Patent Publication Number: US-2023143255-A1

Title: Multilayer varistor and method of manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is based upon and claims the benefit of priority to Japanese Patent Application No. 2021-184419, filed on Nov. 11, 2021, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to multilayer varistors and methods of manufacturing the multilayer varistors, and specifically, to a multilayer varistor including a sintered body, an internal electrode, and an external electrode and a method of manufacturing the multilayer varistor. 
     BACKGROUND ART 
     A varistor is used, for example, to protect various types of electronic apparatuses, electronic devices, and the like from abnormal voltages caused by lightning surges, static electricity, and the like, and to prevent the electronic apparatuses, electronic devices, and the like from malfunctioning due to noises generated in their circuits. 
     JP 2003-151805 A discloses a chip-type electronic component including: a ceramic body; a glass-coating layer coated on at least part of surfaces of the ceramic body; and external electrodes on both end surfaces of the ceramic body. JP 2003-151805 A describes that setting the thickness of the glass-coating layer to be greater than or equal to a predetermined value can suppress plating from depositing on the surface of the ceramic body during plating. 
     Similarly to the chip-type electronic component described above, a multilayer varistor generally includes a high-resistance layer such as a glass-coating layer, a primary electrode as the external electrode; a plating electrode, and the like. 
     Therefore, providing the multilayer varistor with the high-resistance layer enables plating to be suppressed from depositing. However, in connection with, for example, the use of the primary electrode including Ag as a major component, migration may occur at the surface of the high-resistance layer under conditions of voltage application and wetting. 
     SUMMARY 
     An object of the present disclosure is to provide a multilayer varistor in which the occurrence of migration at a surface of a high-resistance layer is suppressed and a method of manufacturing the multilayer varistor. 
     A multilayer varistor according to an aspect of the present disclosure includes: a sintered body; an internal electrode disposed in the sintered body; a high-resistance layer covering at least part of the sintered body; and an external electrode disposed to cover part of the high-resistance layer, the external electrode being electrically connected to the internal electrode. An arithmetic mean roughness of a surface of the high-resistance layer is greater than or equal to 0.06 μm. 
     A multilayer varistor according to an aspect of the present disclosure includes: a sintered body having a pair of main surfaces opposite to each other, a pair of side surfaces opposite to each other, and a pair of end surfaces opposite to each other; an internal electrode disposed in the sintered body, the internal electrode facing the main surfaces; a high-resistance layer covering at least part of the sintered body; and an external electrode covering part of the high-resistance layer on one of the end surfaces, the external electrode being electrically connected to the internal electrode. an arithmetic mean roughness of a surface of the high-resistance layer is greater on the side surfaces than on the main surfaces. 
     A method of manufacturing a multilayer varistor according to an aspect of the present disclosures includes a first step, a second step, a third step, and a fourth step. The first step includes preparing a sintered body including ZnO as a major component, an internal electrode being disposed in the sintered body. The second step includes forming a high-resistance layer covering at least part of the sintered body. The third step includes applying a primary electrode paste such that the primary electrode paste covers part of the high-resistance layer and is in contact with part of the internal electrode. The fourth step includes forming a plating electrode covering at least part of a primary electrode formed from the primary electrode paste. An arithmetic mean roughness of a surface of the high-resistance layer after the second step is greater than or equal to 0.06 μm and less than or equal to 0.9 μm. 
     A method of manufacturing a multilayer varistor according to an aspect of the present disclosures includes a first step, a second step, a third step, and a fourth step. The first step includes preparing a sintered body including ZnO as a major component and including internal electrode disposed in the sintered body. The second step includes forming a high-resistance layer covering at least part of the sintered body. The third step includes applying a primary electrode paste covering part of the high-resistance layer and being in contact with part of the internal electrode. The fourth step includes forming a plating electrode covering at least part of a primary electrode formed from the primary electrode paste. The second step includes spraying, while stirring a plurality of the sintered bodies, a solution including a precursor of the high-resistance layer toward the sintered bodies, and thermally treating each of the sintered bodies provided with the precursor to form the high-resistance layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures depict one or more implementation in accordance with the present teaching, by way of example only, not by way of limitations. In the figures, like reference numerals refer to the same or similar elements. 
         FIG.  1    is a schematic sectional view of a multilayer varistor according to an embodiment of the present disclosure; and 
         FIG.  2    is a schematic perspective view of the multilayer varistor. 
     
    
    
