JP2007153636A - Dielectric porcelain - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a dielectric porcelain having a high dielectric constant even if the main crystal particle is a fine particle. <P>SOLUTION: The dielectric porcelain is provided with a crystal particle represented by compositional formula of Ba<SB>100-x</SB>M<SB>x</SB>TiO<SB>3</SB>(M=Ca, Er, Tb, Sm, x=0.05-0.2), wherein by allowing the barium titanate-based dielectric material to have such a composition, the ratio of R(M)/R(O) lies in a range of 0.686-0.764 when R(M) is designated to be an ionic radius of M and R(O) is designated to be an ionic radius of oxygen, wherein the ionic radii fall in such a range that the ratio of the number of main crystal particles having a domain structure caused by the spontaneous polarization against the number of the total main crystal particles in a field of view of a transmission electron microscope image where at least 10 or more of a plurality of main crystal particles exist becomes 50% or higher. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a dielectric ceramic, and more particularly, to a dielectric ceramic having fine particles and crystal grains having a high dielectric constant.

  In recent years, with the miniaturization and high performance of electronic devices, there has been an increasing demand for miniaturization and large capacity of multilayer ceramic capacitors. In order to meet such demands, in multilayer ceramic capacitors (MLC), the dielectric layer is made thinner to increase the capacitance, and by increasing the number of layers, the size and capacity can be reduced. It has been.

Here, regarding the crystal particles constituting the dielectric layer, for example, according to Patent Document 1, instead of BaTiO ₃ conventionally used as a dielectric material having a high dielectric constant, a Ca component is added to the Ba site of this BaTiO _3. A dielectric material represented by a composition formula Ba _1-x M _x TiO ₃ (M = Ca) in which is dissolved is proposed.
JP 2000-58377 A

However, the Ba _1-x M _x TiO ₃ (M = Ca) -based dielectric material disclosed in Patent Document 1 is 0.003 ≦ x ≦ 0.22 and the average particle size is about 0.5 μm. Although a high relative dielectric constant of 3360 at the maximum can be obtained, there is a problem that when the crystal grains are atomized with the composition of the dielectric material, the relative dielectric constant is extremely lowered.

  Accordingly, an object of the present invention is to provide a dielectric ceramic comprising barium titanate-based crystal particles having a high dielectric constant even if they are fine crystal particles.

The dielectric ceramic of the present invention is a crystal whose composition formula is represented by Ba _1-x M _x TiO ₃ (M is one selected from Ca, Er, Tb and Sm, x = 0.005 to 0.002). It is characterized by comprising particles.

  In the dielectric ceramic, the average particle size of the crystal particles represented by the composition formula is in the range of 0.1 to 0.2 μm, and a transmission electron microscope image in a field where at least 10 or more of the crystal particles are present. The ratio of the number of crystal grains having a domain structure resulting from spontaneous polarization to the total number of crystal grains is 50% or more. In such a dielectric ceramic, the relative permittivity has an AC electric field dependency. It is desirable that it is 1.4 times or more.

The dielectric ceramic of the present invention has a composition formula represented by Ba _1-x M _x TiO ₃ (M is one selected from Ca, Er, Tb and Sm, x = 0.005 to 0.002). In this case, if the ionic radius of M is R (M) and the ionic radius of oxygen is R (O), the ratio of R (M) / R (O) is in the range of 0.686 to 0.764. In such a range of ionic radii, the ratio of the number of crystal grains having a domain structure resulting from spontaneous polarization to the total number of crystal grains in a transmission electron microscope image in a field where at least 10 crystal grains exist Is 50% or more.

  Therefore, the dielectric material of the present invention has AC dependency with a high relative dielectric constant, and can achieve a high dielectric constant even if the average particle diameter of crystal grains is in the range of 0.1 to 0.2 μm. This is suitable for a small-sized and high-capacity monolithic ceramic capacitor having a thin body layer.

The dielectric ceramic of the present invention comprises crystal grains represented by Ba _1-x M _x TiO ₃ (M is one selected from Ca, Er, Tb and Sm, x = 0.005 to 0.002). It is characterized by that.

  In particular, M is more preferably Ca or Tb in that the high dielectric constant and the relative dielectric constant have a higher AC electric field dependency. The reason why Ca and Tb are dissolved is that the existence ratio of crystal grains having a domain structure is 60% or more, and the existence ratio of crystal grains having a high dielectric constant in a dielectric ceramic can be increased.

  In the above composition, x is more preferably 0.0005 to 0.001. When x is 0.0005 or more, there is an advantage that the domain region can be enhanced by an element that dissolves in fine crystal grains. When x is 0.001 or less, there is an advantage that many domain structures can be formed in the barium titanate crystal particles. When x is lower than 0.0005, the solid solution amount of the element in barium titanate decreases, and the crystal grains cannot form a domain structure, resulting in a low dielectric constant. When x is more than 0.002, since a large number of crystal grains having a domain structure cannot be formed in the barium titanate crystal grains, the dielectric constant is low.

