JP2010006623A - Piezoelectric ceramic and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a barium titanate-based piezoelectric ceramic which can be fired at a low temperature of ≤1,200°C without deteriorating the piezoelectric characteristics and allows a phase transformation temperature t<SB>OT</SB>to become equal to or below 0°C and a method of manufacturing the same. <P>SOLUTION: The piezoelectric ceramic consists essentially of BaTiO<SB>3</SB>and CaTiO<SB>3</SB>in a mixing ratio corresponding to a chemical formula, (1-x)BaTiO<SB>3-x</SB>-CaTiO<SB>3</SB>(wherein 0.01≤x≤0.08) and is formed by adding 0.10-0.30 wt.% LiF as an auxiliary component and firing at ≤1,200°C. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to piezoelectric ceramics used for piezoelectric devices such as vibrators, actuators, and sensors, and a method for manufacturing the same.

  Piezoelectric materials such as piezoelectric ceramics and piezoelectric crystals are substances that generate electrical polarization when strain is applied, and conversely, when electric fields are applied, reversible conversion between electrical and mechanical signals is possible. Utilizing the possible properties, it is used in piezoelectric devices such as various sensors, filters, and actuators.

In particular, lead-containing piezoelectric ceramics such as lead zirconate titanate (Pb (Ti, Zr) O ₃ ) have not only excellent piezoelectric properties but also good temperature properties and can be fired at low temperatures. It has advantages and is currently used in the widest area as a material for piezoelectric devices.

  However, since it has been confirmed that lead is harmful to the human body, the development of lead-free piezoelectric materials for piezoelectric devices that do not contain lead, which can be practically substituted for lead-containing piezoelectric materials as described above, has been developed worldwide. Has been done.

In such a background, barium titanate (BaTiO ₃ ) is one of lead-free piezoelectric materials having good piezoelectric characteristics. According to Patent Document 1, barium titanate powder having an average particle size of 0.1 μm is used as a raw material, and after molding it, it is fired at 1110 ° C. in a resistance heating furnace, so that the electromechanical coupling coefficient k _p is 0.1. A piezoelectric ceramic of 25 is obtained.

According to Patent Document 2 and Non-Patent Document 1, a raw material is barium titanate powder having an average particle size of 0.1 μm, and after molding, it is fired at 1320 ° C. by microwave heating, so that k _p = 0 A piezoelectric ceramic of 36 is obtained.

According to Non-Patent Document 2, by using a barium titanate powder having an average particle size of 0.1 μm as a raw material and molding, two-stage firing at 1320 ° C. and 1150 ° C. is performed in a resistance heating furnace, k _p = A piezoelectric ceramic of 0.42 is obtained.

  However, when barium titanate is intended to be used as a piezoelectric ceramic product, the crystal structure undergoes phase transformation from orthorhombic to tetragonal at around room temperature, which significantly changes the characteristics in the product operating temperature range. is there.

One measure for lowering the orthorhombic-tetragonal structural phase transformation temperature (t _OT ) existing near room temperature to below zero degrees is the addition of a small amount of calcium titanate (CaTiO ₃ ). According to Non-Patent Document 3, it is shown that _tOT sequentially shifts to the low temperature side according to the amount of calcium titanate added to barium titanate. However, a decrease in piezoelectric characteristics is observed corresponding to the lowering of t _OT .

  Further, in order to produce a barium titanate ceramic having a high electromechanical coupling coefficient, as described in Non-Patent Documents 1 and 2, a method such as microwave heating or two-stage heating is used. It is necessary to bake at a high temperature of at least ° C. However, the above two methods are not very mass-productive, and in addition, when high-temperature firing at 1300 ° C. or higher is performed, expensive products such as heat-resistant palladium and platinum are used as internal electrodes to produce a multilayer ceramic device. There is a concern that the cost will rise due to the use of new precious metals.

