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Tantalum carbide, as a Ultra-High Temperature Ceramic, is known to be characterized by intrinsic spectral selectivity, which makes it attractive for novel high-temperature thermal solar absorbers. However, a weakness point of TaC is the solar absorbance, which, in absence of any treatment, is usually lower than that of other materials currently under development for solar absorber applications. In the present work we have investigated the effect of surface texturing by femtosecond laser machining on this superhard material. Laser-induced microstructural and optical property changes have been characterized in two samples with different starting roughness and for four laser treatments each, as a function of the accumulated laser fluence. As the interaction with the laser beam takes place and gets stronger, both surface and chemical composition changes appear. Optical properties are heavily affected: solar absorbance considerably increases, at a slight expense of spectral selectivity. [ABSTRACT FROM AUTHOR]

Abstract: In2O3 and SnO2 multilayered semiconducting thin-films have been deposited on Si substrates by reactive pulsed laser ablation (RPLA), with the aim of evaluating their photoconductive and photovoltaic properties. A photo-stimulated electrical study has been performed on the films as a function of oxygen pressure during the deposition process and compared to their microstructure. Temperature-dependent resistivity measurements have been performed to determine the charge carrier transport mechanisms. The experimental demonstration of active photogeneration has been achieved and the influence of deposition parameters on the film structural properties has been correlated to the photovoltaic performance (open-circuit voltage, short-circuit current and output power). [Copyright &y& Elsevier]

Abstract: New solid-state detectors, based on chemical vapour deposited (CVD) polycrystalline diamonds produced by hot-filament (HF) or microwave plasma (MW) assisted deposition methods, were constructed for radiation therapy dosimetry. Properties of diamond crystals, such as high radiation sensitivity, resistance to radiation damage and tissue-equivalence giving a low-energy dependence are very advantageous for clinical dosimetry. Therefore the encapsulation was specially designed for these detectors to have as little influence as possible on the radiation response. The prototypes were irradiated with use of a wide range of photon beam qualities (60Co γ-rays, 6 and 18MV X-rays). The radiation sensitivity varied considerably between samples deposited with HF (9nCGy−1 mm−3) and MW (66 and 144nCGy−1 mm−3) methods. For all detectors the leakage current was of the order of 10% of the radiation-induced current (bias voltage 100V, dose rate 0.3Gy/min). When irradiated with 60Co γ-rays, the detectors showed a dose-rate linearity with an exponential Δ parameter close to unity. However, a difference of 8% was found between Δ values for the different beam qualities. A small energy dependence was observed, for which the most probable sources are interface effects due to the silver electrodes and partly the geometry of the encapsulation which needs to be further optimized. Despite some limitations in the performance of present prototype detectors, with an improved CVD technique producing crystals of better electrical and dosimetric properties, and with a well-designed tissue-equivalent encapsulation, CVD-diamonds could serve as very good dosimeters for radiotherapy. [Copyright &y& Elsevier]

Abstract: We analyzed the feasibility of CVD diamond to operate as the main component in active devices for conversion of high-energy radiation into electrical power. A self-sustained radiation dosimeter based on the electron emission effect was designed and tested under low-energy X-ray beam (Mg X-ray tube). The device operative conditions (absence of applied bias voltage) represented also the first experimental test towards the development of diamond energy conversion systems. On this basis, we designed a CVD diamond vacuum radiation energy converter and analyzed it using electron beams. An analysis of the performance was obtained letting both electron flux I 0 and kinetic energy E 0 to vary. For E 0 =1 keV the power exploited by a load was estimated as tenths of nW and the total conversion efficiency was between 0.2 and 0.4%. This performance makes the device nominally competitive if compared to other similar solid-state converters. A discussion about the device design and possible improvements was performed in order to rationalize the conditions able to maximize the energy conversion efficiency. [Copyright &y& Elsevier]

Abstract: Aiming at the production of image-enhancing devices (electron multipliers) to amplify the backscattered electron signal in scanning electron microscopy, a study of the stationary secondary electron emission from CVD diamond films, irradiated by a high-energy primary electron beam, was performed in order to deepen the understanding of the specific material characteristics that appear to control the electronic emission. An extensive analysis of the secondary electron emission yield δ, as a function of the primary electron beam energy, was performed on polycrystalline CVD samples of different thickness. The emission yield was observed to increase up to a maximum value corresponding to a film thickness of about 15 μm and to decrease at larger thickness values. The experimental results are analyzed according to a theoretical model previously developed [P. Ascarelli, E. Cappelli, F. Pinzari, M.C. Rossi, S. Salvatori, P.G. Merli, A. Migliori, J. Appl. Phys. 89 (2001) 689] and extended within this work. We realized that the most important material parameter controlling the secondary electron emission yield as a function of the primary electron energy is the mean escape depth λ S of the secondary electrons. The obtained λ S values were found to be adequate to explain the film emission performance dependence on its thickness d. Moreover, using the atomic force microscopy technique and developing an algorithm based on a three-points method to identify the mean orientation of crystal facets in space, we observed that the λ S(d) (and consequently the secondary electron emission performance) evolution during the growth process may be largely explained by a variation of the film surface mean orientation and a concomitant variation of the film quality. [Copyright &y& Elsevier]

