JP3878929B2 - Varistor and varistor manufacturing method - Google Patents

Description

  The present invention relates to a varistor and a varistor manufacturing method.

Conventionally, ZnO-based varistors are known in which a dielectric containing ZnO as a main component is sandwiched between a pair of electrodes in order to absorb and suppress surge in an electronic circuit (see Patent Document 1).
JP-A-6-13260

  In recent years, it has been necessary to use such ZnO-based varistors in electronic circuits that transmit high-frequency signals such as 240 MHz signals of USB 2.0, for example, as communication speeds in communication devices increase. is there. In an electronic circuit to which such a high-frequency signal is transmitted, it is required to increase the impedance of the varistor by reducing the capacitance of the ZnO-based varistor so as to reduce the deterioration of the waveform with respect to the high-frequency signal. Yes.

  However, in conventional ZnO-based varistors, even if the area of the opposing electrode is simply reduced to reduce the impedance, the current density per unit area of the electrode is increased, and the resistance to electrostatic discharge (ESD) (ESD) (ESD tolerance) decreases, which is not preferable.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a ZnO-based varistor having a high ESD tolerance and less waveform deterioration with respect to a high-frequency signal, and a varistor manufacturing method.

  As a result of intensive studies, the inventors of the present invention have found that a varistor having a ceramic composition including a first crystal grain mainly composed of ZnO and a second crystal grain mainly composed of an oxide containing Zn, Co, and Si. Has found that the capacitance decreases as the frequency increases, and the present invention has been conceived.

  A varistor according to the present invention sandwiches a porcelain composition including a first crystal grain mainly composed of ZnO and a second crystal grain mainly composed of an oxide containing Zn, Co, and Si, and the porcelain composition. And at least a pair of electrode plates.

  A ZnO-based varistor having such a porcelain composition has a lower capacitance as the frequency increases. For this reason, even if the electrode area is set to the same level as the conventional one and the ESD tolerance is maintained, the electrostatic capacity for the high-frequency signal is lower than the conventional one, so that the impedance for the high-frequency signal can be increased.

  Here, the first crystal grains contain Co and Al, and it is preferable that an oxide containing Pr, K, Cr, Ca and Si exists at the grain boundary between the first crystal grains. .

  Co forms an interface state at the grain boundary of the ZnO-based first crystal grains, and greatly contributes to the expression of voltage nonlinearity. Moreover, Al suitably acts as a semiconducting agent for the ZnO-based first crystal grains. Also, when an oxide containing Pr, K, Cr, Ca and Si is present at the grain boundaries between the first crystal grains, the oxygen diffusion rate into the crystal grain boundaries of the ceramic composition during sintering is fast. High sinterability. In addition, it becomes easy to control the sintering to increase the uniformity of crystal grain size.

Here, as a suitable composition of the oxide of the second crystal grains for sufficiently reducing the capacitance of the varistor when the frequency is increased, (Zn _1-x Co _x ) ₂ SiO ₄ is Can be mentioned. Here, 0 <x <1. In particular, (Zn _0.9 Co _0.1 ) ₂ SiO ₄ where x = 0.1 is preferable.

  Moreover, it is preferable that the volume fraction of the 2nd crystal grain with respect to the total volume which put together the 1st crystal grain and the said 2nd crystal grain is 5% or more and less than 60%.

  According to this, it is preferable that the nonlinear constant α can be increased while having the property that the electrostatic capacity is sufficiently lowered when the frequency is increased. Here, when the volume fraction of the second crystal grains is 5% or less, there is a tendency that the electrostatic capacity is not sufficiently lowered even if the frequency is increased. On the other hand, when the volume fraction of the second crystal grains is 60% or more, the nonlinear constant tends to be small and it becomes difficult to function as a varistor.

  A plurality of element bodies including the porcelain composition and a pair of electrode plates are preferably laminated, and such a laminated varistor can be miniaturized.

