JP2005093574A - Multilayer positive characteristic thermistor and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve withstand voltage in a multilayer positive characteristic thermistor by hardly causing breakdown at the center of ceramic raw material when testing withstand voltage. <P>SOLUTION: Gradient distribution is given to gap rate so that a gap rate of a thermistor layer 5 provided at the center in the laminating direction of a plurality of thermistor layers 3 to 7 of effective layers in the ceramic raw material 11 becomes higher than that of the thermistor layers 3, 7 provided at the external side in the laminating direction. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a laminated positive temperature coefficient thermistor and a method for manufacturing the same, and more particularly to an improvement for improving withstand voltage performance.

  In recent years, in the field of electronic equipment, miniaturization and high integration have progressed, and accordingly, for example, positive characteristic thermistors are also required to be miniaturized and surface-mounted. A multilayer positive temperature coefficient thermistor has been proposed as one that can advantageously satisfy such a requirement.

A multilayer positive characteristic thermistor is generally formed by laminating a plurality of thermistor layers made of, for example, a semiconductor ceramic mainly composed of BaTiO ₃ and a plurality of internal electrodes formed along a predetermined interface between the thermistor layers. It has a ceramic body. For the internal electrode, a metal such as nickel is used so that an ohmic contact is obtained with the thermistor layer.

  The ceramic body described above is obtained by integrally sintering the thermistor layer and the internal electrode. In the firing step for sintering the ceramic body, oxidation of nickel or the like contained in the internal electrode is performed. In order to avoid this, a reducing atmosphere is applied. Therefore, in order to sufficiently develop the thermistor characteristics in the thermistor layer, the sintered ceramic body is subjected to a re-oxidation treatment after the firing step under conditions such that nickel or the like is not oxidized.

  However, this reoxidation treatment tends to be uneven. Therefore, if the reoxidation condition is too weak, the oxidation does not proceed to the inside of the ceramic body, while if the reoxidation condition is too strong, the internal electrode is also oxidized and its ohmic property is impaired.

Therefore, in order to solve the above-mentioned problem, the ceramic body, in particular, the thermistor layer has a structure having voids, and by setting the porosity to 3 to 15% by volume, the reoxidation conditions are controlled well, and as a result, It has been proposed to prevent uneven reoxidation (see, for example, Patent Document 1).
JP-A-6-302403

  However, when a withstand voltage test is performed on a stacked positive temperature coefficient thermistor, the thermistor element is destroyed, but this destruction is likely to occur at the center of the thermistor element, and thus the destruction at the center. Withstand voltage performance is low for those in which. This is considered to be because the volume resistivity and resistance change rate of the central part of the ceramic body are lower than those of the outer peripheral part.

  As described above, it can be presumed that the lower volume resistivity and resistance change rate in the central part than in the outer peripheral part are caused by unevenness in the reoxidation treatment. That is, since the reoxidation process for the ceramic body is less likely to proceed at the center of the ceramic body than at the outer periphery of the ceramic body, the volume resistivity and resistance change rate at the center are higher than those at the outer periphery. It will be lower. As a result, thermal runaway occurs at a lower temperature in the central portion of the ceramic body, so that the entire ceramic body can only withstand a lower voltage.

  Even in a laminated positive temperature coefficient thermistor including a ceramic body with a selected porosity as described in Patent Document 1, unevenness in the reoxidation process may not be completely prevented. This is presumed from the fact that even when a withstand voltage test is performed on a laminated positive temperature coefficient thermistor having a void ratio as described in Patent Document 1, the center portion of the thermistor body is easily broken. be able to.

  Accordingly, an object of the present invention is to provide a laminated positive temperature coefficient thermistor that can further increase the volume resistivity and the resistance change rate at the center of the ceramic body, thereby improving the withstand voltage performance. It is to be.

  The present invention relates to a ceramic body formed by laminating a plurality of thermistor layers having a positive resistance temperature coefficient and a plurality of internal electrodes formed along a predetermined interface between the thermistor layers, and a predetermined one of the internal electrodes. In order to solve the above technical problem, the present invention is directed to a multilayer positive temperature coefficient thermistor having an external electrode formed on the outer surface of a ceramic body so as to be electrically connected. It is characterized by having a simple structure.

  That is, the laminated positive temperature coefficient thermistor according to the present invention is a thermistor at the center in the laminating direction among a plurality of thermistor layers which are effective layers between the two internal electrodes located on the outermost sides in the laminating direction. It is characterized in that the porosity of the layer is higher than the porosity of the thermistor layer on the outside in the stacking direction.

