Dielectric properties tapes

The exceUent adhesion to primed films of polyester combined with good dielectric properties and good surface properties makes the vinyhdene chloride copolymers very suitable as binders for iron oxide pigmented coatings for magnetic tapes (168—170). They perform very weU in audio, video, and computer tapes. 

These transformers may be PVC taped, thermoplastic (polypropylene) moulded, fibreglass taped, polyester resin cast or epoxy resin cast depending upon the system voltage and the surroundings. HT indoor transformers, for instance, are generally polyester or epoxy resin cast, and are economical with good dielectric properties. They are resistant to humid, chemically contaminated and hazardous areas. Outdoor HT transformers, how-ever. may be epoxy... 

PSA coated tapes are used for a variety of applications in the electrical industry, but they have to meet additional requirements to be accepted in this industiy The adhesive are typically expected to be non-corrosive, they need to have good dielectric properties, they need to hold out moisture, and they need to show good flame retardaney. Espeeially for eleetronic applieations, little or no outgassing is aeceptable, and the adhesive should not introduce migratory contaminants resulting from exposure to release coatings or the presence of additives. 

Of primary interest is pressure-sensitive type used for tape. High strengths for other forms are reported from —100 to 500°F limited service to 700°F. Excellent dielectric properties... 

Initial evaluation of commercially available plastic films disclosed that none simultaneously satisfied all our requirements. Dielectrically acceptable tapes were mechanically weak, and mechanically strong tapes had unacceptable dielectric properties (see Table II). The Teflons, Kaptons, and other exotic tapes had attractive properties but were set aside because of their very high costs. Attempts to reduce the 60 Hz, 4.2 K loss tangents of polysulfone and polycarbonate by altering their chemical construction were unsuccessful. Consequently, the decision was made to modify the dielectric and mechanical characteristics of the less expensive, intrinsically lower loss polyolefins. This development work is described in the following sections. 

Ferroelectric ceramics (such as barium titanate, lead zircanate titanate) Sensors and actuators, electronic memory, optical applications Tape casting, sputtering, pressing, templated grain growth Improved dielectric and piezoelectric properties... 

Tapes. A great variety of tapes find application in electrical equipment. Some tapes contain filler materials in macroscopic form such as glass fibers, mica flakes, and cloth others have finely divided filler particles or no fillers at all. In the heavily filled materials the polymeric binders are present in small fractions, and the major emphasis may be on their adhesive capabilities rather than on their properties as dielectric materials. Most of the polymers used in tapes have already been mentioned in connection with other insulation applications, for example, polyesters, aromatic polyamides, polyimides, and polypropylene. Other polymers frequently used for electrical tapes are vinyls, including poly(vinyl fluoride) these are particularly well suited as conformable tapes. Polytetrafluoroethylene (Teflon TFE) has also been fabricated into tape constructions, frequently in combination with adhesives to provide a bondable material. 

Table II. Dielectric and Tensile Properties of Dielectric Tape Candidates. 

In many cases the powder is the one ingredient over which the materials engineer or scientist has the least control, since it is usually selected to provide specific properties in the final product. In other cases, it is selected for its ability to be processed or sintered under specified conditions. In our laboratory, about 80% of the materials we are asked to process into tape-cast products are preselected by our clients. In the rare cases where we have the liberty to select the starting powder (s), we can define several powder properties to make the tape casting formulation more forgiving and easier to process. These powder characteristics will be reviewed in the sections that follow. It is often, but not always, necessary to combine different powder chemistries in order to achieve the desired strength, resistivity, dielectric constant or dielectric strength, chemical resistance, porosity, or other fired property or characteristic. 

Equipment is now available to produce the thin tapes (less than 3 pm). There is also equipment available to screen-print and laminate these very thin tapes. Multilayered capacitors with 5-pm layers that are 300 layers thick are currently being manufactured. Maher states that there is a practical limitation where the dielectric constant of the material itself reaches its peak value as a function of the sintered grain size. If the grain size is in the range of 0.8 to 1.1 pm then there is a limitation on the minimum tape thickness at about 4 to 5 pm in order to maintain the dielectric breakdown properties of the chip capacitor. Maher also contends that a minimum of about 5 grains in series is desirable for reliability. It is generally believed that the 3-pm limitation on tape thickness will be the norm in the future. 

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