JPH0430732B2 - - Google Patents

Description

[Detailed description of the invention]

FIELD OF THE INVENTION The present invention relates to an improvement in a multifunctional element having both the functions of a capacitor and a varistor, and in particular to a multifunctional element suitable for low voltage applications. Conventional technology Mainly made of strontium titanate, Nb,
Semiconductor porcelain obtained by adding Ta or rare earths as a semiconducting agent and firing in a neutral or reducing atmosphere is heat-treated in air at 1000℃ or higher to make the grain boundaries high resistance, and then electrode An element provided with this is known as a multifunctional element that has both a capacitor function and a varistor function. FIG. 1 is a sectional view for explaining the schematic structure of this multifunctional element 1. As shown in FIG. Also, Figure 2 shows
2 is a diagram showing an electrical equivalent circuit of the multifunctional element 1 of FIG. 1. FIG. In the figure, a multifunctional element 1 is constituted by a combination of crystal grains 2 of semiconductor ceramics. Between crystal grains 2 and 2, that is, grain boundary 3
The resistance has been increased by heat treatment. therefore,
The equivalent circuit seen from the two electrodes 4, 4' is one in which a large number of varistors 5 are connected in series and parallel, as shown in FIG. The multifunctional element 1 having such a structure generally has voltage-current (VI) characteristics as shown in FIG. That is, if the voltage applied between the electrodes 4 and 4' is within the predetermined range 6, almost no current flows between the electrodes 4 and 4', and the multifunctional element 1 functions as a capacitor. On the other hand, when the applied voltage exceeds the predetermined threshold voltage Vth and reaches a range of 7, a nonlinear current flows between the electrodes 4 and 4', and the multifunctional element 1 functions as a varistor. For the function as a varistor, it is preferable that the rise of the current flow when the voltage applied between the electrodes 4 and 4' exceeds the threshold voltage Vth, that is, the nonlinear coefficient α is large. Here, the nonlinear coefficient α is α=1/(logE ₁₀ /E ₁ ). However, _E10 is the voltage when a current of 10 mA is passed. _E1 is the voltage when a current of 1 mA is passed. FIG. 4 is a plan view of a so-called varistor ring 10, which is one application of a multifunctional element. The varistor ring 10 includes, for example, three electrodes 1 on one main surface of a semiconductor ceramic 11 formed in a ring shape.
2, 13, and 14 are formed.
This varistor ring 10 is used, for example, in a three-pole micromotor, and is used as a starting capacitor when starting the motor, and as a varistor for noise prevention by absorbing sparks when the motor is rotating. Therefore, the varistor ring 10 that has a low threshold voltage Vth is generally used. However, in the conventional ring-shaped multifunctional element represented by the varistor ring 10, the fifth
As shown in the figure, when the threshold voltage Vth is lowered, the nonlinear coefficient α also becomes a lower value,
The drawback was that it did not provide satisfactory characteristics as a varistor. In particular, when using the varistor ring 10 in a micromotor, the threshold voltage Vth should be set to 6
It has to be set to about volts, but if this is done, the nonlinear coefficient α will be around 3 (the 5th
(see figure), the drawback was that the spark absorption effect of the micromotor was not desirable in practice. Object and Structure of the Invention Therefore, the present invention provides a multifunctional element having a capacitor function and a varistor function. The purpose is to provide a multifunctional element with a large coefficient α. That is, the object is to provide a low-voltage multifunctional element with improved characteristics. The gist of this invention is to use strontium titanate as a main ingredient, and one or more types of Nb, Ta, W, Sb, or Y as a semiconductor agent.
An electrode mainly composed of silver is formed by baking on the surface of semiconductor porcelain containing rare earth elements such as La and having an average crystal grain size of 10 to 100μ, which is fired in a neutral or reducing atmosphere. This is a multifunctional element in which the crystal grain boundaries near the surface have a high resistance state. The specific structure of this invention will be explained with reference to FIGS. 6 and 7. FIG. 6 is a cross-sectional view for explaining the schematic structure of the multifunctional device 20 according to the present invention. Multifunctional element 20 shown in FIG.
However, the major difference from the conventional multifunctional element 1 shown in FIG.
The area between the grain boundaries 22 near the surface immediately below is a high-resistance grain boundary 23. In other words, unlike the conventional multifunctional element 1 (Fig. 1), not all the grain boundaries of the porcelain are made to have high resistance through heat treatment, but only the necessary grain boundaries are selectively made high. This means that the resistance is high. Therefore, since the crystal grains 22 constituting the semiconductor porcelain and the crystal grain boundaries other than near the surface directly under the electrodes 21 and 21' remain semiconductors, this equivalent circuit can be expressed as shown in FIG. . That is, a varistor 5 is provided in association with each electrode 21, 21', and the varistor 5 is connected in series. Therefore, according to the multifunctional element 20, the number of formed varistors is small and the connection thereof is not complicated, so that the multifunctional element 20 has a relatively low threshold voltage Vth and a large nonlinear coefficient α. It can be made into an element. The multifunctional element 20 having such a structure can be manufactured as follows. In other words, on the surface of semiconductor porcelain, mainly silver,
Print silver paste containing glass frit etc.
By baking in air, the grain boundaries near the surface directly under the silver electrode are selectively oxidized, and the oxidized grain boundaries become highly resistive. As the above-mentioned porcelain paste, for example, one prepared by adding 2 to 20% by weight of an inorganic substance such as lead borosilicate glass frit or bismuth oxide to silver powder is suitable. Moreover, the baking temperature is preferably 850 to 900°C. In the configuration of this invention, the average crystal grain size of the fired semiconductor porcelain was 10 to 100 microns. This is based on an experimental result that if the average crystal grain size is less than 10 μm, the threshold voltage Vth of the formed multifunctional element increases, which is not preferable. Also,
Conversely, if the average crystal grain size exceeds 100μ,
It was confirmed that the pulse resistance of the formed multifunctional element deteriorated. In addition to the above configuration, the following improvements may be made in order to improve the characteristics of semiconductor ceramics from a material standpoint. That is, one or more of Mn, Fe, Ni, Co, and Cr may be added as a property improving agent for semiconductor ceramics. In addition, SiO ₂ ,
Appropriate amounts of mineralizers such as Al ₂ O ₃ , B ₂ O ₈ , GeO ₂ and the like may be added. By doing so, the sinterability of the porcelain is improved. Note that the multifunctional elements 1 and 2 shown in FIGS. 1 and 6
0, or when the electrodes are formed on the same plane as in the ring varistor 10 shown in FIG.
Although the threshold voltage Vth is higher than that of a device having a counter electrode manufactured under the same processing conditions, a practically sufficient threshold voltage Vth can be obtained as long as the structure of the present invention is followed. If a multifunctional element with a large capacity is required, the electrodes 21 and 21' may be used as opposing electrodes, as shown in FIG. Hereinafter, specific examples of the present invention will be described. Examples of the invention Example 1 SrTiO ₈ : 99.67% by weight, Y ₂ O ₈ : 0.3% by weight,
A composition consisting of 0.03% by weight of SiO ₂ was formed into a ring shape, and after calcining at 1100°C, nitrogen: 97% by volume,
Hydrogen: 1420 in a reducing atmosphere consisting of 3% by volume
A ring-shaped semiconductor porcelain was obtained by firing at ℃ for 2 hours. The obtained semiconductor porcelain had an outer diameter of 12 mm, an inner diameter of 8 mm,
It had a wall thickness of 0.8 mm. On the semiconductor porcelain, as an electrode, a paste consisting of silver powder, 5% by weight of glass frit relative to the silver powder, and an appropriate amount of varnish was printed in the pattern shown at 12, 13, and 14 in Figure 4, and heated to 860°C. Burnt with. As a result, the V-I characteristics between the three electrodes were as shown in Table 1. In order to compare with the above-mentioned example, a ring-shaped porcelain having the same shape and dimensions as above was fired, and it was heat-treated in air at 1000°C to make the grain boundaries high in resistance.
Then, silver paste with the exact same composition as above was printed in the same pattern on the porcelain and baked at 800 degrees Celsius.
As a result, the V-I characteristics between the three electrodes were as shown in Table 2. As is clear from the results shown in Tables 1 and 2, it can be seen that the nonlinear coefficient α is significantly improved according to the examples of the present invention.

