This invention relates to an electrical resistance device and, more particularly, to a resistance device having specific properties of sharply increased electrical resistance, as temperature increases, within a relatively narrow temperature range, i.e., to a PTC (positive temperature coefficient) device.
Materials having PTC characteristics can be utilized in a control device by which heat generation is ceased when a heater reaches a high temperature, in a PTC thermistor, in a heat-sensitive sensor, or in a protection device wherein when an excessive current flows through a circuit due to short or the like the current is increased and therefore self-heating is developed by Joule heat. In such a circuit, the resistance of the PTC protection device is increased to restrict the current to a predetermined value or less, whereas when the short is released the circuit is restored. A variety of materials have been developed as the materials having PTC characteristics. For example, one type of material having PTC characteristics is a ceramic-type material comprising BaTiO.sub.3 having a monovalent or trivalent metal oxide incorporated therein, and a polymer-type material comprising a polymer such as polyethylene having an electrically conductive material such as carbon black dispersed therein.
As shown in FIG. 3, a PTC device generally comprises a material having PTC characteristics 2 consisting of a polymer having an electrically conductive material dispersed therein (a PTC composition), metallic electrode plates 3a and 3b having the PTC composition sandwiched or interposed therebetween, and lead plates 4a and 4b connected to the electrode plates 3a and 3b, respectively. Each electrode plate is connected to a separate device, apparatus, power source or the like via each lead plate.
The PTC device is obtained by first preparing a PTC composition, forming this PTC composition into a film, hot pressing metallic foil electrodes to upper and lower surfaces of the film to form a laminate, cutting this laminate into a predetermined size, and providing a lead plate on the surface of each of the electrodes by soldering, welding or the like. The joining between the PTC composition and the electrode plates is carried out by hot pressing the PTC composition to the electrode plates at a temperature close to the melting point of the PTC composition.
It is desirable that the PTC device exhibits a resistance value as low as possible at room temperature (a room temperature resistance) and a resistance value as high as possible at a high temperature (a peak resistance). The room temperature resistance is primarily dependent on the type of the PTC composition and the adhesion between the PTC composition and the surface of each electrode. In order to reduce the room temperature resistance, the amount of the electrically conductive particles packed in the PTC composition can be increased. However, in this case, the peak resistance is decreased and therefore it is impossible to obtain a high ratio of peak resistance to room temperature resistance. In order to improve the adhesion between the PTC composition and the surface of each electrode, a process for decreasing the contact resistance between the PTC composition and each electrode has been proposed (U.S. Pat. Nos. 4,238,812 and 4,426,339).
In electrically connecting the lead plates to each electrode of the PTC device by soldering, welding or the like, the whole of the PTC device is heated. Because of this heat, a portion of the PTC composition is pyrolyzed, causing damage such as gas evolution, heat deterioration or weakening of the bond between the PTC composition and the electrode plates. Because of this heat damage, the adhesion between the PTC composition and the electrodes is impaired, thus increasing the contact resistance therebetween.