GB2111862A - Electrode coating process - Google Patents

Abstract

Disclosed is a method of coating electrodes in spark gap devices which minimizes filament formation. A coating such as carbon is deposited on the electrode surfaces. A signal is then applied so that the device conducts in the arc mode for several short periods. A small spot of the coating bonds with the negatively biased electrode during each conduction. This operation continues at alternating polarities until essentially the entire surface area of both electrodes is bonded.

Description

SPECIFICATION Electrode coating process This invention relates to coating electrode surfaces, and in particular to a coating method for rigidly adhering a coating to the electrode.

Surge limiters have for many years been used to protect apparatus from high voltage surges resulting from various causes, such as lightning strikes. The devices basically comprise a pair of electrodes with a spark gap therebetween. The device, which is coupled in parallel with the protected apparatus, is nonconducting during normal operation of the apparatus. However, when a voltage surge of sufficient magnitude appears at the electrodes, a spark is produced across the gap and the surge is shunted from the protected apparatus. In the sealed gas surge limiter, the electrodes are placed in a hermetically sealed housing along with an inert gas. The device fires when the gas in the gap area is sufficiently ionized to produce a spark.

It has been recognized in such devices that a coating of graphite on the surface of the electrodes will improve device performance by increasing electron emission from the electrode thereby enhancing formation of the plasma discharge in the gap. One problem associated with such coatings, however, is the formation of carbon filaments on the surface after a few discharges of the device, which results in leakage currents and short circuits in extreme cases.

This problem can be avoided by more firmly adhering the electrode coating to the electrode surface.

According to the present invention there is provided a method of bonding a coating to one electrode, comprising the step of depositing coating material on at least a portion of the said one electrode and applying a signal between the said one electrode and an adjacent electrode forming a spark gap with said one electrode, said signal serving to cause conduction in the arc mode for several relatively short periods of time so that for each of said periods a different portion of the coating bonds with said one electrode.

In an embodiment of the invention, a coating is first deposited on the surface of the electrodes. A signal is then applied to the electrodes to cause conduction in the arc mode for several short periods of time so that for each of said periods a different portion of the coating bonds to the electrodes.

For a better understanding of the invention, reference is made to the accompanying drawings, in which: Figure 7 is a cross-sectional view of a typical sealed gas surge limiter fabricated in accordance with one embodiment of the invention; Figure2 is a current-voltage characteristic of the device in Figure 1 illustrating arc mode conduction; Figures 3 and 5 are circuit diagrams of examples of circuits which are used in the practice of this invention; Figure 4 is an illustration of the voltage across the electrodes and current through the electrodes during the application of the signal from the circuit in Figure 3; Figure 6 shows the voltage across the electrodes using the circuit of Figure 5; Figure 7 is an enlargement of a portion of Figure 6 and further shows the current through the electrodes;; Figure 8 shows a circuit for performing an additional processing step; and Figure 9 shows the current through the device using the circuit of Figure 8.

Figure 1 illustrates a sealed gas surge limiter. The device includes two electrodes, 11 and 12, defining a narrow spark gap 19 therebetween. The electrodes were bonded to flanges, 14 and 15, which were, in turn, bonded to opposite ends of an insulating housing, 13. Also bonded to the flanges and electrically coupled to the electrodes were terminals, 16 and 17. The housing was filled with argon gas and hermetically sealed utilizing a fusible metal 18 for all bonding between electrodes, flanges, terminals and the insulating housing.

A spring, 20, was included between electrode 12 and terminal 17 to aid in achieving a uniform gap.

In this example, the electrodes were made of copper and included a coating, 21, of carbon (graphite) on the portion of the electrode surfaces which face each other. The electrode surfaces also included grooves, 22, to inhibit deterioration of the carbon coating. (See, for example, U.S. Patent No. 4,037,266 issued to English et al). The insulating housing was made of ceramic, the flanges were made of copper, and the terminals comprised an iron-nickel alloy plated with nickel. The fusible metal was a silver-copper eutectic. Except for the process used to adhere the coating, 21, to the electrodes, 11 and 12, the device and its method of manufacture are known.

In accordance with one embodiment of the invention, the carbon coating, 21, was formed on the electrode surface by first depositing the coating by a standard spraying of colloidal graphite (a suspension of graphite in alcohol and water). In this example, the coating was approximately 3thick but will generally fall within the range of 1.5-5 Fm. The device was then completely assembled according to standard fabrication techniques.

