JP4915153B2 - Multilayer varistor - Google Patents

Description

  The present invention relates to a laminated varistor in which a plurality of internal electrodes are arranged in a ceramic body having voltage nonlinear resistance characteristics, and more specifically, an internal structure formed using a ZnO-based semiconductor ceramic and an Ag-based electrode material. The present invention relates to a laminated varistor having an electrode.

  Conventionally, various varistors having voltage non-linear resistance characteristics have been proposed in order to protect against overvoltage and noise and static electricity. In addition, as electronic devices are miniaturized and the rated voltage of a circuit is lowered, varistors are strongly required to be small and to be driven at a lower voltage. Therefore, a multilayer varistor using a ceramic body in which a plurality of internal electrodes are laminated via a semiconductor ceramic layer has been widely used.

  However, in the multilayer varistor, when the size is reduced and the driving voltage is lowered, the thickness of the ceramic layer between the internal electrodes has to be reduced. That is, since the varistor voltage depends on the number of grain boundaries between the electrodes, in order to reduce the varistor voltage, the number of grain boundaries between the internal electrodes had to be considerably reduced to about ten or less.

  Therefore, when an ultra-high voltage pulse such as ESD is applied to the laminated varistor, there is a problem that the stress per grain boundary becomes large and the characteristics are likely to deteriorate. Further, when the number of grain boundaries is reduced, it becomes easy to be affected by the variation in the particle size, so it is difficult to reduce the variation in the characteristics of the laminated varistor. Furthermore, when downsizing is promoted, there is a problem that it is easily affected by the atmosphere at the time of degreasing and firing at the time of manufacture, and the characteristics vary due to fluctuations in the firing conditions at the time of manufacture.

On the other hand, in Patent Document 1 below, zinc oxide is the main component, and as a subcomponent, praseodymium is 0.05 to 3.0 atomic percent of the whole, cobalt is 0.5 to 10 atomic percent of the whole, potassium, sodium. , And at least one of lithium in a total amount of 0.005 to 0.5 atomic percent of the total, and at least one of aluminum, gallium, and indium in a total amount of 2 × 10 ^{−5 to} 0.5 atomic percent of the total A multilayer varistor using a porcelain composition for varistors containing zirconium in a total range of 0.005 to 5.0 atomic% is disclosed. Compared with ZnO-based semiconductor ceramic materials containing Bi-based subcomponents, ZnO-based semiconductor ceramic materials containing Pr-based subcomponents contain Bi ₂ O ₃ that forms a liquid phase or volatilizes at a low temperature. It has not been. Therefore, in the multilayer varistor described in Patent Document 1, the characteristics can be stabilized. Moreover, since it has the above-mentioned specific composition, it is said that in the multilayer varistor, leakage current can be suppressed and surge resistance and ESD resistance can be increased.

  In Patent Document 1, as an internal electrode of a laminated varistor, an internal electrode made of Pt is shown in the embodiment.

On the other hand, the following Patent Document 2 discloses a multilayer varistor using a ZnO-based semiconductor ceramic composition containing Pr as an accessory component. That is, an internal electrode made of an Ag—Pd alloy is formed in a sintered body made of ceramics containing ZnO as a main component and containing 0.005 to 5 mol% of Pr oxide in terms of Pr. A laminated varistor is disclosed. Here, by using the ZnO-based semiconductor ceramic material having the composition containing the specific Pr as a subcomponent, the deterioration of the electrical characteristics of the multilayer varistor is avoided, and the cost of the metal material used for the internal electrode is reduced. It is supposed to be possible.
JP 2004-140334 A Japanese Patent Laid-Open No. 5-283209

  In the multilayer varistor described in Patent Document 1, the internal electrode is made of Pt, so the cost has to be high. Further, Pt has a high melting point of about 1700 ° C., and even if the internal electrode material made of Pt and the ceramic material are integrally fired, Pt hardly diffuses into the ceramic layer. For this reason, no consideration was given to the diffusion of the internal electrode material into the ceramic layer during firing.

  On the other hand, Patent Document 2 discloses a multilayer varistor in which an internal electrode made of an Ag—Pd alloy is provided in a ceramic body using a ZnO-based semiconductor ceramic material containing Pr as an auxiliary component. Since the internal electrode is made of an Ag—Pd alloy, the cost is reduced.

  However, when the ZnO-based semiconductor ceramic having the composition described in Patent Document 2 was used and an internal electrode made of an Ag—Pd alloy was formed as the internal electrode, the ESD resistance was not sufficiently increased.

  That is, in Patent Document 2, by using the specific composition, even when an internal electrode made of an Ag—Pd alloy is employed, the ESD resistance cannot be sufficiently increased.

