Field of the Invention
[0001] The present invention relates to a zinc oxide varistor which absorbs dielectric lightning
surge, electrostatic surge, burst surge or the like, and a method of manufacturing
same.
Background of the Invention
[0002] As a conventional zinc oxide varistor, the following zinc oxide varistor is generally
known.
[0003] First, a material based on zinc oxide is sintered to make a varistor element. A first
external electrode is formed on the surface of the sintered varistor element. Next,
the varistor element is buried into a mixture based on SiO
2 and is subjected to heat treatment. Thus, Zn
2SiO
4 film having acid and alkali resistance is formed on the surface of the varistor element.
To have acid and alkali resistance means to have plating resistance. Then, Zn
2SiO
4 film is also formed on the first external electrode, resulting in generation of irregularities
thereon. In order to eliminate such irregularities and to assure electrical connection
with external circuits, a second external electrode is formed on the first external
electrode. After that, Ni plating and solder plating are performed on the second external
electrode.
[0004] However, in the conventional configuration as described above, it is necessary, after
forming the first external electrode, to again perform heat treatment in SiO
2, to remove deposits, and to form the secondary external electrode. Accordingly, there
has been a problem that the manufacturing process becomes very complicated.
[0005] In order to solve such problem, the present invention is intended to provide a zinc
oxide varistor having a Zn
2SiO
4 film on the surface of the varistor element, requiring no heat treatment in SiO
2 after forming the first external electrode, that is, after sintering the varistor
element.
Summary of the Invention
[0006] A method of manufacturing a zinc oxide varistor of the present invention comprises:
(a) a first process of forming a varistor element whose main component is zinc oxide,
and
(b) a second process of sintering the varistor element,
wherein by sintering the varistor element, the varistor element is sintered, and
zinc compound having at least one of acid resistance and alkali resistance is precipitated
and formed on the surface of the varistor element.
[0007] Preferably, the method of manufacturing the zinc oxide varistor further comprises:
(c) a process of attaching an external electrode to the varistor element, wherein
the external electrode attaching process is performed after finishing the process
of sintering the varistor element.
[0008] A zinc oxide varistor of the present invention comprises:
a varistor element whose main component is zinc oxide; and
a precipitate film formed on the surface of the varistor element;
wherein the precipitate film is more excellent in alkali resistance or acid resistance
than the varistor element.
[0009] Preferably, the zinc oxide varistor further comprises an external electrode disposed
on the surface of the varistor element.
[0010] By this configuration, a precipitate film having plating resistance may be formed
on the surface of the varistor element during sintering process. As a result, it is
possible to shorten the manufacturing process, and also, to improve the productivity.
Plating resistance means that no deterioration occurs during plating process.
Brief Description of the Drawings
[0011] Fig. 1 is a sectional view of a laminate chip varistor being a zinc oxide varistor
in an embodiment of the present invention.
[0012] Fig. 2 shows a process of manufacturing a laminate chip varistor being a zinc oxide
varistor in an embodiment of the present invention.
[0013] Fig. 3 is a view of varistor element grain size and precipitate film when aluminum
compound is not added as a sub-component for a varistor element in an embodiment of
the present invention.
[0014] Fig. 4 is a view of varistor element grain size and precipitate film when aluminum
compound is added as a sub-component for a varistor element in an embodiment of the
present invention.
Description of Reference Numerals
[0015]
- 1
- Varistor element
- 2
- Precipitate film
- 3
- Internal electrode
- 4
- External electrode
- 5
- Ni layer
- 6
- Solder layer
- 30
- Zn2SiO4
Detailed Description of the Invention
[0016] A method of manufacturing a zinc oxide varistor of the present invention comprises:
(a) a first process of forming a varistor element whose main component is zinc oxide,
and
(b) a second process of sintering the varistor element,
wherein by sintering the varistor element, the varistor element is sintered, and
zinc compound having at least either acid resistance or alkali resistance is precipitated
and formed on the surface of the varistor element.
[0017] By this configuration, it is possible to obtain a zinc oxide varistor having an acid
or alkali resisting film on the surface of the varistor element without requiring
heat treatment in SiO
2 after sintering the varistor element. Such acid or alkali resisting film is free
from damage, breakage, and deterioration during plating process. That is, the acid
or alkali resisting film ensures plating resistance. In the above manufacturing method,
a precipitate film having plating resistance may be formed on the surface of a varistor
element during sintering process. As a result, it is possible to shorten the manufacturing
process, and also, to improve the productivity.
[0018] Preferably, in the first process, the varistor element further contains bismuth compound
and silicon compound as sub-components. Thus, due to the bismuth compound, it is possible
to promote the precipitation of zinc compound film on the varistor element surface
during sintering process. As a result, a zinc oxide varistor having plating resistance
can be obtained.
[0019] Preferably, the second process includes a step of precipitating Zn-Si-O based compound
as zinc compound. Thus, Zn-Si-O based compound is produced in the varistor element,
and consequently, a zinc oxide varistor having plating resistance can be obtained.
[0020] Preferably, the silicon compound contained ranges from 1 mol% to 15 mol% in terms
of Si. Thus, it is possible to precipitate Zn-Si-O based compound having plating resistance
on the varistor element surface without causing hindrance to the sintering effect.
[0021] Preferably, the sintering temperature in the second process ranges from 1000°C to
1400°C. Thus, it is possible to precipitate Zn-Si-O based compound having plating
resistance on the surface thereof and to obtain a zinc oxide varistor having the desired
electric characteristics.
[0022] Preferably, in the first process, the varistor element further contains aluminum
compound as a sub-component. Thus, it is possible to reduce generation of irregularities
on the varistor element surface and to lessen the portion where Zn-Si-O based compound
is not precipitated.
