Reticulated porous silicon nitride-based ceramics

The interest towards the production of porous silicon nitride originates from the unique combination of light weight, of mechanical and physical properties typical of this class of ceramics that make them attractive for many engineering applications. Although pores are generally believed to deteriorate the mechanical properties of ceramics (the strength of porous ceramics decreases exponentially with an increase of porosity), the recent literature reports that porous silicon nitride can exhibit outstanding mechanical properties even at certain porosity levels, such as heat resistance and chemical resistance, damage tolerance, the thermal shock resistance and the strain tolerance. A study on silicon nitride characterized by high porosity content (more than 70%) has been carried out, focusing on the comparison of different processing procedures to produce silicon nitride with tailored novel porous structures and a high porosity content. Three technologies were compared to produce reticulated porous silicon nitride: 1) impregnation of templating structures (sponges) with ceramic powder slurries. Production of highly porous ceramic bodies using pyrolizable porous templates is quite a common process for almost any kind of ceramic material. Open porosities over 50% vol. can be easily achieved by using solid foamed sponges, both in polymeric materials and cellulose. 2) using a consolidator/binder and a pore former, 3) from foamed suspensions stabilized through organic additives. Some mechanical properties are reported and discussed in terms of microstructure and fabrication technique

TaB2-based ceramics: microstructure, mechanical properties and oxidation resistance

Among ultra-high temperature ceramics, major attention has been devoted to zirconium and hafnium borides and carbides. Tantalum composites remain a less explored class of ceramics. In this contribution, TaB2-based ceramics were hot pressed with addition of 5-10 vol% MoSi2. Temperatures in the range of 1680-1780?C led to relative density around 90-95%. The microstructure was studied through X-ray diffraction, scanning and transmission electron microscopy and the results enabled the rebuilding of the densification mechanisms occurring upon sintering. The hardness was about 18 GPa, the fracture toughness 4.6 MPam1/2 and the room temperature flexural strength was around 630 MPa, but abruptly decreased to 220 MPa at 1200?C. The composite containing 10 vol% of MoSi2 was tested in a bottom-up furnace in the temperature range 1200-1700?C for 30 minutes. The microstructure appeared covered by a SiO2 layer, but the bulk remained unaltered up to 1600?C. At 1700?C the specimen vaporized. Nanoindentation was employed on the oxidized cross section to detect eventual mechanical properties modification associated to chemical/microstructural chang

Toughened ZrB2-based ceramics with addition of SiC whisker or short fiber

In order to improve the fracture toughness, SiC whiskers or SiC chopped fibers were added to a ZrB2 matrix in volumetric fraction of 10 and 20 vol.%. The composites were hot-pressed between 1650 and 1730?C and their final relative densities were higher than 95%. Even at the lowest sintering temperature, the whiskers showed an evident degradation. On the other hand, the fibers maintained their initial shape and a strong interface formed between matrix and reinforcement. The fracture toughness of the composites increased from 30 to 50% compared to the baseline material, with the fibers showing a slightly higher toughening effect. In the whiskers-reinforced composites, the room-temperature strength increased when 10 vol.% whiskers were added. In the fibers-reinforced composites, the room-temperature strength decreased regardless the amount of fibers added. The high-temperature strength of the composites was higher than that of the baseline material for both types of reinforcemen

Ultrahigh temperature ceramics for aerospace and solar energy applications

Borides and carbides of early transition metals are considered a class of promising materials for several applications, the most appealing ones being in the aerospace and energy sectors. Beside the well known characteristics that make UHTCs attractive as TPS, there is a strong interest in their applications as sunlight absorbers for solar concentrating systems that can operate in the high temperature regime. The first part of this work is focused on toughening of UHTCs, which is a crucial issue that needs to be addressed for application in the aerospace sector. Different strategies have been developed to increase the fracture toughness, through either incorporation of elongated reinforcement (SiC chopped fibers, SiC whiskers) or in-situ development of SiC platelet-reinforced materials. The mechanical properties are compared to those of the un-reinforced materials and the effect of different kinds of sintering aids amongst Si3N4, MoSi2 and ZrSi2 is studied. Toughening mechanisms such as crack deflection, bridging, bowing and residual stresses are explored through analysis of crack propagation. Experimental increases of toughness are compared to those predicted by theoretical models. Addition of fibers or whiskers allows toughness to be increased from 3-4 MPa m1/2 (for unreinforced materials) to 5.0-6.3 MPa m1/2. On the other hand, quite often, the improvement of fracture toughness is accompanied by a decrease of strength, due to a change of the defects population. The second part of this work is dedicated to the characterization of various carbides and borides, in terms of room temperature and high temperature mechanical, thermal and optical properties. The discussion on properties relevant to solar energy applications will help to select the most promising matrices and/or composites for concentrating solar power (CSP) technologie

Processing and properties of ultra-refractory composites based on Zr- and Hf-borides: state of the art and perspectives