  Further, it has been found that, for example, when the voltage of 65% of the varistor voltage is applied at a high temperature of 125 ° C. and left for 1000 hours, the rate of change with time is high, and sufficient reliability cannot be obtained.

  An object of the present invention is to solve the above-described disadvantages of the prior art, and to have a ceramic body composed of a ZnO-based semiconductor ceramic material containing Pr as an auxiliary component, and further, Ag and Pd whose internal electrodes are inexpensive. Despite being made of the material included as the main component, it not only can reduce variation in characteristics and cost, but also suppresses deterioration of electrical characteristics such as ESD resistance and has excellent reliability at high temperature load It is to provide a laminated varistor.

According to the present invention, a ceramic body formed by laminating a plurality of ceramic layers made of a semiconductor ceramic material and a plurality of internal electrodes formed along a predetermined interface between the ceramic layers; A multilayer varistor including external electrodes formed so as to be connected to each other, wherein the ceramic layer has ZnO as a main component, Pr as a subcomponent, 0.1 to 5.0 atomic% of the whole, Co 1.0 to 5.0 atomic percent, at least one of Al, Ga, and In in a total amount of 0.002 to 0.2 atomic percent, and at least one of Ca, Sr, and Ba in a total amount of 0.0. 005 to 0.5 atomic%, wherein the Zr 0.01 to 0.2 atomic%, and the internal electrodes 10 atomic% to 60 atomic% Ag content of Ag and Pd in unrealized said internal electrodes and characterized in that Laminated varistor is provided.

  In the multilayer varistor of the present invention, the ESD resistance can be effectively obtained as described above because of the following reasons.

Compared to ZnO-based semiconductor ceramics containing Bi as a subcomponent, ZnO-based semiconductor ceramics containing Pr as a subcomponent had to be fired at a higher temperature. This is because in ZnO-based semiconductor ceramics containing Bi as a subcomponent, Bi ₂ O ₃ forms a liquid phase at a temperature of 800 ° C. or higher, and promotes grain growth and sintering of ZnO. Cheap. On the other hand, in a ZnO-based semiconductor ceramic having Pr as a subcomponent, Pr ₆ O ₁₁ inhibits ZnO grain growth up to about 1100 ° C. In addition, since a material that forms a liquid phase at a low temperature is not contained, it is generally necessary to fire at a relatively high temperature of 1000 ° C. or higher, preferably 1150 ° C. to 1250 ° C.

  The inventor of the present application has found that the decrease in ESD resistance of ZnO-based semiconductor ceramics containing Pr as a subcomponent and the decrease in reliability at high temperature load are the internal electrodes constituting the internal electrode by firing at a high temperature as described above. -It was thought that Ag of Pd electrode material might be concerned with spreading | diffusion to a ceramic layer.

  Therefore, as a result of studying to solve such problems, the inventor of the present application, as described above, in the ZnO-based semiconductor ceramics containing Pr as an accessory component, the specific ceramic composition, that is, Pr is 0% of the entire ceramic layer. 0.1-5.0 atomic%, Co: 1.0-5.0 atomic% of the entire ceramic layer, and at least one of Al, Ga and In in total amount of 0.002-0.2 atomic% of the entire ceramic layer %, Ca, Sr, and Ba in a total amount of 0.005 to 0.5 atomic% of the entire ceramic layer and Zr of 0.01 to 0.2 atomic% of the entire ceramic layer. The present inventors have found that it is possible to effectively suppress a decrease in ESD resistance due to diffusion of Ag and to improve the reliability at high temperature load, thereby achieving the present invention.

  By having the above-described configuration, it is possible to effectively suppress the diffusion of Ag, suppress a decrease in ESD resistance, and prevent a decrease in reliability at a high temperature load. This is presumably because two diffusions of Ag, namely, the grain boundary diffusion in which Ag diffuses through the ZnO grain boundary and the intragranular diffusion that diffuses while dissolving in the ZnO grain can be suppressed.

More specifically, the present inventor considered that it is necessary to suppress both diffusion of Ag in ZnO grain boundaries and diffusion in ZnO grains in order to suppress a decrease in reliability by diffusion of Ag. Conventionally, ZnO-based semiconductor ceramics containing Pr as an auxiliary component required a higher firing temperature than Bi-based materials in order to obtain a dense microstructure. When it is desired to lower the firing temperature, a method is known in which a large amount of an alkaline earth metal such as Ca that forms a liquid phase at a low temperature is added. However, when such a substance was added in a large amount, it was thought that although the firing temperature could be lowered, the diffusion of Ag was promoted by the presence of the liquid phase. Further, among Co and Al dissolved in ZnO, in particular, if Co is added in a large amount of materials added as a subsidiary component and dissolved in ZnO, solid solution of Ag into ZnO particles is allowed. I thought the capacity would be lower. Therefore, in order to suppress the above-mentioned grain boundary diffusion, the content ratio of the liquid phase forming substance is reduced as much as possible, and Co which is easily dissolved in the ZnO grains is set within the predetermined range of the present invention, whereby the diffusion of Ag is reduced. We think that we were able to suppress effectively.

