Method for producing zirconium tungstate

Abstract

This method for producing zirconium tungstate comprises: a reaction step for heating and reacting raw materials including zirconium and tungsten under the action of pressure in a pressure-resistant container to obtain a precursor; and a heat treatment step for heating the precursor to obtain a product containing ZrW2O8, wherein, in the reaction step, the raw materials are reacted in a state in which sulfate ions are present in the pressure-resistant container.

Description

[Technical Field]

[0001] This specification discloses a method for producing zirconium tungstate. [Background technology]

[0002] While normal materials expand when heated, zirconium tungstate (ZrW2O8) is one material that exhibits negative thermal expansion, contracting when heated. Such negative thermal expansion materials can be mixed with positive thermal expansion materials such as resins as fillers to create composite materials with reduced thermal expansion, and these are expected to be used as encapsulants for semiconductor chips mounted on electronic components.

[0003] Some applications require fine powdered zirconium tungstate, for example, on the submicron order, and methods for producing such fine zirconium tungstate include the hydrothermal method.

[0004] For example, Patent Document 1 cites "a method in which a precursor is synthesized by hydrothermal synthesis, in which a mixed solution consisting of an aqueous solution of a tungstate salt, an aqueous solution of a zirconium compound having a Zr=O structure, and concentrated hydrochloric acid is heated, and this is then heat-treated at a temperature of about 600°C," and then focuses on the following problem: "Hydrothermal synthesis has the advantage of being able to synthesize small particles of 1 micrometer or less. However, because the precursor synthesis requires synthesis conditions of 130°C or higher in an acidic hydrochloric acid solution, a pressure-resistant autoclave with an acid-resistant fluororesin inner cylinder is used. Furthermore, the precursor yield does not exceed 50% unless the reaction is allowed to proceed for a long time (for example, 6 hours or more if the reaction temperature is 150°C). This makes mass production difficult and poses problems of high manufacturing costs." To solve this problem, Patent Document 1 proposes "a method for producing ZrW2O8 particles, which comprises heat-treating a mixed solution containing an aqueous solution of a tungstate, an aqueous solution of a zirconium compound having a Zr=O structure, and concentrated hydrochloric acid in a reaction vessel while providing a temperature difference between the liquid phase and the gas phase in the reaction vessel to produce ZrW2O8 precursor particles, and then heat-treating the ZrW2O8 precursor particles to produce ZrW2O8 particles." [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-179915 Summary of the Invention [Problem to be solved by the invention]

[0006] As described in Patent Document 1, hydrochloric acid systems are often used in synthesis by hydrothermal methods and the like. However, when using hydrochloric acid systems, crystals grow into needle-like shapes when precursors such as ZrW2O7(OH)2(H2O)2 are synthesized from raw materials containing zirconium and tungsten, resulting in a needle-like precursor. When such needle-like precursors are heated to produce zirconium tungstate, the zirconium tungstate also tends to become needle-like. There is a concern that needle-like zirconium tungstate may increase viscosity and deteriorate fluidity (thixotropy) when mixed with resins or the like as a filler.

[0007] This specification provides a method for producing zirconium tungstate that can suppress the formation of needle-shaped products and obtain granular products. [Means for solving the problem]

[0008] The method for producing zirconium tungstate disclosed in this specification includes the steps of: More than self-generated pressure Under the action of pressure 100℃~300℃ for 30 minutes or more The method includes a reaction step in which a precursor is obtained by heating and reacting, and a heat treatment step in which the precursor is heated to obtain a product containing ZrW2O8, and in the reaction step, the raw materials are reacted in the presence of sulfate ions in the pressure-resistant vessel. [Effects of the Invention]

