Laminated Barista

Abstract

To provide a multilayer varistor with low variation in varistor characteristics and excellent voltage nonlinearity. [Solution] The multilayer varistor 1 comprises a sintered body 11 containing Zn oxide as a main component and Pr oxide as a secondary component, at least a pair of internal electrodes 12 provided inside the sintered body 11 and containing Pd as a main component, and at least a pair of external electrodes 13 provided so as to cover a part of the sintered body 11 and electrically connected to each of the at least pair of internal electrodes 12. In the region sandwiched between the pair of internal electrodes 12, the concentration of Pr relative to Zn in the vicinity region 11a of the internal electrodes 12 is 1.1 times or more and 2 times or less than the concentration of Pr relative to Zn in the region 11b adjacent to the vicinity region 11a.

Description

Technical Field

[0001] The present disclosure relates to a multilayer varistor, and more particularly to a multilayer varistor including a sintered body, internal electrodes, and external electrodes.

Background Art

[0002] Multilayer varistors are used for the purpose of protecting various electronic devices, electronic components, etc. from abnormal voltages caused by lightning surges, static electricity, etc., and preventing malfunction of electronic devices, electronic components, etc. due to noise generated in the circuit.

[0003] Patent Document 1 discloses a multilayer varistor including a ceramic element formed by laminating a plurality of ceramic layers and a plurality of internal electrodes, and an external electrode, wherein the ceramic layer contains ZnO as a main component and, as sub-components, Pr, Co, and at least one of Al, Ga, and In, and the internal electrode contains Ag and Pd. Further, it is described that the firing for obtaining the ceramic element is performed under a nitrogen atmosphere in all of the temperature rising processes.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in the multilayer varistor using the sintered body obtained by firing under a nitrogen atmosphere by the method described in Patent Document 1, there are disadvantages such as large variations in varistor characteristics represented by the V1mA variation coefficient, etc., and poor voltage non-linearity represented by the voltage non-linear index (α), etc.

[0006] The object of this disclosure is to provide a method for manufacturing a multilayer varistor and a multilayer varistor that can be obtained with low variation in varistor characteristics and excellent voltage nonlinearity. [Means for solving the problem]

[0007] A method for manufacturing a laminated varistor according to one aspect of the present disclosure comprises a first step and a second step. In the first step, a laminate is prepared by alternately stacking a green sheet layer containing Zn oxide powder as a main component and Pr oxide powder as a secondary component, and an internal electrode paste layer containing Pd powder. In the second step, the laminate is fired to obtain a sintered body having internal electrodes inside. The second step includes a second B step and, after the second B step, a second C step. In the second B step, firing is performed with an oxygen concentration in the atmosphere of 1000 ppm by volume or less during the heating process from 500°C to 800°C. In the second C step, firing is performed with an oxygen concentration in the atmosphere of 1000 ppm by volume or more during the heating process to the maximum temperature reached.

[0008] A multilayer varistor according to one aspect of the present disclosure comprises a sintered body, at least a pair of internal electrodes, and at least a pair of external electrodes. The sintered body contains Zn oxide as a main component and Pr oxide as a minor component. The internal electrodes are provided inside the sintered body and contain Pd as a main component. The external electrodes are provided so as to cover a part of the sintered body and are electrically connected to each of the at least pair of internal electrodes. In the region sandwiched between the pair of internal electrodes, the concentration of Pr relative to Zn in the region near the internal electrodes is 1.1 times or more and 2 times or less than the concentration of Pr relative to Zn in the region adjacent to the region near the internal electrodes. [Effects of the Invention]

[0009] According to this disclosure, a multilayer varistor with low variability in varistor characteristics and excellent voltage nonlinearity can be obtained. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a schematic cross-sectional view of the stacked varistor according to this embodiment. [Modes for carrying out the invention]

[0011] (1) Overview Hereinafter, a method for manufacturing a laminated varistor and a laminated varistor according to one embodiment of the present disclosure will be described with reference to the drawings. Note that the figures described in the following embodiment are schematic, and the ratios of the size and thickness of each component in the figures do not necessarily reflect the actual dimensional ratios.

