JP2772528B2 - Dielectric porcelain composition - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a laminated ceramic capacitor having an internal electrode made of an inexpensive Ag-Pd alloy which has excellent DC bias characteristics (VC characteristics) and can be fired at a low temperature. The present invention relates to a dielectric ceramic composition suitable for:

[Conventional technology and problems]

In a laminated ceramic capacitor that operates by applying a high DC voltage to a communication device or the like, the dielectric layer is generally thin, such as 20 to 30 μm, due to the bias voltage. Inhibits. Therefore, it is necessary to increase the thickness of the dielectric layer, specifically, to 50 to 80 μm, and the element becomes large. However, as electronic components have been miniaturized, there is a demand in the market for a dielectric material whose capacitance is not greatly reduced by a bias voltage even when the thickness of the dielectric is about 20 μm.

The relative dielectric constant of such a dielectric material is required to be 1500 or more in practical use. Therefore, a dielectric composition containing barium titanate as a main component is used, and the size of polycrystalline crystal particles is reduced. Based on the design philosophy of improving the VC characteristics as a whole by reducing the electric field intensity per particle by making it smaller, various additives were studied for the main component barium titanate, and a certain result was obtained. However, a decrease in the relative dielectric constant due to the application of a bias voltage, which is essentially caused by the ferroelectric properties of barium titanate, cannot be avoided, and has not yet met the demands of the market.

[Object of the invention]

The present invention has been made to solve the above problems, and is specifically a dielectric porcelain composition that can be fired at a low temperature of 1150 ° C. or less. Loss less than 2.5%, insulation resistance 1.0 × 10 ¹² Ωcm at 25 ° C
It is an object of the present invention to provide a dielectric ceramic composition having a characteristic of 1.0 × 10 ¹¹ Ω · cm or more at 125 ° C. and a VC characteristic of −15% or more.

[Means for solving the problem]

According to the present invention, the crystal grains of the barium titanate polycrystal are reduced, and the surface layer of the crystal grains is formed of a paraelectric substance having a small relative dielectric constant. That is, most of the electric field applied to each crystal grain is applied to a particle surface layer having a relatively small relative dielectric constant (εr50) having a paraelectric property, and a relatively large relative dielectric constant having a ferroelectric property. The electric field applied inside the crystal grain having As a result, the crystal grain surface composed of a paraelectric phase having essentially no electric field dependence of the relative dielectric constant bears most of the applied voltage (applied electric field). Can be reduced. Based on this technical idea, as a result of exploration research, a composition system that can be restricted to the surface of barium titanate crystal grains and can be modified into a paraelectric phase was found, and as a result of adding a substance represented by Sr ₂ Bi ₄ Ti ₅ O ₁₈ , And found that it can be realized.

Therefore, the present invention comprises most of BaTiO ₃ crystal grains and Sr ₂ Bi ₄ Ti ₅ O ₁₈ present at the grain boundaries, and is represented by the following formula (1-x) BaTiO ₃ + xSr ₂ Bi ₄ Ti ₅ O ₁₈ Then, Ba so that the value of x is 0.025 to 0.075
It is composed of TiO ₃ powder and Sr ₂ Bi ₄ Ti ₅ O ₁₈ synthetic powder. In particular, the value of x is 0.025 to 0.05 in the porcelain composition, which satisfies the above-mentioned object of sufficiently satisfying the market requirements,
A dielectric ceramic composition suitable for a laminated ceramic capacitor using an Ag-Pd alloy as an internal electrode can be provided.

In the structure of the present invention, when the value x indicating the compounding ratio of Sr ₂ Bi ₄ Ti ₅ O ₁₈ is less than 0.025, the insulation resistance is less than 1 × 10 ¹² Ω · cm, and the VC characteristic is less than −15%. Not fit for purpose. On the other hand, when the value x exceeds 0.075, the relative dielectric constant falls below 1500, which causes a problem that it is not suitable for practical use.

