and Characterization of Based Polymeric Magnesium...

FabricationandCharacterizationof

EthylcelluloseBasedPolymericMagnesium

DiborideSuperconductingTapes

By

YingLingLin

DepartmentofMiningandMaterialsEngineering

McGillUniversity,Montral,Qubec,Canada

August2008

AthesissubmittedtotheFacultyofGraduateStudiesandResearchinpartial

fulfillmentoftherequirementforthedegreeofMasterofEngineering

YingLingLin,2008

ABSTRACT

i

ABSTRACT

Magnesiumdiboridewas found tobeasimple intermetallicsuperconductor in2001with

the highest critical temperature to date at 39 K. Following this discovery thousands of

studies have been conducted into the synthesis andmodification of this simple binary

compound.Additionally,magnesiumdiboridehasbeenstudied inordertounderstandthe

fundamental physics of superconductivity. However, implementation of commercial

applicationshasbeenlimitedduetotheassociateddifficultyofproduction.Thecompound

itself is relatively cheap toproducehowever, standardpowderintubemethods forwire

production require multiple steps and could prove to be difficult to incorporate into

automatedproduction.

In this thesis project, twophase superconductor tapeswere produced by blending high

puritymagnesiumdiboridepowderwithaliquidethylcellulosebasedpolymericbinderand

simply leaving them to dry. Shaping the tapes required a simple cutting tool and some

peeling from the flexible aluminium substrate. The objective was to produce robust,

superconductive coatings which can potentially be shaped into any geometry including

wiresandtapeswithoutnecessitatingsinteringandpressingstepsforeventualcommercial

applications.Thetransitiontemperatureaswellasthecriticalfieldsweredeterminedusing

electrical transport and magnetization measurements. Fouriertransform infrared

spectroscopyusingaphotoacousticcellwasusedtodeterminethenormalstatevibration

modes of the two main components,MgB2 and ethylcellulose, in the superconducting

tapes.

All samples producedwith this newmethodwere found to be superconductive. Results

from transport measurements in normal atmosphere (1000 mbar) and magnetization

measurementsrevealedatransitiontemperatureof37.5K0.7K.Thecriticalcurrentand

ABSTRACT

ii

criticalcurrentdensitywerevery lowforallsamples,measuredatmosttobe4.21x103A

and1.85x101A/cm2.Theuppercriticalmagneticfieldat4.2Kwasashighas6.38Tforthe

besttapesandthe lowercriticalmagneticfieldwas0.27T.Thetransportpropertieswere

foundtobestronglydependentonthepressureoftheheliumatmospheresurroundingthe

samples.Thenormalresistanceimprovedatlowpressure.

Thesetapes,producedusingawetanddrymixingmethod,weresuperconductiveandeasy

to produce and demonstrate that superconductivity persists in a composite twophase

material. However, the low critical current density points to the presence of many

Josephson junctions. No vortexmotionwas observed and implies strong pinning forces

probablyduetothepolymercomponentwhichrestrictsvortexmotion.

RSUM

iii

RSUM

Le diboride de magnsium a t dcouvert comme tant un simple supraconducteur

intermtalliqueen2001avec laplushaute tempraturede transitiondatede39K.En

raisondesahautetempraturedetransition,denombreusesrecherchesonttfaitessur

la synthse et la modification de ce compos binaire et pour mieux comprendre les

principes fondamentauxde lasupraconductivit. Lecomposnestpascher;parcontre,

lesapplicationscommercialestant limites ladifficultde la facilitdeproduction, les

mthodes de production courantes, dont la poudre en tube, ncessitent des tapes

multiplesetpourraienttredifficilesintroduiredanslaproductionautomatise.

Dansceprojetdethse,desrubanssupraconducteursdedeuxphasesonttproduitsen

mlangeant la poudre de diboride de magnsium de haute puret avec un liant

polymriqueenformeliquidebasedthylcelluloseetpuisenlaissantscher.Lamiseen

formedesrubanssefaitsimplementencoupantavecunustensileetunetapedcaillage

du substrat daluminium. Lobjectif a t de produire des couchages supraconducteurs

robustesavecunpotentieldemiseenformedansnimportequellegomtriencessaire,

incluant fil ou ruban, sans les tapes de frittage et compression traditionnels pour des

applicationscommercialesventuelles.Latempraturedetransitionatdtermine lors

dexpriencesde transportlectriqueetdaimantation.Laspectroscopie linfrarougede

transforme de Fourier avec accessoire photo acoustique a t utilise pour la

dterminationdesmodesdevibrationdesdeuxcomposants,soitlediboridedemagnsium

etlthylcellulosedanslesrubanssupraconducteurs.

Tous les chantillons produits par cette nouvelle mthode ont montr de la

supraconductivit. Les rsultats des expriences de transport lectrique en atmosphre

normal (1000mbar) et des expriences daimantation ont dtermin la temprature de

RSUM

iv

transition tant de 37.5 K 0.7 K. Le courant critique et la densit de courant critique

dterminssontcependanttrsbas,4.21x103Aand1.85x101A/cm2,respectivement.Des

champs critiques infrieurs et suprieurs, 4.2 K, de 0.27 T et jusqu 6.38 T,

respectivement,onttobservs.Lespropritsde transportlectriquessontcependant

trs dpendantes sur leffet de pression de lhlium atmosphrique qui entoure les

chantillons.Larsistancenormalesestamliorebassepression.

Ces rubans, produits par une mthode de mlange de composs humides et secs,

supraconducteursetfacilesfabriquerdmontrequelasupraconductivitpersistedansun

matrieldeuxphases.Cependant,ladensitdecourantcritiquebassesuggrelaprsence

de plusieurs jonctions de Josephson. Lemouvement de vortex na pas t observ et

impliquequedepuissantspointsdancrageprobablementdusaucomposantpolymrique

quirestreignentlemouvementdevortextprsents.

ACKNOWLEDGEMENTS

v

ACKNOWLEDGEMENTS

Id liketothankmywiseand imaginativesupervisors,ProfessorMihribanPekguleryuzand

ProfessorMichaelHilke.IthankMihribanformentoringmeforthesecondtimeandnever

ceasing toencouragemeandmake time formewhen Ineededguidanceandadviceand

historylessonsofthefareast,whennecessary;shehasbeenbothasupervisorandagood

friend.IthankMichaelforhisunendingsearchtomakesenseoftheseeminglynonsensical,

patienceandhishighqualitylowtemperaturelabandequipment.

I thank the lightmetals researchgroup includingMr.PierreVermette,whoseknowledge

andexperiencehavehelpedtheprojectrunsmoothly,Erol,Xin,Mert,ElviandAna.Aswell,

Id liketothankDr.MirelaBarsanandPetrFiurasek intheChemistrydepartmentfortheir

help and access to theirATR FTIR spectroscope.A completepictureof the FTIR aspect

couldnotbepossiblewithoutthehelpofDr.SamirElouatikatlUniversitdeMontralfor

hishelpandpatiencewithRamanspectroscopyandPAFTIR.

Iwould also like to thankMichaels research group including Sophie andAlistairBrown

Armstrongandforwelcomingintotheirgroup.IdalsoliketothankProfessorDominicRyan

and Ph.D. candidate Chris Voyer for access to their essential lab, help, patience and

guidancewithmagnetizationexperiments.

MydeepestgratitudegoestoPh.D.candidate,JosianneLefebvre,forshowingmeintheins

andouts,valveopeningsandclosings,analogiesofthelowtemperaturecondensedphysics

labandforherunendingpatience,helpandsupervision,herhelpacceleratedmyprogress

throughthisvery interdisciplinaryprojectand itwouldnothavebeenpossibleto learnso

muchsofastandconductexperimentsofsuchqualitywithouther.

ACKNOWLEDGEMENTS

vi

IndustrialhelpwasgivenbyDowChemicaland theNationalResearchCouncilAerospace

Manufacturing Technology Centre and I thank them as they helped explore different

production routes.Aswell,DoctorGeorgeVanderVoortatBuehler Inc.helpedmegeta

clearpictureofthedifficulttoprepareMgB2particlesthroughhismetallographyexpertise

atBuehler.

Iwouldalsoliketothankmygoodfriends,ErolOzbakir,GenellTongeandAnnaLabarias,for

theirsupportandwhohaveenduredmyrantingandravingsduringcrunchtimes.Lastlyand

hardly leastof all, Id like to thankmyparents for their support, readymademeals and

laundryservicewhenthingsgottoobusy.

TABLEOFCONTENTS

vii

Table of Contents

Chapter1INTRODUCTION........................................................................................................1

1.1Superconductors.............................................................................................................1

1.2MagnesiumDiboride.......................................................................................................3

Chapter2THEORETICALBACKGROUND...................................................................................6

2.1SuperconductorMaterials...............................................................................................6

2.2MagnesiumDiboride.......................................................................................................8

2.3BackgroundTheory.......................................................................................................12

2.3.1SuperconductivityHistoryofDiscoveries............................................................12

2.3.2MeissnerOchsenfeldEffect...................................................................................14

2.3.3ElectrodynamicsofSuperconductivityandtheLondonEquation.........................15

2.3.4ThermodynamicsofSuperconductivity,GinzburgLandauTheory........................16

2.3.5BardeenCooperSchrieffer(BCS)Theory...............................................................17

2.3.6Shubnikov(Mixed)StateinTypeIIsuperconductors............................................20

2.3.7VortexPinning........................................................................................................22

2.3.8SingleandDoubleEnergyGaps..............................................................................24

2.3.9JosephsonJunctionEffectandJosephsonJunctionArrays....................................26

TABLEOFCONTENTS

viii

2.3.10DirtySuperconductors..........................................................................................30

2.4BackgroundonPolymers...............................................................................................31

2.4.1BackgroundonConductivePolymers.....................................................................31

2.4.2BackgroundonEthylcelluloseBasedBinder..........................................................32

2.5Summary.......................................................................................................................35

Chapter3ExperimentalMethod............................................................................................37

3.1Materials........................................................................................................................37

3.2SampleFabrication........................................................................................................38

3.2.1PreliminarySampleProductionRoute...................................................................38

3.2.2Polymer/MgB2Tapes..............................................................................................41

3.3MaterialsCharacterizationofCoatings.........................................................................43

3.3.1Stereoscopy............................................................................................................43

3.3.2FieldEmissionGunScanningElectronMicroscope(FEGSEM)...............................43

