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

The present work reports the extrusion characteristics of alumina pastes involving 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 further subjected to extrusion for fabricating alumina shapes such as rods and tubes. The paste rheology, workability, green strength and sintered properties are studied and reported. The paste rheology was studied by torque rheometer and the results on the development of torque, viscosity and the yield stress are presented. The alumina paste containing 18 vol. % boehmite binder 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 viscosity 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 sintering has a theoretical sintered density of about 98% at 1500 degreesC. The microstructure of sintered alumina showed an average grain size approx. 2 mum. The combination of colloidal processing and gel assisted extrusion yields highly dense, fine grained ceramics. Boehmite gel as a binder has advantages such as matrix compatibility, good workability and better green strength. (C) 2001 Elsevier Science Ltd and Techna S.r.l. All rights reserved.