Effects of Humidity on the Electro-Optical-Thermal Characteristics of High-Power LEDs

LEDs are subjected to environments with high moisture in many applications. In this paper, the experiments reveal photometric and colorimetric degradation at high humidity. Corresponding spectral power analysis and parameter extraction indicate that the flip-chip bonded LED samples show accelerated chip failure compared to the conventionally bonded samples. The chip-related failure induces greater heat accumulation, which correlates with the increase in heating power observed in the package. However, the temperature rise and thermal resistance for the flip-chip bonded LEDs do not increase substantially as compared to the conventionally bonded LEDs. This is because the junction temperature can be reduced with a flip-chip die-bonding configuration where the heat generated in the LED chip is dissipated effectively onto the AlN substrate, thereby reducing the increase in temperature rise and thermal resistance. The experimental results are supported by evaluation of the derivative structure functions. In addition, as the thermal resistance of the LED package varies with different humidity levels, there is a need to specify the conditions of humidity in data sheets as LED manufacturers routinely specify a universal thermal resistance value under a fixed operating condition

Sediment transport processes at the head of Halibut Canyon, Eastern Canada margin: An interplay between internal tides and dense shelf water cascading

European Geosciences Union General Assembly 2013, 7-12 April, Vienna, Austria.-- 1 pageTo investigate the processes by which sediment is transported through a submarine canyon incised in a glaciated margin, the bottom boundary layer quadrapod RALPH was deployed at 276-m depth in the West Halibut Canyon (off Newfoundland) during winter 2008–2009. Two main sediment transport processes were identified throughout the deployment. Firstly, periodic increases of near-bottom suspended-sediment concentrations (SSC) were recorded associated with the up-canyon propagation of the semidiurnal internal tidal bore along the canyon axis, carrying fine sediment particles resuspended from deeper canyon regions. The recorded SSC peaks, lasting less than 1 h, were observed sporadically and were linked to bottom intensified up-canyon flows (~ 40 cm s− 1) concomitant with sharp drops in temperature. Secondly, sediment transport was also observed during events of intensified down-canyon current velocities that occurred during periods of sustained heat loss from surface waters, but were not associated with large storm waves. High-resolution velocity profiles throughout the water column during these events revealed that the highest current speeds (~ 1 m s− 1) were centered several meters above the sea floor and corresponded to the region of maximum velocities of a gravity flow. Such flows had associated low SSC and cold water temperatures and are interpreted as dense shelf water cascading events channelized along the canyon axis. Sediment transport during these events was largely restricted to bedload and saltation, producing winnowing of sands and fine sediments around larger gravel particles. Analysis of historical hydrographic data suggests that such gravity flows are not related to the formation of coastal dense waters advected towards the outer shelf that reached the canyon head. Rather, the dense shelf waters appear to be generated around the outer shelf, where convection during winter is able to reach the sea floor and generate a pool of near-bottom dense water that cascades into the canyon during one or two tidal cycles. A similar transport mechanism is likely to occur in other submarine canyons along the eastern Canadian margin, as well in other canyoned margins where winter convection can reach the shelf-edgePeer Reviewe

Thinking about the future of global water governance

Global water problems are likely to increase in severity, rendering existing governance approaches unable to cope with the resulting problems. We inquire into the relationship between global water governance structures, particularly those involving the United Nations, and look at how those structures are likely to respond to and shape projected water futures. Building on story lines of possible water futures taken from existing scenarios, we discuss the functions to be performed by global water governance. We aim to open a discussion about four global water governance options and to introduce the constraints and possibilities for each option. We argue that the nature of the water problem calls for structural changes. However unfeasible these may appear today, such transitions do occur, and, if the narrative is sufficiently sound, it can be used by social movements and networks to mobilize policy entrepreneurs and directional leaders to work for such changes

Integral circulant graphs of prime power order with maximal energy

The energy of a graph is the sum of the moduli of the eigenvalues of its adjacency matrix. We study the energy of integral circulant graphs, also called gcd graphs, which can be characterized by their vertex count n and a set D of divisors of n in such a way that they have vertex set Zn and edge set {{a, b} : a, b in Zn; gcd(a - b, n) in D}. Using tools from convex optimization, we study the maximal energy among all integral circulant graphs of prime power order ps and varying divisor sets D. Our main result states that this maximal energy approximately lies between s(p - 1)p^(s-1) and twice this value. We construct suitable divisor sets for which the energy lies in this interval. We also characterize hyperenergetic integral circulant graphs of prime power order and exhibit an interesting topological property of their divisor sets.Comment: 25 page

RPA approach to rotational symmetry restoration in a three-level Lipkin model

We study an extended Lipkin-Meshkov-Glick model that permits a transition to a deformed phase with a broken continuous symmetry. Unlike simpler models, one sees a persistent zero-frequency Goldstone mode past the transition point into the deformed phase. We found that the RPA formula for the correlation energy provides a useful correction to the Hartree-Fock energy when the number of particle N satisfies N > 3, and becomes accurate for large N. We conclude that the RPA correlation energy formula offers a promising way to improve the Hartree-Fock energy in a systematic theory of nuclear binding energies.Comment: RevTex, 11 pages, 3 postscript figure

Quasi-Local Density Functional Theory and its Application within Extended Thomas-Fermi Approximation

A generalization of the Density Functional Theory is proposed. The theory developed leads to single-particle equations of motion with a quasi-local mean-field operator, which contains a quasi-particle position-dependent effective mass and a spin-orbit potential. The energy density functional is constructed using the Extended Thomas-Fermi approximation. Within the framework of this approach the ground-state properties of the doubly magic nuclei are considered. The calculations have been performed using the finite-range Gogny D1S force. The results are compared with the exact Hartree-Fock calculations

Critical current degradation in HTS wires due to cyclic mechanical strain

HTS wires, which may be used in many devices such as magnets and rotating machines, may be subjected to mechanical strains from electromagnetic, thermal and centripetal forces. In some applications these strains will be repeated several thousand times during the lifetime of the device. We have measured critical current degradation due to repeated strain cycles for both compressive and tensile strains. Results for BSCCO-2223 HTS conductor samples are presented for strain values up to 0.5% and cycle numbers up to and beyond 10/sup 4/

Second Harmonic Generation for a Dilute Suspension of Coated Particles

We derive an expression for the effective second-harmonic coefficient of a dilute suspension of coated spherical particles. It is assumed that the coating material, but not the core or the host, has a nonlinear susceptibility for second-harmonic generation (SHG). The resulting compact expression shows the various factors affecting the effective SHG coefficient. The effective SHG per unit volume of nonlinear coating material is found to be greatly enhanced at certain frequencies, corresponding to the surface plasmon resonance of the coated particles. Similar expression is also derived for a dilute suspension of coated discs. For coating materials with third-harmonic (THG) coefficient, results for the effective THG coefficients are given for the cases of coated particles and coated discs.Comment: 11 pages, 3 figures; accepted for publication in Phys. Rev.