Introduction

Table of Contents

1.1 introduction 3

1.2 Superconductivity 4

1.3 High T superconductor 4

1.4 Diamagnetic properties of High TL superconductors 6 1.4.1 Meissner effect 6

1.4.2 Levitation properties 7

1.5 Electromagnetic shielding

1.6 Superconductor/polymer composites

1.7 Polymers

1.8 Copolymerization 1.9 Polyethylene-A highly applicable material

1.10 Classification of polyethylenes

1.11 Linear Low Density Polyethylene (LLDPE)

1.12 Bibliography

1.13 Index

Introduction 3

The ever growing field of sclisitive electronic gadgets warrants effective ways

and means to shield them froni all penading e l e~ t ro~gne t i c radiations. As the

electronic and optoelectronic industries nuke quzntum l aps by showcasing h@y

sophisticated electronic podpcrs for test, tneasurement and medical diagnosis,

electromagneticallyill shielding rhcm also require sophisticated technology and

materials. Superconductors are important matertals in t h s context not for their

zero resistivity property but ih their other independent characteristics, viz.,

Meissner effect. Also polymers can be effectively used to bypass the hindrances

in utilizing superconductors for this purpose. This study is all about these two

aspects.

The trend in superconductivih research has been chzngmg gradually sulce Dutch

physicist Heike Kamerlingh ( h s [ I . 11 discovered it in mercury. Since the

discovery [ I .2] of the YBa,Cu,O,.+ superconductor with Tc above the liquid

nitrogen temperature, various attempts lbzve k e n made to improve two of the

most critical parameters, namely, current density and mechanical or fracture

strength that limit the applications of a bulk ceramic conductor. There have been

different studies regarding the ~mprovenlent of the latter property reported in the

literature. For instance, Nics et al. [1.3] tirst descrikd the preparation and

properties of certain pl;iss bonded composites of superconducting YB~,CU,O,~

that showed large dimnngnetic susceptibility. Fuierer et al. j1.41 reported the

physical and microstructural 131-operties of comnpsitrs formed from YB~,CU,O,.~

and silicon r u b k . A number of workers have studied [1.5, 1.61 the effects of

adding silver to hgh Tc superco~~ducting ceramics. It has been shown that addition

of silver to these ceramic materials enhanced the connectivity between the

superconducting grains and increased the grain size. But mechanical property

enhancement of hgh Tc superc~~nductor with silver addition was rather poor. These

studies suggest that the practical applications of ceramic kyh Tcsuperconductors

are likely to be in the form of composite materials.

4 Chapter 1

The goal of the current study is to investigate the Y-I23iZinear Low Density

Polyethylene composites for magnetic levitation and shielding applications.

The superconductivity research is ever growing. As mentioned earlier, here we

are interested in its diamagnetic properties. Before going in to the details of the

work done, it is pertinent to have a discussion on superconductivity and its

evolution.

1.2 Superconductivity

Over ten Nobel prizes being awarded so far in low temperature physics, in

particular, superconductivity and superfluidity, which occur at very low

temperatures, exemplify this area to be one of the most interesting areas of

condensed matter physics. This goes on unabated. The Nobel Prize in physics

2003 went to Alexei Abrikosov and Vitaly Ginzburg for their work [1.7] on the

theory of superconductivity together with Anthon! Leggrt for his explanation on

one type of superfluidity. In fact, the history of superconductivity started after

the successful liquefaction of helium by the Dutch physicist Heike Kamerlingh

Onnes in 1908. He investigated the low temperature resistivity of mercury in

1911 and found that the resistivity suddenly dropped to zero at 4.2K, a phase

transition to a zero resistance state; for this Onnes was awarded the Nobel Prize

1913 in Physics [I 81. This phenomenon was called superconductivity, and the

temperature at which it occurred is called its critical temperature.

