NASA Technical Reports Server (NTRS) 20050080710: Boron Nitride Nanotubes for Engineering Applications

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by  NASA Technical Reports Server (NTRS)

SYNIMESIS OF RORON NITRIDE. NANOTUBES FOR ENGINEERING
APPLICATIONS

net Hurst
David Lill

NASA Glenn Research Cancer
21000 Brookpark Itd
Clevsand, Ohio 44135

Duciel Gariean
58 Group

21600 Brookpark Rd
Cleveland. Ohio 44135

ABSTRAC
‘Boron nitride nanotubes (BNNT) an: of signifisent interest 9 the selcaisic and techie
ccuonmunites for many of the same reasons thl urbon nanotubes (CNT) bave attracted,
‘wide attention, Bor materials have pocemially unique and important properties for
sieuctcal an clevtroniv upplieasions, However of even snore consequence than their
similarities may be the complementary differeuees bewwenn catbou and boron nitride
nanotubes. While BNNT passess a very high modulus similar to CNT they also pasecas
supstior chemical and therunal stability, Additionally, BNNT howe mame uniform
Teoironic properties. with « uniform band gap of 5.5 eV while CNT vary (roms seri
anduclive kr highly eonduvsive behavior.

otor nitride nanotubes have been synthesize] both inch litenture and at NASA Glen
Research Come. by a varicty of methods such as cheznieal wapor depasilion, are
discharge und reactive milling. Consistent large scale production ol aceliuble product
has proven difficull. Progress ia the reproducible syethesis of 1-2 gram sized basshes a
‘boron nitride naniubes ill he discuss! 9 well as porential uses for this que
note

INTRODUCTION,

Inthe last decade, sigifivam attention fe the sieatife comaunniy as been focused
cnr ava of naotechnalogy and spacifieally upon mnoiuhe synthesis, While ebon
nanotubes have generated the bulk of interest to date, other vorpnitions ofr promise
‘wel and may have achmtages ox complementary properties eelauve to caehoo nanatubes
for various appligatens.. ACNASA Glenn Rewsrch Center, were application iotereste
ane often foctsed on high temperature propulsion, both BN and SiC nanotube synthesis
axe curemly under investization for hig temperturssrvetural and cleetonie materials
(1.3), The focus of tc eurtoneeffor is BRN eemesis. While heron aire minotubes
«rc known to bs structusll simile to cacbon nanotubes inasrauch as hol are lormial
fam graphene sets, mnch les is katown abont BNA Jn large part this is due to the
difficulty in spathesiing this material rahe toa lack of utes, Is been fou that
BNI have excellent mechanical properties with a measured Young's modulus of 1.22
+5024 BAG). BNNT also have a constant baad yop of shoul 5.5 eV (4). in comin,

Ins preprint or epar 2 epoine derprasersation ata
‘omfoenes scape enanges nay bo made ters fs
Duotone ase avails ite undarstanng that
"otos cledor predused thou! Is remission oe anor
COL vary thom semi-conducting to condueting behavior depending, on chitality ond
ameter of the product. Little proyress has been demonstrated in the conzal of the chiral
angle and honee the clestzonio propertics ef CWT, On the ochor hand, BUNT
preferentially toms the igeag etuuctas rah than the ammichair or chiral stmetures duc
to the polar aanue pl the BEN hand (5). Recenlly. it has phic heen shaven tha NT
systcuns have exccllemr piezoelectric pruperties, superior ta those ol pievpelectric
polymers (6), Ackltioually. the cxpeeted oxidetion resistance of BNI relative to CNL
suggests BNNT riyy he stable for high temperature stuctaral applicatious. This
srability may be an imsportanesalety consideration (ir some applications, ssh 2s
hydrogen storage, as carbon uauotubes ecadily busn in aie. Many aynchesis approaches
have heen Wed with varying degrews of success, Among these approaches arc pyrolysis
caver Cr (7), CVD inethuads (8), are discharge (9) laser ablation (10), and reactive mil
techniques 1,12}, The reported approach develope! wf NASA Glenn Reseurch Center
produced BNNT of significant length and abundance.

EXPRRIMENTAL PROCEDURE

TINNT were prepared by reacting amorphous boron powder in a flowing atmosphere of
hicrngen with a small amonat of NTs. Prier to heat toatmacnt, fine iron eatalyst particles
‘yore added inthe range of up tp several wefght percent and brietly med in
polyetiylene bottcs with a hydrocarbon solvent and ceric grialing media. Bauch
Sizes of 2 grams are rypivilly produced bur the process should be casily sesleable to
Targer size. Milled materiel sass upplied 10 varigus high lemperature substruics such 4s
‘ltunina silicon carbide, platinum and molybdenum, Nunolubes were larmed during
beat treatments to temperanines rauging ftom 1109 € to 1400 € tor brief times, 20
rime 2 hones

Nanovulbes were imaged with a Itachi $4700 field emission searing electron
amieroscope with a super shin window MDAX Genesis System energy dispersive
spectrometer (HDS) or Phillips CM2U0 ansmission election mictoscope operated at 200
EY, with Catan clectran onergy Ieee spoetumetsr (EELS), Lhemogravimerdc aualysis
ETGA) was done iat gir up us 1000 €.

