JPH0362005B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing semiconductor ceramics for voltage nonlinear resistors. In recent years, resistors with non-linear voltage-current characteristics, so-called varistors, whose element bodies are made of strontium titanate-based semiconductor ceramics have been developed.
This type of voltage nonlinear resistor is manufactured by (a) applying a paste containing metal oxides such as Mn, Zn, and Co to the surface of semiconductor porcelain made by converting strontium titanate into a semiconductor, and The element body is an insulating layer formed on the crystal grain boundaries of the semiconductor porcelain by heat treatment at a temperature of 1200 to 1300°C in an atmosphere, and an electrode is attached to this, or (b) a main body made of strontium titanate. Ingredients include metal oxides for promoting semiconductor formation, such as Nb ₂ O ₅ , Ta ₂ O ₅ , La ₂ O ₃ ,
CeO ₂ , Nd ₂ O ₃ , WO _{3 ,} etc., and V ₂ O ₅ , Cr ₂ O ₃ , CuO, which are metal oxides for improving voltage-current nonlinear characteristics.
It is known to have an element body containing CuO ₂ , MoO ₃ , MnO _{2 ,} etc., and to which electrodes are attached. This voltage nonlinear resistor has a perovskite crystal structure and exhibits ferroelectricity, so it functions not only as a varistor but also as a capacitor. It has the advantage of being able to absorb voltage (surge) and stabilize voltage. however,
In the case of conventional strontium titanate-based semiconductor ceramics, for example, in the case of (a) above, metal oxides such as Mn, Zn, Co, etc. are added to the surface of the small semiconductor ceramics in order to insulate the grain boundaries. The manufacturing process is complicated and involves processes that are difficult to control, such as the need to apply the oxide to the grain boundaries of the semiconductor porcelain, and a heat treatment to uniformly diffuse the oxide into the grain boundaries of the semiconductor porcelain. There is a drawback that characteristics such as the nonlinear index (α) and the threshold voltage (Vth) of the voltage nonlinear resistor vary widely due to the fact that the voltage is easily influenced. Also, in the latter (b),
When firing in a neutral or reducing atmosphere, the metal oxide for improving voltage-current non-linear characteristics may metallize and evaporate, damaging the furnace material of the firing furnace or making it difficult to obtain the desired composition. The drawback was that characteristics such as nonlinearity index (α) and threshold voltage (Vth) varied. The present invention makes it possible to easily manufacture semiconductor ceramics for voltage nonlinear resistors whose composition and characteristics do not vary depending on manufacturing conditions and whose characteristics such as threshold voltage and nonlinear index have little variation. The general formula is: (Sr _1-xy Ca _x ) (Ti _1-y Zr _y ) O ₃ (where x and y are the mole fractions of each component, and 0.03≦x
≦0.30, 0.02≦y≦0.20) and 99.0 to 99.9 mol% of the main component, and 0.1 to 1.0 mol% of at least one semiconductor forming agent selected from the group consisting of oxides of rare earth elements, Nb, W, and Ta. The main component raw material of semiconductor porcelain and a semiconductor-forming agent are mixed to have a composition consisting of mol%, the resulting mixture is molded into a predetermined shape, and then fired in a neutral or reducing atmosphere to produce semiconductor porcelain. A method for manufacturing a semiconductor ceramic for a voltage nonlinear resistor, characterized in that the crystal grain boundaries of the semiconductor ceramic are insulated by heat-treating the semiconductor ceramic in a natural atmosphere or an oxidizing atmosphere. In one embodiment of the invention, the composition contains up to 5 mol% of SiO ₂ and/or Al ₂ O ₃ as a mineralizer. Also,
In other embodiments, up to 1 mol% _MnO2
Other components may also be included. The content of these components was limited as described above because
This is because if the content exceeds the above, the properties will be adversely affected. The reason why the composition of the semiconductor ceramic according to the present invention is limited as described above is as follows. That is, when the mole fraction x of Ca in the main component is less than 0.03, the nonlinear index is small, and when it exceeds 0.30, the threshold voltage becomes high, which is not practical, so the above range was set. The mole fraction y of Zr in the main component is set to 0.02 to 0.20 because if y is less than 0.02, the nonlinear index will be small, and if it exceeds 0.20, the threshold voltage will be high, and both are not practical. The content of the semiconducting agent was set at 0.1 to 1.0 mol% because the content was
This is because if it is less than 0.1 mol%, the threshold voltage is high, and if it exceeds 1.0 mol%, varistor characteristics cannot be obtained. The semiconductor ceramic for a voltage nonlinear resistor according to the present invention is produced by mixing the main component of the strontium titanate ceramic composition with the above composition and a semiconducting agent in a predetermined ratio, and adding an appropriate organic binder to the mixture. After granulating and molding under pressure of 750-2500Kg/ ^cm2 , pre-baking at 1000-1200℃ in a natural atmosphere, further baking at 1350-1400℃ in a neutral or reducing atmosphere to convert into a semiconductor, then natural Heat treatment at 1000 to 1200° C. in an atmosphere or an oxidizing atmosphere results in a semiconductor ceramic with insulated grain boundaries, and by forming electrodes on the surface of this semiconductor ceramic it can be made into a voltage nonlinear resistor. The semiconductor ceramic composition according to the present invention makes it possible to manufacture a voltage nonlinear resistor that has a large nonlinear index in a low threshold voltage region and has little variation in characteristics, and also has crystal grain boundaries. There is no need for the process of applying and diffusing gold layer oxide to form an insulating layer on the surface, and in addition, during the reduction treatment to convert it into a semiconductor, the components rarely become metallized and evaporate, making it easier to manufacture. It has excellent advantages such as ease of use, high yield, and no damage to the firing furnace. Examples SrCO ₃ , CaCO ₃ , TiO ₂ and ZrO ₂ as raw materials
Weigh these to obtain the main components of the composition shown in Table 1, wet mix them in a ball mill for 10 hours, dehydrate and dry them, and then heat them in the air at 1100 to 1250℃.
Calcined for 2 hours to obtain a strontium titanate ceramic composition having the composition shown in Table 1, which was mixed with each semiconductor agent shown in Table 1 in the composition ratio shown in the same table.
After thorough wet mixing in a ball mill and dehydration, 6% by weight of an organic binder was added and granulated, and a pressure of 2000 kg/cm ² was applied to form into a disk. This molded product was pre-fired at 1150°C for 1 hour in a natural atmosphere, and then heated to 1350-1400°C in a reducing atmosphere (95% N ₂ + 5% H ₂ ).
C. for 2 hours to obtain a semiconductor porcelain having a diameter of 8 mm and a wall thickness of 1 mm, which was then heat treated at 1000 to 1200 DEG C. in a natural atmosphere to obtain a semiconductor porcelain with insulated grain boundaries. Silver paste was printed and coated on opposing surfaces of this semiconductor porcelain and baked at 800°C in a natural atmosphere to form electrodes and obtain a voltage nonlinear resistor. The threshold voltage (Vth) and nonlinear index (α) of each voltage nonlinear resistor obtained in this manner were determined. The results are also shown in Table 1. 1st
In the table, the samples with numbers marked with 〓 are outside the scope of the present invention, and Vth is 10mA to the voltage nonlinear resistor.
It is the voltage (V ₁₀ ) when flowing, and α is the voltage (V _{10 )}
This is the value obtained from the following formula from and the voltage (V ₁ ) when 1mA is applied. α=1/log( _V10 / _V1 )

