International Microwave Power Institute 43-1-�

Avijit Mondal1*, Dinesh Agrawal2 and Anish Upadhyaya1

1Department of Materials & Metallurgical EngineeringIndianInstituteofTechnology,Kanpur208016,India

2ThePennsylvaniaStateUniversity,UniversityPark,PA16802,USA*avijitm@iitk.ac.in

In recent years, microwave processing of metal/alloy powders have gained considerable potential in the field of material synthesis. Microwave heating is recognized for its various advan-tages such as: time and energy saving, rapid heating rates, considerably reduced processing cycle time and temperature, fine microstructures and improved mechanical properties, better product performance, etc. Microwave material interaction for materials having bound charge are well es-tablished, but for highly conductive materials like metals, there is not much information available to interpret the mechanism of microwave heating and subsequent sintering of metallic materials. The present study describes how the thermal profile of electrically conductive powder metal like copper changes with particle size and also with porosity content; in other words, initial green density when the material is exposed to 2.45 GHz microwave radiation in a multimode microwave furnace.

Submission Date: �August2008 Acceptance Date: 29December2008

Publication Date: 16January2009

Microwave Heating of Pure coPPer Powder witH varying Particle Size and PoroSity

Keywords: Cu metal powder, microwave sintering, particle size, porosity

INTRODUCTION

Microwave processing technology has beendeveloped in the field of ceramics and compos-ites.Microwaveheatingallowsaninstantaneousvolumetric,andthereforeamorerapid,heatingin comparison with the conventional heatingprocess. It also provides enhanced sinteringkinetics.As a result, it allows a reduction ofprocess temperature and time which leads toan increased productivity and a reduction ofenergyconsumption.Duringthelastdecade,themicrowavehasbeenusedforsinteringparticu-latemetals.The reportedmaterialsof interestincludedhardmetals,metal–ceramicfunctional

gradedmaterials,metalsandalloys[Rodigeret al.,1998;PoradaandBorchert,1996-97;Jainet al.,2006;Sethiet al.,2003;Anklekaret al.,2001]. Microwave sintering of metal powderswhich have high electrical conductivity is anew area with growing interest. It was first reportedin1999byRoyandcoworkers[1999]thataporous,powdermetalcompactcouldbeheated and sintered in a microwave field. At that time,thiswasconsideredsurprisingbecausetheelectricallyconductingmaterialsweresupposedto reflect microwave radiation. Later, otherresearchersalsodemonstrated thatallpowdermetalsatroomtemperatureabsorbmicrowavesand only bulk metals reflect the microwaves, allowingonlysurfacepenetration. However, these empirical studies did notattempttoexplaintheobservedheatingtrendsand presently there are few reports regarding

Guest Editor: Dr. Satoshi Horikoshi, Tokyo University of Science, Chiba, Japan

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the details of the direct interaction of micro-waveswithpowdermetalcompacts.SinteringmechanismsandactivationenergyfordiffusionwerestudiedbySaitou[2006]andheconcludedthatmicrowaveradiationdoesnotchange thesinteringmechanism fordiffusion in the caseofmetallicmaterialsintering.Thus,thereisnotmuchinformationavailablebywhichtointer-pretthemechanismofmicrowaveheatingandsubsequentsinteringofmetallicmaterials. Microwave heating of ceramic and otherdielectricallylossymaterialshavebeenwidelyinvestigated and the mechanism of micro-wave-materialinteractioniswelldocumented.Microwavepenetratesandpropagatesthroughdielectricmaterial,suchasSiC.Thisgeneratesan internal electric field (E) within a specific volume,whichinturninducespolarizationandmovementofcharges.Theresistance to theseinduced motions due to internal, electric andfrictional forces attenuates the electric field. Theselossesresultinvolumetricheating. The microwave-metal interaction is quitedifferent than that of ceramics. Being goodelectrical conductors, no internal electrical field is induced in metals. Microwave interactionwithmetalsisrestrictedtoitssurfaceonly.Thedepthofpenetrationinmetals,alsoknownasskin depth, is defined as the distance into the materialatwhichtheincidentpowerdropsto1/e (36.8 %) of the surface value. In general, theskindepthisrelativelysmallinmetals,sinceinthemicrowaveregime,theparticlesizesaremuchsmallerthanthewavelengthofmicrowaveradiation; the field across the particle is uniform andcausesvolumetricheating[Takayamaet al.,2006].However, for relativelycoarseparticle(>100µm), the heating may be conductive from outsidetotheinteriorofthepowder.AccordingtotheFaraday’seffectinaconductivematerial,a varying magnetic field generates an electric field that gives rise to eddy currents and sub-sequentresistivelosses.Henceametalpowdercompactwillstartabsorbingmoremicrowaveradiationwithanincreaseintemperaturedueto

