| The incorporation of gradient porous structure in synthetic materials may lead to thecreation of new functional materials for many innovative applications. Due to thecontinuous change of gradient in porosity and pore size, filter efficiency and highselectivity of pore-gradient ceramic foams (PGCF) are higher than that of traditionalceramic foams. PGCF offer an interesting potential for being used as bone tissuematerials, thermal barrier materials and medical implants. However, few techniqueshave been reported on the fabrication of PGCF with fully interconnected open cell.Creation of a continuously varying and spatially controlled gradient in porosity withinterconnected open pores is still a challenge for these processing steps.The forming process and their characteristic of porous ceramics and pore-gradientceramics are generalized in the paper. A novel approach was introduced to preparepore-gradient Al2O3ceramic foams with association of gelcasting process andpolymeric sphere template. Gelcasting process, based on the in situ polymerization oforganic monomers, shows the ability to fabricate homogeneous, near-net-shape andcomplex-shaped bodies with high strength required for machining. The approach can beadapted for many different compositions and systems, and allow the design of poreconnectivity and gradient height of ceramic foams by the appropriate selection of spheresizes and numbers. The limitation of ceramic foams fabricated by polymeric spongeprocess in sponge carrier can be resolved by this approach. The main results of studiesare as follows:(1) Influence of shear rate, dispersant, solid content, ball milling time and pH valueon the viscosity of Al2O3slurry was studied. Results show that viscosity of different solid content slurries is comparatively high at low shear rates. The viscosity decreasesdramatically and hardly changes after60s-1of shear rate. The viscosity decreases withthe increase of dispersant and slightly enhances with excessive dispersant.0.5-0.7%dispersant can effectively decreases viscosity of slurries. The viscosity increases withthe increase of solid content and enhances quickly at60Vol.%solid content. Slurryparticles can disperse well and accordingly viscosity is very low when ball milling timeis8h and pH value is9. Samples with55Vol.%solid content slurry maintaining goodfluidity perform well in both macro-structure and micro-performance.(2) Studies of different modification methods on EPS surfaces show thatmodification by triethylolamine is ineffective and modification by oxygen plasma andVacuum Ultraviolet is effective. Compared with treated sphere surface, wettability ofslurry for untreated surface is not good and micro-pores were found in gelcasting bodies.From the stress-strain simulation pictures of spheres in the template and experimentalresults we know, in order to enhance the porosity of PGCF, it is needed not only toenhance the applied force on template for reducing pores in the template, but to avoidthe reducing of slurry permeability due to excessive deformation of template, and thecrack of bodies linked to flexibility of formed template.(3) The influence of pre-removal of EPS template on the integrity of networks wasinvestigated. Some defects such as edge-delamination and micropores disappeared bysurface modification. The dendritic solidification structure, flawless filamentary of thenetwork and spherical-shaped walls perfectly maintained through template pre-removedwith organic solvent. This method was better than direct pre-removal of template byheating, because closely ordered spheres bonded together by adhesive force, which willcause strain to struts of sample and result in rupture of solid struts and cracks ofreticulation points.(4) As for the same sintering temperature and different gradient layers, compressivestrength enhanced along with the increase of numbers of gradient layers and intersectionof gradient structure. When gradient layer is same and diameter of spheres increases,compressive strength enhanced slightly due to the increase of strut thickness. The hierarchical pore structure was fabricated with EPS spheres of2.1mm,2.6mm and3.2mm diameters. Porosity and compressive strength of the pore-gradient Al2O3ceramicfoam were68.5%and3.06MPa at1500℃, respectively. |
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