  The average particle size of the crystal particles according to the present invention is preferably in the range of 0.1 to 0.2 μm. When the average particle size of the crystal particles is 0.1 μm or more, particularly 0.15 μm or more, the tetragonal nature of the crystal particles can be increased, the dielectric constant can be increased, and the dependency of the relative permittivity on the AC electric field can be increased. There is. If the average grain size of the crystal grains is 0.2 μm or less, a large number of grain boundaries can be formed even if the dielectric layer is thinned, so that there is an advantage that it is possible to contribute to the improvement of insulation by the grain boundaries.

  According to the composition of the dielectric ceramic of the present invention, when the ion radius of M is R (M) and the ion radius of oxygen is R (O), the ratio of R (M) / R (O) is 0.686. It becomes the range of -0.764.

  This ion radius ratio can be explained as follows. That is, in an ionic crystal in which the dielectric ceramic according to the present invention is classified, the radius ratio between the cation and the anion can be calculated geometrically, and a theoretical ion radius ratio is given. When the ion radius ratio is smaller than the theoretical ion radius ratio, covalent bonding is required, and ions smaller than a certain value cannot stably enter the lattice. When R (M) / R (O) is smaller than 0.686, the cubic (or tetragonal) perovskite structure is unstable, so that it is difficult to form a solid solution and it is difficult to form a desired solid solution. Become.

  On the other hand, when R (M) / R (O) is greater than 0.764, it is easily replaced by solid solution and enters the crystal lattice. However, since the ionic bond is large, the substitution effect is small and the displacement polarization is improved. It is difficult to do.

  When a part of barium titanate is replaced by the element having the ionic radius ratio, since the ions are smaller than the theoretical ionic radius ratio, the covalent bond is generated and the spontaneous polarization is easily induced.

  In addition, by introducing an element with a small ion radius, a local strain field is generated, and the crystal lattice is easily deformed. Therefore, the symmetry of the crystal structure is easily lowered, and the generation of spontaneous polarization is facilitated. . Even if the particle size is small, the domain structure can be easily generated by improving the spontaneous polarization accompanied by lattice deformation. The ionic radius of the element M according to the present invention is a 12-coordinate ionic radius based on the perovskite structure (for example, shown by Ceramic Applied Chemistry Series 2 (by Yanagida, Maruzen Co., Ltd.)).

  In addition, any of the above-described element substitution amounts may be small, and spontaneous polarization can be induced by a strain field caused by such a small substitution. When the substitution amount is large, the behavior of the phase transition is changed, and a high dielectric constant and a flat temperature characteristic cannot be realized.

  FIG. 1 is a schematic view showing crystal grains having a domain structure resulting from spontaneous polarization observed in a transmission electron microscope, with respect to the dielectric ceramic of the present invention.

  The dielectric ceramic according to the present invention, as described above, transmits in a field of view where at least 10 crystal grains 1 are present by substituting the element having the ionic radius ratio with barium titanate in a very small amount as a solid solution. The ratio of the number of crystal grains having the domain structure 3 resulting from spontaneous polarization to the total number of crystal grains in the electron microscope image can be 50% or more. A dielectric constant can be obtained, and further, an effect that the AC electric field dependency of the relative dielectric constant can be increased to 1.4 times or more can be obtained.

  As a result, when the above-described dielectric ceramic of the present invention is used as a dielectric layer, the multilayer ceramic capacitor can be made thinner and highly laminated due to its high relative permittivity and high AC dependency of relative permittivity. It is suitable for.

Table 1 shows the composition and synthesis temperature of the prepared barium titanate raw material powder. As shown in Table 1, BaCO ₃ , TiO ₂ , SrCO ₃ , CaCO ₃ and oxides of various rare earth elements are mixed in a predetermined amount based on the composition formula (Ba _1-x M _x ) TiO ₃ , and zirconia having a diameter of 5 mm is obtained. The balls and IPA were used as solvents and mixed for 15 hours, and the dry powder was heat treated at 1050 ° C. in the atmosphere for 2 hours at the temperature shown in Table 1.

Table 2 shows the average particle diameter of the barium titanate-based synthetic raw material used, the sintering aid and its addition amount, the firing temperature, and the characteristics of the dielectric ceramic. As a sintering aid, 1.2 parts by mass of BaSiO ₃ was added to 100 parts by mass of the barium titanate synthetic raw material. In this example, MgO, Y ₂ O ₃ and MnCO ₃ used in conventional dielectric ceramics for nickel internal electrode capacitors were used to increase the grain growth and reduction resistance of the main crystal grains. MgO was 0.2 parts by mass, Y ₂ O ₃ was 0.5 parts by mass, and MnCO ₃ was 0.16 parts by mass with respect to 100 parts by mass of the barium titanate powder.

  Next, the mixed powder was wet-mixed by adding a mixed solvent of toluene and alcohol as a solvent using zirconia balls having a diameter of 5 mm.