3.3.3XrayDiffraction......................................................................................................44

3.4ElectricalContacts.........................................................................................................44

3.4.1FourProbeTransportMeasurements....................................................................45

3.4.2MagnetizationExperiments...................................................................................55

TABLEOFCONTENTS

ix

3.5FourierTransformInfraredandRamanSpectroscopy.................................................56

Chapter4RESULTSANDDISCUSSION.....................................................................................57

4.1StereoMicroscopyandFieldEmissionGunScanningElectronMicroscopy................57

4.1.1EnergyDispersiveSpectrometry............................................................................59

4.2XRayPowderDiffraction..............................................................................................61

4.2.1XRDofMgB2powder..............................................................................................61

4.2.2XRDofMgB2/PolymerFilm.....................................................................................63

4.3SuperconductorCharacterization.................................................................................64

4.3.1PressureDependence.............................................................................................65

4.4SuperconductiveTransportProperties.........................................................................67

4.4.1CriticalTemperature,Tc..........................................................................................67

4.4.2MagnetizationMeasurements...............................................................................87

4.5RamanandFourierTransformInfraredSpectroscopy.................................................91

4.6Synopsis.........................................................................................................................98

Chapter5CONCLUSIONS......................................................................................................100

Chapter6RECOMMENDATIONSFORFUTUREWORK..........................................................103

6.1Mechanicaltesting......................................................................................................103

TABLEOFCONTENTS

x

6.2SuperconductiveCharacterization..............................................................................103

6.3XRD,FTIRandRamanSpectroscopy..........................................................................104

6.4Conductivepolymer,polyacetylene,variation...........................................................105

6.5Dopants.......................................................................................................................105

REFERENCES.....................................................................................................................118

LISTOFFIGURES

xi

List of Figures

Figure 1.1. Superconductor commercial applications showing (a) Philips 3T Achieva, high

fieldclinicalMRIscannerand(b)JRMaglevhighspeedtraininJapan...................................1

Figure1.2.Publishedpicturesshowing(a)MgB2wiresastheyappearafterremovalfromthe

tantalum tube and (b) SEMmicrograph of theMgB2wire compared to boron precursor

filamentintheuppercorner....................................................................................................4

Figure2.1.StructureofBSCCO.................................................................................................7

Figure2.2.Representationofa)thecrystalstructureofMgB2andb)opticalmicrographof

MgB2powderunderpolarizedlight..........................................................................................9

Figure2.3.Publishedphasediagrams:a)CalculatedPhaseDiagramforBMgSystem(2004)

andb)MgBphasediagramat4.5GPa(2003).......................................................................10

Figure2.4.Criticalsurfaceofasuperconductorwithcriticaltemperature,carryingcapacity

andmagneticfieldindicatedintheaxes................................................................................13

Figure2.5.RepresentationofavortexcoregeneratedbyappliedmagneticfieldandtheBCS

characteristiclength...............................................................................................................16

Figure2.6.MagnetizationCurvefora)TypeIandb)TypeIISuperconductors....................20

Figure2.7.Crosssectionofatype IIsuperconductorandthechangeasexternalmagnetic

fieldisincreased.....................................................................................................................21

Figure2.8.BTphasediagramsfortypeI(a)andtypeII(b)superconductors......................22

Figure2.9.DoublebandgapsofMgB2...................................................................................26

LISTOFFIGURES

xii

Figure2.10.SchematicofaCooperpaircrossingaJosephsonjunctionfromlefttoright....28

Figure2.11.Chemicalbondstructureofpolyacetylene.........................................................31

Figure2.12.Chemicalbondstructureofcellulose.................................................................32

Figure2.13.Chemicalbondstructureofethylcellulose.........................................................33

Figure2.14.Chemicalbondstructureofbutylcellosolveacetate.........................................33

Figure2.15.LoadElongationCurvesforEthocelpolymers....................................................35

Figure3.1ColdpressedMgB2/MgpelletimmersedinMg....................................................39

Figure3.2.Pelletof90wt.%Mgto10wt.%MgB2ratioa)beforeandb)afterhotpressing.

.................................................................................................................................................39

Figure3.3LasermeltedBpasteonMgsubstrateshowingdifferentpasses.........................40

Figure3.4.Wetpolymerbinderbeforeadditionofmetallicpowder....................................42

Figure3.5.FourProbetransportmeasurementconfiguration..............................................45

Figure3.6.Threetypesofcontactsused:a)layeredAgpaintandAgwiressolderedtoInb)

AgpaintconnectedtoAgwiresandc)AgwiresconnectedtosemisolidpolymerMgB2tape.

.................................................................................................................................................46

Figure3.7.StereoscopicImageofSample102withfivecontactsattachedtopins14,12,4,

10and9andaPtRTDwithtwobridgedpins(11and13)and(1and2)...............................46

Figure3.8.Locationschosenwithrespecttotheendcontactsdepositedonthesampleand

usedtominimizeerrorwhenmeasuringlength,widthandthickness..................................47

LISTOFFIGURES

xiii

Figure3.9.14pinspecimenholdershowing fourcontactsat10,4,2and13withbridged

pins8and9and7and6forPtRTDconnections...................................................................48

Figure3.10.Schematicinternalcomponentsoftheclosed3Hesystem................................49

Figure3.11.Lowtemperaturephysics laboratorysetupshowingthedewardippingprobe

(I),the3Hesystemdippingprobe(II),aLHedewar(III),magnet(IV),andresistancebridges

(V)............................................................................................................................................50

Figure 3.12. Schematic of the sourcemeter, sample and data acquisition system

configurationfortemperaturesweeps..................................................................................51

Figure3.13.Schematicof thesourcemeter,sample,dataacquisitionsystemconfiguration

andmagnetic field source showingdirectionofvectorsas theypenetrate the sample for

magneticfieldsweeps............................................................................................................53

Figure3.14.Schematicofsourcemeters,sampleanddataacquisitionsystemconfiguration

forcurrentsweeps..................................................................................................................54

Figure 3.15. Dried coatings formagnetizationmeasurements a) pieces ofMgB2 polymer

coatingsandb)piecesplaced intogelcapsandc)WetmixtureofMgB2andethycellulose

polymerformagnetizationmeasurements............................................................................55

Figure4.1.Au/PdSputterCoatedPolymerCoating...............................................................57

Figure4.2.FEGSEMImageofthetopofanMgB2coating.....................................................58

Figure4.3.FEGSEMImageofthebottomofanMgB2coating...............................................59

LISTOFFIGURES

xiv

Figure4.4.EnergydispersivespectrumofasquarerasterofMgB2polymercoatingsupto5

keVwiththeKpeakslabelled..............................................................................................60

Figure4.5.EnergydispersivespectrumofalargeareaofMgB2polymercoatingsupto5keV

withtheKpeakslabelled.....................................................................................................61

Figure4.6.XraydiffractionpatternofMgB2powdershowingthephasespresent.............62

Figure4.7.XraydiffractionpatternofMgB2polymercoating..............................................63

Figure4.8.Stereographsofsamplesa)64b)67andc)84.....................................................65

Figure 4.9. Pressure effect on resistance of selected samples 64, 67 and 84 at room

temperaturewithoutcooling.................................................................................................66

Figure4.10.Stereographsofa)sample19andb)sample102..............................................68

Figure4.11.Temperaturetracesofsample19inopenatmosphere.....................................69

Figure4.12.Stereographsofsamplesa)26andb)111.........................................................71

Figure4.13.Temperaturetraceforsample26duringthefirstcoolingcycle........................71

Figure4.14.Magneticfieldsweepcurveforselectedsamples..............................................73

Figure4.15.Magneticfieldsweepcurveofsample19undervacuum..................................74

Figure4.16.Magneticfieldsweepforsample26...................................................................76

Figure4.17.Magneticfieldsweepsforsample38usingdrivingcurrentsof0.3,1,3and10

A............................................................................................................................................77

LISTOFFIGURES

xv

Figure 4.18. VI characteristic curve of sample 102 in liquid helium at 4.2 Kwith a 10x

zoomed insection located inthe lowerrightcorner illustratingtheconstruction linesused

todetermineIc........................................................................................................................80

Figure4.19.VIcharacteristiccurvesofsample19ina)liquidheliumandb)undervacuum

at4.2K....................................................................................................................................80

Figure 4.20. VI curves for all samples tested under vacuum with the approximate

temperaturemeasuredbythecryostatsCernoxRTD.Allcurvesrananticlockwise...........82

Figure4.21.ZoomedinviewofthepreviousVIcurvesshowingmoreclearlysamples19,23,

26and111undervacuumwith theapproximate temperaturemeasuredby thecryostats

CernoxRTD.Allcurvesrananticlockwise,includingthetwolowestcurves........................82

Figure4.22.VIcharacteristiccurveofsample26undervacuumat4.2Katzerofieldand2T

magneticfieldwithallfourcurvesrunninganticlockwise....................................................84

Figure4.23.MagneticfieldsweepofMgB2powderinethylcellulosepolymer.....................88

Figure4.24.Magnetizationcurveshowingtheupperandlowercriticalmagneticfields.....89

Figure4.25.TemperaturesweepofwetanddryMgB2/polymersamples............................90

Figure 4.26. Raman spectra for ethylcellulose binder showing characteristic peaks under

different operating conditions (dwell time, laser wavelength,magnification and percent

power).....................................................................................................................................91

Figure4.27.ATRFTIRspectrumforMgB2powder................................................................92

Figure4.28.PAFTIRspectrumofethylcellulosebindershowingcharacteristicpeaks........93

LISTOFFIGURES

xvi

Figure4.29.PAFTIRspectrumofMgB2powdershowingcharacteristicpeaksfortherange

of2000to450cm1.................................................................................................................94

Figure4.30.PAFTIRspectrumofethylcellulosepolymerbinder,MgB2powderandcrushed

MgB2polymercoating............................................................................................................96

FigureA.1.Fitfor0to0.01662Z..........................................................................................106

FigureA.2.Fitfor0.01662to0.08237Z...............................................................................106

FigureA.3Fitfor0.08237to0.15605Z................................................................................107

FigureA.4Fitfor0.15605to10Z.........................................................................................107

FigureC.1.Magneticfieldtraceofsample15......................................................................113




FigureC.5.VIcurveofsample38atzerofield,1Tand2Tmagneticfields.......................115

