Roberta Licheri

Self-propagating combustion synthesis and plasma spraying deposition of TiC–Fe powders

Abstract The synthesis of titanium carbide/iron composite from elemental powders by means of self-propagating reactions to be subsequently employed for plasma spray deposition is investigated. The combustion temperature and velocity of propagating front are found to decrease as the amount of iron in the starting mixture increased. In addition, the maximum value of the iron content in the initial mixture allowable for guaranteeing the self-propagating character of the combustion synthesis process is identified. Below this threshold, i.e. 60 wt.%, independently of the iron content, the final products resulted constituted by titanium carbide and iron, being the latter one found as a binder distributed around the carbide grains. In particular, a strong dependence of the grains size of the obtained titanium carbide on the iron content is observed. Once reduced in powder form, the obtained composite, specifically TiC–30 wt.% Fe, is subsequently used for thermal spraying coatings deposition. Adhesion performance, hardness and wear resistance tests results for the obtained coatings by vacuum plasma spraying are also reported.

Optical properties of dense zirconium and tantalum diborides for solar thermal absorbers

Ultra-high temperature ceramics (UHTCs) are interesting materials for a large variety of applications under extreme conditions. This paper reports on the production and extensive characterization of highly dense, pure zirconium and tantalum diborides, with particular interest to their potential utilization in the thermal solar energy field. Monolithic bulk samples are produced by Spark Plasma Sintering starting from elemental reactants or using metal diboride powders previously synthesized by Self-propagating High-temperature Synthesis (SHS). Microstructural and optical properties of products obtained by the two processing methods have been comparatively evaluated. We found that pure diborides show a good spectral selectivity, which is an appealing characteristic for solar absorber applications. No, or very small, differences in the optical properties have been evidenced when the two investigated processes adopted for the fabrication of dense TaB2 and ZrB2, respectively, are compared.

Eu3+-doped Y2O3-SiO2 nanocomposite obtained by a sol-gel method

A Y 2 O 3 -SiO 2 nanocomposite doped with Eu 3+ was obtained by a sol-gel method and characterized by X-ray diffraction, IR, 29 Si NMR and laser-excited luminescence spectroscopy. It was found that small (2-3 nm) yttria nanoparticles are homogeneously dispersed in, and interacting with, the amorphous silica matrix. Luminescence spectroscopy indicates that the Eu 3+ ion is preferentially located inside or at the surface of highly disordered Y 2 O 3 nanoparticles. These luminescent nanocomposites form a class of materials which could find applications in the field of phosphors.

Spark plasma sintering of ZrB2- and HfB2-based Ultra High Temperature Ceramics prepared by SHS

The combination of the SHS technique and the Spark Plasma Sintering (SPS) technology was adopted in this work for the fabrication of fully dense MB2-SiC and MB2-MC-SiC (M = Zr, Hf) Ultra High Temperature Ceramics (UHTCs). Specifically, Zr or Hf, B4C, Si, and (for the cases of ternary systems) graphite powders were first reacted by SHS to successfully form the desired composites. The resulting powders were then subjected to consolidation by SPS. In particular, by setting a dwell temperature level of 1800°C, a mechanical pressure of 20 MPa, and a non-isothermal heating time of 10 min, products with relative densities ≥98.5% were obtained for the all systems investigated within 30 min of total processing time. The characteristics of the resulting dense UHTCs, i.e. hardness, fracture toughness, and oxidation resistance, are similar to, and in some cases superior than, those related to analogous products synthesized by alternative, less rapid, methods.

Processing, Mechanical and Optical Properties of Additive-Free ZrC Ceramics Prepared by Spark Plasma Sintering

In the present study, nearly fully dense monolithic ZrC samples are produced and broadly characterized from microstructural, mechanical and optical points of view. Specifically, 98% dense products are obtained by Spark Plasma Sintering (SPS) after 20 min dwell time at 1850 °C starting from powders preliminarily prepared by Self-propagating High-temperature Synthesis (SHS) followed by 20 min ball milling. A prolonged mechanical treatment up to 2 h of SHS powders does not lead to appreciable benefits. Vickers hardness of the resulting samples (17.5 ± 0.4 GPa) is reasonably good for monolithic ceramics, but the mechanical strength (about 250 MPa up to 1000 °C) could be further improved by suitable optimization of the starting powder characteristics. The very smoothly polished ZrC specimen subjected to optical measurements displays high absorption in the visible-near infrared region and low thermal emittance at longer wavelengths. Moreover, the sample exhibits goodspectral selectivity (2.1–2.4) in the 1000–1400 K temperature range. These preliminary results suggest that ZrC ceramics produced through the two-step SHS/SPS processing route can be considered as attractive reference materials for the development of innovative solar energy absorbers.

