IE47121B1 - Stabilized varistor - Google Patents

Abstract

The object of the proposed process is to inhibit the tendency to thermal runaway in a metal oxide variator which has to pass full load current for extended periods. A satisfactory stabilisation of the material is achieved by coating the whole external surface of the varistor element with silver and then heating it to a temperature in excess of 500 deg.C for a length of time. During this process the silver is diffused into the outer regions of the metal oxide. The process may be extended to include the production, simultaneously with the coating and diffusion, of electrical contact surfaces on the varistor. These surfaces have a thicker layer of silver. Alternatively the contact surfaces can be added in a further manufacturing stage.

Description

This invention relates in general to metal oxide varistors and more particularly to varistors having improved stability over prior art varistors and a method for the manufacture of improved stability varistors.

Metal oxide varistors have become a widely used electrical device for the suppression of over voltage t-ransients in electrical systems. Varistors are used in a large number of applications ranging from suppressors used to eliminate the effects of line voltage transients from electrical equipment to lightning arrestors for protecting high voltage electrical transmission lines. It has been a problem in certain applications of metal oxide varistors that prolonged exposure to electrical stress has resulted in undesirable degradation of the properties of the varistors themselves.

Specifically, after an extended period of operation under load, the power dissipation of certain varistors tends to increase· This increase, after a critical level of dissipation is reached, may lead to thermal runaway of the varistor device and the subsequent destruction thereof. This runaway is due to the dependence of the electrical characteristics of metal oxide varistors upon the temperature thereof. As the temperature increases the electrical resistance of varistors decreases and the current therethrough at a particular operating voltage is likewise increased. As the current in25 creases the power dissipation likewise increases and the temperature is further increased. It is essential in varistor applications wherein long lifetime is a requirement that - la47121 the increase in power dissipation of varistors as a function of time be as low as possible in order that the varistors may be expected to function in a desirable fashion over a suitable lifetime.

Accordingly, it is an object of this invention to provid'' an improved metal oxide varistor having higher stability than prior art devices. It is another object of this invention to provide an improved,high stability metal oxide varistor which may be manufactured at little increase in either effort or cost, over prior art devices.

It is yet another object of this invention to provide a method for fabricating high stability metal oxide varistors which may be readily practised in combination with presently existing methods for fabricating varistors.

Briefly stated and in accordance with a presently preferred embodiment of this invention, a high stability, metal oxide varistor is produced by providing a metal oxide varistor body, applying a source of silver to essentially the entire surface of said body and heating said body to diffuse said silver at least partially into said body, thereby forming at least a layer of silver-containing metal oxide varistor material over essentially the entire surface of the metal oxide varistor body.

It is conventional in the fabrication of metal oxide varistors to apply contacts which are typically metal contacts to opposite faces of a substantially cylindrical varistor body. It is likewise conventional to utilize silver as a contact material and also, in certain instances, to fire the silver contacts at a relatively high temperature after the application thereof to the varistor body. In accordance with another embodiment .of this invention , wherein silver contacts are employed, a relatively thin layer of silver-bearing material is applied to essentially only the non-contact areas of a varistor body and subsequently fired either separately or along with the silver contact material to produce a layer of silver-containing varistor material essentially surrounding the entire varistor body. A major portion of this silver-containing varistor material is produced as an incident to the application of silver contacts to the device while the remaining portion of the silver layer which surrounds the device'is formed by the application and firing of the relatively thin layer of silverbearing material to what is essentially the edge portion of the device. A varistor is thereby produced having essentially the entire surface thereof diffused with an attendant silver-containing layer.

In accordance with still other embodiments of this invention, various combinations of relatively thin silver bearing layers and relatively thick contact forming layers are applied to varistor bodies and fired, the common result sought to be obtained being a device having a layer containing silver extending from the surfaces of the metal oxide varistor body down into the bulk thereof through at least a portion of the thickness of the body. On those portions of the varistor body where no contact is desired to be formed, the silver is diffused into the body to an extent that essentially no silver remains at the surface which would form a low impedance surface path between contacts.

