Title:
THICK FILM VARISTOR AND METHOD FOR MAKING THE SAME
United States Patent 3725836
Abstract:
A thick film varistor having a thick film of finely divided particles of zinc oxide dispersed in a glass frit which bonds the particles together. Electrodes are provided on the thick film. The varistor is made by preparing a paste of the zinc oxide particles, the frit, and a liquid vehicle, and applying the paste to an insulating base. The paste is heated to evaporate the liquid vehicle and then melt the particles of frit to bond the particles of zinc oxide together. Thereafter the electrodes are applied. Alternatively, one electrode can be applied to the insulating base, the thick film formed thereon, and a second electrode applied to the surface of the thick film.
US Patent References:
Resistor and resistor composition
Dumesnil - September 1962 - 3052573
Method of making a non-linear resistor
Yando - November 1965 - 3220881
Resistor and resistor composition
Dumesnil - September 1962 - 3052573
Method of making a non-linear resistor
Yando - November 1965 - 3220881
Inventors:
Wada, Mitsuo (Suita-shi, JA)
Nishimoto, Kazuyuki (Osaka, JA)
Aoki, Masaki (Osaka, JA)
Iida, Yoshio (Osaka, JA)
Nishimoto, Kazuyuki (Osaka, JA)
Aoki, Masaki (Osaka, JA)
Iida, Yoshio (Osaka, JA)
Application Number:
05/146624
Publication Date:
04/03/1973
Filing Date:
05/25/1971
View Patent Images:
Export Citation:
Assignee:
Matsushita Electric Industrial Co., Ltd. (Kadoma, Osaka, JA)
Primary Class:
Other Classes:
29/620, 252/519.540, 252/519.500
International Classes:
H01C17/065; H01C17/06; H01C7/12
Field of Search:
338/13,20,21,262,308,332 117/201,217 252/501 29/620,621,610
Primary Examiner:
Albritton C. L.
Claims:
What we claim is
1. A thick film varistor consisting essentially of a thick film having 30 to 95 weight percent of finely divided particles of zinc oxide dispersed in 5 to 70 weight percent of glass frit, said zinc oxide having incorporated therein 0.1 to 8 mole percent of one member selected from the group consisting of bismuth oxide, lead oxide and barium oxide, and a pair of electrodes applied to said thick film, said finely divided particles of zinc oxide being obtained by heating zinc oxide powder at a temperature of 1100° to 1500°C and crushing the heated zinc oxide into finely divided particles.
2. A thick film varistor consisting essentially of a thick film having 30 to 95 weight percent of finely divided particles of zinc oxide dispersed in 5 to 70 weight percent of glass frit and a pair of electrodes applied to said thick film, said finely divided particles of zinc oxide being obtained by heating zinc oxide powder at a temperature of 1100° to 1500°C and crushing the heated zinc oxide into finely divided particles.
3. A thick film varistor as claimed in claim 2, wherein said glass frit consists essentially of a glass selected from the group consisting of borosilicate glass, lead borosilicate glass, zinc leadborate glass and bismuth borosilicate glass.
4. A thick film varistor as claimed in claim 3, wherein said zinc leadborate glass consists essentially of 55 to 85 weight percent of lead oxide, 10 to 25 weight percent of boron oxide and 5 to 20 weight percent of zinc oxide.
5. A thick film varistor as claimed in claim 2, wherein at least one of said two electrodes is a silver paint electrode.
6. A thick film varistor as claimed in claim 5, wherein said silver paint electrode consists essentially of 50 to 95 weight percent of silver powder and 5 to 50 weight percent of glass frit, said glass frit consisting essentially of 60 to 80 weight percent of bismuth oxide, 10 to 20 weight percent of boron oxide and 10 to 20 weight percent of silica.
7. A method for making a thick film varistor comprising providing a varistor paste comprising, as solid ingredients, 30 to 95 weight percent of finely divided particles of zinc oxide and 5 to 90 weight percent of finely divided particles of glass frit dispersed in a liquid vehicle; applying said varistor paste to an insulating base; heating the applied varistor paste to evaporate said liquid vehicle and to melt said finely divided particles of glass frit, whereby the melted glass frit bonds said finely divided particles of zinc oxide together and forms a thick film upon hardening; and providing said thick film with two electrodes.
8. A method for making a thick film varistor as claimed in claim 7, wherein said varistor paste is applied to an insulating base having one of said two electrodes formed thereon.
9. A method for making a thick film varistor as claimed in claim 7, wherein said finely divided particles of glass frit consist essentially of a member selected from the group consisting of borosilicate glass, lead borosilicate glass, zinc leadborate glass and bismuth borosilicate glass.
