification is a
solution to the problem along with the derivative of the solution with respect
to the parameter of interest (the sensitivity). We call the new technique to
perform sensitivity analysis within this context SA-PINN. We show the
effectiveness of the technique using 3 examples: the first one is a simple 1D
advection-diffusion problem to show the methodology, the second is a 2D
Poisson's problem with 9 parameters of interest and the last one is a transient
two-phase flow in porous media problem.Comment: 22 pages, 11 figure
Facile silane functionalization of graphene oxide
The facile silane functionalization of graphene oxide (GO) was achieved yielding vinyltrimethoxysilane-reduced graphene oxide (VTMOS-rGO) nanospheres located in the inter-layer spacing between rGO sheets via an acid–base reaction using aqueous media. The successful grafting of the silane agent with pendant vinyl groups to rGO was confirmed by a combination of Fourier-transform infrared (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The structure and speciation of the silane-graphene network (nanosphere) and, the presence of free vinyl groups was verified from solid-state magic angle spinning (MAS) and solution 13C and 29Si nuclear magnetic resonance (NMR) measurements. Evidence from Scanning Electron Microscopy (SEM), High-Resolution Transmission Electron Microscopy (HRTEM) and TEM-High-Angle Annular Dark-Field (TEM-HAADF) imaging showed that these silane networks aided the exfoliation of the rGO layers preventing agglomeration, the interlayer spacing increased by 10 Å. The thermal stability (TGA/DTA) of VTMOS-rGO was significantly improved relative to GO, displaying just one degradation process for the silane network some 300 °C higher than either VTMOS or GO alone. The reduction of GO to VTMOS-rGO induced sp2 hybridization and enhanced the electrical conductivity of GO by 105 S m−1
Simultaneous MQMAS NMR experiments for two half-integer quadrupolar nuclei
A procedure to acquire two Multiple-Quantum Magic Angle Spinning (MQMAS) NMR experiments with the same instrument time is presented. A triply tuned probe is utilized with multiple receivers to collect data with staggered acquisitions and thus more efficiently use the instrument time. The data for one nucleus is collected during the recovery delay of the other nucleus, and vice versa. The instrument time is reduced to 60-80% of the time needed for the single acquisition collection Specifically our approach is presented for recording triple-quantum (3Q) 17O and either 3Q or quintuple-quantum (5Q) 27Al MAS NMR spectra of a 1.18Na2O•5SiO2•Al2O3 glass gel
A multinuclear 1H, 13C and 11B solid-state MAS NMR study of 16- and 18-electron organometallic ruthenium and osmium carborane complexes
YesThe first 1H, 13C, 31P and 11B solid state MAS NMR studies of electron-
deficient carborane-containing ruthenium and osmium
complexes [Ru/Os(p-cym)(1,2-dicarba-closo-dodecaborane-1,2-
dithiolate)] are reported. The MAS NMR data from these 16-electron
complexes are compared to those of free carborane-ligand
and an 18-electron triphenylphosphine ruthenium adduct, and
reveal clear spectral differences between 16- and 18-electron
organometallic carborane systems in the solid state.We thank the Swiss National Science Foundation (grant no. PA00P2-145308 to NPEB), the ERC (grant no. 247450 to PJS), EPSRC (grant no. EP/F034210/1) and EC COST Action CM1105 for support. JVH thanks EPSRC and the University of Warwick for partial funding of the solid state NMR infrastructure at Warwick, and acknowledges additional support obtained through Birmingham Science City: Innovative Uses for Advanced Materials in the Modern World (West Midlands Centre for Advanced Materials Project 2), with support from Advantage West Midlands (AWM) and partial funding by the European Regional Development Fund (ERDF)
Multinuclear solid-state NMR investigation of Hexaniobate and Hexatantalate compounds
This work determines the potential of solid-state NMR techniques to probe proton, alkali, and niobium environments in Lindqvist salts. Na7HNb6O19·15H2O (1), K8Nb6O19·16H2O (2), and Na8Ta6O19·24.5H2O (3) have been studied by solid-state static and magic angle spinning (MAS) NMR at high and ultrahigh magnetic field (16.4 and 19.9 T). 1H MAS NMR was found to be a convenient and straightforward tool to discriminate between protonated and nonprotonated clusters AxH8–xM6O19·nH2O (A = alkali ion; M = Nb, Ta). 93Nb MAS NMR studies at different fields and MAS rotation frequencies have been performed on 1. For the first time, the contributions of NbO5Oμ2H sites were clearly distinguished from those assigned to NbO6 sites in the hexaniobate cluster. The strong broadening of the resonances obtained under MAS was interpreted by combining chemical shift anisotropy (CSA) with quadrupolar effects and by using extensive fitting of the line shapes. In order to obtain the highest accuracy for all NMR parameters (CSA and quadrupolar), 93Nb WURST QCPMG spectra in the static mode were recorded at 16.4 T for sample 1. The 93Nb NMR spectra were interpreted in connection with the XRD data available in the literature (i.e., fractional occupancies of the NbO5Oμ2H sites). 1D 23Na MAS and 2D 23Na 3QMAS NMR studies of 1 revealed several distinct sodium sites. The multiplicity of the sites was again compared to structural details previously obtained by single-crystal X-ray diffraction (XRD) studies. The 23Na MAS NMR study of 3 confirmed the presence of a much larger distribution of sodium sites in accordance with the 10 sodium sites predicted by XRD. Finally, the effect of Nb/Ta substitutions in 1 was also probed by multinuclear MAS NMR (1H, 23Na, and 93Nb)
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