  In this invention, the porosity of the plurality of thermistor layers that are effective layers is such that the porosity of the thermistor layer in the center in the stacking direction is higher than the porosity of the thermistor layer on the outside in the stacking direction. It is preferable that it is inclined.

  Further, in the present invention, in a plurality of thermistor layers serving as effective layers, the maximum porosity of the thermistor layer at the center in the stacking direction is 1.5 times the minimum porosity of the thermistor layer outside in the stacking direction. The above is preferable.

  Moreover, in this invention, it is preferable that the porosity of a ceramic element | base_body is 5-40 volume% as a whole.

  The present invention also provides a ceramic body formed by laminating a plurality of thermistor layers having a positive resistance temperature coefficient and a plurality of internal electrodes formed along a predetermined interface between the thermistor layers, and a predetermined internal electrode The present invention is also directed to a method of manufacturing a laminated positive temperature coefficient thermistor comprising external electrodes formed on the outer surface of a ceramic body so as to be electrically connected to the object. The manufacturing method of the laminated positive temperature coefficient thermistor according to the present invention is characterized in that the following steps are performed.

  That is, a plurality of types of ceramic green sheets having different organic material content ratios are prepared, and each step of forming a conductive paste film for internal electrodes on each ceramic green sheet is performed.

  In addition, a plurality of types of ceramics are provided so that a relatively high organic material content is positioned in the center in the stacking direction and a relatively low organic material content is positioned outside in the stacking direction. The process of laminating green sheets and thereby obtaining a green laminate is carried out.

  In addition, the raw laminate is fired in a reducing atmosphere, whereby a step of obtaining a ceramic body in which the thermistor layer is provided by a ceramic green sheet and the internal electrodes are provided by a conductive paste film is performed. Is done.

  Further, a step of reoxidizing the fired ceramic body is performed.

  In addition, a step of forming an external electrode electrically connected to a predetermined one of the internal electrodes is performed on the outer surface of the ceramic body.

  According to this invention, since the porosity of the thermistor layer at the center in the stacking direction is higher than the porosity of the thermistor layer at the outside in the stacking direction, reoxidation at the center proceeds during the reoxidation process. It becomes easy to increase the volume resistivity and the resistance change rate in the central portion more reliably. As a result, thermal runaway at the center of the ceramic body provided in the multilayer positive characteristic thermistor can be prevented, and as a result, the withstand voltage performance of the multilayer positive characteristic thermistor can be enhanced.

  In this invention, when the porosity of the plurality of thermistor layers serving as the effective layers is increased from the outside toward the center, the difference in porosity between adjacent thermistor layers is further reduced. This can facilitate re-oxidation. Moreover, although there is a possibility of causing delamination (cracking) at the interface between the thermistor layers due to the difference in porosity, this can be prevented.

  In this invention, when the maximum porosity of the thermistor layer at the center in the stacking direction is 1.5 times or more than the minimum porosity of the thermistor layer outside in the stacking direction, the reoxidation treatment is performed. In this case, the re-oxidation at the central portion can be more surely promoted, and the withstand voltage performance can be improved more reliably.

  In this invention, when the porosity of the ceramic body is 5 to 40% by volume as a whole, the reoxidation in the reoxidation process can be promoted more smoothly, and the thermistor characteristics are more fully expressed. And the strength of the thermistor body can be maintained high. That is, when the overall porosity is less than 5% by volume, reoxidation in the reoxidation treatment process is difficult to proceed, and the thermistor characteristics cannot be sufficiently exhibited. The re-oxidation at the central portion also does not proceed smoothly, resulting in a large difference in volume resistivity between the central portion and the outer peripheral portion, and breakage due to thermal runaway tends to occur. On the other hand, when the porosity as a whole exceeds 40% by volume, the strength of the ceramic body is lowered, which is not preferable.

  Further, according to the method for manufacturing a laminated positive temperature coefficient thermistor according to the present invention, a raw laminate is produced using a plurality of types of ceramic green sheets having different organic material content ratios, and is contained in the ceramic green sheet. Since the desired porosity is obtained by varying the ratio of the organic material, a laminated positive temperature coefficient thermistor whose porosity is controlled as described above can be easily manufactured.

  FIG. 1 is a sectional view showing a laminated positive temperature coefficient thermistor 1 according to an embodiment of the present invention.

