JP2011045859A - Method for synthesizing powder and method for manufacturing electronic component - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for synthesizing powder with high productivity. <P>SOLUTION: The method for synthesizing powder comprises a step of pressurizing and heating raw material slurry prepared by mixing raw materials with a solvent; a reaction step of supplying the pressurized and heated raw material slurry and an accelerant including water separately to a reaction path to perform a reaction in a subcritical or supercritical state; and a cooling step of cooling the reaction slurry to stop the reaction. Critical conditions to come into the supercritical state are alleviated in comparison with the case of water alone and the ratio of the slurry is made higher. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention belongs to the fields of powder synthesis for manufacturing metal oxide powder and manufacturing technology of electronic parts, and in particular, dielectric materials used for manufacturing electronic parts such as capacitors and PTC elements, The present invention relates to a powder synthesis method and an electronic component manufacturing method for manufacturing barium titanate fine particles to be used as an electronic material such as a piezoelectric material and a semiconductor.

Conventionally, tetragonal barium titanate (BaTiO ₃ ) has a very high relative dielectric constant, and therefore, when applied to a multilayer ceramic capacitor, the thickness of the dielectric layer can be suppressed to about several μm. It is known that capacitors can be made smaller and larger in capacity. And with the miniaturization of capacitors, the thickness of the dielectric layer also tends to become thinner, and for this reason, barium titanate powder, which is a dielectric material used for the dielectric layer, is nano-sized as a nanoparticle. Various proposals to make fine particles on the order have been made.

  As a method for producing such barium titanate fine particles, various methods such as a solid phase reaction method, an oxalic acid method, and a sol-gel method have been proposed. Temperature and pressure are important parameters for chemical reactions, but the reaction temperature is heated above the boiling point of the liquid medium, the system pressure is raised above atmospheric pressure, and the liquid phase reacts. There is a reaction. Examples of such a liquid phase reaction include a hydrothermal synthesis method, a solvothermal method (reaction), and a supercritical hydrothermal method. In the liquid phase reaction in which the reaction temperature is heated above the boiling point of the liquid medium, the system pressure is increased to be higher than atmospheric pressure, and the liquid phase is reacted, when using water, “hydrothermal reaction”, If another organic solvent is used as a reaction medium, it is called “solvothermal reaction”. The solvothermal method can produce nanoparticles of several nanometers, but it often takes several hours to several days to produce nanoparticles. For this reason, it is difficult to efficiently produce fine particles of about several tens of nanometers.

  In contrast, the supercritical hydrothermal method that exceeds the critical temperature and pressure of the liquid has a high ion reaction rate, facilitates the production of nanoparticles, and can be applied to a flow-type manufacturing process suitable for mass production. There is an advantage. In the supercritical hydrothermal method, a titanium compound aqueous solution and a barium salt aqueous solution are mixed, an alkali aqueous solution is added, and then hydrothermal reaction is performed in subcritical or supercritical water, thereby producing a cubic crystal of 30 nm or less or 50 nm or less. The tetragonal barium titanate nanoparticles are produced (see, for example, Patent Documents 1 and 2).

JP 2003-261329 A JP 2005-289737 A

  When synthesizing powders by the conventional flow-type supercritical hydrothermal method shown in Patent Documents 1 and 2, etc., a large amount of high-temperature water is required to rapidly heat the raw material slurry at room temperature to the supercritical state. Become. For example, when mixing a slurry at room temperature and high-temperature water at about 500 ° C., the critical temperature of water exceeds 374 ° C., so that supercritical water that is about four times the raw material slurry flow rate is required. For this reason, the density | concentration of a reaction material will become low and synthesis efficiency will fall. On the other hand, when the slurry is heated before mixing, hydrothermal synthesis occurs at the time of heating, and there is a problem that a supercritical reaction cannot be realized.