[0023] Preferably, the aluminum compound is contained by 3 mol% or less. Thus, it is possible
to suppress the generation of irregularities on the varistor element surface and to
lessen the portion where Zn-Si-O based compound is not precipitated.
[0024] Preferably, in the second process, the bismuth compound is disposed around the varistor
element when the varistor element is sintered. Thus, the bismuth compound disposed
around the varistor element is scattered during sintering and some of the scattered
bismuth compound sticks to the surface of the varistor element during the temperature
lowering process. Accordingly, it is possible to promote the precipitation of Zn-Si-O
based compound onto the surface thereof the same as for the bismuth component in the
varistor element.
[0025] Preferably, during sintering in the second process, the temperature becomes lowered
at a speed so as to suppress the grain growth of the varistor element. Thus, it is
possible to suppress the generation of irregularities on the surface of the varistor
element and to lessen the portion where Zn-Si-O based compound is not precipitated.
[0026] Preferably, the silicon compound used is Zn
2SiO
4. Thus, it is possible to efficiently precipitate Zn-Si-O based compound on the surface
of the varistor element during sintering process.
[0027] Preferably, the second process includes a step of storing the varistor element into
a sheath and sintering same while rotating the sheath. Thus, even when a large quantity
of varistor element is sintered, the heat distribution and the sintering atmosphere
can be uniformed. As a result, it is possible to prevent variation in precipitation
of zinc compound having plating resistance.
[0028] Preferably, the sheath stores at least one powder selected from the group consisting
of Al
2O
3, MgO, ZrO
2, ZnO and NiO together with the varistor element. Thus, it is possible to prevent
varistor elements from sticking to each other during sintering process.
[0029] Preferably, the first process includes a step of obtaining a mixture by mixing the
main component and the sub-component before forming the varistor element, and then
a step of calcining the mixture. Thus, due to calcining, zinc compound may be precipitated
as previously intended. And, during sintering process, zinc compound can be efficiently
precipitated on the surface of the varistor element.
[0030] Preferably, in the first process, the varistor element further contains bismuth compound
and antimony compound as sub-components, and the second process includes a step of
precipitating Zn-Sb-O based compound as zinc compound. Thus, it is possible to produce
Zn-Sb-O based compound in the varistor element by sintering, and to promote film precipitation
on the surface of the varistor element by bismuth compound. As a result, zinc oxide
varistor having plating resistance can be obtained.
[0031] Preferably, the antimony compound is contained in a range from 1 mol% to 10 mol%
in terms of Sb. The antimony compound is contained in a range of 1 mol% to 10 mol%
in terms of Sb. Thus, it is possible to precipitate Zn-Sb-O based compound having
plating resistance on the surface of the varistor element without causing hindrance
to the sintering effect.
[0032] Preferably, in the first process, the varistor element further contains aluminum
compound as a sub-component. Thus, it is possible to suppress the generation of irregularities
on the surface of the varistor element and to lessen the portion where Zn-Si-O based
compound is not precipitated.
[0033] Preferably, the aluminum compound is contained by 3 mol% or less. Thus, it is possible
to suppress the generation of irregularities on the surface of the varistor element
and to lessen the portion where Zn-Si-O based compound is not precipitated.
[0034] Preferably, a method of manufacturing a zinc oxide varistor of the present invention
further comprises:
(c) a process of attaching an external electrode to the varistor element, wherein
the external electrode attaching process is executed after finishing the step of sintering
the varistor element.
[0035] Preferably, the external electrode attaching process includes a step of disposing
an external electrode material, and a step of forming a plated layer by a plating
method on the surface of the external electrode material.
[0036] Preferably, the plated layer is at least one selected from the group consisting of
a nickel layer, tin layer, and solder layer.
[0037] Preferably, the plated layer contains at least two layers which have the nickel layer
and one of tin layer and solder layer.
[0038] Preferably, the process of forming the varistor element includes a step of forming
a laminate varistor element having an internal electrode in the varistor element.
[0039] Preferably, the process of forming the varistor element includes:
a step of manufacturing a plurality of sheet varistor materials;
a step of disposing internal electrodes on the surface of each sheet varistor material;
and
a step of laminating the sheet varistor materials respectively having the internal
electrodes.
[0040] Preferably, the first process includes:
(i) a step of preparing a mixture by mixing ZnO as main component, SiO2 and at least one selected from the group consisting of Bi2O3, Sb2O3, Co3O4, MnO2, NiO, Cr2O and Al (NO3)3 as sub-component, and
(ii) a step of forming the mixture into a predetermined shape to form the varistor
element,
wherein the second process includes a step of precipitating Zn-Si-O based compound
as zinc compound on the surface of the varistor element.
[0041] Preferably, the first process
(i) a step of preparing a mixture by mixing ZnO as main component, Sb2O3 and at least one selected from the group consisting of Bi2O3, Co3O4, MnO2, NiO, Cr2O and Al (NO3)3 as sub-component, and
(ii) a step of forming the mixture into a predetermined shape to form the varistor
element,
wherein the second process includes a step of precipitating Zn-Sb-O based compound
as zinc compound on the surface of the varistor element.
[0042] Preferably, the first process includes
(iii) a step of calcining of the mixture;
(iv) a step of forming the temporarily burnt mixture into a predetermined size of
the calcined powder; and
(v) a step of preparing a slurry by using the calcined powder,
wherein the slurry is used to form the varistor element into a predetermined shape.
[0043] Preferably, the first process includes:
(i) a step of preparing a mixture by mixing ZnO as main component and at least one
of Zn-Si-O based compound and Zn-Sb-O based compound as sub-component;
(ii) a step of preparing a slurry by using the mixture; and
(iii) a step of forming the mixture into a predetermined shape to form the varistor
element,
wherein the second process includes a step of precipitating at least either Zn-Si-O
based compound or Zn-Sb-O based compound as zinc compound on the surface of the varistor
element.
