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Thermal shockAdditional recommended knowledge
Thermal shock in mechanical models
Thermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, and high thermal expansion coefficients. However, they are used in many high temperature applications due to their high melting point. Thermal shock occurs when a thermal gradient causes different parts of an object to expand by different amounts. This differential expansion can be understood in terms of stress or of strain, equivalently. At some point, this stress overcomes the strength of the material, causing a crack to form. If nothing stops this crack from propagating through the material, it will cause the object's structure to fail. Thermal shock can be prevented by:
Borosilicate glass such as Pyrex is made to withstand thermal shock better than most other glass through a combination of reduced expansion coefficient and greater strength, though fused quartz outperforms it in both these respects. Some glass-ceramic materials include a controlled proportion of material with a negative expansion coefficient, so that the overall coefficient can be reduced to almost exactly zero over a reasonably wide range of temperatures. Reinforced carbon-carbon is extremely resistant to thermal shock, due to graphite's extremely high thermal conductivity and low expansion coefficient, the high strength of carbon fiber, and a reasonable ability to deflect cracks within the structure. To measure thermo shock the impulse excitation technique proofed to be a useful tool. It can be used to measure Young's modulus, Shear modulus, Poisson's ratio and damping coefficient in a non destructive way. The same test-piece can be measured after different thermo shock cycles and this way the detoriation in physical properties can be mapped out. Thermal shock parameter in the physics of solid-state lasersThe laser gain medium generates heat. This heat is drained through the heat sink. The transfer of heat occurs at certain temperature gradient. The non-uniform thermal expansion of a bulk material causes the stress and tension, which may break the device even at slow change of the temperature. (for example, continuous-wave operation). This phenomenon is also called thermal shock. The robustness of a laser material to the thermal shock is characterized with the thermal shock parameter ^{[1]}
where
Roughly, at the efficient operation of laser, the power P_{h} of heat generated in the gain medium is proportional to the output power P_{s} of the laser, and the coefficient q of proportionality can be interpreted as heat generation parameter; then, P_{h} = qP_{s}. The heat generation parameter is basically determined by the quantum defect of the laser action, and one can estimate q = 1 − ω_{s} / ω_{p}, where ω_{p} and ω_{s} are frequency of the pump and that of the lasing. Then, for the layer of the gain medium placed at the heat sink, the maximal power can be estimated as where h is thickness of the layer and L is the transversal size. This estimate assumes the unilateral heat drain, as it takes place in the active mirrors. For the double-side sink, the coefficient 4 should be applied. Thermal loadingThe estimate above is not the only parameter which determines the limit of overheating of a gain medium. The maximal raise ΔT of temperature, at which the medium still can efficiently lase, is also important propertiy of the laser material. This overheating limits the maximal power with estimate Combination of the two estimates above of the maximal power gives the estimate where is thermal loading; parameter, which is important property of the laser material. The thermal loading, saturation intensity Q and the round-trip loss β determine the limit of power scaling of the disk lasers ^{[2]}. Roughly, the maximal power at the optimised sizes L and h, is of order of . This estimate is very sensitive to the loss β. However, the same expression can be interpreted as a robust estimate of the upper bound of the loss required for the desirable output power P: All the disk lasers reported seem to work at the round-trip loss below this estimate. The thermal shock parameter and the loading depend of the temperature of the heat sink. Certain hopes are relates with a laser, operating at cryogenic temperatures. The corresponding Increase of the thermal shock parameter would allow to softer requirements for the round-trip loss of the disk laser at the power scaling. Examples of thermal shock failure
See also
References
Categories: Materials science | Lasers |
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Thermal_shock". A list of authors is available in Wikipedia. |