This work presents the preparation of porous alumina ceramics through the sacrificial phase method, using an eco-friendly material, namely waste coffee grounds, as a pore-forming agent. The effects of coffee grounds content in the green ceramic bodies on the linear and volumetric shrinkage, as well as the total and open porosity of the sintered product, were evaluated. The influence of the resulting porosity on mechanical properties of the prepared porous alumina was determined using Brazilian disk compression test for the determination of the indirect tensile strength of the prepared samples. Microstructure and pores morphology were characterized by scanning electron microscopy. Porosities in the range 35-54 vol % were achieved, by varying the coffee grounds content from 0 to 50 wt% in the green bodies. The indirect tensile strength of the final obtained porous alumina ceramic decreased accordingly from 57.4 MPa to 17.7 MPa. K E Y W O R D S alumina, Brazilian test, green body, porous ceramics, sacrificial phase | INTRODUCTIONPorous ceramics possess interesting features that cannot be achieved by their metallic or polymeric counterparts, such as high thermal and chemical stability, high thermal insulation, and mechanical resistance. 1-3 These unique properties, along with the high surface area, low density, high permeability, high specific strength, low specific heat, and low thermal conductivity (which are associated with the presence of pores, and therefore cannot be offered by the dense state), in addition to biocompatibility; make them typical for applications such as high-temperature insulation, gas combustion burners, catalyst carriers, filters for molten metals and hot corrosive gases, bone scaffolds, bioreactors, electrodes in fuel cells, chemical sensors, metal and polymer matrix composites, water filtration as well as lightweight building materials. [2][3][4] There exists a number of methods for the fabrication of these materials, such as partial sintering, replication technique (using natural and artificial templates), sacrificial fugitives (sacrificial phase or space holder technique), direct foaming, paste extrusion of ceramic powders, rapid prototyping techniques, sintering of hollow spheres, gel casting, connected rods and fibers, bonding techniques, sol-gel and starch consolidation. 2,[5][6][7][8][9][10] In the sacrificial phase technique, a mixture consisting of the ceramic powder, or its precursors, and a fugitive material, acting as a pore-forming agent (PFA), is prepared, and a green body is formed. To obtain a porous structure, the PFA, depending on its nature, is removed by several methods, including thermal methods such as pyrolysis, evaporation, and sublimation, or chemical methods such as washing by water and leaching by acids. 3,8,11 Then, the resulting product is sintered to obtain a porous ceramic. Many types of synthetic and natural materials, employed by the sacrificial phase technique and used as fugitives or pore formers (porogen agents, space-holders, or PFAs), are reported in the...
Porous alumina ceramics were prepared through the space holder technique, using ground sunflower seed shells as a fugitive material and uniaxial pressing for forming the green ceramics. Influences of the sunflower seed shell content on the shrinkage of the green bodies and porosity of sintered products were evaluated. The prepared ceramics were characterized for mechanical properties using the Brazilian disk test, and the porosity effect on the measured strength was determined. The microstructure was characterized by SEM. The sunflower seed shell content was varied from 0 to 60 wt%. Porosities within 29.9-71.0 vol% were achieved, and the strength of the obtained alumina ceramics decreased accordingly from 59.7 to 4.0 MPa. Additional samples, prepared with different compaction pressure, were characterized for electrical properties and showed high electrical insulation capability, which increased with porosity. Mechanical and electrical data were discussed based on theoretical models, namely the Gibson-Ashby model and/or the minimum solid area model.
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