Abstract
Motivated by the hierarchical micro and nanoscale features in terms of porosity of diatomite, the production of ceramic-graded porous foams with tailored porosity, obtained by using it as raw material, has been proposed. The main challenge during the foam-production process has been the preservation of diatomite nanometric porosity and the addition of other levels of hierarchical porosity. The coupled use of two techniques of direct foaming (chemical and mechanical), combined with the use of 3D printing inverse replica method, assured the achievement of porosity of, respectively, microscopic and macroscopic dimensions. Optical and scanning electron microscopies have been performed for an in-depth characterization of the final microstructure. XRD analysis has been carried out to check the influence of sacrificial templates on the matrix mineralogical composition. The porosity of the diatomite-based foams has been investigated by means of nitrogen-adsorption analysis and mercury-intrusion porosimetry. The experimental tests confirmed the presence of different porous architectures ranging over several orders of magnitudes, giving rise to complex systems, characterized by hierarchical levels of porosity. The presence of porosity of graded dimensions affects the final mechanical performances of the macroporous diatomite-based foams, while their mineralogical composition does not result to be affected by the addition of templates.
Highlights
The fabrication of foam materials with a porosity gradient is widely desired from scientists in order to achieve an enhanced performance (3D functionally-graded properties), mainly due to their low density, high strength and specific functional properties
chemically bonded ceramics (CBCs) are produced through a chemical reaction, consisting of a polycondensation of silica phases dissolved in a strong alkaline environment at low temperature, in contrast to traditional ceramic materials that are usually produced using fusion or sintering processes at elevated temperature
The design and synthesis of ceramic graded porous foams with hierarchical porosity starting from a natural nanoporous material, the diatomite, has been proposed in this paper
Summary
The fabrication of foam materials with a porosity gradient (functionally-graded porous materials-FGPMs) is widely desired from scientists in order to achieve an enhanced performance (3D functionally-graded properties), mainly due to their low density, high strength and specific functional properties. FGPMs are innovative materials suitable for specific and advanced functions, in which a spatial gradation in structure and/or composition leads to tailored properties[1] For this reason, these materials find application in a broad range of high-tech fields such as energy, building, aerospace, filtration and bioengineering. Apart from the aforementioned conventional processes, the use of advanced techniques such as additive manufacturing, which permits the fabrication of complex geometries with high accuracy, to make graded porosity (with predicted distributed porosities) and compositions, still remains a challenge[5] This is even more a challenge if the “starting material” itself contains an intrinsic gradient porosity, obtained through physical and chemical blowing reactions and in specific environmental conditions[6,7]. The innovative manufacturing process proposed in the present paper guarantees the presence of both microscopic- and macroscopic-scale porosity, preserving at the same time the nanoscale porosity of the starting diatomite, leading to ceramic foams with graded and tunable porosity
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