  That is, at least one of Ca, Sr, and Ba as an alkaline earth metal in a total amount of 0.005 to 0.5 atomic%, and Co is 1.0 atomic% to 5.0 atomic%. In addition, by making the additive material and addition amount of the subcomponents as described above, the diffusion of Ag is suppressed, the deterioration of the ESD resistance is suppressed, and the reliability in the high temperature load test is as described above. We have found that a laminated varistor with excellent properties can be obtained.

  In addition, when Co exceeds 5.0 atomic% of the whole, as will be apparent from Examples to be described later, ESD resistance is reduced, and defective products are generated in a high-temperature load test. Therefore, the Co content is in the range of 1.0 to 5.0 atomic% of the whole.

  In addition, since Ag diffused in the vicinity of the internal electrode acts as an acceptor, it is desirable to add Al, Ga, and / or In as a donor in advance in order to suppress the diffusion of Ag. Then, at least one of Al, Ga and In is added in a total amount of 0.002 to 0.2 atomic%.

  In addition, when these addition amounts are less than 0.002 atomic%, the effect of adding at least one of Al, Ga and In is not sufficient. As described above, the excessive increase in the donor concentration lowers the grain boundary barrier and causes deterioration in the high temperature load test or ESD test.

  As described above, in the present invention, the proportion of Co dissolved in ZnO particles is set to 1.0 to 5.0 atomic%, and alkaline earth metals such as Ca ejected to the ZnO grain boundary are added to 0.005 to 0. By setting the content to 0.5 atomic%, it is considered that diffusion of Ag into the ZnO particles or ZnO grain boundaries is suppressed. Further, Ag is easily diffused in the vicinity of the internal electrode, but by adding a predetermined amount of Al, Ga and / or In as a donor, the influence of Ag acting as an acceptor is suppressed, and the reliability is further enhanced. It is thought that.

In addition, in Patent Document 2, sufficient reliability was not obtained because Pr ₂ O ₃ was added without adding Bi ₂ O ₃ which easily reacts with Pd in order to use an Ag—Pd electrode, and the firing temperature. However, it is considered that a sufficiently dense sintered body was hardly obtained. Further, Co is not included in the range of 1.0 to 5.0 atomic% and alkaline earth metal in the range of 0.005 to 0.5 atomic%, and Al, Ga, In and the like as a donor element are not added. It is considered that the intragranular resistance is high and the grain boundary barrier is not made uniform, so that electrical stress is concentrated locally, causing the above-described decrease in reliability.

  On the other hand, the present invention was thought to promote the diffusion of Ag due to the high firing temperature. However, by using a predetermined amount of Co and alkaline earth metal as in the present invention, for example, the firing temperature It is a new finding that even in a high state, diffusion of Ag can be suppressed, deterioration of electrical characteristics such as ESD resistance can be suppressed, and excellent reliability can be obtained at high temperature load. is there.

  In addition, in the present invention, since the internal electrode is mainly composed of Ag and Pd, not only can the cost be reduced, but addition of Ag to Pd suppresses oxidation and reduction of Pd, and the characteristics. Variations can be reduced. Unlike Pt, Pd tends to cause an oxidation-reduction reaction during firing. By this oxidation-reduction reaction, oxygen contributing to the formation of the grain boundary barrier is deprived, the insulation resistance and the voltage non-linearity are lowered, and characteristic variation occurs. Therefore, firing is generally performed in an oxygen atmosphere, but it is difficult to make the atmosphere in the entire firing furnace uniform due to the influence of the convection of the firing furnace. It tends to be large and requires careful atmosphere control. This occurs more remarkably when the miniaturization and the low varistor voltage are promoted.

  In the present invention, the internal electrode containing not only Pd but also Ag can shift the temperature at which Pd is oxidized / reduced to a lower temperature side, so that such Pd oxidation / reduction is suppressed, thereby insulating the Pd. It is possible to suppress a decrease in resistance and voltage nonlinearity and variation in characteristics. In addition, since there is no need for oxygen atmosphere firing, problems such as characteristic variations when producing in large quantities are reduced, and stable production becomes possible.

  By having the internal electrodes as described above, the multilayer varistor according to the present invention can suppress and improve the electrical characteristics such as ESD resistance. Further, it is possible to reduce the variation in characteristics and the cost of the electrode material.