The laminated positive temperature coefficient thermistor 1 is formed along a predetermined interface between a plurality of thermistor layers 2 to 8 and the thermistor layers 2 to 8 made of, for example, a semiconductor ceramic mainly composed of BaTiO ₃ and having a positive resistance temperature coefficient. The ceramic body 11 is formed by laminating a plurality of internal electrodes 9 and 10.

  Internal electrodes 9 and 10 are formed using a conductive paste containing, as a conductive component, powder made of a base metal such as nickel. The internal electrodes 9 and 10 are formed so as to reach the outer surface of the ceramic body 11, but are extended to the internal electrode 9 and the other end face 13 that are drawn to one end face 12 of the ceramic body 1. The internal electrodes 10 are alternately arranged inside the ceramic body 11.

  External electrodes 14 and 15 are formed on end surfaces 12 and 13 which are parts of the outer surface of the ceramic body 11, respectively. The external electrodes 14 and 15 are electrically connected to the respective edges of the internal electrodes 9 and 10 on the end faces 12 and 13 of the ceramic body 11, respectively. The external electrodes 14 and 15 include, for example, a conductive paste containing, as a conductive component, a powder made of a noble metal such as silver, palladium, a silver-palladium alloy or platinum, or a base metal such as nickel or copper. It is formed by applying and baking this.

  The external electrodes 14 and 15 are only required to have an ohmic connection with the internal electrodes 9 and 10, and therefore, the formation method is not limited to the above-described conductive paste baking method. Such a dry plating method may be used.

  If necessary, first plating films 16 and 17 made of nickel, for example, are formed on the external electrodes 14 and 15, respectively. Further, a second plating film 18 made of tin, solder, or the like is further formed thereon. And 19 are formed respectively.

  Although not shown in FIG. 1, a glass layer is preferably formed on the outer surface of the ceramic body 11 where the external electrodes 14 and 15 are not formed.

  In the laminated positive temperature coefficient thermistor 1 having the above-described structure, among the plurality of thermistor layers 3 to 7 which are effective layers between the two internal electrodes 9 and 10 located on the outermost sides in the laminating direction, The porosity of the thermistor layer at the center in the direction, for example, the thermistor layer 5 is higher than the porosity of the thermistor layers at the outside in the stacking direction, for example, the thermistor layers 3 and 7.

  In this case, the porosity of the plurality of thermistor layers 3 to 7 serving as the effective layers is such that the porosity of the central portion in the stacking direction is higher than the porosity of the outer one in the stacking direction. It is preferable that it is inclined. That is, with respect to the porosity of the plurality of thermistor layers 3 to 7 serving as the effective layer, there is an inclination that the porosity becomes higher as it goes from the outer thermistor layers 3 and 7 to the central thermistor layer 5. preferable.

  In FIG. 1, five thermistor layers 3 to 7 serving as effective layers are illustrated, but the number of thermistor layers serving as effective layers can be increased or decreased as necessary.

  Further, in the thermistor layers 3 to 7 as effective layers, the thermistor layer 5 at the center in the stacking direction has the maximum porosity, and the thermistor layers 3 and 7 on the outside in the stacking direction have the minimum porosity. When it has, it is preferable that the said maximum porosity is 1.5 times or more of the said minimum porosity.

  The ceramic body 11 preferably has a porosity of 5 to 40% by volume as a whole.

  Such a laminated positive temperature coefficient thermistor 1 is preferably manufactured as follows.

  First, as the ceramic green sheets to be the thermistor layers 2 to 8, a plurality of types having different organic material content ratios are prepared. The content ratio of the organic material governs the porosity of each of the thermistor layers 2 to 8 after firing. The higher the content ratio, the higher the porosity. As the organic material, for example, polystyrene is used, or a resin such as a methacrylic resin different from the binder may be used. In addition, when using the particle | grains which consist of resin as an organic material, a thing with a particle size comparable as a ceramic powder is preferable.

  Next, a conductive paste film for the internal electrodes 9 and 10 is formed on a predetermined ceramic green sheet.

  Next, a plurality of kinds of organic materials are positioned so that a relatively high organic material content ratio is located at the center in the stacking direction, and a relatively low organic material content ratio is positioned outside the stacking direction. Ceramic green sheets are laminated, thereby obtaining a raw laminate.

  Next, the raw laminate is fired in a reducing atmosphere. As a result, a ceramic body 11 is obtained in which the thermistor layers 2 to 8 are provided by the ceramic green sheets and the internal electrodes 9 and 10 are provided by the conductive paste film.

