JOM • June 200646

Rapid/Pulse Thermal ProcessingResearch Summary

Pulse-thermal processing (PTP) based on high-density plasma arc lamp technology has been utilized to crystallize melt-spun NdFeB-based amorphous ribbons to form magnetic nanocomposites consisting of Nd

2Fe

14B

and α-Fe phases. After applying suitable pulses, the NdFeB-based ribbons were developed with hard magnetic properties. The highest coercivity can be obtained for ribbons with a thickness of 40 µm after PTP treatments consisting of a 400 A pulse for 0.25 s for ten times. The correlation between PTP parameters and magnetic properties indicates that PTP is an effective approach to control the structure and properties of nanostruc-tured magnetic materials. Transmission-electron microscopy analysis revealed that the observed decoupling between the hard and the soft phases is related to large grain size in the samples, which is in turn related to different heating condi-tions in different regions of samples.

InTRoducTIon

Nanocrystalline materials possessphysicalandchemicalpropertiesquite

The Pulse-Thermal Processing of ndFeB-Based nanocomposite Magnets

Z.Q. Jin, V.M. Chakka, Z.L. Wang, J.P. Liu, P. Kadokar, and R.D. Ott

different fromthoseofconventionallycoarse-grained materials. As a subsetof nanomaterials, nanocomposite per-manentmagnetshavebeenahotspotinalong-termsearchforhigh-performancemagnets. Coehoorn et al.1 in 1989observedmagneticexchangeinteractionat nanoscale between a magneticallyhard phase with high anisotropy anda magnetically soft phase with highmagnetization,whichmayleadtoveryhigh magnetic energy products in theexchange-couplednanocompositemag-nets.Numerousinvestigationshavebeenmadesincethen.Ithasbeenrevealedthatadeliberatecontrolofnanostructureisthepreliminaryassuranceofimprovedmagneticpropertiesinthenanocompos-ites.2–8Thenanostructuredcompositescan be obtained by using techniquesincluding melt spinning, mechanicalmilling, sputtering, atomization, andchemicalsynthesis.Withmostofthesetechniques,subsequentheattreatmentsarenecessarytocontrolphasestructureandmorphology. Rapidthermalprocessing(RTP)tech-niques9–13havebeenappliedtoprocess

nanocompositemagnets.Highheatingratesuptoseveralhundredsofdegreesper second, the utilization of directcurrent,orextremelyshortcompactiondurations of microseconds distinguishthem from conventional processingtechnologiesthatleadtoexcessivegraingrowth.HighcoercivityvalueshavebeenachievedinRTP-processednanostruc-tured NdFeB,9 SmCo,10 and FePt thinfilms.14Arecentinvestigationrevealedthatheatingtimeasshortas1sresultsinahighdegreeofmagnetichardeningand complete crystallization in amor-phousNdFeBribbons.15Thisinterestingfindingleadstoakeyquestion:howfastcanmagnetichardeningberealizedinagivenmagneticmaterial?Thispaperreports preliminary results achievedusingpulse-thermalprocessing(PTP)ofanNdFeB-basednanocompositemagnet.ThePTPtechniquehasthecapabilityofreachingextremelyhighheatingrates(upto600,000°C/s)byutilizinghigh-densityplasmaarclamptechnology.16,17Thehighheatingratecombinedwithshortdwelltimeatmaximumtemperatureensuresacontrolleddiffusiononthenanoscaleinprocessedmaterials. Seethesidebarforexperimentalpro-cedures.

ReSulTS and dIScuSSIon

Phase Structure and Magnetic Properties of the PTP Samples

The initial objective of this workis to verify the feasibility of the PTPtechnique for processing of magneticnanocomposites,thustheinvestigationwas first done on the crystallizationof amorphous ribbons by changingtheincidentpowerandexposuretime.The thermophysical properties of thesamples, includingemissivity,specificheat,density,andthermalconductivity

Figure 1. X-ray diffraction patterns of (a) Nd12 and (b) Nd9 samples subjected to pulse-thermal processing with 15 pulses of 0.25 s dwell time at maximum currents Imax of 400 A and 450 A, respectively.

