WO2009043652A1

WO2009043652A1 - Unleaded piezoceramic material of the potassium/sodium/niobate system having a iron/lanthanum doping, method for the production of a component comprising the piezoceramic material and use of the component

Info

Publication number: WO2009043652A1
Application number: PCT/EP2008/061377
Authority: WO
Inventors: Robert Bathelt; Katrin Benkert; Aurelie Cardin; Carsten Schuh
Original assignee: Siemens Aktiengesellschaft
Priority date: 2007-09-28
Filing date: 2008-08-29
Publication date: 2009-04-09
Also published as: DE102007046450A1

Abstract

The invention relates to a piezoceramic material, having a Perowskit phase comprising the stoichiometric composition (K1-xLax) (Nb1-yFey) O3 where 0.002 ≤ x ≤ 0.1 and 0.002 ≤ y ≤ 0.1. Said material preferably comprises a further alkali metal is contained in addition to potassium sodium at a proportion of 0.4 mol-% to 0.6 mol-%. This composition creates a system having a morphotropic phase threshold. Said system is accompanied by excellent piezoelectric properties. The invention further relates to a method for the production of a piezoceramic component using the piezoceramic material having the following process steps: a) providing a green body, wherein the piezoceramic material of the component is created from the piezoceramic basic material, b) heat treatment of the green body, wherein the piezoceramic basic material of the component results from the piezoceramic basic material. The thermal treatment comprises a calcinating and/or sintering of the piezoceramic composition. The piezoceramic component is, for example, an ultrasonic transducer, or a piezoceramic bending transducer. Particularly, the piezoceramic component is a multi-layer piezoactuator, which is used for activating a fuel valve of an internal combustion engine of a motor vehicle.

Description

Lead-free piezoceramic material of the

Potassium-sodium niobate system with iron-lanthanum doping, method for producing a component with the piezoceramic material and use of the component

The invention relates to a piezoceramic material having a perovskite phase of the potassium-sodium niobate system (KNN). In addition, a method for producing a piezoceramic component with the piezoceramic material and a use of the component are specified.

Lead-containing piezoceramic materials based on the binary mixed system of lead zirconate and lead titanate, so-called lead zirconate titanate (Pb (Ti, Zr) O ₃ , PZT), are currently being used for their excellent mechanical and piezoelectric properties, for example a high Curie temperature T _c of over 300 ° C. or high d ₃₃ coefficient in the large and

Small signal range, used in many fields of technology. Piezoceramic components with these materials are, for example, bending transducers, multilayer actuators and ultrasonic transducers. These components are used in actuators, medical technology, ultrasound technology or automotive engineering.

With a view to improved environmental compatibility, lead-free piezoceramic materials are to be used in the future. From Y. Saito et al. , Lead-free piezoceramics, Nature, vol. 432, pages 84 to 87, for example, a lead-free, phase-pure piezoceramic material with good piezoelectric properties is known. The material consists of a perovskite phase based on a

Potassium sodium niobate. The empirical formula of the piezoceramic material is

(Nbo, 86Ta _o , iSb _o , o4) O ₃ . The Curie temperature is 253 ° C. The d ₃₃ coefficient in the large signal range is about 300 pm / V (polarity at 5 kV / mm). The object of the invention is to provide an alternative to the known lead-free piezoceramic material, which has similarly good piezoelectric properties.

To achieve the object a piezo-ceramic material is provided, comprising a perovskite phase having the stoichiometric composition (K ₁ - _J5 La _x) (Nbi_ _y Fe _y) O _3, with 0.002 <x <0.1 and 0.002 <y <0, 1.

To achieve the object, a method for producing a piezoceramic component with the piezoceramic material is also specified with the following method steps: a) providing a green body with a piezoceramic starting composition of the piezoceramic material and b) heat treating the green body, wherein the piezoceramic material of the piezoceramic starting material Component arises.

A lanthanum content and an iron content are between 0.2 mol% and 10 mol%. Iron and lanthanum are contained in equal or approximately equal proportions. These proportions lead to a system with a new morphotropic phase boundary. Near the morphotropic phase boundary, the piezoelectric properties of a material improve significantly.

