We measure the time to explosion of 12.7 mm diameter spheres of ultra fine TATB and PBX-9502 (95 wt% TATB, 5 wt% Kel-F 800) at 85.0, 92.5, and 98.0 percent of theoretical maximum density (TMD) in confined and unconfined configurations and at several elevated temperatures with the Lawrence Livermore National Laboratory (LLNL) One Dimensional Time to Explosion (ODTX) apparatus. Time to explosion data provide insight into the relative ease of thermal ignition and allow for the calibration of kinetic parameters. The measurements show that PBX-9502 is more thermally stable than ultra fine TATB, that unconfined samples are slightly more thermally stable than confined ones, and that lower density samples are more thermally stable than higher density ones. 'Go/no go' data at the lowest temperatures yield an experimental measurement of the critical temperature, which is the temperature at which an explosive can be heated indefinitely without undergoing self-heating and concomitant rapid and violent decomposition. Critical temperatures ranges for 12.7 mm diameter spheres of 98% TMD ultra fine TATB and PBX-9502 are 213-230 C and 234-239 C, respectively. Experimental data are modeled with ALE3D and kinetic parameters are determined. These kinetic parameters, when coupled with thermal property data, provide good prediction of the time to explosion

Burnham, A

Koerner, J

Maienschein, J

Wemhoff, A.

