Conduction electronic

The electronic and optical properties ofTT-conjugatedpolymers result from a number of states around the highest occupied and the lowest unoccupied levels. The highest occupied band, which originates from the HOMO of each monomer unit, is referred to as the VB. The corresponding lowest unoccupied band, originating from the LUMO of the monomer, is the CB (Fig. 3.1]. 

Conjugated polymers have a conjugated n system and n bands. As a result, they have a low ionization potential (usually lower than 6 eV) and/or a high electron affinity (lower than 2 eV). 

They can be easily oxidized by electron-accepting molecules (I2, AsFs, SbFs] and/or easily reduced by electron donors (alkali metals Li, Na, K], 

Charge transfer between the polymer chain and dopant molecules is easy after doping neutral conjugated molecules n-doping corresponds to reduction (addition of electron], and p-doping corresponds to oxidation (removal of electron]. 

The soliton model was first proposed for degenerated CPs (polyacetylene [PA] in particular] and it was noted for its extremely one-dimensional character, each soliton being confined to one polymer chain. Thus, there was no conduction via interchain hopping. Furthermore, solitons are very susceptible to disorder and any defect such as impurities, twists, chain ends, or crosslinks will localize them. 

Electronic conduction plays a limited role, if any, in anodic oxide formation, since under the anodization conditions and with a high 

However, most of this work has avoided consideration of some complicating factors, arising especially from the limited thickness and real structure of anodic dielectric films. The latter causes the following effects  

Both classical268 and more recent269,270 papers have paid little, if any, attention to these complications. 

The modeling of hopping conductivity of real amorphous dielectrics of limited thickness, with or without the incorporated space charge, has recently been done by Parkhutik and Shershulskii.62 

The modeling of conductivity of thin disordered dielectrics was based on the following assumptions  

We saw in Chapter 20 that the electron-hole product in an intrinsic semiconductor is given by np (2.5 x 10 /m ) exp(—Eg/fcT) so the number of charged carriers will be (5 X 10 /m ) exp( Eg/2fcT). Even with a bandgap as low as 3 eV, the number of carriers present at ambient temperature would be less than 1 electron or hole/m. However, if electrically active impurities are present that could act as donors or acceptors, the electron or hole population could increase dramatically. For example, 1 ppm of a donor impurity, if fully ionized, would produce a carrier concentration of 10 electrons/m.  

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AFM measures the spatial distribution of the forces between an ultrafme tip and the sample. This distribution of these forces is also highly correlated with the atomic structure. STM is able to image many semiconductor and metal surfaces with atomic resolution. AFM is necessary for insulating materials, however, as electron conduction is required for STM in order to achieve tiumelling. Note that there are many modes of operation for these instruments, and many variations in use. In addition, there are other types of scaiming probe microscopies under development. 

At low currents, the rate of change of die electrode potential with current is associated with the limiting rate of electron transfer across the phase boundary between the electronically conducting electrode and the ionically conducting solution, and is temied the electron transfer overpotential. The electron transfer rate at a given overpotential has been found to depend on the nature of the species participating in the reaction, and the properties of the electrolyte and the electrode itself (such as, for example, the chemical nature of the metal). 

Fig. 3. An overview of atomistic mechanisms involved in electroceramic components and the corresponding uses (a) ferroelectric domains capacitors and piezoelectrics, PTC thermistors (b) electronic conduction NTC thermistor (c) insulators and substrates (d) surface conduction humidity sensors (e) ferrimagnetic domains ferrite hard and soft magnets, magnetic tape (f) metal—semiconductor transition critical temperature NTC thermistor (g) ionic conduction gas sensors and batteries and (h) grain boundary phenomena varistors, boundary layer capacitors, PTC thermistors.

Flaws in the anodic oxide film are usually the primary source of electronic conduction. These flaws are either stmctural or chemical in nature. The stmctural flaws include thermal crystalline oxide, nitrides, carbides, inclusion of foreign phases, and oxide recrystaUi2ed by an appHed electric field. The roughness of the tantalum surface affects the electronic conduction and should be classified as a stmctural flaw (58) the correlation between electronic conduction and roughness, however, was not observed (59). Chemical impurities arise from metals alloyed with the tantalum, inclusions in the oxide of material from the formation electrolyte, and impurities on the surface of the tantalum substrate that are incorporated in the oxide during formation. 

A second type of soHd ionic conductors based around polyether compounds such as poly(ethylene oxide) [25322-68-3] (PEO) has been discovered (24) and characterized. These materials foUow equations 23—31 as opposed to the electronically conducting polyacetylene [26571-64-2] and polyaniline type materials. The polyethers can complex and stabilize lithium ions in organic media. They also dissolve salts such as LiClO to produce conducting soHd solutions. The use of these materials in rechargeable lithium batteries has been proposed (25). 

