The carbothermal nitridation synthesis of ␣-Si 3 N 4 is studied using electron microscopy techniques (FEG/SEM and TEM) and chemical composition analysis to characterize the reaction at various degrees of conversion. The reaction follows a nucleation-growth mechanism. Without ''seed'' ␣-Si 3 N 4 in the precursor, the reaction rate is controlled by the formation of nuclei which are associated with a Si-O-C intermediate phase. In the presence of ''seed,'' the limiting step is growth of ␣-Si 3 N 4 onto the ''seed'' nuclei. Growth appears to follow a gas-phase route and is characterized by an irregular porous layer which grows onto the ''seed.'' The porous structure is the result of reaction around carbon particles which are consumed during the process. The presence of admixed ''seed'' Si 3 N 4 in the precursor formulation increases the reaction rate since the nucleation step is eliminated. An activation energy of E = 457 ± 55 kJ/mol for the overall reaction closely approximates that previously reported for the formation of SiO. This result, along with the finding that residual crystalline SiO 2 is present at all stages of the reaction, indicates that the overall reaction rate is controlled by the reduction of SiO 2 . Since reaction at the carbon and SiO 2 contact points is fast, the rate-limiting step is most likely the gas-phase carbon reduction of SiO 2 with CO.

Solid-state reactions between bulk samples of copper oxide and alumina have been studied using scanning electron microscopy and electron microprobe analysis. Both CuAl 2 O 4 and CuA10 2 were found to form during reactions in air at 1100 °C between CuO powder and single-crystal alumina substrates. The relative position of the CuAl 2 O 4 and CuA10 2 layers was observed to depend on the crystallographic orientation of the surface of the alumina substrate: CuAl 2 O 4 formed in contact with (0001) alumina substrates while CuA10 2 formed when the alumina substrate surface was (1120). Faceted Cu-amminate/alumina phase boundaries were observed to develop when single-crystal alumina rods were reacted with CuO, although the interfaces invariably tended to be wavy.

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The possibility of having Sr as an interstitial metal cation in α‐SiAION has been investigated in two systems: a single‐cation system (Si3N4‐SrO‐AlN) and a multication system (Si3N4‐(Y2O3/SrO/CaO)‐AlN). It was found that Sr alone does not form α‐SiAlON and that Sr could only be accommodated in α ‐SiAION in conjunction with Y and Ca. The Sr content of α‐SiAION increased as the total content of (Y + Ca) increased and appeared to reach a limit at 0.5 at.%, or 0.15 atom per α‐SiAlON. Unexpectedly, some of the α‐SiAlON that contained (Sr + Y + Ca) was present as laths or fibers with the c‐axis perpendicular to the hot‐press direction.