1.3 High Tc superconductor

The discovery of superconductivity in La,.xBa,CuO, with superconducting

transition temperature, T of 30K by Bednorz and Muller [1.9] in 1986 was a

milestone in the history of superconductivity. It immediately led to the discovery

of a new class of high temperature superconductors having Tcs in excess of 1OOK

* wherein the basic structure comprises of metallic CuO plane separated by

insulating charge reservoir buffer layers. Most popular cuprate is YB~,CU,O,-~

Introduction 5

(Y-123:)[1.2, 1.10, 1.11] withamaximumTc- 95KandxcanvaryfromOto-

0.4 (leading to variation in Tc from 95K to OK) [I. 12 - 1.151. Therefore, cuprate

superconductor or simply cuprates were named high Tc superconductor (HTSC),

to contrast them with the conventional low T superconductors which supr:rconduct

at boiling helium temperatures (4.2K). The Y-123 compound can be considered

as the representative compound of the system.

Wlule the copper oxide superconductors (YBCO) are the system of choice here,

they have the drawback that they are stoichiometrically and structurally complex.

Illustrating this point will require knowledge ofthe crystal structure of these high

Tc superconductors.

YB~,CU,O,-~ (Y-123) is an oxygen-deficient triple perovskite ox~de. The

YBa,CyO, unit cell consists of the rare earth ion Y sandwiched between Cu-0,

sheets (figure 1.1). These sheets are surrounded by the charge reservoirs [I. 161

which consist of the Ba-0 and Cu-0 chain layers. Cu-0 is a one dimensional

chain and Cu-0, is a two dimensional buckled plane. The oxygen of the chain

layer is labile. The structural evolution of the orthorhombic from the tetragonal

in YBa&u,07_ in terms of oxygen stoichiometry is interesting. Here x is the

variable oxygen content which undergoes drastic change in its structure and

properties when its value changes from 0 to 1 [ l . 17,l. 181. When x = 0, substance

is a perfect superconductor with orthorhombic crystal symmetry and when x = 1,

substance is a perfect semiconductor with tetragonal symmetry. To overcome

the problem of oxygen loss and to form YBa,Cu,O, which exhibits the optimal

superconducting properties, it is necessary to anneal the sample in an oxygen

rich environment at temperatures around 450°C. Since the structure is stable

over the entire range, x = 1 to 0 and the transition temperature is highly dependent

L1.19, 1.201 on the oxygen level, great care must be taken to ensure that the

multigrain structure is fully oxidized. The formation of the chain Cu-0 layer (at

x = 0) reduces the crystal symmetry from tetragonal to orthorhombic. The

6 Chapter 1

vacancies in the Cu-0 layer form channels, throuph which small molecules may

enter OF leave the structure, The structure of YB$CqO, is shown in figure 1.1.

[1.16].

Figurel. 1 [1.16] : The crystal structure of YBa,Cu,O,.

(Shaded area: charge reservoirs)

The copper oxide high temperature superconductors, seem to be uniquely suited

for the study of LLDPEI high Tc superconductor composites for electromagnetic

shielding and levitation applications, as they exhibit diamagnetic flux exclusion

and levitation properties.

1.4 Diamagnetic properties of high Tc ~uperconductors

1.4.1 Meissner effect

The Meissner effect is at the heart of the work presented in this dissertation as it

is ths effect that allows for producing superconductor/polymer composites for

electromagnetic and levitation applications.

Superconductors are perfectly diamagnetic in a particular range of magnetic field

strength; superconducting state will exist up to a range of temperature (1.211 and

field strength. Tt will disappear if the specimen is raised above its critical

temperature or if a sufficiently strong magnetic field is employed. In other words

it expels weak magnetic fields from their interior when they are cooled below

Introduction I

their transition tenipcrat1u.e. 1111s 1s called as the Meissner etiect and is illwtrated

in figure 1.2

- - - p~ 6- -A

...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...... . . . . . . . . . . . . . . . . . . .......................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . --- - - - - - - - -

Pcr iec i T r h n r l i i - , o lo 'juprrcon~i,lctor n T r e n v t l o n to ,jlarna,Jnit $- x r - o i- i , tdn,.i , zero reaslarcc