10. A method for making a thick film varistor as claimed in claim 9, wherein said zinc leadborate glass consists essentially of 55 to 85 weight percent of lead oxide, 10 to 25 weight percent of boron oxide and 5 to 20 weight percent of zinc oxide.
11. A method for making a thick film varistor as claimed in claim 7, wherein said finely divided particles of zinc oxide are made by heating zinc oxide powder at a temperature of 1100° to 1500°C and crushing the heated zinc oxide into finely divided particles.
12. A method for making a thick film varistor as claimed in claim 11, wherein said zinc oxide powder has incorporated therein 0.1 to 8 mole percent of a member selected from the group consisting of bismuth oxide, lead oxide and barium oxide.
13. A method for making a thick film varistor as claimed in claim 7, wherein at least one of said two electrodes is a silver paint electrode.
14. Amethod for making a thick film varistor as claimed in claim 13, wherein said silver paint electrode consists essentially of 50 to 95 weight percent of silver powder and 5 to 50 weight percent of glass frit, said glass frit consisting essentially of 60 to 80 weight percent of bismuth oxide, 10 to 20 weight percent of boron oxide and 10 to 20 weight percent of silica.
1. A thick film varistor consisting essentially of a thick film having 30 to 95 weight percent of finely divided particles of zinc oxide dispersed in 5 to 70 weight percent of glass frit, said zinc oxide having incorporated therein 0.1 to 8 mole percent of one member selected from the group consisting of bismuth oxide, lead oxide and barium oxide, and a pair of electrodes applied to said thick film, said finely divided particles of zinc oxide being obtained by heating zinc oxide powder at a temperature of 1100° to 1500°C and crushing the heated zinc oxide into finely divided particles.
2. A thick film varistor consisting essentially of a thick film having 30 to 95 weight percent of finely divided particles of zinc oxide dispersed in 5 to 70 weight percent of glass frit and a pair of electrodes applied to said thick film, said finely divided particles of zinc oxide being obtained by heating zinc oxide powder at a temperature of 1100° to 1500°C and crushing the heated zinc oxide into finely divided particles.
3. A thick film varistor as claimed in claim 2, wherein said glass frit consists essentially of a glass selected from the group consisting of borosilicate glass, lead borosilicate glass, zinc leadborate glass and bismuth borosilicate glass.
4. A thick film varistor as claimed in claim 3, wherein said zinc leadborate glass consists essentially of 55 to 85 weight percent of lead oxide, 10 to 25 weight percent of boron oxide and 5 to 20 weight percent of zinc oxide.
5. A thick film varistor as claimed in claim 2, wherein at least one of said two electrodes is a silver paint electrode.
6. A thick film varistor as claimed in claim 5, wherein said silver paint electrode consists essentially of 50 to 95 weight percent of silver powder and 5 to 50 weight percent of glass frit, said glass frit consisting essentially of 60 to 80 weight percent of bismuth oxide, 10 to 20 weight percent of boron oxide and 10 to 20 weight percent of silica.
7. A method for making a thick film varistor comprising providing a varistor paste comprising, as solid ingredients, 30 to 95 weight percent of finely divided particles of zinc oxide and 5 to 90 weight percent of finely divided particles of glass frit dispersed in a liquid vehicle; applying said varistor paste to an insulating base; heating the applied varistor paste to evaporate said liquid vehicle and to melt said finely divided particles of glass frit, whereby the melted glass frit bonds said finely divided particles of zinc oxide together and forms a thick film upon hardening; and providing said thick film with two electrodes.
8. A method for making a thick film varistor as claimed in claim 7, wherein said varistor paste is applied to an insulating base having one of said two electrodes formed thereon.
9. A method for making a thick film varistor as claimed in claim 7, wherein said finely divided particles of glass frit consist essentially of a member selected from the group consisting of borosilicate glass, lead borosilicate glass, zinc leadborate glass and bismuth borosilicate glass.
10. A method for making a thick film varistor as claimed in claim 9, wherein said zinc leadborate glass consists essentially of 55 to 85 weight percent of lead oxide, 10 to 25 weight percent of boron oxide and 5 to 20 weight percent of zinc oxide.
11. A method for making a thick film varistor as claimed in claim 7, wherein said finely divided particles of zinc oxide are made by heating zinc oxide powder at a temperature of 1100° to 1500°C and crushing the heated zinc oxide into finely divided particles.
12. A method for making a thick film varistor as claimed in claim 11, wherein said zinc oxide powder has incorporated therein 0.1 to 8 mole percent of a member selected from the group consisting of bismuth oxide, lead oxide and barium oxide.
13. A method for making a thick film varistor as claimed in claim 7, wherein at least one of said two electrodes is a silver paint electrode.
