Electrical resistors – Resistance value responsive to a condition – Current and/or voltage
Patent
1994-06-09
1997-06-17
Hoang, Tu B.
Electrical resistors
Resistance value responsive to a condition
Current and/or voltage
338 21, 252521, 264617, H01C 710
Patent
active
056401361
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to voltage-dependent nonlinear resistors.
BACKGROUND OF THE INVENTION
In accordance with the rapid advance of semiconductor elements and semiconductor circuits such as thyristors, transistors and integrated circuits and their applications, the use of semiconductor elements and semiconductor circuits in instrumentation, control and communication equipment and power equipment is now widespread, and these equipment make rapid progress toward miniaturization and higher performance. On the other hand, despite such advances, these equipment and parts used therein are not regarded satisfactory in withstand voltage, surge rating and noise immunity. It is then a very important task to protect such equipment and parts from abnormal surge and noise or to establish a stable circuit voltage. For solving these problems, there is a demand for the development of a voltage-dependent nonlinear resistor material which has substantial voltage-dependent nonlinearity, great discharge withstand current rating, and improved life property and is inexpensive.
Used for such purposes are varistors containing silicon carbide (SIC), selenium (Se), silicon (Si), ZnO or the like as a major component. Among others, the varistors based on ZnO are generally characterized by a low clamping voltage and a great voltage-dependent nonlinearity index. These varistors are then suitable for protection again overvoltage of equipment composed of elements having a low overcurrent rating such as semiconductor elements and have been widely utilized as a substitute for SiC-based varistors.
By the way, such ZnO-based voltage-dependent nonlinear resistors are generally prepared, like voltage-dependent nonlinear resistors based on other materials, by firing a compact of a voltage-dependent nonlinear resistor-forming source powder containing ZnO as a major component according to a firing process including a heating step, a high temperature holding step and a cooling step. In the prior art, the entire firing process was carried out in an atmosphere having a constant oxygen partial pressure (typically ambient air), but no varistors thus obtained had a nonlinearity index .alpha. in excess of 100, with .alpha. being normally about 50.
JP-A 106102/1984 proposes a method for preparing a ZnO-based varistor wherein the oxygen partial pressure of the firing atmosphere used in the firing process is switched from below to above 2.times.10.sup.-1 atm (air's oxygen partial pressure) in a time region from a point in a later stage of the high-temperature holding step to a point immediately after transition to the cooling step, for the purpose of providing an increased .alpha. value.
DISCLOSURE OF THE INVENTION
However, the prior art ZnO-based varistors are likely to degrade in a load life test at high temperature and humidity and must be provided with glass coatings or the like. A problem also arises with respect to degradation by DC voltage application that the volt-ampere characteristic becomes asymmetric depending on the direction of voltage application. The prior art ZnO-based varistors have another problem that grain growth is accelerated and leakage current is increased particularly when they are manufactured under high-temperature firing conditions.
Further, in the prior art manufacturing technology, no research work has been made on the relationship of varistor properties except for .alpha. to the oxygen partial pressure of the firing atmosphere. When varistors were actually manufactured by the method of the above-referred JP-A 106102/1984, there occurred a surge life problem as shown by a change rate of varistor voltage approaching to -4.0% or more.
Disk varistors having a thickness in excess of about 2 mm suffer from the problem of a deteriorated surge life whichever technique is selected for firing among conventional ones. This is because in thicker varistors, grains have a smaller diameter in the interior than at the surface so that when current flow is conducted, most of the current flows solely along the surface t
REFERENCES:
patent: 4077915 (1978-03-01), Yodogawa et al.
patent: 4160748 (1979-07-01), Yodogawa et al.
patent: 4254070 (1981-03-01), Yodogawa et al.
patent: 4290041 (1981-09-01), Utsumi et al.
patent: 4319215 (1982-03-01), Yamazaki et al.
patent: 4320379 (1982-03-01), Yodogawa
patent: 4477793 (1984-10-01), Mukae et al.
patent: 5107242 (1992-04-01), Imai et al.
patent: 5225111 (1993-07-01), Imai
Furukawa Masahito
Matsuoka Dai
Naitou Hitomi
Yamazaki Toshiyuki
Yodogawa Masatada
Hoang Tu B.
TDK Corporation
LandOfFree
Voltage-dependent nonlinear resistor does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Voltage-dependent nonlinear resistor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Voltage-dependent nonlinear resistor will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2160795