  Next, for example, barrel polishing is performed on the fired ceramic body 11, thereby rounding the corners of the ceramic body 11 and the corners of the ridge line as shown in FIG. 1.

  Next, the ceramic body 11 is reoxidized.

  Next, external electrodes 14 and 15 are formed on the end faces 12 and 13 of the ceramic body 11, respectively. When the external electrodes 14 and 15 are formed by baking a conductive paste, this baking process may also serve as the reoxidation process.

  Next, if necessary, first and second plating films 16 and 17 are formed on the external electrodes 14 and 15, and further, second plating films 18 and 19 are formed. 1 is completed.

  Below, the experiment example implemented in order to confirm the effect by this invention is demonstrated. In this experimental example, the stacked positive temperature coefficient thermistor 1 shown in FIG. 1 was prepared as a sample.

First, using BaCO ₃ , TiO ₂ and Sm ₂ O ₃ powders as raw materials, (Ba _0.9998 Sm _0.0002 ) TiO ₃ was prepared. Next, pure water was added to the prepared powder, mixed and pulverized with zirconia balls for 10 hours, dried, and calcined at 1100 ° C. for 2 hours.

  Next, polystyrene particles were added to the obtained powder after calcination together with an organic binder, a dispersant and water, and mixed with zirconia balls for several hours to prepare a slurry. Here, the addition amount of the polystyrene particles is different so that the porosity in the thermistor layer after firing described later is 5% by volume, 10% by volume, 15% by volume, 20% by volume, 30% by volume and 40% by volume. Six types of slurries were prepared.

  Next, a ceramic green sheet having a thickness of 20 to 30 μm was formed using each slurry. Here, the thickness of the ceramic green sheet was varied in the range of 20 to 30 μm because the thermistor layer after firing was given by adjusting the thickness despite the difference in porosity affecting the resistance value. This is to make the resistance values all the same.

  Next, a conductive paste film containing nickel as a conductive component was printed on a predetermined ceramic green sheet by screen printing to form a conductive paste film.

  Next, a plurality of ceramic green sheets are stacked so that the conductive paste films face each other through the ceramic green sheets. Cut to obtain a raw laminate chip.

  Here, as the ceramic green sheet for each of the thermistor layers 3 to 7 serving as the effective layers, a ceramic green sheet that gives a porosity as shown in the column “Porosity distribution state” in Table 1 was used. As an example, in sample 1, as the ceramic green sheets for thermistor layer 3 (outside), thermistor layer 4, thermistor layer 5 (center), thermistor layer 6 and thermistor layer 7 (outside), 5 Ceramic green sheets giving respective porosity of volume%, 15 volume%, 30 volume%, 15 volume% and 5 volume% were used.

  In each sample, the same ceramic green sheet as the ceramic green sheets for the outer thermistor layers 3 and 7 was used as the protective ceramic green sheet.

Next, the raw laminate chip was degreased in the atmosphere at a temperature of 350 ° C. for 20 hours, and fired at a temperature of 1250 ° C. for 2 hours in a reducing atmosphere of H ₂ / N ₂ = 3%. As a result, a sintered ceramic body 11 in which the thermistor layers 2 to 8 were provided by the ceramic green sheets and the internal electrodes 9 and 10 were provided by the conductive paste film was obtained.

  As a result of the above-described firing step, the polystyrene particles contained in the ceramic green sheet were burned away, whereby voids were formed in each of the thermistor layers 2 to 8 included in the obtained ceramic body 11. The volume ratio occupied by the voids, that is, the void ratio, had a distribution as shown in the column “Porosity distribution state” in Table 1.

  Next, barrel polishing using polishing media made of Si and Al is applied to the sintered ceramic body 11 to round the corners of the ceramic body 11 and the corners of the ridge lines. Then, heat treatment for reoxidation was performed at a temperature of 600 ° C. for 1 hour.

  Next, external electrodes 14 and 15 made of silver are formed on the end faces 12 and 13 of the ceramic body 11 by a stapling method, and then first plated films 16 and 17 made of nickel are formed on the external electrodes 14 and 15. In addition, second plating films 18 and 19 made of tin were formed.

  As described above, the laminated positive temperature coefficient thermistor 1 according to each sample was obtained. The obtained laminated positive temperature coefficient thermistor 1 has a planar dimension of 2.0 mm × 1.2 mm for each of the samples 1 to 6, has a room temperature resistance of 2Ω, and has a resistance change rate equal to each other. there were.