     DETAILED DESCRIPTION 
     1. Overview 
     A multilayer varistor according to an embodiment of the present disclosure will be described below with reference to the drawings. Note that the drawings to be referred to in the following description of the embodiment are schematic representations. Thus, the sizes, thicknesses, and other attributes of the respective constituent elements illustrated on those drawings are not always to scale, compared with actual ones. 
     As shown in  FIG.  1   , a multilayer varistor  1  of the present embodiment includes a sintered body  11 , internal electrodes  12 , a high-resistance layer  13 , and external electrodes  14 . A feature of the multilayer varistor  1  is that the arithmetic mean roughness (hereinafter also referred to as Ra) of a surface of the high-resistance layer  13  is greater than or equal to 0.06 μm. As used herein, the term “surface” refers to a surface in an exposed area that is not covered with another layer or the like. 
     The value of the Ra of a surface of the sintered body  11  is controllable by adjusting raw materials and/or baking conditions, but electrical characteristics such as a varistor voltage change depending on the value of the Ra. Therefore, controlling both the electrical characteristics and the Ra to be desired values is difficult. However, the inventors found that both the electrical characteristics and the Ra are controllable to be desired values by controlling the value of the Ra of the surface of the high-resistance layer  13  formed on the surface of the sintered body  11 . That is, the Ra of the surface of the high-resistance layer  13  can be changed while the electrical characteristics are secured. The inventors found that setting the Ra of the surface of the high-resistance layer  13  to be larger than or equal to a specific value can suppress the occurrence of migration at the surface of the multilayer varistor  1 . The reason why the multilayer varistor  1  configured as described above provides the effect described above is not necessarily clear but can be inferred as indicated below. The migration in the multilayer varistor  1  may be caused due to elution and movement of metal ions such as Ag ions from the external electrodes  14 , and deposition of metal. To take care of this, the Ra of the surface of the high-resistance layer  13  of the multilayer varistor  1  is controlled to be larger than or equal to a specific value to increase the creepage distance between the external electrodes  14 , that is, the distance of a path along the surface of the high-resistance layer  13  between the external electrodes  14 , thereby increasing a distance that ions and the like have to move to cause the migration. This enables in the multilayer varistor  1 , a mobility barrier to be increased, which consequently enables the occurrence of the migration at the surface of the high-resistance layer  13  to be suppressed. 
     The inventors further studied the configuration of the multilayer varistor  1  of the present embodiment and found that performing control such that arithmetic mean roughnesses of respective surfaces of the high-resistance layer  13  are in a specific relationship also enables the occurrence of the migration at the surface of the high-resistance layer  13  to be suppressed. 
     As shown in  FIG.  1   , the multilayer varistor  1  includes the sintered body  11 , the internal electrodes  12 , the high-resistance layer  13 , and the external electrodes  14 . The sintered body  11  has a pair of main surfaces opposite to each other (an upper surface and a lower surface in  FIG.  1   ), a pair of side surfaces opposite to each other, and a pair of end surfaces opposite to each other (a right surface and a left surface in  FIG.  1   ). The external electrodes  14  are disposed on the end surfaces to cover part of the high-resistance layer  13  and are electrically connected to the internal electrodes  12 . A feature of the multilayer varistor  1  is that the Ra of a surface of a high-resistance layer (hereinafter, also referred to as a side surface high-resistance layer  13   b ) on each side surface is greater than the Ra of a surface of a high-resistance layer (hereinafter, also referred to as a main surface high-resistance layer  13   a ) on each main surface. 
     The inventors found that when a high-resistance layer is formed on the sintered body  11  produced by baking a rectangular parallelepiped piece obtained by cutting, the Ra of the surface, which is a cut surface side of the sintered body  11 , of the side surface high-resistance layer  13   b  is greater than the Ra of the surface of the main surface high-resistance layer  13   a . Increasing the Ra of the surface of the side surface high-resistance layer  13   b  could be considered to suppress the occurrence of the migration in the multilayer varistor  1 . 
     A multilayer varistor  1  according to the present embodiment includes a first step, a second step, a third step, and a fourth step. The first step includes preparing the sintered body  11  including ZnO as a major component, the internal electrodes  12  being disposed in the sintered body  11 . The second step includes forming the high-resistance layer  13  covering at least part of the sintered body  11 . The third step includes applying a primary electrode paste covering part of the high-resistance layer  13  and being in contact with part of the internal electrodes  12 . The fourth step includes forming plating electrodes  16  covering at least part of primary electrodes  15  formed from the primary electrode paste. An arithmetic mean roughness of a surface of the high-resistance layer  13  after the second step is greater than or equal to 0.06 μm and less than or equal to 0.9 μm. 
     According to the method of manufacturing the multilayer varistor  1  of the present embodiment, the Ra of the surface of the high-resistance layer  13  can be within a specified range, which enables the multilayer varistor  1 , in which the occurrence of the migration at the surface of the high-resistance layer  13  is suppressed, to be manufactured. 
     A method of manufacturing the multilayer varistor  1  according to the present embodiment includes the first to fourth steps as described above. The second step includes spraying a solution including a precursor of the high-resistance layer  13  onto a plurality of sintered bodies  11  while stirring the sintered bodies  11 , and thermally treating each of the sintered bodies  11  provided with the precursor to form the high-resistance layer  13 . 
     According to such a method, the high-resistance layer  13  having a large number of raised portions can be formed, and therefore, the Ra of the surface of the high-resistance layer  13  can be increased, which consequently enables the multilayer varistor  1 , in which the occurrence of the migration at the surface of the high-resistance layer  13  is suppressed, to be manufactured. 
