NWSC/CR/RDTR-1 92

THERMAL ANALYSIS OF REACTIVITY OF PYROPHORICFUELS IN A CHEMICALLY SECURE ENVIRONMENT

HARLAN L. LEWIS

18 November 1982

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Prepared for:CommanderNaval Air Systems CommandWashington, D. C. 20361

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APPLIED SCIENCES DEPARTMENT

• NAVAL WEAPONS SUPPORT CENTER, CRANE, INDIANA

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*. Applied Sciences Department

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Thermal Analysis of Reactivity of PyrophoricFuels in a Chemically Secure Environment 6. PERFORMING ORG. REPORT NUMBER

7. AUTHOR(e) 8. CONTRACT OR GRANT NUMBER(s)

Harlan L. Lewis

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IS. KEY WORDS (Continue on reverse aide If i et eessa'y and identtfy b, block number)

Alloys, Pyrophoric Metals, Inert Atmosphere Enclosures

20\ ABSTRACT (Continue an reverse sde It necea.. and identify by block tumber)The conditions der which air and/or moisture sensitive materials can be

prepared, purified, and studied for reactivity characteristics are outlined,

using the pyrophoric materials Li and L17 B6 ' as examples. Anexperimental arrangement for examining reactivity in flowing gas atmospheresis described

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SUMMARY

Investigations into the modes and rates of reaction of pyrophoric solids withpotential use as fuels in decoy flares are particularly difficult to conductwhen it is desired to study a variety of atmospheric and temperatureconditions. Sample integrity prior to exposure to a given reactionenvironment is also difficult to guarantee. Control of reaction rate byexposure to diluted atmospheres is also a problem.

This report provides details on a technique wherein sample preparation,handling, and exposure to a reactive environmert are all completed within asecure environment. The overall system involves thermogravimetric analysisof materials in an inert atmosphere enclosure. The samples can be studiedunder a variety of atmospheric, changing atmospheric, and temperatureconditions, with static or flowing environments with adjustable flow rates.The procedures are rapid and efficient and provide reasonable turnaroundtimes, and they guarantee quantitative analytical information on mass changeand temperature effects during the actual reaction period. Further,calorimetric data and heats of reaction under a variety of reactionconditions, and differential thermal analytic data may be easily obtained aswell. Finally, the reaction products are then available for subsequentanalysis for surface effects, ratios of products, etc.

The basic procedure is to effect sample preparation or synthesis in an inertatmosphere environment, transfer a measurable quantity of sample to a giventhermoanalytic apparatus inside the inert atmosphere enclosure, and proceedwith setting up of experimental conditions. Then the desired reactiveatmosphere is introduced to an isolated sample compartment, the quantitativedata gathered, and the sample compartment purged prior to recovery of theproducts and loading of a new sample. The procedure is quick, reliable, andcompletely reproducible.

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PREFACE* (U)

(U) This report of NAVAIR-sponsored research consists entirely of workdone at the Naval Weapons Support Center (NAVWPNSUPPCEN), Crane, and wassupported by funds from AIR-32R.

* In order to specify procedures adequately, it has been necessary to iden-tify commercial materials and equipment in this report. In no case doessuch identification imply recommendation or endorsement by the Navy, nordoes it imply that the material or equipment identified is necessarily

IN the best available for the purpose.

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"TABLE OF CONTENTS

Page

Summary ---a------------------------------------------------------ 1

VPreface--------------------------------------------------------- 2

"List of Figures -------------------------------------------------- 4

Background ------------------------------------------------ 5

A Experimental -------------------------------------------------- 5

Results and Discussion ---------------------------------------- 10

Conclusions -------------------------------------------------- 10,'

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LIST OF FIGURES

Page

Figure 1. Diagram of Thermogravimetric AnalysisSetup in the Dri-Lab. 6

Figure 2. Reactivity of Lithium Metal with anAtmosphere of Dry Air at 175 0 C. 8

Figure 3. Reactivity of Lithium Metal with Moist Ar,Triplicate Run. 9

Figure 4. Reactivity of Lithium Metal and Li7 B6Alloy with Atmospheres of Moist Ar or N2at Ambient Temperatures. 11

Figure 5. Reactivity of Lithium Metal and Li 7 B6Alloy with Atmospheres of Moist Air or 02at Ambient Temperatures. 12

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Background

"Pyrophoric solids present particular difficulties for handling when it is¾• desired to prepare, purify, and study their properties in a variety of ways.

