Ultra-high temperature ceramics (UHTCs) are interesting materials for a large variety of applications under extreme conditions. This paper reports on the production and extensive characterization of highly dense, pure zirconium and... more
Ultra-high temperature ceramics (UHTCs) are interesting materials for a large variety of applications under extreme conditions. This paper reports on the production and extensive characterization of highly dense, pure zirconium and tantalum diborides, with particular interest to their potential utilization in the thermal solar energy field. Monolithic bulk samples are produced by Spark Plasma Sintering starting from elemental reactants or using metal diboride powders previously synthesized by Self-propagating High-temperature Synthesis (SHS). Microstructural and optical properties of products obtained by the two processing methods have been comparatively evaluated. We found that pure diborides show a good spectral selectivity, which is an appealing characteristic for solar absorber applications. No, or very small, differences in the optical properties have been evidenced when the two investigated processes adopted for the fabrication of dense TaB 2 and ZrB 2 , respectively, are compared.
Porous ceramic materials are currently used as volumetric sunlight absorbers in concentrating solar power systems. As the efficiency of thermodynamic cycles rapidly increases with the operating temperature, the favorable characteristics... more
Porous ceramic materials are currently used as volumetric sunlight absorbers in concentrating solar power systems. As the efficiency of thermodynamic cycles rapidly increases with the operating temperature, the favorable characteristics of so-called ultra-high temperature ceramics (UHTCs) can be successfully exploited in novel solar absorbers. The present work reports, for the first time to the best of our knowledge, on optical properties and microstructural analysis of novel ice-templating porous ZrB2 UHTCs, to evaluate their potential as volumetric solar receivers. We demonstrate that the different complex structures that can be obtained with the freeze casting technique show promising optical properties. The idea of conjugating an highly tailorable morphology, useful for optimizing gas fluxes and heat exchanges between absorber and gas, to the spectral selectivity which is a characteristics of ZrB2 can be a promising route for increasing the efficiency of thermal solar systems.
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... more
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.
It is a known rule that the efficiency of thermodynamic solar plants increases with the working temperature. At present, the main limit in temperature upscaling is the absorber capability to withstand high temperatures. The ideal solar... more
It is a known rule that the efficiency of thermodynamic solar plants increases with the working temperature. At present, the main limit in temperature upscaling is the absorber capability to withstand high temperatures. The ideal solar absorber works at high temperatures and has both a low thermal emissivity and a high absorptivity in the solar spectral range. The present work reports on the preparation and optical characterization of hafnium and zirconium diboride ultrahigh-temperature ceramics for innovative solar absorbers operating at high temperature. Spectral hemispherical reflectance from the ultraviolet to the mid-infrared wavelength region and high-temperature hemispherical emittance reveal their potential for high-temperature solar applications. Boride samples are compared with silicon carbide (SiC), a material already used in solar furnaces.
The controlling densification mechanisms of hot pressed monolithic ZrB2 ceramics and ZrB2-based composites, containing 15 and 30 vol. % SiC, at different consolidating temperatures were investigated, based on scanning electron microscopy... more
The controlling densification mechanisms of hot pressed monolithic ZrB2 ceramics and ZrB2-based composites, containing 15 and 30 vol. % SiC, at different consolidating temperatures were investigated, based on scanning electron microscopy micrographs of fracture surfaces, relative densities, and average grain size of ZrB2. For the hot pressed samples at 1700 °C, particles fragmentation in the composite samples, mechanical interweaving, and rearrangement without sizeable chemical bonding were appointed as dominant densification mechanisms. Neck formation between ZrB2/ZrB2 was observed at 1850 °C and plastic deformation of ZrB2 grains were nominated as controlling densification mechanisms. Reduction of porosity in the hot pressed specimens at 2000 °C was related to grain boundary diffusion mechanism. Colossal grain growth in monolithic ZrB2 ceramic proposed the occurrence of detrimental mechanisms such as grain coarsening and evaporation/condensation. Presence of intergranular SiC particles between ZrB2 grains impeded extremist grain growth.
