Alkali Metal Niobates for Piezoelectric Applications

Abstract

The present invention provides a niobate powder of general composition Li(Na / K)NbO3 for piezoelectric applications, with a BET surface area of ​​10-100 ppm / (m 2 The present invention further relates to a method for producing said niobate powder, and to its use for producing a piezoelectric material.

Description

[Technical Field]

[0001] The present invention provides a niobate powder of the general composition Li(Na / K)NbO3 for piezoelectric applications, with a BET surface area of ​​10-100 ppm / (m 2 / g)。 The present invention further relates to a method for producing said niobate powder, and its use for producing piezoelectric materials. [Background technology]

[0002] Piezoelectricity describes the change in electric polarization, and thus the generation of voltage, when a solid is elastically deformed. This phenomenon of generating a piezoelectric charge upon mechanical deformation is typically utilized in force, pressure, and acceleration sensors, which are used, for example, in medical, ultrasound, and automotive technology. Piezoelectric elements used in industry are often ceramics made from synthetic, inorganic, ferroelectric, and polycrystalline ceramic raw materials. Typical base materials are modified lead zirconate titanate (PZT) and lead magnesium niobate (PMN).

[0003] Alternative materials are needed as changes in legal regulations, in particular the entry into force of the RoHS (Restriction of Hazardous Substances) Directive, have severely restricted the legally permitted content of heavy metals in electrical and electronic components within the European Union. Potential lead-free replacements for conventional ceramics include lead-free ceramics, in particular those with the general composition {Li(Na / K)}Ta u Nb 1-u Alkali metal niobates have been identified in O3 (abbreviated as LNKTN for u>0 and LNKN for u=0) that have piezoelectric properties similar to lead-containing compounds.

[0004] However, these alternatives have the drawback that they tend to decompose partially at high air humidity and in water, which leads to the formation of electrically conductive compounds, making the ceramic unsuitable for piezoelectric applications. Therefore, much work has already been done in the prior art to improve the water resistance of such compounds.

[0005] In this regard, U.S. Patent Application Publication No. 2014 / 0339458 (US 2014 / 0339458) describes a piezoelectric ceramic whose main component is sodium potassium niobate and whose carbon content after sintering is 55-1240 ppm, particularly exhibiting excellent bending resistance. According to the example in U.S. Patent Application Publication No. 2014 / 0339458, the ceramic is produced by wet-milling alkaline earth metal carbonates with NbO, TaO, and ZrO, followed by sintering. The resulting powder is mixed with a PVA binder solution and 0.1-1.5% carbon powder, pressed, heat-treated at 300-700°C, and subsequently sintered at 1000-1250°C, where the addition of carbon adversely affects the sintered density.

[0006] Patent No. 5588771 (JP5588771) describes the composition Li x K y Na (1-x-y) Nb a Ta b Sb (1-a-b) O3, where x+y<1; 0≦x≦0.3; 0.1≦y≦0.7; 0.3≦a≦0.9, and 0≦b≦0.2, is disclosed. The material is coated with glass to enhance moisture resistance.

[0007] US Patent No. 10,193,054 (US 10,193,054) relates to a piezoceramic having an alkali metal niobate as the main component with 0.005 to 0.1 mol % Sn at the A site of the lattice and 0.005 to 0.1 mol % Zr at the B site of the lattice. 2+ Occupation of the A-sites of the lattice by ions has been concluded from EXAFS recordings, which, according to US Pat. No. 10,193,054, improves the resistance of the ceramic to air humidity.

[0008] JP2008 / 160045 describes a piezoelectric powder that is coated with a hydrophobic organic coating, thereby reducing its moisture sensitivity.

