Enhancing sensitivity of quadrupolar nuclei in solid-state NMR with multiple rotor assisted population transfers

Rotor-assisted population transfer (RAPT) was developed as a method for enhancing MAS NMR sensitivity of quadrupolar nuclei by transferring polarization associated with satellite transitions to the central m=12-->-12 transition. After a single RAPT transfer, there still remains polarization in the satellite transitions that can be transferred to the central transition. This polarization is available without having to wait for the spin system to return to thermal equilibrium. We describe a new RAPT scheme that uses the remaining polarization of the satellites to obtain a further enhancement of the central transition by performing RAPT-enhanced experiments multiple times before waiting for re-equilibration of the spin system. For 27Al (I=5/2) in albite we obtain a multiple RAPT enhancement of 3.02, a 48% increase over single RAPT. For 93Nb (I=9/2) in NaNbO(3) we obtain a multiple RAPT enhancement of 5.76, an 89% increase over single RAPT. We also describe a data processing procedure for obtaining the maximum possible signal-to-noise ratio.     

Investigations on Pd:SnO2/porous silicon structures for sensing LPG and NO2 gas

P-type porous silicon (PS) structure has been prepared by anodic electrochemical etching process under optimized conditions. Photoluminescence studies of the PS structure show emission at longer wavelengths (red) for the excitation at 365 nm. Scanning electron microscope investigations of the PS surface confirm the formation of uniform porous structure, and the pore diameter have been estimated as 25 μm. Pd:SnO₂/PS/p-Si heterojunction with top gold ohmic contact developed by conventional methods has been used as the sensor device. Sensing properties of the device towards liquefied petroleum gas (LPG) and NO₂ gas have been investigated in an indigenously developed sensor test rig. The response and recovery characteristics of the sensor device at different operating temperatures show short response time for LPG. From the studies, maximum sensitivity and optimum operating temperature of the device towards LPG and NO₂ gas sensing has been estimated as 69% at 180 °C and 52% at 220 °C, respectively. The developed sensor device shows a short response time of 25 and 57 s for sensing LPG and NO₂ gases, respectively. Paper presented at the Third International Conference on Ionic Devices (ICID 2006), Chennai, Tamilnadu, India, Dec. 7–9, 2006.     

Finite element simulation of wave propagation in an axisymmetric bar

An accurate solution for high-frequency pulse propagation in an axisymmetric elastic bar is obtained using a new finite element technique that yields accurate non-oscillatory solutions for wave propagation problems in solids. The solution of the problem is very important for the understanding of dynamics experiments in the split Hopkinson pressure bar (SHPB). In contrast to known approaches, no additional assumptions are necessary for the accurate solution of the considered problem. The new solution helps to elucidate the complicated distribution of parameters during high-frequency pulse propagation down the bar as well as to estimate the applicability of the traditional dispersion correction used in the literature for the analysis of wave propagation in a finite bar. Due to the dimensionless formulation of the problem, the numerical results obtained depend on Poisson's ratio, the length of the bar and the pulse frequency, and are independent of Young's modulus, the density and the radius of the bar.     

A cold thermionically emitting dusty air plasma formed by radiativeheating of graphite particulates

Formation of an atmospheric pressure dusty air plasma is explored experimentally in this paper. The plasma is created by seeding an air flow with graphite particles and irradiating the particulates with a focused CO₂ laser beam. The graphite particles are, thus, heated to thermionically emitting temperatures, and average particle temperatures and average particle number densities are measured. The presence of charges is inferred both from these measured quantities using a simple theoretical transient model, and experimentally by applying a dc bias across the irradiated region. It is found that an electron density of ~6.7 × 10⁵ cm^-3 (6.7 × 10¹¹ m^-3) can be produced at steady state in the presence of O₂. This value can be increased to 3.6 × 10⁷ cm^-3 (3.6 × 10¹³ m ^-1) in the ideal case where an electron attachment to O₂ is suppressed and where a lower work function particulate is used     

Inclusive ϕ production inK − p interactions at 110 GeV/c and search for structure in φ π−

Results on inclusive ? production inK ^? p interactions at 110 GeV/c are presented. The production cross section is found to be larger than in πp andpp interactions at similar energies, suggesting OZI allowed \(s\bar s\) fusion to be the dominant mechanism in ? production. Thex distributions of ? and \(\bar K^{*0} \) are found to be similar to each other over the entirex range suggesting an overall strangeness suppression factor of 0.20±0.04 in the sea to be the dominant source of the difference in the cross section for ? and \(\bar K^{*0} \) . There is no evidence of a narrowφπ ^? state around 2.1 GeV/c² as suggested byK ⁺ experiments, but there is some excess of events in the region 1.94?1.98 GeV/c² consistent with theF-meson mass as observed ine ⁺ e ^? experiments.     