Most battery electrodes are porous stmctures in which an interconnected matrix of soHd particles, consisting of both nonconductive and electronically conductive materials, is filled with electrolyte. When the active mass is nonconducting, conductive materials, usually carbon or metallic powders, are added to provide electronic contact to the active mass. The soHds occupy 50% to 70% of the volume of a typical porous battery electrode. Most battery electrode stmctures do not have a well defined planar surface but have a complex surface extending throughout the volume of the porous electrode. MacroscopicaHy, the porous electrode behaves as a homogeneous unit. 

In a battery, the anode and cathode reactions occur ia different compartments, kept apart by a separator that allows only ionic, not electronic conduction. The only way for the cell reactions to occur is to mn the electrons through an external circuit so that electrons travel from the anode to the cathode. But ia the corrosion reaction the anode and cathode reactions, equations 8 and 12 respectively, occur at different locations within the anode. Because the anode is a single, electrically conductive mass, the electrons produced ia the anode reaction travel easily to the site of the cathode reaction and the 2iac acts like a battery where the positive and negative terminals are shorted together. 

There are many appHcations in which glass is used as an electrical insulator. One example is glass-to-metal seals. Moreover, other glasses are useful as a result of ionic or electronic conductivity. 

Similarly, electronic conduction typically arises in these oxide materials from the natural loss of oxygen, which occurs in oxides on heating to high temperatures. 

Electrons trapped at the vacancy can become partially or fully ionized, leading to weak n-ty e electronic conduction in an electric field. Again, the conductivity is low. 

Electrical conductivity of copper is affected by temperature, alloy additions and impurities, and cold work (9—12). Relative to temperature, the electrical conductivity of armealed copper falls from 100 % lACS at room temperature to 65 % lACS at 150°C. Alloying invariably decreases conductivity. Cold work also decreases electrical conductivity as more and more dislocation and microstmctural defects are incorporated into the armealed grains. These defects interfere with the passage of conduction electrons. Conductivity decreases by about 3—5% lACS for pure copper when cold worked 75% reduction in area. The conductivity of alloys is also affected to about the same degree by cold work. 

The three elements necessary for corrosion are an aggressive environment, an anodic and a cathodic reaction, and an electron conducting path between the anode and the cathode. Other factors such as a mechanical stress also play a role. The thermodynamic and kinetic aspects of corrosion deterrnine, respectively, if corrosion can occur, and the rate at which it does occur. 

This article addresses the synthesis, properties, and appHcations of redox dopable electronically conducting polymers and presents an overview of the field, drawing on specific examples to illustrate general concepts. There have been a number of excellent review articles (1—13). Metal particle-filled polymers, where electrical conductivity is the result of percolation of conducting filler particles in an insulating matrix (14) and ionically conducting polymers, where charge-transport is the result of the motion of ions and is thus a problem of mass transport (15), are not discussed. 

The anode material in SOF(7s is a cermet (rnetal/cerarnic composite material) of 30 to 40 percent nickel in zirconia, and the cathode is lanthanum rnanganite doped with calcium oxide or strontium oxide. Both of these materials are porous and mixed ionic/electronic conductors. The bipolar separator typically is doped lanthanum chromite, but a metal can be used in cells operating below 1073 K (1472°F). The bipolar plate materials are dense and electronically conductive. 

One feature of oxides is drat, like all substances, they contain point defects which are most usually found on the cation lattice as interstitial ions, vacancies or ions with a higher charge than dre bulk of the cations, refened to as positive holes because their effect of oxygen partial pressure on dre electrical conductivity is dre opposite of that on free electron conductivity. The interstitial ions are usually considered to have a lower valency than the normal lattice ions, e.g. Zn+ interstitial ions in the zinc oxide ZnO structure. 

Graphite has an electron conductivity of about 200 to 700 d cm is relatively cheap, and forms gaseous anodic reaction products. The material is, however, mechanically weak and can only be loaded by low current densities for economical material consumption. Material consumption for graphite anodes initially decreases with increased loading [4, 5] and in soil amounts to about 1 to 1.5 kg A a at current densities of 20 A m (see Fig. 7-1). The consumption of graphite is less in seawater than in fresh water or brackish water because in this case the graphite carbon does not react with oxygen as in Eq. (7-1),... 

Electronic conductivity Flexible conductor of electricity heating elements (resistance heating), shielding of electromagnetic radiation field flattening (high-voltage cables), materials with antistatic capability... 

After briefly introducing the main electronic features of CNTs (Sec. 2) and some general aspects of electronic conduction and transmission (Sec.. 1), we will show how complex electrical measurements to perform on such tiny entities are (Sec. 4). Then we will present the main experimental results obtained on the electrical resistivity of MWCNT and SWCNT and the very recent data relative to the thermopower of SWCNT bundles (Sec. 5). We will also discuss the effect of intercalation on the electrical resistivity of SWCNT bundles (Sec. 6). Finally, we will present some potential applications (Sec. 7). 

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