2006 June • JOM 47

exPeRIMenTal PRoceduReS Thestartingmaterialsusedforthisinvestigationweremelt-spunamorphousribbons2–4 mm in width. A thin ribbon sample (40 µm in thickness, noted as Nd9) had acompositionofNd

2Fe

14Bastheprimaryphasewithanadditional20vol.%α-Fe.Athick

ribbonsample(200µminthickness,notedasNd12)hadastoichiometriccompositionclosetoNd

2Fe

14B.Thepulse-thermalprocessingwascarriedoutatOakRidgeNational

Laboratoryusingahigh-densityplasmaarclamptoheatribbonsamplesinargonwithinseveralshortpulseswithcontrolledpoweroutputanddwelltime.Theutilizationoftheplasmaarclampgeneratesapowerdensityof≤3.5kW/cm2overanareaof1×10cm2.Sincesingle-pulseheatingisnotenoughforthecrystallizationofamorphoussamples,amulti-pulseheatingmodewasusedwithapreheatingwitha50Apulsecurrentfor3sandfollowedbymulti-pulseheatingwithpulsecurrentsof400A,450A,500A,525A,550A,andamaximumpoweroutputof600A.Theexposuretimeofeachpulseis0.25sforacurrentof400Aand450A(five,ten,and15pulses),and0.1sforcurrentsabove525A(fivepulses),respectively. A Siemens D500 powder x-ray diffraction (Philip-Cu Kα

radiation) and a JEOL-2010transmission-electronmicroscopewereusedtoexaminethestructuralandgrainmorphologies. The magnetic properties of crystallized samples were measured usingan alternating gradient magnetometer at a low applied field of 1.4 T. After severalmeasurements,itwasfoundthattheribbonspreparedinthesamemelt-spinningprocesswith the same composition exhibited minor differences in their magnetic properties,whichmaybeduetonon-uniformityintheirphasestructureandcomposition.Therefore,toensurethereproducibilityofthemeasurements,eachmeasurementwasbasedonanaverageofseveralsamples.

all combine to determine the heatingtemperatureandaredifficulttomeasure.Therefore,thepulsepoweroutputwasusedtocharacterizetheheatingcondi-tionsandcontrolthecrystallizationoftheamorphousphase.Figure1 showsthe x-ray diffraction (XRD) profilesforNd9andNd12samplesexposedtomulti-pulses with maximum currentsI

maxof450Aand400A,respectively.

Before the PTP treatment, all of thesampleswereamorphous.AfterthePTPtreatment,thethickNd12ribbonsshownosignificantcrystallizationduetotheirlargethickness(above200µm)andthelowheatingpoweroutputofI

max=400

A. The amorphous structure of Nd12ribbons showsnomagnetic coercivityforpoweroutputupto550Aasevidentfromthemagneticmeasurements.ThinNd9ribbons(around40µmthick)showacrystallizednanocompositestructurecontaining ahardmagnetic tetragonalNd

2Fe

14Bphaseandsoftmagneticα-Fe

phase.TheintensitydistributionoftheNd

2Fe

14B peaks indicates no texture

in the samples. Higher-power pulsesresultedinoverheatingandevenmelting,whichledtosignificantgraingrowthandsubsequentmagneticsoftening. Magneticmeasurementwasthencar-riedouttoanalyzethephasestructureand magnetic property evolution. Themeasurement showed that theas-spunribbonsofamorphousstructurearemag-

neticallyisotropicwithnearlyzerocoer-civity.Afterheat treatment,coercivitywasdeveloped.Figures2and3presentthehysteresisloopsobtainedforNd12andNd9samplestreatedwithdifferentPTPvariables.BecauseoftheextremelyshortprocessingtimeofPTPtechniquescompared to furnace processing, theannealingtime(i.e.,thedwelltimeandpulsenumber)willbeakeyparameterforcontrollingthemagneticproperties.ItcanbeseenthatforNd12samples,thestartingribbonmaterialsarecompletelysoft with undeveloped coercivity (<3Oe). After the PTP processing, mostsampleswerestillsoftwhileonlythose

processedwithapulsecurrentImax

above550Aandapulsedwelltimetof0.1shadbeendevelopedwithhardmagneticprop-ertiesalthoughtheircoercivityvaluesarequitemodest(200OeforI=525Aand500OeforI=550A).Prolongeddwelltimesdonotpromotethedevelopmentof magnetic hardening for the pulsecurrentbelow500A.Itwasconfirmedthatapulsecurrentgreaterthan550AisnecessaryforinducingcrystallizationoftheNd12samplewhilea600Apulsecurrenthasresultedintheburningtraceand reduction of magnetic properties,revealingaverynarrowPTPwindowfortheoptimizationofmagneticproperties.Thisnarrowprocessingwindowshouldberelatedtothelargethicknessoftheribbonsduetothelowthermalconduc-tion performance of materials relativeto the short pulse process time. Withreduced ribbon thickness, the exten-sionoftheoptimalprocessingwindowshouldbeexpected,asevidencedbytheresultforNd9samplesofaround40µmthick.Thehalfhysteresisloops(Figure3)showthatwhenthecurrentisabove550A,thesamples(B1andB2),whichhadaburnedappearance,showaverylowcoercivity,apparentlyduetograincoarsening.Acurrentbelow525A isnecessaryforachievinghighcoercivity.Thehighestcoercivityof5.2kOewasobtainedforthesampleprocessedwithconditionB8,whereapulsecurrentof400Awasappliedwitha shortpulseduration(10pulsesof0.25s).ItisnotedthatmostPTP-processedNd9sampleshavestepsonthehysteresisloops.Thisindicates that the exchange coupling

