JPH0126168B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a voltage-sensitive capacitive element made of ferroelectric polycrystalline ceramic whose main component is barium titanate (BaTiO ₃ ). In general, ABO ₃ type ferroelectric ceramics, which have a perovskite crystal structure centered on BaTiO ₃ type porcelains, are mainly applied to the use of high dielectric constant and piezoelectric properties. Among these, ferroelectric ceramics are widely used as dielectric materials for ceramic capacitors, and are most commonly used in all fields. The reason for this is that other capacitors, such as those made of mica, paper, or film, have a dielectric constant εs of 3.
In contrast, in the BaTiO ₃ system, εs is around ~15.
20,000 is in practical use, and because it has a much higher relative dielectric constant than other materials, it is small and has a simple structure, but a high capacitance value can be obtained. On the other hand, since porcelain compositions are made from ceramics, they are difficult to wind and bend like the structure of capacitors using film or paper, and when compared in terms of volume, they do not provide high capacitance. . Generally, the capacitance value is given by the following formula and parameters. C: Capacitance value (F) εo: Vacuum permittivity C=εo×εs×S/tεs: Relative permittivity (permittivity of dielectric material) S: Overlapping area of electrodes (m ² ) t: Dielectric thickness (m) In order to obtain a large capacity as shown in this equation, it is necessary to reduce the thickness t, use a dielectric material with a high relative dielectric constant, and increase the overlapping area S of the electrodes. However, the general structure of the ceramic capacitor shown in Figure 1 is difficult to put into practical use due to limitations in mechanical strength and area occupied.
It has a capacitance of about 0.05μF. If a capacitance value higher than this can be obtained with a ceramic capacitor, the second
There is a multilayer ceramic capacitor as shown in the figure. In such cases, even if the thickness of one dielectric layer is sufficiently thin, it is made up of multiple layers, which increases the total thickness and provides mechanical strength, and by making the electrodes multi-layered, the electrode area In other words, it is possible to obtain a large specific surface area, and as a result, a large capacity can be obtained. In this way, multilayer ceramic capacitors can obtain large capacity with a small volume, but on the other hand, they are very expensive because precious metals are used for the multilayer internal electrodes, and the dielectric layer is thin, so they have a high withstand voltage. The disadvantage is that it is difficult to manufacture. In other words, multilayer ceramic capacitors are obtained through a manufacturing process in which the dielectric and electrodes are simultaneously fired and formed, and this firing temperature is usually 1300 to 1450.
The temperature is as high as ℃, and Pt and Pt are used as internal electrode materials.
Very expensive precious metals such as Au, Pd, Ag or alloys thereof are used. For this reason, multilayer ceramic capacitors have become very expensive, which is a major problem that hinders their widespread use. Furthermore, in order to obtain a multilayer capacitor with high withstand voltage and large capacity, the dielectric must be made thicker, and the capacitance per layer becomes smaller, requiring an increase in the number of electrodes, which further increases the cost. Currently, it is not used for anything other than special purposes. However, the demand for high reliability, small size, high withstand voltage, and large capacity has recently become particularly strong, and it has been necessary to respond to these demands. For example, capacitors of about 0.1 μF for absorbing surges of about 500 to 600 V (op) are often used in protection circuits for motor control thyristors to save energy, but they must be highly heat resistant. Therefore, even though multilayer porcelain capacitors are optimal, they cannot be used completely due to cost. As a result of intensive research aimed at providing a small, high-withstand-voltage, large-capacity, low-cost capacitor suitable for these applications, we have discovered a voltage-sensitive capacitive element that can completely meet these requirements. It is. The gist of the present invention will be described below. In order to obtain a high withstand voltage, it is difficult to keep the thickness of the dielectric material below a certain limit, and considering the cost, we decided to adopt a structure as shown in Figure 1. ) As a result of repeated studies on various ferroelectric ceramic compositions with the aim of obtaining large capacity with a composition system with a high There was nothing I could do but get it. Therefore, after various discussions and examinations of the results, we focused on the AC voltage characteristics of BaTiO ₃ -based porcelain. As cited in Japanese Patent Publication No. 52-44440 regarding BaTiO ₃ -based high permittivity ceramic compositions, the apparent εs increases by approximately 100 to 140% (approximately 2 to 2.4 times) by applying an AC voltage. It is well known to do so. However, such an increase rate is extremely unsatisfactory, and it is desirable to have an increase rate of at least 10 times. Therefore, with such characteristics of the conventional example,