[0012] The manufacturing method for the laminated varistor 1 of this embodiment comprises a first step and a second step. In the first step, a laminate is prepared by alternately stacking a green sheet layer containing Zn oxide powder as the main component and Pr oxide powder as a secondary component, and an internal electrode paste layer containing Pd powder. In the second step, the laminate is fired to obtain a sintered body having an internal electrode 12 inside. The second step includes a second B step and, after the second B step, a second C step. In the second B step, firing is performed with an oxygen concentration in the atmosphere of 1000 volume ppm or less during the heating process from 500°C to 800°C. In the second C step, firing is performed with an oxygen concentration in the atmosphere of 1000 volume ppm or more during the heating process to the maximum temperature reached.

[0013] In their diligent research into manufacturing methods for multilayer varistors, the inventors discovered that when the sintered body contains Pr oxide as a minor component and the internal electrodes contain Pd, the variation in varistor characteristics can be reduced by (1) keeping the oxygen concentration in the atmosphere below 1000 volume ppm during the heating process from 500°C to 800°C. Furthermore, they discovered that (2) by keeping the oxygen concentration above 1000 volume ppm during the heating process to the highest temperature after (1), excellent voltage nonlinearity can be achieved. Thus, according to the manufacturing method of the multilayer varistor 1 of this embodiment, a multilayer varistor with low variation in varistor characteristics and excellent voltage nonlinearity can be obtained.

[0014] The reason why the variation in varistor characteristics can be reduced by keeping the oxygen concentration below 1000 ppm by volume during the heating process from 500°C to 800°C in (1) is not entirely clear, but it can be inferred as follows. One of the causes of variation in varistor characteristics is thought to be the uneven distribution of Pr oxide, and this uneven distribution of Pr oxide is thought to occur because, during sintering, the Pr oxide in the green sheet layer reacts with the Pd in ​​the internal electrode paste layer, making it easier for it to move towards the internal electrode. The inventors have found that this reaction between Pr oxide and Pd can be suppressed by keeping the oxygen concentration in the atmosphere relatively low during the heating process of firing.

[0015] Furthermore, the reason why the voltage nonlinearity can be improved by keeping the oxygen concentration below 1000 ppm in the heating process to the highest temperature after (1) in (2) is not entirely clear, but for example, when ZnO is sintered in an atmosphere with a low oxygen concentration, Zn is released into the ZnO. + It is thought that as the oxygen concentration increases, the insulation resistance at the grain boundaries decreases, and voltage nonlinearity decreases. However, by keeping the oxygen concentration above a certain value, it is thought that this decrease in insulation resistance at the grain boundaries can be suppressed, and voltage nonlinearity can be improved.

[0016] As shown in Figure 1, the multilayer varistor 1 of this embodiment comprises a sintered body 11, at least a pair of internal electrodes 12, and at least a pair of external electrodes 13. The sintered body 11 contains Zn oxide as a main component and Pr oxide as a minor component. The internal electrodes 12 are provided inside the sintered body 11 and contain Pd as a main component. The external electrodes 13 are provided so as to cover a part of the sintered body 11 and are electrically connected to each of the at least pair of internal electrodes 12. In the region sandwiched between the pair of internal electrodes 12, the concentration of Pr relative to Zn in the vicinity region 11a (hereinafter also referred to as the vicinity region 11a) of the internal electrodes 12 is at least twice the concentration of Pr relative to Zn in the region 11b (hereinafter also referred to as the adjacent region 11b) adjacent to the vicinity region 11a.

[0017] The multilayer varistor 1 of this embodiment has little variation in varistor characteristics. Also, it is excellent in voltage non-linearity.

[0018] (2) Details (Manufacturing method of multilayer varistor) The manufacturing method of the multilayer varistor 1 of this embodiment includes a first step and a second step.

[0019] The manufacturing method of the multilayer varistor 1 of this embodiment may further include a step of forming an insulating coat layer so as to cover at least a part of the sintered body 11 obtained in the second step (hereinafter also referred to as the insulating coat layer forming step), a step of forming an external electrode 13 so as to cover a part of the sintered body 11 or the insulating coat layer and be electrically connected to the internal electrode 12 (hereinafter also referred to as the external electrode forming step), a step of forming a plating electrode so as to cover at least a part of the external electrode 13 (also referred to as the plating electrode forming step), and the like. Hereinafter, each step will be described.