The features of the invention which are believed to be novel are pointed out with particularity in the appended claims. The invention itself, however, both as to its or-? -347121 ganization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings in which: Figure 1 is a graphical representation of the relation bet aen voltage and current with time as a parameter in an unstable metal oxide varistor.

Figure 2 is a graphical representation of the relation between power dissipation and time for an unstable metal oxide varistor and for a stable metal oxide varistor provided in accordance with the teachings of this Invention. J Figure 3 is a graphical representation of the relation between voltage and current for a metal oxide varistor made | I in accordance with this invention and also for a varistor I having high internal resistance, I Figure 4 is a view of a metal oxide varistor of the 1 prior art.

Figure 5 is a section view of another metal oxide varistor constructed in accordance with the prior art and showing the presence of silver in the body of the varistor.

Figure 6 is a section view illustrating a step in the formation of a metal oxide varistor in accordance with this invention.

Figure 7 is a section view of a metal oxide varistor body in accordance with this invention.

Figure 8 is a section view of a metal oxide varistor body in accordance with this invention and having contacts affixed thereto.

Figure 9 is a section view illustrating a step forming a metal oxide varistor in accordance with another aspect of -447121 this invention.

Figure 10 is a section view of a metal oxide varistor in accordance with this invention showing the presence of silver therein.

Referring now to Figure 1, there is illustrated in graphical form a family of curves showing the relation between a voltage impressed across a body of varistor material and the current flowing therethrough, as a function of time. While no particular values for voltage or current are shown in Figure 1, it will be appreciated that the curves illustrate, in general, the relation between voltage and current for varistor materials wherein the log of voltage is plotted along the ordinate while the log of current is plotted along the abscissa of the graph. Curves 10, 12, 14, and 16 illustrate the degradation of an unstable varistor as a function of time. Typically, curve 10 may represent the varistor at a stage in which it has not been subjected to an electrical stress and therefore exhibits optimum voltage-current characteristics. Curves 12, 14, and 16 represent the change in voltage-current characteristics as the varistor ages in the presence of electrical stress. It will be appreciated by those skilled in the art that as'the characteristics of the metal oxide varistor change from those illustrated by curve 10 to those of curve 16 in the manner illustrated, that the quiescent power dissipation, in the absence of overvoltage conditions, increases. In essence, the varistor exhibits a decreasing maximum resistance.

The degradation of the characteristics of unstable varistors may be further appreciated by referring now to Figure 2 wherein the power dissipation of a varistor in watts ' -54 7121 is plotted as a function of time for stable and unstable devices. Both devices begin with essentially the same power dissipation which depends primarily upon the nature of the device. Curve 18 illustrates the power dissipation of an unstable device while curve 20 illustrates the power dissipation of a stable device. It will be seen that during the initial portion of the life of both the stable and unstable devices, that the power dissipation of each is acceptable.

Only after the passage of a period of time which depends upon the degree of stress applied to the device and upon the ι particular characteristics of the device, does instability j begin to produce unacceptable power dissipation. Curve 18, ι t therefore, reflects an increasing power dissipation with time which after a sufficient time begins to increase rapidly as thermal runaway occurs. Reference-to curve 20 indicates I that the power dissipation for a stable device remains low. J Figure 2 is intended to indicate, only in general, the distinction between stable and unstable devices. No particular values for wattage and time are indicated, it being understood that these will vary depending upon the particular type, size, and operating conditions of the varistor involved.

Figure 3 illustrates the effect upon a varistor of otherwise desirable operating characteristics of high grain, resistivity. Figure 3 illustrates the relation between voltage and current in two varistors by curves 22 and 24 which are identical for low values of voltage and current but which diverge for increasing values. Curve 22 illustrates the effect of high grain resistivity on the high voltage performance of the device. It will be seen that the -647121 device having characteristics corresponding to curve 22 is less effective in limiting high voltages than the device corresponding to curve 24. The particular characteristic of the device which distinguishes devices having characteristics such as illustrated by curve 22 from those illustrated hy curve 24 is the series resistance of the device. Insofar as the characteristics of metal oxide varistors and the physical bases therefor are understood, the intergranular structure of varistors is primarily responsible for the nonlinear characteristics thereof while the series resistive characteristics illustrated in Figure 3 are the result of properties of the zinc oxide grains themselves. As will be hereinbelow described, the characteristics of zinc oxide varistors may be unintentionally modified from the type illustrated at curve 24 to the less desirable type illustrated by curve 22 by the diffusion of an excess amount of silver into the body of the device.