【table】

[Table] Example 2 (Sr _0.80 Ca _0.20 ) TiO ₅ : 99.67% by weight, _{Er 2} O _{5 :}
0.3% by weight, MnO ₂ :0.03% by weight, a ring-shaped element having the same shape and size was fired through the same process as in Example 1 above. Then, on the ring-shaped element, a paste made by adding silver powder, 8% by weight of bismuth oxide, and an appropriate amount of varnish to the silver powder was printed in the pattern shown in Figure 4 in the same manner as described above, and baked at 880°C. Ta. As a result of measuring the V-I characteristics between the three electrodes of the completed multifunctional device, the values shown in Table 3 were obtained.

[Table] Effects of the Invention As described above, according to the present invention, a multifunctional element with a low threshold voltage, which is the boundary voltage between the capacitor function and the varistor function, and a relatively large nonlinear coefficient α, is realized. can do.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view for explaining the schematic structure of a conventional multifunctional element. FIG. 2 is an equivalent circuit of the multifunctional element shown in FIG. FIG. 3 is a diagram showing general voltage-current characteristics of a multifunctional element. FIG. 4 is a plan view of the ring varistor. Figure 5 shows the threshold voltage of the ring varistor shown in Figure 4.
FIG. 3 is a diagram showing the relationship between Vth and nonlinear coefficient α. FIG. 6 is a cross-sectional view for explaining the schematic structure of a multifunctional element according to an embodiment of the present invention. Figure 7 shows
This is an equivalent circuit of the multifunctional element shown in FIG. 6. 8th
The figure is a sectional view showing a schematic structure of another embodiment of the present invention. In the figure, 1 and 20 are multifunctional elements, 2 and 22
are crystal grains, 3 and 23 are grain boundaries with high resistance, 4, 4', 12, 13, 14, 21, and 21' are baked electrodes, 5 is a varistor symbol, 10 is a ring varistor, and Vth is a threshold value voltage, α indicates the nonlinear coefficient.

Claims (1)

[Scope of Claims] 1 Mainly strontium titanate or strontium titanate and its homologs, and one or more rare earths such as Nb, Ta, W, Sb, Y, and La as a semiconductor agent. An electrode mainly composed of silver is formed by baking on the surface of semiconductor porcelain with an average crystal grain size of 10 to 100μ, which is fired in a neutral or reducing atmosphere, and the crystal grains near the surface directly under the baked electrode are formed by baking. A multifunctional element consisting of a high resistance field. 2. The electrode mainly composed of silver is composed of a plurality of electrodes,
2. The multifunctional element according to claim 1, wherein the plurality of electrodes are provided on the same plane of the semiconductor ceramic.