JP2007261847A - Method for producing bismuth niobate-based fine particles - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing bismuth niobate-based fine particles easy to produce, excellent in uniformity of composition and particle diameter, and having particularly high crystallinity. <P>SOLUTION: The method for producing the bismuth niobate-based fine particles includes: a step of obtaining a melt comprising, by mol on an oxide basis, 25-60% MO (M is one or more selected from the group consisting of Mg, Ca, Sr and Ba), 3-25% Bi<SB>2</SB>O<SB>3</SB>, 2-25% Nb<SB>2</SB>O<SB>5</SB>and 15-60% B<SB>2</SB>O<SB>3</SB>; a step of rapidly cooling the melt to form an amorphous material; a step of depositing bismuth niobate-based crystals from the amorphous material; and a step of separating the bismuth niobate-based crystals from the obtained deposition, in this order. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing bismuth niobate fine particles, and more particularly to a method for producing bismuth niobate fine particles that are easy to produce, excellent in composition and particle size uniformity, and particularly preferably have high crystallinity.

  Ferroelectric materials have a variety of excellent properties such as insulation, ferroelectricity, piezoelectricity, pyroelectricity, and electro-optic effect, so electronic parts such as capacitors, ferroelectric memories, filters, and vibrators It is widely used as a constituent material.

  As ferroelectric materials, perovskite type compounds such as lead zirconate titanate ferroelectrics (PZT, PZTN) have been known so far. Among these, PZT is widely used as a constituent material of a nonvolatile ferroelectric memory because it has a large spontaneous polarization value. However, PZT has a problem of low resistance to spontaneous polarization characteristics due to polarization reversal (voltage application) cycle, so-called fatigue resistance.

Examples of ferroelectric materials having excellent fatigue resistance include Bi ₄ Ti ₃ O ₁₂ , (La _x Bi _1-x ) ₄ Ti ₃ O ₁₂ [0 ≦ x ≦ 0.5], and SrBi ₂ Ta ₂ O ₉ . A representative bismuth-based layered perovskite ferroelectric (BLSF) is known. This BLSF has a layered perovskite structure, and a pseudo perovskite structure layer (A _n-1 B _n O _3.5n-0.5 ) and another crystal structure insertion layer (Bi ₂ O ₂ ) Since it has a crystal structure in which the layers are alternately laminated at a constant period, there is an advantage that it is very excellent in fatigue resistance.

  Bismuth niobate-based ferroelectric, which is one of the ferroelectric oxides, is an environmentally friendly substance that does not contain lead, and has excellent properties such as high remanent polarization, as well as the above BLSF. As a layered perovskite-type structure, it is excellent in fatigue resistance, is not easily deteriorated even when thinned, and is a next-generation ferroelectric memory material and oscillator material that has a small temperature change (frequency temperature change) of the resonance frequency. It is expected (Non-Patent Document 1).

  In recent years, many attempts have been made to obtain a ferroelectric thin film cheaply and easily by a chemical solution film-forming method (solution method) (see Patent Documents 1 to 3). Therefore, the crystallinity is high and the particle size is small. There is also a need to provide a method for producing bismuth niobate fine particles that are excellent in particle size uniformity and suitable for application to a solution method.

US Pat. No. 5,468,679 US Pat. No. 5,519,234 JP 2003-192431 A (Claims) Ceramics (Vol.40, 627-630, 2005)

  The present invention relates to a method for producing bismuth niobate fine particles, and provides a method for producing bismuth niobate fine particles that are easy to produce, excellent in composition and particle size uniformity, and particularly preferably have high crystallinity. Objective.

The present invention is expressed in mol% on the basis of oxide, and MO (M = Mg, Ca, Sr and Ba selected from the group consisting of 25 to 60%), Bi ₂ O ₃ is ₃ to 25%, A step of obtaining a melt containing ₂ to 25% of Nb ₂ O ₅ and 15 to 60% of B ₂ O ₃ , a step of rapidly cooling the melt to an amorphous material, Provided is a method for producing bismuth niobate fine particles, comprising a step of precipitating a bismuth niobate crystal and a step of separating the bismuth niobate crystal from the obtained precipitate in this order.

