Hafnium and tantalum carbides for high temperature solar receivers
Elisa Sani, Luca Mercatelli, Daniela Fontani, Jean-Louis Sans, and Diletta Sciti
Citation: J. Renewable Sustainable Energy 3, 063107 (2011); doi: 10.1063/1.3662099
View online: http://dx.doi.org/10.1063/1.3662099
View Table of Contents: http://jrse.aip.org/resource/1/JRSEBH/v3/i6
Published by the American Institute of Physics.
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JOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 3, 063107 (2011)
Hafnium and tantalum carbides for high temperature solar
receivers
Elisa Sani,1,a) Luca Mercatelli,1 Daniela Fontani,1 Jean-Louis Sans,2
and Diletta Sciti3
1
CNR-INO National Institute of Optics, largo E. Fermi, 6, 50125 Firenze, Italy
PROMES-CNRS Processes, Materials and Solar Energy Laboratory, 7 rue du Four Solaire,
66120 Font Romeu, France
3
CNR-ISTEC Institute of Science and Technology for Ceramics, Via Granarolo, 64, 48018
Faenza (RA), Italy
2
(Received 19 August 2011; accepted 24 October 2011; published online 28 November 2011)
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
C 2011 American Institute of Physics. [doi:10.1063/1.3662099]
solar receivers. V
INTRODUCTION
Supplying with energy a growing world population while fossil fuel reserves are declining
is one of the main challenges of the present century. Moreover, the increased attention worldwide to pollution and global climate change problems has resulted in a huge growth of the interest in renewable energies, and in particular in solar energy exploitation, as being underlined
by several authors (see, for example, Refs. 1–3 and references therein). In this framework, the
efficiency raise of sunlight exploiting systems is a key issue. In the field of photovoltaic sunlight exploitation, different systems are being studied, including new approaches for siliconbased devices,4,5 new kinds of solar cells,6,7 and luminescent solar concentrators.8,9 For solar
thermodynamic approach, efficiency increase is strongly connected to the development of materials and technologies for plants that can be operated at ever higher temperatures.
Different concepts for receivers for thermal solar systems have been developed in the
past.10 Each architecture has advantages and drawbacks, and the best tradeoff depends on the
specific application. The bulk absorber cavity scheme appear interesting for high-temperature
large-scale plants because of its low heat loss, easy controlling, high heat capacity, and the possibility to successfully exploit mature technologies of conventional fossil fuel power plants.11
The maximum operating temperatures of a solar power plant are usually less than 900 K
because of the rapid degradation of its components. Commercial absorber materials are indeed
able to operate at temperature only up to 800 K, with very few prototypal exceptions.12 However, as the efficiency of solar thermal power plants increases rapidly with increasing working
temperatures, the main challenge is improving the high temperature stability, especially of the
receiver. This can be achieved developing materials able to resist to damage at very high temperatures (in perspective up to 1800 K, even if temperature of 1400 K can be considered already
very satisfying) while keeping good thermal conductivity and favorable radiative properties.
Diborides and carbides of zirconium, hafnium, and tantalum (ZrB2, ZrC, HfB2, HfC, TaC,
etc., melting points well above 3200 K), are referred as ultra-high-temperature-ceramics
(UHTCs) and are considered the best-emerging materials for applications in aerospace and
a)
Author to whom correspondence should be addressed. Electronic mail: [email protected]. Tel.: þ39(0)55.23081.
FAX:þ39(0)55.233775.
1941-7012/2011/3(6)/063107/6/$30.00
3, 063107-1
C 2011 American Institute of Physics
V
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063107-2
Sani et al.
