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Recent advances within the field of materials science in Spain

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Recent advances within the field of materials science in Spain

Monografía materia: Ciencias Editores/as: Berenguer Murcia, A., Caturla Terol, M. J., Molina Jorda, J. M., Morallon Nuñez, E., Quijada Tomás, C., Román Martínez, M. C., Sancho Garcia, J. C., Vidal Martinez, L. ISBN: 978-84-9717-346-9 edición: 2015 idioma: inglés formato: 24x17 nº págs: 468 PVP sin IVA: 21,15 € PVP con IVA: 22,00 € Recent advances within the field of materials science in Spain This manuscript originates from the different contributions presented in the Scientific Meeting of the Materials Institute throughout its first 10 years of existence. The Materials Institute “Instituto Universitario de Materiales de Alicante” (IUMA) organizes a scientific meeting each year (Scientific Meeting of the IUMA) with the objective of gathering outstanding researchers in the field of Materials Science in Spain, and get to know and discuss on the main research guidelines in this field. In the year 2014 these meetings reached their tenth http://publicaciones.ua.es/publica/ficha.aspx?fndCod=LI9788497173469

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edition with an international perspective.

The book content is organized in five sections which represent current strategic research sectors in the field of Materials Science: Materials for Energy Applications, Nanomaterials, Materials Modeling, Catalysis, Functional Materials and Biomaterials. The thirty-six contributions presented here, distributed in these five relevant áreas of materials field make this book a suitable consultation manual, which will allow knowing in depth both physical and chemical aspects of different materials, as well as processes for their application. Añadir a la cesta Versión imprimir Mandar a un amigo

Detalles del libro » Index (PDF) » Fragment (PDF) Información legal - copyright© 2010 Todos los derechos reservados sobre las publicaciones. [email protected]

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INDEX ENERGY APPLICATIONS Multifunctional M/Si3N4 (M = metal) multilayers: From nanoparticle magnetism to the design of selective coatings for concentrated solar power systems................................................. 19 E. Céspedes, M. Vila, F. Jiménez-Villacorta, J. Sánchez-Marcos, L. Álvarez-Fraga, C. Prieto New Generation of Materials for More Efficient Solar Energy Use: Quantum Modelling and Experimental Realizations............................................................................................... 30 P. Wahnon, P. Palacios, I. Aguilera, Y. Seminovski, R. Lucena, J.C. Conesa Light-induced Trapping and Pattering of Micro- and NanoParticles on the Surface of Iron Doped LiNbO3.................................. 44 M. Carrascosa, H. Burgos, J. Matarrubia, M. Jubera, A. García-Cabañes, F. Agulló-López Advanced Polymer Membrane Materials for Fuel Cell Applications............................................................................................... 54 C.del Río, P.G. Escribano Advanced materials as electrodes for energy storage...................... 67 E. Morallón, D. Cazorla-Amorós, F. Huerta, C. Quijada, F. Montilla Advanced Carbon Materials for Gas Storage and Space Cryocoolers............................................................................................... 81 D. Lozano-Castelló, J. Alcañiz-Monge, F. Suarez-García, D. Cazorla-Amorós, A. Linares-Solano

Zeolite membrane reactors for energy processes................................ 94 M. Menéndez Carbons containing heteroatoms as electrodes for supercapacitors........................................................................................ 107 E. Raymundo-Piñero Catalytic routes for the conversion of biomass-derived molecules into liquid fuels................................................................... 117 J.C. Serrano Ruiz Developments in Co-based catalysts for H2 production by means of bioalcohol reforming............................................................ 124 N. Homs, P. Ramírez de la Piscina Photocatalytic Activation of CO2 for the Production of Chemicals and Fuels............................................................................... 135 V. de la Peña O’Shea, D. Serrano, J.M. Coronado NANOMATERIALS Nanomaterials in Catalysis.................................................................... 151 I. Rodríguez-Ramos Synthesis and applications of graphene.............................................. 161 R. Menéndez, C. Blanco, N.G. Asenjo Porous nanoarchitectures based on clays and other nanoparticles: design, synthesis and applications............................. 175 P. Aranda Gold Nanoparticles as Useful Elements in Functionalized Molecular Platforms............................................................................. 190 A.J. Viudez, R. Madueño, J.M. Sevilla, T. Pineda, M. Blázquez Nanomaterials as disposable transducers for lead and mercury sensors....................................................................................... 201 D. Martín, A. Costa Mechanical, Electrical and Chemical Properties of Metal Nanowires................................................................................................ 213 F. Flores, J. I. Martínez, J. Ortega, A. Zanchet