[0009] According to the above-described method for producing zirconium tungstate, it is possible to suppress the formation of needle-like products and obtain granular products. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a flow chart showing an example of a method for producing zirconium tungstate according to one embodiment, in which a solvothermal method is used in a reaction step. [Figure 2]FIG. 10 is a flow chart showing an example of a method for producing zirconium tungstate according to another embodiment, in which a hydrothermal method or a dry gel conversion (DGC) method is used in the reaction step. [Figure 3] 1 is an X-ray diffraction profile of the product obtained in Example 1. [Figure 4] 1 is an SEM image of the product obtained in Example 1. [Figure 5] 1 is an X-ray diffraction profile of the product obtained in Example 2. [Figure 6] 1 is an SEM image of the product obtained in Example 2. [Figure 7] 1 is an X-ray diffraction profile of the product obtained in Example 3. [Figure 8] 1 is an SEM image of the product obtained in Example 3. [Figure 9] 1 shows X-ray diffraction profiles of the products obtained in Example 4. [Figure 10] 1 is an SEM image of a product obtained in Example 4 under the condition of a sulfuric acid concentration of 1.0 M. [Figure 11] 1 is an SEM image of a product obtained in Example 4 under the condition of a sulfuric acid concentration of 0.5 M. [Figure 12] 1 is an SEM image of a product obtained in Example 4 under the condition of a sulfuric acid concentration of 0.25 M. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the method for producing the above-mentioned zirconium tungstate will be described in detail. A method for producing zirconium tungstate according to one embodiment includes a reaction step of synthesizing a precursor (such as ZrW2O7(OH)2(H2O)2) from raw materials containing zirconium and tungsten, and a heat treatment step of heating the precursor to obtain a product containing ZrW2O8.

[0012] The reaction step can be carried out using, for example, a solvothermal method, a hydrothermal method, or a dry gel conversion (DGC) method. In such synthesis methods, the above-mentioned raw materials are reacted by heating them under pressure in a pressure-resistant vessel (autoclave). In the solvothermal or hydrothermal method, the raw materials are stirred in a liquid phase of an organic solvent or water as a solvent during pressurization and heating. In the DGC method, the raw materials are pressurized and heated in a gas phase containing solvent vapor.

[0013] Whether a solvothermal method, hydrothermal method, or DGC method is employed, in this embodiment, sulfate ions are present in a pressure vessel during the reaction process, and the raw materials are reacted in the presence of sulfate ions. This suppresses the formation of a needle-shaped precursor, resulting in a granular precursor. The sulfate ions in the pressure vessel are believed to inhibit crystal growth, rather than inhibit nucleation, which is why the precursor crystallizes into granular crystals. In many cases, after nucleation, the crystals grow vertically due to the characteristics of the tetragonal crystal structure. In contrast, in this embodiment, sulfate ions act as an inhibitor of crystal growth. However, this theory is not limited to this. It should be noted that the use of hydrochloric acid or chlorides in a hydrothermal method results in a needle- or rod-shaped product, as shown in “Hydrothermal synthesis of ZrW2O8 nanorods” (X. Xing et al., January 2006, Physica B 371, pp. 81-84).

[0014] (raw materials) Zirconium compounds and tungsten compounds can be used as raw materials. Zirconium compounds include zirconium sulfate (Zr(SO4)2). Tungsten compounds include sodium tungstate (Na2WO4) and ammonium paratungstate ((NH4) 10 W 12 O 41 Such compounds may be in the form of a hydrate.

[0015] The zirconium compound and / or tungsten compound used as the raw material preferably contains sulfuric acid, such as sulfate salts. This makes it easier to heat and pressurize the raw material in the reaction step in the presence of sulfate ions. From this perspective, it is preferable that the raw material contains zirconium sulfate or a hydrate thereof as the zirconium compound. However, as long as sulfate ions can be present in the pressure vessel used in the reaction step, it is not essential that the raw material contain sulfuric acid. Instead of or in addition to this, sulfuric acid can be added to the pressure vessel and / or the raw material gel can be impregnated with sulfuric acid, as described below.

[0016] Regarding the ratio of the zirconium compound to the tungsten compound in the raw materials, the ratio (W / Zr) of the atomic percentage (atm%) of tungsten in the tungsten compound to the atomic percentage (atm%) of zirconium in the zirconium compound can be set to 1.0 to 3.0.

