Thermal Conductance (thermal + conductance)

Distribution by Scientific Domains


Selected Abstracts


Thermal Conductance of Delamination Cracks in a Fiber-Reinforced Ceramic Composite

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2000
Kathleen R. McDonald
The thermal conductance of delamination cracks in a unidirectionally reinforced ceramic composite is investigated. A phase-sensitive photothermal technique is used to measure the crack conductance in situ under load. Special emphasis is given to the effects of the local crack opening displacement (,). A crack conductance model that considers the contributions from both the air and the fibers within the crack is developed and compared with the measurements. Despite considerable scatter in the experimental data, the model adequately predicts the increased conductance that is associated with fiber bridging, as well as the overall trend that is observed with ,. [source]


Thermal conductance of the AlN/Si and AlN/SiC interfaces calculated with taking into account the detailed phonon spectra of the materials and the interface conditions

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2010
M. Kazan
Abstract We present a calculation of the thermal conductance (TC) of the interface between aluminium nitride (AlN) and silicon (Si) and that between AlN and silicon carbide (SiC) with taking into account the detailed phonon spectra of the materials, as obtained from first principles calculations, and the interface conditions. On the basis of the results obtained, we discuss the relation between the interface TC, the interface conditions, and the mismatches between the acoustic waves velocities and the phonon densities of states of the materials in contact. Our calculation method is expected to provide a reliable tool for thermal management strategy, independently from the substrate choice (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Carbon Nanotubes: (Thermal and Structural Characterizations of Individual Single-, Double-, and Multi-Walled Carbon Nanotubes) Adv.

ADVANCED FUNCTIONAL MATERIALS, Issue 24 2009
Funct.
Here, M. T. Pettes and L. Shi report for the first time the thermal conductance, diameter, and chiral angle for a single single-walled carbon nanotube (SWCNT). A scanning electron micrograph of the suspended micro-thermometer device and transmission electron microscopy images used to determine the SWCNT's (22, 12) chirality are shown in this frontispiece image, along with the rendered unit cell. [source]


Is ,nocturnal hypothermia' a valid physiological concept in small birds?: a study on Bronze Mannikins Spermestes cucullatus

IBIS, Issue 4 2003
Barry G. Lovegrove
The thermoregulatory capacity and metabolic responses to light,dark cycles under various mild food-deprivation treatments were measured in Bronze Mannikins Spermestes cucullatus (10,11 g). We measured the response of minimum oxygen consumption to ambient temperature in order to determine the basal metabolic rate (BMR), thermal conductance and limits of thermoneutrality of the Mannikins. In addition, we measured oxygen consumption in response to light,dark cycles and three mild food-deprivation treatments. Bronze Mannikins have a low BMR (1.67 mlO2/g/h) that is c. 50,60% of that predicted from phylogenetically independent allometric curves for all birds. A low BMR resulted in amplitudes of metabolism between the active and rest phases that were double those predicted allometrically from body mass. The reduced nocturnal metabolic rate did not represent torpor. Typically, Mannikins would need to reduce their metabolic rate during the rest phase to c. 17% of BMR to attain the average torpor metabolic rate of other birds. The data are, however, consistent with those of other group-living Afrotropical birds that benefit energetically from group huddling in environments in which moderate seasonality is accompanied by unpredictable climates , and thus unpredictable energy inputs in time and space. When food-deprived and placed under moderate cold stress (20 °C), Mannikins decreased their rest-phase metabolic rates to the same magnitude as several small Holarctic birds. We suggest that, in the context of the progress made to quantify and define proximate heterothermic responses in endotherms, such as torpor and hibernation, the term nocturnal hypothermia often applied to moderate nocturnal reductions in metabolic rate is vague, misleading and inappropriate. [source]


Thermodynamic optimization of global circulation and climate

INTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 4 2005
Adrian Bejan
Abstract The constructal law of generation of flow structure is used to predict the main features of global circulation and climate. The flow structure is the atmospheric and oceanic circulation. This feature is modelled as convection loops, and added to the earth model as a heat engine heated by the Sun and cooled by the background. It is shown that the dissipation of the power produced by the earth engine can be maximized by selecting the proper balance between the hot and cold zones of the Earth, and by optimizing the thermal conductance of the circulation loops. The optimized features agree with the main characteristics of global circulation and climate. The robustness of these predictions, and the place of the constructal law as a self-standing principle in thermodynamics, are discussed. Copyright © 2005 John Wiley & Sons, Ltd. [source]


Effect of Altitude on Energy Exchange Characteristics of Some Alpine Medicinal Crops from Central Himalayas