Abstract: CVD diamond shows interesting perspectives for the production of high-performance radiation detectors and electronic devices. However, due to a polycrystalline structure, the performance of CVD diamond-based devices may be hampered by the low signal-to-noise ratio associated with high level of conductivity. We consider that the level of conductivity correlates with the presence of graphitic impurities within the polycrystalline samples. Assuming that this graphitic phase is concentrated in the free volume of the interfacial crystal grain-boundaries, we show that the graphitic contamination and bulk leakage conductivity can be reduced by increasing the nucleation density. This effect is mainly due to a better filling of the interfacial space by smaller grains induced during the first stage of CVD deposition process. The 60 μm-thick films were structurally characterized, using Raman spectroscopy and X-ray diffraction (XRD), and electrically by the analysis of room temperature (RT) conductivity and charge collection efficiency, extracted from low-energy X-ray irradiation (8.05 keV). [Copyright &y& Elsevier]

Abstract: Diamond dosimeters for intense ionizing radiation were assembled by developing Ohmic contacts on single-crystal high-purity diamond films. Leakage electrical resistivity of the order of 1014 Ωcm was measured. Devices were tested with a low-intensity 17.48keV X-ray beam to probe their maximum sensitivity and the minimum detectable dose-rate. Diamond detectors provided a linearity coefficient to X-ray dose-rate of 1.023±0.07, whose dependence on operative bias voltage is here analyzed. [Copyright &y& Elsevier]

Thin films of nanocrystalline diamond with thickness around 100 nm were deposited on highly doped p-type silicon substrates to evaluate the electron emission performance of these structures under illumination of concentrated sunlight in the temperature range 500–700 °C. By comparing the emitted current densities measured using a pure thermal source and a concentrated light source simulating the solar radiation spectrum (Xe lamp), an increase up to about 80 times at 600 °C was found using the concentrated light source, thus demonstrating the boost on the thermionic emission thanks to the sunlight absorption. At temperatures higher than 600 °C the action of the photon-enhanced thermionic emission (PETE) mechanism begins to vanish, starting the regime of pure thermionic emission. The opening of the quasi-Fermi levels reducing the barrier height down to 0.33 eV for electron emission is considered to explain the overall behavior of the diamond-silicon system in the PETE regime. [ABSTRACT FROM AUTHOR]

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%MoSi 2 . 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. [ABSTRACT FROM AUTHOR]

The DC electronic response of a small area polycrystalline diamond film irradiated by a low energy X-ray beam has been analysed. The relationship between the induced photocurrent and the X-ray flux has been investigated making use of an X-ray absorber method. Aluminium foils have been used to attenuate the direct beam and the metal–diamond–metal device response tested at several electric field values. The X-ray photocurrent shows a linear response of the device that has been observed for a large range of X-ray flux values. [Copyright &y& Elsevier]

Polycrystalline lanthanum hexaboride (LaB 6) discs were sintered by hot pressing of commercial powders and successively polished, in order to investigate their thermionic emission capability. LaB 6 samples presented values of work function in the range 2.55–2.61 eV. Discs with polished surface, independently from the composition and density, demonstrated to provide a Richardson constant around 45 A cm−2 K−2. These results are promising for the use of sintered LaB 6 discs in thermionic energy converters operating at temperatures in the 1400–1700 °C range. Moreover, since the properties of solar absorption and spectral selectivity of LaB 6 make them appealing for the conversion of concentrated solar radiation, a modelling study to estimate the energy conversion efficiency was performed, aimed at identifying the proper engineering strategies able to make the resulting devices competitive for electrical conversion of concentrated solar radiation. [ABSTRACT FROM AUTHOR]

We report on fs laser structuring and graphitization of diamond and experimental characterization of its THz response. A full characterization of graphitized, conductive layer generated by laser irradiation is carried out by performing scanning-electron microscopy, Raman spectroscopy and electrical measurements. The transmittance of the laser textured diamond samples, both with the graphitic overlayer and after selective oxidizing etching, is analyzed in the (0.25 ÷ 6.0) THz spectral range. A significant selective absorption of the graphitized overlayer towards polarized THz radiation is demonstrated, which is associated to the formation of graphitic laser induced periodic surface structures. This anisotropy allows conceiving compact passive metasurfaces based on conductive/dielectric patterns on the diamond plate surface for the development of robust, lightweight and broadband THz optical components. [ABSTRACT FROM AUTHOR]

