Method for producing a PTC thermistor

Abstract: Disclosed is a PTC thermistor which is coated with a glass material except for the area where a silver electrode is formed. The PTC thermistor is placed in a case in a manner to be nipped by a pair of terminals, and the encased PTC thermistor is employed in a resistor. Further, a combination of the PTC thermistor with radiating fins can be used as a radiator.

Description: The present invention relates to a method of producing PTC thermistors. Figs. 20 and 21 show existing PTC thermistors. Referring to Fig. 20, electrodes 81 and 82 made of a silver compound which performs ohmic contact, such as a compound of silver with zinc, a compound of silver with tin, etc., are formed on upper and lower surfaces of a substrate 80. Referring to Fig. 21, double-layered electrodes 91 and 92 are formed on upper and lower surfaces of a substrate 90. The electrode 91 is composed of an underelectrode 91a formed on the surface of the substrate 90 and a silver electrode 91b formed thereon. The electrode 92 is composed of an underelectrode 92a and a silver electrode 92b likewise. The reasons why a silver compound is used as the electrodes 81, 82, 91b and 92b are that a lead terminal is soldered with silver easily and that silver is excellent in ohmic contact. Using a silver compound as an electrode, however, has a disadvantage that the main constituent of the electrode, silver is likely to react on chlorine ions of water in air and be ionized, in which case, silver ions migrate. More specifically, the substrates 80 and 90 are ceramic, and the surface potential of each substrate 80 or 90 is not uniform, whereby ionized silver of the electrodes 81 and 82, or 91 and 92 migrates on the surface from a higher potential area to a lower potential area. This migration of silver ions may cause a short circuit. Also, since the substrates 80 and 90, and the electrodes 81, 82, 91 and 92 are exposed, these thermistors are likely to be affected by external force and become defective easily. For example, the substrates 80 and 90 may get cracks and chips during the production or the transport. Also, the electrode 81, 82, 91 or 92 may peel off the substrate 80 or 90, which causes a spark at the time of impressing a voltage. Thus, from an electrical point of view, such thermistors are not reliable. Further, since the sides of the substrates 80 and 90, which are porous, are exposed, harmful gasses such as a chlorine gas, a halogen gas, etc. are likely to penetrate into the substrates 80 and 90 through the sides, which deteriorates the substrates 80 and 90. Such a deteriorate substrate impairs the performance of the PTC thermistor. In thermistors as shown in Figs. 20 and 21, generated heat leaks therefrom, and when such a thermistor is employed in a magnetic erasure circuit for a television, a picture received by the television shakes. When such a thermistor is employed in a radiator for a fan forced heater, a water-proof heater or the like, the themistor interferes with an improvement of the radiator in temperature-rise characteristic. Further, a PTC thermistor is conventionally housed in an insulating case having holes through which terminals are protruded, and there is fear that substances which deteriorate the PTC thermistor, such as flux, chlorine, etc., may penetrate into the case through the holes. Furthermore, the PTC thermistor may be displaced from a set position and may contact with the inner wall of the insulating case. In this case, a gas is generated from the insulating resin of the case with the generation of heat by the PTC thermistor, and this gas may deteriorate the PTC thermistor. EP-A-0129997 discloses a process for producing a glass sealed type PTC thermistor. A semi-conductor barium titanate ceramic material 1 is first sliced to a suitable thickness having regard to the length of a glass tube in which the finished thermistor element is to be sealed. Then, two silver electrodes are applied to both main surfaces of the thus obtained element. Subsequently, the element is placed in the tube consisting of a glass having a softening point below 560°C into both ends of which wires for contacting the element are inserted. Then, the tube is sealed by means of a heater. The present invention is based on the object of providing a method for producing a PTC thermistor having reliable electrical properties. This object is achieved by the method in accordance with claim 1. [Brief Description of Drawings] These and other objects and features of the present invention will become apparent from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, in which: Figs. 1 through 4 show a first embodiment of the present