     Thus, the present disclosure provides a multilayer varistor in which the occurrence of migration at a surface of a high-resistance layer is suppressed and a method of manufacturing the multilayer varistor. 
     2. Details 
     &lt;Multilayer Varistor&gt; 
       FIG.  1    is a sectional view of the multilayer varistor  1  according to the embodiment of the present disclosure. The multilayer varistor  1  includes the sintered body  11 , the internal electrodes  12 , the high-resistance layer  13 , and the external electrodes  14 . 
     The sintered body  11  includes a semiconductor ceramic component having nonlinear resistance characteristics. 
     The multilayer varistor  1  is provided with at least one pair of external electrodes  14 . Here, the pair of external electrodes  14  include a first external electrode  14 A on one of the end surfaces of the sintered body  11 , and a second external electrode  14 B on the other of the end surfaces of the sintered body  11 . When a voltage is applied between the first external electrode  14 A and the second external electrode  14 B, one of the first external electrode  14 A and the second external electrode  14 B serves as a high-potential electrode, and the other of the first external electrode  14 A and the second external electrode  14 B serves as a low-potential electrode. 
     In the multilayer varistor  1  shown in  FIG.  2   , the pair of external electrodes  14  are disposed on the pair of end surfaces opposite to each other. The number and the locations of the external electrodes  14  are not limited to this example, but a pair of external electrodes  14  may be provided on the side surfaces, or a pair of external electrodes  14  may be provided on the end surfaces and a pair of external electrodes  14  may be provided on the side surfaces. 
     The internal electrodes  12  are at least provided such that one or more of the internal electrodes  12  are electrically connected to each of the external electrodes  14 . The multilayer varistor  1  shown in  FIG.  1    includes two internal electrodes  12 . That is, the internal electrodes  12  include a first internal electrode  12 A and a second internal electrode  12 B. The first internal electrode  12 A is electrically connected to the first external electrode  14 A. The second internal electrode  12 B is electrically connected to the second external electrode  14 B. 
     The at least one pair of external electrodes  14  are mounted on a printed circuit board on which an electric circuit is to be formed. The multilayer varistor  1  is to be connected, for example, to an input side of the electric circuit. When a voltage exceeding a predetermined threshold voltage is applied between the first external electrode  14 A and the second external electrode  14 B, the electric resistance between the first external electrode  14 A and the second external electrode  14 B rapidly decreases, and a current flows through a varistor layer, and therefore, the electric circuit downstream of the multilayer varistor  1  can be protected. 
     [Sintered Body] 
     The semiconductor ceramic component having nonlinear resistance characteristics and constituting the sintered body  11  includes, for example, ZnO as a major component, and Bi 2 O 3 , Co 2 O 3 , MnO 2 , Sb 2 O 3 , Pr 6 O 11 , Co 2 O 3 , CaCO 3 , Cr 2 O 3  or the like as a minor component. The varistor layer constituting the sintered body  11  is formed by, for example, baking ceramic sheets including these components, whereby the major component such as ZnO and some of the minor components are sintered to form a solid solution, and the remaining minor components deposit on grain boundaries between the major component and the minor components. 
     More specifically, ceramic sheets each including the components described above are stacked on each other to obtain a laminate, which is then cut perpendicularly to a lamination surface of the laminate to obtain a piece, and the piece is baked, thereby producing the sintered body  11 . The sintered body  11  thus produced includes, for example, a shape having a pair of main surfaces opposite to each other, a pair of side surfaces opposite to each other, and a pair of end surfaces opposite to each other. The “main surfaces” are lamination surfaces, and of two types of cut surfaces, surfaces having larger areas are the “side surfaces”, and surfaces having smaller areas are the “end surfaces”. The shape of the sintered body  11  is, for example, a rectangular parallelepiped having two each of these surfaces, that is, a total of six surfaces. 
     [Internal Electrode] 
     The internal electrodes  12  are provided in the sintered body  11 . The internal electrodes  12  include, for example, Ag, Pd, PdAg, or PtAg, and are usually formed by stacking, on each other, ceramic sheets provided with the internal electrode paste and by baking the ceramic sheets. 
     [High-Resistance Layer] 
     The high-resistance layer  13  has higher resistance than the sintered body  11 . The high-resistance layer  13  is provided to cover at least part of the sintered body  11 . The surface shape, such as the Ra of the surface, of the high-resistance layer  13  is controllable by appropriately selecting, for example, a later-described method of forming the high-resistance layer  13  and a later-described method of producing the sintered body  11 . 
     The Ra of the surface of the high-resistance layer  13  is greater than or equal to 0.06 μm. This enables the occurrence of the migration at the surface of the high-resistance layer  13  to be suppressed. When the Ra is smaller than the value described above, the creepage distance between the external electrodes  14  is shortened, and migration is thus likely to occur. The Ra is preferably greater than or equal to 0.08 μm, more preferably greater than or equal to 0.15 μm, and much more preferably greater than or equal to 0.25 μm. The Ra is preferably less than or equal to 0.9 μm. In this case, exposed portions of the sintered body  11  could be considered to further decrease, so that the occurrence of the migration can be further suppressed. When the Ra exceeds the value described above, the exposed portions of the sintered body  11  further increase, and plating is likely to deposit, and thus, the occurrence of the migration may not be suppressed in some cases. Further, a flux component of solder may accumulate on the surface. The Ra is more preferably less than or equal to 0.7 μm, and much more preferably less than or equal to 0.4 μm. The Ra of the surface of the high-resistance layer  13  is measurable in accordance with, for example, a method defined by JIS-B0601: (2013), and specifically, is measurable by using Surfcorder (ET4000A manufactured by Kosaka Laboratory Ltd.) which is a highly accurate microfigure measuring instrument. The Ra may also be measured by, for example, a scanning probe microscope or a non-contact laser microscope. 
