Title:
Metal oxide varistor with discrete bodies of metallic material therein and method for the manufacture thereof
United States Patent 3928242
Abstract:
Disclosed is a metal oxide varistor and a method for the manufacture thereof. Discrete bodies of metallic matter are combined with a mixture of a metal oxide and at least one preselected additive. Following the combining operation a varistor body portion is formed and sintered. Typically, the metal oxide is zinc oxide and the preselected additive or additives comprise at least one member of the group consisting of the oxides of bismuth, manganese, cobalt, antimony, barium, titanium, lithium, chromium, germanium, nickel and silicon. The discrete bodies of metallic matter comprise at least one member of the group consisting of bismuth, antimony, barium, boron, germanium, silicon, beryllium, copper, gadolinium, indium, selenium, tin, the precious metals and the transition metals. The size of the bodies of metallic matter can vary over broad ranges. For example, particles on the order of 100 microns have been found to yield substantially improved devices. Or, bodies with a dimension of approximately one-tenth of the thickness of the varistor body portion will improve device characteristics as hereinafter set forth.
Application Number:
05/417274
Publication Date:
12/23/1975
Filing Date:
11/19/1973
View Patent Images:
Export Citation:
Assignee:
General Electric Company (Syracuse, NY)
Primary Class:
Other Classes:
252/517, 252/519.540, 252/515, 252/514, 338/20, 252/519.500, 252/512
International Classes:
H01C7/112; H01C7/105; H01B1/02
Field of Search:
252/512,518,517,520,513,514,515 338/20
Primary Examiner:
Padgett, Benjamin R.
Assistant Examiner:
Parr, Suzanne E.
Attorney, Agent or Firm:
Mooney, Stoner R. J. D. E.
Claims:
What is claimed is
1. A method for making a metal oxide varistor comprising the steps of:
2. A method according to claim 1 wherein said discrete bodies of metallic matter comprise at least one member selected from the group consisting of bismuth, antimony, barium, boron, tin, germanium, silicon, beryllium, copper, gadolinium, indium, selenium, strontium, tantalum, thorium, tungsten, magnesium, the precious metals and the transition metals.
3. A method according to claim 2 wherein said preselected additive comprises at least one member selected from the group consisting of the oxides of bismuth, manganese, cobalt, antimony, barium, titanium, tin, lithium, chromium, germanium, nickel and silicon.
4. A method according to claim 3 wherein said sintering step is carried out at a temperature in the range of 800° to 1350°C.
5. A method according to claim 3 further comprising the step of applying metallic contacts to said body, wherein the minimum separation between said contacts defines a minimum current path length and the maximum dimension of each of said discrete bodies of metallic matter in the direction of contact separation is smaller than approximately one-tenth of the minimum current path length.
6. A metal oxide varistor having a sintered body comprising zinc oxide powder and a small percentage of at least one additive selected from the group consisting of metal oxides and metal fluorides, said zinc oxide and at least one additive forming a varistor phase and wherein said body portion further comprises discrete bodies of metallic matter distributed therethrough and wherein said discrete bodies of metallic matter are separated by said varistor phase.
7. A varistor according to claim 6 wherein said discrete bodies of metallic matter comprise at least one member of the group consisting of bismuth, antimony, tin, barium, boron, germanium, magnesium, silicon, beryllium, copper, gadolinium, indium, selenium, strontium, tantalum, thorium, tungsten, the precious metals and the transition metals.
8. A varistor according to claim 7 wherein said additive comprises at least one member of the group consisting of the oxides of bismuth, manganese, cobalt, antimony, barium, titanium, lithium, chromium, germanium, tin, nickel and silicon.
9. A varistor according to claim 8 further comprising two metallic contacts on said sintered body, wherein the minimum separation between said contacts defines a minimum current path length and the maximum dimension of each of said discrete bodies of metallic matter in the direction of contact separation is smaller than approximately one-tenth of the minimum current path length.
10. A varistor according to claim 9 wherein said discrete bodies of metallic matter are of a substantially uniform size.
11. A varistor according to claim 9 wherein said discrete bodies of metallic matter comprises, by volume, less than approximately 75 percent of said body.
12. A varistor according to claim 8 wherein said discrete bodies of metallic matter are of a planar shape and in a stacked relationship and separated by layers of said varistor phase.
