Oxidation and ignition study

The oxidation kinetics, the ignition reactions, the intervening physiochemical reactions occurring at different temperature ranges, physical transitions and the thermal stabilities of the nanoparticles were studied using STA 1500 thermal analyser (Rheometric Scientific, Germany), similar to our previous studies [51, 52]. Simultaneous Thermal Analysis (STA) is an analytical technique used to monitor the behaviour of the matter

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as a function of temperature. This technique (TGA/DSC) is often used to investigate the thermodynamic properties of the matter, due to its reliability as well as its simplicity. Thermogravimetric analysis (TGA) is an experimental technique where changes of the mass of the sample are recorded with respect to the temperature. The sample is loaded into a pan, made of platinum or alumina, placed in furnace whose temperature is controlled by a computer program. The temperature can be raised from room

temperature to 1500 oC. Microbalance having a resolution of 0.01 g is used to measure the changes of weight. DSC is a thermoanalytical technique used to measure the energy emitted or absorbed by the sample. Endothermic or exothermic event will give rise to a peak in the DSC curve. A small amount of the sample is taken to ensure that within the sample the heat from the furnace to the sample is conducted homogenously without developing any heat barrier. To monitor weight and heat changes of the same process simultaneously, both DSC and TGA techniques are coupled into one set of instrument. Differential thermal analyses (DTA) can also be performed by taking the first derivative of the change of mass and heat, i.e., dm/dT and dh/dT. Various steps of reaction are analysed with the help of DTA. The atmosphere of the furnace is generated by a continuous supply of the gases such as air, N2, CO, CO2, O2 and Ar. The atmosphere may be purged with an inert gas to prevent any reaction or desired gas to allow reaction to take place. The schematic diagram of the apparatus is shown in Figure 2.2.

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Figure 2.2 Block diagram of thermobalance

Thermal oxidation and nitridation experiments were performed at the atmospheric pressure using dry air and nitrogen, respectively, Table 2.1. The changes of mass during the whole oxidation process were monitored with thermo-gravimetric analysis (TGA) and the changes of energy were surveyed with differential scanning calorimetery (DSC). Temperature calibration of the thermo-balance was conducted with the melting points standards of various metals (Zn, Sn, In, Pb etc.), and the weight calibration was done before the start of each experiment. A small amount (~6 mg) of the samples was loaded uniformly in the pan Table 2.1. With such a small quantity of the sample, the tendencies of developing the internal temperature gradient in the pan were reduced. Platinum crucibles were selected to ensure that there were no reactions between the pan and the products of on-going reactions. The pans were cleaned with 10% HNO3 solution after each experiment to ensure that there were no residues left for the next experiment.

Temperature control Sample Pan Ref. Pan Heater Heater Atmosphere control Computer Recording balance

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Table 2.1 Experimental Conditions.

Sample Tiso i t Tf10  Tf mi Q

o_{C Kmin}-1 _Hrs. o_{C Kmin}-1 o_{C mg ml/min}

nAl 400, 450, 500, 550, 720 30 10 525, 630, 665, 925, 1000, 1050, 1175, 500, 600, 700, 800 2-10, 12, 15, 18, 20, 30 1175 6 ± 0.05 20 ± 1 nAlZn 450, 500, 600, 650 30 10 400, 460, 625, 710, 910, 1200 2, 5-7, 10, 12, 15, 20, 30 1200 6 ± 0.1 23 ± 2 nAlCu - 30 10 570, 900, 1100, 1200 2, 5, 7, 10, 12, 15, 20, 30 1200 6.1 ± 0.1 21 ± 2 Note:

Tiso is isothermal temperature.

is heating rate or temperature ramp, iis initial heating rate to attain the required isothermal temperature conditions t is total experimentation time for one set of isothermal temperature conditions

Tf10 is the termination temperature for DSC/TGA test when K/min

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To understand the effects of heating rate and temperature on the oxidation and the thermal stability of the aluminum nanoparticles and its nanoalloys, two sets of the experiments were performed in the thermal analyser. In the first set of experiments, the samples were thermally treated at heating rates of 2-30 K/min from room temperature to 1175-1200 oC. This helped in understanding the effect of heating rate on the oxidation and the ignition process. In the second set of experiments, the samples were heated from the room temperature at a heating rate of 10 K/min and the reactions were terminated at ‘Tf10’, Table 2.1. These temperatures were selected where sudden changes were

observed on the TGA/DSC curves. This helped in revealing the various polymorphic phases of the products developed, the intervening oxidation steps and the path of the ongoing oxidation reaction. The products oxidation were cooled in the inert atmosphere of nitrogen for ex-situ XRD, SEM and TEM analyses. In addition to that, two sets of experiments were performed in the thermobalance to quantify the effects of the heating rate and the temperature on the changing morphology of the particles. In the first set, the particles were oxidized at a constant heating rate of 5 K/min and the temperature of furnace was raised from room temperature to 600, 700 and 800 oC. This set of experiments was helpful in understanding the evolution of the voids in relation to the change of temperature and the physical state of the particles. It also reflects the effect of melting process on the development of the hollow structure. In the second set of experiments, the particles were heated at various heating rates (2, 8, 20 K/min). The products of oxidation were cooled in nitrogen gas and later analysed with TEM and the effect of the heating rate on the size and shape of the particles and the voids was studied. For each set of the experimentations, the parameters (i.e., reaction gas environment, crucible material and mass of the sample) that may affect the TGA/DSC experimental results were kept the same.

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