Dielectric Materials
A perfect dielectric is a material in which there are no free electrical charges, and which therefore does not conduct electricity. Real dielectrics, also known as insulators, are never perfectly insulating, with the result that there has been a 150-year search for ever better dielectric materials.
The first practical need for dielectric materials arose out of the discovery of electricity, which in turn led to the invention of the light bulb, telegraph, telephone, and other electricity-oriented innovations. Electric line wires in service must be kept as dry as possible in order to function efficiently, and to cut down on loss of current. As electric technology developed, insulators were needed for telephone lines, electric power lines, and other applications. Also, as railroads began crossing the continent, there came the need for signal devices. Electricity had to be moved economically from one place to another to meet the increasing demands generated by these new inventions.
An early challenge was the possibility of running electrical cables under the sea. The only materials for insulating and waterproofing wire in the mid-1800s, however, were such materials as oakite and asphaltum. These materials did not last long for even short, shallow ocean crossings. The discovery of a close relative of rubber called gutta-percha (an inelastic latex) in 1843, which English physicist and chemist Michael Faraday (1791-1867) determined to be an electrical insulator and that proved reliable in salt-water, led to electricians and telegraphers developing a suitable means of extending telegraph lines undersea, the first line being placed between England and France in 1850. A screw extruder was developed to coat the copper conductor, thus laying the foundations for today's plastics extrusion industry.
In 1848, the first U.S. factory to insulate wire with gutta-percha started operation. In Germany, Ernst Werner von Siemens (1816-1892) developed a gutta-percha insulating machine in 1847, and succeeded in laying a gutta-percha cable in Kiel Harbor a year later. In 1921, the Key West-Havana telephone cable began service, using a blend of rubber and gutta-percha for cable insulation. By 1947, polyethylene had replaced rubber and gutta-percha as the preferred underwater insulator. In 1949, AT&T developed a submarine cable insulated by polyethylene and 5% butyl rubber. Modern undersea cables are comprised of steel strength members protecting multiple hair thin glass communication fibers, with the whole package surrounded by a waterproof electrical insulator such as polyethylene.
Meanwhile, across land surfaces, communication and electric line wires were kept off of the ground by being strung between poles. But something was needed to keep the wires and poles apart. This device was the glass or porcelain insulator. Glass insulators were first produced in the 1850s for use with telegraph lines. Porcelain insulators had their start when local potteries began making telegraph insulators in the 1850s and 1860's. With the advent of electric power distribution in the 1880's, larger and more reliable insulators were needed to carry the higher voltages of power lines. Glass at that time was not sufficient. Glaze-welding in which insulators were produced in two or three pieces and cemented together became popular in the late 1890s and early 1900s and was used by most major insulator manufacturers. From around 1915 on, the porcelain insulator virtually replaced glass on all electrical distribution, even at low voltages, as the superiority was demonstrated in both insulation quality and strength. Other types of insulators such as rubber, and a very dense, oily wood called lignum vitae were used extensively from the 1930s to the 1970s.
The addition of sulphur under heat to natural rubber to produce a hard, rigid, thermoplastically moldable product was patented by Hancock (1786-1865) in 1843. Termed vulcanite, ebonite, or hard rubber, the material soon came to be used as an insulating solid in the electrical industry. Rubber provided the electrical and communications industries with the effective insulation they badly needed. In the years since Edison's (1847-1931) revolutionary inventions, several different wiring styles have come and gone. The oldest wiring system, the knob and tube type, was made up of individual conductors with a cloth insulation. Following knob and tube insulation came multi-conductor cable. The earliest multi-conductor cable used cloth and varnish insulation. It was employed in the 1940s and 1950s in the U.S. Afterwards, the more modern asphalt-impregnated romex cloth was invented. Despite its stickiness, romex was as safe as its modern replacement, thermoplastic insulation wire.
For the first 10 years or so following Bakelite's introduction in 1909, it was used primarily for electrical and automobile insulators, and in heavy industrial products.
High pressure laminates of paper and phenol formaldehyde resins for electrical insulation date back to 1913.
Cables connecting to oil-filled units that are not completely sealed, such as transformers, condensers, oil coolers, and high-voltage switches, require liquid or gas insulation. Polychlorinated biphenyls (PCBs) were commonly used as liquid dielectrics in the 1930s. Colorless oily compounds, PCBs are resistant to dielectric breakdown. They are chemically stable and resistant to heat and burning, making them especially useful in industry. However, adults who have been exposed to high concentrations of PCBs have suffered debilitating stomach pain and disfiguring skin eruptions, while their children have suffered physical, behavior, and cognitive problems. Consequently PCBs have been banned by the Environmental Protection Agency for most uses. Alternatives to PCB insulators today include hydrocarbon oils, askarel, and silicone oils. Gas insulation, such as SF6, is another alternative.
This is the complete article, containing 881 words
(approx. 3 pages at 300 words per page).
