Jagga, C. R.

Yoganarasimhan, S. R.

English

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Indian Journal ot CbemistryVol. 14A, Auaust 1976, pp. 671-674Thermal Decomposition of UV-irradiated Barium AzideS. R. YOGANARASIMHAN* & C. R. JAGGADepartment of Chemistry, Indian Institute of Technology, New Delhi 110029Received 22 May 1974; accepted 10 November 1975The thermal decomposttion of three differently obtained anhydrous samples of barium azideand the effect of short-period pre-irradiation with UV light on their subsequent thermal decorn-posttlon have been studied. Theakinetic data are best fitted into the Avrami-Erofeyeev equationwith n=4. The observed rates of decomposition of samples obtained from hydrates are lowerthan that of the sample obtained anhydrous. The role of deep surface traps and excess cationvacancies which scatter the excitons has been invoked to explain the observed difIerences in therates of decomposition. The effect of pre-irradiation in activating the deep surface traps andaccelerating the rate of growth of small nuclei has been discussed. The effect of radiation induceddefects N; and N. on decomposition has also been discussed.THE effect of pre-irradiation on thermaldecomposition of barium azide has beenextensively studied>+ Garner and Maggs!studied the effect of pre-irradiation with UV lightand noted a shortening of induction period andincrease in the maximum velocity of the reactionwith increasing dose of irradiation. Thomas andTompkins- employed power law for the analysis ofkinetic data.. They reported a 100-folL! increasein the rate of thermal decomposition of Ba(Ns)2pre-irradiated with UV light. They further notedthat pre-irradiation is more effective, beyond acertain critical dose, in shortening the inductionperiod and decreasing the exponent in the powerlaw from 6 to 3. Analysts of the kinetics of thermaldecomposition of unirradiated and Y-irradiatedBa(Ns)z by Prout and Moore", using the Avrami-Erofeyeev equation with 1~= 4 showed that theincreased dose brought about Q. drastic reduction inthe induction period, increase in the velocity constantand steady fall in the inflexion points of the pressure-time plots.So far only the anhydrous Ba(Na)2 obtained fromthe monohydrate has been the subject of such astudy and the effect of pre-irradiation on thedecomposition of the sample prepared as anhydroussis not reported in the literature. It was consideredworthwhile to study the thermal decompositionof such a sample pre-irradiated with UV light.Materials and MethodsBa(Na)2' Ba(Nsh·H20 and Ba(Ns)2.1! H20 wereprepared by the method reported by Torkar et al»,The samples were completely dehydrated by eva-cuation over P20j)' [The samples so obtained arereferred to in the text as Ba(Na)2 I, II and IIIrespectively]. The samples were all uniformly aGedfor about three months and the particle size wasbetween -275 and +325 mesh (Tyler standard).*Present address: Explosives Research & DevelopmentLaboratory. Pashan, Poona 411021.Chemical and spectrographic analysis showed thatthe azide samples were better than 99·97% pure.The percentage of some of the major impurities wereFe, 0·0005; Ca, 0·005; Sr, 0·003; Na, 0·004; and ci-,0·003.For a typical pre-irradiated thermal decompositionrun, the sample (1-1'5 mg) was spread uniformlyat the bottom of the pyrex bucket and irradiated,at 0°, with a low-pressure mercury lamp having astrong emission at 2537A. For each pre-irradiatedrun, the sample was irradiated for the required time,evacuated for 30 rnin to out-gas the accummulated gasand then raised to the decomposition chamber main-tained at a constant temperature. The nitrogenpressures during the subsequent thermal decomposi-tion were recorded as a function of time.The degree of decomposition, (1.., was calculatedfrom the measured nitrogen pressures, consideringthat 75% of the azide decomposed to give nitride".The samples were pre-irradiated to different dosagesand their subsequent thermal decomposition wasstudied at 105°.ResultsUnirradiated barium azide - A knowledge of thedecomposition of the unirradiated salt was necessarybefore studying the effect of UV irradiation. Thepressure-time plots were fitted into power lawequation with different exponents ranging from 3 to6, and also Avrarni-Erofeyeev equations-v,1((1.) = [-In(l-e<:)]l/n = k(t-ti) ... (1)where n = 4, (1. = P IPI and ti is the length of theinduction period, PI being the final pressure theoreti-cally possible after complete decomposition of thematerial taken. The decomposition was followedup to about rJ. = 0·3 and ti has been taken to be thetime corresponding to 1((1.) = 0'1, during which nomeasurable amount of gas is evolved. The resultsshow that the rate of decomposition f0110ws theorder Ba(Ns)2 I > II > III.571iNDiAN J. cH:EM.