Plastic and nonmetallic article shaping or treating: processes – Including step of generating heat by friction
264 66, C04B 3564
A process for preparing a positive temperature coefficient thermistor is disclosed. This thermistor is comprised of at least about 60 weight percent of undoped, crystalline, ferroelectric material, and it has a room-temperature resistivity of less than 5,000 ohm-centimeters, an undegraded resistivity of at least about 1,000,000 ohm-centimeters, a degraded resistivity of at least about 900,000 ohm-centimeters, a ratio between its undegraded resistivity and its room-temperature resistivity of at least about 1,000, and a ratio between its undegraded resistivity and its degraded resistivity not exceeding 2.0.
The process of this invention is comprised of the steps of sequentially: (1) providing a powder composition comprised of at least about 60 weight percent of undoped, crystalline, ferroelectric material; (2) fabricating the powder composition into a shaped powder compact; (3) sintering said powder compact by subjecting it to a temperature of from about 1150 to about 1450 degrees centigrade for at least about 1 hour, thereby forming a consolidated, sintered body; (4) reducing said sintered body by subjecting it to a temperature of from about 700 to about 1,400 degrees centigrade for at least about 30 minutes while maintaining said sintered body under a reducing atmosphere comprised of hydrogen; and (5) halogenating said reduced, sintered body by subjecting it to a source of halogen while maintaining it in a closed container at a temperature of from about 700 to about 1200 degrees centigrade for at least about 5 minutes.
"Instabilities in PTC Resistors", B. M. Kulwicki (Proceedings of the 6th IEEE International Symposium on Applications of Ferroelectrics, Bethlehem, Pa., Jun. 1986), pp. 656-664.
"PTC Materials Technology, 1955-1980", Bernard M. Kulwicki (Advances in Ceramics, vol. 1, Grain Boundary Phenomena in Electronic Ceramics, publ. by American Ceramic Society, Columbus, OH, 1981), pp. 138-155.
"The PTC Effect of Barium Titanate", J. Daniels, K. H. Hardtl & R. Wernicke (Philips Technical Review, vol. 38, No. 3, 1978/79), pp. 73-82.
"Oxidation Phenomena in Semiconducting BaTiO.sub.3 ", Ichiro Ueda and Seiji Ikegami (Journal of the Physical Society of Japan, vol. 20, No. 4, Apr. 1965), pp. 546-552.
"Halogen Treatment of Barium Titanate Semiconductors", G. H. Yonker (Mat. Res. Bulletin, vol. 2, Pergamon Press, Inc.), pp. 401-407.
"The Effect of Firing Atmosphere on the PTCR Effect in Barium Titanate", Christopher S. Best (Abstract of B.S., Alfred University, 1984).
Alles Aldo B.
Amarakoon Vasantha R. W.
Burdick Vernon L.
Greenwald Howard J.
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