  According to the present invention, bismuth niobate-based fine particles having a small crystallite size, high crystallinity, and excellent uniformity in composition and particle size can be produced. Since the fine particles are fine particles with excellent ferroelectricity, piezoelectricity, pyroelectricity, electro-optic effect, etc., they are useful as a constituent material for obtaining dielectric elements and ferroelectric thin films for piezoelectric elements by the solution method. It is.

  In the method for producing bismuth niobate-based fine particles of the present invention, the melt contains an M (M = one or more selected from the group consisting of Mg, Ca, Sr, and Ba) source, Bi source, Nb source, and B source. It is preferable to obtain the mixture by melting.

First, as the M source, it is preferable to use one or more selected from the group consisting of M oxide (MO) and carbonate (MCO ₃ ). Further, 1 selected from the group consisting of M nitrate (M (NO ₃ ) ₂ ), chloride (MCl ₂ · mH ₂ O), sulfate (MSO ₄ · mH ₂ O) and each fluoride (MF ₂ ). More than one species may be used. Furthermore, for the purpose of lowering the melting temperature or facilitating vitrification by rapid cooling described later, a part of MO may be substituted with ZnO. Among these, when M = Sr, bismuth niobate fine particles having excellent frequency temperature characteristics can be obtained, which is preferable.

Next, as the Bi source, one or more selected from the group consisting of bismuth oxide (Bi ₂ O ₃ ), bismuth carbonate (Bi ₂ CO ₃ ) ₃ ) and bismuth hydroxide (Bi (OH) ₃ ) should be used. Is preferred. 1 selected from the group consisting of bismuth nitrate (Bi (NO ₃ ) ₃ .mH ₂ O), bismuth chloride (BiCl ₃ ), bismuth sulfate (Bi ₂ (SO ₄ ) ₃ ) and bismuth fluoride (BiF ₃ ). More than one species may be used (the above bismuth compounds also include their respective oxy salts, such as (BiO) ₂ CO ₃ , where m is the hydration number and m = 0 anhydride Also included).

In addition, niobium oxide (Nb ₂ O ₅ or Nb ₂ O ₃ ) is preferably used as the Nb source. Alternatively, niobium chloride (NbCl ₅ ) or niobium fluoride (NbF ₅ ) may be used.

Furthermore, boron oxide (B ₂ O ₃ ) or boric acid (H ₃ BO ₃ ) is preferably used as the B source, but M borate, bismuth borate, or niobium borate may be used.

  The purity of the constituent materials in the mixture is not particularly limited as long as the desired properties are not deteriorated, but the purity excluding hydration water is preferably 99% or more, and more preferably the purity is 99.9% or more. Use a good one. Further, the particle size of the constituent material is not particularly limited as long as it is within a range in which a uniform melt can be obtained by melting. The constituent materials are preferably melted after being mixed dry or wet using a mixing / pulverizing means such as a ball mill or a planetary mill.

  Melting may be performed in an air atmosphere, but is preferably performed while controlling the oxygen partial pressure and the oxygen flow rate. The crucible used for melting is preferably made of alumina, platinum, or platinum containing rhodium, but a refractory can also be used. Further, a crucible with a lid may be used. Further, it is preferable to perform melting using a resistance heating furnace, a high frequency induction furnace or a plasma arc furnace. The resistance heating furnace is preferably an electric furnace including a heating element made of a metal such as a nichrome alloy, silicon carbide, or molybdenum silicide. The high frequency induction furnace may be provided with an induction coil and can control the output, and the plasma arc furnace may be any one that uses carbon or the like as an electrode and can use a plasma arc generated thereby. Further, it may be melted by infrared or laser direct heating.

  In addition, the mixture in which the constituent materials are mixed may be melted in a powder state, or a previously molded mixture may be melted. In the case of using a plasma arc furnace, a previously molded mixture can be melted as it is and further rapidly cooled.

