Thermal stability, nonstoichiometric

The interaction of the actinides or of their hydrides with chalcogens or H2X, respectively, give a number of nonstoichiometric phases with a composition close to AnX, An2X3, A11X2, An2Xs, or A11X3. Their thermal stability decreases in the order S > Se > Te. The magnetic susceptibility and Mossbauer spectroscopy show that the latter compounds may be formulated as [An +]2(S )2(S-Sf-, [An +]2(S -)3(S-S)2-, and [An +](S2-)(S-S)2-. These compounds have coordination numbers of 8, 10, and... 

However, the decomposition temperature ranges differ widely in accordance with the nature of the metal involved and the wide variation in thermal stability of the intermediate metal carbonates. Aluminium oxalate is anomalous in that its nonstoichiometric water of hydration is retained in the final residue, which corresponds to hydrated alumina. ... 

The thermal stability of all of these nonstoichiometric pyrochlores is limited and is inversely dependent upon the extent of substitution of noble metal cations on the B-site by post transition element cations (1). For example, Pb2[Ru2]05,5 is stable to 850 C in air while Pb2[RuPK]06,5 is only stable to about 400 C. 

The nonstoichiometric composition coupled with the microporosity and amorphous phase in the microstructure reduces the thermal stability of the fibers. Depending on the magnimde and duration of the applied stress, the use of the fibers may be limited to temperatures lower than 1000°C. To overcome this deficiency, recent efforts have been devoted to producing dense, polycrystalline, stoichiometric SiC through modification of the synthesis and processing conditions or through the synthesis of more useful preceramic polymers (63,67,68). 

---