invention; Fig. 1 is a vertical sectional view of a substrate of a PTC thermistor; Figs. 2, 3 and 4 are vertical sectional views of the PTC thermistor showing a process of forming a glass coating and silver electrodes; Figs. 5 through 10 show a second embodiment of the present invention; Fig. 5 is a vertical sectional view of a substrate of a PTC thermistor; Figs. 6 and 7 are vertical sectional views of the PTC thermistor showing a process of forming underelectrodes; Figs. 8, 9 and 10 are vertical sectional views of the PTC thermistor showing a process of forming a glass coating and silver electrodes; Fig. 11 is a vertical sectional view of a PTC thermistor according to a third embodiment of the present invention; Fig. 12 through 15 show a fourth embodiment of the present invention; Fig. 12 is a vertical sectional view of a resistor; Fig. 13 is a plan view of a common terminal; Fig. 14 is a plan view of a PTC thermistor and a spring terminal showing the positional relationship; Fig. 15 is a diagram of a magnetic erasure circuit in which the resistor is employed; Fig. 16 through 19 show a fifth embodiment of the present invention; Fig. 16 is a vertical sectional view of a fan forced heater; Fig. 17 is a perspective view of a sample device used for measuring the temperature-rise characteristic of a PTC thermistor; Fig. 18 is a diagram of an electric circuit used for measuring the temperature-rise characteristic; Fig. 19 is a graph showing the result of the measure; and Figs. 20 and 21 vertical sectional views of existing PTC thermistors. [Best Mode for Carrying out the Invention] The following describes preferred embodiments of the present invention referring to the drawings. [First Embodiment: Figs. 1 - 4] Fig. 1 is a vertical sectional view of a substrate 1 of a PTC thermistor. The substrate 1 is rectangular and is made of ceramics such as BaTiO₃. First, as shown in Fig. 2, the substrate 1 is entirely coated with a bismuth glass material 3 by barrel processing or spraying. Silver paste is spread on the glass coating 3, and silver layers 4 and 5, whose thickness is 2 - 6µm, are fixed thereon by printing (see Fig. 3). Next, the substrate 1 is subjected to sintering in the temperature of 500°C for 30 minutes. In this processing, the glass material 3 around the silver layers 4 and 5 fuses therewith. Thus, the silver layers 4 and 5 change into electrodes 4a and 5a (see Fig. 4). The electrodes 4a and 5a whose main constituent is silver firmly adhere to the substrate 1. The rest of the glass material 3 forms a glass coating 3a for protecting the sides and corners of the substrate 1. The glass coating 3a is insulating and firmly adheres to the substrate 1. Then, the PTC thermistor is finished with soldering lead terminals with the electrodes 4a and 5a. [Second Embodiment: Figs. 5 - 10] Figs. 5 through 10 show a second embodiment of the present invention, and in the drawings, the same components as used in the first embodiment are numbered the same. Fig. 5 is a vertical sectional view of a substrate 1 of a PTC thermistor. The substrate 1 is rectangular and is made of ceramics such as BaTiO₃. First, the substrate 1 is entirely coated with nickel by electroless plating (see Fig. 6). In view of cost and reliability, preferably the nickel coating 2 is about 1 µm thick. The unnecessary part of the nickel coating 2, in this embodiment the nickel on the sides of the substrate 1, is removed by chemical etching, rubbing or blasting. The nickel reminding on the upper and lower surfaces of the substrate 1 becomes underelectrodes 2a and 2b (see Fig 7). Next, as shown in Fig. 8, the substrate 1 with the underelectrodes 2a and 2b is entirely coated with a bismuth glass material 3 by barrel processing or spraying. Silver paste is spread on the parts of the glass coating 3 under which the underelectrodes 2a and 2b are formed, and silver layers 4 and 5 whose thickness is 2 - 6 µm are fixed thereon by printing. Then, the substrate 1 is subjected to sintering in the temperature of 500°C for 30 minutes. In this processing, the glass material 3 around the silver layers 4 and 5 fuses therewith, and thus the silver layers 4 and 5 change into electrodes 4a and 5a (see Fig. 10). The electrodes 4a and 5a whose main constituent is silver firmly adhere to the underelectrodes 2a and 2b. The rest of the glass material 3 forms a bismuth glass coating 3a for protecting the sides and corners of the substrate 1. The glass coating 3a is insulating and firmly adheres to the substrate 1. Then, the PTC thermistor is finished with soldering lead terminals with the electrodes 4a and 5a. With the structure according to the first or the second embodiment, even if silver which is a