Figure 4 is a section view of a metal oxide varistor in accordance with the prior art illustrating the elements thereof which will be important in the description of the instant invention. The varistor,indicated generally at 25,includes a body portion 28 and first and second contact portions 30 and 32. To the extent that metal oxide varistors are devices well known in the art, extensive descriptions of the structure and method for formation thereof will not be presented herein. For the purpose of understanding the present invention, it will be recalled that metal oxide varistors include, within body portion 28, metal oxide grain regions which are typically zinc oxide grains separated by intergranular regions of a composition depending 121 upon the particular choice of additives utilized to form the varistor body. Body portion 28 is conventionally formed by compacting a powder mixture of zinc oxide and a plurality of additives into a green pellet which is subsequently sintered and cooled to form the varistor body itself. Subsequently, contacts are applied to the body which is conventionally in the form of a cylinder. Contacts may be applied to the varistor body in a number of ways. Conventionally, silver contacts are usefully enployed and may be either evaporated onto the varistor body or applied in a paste-like form as, for example, by silk-screening, and subsequently fired. While silver contacts provide many advantages, other types of contacts such as aluminum contacts, or a variety of other contacts such as are familiar to those skilled in the art may be enployed. Heretofore, the method selected for applying contacts to the varistor body has been merely a matter of design choice depending upon the ultimate form of the varistor package. It has not been recognized heretofore that the method for forming contacts on •the varistor body substantially affects the characteristics of the varistor material itself.

Referring now to Figure 5, it has been discovered in accordance with the present invention that when silver contacts are applied to a varistor body by firing, silver diffuses from the contacts into the body of the varistor a distance dependent upon the temperature and time of firing. It has been further discovered according to the present invention that the presence of silver in the varistor body tends to stabilize the varistor material. Figure 5 is a section view of a varistor similar to the varistor 847121 of Figure 4, wherein fired silver contacts are utilized. Stippled regions 34 and 36 underlying contacts 30 and 32, respectively, are the regions of the device containing silver.

The application of fired silver contacts to a varistor device as illustrated in Figure 5 causes a number of effects. In the area underlying the contacts, silver diffuses into the device causing locally stable device characteristics.

In the peripheral area of the device wherein no silver is present, it is thought that the leakage of the varistor material increases as theresult of the firing temperature.

For the purpose of understanding the operation of this invention, three examples can be considered. In a first example, non-fired silver contacts are applied to a sintered varistor body. In a second example, fired silver contacts are applied to a sintered varistor body, the contacts being fired at a temperature of about 800°C; and a third example wherein sintered discs are fired without the application of silver contacts thereto at the same temperature as in example 2. The relative characteristics of the varistors produced by these three examples described illustrate the effect of silver on the vpristor. In the first two examples, devices are obtained having a relatively low leakage current while in the third example, relatively high leakage current is obtained. In the first and third examples, devices are obtained which are unstable as hereinabove described with respect to time, while in the second example, stable devices are produced.

Xt will be apparent from these three examples that at least two phenomena are simultaneously occurring at the 800 °C firing temperature. First, where no silver .is present, the -947121 exposure of the sintered disc to the 800eC firing temperature substantially degrades the leakage current of the device without producing any increase in stability; second, where silver is present, however, the degradation in leakage is substantially inhibited while the stability is increased. Applying these findings to the structure of Figure 5, it will be appreciated that the device illustrated therein is not entirely satisfactory either from a leakage or from a stability aspect. While locally stable regions are produced beneath the fired silver electrodes, the stability of the edge regions of the device is not enhanced. Further, the leakage of the edge regions, which are exposed to the effects of heating at an elevated temperature without the presence of silver therein, is increased.