J. Renewable Sustainable Energy 3, 063107 (2011)
advanced energy systems (turbine blades, combustors, scramjet engines, nuclear fusion reactors). The increasing interest in these materials is due to their unique combination of properties
including the highest melting points of any group of materials, high strength, high thermal and
electrical conductivities and chemical stability. Recently, it has been found that most of these
compounds also have the characteristic of being intrinsic solar spectrally selective materials,
but the knowledge on the optical properties of these materials is still very poor all over the
whole range of temperatures which are of interest for the present application (i.e., up to 1800 K
in perspective). UHTCs have thus the potential to be remarkably suited for application in high
temperature solar receivers, once their basic properties have been properly characterized. Very
recently, we measured the emissivity of zirconium carbide-based UHTCs, obtaining promising
results.13 The present work investigates the high-temperature emissivity properties of hafnium
and tantalum carbides, to assess their potential as novel solar absorbers in thermodynamic solar
plants. The results are compared with those obtained of a silicon carbide sample in the same
conditions. It should be emphasized that SiC material is under study for volumetric sunlight
absorbers.14
MATERIALS AND METHODS
The materials analyzed in the present work are a specimen of pure HfC and a couple of
TaC monoliths with different density levels. Moreover, for a more meaningful characterization,
the samples have been also compared with a specimen of SiC. The pellets were densified by
hot pressing of as-received powders at temperatures in the 2070–2220 K range. Flat surfaces
have been prepared by conventional mechanical machining by diamond tools with the same
roughness (300 nm) for all samples.
Spectral selectivity of samples at room temperature has been evaluated by acquiring the
room-temperature specular reflectance spectra. A double-beam UV-VIS-NIR spectrophotometer
(Perkin Elmer Lambda900) equipped with a Spectralon integrating sphere was used in the range
from 0.2 to 2.4 lm. In this wavelength range, the specular reflectance is calculated after the
separate measurement of both the purely diffuse and the total hemispherical reflectance. For the
wavelength range 2.4–25.0 lm, we used a Fourier transform infrared spectrometer (Bio-Rad
Excalibur) equipped with a specular reflectance accessory.
High-temperature characterization has been carried out using the MEDIASE (Moyen
d’Essai et de Diagnostic en Ambiance Spatiale Extrême) set-up of the MegaWatt Solar
Furnace15–17 in Font Romeu (France). MEDIASE is composed by a high-vacuum chamber (ultimate pressure limit of 106 mbar), equipped with a hemispherical silica glass window of 35 cm
diameter. The water-cooled sample holder is set to keep the specimen at the focus of the 1MW solar furnace. Care is taken in mounting the sample in such a way to minimize the thermal
losses towards the holder, so that the only relevant thermal energy loss channel is thermal radiation. Directional radiance was measured on the back face of the sample at different angles
thanks to a movable, computer-controlled three-mirror system, using a radiometer as light detector. The spectral response of the whole optical system was calibrated against a reference
blackbody. The characterization was performed by measuring both the total (0.6–40 lm) and
spectral radiance in various spectral bands (0.6–2.8 lm, 2.7 lm, 5 lm, 5.5 lm, 8–14 lm). For
the first measurement (total radiance), the whole sensitivity range of the radiometer was
exploited, while spectral measurements were performed by adding proper optical filters in front
of the detector to select the spectral band. The directional emissivity in each wavelength band
of interest is then calculated as the ratio between the measured directional radiance and the theoretical blackbody radiance in the same spectral band and at the same temperature. The temperature being a critical parameter, the specimen surface temperature was measured using an especially developed pyro-reflectometer, taking care of reading the temperature near the sample
center to minimize border effects. The method, called bicolor pyroreflectometry, is based on
Planck’s law, Kirchhoff’s law, and the assumption of identical reflectivity indicatrixes for the
target surface at two different close wavelengths.18,19 The hemispherical emissivity is obtained
by numerical integration of the directional values. The relative standard uncertainties of the
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063107-3
HfC and TaC for HT solar receivers
J. Renewable Sustainable Energy 3, 063107 (2011)
temperature measurement are typically from 0.5% to 1%, while the relative standard uncertainties of the measurement of emissivity are 1% for measurements in the 2.7 lm, 5 lm, and
5.5 lm spectral bands and 2.5% for measurements in the ranges 0.6–2.8 lm, 8–14 lm, and
0.6–40 lm.
RESULTS AND DISCUSSION
Figure 1 shows some morphological details of the materials under investigation. Monolithic
HfC sample (HC) was consolidated by hot pressing at 2200 K, reaching a final density of 98%.