  The varistor according to the present invention is a varistor comprising a porcelain composition and a pair of electrode plates sandwiching the porcelain composition from both sides, and the capacitance at a frequency of 1 MHz is 90% or less of the capacitance at a frequency of 1 kHz. It is.

  Since such a varistor has a lower capacitance as the frequency is higher, it is possible to increase the impedance to a high-frequency signal while maintaining the ESD tolerance by making the electrode area the same as the conventional one.

The method for producing a varistor according to the present invention includes a step of forming a mixture of a powder containing an oxide containing Zn, Co and Si as a main component and a powder containing Zn into a formed body, Firing the body to obtain a ceramic composition having first crystal grains mainly composed of ZnO and second crystal grains mainly composed of oxides containing Zn, Co and Si.

  According to this, the above-mentioned varistor can be suitably manufactured.

  According to the present invention, it is possible to provide a varistor having high ESD tolerance and less waveform deterioration with respect to a high-frequency signal.

  Hereinafter, preferred embodiments of a varistor according to the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same or corresponding elements are denoted by the same reference numerals, and redundant description is omitted.

(First embodiment)
FIG. 1 is a schematic cross-sectional view illustrating a varistor 2 according to this embodiment. The varistor 2 includes a voltage nonlinear resistor layer 10 made of a porcelain composition, a pair of flat internal electrode layers (electrode plates) 4 and 4 formed in the voltage nonlinear resistor layer 10, and And external electrodes 12 and 14 respectively formed on both end faces of the voltage nonlinear resistor layer 10.

  The voltage non-linear resistance layer 10 is formed in a substantially rectangular parallelepiped shape. Details of the porcelain composition of the voltage nonlinear resistor layer 10 will be described later.

  The internal electrode layers 4 and 4 are conductive layers formed in the voltage non-linear resistor layer 10, and are opposed to each other in the thickness (up and down) direction of the voltage non-linear resistor layer 10. The central part 10c of the linear resistor layer 10 is sandwiched. Here, the central portion 10 c of the pair of internal electrode layers 4, 4 and the voltage nonlinear resistance layer 10 forms the varistor element body 50.

  One end (upper) of the internal electrode layer 4 reaches one end (left side) of the voltage nonlinear resistor layer 10 and is electrically connected to the external electrode 12. The other (lower) internal electrode layer 4 has the other end reaching the other (right) end face of the voltage nonlinear resistor layer 10 and is electrically connected to the external electrode 14. The internal electrode layers 4 and 4 are conductive materials, and for example, a metal material such as Pd can be used. Here, the area of the portion where the internal electrode layers 4 and 4 face each other is defined as an overlapping area S.

  The external electrodes 12 and 14 are respectively formed on the side surface 10a and the side surface 10b facing each other in the voltage nonlinear resistor layer 10. The external electrodes 12 and 14 are conductive materials, and for example, a metal material such as Ag can be used. Further, the external electrode 12 and the external electrode 14 are formed so as to protrude in the vertical direction from the upper surface 10c and the lower surface 10d of the voltage nonlinear resistor layer 10, respectively.

  As shown in FIG. 2, the voltage nonlinear resistor layer 10 includes a first crystal grain 100 mainly composed of ZnO and a second crystal grain mainly composed of an oxide containing Zn, Co, and Si. 120, and a porcelain composition as a polycrystalline body. The second crystal grains 120 are dispersed between the first crystal grains 100. Here, a boundary portion between the first crystal grain 100 and the first crystal grain 100 is defined as a grain boundary 130.

  The varistor 2 having such a voltage non-linear resistance layer 10 tends to have a lower capacitance as the frequency increases. Therefore, even if the overlapping area S of the electrodes is set so that the ESD tolerance comparable to that of the conventional case can be maintained, the capacitance at high frequency can be made sufficiently lower than that of the conventional ZnO-based varistor. For this reason, the varistor which can make the impedance with respect to a high frequency signal high, has high ESD tolerance, and has little waveform deterioration with respect to a high frequency signal is implement | achieved.

