IE42714B1 - Improvements in varistors - Google Patents

Abstract

PURPOSE: To extrude always normally without generation of surging even in the extrusion of a mixture containing a large amount of a scrap material with a virgin material. CONSTITUTION: A raw material supply apparatus for scrap mixing in which a mixture of a virgin material (5) and a scrap material (6) is supplied into an extruder (20) with a screw (2) through a hopper (3) is equipped with a quantitative feeder (22) for the virgin raw material connected to the first driving motor (30) for supplying the virgin material (5), a quantitative feeder (24) for the scrap raw material connected to the second driving motor (31) for supplying the scrap material (6), an accumulation preventing controller (33) connected to a level sensor (25) installed in the hopper (3). The virgin material (5) and the scrap material (6) are independently supplied to the hopper (3), and their accumulation is detected by the level sensor (25)[JPS524096A]

Description

This invention relates to methods of manufacturing thick film varistors from polycrystalline ceramics, such as polycrystallihe zinc oxide compounds.

There are few knovm materials which exhibit nonlinear resistance characteristics and which require resort to the following equation to relate quantitatively current and voltage: where V is the voltage between points separated by a body of the material under consideration, I is the current flowing between the two points, C is a constant, and a is an exponent greater than 1.

A new family of varistor materials having a values -3 2 in excess of 10 at a current density range of 10 to 10 amperes per square centimeter has recently been produced from metal oxides. Metal oxide varistor material is a polycrystalline ceramic material formed of a particular metal oxide with small quantities of one or more other metal oxides or halides being added. As one example, the predominant metal oxide is a zinc oxide with small quantities of bismuth oxide and other transition and post transition metal oxides being added. Further examples of such materials are described in United States Patent 3,682,841 to Matsuoka et al and United States Patent 3,687,871 to Masuyama et al.

The above-described polycrystalline ceramic materials are sintered at temperatures in excess of 1100°C. These temperatures are generally incompatible with circuit integration techniques.

A technique for producing metal oxide varistor circuit components which are compatible with thick film circuit integration techniques and exhibit electrical characteristics which are similar to, although somewhat degraded from, those of the above-described polycrystalline ceramic material and is described in United States Patent 3,725,836 to Wada et al.

The Wada method comprises grinding ceramic varistor materials; forming a paste from the resulting powder, glass frit, and a suitable binder compound; and firing the paste, using conventional thick film techniques, at temperatures between 400 and 850°C.

The electrical properties of euch thick film reconstituted zinc oxide varistors may, obviously, be controlled by varying the thickness and area of the thick film circuit component.

Often, however, the electrical or physical constraints imposed by circuit operation or packaging parameters necessitate the use of varistor materials having specifically defined electrical properties. Thus, the clamping voltage of a varistor element having a specified thickness and area is proportional to the breakdown voltage of the varistor material used in its - 3 42714 construction. Likewise, the capacitance of a varistor element of constant thickness and area varies as the dielectric constant of the varistor material used.

According to the present invention there is provided a method of producing a thick film varistor comprising the steps: forming a powder from a polycrystalline ceramic suitable for use in a varistor; mixing said powder with glass frit and an organic binder to form a paste; □ applying said paste on a dielectric substrate; firing said paste at a temperature between substantially 65O°c. and substantially 1100°C whereby a thick film varistor is formed; and choosing the maximum temperature of said firing step to control the breakdown voltage and/or the dielectric constant of said thick film varistor.

The present invention will be further described, by way of example only, with, reference to the accompanying drawings, in which:0 Figure 1 is a logarithmic plot of the voltage-current characteristics of a family of thick film polycrystalline varistors produced in accordance with the present invention.

Figure 2 is a logarithmic plot of the dielectric constant of varistor material produced in accordance with the present invention as an inverse function of firing temperature.

Figure 3 is a plot of breakdown voltage vs. firing temperature for exemplary varistor films. - 4 42714 A preferred embodiment of a polycrystalline varistor material comprises a ceramic formed by sintering approximately 97 mol percent zinc oxide, 1/2 mol percent bismuth oxide, mol percent antimony oxide, and traces of tin oxide, cobalt oxide, manganese oxide, barium carbonate, and boric acid at a temperature between approximately 1100 °C and 1400°C for one hour. The microstructure of these ceramic materials is known to comprise zinc oxide grains separated by a thin amorphous intergranular phase. The non-linear varistor properties characteristic of the material occur at the boundaries formed between the zinc oxide grains and the intergranular phase.