Accordingly, referring now to Figure 6, there is illustrated in cross section a process step according to the present invention for forming a metal oxide varistor body having both low leakage and high stability. After sintering, body 36 is coated with a relatively thin layer of silver-containing material 38 which essentially covers the entire surface of the device. Both the contact areas and the edge areas are coated. In accordance with this invention, the precise method for providing silver-containing layer 38 is not critical. Any number of methods for providing a source of silver on the surface of a varistor body may be equally well employed. By way of example, but not of limitation, the evaporation of high purity silver on the device, or the application of a thin layer of a material like that which is utilized to form silver contacts to the entire surface of the device (for example,.commercially 104?lSl available silver formulations), or other methods for providing silver in a form which is available to be diffused into the varistor body may be employed. In order to insure that, during heating, silver is diffused into the varistor body, it is essential that certain elements whicn tend to inhibit diffusion not be present at the surface of the device. For example, it is thought that the presence of silica in some glasses inhibits the diffusion of silver into the metal oxide varistor body. In accordance with a presently preferred embodiment of this invention, fine silver powder may be advantageously applied in combination with a liquid carrier vehicle which may preferably consist of N-butyl acetate and N-butyl carbitol with an ethyl cellulose binder in which the silver may be either readily suspended or dissolved. In addition, the solution or suspension may contain oxides of bismuth. Further, the silver may be either in the elemental form or present as a compound which decomposes at a temperature lower than the firing temperature, such as silver carbonate or silver nitrate.

Conveniently, the silver containing suspension may be applied to the varistor body by either dipping, spraying, or the like and then dried and fired either prior to tlie contact firing, subsequent to the contact firing, or along with the contact firing operation.

I Figure 7 illustrates a varistor body produced in accordance with a method of this invention. Body 40 is pro duced after the diffusion of the silver-containing,suspensiencoated body of Figure 6. As has been described, according to a preferred embodiment of this invention, the body of -11· Figure 6 ia fired for approximately one hour at a temperature of approximately 860’C to diffuse the silver applied to the surfaces thereof into the body as indicated by stippled region 42 surrounding the periphery of body 40. Firing is carried out for a time and at a temperature sufficient to insure that no conducting metallic silver material remains at the surface of the device. This is necessary in order to insure that after contacts are applied to faces 44 and 46, no low resistance paths will exist between the faces which would tend to short circuit the device.

Figure 8 illustrates, the application of contacts 48 and 50 to the device. In accordance with this invention, contacts 48 and 50 need not be fired silver contacts or any other particular type of contacts but rather may be essentially any form of contact known to those skilled.in the art and which is particularly adapted to the intended varistor use. The enhanced stability in accordance with this invention is provided by silver containing peripheral varistor region'42 which is formed as described in the foregoing steps. It will be understood, however, that where desired, electrodes 48 and 50 may be formed as is conventional by applying a silver containing suspension and firing the device.

Figure 9 illustrates the simultaneous application of both contact-forming and stability-enhancing silver-bearing layers to a varistor body before firing in accordance with a method of this invention. The silver-containing material is applied in two thicknesses: a first thickness, in the area where it is desired to form contacts 52 and 54, and a second thickness, around the periphery of the device as indicated by regions 56 and 58. It will be appreciated that 12-. varistors are typically cylindrical and therefore that regions 56 and 58 are in fact the same region. It is necessary to form a silver-bearing layer in two thicknesses prior to firing so that when the device is in fact fired, the material in regions 56 and 58 will completely diffuse into the varxstor body while the material in contact regions 52 and 54 will not completely diffuse but rather partially diffuse to provide silver contacts at the surfaces of the device, along with a silver containing varistor layer thereunder.

