A damper door unit is swingably disposed in an air intake duct for selectively permitting outdoor intake introduction and indoor air introduction. The damper door unit comprises a main door which is swingably connected to the air intake duct to selectively close an outdoor air intake opening and an indoor air intake opening, and an auxiliary door which is swingably connected to the main door to se...

The porosity and surface roughness of ceramic substrates are improved by coating the surface with silicon compounds, drying the coated substrate, and then subjecting the substrate to elevated temperature so as to convert the silicon compounds to silicon oxides. Coated substrates obtained by the above process as well as specific preferred organosilane compositions are provided.