  The present invention has been made in view of the above, and the concentration of the raw material in the reaction region is higher (for example, about one digit higher than in the case of the conventional flow-type supercritical hydrothermal synthesis, and a larger amount can be obtained. An object of the present invention is to provide a method for producing a nanoparticle and a method for producing an electronic component using the powder.

  In order to solve the above-mentioned problems and achieve the object, the method for synthesizing powder according to the present invention is a mixture of a raw material and a solvent that becomes a supercritical state at a lower pressure and lower temperature than water alone. A first step of pressurizing and heating the raw material slurry mixed with the raw material slurry, a pressurized and heated raw material slurry, and a reaction accelerator containing water are separately supplied to the reaction path, and the reaction path A second step of generating a reaction environment in a subcritical or supercritical state by pressurization; and the raw slurry is allowed to stay in the reaction field for a predetermined time using the generated reaction environment in the subcritical or supercritical state as a reaction field. A third step of generating fine powder particles, and a fourth step of cooling the generated fine powder particles and stopping the growth of the fine powder particles.

  In the method for synthesizing powder according to the present invention, in the above invention, in the second step, the heated raw material slurry is supplied to the reaction path at a higher volume ratio than water contained in the reaction accelerator. It is characterized by that.

  The method for synthesizing a powder according to the present invention further includes a step of heating the reaction accelerator in the above invention, and the second step uses the heated reaction accelerator as a reaction path. It is characterized by supplying.

  In the method for synthesizing a powder according to the present invention, in the above invention, the solvent is a solvent that becomes a supercritical state at a lower pressure than that of water alone, and It is characterized by producing a critical or supercritical state.

  Moreover, the method for synthesizing powder according to the present invention is characterized in that, in the above invention, the solvent contains an organic solvent that can be continuously dissolved in water in a supercritical state.

  The powder synthesis method according to the present invention is characterized in that, in the above invention, the reaction accelerator contains water.

  The electronic component manufacturing method according to the present invention is characterized in that an electronic component including an electronic material manufactured using powder fine particles generated by the powder synthesis method of the above invention as a constituent element is manufactured. .

  According to the present invention, since a reaction environment in a subcritical or supercritical state is generated using a reaction accelerator containing water, a critical temperature and / or a critical pressure for achieving a subcritical or supercritical state is obtained. Therefore, by relaxing the critical conditions, it is possible to reduce the corrosion of the reaction vessel, contamination of impurities in the product due to the corrosion, and widen the choice of materials for the reaction vessel. There is an effect that can be done. Also, by increasing the ratio of alcohol in the mixed solvent, the extremely high reaction rate of pure water synthesis is greatly suppressed, and the synthesized particles can be quickly diffused by the blanc motion etc. by the solvent to obtain a better dispersion state. Is possible.

  Furthermore, utilizing the fact that the raw material slurry adjusted with a solvent such as alcohol does not react very much even if it is first heated to high temperature and pressure, the raw material slurry is pressurized and heated and then put into the reaction path to accelerate a large amount of reaction. A subcritical or supercritical state can be generated without heating the raw material slurry by the agent. As a result, the mixing ratio of the conventional supercritical water and the raw material slurry can be reversed, that is, the amount of the raw material slurry can be larger than that of the reaction accelerator, and the concentration of the raw material in the reaction region can be increased compared to the conventional supercritical hydrothermal synthesis. Can be increased by an order of magnitude, and more nanoparticles can be made. Moreover, the temperature of the reaction accelerator thrown into the reaction path can be lowered by adopting the high temperature raw material slurry. For example, when the temperature after mixing is 300 ° C., it is not necessary to heat the preheating temperature of the slurry and water to 300 ° C. or higher. In particular, when the water ratio is several percent, room temperature water can be used. Thus, since it is not necessary to preheat the reaction accelerator containing water to 400 ° C. to 500 ° C., corrosion due to high-temperature water can be reduced, and the safety of the reaction region at high temperature and high pressure can be improved.