  Here, the ESD tolerance is a measure of the magnitude of static electricity that can be absorbed by the varistor, and can be measured, for example, by an electrostatic discharge immunity test defined in IEC (International Electrotechnical Commission) standard IEC61000-4-2.

  In the varistor 2 having the voltage nonlinear resistor layer 10 configured as in the present embodiment, the reason why the capacitance decreases as the frequency increases can be considered as follows, for example.

  First, as shown in FIG. 3, the varistor is considered that the first crystal grain 100, the second crystal grain 120, and the grain boundary 130 are connected in series, and the first crystal grain 100 is connected to the resistor R1 and the capacitor. The parallel circuit with C1, the second crystal grain 120 is assumed to be a parallel circuit with a resistor R2 and a capacitor C2, and the grain boundary 130 is assumed to be a parallel circuit with a resistor Rb and a capacitor Cb.

Then, in the equivalent circuit of the varistor as shown in FIG. 3, fitting was performed using “Spice” of the circuit simulator to obtain the capacitance change rate ΔC of the varistor.
Here, the resistance of the resistor R1 in the first crystal grain 100 containing ZnO as a main component is 1Ω, the capacitance of the capacitor C1 in the first crystal grain 100 is 0.1 pF, and the resistance of the resistor Rb in the grain boundary 130 Was fixed at 600 MΩ, and the capacitance of the capacitor Cb at the grain boundary 130 was 4 pF.
FIG. 4 shows the capacitance change rate ΔC of the varistor 2 when the resistance of the resistor R2 of the second crystal grain 120 is changed when the capacitance of the capacitor C2 of the second crystal grain 120 is 8 pF. FIG. FIG. 5 shows a capacitance change rate ΔC when the capacitance of the capacitor C2 of the second crystal grain 120 is changed when the resistance of the resistor R2 of the second crystal grain 120 is 60 kΩ. FIG.

The capacitance change rate ΔC is a value determined by ΔC = (CH−CL) / CL, where CH is the capacitance measured at a frequency of 1 MHz and CL is the capacitance measured at a frequency of 1 kHz. It was. The lower the capacitance change rate ΔC (negative value), the more the capacitance decreases when the frequency increases.
Then, in such a varistor 2, by setting the resistance of the resistor R2 in the second crystal grain 120 and the capacitance of the capacitor C2 in the second crystal grain 120 within a predetermined range, the varistor 2 increases as the frequency increases. It can be seen that the capacitance of can be reduced.
Specifically, in order to sufficiently reduce the capacitance change rate ΔC, the resistance of the second crystal grain resistor R2 and the capacitance of the capacitor C2 are set to 0.01 MΩ ≦ R2 ≦ 50 MΩ and 0 It is preferable that 1 ≦ C2 / Cb ≦ 10.

  And since the 2nd crystal grain 120 which is an oxide containing Zn, Co, and Si which concerns on this embodiment shows the resistance and electrostatic capacitance of the above-mentioned conditions, the varistor 2 which concerns on this embodiment becomes so that a frequency becomes high. It is considered that the capacitance tends to decrease sufficiently.

  Here, the first crystal grain 100 preferably contains Al atoms and Co atoms. The Co atom forms an interface state of the grain boundary 130 of the ZnO-based first crystal grain 100 and greatly contributes to the expression of the voltage nonlinearity of the voltage nonlinear resistance layer 10. In addition, Al atoms preferably act as a semiconducting agent for the ZnO-based first crystal grains 100. Note that the operation is possible even if at least one element selected from B, Ga, and In is included instead of Al.

  Moreover, it is preferable that an oxide containing Pr, K, Cr, Ca, and Si exists in the grain boundary 130 between the first crystal grains 100. When an oxide containing Pr, K, Cr, Ca and Si is present at the grain boundary 130, the oxygen diffusion rate to the crystal grain boundary of the voltage nonlinear resistance layer 10 can be increased, and the sinterability is good. Become. Moreover, since the sinterability at the time of sintering can be controlled suitably, the uniformity of the grain size of each crystal grain becomes high.

