1. Field of the Invention
This invention relates to a method for producing a thermoplastic electric element by attaching electrodes on required parts of the surfaces of a thermoplastic film which has been polarized to acquire piezoelectricity, pyroelectricity, or a semipermanent external electric field.
2. Description of the Prior Art
It has been widely known that, when a thermoplastic formed substance is polarized by applying a D.C. high electric field under a polarizing temperature, the electrical properties of the formed substance are very much different from those of the original substance. The first observable variation in electrical property of the polarized substance is that the substance has surface electric charges. More specifically, the thermoplastic formed substance, just after the polarization, exhibits extremely high surface electric charges even after the removal of the electric field, thus presenting an external electric field. Although the external electric field tends to decrease when the formed substance is heated or left as it is for a considerable period, or sometimes tends to exhibit a polarity opposite to that just after the polarization, some kinds of thermoplastic substances retain surface electric charges of extremely stable nature after disappearance of unstable surface electric charges, and therefore exhibit a semi-permanent stable external electric field. Those having such stable external electric field are generally called electrets. As for the thermoplastic substances which can produce electrets, there are polytetrafluoroethylene, polystyrene, polycarbonate, tetrafluoroethylene-ethylene copolymer, polyethylene, polypropylene, a mixture of polyvinylidene fluoride and methylmethacrylate, and the like.
Another variation in electrical properites of the formed substance is the appearance of piezoelectricity or pyroelectricity. For instance, polarization of some polar thermoplastic substances such as polyvinylidene fluoride, polyvinyl fluoride, and polyvinyl chloride can frequently produce substances having extremely high piezoelectricity or pyroelectricity.
The piezoelectricity and pyroelectricity are considered to be properties caused by latent polarization internally of the polar thermoplastic substances, while the electret is considered to be caused by distinct polarization thereof. For this reason, broadly all of these properties are frequently called electret phenomena, although some people oppose such denomination because stable piezoelectricity and pyroelectricity sometimes appear after the disappearance of the surface electric charges.
The present invention is related to a method for producing electrical elements obtained by attaching electrodes to a polarized thermoplastic film which exhibits the "electret" property in the broader meaning inclusive of the above-described three properties. However, the term "electret" will be used hereinbelow in its narrow meaning to designate those elements having stable surface electric charges.
Various applications, such as the diaphragm in an electroacoustic transducer, elements for oscillation measuring purposes, piezoelectric switches, and the like have been considered for utilizing the piezoelectricity of a thermoplastic film, and in U.S. Pat. Application 506,814, now U.S. Pat. No. 3,935,485, (B. Patent Application 40566/74), there has been proposed a co-ordinate input device having a plurality of piezoelectric conversion elements provided on a sheet of piezoelectric thermoplastic film. For instance, when the input ordinate of this input device is composed of a plurality of switches, a keyboard switch can be obtained, and when a plurality of thermoplastic piezoelectric elements acting as a plurality of input co-ordinates are attached on an oscillating body, oscillations at various positions can be measured thereby indicating a distribution of the oscillation.
As for the application utilizing the pyroelectricity of a thermoplastic film, various fields relating to a detector for infrared rays, a measuring unit for temperature variation, a fire-alarm device, a pyroelectric switch, and the like are considered. An application thereof as a co-ordinate input device also has been proposed in U.S. Pat. No. 3,772,518 or British Pat. No. 1,355,783.
On the surfaces of the piezoelectric or pyroelectric thermoplastic films to be used in the co-ordinate input devices, electrodes are provided on both surfaces thereof for collecting electric charges generated on the surfaces by the piezoelectric or pyroelectric conversion, and for supplying signals related to the electric field to respective electric circuits. The electrodes to be provided on one surface of the film are formed as a plurality of separate lines or spots so that the input co-ordinates are electrically isolated from each other. On the other hand, the electrodes to be provided on the other surface of the film may be formed as a single continuous electrode to be grounded or as a separate lines or spots provided at the opposite positions to the above-mentioned signal generating electrodes. Otherwise, the electrodes on the two surfaces of the film may be formed as lines extended in different directions, and intersecting positions of these groups of lines, one being on the front surface and the other being on the rear surface, may be used as separate co-ordinate inputs. In any one of the above-described cases, electrodes on at least one surface of the piezoelectric or pyroelectric thermoplastic film, which are to be used as co-ordinate inputs, must be attached to the surface in a discontinuous manner at separate positions on the surface.