  In a specific aspect of the multilayer varistor according to the present invention, the total amount of Zr contained in the ceramic layer includes 0.01 to 0.08 atomic% of the whole.

  In still another specific aspect of the multilayer varistor according to the present invention, the Co content (atomic%) contained in the ceramic layer is x, and the Ag content (atomic%) contained in the internal electrode is y. Then, 0.05 ≦ x / y ≦ 0.5.

  In the multilayer varistor according to the present invention, a ceramic body formed by laminating a plurality of ceramic layers made of a semiconductor ceramic material and a plurality of internal electrodes formed along a predetermined interface between the ceramic layers, and the internal electrodes In the multilayer varistor including the external electrode formed so as to be electrically connected to the ceramic body, since the ceramic body has the specific composition, when the internal electrode containing Ag and Pd as main components is formed, Not only can the cost of the internal electrode material be reduced, but it is also possible to suppress the diffusion of Ag during firing, thereby effectively increasing the ESD resistance.

  In other words, according to the present invention, since the ceramic body is formed using ZnO-based semiconductor ceramics containing Pr as a subcomponent, the internal electrode mainly composed of Ag and Pd is employed with little variation in characteristics. In addition to the cost being reduced, the ZnO-based semiconductor ceramic material has ZnO as a main component and, as a subcomponent, Pr is 0.1 to 5.0 atomic% of the whole and Co is the whole. 1.0-5.0 atomic% of the total, at least one of Al, Ga, and In in total amount 0.002-0.2 atomic% of the total, and at least one of Ca, Sr, and Ba in total amount Since it has a composition containing 0.005 to 0.5 atomic% of the whole and 0.01 to 0.2 atomic% of the whole of Zr, ESD resistance can be effectively increased. Therefore, it is possible to provide a laminated varistor having excellent reliability, low cost, and excellent characteristics.

  In particular, in the ceramic body, when Zr is contained in the range of 0.01 to 0.08 atomic% of the whole, it is preferable because ESD resistance is improved, and the reliability of the laminated varistor is effectively improved. It becomes possible to raise.

Further, since the proportion of Ag in the internal electrodes is in the range of 10 to 60 atomic%, it is possible to improve the insulation resistance and voltage nonlinearity, is possible to further reduce the variation in the varistor voltage V ₁ mA it can. Therefore, it is possible to provide a laminated varistor that is excellent in various electrical characteristics.

Further, when the Co content (atomic%) contained in the ceramic layer is x and the Ag content (atomic%) contained in the internal electrode is y, a range of 0.05 ≦ x / y ≦ 0.5. Is preferred. As the Ag ratio increases, a part of Ag is significantly scattered from the internal electrode lead-out portion to the outside of the ceramic body. At this time, in order to deprive the ceramics of oxygen, a new problem arises that the grain boundary barrier is lowered and the insulation resistance is lowered. This problem appears more prominently when the voltage is further lowered, for example, when the varistor voltage is 9 V or less. On the other hand, as the Ag ratio of the internal electrode is increased, if more Co is included in the ceramic layer, the decrease in insulation resistance is suppressed, and even when the internal electrode having a high Ag ratio is used, the insulation resistance and voltage ratio are reduced. It becomes possible to provide a highly reliable multilayer varistor having excellent linearity and small variations in V ₁ mA.

  Hereinafter, the present invention will be clarified by describing specific examples of the present invention.

(Experimental example 1)
As starting materials, ZnO, Pr ₆ O ₁₁ , CoCO ₃ , Al ₂ O are used so that Co, Pr, Ca, Al, and Zr have the composition ratio shown in Table 1 below after firing with respect to ZnO as the main component. ₃ , each material of CaCO ₃ and ZrO ₂ , a dispersant and ion-exchanged water were weighed and wet-mixed for 24 hours using PSZ balls. In this experimental example, the additive element is added in the form of an oxide or carbonate, but may be added in another form such as a metal or a hydroxide.

  The mixed slurry obtained as described above was dehydrated, dried, and then calcined in the atmosphere at a temperature of 700 to 900 ° C. An organic solvent and a dispersant were added to the calcined raw material thus obtained, and the mixture was pulverized with PSZ balls and mixed. Thereafter, an organic solvent, an organic binder resin, and an organic plasticizer are added separately, and the resulting organic binder solution is added so that the solid content of the binder resin is 6% by weight with respect to the ceramic, thereby obtaining a sheet forming slurry. .

  A green sheet having a thickness of 60 μm was obtained by the doctor blade method using the sheet forming slurry, and the green sheet was cut into strips.