  It should be noted that the operation is performed even if at least one rare earth element selected from Y, La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu is included instead of Pr. Is possible. Further, the operation is possible even if at least one element selected from Na, Rb, and Cs is included instead of the K atom. Further, instead of Cr, operation is also possible with Mo atoms. Furthermore, instead of Ca, operation is possible with at least one element selected from Mg, Sr, and Ba.

Here, the composition of the second crystal grains 120 is preferably (Zn _1-x Co _x ) ₂ SiO ₄ (0 <x <1), and x = 0.1 (Zn _0.9 Co _{0.1 2} SiO ₄ is more preferred. At this time, the composition ratio of (Zn _1-x Co _x ): Si: O is preferably 2: 1: 4, but a slight deviation in the composition ratio may be acceptable.

  Moreover, it is preferable that the volume fraction of the 2nd crystal grain 120 with respect to the total volume which put together the 1st crystal grain 100 and the 2nd crystal grain 120 shall be 5% or more and less than 60%. This is preferable because the nonlinear constant α of the varistor 2 can be increased while the capacitance change rate ΔC is set to a sufficiently low value. Here, when the volume fraction of the second crystal grains 120 is 5% or less, the capacitance change rate ΔC tends not to be sufficiently low. On the other hand, when the volume fraction of the second crystal grain 120 is 60% or more, the nonlinear constant α tends to be small, and it becomes difficult to function as a varistor.

Note that the nonlinear constant α indicates α when the relationship between the voltage V and the current I when the voltage V is applied to the varistor 2 is approximated by I = (V / C) ^α . Here, C is a constant. If α is 1, it indicates an ohmic resistor, and the larger the value exceeds 1, the more the voltage non-linear characteristic that the current hardly flows below a predetermined voltage, that is, the varistor characteristic sufficiently appears. Indicates.

  Moreover, the suitable diameter of the 1st crystal grain 100 is about 2-20 micrometers, and the suitable diameter of the 2nd crystal grain 120 is 2-20 micrometers.

  Furthermore, when the internal electrode layers 4 and 4 contain palladium, it is preferable that the voltage non-linear resistance layer 10 does not contain bismuth. In this case, bismuth reacts with palladium to form a high resistance layer of palladium oxide, thereby preventing adverse effects on the varistor voltage and the nonlinear constant. Moreover, since sintering can be performed at a sufficiently high temperature, the voltage nonlinear resistor layer 10 and the electrode layer can be sufficiently sintered.

  Next, an example of a method for manufacturing such a varistor 2 will be described.

  First, a raw material slurry containing the raw material powder of the voltage nonlinear resistor layer 10 is prepared. Specifically, raw material powder containing atoms of Zn, Co, and Si is wet-mixed with a ball mill or the like to prepare a raw material slurry. Examples of the powder constituting such a raw material slurry include oxides, carbonates, carbonate hydrates and the like for each element, and oxides, carbonates, carbonate hydrates having a plurality of elements, etc. May be used.

  Here, in order to suitably form the first crystal grain 100 and the second crystal grain 120 after sintering, when the total number of moles of Zn, Co, Si is 100%, Each atomic fraction of Zn, Co, and Si is preferably about Zn: 20 to 98.95%, Co: 0.05 to 50%, and Si: about 1 to 30%.

  Further, the raw material powder in this raw material slurry preferably contains Al atoms from the above viewpoint, and preferably contains Pr, K, Cr and Ca atoms. Suitable content ratios of Al, Pr, K, Cr, and Ca are 0.0001 to 1%, 0.01 to 10%, and 0.005, respectively, where the number of moles of Zn atoms is 100%. -5%, 0-2%, 0.05-5%.