anincreaseinskindepth. Manyresearchershaveaddressedtheroleofseveralparameterssuchassampleconductivityandfrequency[Maet al.,2007],andeffectofEand H field [Rybakov et al.,2006]uponthermalprofile which affect the microwave absorption bypurepowderedmetals.Intheliterature,theresearchershavealsoaddressedtheroleofpar-ticlesizeandinitialporosityuponthethermalprofile during microwave heating of different conductivemetalpowders[Rybakovet al.,2006;Mishra et al., 2006;Vaidhyanathan and Rao,1997]. This paper reports how the thermal profile of electrically conductive metal like copperchangeswithparticlesizeandalsowithporos-ity content (i.e., initial green density) when the material is exposed to 2.4� GHz microwaveradiationinamultimodemicrowavefurnace.

EXPERIMENTAL PROCEDURE

For the present study gas-atomized copperpowders of different particle sizes were sup-pliedbyAmericanChemetCorporation,USA.The average (d�0) particle sizes were 6, 12, 18, 63 and 383 µm respectively. The experimental part consists of two subsets. The first experiment investigates the effect of particle size duringtheheatingofpowderparticleinamultimodemicrowave furnace. The second experimentevaluatestheroleofinitialdensityduringtheheatingofmetalpowderinamultimodemicro-wave furnace. In the first experiment powders were uniaxially compacted at 400 MPa in toa cylindrical pellet (12.7 mm diameter and 5 mm average height) of green densities ranging between 60 to 83% of the theoretical density (8.9 g/cm3). For the second part of the experi-ment,cylindricalpelletsofthesamedimensionsof as-received gas-atomized copper powders(particle size 18 µm) were compacted using the samecompactionpressat90to3�0MPaintoa green densities ranging between 56 to 76% ofthetheoreticaldensity.Microwavesintering

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ofthegreencompactswascarriedoutusingamultimodecavity2.4�GHz,6kWcommercialmicrowavefurnace.TheexperimentalsetupisshowninFigure1.Amulti-layeredinsulationpackage was used to provide sufficient insula-tion to obtain high and uniform temperaturesthroughoutthesample.Theouterpackagewasmade of thick ceramic fiber (alumina and silicon carbide) sheets. A mullite tube was placed at the centreofthepackage,andsampleswereplacedinsidethemullitetube.Theentirepackagewasplacedonaturntabletoensureuniformexposureof the sample to the microwave field. A reduced atmospherewasmaintainedduringthesinteringby first creating a vacuum of 8-10 torr inside the furnace followed by back filling ultra high purity (UHP) hydrogen. Throughout the sinter-ing, hydrogen gas flow was maintained at 2 lpm. Thisgaswasdilutedwithnitrogengasbeforeventingtotheatmosphere.Forthisexperimentathinlayerofgraphitecoatingoutsidethemullitetube was used as a susceptor.The susceptorsusuallycoupleverywellwiththemicrowavesandareusedforinitiallyraisingthetemperatureofthecompact.Sinteringwascarriedoutfor30mininhydrogenatmospherewithdewpoint-

35°C. The power setting for the first experiment was 0.� kW starting power followed by 0.2kWincrementaftereach�min.Themaximumpower was 1.2 to 1.3 kW. The final temperatures achievedforthedifferentparticlesizesrangedfrom927°Cto100�°C,whilethetotaltimeof60min and power setting were fixed for all experi-ments.Forthesecondpartoftheexperiement,apowerof1.�kWwasmaintainedfromthestartto the finish of the experiement. Temperature was103�°Candtotaltimespan13min.Aftersintering, themicrowavepowerwasswitchedoffandsampleswereallowedtofurnacecool.Unlikeaconventionalfurnace,thetemperatureofthesamplesinsideamicrowavefurnacecan-notbemonitoredusingathermocouple[Pertet al.,2001].Thetemperatureofthesamplewasmonitored using an infrared pyrometer (Type: RAYMA25CCF; manufacturer: Raytek Co., Santa Cruz, CA, USA). The infrared pyrometer wascoupledwithdataacquisitionanddisplaysoftwareonapersonalcomputer.Thepyrom-eter is emissivity based and the temperaturemeasurementsforallthecompactsweredoneby considering emissivity of copper (0.65) [Nayer,1997].Typically,emissivityvarieswith

Figure 1. Schematic diagram showing the inside of the multimode microwave furnace used in the present study.

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temperature.However,asverylittlevariationintheemissivitywasreportedinthetemperaturerangeusedinthepresentstudy,hence,theef-fectofvariationinemissivitywasignoredinthepresentinvestigation.