Fiprel .2 11.21 j: Meissner effect in superconductors

1.3.2 Levitation properties

Levitation of a magnet above ;I lugti T< su~perwndudor is a universally observed

phenomenon and it is one of thc most attractive properties for superconducting

applications. It could be used to construct a levitated vehicle or bearings in

rotating machi~my. A high sspccd levitating train (-500 km per hour on average)

rnalclng use of a convent~onal liquid h e l i ~ u ~ ~ superconductor has been tested in

Japan [1.22). Japan cspects ti, have a le~itating passenger train between Osah

and Tokyo before the end of 111is century. There have lmen several studies [1.23

- 1.341 on superconductor bc:irings and some simple proto~yjx l x a q debices

have been made. Thm iise the levitation capabiliv and lateral sti&ess of ceramic

superconductors to spin mnp~ictic bearings at high speeds with a stal~le spstmn

configuration.

In HTSC, which are. type I1 sulierconduclors, the complete Meissner eff'ect cwcurs

only with macnetic field Iowci than the critical field. For higher magnetic field

the dianmgnetism arises from 11tc shielding currents [ 1.351. In fact the flus pinning

(1.361 can sustain a magnetic field gradient which is related to a mcroscopic

current. The strong diamagnetic behavior leads to remarkable phenomenon such

8 Chapter 1

as magnetic levitation. To achieve a large interaction force between a

superconductor and a field source, it is impclrtdnt to obtain superccmducting

materials allowing for high current density exfended to electrical paths as large

as possible. Owing to the hysteretic magnetic klmvior of HTSC the interaction

magnetic force can be either repulsive or attractive [1.37 - 1.39) leading

respectively to magnetic le&ation and suspensintl.

1.5 Electromagnetic shielding

Electromagnetic shielding is a more widespread technique [1.40] in the era of

electricity and electronics, than it is usually ac!alo~vledged. Compliance to

internationally agreed electrom~gnetic compatibilic codes is an integral part of

all electronic and of many electrical devices in the market. Compatibility means

[1.40] that a device must complv with given limits for electromagnetic radiation

to get the generated interference below the limits (in anlplitude or frequency)

stated by the agreed codes. Most domestic appliances, like those containing

electrical motors, magnetic actuators and florescent lights, radio and television

sets, computers, computer monitors and telephones etc. are UI tlus category. From

the steep increase in the preference given to optical methods for signal

tratlsmissions (example, fiber optics for telephorle cables), we can understand

the extent and importance of electromagnetic interference. Shielding is therefore

necessary in cases where a device must hc prevented from radiating

electromagnetic energy by itself and protected from p i c k up electromagnetic

energy radiated by other devices.

The type and extent of the necessary shelding will be restricted [ I .4 I] to the case

of malung an electromagnetic vacuum, where the techniques based on the new

materials are almost mature for applications like biomedical measuren1ents of

weak magnetic field?, electromagnetic compatibilitv measurements and electronic

security. The solution to be adopted will depend un the size of the shield and on * whether the internal temperature must be d~tferent from the refrigerator

temperature or not.

Introduction 9

A shielding surface m d e of high Tc mtcrials is etrective because of the intergrain

currents: the contribution of inliagrain currents is very low, except when the fill1

shielding properties are very mdest, which is not of technological interest. The

shelding currents fully d e p r c l on the strax fields, whose directions [1.40] are

generally unhovin and randoril. Therefore, w~ th the exception of very special

cases where the field direction is hio\v11, the shielding properties of the nmterial

must be' isotropic.

For applications where the attainment of all electromagnetic vacuum is the aim,

the use of high-Tc supercond~~cting materials showed far superior results on

monolithic shields with respect to traditional high-permeability nmterials and a

peat simplification in practical use with respect to the low-Tc superconducting

materials. Moreover. this magnetic shielding techrucp is mature to scale up to

the medium to large size industrial prototype stage, where it is likely to show

good competitiveness with respect to the existing techniques.