According to a particular embodiment, the perovskite phase has a sodium content selected from the range from 0.4 mol% to 0.6 mol% and in particular from the range from 0.5 mol% to 0.55 mol%. The stoichiometric composition is as follows: (K ₁ - _{^ 2} _x La Na ₂₎ (Nbi_ _y Fe _y) O _3, with 0.002 <x <0.1, 0.002 <y <0.1, and 0.004 ≤ z ≤ 0.006. Potassium and sodium are present in approximately equal molar proportions. The ceramic material is a potassium-sodium niobate. Particularly good electrical or piezoelectric properties are achieved in the presence of both alkali metals. The proportions are chosen so that the system is near the morphotropic phase boundary. Incidentally, lithium may also be included. According to a further aspect of the invention, a method for producing a piezoceramic component with the piezoceramic material is specified with the following method steps: a) providing a green body with a piezoceramic starting composition of the piezoceramic material and b) heat treating the green body, wherein from the piezoceramic starting composition of the piezoceramic material of the component arises. The green body is a shaped body which consists, for example, of homogeneously mixed, pressed-together oxides of the specified metals. Likewise, the green body may have an organic additive, which is processed with the oxides of the metals to a slurry. The organic additive is, for example, a binder or a dispersant. From the slurry, a green body is produced in a shaping process. The green body is, for example, a green sheet produced by the forming process (film drawing). The green body with the piezoceramic starting composition produced in the shaping process is subjected to a heat treatment. The heat treatment of the green body includes calcination and / or sintering. It comes to the formation and compression of the forming piezoceramic material.

To provide the green body, according to a particular embodiment, mixing is more powdery, oxidic

Metal compounds of the metals required for the material performed. Oxides of the required alkali metals (K ₂ O, optionally Na ₂ O), of niobium (Nb ₂ O ₅ ), of lanthanum (La ₂ O ₃ ) and of iron (Fe ₂ O ₃ ) are used. It is also possible to use precursors of the oxides of the metals, for example carbonates (Na ₂ CO ₃ , K ₂ CO ₃ ) or oxalates. Both types of metal compounds, ie the precursors of the oxides and the oxides themselves, can be referred to as oxidic metal compounds.

The powders of the oxidic metal compounds can be prepared by known methods, for example according to

Sol-gel, citrate, hydrothermal or the

Oxalate process. In this case, oxidic metal compounds with only one kind of metal can be made. It is also conceivable, in particular, that oxidic metal compounds are used with a plurality of types of metals (mixed oxides).

The oxidic metal compound having at least two of the metals may also be the perovskite phase itself. To provide these mixed oxides can be made of the mentioned precipitation reactions. It is also conceivable

Mixed oxide method for providing a mixed oxide. In this case, powdery oxides of the metals are mixed together and calcined at higher temperatures. Calcination results in mixed oxides.

According to a particular embodiment, therefore, a piezoceramic starting composition with at least one oxidic metal compound is used with at least two of the metals. Examples include potassium niobate (KNbO ₃ ) or sodium niobate (NaNbO ₃ ). The aforementioned alkali niobates and lanthanum iron oxide (LaFeO ₃ ) are used with particular advantage. Lanthanum iron oxide forms together with the

Alkali niobate a mixed crystal that crystallizes in the tetragonal system.

The workup of the metal oxides with the conversion into the piezoceramic material can be done in various ways. It is conceivable, for example, that first the powders of the oxidic metal compounds are homogeneously mixed. The piezoceramic starting composition is formed in the form of a homogeneous mixture of the metal oxides. Subsequently, the piezoceramic starting composition is transferred into the piezoceramic material by heat treatment, eg by calcination. The piezoceramic material is ground to a fine piezoceramic powder. Subsequently, a ceramic green body with an organic binder and further organic additives is produced from the fine piezoceramic powder in the shaping process. This ceramic green body is debinded and sintered. In this case, the piezoceramic component is formed with the piezoceramic material. As an alternative to the procedure described, the powders of the oxidic metal compounds can be homogeneously mixed and processed in the shaping process into a ceramic green body with organic binder. This green body also has the piezoceramic starting composition. Subsequent sintering leads to the piezoceramic component with the piezoceramic material.

