A Comparative Study of Si-BaSO4 and Si-CaSO4 Pyrotechnic Time Delay Compositions

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Choice of Fuel and Oxidiser

The choice of fuel(s) and oxidiser(s) significantly affects the activation energy, the heat of reaction and the efficiency of energy feedback. Meyerriecks and Kosanke (2003) illustrated the effect of oxidiser type by reporting the variations in heat of reaction and burn rates observed for boron- and zirconium-fuelled compositions reacted with different oxidisers. Berger (2005) showed the variability in energy output and burn rate when boron, titanium and zirconium are coupled with KClO4. The effect of the choice of reactants on the activation energy can be explained in terms of the amount of energy required for an oxidant to make its oxygen available to react with fuel. These oxidisers may release oxygen to the reducing fuel via lattice destabilisation, melting, sublimation and thermal decomposition (McLain, 1980; Conkling, 1985; Laye and Charsley, 1987). Some oxidisers require the input of a large amount of energy, while others actually produce energy in the process of releasing their oxygen. Potassium nitrate, for example, requires energy for it to decompose, while KClO3 produces energy when it decomposes. Therefore, if a similar fuel is used, the KNO3 composition would have a higher activation energy compared with the KClO3 composition (Shidlovskiy, 1997).
Since burning propagates by re-ignition from layer to layer along the burn path and the efficiency of energy feedback from reacting to unreacted material is influenced by conduction, convection and radiation, the choice of chemicals can affect the efficiency of all three feedback mechanisms. Mixtures prepared from metal fuels have a higher conductivity compared nonmetals and thus they have better conductive feedback (Hill et al., 1950; Khaikin and Merzhanov, 1966; McLain, 1980; Kosanke et al., 2004, Yoganarasimhan and Josyulu, 2014).
Gas-producing fuels and oxidisers increase convective energy transfer, while dark-coloured fuels, such as charcoal, can increase the absorption of radiant thermal energy (Kosanke et al., 2004).

Fuel to Oxidiser Ratio

In any pyrotechnic composition there exists an optimal stoichiometric composition that gives rise to the highest energy output. This often corresponds to the situation where the reaction will be essentially complete, with little fuel or oxidiser remaining after the reaction (Kosanke et al , 2004; Berger, 2005). Deviation from this optimum value leads to reduced energy outputs. In such situations the excess fuel or oxidiser acts as an inert diluent. Although a decrease in energy output is usually accompanied by a decrease in the burn rate, in compositions where metal fuels are used it is not uncommon for the maximum energy output and the highest burn rates to occur at different ratios. This is because metal fuels have a higher thermal conductivity and diffusivity than solid oxidisers (Yen and Wang, 2012). Thus when more metal is added, the efficiency of energy feedback and ultimately the burn rate increase (Ellern, 1968; McLain, 1980; Conkling, 1985). Changes in the fuel to oxidiser ratio can also influence the product spectrum for a given pyrotechnic system. The Si-PbO2 system was used by McLain (1980) to illustrate this effect and the results are presented in Table 2-2.

Degree of Mixing

The effect of mixing plays a very critical role in the burn rate of pyrotechnic compositions. Since these compositions are primarily mixtures of powders, situations can arise where the entire volume of a poorly mixed pyrotechnic composition may have the optimum fuel to oxidiser ratio, but there will be many small regions where the fuel to oxidiser ratio is far from optimum (Kosanke et al., 2004). When burning takes place, the burn rate will be determined by the fuel to oxidiser ratio at each of those small regions rather than by that of the bulk. Poorly mixed compositions will therefore have slower burn rates compared with well-mixed, homogeneous mixtures. Kosanke et al. (2004) demonstrated this using black powder which was processed by different methods, resulting in varying degrees of mixing. One sample was dry mixed by passing it several times through a 60-mesh screen and burned at a rate of 0.2 g s- 1. For another sample the charcoal and sulphur were dry ball-milled for 4 hours, followed by the addition of the potassium nitrate and wet ball-milled for 8 hours. Finally, the sample was dried and crushed to –100 mesh with a mortar and pestle. This sample had a burn rate of approximately 0.5 g s-1. Several methods of improving the homogeneity of pyrotechnics are currently in use. These include ultrasonics, resonant acoustic mixing, arrested reactive mixing (ARM) and spray drying (Umbrajkar et al., 2006; Morgan and Rimmington, 2012; Osorio and Muzzio, 2015).

