High-performance piezoelectric ceramic

Compositions – Piezoelectric – Lead – zirconium – titanium or compound thereof containing

Reexamination Certificate

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Details

C501S134000, C501S136000, C501S135000, C310S311000

Reexamination Certificate

active

06692651

ABSTRACT:

The subject of the present invention is a piezoelectric ceramic material based on lead zirconate titanate, which is distinguished by an excellent thermal and temporal stability of the functional characteristics.
Materials with high piezoelectric activity along with high thermal and temporal stability (high Curie temperature, low temperature coefficients and low ageing rates) of the functional characteristics are required for piezoelectric ceramics to be used for sensors, in particular in automotive engineering (knocking sensors, rotation-rate sensors, reversing sensors).
Piezoelectric ceramics with large deformation defects and high Curie temperature are required for the furthering of development in the field of multilayer actuators.
Piezoelectric ceramics materials have long been made of compositions based on the solid solutions (mixed crystals) of lead zirconate (PbZrO
3
) and lead titanate (PbTiO
3
). A variety of modifications of the basic system is possible as a result of substitution and/or addition of metal ions in limited concentrations if the respective ions fulfil prerequisites with respect to valency, ionic radius and character of chemical bonding.
By substitution in the original sense of the word is to be understood the partial replacement of the Pb
2+
or Zr
4+
and Ti
4+
ions with ions of the same valency and similar ionic radii, such as Ba
2+
, Sr
2+
, Mg
2+
and Sn
4+
. Such substitutions effect, on the one hand, an increase in the piezoelectric activity, but on the other hand, however, can also impair the thermal stability of the piezoelectric state.
A modification of the basic composition by doping with ions with a valency which is different from that of the original ions leads to a further diversification of the dielectric and electromechanical properties.
Ions from the “softener” group La
3+
, Bi
3+
, Sb
5+
, Nb
5+
act in the basic system as donors and produce piezoelectric ceramics which are distinguished by a high dielectric constant and a high electromechanical activity, but are also characterised by high dielectric and mechanical losses as well as a dependency of the specific characteristics on strong electrical fields and mechanical loads.
A stabilisation of the piezoelectric ceramics based on lead zirconate titanate results from the doping with ions from the “hardener” group K
+
, Fe
3+
, Al
3+
: these ions act as acceptors and, in interacting with the ions of the basic system, effect a reduction of the dielectric and mechanical losses, but lead to a reduction in the dielectric constant, piezoelectric activity and specific electrical resistance.
As a result of coupled substitution of ions from the group of the “softeners” with ions from the group of the “hardeners”, it is, on the other hand, possible clearly to increase the stability of piezoelectric ceramics based on lead zirconate titanate whilst maintaining the piezoelectric activity and the high dielectric constant.
In conclusion, attempts have been made to do justice to the increased demands of the practical application of piezoelectric ceramics by means of multi-component systems, in which lead-containing ion complexes (“complex compounds”) with the general formulation PbB′
1−&agr;
B″
&agr;
O
3
, with B′: 5-valent or 6-valent cations and B′: 2-valent cations (&agr;=⅓ or ½ depending on the valency of the cation B′), partially replace the ion complex Pb
2+
(Zr
4+
, Ti
4+
)O
3
.
As a result of substitution of one or more of the complex compounds, which are also known as so-called relaxor-ferroelectrics, single-phase multi-component systems (for example as ternary or quaternary solid solutions) with perovskite structure result.
The substitution by lead-free compounds with perovskite structure, such as BiFeO
3
, KNbO
3
, NaNbO
3
, Na
0.5
Bi
0.5
TiO
3
also leads to improvement in the properties of lead zirconate titanate ceramics.
These piezoelectric ceramics belong to the large family of (ceramic) ferroelectrics. Lead-free compositions such as (K, Na)NbO
3
, (Sr
1−x
Ba
x
)Nb
2
O
6
are also known as ceramic ferroelectrics.
In all, there exists, as a results of the widest variety of modifications of the basic system of the solid solutions of lead zirconate titanate, a large multiplicity of compositions with which it has been possible in many cases to realise a specification of the dielectric and electromechanical properties of piezoelectric materials for different transformer functions that is suitable for the respective use.
From 1 Eyraud, L., Eyraud, P., Mathieu, J. C., Claudel, B. “Effect of Simultaneous Heterovalent Substitutions on Both Cationic Sites on the Electrical Conductivity and Ageing of PZT Type Ceramics” (Ferroelectrics 50(1983)103-1 10), 2 Eyraud, L., Eyraud, P., Claudel, B. “Influence of Simultaneous Heterovalent Substitutions in Both Cationic Sites on the Ferroelectric Properties of PZT Type Ceramics” (J. Solid State Chem. 53(1984)266-272), 3 Ohenassion, H., Gonnard, P., Troccaz, L., Eyraud, L., Eyraud, P. “Characterisation de la stabilité d'un element piézoélectrique du type PZT sous compression uniaxiale rapide” (Revue Phys. Appl. 18(1983)479-486), and 4 Eyraud, L., Eyraud, P., Bauer, F. “Current Research in the Field of PZT Ceramics and Ferroelectric” (Polymers Adv. Cer. Mat. 1(1986)3, 223-231) is known the series of compositions
Pb
0.995
Sr
0.015
Ba
0.005
K
0.0025
Na
0.0025
(Zr
x
Ti
1−x
Nb
0.02
)O
3(1+0.02)
x=0.52-0.54
The solid compositions were prepared unconventionally by oxalate mixed precipitation in a wet-chemical process. The aim of these investigations was the preparation of compositions with very low electrical conductivity as a result of compensation of the valency of heterovalent substituents, and ultimately in the stabilisation of the functional properties with respect to comparatively great mechanical loads. The optimal concentration of the substituents was found by trial and error, and as the most stable composition with respect to the mechanical loading capacity emerged the formulation with the Zr
4+
content x=0.53. The compositions are characterised in 3 and 4. The stability criterion for these compositions was the resistance with respect to mechanical load. Details about the Curie temperature, temperature coefficients and ageing rates were not given. The optimal sintering temperature was given as 1230° C. The wet-chemical preparation used in these works in order to prepare the compounds can be converted to a commercial scale only at considerable expense.
The underlying object of the present invention has been to prepare modified piezoelectric ceramics based on lead zirconate titanate with high depolarisation strength in the case of high mechanical (impact) loads (for gas igniters) and in particular with low temperature coefficients and ageing rates of the functional characteristics, high Curie temperature (for sensors) and large deformation effect (for actuators).
In this connection, these piezoelectric ceramics are to be able to be synthesised by way of the conventional mixed-oxide route and sintered at temperatures below 1150° C.
This object has been achieved by a piezoelectric ceramic material based on lead zirconate titanate having the characterising features of the first or second claim. Preferred developments are characterised in the subclaims.
Surprisingly, it has been established that, in the case of lead zirconate titanate (with perovskite structure A
2+
B
4+
O
3
), the desired stabilisation of the materials can be achieved, and simultaneously the reduction of the sintering temperature can be rendered possible, by partial substitution with ferroelectrically active compounds.
In accordance with the invention, in order to do this, alkaline earth metals, preferably Sr
2+
and/or Ba
2+
, alkali metals K
+
, Na
+
, and metals Nb
5+
, Sb
5+
, Ta
5+
, known per se in modifications of lead zirconate titanate (perovskite structure A
2

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