constituent of the electrodes 4a and 5a reacts on chlorine ions of water in air and is ionized, the insulating coating 3a formed between the electrodes 4a and 5a prevents migration of the silver ions. Also, since the glass coating 3a and the substrate 1 are firmly joined together, the silver ions cannot migrate on the interface between the glass coating 3a and the substrate 1. Since the glass coating 3a covers the substrate 1 except for the parts where the electrodes 4a and 5a are formed, the substrate 1 does not receive external force directly, which eliminates such fear that the substrate 1 may get cracks and chips and that the underelectrodes 2a and 2b may peel off. The glass coating 3a also protects the substrate 1 from harmful gasses in outside air. That is, there is no fear that the characteristic of the PTC thermistor may change due to harmful gasses. The electrodes 4a and 5a whose main constituent is silver functions as contactors with terminals. In the first and the second embodiments, the electrodes 4a and 5a firmly adhere to the substrate 1 or the undercoated electrodes 2a and 2b, and therefore the PTC thermistors according to these embodiments are reliable in respect to electrical connection. [Third Embodiment: Fig. 11] Fig. 11 shows a PTC thermistor according to a third embodiment of the present invention. The PTC thermistor has two electrodes 13 and 14 whose main constituent is silver, the electrodes 13 and 14 paralleling to each other on the upper surface of a substrate 7. Two underelectrodes 7a and 7b are disposed on the upper surface of the substrate 7 in a manner to parallel to each other, and the electrodes 13 and 14 are formed thereon. A bismuth glass material 12 is coated on the substrate 7 except for the parts where the electrodes 13 and 14 are formed. Even if silver which is a main constituent of the electrodes 13 and 14 is ionized, the insulating glass coating 12 prevents migration of the silver ions. The PTC thermistor according to the third embodiment may be modified so as not to have the underelectrodes 7a and 7b. The modified PTC thermistor brings the same effect as the PTC thermistor having the underelectrodes 7a and 7b. [Fourth Embodiment: Figs. 12 - 15] A fourth embodiment of the present invention is a double-type resistor 8 comprising two PTC thermistors. The resistor 8 is employed in a magnetic erasure circuit for a television as shown in Fig. 15. The resistor 8 has an insulating case 9 in which a pair of spring terminals 15 (see Fig. 14) and a common terminal 16 (see Fig. 13) are housed. A magnetic erasing PTC thermistor 20 is placed between one of the spring terminals 15 and the common terminal 16 in a manner to be stuck between tabs 15a of the spring terminal 15 and a heat exchanging part 16a of the common terminal 16. A heating PTC thermistor 25 is placed between the other spring terminal 15 and the common terminal 16 in the same manner as the magnetic erasing PTC thermistor 20. The thermistors 20 and 25 are of the type shown in Fig. 10. The insulating case 9 is made of phenol resin or the like. Leads 15b and 16b of the respective terminals 15 and 16 are protruded from the case 9 through holes 10 made in the bottom thereof. The spring terminal 15, as an example, is produced by punching a stainless plate whose thickness is 0.25mm and coating the surface with tin. The common terminal 16, as an example, is produced by punching a stainless plate whose thickness is 0.10mm and coating the surface with tin. The PTC thermistors 20 and 25 have the glass coating 3a on the surfaces. A chlorine gas or other harmful gasses may penetrate into the case 9 through the holes 10, but even in this case, the PTC thermistors 20 and 25 are protected from the gasses by the glass coating 3a. Also, if the PTC thermistor 20 or 25 is displaced from the set position and contacts with the inner wall of the case 9, the case 9 is less heated by the thermistor 20 or 25 than by an existing thermistor without a glass coating. Even if a harmful gas is generated from the case 9 by heat of the PTC thermistors 20 and 25, the PTC thermistors 20 and 25 are protected from the gas by the glass coating 3a. Further, even if the PTC thermistor 20 or 25 is displaced from the set position in a manner that one of the tabs 15a keeps contacting with the electrode 4a and that the other tab 15a goes away from the electrode 4a and comes in contact with another part of the thermistor 20 or 25, a spark will not occur because the glass coating 3a prevents the tab 15a from contacting with the nickel electrode 2a. The resistor 8 with the structure is employed in a magnetic erasure circuit as shown in Fig. 15. In the magnetic erasure circuit, when a switch