Material properties electrical resistivity

Physical Properties Electrical. Electrical properties have been the main focus of study of organic semiconductors, and conductivity studies on organic materials have led to the development of materials with extremely low resistivities and large anisotropies. A discussion of conductivity behaviors for various classes of compounds follows. 

Spinel ferrites, isostmctural with the mineral spinel [1302-67-6] MgAl204, combine interesting soft magnetic properties with a relatively high electrical resistivity. The latter permits low eddy current losses in a-c appHcations, and based on this feature spinel ferrites have largely replaced the iron-based core materials in the r-f range. The main representatives are MnZn-ferrites (frequencies up to about 1 MH2) and NiZn-ferrites (frequencies 1 MHz). 

Electrical Properties. Generally, deposited thin films have an electrical resistivity that is higher than that of the bulk material. This is often the result of the lower density and high surface-to-volume ratio in the film. In semiconductor films, the electron mobiHty and lifetime can be affected by the point defect concentration, which also affects electromigration. These effects are eliminated by depositing the film at low rates, high temperatures, and under very controUed conditions, such as are found in molecular beam epitaxy and vapor-phase epitaxy. 

Thermal Conductivity. The value of 2000 W/(m-K) at room temperature for Type Ila natural stones is about five times that of Cu, and recent data on 99.9% isotopicaHy pure Type Ila synthesized crystals ate in the range of 3300—3500 W/(m-K) (35). This property combined with the high electrical resistance makes diamond an attractive material for heat sinks for electronic devices. 

Nonferrous alloys account for only about 2 wt % of the total chromium used ia the United States. Nonetheless, some of these appHcations are unique and constitute a vital role for chromium. Eor example, ia high temperature materials, chromium ia amounts of 15—30 wt % confers corrosion and oxidation resistance on the nickel-base and cobalt-base superaHoys used ia jet engines the familiar electrical resistance heating elements are made of Ni-Cr alloy and a variety of Ee-Ni and Ni-based alloys used ia a diverse array of appHcations, especially for nuclear reactors, depend on chromium for oxidation and corrosion resistance. Evaporated, amorphous, thin-film resistors based on Ni-Cr with A1 additions have the advantageous property of a near-2ero temperature coefficient of resistance (58). 

Industrial equipment is a continuing area of development for plastics. Pipes, pumps, valves and sight glasses, made from such materials as PVC, PTFE and poly-4-methylpent-l-ene, have become well established on account of their corrosion resistance. The nylons are used for such diverse applications as mine conveyor belts and main drive gears for knitting machines and paper-making equipment. These and other materials are widely used where such features as toughness, abrasion resistance, corrosion resistance, non-stick properties, electrical insulation capability and transparency are of importance. 

Electrical Properties Traditionally plastics have established themselves in applications which require electrical insulation. PlFt and polyethylene are among the best insulating materials available. The material properties which are particularly relevant to electrical insulation are dielectric strength, resistance and tracking. 

Instruments based on the contact principle can further be divided into two classes mechanical thermometers and electrical thermometers. Mechanical thermometers are based on the thermal expansion of a gas, a liquid, or a solid material. They are simple, robust, and do not normally require power to operate. Electrical resistance thermometers utilize the connection between the electrical resistance and the sensor temperature. Thermocouples are based on the phenomenon, where a temperature-dependent voltage is created in a circuit of two different metals. Semiconductor thermometers have a diode or transistor probe, or a more advanced integrated circuit, where the voltage of the semiconductor junctions is temperature dependent. All electrical meters are easy to incorporate with modern data acquisition systems. A summary of contact thermometer properties is shown in Table 12.3. 

The toughness of a material is a design driver in many structures subjected to impact loading. For those materials that must function under a wide range of temperatures, the temperature dependence of the various material properties is often of primary concern. Other structures are subjected to wear or corrosion, so the resistance of a material to those attacks is an important part of the material choice. Thermal and electrical conductivity can be design drivers for some applications, so materials with proper ranges of behavior for those factors must be chosen. Similarly, the acoustical and thermal insulation characteristics of materials often dictate the choice of materials. 

Applications Plasma spraying is used to apply coatings for protection against wear and corrosion, to prevent erosion or cavitations, and to provide electrical insulation or conductivity. It can also be employed to produce bearing surfaces, abrasive properties or resistance to wetting by molten metals. The coatings can also be applied to facilitate the joining of different materials. 

The resistance of most plastics to the flow of direct current is very high. Both surface and volume electrical resistivities are important properties for applications of plastics insulating materials. The volume resistivity is the electrical resistance of the material measured in ohms as though the material was a conductor. Insulators will not sustain an indefinitely high voltage as the applied voltage is increased, a point is reached where a drastic decrease in resistance takes place accompanied by a physical breakdown of the insulator. This is known as the dielectric strength, which is the electric potential in volts, which would be necessary to cause the failure of a 1/8-in. thick insulator (Chapter 4, ELEC-TRICAL/ELECTR ONICS PRODUCT). 

The compact structure of diamond accounts for its outstanding properties. It is the hardest of all materials with the highest thermal conductivity. It is the most perfectly transparent material and has one of the highest electrical resistivities and, when suitably doped, is an outstanding semiconductor material. The properties of CVD and single-crystal diamonds are summarized in Table 7 2.[1][18]-[20]... 

In practice the situation is less favorable due to losses associated with overpotentials in the cell and the resistance of the membrane. Overpotential is an electrochemical term that, basically, can be seen as the usual potential energy barriers for the various steps of the reactions. Therefore, the practical efficiency of a fuel cell is around 40-60 %. For comparison, the Carnot efficiency of a modern turbine used to generate electricity is of order of 50 %. It is important to realize, though, that the efficiency of Carnot engines is in practice limited by thermodynamics, while that of fuel cells is largely set by material properties, which may be improved. 

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