〔Example〕

SrCO ₃ , Bi ₂ O ₃ and TiO ₂ with a purity of 99.5% or more in a molar ratio of 2:
The mixture is weighed so as to have a ratio of 2: 5, the total amount is 200 g, and it is put in a polyethylene container having an internal volume of 1 together with 400 g of agate cobblestone and 400 cc of ion-exchanged water, and mixed by a ball mill. The mixed powder was dried, placed in an alumina crucible, and calcined at 900 ° C. for 2 hours to synthesize Sr ₂ Bi ₄ Ti ₅ O ₁₈ powder. The powder obtained is X
It was confirmed by line diffraction that no crystal phase other than Sr ₂ Bi ₄ Ti ₅ O ₁₈ was contained.

The Sr ₂ Bi ₄ Ti ₅ O ₁₈ synthetic powder obtained in this way and a purity of 99% or more
BaTiO ₃ powder was weighed to the composition shown in Table 1 and the total amount was 200 g.
Was mixed in the same manner as described above, and calcined at 950 ° C. for 2 hours. After pulverizing with a ball mill, 2 parts by weight of vinyl acetate was added to 100 parts by weight of the powder in a Reika apparatus, and the mixture was granulated, and then pressed to a diameter of 12 mm and a thickness of about 0.8 mm.

 This disk-shaped compact was fired at 1120 ° C to 1450 ° C.

The obtained sintered body is polished to a thickness of about 0.35 mm and on both sides
An In-Ga alloy was applied to prepare a parallel disk capacitor, which was used as an evaluation sample.

The capacitance and dielectric loss (DF) of the obtained sample were measured using a 1 KHz, 1.0 Vrms sine wave input signal using an LCR meter (YHP4274A), and 50 V _DC was measured using an insulation resistance meter.
The temperature was measured at 25 ° C. and 125 ° C. by calculating the insulation resistivity (IR) from the obtained resistance value and the dimensions of the sample. Also, at room temperature, the capacitance was measured by applying a DC voltage from the outside so that the DC electric field was 2.5 KV / mm, and the rate of change with no DC voltage applied was determined. And

In Table 1, for samples Nos. 1, 2, and 4, the rate of change of capacitance ΔC / C with respect to application of DC bias 0 to 3.0 (KV / mm)
(VC characteristics) are shown in FIG.

As is clear from the table, the sample No. 1 which does not contain Sr ₂ Bi ₄ Ti ₅ O ₁₈ (SBT) has a VC characteristic lower than −30% and 125%.
IR at ℃ is too small to measure.

Sample Nos. 2, 3, 4 and 6 have εr of 1500 or more and DF of 2.5%
Hereinafter, the IR at 25 ° C. and 125 ° C. is 1.0 × 10 ¹² Ω · cm or more and 1.0 × 10 ¹¹ Ω · cm or more, respectively, and the VC characteristic is −15.
% And the sintering temperature is 1150 ° C. or less, which satisfies the object of the present invention.

Sample No. 5 has good εr, DF and VC characteristics,
The IR at 125 ° C is too small to be measured and cannot be put to practical use. FIG. 1 clearly shows the effect of SBT on the VC characteristics.

Sample No. 7 is a sample in which Sr ₂ Bi ₄ Ti ₅ O ₁₈ exceeds 7.5 mol%, and has a small relative dielectric constant εr.

〔The invention's effect〕

A dielectric ceramic made of BaTiO ₃ containing 2.5 to 7.5 mol% of Sr ₂ Bi ₄ Ti ₅ O ₁₈ has a high insulation resistance, a very small decrease in the relative dielectric constant due to an applied DC bias voltage, and a sintering temperature of 11%.
As low as 20 to 1150 ° C., when applied as a laminated ceramic capacitor, the Ag-Pd alloy used as an electrode material can achieve an Ag fraction of up to 70% by weight, which is economical.

Therefore, as a capacitor element used as a circuit element to which a high DC bias voltage is applied, the thickness of the dielectric is about
Even with a thickness of about 20 μm, it is possible to meet the requirements of circuit characteristics and to provide a highly economical laminated ceramic capacitor.

[Brief description of the drawings]

FIG. 1 shows the DC bias application voltage and the change rate of the capacitance (Δ
FIG.

Claims (1)

(57) [Claims] 1. A method according to claim 1, wherein the BaTiO ₃ powder and the Sr ₂ Bi ₄ Ti ₅ O ₁₈ synthetic powder have a composition formula of (1-x) BaTiO ₃ + xSr ₂ Bi ₄ Ti ₅ O _18.
A dielectric ceramic composition having a value of 0.025 to 0.075.