  The step of obtaining the melt is preferably performed at 900 ° C to 1500 ° C. When the temperature is lower than 900 ° C., it is difficult to obtain a uniform melt. On the other hand, when the temperature exceeds 1500 ° C., the evaporation of the raw material becomes severe, which is not preferable. Especially, it is preferable to perform melting at 1100 ° C. to 1400 ° C. Moreover, you may stir in order to improve the uniformity of the obtained melt.

The composition of the melt is expressed in mol% on the basis of oxide, MO is 25 to 60%, Bi ₂ O ₃ is ₃ to 25%, Nb ₂ O ₅ is 2 to 25%, B ₂ O ₃ is 15 to 60%. % Included. As a result of repeated studies by the present inventors, the chemical composition of the melt is in the above range, the melt is rapidly cooled to form an amorphous substance, and further a crystallization step is performed. Almost all of the Bi source and Nb source in the constituents of bismuth are taken into the bismuth niobate crystal (hereinafter also referred to as target crystal component), and almost all of the B source is a component other than the target crystal component (hereinafter referred to as matrix). It was also found that the M source can be a target crystal component or a matrix component. The melt having the above composition range is preferable because it has an appropriate viscosity and can be vitrified without crystallization by the subsequent rapid cooling operation to obtain an amorphous substance. This composition also corresponds to the chemical composition (as oxide) of the constituent material before melting. When volatilization of the constituent materials, particularly Bi and / or B, occurs during the melting operation, and a melt having a desired composition cannot be obtained, the addition ratio of the constituent materials as raw materials may be adjusted.

When the Nb ₂ O ₅ content in the melt exceeds 25%, or the MO content is less than 25% or the B ₂ O ₃ content is less than 15%, the melt is cooled by rapid cooling. Since it is easy to crystallize and it is difficult to vitrify to an amorphous substance, it is not preferable because it is difficult to obtain bismuth niobate-based fine particles having a target composition. On the other hand, when the content ratio of Nb ₂ O ₅ is less than 2% or Bi ₂ O ₃ is less than 3%, or the content ratio of MO or B ₂ O ₃ exceeds 60%, in the subsequent crystallization step , Because the target crystal component may not be sufficiently precipitated.

In particular, when MO is 30 to 50%, Bi ₂ O ₃ is 5 to 15%, Nb ₂ O ₅ is 5 to 15%, and B ₂ O ₃ is 35 to 50%, bismuth niobate having a desired composition is included. It is preferable because system-based fine particles can be easily obtained and the yield can be increased.

Further, the chemical composition of the melt is expressed in mol% based on oxide, expressed in mol% based on oxide, MO is 25 to 60%, Bi ₂ O ₃ is ₃ to 25%, and Nb ₂ O ₅ is It is preferable that ₃ to 25% and B ₂ O ₃ to be 15 to 60% because fine particles of a bismuth niobate crystal having a layered perovskite crystal structure having a desired composition including M can be easily obtained. Here, particularly when (Bi ₂ O ₃ + Nb ₂ O ₅ ) is contained in an amount of 7 to 35% and (MO + B ₂ O ₃ ) is contained in an amount of 65 to 93%, vitrification of the melt is facilitated, and the amount of precipitation of the target crystal component As a result, the yield of bismuth niobate fine particles can be increased, which is preferable.

Furthermore, the chemical composition of the melt is expressed in mol% based on oxide, SrO is 25 to 60%, Bi ₂ O ₃ is ₃ to 25%, Nb ₂ O ₅ is 3 to 25%, B ₂ O ₃ 15 to 60% is preferable because fine particles of bismuth strontium niobate crystals having a target composition can be easily obtained in a high yield.

In addition, the chemical composition of the melt is Bi ₂ O ₃ / Nb ₂ O ₅ = 40/60 to 60/40 in terms of mol% on the basis of oxide, and the bismuth niobate system having the desired composition It is preferable because fine particles of crystals are easily obtained. Particularly preferably, the chemical composition of the melt is Bi ₂ O ₃ / Nb ₂ O ₅ = 50/50 to 60/40.

