GB2044531A - Non-linear resistance elements and method for manufacturing same - Google Patents

Abstract

A non-linear resistance element or varistor is provided by the invention. The body of the element is a ceramic sintered body mainly composed of titanium dioxide admixed with small amounts of oxides of niobium, tantalum and/or antimony. Each electrode is of a metallic material bonded to the sintered body in ohmic contact by flame spraying or electroless plating depending on the metallic material for the electrode, for which silver, aluminium or nickel is preferred. It is further proposed to facilitate the soldering for wires to the electrodes by providing for the electrode at least a partial coat of a material having an affinity for solder. Such a layer also serves for inhibiting the oxidation of the electrode.

Description

SPECIFICATION Non-linear resistance elements and method for manufacturing same The present invention relates to a non-linear resistance element having a large voltagedependent electrical resistance and more particularly, to a so-called varistor comprising a titanium dioxide-based sintered body and a method for manufacturing same.

One form of a non-linear resistance element is, for example, 9 ceramic varistor, which exhibits a non-linear relationship between the value of the electric current across a sintered ceramic body and the voltage applied between the electrodes provided on opposite surfaces of the ceramic body. Accordingly, the electrical resistance of the body is not con stant; normally it drastically decreases for a region where the voltage applied to the electrodes exceeds a particular threshold called the "varistor voltage". The non-linear property of a varistor makes it useful in, for example, the noise suppression of small DC motors used in or relating to acoustic instru ments, protection of relay contacts, discharge absorption of Braun tube circuits of colour television sets and elsewhere.

The usual materials for making ceramic var istors are stannic oxide (SnO2), ferric oxide (Fe203), and silicon carbide (SiC). Sintered bodies of stannic oxide or ferric oxide are normally linearly resistive elements and the non-linearity characteristic of a varistor ele ment is imparted by forming a potential bar rier between the surface of the sintered body and a specific electrode provided on the surface of the body. Such an expedient is not required for varistors made of silicon carbide because the non-linearity of the element is caused by an interfacial phenomenon at the granular boundaries of the silicon carbide.

It has usually been difficult to make the required sintered bodies and to form the electrodes; moreover the varistors' linearity has deteriorated with the elapse of time and varistors have usually exhibited varistor voltages too high for noise suppression in small DC motors driven at low voltages. It has been proposed in United States Patent Specification No. 371 5701 to make a varistor using for the main ingredient titanium dioxide admixed with traces of bismuth oxide and an oxide of either antimony, niobium or tantalum. Such a varistor has quite a low varistor voltage.However, in such a varistor, the junction of the sintered body of titanium dioxide and an electrode thereof is non-ohmic and the consequential rectifying characteristic tends to mask the non-linearity which is characteristic of the sintered body which constitutes the varistor and the otherwise excellent non-linear characteris -tics of the sintered body cannot be fully exploited.

It is therefore an object of the present invention to provide a novel and improved non-linear resistor or varistor.

The non-linear resistance element of the invention comprises a ceramic sintered body composed mainly of titanium dioxide and at least one electrode made of a metallic material and bonded to the surface of the ceramic sintered body in ohmic contact. The metallic material may be one or an alloy of metals such as silver, aluminium, zinc, tin, copper, lead, bismuth, nickel and the like.

The preferred method for providing the electrode depends on the material selected for it. Electrodes of most of the above named metals can be formed by flame spraying but electrodes made of nickel are most suitably formed by electroless plating.

A metallic layer which is receptive to solder may be provided on the electrode in order to facilitate the attachment of wires to the electrode.