[0017] In the solvothermal method, as shown in Figure 1, raw materials containing a zirconium compound and a tungsten compound are placed in a pressure-resistant vessel and the reaction process is carried out. This is because the zirconium compound and tungsten compound placed in the pressure-resistant vessel are dissolved in the solvent under the high temperature and pressure of the reaction process, and the synthesis reaction occurs in this state. In contrast, in the DGC method and hydrothermal method, as shown in Figure 2, the zirconium compound and tungsten compound are subjected to the gel preparation process described below, and the resulting raw gel can be used as the raw material in the reaction process. In the DGC method and hydrothermal method, for example, when an aqueous solution of zirconium sulfate as the raw material zirconium compound and an aqueous solution of sodium tungstate as the raw material tungsten compound are mixed, they immediately precipitate or co-precipitate to form a gel. After this gel preparation process, the raw gel obtained can be used in the reaction process. In either method or process, the formation of needle-shaped crystals can be suppressed by carrying out the reaction in the presence of sulfate ions.

[0018] (Gel preparation process) In the case of the hydrothermal method or the DGC method, in the gel preparation step, raw materials including a zirconium compound and a tungsten compound are mixed in a liquid such as water to prepare a raw material gel.

[0019] Specifically, for example, a zirconium compound such as zirconium sulfate may be added to pure water to prepare a zirconium sulfate aqueous solution, and a tungsten compound such as tungstate may be added to pure water to prepare a tungstate aqueous solution, and then these solutions may be mixed and stirred. The stirring speed is not particularly limited as long as the two aqueous solutions can be mixed uniformly. Furthermore, the liquid temperature does not need to be so high or low that it changes the pH, and can be room temperature or ordinary temperature. The conditions can be changed as appropriate.

[0020] This can produce a gel-like coprecipitate. Thereafter, filtration such as suction filtration or vacuum filtration or other solid-liquid separation is performed, followed by washing with pure water as necessary and drying, for example, under a vacuum atmosphere, to obtain a raw gel. The raw gel can contain a zirconium compound such as zirconium sulfate and a tungsten compound such as tungstate, each dispersed in the raw gel.

[0021] (Reaction step) In the reaction step, a precursor is synthesized from a zirconium compound and a tungsten compound in the raw material or raw material gel by a solvothermal method, a hydrothermal method, a DGC method, or the like.

[0022] The solvothermal method, hydrothermal method, and DGC method involve dissolving and precipitating raw materials in a liquid or gas phase containing a solvent, and have advantages such as being able to synthesize more stably and easily than solid-state reactions, and also being able to produce reaction products with high crystallinity and compositional uniformity.

[0023] In the solvothermal method, raw materials containing a zirconium compound and a tungsten compound are placed in a pressure-resistant vessel together with an organic solvent. The raw materials are heated and reacted under pressure (e.g., the autogenous pressure described below or a higher pressure) while being stirred in the liquid phase of the organic solvent. The organic solvent can contain a diol such as 1,4-butanediol. In particular, when an organic solvent containing 1,4-butanediol is used, it is believed that tetrahydrofuran (THF) and water are obtained under high temperature and pressure through the dehydration reaction of 1,4-butanediol with sulfuric acid. In this case, the dehydration reaction of 1,4-butanediol and the dissolution of the raw materials into the organic solvent occur simultaneously or sequentially under high temperature and pressure, accelerating the reaction. For this reason, it is preferable that the organic solvent contains 1,4-butanediol. For example, if a magnetic stirrer or other stirring bar is used to stir the liquid phase during the reaction, the stirring speed may be set to 600 rpm or higher to ensure sufficient stirring. Alternatively, if a relatively large stirring blade or paddle is used, the stirring speed may be lower. The stirring speed can affect the control of particle shape.

[0024] The hydrothermal method can be carried out in substantially the same manner as the solvothermal method, except that the raw material is the raw material gel described above and that the solvent is water such as pure water instead of the organic solvent used in the solvothermal method.

[0025] In the DGC method, a dried raw gel is placed in a pressure vessel containing a liquid solvent, without coming into contact with the liquid. Then, in the pressure vessel, the solvent is evaporated, and the raw material is reacted in a gas phase containing the solvent vapor by pressurizing (for example, the autogenous pressure described below or a higher pressure) and heating. The solvent can be water or a solution of water with sulfuric acid added.

