DE102010025659A1 - Piezoelectric component e.g. monolithic piezoceramic multilayer actuator, for controlling fuel injection valve of combustion engine for motor car, has piezoceramic layer comprising sodium, lithium, silver and ozone in specified composition - Google Patents

Abstract

The component e.g. monolithic piezoceramic multilayer actuator (1), has piezo elements (10) comprising electrode layers (11, 12) with electrode materials. A piezoceramic layer (13) is arranged between the electrode layers. One of the electrode materials comprises silver. The piezoceramic layer comprises potassium, sodium, lithium, silver, niobium, tantalum and ozone in specified stoichiometric composition. The piezo elements are arranged one above the other to form a piezo element stack. A mixed oxide of silver niobate is utilized in piezoceramic output composition of a piezoceramic material. An independent claim is also included for a method for manufacturing a piezoelectric component.

Description

The invention relates to a piezoelectric component with a lead-free, piezoceramic material based on a silver-doped potassium-sodium niobate and a method for producing the component.

Piezoceramic materials based on the binary mixed system of lead zirconate and lead titanate, so-called lead zirconate titanate ceramic (Pb (Ti, Zr) O ₃ , PZT), are used because of their very good mechanical and piezoelectric properties, for example high Curie temperature T _c of over 300 ° C or high d ₃₃ coefficient in the large and small signal range, used in many areas of technology. Piezoelectric 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 the entry into force of the Restriction of Hazardous Substances (RoHS) directive in 2006, the legally permitted levels of heavy metals in electrical and electronic components within the European Union (EU) were severely restricted. This also applies to the above-described piezoelectric components. The use of components with PZT as a piezoceramic material is currently only possible through a temporary EU exemption.

With regard to an improved environmental performance is for example from the US Pat. No. 7,101,491 B2 a promising lead-free, phase-pure piezoceramic material with good piezoelectric properties known. The material consists of a perovskite phase based on potassium niobate (KNN). To improve the piezoelectric properties of the piezoceramic material, a plurality of dopants may be present. Particularly good piezoelectric properties are obtained with lithium, tantalum and / or antimony as dopants.

However, these new materials have lower electrical insulation resistance compared to the PZT. The insulation resistance of PZT is typically 10 GΩcm to 20 GΩcm (100 V, 60 s measurement). By contrast, the insulation resistances of the novel lead-free piezoceramic materials are in the range of 0.5 GΩcm to 3 GΩcm under the same measuring conditions.

In addition, these novel materials are hygroscopic, i. H. they tend to absorb moisture from the environment (eg humidity). This leads to a further reduction of the insulation resistance. Thus, a use of piezoelectric components with these novel piezoceramic materials is limited.

The object of the invention is to provide a piezoelectric component with a lead-free piezoceramic material, which can also be used in a humid environment.

• – 0 ≤ x ≤ 0,75;
• – 0 ≤ y ≤ 0,75;
• – 0 ≤ z ≤ 0,07;
• – 0 ≤ a ≤ 0,5;
• – 0,95 ≤ b ≤ 1,05;
• – x + y + z + a = 1;
• – 0 ≤ u ≤ 1;
• – 0 ≤ v ≤ 0,4;
• – u + v = 1.

To achieve the object, a piezoelectric component with at least one piezo element is specified, 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. At least one of the electrode materials has elemental silver. The piezoceramic material has a stoichiometric composition [K _x Na _y Li _z ) _1-a Ag _a ] _b (Nb _u Ta _v ) O ₃ . In addition, the following relationships apply:

• - 0 ≤ x ≤ 0.75;
• - 0 ≤ y ≤ 0.75;
• - 0 ≤ z ≤ 0.07;
• - 0 ≤ a ≤ 0.5;
• - 0.95 ≤ b ≤ 1.05;
• - x + y + z + a = 1;
• - 0 ≤ u ≤ 1;
• - 0 ≤ v ≤ 0.4;
• - u + v = 1.

• a) Bereitstellen mindestens einer keramischen Grünfolie mit einer piezokeramischen Ausgangszusammensetzung des piezokeramischen Werkstoffs und mit beidseitig an den Hauptflächen der Grünfolie angeordneten Elektrodenmaterialien;
• b) Wärmebehandeln der Grünfolie, wobei aus der piezokeramischen Ausgangszusammensetzung der piezokeramische Werkstoff des Piezoelements und aus des Elektrodenmaterialien die Elektrodeschichten des Piezoelements entstehen.