No other applications can be considered, and the gist of Japanese Patent Publication No. 52-44440 is that if the rate of increase is small and the capacitance value is stable with respect to voltage, other applications can be opened up. On the contrary, the present invention cannot sufficiently increase the rate of increase in capacitance value with respect to the application of alternating current voltage, but as a result, research is being conducted based on the pursuit of the possibility of opening up even more applications. It is something that was given. The fact that the dielectric constant (εs) changes with the measurement voltage (applied voltage) was considered to indicate that the dielectric constant (εs), that is, the polarization phenomenon, has voltage dependence. In a normal ceramic dielectric material, the voltage dependence of the relative permittivity εs can be ignored, and the electric flux density D in the dielectric material is determined by the following equation. D: Electric flux density (C/m ² ) D=εE ε: Dielectric constant εo: Dielectric constant of vacuum ε=εoεs εs: Relative permittivity (permittivity of dielectric material) E: Applied electric field (v/mm ) It is common to observe DE hysteresis in the Sawyer Tower circuit as a means of observing this relationship. FIG. 3 shows the DE hysteresis loop of a TiO ₂ -based high permittivity ceramic dielectric as a normal dielectric. Looking at this hysteresis loop, it is clear that in a normal dielectric material, DE has a linear relationship, but in the case of BaTiO ₃ ceramic dielectric material, there is no linear relationship and a dielectric constant (ε) is applied. This clearly indicates that the change is caused by the electric field E. Therefore,
Using this BaTiO ₃ based high dielectric constant porcelain as a capacitor, we measured the capacitance voltage characteristics using a shearing bridge, and the results were as shown in Figure 4.
It has been reconfirmed that the capacity increase rate is comparable to that cited in 44440. Therefore, in order to increase the capacity increase rate, at least 10 times the capacity increase rate, in other words, to increase the sensitivity as a voltage-sensitive capacitive element, the present inventors developed a ferroelectric ceramic composition.
As a result of intensive research, BaTiO ₃ alone or Pb, Sn,
Good results were obtained as a basic composition in which part of Ti or Ba in BaTiO ₃ was replaced with subcomponents such as Zr and Sr. For materials other than BaTiO ₃ -based materials, the rate of increase in capacity is low, or the rate of increase in capacity does not increase unless a very high alternating current electric field is applied, making them impractical due to the relationship with the withstand voltage of porcelain. The results are shown in Table 1. Furthermore, by adding small amounts of Mn and Cr oxides as mineralizers to this basic composition, reduction can be prevented during porcelain sintering without impairing the properties, and the sinterability can be improved, resulting in more dense and homogeneous ceramics. It has been found that excellent polycrystalline materials can be obtained. As shown in Figures 6 and 7 and Table 2, the characteristics vary depending on the added elements and their amounts, and Mn and Cr are the best. It was 0.3% by weight. Good results were also obtained even when both were added in combination. This amount of addition is somewhat different from that of ordinary porcelain dielectrics.

【table】

[Table] Furthermore, the good range of these compositions is as follows:
Discovered a common phenomenon in which the crystal grain size of BaTiO ₃ or a solid solution of BaTiO ₃ and subcomponents is significantly larger and unique than that of conventional ferroelectric porcelain or compositions with low capacity increase rates. did.
That is, 10 μ on the porcelain surface of these good ones.
Compared to the high-permittivity dielectric ceramics used in the above-mentioned conventional ceramic capacitors, the number of particles that have grown to be significantly larger is the capacity increase rate and the number of giant particles, i.e., the area occupied by the giant particles on the flat ceramic surface. It was discovered that the proportion of
When the ratio exceeds 65%, the capacity increase rate increases.
It was confirmed that the increase was more than 10 times. As a result, the voltage-sensitive capacitive element of the present invention has extremely sensitive capacitance-voltage characteristics as a novel product, and when the capacitance increase rate is 0.5v/mm as a reference, the capacitance increase rate increases by up to 455 times in an applied AC electric field of 500v/mm. Converting this into a dielectric constant of ¹⁰⁶ , it is 75 times higher than the previous best value, making it extremely groundbreaking. In addition, with this composition system, it is possible to arbitrarily control the capacity increase rate by selecting subcomponents depending on the application, with a minimum magnification of 10 times and a maximum of 455 times.
