CERAMIC MATERIAL, METHOD FOR PRODUCING THE CERAMIC MATERIAL, AND ELECTROCERAMIC COMPONENT COMPRISING THE CERAMIC MATERIAL

Abstract

The invention relates to a ceramic material, comprising lead zirconate titanate, which additionally contains K and optionally Cu. The ceramic material can be used in an electroceramic component, for example a piezoelectric actuator. The invention also relates to methods for producing the ceramic material and the electronic component.

Claims

1. A ceramic material, comprising lead zirconate titanate, which additionally contains Nd, and K, according to the following formula:
Pb.sub.1-(3x/2)-aNd.sub.XK.sub.2aV.sub.Pb(x/2)-a(Zr.sub.1-yTi.sub.y)O.sub.3, within the limits 0.0001≦x≦0.06, 0.0001≦a≦0.03, 0.35≦y≦0.60, preferably in the composition range.

2. The ceramic material according to claim 1, in which the lead zirconate titanate has the general composition ABO.sub.3 of a perovskite lattice, wherein A represents the A sites and B represents the B sites of the perovskite lattice and Nd, and K occupy A sites in the perovskite lattice.

3. An electroceramic component comprising a ceramic material according to claim 1 or 2.

4. The electroceramic component according to claim 3, in the form of a piezoelectric actuator.

5. The piezoelectric actuator according to claim 4, comprising: at least two layers of the ceramic material; and an inner electrode, preferably a Cu inner electrode, between the at least two layers.

6. A method for producing a ceramic material according to claim 1 or 2, comprising the following steps: A) producing a ceramic raw material mixture by comminuting and mixing starting materials containing Pb, Zr, Ti, Nd and oxygen; B) introducing a K-containing compound; C) calcining the raw material mixture; and D) sintering to form the ceramic.

7. The method according to claim 6, wherein an additional method step A1), in which the ceramic raw material mixture is calcined and then comminuted, is carried out after method step A) and before method step B).

8. The method according to claim 6, wherein Nd is added in a range of from 0.01 to 6 mol %.

9. The method according to claim 6, wherein a K-containing compound is introduced before the calcining in method step C).

10. The method according to claim 6, wherein a K-containing compound is introduced after the calcining in method step C).

11. The method according to claim 6, wherein an additional method step C1), in which the calcined ceramic raw material mixture is comminuted and mixed with up to 1.5 mol % PbO, is carried out after method step C) and before method step D).

12. The method according to claim 6, wherein K are added in a range of from 0.01 to 6 mol %.

13. The method according to claim 6, wherein, before method step D), ceramic green sheets are formed from the ceramic raw material mixture comprising a binder.

14. A method for producing a multilayer component, comprising: A) producing a first ceramic raw material mixture by comminuting and mixing starting materials containing Pb, Zr, Ti, Nd and oxygen; B) calcining the first ceramic raw material mixture; C) comminuting the calcined first ceramic raw material mixture and mixing with a K-containing compound to produce a second ceramic raw material mixture; D) calcining the second ceramic raw material mixture; E) adding a binder to the ceramic raw material mixture of D3); F) forming green sheets from the mixture of E3); G) printing inner electrodes on the green sheets of F3, the material of the inner electrodes comprising Cu, H) layering a multiplicity of the ceramic green sheets of G) on top of one another to form a stack; I) consolidating the stack by lamination, as a result of which green parts are formed; J) debindering the green parts; and K) sintering the green parts, some of the Cu being transferred from the inner electrodes into the ceramic material and being incorporated therein.

15. The method according to claim 14, wherein the material of the inner electrodes comprises metallic Cu or Cu oxide.

Description

[0065] FIG. 1 schematically shows the structure of a PZT multilayer component (1) having Cu inner electrodes (20), ceramic layers (10) and outer contacts (30).

[0066] FIG. 2 shows the process window with the equilibrium lines for Cu.sub.2O/Cu and PbTiO.sub.3/Pb, TiO.sub.2. It is permissible for the oxygen partial pressure to vary only within these limits, when oxidation of Cu to form Cu.sub.2O or reduction of PbTiO.sub.3 to form Pb and TiO.sub.2 are to be avoided.

[0067] FIG. 3A shows the ceramic microstructure which is obtained in actuators with combined Nd/K, Cu doping on the A sites of the PZT perovskite structure.

[0068] FIG. 3B shows the microstructure of actuators having the ceramic composition

Pb.sub.0.9735Nd.sub.0.02Cu.sub.0.003V.sub.0.0035[(Ti.sub.0.4485Zr.sub.0.5515).sub.0.995Ni.sub.0.005]O.sub.3,

in which, in addition to Nd donors, Cu acceptors are incorporated on the A sites and at the same time Ni acceptors are incorporated on the B sites.

