Compositions – Electrically conductive or emissive compositions – Free metal containing
Reexamination Certificate
1999-06-02
2001-05-01
Gupta, Yogendra (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Free metal containing
C252S521100, C252S519130, C252S06290R, C501S001000
Reexamination Certificate
active
06224790
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a conductive ceramic-metal composite body exhibiting positive temperature coefficient (PTC) behavior, which is used to protect electrical and electronic components from damage due to overcurrent conditions.
It is known that ceramic materials which exhibit PTC behavior/characteristics can be used to protect electrical and electronic components against overcurrent conditions, because the resistivity of those materials increases dramatically at specific temperatures. Traditionally, materials like barium titanate have been used in this regard, because the material exhibits an exponential increase in resistivity at its Curie point temperature. However, such materials also have relatively low conductivity at room temperature, thus rendering them unsuitable for many applications, such as consumer electronics.
In view of the drawbacks associated with barium titanate PTC products, the industry has turned to polymer PTC materials for use in electronic components where currents of several tens of milliamperes can be expected. In such polymer materials, conductive particles are dispersed in a polymer matrix to form a conductive path from one side of the matrix to the other. When an overcurrent condition occurs, the polymer matrix is heated above its phase transition temperature (e.g., 120° C. for polyethylene), at which time the volume of the polymer matrix expands and disrupts the conductive path of particles formed therethrough. As a result, the resistivity of the overall material increases substantially and thus prevents the overcurrent condition from damaging downstream electronic components. These materials are attractive in that they have high conductivity and high insulation breakdown strength at room temperature.
One drawback associated with polymer PTC devices is that the trip-point temperature of the device is dictated solely by the phase transition temperature of the polymer used as the matrix. In the case of polyethylene, the phase transition temperature of that polymer material is about 120° C. and thus the trip-point temperature of any PTC device made of polyethylene is limited to about 120° C. Consequently, it is difficult to change the trip-point temperature to account for different overcurrent conditions in different electronic devices.
Another drawback associated with polymer PTC devices is that the PTC effect occurs due to a phase transformation in the matrix material itself, and not in the conductive particles held within the matrix. Accordingly, every time the matrix goes through a phase transformation, the network of conductive particles changes. Consequently, the room temperature resistivity after a trip condition rarely matches the room temperature resistivity before the trip condition. This is undesirable, since circuit designers would like the room temperature resistivity of the PTC device to be the same after every trip condition.
Yet another drawback associated with polymer PTC devices is that, in severe overcurrent conditions, the polymer matrix material can be decomposed to elemental carbon thus leaving a permanent conductive path through the device. Such a permanent conductive path, of course, would allow the overcurrent condition to reach downstream electronic components.
There have been recent reports of ceramic-metal composite PTC devices wherein metal particles, such as bismuth, are disposed in a ceramic matrix to form a conductive path therethrough. Materials such as silica and alumina have been used as the matrix material for these composites, and it is has been demonstrated that these composites show an exponential increase in resistivity at about 280° C. However, the room temperature resistivity is on the order of 1000 &OHgr;·cm, which is much too high for use in practical applications. Acceptably low room temperature resistivities have been realized only by using semi-insulating materials for the matrix.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a conductive composite (preferably ceramic-metal composite) body that exhibits PTC behavior over a wide range of selectable temperatures, and exhibits sufficiently low room temperature resistivity so as to allow its use in the protection of high current electrical and electronic components.
To meet the above-stated object, the inventor discovered that a specific relationship between the average distance between the conductive particles dispersed in the insulating matrix and the average particle diameter of those particles must exist in order for sufficiently low room temperature resistivity to be realized. At the same time, this relationship ensures an exponential increase in resistivity at specific trip point temperatures, and the ratio between the high temperature resistivity and the room temperature resistivity can easily exceed 10, 100, or more.
In accordance with one embodiment of the present invention, the conductive composite sintered body includes a high electrical resistance matrix and 20 vol %-40 vol % electrically conductive particles dispersed in the matrix to form an electrically conducting three-dimensional network therethrough. The particles are selected from bismuth, gallium, or alloys thereof. An average distance between the particles, when viewed in an arbitrary cross-section through the sintered body, is no more than 8 times, preferably no more than 4 times, the average particle diameter of the particles. The resistivity of the sintered body is low at temperatures below the melting point of the electrically conductive material and increases substantially at or above the melting point.
Preferably, the resistivity of the sintered body is no more than 5 &OHgr;cm below the melting point of the electrically conductive material and at least 1 k&OHgr;cm at or above the melting point of the electrically conductive material.
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T. Sawaguchi, et al., “Effect of Microstructure of Bismuth Metal Particle Filled Ceramic Composite on the PTCR Property,” Proceedings of the Annual Meeting, 1998, The Ceramic Society of Japan, Mar. 3, 1998, p. 319.
T. Sawaguchi, et al., “Effect of Resistivity of the Matrix Ceramics in Bismuth Metal Particle Filled Ceramic Composite on the PTCR Property,” Proceedings of the Fall Meeting, 1997, The Ceramic Society of Japan, Oct. 2, 1997, vol. 10, p. 197.
T. Sawaguchi, et al., “The PTCR Property of the Bismuth Metal Particle Filled Ceramic Composite Sintered by Hot Pressing,” Proceedings of the Fall Meeting, 1998, The Ceramic Society of Japan, Oct. 1, 1998, vol. 11, p. 106.
Burr & Brown
Gupta Yogendra
Hamiln Derrick G
NGK Insulators Ltd.
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