KR100390389B1 - Process for Preparing Nanocrystalline Dielectric Ceramics - Google Patents

Abstract

The present invention relates to a method for producing ultrafine BaTiO ₃ -based dielectric ceramics to which rare earth elements and CuO are added. According to the conventional solid-phase reaction method, the method of preparing the ultrafine dielectric ceramics of the present invention is performed on BaTiO ₃ powders with RE ₂ O ₃ (RE is La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and Yb At least one rare earth element selected from the group consisting of) powder and CuO powder is added to prepare a mixed powder, it is characterized by sintering in an oxygen atmosphere. According to the present invention, by adding a rare earth element and CuO to BaTiO ₃ and controlling the sintering process to an oxygen atmosphere, a high density, ultrafine BaTiO ₃ based dielectric ceramic can be obtained at low temperature.

Description

Process for Preparing Nanocrystalline Dielectric Ceramics

The present invention relates to a method for producing ultrafine dielectric ceramics. More specifically, by adding CuO and rare earth metal oxides to BaTiO ₃ and controlling the sintering process in an oxygen atmosphere, high density and ultrafine barium titanate dielectric ceramics are produced at low temperature. It relates to a manufacturing method.

Multi-Layer Ceramics Capacitors are an essential passive component for building small, lightweight electronic circuits. To date, titanate based on BaTiO ₃ has been mainly used in the manufacture of multilayer ceramic capacitors. However, these materials are generally manufactured at high sintering temperatures of 1300 ° C. or higher, requiring expensive precious metal internal electrodes such as Pd, Pt and the like. In order to reduce the cost of using such an expensive electrode, there is a need for a low-temperature baking dielectric ceramic composition that can use a cheap electrode such as Ag, Ag-Pd.

On the other hand, in accordance with the trend of miniaturization of components due to the recent miniaturization of various electronic devices and high integration of electronic circuits, the necessity of developing multilayer ceramic capacitors as micro devices is also rapidly increasing. In order to manufacture a compact multilayer ceramic capacitor, development of a dielectric ceramic composition capable of maintaining ultrafine particles after sintering should be made in advance. That is, a low-temperature firing and ultrafine dielectric ceramic composition is required after firing.

Sintering aids such as Pb-based, Cd-based, Bi-based, B-based, Li-based, etc. are added to keep BaTiO ₃ -based dielectric ceramics, which have been used as a main raw material of multilayer ceramic capacitors, at low temperature and maintain ultra-fine particles after firing. Attempts have been made to suppress particle growth by lowering the sintering temperature (cf. Japanese Patent No. Hei 5-120915, Hei 1-92762). However, these sintering aids are all toxic, not environmentally friendly, and have problems such as not only reacting with the dielectric material but also reacting with water used as a solvent in the water system. In order to solve such a problem, an environmentally friendly and chemically stable ultrafine BaTiO ₃ based dielectric ceramic composition for low temperature firing is required.

A method for producing ultrafine BaTiO ₃ dielectric ceramics by adding environmentally friendly and chemically stable rare earth elements such as La ³⁺ , Yb ³⁺ , and Dy ³⁺ has been proposed. See, AF Shimanskij, M. Drofenik and D. Kolar, "Subsolidus Grain Growth in Donor Doped Barium Titanate", J. Mater. Sci. 29, 6301-6304 (1994); A. Yamaji, Y. Enomoto, K. Kinoshita and T. Murakami, "Preparation, Characterization, and Properties of Dy-Doped Small-Grained BaTiO ₃ Ceramics", J. Am. Ceram. Soc., 60, 97-101 (1977); "Dielectric and X-Ray Diffraction Studies of Barium Titanate Doped with Ytterbium" by NM Molokhia, MAA Issa and SA Nasser, J. Am. Ceram. Soc., 67, 289-291 (1984)]. This is understood as the rare earth element substitutes for the Ba ²⁺ site and reduces the concentration of oxygen ion vacancy required for grain growth. However, BaTiO ₃ -based ceramics containing rare earth elements have not been widely used in the manufacture of multilayer ceramic capacitors due to the disadvantage that grain size is suppressed and densification of the sintered body is suppressed as much as possible.

In order to solve this problem, the present inventors have proposed an ultrafine dielectric ceramic composition for low-temperature sintering at the same time adding a divalent oxide of CuO and rare earth elements to form a liquid phase at a sintering temperature in BaTiO ₃ to promote densification. (Korean Patent Application No. 2000-1094). However, this composition also has a problem that when the average particle diameter is 0.1 μm or less, only a sintered density of 5.2 ± 0.2 g / cm ³ can be obtained, and thus sufficient mechanical strength and high dielectric constant for use as a multilayer ceramic capacitor are difficult to obtain.