[0026] In the reaction process, the temperature inside the pressure vessel is preferably maintained at 100°C to 300°C for at least 30 minutes, at least 1 hour, or even at least 5 hours. By increasing the temperature to a certain extent, crystals tend to form in a relatively short time, and the product tends to have a more desirable shape, such as non-acicular granules, such as angular, spherical, or even cubic. The temperature can be set taking into consideration the material of the pressure vessel, which may be made of polytetrafluoroethylene or stainless steel. If a certain amount of liquid is present in the pressure vessel, the solvent will evaporate upon heating, increasing the pressure inside the pressure vessel. In the reaction process, the pressure inside the pressure vessel does not need to be intentionally controlled; the reaction can be carried out under the action of the vapor pressure (self-generated pressure of the solvent) when heated to a predetermined temperature. However, the pressure can also be increased by supplying another gas, such as nitrogen. The pressure inside the pressure vessel during the reaction can be set to at least the self-generated pressure, for example, 2 MPa. If the pressure becomes too high, there is a risk of decomposition of the solvent. Depending on the specifications of the pressure vessel and other conditions, the pressure inside the pressure vessel may be set to 30 MPa or less, or even 20 MPa or less. The time for maintaining the above temperature can be set appropriately taking into consideration economic efficiency, mass productivity, etc., but may be set to 24 hours or less. By carrying out the reaction process in this manner, the raw materials are dissolved in the liquid phase or gas phase and a reaction product is obtained.

[0027] The precursor obtained in the reaction step often contains ZrW2O7(OH)2(H2O)2, but in some cases, such hydroxides are not produced by the DGC method, etc. In such cases, the precursor may not be crystallized and may remain in an amorphous state.

[0028] In the reaction process described above, when the raw material or raw material gel is reacted in the pressure vessel, sulfate ions are present in the pressure vessel, which tends to suppress the generation of a needle-shaped precursor and make it easier to obtain a granular precursor.

[0029] For example, in the solvothermal method, when the raw material contains sulfate, sulfate ions derived from the sulfate may be present in the pressure vessel in the reaction step.

[0030] In the hydrothermal method, sulfate ions are present in the pressure vessel during the reaction step after the gel preparation step, so sulfuric acid can be added to the pressure vessel as an aqueous sulfuric acid solution, etc., as needed during the reaction step. In the DGC method, the raw gel can be impregnated with sulfuric acid before the reaction step, for example, by immersing the raw gel in an aqueous sulfuric acid solution. In this case, the raw gel contains sulfuric acid, and when subjected to the reaction step, sulfate ions are generated in the pressure vessel.

[0031] In particular, in the DGC method, it is preferable that the sulfate ions are present in the pressure vessel during the reaction in an amount of 8.2 to 49.2% by mass. This makes it easier to obtain a fine precursor. If the amount of sulfate ions is too small, sufficient micronization may not be possible. The amount of sulfate ions in the pressure vessel may be preferably 32.8% by mass or less and 16.4% by mass or more. In the solvothermal method, if the amount of sulfate ions is too large, the yield may decrease, and if the amount of sulfate ions is too small, there is a concern that the generation of needle-shaped precursors may not be sufficiently suppressed. In the hydrothermal method, the amount of sulfate ions increases, which tends to result in finer precursors. It may be preferable that the amount of sulfate ions in the pressure vessel be 32.8% by mass or less and 16.4% by mass or more.

[0032] After the reaction, the precursor obtained by solid-liquid separation by suction filtration or the like can be dried, for example, under a vacuum atmosphere.

[0033] (Heat treatment process) The precursor produced in the reaction step is heated in the heat treatment step to become oxides containing ZrW2O8 and other products.

[0034] The conditions for the heat treatment process can be varied as appropriate. For example, the precursor may be heated to 600°C to 700°C for 0.5 to 3 hours in air or in an inert atmosphere such as argon. If the temperature is too low or the time is too short, crystallization may not occur. On the other hand, if the temperature is too high or the time is too short, there is a risk that the crystallized ZrW2O8 may decompose into ZrO2 and WO3.

