JP2019057726A - Varistor having multilayer coating and manufacturing method - Google Patents

Abstract

To provide a varistor that suppresses increase of leakage current due to migration of an electrode material, or a coating material, and a manufacturing method thereof.SOLUTION: A varistor 100 comprises a ceramic body 102 and a multilayer coating 104 arranged around the ceramic body. The multilayer coating includes an outer layer 108 including an epoxy material and an inner layer 106 that is adjacent to the ceramic body and is arranged between the outer layer and the ceramic body. The inner layer includes 3 resistance lacquer of moisture-proof, corrosion-proof and mildew-proof being composed of a polymer material which is composed of a mixture of acrylic component resin, amino resin, a xylol solvent and a curing agent, and a total thickness of the multilayer coating does not substantially change due to a thickness of the inner layer.SELECTED DRAWING: Figure 2A

Description

BACKGROUND Embodiments of the present invention relate to the field of circuit protection elements. More specifically, the present invention relates to metal oxide varistors for surge protection.

Overvoltage protection elements are used to protect electronic circuits and electronic components from damage due to overvoltage fault conditions. These overvoltage protection elements may include a metal oxide varistor (MOV) connected between the circuit to be protected and the ground line. MOVs have unique current-voltage characteristics that allow them to be protected from catastrophic voltage surges using MOVs. However, since varistor elements are very widely arranged to protect many different types of devices, there is a continuing need to improve the characteristics of varistors.

MOV elements are generally ceramic disks (often ZnO based), Ag (silver) electrodes,
And a first metal lead and a second metal lead connected by a first surface and a second surface opposite to the first surface. In many cases, MOV elements are also provided with an insulating coating that covers ceramic disks and other materials. An example of an MOV found in the current market is a ceramic disc coated with an epoxy insulator having a high dielectric strength.

However, this type of MOV is typically limited to operation at relatively low temperatures, such as less than 85 ° C., more specifically 85 ° C., 85% relative humidity (RH), high DC operating voltage, etc. When operated under bias humidity conditions, it presents reliability problems. Reliability problems encountered under such bias humidity conditions are due to the migration of the silver electrode material used to contact the surface of the MOV ceramic body and the interaction between the epoxy coating and the ZnO ceramic. It is thought to occur from. An example of a reliability problem is an increase in leakage current through the interface when operating an epoxy-coated MOV under high temperature conditions (at least 85 ° C.) and high humidity conditions while applying a DC operating voltage. With respect to these and other problems, this improvement may be desirable.

Exemplary embodiments relate to an improved varistor. In one embodiment, the varistor can include a ceramic body. The varistor may further include a multi-layer coating disposed around the ceramic body. The multilayer coating may include an outer layer that includes an epoxy material. The multilayer coating can also include an inner layer disposed adjacent to and between the outer layer and the ceramic body. The inner layer is
A polymer material composed of an acrylic component may be included.

In another embodiment, a method of forming a varistor can include providing a ceramic body and applying a first layer comprising an acrylic component on the ceramic body. This method
It may further comprise applying a second layer comprising an epoxy material to the first layer.

FIG. 1 shows an infrared spectrum of an exemplary lacquer layer that may be used as the inner layer of a two-layer coating of a metal oxide varistor (MOV) according to an embodiment of the present disclosure. FIG. 2A shows a plan view of an MOV according to an embodiment of the present disclosure. FIG. 2B shows a plan view of another MOV according to an embodiment of the present disclosure. FIG. 2C shows a side cross-sectional view of the MOV of FIG. 2B. FIG. 3 depicts a plan view of a conventional MOV. FIG. 4A provides the results of electrical measurements at an early stage of a MOV placed with a two-layer coating according to this embodiment. FIG. 4B provides the results of electrical measurements of the MOV of FIG. 4A after 168 hours under bias conditions. FIG. 4C provides the results of electrical measurements of the MOV of FIG. 4A after 336 hours under bias conditions. FIG. 4D provides the results of an electrical measurement of the MOV of FIG. 4A after 500 hours under bias conditions. FIG. 5A provides the results of electrical measurements at an early stage of a conventional MOV placed with a single layer epoxy coating. FIG. 5B provides the results of electrical measurements of the MOV of FIG. 5A after 168 hours under bias conditions. FIG. 5C provides the results of electrical measurements of the MOV of FIG. 5A after 336 hours under bias conditions. FIG. 5D provides the results of an electrical measurement of the MOV of FIG. 5A after 500 hours under bias conditions.