In the present article we describe calibration measurements of the compact neutron spectrometer at the PTB quasi-monoenergetic neutron reference fields. The spectrometer is based on two diamond detectors enclosing a 6 Li neutron converter. The conversion occurs through the 6 Li(n,t) 4 He reaction, featuring the Q -value of 4.8 MeV. The incident neutron energy is measured through the sum of energies of the two conversion reaction products: t and α. The prototype used in this work was based on two 4 × 4 mm2 100 μ m thick single crystal CVD commercial diamond plates. The measurements were performed in the fission spectrum domain at five different neutron energies varying from 0.3 up to 2 MeV. The obtained data showed a very good neutron energy reconstruction, i.e. in agreement with its reference values within statistical uncertainties or with absolute deviations below 11 keV. The energy resolution of the spectrometer was found to be about 100 keV (RMS) with a small (about 9 keV/MeV) rising trend towards higher energies. The measured detection efficiency was found to be compatible with predictions based on analytical calculations and on Geant4 simulations within 3% statistical and 4% systematic uncertainty. [ABSTRACT FROM AUTHOR]

Double-nanotextured black diamond films with different geometries were fabricated by double-step femtosecond laser treatments at different split ratios of accumulated laser fluence. A “2D-like” pseudo-periodic nanostructure was obtained for the first time when the split ratio was slightly unbalanced in favour of the first step of the treatment, as inferred by scanning electron microscopy. Raman analysis showed that a residual biaxial stress, composed by a superposition of a tensile and a compressive component, is always present after the laser writing process, and that the two components tend to balance each other in the 2D pseudo-periodic case. Spectrophotometric measurements in the 200–2000 nm wavelength range returned outstanding solar absorptance values for all the fabricated films (reaching the unprecedented value of 99.1% in the “2D-like” structure), launching double-nanotextured black diamond as a possible alternative to black silicon as absorbing layer for high-efficiency solar cells. [ABSTRACT FROM AUTHOR]

The thermal oxidation of the β -Si 3 N 4 (0 0 0 1)-8 × 8 surface has been carried out. The spectroscopic characterization has been performed by high resolution core-level and by angle-resolved photoemission spectroscopy. Our findings demonstrate that the surface shows a low but clear reactivity with oxygen. The known resonance ascribed to the 8 × 8 reconstruction results strongly affected by the oxygen dosing, demonstrating the involvement of the topmost nitride atoms. [ABSTRACT FROM AUTHOR]

Thermionic-photovoltaic (TIPV) converters exploit both electrons and photons emitted by a hot cathode to produce electric power. In this work, a TIPV converter structured with a tungsten cathode and an In 0.53 Ga 0.47 As photovoltaic (PV) anode (0.75 eV bandgap) is demonstrated to provide an increased output voltage with respect to the reference thermionic energy converter made of the same materials and operating under similar conditions. The higher voltage, measured to be from 0.3 to 0.5 V in the explored 1150–1450 °C temperature range, is explained by the additional contribution of the PV anode. Another work function engineered TIPV converter, obtained by coating the W cathode with LaB 6 and the In 0.53 Ga 0.47 As PV anode with BaF x , has led to an output power 2 orders of magnitude higher than the uncoated TIPV converter at 1200 °C cathode temperature. This opens the route towards future optimized TIPV electrodes. • Thermionic-photovoltaic (TIPV) devices with In 0.53 Ga 0.47 As anodes were assembled. • TIPV converter showed output voltages up to 0.5 V higher than the thermionic one. • A TIPV converter with low work function coatings (LaB 6 and BaF x) was fabricated. • The coated TIPV energy converter provided an output power 100-fold than the uncoated one. [ABSTRACT FROM AUTHOR]

Abstract: The present paper describes an experimental apparatus suitable to create and study free clusters by combining laser ablation and synchrotron radiation. First tests on sulfur samples, S, showed the production, through laser ablation, of neutral S n clusters (n =1–8). These clusters were ionized using synchrotron radiation at photon energies from 160eV to 175eV, across the S 2p core edge. The feasibility of such combined ablation–synchrotron radiation experiments is demonstrated, opening new possibilities on the investigation of free clusters and radicals. [Copyright &y& Elsevier]