  Next, a withstand voltage test was performed on the positive temperature coefficient thermistor 1 according to each sample. In the withstand voltage test, a laminated positive temperature coefficient thermistor as each sample is sandwiched between terminals connected in series to a DC power source, and in this state, the voltage applied from the DC power source is boosted every 20V to 2V. At the same time, step-up boosting was held for 1 minute at each voltage until the stacked positive temperature coefficient thermistor 1 according to each sample was destroyed. And the voltage just before destruction was made into the withstand voltage of the lamination type positive temperature coefficient thermistor 1 concerning each sample. Such a withstand voltage test was performed for each of the 20 samples, and the average value, the maximum value, and the minimum value were obtained. These results are shown in Table 1.

  In Table 1, Samples 1 to 4 are examples within the scope of the present invention, and Samples 5 and 6 are comparative examples outside the scope of the present invention.

  First, as can be seen by comparing Samples 1 to 4 as Examples and Samples 5 and 6 as Comparative Examples, according to Samples 1 to 4, the withstand voltage is improved as compared with Samples 5 and 6. Can do. This is presumably because the re-oxidation at the center of the ceramic body 11 was promoted, and as a result, the volume resistivity and the resistance change rate were improved.

  In particular, Samples 1 and 4 have a distribution in which the porosity is gradually inclined from the outside toward the center, so that the withstand voltage is higher than that of Samples 2 and 3 within the scope of the present invention. It can be improved further.

  Since the sample 5 as a comparative example has a uniform porosity, the volume resistivity at the center is lower than the volume resistivity at the outside, and as a result, the withstand voltage is low. . This is presumably because a thermal runaway occurred in the central part.

  Further, in the sample 6 as a comparative example, the porosity is high at the outside in the inclination direction of the porosity distribution state, so that the volume resistivity at the center portion remains low, thermal runaway occurs, and the withstand voltage Is low.

1 is a cross-sectional view showing a laminated positive temperature coefficient thermistor 1 according to an embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Stack type positive temperature coefficient thermistor 2-8 Thermistor layer 9, 10 Internal electrode 11 Ceramic body 14, 15 External electrode

Claims (5)

A ceramic body formed by laminating a plurality of thermistor layers having a positive temperature coefficient of resistance and a plurality of internal electrodes formed along a predetermined interface between the thermistor layers, and electrical connection with a predetermined one of the internal electrodes A laminated positive temperature coefficient thermistor comprising external electrodes formed on the outer surface of the ceramic body so as to be connected to the ceramic body,
Among the plurality of thermistor layers that are effective layers between the two internal electrodes located on the outermost sides in the stacking direction, the porosity of the thermistor layer at the center in the stacking direction is A laminated positive temperature coefficient thermistor characterized by being higher in porosity than the thermistor layer on the outside. Each porosity of the plurality of thermistor layers to be the effective layer is such that the porosity of the thermistor layer at the center in the stacking direction is higher than the porosity of the thermistor layer at the outside in the stacking direction. The laminated positive temperature coefficient thermistor according to claim 1, wherein the laminated positive temperature coefficient thermistor is inclined. In the plurality of thermistor layers serving as the effective layer, the maximum porosity of the thermistor layer in the center in the stacking direction is 1.5 times or more the minimum porosity of the thermistor layer outside in the stacking direction. The laminated positive temperature coefficient thermistor according to claim 1, wherein the laminated positive temperature coefficient thermistor is provided. 4. The laminated positive temperature coefficient thermistor according to claim 1, wherein the ceramic body as a whole has a porosity of 5 to 40% by volume. 5. A ceramic body formed by laminating a plurality of thermistor layers having a positive temperature coefficient of resistance and a plurality of internal electrodes formed along a predetermined interface between the thermistor layers, and electrical connection with a predetermined one of the internal electrodes A laminated positive temperature coefficient thermistor comprising external electrodes formed on the outer surface of the ceramic body so as to be connected to
Preparing a plurality of types of ceramic green sheets having different organic material content ratios;
Forming a conductive paste film for the internal electrode on each ceramic green sheet;
A plurality of types of the organic materials are positioned so that a relatively high content of the organic material is located in the center in the stacking direction and a relatively low content of the organic material is positioned outside in the stacking direction. Laminating ceramic green sheets, thereby obtaining a raw laminate,
Firing the raw laminate in a reducing atmosphere to obtain the ceramic body having a thermistor layer and internal electrodes;
A step of reoxidizing the ceramic body after firing;
Forming an external electrode electrically connected to a predetermined one of the internal electrodes on an outer surface of the ceramic element body.

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