JPS628930B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a dielectric ceramic for use in high-frequency, high-voltage capacitors that has a large dielectric constant and a small rate of change in dielectric constant with respect to electric field and temperature. A dielectric for a high-frequency, high-voltage capacitor must have a large dielectric constant and be stable with respect to electric field, frequency, and temperature. Porcelain, paper, etc. have been considered as dielectric materials for such high-frequency, high-voltage capacitors, but paper has a small dielectric constant, so it has the disadvantage of requiring a large shape in order to obtain the desired dielectric constant. ing. On the other hand, BaTiO ₃ -based porcelain is widely used as a material for capacitors. Although this porcelain has a high dielectric constant and can be said to be a good material in that sense, it has the disadvantage that the dielectric constant has a large dependence on electric field and temperature. Therefore, in order to improve this drawback, BaTiO ₃ −PbTiO ₃ system,
BaTiO ₃ −PbTiO ₃ −La ₂ O ₃ system or BaTiO ₃ −
BaSnO ₃ −Bi ₂ O ₃ systems, etc. have been studied. However, the electric field dependence of the dielectric constant has not yet been sufficiently reduced, and satisfactory characteristics have not been obtained. In view of the above-mentioned current situation, the present invention provides a material for use in high-frequency, high-voltage capacitors that has a high dielectric constant and
The purpose of this study is to obtain dielectric ceramics whose permittivity is less dependent on electric fields. Therefore, the first dielectric ceramic for high frequency and high voltage capacitors according to the present invention is a sintered body basically consisting of 90 to 97 mol% of BaTiO ₃ and 3 to 10 mol% of Ba ₂ Bi ₄ Ti ₅ O ₁₈ . Then, the BaTiO ₃ grains are transformed into Ba ₂ Bi ₄ Ti ₅ O ₁₈
It is characterized by being surrounded by Further, the second dielectric ceramic for high frequency and high voltage capacitors according to the present invention has Ba _1-x Dy _x TiO ₃ (where x≦
0.03) 90-97 mol%, Ba ₂ Bi ₄ Ti ₅ O ₁₈ 3-10 mol%
A sintered body basically consisting of
It is characterized in that Ba _1-x Dy _x TiO ₃ crystal grains are surrounded by Ba ₂ Bi ₄ Ti ₅ O ₁₈ . The ceramic dielectric material for a high-frequency, high-voltage capacitor according to the present invention has a structure in which crystal grains of BaTiO ₃ or Ba _1-x Dy _x TiO ₃ are surrounded by Ba ₂ Bi ₄ Ti ₅ O ₁₈ . Therefore, most of the applied electric field is applied to Ba ₂ Bi ₄ Ti ₅ O ₁₈ whose dielectric constant has a small electric field dependence, and the electric field dependence of the permittivity of the entire porcelain becomes small, and the dielectric constant is the main component. It exhibits certain characteristics of BaTiO ₃ or Ba _1-x Dy _x TiO ₃ and has a high dielectric constant of 1500-2800. In addition to these advantages, it also has the characteristics of low temperature dependence of dielectric constant and high dielectric strength. Therefore, the dielectric ceramic for high-frequency, high-voltage capacitors according to the present invention is suitable for use in high-frequency, high-voltage capacitors that require miniaturization. The present invention will be explained in more detail below. The first porcelain dielectric material for high frequency and high voltage capacitors according to the present invention is composed of BaTiO ₃ and Ba ₂ Bi ₄ Ti ₅ O ₁₈ as described above.
It is more basic, as shown in Figure 1.
It has a structure in which BaTiO ₃ crystal grains 1 are surrounded by Ba ₂ Bi ₄ Ti ₅ O ₁₈ 2. This is Ba ₂ Bi ₄ Ti ₅ O ₁₈
Considering its lower melting point than _BaTiO3 ,
It is produced by firing near the melting point of Ba ₂ Bi ₄ Ti ₅ O ₁₈ . The same applies to the second invention according to the present invention, where BaTi ₅ O ₁₈ 2 is surrounded by Ba _1-x Dy _x TiO ₃ crystal grain 1.