[0009] The alternatives proposed in the prior art have the drawback that the stabilization approach sometimes leads to very cumbersome manufacturing methods, and the resulting materials are in some cases very sensitive to oxidation, or require the introduction of foreign elements to achieve the desired stabilization. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] US Patent Application Publication No. 2014 / 0339458 [Patent Document 2] Patent No. 5588771 [Patent Document 3] U.S. Patent No. 10,193,054 [Patent Document 4] Japanese Patent Application Laid-Open No. 2008 / 160045 Summary of the Invention [Problem to be solved by the invention]

[0011] It is therefore an object of the present invention to provide a niobate for producing lead-free piezoelectric materials which overcomes the drawbacks of the prior art and which can be obtained by an industrially viable process. [Means for solving the problem]

[0012] Surprisingly, it has been found that this problem is solved by a niobate powder of the general composition Li(Na / K)NbO3, which is characterized in particular by the ratio of carbon to BET surface area.

[0013] Therefore, a first object of the present invention is a niobate powder of the general formula Li(Na / K)NbO3 for piezoelectric applications, wherein the niobate has a carbon content of 10 to 100 ppm / (m 2 / g) with respect to its BET surface area, where the BET surface area is in accordance with DIN ISO 9277, and the carbon content is determined by non-dispersive infrared absorption, and the ppm relates to the mass ratio.

[0014] The niobate powder according to the present invention exhibits a clearly reduced reactivity towards water and moisture compared to the hitherto known LNKN powder or LNKTN powder. Due to the improved stability, the niobate powder according to the present invention can further process piezoelectric materials even in processes based on humid indoor air and water, while hitherto known niobate powders of this type can only be worked under low humidity conditions and using organic solvents, which is usually associated with high costs, for example for explosion protection equipment.

[0015] In the context of the present invention, the BET surface area is understood as the specific surface area with respect to mass, as determined by regression to the BET model using gas adsorption, which is based on the description by S. Brunauer, P. H. Emmett and E. Teller in the Journal of the American Chemical Society, volume 60, No. 2, February 1938, pages 309 - 319.

[0016] In a preferred embodiment, the niobate powder according to the present invention has the following composition: {Li x (Na 1-y K y ) 1-x} 1+z Nb 1-u Ta u O3 [where 0.02 < x < 0.12; 0.4 < y < 0.6; -0.05 < z < 0.05 and 0 ≦ u ≦ 0.25].

[0017] Here, x represents the proportion of lithium, where 0.02 < x < 0.12, preferably 0.04 ≤ x ≤ 0.08. Within the scope of the present invention, by appropriately adjusting the lithium content, a higher dielectric constant can be achieved, and it has been shown that this has a beneficial effect on the piezoelectric properties of the subsequent material.

[0018] The contents of sodium and potassium in the composition are represented by 1 - y or y, where 0.4 < y < 0.6, preferably 0.43 ≤ y ≤ 0.53.

[0019] The niobate powder according to the present invention preferably exists in a perovskite crystal structure of the general formula ABO3. Here, the composition A 1+z In BO3, z represents the deviation from the stoichiometric composition of the element in the A site of the lattice, such as potassium, sodium, and lithium, with respect to the element in the B site of the lattice, such as niobium and tantalum. According to the present invention, preferably -0.05 < z < 0.05, particularly preferably 0 < z < 0.05 applies.

[0020] According to the composition of the niobate powder according to the present invention, u represents the proportion of tantalum, and preferably 0 ≤ u ≤ 0.25 applies. For specific applications, it may be desirable that the proportion of the B site of the lattice within the crystal lattice of the niobate powder is occupied by tantalum.

[0021] Without being bound by a particular theory, in the niobate powder according to the present invention, carbon is concentrated on the surface of the niobate powder, where it exists in the form of niobium carbonate oxide bonded to the surface of the niobate particles, and is not assumed to exist in the form of alkali metal carbonate as in the case of conventional niobate powder. The niobate powder according to the present invention is considered to be protected against decomposition by the action of water by the layer of niobium carbonate oxide.

[0022] In a preferred embodiment, the carbon content of the niobate powder according to the present invention with respect to its BET surface area is 30 - 90 ppm / (m 2 / g), advantageously 40 to 90 ppm / (m 2 / g), where the BET surface area and carbon content were determined as above.

