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ISTEC R&D activities span from the fundamental understanding of the process-microstructure-property correlations, to the materials functionalization through suitable procedures, up to the validation of demonstrators in relevant environment.

The thermionic-thermoelectric solid-state technology, characterized by solar-to-electric conversion efficiency feasibly > 40%, is comprehensively proposed and discussed for conversion of concentrating solar power. For the first time, the related solar generator prototype is designed and fabricated by developing advanced materials functionalized for the specific application, such as thermally resistant hafnium carbide-based radiation absorbers, surface-textured at the nanoscale to obtain a solar absorptance > 90%, and chemical vapor deposition diamond films, acting as low- work-function (2.06 eV) thermionic emitters. Commercial thermoelectric generators and encapsulation vacuum components complete the prototype. The conversion efficiency is here evaluated under outdoor concentrated sunlight, demonstrating thermionic stage output power of 130 mW at 756 °C, combined to the maximum thermoelectric output power of 290 mW. The related solar-to-electric conversion efficiency is found to be 0.4%, but, once the net thermal flux fed to the conversion stages is considered, a thermal-to-electric efficiency of 6% is revealed. Factors affecting the performance of the present prototype are analyzed and discussed, as well as a strategy to rapidly overcome limitations, in order to prepare an efficient and highly competitive solid-state conversion alternative for future concentrating solar plants

Ceramics based on group IV-V transition metal borides and carbides possess melting points above 3000 degrees C, are ablation resistant and are, therefore, candidates for the design of components of next generation space vehicles, rocket nozzle inserts, and nose cones or leading edges for hypersonic aerospace vehicles. As such, they will have to bear high thermo-mechanical loads, which makes strength at high temperature of great importance. While testing of these materials above 2000 degrees C is necessary to prove their capabilities at anticipated operating temperatures, literature reports are quite limited. Reported strength values for zirconium diboride (ZrB2) ceramics can exceed 1 GPa at room temperature, but these values rapidly decrease, with all previously reported strengths being less than 340 MPa at 1500 degrees C or above. Here, we show how the strength of ZrB2 ceramics can be increased to more than 800 MPa at temperatures in the range of 1500-2100 degrees C. These exceptional strengths are due to a core-shell microstructure, which leads to in-situ toughening and sub-grain refinement at elevated temperatures. Our findings promise to open a new avenue to designing materials that are super-strong at ultra-high temperatures.

Accepted manuscript

Hypersonic vehicles need sharp leading edges to enable vehicle manoeuvrability during atmospheric exit and re-entry. The edges are expected being hit with corrosive plasmas from atmosphere at ablative speeds with temperatures inexcess of 2000°C. Such extreme conditions limit the field of material candidates to ultra-high temperature ceramics (UHTCs). These are typically non-oxides with melting/decomposition temperatures in excess of 3000°C: borides of the group IV transition metals like ZrB2 and HfB2 are currently the most studied systems. Bulk single-phase UHTCs for these high-temperature structural applications are limited by their poor oxidation resistance, as well as lacking damage tolerance. Recent development of composite UHTC systems has focused on additions of SiC in form of fiber in order to improve not only resistance to oxidation but also resistance to thermal shock. In the present contribution, leading edges with a sharp profile were produced in the system MB2-SiC fibers, M= Zr and Hf. The dynamic response to oxidation was studied under aero-thermal heating using a supersonic flow (enthalpy up to xx MJ/kg aggiungi quanto serve) using a arc-jet plasma gallery. Temperature and surface emissivity were on-line monitored. Microstructural modifications upon oxidation were analyzed and correlated to test conditions through Computational Fluid Dynamics simulations.