14. Amethod for making a thick film varistor as claimed in claim 13, wherein said silver paint electrode consists essentially of 50 to 95 weight percent of silver powder and 5 to 50 weight percent of glass frit, said glass frit consisting essentially of 60 to 80 weight percent of bismuth oxide, 10 to 20 weight percent of boron oxide and 10 to 20 weight percent of silica.
Description:
This invention relates to thick film varistor having finely divided particles of zinc oxide dispersed in glass and a pair of electrodes applied to said thick film and to a method for making the same.
There have been known various varistors such as silicon carbide varistor, silicon diode and selenium varistor. Usually a varistor is defined as a non-ohmic resistor the electrical resistance of which varies with the applied voltage. The electric characteristics of such a varistor are expressed by the following equation:
I = (V/C) n
Where V is the voltage across the varistor, I is the current flowing through the varistor, C is a constant equivalent to the electrical resistance at a given voltage and n is a numerical value greater than 1. The value which it is desired that C have depends upon the particular use to which the varistor is to be put. It is ordinarily desirable that the value of n be as large as possible since this exponent determines the degree to which the varistor departs from ohmic characteristics. Generally a varistor voltage V c is defined as a voltage at a flowing current of I c milliampere through the varistor. The varistor voltage referred to hereinafter is a voltage at 10 milliamperes of flowing current. The value of n is calculated from the relation 1:
n =[log (I 2 /I 1 )/log (V 2 /V 1 )] (1) where V 1 and V 2 are the voltages at the currents I 1 and I 2 , respectively.
Conventional varistors are usually of a discrete type component. Varistors of the thick film type have not been known in the art. Recently so-called IC, integrated circuits, have developed remarkably. Along with such development, resistors and capacitors have been constructed in such circuits in the form of a thick film. Consequently, the electronic industry has generated a demand for a thick film varistor for use in integrated circuits.
Therefore, a principal object of this invention is to provide a thick film varistor which can be fired on an insulating base.
Another object of this invention is to provide a method for making a thick film varistor.
These objects are achieved by providing a thick film varistor having a thick film of finely divided particles of zinc oxide dispersed in a glass frit which bonds the particles together, and by providing electrodes on the thick film. The varistor can be made by preparing a paste of the zinc oxide particles, the frit, and a liquid vehicle, applying the paste to an insulating base, and heating the paste to evaporate the liquid vehicle and then melt the particles of frit to bond the particles of zinc oxide together, and thereafter applying the electrodes. Alternatively, one electrode can be applied to the insulating base, the thick film formed thereon, and a second electrode applied to the surface of the thick film.
Other and further features of the present invention will be apparent upon consideration of the following detailed description taken together with the accompanying drawings, wherein
FIG. 1 is a cross section, on greatly enlarged scale, of a varistor according to the present invention; and
FIG. 2 is a cross section, on a greatly enlarged on a highly exaggerated scale, of a varistor according to another embodiment of the present invention.
Referring to FIG. 1, a thick film varistor according to the present invention comprises a thick film 3 applied to an insulating base 1 and a pair of electrodes 2 and 2'. Said thick film 3 has finely divided particles of zinc oxide 4 dispersed in glass frit 5.
The glass 5 is a bonding material for finely divided particles of zinc oxide 4 and may be composed of any glass capable of bonding said finely divided particles of zinc oxide 4 upon hardening. An operable composition for said thick film 3 is 30 to 95 wt.% of finely divided particles of zinc oxide and the remainder glass. A high amount of glass makes the electrical resistance of the resultant thick film 3 higher and a lower amount of glass results in poorer adherence of the thick film to the insulating base.
The structure of the thick film varistor of FIG. 1 can be modified as shown in FIG. 2. Referring to FIG. 2 wherein similar reference characters designate similar components to those of FIG. 1, a thick film 3 having finely divided particles of zinc oxide 4 dispersed in glass frit 5 is sandwiched between two electrodes 2 and 2' one of which is formed on an insulating base 1. In this structure, said one electrode 2 formed on said insulating base may be replaced by a suitable and available metal plate such as silver, platinum, titanium or nickel.
A method for making a thick film varistor contemplated by the present invention comprises the following steps; providing a varistor paste having finely divided particles of zinc oxide and finely divided particles of glass frit, as a solid ingredients, dispersed in a liquid vehicle; applying said varistor paste to an insulating base; heating the applied varistor paste to evaporate said liquid vehicle and to melt and finely divided particles of glass frit whereby the melted glass frit bonds said finely divided particles of zinc oxide together and forms a thick film upon hardening; and providing said thick film with two electrodes.
This method can be modified in the following way: The varistor paste is applied to an electrode formed on an insulating base or to a metal plate acting as an electrode. The subsequent steps are similar to those mentioned above.