Next, Ag-Pd is formed by mixing Ag and Pd on the strip-shaped green sheet formed as described above so that Ag: Pd = 30: 70 (atomic%) after firing. The paste was screen printed to form an internal electrode pattern. After forming the internal electrode pattern in this way, the ceramic green sheets on which the internal electrode pattern is formed are stacked so that the internal electrode patterns are alternately drawn to the end face, and the internal electrode pattern is not printed vertically. The ceramic green sheets were laminated and pressed in the thickness direction to obtain a mother laminate. The mother laminate was cut so that the dimensions after firing would be a size of length (L) 1.0 mm × width (W) 0.5 mm × thickness (T) 0.5 mm. In this way, a laminate chip was obtained. The total area of the internal electrodes in the multilayer chip was 0.4 mm ² , and the thickness of the ceramic layer between the internal electrodes was about 50 μm after firing.

The laminate chip obtained as described above was heated in the atmosphere at 500 ° C. for 12 hours to perform a binder removal step. Next, the firing temperature was variously changed so that the varistor voltage V ₁ mA was about 27 V, and firing was performed. The firing temperature was a temperature in the range of 1100 ° C to 1300 ° C. Further, the temperature raising process during firing was performed in a nitrogen atmosphere, and the firing maximum temperature holding process and the temperature lowering process were performed in an air atmosphere.

  After barrel-polishing the sintered body thus obtained, an Ag—Pd paste is applied to form external electrodes on both end faces, and baking is performed at a maximum temperature of 820 ° C. for 60 minutes to obtain an Ag—Pd external electrode. It was. Thereafter, a Ni plating film and a Sn plating film were formed on the surface of the external electrode so as to have thicknesses of 1.5 μm and 3 μm, respectively.

  The structure of the laminated varistor obtained as described above is shown in a schematic front sectional view in FIG. FIG. 2 is an exploded perspective view of the ceramic body of the multilayer varistor. The multilayer varistor 1 has a ceramic body 4 in which a plurality of internal electrodes 3 a and 3 b are laminated via a ceramic layer 2. In the ceramic body 4, the first and second internal electrodes 3 a and 3 b overlap with each other with the ceramic layer 2 interposed therebetween. Among these, at least one first internal electrode 3a is drawn out to the first end face of the ceramic body 4, and a first external electrode 5a is formed on the first end face and is electrically connected. Yes. Further, at least one second internal electrode 3b is drawn out to the second end face, and a second external electrode 5b is formed on the second end face and is electrically connected.

  Although not shown in FIG. 1, a Ni plating film and a Sn plating film are further formed on the surfaces of the external electrodes 5a and 5b as described above.

  As described above, laminated varistors indicated by sample numbers 1 to 45 in Table 1 below were obtained. The composition of the fired ceramic body was analyzed by pulverizing the fired ceramic body and using inductively coupled plasma emission spectroscopy and inductively coupled plasma mass spectrometry.

  20 laminated varistors obtained as described above were prepared, and a high temperature load test and an ESD test were performed as follows as tests for evaluating reliability.

  High temperature load test: A voltage of 65% of the varistor voltage of the laminated varistor was applied for 1000 hours at a temperature of 125 ° C., and then the insulation resistance and varistor voltage of the laminated varistor were measured.

  ESD test: An ESD pulse conforming to IEC61000-4-2 was applied 10 times from both ends of the multilayer varistor, and then the insulation resistance and varistor voltage of the multilayer varistor were measured.

In any of the above tests, when the insulation resistance was less than 1 MΩ after the test, or the change rate of the varistor voltage (ΔV ₁ mA / V ₁ mA) was 10% or more, it was judged as a defective product. .

  In the high temperature load test, the ratio of defective products was defined as the defective rate. In the ESD test, the maximum ESD amount that can satisfy the above criteria was measured.

  Further, with respect to the laminated varistor obtained as described above, the end face of the ceramic body is polished by using an Ar etching apparatus to determine whether or not Ag contained in the internal electrode is diffused in the ceramic layer. Then, the ceramic layer between the internal electrodes was analyzed by a wavelength dispersive X-ray spectrometer (WDX). By this analysis, the presence or absence of Ag diffusion was investigated. The results are shown in Table 1 below.

  In Table 1 below, the samples marked with x indicate that the samples are outside the scope of the present invention. Further, the sample indicated by ☆ indicates that it is the same sample as the sample number 3.

  As is apparent from Table 1, all of Samples 2 to 4, 7 to 10, 12 to 17, 20 to 23, 25 to 28, 30 to 34, and 38 to 44, which are within the scope of the present invention, are not acceptable in the high temperature load test. It was found that good products were not generated and ESD resistance of 15 kV or higher was obtained. On the other hand, the addition amount of Co having a large solid solubility limit with respect to ZnO is less than 1.0 atomic%. In Sample No. 1, since the amount of Co dissolved in ZnO particles is small, Ag diffusion occurs, Defects occurred in the ESD test and the high temperature load test.

