Piezoelectric ceramic material, method for production thereof and electroceramic multi-layer component

Abstract

A piezoelectric ceramic material having the molar composition Pb(1+c)−x−(3/2a)−1/2(x·b)A1xA2a (Zry(1−x−b−x·z)Ti(1−y)(1−x·b−x·w))(B1bB2zB3w)xO3 is described

Description

[0001] The present invention relates to a piezoelectric ceramic material, a method of manufacturing same, and an electroceramic multilayer component having such a piezoelectric ceramic material according to the preambles of the main claims.

BACKGROUND INFORMATION

[0002] Piezoelectric ceramics which are used in actuators, for example, are often manufactured on the basis of the lead zirconate titanate (PZT) mixed crystal and, if suitable additives or dopants are used, have very good property combinations such as high temperature resistance, high piezoelectric charge constant, high Curie temperature, low dielectric constant, and low coercive field intensity. It is, however, disadvantageous that-some of the maximally achievable and desired properties of these PZT ceramics are not obtained when the known additives are used.

[0003] Considering a PZT structure as an A2+B4+O2−3 structure, ceramics which undergo A or B site substitution by a higher-valency cation (donor) are designated “soft ceramics.” This substitution produces lead vacancies, so that such ceramics have primarily a high piezoelectric charge constant, high dielectric constant, high dielectric and mechanical loss, low coercive field intensity, and easy polarizability. Typical additives producing these properties include, for example, La3+ or, in general, oxides of the rare-earth elements or also Bi3+ for A site substitution. Ta5+, Nb5+, W6+, or Sb5+ may be considered for B site substitution.

[0004] On the other hand, PZT ceramics which undergo A or B site substitution by a lower valency cation (acceptor) are designated “hard ceramics.” In this case, oxygen vacancies are produced, so that such ceramics have a low piezoelectric charge constant, low dielectric and mechanic loss, high coercive field intensity, and low electrical resistance. Furthermore, such ceramics are usually difficult to polarize. Typical additives inducing such properties in such “hard ceramics” are, for example, K1+ or Na1+ for A site substitution. Primarily Ni2+, Zn2+, Co2+, Fe3+, Sc3+, or Mg2+ may be considered for B site substitution.

[0005] In summary, primarily materials having a combination of the properties of “soft ceramics” and “hard ceramics” are relevant for use in a piezoelectric actuator as PZT ceramics. In particular, the piezoelectric charge constant and the Curie temperature should be as high as possible, i.e., d33>500 1012 m/V and Tc>300° C. Furthermore, the dielectric constant of the material obtained should-be as low as possible, i.e., ε33/ε0 should be less than 2000.

[0006] In order to meet these requirements, it has been proposed that the PZT ceramic be codoped, resulting in the formation of both Pb vacancies and oxygen vacancies. Thus, German Patent 196 15 695 C1 proposes that the surface of pure donor-doped PZT green ceramics be provided with an Agx/Pd1−x paste (x=0.7) and that these ceramics be stacked, with silver diffusing into the adjacent ceramic layers in a subsequent joint sintering (cofiring) of the green ceramic which has the Ag/Pd paste and being built into an A site as an acceptor. Furthermore, European Patent 0 619 279 B1 proposes that doping be performed using complex compounds having the general formula A(B11−xB2x) where A=Pb and B1=monovalent, bivalent, or trivalent cations, and B2=trivalent, pentavalent, or hexavalent cations, i.e., compounds of the type A(W1/3Ni2/3)O3 or A(Mg1/3Nb2/3)O3, for example, where A may be lead, strontium, calcium, or barium.

[0007] Finally, WO 99/12865 proposed doping a PZT ceramic with lead-free complex compounds which have a Perowski structure like the PZT mixed crystal and which react to yield a single-phase mixed crystal when added in small amounts and jointly calcined. Such compounds have the general composition A2+B10.251+B20.755+O3 where A=barium and/or strontium, B1=potassium and/or sodium, and B2=niobium, tantalum, or antimony. The required sintering temperatures for producing an electroceramic multilayer component having such a piezoelectric ceramic material are, according to WO 99/12865, below 1150° C.

[0008] The object of the present invention was to provide a piezoelectric ceramic material having the highest possible thermal stability, piezoelectric charge constant, and Curie temperature, and the lowest possible dielectric constant, electromechanical losses, a low coercive field intensity, and a low electrical conductivity. Furthermore, the object was to provide a material sinterable at temperatures below 1000° C. so that by using this material a more cost-effective internal electrode paste could be used in the manufacture of electroceramic multilayer components than previously.

