Engineered thin films by Pulsed Laser Deposition for
Engineered thin films by Pulsed Laser Deposition for thermionic applications A. 1* Bellucci , M. 1 Girolami , M. 1 Mastellone , S. 2 Orlando , A. 3 Mezzi , S. 3 Kaciulis , and D. M. 1 Trucchi 1 Istituto di Struttura della Materia del Consiglio Nazionale delle Ricerche Sez. Montelibretti – Via Salaria km 29. 300 00015 Monterotondo (RM), Italy 2 Istituto di Struttura della Materia del Consiglio Nazionale delle Ricerche Sez. Tito Scalo – Contrada Santa Loja, 85050 Tito Scalo (Pz), Italy 3 Istituto per lo Studio dei Materiali Nanostrutturati del Consiglio Nazionale delle Ricerche Sez. Montelibretti – Via Salaria km 29. 300 00015 Monterotondo (RM), Italy Email: alessandro. bellucci@ism. cnr. it INTRODUCTION Borides (La- and Ce-based) and nitrides (Al- and C-based) thin-films were grown by ns (Ar. F, 193 nm) Pulsed Laser Deposition. Our aim is the production of thin layers with low work function (<2. 7 e. V), that can act as efficient electron emitters for high temperature (2000 °C) thermionic cathodes. A study of the physical properties of the developed thin-films was carried out by using x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Thermionic (TI) emission measurements were performed to evaluate the capability of emit electrons by thermal stimulation of the thin layers deposited on W substrates. Thin-Film Deposition: Experimental setup PLD set up Laser source parameters: • COMPex Lambda Physik Ar. F laser • = 193 nm, h = 6. 42 e. V • Pulse width = 7 ns • Target-to-sample distance: 50 mm • Repetition rate f = 10 Hz • Fluence ~ 1 - 2 J/cm 2 Operating Pressure (mbar) Pulse energy (m. J) Ar-N 2 flux (sccm) RF Power (W) Thickness (nm) Sample Target Tsubstrate (°C) La-B La. B 6 500 6. 5 e-7 75 - - 5. 60 Ce-B Ce. B 6 500 1. 2 e-6 75 - - 11. 75 Al-N Al. N 500 3. 5 e-6 60 - - 2. 25 C-N C 300 4. 5 e-1 60 32 -16 20 0. 25 Characterization General Aim Development of an efficient thermionic-photovoltaic (TIPV) converter for high-temperature applications (1700 -2000 °C) Development of a specific thin emitting layer, able to: 1) efficiently emit electrons by thermionic emission at 2000 °c XPS investigation: elemental quantification Sample X/Y (%at. ) X Y Oxygen/Y (%at. ) Composition (%at. ) La-B 1. 97 B La 6. 67 Ce-B 2. 98 B Ce 6. 48 Al-N 0. 17 N Al 1. 73 C-N 0. 16 N C 6. 04 La 10. 37 B 20. 51 O 69. 12 Ce 9. 56 B 28. 49 O 61. 95 Al 34. 46 N 5. 96 O 59. 58 C 13. 8 N 2. 28 O 61. 95 UPS measurements: evaluation of the work function by cut-off method @ room temperature La-B • A WORK FUNCTION AS LOW AS POSSIBLE, BUT NOT LOWER THAN THAT OF THE ANODE • A HIGH MELTING POINT TO OPERATE AT 2000 °C Al. N 2) filter in a selective way thermal irradiance emitted by thermionic cathode component for an optimized collection of photons by the underlying photovoltaic (PV) cell. Ce-B 3 CN 4 3. 1 La-B 2. 8 0 A SPECTRALLY SELECTIVE EMISSIVITY MATCHED WITH THE PV CELL 1 2 3 4 Work function (e. V) The primary project specification is obtaining an effective low work function of the emitter (< 2. 7 e. V), which has to be stable at 1700 -2000 °C. Thermionic measurements T = 1000 - 1300 K Emitter Melting point Theoretical candidates (°C) Φ (e. V) 2210 2. 6 La. B 6 2552 2. 5 Ce. B 6 2200 3. 1 -37 Al. N >2300 0. 64 -3. 81 CN Richardson-Dushmann fit: J= ART 2 exp(-Φ/KBT) Sample La-B Ce-B Al-N C-N All the selected materials are usually studied as efficient cathodes in field emission applications, with a particular attention to the effect of nanostructuring on the emission properties. THIS WORK AIMS AT FINDING THE BEST DEPOSITION CONDITIONS FOR MAXIMZING THERMALLYSTIMULATED ELECTRON EMISSION A* (A/cm 2 K 2) 10. 14 9. 63 77. 30 5. 47 e-5 Φ (e. V) 2. 82 2. 96 3. 19 1. 58 J @ 2000 K (A/cm 2) 3. 19 1. 34 2. 83 0. 023 The emission capability remains unaltered after many thermal cycles from 300 to 1300 K Sample Conclusions ✓ Thin films deposited starting from La. B 6 target show the best performances in terms of thermionic emission. ✓ Oxygen has been found in the composition of borides and nitride films. XRD characterization is necessary for investigating the structural properties of these ternary compounds. ✓ Thermionic measurements allowed to extrapolate the value of work function by applying the Richardson-Dushmann’s fit: values comparable to those measured by UPS were found, less than for C-N sample. www. amadeus-project. eu The project AMADEUS has received funds from the European Union’s Horizon 2020 research and innovation program, FET-OPEN action, under grant agreement 737054. The sole responsibility for the content of this publication lies with the authors. It does not necessarily reflect the opinion of the European Union. Neither the REA nor the European Commission are responsible for any use that may be made of the information contained therein.
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