Improved electrochemical properties of Sr2+-doped LiNi0.8Co0.2O2 synthesized via a sol-gel route using tartaric acid as a chelating agent

Single-phase undoped LiNi_0.8Co_0.2O₂ and Sr²⁺-doped LiNi_0.8Co_0.2O₂ were synthesized by a low temperature tartaric acid assisted sol-gel method. Small quantities of Sr²⁺ were used as dopants in order to improve the electrochemical characteristics, especially the capacity and cycling performance of LiNi_0.8Co_0.2O₂. The electrochemical performance of the undoped material was promising with a first discharge capacity of 174 mAh/g and 165 mAh/g after 10 cycles with a 100% cycling efficiency in the tenth cycle. Addition of Sr²⁺ for Li in minimum quantities with the Sr²⁺/Li⁺ dopant mole ratio ranging from 10⁻⁴ to 10⁻⁸ resulted in improved electrochemical properties for dopant mole ratio of 10⁻⁶. The first discharge capacity was 182 mAh/g and the tenth was 174 mAh/g at the 10th discharge. The synthesis of Sr²⁺-doped LiNi_0.8Co_0.2O₂ and its improved electrochemical properties have been discussed for the first time. The improved electrochemical properties of Sr²⁺-doped LiNi_0.8Co_0.2O₂ system are explained based on defect models.     

Numerical simulation of partially coherent broadband optical imaging using the finite-difference time-domain method

Rigorous numerical modeling of optical systems has attracted interest in diverse research areas ranging from biophotonics to photolithography. We report the full-vector electromagnetic numerical simulation of a broadband optical imaging system with partially coherent and unpolarized illumination. The scattering of light from the sample is calculated using the finite-difference time-domain (FDTD) numerical method. Geometrical optics principles are applied to the scattered light to obtain the intensity distribution at the image plane. Multilayered object spaces are also supported by our algorithm. For the first time, numerical FDTD calculations are directly compared to and shown to agree well with broadband experimental microscopy results.     

Cosmic microwave background polarization signals from tangled magnetic fields

Tangled, primordial cosmic magnetic fields create small rotational velocity perturbations on the last scattering surface of the cosmic microwave background radiation. For fields which redshift to a present value of B0 = 3 x 10(-9) G, these vector modes are shown to generate polarization anisotropies of order 0.1-4 microK on small angular scales (500相似文献   

Influence of Sm<Superscript>3+</Superscript> ion in structural,morphological, and electrochemical properties of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> synthesized by microwave calcination

LiSm_xMn_2–xO₄ samples were synthesized via co-precipitation technique. The structural properties of the synthesized materials were studied using X-ray diffraction analysis and it confirmed the cubic spinel structure for all the compounds. The lattice parameter of LiMn₂O₄ was observed to be 8.2347 Ǻ and it decreased with Sm³⁺ concentration, due to the shrinkage in cell volume aided by higher binding energy between Sm-O bond. The SEM micrographs were analyzed using Image processing software (Image-J) to ascertain the pore and grain properties. The microwave synthesis had been observed to control the bulk grain formation and had yielded lesser porous and nanoparticles. The particle size distributions obtained through photocross correlation laser diffraction analysis had shown that LiMn₂O₄ with 60 nm and Sm-doped compounds with ∼30 nm, respectively. The cyclic voltammetry studies had revealed the decrease in electrocatalytic behavior in the initial cycle for compounds doped with Sm³⁺ ion. The initial capacities of LiMn₂O₄, LiSm_0.05Mn_1.95O₄ and LiSm_0.10Mn_1.90O₄ substituted compounds were observed to be 134.87 mAhg⁻¹, 132.22 mAhg⁻¹ and 126.41 mAhg⁻¹, respectively. The cells were simulated using 1D model namely Dualfoil5.1 program. The simulated results coincide well with the measured results. The cycle life studies reveal 93% capacity retention of samarium-0.05-doped samples when compared with 78.4% of the LiMn₂O₄.