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Heat Dissipation (heat + dissipation)
Selected AbstractsDiscontinuous deformation in an elastic material.POLYMER ENGINEERING & SCIENCE, Issue 10 2007Part 1. The concept of energy or work dissipation from a perfectly elastic material, due to a discontinuous deformation mechanism, is developed in this study. Dissipation occurs even from a perfectly elastic material, preferably an elastomer, when subjected to a discontinuous "jump" stretch or "jump" contraction. Stretching an elastomeric member through free extension requires a large amount of work. Such a sudden jump stretch of an elastic material is difficult to accomplish and is equivalent to thermodynamic free compression of a gas. The amount of work required can greatly exceed the strain energy stored in the material if the extension were applied without the jump or "shock" process. Interestingly, only part of the stored energy is recovered on unloading the elastomer the same way (through contraction). Excess work lost in contraction dissipates as heat but is not due to the common viscoelastic/plastic losses associated with internal friction in solids. Dissipation is possible even from a perfectly elastic material. Energy values associated in this jump deformation process are independent of the stress,strain curve path, and depend only on initial and final states for the material. Heat dissipation from an elastic rubber belt is examined and some applications extended from the developed principle are enunciated. POLYM. ENG. SCI., 47:1511,1520, 2007. © 2007 Society of Plastics Engineers [source] Exposure to a hot environment can activate rostral ventrolateral medulla-projecting neurones in the hypothalamic paraventricular nucleus in conscious ratsEXPERIMENTAL PHYSIOLOGY, Issue 1 2008Joo Lee Cham A major integrative site within the brain for autonomic function is the hypothalamic paraventricular nucleus (PVN). Several studies have suggested that the PVN may be involved in the responses regulating body temperature. Hyperthermia elicits redirection of blood flow from the viscera to the periphery and involves changes in sympathetic nerve activity mediated by the central nervous system. The hypothalamic PVN includes neurones that project to the rostral ventrolateral medulla (RVLM), an important autonomic region involved in the tonic regulation of sympathetic nerve activity. This pathway could contribute to the cardiovascular changes induced by hyperthermia. The PVN has a high concentration of nitrergic neurones and it is known that nitric oxide within the brain mediates heat dissipation. Thus the aims of this study were to determine whether RVLM-projecting neurones in the PVN are activated by heat and whether those neurones are also nitrergic. The results show that, compared with control conditions, exposure of conscious rats to a hot environment of 39°C significantly increased the number of neurones containing a Fos-positive nucleus (a marker of activation) and significantly increased the number of activated RVLM-projecting neurones in the PVN. Also, although heating significantly increased the number of activated nitrergic PVN neurones, triple-labelled neurones (i.e. activated, nitrergic and RVLM projecting) in the PVN were rarely observed. The results suggest that RVLM-projecting neurones in the PVN may play a role in responses to heat exposure but these are not nitrergic. [source] Mathematical Modelling and Simulation of Polymer Electrolyte Membrane Fuel Cells.FUEL CELLS, Issue 2 2002Part I: Model Structures, Solving an Isothermal One-Cell Model Abstract Amongst the various types of fuel cells, the polymer electrolyte membrane fuel cell (PEM-FC) can be used favourably in vehicles and for in house energy supply. The focus of the development of these cells is not only to provide cost-effective membranes and electrodes, but also to optimise the process engineering for single cells and to design multi-cell systems (cell stacks). This is a field in which we have successfully applied the methods of mathematical modelling and simulation. Initially, in this work, a partial model of a single membrane-electrode unit was developed in which the normal reaction technology fields (concentration, temperature, and flow-speed distributions) were calculated, but also the electrical potential and current density distribution in order to develop model structures for technically interesting PEM-FC. This allows the simulation of the effects that the geometric parameters (electrode and membrane data and the dimensions of the material feed and outlet channels) and the educt and coolant intake data have on the electrical and thermal output data of the cell. When complete, cell stacks consisting of a number of single cells, most of which have bipolar switching, are modelled the distribution of the gas flows over the single cells and the specific conditions of heat dissipation must also be taken into consideration. In addition to the distributions mentioned above, this simulation also produces characteristic current-voltage and power-voltage curves for each application that can be compared with the individual process variations and cell types, thus making it possible to evaluate them both technically and economically. The results of the simulation of characteristic process conditions of a PEM-FC operated on a semi-technical scale are presented, which have been determined by means of a three-dimensional model. The distributions of the electrical current density and all component