Electrical resistors – Resistance value responsive to a condition – Current and/or voltage
Reexamination Certificate
1999-01-28
2002-09-17
Enad, Elvin (Department: 2832)
Electrical resistors
Resistance value responsive to a condition
Current and/or voltage
C338S020000, C252S511000, C252S513000
Reexamination Certificate
active
06452476
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Prior Art
The present invention relates to an organic positive temperature coefficient thermistor that is used as a temperature sensor or overcurrent-protecting element, and has PTC (positive temperature coefficient of resistivity) characteristics or performance that its resistance value increases with increasing temperature.
2. Background Art
An organic positive temperature coefficient thermistor having conductive particles dispersed in a crystalline thermoplastic polymer has been well known in the art, as typically disclosed in U.S. Pat. Nos. 3,243,753 and 3,351,882. The increase in the resistance value is thought as being due to the expansion of the crystalline polymer upon melting, which in turn cleaves a current-carrying path formed by the conductive fine particles.
An organic positive temperature coefficient thermistor can be used as a self control heater, an overcurrent-protecting element, and a temperature sensor. Requirements for these are that the resistance value is sufficiently low at room temperature in a non-operating state, the rate of change between the room-temperature resistance value and the resistance value in operation is sufficiently large, and the resistance value change upon repetitive operations is reduced.
To meet such requirements, it has been proposed to use a low-molecular organic compound such as wax and employ a thermoplastic polymer matrix for a binder. Such an organic positive temperature coefficient thermistor, for instance, includes a polyisobutylene/paraffin wax/carbon black system (F. Bueche, J. Appl. Phys., 44, 532, 1973), a styrenebutadiene rubber/paraffin wax/carbon black system (F. Bueche, J. Polymer Sci., 11, 1319, 1973), and a low-density polyethylene/paraffin wax/carbon black system (K. Ohe et al., Jpn. J. Appl. Phys., 10, 99, 1971). Self control heaters, current-limiting elements, etc. comprising an organic positive temperature coefficient thermistor using a low-molecular organic compound are also disclosed in JP-B's 62-16523, 7-109786 and 7-48396, and JP-A's 62-51184, 62-51185, 62-51186, 62-51187, 1-231284, 3-132001, 9-27383 and 9-69410. In these cases, the resistance value increase is believed to be due to the melting of the low-molecular organic compound.
One of advantages to the use of the low-molecular organic compound is that there is a sharp rise in the resistance increase with increasing temperature because the low-molecular organic compound is generally higher in crystallinity than a polymer. A polymer, because of being easily put into an over-cooled state, shows a hysteresis where the temperature at which there is a resistance decrease with decreasing temperature is usually lower than the temperature at which there is a resistance increase with increasing temperature. With the low-molecular organic compound it is then possible to keep this hysteresis small. By use of low-molecular organic compounds having different melting points, it is possible to easily control the temperature (operating temperature) at which there is a resistance increase. A polymer is susceptible to a melting point change depending on a difference in molecular weight and crystallinity, and its copolymerization with a comonomer, resulting in a variation in the crystallographic state. In this case, no sufficient PTC characteristics are often obtained.
In the organic positive temperature coefficient thermistors set forth in the above publications, however, no sensible tradeoff between low initial (room temperature) resistance and a large rate of resistance change is reached. Jpn. J. Appl. Phys., 10, 99, 1971 shows an example wherein the specific resistance value (&OHgr;·cm) increases by a factor of 10
8
. However, the specific resistance value at room temperature is as high as 10
4
&OHgr;·cm, and so is impractical for an overcurrent-protecting element or temperature sensor in particular. Other publications show resistance value (&OHgr;) or specific resistance (&OHgr;cm) increases in the range between 10 times or lower and about 104 times, with the room-temperature resistance being not fully decreased.
A problem associated with using the thermoplastic polymer for the matrix is that because the matrix melts and fluidizes at the melting point of the polymer, the dispersion state of the system changes upon exposure to high temperature in particular, resulting in unstable performance.
On the other hand, JP-A's 2-156502, 2-230684, 3-132001 and 3-205777 disclose an organic positive temperature coefficient thermistor using a low-molecular organic compound and a thermosetting polymer behaving as a matrix. Since carbon black, and graphite are used as conductive particles, however, the rate of resistance change is as small as one order of magnitude or less and the room-temperature resistance is not sufficiently reduced or about 1 &OHgr;·cm as well. Thus, no compromise is made between the low initial resistance and the large rate of resistance change.
JP-A's 55-68075, 58-34901, 63-170902, 2-33881, 9-9482 and 10-4002, and U.S. Pat. No. 4,966,729 propose an organic positive temperature coefficient thermistor constructed solely of a thermosetting polymer and conductive particles without recourse to a low-molecular organic compound. In these themistors, either, no compromise is achieved between a room-temperature resistance of up to 0.1 &OHgr;·cm and a large rate of resistance change of 5 orders of magnitude greater, because carbon black, and graphite are used as the conductive particles. Generally, thermistor systems composed merely of a thermosetting polymer and conductive particles have no distinct melting point, and so many of them show a sluggish resistance rise in temperature vs. resistance performance, failing to provide satisfactory performance in overcurrent-protecting element, temperature sensor, and like applications in particular.
In many cases, carbon black, and graphite have been used as conductive particles in prior art organic positive temperature coefficient thermistors including those set forth in the above publications. A problem with carbon black is, however, that when an increased amount of carbon black is used to lower the initial resistance value, no sufficient rate of resistance change is obtainable; no reasonable tradeoff between low initial resistance and a large rate of resistance change is obtainable. Sometimes, particles of generally available metals are used as conductive particles. In this case, too, it is difficult to arrive at a sensible tradeoff between the low initial resistance and the large rate of resistance change.
One approach to solving this problem is disclosed in JP-A 5-47503 that teaches the use of conductive particles having spiky protuberances. More specifically, it is disclosed that polyvinylidene fluoride is used as a crystalline polymer and spiky nickel powders are used as conductive particles having spiky protuberances. U.S. Pat. No. 5,378,407, too, discloses a thermistor comprising filamentary nickel having spiky protuberances, and a polyolefin, olefinic copolymer or fluoropolymer. However, these thermistors are still insufficient in terms of hysteresis and so are unsuitable for applications such as temperature sensors, although the effect on the tradeoff between low initial resistance and a large resistance change is improved. This is because no low-molecular organic compound is used as a working or active substance. Another problem with these thermistors is that when they are further heated after the resistance increase upon operation, they show NTC (negative temperature coefficient of resistivity) behavior that the resistance value decreases with increasing temperature. It is to be noted that the above publications give no suggestion about the use of a low-molecular organic compound at all.
JP-A 5-198403 and 5-198404 disclose an organic positive temperature coefficient thermistor comprising a mixture of a thermosetting resin and conductive particles having spiky protuberances, and show that the rate of change resistance obtained is 9 orders of magnitude greater. How
Enad Elvin
Lee Kyung S.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
TDK Corporation
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