In the process of rapidly cooling the melt obtained as described above to an amorphous substance, a method of obtaining a flake-like amorphous substance by dropping the melt between twin rollers rotating at high speed Alternatively, a method of continuously winding a fiber-like amorphous substance (long fiber) from the melt with a drum rotating at high speed is preferably used. The temperature at the time of rapid cooling is, for example, 100 ° C./second or more, preferably 1 × 10 ⁴ ° C./second or more. Here, as the double roller and the drum, those made of metal or ceramics are used. Moreover, you may obtain a fiber-like amorphous substance (short fiber) using the spinner which rotated at high speed and provided the pore on the side wall. By using these apparatuses, the melt can be effectively rapidly cooled to a high purity amorphous material.

  When the amorphous substance is flaky, its thickness is 200 μm or less, more preferably 100 μm or less. When it is fibrous, its diameter is 50 μm or less, more preferably 30 μm or less. It is preferable to cool rapidly so that. Rapid cooling so as to form an amorphous material having a thickness or diameter larger than this is preferable because the crystallization efficiency in the subsequent crystallization process can be increased, and an amorphous material having a thickness or diameter larger than the above is obtained. In such a case, it is preferable to use the crystallization process after pulverization.

  Next, bismuth niobate crystals are precipitated from the amorphous material. The step of depositing the bismuth niobate crystal from the amorphous substance is preferably performed at 600 to 900 ° C. in the atmosphere. Even if the heating temperature is less than 600 ° C. for about 24 hours, it is difficult for crystals to precipitate, and if it exceeds 900 ° C., there is a possibility that a crystallized material containing an amorphous substance may melt. It is not preferable. More preferably, it is performed at 650 to 800 ° C. Since this crystal precipitation step is microscopically composed of two stages of nucleation and subsequent crystal growth, these two stages may be performed at different temperatures. Note that as the heating is performed at a higher temperature, the amount of crystals to be precipitated and the particle diameter of the precipitated crystals tend to increase. Therefore, the crystallization temperature may be set according to the desired particle diameter.

  In the present invention, bismuth niobate and M borate are mainly precipitated as crystals by the crystallization process of the amorphous material. The trace amounts of Bi borate and Nb borate that may be formed by the composition of the M borate and the melt can be removed simultaneously by the subsequent leaching process.

  In crystallization, it is preferable to keep the above temperature range for 4 hours to 96 hours because the bismuth niobate-based substance can be sufficiently crystallized. At that time, the longer the holding time, the larger the amount of crystals to be precipitated, and the larger the particle size of the precipitated crystals, so the holding time should be set according to the desired crystal precipitation amount and particle size. That's fine.

  Next, the bismuth niobate crystal is separated from the crystallization product containing the bismuth niobate crystal obtained as described above. If an acid is used, substances other than the bismuth niobate crystal can be easily leached and removed from the crystallized product. As the acid, inorganic acids such as acetic acid, hydrochloric acid and nitric acid, and organic acids such as oxalic acid and citric acid can be used. In particular, an acetic acid aqueous solution of 3 mol / L or more is preferable because even when a bismuth hydroxide salt or a bismuth oxyhydroxide salt is generated by a chemical reaction accompanying the leaching treatment, these can be redissolved and simultaneously removed. In order to accelerate the leaching treatment and adjust the particle size of the target crystal component to a desired range, the crystallization product containing the bismuth niobate crystal may be pulverized dry or wet before the leaching treatment. When pulverizing, it is preferable to use a medium such as a ball mill. Further, in order to promote the leaching reaction, the acid may be warmed and used, or ultrasonic irradiation may be used in combination. Although this leaching treatment may partially dissolve some of the bismuth niobate crystals, it is preferable in that the particle diameter can be made uniform. Furthermore, this leaching process may be repeated several times.

