DE102011078225B4 - Method of producing texture seeds for a piezoceramic material from Molten Salt Synthesis (MSS) - Google Patents

Abstract

Process for the preparation of sodium niobate (NN) texture target germs in a two-stage Molten Salt Synthesis (MSS), wherein in a first process step a molten salt intermediate is made having a layered structure comprising a number of layers of metal NaNbO3 -Keimvorstufen is separated by an interposition of another metal oxide, and then in a second process step, a topochemical conversion (TMC), by which the NN-texture target germs are produced, wherein for topochemical conversion as starting material either metal NN seed precursors with varying Number of layers of NaNbO3 seed precursors between the layers of the other metal oxide, so-called non-phase pure metal NN seed precursors, and / or a mixture of phase-pure metal NN seed precursors are used, only by the number of Perowskitlagen between differentiates between the liners.

Description

The invention relates to lead-free, piezoceramic texture nuclei and to a method for producing these texture nuclei via Molten Salt Synthesis.

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 piezoelectric components described above. 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). In particular, textured lead-free CNN piezoceramics have performance data comparable with respect to PZT. The texturing can be achieved by introducing monocrystalline NN nuclei into a polycrystalline KNN matrix with subsequent thermal processing. The production of high-quality NN seeds is a necessary prerequisite for a successful texturing of the KNN ceramics.

Out DE 10 2008 042 955 A1 For example, a method is known in which a layered perovskite-based compound includes a bismuth layer type perovskite-based compound.

High-quality monocrystalline sodium niobate nuclei are currently produced by the Molten-Salt Synthesis process, whereby the objective of the process is usually as an intermediate, the formation of as phase-pure BNN5 seed precursors.

The object of the present invention is to provide an optimized process for the production of high-grade monocrystalline NN target germs, which are particularly suitable for the texturing of KNN piezoceramics.

A solution to the problem and object of the present invention is therefore a process for the preparation of sodium niobate (NN) texture target germs in a two-stage Molten Salt Synthesis (MSS), wherein in a first process step from an molten salt, an intermediate product having a layer structure is produced in which a number of layers of perovskite are separated by an interlayer of another metal oxide, and then in a second process step a topochemical conversion (TMC) is carried out by which the NN-texture target germs can be produced, with either not being used for topochemical conversion as starting material phase-pure metal NN seed precursors with different numbers of internal perovskite layers and / or a mixture of phase-pure metal NN seed precursors is used, which differs only by the number of Perowskitlagen between the intermediate layers.

By the simultaneous presence of different (intragranular and / or separate) BNN _m phases (each with inherently different relative contents of the other metal oxide, such as bismuth) in the starting material, the amount of other metal oxide, such as bismuth, in the final NN-texture target germs (after the corresponding transformation in the context of the second process step, the TMC process) can be manipulated in a targeted manner. Moreover, a suitable choice of the intragranular layer sequence (for example by core-shell structures) makes it possible to increase the reactivity and rate as well as to facilitate the defined separation / leaching of the by-produced metal oxide, for example the bismuth oxide.

The two-stage MSS anisometric particles is a universal method for the production of tabular texture germs. An anisometric intermediate with, for example, bismuth layer or Ruddlesden popper structure is first prepared in an alkali / alkaline earth molten salt melt , Both last-mentioned types of connection are distinguished by the fact that they consist of several stacked perovskite layers, which are delimited from one another by intermediate layers of different construction. Due to a strong anisotropy in surface energies established herein, these materials preferentially crystallize in a platelike habit.

According to the invention, first several intermediates are produced, which differ only by the number of stacked Perowskitlagen. The formation of various intermediates, such as BNN ₂ , BNN ₄ , BNN ₅ depending on the temperature and hold time, is likely to affect the amount of Bi ₂ O _{3 provided} .

In the second step of the MSS, then again in a molten salt, the topochemical conversion (TMC) of the tabular intermediates to the NN target germs, wherein morphology preservation, the layer structure is converted into a pure perovskite lattice. Based on the layer compounds Sr ₃ Ti ₂ O ₇ , Na _0.5 Bi _4.5 Ti ₄ O ₁₅ and Ba ₆ Ti ₁₇ O ₄₀ , SrTiO ₃ , Bi _0.5 Na _0.5 TiO ₃ and BaTiO ₃ could be successfully synthesized by MSS Template prepared and used for texturing piezoceramics.

