Contact materials

Rhenium is also used as an electrical contact material because it has good wear resistance and withstands arc corrosion. Thermocouples made of Re-W are used for measuring temperatures up to 2200C, and rhenium wire is used in photoflash lamps for photography. 

Polysilicon. Polysihcon is used as the gate electrode material in MOS devices, as a conducting material for multilevel metallization, and as contact material for devices having shallow junctions. It is prepared by pyrolyzing silane, SiH, at 575—650°C in a low pressure reactor. The temperature of the process affects the properties of the final film. Higher process temperatures increase the deposition rate, but degrade the uniformity of the layer. Lower temperatures may improve the uniformity, but reduce the throughput to an impractical level. 

Conta.ctMa.teria.ls, Electrical contact materials are produced by either slicing rod made from metal powder, infiltrating a porous refractory skeleton, or compaction and sintering of powders (see Electrical CONNECTORS) (51—53). 

Migration and Extraction. When plasticized PVC comes into contact with other materials, plasticizer may migrate from the plasticized PVC into the other material. The rate of migration depends not only on the plasticizer employed but also on the nature of the contact material. 

Biomaterials with Low Thrombogenicity. Poly(ethylene oxide) exhibits extraordinary inertness toward most proteins and biological macromolecules. The polymer is therefore used in bulk and surface modification of biomaterials to develop antithrombogenic surfaces for blood contacting materials. Such modified surfaces result in reduced concentrations of ceU adhesion and protein adsorption when compared to the nonmodifted surfaces. 

Resistance Heating of Contacts. The contact material, contact area, and heat dissipating abihty, as well as the heat dissipating abihty of the stmcture to which the material is attached, limit the amount of current that a contact can transport. Excessive current heats and softens the metal contact. This softening results in an increase in the surface area of the contact and a corresponding reduction in contact resistance. 

They may inherit current chopping tendencies at very low currents of 3-5 A, varying from one manufacturer to another and depending upon the contact material used. This is due to their extremely fast operation as a result of a high vacuum pressure of the order of 10 Torr (1.333 x 10 N/nr) or more (one Torr being the pressure equivalent to hold a column of mercury... 

Furthermore, u, and , refer to the Poisson s ratio and Young s modulus of each of the contacting materials. Finally, iua represents work of adhesion. It is related to the surface energies y and yi of the two materials and their interfacial energy Fiiby... 

Up to this point, the discussion has been limited to cases where the adhesion-induced stresses result in elastic deformation. However, as previously discussed, this need not be the case. Indeed, these stresses can be, in many instances, comparable to the Young s moduli of the contacting materials, often resulting in plastic deformations. This area has been most notably explored by Maugis and Pollock [63]. 

According to the theory proposed by Maugis and Pollock, hereafter referred to as the MP model, if the adhesion induced stresses cause at least one of the contacting materials to yield and undergo a totally plastic response, the contact region formed will increase in size until the force causing the yielding is balanced... 

Upon combining Eqs. 88 and 85, one finds that the total work of adhesion is related to the surface energies of the contacting materials as well as the enthalpy associated with acid-base interactions by... 

As previously discussed, many, if not most, cases of particles adhering to substrates involve at least one of the contacting materials deforming plastically, rather than elastically. Under such circumstances, it would be expected that the extent of the contact should increase with time and, with it, the force needed to detach a particle from a substrate. Moreover, material flow can occur, resulting in the engulfment or encapsulation of the particles. 

As a replacement for bulk oil circuit breakers for distribution purposes, vacuum is a rival to SFg. The contact interruption chamber is a sealed unit with a small contact-separation distances (typically, 16mm for 24kV). Choice of contact material is critical since the material affects ... 

Avoid metal-metal or metal-non-metallic contacting materials that facilitate corrosion such as... 

Description of the cell composition is based - as far as possible - on the Stockholm convention (1953), i.e. the left-hand electrode constitutes the negative terminal of the cell. Cells are listed according to the metallic constituent of the electrode mentioned first which is involved in the electrode reaction establishing the respective electrode potential. Contact materials and conductive additives may be mentioned first before the actual element of interest only for the sake of correct materials sequence. The sequence of electrode components is stated as reported in the original publications. When an oxygen electrode is used as reference electrode an oxygen partial pressure of 0.21 atm is assumed. 

Both thermosets and thermoplastics are used as food-contact materials, though thermoplastics predominate in this appfication. Examples of the former are phenol- and urea-formaldehyde, while probably the best known example of the latter is low-density poly(ethylene). Other linear polymers are used include high-density poly(ethylene), poly(propylene), and PVC, all of which find quite extensive use. Polymers for food packaging may be in the form of films and other flexible items, or in the form of rigid containers, such as clear drinks bottles or opaque cartons for dairy products. 

Additional regulations that provide specific requirements for color additives in foods are found in other parts of the CFR. Labeling of food products is found at 21 CFR 101.22(k). Color additives are sometimes called artificial colors or artificial colorings [21 CFR 101.22(a) (4)]. From a regulatory standpoint, a colorant is a dye or pigment used in a food contact material such as a polymer that does not migrate to food. Such materials are regulated as food additives [21 CFR 178.3297(a)], not as color additives. ... 

Boenke a (1997) Activities and current research from the EC Standards, Measurement and Testing Programme (SMT) in the area of food contact materials. Food AdditContamin 14 561-569. 

Fitness for purpose (food/water contact materials, toys, medical)... 

Physico-chemical instrumental analysis nowadays has greatly suppressed such chemical handwork. An internet website disseminates methods of analysis and supporting spectroscopic information on monomers and additives used for food contact materials (principally packaging). 

D. vanBattumandJ.B.H. van Lierop, Materials and Articles in Contact with Foodstuffs, Guide for Examination of Plastic Food Contact Materials. CEN TC 194/SC1/WG2 Document N118 (1997). 

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