Fig 6 - uploaded by Elisa Sani
Content may be subject to copyright.
Details of surface morphology of porous samples: a) ZrB 2 , b) HfB 2 , c) HfC. Residue of the final machining are sometimes visible on the edges of sample a).
Source publication
The present work reports on the comparative characterization of optical properties of hafnium and zirconium-based ultra-refractory ceramics at room and high temperature, in view of their possible use in novel solar receivers for thermal solar plants operating at higher temperatures. We show how porosity and surface finishing affect both the spectra...
Contexts in source publication
Context 1
... emissivity tests the morphology of the irradiated samples was controlled by SEM-EDS. No significant variation of surface morphology was observed, apart from a general blackening of the irradiated surfaces. Typical morphologies of the porous samples are shown in Fig. 6. As for ZrB 2 -based specimens, the dense one (ZrB 2 _d) has been tested in the solar furnace up to 1450 K. SEM-EDS post-test analyses revealed the formation of Mo-rich species such as Mo 5 Si 3 , MoB and MoSiB, and traces of oxygen. traces of amorphous phases could be detected on the surface, likely explained as machining residuals. ...
Context 2
... of amorphous phases could be detected on the surface, likely explained as machining residuals. In agreement with microstructural observations, pre-test optical spectrum shows the typical MoSi 2 shoulder at around 2.7 µm, that disappeared in the post-test spectrum, thus confirming the reaction of a significant part of MoSi 2 at least on the surface (Figure 6). Although emissivity tests were carried out in high vacuum, formation of Mo-rich species is ascribed to the presence of small traces of oxygen according to the following chemical reactions: 5MoSi 2 + 3.5O 2 (g) = Mo 5 Si 3 + 7SiO(g) ZrB 2 +2MoSi 2 +3O 2 (g) = ZrO 2 + 2MoB + 4SiO(g) Both these reactions lead to MoSi 2 consumption. ...
Citations
... The addition of the group VI element molybdenum to Hf and Zr based UHTCs has mainly been in the form of MoSi 2 [23][24][25][26][27][28]. The oxidation behavior of MoSi 2 containing boride materials has typically been characterized by the observation of silica along with the group IV rich condensed phase oxide [29], although small amounts of condensed MoO x and MoO 3 have been observed [30]. ...
... The cited work dealt with a monophasic and fully dense TiB 2 pellet. However, it is known that the material composition with the possible presence of additives and byproducts, as well as the porosity, have a critical influence on the overall optical properties of UHTC ceramics [54][55][56][57]. ...
TiB2 is a promising material for several fields including impact-resistant armor, wear-resistant coatings, cutting tools and crucibles given its physical, mechanical and chemical properties, especially due to the combination of high hardness and exceptional wear resistance. It is however very difficult to sinter below 2000 °C, even under mechanical pressure; moreover, the low fracture toughness limits the applicability of the ceramic material. By using sintering additives, it is possible to improve the sintering process and increase the mechanical properties since the additives react with oxidized layers and form secondary phases. In this study, different preparation methods and various combinations of additives (B4C, Si3N4 and MoSi2) via hot pressing sintering have been explored. Through the synergy between optimized process and tailored composition, an almost fully dense material was obtained at 1700 °C with hardness of 24.4 ± 0.2 GPa and fracture toughness of 5.4 ± 0.2 MPa m1/2. However, the highest hardness (24.5 ± 0.2 GPa) and density values were obtained for only the high-energy-milled sample with WC-Co media, featuring a core–shell grain structure. Finally, optical properties for selected samples were measured, identifying the high-energy-milled TiB2 as the sample with the highest spectral selectivity α/ε and solar absorptance.
... However, producing UHTCs with high porosity through the partial sintering process is challenging, and their porosity typically does not exceed 50%. Sani et al. [82] employed ultra-high temperature ceramic powder as a raw material to obtain porous ceramics of ZrB 2 , HfB 2 , and HfC with a porosity range of 13-40% through a pressureless sintering process. The pores were formed by the accumulation of pores between the particles and the sintering neck. ...
... In the first case, the corrosion resistance is the main limitation, whereas in the second it is the oxidation resistance and optical response. Recently, ultra-high temperature ceramics (UHTCs), and in particular diborides [1][2][3][4], carbides [3,5], and carbide-based nanocomposites [6,7] have been suggested as suitable alternatives to the more conventional graphite [8] absorbers. Other promising materials which fulfill the specification requirements and which have yet to be fully investigated are MAX phases [9][10][11][12]. ...
... In the first case, the corrosion resistance is the main limitation, whereas in the second it is the oxidation resistance and optical response. Recently, ultra-high temperature ceramics (UHTCs), and in particular diborides [1][2][3][4], carbides [3,5], and carbide-based nanocomposites [6,7] have been suggested as suitable alternatives to the more conventional graphite [8] absorbers. Other promising materials which fulfill the specification requirements and which have yet to be fully investigated are MAX phases [9][10][11][12]. ...