[0020] [First step] The first step is a step of preparing a laminate in which green sheet layers and internal electrode paste layers are alternately laminated.

[0021] (Green sheet layer) The green sheet layer contains ZnO powder as the main component and Pr oxide powder as the subcomponent.

[0022] Examples of ZnO include ZnO and the like. Examples of Pr oxide include Pr6O 11 and the like.

[0023] The green sheet layer can be produced by sheet-forming a slurry prepared by mixing ZnO powder, Pr oxide powder, and organic components such as an organic solvent and a binder using a coater or the like.

[0024] (Internal electrode paste layer) The internal electrode paste layer contains palladium powder. Examples of palladium powder include palladium powder and silver-palladium powder.

[0025] The internal electrode paste layer can be formed on the green sheet layer by preparing an internal electrode paste containing Pd powder and printing this internal electrode paste onto the green sheet layer. Preferably, the internal electrode paste is substantially free of at least one element selected from the group consisting of Al, In, and Ga. If the formed internal electrode 12 contains these elements, the voltage nonlinearity of the multilayer varistor 1 may decrease. "Substantially free" means that the element is not actively added unless it is inevitably mixed in. Specifically, the content of these elements relative to the Pd constituting the internal electrode 12 is preferable. This means that the amount is 0.00001% by mass or less.

[0026] A laminate can be obtained by stacking the green sheet layer prepared in this way with a green sheet layer on which an internal electrode paste layer has been formed. This laminate has at least one pair of internal electrode paste layers inside, which become internal electrodes 12 upon firing.

[0027] [Second process] The second step is to fire the laminate obtained in the first step to obtain a sintered body having internal electrodes 12 inside. The laminate obtained in the first step is cut and subjected to firing as green chips. Known firing equipment such as a ceramic setter can be used for firing. The internal electrode paste layer in the laminate becomes the internal electrode 12 by firing.

[0028] The second step includes the second B step, followed by the second C step. The pressure (absolute pressure) during the firing of the second step is preferably between 0.01 atmospheres and 2 atmospheres, and more preferably between 0.5 atmospheres and 1.5 atmospheres. The firing of the second step is usually carried out under atmospheric pressure. Examples of gaseous components other than oxygen in the atmosphere during the firing of the second step include inert gases such as nitrogen and argon.

[0029] The second step preferably includes the second A step before the second B step. The pressure and gas components other than oxygen used during the firing in the second A, second B, and second C steps may be the same or different from each other. The following describes each step.

[0030] (2nd A process) Step 2A is a firing process that takes place during the heating process preceding Step 2B. The heating process in Step 2A is, for example, from room temperature (25°C) to 500°C.

[0031] In the calcination process of step 2A, the oxygen concentration in the atmosphere is preferably 100 ppm by volume or more. In this case, during calcination, Zn is added to ZnO. + This allows for better sintering of the laminate while further suppressing the decrease in insulation resistance at grain boundaries due to an increase in the material, and as a result, the voltage nonlinearity can be further improved. In this specification, "volume ppm" means ppm on a volume basis of oxygen relative to the total volume of the gas, and is approximately equal to ppm on a molar basis of oxygen relative to the total number of moles of molecules constituting the gas (molar ppm).

[0032] The oxygen concentration in the atmosphere is more preferably 200 ppm by volume or more, even more preferably 300 ppm by volume or more, and particularly preferably 400 ppm by volume or more. The oxygen concentration in the atmosphere is, for example, 1000 ppm by volume or less, preferably 800 ppm by volume or less, and more preferably 600 ppm by volume or less.

[0033] In step 2A, it is preferable to set the average heating rate to 10°C / h or more and 100°C / h or less, and more preferably to 30°C / h or more and 70°C / h or less.

[0034] (2nd B process) Step 2B is a firing process that takes place during the heating process from 500°C to 800°C. The firing in Step 2B may be carried out at temperatures exceeding 800°C, for example, up to 850°C, 900°C, etc.

