Enrico Bernardo

Macro- and micro-cellular porous ceramics from preceramic polymers

Macrocellular and microcellular SiOC open cell ceramic foams were fabricated from a preceramic polymer. Macrocellular foams, with a cell size ranging from about 100–600 μm and a bulk density ranging from about 0.25–0.58 g/cm3, depending on the processing parameters, were fabricated using a direct foaming approach. Microcellular foams, with a cell size of about 8 μm, were fabricated using poly(methyl methacrylate)microbeads as sacrificial templates. The bulk density ranged from about 0.31–0.48 g/cm3, depending on the amount of microbeads in the starting material. The compression strength of the foams increased with increasing relative density, and microcellular foams possessed a 2–5 times higher crushing strength than macrocellular foams of similar density.

Mechanical properties of metal-particulate lead-silicate glass matrix composites obtained by means of powder technology

The great quantity of waste glasses leads to the need for new applications. The realization of matrices for innovative and cost-effective materials is one possible use. In the present work, lead silicate glasses, recovered from cathode ray tubes (CRTs) are investigated. A low cost powder processing route is proposed for the manufacture of particulate aluminium reinforced glass matrix composites. These composites exhibit an anomalous mechanical behaviour which is thought to be due to a complex metal/glass interaction. In the case of limited metal/glass interaction, good bending strength and fracture toughness are achieved. The obtained KIC level of about 1.20 MPa m0,5, together with the observed crack control behaviour, appears promising.

3D-printed silicate porous bioceramics using a non-sacrificial preceramic polymer binder

Silicate bioceramics possess an excellent bioactivity; however, shaping them into complex geometries is still challenging. Therefore, this paper aims to present a new strategy for the shaping of a bioglass-ceramic with controlled geometry and properties starting from a glass powder combined with a preceramic polymer, i.e. a silicon resin, and reactive fillers. The powder-based three-dimensional (3D)-printing of wollastonite (CaSiO3)-based silicate bioceramic parts was demonstrated in this work. The resin plays a dual role, as it not only acts as a non-sacrificial binder for the filler powders in the printing process but it also reacts with the fillers to generate the desired bioceramic phases. The mechanical and physical properties, i.e. ball-on-three-balls test, density, porosity and morphology, were evaluated in 3D-printed discs. These samples possessed a total porosity around 64 vol% and a biaxial flexural strength around 6 MPa. The raw materials used in this work also enabled the 3D-printing of scaffolds possessing a designed multi-scale porosity, suitable bioceramic phase assemblage and a compressive strength of 1 MPa (for cylindrical scaffolds with total porosity ~80 vol%). Solubility in TRIS/HCl and in vitro assays, i.e. viability, cytotoxicity and apoptosis assays, were also performed. In vitro tests indicated good cell viability and no cytotoxicity effect on the cells.

Bioactive Wollastonite-Diopside Foams from Preceramic Polymers and Reactive Oxide Fillers

Wollastonite (CaSiO3) and diopside (CaMgSi2O6) silicate ceramics have been widely investigated as highly bioactive materials, suitable for bone tissue engineering applications. In the present paper, highly porous glass-ceramic foams, with both wollastonite and diopside as crystal phases, were developed from the thermal treatment of silicone polymers filled with CaO and MgO precursors, in the form of micro-sized particles. The foaming was due to water release, at low temperature, in the polymeric matrix before ceramic conversion, mainly operated by hydrated sodium phosphate, used as a secondary filler. This additive proved to be “multifunctional”, since it additionally favored the phase development, by the formation of a liquid phase upon firing, in turn promoting the ionic interdiffusion. The liquid phase was promoted also by the incorporation of powders of a glass crystallizing itself in wollastonite and diopside, with significant improvements in both structural integrity and crushing strength. The biological characterization of polymer-derived wollastonite-diopside foams, to assess the bioactivity of the samples, was performed by means of a cell culture test. The MTT assay and LDH activity tests gave positive results in terms of cell viability.

Bioactive glass-ceramic scaffolds from novel 'inorganic gel casting' and sinter-crystallization

Highly porous wollastonite-diopside glass-ceramics have been successfully obtained by a new gel-casting technique. The gelation of an aqueous slurry of glass powders was not achieved according to the polymerization of an organic monomer, but as the result of alkali activation. The alkali activation of a Ca-Mg silicate glass (with a composition close to 50 mol % wollastonite—50 mol % diopside, with minor amounts of Na2O and P2O5) allowed for the obtainment of well-dispersed concentrated suspensions, undergoing progressive hardening by curing at low temperature (40 °C), owing to the formation of a C–S–H (calcium silicate hydrate) gel. An extensive direct foaming was achieved by vigorous mechanical stirring of partially gelified suspensions, comprising also a surfactant. The open-celled structure resulting from mechanical foaming could be ‘frozen’ by the subsequent sintering treatment, at 900–1000 °C, causing substantial crystallization. A total porosity exceeding 80%, comprising both well-interconnected macro-pores and micro-pores on cell walls, was accompanied by an excellent compressive strength, even above 5 MPa.

