U.S. patent number 3,611,073 [Application Number 04/880,759] was granted by the patent office on 1971-10-05 for diode comprising zinc oxide doped with gallium oxide used as a voltage variable resistor.
This patent grant is currently assigned to Matsushita Electric Industrial Co. Ltd.. Invention is credited to Kazuo Hamamoto, Takeshi Masuyama, Michio Matsuoka.
United States Patent |
3,611,073 |
Hamamoto , et al. |
October 5, 1971 |
DIODE COMPRISING ZINC OXIDE DOPED WITH GALLIUM OXIDE USED AS A
VOLTAGE VARIABLE RESISTOR
Abstract
Voltage variable resistors having nonohmic resistance comprising
a sintered wafer consisting essentially of zinc oxide with 0.05 to
10.0 mole of gallium oxide and two electrodes applied to opposite
surfaces of said sintered wafer, at least one of said two
electrodes being a silver paint electrode.
Inventors: |
Hamamoto; Kazuo (Osaka,
JA), Matsuoka; Michio (Osaka, JA),
Masuyama; Takeshi (Takatsuki-shi, JA) |
Assignee: |
Matsushita Electric Industrial Co.
Ltd. (Osaka, JA)
|
Family
ID: |
13953722 |
Appl.
No.: |
04/880,759 |
Filed: |
November 28, 1969 |
Foreign Application Priority Data
|
|
|
|
|
Dec 2, 1968 [JA] |
|
|
43/88826 |
|
Current U.S.
Class: |
257/2; 257/43;
257/471; 338/20 |
Current CPC
Class: |
H01B
1/08 (20130101); H01C 7/112 (20130101); H01C
17/285 (20130101) |
Current International
Class: |
H01C
7/105 (20060101); H01C 7/112 (20060101); H01C
17/28 (20060101); H01B 1/08 (20060101); H01l
003/22 (); H01l 007/62 (); H01l 003/00 () |
Field of
Search: |
;317/235AP,238
;252/62.3ZT,62.9 ;338/20 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Huckert; John W.
Assistant Examiner: Larkins; William D.
Claims
What we claimed is:
1. A voltage variable resistor comprising a sintered wafer
consisting essentially of zinc oxide and a member taken from the
group consisting of (a) 0.05 to 10.0 mole percent of gallium oxide
(Ga.sub.2 O.sub.3) and (b) mixtures of 0.05 to 10.0 mole percent of
gallium oxide (Ga.sub.2 O.sub.3) and 0.05 to 8.0 mole percent of
bismuth oxide (Bi.sub.2 O.sub.3,) and two electrodes applied to
opposite surfaces of said sintered wafer, at least one of said two
electrodes consisting of silver paint electrodes.
2. A voltage variable resistor according to claim 1, wherein one of
said electrodes is a silver paint electrode and the other is an
ohmic electrode.
3. A voltage variable resistor according to claim 1, wherein said
sintered wafer consists essentially of 99.9 to 97.0 mole percent of
zinc oxide (ZnO) and 0.1 to 3.0 mole percent of gallium oxide
(Ga.sub.2 O.sub.3).
4. A voltage variable resistor according to claim 1, wherein said
sintered wafer consists essentially of 99.8 to 94.0 mole percent of
zinc oxide (ZnO), 0.1 TO 3.0 mole percent of gallium oxide
(Ga.sub.2 O.sub.3) and 0.1 TO 3.0 mole percent of bismuth oxide
(Bi.sub.2 O.sub.3).
5. A voltage variable resistor according to claim 1, wherein said
silver electrode has a composition comprising 70 to 99.5 wt.
percent of silver, 0.3 to 27 wt. percent of load oxide (PbO), 0.1
to 15 wt. percent of silicon dioxide (SiO.sub.2) and 0.05 to 15 wt.
percent of boron trioxide (B.sub.2 O.sub.3).
6. A voltage variable resistor according to claim 1, wherein said
silver electrode has a composition comprising 70 to 99.5 wt.
percent of silver 0.3 to 27 wt. percent of bismuth oxide (Bi.sub.2
O.sub.3), 0.03 to 15 wt. percent of silicon dioxide (Si.sub.2) and
0.05 TO 15 wt. percent of boron trioxide (B.sub.2 O.sub.3).
7. A voltage variable resistor according to claim 1, wherein said
silver electrode has a composition comprising 70 to 99.5 wt.
percent of silver, 0.3 to 27 wt. percent of lead oxide (PbO), 0.1
to 15 wt. percent of silicon dioxide (SiO.sub.2), 0.05 to 15 wt.
percent of boron trioxide (B.sub.2 O.sub.3) and 0.05 to 6.0 wt.
percent of at least one member selected from the group consisting
of cobalt oxide (CoO) and manganese oxide (MnO).