JOURNAL OF AGRONOMY AND CROP SCIENCE, Issue 1 2004
S. Chandra
Abstract To explore the conservation and cultivation of endangered alpine medicinal crops at comparatively lower altitudes, a study on variations in morphological parameters and energy exchange characteristics was conducted on five herbaceous medicinal crops from the alpines of Central Himalayas. Plants of same age were selected from the alpine medicinal crop nursery, Tungnath (3600 m), and were planted at the nurseries at 2100 and 550 m altitudes. After well acclimatization at lower altitudes, plants were examined for morphological and energy exchange studies during their active growth period. The energy balance sheet of these plant species indicates that most of the energy absorbed by the leaves dissipates by re-radiation, transpiration and thermal conductance across leaf surfaces. All species maintained leaf temperature below the surrounding air temperature at all altitudes and therefore gained energy by convection of heat as well as by boundary layer thermal conduction. Leaf-to-air temperature difference, gain of energy by convection of heat and boundary layer thermal conduction was maximum at an altitude of 2100 m in all the species. Boundary layer thermal conductivity, boundary layer thickness, thermal conductivity of the leaf and therefore, total energy absorbed by the leaves of these species increase significantly with decreasing altitude. Leaf thickness significantly decreases with decreasing altitude, which in turn enhances total energy absorption (r = ,0.975, P < 0.005) of the leaves in all the species. The results indicate that all these species absorb higher amount of energy at lower altitudes, which indicates their adaptability to warm temperatures at low altitudes (up to 550 m). Therefore, these species can be cultivated at relatively lower altitudes. However, a proper agronomic methodology needs to be developed for better yields. [source]


Effects of Matrix Cracks on the Thermal Diffusivity of a Fiber-Reinforced Ceramic Composite

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2001
Kathleen R. McDonald
Effects of matrix cracks and the attendant interface debonding and sliding on both the longitudinal and the transverse thermal diffusivities of a unidirectional Nicalon/MAS composite are investigated. The diffusivity measurements are made in situ during tensile testing using a phase-sensitive photothermal technique. The contribution to the longitudinal thermal resistance from each of the cracks is determined from the longitudinal diffusivity along with measurements of crack density. By combining the transverse measurements with the predictions of an effective medium model, the thermal conductance of the interface (characterized by a Biot number) is determined and found to decrease with increasing crack opening displacement, from an initial value of ,1 to ,0.3. This degradation is attributed to the deleterious effects of interface sliding on the thermal conductance. Corroborating evidence of degradation in the interface conductance is obtained from the inferred crack conductances coupled with a unit cell model for a fiber composite containing a periodic array of matrix cracks. Additional notable features of the material behavior include: (i) reductions of ,20% in both the longitudinal and the transverse diffusivities at stresses near the ultimate strength, (ii) almost complete recovery of the longitudinal diffusivity following unloading, and (iii) essentially no change in the transverse diffusivity following unloading. The recovery of the longitudinal diffusivity is attributed to closure of the matrix cracks. By contrast, the degradation in the interface conductance is permanent, as manifest in the lack of recovery of the transverse diffusivity. [source]


Simplified heat exchange model for semiconductor laser diodes thermal parameters extraction

LASER PHYSICS LETTERS, Issue 11 2005
P. S. André
Abstract By investigating the heat flow mechanism in a semiconductor laser diode, we demonstrate a comprehensive technique for optical device thermal parameters extraction to be used in the prediction of the laser performance This accurate and precise heat exchange model takes into account the relevant heat exchange mechanism and mechanical considerations of the laser diode mounting. We measured the thermal response of a semiconductor laser diode attach to a substrate, deriving from those the device thermal parameters such as heat capacity and thermal conductance for the device and subtract. From the estimated values a prediction of the real laser temperature response is obtained directly from the measurements realized in the substrate. (© 2005 by Astro, Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source]


Determination of in-plane thermal conductivity of Nax Co2O4 single crystals via a parallel thermal conductance (PTC) technique

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 5 2008
Xiaofeng Tang
Abstract A novel parallel thermal conductance ("PTC") system was specifically developed to conduct the steady state thermal conductivity measurement on small size samples such as single crystals of Nax Co2O4 as reported here. The accuracy and reproducibility of the PTC system has been confirmed by measuring several standard reference materials. The in-plane thermal conductivity , of Nax Co2O4 single crystals grown by a NaCl flux method was measured from 10 K to 300 K and , was found to be ,5 W m,1 K,1 at 300 K. The phonon mean free path (MFP) is estimated to be lph , 9 Ĺ at 300 K, which is comparable to the lattice constant but much smaller than the reported MFP of conducting carriers. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Thermal conductance of the AlN/Si and AlN/SiC interfaces calculated with taking into account the detailed phonon spectra of the materials and the interface conditions

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2010
M. Kazan
Abstract We present a calculation of the thermal conductance (TC) of the interface between aluminium nitride (AlN) and silicon (Si) and that between AlN and silicon carbide (SiC) with taking into account the detailed phonon spectra of the materials, as obtained from first principles calculations, and the interface conditions. On the basis of the results obtained, we discuss the relation between the interface TC, the interface conditions, and the mismatches between the acoustic waves velocities and the phonon densities of states of the materials in contact. Our calculation method is expected to provide a reliable tool for thermal management strategy, independently from the substrate choice (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]