KR20170051216A - Manufacturing Methods of ZnO Varistors With High Surge Energy Capability - Google Patents

Abstract

The present invention provides a method for preparing zinc oxide (ZnO) varistors with a high surge energy tolerance. According to the present invention, the large-capacity zinc oxide varistor preparing method includes the steps of: blending an additive including bismuth oxide powder, antimony oxide powder, iron oxide powder, manganese oxides, and alumina powder; pulverizing the blended additive primarily; mixing zinc oxide powder to the pulverized additive to blend an ingredient composition for a varistor; pulverizing the ingredient composition secondarily; forming the ingredient composition; and sintering the formed ingredient composition. The present invention can prepare the zinc oxide varistor with high reliability and high surge energy tolerance for surge energy such as high power electromagnetic pulses (HPEMP) and direct lightning.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a ZnO varistor,

More particularly, the present invention relates to a method of manufacturing a ZnO varistor having a high surge energy tolerance such as a direct lightning discharge or a high power electro-magnetic pulse (HPEMP).

The zinc oxide (ZnO) varistor is a zinc oxide (ZnO) varistor which can be obtained by firing a zinc oxide raw material mixture containing zinc oxide and basic additives bismuth oxide, manganese dioxide and cobalt oxide and various oxides added for further performance improvement (Sintered body). It is known that the varistor voltage of a zinc oxide varistor rises almost in proportion to the number of grain boundaries existing between electrodes.

Conventionally, a method of suppressing the growth of ZnO particles by adding a grain growth inhibitor such as antimony oxide (Sb ₂ O ₃ ) has been used to produce a high-voltage zinc oxide varistor.

However, some of the additives added to the zinc oxide raw material powder react with zinc oxide or a part of other additives, resulting in non-uniformity, and as a result, there is a tendency that other parts of the sintered body have different grain growth sites. Therefore, there is a problem that it is difficult to manufacture a ZnO varistor having a uniform particle size distribution in the conventional manufacturing method. In addition, even when the grain growth of ZnO is controlled, there is a problem that the electric characteristics and reliability of the zinc oxide varistor are largely uneven in each lot.

In order to solve the problems of the prior art, it is an object of the present invention to provide a method of manufacturing a ZnO varistor having a high surge energy amount.

It is another object of the present invention to provide a method of manufacturing a ZnO varistor which is excellent in reproducibility and reliability against surge energy tolerance.

It is another object of the present invention to provide a large-capacity ZnO varistor produced by the above-described production method.

According to an aspect of the present invention, there is provided a method of manufacturing a semiconductor device, the method comprising: mixing an additive comprising bismuth oxide powder, antimony oxide powder, iron oxide powder, manganese oxide powder and alumina powder; Firstly pulverizing the blended additives; Mixing the powdered additive with zinc oxide powder to form a varistor raw material composition; Secondarily pulverizing the raw material composition; Molding the raw material composition; And sintering the molded body. The present invention also provides a method of manufacturing a direct-fired zinc oxide varistor.

In one embodiment of the present invention, the powder additive of the blending step preferably has a bimodal distribution having a first peak of the first grain size and a second peak of the second grain size lower. In this case, the frequency of the second peak is preferably equal to or larger than the first peak frequency.

Also, in the embodiment of the present invention, the first peak is 1 to 10 micrometers and the second peak is less than 1 micrometer.

In one embodiment of the present invention, the sintering step includes: placing an alumina crucible in a firing furnace; Disposing the formed body in the alumina crucible; And sintering the shaped body.

Alternatively, the sintering step may include the steps of disposing the magnesia crucible in the calcining furnace; And sintering the shaped body.

It is also preferable that the present invention includes a step of placing zinc oxide powder in the crucible, sintering the formed body in the zinc oxide powder atmosphere, disposing a zinc oxide plate on the crucible, and sintering the formed body on the plate.

At this time, the sintering step may be performed through a two-step process. That is, the sintering step may include degreasing the formed body with the crucible in an open state, and sintering the degreased molded body while keeping the crucible in a closed state.