The present invention will now be described in more detail with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. However, the present invention can be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numerals refer to like elements throughout.

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) "Can be used in the following description and claims. "On (On)", "
“Overlying”, “disposed on”, and “over” are used to indicate that two or more elements are in direct physical contact with each other. Can be used. However, “on”, “overlyi”
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Can also mean that two or more elements are not in direct contact with each other. For example, “Upward (o
ver) "may mean that one element is above but not in contact with another element and may have another element (s) between the two elements. In addition, “and / or (and /
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, But the scope of claimed subject matter is not limited in this respect.

This embodiment generally relates to a metal oxide varistor (MOV) based on a zinc oxide material. As is known, this type of varistor comprises a ceramic body having a microstructure comprising zinc oxide particles and may contain various other components such as other metal oxides disposed within the ceramic microstructure. . As a background, MOV is mainly composed of zinc oxide granules that are sintered to form a disk, while zinc oxide granules as a solid are highly conductive materials, while intergranular grains formed from other oxides The boundary is high resistance. Only at the point where the zinc oxide granules meet,
Sintering produces a “microvaristor” corresponding to a symmetric Zener diode. The electrical behavior of metal oxide varistors stems from the number of microvaristors connected in series or in parallel. The sintered body of MOV is responsible for its high electrical load capability that allows high energy absorption, and therefore also an exceptionally high surge current handling capability.

The material used to contact or encapsulate the ceramic body of the varistor is:
It is a potential cause of device degradation, especially when operated under high temperature, high humidity and / or high pressure conditions. In various embodiments, an improved varistor is provided that is resistant to degradation under conditions such as high temperature, high humidity, or high pressure. In various embodiments, an MOV is provided having a coating composed of a multilayer structure, in particular a two-layer structure composed of an outer layer composed of epoxy and an inner layer composed of lacquer. This multilayer coating may improve resistance to leakage currents and other electrical degradations compared to conventional MOVs where the ceramic is in direct contact with the epoxy coating.

An example of a suitable lacquer layer that functions as an inner layer in the two-layer coating includes a layer composed of a mixture of an acrylic resin and another resin such as an amino resin. In a particular embodiment, the lacquer layer can be composed of so-called tri-resistant lacquers that are moisture, corrosion and anti-corrosive. One exemplary formulation of lacquer used as the inner layer of a two-layer coating is 40% acrylic resin, 7% amino resin, 35% xylol, 16% additional solvent, and 2% cure. It is an agent. After curing, solvents such as xylol and other solvents can be removed from the resulting lacquer layer. The acrylic resin and the amino resin can react to form a lacquer layer composed of a polymer material such as a thermosetting polymer, and the polymer is composed of an acrylic component and an amino component. The ratio of the acrylic component to the amino component may be similar or the same as the ratio of the acrylic resin to the amino resin used to form the lacquer. Therefore, the ratio of acrylic component to amino component in the cured lacquer layer is 40: 7 or about 6
: 1. In other embodiments, the ratio of acrylic to amino component is 3: 1
It can vary between ˜19: 1. Embodiments are not limited in this context. For example, this embodiment includes other ratios of the acrylic component and the amine component that the amine component is sufficient to provide a crosslinked thermoset polymeric material after curing.