[0035] In step 2B, firing is performed with an oxygen concentration of 1000 ppm by volume or less in the atmosphere. By setting the oxygen concentration in the atmosphere to 1000 ppm by volume or less during firing in step 2B, the reaction between Pd in ​​the internal electrode paste layer and Pr oxide, a by-component in the green sheet layer, is suppressed, thereby preventing Pr oxide from migrating to the internal electrode paste layer. As a result, the non-uniformity of the presence of Pr oxide is suppressed, and variations in varistor characteristics can be reduced. The oxygen concentration in the atmosphere is preferably 800 ppm by volume or less, more preferably 700 ppm by volume or less, and even more preferably 600 ppm by volume or less. The oxygen concentration in the atmosphere is, for example, 0.1 ppm by volume or more, preferably 1 ppm by volume or more, and more preferably 5 ppm by volume or more.

[0036] In step 2B, when the oxygen concentration in the atmosphere is 100 ppm or more and 1000 ppm or less by volume, it is preferable to set the average heating rate to 100°C / h or more and 1000°C / h or less. In step 2B, even when the oxygen concentration is relatively high, increasing the heating rate further can suppress the movement of Pr oxide towards the internal electrode paste layer, and as a result, the variation in varistor characteristics can be reduced. It is more preferable that the average heating rate be 200°C / h or more and 800°C / h or less, even more preferable that it be 300°C / h or more and 700°C / h or less, and particularly preferable that it be 400°C / h or more and 600°C / h or less.

[0037] In step 2B, when the oxygen concentration in the atmosphere is 0.1 ppm or more and 100 ppm or less, it is preferable to set the average heating rate to 25°C / h or more and 200°C / h. In step 2B, when the oxygen concentration is relatively low, by further reducing the heating rate, the movement of Pr oxide towards the internal electrode paste layer can be further suppressed, and as a result, the variation in varistor characteristics can be further reduced. It is more preferable that the average heating rate be 30°C / h or more and 150°C / h or less, even more preferable that it be 35°C / h or more and 100°C / h or less, and particularly preferable that it be 40°C / h or more and 60°C / h or less.

[0038] (2nd C process) Step 2C is a firing process that takes place after Step 2B, during the heating process up to the maximum temperature. The heating process in Step 2C starts from, for example, 800°C, 850°C, 900°C, etc. The maximum temperature is preferably 1000°C to 1500°C, more preferably 1100°C to 1450°C, and even more preferably 1200°C to 1400°C.

[0039] In step 2C, the oxygen concentration in the atmosphere is set to 1000 ppm by volume or higher. By setting the oxygen concentration in the atmosphere to 1000 ppm by volume or higher during the firing in step 2C, the decrease in grain boundary insulation resistance that occurs during the sintering of Zn oxide can be suppressed, and as a result, excellent voltage nonlinearity can be achieved. The oxygen concentration in the atmosphere is preferably 1500 ppm by volume or higher, more preferably 1800 ppm by volume or higher, and even more preferably 2000 ppm by volume or higher.

[0040] In the second C step, it is preferable to set the average heating rate to 100°C / h or more and 400°C / h or less. By setting the average heating rate within the above range, fluctuations in the oxygen concentration in the atmosphere are small and stable, which further suppresses the decrease in grain boundary insulation resistance during sintering of Zn oxide, and as a result, the voltage nonlinearity can be made even better. It is more preferable that the average heating rate be 130°C / h or more and 350°C / h or less, even more preferable that it be 150°C / h or more and 300°C / h or less, and particularly preferable that it be 180°C / h or more and 250°C / h or less.

[0041] In the second step, by performing firing as described above, a sintered body 11 having an internal electrode 12 inside can be obtained.

[0042] [Protective layer formation process] The protective layer formation step is a step of forming a protective layer (insulating coating layer, high resistance layer) so as to cover at least a portion of the sintered body 11. The protective layer includes, for example, silicon oxide, zinc silicate, or glass components. The protective layer can be formed by methods such as applying a solution containing a silicon oxide precursor or applying glass components.

[0043] [External electrode formation process] The external electrode formation step is a step in which an external electrode paste is applied to cover a portion of the sintered body 11 obtained in the second step or the protective layer obtained in the protective layer formation step, and to contact a portion of the internal electrode 12, thereby forming an external electrode 13.

