155 research outputs found
Direct measurement of doping density and barrier lowering effect with bias in quantum wells
An experimental method for determining the doping density in thin-sheet semiconductor material such as quantum wells (QWs) is demonstrated in GaAs/AlGaAs multiquantum-well infra-red photodetectors. The results agree very well with the conventional Hall measurement method. Barrier lowering effect with bias in QWs is determined experimentally
Ultrafast Temperature Profile Calculation in Ic Chips
One of the crucial steps in the design of an integrated circuit is the
minimization of heating and temperature non-uniformity. Current temperature
calculation methods, such as finite element analysis and resistor networks have
considerable computation times, making them incompatible for use in routing and
placement optimization algorithms. In an effort to reduce the computation time,
we have developed a new method, deemed power blurring, for calculating
temperature distributions using a matrix convolution technique in analogy with
image blurring. For steady state analysis, power blurring was able to predict
hot spot temperatures within 1 degree C with computation times 3 orders of
magnitude faster than FEA. For transient analysis the computation times where
enhanced by a factor of 1000 for a single pulse and around 100 for multiple
frequency application, while predicting hot spot temperature within about 1
degree C. The main strength of the power blurring technique is that it exploits
the dominant heat spreading in the silicon substrate and it uses superposition
principle. With one or two finite element simulations, the temperature point
spread function for a sophisticated package can be calculated. Additional
simulations could be used to improve the accuracy of the point spread function
in different locations on the chip. In this calculation, we considered the
dominant heat transfer path through the back of the IC chip and the heat sink.
Heat transfer from the top of the chip through metallization layers and the
board is usually a small fraction of the total heat dissipation and it is
neglected in this analysis.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
Method of Images for the Fast Calculation of Temperature Distributions in Packaged VLSI Chips
Thermal aware routing and placement algorithms are important in industry.
Currently, there are reasonably fast Green's function based algorithms that
calculate the temperature distribution in a chip made from a stack of different
materials. However, the layers are all assumed to have the same size, thus
neglecting the important fact that the thermal mounts which are placed
underneath the chip can be significantly larger than the chip itself. In an
earlier publication, we showed that the image blurring technique can be used to
calculate quickly temperature distribution in realistic packages. For this
method to be effective, temperature distribution for several point heat sources
at the center and at the corner and edges of the chip should be calculated
using finite element analysis (FEA) or measured. In addition, more accurate
results require correction by a weighting function that will need several FEA
simulations. In this paper, we introduce the method of images that take the
symmetry of the thermal boundary conditions into account. Thus with only "two"
finite element simulations, the steady-state temperature distribution for an
arbitrary complex power dissipation profile in a packaged chip can be
calculated. Several simulation results are presented. It is shown that the
power blurring technique together with the method of images can reproduce the
temperature profile with an error less than 0.5%.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
Synthesis, Characterisation and 3D Printing of an Isosorbide Based, Light Curable, Degradable Polymer for Potential Application in Maxillofacial Reconstruction
Although emergence of bone tissue engineering techniques has revolutionised the field of maxillofacial reconstruction, the successful translation of such products, especially concerning larger sized defects, still remains a significant challenge. Light curable methacrylate based polymers have ideal properties for bone repair. These materials are also suitable for 3D printing which can be applicable for restoration of both function and aesthetics. The main objective of this research was to synthesise a mechanically stable and biologically functional polymer for reconstruction of complex craniofacial defects. The experimental work initially involved synthesis of (((3R,3aR,6S,6aR)-hexahydrofuro[3,2-b]furan-3,6-diyl)bis(oxy))bis(ethane-2,1-diyl) bis((4-methyl-3-oxopent-4-en-1-yl)carbamate), CSMA-1, and ((((((((((((3R,3aR,6S,6aR)-hexahydrofuro[3,2-b]furan-3,6-diyl)bis(oxy))bis(ethane-2,1 diyl))bis(oxy))bis(carbonyl))bis(azanediyl))bis(methylene))bis(3,3,5-trimethylcyclohexane-5,1-diyl))bis(azanediyl))bis(carbonyl))bis(oxy))bis(ethane-2,1-diyl) bis(2-methylacrylate), CSMA-2; Nuclear Magnetic Resonance (NMR) analysis confirmed formation of the monomers and composite samples were fabricated respectively by exposing 11 mm diameter discs to blue light. Modulus of the tensile elasticity was tested using a biaxial flexural test and the values were found to be between 1 and 3 GPa in CMA-1, CSMA-2 and their composites. In vitro cell culture, using human Bone Marrow Derived Mesenchymal Stem Cells (BMSCs), confirmed non-toxicity of the samples and finally 3D printing allowed direct extrusion and setting of the bio ink into a mesh-like construct
Efficiency in nanostructured thermionic and thermoelectric devices
Advances in solid-state device design now allow the spectrum of transmitted
electrons in thermionic and thermoelectric devices to be engineered in ways
that were not previously possible. Here we show that the shape of the electron
energy spectrum in these devices has a significant impact on their performance.
