The primary objective of this thesis is the characterization of innovative metal-based energetic additives for hybrid rocket propulsion. With respect to the existent literature, this work extends the investigation of coating and mechanical activation techniques in the frame of metal powders reactivity enhancement, coupling metals and oxidizers in the creation of oxidizer-fuel complexes: ALEX and Ammonium Perchlorate (AP) in nanomaterial-based complexes, spherical Aluminum (μAl) and PTFE in mechanically activated micrometric complexes. The produced powders are characterized and compared according to their relative particle size and specific surface area (observed by means of optical microscopy and laser diffractometry), particle morphology and texture (qualitatively analyzed by optical microscopy), active metal content (Al_0, evaluated by monitoring the hydrogen evolution in water reaction), ignition temperature (T_ign, evaluated by an hot-wire technique) and finally, according to their effects on the regression rate (r_f) of HTPB-based solid fuels (as evaluated at lab-scale level by a 2D radial micro-burner). With respect to the virgin μAl, activated powders present different particle size distributions, specific surface areas, morphologies and textures due to the different PTFE mass fractions which differently contribute to the different phenomena of inter-particle cold-cohesion, fracture and lubrication in the mechanical activation process. Base ALEX powders show an Al_0 of 88%. In the case of coated powders, at the lower investigated AP mass fraction (10%, AP10-ALEX powders), powders Al_0 is 80-82%, stable upon the investigated time (up to 5 months) while at the higher investigated AP mass fraction (30%, AP30-ALEX powders), the complex Al_0 increases in time, indicating a possible partial AP coating detachment. In both the AP-coated powders, no Al_0 reduction due to a coating-nAl interaction was observed, confirming the protective action of Alumina shell on the metal core. Conversely, micron-sized powders show the highest Al_0, stable upon the investigated time (up to 6 months) and ranging from 69% to 97% according to the PTFE inclusions mass fraction (ranging from 0% to 30%). Nano- and coated nanopowders show relatively low T_ign, ranging from 864.9 K to 772.8 K respectively. In this case, the AP action on powders reactivity enhancement is well visible. On the other hand, micrometric powders T_ign ranges between 854.4 K and 1472.2 K according to the PTFE mass fraction. The relative ballistic grading between the envisaged formulations was carried out through combustion tests on HTPB-based solid fuels and loaded with a certain powders percentage, maintaining constant (or halving) the Aluminum molar content. Combustion chamber pressures (p_c) of 1.0 and 1.9 MPa were investigated. Data reduction was performed using a non-intrusive, optical time-resolved technique. The formulations loaded with ALEX-based powders showed the highest percent r_f enhancements at high G_ox. For G_ox = 300 kg/m2s and p_c = 1.0 MPa, ALEX- and AP30-ALEX-loaded fuels yielded percent r_f increases with respect to the baseline (pure HTPB) of 61.8% and 42.7% respectively. However, these fuel formulations also suffer from the highest G_ox sensitivity. In this frame, AP coating reduces the loaded fuel G_ox sensitivity. Conversely, at the same G_ox and p_c, activated powders-loaded formulations showed percent r_f enhancements lower than ALEX-based powders-loaded ones (+7.6% for A-PTFE(1)10-Al(15) and +24.1% for A-PTFE(1)30-Al(15) additives) but also a lower G_ox sensitivity. In this work, the enhanced reactivity of coated and activated complexes (with respect to the base powders) demonstrated to be important in the enhancement of the studied HTPB-based solid fuels ballistic performance.
L’obiettivo primario di questo lavoro è la caratterizzazione di additivi energetici innovativi a base metallica per propulsori ibridi. Rispetto alla letteratura esistente, questo lavoro estende l’indagine riguardo le tecniche di rivestimento e attivazione meccanica per aumentare la reattività delle polveri metalliche, accoppiando metalli e ossidanti in complessi metallo-ossidante: ALEX e Perclorato d’Ammonio (AP) in complessi basati su nanopolveri, Alluminio sferico (μAl) e PTFE in complessi micrometrici attivati meccanicamente. Le polveri prodotte sono caratterizzate e comparate rispetto alla loro relativa granulometria e superficie specifica (osservate mediante microscopia ottica e tecnica di misura laser), morfologia e struttura superficiale (analizzate qualitativamente mediante microscopia ottica), contenuto attivo di Alluminio (Al_0, valutato monitorando l’evoluzione di idrogeno liberato dalla reazione con acqua), temperatura d’ignizione (T_ign, valutata attraverso la tecnica del filo caldo) e infine, in relazione ai loro effetti sul rateo di regressione (r_f) di combustibili a base di HTPB (valutati a livello di laboratorio mediante un micro bruciatore radiale 2D). Le polveri attivate presentano distribuzioni granulometriche, valori di superficie specifica, morfologie e strutture superficiali diversi rispetto al μAl a causa delle diverse frazioni massiche di PTFE, le quali contribuiscono a diversi fenomeni di coesione a freddo, frattura e lubrificazione tra le particelle nel processo di attivazione meccanica. L’ALEX presenta un Al_0 dell’88%, ridotto all’80-82% e stabile nel periodo investigato (fino a 5 mesi) per le polveri ricoperte al 10% in massa (AP10-ALEX). Nel caso di polveri rivestite al 30% in massa (AP30-ALEX), il valore di Al_0 aumenta nel tempo, indicando un possibile parziale distacco del rivestimento. In entrambe le polveri ricoperte, non è stata osservata alcuna riduzione di Al_0 dovuta all’interazione tra il rivestimento e l’Alluminio della particella, segno che il guscio di allumina esercita la sua azione protettiva sul cuore metallico della