It has a structure surrounded by. When a high DC electric field is applied to a dielectric ceramic having such a structure, the resistivity of Ba ₂ Bi ₄ Ti 5 O ₁₈ is higher than that of Ba 2 Bi 4 Ti ₅ O 18.
Because the resistivity is higher than that of BaTiO ₃ or Ba 1 _-x Dy _x TiO ₃ and because Ba ₂ Bi ₄ Ti ₅ O ₁₈ exists around the crystal grains of BaTiO ₃ or Ba 1 _-x Dy _x TiO ₃ , The applied electric field is divided so that a high voltage is applied to Ba ₂ Bi ₄ Ti ₅ O ₁₈ , which has a high resistance, and not so high voltage is applied to BaTiO ₃ or Ba _1-x Dy _x TiO ₃ . Ba ₂ Bi ₄ Ti ₅ O ₁₈ has a small dependence of dielectric constant on electric field, and BaTiO ₃ or Ba _1-x Dy _x TiO ₃ has a large dependence of dielectric constant on electric field, but due to the above-mentioned partial pressure effect, high electric field is Therefore, by controlling the amount of Ba ₂ Bi ₄ Ti ₅ O ₁₈ , it is possible to reduce the dependence of the dielectric constant on the electric field while suppressing the decrease in the dielectric constant as much as possible for the dielectric ceramic as a whole. It can be done. In order to improve the above-mentioned effect, in the first invention, the sintered body contains 90 to 97 mol% of BaTiO ₃ ,
It has been found that a composition of 3 to 10 mol % Ba ₂ Bi ₄ Ti ₅ O ₁₈ is required. This is because when BaTiO ₃ exceeds 97 mol%, the dielectric constant decreases to the same level as that of normal BaTiO ₃ porcelain, and the stability of the dielectric constant against temperature and electric field also decreases to the same level as normal BaTiO ₃ porcelain. be. Moreover, if BaTiO ₃ is less than 90 mol %, the dielectric constant decreases and sintering becomes difficult. In the second invention according to the present invention, Ba _1-x Dy _x TiO ₃ is used instead of BaTiO ₃ . In this case, the composition of Ba _1-x Dy _x TiO ₃ and Ba ₂ Bi ₄ Ti ₅ O ₁₈ is
It is the same as for BaTiO ₃ and the reason is also the same. In this second invention, Dy is used to further reduce the electric field dependence and frequency dependence of the dielectric constant, and is 3.0% or less (x
≦0.03). The amount of Dy added is 3.0
% (x=0.03), the electric field dependence of the dielectric constant becomes large again. To further describe the relationship between the amount of Dy added, the dielectric constant, and the electric field stability of the dielectric constant, the amount of Dy is 0.3 to 1.0% (x =
The electric field stability of the dielectric constant is best in the range of 0.003 to 0.01). In the range of 0.3% or less (x≦0.003), the electric field stability of the dielectric constant is slightly worse than the above (x = 0.003 to 0.01), but it is still significantly better than the conventional one. ,
The dielectric constant increases. Next, embodiments of the present invention will be described. Example 1 Ba _1-x Dy _x TiO ₃ (x=0.003),
BaCO ₃ , TiO ₂ , and Dy ₂ O ₃ were weighed, mixed, and calcined at 1200° C. for 1 hour to produce Dy-added BaTiO ₃ .
Next, BaCO ₃ , Bi ₂ O ₃ , and TiO ₂ were weighed out at a molar ratio of 2:2:5, mixed thoroughly, and heated to a temperature of 900 to 1000°C.
Ba ₂ Bi ₄ Ti ₅ O ₁₈ was produced by calcining for an hour. this
After mixing BaTiO ₃ and Ba ₂ Bi ₄ Ti ₅ O ₁₈ at the molar ratio shown in Table 1, the mixture was heated to 24 mm at a pressure of 1 to 35 t/cm ² .