UNT Digital Library

UCRL-CONF-232590ODTX Measurements andSimulations on Ultra Fine TATBand PBX-9502Jake Koerner, Jon Maienschein, Alan Burnham,Aaron WemhoffJuly 9, 2007North American Thermal Analysis Society 35th AnnualConferenceEast Lansing, MI, United StatesAugust 26, 2007 through August 29, 2007Disclaimer   This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes.  ODTX Measurements and Simulations on Ultra Fine TATB and PBX-9502Jake Koerner*, Jon Maienschein, Alan Burnham, Aaron WemhoffLawrence Livermore National Laboratory7000 East Avenue L-282Livermore, CA  94550koerner2@llnl.govABSTRACTWe measure the time to explosion of 12.7 mm diameter spheres of ultra fine TATB and PBX-9502 (95 wt% TATB, 5 wt% Kel-F 800) at 85.0, 92.5, and 98.0 percent of theoretical maximum density (TMD) in confined and unconfined configurations and at several elevated temperatures with the Lawrence Livermore National Laboratory (LLNL) One Dimensional Time to Explosion (ODTX) apparatus.  Time to explosion data provide insight into the relative ease of thermal ignition and allow for the calibration of kinetic parameters.  The measurements show that PBX-9502 is more thermally stable than ultra fine TATB, that unconfined samples are slightly more thermally stable than confined ones, and that lower density samples are more thermally stable than higher density ones.  “Go/no go” data at the lowest temperatures yield an experimental measurement of the critical temperature, which is the temperature at which an explosive can be heated indefinitely without undergoing self-heating and concomitant rapid and violent decomposition.  Critical temperatures ranges for 12.7 mm diameter spheres of 98% TMD ultra fine TATB and PBX-9502 are 213-230 °C and 234-239 °C, respectively.  Experimental data are modeled with ALE3D and kinetic parameters are determined.  These kinetic parameters, when coupled with thermal property data, provide good prediction of the time to explosion.INTRODUCTIONMeasurement and prediction of the time to explosion as a function of temperature for a finite body of exothermically reacting material is of broad interest for industrial and military applications. We at LLNL are primarily interested in these measurements and predictions on energetic materials, namely, high explosives (HE). Time to explosion measurements have been made on a wide variety of HEs using the LLNL ODTX apparatus.  The ODTX apparatus was first reported in 1976 by Catalano (1).  A new system, which is geometrically identical to the old one, but incorporates new components, modern equipment, and expanded diagnostic capabilities, was deployed in 2001 and is used in this work (2).  EXPERIMENTS AND CALCULATIONSLLNL ODTX ApparatusODTX experiments measure times to explosion and minimum ignition temperatures of high explosives.  These measurements provide insight into the relative ease of thermal ignition and allow for the determination of kinetic parameters.  The experiment involves isothermally heating a 12.7 mmdiameter spherical sample in a 12.7 mm diameter spherical cavity between two aluminum anvils.  The apparatus is pictured in Figure 1 and key components are labeled.Figure 1. LLNL ODTX apparatus with key components labeled.The sample is remotely delivered to the anvil cavity with the sample delivery system.  A cross-sectional view of the anvil cavity is shown in Figure 2.  Figure 2. Cross-sectional view of 12.7 mm diameter spherical anvil cavity.The hydraulic piston drives the top heater and anvil downward towards the bottom heater and anvil.  A copper o-ring, if present, provides approximately 150 MPa of confinement pressure when the two knife-edges on the aluminum anvils compress it.  The top and bottom anvil temperatures are recorded to 0.1 °C by resistance temperature detectors and a microphone measures a sound signal which indicates the time at which an explosion occurs.  The sample is heated by the anvils until it violently reacts.MaterialsThe ultra fine TATB used is 100% ultra fine TATB while the PBX-9502 is 95 wt% dry aminated TATB and 5 wt% Kel-F 800.  Particle sizes for ultra fine TATB and dry aminated TATB are 3.8 and 70 μm, respectively (3).  LLNL lot identification numbers for ultra fine TATB and PBX-9502 are C-295and C-382, respectively.Both materials were uniaxially pressed in a spherical mechanical pressing die at 207 MPa (30,000 psi) and at 105 °C to densities of 85.0, 92.5, and 98.0% TMD.  