  The raw material powder may contain at least one element selected from B, Ga, In instead of Al atoms, and instead of Pr, Y, La, Ce, Nd, Sm, Eu, Gd , Tb, Dy, Ho, Er, Tm, Yb, Lu may be included, and at least one element selected from Na, Rb, Cs may be included instead of the K atom. In addition, it may contain Mo atoms instead of Cr, and may contain at least one element selected from Mg, Sr, and Ba instead of Ca, and any of them can operate as a varistor after sintering. .

  Further, in the wet mixing of the raw material powder in a ball mill or the like, it is preferable to add an organic binder such as ethyl cellulose or polyvinyl petital, and it is preferable to use an organic solvent such as acetone or toluene as a mixing solvent. Furthermore, it is preferable to add an organic plasticizer such as dibutyl phthalate.

  A zirconia ball etc. are mentioned as a ball | bowl in the case of mixing with a ball mill. The mixing time in the ball mill is preferably about 20 hours.

  Here, a powder having the same crystal composition as the first crystal grain 100 and a powder having the same crystal composition as the second crystal grain 120 are prepared in advance, and a raw material slurry is prepared using this powder. Also good.

For example, assuming a crystal composition of (Zn _0.9 Co _0.1 ) ₂ SiO ₄ as the second crystal grain 120, the powder having such a crystal composition has Zn: Co, Si atoms of 1.8: The raw material powder is weighed to contain 0.2: 1, mixed in a ball mill using zirconia balls or the like in a solvent such as water, dried, sintered at about 1200 ° C. for about 2 hours, Can be obtained by pulverizing in a ball mill using zirconia balls as a solvent for about 20 hours and drying. A powder having the same crystal composition as that of the first crystal grains 100 can be produced in the same manner.

  Subsequently, in order to form the internal electrode layers 4 and 4, a powder paste for internal electrodes such as palladium, which becomes a conductive material when fired, is prepared.

  And the laminated body which pinched | interposed the ceramic composition raw material powder layer with a pair of powder layer for internal electrodes is formed on a base material. Here, the porcelain composition raw material powder layer can be formed by applying a raw material slurry by a doctor blade method or the like. On the other hand, the internal electrode powder layer can be formed by laminating the internal electrode powder paste on the porcelain composition raw material powder layer by a printing method or the like. And this laminated body is baked, for example at about 1200 degreeC, and a sintered compact is obtained. And a laminated varistor 2 as shown in FIG. 1 can be manufactured by forming an external electrode on the side surface of the sintered body.

(Second embodiment)
Next, the varistor 202 according to the second embodiment will be described with reference to FIG. The varistor 202 of this embodiment is different from the varistor 2 according to the first embodiment in that a plurality of varistor element bodies 50 are laminated in the direction in which the internal electrode layers 4 of the varistor element bodies 50 are laminated. The lower electrode plate 4 of each varistor element body 50 also serves as the upper electrode plate 4 of the varistor element body 50 adjacent thereto below.

  In the laminated varistor 202 of the present embodiment, in addition to the same effects as described above, the varistor can be miniaturized by adopting a multilayer structure.

  Hereinafter, although the present invention is explained based on a more detailed example, the present invention is not limited to these.

Example 1
First, a powder for second crystal grains was prepared. That is, ZnO, CoO and SiO ₂ powders were weighed at a molar ratio of 18: 2: 10, mixed in a ball mill of ZrO ₂ media using water as a solvent for 20 hours, and dried in total. Furthermore, the dried mixture is molded to obtain a temporary molded body, and this temporary molded body is fired in the atmosphere at 1200 ° C. for 2 hours to form (Zn _0.9 Co _0.1 ) ₂ SiO ₄ crystals. A temporary sintered body was obtained. The pre-sintered body was pulverized in a ZrO ₂ media ball mill for 20 hours and dried to obtain a second crystal grain powder.

