JP3080277B2 - Method for producing bismuth layered compound - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a bismuth layered compound having piezoelectricity, which can be used as a pressure sensor, a high frequency filter or the like.

[0002]

2. Description of the Related Art Piezoelectric ceramics include:
Barium titanate (BaTiO ₃ ), lead titanate (Pb
TiO ₃ ), lead zirconate titanate solid solution (PZT),
Bismuth layered compounds (SrBi ₄ Ti ₄ O ₁₅ ) and the like are known. For example, PZT has particularly high sensitivity and is therefore used as a piezoelectric actuator. Further, the bismuth layered compound is not as high in sensitivity as PZT, but has high strength, and has a Curie temperature as high as 500 to 800 ° C., and can be used in a wide temperature range, and thus is expected to be used as a pressure sensor.

[0003] Pressure sensors made of a bismuth layered compound include bismuth oxide (Bi ₂ O ₃ ) and titanium oxide (Ti).
Powdered raw materials such as O ₂ ) and strontium carbonate (SrCO ₃ ) are mixed at a predetermined ratio to prepare a powdered raw material. A molded body having a predetermined shape is formed from the powdered raw material, and then sintered to form a sintered body. Is manufactured by polarization processing. By the way, the bismuth layered compound is a piezoelectric element having excellent high-temperature characteristics at a high Curie temperature, but has a small crystal anisotropy but is sintered in a state in which crystals are randomly oriented, so that the sensitivity of piezoelectricity is small. There are drawbacks. Therefore, Japanese Patent Application Laid-Open No. 52-86
Japanese Patent Publication No. 198 discloses a manufacturing method in which the pressure is applied in one direction at the time of sintering using a hot press or the like to align the crystal axes of the plate-like crystals in one direction, thereby improving the sensitivity. . According to this manufacturing method, the plate crystals of the bismuth layered compound are sintered in a state of being oriented in the direction perpendicular to the pressing direction, and the axes of easy polarization are aligned in the direction perpendicular to the pressing direction, so that the piezoelectric sensitivity is improved. I do.

[0004]

However, in the hot pressing method disclosed in the above publication, not only the apparatus itself is expensive, but also a sintered body which can be pressed at one time is usually used.
The production cost was high because of low productivity. The present invention has been made in view of such circumstances, and it is an object of the present invention to increase the orientation of the axis of easy polarization of a bismuth layered compound by a method simpler than the hot press method.

[0005]

Means for Solving the Problems A first method for solving the above problems is described below.
The method for producing a bismuth layered compound according to the present invention comprises a mixing step of mixing a plurality of raw materials at a composition ratio of a bismuth layered compound to obtain a powder raw material, a forming step of forming a formed body from the powder raw material, From a pre-sintering step of pre-sintering the pre-sintered body into a pre-sintered body while pressing, and a main sintering step of sintering the pre-sintered body while uniaxially pressing in a direction perpendicular to the pressing direction of the pre-sintering step. It is characterized by becoming.

[0006] The mixing step and the molding step can be performed in the same manner as in a conventional manufacturing method. As the bismuth layered compound, a general compound represented by the general formula (Mx Biy) Ti ₄ O ₁₅ can be used. Here, M is a monovalent to pentavalent metal element, and Sr, Be, Mg, Ca, Ba, R
alkaline earth metals such as a, Li, Na, K, Rb, C
Various types can be selected from alkali metals such as s and Pb. Further, Mn, Ni, Cr within a range that does not impair the performance.
Such a metal element may coexist.

[0007] The pre-sintering step is desirably performed at a temperature of 900 to 1050 ° C. If this temperature is lower than 900 ° C., the formation of the bismuth layer compound in the preliminary sintering step becomes insufficient, and it is difficult to obtain a desired effect. On the other hand, when the temperature is higher than 1050 ° C., sintering proceeds too much, and the crystal orientation in the main sintering step becomes insufficient, so that it is difficult to obtain a desired effect. In addition, sintering may be insufficient, and a problem may occur in the strength of the sintered body.

[0008]

The bismuth layer compound is a material having a very strong anisotropy due to its crystal structure, and the direction of easy polarization is limited to a specific direction. Further, the shape of the crystal is plate-like, and the crystal is easily oriented in one direction. However, in molding and sintering by a normal manufacturing method, the axis of easy polarization becomes random, and improvement in piezoelectric characteristics cannot be expected.

[0009] In Therefore the production method of the present invention, as shown in FIG. 1, the preliminary sintering while applying a uniaxial pressure to the molded body from the P ₁ direction. Thereby, the plate-like crystal of the bismuth layered compound is
The uneven plane grows while being oriented in a direction perpendicular to the pressing direction. In the obtained pre-sintered body, as shown in FIG. 2, the crystals overlap in layers, and in one layer, the easy polarization axis 10 is in the same plane, but has a two-dimensional random orientation. .

Further, in the present invention, the main sintering step is performed while the pre-sintered body is pressed from a direction P ₂ perpendicular to the pressing direction P _{1 in the} pre-sintering step. As a result, the crystal becomes a needle-like crystal as shown in FIG. 2 and is oriented one-dimensionally. Therefore, the orientation of the axis of easy polarization of the crystal of the sintered body is extremely improved, and the piezoelectric characteristics are improved.