The IV and V group transition metals borides, carbides and nitrides are widely known as ultra-high temperature ceramics (UHTCs), thanks to their high melting point above 2500°C. These ceramics possess outstanding physical and engineeristic properties, such as high hardness and strength, low electrical resistivity and good chemical inertness. The materials more deeply investigated are the ZrB2-based ones, in view of the lower density, compared to Hf and Ta composites, and the better oxidation resistance of the borides compared to the carbides and nitrides. So far, the research activity has mainly focused on the achievement of high strength UHTCs usually based on ZrB2 and HfB2 composites and SiC particles in amounts of 20-30vol%. These ceramics can in fact reach strength up to 1 GPa and even higher, through careful tailoring of processing parameters. However, these strong ceramics still have very low fracture toughness, with values ranging from 2.5 to 4 MPam1/2 for ZrB2-based materials. The demand to design and realize reinforced UHTC materials comes from the need to overcome this brittleness, which makes the scale-up of big components a technological issue. The basic concept is the improvement of the fracture toughness through the introduction of elongated reinforcing phases, like SiC fibers (macro-reinforcement), or whiskers (micro-reinforcement) into UHTC matrices. A number of critical issues rise when elongated secondary phases are added to the matrix: it is difficult to obtain a homogeneous dispersion, fracture of the reinforcing phase occurs during milling and it is problematic to achieve dense ceramics without deteriorate the reinforcing phase. For example, whiskers tend to degenerate into particles if the sintering temperature is too high and then lose their reinforcing action. For carbon or silicon carbide fibers there is a strong interface reaction with the ZrB2 matrix, which should be instead avoided to promote significant pull out. This chapter presents the latest results on ZrB2-based materials reinforced with SiC fibers or whiskers. Several sintering additives are experienced in order to preserve the integrity of the reinforcing element, thus to exploit at best their toughening action. The microstructure of the dense materials is studied by SEM and TEM techniques to investigate the effect of the sintering additive and the interface between matrix and reinforcing phase. The mechanical properties are compared to those of the reference material to assess the real variation obtained by the introduction of these reinforcing agents. Following the inspection of the crack/microstructure interaction, the experimental fracture toughness is quantitatively compared to theoretical models. The addition of such reinforcements can improve the fracture toughness, from 3.7 to 5.5-6.3 MPam1/2 upon a proper choice of sintering additive and right amount of reinforcing phase. The main mechanisms invoked for explaining such improvements are crack deflection and crack bridging. The values of high temperature strength and fracture toughness up to 1500 °C are also presented and discussed. Mention is also made to the effect of the introduction of SiC short fibers to other UHTCs, namely to HfB2 and TaC.

Some borides-based composites are conventionally classified as ultra-high temperature ceramics (UHTCs) for their extremely high melting points. UHTCs are actively studied as key enabling technology for thermal protection structures (TPS) like wing leading edges, surface control components to be applied in the next generation of space vehicles flying at hypersonic speed or/and re-entering the Earth's atmosphere: increased performances and better maneuvrability are gained only through the design of sharp profile. For the present contribution, a fully dense ZrB2-based composite containing short SiC fibres was produced by hot pressing, and thus characterized. Nominally identical test articles with an hemispheric shape (5 mm radius of curvature) were machined and exposed to high enthalpy supersonic flows for different duration up to 19 MJ/kg using an arc-jet plasma wind tunnel. Surface temperature and optical properties were on-line monitored. Post-test microstructural changes were analyzed by SEM-EDX and correlated to test conditions through Computational Fluid Dynamics (CFD) simulations. Specific samples were also produced to evaluate the catalytic recombination of oxygen by optical emission spectroscopy and actinometry.

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) technologies

The objectives of the present report include the microstructural analysis of commercial allumina products for identification of residual porosity, mean grain size, secondary phases. Hardness and eleastic modulus are determined by nanoindentation

The contribution is addressed to offer an overview of recent achievements on some borides-based composites that are conventionally classified as ultra-high temperature ceramics (UHTCs) for their extremely high melting points. UHTCs are currently the most studied key enabling technology for thermal protection structures (TPS) like wing leading edges, surface control components to be applied in the next generation of space vehicles flying at hypersonic speed or/and re-entering the Earth's atmosphere. They are characterized by sharp profiles to increase performances and maneuvrability. Sharp wedges and blunt(er) hemispheric test articles were fabricated in the ZrB2 base system, using SiC particulate or SiC short fibers as second phase. Microstrcture evolutions and fundamental thermo-mechanical characteristics were determined. The dynamic response to oxidation of such UHTCs was studied under aero-thermal heating using high enthalpy supersonic flows in arc-jet plasma wind tunnel. Microstructural changes were analyzed and correlated to test conditions through Computational Fluid Dynamics (CFD) simulations

Study of the effect of sintering aid and SiC chopped fibres on ZrB2 massive ceramics by analyzing the fracture toughness, crack propagation, strength and resistance to oxidation under aerodynamic heating

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 material

This technical report aims at the characterization of structural modifications induced by two different kinds of machining on SiC-based rings, namely moulding and laser machining. The microstructure of the rings is analysed by SEM-EDS and hardness and Young's modulus by nanoindentation

