Chapter 7 Electrical Properties Hong-Wen Wang

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 ?

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

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.

Doped polyacetylene The polyacetylene has the conjugated long-chain which is potential for electrical conductivity Doping suitable inorganic compounds –Acceptor : Br 2, SbF 5, WF 6 and H 2 SO 4 –Donor: alkali metals –Conductivity as high as 10 3 ohm -1 cm -1 in trans-polyacetylene can be achieved. –Synthetic metal.

聚對伸苯基 聚砒硌 Doped with FeCl 3, 0.3 S/cm at R.T. Oxidized to 10 2 S/cm

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

TCNQ, a  electron acceptor Chloroanil, a  electron acceptor

Paraphenyle nediamine, a  electron donor TTF, a  electron donor BEDT-TTF, a  electron donor

Superconductivity At the end of 1986, superconductivity oxide La 2-x Ba x CuO 4-x and YBa 2 Cu 3 O 7 were discovered. YBa 2 Cu 3 O 7 can be superconducting at Tc=92 K, which is easily achieved by liquid N 2. Superconductor are characterized by two phenomena. –Zero resistivity –Perfect diamagnetic.

Superconductivity - phenomenon 1, zero resistance

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

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.

What is diamagnetic ?

Phenomenon 2, perfect diamagnetic

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)

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.

High-temperature superconductor – Ceramic superconductor All high-temperature superconductor are ceramics –Challenges to produce wires, tapes …. Four categories of ceramics: –YBa 2 Cu 3 O 7 93 K –Bi 2 Sr 2 Ca 2 Cu 3 O K –HgBa 2 Ca 2 Cu 3 O K –Tl 2 Ba 2 Ca 2 Cu 3 O K Under high pressure, Tc might increase to higher temperature.

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.

Type 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 superconductors Zero electrical resistance  power transmission over long distances Perfect diamagnetism  SQUID, Levitation for transportation.

Levitation Car or Train

Superconducting wires

7.4 Semiconductivity Conductivity of semiconductivity is given by :   = ne Number of electron Charge Mobility

Semiconductivity n = n o exp (-E/kT) Depend on the dopant level

Ionic Conductivity