FIG. 1-1 is a characteristic diagram showing critical temperature conditions of water and ethanol mixed solvent for explaining the outline of the present invention. FIG. 1-2 is a characteristic diagram showing the critical pressure condition of water and ethanol mixed solvent for explaining the outline of the present invention. FIG. 2 is a schematic configuration diagram schematically showing the continuous flow manufacturing apparatus applied to the first embodiment of the present invention. FIG. 3 is a schematic process diagram showing the method for producing barium titanate according to Embodiment 1 of the present invention using a continuous flow production apparatus. FIG. 4 is a cross-sectional view showing a configuration example of a multilayer ceramic capacitor to which the second embodiment of the present invention is applied.

  Hereinafter, the best mode for carrying out the method for synthesizing powder and the method for producing electronic parts of the present invention will be described in detail. The present invention is not limited to the embodiment, and various modifications can be made without departing from the spirit of the present invention.

(Outline of the present invention)
The powder synthesis method of the present invention uses a mixed solvent of water and a solvent that becomes a supercritical state at a lower temperature and lower pressure than in the case of water alone, and a supercritical reaction environment by heating and pressurization. And the powder is synthesized in the reaction environment. Thereby, the critical condition for setting to a subcritical or supercritical state is relaxed rather than the case of water alone.

  As such a solvent, there is a polar organic solvent having a critical temperature and a critical pressure lower than pure water. Examples of the organic solvent that can be used in the present invention include ethanol (about 241 ° C., 6 MPa), methanol (about 239 ° C., 8 MPa), isopropanol (about 235 ° C., 4.8 MPa), methyl ether (127 ° C., 5.MPa). 3 MPa) and acetone (235 ° C., 4.6 MPa). Here, a case where ethanol is used as a solvent will be described as an example. FIGS. 1-1 and 1-2 are characteristic diagrams showing critical conditions of a mixed solvent in which water and ethanol are mixed. FIG. 1-1 shows the relationship between the critical temperature and the ethanol concentration, and FIG. 1-2 shows the relationship between the critical pressure and the ethanol concentration. The critical conditions are described in the literature “AA Abdurashidova, AR Bazaev, EA Bazaev and IM Abdulagatov,“ The thermal properties of water-ethanol system in the near-critical and supercritical states ”, High Temperature Vol. 45 No. 2, 2007, p178-186 "(hereinafter referred to as Reference 1) was used for the calculation. First, the critical temperature Tw in the case of water alone is about 374 ° C., and the critical pressure Pw is about 22 MPa. On the other hand, in the case of ethanol alone, the decomposition power of supercritical ethanol is slightly lower than that of supercritical water, but the critical temperature Te is about 241 ° C. and the critical pressure Pe is about 6.1 MPa. Have. In such a mixed solvent in which water and ethanol are mixed, depending on the weight fraction of ethanol, as shown in FIGS. 1-1 and 1-2, the critical temperature Tm and the critical pressure Pm are Change. Specifically, by mixing ethanol with water, the critical temperature and critical pressure are lower than in the case of water alone, and the critical temperature and critical pressure decrease as the weight fraction of ethanol increases.

Therefore, by adjusting the ratio of the composition of the mixed solvent (water and ethanol), the critical temperature and critical pressure of the mixed solvent are adjusted to control the reaction rate, particle growth rate, crystallinity, and product powder dispersibility. Etc. can be realized. For example, even when the raw material barium titanyl oxalate was put in 100% ethanol and kept in a supercritical state at 300 ° C. and 20 MPa for 1 hour or more, the decomposition reaction hardly occurred, and synthesis of barium titanate BaTiO ₃ could not be performed. . On the other hand, when placed in supercritical water at 400 ° C. and 30 MPa, barium titanate was produced even for 1 second. With this characteristic, by increasing the ratio of water, it is possible to increase the decomposition rate of the organic matter and the reaction rate of the reactant, and it is possible to generate barium titanate fine particles and the like at high speed. In addition, the ionic product [H ⁺ ] [OH ⁻ ] of the high-temperature and high-pressure water has a maximum value of about 10 ⁻¹¹ in the temperature range of 200 ° C. to 300 ° C., and the water decomposition ability is maximized and promotes the ion reaction. You can also. In addition, as shown in FIG. 1, when the molar fraction of ethanol is about 30% or more, the mixed solvent of water and ethanol has a sufficient decomposing power, and is considerably lower than about 300 ° C. or less than 10 MPa pure water. Low critical conditions can be achieved.