[0023] The niobate powder according to the present invention preferably has a perovskite-type crystal structure. This is confirmed by the X-ray diffraction pattern of the niobate powder according to the present invention, in which the two peaks with the highest intensity, referred to herein as the main phase peaks, are preferably in the ranges of 21.5 to 23.2° 2θ and 30.5 to 33.1° 2θ (see FIG. 5). In addition to the main perovskite-type crystal phase, the niobate powder according to the present invention may have a secondary crystal phase, preferably a tungsten bronze-type crystal structure, whose peak with the highest intensity is located between the two main phase peaks. In a preferred embodiment, the proportion of the secondary phase in the niobate powder according to the present invention, expressed as a percentage of the absolute intensity ratio of the highest peak of the secondary phase between the two main phase peaks to the highest main phase peak in the X-ray diffraction pattern, is 8.5% or less, preferably 6.5% or less, particularly preferably 4.5% or less.

[0024] The niobate powder according to the invention is characterized in particular by its carbon content relative to its BET surface area. In a preferred embodiment, the niobate powder has a surface area of ​​2 to 8 m, as specified in accordance with DIN ISO 9277. 2 / g BET surface area.

[0025] The niobate powder according to the present invention is particularly intended as a replacement for conventional lead-containing piezoceramics.In order to enable unrestricted use even after new regulations come into force in the EU, the niobate powder according to the present invention is correspondingly lead-free.The lead content in the niobate powder according to the present invention is preferably less than 0.01% by weight, particularly preferably less than 0.001% by weight, based on the total weight of the niobate powder.

[0026] A scoping study of the present invention has resulted in niobate powders according to the present invention having improved stability to water over comparable niobate powders of the prior art.

[0027] Without being bound by any particular theory, it is postulated that the surface of the niobate powder is covered by a niobium oxide carbonate layer, which protects the underlying alkali metal niobate powder from decomposition by water.

[0028] Generally, the partial decomposition of any alkali metal niobate powder suspended in water is assumed to proceed on the particle surface according to the following reaction: ANbO3+xH2O→A 1-x H x NbO3+xA + OH - [Wherein A=Li, Na, K].

[0029] As the decomposition of the niobate powder progresses due to the generated ions, the conductivity and OH concentration of the suspension, and therefore the pH value, increase. Correspondingly, the conductivity and pH value of a given alkali metal niobate powder suspension, respectively relative to the BET surface area of ​​the niobate powder, can be used as a measure of the stability of the niobate powder. The slower the increase in conductivity and / or pH value, the more stable the niobate powder. Therefore, the niobate powder according to the present invention has a conductivity of 10 to 90 (μS / cm) / (m), expressed as the conductivity of a suspension of the niobate powder relative to its BET surface area (2 g of niobate powder per 100 ml of water, 25°C, 2 minutes of reaction time). 2 / g), advantageously 10 to 70 (μS / cm) / (m 2 / g), particularly preferably 10 to 40 (μS / cm) / (m 2 / g), where the BET surface area of ​​the niobate powder was determined according to DIN ISO 9277, while the conductivity was determined by measuring the conductivity of a suspension of the niobate powder after reaction with water at 25° C. for 2 minutes (2 g of niobate powder per 100 ml of water).

[0030] Furthermore, it has been found, unexpectedly, that the increase in the conductivity of the aqueous suspension of the niobate powder according to the invention occurs significantly slower than expected, which is taken as a further indication of its unexpected stability. Thus, it is believed that the niobate powder has a conductivity of 10-100 (μS / cm) / (m), expressed as the conductivity of the suspension of the niobate powder relative to its BET surface area (2 g of niobate powder per 100 ml of water, 25°C, 32 minutes of reaction time). 2 / g), advantageously 10 to 80 (μS / cm) / (m 2 / g), particularly preferably 10 to 50 (μS / cm) / (m 2 Preferred is an embodiment of the niobate powder according to the invention having a stability of 0.05 g / g, where the BET surface area of ​​the niobate powder was determined in accordance with DIN ISO 9277, while the conductivity was determined by measuring the conductivity of the suspension after reaction with water at 25° C. for 32 minutes (2 g niobate powder per 100 ml water).