Reference will hereinafter be made to the accompanying drawings, in which: Figure 1 is a cross-sectional view of a typical varistor; Figure 2 is a graph showing voltage vs.

current in a varistor; Figure 3 is a graph showing noise voltages in a DC motor as a function of time; Figure 4 is a graph illustrating the overlapping of the varistor and rectifier action in a varistor; Figure 5 is a schematic illustration of the flame spraying of a sintered body; Figure 6(a) and Figure 6(b) are plan views of the top surface and the bottom surface, respectively, of an annular sintered body for use as a noise suppressor in DC motors; Figure 7 is a schematic cross-sectional view of a DC motor and a varistor mounted on the shaft of the rotor; Figure 8 is an equivalent circuit of the motor's rotor with a varistor in star connection; Figure 9 is an equivalent circuit of the motor's rotor with a varistor in delta connection; Figure 10 is a cross-sectional view of a varistor provided with the solder-receptive layers on its electrodes;; Figure 11 is a cross-sectional view of a varistor element provided with dual solderreceptive layers on its electrodes; Figure 12(a) and Figure 12(b) are plan views of the top surface and the bottom surface respectively of an annular varistor having three sector shaped electrodes on each surface; and Figure 13 is an equivalent circuit of the rotor of a DC motor with the varistor of Fig.

12.

A ceramic varistor element has typically the structure shown in Fig. 1, although other forms may be determined, of course, by the particular use to which the varistor is put. As is shown in Fig. 1, a sintered ceramic disc 1 has two opposite broad surfaces bearing electrodes 2, 2' respectively; leads 3,3' are bonded to the electrodes 2,2' respectively by solder 4,4'.

Fig. 2 shows voltage vs current in a varistor in which the current I increases non-linearly as the voltage V applied to the electrodes is increased. When the voltage V exceeds a certain critical value VT, the current I increases rapidly owing to the decrease of the electrical resistance of the varistor.

Usually, the widths of the commutator segments and the brushes in small sized DC motors are small and the rate of change in the current through the armature coils is often- large; sparks are very frequently produced between the surface of the commutator and the brush when the brush moves from one commutator segment to the next. These sparks cause spike-like noise voltages, accelerate the wearing of the commutator and the brushes and consequently shorten the life of the motor. The noise voltage is, as is shown in Fig. 3, a bipolar voltage and, having a peak value which may be at least an order of magnitude tens of times larger than the line voltage, can adversely influence instruments electrically connected to the motor. Varistors may be used to eliminate these noise voltages; the noise voltages N1, N2 are absorbed if they exceed the varistor voltage VT.The varistor voltage VT should normally be slightly greater than the supply voltage of the motor.

A ceramic varistor comprising titanium dioxide is capable of exhibiting the requisite nonlinearity, as shown by the curve L, in Fig. 4.

When the junction between the sintered body and an electrode is non-ohmic, the phenomenon of rectification is produced; the curve L2 in Fig. 4 illustrates the voltage-current characteristic. The varistor then exhibits the characteristic L3, the summation of the curves L1 and L2. Owing to the substantial resistance which the varistor possesses for applied voltages greater than VT and the lowered resistance that it exhibits for applied voltages less than VT, the varistor is far from ideal. It is now proposed to improve such varistors by making the contact between the sintered body and its electrodes ohmic, the electrodes being metallic.

A ceramic body for a varistor according to the invention may be prepared by sintering titanium dioxide which may be admixed with small amounts of one or more of niobium oxide, tantalum oxide and antimony oxide.

Each of a pair of electrodes is made of metallic material, such as aluminium, zinc, tin, silver, copper, lead, bismuth and nickel or alloys of these metals. Most preferred is silver but it should be noted that very pure silver is not suitable for electrodes bonded to the sintered body in ohmic contact and if silver is used it should contain from 2 to 20% by weight of at least one auxiliary metal selected from the group consisting of indium, gallium, tin, antimony, cadmium, zinc and aluminium.

Various techniques can be employed for forming the electrodes on the surface of the sintered body but particular materials normally require particular methods for forming the electrode in order to ensure ohmic contact between the electrode and the sintered body.

Vacuum deposition is generally feasible but is expensive. A less expensive method for forming silver electrodes on the sintered body is the use of a silver paste containing an auxiliary metal and a frit. Silver electrodes can readily be formed from such a paste by screen printing followed by baking. This method is quite suitable for mass production.