  Next, a mixed solvent of polyvinyl butyral resin and toluene / alcohol is added to the wet-mixed powder, and wet-mixed using the same zirconia balls with a diameter of 5 mm to form a tablet with a diameter of 16 mm and a thickness of 1 mm. A sample was prepared by firing in an atmosphere. This condition is that the rate of temperature increase is 300 ° C./h, the temperature shown in Table 2 is 2 hours in hydrogen-nitrogen, and then cooled to 1000 ° C. at a rate of temperature decrease of 300 ° C./h. A re-oxidation treatment was performed at 1000 ° C. for 4 hours in a nitrogen atmosphere, and the sample was cooled at a temperature decrease rate of 300 ° C./h to produce a dielectric ceramic as a sample.

  Next, the average particle diameter of the crystal particles constituting the dielectric ceramic was determined by a scanning electron microscope (SEM). The polished surface was etched, 20 crystal particles in the electron micrograph were arbitrarily selected, the maximum diameter of each crystal particle was determined by the intercept method, and the average value (D50) was determined.

  Next, the dielectric ceramic was polished to a thickness of 400 μm, and Au electrodes were formed on the upper and lower surfaces of the sample by sputtering. For the electrical characteristics, the relative dielectric constant was calculated by measuring the capacitance under the conditions of AC: 1 V and measurement frequency: 1 kHz in the temperature range of −25 ° C. to 85 ° C. using an LCR meter.

  The temperature change rate TCC of the relative dielectric constant was determined from the following formula: TCC (%) = {ε (T) −ε (20 ° C.)} × 100 / ε (20 ° C.). The reference temperature is 20 ° C.

  Further, the AC voltage dependence of the relative permittivity was obtained by measuring a polarization-electric field curve by a virtual ground method at 100 Hz and calculating the permittivity from the slope of the curve.

The presence of domains in crystal grains was determined by changing the observation conditions in the same field of view by changing the tilt of the sample surface with respect to the electron beam in bright field image observation in transmission electron microscope observation. The results are shown in Table 2.

  As is apparent from the results in Table 2, the sample No. using a barium titanate-based material in which an element having an ionic radius ratio of 0.664 was dissolved. In No. 12, the existence ratio of the domain was as small as 22%, and the relative dielectric constant increase rate when the AC electric field was increased was small. For this reason, the relative dielectric constant at an AC electric field of 1 V / μm was as small as 2350. Sample No. using a barium titanate-based raw material substituted with an element having an ionic radius ratio of 0.8. In No. 13, the abundance ratio of the crystal grains showing the domain structure was as small as 17%, the relative dielectric constant increase rate when the AC electric field was increased, and the relative dielectric constant when the AC electric field was 1 V / μm was as low as 2400.

  Sample No. with a solid solution component of La. In No. 14, the existence ratio of crystal grains having a domain structure was as low as 13% and the relative dielectric constant was as low as 2800. In this case, the temperature characteristics of the relative dielectric constant were as large as −20% and −25%.

  Sample No. whose solid solution component is Sr. Also in No. 15, the existence ratio of the crystal grains showing the domain structure was as low as 15%, and the relative dielectric constant was as low as 2150.

  For porcelain in which no substitution element is added to barium titanate, sample no. As shown in FIG. 16, the existence ratio of the crystal particles showing the domain structure was as small as 15% and the relative dielectric constant was as small as 1900.

  In addition, sample No. In No. 17, the temperature characteristic of the relative dielectric constant was changed, and the temperature change rate was as large as −12% and −32%.

  On the other hand, Sample No. which is a dielectric ceramic of the present invention using a barium titanate-based raw material in which an element having an ionic radius ratio of 0.686 to 0.764 is dissolved. 1 to 11, the existence ratio of the domain is 56% or more, the relative dielectric constant is 2700 or more, the AC electric field dependence of the relative dielectric constant is as large as 1.4 times, and the relative dielectric constant The rate of change in temperature was also as excellent as ± 10% or less.

  In particular, sample Nos. In which the average particle size of the crystal particles is in the range of 0.15 to 0.2 μm. In 1-4, 7-8, 10, and 11, the AC electric field dependence of the relative permittivity is as large as 1.6 times, the relative permittivity is 3000 or more, and the temperature characteristics of the relative permittivity is as low as −8% or less. It was a thing.

It is the schematic diagram showing the crystal grain which has the domain structure resulting from the spontaneous polarization seen in transmission electron microscope observation about the dielectric material ceramic of this invention.

Explanation of symbols

1 Crystal grain 3 Domain structure

Claims (4)

Characterized by comprising crystal grains represented by the composition formula Ba _1-x M _x TiO ₃ (M is one selected from Ca, Er, Tb and Sm, x = 0.005 to 0.002). Dielectric porcelain. The dielectric ceramic according to claim 1, wherein an average particle diameter of the crystal particles is in a range of 0.1 to 0.2 μm. The ratio of the number of crystal grains having a domain structure due to spontaneous polarization to the total number of crystal grains in a transmission electron microscope image in a field of view in which at least 10 crystal grains are present is 50% or more. Dielectric porcelain described in 1. 4. The dielectric ceramic according to claim 1, wherein the AC electric field dependency of the dielectric constant is 1.4 times or more.

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