  On the other hand, research on the low-temperature firing of barium titanate is being conducted mainly in the field of ceramic capacitor development. One attempt is to add a small amount of lithium fluoride (LiF). According to Patent Document 3, it is disclosed that baking can be performed at a temperature of 1250 ° C. or lower by adding lithium fluoride of 0.25 wt% to 10.0 wt% to barium titanate. According to Patent Document 4, low-temperature firing is promoted by using barium-excess barium titanate powder as a raw material and adding 1.5 wt% or more and 10.0 wt% or less of lithium fluoride thereto. Has been found, and a multilayer ceramic capacitor using the same has been proposed. According to Patent Document 5, barium-excess barium titanate powder having an average particle size of 0.5 μm or less is used as a raw material, and 0.03 wt% or more and 0.40 wt% or less of lithium fluoride is added thereto. Discloses that low-temperature firing at 1200 ° C. or lower is possible.

  However, in any of the above patent documents, the purpose is only to achieve the dielectric constant and its temperature characteristics for application to a ceramic capacitor, and it is essential for the application of the obtained barium titanate fired body to a piezoelectric device. The piezoelectric characteristics are not described.

JP 2007-277031 A JP 2006-315927 A U.S. Pat. No. 4,082,906 Japanese Unexamined Patent Publication No. 57-160963 JP-A-1-115868 Hirofumi, T. Jpanese Journal of Applied Physics, 2006, Vol. 45, p. L30-L32 Tomoaki, K.M. Others, Japanese Journal of Applied Physics, 2007, Vol. 46, p. L97-L98 Okazaki Kiyoshi, 4th edition Ceramic Dielectric Engineering, Academic Consortium, published June 1, 1992

As described above, piezoelectric materials mainly composed of barium titanate as an alternative to lead-containing piezoelectric materials are considered to be promising. Conventionally, however, the phase transformation temperature is maintained while maintaining sufficient piezoelectric properties to withstand practical use. lowering the t _OT, and lowering the firing temperature is reduced significantly, usually, barium titanate-based ceramics for producible piezoelectric devices fired has not been shown.

In view of such a situation, an object of the present invention is to provide a barium titanate-based piezoelectric ceramic that can be fired at a low temperature of 1200 ° C. or less without impairing piezoelectric characteristics, and has a phase transformation temperature t _OT of zero degrees or less, and its It is to provide a manufacturing method.

In order to solve the above problems, the piezoelectric ceramic according to the present invention has an average particle size of 0 corresponding to a chemical formula of (1-x) BaTiO ₃ —xCaTiO ₃ (where 0.01 ≦ x ≦ 0.08). The main component is a mixture of BaTiO ₃ powder of less than 2 μm and CaTiO ₃ powder having an average particle size of less than 0.2 μm, and LiF is added to this as a subcomponent and baked by adding 0.10 wt% to 0.30 wt%. It is produced.

In addition, the method for producing the piezoelectric ceramic according to the present invention provides an average amount corresponding to a molar ratio represented by a chemical formula of (1-x) BaTiO ₃ —xCaTiO ₃ (where 0.01 ≦ x ≦ 0.08). To a BaTiO ₃ powder having a particle size of 0.1 μm or less and a CaTiO ₃ powder having an average particle size of 0.1 μm or less, LiF of 0.10 wt% or more and 0.30 wt% or less is added to the total amount of the powder. After the mixed raw material is formed, it is fired in a temperature range of 950 ° C. or more and 1200 ° C. or less. Here, LiF to be added may be powder, and the average particle size thereof may be 0.1 μm or less.

  In the present invention, as shown in the experimental results of Examples and Comparative Examples later, the inventors of the present application have found an optimal raw material composition of barium titanate ceramics and a method for producing the same for solving the above-described problems. It is based on.

In the present invention, the substitution amount x of CaTiO ₃ with respect to BaTiO ₃ is x <0.01, the orthorhombic-tetragonal structure phase transformation temperature t _OT is zero degrees or more, and the piezoelectricity is x> 0.08. Since the characteristics deteriorate, the composition range is set to 0.01 ≦ x ≦ 0.08.

The amount of LiF added as an accessory component is in the range of 0.10 wt% to 0.30 wt% with respect to (1-x) BaTiO ₃ -xCaTiO ₃ when the LiF amount is less than 0.10 wt%. For densification, it must be fired at a temperature higher than 1200 ° C. On the other hand, if it exceeds 0.30 wt%, the piezoelectric characteristics will deteriorate.

