N.N. Thadhani et al- Shock-Induced Chemical Reactions and Synthesis of Binary Compounds

8/3/2019 N.N. Thadhani et al- Shock-Induced Chemical Reactions and Synthesis of Binary Compounds

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SHOCK- INDUCED CHEMICAL REACTIONS ANDSYNTHESIS OF BINARY COMPOUNDS

N.N. THADHANI, A. ADVANI, I. SONG, E. DUNBAR, AND A. GREBE,CETR, New Mexico Tech, Socorro, NM 87801, U.S.A.,

and

R.A. Graham

Sandia National Laboratories, Albuquerque, NIM 87185, U.S.A.

(PRESENTED AT THE EXPLOMET '90 INTERNATIONAL CONFERENCE ON_HOCK WAVES AND HIGH STRAIN RATE PHENOMENA IN MATERIALS

SAN DIEGO, AUGUST 12-16, 1990)

DISCLAIMER

This reporl was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orproce._is disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, prcx_ess, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the Umted States Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof.

OlSTI_IIILITIONOF Tills DOCUMENTIS UNLIMIT@


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N.N. T_ADHANI, I A. ADVANI, I I. SONG, I E. DUNBAR, I A. GREBE I AND R.A. GRA]IAM 2

ICETR, New Mexico Tech, Socorro, New Mexico 87801, USA.

2Sandia National Laboratories, Albuquerque, New Mexico 87185, USA.

The results of an experimental proEram on shock-induced chemicalreactions and s_nchesis of binary compounds are presented. Binary powdermixture systems char are invesriEated include: (i) intermetallic formin Ecompounds (e.E., Ni-AI, Ni-Si, Nb-Si, err.) associated with larEe neEativeheats of reaction; and (ii) isomorphous (e.E., Ni-Cu) and fully immiscible(e.E., Nb-Cu) systems associated with zero (or positive) heat of reaction.The extent of shock induced chemical reactions and the type of shocksynthesized compounds formed in these systems are observed to be dependenton (i) shock-loadin E conditions, (ii) the relative volumetric distributionof the mixture constituents, and (iii) differences in respective materialproperties which affect relative particle flow.

I. INTRODUCTION AND BACKGROUND

Shock compression of powder mixtures can induce extensive plasticdeformation and fluid-like flow of individual particles, leading toenhancement of reactivity and sufficiently intimate contact between cleansurfaces to permit combustion-type chemical reactions within the durationof shock loading. These features in fact form the basis of a conceptualframework characterizing shock-induced chemical synthesis, as identified byGraham [I]. Shock compression conditions control the degree of plasticflow and mechanical mixing between reactant powders and the extent of


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enhancement of reactivity. Constituent powder properties such asmorphology, density, melting temperature, strength, sound speed, etc. andthe energetics of the reaction (e.g., heat of reaction) also stronglyaffect the synthesis process [2,3]. Thus, the objectives of our overallprogram are" (a) to study shock-induced chemical reactions in variousbinary alloys; (b) establish the mec'hanisms of chemical reactions; and (c)investigate shock synthesis of novel compounds unattainable by otherconventional techniques.

II. EXPERIMENTAL APPROACH AND PROCEDUREThe selection of materials was based on the objective of understanding themechanisms of shock-induced chemical reactions in"(a) intermetallic forming systems associated with a large negative heat

of reaction (HR _ 0), e.g., Cu-Ni and Cu-Nb.Thus, the Ni-Al system was used for establishing the effects of shockconditions, powder particle morphology, and the volumetric distribution ofstarting reactant powders. The Ni-Si and Nb-Si systems were used toinvestigate the effects of physical (melt temperature) and mechanicalproperty (hardness) differences between constituents in systems havingotherwise similar reaction initiation characteristics at ambient pressure.Cu-Ni was selected as a typical isomorphous system, while Cu-Nb wasselected as a typical immiscible system, both associated with zero orpositive heats of reaction. The shock synthesis experiments were conductedusing the CETR/Sawaoka 12-capsule plate impact system as well as the SandiaMomma Bear A (with Comp B explosive) fixtures [2-4]. Two dimensionalradial effects (due to impedance mismatch between the steel holder and theporous compact) dominate the shock loadi_g process in these and similarrecovery fixtures. Numerical simulations conducted on the CETR/Sawaokafixtures [4] show pressure and temperature conditions far in excess ofthose in the Sandia Momma Bear fixture.

