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A partially porous SiC ceramic, reinforced with 30 vol% short carbon fibers, was hot pressed and characterized as potential ISOL target for nuclear applications. Powder milling and hot pressing were effective for the realization of a ceramic with about 40% interconnected porosity in the 0.6–0.8 µm size range. A fiber-free porous SiC material was also synthesized for the sake of comparison. Compression strength of the fiber-rich SiC passed from about 200 MPa at room temperature to about 120 MPa upon testing at 1200 °C. The thermal conductivity was higher than the fiber-free SiC and other state-of-art ISOL target materials and was 48 W/m·K at 600 °C and decreased to 17 W/m·K at 1400 °C, owing to the porosity. Remarkably, this fiber-rich ceramic in form of thin disk, possessed suitable thermo-mechanical behavior to successfully withstand a 350 °C thermal gradient without failure. [Display omitted] • Hot pressing of a SiC material with 30 vol% short carbon fiber and 40 vol% porosity. • Compressive strength of ~200 MPa at room temperature and ~120 MPa at 1200 °C. • Thermal conductivity of 48 W/m·K at 600 °C and of 17 W/m·K at 1400 °C. • Successful exposition to high thermal loads without fracture. • Porous SiC-Cf has the suitable thermo-mechanical properties for being a suitable ISOL target in nuclear applications. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: A zirconium carbide ceramic was hot pressed to full density thanks to the addition of TaSi2, which enabled the densification to occur at 1970 K and improved the mechanical properties as compared to monolithic ZrC. The microstructure was analysed by combined X-ray diffraction, scanning and transmission electron microscopy to investigate the effective role of the sintering additive. In addition, high temperature oxidation was performed using the reactor REHPTS (Réacteur Hautes Pression et Température Solaire) from 1800 to 2200 K for 20 min and this composite demonstrated to resist towards the highly oxidative conditions better than other carbides, thanks to the chemical modification of the oxide formed upon Ta addition. However from 2000 K, the specimen resulted very damaged. [Copyright &y& Elsevier]

Abstract: Transient-liquid-phase (TLP) bonding enables joining at reduced temperatures relative to those required by more traditional joining routes while preserving the potential for high-temperature applications of ultrarefractory carbides. Pure ZrC, HfC and TaC ultra-high temperature ceramics (UHTCs) were joined using a multilayer Ni/Nb/Ni interlayer by executing a 30-min bonding cycle under low load (<1.3kPA) at 1400°C in a high-vacuum furnace. Microstructural and microchemical analyses of the resulting joints were conducted using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The SEM/EDS analysis suggests that the (Ni,Nb)-containing liquid formed during bonding wets the carbide matrices properly, resulting in well-bonded crack-free interfaces between the UHTCs and the Nb-rich interlayer. Although there were many similarities between the resulting joints, the microstructures in the interlayer-carbide transition regions also exhibited considerable diversity. [Copyright &y& Elsevier]

Abstract: The microstructure of hot pressed Hf- and Ta-carbides with 15vol% 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 Ta4.8Si3C0.3, 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. [Copyright &y& Elsevier]

Nearly fully dense carbides of zirconium, hafnium, and tantalum were obtained by pressureless sintering at 1950 °C with the addition of 5-20 vol% of MoSi2. Increasing the amount of sintering aid, the final density increased too, thanks to the formation of small amounts of liquid phase constituted by M-Mo-Si-O-C, where M is either Zr, Hf, or Ta. The matrices of the composites obtained with the standard procedure showed faceted squared grains; when an ultrasonication step was introduced in the powder treatment, the grains were more rounded and no exaggerated grains growth occurred. Other secondary phases observed in the microstructure were SiC and mixed silicides of the transition metals. Among the three carbides prepared by pressurless sintering, TaC-based composites had the highest mechanical properties at room temperature (strength 590 MPa, Young's modulus 480 GPa, toughness 3.8MPa·m1/2). HfC-based materials showed the highest sinterability (in terms of final density versus amount of sintering aid) and the highest high-temperature strength (300 MPa at 1500 °C). [ABSTRACT FROM AUTHOR]