New generation concentrated solar power (CSP) plants require new solar receiver materials with selective optical properties and excellent corrosion resistance against molten salts. MAX phases are promising materials for CSP applications due to their optical properties and resistance to thermal shocks. Herein, we report a solar absorp-tance ≥0.5 and a thermal emittance of 0.17-0.31 between 600 and 1500 K for Cr 2 AlC, Ti 2 AlC, and Ti 3 AlC 2. These compositions were also exposed to solar salt corrosion at 600 • C for up to 4 weeks. Cr 2 AlC exhibited superior corrosion resistance due to the formation of a protective nanometric layer.
... It is possible to see a significant increase in the solar absorptance, which is advantageous, together with the above-mentioned increase in the thermal emittance. The porous sample shows both the highest absorptance and thermal emittance, as expected [33,34]. The dependence of these parameters on the laser fluence is monotonic up to φ = 0.51 kJ/cm 2 , whereas P0.97 shows a different trend, with values of absorptance and emittance lower than those of P0.51. ...
... It is possible to see a significant increase in the solar absorptance, which is advantageous, together with the above-mentioned increase in the thermal emittance. The porous sample shows both the highest absorptance and thermal emittance, as expected [33,34]. The dependence of these parameters on the laser fluence is monotonic up to ϕ = 0.51 kJ/cm 2 , whereas P0.97 shows a different trend, with values of absorptance and emittance lower than those of P0.51. ...
Tantalum boride is an ultra-refractory and ultra-hard ceramic known so far for its favorable high-temperature thermo-mechanical properties and also characterized by a low spectral emittance, making it interesting for novel high-temperature solar absorbers for Concentrating Solar Power. In this work, we investigated two types of TaB2 sintered products with different porosities, and on each of them, we realized four femtosecond laser treatments differing in the accumulated laser fluence. The treated surfaces were then characterized by SEM-EDS, roughness analysis, and optical spectrometry. We show that, depending on laser processing parameters, the multi-scale surface textures produced by femtosecond laser machining can greatly increase the solar absorptance, while the spectral emittance increase is significantly lower. These combined effects result in increased photothermal efficiency of the absorber, with interesting perspectives for the application of these ceramics in Concentrating Solar Power and Concentrating Solar Thermal. To the best of our knowledge, this is the first demonstration of successful photothermal efficiency enhancement of ultra-hard ceramics using laser machining.
... Porous ultra-high temperature ceramics (UHTCs) are considered as promising materials for aerospace thermal protection systems (TPS) [1][2][3] because they not only possess the inherent property of UHTCs (such as high melting point, outstanding ablation resistance, and excellent high temperature stability) [4][5][6][7][8] but also present the distinguished characteristics of porous ceramics (such as low density and low thermal conductivity) [9][10][11]. ...
Porous (Ta0.2Nb0.2Ti0.2Zr0.2Hf0.2)C high-entropy ceramics (HEC) with a dual-porosity structure were fabricated by pressureless sintering using a mixture powder of ceramic precursor and SiO2 microspheres. The carbothermal reduction in the ceramic precursor led to the formation of pores with sizes of 0.4–3 μm, while the addition of SiO2 microspheres caused the appearance of pores with sizes of 20–50 μm. The porous HECs exhibit competitive thermal insulation (4.12–1.11 W·m−1 k−1) and extraordinary compressive strength (133.1–41.9 MPa), which can be tailored by the porosity of the ceramics. The excellent properties are ascribed to the high-entropy effects and dual-porosity structures. The severe lattice distortions in the HECs lead to low intrinsic thermal conductivity and high compressive strength. The dual-porosity structure is efficient at phonon scattering and inhabiting crack propagations, which can further improve the thermal insulation and mechanical properties of the porous HECs.
... It has been reported that the total hemispherical emissivity can notably increase with the increase of the surface roughness/porosity. 46 For instance, an increase of the porosity from 5 to 30 vol% in HfC-based materials led to significant gain of emissivity from 0.4 to 0.55 46 ; thereby a decrease of the selectivity. As a consequence, we can reasonably expect an increase of the RT selectivity if dense materials can be prepared. ...
... It has been reported that the total hemispherical emissivity can notably increase with the increase of the surface roughness/porosity. 46 For instance, an increase of the porosity from 5 to 30 vol% in HfC-based materials led to significant gain of emissivity from 0.4 to 0.55 46 ; thereby a decrease of the selectivity. As a consequence, we can reasonably expect an increase of the RT selectivity if dense materials can be prepared. ...
TiC/SiC nanocomposites have been prepared and fully characterized at various length scales before exploring their spectral selectivity for their exploitation as solar selective absorbers. The full design process consists of (i) performing the reaction between a polysilazane, namely Durazane® 1800, and tetrakis(dimethylamino)titanium, (ii) shaping the as-synthesized polymer into pellets, (iii) pyrolyzing in flowing argon the raw pellets in order to achieve the conversion of the polymer into a single-phase ceramic and (iv) performing a heat-treatment at higher temperature to generate nanocomposite structures while developing the porosity of the pellets. The evolutive material is characterized at each step of the process via a combination of complementary techniques including FTIR and solid-state NMR spectroscopies, elemental analyses, thermogravimetric analysis (TGA) coupled with mass spectroscopy (MS), X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM). The optical properties (0.25–25 μm wavelength range) of the materials have been explored as a function of the temperature at which the final material has been isolated (1000–1800 °C). The material prepared at 1800 °C made of TiC nanocrystals in situ immobilized in a highly crystallized SiC matrix with an open porosity of 56% exhibited a boosted room temperature spectral selectivity of 1.91 which has been so far the highest referenced among SiC-based materials.