It is formed into a disc shape with a diameter of φ, and is fired at an appropriate temperature of 1170 to 1300°C depending on the amount of Ba ₂ Bi ₄ Ti ₅ O ₁₈ added.
A dielectric ceramic with a high dielectric constant and a small electric field dependence of the dielectric constant was obtained. Table 1 shows the dielectric constant, tan δ, electric field dependence of the dielectric constant, and volume resistivity of the obtained dielectric ceramic at room temperature. The dielectric constant and tanδ were measured using a bridge at a measurement frequency of 1KHz. Regarding the electric field dependence of the dielectric constant, electric fields of 1 kV/mm and 3 kV/mm were applied, and the rate of change obtained from the measured values 10 minutes later is shown. Volume resistivity is 1kV applied 2
It was determined from the resistance value after minutes. As is clear from Table 1 below,
Samples with a small amount of Ba ₂ Bi ₄ Ti ₅ O ₁₈ added (No. 1, No. 2)
The dielectric constant of the samples according to the present invention ( _{No. 3 , No. 4} , _No. ) has a large dielectric constant of 1480 to 2340. However, when the dielectric constant increases further, the dielectric constant becomes smaller again. Figure 2 shows the temperature dependence of the dielectric constant. Figure 2 also shows, for comparison, the temperature dependence of the dielectric constant of BaTiO ₃ porcelain, which is classified as YY according to the JIS standard and has little temperature change around room temperature. It can be seen that the sample according to the present invention has a small temperature dependence of dielectric constant over a wide temperature range. The sample in the present invention also has a tan δ of 1.0 to 1.3%, which is small for BaTiO ₃ porcelain. Also, the volume resistivity is normal
It is one order of magnitude larger than the approximately 10 ¹² Ωcm of BaTiO ₃ porcelain.
Regarding the electric field dependence of the dielectric constant, the sample according to the present invention has |Δε/ε _E=0 | at an electric field strength of 1 kV/mm.
It can be seen that the value of is very small, less than 1%. In order to see more clearly, the dependence of the dielectric constant on the electric field is shown in FIG. For comparison, Table 1 and Figure 3 show samples of microcrystalline BaTiO ₃ porcelain (published in 1973), which is used as a dielectric material for high-voltage capacitors.
34905) (sample No. 6). The structure of the dielectric ceramic of the present invention will be described. The structure of the dielectric ceramic of the present invention is based on the following points.
It is thought to have a structure in which Ba ₂ Bi ₄ Ti ₅ O ₁₈ surrounds crystal grains of BaTiO ₃ (or Ba _1-x Dy _x TiO ₃ ). The result of X-ray diffraction is as shown in Figure 4.
The diffraction pattern of BaTiO ₃ is mixed with the diffraction pattern of Ba ₂ Bi ₄ Ti ₅ O ₁₈ , and the porcelain according to the present invention
Become a mixture of BaTiO ₃ and Ba ₂ Bi ₄ Ti ₅ O ₁₈ . The volume resistivity is ~5×10 ¹³ Ωcm,
One order of magnitude larger than BaTiO ₃ porcelain's 10 ¹² - 10 ¹³ Ωcm. The electric field dependence of the dielectric constant is significantly smaller than that of BaTiO ₃ porcelain to which Ba ₂ Bi ₄ Ti ₅ O ₁₈ is not added (for example, the electric field strength at which Δε/ε(E=0)=0=−0.05 is BaTiO ₃ Compared to 0.7KV/mm for porcelain, it is ~2.3KV/mm for the porcelain according to the present invention). The melting points of BaTiO ₃ and Ba ₂ Bi ₄ Ti ₅ O ₁₈ are approximately
The temperatures are 1,600°C and 1,150°C, and the firing temperature for producing the porcelain of the present invention is 1,200 to 1,250°C, at which Ba ₂ Bi ₄ Ti ₅ O ₁₈ is thought to be in a liquid phase. First, from the above, it is clear that the present invention combines BaTiO ₃ and
Rather than Ba ₂ Bi ₄ Ti ₅ O ₁₈ forming a solid solution,
It was revealed that it was a mixture of BaTiO ₃ and Ba ₂ Bi ₄ Ti ₅ O ₁₈ . In the case of a mixture, there are two possibilities: the above crystal grains and their grain boundaries, and a simple mixture (both are combined in a disordered manner). The above fact is that the porcelain of the present invention is BaTiO ₃
This shows that the crystal grains are surrounded by Ba ₂ Bi ₄ Ti ₅ O ₁₈ . I will explain this in a little more detail. The above is because Ba ₂ Bi ₄ Ti ₅ O ₁₈ becomes a liquid phase during firing, and BaTiO ₃ particles are in a solid phase.
