LA-7358-MS

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LA-7358-MSInformal ReportC*3REPRODUCTIONCOPYIS-4 REPORT SECTIONThe Ignition and Initiation of Potassium Picrateand Potassium Picrate/Explosives Mixtures:Nonprimary, Hot-wire Detonators .-Cn-iii).- C3L%%LOS ALAMOS SCIENTIFIC LABORATORYPost Office Box 1883Los Alamos, New Mexico 87545

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LA-7358-MSInformal ReportUC-45Issued: August 1978The Ignition and Initiation of Potassium Picrateand Potassium Picrate/Explosives Mixtures:Nonprimary, Hot-Wire DetonatorsRobert H. Dinegar—-r

THE IGNITION AND INITIATION OF POTASSIUM PICRATE ANDPOTASSIUM PICRATWEXPLOSIVESMIXTURES:NONPRIMARY, HOT-WIRE DETONATORSbyRobert H. DinegarARSTRACTPotasshm picrate (KP) and KP blended with the explosives PETN orHMX can be ignited by a hot wire. Maximum gas pressure generation occurs in several milliseconds and is about 0.3 GPa in a volume of 0.3 cm’. Theshock tkom detonating PEI’N initiatas small, brass-confined pressings ofEP to steady detonation velocities of 3.5 to 6.6 km/sin the density range 1.0to 1.6 Mg/m8. KP mixes can be employed as deflagrating donor mixtures intwo types of electric detonators. In one the high-pressure deflagrationdrives a stress wave into an explosive in which the deflagration is transformed into a detonation. In the other the high pressure accelerates a flyingplate, which initiates detonation in an accaptor explosive upon impact.I. INTRODUCTION.Salts of picric acid are crystalline substances thatcan be exploded by heat and shock, Most, however,possess inherent drawbacks to their use. For example: (1) the performance of the heavy metal salts(lead, barium, iron, and nickel) is erratic and unreliable; (2) lithium and sodium picrates formhydrates that decrease their sensitivity to ignition;and (3) ammonium picrate occurs in two forms atroom temperature, the stable yellow and metastablered.lThe potassium salt of picric acid - potassiumpicrate (KP) - does not form a hydrate and normalpreparation gives only one form. It aleo possesses ahigh crystal density as well as low drop-weight impact and spark sensitivities,This report discusses: (1) the hot-wire, lowvolt.age ignition of KP; (2) the shock initiation todetonation of KP; (3) the hot-wire, low-voltage ignition of KP mixed with common secondary explosives; and (4), two schemes in which rapidly burning KP mixes may be used to bring about detonation of explosives.II. IGNITIONOF KPThe test vehicle for the ignition experiments ieshown in Fig. 1. The ignition wire is Nichrome V,80% Ni and 20% Cr. It is soldered between two electrodes 1 mm apart. The wire diameter was aparameter in these experiment and ranged between0.038 and 0.508 mm.The KP used as the ignition charge was obtainedse a powder 99.5% pure. The particles had a permeametrically determined specific surface (f%) ofless than 100 m’/kg. Most of our experiments were

LTo FiringUnitPla8!icKP ProssiNlchrome SCBrldgewkerassPlastlFig. 1.Hot-wire ignition-thresholdassembly.performed using this large-particle powder comminuted by grinding with porcelain balls to an S:value of -250 mYkg. The mass of the charge variedbetween 300 and 525 mg; the charge diameter was7.6 mm; the length, 6.8 mm. The loading densitieswere between 1.0 and 1.6 Mg/m’.The wires were heated rapidly by passing directcurrent through the circuit from an external source.The voltages at which the devicee fired were recorded. The voltagelcurrent relationships in thebridgewires were measured to establish the values ofthe electrical parameters of the system.Table I shows, as functions of wire diameter, thethreshold (minimum) ignition voltage, current,and power required to ignite KP to burning.The data lead to several conclusions: (1) bothcoarse and ground KP have the same threshold ofignition; (2) the threshold is independent of loadingdensity; and (3) the wire diameter determines thevalues of the threshold minima. Since there isprobably a fixed ignition temperature for KP, therough linearity of power versuB the square of thewire diameter simply reflects that the more massivewires require more energy.All loading densities of KP have approximatelythe same ignition threshold, for a given wire size.This does not mean that the ignition results werethe same in all cases. Low-density (1.0 Mg/m’)pressings were only marginally ignited and the reaction was quite weak. The brisance of the ignitedmaterial increased with charge density.The threshold values of Table I are for promptreaction (essentially instantaneous) of KP pressings. Delayed ignitions were also observed. Theywere most common with large wire diameters. b.TABLE ITHRESHOLD IGNITION VOLTAGE,CURRENT, AND POWER OF KPSZ 100 and 240 m2/kgDensities 1.0; 1.4; 1.6 Mg/m’Nichrome ,50821.150.600.100.050.02.