... The second CSP approach uses volumetric air receivers, where air is the HTF and the main challenge in terms of material properties lies in the optical response and oxidation resistance. Recently, ultra-high temperature ceramics (UHTCs), and in particular diborides [1][2][3][4] and carbides [3,5] have been suggested as suitable alternatives to the more conventional graphite [6] absorbers. Other promising materials which fulfill the specification requirements and which have yet to be fully investigated, are MAX phases. ...
... The second CSP approach uses volumetric air receivers, where air is the HTF and the main challenge in terms of material properties lies in the optical response and oxidation resistance. Recently, ultra-high temperature ceramics (UHTCs), and in particular diborides [1][2][3][4] and carbides [3,5] have been suggested as suitable alternatives to the more conventional graphite [6] absorbers. Other promising materials which fulfill the specification requirements and which have yet to be fully investigated, are MAX phases. ...
New generation concentrated solar power (CSP) plants require new solar receiver materials with selective optical properties and excellent corrosion resistance against molten salts. MAX phases are promising materials for CSP applications due to their optical properties and resistance to extreme environments. Herein, we report a solar absorptance above or equal to 0.5 and a thermal emittance of 0.17-0.31 between 600 and 1500 K for Cr2AlC, Ti2AlC, and Ti3AlC2. These compositions were also exposed to solar salt corrosion at 600 {\deg}C for up to 4 weeks. Cr2AlC exhibited a superior corrosion resistance due to the formation of a protective nanometric layer.
... Similarly, ZrB 2 reinforced Al 2 O 3 -10% SiC composite processed through Self-propagating high-temperature synthesis (SHS) showed reduced wear loss than Al 2 O 3 -SiC at high temperature due to the grain boundary refining and strengthening [65]. The addition of ultra-high temperature ceramics like TiB 2 , ZrB 2 , HfB 2, and TiC may boost the Al 2 O 3 -SiC suite for space applications [67][68][69][70][71]. In this way, various reinforcements transform the properties of the Al 2 O 3 -SiC. ...
Recent innovations in materials technology upsurge the use of Al2O3 and SiC based composites in aerospace, automotive, defence, transportation, and sporting industries. The reinforcements like Al, Ni, Cr, Gr, TiB2, ZrB2, MgO, HfB2, CNTs, B4C, Y2O3, GFRP and metal alloys are improving the elevated temperature thermal stability, corrosion resistance, wear resistance, fracture toughness, tensile strength, solar absorbance, fracture strength, flexural strength and crack healing ability respectively. The manufacturing methods like hot isostatic pressing, hot pressing, spark plasma sintering, electro stir forging process the Al2O3-SiC based material with a temperature range of 1400 °C–2000 °C and a pressure range of 2–100 Mpa to make the material suitable for ultra-high temperature applications. Reinforcements, the shape of the reinforcements, processing methods, processing routes, heat treatment methods, and surface finishing techniques are the prime factors that influence the strength, durability, reliability, and structural integrity of any material at ambient as well as elevated temperature. Especially, reinforcements and the manufacturing methods are playing a vital role in property enhancement of Al2O3 and SiC based composites. In this paper, a close review of various reinforcements, manufacturing techniques, behavioural enhancements, respective mechanisms, applications, and futuristic scope of Aluminium oxide-Silicon carbide based composites are presented.
... The covalent nature of the interatomic bonds of metal oxides, nitrides, carbides, and borides not only results in the high melting point of ceramics and oxidation resistance but also shifts the strength loss homologous temperature (the ratio of the current temperature to the material melting point) from 0.2 ÷ 0.3 for metals to 0.5 ÷ 0.7. This makes ceramics a key candidate for high-temperature applications [1][2][3][4][5], despite well-known drawbacks of susceptibility to sharp temperature variations, sintering challenges, and shaping/forming difficulties [6,7]. ...
Soft TiB2-BN-C hetero-modulus ceramics were sintered with the assistance of in-situ reactions during the hot pressing of TiN-B4C precursors. TiB2 formation was observed already after the hot pressing at 1100 °C, remaining the only phase identifiable by XRD even after sintering at 1500 °C. Analysis of reaction kinetics allows us to assume that the most probable reaction controlling stage is boron atoms sublimation and gas phase transfer from B4C to TiN. Reactive sintering route allows almost full densification of TiB2-BN-C composite ceramics at 1900 °C. The processes enable the formation of multilayer h-BN nanosheets inside the TiB2 matrix. The manufactured TiB2-33BN-13C ceramic with K1C = 5.3 MPa·m1/2 and HV = 1.6 GPa is extremely thermal shock-resistant at least up to quenching temperature differential of 800 °C. The sintered UHTC composite can be machined into complex geometry components.