In the production method, the average particle diameters of BaTiO ₃ and CaTiO ₃ are preferably as small as possible. When the average particle diameter is as large as 0.2 μm or more, the additive amount of LiF as a secondary component is 0.10 wt% or more and 0.0. In order to obtain a dense BaTiO ₃ —CaTiO ₃ ceramic in a range of 30 wt% or less, firing at 1200 ° C. or higher is necessary, and in order to heat it at a low temperature, it is necessary to increase the amount of LiF added, As a result, there arises a problem that the piezoelectric characteristics are deteriorated. Therefore, in the production method of the present invention, the average particle size of BaTiO ₃ and CaTiO ₃ was set to 0.1 μm or less.

As described above, according to the present invention, the phase transformation temperature near room temperature, which has been a problem for practical use in piezoelectric ceramics mainly composed of BaTiO ₃ , is lowered to below zero degrees, and the piezoelectric characteristics are deteriorated. Thus, firing at a low temperature can be realized. That is, a barium titanate-based piezoelectric ceramic that can be fired at a low temperature of 1200 ° C. or less without impairing the piezoelectric characteristics and has a phase transformation temperature t _OT of zero degrees or less, and a method for manufacturing the same.

In the present invention, barium titanate BaTiO ₃ powder having an average particle size of less than 0.2 μm and calcium titanate CaTiO ₃ powder having an average particle size of less than 0.2 μm are (1-x) BaTiO ₃ -xCaTiO ₃ 01 ≦ x ≦ 0.08) as a main component, and a mixture ratio corresponding to the chemical formula of 01 ≦ x ≦ 0.08), and with respect to this main component, lithium fluoride LiF as a subcomponent is 0.10 wt% or more and 0.30 wt% or less. Piezoelectric ceramics added and fired. The substitution amount x of CaTiO ₃ with respect to BaTiO ₃ is x <0.01, the orthorhombic-tetragonal structure phase transformation temperature t _OT is zero degrees or more, and the piezoelectric property is deteriorated when x> 0.08. Therefore, the composition range is preferably 0.01 ≦ x ≦ 0.08. In addition, when the additive amount of LiF as an auxiliary component is less than 0.10 wt%, it must be fired at a temperature higher than 1200 ° C. for densification of ceramics, while it is added exceeding 0.30 wt%. In this case, since the piezoelectric characteristics are deteriorated, the amount of LiF added is desirably 0.10 wt% or more and 0.30 wt% or less.

In addition, since the raw material powder used in manufacturing the piezoelectric ceramic according to the present invention has a smaller average particle size, the firing temperature for densification can be lowered. Therefore, the average particle size of the BaTiO ₃ powder is 0.1 μm or less, and the CaTiO ₃ powder. It is desirable to use an average particle size of 0.1 μm or less. Moreover, when baking, it is desirable to bake in the temperature range of 950 degreeC or more and 1200 degrees C or less. Furthermore, it is desirable to use LiF added as an accessory component with a powder having an average particle size of 0.1 μm or less.

  Hereinafter, the piezoelectric ceramic according to the present invention and the manufacturing method thereof will be described in detail based on examples.

Piezoelectric ceramics serving as examples of the present invention were manufactured by the following manufacturing process. First, high-purity BaTiO ₃ and CaTiO ₃ powder having an average particle diameter of 0.1 μm or less are used as starting materials, and the chemical formula (1-x) BaTiO ₃ —xCaTiO ₃ , where 0.01 ≦ x ≦ 0. Weighed at a predetermined ratio to be 08. Next, LiF was added as an auxiliary component in an amount of 0.10 wt% or more and 0.30 wt% or less with respect to the above weighed value, ethanol was added, and wet mixing was performed for 24 hours using a zirconia ball mill. Here, LiF to be added is a powder, and its average particle size is set to 0.1 μm or less in order to increase dispersibility when mixed. In preparation, as a comparative example, a sample using x = 0 and x = 0.085, which is outside the scope of the present invention, a sample using BaTiO ₃ and CaTiO ₃ powder having an average particle diameter of 0.2 μm, and A sample in which the amount of LiF added was outside the above range was also produced.

