Tiziano Manfredini

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.

The role of sodium hexametaphosphate in the dissolution process of kaolinite and kaolin

Suspensions of kaolinite and ceramic grade kaolin in sodium hexametaphosphate solutions at different concentrations were prepared and kept at constant temperature for 4 h: the solutions obtained after centrifugation and filtration were analysed using a ICP technique to determine the concentration of significant cations. The results allowed to stress the role of NaHMP in the dissolution process of kaolinite and ceramic grade kaolin. The role of accessory minerals in ceramic grade kaolin dissolution was discussed in the light of the obtained data.

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.

Utilisation of municipal incinerator grate slag for manufacturing porcelainized stoneware tiles manufacturing

Porcelainized stoneware tiles containing up to 20 wt.% of municipal incinerator grate slag have been fabricated by cold uniaxial pressing and conventional fast firing cycles. The sinterability of these samples was investigated from density and shrinkage measurements together with mechanical (Vickers microhardness and Youngs modulus) and esthetical (spot resistance and colour parameters analysis) properties. The comparison with the porcelainized stoneware tiles containing no waste suggest a good compatibility between the ceramic body and the waste that does not significantly change the properties of the final products and the conditions of the firing cycle. Nevertheless, the porosity increase, proportional to the waste content, causes a decrease in density, shrinkage, spot resistance and whiteness.

Synthesis and characterization of praseodymium-doped ceria powders by a microwave-assisted hydrothermal (MH) route

Nanocrystalline Pr-doped ceria powders were prepared for the first time by a microwave-assisted hydrothermal route. The effect of the microwave treatment in relation to the conventional hydrothermal technique was evaluated. The samples prepared were characterized in terms of composition, crystalline structure, particle shape and size distribution by X-ray diffraction, transmission electron microscopy (TEM) and specific surface area analysis (BET). The color properties of these solid solutions were also evaluated as a function of synthesis conditions and composition. Finally the oxidation state of the Pr cations and the relative quantities of Pr(IV) and Pr(III) were investigated by X-ray absorption at the M4,5 Pr absorption edge.

Comparison between plasma- and HVOF-sprayed ceramic coatings. Part I: microstructure and mechanical properties

Few papers deal with High Velocity Oxygen-Fuel (HVOF) sprayed ceramics. This two-part study thoroughly compares HVOF sprayed Al2O3, nanostructured Al2O3, Cr2O3 to Atmospheric Plasma Sprayed (APS) ones. The first part discusses microstructure and micromechanics. HVOF-sprayed ceramics achieve superior cohesion (lower porosity and lower average pore area). Size effects in Vickers microindentation tests are different for HVOF and APS ceramics. At 1 N load, hardness is higher for HVOF coatings (no cracking). Under 5 N and 10 N loads, hardness decreases gradually for HVOF coatings; APS coatings are largely cracked at 5 N. HVOF-sprayed ceramics are tougher than APS ones and have higher elastic modulus.