DE102006008742B4 - Lead-free piezoceramic material with Erdalkalidotierung, method for producing a piezoceramic component with the material and use of the component - Google Patents

Abstract

Piezoceramic material, comprising
At least one perovskite phase having the empirical formula Ap (Nb _1-w Ta _w ) O ₃ , where Ap is at least one alkali metal of the perovskite phase and the following relationship applies:
0 ≤ w ≤ 0.15,
and
- At least one tungsten bronze phase having the empirical formula AwEw ₂ Nb ₅ O ₁₅ , wherein Aw is at least one alkali metal of the tungsten bronze phase and Ew is at least one alkaline earth metal of the tungsten bronze phase, wherein
The proportion of the tungsten bronze phase is selected on the solid of the piezoceramic material from the range of from 0.01% by volume up to and including 25.0% by volume.

Description

The The invention relates to a lead-free piezoceramic material Erdalkalidotierung, a method for producing a piezoceramic Component with the material and a use of the component.

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 areas 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 potassium niobate (KNN). The empirical formula of the piezoceramic material is (Li _0.04 K _0.44 Na _0.52 ) (Nb _0.86 Ta _0.1 Sb _0.04 ) 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).

From Y. Miyauchi et al., Piezoelectric Properties of NaNbO ₃ + MeNb ₂ O ₆ (Me = Ba, Sr, Ca) ceramics, Journal of the Japan Society of Powder and Powder Metallurgy, Vol. 50, No. 5 (2003) , Pages 341-346 and from SV Alchangyan et al., Phase Diagram of the System NaNbO ₃ -SrNb ₂ O ₆ , Inorganic Materials, Vol. 13, No. 9 (1977), pages 1331 to 1335 is each a biphasic, lead-free piezoceramic material with sodium niobate and a special tungsten bronze known.

From the EP 1 405 836 A1 is a lead-free piezoceramic material having a perovskite phase (Na _1-xyz K _x Li _y Ag _z Nb _1-w Ta _w O ₃ ) and a tungsten bronze phase (M (Nb _1-y Ta _y ) ₂ O ₆ where M an alkaline earth metal) is known.

task The invention is an alternative to the known, lead-free specify piezoceramic materials, the similar good piezoelectric Features.

To achieve the object, a piezoceramic material is specified, comprising at least one perovskite phase with the empirical formula Ap (Nb _1-w Ta _w ) O ₃ , where Ap is at least one alkali metal of the perovskite phase and the following relationship holds: 0 ≦ w ≦ 0.15, and at least one tungsten bronze phase having the empirical formula AwEw ₂ Nb ₅ O ₁₅ wherein Aw is at least one alkali metal of the tungsten bronze phase and Ew is at least one alkaline earth metal of the tungsten bronze, wherein a tungsten bronze portion of the tungsten bronze phase on the solid of the piezoceramic material is selected from the range of 0.01% by volume to 25.0% by volume.

Of the piezoceramic material includes a lead-free, at least two-phase System containing a perovskite phase based on an alkali niobate and a tungsten bronze phase based on an alkali-alkaline-earth niobate having.

It has surprisingly shown that the piezoceramic material then very good piezoelectric Properties shows, if he does not like it from the state of the art is known, single-phase (pure phase), but two- or multi-phase is. In addition to at least one perovskite phase at least one Tungsten bronze phase available. In addition, more can (fixed) phases may be present.

Either the perovskite phase as well as the tungsten bronze phase Alkali metal or more alkali metals. The two phases can do that have the same alkali metal or the same alkali metals. It is also grateful that the alkali metals of the two phases differ are. This means that Ap and Aw are unequal.