[0044] Preferably, in the second process, the zinc compound contains at least one of Zn-Si-O
based compound and Zn-Sb-O based compound.
[0045] A zinc oxide varistor of the present invention comprises:
a varistor element whose main component is zinc oxide, and
a precipitate film formed on the surface of the varistor element,
wherein the precipitate film is more excellent in alkali resistance or acid resistance
than the varistor element. Thus, it is possible to obtain a precipitate film having
plating resistance on the surface of the varistor element without heat treatment in
SiO
2 after sintering process.
[0046] Preferably, the precipitate film contains at least one of Zn-Zi-O based compound
and Zn-Sb-O based compound. Thus, it is possible to further improve the plating resistance.
[0047] Preferably, the varistor element contains aluminum compound as sub-component. Thus,
it is possible to suppress the generation of irregularities on the surface of the
varistor element and to lessen the proportion where Zn-Si-O based compound is not
precipitated.
[0048] Preferably, the varistor element includes an internal electrode disposed in the varistor
element, and an external electrode disposed on the surface of the varistor element.
The external electrode is electrically connected to the internal electrode. More preferably,
the internal electrode is made of platinum. Thus, the percentage of contraction of
the internal electrode becomes smaller. Further, zinc compound is precipitated out
of the varistor element. Accordingly, it is possible to establish electrical connection
between the internal electrode and the external electrode without executing a step
of exposing the internal electrode after forming a precipitate film.
[0049] Preferably, the varistor element includes a plurality of varistor materials, a plurality
of internal electrodes disposed inside the varistor materials, and external electrodes
disposed on the surface of the varistor materials.
[0050] Preferably, the external electrode includes an external electrode material and a
plated layer disposed on the surface of the external electrode material.
[0051] Preferably, the internal electrode includes a material whose main component is platinum,
and the external electrode material includes at least one selected from the group
consisting of Pt, Pt-Ag, Ag-Pd, and resin containing Ag. And the plated layer includes
at least one selected from the group consisting of nickel plated, solder plated, and
tin plated layers.
[0052] Preferably, the plated layer includes contains at least two layers which have a nickel
plated layer and one of a solder plated layer and a tin plated layer disposed on the
nickel plated layer.
Exemplary Embodiment 1:
[0053] A zinc oxide varistor in an exemplary embodiment of the present invention will be
described in the following. Fig. 1 is a sectional view of a laminate chip varistor
as a zinc oxide varistor.
[0054] In Fig. 1, varistor element 1 whose main component is zinc oxide has internal electrodes
3 whose main component is Pt. Also, precipitate film 2 whose main component is Zn
2SiO
4 is formed on the surface of the varistor element 1. External electrode 4 whose main
component is Ag is disposed on the exposed ends of the internal electrodes 3. Further,
Ni layer 5 and solder layer 6 are disposed on the external electrode 4.
[0055] Fig. 2 is a manufacturing process chart of a laminate chip varistor in the present
exemplary embodiment.
[0056] Fig. 3 is a view of varistor element grain size and precipitate film when aluminum
compound is not applied as a sub-component of the varistor element in the present
embodiment.
[0057] Fig. 4 is a view of varistor element grain size and precipitate film when aluminum
compound is applied as a sub-component of the varistor element in the present embodiment.
That is, Fig. 3 and Fig. 4 are sectional views that show the states of irregularities
and precipitate film 2 formed on the surface of varistor element 1 with and without
aluminum compound applied into the varistor element 1. In Fig. 3 and Fig. 4, Zn
2SiO
4 30 is formed in the varistor element 1.
[0058] First, in the step No. 8 of Fig. 2, ZnO as main component and SiO
2, Bi
2O
3, Sb
2O
3, Co
3O
4, MnO
2, NiO, Cr
2O
3, Al (NO
3)
3 as sub-components are subjected to wet mixing. Next, the mixture is dried in the
step No. 9. Thus, material powder may be obtained. In that case, if silicon compound
is insufficient, precipitate film 2 cannot be formed on the surface of varistor element
1, and if silicon compound is excessive, it will affect the sintering effect. Accordingly,
the quantity of silicon compound added is adjusted to 1 mol% to 15 mol% or preferably
to 5 mol% to 10 mol% in terms of Si.
[0059] Also, in case of adding aluminum compound, it is possible to suppress the generation
of irregularities on the surface of varistor element 1 and to lessen the portion where
precipitate film 2 is not formed and to further improve the plating resistance.
[0060] The quantity of aluminum compound added is adjusted to 3 mol% max. or preferably
to 1 mol% or less in terms of Al. Further, by adding aluminum compound, it is also
possible to obtain the effect of improving the plating resistance inside the varistor
element 1.
[0061] Next, in the step No. 10 of Fig. 2, dry powder grain size is adjusted. Subsequently,
in the step No. 11 of Fig. 2, the powder is put into a sheath and is calcined at a
temperature of 800°C to 1000°C. After that, in the step No. 12 of Fig. 2, the calcined
powder is crushed until becoming 1.0 ± 0.5 µm in grain size on the average. Then,
in case the crushed powder is smaller in grain size, the excellent life under high
temperature is obtained, and the precipitation of Zn
2SiO
4 onto the surface of varistor element 1 can be promoted. And the powder is finely
crushed in the step No. 13, and is sufficiently dried in the step No. 14. The powder
is again crushed in the step No. 15, and then, powder of larger gain sizes is eliminated
in order to obtain a uniform slurry.
[0062] Next, in the step No. 16, the crushed powder is mixed with butyl acetate as a solvent,
benzene butyl phthalate as a plasticizer, and butyral resin as a binder, thereby manufacturing
a slurry.