According to a particular embodiment, a piezoceramic component having at least one piezoelectric element is produced which has an electrode layer with electrode material, at least one further electrode layer with a further electrode material and at least one piezoceramic layer arranged between the electrode layers with the piezoceramic material. A single piezoelectric element represents the smallest unit of the piezoceramic component. To produce the piezoelectric element, for example, a ceramic green sheet with the piezoceramic starting composition is printed on both sides with the electrode materials. The electrode materials may be the same or different. Subsequent debindering and sintering results in the piezoelectric element.

In a particular embodiment, a piezoelement is used in which the electrode material and / or the further electrode material have at least one elementary metal selected from the group silver, copper and palladium. The piezoceramic material or the piezoelectric element is produced in particular by co-sintering the piezoceramic starting composition and the electrode material (cofiring). The electrode material may consist of the pure metals, for example, only of silver or only of copper. An alloy of said metals is also possible, for example an alloy of silver and palladium.

The sintering to the piezoceramic material can be carried out both in a reducing or oxidizing sintering atmosphere become. In a reducing sintering atmosphere, almost no oxygen is present. An oxygen partial pressure is less than 1-10 ^"2 mbar, and preferably less than 1-10 ^{" 3} mbar. By sintering in a reducing sintering atmosphere inexpensive copper is possible as an electrode material.

In principle, any piezoceramic component can be produced with the piezoceramic material with the aid of the piezoceramic starting composition. The piezoceramic component has primarily at least one piezoelectric element described above. Preferably, the piezoceramic component is selected with the piezoelectric element from the group piezoceramic bending transducer, piezoceramic multilayer actuator, piezoceramic transformer, piezoceramic motor and piezoceramic ultrasonic transducer. The piezoelectric element is for example part of a piezoelectric bending transducer. By

Stacking a plurality of green films printed on one or both sides with electrode material, subsequent debinding and sintering results in a monolithic stack of piezoelements. With suitable dimensioning and shape results in a monolithic piezoceramic

Multilayer actuator. This piezoceramic multilayer actuator is preferably used to control a fuel injection valve of an internal combustion engine. Due to the stack-shaped arrangement of the piezoelectric elements, a piezoceramic ultrasonic transducer is also accessible, with suitable dimensioning and shape. The ultrasonic transducer is used for example in medical technology or for material testing.

Reference to an embodiment and the associated figures, the invention will be described in more detail below. The figures are schematic and do not represent true to scale figures.

Figure 1 shows a ceramic piezoelectric element with a piezoceramic material, which by means of suitable piezoceramic starting compositions was prepared, in a lateral cross-section.

FIG. 2 shows a piezoceramic component with a multiplicity of piezoelements in a lateral cross section.

The piezoceramic material has a perovskite phase having the following composition: (K _x _ _ _z La _x Na _{_z!)} (Nbi_ _y Fe _y) O _3. The piezoceramic material is obtained as follows: A green body with a piezoceramic starting composition is provided. For this purpose, a mixing of pulverulent, oxidic metal compounds (starting powder) is first carried out. These oxidic metal compounds are in detail: KNbO ₃ , NaNbO ₃ and LaFeO ₃ .

The starting powders are weighed in appropriate proportions and mixed in hexane or alcohol for two minutes (with a speed mixer). The resulting fine powder mixture is calcined at 750 ^° C. for five hours. The piezoceramic composition formed in this heat treatment is ground in water for 3 hours and pressed in a molding process into a green body in the form of a powder compact of about 6 to 12 mm in diameter. This powder compact is sintered at a temperature of 1000 ⁰ C to 1150 ° C for one hour to a sample.

