2,971 research outputs found
Thermodynamics of Thermoelectric Phenomena and Applications
Fifty years ago, the optimization of thermoelectric devices was analyzed by considering the relation between optimal performances and local entropy production. Entropy is produced by the irreversible processes in thermoelectric devices. If these processes could be eliminated, entropy production would be reduced to zero, and the limiting Carnot efficiency or coefficient of performance would be obtained. In the present review, we start with some fundamental thermodynamic considerations relevant for thermoelectrics. Based on a historical overview, we reconsider the interrelation between optimal performances and local entropy production by using the compatibility approach together with the thermodynamic arguments. Using the relative current density and the thermoelectric potential, we show that minimum entropy production can be obtained when the thermoelectric potential is a specific, optimal value
Produktivitätsorientierte Arbeitszeitverkßrzung als beschäftigungspolitisches Instrument
"Vollbeschäftigung ist keineswegs der Normalzustand unserer Wirtschaftsordnung. Steigende Produktivität mit dem Ziel steigender Realeinkommen verursacht vielmehr einen stetigen Freisetzungsdruck. ... Da die keynesianischen Strategien mangels ausreichender Dimensionierung und die neoklassischen Strategien mangels ausreichender Elastizitäten keinen arbeitsmarktpolitischen Erfolg versprechen, empfiehlt sich die Reduzierung des Faktoreinsatzes. .. Weil die Reduzierung der Normalarbeitszeit ohne Lohnausgleich, also unter Hinnahme von EinkommenseinbuĂen, politisch nicht durchsetzbar ist, bleibt als einzige in grĂśĂerem MaĂstab realisierbare Form die ArbeitszeitverkĂźrzung im Rahmen der Produktivitätssteigerung und an Stelle von RealeinkommenserhĂśhungen. Sie ist zugleich ArbeitszeitverkĂźrzung mit vollem Lohnausgleich je Arbeitsstunde. Diese Variante der ArbeitszeitverkĂźrzung verhindert Freisetzungen und ist insofern fĂźr eine langfristig orientierte Arbeitsmarktpolitik von entscheidender Bedeutung. Zu steigender Nachfrage nach Arbeitskräften fĂźhrt die produktivitätsorientierte ArbeitszeitverkĂźrzung erst dann, wenn der Konsum der privaten Haushalte im Verhältnis zum Einkommen steigt. ... Ferner fĂźhrt die Vermehrung der Feizeit zu steigendem Verbrauch von GĂźtern und Diensten. Durch die produktivitätsabhängige ArbeitszeitverkĂźrzung werden die LĂśhne und die lohnproportionalen Kosten der Unternehmen im Durchschnitt nicht stärker erhĂśht als bei anderen Tariflohnsteigerungen. ... Entscheidend fĂźr die Kostenwirkung und den Absorptionseffekt bei gegebener Nachfrage ist die Auslastung der Betriebe. Es werden insgesamt 5 Betriebstypen nach dem Grad der Auslastung unterschieden und die Art ihrer Reaktion auf die ArbeitszeitverkĂźrzung mit und ohne Lohnausgleich untersucht."ArbeitszeitverkĂźrzung, Beschäftigungspolitik - Instrumentarium
Improved Thermoelectric Cooling Based on the Thomson Effect
Traditional thermoelectric Peltier coolers exhibit a cooling limit which is
primarily determined by the figure of merit, zT. Rather than a fundamental
thermodynamic limit, this bound can be traced to the difficulty of maintaining
thermoelectric compatibility. Self-compatibility locally maximizes the cooler's
coefficient of performance for a given zT and can be achieved by adjusting the
relative ratio of the thermoelectric transport properties that make up zT. In
this study, we investigate the theoretical performance of thermoelectric
coolers that maintain self-compatibility across the device. We find such a
device behaves very differently from a Peltier cooler, and term self-compatible
coolers "Thomson coolers" when the Fourier heat divergence is dominated by the
Thomson, as opposed to the Joule, term. A Thomson cooler requires an
exponentially rising Seebeck coefficient with increasing temperature, while
traditional Peltier coolers, such as those used commercially, have
comparatively minimal change in Seebeck coefficient with temperature. When
reasonable material property bounds are placed on the thermoelectric leg, the
Thomson cooler is predicted to achieve approximately twice the maximum
temperature drop of a traditional Peltier cooler with equivalent figure of
merit (zT). We anticipate the development of Thomson coolers will ultimately
lead to solid state cooling to cryogenic temperatures.Comment: The Manuscript has been revised for publication in PR
Drivers and patterns of land biosphere carbon balance reversal
The carbon balance of the land biosphere is the result of complex interactions between land, atmosphere and oceans, including climatic change, carbon dioxide fertilization and land-use change. While the land biosphere currently absorbs carbon dioxide from the atmosphere, this carbon balance might be reversed under climate and land-use change (âcarbon balance reversalâ). A carbon balance reversal would render climate mitigation much more difficult, as net negative emissions would be needed to even stabilize atmospheric carbon dioxide concentrations. We investigate the robustness of the land biosphere carbon sink under different socio-economic pathways by systematically varying climate sensitivity, spatial patterns of climate change and resulting land-use changes. For this, we employ a modelling framework designed to account for all relevant feedback mechanisms by coupling