Ultra fine TATB and PBX-9502 TMDs are 1.940 and 1.943 g/cm3, respectively.Copper O-RingKnife EdgeAl AnvilHeaterSample Delivery SystemAluminum AnvilResistance Temperature DetectorHydraulic PistonHE SampleMicrophoneALE3D CalculationsThe Arbitrary Lagrangian-Eulerian in Three Dimensions (ALE3D) computer code is used in many LLNL applications to predict both the timing and violence of thermal ignition events (4).  ALE3D includes the calculation of chemical reactions, thermal transport, and material movement and deformation.  Most previous applications of ALE3D have used chemical reactions that exhibit Arrheniustemperature dependence and that depend on the concentration of one or more materials to the nth power, where n is an integer.  Recently, an autocatalytic model adapted from Prout and Tompkins was installed into ALE3D, since explosives tend to have autocatalytic mechanisms (5).  The equation statesmn qxkxdtdx )1( --=where÷øöçèæ-=RTEATk exp)(andx = mass fraction of reactant remainingA = frequency factorE = the activation energyR = universal gas constantT = temperaturen, m, q = Prout-Tompkins model parametersThis model has been applied successfully to HMX cookoff (6).  Here, we apply it to ultra fine TATB and PBX-9502 to determine kinetic parameters A and E.RESULTS AND DISCUSSIONODTX Experimental ResultsODTX test results for ultra fine TATB and PBX-9502 are shown in Figures 3 and 4.  Because times to explosion are fairly reproducible, some data overlap.  Raw time to explosion data are included in the Appendix for clarity.  For each experiment, sample density, confinement, and temperature arevaried.Observation of post-shot anvils indicates that the presence of the copper gasket may not completely confine the sample in some cases, especially at higher temperatures.  Post-shot anvil images of different confinement scenarios are shown in Figure 3 to illustrate this.Figure 3.  Post-shot anvil images illustrating a) successful confinement b) possible failed confinement c) no confinementFigure 3.a shows successful confinement.  Part of the copper gasket is seen outside the anvilcavity, and its impression is seen on that anvil’s pair.  When the explosion occurs, rapidly expanding gas pushes the anvils apart and the copper gasket away from the anvil cavity. Once the gas escapes, the anvils, still under pressure, rapidly close.  Figure 3.b shows an intact copper gasket and a channel (1)(2)a b cleading from the anvil cavity across the anvil surface.  Here, it appears as if the copper gasket failed not by rupture, but by the formation of a hole between the anvil surface and the copper gasket.  If the holeforms near the end of the experiment or if it is small in size, decomposition gasses escape very slowly, and confinement exists.  However, if the hole is large or forms near the beginning of the experiment, a larger volume of decomposition gasses may leak, and confinement may not exist. In Figure 3.c, no copper gasket is present during the experiment, and a dark, thin coating of reacted explosive residue is seen on the anvil surface.These three images are characteristic of all ultra fine TATB and PBX-9502 confinement scenarios.  Data tabulated in the Appendix from runs which were confined, but do not show gasket rupture, as in Figure 3 are marked with an asterisk.Figure 3. ODTX data for ultra fine TATBIn general, ultra fine TATB exhibits shorter times to explosion than historic TATB.  Some systematic error is present in the historic TATB data, as historic TATB and ultra fine TATB were tested on two different apparatuses.  Tran showed that the temperature in the center of the cavity of the apparatus used to test historic TATB is 2-3°C lower than that of the new apparatus; however, if the historic data are corrected for this difference, the result is only a 0.01 1/K shift to the right of the historic TATB data.  Also, one might expect a lower differential scanning calorimetry (DSC) exotherm onset temperature for historic TATB to explain the differences in times to explosion.  While no DSC data could be located from historic TATB lots, DSC data on similarly formulated TATB does exist and does not vary significantly from that of ultra fine TATB. Unless the historic TATB lots are significantly different than the ones for which DSC data exists, some other phenomena are causing the reduction in time to explosion of ultra fine TATB.Ultra fine TATB times to explosion increase as temperature, sample density and confinement decrease.  Increases in time to explosion due to reduced density and confinement seem to be more pronounced at lower temperatures.  Figure 4. ODTX data for PBX-9502In general, PBX-9502 exhibits longer explosion times than historic TATB.  The presence of 5 wt% Kel-F 800 binder is one likely contributor.  