Rheology and packing characteristics of alumina extrusion using boehmite gel as a binder

The present work reports the extrusion characteristics of alumina pastes in volving boehmite gel as a binder. Submicron sized alumina (average particle size 0.35 mum) is dispersed in boehmite sol and converted to form a thick- alumina paste by controlled flocculation and centrifugation. The paste is f urther subjected to extrusion for fabricating alumina shapes such as rods a nd tubes. The paste rheology, workability, green strength and sintered prop erties are studied and reported. The paste rheology was studied by torque r heometer and the results on the development of torque, viscosity and the yi eld stress are presented. The alumina paste containing 18 vol. % boehmite b inder resulted in 53.4 vol. % solids loading with suitable consistency for low pressure extrusion. The system exhibits a low torque value of 51.85 KPa at shear rate 28.5 s(-1) and showed plastic behavior. The apparent viscosi ty of the system showed a shear dependent flow behavior. Alumina-18 vol. % boehmite resulted in a theoretical green density value of 61.4% and a green strength of about 1.05 N/mm(2) on extrusion. The extruded alumina on sinte ring has a theoretical sintered density of about 98% at 1500 degreesC. The microstructure of sintered alumina showed an average grain size approx. 2 m um. The combination of colloidal processing and gel assisted extrusion yiel ds highly dense, fine grained ceramics. Boehmite gel as a binder has advant ages such as matrix compatibility, good workability and better green streng th. (C) 2001 Elsevier Science Ltd and Techna S.r.l. All rights reserved.