[0035] The zirconium tungstate produced in this manner is often granular rather than acicular, since the generation of acicular precursors is suppressed in the reaction step. When such granular zirconium tungstate is mixed as a filler in a resin or the like, the increase in viscosity is suppressed, and the required fluidity (thixotropy) can be exhibited.

[0036] The presence of ZrW2O8 in the product obtained in the heat treatment step and ZrW2O7(OH)2(HO)2 in the precursor obtained in the reaction step can be confirmed by X-ray diffraction analysis. For X-ray diffraction analysis, for example, a Rigaku SmartLab X-ray diffractometer can be used. The analytical conditions include measurement with an X-ray tube (lab apparatus), 40 kV / 30 mA, a Cu source (Kα), and a focusing method for θ-2θ measurement. In the examples described below, such an X-ray diffractometer and analytical conditions were used. [Example]

[0037] The above-described method for producing zirconium tungstate was experimentally carried out and is described below, however, the description is for illustrative purposes only and is not intended to be limiting.

[0038] Example 1 400 mg of tungsten compound (NH4) 10 W 12 O 41273 mg of Zr(SO4)2·5H2O and 273 mg of Zr(SO4)2·4H2O as a zirconium compound were placed in an autoclave along with the organic solvent 1,4-butanediol, and synthesis was performed using the solvothermal method. The autoclave was heated on a hot plate for 6 hours. The actual temperature of the hot plate was measured at 156°C. The relationship between temperature and pressure was examined using another autoclave equipped with a pressure gauge, and the pressure was 2 MPa at 200°C. Therefore, it is estimated that the pressure inside the autoclave during this reaction was 2 MPa or less. During the reaction, the mixture was stirred at 600 rpm using a cylindrical rotor (Sm·Co). After heating and pressurization, the mixture was rapidly cooled in an ice bath.

[0039] The precursor thus obtained was a solid powder with a gray-white to pale blue color, and when analyzed by X-ray diffraction, the peaks of ZrW2O7(OH)2(H2O)2 were confirmed.

[0040] The precursor was then heat-treated at 700°C for 3 hours to obtain a product. This product was a white solid powder, and the X-ray diffraction profile (sample) shown in Figure 3 indicated that zirconium tungstate (ZrW2O8) had been produced. Figure 4 shows an SEM image of the product after heat treatment. Figure 4 shows that the product contained a large amount of granular material, with needle-like particles suppressed.

[0041] Example 2 The precursor was synthesized and heat-treated in the same manner as in Example 1, except that the heating time in the solvothermal method was changed to 8 hours and the measured temperature of the plate was 161°C, to obtain a product. The measured temperature of this plate was approximately the same as that of Example 1, and it is presumed that the temperature inside the pressure vessel during the reaction was also approximately the same as that of Example 1. The X-ray diffraction profile and SEM image of the product thus obtained are shown in Figures 5 and 6, respectively.

[0042] Figure 5 shows that this product also contained zirconium tungstate (ZrW2O8), and Figure 6 shows that the product was more granular than acicular.

[0043] Example 3 A tungsten compound (11,742.7 mg of Na2WO4·2H2O) was added to 50 mL of distilled water to prepare a tungstate aqueous solution. A zirconium compound (6,325.9 mg of Zr(SO4)2·4H2O) was added to 50 mL of distilled water to prepare a zirconium salt aqueous solution. These aqueous solutions were mixed and stirred at 600 rpm in an ion exchange solution at 20°C, then suction filtered and washed with 100 mL of distilled water to obtain a raw gel.

[0044] Next, the raw gel was impregnated with 16.4% by mass of sulfuric acid, and the raw gel was placed in an autoclave together with 16.4% by mass of sulfuric acid, where synthesis was carried out using the DGC method. Here, distilled water was used as the solvent, the heating time was 12 hours, and the autoclave temperature was set to 150°C. During the reaction, the temperature inside the autoclave was 150°C, and the pressure was estimated to be 0.5 MPa based on the vapor pressure curve. After heating and pressurization, the mixture was rapidly cooled in an ice bath.

[0045] The precursor thus obtained was a gray solid, and when analyzed by X-ray diffraction, the peaks of ZrW2O7(OH)2(H2O)2 were confirmed.