8. A voltage variable resistor according to claim 1, wherein said
silver electrode has a composition comprising 70 to 99.5 wt.
percent of silver, 0.3 to 27 wt. percent of bismuth oxide (Bi.sub.2
O.sub.3), 0.03 to 15 wt. percent of silicon dioxide (SiO.sub.2),
0.05 to 15 wt. percent of boron trioxide (B.sub.2 O.sub.3) and 0.05
to 6.0 wt. percent of at least one member selected from the group
consisting of cobalt oxide (CoO) and manganese oxide (MnO).
Description
This invention relates to voltage variable resistors having
nonohmic resistance and more particularly relates to varistors
comprising zinc oxide and having silver electrodes applied
thereto.
Various voltage variable resistors such as silicon carbide
varistors, selenium or cuprous oxide rectifiers and germanium or
silicon PN junction diodes, are known. The electrical
characteristics of such voltage variable resistors are expressed by
the relation:
I=(V/C).sup.n
Where V is the voltage across the resistor, I is the current
flowing through the resistor, C is a constant equivalent to the
voltage at a given current and exponent n is a numerical value
greater than 1. The value of n is calculated by the following
equation:
n=(log.sub.10 (I.sub.2 /I.sub.1)/log.sub.10 (V.sub.2 /V.sub.1)
where V.sub.1 and V.sub.2 are the voltages at given currents
I.sub.1 and I.sub.2, respectively. Conveniently, I.sub.1 and
I.sub.2 are 2 are 10 ma. and 100 ma; respectively. The desired
value of C depends upon the use to which the resistor is to be put.
It is ordinarily desired that the value of n be as large as
possible since this exponent determines the degree to which the
resistors depart from ohmic characteristics.
Silicon carbide varistors are most widely used as voltage variable
resistors and are manufactured by mixing fine particles of silicon
carbide with water, a ceramic binder and/or conductive material
such a graphite or metal powder, pressing the mixture in a mold to
the desired shape, and then drying and firing the pressed body in
air or a nonoxidizing atmosphere. Silicon carbide varistors with
conductive materials are characterized by a low electric
resistance, i.e. a low valve of C and a low value of n whereas
silicon carbide varistors without conductive materials have a high
electric resistance, i.e. a high value of C and a high value of n.
It has been difficult to manufacture silicon carbide varistors
characterized by a high n and a low C. For example, silicon carbide
varistors with graphite have been known to exhibit n-valves from
2.5 to 3.3 and C-values from 6 to 13 at a given current of 100 ma.
and silicon carbide varistors without graphite shown n-values from
4 to 7 and C-values from 30 to 800 at a given current of 1 ma. with
respect for a given size of varistor, e.g. 30 mm. in diameter and 1
mm. in thickness.
Conventional rectifiers comprising selenium or cuprous oxide have
an n-value of 5 to 10 and a C-valve less than 2 at a given current
of 100 ma. with respect for a rectifier size of 20 mm. in diameter.
In this case, the thickness of the rectifier does not effect the
C-value.
A germanium or silicon PN junction resistor has an extremely high
value of n but its C-value is constant, e.g. on the order of 0.3 or
0.7 at a given current of 100 ma. because its diffusion voltage in
the V-I characteristic is constant and cannot be changed greatly.
It is necessary for obtaining a desirable C-value to combine
several diodes in series and/or in parallel. Another disadvantage
of such diodes is the complicated steps involved in their
manufacture, with resultant high cost. As a practical matter, the
use of diode resistors is not widespread at the present because of
their high cost even though they may have a high value of n.
An object of this invention is to provide a voltage variable
resistor having a high value of n and a low value of C.
A further object of this invention is to provide a voltage variable
resistor capable of being made by a simple manufacturing method at
a low cost.
A further object of this invention is to provide a voltage variable
resistor characterized by a high stability with respect to
temperature, humidity and electric load.
Another object of this invention is to provide a voltage variable
resistor, the C-value of which can be controlled.