In a particular embodiment, the alkali metal Ap of the perovskite phase is selected from the group consisting of lithium, sodium and potassium. The following relationships apply here: Li _x K _1-xy Na _y , 0 ≦ x ≦ 0.15 and 0.25 ≦ y ≦ 0.75. The molecular formula of the perovskite phase is accordingly (Li _x K ₁ Na _y ) (Nb _1-w Ta _w ) O ₃ . The perovskite phase is based on the potassium-sodium niobate system. Besides potassium and sodium can be further Alka limetall Ap of the perovskite phase lithium may be present.

The alkaline earth metal Ew of the tungsten bronze phase is selected, in particular, from the group consisting of calcium, strontium and barium, where the following relationships apply: Ba _1-xy Sr _x Ca _y , 0 ≤ x ≤ 1 and 0 ≤ y ≤ 1. The empirical formula of the tungsten bronze Phase is therefore Aw (Ba _1-xy Sr _x Ca _y ) ₂ Nb ₅ O ₁₅ .

Particularly good piezoelectric properties can be achieved, for example, by combining the perovskite phase on the basis of the abovementioned potassium sodium nobate system with the tungsten bronze phase of the composition Na (Sr _0.925 Ca _0.075 ) ₂ Nb ₅ O ₁₅ . The alkali metal Aw of the tungsten bronze is sodium. The alkaline earth metals Ew of the tungsten bronze are calcium and strontium.

The Shares of the two phases can be very different. With regard to good piezoelectric properties It is particularly advantageous if the proportion of the tungsten bronze phase on the solid of the piezoceramic material from the range of including 0.05% by volume up to and including 15.0 vol.% Selected is. Preferably, the tungsten bronze content is up to and including 10 Vol.% Selected. For a material consisting only of the two specified phases, arises for a perovskite portion of the perovskite phase on the solid of the piezoceramic Material a value within the range of 99.99 Vol.% Inclusive until finally 75.0% by volume and in particular a value within the range of 99.95 inclusive Vol.% Up to and including 85 vol.% Or 90 vol.% Essential to the good piezoelectric Phases is the combination of these two phases. Next to the two Phases can but also other solid phases may be present.

Either the perovskite phase as well as the tungsten bronze phase can be further Have dopants. These further dopants are, for example Rare earth metals. As a rare earth metal can be any Element of the lanthanide or actinide group are used, for example europium, gadolinium, lanthanum, neodymium, praseodymium and samarium. Further, special dopants, for example antimony in the perovskite phase or tantalum in the tungsten bronze phase also conceivable.

In a particular embodiment is a molar lead content the piezoceramic composition below 0.1 mol .-% and in particular below 0.01 mol%. This means that the tungsten bronze phase and the piezoceramic phase contain essentially no lead ions. This also applies to possibly further phases contained in the piezoceramic material. Despite the absence of lead ions, especially despite the absence of lead ions Lead ions in the perovskite phase, become very good piezoelectric Achieved properties.

According to one second aspect of the invention is a method for producing a piezoceramic component with the piezoceramic material with the following method steps are given: a) Provision of a Green body with a piezoceramic starting composition of the piezoceramic material and b) heat treating of the green body, wherein from the piezoceramic starting composition of the piezoceramic Material of the component is created. The green body is a shaped body that for example, homogeneously mixed, co-compressed oxides the specified metals. Likewise, the green body can have organic additive that with the oxides of the metals to a slip is processed. The organic additive is for example a binder or a dispersant. From the slip is in one Forming process generates a green body. The green body is for example, a green sheet, which is produced by the forming process (film drawing). The green body produced with the piezoceramic during the molding process Starting composition is subjected to a heat treatment. The heat treatment of the green body calcining and / or sintering. It comes to education and to Compacting the forming piezoceramic material.