Figure 10 illustrates a stable varistor in accordance with this invention wherein the silver containing portion of the varistor body is shown in the drawing as a stippled area. In Figure 10, silver diffusion is indicated to have taken place to different depths beneath the contact regions 60 and 62 and the edge regions 64 and 66. This would be the case for a varistor formed as illustrated in Figures 4-8 Where contacts 48 and 50 of Figure 8 are in fact fired. It Will be understood that where a device is formed as illustrated in Figure 9, a more uniform depth of diffusion of silver is obtained.

It will be apparent from the foregoing that while certain elements and processes are required in accordance with this invention, that various, modifications and changes both in the details of the invention and in the order of performing the steps thereof may occur to those skilled in the art and still lie within the true spirit and scope of the invention. For example, as has been described, the application of contacts to the oontactrreceiving surfaces of the varistor body and the application of silver-bearing materials to the entire or substantially entire surface -1347121 of the body may be performed in a number of ways, either application being performed before the other and the desired layer of silver-bearing material being applied in one or more steps.

Figure 11 illustrates in block diagram, flow chart form the essential elements of the invention in accordance with one particular embodiment thereof. A varistor body is first provided and silver is applied to its surface. After the application of silver to the body, an optional firing step may be provided. Contacts are then applied to the body followed by a second optional firing step. While the firing steps are characterized as optional, it will be understood that one or the other of the firing steps must be provided. This provides for a diffused layer of silver surrounding the entire metal oxide varistor body, the source of which silver is either the contacts, the silver-containing layer, or some combination thereof.

While the invention has been particularly shown and described with reference to several preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the true spirit and scope of the invention as defined by the appended claims.

Claims (17)

1. A method for the manufacture of metal oxide varistors comprising: providing a body of metal oxide varistor material; coating said body with a source of silver, said. 5 source of silver covering essentially all of said body; heating said body to a temperature in excess of 500 °C for a time sufficient to significantly increase the stability of said metal oxide varisto-.

2. The method of Claim 1 wherein said coating step comprises: coating a contact area on said varistor body with a relatively thick contact forming source of silver; and 5 coating the remainder of said body with a relatively thin source of silver.

3. The method of Claim 1 wherein said coating step comprises: coating essentially all of said varistor body with a relatively thin layer of silver.

4. The method of Claim 1 wherein said coating step comprises: coating essentially all of said varistor body with a relatively thin layer of silver; and S coating a contact area of said varistor body with a relatively thick layer of silver.

5. The method of Claim 1 wherein said heating step comprises heating said body to a temperature between about 800° and 950’C. -1547121

6. The method of Claim 1 wherein said time is approximately one hour.

7. Hie method of Claim 1 wherein coating said body with a source of silver comprises: applying a fine silver powder in a liquid carrier vehicle and further including a binder.

8. The method of Claim 5 wherein said temperature is in the range of about 840® to 860°C.

9. A metal oxide varistor-having stable operating characteristics comprising: a metal oxide varistor body having a silver bearing surface adjacent region extending into said body 5 and including substantially the entire surface of said body and extending therefrom into the bulk of said body.

10. The varistor of Claim 9 wherein said silver bearing region is characterized by a concentration of silver therein which is greatest at the surface of said varistor body and which decreases with increasing distance 5 from said surface.

11. The varistor of Claim 10 wherein said varistor body includes first and second generally opposing major surfaces and further comprising first and second electrical contacts on said first and second surfaces.

12. The varistor of Claim 11 wherein said contacts comprise silver contacts. 47131

13. The varistor of Claim 12 wherein said silver bearing region is characterized by a first thickness underlying said silver contacts and a second, lesser thickness in the other portions of the device. 5

14. The varistor of Claim 12 wherein said silver bearing layer is characterized by a substantially uniform thickness.

15. A metal oxide varistor having stable operating characteristics comprising: 10 a body of varistor material, said body characterized by a peripheral region thereof covering essentially the entire surface of said body of varistor material, said peripneral region containing silver.

16. A method for the manufacture of metal oxide varistors 15 substantially as herein described with reference to or as illustrated in Figure 11 of the accompanying drawings.

17. A metal oxide varistor substantially as herein · described with reference to or as illustrated in Figures 6, 7, 8, 9 and/or 10 of the accompanying drawings.