13. A varistor according to claim 12 comprising metallic contacts outside of said sintered body.
1. A method for making a metal oxide varistor comprising the steps of:
2. A method according to claim 1 wherein said discrete bodies of metallic matter comprise at least one member selected from the group consisting of bismuth, antimony, barium, boron, tin, germanium, silicon, beryllium, copper, gadolinium, indium, selenium, strontium, tantalum, thorium, tungsten, magnesium, the precious metals and the transition metals.
3. A method according to claim 2 wherein said preselected additive comprises at least one member selected from the group consisting of the oxides of bismuth, manganese, cobalt, antimony, barium, titanium, tin, lithium, chromium, germanium, nickel and silicon.
4. A method according to claim 3 wherein said sintering step is carried out at a temperature in the range of 800° to 1350°C.
5. A method according to claim 3 further comprising the step of applying metallic contacts to said body, wherein the minimum separation between said contacts defines a minimum current path length and the maximum dimension of each of said discrete bodies of metallic matter in the direction of contact separation is smaller than approximately one-tenth of the minimum current path length.
6. A metal oxide varistor having a sintered body comprising zinc oxide powder and a small percentage of at least one additive selected from the group consisting of metal oxides and metal fluorides, said zinc oxide and at least one additive forming a varistor phase and wherein said body portion further comprises discrete bodies of metallic matter distributed therethrough and wherein said discrete bodies of metallic matter are separated by said varistor phase.
7. A varistor according to claim 6 wherein said discrete bodies of metallic matter comprise at least one member of the group consisting of bismuth, antimony, tin, barium, boron, germanium, magnesium, silicon, beryllium, copper, gadolinium, indium, selenium, strontium, tantalum, thorium, tungsten, the precious metals and the transition metals.
8. A varistor according to claim 7 wherein said additive comprises at least one member of the group consisting of the oxides of bismuth, manganese, cobalt, antimony, barium, titanium, lithium, chromium, germanium, tin, nickel and silicon.
9. A varistor according to claim 8 further comprising two metallic contacts on said sintered body, wherein the minimum separation between said contacts defines a minimum current path length and the maximum dimension of each of said discrete bodies of metallic matter in the direction of contact separation is smaller than approximately one-tenth of the minimum current path length.
10. A varistor according to claim 9 wherein said discrete bodies of metallic matter are of a substantially uniform size.
11. A varistor according to claim 9 wherein said discrete bodies of metallic matter comprises, by volume, less than approximately 75 percent of said body.
12. A varistor according to claim 8 wherein said discrete bodies of metallic matter are of a planar shape and in a stacked relationship and separated by layers of said varistor phase.
13. A varistor according to claim 12 comprising metallic contacts outside of said sintered body.
Description:
BACKGROUND OF THE INVENTION
This invention relates to metal oxide varistors and, more particularly, to a method of manufacturing metal oxide varistors with pure metallic additives so as to provide improved devices.
In general, the current flowing between two spaced points is directly proportional to the potential difference between those points. For most known substances, current conduction therethrough is equal to the applied potential difference divided by a constant, which has been defined by Ohm's law to be its resistance. There are, however, a few substances which exhibit non-linear resistance. Some devices, such as metal oxide varistors, utilize these substances and require resort to the following equation (1) to quantitatively relate current and voltage: ##EQU1## where V is the voltage applied to the device, I is the current flowing through the device, C is a constant and α is an exponent greater than 1. Inasmuch as the value of α determines the degree of non-linearity exhibited by the device, it is generally desired that α be relatively high. α is calculated according to the following equation (2): ##EQU2## where V 1 and V 2 are the device voltages at given currents I 1 and I 2 , respectively.
At very low currents and very high currents metal oxide varistors deviate from the characteristics expressed by equation (1) and approach linear resistance characteristics. However, for a useful current and voltage range the response of metal oxide varistors is as expressed by equation (1).
The values of C and α can be varied over wide ranges by changing the varistor formulation or the manufacturing process. Another useful varistor characteristic is the varistor voltage which can be defined as the voltage across the device when a given current is flowing through it. It is common to measure varistor voltage at a current of one milliampere and subsequent reference to varistor voltage shall be for voltage so measured. The foregoing is, of course, well known in the prior art.