Accordingly, the present invention relates to the improvement of the above-described Korean Patent Application No. 2000-1094, and an object thereof is to improve the sintering process by simultaneously adding environmentally friendly and chemically stable rare earth element and Cu divalent oxide CuO to BaTiO ₃ . The present invention provides a method of manufacturing BaTiO ₃ -based dielectric ceramics having high density and ultrafine grain at low temperature.

In order to achieve this object, according to the present invention, BaTiO ₃ + xCuO + yRE ₂ O ₃ (wherein 0.00 <x, high-purity BaTiO ₃ powder, CuO powder and RE ₂ O ₃ powder are weighed quantitatively). ≤ 0.05, 0.00 <y ≤ 0.05, RE is at least one rare earth element selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and Yb) There is provided a method of producing ultrafine dielectric ceramics comprising the steps of obtaining, calcining the slurry, molding the BaTiO ₃ + xCuO + yRE ₂ O ₃ calcined powder, and sintering the obtained molded body in an oxygen atmosphere. .

Hereinafter, a method of manufacturing ultrafine dielectric ceramics according to the present invention will be described in detail.

Starting materials for producing ultrafine dielectric ceramics according to the method of the present invention include BaTiO ₃ powder and CuO powder and RE ₂ O ₃ (RE is La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, At least one rare earth element selected from the group consisting of Er and Yb), and a powder having a high purity of about 99.9% or more is preferably used.

First, BaTiO ₃ powder, CuO powder, and RE ₂ O ₃ powder are finally weighed according to the composition of BaTiO ₃ + xCuO + yRE ₂ O ₃ dielectric ceramic, which is to be finally obtained according to the solid phase reaction method, and then ethyl alcohol and zirconia ball are Wet mixing to prepare slurry. The mole fractions x and y of CuO and RE ₂ O ₃ have a value of 0.05 or less, respectively. When the mole fraction of CuO exceeds 0.05, excessive liquid phase is generated during sintering, and abnormal grain growth occurs. When the mole fraction of RE ₂ O ₃ exceeds 0.05, densification does not occur, which is not preferable.

The mixed slurry is dried and then calcined in an air atmosphere at about 900 to 1100 ° C. below the sintering temperature.

The BaTiO ₃ + xCuO + yRE ₂ O ₃ calcined powder thus prepared is molded by press molding followed by hydrostatic molding, and then sintered by raising the temperature to 1100 ° C. at a rate of about 360 ° C./hr from normal temperature in an oxygen atmosphere.

In the present invention, the high-density ultrafine BaTiO ₃ -based dielectric ceramics can be manufactured by performing sintering in an oxygen atmosphere. By sintering under an oxygen atmosphere, the concentration of oxygen ions vacancies necessary for grain growth of the sintered compact is reduced while the concentration of Ba ion vacancy necessary for densification is increased.

The flow rate of oxygen for sintering oxygen is 1.1 times or more per minute, preferably 3.2 times or more, based on the volume of the sintering reactor. If the flow rate of oxygen is less than 1.1 times per minute, there is no big difference from the sintering of the air atmosphere, and it is difficult to expect a substantial contribution to the concentration reduction of the oxygen ion vacancies. In addition, according to the results of the present inventors, the structure of the sintered compact was ultrafine at an oxygen flow rate of 3.2 times per minute, and there was no significant change in the flow rate above.

BaTiO ₃ -based dielectric ceramics prepared according to the method of the present invention are high density ultrafine dielectric ceramics having an average particle diameter of about 100 to 90 nm and a sintered density of about 5.2 to 5.8 g / cm ³ .

Hereinafter, the present invention will be described in more detail with reference to Examples.

<Examples 1 to 36>

First, BaTiO ₃ powder, CuO powder, and La ₂ O ₃ powder having a purity of about 99.9% were weighed according to the composition shown in Table 1, followed by wet mixing for about 36 hours using ethyl alcohol and zirconia balls. The mixed slurry was dried and then calcined at about 1000 ° C. for about 2 hours under an air atmosphere.

