Emission spectroscopy on pyrotechnic mixtures

Pyrotechnic mixtures mostly consist of systems of fuel and oxidizer particles with defined mixture ratios and particle sizes. After ignition by a defined energy input, they react highly exothermic with a high energy release in a combustion process and produce a bright shining reaction zone with light emission from UV till the far MIR. Especially for colorful emission and high temperatures, metals or metal compounds are used. Emission spectroscopy is a well-known experimental measurement technique to examine the reaction zone and burning behavior of such mixtures. By analyzing the resulting emission spectra, valuable information can be obtained: the intermediate and end product occurrence, the particles' surface and gas phase temperature, the emissivity, the radiation of energy, the ClE-color, and the reaction mechanism. In the case of pyrotechnic mixtures, most of this information can be received from the emitted light in the UV NIS wavelength range. In the gas phase of the reaction, the intermediate products, e.g. atoms and diatomic molecules, emit because of a stimulated electronic transition. In the UV NlS wavelength range, they show atomic lines and diatomic molecule band systems. The background emission is usually gray body radiation of the solid particles. Because of the high temperature, the maximum of the gray body function is in the near-infrared wavelength range and therefore, it is still visible in the UV NIS range. A valuable experimental device to investigate pyrotechnic mixtures is a window bomb. At lCT, a color high-speed camera and a fast grating UV NIS spectrometer are used to monitor the reaction process. The window bomb is a chimney type high pressure autoclave, which can be operated with different gases and pressured up to 15 MPa. For the correction of the intensity ratios of atomic lines and diatomic molecule band systems and to obtain information about the radiated energy, an intensity calibration is necessary. It corrects for different sensitivities of the detector and correlates the measured intensities with the radiance. The calibration was realized using a calibrated light source with known radiation, in our case a Tungsten strip lamp. In addition, the optical setup was adapted to limit the view angle. By analyzing the received UV NIS spectra of the experiments, the chronological occurrence of different species can be reconstructed. The gas phase temperature and the molecules emissivity can be obtained by modeling the spectrum from the atomic lines and the diatomic molecule band systems. In this work, we present how emission spectra are obtained in the UV /VIS range, as well as how the spectra are analyzed. We demonstrate the technique in various examples.