Said varistor paste can be prepared by dispersing a uniform mixture of glass frit powder and zinc oxide powder, as solid ingredients, homogeneously in a liquid vehicle. The weight proportion of zinc oxide powder to glass frit powder in said mixture is 30 to 95 wt.% of zinc oxide and 5 to 70 wt.% of glass frit powder.
The liquid vehicle may vary widely in composition. Any inert liquid can be employed for this purpose, for example, water, organic solvents, with or without thickening agents, stabilizing agents, or the like, for example, methyl, ethyl, butyl, propyl or higher alcohols, the corresponding esters such as the carbitol acetates, propionates, etc., the terpenes and liquid resins, for example, pine oil, alpha-terpenol, and the like, and other liquids without limitation, the function of the liquid vehicle being mainly to form a liquid or paste of the desired consistency for application purposes. The liquid vehicles may contain volatile liquids to promote fast setting after application, or they may contain waxes, thermoplastic resins such as cellulose acetate butyrate, or wax-like materials which are thermofluid by nature whereby the composition can be applied to an insulating base while at an elevated temperature so as to set immediately upon contact with the insulating base.
The amount of said liquid vehicle relative to solid ingredient can vary with the manner of application. For example, in a stencil screen printing method, the operable weight ratio of liquid vehicle to solid ingredient is 10 to 45 wt.% of liquid vehicle and 55 to 90 wt.% of solid ingredient. An advantageous weight ratio is 15 to 30 wt.% of liquid vehicle and 70 to 85 wt.% of solid ingredient. It is preferable that the viscosity of the resultant paste be 500 to 2,000 poises.
The varistor paste is applied in a uniform thickness to the insulating base or to the electrode surface. This may be done by any application method such as a stencil, spray, print, dip or brush method.
The varistor paste applied to the insulating base is dried, if necessary, to remove the liquid vehicle and then fired in an electrical furnace at a temperature at which the glass frit fuses so as to bond the zinc oxide powder particles and to adhere firmly to the insulating base. The firing temperature may vary with the composition of glass frit. It is preferable to adjust the firing temperature so as to be 400° to 850°C.
Finely divided zinc oxide powder is prepared by pulverization of sintered zinc oxide which is heated at a temperature of 1100° to 1500°C for 0.5 to 10 hours. The pulverization of zinc oxide powder can be achieved in accordance with well known techniques. Said sintered zinc oxide may be pre-crushed into granules having a diameter of few millimeters by a crushing machine equipped with a steel or iron pestle and mortar. The granules are further pulverized into fine powder with a fine crusher such as a ball mill or vibration mill etc. The preferable particle size of the zinc oxide powder is 0.5 to 10 microns.
It has been discovered according to the present invention that the n is improved when said zinc oxide powder has incorporated therewith 0.1 to 8 mole percent of one member selected from the group consisting of bismuth oxide (Bi 2 O 3 ), lead oxide (PbO) and barium oxide (BaO). The incorporation can be achieved by a well known ceramic technique. A mixture of zinc oxide powder and additives of a given composition is heated at a high temperature of 1100° to 1500°C and then crushed into fine powder in a manner similar to that described above.
An operable glass frit for use in the varistor paste can be borosilicate glass, lead borosilicate glass, zinc leadborate glass and bismuth borosilicate glass. Most advantageous is zinc leadborate frit having a composition consisting essentially of 55 to 85 wt.% of PbO, 10 to 25 wt.% of B 2 O 3 and 5 to 20 wt.% of ZnO.
Said glass frit can be prepared in accordance with a per se well known glass frit technique. A mixture including desired starting materials is heated to a high temperature so as to form a glass frit and is quenched in water. The quenched glass frit is pulverized into powder having a desired particle size by using, for example, a wet ball mill. An advantageous average particle size for the particles of the glass frit is 0.5 to 20 microns.
The electrodes 2 and 2' may be formed by any suitable and available method, for example, evaporating or metallizing silver, gold, aluminum, copper and zinc or electroless-plating of nickel metal. It has been discovered according to the invention that a higher n is obtained by using a silver paint electrode which has finely divided particles of silver dispersed in bonding glass. Care should be taken that the softening temperature of said bonding glass is not higher than that of the glass frit of the varistor paste.
The silver paint is prepared by dispersing a mixture of silver powder and bonding glass frit powder in a liquid vehicle. Said mixture is preferably composed of 50 to 95 wt.% of silver powder and 5 to 50 wt.% of bonding glass frit powder. The composition of the bonding glass frit powder is controlled of 60 to 80 wt.% so that it is bismuth oxide, 10 to 20 wt.% of boron oxide and 10 to 20 wt.% of silica. The method of preparing the silver paint are essentially similar to those for the varistor paste mentioned above.