FIG. 1 shows an infrared spectrum 10 of an exemplary lacquer layer that may be used as an inner layer of a MOV bilayer coating according to an embodiment of the present disclosure. As shown, the infrared spectrum 10 is
It includes a plurality of absorption bands characteristic of a polymer material composed of an amino component and an acrylic component.

In one embodiment, a lacquer layer is applied on the ceramic varistor body to form a MOV, which lacquer layer may be a tri-resistant lacquer based on acrylic and amine resins as described above. In some embodiments, the lacquer formulation may be a prepared commercial formulation that is applied during application of the ceramic body of the varistor, while in other embodiments, the lacquer formulation may be prepared during coating of the varistor. In one embodiment, the lacquer layer may be applied in a manner that covers the exposed surface of the ceramic body such that subsequent layer (s) do not contact the ceramic body. An advantage of lacquer formulations, such as the exemplary formulations disclosed above, is that lacquer formulations that can be applied by brushing, spraying, dip coating, curtain coating, or other methods have a low viscosity. Moreover, such a formulation can exhibit good adhesion. In addition, solidification into a solid lacquer layer can occur at a relatively fast rate.

Thereafter, an epoxy layer may be applied to cover the lacquer layer. Examples of suitable epoxies for the epoxy layer include known epoxy materials used to form conventional MOV devices. The epoxy layer may encapsulate the lacquered ceramic body so as to protect the ceramic body, such as by providing high dielectric strength.

FIG. 2A shows a plan view of a MOV, varistor 100 according to an embodiment of the present disclosure. For clarity, a portion of the varistor coating has been removed to illustrate the structure of the coating. As shown, the varistor 100 includes a ceramic body 102 that may have a flat shape, with the ceramic body 102 generally present in the XY plane as shown. Ceramic body 102
May have a conventional shape, such as a generally rectangular shape having a length A and a width D as shown. However, in other embodiments, the ceramic body can have an oval shape, a circular shape, or other shapes known in the art. Embodiments are not limited in this context. As shown in FIG. 2, the first lead 110 can contact the upper surface of the ceramic body 102, while the second lead 112 contacts the lower surface (not visible) of the ceramic body 102. The ceramic body 102 is encapsulated by a two-layer coating 104 as shown. The two-layer coating 104 is
It will be appreciated that the ceramic body 102 may extend to encapsulate it on all sides. The bilayer coating 104 includes an inner layer 106 and an outer layer 108. In various embodiments, the outer layer 108 is comprised of a conventional epoxy material that can be used to coat a conventional MOV element. The outer layer 108 may further have the thickness characteristics of a conventional MOV element. In some examples, the thickness of the outer layer can range from 0.3 mm to 3 mm, more specifically from 0.5 mm to 1.2 mm. For a given sample, the thickness of outer layer 108 may be uniform. However, the thickness of the outer layer 108 can vary over different regions of the MOV element, as in a conventional MOV element. Embodiments are not limited in this context.

The inner layer 106 may be composed of a lacquer such as a lacquer formed from an acrylic resin and an amine resin as described above. In some embodiments, the thickness of the inner layer 106 is 3μ.
The range may be m to 100 μm, particularly 5 to 50 μm. The embodiment is
It is not limited in this context. It can therefore be seen that the application of the inner layer does not substantially change the overall thickness of the two-layer coating according to this embodiment compared to a single-layer conventional epoxy coating. In other words, in some examples, the inner layer 106 has a thickness that can range from about 0.4% to 10% of the thickness of the outer layer 108.

FIG. 2B shows a plan view of another MOV, varistor 120, according to an embodiment of the present disclosure. FIG.
C shows a side cross-sectional view of the varistor 120. For clarity, a portion of the varistor coating has been removed to illustrate the structure of the coating. In this embodiment, the ceramic body 122 has a round shape. As shown in FIGS. 2B and 2C, the first lead 130 can contact the upper surface of the ceramic body 122, while the second lead 132 contacts the lower surface of the ceramic body 122. The two-layer coating 124 includes an inner layer 126 and an outer layer 128 that may be composed of materials similar to the inner layer 106 and the outer layer 108, respectively. The thickness of the inner layer 126 is also 3 μm.
The outer layer 128 may have a thickness in the range of 0.3 mm to 3 mm.