- Their air-reactivity requires an inert atmosphere such as dry nitrogen orargon to be used during all manipulations. Thus, so-called dry-labenclosures are used for their study. In situations where it is desirable tostudy the actual behavior of such materials in reactive enviror,ments, newproblems quickly arise.

When pyrophoric materials are removed from a secure environment to theambient world in order to perform experiments, there is a significant dangerof sample contamination and degradation during all transfer operations. Thenthe sample integrity at the time of a test procedure is always in doubt. If,on the other hand, the reactive environment is taken to the sample and ifcontamination of the secure environment can be prevented, then a morerel,able experimental situation prevails. Of course, there may still be"difficulties associated with modifications in the reactive environment duringexperiments. Nevertheless, controlled, reproducible, reactive situations aredesirable when it is necessary to characterize the reactive behavior of apyrophoric material which has potential use in flare devices.

We have utilized an approach involving performance of the reactionstudies in a dry-lab facility such that sample integrity is maintained priorto a reaction situation. During the actual experimental runs, the dry-labatmosphere is secure, such that sample preparation is not threatened. Thus,samples for study can be prepared, stcred, and run simultaneously in a secureenvironment, reproducibly, without concern for the sample integrity or forcontamination of the enclosure atmosphere.

Experimental

The work to be discussed will be confined to thermogravimetric analysis(TGA) in which the mass and temperature changes in pyrophoric samplematerials were observed as functions of time, when reactive gas atmosphereswere introduced to the sample environment. The entire system is shownschematically in Figure 1. A Du Pont Model 951 TGA balance was used forthese studies and was placed in a Vacuum Atmospheres Inert AtmosphereEnclosure which was routinely operated with an argon atmosphere, whichcontained less than 0.3 ppm 02 and H20, and less than 3 ppm N2 The"TGA balance was connected to a Du Pont Model 990 Thermal Analysis consoleoutside the enclosure, and all the operating conditions were controlled fromthe console. The mass balance was set up so that a gas flow could bemaintained through the apparatus. Then a gas proportioner was used to allowrapid switchover from one sample atmosphere to another, as well as to allowmixing of two gases in any proportions. For example, a mixture of 20% 02in Ar could be used to simulate Air without N2 to eliminate potentialreaction conflicts with nitrogen.

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The sample materials to be discussed were pyrophoric: Lithium metal"powder (-140 mesh) and lithium-boron alloy, Li 7 B6 (-50 mesh). Samplequantities were routinely 5-10 mg in size and were placed in a platinum pansuspended from a quartz balance arm. The loaded balance wps allowed to reachequilibrium, under a flow of dry argon at about 15 ml-min-m , at the desiredreaction temperature. Then the reactive gas atmosphere was introduced intothe apparatus, flowing also at 15 ml-min-, and mass and temperaturechanges with time were monitored. Typical run times were 3-4 hours, althoughsome were much longer. The experimental arrangement also allowed theintroduction of moisture into the gas flow, so that the effects of humidityon reactive atmospheres could be studied, and moisture alone in an inert gas

- stream could be examined.

The gases used as "dry" were Matheson Purity Air, Oxygen, Nitrogen, and* Argon, with moisture contents less than 5 ppm, and the Air was C02 free as

well. Gases used during humidity studies were just High Purity grade.