Metal oxide varistors are usually manufactured as follows. A plurality of additives is mixed with a powdered metal oxide, commonly zinc oxide. Typically, four to twelve additives are employed, yet together they comprise only a small portion of the end product, for example less than 5 to 10 mole percent. In some instances the additives comprise less than one mole percent. The types and amounts of additives employed will vary with the properties sought in the varistor, although the additives are generally oxides or fluorides. Copious literature describes metal oxide varistors utilizing various oxide additive combinations. For example, see U.S. Pat. No. 3,663,458. A portion of the metal oxide and additive mixture is then pressed into a body of a desired shape and size. The body is then sintered for an appropriate time at a suitable temperature as is well known in the prior art. Sintering causes the necessary reactions among the additives and the metal oxide and fuses the mixture into a coherent pellet. Leads are then attached and the device is encapsulated by conventional methods.
A problem encountered when varistors are manufactured in accordance with the prior art technique is that the devices are sometimes unable to meet the rigorous requirements of certain demanding surge protection applications. Thus, the size and economy of metal oxide varistor voltage transient protection is not available in some applications. An example of a varistor characteristic that sometimes limits the utility of a varistor for a given application is the breadth of the useful voltage range for which the device performs as expressed by equation (1). If its range is not broad enough, a varistor may be unsuitable for certain applications.
It is, therefore, an object of this invention to provide a varistor with improved electrical properties such that it is suitable for use in various applications for which varistors were heretofore thought unsuitable. It is a further object of this invention to provide a method by which the subject varistor can be manufactured.
SUMMARY OF THE INVENTION
This invention is characterized by a metal oxide varistor and a method for the manufacture thereof. The subject varistor includes a sintered body portion that is made from a mixture of discrete bodies of metallic matter and a combination of a metal oxide and at least one additive. A preferred metal oxide that is utilized in the process disclosed below is zinc oxide. When utilizing zinc oxide, the preselected additive, or additives if more than one is selected, can be chosen from the oxides of bismuth, manganese, cobalt, antimony, barium, titanium, lithium, chromium, germanium, tin, nickel and silicon. The discrete bodies of metallic matter to be used with zinc oxide can be selected from the group consisting of bismuth, antimony, barium, boron, germanium, silicon, beryllium, tin, copper, gadolinium, indium, selenium, strontium, tantalum, thorium, tungsten, the precious metals and the transition metals. The size of the discrete bodies of metallic matter and the percentage of the total of the sintered body that they comprise can vary over wide ranges. Specific examples illustrating the breadth of the ranges will be described below.
DESCRIPTION OF THE DRAWINGS
These and other features and objects of the present invention will become more apparent upon perusal of the following description taken in conjunction with the accompanying drawings wherein:
FIG. 1 is a plot of voltage versus current representing varistor response;
FIG. 2 is a sectional elevation view of a metal oxide varistor;
FIG. 3 is a detail sectional view of a portion of a preferred varistor fabricated in accordance with the subject method; and
FIG. 4 is a sectional elevation view of a portion of an alternate varistor body fabricated in accordance with the subject technique.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to FIG. 1 there is shown a plot of metal oxide varistor voltage response. The abcissa in FIG. 1 represents the logarithm of the current passing through the varistor and the ordinate represents the logarithm of the voltage impressed across the varistor. In general, the curve can be divided into three portions.
The portion OA extending from the origin to the knee at A represents the low voltage characteristic and the high resistance the varistor exhibits in response to low voltage. Portion OA does not represent any significant varistor action. The portion AB is the useful voltage range mentioned above and is expressed by equation (1). It is, of course, the breadth of the range AB that was referred to above as limiting the applicability of metal oxide varistors in some transient protection applications. The final portion BC is again characterized by high resistance. As mentioned previously, the conduction process in metal oxide varistors is not fully understood. However, it is believed that the high resistance of portion BC is produced by the following mechanism. Conventional metal oxide varistors include a body of granular metal oxide crystals separated by intergranular regions. It is believed that the varistor action takes place in the intergranular regions at the grain boundaries and that the granular region, typically primarily zinc oxide, has little part in the varistor action. It is believed that at a threshold current density, represented by point B, the current is of a level sufficient to cause a significant voltage drop across the metal oxide grains and thus the voltage increases rapidly with increases in current.
