Chapter 7 Chapter 7 Electrical Electrical PropertiesProperties

Hong-Wen WangHong-Wen Wang

Basic of electrical propertiesBasic of electrical properties• What is characteristics of metallic condu

ctivity ?• What is characteristics of superconducti

vity ?• What is semiconductivity ?• What is ionic conductivity ?• What is dielectrics ? Ferroelectrics ? Piez

oelectric ? Pyroelectrics ?

7.2 Metallic conductivity: 7.2 Metallic conductivity: organic metalsorganic metals

• Characteristics of organic metals– Flexibility– Easy fabrication– High conductivity as metal

• Two main categories:– Conjugated system– Charge transfer complexes

Conjugated systemsConjugated systems• Organic solid are ususlly insulators• Polymers such as polyethylene are insul

ators - only C-C single bonds.• However, polymers have conjugated co

uld be electrical conductive such as polyacetylene.

• There are cis and trans for polyacetylene.

Doped polyacetyleneDoped polyacetylene• The polyacetylene has the conjugated lo

ng-chain which is potential for electrical conductivity

• Doping suitable inorganic compounds– Acceptor : Br2, SbF5, WF6 and H2SO4

– Donor: alkali metals– Conductivity as high as 103 ohm-1cm-1 in tran

s-polyacetylene can be achieved. – Synthetic metal.

聚對伸苯基 聚砒硌

Doped with FeCl3, 0.3 S/cm at R.T.

Oxidized to 102 S/cm

Charge transfer complexesCharge transfer complexes• Two-component organic system in

which one is a electron donor and the other an electron acceptor– Donor – acceptor form separate,

alternating stacks– Electron transfer take place– conducting behavior

TCNQ, a electron acceptor

Chloroanil, a electron acce

ptor

Paraphenylenediamine, a electron do

nor

TTF, a electron donor

BEDT-TTF, a electron donor

SuperconductivitySuperconductivity• At the end of 1986, superconductivity oxide La2-x

BaxCuO4-x and YBa2Cu3O7 were discovered. • YBa2Cu3O7 can be superconducting at Tc=92 K,

which is easily achieved by liquid N2. • Superconductor are characterized by two phen

omena. – Zero resistivity– Perfect diamagnetic.

SuperconductivitySuperconductivity- phenomenon 1, - phenomenon 1, zero resistancezero resistance

The properties of zero The properties of zero resistanceresistance

• Superconductor are zero resistance to the flow of electrical current below Tc (90 K, for YBaCuO)

• Above Tc (92 K, for YBaCuO), materials resistance gradually rises with increasing temperature and is normal metallic state.

• Resistance is from electron-phono collisions

The properties of zero The properties of zero resistanceresistance

• Superconductivity could be explained by BCS theory but need modification.

• A loose associated electron pairs (Cooper pairs) more cooperatively through the lattice in such a way that electron-phonon collisions are avoided.

• More works need to be done to understand ceramic superconductors.

What is diamagnetic ? What is diamagnetic ?

Phenomenon 2, perfect diamagneticPhenomenon 2, perfect diamagnetic

Perfect Diamagnetic :Perfect Diamagnetic :The Meissner EffectThe Meissner Effect

• Superconductor exhibits “perfect diamagnetism” and expel a magnetic field (< Hc)

• The is called the Meissner effect. • Fig. 7.5 (a) ~ (f)

Critical temeprature Tc, critical magnetic fCritical temeprature Tc, critical magnetic field Hc, and critical current density Jc for sield Hc, and critical current density Jc for s

uperconductivityuperconductivity

• The superconductivity is lost when either following happened:– Heating above Tc– Appling the magnetic field higher than Hc.– Increasing the electrical current beyond Jc. – These are called critical temperature, critica

l magnetic field, and critical current density.

High-temperature High-temperature superconductor – superconductor –

Ceramic superconductorCeramic superconductor• All high-temperature superconductor are ceramics

– Challenges to produce wires, tapes….• Four categories of ceramics:

– YBa2Cu3O7 93 K– Bi2Sr2Ca2Cu3O10 110 K– HgBa2Ca2Cu3O10 134 K– Tl2Ba2Ca2Cu3O10 125 K

• Under high pressure, Tc might increase to higher temperature.

Type I, and Type II Type I, and Type II superconductorssuperconductors

• Type I superconductor– With increasing H or T, an abrupt

change from a superconducting to a non-superconducting state occurs.

• Type II superconductor– There is a transition state, so called

vortex state, or mixed state, between superconductor and normal metallic region.

Type II superconductorType II superconductor• In vortex (mixed) state

– Magnetic line are bunched together through vortex regions.

– No lateral displacement– Levitation , non-contact vehicle is possible.

定子為超導材 ,轉子為永磁 .轉子利用超導材料與永久磁鐵間之作用力 ,懸浮 , 旋轉於空氣中 . 高效率 ,壽命長 ,低維修轉速可達 520,000rpm, 振幅僅 5μm.

Applications of Applications of superconductorssuperconductors

• Zero electrical resistance power transmission over long distances

• Perfect diamagnetism SQUID, Levitation for transportation.

Levitation Car or TrainLevitation Car or Train

Superconducting wiresSuperconducting wires

7.4 Semiconductivity7.4 Semiconductivity• Conductivity of semiconductivity is give

n by : = n•e•

Number of electron

Charge

Mobility

SemiconductivitySemiconductivityn = no exp (-E/kT)

Depend on the dopant level

Ionic ConductivityIonic Conductivity