The microstructure of two pressureless-sintered ultra-high-temperature ceramics, namely ZrC+20 vol% MoSi2 and HfC+20 vol% MoSi2, was characterized by scanning and transmission electron microscopy. With regard to the ZrC–MoSi2 system, Zr xSi y compounds and SiC were detected. In the HfC–MoSi2 system, a mixed phase was detected at the triple points and identified as (Mo,Hf)5Si3. For both the systems investigated, the high wettability of the silicide-based phases on the matrix grains suggests that sintering is assisted by a liquid phase. This contribution reports for the first time on the sintering mechanisms of early transition metal carbides doped with MoSi2 as a sinter additive, on the basis of the microstructural evolution observed upon sintering and in the light of phase diagrams and thermodynamical calculations. [ABSTRACT FROM AUTHOR]

The microstructure of two pressureless-sintered ultra-high-temperature ceramics, namely ZrC+20 vol% MoSi2 and HfC+20 vol% MoSi2, was characterized by scanning and transmission electron microscopy. With regard to the ZrC–MoSi2 system, Zr xSi y compounds and SiC were detected. In the HfC–MoSi2 system, a mixed phase was detected at the triple points and identified as (Mo,Hf)5Si3. For both the systems investigated, the high wettability of the silicide-based phases on the matrix grains suggests that sintering is assisted by a liquid phase. This contribution reports for the first time on the sintering mechanisms of early transition metal carbides doped with MoSi2 as a sinter additive, on the basis of the microstructural evolution observed upon sintering and in the light of phase diagrams and thermodynamical calculations. [ABSTRACT FROM AUTHOR]

Hafnium carbide (HfC)-(5, 10, and 20 vol%) MoSi2 ceramics were pressureless sintered at 1950°C in an argon flux. The materials had nearly full density (96%–98%), with mean grain sizes in the range of 3–4 μm. Depending on the MoSi2 amount (5–20 vol%), the mechanical properties were in the following ranges: hardness 16–15 GPa, Young's modulus 434–385 GPa, fracture toughness 3.6–3.4 MPa·m1/2, and room-temperature 4-point flexural strength 465–383 MPa. [ABSTRACT FROM AUTHOR]

The article tackles the processing, mechanical properties and oxidation behavior of tantalum carbide (TaC) and hafnium carbide (HfC) composites containing 15 vol% tantalum silicide (TaSi2) or molybdenum silicide (MoSi2). The sinterability of the composites was improved by TaSi2. It notes the better performance of the TaC-based composites than its HfC counterparts at room temperature. HfC-based composites are said to maintain a superior high temperature stability compared to TaC-based ones.

ZrC-based composites were produced by pressureless sintering thanks to the addition of MoSi2 as sintering aid. After preliminary tests, a baseline ZrC material and two mixed ZrC-HfC and ZrC-ZrB2 composites with 20 vol% MoSi2 were densified at 1900 to 1950 °C reaching final relative densities of 96%-98%. Mean particle size of the dense bodies ranged from 5 to 9 µm. Secondary phases were found to form during sintering, such as SiC and Zr-Mo-Si-based compounds. Room-temperature mechanical properties were in the range of the values reported in the literature for similar materials densified by pressure-assisted techniques. The flexural strength was tested at room temperature, 1200 and 1500 °C. [ABSTRACT FROM AUTHOR]

Besides ultra-refractoriness and favorable mechanical and chemical characteristics, carbides of early transition metals show intrinsic spectral selectivity, making them appealing for high-temperature solar absorber applications. However these kinds of ceramics can be produced using many processing methods resulting in different compositions, density and surface finishing. Thus the present work reports on the systematic study of microstructural, mechanical and optical properties of dense zirconium, hafnium and tantalum carbides as a function of the sintering method (high pressure or pressureless), implying use of 10 or 20 vol% of MoSi 2 as sintering aid. The spectral hemispherical reflectance of Zr-, Hf- and Ta-carbides has been measured in the 0.25–16.5 μm wavelength range and correlated to the surface microstructure and roughness. Room and high temperature fracture strength has been measured as well. [ABSTRACT FROM AUTHOR]

Highlights: [•] Dense and porous Hf and Zr-based UHTCs are sintered and structurally characterized. [•] They are spectrally selective, according to RT optical characterization. [•] Thermal emittance in the 1100–1800K temperature range shows promising properties. [•] Thus they appear promising for novel solar absorbers operating at high temperature. [ABSTRACT FROM AUTHOR]