Electromagnetic shielding using [1.42 - 1.441 hidl temperature superwnductor

f,HTSC) is one of the large scalc industrial application with peculiar requirements.

It is the property of diamagnetic tlux exclusion [1.45] and not the zero resistivity

of the h g h TC superconductors umkes it a potential candidate for shelding.

However in spite of all thesc. the ditficulties involved in processing HTSC

materials in to desired shapes and sixes as \vcll as their fragile nature are the

main drawbacks in using thesc materials for shielding applications. In the next

section this aspect is discussed and the role of polymer/supercondu~ctor w~~iposites

as an altemati\e is highlighteL

1.6 Superconductor/pol~r composites

High Tcsuperconductmg cerrin~ics have an inherent brittleness which then1

difficult when they are to be ri~olded and die-cast in to pieces of even the simple

shapes, let alone the case of lahricating it in to flexible shielding sheets. Their

10 Chapter 1

mechanical properties cannot compete with those of conventional metallic

superconductors. Therefore the use of HI'SC ~mterial for large scale shieldmg or

levitation has been proved [I ,461 not an easy task. l'wther high Tcsuperconductors

are plagued with compositional instability and susceptibility to environmental

degradation. These problems hive impeded thc iievelopnent of application for

h g h Tc superconductors.

The large electromagnetic shelds are generally &signed with the superconducting

nlrtterial deposited as a thck film on a stmct~~ri~l substrate. ( h e of the most

important criteria for the selection of any material as substrate for high Tc

superconducting film is their chemical compatibility with the film. The choice

of the substrate, whch is necessav to support a thick film muses several constrains

like high cost, diffic~ilty in rebgeration and thennal expansion mismatch.

One of the most efficient approaches [1.47 - 1.531 to circumvent these

disadvantages is presumably the introduction of flexible materials lke polymers

as a structural support and protective matrix in to powder oxide superconductor

filler to form a composite with good mechanical flexibility. Polyner nutrices

provide great mecha~ucal integriw and chemical stability Superconductmgl

polymer composites h7ve supenor mechanical properties, ktter ~michiibility

and processing flexibility in comparison with c e r a ~ ~ c s alone. Encapsulation

also protects the ceramics from environmental deterioration and owgen loss.

It has also been proved that composites formed from ceramic superconductor

and polper exhibited good magnetic levitation at liquid nitrogen temperature,

though dc zero resistivity was generally lost in the composites due to poor contact

lxtween superconducting grains. Also these composites showed &cent flexibility

and improved toughness. Hence these coinp<~sites are ideal for potential

applications where zero resistivity or contact kt\veen superconducting grains is

not essential to the function such as magnetic shielding (1.541 and levitation

lntroduclion 11

Therefore i t is very imponant to evaluate mapet ic levitation properties for composite

materials. Weinbergeret al. [1.52, 1.531 havc studied thc levitation force fot- YBaCuO1

paraffin wax con~posite and suggested a prototype bearing using this typc ofcompositc

[ I 531. More efforts are necessary. improving the superconductor magnclic

prope~ties, to achieve higher load and stiffness capacity.

In this context this proposal suggests a versatile thermoplastic Linear Low Density

Polyethylerie (LLDPE) to bc used as a matlix for the superconducting ccranic mate~ial

YB~,CU,O,~~ . LLDPE is selected [ I .4] for the matrix material because it has versatile

properties like flexibility, good mechanical strength, elongation at break anti puncture

resistance compared to other bl-anchcs ofthe polyethylene. They are readily workcd

at room temperature since they posses the lowest glass transition temperature ofthe

common thcrmoplastics. Also they havc well defined crystallitie phase. O~thorhornbic

sh1ichlre ofLLDPE and the sizeoftllccrystallized domains can easily be corrtrollcd by

XRD and crystallinity studics using Differential Scanning Calorimetry (DSC).