FigureC.6.VICurveofsample44atzerofield,1Tand2Tmagneticfields.......................115

Figure D.1. ATR FTIT spectra ofwet (ethocelmixwet) and dry (dried ethocel) polymer

binderandcomponents(cellosolve,terpineolandethocelstd45)..116

FigureD.2. ATR FTIR spectra ofwet (ethocelmixwet) and dry (dried ethocel) polymer

binder, MgB2 powder (MgB2 pwd) and dried MgB2/polymer coating (dried MgB2

coating)...117

LISTOFTABLES

xvii

List of Tables

Table1.1.CURRENTANDEMERGINGAPPLICATIONSOFSUPERCONDUCTORS......................2

Table2.1.BASICPROPERTIESOFMAGNESIUMANDBORON..................................................9

Table2.2.MATERIALPROPERTIESOFMgB2...........................................................................10

Table2.3.LISTOFSUPERCONDUCTORPROPERTIESOFMgB2...............................................11

Table2.4.GENERALPROPERTIESOFETHOCELSTANDARD45...............................................34

Table3.1SUMMARYOFTHEMATERIALSUSEDTOPRODUCECOATINGS.............................38

Table3.2.WEIGHTLOSSOFETHYLCELLULOSEPOLYMER(UPTO12DAYS)..........................42

Table3.3.WEIGHTLOSSOFMgB2POLYMERCOATINGRECORDEDUPTO3DAYS...............43

Table4.1. SUMMARYOFRESISTANCEANDRESISTIVITYOF SAMPLESBEFOREANDAFTER

HIGHVACUUMPUMPING.......................................................................................................67

Table4.2. SUMMARYOF TRANSITION TEMPERATUREANDWIDTHSOF SAMPLES19AND

102..........................................................................................................................................69

Table 4.3. GEOMETRIC INFORMATION ON SAMPLES 19 AND 102 USED IN OPEN

ATMOSPHERETEMPERATURETRACES...................................................................................70

Table 4.4. SUMMARYOF THE PROPERTIESOF SAMPLES 26 AND 111 ALL TAKENUNDER

VACUUM.................................................................................................................................72

Table4.5.UPPERCRITICALMAGNETICFIELD,Bc2,VALUESFORSELECTEDSAMPLESAT4.2K

.................................................................................................................................................78

LISTOFTABLES

xviii

Table4.6.EXTRACTEDGINZBURGLANDAUCOHERENCELENGTHSFORSELECTEDSAMPLES

.................................................................................................................................................78

Table4.7DIMENSIONSOFSAMPLE19FORTWOCONTACTCONFIGURATIONS...................81

Table 4.8. SUMMARY OF Ic AND Jc VALUES FOR SELECTED SAMPLES, * DENOTES LHE

WHEREASIFNOTLABELLEDCORRESPONDTOVACUUMCONDITIONS.................................84

Table4.9.IcRnPRODUCTFORSELECTEDSAMPLES,*DENOTESOPENATMOSPHERE(IFNOT

LABELLEDCORRESPONDTOVACUUMCONDITIONS)............................................................85

Table4.10.DIMENSIONSOFSAMPLE102FORTWOCONTACTCONFIGURATIONS..............86

Table 4.11. WAVENUMBERS FOR ETHYLCELLULOSE AND MgB2 WITH EXPERIMENTAL

WAVENUMBERSDETERMINEDINTHECRUSHEDMgB2/POLYMERTAPESPECTRUM...........95

Table 4.12 PUBLISHED AND EXPERIMENTALWAVENUMBERSOF ETHYCELLULOSE BINDER

AND CRUSHED MgB2/POLYMER TAPE DETERMINED FROM FTIR AND RAMAN

SPECTROSCOPY.......................................................................................................................97

Table4.13.SUMMARYOFTHESUPERCONDUCTORPROPERTIESOFMgB2POLYMERICTAPES

.................................................................................................................................................98

TableB.1MgB2POWDERPEAKLIST.....................................................................................108

TableB.2IDENTIFIEDPATTERNSLISTOFMgB2POWDER....................................................108

TableB.3MgB2POWDERANDPOLYMERBINDERPEAKLIST...............................................109

TableB.4.IDENTIFIEDPATTERNSLISTOFMgB2POWDERANDPOLYMERBINDER.............109

LISTOFTABLES

xix

TableB.5.MgB2PEAKLIST,REFERENCECODE000381369................................................110

TableB.6.MgOPEAKLIST,REFERENCECODE000040829.................................................110

TABLEB.7.MGPEAKLIST,REFERENCECODE000350821.................................................111

TABLEB.8.CPEAKLIST,REFERENCECODE000261081.....................................................112

CHAPTER1INTRODUCTION

1

Chapter 1 INTRODUCTION

1.1 Superconductors

Superconductors are fascinating materials not yet fully understood but gaining ever

increasing use (Table 1.1). Superconductors find theirmain uses inmagnetic resonance

imaging devices and highspeed trains. Magnetic resonance imaging (MRI) devices in

medicineproducethehighmagneticfieldsneededtogeneratedetailedthreedimensional

imagesofthehumanbodyandmakeuseofsuperconductors(Figure1.1a)[1].Currently,a

prototypemagneticlevitatingtrainutilizingsuperconductorshasbeenconstructedinJapan

(Figure1.1b)[1]).Thecurrentrecordforfastesttrain,heldbyamagneticlevitatingtrain,is

581km/hour.

(a) (b)

Figure1.1.Superconductorcommercialapplicationsshowing(a)Philips3TAchieva,highfieldclinicalMRIscannerand(b)JRMaglevhighspeedtraininJapan.

The largehadroncollider located inCERN,Switzerland, isa largescaleparticleaccelerator

whichhasbeenscheduledtocomeonlineinAugustof2008andmeasures27kilometresin

diameterandutilizesthemostnumberofsuperconductorsintheworldandcanaccelerate

particlesuptoenergiesof1,150TeV(1.15x1015eV).Superconductorscanalsobeusedfor


2

applications includingkineticenergystoragedevices (flywheels)andconductors inenergy

powergrids.Theabilityofsuperconductorstoconductelectricitywithzeroresistancecan

beexploited in theuseofelectrical transmission lines.The JosephsonEffectwhich is the

tunnelling of a pair of electrons between superconductors separated by an insulating

barrieristhebasisoftheJosephsonjunctionwhichcanbeusedasswitchingdevices(e.g.in

computermicrowavedetectors,magnetometersandSQUIDS).Table1.1 listspossibleand

currentapplicationsofsuperconductorsforvariousscientificfields.

Table1.1.CURRENTANDEMERGINGAPPLICATIONSOFSUPERCONDUCTORS

Field Application Current Emerging

Medical magneticresonanceimaging x biotechnicalengineering X

Electronics

SQUIDs x transistors x JosephsonJunctiondevices x circuitryconnections x particleaccelerators x sensors x

PowerGeneration

Motors XGenerators XEnergyStorage XTransmission XTransformersandInductors x Fusion X

Transportation Magneticallylevitatedvehicles XMarinepropulsion X

Industrial

separation x magnets x sensorsandtransducers Xmagneticshielding X

The limiting factor for the widespread use of conventional lowtemperature

superconductors(LTS),suchasniobiumtinandniobiumtitanium intermetallics isthecost

ofcoolingthemtoaround4.2Kwith liquidheliumtechnology.Thekey factor thatwould


3

increaseapplicationsof superconductors is increasing the critical temperature. With the

adventofhightemperaturesuperconductors (HTS)andthediscoveryofmaterialssuchas

mercurybariumcalciumcopper oxide (HgBa2Ca2Cu3O8), which are superconducting at

temperaturesashighas134K,applicationscouldbemorefeasible.Liquidnitrogen(77K)

cooled superconductors have provided industry with more flexibility to utilize

superconductivityascomparedtoliquidheliumsuperconductors.

1.2 Magnesium Diboride

Thousands of elements and compounds have been found to exhibit superconductive

behaviour.Amongthese,isthesimple intermetalliccompound,magnesiumdiboride,from

hereonreferredtoasMgB2.TheadvantageofMgB2 isanoptimalcombinationofsimple

crystal structure and a medium transition temperature. It is also a surprising

superconductormaterialsinceitismetallic.

Japanesescientistsdiscoveredin2001thatMgB2issuperconductiveatupto39K[2],hence

MgB2 joins a small group ofmaterials known as lowtemperature superconductors (LTS)

includingniobiumtin(Nb3Sn)discoveredinthe1960s.Butwhileniobiumtinisexpensiveto

produceandmustbekeptcooledusingcostly liquidhelium,whichhasatemperature just

aboveabsolutezero4.2K,magnesiumdiborideoperatesatamuchhighertemperature

(39Kvs.18K)thanmanyothermagnetmaterials.Althoughthetransitiontemperatureis

notashighasliquidnitrogentemperature,advancesincryogenictechnologies,forexample

cryocoolers operating at 20 K, sufficiently cool and promote the future use of MgB2

superconductors.Recently,acryogenfree,MgB2basedopenatmospheremagnethasbeen

developed foruse inMRIandoperatesat20Kwithcryocoolerswhosecoolingpower is

derivedfromelectricalpower[3].TheMgB2MRIdevice iscapableofgeneratingmagnetic

fieldsupto0.5Teslaandutilizes18kmofMgB2superconductingwires.


4

The poormechanical properties of bulk and brittleMgB2mean that currently, themost

practical and widely studied form of thematerial is in a powderintube configuration.

StudieshavebeendoneonsingletubesofMgB2containedinashellofsomesheathmetal

orseveraltubesspacedequallyapartandsetinanarray.Ametal/polymercoatedwirehas

not been studied yet however polymers have been used inmelt suspension spinning of

MgB2wiresasafirststepinthetechniqueofsingletubes.

(a)(b)

Figure1.2.Publishedpicturesshowing(a)MgB2wiresastheyappearafterremovalfromthetantalumtubeand(b)SEMmicrographoftheMgB2wirecomparedtoboronprecursorfilamentintheuppercorner.

