Compositions – Electrically conductive or emissive compositions
Reexamination Certificate
1999-07-23
2002-03-19
Gupta, Yogendra N. (Department: 1751)
Compositions
Electrically conductive or emissive compositions
C252S512000, C252S518100, C252S519130, C427S101000, C429S006000
Reexamination Certificate
active
06358436
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to resettable PTC devices made of inorganic-metal composite materials, and more particularly to a body of such composite material having a room temperature resistivity of less than 10 &OHgr;·cm and a high temperature resistivity of at least 100 &OHgr;·cm.
BACKGROUND OF THE INVENTION
Positive Temperature Coefficient (PTC) materials exhibit a sharp increase in resistivity over a particular temperature range. As such, these materials have been used widely as resettable fuses for protecting circuits against overcurrent conditions.
Two types of PTC materials have been proposed in the past: ceramic-based PTCs and polymer-based PTCs. Ceramic PTCs made of, for example, barium titanate, have been used in heaters and in some circuit protection applications. Ceramic PTCs have not been widely adopted for circuit protection devices, however, since the room temperature resistivity of those materials is too high for use in circuits of consumer electronic products, for example.
In view of the problems associated with ceramic PTC materials, the industry has adopted polymer-based materials. Such polymer-based PTC materials include a matrix of polymer material in which conductive particles, such as carbon black, are uniformly dispersed to form a conductive network through the material. The resistivity of the polymer PTC is controlled by varying the content of conductive particles. The range of conductive particle content within which the polymer composite material exhibits PTC behavior is known as the percolation threshold range.
FIG. 1
is an operating curve for a typical polymeric PTC device. The PTC device will generate heat as current passes therethrough. The device will operate in region
1
as long as the amount of heat generated in the device can be dissipated to the ambient environment. In an overcurrent condition, the heat generated by the device exceeds the ability of the ambient environment to absorb that heat, and, consequently, the temperature of the device increases. When the temperature of the device reaches the melting point temperature of the polymer matrix, the polymer melts, expands and disrupts the conductive network of carbon black particles formed therein. Once the conductive network is disrupted, the resistivity of the polymeric material increases sharply as shown in
FIG. 1
, to thus allow only a very small amount of current to pass therethrough. Region
3
shown in
FIG. 1
basically represents the resistivity of the polymeric composite material in the melted state. Once the overcurrent condition is terminated (e.g., by switching off the electronic device), the polymer recrystallizes and effectively reconstructs the conductive network of carbon black particles. The device then operates in region
1
of
FIG. 1
until a subsequent overcurrent condition occurs.
While polymeric PTC devices have been widely adopted in industry, there are several problems associated with these devices.
First, while the magnitude of resistivity in region
1
of a polymeric PTC device can be adjusted by changing the amount of conductive particles added to the polymer matrix, the trip point temperature (T
TP
) is dependent solely upon the melting point of the polymer. Polyethylene is the material of choice in polymeric PTC devices, and melts at about 150° C. Accordingly, all polymeric PTC devices employing polyethylene as the matrix material will trip when the device temperature reaches 150° C.
Second, the breakdown voltage of polymeric PTC devices is relatively low (e.g., less than 100 V/mm), primarily due to the relatively low breakdown voltage of polymer materials such as polyethylene.
Third, there is a time lag between the occurrence of an overcurrent condition and the tripping of the polymeric PTC device. Specifically, the “trip time” of a polymeric PTC device is on the order of 100 milliseconds. Consequently, some or all of the overcurrent could be transmitted to downstream electronic components within this time lag.
Fourth, polymeric PTC devices do not return to their initial resistivity value after tripping. Specifically, the first time a polymeric PTC device trips, and the polymer matrix melts as explained above, the initial conductive network of carbon black particles is disrupted. The carbon black particles do not assume the same network when the polymeric matrix cools to region
1
of
FIG. 1
since the structure of the polymer matrix changes slightly. Consequently, the magnitude of resistivity in region
1
essentially doubles after the polymeric PTC device is tripped for the first time. Such an increase in region
1
resistivity is unacceptable, especially in devices where the initial resistivity of the polymeric PTC device plays an important role in the design of the electronic circuit.
Fifth, polymeric PTC devices require several hours, if not several days, to reset. Specifically, once the polymeric matrix melts as a result of an overcurrent condition, it could take several hours or days for the polymeric matrix to recrystallize and again become conductive (by restoration of the conductive network of carbon particles). This is unacceptable since an electronic device in which the polymeric PTC device is disposed cannot operate until the PTC device resets.
Sixth, the heat resistance of polymeric PTC devices is unacceptably low (i.e., less than 200° C.). As explained above, the polymeric matrix, if formed of polyethylene, will melt at about 150° C. to disrupt the conductive network of carbon black particles in the device. However, in certain severe overcurrent conditions, the PTC device itself can be heated above the melting point of the polymer and perhaps even above the decomposition temperature of the polymer itself. That is, a severe overcurrent condition can cause decomposition of the polymer matrix if the current flowing through the device generates excessive Joule heating. Decomposition of a polymeric material essentially forms carbon (which is electrically conductive) and essentially renders the device permanently inoperative. Accordingly, the PTC device is no longer resettable.
Finally, certain overcurrent conditions can cause shorting around the ends of the polymeric material (known as “tracking”) and even through certain local regions of the polymeric material. These short circuit conditions create local areas of decomposition in the polymeric material, which in turn result in permanent conductive paths of carbon in the device. Such conductive paths are, of course, unacceptable, as the device will no longer exhibit a sharp increase in resistivity at the trip point temperature.
It would be desirable to develop a PTC material that does not suffer from the excessive resistivity problems of traditional ceramic PTC materials and also does not suffer from the numerous drawbacks associated with polymeric PTC materials.
While extensive research has been conducted in the area of polymeric PTC devices in an attempt to overcome some of the above problems, the industry, until recently, had not been able to provide a PTC material that overcomes all of the problems discussed above with respect to both traditional ceramic and polymeric PTC materials. There has been recent disclosure, however, of a PTC thermistor material including a ceramic matrix and conductive particles dispersed therein. Specifically, WO 98/11568 (EP0862191) discloses such a composite material device that purports to exhibit reliable PTC behavior. However, the device must make use of a semi-insulating matrix material in order to attain acceptably low room temperature resistivity. While insulating ceramic matrix materials (e.g., Al
2
O
3
) are disclosed, the room temperature resistivity of the devices employing these materials is unacceptably high (~1000 &OHgr;·cm). Moreover, the use of semi-insulating matrix materials often results in unacceptably low high temperature resistivities (above the trip point temperature of the device), and the cost of such semi-insulating materials tends to be prohibitive. Accordingly, WO '568 does not disclose a device that simul
Gupta Yogendra N.
Hamlin Derrick G.
NGK Insulators Ltd.
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