Electrical creep resistance

Ultem polyetherknides have appHcations in areas where high strength, dimensional stabiUty, creep resistance, and chemical stabiUty at elevated temperatures are important. Uses include electrical coimectors, wave guides and printed ckcuit boards for electronic equipment, food appHcations (microwaveable containers, utensils, and films), akcraft interior materials, and stetilizable medical equipment. 

Solders. In spite of the wide use and development of solders for millennia, as of the mid-1990s most principal solders are lead- or tin-based alloys to which a small amount of silver, zinc, antimony, bismuth, and indium or a combination thereof are added. The principal criterion for choosing a certain solder is its melting characteristics, ie, soHdus and Hquidus temperatures and the temperature spread or pasty range between them. Other criteria are mechanical properties such as strength and creep resistance, physical properties such as electrical and thermal conductivity, and corrosion resistance. 

Phenolics. Phenol-formaldehyde (Bakelite) is one of the oldest synthetic materials available. It is a strong, hard, brittle material with good creep resistance and excellent electrical properties. Unfortunately the material is only available in dark colours and it is susceptible to attack by alkalis and oxidising agents. Typical applications are domestic electrical fittings, saucepan handles, fan blades, smoothing iron handles and pump parts. 

Polytetrafluorethylene (p.t.f.e.) This polymer does not absorb water, has no solvents and is almost completely inert to chemical attack molten alkali metals and sodium in liquid ammonia are the rare exceptions. Furthermore it does not soften below 320°C, is electrically inert and has a very low coefficient of friction. It is more expensive than general purpose plastics, requires special fabrication techniques, is degraded by high energy radiation, and has a low creep resistance. 

Good ratios for cost versus mechanical and electrical properties fair creep resistance at room temperature low moisture absorption fair heat and cold behaviours moisture and hot-water resistances low shrinkage. 

Good mechanical and electrical properties fair creep resistance fatigue behaviour fair shrinkage fair moisture uptake heat and cold behaviours with a broad range of continuous use temperatures -100°C to +150/180°C optical and UHF transparency food contact and sterilization for suitable grades. 

Good mechanical, chemical and electrical properties rigidity good creep resistance fatigue behaviour fair moisture uptake fair shrinkage heat behaviour with continuous use temperature up to 250°C high-energy radiation behaviour. 

Good thermo-mechanical, chemical and electrical properties rigidity gamma irradiation resistance UHF transparency good creep resistance and fatigue behaviour low moisture uptake low shrinkage heat behaviour fire resistance low coefficient of thermal expansion. 

To improve the properties of the raw polymer (wear resistance, creep resistance, thermal and electrical conductivity), various fillers, such as glass fibers, powdered metals, and graphite, are combined with all three types of PTFE polymers mostly by intimate mixing. Filled fine powders are produced mostly by adding fillers into a dispersion and then coagulating the mixture. 

Hardness and a ductile-to-brittle transition temperature (DBTT) have also been noted to follow a Hall-Petch relationship (Meyers, and Chalwa, 1984). Ductility increases as the grain size decreases. Decreasing grain size tends to improve fatigue resistance but increases creep rate. Electrical resistivity increases as grain size decreases, as the mean free path for electron motion decreases. 

Talc or hydrated magnesium silicate is another mineral that is used to reinforce epoxy adhesives. It has a platelike structure that provides good stiffness and creep resistance at elevated temperatures. It also provides good electrical and chemical resistance characteristics.26 It is relatively inexpensive and disperses well in the resin. 

Arsenic is added to Pb-Sb alloy for age hardening As helps to improve resistance to bending and creep of electric power cable sheathing alloys most widely used alloy is Pb-0.15% As - 0.1% Sn - 0.1% Bi... 

Acetal translucent crystalline polymer is one of the stiffest TPs available. It provides excellent hardness and heat resistance, even in the presence of solvents and alkalies. Its low moisture sensitivity and good electrical properties permit direct competition with die-cast metal in a variety of applications. In addition, acetal has extremely high creep resistance and low permeability. Acetal is also available as a copolymer (Hoechst Celanese Corp. s Celcon) for improved processability. The homopolymer (DuPont s Delrin) has a very low coefficient of friction and its resistance to abrasion is second only to nylon 6/6. Acetals are frequently blended with fibers such as glass or fluorocarbon to enhance stiffness and friction properties. Acetal is not particularly weather-resistant, but grades are available with UV stabilizers for improved outdoor performance. Acetal, whether homopolymer or copolymer, is not used to any significant degree in forming structural foams. 

This chapter is the first in a series that will make the case that many of the important features of real materials are dictated in large measure by the presence of defects. Whether one s interest is the electronic and optical behavior of semiconductors or the creep resistance of alloys at high temperatures, it is largely the nature of the defects that populate the material that will determine both its subsequent temporal evolution and response to external stimuli of all sorts (e.g. stresses, electric fields, etc.). Eor the most part, we will not undertake an analysis of the widespread electronic implications of such defects. Rather, our primary charter will be to investigate the ways in which point, tine and wall defects impact the thermomechanical properties of materials. 

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