[0031] The conductivity of an aqueous suspension of niobate powder according to the present invention (2 g of niobate powder per 100 ml of water) was found to be 10 μS / cm (m) at 25° C. during a reaction time of 30 minutes. 2 Particularly preferred are embodiments in which the saturation level increases by no more than 1 / g.

[0032] As a further measure of the stability of the niobate powder according to the invention, in accordance with the above considerations, the pH value of the aqueous suspension of the niobate powder and, therefore, the concentration of OH ions (OH concentration) can be used. Thus, it is possible to determine whether the niobate powder according to the invention has a BET surface area of ​​2.0×10, expressed as the OH concentration of an aqueous suspension of the niobate powder according to the invention (2 g of niobate powder, 25° C., 2 minutes of reaction time). -5 ~8×10 -5 (mol / l) / (m 2 / g) is preferred. The BET surface area was determined in accordance with DIN ISO 9277, while the OH concentration was determined from a pH measurement of the suspension (2 g of niobate powder per 100 ml of water) after 2 minutes of reaction at 25°C. Investigations have shown that the OH concentration also surprisingly increases only slightly after longer reaction times. It is therefore possible to determine that the niobate powder according to the invention has a BET surface area of ​​2.0 × 10 for its BET surface area, expressed as the OH concentration of an aqueous suspension of the niobate powder according to the invention (2 g of niobate powder, 25°C, 32 minutes of reaction time). -5 ~9×10 -5 (mol / l) / (m 2 / g), wherein the BET surface area is determined in accordance with DIN ISO 9277, while the OH concentration is calculated from a pH value measurement of a suspension (2 g of niobate powder per 100 ml of water) after reaction for 32 minutes at 25°C.

[0033] The niobate powder according to the invention advantageously has a particle size D50 of 0.3 to 1.5 μm, preferably 0.5 to 1.0 μm, determined by laser diffraction after 5 minutes of ultrasonic pretreatment according to ASTM B822.

[0034] Within the scope of the present invention, it has been surprisingly found that the advantageous properties of the niobate powder according to the invention arise in particular when the production of the niobate powder is carried out in a CO2-free atmosphere. Thus, the niobate powder according to the invention can be produced by the following steps: i) preparing an aqueous solution of salts of lithium, sodium and potassium, said salts being selected from the group consisting of oxides, hydroxides, peroxides, superoxides, nitrates and nitrites of the elements lithium, sodium and potassium, and mixtures thereof, said solution being produced under the exclusion of CO2; ii) preparing an aqueous suspension of a second starting material, said second starting material being selected from the group of oxides and oxide hydrates of niobium, said suspension being produced under the exclusion of CO2; iii) mixing the aqueous solution from step i) and the suspension from step ii) under exclusion of CO2 to obtain a mixed suspension; iv) drying the mixed suspension obtained in iii) under exclusion of CO2 to obtain granules; v) calcining the granules obtained in iv) under exclusion of CO2, vi) conditioning the surface of the calcined granules in the presence of CO2 Preferred are embodiments obtained by a process comprising:

[0035] In a preferred embodiment, the second starting material may further contain oxides and / or oxide hydrates of tantalum.

[0036] Typically, alkali metal niobate powders for use in lead-free piezoceramics are produced from carbon-containing precursors, such as alkali metal carbonates. Most process steps are carried out under room air, allowing unimpeded infiltration of CO2 contained in room air. The niobate powders thus obtained have a high carbon content and a high tendency to decompose in water. It is believed that the carbon is bound in the material in the form of alkali metal carbonates. However, these alkali metal carbonates are hygroscopic and water-soluble, which accelerates the decomposition of the alkali metal niobate powder and thus adversely affects its piezoelectric properties. In contrast, production in a CO2-free, or at least CO2-poor, atmosphere, and controlled tempering in the presence of CO2, is believed to result in the carbon being present on the surface of the niobate particles in the form of niobium oxide carbonate, thereby achieving the unexpected stability observed in the niobate powders of the present invention.