the integrated assessment model IMAGE with the process-based dynamic vegetation, hydrology and crop growth model LPJmL. We find that carbon balance reversal can occur under a broad range of forcings and is connected to changes in tree cover and soil carbon mainly in northern latitudes. These changes are largely a consequence of vegetation responses to varying climate and only partially of land-use change and the rate of climate change. Spatial patterns of climate change as deduced from different climate models, substantially determine how much pressure in terms of global warming and land-use change the land biosphere will tolerate before the carbon balance is reversed. A reversal of the land biosphere carbon balance can occur as early as 2030, although at very low probability, and should be considered in the design of so-called peak-and-decline strategies.Bundesministerium fĂźr Bildung und Forschung http://dx.doi.org/10.13039/501100002347Peer Reviewe
Hip joint load and muscle stress in soccer inside passing
Studies investigating the mechanisms of adductor injuries in soccer have concentrated on full effort kicks. Purpose of this study was a kinetic analysis of the inside pass. Using infrared cameras and inverse dynamics, hip joint moments and adductor muscle stress was calculated during the swing phase of the pass. Moments in the transverse plane were nearly as high as in full effort kicks reported previously. Muscle stress in the m. gracilis reached up to 450 kPa. Considering the repetitive nature of inside passes in modern sot; cer, adductor muscles undergo high loads in matches and training. This might contribute to the explanation of the high incidences of adductor injuries. Practitioners should therefore consider the load-recovery-relation even in inside pass training. Specific strength training programs for the adductor and abductor muscle groups should be developed
Diamagnetic response of cylindrical normal metal - superconductor proximity structures with low concentration of scattering centers
We have investigated the diamagnetic response of composite NS proximity
wires, consisting of a clean silver or copper coating, in good electrical
contact to a superconducting niobium or tantalum core. The samples show strong
induced diamagnetism in the normal layer, resulting in a nearly complete
Meissner screening at low temperatures. The temperature dependence of the
linear diamagnetic susceptibility data is successfully described by the
quasiclassical Eilenberger theory including elastic scattering characterised by
a mean free path l. Using the mean free path as the only fit parameter we found
values of l in the range 0.1-1 of the normal metal layer thickness d_N, which
are in rough agreement with the ones obtained from residual resistivity
measurements. The fits are satisfactory over the whole temperature range
between 5 mK and 7 K for values of d_N varying between 1.6 my m and 30 my m.
Although a finite mean free path is necessary to correctly describe the
temperature dependence of the linear response diamagnetic susceptibility, the
measured breakdown fields in the nonlinear regime follow the temperature and
thickness dependence given by the clean limit theory. However, there is a
discrepancy in the absolute values. We argue that in order to reach
quantitative agreement one needs to take into account the mean free path from
the fits of the linear response. [PACS numbers: 74.50.+r, 74.80.-g]Comment: 10 pages, 9 figure
Frustration induced Raman scattering in CuGeO_3
We present experimental data for the Raman intensity in the spin-Peierls
compound CuGeO_3 and theoretical calculations from a one-dimensional frustrated
spin model. The theory is based on (a) exact diagonalization and (b) a recently
developed solitonic mean field theory. We find good agreement between the
1D-theory in the homogeneous phase and evidence for a novel dimerization of the
Raman operator in the spin-Peierls state. Finally we present evidence for a
coupling between the interchain exchange, the spin-Peierls order parameter and
the magnetic excitations along the chains.Comment: Phys. Rev. B, Rapid Comm, in Pres
Maintenance of Leukemia-Initiating Cells Is Regulated by the CDK Inhibitor Inca1
Functional differences between healthy progenitor and cancer initiating cells may provide unique opportunities for targeted therapy approaches. Hematopoietic stem cells are tightly controlled by a network of CDK inhibitors that govern proliferation and prevent stem cell exhaustion. Loss of Inca1 led to an increased number of short-term hematopoietic stem cells in older mice, but Inca1 seems largely dispensable for normal hematopoiesis. On the other hand, Inca1-deficiency enhanced cell cycling upon cytotoxic stress and accelerated bone marrow exhaustion. Moreover, AML1-ETO9a-induced proliferation was not sustained in Inca1-deficient cells in vivo. As a consequence, leukemia induction and leukemia maintenance were severely impaired in Inca1â/â bone marrow cells. The re-initiation of leukemia was also significantly inhibited in absence of Inca1â/â in MLLâAF9- and c-myc/BCL2-positive leukemia mouse models. These findings indicate distinct functional properties of Inca1 in normal hematopoietic cells compared to leukemia initiating cells. Such functional differences might be used to design specific therapy approaches in leukemia
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