ADVANTAGES OF THE INVENTION

[0009] The piezoelectric ceramic material according to the present invention has the advantage over the related art in that it has a high thermal stability, a piezoelectric charge constant d33 greater than 500 1012 m/V, a Curie temperature Tc higher than 300° C., a dielectric constant ε33/ε0 less than 2000, and a coercive field intensity less than 1.2 kV/mm with low electromechanical losses, and low electrical conductivity. Furthermore, this piezoelectric ceramic material is sinterable, for example, together with copper-based or AgtPd1−t-based electrode paste layers (where t≧0.7 and t is the proportion by weight) at temperatures below 1000° C. in a cofiring process to yield an electroceramic multilayer component, this sintering advantageously being able to take place under air, nitrogen, or a nitrogen-containing gas atmosphere.

[0010] In particular, this lower sintering temperature allows the use of a silver-palladium mixture as the material for the electrode paste layers, whose Ag content is considerably greater than 70% by weight and is less expensive than platinum-based internal electrode materials, for example, or internal electrodes having a higher palladium content.

[0011] The use of precious metal-free electrode paste layers, i.e., precioust metal-free pastes such as copper pastes, for example, offers considerable cost advantages, an undesirable diffusion of copper from the internal electrode layers into the adjacent ceramic material being avoidable at the same time by adding copper oxide to the piezoelectric ceramic material as an additive or dopant. In this case, the PZT ceramic is already saturated with copper ions due to the copper oxide addition to the PZT ceramic. In addition, the possibility of using copper as an alternative to silver/palladium limits the dependence of the price of the resulting electroceramic multilayer components on the highly speculative prices of palladium and platinum.

[0012] In the method of manufacturing the piezoelectric ceramic material according to the present invention, it is advantageous that all additives and dopants used in addition to lead, zirconium, and titanium may be used as powdered oxides or carbonates, which are available at a reasonable cost and in large quantities.

[0013] Advantageous refinements of the present invention are derived from the measures named in the subclaims.

[0014] Thus, it is advantageous to add copper in the form of a Cu1+ and/or Cu2+ ion as a dopant to the piezoelectric ceramic material, which makes sintering of the green ceramic, to which this material is added as a ceramic component together with a copper-containing electrode paste layer, under air, nitrogen, or a nitrogen-containing atmosphere possible.

[0015] The piezoelectric material obtained may also have either a e stoichiometric composition-or a composition containing a lead oxide, in particular PbO, in a stoichiometric excess, which causes a further reduction in the sintering temperature.

[0016] In addition, it is processable simply in a known manner to produce a castable slurry or an extrudable compound for the manufacture of green ceramics.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0017] The present invention is based on a lead zirconate titanate-based (PZT) piezoelectric ceramic material having the general composition A2+B4+O2−3, which is to be codoped using donors and acceptors. This codoping is achieved by the addition of powdered oxides and/or powdered carbonates containing the corresponding metal ions and takes place at both the A2+ sites and B4+ sites of the PZT ceramic. In this manner, the overall defect concentration remains low in the piezoelectric ceramic material obtained due to a compensation of charges between the lead vacancies and the oxygen vacancies, which enhances the stability of its structure and thus the piezoelectric activity and thermal stability. In the stoichiometric case, the general formula of the piezoelectric ceramic material is the following:

Pb1−x−(3/2a)−1/2(x·b)A1xA2a(Zry(1−x·b−x·z)Ti(1−y)(1−x·b−x·w))(B1bB2zB3w)xO3

[0018] where A1 is selected from the group Ca, Mg, Sr, Ba, or their mixtures; A2 is selected from the group of rare-earth elements, in particular La, or their mixtures; B1 is selected from the group Nb, Ta, or Sb, or their mixtures; B2 is Cu or a mixture of Cu with at least one element selected from the group Zn, Ni, Co, or Fe, and B3 is Fe, under the condition that the following applies to a, b, c, x, y, z, and w:

0.001≦a≦0.05

0.05≦b≦0.90

0.005≦x≦0.03

0.5≦y≦0.55

0.05≦z≦0.90

0≦w≦0.5.

[0019] The numerical values for a, b, x, y, z, and w are always to be understood as referring to mols.

[0020] As an alternative to the above composition, a lead oxide, in particular PbO, may be added in stoichiometric excess, so that the piezoelectric ceramic; material obtained has a non-stoichiometric overall composition. In this case, the general formula of the piezoelectric ceramic material is:

Pb(1+c)−x−(3/2a)−1/2(x·b)A1xA2a(Zry(1−x·b−x·z)Ti(1−y)(1−x·b−x·w))(B1bB2zB3w)xO3

[0021] where A1, A2, B1, B2, and B3 are as above and under the condition that the following applies to a, b, c, x, y, z, and w:

0.001≦a≦0.05

0.05≦b≦0.90

0≦c≦0.04

0.005≦x≦0.03

0.5≦y≦0.55

0.05≦z≦0.90

0≦w≦0.5.