Following assembly, the device was then subjected to a signal which caused the device to conduct in the arc mode for several short periods of time. Arc mode conduction may be understood by referring to Figure 2 which shows a voltage-current curve for a typical device when a slowly rising voltage is applied to the electrodes, e.g., a slope of approximately 2000 v./sec. At some point, the device will reach the breakdown voltage VB after which the device conducts in the "glow mode" from Ii to 12 at a fairly constant voltage below breakdown. As current increases beyond 12 in the interval 12-13, the device will operate in the arc mode where the voltages across the electrodes will be at a fairly constant, but much lower value.In the example, the breakdown voltage, VB, was 300v, the glow mode voltage VG was approximately 1 80v, and the arc mode voltage, VA, was approximately 15v. The lower current (II) for the glow mode was of the order of microamps and the onset of the arc mode was at 200 milliamps.

It was discovered that each time the device was operated in the arc mode, the temperature reached was sufficient to cause a reaction between the cathode coating and the underlying electrode surface to form a stronger bond. The mechanism was therefore distinctly different from standard prior art aging processes which drove the limiter primarily in the glow mode and caused sputtering of particles from the electrode surface. (See, for example, U.S. Patent 3,454,811 issued to Scudner). (It will be noted that some sputtering will occur during the embodiment method, but this is not the primary reaction).It was also discovered that if discrete pulses of relatively short duration were utilized (for example, less than 200 Rsec), the spark produced during each firing would occur at random at unreacted areas around the surface of the electrode to substantially cause a different portion of the coating to react during each firing. Further, by reversing polarity of the pulses, the other electrode in the limiter could be so treated. Thus, if a signal comprising many relatively short pulses of sufficient magnitude to cause the device to conduct in the arc mode was applied to the device and the polarity of the signal reversed at certain intervals, essentially the entire carbon coating on the surface of both electrodes 11 and 12 could form a strong bon with the electrodes.

For purposes of illustration, two different circuit arrangements are described for effecting the aforedescribed processing step.

The first is shown in Figure 3. In this circuit, current was supplied by an AC signal source, 24, which produced a 60 cycle/second signal with voltage of 1000 volts RMS. The source was coupled via resistors R1 and R2 to a socket S (in which is inserted the surge limiter device being processed). Coupled between the two resistors and in a discharge path in series with one of the resistors (R2) and the socket was a capacitor, C. The circuit was designed to operate in the relaxation oscillator mode with sufficient current supplied to the limiter so that each time the device conducts it will switch to the arc mode, and with sufficiently short pulses to ensure reaction of a different portion with each pulse.Thus, C stores charge until the voltage across the limiter reaches breakdown, at which time C discharges through R2 to the limiter with sufficient current to cause arc mode conduction for a short period. The current is a pulse determined by C, R2, and the inductance of R2. After thins period, conduction stops and the voltage across the limiter begins increasing until breakdown is again achieved and the process repeats. This operation of the limiter is illustrated in Figure 4 which is an approximate representation of the voltage across the device and the current therethrough for one cycle of the AC signal. It will be noted that the device fired several times during each half-cyle. In one embodiment, 17 firings, on average, occurred during each half-cycle.During each firing, arc mode conduction was achieved with a peak current of approximately 1.4 amps resulting in surface temperatures of several thousand degrees. This was sufficient to cause a reaction between the copper and carbon at a different, small area of the negatively biased electrode during each firing. During the second half of the cycle, when the polarity was reversed, the reactions occurred on the other electrode. This procedure was repeated for several cycles (approximately 20 seconds in this example) until the entire carbon coating on both electrode surfaces had thus reacted.

Using the circuit of Figure 3, it is preferred that the time of each conduction in the arc mode be within the range 1 lisec-200 lisec, but time up to 400 Fsec are feasible. If the period is too short, the spot size is too small making it difficult to produce a complete reaction of the electrode surface, and if it is too long the electrode may be damaged.The actual time period for each arc mode conduction in this example was approximately 20 usec for the main pulses, (and less for random parasitics). It should be realized that these conductions may be obtained by isolated pulses having the proper width as described here, or by a rapid sequence of current spikes which individually have very short widths but the effect of which is apparently to keep the gas ionized for the duration of the sequence of current spikes.

To ensure that a different area will be reacted during each current pulse, it is preferred that each current pulse be terminated with a voltage polarity reversal of a few volts, as shown in Figure 4 (and described further hereinafter), to assure turn-off of the device. For the same purpose, it is recommended that the current pulses be at least a millisecond apart. A peak current of at least 1 amp is preferred.

