Microstructure evolution upon annealing of a ZrB2–SiC composite containing lanthana and magnesia

The purpose of this work was to produce a dense ZrB₂–SiC ceramic and to identify a suitable annealing cycle to crystallize the glassy phase and promote SiC growth along the c axis. The concept behind this work exploits the irreversible SiC β → α transformation occurring at temperature above 1900 °C, in such a way to have SiC platelets able to trigger effective toughening mechanisms. La₂O₃ and MgO were used as sintering additives. The as sintered and annealed materials were examined through X-ray diffraction (XRD), to identify the crystalline phases, scanning electron microscope (SEM), to study the distribution of the secondary phases, and transmission electron microscope (TEM), to analyze the microstructure at nanoscale level, with particular attention to new crystalline phases and to the interfaces in high resolution mode. A model for the microstructure evolution during densification and upon annealing is presented.     

Nanoindentation-based study of the mechanical behavior of bulk supercrystalline ceramic-organic nanocomposites

Bulk poly-supercrystalline ceramic-organic nanocomposites were produced and characterized with a nanoindentation-based study. These nanocomposites were processed using two different routines, to compare their properties with and without crosslinking the organic ligands interfacing the ceramic nanoparticles. Together with the expected material strengthening induced by crosslinking, a distinct response emerges when using Berkovich and cube-corner indenters. The supercrystalline materials are prone to compaction, cracking and chipping phenomena that become more severe when a sharper tip is employed, implying that a Berkovich indenter is more suitable for the evaluation of elastic modulus and hardness. The cube-corner tip, on the other hand, is employed for the investigation of fracture toughness, comparing two methods and multiple models available from the literature. The fracture toughness outcomes suggest that cracks evolve with a quarter-penny shaped profile at the indent’s corners, and that extrinsic toughening mechanisms, such as plastic-like deformation and crack deflection, play a significant role.     

An agent-based multilayer architecture for bioinformatics grids

Due to the huge volume and complexity of biological data available today, a fundamental component of biomedical research is now in silico analysis. This includes modelling and simulation of biological systems and processes, as well as automated bioinformatics analysis of high-throughput data. The quest for bioinformatics resources (including databases, tools, and knowledge) becomes therefore of extreme importance. Bioinformatics itself is in rapid evolution and dedicated Grid cyberinfrastructures already offer easier access and sharing of resources. Furthermore, the concept of the Grid is progressively interleaving with those of Web Services, semantics, and software agents. Agent-based systems can play a key role in learning, planning, interaction, and coordination. Agents constitute also a natural paradigm to engineer simulations of complex systems like the molecular ones. We present here an agent-based, multilayer architecture for bioinformatics Grids. It is intended to support both the execution of complex in silico experiments and the simulation of biological systems. In the architecture a pivotal role is assigned to an "alive" semantic index of resources, which is also expected to facilitate users' awareness of the bioinformatics domain.     

Feasibility of in situ de-agglomeration during powder consolidation

Consolidation of nano-sized powders is a growing area in manufacturing of advanced materials, thanks to the reduced processing times, enhanced mechanical properties and high potential for the introduction of multi-functionality enabled by such reduced particle sizes. Nanopowders, however, are particularly prone to the agglomeration phenomena, and thus to the formation of hierarchical porous structures. The presence of pores differing up to several orders of magnitude in size leads to undesired differential shrinkage and localized grain growth. In order to avoid such issues, strategies for in situ de-agglomeration are proposed here. These optimization strategies are based on the development of an analytical model for shrinkage kinetics and mechanical properties of a hierarchical porous structure, containing both small-size intra-agglomerate pores and large-size inter-agglomerate ones. The modeling approach is an expansion of the continuum theory of sintering to the case of biporous materials presenting nonlinear viscous rheology, as expected for nano-sized crystalline powders. Considering the nonlinear viscous constitutive behavior of the solid phase also allows assessing the influence of the temperature on the microstructural evolution during processing, due to the dependence of the creep characteristic parameter, strain-rate sensitivity, on the thermal history. Material structure optimization strategies, aimed at de-agglomeration or at the design of tailored porous structures, become then possible and are here explored.     

Spark plasma sintering of ultra refractory compounds

Spark plasma sintering experiments were conducted on Zr- and Hf-based borides and carbides with the addition of 1, 3, and 9 vol% MoSi₂ as sintering aid. For comparison, as-received ZrC, HfC, ZrB₂, HfB₂ powders were also sintered. The microstructural features were investigated by means of scanning electron microscop–energy dispersive spectroscopy technique. Silicon carbide was detected in all the doped compositions along with significant amounts of oxide species (Hf/ZrO₂, and SiO₂). The effect of the MoSi₂ content on densification, microstructure, and mechanical properties is analyzed.     