This article reports on the study of the oxidation of ZrC-20 vol% MoSi2 in the temperature range 1800-2400 K in air, in order to partially reproduce the operating conditions of a high-temperature receiver for concentrated solar radiation. Such receivers are used in solar tower power plants, this technology being likely to grow in scale in the future due to environmental concerns. A thermodynamic calculation showed the existence of a limit temperature at about 1600 K, above which CO formation is significantly increased. During oxidation experiments carried out in a solar furnace, Bubble Burst Events appeared around 2000 K. The formation of these bubbles was accompanied by an increase in oxidation damage for the material. It was assumed that these bubbles formed because of the surface melting of samples in addition to an increased release of CO. However ZrC was found to undergo less oxidation damage than SiC under same conditions, as demonstrated in this study. Two different ZrC surface states were studied, but no major differences were observed in terms of oxidation behavior. Results of the thermodynamic calculation and characterization of the oxide layer let us think that the addition of such a high amount of MoSi2 was detrimental to the oxidation behavior above 1800 K, because of its dissociation and its role in the surface melting

The microstructure of fully dense hot pressed ultra-high-temperature ceramics (UHTCs), namely ZrB2 and HfB2 containing 15 vol% of TaSi2, was characterized by x-ray diffraction, scanning and transmission electron microscopy. ZrB2 and HfB2 grains displayed a core-shell structure: the core was constituted by the original MB2 grain and the shell by a (M,Ta)B2 solid solution which grew epitaxially on the core. The compositional misfit between core-shell was accommodated by low angle grain boundaries and dislocations pile-up, especially pronounced in the ZrB2-based composite, where a higher amount of Ta entered the boride lattice. Ta5Si3, Ta4.8Si3C0.3 and Ta5SiB2, with Zr or Hf impurities, were detected at the triple points and wetting of the grain boundaries by a Ta-Si-B-C-O amorphous phase was observed. Based on the new microstructural features detected by TEM, thermodynamic calculations and the available phase diagrams, a densification mechanism for ZrB2 and HfB2 with addition of TaSi2 is proposed. The microstructures of the UHTC composites presented here are compared to composites sintered with the addition of MoSi2 in the same amount

The present technical report is mainly focused on the microstructural analysis of SiC commercial powders and the resulting SiC sintered products. The main objectives are: -determination of SiC powder mean grain size and particle morphology; -analysis of surface and inner morphology, type and amount of secondary phases, defects and porosity in dense products

The present Technical Report includes the activities within the WP3 of project E2PHEST2US that concerns the design, development and ptimization of the ceramic absorber material. According to previous analyses the best candidate material for the present application should possess a combination of properties including appropriate absorbance, good electrical conductivity and good thermal stability. The characterizations performed so far have identified SiC-based materials and the ternary composites based on AlN-SiC-MoSi2 as the most promising. These composite materials offer the possibility to tailor microstructure and properties through an appropriate choice of composition. They also proved to have superior oxidation resistance thanks to the development of a silica-based protective layer. Optical measurements evidenced that the amount of MoSi2 should be minimal because it increases the material's reflectance. On the other hand, MoSi2 content should be high enough (around 30 vol%) to obtain an electrically conductive composite, which is another requirement for the kind of absorber designed. These requirements can be fulfilled designing a graded composite, constituted of at least two different compositions as follows: - the side that works as the receiver contains a low amount of MoSi2 (typically 10 vol%); - the opposite side that works as emitter (with a suitable diamond coating), contains a high amount of MoSi2 (typically 30 vol%). On these bases, the activity was concentrated on the design of optimized pellets with a double composition: compositions based on SiC and ternary composites containing 10 vol% of MoSi2 were coupled to compositions containing 30 vol% of MoSi2 to combine the beneficial effect of low amount of reflective phase in the external surface and a conductive material in the inner surface. Additional pellets of best performing materials, SAYM10 and ASMY30 were also produced to carry out further tests with diamond deposition

The present technical report is mainly focused on the microstructural analysis of SiC sintered products. The main objectives are the analysis of analysis of surface and inner morphology, type and amount of secondary phases, defects and porosity in dense products by scanning electron microscopy.