Referring now to FIG. 2 there is shown a sectional view of a metal oxide varistor 10 which includes as its active element a sintered body portion 11 having a pair of metallic electrodes 12 and 13 in ohmic contact with the opposite surfaces thereof. The body 11 is made as hereinafter set forth and can be in any form such as circular, square or rectangular. Wire leads 15 and 16 are conductively attached to the electrodes 12 and 13, respectively, by a connection material 14 such as solder.
Fabrication of the varistor body 11 in accordance with the subject method proceeds as follows. A metal oxide powder is mixed with at least one preselected additive. This mixture, when sintered, forms a varistor phase which exhibits varistor action. More than one additive can be utilized if desired. The additive, or additives, can include any member of the group consisting of the oxides of bismuth, manganese, cobalt, antimony, barium, tin, titanium, lithium, chromium, germanium, nickel and silicon. A metal oxide that is commonly used in varistors and works well in the subject method is zinc oxide. The mixture is combined with discrete bodies of metallic material. The metallic material is selected from the group consisting of bismuth, antimony, tin, barium, boron, germanium, nickel, magnesium, silicon, beryllium, copper, gadolinium, indium, selenium, strontium, tantalum, thorium, tungsten, the precious metals and the transition metals. The bodies of metallic material can vary widely in size and shape as will become more apparent below. For example, the metallic material can be in powder form with each individual particle being approximately the size of the particle of metal oxide and additives, which is about 100 microns.
A portion of the mixture described above is pressed to form a varistor body and sintered at a temperature between 800° to 1350°C. The metallic contacts 12 and 13 are then applied. Examples of varistors fabricated in accordance with the above described method utilizing powdered metallic material are presented in the following table.
______________________________________ Metallic Sample Additive Varistor Number mole % Voltage α ______________________________________ Control None 30 20 1 1 Cr 38 25 2 1 Ni 25 20 3 1.5 B 72 21 4 * 1.5 B 79 26 5 * 1.5 B + 107 29 1.5 MnO 2 6 0.5 Ti 70 20 7 10 Ni 55 20 8 10 Cr 100 15 ______________________________________
The control composition consisted of 98 mole percent zinc oxide, 0.5 mole percent bismuth oxide, 0.5 mole percent cobalt oxide, 0.5 mole percent manganese oxide, and 0.5 mole percent titanium oxide. In samples 1 through 8 the additives shown were added to the control mixture and, following sintering, provided a varistor with the characteristics shown. Sintering was carried out at a temperature of 1200°C for two hours, except for those samples marked with an asterisk for which sintering was carried out at 1180°C. Observation of the Table will indicate the range of device characteristics available by the practice of this phase of the subject method. For example, the varistor voltages can be raised or lowered as compared to the control sample. The exponents are generally as good as or better than the exponent of the control mixture.
Referring now to FIG. 3 there is shown a detail sectional view of a portion of a varistor 10A made in accordance with the subject method. Discrete bodies 17A of metallic material are dispersed through the body 11A and separated by a varistor phase 18A that results from the sintering of a combination of a metal oxide and an additive or additives. A grain boundary is formed at the surface of each body of metallic material 17A and this boundary provides varistor action. As shown in FIG. 3, the bodies 17A are of a substantially uniform size. They need not be as uniform as shown. However, it is felt that more uniform production of varistors and thus more precise control of device properties is possible if uniform bodies 17A are employed. Bodies of metallic material can be separated into groups of substantially uniform size by passing them through a plurality of meshes.
A metal contact 12 is shown on the varistor body 10. A corresponding contact is not shown in FIG. 3; however, inasmuch as the body 10A was stated to be similar to the body 10, it will be appreciated that another metallic contact is disposed on the upper (unshown) surface of the varistor body 10A. In other words, if the entirety of the varistor 10A were shown, it would appear similar to the varistor 10. A minimum current path length in the varistor 10A is established by the smallest separation between the metal contact 13A and the contact that is not shown. In order to assure appropriate uniformity and homogeneity, it is felt that the dimension of the bodies of the metallic material 17A in the direction of current flow should be no greater than one-tenth of the minimum current path length. Thus it is assured that approximately 10 regions of the varistor phase 18A will be in any given current path.