[0044] External electrode paste can be prepared by mixing a metal component, such as Ag powder, AgPd powder, or AgPt powder, with a glass component, such as Bi2O3, SiO2, or B2O5, and a solvent. Alternatively, an external electrode paste with Ag as the main component and containing a resin component can also be used.

[0045] By applying the external electrode paste and then baking it at a temperature between 700°C and 800°C, alloying with the internal electrode 12 can be promoted, and an external electrode 13 with improved adhesion can be formed.

[0046] [Plating electrode formation process] The plating electrode formation process is a process of forming a plated electrode so as to cover at least a portion of the external electrode 13.

[0047] The plated electrode can be formed, for example, by performing Ni plating followed by Sn plating using an electroplating method.

[0048] <Laminated Barista> The multilayer varistor 1 of this embodiment can be manufactured by the manufacturing method of the multilayer varistor 1 described above. The multilayer varistor 1 comprises a sintered body 11, at least a pair of internal electrodes 12, and at least a pair of external electrodes 13.

[0049] The sintered body 11 is composed of semiconductor ceramic components having nonlinear resistance characteristics.

[0050] The multilayer varistor 1 only needs to have at least one pair of internal electrodes 12. In the multilayer varistor 1 shown in Figure 1, one pair of internal electrodes 12 is provided. That is, the internal electrodes 12 include a first internal electrode 12A and a second internal electrode 12B.

[0051] The multilayer varistor 1 is provided with at least one pair of external electrodes 13. One of the external electrodes 13 is provided to be electrically connected to one or more internal electrodes 12. In the multilayer varistor of Figure 1, there is a pair of external electrodes 13. That is, the external electrodes 13 include a first external electrode 13A provided on one end face of the sintered body 11 and a second external electrode 13B provided on the other end face of the sintered body 11. When a voltage is applied between the first external electrode 13A and the second external electrode 13B, one of the first external electrode 13A and the second external electrode 13B becomes the high-potential electrode, and the other of the first external electrode 13A and the second external electrode 13B becomes the low-potential electrode.

[0052] At least one pair of external electrodes 13 are mounted on a printed circuit board on which an electrical circuit is formed. The multilayer varistor 1 is connected, for example, to the input side of the electrical circuit. When a voltage exceeding a predetermined threshold voltage is applied between the first external electrode 13A and the second external electrode 13B, the electrical resistance between the first external electrode 13A and the second external electrode 13B decreases sharply, and current flows through the varistor layer. Therefore, it can protect the electrical circuit downstream of the multilayer varistor 1.

[0053] The multilayer varistor 1 may also include a protective layer, plated electrodes, etc., in addition to the sintered body 11, internal electrode 12, and external electrode 13. The following describes each component.

[0054] [Sintered body] The semiconductor ceramic component having nonlinear resistance characteristics that constitutes the sintered body 11 includes Zn oxide as the main component and Pr oxide as a secondary component.

[0055] Examples of Zn oxides include ZnO. Examples of Pr oxides include Pr6O. 11 These are some examples.

[0056] [Internal electrode] The internal electrode 12 is provided inside the sintered body 11 and contains Pd as its main component. Examples of Pd as the main component include Pd, Ag-Pd, etc.

[0057] It is preferable that the internal electrode 12 substantially does not contain at least one element selected from the group consisting of Al, In, and Ga. If the internal electrode 12 contains these elements, the voltage nonlinearity of the multilayer varistor 1 may decrease.

[0058] (Distribution of Pr oxide in sintered body) As an indicator of the non-uniformity of the distribution of Pr oxide in the sintered body 11, as shown in Figure 1, in the region sandwiched between a pair of internal electrodes 12 inside the sintered body 11, the ratio of the concentration of Pr relative to Zn (Pr concentration (1)) in the region near the internal electrodes 12 (hereinafter also called the nearby region 11a) to the concentration of Pr relative to Zn (Pr concentration (2)) in the region adjacent to the nearby region 11a (hereinafter also called the adjacent region 11b) (Pr concentration (1) / Pr concentration (2)) (hereinafter also called the Pr concentration ratio (times)) is given. In other words, the Pr concentration ratio is the value of the ratio of the Pr concentration in the nearby region 11a to the Pr concentration in the adjacent region 11b (neighboring region / adjacent region). The thickness of the internal electrode 12 in the direction normal to the nearby region 11a is, for example, 5 μm. The thickness of the internal electrode 12 in the direction normal to the adjacent region 11b is, for example, 10 μm.

