Non-Linear Electrical Resistor Having Varistor Characteristics

The present invention relates to a ceramic composition for non-linear electrical resistors. More particularly, the present invention is concerned with a ceramic composition which is comprised mainly of strontium titanate and which provides electrical resistors having good non-linear resistance characteristics and a high flexural strength.

European Patent 70,540, and West German Patent Application Nos. 3,121,289 and 3,121,290 describe resistors (hereinafter referred to as "varistors") obtained by increasing the resistance at grain boundaries of strontium titanate-base semiconductor ceramics. These varistors are used as ring varistors for absorbing sparks in micromotors. Ring varistors must have a high flexural strength because they are usually fitted in position without a resin dip. This high flexural strength is essential, particularly for varistors for micromotors because of their small size.

Varistors made of the conventional strontium titanate-base semiconductor ceramic described in the above references are low in flexural strength. In order to provide a flexural strength sufficient for practical use, therefore, it is necessary to increase the thickness of varistors to about 1 mm. Such varistors of increased thickness are not suitable for use in micromotors which are desired to be miniaturized.

An object of the present invention is to provide a ceramic composition which permits production of varistors having good varistor characteristics.

Another object of the present invention is to provide a ceramic composition which permits production of varistors having a high flexural strength.

These and other objects of the present invention will become more apparent from the preferred embodiments and examples.

The present invention relates to a ceramic composition particularly useful for non-linear electrical resistors. It is comprised of strontium titanate and 0.1 to 1.0 mol % of Er.sub.2 O.sub.3 or Ho.sub.2 O.sub.3 or mixtures thereof. Such a material has crystal grains that are semiconductors and increased electrical resistance at grain boundaries.

FIG. 1 is a perspective view of a ring-shaped resistor disposed in a three point bending test to determine its flexural strength.

The ceramic composition of the present invention comprises strontium titanate (or a mixture composed mainly of strontium titanate and further containing other titanates, zirconates or stannates) and 0.1 to 1.0 mol % of at least one member of the group consisting of Er.sub.2 O.sub.3 and Ho.sub.2 O.sub.3. The ceramic composition may contain more than 60 mol % strontium titanate, and even over about 98 mol % strontium titanate. Preferably strontium titanate comprises from 99.0 to 99.9 mol % of the composition.

If desired, at least one member of the group consisting of MnO.sub.2 and Co.sub.2 O.sub.3 may be added to the above-described ceramic composition in a proportion of from 0.02 to 0.2 mol %. In this preferred embodiment of the present invention, the composition comprises 98.80 to 99.88 mol % of strontium titanate (or a mixture composed mainly of strontium titanate and further containing other titanates, zirconates or stannates), 0.1 to 1.0 mol % of at least one member of the group consisting of Er.sub.2 O.sub.3 and Ho.sub.2 O.sub.3, and 0.02 to 0.2 mol % of at least one member of the group consisting of Mn0.sub.2 and Co.sub.2 O.sub.3.

If the amount of at least one member of the group consisting of Er.sub.2 O.sub.3 and Ho.sub.2 O.sub.3 being compounded in the ceramic composition of the present invention is less than 0.1 mol %, the flexural strength undesirably drops, whereas if it is in excess of 1.0 mol %, the threshold level voltage (Vth) increases. Thus, the amount of at least one member of the group consisting of Er.sub.2 O.sub.3 and Ho.sub.2 O.sub.3 is chosen within the range of from 0.1 to 1.0 mol %.

In the latter embodiment where Er.sub.2 O.sub.3 and Ho.sub.2 O.sub.3 are added, the Mn0.sub.2 and Co.sub.2 O.sub.3 are added to increase the non-linear factor (.alpha.). If the amount of at least one member of the group consisting of Mn0.sub.2 and Co.sub.2 O.sub.3 is less than 0.02 mol %, the effect of increasing the non-linear factor (.alpha.) is insufficient; whereas if it is in excess of 0.2 mol %, the threshold level voltage (Vth) undesirably increases.

In still another embodiment of the present invention, no more than 5 mol % of a mineralizer, such as Al.sub.2 O.sub.3, SiO.sub.2 or B.sub.2 O.sub.3, is added to the above-described ceramic composition. If the amount of the mineralizer added is more than 5 mol %, electrical characteristics of the composition are adversely affected.

In another preferred embodiment of the present invention, the ceramic composition comprises 99.75 to 99.85 mol % of strontium titanate (or a mixture composed mainly of strontium titanate and further containing other titanates, zirconates or stannates) and 0.15 to 0.25 mol % of at least one member of the group consisting of Er.sub.2 O.sub.3 and Ho.sub.2 O.sub.3.

In still another preferred embodiment of the present invention, the ceramic composition comprises 99.58 to 99.80 mol % of strontium titanate (or a mixture composed mainly of strontium titanate and further containing other titanates, zirconates or stannates), 0.15 to 0.25 mol % of at least one member of the group consisting of Er.sub.2 O.sub.3 and Ho.sub.2 O.sub.3, and 0.05 to 0.17 mol % of at least one member of the group consisting of MnO.sub.2 and Co.sub.2 O.sub.3.

Preferably, the amount of binder used in the present invention is in the range of from 3 to 10% by weight based on the weight of powder mixture. Preferably the binder is a water soluble binder such as vinyl acetate polyvinyl alcohol although a solvent based binder such as butyral may be used. It is particularly preferred that the addition of binder used in the present invention amount to from 5 to 8% by weight based on the weight of powder mixture. Thus, the amount of at least one member of the group consisting of MnO.sub.2 and Co.sub.2 O.sub.3 should be controlled to the range of from 0.02 to 0.2 mol %.