  As is clear from Sample Nos. 5 and 6, when Co is more than 5 atomic%, Co cannot be completely dissolved, and part of it segregates at the ZnO grain boundary, resulting in defective products in the high temperature load test and ESD test. You can see that

  Further, as apparent from the results of Sample No. 11, when Al was less than 0.002 atomic%, defects occurred in both the high temperature load test and the ESD test. When Al was more than 0.2 atomic%, as apparent from Sample No. 18, the donor was excessive, so that defects occurred in the ESD test and the high temperature load test.

  On the other hand, when the addition amount of Pr is less than 0.1 atomic%, as is clear from Sample No. 19, a sufficiently high grain boundary barrier cannot be formed, resulting in a failure in the high temperature load test or ESD test. When the amount was more than atomic%, as is clear from Sample No. 24, Pr was excessively segregated at the ZnO grain boundary, which promoted the diffusion of Ag. Therefore, remarkable defective products were generated in the high temperature load test and the ESD test.

  Further, when the addition amount of Ca as an alkaline earth metal that forms a liquid phase at the ZnO grain boundary to enhance the sinterability is less than 0.005 atomic%, it is apparent from the result of the sample number 29. In addition, the sinterability decreased, and many defective products were generated in the high temperature load test and the ESD test. When the amount was more than 0.5 atomic%, the liquid phase was excessively formed to promote the diffusion of Ag, resulting in defective products in the high temperature load test and the ESD test. Even if a large amount of Co, which has a high Ag diffusion suppressing effect, is added, as is apparent from Sample No. 35, if Ca is added excessively, Ag diffuses due to grain boundary diffusion, resulting in a high temperature load test or ESD test. A defective product was produced.

  Further, when the Zr content was in the range of 0.01 to 0.2 atomic%, no defective product was generated in the high temperature load test, and the ESD resistance was 15 kV or more. In samples 2 to 4, 7 to 10, 12 to 17, 20 to 23, 25 to 28, 30 to 34, and 38 to 41 having a Zr content of 0.01 to 0.08 atomic%, the ESD resistance is 30 kV or more. It turned out that it was more preferable. When Zr is more than 0.2 atomic%, as is clear from the sample 45, Ag diffuses to a level that can be detected by WDX, and the reliability decreases.

  On the other hand, if the Zr content is less than 0.01 atomic%, abnormal grain growth is likely to occur, and the variation in the grain size becomes large, and defective products appear significantly in the reliability test. When the Zr content is more than 0.08 atomic%, segregation may occur, so that ESD is concentrated locally and ESD resistance seems to be slightly reduced.

(Experimental example 2)
Next, the effect of adding Ga, In or a combination of Al, Ga and In instead of Al was similarly evaluated. The results are shown in Tables 2 to 4 below.

  As is apparent from Tables 2 to 4, it can be seen that the same effect as in the case of using Al can be obtained when Ga and In are used instead of Al. Further, even when Al, Ga, and In are appropriately combined, in the case of Samples 47 to 52, 55 to 60, and 63 to 68 that are within the scope of the present invention, the defect rate is 0 in the high-temperature load test, and 15 kV or more It can be seen that the ESD resistance can be obtained. That is, within the scope of the present invention, by adding Al, Ga and In, even when an Ag—Pd internal electrode is used, the diffusion of Ag is suppressed, whereby the high temperature load life and ESD resistance are reduced. It can be seen that can be effectively increased.

  Next, the effect of using Sr, Ba instead of Ca, or adding Ca, Sr, and Ba in combination was evaluated in the same manner as in Experimental Example 1. The results are shown in Tables 5 to 7 below.

  As is apparent from Tables 5 to 7, it can be seen that the same effect can be obtained when Sr or Ba is used instead of Ca. In the case of the samples 71 to 75, 79 to 83, and 87 to 91 that are within the scope of the present invention, it can be seen that the defect rate is 0 in the high temperature load test, and ESD resistance of 15 kV or more is obtained. Further, in the samples 77, 85, and 93 in which the amount of Co, which is excellent in the intragranular diffusion suppressing effect of Ag, is increased to 5 atomic%, when the alkaline earth metal is added in a larger amount than the range of the present invention, Ag is added. It can be seen that it is not possible to suppress the diffusion of.

(Experimental example 3)
Next, a laminated varistor was produced and evaluated in the same manner as in Experimental Example 1 except that the Zr content was changed. The composition ratio and the evaluation results are shown in Table 8 below.