The following examples are given to illustrate certain preferred details of the invention, it being understood that the details of the examples are not to be taken as in any way limiting the invention thereto.
Example 1
Zinc oxide powder of a chemical grade is heated at a temperature of 1350°C for 1 hour and is pulverized into fine powder having an average particle size of 2 microns in a manner described above. Glass frit block having a composition of 74.6 wt.% of PbO, 11.6 wt.% of B 2 O 3 and 13.8 wt.% of ZnO is pulverized into a fine powder having an average particle size of 3 microns. A uniform mixture of 75 wt .% zinc oxide powder and 25 wt.% of glass frit is prepared by a well known method. The uniform mixture of these solid ingredients is mixed well with a liquid vehicle consisting of 15 wt.% of ethyl cellulose and 85 wt.% of carbitol acetate to form a varistor paste having a composition consisting of 80 wt.% of solid ingredient and 20 wt.% of liquid vehicle.
Commercially available silver paint No. 6730 (silver paint 1) from Dupont Co. in U.S.A. is applied to an aluminum oxide ceramic insulating base by a stainless steel screen stencil with a 200 mesh and is fired in air at 800°C for 10 minutes by a tunnel type kiln so as to form a silver paint electrode.
The varistor paste is applied to said silver paint electrode and is fired in air at 800°C for 10 minutes by the tunnel kiln. The resultant thick film has a thickness of about 20 microns. The silver paint 1 is applied to the varistor film and is fired at 800°C in a manner similar to that described above to form an upper silver paint electrode having an active area of 6×6 mm 2 . The thick film varistor has electrical properties shown for sample No. 1 in Table 1. In Table 1, exponent n is calculated from relation 1 by using I 1 =1mA. and I 2 =10mA. and V c is the varistor voltage at the current I c =10mA.
Example 2
A varistor paste similar to that of Example 1 is applied to an aluminum oxide ceramic insulating base in the same manner as in Example 1. The produced varistor film is provided with a pair of silver paint electrodes 6 mm wide and spaced by 0.5 mm from each other. The preparation of silver paint electrodes is similar to that of Example 1.
Resultant electrical characteristics of n and V c of this thick film varistor are shown for sample No. 2 in Table 1.
Example 3
Example 3 differs from Example 2 only in that it has a different electrode from Example 2. After the thick film has been formed on the insulating base in a manner similar to that of Example 2, the thick film is provided with two electrodes of zinc film prepared by a well known vacuum evaporation technique. The dimensions of the electrodes are similar to that of Example 2.
The resultant electrical characteristics of n and V c for this thick film varistor are shown for sample No. 3 in Table 1.
Example 4
Compositions of zinc oxide with 2 mol % of bismuth oxide (Bi 2 O 3 ), 2 mol % of lead oxide (PbO), and 2 mol % of barium oxide (BaO), respectively, are heated at 1350°C for 1 hour and then pulverized into fine powder having an average particle size of 2 microns by using a stamp mill for pre-crush and a ball mill for fine pulverization. The zinc oxide powder compositions are used for formation of thick film varistors. The other formation steps are exactly similar to those of example 1.
The resultant electrical characteristics of n and V c of these thick film varistors are shown for samples No. 4, 5 and 6 in Table 1. It can be seen that the incorporation of Bi 2 O 3 improves the n value and increases the V c as compared with the absence of zinc oxide from the thick film varistor of sample No. 1. The incorporation of PbO or BaO improves the n value and decreases the V c .
Example 5
Example 5 utilizes a different silver paint from Example 1. The silver paint (silver paint 2) is composed of bonding glass frit powder, liquid vehicle and silver particles having in an average particle size of 1 micron. The weight proportions of said silver particles and glass frit are 85 wt.% and 15 wt.%, respectively. The amount of liquid vehicle can be adjusted to provide a viscosity suitable for screen printing. The glass frit powder is of bismuth borosilicate glass having an average particle size of 4 microns and has a a composition of 15 wt .% of B 2 O 3 , 15 wt.% of SiO 2 and 70 wt.% of Bi 2 O 3 .
The thick film varistor is formed in the same manner as that of Example 1 by using said silver paint 2.
The resultant electrical characteristics of n and V c of this thick film varistor is shown for sample No. 7 in Table 1. The silver paint 2 gives an improved n value as compared with the commercially available silver paint of Example 1.