Nearly any shape bodies 17A can be employed but how closely packed they can be made is dependent, in part, on their shape. It is felt that fabricating a varistor body 11A from a mixture including bodies of metallic material 17A in excess of 75 volume percent is unwise inasmuch as there is a danger of an excessive number of direct metal to metal contact points among the several bodies creating shorted or partially shorted devices.
The embodiment depicted in FIG. 3 is advantageous inasmuch as the useful voltage range (AB in FIG. 1) is broadened. It will be recalled that the varistor action takes place at the grain boundaries in the intergranular region and that the upturn in the curve, point B, is a result of a current density great enough to cause a significant voltage drop across the zinc oxide grains. In the body 11A the point B occurs at a higher current level inasmuch as the resistivity of the bodies of metallic material 17A is several orders of magnitude lower than that of metal oxides such as zinc oxide.
It is uncertain whether the mechanism that provides improved devices when utilizing powdered metals is similar to the mechanism in evidence when utilizing larger metallic bodies. For example, the powdered metal may oxidize and scavenge oxygen from the remainder of the body 11A. This overall lowering of oxygen content, if it in fact occurs, may affect a change in device characteristics. Furthermore, if there is a difference, it is difficult to determine at what body size range that difference manifests itself. While the mechanism is not fully understood, it is believed that the smaller particles are more effective in relatively low concentrations, for example, comprising less than 10 mole percent of the total sintered body. Conversely, the larger bodies are most effective when they constitute a substantial portion of the varistor body, for example, 50 or more percent.
Referring now to FIG. 4 there is shown another varistor body portion 11B. Substantially planar bodies of metallic material 17B are separated by layers of the varistor phase 18B. Only a portion of the body 11B is shown. In practice, it is felt that there should be at least about ten bodies of metallic material 17B in the sintered body 11B. The body 11B has contacts applied that are similar to the contacts 12 and 13 depicted above. The advantage of the body portion 11B depicted in FIG. 4 is that a substantially more uniform conductivity and varistor action is provided across the entire conductive area of the body by the clearly uniformly layered structure.
It is believed that the above described examples will impart a realization of the wide range of specific processes that can be performed under the present teaching. For example, the total concentration of metallic matter can range from a fraction of a mole percent to about 75 volume percent of the sintered body. Furthermore, sintering can be performed over a broad temperature range as, for example, between 800° and 1350°C. Also, it is believed that the group of metals delineated above will yield favorable results when incorporated in a metal oxide varistor body for the following reasons. It will be noted that the Table includes the transition metals nickel, chromium and titanium. Thus it is felt that the other transition metals manganese, cobalt, vanadium, iron, yttrium, zirconium, niobium and molybdenum will also work well. At least with respect to the embodiments of FIGS. 3 and 4, it is desirable that the bodies do not oxidize to a great extent. Thus the precious metals silver, gold, palladium, platinum, rhodium and ruthenium are expected to yield excellent devices. The remainder of the metals listed were chosen after individual consideration of such factors as free energy of oxidation, compatibility with the other constituents and the boiling point as compared to the sintering temperature range. For example, lithium, calcium and arsenic were omitted due to handling and safety problems and cadmium was omitted due to a low boiling point.
Consequently, in the light of the above teachings, many modifications and variations of the present invention will be apparent to those skilled in the art. For example, the bodies can be glass passivated if desired or, in order to prevent oxidation, sintering may be performed in an inert atmosphere. Therefore, the invention can be practiced in ways other than as specifically described.
This invention relates to metal oxide varistors and, more particularly, to a method of manufacturing metal oxide varistors with pure metallic additives so as to provide improved devices.
In general, the current flowing between two spaced points is directly proportional to the potential difference between those points. For most known substances, current conduction therethrough is equal to the applied potential difference divided by a constant, which has been defined by Ohm's law to be its resistance. There are, however, a few substances which exhibit non-linear resistance. Some devices, such as metal oxide varistors, utilize these substances and require resort to the following equation (1) to quantitatively relate current and voltage: ##EQU1## where V is the voltage applied to the device, I is the current flowing through the device, C is a constant and α is an exponent greater than 1. Inasmuch as the value of α determines the degree of non-linearity exhibited by the device, it is generally desired that α be relatively high. α is calculated according to the following equation (2): ##EQU2## where V 1 and V 2 are the device voltages at given currents I 1 and I 2 , respectively.