The microstructure consists of rounded grains with dimensions around 20 lm. The tantalum
carbide-based materials (TC76 and TC92) were consolidated at 2070 K and 2170 K reaching
densities of 76% and 92%, respectively. TC76 is characterized by a fine microstructure and fine
porosity. TC92 contains grains with size around 10–20 lm along with an amount of very fine
closed porosity in the range 0.3–1 lm. Finally, the SiC-based material (SC) (not shown) was
hot pressed at 2150 K, it contains small percentages of metal oxides as sintering aids (Al2O3
and Y2O3), and reaches a final density of 88%. Further details on materials’ preparation are
available in Refs. 20–22. Table I summarizes the investigated samples.
For optimum efficiency, the solar absorber should have the maximum possible absorbance
at solar spectrum wavelengths while exhibiting a minimum infrared emissivity. This may be obtained
using intrinsic selective absorbers. A solar selective surface efficiently captures solar energy in the
high intensity visible and the near infrared, up to about 2.5 lm wavelength, while maintaining poor
radiating properties (i.e., a high reflectivity) in the thermal infrared spectral region.
To give a qualitative comparison of spectral selectivity properties of the samples, in Figure 2
we show the specular spectral reflectance, measured at room temperature and before the samples
underwent the high temperature heating. The narrow small peaks at around 4.2 and 15 lm in
some of the spectra are instrumental artifacts due to unbalanced residual absorption by water
vapor and CO2 molecules within the unpurged spectrometer chamber. From Fig. 2, we can
clearly distinguish the optical properties of silicon carbide from those of UHTCs. In particular, in
the spectrum of SC, we can identify the typical reflectivity curve of a semiconductor, with its
reststrahlen reflectance peak at around 12 lm,23 while UHTCs show a metal-like reflectivity
behavior with smoothly rising, almost monotonic curves and a high reflectivity value in the infrared. The lower reflectivity of TC76 with respect to the denser TC92 can reasonably be ascribed
to a porosity effect. Fig. 2 qualitatively suggests that tantalum and hafnium carbides may show a
more pronounced spectral selectivity against thermal re-radiation than silicon carbide. It should
be noticed that the spectra in Fig. 2 help to discriminate the basics optical properties of samples,
even if their aim is only to give a qualitative comparison. On the other hand, the quantitative
assessment must be done by measuring the hemispherical reflectivity or emissivity at the absorber
operating temperature.
For this reason, for all samples, we measured the total hemispherical emissivity (representing the hemispherical spectral emissivity integrated in the 0.6-40 lm wavelength range) as a
function of the temperature, as shown in Fig. 3. We can see that UHTC samples retain a considerably lower emissivity than silicon carbide at all temperatures. HC shows a linear emissivity
increase versus temperature, with the highest temperature dependence among the samples. In
FIG. 1. Microstructural features of polished samples of (a) HC, (b)TC76, and (c)TC92.
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063107-4
Sani et al.
J. Renewable Sustainable Energy 3, 063107 (2011)
TABLE I. Investigated samples.
Material full name
Material chemical formula
Sample labeling
Hafnium carbide
Tantalum carbide
HfC
TaC
Silicon carbide
SiC
HC
TC76 (76% density)
TC92 (92% density)
SC
fact, its emissivity ranges from 0.35 at 1100 K to about 0.60 slightly above 1400 K. TC92 sample has the lowest emissivity at all temperatures, with values that are about a half of those of
HC and as low as about a third than those of SC and have the weakest temperature dependence.
TC76 shows an intermediate behavior: emissivity values are similar to those of HC but with a
temperature trend similar to that of TC92. As both tantalum carbide samples contain only pure
TaC and they have been machined with the same surface finishing, the emissivity differences
among them can likely be ascribed to their different porosity. In fact, emissivity is a surface
property (thus it is dependent on the surface finishing24,25) and pores produce an increase of the
effective sample surface available for radiating. On the other hand, proper surface pores are
also expected to raise the solar absorptivity.26 Therefore, for the best solar absorber and energy
storage material, an optimal tradeoff must be found between these conflicting phenomena.