To achieve the object, a method for producing the piezoelectric component is also specified with the following method steps:

• a) providing at least one ceramic green sheet with a piezoceramic starting composition of the piezoceramic material and with electrode materials arranged on both sides on the main surfaces of the green sheet;
• b) heat treating the green sheet, wherein the piezoceramic material of the piezoelectric element is formed from the piezoceramic starting composition and the electrode layers of the piezoelectric element are formed from the electrode materials.

The piezoelectric component has a lead-free piezoceramic material. Lead-free means that very small, detectable impurities can be present on lead, for example in the pm range.

The fundamental finding of the invention lies in the fact that the electrical insulation resistance to the electrical insulation resistance of an undoped KNN ceramic can be improved by doping the lead-free CNN ceramic with silver. Moreover, such a material tends to absorb moisture from the environment to a much lesser extent compared to the undoped KNN ceramic. This leads to an additionally improved electrical insulation resistance.

At least one of the electrode layers has silver as electrode material. Preferably, however, both electrode layers have silver as the electrode material. In this case, pure silver or a silver alloy (eg palladium-silver) can be used.

The piezoceramic material of the piezoelectric element can consist of several phases. A major phase of the material has the stoichiometric composition. In this case, the main phase means that a proportion of this phase in the entire piezoceramic material is more than 50% by volume, preferably more than 75% by volume. For example, the proportion of the main phase is 90 vol.%.

Preferably, however, the piezoceramic material is single-phase. The material essentially has a single phase with the stoichiometric composition. Impurities in the range of a few vol.% Are possible. The fact that the piezoceramic material is single-phase results in a highly efficient piezoceramic material.

The piezoelectric component may have a single piezoelectric element. In particular, a multiplicity of piezo elements arranged one above the other to form a piezoelement stack is present. Preferably, the piezo elements are stacked flush. The piezo elements form a common, substantially planar stack side surface.

In the piezoelectric element stack, adjacent piezoelectric elements may have common electrode layers. Thus results in a piezoelectric component in multi-layer construction with alternately stacked piezoceramic layers and electrode layers.

In principle, any piezoelectric component can be equipped with the piezoceramic material. Preferably, the piezoelectric component with the piezoelectric element is selected from the group of piezoceramic bending transducers, piezoceramic multilayer actuator, piezoceramic transformer, piezoceramic motor and piezoceramic ultrasonic transducer. The piezoelectric element is for example part of a piezoelectric bending transducer. It is also conceivable that the piezoelectric element is part of 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 stacked 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.

Any starting materials can be used for the production of the piezoceramic material. Advantageously, to provide the green body, a mixing of powdered oxidic metal compounds of metals of the stoichiometric composition to form the piezoceramic starting composition is performed. Besides oxides of the metals, such as sodium oxide (Na ₂ O), potassium oxide (K ₂ O) or niobium trioxide (Nb ₂ O ₃ ), it is also possible to use precursors of the oxides of the metals, for example carbonates 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 processes, for example by the sol-gel, citrate, hydrothermal or oxalate process. In this case, oxidic metal compounds can be produced with only one kind of metal. It is also conceivable, in particular, that oxidic metal compounds are used with a plurality of types of metals (mixed oxides). A piezoceramic starting composition having at least one mixed oxide with at least two of the metals of the stoichiometric composition is used. According to a particular embodiment, silver niobate (AgNbO ₃ ) is used as the mixed oxide.

For providing mixed oxides, it is also possible to resort to the abovementioned precipitation reactions. Also conceivable is a mixed-oxide process. In this case, powdery oxides of the metals are mixed together and calcined at higher temperatures. Calcination results in mixed oxides.

The green body, for example a green sheet, may have at least one organic additive. The organic additive is, for example, a binder or a dispersant. Thus, such an additive is processed with the oxides of the metals to a slurry. The green film is produced from the slurry in a shaping process. The green sheet is made by the forming process "film drawing".

The electrode materials are applied, for example in the form of a silver-containing paste by means of a screen printing process on at least one of the main surfaces of the green sheet. In addition to debindering the green sheet, the heat treatment comprises a common sintering of the ceramic material used and of the electrode material used. In such a process called "cofiring", a monolithic (one-piece) piezoelectric element is created. With regard to a plurality of stacked piezoelectric elements, for example, ceramic green sheets are printed on one side with electrode material, stacked on top of one another, debinded and sintered. The result is a piezoelectric component in a monolithic multilayer construction. The electrode layers are referred to herein as internal electrodes.

To produce the piezoceramic material, use is preferably made of the so-called "templated grain growth process" (TGG) process. For example, texture nuclei with the stoichiometric composition of the main phase are introduced into the ceramic green sheet. During the heat treatment (sintering) epitaxial growth of crystallites with the stoichiometric composition takes place on the texture germs. The texture germs are crystallization nuclei and act as templates. They form a matrix by means of which crystallites of ceramics with the stoichiometric composition are grown in the course of the sintering process.