MgB2 iscurrentlyonlyavailable inapurepowderform.MgB2powdercanbesynthesized

bysinteringstoichiometricamountsofMg(99.9%)andfinecrystallineBpowder(99.5%

100m) inTatubesandsealedattheendsunderapartialpressureofArandexcessMg

[4].MgB2wireshavebeen synthesized fromboron fibres (TextronSystems)withexcess

magnesium powder placed in tantalum tubes, under partial pressure of Ar, and then

sealing the tubes at both ends, Figure 1.2. The Ta tubes are then placed in quartz

ampoules,areagainsealed,with~175mbarofAr,andheated inabox furnaceat950C

[5]. After reaction, the quartz tubes are quenched in cold running water. S. Jin and

coworkersofBellLabshavefabricatedironcladMgB2byswagingthepowderinFetubes


5

andthensintering.Fromdirectmeasurementsafterremovalofthecladding,ahighcritical

currentdensity,Jc,wasconfirmedat85,000A/cm2withatransitiontemperature,Tc,of39

K and the formability of Fewas an advantage over Ta in these studies [6]. There are

ongoing efforts (Hyper Tech Research Inc.) to develop lowcost long length (50100m)

MgB2superconductorwireformedical(MRI)andpowerutilityapplications.

Thisthesisprojectwillfocusonanewcompositeformofthesuperconductivematerial:that

ofmagnesiumdiboridepolymercoatings.Thesepolymersuperconductivecompositesare

not limited to coatingsbut canbe shaped intowireand tape (thick coatings) forms.The

majorfocuswillbetodetermineifMgB2powdersconsolidatedinapolymerbinder(which

doesnotexhibitsuperconductivity)canmaintainsuperconductivebehaviour.

CHAPTER2THEORETICALBACKGROUND

6

Chapter 2 THEORETICAL BACKGROUND

BACKGROUND ON SUPERCONDUCTOR MATERIALS, MAGNESIUM DIBORIDE AND THEIR

APPLICATIONS

2.1 Superconductor Materials

Thefirstsuperconductormaterialdiscoveredwaspurifiedmercuryandwasdiscoveredby

HeikeKamerlinghOnnesandhisdoctoral student,GillesHolst, in1911 followingOnnes's

successful liquefactionofhighpurityheliumgasobtainedfrommonazitesand in1908[1].

The experiment showed that for certainmetals in very narrow temperature ranges the

value of the resistance becomes zero. The transition to superconductivity occurs very

sharplyascontrastedtoagradualdecreasetoa limitingresidualvalue,asseen innormal

metals.

Presently, several superconductive materials exist and they can be classified into the

following general categories: pure metals (Hg), intermetallics (Nb3Sn), high critical

temperatureceramics(YBa2Cu3O(7x))and,organicmolecules.Certainorganicmoleculesthat

exhibitsuperconductivityincludesinglewalledcarbonnanotubes,aswellasfullerene,with

a complex spherical structuremade up of 60 carbon atoms arranged in hexagons and

pentagons[7,8].Surprisingly,themostefficientnormalconductorsincludingCu,AgandAu

havenotbeen foundtobesuperconductive.Electronsmoving ingoodconductorsdonot

interactmuchwith the lattice, this ispreciselywhy theyaregoodconductors,so there is

notenoughinteractionbetweentheelectronsinagoodconductorandthelatticevibrations

(phonons) to induce strongelectronphonon interactionsnecessary to create theCooper

pairs, which are responsible for the superconducting behaviour in conventional

superconductors.


7

The impediment to thewide use of superconductors is the requirement for low (liquid

helium) temperature conditions.With the advent of hightemperature superconductors

(HTS) and the discovery of materials including mercurybariumcalciumcopper oxides

(HgBa2Ca2Cu3O8),which are superconducting at temperatures as high as 134 K, practical

applications became more feasible. Liquid nitrogen cooled copper oxide based

superconductors havemade possible wider use of superconductors compared to liquid

heliumcooledsimplersuperconductors.

High temperature superconductors have their own

technical challenges.HTS are brittle ceramicmaterials

withcomplexcrystalstructuresinwhichelementssuch

asyttriumandbarium,orlanthanumandstrontium,are

sandwiched between layers of copper and oxygen

atoms. This layered atomic structure causes the

materials to have highly anisotropic physical and

superconductingproperties.Itispossibletoformsingle

crystals, thin films or polycrystalline structures from

these materials but their properties generally

deterioratewiththelengthofthewireorsufferbreaks

duetothebrittlenatureoftheceramic;also,grains in

HTSimpedesupercurrentflow.Inordertobeusefulfor

largescaleapplications,longflexiblewiresarerequired

buttheseareoftendifficulttoproduceand/oruse.

Figure2.1.StructureofBSCCO.

There are a number of manufacturing methods for HTS (i) thin epitaxial films of

superconductormaterialgrownon long flexiblesubstrates,or (ii)polycrystalline filaments

of superconductor supported in ametallic wirematrix, which need to ensure that the

supercurrentswill flowadequately fromgraintograin.Bismuthstrontiumcalciumcopper


8

oxides (BSCCO), Bi2Sr2Ca2Cu3O10, and Bi2Sr2CaCu2O8, Figure 2.1, have been successfully

textured (grainaligned) as wires [9]. However, more work is required to develop

superconductors with the combined optimum characteristics of LTS (simple crystal

structures)andHTS(hightransitiontemperatures).

The possible discovery of room temperature superconductorswould bring deviceswith

superconductor components into everyday life as theywould start to replace even the

mostefficientmetal conductors. InMarchof2008,amaterial superconductiveat185K

withaproposedchemicalformulaof(Sn1.0Pb0.5In0.5)Ba4Tm5Cu7O20+hasbeendiscovered[8]

butonly timewill tell if theseHTSmaterialswillbesufficientlyrobust to findcommercial

applications.

2.2 Magnesium Diboride

As themolecular formula states,MgB2s HCP structure is composed of a ratio of one

magnesiumatomtotwoboronatomsandthebondingbetweentheatomsisamixofionic,

covalentandmetallicbonds[10].ThepropertiesoftheseparateelementsaregiveninTable

2.1.Itsstructureissimplehexagonalclosepacked,AlB2typewithspacegroupP6/mmm.

Structurally, magnesium diboride is a simple intermetallic compound. The three

dimensionalstructureoftheunitcellisthatofahexagonalclosepackedsystemcomprising

of alternating layersofMg andB. It is represented in Figure 2.2 a) [11].And an optical

micrographofMgB2powder ispresented inFigure2.2b) [12].Thiscompoundwhichwas

first synthesized in the 1950s was discovered to be superconductive by a team of

researchersledbyJunAkimitsuin2001[11]andthus,initiatedaworldwideeffortintothe

studyofthissimple intermetallicsuperconductorwithahighTcfor itsclass.Followingthis

discovery,researchersaroundtheworldhavestudiedandpublishedkeyfindings.


9

Table2.1.BASICPROPERTIESOFMAGNESIUMANDBORON

Properties Boron MagnesiumAtomicnumber 5 12Valence 3+ 2+Atomicweight(amu) 10.81 24.31Densityat293K(g/cm3) 2.34 1.738Crystalstructure rhombohedral HCPAtomicRadius(nm) 0.085 0.160AtomicVolume(cm3/mol) 4.6 13.97MeltingPoint(K) 2300 1378HeatofFusion(Kj/mol) 50.20 8.954HeatofVaporization(kJ/mol) 489.70 127.40Hardness(mohs) 9.5 2ThermalConductivity(J/msecdeg) 27.4 156ElectricalResistivityat20C(m) 1.5104 43.9x109

(a) (b)

Figure2.2.Representationofa)thecrystalstructureofMgB2andb)opticalmicrographofMgB2powderunderpolarizedlight.

Thecompoundformsabove800Cfromelementalpowders;thecalculatedMgandBbinary

phase diagram, Figure 2.3 a) [13], is presented and is an improvement over the first

proposedphasediagramfrom1988[14]byA.A.NayebHashemiandJ.B.Clark.Thephase

diagramandtheformationoftheMgB2phaseisfurthereffectedbypressureanditsMgB

phasediagramunder4.5GPaofpressureispresentedinFigure2.3b)[15].Thematerialhas


10

been studied extensively andmost researchers have produced the compound bymixing

stoichiometricamountsofBandMg (inslightexcess) ina reducingatmosphereorunder

inertgas.The selectedmaterialpropertiesof thebulkMgB2 intermetallic compound are

presentedinTable2.2.

a) b)

Figure2.3.Publishedphasediagrams:a)CalculatedPhaseDiagramforBMgSystem(2004)andb)MgBphasediagramat4.5GPa(2003).

Table2.2.MATERIALPROPERTIESOFMgB2

Property ValuesMeltingTemperature(C) Tm =1073MolecularWeight(g/cm3) Wm=45.93Density(g/cm3) 2.57Hardness(kg/cm2) 17002800Nanohardness(GPa) 35.60.9ElectricalResistivityat20C(m) 1.5x106

PreviousworkwasconductedonpressedMgB2powdersinpelletformusinghighpressure

sintering[16]orintubesusingthewellknownpowderintube(PIT)process,ineitherwire

[17,18]ortape[19]geometries.MgB2powdershaveonlybeenusedthus far inwireand

tapegeometriesmadeofeitherstainlesssteel[20,21],silver[17]orcopper[17,20,22].PIT

tapes aregenerally rolled tubes containingMgB2 and are shapedduring compaction.Ni

sheathed tapespropertieshavebeen studiedwith10%vol.of Inasaconductivebinder


11

[23,24] to link the individualparticles togetherhoweverno researchhasbeen foundon

MgB2powderswithpolymerorsolgel(glass)bindersandourworkwillfocusonthisaspect.

Thinfilmshavealsobeenproducedbychemicalvapourdeposition[25].

Table2.3.LISTOFSUPERCONDUCTORPROPERTIESOFMgB2

Parameter ValuesCriticaltemperature Tc=3940K

Hexagonallatticeparametersa=0.3086nmb=0.3524nm

Theoreticaldensity =2.55g/cm3Pressurecoefficient dTc/dP=1.12K(GPa)

1Carrierdensity nS=1.72.8x10

23holescm3Isotopeeffect T=B+Mg=0.30+0.02

ResistivitynearTc (40K)=0.416cmResistivityratio RR=(40K)/(300K)=127

Uppercriticalfield*Hc2||ab(0)=1439THc2||c(0)=224T

Lowercriticalfield* Hc1(0)=2748mTIrreversiblefield* Hirr(0)=635T

BCSCoherencelengthsab(0)=3.712nmc(0)=1.63.6nm

Penetrationdepths (0)=85180nmEnergygap (0)=1.87.5meV

Debyetemperature D=750880K

CriticalcurrentDensities

Jc(4.2K,0T)>107Acm2

Jc(4.2K,4T)=106Acm2

Jc(4.2K,10T)>105Acm2

Jc(25K,0T)>5x106Acm2

Jc(25K,2T)>105Acm2

Themostcloselyrelatedexampleofpolymerandsuperconductorpowderwirefabrication

is that of suspension spinning. Suspension spinning is a wellknown polymer thread

productionmethod.IntworecentstudiesonsuspensionspinningforMgB2wirefabrication,

oxidizedwireswereproduced.Theprocedure involvedusing crushedMgB2powder finer

than350meshwhichwassuspended inapolymericmixedpoly (vinylalcohol)solutionof

dimethylsulfoxide and hexamethylphosphoric triamide [26]. The viscous solution was

extrudedasafilamentintoaprecipitatingmediumofmethylalcoholandcoiledonadrum.


