Compositions – Piezoelectric
Reexamination Certificate
2002-05-20
2004-03-09
Koslow, C. Melissa (Department: 1755)
Compositions
Piezoelectric
C501S137000, C501S138000
Reexamination Certificate
active
06702952
ABSTRACT:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to a piezoelectric/electrostrictive material made of a porcelain obtained by firing, for example, to a piezoelectric/electrostrictive material used as an actuator or a sensor both assembled as an electromechanical transducer for positioning in precision machine tool or length control of optical path in optical instrument or in valve for flow rate control, etc. More particularly, the present invention relates to a piezoelectric/electrostrictive material suitably used in a very small sensor or a highly integrated very small actuator both used in an element for measurement of liquid property or very small weight.
As piezoelectric/electrostrictive materials, there have been known Pb(Zr,Ti)O
3
(hereinafter referred to as PZT), BaTiO
3
, etc. They are in use in actuators, filters, various sensors, etc. PZT type piezoelectric/electrostrictive materials have been used mainly because they are superior in overall piezoelectric properties.
Pb contained in PZT, etc. is stabilized and essentially generates no problem caused by decomposition or the like. However, there are cases that a Pb-free material is required depending upon its application. Further, since Pb-containing porcelains such as PZT, PLZT [(Pb,La)(Zr,Ti)O
3
] and the like give rise to vaporization of small amount of Pb in high-temperature firing, they have had, particularly when used in applications requiring a thin or thick film, a problem that they hardly show stable properties owing to the compositional change during firing.
Meanwhile, BaTiO
3
contains no Pb and offers a promising material for such a need. BaTiO
3
viewed as a piezoelectric/electrostrictive material, however, is inferior in piezoelectric/electrostrictive properties to a PZT type material, and has seldom been used as an actuator or as a sensor.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-mentioned problems of the prior art and aims at providing a BaTiO
3
-based piezoelectric/electrostrictive material which is superior in piezoelectric/electrostrictive properties to conventional products and which can be suitably used in an actuator or a sensor, and a process for producing such a piezoelectric/electrostrictive material.
A piezoelectric/electrostrictive material, when used in an actuator, is required to show a large displacement to a voltage applied. A study by the present inventor, made on the piezoelectric/electrostrictive properties of BaTiO
3
-based porcelain indicated that by controlling the fine structure of BaTiO
3
-based porcelain, particularly the distribution of the crystal grain constituting the BaTiO
3
-based porcelain, a piezoelectric/electrostrictive material showing a large displacement can be obtained. This finding has led to the completion of the present invention.
According to the present invention, there is provided a piezoelectric/electrostrictive material made of a BaTiO
3
-based porcelain composed mainly of BaTiO
3
and containing CuO and Nb
2
O
5
, characterized in that 85% or more of the crystal grains constituting the porcelain are grains having particle diameters of 10 &mgr;m or less and the maximum particle diameter of the grains is in a range of 5 to 25 &mgr;m.
In the piezoelectric/electrostrictive material of the present invention, at least Dart of the Ba may be substituted with Sr. Also in the present invention, the Ba/Ti ratio or the (Ba+Sr)/Ti ratio is preferably in a range of 1.001 to 1.01 because such a ratio can easily prevent the growth of abnormal grains occurring during the firing for porcelain formation and can easily control the particle diameters of the crystal grains constituting the porcelain.
According to the present invention, there is also provided a process for producing a piezoelectric/electrostrictive material made of a BaTiO
3
-based porcelain composed mainly of BaTiO
3
and containing CuO and Nb
2
O
5
, characterized by weighing individual raw materials so as to give a predetermined composition, mixing and grinding them, calcinating the resulting mixed powder in the air at 850 to 950° C., then grinding the resulting calcinated material until the ground material comes to have a specific surface area of 7 m
2
/g or less, and molding and firing the ground material.
DESCRIPTION OF PREFERRED EMBODIMENTS
The piezoelectric/electrostrictive material according to the present invention is described in more detail below. The piezoelectric/electrostrictive material according to the present invention is made of a BaTiO
3
-based porcelain composed mainly of BaTiO
3
and containing CuO, Nb
2
O
5
, etc.
A specific composition of the porcelain of the present invention may be such wherein BaTiO
3
is the main component and part of the Ba, for example, 0.1 to 10 mole % may be substituted with Sr. Also, the porcelain of the present invention may inevitably contain Zr, Si, Al, etc. in an amount of 0.5% by weight or less based on the total weight. Further in the BaTiO
3
-based porcelain of the present invention, the A/B ratio, which is a (Ba+Sr)/Ti, is preferably larger than 1, more preferably in a range of 1.001 to 10.1. Also, to the present porcelain are preferably added Nb
2
O, and CuO each in an amount of 0.05 to 0.5% by weight, more preferably each in an amount of 0.1 to 0.3% by weight based on the porcelain components excluding these components. Further, to the porcelain of the present invention may be added rare earth metals and/or transition metals other than the above components, in a total amount of 0.5% by weight or less in terms of their metal oxides. Incidentally, the forms of the components added are ordinarily oxides, carbonates or sulfates thereof.
The individual crystal grains constituting the porcelain of the present invention have crystal lattices of perovskite structure. The porcelain of the present invention is characterized in that the particle diameter distribution of the crystal grains constituting the porcelain is controlled as predetermined; specifically, 85% or more of the crystal grains are constituted by grains having particle diameters of 10 &mgr;m or less and the maximum particle diameter of the grains is in a range of 5 to 25 &mgr;m. In a preferred particle diameter distribution of the crystal grains, 90% to less than 100% of the crystal grains have particle diameters of 10 &mgr;m or less and the maximum particle diameter of the grains is in a range of 10 to 25 &mgr;m.
The action mechanism for why a porcelain having the above particle diameter distribution shows superior piezoelectric/electrostrictive properties, is not clear. However, from the results shown in Examples described later, it is clear that a porcelain constituted by crystal grains having a particle diameter distribution in the above mentioned range is superior in piezoelectric/electrostrictive properties to a porcelain having a particle diameter distribution in other range.
Next, description is made on the process for producing a piezoelectric/electrostrictive material according to the present invention.
First, raw materials (oxides, hydroxides and carbonates of metal elements) are weighed as so as to give a compositional range of the present invention and are mixed using a mixer such as ball mill or the like. In this mixing, it is preferred to allow the primary particles of each raw material after mixing to have particle diameters of 1 &mgr;m or less, in order to allow the porcelain obtained to have a particle diameter distribution specified in the present invention.
Then, the resulting mixed powder is calcinated in the air at 850 to 950° C. to obtain a calcinated material. An appropriate calcination temperature is 850 to 950° C. With a calcination temperature above 950° C., the resulting sintered material is nonuniform and, with a calcination temperature below 850° C., an unreacted phase remains in the resulting sintered material, making it impossible to obtain a dense porcelain.
Next, the calcinated material obtained is ground using a grinder such as ball mill or the like until the ground ma
Koizumi Takaaki
Yamaguchi Hirofumi
Burr & Brown
Koslow C. Melissa
NGK Insulators Ltd.
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