Table 1
Sample Electrode Electrode No. ZnO Composition 2* 2'* n V c (volt) 1 pure ZnO Silver Silver 5 20 Paint 1 Paint 1 2 pure ZnO " " 5 450 3 pure ZnO Zinc film Zinc 4 400 film 4 Bi 2 O 3 2 mol.% Silver Silver 6 25 incorporated Paint 1 Paint 1 5 PbO 2 mol.% " " 7 15 incorporated 6 BaO 2 mol.% " " 7 10 incorporated 7 pure ZnO Silver Silver 8 20 Paint 2 Paint 2 *: See FIG. 1 and 2
There have been known various varistors such as silicon carbide varistor, silicon diode and selenium varistor. Usually a varistor is defined as a non-ohmic resistor the electrical resistance of which varies with the applied voltage. The electric characteristics of such a varistor are expressed by the following equation:
I = (V/C) n
Where V is the voltage across the varistor, I is the current flowing through the varistor, C is a constant equivalent to the electrical resistance at a given voltage and n is a numerical value greater than 1. The value which it is desired that C have depends upon the particular use to which the varistor is to be put. It is ordinarily desirable that the value of n be as large as possible since this exponent determines the degree to which the varistor departs from ohmic characteristics. Generally a varistor voltage V c is defined as a voltage at a flowing current of I c milliampere through the varistor. The varistor voltage referred to hereinafter is a voltage at 10 milliamperes of flowing current. The value of n is calculated from the relation 1:
n =[log (I 2 /I 1 )/log (V 2 /V 1 )] (1) where V 1 and V 2 are the voltages at the currents I 1 and I 2 , respectively.
Conventional varistors are usually of a discrete type component. Varistors of the thick film type have not been known in the art. Recently so-called IC, integrated circuits, have developed remarkably. Along with such development, resistors and capacitors have been constructed in such circuits in the form of a thick film. Consequently, the electronic industry has generated a demand for a thick film varistor for use in integrated circuits.
Therefore, a principal object of this invention is to provide a thick film varistor which can be fired on an insulating base.
Another object of this invention is to provide a method for making a thick film varistor.
These objects are achieved by providing a thick film varistor having a thick film of finely divided particles of zinc oxide dispersed in a glass frit which bonds the particles together, and by providing electrodes on the thick film. The varistor can be made by preparing a paste of the zinc oxide particles, the frit, and a liquid vehicle, applying the paste to an insulating base, and heating the paste to evaporate the liquid vehicle and then melt the particles of frit to bond the particles of zinc oxide together, and thereafter applying the electrodes. Alternatively, one electrode can be applied to the insulating base, the thick film formed thereon, and a second electrode applied to the surface of the thick film.
Other and further features of the present invention will be apparent upon consideration of the following detailed description taken together with the accompanying drawings, wherein
FIG. 1 is a cross section, on greatly enlarged scale, of a varistor according to the present invention; and
FIG. 2 is a cross section, on a greatly enlarged on a highly exaggerated scale, of a varistor according to another embodiment of the present invention.
Referring to FIG. 1, a thick film varistor according to the present invention comprises a thick film 3 applied to an insulating base 1 and a pair of electrodes 2 and 2'. Said thick film 3 has finely divided particles of zinc oxide 4 dispersed in glass frit 5.
The glass 5 is a bonding material for finely divided particles of zinc oxide 4 and may be composed of any glass capable of bonding said finely divided particles of zinc oxide 4 upon hardening. An operable composition for said thick film 3 is 30 to 95 wt.% of finely divided particles of zinc oxide and the remainder glass. A high amount of glass makes the electrical resistance of the resultant thick film 3 higher and a lower amount of glass results in poorer adherence of the thick film to the insulating base.
The structure of the thick film varistor of FIG. 1 can be modified as shown in FIG. 2. Referring to FIG. 2 wherein similar reference characters designate similar components to those of FIG. 1, a thick film 3 having finely divided particles of zinc oxide 4 dispersed in glass frit 5 is sandwiched between two electrodes 2 and 2' one of which is formed on an insulating base 1. In this structure, said one electrode 2 formed on said insulating base may be replaced by a suitable and available metal plate such as silver, platinum, titanium or nickel.
A method for making a thick film varistor contemplated by the present invention comprises the following steps; providing a varistor paste having finely divided particles of zinc oxide and finely divided particles of glass frit, as a solid ingredients, dispersed in a liquid vehicle; applying said varistor paste to an insulating base; heating the applied varistor paste to evaporate said liquid vehicle and to melt and finely divided particles of glass frit whereby the melted glass frit bonds said finely divided particles of zinc oxide together and forms a thick film upon hardening; and providing said thick film with two electrodes.
This method can be modified in the following way: The varistor paste is applied to an electrode formed on an insulating base or to a metal plate acting as an electrode. The subsequent steps are similar to those mentioned above.
Said varistor paste can be prepared by dispersing a uniform mixture of glass frit powder and zinc oxide powder, as solid ingredients, homogeneously in a liquid vehicle. The weight proportion of zinc oxide powder to glass frit powder in said mixture is 30 to 95 wt.% of zinc oxide and 5 to 70 wt.% of glass frit powder.