READ Hetarding Effects Of The Products Of Corrosion

Effect of Particle Size and Surface Area

The influence of particle size on the reactivity of gas–solid or liquid–solid chemical reactions is well documented (Peacock and Richardson, 2012; Levenspiel, 1999). Similarly, this effect applies to solid–solid reactions in general and pyrotechnic reactions in particular (McLain, 1980; Pantoya and Granier, 2005; Piercey and Klapoetke, 2010). In the case of the burn rate, the general trend is that a decrease in particle size of either the fuel or the oxidiser increases the burn rate (Rugunanan, 1992; Kosanke et al., 2004; Ricco et al., 2004; Kalombo et al., 2007). The particle size effect is brought about by a reduction in the effective activation energy of the system since smaller particles require less energy to be heated to the ignition temperature.
Also, as the particle size decreases, the specific surface area increases and the number of contact points between the reactants also increases (Shimizu et al., 1990; Brown et al., 1998; Valliappan and Puszynski, 2003)

CHAPTER 1 INTRODUCTION
1.1 Introduction
1.2 Aims and Objectives
1.3 Outline of Thesis
CHAPTER 2 LITERATURE REVIEW
2.1 Introduction
2.2 Ignition and Propagation of Pyrotechnic Compositions .
2.3 Factors Affecting the Burn Rate
2.4 Reaction Mechanisms
2.5 Theoretical Predictions
2.6 Sensitivity
2.7 Previous Work on Selected Pyrotechnic Compositions
2.8 Silicon-Barium Sulfate (Si-BaSO4) Composition
2.9 Calcium Sulfate as a Candidate Replacement Oxidiser
CHAPTER 3 EXPERIMENTAL
3.1 Calcium Sulfate as a Possible Oxidant in “Green” Silicon-based Pyrotechnic Time Delay Compositions
3.2 A Comparative Study of Si-BaSO4 and Si-CaSO4 Pyrotechnic Time Delay Compositions
3.3 The Effect of Additives on the Burn Rate of the Silicon-Calcium Sulfate Pyrotechnic Delay Compositions
3.4 EKVI Combustion Modelling
CHAPTER 4 RESULTS: CALCIUM SULFATE AS A POSSIBLE OXIDANT IN “GREEN” SILICON-BASED PYROTECHNIC TIME DELAY COMPOSITIONS.
4.1 Characterisation of Reactants .
4.2 Thermal Stability of Reactants
4.3 Experimental and Theoretical Energy Output Measurements
4.4 Pressure–Time Analysis .
4.5 Burn Rates
4.6 XRD Analysis of Reaction Products
4.7 Discussion
CHAPTER 5 RESULTS: A COMPARATIVE STUDY OF Si-BaSO4 AND Si+CaSO4 PYROTECHNIC TIME DELAY COMPOSITIONS
5.1 Characterisation of Reactants
5.2 Thermal Behaviour of Reactants in Nitrogen
5.3 Experimental and Theoretical Energy Output Measurements
5.4 Pressure–Time Analysis
5.5 Burn Rates
5.6 XRD Analysis of Reaction Products
5.7 Simulated Reaction Products
5.8 Sensitivity Testing
5.9 Discussion
CHAPTER 6 RESULTS: THE EFFECT OF ADDITIVES ON THE BURN RATE OF THE SILICON-CALCIUM SULFATE DELAY COMPOSITIONS
6.1 Effect of Fuel Particle Size on the Si-CaSO4 Pyrotechnic Reaction .
6.2 Influence of Additives on the Si + CaSO4 Pyrotechnic Reaction
CHAPTER 7 CONCLUSIONS AND RECOMMENDATIONS .
REFERENCES
PUBLICATIONS
APPENDICES