S1 is turned on, an alternating current source 28 provides a magnetic erasing coil 29 with an electric current, thereby causing a magnetic flux. Simultaneously the magnetic erasing PTC thermistor 20 and the heating PTC thermistor 25 are electrified. Then, as the thermistor 20 heats up, the resistance of the thermistor 20 increases. Thereby the current flowing from the thermistor 20 to the magnetic erasing coil 29 decreases, and the magnetic erasing coil 29 performs magnetic erasure. The heating PTC thermistor 25 heats up likewise when it is electrified. Since the thermistors 20 and 25 are connected by the heat exchanging part 16a of the common terminal 16, the thermistor 20 is further heated by the heat sent from the thermistor 25. Thereby, the current flowing to the magnetic erasing coil 29 further decreases. We conducted an experiment to examine the effect of the glass coating on PTC thermistors. A bare PTC thermistor whose configuration is a disk of 14mm in diameter and 2mm in thickness, whose Curie point is 50°C, and whose resistance under a room temperature (25°C) is 5.0Ω was used as the magnetic erasing thermistor 20, and a bare PTC thermistor whose configuration is a disk of 12mm in diameter and 2mm in thickness, whose Curie point is 143°C and whose resistance under the room temperature (25°C) is 180Ω was used as the heating PTC thermistor 25. In the conditions, an electric current flowing to the magnetic erasing coil 29 60 seconds after a turning-on of the switch S1 was measured to be 1.5mA. Then, the same thermistors were coated with a glass material, and the coated thermistors were used as the magnetic erasing PTC thermistor 20 and the heating PTC thermistor 25. In the conditions, an electric current flowing to the magnetic erasing coil 29 60 seconds after a turning-on of the switch S1 was measured to be 1.0mA. As is evident from the experiment, using glass-coated PTC thermistors brings a decrease of an electric current flowing to the magnetic erasing coil 29. Therefore when a resistor comprising such glass-coated PTC thermistors is employed in a magnetic erasure circuit for a television, the television can be improved in picture shaking. [Fifth Embodiment: Figs. 16 - 19] A fifth embodiment of the present invention is a fan forced heater in which a PTC thermistor is employed. As shown in Fig. 16, a fan forced heater 40 has a metal case 41 in which a PTC thermistor 30 and metal terminals 31 are housed. The PTC thermistor 30 is of the type shown in Fig. 10. The metal case 41 acts as a heat radiator, and it consists of a container 42 and a lid 47. The container 42 is divided by walls 42a, and the upper part of each wall 42a is formed into a bracket 42b. The lid 47 has engaging members 47a on right and left sides, and the engaging members 47a enclose the respective spring brackets 42b. The container 42 contains the PTC thermistor 30, the metal terminals 31, resin spacers 32 and insulating sheets 33 in a manner that the PTC thermistor 30 is arranged between the metal terminals 31 and that the insulating sheets 33 insulate the metal terminals 31 from the case 41. A spring 34 pressed and shaped like C is disposed in each space between the engaging members 47a and the spring brackets 42b. The container 42, the lid 47, the PTC thermistor 30, the metal terminals 31 and the insulating sheets 33 are kept in position by the force of the springs 34. Heat generated by the PTC thermistor 30 is transmitted to the case 41 via the metal terminals 31 and the insulating sheets 33. Several radiating fins 45 are disposed around the case 41 at uniform intervals so that the heat of the case 41 can be radiated effectively. The PTC thermistor 30 employed in the heater 40 is coated with the glass material 3a except for the parts where the electrodes are formed, and the glass coating 3a protects the PTC thermistor 30 from harmful gasses such as chlorine gas which may penetrate into the case 40. We conducted an experiment to figure out the temperature-rise characteristic of the PTC thermistor 30. In the experiment, as shown in Fig. 17, a test thermistor was disposed on one side of an aluminum plate 51, which acts as a radiator, at the center. A PTC thermistor whose configuration is a disk of 8.0mm in diameter and 3.0mm in thickness, whose Curie point is 216°C, whose resistance under a room temperature (25°C) is 200 - 500Ω was used as the test thermistor in the experiment. The length L of the aluminum plate 51 was 40mm, and its width W was 30mm. Fig. 18 illustrates a circuit used in the experiment to measure the temperature-rise characteristic of the PTC thermistor 30. In the circuit, when a switch S2 is turned on, an electric current flows from an