In another mode of application of the electret film utilizing the surface electric charges, the outer electric field of the electret is used as a bias voltage of a capacitor, and the variation in the capacitance of the capacitor is employed for various purposes. A capacitor type microphone utilizing this principle is already available on the market. As another application of an electret film, there are various switching elements, and a type of keyboard switches utilizing this principle has been proposed in, for instance, U.S. Pat. No. 3,668,417.
When the external electric field of an electret film is used as a bias voltage of a capacitor, the surface of the electret facing the opposing electrode of the capacitor, and which presents the external electric field, should be kept bare without having any electrode, and the other surface of the electret is provided with an electrode connected to an electric circuit. In the case of the keyboard switches, the other surface of the electret may be provided with spot-like non-continuous electrodes.
Thus, it is apparent that in the application as an electret film, only one surface of the electret film should be provided with a single electrode covering the entire surface or a plurality of separate electrodes discontinuous with each other.
An example wherein the piezoelectricity and surface electric charges of an electret also having a piezoelectric property are both utilized has been proposed in the Japanese Patent Application No. 115190/1973, and in this case also the electrode must be provided on one surface only of the electret film.
Summarizing the above description, when a polarized thermoplastic film having electret, piezoelectric, or pyroelectric property is used for producing electric elements, the electrode or electrodes are provided on one surface of the thermoplastic film, or else in the form of lines or spots of a discontinuous nature on both surfaces of the thermoplastic film.
The polarization of the thermoplastic film is realized by inserting the film between two electrodes and applying a D. C. electric field across the electrodes with the film maintained at a polarizing temperature. In this case, if other polarizing conditions remain the same, a polarizing film having a high concentration of surface electric charges, or a higher degree of piezoelectricity or pyroelectricity, can be ordinarily obtained when the intensity of the applied electric field is higher. The temperature for the polarization is preferably selected at a higher value so long as the nature of the thermoplastic film (such as crystalization and form of the crystals, particularly when the aim is to produce piezoelectricity and pyroelectricity) is not changed widely. However, since the insulating resistance of the film is lowered at higher temperatures, an optimum value of temperature should be selected in relation to the electric field applied thereto. An advantageous result is obtained when a high field, near the break-down voltage, is applied to the thermoplastic film at a predetermined polarizing temperature. At the time of the polarization, if the electrodes are not attached tightly onto the film, an air-gap might exist between the electrodes and the film, and corona discharge tends to occur in the air-gap because the break-down voltage in air is lower than that of the plastic, and pin holes are frequently created through the thin film. Furthermore, electric conductivity in the thin film is ordinarily increased in accordance with the elevation of the temperature, and when the conductivity in the thin film exceeds that of the air-gap, a voltage higher than the intensity of the applied electric field (KV/cm) is created in a concentrated manner in the air-gap, thereby accelerating the occurrence of the corona discharge in the air-gap. Once the corona discharge occurs, the electric resistance in the air gap decreases, and since the energy of the corona discharge is not so high, the occurrence of the corona discharge does not constitute any serious problem when the thickness of the film is sufficiently great. However, if the film is quite thin, and when such a thin film is polarized at a high temperature under a high voltage, the occurrence of the corona discharge sometimes poses a serious problem. In order to obviate such a difficulty, the thermoplastic film must be polarized under a relatively low voltage, and therefore a satisfactory polarization of the thin film cannot be attained. In the case where no break-down due to discharge occurs, the presence of the air gap lowers the applied voltage, and the polarization thereby obtained becomes irregular.
At the time of the production of elements, each having electrodes on both surfaces of the film with each of the electrodes covering the substantial part of the surface except peripheral portions thereof, the polarization of the thermoplastic film can be effectuated with use being made of these electrodes without any accompanying possibility of discharges due to the air gap or of producing uneven polarization due to discharges. However, when it is desired to produce an element having a single electrode on one surface thereof or a plurality of electrodes of discontinuous configurations on both surfaces of the film, it has been a common practice to provide a separate electrode or electrodes over the surface having no electrode or the surfaces having the discontinuous electrodes, and to polarize the thermoplastic film through the single separate electrode or plurality of separate electrodes. In the latter case, however, the above-described trouble due to the discharges occurs. Furthermore, it may also be considered that the film having a plurality of discontinuous electrodes be polarized by respectively connecting these electrodes with the voltage source. However, such a procedure is extremely troublesome and is not suitable for the production of such electric elements on an industrial scale.