The range of applications has been greatly expanded. The present invention will be explained in detail below with reference to Examples. FIG. 1A is a structural diagram of a conventionally used disk-shaped ceramic capacitor, and FIG. 1B is a sectional view thereof. In FIG. 2, A is a structural diagram of a multilayer ceramic capacitor, and B is a cross-sectional view thereof. FIG. 3 shows a DE hysteresis loop of a conventional ceramic dielectric (TIO ₂ type). FIG. 4 shows the DE hysteresis loop of a currently used high dielectric constant ceramic capacitor (εs=18000) whose main component is BaTiO ₃ . FIG. 5 shows typical examples of capacitance-voltage characteristics among the compositions shown in Table 1. FIG. 6 shows typical examples of capacitance-voltage characteristics among the compositions shown in Table 2. Figure 7 is a 400x magnification surface crystal photo of typical compositions shown in Table 2. A and B have capacitance-voltage characteristics of about 3 to 5 times, and C and D have 334x - It is 220 times larger. In addition, in Figure 7, A is No. 5 in Table 2, B is No. 9,
C corresponds to No. 2 and D corresponds to No. 7. FIG. 8 shows the crystal size of BaTiO ₃ --BaSnO ₃ -based ceramic flat surface and the rate of increase in capacity in terms of capacitance-voltage characteristics. Figure 9 shows the number of grains by size and the ratio of the grains to the total area corresponding to the crystal photographs of the porcelain flat surfaces shown in Figures 7A, B, C, and D, which are analyzed using an image analysis device. This is expressed in a histogram. Example 1 Raw material powder BaCO ₃ , TiO ₂ , SnO ₂ , SrCO ₃ ,
One or more of ZrO ₂ , PbO, MnCO ₃ and Cr ₂ O ₃ were wet-mixed using a polyethylene pot and an agate ball to obtain the desired composition shown in Table 1.
After dehydration and drying, it was held at 1150℃ for 2 hours and calcined.
Thereafter, pulverization was performed again using a polyethylene pot and an agate ball. After the water was evaporated, an appropriate amount of binder was added to the mixture, and the mixture was pressure-molded into a 16.5φ, 0.45m/m disc using a 10-ton press. Then, it was baked at 1300 to 1400°C for 2 hours. A silver electrode was baked onto the thus obtained ceramic element, the lead wires were then fixed with solder, and after cleaning, a 1KHz
The value measured with an LCR meter at a voltage of 0.5v/mm was used as the reference value. Thereafter, the capacitance value was measured by changing the measurement voltage from several volts to 500 volts (r, m, s) using a Schering bridge at AC 50 Hz, and the value obtained by subtracting this value by the reference value was taken as the capacity increase rate. The results are as shown in Table 1 and Figure 5.
Those within the scope of the present invention, that is, those containing BaTiO ₃ as the main component, exhibit sufficiently high voltage sensitivity, that is, the rate of increase in capacity when AC voltage is applied. this is,
This is a completely new discovery, showing that BaTiO ₃ has unique properties compared to other ferromagnetic compositions. Example 2 Regarding the product that showed a good capacity increase rate in Example 1, that is, the product whose main component was BaTiO ₃ , an investigation and study was conducted regarding the type of additive and the range of the amount added. Examples of the most average among them are shown in Table 2 and Figure 6. That is,
The same procedure as in Example _{1 was carried out by adding one or both of MnCO 3 and} Cr ₂ O ₃ as additives to 94 mol % of BaTiO _{3 and} 6 mol % of BaSnO 3 as shown in Table 2 and Figure 6. It was measured. Table 2 shows that the capacity increase rate can be significantly improved depending on the amount of additive added. When adding these additives, for example, Mn, MnCO ₃ ,
Although Cr was added as Cr ₂ O ₃ , it is not limited to this; similar effects can be seen even when used as other compounds, and the amount added should also take into account the state of the compound to be added. However, 0.01 to 0.3% by weight of Cr ₂ O ₃ based on the main component is considered to be within the practical range, and if it exceeds 0.3% by weight, the capacity increase rate will deteriorate significantly, which is not preferable. Figure 6 is a graph showing the capacity increase rate and addition amount. From this figure, it can be seen that the range of addition amount of 0.001 to 0.3% by weight is the range that satisfies various electrical properties, especially the capacity increase rate. It is safe to say that there is. In this case, if it is less than 0.01% by weight, the element body will not be sintered and no effect of addition will be observed. On the other hand, similarly, 0.01 to 0.6% by weight of MnCO ₃ is considered to be in the practical range, and if it is less than 0.01% by weight, no effect was observed and sintering was difficult. Furthermore, the addition of MnCO ₃ and CrO ₂ O ₃ prevents reduction of the porcelain or promotes more dense sintering during sintering. Samples Nos. 5 and 9 "in Table 2" are outside the scope of the present invention. Example 3 As a result of observing the size of crystal grains on the surface of the porcelain in Examples 1 and 2 described above, it was found that there is a relationship between the size of the grains, their distribution, the proportion of the surface area occupied by the grains, and the rate of increase in capacity. It turned out that there is a close relationship. In all of the samples showing a good capacity increase rate, the particles grew well and many giant crystal grains, which are not seen in conventional high dielectric constant porcelains, were observed. The surface of each sample was photographed using an electron microscope at 400x magnification.