Another method of forming the electrodes is flame spraying the metal. Flame spraying is a method in which the metallic material is continuously melted in a flame spraying machine where the molten metal is blown with a gas jet and sprayed in the form of fine molten particles on to the surface of a substrate to form continuous metallic film thereon. This method of flame spraying can readily provide metallic electrodes of any desired purity in good ohmic contact with the surface of the sintered body. Moreover, the characteristics of varistors made thus are uniformly reproducible. In addition, the strength of the bond of the electrode to the sintered body is great and the electrode does not flake off. Flame spraying can be used for electrodes made of aluminium, zinc, tin, silver, copper, lead and bismuth.These materials are cheaper than the indium-gallium alloys usually used for electrodes.

In carrying out the method of flame spraying on to the surface of a ceramic sintered body, the sintered body is overlaid by a mask which has an aperture corresponding to the desired pattern of the electrode and the moten metal is sprayed thereon to form the electrode.

Fig. 5 illustrates a flame spraying machine and a sintered body under flame spraying covered with a mask and mounted on the machine. As is shown in the figure, the body 11 of the machine is equipped with a spray nozzle 1 2 and a pair of metal wire feeders 13a, 1 3b through which metal wires 19a, 1 9b are fed by rollers 1 4a, 1 4b from reels 1 6a, 1 6b continuously into the spraying zone at the spraying end of the spray nozzle 1 2.

Compressed air or other suitable gas is supplied into the air inlet 1 5 and blow in a jet out of the spraying nozzle 1 2 into the spraying zone. Alternating or direct current is supplied to the wires 1 9a, 1 9b from the lines 1 8a, 1 8b supported by a frame 1 7. When the advancing ends 19, 9bl of the metal wires 1 9a, 1 9b contact each other and are then pulled slightly apart, an electric arc is pro duced in the space between the advancing ends of the metal wires 19a, 19b. The ends of the wires 1 9a, 1 9b melt and the molten metal in the spraying zone is immediately sprayed by the jet of air onto the surface of the sintered body 20 through the apertures 21a, 21 b of the mask 21, the electrode being formed by the metal layer 22.

The use of a mask as described is convenient because in general any desired pattern, however complex, can be prepared by machining, moulding or otherwise forming the mask, and the method yields uniformly reproducible results.

The mask 21 is removed from the surface of the sintered body 20 after completion of flame spraying and can be used- repeatedly.

The mask 21 should be of a metal to which the molten metal sprayed thereon does not adhere. The mask may be of brass and may be coated with a layer of a releasing agent such as a carbonaceous material or ethyleneglycol so that the layer of metal which is sprayed onto the mask can be readily removed without damage to the mask.

After the provision of its electrodes the sintered body may be artificially aged at a temperature of 100 to 300"C for 30 to 1 80 minutes so as to improve the stability of the ohmic contacts between the sintered body and the electrodes.

Particular patterns of the electrodes provided on the surface of the sintered body naturally depend on the particular applications of the varistor elements with the electrodes.

One typical pattern of the electrodes in a varistor element used for the noise suppression in small sized DC motors is shown in Fig.

6(a) and Fig. 6(b) for each of the opposite surfaces of a sintered body, respectively. An annular sintered body 6 is provided with a circular central hole 6 through which the rotating shaft of the motor is to be inserted.

On one surface of the sintered body 6 are provided three electrodes 7a, 7b, 7cin equal sectors. On the opposite surface of the same sintered body 6 is provided a single annular electrode 8 as shown in (Fig. 6(b) Fig. 7 is a cross-sectional illustration of a small DC motor and a varistor as shown in Fig. 6(a) and Fig. 6(b). The motor has stator poles 1 3a, 1 3b in the field of which is an armature 9 mounted on a shaft 10 which also carries a commutator ring 11.A varistor 6 is mounted between the armature 9 and the commutator 11 on the shaft 1 0. Each of the sector shaped electrodes 7a, 7b, 7 c of the varistor is electrically connected to a respective one of three commutator segments 11 a, 11 b, 11 cwhich come into intermittent contact with the brushes 1 2a, 1 2b as the rotor rotates.