BaTiO ₃ + xCuO + yLa ₂ O ₃ (where x = 0.03 and 0.00 <y ≤ 0.05) prepared in this way was uniaxially press-molded at a pressure of 1 ton / cm 2 in a mold having a diameter of 10 mm, followed by 3 Hydrostatic molding was carried out at a pressure of ton / cm 2. The obtained molded object was sintered by raising the temperature to 1100 ° C. at about 360 ° C./hr from normal temperature in an alumina tube having a diameter of 6 cm. The sintering atmosphere at this time was as nitrogen, air or oxygen at 1 atm as shown in Table 1 below, the purity of each gas was about 99.9% or more, and the flow rate was adjusted within the range of 300 to 1500 cm ³ / min. .

Here, the oxygen flow rate, 300 cm ³ / min corresponds to 1.1 times per minute based on the volume of the sintering reactor, considering that the hot zone of the sintering reactor consisting of a tube of 6 cm diameter is 10 cm ( π × 3 ² × 10 = 283 cm ³ ).

After sintering, polishing was performed using SiC abrasive paper (# 1000) so that the final specimen thickness was 1 mm. After polishing, silver paste was applied to both sides of the specimen and heat treated at about 600 ° C. for about 10 minutes to form electrodes. Dielectric properties of the obtained specimens were measured at 1.0 V _rms , 1 Hz using an LCR meter (model name 4263B manufactured by Hewlett Packard).

Subsequently, after removing all electrodes on both sides of the specimen, the sintered density was measured, and one side was polished with SiC abrasive paper (# 2000) and diamond paste (9, 3, 1 μm), followed by scanning electron microscope (manufactured by Hitachi, S-4200), the average particle diameter of the sintered compact was measured.

The results are shown in Table 1 below.

Dielectric and Sintering Properties of BaTiO ₃ + xCuO + yLa ₂ O ₃ Dielectric Ceramics. No. y Sintering atmosphere Gas flow rate (cm ³ / min) Sintering Temperature (℃) Dielectric constant Average particle diameter (nm) Sintered Density (cm ³ / min) One 0 air 300 1100 2480 150 6.1 2 0.01 nitrogen 300 1100 2250 140 5.6 3 0.01 air 300 1100 1880 120 6.0 4 0.01 Oxygen 300 1200 1720 120 6.0 5 0.01 Oxygen 600 1200 1660 110 6.0 6 0.01 Oxygen 900 1200 1620 100 6.0 7 0.01 Oxygen 1200 1200 1590 100 6.0 8 0.01 Oxygen 1500 1200 1590 100 6.0 9 0.02 nitrogen 300 1100 2250 180 5.3 10 0.02 air 300 1100 1720 100 5.6 11 0.02 Oxygen 300 1200 1650 100 5.2 12 0.02 Oxygen 600 1200 1630 90 5.2 13 0.02 Oxygen 900 1200 1600 90 5.3 14 0.02 Oxygen 1200 1200 1580 90 5.4 15 0.02 Oxygen 1500 1200 1530 90 5.3 16 0.03 nitrogen 300 1100 2160 140 4.9 17 0.03 air 300 1100 1690 100 5.1 18 0.03 Oxygen 300 1200 1680 90 5.1 19 0.03 Oxygen 600 1200 1620 90 5.1 20 0.03 Oxygen 900 1200 1590 80 5.2 21 0.03 Oxygen 1200 1200 1600 80 5.2 22 0.03 Oxygen 1500 1200 1580 80 5.3 23 0.04 nitrogen 300 1100 2050 140 4.7 24 0.04 air 300 1100 1670 90 5.0 25 0.04 Oxygen 300 1200 1600 90 5.3 26 0.04 Oxygen 600 1200 1520 90 5.3 27 0.04 Oxygen 900 1200 1500 90 5.3 28 0.04 Oxygen 1200 1200 1490 80 5.3 29 0.04 Oxygen 1500 1200 1490 80 5.2 30 0.05 nitrogen 300 1100 2010 130 4.9 31 0.05 air 300 1100 1690 90 5.0 32 0.05 Oxygen 300 1200 1510 90 5.2 33 0.05 Oxygen 600 1200 1420 90 5.3 34 0.05 Oxygen 900 1200 1370 80 5.3 35 0.05 Oxygen 1200 1200 1330 80 5.3 36 0.05 Oxygen 1500 1200 1340 80 5.3 x = 0.03

In the results of Table 1, when the amounts of CuO and La ₂ O ₃ were the same, dielectric ceramics having a relatively small dielectric constant but a high sintered density and a small average particle diameter were obtained in order of nitrogen, air, and oxygen atmosphere. In addition, when sintered in an oxygen atmosphere, the most dense and ultrafine BaTiO ₃ dielectric ceramics were obtained at a flow rate of 900 cm ³ / min or more.

