Temperature | EPFL Graph Search

Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied on various reference points and thermometric substances for definition. The most common scales are the Celsius scale with the unit symbol °C (formerly called centigrade), the Fahrenheit scale (°F), and the Kelvin scale (K), the latter being used predominantly for scientific purposes. The kelvin is one of the seven base units in the International System of Units (SI). Absolute zero, i.e., zero kelvin or −273.15 °C, is the lowest point in the thermodynamic temperature scale. Experimentally, it can be approached very closely but not actually reached, as recognized in the third law of thermodynamics. It would be impossible to extract energy as heat from a body at that temperature. Temperature is important in all fields of natural science, including

Quantitative microstructural characterisation of concrete cured under realistic temperature conditions

Xinyu Zhang

The curing temperature is known to influence the strength and durability of concrete. In general, curing at elevated temperature results in high early strength gain, but impairs the long-term strength and transport properties. Previous investigations have not provided a complete understanding of these effects; in particular, no quantitative relation between microstructural and macroscopic changes has been developed. This work has adopted a combination of macro and micro characterisation to further a comprehensive and quantitative understanding of the property development under different curing conditions, through approaches which include both qualitative and quantitative microstructural analysis with SEM, EDS, XRD, NMR and TG. A systematic study on several mix designs and curing regimes has been carried out, to relate microstructural properties such as degree of hydration, C-S-H composition, C-S-H relative density and capillary porosity to compressive strength and water sorptivity. The results show that the Ca/(Si+Al) ratio of C-S-H is constant with temperature, and the increase in relative C-S-H brightness is thus attributed to the higher C-S-H density and lower chemically bound water at higher temperatures. The early hydration of concretes is accelerated by high temperatures, but the ultimate value is not limited, as opposed to widely reported lower values in the literature. This discrepancy is explained by the fact that previous workers have assumed that the bound water is constant for different curing temperatures whereas this is not the case. Linear relationships were established between the degree of hydration and the compressive strength of concretes, which depend on curing temperatures and mix design. Through their influence on the density and distribution of hydration products, elevated temperatures increase the ultimate capillary porosity of concretes. The relationship between capillary porosity and compressive strength seems independent of temperature to a first approximation, but no unique relationship is shown between sorptivity and compressive strength. Materials which were exposed to higher temperatures for only short periods of time, such as might be experienced in prefabrication or in the curing of large masses, showed some recovery in both microstructure and mechanical properties from the detrimental effects of high temperature.

EPFL2007

Control Scenarios for an SOFC System

Joel Albrektsson

This report is intended to summarize the work of a semester project conducted on a gPROMS model (developed in LENI) of an existing stand-alone 1kW solid oxide fuel cell (SOFC) system. Different means of controlling the cathode gas temperature with single-input-single-output PID controllers were investigated. Particular attention was directed to the issue of erroneous thermocouple readings due to radiation and how this error can be minimized. It was shown that the discrepancy between thermocouple reading and real temperature depends strongly on the method used to control the temperature. In some cases, the incorrect thermocouple reading can be fatal for the system even in regular operation. With this basic type of PID controller, lambda control was shown to be the scenario with the smallest discrepancy. However, it is still non-negligible and a more advanced control approach should be investigated before an implementation on the physical system.

2011

Etude des propriétés morphologiques et catalytiques d'agrégats d'or triés en taille et déposés sur le TiO2(110)

Raphaël Vallotton

This thesis aims to find, for the first time, a direct relation between the size and morphology of small metallic nanostructures (gold in this case) supported on a metal-oxide surface to their catalytic activity. In this perspective, three main topics have been treated during this thesis. The first part concerns the design and realization of a Scanning Tunneling Microscope (STM) supposed to work over a wide temperature range (4K < T ≤ 300K). This new device replaces an existing solution. The coarse approach of the scanning tip towards the sample is realized by an axial motor based on a sapphire prism gliding on shear piezos in the stick&slip mode. The new STM is very rigid moving the resonance frequencies to higher values with respect to the existing solution. Operation of the motor down to T = 8K has been proven, however topographic imaging has been performed only in the temperature range 77K < T ≤ 300K. The second part of this work focuses on a study of the evolution of the morphology of gold nanoparticles on a TiO2(110) surface. Size-selected clusters Aun+ (n = 5, 7) are deposited at a well defined kinetic energy on the surface held at room temperature. Subsequent annealing of the sample has been performed stepwise. After each temperature increase, the morphology has been determined by STM. The evolution as a function of surface temperature has been studied for two different surface reconstructions, TiO2(110)-(1×1) and TiO2(110)-(2×1). The deposition process leads only to small fragmentation and the morphology is stable up to T = 400K. Further increase of the surface temperature leads to sintering of the particles by Ostwald ripening as shown by an exponential decrease of the island density with temperature. The main topic of this thesis, the correlation between morphology and catalytic activity, is described in the last part of this manuscript. For the first time we are able to relate the onset of CO2 production from CO and O2 to a clear change in the morphology. The catalytic activity of the particles strongly depends on their size and dimensionality. The relative activity per particle has been determined and we find a clear maximum for clusters containing 60 atoms and are 3 to 4 monolayers high. These results are discussed in contrast of literature data on the same but also on different metal-oxide surfaces.

EPFL2009