Compositions – Piezoelectric – Lead – zirconium – titanium or compound thereof containing
Reexamination Certificate
2002-11-13
2004-10-26
Koslow, C. Melissa (Department: 1755)
Compositions
Piezoelectric
Lead, zirconium, titanium or compound thereof containing
C310S364000, C501S135000, C501S136000, C501S134000
Reexamination Certificate
active
06808649
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a soft piezoelectric ceramic composition and, more particularly, to a soft piezoelectric ceramic composition sinterable at a reduced temperature with high piezoelectric parameters and a piezoelectric device using the same.
2. Description of the Prior Art
With a successful application of piezoelectric actuators for valve control of fuel injection system in diesel engine, multi-layer piezoelectric actuators finally find a major application. Soft PZT (lead zirconium titanate, Pb(Zr Ti)O
3
) materials with a big piezoelectric constant, d
33
, are generally used for piezoelectric actuators to get a larger displacement. Monolithic multi-layered structures are always chosen to get a sufficient displacement at a low input voltage as in automobiles.
When producing multi-layer piezoelectric actuators with conventional soft PZT materials, expensive Ag—Pd or Pt alloys are usually used for internal electrodes due to a high cofiring temperature. Pd contents are dependent on the cofiring temperature and usually in the range of 20~70% by weight when sintering at 1150~1300° C. Therefore, there is a big technical push to decrease sintering temperature of piezoelectric ceramics, preferably below the melting point of Ag (Silver), to reduce a cost of internal electrodes in multi-layer piezoelectric actuators.
Soft piezoelectric ceramic materials have typical characteristics of:
high piezoelectric constant
high electromechanical coupling coefficient, Kp
high dielectric constant
high dielectric loss
low mechanical quality factor, and
high residual polarization with a low coercive field.
In PZT based materials, these properties are achieved by substituting (Zr
4+
or Ti
4+
) with ions of a higher valence such as Nb
5+
, Ta
5+
, W
6+
, etc. or by substituting Pb
2+
with La
3+
. However, sintering temperature of conventional soft piezoelectric ceramic materials are in the range of 1150~1200° C. (Refer to the following: 1) H. Zheng et. al., “Effects of Octahedral Tilting on the Piezoelectric Properties of Strontium/Barium/Niobium-doped Soft Lead Zirconate Titanate Ceramics”, J. of Am. Ceram Soc. 85[9], 2337(2002). 2) U.S. Pat. No. 5,423,995) Therefore, expensive Ag—Pd alloys have to be used to build multi-layered piezoelectric actuators by cofiring green sheets with internal electrodes.
Sintering temperature of PZT materials can be lowered by adding sintering aids such as B
2
O
3
, V
2
O
5
, Bi
2
O
3
, CuO, which form liquid phase enhancing densification at a reduced temperature. (Refer to the followings; 1)U.S. Pat. No. 5,423,995, 2)U.S. Pat. No. 5,433,917, 3)U.S. Pat. No. 5,792,379, 4) K. Murakami et. al. “Morphotropic Phase Boundary and Microstructure of Low-Temperature sintered PZT ceramics with BiFeO
3
and Ba(Cu
0.5
W
0.5
)O
3
”. Proceedings of the Ninth IEEE International Symposium on Applications of Ferroelectrics 5) X. Wang et. al., “The Mechanism of Low Temperature Sintering PZT Ceramics with additives of Li
2
O
3
—Bi
2
O
3
—CdO”, Proceedings of Eighth IEEE International Symposium on Applications of Ferroelectrics.) However, these sintering aids often remain as glassy phases in the sintered ceramics, increasing dielectric loss tangent and deteriorating piezoelectric parameters. In addition, these liquid phases formed during sintering react easily with Ag or Ag/Pd electrode and make it difficult to cofire multi-layered piezoceramic elements.
BRIEF SUMMARY OF THE INVENTION
The present invention is intended to solve the above-described problems of the conventional techniques.
Therefore, an object of the present invention is to provide soft piezoelectric ceramic composition which can be sintered at a reduced temperature, preferably further below the melting point of pure Ag so that they can be co-fired with pure Ag electrode.
Another object of the present invention is to produce piezoelectric device using the same materials.
Still another object of the present invention is to produce multi-layered piezoelectric device using the same materials.
In achieving the above objects, the soft piezoelectric ceramic composition according to the present invention includes: [(Pb
1-m-n
Sr
m
Ba
n
)
(1-y
)Bi
y
][(Zr
x
Ti
1-x
)
1-a-b
Ni
a
W
b
]O
3
+pCdO, wherein m, n, x, y, a and b are molar ratio, p is wt % and are in ranges, respectively,
0.00% m≦0.18, 0.00≦n≦0.18,0.00≦(m+n)≦0.21,0.40≦x≦0.60 0.00≦y≦0.04, 0.00≦a≦0.02, 0.00<b<0.02, 0.00<p≦2.00.
In another aspect of the present invention, the piezoelectric ceramic device according to the present invention comprises: a piezoelectric ceramic comprising said piezoelectric ceramic composition, sintered below 980° C.
In still another aspect of the present invention, the multi-layered piezoelectric ceramic device according to the present invention comprises: a piezoelectric ceramic layer, wherein the piezoelectric ceramic layer comprising said piezoelectric ceramic composition, co-fired with pure Ag internal electrode below 960° C.
In still another aspect of the present invention, the multi-layered piezoelectric ceramic device according to the present invention comprises: a piezoelectric ceramic layer, wherein the piezoelectric ceramic layer comprising said piezoelectric ceramic composition, co-fired with Ag—Pd internal electrode below 1050° C.
According to this invention, it is possible to provide soft piezoelectric ceramic composition which have high piezoelectric constants and electromechanical coupling coefficients and which can be sintered at a reduced temperature below 1000° C. and further soft piezoelectric ceramic materials which can be co-fired with pure Ag electrode below 960° C.
Based on the piezoelectric ceramic compositions according to this invention, multi-layer piezoelectric actuators are built with pure Ag internal electrode, saving a cost of electrode as much as 90% compared to Ag—Pd (30%) internal electrode.
REFERENCES:
patent: 3434122 (1969-03-01), Land et al.
Derwent abstract for SU 528284, Oct. 1, 1976.*
Derwent abstract for SU 1073227, Feb. 15, 1984.*
Derwent abstrct for SU 1502537, Aug. 23, 1989.
Dongil Technology Co. Ltd.
Koslow C. Melissa
Vigil Thomas R.
Welsh & Katz Ltd.
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