Topic
Thermal expansion
About: Thermal expansion is a research topic. Over the lifetime, 21040 publications have been published within this topic receiving 349407 citations. The topic is also known as: heat expansion.
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01 Jan 2008
TL;DR: In this article, the authors used the Weibull analysis to evaluate the structural stability of planar solid oxide fuel cells (SOFC) in both steady-state and dynamic operations.
Abstract: Structural stability issues in planar solid oxide fuel cells (SOFC) arise from the mismatch between the coefficients of thermal expansion (CTE) of the components. The stress state at operating temperature is the superposition of several contributions, which differ depending on the component. First, the cells undergo residual stresses due to the sintering phase during the manufacturing process. Furthermore, the load applied during the assembly of the stack to ensure the electric contact and flatten the cells prevents a completely stress-free expansion of each component during the heat-up. In operation, finally, thermal gradients cause additional stresses. The temperature profile generated by a thermo-electro-chemical model implemented in an equation oriented process modeling tool (gPROMS) was imported into finite-element software (ABAQUS) to calculate the stress distribution in all components of a representative SOFC repeat element. An uncoupled approach was used, since no direct feedback from the stress calculation to the thermo-electro-chemical model exists. The thermal stresses in the components of the repeat element were simulated in both steady-state and dynamic operations. Particular conditions such as current load shutdown, and cooling to room temperature after operation, were investigated as well. The different layers of the cell, i.e. anode, electrolyte, cathode, compensating layer and compatibility layer, were considered in the analysis by using the submodelling capabilities of the finite-element tool. Assessment of the risks of failure was performed by the widely used Weibull analysis. The occurrence of plastic deformation and the dependence on temperature of both CTE and Young’s modulus of the metallic parts as well as the orthotropic nature of the compressive sealant were implemented in the finite-element model. The residual stresses were dominating the stress state in the cell, except in severe operation conditions. Thus the cell at room temperature after the reduction procedure was revealed as the most critical case. On the contrary, thermal gradients induced irreversible deformation of the metallic interconnector in the area submitted to the highest temperature.
144 citations
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TL;DR: The positive coupling between compression and thermal expansion in this material enhances its piezo-mechanical response in adiabatic process, which may be used for designing new artificial composites and ultrasensitive measuring devices.
Abstract: Materials with negative linear compressibility are sought for various technological applications. Such effects were reported mainly in framework materials. When heated, they typically contract in the same direction of negative linear compression. Here we show that this common inverse relationship rule does not apply to a three-dimensional metal-organic framework crystal, [Ag(ethylenediamine)]NO3. In this material, the direction of the largest intrinsic negative linear compression yet observed in metal-organic frameworks coincides with the strongest positive thermal expansion. In the perpendicular direction, the large linear negative thermal expansion and the strongest crystal compressibility are collinear. This seemingly irrational positive relationship of temperature and pressure effects is explained and the mechanism of coupling of compressibility with expansivity is presented. The positive coupling between compression and thermal expansion in this material enhances its piezo-mechanical response in adiabatic process, which may be used for designing new artificial composites and ultrasensitive measuring devices.
144 citations
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TL;DR: In this paper, an unexpected mechanism for fast reaction of Alnanoparticles covered by a thin oxide shell during fast heating is proposed and justified theoretically and experimentally, in which the volume change due to melting induces pressures of 1-2 GPa and causes dynamic spallation of the shell.
Abstract: An unexpected mechanism for fast reaction of Alnanoparticles covered by a thin oxide shell during fast heating is proposed and justified theoretically and experimentally. For nanoparticles, the melting of Al occurs before the oxide fracture. The volume change due to melting induces pressures of 1–2 GPa and causes dynamic spallation of the shell. The unbalanced pressure between the Al core and the exposed surface creates an unloading wave with high tensile pressures resulting in dispersion of atomic scale liquid Al clusters. These clusters fly at high velocity and their reaction is not limited by diffusion (this is the opposite of traditional mechanisms for micron particles and for nanoparticles at slow heating). Physical parameters controlling the melt dispersion mechanism are found by our analysis. In addition to an explanation of the extremely short reaction time, the following correspondence between our theory and experiments are obtained: (a) For the particle radius below some critical value, the flame propagation rate and the ignition time delay are independent of the radius; (b) damage of the oxide shell suppresses the melt dispersion mechanism and promotes the traditional diffusive oxidation mechanism; (c) nanoflakes react more like micron size (rather than nanosize) spherical particles. The reasons why the melt dispersion mechanism cannot operate for the micron particles or slow heating of nanoparticles are determined. Methods to promote the melt dispersion mechanism, to expand it to micron particles, and to improve efficiency of energetic metastable intermolecular composites are formulated. In particular, the following could promote the melt dispersion mechanism in micron particles: (a) Increasing the temperature at which the initial oxide shell is formed; (b) creating initial porosity in the Al; (c) mixing of the Al with a material with a low (even negative) thermal expansion coefficient or with a phase transformation accompanied by a volume reduction; (d) alloying the Al to decrease the cavitationpressure; (e) mixing nano- and micron particles; and (f) introducing gasifying or explosive inclusions in any fuel and oxidizer. A similar mechanism is expected for nitridation and fluorination of Al and may also be tailored for Ti and Mg fuel.
143 citations
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TL;DR: In this article, the Grueneisen theory has been compared with the linear thermal expansion of room temperature to liquid nitrogen for Mo, Pd, Ag, Ta, W, Pt, and Pb.
Abstract: Extremely accurate determinations of the linear thermal expansions have been made interferometrically from room temperature to the temperature of liquid nitrogen for Mo, Pd, Ag, Ta, W, Pt, and Pb. Comparisons are made with the Grueneisen theory.
143 citations
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TL;DR: In this paper, the authors evaluated and compared thermal shock behavior of plasma-sprayed nanostructured and conventional yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs).
143 citations