RESULTS

TIKNT synthesived by this msthod are shown in Figure 1. The low magnification photo in
Figure La shows an us-preduced Tlake of BNNT, The flake was removed by tweeze08
hom a 25 mmx 5 mam subsite oe crucible, i rshust und easily undled Uh Fora,
Also, as the nuotubes are anchored wichin a growth medi, there is 00 respiration
hard, BN nunetubes urow extensively both Hom the “top” and “bottom of the growth
rmedia, where “up” relirs ly Ure side exposed ky the atmosphere, Rolh top and bextom
laycis can bo obscrved in Figure Ta as the edges were slightly rolled during handling.
‘The nanotubes az quite long. 100 microns being common as shown in Figs 1b and Ie.
(Gin the bostam «the growth r:=dia, 4 lyyer of short nanocabes develons berweeu the
film ad subsieate or crucible, TTDS ul showed Cha the NNT gras per a ffm
composed primarily of 8, N, ©, Fe and some purities. Nunolube diacasters wun he very
zat throughout thele length; however tis is processing temperature dependent,
‘Not surprisingly. variation in the heat ueatnent temperature resulted in somewhat
‘igierent prodnct, Lower processing temperature, resillad in more Tne, uaiforin
ranolubes, as those show in Figure 1h, fumeters of 20mm to 50 aan were typical.
Tigher iemperaiures enor longer times resulted in lage “uanombs" growth as shown in
Figure 2b, Growth originated thom finc uuelei but vith cxocssive tempat, large
strucnucs quickly devcloped. up ro a ow microns in diameter, Ax shosen in Figures 2a
and 2b, sscondary nucleation of small dismeter BNNT also necurred on these larger
‘iructures. Tigher processing lemperatures, as wel as excess ezralyst concentration,
resulted in interesting structures, such as nanohorns or uanoflowers, as seen in figure 2a
“These serucures also generally had considerable amounts of oxygen found by TIDS, up ta
6 wi.

ais! Fe castbnstatagsovin mentane meget 9) alate pate manera) Te
‘epvrsmon rane toe gin Some

Aransotssion sloetron mioreseopy reauits ars shown in Figures 3 and 4. TEM results
showed fhe nanotubes to be nearly stnichiometric BN and a mixture of both straight
‘walled naneituhes, 2s shown in Figure 3, and the "bamboo" steuetues, Figure 4.
Predominately, the product from this method is multiwalled, commonly composed of 15-
30 lattice layers, although this ean be affected by processing conditions, Diameters af the
Figure 2. Examples of ess ypical srusraressynthesiand st provessing oxromes,
2) Adjacent enirems insias 5) upen sed nanepode¢) ine BNST nucleus un lrg tubes
nations,

Figure 3 TEM photos of ties! straight walled BNNT

_mltivslled BNL were often in the 20-401 nm range, again determined by processing
‘conditions wn extalysts eoneouraion, Figure 3 shows the atomic planes within the
straight walled sanctubes exhibited lattice fringes at an angle of 12.5° wilh respect to the
‘ube ais. Lhis hes also aeen noted elsewhere (§, 13) and may he wn neation of
vocmohedrel sacking onder (12). In Figure 4 tho iypially highly Felted lattice wall of
the bamboo aucture are evident, These lauted shor walls, with their open edge lavers
hve been sugserted i he superoe foe hydrogen storage (14,15). ‘he stuctse ofthe BN
layers are unslagous to stacked paper cups with potential hydrogen storege sites on the
surface and bervcen lattice panes. ad slso pecaars within tho isolated voids.
Predominatsly bamboo sroctures can bo eensisteily proshuced hy this proces
rthod, Figure 5 shanssu region of exclusively hamboo BNNT:

igure 4 TEM photos of gpl bamboo BNNT.

‘The sompernure subility of as-produced| BN nunoluhes was investigated by nen
gavimelrie anulysis, The revulls were compared ta corumercial asspeoduced carbon.
nanotubes*. Photos of the as-produecd CNT and BNNT aaterials arc shown in Figurs 6
‘swell as these following heating in the TOA, The BNNT sucture is elesly intact with
the CNT decomposed, leuving hehind he extensively sided jn catalyst. Faure 7
shows the TGA dau in ir, ecnfiring That the earbon nasonubes have decomposed b
400 C. However, the BNNT are unaflected by the heat ceaument with the exception of
some Slight weighr gain from oxidation above LUDU C.
oF

CNT sample folowing TGA,

we 8, Asproosssey BNNT aixl CNT saps eis exaniinod Defoe ard as
130 Cina TGA. Photos onthe lft are as-processed sarrpiss, On tke right are paolo aleial
emaning flowing a TCA ru ie 1000C

| 2 0 tan 1500
| Temperature (©)

Figure 7. TGA results for NNT and CNT samples
shasving the superior slability of BNNT relative to CNT
CONCLUSION

Boron niteide nanotubes were successfully ard reproducibly grivn by a NASA Glenn
Ressavcl Center developed proccss. Curtently 1-2 aramn batches ace being synthesized:
hhomever, the process is scaleable to umuch lager batch sizes. Suificicut smouats ate aow
available s0 that haram ritride nanoluhes are eirrenily being ineorporaled inla eomposives
1 provide high steength heliavie at high temperature. [cis also possible that this watered
andy have many applications for sensors, cleewouies, piezoelecties, among other
ugplicuions a interes! to NASA snd the teclnies] cocamunity.

A preponderance of bamboo structured nanotubes could be achieved hy careful altions
of catalyst materials and coatol of processing conditions, These bamboo steuctures are

cof inieres! fhe hydrngen slorage applications, ‘Ibe boron nitride nanotubes were found to
be much mone sable ul high lemparalure hem exthxn nanouhes

ACKNOWLEDGEMENTS

‘This work-was sponsoved by the Alscenate Fue] Foundation ‘Technologies (AEE)
Suisgnojest of the Low Emissions Alterastive Power (LEAP) Project at tho NASA Gloan_
Research Center

Csrbon Nanotechnologics, Ino.16200 Park Row, Houston, 1X
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