According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising the steps of: preparing a molded body of an additive comprising bismuth oxide powder, antimony oxide powder, iron oxide powder, manganese oxide and alumina powder and zinc oxide powder; Disposing a molded body container in a reaction furnace; Disposing a zinc oxide plate in the molded body container; Placing the shaped body on the zinc oxide plate; And sintering the formed body. The present invention also provides a method of manufacturing a large-capacity zinc oxide varistor.

In the present invention, the molded body container is preferably made of alumina or magnesia.

According to an embodiment of the present invention, the step of sintering the molded body includes degreasing the molded body; And sintering the degreased shaped body in a closed state of the vessel.

According to another aspect of the present invention, there is provided a zinc oxide varistor produced by blending a bismuth oxide powder, an antimony oxide powder, an iron oxide powder, a manganese oxide powder, and an alumina powder additive, wherein the zinc oxide varistor has a current density Zinc oxide is 0.5 kA / cm < ² > or more.

In the present invention, the current density of the zinc oxide varistor may be 0.6 kA / cm ² or more, more preferably 0.7 kA / cm ^{2 or} more.

According to the present invention, ZnO varistors having a high surge capacity of 35 kA or more can be manufactured by controlling the particle size and the sintering atmosphere of the raw material powder.

Further, according to the present invention, a ZnO varistor having high reliability and reproducibility can be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showing a procedure of a method of manufacturing a ZnO varistor according to an embodiment of the present invention; FIG.
FIG. 2 and FIG. 3 are graphs showing the particle size distribution of the additive according to the grinding time according to an embodiment of the present invention.
4 is a diagram schematically showing a sintering atmosphere according to an embodiment of the present invention.
5 is a diagram schematically showing an embodiment according to the opening and closing state of the container in the present invention.
6 and 7 are graphs showing the results of the surge test of the samples # 5 and # 6 of the embodiment, respectively.
8 is a graph showing the results of measuring the varistor voltage before and after the surge test of sample # 6 of this embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to the preferred embodiments of the present invention with reference to the drawings.

The microstructure of ZnO varistor sintered with metal oxide as additive contains polycrystalline ZnO particles and intergranular phase between these particles. ZnO varistors exhibit excellent voltage dependence, which is called varistor effect. In other words, the grain of ZnO has a very high electrical conductivity while the other oxide forming the grain boundary has a very high resistance. The electrical properties of metal oxide varistors can be expressed as a number of fine varistors connected in series or in parallel.

For example, increasing the thickness of the varistor increases the varistor voltage. This is the same as when the fine varistors are connected in series. Also, if the area of the varistor is increased, the surge current capacity increases. This is similar to the case where the fine varistors are connected in parallel with each other. Also, doubling the volume of the varistor almost doubles the energy tolerance. This is because the number of energy absorbing ZnO particles doubles.

The distribution of ZnO particle size in microstructure is very important to determine the threshold voltage of ZnO varistor. That is, the voltage of the varistor varies with the number of series of the grain boundaries, and the surge capacity varies with the number of parallelism of the grain boundaries.

According to one embodiment of the present invention, the grain growth of the ZnO varistor is affected by the starting material and sintering conditions. ZnO varistors doped with an oxide additive are predominant in liquid phase sintering mechanism rather than solid phase sintering, and thus the content and distribution of additives affect the particle size and distribution of the sintered body. In order to fabricate a ZnO varistor having a high surge capacity, it is necessary to precisely control the ZnO particle size and grain size distribution.

Hereinafter, a method of manufacturing a ZnO varistor according to an embodiment of the present invention will be described.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showing a procedure of a method of manufacturing a ZnO varistor according to an embodiment of the present invention; FIG.

In this embodiment, bismuth oxide, antimony oxide, and iron oxide are used as additives. The iron oxide is an oxide of at least one element selected from the group consisting of Fe, Co and Ni. In this embodiment, additives such as manganese oxide and alumina are added to the additive.

The above series of additives are compounded and mixed in an appropriate amount. The mixed additive raw material is first pulverized (S100). As a result of the first milling, the additive raw material has an appropriate particle size range.

In the present invention, the volume particle size distribution of the additive raw material powder has a bimodal distribution including a first peak and a second peak lower than the first peak. According to an embodiment of the present invention, the first peak is at least 1 micrometer and the second peak is at less than 1 micron meter. According to the embodiment of the present invention, the intensity (frequency) of the second peak is preferably at least 1/2 of the intensity (frequency) of the first peak, more preferably, the intensity of the first peak and the intensity of the second peak are substantially equal Or larger than that.