Sintered glass ceramic articles from plasma vitrified asbestos containing waste

Abstract Abstract Plasma vitrification is a safe technique for eliminating the environmental impact of asbestos containing materials. Additional advantages may arise from the obtainment of valuable ceramics from vitrified waste if low cost treatments, comparable to those applied to traditional ceramics, are feasible. In the present paper, the application of a fast heating rate (40°C min−1) to produce dense sinter crystallised materials from vitrified asbestos containing waste, having strength in excess of 100 MPa, is discussed. Sinter crystallisation, with fast heating processes, constituted also the basis of cellular glass ceramics and a new type of stoneware, with waste glass replacing conventional feldspar fluxes.

Waste derived glass ceramic composites prepared by low temperature sintering/sinter-crystallisation

Glass-ceramics based on iron rich wastes were produced by direct sintering and by following an innovative approach, combining direct sintering and sinter-crystallisation processes. According to the second method, a layered tile was manufactured by single firing at 900°C using a selected combination of wastes for both the porous body and the dense coating layer. The coating layer (‘glaze’) results from the sinter-crystallisation of a waste derived glass mixed with zircon and recycled borosilicate glass. The glaze sealed the porosity of the body and enhanced both mechanical properties and chemical stability. The results show a near to zero water absorption rate, despite a low geometric density (∼2 g cm− 3), accompanied by a Youngs modulus of ∼40 GPa and a bending strength of ∼30 MPa. The chemical stability of the glass-ceramics thus developed was assessed by the application of a toxicity control leaching procedure. Furthermore, cell culture tests were carried out to evaluate the potential cytotoxicity of the materials.

Microporous glass ceramics from combination of silicate, borate and phosphate wastes

Abstract Borate mineral wastes and phosphate ash resulting from the incineration of meat and bone meal represent two particularly abundant inorganic wastes. This paper is dedicated to the combination of such wastes, together with kaolin clay, focused on the development of highly porous ceramic bodies. Borate waste has a multiple effect, providing liquid phase at the sintering temperature (1050°C), gas release (from the decomposition of its calcite fraction) and a CaO source, which reacts with residues from clay and promotes the formation of anorthite crystals as a newly formed phase. Control of the heating rate, i.e. adoption of fast heating (20°C min−1) and, above all, introduction of recycled soda–lime–silica glass as secondary additive, allowed obtaining lightweight microporous bodies (density below 0·45 g cm−3) with uniform pore structure that could be useful for thermal and acoustic insulations.

Sintered silicophosphate glass ceramics from MBM ash and recycled soda-lime-silica glass

Abstract Abstract Meat and bone meal ash, mixed with recycled soda–lime–silica glass and small amounts of additives, was successfully valorised in the processing of sintered glass ceramics, after melting and forming two calcium phosphate glasses. Sintering was applied to fine powders (<37 μm) at temperatures of 700–1070°C for 0·5–2 h, after very rapid heating (40°C min−1). Mixtures with small additions of CaO and CaF2 led to fluorapatite–wollastonite glass ceramics, which retained a significant porosity even at 1070°C, due to the delay in viscous flow caused by rapid crystallisation. This feature was exploited for strong open celled macrocellular glass ceramics, obtained by sintering glass powders mixed with polyethylene sacrificial templates. Mixtures with small additions of CaO and Na2O led to dense and strong combeite glass ceramics (bending strength, >100 MPa), sinterable at particularly low temperatures (800°C). Both porous and dense glass ceramics could be exploited as low cost and high strength materials, or even as biomaterials, due to the biocompatibility of the crystal phases.

Biocompatibility and bioactivity of porous polymer-derived Ca-Mg silicate ceramics.

Magnesium is a trace element in the human body, known to have important effects on cell differentiation and the mineralisation of calcified tissues. This study aimed to synthesise highly porous Ca-Mg silicate foamed scaffolds from preceramic polymers, with analysis of their biological response. Akermanite (Ak) and wollastonite-diopside (WD) ceramic foams were obtained from the pyrolysis of a liquid silicone mixed with reactive fillers. The porous structure was obtained by controlled water release from selected fillers (magnesium hydroxide and borax) at 350°C. The homogeneous distribution of open pores, with interconnects of modal diameters of 160-180μm was obtained and maintained after firing at 1100°C. Foams, with porosity exceeding 80%, exhibited compressive strength values of 1-2MPa. In vitro studies were conducted by immersion in SBF for 21days, showing suitable dissolution rates, pH and ionic concentrations. Cytotoxicity analysis performed in accordance with ISO10993-5 and ISO10993-12 standards confirmed excellent biocompatibility of both Ak and WD foams. In addition, MC3T3-E1 cells cultured on the Mg-containing scaffolds demonstrated enhanced osteogenic differentiation and the expression of osteogenic markers including Collagen Type I, Osteopontin and Osteocalcin, in comparison to Mg-free counterparts. The results suggest that the addition of magnesium can further enhance the bioactivity and the potential for bone regeneration applications of Ca-silicate materials. STATEMENTS OF SIGNIFICANCE Here, we show that the incorporation of Mg in Ca-silicates plays a significant role in the enhancement of the osteogenic differentiation and matrix formation of MC3T3-E1 cells, cultured on polymer-derived highly porous scaffolds. Reduced degradation rates and improved mechanical properties are also observed, compared to Mg-free counterparts, suggesting the great potential of Ca-Mg silicates as bone tissue engineering materials. Excellent biocompatibility of the new materials, in accordance to the ISO10993-5 and ISO10993-12 standard guidelines, confirms the preceramic polymer route as an efficient synthesis methodology for bone scaffolds. The use of hydrated fillers as porogens is an additional novelty feature presented in the manuscript.