Subsequently, each powder of ZnO, Pr ₆ O ₁₁ , CoO, Al ₂ O ₃ , K ₂ CO ₃ , Cr ₂ O ₃ , CaCO ₃ , SiO ₂ has a molar ratio of each metal of Zn: 97.965%. Pr: 0.5%, Co: 1.5%, Al: 0.005%, K: 0.05%, Cr: 0.1%, Ca: 0.1%, Si: 0.005% The mixture powder for sintering was obtained by weighing. Then, the powder for second crystal grain, the organic binder, the organic solvent, and the organic plasticizer are added to the powder mixture for main sintering, and mixed in a ball mill of ZrO ₂ media for 20 hours. A raw slurry was obtained.

  Here, the mixing ratio of the powder for the second crystal grain and the mixed powder for the main sintering is set so that the volume fraction of the second crystal grain after sintering is 5%, as will be described later. did. The volume fraction was determined by preparing 10 × 10 meshes on an electron micrograph and counting which crystal grains exist at the intersections.

  Here, polyvinyl butyral was used as the organic binder, a mixture of ethanol and acetone was used as the organic solvent, and dibutyl phthalate was used as the organic plasticizer.

  Subsequently, this raw material slurry was applied to a thickness of 30 μm on a PET film using a doctor blade and dried to form a large number of green sheets. Further, a palladium paste is applied to a part of the green sheet by screen printing to form an electrode layer to be an internal electrode, and this is dried and peeled off from the PET film. Got. On the other hand, a plurality of green sheets having no electrode layer formed thereon were stacked to obtain a protective green sheet laminate. Then, two laminates described above are laminated on the protective green sheet laminate, and further, the protective green sheet laminate is laminated, and these are crimped to obtain a laminate having the structure of FIG. It was.

  Subsequently, such a laminate was heated and pressure-bonded, and then cut into a predetermined chip shape to obtain a green chip.

The green chip was heat-treated at 350 ° C. for 2 hours to remove the binder, and then sintered in the air at 1200 ° C. for 2 hours to form a pair of internal electrode layers 4 and 4 and a voltage nonlinear resistor layer. A varistor element body containing 10 was obtained. Subsequently, a paste containing silver was applied to both ends of the varistor element body and baked at 800 ° C. to form the external electrodes 12 and 14, thereby obtaining the varistor 2 according to the first embodiment. In the obtained varistor 2, the distance between the internal electrode layers 4 and 4 was 20 μm, and the overlapping area S by the internal electrode layers 4 and 4 was 0.05 mm ³ .

(Examples 2 to 6)
The mixing ratio of the powder for the second crystal grain and the mixed powder for main sintering is such that the volume fraction of the second crystal grain after sintering is 10%, 15%, 30%, 45%, 60%. The varistors of Examples 2 to 6 were obtained in the same manner as in Example 1 except that setting was made.

(Comparative Example 1)
A varistor of Comparative Example 1 was obtained in the same manner as in Example 1 except that the second crystal grain powder was not used.

  The varistor thus obtained was measured for nonlinear constant α and capacitance change rate ΔC.

また、静電容量変化率ΔＣは、前述の定義に基づいて測定した。ここで、１ｋＨｚ及び１ＭＨｚでの静電容量は、ＨＰ製の４２８４Ａ装置により測定した。測定結果を図６の表、及び、図７，図８のグラフに示す。 Here, the non-linear constant α was obtained using the equation (1). Here, V _{1 mA} is a voltage at a current of 1 _mA , and V _{0.1 mA} is a voltage at a current of 0.1 mA.

The capacitance change rate ΔC was measured based on the above definition. Here, the electrostatic capacity at 1 kHz and 1 MHz was measured by a 4284A apparatus manufactured by HP. The measurement results are shown in the table of FIG. 6 and the graphs of FIGS.