Rather than becoming a simple mixture when the sample is fired, the
This shows that Ba ₂ Bi ₄ Ti ₅ O ₁₈ can easily cover the BaTiO ₃ particles. Next, we will discuss why the characteristics of can't be explained by simple mixtures. Structural models and their equivalent circuits for the dielectric ceramic structure and simple mixture according to the present invention are shown in the fifth section.
It can be displayed as shown in FIG. First, let's talk about volume resistivity. Resistivity (volume specific resistance) ρ _a in the case of Figure 6 a
can be written as below. ρ _a = S/lR _a ......(1) (l is the total thickness, S is the total surface area, and Ra is the total resistance) Here, R _a = R ₁ + R ₂ = ρ ₁ l ₁ /S + ρ ₂ l ₂ /S=1/S(
ρ ₁ l ₁ + ρ ₂ l ₂ ) ………(2) Therefore, ρ _a = ρ ₁ l ₁ /S+ρ ₂ l ₂ /S ………(3) (ρ ₁ and ρ ₂ are the Resistivity, l ₁ and l ₂ are the thicknesses of substances 1 and 2 on the model) On the other hand, the resistivity ρ _b in the case of Figure 6b is ρ _b =S/lR _b ......(4) (R _b is the overall resistance) 1/R _b = 1/R ₁ + 1/R ₂ = S ₁ /ρ ₁ l+S ₂ /
Since ρ ₂ l=1/l [S ₁ /ρ ₁ +S ₂ /ρ ₂ ]……(5), (S ₁ and S ₂ are the surface areas of substance 1 and substance 2, and R ₁ and R ₂ are the resistances of substance 1 and substance 2.) Now, substance 1 is BaTiO ₃ and substance 2 is
Assume that Ba ₂ Bi ₄ Ti ₅ O ₁₈ and the ratio of substance 1 to substance 2 is 9:1. The resistivity of _BaTiO3 is 10 ¹² ~ 10 ¹³ Ωcm,
Since the resistivity of Ba ₂ Bi ₄ Ti ₅ O ₁₈ is 10 ¹⁴ to 10 ¹⁵ Ωcm, ρ ₁ = 5×10 ¹² and ρ ₂ = 5×10 ¹⁴ , and l ₁ /
l=0.9, _l2 /l=0.1, _S1 /S=0.9, _S2 /S=0.1
Substituting ρ _a and ρ _b become ρ _a =5.5×10 ¹³ (Ωcm) and ρ _b =5.6×10 ¹² (Ωcm), respectively. No. 3 to No. 5 in Table 1 of Example 1 according to the present invention
The volume resistivity (resistivity) of is 2.5 to 5×10 ¹³ Ω.