In the circuit shown in Figure 3, wire-wound (inductive) resistances consisting of six resistors of 1470 ohms each coupled in series R1, a commercial wire-wound resistor of 215 ohms for R2, and a capacitor having a capacitance of 0.1 lif were used. The inherent inductances of the wire-wound resistors provide the slight voltage polarity reversal for turn-off at the end of each pulse. The capacitance will determine the amount of energy delivered to the limiter when breakdown is achieved, and the value of R2 will control the size of the area reacted in each arc mode conduction by controlling the peak current of the pulse. R1 will determine the time necessary for charging the capacitor and therefore controls the pulse repetitition rate.

Another illustrative circuit for performing the coating bonding process, but in a somewhat different fashion, is shown in Figure 5. It is found, that for even further improving the process, the coating should be subjected to a rapid sequence of current spikes each having a rapidly rising leading edge and a high amplitude. During arc initiation (i.e., within the first 50 nanoseconds of the onset of discharge), extremely high current densities occur across the gap because, at least initially, there is a very narrow lateral extension of the arc. Each high density arc initiation causes a minute area of the electrode surface to react with the coating. Because the arc spreads, the desired surface reaction is produced only during arc initiation and so a high amplitude during arc initiation is preferred. Further, the current spikes are preferably sufficiently rapid so that the device fires several times before the plasma is completely extinguished. This results in reactions which are produced at random along the electrode surface since the locations are determined by the drift of remnant charges from the previous discharge and not by surface conditions. Such a treatment produces a uniform reaction on at least the flat portions of the electrodes, which are the significant portions of the electrodes since they determine the value of the surge limiting voltage.

In the circuit shown in Figure 5, the surge limiter is represented by S. Current was supplied by an AC signal source, 123, which produced a 60 cycle/second signal with a voltage of 1,000 volts RMS. For purposes of discussion, the remainder of the circuit is divided into portions 1, II and Ill and their basic functions will be described for illustrative purposes and not by way of limitation.

Portion I included a series connection of resistors R and R72 and inductor L11 between source 123 and one electrode of the limiter S, and a resistor R13 between the other electrode of the limiter and the source 123.

Coupled in a series discharge path to one end of resistors R11 and R13 was a capacitor C11, and coupled to the other end of R11 and R13 in a series discharge path with Rr2 and L11 was a capacitor C12. R, R13, C12 and the surge limiter, S, acted as a relaxation oscillator to produce a desired number of sawtooth voltage waveforms per half cycle of the applied 60 cycle voltage, in this case approximately 45-60. This is illustrated in the curve of Figure 6, which shows the approximate voltage waveform across the device. The dashed curve represents the voltage supplied by source 123.As a result of this voltage, C12 will charge at a rate determined by its capacitance as well as resistances R11 and R13. When the voltage across the limiter, S, reaches breakdown voltage, V5, the capacitor C12 will discharge. When the limiter turns off, C12 will again charge and the process is repeated. As shown in Figure 6, the oscillation frequency will vary with the applied voltage. (Not all breakdowns are shown in the Figure for the sake of clarity). C11 serves as a by-pass capacitor, R12 limits the discharge current, and L11 slows the discharge from C12 to permit functioning of the other portions of the circuit several times for each period of oscillation.In this example, the period of oscillation, T, will vary with the voltage but will be greater than 80 microseconds.

Portion II of the circuit included a capacitor C13 and inductor L12 also in a series discharge path with the limiters, with C13 coupled between the two inductors L11 and L12. This portion forms a shocked resonant oscillator with S, the effect being to cause the limiter to turn off several times with C12 is discharging. In fact, the circuit causes a slight voltage polarity reversal each time the device discharges to ensure turn off of the device. This happens because, on breakdown, C13 will discharge through L12 until the voltage across C13 reverses. The oscillations of this portion are short-lived because the device will turn off after a half-cycle and the circuit will be loaded down by R14 and C14.However, the charging and discharging of C13 will repeat several times while C12 is discharging. Thus, the period of oscillation of this portion should be less than that of portion I to ensure multiple breakdown of the limiter for each period z. In this example, the period of oscillation for L12 and C13 was caiculated to be .38 microsecond. Interaction with the limiter and other circuit components actually resulted in periods which in general fall within the range 1-20 microseconds.

Portion Ill of the circuit included a resistor, R14, and capacitor, C14 in a series discharge path with the limiter. At each breakdown of the limiter, the capacitor discharges through the resistor a high current, in this example, approximately 30 amps. The response time of this portion (the time required for peak current from capacitor C14 to be supplied to the limiter) should be very short to insure a very high current density across the gap of the limiter during arc initiation. In this example, the response time was less than 50 nanoseconds.