     In the high-resistance layer  13  formed on the sintered body  11  produced as described above and in the shape of a rectangular parallelepiped, the Ra of each of the lamination surfaces (main surfaces) can be greater than or equal to 0.06 μm and less than or equal to 0.85 μm, and the Ra of each of the cut surfaces (the side surfaces and the end surfaces) can be greater than or equal to 0.11 μm less than or equal to 0.9 μm, and thus, the Ra of each of the lamination surfaces and the cut surfaces are controllable. In such a multilayer varistor  1 , the Ra of the surface of the side surface high-resistance layer  13   b  can be greater than the Ra of the surface of the main surface high-resistance layer  13   a , which consequently enables the occurrence of the migration caused by the movement of ions and the like at the side surfaces to be further suppressed. In particular, the occurrence of the migration is more effectively suppressed in a multilayer varistor  1  in which external electrodes  14  are provided on the side surfaces in addition to on the end surfaces and the distance between each of the external electrodes  14  on the end surfaces and each of the external electrodes on the side surfaces is therefore shorter than the distance between the pair of external electrodes  14  on the end surfaces. 
     The high-resistance layer  13  preferably has a raised portion. The “raised portion” is a portion which is part of the high-resistance layer  13  and which has a thickness greater than 1 μm. Since the high-resistance layer  13  has the raised portion, the creepage distance between the external electrodes  14  is further increased, which enables the occurrence of the migration to be further suppressed. 
     When the high-resistance layer  13  has a plurality of raised portions, the average length of major axes of the raised portions is preferably greater than or equal to 10 μm and less than or equal to 50 μm. Setting the average length of the major axes of the raised portions to be within the range described above can further increase the creepage distance between the external electrodes  14 , which consequently enables the occurrence of the migration to be further suppressed. The average length of the major axes is more preferably greater than or equal to 15 μm and less than or equal to 45 μm, and much more preferably greater than or equal to 20 μm and less than or equal to 40 μm. The “the major axis” of a raised portion refers to the longest length of the shape of the raised portion, which is part of the high-resistance layer  13  and which has a thickness greater than 1 μm, in plan view. The “average length of the major axes” refers to the arithmetic mean value of measured lengths of the major axes of a plurality of raised portions (e.g., any ten raised portions). The average length of the major axes of the elevated portions is measurable by observing an elemental mapping image by using the scanning probe microscope or an EPMA. 
     In the case of a plurality of raised portions, the total area of the plurality of raised portions is preferably greater than or equal to 5% and less than or equal to 30% of the whole area of the surfaces of the high-resistance layer  13 . Setting the total area of the plurality of raised portions to be within the range described above can form a roughened surface that cannot be controlled only by the production of the sintered body  11 , which consequently enables the occurrence of the migration to be further suppressed. The total area is more preferably greater than or equal to 7% and less than or equal to 27%, and much more preferably greater than or equal to 10% and less than or equal to 25%. The “whole area of the surfaces” of the high-resistance layer  13  is the sum of areas of exposed portions of the high-resistance layer  13 , the exposed portions not being covered with the external electrodes  14  and the like. The total area of the plurality of raised portions is measurable based on an observation image of the elemental mappings by using the EPMA. 
     A method preferably used to form an increased number of raised portions is spraying a later-described solution including the precursor of the high-resistance layer  13  when forming the high-resistance layer  13 . 
     The average thickness of the high-resistance layer  13  is preferably greater than or equal to 0.01 μm. In this case, exposed portions of the sintered body  11  could be considered to further decrease, so that the occurrence of the migration can be further suppressed. The average thickness is more preferably greater than or equal to 0.05 μm, and much more preferably greater than or equal to 0.1 μm. The average thickness of the high-resistance layer  13  is preferably less than or equal to 5 μm. In this case, the high-resistance layer  13  could be considered to further suppress ions or the like from moving, so that the occurrence of the migration can be further suppressed. The average thickness is more preferably less than or equal to 3 μm, and much more preferably less than or equal to 1 μm. The “average thickness” refers to an arithmetic average value of the thicknesses of a high-resistance layer  13  which are measured at a plurality of points (for example, any ten points) of the high-resistance layer  13 . 
     [External Electrode] 
     The external electrodes  14  are disposed to cover part of the high-resistance layer  13 . The external electrodes  14  are electrically connected to the internal electrodes  12 . 
     Each of the external electrodes  14  includes, for example, the primary electrode  15  and the plating electrodes  16 . A secondary electrode may be further provided on the primary electrode  15 . The secondary electrode is preferably formed to cover the primary electrode  15 . As described above, each of the external electrodes  14  (the first external electrode  14 A and the second external electrode  14 B) may have a multi-layer configuration. 
     (Primary Electrode) 
     The primary electrodes  15  are provided to cover part of the high-resistance layer  13  and to be electrically connected to the internal electrodes  12 . The primary electrodes  15 , for example, include a metallic component such as Ag, AgPd, or AgPt, and a glass component such as Bi 2 O 3 , SiO 2 , or B 2 O 5 . The primary electrodes  15  preferably include metal as a major component, and more preferably include silver as the major component. When the primary electrodes  15  include silver as the major component, the migration easily occurs in the multilayer varistor, but the present disclosure enables the occurrence of the migration to be suppressed, and therefore, the present disclosure provides enhanced benefits. The primary electrodes  15  are usually formed by applying a primary electrode paste to the part of the high-resistance layer  13 . 
