Thermistor device

Abstract: A thermistor device manufactured at low cost in which atoms of Ag do not migrate substantially into the thermistor body. The device comprises a disk-like thermistor body (1) and annular first electrodes (2, 3) formed in peripheral portions of the front and back surfaces, respectively, of the thermistor body (1). The first electrodes (2, 3) are made from a conductive material not containing silver. Second electrodes (4) are formed in central portions of the front and back surfaces, respectively, of the thermistor body (1). The second electrodes (4) are in ohmic contact with the thermistor body (1), and are made from a conductive material made mostly of silver.

Description: The present invention relates to thermistor devices and, more particularly, to a positive-characteristic thermistor device used in a demagnetizing circuit incorporated in a TV receiver and also to a negative-characteristic thermistor device used in a temperature-compensating circuit or the like. A known thermistor device having a positive or negative temperature coefficient is shown in Figs. 7 and 8. The body of the thermistor is indicated by numeral 30. Electrodes 31 and 32 made from a conductive material consisting mainly of silver (Ag) are formed on the front and back surfaces, respectively, of the thermistor body 30. The electrodes 31 and 32 are in ohmic contact with the thermistor body 30. In the thermistor device of this construction, if a potential difference is developed between the electrodes 31 and 32, some Ag atoms forming the material of the electrodes 31 and 32 migrate across the surface of the thermistor body 30, thus deteriorating the insulating performance. In the worst case, the electrodes 31 and 32 are shorted together. Referring to Fig. 8, A and D refer to the outer ends of the electrodes 31 and 32, respectively. B and C refer to the left and right edges, respectively, of the outer end surface of the thermistor body 30. Because of the resistive component of the thermistor body 30, potential differences are produced between A and B, between B and C, and between C and D on the surface of the thermistor body 30. These potential differences cause migration of the Ag atoms forming the electrodes 31 and 32. Another thermistor device equipped with means for reducing or slowing this problem has been proposed, and is shown in Figs. 9 and 10. This thermistor device is similar to the known thermistor device already described in conjunction with Figs. 7 and 8 except that the surface of the thermistor body 30, excluding the portions covered by the electrodes 31 and 32, is coated with an insulating film 33 made of a resin, glass, or the like. As shown in Figs. 9 and 10, the Ag migration entails the movement of metal caused by a potential difference between A and B, between B and C, and between C and D. In addition, if there is a potential difference, the migration velocity is accelerated when the thermistor device is operated in a moist atmosphere, and the electrolytic ion such as chloric ions, sulfurate ions, or the like are absorbed onto the thermistor surface on operating. Coating the thermistor body with resin or glass will prevent water and the electrolytic ions from being absorbed onto the thermistor surface, thus maintaining the migration at a low velocity. However, it is costly to fabricate this thermistor device shown in Figs. 9 and 10, because it is cumbersome to coat the outer surface of the thermistor body 30 with the insulating film 33 made of a resin or glass. EP 0 749 132 A1 which is a document pursuant to Article 54 (3) and (4) EPC discloses a positive temperature coefficient thermistor in which silver does not migrate, even in hot and humid environments. The thermistor comprises first electrodes constituted of silver layers that are formed on both principle sides of the thermistor body such that the edges thereof recede inward from the outer rims of the thermistor body. Second electrodes containing aluminium are formed in such a way to cover the exposed surfaces of the first electrodes. It is the object of the present invention to provide an improved thermistor device which is economical to fabricate and is free, or is substantially free, from migration of silver atoms. This object is achieved by a thermistor device according to claim 1. In accordance with the invention a thermistor device comprises a thermistor body, first electrodes formed in peripheral edge portions of the front and back surfaces, respectively, of the thermistor body, and second electrodes formed at least in central portions of the front and back surfaces, respectively, of the thermistor body. The first electrodes are made from a conductive material not containing silver (Ag). The second electrodes are made from a conductive material principally including silver (Ag). In this construction, the outer surface of the thermistor body is not required to be coated with an insulating film. Even if a potential difference is produced between the second electrodes formed on the front and back surfaces, respectively, of the thermistor body, the first electrodes made from the conductive material not containing Ag prevents migration of Ag atoms from the second electrodes for reasons explained below. The invention will now be described by way of exemplary embodiments illustrated in the accompanying drawings in which: Fig. 1 is a perspective view of a thermistor device according to the present invention; Fig. 2 and 2' are cross-sectional views of the thermistor device shown in Fig. 1; Fig. 3 is a perspective view of another thermistor device according to the invention; Fig. 4 and 4' are