[0059] In conventional multilayer varistors 1, the Pr oxide migrates to the internal electrode paste side during firing to obtain the sintered body, resulting in a large Pr concentration ratio. However, in the multilayer varistor 1 of this embodiment, the migration of Pr oxide is suppressed, and the non-uniformity of the Pr oxide distribution is reduced, resulting in a smaller Pr concentration ratio.

[0060] The "concentration of Pr relative to Zn" in each of the neighboring region 11a and the adjacent region 11b can be determined by analyzing the cross-sections obtained by cutting the multilayer varistor 1 so that each region is exposed, using an XMA (X-ray microanalyzer), and measuring the X-ray intensity originating from Zn or Pr in each region.

[0061] In the multilayer varistor 1 of this embodiment, the Pr concentration ratio is 2 times or less. In other words, the migration of Pr oxide toward the internal electrode paste layer has not progressed much, and the Pr oxide is present almost uniformly in the region sandwiched between the pair of internal electrodes 12, so the multilayer varistor 1 is considered to have excellent voltage nonlinearity.

[0062] The Pr concentration ratio is preferably 1.5 times or less, more preferably 1.3 times or less, even more preferably 1.2 times or less, and particularly preferably 1.1 times or less.

[0063] [Protective layer] A protective layer (insulating coating layer, high-resistance layer) is provided to cover at least a portion of the sintered body 11. The protective layer includes, for example, silicon oxide, zinc silicate, glass components, etc.

[0064] [External electrode] The external electrode 13 is provided so as to cover a portion of the sintered body 11 or the protective layer, and is electrically connected to each of at least one pair of internal electrodes 12. The external electrode 13 contains, for example, a metallic component such as Ag, AgPd, or AgPt, and a glass component such as Bi2O3, SiO2, or B2O5.

[0065] [Plating electrodes] The plated electrode is provided so as to cover at least a portion of the external electrode 13. Examples of plated electrodes include Ni-plated electrodes and Sn-plated electrodes. [Examples]

[0066] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited to the examples.

[0067] <Manufacturing of multilayer varistors> The multilayer varistors of Examples 1 and 2 and Comparative Examples 1 to 3 were manufactured according to the following procedure. [Fabrication of sintered bodies] (Preparation of slurry) The main raw material is ZnO (98.5% by mass), and the secondary raw material is Pr6O 11 A slurry was prepared by adding organic components such as an organic solvent and a binder to (0.5 mass%), Co2O3 (0.5 mass%), and CaO (0.5 mass%).

[0068] (Preparation of the green sheet layer) Using the prepared slurry, a green sheet layer was produced by forming it to a predetermined thickness of 20 μm to 50 μm using a coating machine.

[0069] (Preparation of a green sheet layer with an internal electrode paste layer formed on it) As the internal electrode paste, a paste containing the components shown in "Composition of Internal Electrode" in Table 1 was used. This internal electrode paste was printed onto the prepared green sheet layer in a predetermined shape to create a green sheet layer with the internal electrode paste layer formed on top. "Ag-Pd(30:70)" means an alloy of Ag and Pd (mixture mass ratio = 30 / 70). "Pd + Al 0.1 mol% added" means "Pd with 0.1 mol% of Al oxide added."

[0070] (Fabrication of laminates) A laminate was fabricated by stacking the aforementioned green sheet layer and the aforementioned green sheet layer on which the internal electrode paste layer was formed.

[0071] (Firing) By firing the laminate under the temperature, heating rate, and oxygen concentration of the heating process shown in "Step 2A," "Step 2B," and "Step 2C" in Table 1, as well as under atmospheric pressure conditions, a sintered body having internal electrodes was obtained. In Table 1, "ppm" for oxygen concentration refers to "volume ppm".