[0069] FIG. 3C reproduces the microstructure of ceramic layers in actuators having the ceramic composition

Pb.sub.0.9685Nd.sub.0.02Cu.sub.0.003V.sub.0.0085(Zr.sub.0.5515Ci.sub.0.4485)O.sub.3,

which, in addition to the Nd/Cu doping, has no further additional doping by K acceptors on A sites or Ni acceptors on B sites.

[0070] In a preferred embodiment, starting compounds containing Pb, Ti, Zr and Nd, for example the oxide components Pb.sub.3O.sub.4, TiO.sub.2, ZrO.sub.2 and Nd.sub.2O.sub.3, according to the Nd-doped PZT composition

Pb.sub.1-(3x/2)Nd.sub.xV.sub.x/2(Zr.sub.1-yTi.sub.y)O.sub.3

in a mixture with the limits 0.35≦y≦0.60 and 0.0001≦x≦0.06 and preferably with the limits 0.45≦y≦0.55 and 0.015≦x≦0.025, are subjected to a first reaction, for example at 925° C. for two hours.

[0071] In a version A, the reaction product is milled to d.sub.50≦1 μm in an aqueous slurry and, after drying by evaporation of the water, is reacted a second time, for example at 950° C. for 2 h, for the purpose of better homogenization of the constituents in the PZT perovskite structure. Up to 1.5 mol % PbO based on the PZT formula unit are optionally added as a sintering aid to the subsequent fine milling, for example to d.sub.50=0.3 to 0.55 μm, using for example ZrO.sub.2 beads (≦0.8 nm).

[0072] After renewed comminuting, optionally with evaporation and sieving, the disperse powder is converted into a slip for producing ceramic sheets with the addition of a quantity of K-containing compounds corresponding to the formula

Pb.sub.1-(3x/2)-aNd.sub.xK.sub.2aV.sub.(x/2)-a(Zr.sub.1-yTi.sub.y)O.sub.3,

for example K.sub.2CO.sub.3, KCH.sub.3COO or K.sub.2C.sub.2O.sub.4 or a mixture thereof, within the limits 0.0001≦a≦0.03 and preferably within the limits 0.002≦a≦0.0125, use preferably being made of a nonaqueous dispersing medium, either an ethanol/toluene mixture in combination with a polyvinyl butyral binder or a methyl ethyl ketone/isopropanol mixture in combination with a polyurethane binder. In this version A, the K acceptors are incorporated in the PZT perovskite structure only after the debindering, over the course of the sintering of the sheet stack of an actuator which is provided with Cu inner electrodes.

[0073] In a version B, after the reaction, for example at 925° C. for two hours, the powder is likewise milled, for example to d.sub.50≦1 μm, in an aqueous slurry, but already at this stage a quantity of K-containing compound corresponding to the formula

Pb.sub.1-(3x/2)-aNd.sub.xK.sub.2aV.sub.(x/2)-a(Zr.sub.1-yTi.sub.y)O.sub.3,

for example K.sub.2CO.sub.3, KCH.sub.3COO or K.sub.2C.sub.2O.sub.4 or a mixture thereof, within the limits 0.0001≦x≦0.03 and preferably within the limits 0.002≦a≦0.0125, is added. In order to avoid losses of K.sup.+ ions, the water is preferably eliminated from the slurry with the aid of a dryer, for example a drum dryer. In this version B, the K.sup.+ ions are already incorporated and thereby fixed in the PZT perovskite structure during the subsequent, second reaction, for example at 950° C. for two hours. Up to 1.5 mol % PbO based on the PZT formula unit are preferably added as a sintering aid to the subsequent fine milling, for example to d.sub.50=0.3 to 0.55 μm, using ZrO.sub.2 beads (≦0.8 nm), in an aqueous slurry. In order to avoid K.sup.+ ion losses on account of partial elution by hydrolysis, use is also made at this point of a dryer, for example a drum dryer, during evaporation of the water. After sieving, the finely disperse powder is converted into a sheet slip in the same way as in the version A.

[0074] In both versions, it is preferable to coordinate the parameter y, which describes the morphotropic phase boundary, and the parameter x, which has been recognized as optimal and denotes the K content, with one another as the result of an experimental investigation.