Ultra-high temperature ceramics (UHTCs) are the ideal material for extreme conditions thanks to very high melting points, high refractoriness and good thermo-mechanical properties at high temperatures. This paper reports, for the first... more
Ultra-high temperature ceramics (UHTCs) are the ideal material for extreme conditions thanks to very high melting points, high refractoriness and good thermo-mechanical properties at high temperatures. This paper reports, for the first time to the best of our knowledge, on the microstructural and optical characterization of several tantalum diboride (TaB2) samples with density values from 67% to full density. Pristine samples have been investigated at room temperature by means of SEM, XRD and spectral hemispherical reflectance measurements. Thermal emittance in the temperature range 1050–1800 K has been measured. Structural, compositional and optical properties after high temperature exposure have been characterized as well and property changes have been explained. The obtained results favorably compare TaB2 over conventional solar absorbers for novel solar plants operating at higher temperatures.
Solar thermal technology is a safe, sustainable, and cost-effective energy supply. The present paper reports on the comparative high-temperature emissivity characterization of hafnium and tantalum carbide ultra-high temperature ceramics... more
Solar thermal technology is a safe, sustainable, and cost-effective energy supply. The present paper reports on the comparative high-temperature emissivity characterization of hafnium and tantalum carbide ultra-high temperature ceramics to evaluate their potential as novel absorbing materials for concentrating solar power plants. For a more meaningful property assessment, ultra-high temperature ceramic samples have been compared also with silicon carbide, a material already used in volumetric solar receivers
Ultra-refractory diborides are currently studied mainly as thermal protection materials for aerospace and military applications. However, their favorable properties (very high melting points and good thermo-mechanical properties at high... more
Ultra-refractory diborides are currently studied mainly as thermal protection materials for aerospace and military applications. However, their favorable properties (very high melting points and good thermo-mechanical properties at high temperatures) can be advantageously exploited to increase the operating temperature of thermodynamic solar plants in concentrating solar power systems. This paper reports on the spectral reflectance characterization of hafnium and zirconium diborides containing MoSi2 as secondary phase to evaluate their potential as novel solar absorbers. To assess the spectral selectivity properties, room-temperature hemispherical reflectance spectra were measured from the UV wavelength region to the mid-infrared, considering different levels of porosity for each system. Moreover, for zirconium diboride and hafnium diboride composites containing 10 vol% of MoSi2 sintering aid, the thermal emittance was measured in the 1100-1400 K temperature range. Room temperature spectral characteristics and high temperature emittance were compared to that of a monolithic silicon carbide.
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... more
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 spectral reflectance and the high-temperature emittance. Moreover, structural and chemical changes occurring at high temperatures are assessed.
HfC-based materials are promising composites for application as solar absorbers. Being a ceramic with some metallic character, HfC shows intrinsic spectral selectivity, but quite a high reflectance at the wavelengths of the Sun spectrum.... more
HfC-based materials are promising composites for application as solar absorbers. Being a ceramic with some metallic character, HfC shows intrinsic spectral selectivity, but quite a high reflectance at the wavelengths of the Sun spectrum. In this work, a femtosecond laser treatment has been specifically tailored to increase the solar absorbance of a composite 70 vol% HfC-30 vol%MoSi2. We investigated the morphological surface changes induced by the femtosecond laser on both phases, proposing a mechanism for surface modifications on the basis of microstructural analysis. Despite the presence of two ceramic phases with different physical properties, the laser was able to modify both phases simultaneously upon specific parameters. The effect of the surface texturing on the optical spectrum was analyzed. By use of specific laser interaction and patterning parameters, the formation of a regular surface pattern allowed the absorbance-over-emittance ratio to be increased from about 1.8 to 2.1. (C) 2014 Elsevier B.V. All rights reserved.
It is well known that the efficiency of thermodynamic solar plants increases with working temperature. At present the main limit to temperature upscaling is the capability of the absorber to withstand high temperatures. The ideal solar... more
It is well known that the efficiency of thermodynamic solar plants increases with working temperature. At present the main limit to temperature upscaling is the capability of the absorber to withstand high temperatures. The ideal solar absorber works at high temperatures, has a low thermal emissivity and a high absorptivity in the solar spectral range. This paper reports on the high-temperature emissivity characterization of ceramic zirconium carbide samples. The investigation has been performed for samples of different chemical compositions
The viability of an all-optical fibre optical thermocoupler for remote sensing of ultra-high temperatures, independent of electronics, is explored. Simple packaging of regenerated fibre Bragg gratings (FBGs) within silica capillaries were... more
The viability of an all-optical fibre optical thermocoupler for remote sensing of ultra-high temperatures, independent of electronics, is explored. Simple packaging of regenerated fibre Bragg gratings (FBGs) within silica capillaries were shown to protect the fibre structure sufficiently to allow the temperature measurement of a furnace element to >1500 °C a temperature at which the regenerated FBG, which was written in relatively soft boron-codoped germano silicate glass, was shown to decay rapidly.