At very low currents and very high currents metal oxide varistors deviate from the characteristics expressed by equation (1) and approach linear resistance characteristics. However, for a useful current and voltage range the response of metal oxide varistors is as expressed by equation (1).
The values of C and α can be varied over wide ranges by changing the varistor formulation or the manufacturing process. Another useful varistor characteristic is the varistor voltage which can be defined as the voltage across the device when a given current is flowing through it. It is common to measure varistor voltage at a current of one milliampere and subsequent reference to varistor voltage shall be for voltage so measured. The foregoing is, of course, well known in the prior art.
Metal oxide varistors are usually manufactured as follows. A plurality of additives is mixed with a powdered metal oxide, commonly zinc oxide. Typically, four to twelve additives are employed, yet together they comprise only a small portion of the end product, for example less than 5 to 10 mole percent. In some instances the additives comprise less than one mole percent. The types and amounts of additives employed will vary with the properties sought in the varistor, although the additives are generally oxides or fluorides. Copious literature describes metal oxide varistors utilizing various oxide additive combinations. For example, see U.S. Pat. No. 3,663,458. A portion of the metal oxide and additive mixture is then pressed into a body of a desired shape and size. The body is then sintered for an appropriate time at a suitable temperature as is well known in the prior art. Sintering causes the necessary reactions among the additives and the metal oxide and fuses the mixture into a coherent pellet. Leads are then attached and the device is encapsulated by conventional methods.
A problem encountered when varistors are manufactured in accordance with the prior art technique is that the devices are sometimes unable to meet the rigorous requirements of certain demanding surge protection applications. Thus, the size and economy of metal oxide varistor voltage transient protection is not available in some applications. An example of a varistor characteristic that sometimes limits the utility of a varistor for a given application is the breadth of the useful voltage range for which the device performs as expressed by equation (1). If its range is not broad enough, a varistor may be unsuitable for certain applications.
It is, therefore, an object of this invention to provide a varistor with improved electrical properties such that it is suitable for use in various applications for which varistors were heretofore thought unsuitable. It is a further object of this invention to provide a method by which the subject varistor can be manufactured.
SUMMARY OF THE INVENTION
This invention is characterized by a metal oxide varistor and a method for the manufacture thereof. The subject varistor includes a sintered body portion that is made from a mixture of discrete bodies of metallic matter and a combination of a metal oxide and at least one additive. A preferred metal oxide that is utilized in the process disclosed below is zinc oxide. When utilizing zinc oxide, the preselected additive, or additives if more than one is selected, can be chosen from the oxides of bismuth, manganese, cobalt, antimony, barium, titanium, lithium, chromium, germanium, tin, nickel and silicon. The discrete bodies of metallic matter to be used with zinc oxide can be selected from the group consisting of bismuth, antimony, barium, boron, germanium, silicon, beryllium, tin, copper, gadolinium, indium, selenium, strontium, tantalum, thorium, tungsten, the precious metals and the transition metals. The size of the discrete bodies of metallic matter and the percentage of the total of the sintered body that they comprise can vary over wide ranges. Specific examples illustrating the breadth of the ranges will be described below.
DESCRIPTION OF THE DRAWINGS
These and other features and objects of the present invention will become more apparent upon perusal of the following description taken in conjunction with the accompanying drawings wherein:
FIG. 1 is a plot of voltage versus current representing varistor response;
FIG. 2 is a sectional elevation view of a metal oxide varistor;
FIG. 3 is a detail sectional view of a portion of a preferred varistor fabricated in accordance with the subject method; and
FIG. 4 is a sectional elevation view of a portion of an alternate varistor body fabricated in accordance with the subject technique.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to FIG. 1 there is shown a plot of metal oxide varistor voltage response. The abcissa in FIG. 1 represents the logarithm of the current passing through the varistor and the ordinate represents the logarithm of the voltage impressed across the varistor. In general, the curve can be divided into three portions.
