Plastic contact conductance

Plastic Contact Conductance Model of Greenwood and Williamson. Sridhar and Yovanovich [109] developed correlation equations for the contact conductance of conforming rough surfaces based on the Greenwood and Williamson [26] surface model using the plastic deformation model described above. The dimensionless contact conductance correlation is... 

The proposed elastoplastic contact conductance model moves smoothly between the elastic contact model of Mikic [66] and the plastic contact conductance model of Cooper, Mikic, and... 

M. R. Sridhar and M. M. Yovanovich, Review of Elastic and Plastic Contact Conductance Models Comparison with Experiment, J. Thermophysics and Heat Transfer (8/4) 633-640,1994. 

A polymer (material containing a long chain of similar molecular structures) is the first and foremost electrical and heat insulator. The idea that polymers or plastics could conduct electricity had been considered absurd. Their wide application as an insulating material is the reason they are studied and developed in the first place. In fact, these materials are commonly used for surrounding copper wires and manufacturing the outer structures of electrical appliances that prevent humans from coming in direct contact with electricity. 

Finally, the effects of thermal contact resistance cannot be neglected in the realm of plastics thermal conductivity. This important heat transfer effect, which is due to the presence of an interface between the specimen and the sensor, needs to be compensated for, or eliminated. The success of a thermal conductivity measurement technique often depends on how well it addresses this issue. This issue is discussed in more detail later. 

Silver is often preferred as an undercoat for rhodium by reason of its high electrical conductivity. A further advantage of silver in the case of the thicker rhodium deposits (0-0025 mm) applied to electrical contacts for wear resistance is that the use of a relatively soft undercoat permits some stress relief of the rhodium deposit by plastic deformation of the under-layer, and hence reduces the tendency to cracking , with a corresponding improvement in protective value. Nickel, on the other hand, may be employed to provide a measure of mechanical support, and hence enhanced wear resistance, for a thin rhodium deposit. A nickel undercoating is so used on copper printed connectors, where the thickness of rhodium that may be applied from conventional electrolytes is limited by the tendency of the plating solution to attack the copper/laminate adhesive, and by the lifting effect of internal stress in the rhodium deposit. 

Electronic marketplace/E-commerce In addition to the many databases available and person-to-person contacts, E-commerce in plastics has been conducted through suppliers web sites or the dot-commerce independent web sites that link material buyers with sellers in transactions or auction formats. During the year 2000 five plastic producers/suppliers and various elastomer producers/suppliers created a new and important business model of a joint-venture web site. It provides multiple companies to join forces to do business. This is a strategy some observers call competition and others regard as just another form of selling in. an electronic format. Regardless of how it is perceived, the model will help propel e-commerce into the mainstream of processor procurement due to the size and wealth of the companies involved. The plastic model example is the largest online business-to-business site todate. 

Figure 4a. Electrochemical cells for microwave conductivity measurements. Cell above microwave conduit (1) electrochemical cell (plastic tube, placed on working electrode material), (2) counter-electrode, (3) reference electrode, (4) electrolyte, (5) space charge layer, (6) diffusion layer, (7) contact to working electrode, (8) waveguide.

Additives are needed not only to make resins processable and to improve the properties of the moulded product during use. As the scope of plastics has increased, so has the range of additives for better mechanical properties, resistance to heat, light and weathering, flame retardancy, electrical conductivity, etc. The demands of packaging have produced additive systems to aid the efficient production of film, and have developed the general need for additives which are safe for use in packaging and other applications where there is direct contact with food or drink. 

Instead, when two solids are pressed together, due to surface irregularity, the real contact area A (see Fig. 4.2) can be much smaller (for metal a factor 10-6) than the nominal contact area A. An applied force results in a strong pressure on the contact area A, and a plastic or permanent deformation occurs. The deformation changes the area A hence the thermal conductance of the contact is proportional to the force. A drawback of the deformation of the lattice is the reduction of the bulk conductivity of the material. 

Electrolytic conductivity is most often measured by placing electrodes in contact with the electrolytic solution which is contained in such a way that the measured electrical conductance between the electrodes can be related to the conductivity of the solution. The conductivity cell most commonly comprises art enclosure made of electrically insulating material, such as glass or plastic, which serves to hold or isolate a portiun of the electrolytic solution and to accommodate the two electrodes. The cell constant of such a device is then used to relate the measured electrical conductance between the electrodes to the actual electrolyuc conductivity. 

The resolution of potentiometric transducers is dependent upon the construction of the resistance element. In the case of a wire-wound resistance, in order to obtain a high resistance in a small space, the resistance wire is wound on to a mandrel or card which is straight or formed into a circle or helix depending upon the motion of the contact. This limits the resolution of the transducer as the wiper moves from one wire to the next on the mandrel. The best resolution that can be obtained is about 0.01 per cent (see Section 6.10.1). Typical wire-wound potentiometers have strokes of between 0.0025 m and 0.5 m and rotational versions from about 10° of arc to 50 turns. An alternative often employed is the conductive plastic film element. This provides a continuous resistance element and thus, a zero resolution, but such elements suffer from a higher temperature coefficient of resistance. A more recent development is a combination of earlier types in which a conductive plastic coating is sprayed on to a wire-wound resistor. 

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