Tianbao Cheng

The temperature dependence of grain boundary free energy of solids

The grain boundary free energy of solids at elevated temperatures has been rarely reported because of the difficulty in determining it from the existing methods. In the present work, a theoretical model for the temperature-dependent grain boundary energy is proposed via an analytical approach from the energy view by relating its temperature dependence to that of specific heat at constant pressure. The accuracy of the model is validated on metals and ceramics. The study shows that the grain boundary energy first remains approximately constant and then decreases almost linearly as temperature changes from 0 K to melting point. Phase transformation can reduce the grain boundary energy. As an example of application, the “brittleness parameter” of solids at elevated temperatures is characterized quantitatively for the first time. The ductility of metals increases rapidly with temperature. The brittleness of ceramics almost holds up to the melting point.

Thermal Shock Resistance of Chemical Vapor Deposited Zinc Sulfide Under Active Cooling

The thermal shock resistance of chemical vapor deposited zinc sulfide (CVD ZnS) infrared side window of high-speed vehicles with convective cooling is studied using finite volume method. The involved factors are the surface heat flux, coolant temperature, convective heat transfer coefficient, and thermal shock initial temperature. All the material properties are temperature-dependent. The study shows that convective cooling can improve the thermal up shock resistance of CVD ZnS caused by aerodynamic heating at the upper surface. On the other hand, it can also lead to thermal down shock failure at the lower surface. The critical failure time corresponding to thermal down shock failure is much less than that corresponding to thermal up shock failure. Thus, thermal down shock failure should be avoided in application. The critical thermal shock initial temperatures, below which convective cooling will not cause thermal down shock failure, for different coolants are calculated.