RESULT AND DISCUSSION

Figure 2 compares the thermal profiles of the copperpowdercompactsofvaryingparticlesize,sinteredinmultimodemicrowavefurnace.Itisinterestingtonotethattheporousmetalcompactcouplewithmicrowavesheatsrapidly.Aspar-ticlesizeincreasestheheatingratedecreasesandaftercertaintimeheatingratebecomesconstantataparticularpowersetting.Itiswellknownthatmicrowavesforelectricallyconductingmaterialssuchascopperdonotpenetrateabulksamplebeyondtheskindepth.Theskindepthisgivenby the well-known expression:

Here, µ is the real part of the permeability taken tobe4× 10-7Tm/Afornonmagneticmaterials.

In theaboveexpression,σ andω are theDCelectrical conductivityandangular frequency,respectively.At2.4�GHz,theskindepthofbulkcopper is about 1.3µm. But, as in case of porous material,electricalconductivitydecreaseswithanincreaseinporosity.Therefore,basedontheaboveequation,effectiveskindepthshouldalsoincrease.Sothedifferenceintheheatingratescanbeattributedtothedifferenceinskindepthandalsothechangeinsurfaceareaperunitvol-umewiththechangeofparticlesize.Theeffectismorepronouncedforthelargersizepowdersbecausetheydonotretaintheirsingle-particlestatus when pressed into a compact and aremore likely to formelectricalcontactsduringdie pressing, making them effectively muchlarger.ThiswasexperimentallyprovedbyMaet al. [2007].Theymeasuredtheconductivityofa green sample made from 22 µm powder and foundthatitwas104timesasgreatasonemadefrom 3 µm powder. Another factor that influences the heat-ingbehavioristheinitialgreendensityofthemetal powder compacts. Figure 3 shows theexperimentaltemperaturerisewithtimeinpurecopper compacts with 56%, 65%, 71% and 76%

Figure 2. Thermal profiles of copper compacts as a function of particle size.

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the temperature range used in the present study,hence, the effect of variation in emissivity wasignored in the present investigation.

RESULT AND DISCUSSION

Figure 2 compares the thermal profiles of the copper powder compacts of varying particlesize; sintered in multimode microwave furnace.It is interesting to note that the porous metalcompact couple with microwaves and get heatedup rapidly. As particle size increases the heatingrate decreases and after certain time heating ratebecomes constant at a particular power setting.It is well known that microwaves for electri-cally conducting materials such as copper donot penetrate a bulk sample beyond the skindepth. The skin depth is given by the well-knownexpression:

Here, µ is the real part of the permeability taken to be 4 _10-7 Tm/A for nonmagnetic materials.In the above expression, _ and _ are the DCelectrical conductivity and angular frequency,respectively.At 2.45GHz, the skin depth of bulkcopper is about 1.3µm. But as in case of porous material electrical conductivity decreases withan increase in porosity so from the above equa-tion effective skin depth should also increase.So the difference in the heating rates can beattributed to the difference in skin depth andalso the change in surface area per unit volumewith the change of particle size.The effect ismore pronounced for the larger size powdersbecause they do not retain their single-particlestatus when pressed into a compact and aremore likely to form electrical contacts duringdie pressing, making them effectively muchlarger. This was experimentally proved by Maet al. [2007]. They measured the conductivityof a green sample made from 22 µm powder; and found that it was 104 times as great as onemade from 3 µm powder. Another factor that influences the heat-

Figure 2. Thermal profiles of copper compacts as a function of particle size.

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theoretical density, respectively, for a specific particlesizewhilekeepingconstantthepowersettingandtotalexposuretime.Itisinterestingto note that the heating rate increases as thegreen density decreases (porosity increases). Thehighertheporosity,thehighertheheatingrate.Itshouldalsobenotedthatforaparticularchoiceofexperimentalvariables,thetempera-tureriseisrestrictedtoacertainlevel.Thiscanbeattributedtothefactthattheheatgeneratedin the compact (due to electromagnetic power absorbed) is balanced by the convective and radiativeheatlosses.

CONCLUSION

Pure copper powders with varying mean sizes (6 to 383 µm) and having a range of initial porosity (24 to 44%) effectively couple with microwaves andheatedtohightemperatures.However,theheatingrateisdependentupontheparticlesizeas well as the initial as-pressed density.Thesmallerthepowdersize,thehighertheheatingrate.Atthesamepowerlevel,compactshavinghigherporosityheatatfasterrates.Irrespective

oftheinitialporosity,allthecompactsattainedthesamesteady-statetemperature.

ACKNOWLEDGMENTSThiscollaborativeresearchwasdoneasapartoftheCenterforDevelopmentofMetal-CeramicComposites through Microwave ProcessingwhichwasfundedbytheIndo-USScienceandTechnology Forum (IUSSTF), New Delhi. This workwasalsopartiallysupportedbyNationalInstitute for Fusion Science, Japan, which isgratefullyacknowledged.

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