The varistor characteristics of the element are exhibited in the portions of the sintered body 6 between each of the electrodes 7a, 7b, 7cand the common electrode 8 on the opposite surface. Thus the varistor is in effect a star (Fig. 8) of three elementary resistors Pa, Pb, Pe connected to the armature coils 9a, 9b, 9c. When noise voltages N1, N2 as shown in Fig. 3 are produced in the motor with this varistor connection, a varistor effect occurs in at least two of the three equivalent elements Pa, Pb and Pc so that the noise voltages N1, N2 are effectively absorbed in the commutator with no adverse influences on the external circuit.

Instead of the star connection shown in the equivalent circuit given in Fig. 8, a delta connection can also be formed as is shown by the equivalent circuit given in Fig. 9. In this case, the varistor has three sector electrodes 7a, 7b, 7caws shown in Fig. 6(a) on one surface of the sintered body 6 but no electrode is provided on the opposite surface. As is shown in Fig. 9, three equivalent elemetary varistors P'a, P'b and P'ceach formed between two of the electrodes 7a, 7b, 7care connected in delta to the commutator segments 11 a, 11 b, 11 C. This delta connection is also effective to suppress noise. The armature coils 9a, 9b, scion Fig. 8 and Fig. 9 are in delta connection but they can of course be connected in star if necessary.

The number of the sector electrodes may be determined by the number of the commutator segments although the above description presumes that there are only three commutator segments.

The method of flame spraying is suitable for most of the metallic materials mentioned, but not ideal for all of them, particularly nickel.

However, a good ohmic contact of a nickel electrode is obtained when the nickel electrode is formed by electroless plating on the surface of the sintered body.

One method for electroless plating of nickel on the surface of a sintered body may proceed as follows. Firstly, a sintered body, which is in any desired form (such as the annular one shown in Fig. 6) provided with a layer of a plating resist where no nickel layer should be deposited by the electroless plating, the areas for the electrodes being exposed. The layer of the plating resist can be formed by screen printing. Various organic polymeric substances insoluble in the undermentioned plating solutions may be used for the plating resist.

The sintered body, provided with the layer of the plating resist, is activated on the exposed areas for electrode formation by submersion in an aqueous solution of tin chloride and palladium chloride (see, for example, Journal of the Electrochemical Society, vol.

107 p. 250, 1960) followed by electroless plating in a plating solution containing nickel chloride, sodium hypophosphite and sodium citrate at a temperature of 80 to 90"C to deposit a layer of nickel containing phosphorus. Thereafter, the layer of the plating resist is removed by a suitable organic solvent. If necessary, the layer of nickel-phosphorus deposited on undesired areas is removed mechanically, for example by centerless grinding or sand blasting to leave the electrodes in an exact pattern.

Ohmic contact between the nickel electrode formed by electroless plating and the surface of the sintered body is more complete when the electrode is composed of 98 to 80% by weight of nickel and 2 to 20% by weight of phosphorus. The weight ratio of nickel and phosphorus in the deposited layer is controlled by adjusting the pH value of the plating solution which should be in the range from 2 to 10; a value of the pH higher than 10 results in a smaller content of phosphorus than 2% by weight and a lower value of pH than 2 results in a higher content of phosphorus than 20% by weight.

The sintered body provided with the nickelphosphorus electrodes as described above is then preferably subjected to artificial aging by heating at about 300"C to stabilize the ohmic contact of the electrodes with the surface of the sintered body.

Another method for forming electrodes on the surface of the sintered body is by the use of an etching resist. In this case, the sintered body is first provided with a deposition of the nickel-phosphorus all over its surface by electroless plating. Then the areas corresponding to the desired electrode pattern are coated with an etching resist by screen printing or other suitable method. The nickel-phosphorus layer on the areas uncoated with the etching resist is removed by etching. One example of a suitable etching solution is a mixture of acetic acid, nitric acid and acetone in equal proportions; the solution may be used at about 40"C. The etching resist covering the electrode areas is removed by washing with an alkaline solution or an organic solvent according to the particular etching resist.Artificial aging may be applied in the same manner as when a plating resist has been used.

Electroless plating for forming nickel-phosphorus electrodes is very advantageous because apart from its cheapness, it provides a strong, excellently ohmic bond. Moreover electrodes of any complicated pattern can readily be formed because the pattern may be determined exactly by printing and the removal of the resist need not be performed mechanically.