[0037] Within the scope of the present invention, it has surprisingly been found that a method for producing a niobate powder, in which the niobate powder is first produced carbon-free or at least carbon-poor, and in a subsequent process step is exposed to a CO2-containing gas stream under defined conditions, produces a niobate powder according to the invention which has surprisingly high water and humidity resistance. A further object of the present invention is therefore to provide a method for producing a niobate powder which comprises the following steps: i) preparing an aqueous solution of salts of lithium, sodium and potassium, said salts being selected from the group consisting of oxides, hydroxides, peroxides, superoxides, nitrates and nitrites of the elements lithium, sodium and potassium, and mixtures thereof, said solution being produced under the exclusion of CO2; ii) preparing an aqueous suspension of a second starting material, said second starting material being selected from the group of oxides and oxide hydrates of niobium, said suspension being produced under the exclusion of CO2; iii) mixing the aqueous solution from step i) and the suspension from step ii) under exclusion of CO2 to obtain a mixed suspension; iv) drying the mixed suspension obtained in step iii) under exclusion of CO2 to obtain granules; v) calcining the granules obtained in step iv) under exclusion of CO2, vi) conditioning the surface of the calcined granules in the presence of CO2 The method for producing the niobate powder according to the present invention comprises:

[0038] It has been found to be particularly advantageous to carry out process steps i) to v) in order to produce the niobate powder according to the invention under the exclusion of CO2, preferably in a CO2-free atmosphere. Corresponding techniques are known to and routinely used by those skilled in the art.

[0039] In a preferred embodiment, the second starting material may further contain oxides and / or oxide hydrates of tantalum.

[0040] The production of niobate powder according to the invention is carried out under controlled conditions. In a preferred embodiment, the calcined granules are tempered using an air stream mixed with CO2, with the proportion of CO2 added to the air stream being preferably 1 to 30% by volume, particularly preferably 5 to 20% by volume, based on the total volume of the air stream. Furthermore, tempering can be advantageously carried out in a calcination furnace, thereby eliminating the need for complicated transportation of the calcined granules. Therefore, an embodiment of the method according to the invention in which tempering in step vi) is carried out in a calcination furnace is preferred. Furthermore, the success of tempering can be influenced by the ambient relative air humidity. It has proven advantageous if the relative humidity of the air / CO2 mixture used for tempering is maintained within a specific range. Therefore, an embodiment in which tempering is carried out in an atmosphere in which the relative air humidity before the introduction of CO2 is, in particular, 40 to 60% at 20°C is preferred. Furthermore, a relatively narrow temperature range below 500°C has proven advantageous for carrying out tempering. Therefore, the tempering in step vi) of the process according to the invention is preferably carried out at temperatures between 200 and 400° C., preferably between 250 and 300° C. Surprisingly, it was found that temperatures below 200° C. increase the formation of undesired bicarbonates, while temperatures above the indicated values ​​make controlled production difficult due to the increased reactivity of the alkali metal niobate powder.

[0041] It has been found to be advantageous for the method according to the present invention if the starting material has as small a particle size as possible.Therefore, in a preferred embodiment of the method according to the present invention, the second starting material has a maximum primary particle size of less than 1.0 μm, preferably less than 0.5 μm, particularly preferably less than 0.3 μm, as determined by image analysis of SEM photographs.

[0042] Contrary to general expectations, it has been observed that in the scope of the method according to the present invention, powder particles have only a slight tendency to agglomerate.Therefore, in a preferred embodiment of the method according to the present invention, the solid content in the mixed suspension in step iii) has a particle size D50 of less than 2.0 μ m, preferably less than 1.5 μ m, particularly preferably less than 1.2 μ m, according to ASTM B822, using laser diffraction and without pre-treatment in an ultrasonic bath.In this case, the proportion D50 in particle size distribution indicates the proportion of particles with particle sizes above or below the indicated value.

[0043] It has proven particularly advantageous when producing the niobate powder according to the invention to carry out the drying in step iv) of the process under the exclusion of CO. Preferably, drying is carried out by means of static drying, spray drying, freeze drying or spray calcination.

