Chemical Synthesis of Microporous Nonoxide Ceramics from Polysilazanes

In a search for a generic synthesis methodology to produce microporous, nonoxide ceramics, pyrolysis of silicon-based preceramic polymers has been studied in inert (He or Ar) and reactive (NH3) atmospheres over the temperature range 25−1300 °C. Pyrolysis of polysilazanes in an inert atmosphere in the absence of additives produced only low-density, nonmicroporous solids. However, four successful approaches to induce microporosity were developed, involving controlled reaction of selected low molecular weight polysilazane preceramic polymers. The first method employs the formation of colloidal polysilazane mixtures with micron-size particles of ceramics such as Si3N4, SiC, and AlN, followed by their pyrolysis in He or NH3 to form a ceramic−ceramic composite. The second method involves the synthesis of a nanoscale, polysilazane-stabilized metal colloid of a noble or transition metal and its conversion to a metal− or cermet−ceramic composite by pyrolysis in He or NH3. In the third method, a polysilazane is pyr...