     (Plating Electrode) 
     The plating electrodes  16  are provided to cover at least part of the primary electrodes  15 . Each plating electrode  16  includes: a Ni electrode provided, for example, such that the Ni electrode covers at least part of the primary electrode or the secondary electrode; and a Sn electrode provided to cover at least part of the Ni electrode. 
     &lt;Method of Manufacturing Multilayer Varistor&gt; 
     The method of manufacturing the multilayer varistor according to the present embodiment includes a first step, a second step, a third step, and a fourth step. Each of the steps will be described below. 
     [First Step] 
     The first step includes preparing the sintered body  11  including ZnO as a major component, the internal electrodes  12  being disposed in the sintered body  11 . 
     The sintered body  11  may be produced by applying the internal electrode paste to ceramic sheets produced by adopting a slurry including ZnO, stacking the ceramic sheets on each other, pressing the ceramic sheets, and cutting the ceramic sheets, and then, debindering and baking the ceramic sheets. The slurry can be prepared, for example, by mixing ZnO which is a main raw material, and Bi 2 O 3 , Co 2 O 3 , MnO 2 , Sb 2 O 3 , Pr 6 O 11 , Co 2 O 3 , CaCO 3 , Cr 2 O 3 , or the like as a minor raw material, and a binder together. 
     Examples of the internal electrode paste include a Ag-paste, a Pd-paste, a Pt-paste, a PdAg paste, and a PtAg paste. 
     A temperature at which the debindering is performed is, for example, higher than or equal to 300° C. and lower than or equal to 500° C. A temperature at which the sintering is performed is appropriately adjustable depending on the configuration, composition, and the like of the sintered body  11  to be obtained, and is, for example, higher than or equal to 800° C. and lower than or equal to 1300° C. 
     The first step includes, for example, a coating step, an internal electrode application step, a lamination step, a cutting step, and a baking step. In the coating step, ceramic sheets including ZnO as a major component are produced. In the internal electrode application step, the internal electrode paste is applied to surfaces of some of the ceramic sheets. Examples of an application method in the internal electrode application step include printing. The lamination step includes stacking, on each other, the ceramic sheets provided with the internal electrode paste and the ceramic sheets not provided with the internal electrode paste to obtain a laminate. The cutting step includes cutting the laminate to obtain a laminate body having lamination surfaces and cut surfaces. In the baking step, the laminate body is baked to obtain a sintered body having lamination surfaces and cut surfaces. 
     Such a method can produce a sintered body  11  having a pair of main surfaces opposite to each other, a pair of side surfaces opposite to each other, and a pair of end surfaces opposite to each other. In the sintered body  11  thus produced, the Ra of each of the cut surfaces can be greater than the Ra of each of the lamination surfaces. 
     [Second Step] 
     In the second step, the high-resistance layer  13  is formed to cover at least part of the sintered body  11  after the first step. 
     Examples of the method of forming the high-resistance layer  13  include a method (i) of applying a solution including a precursor of the high-resistance layer  13  to the sintered body  11 , a method (ii) of reacting SiO 2  with the sintered body  11  including ZnO as a major component, and a method (iii) of thermally diffusing alkali-metal into the sintered body  11 . 
     The method (i) includes applying the solution including the precursor of the high-resistance layer  13  to the sintered body  11  and then performing dehydration and curing, thereby forming the high-resistance layer  13  on the surface of the sintered body  11 . Examples of the precursor of the high-resistance layer  13  include a glass component such as polysilazane having Si in its main chain. Using the glass component such as polysilazane having Si in its main chain as the precursor of the high-resistance layer  13  enables a continuous high-resistance layer  13  including SiO 2  as a major component to be formed. Such a high-resistance layer  13  could be considered to further reduce the exposed portions of the sintered body  11 , which consequently enables the multilayer varistor  1 , in which the occurrence of the migration at the surface of the high-resistance layer  13  is further suppressed, to be manufactured. 
     Examples of the application method include spraying, immersion, and printing. Among these methods, the spraying is a desirable method because the spraying can form the high-resistance layer  13  having an increased number of raised portions each having a thickness of greater than 1 μm, and can thus further increase the Ra of the surface of the high-resistance layer  13 . The spraying in this case is preferably performed on a plurality of sintered bodies  11  mixed by being stirred. 
     The method (ii) includes reacting the sintered body  11  including ZnO as the major component with SiO 2  to change a surface region of the sintered body  11  to a high-resistance layer  13  including Zn 2 SiO 4  as a major component, thereby forming the high-resistance layer  13 . Specifically, this method can be performed by, for example, bonding powder or a liquid including SiO 2  to the sintered body  11  including ZnO as the major component and then performing thermal treatment or the like. 
     The method (iii) includes thermally diffusing alkali metal into the sintered body  11  to change the surface region of the sintered body  11  to the high-resistance layer  13 , thereby forming the high-resistance layer  13 . This method can specifically be performed by, for example, mixing the sintered body  11  with a liquid including alkali metal powder or alkali metallic salt as a major component, and then performing thermal baking. 
     The second step preferably includes a spraying step and a thermal treatment step in a manner similar to the method (i). The spraying step includes spraying a solution including a precursor of the high-resistance layer  13  onto a plurality of the sintered bodies  11  while stirring the sintered bodies  11 . The thermal treatment step includes thermally treating each of the sintered bodies  11  provided with the precursor to form the high-resistance layer  13 . This method enables the high-resistance layer  13  having many raised portions to be formed, which consequently enables the multilayer varistor  1 , in which the occurrence of the migration is suppressed, to be manufactured. 