The thermal and electrical performance of the innovative solar thermionic-thermoelectric generator (ST2G) has been carefully analyzed by using a high-flux solar simulator. Specific technological solutions have been integrated with respect to the first prototype for the operational testing at the moderate temperature range of 700–1000 K. The influence of spacers with different thickness has been studied to quantify the reduction of output current due to space-charge effect, whereas a BaF 2 -based coating has been deposited on the anode to make its work function (~2.1 eV) approach the cathode one, on which a hydrogen-terminated nitrogen doped nano-diamond film acts as emitter (work function of ~2.0 eV). The engineered ceramic absorber has demonstrated to be a reliable selective solar absorber with a high solar-to-thermal efficiency of 84% for solar concentration ratio of 200 suns, extrapolated to 83% at 500 suns. As expected, the thermionic energy converter still provides a small converted power. However, the minimum inter-electrode spacing used (50 μm) and the engineered anode allow to better figure out the present limitations in the electrical power generation and how to surpass them in the near future. • The combined solar thermionic-thermoelectric technology was improved and tested. • Engineered ceramic absorbers has a solar-to-thermal efficiency of 84% at 200 suns. • Thermionic conversion stages need to be still improved to result competitive. • Inter-electrode spacing <50 μm and work function engineering are necessary. • Thermoelectric stage allows obtaining thermal-to-electric conversion of 4% at 700 K. [ABSTRACT FROM AUTHOR]

• Thin films based on LaB 6 were produced for novel hybrid thermionic converters. • Deposition assisted by fs laser was used for the first time for films' fabrication. • A detailed physical-chemical study on surface and bulky properties was performed. • Experimental conditions were optimized for enhancing the material's properties. • Optimal work function and emissivity values were achieved for practical applications. Lanthanum hexaboride (LaB 6)-based thin films were successfully synthesized via femtosecond Pulsed Laser Deposition (fs-PLD) at room temperature and at high growth rate (>110 nm/min) for acting as electron and photon emitters in hybrid thermionic-photovoltaic devices applied in thermal-to-electrical energy conversion. The physical and chemical properties of the films were systematically investigated as a function of the most relevant deposition parameters (laser pulse repetition rate, pressure of the reactor, and deposition time) aimed at optimizing the material's microstructure and functional optical and electronic properties. Specific growth conditions allowed for the achievement of thin films consisting of nanocrystalline LaB 6 (10–20 nm grain-size) with enhanced spectral emissivity and low work function of (2.64 ± 0.03) eV, evaluated by thermionic emission measurements up to 1860 K, where a thermionic current density of 1.78 A cm−2 was measured. The obtained results demonstrate that fs-PLD represents a novel and rapid method for preparing efficient, and low-cost thermal electron emitters for innovative hybrid thermionic conversion devices operating at temperatures close to 2000 K. [ABSTRACT FROM AUTHOR]

• Experimental investigation and CFD simulation of a copper plate heat spreader. • The heat spreader cools down a novel solid-state heat-to-power converter. • A mixture of water/ethylene or cryogenic liquid nitrogen (LN) is used as coolant. • A parametric analysis is conducted of various operating conditions of the system. • The cooling system can dissipate heat fluxes up to 600 W·cm−2 with LN as coolant. Hybrid thermionic-photovoltaic (TIPV) converters are efficient and clean solutions for the direct conversion of thermal energy to electricity, taking advantage of both the photovoltaic and thermionic phenomena. An important hurdle for their efficient operation is the overheating of the PV cell integrated within the TIPV anode, due to partial conversion of the emitted electron and photon fluxes to thermal heat. This obstacle needs to be overcome with an efficient, yet practical, cooler. In this work, a copper plate heat spreader is experimentally tested for TIPV cathode temperatures up to 1450 °C, whilst its performance is also assessed using a validated CFD model for temperatures up to ~2000 °C. A multi-parametric analysis is conducted testing two coolants: i) a water/ethylene glycol mixture at various temperatures (−5–40 °C) and mass flow rates (0.05–0.4 kg·s−1), and, ii) cryogenic liquid nitrogen at a temperature of −196 °C and mass flow rate of 0.074 kg·s−1. Numerical results reveal that with water/ethylene mixture the PV can withstand heat fluxes up to 360 W·cm−2, without its temperature exceeding 100 °C. For higher thermal fluxes (360–600 W·cm−2), cryogenic liquid nitrogen is found to prevent the PV overheating and, therefore, is an attractive coolant; however, it poses safety concerns due to its possible boiling. Finally, two additional cooling system designs are proposed, a heat sink with straight fins and another with copper pipes, which offer higher heat transfer areas, but are more difficult to manufacture, than the copper plate heat spreader. [ABSTRACT FROM AUTHOR]