Figure 2. Hysteresis loops for Nd12 samples with different PTP variables. (A0) starting amorphous ribbons; (A1) five pulse currents with Imax of 600 A and 0.1 s dwell time for each pulse; (A2) five pulses, 550 A, 0.1 s; (A3) five pulses, 525 A, 0.1 s; (A4) 15 pulses, 450 A, 0.25 s; (A5) 15 pulses, 400 A, 0.25 s.

JOM • June 200648

betweenthehardandsoftphasesisnotstrong,whichmaybeduetothelargegrainsizeofironortothenon-uniformityinmicrostructurealongthecrosssection,which isvery sensitive to the thermalconductivity of the materials duringthethermalprocessing.Thisstudyalsorevealed that five heating pulses withdwelltimesof0.1sarenotenoughtoachieveoptimalmagneticpropertiesatthiscurrent(B7).Thecoercivityisonlyonefourthofthehighestvalue. With the comparison of magneticpropertiesofPTP-processedthickandthin ribbons, it isbelieved thatduringthe PTP treatment extremely high

energywithaveryshortdwelltimewasemployedinitiallyontheribbonsurfacetoproducealargethermalgradientwithinthesamples.Subsequentannealingwasrealizedbythermalconductionfromthesurfacetotheinsideoftheribbons.Thehuge vertical and lateral temperaturegradientwithintheribbonleadstothenon-uniformityofmicrostructurewherecrystallized and amorphous regionscoexist, resulting in theoccurrenceofstepsonthehysteresisloops.Thelargetemperaturegradientisrelatedtother-mal conductivity and capacity of thematerials.Athighcurrent,localelevatedtemperatureincurredatthesurfacemight

beabovethemeltingpoint,resultingintheoccurrenceofaburningtrace.Atlowcurrent, the thermal conductivity maynot be enough to fully crystallize theamorphousregionswithintheribbons,giving rise to thedecouplingbetweenthehardandsoftphases.

Morphology of the PTP Samples

Figure4showstransmission-electronmicroscopy(TEM)imagesofPTP-pro-cessedNd9samplescorrespondingtoanI

maxof525Awithfivepulsesof0.1s,an

Imax

of450Awith15pulsesof0.25s,andanI

maxof400Awithtenpulsesof

0.25s. It is interesting tonote that thePTP-processedsamplesapparentlyhavevery large grain sizes (100–200 nm),significantly larger than those (30–90nm) of conventional furnace-annealedsamplesandthose(20–50nm)ofRTPsamples.15 It seems that a high-pulse-current-processed sample has a morecleargrain interface than low-current-processed samples. The latter has anon-uniform size distribution, whichmayaccount for the large stepon theloopalthoughrelativelylargecoerciv-ity was obtained. Most of the grainsdonotshowsphericalmorphologybutan irregularandevenelongatedshapeof200nmgrainsize.Thisisdifferentfrom conventionally furnace-treatedandrapidlythermal-processedsampleswheremuchuniformmorphologywasobserved.High-resolutionTEMrevealedtheexistenceofremainingamorphousphaseasshowninFigure5,eveninthecaseof450A.This indicates that therapid crystallization and grain growthareverylocalized,possiblyduetothe

Magnetic Field (kOe)

150

100

50

150

100

50

150

100

50

012840–4–8

Mag

netiz

atio

n (c

mu/

g)

300 nm200 nm200 nm

Figure 4. Transmission-electron microscopy images of Nd9 samples subjected to PTP with different parameters. (a) Imax of 525 A with five pulses of 0.1 s, (b) Imax of 450 A with 15 pulses of 0.25 s, and (c) Imax of 400 A with ten pulses of 0.25s.

a b c

Figure 3. Half-hysteresis loops for Nd9 samples with different PTP variables. (B1, B2, B3) are five pulse currents of 0.1 s dwell time for each pulse and with Imax = 600 A, 550 A, and 525 A, respectively; (B4, B5, B6) are five pulses, ten pulses, and 15 pulses, respectively, of 0.25 s and with Imax of 450 A; (B7) five pulses, 400 A, 0.1 s; (B8, B9) ten and 15 pulses, respectively, with 400 A and 0.25 s.

