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VARTEK ASSOCIATES LLC Consultants in Varistor Technology and Electronic Materials Vartek Associates LLC
Dr. Lionel M. Levinson 6 Franklin Court South, Unit 433
Saint Petersburg, Florida 33711 Tel: 518-869-5192 E-mail: Lionel@LionelLevinson.com
(Varistor ceramics have been widely applied in surge protection for more than 30 years. A preprint of a forthcoming review on varistor technology can be obtained by contacting Vartek Associates LLC.)
A Brief Description of ZnO Varistor Technology Metal oxide varistors are novel ZnO-based ceramic semiconductor devices with highly nonlinear current-voltage characteristics similar to back-to-back Zener diodes. Over the last 30 years these varistor devices have become the preferred approach to protecting electronic, electrical and power distribution and transmission circuits from the destructive voltage levels induced by lightning impulses or switching surges. The varistors are produced by a ceramic sintering process that provides a structure largely comprised of conductive ZnO grains surrounded by thin electrically insulating barriers. ZnO varistors have now been in widespread use for more than 30 years. Applications of ZnO varistors are predominantly in the field of circuit overvoltage protection. Overvoltage protection is a necessity for both electronic circuits and in the electrical power distribution and transmission industries. The technology has supplanted generally less useful prior approaches to voltage surge protection such as silicon carbide arresters, gas discharge tubes and to some extent semiconductor-based surge suppressors such as Zener diodes. A significant advantage of ZnO varistors as a transient protective device derives from the ceramic nature of the material. Since the material is polycrystalline with energy absorption occurring essentially at the grain boundaries distributed throughout the volume of the material, ZnO varistors are inherently able to absorb more energy than single-junction protective devices such as Zener diodes. More than one billion ZnO varistor devices are in service. They protect circuits with voltages ranging from around 10 volts to millions of volts. The larger devices are capable of surviving transient currents greater than 100,000 amps and can absorb energies greater than 10,000 joules (energy absorption capability > 200 J/cm3 is routinely available). Consumer-purchased surge-suppressor electrical outlets based on ZnO varistor technology are commonly used to protect computers and other high value electronic equipment. In addition to the protection of electronic equipment, a major application of ZnO varistors is for the protection of electric power distribution and transmission systems. In these applications, a surge suppressor is required to function reliably on systems with voltages ranging up to a megavolt and to absorb transient energies in the megajoule range. Large volumes of varistor material are needed to meet these requirements. ZnO varistors have found acceptance as a simple, cost-effective way to protect electrical systems from transient voltage surges. By virtue of their ceramic nature, they can be fabricated into a variety of sizes and shapes, and this feature facilitates a high degree of user flexibility. Varistors are inherently multijunction grain-boundary devices and any transient surge energy to be absorbed when the device acts in a protective mode is distributed among the many ZnO grain junctions. The multijunction feature of varistor behavior is the heart of the high-current and energy-absorption capability of the material. Varistor Technology Slide PresentationYou may download a 3 megabyte slide presentation on Varistor Technology. A few slides from the presentation are also given below. A preprint of a forthcoming review on varistor technology can be obtained by contacting Vartek Associates LLC.
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|  BiographyDr. Levinson is principal and owner of Vartek Associates LLC, a consulting company active in electronic ceramics and materials. He has consulted for a variety of government and industrial institutions, including Sandia National Laboratories, General Electric Company, AVX Corporation, and TAL Materials Inc. He is a fellow of the American Ceramic Society. You may download a biography of Dr. Levinson. Patents and PublicationsDr. Levinson has written or edited 4 books, approximately 87 technical papers and articles pertaining to his fields of work, and has been awarded 63 U.S. patents. You may download a list of Dr. Levinson's patents and publications. |