Results and Discussion

In a typical procedure, the TGA balance would be under dry Ar at thestart of a run. The empty Pt pan was first tared to zero mass at the desiredreaction temperature and gas flow rate. Then the balance compartment wasloaded with a sample and a period of time allowed for the balance toequilibrate in the dry Ar stream. Next, the sample atmosphere was changed tothe desired reactive gas. Usually, a priod of 3-5 minutes was required toestablish a reactive atmosphere before any indication of mass change wasobserved.

In Figure 2 are the data from reaction of Li metal with dry 02 and Air,at 1900 C, above the Li fusion temperature. In both gases, the reactionprocess occurs readily, and the data are very similar. This similarity meansthat Li20 is the product from dry Air, and, therefore, that Li 3N formationis not a significant competing reaction. The slight differences in the datacan be attributed to differences in gas flow rates and 02 concentration.The significant information is the similarity in results from two experimentsusing different gases. The fact that the sample handling is accomplisned in asecure environment allows us to infer product and reaction similaritieswithout having to be concerned about a possible role of catalytic contaminantpicked up during transfer operations of such air- and moisture-sensitivematerials.

*, in Figure 3 are the data from three experimental runs on the reaction ofLi metal powder with humid Ar. The only reactive naterial in such a gas flowis moisture, and the predicted final product is LiOH-H 2O. The dataclearly show this product formation, along with an earlier inflection in thecurves corresponding to initial formation of LiOH. Further, the data showthat when the atmosphere over the sample was changed back to dry Ar, the water"of hydration was quickly lost, with the product returning to LiOH. Theobserved mass changes correspond to:

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and all of this occurs readily at ambient temperature (32 0 C). Finally, thethree runs indicate the reproducibility of the reaction processes. Thus,there is freedom from concern about sample contamination. The processeswhich lead to changes in sample mass must be a consequence of only thecomponents of the atmosphere over the sample. The confidence about this factleads in a natural way to the conclusions drawn from the data in the next twofigures.

In Figure 4 are presented the data from reactions of Li metal andLi 7 B6 alloy with humid Ar and N2, at ambient temperatures. These dataallow two sets of comparisons to be made. First, it has been reported thatLi 3 N formation occurs readily from moisture catalyzed reaction with N2gas. Second, we wish to know the comparative reactivity of Li and Li 7 B6in similar atmospheres. With regard to the first, using the data fromreaction of Li metal, it seems evident that the reaction process in bothatmospheres is the one outlined above in the equations. Thus, the process isformation of LiOH, conversion to LiOH.H 20, and subsequent dehydration toLiOH in the dry Ar stream. Li 3 N does not appear to form.

Next, we address the second question about the comparative reactivebehavior of the two Li materials in the respective atmospheres. The data,which are in percent mass change for Li in both materials, clearly show thesimilarities in the behavior of the two materials. This allows us to saythat Li7B6 behaves essentially like Li metal, that the Li component ofthe alloy is the reactive portion, and that its behavior at ambienttemperature has been modified only slightly by incorporation into the alloy.This modification amounts to a slight reduction in overall reactivity.

Then, in Figure 5, we examine the data for the same two materials inhumid Air and 02, at ambient temperature. These data once again show thatthe preponderant and probably the only significant reaction is with watervapor. Lithium is simply not reactive toward 02 or N2 in humidenvironments at ambient temperatures. Also, as a consequence of thepreferred reactions with H20, Li in both materials behaves essentiallyidentically.

Conclusions

We have been able to investigate the reactivity of two chemically relatedpyrophoric materials with a variety of atmospheres and to draw conclusionsabout pathways and relative reactivities without concern about confusion ofthe data with unwanted catalytic contamination of the samples. This wasachieved through the use of a secure environment in which all stages ofsample preparation and study are reliable and reproducible. Thus, we canarrive at a significant understandirg of the reaction processes for materialswhich are inherently difficult to handle, in a straightforward fashion. Thisunderstanding can then be transferred to the behavior of these materials whenthey are exposed to an ambient environment as they are used as fuels in flaredevices.

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