cross-sectional views of the thermistor device shown in Fig. 3; Fig. 5 is a perspective view of a further thermistor device according to the invention; Fig. 6 is a cross-sectional view of the thermistor device shown in Fig. 5; Fig. 7 is a perspective view of a conventional thermistor device; Fig. 8 is a cross-sectional view of the conventional thermistor device shown in Fig. 7; Fig. 9 is a perspective view of a known thermistor device; and Fig. 10 is a cross-sectional view of the known thermistor device shown in Fig. 9. Referring to Figs. 1 and 2, there is shown a thermistor device according to the present invention. This thermistor device comprises a disk-like thermistor body 1. First annular electrodes 2 and 3 are formed at peripheral portions of the front and back surfaces, respectively, of the thermistor body 1. The first electrodes 2 and 3 are made from a conductive material not containing silver (Ag), such as a metallic paste including mainly nickel (Ni). The first electrodes 2 and 3 may be made up of other materials containing aluminum, indium, gallium, chromium, zinc, or copper, and alloys thereof. This metallic paste is applied to the front and back surfaces of the thermistor body 1 by screen printing or other methods. Where the thermistor device has a positive temperature coefficient, a ceramic material such as BaTiO 3 is used as the material of the thermistor body 1. Where the thermistor device has a negative temperature coefficient, a ceramic material such as Mn 2 O 3 or Co 2 O 3 is employed as the material of the thermistor body 1. Second electrodes 4 and 5 are formed in central portions of the front and back surfaces, respectively, of the thermistor body 1. The second electrodes 4 and 5 are in ohmic contact with the thermistor body 1. The outer ends of the second electrodes 4 and 5 are in contact with the inner ends of the first electrodes 2 and 3, respectively. It is not always necessary that the first electrodes 2 and 3 be in ohmic contact with the thermistor body 1. However, where a material making ohmic contact with the thermistor body 1 is used as the material of the first electrodes 2 and 3, variations in the resistance values of different thermistor devices are reduced with desirable results. The reason variations in the resistance values of manufactured thermistors are reduced if the first electrodes 2 and 3 are made of material making ohmic contact with the thermistor body 1 is as follows: As shown in Fig. 2', the second electrodes 4' and 5' have shifted relative to the first annular electrodes 2' and 3'. In this case, if the first electrodes 2' and 3' are not in ohmic contact with the thermistor body, the resistance value is increased because the average current path becomes longer compared to the case where the second electrodes 4 and 5 are formed centered in the first annular electrodes 1 and 2 as shown in Fig. 2. Such shifts in the registration of the two sets of electrodes can happen anytime as a result of the manufacturing process. Where the thermistor device has a positive temperature coefficient, a conductive material consisting principally of Ag, such as Ag, Ag-Zn, Ag-In, Ag-Ga, Ag-Zn, or Ag-Sb, is used as the material of the second electrodes 4 and 5. Paste of this material is applied to the front and back surfaces of the thermistor body 1 by screen printing or another suitable method. Where the thermistor device has a negative temperature coefficient, a conductive material consisting mainly of Ag, such as Ag or Ag-Pd, is used of the second electrodes 4 and 5. Paste of this material is applied to the front and back surfaces of the thermistor body 1 by screen printing or another suitable method. The thermistor body 1 constructed as described above is baked at a temperature of about 900°C for 30 minutes in a nitrogen atmosphere. The outer surface of the resulting thermistor body 1 is not required to be coated with an insulating film and this cumbersome operation can be dispensed with. Hence, this thermistor device can be manufactured at a lower cost than the prior art. Since the first electrodes 2 and 3 are made from a conductive material not containing silver (Ag), if a potential difference is produced between the second electrodes 4 and 5, the atoms of the silver forming the second electrodes 4 and 5 do not migrate, for the following reasons. Referring to Fig. 2, A and D represent the outer ends of the second electrodes 4 and 5, respectively and B and C represent the left and right edges, respectively, of the outer end surfaces of the thermistor body 1. A potential difference due to the resistive component of the thermistor body 1 is produced only between the edges B and C on the surface of the thermistor body 1. No potential difference is created between A and B or between C and D because of the uniform potential caused by the first electrodes 2 and 3. Therefore, the Ag atoms in the second electrodes 4 and 5 are prevented from migrating by the first electrodes 2 and 3 which surround the second electrodes 4 and 5. As a consequence, the reliability of the insulating performance of the thermistor device is enhanced. Referring next to Figs. 3 and 4, there is shown a further thermistor device according to the invention. This thermistor device has a disk-like thermistor body 11. Annular first electrodes 12 and 13 are formed in peripheral portions of the front and back surfaces, respectively, of the disk-like thermistor body 11. Second electrodes 14 and 15 are formed in central portions of the front and back surfaces, respectively, of the thermistor body 11. The second electrodes 14 and 15 are in ohmic contact with the thermistor body 11. Outer