cm, which is larger than normal BaTiO ₃ and roughly coincides with ρ _a . As stated above, this increase in volume resistivity cannot be explained by a simple mixture model, but can be explained by a structure like the model described in the present invention. Next, the characteristics of the electric field dependence of the dielectric constant will be described. First, consider a model according to the present invention. In this case, the equivalent circuit can be written as shown in FIG. 6a. substance 1,2
Assume that the dielectric constants are ε ₁ and ε ₂ , the capacitances are C ₁ and C ₂ , and the thicknesses are l ₁ and l ₂ . Further, the volume resistivity ρ ₁ and ρ ₂ of the dielectrics 1 and 2 may be considered as ρ ₁ , ρ ₂ >>1. Further, let S be the area of substances 1 and 2. At this time, the dielectric constant ε _a of this porcelain can be written as follows. DC voltage V (electric field strength E) between PQ in Figure 6a
Immediately after applying a DC electric field, ρ 1 and ρ 2 are sufficiently large, as in the case of applying an AC electric field. Based on the assumption that ρ ₁ and ρ ₂ are sufficiently large, the electric field distribution applied to substances 1 and 2 is determined by the capacitance, respectively. It will be decided. V ₁ + V ₂ = V = lE = (l ₁ + l ₂ ) E C ₁ V ₁ = C ₂ V ₂ (V ₁ and V ₂ are the voltages applied to substances 1 and 2, respectively)
......(8) Also, C ₁ = ε ₀ ε ₁ /l ₁ S, C ₂ = ε ₀ ε ₂ /l ₂ S...
...(9) (ε ₀ is the dielectric constant of vacuum) From (8) and (9), the electric fields E ₁ and E 2 applied to substances 1 and ₂
(E ₁ ≡V ₁ /l ₁ , E ₂ ≡V ₂ /l ₂ ) is It can be written as On the other hand, after a sufficient period of time has passed after the application of the DC electric field, the voltage distribution applied to the substances 1 and 2 is determined by their respective resistance components. V ₁ ∞＋V ₂ ∞=(l ₁ +l ₂ )E V ₁ ∞=ρ ₁ ρ ₁ /SI, V ₂ ∞=ρ ₂ ρ ₂ /SI……
It can be written as (12). Next, consider the case of a simple mixture as shown in Figure 5b. The dielectric constant ε _b of the porcelain at this time
can be written as follows. ε _b = l/ε ₀ SC _b = l/ε ₀ S (C ₁ + C ₂ ) ε _b = l/ε ₀ S [ε ₀ ε ₁ /lS ₁ +ε ₀ ε ₂ /lS ₂ ] = [ε ₁ S ₁ /S+ε ₂ S ₂ /S] ......(15) Also, the voltages E _1a and E _2b applied to substances 1 and 2 are:
This is the applied voltage itself, and E _1b = E _2b = E (16). Here, BaTiO 3 is used as substance 1, and BaTiO ₃ is used as substance 2.
Considering Ba ₂ Bi ₄ Ti ₅ O ₁₈ , ε ₁ = 3000, ε ₂ = ~
300, ρ ₁ = 5×10 ¹² , ρ ₂ = 5×10 ¹⁴ , and l ₁ /
l=0.9, _l2 /l=0.1, _S1 /S=0.9, _S2 /S=0.1
Try substituting (7), (10), (11), (13), (14), and (15) as follows. First, in the case of the model of the present invention, ε _a = ~1500≒0.5ε ₁ E _1a = 1/1.9E = 0.52E E _2a = 5.2E E ₁ ∞ = 1/10.9E = 0.09E E ₂ ∞ = 9.2E ...... (17) On the other hand, in the simple model, ε _b =~2700≒0.9ε ₁ E _1b = E _2b = E (18). As can be seen from (18), in the simple mixture model, the dielectric constant of the porcelain is the main component
Since the dielectric constant of BaTiO ₃ is almost directly reflected, and the voltage applied to BaTiO ₃ is the same as the applied voltage, the dependence of the dielectric constant of the mixture porcelain is
It should be almost the same as BaTiO ₃ porcelain.
