GB2111748A - Method of manufacturing non-linear resistance elements - Google Patents

Abstract

A method of manufacturing a non- linear resistance element comprising sintering a body of titanium dioxide (1) and depositing a layer of nickel containing phosphorus (2) by electroless plating on the surface of the sintered body to form at least one electrode in ohmic contact with the body. The titanium dioxide (1) body may be admixed with small amounts of other materials. The electrode(s) may be patterned and the varistor used for noise suppression in small electric motors, with a motor shaft inserted through a hole in the body (1). <IMAGE>

Description

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

One form of a non-linear resistance element is, for example, a ceramic varistor, which exhibits a not 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 constant; 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 instruments, protection of relay contacts, discharge absorption in Braun tube circuits of colourtelevision sets and elsewhere.

The usual materials for making ceramic varistors are stannic oxide (SnO2), ferric oxide (Fe2O3), and silicon carbide (SiC). Sintered bodies of stannic oxide of ferric oxide are normally linearly resistive elements and the non-linearity characteristic of a varistor element is imparted by forming a potential barrier 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; moreovear the varistors' linearity has deteriorated with the elapse of time and varistors have usually exhibited varistor voltages too high for noise suppressing in small DC motors driven at low voltages. It has been proposed in United States Patent Specification No.

3715701 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 characteristics tends to mask the non-linearity which is characteristic of the sintered body which constitutes the varistor and the otherwise excellent non-linear characteristics of the sintered body cannot be fully exploited.

Our copending Patent Application No. 8004324 describes and claims a non-linear resistance element which comprises a sintered body mainly composed of titanium dioxide, and at least one electrode made of silver containing from 2 to 20% by weight of one or more indium, gallium, tin, antimony cadmium, zinc or aluminium, the or each electrode being bonded in ohmic contact with the surface of the sintered body.

We have now developed a method of manufacturing a non-linear resistance element which comprises 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.

The lay 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.

A metallic layer which is respective 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; Figure2 is a graph illustrating the overlapping of the varistor and rectifier action in a varistor; Figures 3(a) and Figure 3(b) are plan views of the top surface and the bottom surface, respectively, or an annular sintered body for use as a noise suppressor in DC motor; A ceramic varistor element has typically the structure shown in Figure 1, although other forms may be determined, of course, by the particular use to which the varistor is put. As is shown in Figure 1, a sintered ceramic disc has two opposite broad surface bearing electrodes 2,2' respectively; leads 3,3' are bonded to the electrodes 2,2' respectively by solder 4,4'.

A ceramic varistor comprising titanium dioxide is capable of exhibiting the requisite non-linearity, as shown by the curve L1 in Figure 2. When the junction between the sintered body and an electrode is non-ohmic, the phenomenon of rectification is produced; the curve L2 in Figure 2 illustrates the voltage-current characteristic. The varistorthen 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 voltage less than VT, the varistor is for 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.

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 Figure 3 (a) and Figure 3 (b) for each of the opposite surfaces of a sintered body, respectively. An annular sintered body 6 is provided with a circular central hole 5 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, 7c in equal sectors. On the opposite surface of the same sintered body 6 is provided a single annular electrode 8 as shown in Figure 3(b).

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.

In the method of the present invention a good ohmic contact of the nickel electrode is obtained 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 Figure 3) is 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.

Claims (7)

1. A method of manufacturing a non-linear 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.

2. A method as claimed in claim 1 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.

3. A method as claimed in claim 1 or claim 2 wherein the layer of nickel containing phosphorus deposited on the surface of the sintered body is coated with an etching resist to the desired electrode pattern and the nickel phosphorus layer on the areas uncoated with the etching resist is removed by etching.

4. A method as claimed in claim 3 wherein the sintered body is coated with the etching resist by screen printing.

5. A method as claimed in claim 3 or claim 4 wherein the etching solution is a mixture of acetic acid, nitirc acid and acetone in equal proportions.

6. A method as claimed in claim 1 substantially as hereinbefore described.

7. A non-linear resistance element whenever made by a method as claimed in any one of the preceding claims.