portions of the second electrodes 14 and 15 overlap inner portions of the first electrodes 12 and 13, respectively. The thermistor device constructed in this way yields the same advantages as the thermistor device described already in connection with Figs. 1 and 2. For example, variations in the resistance values of manufactured thermistors are reduced when the first electrodes 12 and 13 make ohmic contact with the thermistor body 11 for the following reasons. As shown in Fig. 4', the first electrode 13' has been shifted relative to the center portion of the thermistor's circular surface. In this case, if the first electrodes 12' and 13' are not in ohmic contact with the thermistor body, a variation in the resistance value is caused because the areas of the ohmic contact which function as electrodes differ from thermistor body to thermistor body. Referring next to Figs. 5 and 6, there is shown a yet other thermistor device according to the invention. This thermistor device comprises a disk-like thermistor body 21. Annular first electrodes 22 and 23 are formed in peripheral portions of the front and back surfaces, respectively, of the thermistor body 21. Second electrodes 24 and 25 are formed in central portions of the front and back surfaces, respectively, of the thermistor body 21. The second electrodes 24 and 25 are in ohmic contact with the thermistor body 21. A gap is created between the outer end of the second electrode 24 and the inner end of the first electrode 22. Similarly, a gap is formed between the outer end of the second electrode 25 and the inner end of the first electrode 23. In the thermistor device constructed as described above, if a potential difference is developed between the second electrodes 24 and 25, atoms of Ag forming the second electrodes 24 and 25 do not migrate for the following reason. Referring to Fig. 6, current paths are represented by arrows 26, A and D represent the outer ends of the second electrodes 24 and 25, respectively, and E and F represent the inner ends of the first electrodes 22 and 23, respectively, and B and C represent the left and right edges, respectively, of the outer end surfaces of the thermistor body 21. A potential difference attributed to the resistive component of the thermistor body 21 is produced between the ends A and E, between the edges B and C, and between the ends F and D on the surface of the thermistor body 21. However, no potential difference is created between B and E or between C and F because of the presence of the first electrodes 22 and 23. Even if the atoms of Ag in the second electrodes 24 and 25 move between A and E or between F and D, the first electrodes 22 and 23 prevent further migration of these Ag atoms. Hence, a thermistor device having highly reliable insulation is obtained. It is to be understood that the invention is not limited to the illustrated examples and that various changes and modifications are possible within the scope of the invention delineated by the accompanying claims. As can be understood from the description given thus far, according to the invention, first and second electrodes are formed on the front and back surfaces, respectively, of a thermistor body. The conventional cumbersome operation of coating the outer surface of the thermistor body with an insulating film can be omitted. As a result, the manufacturing cost can be reduced. Furthermore, the first electrodes made from a conductive material not containing Ag are formed in peripheral portions of the front and back surfaces, respectively, of the thermistor body. The second electrodes made from a conductive material consisting mainly of Ag are formed at least in central portions of the front and back surfaces, respectively, of the thermistor body. Therefore, even if a potential difference is produced between the second electrodes, the first electrodes prevent the atoms of Ag in the second electrodes from migrating. Consequently, a thermistor device exhibiting highly reliable insulation is derived.

Claims: A thermistor device comprising: a thermistor body (1; 11; 21) having a front surface and a back surface; annular first electrodes (2, 3; 12, 13; 22, 23) made from a conductive material not containing silver and located at peripheral edge portions of the front and back surfaces, respectively, of said thermistor body (1; 11; 21); and second electrodes (4, 5; 14, 15; 24, 25) made from a conductive material including silver and located at least at central portions of the front and back surfaces, respectively, of said thermistor body (1; 11; 21), said annular electrodes surrounding said second electrodes. The thermistor device of claim 1, wherein said first electrodes (2, 3; 12, 13; 22, 23) are made from a material containing at least one of the materials selected from a group consisting of nickel, aluminum, indium, gallium, chromium, zinc, copper, and alloys thereof. The thermistor device of claim 1 or 2, wherein said second electrodes (4, 5; 14, 15; 24, 25) are in ohmic contact with said thermistor body (1; 11; 21). The thermistor device of claim 1 or 2, wherein said second electrodes (4, 5; 24, 25) do not overlap said first electrodes (2, 3; 22, 23). The thermistor device of claim 1 or 2, wherein said second electrodes (14, 15) overlap said first electrodes (12, 13). The thermistor device of claim 1 or 2, wherein gaps are located between said first electrodes (22, 23) and said second electrodes (24, 25), respectively.

Inventors: INOUE HIDEHIRO, INOUE, HIDEHIRO, Inoue, Hidehiro, c/o Murata Man. Co., Ltd,
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1996
Main classification: H01C1
IPC codes: H01C1, H01C7