Petroleum-based carbon materials for CO2 adsorption................... 228 J. Silvestre-Albero, M. Martínez-Escandell, M. Molina-Sabio, F. Rodríguez-Reinoso MATERIALS MODELING Modeling Nanocontacts and nanowires with Molecular Dynamics................................................................................................... 237 M. J. Caturla, G. Chiappe, E. Louis, C. Sabater, E. San Fabián, C. Untiedt Polyatomic Systems.The Quest for Potential Energy Surfaces....... 248 J. Espinosa García Subphthalocyanines: molecular electron structure, aromaticity and NLO properties........................................................... 259 V. R. Ferro, L. A. Poveda, J. M. García de la Vega Determining the lattice energy of molecular crystals by theoretical methods............................................................................... 273 J. C. Sancho-García Molecular electron density as a support of empirical structural chemistry............................................................................. 280 J. Fernández Rico, R. López, I. Ema, G. Ramírez Understanding heterogeneous catalysis at the molecular level: the role of theory...................................................................... 290 N. López, N. Almora-Barrios Vibrational relaxation of peptides in aqueous solution.................. 314 A. Bastida, J. Zúñiga, A. Requena, B. Miguel, M. A. Soler CATALYSIS Mo-containing mixed oxides bronzes as catalysts in oxidation reactions.................................................................................................. 329 J.M. López Nieto, M. Dolores Soriano, F. Ivars Barceló New catalytic applications of cerium oxides...................................... 341 A. García García, M. J. Illán Gómez, A. Bueno López

Carbon Materials as Nanocatalysts Supports.................................... 349 A. Berenguer-Murcia, M.A. Lillo-Ródenas, M. C. Román-Martínez Plasma-assisted preparation of polypyrrole-supported catalysts. Application to nitrate removal in water........................... 366 M. J. García-Fernández, R. Buitrago-Sierra, M. M. Pastor-Blas, A. Sepúlveda-Escribano FUNCTIONAL MATERIALS AND BIOMATERIALS Third-generation bioceramics............................................................... 381 M. Vallet-Regí Ceramic composites containing graphene fillers.............................. 391 M. Isabel Osendi Biomimetic, Multifunctional and Reactive Materials and Devices...................................................................................................... 402 T. F. Otero, J. G. Martínez Lignin as a raw material for the preparation of advanced carbon materials..................................................................................... 413 J. Rodríguez-Mirasol, T. Cordero Electrochemical strategies for tuning the properties of Polyaniline-based conducting polymers.............................................. 429 C. Quijada, E. Morallón, F. Huerta, F. Montilla Last advances in organic distributed feedback lasers..................... 444 M. A. Díaz-García, J. M. Villalvilla, P. G. Boj, J. A. Quintana, E. M. Calzado, V. Navarro-Fuster, M. G. Ramírez, I. Vragovic, M. Morales-Vidal Wettability in gas-pressure infiltration of liquid metals into porous preforms....................................................................................... 455 J.M. Molina, J. Narciso, E. Louis

Multifuntional M/Si3N4 multilayers

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Multifunctional M/Si3N4 (M = metal) multilayers: From nanoparticle magnetism to the design of selective coatings for concentrated solar power systems. E. Céspedes, M. Vila, F. Jiménez-Villacorta, J. Sánchez-Marco, L. Álvarez-Fraga and C. Prieto

Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas. Cantoblanco, 28049 – Madrid (Spain) 1. Introduction Advances in the preparation and characterization of magnetic metal-insulator layered and nanostructured systems have boosted their research mainly focusing in sensor applications and materials for information technologies (Dormann et al. 1997). Control of preparation conditions allows tuning of the structural features of such systems and provides an indispensable tool to fabricate metal/ceramic multilayer or embedded nanoparticles in transparent matrices (Céspedes et al. 2010a). Structural changes and proximity effects in the contact regions at the interface may lead to modifications of their magnetic (JiménezVillacorta et al. 2010) or optical properties (Céspedes et al. 2010b). Also, changes in the structure at the interface or the chemical bonding between metal atoms and the ceramic matrix can lead to the rise up of ferromagnetic phases (Céspedes et al. 2008). Silicon nitride is a promising material to be used as an insulating and transparent matrix in metal/ceramic granular systems and multilayers. It is a good candidate for applications in electronic devices due to its optical and transport properties and its high temperature chemical inertness (Remashan et al. 2007). This chapter summarizes structural and physical properties of thin films prepared by the alternative deposition of silicon nitride and a metal (M = Mn, Fe, Co, Ni, Ag, Au and Mo). For sufficiently large metal thickness, continuous layers forming multilayers are obtained; for lower thicknesses, the film becomes a hybrid material in which nanoparticles are embedded in the Si3N4 matrix. Special attention is paid to correlations between microstructure, composition of nanoparticles and their optical and magnetic properties.

2. Materials, preparation and characterization techniques Independently of the metal nature, all the here studied systems may be formulated as

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[M(tM)/Si3N4(tSiN =3nm)]n films (being t the nominal layer thickness and n the number of repetition). Series have been prepared by sequential magnetron sputtering at room temperature on Si(001) substrates. Typically, tM has ranged from 0.2 to 5 nm. Two planar 2-inch magnetrons, operated by DC- and RF- power supplies were used for metals and Si3N4 deposition, respectively. The set-up was controlled by a home-made computer software that allows preparation with great precision in the deposition parameters. Si3N4 layers were grown from a Si target and pure N2 as reactive gas to obtain non-oxidized silicon nitride (Vila et al. 2003). In these systems, the element composition is determined by Rutherford backscattering spectroscopy (RBS) and the microstructure by X-ray reflectivity (XRR) and transmission electron microscopy (TEM). X-ray absorption spectroscopies (XAS) are used both, to determine the compounds present in the film and also to evaluate its characteristic size of nanoparticles (NPs). Magnetic and optical measurements are essential to characterize these systems. The magnetic properties have been studied by a SQUID magnetometer (MPMS-5T from Quantum Design) and the optical properties are mainly characterized by measuring the optical absorption with a UV-Vis-NIR spectrophotometer in the wavelength range of 190 2600 nm and with a FTIR spectrometer in the 1.5 - 25 µm one.

3. Results and discussion The chapter has been structured in five different sections according to the main scientific area of interest of the investigated M/Si3N4 systems. To summarize the large spectra of applications, selected examples have been included related to (i) Production of thin films with embedded NPs, (ii) Magnetism of NPs, (iii) NPs for magneto-optics, (iv) Production of new phases, and (v) Optical properties of NPs and cermets. 3.1 Production of thin films with embedded nanoparticles. Owing to the chemical properties of gold, Au/Si3N4 multilayers are among the best candidates study the production of NPs embedded in transparent Si3N4. The formation and self-organization of gold NPs have been studied in Au/Si3N4 films prepared by the sequential sputtering deposition of gold and silicon nitride (Céspedes et al. 2010a). Independently of the gold layer thickness, the characterization by extended X-ray absorption fine structure spectroscopy (EXAFS) shows that gold appears in metallic form. Typically, the formation of metallic NPs in a multilayered system is expected when the interface roughness and metallic layer thickness become comparable. To confirm this assumption, a combined Rutherford backscattering spectroscopy (RBS) and X-ray reflectivity (XRR) study has been summarized in Fig. 1. This figure corresponds to Au/Si3N4 multilayers prepared with constant sputtering time to deposit each Si3N4 layer while the sputtering time to grow the Au layer was varied. Despite the amount of gold measured by RBS is expected to be proportional to its sputtering time, a discontinuous behaviour around 2 x 1016 at/cm2 is clearly observed in Fig. 1(a). A qualitative explanation can be given as follows. For large enough amount of gold, Au layers are continuous and the bilayer thickness is the sum of both Au and Si3N4. However, since the most favourable Au nucleation sites lay at the topmost topography features of the