12

Theresultingfilamentwascutandheatedat500Cfor30minutestoevaporatethevolatile

compounds. The sampleswere thenuniaxially pressedunder a forceof 200 kg/cm2 and

thenrolledintoanironsheetcontainingapelletofMgandBpowder.Varioussampleswere

sealedinquartztubesandheatedatvarioustemperatures.

Thismethodwas used to carbon dopeMgB2 aswell [27]. These same researchers have

foundthatthepolymericsuspensionspunsamplesshowednodeteriorationoftheTcand

an increased critical current Jc. However this method requires several steps and the

suspensionspinningrequirescompleteevaporationofthepolymerandseveralheatingand

pressingstepsinmetaltubingresultinginafinalproductsimilartothoseproducedbythe

PIT method. Many studies have been undertaken to determine the superconductivity

propertiesofMgB2.Table2.3isasummaryofthemostimportantproperties[28]however

these stated propertiesmay not taken into account the double band gap nature of the

materialdiscussedlater.

2.3 Background Theory

2.3.1 Superconductivity History of Discoveries

Followingthediscoveryofperfectelectricalconductivitybelowacriticaltemperature,Tc,in

1911, in 1933,WaltherMeissner and RobertOchsenfeld discovered the second defining

characteristic of a superconductor, that they exhibit perfect diamagnetism behaviour by

beingabletocompletelyexcludeandexpelmagneticfieldfromitsinterior.Followingthese

two surprisingdiscoveries, several important theorieshavebeenproposed toexplain the

puzzlingphenomenabothmicroscopicallyandmacroscopically.Macroscopically, the topic

was investigatedandexplained fromanelectrodynamicspointofviewbyFritzandHeinz

Londonin1935andfromathermodynamicspointofviewbyVitalyLazarevichGinzburgand

LevDavidovichLandau in1950.Microscopically,JohnBardeen,LeonNeilCooperandJohn

RobertSchrieffertogether in1957developedthemostcomplete,unifyingtheorytodate.


13

Furtherdiscoveriescamein1957withthevortexstateintypeIIsuperconductors,inwhich

magneticfluxisallowedintothesuperconductorbutonlyinaquantizedform.Lastbutnot

least, tunnelling currents between two superconductors separated by a non

superconductingbarrierpredictedin1962byBrianD.JosephsonandtermedtheJosephson

effect conclude some of themost notable aspects of the phenomenon first discovered

nearlyacenturyago.

Figure2.4.Criticalsurfaceofasuperconductorwithcriticaltemperature,carryingcapacityandmagneticfieldindicatedintheaxes.

Three importantparametersneed tobedefined, the critical temperature,Tc, the critical

magnetic field,Bc, and the critical current, Ic. These threeparametersmakeup a critical

surface shown inFigure2.4.The transition temperature isdefinedas the temperatureat

whichthematerialceasestoresistcurrentflowinzeromagneticfield.Perfectcurrentflow

up toa limit termed the critical current, Ic,oroveranuniform crosssection, termed the

criticalcurrentdensity,Jc,andcompleteexpulsionofmagneticfielduptoacriticalfield,Bc,

are often quoted at a constant temperature below the transition temperature. Studying

theseparameters as a functionof theother two yields the critical surface, fora specific

superconductor,belowwhichsuperconductivityexists.

Bc

Jc

Tc


14

2.3.2 Meissner-Ochsenfeld Effect

TheMeissnerOchsenfeld effect is defined as the complete expulsion of any externally

imposed magnetic field from the interior of a superconductor resulting in perfect

diamagnetism. The imposed external magnetic field generates permanent currents by

inductionwhich screensmagnetic field from the interior of thematerial. TheMeissner

Ochsenfeldeffectisaconsequenceoftheminimizationoftheelectromagneticfreeenergy

carriedbythesupercurrent[1].Innormalconductors,assoonastheexternalmagneticfield

isstablethecurrentsdecayaccordingtothefollowingequation:

tLR

eItI

= 0)( Equation2.1

whereIisthepermanentcurrentattimet,I0istheinitialpermanentcurrentvalue,Risthe

finiteresistanceandListheselfinductioncoefficient.Eventually,themagneticfieldwithin

andoutsideequalize.However,insuperconductors,regardlessofhowmuchmagneticfield

is trapped in thesuperconductoras itcools, it remains trappedbelowTc.Experimentally,

the content of the superconductive phase can be measured through magnetization

experiments.Bymeasuringhowmuchof the applied external field ispushedoutof the

interior of the sample, one canmeasure the volume fraction of superconductivity. This

volumefractionofsuperconductivityisthesusceptibility, ,andcanbenormalizedonthe

basisofmassandrenamed,masssusceptibility,,andforaperfectsuperconductorwould

haveavalueof1.

HM

=

Equation2.2

]/[ 3 kgmdensityH

==

Equation2.3


15

2.3.3 Electrodynamics of Superconductivity and the London Equation

In1935, following thediscoveryof theMeissnerOchsenfeldeffect, the Londonbrothers

contributed to the understanding of superconductivity by their treatment of the

phenomenonfromanelectrodynamicspointofviewbychangingthetraditionaltheoryof

normal conductivity,Ohm's Law,where I=V/R, to suit superconductivity.Theyassumeda

twofluid system for the entire superconductor as having charge carriers from a

superconductingphaseandfromanormalconductingphase[29].Theirfindingsintroduced

theconceptoftheLondonpenetrationdepth,L,whichisthelengthoverwhichanexternal

magneticfieldpenetratesthesuperconductor.ItisgivenbythefollowingEquation2.4.

sL nq

m2

0 = Equation2.4

wheremisthemassofthechargecarriers,eisthechargeofanelectron,nsisthenumber

ofchargecarriers,q=2e,isthechargeofaCooperpair,and0isthemagneticpermeability

[29,30]. Inaddition, the initialapplied field,Ba,decaysover the length,L,exponentially.

Thefield,B,atanypoint,x,alongthepenetrationdepthcanbeexpressedbythefollowing

relationship:

=

La

xBB

exp Equation2.5

Ingeneral, todescribe thesuperconductivityofamaterialboth thepenetrationdepth,L

and the size of theCooper pairor the coherence length, 0, are used. First, the London

penetrationdepth,L,isthedepthwithinwhichtheinternalmagneticfieldis1/etimesthe

external appliedmagnetic field, Ba. Second, the Cooper pair correlation is active for an

average distance called the coherence length 0, calculated from the BCS theory. As

mentionedpreviously,theGinzburgLandaucoherencelength,GL,isthelengthscalewithin


16

whichthetotalsystemofCooperpairscanchangeand isanalogoustotheBCScoherence

length,asshowninFigure2.5.

BILF = ;LorentzForcerequiredtomovethevortexwhenafinitecurrentIisreachedforawireoflengthLundermagneticfieldBa(Equation2.6)

Figure2.5.RepresentationofavortexcoregeneratedbyappliedmagneticfieldandtheBCScharacteristiclength.

2.3.4 Thermodynamics of Superconductivity, Ginzburg-Landau Theory

The GinzburgLandau theory introduced two concepts when treating superconductivity.

First, the spatial variations of the superconductive state and, second, its state as a

macroscopicwavefunction.GinzburgandLandauintroducedthefirstaspect,nonlocalityof

thesuperconductorpropertiesbyintroducingasuperconductingorderingparameter,(r),

where r is thevectorpositionof thechargecarrierbelowTc.The theoryassumes that

increases from01astemperaturegoesfromTc0.|(r)|2canbe interpretedasthe

densityofthesuperconductingchargecarriers.Byconsideringthefreeenergyofthesystem

anewmethod fordetermining the coherence lengthwasdevelopedanda characteristic

lengthoverwhich the superconductororderparameter varieswas introduced. From this

theory, the GinzburgLandau relation, Equation 2.7, can be used to determine the

coherencelengthofthesample.

0

vortexcore(nosupercurrent,normalconducting)

L1/e*Ba

Externalfield,Ba

superconductor

F

GL


17

( )2

1

2

0

02

=

cGL B

Equation2.7

Where0isequalto2.0678x105Tm2andBc2istheuppercriticalmagneticfield.

TheGinzburgLandaucriterion,,isanimportantparameterstemmingfromthistheoryand

isdefinedby Equation2.8. The approximation fromGorkov andGoodmanwas found to

agreewellwiththeGinzburgLandauequation[30].

2/130 105.7

+=

GL

L Equation2.8

where0istheGLparameterofthepuresuperconductorinwhichtheelectronmeanfree

path,l*,isnotreducedbyimpuritiesandl*.Experimentally,GLisdeterminedfromthe

uppercriticalfield,Bc2oftypeIIsuperconductors(Equation2.7).

2.3.5 Bardeen-Cooper-Schrieffer (BCS) Theory

Themicroscopic theoryof superconductivitydeveloped in 1957byBardeen,Cooper and

Schrieffer, termed the BCS theory, unified all the previous theories and described the

natureof thechargecarriers in superconductors.The theoryexplained the resistanceless

current flow, observed in certainmaterials and below a characteristic temperature, by

postulatingthatatomiclatticevibrations(phonons)causetheelectrons,whichmakeupthe

electric current to pair up. The BCS theory showed that ordering in conventional

superconductorsismediatedbyphononsleadingtotheformationofthesesocalledCooper

pairs leadingtoacondensedandcoherentstate.ACooperpair isformallydefinedastwo

electrons with opposite momentum of equal magnitude and with opposite spins. The

angularmomentumofthepairaswellas thatofthetotalsuperconductingcondensate is

zero[30].