[0022] Thus, in the stoichiometric case, the condition c=0 applies to the piezoelectric ceramic material obtained, while in the non-stoichiometric case, the condition 0.05≦c≦0.04 applies for c.

[0023] One preferred piezoelectric ceramic material, which is particularly well-suited for application in piezoelectric actuators, is a material of the formula

Pb(1−x)−(3/2a)−1/2(x·b)CaxLaa(Zry(1−x·b−x·z)Ti(1−y)(1−x·b−x·w))(NbbCuzFew)xO3

[0024] where

0.0025≦a≦0.01

0.5≦b≦0.9

0.01≦x≦0.025

0.05≦w≦0.3

0.51≦y(1−(x·b)−(x·z))≦0.55:

0.45≦(1−y)(1−(x·b)−(x·w))≦0.49,

0.05≦w≦0.3.

[0025] It should be emphasized here that the oxygen content in all the above formulas of the piezoelectric ceramic material as a result of oxygen vacancies incorporated or generated by sintering is usually to be described as O3−δ rather than exactly as O3, where δ is less than 0.01, in particular less than 0.004. Since the formation of oxygen vacancies always occurs in such materials, and it is not accurately measurable within the above limits, in this Patent Application the formula

Pb1+c)−x−(3/2a)−1/2(x·b)A1xA2a(Zry(1−x·b−x·z)Ti(1−y)(1−x·b−x·w))(B1bB2zB3w)xO3

[0026] is to be understood, according to the general understanding of those skilled in the art, as a material having oxygen vacancies within the above-mentioned limits.

[0027] Specific examples of the materials included in the above-mentioned formulas are:

Pb0.965Ca0.02La0.005Zr0.5194Ti0.4606Nb0.015Fe0.003Cu0.002O3−δ

[0028] where x=0.02, a=0.005, y=0.53, b=0.75, z=0.1, c=0, and w=0.15, with δ less than 0.004.

Pb0.9685Ca0.02La0.002Zr0.5194Ti0.4606Nb0.017Fe0.002Cu0.001O3−δ

[0029] where x=0.02, a=0.002, y=0.53, b=0.85, z=0.05, c=0, and w=0.1, with δ less than 0.004.

Pb0.9687Ca0.02La0.0025Zr0.5194Ti0.4606Nb0.015Zn0.002Co0.002Cu0.001O3−δ

[0030] where x=0.02, a=0.0025, y=0.53, b=0.75, z=0.25, c=0, and w=0, with δ less than 0.004.

Pb0.9678Ca0.018La0.005Zr0.5203Ti0.4614Nb0.0135Zn0.003Cu0.0018O3−δ

[0031] where x=0.02, a=0.0025, y=0.53, b=0.75, z=0.25, c=0, and w=0, with δ less than 0.004.

Pb0.9978Ca0.018La0.005Zr0.5203Ti0.4614Nb0.0135Zn0.003Cu0.0018O3−δ

[0032] where x=0.02, a=0.0025, y=0.53, b=0.75, z=0.25, c=0.03, and w=0, with δ less than 0.004.

Pb0.9978Ca0.02La0.0025Zr0.2Ti0.46Nb0.015Zn0.002Co0.002Cu0.001O3−δ

[0033] where x=0.02, a=0.0025, y=0.53, b=0.75, z=0.25, c=0.02, and w=0, with δ less than 0.004.

[0034] In order to further process such a piezoelectric ceramic material into an electroceramic multilayer component, in particular a piezoelectric actuator, a thermistor, or a capacitor having a plurality of insulating layers arranged one above the other and made of the piezoelectric ceramic material and internal electrode layers located in some areas between those insulating layers, the internal electrode layers being made largely of copper or Agt/Pd1−t where t≧0.7, in particular >0.7, and preferably >0.8, one of the above-described materials is initially processed into a castable sludge or an extrudable compound in a known manner and then shaped into a green ceramic using film casting or extrusion, is dried and its surface is provided with a layer of a conductive internal electrode paste in some areas. This internal electrode paste is preferably a paste based on copper or on one of the above-described silver/palladium mixtures.

[0035] After the green ceramics, which have a thickness of 30 μm to 150 μm for example, are impressed with the conductive internal electrode paste, they are punched, stacked, and laminated, the number of insulating layers having the piezoelectric ceramic material usually being between 10 and 500 layers. After lamination, a known cofiring process follows under air, nitrogen, or a nitrogen-containing gas atmosphere at sintering temperatures of less than 1000° C.

