The authors would like to express their sincere thanks to DRDO (TBR-1251) for funding and awarding the Project. The authors are expressed their sincere thanks to Sh. C. Sarkar, Sh. N. Mukherjee, Dr. P. K. Soni and Dr. Manjit Singh, Distinguished Scientist/Director TBRL for their motivation, guidance and constant help provided during the execution of this work.
Pyrotechnic compositions have wide range of applications including gas generator, smoke, noise, heat, and colored flame [1–5]. The production of bright light with single wave length is the primary purpose of colored flame compositions [6–8]. Certain elements and compounds when heated to high temperature have the unique property of emitting lines or narrow bands of light in the visible region (380–780 nm) [9–13]. These elements are called the color source, for instance strontium (red), barium (green), copper (green or blue), and sodium (yellow) [14–17]. Strontium, barium, and copper emit color by forming their halides; this emission type is known as molecular emission which is characterized by broad band emission . In the meantime, atomic emission is characterized by sharp discrete wave length [19,20]. The production of a vividly colored flame is a challenging problem which need a delicate balance between different factors including [21,22].
The combustion wave of colored flame was demonstrated to consist of five distinctive zones as demonstrated in Fig. 1[23–25].
The vapors of the atomic or the molecular emitting species are excited by the thermal energy of the secondary luminous combustion zone . The excited levels of atoms, or molecules relaxed to the ground state with the emission of the visible light. Yellow flame presents no color problem considering the very strong atomic emission from excited sodium atoms [15,27]. The wavelength of sodium light actually consists of two wavelengths called D lines (D1 and D2) . The sodium spectrum is dominated by the bright doublet known as the sodium D-lines at 584 ± 2 and 588 ± 1 nm . The transition which gives rise to the doublet is from the 3P to the 3S level [30,31]. The 3P level is split into states with total angular momentum j = 3/2 and j = 1/2 by the magnetic energy of the atp gamma s spin in the presence of the internal magnetic field caused by the orbital motion; this effect is called the spin-orbit effect . The difference in energy for the 3P3/2 and 3P1/2 is 0.0021 eV (Fig. 2).
Magnesium metal fuel is broadly used in many colored flame compositions. In an oxidizing flame, magnesium is converted to magnesium oxide (MgO), which is an excellent white light emitter by incandescence which may lower the color purity [10,33–36]. In yellow flares the emission intensity at D1 and D2 lines increases as the reaction temperature is raised; there is no molecular emitting species to decompose. However, there is an upper limit of temperature that must be avoided for maximum color quality (5000 K) as demonstrated in Fig. 3[14,37,38].
The chromaticity diagram describes colors in terms of rectangular x and y dimensionless coordinates, further details can be found in the following reference . The pure colors are ranged along the upper edge of the diagram, their wavelengths indicated in nm. The colors displayed by sources of blackbody radiation at different temperatures (in kelvin) lie along a line that extends into the center of the diagram [38,40]. As the temperature increases the yellow color fades until 5000 K, above 5000 K the flame becomes white.
Results and discussion
Arabic gum was found to be a novel binder for the development of yellow flares with superior spectral performance; as it has a dual function as a binder and prevent the formation of NaCl in the flame zone. Upon combustion, arabic gum exhibited an effective rule to support the formation of Na atom (the main yellow color emitting specimen). Al fuel was found to eliminate any incandescent emotions from (MgO). Furthermore, Al provides a higher heat of combustion which has a substantial rule to excite the Na vapors. Consequently, yellow colored flame with high intensity and quality was developed. Thanks to the optimum ratio of color source (NaNO3) to novel binder (arabic gum) using Al fuel, this approach secured yellow tracer with superior spectral performance. It exhibited an increases the luminous intensity by 287. The yellow band spectrum (575–600 nm) was enhanced by 170% to the standard reference flare. Furthermore, yellow flare formulations based on aluminum fuel can exhibit extended service life without loss of reactivity in virtue of the protective oxide layer on aluminum.