Analyses of the thermal decomposition of urea nitrate at high temperature

The decomposition of urea nitrate (UN) was studied using adiabatic and non-isothermal calorimetry techniques. Gas species released were identified and quantified in situ using TG-infrared spectroscopy/mass spectrometry and molar proportions of these gases were evaluated. A decomposition mechanism at high temperature was proposed based on the nature and sequences of gaseous species observed combined with literature data on decomposition of intermediate products formed. Non-isothermal decomposition kinetics of urea nitrate were measured using variable heating rates to give activation energies E/kJ mol−1 = 206 and 113 with preexponential factors Ln Z/min−1 = 47 and 21, in a closed and open system, respectively. In these systems the major UN decomposition step is strongly coupled to an endothermic dissociation reaction. Species remaining after this exothermic decomposition showed only minor exothermicity at higher temperatures. This is contrasted with the onset to adiabatic decomposition which occurred ∼30 °C below the apparent melting point (155–156 °C), and where solid (condensed) species are available.

► Decomposition mechanism of urea nitrate at high temperature.
► Gaseous species produced and their molar proportions released by the whole decomposition process.
► Decomposition kinetics and activation energies of the exothermic decomposition.
► We demonstrated that the nature of the heating process used has implications on the decomposition pathway of urea nitrate.