Catalyst deactivation in on-board H2 production by fuel dehydrogenation

The production of H₂ for on-board application is a very interesting challenge for industrial and academic researchers. The aim is the application of on-board hydrogen production on the airplanes using kerosene as H₂ source. In this work an in depth study into the partial dehydrogenation (PDH) of two hydrocarbons blends and desulfurized JetA1 fuel has been performed by using 1 wt.%Pt–1 wt.%Sn/γ-Al₂O₃ and 1 wt.%Pt–1 wt.%Sn–0.5%K/γ-Al₂O₃ to find a way to produce H₂ “on-board” for the feeding of the fuel-cell apparatus. The mechanism of deactivation by coke was studied in depth combining Raman spectroscopy and Temperature-programmed oxidation (TPO) analyses. Microstructure analysis of metallic particles in fresh and deactivated catalysts was investigated by HRTEM. Relatively high H₂ partial pressure increases catalyst life by controlling full dehydrogenation coke-forming reaction. By feeding model organic molecules, it was possible to identify the contribution of each class of compounds to the H₂ production as well as the amount and type of coke formed. A relatively complex reaction pathway, which is able to evidence the role of different sites and reactions involved in PDH processes, was proposed.     

Oxidation behaviour of a pressureless sintered HfB2–MoSi2 composite

The thermal stability of a 80-vol.% HfB₂ + 20 vol.% MoSi₂ composite is tested under oxidizing environment. Oxidation tests are carried out in flowing synthetic air in a TG equipment from 1000 to 1400 °C with exposure time of 30 h. At temperatures ≥1200 °C the silica resulting from oxidation of molybdenum disilicide seals the sample surface, preventing hafnium diboride from fast degradation. Analysis of the kinetics is carried out through fitting of the thermogravimetric curves. Between 1200 and 1400 °C, the kinetic curves deviate from a parabolic behaviour, being more close to a logarithmic–parabolic behaviour.     

Densification and Mechanical Behavior of HfC and HfB2 Fabricated by Spark Plasma Sintering

Hafnium diboride (HfB₂)- and hafnium carbide (HfC)-based materials containing MoSi₂ as sintering aid in the volumetric range 1%–9% were densified by spark plasma sintering at temperatures between 1750° and 1950°C. Fully dense samples were obtained with an initial MoSi₂ content of 3 and 9 vol% at 1750°–1800°C. When the doping level was reduced, it was necessary to raise the sintering temperature in order to obtain samples with densities higher than 97%. Undoped powders had to be sintered at 2100°–2200°C. For doped materials, fine microstructures were obtained when the thermal treatment was lower than 1850°C. Silicon carbide formation was observed in both carbide- and boride-based materials. Nanoindentation hardness values were in the range of 25–28 GPa and were independent of the starting composition. The nanoindentation Young's modulus and the fracture toughness of the HfB₂-based materials were higher than those of the HfC-based materials. The flexural strength of the HfB₂-based material with 9 vol% of MoSi₂ was higher at 1500°C than at room temperature.     

Ice templating of ZrB2 porous architectures

For the first time, freeze casting of aqueous ZrB₂ based suspension was performed in order to produce porous architectures with main unidirectional anisotropic pores interconnected by diffuse globular-isotropic pores. The porosity characterizing the green bodies after the sublimation process is the replica of the ice crystals. The effects of solid content, suspension stabilization and cold transmission depending on mold type were studied since micro and macrostructure and, hence, the properties of the sintered body are determined by controlling the growth direction of the ice crystals during freezing. Improved mechanical performances are obtained by changing the above mentioned parameters, in virtue of the formation of thinner and more homogenously distributed ceramic lamellae. Possible applications of these materials include high temperature porous volumetric absorbers for concentrating solar power systems.     

Transmission electron microscopy on Hf- and Ta-carbides sintered with TaSi2

The microstructure of hot pressed Hf- and Ta-carbides with 15 vol% of TaSi₂ was characterized by X-ray diffraction, scanning and transmission electron microscopy in order to investigate the densification mechanisms.The microstructure of the carbides was constituted by squared grains and subgrains were recognizable only by transmission electron microscopy: the inner part was constituted by the original MC grain and the outer area by a (M,Ta)C solid solution which grew epitaxially on it. The compositional misfit and the difference of the coefficients of thermal expansion between the two regions were accommodated by 45° grain boundaries and dislocations. At the triple junctions, Ta₅Si₃ and Ta_4.8Si₃C_0.3, with Hf impurities were detected. The grain boundaries were observed to be clean.The microstructure of the composites containing TaSi₂ was subsequently compared to composites sintered with addition of the same amount of MoSi₂.