Light emitting diodes, LEDs, historically have been used for indicators and produced low amounts of heat. The introduction of high brightness LEDs with white light and monochromatic colors have led to a movement towards general illumination. The increased electrical currents used to drive the LEDs have focused more attention on the thermal paths in the developments of LED power packaging. The luminous efficiency of LEDs is soon expected to reach over 80 lumens/W, this is approximately 6 times the efficiency of a conventional incandescent tungsten bulb. Thermal management for the solid-state lighting applications is a key design parameter for both package and system level. Package and system level thermal management is discussed in separate sections. Effect of chip packages on junction to board thermal resistance was compared for both SiC and Sapphire chips. The higher thermal conductivity of the SiC chip provided about 2 times better thermal performance than the latter, while the under-filled Sapphire chip package can only catch the SiC chip performance. Later, system level thermal management was studied based on established numerical models for a conceptual solid-state lighting system. A conceptual LED illumination system was chosen and CFD models were created to determine the availability and limitations of passive air-cooling.

Thermal management of LEDs: Package to system

Spatial encoding and spatial selection methods in high-resolution NMR spectroscopy.

Selective measurements of long-range homonuclear J-couplings in solid-state NMR

A quarter of a century of SERF: The progress of an NMR pulse sequence and its application

Tetramantanes, and all diamondoid hydrocarbons, possess carbon frameworks that are superimposable upon the cubic diamond lattice. This characteristic is invaluable in assigning their (1)H and (13)C NMR spectra because it translates into repeating structural features, such as diamond-cage isobutyl moieties with distinctively complex methine to methylene signatures in COSY and HMBC data, connected to variable, but systematic linkages of methine and quaternary carbons. In all tetramantane C22H28 isomers, diamond-lattice structures result in long-range (4)JHH, W-coupling in COSY data, except where negated by symmetry; there are two highly symmetrical and one chiral tetramantane (showing seven (4)JHH). Isobutyl-cage methines of lower diamondoids and tetramantanes are the most shielded resonances in their (13)C spectra (<29.5 ppm). The isobutyl methylenes are bonded to additional methines and at least one quaternary carbon in the tetramantanes. W-couplings between these methines and methylenes clarify spin-network interconnections and detailed surface hydrogen stereochemistry. Vicinal couplings of the isobutyl methylenes reveal positions of the quaternary carbons: HMBC data then tie the more remote spin systems together. Diamondoid (13) C NMR chemical shifts are largely determined by α and β effects, however γ-shielding effects are important in [123]tetramantane. (1)H NMR chemical shifts generally correlate with numbers of 1,3-diaxial H-H interactions. Tight van der Waals contacts within [123]tetramantane's molecular groove, however, form improper hydrogen bonds, deshielding hydrogen nuclei inside the groove, while shielding those outside, indicated by Δδ of 1.47 ppm for geminal hydrogens bonded to C-3,21. These findings should be valuable in future NMR studies of diamondoids/nanodiamonds of increasing size.

NMR spectral properties of the tetramantanes - nanometer-sized diamondoids.

Small J coupling constants are closely relevant to the assignment of conformational preferences for medium- to large-sized molecules and the valuable structural information about the backbone dihedral angles of biological molecules. Thus, accurate measurement for small J coupling constants is extremely important, which has long been recognized. However, the accurate measurement of small J coupling constants is limited by the natural linewidth since the line splitting arising from small J couplings are usually merged into the related peaks. A cornucopia of NMR spectral techniques has been designed for the achievement of small J coupling information. Generally, the existing techniques can be divided into two types. One is to directly extract small J coupling constants without J multiplication, while another is on the basis of J multiplication. Performance of the existing techniques has been evaluated and the results show that these approaches can be adapted for a wide range of samples, ranging from complex chemical molecules to biological samples, even for the samples under inhomogeneous magnetic fields. These spectral editing techniques provide complementary strategies for the quantification of small J coupling constants. In this review, the mechanisms of these approaches are presented with the analysis of their corresponding applicability and efficiencies.

Accurate Measurement of Small J Couplings

Intermolecular zero-quantum coherences (iZQCs) originating from distant dipolar interactions between spins in different molecules provide an innovative way for nuclear magnetic resonance (NMR) spectroscopy. Since the field experienced by iZQC signals is in the dipolar correlation distance, it is naturally to apply iZQCs for resolution enhancement in NMR spectroscopy, especially under the spatially and/or temporally varying magnetic fields. Two theoretical frames, classical distant dipolar field and quantum-mechanical intermolecular multiple-quantum coherence treatments, are available for the description of iZQC signal evolution. A variety of iZQC spectroscopic techniques have been established and most of them take advantage of two-dimensional acquisition to recover a one-dimensional high-resolution spectrum with information on chemical shifts, relative peak areas, J -coupling constants, and multiplet patterns. In this review, two comprehensive descriptions of an iZQC signal are presented first. The existing iZQC techniques are then systematically described. The details and underlying mechanisms of these techniques are discussed. Finally, the in vivo applications of iZQC spectroscopic techniques are given.

Intermolecular Zero Quantum Coherence in NMR Spectroscopy

Conference Name:2nd International Conference on Opto-Electronics Engineering and Materials Research, OEMR 2013. Conference Address: Zhengzhou, Henan, China. Time:October 19, 2013 - October 20, 2013.

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陈忠

Papers

Accurate Measurement of Small J Couplings

Intermolecular Zero Quantum Coherence in NMR Spectroscopy

Heat Dissipation Simulation Research of LED Downlight and Optimization of Light Source Layout