voltage drops that are important for optimising the conditions of the process are determined and also the water concentration in the membrane as an important factor that influences the cell's momentary output and the PEM-FC's long-term stability. [source] Rapid contrasting of ultrathin sections using microwave irradiation with heat dissipationJOURNAL OF MICROSCOPY, Issue 2 2001F. Hernández-Chavarría The use of microwave irradiation (MWI) to accelerate fixation, dehydration and contrasting (staining) for electron microscopy has been applied to the development of rapid methods to process biological samples in electron microscopy. A simple explanation is that the reduced time in those procedures is due to heating. In this paper we propose a contrasting method for thin sections that avoids the thermal effects of MWI. Grids with thin sections of mouse kidney, the dinoflagellate Alexandrium monilatum, spermatophores of the fly Archicepsis diversiformis, the bacteria Acinetobacter calcoaceticum and Enterobacter cloacae were placed into Beem capsules and stained with uranyl acetate and lead citrate, while immersed in an ice-water bath, and irradiated for periods ranging from 30 s to 2 min. After each contrasting procedure, the Beem capsule was filled with distilled water to wash the grids under MWI with the same irradiation time as used to contrast. Good results were obtained on irradiating for 1 min and the temperature of the Beem capsule was maintained around 5 °C. [source] Heat Transfer in Gas Phase Olefin PolymerisationMACROMOLECULAR SYMPOSIA, Issue 1 2009Estevan Tioni Abstract A fixed bed microreactor has been used to study heat transfer during the initial transient state of gas phase olefin polymerization on a supported catalyst. It has been shown that heat transfer during this stage of the polymerisation is critical, and under conditions found commercially problems can arise with hot spots and polymer melting. It is proven how the thermal properties of the gas mixture flowing on the catalytic bed exert great influence on heat dissipation reducing the sudden increase in temperature by as much as a factor of 5. Flow rate and especially the process gas composition are the key factors in controlling the bed temperature. [source] Inside Back Cover (Phys. Status Solidi A 5/2010)PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 5 2010J. H. Leach The Feature Article by Morkoç and co-workers (pp. 1091,1100) centers around the not so intuitive phenomena in two types of GaN based devices, namely InGaN based LEDs and InAlN barrier GaN heterojunction FETs. In terms of the LEDs, the paper uncovers that the quantum efficiency degradation observed at high current injection levels is not necessarily of Auger recombination origin. Furthermore, nearly similar behavior of LEDs on c-plane and mplane suggests that the main driving force for the efficiency degradation is not polarization induced field either. The data along with their interpretation should set the stage for an accurate physics- based model to be developed. In terms of the FETs, the authors show that there is an optimum sheet density, which depends on drain bias or the electric field in the channel, at which the LO phonon lifetime is shortest, the velocity is highest, and the device degradation is least. The average optimum density is near 7 × 1012 cm,2 which challenges the proverbial notion that the higher the sheet density the better it is. Another outcome of this discussion is that heat dissipation takes the route of hot electrons giving off heat to LO phonons which in turn give it to LA phonons when they decay. Naturally, the shortest LO phonon lifetime is best for heat removal and thus the devices are more reliable in addition to electrons traversing at the highest velocity. [source] Low thermal resistance, high-speed 980 nm asymmetric intracavity-contacted oxide-aperture VCSELsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 7 2009Y. M. Song Abstract We demonstrated high-speed characteristics of an oxide-aperture vertical-cavity surface-emitting laser (VCSEL) with intracavity structures for both p- and n-contacts, based on InGaAs/GaAs multiple quantum wells operating at , , 980 nm, indicating a low thermal resistance (Rth). The asymmetric current injection scheme is employed for reducing current crowding around the rim of the oxide aperture. A high aluminium content undoped Al0.88Ga0.12As and GaAs distributed Bragg reflector (DBR) mirror is used for efficient heat dissipation. The VCSEL with a 7 ,m oxide aperture exhibited an output power of 2.5 mW and a threshold current of 0.8 mA with a slope efficiency of 0.39 mW/mA at 20 °C under continuous-wave operation and it still worked with 1.3 mW at 90 °C. The temperature tuning coefficient of 0.081 nm/°C and dissipated electrical power tuning coefficient of 0.104 nm/mW were observed, leading to a low Rth of 1.28 °C/mW. A high modulation bandwidth up to 13 GHz with a modulation current efficiency factor of 6.1 GHz/mA1/2 was achieved. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Realization of AlGaN/GaN HEMTs on 3C-SiC/Si(111) substratesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2008Y. Cordier Abstract Cubic SiC/Si (111) template is an interesting alternative for growing GaN on silicon. As compared with silicon, this substrate allows reducing the stress in GaN films due to both lower lattice and thermal expansion coefficient mismatch, and can provide better heat dissipation. In this work, we first developed the epitaxial growth of 3C-SiC films on 50 mm Si(111) substrates using chemical vapor deposition. AlGaN/GaN high electron mobility transistors were grown by molecular beam epitaxy on these films. Both the structural quality and the behavior of transistors realized on these structures show the feasibility of this approach. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Reliable hot-spot classification in 10,ms using ultra-fast lock-in thermographyPROGRESS IN PHOTOVOLTAICS: RESEARCH & APPLICATIONS, Issue 7 2009Martin Kasemann Abstract We propose and demonstrate a reliable and non-destructive spatially resolved measurement technique for ultra-fast hot-spot classification of solar cells. The method can deliver quantitative images of the local heat dissipation in hot-spots in measurement times below 10,ms. The high accuracy and sensitivity allow for reliable hot-spot testing and provide the basis for a reliable classification of solar cells into different hot-spot categories. The method can be applied to wafer-based silicon solar cells and in principle also to thin-film solar cells of all material compositions. This paper explains the measurement principle, gives a detailed step-by-step description of viable automated evaluation procedures, and assesses the sensitivity and accuracy of the method. Copyright © 2009 John Wiley & Sons, Ltd. [source] Ecogeographic variation in human nasal passagesAMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY, Issue 1 2009Todd R. Yokley Abstract Theoretically, individuals whose ancestors evolved in cold and/or dry climates should have greater nasal mucosal surface area relative to air volume of the nasal passages than individuals whose ancestors evolved in warm, humid climates. A high surface-area-to-volume (SA/V) ratio allows relatively more air to come in contact with the mucosa and facilitates more efficient heat and moisture exchange during inspiration and expiration, which would be adaptive in a cold, dry environment. Conversely, a low SA/V ratio is not as efficient at recapturing heat and moisture during expiration and allows for better heat dissipation, which would be adaptive in a warm, humid environment. To test this hypothesis, cross-sectional measurements of the nasal passages that reflect surface area and volume were collected from a sample of CT scans of patients of European and African ancestry. Results indicate that individuals of European descent do have higher SA/V ratios than individuals of African descent, but only when decongested. Otherwise, the two groups show little difference. This pattern of variation may be due to selection for different SA/V configurations during times of physical exertion, which has been shown to elicit decongestion. Relationships between linear measurements of the skeletal nasal aperture and cavity and cross-sectional dimensions were also examined. Contrary to predictions, the nasal index, the ratio of nasal breadth to nasal height, is not strongly correlated with internal dimensions. However, differences between the nasal indices of the two groups are highly significant. These results may be indicative of different adaptive solutions to the same problem. Am J Phys Anthropol, 2009. © 2008 Wiley-Liss, Inc. [source] Energy scavenging for energy efficiency in networks and applicationsBELL LABS TECHNICAL JOURNAL, Issue 2 2010Kyoung Joon Kim Telecommunication networks will play a huge part in enabling eco-sustainability of human activity; one of the first steps towards this is to dramatically increase network energy efficiency. In this paper we present two novel approaches for energy scavenging in networks. One involves thermal energy scavenging for improving wireless base station energy efficiency, and the other involves mechanical energy scavenging for powering sensors in sensor networks, for machine-to-machine (M2M) communications, and for smart grid applications. Power amplifier (PA) transistors in base stations waste 30 percent of the total energy used in a wireless access network (WAN) as heat to the environment. We propose a thermoelectric energy recovery module (TERM) to recover electricity from the waste heat of PA transistors. A fully coupled thermoelectric (TE) model, combining thermoelectricity and heat transfer physics, is developed to explore the power generation performance and efficiency as well as the thermal performance of the TERM. The TE model is comprehensively used to determine optimized pellet geometries for power generation and efficiency as a function of PA transistor heat dissipation, heat sink performance, and load resistance. Maximum power generation and efficiency for various parametric conditions are also explored. Untapped kinetic energy is almost everywhere in the form of vibrations. This energy can be converted into electrical energy by means of transducers to power wireless sensors and mobile electronics in the range of microwatts to a few milliwatts. However, many problems limit the efficiency of current harvesting generators: narrow bandwidth, low power density, micro-electro-mechanical system (MEMS) scaling, and inconsistency of vibrating sources. We explore energy scavenger designs based on multiple-mass systems to increase harvesting efficiency. A theoretical and experimental study of two degrees-of-freedom (2-DOF) vibration-powered generators is presented. Both electromagnetic and piezoelectric conversion methods are modeled by using a general approach. Experimental results for the multi-resonant system are in agreement with the analytical predictions and demonstrate