After drying, the obtained powder was granulated by mixing polyvinyl alcohol as a binder, and a disk-shaped sample having a diameter of 20 mm and a thickness of 5 mm was formed by uniaxial pressure molding of a pressure of 1.0 t / cm ² . . This molded body was fired at a temperature of 950 ° C. or higher and 1200 ° C. or lower in the examples, and at a temperature of 950 ° C. or higher and 1300 ° C. or lower where the firing was possible in the comparative example, for 3 hours.

  The produced disk-shaped ceramic was measured for sample density by the Archimedes method, then processed to a thickness of 1 mm, and silver electrodes were baked on both sides thereof. Each sample thus obtained was subjected to polarization treatment by applying a DC electric field of 1 kV / mm for 30 minutes in 80 ° C. silicone oil.

The sample subjected to the polarization treatment is allowed to stand at room temperature for 24 hours to stabilize the piezoelectric characteristics, and then, using an impedance analyzer, electromechanical coupling coefficients k _p and k ₃₁ that serve as indices of the piezoelectric characteristics by the resonance-antiresonance method. Was measured. Furthermore, the orthorhombic-tetragonal phase transformation temperature t _OT of the sample was determined by measuring the temperature change of the capacitance. Table 1 shows the composition, sintering temperature, relative density, orthorhombic-tetragonal structural phase transformation temperature t _OT , and electromechanical coupling coefficients k _p , k ₃₁ of the piezoelectric ceramics of Examples and Comparative Examples that were prepared. . The denseness of the obtained sample can be judged to be dense when the relative density is 95% or more.

As is apparent from Table 1, in (1-x) BaTiO ₃ -xCaTiO ₃ , a BaTiO ₃ powder having an average particle size of 0.1 μm or less and a CaTiO ₃ powder having an average particle size of 0.1 μm or less are used as starting materials. In which x is in the range of 0.01 ≦ x ≦ 0.08 and LiF is added in the range of 0.10 wt% or more and 0.30 wt% or less as the accessory component, the firing temperature is 950 ° C. or more. It is sufficiently densified in the range of 1200 ° C. or less, and the electromechanical coupling coefficients k _p and k ₃₁ have the same or better characteristics than the additive-free product with x = 0, and the orthorhombic-tetragonal phase transformation It was confirmed that the temperature t _OT dropped below zero degrees. On the other hand, in the sample of the comparative example, characteristics satisfying all of the electromechanical coupling coefficient, the firing temperature, and the phase transformation temperature _tOT are not obtained.

As described above in detail, the piezoelectric ceramic according to the present invention and the manufacturing method thereof can be fired at a low temperature of 1200 ° C. or less without impairing the piezoelectric characteristics, and the titanic acid having a phase transformation temperature t _OT of zero degrees or less. A barium-based piezoelectric ceramic was obtained.

  The manufacturing method of the present invention can be applied to the manufacture of lead-free piezoelectric ceramics, and the resulting piezoelectric ceramics of the present invention can be widely applied to piezoelectric devices as lead-free, environment-friendly, low-cost piezoelectric ceramics. Can be expected.

Claims (3)

(1-x) BaTiO ₃ -xCaTiO ₃ (0.01 ≦ x ≦ 0.08) BaTiO ₃ powder having an average particle size of less than 0.2 μm and an average particle size of 0. A piezoelectric ceramic produced by firing by adding a mixture of CaTiO ₃ powder of less than 2 μm as a main component and adding 0.10 wt% or more and 0.30 wt% or less of LiF as an accessory component thereto. (1-x) BaTiO ₃ -xCaTiO ₃ (where 0.01 ≦ x ≦ 0.08) BaTiO ₃ powder having an average particle size of 0.1 μm or less in an amount corresponding to the molar ratio represented by the chemical formula; After forming a raw material obtained by adding 0.10 wt% or more and 0.30 wt% or less of LiF to a CaTiO ₃ powder having an average particle size of 0.1 μm or less with respect to the total amount of these powders, A method for producing a piezoelectric ceramic, characterized by firing within a temperature range of less than or equal to ° C.   The method for producing a piezoelectric ceramic according to claim 2, wherein the LiF to be added is a powder and has an average particle size of 0.1 µm or less.