  As apparent from Table 8, the samples 94 to 117 having a Zr content of 0.01 atomic% or more and 0.2 atomic% or less have a defect rate of 0 in the high-temperature load test and an ESD resistance of 15 kV or more. I understand that Particularly, samples 94, 95, 97, 98, 100, 101, 103, 104, 106, 107, 109, 110, 112, 113, 115 having a Zr content of 0.01 atomic% or more and 0.08 atomic% or less. , 116 shows that no defect occurs in the high-temperature load test, and excellent ESD resistance of 30 kV or more is realized.

  Therefore, in order to suppress the diffusion of Ag and improve the high temperature load life, it is necessary to suppress both the intragranular diffusion and the grain boundary diffusion of Ag, and Co, Pr, alkaline earth metal, Al, Ga, or It can be seen that by blending In and Zr within the scope of the present invention, the insulation resistance and high temperature load life can be improved even when an Ag—Pd internal electrode is used. Furthermore, it turns out that ESD tolerance can be improved further by making the content rate of Zr 0.01-0.08 atomic%.

  That is, by adjusting the addition amount of Co, which has a large solid solubility limit in zinc oxide, and the content of Ag, particularly alkaline earth metals and Zr among elements that segregate at ZnO grain boundaries or form a liquid phase, By suppressing both the intragranular diffusion and the grain boundary diffusion, it is possible to suppress a decrease in reliability due to the diffusion of Ag into the ceramic layer, and to obtain excellent varistor characteristics.

(Experimental example 4)
A laminated varistor was produced in the same manner as in Experimental Example 1. The composition of the ceramic layer was adjusted so as to be the same as that of Sample 3 in Table 1, and for the internal electrode, an internal electrode paste having a Ag: Pd of 5 to 70:95 to 30 (atomic%) after firing was used. Using. Then, the obtained laminated varistor was evaluated in the same manner as in Example 1, and the presence or absence of Ag diffusion was also evaluated in the same manner.

  The Ag: Pd ratio of the fired internal electrode is between the outermost internal electrodes on the most principal surface side after polishing the LT surface of the fired ceramic body until all the internal electrodes are exposed, and The WDX analysis was performed using the portion where the opposing internal electrodes overlap (internal electrode effective portion) as the analysis range, and the ratio of Ag and Pd was analyzed.

Furthermore, as the characteristics of the multilayer varistor, 20 multilayer varistors each having the composition of Sample 3 shown in Table 1 were prepared, and the variations in insulation resistance, voltage nonlinearity, and varistor voltage V ₁ mA were evaluated. The varistor voltage is obtained by measuring the voltage across each test piece when a direct current of 1 mA is passed. Further, the voltage nonlinear coefficient α is calculated based on the formula (1) from the both-end voltage of each test piece and a varistor voltage V ₁ mA when a direct current of 0.1 mA is passed.

The insulation resistance was defined as the resistance after applying 65% of the varistor voltage for 0.1 seconds. The varistor voltage 3C. V. Was calculated as the variation of the varistor voltage V ₁ mA.

The insulation resistance was determined to be good when it was 10 MΩ or more, and when the voltage ratio linearity was 25 or more, it was determined to be good when the varistor voltage V ₁ mA variation was 10% or less.

In Table 9, from the samples 118 and 119, when the Ag content is less than 10%, the ESD resistance and the reliability in the high temperature load test are good, but even if the oxygen concentration atmosphere is adjusted, the insulation resistance and It was difficult to achieve both V ₁ mA variation. On the other hand, in the case of samples 120 to 123 in which Ag is contained at a ratio of 10 to 60 atomic%, the Ag-Pd internal electrode has high insulation resistance (IR) and voltage ratio linearity without performing special atmosphere control. It was found that (α) can be realized and is effective for V ₁ mA variation (3 CV). When the content ratio of Ag is more than 60 atomic%, the Ag ratio of the internal electrode is increased, so that it becomes impossible to completely suppress the diffusion of Ag, and the low voltage varistor with a small number of grain boundaries diffuses. The reliability is somewhat lowered due to the influence of Ag, and varistor characteristics may not be obtained satisfactorily.

  Therefore, it can be seen from the results of Table 9 that it is preferable that the total amount of Ag and Pd is in the range of 100 atomic% and the ratio of Ag is in the range of 10 to 60 atomic%.

(Experimental example 5)
A laminated varistor was produced in the same manner as in Experimental Example 1 so as to obtain the internal electrode composition shown in Table 10 below. However, in this example, the green sheet was formed with a molding thickness of about 30 μm and the internal electrode surface area was 1 mm ² . The firing temperature was adjusted in the range of 1100 to 1300 ° C. so that the varistor voltage was 9V. The total area of the internal electrodes in the multilayer chip was 0.4 mm ² , and the thickness of the ceramic layer between the internal electrodes was about 20 μm after firing.