III. RESULTS AND DISCUSSIONA. SHOCK SYNTHESIS OF INTERMETALLIC TYPE SYSTEMSNi-Al system" It was shown in our earlier work [3] that the extent ofshock-induced chemical reactions and type of the shock synthesized productsformed are influenced by the morphology of starting powders and the


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intensity of shock loading conditions. Tb _e results show that higher(more in tense)shock conditions and a flaky (irregular) powder morphology,promote plastic deformation and flow, and mixing of dissimilar particles.In such an intimately mixed and activated configuration, and with rapidshock-induced increases in temperature, combustion reactions are initiated

and self-sustained during the high pressure state.In experiments which build upon prior work, mixtures of three

different volumetric distributions of coarse Ni and A1 powders (similar tothose used in earlier work) [3] were shock treated under identicalconditions using the Sandia Momma Bear A Comp B set-up, lt was observed(as shown in the XRD res_?,ts in Figure I) that the mixtures with volumetricratios which yield an Ni3AI and NiAI atomic stoichiometry, undergo bulkchemical reaction. Powders mixed in a volumetric distribution yieldingNi3AI [Figure l(a)] show a greater degree of bulk chemical reaction, incontrast to powders mixed in the NiAI distribution [Figure l(b)] [based oncomparison of peak heights of NiAI (Ii0), Ni (200), and A1 (Ii0)].Practically no reaction is observed in the powders mixed in NiAI 3distribution [Figure l(c)]. Due to the vast density difference between Niand Al, the starting Ni3AI stoichiometry powder mix has almost 67 vol% Niand balance Al, the NiAI stoichiometry powder mix has approximately 40 vol%Ni and balance Al, while the NiAl3-stoichiometry powder mix has only 18% Niand 87% Al. Thus, with the NiAI 3 stoichiometry mixture there is very littleNi available which can participate in the mixing and form an intimatemixture with Al, thereby undergoing chemical reaction.Ni-Si and Nb-Si: Similar to the intermetallic-forming Ni-Al system, arethe Ni-Si and N-b-Si systems, however, the melt temperature of Si is knownto be substantially reduced at higher pressures. The differences inmelting temperature and hardness of the elemental constituents in the N-b-Sisystem are larger than those in the Ni-Si system. However, differentialthermal analysis of unshocked Ni-Si and Nb-Si powder mixtures revealedidentical reaction behavior for both with reaction initiating atapproximately 1250C, which corresponds to the eutectic melting temperatureof a few percent Ni (or Nb) in Si, typical of self-propagating high-temperature combustion reactions. Shock synthesis experiments on Ni-Siand Nb-Si conducted using the 12 capsule CETR/Sawaoka fixture, at an impactvelocity of 0.9 km/s, revealed that complete chemical reaction to NiSi 2 andNi2Si compounds was observed in the Ni-Si powder mixture [Figure 2(a,b,c)],while no bulk chemical reaction was evident in the Nb-Si powder mixture


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[Figure 3(a,b,c)]. In the Nb-Si compact [Figure 3 (a,b,c)] Si powderparticle boundaries were totally absent indicating that most of the Si hadmelted and resolidified. The individual Nb particles appear to becontained in the matrix of molten and resolidified Si, although localized 1#m thick reaction layer around the Nb particles is observed. Theseobservations suggest that the presence of a melt phas_ may not be necessaryfor shock induced chemical reactions. In fact, it is also possible thatmelting of Si may limit particle flow and intimate mixing between Si andthe harder hrb particles which can otherwise be attained while both Nb andSi are in the solid state.