[Table] Comparative examples SrTiO ₃ 99.3 mol%, Y ₂ O ₃ 0.2 mol% and
Weigh and mix 0.5 mol% of CuO, thoroughly wet mix in a ball mill, dehydrate and dry, add 6% by weight of an organic binder, granulate, and apply a pressure of 2000 kg/cm ² to form a disc. It was molded into. Next, this molded product was pre-fired at 1150°C in a natural atmosphere for 1 hour, then fired at 1380°C in a reducing atmosphere (95% N ₂ + 5% H ₂ ) for 2 hours, and then heat treated at 1100°C in a natural atmosphere. ,
A semiconductor porcelain with a diameter of 8 mm and a wall thickness of 1 mm with insulated grain boundaries was obtained, and electrodes were formed thereon in the same manner as in the examples to obtain a voltage nonlinear resistor. Threshold voltage (Vth) and nonlinear index before and after applying a pulse voltage of 300 V for the voltage nonlinear resistors of sample numbers 3 and 6 obtained in the example and the voltage nonlinear resistor obtained in the comparative example (α) was calculated. The results are shown in Table 2. In addition, Table 2 shows the average value ( ) and deviation value (σ) for each 100 samples. In the column for each sample, the upper row is the result before pulse application, and the lower row is the result after pulse application. It is.

[Table] As is clear from the results in Tables 1 and 2, the voltage nonlinear resistor using the semiconductor ceramic composition according to the present invention has a low threshold voltage and a large nonlinear index. Moreover, compared to the conventional one shown in the comparative example, the deviation values of the threshold voltage and nonlinear index are less than half, and the variation in characteristics is very small. As is clear from the above description, the present invention eliminates the disadvantage of large variations in characteristics in conventional voltage nonlinear resistors using strontium titanate-based semiconductor ceramic as an element body, and achieves low threshold voltage and This makes it possible to produce low-voltage nonlinear antibodies with large nonlinearity indexes and little variation in properties, and also eliminates the need for the process of coating and diffusing metal oxides to form an insulating layer at grain boundaries. Moreover, during the reduction treatment, the components hardly evaporate, and therefore, it has excellent effects such as easy production, good yield, and no damage to the firing furnace.

Claims (1)

[Claims] 1 General formula: (Sr _1-xy Ca _x ) (Ti _1-y Zr _y ) O ₃ (where x and y are the mole fractions of each component, and 0.03≦x
≦0.30, 0.02≦y≦0.20) and 99.0 to 99.9 mol% of the main component, and 0.1 to 1.0 mol% of at least one semiconductor forming agent selected from the group consisting of oxides of rare earth elements, Nb, W, and Ta. The main component raw material of semiconductor porcelain and a semiconductor-forming agent are mixed to have a composition consisting of mol%, the resulting mixture is molded into a predetermined shape, and then fired in a neutral or reducing atmosphere to produce semiconductor porcelain. A method for producing a semiconductor ceramic for a voltage nonlinear resistor, characterized in that the crystal grain boundaries of the semiconductor ceramic are made into an insulator by heat-treating the semiconductor ceramic in a natural atmosphere or an oxidizing atmosphere.