NITRIDES OF INTERMETALLIC COMPOUNDS PREPARED FROM MIXTURES OF OXIDES USING SINTERING AND REACTIVE GAS TREATMENT, IN A SINGLE STEP

We examine a method for obtaining nitrides of intermetallic compounds containing up to four cations. The method utilizes a mixture of the relevant monocation oxides and a nitridization process at elevated temperatures induced by a reducing gas which also contains nitrogen: (1) N2 + H-2, (2) NH3 + N2, or (3) NH3. The cations considered are barium, bismuth, lanthanum, niobium, yttrium and zirconium. Their corresponding oxides cover a wide range of stabilities. We look also for any solid solution oxides that may form. The effect of the stability of the monocation oxides and possible solid solution oxides on the nitridization process is examined. A theoretical analysis shows that the nitridization process should proceed, with all the oxides involved, at an oxygen partial pressure P(O2) below about 10(-34) atm. We find partial nitridization at experimental P(O)2 values of 10(-21)-10(-23) atm. The results support the theoretical suggestion that the method can only be applied lo cations having oxides with a decomposition P(O2) exceeding the lowest P(O2) value attainable experimentally. For a mixture of stable and less stable oxides one cannot always rely on the formation of intermediate solid solution oxides with relatively high decomposition P(O2). In these cases one should avoid the use of oxides as the starting material.