However, it is also possible to use texture germs which have a texture-germ composition deviating from the composition of the main phase. However, this creates an at least two-phase ceramic material.

• – Durch die Dotierung der bleifreien KNN-Keramik mit Silber wird der elektrische Isolationswiderstand gegenüber dem elektrischen Isolationswiderstand einer undotierten KNN-Keramik erhöht.
• – Im resultierenden Gefüge des piezokeramischen Werkstoffs des Piezoelements erhöhen sich im Vergleich zur undotierten KNN-Keramik eine Sinterdichte und eine mittlere Korngröße der entstehenden Keramik-Kristallite.
• – Es stellt sich ein im Vergleich zur undotierten KNN-Keramik verbessertes Alterungsverhalten ein. Piezoelektrische Kennwerte ändern sich im Verlauf der Zeit nicht oder nur geringfügig und bleiben im Wesentlichen gleich.
• – Ein piezoelektrisches Bauteil in Vielschichtbauweise weist sehr unterschiedliche Bereiche auf (piezoelektrisch aktive und inaktive Bereiche, eventuell Deckpakete oder gradierte Bereiche mit unterschiedlichen Elektrodenstrukturen, etc.) Bezüglich dieser unterschiedlichen Bereiche können prinzipiell unterschiedliche Verdichtungs- und Schwindungsverhalten der Piezokeramik beim Sintern auftreten. Durch die Kombination von Silber haltigem Elektrodenmaterial und Silber haltiger KNN-Keramik gelingt eine Anpassung des Schwindungs-Verhaltens der verschiedenen Bereiche aneinander.
• – Durch die Dotierung der KNN-Keramik mit Silber wird eine Eindiffusion von Silber aus den Innenelektroden in die KNN-Keramik während des Sinterns vermindert bzw. nahezu ganz unterdrückt. Dadurch wird die Gefahr einer Degradation der Innenelektroden verringert.

The following advantages are with the invention:

• - By doping the lead-free CNN ceramic with silver, the electrical insulation resistance to the electrical insulation resistance of an undoped KNN ceramic is increased.
• - In the resulting structure of the piezoceramic material of the piezoelectric element increase in comparison to the undoped KNN ceramic, a sintering density and an average grain size of the resulting ceramic crystallites.
• - There is an improved compared to the undoped KNN ceramic aging behavior. Piezoelectric characteristics do not change or only slightly over time and remain essentially the same.
• - A piezoelectric component in multilayer construction has very different areas (piezoelectrically active and inactive areas, possibly deck packages or graded areas with different electrode structures, etc.) With respect to these different areas in principle different compression and shrinkage behavior of the piezoceramic can occur during sintering. The combination of silver-containing electrode material and silver-containing KNN ceramics allows an adaptation of the shrinkage behavior of the different areas to one another.
• - By doping the KNN ceramic with silver, a diffusion of silver from the inner electrodes into the KNN ceramic during sintering is reduced or almost completely suppressed. This reduces the risk of degradation of the internal electrodes.

With reference to several embodiments 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.

1 shows a ceramic piezoelectric element in a lateral cross-section.

2 shows a piezoelectric component having a plurality of piezo elements in a lateral cross-section.

3 ₃₃ shows the d coefficients of the piezoceramic materials that were sintered at different sintering temperatures at a field strength of 2 kV / mm.

4 shows the loss angle tanδ of the embodiments.

5 shows the permittivity of the embodiments.

Given is a single-phase, lead-free piezoceramic material. The material has the following stoichiometric composition: K _x Na _y Li _z ) _1-a Ag _a ) _b (Nb _u Ta _v ) O ₃ . The material is a CNN ceramic doped with silver (Ag doping).

The embodiments include the following effective compositions: Ag doping [mol.%] x y z a u v Sintered density [g / cm ³ ] 0 .4462 .5238 0.0300 0 .8100 .1900 4.77 0.5 .4440 .5212 0.0299 0.0050 .8109 0.1891 4.81 1.0 0.4418 .5186 0.0287 0.0099 .8119 .1881 4.84 3.0 .4332 .5085 0.0291 0.0291 .8155 0.1845 4.84 5.0 .4250 .4989 0.0286 0.0476 .8190 .1810 4.91

The 0 mol% Ag doping is not covered by the invention and is for reference only. Sintering was carried out at a temperature of 1140 ° C, 1150 ° C or 1160 ° C. This resulted in each case a sintering density of about 4.8 g / cm ³ .