These and other objects of the invention will become apparent upon
consideration of the following description taken together with the
accompanying drawing in which the single figure is a partly
cross-sectional view through a voltage variable resistor in
accordance with the invention.
Before proceeding with a detailed description of the voltage
variable resistors contemplated by the invention, their
construction will be described with reference to the aforesaid
figure of drawing wherein reference character 10 designates, as a
whole, a voltage variable resistor having, as its active element, a
sintered wafer 1 of electrically conductive ceramic material
according to the present invention.
Sintered wafer 1 is prepared in a manner hereinafter set forth, and
is provided with a pair of electrodes 2 and 3 having specified
compositions and applied in a suitable manner hereinafter set
forth, on two opposite surfaces of the wafer.
The wafer 1 is a sintered plate having any one of various shapes
such as circular square, rectangular, etc. Wire leads 5 and 6 are
attached in conductive relationship to the electrodes 2 and 3,
respectively, by a connection means 4 (solder or the like).
According to the present invention, sintered wafer 1 consists
essentially of zinc oxide (ZnO) and a minor portion of an additive
of gallium oxide (Ga.sub.2 O.sub.3) It is preferable that said
sintered wafer have further a minor proportion of an additive of
bismuth oxide (Bi.sub.2 O.sub.3).
It has been discovered according to the invention that said
sintered body 1 has electrical characteristics of superior
nonlinearity when it is provided with silver electrodes prepared by
applying silver paint to at least one of the opposite surfaces
thereof and firing at 300.degree. C. to 900.degree. C. in an
oxidizing atmosphere such as air or oxygen. The n-valve and C-value
of voltage variable resistors produced in this manner vary with the
compositions of the sintered body and the electrodes, and the
method of their preparation.
Since the nonlinearity of the novel resistors is attributed to the
nonohmic contact between said sintered body 1 and electrodes 2 and
3, it is necessary for obtaining a desirable C-value and n-value to
control the compositions of the sintered body 1 and the electrodes
2 and 3. The control of the C-value and the n-value can also be
obtained by applying an ohmic electrode as the electrode 3 in place
of the silver electrode.
It is necessary for achieving a low value of C for the resultant
voltage variable resistors that the sintered body have an
electrical resistivity less than 10 ohm-cm., said electrical
resistivity being measured by a four point method in a per se
conventional way.
According to the present invention, a voltage variable resistor
with low C and high n can be obtained when said resistor comprises
a sintered wafer consisting essentially of 90 to 99.95 mole percent
of zinc oxide (ZnO) and 0.05 to 10.0 mole percent of gallium oxide
(Ga.sub.2 O.sub.3) and two electrodes are applied to opposite
surfaces of said sintered wafer, at least one of said two
electrodes being a silver paint electrode.
It has been discovered according to the invention that the C-value
is further lowered when one of said electrodes applied to said
sintered wafer is a silver paint electrode and the other is an
ohmic electrode
The n-value is further elevated when said sintered wafer is 99.9 to
97.0 mole percent of zinc oxide (ZnO) and 0.1 to 3.0 mole percent
of gallium oxide.
According to the invention the combination of a lower C and a
higher n can be obtained when said sintered wafer is 99.9 to 82.0
mole percent of zinc oxide (ZnO), 0.05 to 10.0 mole percent of
gallium oxide (Ga.sub.2 O.sub.3) and gallium to 8.0 mole percent of
bismuth oxide (Bi.sub.2 O.sub.3).
The stability during an electric load life test and at ambient
temperature is improved when said silver electrode or electrodes
has a composition of 70 to 99.5 wt. percent of silver 0.3 to 27 wt.
percent of lead oxide (PbO), 0.1 to 15 wt. percent of silicon
dioxide (SiO.sub.2) and 0.05 to 15 wt. percent of boron trioxide
(B.sub.2 O.sub.3).
The stability during electric load life test and at ambient
temperature is also improved when said silver electrode or
electrodes have a composition of 70 to 99.5 wt. percent of silver,
0.3 to 27 wt. percent of bismuth oxide (Bi.sub.2 O.sub.3), 0.3 to
15 wt. percent of silicon dioxide (SiO.sub.2) and 0.05 to 15 wt.
percent of boron trioxide (B.sub.2 O.sub.3). It has been discovered
according to the invention that the n-value and the stability
during an electric load life test and at ambient temperature of the
resistors are greatly improved when said silver electrodes have a
composition of 70 to 99.5 wt. percent silver, 0.3 to 27 wt. percent
of bismuth oxide (Bi.sub.2 O.sub.3), 0.03 to 15 wt. percent of
silicon dioxide (SiO.sub.2), 0.05 to 15 wt. percent of boron
trioxide (B.sub.2 O.sub.3) and 0.05 to 6 wt. percent of at lest one
member selected from the group of consisting of cobalt oxide (CoO)
and of manganese oxide (MnO).
