This invention relates to electronics, and more particularly to thermistors, and powder compositions for making thermistors.
Thermistors are semiconductors exhibiting large variations of resistance with temperature, that is, a large temperature coefficient of resistance (TCR). When the resistance varies negatively with temperature, the thermistor is said to have a negative TCR; when the resistance varies positively with temperature, the thermistor is said to have a positive TCR. There exists a need for negative TCR thermistors and compositions for producing the same. The applications for NTC (negative temperature coefficient) thermistors are principally in temperature sensing, environmental sensing, current control and power.
There is a need in the electronics industry for both discrete (bulk) and thick-film thermistors. By "thick film" is meant films obtained by printing dispersions of powders (usually in an inert vehicle) on a substrate using techniques such as screen and stencil printing, as opposed to the so-called "thin" films deposited by evaporation or sputtering. Thick-film technology is discussed generally in Handbook of Materials and Processes for Electronics, C. A. Harper, Editor, McGraw-Hill, New York, 1970, Chapter 11.
By discrete or bulk thermistors is meant thermistors which are not deposited on a substrate, as in thick-film technology, but rather thermistors made by mixing together various powders, pressing them to the desired shape, and firing or sintering to make the body physically and electrically continuous. Usually, such sintering is not accompanied by melting of all the particles.
Pyrochlore is a mineral of varying composition generally expressed as (Na,Ca).sub.2 (Nb,Ti).sub.2 (O,F).sub.7, but which approaches the simpler formulation NaCaNb.sub.2 O.sub.6 F. The structure of the mineral, established by characteristic X-ray reflections, has a cubic unit cell with dimensions of about 10.4 Angstroms and contains eight formula units of approximate composition A.sub.2 B.sub.2 X.sub.6-7. The term pyrochlore is used interchangeably herein with the term pyrochlore-related oxide to mean oxides of the pyrochlore structure with the approximate formula A.sub.2 B.sub.2 O.sub.6-7. Certain compounds of the pyrochlore-related (cubic) crystal structure are known to be useful as resistors. See, for example, Schubert U.S. Pat. No. 3,560,410, issued Feb. 2, 1971; Hoffman U.S. Pat. No. 3,553,109, issued Jan. 5, 1971; Bouchard U.S. Pat. No. 3,583,931, issued June 8, 1971; Popowich U.S. Pat. No. 3,630,969, issued Dec. 28, 1971; Bouchard U.S. Pat. No. 3,681,262, issued Aug. 1, 1972; and Bouchard U.S. Pat. No. 3,775,347, issued Nov. 27, 1973; each of which is incorporated by reference herein.
Pyrochlores which are highly conductive or metallic-like are known; see, e.g., Bouchard U.S. Pat. No. 3,583,931. Pyrochlores which are semiconducting, i.e., of low conductivity or insulating, are known; Cd.sub.2 Nb.sub.2 O.sub.7 is disclosed by W. R. Cook and H. Jaffe, Phys. Rev. 88, 1426 (1952). Semiconducting or insulating pyrochlores are also disclosed in commonly assigned copending application Bouchard U.S. Ser. No. 387,479, filed Aug. 10, 1973, now U.S. Pat. No. 3,847,829. Solid solutions between pyrochlores having the same B site cation (in A.sub.2 B.sub.2 O.sub.7), Bi.sub.2 Ru.sub.2 O.sub.7 and Nd.sub.2 Ru.sub.2 O.sub.7, have been disclosed by Bouchard and Gillson in Mat. Res. Bull. 6, 669 (1971).
There is a need for both discrete and thick-film resistors which have NTC characteristics, which can be fired in air and yet withstand temperatures such as 750.degree.-950.degree.C. In thick-film technology, since temperatures in this range are typical firing temperature for other thick-film components (e.g., conductors, switches, etc.), there is a special need for NTC thermistor compositions fireable there. In discrete thermistor technology, thermistors fireable at lower temperatures such as 850.degree.C. require less power.