With regard to the texturing of KNN-based materials, NaNbO ₃ nuclei in particular have proved to be promising. Here, too, the synthesis takes place via the MSS method, the starting point being, for example, the bismuth layer compounds Bi ₂ O ₂ [Bi _0.5 Na _m-1.5 NbmO _{3m + 1} ] (abbreviated to BNN _m ). In the conversion step, an exchange of the Bi ³⁺ ions then takes place against Na ⁺ ions and an associated rearrangement of the bismuth layer structure into the NaNbO ₃ perovskite lattice.

In this case, m indicates the number of perovskite layers between the adjacent or delimiting or delimiting layers of the other metal oxide, which may be, for example, Bi ₂ O ₂ ²⁺ . According to the invention, m is in the reaction (here with Bi ₂ O ₂ ²⁺ as "other metal oxide") 4Bi ₂ O ₂ [Bi _0.5 Na _m-1.5 Nb _m O _{3m + 1} ] + 3Na ₂ CO ₃ -> 4mNaNbO ₃ + 5Bi ₂ O ₃ + 3CO ₂ not constant, but different starting compounds 4Bi ₂ O ₂ [Bi _0.5 Na _m-1.5 Nb _m O _{3m + 1} ], which differ only in the value of m from each other, used according to the invention.

"M" can take values between 1 and 20, values between 1 and 7 being preferred, values between 2 and 5 being particularly preferred.

According to one embodiment of the process, a compound Me ₂ O ₂ [Me _0.5 Na _m-1.5 NbmO _{3m + 1} ] is used as starting material for the topochemical conversion, in which "m" varies in the range from 1 to 20 and " Me "stands for titanium, strontium, barium, antimony, arsenic, rare earth and / or bismuth.

As alternative metal oxides as "adjacent layers", for example, other 3-valent cations such as Sb, As or rare earths could be used. For titanates, for example, strontium and barium would be considered as alternatives.

To produce the piezoceramic, the texture nuclei are introduced into the starting materials and aligned there. Due to the shape anisotropy, an alignment of the texture nuclei in the green body is possible.

The green body is a shaped body which has the starting materials in addition to the texture germs. These starting materials are, for example, powdered oxides of the metals used. The powdery oxides and the texture nuclei are homogeneously, ie uniformly mixed. Subsequently, the texture germs are aligned. Pressing the resulting mixture results in the green body.

It is also conceivable that the green body has at least one organic additive. The organic additive is, for example, a binder or a dispersant. Thus, such an additive with the oxides of the metals or mixed oxides of the metals is processed into a slurry. The green body is produced from the slurry in a shaping process. The texture germs are aligned. The green body is therefore preferably a green sheet. The green sheet is made by the forming process "film drawing".

The green body having the aligned texture nuclei and the starting materials prepared in the molding process is subjected to a heat treatment. The heat treatment of the green body optionally includes debinding, calcination and sintering. It comes to the formation and compression of the piezoceramic material. The TGG process takes place here. An initial epitaxial growth of the piezoceramic material takes place on the texture germs. There is an oriented growth of at least one of the phases, preferably the main phase. The result is a multi-phase piezoceramic material with texturing.

According to a particular embodiment of the invention, the texture germs contain bismuth-containing inclusions.

According to a further embodiment of the invention, the inclusions are intragranular.

The proportion of inclusions is preferably between 3 and 8% by weight, in particular between 4 and 7% by weight and particularly preferably between 4.5 and 6.5% by weight.

According to a particular embodiment, a proportion of the texture seed phase on the piezoceramic material is selected from the range of 0.01% by volume to 35% by volume. The proportion of texture nuclei on the piezoceramic material can be chosen very differently. It plays a role, for example, whether the texture germs themselves consist of piezoceramic material. An efficiency with which the texturing of the piezoceramic material can be achieved also enters into the proportion of texture nuclei on the piezoceramic material.