     The Ra of the surface of the high-resistance layer  13  after the second step is preferably greater than the Ra of the surface of the sintered body  11  after the first step. Appropriately selecting the method of forming the high-resistance layer  13  enables the Ra of the surface of the high-resistance layer  13  to be increased, which consequently enables the multilayer varistor  1 , in which the occurrence of the migration is further suppressed, to be manufactured. 
     The average thickness of the high-resistance layer  13  after the second step is preferably greater than the Ra of the surface of the sintered body  11  after the first step. In this case, exposed portions of the sintered body  11  could be considered to further decrease, so that the occurrence of the migration can be further suppressed. When the average thickness of the high-resistance layer  13  is less than the Ra of the sintered body  11 , part of the sintered body  11  of the multilayer varistor  1  is exposed, and plating deposition and/or the migration are/is likely to occur. Further, the Ra of the surface of the high-resistance layer  13  after the second step is preferably greater than or equal to 0.06 μm and less than or equal to 0.9 μm. 
     Furthermore, the Ra of the surface of the high-resistance layer  13  after the second step is controllable by, for example, a method of performing surface polishing by a rotating pot containing polishing powder, a method adopting blasting, or the like. The Ra of the surface of the sintered body  11  after the first step is controllable by, for example, a method of performing dissolution treatment on the surface of the sintered body  11  by acid treatment. The dissolution treatment causes elution of some particles of the sintered body  11  and formation of grain boundaries, thereby increasing the Ra of the surface of the sintered body  11 , and therefore, adopting the sintered body  11  enables the surface of the high-resistance layer  13  to have an increased Ra after the second step. 
     [Third Step] 
     The third step includes applying the primary electrode paste such that the primary electrode paste covers part of the high-resistance layer  13  and comes into contact with part of the internal electrodes  12 . 
     The primary electrode paste can be prepared by mixing a metallic component including, for example, Ag powder, AgPd powder, AgPt powder, or the like, a glass component including Bi 2 O 3 , SiO 2 , B 2 O 5 , or the like, and a solvent together. As the primary electrode paste, a paste including Ag as a major component and including a resin component may be used. After the application of the primary electrode paste, baking is performed at a temperature higher than or equal to 700° C. and lower than or equal to 800° C., thereby promoting alloying with the internal electrodes  12 , and the primary electrodes  15  having improved adhesion can be formed. 
     [Fourth Step] 
     The fourth step includes forming plating electrodes such that the plating electrodes cover at least part of the primary electrodes  15  formed from the primary electrode paste. A method of forming the plating electrodes is, for example, sequentially performing Ni plating and Sn plating by an electrolytic plating method. 
     Examples 
     The present disclosure will be more specifically described below with reference to examples, but the present disclosure is not limited to the examples below. 
     &lt;Manufacturing of Multilayer Varistor&gt; 
     Multilayer varistors of Examples 1 and 2 and Comparative Example 1 were manufactured by the following steps. 
     (Preparation of Slurry) 
     ZnO which is a major component, Pr 6 O 11 , Co 2 O 3 , CaCO 3 , Cr 2 O 3 , or the like which is a minor component, and a binder were mixed together, thereby preparing a slurry. 
     (Production of Ceramic Sheet) 
     The slurry thus prepared was molded into a predetermined thickness of greater than or equal to 20 μm and less than or equal to 50 μm, thereby producing ceramic sheets. 
     (Production of Laminate Body) 
     As the internal electrode paste, a Pd paste was used. The internal electrode paste was printed, in a predetermined shape, onto some of the ceramic sheets thus produced. The ceramic sheets on which the internal electrode paste was printed and the ceramic sheets on which the internal electrode paste was not printed were stacked on each other, thereby obtaining a laminate having a predetermined electrode structure. The laminate thus obtained was pressed to a predetermined thickness and was then cut into a length of 1.0 mm, a width of 0.5 mm, and a height 0.5 mm, thereby producing a laminate body. 
     (Production of Sintered Body) 
     The laminate body thus produced was debindered at a temperature of higher than or equal to 300° C. and lower than or equal to 500° C. and was then baked at a temperature of higher than or equal to 800° C. and lower than or equal to 1300° C., thereby producing a sintered body. 
     (Formation of High-Resistance Layer) 
     Onto the sintered body thus produced, a coating liquid containing polysilazane was sprayed by using a spray, and then the precursor adhered to the sintered body was cured at a temperature of higher than or equal to 400° C. and lower than or equal to 600° C., thereby forming a high-resistance layer. 
     (Formation of Primary Electrode) 
     Ag powder, glass frit, and a solvent were mixed together, thereby preparing a primary electrode paste. The primary electrode paste was applied to the end surfaces of the sintered body provided with the high-resistance layer and was then baked at 800° C., thereby forming primary electrodes. 
     (Formation of Plated Electrode) 
     On the primary electrodes thus formed, Ni plating electrodes having a predetermined thickness were formed by electrolytic plating, and then, Sn plating electrodes were formed on the Ni plating electrodes. 
     Conditions of the concentration, the rate of spraying, and the like of the coating liquid in forming the high-resistance layer were selected, thereby producing a multilayer varistor of Example 1 in which the arithmetic mean roughness Ra of the surface of the high-resistance layer is 0.3 μm and a multilayer varistor of Example 2 in which the arithmetic mean roughness Ra of the surface of the high-resistance layer is 0.09 μm. In addition, the sintered body was immersed in a coating liquid, and then, the coating liquid was cured, thereby producing a multilayer varistor of Comparative Example 1 in which the Ra is 0.03 μm. 
     &lt;Evaluation&gt; 
     The multilayer varistors thus produced were evaluated for the occurrence of migration by a wet load test under the following conditions. 
     (Conditions)
         Temperature: 85° C., relative humidity: 85% RH, load voltage: 18 V, test time period 1000 h       

