PergamonCarbon, Vol. 32, No. 3, pp. 507-513, 1994

Copyright © 1994 Elsevier Science LtdPrinted in Great Britain. All rights reserved

0008-6223/94 $6.00 + .00

MAGNETIC SEPARATION OF GdC, ENCAPSULATED INCARBON NANOPARTICLES

kll

SHEKHAR SUBRAMONEYE. I. du Pont de Nemours & Company, Experimental Station, P.O. Box 80228.

Wilmington, DE 19880-0228. U.S.A.

RODNEY S. RUOFF, DONALD C. LORENTS, BRYAN CHAN,RIPUDAMAN MALHOTRA, MARK J. DYER

Molecular Physics Laboratory. SRI International. 333 Ravenswood Avenue.Menlo Park, CA 94025, U.S.A.

and

K. PARVINPhysics Department, San Jose State University, San Jose, CA 95192-0106, U.S.A.

(Received 29 October 1993; accepted in revised form 13 December 1993)

Abstract—Single crystal particles of a-GdCi have been encapsulated into carbon polyhedral shells bythe carbon arc-discharge technique. These particles range in size from 20 to 50 nm and are paramagneticin nature. The phase and stoichiometry of these crystals have been determined using a combinationof techniques, such as X-ray diffraction and high-resolution electron microscopy. As reported for«-LaC2 earlier, the crystal structure of bulk «-GdC: is also tetragonal, and it is metallic and undergoeshydrolysis. However, the encapsulation of the nanoscale crystals in carbon shells prevents theirhydrolysis indefinitely. These nanometer-sized particles, which can be preferentially extracted usingpowerful magnetic fields, may have useful applications in several fields of science.

Key Words-Gadolinium carbide, nanotubes, carbon shells, encapsulation.

The discovery in 1991 by Iijima[l,2] of nested car-bon nanotubes and nanopolyhedra as a by-productof fullerene production has sparked a significantamount of interest into the research of these novelforms of carbon. Theoretical calculations[3-8] haveshown that these materials would possess uniqueelectrical, thermal, and mechanical properties thatcould be of value to several diverse fields of science.The successful production of endohedral fullerenesby several researchers[9-12] prompted our first at-tempt at filling the cavities of the tubes and poly-hedra with highly electropositive elements. Ourresults, reported previously, showed successfulencapsulation of single-crystal, nanometer-sizedparticles of a-LaC2 into about 50% of the carbonpolyhedral shells using arc-discharge conditions thatfavored the growth of carbon nanotubes and poly-hedra[13].

In order to effectively utilize the benefits of suchhigh-density nanoparticles (density of a-LaC2 is 5.32gm/cc), it would be desirable to extract them prefer-entially from the rest of the products of the arc-discharge experiments. Encapsulated materials of avariety of sizes and morphologies are difficult toseparate in bulk using either physical or chemicalmeans. Magnetic means of extracting a-LaC2 parti-cles would not be successful, as they are onlyweakly diamagnetic. Our initial attempts at encapsu-

lating a ferromagnetic element (Fe) were not suc-cessful, and this may be due to several factors,among them the higher ionization potential of Fecompared to that of La, or the lower melting temper-ature and higher vapor pressures of the carbides ofFe compared to those of a-LaC2. In this paper wereport the successful encapsulation of Gd, one ofthe four ferromagnetic elements. The paramagneticproperties of the carbon-shell encapsulated, singlecrystals of a-GdC2 are also reported.

Analogous to our production of a-LaC2 encapsu-lated in carbon nanoparticles[13], the arc-dischargeexperiments were run using 7.9-mm-diameter, 30.5-cm-long graphite anodes, which were drilled to adepth of 23 cm with a 3.2-mm drill and packed witheither Gd,O3, which has negligible magnetic suscep-tibility, or pure Gd metal. The carbon arc conditionswere a DC current of 75 A, and gap distances of2-5 mm and 2-8 mm from the 12.7-mm-diametercathode for the Gd2O3 and Gd runs, respectively.The corresponding He pressures were 1,000 Torr(Gd2O3) and 500 Torr (Gd), and the Gd: C molarratios were 2.3 :100 (Gd2O3) and 8.2 :100 (Gd). Un-der these conditions a cylindrical boule grows out-ward from the end of the graphite cathode. Thecentral core section obtained from this cylindricalgrowth on the end of the cathode was powdered. Asample of the Gd-containing powder (ca. 50 mg)

507

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S. SUBRAMONEY el al.Magnetic separation of GdCi encapsulated carbon nanoparticles 509

508

obtained from

,„ be pure OcP,. »» ™^^;iighrty higher heights we = "Jamoams of amorphous

L°rJ^« roi Resent as O.O., I £SKSj«SS£

fication, and then uepusn^v. .., _,Cu grids, and air-dried for characterization . „mission electron microscopy (TEM). Most of theelectron microscopy used to study these materialswas carried out on a JEOL 2000-FX TEM operatedat 200 kV. The Atomic Resolution Microscope atthe National Center for Electron Microscopy, Law-rence Berkeley Laboratories, as well as a PhilipsCM-30 TEM equipped with a Link windowless de-tector, were also used for high-resolution micros-copy and elemental analysis, respectively.