[0072] (Formation of external electrodes) An external electrode paste was prepared by mixing Ag powder, glass frit, and a solvent. This external electrode paste was applied to the end face of the fabricated sintered body, and then baked at 800°C to form the external electrode.

[0073] (Formation of plated electrodes) On the external electrode formed as described above, a Ni-plated electrode of a predetermined thickness was formed by electroplating, and then a Sn-plated electrode was formed on top of that.

[0074] <Rating> The coefficient of variation and voltage nonlinearity of the fabricated multilayer varistors were evaluated using the methods described below. The Pr concentration ratio (the ratio of the Pr concentration in the near region to the adjacent region relative to the Zn concentration (adjacent region / near region)) was also measured. Furthermore, the sintering state of the sintered body constituting the multilayer varistor was evaluated.

[0075] (V1mA coefficient of variation) The V1mA coefficient of variation was evaluated as an indicator of the variability of varistor characteristics. For a multilayer varistor with V1mA = 27V, the V1mA coefficient of variation was calculated by measuring the standard deviation (σ) of the voltage (V1mA) variation and using the formula V1mA coefficient of variation = σ × 100 / V1mA (%). The V1mA coefficient of variation can be evaluated as "good" if it is between 0.4% and 3.7%, and as "poor" if it exceeds 3.7%.

[0076] (Voltage nonlinearity) As an indicator of voltage nonlinearity, the voltage nonlinearity index (α) was evaluated. The voltage nonlinearity index (α) was calculated by measuring the varistor voltage (V1) when a current I1 (1 mA) was applied and the varistor voltage (V2) when a current I2 (0.01 mA) was applied, and using the formula α = log(I1 / I2) / log(V1 / V2). Voltage nonlinearity is better the larger the value of α. If α is 14 or higher, it can be evaluated as "good," and if α is less than 14, it can be evaluated as "poor."

[0077] (Pr concentration magnification) The Pr concentration ratio was calculated by analyzing cross-sections obtained by cutting a multilayer varistor so that the surrounding and adjacent regions were exposed, using an XMA (X-ray microanalyzer), measuring the X-ray intensity originating from Zn or Pr in each region, determining the Pr concentration (Pr / Zn) in each region, and then using the formula Pr concentration ratio = Pr concentration in the surrounding region / Pr concentration in the adjacent region. The Pr concentration ratio can be evaluated as "good" if it is 2 times or less, and as "poor" if it is greater than 2 times.

[0078] (Sintered state) The sintering state of the sintered body was evaluated by measuring the shrinkage rate during firing (= thickness of the laminate after firing × 100 / thickness of the laminate before firing) (%), as follows. ○ (Good): The contraction rate is 90% or less. × (Defective): The shrinkage rate is over 90%.

[0079] [Table 1]

[0080] The results in Table 1 show that the multilayer varistors manufactured using the manufacturing methods of Example 1 and Example 2 exhibited low variation in varistor characteristics and excellent voltage nonlinearity. On the other hand, the multilayer varistors manufactured using the manufacturing methods of Comparative Example 1 and Comparative Example 2 had poor coefficient of variation and / or voltage nonlinearity. The multilayer varistor of Reference Example 1 used Al-doped Pd for its internal electrodes, which resulted in reduced voltage nonlinearity.

[0081] (summary) As is clear from the embodiments and examples described above, a method for manufacturing a laminated varistor according to a first aspect of the present disclosure comprises a first step and a second step. In the first step, a laminate is prepared by alternately stacking a green sheet layer containing Zn oxide powder as a main component and Pr oxide powder as a secondary component, and an internal electrode paste layer containing Pd powder. In the second step, the laminate is fired to obtain a sintered body. The second step includes a second B step and, after the second B step, a second C step. In the second B step, firing is performed with an oxygen concentration in the atmosphere of 1000 ppm by volume or less during the heating process from 500°C to 800°C. In the second C step, firing is performed with an oxygen concentration in the atmosphere of 1000 ppm by volume or more during the heating process to the maximum temperature reached.