2006 June • JOM 49

extremelyhighincidentenergy,whilenostructuraldefectwasobservedincrystal-lizedNd

2Fe

14Bgrains.Theexistenceof

largesoftgrainsandamorphousmate-rial easesdomainwallmovementandnucleationofreversaldomains,leadingtoreducedmagnetichardening.Whenthedimensionofthesoftmagneticregionis larger than the required exchangelength(~10nm),significantdecouplingwilloccurasrevealedinFigure3.ThisshowsafinetuningofPTPparametersisnecessarytoexcavatethepotentialinprocessingnanocompositemagnets.

concluSIonS

This study found that an annealingtimeasshortas0.5sinitiatesacrystal-lization transition froman amorphousphasetomagneticnanocomposite,whileprolonged processing is required foroptimizationofmagnetichardeningintheribbons.Thepoweroutputrequiredfor thin ribbons is apparently smallerthanthatforthickribbons.Atlowpower

output,theoptimizationofexperimentalparametersismorecontrollable.Crys-tallizedsamplesshowelongatedgrainmorphologywithameangrainsizeabove200nm.Althoughmoreworkneedstobedoneintheoptimizationofprocess-ingparameters,thecorrelationbetweenthe PTP parameters and the magneticproperties shows thatPTP iseffectiveforinvestigatingtherapidevolutionofmicrostructureandpropertiesofnano-structuredmagneticmaterials.

acknowledgeMenT

This work was supported by the U.S. Department of Defense/Defense Advanced Research Projects Agency through the Army Research Office under grant DAAD19-03-1-0038 and by the Multidisciplinary University Research Initiative program under grant N00014-05-1-0497. Portions of this research are sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, man-

aged by UT-Battelle, LLC for the U.S. Department of Energy under Contract No. DE-AC05-00OR22725.

References

1. R. Coehoorn, D.B. de Mooij, and C. DeWaard, J. Magn. Magn. Ma���.Ma���., 80 (1989), pp. 101–104.2. S.D. Li et al., J. Appl. Phys., 92 (2002), pp. 7514–7518.3. R. Skomski and J.M.D. Coey, Phys. R�v. B, 48 (1993), pp. 15812–15816.4. R.H. Yu et al., J. Appl. Phys.,Phys., 85 (1999), pp. 6034–6036.5. Z.M. Chen et al., J. Appl. Phys.,Phys., 89 (4) (2001), pp. 2299–2303.6. V. Neu and L. Schultz, J. Appl. Phys., 90 (2001), pp. 1540–1544.7. W. Liu et al.,Liu et al., J. Appl. Phys., 93 (2003), pp. 8131–8133.8. T. Schrefl, J. Fidler, and H. Kronmüller, Phys. R�v. B, 49 (9) (1994), pp.6100–6110.9. M. Yu, J. Appl. Phys., 83 (1998), pp. 6611–6613.10. J.P. Liu et al., J. Appl. PhysPhys., 85 (1999), pp. 4812–4814.11. J. Zhang et al., J. Appl. Phys., 89 (2001), pp. 5601–5605.12. Z.Q. Jin et al., Appl. Phys. ����.,Phys. ����., 84 (2004) pp. 4382–4384.13. Z.Q. Jin et al., Ac�a Ma���ialia, 52 (2004), pp. 2147–2154.14. Y. Shao, M.L. Yan, and D.J. Sellmyer, J. Appl. Phys., 93 (2003), pp. 8152–8154.15. K.T. Chu et al., J. Phys. D: Appl. Phys., 38 (2005), pp. 4009–4014.16. R.D. Ott et al., JOM, 56 (10) (2004), pp. 45–47.17. J.D.K. Rivard et al., Su�fac� Engin���ing, 20 (2004), pp. 220–228.

Z.Q. Jin, V.M. Chakka, and J.P. Liu are with the Department of Physics at the University of Texas at Arlington. Z.L.�angiswith theSchoolofMaterialsZ.L. �ang is with the SchoolofMaterialsSchool of Materials Science and Technology at the Georgia Institute of Technology in Atlanta, Georgia. P. �adokarP. �adokar and R.D. Ott are with the Materials ProcessingMaterials Processing Group, Metals and Ceramics Division at Oak Ridge National Laboratory in Tennessee.

For more information, contact Z.Q. Jin, �rno����rno��� Magnetic Techno�ogies Corp., 770 Lin��en �venue, Rochester, NY 14625; e-mai� jjin@arno���magnetics.com.

Figure 5. A high-resolution TEM image of Nd9 samples subjected to PTP with 15 pulses of 0.25 s dwell time at Imax= 450 A.

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