[0046] The precursor was then heat-treated at 600°C for 3 hours to obtain a product. This product was a white solid, and had the X-ray diffraction profile shown in Figure 7, indicating that zirconium tungstate (ZrW2O8) had been formed. Figure 8 shows an SEM image of the product after heat treatment. Figure 8 shows that the product contained a large amount of granular material, with needle-like particles being suppressed.

[0047] Example 4 A raw gel was obtained in the same manner as in Example 3. Next, the raw gel was not impregnated with sulfuric acid, but was placed in an autoclave together with an aqueous sulfuric acid solution, and synthesis was carried out by a hydrothermal method. The heating time was 12 hours, and the autoclave temperature was set to 150°C. During the reaction, the temperature inside the autoclave was 150°C, and the pressure was estimated to be 0.5 MPa based on the vapor pressure curve. During the reaction, the mixture was stirred at 600 rpm using a cylindrical rotor (Sm·Co). After heating and pressurization, the mixture was rapidly cooled in an ice bath. This synthesis was carried out for sulfuric acid aqueous solutions with molar concentrations of 1.0 M, 0.5 M, and 0.25 M, and precursors were produced under each condition.

[0048] All of the precursors obtained in this way were gray solids, and when analyzed by X-ray diffraction, the X-ray diffraction profile shown in Figure 9 was obtained, and the peaks of ZrW2O7(OH)2(H2O)2 were confirmed.

[0049] SEM images of each of the precursors are shown in Figures 10 to 12. Figures 10 to 12 show that the precursors contain fewer needle-shaped particles and more granular particles. Therefore, it is estimated that heat treatment of the precursors will produce granular zirconium tungstate (ZrW2O8).

[0050] From the above, it was found that the above-mentioned method for producing zirconium tungstate suppresses the formation of needle-like products and produces granular products.

Claims

1. 1. A method for producing zirconium tungstate, comprising: a reaction step of heating raw materials containing zirconium and tungsten in a pressure-resistant vessel at 100°C to 300°C for 30 minutes or more under the action of a pressure equal to or greater than the self-generated pressure to cause a reaction, thereby obtaining a precursor; The precursor is heated to form ZrW 2 O 8 a heat treatment step to obtain a product comprising Including, In the reaction step, the raw materials are reacted in the presence of sulfate ions in the pressure vessel.

2. In the reaction step, an organic solvent is used as a solvent, and the raw materials are reacted in a liquid phase of the organic solvent; the raw material includes a zirconium compound and a tungsten compound, The method for producing zirconium tungstate according to claim 1, wherein the zirconium compound and the tungsten compound are each placed in the pressure-resistant vessel together with the solvent, and the reaction step is carried out.

3. The method for producing zirconium tungstate according to claim 2 , wherein the organic solvent comprises a diol.

4. The method for producing zirconium tungstate according to claim 2, wherein the organic solvent comprises 1,4-butanediol.

5. The method for producing zirconium tungstate according to claim 2 , wherein the zirconium compound and / or the tungsten compound includes a sulfate.

6. In the reaction step, the raw materials are reacted while being stirred in a liquid phase of water as a solvent or in a gas phase containing vapor of the solvent, The method further includes, before the reaction step, a gel preparation step of mixing a zirconium compound and a tungsten compound in a liquid and then drying the mixture to prepare a raw gel in which the zirconium compound and the tungsten compound are dispersed, The method for producing zirconium tungstate according to claim 1 , wherein the raw material gel is used as the raw material in the reaction step.

7. The method for producing zirconium tungstate according to claim 6, wherein in the reaction step, sulfuric acid is introduced into the pressure vessel together with the raw material gel and the solvent.

8. The method for producing zirconium tungstate according to claim 6, wherein the zirconium compound and / or the tungsten compound used in the gel preparation step contains a sulfate.

9. In the reaction step, the raw materials are reacted in a gas phase containing solvent vapor; The method for producing zirconium tungstate according to claim 6, wherein the raw material gel is impregnated with sulfuric acid and used in the reaction step.

10. The precursor is ZrW 2 O 7 (OH) 2 (H 2 O) 2 The method for producing zirconium tungstate according to any one of claims 1 to 9, comprising:

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