The objectives of the present report include the analyses of commercial powders of Silicon carbide, Allumina, Yttrium oxide in terms of mean grain size distribution and particle morphology by scanning electron microscopy

The ultra-high-temperature ceramics (UHTCs) are currently the most studied key enabling technology for thermal protection structures (TPS) like wing leading edges, surface control components to be applied in the next generation of space vehicles flying at hypersonic speed or/and re-entering the Earth's atmosphere. They are characterized by sharp profiles to increase performance and maneuvrability. Wedges with a very sharp profile (0.2 mm radius of curvature) and blunt hemispheric articles (5 mm radius of curvature) were produced in the system ZrB2-SiC. The dynamic response to oxidation of such UHTC demonstrators was studied under aero-thermal heating using a high enthalpy supersonic flow of a N2/O2 gas mixture in a plasma wind tunnel. Microstructural features of the reaction scale developed upon oxidation were analyzed and correlated to test conditions through Computational Fluid Dynamics (CFD) simulations. The outputs of CFD simulations matched the in-situ determinations and the materials evolution during arc-jet testing

Accepted manuscript

The main reasons for the attention focused on ceramics as possible structural materials are their wear resistance and the ability to operate with limited oxidation and ablation at temperatures above 2000°C. Hence, this work is devoted to the study of two classes of materials which can satisfy these requirements: silicon carbide -based ceramics (SiC) for wear applications and borides and carbides of transition metals for ultra-high temperatures applications (UHTCs). SiC-based materials: Silicon carbide is a hard ceramic, which finds applications in many industrial sectors, from heat production, to automotive engineering and metals processing. In view of new fields of uses, SiC-based ceramics were produced with addition of 10-30 vol% of MoSi2, in order to obtain electro conductive ceramics. MoSi2, indeed, is an intermetallic compound which possesses high temperature oxidation resistance, high electrical conductivity (21·10-6 Ω·cm), relatively low density (6.31 g/cm3), high melting point (2030°C) and high stiffness (440 GPa). The SiC-based ceramics were hot pressed at 1900°C with addition of Al2O3-Y2O3 or Y2O3-AlN as sintering additives. The microstructure of the composites and of the reference materials, SiC and MoSi2, were studied by means of conventional analytical techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (SEM-EDS). The composites showed a homogeneous microstructure, with good dispersion of the secondary phases and low residual porosity. The following thermo-mechanical properties of the SiC-based materials were measured: Vickers hardness (HV), Young’s modulus (E), fracture toughness (KIc) and room to high temperature flexural strength (σ). The mechanical properties of the composites were compared to those of two monolithic SiC and MoSi2 materials and resulted in a higher stiffness, fracture toughness and slightly higher flexural resistance. Tribological tests were also performed in two configurations disco-on-pin and slideron cylinder, aiming at studying the wear behaviour of SiC-MoSi2 composites with Al2O3 as counterfacing materials. The tests pointed out that the addition of MoSi2 was detrimental owing to a lower hardness in comparison with the pure SiC matrix. On the contrary, electrical measurements revealed that the addition of 30 vol% of MoSi2, rendered the composite electroconductive, lowering the electrical resistance of three orders of magnitude. Ultra High Temperature Ceramics: Carbides, borides and nitrides of transition metals (Ti, Zr, Hf, Ta, Nb, Mo) possess very high melting points and interesting engineering properties, such as high hardness (20-25 GPa), high stiffness (400-500 GPa), flexural strengths which remain unaltered from room temperature to 1500°C and excellent corrosion resistance in aggressive environment. All these properties place the UHTCs as potential candidates for the development of manoeuvrable hypersonic flight vehicles with sharp leading edges. To this scope Zr- and Hf- carbide and boride materials were produced with addition of 5-20 vol% of MoSi2. This secondary phase enabled the achievement of full dense composites at temperature lower than 2000°C and without the application of pressure. Besides the conventional microstructure analyses XRD and SEM-EDS, transmission electron microscopy (TEM) was employed to explore the microstructure on a small length scale to disclose the effective densification mechanisms. A thorough literature analysis revealed that neither detailed TEM work nor reports on densification mechanisms are available for this class of materials, which however are essential to optimize the sintering aids utilized and the processing parameters applied. Microstructural analyses, along with thermodynamics and crystallographic considerations, led to disclose of the effective role of MoSi2 during sintering of Zrand Hf- carbides and borides. Among the investigated mechanical properties (HV, E, KIc, σ from room temperature to 1500°C), the high temperature flexural strength was improved due to the protective and sealing effect of a silica-based glassy phase, especially for the borides. Nanoindentation tests were also performed on HfC-MoSi2 composites in order to extract hardness and elastic modulus of the single phases. Finally, arc jet tests on HfC- and HfB2-based composites confirmed the excellent oxidation behaviour of these materials under temperature exceeding 2000°C; no cracking or spallation occurred and the modified layer was only 80-90 μm thick.