Both with regard to a processing, for example as part of a film-drawing process, as well as with regard to the piezoelectric properties of the resulting piezoceramic material, it is particularly advantageous if the texture nuclei a platelet-shaped morphology with an edge length in the range of 1 microns to 50 microns and a height of 0.1 microns to 5 microns. For example, the texture nuclei have a mean edge length of about 20 microns and an average height of about 2 microns. With such small dimensions, it is ensured that the texture nuclei are characterized by a relatively large "reactive" surface on which epitaxial growth of the major or minor phase can occur. In addition, the small dimensions make it possible to produce relatively thin green sheets. Such small texture germs also have the advantage that a volume fraction of the texture germs can be kept small.

Any starting materials can be used for the production of the piezoceramic material. Advantageously, for the preparation and bringing together of the starting materials, a mixing of pulverulent, oxidic metal compounds of the required metals of the main and secondary phase is carried out. Besides oxides of the metals, such as sodium oxide (Na ₂ O), potassium oxide (K ₂ O) or niobium pentoxide (Nb ₂ O ₅ ), 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 with several types of metals are used (mixed oxides). To provide these mixed oxides can also be resorted to the above-mentioned 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 sintering can be carried out both in reducing and in oxidizing sintering atmosphere. There is almost no oxygen in a reducing sintering atmosphere. 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 can be used as the electrode material.

In principle, any piezoceramic component can be equipped with the piezoceramic material. 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. Stacking a large number of green films printed on one or both sides with electrode material, subsequent debinding and sintering results in a monolithic stack of piezo elements. With suitable dimensioning and shape results in a monolithic piezoceramic multilayer actuator. This piezoceramic multilayer actuator is used for example for controlling 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.

According to a further advantageous embodiment, there are several varieties of anisometric texture germs, wherein in addition to texture germs with inclusions and phase-pure texture germs can occur.

Preferably, the texture nuclei are crystalline, particularly preferably monocrystalline.

The piezoceramic material is lead-free. Lead-free means that very small, detectable impurities can be present on lead, for example in the ppm range.

By influencing the microstructure development (that is to say, in particular the crystallographic orientation of the grains) of conventionally prepared piezoceramics by introducing a texture, utilization of the single-crystal power potential can be achieved while maintaining the established and cost-optimized process technology.

Incidentally, based on the basic patent application on the subject DE 10 2010 009 461 A1 the content of which is hereby also to be added to the disclosure content of the present application.

The present invention optimizes the texture seed preparation process for texturing lead-free piezoceramic materials. In this case, in the two-stage MSS process as starting materials for the second process step, not just the intermediates of the first process step are used, but precisely selected seed precursors having a certain number of perovskite interlayers between other metal oxide layers.

Claims (3)

Process for the preparation of sodium niobate (NN) texture target germs in a two-stage Molten Salt Synthesis (MSS), wherein in a first process step a molten salt intermediate is produced having a layered structure comprising a number of layers of metal NaNbO ₃ -Keimvorstufen by an intermediate layer of another metal oxide is present separately, and then in a second process step, a topochemical conversion (TMC), by which the NN-texture target germs can be produced, wherein for topochemical conversion as starting material either metal NN seed precursors varying number of layers of NaNbO ₃ -Keimvorstufen between the layers of the other metal oxide, so-called non-phase-pure metal NN seed precursors, and / or a mixture of phase-pure metal NN seed precursors are used, which only by the number Perowskitlagen distinguishes between the liners. The process according to claim 1, wherein non-phase-pure bismuth layer compounds Bi ₂ O ₂ [Bi _0.5 Na _m-1.5 Nb _m O _{3m + 1} ] (abbreviated to BNN _m ) seed precursors are used as starting material for the topochemical conversion which have a variation in the number of metal-NaNbO ₃ seed precursor liners expressed by the index "m" of 1 to 20. The process of claim 1, wherein a compound Me ₂ O ₂ [Me _0.5 Na _m-1.5 Nb _m O _{3m + 1} ] is used as a starting material for the topochemical conversion in which m varies in the range of 1 to 20 and "Me" for antimony, arsenic, rare earths, strontium, barium and / or bismuth, in each case by approximation of the oxides contained in the empirical formula.