From the obtained data, we can conclude that the investigated material with the best thermal
emission characteristics for solar applications is tantalum carbide. Its emissivity properties are
by far much favorable than those of silicon carbide, thanks to its lower energy losses due to
thermal emission and thus to its superior energy storage capability. Moreover, TC92 sample
favorably compares, at all temperatures, also to zirconium carbide-based UHTCs we previously
investigated.13 On the other hand, if we compare also HC and TC76 to ZrC, hafnium and tantalum carbides result still preferable to ZrC-based materials for temperatures higher than about
1300 K, while in the 1100-1300 K range, they are only slightly less performing than ZrC.
Finally, for a more comprehensive high-temperature emissivity characterization, we
evaluated the spectral differences of samples acquiring their spectral emissivity in a series of
wavelength bands. The hemispherical emissivity measured in the wavelength range (k1, k2) is
given by
ð\
e\k1;k2 ðTÞ ¼
ð k1
sin hdhdu
Iðk; h; TÞdk
k2
;
ð k1
IB ðk; TÞdk
2p
(1)
k2
FIG. 2. Specular reflectance spectra of the samples before the high temperature heating.
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063107-5
HfC and TaC for HT solar receivers
J. Renewable Sustainable Energy 3, 063107 (2011)
FIG. 3. Measured total hemispherical emissivities as a function of the temperature.
where I(k, h, T) is the directional spectral radiance of the sample, IB(k, T) is the blackbody
directional spectral radiance at temperature T, and hemispherical radiance values are calculated
integrating on a half space (as indicated by the symbol \). The integral at numerator is given
by the measured radiance in the interval (k1, k2), being (k1, k2) the bandpass of the filter.
According to the definition in Eq. (1), the emissivity in the interval (k1, k2) actually represents
the mean value of spectral emissivity in the considered wavelength range. Thus, we can obtain
information about spectral emissivity differences among samples by comparing the emissivity
values acquired in a series of spectral bands. In Fig. 4, we show the result of the measurement
in the various spectral bands. Due to the fact that each sample shows different emissivity
values for the different considered spectral ranges, we can conclude that both tantalum and
hafnium carbides have a non-grey body behavior and a temperature dependence of spectral
characteristics.
From Fig. 4, we can appreciate the similarity between the two Tantalum Carbide samples
as for the emissivity trend of the various spectral bands versus the temperature, while absolute
values of emissivities are different reasonably because of their different porosities, as discussed. On the other hand, Hafnium Carbide shows a different trend: the relative heights of
practically all bands over the total 0.6–40 lm one decrease with the temperature, allowing to
infer that the relative weight of emissivity at wavelengths longer than 14 lm on the overall
emissivity increases with increasing temperature more markedly than what happens for tantalum carbide.
FIG. 4. Measured hemispherical emissivities in various spectral bands as a function of the temperature. First column: HC;
second column: TC76; third column: TC92.
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063107-6
Sani et al.
J. Renewable Sustainable Energy 3, 063107 (2011)
CONCLUSIONS
The main challenge in present solar energy exploitation is the efficiency increase that, for
thermodynamic solar plants, translates in a higher temperature operation. Thus the ideal solar
receiver material should withstand high temperatures with good oxidation properties, while having both a high solar absorption and a low emissivity at the furnace operating temperatures.
The present work reports on the emissivity characterization of hafnium and tantalum carbide
UHTCs. Up to now, this class of materials has been widely studied for space and military
applications, but proposing them for solar thermal technology is quite a new use. For a more
significant assessment of UHTC emissivity performances, we compared them with the results of
measurements on silicon carbide in the same experimental conditions. It should be emphasized
than silicon carbide is already used as solar absorber in thermodynamic plants. Our hightemperature investigation shows that hafnium and tantalum carbides are very promising for
solar energy exploitation. We measured for them a considerably lower thermal emissivity than
silicon carbide, with subsequent superior energy storage capability. Other aspects will be investigated, like high-temperature oxidation properties in air and thermal emissivity stability under
heat cycling. Anyway the present work demonstrates that UHTCs are undoubtedly worth of
further studies in the perspective to use them as sunlight receivers in new-generation thermodynamic plants.
ACKNOWLEDGMENTS
Financial support by the Access to Research Infrastructures activity in the 7th Framework
Programme of the EU (SFERA Grant Agreement No. 228296) is gratefully acknowledged. Thanks
are due to the PROMES Director and PROMES Researchers for the use of facilities.
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