Cristina Siligardi

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.

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.

Densification of glass powders belonging to the CaO–ZrO2–SiO2 system by microwave heating

Abstract Densification and microstructural changes of two glassy compositions belonging to the wollastonite and zirconia stability fields in the ternary CaO–ZrO 2 –SiO 2 system were studied in a 2.45 GHz multimode microwave cavity. The effect of microwaves is to lower the sintering and devitrification temperature with stronger influence for high zirconia content composition. Correlation was found between dielectric properties and heating rate, showing lower interaction temperature for high zirconia content composition which starts to absorb microwave energy at about 400°C compared to 800°C for the low-zirconia one. Sintering and crystallization processes evolved in complex ways during heat treatment so that the two final glass-ceramic materials exhibit different microstructures, crystalline phases and mechanical properties.

Effect of aging processes on solar reflectivity of clay roof tiles

Clay roof tiles are widely used as roofing materials because of their good mechanical and esthetical properties. The exposure to atmospheric agents and, most of all, to pollutants and smog affects negatively the solar reflectance of a tile surface. The aim of this study is to analyze the influence of aging on the solar reflectance of clay roof tiles. We studied samples provided by manufacturer in Greece and USA. Samples were coated with either organic or inorganic coatings. Natural aging processes were used for samples with inorganic coating, and artificial aging simulation was performed on all samples. Samples were naturally aged in a test farm in Arizona, with an exposure time of three years. In artificial aging processes, the surface of the tiles was subjected to the application of two different mixtures simulating exposure to (i) Arizona weathering agents such as clay, salts and soot and (ii) Arizona, Florida and Ohio weathering agents through an average mixture made by clay, salts, particulate organic matter and soot. The amount of soiling mixture deposited on the surface of the samples was aimed at reproducing a three-year exposure. Soiled samples were subjected to air blowing and rinsing under running water to simulate the wind and rain effects, respectively. The effects of both natural aging and artificial soiling on the surface reflectivity of the clay roof tiles were assessed in the ultraviolet–visible–near infrared range (range from 300 to 2500 nm). The two different soiling conditions were found to affect significantly the solar reflectance of the samples, in particular the samples soiled with the average mixture present a decrease up to 0.20, while Arizona weathering condition affects the solar reflectance up to 0.05, and neither air blowing nor rinsing seem to permit a significant recovery of the surface properties. All solar reflectance measurements were computed by averaging the spectral reflectivity weighted by the air-mass 1 global horizontal solar spectral irradiance.

The structure of ZrO2 phases and devitrification processes in a Ca–Zr–Si–O-based glass ceramic: a combined a-XRD and XAS study

The structure of the Zr atomic environment in a CaO-ZrO 2 -SiO 2 glass ceramic as a function of thermal treatments has been studied, combining X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD) and anomalous XRD (a-XRD) techniques. The analysis of XRD patterns demonstrates that the devitrification process proceeds through the partial segregation of Zr-depleted phases (wollastonite-like) and Zr-rich phases (Zr oxides). The XAS and a-XRD measurements at the Zr K-edge have been exploited in order to obtain a closer insight into the atomic structure around the Zr atoms. In the as-quenched glass the Zr atom is sixfold coordinated to O atoms in an amorphous environment rich in Ca and Si. Thermal treatment firstly (T = 1273-1323 K) causes partial segregation of Zr in the form of an oxide with a tetragonal zirconia (t-ZrO 2 ) crystalline structure. Raising the temperature (T = 1373 K) causes the formation of ZrO 2 crystallites in the monoclinic crystallographic phase (baddeleyite, m-ZrO 2 ). Analysis of the XAS data shows that a considerable amount of Zr remains in an amorphous calcium silicate phase.

Structure, magnetic and positron lifetime studies on CaO–ZrO2–SiO2 glass system doped with vanadium oxide

The structure of CaO–ZrO2–SiO2 glasses doped with V2O5 (0.1–2 mole%) was studied using density and molar volume measurements. It was observed that the molar volume increased while the density decreased with increasing V2O5 content. Magnetic susceptibility measurements showed the diamagnetic behaviour of this glass system. The free volume was probed using ortho-positronium pick-off annihilation lifetime parameters. The mean free volume and fractional free volume have also been calculated from the positron annihilation lifetime data. The overall measured parameters from the structural and/or positron annihilation spectra indicated that the majority of the vanadium ions in the range 0–0.5 mole% of V2O5 are in the V5+ valence state which act as a glass former. On the other hand, the majority of the vanadium ions in the range 0.7–2 mole% of V2O5 are the V3+ and V4+ states acting as a glass modifier.

Characterisation of the surface conductivity of glassy materials by means of impedance spectroscopy measurements

Abstract Abstract The phenomenon of tin dioxide doping by antimony, Sb(V), atoms in semiconductive glazes is discussed. In particular, the impedance spectra of the glaze surface at room temperature were experimentally determined, and the results were analysed in terms of equivalent circuits. The parameters resulting from the fitting procedure were related to the SnO 2 Sb(V) relative content, the SnO 2 Sb (V) percentage in the glaze, and the microstructure as observed by SEM. It is proposed that the addition of SnO 2 and Sb 2 O 3 to a common glaze for ceramic tile results in a semiconductive continuous phase, whose electrical characteristics fulfil the antistatic floor regulation, as far as surface conductivity is concerned. As a whole the obtained results suggest that the thermal cycle used in fast firing technology is capable to promote the oxidation of Sb 2 O 3 to Sb 2 O 4 , resulting in a sufficient amount of Sb(V) capable of generating a semiconductive behaviour of the SnO 2 crystalline phase dispersed in the glaze.

Comparison of precipitated calcium carbonate/polylactic acid and halloysite/polylactic acid nanocomposites

PLA nanocomposites with stearate coated precipitated calcium carbonate (PCC) and halloysite natural nanotubes (HNT) were prepared by melt extrusion. The crystallization behavior, mechanical properties, thermal dynamical mechanical analysis (DMTA), and the morphology of the PCC/PLA, HNT/PLA, and HNT/PCC/PLA composites were discussed. Compared to halloysite nanotubes, PCC nanoparticles showed a better nucleating effect, which decreased both the glass transition and cold crystallization temperatures. The tensile performance of PLA composites showed that the addition of inorganic nanofillers increased Youngs modulus but decreased tensile strength. More interestingly, PLA composites with PCC particles exhibited an effectively increased elongation at break with respect to pure PLA, while HNT/PLA showed a decreased ultimate deformation of composites. DMTA results indicated that PLA composites had a similar storage modulus at temperatures below the glass transition and the addition of nanofillers into PLA caused Tg to shift to lower temperatures by about 3°C. The morphological analysis of fractures surface of PLA nanocomposites showed good dispersion of nanofillers, formation of microvoids, and larger plastic deformation of the PLA matrix when the PCC particles were added, while a strong aggregation was noticed in composites with HNT nanofillers, which has been attributed to a nonoptimal surface coating.

Differential anomalous wide-angle X-ray scattering and X-ray absorption experiments to investigate the formation of glass ceramics in the CaO–SiO2–ZrO2 system

The structure of a CaO–SiO2–ZrO2-based glass ceramic has been investigated by X-ray diffraction, X-ray absorption spectroscopy and differential anomalous scattering techniques as a function of the thermal treatment of the sample. The microstructure of the glass has been investigated at room temperature, before the recrystallization of the glass ceramic, and on two samples annealed at 1073 and 1273 K for 1 h to follow the early stages of nucleation of the quartz and wollastonite crystalline phases. Indications on the roles of Ca, Si and Zr during the devitrification process are given.