The time constant for discharging of capacitor C14 was approximately 0.5 lisec, but depending on the characteristics desired for the limiter, time constants up to 0.1 microsecond should generally be useful.

Figure 7 shows a more detailed view of a typical voltage across the device during one period of the relaxation oscillation shown in Figure 6. Since the voltage waveform will vary from device to device and with the aging time, it should be appreciated that this waveform is shown for illustrative purposes only. It will be noted that the limiter typically breaks down several times at each sawtooth portion. This is caused by the action of portions il and Ill of the circuit as previously described. It will also be noted that there is a slight polarity reversal at each breakdown as previously described. Figure 7 also illustrates typical current spikes through the device corresponding to the illustrative voltage. A current spike will occur each time the device breaks down.It is one aspect of the use of this circuit that the current spikes have a high amplitude at least during arc initiation and are produced in rapid sequence, in order to achieve a uniform reaction over the entire interface between the coating and flat portion of the electrode. The precise amplitude and frequency will vary with aging and from device to device. In general, current spike amplitude is limited by R14 the value of which is determined by the desired limiter characteristics. Spike amplitudes should generally be in the range 10-1000 amperes, and current spikes during a period of oscillation should be less than 20 lisecs apart.

In this example, the amplitude was 25-30 amperes and spikes were less than 10 lisecs apart.

In this particular example, the following circuit parameters were utilized (intrinsic parasitic inductances are included in parenthesis): R17 = 4k ohms (203 H) R12 = 215 ohms (16 uH) R13 = 4k ohms (203 pH) R14 = 10 ohms C11 = 500 pF C12 = .031lF C13 = 1,000 pF C14 = 5,000 pF L11 = 27,uH L12 = 3.6RH It will be understood that these values are presented for purposes of illustration and can be varied according to particular needs.

It is theorized that the high current density produced during initiation of the arc of the surge limiter (within 50 nanoseconds of the beginning of the discharge) causes the drive-in of the carbon coating and results in good bonding. Further, it takes several micro-seconds for the plasma produced in the area of the gap to be dissipated. By creating a rapid sequence of pulses, some ions will remain in the gap for the next succeeding discharge of the device. (This is evidenced by the fact that succeeding discharges occur at lower voltages as shown in Figure 7). It is believed that because some of these ions migrate between discharges, there is a greater tendency for subsequent discharges to be spread over the area of the electrode and a more uniform reaction over the surface of the electrode results.That is, the reactions will occur at random over the electrode surface and the locations will not be dependent upon surface properties. It should be noted that the precise mechanism is not well understood, and the above is presented only as a possible explanation of the results achieved.

The above-described reaction occurs at the negatively charged electrode (cathode). Thus, the reversal of polarity supplied by the AC source 123 allows both electrodes to be treated.

The total time needed to apply the pulsed signal to the limiter using either of the circuits shown in Figures 3 and 5, or other circuits suitable to practice the invention, can be determined by a visual inspection of the coating since the reacted area will be covered with contiguous spots. The time can also be determined empirically for each type of device by looking at the distribution of breakdown voltages and surge limiting voltages for groups of such devices aged at various times. If the time is too short, there will be a wide variation in these values, and if it is too long, the medium surge limiting voltage will increase. Using the circuit of Figure 5, the 60 cycle current source provided nine pulses with durations of 1 second each. In general, it is desirable in commercial production to subject the limiter to the pulsed signal for less than 10 seconds.

The above technique will create a uniformly bonded coating over at least the flat area of the electrodes.

However, it is desirable in certain circumstances to leave some particles of the coating unbound and the electrode surfaces in a roughened condition. The unbound particles aid in producing surface asperities. Too few asperities, for example, may result in high surge limiting voltages (on the other hand, too much free carbon may result in low device resistance).