Multifuntional M/Si3N4 multilayers

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Measured bilayer thickness (nm)

preceding Si3N4 layer; if deposition time gets short enough, the formation of NPs becomes straightforward. NPs formation, linked to enhanced roughness, entails important effects in the growth process, boosting shadow effects for subsequent Si3N4 deposition and reducing its compactness (density) what leads to an effective larger bilayer thickness. Additional GISAXS characterization has corroborated this change while providing further details of Au NPs morphology. Although formation of isolated spherical-like Au NPs is found below a critical value (tAu ≤ 2.9 nm), either NPs with in-plane spatial correlation or without (for smallest amounts) can be obtained, as schematized in Fig. 1(a) and shown in Fig. 1(b) & (c). 14

(a)

(b)

continuous Au-layer

(c)

12

Au NPs

10

Percolation regime

8 6

Multilayers

Au NPs systems

4 2 0 0

1

2

3

4 16

5 2

Au-layer density (10 at./cm )

Fig. 1. (a) Bilayer thickness values (averaged XRR-Λ and GISAXS-Λ) vs. the number of Au atoms per square cm2 in each Au-layer measured by RBS. Au continuous layers are obtained for Au-layer atomic densities 2.75 x 1016 at/cm2 (layer thickness, tAu 4.7 nm) leading to Si3N4/Au multilayered samples. For Au-layer atomic density = 1.73 x 1016 at/cm2 (tAu = 2.9 nm), Au NPs with short-range order are obtained while no in-plane correlation is obtained for the smallest Au-layer atomic density, 9.4 x 1015 at/cm2 (tAu = 1.6 nm). (b)&(c) Crosssection and plane-view TEM images showing gold nanoparticles embedded in Si3N4. 3.2 Magnetism of nanoparticles. Analogue to the preparation conditions of Au/Si3N4 multilayers, the magnetic behavior of granular magnetic materials formed by metallic particles (M = Fe, Co or Ni) embedded in a nonmagnetic matrix can be modified, by varying some of its structural features, such as the metal layer thickness or the deposition rate. M/Si3N4 multilayers display a ferromagnetic character for tM values above a critical thickness, of the order of the average layer roughness. Below this critical thickness, granular magnetic behavior emerges. Also, it has been shown that, unlike free clusters, fabrication of these hybrid systems may lead to the formation of a non-magnetic intermediate layer between the metallic clusters and the insulating matrix, with a subsequent reduction of the magnetic moment per atom compared to bulk values (Vila et al. 2005, Jiménez-Villacorta et al. 2008, Jiménez-Villacorta et al. 2011). This intermediate phase formation can affect critically to the magnetic behavior of the granular systems, reducing the magnetic cluster size, with the subsequent development of non-interacting superparamagnetic nanoparticles with reduced magnetization. Structural and magnetic features can be controlled as well by post-deposition processes, such as ion irradiation and temperature annealing. For instance, as-prepared Ni/Si3N4 system produces paramagnetic Ni atoms diluted in the silicon nitride matrix and ultrasmall NPs. Irradiation with He or P atoms of 1.0 and 0.43 MeV energy, respectively, yields the

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formation of single-domain Ni aggregates with increased size and an enhanced magnetic signal (Vila et al. 2005). On the other hand, annealing reduces significantly the presence of nonmagnetic interface regions, with a concomitant increase in magnetization. As a consequence, the magnetic interparticle interactions (exchange, dipolar) between the metallic clusters can be controlled and tailored in these systems solely through the metal content (as observed in Fe and Co), leading to a variety of magnetic behaviors, from superferromagnetism to superspin glass state (Jiménez-Villacorta et al. 2010). Fig. 2 show magnetothermal curves for Fe/Si3N4 systems revealing a variety of magnetic behaviors.

