Synthesis of Si3N4-celsian composite materials

The synthesis of Si3N4-celsian composite materials was studied in this pape r. To obtain celsian, two different types of additives were used. In the fi rst case the appropriate mixture of BaCO3, Al2O3 and SiO2 was added to Si3N 4, while in the second case a Ba-A-zeolite, previously heated at 850 degree s C, was used as an additive. Mixtures of Si3N4 and the sintering aids were homogenized in a vibratory mill in the presence of isopropanol. The green bodies were pressurelessly sintered in nitrogen atmosphere at temperatures of 1650-1750 degrees C. Densification degree was followed, as well as micro structural changes, as a function of the sintering conditions and the addit ive used. Reaction of BAS formation was followed by X-ray analysis during t he heating up period. It was found that with 10% zeolite as a sintering aid , full densification was achieved at 1750 degrees C but with the same amoun t of oxide mixture, only 96% TD was attained. Phase changes were followed a lso during cooling down the samples from sintering temperature. Two crystal line phases, beta-Si3N4 and hexacelsian, were present in both types of sint ered samples. (C) 1999 Elsevier Science Ltd and Techna S.r.l. All rights re served.