The n-value and the stability during an electric load life test and
at ambient temperature are further improved when said silver
electrodes have a composition of 70 to 99.5 wt. percent of silver,
0.3 to 27 wt. percent of lead oxide (PbO), 0.1 to 15 wt. percent of
silicon dioxide (SiO.sub.2), 0.05 to 15 wt. percent of boron
trioxide (B.sub.2 O.sub.3) and 0.05 to 6 wt. percent of at least
one member selected from the group of consisting of cobalt oxide
(CoO) and manganese oxide (MnO).
The sintered body 1 can be prepared by a per se well-known ceramic
technique. The starting materials in the compositions described in
the foregoing description are mixed in a wet mill so as to produce
a homogeneous mixtures. The mixtures are dried and pressed in a
mold into desired shapes at a pressure from 100 kg./cm..sup.2 to
1000 kg./cm.sup.2. The pressed bodies are sintered in air at
1100.degree.to 1450.degree. C. for 1 to 3 hours, and then
furnace-cooled to room temperature (about 15.degree.to about
30.degree. C.). The pressed bodies are preferably sintered in a
nonoxidizing atmosphere such as nitrogen and argon when it is
desired to reduce the electrical resistivity. The electrical
resistivity also can be reduced by air-quenching from the sintering
temperature to room temperature even when the pressed bodies are
fired in air.
The mixtures may be preliminarily calcined at 600.degree.to
1000.degree. C. and pulverized for easy fabrication in the
subsequent pressing step. The mixture to be pressed can be admixed
with a suitable binder such as water, polyvinyl alcohol, etc.
It is advantageous that the sintered body, have the opposite
surfaces lapped with an abrasive powder such as silicon carbide
having a particle size of 300 mesh to 1500 mesh.
The sintered bodies are coated on one or both surfaces thereof by a
silver electrode paint in a per se conventional manner such as by a
spray method, screen printing method or brushing method. Solid
ingredients having compositions described in the foregoing
description can be prepared in a per se conventional manner by
mixing commercially available powders with organic resin such as
epoxy, vinyl and phenol resin in an organic solvent such as butyl
acetate, toluene or the like so as to produce silver electrode
paints.
The silver powder can be in the form of metallic silver, or in the
form of silver carbonate or silver oxide, or in any other form
which in firing at the temperatures employed will be converted to
metallic silver. Therefore, the term "silver" as used throughout
this specification and the claims appended hereto in connection
with the silver composition before it is fired, is meant to include
silver in any form which on firing will be converted to metallic
silver. The viscosity of the resultant silver electrode paints can
be controlled by the amounts of resin and solvent. Particle size of
solid ingredients also are required to be controlled so as to be in
the range of 0.1.mu. to 5.mu..
The sintered bodies with a silver electrode on only one surface
thereof have applied to another surfaces an ohmic electrode in a
manner such as by a spray method using Al, Zn and Sn, a vacuum
evaporation method using Al. In on Zn and an electrolytic or
electroless method using Ni, Cu and Sn.
Lead wires can be applied to the silver electrodes in a per se
conventional manner by using conventional solder having a low
melting point. It is convenient to employ a conductive adhesive
comprising silver powder and resin in an organic solvent for
connecting the lead wires to the silver electrodes.
Voltage variable resistors according to this invention have a high
stability with respect to temperature and in a load life test,
which is carried out at 70.degree. C. at a rating powder for 500
hours. The n-value and C-value do not change greatly after heating
cycles and the load life test. It is preferable for achieving a
high stability with respect to humidity that the voltage variable
resistors be embedded in a humidity proof resin such as epoxy resin
or phenol resin in a per se well known manner.
According to the invention, it has been discovered that the method
of curing the applied silver electrode paint has a great effect on
the n-value of the nonlinear resistors. The n-value will not be
optimal when the applied silver electrode paint is heated in a
nonoxidizing atmosphere such as nitrogen and hydrogen for curing.