• vot, 14A. AUGUsT 1976P1'e-irradiated salt - The three samples wereirradiated at constant intensity for different periodsof time (up to 2 min) and then thermally decomposedat the same temperature as the unirradiated samples.The typical pressure-time curves for different dosagesin the case of Ba(N')2 II are shown in Fig. 1. Thekinetic data for all the samples have been plottedusing the A-E equation. The effect of pre-irradia-tion on the rate constant and induction period asa function of dosage in all the three samples is shownin Fig. 2. The significant results are:(i) The values of the rate constant, k, increasewith increase in dosage and eventually reach asaturation value.(ii) The induction period remains practically con-stant in the case of sample I. With the other twosamples, however, there is an initial sharp fall in theinduction period which subsequently approachessaturation with increase in dosage.(iii) The deviation from the A-E equation.manifest as lowering of the inflexionpoints followstheorder III > II > I for the same time of irradiation.o,~,~-------------------------~------------------__~0-240·20TIME (min)Fig. 1 - a.-t plots for thermal decomposition of sample-Ll pre-irradiated for different periods6IIc!o• •-----.-- --------------0~----------~----------~----------~~----------200~~0SO 100 ISOOOSE:--Intensity X Time at irradiation (minutes) ARBITRARY UNITSFig. 2 - Plots of induction period vs dose (broken linea) and k VS dose (continuous lines) for the three samples I. IIand III572YOGANARASIMHAN & JAGGA: tHERMAL BEHAVioUR 0];' UV-IRRADlA'l'El> Ba(NalaDiscussionThe large band gap, about 8·5 eV, of barium azide'"forbids conduction electrons to be involved inthermal decomposition. Consistent with their newexperimental data, Torkar, Spath and Herzog modi-fied an earlier theory proposed by Tompkins andcoworkers- involving the formation of mobile ex;-citons, N~*, by thermal excitation process. Theseexcitons can either return to the ground state orafter collision, be reversibly trapped at somelattice imperfections. The trapped excitons under-go, through thermal energy, electron transfer givingtrapped electron - shared among surrounding cation- and holes bound to the complex. These com-plexes act as germ nuclei. The growth of thesesmall nuclei takes place by electron transfer fromthe adjacent azide ion to the nucleus, giving a Baatom (and two anion vacancies) which is stabilizedby interaction with adjacent 13a2+ ion. The pro-pagation of these electron transfer steps leads toaggregation of small nuclei to the metal speck ofcritical size which acts as an active growth nucleus.N;* +Phr=N;*N;* +T~[TN~*]-+[TeJN;[TeJN;;+Ba++-+[TeBa++]N~-+[TBa+]N;N;+N;-+3NII+2DPh = phonon, T = trap and [] = anion vacancyChart 1.Such a mechanism (Chart 1) along with a growthprocess involving an electron transfer from an ad-jacent Nil ion resulting in a Ba atom and two anionvacancies gives rise to a power Iaw-", a: = kt4. Forsmall values of IX as we have in the present case,namely in the acceleratory period, the power lawis equivalent to the A. E. equation with n = 4.Y oung+' has stressed the importance of amorphousregions of the lattice in the process of nucleusformation. A hydrate crystal when exposed tovacuum over Pa06 is supposed= to give a metastable,partly amorphous, anhydrous phase which under-goes further rearrangement thereby creating cracks,channels and surfaces. Sample HI is more suscep-tible to the process of rupture and rearrangementof lattice because of larger water of hydration. Thesamples obtained from hydrate, therefore, havegreater number of amorphous regions and hencegreater number of small nuclei around a growingnucleus. This has the effect of increasing the dis-order in the lattice by increasing the strain at thereaction interface, with a consequent increase in theentropy of transition state. The experimentallydetermined pre-exponential factors for the samplesI, II and III are 1013, 1019, 1019.6 respectively(unpublished work of Tompkins and Yoganara-simhan}. At least a part of increase in this factorin samples U and III must be due to increase inentropy of activation. It is possible that about1018 nuclei are in normal positions distributed fairlyuniformly in addition to a large number of nucleiin special positions. Because of incoherence ofreaction interface in highly disordered samples IIand III, the growth of satellite nuclei takes placemore slowly in areas surrounding these specialpositions. This manifests as slower rates 01 uecorn-position and longer induction perious 111 samples 11and III.Another factor which is responsible for the slowrate 01 decomposition 01 samples II and III ISprobably the existence 01 ueep surtace traps. Inaddition, several ionic crystals have a negativepotential between the surface and the bulk Que tothe presence ot excess cation vacancies at thesurface. The cation vacancies by virtue ot theirnegative charge can scatter electrons very stronglythereby preventing the electron transter from ex-citons to the BaH ion. The result would be alowering in the rate of nucleation in samplesof barium azide which have high internal surfaces.Effect of pre-irradiation - Interaction 