?’III. GAS PRESSUREDEFLAGRATING KPGENERATIONINThe development of the deflagration of ignition KPdepends upon many factors; e.g., the specific surface of the material, the porosity of the pressing, aswell as the degree of confinement of the charge.Most of our pressure measurement were made withlarge-particle KP and KP comminuted to a surfaceper unit mass of 240-250 mz/kg by grinding and at aporosity of slightly less than 0.2. Hot-wire, ignitionthreshold assemblies (without brass/plastic plugand brass coupling ring) loaded with the powderwere cotilned in steel bodies which, in turn, fittedinto a pressure-measuring and recording setup, Theassembly is shown schematically in Fig. 2. Thepressure detector was a Kistler 216 MPa (30-kpsi)quartz transducer. The pressure signal received bythe transducer as a function of time was transmittedto a recording system as a voltage/time output,digitized and plotted as a pressure/time graph usinga previously obtained pressure-transducer calibration.The data of Table II reveal that deflagrating KPgenerates pressure maxima of about 0.3 GPa in thetest device. The values of pressure maxima seem tofunitFig. 2.Gas pressure measurementMaterialKPKPKPKP-6KP-7KP-6(P)/TIME (t) DATA FOR IGNITED KPDensity 1.6 Mg/m”Ignition Voltage 2.5 Vs%(m’/kg)Nichrome 13.42.112.20.20.30.30.20.30.2 100 1oo 1oo240250240tlcnNotes:tic. Total time to KP ignition.k., Total time to maximum pressure.P m., Maximumassembly.be relatively unaffected by a change in particlespeciilc surface or the diameter of the igniting wire.The time to ignition of the KP (tiaJ, as indicated bya pressure rise, is on the order of milliseconds anddoes depend upon the diameter of the hot wire andthe KP specific surface. The same statement can bemade of the times associated with pressure maxima(t J. The differences in time between the t,.n andk,, me small d constant at 0.3 and 0.4 ms. Thefast rise in pressure is shown in Fig. 3, a representative trace.TABLE ’preaaure generated in test device.tmxPmnx

5.5t5.0moxE.t11.1LoI11.s13t1.4!15!1.6CJwrgc Dcn:ny (g)Detonationdensity.o2002503,00TIMEGas pressureversus time.3.504.00ignitedKPThe t gnand t .x values appear to go throughminima at 0.050-mm wire diam. We stronglysuspect that the longer times with larger wires againsimply reflect an increased time of wire heating. Theslight increase in ignition time with the wire havingthe smallest diameter could be associated withpoorer heat transfer to the KP crystals.IV. INITIATIONOF KP TO DETONATIONKP can be initiated to detonation by a strongshock such as emanates from the face of adetonating explosive.Cylindrical pressings with diameter and length 12.7 mm and loading density 1.0 Mg/ma, inMicarta holders (wall thickness 0.64 mm), wereinitiated by the detonation from a 9407 PBX pellet(P 1.6 Mg/m8), Steady state detonation was established in the KP within 1 mm of charge length.The stable velocity was 3.8 km/s.The variation of the detonation velocity withloading density is shown in Fig. 4. In this experiment the pressings were 7.62-mm diam by 5.16 mm4/,versuschurge4,50(ins)Fig. 3.genemtion fromFig. 4.velocity of KPlong and were pressed in brass sleeves 2 mm thick.The KP particles had a specific surface of 240 msfkg.The KP was initiated by the shock from detonatingPETN of three different densities -0.85, 0.90, and0.95 Mg/m*. All three initiated KP to detonationwith little delay. Significant delay times in 1.3Mg/m8 KP begin to occur with PETN densities of0.8 Mg/m”. This sets the minimum PETN detonation velocity for prompt initiation of KP as around 4km/s. Scaling the data of Dobratz,’ this correspondsto a Chapman-Jouget pressure (Pa) of about 3 GPain PETN.The plot of the detonation velocity (D) versusreciprocal of charge radius (r) for two pressingdiameters at two densities is shown in Fig. 5. An extrapolation of these data gives 4.1 km/s and 4.8 km/sfor the infinite-diameter detonation velocities of KPat 1.0 and 1.3 Mg/ms. The calculated values’ are 4.5and 5.3 kmls. This is considered good agreement - itis about as close as is found for PETN, for example.The Eyring reaction zone lengths, “a,” are 0.9 and0.8 mm, calculated from the equation’Dr Dr -aDr r“.The depth of a dent in a metal plate is a roughmeasure of the detonation pressure developed in anexplosive. The average depth of the dent generated