Tarver suggests that the longer thermal explosion times for plastic bonded explosives with endothermic binders are most likely due to endothermic reactions of the binders with the gaseous decomposition products of the high explosive (7).  Increases in times to explosion due to decreased density and confinement for PBX-9502 are similar to those of ultra fine TATB.Historically, ODTX measurements have been conducted at temperatures high enough to cause the test material to violently react within three hours.  For these experiments, we test at lower temperatures and thereby extend the heating time of the test material to up to 11 days.  The benefits of this test method are two fold.  First, kinetic parameters used in ALE3D computer codes to fit ODTX data can be more accurately calibrated.  Second, we gain insight into the critical temperature, which is the temperature at which an explosive can be heated indefinitely without undergoing self-heating and concomitant rapid and violent decomposition.  The critical temperature ranges were determined to be 213-230 °C and 234-239 °C for 98% TMD ultra fine TATB and 98% TMD PBX-9502, respectively. It is important to note that critical temperature is geometry dependent, and that these values are reported for 12.7 mm diameter spherical samples tested in sealed configurations.ALE3D Modeling ResultsWe reported experimental times to explosion on ultra fine TATB and PBX-9502 pressed tovarious densities and tested in sealed and unsealed configurations.  We calibrate kinetic parameters and compare modeling results to experimental results using the ALE3D computer code.  To calculate times to explosion in ALE3D, thermal conductivities and heat capacities of the materials are needed.  Thermal conductivities were calculated using an ALE3D thermal conductivity estimator tool and heat capacities were calculated by mass averaging material components. The values are shown in Table 1 (8).Table 1. Thermal transport properties for ultra fine TATB and PBX-9502 at various densitiesMaterial %TMD Density Heat CapacityThermal ConductivityThermal Diffusivity x 106(g/cm3) (J/kg·K) (W/m·K) (m2/s )98.0 1.9012 1000.1 0.522 0.27592.5 1.7945 1000.5 0.466 0.260ultra fine TATB 85.0 1.6490 1001.1 0.395 0.23998.0 1.9041 1100.5 0.563 0.26992.5 1.7973 1095.3 0.500 0.254PBX-950285.0 1.6516 1088.1 0.412 0.229The ALE3D results for ultra fine TATB and PBX-9502 in sealed configurations are shown in Figures 5 and 6, respectively.Figure 5. ODTX data and ALE3D modeling results for ultra fine TATB in sealed configurationFigure 6. ODTX data and ALE3D modeling results for PBX-9502 in sealed configurationALE3D modeling results for 98% TMD materials fit the experimental data well, as they should.  98% TMD data were used to calibrate the kinetic parameters.  The ALE3D curves for 92.5 and 85.0% TMD samples essentially overlap the 98% TMD curve and do not fit the experimental data as well.  One reason that the reduced density does not seem to affect the time to explosion calculated by ALE3D is because the reduced density is counterbalanced by a lower thermal conductivity.  This results in only a modest change in thermal diffusivity and, consequently, only a modest change in heat transfer into the sample (9).  Also, if TATB kinetics are pressure dependent, the ALE3D code may not be modeling them properly.  Consider that as TATB is heated, it slowly generates decomposition gases.  For 92.5 and 85.0% TMD samples, significant void space exists and may allow for additional decomposition gas generation inside the anvil cavity.  If kinetics are pressure dependent, the additional void space and subsequent reduction in pressure would slow reactions leading to further decomposition and eventual explosion.For unconfined samples, the void space is essentially infinite, so an increase in time to explosion is also expected for this test configuration.Frequency factors, activation energies, Prout-Tompkins parameters, critical temperature calculations, and average fit errors are determined from the ALE3D fits to the 98% TMD ultra fine TATB and PBX-9502 curves and shown in Table 2 below.Table 2.  ALE3D-determined frequency factors, activation energies, Prout-Tompkins parameters, and critical temperaturesA E/R m/n/q Tcritical Average Fit Error(1/s) (K) (d’less) (°C) (%)ultra fine TATB 2.41·109 15389 0.656/1/0.999999 221 6PBX-9502 1.85·1010 15618 1/1/0.999999999 199 9Literature values for E/R for ultra fine TATB and PBX-9502 based on DSC data are reported as24176 and 23214 K, respectively (10). It is important to note that these values are reported for smaller samples at different pressures, in a different test configuration, and using different kinetic analysis techniques, so some variation is expected.SUMMARY AND CONCLUSIONSWe measured the time to explosion of 12.7 mm diameter spheres of PBX-9502 and ultra fine TATB at 85.0, 92.5, and 98.0% TMD in confined and unconfined configurations and at several elevated temperatures with the LLNL ODTX apparatus.  