[0044] It has been surprisingly shown that sintering of the granules can be carried out at significantly lower temperatures than is customary in the prior art. It has therefore proven advantageous to carry out the calcination in step v) of the process according to the invention at a temperature in the range of 500 to 1000° C. Preferred is therefore the embodiment in which the calcination in step v) of the process according to the invention is carried out at a temperature of 500 to 1000° C., advantageously 650 to 800° C., for a time period advantageously of 0.5 to 2 hours.

[0045] To allow the granules obtained after calcination to be tempered as desired, they are advantageously cooled after calcination, and therefore preferred is an embodiment in which calcination is followed by a cooling step, advantageously achieving a temperature in the granules of 200 to 400° C., preferably 250 to 300° C.

[0046] Due to its unexpected stability, the niobate powder according to the present invention is particularly suitable for producing piezoelectric materials.Therefore, a further object of the present invention is the use of the niobate powder according to the present invention for producing piezoelectric materials.These materials are preferably ceramic materials, composite materials and composite materials.

[0047] Another object of the present invention is a piezoelectric material, preferably a piezoelectric ceramic or piezoelectric composite, produced from the niobate powder according to the present invention. The ceramic according to the present invention is typically produced by applying a green molding technique, such as pressing, (screen) printing, or film drawing, to the niobate powder according to the present invention, using binders, solvents, rheological additives, and optionally sintering aids to obtain the desired size and shape for the function, and then sintering it into a ceramic, i.e., a polycrystalline aggregate of grains. In the case of multilayer actuators, green films of niobate powder and binder with a metal paste are alternately printed, stacked, cut, and then sintered together. After metallizing the outer surface, the sintered ceramic is subsequently poled in a high electric field to obtain its piezoelectric properties.

[0048] The piezoelectric material can be used to manufacture, for example, piezoelectric elements, such as multilayer actuators, bending transducers, ultrasonic sensors and ultrasonic transducers, as are used in medical technology, ultrasound technology and automotive technology.

[0049] The present invention will be explained in more detail using the following examples, which should not be understood as limiting the concept of the present invention. [Brief explanation of the drawings]

[0050] [Figure 1] 1 is a graph of the OH concentration of powder suspensions. [Figure 2] 1 is a graph of the conductivity of a powder suspension. [Figure 3] 1 is an SEM photograph of a niobate powder according to the present invention. [Figure 4] 1 is an SEM photograph of a niobate powder according to a comparative example. [Figure 5] 1 is an XRD spectrum of a niobate powder according to the present invention. [Example]

[0051] Composition (Li 0.07 (Na 0.50 K0.50 ) 0.93 ) 1.02 NbO3 (Examples 1-6 and 8-10), and composition (Li 0.07 (Na 0.50 K 0.50 ) 0.93 ) 1.02 Nb 0.80 Ta 0.20 Various niobate powders of O3 (Example 7) were prepared by the methods described below, where operation was carried out under CO2 exclusion unless otherwise stated. First, for Example 1, a mixture of an aqueous solution containing 21.62 g of lithium nitrate (LiNO3), 177.02 g of sodium nitrate (NaNO3), and 210.59 g of potassium nitrate (KNO3) with 1562 g of an aqueous suspension of niobium hydroxide (Nb(OH)5) (containing 26.12% by mass of Nb) was prepared. For Examples 2-6 and 8-10, the same molar ratios as in Example 1 were used. For Example 7, 20 mol% of the niobium hydroxide was replaced by tantalum hydroxide. The mixtures were each dried at 95 °C in a drying cabinet under reduced pressure and a CO2-free air atmosphere. The granules thus obtained were calcined in a calciner at 700-800°C for 1.5 hours and then cooled to 300 or 250°C in a stream of dry, CO2-free air. The products thus obtained were treated at 300 or 250°C for 40 minutes in a stream of air to which 5-15% by volume of CO2 had been added. The relative humidity of the air used for conditioning was 45%, specified at 20°C before the introduction of CO2. The respective reaction conditions are summarized in Table 1.