On the other hand, in the model of the present invention, the dielectric constant of this porcelain is lower than that of BaTiO ₃ porcelain;
The electric field applied to the ₃ parts (crystal grains) of BaTiO is
1/10 of the applied electric field because it is distributed by resistivity
The electric field dependence of the permittivity of Ba ₂ Bi ₄ Ti ₅ O ₁₈ and the breakdown voltage performance are significantly better than that of BaTiO ₃ , so the electric field dependence of the permittivity of this porcelain is improved. Ru. The above characteristics cannot be explained by a simple mixture, but can be explained by the model of the present invention, that is, by considering that BaTiO ₃ crystal grains are surrounded by Ba ₂ Bi ₄ Ti ₅ O ₁₈ as grain boundaries. Become. For the reasons mentioned above, it is considered that in the dielectric ceramic of the present invention, BaTiO ₄ Ti ₅ O ₁₈ surrounds BaTiO ₃ (or Ba _1-x Dy _x TiO ₃ ) crystal grains. Example 2 Ba _1-x Dy _x TiO ₃ 95 mol%, Ba ₂ Bi ₄ Ti ₅ O ₁₈ 5 mol%
Four types of samples were prepared in the same manner as shown in Example 1, with the composition and the amount of Dy being x=0, 0.003, 0.008, and 0.02. The permittivity of these four samples, tan
δ, electric field dependence of permittivity, and volume resistivity were measured, and the results are shown in Table 2. As is clear from Table 2, sample No. 8 in which Dy was added to BaTiO ₃ ,
Although the dielectric constant of No. 9 is slightly smaller than that of the sample (No. 7) without the addition of Dy, the electric field dependence of the dielectric constant becomes smaller by adding Dy. However, when the amount of Dy added becomes 2.0% (No. 10), the dependence of the dielectric constant on the electric field becomes about the same as in the case without Dy. Note that tanδ and volume resistivity are Dy
It has good properties and is hardly affected by the amount of addition. Figure 2 shows the temperature dependence of the permittivity of samples No. 7, No. 8, and No. 10. Although the dielectric constant is affected by the amount of Dy added, it can be seen that the temperature changes are almost the same.

【table】

【table】 [Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing a model of the structure of the dielectric ceramic of the present invention, in which the periphery of BaTiO ₃ crystal grains is
This shows that Ba ₂ Bi ₄ Ti ₅ O ₁₈ is surrounding it. 1...BaTiO ₃ (or Ba _1-x Dy _x TiO ₃ ), 2
...Ba ₂ Bi ₄ Ti ₅ O ₁₈ . Figure 2 is a graph showing the temperature dependence of permittivity of the present invention and conventional dielectric ceramics (the numbers in the figure indicate the numbers of the samples shown in Tables 1 and 2). ). Figure 3 is a graph showing the electric field dependence of the dielectric constant of the present invention and conventional dielectric ceramics (the numbers in the figure indicate the numbers of the samples shown in Tables 1 and 2). ). Figure 4 shows the X-ray diffraction pattern of the dielectric ceramic of the present invention (the X-ray diffraction pattern used
The line is CuK α line). FIG. 5 shows a comparison between the structural model of the porcelain of the present invention and the structural model of a simple mixture. FIG. 6 shows an equivalent circuit of the porcelain structural model of the present invention and an equivalent circuit of a simple mixture structural model. C ₁ ...Capacitance of substance 1, C ₂ ...Capacitance of substance 2, R ₁ ...Resistance of substance 1, R ₂ ...Resistance of substance 2.

Claims (1)

[Scope of Claims] 1 A sintered body basically consisting of 90 to 97 mol% of BaTiO ₃ and 3 to 10 mol% of Ba ₂ Bi ₄ Ti ₅ O ₁₈ , wherein BaTiO ₃
A dielectric ceramic for high frequency and high voltage capacitors, characterized in that crystal grains are surrounded by Ba ₂ Bi ₄ Ti ₅ O ₁₈ . 2 A sintered body basically consisting of Ba _1-x Dy _x TiO ₃ (x≦0.03) 90 to 97 mol%, Ba ₂ Bi ₄ Ti ₅ O ₁₈ 3 to 10 _mol %, Dy _x TiO ₃ grains
A dielectric ceramic for high frequency and high voltage capacitors, characterized in that it is surrounded by Ba ₂ Bi ₄ Ti ₅ O ₁₈ .