FIG. 3 illustrates a conventional MOV 150 that may be composed of components similar to MOV 100, except that the ceramic body 102 is covered by a single layer (epoxy layer 152) that may be similar or the same as the outer layer 108 of the MOV 100. I'm drawing.

Advantages provided by the MOV device according to the present embodiment are a high temperature load test (DC 1500 V applied at 150 ° C., DC 970 V applied at 125 ° C.), a bias humidity load test (85 ° C., 8
Improved performance under various conditions, including improved performance in 5% RH (applied voltage up to 1500V DC) and high potential test (applied AC> 2500V). FIG.
FIGS. 4A-4D provide electrical measurement results for a set of MOV samples arranged with a two-layer coating according to this embodiment. Various measurements were made on MOV samples at intervals of about 168 hours while subjected to an applied bias. In particular, in one set of tests, the MOV sample was exposed to a 970 V flat current bias in an environment of 85 ° C. and 85% relative humidity, while in another set of tests, the sample was applied with a flat current of 970 V applied. Maintained at 125 ° C. 4A to 4D and FIGS.
5D shows the results for a sample exposed to a 970 V cross-current bias in an environment of 85 ° C. and 85% relative humidity. As described, samples were removed at approximately 168 hour intervals and measurements were taken. In the data shown, Vnom represents the voltage drop across the MOV when a 1 mA current is passed through the MOV, and the leakage current is measured at 80% of Vnom.

In FIG. 4A, the varistor voltage (Vnom) at a current of 1 ma was measured for a set of samples 42, 43, 44, 45, and 46 under forward and reverse bias conditions.
A measurement of leakage current under forward and reverse bias conditions is also shown. Vn
The initial value of om exhibits an average of about 1190 under forward bias and 1200 under reverse bias. These values increase slightly with about 1.3% and 2.5% respectively over time up to 500 hours. Leakage current (shown in microamps) is measured with a bias voltage of 80% of Vnom and both forward and reverse voltages are recorded. The initial value of leakage current under non-biased conditions exhibits an average value of about 32 and decreases slightly as a function of time. The initial value of the leakage current under bias exhibits an average value of about 34 and varies slightly as a function of time, but does not show a regular shift. These results indicate that the MOV is stable up to at least 500 hours under the test conditions.

5A-5D provide the results of a conventional MOV electrical measurement placed with a coating comprising an epoxy monolayer. A set of samples 47 using the same measurement conditions as shown in FIGS.
, 48, 49, 50, and 51 were measured. As illustrated in FIG. 5A, the initial measurements of Vnom and leakage current exhibit substantially the same results as the sample measurements of FIG. 4A, as expected. However, as shown in FIGS. 5B, 5C, and 5D, the electrical characteristics vary significantly as a function of time. For example, after 500 hours, Vnom decreases about 8% under reverse bias conditions and about 54% under forward bias conditions. Moreover, after 500 hours, under both unbiased and biased conditions, the leakage current increases more than 10 times, indicating severe performance degradation.

In addition to the above-mentioned advantages shown in the measured values of the electrical characteristics of FIGS. It can be expected to exhibit properties, aging resistance, and corona resistance.

The above results in FIGS. 4A-4D show that the inner layer is a mixture of amine resin and acrylic resin, specifically 40% acrylic resin, 7% amino resin, 35% xylol, 16% additional solvent. Note that it provides measurements of a two-layer MOV formed from a mixture of 2% and 2% curing agent. However, in other embodiments, the two-layer coating may consist of an inner layer of lacquer in which the relative amounts of amino resin and acrylic resin are different from the above composition. Moreover, in a further embodiment, a two-layer coating wherein the outer layer is composed of epoxy and the inner layer is composed of other thermosetting materials formed by a combination of precursors other than amine resins and acrylic resins. Is included.