Subsequently, the first milled additive raw material is mixed with the ZnO powder, and the mixed raw material is secondarily milled (S110). In the present invention, the volume particle size distribution of the raw powder as a result of the second pulverization has a bimodal distribution. Preferably, the particle size distribution of the raw material powder has a bimodal distribution having a third peak of the third grain size and a fourth peak of the fourth grain size, and the frequency of the third peak is smaller than the frequency of the fourth peak.

Next, the blended raw material powder is dried and shaped into a desired varistor shape, e.g., a circular plate shape (S120), and fired in a firing furnace (S130).

Hereinafter, embodiments of the present invention will be described in detail.

≪ Crushing and particle size distribution of additive powder >

The particle size distribution according to the pre-milling time of the additives was evaluated. The raw material powders were formulated according to the blending ratio shown in Table 1 below. As the raw material powders, ZnO (Noahtech, -200 mesh), Bi ₂ O ₃ (monoclonal industry, Max. 4 μm), Sb ₂ O ₃ (Noahtech, -325 mesh), Co ₃ O ₄ (Kosundo), Mn ₃ O ₄ ), which was used for _{NiO (Kosundo), Al 2 O} 3 (Kosundo, average particle size of 1 ㎛).

Furtherance Bi ₂ O ₃ Sb ₂ O ₃ Co ₃ O ₄ Mn ₃ O ₄ NiO Al ₂ O ₃ ZnO Mixing ratio 0.26-0.42 0.41 to 0.75 0.51-0.77 0.04 to 0.08 0.18-0.22 0.001 to 0.0014 97.92-98.46

First, the additives other than ZnO were weighed and pulverized using a 3 mm diameter zirconia ball for 0 hour, 1 hour, 10 hours, 30 hours, and 60 hours.

FIG. 2 and FIG. 3 show the particle size distribution according to the milling time.

As shown in the sample without ball milling (Fig. 2A), the sample with 10 hours of milling (Fig. 2B), the sample with milling 30 hours (Fig. 3A) As the milling time increased, the average particle size decreased. However, when the pulverization time is prolonged for 60 hours or more, coagulation between particles occurs rather (see Fig. 3 (b)). On the other hand, in the case of the sample milled for 10 hours, the frequency of the low grain size peak is almost the same as the frequency of the high grain size peak, and in the case of the sample milled for 30 hours, the peak frequency of the low grain size is higher than that of the high grain size. .

The additive powder after the first pulverization is mixed with ZnO powder and then subjected to second pulverization. The pulverized powder is molded into a suitable shape and then sintered. The sintering conditions in the present invention have a great influence on the varistor characteristics of ZnO powder.

4 is a diagram schematically showing a sintering atmosphere according to an embodiment of the present invention.

Referring to Fig. 4 (a), the molded body 10 molded in the form of a circular plate is sintered in a crucible 20 of a different material. In the embodiment of the present invention, a crucible made of alumina or magnesia is used as the crucible.

4 (b), the formed body 10 uses the atmosphere powder 30 at the bottom of the crucible such as a sagger 20, as shown in Fig. 4 (b). Here, ZnO powder having substantially the same composition as the varistor composition is used as the atmosphere powder.

In another embodiment, as shown in Fig. 4C, a ZnO plate 40 is used in the lower portion of the molded body 10 in place of the atmosphere powder.

Hereinafter, the results of the surge energy tolerance measurement of the ZnO varistor produced according to the above sintering conditions will be described.

<Evaluation of surge resistance according to sintering atmosphere>

Only the additive was milled for 10 hours, then mixed with ZnO powder and then subjected to secondary milling. Similar to the first milling, the second milling was carried out under the same conditions. Subsequently, the mixed raw material powder was formed, heated to a temperature of 500 ° C to remove the binder, and sintered at a temperature of 1200 ° C for 3 hours.

After electrodes were formed on both sides of the sintered body, a pulse current was applied using a lightning impulse current generator (ICG) generating a waveform of a 10/350 μs standard direct current transmission current waveform in order to evaluate the surge energy tolerance . The device charges the capacitor and then instantaneously applies a current through the resistor and the inductor to the varistor in response to the switch operation.

