Cristina Leonelli

Syntheses of Fe2O3/Silica Red Inorganic Inclusion Pigments for Ceramic Applications

Abstract The work focuses on the syntheses of red inorganic ceramic pigments by inclusion of hematite in a fumed silica matrix. Leaching tests in boiling solutions of concentrated (36 wt%) hydrochloric acid demonstrated that, starting from mixtures of fumed silica and synthetic goethite, the occlusion of hematite occurs by calcination at 1150–1200°C for 2–4 h. Better results were obtained by using fumed silica having surface area ranging from 300 to 400 m 2 /g. A continuos change in colour was measured by comparing L-a-b values of the calcined samples and more information were obtained by the Kubelka-Munk absorption function. Because its chemical and thermal stability, the obtained heteromorphic pigment may be considered a suitable red pigment for ceramic manufacturing by fast firing cycles.

Enhancing the mechanical properties of porcelain stoneware tiles: a microstructural approach

Abstract This paper focuses on the complexities of the microstructure and phase development in porcelain stoneware tiles produced following industrial fast single firing cycles. A microstructural investigation was conducted to determine if the addition of selected low cost minerals would improve mechanical properties. The minerals tested were quartz, mullite and kyanite. Uniaxially pressed samples were submitted to the same industrial firing schedule and tested according to the European tile standards before further microstructural analysis. All the requirements specified in UNI EN normative concerning BIa class tiles were fulfilled; moreover, mullite and kyanite added formulations showed sensible increases in mechanical properties, especially as far as flexural strength and abrasion resistance are concerned.

Chemical stability of geopolymers containing municipal solid waste incinerator fly ash

Municipal solid waste incinerators every year produce tons of fly ashes which, differently from coal fly ashes, contain large amounts of toxic substances (heavy metals, dioxins, furans). The stabilization/solidification (S/S) technology known as geopolymerization is proposed with the purpose to bond physically and chemically incinerator fly ashes (IFA) in a solid matrix, in order to reduce pollutant mobility. The chemical stability of geopolymers with Si/Al ratio of 1.8-1.9 and Na/Al ratio of 1.0, synthesized by alkali activation of metakaolin and the addition of 20wt% of two different kinds of IFA, is presented. The concentration of the alkaline solution, water to solid ratio and curing process have been optimized. The room temperature consolidation of IFA containing geopolymers has been tested for leachability in water for 1day, accordingly to EN 12457 regulation and extended to 7days to increase the water attack on solid granules. Leachable metals in the test solution, determined by ICP_AES, fall within limit values set by regulation for non-dangerous waste landfill disposal. Geopolymeric matrix evolution with leaching time has been also evaluated in terms of pH and electrical conductivity increase in solution.

Synthesis and characterization of cerium-doped glasses and in vitro evaluation of bioactivity

Abstract The results of preparation, characterization and in vitro bioactivity evaluation of phosphosilicate glasses based on Bioglass® 45S5 (SiO2 45; Na2O 24.5; CaO 24.5; P2O5 6 wt%) doped during melting with (1.5–13.5 wt%) cerium dioxide (CeO2), has been reported. The choice of cerium was related to its low toxicity associated with bacteriostatic properties; cerium-doped bioactive glasses could be useful when implantation concerns local infected areas. The maximum value that permitted forming a homogeneous glass was 13.5 wt% and enabled us to get a better insight into CeO2 effect on the chemical behaviour of glasses. The as-quenched glasses were characterized by means of magnetic and spectroscopic measurements that revealed the prevailing presence of cerium (III). The bioactivity of the glasses was tested by soaking them in a simulated body fluid at 37 °C, under continuous stirring. ICP measurements were carried out for ion concentration determinations and the solution/glass interface was investigated by scanning electron microscopy equipped with energy dispersive analyzer (SEM/EDS technique) to check morphological modifications; the solids were investigated by means of X-ray powder diffraction and IR techniques. The results indicated that at low cerium content the glass degradation and repolymerization gave rise to an internal silicon-based layer and an external calcium-phosphate-based layer formed on the glass surface; high cerium content retards the glass degradation and gave rise to cerium–phosphate layer instead of calcium one. The cerium was never found in solution. For the reacted glasses, in the external layer, at 10% and 13.5% CeO2 contents the molar ratio Ca/P were ≈1 and 0.8 respectively and the molar ratio Ce/P was ≈0.3 in both cases. On the surface of the glasses with the highest cerium content it could distinguish new regular aggregates mainly formed by cerium and phosphate (Ce/P≈3.5, Ca/P≈0.6). In the cerium-free glass and with low cerium content (1.5 wt%) the calculated Ca/P ratio was 1.67, as calculated for pure hydroxyapatite. At low cerium content (BG-1.5 Ce) the glass behaviour was strictly similar to that of Bioglass® 45S5.