     (Migration Evaluation)
         After the wet load test, the deposition of Ag on the surface of the high-resistance layer, i.e., whether or not the migration occurs was observed by visual observation and elemental analysis.       

     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Arithmetic 
                   
                 The 
                   
               
               
                   
                 Mean 
                   
                 Number of 
                 Percent- 
               
               
                   
                 Roughness 
                 The 
                 Varistors 
                 age of 
               
               
                   
                 of Surface of 
                 Number of 
                 in which 
                 Occur- 
               
               
                   
                 High-Resistance 
                 Tested 
                 Migration 
                 rence of 
               
               
                   
                 Layer (Ra) 
                 Varistors 
                 Occurred 
                 Migration 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 1 
                 0.3 
                 μm 
                 10 
                 0 
                 0% 
               
               
                 Example 2 
                 0.09 
                 μm 
                 10 
                 0 
                 0% 
               
               
                 Comparative 
                 0.03 
                 μm 
                 10 
                 10 
                 100%  
               
               
                 Example 1 
               
               
                   
               
            
           
         
       
     
     Results in Table 1 shows that in the multilayer varistors of Example 1 and Example 2, the arithmetic mean roughnesses Ra are respectively 0.3 μm and 0.09 μm which are within the scope of the present disclosure, showing that the occurrence of the migration is suppressed. In addition, in the multilayer varistor of Comparative Example 1, the Ra of the surface of the high-resistance layer is 0.03 μm, which is outside the scope of the present disclosure, and the migration occurred. 
     (Summary) 
     As can be seen from the embodiment and Examples above, a multilayer varistor ( 1 ) of the first aspect includes: a sintered body ( 11 ); an internal electrode ( 12 ) in the sintered body ( 11 ); a high-resistance layer ( 13 ) covering at least part of the sintered body ( 11 ); and an external electrode ( 14 ) covering part of the high-resistance layer ( 13 ) and electrically connected to the internal electrode ( 12 ). An arithmetic mean roughness of a surface of the high-resistance layer ( 13 ) is greater than or equal to 0.06 μm. 
     According to the first aspect, an increase in the creepage distance between the external electrodes ( 14 ) and an increase in a distance that ions or the like have to move to cause the migration enable a migration barrier to be increased, which consequently enables the occurrence of the migration at the surface of the high-resistance layer ( 13 ) to be suppressed. 
     In a multilayer varistor ( 1 ) of the second aspect referring to the first aspect, the arithmetic mean roughness of the surface of the high-resistance layer ( 13 ) is less than or equal to 0.9 μm. 
     According to the second aspect, exposed portions of the sintered body ( 11 ) could be considered to further decrease, so that the occurrence of the migration is further suppressed. 
     In a multilayer varistor ( 1 ) of a third aspect referring to the first or second aspect, a mean thickness of the high-resistance layer ( 13 ) is greater than or equal to 0.01 μm and less than or equal to 5 μm. 
     According to the 3 aspect, exposed portions of the sintered body ( 11 ) could be considered to further decrease, and the high-resistance layer ( 13 ) could be considered to further suppress the movement of ions and the like, so that the occurrence of the migration is further suppressed. 
     In a multilayer varistor ( 1 ) of a fourth aspect referring to any one of the first to third aspects, the high-resistance layer ( 13 ) has a plurality of raised portions each having a thickness of greater than 1 μm, and an average length of major axes of the plurality of raised portions is greater than or equal to 10 μm and less than or equal to 50 μm. 
     According to the fourth aspect, setting average length of the major axes of the plurality of raised portions to a specific range further increases the creepage distance between the external electrodes ( 14 ), which consequently enables the occurrence of the migration to be further suppressed. 
     In a multilayer varistor ( 1 ) of a fifth aspect referring to the fourth aspect, a total area of the plurality of raised portions is greater than or equal to 5% and less than or equal to 30% of a whole area of the surface of the high-resistance layer ( 13 ). 
     According to the fifth aspect, setting the total area of the plurality of raised portions to a specific range enables a roughened surface, which cannot be controlled only by the production of the sintered body ( 11 ), to be formed, it is possible to form a roughened surface, which consequently enables the occurrence of the migration to be further suppressed. 
     In a multilayer varistor ( 1 ) of a sixth aspect referring to any one of the first to fifth aspects, the external electrode ( 14 ) includes a primary electrode ( 15 ) covering part of the high-resistance layer ( 13 ) and a plating electrode ( 16 ) covering at least part of the primary electrode ( 15 ). The primary electrode ( 15 ) includes silver as a major component. 
     When the primary electrode ( 15 ) includes silver as the major component, the migration easily occurs in the multilayer varistor, but according to the sixth aspect, the present disclosure enables the occurrence of the migration to be suppressed, and therefore, the present disclosure provides enhanced benefits. 
     In a multilayer varistor ( 1 ) of a seventh aspect referring to any one of the first to sixth aspects, the sintered body ( 11 ) has a pair of main surfaces opposite to each other, a pair of side surfaces opposite to each other, and a pair of end surfaces opposite to each other. The internal electrode ( 12 ) faces the main surfaces. The external electrode ( 14 ) covers one of the end surfaces. the arithmetic mean roughness of the surface of the high-resistance layer ( 13 ) is greater on the side surfaces than on the main surfaces. 
     The seventh aspect enables the migration to be further suppressed from being caused by the movement of ions and the like at the side surfaces and is particularly effective in suppressing the occurrence of the migration in the multilayer varistor ( 1 ) having the external electrodes ( 14 ) on the end surface and the side surface and a reduced distance between the external electrodes ( 14 ). 
     In a multilayer varistor ( 1 ) of an eighth aspect referring to any one of the first to seventh aspects, the high-resistance layer ( 13 ) includes SiO 2  as a major component. 
     According to the eighth aspect, the high-resistance layer ( 13 ) includes SiO 2  as a major component, and therefore, the high-resistance layer ( 13 ) can be a continuous high-resistance layer ( 13 ), and such a high-resistance layer ( 13 ) could be considered to further reduce the exposed portions of the sintered body ( 11 ), so that the occurrence of the migration is further suppressed. 
     A multilayer varistor ( 1 ) of the ninth aspect includes: a sintered body ( 11 ) having a pair of main surfaces opposite to each other, a pair of side surfaces opposite to each other, and a pair of end surfaces opposite to each other; an internal electrode disposed in the sintered body ( 11 ), the internal electrode facing the main surfaces; a high-resistance layer ( 13 ) covering at least part of the sintered body ( 11 ); and an external electrode ( 14 ) covering part of the high-resistance layer ( 13 ), the external electrode ( 14 ) being electrically connected to the internal electrode ( 12 ). An arithmetic mean roughness of a surface of the high-resistance layer ( 13 ) is greater on the side surfaces than on the main surfaces. 