YB~,CU,O,.~ is selected [1.55] Tor the supel-conductor pan since this ccramic material

shows a more abrupt ficld pent.11-ation and they crystallize in orthorhornbic oxygeli

deficient perovskitc structure with a transition tcmperaturc ofaround 95K.

Bcfore going in to furtlicr details. 3 discussion on polyrtiers would not be out ofplace.

1.7 Polyrncrs

Polymet- science has cnierged as an active disciplilii- ol~matcrial science. This field

impinges on areas ofcommodity, engineering and specialty polyrnm, thereby stimulating

interest all over the globe inexploiting newer domains. There is a long histo~y ofst~ccessfi~l

development, which came bonr the enormous conhibutions ofnulnerous pmplc [ 1.561.

A polymer is a gencric tern1 ~ s e d to describe a substantially long molecule. I'his lony

molecule consists ~Ss t ruc tu~al~~~i i t s and repeating units stn~ng together through chemical

bonds. These units ofpolymer arc called tnonomers [1.57 - 1.591 and thcproccss of

combination is called polyrncrization. Polyn~erization is a process of rcactirig monomer

molecules together in a chemical reaction to fom~ three-dimensional nctworks or polymer

chains [I ,601. The monomers are trpically small molecules of low molecular weight.

The tnonomers can be ider~tical or thcy can have one or more substituted cheniical

groups. When the monomers arc ofsarne kind, the product ofpolymerization is called

homopolymer and when differctit typcs of monomers are involved, the polymers are

called copolymer. These differences between monomers can affect properties [I .6 I ]

such as solubility, flexibility or strength.

1.8 Copolymerization

Copolymcritation is polymel-ization with two or more different monomcrs [I ,621.

Copolymerization of different nlonolners can result in varied properties [ 1.631 of

polynicrs. For example, copolymerising ethene with small amounts ofhcxene- l is one

way to form lincar low density polyethene (1,I.DPE). In LLDPE, thc C, branches

resulting frorn the hcxcnc lower the density and prevent such largc crystalline regions

within the polyrner as in HDPE. This means that I.LDPE can withstand strong tearing

fol-ces wllilst remaining flcxiblc

In this work, L<LDPE lias heen lried as tllc ~natl-ix matenat ofHTS(' arid a discussio~~

o n polyethylene follo\,\i tins src\io~i.

'I'l~e discovcry and dcvelopn~ent of polyethylene provides an excrllent lesson i l l

the value of observing and following up an ur~expected experiniental result

Polyethylcnc is the most popular plastic in the world. I t is one of the most

important polyolefins in terms of commercial production and technological

Introduction 13

applications. Figure 1.3 shows the space filling model [1.64] of a polyethylene

chain

Figurel.3 11.641 : Space-filling model of a polyethylene chain

Its name originates from the monomer ethene, also

known as ethylene, used to create the polymer. 1n

most scientific publicatjons it is known as

polyethylene - an indication that it is a polymer of

ethylene [1.64]. Figure 1.4 [1.64] shows the three

dimensional image of polyethylene. Figure 1.4 11.64):

The three dimensional image of polyethylene

Polymers formed by the repeated addition of monomer units [1.65] without the

elimination ofany by product molecules are called addition polymers. This occurs

during polymerization, in whlch many monomer molecules link to each other.

The monomers are unsaturated compounds and are usually the derivatives of

ethene. The ethene molecule, C,H, is CH, = CH,, two CH, connected by a double

bond, as shown below,

A molecule of polyethylene is nothing more than a long chain of carbon atoms,

with two hydrogen atoms attached to each carbon atom as shown below [1.64].

14 Chapter 1

H H H H H H H H H H H I & '

N I . y l f - - & - & - ~ - - & - - C - C - -C-@..",w I l l l l l l l I l l

H H H H H H H H H H H

Polyethylene or polyethene is a thermoplastic heavily used in consumer products.

