Chapter 7 Electrical Properties

1. Chapter 7 Electrical Properties Hong-Wen Wang

2. Basic of electrical properties • What is characteristics of metallic conductivity ? • What is characteristics of superconductivity ? • What is semiconductivity ? • What is ionic conductivity ? • What is dielectrics ? Ferroelectrics ? Piezoelectric ? Pyroelectrics ?

10. 7.2 Metallic conductivity: organic metals • Characteristics of organic metals • Flexibility • Easy fabrication • High conductivity as metal • Two main categories: • Conjugated system • Charge transfer complexes

11. Conjugated systems • Organic solid are ususlly insulators • Polymers such as polyethylene are insulators - only C-C single bonds. • However, polymers have conjugated could be electrical conductive such as polyacetylene. • There are cis and trans for polyacetylene.

13. Doped polyacetylene • The polyacetylene has the conjugated long-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 trans-polyacetylene can be achieved. • Synthetic metal.

14. Doped with FeCl3, 0.3 S/cm at R.T. Oxidized to 102 S/cm 聚對伸苯基 聚砒硌

15. Charge 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

16. Chloroanil, a  electron acceptor TCNQ, a  electron acceptor

17. TTF, a  electron donor Paraphenylenediamine, a  electron donor BEDT-TTF, a  electron donor

19. Superconductivity • At the end of 1986, superconductivity oxide La2-xBaxCuO4-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 phenomena. • Zero resistivity • Perfect diamagnetic.

20. Superconductivity- phenomenon 1, zero resistance

21. The properties of zero resistance • 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

22. The properties of zero resistance • 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.

23. What is diamagnetic ?

24. Phenomenon 2, perfect diamagnetic

25. Perfect Diamagnetic :The Meissner Effect • Superconductor exhibits “perfect diamagnetism” and expel a magnetic field (< Hc) • The is called the Meissner effect. • Fig. 7.5 (a) ~ (f)

27. Critical temeprature Tc, critical magnetic field Hc, and critical current density Jc for superconductivity • 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, critical magnetic field, and critical current density.

29. High-temperature superconductor –Ceramic 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.

30. Type I, and Type II superconductors • 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.

31. Type II superconductor • In vortex (mixed) state • Magnetic line are bunched together through vortex regions. • No lateral displacement • Levitation , non-contact vehicle is possible.

32. 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.

33. Levitation Car or Train

34. Superconducting wires

45. Applications of superconductors • Zero electrical resistance  power transmission over long distances • Perfect diamagnetism  SQUID, Levitation for transportation.

46. Semiconductivity