In further embodiments, a two-layer coating may be applied to protect other electronic components from degradation under high pressure, high temperature, or high humidity conditions. Such electronic components include a positive temperature characteristic thermistor (PTC thermistor), a negative temperature characteristic thermistor (
NTC thermistor), resistors, capacitors, filters, ferroelectric components, and piezoelectric components.

Although the invention has been disclosed with reference to particular embodiments, numerous variations to the described embodiments, without departing from the scope of the invention, as defined in the appended claims,
Modifications and changes are possible. Thus, the present invention is not limited to the described embodiments, but is intended to have the greatest scope defined by the following claims and their equivalents.

Claims (21)

A ceramic body;
A multi-layer coating disposed around the ceramic body, wherein the multi-layer coating is
An outer layer comprising an epoxy material;
A polymer material 3 adjacent to the ceramic body and disposed between the outer layer and the ceramic body, the polymer material comprising a mixture of an acrylic component resin, an amino resin, a xylol solvent, and a curing agent An inner layer comprising a resistant lacquer , and
A varistor in which the overall thickness of the multilayer coating does not substantially vary with the thickness of the inner layer .   The varistor of claim 1, wherein the ceramic body comprises ZnO ceramic.   The varistor of claim 1, wherein the inner layer comprises a thickness of 3 μm to 100 μm.   The varistor according to claim 1, wherein the ratio of the acrylic resin to the amino resin is 3: 1 to 19: 1.   The varistor according to claim 4, wherein the ratio of the acrylic resin to the amino resin is 6: 1.   The varistor according to claim 1, wherein the outer layer has a thickness of 0.3 mm to 3 mm.   The varistor of claim 1, wherein the outer layer does not contact the ceramic body. Providing a ceramic body;
Applying a first layer comprising a 3-resistant lacquer composed of a mixture of an acrylic component resin, an amino resin, a xylol solvent, and a curing agent on the ceramic body;
Seen containing a applying a second layer comprising an epoxy material to said first layer, and
A method of forming a varistor, wherein the overall thickness of the multilayer coating is substantially unchanged by the thickness of the inner layer .   The method of claim 8, wherein the ceramic body comprises ZnO ceramic.   The method of claim 8, wherein the first layer comprises a thickness of 5 mm to 100 mm.   The method of claim 8, wherein the applying of the first layer comprises curing the mixture to form a solid layer.   The method according to claim 11, wherein the ratio of the acrylic resin to the amino resin is 3: 1 to 19: 1.   The method according to claim 12, wherein the ratio of the acrylic component to the amino component resin is 6: 1.   The method of claim 8, wherein the second layer does not contact the ceramic body.   9. The method of claim 8, wherein the applying of the first layer comprises applying the first layer by brush application, spray application, dip application, or curtain application. The 3 resistant lacquer is
40% acrylic resin,
7% amino resin,
35% xylol,
The varistor according to claim 1, comprising 2% curing agent.   17. A varistor according to claim 16, further comprising 16% additional solvent.   18. A varistor according to claim 17, wherein xylol and further solvent are removed from the tri-resistant lacquer upon curing.   19. A varistor according to claim 18, wherein the acrylic resin and the amino resin are capable of reacting to form the 3 resistant lacquer composed of a thermosetting polymer material. A ceramic body;
A multi-layer coating disposed around the ceramic body, wherein the multi-layer coating is
An outer layer comprising an epoxy material;
An inner layer disposed adjacent to the ceramic body and between the outer layer and the ceramic body, including a thermosetting material formed by a combination of precursors other than an amine resin and an acrylic resin; A varistor comprising an inner layer. The varistor according to claim 1, wherein the thickness of the inner layer is 0.4% to 10% of the thickness of the outer layer.

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