Crystallization of some anorthite-diopside glass precursors

Anorthite and diopside have been obtained from complete devitrification of glasses belonging to the quaternary system MgO-CaO-Al2O3-SiO2. Microstructure, the natural trend of the nucleation mechanism and kinetic studies on the crystallization phenomenon have been investigated by means of optical and electron microscopies, thermal and thermomechanical techniques and X-ray powder diffractometry. All the glasses investigated show a complete crystallization starting from a simple surface nucleation process. The activation energy for the crystallization process proved to be higher than that for viscous flow, leading to an important aspect modification in the sample during ceramization. Thermal stability and physical properties of both glass and glass-ceramic materials have been tested, suggesting the possible use of these materials in industrial application.

Effects of nucleating agents on diopside crystallization in new glass-ceramics for tile-glaze application

The effect of crystallization produced by addition of TiO2, ZrO2 and P2O5 oxides to glass-ceramic of the system CaO-MgO-SiO2 was studied using structural and thermal techniques. The devitrification process was independent of thermal treatment. X-ray diffraction studies performed on the glass-ceramic system indicated that diopside crystalline phase was more thermodynamically favourable than other phases. The effect of the nucleating agent depends on its nature: TiO2 decreased the activation energy while P2O5 and ZrO2 did not. SEM analysis of the doped glass-ceramics showed randomly distributed crystals with significant dimensional variations from those of the undoped system. All these formulations, showing a high crystallization rate, and a fast heating rate, can be used as tile glazes and/or tile-glaze components.

Effect of TiO2 addition on the properties of complex aluminosilicate glasses and glass-ceramics

The nucleating effect of titania during glass crystallization has been studied in a complex glassy system where some particular oxides, such as ZnO and MgO, which present chemical and thermodynamic affinity for titanium have been added. Such additions tend to produce phase separation in the glass and leads to titanate phases formation in the glass-ceramic. Moreover, the presence of lithia has a promoting effect on both mechanisms because the lowered viscosity increases cation diffusion. Various thermal, microscopic, and diffractometric techniques have been used to investigate the amorphous and the crystalline phase.

NANOSIZED CeO2 POWDERS OBTAINED BY FLUX METHOD

Cerium(IV) oxide (CeO2) powders were prepared by the flux method, adding cerium ammonium nitrate ((NH4)2Ce(NO3)6) to an eutectic mixture of molten salts, followed by washing and drying. To evaluate the effect of the molten salts on the powders, three different fluxes were used: KOH/NaOH, NaNO 3/ KNO3, and LiCl/KCl eutectic mixtures. The temperature and the stoichiometry of each reaction were determined by thermogravimetric analysis. Specific surface area analysis (BET), transmission electron microscopy (TEM), and X-ray techniques were used to study the morphology and particle size distribution of the solid products. Results showed the presence, in the reaction products, of homogeneously sized and shaped particles of a single nanosized CeO2 phase. The powder obtained with chlorides out of molten salts had the finer particle size distribution, with a very narrow dimensional range of 20 ‐10 nm.

Solubility, reactivity and nucleation effect of Cr2O3 in the CaO-MgO-Al2O3-SiO2 glassy system

The effect of Cr2O3 on some anorthite-diopside glass-ceramics has been investigated up to amounts of 5 mol%. The solubility in the glassy compositions analysed is total for the oxide, but for amounts higher than 0.5 mol%, an insoluble spinel form, MgCr2O4, precipitates. Ultraviolet-visible spectroscopy has proved to be the most sensitive technique to the presence of Cr(III) in a crystalline spinel site, followed by X-ray diffraction and scanning electron microscopy observations. Electron spin resonance and X-ray photoelectron spectroscopy techniques excluded any oxidation state, other than Cr3+. The influence of the transition cation on glass nucleation is that of an increasing bulk effect with chromium, and thus chromium-spinel, content. The magnesium content affects spinel formation, while heat treatments up to 1100 °C do not. The spinel formation influences the anorthite-diopside ratio in the glass-ceramic, with a large favour towards the pyroxene.

Numerical models for thermal residual stresses in Al2O3 platelets/borosilicate glass matrix composites

Abstract In this paper, we model and numerically study thermal residual stresses in a borosilicate glass reinforced with Al 2 O 3 platelets. This composite material was experimentally characterized in a previous work. The aim of this paper is to investigate further the thermal residual stresses in these composites that are responsible for toughening and to assess the variation of the thermal residual stresses with platelets content. A numerical model based on finite element simulations of the given microstructures is used. Computational tests are compared both with previously obtained experimental data and with simplified theoretical equations. The model provides consistent results which are in agreement with experimental measurements. Moreover, the numerical approach presented here is able to take into account the effect of particle shape on residual thermal stresses, which could not be computed easily by theoretical equations.