In practice, a terminal wire has to be bonded to each electrode of the varistor. However, there are difficulties associated with soldering of such wires to the electrodes. For example, aluminium or nickel electrodes have little affinity for solder regardless of the method by which they are made, owing to their acquisition of a film of oxide. Furthermore oxidation of the electrode's material may sometimes adversely affect the ohmic contact between the electrode and the surface of the sintered body, the varistor having thereby a shorter useful life. Moreover, an electrode of silver formed by the method of flame spraying is also unreceptive to solder.This lack of affinity for solder is more apparent when the electrode is formed of a silver mixed with small amounts of the auxiliary elements such as indium, gallium, antimony, cadmium, zinc, aluminium and the like as mentioned before than it is when the electrodes are made of pure silver.

The above-mentioned difficulties in soldering can be overcome by providing a layer of a metallic material having good affinity for solder on the electrode formed on the sintered body, as shown in Fig. 10. In this Figure, a sintered body 1 is a disc similar to that shown in Fig. 1 and is provided on each of two opposite surfaces with electrode 2, 2' formed by a suitable method such as printing with a silver paste, flame spraying with molten aluminium or electroless plating of nickel-phosphorus. Covering the electrodes 2, 2' are solder-receptive metallic layers 21, 21' to which the lead wires 3 3' are bonded with solder 4,4'.

The layers 21 and 21' are formed most conveniently by printing, e.g. screen printing, with a silver paste containing high purity silver particles dispersed therein. It is important in printing with the silver paste that the silver paste never spreads (by misregistered printing) onto the areas where the electrode layer is not formed. The pattern for the solderreceptive layer 21 may be such that margins g1 are left around the peripheries of the electrodes 2, 2'.

The provision of layers which are receptive to solder is beneficial not only by virtue of the increased facility of attachment of wires to the electrodes but also because the layers inhibit the oxidisation of the electrodes, and consequently preserve the ohmic contact between the electrodes and the sintered body.

The solder-receptive layers can be formed also by the electrolytic plating of tin or other suitable metals or plating with a solder alloy.

One of the advantages of this electrolytic plating over the printing method with a silver paste is that the metallic plating is obtained all over the surfaces of the electrodes without the danger of overspreading of the solderreceptive layers; the coverage of the electrodes may be more complete.

Fig. 11 is a cross-sectional view of a varistor of which the sintered body 1 is first provided with ohmic-contact electrodes 2,2' on its opposite surfaces. Solder-receptive layers 21, 21' are formed on the electrode surfaces by printing with a silver paste leaving marginal gaps 91 around the solder-receptive layers 21, 21'. Next, second solder-receptive layers 22, 22' are formed by the electrolytic plating of, for example, tin to cover whole areas of the first solder-receptive layers 21, 21' and the surfaces of the electrodes 2, 2' not covered by the first solder-receptive layers 21, 21'. An advantage of such a double-layer coating is the further improved prevention of the electrodes' oxidation as well as the increase in the area of receptivity to solder.

Fig. 12(a) and Fig. 12(b) are plan views of the top surface and the bottom surface, respectively, of an annular varistor similar to that shown in Fig. 6 used for the noise suppression in small sized DC motors. As is shown in Fig. 12(a), the sintered body 1 is provided on the top surface with three equal sector electrodes 2a, 2b, 2c in ohmic contact with the surface of the sintered body 1. Each of the electrodes 2a. 2b, 2c subtends somewhat less than 120 to the centre, gaps g2 being provided between the adjacent electrodes. Each of the electrodes 2a, 2 b, 2 c is covered with a respective solder-receptive layer 21 a, 21 band 21 c as described above, the lateral dimensions of the solder-receptive layers 21 a, 21 b and 21 c being somewhat smaller than the respective electrode 2a, 2b or 2c.Three lead wires 3a, 3b, 3care soldered to the layers 21 a, 21 b, 21 c, respectively.