This is the polymer that makes grocery bags, shampoo bottles, children's toys,

and even bullet proof vests. Its many applications include films or sheets for

packaging, shower curtains, unbreakable bottles, pipes, pails, drinking glasses,

and insulation for wire and cable.

Polyethylene is resistant to water, acids, alkalies, and most solvents. It has good

mechanical and electrical properties, resistance to cold flow, ease of processing

and ail excellent cost-performance relation. These characteristics of polyethylene

have generated great interest in the development of polyethylene composites with

other materials so as to improve some specific property.

1.10 Classification of polyethylenes

Polyethylene is classified into several different categories based mostly on its

density and branching. The mechanical properties ofPE depend significantly on

variables [1.64] such as the extent and type of branching, the crystal structure,

and the molecular weight. Some of them are:

O HDPE (high density PE)

.:. HDXLPE (high density cross-linked PE)

*:* PEX (cross-linked PE)

MDPE (medium density PE)

-3 LDPE (low density PE)

*:* LLDPE (linear low density PE)

VLDPE (very low density PE)

Introduction 75

1 . 1 Linear Low Density Polyethylene (LLDPE)

Linear Low Density Polyethylene is a linear microstructure version of the LDPE.

LLDPE, defined by a density range of 915 - 925 kg/m3 is a subat id ly linear

polymer, with unbranched chains, commonly made by copolymerization of

ethylene with short-chain alpha-olefins (e.g. 1-butene, 1-hexene, and I-octene).

Branchng can influence [1.66] a nwnber of physical propxbes like tensile strength

and crystallinity. The more branched a molecule is, the lower is its tensile strength

and crystallinity. LLDPE is a semicrystalline polymer and have better elonjption

performance and stress craclung resistance. It has glass transition temperature,

(TJ much less than room temperature and crystalline melting temperature (Tm)

much higher than room temperature. At room temperature, there is a cqstalline

fraction that can bear load and an amorphous fiaction that is mobile and adds

toughness. Therefore LLDPE hiis very high tensile strength and crystallinity.

LLDPE are made by Ziegler-Natta vinyl polymerization, a method that uses a

transition metal catalyst, like TiCI,, to initiate polymerization. KarI Ziegler and

Giulio Natta received the 1963 Nobel Prize in Chemistry for dewsoping this

method [ 1.661.

H2C=CH2 m m m

H2CtCH Y ~ - m n f h \ CH2 examp&: &ut-fime 1

1 CH3

TC'X trtrlystrrC13~1"'w

LLDPE has higher tensile strength and higher impact and puncture resistance

than low density polyethylene (LDPE). It is very flexible and elongates under

stress. It can be used to make thinner films, with better environmental stress

cracking resistance compared to LDPE. It has good resistance to chemicals and

to ultraviolet radiation and has good electrical properties.

LLDPE is used in flexible tubing and in bags either neat or blended with LDPE.

It is also used for plastic wrap, pouches, toys, lids, pipes, buckets and containers,

covering of cables, and geornembranes.

Coming back to the goals and aims of this study, it must be noted here that there

are reports about the studies on various superconductor/polymer composites.

However optimum properties for such composites have not yet been accomplished

due to various reasons like the effect of reinforcement on the polymer matrix,

high insulating nature of the matrix, lack of interconnectivity of superconducting

grains in the matrix etc. In this study, the preparation and characterization of

LLDPEIhigh Tcsuperconductor composites and further studies to bridge the gaps

in the available reports on similar blends are made.

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Introduction 17

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18 Chapter 1

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Introduction 19

1.35 h~p:lh~vw.googlr.cotd~opicflaitation properties for superwnduct ors

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Introduction 2 1

1.13 Index

Kamerlingh Onnes

supcrconductivity

high '1'" superconductor

perovskite oxide

diamagnetic properties

Mcissncr effect

critical tempcraturc

levitation properties

electromagnetic shielding

superconductor/pol~~~~er composite

polymer

polymerization

copolymerization

polycthylcnc

lincar low dcnsity polyethylene

Ziegler-Natta vinyl polyrncrization