The portion OA extending from the origin to the knee at A represents the low voltage characteristic and the high resistance the varistor exhibits in response to low voltage. Portion OA does not represent any significant varistor action. The portion AB is the useful voltage range mentioned above and is expressed by equation (1). It is, of course, the breadth of the range AB that was referred to above as limiting the applicability of metal oxide varistors in some transient protection applications. The final portion BC is again characterized by high resistance. As mentioned previously, the conduction process in metal oxide varistors is not fully understood. However, it is believed that the high resistance of portion BC is produced by the following mechanism. Conventional metal oxide varistors include a body of granular metal oxide crystals separated by intergranular regions. It is believed that the varistor action takes place in the intergranular regions at the grain boundaries and that the granular region, typically primarily zinc oxide, has little part in the varistor action. It is believed that at a threshold current density, represented by point B, the current is of a level sufficient to cause a significant voltage drop across the metal oxide grains and thus the voltage increases rapidly with increases in current.
Referring now to FIG. 2 there is shown a sectional view of a metal oxide varistor 10 which includes as its active element a sintered body portion 11 having a pair of metallic electrodes 12 and 13 in ohmic contact with the opposite surfaces thereof. The body 11 is made as hereinafter set forth and can be in any form such as circular, square or rectangular. Wire leads 15 and 16 are conductively attached to the electrodes 12 and 13, respectively, by a connection material 14 such as solder.
Fabrication of the varistor body 11 in accordance with the subject method proceeds as follows. A metal oxide powder is mixed with at least one preselected additive. This mixture, when sintered, forms a varistor phase which exhibits varistor action. More than one additive can be utilized if desired. The additive, or additives, can include any member of the group consisting of the oxides of bismuth, manganese, cobalt, antimony, barium, tin, titanium, lithium, chromium, germanium, nickel and silicon. A metal oxide that is commonly used in varistors and works well in the subject method is zinc oxide. The mixture is combined with discrete bodies of metallic material. The metallic material is selected from the group consisting of bismuth, antimony, tin, barium, boron, germanium, nickel, magnesium, silicon, beryllium, copper, gadolinium, indium, selenium, strontium, tantalum, thorium, tungsten, the precious metals and the transition metals. The bodies of metallic material can vary widely in size and shape as will become more apparent below. For example, the metallic material can be in powder form with each individual particle being approximately the size of the particle of metal oxide and additives, which is about 100 microns.
A portion of the mixture described above is pressed to form a varistor body and sintered at a temperature between 800° to 1350°C. The metallic contacts 12 and 13 are then applied. Examples of varistors fabricated in accordance with the above described method utilizing powdered metallic material are presented in the following table.
______________________________________ Metallic Sample Additive Varistor Number mole % Voltage α ______________________________________ Control None 30 20 1 1 Cr 38 25 2 1 Ni 25 20 3 1.5 B 72 21 4 * 1.5 B 79 26 5 * 1.5 B + 107 29 1.5 MnO 2 6 0.5 Ti 70 20 7 10 Ni 55 20 8 10 Cr 100 15 ______________________________________
The control composition consisted of 98 mole percent zinc oxide, 0.5 mole percent bismuth oxide, 0.5 mole percent cobalt oxide, 0.5 mole percent manganese oxide, and 0.5 mole percent titanium oxide. In samples 1 through 8 the additives shown were added to the control mixture and, following sintering, provided a varistor with the characteristics shown. Sintering was carried out at a temperature of 1200°C for two hours, except for those samples marked with an asterisk for which sintering was carried out at 1180°C. Observation of the Table will indicate the range of device characteristics available by the practice of this phase of the subject method. For example, the varistor voltages can be raised or lowered as compared to the control sample. The exponents are generally as good as or better than the exponent of the control mixture.
Referring now to FIG. 3 there is shown a detail sectional view of a portion of a varistor 10A made in accordance with the subject method. Discrete bodies 17A of metallic material are dispersed through the body 11A and separated by a varistor phase 18A that results from the sintering of a combination of a metal oxide and an additive or additives. A grain boundary is formed at the surface of each body of metallic material 17A and this boundary provides varistor action. As shown in FIG. 3, the bodies 17A are of a substantially uniform size. They need not be as uniform as shown. However, it is felt that more uniform production of varistors and thus more precise control of device properties is possible if uniform bodies 17A are employed. Bodies of metallic material can be separated into groups of substantially uniform size by passing them through a plurality of meshes.
