Engineering, 19.09.2019 00:30 leannehounschell

Ayoung engineer is asked to design a thermal protection barrierfor a sensitive electronic device that might be exposed toirradiation from a high-powered infared laser. having learned as astudent that a low thermal conductivity material provides goodinsulating characteristics, the engineer specifies use of ananostrutured aerogel, characterized by a thermal conductivity ofka = 0.005 w/m-k, for the protective barrier. the engineer'sboss questions the wisdom of selecting the aerogel because it has alow thermal conductivity. consider the sudden laserirradiation of (a) pure aluminum, (b) glass, and (c) aerogel. the laser provides irradiation of g = 10x10^6 w/m^2. theabsortivities of the materials are alpha = 0.2, 0.9, and 0.8 for the aluminum, glass, andaerogel, respectively, and the initial temperature of the barrieris ti = 300 k. explain why the boss is concerned. hint: all materials experience thermal expansion (or contraction), andlocal stresses that develop within a material are, to a firstapproximation, proportional to the local temperature gradient. kglass = 1.4 w/m-kkaluminum = 237 w/m-k

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