Fig. 2. ZFC-FC curves at different fields of Fe/Si3N4 samples showing four different magnetic behaviors: (a) Ferromagnetic continuous layers (as-deposited tFe=6nm), (b) Superferromagnetic aggregates (annealed tFe=1.3nm), (c) Superspin glass system (annealed tFe=0.7nm), (d) Superparamagnetic nanoparticle system (as-deposited tFe=2.5nm). 1.0

(b)

Magnetization ( B/at.Fe)

(a)

2.5

Optical density

[Au(1.5nm)/Fe(0.5nm)/Au(1.5nm)/Si3N4(3nm)] 2.0

"as-grown" o annealed 500 C 1.5

0.5

T = 4K

"as-grown" "as-grown" o annealed 500 C o annealed 500 C

0.0

T = 300K

-0.5

1.0 200

-1.0 300

400

500

600

700

800

Wavelength (nm)

900

-40

-20

0

20

40

Magnetic Field (kOe)

Fig. 3. Annealing effect in Au-Fe-Si3N4 system. (a) Optical absorption. (b) Hysteresis loops. 3.3 Nanoparticles for magneto-optics. The aforementioned nanostructured morphology shown in very thin gold layers in the Au/Si3N4 system may be used to produce gold coated iron NPs inside the transparent Si3N4 matrix. We have prepared Au/Fe/Au/Si3N4 multilayers, where the deposited amount of gold has become essential for tailoring such NPs because below 1.5 x1016 at./cm2 gold

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appears as NPs that catalyses the iron growth on top of them (Sánchez-Marcos et al. 2013). By varying the iron content per layer on top of gold NPs, magnetic characterization evidences a transition from continuous iron layers to iron NPs with higher saturation magnetization values compared to analogue Fe/Si3N4 systems without gold. However, a more interesting result is the development of combined ferromagnetic and transparent films (with the characteristic gold plasmon resonance) for precise preparation conditions and post-deposition treatments, as shown in Fig. 3, which opens the possibility of using this system for magnetoplasmonic. It is worth mentioning that transparence is possible because of the NP character of both metals as well as of the small concentration of them. Hence, ferromagnetism is obtained by the smallest amount of iron without any appreciable diffusion in the dielectric matrix, preventing from magnetization signal loss and opacity. 3.4 Diluted magnetic semiconductors. For certain metals, such as manganese, the metal – silicon nitride reactivity promotes the stabilization of new promising phases. In the case of Mn/Si3N4, silicon nitride at the interface leads to partial nitridation of manganese. In particular, manganese nitrides, apart from their variety of electronic and magnetic properties (Granville et al. 2005), have generated interest in recent years in the field of diluted magnetic semiconductors (DMS) with high Curie temperature. They have been reported as potentially magnetic secondary phases in Mn-doped III-V semiconductors such as Mn:GaAs, MnInN and Mn:GaN (Rao et al. 2002, Marques et al. 2005). Also, calculations suggest that they can be used for new spintronics devices, as they can provide spin-polarized carriers in the wide-gap nitride systems (Marques et al. 2006). In the Mn/Si3N4 system, for thick enough Mn layers (tMn > 3.4 nm) major contributions of metallic Mn and Mn2+ have been found by XPS and XAS, while mostly Mn2+ appears for thinner ones, evidencing total Mn nitridation for tm 1.5 nm. Coercive fields between 80 and 130 Oe at 5 K and ordering temperature well above 400 K have been inferred by magnetic measurements. Saturation magnetization at room temperature increases with tMn, exhibiting a maximum for Mn layer thickness similar to that of Si3N4 (~0.4 B/at.Mn for tMn ~3.5 nm, Fig. 4a) and decreases for further tMn increase (Céspedes et al. 2009). 5K

