Design a Fire resistant safety sphere with diameter of 0.5m that will protect its combustible contents from fire which may last up to 2hr. Assume the box will be exposed to an environment at an average temperature of 700C with a combined heat transfer coefficient of 70W/m^2*K and the temperature inside the sphere must be below 150C at the end of 2hr. The cavity of the sphere must be as large as possible while meeting the design constraints and the insulation material selected must withstand the high temperatures to which it will be exposed. Cost, durability and strength are also important considerations in the selection of insulation materials.