In the case of varistors for class 2 surge protector, which is currently used to protect the induction furnace (8 / 20μs) on the low - voltage line, the current density, which is an indicator of energy tolerance, is on the order of several tens A / ㎠. However, the current density of a class 1 surge protector used for protection of direct current (10 / 350μs) is several hundred A / ㎠ and requires about ten times more energy than the varistor for class 2 surge protector.

When the current applied to the varistor is above the surge energy tolerance, the varistor is mainly in the form of Flash Over, Cracking and Puncture. If the varistor voltage (V _{1 mA} ) is measured within 10% of the varistor voltage (V _{1 mA} ) before application, even if there is no apparent destruction, it is regarded as failure. These standards are referred to in "IEC 61643-1 and IEC 60060-1".

Table 2 shows the measurement results of surge energy tolerance by atmosphere. In Table 2, ZnO powder and ZnO plate were sintered in the same composition as the raw material powder.

Sample
Vessel
atmosphere
Surge energy capacity
(10/350)) Peak current
Ip [kA] Current density
J [kA / cm ² ] #One Alumina
none 29 0.536 #2 ZnO powder 32 0.591 # 3 magnesia

none 35 0.647 #4 ZnO powder 38 0.702 # 5 ZnO plate 42 0.776

As shown in Table 2, it can be seen that it is possible to manufacture a varistor having a higher surge energy tolerance when a magnesia vessel is used than when an alumina vessel is used.

In addition, when ZnO is used as the atmosphere powder, it exhibits a higher surge energy tolerance value than when ZnO is not used. In particular, it can be seen that the use of a ZnO plate instead of the atmosphere powder is useful for increasing the surge energy tolerance value.

<Additive particle size distribution and surge energy content>

The relationship between the particle size distribution of the additive powder and the surge energy tolerance was examined. ZnO varistors were prepared by varying the grinding time of the additives from 0 to 100 hours, and the surge energy tolerance was measured. Sintering was carried out on a MgO container and a ZnO plate in the same manner as in Fig. 4 (c).

Table 3 below shows the results of measurement of surge energy tolerance of ZnO varistor according to grinding time.

Grinding time
(hrs) Surge energy capacity
(10/350 &lt; Peak current
Ip [kA] Current density
J [kA / cm ² ] 0 35 0.646 10 42 0.776 30 36.5 0.674 60 35 0.646 100 32.5 0.600

Table 3 shows the surge energy tolerance according to the grinding time of the additives. As can be seen from Table 3, when the additive is pulverized for 10 to 30 hours, it shows a high surge energy tolerance value. In particular, when the grinding time is 10 hours, the surge energy tolerance is the highest (Ip: 42 kA, J: 0.776 kA / cm ² ).

&Lt; Relation between whether the container is opened or closed during sintering and the surge energy tolerance >

The relationship between the opening and closing of the container and the surge energy tolerance was examined. Fig. 5 is a diagram schematically showing the open / closed state of the container in this embodiment.

The production of the sample of this embodiment is carried out through a two-step process. First, as shown in Fig. 5 (a), the molded body is placed on a ZnO plate in a container and subjected to a degreasing process. At this time, the container is kept in the furnace in an open state. Next, as shown in Fig. 5 (b), the lid of the container is covered and held in a closed state, and then the degreased molded body is sintered. At this time, the sealed state did not reach a complete seal, and the container was sealed in such a manner that the lid and the container were joined by mechanical fitting. The vessel and the lid were made of alumina having a theoretical density of 99% or more for sealing the vessel.

The molded body was subjected to a 10-hour milling process. The first-stage degreasing process was performed at 500 ° C for 30 minutes, and the second-stage sintering process was performed at 1275 ° C for 6 hours, which is somewhat higher than in the previous example.

Table 4 below shows the measurement results of the surge energy tolerance of the samples prepared in this Example.

Sample Vessel atmosphere Container opening and closing state Surge energy capacity
(10/350)) # 6 Alumina
(Al ₂ O ₃₎ ZnO plate closeness 55 0.864

As can be seen from the above table, the fabricated sample exhibits a very high surge energy capacity of about 55 kA and exhibits good values falling within the range of 10% of the varistor voltage before the varistor voltage (V1 mA) after the surge test.