18

FromtheBCStheory,thechargecarriers,Cooperpairs,possessadoubleelectronchargeof

q=2e and ns is proportional to the square of thewave function and transforms to the

Ginzburg Landau result forpenetrationdepth into the Londonpenetrationdepth [29]. In

BCStheory,thesizeoftheCooperpairistermedthecoherencelength,0.

Also, inthesuperconductingstatetheCooperpairshavetranslationalsymmetry,meaning

they transfer their charge exactly and repeatedly as theymove through the solid. Two

electrons form a Cooper pair over a given distance; past the coherence length the two

electronsare incoherentandcease tobeapair.Thispairingcauses theelectrons topass

withoutresistancethroughthesuperconductorslattice.

InadditiontheBCStheory introducedtheconceptofanenergygap,0,betweentheBCS

groundstateandthefirstexcitedstate.Thisdeterminestheminimumenergyrequiredto

forma singleelectron (hole)excitation from the superconductingground state [29].And

fromthis,thebindingenergyof,ortheenergyrequiredtobreak,theCooperpair,whichis

twotimestheenergygap(energygapEg=20).Thisorderingoftheelectronsputsthemina

lowerenergystatebyanamountequivalenttothebindingenergyoftheelectrons inthe

Cooperpair. Inexperimental terms, this couldbe theenergy suppliedbyaddingheat to

increase temperature, increasingthecurrentrunningthroughthesuperconductorcausing

scattering and Joule heating or imposing an externalmagnetic field causing circulating

currentsofthevorticestospinfasterandexpelenergy.Inallcases,energyissuppliedand

thechargecarriershave sufficientenergy toescape the superconductingground state to

become free single electrons. The BCS theory relates this energy gap between the

superconducting state and the free electron state at zerotemperature, 0, and the

transitiontemperature,Tc,bythefollowingrelationships(Equation2.9andEquation2.10)

[30]:

cBTk5.3)0(2 = (Equation2.9)and cc

TTTTT

=

,174.1

)0()( 2

1

(Equation2.10).


19

where kB is the Boltzman constant (8.62x105eV/K). The concept of the energy gap in a

superconductorwascorrectlypredictedbytheBCStheory.

Inalloys,themean freepathandhencethecoherence lengthoftheCooperpair ismuch

smaller than inpuremetals. In the caseofpuremetals thepenetrationdepth is smaller

thanthecoherencelengthwhereasinalloystheoppositeistrue.

Inparticular, theeffectof isotopesor theeffectofvarying theatomicmasswasused to

confirmthephononmediatedsuperconductivitybehaviourasdictatedbytheBCStheory.

The effect of slower Cooper pair formation is seen through a decrease in the transition

temperature.Withlighteratomicmasses,morephononsareproducedandstrongerCooper

pairsformbecausethebindingenergy issmaller.Aparticularlysuccessfulexperimentwas

withSnsinceawiderangeofSnisotopescanbefound,rangingfromatomicmassesof113

to 123 [30]. Frhlich and Bardeen suggested that the transition temperature should be

inverselyproportionaltotheatomicmass,M.

2/1 MTc

Thisagreedwellwith theexperimentof theSn isotopesandmanynontransitionmetals.

However transitionmetal isotopes deviated from the coefficient , of 1/2, and amore

accuraterelationshipwasdeveloped[30]:

( )

++

D

DcT

/*1**1*exp Equation2.11

where*istheelectronphononinteraction,*istheCoulombinteractionisaveragevaluetakenoverallfrequenciesofthecrystallatticeDistheDebyefrequency

For theMgB2 superconductor consisting ofmetal (Mg) and nonmetal (B), experiments

wereconductedtoseetheeffectsofnontransitionelementisotopesanditwasfoundthat

(10B11B)isotopesyieldedacoefficientBof0.3and(26Mg27Mg)isotopesyielded


20

acoefficientMgof0.02[28,30].Thus,indicatingthatthevibrationoftheBionsplayan

importantrolefortheCooperpairing inMgB2[30].Thetotal isotopeeffectforMgB2T=

Mg + B 0.3 differs from the BCS of 0.5 and illustrates deviations from BCS, but

suggeststhatMgB2ssuperconductivityisphononmediated[31]

TheBCStheory isoftenusedtopredictmanypropertiesofconventionalsuperconductors.

ThetheorywasalsousedtopredictthesuperconductivityofhigherTc incompoundswith

lighterelementsandthiswasconfirmedbystudyingLis,BesandMgB2ssuperconductive

behaviour[28].

2.3.6 Shubnikov (Mixed) State in Type-II superconductors

Generally, thematerial ceases to be superconductive if it is subjected to a temperature

abovethetransitiontemperature,Tc,oramagneticfieldabovethecriticalmagneticfield,

Hc(typeI)orHc2(typeII),oriftheappliedcurrentsurpassesthecriticallycurrentcapacity,

Jc.These limitingvariablescanbevisualizedasacritical surface in theFigure2.4.Typical

magnetizationcurvesfortypeIandtypeIIsuperconductorsareshowninFigure2.6.

Figure2.6.MagnetizationCurvefora)TypeIandb)TypeIISuperconductors

TheGinzburgLandauparameter,,determineswhetherasuperconductor isa typeIora

typeIIsuperconductor.For 2/1


21

2/1> , it isnegativeand is typeII [7,32]. In the lattercase, itbecomesenergetically

morefavourableformagneticfluxtopenetratethesuperconductorandproduceamaterial

withamixofnormalandsuperconductingregions.FortypeIIsuperconductorswhereatthe

lowercriticalfield,Bc1,amixedstateisobserved.Vorticesappearwithinthematerialwith

supercurrent flowingaround thevorticesandnosupercurrent in thecore.As theapplied

field is increased to theupper critical field,Bc2,morevorticesappearuntilall thevortex

cores start to touchandoverlapeventuallymakingall thematerialnonsuperconducting.

Withdecreasingtemperature,theelectroniccontributiontothermalconductivitydecreases

becausefewerfreeelectronsareavailabletoconductsincetheyarelockedinCooperpairs

andaredecoupledfromthethermalbehaviour[30].Inthissamerespect,superconductivity

isnotobservedinthecentreofavortexaxiswhereitisnormalconducting.

Figure2.7.CrosssectionofatypeIIsuperconductorandthechangeasexternalmagneticfieldisincreased.

Figure2.7servestoillustratethecurrentflowinacrosssectionofatypeIIsuperconductor

starting from theMeissner state to thebreakdownof superconductivityatapplied fields

abovetheuppercriticalfield,Bc2.Fromthediagramshown inFigure2.7,thebehaviourof

vorticesaswellasthedestructionofsuperconductivityaboveBc2canbevisualized.Above

Bc1, the lower criticalmagnetic field,magnetic fields begin to penetrate in the form of

vortices.There isamagneticflux inthecoreofthevorticesandeachvortex isamagnetic

singlefluxquantum.Foranymaterial,atagivenimposedmagneticfieldthevortexdensity

isthesame.Thesizeofthevortices isdictatedbythematerialand isapproximatelygiven

Supercurrent,Is

MeissnereffectatB


22

bytheLondonpenetrationdepth.Foragivennumberofvortices,ifthesizeofthevortexis

largetheyoverlapanddestroysuperconductivity.ThemagneticfieldBatwhichthevortices

overlapisthecriticalmagneticfieldBc2.

Tc Temperature Tc

Bc

Bc1

Bc2

Temperature

Superconductor Superconductor

Normal Normal

Vortex State

Type II Type I

Figure2.8.BTphasediagramsfortypeI(a)andtypeII(b)superconductors.

Generally, type I and type II superconductors can be differentiated by the BT phase

diagram (Figure2.8)and themagnetizationcurves (Figure2.6).Whether thematerial isa

typeIoratypeIIsuperconductorcanbedeterminedfromtheshapeofthecurve.Aswell,

thecriticalfieldBcortheloweranduppercriticalfields,Bc1andBc2,canbeobtained.

2.3.7 Vortex Pinning

Forpracticalapplications,ahighcriticalcurrent,Ic,andconsequentlyahighcriticalcurrent

density,Jc,isdesired.Inordertoachievethis,latticedefectsareintroducedbytheaddition

of foreign atomsorof foreignparticles to theperfect lattice and these serve topin the

vortices present in amagnetic field.Generally, [i]n superconductors the ability to carry

currentwithout dissipation is enhanced if thematerial contains impurities or defects of

suitablecharacteristicsinorderto"pin"themagneticflux[8].

(a) (b)


23

Vorticesaremesoscopicswirlingtubesofelectricalcurrentinducedbyanexternalmagnetic

field.Vorticesarepresent in themixedstate in typeII superconductors; theirmovement

producesresistanceandshouldbepreventedforstabilityintechnologicalapplications.One

way isthroughvortexpinningthroughthe introductionofdefects inthecrystalstructure.

HighpurityMgB2 requires impurities (or substitutions) forvortexpinningand to increase

the critical current density. Magnetic properties of the material can be improved by

introducingdisorder in the systemwhichpromotesvortexpinning [33].As thesevortices

remain pinned, themagnetic fields can penetrate while stillmaintaining zero electrical

resistivity paths through thematerial. The size of the cores ismaterial dependent and

varies. In these superconductors, the vortices of the Shubnikov phase are energetically

strongly bound to favourable locations through the introduction of enough defects by

impurityaddition.

IntheShubnikovphaseormixedstate,pinningpreventsthedissipationofenergywhena

superconductingcurrentflowsbypreventingthemovementofthefluxesundertheLorentz

force [34]. This is due to the spatial variation of themagnetic field in the vortex. The

mechanism canbe explainedby the appearanceof localelectric fields generatedby the

moving vortices, these fields accelerate theunpairedelectrons and theenergy from this

accelerationisthentransferredtothelatticeandhenceheatisgenerated.

IntypeIIsuperconductorsatmagneticfieldsaboveBc1,vorticesappearsincethemagnetic

field penetrates the superconductor and it enters the Shubnikov/mixed phase. When

precipitatesareintroduced,thevortexpassingthroughtheprecipitatewillloweritsenergy.

