Chapter Summary

This chapter studies the exothermic reactions such as oxidation, ignition and combustion of nAl and its nanoalloys, i.e., nAlCu and nAlZn under controlled environments. The particles are oxidised under the heating rates of 2-30 K/min in a simultaneous TGA/DSC system in the atmosphere of air and nitrogen and thermodynamics, kinetics, morphological and structural characteristics of the particles along with the energy transfer and conversion processes at nanometer scale are explored. In the preliminary combustion analysis, the relationship of the combustion time with the diameter of the particles is also discussed. Some unique features of oxidation, ignition and combustion aluminum based nanomaterials are observed, as summarized below:

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 In case of nAl, a three- stage oxidation scenario is observed under all heating rates: the main reaction occurs before the melting of nAl and other two occurs after.

 Early ignition of nAl is observed for heating rates higher than 8 K/min, which is characterized by a sudden temperature runaway, a fast mass gain and a rapid heat release. The ignition occurs before the bulk melting temperature of nAl, which is likely caused by the partial melting of the aluminum core and the phase transition during the rapid self-heating period. The ignition temperature increases approximately with the heating rates.

 XRD shows that there is a co-existence of different polymorphs of alumina after the first stage rapid oxidation, caused by the early ignition, which continues to the 2nd stage, and all are converted to the stable alpha phase at the end of the oxidation. The early ignition is responsible for the co-existence of different polymorphs of alumina.

 When the particles are heated in the nitrogen atmosphere, no such ignition behaviour is observed.

 Different to the shrink-core model, a phenomenal model is proposed to explain the formation of hollow structure in aluminium nanoparticles, which is associated with the melting, phase transition of the oxides and the nanoscale Kirkendall effect.

 The unique step-wise cavity development in relation to the change of temperature and heating rates shows that after the first oxidation stage at ~600 o_{C, the particles are still solid. The void is generated after the melting process,}

i.e., after 660 oC, and the particles are a combination of solid and hollow structures at 700 o_{C. The size of the void increases with the rise of the}

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temperature. The void development process completes at ~800 oC where all the examined particles are hollow.

 It is also observed that at the higher heating rates when the early ignition phenomenon is observed, the size of the void is larger as compared to those oxidised without ignition, and the oxide shell thickness remains the same regardless of the heating rate.

 The oxidation, ignition and chemical kinetics of the nano alloy (NA) of Al and Cu are investigated from 2 to 30 K/min, up to 1200 oC in the presence of air shows that the complete oxidation scenario of nano Al-Cu alloys can be characterized by two exothermic and two endothermic processes, associated with the melting and different reaction paths.

 A unique early ignition phenomenon was observed for nanoalloys (nAl-Cu) particles at heating rates K/min, which is characterised with sudden change of mass and heat released, and the ignition temperature is identified in the region of 564.7 ± 10.8 oC. The eutectic melting temperature found to be 545.6 oC ± 1.4 o_{C, similar to its bulk value, and played a key role for the early ignition of the}

NA.

 Complete reaction paths up to 1200 o_{C for nano Al-Cu alloy are proposed in} conjunction with the XRD analysis. The weak peaks of copper aluminate spinel CuAl2O4 in the diffractograms at 900 oC suggest that its formation is not thermodynamically favourable due to the slow diffusion of metallic ions. The reduction of the weight at higher temperature ~1175 oC is due to the liberation of oxygen gas during the reduction of CuO to Cu2O.

 The thermal analysis combined with elemental, morphology and crystalline structure analysis of Zn and Al nanoalloys (nAlZn) having a BET equivalent

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diameter of 141 nm elucidates that the complete oxidation of nAlZn in air can be characterised by a three-stage scenario, including two endothermic and three exothermic processes.

 The first stage occurs between room temperature and ~ 400 o_{C, includes two} endothermic reactions, which is due to the phase transformation and the eutectic melting.

 The second stage extends to ~750 o_{C, involving the first and the second} exothermic reactions, where the 2nd peak is associated with the melting of Al component.

 The third stage includes the third exothermic peak until the completion of the reaction at ~ 1050 oC, and is associated with the conversion of the formed alumina to ZnAl2O4.

 The reaction pathways forming intermediate products of ZnO and -Al2O3 are proposed with the help of ex-situ XRD, leading to the end products of ZnO and ZnAl2O4.

 A preliminary investigation of the combustion characteristics of three particles, Al, Fe and Si, having variable dimensions in the methane/ air stream in a Bunsen burner setup shows that most of the particles are burned after passing through the inner flame cone. The chemical compositional analyses (EDS) show no carbon signals for the micrometric particle, which suggests small number of particles being burned during the experiment.

 The comparison of silicon particle with different sizes show that the reactivity of the particle increases with the decrease of the particle size and the

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Chapter 4