We report on the preparation, room temperature spectral reflectance and high-temperature thermal emittance characterization of different boride and carbide Ultra-High Temperature Ceramics (UHTCs). The investigated samples are compared... more
We report on the preparation, room temperature spectral reflectance and high-temperature thermal emittance characterization of different boride and carbide Ultra-High Temperature Ceramics (UHTCs). The investigated samples are compared with a reference material for solar absorber applications, i.e. silicon carbide. We show that spectral and thermal emittance properties of UHTCs are promising for novel solar receivers. [DOI: http://dx.doi.org/10.2971/jeos.2012.12052]
Ultrasonic and oscillating rheological methods were applied to investigate the effects of heat pre-treatment at Ultra-High-Temperatures on the rennet gel formation of a whey-protein-free casein solution. By comparison, the ultrasonic... more
Ultrasonic and oscillating rheological methods were applied to investigate the effects of heat pre-treatment at Ultra-High-Temperatures on the rennet gel formation of a whey-protein-free casein solution. By comparison, the ultrasonic velocity was found to be sensitive to measure the enzymatic hydrolysis by rennet and the following aggregation process, but not as sensitive to detect the formation of a casein gel.
Multiphase composite alloys based on the Mo-Si-B system are candidate materials for ultra-high temperature applications. In non load-bearing uses such as thermal barrier coatings or heat exchangers in fossil fuel burners, these materials... more
Multiphase composite alloys based on the Mo-Si-B system are candidate materials for ultra-high temperature applications. In non load-bearing uses such as thermal barrier coatings or heat exchangers in fossil fuel burners, these materials may be ideally suited. The present work investigated the effect of water vapor on the oxidation behavior of Mo-Si-B phase assemblages. Three alloys were studied: Alloy 1 = Mo5Si3Bx (T1)- MoSi2-MoB, Alloy 2 = T1- Mo5SiB2 (T2)- Mo3Si, and Alloy 3 = Mo- T2- Mo3Si. Tests were conducted at 1000° and 1100°C in controlled atmospheres of dry air and wet air nominally containing 18, 55, and 150 Torr H2O. The initial mass loss of each alloy was approximately independent of the test temperature and moisture content of the atmosphere. The magnitude of these initial losses varied according to the Mo content of the alloys. All alloys formed a continuous, external silica scale that protected against further mass change after volatilization of the initially formed ...
The behaviour of pressureless sintered HfC and HfB2 ceramics, when exposed to high enthalpy plasma flows typical of atmospheric re-entry environment, was investigated with an arc-jet facility at temperatures exceeding 2000°C. The surface... more
The behaviour of pressureless sintered HfC and HfB2 ceramics, when exposed to high enthalpy plasma flows typical of atmospheric re-entry environment, was investigated with an arc-jet facility at temperatures exceeding 2000°C. The surface temperature and emissivity of the materials were evaluated during the test. The microstructure modifications were analysed after exposure. Fluid dynamic numerical simulations were carried out to evaluate
The controlling densification mechanisms of hot pressed monolithic ZrB2 ceramics and ZrB2-based composites, containing 15 and 30 vol. % SiC, at different consolidating temperatures were investigated, based on scanning electron microscopy... more
The controlling densification mechanisms of hot pressed monolithic ZrB2 ceramics and ZrB2-based composites, containing 15 and 30 vol. % SiC, at different consolidating temperatures were investigated, based on scanning electron microscopy micrographs of fracture surfaces, relative densities, and average grain size of ZrB2. For the hot pressed samples at 1700 °C, particles fragmentation in the composite samples, mechanical interweaving, and rearrangement without sizeable chemical bonding were appointed as dominant densification mechanisms. Neck formation between ZrB2/ZrB2 was observed at 1850 °C and plastic deformation of ZrB2 grains were nominated as controlling densification mechanisms. Reduction of porosity in the hot pressed specimens at 2000 °C was related to grain boundary diffusion mechanism. Colossal grain growth in monolithic ZrB2 ceramic proposed the occurrence of detrimental mechanisms such as grain coarsening and evaporation/condensation. Presence of intergranular SiC part...