[0052] [Table 1]

[0053] Composition (Li 0.07 (Na 0.50 K 0.50 ) 0.93 ) 1.02The powder of Comparative Example 1 of NbO was prepared in a first step by ball milling 22.25 g of lithium carbonate (LiCO), 212.01 g of sodium carbonate (NaCO), and 276.46 g of potassium carbonate (KCO) with 1121.0 g of niobium oxide (NbO) in ethanol for 24 hours. For Comparative Examples 2 to 8, the same molar ratios as in Comparative Example 1 were set. The mixture so obtained was dried under room air without CO2 exclusion, homogenized, and processed by calcination at various temperatures ranging from 750 to 950 °C (see Table 2) under room air without CO2 exclusion. Conditioning of the air used for conditioning with 5% CO by volume in air (Comparative Example 7) or 15% CO by volume in air (Comparative Example 8) at 300°C for 40 minutes at a relative air humidity of 45% at 20°C before the addition of CO did not lead to an improvement in the water stability of these materials, and the conductivity and OH of aqueous suspensions of the comparative materials were significantly improved. - The concentration did not change significantly with conditioning in a CO2-containing atmosphere.

[0054] [Table 2]

[0055] The powders thus produced were analyzed by chemical analysis of major elements. Physical properties were evaluated by XRD, SEM particle size distribution, and BET surface area measurements according to DIN ISO 9277. Carbon content was determined by non-dispersive infrared absorption (NDIR) analysis, where the sample material was weighed into a biscuit-fired porcelain boat and then combusted in the presence of oxygen in a tube furnace. The analyzed gas, CO2, was detected by NDIR. The analysis was performed using a carbon-sulfur analyzer from Leco Instrumente GmbH. The water stability of the resulting niobate powders was determined by suspending 2 g of niobate powder in 100 ml of deionized water, stirring at 25 °C for 2 or 32 minutes, and measuring the conductivity and pH of the suspension, where the determined pH served as the basis for calculating the OH concentration. The results are summarized in Table 3.

[0056] [Table 3]

[0057] Figure 1 shows Calculated OH concentration after 2 or 32 minutes based on the corresponding pH value for the powder BET surface area of ​​the powder suspension (2 g powder in 100 ml water at 20°C) as a function of the carbon content per BET surface area of ​​the powder The graph shows:

[0058] Figure 2 shows Measured conductivity of powder suspensions (2 g of powder in 100 ml of water at 20°C) as a function of the carbon content per BET surface area of ​​the powder after 2 or 32 minutes. The graph shows:

[0059] As can be seen from Figures 1 and 2, powders having carbon content relative to BET surface area within the range of the present invention exhibit clearly improved stability to water, as expressed as suspension conductivity and OH concentration, over comparable powders produced by conventional methods.

[0060] 3 and 4 show SEM photographs of the niobate powder according to the present invention according to Example 8 (FIG. 3) or the niobate powder according to Comparative Example 6 (FIG. 4), respectively. A comparison of the two photographs clearly shows the difference in the surface morphology of the powders, which is believed to be due to the controlled refinement of the powders according to the present invention.

[0061] FIG. 5 shows the XRD spectrum of the niobate powder according to the present invention from Example 8, in which the proportion of the secondary phase (the intensity ratio of the highest peak of the secondary phase to the highest peak of the main phase in absolute value as a percentage) was 4.96%.

[0062] As the examples show, the method according to the present invention allows for the production of fine, largely crystallized, and only slightly agglomerated alkali metal niobate powders with high homogeneity under mild conditions, and their reactivity to water can be reduced by CO2 treatment. In this way, highly sinterable, fine alkali metal niobate powders with controlled carbon content are obtained, which have significantly reduced reactivity to water and (air) moisture, and therefore better storage stability, more stable processability during pressing and sintering, and offer the possibility of aqueous formulations for producing casting slips for producing multilayer actuators. Furthermore, the niobate powders according to the present invention exhibit very small primary particle sizes and significantly narrower primary particle size distributions than previously known powders.