significantly better performance in terms of maximum power density per total mass and a wider bandwidth compared to single DOF (1-DOF) generators. © 2010 Alcatel-Lucent. [source] Why do Anatolian ground squirrels exhibit a Bergmannian size pattern?BIOLOGICAL JOURNAL OF THE LINNEAN SOCIETY, Issue 3 2010A phylogenetic comparative analysis of geographic variation in body size A phylogenetic comparative analysis of geographic variation in body size of an obligately hibernating marmotine species (Anatolian ground squirrels, Spermophilus xanthoprymnus) is presented in relation to environmental variables that pertain to four principal hypotheses (heat conservation, heat dissipation, primary productivity, and seasonality hypotheses). Adult Anatolian ground squirrels (78 males and 90 females) were collected from ten geographic localities in Anatolia for use in morphometric analyses. First, the study tested whether significant variation in body size occurs over the geographic range of S. xanthoprymnus. Then, to understand the possible cause(s) of the observed pattern of geographic variation in body size of Anatolian ground squirrels, four hypotheses were tested, separately and simultaneously, using a phylogenetic comparative method. Overall, food availability (primary productivity hypothesis) and, especially in males, over-winter fasting endurance (seasonality hypothesis) are likely the primary underlying mechanisms generating the observed pattern of increasing body size towards colder, more seasonal environments, with higher summer precipitation and productivity (or a Bergmannian size pattern). © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100, 695,710. [source] Do physical forces contribute to cryodamage?BIOTECHNOLOGY & BIOENGINEERING, Issue 4 2009Joseph Saragusty Abstract To achieve the ultimate goal of both cryosurgery and cryopreservation, a thorough understanding of the processes responsible for cell and tissue damage is desired. The general belief is that cells are damaged primarily due to osmotic effects at slow cooling rates and intracellular ice formation at high cooling rates, together termed the "two factor theory." The present study deals with a third, largely ignored component,mechanical damage. Using pooled bull sperm cells as a model and directional freezing in large volumes, samples were frozen in the presence or absence of glass balls of three different diameters: 70,110, 250,500, and 1,000,1,250,µm, as a means of altering the surface area with which the cells come in contact. Post-thaw evaluation included motility at 0,h and after 3,h at 37°C, viability, acrosome integrity, and hypoosmotic swelling test. Interactions among glass balls, sperm cells, and ice crystals were observed by directional freezing cryomicroscopy. Intra-container pressure in relation to volume was also evaluated. The series of studies presented here indicate that the higher the surface area with which the cells come in contact, the greater the damage, possibly because the cells are squeezed between the ice crystals and the surface. We further demonstrate that with a decrease in volume, and thus increase in surface area-to-volume ratio, the intra-container pressure during freezing increases. It is suggested that large volume freezing, given that heat dissipation is solved, will inflict less cryodamage to the cells than the current practice of small volume freezing. Biotechnol. Bioeng. 2009; 104: 719,728 © 2009 Wiley Periodicals, Inc. [source] The pharmacology of the acute hyperthermic response that follows administration of 3,4-methylenedioxymethamphetamine (MDMA, ,ecstasy') to ratsBRITISH JOURNAL OF PHARMACOLOGY, Issue 1 2002Annis O Mechan The pharmacology of the acute hyperthermia that follows 3,4-methylenedioxymethamphetamine (MDMA, ,ecstasy') administration to rats has been investigated. MDMA (12.5 mg kg,1 i.p.) produced acute hyperthermia (measured rectally). The tail skin temperature did not increase, suggesting that MDMA may impair heat dissipation. Pretreatment with the 5-HT1/2 antagonist methysergide (10 mg kg,1), the 5-HT2A antagonist MDL 100,907 (0.1 mg kg,1) or the 5-HT2C antagonist SB 242084 (3 mg kg,1) failed to alter the hyperthermia. The 5-HT2 antagonist ritanserin (1 mg kg,1) was without effect, but MDL 11,939 (5 mg kg,1) blocked the hyperthermia, possibly because of activity at non-serotonergic receptors. The 5-HT uptake inhibitor zimeldine (10 mg kg,1) had no effect on MDMA-induced hyperthermia. The uptake inhibitor fluoxetine (10 mg kg,1) markedly attenuated the MDMA-induced increase in hippocampal extracellular 5-HT, also without altering hyperthermia. The dopamine D2 antagonist remoxipride (10 mg kg,1) did not alter MDMA-induced hyperthermia, but the D1 antagonist SCH 23390 (0.3 , 2.0 mg kg,1) dose-dependently antagonized it. The dopamine uptake inhibitor GBR 12909 (10 mg kg,1) did not alter the hyperthermic response and microdialysis demonstrated that it did not inhibit MDMA-induced striatal dopamine release. These results demonstrate that in vivo MDMA-induced 5-HT release is inhibited by 5-HT uptake inhibitors, but MDMA-induced dopamine release may not be altered by a dopamine uptake inhibitor. It is suggested that MDMA-induced hyperthermia results not from MDMA-induced 5-HT release, but rather from the increased release of dopamine that acts at D1 receptors. This has implications for the clinical treatment of MDMA-induced hyperthermia. British Journal of Pharmacology (2002) 135, 170,180; doi:10.1038/sj.bjp.0704442 [source] | |||||||||||||||||||