The liquid vehicle may vary widely in composition. Any inert liquid can be employed for this purpose, for example, water, organic solvents, with or without thickening agents, stabilizing agents, or the like, for example, methyl, ethyl, butyl, propyl or higher alcohols, the corresponding esters such as the carbitol acetates, propionates, etc., the terpenes and liquid resins, for example, pine oil, alpha-terpenol, and the like, and other liquids without limitation, the function of the liquid vehicle being mainly to form a liquid or paste of the desired consistency for application purposes. The liquid vehicles may contain volatile liquids to promote fast setting after application, or they may contain waxes, thermoplastic resins such as cellulose acetate butyrate, or wax-like materials which are thermofluid by nature whereby the composition can be applied to an insulating base while at an elevated temperature so as to set immediately upon contact with the insulating base.
The amount of said liquid vehicle relative to solid ingredient can vary with the manner of application. For example, in a stencil screen printing method, the operable weight ratio of liquid vehicle to solid ingredient is 10 to 45 wt.% of liquid vehicle and 55 to 90 wt.% of solid ingredient. An advantageous weight ratio is 15 to 30 wt.% of liquid vehicle and 70 to 85 wt.% of solid ingredient. It is preferable that the viscosity of the resultant paste be 500 to 2,000 poises.
The varistor paste is applied in a uniform thickness to the insulating base or to the electrode surface. This may be done by any application method such as a stencil, spray, print, dip or brush method.
The varistor paste applied to the insulating base is dried, if necessary, to remove the liquid vehicle and then fired in an electrical furnace at a temperature at which the glass frit fuses so as to bond the zinc oxide powder particles and to adhere firmly to the insulating base. The firing temperature may vary with the composition of glass frit. It is preferable to adjust the firing temperature so as to be 400° to 850°C.
Finely divided zinc oxide powder is prepared by pulverization of sintered zinc oxide which is heated at a temperature of 1100° to 1500°C for 0.5 to 10 hours. The pulverization of zinc oxide powder can be achieved in accordance with well known techniques. Said sintered zinc oxide may be pre-crushed into granules having a diameter of few millimeters by a crushing machine equipped with a steel or iron pestle and mortar. The granules are further pulverized into fine powder with a fine crusher such as a ball mill or vibration mill etc. The preferable particle size of the zinc oxide powder is 0.5 to 10 microns.
It has been discovered according to the present invention that the n is improved when said zinc oxide powder has incorporated therewith 0.1 to 8 mole percent of one member selected from the group consisting of bismuth oxide (Bi 2 O 3 ), lead oxide (PbO) and barium oxide (BaO). The incorporation can be achieved by a well known ceramic technique. A mixture of zinc oxide powder and additives of a given composition is heated at a high temperature of 1100° to 1500°C and then crushed into fine powder in a manner similar to that described above.
An operable glass frit for use in the varistor paste can be borosilicate glass, lead borosilicate glass, zinc leadborate glass and bismuth borosilicate glass. Most advantageous is zinc leadborate frit having a composition consisting essentially of 55 to 85 wt.% of PbO, 10 to 25 wt.% of B 2 O 3 and 5 to 20 wt.% of ZnO.
Said glass frit can be prepared in accordance with a per se well known glass frit technique. A mixture including desired starting materials is heated to a high temperature so as to form a glass frit and is quenched in water. The quenched glass frit is pulverized into powder having a desired particle size by using, for example, a wet ball mill. An advantageous average particle size for the particles of the glass frit is 0.5 to 20 microns.
The electrodes 2 and 2' may be formed by any suitable and available method, for example, evaporating or metallizing silver, gold, aluminum, copper and zinc or electroless-plating of nickel metal. It has been discovered according to the invention that a higher n is obtained by using a silver paint electrode which has finely divided particles of silver dispersed in bonding glass. Care should be taken that the softening temperature of said bonding glass is not higher than that of the glass frit of the varistor paste.
The silver paint is prepared by dispersing a mixture of silver powder and bonding glass frit powder in a liquid vehicle. Said mixture is preferably composed of 50 to 95 wt.% of silver powder and 5 to 50 wt.% of bonding glass frit powder. The composition of the bonding glass frit powder is controlled of 60 to 80 wt.% so that it is bismuth oxide, 10 to 20 wt.% of boron oxide and 10 to 20 wt.% of silica. The method of preparing the silver paint are essentially similar to those for the varistor paste mentioned above.
The following examples are given to illustrate certain preferred details of the invention, it being understood that the details of the examples are not to be taken as in any way limiting the invention thereto.
