Electrical resistors – Resistance value responsive to a condition – Current and/or voltage
Patent
1991-03-22
1993-05-25
Lateef, Marvin M.
Electrical resistors
Resistance value responsive to a condition
Current and/or voltage
338 28, H01C 710
Patent
active
052147380
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thermistor having a positive coefficient (hereinafter referred to as PTC (positive temperature coefficient) characteristic) where electric resistance remarkably increases with an increase in temperature, particularly relates to a thin-film thermistor having the PTC characteristic, and more particularly relates to a PTC thin-film thermistor obtained by utilizing a barium titanate based composition.
2. Description of the Related Art
A PTC characteristic has conventionally been known in bulk materials of barium titanate based semiconductor ceramics obtained by adding rare earth elements such as Y and La to bulk barium titanate and burning the mixture in the air at 1200.degree.-1400.degree. C. Heaters and temperature sensors have been prepared by utilizing the characteristic. The maximum resistance variation rate has been at most about 0.1 order/.degree.C. and has been very unsatisfactory. The temperature where electric resistance increases can be shifted to a low temperature side or a high temperature side by replacing a portion of the Ba sites in said ceramic materials with Sr or Pb, respectively. Thus said temperature can be arbitrarily changed in the range of from -30.degree. C. to 300.degree. C.
However, according to the information of the inventors, conventional PTC thermistors have been very small in maximum resistance variation rate and prepared by mixing and burning oxide of each constituting element such as Ti and Ba in prescribed concentration. Consequently, these thermistors inevitably have a large thickness and also result in large resistance at room temperature. These problems must be overcome by enlarging the area of electric circuit to reduce resistance.
SUMMARY OF THE INVENTION
The present inventors have found that, even in a very thin thickness, for example, a film thickness of 5 .mu.m or less, a thin-film thermistor has a satisfactory PTC characteristic and surprisingly exhibits a resistance variation in a transition region of from 1 to 10 orders of magnitude and a maximum resistance variation rate to temperature change of from 1 to 20 orders/.degree. C. which values are steep PTC characteristics far exceeding the anticipation of persons who are skilled in the art. Thus, the present invention has been completed.
One aspect of the present invention is a positive coefficient thin-film thermistor comprising a thin-film which exhibits a PTC characteristic and has a thickness of from 0.005 to 5 .mu.m and electrodes, particularly is a positive coefficient thin film thermistor having resistance variation in the transition region of from 1 to 10 orders of magnitude and a maximum resistance variation rate to temperature change of from 1 to 20 orders/.degree.C., and preferably is a positive coefficient thin-film thermistor comprising a thin film of a barium titanate based composition.
Generally, ceramic semiconductors conventionally obtained by sinter-burning of oxide powder have a considerably large size and can only form a thin film having a thickness of at least about 1 mm. Even though the thickness can be further decreased to a certain extent, the thickness becomes irregular and the resulting thermistor cannot exhibit satisfactory performance.
On the other hand, the thermistor of the present invention uses a thin film having a thickness of from 0.005 to 5 .mu.m and a PTC characteristic and thus exhibits a resistance variation in a transition region of from 1 to 10 orders of magnitude and a maximum resistance variation rate to temperature change of from 1 to 20 order/.degree.C., which PTC characteristics are far exceeding the anticipation of persons who skilled in the art.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic diagram conceptually illustrating a typical resistance temperature dependence of a PTC characteristic.
FIG. 2(a), (b) and (c) are schematic diagrams practically illustrating an example of a thin-film thermistor of the present invention.
FIG. 3 is a graph illustrating the relationship
REFERENCES:
patent: 4906968 (1990-03-01), Gersherfeld et al.
patent: 4951028 (1990-08-01), Tuller
Patent Abstracts of Japan, vol. 13, No. 110 (E-728), Mar. 16, 1989 and JPA-63281401.
Journal of Materials Science Letters, vol. 8, No. 4, Apr. 1989, pp. 411-414.
Fukuda Nobuhiro
Hyakutake Hiroyuki
Kikuzawa Masanaga
Nakajima Shigeaki
Waki Hiroshi
Lateef Marvin M.
Mitsui Toatsu Chemicals Inc.
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