JPS584801B2 - How to make thick film varistors - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing thick film barrisks from polycrystalline zinc oxide compounds.

More particularly, the present invention relates to a method for controlling varistor properties, particularly the voltage breakdown properties and dielectric constant of reconstituted polycrystalline zinc oxide thick film varistors.

A few materials are known that exhibit nonlinear low resistance characteristics and are expressed by the following equation that quantitatively relates current and voltage.

Here, ■ is the two parts separated by the body of the material under consideration.
Voltage between points, ■ is current flowing between the two points, C is constant, α
is an exponent greater than 1.

A new family of varistor materials with values of α greater than 10 at current densities in the range of 10 -3 to 102 Amps per square centimeter have recently been made from metal oxides.

Metal oxide varistor materials are polycrystalline ceramic materials formed from specific metal oxides and small amounts of one or more other metal oxides or halides added thereto.

In one example, the predominant metal oxide is zinc oxide, to which bismuth oxide and small amounts of other transition metals or oxides of metals subordinate to transition metals are added.

Further examples of such materials are described in US Pat. No. 3,682,841 and US Pat. No. 3,687,871.

The polycrystalline ceramic materials described above are fused at temperatures above 1100°C.

These temperatures are generally incompatible with circuit integration techniques.

U.S. Pat. No. 3,725,836 describes a technique for making metal oxide varistor circuits and components that are compatible with thick film circuit integration techniques, and the varistor elements are much more complex than the polycrystalline ceramic materials mentioned above. It exhibits similar electrical characteristics, although it is slightly inferior.

The method of the patent includes pulverizing ceramic varistor material;
It consists of forming a paste from the powder thus obtained and the glass frit and a suitable binder compound and firing the paste at a temperature of 400 to 850 DEG C. using conventional thick film techniques.

The electrical properties of such thick film reconstituted zinc oxide varistors can clearly be controlled by varying the thickness and area of the thick film circuit elements.

However, often electrical or physical constraints imposed by circuit operation or implementation parameters require the use of varistor materials with specially defined electrical properties.

Therefore, the clamping voltage of a varistor element of a specified thickness and area is proportional to the breakdown voltage of the varistor material used in its construction.

Similarly, the capacitance of a varistor element of constant thickness and area varies according to the dielectric constant of the varistor material of the material used.

In accordance with the present invention, the breakdown voltage characteristics and dielectric constant of thick film reconstituted polycrystalline varistor materials can be specifically controlled as a function of the firing temperature of those materials.

Accordingly, it is an object of the present invention to disclose a method for making thick film reconstituted polycrystalline varistors in which the breakdown voltage characteristics of the varistor can be controlled and, as a result, the dielectric constant can be controlled.

Another object of the invention is to disclose a method of making thick film varistors from a common starting material with the same physical dimensions and varying electrical properties.

The polycrystalline varistor material of the preferred embodiment is comprised of approximately 97 mol% zinc oxide, 1/2 mol% bismuth oxide, 1 mol%
of antimony oxide, as well as trace amounts of tin oxide, cobalt oxide, manganese oxide, barium carbonate and boric acid.
It is composed of ceramic formed by sintering at temperatures between 0°C and 1400°C for 1 hour.

The microstructure of these ceramic materials is known to consist of particles of zinc oxide separated by a thin amorphous intergranular phase.

The non-linear varistor customization characteristic of the material occurs at the boundaries formed between the zinc oxide particles and the intergranular phase.

Thick film circuit components are produced by grinding the ceramic described above and pulverizing it by jet milling, forming a mixture of the resulting powder with a glass frit and a suitable organic binder, and then processing the resulting material in a conventional manner. It can be formed by firing between thick film electrodes.

It has been found that the breakdown voltage characteristic of thick film materials formed by the process described above varies as a function of the maximum thick film firing temperature between the firing temperature range of approximately 650°C to approximately 1100°C.

FIG. 1 illustrates a set of electrical characteristic curves resulting from firing thick film varistors approximately 0.1 millimeters thick at various firing temperatures within the above temperature range.

