Direct production of Si3N4 foams by carbothermal reduction and nitridation of SiO2


Topates G.

Ceramics International, cilt.44, ss.20545-20550, 2018 (SCI Expanded İndekslerine Giren Dergi) identifier

  • Cilt numarası: 44 Konu: 16
  • Basım Tarihi: 2018
  • Doi Numarası: 10.1016/j.ceramint.2018.08.052
  • Dergi Adı: Ceramics International
  • Sayfa Sayıları: ss.20545-20550

Özet

In this study, Si3N4 foams are successfully produced by a combination of replica and carbothermal reduction and nitridation (CRN) methods. Slurries prepared from C, SiO2 (molar ratio of C:SiO2 was kept to 2.2), bentonite and three types of oxide additives (Y2O3, CeO2 and Yb2O3) are impregnated into polyether sponges. With the addition of bentonite, sufficient green strength is provided and durable foams are produced after the sponge burnout process. Bentonite also enhances the CRN process by increasing the amount of eutectic melt and, hence, a higher amount of beta-Si3N4 is obtained. In addition to bentonite, the yield of beta-Si3N4 is highly dependent on the type of oxide additive. The highest yield is achieved when Y2O3 is used as a result of the formation of a lower viscosity eutectic melt. Vapor-liquid-solid mechanism is favored by the lower viscosity. Foams have beta-Si3N4 as the major phase with minor amounts of alpha-Si3N4 and secondary phases originate from the eutectic melt. The formation and growth mechanisms of beta-Si3N4 are directly affected by the eutectic melt. SiO is deposited in the eutectic melt and super-saturation of beta-Si3N4 grains occur. During the growth of these grains, they contact each other, hindering the formation of typical elongated beta-grains. The final microstructures of the foams consist of hexagonal beta-grains with a low aspect ratio.