To provide the green body, according to a particular embodiment, a mixing of powdered, oxidic metal compounds of the metals alkali metal Ap the perovskite phase, alkali metal Aw tungsten bronze phase, alkaline earth metal Ew of the tungsten bronze phase, niobium and optionally tantalum is carried out to form the piezoceramic starting composition. In addition to oxides of metals, such as niobium oxide (Nb ₂ O ₅ ) and tantalum oxide (Ta ₂ O ₅ ), and precursors of the oxides of metals, such as carbonates (Li ₂ CO ₃ , K ₂ CO ₃ , BaCO ₃ , CaCO 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 by the sol-gel, citrate, hydrothermal or oxalate method. In this case, oxidic metal compounds can be produced with only one kind of metal. It is also conceivable that oxide metal compounds are used with several types of metals (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 of these are sodium niobate (NaNbO ₃ ), potassium niobate (KNbO ₃ ) or potassium tantalate (KTaO ₃ ). The oxide metal compound having at least two of the metals may also be the perovskite phase or the tungsten bronze phase itself. For providing these mixed oxides, use may also be made of the above-mentioned methods for producing the oxide metal compounds. 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 Workup of the metal oxides with the transfer into the piezoceramic Material can be done in different ways. For example, it is conceivable that first the powders of the oxidic metal compounds are homogeneously mixed. The piezoceramic starting composition is formed in the form a homogeneous mixture of metal oxides. Subsequently, will the piezoceramic starting composition by heat treatment, z. B. by calcination, transferred to the piezoceramic material. Of the piezoceramic material is crushed to fine piezoceramic powder. Subsequently is made from the fine piezoceramic powder in the molding process ceramic green body with an organic binder and other organic additives. This ceramic green body is debinded and sintered. The piezoceramic forms Component with the piezoceramic material.

alternative to the procedure described can the powders of the oxidic metal compounds homogeneously mixed and in the forming process to ceramic green body with organic binder are processed. Also this green body has the piezoceramic Starting composition. Subsequent sintering leads to piezoceramic component with the piezoceramic material.

According to one special embodiment is a piezoceramic component with at least a piezoelectric element which has an electrode layer with electrode material, at least one further electrode layer with a further electrode material and at least one disposed between the electrode layers Piezoceramic layer having the piezoceramic material. A single piezo element represents the smallest unit of piezoceramic Component is. For producing the piezoelectric element, for example a ceramic green sheet with the piezoceramic starting composition on both sides with printed on the electrode materials. The electrode materials can thereby be the same or different. By subsequent debinding and sintering results in the piezo element.

According to one special embodiment, a piezoelectric element is used, in which the electrode material and / or the further electrode material at least a selected from the group silver, copper and palladium elementary Metal. The piezoceramic material or the piezoelectric element is in particular by a common sintering of the piezoceramic Made of the starting composition and the electrode material (Cofiring). The electrode material can be made of the pure metals consist, for example, only of silver or only of copper. A Alloying of the metals mentioned is also possible, for example one Alloy of silver and palladium.

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

in principle can with the help of piezoceramic starting composition of each any piezoceramic component with the piezoceramic material getting produced. The piezoceramic component predominantly at least one piezoelectric element described above. Preferably is the piezoceramic component with the piezoelectric element from the group piezoceramic bending transducer, piezoceramic multilayer actuator, piezoceramic transformer, piezoceramic motor and piezoceramic ultrasonic transducer selected. The Piezo element is for example part of a piezoelectric Bender. By stacking a variety of single-sided or double-sided with electrode material printed green sheets, below Debinding and sintering creates a monolithic stack of piezo elements.

With suitable dimensioning and shape results by the method described above, a monolithic piezoceramic multilayer actuator. This piezoceramic multilayer actuator is preferably used to control a fuel injection valve of an internal combustion engine. Through the stack-shaped Arrangement of the piezoelectric elements is also accessible, with suitable dimensioning and shape, a piezoceramic ultrasonic transducer. The ultrasonic transducer is used for example in medical technology or for material testing.

• – Es ist ein piezokeramischer Werkstoff zugänglich, der kein bzw. nahezu kein Blei aufweist.
• – Der piezokeramische Werkstoff weist sehr gute piezoelektrische Eigenschaften auf. Dies betrifft beispielsweise eine relativ hohe Dehnung (d₃₃-Koeffizient) im Klein- und Großsignalbereich oder die Curietemperatur T_c, die bei über 200°C liegt.