Such high-density ultrafine atomization of BaTiO ₃ decreases the concentration of oxygen ion vacancies required for particle growth due to sintering under oxygen atmosphere, increases the concentration of Ba ion vacancies required for densification, and results in surface exchange reaction at a flow rate of 900 cm ³ / min or more. It appears to be due to the sufficient supply of necessary oxygen ions. That is, by the oxygen atmosphere sintering reduces the concentration of oxygen ions public necessary for the grain growth and increasing the concentration of the Ba ion public necessary for densification by being a high density as compared with a nitrogen and air atmosphere sintering ultrafine a low-temperature co-fired BaTiO ₃ based dielectric ceramics for It is judged that can be obtained.

<Examples 37-66>

The compositions of the mixed powder were prepared as shown in Table 2 below, except that the powders were sintered under a gas flow rate of 1200 cm ³ / min under an oxygen atmosphere, and the methods of Examples 1 to 36 were repeated, and the results are shown in Table 2 together. It was.

As shown in Table 2, it can be seen that even when the rare earth oxide is variously changed and the gas flow rate is set to 1200 cm 3 / min, which is 900 cm 3 / min or more, the results obtained in Table 1 do not show a significant difference.

Dielectric and Sintering Characteristics of BaTiO ₃ + xCuO + yRE ₂ O ₃ Dielectric Ceramics. No. x y RE Dielectric constant Average particle diameter (nm) Sintered Density (cm ³ / min) 37 0.01 0.02 La 1650 100 5.5 38 0.01 0.03 La 1690 100 5.4 39 0.01 0.04 La 1570 90 5.3 40 0.02 0.02 La 1600 100 5.3 41 0.02 0.03 La 1410 90 5.2 42 0.02 0.04 La 1550 90 5.2 46 0.03 0.02 Pr 1610 90 5.3 47 0.03 0.02 Nd 1540 90 5.4 48 0.03 0.02 Eu 1500 90 5.2 49 0.03 0.02 Dy 1580 90 5.3 50 0.03 0.02 Yb 1630 90 5.2 51 0.03 0.03 Pr 1400 80 5.2 52 0.03 0.03 Nd 1590 90 5.3 53 0.03 0.03 Eu 1620 90 5.3 54 0.03 0.03 Dy 1680 90 5.2 55 0.03 0.03 Yb 1520 80 5.3 56 0.03 0.04 Pr 1530 90 5.3 57 0.03 0.04 Nd 1420 80 5.2 58 0.03 0.04 Eu 1590 80 5.2 59 0.03 0.04 Dy 1640 90 5.3 60 0.03 0.04 Yb 1480 80 5.2 61 0.04 0.02 La 1720 100 5.3 62 0.04 0.03 La 1640 100 5.2 63 0.04 0.04 La 1600 90 5.2 64 0.05 0.02 La 1830 100 5.2 65 0.05 0.03 La 1770 100 5.3 66 0.05 0.04 La 1520 100 5.3 Sintering temperature = 1200 ° C, oxygen atmosphere, oxygen flow rate = 1200 cm ³ / min

According to the present invention, by adding CuO and rare earth metal oxides to BaTiO ₃ and controlling the sintering process in an oxygen atmosphere, high density, ultrafine dielectric ceramics can be obtained at low temperatures.

Claims (4)

Quantitatively weighing high purity BaTiO ₃ powder, CuO powder and RE ₂ O ₃ powder and then ball milling to obtain a mixed slurry consisting of BaTiO ₃ + xCuO + yRE ₂ O ₃ , Calcining the slurry, Molding the calcined powder of BaTiO ₃ + xCuO + yRE ₂ O ₃ , and Sintering the obtained compact under an oxygen atmosphere, wherein at BaTiO ₃ + xCuO + yRE ₂ O ₃ , 0.00 <x ≦ 0.05, 0.00 <y ≦ 0.05, and RE is La, Pr, Nd, Sm, Eu, A method for producing ultrafine dielectric ceramics, characterized in that it is at least one rare earth element selected from the group consisting of Gd, Tb, Dy, Ho, Er and Yb. The method of claim 1, wherein the flow rate of oxygen for producing the oxygen atmosphere is to supply oxygen at least 1.1 times per minute based on the volume of the sintering reactor. The method of claim 2, wherein the flow rate of oxygen for making the oxygen atmosphere is at least 3.2 times per minute based on the volume of the sintering reactor. The method of claim 1, wherein the calcination step is performed at 900 to 1100 ° C. for 0.5 to 2 hours.