As can be seen from the figure, the clamping voltage of these varistors (which can normally be defined as the voltage at a current of 10-3 amperes) increases as an inverse function of firing temperature by approximately 175 amps.
volts and approximately 30 volts.

It will be appreciated that the firing temperature of these thick film compositions is below the sintering temperature of the polycrystalline varistor ceramic prepared earlier.

Therefore, changes in the voltage breakdown characteristics of the varistor material are due to changes in the reconstituted thick film composition and not to changes in the underlying varistor material.

The basic varistor material is 1100℃ to approximately 1400℃
Changes will only occur at sintering temperatures within the range of .

FIG. 2 is a logarithmic scale plot of the dielectric constant of a reconstituted varistor material formed in accordance with the present invention as an inverse function of firing temperature.

As can be seen from the figure, the dielectric constant is between 650°C and 1100°C.
The firing temperature varies over the range from about 2000 to about 50 as a linear function inversely proportional to the absolute temperature.

To make the present invention easier to practice, a typical membrane varistor structure can be constructed using a powder made of varistor ceramic formed by jet milling to a particle size of approximately 1 micron to approximately 3 microns and approximately 1 micron. Prepared by mixing a glass frit with a particle size of approximately 2 microns and a binder solution containing 87.49% pine oil in 12.51% ethyl cellulose by weight to form a paste. I can do it.

A parallel plate capacitor type thick film structure is then formed by screen printing technique in the following manner.

(1) Apply Pt-Au conductor paste onto an Al2O3 substrate using a screen, dry at 110°C for 10 minutes,
The bottom electrode is formed by firing at 850°C.

(2) Apply the above-mentioned varistor paste on the bottom electrode twice using a screen, dry it, and heat it at 650°C and 1100°C.
Firing at a selected temperature between ℃.

(3) Further, apply one coat of the above paste on top of this, dry it, and bake it at the same temperature.

(4) A conductive paste is then applied, dried and fired at a temperature equal to or lower than the firing temperature of the varistor paste as determined from FIG.

The Pt-Au paste is fired at 850°C, where the varistor paste is fired at 850°C or higher.

When the varistor paste is fired at a temperature lower than 850°C, an A2 conductor paste fired at 450°C is used.

Alternatively, the Pt-Au paste in steps (1) and (4) above may be replaced with Ni paste.

Examples will be shown below in relation to the clamp voltage.

Examples ■ 97 mol% zinc oxide, 1/2 mol% bismuth oxide,
97 parts of a varistor formed from 1 mole % antimony oxide and trace amounts of tin oxide, cobalt oxide, manganese oxide, barium carbonate and boric acid and fired at 1325°C. A paste is made up of the ceramic, three parts of the glass frit, and the binder described above.

The paste is processed in the manner previously described at temperatures ranging from 650°C to 1100°C.

The breakdown voltage of the resulting film varies with firing temperature, as shown by curve A in FIG.

Example 2 A paste is made up of 90 parts of the varistor powder of Example I and 10 parts of 7 liters of glass.

The breakdown voltage of the film treated as a function of firing temperature as in Example 2 is as shown by curve B in FIG.

EXAMPLE 1 A membrane is prepared according to the method of Example I from a varistor material consisting of 98 mol % zinc oxide, 1/2 mol % bismuth oxide, and 1/2 mol % each of antimony oxide, titanium dioxide and cobalt oxide.

The breakdown voltage of this film is as a function of firing temperature as curve C in FIG.

The dielectric constant of the varistor device made according to the above method is:
Similarly, the firing temperature can be varied by selecting the firing temperature with reference to FIG.

An example is shown below.

Examples ■ 97 mol% zinc oxide, 1/2 mol% bismuth oxide,
Varistors were made by sintering 1 mol% antimony oxide and trace amounts of tin oxide, cobalt oxide, manganese oxide, boron oxide and barium oxide at 1325°C for 1 hour.
Form a ceramic.

Using 95 parts of powder formed from this ceramic, 5 parts of glass frit, and 28 organic binders, a thick film varistor with a thickness of 0.1 mm is made by the method described above.