Forasamplevolumecontainingmanyprecipitates,thevorticeswillbendinordertooccupy

the energetically most favourable locations, a minimum value of total energy. For a

continuous flux line, with length increment caused by the bending must be

overcompensatedbytheeffectiveshorteningwithinthenormalconductingregionsofthe

precipitates[30].Inaddition,therepulsiveforceofthefluxlinesrelativetoeachothermust


24

alsobe taken intoaccount in the totalenergybalance. Ingeneral,anydefect lowers the

order parameter, , within the defect and effectively favours the path of the current

throughthedefect.Inthiswaythevortexfollowsonepinnedpathwhichisenergetically

morefavourable.

Analogous todislocationpinningatphaseboundaries,vortexpinning is the restrictionof

vortices ofmagnetic flux by some trace atomic element (Al[35], Ti [25], C[36], Cu[37],

Au[38])orcompound(Al2O3[39],MgO[40],WSi2[41]andSiC[42]).

2.3.8 Single and Double Energy Gaps

Transitionsbetweenthenormalandsuperconductivestatesarecausedbythecompetition

between two energies, the energy gain from the condensation of Cooper pairs and the

energy lossdue to themagnetic fieldexpulsion from the interiorof the superconductor.

From a thermodynamic standpoint, the GinzburgLandau theory was developed in the

1950stomodelthebehaviourofsuperconductivity.ForMgB2,thevalueofthecoherence

length, GL, for a noncubic structure, specifically hexagonal closepacked structures, is

differentdependingontheaxisalongwhichthemagneticfieldisappliedrelativetotheaxis

oforientation.Forthemagneticfieldappliedalongthecaxistherelationshipbetweenthe

upper critical field and the coherence length is given by the following anisotropic GL

relationship[28]:

20||

2 2 abc

cH

= Equation2.12

similarlyforthemagneticfieldappliedalongtheabaxistherelationshipbecomes

cab

abcH

2

0||2 = Equation2.13

where0=2.07x1015Tm2isasinglefluxquantum.


25

Howeverthisrelationship isvalidonlyforsuperconductorswithasingleenergygap.Since

theBCStheoryassumesasphericalFermisurface,themagnitudeofthegapisassumedto

bethesameatallpointsoftheFermisurfaceandassumesaperfectlysymmetricalcrystal.

MgB2possessesatwobandgapanddeviatesinsomewaysfromclassicBCStheorybecause

ofthis.

BeforeMgB2,theexistenceofanenergygap,20,oraforbiddenenergyrange,wasusedto

explainwhybelowacriticalkineticexcitationenergy,Cooperpairscannotinteractwiththe

crystal lattice [30]. The energy gap is also known as the binding energy that induces

electronstoformpairsandisdirectlyrelatedtothematerialstransitiontemperature[43].

Adoublebandgaptheorywasproposedinordertoexplainthesuperconductivebehaviour

ofthecompound[43].Usingbasicatomicdataandphysicallaws,Choietal,havefoundthat

MgB2isofatwobandstructure[44]andisrepresentedinFigure2.9,recreatedfrom[44].

MgB2hasa twobandenergygapwitheachband corresponding toadifferent transition

temperature. Also using computational techniques they have correctly determined the

valueforthetransitiontemperatureofMgB2.Thisdoubleenergygapconsistsofalarge

(sometimes referred as s) gapdue to strongelectronphonon coupling and a smaller

(sometimesreferredasp)gapduetoweakcoupling[22].

Thetwoenergygapscorrespondtotwotransitiontemperatures,oneat15Kandtheother

at 45 K, together they lead to an observed transition temperature of 39 K [44]. The

subsequent investigation in 2003 was conducted using angleresolved photoemission

spectroscopy(ARPES)betweenthetemperatures15Kand45K[43].Itwasindicatedthata

superconductinggapopensatthe lowertemperatureof17K[6].Thesigmabandshavea

largegapmeasuring67meVand thepibandhasa smallerbandof12meV [6]. Itwas

concludedthatthelargeenergygapat45K,sigma,wasduetostrongcouplingofphonons

andwasdominant.Thesmallerpigapopensat17Kandissmallerandlessimportantdue

toweakercoupling.


26

MgB2 is of an HCP structure and shows anisotropic behaviour and a confirmed double

energy gap. For energy gapmeasurements, a steep increase in the current when two

superconductorsareplacedsidebyside for tunnellingexperimentsmakedensityofstate

measurementspossible.Inordertomeasuretheenergygap,otherdeterminationmethods

canbeused inadditiontotunnel junctions, includingultrasound, lightabsorption,nuclear

spinresonance,andspecificheat.

Figure2.9.DoublebandgapsofMgB2.

2.3.9 Josephson Junction Effect and Josephson Junction Arrays

2.3.9.1 Josephson Junction Effect

Themicroscopictheoryofsuperconductivitydepictsapictureofthecurrentacrossabarrier

asthebreakingupofaCooperpairinthefirstsuperconductor.Thetwoelectronscrossing

thebarrier independentlyofeachotherarephasecoherent,and,uponreaching thenext

superconductor, finally recombine to form a pair, Figure 2.10. Due to the interactions

EnergygapwithTc=15K,12meV

Singleparticleenergy

EnergygapwithTc =45K,67meV

Den

sityofstates

0


27

betweentheCooperpairs,thedoubleprocessofbreakingupandrecombininghasabout

thesameprobabilityasthetunnellingofindividualparticlesthroughtheinsulatingbarrier.

Thebarriermaybeaninsulator,I,ornormalconductor,N.Ifthetwosuperconductorsare

identical and the pairwave function shows swave symmetry, in the case of direct

tunnelling at the superconductorinsulator interface, the following AmbegaokarBaratoff

relation,Equation2.14, [30] isvalidand is independentofthetransmissioncoefficient.N.

Equation 2.15 is the case of a thin layer of normal conductor placed between two

superconductors.

( ) ( )

=

TkT

Te

RIB

nc 2tanh

20

0

Equation2.14

( ))/sinh(

/023 20

N

N

cBnc d

dTkx

eRI

=

= Equation2.15

whereIcisthecriticalcurrentormaximumsupercurrentRn is the resistance in the normal state, tunnelling resistance in the absence of pairinteractioneistheelementarychargeoftheelectron0istheenergygapinthesuperconductorTisthetemperatureTcisthecriticaltemperaturekBistheBoltzmannsconstantdisthethicknessofthenormalconductor0(x=0)isthevalueoftheenergygapinthesuperconductorneartheinterfaceNisthecharacteristiclengthscaleawayfromthenormalconductorinterface.

However, a limiting factor is that this layer must be sufficiently thin, usually a few

nanometers. Quantum mechanical tunnelling is responsible for this effect. It has been

shown thatplatinumpowdersat the submicron levelare superconductingat20mK [45]

with smallbarriersof spacebetweenPtparticles.The additionof In, a low temperature

materialsuperconductivebelow3.4K,wasaddedtoimprovethelinkagecharacteristicsof

theoverallmaterialcompositeofregularconductive InandsuperconductiveMgB2[23].A

minimum particle size is required to ensure a sufficiently thin barrier in order for the


28

Josephsoneffect to takeplace.From thisMgB2producedby thepowder in tubemethod

whereinpowdersarearrangedinclosecontactwitheachotherandstillbesuperconducting

isthoughttobeaviableproductionroute.Variationsarealsopossibleandbulk,continuous,

solidsamplesneednotbeproduced toachievesuperconductivity.The Josephsoncurrent

canhelptoexplainthepassingofasuperconductingcurrentthroughparticles.

Figure2.10.SchematicofaCooperpaircrossingaJosephsonjunctionfromlefttoright.

2.3.9.2 Josephson Junction Arrays

Josephson junction arrays or networks are systems of layered superconductors, S,

alternating with normal, N, or insulator, I, in S(IS)n or S(NS)n configurations or any

combination of these. These systems are useful in understanding bulk discontinuous

superconductors or even inhomogeneous superconductors in which regions of a

superconductive phase existwithin nonsuperconductive phases. Theoretically, arrays in

zeromagneticfieldwithasquaregeometryoffourjunctionswillpossessatotalenergy,E,

andcanbeestimatedusingthefollowingequation,Equation2.16

=

aREE J ln Equation2.16

whereEJisthecouplingenergyofaJosephsonlink,Risroughlytheradiusofthearrayanda

isthelatticespacing[32].

In a highly disordered system in whichmany vortices are present, pairs of vortices of

opposite sign exhibit an attractive forcewith each other [32]. At T=0, the system at its

S:superconductorI:insulatorornormalconductor

S I S

Cooperpair

Separateparticles


29

lowestenergystatewillpossessnovortices;withincreasingtemperatureandtherefore,an

increaseinthermalenergy,vorticesaregeneratedandconsequentlypairsoftightlybound

antiparallelvorticesaregenerated.As temperature increases, thenumberofvortexpairs

increasesandsodoes theenergyofeachofthevortexpairs.Assumingaminimizationof

the freeenergyofthesystem,a transition fromboundpairsofvorticestounboundpairs

leads to thedeterminationofaspecific transition temperature termedTKT, theKosterlitz

Thoulesstransition,atwhichthepairsofvorticesstarttounbind.Thistemperaturecanbe

estimatedby JTK EkT [32]wherekisBoltzmannsconstant.

The concept ofmany Josephson junctions in sequence has beenmodelled for granular

superconductorsandisalsotermedJosephsonJunctionArraysorJJAs[46,47]orJosephson

junctionnetworks.Becausemodellingof JJAscan result incomplexsystems,baremodels

are often used and they reduce the grains to points.However, twodimensional square

arrayshavealsobeenused.IncontrastbyusingdressedJJA's,thedetailsofthesystem,say

forgrainsassumed tobeperfectspheresandarranged inacubic formationpossessing8

grains, would result in 12 Josephson junctions to be studied [46]. One can foresee an

increaseddegreeofcomplexityasthenumberofgrainsmodelledincreases.

Experimentally, JJ networks have been studied by producing a network using niobium

spheresmelted fromasaucepanwithanelectronbeamandarrange into triangularand

squarelatticeconfigurationsusingAuelectrodesinasampleholderandsupplyingpressure

bymeansofaneternalscrew[48].Currently,upto7.7x105Tl2212intrinsicJJinserieshave

been synthesized and studied and the inductance, L, of the series was found to stem

primarilyfromtheJosephsoneffect[49]leadingtoareductioninthecriticalmagneticfield

andabroadeningofthetransitiontosuperconductivity.