A thick film circuit component may be formed by grinding and jet-milling the above-described ceramic; forming a mixture of the resultant powder with glass frit and a suitable organic binder; and firing the resultant material between thick film electrodes in a conventional manner.

We have found that the breakdown voltage characteristic of thick film material formed in the above-described process varies as a function of the maximum thick film firing temperature over a firing temperature range from approximately 650°C to approximately 1100°C. FIG. 1 illustrates a family of electrical characteristic curves produced by firing thick film varistors approximately 0.1 millimeter thick at various firing temperatures in the above-described range. The clamping voltage of these varistors, which may be conveniently _3 defined at a current of 10 amps, may be seen t^ vary hetwepn approximately 175 volts and approximately 30 volts as an inverse function of the firing temperatures.

It is to be understood that the firing temperatures of these, thick film compositions are below the sintering - 5 temperatures of the precursor polycrystalline varistor ceramic. The changes in the voltage breakdown characteristics of the varistor material are, therefore, attributable to changes in the reconstituted thick film composition and not to changes in the basic varistor material which changes would only occur at temperatures in the sintering range from 1100°C to approximately 1400°C.

FIG. 2 is a logarithmic plot of the dielectric constant of varistor material formed in accordance with the present invention as a function of inverse firing temperature. The dielectric constant may be seen to vary over a range from approximately 2000 to approximately 50 as a linear function of inverse absolute firing temperature over the range from 650°C to 1100°C.

A typical film varistor structure may be prepared by mixing a paste from a powder formed by jet-milling a varistor ceramic to a particle size between approximately 1 micron and approximately 3 microns, glass frit with a particle size between approximately 1 micron and approximately 2 microns, and a binder solution containing 87.49 weight percent pine oil in 12.51 weight percent ethyl cellulose. A parallel plate capacitor type thick film structure is then formed by screen printing techniques in the following manner: (1) a Pt-Au conductor paste is screened on an A^O-j substrate, dried for 10 minutes at 110°C and fired at 850'O to form a bottom electrode? (2) two coats of the above-described varistor paste are screened over the bottom electrode, dried, and fired at a temperature chosen between 650’C and 1100 °C; - 6 43714 (3) an additional coat of the above-described paste is screened on, dried, and fired at a similar temperature; and (4) a conductor paste is then screened on, dried, and fired at a temperature which is the equal to or less than the firing temperature of the varistor paste as determined from figure 2, A Pl-Au paste ie fired at 850°Ο for varistor pastes fired at 850°C or higher and an Ag conductor paste fired at 450°C is used for varistor pastes fired below 850°C, Alternately a Ni paste may be substituted for the Pt-Au paste in steps 1 and 4, The following Examples relate to the clamping voltage.

Example I The paste comprises: 97 parts of a varistor ceramic formed from 97 mol percent zinc oxide, 1/2 mol percent bismuth oxide, 1 mol percent antimony oxide, and traces of tin oxide, cobalt oxide, manganese oxide, barium carbonate, and boric acid , fired together at 1325°c,· 3 parts glass frit; and the above described binder solution. The paste is processed in the manner described above at temperatures ranging from 650 °C to 1100'JC.

The resultant film breakdown voltage varies with firing temperature in the manner depicted in FIG. 3 as curve A.

Example II The paste comprises 90 parts of the varistor powder of Example I and 10 parts glass frit. The breakdown voltage of a film processed as In Example I as a function of firing temperature is depicted in FIG. 3 as curve B.

Example III A film is produced in the manner of Example I from a varistor material comprising 98 mol percent zinc oxide, 1/2 mol percent bismuth oxide, 1/2 mol percent each antimony oxide, titanium oxide and cobalt oxide. The breakdown voltage of the film as a function of firing temperature is depicted in FIG. 3,as curve C.

The dielectric constant of the varistor devices produced in accordance with the above-described method may, likewise, be controlled by choosing firing temperature by reference to FIG. 2. As i$ pointed out by the following examples.