= 3.5 nm

300 K

200 Brillouin fits

0

t

300 K Mn

t

Mn

3.4 nm

= 0.14 nm

-200 t

-400

Mn

0.86 nm

Mn

t

t = 0.28 nm

MnN Td Mn d

635

640

645

650

655

5

Mn3N2 Simulations

660

Energy (eV) Mn3N2

5K

-40

-20

0

H (kOe)

20

40

(b)

Mn

= 0.28 nm

XMCD

Mn

t

Normalized Absorption

(a)

3

Magnetization (emu/ cm )

400

-50

-25

0

MnN 25

50

E - E0 (eV)

Fig. 4. (a) Hysteresis loops of two Mn/Si3N4 samples (○) and Brillouin fits (continuous lines). The inset shows a scheme of samples depending on the Mn layer thickness (tMn). (b) Mn Kedge XANES of the tMn = 0.28 nm sample (○) and simulation performed for MnN (blue dotted line) and Mn3N2 (red solid line). Inset shows the RT XMCD spectrum of Mn and the calculations for Mn d5 (Mn2+) in Td symmetry.

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The stabilization of a novel ferromagnetic Mn3N2 phase in the Mn/Si3N4 system was reported for the first time by this group (Céspedes et al. 2008). Investigations confirm the existence of a slightly distorted -Mn3N2 phase at the interface, differing from the related MnN. While -MnN has a tetragonally distorted rocksalt structure with an antiferromagnetic ground state and a considerable amount of N vacancies, the closely related -Mn3N2 phase exhibits an ordered array of N vacancies. XANES computations for -MnN and -Mn3N2 (Fig. 4b) have allowed distinguishing between both similar phases, which show analogous oscillations in the high energy spectral region. Nonetheless, the experimental observation of a pre-edge peak, which is reproduced by ab-initio calculations of -Mn3N2 (Mn in fivefold pyramids and linear twofold coordination with a ratio of 2:1) while it is absent for MnN (Mn Oh environment), confirms the existence of -Mn3N2 phase in the Mn/Si3N4 films and reproduce both -Mn3N2 and Mn contributions for tm > 3.4 nm (see Ref. for details). Furthermore, despite the existence of some minor Mn3+ and Mn4+ contributions, Mn L3,2 edge XAS and XMCD measurements have evidenced that magnetism in these films is originated from the Mn d5 (Mn2+) state. Regarding the AF ground state of -Mn3N2, the origin of the observed ferromagnetism has been ascribed here to the formation of a slightly distorted Mn3N2 phase. Supporting that, the presence of a non-centrosymetric Mn3N2 phase has been verified by EXAFS and XANES simulations at the Mn K-edge. Moreover, the Mn-N distances obtained by EXAFS (2.05 Å) are slightly shorter than the average ones in -Mn3N2 (2.1 Å), indicating a compressed lattice, most likely influenced by the surrounding Si3N4 lattice. Similar types of unit cell changes have been theoretically considered in MnN to obtain transitions from the AFM ground state to a FM phase (Marques et al. 2005, Marques et al. 2006). Nonetheless, these investigations constitute the first experimental observation of RTFM from Mn3N2. Supporting these investigations, recent experiments of Mn3N2 films grown by molecular beam epitaxy onto LaAlO3 and LaSrAlO4 have shown coexistence of two types of -Mn3N2 with different lattice constants due to lattice mismatches, showing RTFM for Mn3N2 onto LaAlO3 (Yu et al. 2013). Furthermore, by using the ab-initio generalized gradient density functional method, the study of -Mn3N2 suggests that a ferromagnetic structure could be stabilized at the (110) surface due to its small surface energy (Kedziorski et al. 2012). 3.5 Optical properties of cermets. In this section a direct application of the M/Si3N4 system in the renewable energy field is reported. Thermal conversion of solar energy is one of the simplest methods to harvest green energy based on direct heating technology. Efficiency here is essentially determined by the solar selective coatings, requiring UV-Vis high solar absorbance (αSol) and low IR thermal emittance ( th). Cermets (composite ceramic and metals blended in a nanometric scale) are among the best solar absorbers. Their optical properties and chemical stability at high temperature are of paramount importance because its optical properties and chemical stability at high temperatures determines the design of concentrated solar power (CSP) plants (Kennedy et al., 2002). Best performance through a simple fabrication procedure has been attained by a four layers stack: (i) a buried infrared reflective metallic layer (IR-mirror), (ii) a high metal volume fraction (HMVF) cermet, (iii) a low metal volume fraction (LMVF) cermet layer, and (iv) an anti-reflective (AR) top layer. Due to the gradual variation of refractive index