When the current applied to the varistor is more than the surge energy tolerance, the varistor mainly shows flashover, cracking and puncture. If the varistor voltage (V _{1 mA} ) is measured within 10% of the varistor voltage (V _{1 mA} ) before application, even if there is no apparent destruction, it is regarded as failure.

6 and 7 are graphs showing the results of the surge test of the samples # 5 and # 6 of the embodiment, respectively.

As shown in FIG. 6, in Sample # 5, a puncture occurred at about 43 kA. However, as shown in FIG. 7, it can be seen that Sample # 6 maintains a good state at 55 kA.

8 is a graph showing the results of measuring the varistor voltage before and after the surge test of sample # 6 of this embodiment.

As shown in FIG. 8, it can be seen that the sample # 6 exhibits a varistor voltage within 10% before and after the surge test.

8 shows the results of the varistor voltage measurement before and after the surge test of a sample (1275 ° C) obtained by sintering a molded body having the same condition, except that it is in the open state. However, it can be seen that the open sintered sample exhibits a voltage drop of about -31.4%.

This appears to be due to the fact that the container is kept closed by the lid during sintering, thereby suppressing the volatilization of the additive added even at high temperature sintering.

10 shaped body
20 Crucible
30 atmosphere powder
40 atmosphere plate

Claims (14)

Mixing an additive comprising bismuth oxide powder, antimony oxide powder, iron oxide powder, manganese oxide and alumina powder;
Firstly pulverizing the blended additives;
Mixing the powdered additive with zinc oxide powder to form a varistor raw material composition;
Secondarily pulverizing the raw material composition;
Molding the raw material composition; And
And sintering the formed molded body. The method according to claim 1,
Wherein the powder additive in the blending step has a bimodal distribution having a first peak of the first grain size and a second peak of the second grain size lower than the first peak of the first grain size. 3. The method of claim 2,
Wherein the frequency of the second peak is equal to or greater than the first peak frequency. 3. The method of claim 2,
Wherein the first peak is 1 to 10 micrometers and the second peak is less than 1 micrometer. The method according to claim 1,
Wherein the sintering step comprises:
Disposing an alumina crucible in the calcining furnace;
Disposing the formed body in the alumina crucible;
And sintering the formed body. &Lt; RTI ID = 0.0 &gt; 11. &lt; / RTI &gt; The method according to claim 1,
Wherein the sintering step comprises:
Disposing a magnesia crucible in a calcining furnace; And
And sintering the formed body. &Lt; RTI ID = 0.0 &gt; 11. &lt; / RTI &gt; The method according to claim 5 or 6,
And placing zinc oxide powder in the crucible,
Wherein the shaped body is sintered in the zinc oxide powder atmosphere. The method according to claim 5 or 6,
Wherein a zinc oxide plate is disposed on the crucible and the formed body is sintered on the plate. 9. The method of claim 8,
Wherein the sintering step comprises:
Leaving the crucible in an open state to degrease the formed body; and
And the degreased compact is sintered while keeping the crucible in a closed state.
Preparing a mixture molded body of an additive composed of bismuth oxide powder, antimony oxide powder, iron oxide powder, manganese oxide and alumina powder and zinc oxide powder;
Disposing a molded body container in a reaction furnace;
Disposing a zinc oxide plate in the molded body container;
Placing the shaped body on the zinc oxide plate; And
And sintering the shaped body. &Lt; RTI ID = 0.0 &gt; 11. &lt; / RTI &gt; 10. The method of claim 9,
Wherein the molded container is alumina. &Lt; RTI ID = 0.0 &gt; 11. &lt; / RTI &gt; 10. The method of claim 9,
Wherein the molded container is a magnesia. 10. The method of claim 9,
The step of sintering the shaped body comprises:
Degreasing the formed body; And
And sintering the degreased molded body in a closed state of the vessel. &Lt; RTI ID = 0.0 &gt; 11. &lt; / RTI &gt; A zinc oxide varistor produced by blending an additive comprising bismuth oxide powder, antimony oxide powder, iron oxide powder, manganese oxide and alumina powder,
Wherein the zinc oxide varistor has a current density of 0.5 kA / cm &lt; ² &gt; or more.

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