Beatrice Cioni

The Role of Interfacial Interactions in the Toughening of Precipitated Calcium Carbonate–Polypropylene Nanocomposites

This paper investigates the effect of the interphase properties and the interfacial interactions between matrix and filler on mechanical properties of precipitated calcium carbonate (PCC)–polypropylene nanocomposites. PCC particles were coated with stearic acid (SA). The weight ratio of SA on the particles (w SA) ranged from 0 to 0.135 g SA/g PCC. The introduction of PCC particles resulted in an increase in stiffness and yield stress compared with the pristine polymeric matrix and, at the same time, it increased the impact resistance. The maximum improvement in the impact behaviour was achieved for the composites with w SA =0.045 corresponding to the theoretical monolayer ratio. A decrease in interfacial interactions between monolayer coated PCCs and the matrix with respect to the uncoated particles was observed by using a semi-empirical equation developed by Pukànszky. The low degree of interfacial interactions between particulate filler and matrix allows a matrix–particle debonding phenomenon, as shown by scanning electron microscopy analysis. Extensive plastic deformations were evident as well, promoting an improvement in toughness. The thickness of the interphase between particles and matrix was evaluated by using the Shen–Li model which is based on the hypothesis of a non-homogeneous interphase. It results that the thickness increased in the order uncoated < monolayer coated < 3% SA coated ≅ 13.5% SA coated particles. The thinner and stronger interphase found for the composite with uncoated particles can be explained with the high interaction between matrix and filler and the consequent low mobility of the polymeric chains.

Modeling and development of a microwave heated pilot plant for the production of SiC-based ceramic matrix composites

This paper outlines the development of a microwave heated apparatus for the production of silicon carbide (SiC) based ceramic matrix composites via chemical vapor infiltration. An innovative pilot scale reactor was designed and built. A coupled thermal and electromagnetic model was developed in order to predict the temperature profile inside the reactor. The results obtained from the model demonstrated that the electric field inside the sample was constant. This fact is particularly important in order to prevent the thermal instabilities (run-aways) that are typical in the case of microwave heating. Therefore the heating was uniform with the aid of a mode stirrer that achieved a better distribution of the microwave power and then improved the process efficiency. The infiltration cycles were carried out on SiC fiber preforms and resulted in an excellent average weight increase with respect to the initial sample. By using microwave heating, the treatment times were considerably reduced with respect to the conventional process times reported in the literature. The microstructure of the SiC composites were observed by scanning the electron microscopy in order to evaluate the quality and the degree of densification which was achieved within the fiber tows. The SiC deposition inside of the sample was sufficiently homogeneous and compact, even if a certain degree of inter-tow porosity was still evident.

Synthesis of Bioactive Hydroxyapatite-Zirconia Toughened Composites for Bone Replacement

Hydroxyapatite (HAp) is a major inorganic component of human hard tissues, such as bones and teeth, and its content determines their microstructures and physical properties. Artificial HAp shows strong biocompatibility and bioactivity and thus it has found broad applications in tissue engineering for replacing damaged hard tissues. The artificial HAp, however, suffers from its intrinsic low mechanical properties, so to meet mechanical requirements, HAp can be incorporated with stiff mineral phases (mullite, zirconia, alumina). The performance and long-term survival of these biomedical devices are also dependent on the presence of bacteria surrounding the implants. In order to reduce the incidence of implant-associated infections, several treatments have been proposed, e.g. introduction of silver or fluoride in the HAp. The objective of this research is the sintering of composites based on calcium phosphate, mainly HAp supported on zirconia, for bone replacement with better microstructural features. In fact the use of zirconia can enhance the mechanical properties of bioceramics. Moreover the introduction of small amounts of silver, which should improve the antibacterial properties, will be taken into consideration since it is expected also to further toughen the whole structure.

Zirconium tin titanate (ZST) for miniaturized high impedance surfaces: Microwave dielectric properties and applications

A new approach for obtaining metasurfaces realized by employing Frequency Selective Surfaces (FSSs) with small periodicity printed on a new class of high dielectric constant materials is presented. The synergic use of ad-hoc engineered materials and innovative electromagnetic design provides an improvement of the overall device in terms of size reduction and radiative properties. Zr0.8Sn0.2TiO4 (ZST) exhibits a high dielectric constant, low dielectric losses, good thermal stability and good high frequency characteristics. Consequently, it is one of the most promising materials for these specific purposes. In particular, in this work the microstructure and the microwave dielectric properties of Zr0.8 Sn0.2TiO4 (ZST) perovskite with NiO addition, prepared with conventional solid-state route, are investigated. The addition of NiO (0.2 wt.%) as doping can efficiently promote the densification and reduce dielectric losses of ZST ceramics. Estimates of the physical and electric characteristics, with particular attention to losses, will be presented through measurements within different range of frequencies. Up to now, ZST materials have been successfully applied in memory devices, filters in cellular base stations, miniaturization of microwave devices, duplexers and general wireless applications [1].