B. SHOCK SYNTHESIS OF ISOMORPHOUS AND IMMISCIBLE SYSTEMS

Cu-Ni: The Cu-Ni system, which has a lens shaped phase diagram that runsfrom the low melting (I083C) Cu end to high melting (1453C> Ni end, is atypical isomorphous-forming system [5]. Shock-compression processing offine (0.3-0.7_m), flaky (0.37 x -44#m) and coarse (-44#m) Ni powders with50 - lO0#m rounded Cu powders was performed for i:I packing stoichiometryusing 65% packed compacts in the CETR/Sawaoka capsules at impact velocitiesof 0.9 and 1.6 km/s. Compacts prepared from fine, flaky and coarse Ni-containing powder mixtures at 0.9 km/s showed etching contrast, as createdby presence of reaction zones, for all morphologies examined. Increasingthe impact velocity to 1.6 km/s resulted in presence of Cu-Ni chemical

reactions in regions close to the center of the non-impact surface (highpeak-pressure regions). The reaction zone in the fine and flakymorphologies appeared to have a solidification structure, and no Cu or Nipowder particles were visible in this region [Figure 4 (a,b)]. _%e coarsespecimen also showed a solidification structure but, in addition, had small(variable) amounts of unmelted Ni particles at isolated locations in themicrostructure (Figure 4c).

SLM analysis of the reaction zone verified presence of a dendriticsolidification structure in the reaction zone (Figure 5). Microchemicalanalysis using SEM-EDS showed that the Cu:Ni ratio in reaction(solidification> zones in fine and flaky samples was approximately I:I, andin the coarse powder morphology sample, it varied from 3:7 to I:I, as shownin Figure 5. XRD analysis confirmed the presence of an additional phase inthe reaction region. As shown in Figure 6, fcc peaks from Cu and Nipowders can be seen along with additional peaks present between theoriginal Cu and Ni peaks. This indicates that the phase formed in the


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reaction region has a lattice parameter in between that of Cu and Ni.Cu-Nb: The Cu-Nb immiscible system [5] has a typical S-shaped soliduscurve, which runs from the low-melting Cu end (at !080C) to the high-melting Nb end (2469C), in the phase diagram. The solid solubility of Nbin Cu is less than 0.I atomic % at I080C, while that of Cu in Nb is amaximum of 1.2 atomic percent. No compounds or intermediate phases occurin this system. Shock synthesis of Cu-Nb samples was carried out in a I:istoichiometry at 65% packing density. Powder sizes used in mixtures wereless than 44 _m, while the impact velocity (for the CETR/Sawaokaexperiments) was approximately 1.9 km/s.

Optical examination of the Cu-Nb compact showed small particles of Cuand Nb in some locations, while other regions showed a splitting of Cu intofiner dendrite-like fragments, as well as a solidification microstructurewith no particles of Cu or Nb at locations particularly near the center ofthe non-impact surface [Figure 7(a),(b)]. SEM-EDS and electron microprobeanalysis (F_MPA) techniques further substantiated reaction/solid-solutionformation in Cu-N'b compacts. A solidification microstructure containingdendrites with variable amounts of Cu and Nb was observed on the non-impactsurface as shown in Figure 8(a). Dendrites analyzed were noted to beprimarily Nb-rich containing as high as 96% Nb versus 4% Cu, while otherdendrites containing significantly higher amounts of copper (up to 60%)were, however, also present [Figure 8(c),(d)]. XRD analysis [Figure9(a),(b)] on the non-impact surface also showed presence of peaks (between35 and 45 ) in addition to those of elemental Nb and Cu, confirming thepresence of phases other than that of Cu and Nb. The intensity of thesereaction product peaks was low, but could be reproduced on subsequent runs,especially for slow scans between 35 and 45 The nature of productphases (solid-solution or intermetallic-type) could not be identified usingthe techniques employed above.

The shock-induced chemical reaction in Cu-Ni and Cu-Nb samplesshocked at 1.6 km/s and 1.9 km/s are produced by melting, mixing andsolidification of the constituent powders. Since there is no heat ofreaction evolved in isomorphous Cu-Ni and immiscible Cu-Nb system, itrequires both Cu and Ni (or Cu and Nb) powders to melt and mix to form auniform solidification region. Impetus for melting and mixing is providedby shock-compression of powders.


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IV. SUMMARY OF SHOCK-INDUCED REACTION MECHANISMS

Shock-induced chemical reactions in two types of binary powdersystems are investigated. Intermetallic forming systems such as Ni-A1, Ni-Si, and Nb-Si, (associated with large negative heats of reaction), undergosolid-state combustion reactions in highly activated and intimately mixedpowders, a configuration produced due to shock-induced plastic deformationand particle flow, mixing of reactants, and temperature increasesassociated with void collapse during shock compression. Critical in suchreactions is the degree of flow and mixing between dissimilar particlespermitted by prevailing shock conditions (.shock pressure and temperature),powder properties (morphology, hardness, etc.) and the volumetricdistribution of reactants. For isomorphous (Cu-Ni) and fully immiscible(Cu-Nb) systems (associated with zero or positive heats of reaction)chemical reactions require complete melting and mixing of powder particlesto form a _niforn solidification volume. The rapid quench rates then makeit possible to rel:ain compounds formed in melt state, thus providing atechnique for ford,ing compounds such as CuNb alloys which are otherwiseimmiscible.