[0075] During the cofiring with Cu electrodes, minor takeup of Cu by the ceramic takes place during the sintering itself under an oxygen partial pressure which has been set in a defined manner and avoids the reduction of PbO and similarly the oxidative formation of Cu.sub.2O. The Cu acceptors which are incorporated in addition to the K acceptors in the quantity range, denoted by the parameter p, of 0<p<(x/2−a) additionally have a promoting effect on the grain growth, and therefore a ceramic having the following general composition forms the basis of the finished multilayer actuator with Cu inner electrodes:

Pb.sub.1-(3x/2)-a-pNd.sub.xK.sub.2aCu.sub.2pV.sub.(x/2)-a-p(Zr.sub.1-yTi.sub.y)O.sub.3.

[0076] Consequently, such additional doping with Cu acceptors within the limits 0<p<((x/2)−a) is also attained by the addition of Cu.sub.2O to the slip during the fine milling after the second reaction.

[0077] The powders which are produced by the version A or B are dispersed in an organic solvent with the addition of the respectively suitable binder and processed as standard to form sheets having a thickness of, for example, approximately 80 μm, and these are then printed with Cu paste and stacked up to several hundred layers and laminated. Actuators in their basic state are obtained by cutting along the edge and the Cu inner electrodes which emerge alternately in the region of the intended external contact-connection.

[0078] The organic binder constituents are preferably removed, for the purpose of avoiding uncontrolled oxidation of the Cu electrodes, with substantial exclusion of oxygen, and this is achieved, for example, with steam, to which a quantity of hydrogen is also added at the end of the debindering at up to approximately 500° C., the polymer chains being degraded by hydrolysis, depolymerization and steam reforming to form smaller molecules, which are carried out with the steam. Here, the oxygen partial pressure must not drop below a lower limit so that the reduction of PbO or PZT to form Pb is avoided. It has been identified, and documented in patents EP1240675B1 and U.S. Pat. No. 7,855,488B2, that binders based on polyurethanes are particularly well suited to debindering up to a very low residual carbon content of approximately 300 ppm on account of their hydrolytic cleavability by steam.

[0079] The sintering of the debindered actuators is carried out with a heating rate of, for example, approximately 1 K/min at, for example, 1000 to 1010° C. for a number of hours holding time under the conditions of a controlled oxygen partial pressure, it being possible for this to be set by steam and hydrogen such that the oxidation of Cu and the formation of Pb as a consequence of the reduction of PbO or PZT are avoided throughout the temperature profile. The oxygen partial pressure to be set can be calculated from thermodynamic data depending on the temperature and can be monitored by an oxygen probe.

[0080] For measurement, the actuators can be contacted by screen printing with Cu paste at the surface in the region of electrodes emerging in alternation which is intended therefor, and this external contact-connection can be burnt in in a short thermal process step under a defined oxygen partial pressure for the purpose of avoiding the oxidation of Cu.

[0081] The invention will be explained in more detail in the following exemplary embodiments.

[0082] In accordance with the general formula

Pb.sub.1-(3x/2)-a-pNd.sub.xK.sub.2aCu.sub.2pV.sub.(x/2)-a-p(Zr.sub.1-yTi.sub.y)O.sub.3,

a selection is made of the parameters x=0.02, a=0.0075, y=0.4485 and p=0, giving rise to the following composition:

Pb.sub.0.9625Nd.sub.0.02K.sub.0.015V.sub.0.0025(Zr.sub.0.5515Ti.sub.0.4485)O.sub.3

[0083] First of all, the raw materials Pb.sub.3O.sub.4, TiO.sub.2, ZrO.sub.2 and Nd.sub.2O.sub.3, the impurity content of which has been checked and the metal content of which has been determined separately in each case, are weighed out in the corresponding molar ratio without the addition of a potassium compound (a=0) and subjected to rotary mixing with ZrO.sub.2 grinding media in an aqueous slip for 24 hours. Following evaporation and sieving, the mixture is reacted at 925° C. with a holding time of two hours in a ZrO.sub.2 capsule, in which case the Nd-doped PZT compound

Pb.sub.0.97Nd.sub.0.02V.sub.0.01(Zr.sub.0.5515Ci.sub.0.4485)O.sub.3

already largely forms.

[0084] In a version A, the reaction product is milled to a mean grain size d.sub.50≦1 μm using ZrO.sub.2 beads (2 mm) in an aqueous slurry, the water is removed by evaporation and, for the purpose of completing the reaction, the residue is reacted a second time, this time for two hours at 950° C. 0.8 mol % PbO based on the PZT formula unit are added as a sintering aid to the subsequent fine milling to d.sub.50=0.3 to 0.55 μm using ZrO.sub.2 beads (≦0.8 mm). After renewed evaporation and sieving, the disperse powder is converted into a slip for producing ceramic sheets with the addition of 1.5 mol % KCH.sub.3COO (a=0.0075) based on the PZT formula unit, the nonaqueous dispersing medium used being a methyl ethyl ketone/isopropanol mixture in combination with a polyurethane binder. The sheets are printed with a Cu paste, stacked up to several hundred layers and laminated and the individual actuators are obtained by being cut out from the compacted stack.