Subsequently, the respective sample is provided on both sides with electrodes by vapor deposition of silver and poled at about 100 ° C with an electric field of about 2.5 kV / mm. Thus, a single piezoelectric element is obtained

Based on the described method, a piezoceramic component 1 is produced with the piezoceramic material. The piezoceramic component 1 is according to a first embodiment, a piezoelectric actuator 1 in monolithic

Multilayer construction (Figure 2). The piezoactuator 1 consists of a multiplicity of piezoelements 10 arranged one above the other in a stack (FIG. 1). Each of the piezoelectric elements 10 has an electrode layer 11, a further electrode layer 12, and a piezoceramic layer 13 arranged between the electrode layers 11 and 12. The piezoelectric elements 10 adjacent in the stack each have a common electrode layer. The electrode layers 11 and 12 comprise an electrode material of a silver-palladium alloy in which palladium is contained in a proportion of 5% by weight. In an alternative embodiment, the electrode layers consist of (approximately) pure silver. According to another alternative, the electrode material is copper.

To produce the piezoactuator 1, green bodies in the form of green sheets with the piezoceramic starting composition are provided. For this purpose, the powder mixture is mixed with the piezoceramic starting composition with an organic binder and other organic additives. From the slip obtained in this way, the ceramic green sheets are cast or drawn. The green sheets are dried, printed with a paste with the electrode material, stacked, laminated, debindered and sintered to the piezoelectric actuator 1 under oxidizing sintering atmosphere (silver or silver-palladium alloy as electrode material) or reducing sintering atmosphere (copper as electrode material).

The resulting monolithic piezoceramic multilayer actuator is used to actuate a

Fuel injection valve of an internal combustion engine of a motor vehicle used.

Further, not shown embodiments such as piezoceramic bending transducer, piezoceramic transformer or piezoceramic ultrasonic transducer are also accessible with the help of the new piezoceramic composition.

Claims

claims

1. Piezoceramic material comprising a perovskite phase having the stoichiometric composition (Ki_ _x La _x ) (Nbi- _y Fe _y ) O ₃ with 0.002 <x <0.1 and 0.002 <y <0.1.

2. Piezoceramic material according to claim 1, wherein the perovskite phase has a sodium content selected from the range from 0.4 mol% to 0.6 mol% and in particular from the range of 0, 5 mol% to 0, 55 mol% ,

3. piezoceramic material according to claim 1 or 2, wherein a molar lead content of the piezoceramic composition is less than 0.1 mol% and in particular less than 0.01 mol%.

4. A method for producing a piezoceramic component (1) with a piezoceramic material according to one of claims 1 to 4 with the following method steps: a) providing a green body with a piezoceramic starting composition of the piezoceramic material and b) heat treating the green body, wherein from the piezoceramic starting composition the piezoceramic material of the component (1) is formed.

5. The method of claim 4, wherein for providing the green body, a mixing powdered, oxidic metal compounds of the metals potassium, lanthanum, niobium, iron and optionally sodium is carried out to form the piezoceramic starting composition.

6. The method of claim 5, wherein a piezoceramic starting composition is used with at least one mixed oxide with at least two of the metals.

7. The method according to claim 6, wherein as mixed oxide

Lanthanum iron oxide with the molecular formula LaFeCh is used.

8. The method according to any one of claims 4 to 7, wherein a piezoceramic component (1) with at least one piezoelectric element (10) is produced, the one electrode layer (11) with electrode material, at least one further electrode layer (12) with a further electrode material and at least a piezoceramic layer (13) arranged between the electrode layers (11, 12) with the piezoceramic material.

9. The method of claim 8, wherein a piezoelectric element (10) is used, in which the electrode material and / or the further electrode material have at least one selected from the group consisting of silver, copper and palladium elemental metal.

10. The method according to any one of claims 4 to 9, wherein the piezoceramic component (1) with the piezoelectric element (10) from the group piezoceramic bending transducer, piezoceramic multilayer actuator, piezoceramic transformer, piezoceramic motor and piezoceramic ultrasonic transducer is selected.

11. Use of a piezoceramic multilayer actuator produced by the method according to claim 10 for controlling a fuel injection valve of an internal combustion engine.