As shown in the figure. Furthermore, the size and distribution of the particles were measured using an image analysis device, and the proportion of the giant particles to the total area was calculated, resulting in a histogram as shown in FIG. The particle size was determined by measuring the area of the particle, and assuming that the particle was a perfect circle, the diameter was defined as the particle diameter.
The relationship between the results of such measurements and the capacity increase rate is shown in FIG. That is, even if the composition is based on BaTiO ₃ as a main component, the rate of increase in capacity will be small if crystal grains have not grown sufficiently. Specifically, the diameter of the particles that accounts for 65% of the total area is
If it is less than 10μ, the sensitivity to AC voltage is low, that is, the rate of increase in capacity is small, and it is not practical. This is a new phenomenon and a new discovery. Various reasons have been speculated about these reasons, but it is thought that the generation of domains within the ferroelectric crystal is related to the growth, or size, of crystal grains, and that this influences the voltage dependence of dielectric polarization. . In addition, in general, when crystal grains are small, the ratio of grain boundaries to the total area increases, which weakens the ferroelectricity as a whole. It is thought that the strain applied to the dielectric polarization weakens the voltage dependence of the dielectric polarization as mentioned above, and that the crystal grains have grown sufficiently.
It is inferred from these results that this is a necessary condition for increasing voltage dependence. From the above results, the voltage-sensitive capacitive element of the present invention is sensitive to alternating voltage, and the voltage increases the capacitance value by more than 10 times or up to 455 times.
In fact, it has a relative dielectric constant of ¹⁰⁶ , making it a revolutionary element.
It has unlimited potential for future applications. As described above, the voltage-sensitive capacitive element according to the present invention has great utility in the electronic industry.

[Brief explanation of drawings]

FIG. 1A is a partially cutaway view of a conventionally used disk-shaped ceramic capacitor, and FIG. 1B is a sectional view thereof. FIG. 2A is a partially cutaway view of a multilayer ceramic capacitor, and FIG. 2B is a sectional view thereof. FIG. 3 is a photograph in place of a drawing and shows the oscilloscope waveform of a DE hysteresis loop of a conventionally used ordinary ceramic dielectric (TiO ₂ system). Fourth
The figure is a photograph in place of a drawing and shows the oscilloscope waveform of a DE hysteresis loop of a currently used high dielectric constant ceramic capacitor (s=18000) whose main component is BaTiO ₂ . FIG. 5 shows typical examples of capacitance-voltage characteristics among the compositions shown in Table 1 of the present invention. FIG. 6 shows typical examples of capacitance-voltage characteristics among the compositions shown in Table 2 of the present invention. Figure 7 is a photograph in place of a drawing, showing 400 typical compositions of the compositions shown in Table 2 of the present invention.
These are photographs of surface crystals magnified, with A and B having capacitance-voltage characteristics of about 3 to 5 times, and C and D having capacitance-voltage characteristics of 334 to 220 times. Figure 8 shows the BaTiO ₂ -BaSnO ₂ of the present invention.
It shows the crystal size of the ceramic flat surface of the system and the capacity increase rate in the capacitance-voltage characteristics. FIG. 9 is a histogram of the number of grains by size and the ratio of the grains to the total area corresponding to the crystal photographs of the flat porcelain surfaces of FIGS. 7A, B, C, and D of the present invention.

Claims (1)

[Scope of Claims] 1. Comprised of ferroelectric polycrystalline porcelain containing barium titanate (BaTiO ₃ ) as a main component, 65% or more of the flat area of the porcelain body is occupied by particles with an average crystal grain size of 10μ or more. When the capacitance value measured in an AC electric field of 0.5V/mm is used as the standard,
In an AC electric field of 500V/mm or less, the capacitance is
A voltage-sensitive capacitive element characterized by a value of 10 times or more. 2 Ferroelectric polycrystalline porcelain whose main component is barium titanate (BaTiO ₃ ), with a part of Ba replaced by Sr,
2. The voltage-sensitive capacitive element according to claim 1, wherein part of Pb and Ti is replaced with Zr and Sn. 3 Mn is added as a mineralizer to ferroelectric polycrystalline porcelain whose main component is barium titanate (BaTiO ₃ ).
Mn or Cr is added in the range of 0.01 to 0.6% by weight, Cr in the range of 0.01 to 0.3% by weight, each alone or in combination, and 65% or more of the flat area of the porcelain body has an average crystal grain size. It is dominated by particles of 10μ or more, and when the capacitance value measured in an AC electric field of 0.5V/mm is used as the standard, the capacitance is more than 10 times as large in an AC electric field of 500V/mm or less. A voltage-sensitive capacitive element characterized by a value of .