Arc-discharge experiments, run with either ametal or an oxide in the anode, produced significantquantities of carbon nanopolyhedra, with roughly50% filled or partially filled with gadolinium car-

bides.X-ray analysis of the product of the arc-discharge

such as

273 K' to extract a component of the

-tSi-ni=;s^^ iSssqSoS—S A^6.1d ,M8e of the

JOOJHTL- £

Fig. 1. Bright-field TEM micrograph ofextracted «-GdC, encapsulated carbon nano-

magnetically extracted material in Fig. 1 shows thatit essentially consists only of encapsulated particlesranging from 20 to 50 nm in size. This implies thatthe magnetic component is single crystal particlesof GdCn encapsulated in the nanopolyhedral shellsof carbon.

Accurate measurement of the lattice fringe spac-ings from several high-resolution images of the mag-netically extracted nanoparticles showed that thespacings matched well with the interplanar spacingsof the (101), (002), and (110) planes of o:-GdC2. Im-aging of higher-index lattice planes was limited bythe line resolution of the 2000-FX. Further verifica-tion of the phase of the encapsulated particles wasobtained by measurement of the angles of differentsets of fringes, which matched very well with thosefrom theoretical calculations for tetragonalcrystals[15].

Specific magnetic properties of these materialswere investigated using a magnetometer (PARmodel 4500 VSM) capable of stabilized sample tem-peratures from 4 to 325 K and magnetic fieldstrengths up to 10,000 Oersted. To accommodateas much material as possible in the VSM probechamber, the samples were pressed and formed intodiscs 3 mm diameter by 2 to 3 mm long, consistingtypically of 5 to 50 milligrams of material. Randomsamplings of Gd-carbon materials were collectedfrom core and soot material, and magnetically ex-tracted samples were pulled at room temperatureusing a 5 kG samarium cobalt magnet.

X-ray diffraction and corresponding magnetiza-tion data (insets) are illustrated in Fig. 2 for ran-

100

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6:

-2C

0.4298K

-0.4-10000 0 10000

Magnetic Field, Oersted |Graphite

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domly selected and magnetically extracted core ma-terial from a run with gadolinium metal rod packing.The purely paramagnetic character of the two sam-ples is apparent, as is the large difference in theratio of paramagnetic material per gram mass. Themagnetic extraction has resulted in a 20-fold in-crease in the concentration of the paramagneticcomponent for this particular sample. Samplestaken from subsequent runs under different cell con-ditions have produced a magnetic component com-prising >80% of the central core. All the samplesgenerated with Gd metal or Gd:O3, and obtainedfrom the core material, have exhibited this paramag-netic behavior.

Temperature-dependent magnetic moment datafor the Gd-carbide encapsulates over the range 4 to300 K are shown in Fig. 3. Based upon the XRDanalyses, the paramagnetic response of the Gd-car-bide sample appears to be the result of gadoliniumdicarbide, which in bulk form has an antiferromag-netic transition temperature of 42 K[16]. Since theGdC2 crystal sizes observed by TEM are on theorder of 100 nm or less, and their moments areessentially isolated and uncoupled, a clear antiferro-magnetic transition temperature for GdC2 at 42 Kis not observed. For GdC2 particles of this size, thethermal energy required to overwhelm the energybarrier of stationary moments and flip them in anexternal field is expected to be relatively small;therefore, the net magnetic moment should followthe Curie law 1/T dependence down through thetransition temperature. These encapsulated GdC,crystals fulfill this requirement, and are thus consid-

10 20Two-Theta

(a)

30 40

100

60

20

-20

7.5= 298K

-7.5-10000 0 10000

Magnetic Field, OerstedI Graphite

Gd2C3,|

*JUvX

GdC2

GdC2

10 20Two-Theta

(b)

30 40

CAM-4576-S

Fig. 2. XRD and magnetization curves (insets) for (a) random and (b) magnetically selected corematerial.

510 S. SUBRAMONEY et al.

•

E2

0)

lOKOersted Field

Experiment •

Fit

100 200

Temperature, Kelvin

300

Fig. 3. Moment vs. temperature for Gd-carbon encapsu-late sample generated from Gd2O3 packing. The fitted curve

is a least-squares fit to 1/T Curie law.

ered superparamagnetic[17]. With the exception ofvariations in magnitude, the magnetization loopsand M vs. T curves of all raw and extracted coresamples appear identical.