  After the leaching treatment, washing with pure water is performed as necessary to obtain bismuth niobate fine particles. The average primary particle diameter of the fine particles obtained (in the case of anisotropic particles, the long diameter is indicated) is preferably 5 to 200 nm. The smaller the average primary particle size of the resulting fine particles, the better the dispersibility in the liquid medium, and as a result, the uniformity and flatness of the ferroelectric thin film formed by the solution method can be improved.

  If the resulting bismuth niobate fine particles have a layered perovskite crystal structure, they are superior in relative dielectric constant compared to fluorite-type bismuth niobate fine particles, and easily exhibit excellent dielectric properties and piezoelectric properties. It is preferable.

The chemical composition of the bismuth niobate fine particles is preferably MBi ₂ Nb ₂ O ₉ because it easily exhibits excellent dielectric properties, piezoelectric properties, and the like. In particular, the frequency temperature is preferably SrBi ₂ Nb ₂ O _9. This is preferable because bismuth niobate fine particles having excellent characteristics can be obtained.

  EXAMPLES Hereinafter, although an Example demonstrates this invention, this invention is not limited by these.

[Examples 1 to 9]
M carbonate (MCO ₃ ), bismuth oxide (Bi) so that the composition of the melt has the ratio shown in Table 1 in terms of mol% based on MO, Bi ₂ O ₃ , Nb ₂ O ₅ and B ₂ O _{3. 2} O ₃ ), niobium oxide (Nb ₂ O ₅ ), and boron oxide (B ₂ O ₃ ) were weighed, mixed and pulverized in a dry process to obtain a raw material mixture.

  The obtained raw material mixture was filled in a platinum crucible containing 20% by mass of rhodium with a nozzle and heated in the electric furnace using molybdenum silicide as a heating element at the temperature shown in Table 1 for 1 hour to complete melting. I let you.

Next, the glass melt is dropped while the lower end of the nozzle is heated in an electric furnace, and the droplet is rapidly cooled at about 1 × 10 ⁵ ° C / second by passing through a twin roller having a diameter of about 15 cm rotating at 300 rpm. A flaky solid was obtained. The obtained flakes were dark brown and were transparent amorphous substances. When the thickness of the flakes was measured with a micrometer, it was 30 to 50 μm.

  A part of the obtained flakes was used to obtain a crystallization temperature in advance by differential scanning calorimetry (DSC), and the flakes were heated for 8 hours at 750 ° C., which is higher than the crystallization start temperature. Crystals were precipitated.

  Next, the flakes after the crystallization treatment were allowed to stand in an 8.5 mol / L acetic acid solution at 70 ° C. for 8 hours or more to leach soluble substances. The leached solution was centrifuged and the supernatant was discarded. After performing this operation 5 times, washing with water was performed 5 times and dried to obtain fine particles having a particle diameter of 5 to 100 nm.

The mineral phase of the obtained bismuth niobate fine particles was identified using an X-ray diffractometer. As a result, all of Examples 1 to 9 are orthorhombic, and Examples 1 to 6 are identical with the diffraction peak of the existing SrBi ₂ Nb ₂ O ₉ (JCPDS card number: 86-1190) having a layered perovskite structure. As a result, it was found to be a highly crystalline particle comprising a single phase of SrBi ₂ Nb ₂ O ₉ . Also, similar results to the diffraction peaks existing SrBi ₂ Nb ₂ O ₉ in the example 7-9 was obtained. The X-ray diffraction pattern of the fine particles obtained in Example 2 is shown in FIG.

  Next, the average primary particle size was determined. Here, the average primary particle diameter is a crystallite diameter, and is a particle diameter calculated based on Scherrer's equation from the spread of X-ray diffraction lines. The results are shown in Table 1 together with the chemical composition [mol%] and the melting temperature [° C.] of the melt. From Table 1, it can be seen that all of the obtained fine particles have a very fine particle size.