Example IV A varistor ceramic is formed by sintering 97 mol percent zinc oxide, 1/2 mol percent bismuth oxide, 1 mol percent antimony oxide with traces of tin oxide, cobalt oxide, manganese oxide, boron oxide and barium oxide at 1325°C for one hour. 95 parts of a powder formed from this ceramic, 5 parts of glass frit and 28 parts of urganic binder are used to produce a 0.1 mm thick film varistor in the manner described above. The variation of the dielectric constant of this varistor film with a maximum firing temperature is depicted in Table 1, Table I Maximum Firing Temperature 1100°C 1050°C 850°C 650°C K (at 1 kHz) 1620 1245 425 - 8 42714 Example ν A varistor film structure is produced in the manner of Example IV using 80 parts of the described varistor powder and 20 parts glass frit. The variation of film dielectric constant with maximum firing temperature for this composition is depicted in Table IX Table II Maximum Firing Temperature 1100°C 1050°C 850 C 650C K (at 1 kHz) 877 693 281 33.4 Example VI A varistor ceramic is formed by sintering 98 mol percent zinc oxide, with 1/2 mol percent bismuth oxide, cobalt oxide, manganese oxide, and titanium oxide. A thick film produced from 97 parts of this ceramic and 3 parts glass in the manner described above exhibits a variation of dielectric constant with maximum firing temperature which is depicted in Table III.

Table III Maximum Firing K Temperature (at 1 kHz) 1050C 2560 870'C 41 650 C 83.9 2714 It will be apparent to those skilled in the art that a device having specified clamping voltage and capacitance characteristics may be produced by choosing a firing temperature using FIGS. 1 and 2 in combination and by tailoring the thickness and area of the structures to yield the desired electrical characteristic.

By this invention we have determined methods for producing thick film varistor materials with controlled voltage breakdown and dielectric constant properties. By these methods, varistors of constant size or shape having variable electrical properties may he produced in integrated circuit form and within the physical and electrical constraints imposed by circuit and packaging considerations.

While the invention has been described in detail herein, in accord with certain preferred embodiments thereof, many modifications and changes therein may he effected hy those skilled in the art. The method for performing the invention has been described, for example, with respect to a specific zinc oxide varistor composition. The method is equally applicable, however, to other compositions described in the above-referenced patents and generally known to the varistor arts. Accordingly, it is intended hy the appended claims to cover all such modifications and changes as fall within the Scope of the invention.

Claims (18)

CLAIMS:

1. A method of producing a thick film varistor comprising the steps of: forming a powder from a polycrystalline ceramic suitable for use in a varistor; 5 mixing said powder with glass frit and an organic binder to form a paste; applying said paste on a dielectric substrate; firing said paste at a temperature between substantially 65O°C and substantially 11OO°C whereby a thick film 10 varistor is formed; and choosing the maximum temperature of said firing step to control the breakdown voltage and/or the dielectric constant of said thick film varistor.

2. A method as claimed in claim 1 wherein said polycrystalline ceramic comprises a sintered mixture of zinc oxide, bismuth oxide, and materials selected from the group consisting of transition metal oxides and post transition metal oxides.

3. A method as claimed in claim 2 wherein said transition metal oxides and post transition metal oxides comprise manganese oxides, cobalt oxides, and antimony oxides.

4. A method as claimed in any one of claims 1 to 3 further comprising the steps of: applying a first electrode composition between said paste and said substrate, and 25 applying a second electrode composition over said paste.

5. A method as claimed in claim 4 wherein said first electrode composition comprises an alloy of gold and platinum. 11 4-2714

6. A method as claimed in claim 4 wherein said first electrode composition comprises materials selected from gold/platinum alloy and nickel.

7. A method as claimed in any one of claims 4 to 6, further comprising the step of firing said first and second electrode compositions.

8. A method as claimed in claim 7 wherein the maximum firing temperature of said second electrode composition is no greater than the maximum firing temperature of said paste.

9. A method as claimed in any one of claims 5 to 8 wherein said second electrode composition comprises materials selected from gold/platinum alloy and nickel.

10. A method as claimed in any one of claims 5 to 8 wherein said second electrode composition comprises silver and wherein said second electrode composition is fired at a temperature below 85O°C.

11. A method as claimed in any one of claims 5 to 10 wherein the step of applying said paste comprises screen printing said paste.

12. A method as claimed in any one of claims 1 to 11 wherein the maximum temperature of the firing step is adjusted to control the breakdown voltage properties of the thick film varistor.

13. A method as claimed in claim 12 wherein said polycrystalline ceramic comprises titanium dioxide.

14. A method as claimed in any one of the preceding claims, wherein the varistor ceramic comprises cobalt oxide, manganese oxide and titanium oxide.

15. A method for producing a thick film varistor as claimed in claim 1 substantially as hereinbefore described in any one of Examples I to III.

16. A method for producing a thick film varistor as claimed in claim 1 substantially as hereinbefore described in any one of Examples IV to VI.

17. A thick film varistor when produced by a method 5 as claimed in any one of claims 1 to 13 and 15.

18. A thick film varistor when produced by a method as claimed in any one of claims 1 to 11 and 16.