[0063] Subsequently, in the step No. 17, the slurry is formed into a sheet having a predetermined
thickness by the doctor blade method after eliminating solid matters contained therein.
After that, the sheet is cut to a predetermined shape in the step No. 18. And in the
step No. 19, Pt paste as internal electrode 3 is printed thereon in a desired form,
followed by lamination.
[0064] In that case, an electrode made of at least one metal out of Pt, Pd, and Ag can be
used as the internal electrode.
[0065] After that, main press operation is performed in the step No. 20. And in the step
No. 21, the work is cut to a predetermined shape. In this way, the varistor element
1 can be obtained.
[0066] Next, the varistor element 1 is inserted into a sheath for binder elimination, which
is thrown into a binder eliminating furnace, and then the temperature is increased
up to 400°C at a temperature increasing rate of 25°C/h. The condition is maintained
for two hours, and further, the temperature is increased up to 700°C, and the condition
is maintained for two hours. Thus, the binder is eliminated in the step No. 22. The
purpose of this is to provide the varistor element 1 with a sufficient strength in
advance since it is necessary to rotate the sheath, storing the varistor element 1,
in the next sintering process.
[0067] In the step No. 23, the varistor element 1 with the binder completely eliminated
is put into a bullet-shape sheath together with Al
2O
3 powder, which is then thrown into a furnace and sintered in the air.
[0068] The sintering process is described in the following. First, the temperature is increased
up to 800°C at a temperature increasing rate of 200°C/h without rotating the sheath.
After that, rotating the sheath is started at the temperature higher than 800°C. Subsequently,
the temperature is increased up to 1000°C to 1400°C max. at a rate of 200°C/h, and
the condition is maintained for two hours at the maximum temperature. Next, the temperature
is lowered at a temperature lowering rate of 100°C/h.
[0069] In the step No. 24, chamfering of the varistor element 1 is performed. Subsequently,
in the step No. 25, external electrode 4 whose main component is Ag is formed on the
exposed ends of the internal electrodes 3. Next, in the step No. 26, baking is performed.
In this case, the external electrode 4 is formed from a paste prepared by dispersing
Ag in Pt, Pt-Ag, Ag-Pd, or thermosetting resin.
[0070] In the steps No. 27 and No. 28, the external electrode 4 is subjected to baking,
followed by Ni-plating, and by solder plating. In this way, Ni layer 5 and solder
layer 6 are formed. A laminate chip varistor is completed through such steps. It is
also possible to perform Sn plating to form an Sn layer instead of solder plating.
[0071] Next, precipitate film 2 whose main component is Zn
2SiO
4, which is formed on the surface of the varistor element 1, is described in the following.
[0072] Zinc oxide is an amphoteric substance that dissolves in both acid and alkali. Therefore,
zinc oxide dissolves in Ni plating solution and solder plating solution which are
acidic or alkaline. A film containing Zn
2SiO
4 as main component is harder to dissolve in acidic and alkaline solution than the
varistor element 1. Accordingly, by coating the surface of varistor element 1 with
precipitate film 2 whose main component is Zn
2SiO
4, it is possible to suppress the intrusion of plating solution into the varistor element
1. Generally, when electrolytic plating is performed with the surface of varistor
element 1 completely exposed, a metal flow is generated since the varistor element
1 is a semiconductor. However, in the present exemplary embodiment, it is possible
to prevent generation of a metal flow because the precipitate film 2 having Zn
2SiO
4 as main component is a high resistance substance.
[0073] Also, in the present exemplary embodiment, Sb
2O
3 as a sub-component of varistor element 1 is also applied. Accordingly, Zn-Sb-O based
compound is also produced due to sintering, and Zn-Sb-O based compound is precipitated
on the surface of varistor element 1 together with Zn
2SiO
4. The Zn-Sb-O based compound also has excellent plating resistance the same as Zn
2SiO
4. Therefore, it is possible to obtain a varistor having excellent plating resistance
which does not affect the plating effect.
[0074] Also, in case Sb compound is not applied as a sub-component of varistor element 1,
Zn-Sb-O based compound will not be formed. However, even in case only Zn
2SiO
4 is applied, a laminate chip varistor having practically sufficient plating resistance
can be obtained.
[0075] In the above embodiment, the material powder was calcined to form Zn
2SiO
4 in advance in order to promote the precipitation on the surface of varistor element
1 after calcining, but it is not limited to this configuration. It is also possible
to use Zn
2SiO
4 as silicon compound instead of calcining. In this way, the same effect as described
above may be obtained. Naturally, it is possible to form precipitate film 2 having
Zn
2SiO
4 as main component on the surface of varistor element 1 without using Zn
2SiO
4 as silicon compound or without calcining of the material powder.
[0076] Further, as shown in Fig. 3, with advance of the grain growth of varistor element
1, the irregularities on the surface thereof increase in size, and as a result, there
may be generated some portion where precipitate film 2 cannot be formed on the surface
of varistor element 1. However, as shown in Fig. 4, with grain growth and generation
of surface irregularities suppressed, it is possible to lessen the portion where precipitate
film 2 is not formed on the surface of varistor element.
[0077] From the result of analysis, it is clear that the addition of aluminum compound as
a sub-component of varistor element 1 increases the amount of substance of the spinel
structure (e.g. Zn-Sb-O based compound, etc. in the present embodiment) existing at
the triple point of grain boundary of varistor element 1, and the substance serves
a wedge-like function to suppress the grain growth. As a result, as shown in Fig.
4, it has resulted in suppressing the generation of irregularities on the surface
of varistor element 1 and lessening the portion where precipitate film 2 is not formed
on the surface of varistor element 1. Thus, it is possible to further improve the
plating effect and enhance the metal flow preventing effect.