To characterize the dielectric properties of the piezoceramic materials, a sample in the form of a tablet of the piezoceramic material is produced. For this purpose, pulverulent, oxidic starting materials are pressed together to form a ceramic green body in the form of a tablet and sintered at temperatures of 1140 ° C, 1150 ° C and 1160 ° C, respectively. For silver, the silver niobate is used. On the main surfaces of each resulting tablet electrode layers of silver are applied, via which an electric field is coupled into the ceramic. The result is a piezoelectric element with electrode layers of silver and arranged therebetween piezoceramic layer with the respective piezoceramic material.

At the sintering temperatures of 1140 ° C. and 1150 ° C., the d ₃₃ coefficients of the piezoceramic materials are increased compared to the undoped KNN ceramics ( 3 ). In contrast, the d ₃₃ coefficients of the piezoceramic materials compared to the undoped KNN ceramic at a sintering temperature of 1160 ° C are the same or slightly lowered.

There are 1140 ° C and 1150 ° C loss angle tanδ from 2% to 3% at the sintering temperatures. On the other hand, the loss angles at the sintering temperature of 1160 ° C are increased.

In comparison to the permittivity of the undoped KNN ceramic, the permittivity of the piezoceramic materials with Ag doping, which was sintered at 1160 ° C., is consistently increased. Sintering at 1140 ° C or 1150 ° C results in permittivities comparable to the permittivity of the undoped and CNN ceramic sintered at the same sintering temperatures.

The electrical insulation resistance of the embodiments is increased compared to the undoped KNN ceramic.

Based on the described method for producing the samples is a piezoelectric component 1 made with the piezoceramic material. The piezoelectric component 1 is according to one first embodiment, a piezoelectric actuator 1 in monolithic multilayer construction ( 2 ). The piezo actuator 1 consists of a plurality of piezo elements arranged one above the other to form a stack 10 ( 1 ). Each of the piezo elements 10 has an electrode layer 11 , another electrode layer 12 and one between the electrode layers 11 and 12 arranged piezoceramic layer 13 on. The piezo elements adjacent in the stack 10 each have a common electrode layer. The electrode layers 11 and 12 consist of (nearly) pure silver. In an alternative embodiment, the electrode layers 11 and 12 an electrode material of a silver-palladium alloy in which palladium is contained in a proportion of 5% by weight.

The green sheets are dried, printed with a paste with the electrode material, stacked on top of one another, laminated, debinded and transferred to the piezoactuator 1 sintered under oxidizing sintering atmosphere (silver or silver-palladium alloy 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.

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

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 7101491 B2 [0004]

Claims (8)

Piezoelectric component ( 1 ) having at least one piezoelectric element which has an electrode layer ( 11 ) with electrode material, at least one further electrode layer ( 12 ) with a further electrode material and at least one between the electrode layers ( 11 . 12 ) arranged piezoceramic layer ( 13 ), wherein at least one of the electrode materials has elemental silver, the piezoceramic material has a stoichiometric composition [K _x Na _y Li _z ) _1-a Ag _a ] _b (Nb _u Ta _v ) O ₃ and the following relationships apply: - 0 ≤ x ≤ 0.75; - 0 ≤ y ≤ 0.75; - 0 ≤ z ≤ 0.07; - 0 ≤ a ≤ 0.5; - 0.95 ≤ b ≤ 1.05; - x + y + z + a = 1; - 0 ≤ u ≤ 1; - 0 ≤ v ≤ 0.4; - u + v = 1. A piezoelectric device according to claim 1, wherein the material has substantially a single phase. Piezoelectric component according to claim 1 or 2 with a plurality of piezo elements arranged one above the other to form a piezoelement stack. Piezoelectric component according to one of claims 1 to 3, wherein the piezoceramic component ( 1 ) is selected from the group piezoceramic bending transducer, piezoceramic multilayer actuator, piezoceramic transformer, piezoceramic motor and piezoceramic ultrasonic transducer. Method for producing a piezoceramic component ( 1 ) according to one of claims 1 to 4, comprising the following process steps: a) provision of at least one ceramic green sheet with a piezoceramic starting composition of the piezoceramic material and with electrode materials arranged on both sides on the main surfaces of the green sheet; b) heat treating the green body, wherein from the piezoceramic starting composition of the piezoceramic material of the component ( 1 ) and from the electrode materials the electrode layers are formed. The method of claim 5, wherein for providing the green body, mixing of powdery oxidic metal compounds of metals of the stoichiometric composition to form the piezoceramic starting composition is performed. The method of claim 6, wherein a piezoceramic starting composition having at least one compound oxide with at least two of the metals of the stoichiometric composition is used. A method according to claim 7, wherein silver niobate is used as the mixed oxide.

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