In summary, the invention provides the following advantages:

• - It is a piezoceramic material accessible, which has no or almost no lead.
• - The piezoceramic material has very good piezoelectric properties. This applies, for example, a relatively high elongation (d ₃₃ coefficient) in the small and large signal range or the Curie temperature T _c , which is above 200 ° C.

Based several embodiments and the associated Figures, the invention is described in more detail below. The figures are schematic and do not represent true to scale illustrations represents.

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

2 shows a piezoceramic component with a plurality of piezo elements in a lateral cross-section.

The piezoceramic material of the exemplary embodiments can be obtained in each case as follows: First, a green body with a piezoceramic starting composition is provided. For this purpose, mixing of pulverulent, oxidic metal compounds (starting powder) is carried out. These oxidic metal compounds are Li ₂ CO ₃ , Na ₂ CO ₃ , K ₂ CO ₃ , BaCO ₃ , CaCO ₃ , SrCO ₃ , Nb ₂ O ₅ and Ta ₂ O ₅ . As oxide starting compounds, the mixed oxides NaNbO ₃ , KNbO ₃ and KTaO ₃ are used.

The starting powders are coated with respective proportions for the perovskite phase (Li _x K _1-xy Na _y ) (Nb _1-w Ta _w ) O ₃ with the perovskite portion P and for the tungsten bronze Na (Sr _0.925 Ca _0.075 ) ₂ Nb ₅ O ₁₅ mixed with the tungsten bronze portion W (Table 1). Further embodiments result from an alternative tungsten bronze phase NaBa ₂ Nb ₅ O ₁₅ (Table 2).

To Mixing is milled in hexane. The resulting, fine powder mixture becomes five hours at 750 ° C long calcined. The resulting from this heat treatment, piezoceramic composition is ground in water for 3 h and in a molding process to a green body in the form of a powder compact compressed with about 10 mm in diameter (isostatic pressing). This powder compact is heated at a temperature of 1050 ° C to 1200 ° C for one hour long sintered into a sample.

Of the Tungsten bronze share of tungsten bronze phase at piezoceramic solid Material is - measured at the weighing in - between 0.5 mol% to 5 mol%. The perovskite content of the perovskite phase is corresponding 95 mol% to 99.5 mol%.

The resulting sample is vapor-deposited on both sides with electrodes of silver and at about 100 ° C with an electric field poled of about 2.5 kV / mm.

The properties of the piezoceramic materials measured on the samples are shown in Tables 1 and 2. Where Eps v and Eps n are the relative dielectric constants before and after polarization, tan v and tan n are the loss angles before and after polarization, respectively, and the d ₃₃ coefficients. Table 1: W sintering conditions Eps v tan v Eps n tan n d33 [pm / V] d33 [pm / V] [Mol .-%] [%] [%] 1 kV / mm 2 kV / mm 0.5 2 h 750 ° C 1 h 1140 ° C + 0.15 g CO 3 710 3.53 630 5.20 235 233 1 484 3.53 491 5.45 144 150 1.5 459 3.60 479 6.43 232 236 2 514 5.76 508 7.36 221 206 2.5 697 4.66 594 4.42 265 269 3 842 3.96 789 4.05 258 293 4 845 3.88 814 3.93 194 242 5 927 3.85 871 4.04 224 246 Table 2: W sintering conditions Eps v tan v Eps n tan n d33 [pm / V] d33 [pm / V] [Mol .-%] [%] [%] 1 kV / mm 2 kV / mm 0.5 2 h 750 ° C 1 h 1150 ° C release agent 480 6.96 444 4.24 163 173 1 484 5.70 473 5.95 209 221 1.5 419 3.93 446 6.78 215 220 2 460 3.90 484 6.00 185 199 2.5 541 7.01 534 4.90 187 198 3 518 6.05 492 4.25 165 179 4 642 4.16 619 3.81 143 158 5 570 379 509 369 191 203

Based on the described method for producing the samples is a piezoceramic component 1 made with the piezoceramic material. The piezoceramic component 1 is a piezoelectric actuator according to a first embodiment 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 comprise an electrode material of a silver-palladium alloy in which palladium is contained in an amount 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.