In Examples 1 to 6, the first crystal grains in the voltage nonlinear resistor layer are mainly composed of ZnO and further contain Co and Al, and the second crystal grains are (Zn _0.9 Co _0.1 ) _2. SiO ₄ was mainly contained, and the grain boundary contained oxides of Pr, K, Cr, Ca, and Si.

  In the varistor (Comparative Example 1) in which the voltage nonlinear resistor layer does not contain the second crystal grain, the capacitance change rate ΔC changes only by 3.0%, which is not sufficient. On the other hand, in the varistors of Examples 1 to 6, it has been clarified that the capacitance change rate ΔC can be set to a sufficiently low value, and the impedance to the high-frequency signal can be increased.

  In the varistor having the composition according to the present embodiment, when the volume fraction of the second crystal grains was 60%, the nonlinear constant α was 1, that is, an ohmic resistor, and it was difficult to function as a varistor. . Accordingly, the volume fraction of the second crystal grains is preferably 5% or more and less than 60%.

Further, the capacitance of the varistor of Example 1 at 200 MHz was 3.1 × 10 ⁻¹² F.

FIG. 1 is a cross-sectional view of a multilayer varistor according to an embodiment of the present invention. FIG. 2 is a schematic diagram showing the structure of the ceramic composition of the laminated varistor of FIG. FIG. 3 is a diagram showing an equivalent circuit of the varistor according to the first embodiment. FIG. 4 shows the capacitance change rate ΔC of the varistor 2 when the resistance of the resistor R2 of the second crystal grain 120 is changed when the capacitance of the capacitor C2 of the second crystal grain 120 is 8 pF. FIG. FIG. 5 is a diagram showing a capacitance change rate ΔC when the capacitance of the capacitor C2 of the second crystal grain 120 is changed when the resistance of the resistor R2 of the second crystal grain 120 is 60 kΩ. is there. FIG. 6 is a table showing the volume fraction and characteristics of the second crystal grains in the varistors of Examples 1 to 6 and Comparative Example 1. FIG. 7 is a diagram showing the relationship between the volume fraction of the second crystal grains and the nonlinear constant α in the varistors of Examples 1 to 6 and Comparative Example 1. FIG. 8 is a diagram showing the relationship between the volume fraction of the second crystal grains and the capacitance change rate ΔC in the varistors of Examples 1 to 6 and Comparative Example 1. FIG. 9 is a cross-sectional view of a varistor according to the second embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 2,202 ... Varistor, 10, 10c ... Porcelain composition, 4 ... Internal electrode layer (electrode plate), 100 ... 1st crystal grain, 120 ... 2nd crystal grain, 130 ... Grain boundary.

Claims (6)

A porcelain composition comprising first crystal grains mainly composed of ZnO and second crystal grains mainly composed of (Zn _1-x Co _x ) ₂ SiO ₄ (where 0 <x <1) ;
At least a pair of electrode plates sandwiching the porcelain composition;
A varistor.   The first crystal grain of the porcelain composition contains Co and Al, and has an oxide containing Pr, K, Cr, Ca and Si at a grain boundary between the first crystal grains of the porcelain composition. The varistor described in 1.   The varistor according to claim 1 or 2, wherein a volume fraction of the second crystal grains with respect to a total volume of the first crystal grains and the second crystal grains in the porcelain composition is 5% or more and less than 60%. . The varistor according to any one of claims 1 to 3 , wherein a plurality of element bodies including the porcelain composition and the pair of electrode plates are stacked.   The varistor according to claim 1, wherein the capacitance at a frequency of 1 MHz is 90% or less of the capacitance at a frequency of 1 kHz. (Zn _1-x Co _x ) ₂ SiO ₄ (where 0 <x <1) as a main component,
Forming a mixture of powder containing Zn and forming a mixture; and
The porcelain having the first crystal grains mainly composed of ZnO and the second crystal grains mainly composed of (Zn _1-x Co _x ) ₂ SiO ₄ (where 0 <x <1) is obtained by firing the compact. And a step of obtaining a composition.

Images