Conductors Semiconductors Thermal

Structural change resulting from a Peierls distortion can have dramatic effects on the physical properties. Look at the half-filled bands of Figure 6.6. There is no HOMO-LUMO gap. Such a situation depicts a metallic electrical conductor. On the other hand, after Peierls distortion there is a band gap at the Fermi level (Figure 6.10) and the material, depending on the width of the gap, is a semiconductor or an insulator. For a semiconductor, thermal excitation of electrons from the... 

Electronic conductors Semiconductors Ionic conductors Mixed conductors Ferrimagnetics Thermal conductors Thermal insulators Chromophores Translucent materials Nonlinear optics Photovoltaics Transparent conductors Active materials Inert materials Biocompatibles... 

Some of the metalloids are considered semiconductors. The term metalloids is used in this reference book because these elements do have characteristics of both metals and non-metals, and the term semiconductor refers only to particular elements somewhere between metals and nonmetals. Semiconductors also have properties of both metals and nonmetals. Therefore, they have the ability to act as conductors of electricity and thermal energy (heat), as well as the ability to act as insulators or nonconductors of electricity and heat, depending upon the kind and amount of impurities their crystals contain. Again, following the zigzag steps on the periodic table, the metalloids having properties of both metals and nonmetals are as follows boron, silicon, germanium, arsenic, antimony, tellurium, and polonium. 

This competition between electrons and the heat carriers in the lattice (phonons) is the key factor in determining not only whether a material is a good heat conductor or not, but also the temperature dependence of thermal conductivity. In fact, Eq. (4.40) can be written for either thermal conduction via electrons, k, or thermal conduction via phonons, kp, where the mean free path corresponds to either electrons or phonons, respectively. For pure metals, kg/kp 30, so that electronic conduction dominates. This is because the mean free path for electrons is 10 to 100 times higher than that of phonons, which more than compensates for the fact that C <, is only 10% of the total heat capacity at normal temperatures. In disordered metallic mixtures, such as alloys, the disorder limits the mean free path of both the electrons and the phonons, such that the two modes of thermal conductivity are more similar, and kg/kp 3. Similarly, in semiconductors, the density of free electrons is so low that heat transport by phonon conduction dominates. 

It will be seen that although normal (or thermal ) electrochemical reactions can be sustai ned at low current densities using semiconductors (Le., they act as electron-poor metals), they real ly do not come onto center stage until their photoelectrochemistry is studied (see Chapter 10). Thus, semi conductor electrodes are responsive to light when metals are almost unreactive to it. 

In Fermi-Dirac statistics, g is the Fermi energy Er, which is such that the probability, that a state of energy is occupied, is 1/2. States with energies higher than Et have a smaller probability of being occupied, those with lower energy, a higher probability. The position of the Fermi level in a semiconductor depends markedly on the temperature and on the concentration of impurities. The Fermi levels of two conductors in electrical contact and in thermal equilibrium are the same. 

Metals are located on the left side of the periodic table. Metals tend to form cations, are generally ductile and malleable, and are good electrical and thermal conductors. Nonmetals are located on the right side of the periodic table. Nonmetals tend to form anions and have a wide variety of physical properties. Metalloids look like metals but have electrical conductivity intermediate between metals and nonmetals. For this reason, metalloids are called semiconductors. 

The thermisters (TMs) are semiconductor device with a high resistance dependence on temperature. They may be calibrated as a thermometer. The semiconductor sensor exhibits a large change in resistance that is proportional to a small change in temperature. Normally TMs have negative thermal coefficients. Like RTDs, they operate on the principle that the electrical resistance of a conductive metal is driven by changes in temperatures. Variations in the conductor s electrical resistance are thus interpreted and quantified, as changes in temperature occur. 

Thermogravimetric analyses (TGA) of poly(dialkyl)stannanes under nitrogen showed onset temperatures for thermal decomposition in the temperature range of 250-280 °C, which is close to the values for related polydialkylsilanes. The TGA analyses of poly(diaryl)stannanes ° show onset temperatures for the decomposition in the range of 203-327 °C. Thermal decomposition of polystannanes leads to tin and tin oxide, and thus application of these materials for the preparation of conductors or coatings in semiconductors may be envisaged. It is worth noting that electronic conductivities of about 0.01 -0.3 S cm were observed for thin films of the polystannanes after exposure to SbFs vapor as an oxidant . ... 

Semiconductors. Substances such as ZnO, ZnS, and PbS, while not very good conductors, have electrons which can be thermally excited at very low activation energies (for example, 10 to 20 Kcal) to give electronic conduction." The surfaces and edges of such solids are good centers for redox and possibly free radical reactions. 

Solid state detectors consist of three layers, a layer of pure silicon sandwiched between a p-type and an n-type conductor. We recall that an example of an n-type conductor is germanium to which is added P or As, an impurity. The extra electron in the phosphorus or arsenic atoms is thought of as being in an energy level close to the conduction band. These electrons are readily thermally excited into the conduction band increasing the conductivity. A p-type semiconductor may be silicon to which a trivalent element such as boron or aluminum is added as an impurity. This creates holes close to the valence band. Electrons are readily promoted to these holes leaving positive holes in the valence band that provide for a conduction pathway. 

The p- and n-type conductors are necessary in the solid state detector because silicon crystals are very difficult to obtain pure. Usually they contain impurities such as boron, which would make it a p-type semiconductor. This situation would allow thermal electrons to cross the silicon energy gap to the low lying hole state. However, we want only electrons that are excited by X-ray photons to act as conductors. Interference of thermal electrons is avoided by a process called drifting in which a small amount of lithium is added, usually by thermal diffusion, to one side of the Si crystal and a larger amount to the opposite side. The lithium ionizes within the silicon... 

At the absolute zero point of temperature a typical intrinsic, ideal monocrystal of a semiconductor like germanium, is virtually an insulator. By "intrinsic" is meant a Ge crystal without any trace of admixture, neither intentionally added, nor inadvertently present. "Ideal" means without any lattice defects. The electrons are all bound to the Ge atoms and therefore immobile. When the temperature is raised, some electrons become free, they can move through the crystal and hence confer a certain conductivity on the crystal. In a semiconductor dK / dT > 0, this in contrast to metallic conductors, in which the electrons are always present and the randomization due to thermal motion opposes their directional displacement with increasing temperature. In electrolyte solutions dK / dT > 0 because the viscosity decreases with Increasing tempera-... 

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