particella. Viceversa, le polveri attivate mostrano il valore più elevato di Al_0, stabile nel periodo investigato (fino a 6 mesi) e compreso nell’intervallo 69%-97% in relazione alla frazione massica di PTFE (da 0% a 30%). Le polveri ALEX e ricoperte mostrano una T_ign relativamente bassa, compresa tra 864.9 K e 772.8 K rispettivamente. In questo caso, l’azione del rivestimento di AP sull’aumento della reattività delle nano polveri è evidente. Nel caso delle polveri micrometriche, la T_ign è compresa tra 854.4 K e 1472.2 K, dipendente dalla frazione massica di PTFE. La classificazione balistica relativa delle diverse formulazioni è stata effettuata attraverso prove di combustione di combustibili solidi a base di HTPB caricati con una certa percentuale di additivo, mantenendo costante (o dimezzando) il contenuto molare di Alluminio. Pressioni in camera di combustione (p_c) di 1.0 e 1.9 MPa sono state investigate. La riduzione dei dati sperimentali è stata effettuata attraverso una tecnica ottica, analitica e non intrusiva. Per un flusso specifico di ossidante (G_ox) = 300 kg/m2s e p_c = 1.0 MPa, i combustibili caricati con ALEX e AP30-ALEX hanno mostrato i maggiori incrementi percentuali di r_f (rispetto ad HTPB puro), pari a 61.8% e 42.7% rispettivamente. Tuttavia, questi combustibili esibiscono anche un’elevata dipendenza dal G_ox, minore per AP30-ALEX grazie alla presenza dell’AP nella matrice del combustibile. Viceversa, nelle stesse condizioni operative, le formulazioni caricate con le polveri attivate hanno mostrato minori incrementi (+7.6% per A-PTFE(1)10-Al(15), +24.1% per A-PTFE(1)30-Al(15)) ma anche una minore dipendenza dal G_ox. L’aumentata reattività dei complessi creati per rivestimento e attivazione meccanica di polveri (rispetto ai materiali base) si è dimostrata importante nel migliorare le prestazioni balistiche dei combustibili solidi a base di HTPB studiati in questo lavoro.
Innovative aluminum-based oxidizer metal complexes for regression rate enhancement
PENAZZO, SIMONE
2013/2014
Abstract
The primary objective of this thesis is the characterization of innovative metal-based energetic additives for hybrid rocket propulsion. With respect to the existent literature, this work extends the investigation of coating and mechanical activation techniques in the frame of metal powders reactivity enhancement, coupling metals and oxidizers in the creation of oxidizer-fuel complexes: ALEX and Ammonium Perchlorate (AP) in nanomaterial-based complexes, spherical Aluminum (μAl) and PTFE in mechanically activated micrometric complexes. The produced powders are characterized and compared according to their relative particle size and specific surface area (observed by means of optical microscopy and laser diffractometry), particle morphology and texture (qualitatively analyzed by optical microscopy), active metal content (Al_0, evaluated by monitoring the hydrogen evolution in water reaction), ignition temperature (T_ign, evaluated by an hot-wire technique) and finally, according to their effects on the regression rate (r_f) of HTPB-based solid fuels (as evaluated at lab-scale level by a 2D radial micro-burner). With respect to the virgin μAl, activated powders present different particle size distributions, specific surface areas, morphologies and textures due to the different PTFE mass fractions which differently contribute to the different phenomena of inter-particle cold-cohesion, fracture and lubrication in the mechanical activation process. Base ALEX powders show an Al_0 of 88%. In the case of coated powders, at the lower investigated AP mass fraction (10%, AP10-ALEX powders), powders Al_0 is 80-82%, stable upon the investigated time (up to 5 months) while at the higher investigated AP mass fraction (30%, AP30-ALEX powders), the complex Al_0 increases in time, indicating a possible partial AP coating detachment. In both the AP-coated powders, no Al_0 reduction due to a coating-nAl interaction was observed, confirming the protective action of Alumina shell on the metal core. Conversely, micron-sized powders show the highest Al_0, stable upon the investigated time (up to 6 months) and ranging from 69% to 97% according to the PTFE inclusions mass fraction (ranging from 0% to 30%). Nano- and coated nanopowders show relatively low T_ign, ranging from 864.9 K to 772.8 K respectively. In this case, the AP action on powders reactivity enhancement is well visible. On the other hand, micrometric powders T_ign ranges between 854.4 K and 1472.2 K according to the PTFE mass fraction. The relative ballistic grading between the envisaged formulations was carried out through combustion tests on HTPB-based solid fuels and loaded with a certain powders percentage, maintaining constant (or halving) the Aluminum molar content. Combustion chamber pressures (p_c) of 1.0 and 1.9 MPa were investigated. Data reduction was performed using a non-intrusive, optical time-resolved technique. The formulations loaded with ALEX-based powders showed the highest percent r_f enhancements at high G_ox. For G_ox = 300 kg/m2s and p_c = 1.0 MPa, ALEX- and AP30-ALEX-loaded fuels yielded percent r_f increases with respect to the baseline (pure HTPB) of 61.8% and 42.7% respectively. However, these fuel formulations also suffer from the highest G_ox sensitivity. In this frame, AP coating reduces the loaded fuel G_ox sensitivity. Conversely, at the same G_ox and p_c, activated powders-loaded formulations showed percent r_f enhancements lower than ALEX-based powders-loaded ones (+7.6% for A-PTFE(1)10-Al(15) and +24.1% for A-PTFE(1)30-Al(15) additives) but also a lower G_ox sensitivity. In this work, the enhanced reactivity of coated and activated complexes (with respect to the base powders) demonstrated to be important in the enhancement of the studied HTPB-based solid fuels ballistic performance.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/95622