[0036] In this sintering process, the green ceramics provide the insulating layers and the electrode paste layers provide the internal electrode layers in the electroceramic multilayer component, which is then present as a thick ceramic electrode composite and is usable, for example, as a piezoelectric actuator after the application of an outer contact of the internal electrodes. During sintering, the electrode paste layers are converted into metallic internal electrode layers, which are then at least largely made of copper or the above-described silver/palladium alloy.

[0037] The “columbite method” as described by T. R. Shrout et al., J. Am. Ceram. Soc., 73, (7), pp. 1862-1867 (1990), is one process that is suitable for manufacturing the above-described piezoelectric ceramic materials. In that method, powdered zirconium dioxide and powdered titanium dioxide are initially calcined to yield ZryTi1−yO2, where 0.5<y<0.55 (in mols); the ZryTi1−yO2 obtained is processed into a powder; this powder, used later as a precursor, is then mixed with powdered oxides and/or the powdered carbonates of the additional ions of lead, iron, A1, A2, B1, and B2, and this powdered mixture is then calcined to yield the piezoelectric ceramic material in the form of a homogeneous PZT mixed crystal. As an alternative, the “mixed oxide” process may also be used as the manufacturing method, i.e., initially all the ions to be used in:the piezoelectric ceramic material to be manufactured are first used as powdered oxides and/or powdered carbonates, mixed, and then calcined to form the piezoelectric ceramic material. The amounts of the respective oxides and/or carbonates to be used are determined by the composition of the piezoelectric ceramic material to be obtained.

Claims

1. A piezoelectric ceramic material having the following molar composition (except for the oxygen vacancies):

2. The piezoelectric ceramic material according to claim 1, wherein 0.005≦c≦0.04.

3. The piezoelectric material according to claim 1, wherein c=0.

4. The piezoelectric material according to one of the preceding claims, wherein A1 is a bivalent ion, A2 is a trivalent ion, B1 is a pentavalent ion, B2 is a monovalent ion, a bivalent ion, or a mixture of at least one monovalent ion and at least one bivalent ion, and B3 is a trivalent Fe ion.

5. The piezoelectric ceramic material according to one of the preceding claims, wherein c=0, A1 is Ca, A2 is La, B1 is Nb, and B2 is Cu.

6. The piezoelectric material according to claim 5, wherein the following applies to a, b, x, y, z, and w:

7. An electroceramic multilayer component, in particular a piezoelectric actuator, thermistor, or capacitor, having a plurality of insulating layers which are arranged one above the other and have a piezoelectric ceramic material according to one of the preceding claims 1 to 6, which are at least in some areas separated by internal electrode layers applied to the surface.

8. The electroceramic multilayer component according to claim 7, wherein the internal electrode layers are at least largely made of Cu or Agt/Pd1−t, where t≧0.7, in particular t>0.7, and designates the proportion by weight.

9. The electroceramic multilayer component according to claim 8, wherein it is manufactured by sintering a stack containing green ceramics having a piezoelectric ceramic material according to one of claims 1 through 6, whose surfaces are at least in some areas separated from one another by electrode paste layers containing Cu or Agt/Pd1−t where t≧0.7, at temperatures below 1000° C. in a cofiring process, the insulating layers being obtained from the green ceramics and the internal electrode layers being obtained from the electrode paste layers.

10. A method of manufacturing a piezoelectric ceramic material according to one of claims 1 through 6, wherein the Pb, Zr, Ti, Fe, A1, A2, B1, and B2 ions are initially used as powdered oxides and/or carbonates, mixed together and then calcined to form the ceramic material.

11. The method according to claim 10, wherein a lead oxide, in particular PbO, is used in stoichiometric excess.

12. The method according to claim 10, wherein the powdered oxides and/or carbonates used are utilized so that the piezoelectric ceramic material has a stoichiometric composition.

13. A method of manufacturing a piezoelectric ceramic material according to one of claims 1 through 6, wherein a powdered ZrO2 and powdered TiO2 are initially calcined to form ZryTi1−yO2 where 0.50<y<0.55, the ZryTi1−yO2 is processed to form a powder, this powder is mixed with powdered oxides and/or powdered carbonates of the ions Pb, Fe, A1, A2, B1, B2, and this powder mixture is then calcined to form the ceramic material.

14. The method according to claim 13, wherein a lead oxide, in particular PbO, is used in stoichiometric excess.

15. The method according to claim 13, wherein the powdered oxides and/or carbonates used are utilized so that the piezoelectric ceramic material has a stoichiometric composition.