The change in dielectric constant with respect to the maximum firing temperature of this varistor film is shown in Table ■. Maximum firing temperature Dielectric constant K (at 1 KHz) 1100°C
1620 1050°C 1245 850°C 425 650°C 68 Example ■ A varistor film structure is made by the method of Example (2) using 80 parts of the varistor powder described in Example (2) and 20 parts of glass frit.

In the case of this composition, the change in dielectric constant of the film with respect to the maximum firing temperature is as shown in Table 3.

Table ■ Maximum firing temperature K (at IKHz) 1100℃
877 1050°C 693 850°C 281 650°C 33.4 Example ■ 98 mol% zinc oxide and 1/2 mol% bismuth oxide,
Varistor by sintering cobalt oxide, manganese oxide and titanium oxide. Form a ceramic.

A thick film made from 97 parts of this ceramic and 3 parts of glass by the method described above exhibits the change in dielectric constant with respect to the maximum firing temperature shown in Table (2).

Table ■ Maximum firing temperature K (at 1KHz) 1050℃
2560 870°C 41 650°C 83.9 The device with the specified clamp voltage and capacitance characteristics can be used in conjunction with Figures 1 and 2 to select the firing temperature and adjust the thickness and area of the structure as desired. It will be clear to those skilled in the art that the electrical characteristics can be created to produce the following electrical characteristics.

In accordance with this invention, a method has been determined for making thick film varistor materials with controlled voltage breakdown and dielectric constant properties.

These methods allow varistors of fixed size or shape with various electrical properties to be made in integrated circuit form and within the physical and electrical constraints imposed by the circuit and packaging requirements. I can do it.

Although the invention has been described in detail in accordance with preferred embodiments, various modifications will occur to those skilled in the art.

Although the method of carrying out the invention has been described by way of example with respect to a particular zinc oxide varistor composition, the method is equally applicable to other compositions described in the above-identified patents and generally known in the varistor art.

Accordingly, all such modifications are included within the scope of this invention.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating on a logarithmic scale the voltage-current customization of a group of thick-film reconstituted polycrystalline varistors made in accordance with the present invention;
FIG. 2 shows the dielectric constant of a reconstituted varistor material made according to the invention on a logarithmic scale as an inverse function of firing temperature, and FIG. 3 shows the breakdown voltage versus firing temperature of a typical varistor film. be.

Claims (1)

[Claims] 1. Polycrystalline varistor. Forming a powder from the ceramic, mixing the powder with a glass frit and an organic binder to form a varistor paste, applying the varistor paste onto an insulating substrate, heating the paste to approximately 650°C and approximately 100°C.
℃ to form a thick film varistor, and controlling the breakdown voltage characteristics or dielectric constant of the thick film varistor by adjusting the maximum temperature of the firing step. How to make a membrane varistor. 2 the polycrystalline varistor ceramic comprises zinc oxide;
2. The method of claim 1, comprising a sintered mixture of bismuth oxide and a material selected from the group consisting of transition metal oxides and oxides of metals subordinate to transition metals. 3. The method according to claim 2, wherein the transition metal oxide and the oxide of a metal subordinate to the transition metal are manganese oxide, cobalt oxide, and antimony oxide. 4. The method of claim 2, wherein the polycrystalline varistor ceramic comprises titanium dioxide. 5. The method of claim 1 further comprising the steps of applying a first electrode composition between the varistor paste and the substrate and applying a second electrode composition over the varistor paste. Method described. 6. The method of claim 5, wherein the first electrode composition is gold and platinum. 7. The method of claim 5, wherein the first electrode composition is comprised of a material selected from the group consisting of gold, platinum, and nickel. 8. The method according to claim 5, comprising the step of firing the first and second electrode compositions. 9. The method according to claim 8, wherein the maximum firing temperature of the second electrode composition is not higher than the maximum firing temperature of the paste. 10. The method of claim 9, wherein the second electrode composition is comprised of a material selected from the group consisting of gold, platinum, and nickel. 11 The second electrode composition is composed of silver, and the temperature is 850°C.
10. A method according to claim 9, comprising firing at a temperature of: 12. The method of claim 5, wherein the step of applying the paste comprises screen printing the paste.