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Three metals (Ag, Au, and Ti) and a carbide (WC) were studied to verify their suitability to act as high‐temperature electric contacts for photon‐enhanced thermionic emission cathodes based on an AlGaInP/GaInP heterostructure. Transfer length method measurements, performed in the temperature range 25–350°C, showed that the lowest specific contact resistivity values were found for Au (4.0 × 10−4 Ω cm−2 @350°C) and Ti (1.2 × 10−3 Ω cm−2 @350°C). A further selection was made on the basis of film adhesion tests performed after the thermal cycles, in which Ti demonstrated an excellent mechanical robustness. [ABSTRACT FROM AUTHOR]

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]

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]

Abstract The chemical-physical properties of thin lanthanum (oxy)boride films deposited on polycrystalline tantalum substrates by different deposition techniques (Pulsed Laser Deposition - PLD - with either nanosecond or femtosecond laser source, and electron beam evaporation) were compared in order to investigate the effect of chemical composition, crystallinity, surface morphology of the thin films on the thermionic electron emission capability. The emission performance of the films was analyzed in the 700–1600 °C temperature range, allowing the determination of work function values ranging from 2.59 to 2.85 eV and an effective Richardson constant at least one order of magnitude lower than the ideal value for all the films. The highest measured thermionic current density value of 1.68 A/cm2 at 1600 °C was provided by the samples grown by femtosecond PLD, indicating a promising performance for practical application in high-temperature thermal-to-electric energy conversion. Highlights • LaB 6 -based thin films were grown by ns-PLD, fs-PLD, e−-beam evaporation. • (Oxy)boride compounds were formed with all the techniques. • The reduction of oxide species is obtained for fs-PLD. • fs-PLD induces the achievement of the lowest work function (2.59 eV). [ABSTRACT FROM AUTHOR]

Nanocrystalline diamond (NCD) films with and without a diamond buffer layer (BL) have been grown on p-type silicon substrates by microwave plasma enhanced chemical vapor deposition technique at different values of deposition temperature (652–884 °C). The photo- and thermionic electron emission properties of NCD films have been investigated, illustrated and explained by analyzing the surface morphology and the grain shape determined by atomic force microscopy, the chemical-structural properties by Raman spectroscopy and nanocrystallites size by X-ray diffraction. The NCD films with BL grown at the highest deposition temperature have shown the highest photo- and thermionic emission currents. [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]