Example 1
Zinc oxide powder of a chemical grade is heated at a temperature of 1350°C for 1 hour and is pulverized into fine powder having an average particle size of 2 microns in a manner described above. Glass frit block having a composition of 74.6 wt.% of PbO, 11.6 wt.% of B 2 O 3 and 13.8 wt.% of ZnO is pulverized into a fine powder having an average particle size of 3 microns. A uniform mixture of 75 wt .% zinc oxide powder and 25 wt.% of glass frit is prepared by a well known method. The uniform mixture of these solid ingredients is mixed well with a liquid vehicle consisting of 15 wt.% of ethyl cellulose and 85 wt.% of carbitol acetate to form a varistor paste having a composition consisting of 80 wt.% of solid ingredient and 20 wt.% of liquid vehicle.
Commercially available silver paint No. 6730 (silver paint 1) from Dupont Co. in U.S.A. is applied to an aluminum oxide ceramic insulating base by a stainless steel screen stencil with a 200 mesh and is fired in air at 800°C for 10 minutes by a tunnel type kiln so as to form a silver paint electrode.
The varistor paste is applied to said silver paint electrode and is fired in air at 800°C for 10 minutes by the tunnel kiln. The resultant thick film has a thickness of about 20 microns. The silver paint 1 is applied to the varistor film and is fired at 800°C in a manner similar to that described above to form an upper silver paint electrode having an active area of 6×6 mm 2 . The thick film varistor has electrical properties shown for sample No. 1 in Table 1. In Table 1, exponent n is calculated from relation 1 by using I 1 =1mA. and I 2 =10mA. and V c is the varistor voltage at the current I c =10mA.
Example 2
A varistor paste similar to that of Example 1 is applied to an aluminum oxide ceramic insulating base in the same manner as in Example 1. The produced varistor film is provided with a pair of silver paint electrodes 6 mm wide and spaced by 0.5 mm from each other. The preparation of silver paint electrodes is similar to that of Example 1.
Resultant electrical characteristics of n and V c of this thick film varistor are shown for sample No. 2 in Table 1.
Example 3
Example 3 differs from Example 2 only in that it has a different electrode from Example 2. After the thick film has been formed on the insulating base in a manner similar to that of Example 2, the thick film is provided with two electrodes of zinc film prepared by a well known vacuum evaporation technique. The dimensions of the electrodes are similar to that of Example 2.
The resultant electrical characteristics of n and V c for this thick film varistor are shown for sample No. 3 in Table 1.
Example 4
Compositions of zinc oxide with 2 mol % of bismuth oxide (Bi 2 O 3 ), 2 mol % of lead oxide (PbO), and 2 mol % of barium oxide (BaO), respectively, are heated at 1350°C for 1 hour and then pulverized into fine powder having an average particle size of 2 microns by using a stamp mill for pre-crush and a ball mill for fine pulverization. The zinc oxide powder compositions are used for formation of thick film varistors. The other formation steps are exactly similar to those of example 1.
The resultant electrical characteristics of n and V c of these thick film varistors are shown for samples No. 4, 5 and 6 in Table 1. It can be seen that the incorporation of Bi 2 O 3 improves the n value and increases the V c as compared with the absence of zinc oxide from the thick film varistor of sample No. 1. The incorporation of PbO or BaO improves the n value and decreases the V c .
Example 5
Example 5 utilizes a different silver paint from Example 1. The silver paint (silver paint 2) is composed of bonding glass frit powder, liquid vehicle and silver particles having in an average particle size of 1 micron. The weight proportions of said silver particles and glass frit are 85 wt.% and 15 wt.%, respectively. The amount of liquid vehicle can be adjusted to provide a viscosity suitable for screen printing. The glass frit powder is of bismuth borosilicate glass having an average particle size of 4 microns and has a a composition of 15 wt .% of B 2 O 3 , 15 wt.% of SiO 2 and 70 wt.% of Bi 2 O 3 .
The thick film varistor is formed in the same manner as that of Example 1 by using said silver paint 2.
The resultant electrical characteristics of n and V c of this thick film varistor is shown for sample No. 7 in Table 1. The silver paint 2 gives an improved n value as compared with the commercially available silver paint of Example 1.
Table 1
Sample Electrode Electrode No. ZnO Composition 2* 2'* n V c (volt) 1 pure ZnO Silver Silver 5 20 Paint 1 Paint 1 2 pure ZnO " " 5 450 3 pure ZnO Zinc film Zinc 4 400 film 4 Bi 2 O 3 2 mol.% Silver Silver 6 25 incorporated Paint 1 Paint 1 5 PbO 2 mol.% " " 7 15 incorporated 6 BaO 2 mol.% " " 7 10 incorporated 7 pure ZnO Silver Silver 8 20 Paint 2 Paint 2 *: See FIG. 1 and 2