Claims

1. General composition Li(Na / K)NbO for piezoelectric applications 3 and a carbon content of 10 to 100 ppm / (m) relative to its BET surface area. 2 / g), wherein the BET surface area is determined in accordance with DIN ISO 9277 and the carbon content is determined by non-dispersive infrared absorption; The niobate powder has the following composition: {Li x (Na 1-y K y ) 1-x} 1+z Nb 1-u Ta u O 3 wherein 0.02<x<0.12; 0.4<y<0.6; −0.05<z<0.05 and 0≦u≦0.

25. The niobate powder is characterized by having

2. The powder has a thickness of 2 to 8 m as specified in accordance with DIN ISO 9277. 2 2. The niobate powder of claim 1, characterized in that it has a BET surface area of ​​1 / g.

3. 3. The niobate powder according to claim 1, wherein the niobate powder is lead-free and the lead content in the niobate powder is less than 0.01% by mass with respect to the total mass of the niobate powder.

4. The niobate powder has a conductivity of 10 to 90 (μS / cm) / (m) relative to its BET surface area, expressed as the conductivity of a suspension of the niobate powder (2 g of niobate powder per 100 ml of water, reaction time of 2 minutes at 25°C). 2 4. The niobate powder according to claim 1, wherein the BET surface area is determined in accordance with DIN ISO 9277 and the conductivity is determined by measuring the conductivity of a suspension after a 2-minute reaction with water (2 g of niobate powder per 100 ml).

5. The niobate powder has a conductivity of 10 to 100 (μS / cm) / (m) relative to its BET surface area, expressed as the conductivity of a suspension of the niobate powder (2 g of niobate powder per 100 ml of water, reaction time of 32 minutes at 25°C). 2 5. The niobate powder according to claim 1, wherein the BET surface area is determined in accordance with DIN ISO 9277 and the conductivity is determined by measuring the conductivity of a suspension after reaction with water for 32 minutes (2 g of niobate powder per 100 ml).

6. The conductivity of an aqueous suspension of the niobate powder (2 g of niobate powder per 100 ml of water) is 10 (μS / cm) / (m 2 6. The niobate powder according to claim 1, wherein the niobate content increases by no more than 1 / g.

7. The niobate powder has a BET surface area of ​​2.0×10, expressed as an OH concentration of a suspension of the niobate powder (2 g niobate powder per 100 ml water, 25° C., 2 min). -5 ~8 x 10 -5 (mol / l) / (m 2 7. The niobate powder according to claim 1, characterized in that it has a stability of 0.05 wt. / g, wherein the BET surface area was determined in accordance with DIN ISO 9277, while the OH concentration was calculated from a pH value measurement of a suspension (2 g of niobate powder per 100 ml of water) after reaction for 2 minutes at 25°C.

8. 8. A method for producing a niobate powder according to any one of claims 1 to 7, comprising the steps of: i) providing an aqueous solution of salts of lithium, sodium and potassium, said salts being selected from the group consisting of oxides, hydroxides, peroxides, superoxides, nitrates and nitrites of the elements lithium, sodium and potassium, and mixtures thereof, and said solution being free of CO 2 the step of: ii) providing an aqueous suspension of a second starting material, said second starting material being selected from the group of niobium oxides and oxide hydrates, said suspension being 2 the step of: iii) The aqueous solution from step i) and the suspension from step ii) are mixed with CO 2 mixing under the exclusion of water to obtain a mixed suspension; iv) The mixed suspension obtained in iii) is added to CO 2 drying under the exclusion of v) The granules obtained in iv) are mixed with CO 2 firing under the exclusion of vi) The surface of the calcined granules is subjected to CO 2 conditioning in the presence of The method comprising:

9. The calcined granules are tempered by CO 2 and CO 2 added to the air stream. 2 9. The method according to claim 8, wherein the proportion of the air flow is 1 to 30% by volume relative to the total volume of the air flow.

10. 10. The method according to claim 8 or 9, characterized in that the calcination in step v) is carried out at a temperature of 500 to 1000°C.

11. 8. Use of the niobate powder according to any one of claims 1 to 7 for producing a piezoelectric ceramic.

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