  With respect to the laminated varistor obtained as described above, the insulation resistance (IR) when a voltage of 6.5 V was applied for 0.1 second and the high temperature load test and ESD resistance test were performed in the same manner as in Example 1. And the presence or absence of diffusion of Ag was evaluated by WDX analysis. Further, the Ag ratio in the internal electrode was measured by WDX analysis in the same manner as in Example 4. In addition, when the insulation resistance IR was 10 MΩ or more, it was determined as a good product. Further, a high temperature load test was conducted in the same manner as in Experimental Example 1. The results are shown in Table 10 below.

  As apparent from Sample Nos. 124 and 125, when Ag diffuses into the ceramic layer, it acts as an acceptor, so that the insulation resistance increases when the amount of Co added is small. However, even with the same ceramic layer composition, the insulation resistance tends to decrease as the Ag content ratio in the internal electrode increases.

  That is, as shown in sample numbers 130, 151, 152, 171, and 172, even in a range where there was no problem in the laminated varistor of Example 1 in which the varistor voltage was 27 V, for example, a ceramic layer In a laminated varistor that generates a low varistor voltage of about 9 V with a small number of grain boundaries, the insulation resistance decreases as the Ag ratio increases.

  Therefore, in the case of an Ag—Pd internal electrode with a high Ag content ratio, the relationship between the Co addition amount x (atomic%) and the Ag ratio y (atomic%) in the internal electrode is expressed as 0.05 ≦ x / y ≦ It can be seen that 0.5 is desirable.

  When the amount of Co added x is large and x / y exceeds 0.5, there is no diffusion of Ag and the insulation resistance is good, but Co is inevitably more than 5.0 atomic%. In the high temperature load test, a defect of about 25% occurs. Therefore, it can be seen that x / y is preferably 0.5 or less.

  In addition, when x / y is less than 0.05, in the low voltage multilayer varistor, the balance between the content of Co contained in the ceramic layer and the content of Ag contained in the internal electrode is poor. Of Ag diffuses out of the ceramic body and remarkably deprives the ceramics of oxygen, so that the grain boundary barrier is lowered and the insulation resistance is significantly reduced.

  As described above, in applications where a low varistor voltage is required, the above x / y relationship is specifically limited to Co contained in the ceramic layer in applications where a varistor voltage of 20 V or less is required. It is preferable to satisfy 0.05 ≦ x / y ≦ 0.5, where x is the content (atomic%) of x and y is the content (atomic%) of Ag contained in the internal electrode. This also shows that it is useful when the thickness of the ceramic layer is 30 μm or less. When the varistor voltage is higher than 20V, the number of grain boundaries is large, so that the Ag of the internal electrode is scattered outside the ceramic body and deprives the ceramic of oxygen, and the insulation resistance is sufficiently lowered even if this relationship is not satisfied. And reliability during ESD test and high temperature load test can be ensured.

  In Examples 1 to 5, the experiment was performed using the laminated varistor having the structure shown in FIGS. 1 and 2. However, if the structure has an Ag—Pd internal electrode, FIGS. It is not limited to the structure of 2.

1 is a schematic front cross-sectional view of a laminated varistor manufactured in an embodiment of the present invention. The typical perspective view which decomposes | disassembles and shows the ceramic body of the laminated varistor shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Laminated varistor 2 ... Ceramic layer 3a, 3b ... Internal electrode 4 ... Ceramic body 5a, 5b ... External electrode

Claims (3)

A ceramic body formed by laminating a plurality of ceramic layers made of a semiconductor ceramic material and a plurality of internal electrodes formed along a predetermined interface between the ceramic layers;
A laminated varistor comprising an external electrode formed to be electrically connected to the internal electrode,
The ceramic layer is mainly composed of ZnO,
As subcomponents, Pr is 0.1 to 5.0 atomic% of the whole, Co is 1.0 to 5.0 atomic%, and at least one of Al, Ga, and In is 0.002 to 0.00 in total. 2 atomic%, at least one of Ca, Sr and Ba in a total amount of 0.005 to 0.5 atomic%, Zr 0.01 to 0.2 atomic%,
And, wherein the internal electrode is not more than 60 atomic% content of 10 atomic% or more Ag in unrealized the internal electrode Ag and Pd, laminated varistor.   2. The multilayer varistor according to claim 1, wherein the total amount of Zr contained in the ceramic layer is 0.01 to 0.08 atomic% of the entire ceramic layer. When the Co content (atomic%) in the ceramic layer is x and the Ag content (atomic%) in the internal electrode is y, 0.05 ≦ x / y ≦ 0.5. The multilayer varistor according to claim 1 or 2 , characterized by

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