KP mixed with PIY1’Nand with HMX have beenstudied. Table III presente the ignition data collected. A comparison of this table with Table I showsthat the threshold ignition value of voltage, current,and power are essentially the same for the mixes asfor pure KP. This constancy extends over a widerange of mix composition for both secondaryexplosives.5.0s— z sco OmsUy. 1.0 Mg/msDaIsify l.3 MIJAn3Dr-m 4.8 km/a0 0.8 mm4.5 ---”-----Or. 4.1 km/s0 0.9 mm4.0 I0.350.40by detonating KP and detonating PETN as a function of the loading density have been measured using a “witness” plate of 20/24 Dural, 18 mm thick.All pressings were 7.62-mm diam by 5.16 mmlong; the S: values were 225 m2/kg for KP and 320for PETN. Pc for PETN is known. The Pc for KPwas estimated from the following relationships1.01.31.6GENERATIONIN.ng. 5.Detonation velocity of KP versus reciprocal ofcharge radius.Density(Mglm’)VI. GAS PRESSUREIGNITED KP MIXESPAP(GPa)5.511.018.8The values at densities of 1.0 and 1.6 Mg/mS are inreasonable agreement with the calculated ones of5.3 and 15.3 GPa.V. IGNITION OF KP MIXED WITH SECONDARY EXPLOSIVESKP mixed with secondary explosives providesmaterial that combines good features of both components. The presence of KP permits the mixture tobe ignited at low voltage levels with moderate confinement whereas the addition of the secondary explosive provides additional brisance.The pressure-generationcharacteristicsofKP/PETN and KPfHMX mixes were measured inthe same manner as for pure KP. One igniter wirediameter and a single loading density were usedwith various compositions of mixes. Table IV showsthe data collected.The KP/PETN mixes gave tig. and %,X that appear to become smaller with increasing amounts ofPETN. KP/PETN mixes of 10/90 composition gavetime values that are a little over one-half those forpure KP. Pn.x increases significantly as the per centof KP in the mixture is reduced from 90 to 10. P a,for KP/PETN mix of 10/90 composition is 1-1/2times that for pure KP.The results with KP/I-IMX are not so clear-cut.The ignition times show an erratic dependence onpercentage of fine-particle HMX. The pressuresgenerated are about the same as with KP/PETNmixes, higher than with pure KP.VII. USE OF KP AND KP MIXES AS DONORCHARGES IN DETONATORSWe have investigated two ways in whichdeflagrating KP and KP mixes function successfullyas donor charges in detonators. In the first,“deflagration-to-detonation-transition” (DDT) initiation, the deflagrating donor charge is coupleddirectly to a confined secondary explosive charge.This transition charge must be of such type, density, and size that the deflagration will change into adetonation. The second, labeled “flying-plate” (FP)initiation, involves an indirect process. The donor isignited and the resulting pressure is used to shear ametal disk, which moves across an air gap impinging on an acceptor explosive. The acceptor explosive5

TABLE IIITHRESHOLD IGNITION VOLTAGE, CURRENT, AND POWER OF KP MIXESKP(9’oPETNby mass)--1050759010090502510KPHMX(% by 0580740850Current(A)Power1.03.01.44.2.4Densities 1.4 and 1.6 Mg/m’Nichrome LE IVPRESSURE (P)/TIME (t) DATA FOR IGNITED KP/SECONDARY EXPLOSIVE MIXESNichrome V Bridgewire Diameter 0.05 mmIgnition Voltage 2.5 VKP S: 910 m’/kgPETN S: 330 m’kgDensity 1.6 Mg/m’HMX % 910 m2/kgMaterialKP:KP:KP:KP:Mix-1Mix-2Mix-4Mix-3-2KP PETN(% by ix-6Mix-5502510507590Notes:ttim Totaltimeto mix ignition.t maxP m8x Total time to maximum pressure.Maximumpressure generated in tmt device.a.8