High performance Ultra-High-Temperature Composites (based on zirconium and hafnium borides) are characterized by relevant and unique thermo-physical and thermo-mechanical properties, suitable for applications in Thermal Protection - Durable TPS and hot structure for reusable vehicles and in Hypersonics - Leading edges, cooled scramjet panels, struts, cowls, and nozzles. In spite of the difficult sinterability Zr- and Hf- diborides, recent results highlighted that these ceramics can be produced with full density, fine microstructure and controlled mechanical and thermal properties, through different procedures: pressureless sintering and hot pressing combined with the use of proper sintering aids, reactive synthesis/sintering procedures starting from precursors, field assisted technologies like spark plasma sintering (SPS). The selection of reinforcing phase (SiC, B4C, TaSi2, MoSi2, etc) is suitable to improve mechanical properties and oxidation resistance of ceramic composites based on ZrB2 and HfB2. Strength as high as ~ 800MPa at room temperature and up to 600 MPa at 1500?C in air were obtained, consequently to tailored compositions and processing control. SPS proved to be a very rapid fabrication process leading to refined microstructure and high properties of ultra-refractory diborides -based composite

Production and characterization of toughened UHTC

Ultra-high temperature ceramics are material candidate for application in aggressive environment, especially in sharp components of future generation of space vehicles, driven by the ultimate goal of major speed with new propulsion and hypersonic models. This class of materials is arising always more interest for wing leading edges and nose tips, as well as propulsion system elements. The most investigated system concerns the ZrB2-SiC ones, owing to a high strength, up to 1 GPa, high hardness, around 20 GPa, and oxidation resistance. The major weak point remains the low fracture toughness, 3 to 5 MPam1/2. It has recently been demonstrated that the introduction of elongated secondary phases and the choice of the proper sintering additive, can lead to almost twofold increase of the fracture toughness. This work presents the last development of toughened ZrB2 ceramics with addition of SiC whiskers or chopped fibers, or through the in-situ elongation of SiC particles. The effect of various sintering additives, MoSi2, Si3N4, ZrSi2 and WSi2, is investigated in relationship to the microstructure evolution upon sintering, to the interface between matrix and reinforcing element and to the high temperature behaviour. Flexural strength, with the 4-point method, and toughness, measured by the CNB technique, are compared to those of reference unreinforced materials. The addition of whiskers to ZrB2 -based materials allows both strengthening and toughening compared to the reference material, whilst the addition of fibers only leads to a toughness increase, but it is accompanied by a decrease of strength, due to a change of the defects population. The oxidation tests conducted in a bottom-up loading furnace at 1200, 1500 and 1700?C show that the reinforced composites behave similarly to the baseline ZrB2-SiC well known materia

Spark plasma sintering of TixTa1-xC0.5N0.5-based cermets: Effects of processing conditions on chemistry, microstructure and mechanical properties

Nanometric powdered TixTa1-xC0.5N0.5-based cermets were fabricated using a mechanically induced self-sustaining reaction and consolidated by spark plasma sintering. Highly dense cermets were obtained, and their chemistry, microstructure and mechanical properties were characterised by X-ray diffraction, scanning electron microscopy, image analysis, microindentation and nanoindentation. The microhardness was found to depend directly on the contiguity and size of the ceramic hard particles. The samples synthesised at the lowest temperature (1150°C) exhibited more homogeneous microstructures and smaller ceramic particles and the best combination of microhardness and fracture toughness.Gobierno de España MAT2011-2298

Knowledge of public health informatics among Italian medical residents: design and preliminary validation of a questionnaire

Background: public health requires strong information skills and competencies, as it is information-intensive and information-driven. Public health informatics has been defined as the “systematic application of information, computer science, and technology to public health practice, research, and learning”. New information and communication technologies offer unprecedented opportunities, such as linking smart-phones and mobiles devices to web based tools for data collection, enabling and enhancing participatory epidemiology. However, being an emerging discipline, despite its potential and importance, public health informatics is often neglected and overlooked, being rarely offered as course. The present study was designed as a pilot study, with the aim of designing and validating a questionnaire on the knowledge of public health informatics among medical residents in public health in Italy.  Methods and Results: thirty-two Italian residents in public health volunteered to take part into the study. Mean age of the sample was 31.44±2.23 years, most responders were males (68.8%), from northern Italy (53.1%), at the third year of residency (34.4%) and currently doing practical training at the clinical management staff/hospital directorate (34.4%). Other places of training were the Prevention Department (21.9%), the Institute of Hygiene (18.8%), the local health units and the territory (12.5%), the occupational health service (6.3%) and the Regional Health Agency (3.1%). Cronbach’s alpha coefficient yielded a value of 0.909, demonstrating excellent psychometric properties of the instrument.  Conclusion: in conclusion, the developed questionnaire seems to be an appropriate and useful tool to detect gaps concerning knowledge, education and practices of public health informatics among residents in public health.&nbsp