In order to create the right amount of asperities, the device was then placed in the circuit shown in Figure 8. Again, the surge limiter S is powered by an AC current source, 125, operating at 60 cycles per second and a voltage of 1,000 volts RMS. Coupled in series between the source and the device was a resistor R15 and inductor L13. Coupled in a series discharge path with the inductor and limiter was a capacitor C15. Also coupled in parallel with the limiter at the other end of the inductor was another capacitor C15. This circuit operates in a manner similar to that of Figure 5 in that Era5, C15 and the limiter form a relaxation oscillator, and the inductance of L13 ensures that the device turns off.As is shown by the current and voltage waveform illustrations of Figure 9, when the applied voltage exceeds breakdown, the device will discharge several times consistent with the relaxation oscillation, and current spikes will be conducted through the device. The magnitude of the spikes is determined by C16, which is a stray capacitance. However, the resistor of the circuit, R15, is chosen to be small enough so that when the voltage exceeds a certain value, there will be sufficient current to the limiter to sustain a nonoscillatory arc mode conduction for most of the period of the applied pulse. At the end of the pulse, as shown, the multiple discharges resume. The low current density through the limiter caused by this circuit produced the asperities for low surge limiting voltage. In this example, a single current pulse of approximately 1 amp rms was supplied for one second, and the period of nonoscillatory conduction extended for approximately 6.5 milliseconds per half cycle. In general, it is preferred that nonoscillatory conduction extends for periods of 5-7 milliseconds per half cycle to achieve the desired amount of apserities. The current amplitude of the applied pulse should preferably be within the range 0.5-1.5 ampere rms. In this particular example, the circuit parameters were as follows: R,5= 1 kohms C15 = 1,000 pF L13 = 271lah C, --100 pF Again, it will be appreciated that the circuit parameters may be varied for particular needs.

Although the invention has been described utilizing a graphite coating on the electrodes, it is applicable with other coatings, for example, molybdenum, tungsten, copper, and emissive glass coatings. Further, the underlying electrode need not be copper, but can be molybdenum, tungsten, or other conductors.

Also, although it is advantageous to react the coating and electrodes after the device is completely assembled, such a process can be performed prior to assembly.

Claims (17)

1. A method of bonding a coating to one electrode, comprising the step of depositing coating material on at least a portion of the said one electrode and applying a signal between the said one electrode and an adjacent electrode forming a spark gap with said one electrode, said signal serving to cause conduction in the arc mode for several relatively short periods of time so that for each of said periods a different portion of the coating bonds with said one electrode.

2. A method according to claim 1, wherein the coating comprises carbon.

3. A method according to claim 1 or 2, wherein the said one or both electrodes comprises copper.

4. A method according to claim 1,2, or 3, wherein the signal is applied by a circuit including an AC current source, a plurality of resistors coupled in series with the source and the electrodes, and a capacitor coupled in a discharge path in series with the electrodes and at least one of said resistors.

5. A method according to claim 1,2,3 or 4, wherein the periods of arc mode conduction are within the range 1 lisec-200 used.

6. A method according to claim 1,2 or 3, wherein the said signal is of alternating polarity and is applied to the electrodes so that arc mode conduction is produced several times during each polarity.

7. A method according to any one preceding claim, wherein the amplitude of spikes of the arc mode conduction is within the range 10-1000 amperes.

8. A method according to any one preceding claim, wherein the time spacing between the majority of current spikes is less than 20 uses.

9. A method according to any preceding claim, wherein the total time for applying the said signal is less than 10 seconds.

10. A method according to claim 1,2 or 3, wherein the circuit includes an AC current source for periodically reversing the polarity of the signal so as to cause bonding of coating material on each of the electrodes for each respective polarity, in order to form a spark gap device.

11. A method according to claim 10, wherein the circuit includes first means for causing a relaxation oscillation waveform across the device, second means for producing a plurality of current spikes through the device at the end of each period of the relaxation oscillation waveform and to produce a small polarity reversal each time the waveform goes to zero, and third means for producing current spikes of high amplitude.

12. A method according to claim 11, wherein the first means includes a first resistor coupled i-n series between the current source and one electrode of the device, a second resistor coupled in series between the current source and the other electrode of the device, first and second capacitors coupled in parallel to one end of both of said resistors, and a third resistor and first inductor coupled in a series discharge path between the second capacitor and one of the electrodes of the device.

13. A method according to claim 11, wherein the second means includes a second inductor and a third capacitor coupled in a series discharge path with the device.

14. A method according to claim 11, wherein the third means includes a fourth resistor and fourth capacitor coupled in series with each other and in a series discharge path with the device.

15. A method according to claim 1, further comprising the step of applying a signal to the electrode by means of a second circuit which causes conduction of at least one current pulse through the electrodes sufficient to produce some asperities on the surface of the electrodes.

16. A method of bonding a coating to an electrode, substantially as hereinbefore described with reference to Figures 1 to 4; 5,6 and 7; or 8 and 9 of the accompanying drawings.

17. Coated electrode(s) prepared by the method according to any one preceding claim.