must be of such type, density, and physical dimensions that it is shock-initiated to detonation by theimpacting flying plate.A. Figure 6 is a diagram of the device used in DDTinitiation experiments. The igniter wire is NichromeV, 0.05-mm diam and 1 mm long. The donor chargeis 7.6-mm diam and 6.8 mm long. The principalfeature is the small explosive transition charge between the donor and acceptor pressings. The smalldiameter of the transition barrel relative to thedonor charge seems to provide an interaction region,which enhances the transition to detonation.Table V shows data we have collected usingKPfP13TN 10/90 (% by mass) mix as the donorcharge. Both PETN and KPfPETN have been usedextensively at different densities as the transitioncharge. Data from other experiments indicate thatHMX, RDX, and HNAB also can be used. The acceptor pellet was high-density PETN of 7.6-mmdiam and 5.O-mm length. The criterion for achievement of detonation in this pellet was the productionof a dent in a 20/24 Dural “witness” plate (18 mmthick) placed across the face of the acceptor charge.Apparently, there is a maximum density of thetransition charge that will build up to detonation.This is not inconsistent with the picture of the DDTmechanism given by Sulimov, one aspect of which iscombustion products penetrating through pores intounreacted explosive, preheating the material andhelping initiation of a low-velocity detonation.’The abrupt change in charge diameter betweenthe donor and the transition pressing could causethe shearing effect shown by Campbell in 1976to benecessary for the transition from burning to detonation in the high-density propellant FKM.7 In oursystem this latter would be helpful but appears notto be absolutely necessary for the DDT reaction.KP/PETN 10/90 mixes of 1.2 Mg/m3 density gavetwo detonations in four experiments when the transition charge was the same diameter and density asthe donor charge, but with a length of 5 mm. Thefailures that occurred appeared to be caused by incomplete burning of the donor pressings. From thislatter we conclude that the DDT that occurs whenthere is no charge-diameter discontinuity is veryclose to failure conditions. With a small-diametertransition pressing we have experienced no difficulty and infer that there is a reasonable marginover threshold conditions.The length of the KPfPETN 10/90 mix donorpressing required to detonate KP/PEI’N 10/90 mixsmall-diameter transition charges also has been investigated. With low-density (1.2 Mg/ms) pressingsthe donor length cannot be decreased significantlybelow 7 mm without failure to ignite. If the donordenstiy is 1.6 Mg/m’, the pressing length may bedecreased to slightly below 4 mm.KP/HMX 10/90 (% by mass) mix also has beenemployed successfully as a donor charge. The onlyloading density we have found to ignite satisfactorily is 1.6 Mg/m8. Table VI shows the data forthese donors with PETN of four different densitiesas the transition charge. High-density PETN againwas the acceptor or ChafgcSteBrPlasSC Bridgewlr6 ToFiringB. Flying-PlateUnitFig. 6.Deflagration-to-detonation-transitionmwembly: small-diameter transition(DDT)churge.Initiation of Acceptor ChargeThe assembly used in flying-plate initiation experimente is shown in Fig. 7. The donor charge and7

TABLE VEFFECT OF TRANSITIONDETONATIONCHARGE EXPLOSIVE TYPE AND DENSITY ONOF PETN ACCEPTOR PELLET.KP/PETN 10/90% by massKP S: 250 m2/kgPETN S: 330 ins/kgIgnition Voltage 2.5 VDiameter 2.5 mmLength 6.4 PETN”KP/PETN 10/90KP/PETN 10/90KP/PETN 10/90from acetone with water;IPETNaDensity 1.6 Mg/m Transition ChargeDonor Charge1.61.61.61.61.61.61.2Acceptor ChargeTransition ChargeDonor onationDetonation and no detonationDetonationNo detonationDetonationb’CS: 33o‘Ignition Voltage: 40 V.CDent observed in Al slug used in place of acceptor pellet.igniter wire have the same dimensions as in theDDT initiation assembly. In this system analuminum disk is sheared by the pressure generatedin the reacting-donor chamber, forming a “flyingplate”, which travels down the barrel and impactsthe acceptor charge.With large (7.6-mm diam by 5.0 mm long) acceptor pellets, pure KP did not work satisfactorily inthis fixture. Only the thinnest of aluminum diskscould be sheared and these did not initiate any ofthe acceptor explosives that were tried.KP/PETN mixes of 50/50 composition or morePETN at loading densities 1.2 Mg/ms or above, alldrove flying plates that did initiate high-densityPETN pellets. Only limited success was achievedwith KP/HMX mixes impacting large-diameter acceptor pellets. Data obtained using KP/HMX 10/90mix to the flying plate are shown in Table VII.Evidently, at least HMX and HNAB acceptors - atseveral loading densities - can be used to initiatelarge (7.6-mm by 5.O-mm) high-density 9407 PBXbooster pellets.Small-diameter (-l-mm)lead-sheathed, highdensity PETN and aluminum-sheathed, highdensity HNS mild detonating fuse (MDF) explosives also have been initiated to detonation byKP/PETN-driven flying plates. These, in turn,detonate 9407 PBX.KP/HMX 25/75 and 10/90 mixes will bring aboutflying-plate initiation of PETN MDF with smalldiameter igniter wires, high-density donors, andwith quite thin aluminum flyers.