Multifuntional M/Si3N4 multilayers

25

S1-reflectivity S2-reflectivity

Solar radiation Black body emission (arb. units)

Reflectivity (%)

80

T = 550 OC

60

2.0 1.5

40

1.0

20

0

2.5

2

100

0.5

T = 450 OC

300

1000

3000

10000

30000

Solar Irradiance (W/m nm)

through the structure, the solar radiation is efficiently absorbed internally and by phase interference between the double cermet and the AR layers. Regarding solar absorbers, a large list based on refractory metals and a variety of oxides, oxynitrides or nitrides, has been reported. Silicon nitride is a promising material here due to its chemical inertness at high temperatures (Jacobson, 1993) and superior mechano-elastic properties when prepared by reactive sputtering from pure silicon (Vila et al. 2003) with respect to common silicon oxynitride. Prompted by that, a novel selective coating based on Mo-Si3N4 has recently been reported (Céspedes et al. 2013), showing great efficiency at high temperature.

0.0

Wavelength (nm)

Fig. 5. Optical reflectivity of two different coatings (“S1” and “S2”) with cermets of slightly different thickness and Mo to Si3N4 ratio. The Solar irradiance received at the Earth surface (AM1.5 spectra) and the black body emission (in arbitrary units) are also given. Optical properties of Mo-Si3N4 cermets and simulations of the whole stack have been investigated to achieve the optimum optical selectivity at high temperatures. A precise experimental control of composition and thickness of the individual component layers by means of XRR and RBS has led to optimized solar absorptivity and thermal emissivity above 450 ºC. Its thermal stability has been tested under moderate vacuum conditions (1 10-2 mbar) to ensure its suitability onto stainless steel tubes for parabolic through concentrators, where vacuum is required to avoid convection heat losses. Fig. 5 shows two examples of stacks (made of Ag as IR mirror, two Mo/Si3N4 multilayers with small Mo thickness as double cermet and Si3N4 as AR layer). Fabrication of a Mo-Si3N4 cermet by alternate Mo/Si3N4 deposition has facilitated tuning the optical characteristics. For instance, “S1” selective stack has been especially designed for a working temperature of 550 ºC. Its reflectivity curve (i. e. low to high R transition wavelength) has been adjusted to enhance its selectivity ( =αSol/ th) at 550 ºC (absorptivity and emissivity parameters are αSol=0.93 and th(550ºC)=0.04, leading to an exceptional (550ºC)=23).

4. Conclusions The opposite behaviors found in Mn/Si3N4 (where MnNx is the main present phase) and in Au/Si3N4 (where no Au dangling bonds are found) can be understood regarding the enthalpy of formation ( H) of the bulk metal nitrides (Jiménez-Villacorta et al. 2011). H is the key parameter in determining the chemical bonds at the metal-ceramic interface and, hence, the microstructure and the NP production in multilayer film systems.

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Recent advances within the field of materials science in Spain

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