[0011]

〔Example〕

(1) Mixing process Strontium oxide (SrCO ₃ ),
Bismuth oxide (Bi ₂ O ₃ ), titanium oxide (TiO ₂ )
And manganese oxide (MnO), and weighed each raw material powder so that the composition ratio became SrBi ₄ Ti ₄ O ₁₅ and MnO contained 0.1% by weight in terms of Mn weight, and together with ethanol. The mixture was wet-mixed in a pot mill for 48 hours.

The mixed powder is dried from ethanol and dried at 800 ° C.
For 2 hours. The calcined powder was again wet-mixed in a pot mill for 48 hours, and dried with ethanol to obtain a powder raw material. (2) Molding process Polyvinyl alcohol (PVA) is added to this powder material for about 3 minutes.
% By weight and granulated, and supplied into a mold having a cubic cavity of 10 mm square and molded at a pressure of 1 t / cm ² . (3) Temporary sintering step The obtained molded body was heated at a temperature rising / falling rate of 200 ° C / h,
Temporary sintering was performed at a temperature of 00 to 1050 ° C. for 2 hours. When the holding temperature is reached, a pressure of 10 MPa is applied in the uniaxial direction P.
The pressure was applied to the molded body from ₁ and the pressure application was maintained during the temperature maintenance. (4) Main sintering step The obtained temporary sintered body 1 is placed on a pad material made of ZrO ₂ ,
It was housed in an alumina container, covered with an alumina lid, and placed in a furnace.
Sintering was performed by heating and holding at 100 to 1300 ° C. for 2 hours in the air. When the temperature reached the holding temperature, a pressure of P ₂ = 10 MPa was applied to the pre-sintered body 1 from the direction P ₂ perpendicular to the above P _1, and the applied pressure was maintained during the temperature holding. (5) Polarization Step The obtained sintered body 2 is cut into a plate having a thickness of 1 mm in parallel with the pressing axes P ₁ and P ₂ as shown in FIG. 3, and silver paste is applied to both surfaces of a square shape. It was coated and baked to form a silver electrode. Then, in silicon oil heated to 200 ° C., a voltage of 8 kV / mm was applied for 10 minutes to perform a polarization treatment.

In the present sintered body 2, the axis of easy polarization 10 is oriented perpendicular to the pressing directions P ₁ and P _2. Therefore, in the cut bismuth layer compound, the axis of easy polarization 10 is parallel to the thickness direction. It is oriented. [Comparative Example 1] A molded article formed in the same manner as in
₂ Place the pad on the pad material, place it in an alumina container, cover it with an alumina cover and place it in the furnace.
Sintering was carried out by heating at a temperature of 1100 to 1300 ° C for 2 hours in the atmosphere at a temperature of 00 ° C / h. No pressure was applied during the preliminary sintering step and during sintering. Then, the sintered body was polished to a thickness of 1 mm, and a polarization step was performed on the polished body as in the example. Comparative Example 2 In the method of Comparative Example 1, when the holding temperature was reached, a pressure of 10 MPa was applied to the molded body, and the applied pressure was maintained during the temperature holding. Then, the obtained sintered body was cut out in a plate shape having a thickness of 1 mm in parallel with the pressing direction, and a polarization process was performed on the plate in the same manner as in the example. (Evaluation) The piezoelectric sensitivity of each of the obtained devices was measured. Table 1 shows the results. Here, as shown in FIG. 4, when the amount of charge generated when a force F is applied to a sample is E, the charge per unit force (E / F, unit pC / N).

[0014]

[Table 1] As shown in Table 1, the sensitivity of the piezoelectric element obtained by the manufacturing method of the present invention is 60 to 100% higher than that of the piezoelectric element obtained by the comparative example which is the conventional manufacturing method. It is apparent that this is the effect of sintering by applying pressure in different directions during the preliminary sintering and the main sintering.

[0015]

According to the production method of the present invention, a bismuth layered compound in which the axis of easy polarization is oriented in one direction can be easily produced as in the hot press method. Since only uniaxial pressing is required in two sintering steps, a large number of compacts can be processed at one time, and since the productivity is excellent, the production cost can be reduced as compared with the hot press method. it can.

The bismuth layered compound produced according to the present invention is completely free from vibration modes from other directions because the axis of easy polarization is completely oriented in one direction. It can be used as a sensor or the like.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a temporary sintered body obtained in an example of the present invention.

FIG. 2 is an explanatory view of the present sintered body obtained in an example of the present invention.

FIG. 3 is an explanatory view showing a cutting position of a test piece from the present sintered body in an example of the present invention.

FIG. 4 is an explanatory diagram showing a relationship between force and charge output, and showing a definition of piezoelectric sensitivity.

[Explanation of symbols]

1: Temporary sintered body 2: Main sintered body 10:
Easy polarization axis

Claims (1)

(57) [Claims] 1. A mixing step in which a plurality of raw materials are mixed at a composition ratio of a bismuth layered compound to obtain a powder raw material; a forming step of forming a compact from the powder raw material; A temporary sintering step of sintering to form a temporary sintered body; and a main sintering step of sintering the temporary sintered body while uniaxially pressing in a direction perpendicular to the pressing direction of the temporary sintering step. A method for producing a bismuth layered compound, comprising: