DE102011079818A1 - Producing sodium niobate texture target germs in a two stage molten salt synthesis comprises preparing crystalline intermediate product from molten salt, and performing topochemical conversion by adding sodium carbonate to the intermediate - Google Patents

Abstract

Producing sodium niobate texture target germs in a two stage molten salt synthesis comprises: (a) preparing a crystalline intermediate product from a molten salt, in which a number of perovskite crystal layers is separated by an intermediate layer of an another metal oxide; and (b) performing a topochemical conversion by adding sodium carbonate to the intermediate product, via the metal cations, which form intermediate layers in the intermediate products by displacing the sodium cations. Producing sodium niobate texture target germs in a two stage molten salt synthesis comprises: (a) preparing a crystalline intermediate product from a molten salt, in which a number of perovskite crystal layers is separated by an intermediate layer of an another metal oxide; and (b) performing a topochemical conversion by adding sodium carbonate to the intermediate product, via the metal cations, which form intermediate layers in the intermediate products by displacing the sodium cations, where the amount of sodium carbonate is added 1.1-20 times to the intermediate products during the topochemical conversion.

Description

The invention relates to lead-free, piezoceramic Textur↔ germs and a method for producing these texture nuclei on the 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 a potassium ↔ sodium 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.

High quality monocrystalline sodium niobate (NN) nuclei are currently produced by the Molten Salt Synthesis (MSS) process.

It is an object of the present invention to provide an optimized process for the production of high-quality, but preferably not necessarily, monocrystalline NN target germs, which are particularly suitable for the texturing of the 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 is prepared, in which a number of layers of perovskite separated by an interlayer of another metal oxide, is present, and then in a second process step, a topochemical conversion (TMC) takes place, is added to the intermediate sodium carbonate, through which the metal cations, the defined in the intermediate places in the Occupied by the sodium cations, wherein 1.1 to 20 times the stoichiometrically required amount of sodium carbonate is added to the intermediate during the topochemical conversion.

The two-stage MSS anisometric particles is a universal process for the production of tabular texture. Germs. An anisometric intermediate with bismuth layer or Ruddlesden-Popper structure is first prepared in an alkali and / or 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.

In the second process step of the MSS, the topochemical conversion (TMC) of the crystalline, in particular even tabular, ie platelet-shaped intermediates to the NN-texture target germs takes place, again in a molten salt melt, whereby morphology preservation converts the layer structure 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.

The reactions of the second process step of the MSS ® texture-seed production method can theoretically be explained by the following simple reaction equation: 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

The CO ₂ evaporates and thus the equilibrium reaction is shifted to the right as desired. According to the invention, sodium carbonate (Na ₂ CO ₃ ) is added in strong excess, whereby the balance of the reaction is shifted even further to the right, so the yield of NN target germs is further increased.

The second process step is preferably carried out in cylindrical corundum ceramic vessels, for example those having a capacity of 200-2000 ml.

The excess of sodium carbonate is for example twice the stoichiometric amount, ie 6 moles of Na ₂ CO ₃ per 4 moles of intermediate from the first step of the MSS, but the excess can be within limits of 1.1 or 1.5 times the stoichiometric value to 10 times or 20 times the stoichiometric value. Preferably, the excess will be between 2 to 6 times the stoichiometric value, more preferably between 3 to 4 times.

With regard to the texturing of KNN-based materials, NaNbO ₃ nuclei in particular have proved to be promising. Here, too, the synthesis via the MSS↔ method being used as the starting point, for example, the layer Bismut↔ compounds Bi ₂ O ₂ [Bi _0.5 Na _1.5 Nb _m-m O _{3m + 1]} (abbreviated BNN _m) can. In the conversion step, the second process 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.

By the addition according to the invention of sodium carbonate in the strong excess, for example factor 2 to 20, for example, the conversion to the target product is favored. Moreover, it is possible to reduce the corrosive side reaction of the liberated metal oxide, such as bismuth oxide with ceramic crucible materials, for example Al ₂ O ₃ (corundum) and / or MgO (magnesium oxide) so far that only a very small or none Aging / Degradation of the crucible is more recognizable. Thus, the platinum crucibles used because of the above-mentioned corrosive side effect can be saved in the production and replaced by the inexpensive ceramic crucible materials corundum and magnesium oxide. This dramatically increases the economics of the NN® texture seed production process.

Apart from that, the service lives of the crucibles are increased and / or the reactions taking place in the course of the second process step can be better controlled because the competition reactions with the crucible material are eliminated.

Here, m indicates the number of perovskite layers between the adjacent or confining 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")

Strontium and / or barium, for example, are under discussion as alternative metal oxides which could take over the functionality of the "adjacent layers". 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.

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

The green body is a shaped body which has the starting materials in addition to the texture. 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, to prepare and bring together the starting materials, a mixing of powdered, oxidic metal compounds of the required metals of the main and the Neben↔ phase is performed. 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. 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. By stacking a plurality of green films printed on one or both sides with electrodes, subsequent debinding and sintering, a monolithic stack of piezo elements is produced. 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, which can occur in addition to texture germs with inclusions and phase-pure texture↔ germs.

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 009 461.7 the content of which is hereby also to be added to the disclosure content of the present application.

The present invention optimizes the NN texture seed production process for texturing lead-free piezoceramic materials. In this case, in the two-stage MSS process as starting materials for the second process step, the intermediate products of the first process step are not reacted stoichiometrically with sodium carbonate, but sodium carbonate is added in excess.

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]
• DE 102010009461 [0037]

Claims (7)

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 a molten salt, a crystalline intermediate is produced, in which a number of crystal layers perovskite by an intermediate layer of a separated from other metal oxides, and then in a second process step a topochemical conversion (TMC) takes place in which sodium carbonate is added to the intermediate product, by which the metal cations which form intermediate layers in the intermediate product are displaced by the sodium cations, from 1.1 times to 20 times the stoichiometrically required amount of sodium carbonate added to the intermediate during the topochemical conversion. The method of claim 1, wherein 3 to 4 times the stoichiometrically required amount of sodium carbonate in the second process step, the topochemical conversion, is added. The method 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 starting material for the second process step, wherein m varies in the range of 1 to 20 and "Me" stands for strontium, barium and / or bismuth. Method according to one of claims 1 to 3, wherein the second step of the topochemical conversion takes place in a cylindrical corundum ceramic vessel. The method of claim 4, wherein the cylindrical corundum ceramic vessel is selected to hold a volume between 200ml and 2000ml. A method according to claim 4 or 5, wherein the cylindrical corundum ceramic vessel is covered over the entire surface during the reaction with a corundum plate. Method according to one of the preceding claims, wherein the amount of molten salt used is such that as a result, an amount between 50 and 500 g of seed material is produced.