01 UVradiation with the structural singularu ies causedby dehydration and the role 01 rauiation induceddefects are to be visualized to explain the eftect otpre-irradiation on thermal decomposition of thethree samples. The ESR absorption study of an-hydrous barium azide-", UV-irradiated at liquidnitrogen temperature suggests the formation 01 Niand N~ centres. The absorption of the 2537 Aradiation produces excited azide ions. While theexcited azide ions formed in the perfect crystal simplyrevert to the ground state, those which are formedadjacent to the existing imperfections like impurityions, vacancies dislocations, etc., where the selectionrules are relaxed, can decompose as shown in Chart 2.N;' +N;'-+N:i+De+NaNa' +N;*-+N;+De+2NsN3' +Na*-+DeD +e+3NIlChart 2On warming the irradiated Ba(Na)a from lowtemperature to the room temperature, the N;species bleach as:N;;+N:-+N'+/lThe N:t centre is stable at room temperature andhas even survived 45 min at 100° without appreci-able annealing. Its stability will depend uponcrystal structure and the trapping centres. Thecrystal structure of the three samples of anhydrousbari um azide is essentially the same (Y oganara-sirnhan and Tompkins, unpublished results). How-ever, the higher defect cencentration in samples IIand III will give rise to greater number of N~ andF-centres. The rate of N2 formation increases withincreasing dose of irradiation, with subsequentsaturation. The influence, if any, of radiation in-duced defect N~ on the subsequent thermal decom-position seems to be negligible, unless these species,on bleaching, interact with the existing defects.The effect of pre-irradiation on thermal decom-position could be explained on the basis of the vastdifference in the defect concentrations of the threesamples. The structural singularities give rise todiscontinuities in the periodic potential of the perfectlattice. The result would be creation of additionallevels in the band gap. Thus several additionalexciton levels will become available favouring theexcitation processes. Secondly, the traps themselves,573INDIAN J. CHEM., VOL. 14A, AUGUST 1976namely cation impurities, anion vacancies and sur-faces may get activated by interaction with theradiation. The deep traps can now function eftec-ti vely for bringing about the bimolecular decomposi-tion of the excitons. With the increase in trappingefficiency, the rate of subsequent thermal decomposi-tion obviously increases. The very large increase(~450% w.r.t. the rate of decomposition of un-irradiated salt) for a 60-second irradiation periodin the case of sample III compared to about 100%in the case of sample I should obviously be clue tothe large concentration of surface traps which canbe activated in the former sample. A similarargument holds good for the sample II.Induction periods - The induction period isattributed to the slow growth of nuclei. It is thetime taken for the nucleus to grow to the criticalsize for normal growth. If the rate of slow growthis accelerated by some means, the indue ion periodwill be decreased. Pre-irradiation helps the processby converting the embryos, i.e. the small nuclei intoenlarged germ nuclei, by photolytic decompositionw,so that they do not have to grow for such a longtime before they reach critical dimension for normalgrowth in the subsequent thermal decomposition.This effect will be more in samples II and III be-cause of greater population of s{u.all nuclei in them.Consequently, t. falls considerably in these samples574after pre-irradiation with lighter doses. In sample Ithere is no effect of pre-irradiation on slow [<rowth,probably because of scarce concentration of micro-nuclei.References1. GARNER, 'V. E. & MAGGS, J., Proo, r, Soc., London,A 172 (1939), 299.2. THOMAS. J. G. N. & TOMPKINS. F. C.•Proc, r, Soc .• London,A 210 (1951). 111.3. PHOUT. E. G. & MOORE. D. J., Special technical publicationASTM. No. 400, 1966. 45.4. GARNER. W. E. & MOON, c. R., J. chem. Soc., (1933)"1398·5. EP-OFEYEEV. B. V. & SVIRIDOV, V. V., Chem. Abstr., 52(1958). 1741h.6. TORKAR. K .• KRISCHNER. R. & RADL R., Mh, Chem.,97 (1966). 3137. SPATH, H. T. & TORKAR. K. Mh. Chem., 98 (1967).1696. 1712; 1734; 2020.8. AVRAMI. M.• J. chem. Pliy s .• 9 (1941). 177.9. EROFEYEEV. B. V. Acad. sa. USSR. 52 (1946) 511.10. TORKAR. K., SPATH, H. T. & HERZOG. G. W., cited inReactivity of solids, edited by J. W. Mitchell. R. C.Devries. R. 'V. Ro bertn & P. Cannon (\Viley Irrterscience,New York), 1969. 287.11. YOUNG. D. A .• Decomposition of solids (Pergamon Press,London}, 1966. 181.12. GARNER \\T. E. Chemistry of the solid stale (Butterworths,London), 1956. 214.13. MARIKKAS. L. & BARTRAM, R. R .• J. cbem. Phys., 48(1968). 927.14. TOMPKINS. F. C. & YoeNG, D. A. Trans. Faraday soe.,61 (1965). 1470.Ij

Thermal Decomposition of UV-irradiated Barium Azide

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Thermal Decomposition of UV-irradiated Barium Azide

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