[0082] According to the first embodiment, in step 2B, the migration of Pr oxide towards the internal electrode paste layer can be suppressed, and in step 2C, the decrease in grain boundary insulation resistance that occurs during sintering can be suppressed. As a result, a laminated varistor with small variations in varistor characteristics and excellent voltage nonlinearity can be obtained.

[0083] In a second aspect of the present disclosure, in the first aspect, the second step further includes a second A step in which, during the heating process prior to the second B step, the oxygen concentration in the atmosphere is set to 100 ppm by volume or more for firing.

[0084] According to the second embodiment, during firing, Zn in ZnO + This method allows for better sintering of the laminate while further suppressing the decrease in insulation resistance at grain boundaries, and as a result, voltage nonlinearity can be further improved.

[0085] In a third aspect of this disclosure, in the first or second aspect, in step 2B, if the oxygen concentration in the atmosphere is 100 ppm by volume or more and 1000 ppm by volume or less, the average heating rate is set to 100°C / h or more and 1000°C / h or less.

[0086] According to the third embodiment, even when the oxygen concentration is relatively high, increasing the heating rate can further suppress the migration of Pr oxide to the internal electrode paste layer, and as a result, variations in varistor characteristics can be reduced.

[0087] In a fourth aspect of this disclosure, in the first or second aspect, in step 2B, if the oxygen concentration in the atmosphere is 0.1 ppm or more and 100 ppm or less by volume, the average heating rate is 25°C / h or more and 200°C / h or less.

[0088] According to the fourth aspect, when the oxygen concentration is relatively low, the movement of Pr oxide towards the internal electrode paste layer can be further suppressed by reducing the heating rate, and as a result, the variation in varistor characteristics can be further reduced.

[0089] In the fifth aspect of this disclosure, in any one of the first to fourth aspects, the average heating rate in step 2C is set to 100°C / h or more and 400°C / h or less.

[0090] According to the fifth embodiment, fluctuations in the oxygen concentration in the atmosphere are small and stable, which further suppresses the decrease in grain boundary insulation resistance during sintering of Zn oxide, and as a result, the voltage nonlinearity can be improved.

[0091] A multilayer varistor (1) according to a sixth aspect of the present disclosure comprises a sintered body (11), at least a pair of internal electrodes (12), and at least a pair of external electrodes (13). The sintered body (11) contains Zn oxide as a main component and Pr oxide as a minor component. The internal electrodes (12) are provided inside the sintered body (11) and contain Pd as a main component. The external electrodes (13) are provided so as to cover a part of the sintered body (11) and are electrically connected to each of the at least pair of internal electrodes (12). In the region sandwiched between the pair of internal electrodes (12), the concentration of Pr relative to Zn in the vicinity region (11a) of the internal electrodes is at least twice the concentration of Pr relative to Zn in the region (11b) adjacent to the vicinity region (11a).

[0092] According to the sixth embodiment, the non-uniformity of the presence of Pr, which is thought to be related to the variation in varistor characteristics, is reduced, and a multilayer varistor (1) with small variation in varistor characteristics can be provided.

[0093] In a seventh aspect of this disclosure, in a sixth aspect, the internal electrode (12) substantially does not contain at least one element selected from the group consisting of Al, In, and Ga.

[0094] According to the seventh embodiment, by not including these elements in the internal electrodes (12), the multilayer varistor (1) can exhibit excellent voltage nonlinearity. [Explanation of Symbols]

[0095] 1. Stacked varistor 11 Sintered body 11a Neighboring region 11b Adjacent region 12 Internal electrode 13 External electrode

Claims

1. A sintered body containing Zn oxide as the main component and Pr oxide as a secondary component, The sintered body is provided with at least one pair of internal electrodes, each containing Pd as its main component, The sintered body is provided so as to cover a portion of it and comprises at least a pair of external electrodes that are electrically connected to each of the at least pair of internal electrodes, In the region sandwiched between the pair of internal electrodes, the concentration of Pr relative to Zn in the region near the internal electrodes is 1.1 times or more and 2 times or less than the concentration of Pr relative to Zn in the region adjacent to the near-internal region. Stacked varistor.

2. The internal electrode substantially does not contain at least one element selected from the group consisting of Al, In, and Ga. The multilayer varistor according to claim 1.

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