We distinguish between traditional thermionic devices where electron momentum
is filtered in the direction of transport only and a second type, in which the
electron filtering occurs according to total electron momentum. Such 'total
momentum filtered' kr thermionic devices could potentially be implemented in,
for example, quantum dot superlattices. It is shown that whilst total momentum
filtered thermionic devices may achieve efficiency equal to the Carnot value,
traditional thermionic devices are limited to efficiency below this. Our second
main result is that the electronic efficiency of a device is not only improved
by reducing the width of the transmission filter as has previously been shown,
but also strongly depends on whether the transmission probability rises sharply
from zero to full transmission. The benefit of increasing efficiency through a
sharply rising transmission probability is that it can be achieved without
sacrificing device power, in contrast to the use of a narrow transmission
filter which can greatly reduce power. We show that devices which have a
sharply-rising transmission probability significantly outperform those which do
not and it is shown such transmission probabilities may be achieved with
practical single and multibarrier devices. Finally, we comment on the
implications of the effect the shape of the electron energy spectrum on the
efficiency of thermoelectric devices.Comment: 11 pages, 15 figure
High temperature thermoreflectance imaging and transient Harman characterization of thermoelectric energy conversion devices
Advances in thin film growth technology have enabled the selective engineering of material properties to improve the thermoelectric figure of merit and thus the efficiency of energy conversion devices. Precise characterization at the operational temperature of novel thermoelectric materials is crucial to evaluate their performance and optimize their behavior. However, measurements on thin film devices are subject to complications from the growth substrate, non-ideal contacts, and other thermal and electrical parasitic effects. In this manuscript, we determine the cross-plane thermoelectric material properties in a single measurement of a 25 mu m InGaAs thin film with embedded ErAs (0.2%) nanoparticles using the bipolar transient Harman method in conjunction with thermoreflectance thermal imaging at temperatures up to 550K. This approach eliminates discrepancies and potential device degradation from the multiple measurements necessary to obtain individual material parameters. In addition, we present a strategy for optimizing device geometry to mitigate the effect of both electrical and thermal parasitics during the measurement. Finite element method simulations are utilized to analyze non-uniform current and temperature distributions over the device area as well as the three dimensional current path for accurate extraction of material properties from the thermal images. Results are compared with independent in-plane and 3 omega measurements of thermoelectric material properties for the same material composition and are found to match reasonably well; the obtained figure of merit matches within 15% at room and elevated temperatures. (C) 2014 AIP Publishing LLC
Tunable nano Peltier cooling device from geometric effects using a single graphene nanoribbon
Based on the phenomenon of curvature-induced doping in graphene we propose a
class of Peltier cooling devices, produced by geometrical effects, without
gating. We show how a graphene nanorib- bon laid on an array of curved nano
cylinders can be used to create a targeted and tunable cooling device. Using
two different approaches, the Nonequlibrium Green's Function (NEGF) method and
experimental inputs, we predict that the cooling power of such a device can
approach the order of kW/cm2, on par with the best known techniques using
standard superlattice structures. The struc- ture proposed here helps pave the
way toward designing graphene electronics which use geometry rather than gating
to control devices.Comment: 12 pages, 5 figure
Poly-ε-Caprolactone/Fibrin-Alginate Scaffold: A New Pro-Angiogenic Composite Biomaterial for the Treatment of Bone Defects
We hypothesized that a composite of 3D porous melt-electrowritten poly-É›-caprolactone (PCL) coated throughout with a porous and slowly biodegradable fibrin/alginate (FA) matrix would accelerate bone repair due to its angiogenic potential. Scanning electron microscopy showed that the open pore structure of the FA matrix was maintained in the PCL/FA composites. Fourier transform infrared spectroscopy and differential scanning calorimetry showed complete coverage of the PCL fibres by FA, and the PCL/FA crystallinity was decreased compared with PCL. In vitro cell work with osteoprogenitor cells showed that they preferentially bound to the FA component and proliferated on all scaffolds over 28 days. A chorioallantoic membrane assay showed more blood vessel infiltration into FA and PCL/FA compared with PCL, and a significantly higher number of bifurcation points for PCL/FA compared with both FA and PCL. Implantation into a rat cranial defect model followed by microcomputed tomography, histology, and immunohistochemistry after 4- and 12-weeks post operation showed fast early bone formation at week 4, with significantly higher bone formation for FA and PCL/FA compared with PCL. However, this phenomenon was not extrapolated to week 12. Therefore, for long-term bone regeneration, tuning of FA degradation to ensure syncing with new bone formation is likely necessary
New observation of two species of sea cucumbers from Chabahar Bay (Southeast Coasts of Iran)
Although, sea cucumbers are well known animals in Eastern Asia, in Iran they are not popular marine animals. Divers recorded these animals in their dives but this is the first scientific approach in identification of holoturians in Southeast coast of Iran. All sea cucumbers were collected with SCUBA diving and species identification was done through morphological keys and review of their dermal ossicles. There are two species of sea cucumber belong to genus Holothuria were collected on subtidal zone of Chabahar Bay in the late of 2007. This is the first report of H. hilla, H. parva from Chabahar Bay (North of Oman Sea). This study is revealed the special characteristics of the presented species in order to just identification of them. In the studied areas, H. parva has known as a rare species
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