The Thermodynamics and Kinetics of Explosives Thomas M. Jerant Cindy D. Spangler

TNAZ 1,3,3-trinitroazetidine (TNAZ) is a the only widely used melt castable explosive. TNAZ can be molded into the liquid state. TNAZ is being considered as an alternative for trinitrotoluene (TNT) because of power and sensitivity. Thermally stable above its melting point TNAZ has approximately 30% more energy than TNT

The Decomposition of TNAZ Heat

The Method - DSC Differential Scanning Calorimetry Usually DSC analysis employs a temperature gradient that increases as a function of time.

Power Compensated DSC Both the sample and the reference are kept at the same temperature while electrical power used by the heaters is monitored Small samples are used to insure that thermal equilibration will occur fast

Heat Flux DSC A reference and sample pan are connected to an allow constantan (Cu & Ni allow. Constant resistance over a wide range of temperatures). Two chromel wafers are used to establish a thermocouple between itself and the constantan.

Heat Flux DSC Heat ReferenceSample Chromel Constantan Heating Disk Signal Out Red = Thermal Energy Difference; ΔU therm Black = Electrical Energy Difference; ΔU elec Thermocouple

DSC Curve

Determination of Kinetics  Finding the accuracy of activation energy evaluations using the isoconversional method o A single step kinetic equation is usually used for solid state decompositions; given by o Where α is the extent of conversion and f(α) is the reaction model

Determination of Kinetics To find α, it requires differentiation of experimental measurements but produces unacceptably noisy data o So the integral form is used o The integral of the reaction model can have useful approximations used in it in order to lead to simple linear equations and estimation of Arrhenius parameters

Reaction Model Importance The activation energy depends on the choice of reaction model –Model is usually chosen based on statistics; however within a confidence level, models can be statistically equivalent

Statistical Procedure Validation Used five heating rates and nine independent sets of experimental measurements at each rate to determine the activation energies

The Method Applications: –The DSC is used to measure specific heat capacity (100 to 1200°C) and heats of transition as well as to detect the temperature of phase changes and melting points in the range of 20 to 1500 ° C. Specific heat capacity can be used in conjunction with thermal diffusivity to obtain thermal conductivity.

Kinetic Analysis The decomposition kinetics of a heterogeneous solid state reaction α is the extent of reaction. f(α) is the reaction model k(T) is the temperature-dependent rate constant

Kinetic Analysis To determine E α, the function S 2 (E α ), the variance, is minimized. n is the number of experiments j and i are indices for heating programs for two separate experiments J is the Arrhenius integral solved by the trapezoid rule T(t) is the temperature dependence on time

Kinetic Analysis α is broken down into 50 segments for the entire process. The plot of E α vs. Α is shown below for the nonisothermal DSC trace where β = 10 °C min -1 for the closed pan sample of TNAZ A built up dependence of α is established for the entire reaction, Af(α).

Kinetic Analysis Decrease in E α leads to evidence of autocatalysis during the thermal decomposition TNAZ. The final products gained in the experiments are not the thermodynamically stable end products of TNAZ combustion.

Results for TNAZ The E α determined in this study matched E α values for the decomposition of TNAZ. It was then determined that the bond cleaved during the initial decomposition was the N-NO 2 bond based on such values. Bond CleavageBond Dissociation EnergyInitial E α N-NO kJ mol kJ mol -1 C-NO kJ mol -1