[Example 10 (comparative example)]
Strontium carbonate (SrCO ₃ ), bismuth oxide (Bi ₂ O) so that the composition of the melt has the ratio shown in Table 2 in terms of mol% based on SrO, Bi ₂ O ₃ , Nb ₂ O ₅ and B ₂ O _{3. 3} ), niobium oxide (Nb ₂ O ₅ ) and boron oxide (B ₂ O ₃ ) were weighed, mixed and pulverized in a dry manner to obtain a raw material mixture. The obtained raw material mixture was heated at the temperature shown in Table 2 for 1 hour to be completely melted, and then cooled to room temperature at a rate of about 300 ° C./hour in an electric furnace. No crystalline material was obtained.

[Examples 11 and 12 (comparative examples)]
Strontium carbonate (SrCO ₃ ), bismuth oxide (Bi ₂ O) so that the composition of the melt has the ratio shown in Table 2 in terms of mol% based on SrO, Bi ₂ O ₃ , Nb ₂ O ₅ and B ₂ O _{3. 3} ), niobium oxide (Nb ₂ O ₅ ) and boron oxide (B ₂ O ₃ ) were weighed, mixed and pulverized in a dry manner to obtain a raw material mixture. The obtained raw material mixture was heated at the temperature shown in Table 2 for 1 hour to be completely melted, and then subjected to a rapid cooling operation in the same manner as in Examples 1 to 9, whereby transparent flakes were obtained. The obtained flakes were crystallized in the same manner as in Examples 1 to 9, and leaching and washing were performed. As a result, almost no crystalline fine particles were obtained.

Since the bismuth niobate fine particles obtained by the present invention have a small crystallite size, high crystallinity, and excellent uniformity in composition and particle size, the fine particles are ferroelectric, piezoelectric, pyroelectric. It is suitable as a constituent material for electronic parts such as sensors, ultrasonic motors, actuators, capacitors, ferroelectric memories, filters, vibrators, etc., which have excellent electro-optical effects.
The fine particles can also be applied as a visible light responsive photocatalytic material.

X-ray diffraction pattern of bismuth niobate fine particles obtained in Example 2

Claims (9)

MO (M = Mg, Ca, Sr and Ba selected from the group consisting of 25 to 60%) Bi ₂ O _{3 3} to 25%, Nb ₂ O ₅ A melt containing ₂ to 25% of B ₂ O ₃ and 15 to 60% of B ₂ O ₃ , a step of rapidly cooling the melt to an amorphous material, and a bismuth niobate system from the amorphous material A method for producing bismuth niobate fine particles, comprising a step of precipitating crystals and a step of separating the bismuth niobate crystals from the obtained precipitates in this order. _{2. The} method for producing bismuth niobate-based fine particles according to claim 1, wherein the melt has a chemical composition of Bi ₂ O ₃ / Nb ₂ O ₅ = 40/60 to 60/40 in terms of mol% based on oxide. .   The method for producing bismuth niobate fine particles according to claim 1 or 2, wherein the step of obtaining the melt is performed at 900 to 1500 ° C.   The method for producing bismuth niobate-based fine particles according to any one of claims 1 to 3, comprising a step of rapidly cooling the melt to obtain a flake-like or fiber-like amorphous substance.   The method for producing bismuth niobate-based fine particles according to any one of claims 1 to 4, wherein the step of precipitating bismuth niobate-based crystals from the amorphous substance is performed at 600 to 900 ° C.   The method for producing bismuth niobate fine particles according to any one of claims 1 to 5, wherein the step of separating the bismuth niobate crystal is performed using an acid.   The average primary particle diameter of the bismuth niobate fine particles is 5 to 200 nm. The method for producing bismuth niobate fine particles according to any one of claims 1 to 6.   The method for producing bismuth niobate-based fine particles according to any one of claims 1 to 7, wherein the bismuth niobate-based crystal has a layered perovskite structure. Method for producing a niobium bismuth-based particles according to any one of claims 1 to 8 chemical composition of the niobium-bismuth-based fine particles are MBi ₂ Nb ₂ O _9.

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