For manufacturing the piezoelectric actuator 1 Green bodies are provided in the form of green sheets with the piezoceramic starting composition. 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. 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) or reducing sintering atmosphere (copper as electrode material).

Of the resulting monolithic piezoceramic multilayer actuator is to operate a fuel injection valve of an internal combustion engine of a Motor vehicle used.

Other, not shown embodiments such as piezoceramic bending transducer, piezoceramic transformer or piezoceramic ultrasonic transducer are using the new piezoceramic Zu also accessible.

Claims (12)

Piezoceramic material, comprising - at least one perovskite phase having the empirical formula Ap (Nb _1-w Ta _w ) O ₃ , wherein Ap is at least one alkali metal of the perovskite phase and has the following relationship: 0 ≤ w ≤ 0.15, and at least one tungsten bronze phase having the empirical formula AwEw ₂ Nb ₅ O ₁₅ , wherein Aw is at least one alkali metal of the tungsten bronze phase and Ew is at least one alkaline earth metal of the tungsten bronze phase, wherein - the proportion of the tungsten bronze phase is the solid of the piezoceramic Material is selected from the range of 0.01% by volume to 25.0% by volume. Piezoceramic material according to claim 1, wherein the alkali metal Ap of the perovskite phase is selected from the group consisting of lithium, sodium and potassium, the following relationships apply: Li _x K _1-xy Na _y , 0 ≤ x ≤ 0.15 and 0.25 ≤ y ≤ 0.75. Piezoceramic material according to claim 1 or 2, wherein the alkaline earth metal Ew is selected from the group calcium, strontium and barium, the following relationships apply: Ba _1-xy Sr _x Ca _y , 0 ≤ x ≤ 1 and 0 ≤ y ≤ 1. Piezoceramic material according to one of claims 1 to 3, wherein the proportion of the tungsten bronze phase on the solid of the piezoceramic Material from the range of 0.05% to 15.0 inclusive Vol.% Selected is. Piezoceramic material according to one of claims 1 to 4, wherein a molar lead content of the piezoceramic composition less than 0.1 mol%, and more preferably less than 0.01 mol%. Method for producing a piezoceramic component ( 1 ) with a piezoceramic material according to one of claims 1 to 5 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 ( 1 ) arises. The method of claim 6, wherein providing of the green body Mixing powdered, metallic oxide compounds of the metals alkali metal Ap of the perovskite phase, Alkali metal Aw of the tungsten bronze phase, alkaline earth metal Ew of the tungsten bronze phase, Niobium and optionally tantalum to form the piezoceramic starting composition carried out becomes. The method of claim 7, wherein a piezoceramic Starting composition with at least one mixed oxide with at least two of the metals is used. Method according to one of claims 6 to 8, wherein a piezoceramic component ( 1 ) with at least one piezo element ( 10 ), 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 Having) with the piezoceramic material. Method according to claim 9, wherein a piezoelement ( 10 ) is used, in which the Elektrodenma terial and / or the further electrode material have at least one selected from the group consisting of silver, copper and palladium elemental metal. Method according to one of claims 6 to 10, wherein the piezoceramic component ( 1 ) with the piezo element ( 10 ) is selected from the group piezoceramic bending transducer, piezoceramic multilayer actuator, piezoceramic transformer, piezoceramic motor and piezoceramic ultrasonic transducer. Use of a method according to the claim 11 produced piezoceramic multilayer actuator for driving a fuel injection valve of an internal combustion engine.