alternating electric power source 52 (100 volts) to the PTC thermistor 30, and the thermistor 30 heats up. The aluminum plate 51 heats up with the increase of the thermistor 30 in temperature. Temperatures of the PTC thermistor 30 are represented by temperatures at a point P on the side of the aluminum plate 51 reverse to the side on which the PTC thermistor 30 is disposed. Temperatures of the point P were measured by a thermocouple thermometer 54 connected with a pen recorder 53. For comparison, we also figured out the temperature-rise characteristic of a PTC thermistor which is not coated with a glass material in the same manner. Fig. 19 is a graph showing the result of the experiment recorded by the pen recorder 53. The solid line indicates the temperature-rise characteristic of the glass-coated PTC thermistor. The dashed line indicates the temperature-rise characteristic of the bare PTC thermistor. Table 1 shows the result of the experiment. As is evident from the experiment, the glass coating 3a on the PTC thermistor 30 prevents heat generated by the PTC thermistor 30 from leaking out, thereby increasing the thermal efficiency on the electrodes 4a and 5a and increasing the calorific power of the PTC thermistor 30. [Other Embodiments] Although the present invention has been described in connection with the preferred embodiments above, it is to be noted that various changes and modifications are apparent to those who are skilled in the art. Such changes and modifications are to be understood as included within the appended claims. For example, the substrate 1 can be shaped into a disk, a cylinder, a ball and others. The silver layers 4 and 5 can be made of other silver compounds such as a compound of silver with aluminum, gallium or the like. Although a bismuth glass material is used as the glass coating in the embodiments above, any kind of insulating glass can be selected as the material. Although the fourth embodiment describes a double-type resistor, the present invention can be applied to a single-type resistor which has a single PTC thermistor, too. Employing at least one glass-coated PTC thermistor in a double-type resistor brings the same effects of the present invention. Also, employing a PTC thermistor which is of the type shown in Fig. 4 in a resistor brings the same effects as employing a PTC thermistor which is of the type shown in Fig. 10. The fifth embodiment describes a fan forced heater, but the present invention can be applied to a VTR heater, a heater for a hair iron, a water-proof heater, etc. A PTC thermistor which is of the type shown in Fig. 4 can be employed in such a heater. [Industrial Applicability] A PTC thermistor according to the present invention is coated with a glass material except for the area where an silver electrode is formed. The glass coating brings such advantages that migration of silver ions is prevented and that the PTC thermistor is excellent in mechanical strength and in adaptability for circumstances. Accordingly, the PTC thermistor can be employed in a heating circuit, an overcurrent protection circuit, a magnetic erasure circuit for a television or the like, and it can be also used as a radiator for a fan forced heater, a water-proof heater, etc.

Claims: A method for producing a PTC thermistor, said method comprising the steps of: coating a PTC thermistor ceramic substrate (1) with a glass material (3); forming two silver layers (4,5) on the coated substrate (1); and fusing the silver layers (4,5) with a part of the glass material (3) therearound by heat so that the silver layer (4,5) changes into electrodes (4a,5a) whose main constituent is silver, the electrodes (4a,5a) adhering to the substrate (1), wherein the rest of the glass material (3a) covers the entire surface of the substrate (1) except for the area where the silver electrodes (4a,5a) are formed. A method according to claim 1, said method comprising the step of: applying underelectrodes (2a,2b) to the surface of the substrate (1), wherein the two silver layers (4,5) are respectively formed on the parts of the glass coating (3) under which the underelectrodes (2a,2b) are formed and the electrodes (4a,5a) are adhered to the underelectrodes (2a,2b) by the fusing step.

Inventors: KATSUKI TAKAYO C O MURATA MANU, SHIKAMA TAKASHI C O MURATA MAN, TADOKORO TOMOAKI C O MURATA MA, TAKAOKA YUICHI C O MURATA MANU, KATSUKI, TAKAYO, C/O MURATA MANUFACTURING CO. LTD, SHIKAMA, TAKASHI, C/O MURATA MANUFACTURING CO. LTD, TADOKORO, TOMOAKI, C/O MURATA MANUFACTURING CO. LT, TAKAOKA, YUICHI, C/O MURATA MANUFACTURING CO. LTD
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1991
Main classification: H01C7
IPC codes: H05B3, H01C1, H04N9