[0085] In this version A, the K acceptors are incorporated in the PZT perovskite structure after the debindering, over the course of the sintering of the sheet stack of an actuator which is provided with Cu inner electrodes. In addition, the ceramic layers take up a quantity of Cu from the inner electrodes as Cu acceptors during the sintering compaction. Where p=0.0015, this gives rise to the composition

Pb.sub.0.961Nd.sub.0.02K.sub.0.015Cu.sub.0.003V.sub.0.008(Zr.sub.0.5515Ti.sub.0.4485)O.sub.3

for the ceramic layers in the actuators.

[0086] In a version B, the reaction product obtained after the first reaction at 925° C. is likewise milled to a mean grain size d.sub.50≦1 μm using ZrO.sub.2 beads (2 mm) in an aqueous slurry, but already at this stage a quantity of K.sub.2CO.sub.3, KCH.sub.3COO or K.sub.2C.sub.2O.sub.4 (a=0.0075) corresponding to the formula

Pb.sub.0.9625Nd.sub.0.02K.sub.0.015V.sub.0.0025(Zr.sub.0.5515Ti.sub.0.4485)O.sub.3

is added. In order to avoid losses of K.sup.+ ions, the water is eliminated from the slurry with the aid of a drum dryer. In this version B, the K.sup.+ ions are already incorporated and thereby fixed in the PZT perovskite structure during the subsequent, second reaction, at 950° C. for two hours. 0.8 mol % PbO based on the PZT formula unit are added as a sintering aid to the subsequent fine milling to d.sub.50=0.3 to 0.55 pm using ZrO.sub.2 beads (=0.8 mm) in an aqueous slurry. In order to avoid K.sup.+ ion losses on account of partial elution by hydrolysis, use is also made at this point of a drum dryer during evaporation of the water. After sieving, the finely disperse powder is converted into a sheet slip in the same way as in the version A and further processed to form actuators with Cu inner electrodes, the composition

Pb.sub.0.961Nd.sub.0.02K.sub.0.015Cu.sub.0.003V.sub.0.008(Zr.sub.0.5515Ti.sub.0.4485)O.sub.3

forming in the ceramic layer system.

[0087] It can be seen that the K acceptor doping on A sites of the PZT perovskite structure in FIG. 3A leads to an increase in the mean grain size comparable to that in the case of the Ni acceptor doping on the B sites in FIG. 3B: d.sub.50 approximately 3 μm.

[0088] The properties compiled in table 1 show that the PZT ceramic modified by Nd/K, Cu doping on the A sites (FIG. 3A) leads to a value for the deflection parameter d.sub.33 which is increased by 33% compared to the Nd/Cu doping on A sites and Ni doping on B sites (FIG. 3B) in the structure of an actuator. Here, an increase in the coupling parameter k.sub.33 and also a decrease in the lost energy are achieved at the same time.

[0089] Table 1: Piezoelectric and piezomechanical properties of the actuators comprising an Nd/K, Cu-doped PZT ceramic of the composition

Pb.sub.0.961Nd.sub.0.02K.sub.0.015Cu.sub.0.003V″.sub.0.008(Zr.sub.0.5515Ti.sub.0.4485)O.sub.3

and also for actuators comprising an Nd/Cu, Ni-doped PZT ceramic of the composition

Pb.sub.0.9735Nd.sub.0.02Cu.sub.0.003V.sub.0.0035[(Ti.sub.0.4485Zr.sub.0.5515).sub.0.995Ni.sub.0.005]O.sub.3.

TABLE-US-00001 Nd/K, Cu-doped Nd/Cu, Ni-doped Property PZT ceramic PZT ceramic d33 650 pm/V 490 pm/V ε 2950 2360 Modulus of elasticity c.sub.33 28 GPa 34 GPa Coupling k.sub.33 69.5% 69.5% Electrical energy losses   29%   31%

[0090] The measurements were made on multilayer actuators having the dimensions 3.8×3.8×30 mm.sup.3. During the measurement, use was made of an electric field strength of 3 kV/mm and a mechanical prestress of 27 MPa with the spring constant for the mechanical loading of the actuator of 1 n/μm.