30

2.3.10 Dirty Superconductors

TheBCS theorybrieflypresentedearlier isvalid forcleansuperconductors; in thissection

theeffectsofdisorderleadingtodeviationsfromBCStheoryarepresented.Thecleanand

dirty limits of superconductors correspond to the overall purity of the superconductive

region, the scale of which is arbitrary.Within the theory of the effect of disorder on

superconductivity,twobranchesexist,namelythosehavingtodowithstrongcoupling[50,

51]orintheweaklylocalizedregimecoupling[52].Experimentalevidencepointstothefact

thatahighHc2isfoundatlowtemperaturesinhighlydisorderedsystems[53].Intheweakly

couplinglocalizedregime,effectsofdisorderleadtoadegradationofTcandHc2according

tocalculationsfromFukuyama,EbisawaandMaekawa[52].

Thecleananddirty limitsofasuperconductoraredescribedbytheratioofthemeanfree

path, l,of thenormal state, to the coherence lengthof the superconductor, 0 [29].This

ratio characterizes thepurityof thematerial.Amaterial is cleanwhen l/0>>1 anddirty

when l/00)as

2/1

74.0)(

=TT

TT

c

c Equation2.17and2/1

71.0)(

=TT

TT

c

cL Equation2.18

and dirty limit (l


31

C C C CC*

H

H

H

H

H

H

H*

H

n

)40(2 Knemvl F

= Equation2.21

WhereforMgB2,vFistheFermivelocity~4.8x107cms1[55],nisthechargecarrierdensity~

6.7x1022cm3[55],m isthefreeelectronmassforquasiparticles(2electronswhichmake

uptheCooperpair),1.822x1030kg,and(40K)isthenormalstateresistivity.

2.4 Background on Polymers

Polymersareunits,mers,ofcarbonbasedsegmentslinkedtogethercovalentlyandforming

long molecules generally held together by hydrogen bonding. They possess a large

molecularmassandcanbenaturalorsynthetic.

2.4.1 Background on Conductive Polymers

Recently discovered conductive polymers are doped with impurities and these are

responsible for theconductionofelectrons ina traditionally insulatingmaterial.Theyare

often referred to as organic semiconductors and possess the same forbidden band gap

located between the insulating and conduction bands similar to other silicon or gallium

based semiconductors. They are electrical insulators but when charge carriers are

introduced they behave like normal conductors and conduction increases substantially.

Some organic semiconductors are of the zero band gap type and behave likemetallic

conductors.Thepolymersused in these types are separated into two categories: charge

transfer complexes and conductive polyacetylenes, polypyrrole, polyaniline and their

derivatives [1].Polyacetylenemayachievehigherconductivityperunitmass thancopper

[56].

Figure2.11.Chemicalbondstructureofpolyacetylene.


32

Inpolyacetylene,shown inFigure2.11,thealternatingsingleanddoublebondscontribute

to an unequal distribution of the bond length and leads to localization of the electrons

aroundthedoublebondandlowerstheoverallenergyofthesystem.Anenergygapopens

in thedensityof statesof theelectronsand this turns thepolymer intoa semiconductor

[56].

2.4.2 Background on Ethylcellulose-Based Binder

Asmentionedpreviously,ourstudyofMgB2inpowderformboundinapolymermatrixhas

notbeenstudiedbefore.However,aproductionrouteforwiremadeofpoly(vinylchloride)

andMgB2bythesuspensionspinningroutewas investigatedyieldingcomparablePITwire

results[26].Theinitialmeltspinningstepisusedtoestablishawiregeometrywhichwillbe

inserted inmetal sheathing and subjected to subsequent heating, pressing and drawing

stepstocompletelyeliminatethepolymerfromtheresultingwire.Thus,throughthemelt

spinning technique the advantageousmechanical properties of thePVCpolymer arenot

conferredtothefinalwire.

Figure2.12.Chemicalbondstructureofcellulose.

Inthiswork,theadvantageofusingapolymermatrixistheinheritedmechanicalproperties

resultinginflexibility,improvedductilityandpossiblyahigherelasticmoduluswhenbound

inapolymermatrixcomparedtothePITwiresofcompactedorsinteredpowdergenerally

relyingonametalsheathforitsstructuralpropertiesandtomaintainthecontactbetween


33

theparticlesduring current transport.AnMgB2polymer compositemaybemore robust

thanthecupratebasedceramicsuperconductors.

Inthisstudy, looseMgB2powderwasboundtogetherwithathermoplasticpolymer,ethyl

cellulose.Celluloseisanaturalcompoundfoundinmanyplantsanditsmolecularstructure

is represented in Figure 2.12 [57]. Its simple structure means that at relatively low

temperatures, itbreaksdowneasily.Thebondingbetweenthepolymerchainsaremostly

hydrogenandifmanychainsarepresenttheseleadtostrongenoughinteractionssuchthat

thebulkpolymerismechanicallyverystrong.

Figure2.13.Chemicalbondstructureofethylcellulose.

Ethylcellulose,Figure2.13 [57],differs fromcellulosebythesubstitutionoftwohydrogen

atomsfromthetwohydroxyl(OH)groupsinthecellulosechainwithtwoethylgroups,C2H5.

The substitution of the free hydroxyl groups of each glucose unit alters the physical

properties of the material by making it, for example, soluble in organic solvents and

allowingthematerialtobemadeintofibresandfilms[58].

CH3CO

OCH2

CH3

CH2CH2

CH2

O CH2

Figure2.14.Chemicalbondstructureofbutylcellosolveacetate.


34

The polymeric binder usedwas amixture of ethylcellulose dissolved in butyl cellosolve

acetate.Thechemicalformulaforbutylcellosolveacetate isC4H9OCH2CH2OC(O)CH3and is

representedinFigure2.14[57]andterpineol.Thespecificcompositionofthemixturewas

suggested by the supplier and is a standardmixture formetallic inks used inmicrochip

networks.Often,theinksaredeposited,driedandfiredofftoevaporatethebinderleaving

only the active component. Generally, the paste can be formulated to be conductive,

resistiveordialectricdependingontheactiveingredientanddependingonthepurityofthe

ethylcellulose,theresultingcoatingwillbeconsequentlycleanersincehigherpurityethocel

burnsoffmorecleanly.

Table2.4.GENERALPROPERTIESOFETHOCELSTANDARD45

Properties ValueDensity(g/cm3) 0.4GlassTransitionTemperature(C) 129133SofteningPoint(C) 133138MeltingPoint(C) 165173RefractiveIndexofFilm 1.47TensileStrengthofFilm SeeFigure2.15DielectricConstantat25C,1Mhz 2.83.9DielectricConstantat25C,1kHz 3.04.1DielectricConstantat25C,60Hz 2.54.0PowerFactorat25C,1kHz 0.0020.02PowerFactorat25C,60Hz 0.0050.02VolumeResistivity,ohmcm 10121014DielectricStrength,V/0.0254mm 1500Viscosity(MPas) 4149

The generalmechanical properties of these binders vary depending on the blend with

solvents, the grade standard and the final dried or firedmixture. The ethocel polymer

binderitselfpossessesthegeneralphysicalandelectricalpropertieslistedinTable2.4.

The tensile strength specifically for standard 45 can be extrapolated from the supplier's

elongation versus load curves for different viscosity grades of ethocel, Figure 2.15 [57].


35

Generally,thepropertiesofthepolymerwiththesolventblenddependonthefinalsolvent

tobeevaporated.

Figure2.15.LoadElongationCurvesforEthocelpolymers.

2.5 Summary

Inconclusion,superconductors,MgB2,polymersandethylcellulosewerebrieflyintroduced

inthissection.Inaddition,several importantaspectsofsuperconductivitywerepresented

in their standard framework in order to guide the reader through the analysis of the

experimentalresults.Manyoftheconceptsintroducedwillbeusedinordertoanalyzethe

experimental findings.Due to thenatureof the coatingsproduced in this thesisproject,

deviationsfromthese idealandwellknowncasesareduetothefollowingtwoaspectsof

superconductivity: the twoband gapofMgB2 and thepresenceof innumerablepolymer

barriers, between MgB2 superconductive particles, which act as Josephson junctions.


36

Deviations from ideal cases exist and this section was presented to highlight how our

material'sbehaviourmaynot fitperfectly thestandard framework.Beforepresenting the

experimentalfindingswithinthisframework,wefirstdescribetheexperimentalmethodsin

thefollowingsection.

CHAPTER3EXPERIMENTALMETHOD

37

Chapter 3 Experimental Method

The experimental technique section consists of one fabrication stage which can be

subdividedintovariousproductionroutesfollowedbythreecharacterizationsteps.Thefirst

ofthethreestages isnamedthematerialscharacterizationsectionand involvedprimarily

visual assessments using various imaging techniques including surface microscopy and

electronmicroscopy, inaddition, the crystalline componentsof the sampleare identified

using xray diffraction and elemental analysis is carried out using energy dispersive

spectroscopy.Thesecondstageisthecharacterizationofthesuperconductivepropertiesof

thesamples.Thissecondstageinvolvedtwomaintypesofsuperconductorevaluation,the

first isthroughtransportmeasurements involvingtheobservationoftheresistanceofthe

samplewhilevaryingtemperatureorexternalmagneticfield;thesecondtypeofevaluation

involved the observation of the Meissner effect and the mixed state of the typeII

superconductorusingmagnetizationexperiments.Thethirdandfinalstageinvolvedtheuse

of infrared spectroscopy tounderstand the chemicalbondnatureof thepolymericMgB2

tapesasawhole.

3.1 Materials

The following, Table 3.1, is a list of thematerials used for the four fabrication routes

investigated.TheboronpowderwasproducedfromBchunksandwasseparatedfromthe

crushed powders into different size ranges using sieves, however these powders were

producedbycrushingwithasteelmortarandpestle.Itisbelievedthatironcontamination

was introducedthroughthispreparationmethodbutthatthepuritywassufficienttotest

preliminaryfabricationroutes.Highpuritystartingmaterialswereusedtoproducethemain

samplesofthisthesisproject.

CHAPTER3EXPERIMENTALMETHOD

38

Table3.1SUMMARYOFTHEMATERIALSUSEDTOPRODUCECOATINGS

Material Purity (%) Supplier Boron chunks 99.5 Alfa Aeser

Magnesium pellets 99.999 Alfa Aeser Magnesium ingot pieces 99.979 Timinco

Commercial Al foil 99.

and Characterization of Based Polymeric Magnesium...

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