It is necessary for obtaining a high n-value that the applied
silver electrode paint be cured by heating in an oxidizing
atmosphere such as air or oxygen.
Silver electrodes prepared by any other method than by silver
painting result in poor n-value. For example, the voltage variable
resistor is not produced when the sintered body is provided with
silver electrodes on the opposite surfaces by electroless plating
or electrolytic plating in a conventional manner. Silver electrodes
prepared by vacuum evaporation or chemical deposition result in an
n-value less than 3.
The following examples are given as illustrative of the presently
preferred method of proceeding according to the present invention.
However it is not intended that the scope of said invention be
limited to the specific examples.
Example 1
Starting material according to table 1 is mixed in a wet mill for 5
hours.
The mixture is dried and pressed in a mold into a disc of 13 mm. in
diameter and 2.5 mm. in thickness at a pressure of 340
kg./cm..sup.2.
The pressed body is sintered in air at 1350.degree. C. for 1 hour,
and then quenched to room temperature (about 15.degree. to about
30.degree. C.). The sintered disc has the opposite surfaces thereof
lapping using silicon carbide having a particle size of 600 mesh.
The thus produced sintered disc is 10 mm. in diameter and 1.5 mm.
in thickness. The sintered disc is coated on the opposite surfaces
thereof with a silver electrode paint by a conventional brushing
method. The silver electrode paint employed is composed of solid
ingredient according to table 2 and is prepared by mixing with
vinyl resin in amyl acetate. The coated disc is fired at the
temperature listed in table 2 for 30 minutes in air.
Lead wires are attached to the silver electrodes by means of silver
paint. The electric characteristics of the resultant resistor and
of other similarly prepared resistors are shown in table 1.
It will be readily understood that the zinc oxide sintered body
having gallium oxide therein in has a low C value and a high
N-value and combined additions of gallium oxide and bismuth oxide
will produce a higher n-value. ##SPC1## ##SPC2##
Example 2
Starting material according to table 3 is mixed in a wet mill for 5
hours. The mixture is dried, pressed, sintered and lapped in the
same manner as in the example 1. The lapped disc is coated on one
surface thereof with the same silver electrode paints as those used
in example 1 by a conventional brushing method. The coated disc is
fired at the temperature listed on table 2 for 30 minutes in air .
The fired disc is provided on another surface thereof with an ohmic
electrode by spraying with aluminum or evaporating aluminum.
Lead wires are attached to the silver electrodes by means of silver
paint. The electric characteristics of the thus produced resistor
and of other similarly prepared resistors are shown in table 3,
wherein electrical characteristics are measured with the polarity
applying high voltage terminal to the silver electrode.
It will be easily realized that the combination of a silver
electrode and an ohmic electrode produces a low C and a high n:
##SPC3##
Example 3
A sintered disc having a comparison listed in table 4 is prepared
in the same manner as in example 1. The sintered disc is 10 mm. in
diameter and 1.5 mm. in thickness after lapping. Various silver
electrode paints are applied to the opposite surfaces of the
sintered disc and fired at the temperature listed in table 4 for 30
minutes in air. The silver electrode paints are composed at
ingredient as shown in table 4 and are prepared by mixing 100
weight parts of said solid ingredient compositions with 1 to 20
weight parts of epoxy resin in 20 to 40 weight parts of butyl
alcohol. The resultant resistors exhibit desirable C-values and
n-values as indicated in table 4. It will be readily understood
that the electrode compositions have a great effect on the
electrical characteristics of the resultant voltage variable
resistors and particularly electrode compositions containing cobalt
oxide and manganese oxide are especially useful obtaining a higher
value of n. ##SPC4##
Example 4
A sintered disc having a composition listed in table 5 is made into
resistor in the same manner as in example 1. The resistors are
tested according to the methods used for electronic component
parts. The load life test is carried out at 70.degree. C. ambient
temperature at 1 watt rating power for 500 hours. The heating cycle
test is carried out by repeating 5 times the cycle in which said
resistors are cooled to -20.degree. C. and then kept at such
temperature for 30 minutes. After the heating cycle and load life
test, the rates of change of the C-value and n-value are shown in
table 5. It will easily realized that the change rates are less
than 10 percent and the composition of silver electrodes containing
cobalt oxide and manganese oxide produce excellent stability.
##SPC5##
* * * * *