Piezoelectric ceramic

Details Piezoelectric ceramic Piezoelectric ceramic

2002-07-16

2004-09-21

Koslow, C. Melissa (Department: 1755)

Compositions

Piezoelectric

C252S06290R, C501S134000, C501S135000, C501S136000, C501S138000, C501S139000

Reexamination Certificate

active

06793843

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a piezoelectric ceramic widely used in the field of actuators, sensors, resonators or the like.
BACKGROUND ART
Piezoelectric materials have an effect of creating a distortion by externally applying an electric field (conversion from an electrical energy into a mechanical energy) and an effect of generating an electric charge on a surface thereof by externally imposing a stress (conversion from a mechanical energy into an electrical energy), and in recent years, the piezoelectric materials have been widely used in various fields. For example, as piezoelectric materials such as lead zirconate titanate (Pb(Zr,Ti)O
3
; PZT) create a distortion substantially proportional to an applied voltage on the order of 1×10
−10
m/V, the piezoelectric materials are superior in fine positioning, etc., therefore, the piezoelectric materials are used for fine adjustments, etc. in optical systems. On the other hand, as the piezoelectric materials generate an electric charge proportional to an imposed stress or the amount of strain in the piezoelectric materials by the imposed stress, the piezoelectric materials are used as sensors for detecting a minute force, deformation or the like. Further, the piezoelectric materials have an excellent response, so that by applying an AC electric field, the piezoelectric materials can excite themselves or an elastic body connected thereto to produce resonance. Therefore, the piezoelectric materials are used as piezoelectric transformers, ultrasonic motors or the like.
Most of piezoelectric materials practically used at present are solid solutions (PZT-based) including PbZrO
3
(PZ)-PbTiO
3
(PT), because superior piezoelectric properties can be obtained by the use of a composition of rhombohedral PZ and tetragonal PT near a crystallographic morphotropic phase boundary (M.P.B.). The PZT-based piezoelectric materials have been widely developed in response to various needs through adding various auxiliary components or additives. There are various PZT-based piezoelectric materials ranging, for example, from those with a small mechanical quality factor (Qm) and a large piezoelectric constant (d
33
) used for DC devices such as actuators, etc. for positioning which can obtain a large amount of displacement to those with a small piezoelectric constant (d
33
) and a large mechanical quality factor (Qm) suitable for AC devices such as ultrasonic generating devices, for example, ultrasonic motors.
Moreover, except for the PZT-based piezoelectric materials, there are other practically used piezoelectric materials, most of which are solid solutions including a lead-based perovskite composition such as lead magnesium niobate (Pb(Mg,Nb)O
3
; PMN) or the like as a main component.
However, these lead-based piezoelectric materials include lead oxide (PbO) with extremely high volatility even at a low temperature as much as the order of 60 to 70% by mass as a main component. For example, approximately 2/3 of PZT or PMN contains lead oxide in mass ratio. Therefore, when manufacturing these piezoelectric materials, in heat treatment such as a firing step for ceramics and a melting step for single crystal products, a very large amount of lead oxide volatilizes and diffuses at an industrial level. Further, although lead oxide released in a manufacturing step can be recovered, it is difficult to recover lead oxide included in piezoelectric products placed on the market as industrial products under the status quo, so if lead oxide is widely released into the environment, there are worries that lead may be leached due to acid rain. Therefore, when the piezoelectric ceramics and single crystals have a wider area of application, and the amount of use thereof increases in future, a very important challenge is to develop lead-free piezoelectric materials.
As piezoelectric materials including no lead, for example, barium titanate (BaTiO
3
) or bismuth layer ferroelectric is known. However, barium titanate has a low Curie point of 120° C., so that a temperature higher than 120° C. destroys the piezoelectric properties thereof. Therefore, for soldering and application for vehicles, barium titanate is not practical. On the other hand, bismuth layer ferroelectric typically has a Curie point of 400° C. or over, so the thermal stability thereof is superior, but the crystal anisotropy thereof is large. Therefore, it is required to orient a spontaneous polarization through hot-forging or the like, so there is a problem in productivity. Further, when lead is not included at all, it is difficult to obtain large piezoelectric properties.
Moreover, the development of bismuth sodium titanate-based materials as novel materials has been proceeding recently. For example, in Japanese Examined Patent Publication No. Hei 4-60073 and Japanese Unexamined Patent Publication No. Hei 11-180769, materials including bismuth sodium titanate and barium titanate are disclosed, and in Japanese Unexamined Patent Publication No. Hei 11-171643, materials including bismuth sodium titanate and bismuth potassium titanate are disclosed. However, the bismuth sodium titanate-based materials have not yet obtained sufficient piezoelectric properties, compared with lead-based piezoelectric materials, so the improvement in piezoelectric properties has been in demand.
In view of foregoing, it is an object of the present invention to provide a piezoelectric ceramic having superior piezoelectric properties and being superior in the point of ultra-low emission, environmental friendliness and ecology.
DISCLOSURE OF THE INVENTION
A first piezoelectric ceramic according to the invention comprises three kinds or more of perovskite structure compounds having different crystal structures.
A second piezoelectric ceramic according to the invention comprises a solid solution including three kinds or more of perovskite structure compounds having different crystal structures.
In the first and the second piezoelectric ceramics according to the invention, by the use of three kinds of compounds having different crystal structures, the piezoelectric properties can be improved. Moreover, it is preferable in the point of ultra-low emission, environmental friendliness and ecology that the content of lead (Pb) is 1% by mass or less.
As the three kinds or more of perovskite structure compounds having different crystal structures, a rhombohedral perovskite structure compound, a tetragonal perovskite structure compound and an orthorhombic perovskite structure compound are preferably comprised.
The composition ratio of these compounds is preferably within a range shown in Chemical Formula 1 in molar ratio. That is, the composition ratios of the rhombohedral perovskite structure compound, the tetragonal perovskite structure compound and the orthorhombic perovskite structure compound represent a, b and c, respectively, and the values of a, b and c are preferably within a range satisfying a+b+c=1, 0.40<a≦0.99, 0<b≦0.40, 0<c<0.20, respectively, and more preferably within a range satisfying a+b+c=1, 0.50<a≦0.99, 0<b≦0.30, 0<c<0.20, respectively.
The composition ratio of these compounds and the composition ratio of an element at the A-site to an element at the B-site in each of these compounds preferably have a relation shown in Mathematical Formula 1. That is, the sum of the products of the composition ratio of each of three kinds of compounds and the composition ratio of an element at the A-site to an element at the B-site in the each of three kinds of compounds is preferably within a range from 0.9 to 1.0 inclusive.
Moreover, as another three kinds or more of perovskite structure compounds having different crystal structures, a rhombohedral perovskite structure compound, a tetragonal perovskite structure compound and a cubic perovskite structure compound are preferably comprised.
The composition ratio of these compounds is preferably within a range shown in Chemical Formula 2 in molar ratio. That is, the composition ratios of the rho

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