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 can exceed 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.
Failure due to thermal shock can be prevented by;
Borosilicate glass 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 (mostly in the lithium aluminosilicate (LAS) system) 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 thermal shock the impulse excitation technique proved 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 thermal shock cycles and this way the deterioration in physical properties can be mapped out.
The laser gain medium generates heat. This heat is drained through the heat sink. The transfer of heat occurs at a certain temperature gradient. The non-uniform thermal expansion of a bulk material causes the stress and tension, which may break the device even at a slow change of 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 by the thermal shock parameter. (see above)
Roughly, at the efficient operation of laser, the power of heat generated in the gain medium is proportional to the output power of the laser, and the coefficient of proportionality can be interpreted as heat generation parameter; then, The heat generation parameter is basically determined by the quantum defect of the laser action, and one can estimate , where and 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 is thickness of the layer and 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.
The estimate above is not the only parameter which determines the limit of overheating of a gain medium. The maximal raise of temperature, at which the medium still can efficiently lase, is also the important property of the laser material. This overheating limits the maximal power with an estimate
Combination of the two estimates above of the maximal power gives the estimate
is thermal loading; parameter, which is important property of the laser material. The thermal loading, saturation intensity and the loss determine the limit of power scaling of the disk lasers . Roughly, the maximal power at the optimised sizes and , is of order of . This estimate is very sensitive to the loss . However, the same expression can be interpreted as a robust estimate of the upper bound of the loss required for the desired output power :
All the disk lasers reported work at the round-trip loss below this estimate. The thermal shock parameter and the loading depend on the temperature of the heat sink. Certain hopes are related with a laser, operating at cryogenic temperatures. The corresponding Increase of the thermal shock parameter would allow to soften requirements for the round-trip loss of the disk laser at the power scaling.
Thermal shock testing exposes products to alternating low and high temperatures to accelerate failures caused by temperature cycles or thermal shocks during normal use. The transition between temperature extremes occurs very rapidly, greater than 15 °C per minute.
Equipment with single or multiple chambers is typically used to perform thermal shock testing. When using single chamber thermal shock equipment, the products remain in one chamber and the chamber air temperature is rapidly cooled and heated. Some equipment uses separate hot and cold chambers with an elevator mechanism that transports the products between two or more chambers.
Glass containers can be sensitive to sudden changes in temperature. One method of testing involves rapid movement from cold to hot water baths, and back.
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