TABLE VIEFFECI’ OF TRANSITION CHARGE DENSITY ON DETONATIONOF PETN ACCEPTOR CHARGEDonor ChargeTransition ChargeKP/HMX 10.90% by massKP S: 220 mg/kgHMX S; 35o m’/kgDonor ChargeDensity(Mg/m8)PETNaDiameter 2.5 mmLength 6.4 N’Density 1.6 r Chargefrom acetoneDetonationbDetonationcDetonationc andno detonationNo detonationwith wate S: 33om’/kg.‘Ignition Voltage 40 V.“Ignition Voltage 2.5 V and 40 V,ACKNOWLEDGMENTSDisk;ying ToFlying-plateFiringPlate”DonorChargee YBridgewireUnitFig.7.initiationBecomesassembly.The author wishes to thank the following persons:G. E. Seay and W. H. Meyers, as well as J. Kirkham(Atomic Weapons Research Establishment, Aldermaston, England) for many helpful discussions andsuggestions; C. Mader for BKW calculations; M.Stammler and T. E. Larson for data on KP; M.Manes and R. DeWitt for the pressure measurement; and M. Mirabal and W. Patterson for skillfulexperimental assistance.

TABLE VIIEFFEC?17OF ACCEPTOR EXPLOSIVE TYPE AND DENSITYON DETONATION OF BOOSTER CHARGEDonor ChargeAcceptor ChargeKP/HMX 10/90% by massDensities 1.4& 1.6 Mg/m”KP S: 220 m’/kgHMX S: 360 m’/kgIgnition Voltage 2.6 VDiameter 2.5 mmLength 6.4 mm,Booster Charge9407PBXDensity 1.6 Mg/ms4Flying PlateFlyer Material: 6061-T6 Al, 0.64 mm thickFlyer Barrel: Diameter 2.6 mmFlyer Barrel: Length 6.4 mmAcceptor ChargeBoosterD BHNAB360360360360 100 100 1oo 1ooREFERENCES1. Las Alamos Scientific Laboratory Group WX-2,unpublished data.2. B. M. Dobratz, Ed., “Properties of Chemical Explosives and Explosive Simulants, ” LawrenceLivermore Laboratory report UCRL-5139 Rev. 1(July 1974).3. C. Mader, Los Alamos Scientific LaboratoryGroup T-14, unpublished data, 1977.4. H. Eyring, R. E. Powell, G. H. Duffey, and R. B.Parlin, “The Stability of Detonation, ” Chem.WV. 45, 69 (1948).10Density0.81.01.21.40.81.01.21.4DDDDDDDD5. M. J. Urizar, Los Alamos Scientific LaboratoryGroup WX-2, unpublished data, 1977.6. A. A. Sulimov, B. S. Ermolaev, A. A. Borisov, A.I. Korotkov, B. A. Khasainov, and V. E.Khrapovsky, “On the Mechanism of Deflagrationto Detonation ‘Ilansition in Gas-Permeable HighExplosive,” in Proc. 6th Symp. (Interruztionol) onDetonation, Coronado, California, 24-27 August1976 (ONR-Dept. Navy, Arlington, Virginia,1977, ACR-221), p. 250.7. W. Campbell, Los Alamos Scientific LaboratoryGroup M-3, unpublished data, 1976,.

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Maximum gas pressure generation oc-curs in several milliseconds and is about 0.3 GPa in a volume of 0.3 cm’. The shock tkom detonating PEI’N initiatas small, brass-confined pressings of EP to steady detonation velocities of 3.5 to 6.6 km/sin the density range 1.0 to 1.6 Mg/m8. K