[0078] Precipitate film 2 having Zn
2SiO
4 as main component can be formed on the surface of varistor element 1 without adding
aluminum compound as a sub-component of varistor element 1. However, from the above
result of analysis, it is clear that precipitate film 2 can be further reliably formed
when aluminum compound is used as a sub-component of varistor element 1.
[0079] Also, it is possible to start the formation of Zn
2SiO
4 at a lower temperature when bismuth is added as a sub-component of varistor element
1. For example, in case of no bismuth, the reaction of 2ZnO+SiO
2-->Zn
2SiO
4 will not take place at a temperature lower than 1000°C. However, under the existence
of bismuth, most of Si will become Zn
2SiO
4 at 1000°C. This phenomenon probably occurs in the course of the following reaction.
[0080] Further, the bismuth is liquefied and dispersed during sintering. Therefore, more
bismuth will exist on the surface of varistor element 1. Accordingly, the precipitation
of Zn
2SiO
4 onto the surface of varistor element 1 is promoted and Zn
2SiO
4 close to the surface of varistor element 1 also moves onto the surface, thereby lessening
the portion where precipitate film 2 is not formed on the surface of varistor element
1.
Exemplary Embodiment 2:
[0081] In a laminate chip varistor in the second exemplary embodiment, precipitate film
2 has Zn-Sb-O based compound as main component. The other configuration is same as
in the laminate chip varistor in the first exemplary embodiment described above.
[0082] First, ZnO as main component and Bi
2O
3, Sb
2O
3, Co
3O
4, MnO
2, NiO, Cr
2O
3, and Al (NO
3)
3 as sub-components are subjected to wet mixing (No. 8 of Fig. 2), followed by drying
(No. 9 of Fig. 2). Thus, the material powder is obtained. In that case, if the amount
of antimony compound added is insufficient, precipitate film 2 cannot be formed on
the surface of varistor element 1, and if the amount of antimony compound added is
excessive, it will affect the sintering effect. Accordingly, the amount of antimony
compound added is adjusted to 1 mol% to 10 mol% or preferably 4 mol% to 10 mol% in
terms of Sb.
[0083] The same as in the first exemplary embodiment, varistor element 1 is obtained through
the steps No. 10 to No. 21 of Fig. 2.
[0084] Next, the varistor element 1 is inserted into a sheath for binder elimination, which
is thrown into a binder eliminating furnace, and then the temperature is increased
up to 400°C at a temperature increasing rate of 25°C/h, and the condition is maintained
for two hours. After that,, the temperature is further increased up to 700°C, and
the condition is maintained for two hours. Thus, the binder is eliminated (No. 22
of Fig. 2). In this way, the strength of varistor element 1 is increased. And it is
possible to prevent the varistor element 1 from being damaged when the sheath, storing
the varistor element 1, is rotated in the next sintering process.
[0085] The varistor element 1 with the binder completely eliminated is put into a bullet-shape
sheath together with Al
2O
3 powder, which is then thrown into a furnace and sintered in the air (No. 23 of Fig.
2).
[0086] The sintering process is described in the following. First, the temperature is increased
up to 800°C at a temperature increasing rate of 200°C/h without rotating the sheath.
After that, the sheath rotation is started at a temperature higher than 800°C. Subsequently,
the temperature is increased up to 1000°C to 1400°C max. at a rate of 200°C/h, and
the condition is maintained for two hours at the maximum temperature. Next, the temperature
is lowered at a temperature lowering rate of 100°C/h.
[0087] After the sintering process, the varistor element 1 is subjected to chamfering (No.
24 of Fig. 2). External electrode 4 whose main component is Ag is formed on the exposed
ends of internal electrodes 3 (No. 25 of Fig. 2). After that, baking is performed
(No. 26 of Fig. 2). In this case, the external electrode 4 is formed by using a paste
prepared by dispersing Ag in Pt, Pt-Ag, Ag-Pd, or thermosetting resin.
[0088] After baking the external electrode 4, Ni plating is performed, followed by solder
plating. Thus, Ni layer 5 and solder layer 6 are formed (No. 27, 28 of Fig. 2). In
this way, a laminate chip varistor can be obtained.
[0089] It is also possible to form an Sn layer by performing Sn plating instead of solder
plating.
[0090] Here, precipitate film 2 whose main component is Zn-Sb-O based compound, which is
formed on the surface of varsistor element 1, will be described in the following.
[0091] Zinc oxide is an amphoteric substance that dissolves in both acid and alkali. Therefore,
zinc oxide dissolves in Ni plating solution and solder plating solution which are
acidic or alkaline. A film containing Zn
2SiO
4 as main component is harder to dissolve in acidic and alkaline solution than the
varistor element 1. Accordingly, by coating the surface of varistor element 1 with
precipitate film 2 whose main component is Zn
2SiO
4, it is possible to suppress the intrusion of plating solution into the varistor element
1. Generally, when electrolytic plating is performed with the surface of varistor
element 1 completely exposed, a metal flow is generated since the varistor element
1 is a semiconductor. However, in the present exemplary embodiment, it is possible
to prevent generation of such metal flow because the precipitate film 2 having Zn
2SbO
4 as main component is a high resistance substance.
[0092] In the above embodiment, the material powder was calcined to form Zn-Sb-O based compound
in advance in order to promote the precipitation on the surface of varistor element
1 during burning, but it is not limited to this configuration. It is also possible
to use Zn-Sb-O based compound as antimony compound instead of calcining. In this way,
the same effect as described above may be obtained. Naturally, it is possible to form
precipitate film 2 having Zn-Sb-O based compound as main component on the surface
of varistor element 1 without using Zn-Sb-O based compound as antimony compound or
without calcining of the material powder.