The bottom surface of the sintered body 1 is provided, as is shown in Fig. 12(b), with three similar sector electrodes 20a, 20b, 20c separated by gaps 93. These electrodes 20a, 20b, 20con the bottom surface are also shown in Fig. 12(a) by the broken lines. As is shown in Fig. 12(a), the electrodes 20a, 20b, 20c are not in register with the electrodes 2a, 2 b, 2 c but are displaced by 604 therefrom.

Thus, the sintered body 1 has six varistic regions S1, 52 S3 S4, S5 and S6 each sandwiched between two respective electrode paires 2a,20a; 2a,20b; 2b,20b; 2b,20c; 2g20c; and 2c20a. The electrodes 20a, 20b, 20con the bottom surface are not covered with solder-receptive layers because (in this example) no terminal wires are bonded to these electrodes.

The mounting of the above described varistor having three sector electrodes on each of two opposite surfaces is just the same as for the varistor shown in Fig. 6 and illustrated in Fig. 7. The equivalent circuit including the armature coils 9a, 9b, 9cand the commutator segments 11 a, 11 b, 11 c is as shown in Fig.

1 3. In this equivalent circuit, series combinations, each of two varistic regions, are connected in delta to the armature coils 9a, 9b, 9c, also in delta connection, and to the commutator segments 11 a, 11 b, 11 c. Therefore, the effect of noise absorption is obtained in all of the varistic regions S, to S6 when a noise voltage is produced so that excellent noise suppression is obtained even for large current ranges.

Claims (14)

1. A non-linear resistance element which comprises a sintered body mainly composed of titanium dioxide, and at least one electrode made of a metallic material and bonded in ohmic contact with the surface of the sintered body.

2. A non-linear resistance element as claimed in claim 1 in which the electrode is at least partly covered by a layer of material which has an affinity for solder.

3. A non-linear resistance element as claimed in claim 1 or claim 2 in which the sintered body contains a small amount of the group consisting of niobium oxide or tantalum oxide or antimony oxide or more than one of these oxides.

4. A non-linear resistance element as claimed in any foregoing claim in which the electrode comprises any one or more of the metals silver, aluminium, zinc, tin, copper, lead, bismuth, nickel or an alloy of them.

5. A non-linear resistance element as claimed in claim 1 or claim 2 in which the electrode is made of silver containing from 2 to 20% by weight of one or more of indium, gallium, tin, antimony, cadmium, zinc and aluminium.

6. A non-linear resistance element as claimed in claim 2 in which the solder-receptive metallic material is silver of high purity.

7. A non-linear resistance element as claimed in claim 2 in which the layer of the solder-receptive metallic material is a layer of tin deposited by electrolytic plating.

8. A non-linear resistance element as claimed in claim 1, in which at least one layer of a first solder-receptive metallic material covers at least part of the outer surface of the electrode and a layer of a second solderreceptive metallic material covers the layer of the first solder-receptive material and at least part of the surface of the electrode not covered by the layer of the first solder-receptive material.

9. A non-linear resistance element as claimed in claim 8 wherein the first layer comprises silver of high purity and the second comprises tin.

1 0. A non-linear resistance element as claimed in any foregoing claim in which the body is provided with at least two electrodes each as aforesaid.

11. A method of manufacturing a nonlinear resistance element, comprising sintering a body of titanium dioxide and flame spraying a molten metallic material on to the surface of the sintered body to form at least one electrode in ohmic contact with the body.

12. A method as claimed in claim 10 wherein the metallic material comprises one or more of aluminium, zinc, tin, silver, copper, lead, bismuth and alloys thereof.

1 3. A method as claimed in claim 1 2 wherein the metallic material is aluminium.

14. The method as claimed in claim 1 2 wherein the molten metallic material is flame sprayed on to the surface of the sintered body through a mask defining the electrode.

1 5. A method of manufacturing a nonlinear resistance element comprising sintering a body of titanium dioxide and depositing a layer of nickel containing phosphorus by electroless plating on the surface of the sintered body to form at least one electrode in ohmic contact with the body.

1 6. The method as claimed in claim 1 5 wherein the layer deposited on the surface of the sintered body contains from 98 to 80% by weight of nickel and from 2 to 20% by weight of phosphorus.

1 7. A non-linear resistance element made by any of the methods claimed in claims 11 to 16.