Influence of mechanical and electric current activation on the mechanism of formation and the properties of bulk B4C-TiB2 composites obtained by reactive sintering

Influence of Mechanical and Electric Current Activation on the Mechanism of Formation and the Properties of Bulk B4C-TiB2 Composites Obtained by Reactive Sintering Leila Nikzad, Roberto Orrù*, Roberta Licheri, Giacomo Cao Dipartimento di Ingegneria Meccanica, Chimica e dei Materiali, Unità di Ricerca del Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM) Università degli Studi di Cagliari, Piazza D’Armi, 09123 Cagliari, Italy Division of Nanotechnology and Advanced Materials, Materials and Energy Research Center, Tehran, Iran, P.O. Box 14155-4777 roberto.orru@dimcm.unica.it

Combustion synthesis of TIC-metal composites and related plasma spraying deposition

The synthesis of titanium carbide/metal (i.e. Fe, Ti) composites from elemental powders by means of combustion reactions is investigated. A combustion booster (i.e. Teflon) to guarantee the self-propagating character of the combustion reaction is taken into account in the case of the TiC/Ti system. The combustion temperature and velocity of propagating front are found to decrease as the amount of metal in the starting mixture increase while both parameters increase as the amount of booster added to the mixture is augmented. The final products result constituted by titanium carbide and metal, being the latter one found as binder distributed around the carbide grains. In particular, a strong dependence of the grains size of the obtained titanium carbide on the iron content is observed. Moreover, the amount of titanium in the final product decreases as the Teflon content is increased, thus demonstrating that the latter one directly participates to the carburisation process. Subsequently, the TiC/Fe composite, once reduced in powder form, is tested as promising candidate for thermal spraying coatings. Results obtained by vacuum plasma spraying coatings are also reported.

Self-propagating combustion synthesis of intermetallic matrix composites in the ISS

Combustion Synthesis experiments have been performed on the ISS (International Space Station) during the Belgian taxi-flight mission ODISSEA in November 2002, in the framework of the ESA-coordinated project COSMIC (Combustion Synthesis under Microgravity Conditions). The main objective of the experiments was to investigate the general physico-chemical mechanisms of combustion synthesis processes and the formation of products microstructure. Within the combustion zone, a number of gravity-dependent phenomena occur, while other phenomena are masked by gravity. Under certain conditions, gravity-dependent secondary processes may also occur in the heat-affected zone after combustion. To study the influence of gravity, a specially dedicated reactor ensemble was designed and used in the Microgravity Science Glovebox (MSG) onboard the ISS. In this work, the experiment design is first discussed in terms of the experimental functionality and reactor ensemble integration in the MSG. To investigate microstructure formation, a sample constituted by a cylindrical portion followed by a conical one, the latter being inserted inside a massive copper block, is used. The experiment focused on the synthesis of intermetallic matrix composites (IMCs) based on the Al-Ti-B system. Depending on the composition, different intermetallic compounds (TiAl and TiAl3) can be formed as matrix phase while TiB2 represents the reinforcing particulate phase. During the ISS mission, six samples with a relatively high green density of 65%TD have successfully been processed. The influence of the composition on the combustion process will be examined.

Coupling SHS and SPS processes

The consolidation of refractory ceramic powders at relatively milder conditions with respect to conventional methods represents an important target to achieve. Based on results recently reported in the literature, it is possible to state that the combination of the Self-propagating High-temperature Synthesis (SHS) with the Spark Plasma Sintering (SPS) technologies provides a useful contribution in this direction. Specifically, the two-steps processing route consisting in the synthesis of the ceramic powders by SHS and their subsequent densification by SPS is successfully utilized to obtain various dense MB2-based materials (M= Zr, Hf, Ta). In this regard, an important role is played by the SHS process, particularly for the synthesis of composite powders. Indeed, stronger interfaces are established among the different phases formed in-situ, so that diffusion phenomena are promoted during SPS. Additional benefits are produced by the use of the latter technology, due to the direct passage of the electric current through the powders undergoing sintering and the die containing them.

Processing and Characterization of Zr-, Hf- and Ta-Based Ultra High Temperature Ceramics

The fabrication of MB2-SiC and MB2-MC-SiC (M=Zr, Hf, Ta) Ultra High Temperature Ceramics (UHTCs) is investigated in this work by combining Self-propagating High-temperature Synthesis (SHS) and Spark Plasma Sintering (SPS). Zr, Hf or Ta, B4C, Si, and graphite powders are first reacted by SHS to successfully form in-situ the desired composites. For the case of the Tabased systems, a 20 min ball milling treatment is also required to mechanically activate the SHS reactions. The resulting powders are subsequently consolidated by SPS at 1800 °C and P=20 MPa, thus obtaining products with densities greater than 96% within 30 min of total processing time. Hardness, fracture toughness, and oxidation resistance of the resulting dense UHTCs are among the best when compared to the corresponding values reported in the literature relative to analogous products synthesized by alternative, more energy-consuming and less rapid methods. Thermogravimetric analysis results evidenced the beneficial effect of SiC on the oxidation resistance of the composite materials, while the presence of transition metal carbides appears to be inconvenient from this point of view. This is because, they rapidly oxidize at high temperature to form MxOy and carbon oxides which lead to an increase in sample porosity thus enhancing product oxidation.