[0093] Also, precipitate film 2 in the second exemplary embodiment probably contains Zn
2.33Sb
0.67O
4 as main component. There exists a possibility that Zn-Sb-O based compound having
another configuration is precipitated on the precipitate film 2. Therefore, it was
expressed by precipitate film 2 having Zn-Sb-O as main component with respect to the
precipitate film 2.
[0094] Further, in the second exemplary embodiment, it is possible to lessen the portion
where precipitate 2 is not formed on the surface of varistor element 1 by suppressing
the grain growth of varistor element and generation of irregularities on the surface.
Accordingly, the same as in the first exemplary embodiment, the addition of aluminum
compound as a sub-component of varistor element 1 increases the amount of substance
of the spinel structure consisting of Zn and Sb and O, existing at the triple point
of grain boundary of varistor element 1, and the substance of the spinel structure
serves a wedge-like function to suppress the grain growth.
[0095] Further, by adding bismuth as a sub-component of varistor element 1, the same as
in the first exemplary embodiment, it is possible to start the formation of Zn-Sb-O
based compound at a lower temperature. Moreover, the bismuth is liquefied and dispersed
during sintering. Therefore, more bismuth will exist on the surface of varistor element
1. In this case, preferably, the bismuth compound is being disposed around the varistor
element when the varistor element is sintered. Accordingly, the precipitation of Zn-Sb-O
based compound onto the surface of varistor element 1 is promoted, and also, Zn-Sn-O
based compound close to the surface of varistor element 1 moves onto the surface.
As a result, it is possible to lessen the portion where precipitate film 2 is not
formed on the surface of varistor element 1.
[0096] Also, precipitate film 2 having Zn-Sb-O based compound as main component can be formed
on the surface of varistor element 1 without adding aluminum compound as a sub-component
of varistor element 1. However, in order to lessen as much as possible the portion
where precipitate film 2 is not formed, it is desirable to add aluminum compound.
[0097] The points of the present invention will be described in the following.
(1) The role of precipitate film 2 of the present invention is to prevent intrusion
of the plating solution into varistor element 1 in the plating process and also to
prevent the generation of a metal flow. Accordingly, it is desirable that the whole
surface of varistor element 1 be completely covered with precipitate film 2. In the
present invention, the component of precipitate film 2 is precipitated out of the
varistor element 1. Therefore, the whole surface of varistor element 1 cannot be completely
covered with the film, and there may sometimes exist a portion where precipitate 2
is not formed as shown in Fig. 3.
However, such portion where precipitate film 2 is not formed will hardly cause the
intrusion of plating solution and generation of a metal flow.
(2) When the varistor element 1 is sintered and the sheath is rotated, the varistor
element 1 and Al2O3 can be well mixed by rotating the sheath with the rotary shaft kept in a horizontal
position. Thus, it is possible to promote the formation of precipitate film 2 and
also to prevent the variation of the forming status.
(3) When the varistor element 1 is sintered, Al2O3 powder is mixed in the sheath. In this case, it is also possible to apply at least
one of Al2O3, MgO, ZrO2, ZnO, and NiO powders together with the material powder of varistor element 1. Thus,
the high temperature loading life can be improved since bismuth is adsorbed out of
the varistor element 1. Further, it is possible to prevent the varistor elements 1
from sticking to each other as bismuth serves a function as adhesive when the temperature
is lowered in the sintering process.
Further, when Al2O3 powder is used, the same effect as obtained by adding aluminum compound as a sub-component
of varistor element 1 can be obtained. Accordingly, even when aluminum compound is
not added as a sub-component of varistor element 1, by storing Al2O3 powder into the sheath together with the varistor element 1 before sintering, it
is possible to decrease the irregularities on the surface of varistor element 1 and
to lessen the portion where precipitate film 2 is not formed.
(4) In each of the above embodiments, when the varistor element 1 is sintered, the
rotation of the sheath is started at a temperature higher than 800°C. It is not limited
to this configuration, and the sheath rotation starting temperature is desirable to
be in a range from 700°C to 1000°C. Most preferably, it is desirable to start the
rotation at a temperature around 800°C. Thus, it is possible to prevent cracking of
the varistor element 1 and to disperse the bismuth as specified.
Also, the sheath is rotated in order to make uniform the atmosphere and temperature
distribution inside the sheath. If the sheath rotating speed is too low, it will be
difficult to make uniform the temperature distribution and atmosphere. If the rotating
speed is too high, a greater damage will be given to the varistor element 1. Therefore,
it is desirable to rotate the sheath at a speed ranging from 0.5 rpm to 5 rpm.
(5) In case the highest sintering temperature is lower than 1000°C, precipitate film
2 will not be formed enough to prevent intrusion of plating solution into varistor
element 1 and to prevent generation of a metal flow. In case the highest sintering
temperature exceeds 1400°C, precipitate film 2 is formed, but the electrical characteristics
of the laminate chip varistor will be deteriorated or delamination will take place.
Accordingly, the highest sintering temperature is desirable to be in a range from
1000°C to 1400°C and, preferably, in a range from 1000°C to 1300°C.
(6) Taking into account the points that the grain growth is suppressed, the varistor
element 1 becomes smaller and uniform in grain size, the surface of varistor element
1 is reduced in irregularity, and the portion where precipitate film 2 is not formed
on the surface of varistor element 1 is lessened, the higher the temperature lowering
rate in sintering process, the better and more desirable it is.
On the other hand, taking into account the life expectancy, that is one of the major
characteristics of a laminate chip varistor, the lower the temperature lowering rate,
the better it is.