     The ninth aspect enables the Ra of the surface of the side surface high-resistance layer ( 13   b ) to be increased to be greater than the Ra of the surface of the main surface high-resistance layer ( 13   a ). Increasing the Ra of the surface of the side surface high-resistance layer ( 13   b ) could be considered to suppress the occurrence of the migration in the multilayer varistor  1 . 
     A method of manufacturing a multilayer varistor ( 1 ) of a tenth aspect includes a first step, a second step, a third step, and a fourth step. The first step includes preparing a sintered body ( 11 ) including ZnO as a major component, an internal electrode ( 12 ) being disposed in the sintered body ( 11 ). The second step includes forming a high-resistance layer ( 13 ) covering at least part of the sintered body ( 11 ). The third step includes applying a primary electrode paste such that the primary electrode paste covers part of the high-resistance layer ( 13 ) and is in contact with part of the internal electrode ( 12 ). The fourth step includes forming a plating electrode ( 16 ) covering at least part of a primary electrode ( 15 ) formed from the primary electrode paste. An arithmetic mean roughness of a surface of the high-resistance layer ( 13 ) after the second step is greater than or equal to 0.06 μm and less than or equal to 0.9 μm. 
     The tenth aspect enables the arithmetic mean roughness of the surface of the high-resistance layer ( 13 ) to be set to a predetermined range, which consequently enables the multilayer varistor ( 1 ), in which the occurrence of the migration is suppressed, to be manufactured. 
     In the manufacturing process of A multilayer varistor ( 1 ) of the eleventh aspect, in the tenth aspect, the arithmetic mean roughness of the surface of the high-resistance layer ( 13 ) after the second step is greater than an arithmetic mean roughness of a surface of the sintered body ( 11 ) after the first step. 
     According to the eleventh aspect, appropriately selecting the method of forming the high-resistance layer ( 13 ) enables the surface of the high-resistance layer ( 13 ) to have an increased Ra, which consequently enables the multilayer varistor ( 1 ), in which the occurrence of the migration is further suppressed, to be manufactured. 
     In a method of manufacturing a multilayer varistor ( 1 ) of a twelfth aspect referring to the tenth or eleventh aspect, the second step includes spraying, while stirring a plurality of the sintered bodies ( 11 ), a solution including a precursor of the high-resistance layer ( 13 ) onto the sintered bodies ( 11 ), and thermally treating each of the sintered bodies ( 11 ) provided with the precursor to form the high-resistance layer ( 13 ). 
     According to the twelfth aspect, the high-resistance layer ( 13 ) formed by such a method has an increased number of raised portions. Therefore, the twelfth aspect enables the arithmetic mean roughness of the surface of the high-resistance layer ( 13 ) to be increased, which consequently enables the multilayer varistor ( 1 ), in which the occurrence of the migration is further suppressed, to be manufactured. 
     In the method of manufacturing the multilayer varistor ( 1 ) of a thirteenth aspect referring to the twelfth aspect, the solution contains polysilazane. 
     According to the thirteenth aspect, using polysilazane, which is a glass component having Si in its main chain, as the precursor of the high-resistance layer ( 13 ) enables a continuous high-resistance layer ( 13 ) including SiO 2  as a major component to be formed. Such a high-resistance layer ( 13 ) could be considered to further reduce the exposed portions of the sintered body ( 11 ), so that the multilayer varistor ( 1 ), in which the occurrence of the migration is further suppressed, is manufacturable. 
     In a method of manufacturing the multilayer varistor ( 1 ) of a fourteenth aspect referring to any one of the tenth to thirteenth aspects, the first step includes a coating step, an internal electrode application step, a lamination step, a cutting step, and a baking step. The coating step includes producing ceramic sheets including ZnO as a major component. The internal electrode application step includes applying an internal electrode paste to some of the ceramic sheets. The lamination step includes stacking, on each other, the ceramic sheets provided with the internal electrode paste and the ceramic sheets not provided with the internal electrode paste to obtain a laminate. The cutting step includes cutting the laminate to obtain a laminate body having a lamination surface and a cut surface. The firing step includes baking the laminate body to obtain the sintered body ( 11 ) having a lamination surface and a cut surface. An arithmetic mean roughness of the cut surface of the sintered body ( 11 ) is greater than an arithmetic mean roughness of the lamination surface of the sintered body ( 11 ). 
     According to the fourteenth aspect, the sintered body ( 11 ) having the pair of main surfaces opposite to each other, the pair of side surfaces opposite to each other, and the pair of end surfaces opposite to each other is produced. In this sintered body ( 11 ), the Ra of the cut surface is greater than the Ra of the lamination surface. Thus, forming the high-resistance layer ( 13 ) on the sintered body ( 11 ) to produce the multilayer varistor ( 1 ) enables the Ra of the surface of the side surface high-resistance layer ( 13   b ) to be greater than the Ra of the surface of the main surface high-resistance layer ( 13   a ). This consequently enables the occurrence of the migration caused by the movement of ions and the like at the side surfaces to be further suppressed. 
     A method of manufacturing the multilayer varistor ( 1 ) of a fifteenth aspect includes a first step, a second step, a third step, and a fourth step. The first step includes preparing a sintered body ( 11 ) including ZnO as a major component, an internal electrode ( 12 ) being disposed in the sintered body ( 11 ). The second step includes forming a high-resistance layer ( 13 ) covering at least part of the sintered body ( 11 ). The third step includes applying a primary electrode paste such that the primary electrode paste covers part of the high-resistance layer ( 13 ) and is in contact with part of the internal electrode ( 12 ). The fourth step includes forming a plating electrode ( 16 ) covering at least part of a primary electrode ( 15 ) formed from the primary electrode paste. The second step includes spraying a solution including a precursor of the high-resistance layer ( 13 ) onto a plurality of the sintered bodies ( 11 ) while stirring the sintered bodies ( 11 ), and thermally treating each of the sintered bodies ( 11 ) provided with the precursor to form the high-resistance layer ( 13 ). 
     According to the fifteenth aspect, the high-resistance layer ( 13 ) having a large number of raised portions is formable, and therefore, the fifteenth aspect enables the Ra of the high-resistance layer ( 13 ) to be increased, which consequently enables the multilayer varistor ( 1 ), in which the occurrence of the migration is suppressed, to be manufactured. 
     In a method of manufacturing the multilayer varistor ( 1 ) of the sixteenth aspect referring to the fifteenth aspect, the solution contains polysilazane. 
     According to the sixteenth aspect, using polysilazane, which is a glass component having Si in its main chain, as the precursor of the high-resistance layer ( 13 ) enables a continuous high-resistance layer ( 13 ) including SiO 2  as a major component to be formed. Such a high-resistance layer ( 13 ) could be considered to further reduce the exposed portions of the sintered body ( 11 ), so that the multilayer varistor ( 1 ), in which the occurrence of the migration is further suppressed, is manufacturable. 
     While the foregoing has described what are considered to be the best mode and/or other examples, it is understood that various modifications may be made therein and that the subject matter disclosed herein may be implemented in various forms and examples, and that they may be applied in numerous applications, only some of which have been described herein. It is intended by the following claims to claim any and all modifications and variations that fall within the true scope of the present teachings.