The present invention is described in greater detail with reference to the following Examples and Comparative Examples, but the present invention should not be construed as being limited thereto.

In this example, SrTiO.sub.3, CaTiO.sub.3 and BaZrO.sub.3 were used singly or in combination with each other as the major component, and Er.sub.2 O.sub.3 and Ho.sub.2 O.sub.3 were used, singly or as a mixture thereof, as the semiconductorizing agent. The major component and the semiconductorizing agent were mixed in the amounts shown in Table 1, and 5% by weight of a binder was added thereto. The resulting mixture was wet-blended and crushed for about 10 hours. The material thus prepared was dehydrated, passed through a 30 mesh screen made of Saran (polychlorovinylidene) to remove coarse grains, and compacted in the form of a ring in a mold measuring 13.5 mm in outer diameter, 8 mm in inner diameter, and 1.2 mm in thickness. The compacted ring was preliminarily fired at 1,100.degree. C. for 1 hour and subsequently sintered in a reducing atmosphere consisting of 97% nitrogen and 3% hydrogen at 1,380.degree. C. for 2 hours. The thus-prepared material was such that the crystal grains of the ceramic material were semiconductive and the resistance at grain boundaries of the ceramic material was increased. The dimensions of the sintered ring were as follows: outer diameter, 11 mm; inner diameter, 6.6 mm; and thickness, 1 mm.

One side of the sintered ring was provided with three silver electrodes with a clearance of 1.5 mm between them, and the other side was entirely covered with a silver electrode to form a varistor. The varistor was connected to a constant-current power source, and its voltage values, E.sub.1 and E.sub.10 at 1 mA and 10 mA, respectively, were measured. The threshold level voltage (Vth) was determined by E.sub.10, and the non-linear factor (.alpha.) was calculated from the following equation: ##EQU1##

The flexural strength was measured by the use of a static three-point bending test as depicted in FIG. 1. Triangular support members 10 and 12 are 7 mm apart and supported the specimen 14. The load is applied to the specimen 14 between the two supporting members 10 and 12 by the load member 16. Just prior to failure the load (in Kg) is recorded and reported in the following data as the flexural strength.

In Table 1, the symbol (*) indicates that the sample does not fall within the scope of the present invention.

The procedure of Example 1 was repeated using SrTiO.sub.3 as the major component, and Y.sub.2 O.sub.3, Nb.sub.2 O.sub.5, WO.sub.3, La.sub.2 O.sub.3 and other rare earth oxides, as semiconductorizing agents. The ingredients were mixed in the amounts shown in Table 2.

The samples were tested in the same manner as in Example 1 for the threshold level voltage (Vth), non-linear factor (.alpha.), and flexural strength. The results are shown in Table 2 below.

A comparison of the data of Tables 1 and 2 shows that incorporation of at least one member of the group consisting of Er.sub.2 O.sub.3 and Ho.sub.2 O.sub.3 according to the present invention produces varistors which are comparable to conventional ones in both the threshold level voltage (Vth) and non-linear factor (.alpha.) but, in addition, are improved in flexural strength by about 20 to 80%.

The ceramic compositions shown in Table 3, consisting of SrTiO.sub.3, CaTiO.sub.3 or BaZrO.sub.3 as the major component, Er.sub.2 O.sub.3 or Ho.sub.2 O.sub.3 as a semiconductorizing agent and MnO.sub.2 or Co.sub.2 O.sub.3 as a varistor characteristic-improving agent, were prepared and, thereafter, treated in the same manner as in Example 1 to produce varistor samples.

These samples were measured in the same manner as in Example 1 for the threshold level voltage (Vth), non-linear factor (.alpha.), and flexural strength. The results are also shown in Table 3. In Table 3, the symbol (*) indicates that the sample does not fall within the scope of the present invention.

The ceramic compositions shown in Table 4, consisting of SrTiO.sub.3 as the major component, Y.sub.2 O.sub.3, Nb.sub.2 O.sub.5, WO.sub.3, La.sub.2 O.sub.3, or the like as the semiconductorizing agent, and MnO.sub.2 or Co.sub.2 O.sub.3 as the varistor characteristic-improving agent were prepared and, thereafter, treated in the same manner as in Example 1 to produce varistor samples.

These samples were measured in the same manner as in Example 1 for the threshold level voltage (Vth), non-linear factor (.alpha.), and flexural strength. The results are also shown in Table 4.

A comparison of the data of Tables 3 and 4 shows that incorporation of at least one member of the group consisting of Er.sub.2 O.sub.3 and Ho.sub.2 O.sub.3 as a semiconductorizing agent and at least one member of the group consisting of MnO.sub.2 and Co.sub.2 O.sub.3 as a varistor characteristic-improving agent produces varistors which are comparable to conventional ones in both the threshold level voltage (Vth) and non-linear factor (.alpha.) but, in addition, are improved in flexural strength by about 30 to 65%. It can further be seen that addition of the varistor characteristic-improving agent in the specified amounts disclosed provides varistors which are improved in the non-linear factor (.alpha.) as compared with the case in which the amount of the varistor characteristic-improving agent added is outside the disclosed range.

As described above, the use of the ceramic composition of the present invention produces varistors which are improved in flexural strength compared with conventional ceramic compositions. This results in an increase in reliability of varistors and makes it possible to reduce the thickness of such varistors.

The invention has been disclosed in terms of preferred embodiments and examples. The invention is not to be limited thereto but is defined by the appended claims and their equivalents.