Accordingly, in order to form precipitate film 2 capable of covering the whole surface
of varistor element 1 as much as possible without affecting the electrical characteristics
of the laminate chip varistor, it is desirable that the temperature lowering rate
be in a range of 50°C/h to 400°C/h and, more preferably, in a range of 100°C/h to
200°C/h.
(7) Internal electrode 3 is formed by using at least one metal out of Pt, Pd, and
Ag, as shown in the first exemplary embodiment. When Pt or a metal having Pt as main
component is used, internal electrodes 3 are exposed at the ends of varistor element
1 after sintering. Accordingly, no grinding is needed for exposing the internal electrodes
3 after sintering process.
The reason for this is that the percentage of contraction of internal electrode 3
using Pt or a metal whose main component is Pt is very small. Or, the portion whose
main component is ZnO becomes greater in percentage of contraction than the internal
electrode 3 since precipitate film 2 is formed through reaction of the substance inside
the varistor element 1.
Naturally, it is possible to form external electrode 4 before sintering. It is required
that the external electrode 4 be formed of a metal which may function as an external
electrode 4 even after heat treatment at the highest sintering temperature.
(8) In each of the above embodiments, the varistor element 1 was sintered in the air.
However, from the result of experiments with the partial pressure of oxygen varied,
it is clear that the lower the partial pressure of oxygen around the varistor element
1, the thicker the precipitate film 2 formed on the surface thereof, making it possible
to improve the plating effect. However, due to sintering under the low partial pressure
of oxygen, the laminate chip varistor characteristics may sometimes become deteriorated.
In that case, the desired characteristics can be restored by performing heat treatment
at 800°C to 1000°C again in the air.
(9) The above exemplary embodiments have referred to a laminated zinc oxide varistor.
It is not limited to this type of varistor only, but a single-plate type varistor
is also usable as a zinc oxide varistor of the present invention. In this case, it
is also possible to reduce the manufacturing steps the same as in the laminate zinc
oxide varistor.
[0098] As described above, by the present invention, a precipitate film whose main component
is zinc compound having plating resistance can be formed on the surface of a varistor
element without another heat treatment in SiO
2 after sintering. Accordingly, the manufacturing process can be shortened. As a result,
it is possible to improve the productivity and, further, to reduce the cost.
1. A method of manufacturing a zinc oxide varistor comprising:
(a) a first process of forming a varistor element, said varistor element contains
zinc oxide as main component, and
(b) a second process of sintering said varistor element,
wherein by sintering said varistor element, said varistor element is sintered,
and zinc compound having at least one of acid resistance and alkali resistance is
precipitated and formed on the surface of said varistor element.
2. The method of manufacturing a zinc oxide varistor of claim 1,
wherein, in the first process, said varistor element further contains bismuth compound
and silicon compound as sub-components, and
the second process includes a step of precipitating Zn-Si-O based compound as zinc
compound.
3. The method of manufacturing a zinc oxide varistor of claim 2,
wherein said silicon compound is contained ranging from 1 mol% to 15 mol% in terms
of Si.
4. The method of manufacturing a zinc oxide varistor of claim 2,
wherein the sintering temperature in the second process ranges from 1000°C to 1400°C.
5. The method of manufacturing a zinc oxide varistor of claim 2,
wherein, in the first process, said varistor element further contains aluminum
compound as a sub-component.
6. The method of manufacturing a zinc oxide varistor of claim 5,
wherein said aluminum compound is contained by 3 mol% or less.
7. The method of manufacturing a zinc oxide varistor of claim 2,
wherein, in the second process, said bismuth compound is disposed around said varistor
element when said varistor element is sintered.
8. The method of manufacturing a zinc oxide varistor of claim 2,
wherein sintering in the second process includes a step of lowering a temperature
at a speed so as to suppress a grain growth of said varistor element.
9. The method of manufacturing a zinc oxide varistor of claim 2,
wherein said silicon compound used is Zn2SiO4.
10. The method of manufacturing a zinc oxide varistor of claim 1,
wherein the second process includes a step of storing said varistor element into
a sheath and sintering same while rotating said sheath.
11. The method of manufacturing a zinc oxide varistor of claim 10,
wherein said sheath stores at least one powder selected from the group consisting
of Al2O3, MgO, ZrO2, ZnO and NiO together with said varistor element.
12. The method of manufacturing a zinc oxide varistor of claim 1,
wherein the first process includes
a step of obtaining a mixture by mixing the main component and the sub-component
before forming said varistor element, and then a step of calcining said mixture.
13. The method of manufacturing a zinc oxide varistor of claim 1,
wherein, in the first process, said varistor element further contains bismuth compound
and antimony compound as sub-components, and
the second process includes a step of precipitating Zn-Sb-O based compound as zinc
compound.
14. The method of manufacturing a zinc oxide varistor of claim 13,
wherein the antimony compound is contained ranging from 1 mol% to 10 mol% in terms
of Sb.
15. The method of manufacturing a zinc oxide varistor of claim 13,
wherein, in the first process, said varistor element further contains aluminum
compound as a sub-component.
16. The method of manufacturing a zinc oxide varistor of claim 15,
wherein the aluminum compound is contained by 3 mol% or less.
17. The method of manufacturing a zinc oxide varistor of claim 1, further comprising:
(c) a process of attaching an external electrode to said varistor element,
wherein said external electrode attaching process is executed after finishing
said varistor element sintering process.
18. The method of manufacturing a zinc oxide varistor of claim 17,
wherein the external electrode attaching process includes
a step of disposing an external electrode material, and
a step of forming a plated layer by a plating method on the surface of said external
electrode material.
19. The method of manufacturing a zinc oxide varistor of claim 18,
wherein the step of forming said plated layer includes the steps of
disposing a nickel plated layer on the surface of said external electrode material,
and
disposing one of a tin layer and a solder layer on said nickel plated layer.