Based on the study of high-resolution images ob-tained from the predominantly amorphous regionsof carbon containing a fine dispersion of Gd, asverified by elemental analysis using energy disper-sive spectroscopy (EDS), we can speculate on apossible encapsulation mechanism for the Oddnanocrystals in the carbon polyhedral shells. Wespeculate that the Gd/C amorphous material (with

only a few atomic percent Gd in carbon), phasesegregates into ultrafine aggregates of Gd carbides ina matrix of amorphous carbon. The high-resolutionimage in Fig. 4 suggests that the carbide may crystal-lize prior to the carbon shell. Specific planes of thecrystallized carbide particles appear to promote thegraphitization of the carbon layer planes from theinside out; the high resolution image in Fig. 5 ilius_trates GdC2 crystals surrounded by carbon shellsthat are incompletely graphitized. The encapsula-tion of the GdC2 crystal within the graphitic carbonshell may occur via one or a combination of twomodes. The completion of the crystallization of thecarbon polyhedral shell may occur from the outsidein, causing a reduction in the volume of the carbonsurrounding the GdC2 crystal. Such a phenomenonwould account for the presence of a void adjacentto the carbide crystal inside the graphitic shell. Morelikely, the placement of the 12 pentagons to closethe polyhedral shell and the polyhedron structureis simply incompatible with the shape of the encagedGdC2 crystal, causing the formation of a void. Figure6 illustrates a high-resolution image of a completelycrystallized carbon polyhedron containing a singlecrystal of «-GdC2 and a void region.

Although the concentration of Gd-filled nanopar-ticles was high, filled nanotubes themselves wereobserved only on extremely rare occasions. An ex-ample of such a metal-filled nanotube is shown inFig. 7.

As is evident from Fig. 7, the end of the

Fig. 4. High-resolution TEM image of Gd/C amorphous component, which appears to suggest thathe carbide crystallizes prior to carbon.

10 nm

Fig. 5. High-resolution TEM image showing partial encapsulation of carbide crystals by graphiticlayer planes.

Fig. 6. High-resolution TEM image of a completely crystallized carbon polyhedral shell containing asingle crystal of a-GdC2 and a void region. The particle is lying adjacent to several carbon nanotubes.

511

512 S. SUBRAMONEY el al.Magnetic separation of GdC, encapsulated carbon nanoparticles 513

g G Overney, W. Zhong, and D. Tomanek, Z. Phys.' £> 27, 93 (1993).

a J R Heath el al.,J. Am. Chem.Soc. 107, 7779 (1985).10 Y Chai et al., J. Phys. Chem. 95, 7564 (1991).ll' R. D. Johnson et al., Nature 355, 239 (1991).

Moro R. S. Ruoff, D. C. Lorents, R. Malhotra," and C. H. Becker, J. Phys. Chem., 97, 6801 (1993).

13 R S. Ruoff, D. C. Lorents, B. Chan, R. Malhotra,and S. Subramoney, Science 259, 346 (1993).

14. G. Adachi et al., In Handbook of the Physics and

Chemistry of Rare Earths, Vol. 15, p. 164. (Edited byK. A. Gschneidner, Jr. and L. Eyring). North-Holland(1991). In Handbook of Chemistry and Physics (Editedby D. R. Lide). CRC Press, Ann Arbor, MI (1992).

15. B. D. Cullity, Elements ofX-Ray Diffraction, p. 502.Addison-Wesley, Reading, MA (1978).

16. R. Lallement, U.S. Dept. Com., Clearinghouse Sci.Tech. Inform. AD 627224, 11 (1965).

17. F. F. Luborsky, J. Appl. Phys. 32, 1918 (1961) andreferences therein.

„»> 100 nm

Fig. 7. Bright-field TEM micrograph of metal-filled carbon nanotube. It appears unlikely that the Gdmaterial was drawn in by capillary action.

multilayer nanotube is capped by many carbon lay-ers. It is therefore unlikely that the Gd material inthe interior was drawn into the tube by capillaryaction. Furthermore, the meniscus indicates a con-tact angle greater than 90° (measured value is about135°), which is indicative of non-wetting behavior.Thus, if the Gd material were drawn in by capillaryaction, one would have to involve the unlikely sce-nario that this material expands upon solidificationto cause an inversion of the meniscus. Since wewere unable to observe lattice fringes in the Gdmaterial inside the tube, we could not assign itsstoichiometry or phase.

In summary, we have been able to encapsulatesingle crystals of a-GdCi in carbon polyhedral shellsby DC arc-discharge techniques. However, unlikeencapsulated polyhedra of a-LaC2, encapsulatedGdC2 is paramagnetic, and can be easily extractedat room temperature. Useful applications of suchnanometer sized particles can be expected in severalfields of technology.

Acknowledgements—The assistance of R. Gloyd, P.Eckler, and L. Hanna of the DuPont Company is acknowl-edged. Part of this work was conducted under the program"Advanced Chemical Processing Technology," which isconsigned to the Advanced Chemical Processing Technol-ogy Research Association by the New Energy and Indus-trial Technology Development Organization, and carriedout under the Industrial Science and Technology Frontier

Project administered by the Agency of Industrial Scienceand Technology, the Ministry of International Trade andIndustry, Japan. We thank the staff of the National Centerfor Electron Microscopy (NECM), Lawrence BerkeleyLaboratory for assistance with TEM measurements per-formed there.

Note added in proof: Another study of encapsulates ofgadolinium carbide has recently appeared, Majetich et al,Phys Rev B 48, 16845 (1993); these authors have studiedencapsulated Gd;C3, in contrast to the work reported hereby us, which is of the more strongly paramagnetic GdC:

encapsulates.

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