A metal contact 12 is shown on the varistor body 10. A corresponding contact is not shown in FIG. 3; however, inasmuch as the body 10A was stated to be similar to the body 10, it will be appreciated that another metallic contact is disposed on the upper (unshown) surface of the varistor body 10A. In other words, if the entirety of the varistor 10A were shown, it would appear similar to the varistor 10. A minimum current path length in the varistor 10A is established by the smallest separation between the metal contact 13A and the contact that is not shown. In order to assure appropriate uniformity and homogeneity, it is felt that the dimension of the bodies of the metallic material 17A in the direction of current flow should be no greater than one-tenth of the minimum current path length. Thus it is assured that approximately 10 regions of the varistor phase 18A will be in any given current path.
Nearly any shape bodies 17A can be employed but how closely packed they can be made is dependent, in part, on their shape. It is felt that fabricating a varistor body 11A from a mixture including bodies of metallic material 17A in excess of 75 volume percent is unwise inasmuch as there is a danger of an excessive number of direct metal to metal contact points among the several bodies creating shorted or partially shorted devices.
The embodiment depicted in FIG. 3 is advantageous inasmuch as the useful voltage range (AB in FIG. 1) is broadened. It will be recalled that the varistor action takes place at the grain boundaries in the intergranular region and that the upturn in the curve, point B, is a result of a current density great enough to cause a significant voltage drop across the zinc oxide grains. In the body 11A the point B occurs at a higher current level inasmuch as the resistivity of the bodies of metallic material 17A is several orders of magnitude lower than that of metal oxides such as zinc oxide.
It is uncertain whether the mechanism that provides improved devices when utilizing powdered metals is similar to the mechanism in evidence when utilizing larger metallic bodies. For example, the powdered metal may oxidize and scavenge oxygen from the remainder of the body 11A. This overall lowering of oxygen content, if it in fact occurs, may affect a change in device characteristics. Furthermore, if there is a difference, it is difficult to determine at what body size range that difference manifests itself. While the mechanism is not fully understood, it is believed that the smaller particles are more effective in relatively low concentrations, for example, comprising less than 10 mole percent of the total sintered body. Conversely, the larger bodies are most effective when they constitute a substantial portion of the varistor body, for example, 50 or more percent.
Referring now to FIG. 4 there is shown another varistor body portion 11B. Substantially planar bodies of metallic material 17B are separated by layers of the varistor phase 18B. Only a portion of the body 11B is shown. In practice, it is felt that there should be at least about ten bodies of metallic material 17B in the sintered body 11B. The body 11B has contacts applied that are similar to the contacts 12 and 13 depicted above. The advantage of the body portion 11B depicted in FIG. 4 is that a substantially more uniform conductivity and varistor action is provided across the entire conductive area of the body by the clearly uniformly layered structure.
It is believed that the above described examples will impart a realization of the wide range of specific processes that can be performed under the present teaching. For example, the total concentration of metallic matter can range from a fraction of a mole percent to about 75 volume percent of the sintered body. Furthermore, sintering can be performed over a broad temperature range as, for example, between 800° and 1350°C. Also, it is believed that the group of metals delineated above will yield favorable results when incorporated in a metal oxide varistor body for the following reasons. It will be noted that the Table includes the transition metals nickel, chromium and titanium. Thus it is felt that the other transition metals manganese, cobalt, vanadium, iron, yttrium, zirconium, niobium and molybdenum will also work well. At least with respect to the embodiments of FIGS. 3 and 4, it is desirable that the bodies do not oxidize to a great extent. Thus the precious metals silver, gold, palladium, platinum, rhodium and ruthenium are expected to yield excellent devices. The remainder of the metals listed were chosen after individual consideration of such factors as free energy of oxidation, compatibility with the other constituents and the boiling point as compared to the sintering temperature range. For example, lithium, calcium and arsenic were omitted due to handling and safety problems and cadmium was omitted due to a low boiling point.
Consequently, in the light of the above teachings, many modifications and variations of the present invention will be apparent to those skilled in the art. For example, the bodies can be glass passivated if desired or, in order to prevent oxidation, sintering may be performed in an inert atmosphere. Therefore, the invention can be practiced in ways other than as specifically described.