20. The method of manufacturing a zinc oxide varistor of claim 1,
wherein the process of forming said varistor element includes a step of forming
a laminate varistor element having internal electrodes in said varistor element.
21. The method of manufacturing a zinc oxide varistor of claim 1,
wherein the process of forming said varistor element includes the steps of
manufacturing a plurality of sheet varistor materials,
disposing internal electrodes on the surface of each of said sheet varistor materials,
and
laminating said sheet varistor materials respectively having said internal electrodes.
22. The method of manufacturing a zinc oxide varistor of claim 21, further comprising:
(c) a process of attaching an external electrode to said varistor element,
wherein said external electrode attaching process is executed after finishing
said varistor element sintering process.
23. The method of manufacturing a zinc oxide varistor of claim 22,
wherein said external electrode attaching process includes the steps of
disposing an external electrode material, and
forming a plated layer by a plating method on the surface of said external electrode
material.
24. The method of manufacturing a zinc oxide varistor of claim 1,
wherein, in the first process, said varistor element further contains bismuth compound
and silicon compound as sub-components.
25. The method of manufacturing a zinc oxide varistor of claim 1,
wherein said first process includes the steps of:
(i) preparing a mixture by mixing ZnO as main component, SiO2, and at least one selected from the group consisting of Bi2O3, Sb2O3, Co3O4, MnO2, NiO, Cr2O, and Al (NO3)3 as sub-component, and
(ii) forming the mixture into a predetermined shape to form said varistor element,
wherein said second process includes:
a step of precipitating Zn-Si-O based compound as zinc compound on the surface of
said varistor element.
26. The method of manufacturing a zinc oxide varistor of claim 1,
wherein said first process includes the steps of:
(i) preparing a mixture by mixing ZnO as main component, Sb2O3 and at least one selected from the group consisting of Bi2O3, Co3O4, MnO2, NiO, Cr2O, and Al (NO3)3 as sub-component, and
(ii) forming said mixture into a predetermined shape to form said varistor element,
wherein said second process includes a step of precipitating Zn-Sb-O based compound
as zinc compound on the surface of said varistor element.
27. The method of manufacturing a zinc oxide varistor of claim 25,
wherein said first process further includes the steps of:
(iii) calcining said mixture;
(iv) forming said mixture, which is calcined, into a predetermined size of calcined
powder; and
(v) preparing a slurry by using said calcined powder,
wherein said slurry is used to form said varistor element into a predetermined
shape.
28. The method of manufacturing a zinc oxide varistor of claim 1,
wherein said first process includes the steps of:
(i) preparing a mixture by mixing ZnO as main component and at least one of Zn-Si-O
based compound and Zn-Sb-O based compound as sub-component;
(ii) preparing a slurry by using said mixture; and
(iii) forming said mixture into a predetermined shape to form said varistor element,
wherein said second process includes:
a step of precipitating at least one of Zn-Si-O based compound and Zn-Sb-O based compound
as zinc compound on the surface of said varistor element.
29. The method of manufacturing a zinc oxide varistor of claim 1,
wherein, in the second process, said zinc compound contains at least one of Zn-Si-O
based compound and Zn-Sb-O based compound.
30. A zinc oxide varistor, comprising:
a varistor element, said varistor element contains zinc oxide as main component, and
a precipitate film formed on a surface of said varistor element,
wherein said precipitate film is more excellent in at least one of alkali resistance
and acid resistance than said varistor element.
31. The zinc oxide varistor of claim 30,
wherein said precipitate film contains at least one of Zn-Zi-O based compound and
Zn-Sb-O based compound.
32. The zinc oxide varistor of claim 30,
wherein said varistor element further contains bismuth compound and silicon compound
as sub-components.
33. The zinc oxide varistor of claim 32,
wherein said silicon compound is contained ranging from 1 mol% to 15 mol% in terms
of Si.
34. The zinc oxide varistor of claim 32,
wherein said varistor element further contains aluminum compound as sub-component.
35. The zinc oxide varistor of claim 34,
wherein said aluminum compound is contained by 3 mol% or less.
36. The zinc oxide varistor of claim 32,
wherein said silicon compound contains Zn2SiO4.
37. The zinc oxide varistor of claim 30,
wherein said varistor element includes
an internal electrode disposed in said varistor element, and
an external electrode disposed on the surface of said varistor element, said external
electrode being electrically connected to said internal electrode.
38. The zinc oxide varistor of claim 36,
wherein said external electrode includes an external electrode material and a plated
layer disposed on the surface of said external electrode material.
39. The zinc oxide varistor of claim 38,
wherein said internal electrode includes a material having as main component at
least one selected from the group consisting of Pt, Pd, and Ag;
said external electrode includes at least one selected from the group consisting
of Pt, Pd, Ag, alloy of these, and resin containing Ag; and
said plated layer includes at least two layers which have a nickel plated layer
and one of a tin layer and solder layer disposed on said nickel plated layer.
40. The zinc oxide varistor of claim 30,
wherein said varistor element includes:
a plurality of varistor materials;
a plurality of internal electrodes disposed in each varistor material; and
an external electrode disposed on a surface of said varistor material.
41. The zinc oxide varistor of claim 40,
wherein said external electrode includes an external electrode material and a plated
layer disposed on the surface of said external electrode material.
42. The zinc oxide varistor of claim 41,
wherein said internal electrode includes a material having as main component at
least one selected from the group consisting of Pt, Pd, and Ag;
said external electrode material includes at least one selected from the group
consisting of Pt, Pd, Ag, alloy of these, and resin containing Ag; and
said plated layer includes at least two layers which have a nickel plated layer
and one of a tin layer and solder layer disposed on said nickel plated layer.