ACKNOWLEDGEMENTS

Funded by NSF Award No. DMR-8713258 and Sandia National LaboratoriesGrant No. 41-5737.

REFERENCES

i. R.A. Graham, "Issues in Shock-induced Solid State Chemistry," inProc. of 3rd Inr. Symposium on High Dynamic Pressures, LaGrandeMotte, France, June 5-9, 175, (1989).

2. R.A. Graham, "Shock Compression of Solids as a Physical-Chemical-Mechanical Process, in S.C. Schmidt and N.C. Holmes, Editors, ShockWaves in Condensed Ma_er - 1987, North Holland, II: II (1988).

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3. N.N. Thadhani, "Shock-induced Chemical Synthesis of IntermetallicCompounds," in Shock Compression of Condensed Matter, eds. S.C.Schmidt, J.N. Johnson, L.W. Davison, North-Holland, 1990, p. 503.

4. F.R. Norwood and R.A. Graham, "Numerical Simulation of a sampleRecovery Fixture for High Velocity Impact," (this volume).

5. T.B. Massalski, "Binary Alloy Phase Diagrams: Volumes I and II,"American Society for Metals, Metals Park, Ohio, 1986.


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12O0 a Ni(111)looo ,,u(2oo) 21 H896AINI(110) 3Ni:AI

800 coarse>-t-.-

I:j;: AINI( 1111400 - _)

AK111) jNIG_2_ Ni_111

t0 -_'-"_- "" J - " L _ _ JJ'L ,',500 b

29H896400 Ni:AIcoarse>-i- 3o0

200

t i j o,,700 C6o0 3 0H8 9650o Ni:3AI

)_ coarse_" 400_ :300Z

20O

10 20 30 40 50 60 70 80 9'(D 1(::)(3TWO THETA

Fig. 1 XRD results of shock synthesized Ni-Al compacrs of coarse powdersmixed in (a) 3Ni'AI, (b) Ni'A1, and (c) Ni:3AI volumetric distribution.


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800 CNt2$.(100)640 "

>" 480 -I--....(nZ NiSi21,1,,I_- (220)z 320 --- NI2SiNIS_2 (200)

_ j.,,J.i,t,,, li ,I. {311) (400) I

O 2,0 40 60 80 100TWO THETA ANGLE

Fig. 2 (a.b) SEM microEraphs and (c) XRD results showin E complete reactionin shock synthesized Ni-Si compacts.


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1300S, c

(1111040 "

>-I-- 780 -03ZI,,,IJI.-..z 52o '-- S, r',J200) Nb (200)

260 (110) Si I Si Nb Si

' i J _.... I ,I ,,i

0 20 40 60 80 100TWO THETA ANGLE

Fig. 3 (a,b) SEM micro&raphs and (c) XRD results showing no bulk reactionin Nb-Si compacts.


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Fig. 4 Optical microEraphs of shock processed Cu-Ni compaccs of (a) fine,(b) flaky, and (c) coarse powder morpholoEy.


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N {2,I

ii

: ! ! _ uC

!i U :'1

{ _ ! ;/ '


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5OO! 11 Cu--Ni-4-:I-Surfsce2

400 - _SS File: z(::)l_c_5._:cC_J II 200

o 300 - Ni BS220 411_ 200 ss

100

0 _ I I l _ _ ,,I ,_10 20 30 40 50 60 70 80 90 100(TWO THETA/THETA)

Fig. 6 XP_D analysis showin E presence of an additional phase (marked 'SS'solid solution in shock processed Cu-Ni compact.


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Fig. 7 (a),(b) Optical micrographs of shock processed Cu-Nb compacts.


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N.N. Thadhani et al- Shock-Induced Chemical Reactions and Synthesis of Binary Compounds