These measurements provided insight into the relative ease of thermal ignition.  In general, thermal stability increases as temperature, density, and confinement decrease.  These experiments also allowed for the determination of kinetic parameters used in the Prout Tompkins equation.  The experiments and modeling techniques described can be applied to a wide variety of energetic materials and will continue to provide valuable insight into their thermal response behavior.ACKNOWLEDGEMENTSWe gratefully acknowledge Sally Weber, Greg Sykora, and Dan Greenwood for their contributions to this experimental effort.This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under contract W-7405-Eng-48.REFERENCES1. E. Catalano, R. McGuire, E.L. Lee, E. Wrenn, D. Omellas, and J. Walton, Sixth International Symposium on Detonation Proceedings, 1995, p. 214, Office of Naval Research, ACR-221, Coronado, CA.2. T.D. Tran, R.L. Simpson, J.L. Maienschein, C. Tarver, Thermal decomposition of trinitrotoluene (TNT) with a new one dimensional time to explosion apparatus, 32nd International Annual Conference of Institute of Chemistry Technology, 2001, UCRL-JC-141597.3. M.D. Hoffman, A.R. Mitchell, S.C DePiero, Characterization of new TATBs, In Preparation for Print, 20074. J.J. Yoh, M.A. McClelland, J.L. Maienschein, J. F. Wardell, C.M. Tarver, Simulating thermal explosion of cyclotrimethylenetrinitramine-based explosives:  model comparison with experiment, J. Appl. Phys., 2005, 97, 083504.5. Burnham A.K., Application of the Sestak-Berggren equation to organic and inorganic materials of practical interest. J. Them. Anal. & Cal., 60, 895-908.6. A.P. Wemhoff, A.K. Burnham, A.L. Nichols, Application of global kinetic models to HMX β-δ transition and cookoff processes, J. Phys. Chem. 2007, 111, 1575-1584.7. C.M. Tarver, J.G. Koerner, Effects of endothermic binders on HMX- and TATB-based plastic bonded explosives, accepted for publication in Journal of Energetic Materials, 2007.8. LLNL Online Explosive Reference Guide, April 2007 Edition, UCRL-WEB-230075 9. F.P. Incropera, D.P. DeWitt, Fundamentals of Heat and Mass Transfer, 5th Edition, 2002, Wiley.10. R.K. Weese, A.K Burnham, H.C. Turner, T.D. Tran, Exploring the physical, chemical and thermal characteristics of a new potentially insensitive high explosive RX-55-AE-5, Journal of Thermal Analysis and Calorimetry, 2007, 89, 465-473APPENDIX:  Raw ODTX DataMaterial TMD Confinement Temperature 1000/T Time(%) (Yes/No) (°C) (1/K) (s)351.9 1.59987 64.147*305.2 1.72906 262.861Yes264.6 1.8596 2469.79*351.9 1.59987 68.419264.4 1.86029 5035.26264.6 1.8596 4833.1285.0No304.9 1.72995 485.613305.3 1.72876 154.963*351.9 1.59987 47.772Yes264.4 1.86029 1515.43*264.75 1.85908 1503.85351.9 1.59987 46.82292.5No305.25 1.72891 152.692304.75 1.7304 140.151*352.15 1.59923 34.399*264.55 1.85977 976.505284.4 1.79356 312.808327.75 1.66417 71.709*246.3 1.92511 3397.5229.4 1.98985 14096.1Yes213.5 2.0549 955390**304.9 1.72995 146.333327.5 1.66486 67.65351.6 1.60064 31.694284.25 1.79404 555.874264.6 1.8596 993.584284.15 1.79437 340.469246.35 1.92493 14173.1Ultra Fine TATB98.0No229.4 1.98985 176079**351.95 1.59974 115.147*305.2 1.72906 880.741264.4 1.86029 12106.6246.3 1.92511 27496.4Yes229.4 1.98985 338659**264.5 1.85995 16464.6352 1.59962 116.717246.3 1.92511 34342.885.0No229.4 1.98985 250009**351.9 1.59987 83.394*304.75 1.7304 570.338*264.5 1.85995 8869.35*246.3 1.92511 20405.7Yes237.7 1.95752 31216.7351.9 1.59987 81.857264.6 1.8596 9773.05246.3 1.92511 27230.192.5No229.4 1.98985 331833**352.15 1.59923 77.95*327.5 1.66486 158.069*305.2 1.72906 484.008*284.3 1.79388 1488.77264.5 1.85995 4382.37246.55 1.92419 10974.1229.4 1.98985 356159**237.7 1.95752 17976Yes233.5 1.97375 23540351.9 1.59987 78.313327.6 1.66459 156.809305.2 1.72906 495.842283.85 1.79533 1866.68264.6 1.8596 4971.16PBX-950298.0No246.3 1.92511 11864.4* possible failed confinement  ** no explosion, represents time at which experiment was aborted

ODTX Measurements and Simulations on Ultra Fine TATB and PBX-9502

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ODTX Measurements and Simulations on Ultra Fine TATB and PBX-9502

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