2.9 A young engineer is asked to design a thermal protec- tion barrier for a sensitive electronic device that might be exposed to irradiation from a high-powered infrared laser. Having learned as a student that a low thermal conductivity material provides good insulating charac- teristics, the engineer specifies use of a nanostructured aerogel, characterized by a thermal conductivity of ka % 0.005 W/m! K, for the protective barrier. The engi- neer’s boss questions the wisdom of selecting the aero- gel because it has a low thermal conductivity. Consider the sudden laser irradiation of (a) pure aluminum, (b) glass, and (c) aerogel. The laser provides irradiation of G % 10 + 106 W/m2 . The absorptivities of the materials are ' % 0.2, 0.9, and 0.8 for the aluminum, glass, and aerogel, respectively, and the initial temperature of the barrier is Ti % 300 K. Explain why the boss is concerned. Hint: All materials experience thermal expansion (or contraction), and local stresses that develop within a material are, to a first approximation, proportional to the local temperature gradient.