  In addition, if the ethanol ratio is increased, the critical temperature and pressure of the mixed solvent can be further lowered, the decomposition rate and the fine particle production rate can be reduced, and the handling is facilitated to improve the operability. It becomes possible. Moreover, the occurrence of corrosion in the reactor can be reduced by further lowering the critical temperature / critical pressure and relaxing the temperature and pressure conditions. For example, by increasing the ratio of ethanol, the critical temperature of the mixed solvent can be reduced from about 374 ° C. to about 260 ° C., and the critical pressure can be reduced from 22 MPa to about 6 to 7 MPa. As a result, even when a reaction vessel or piping made of Fe or Ni-based material such as SUS or corrosion-resistant Hastelloy material is used, its corrosiveness is weakened and the content of impurities in the product is reduced. Alternatively, the critical temperature and critical pressure of the mixed solvent are greatly reduced. For example, the critical temperature can be lowered to 300 ° C. or lower, so the choice of materials for reaction vessels and the like can be expanded, and Ti having excellent corrosion resistance can be used. It becomes. Even if the ratio of ethanol is increased, it is necessary to adjust so that water is always included. This is because if water is not included, the decomposition power may be reduced or the ion reaction rate may be insufficient.

  That is, a mixed solvent of water and ethanol can provide a single-phase supercritical reaction environment state while maintaining the same decomposition power as a simple subcritical water environment even at the same temperature and pressure. It can be done. Therefore, for example, by using it for the production of barium titanate, it is advantageous for the formation of highly uniform barium titanate, and it is possible to produce highly crystalline barium titanate nanoparticles without subsequent high-temperature sintering. it can.

  In addition, alcohols such as ethanol have a better dispersion power for fine particles than water, which helps to disperse raw materials and products. For example, in the case of fine particles of barium titanate synthesized only with supercritical water, the primary particles are several tens of nanometers, but there are cases in which they are agglomerated on the order of submicrons due to aggregation. On the other hand, by setting the reaction environment in a supercritical state with a mixed solvent to which ethanol is added, the massive barium titanate particles disappear and can be synthesized into almost dispersed tens of nanometers of barium titanate particles. . That is, the occurrence of aggregation of barium titanate fine particles can be suppressed.

  The solvent to be mixed with water is not limited to ethanol, and any organic solvent that can lower the critical temperature and the critical pressure as compared with pure water and is soluble in water may be used. For example, alcohol (methanol, ethanol, propanol, IPA, (isopropanol) butanol, etc.) that is inexpensive and low in toxicity, or ketones such as acetone can be used.

  Here, when the raw material slurry prepared with water is preheated, a hydrothermal reaction occurs in the raw material slurry. For this reason, in the conventional flow-type supercritical reaction, a supercritical reaction environment is rapidly realized by mixing high temperature water with a raw material slurry at room temperature. However, in order to mix high-temperature water into a supercritical state in this way, high-temperature water several times as much as the raw material slurry at room temperature is required. That is, in order to achieve a supercritical state, there is a limit to the ratio of the raw material slurry that can be mixed with high-temperature water. Therefore, there is a limit to the amount of raw materials that can be synthesized. In contrast, the raw material slurry prepared with an organic solvent such as alcohol has a very low reaction rate of the raw material slurry compared to the high-speed reaction between the high-temperature and high-pressure water and the raw material, even if it is supercritical as long as it does not contain water. . In the present invention, by utilizing this difference in reaction rate, the raw material slurry not containing water is heated and pressurized to a target temperature and a target pressure, and then mixed with a reaction accelerator containing water and reacted. In this way, by preheating the raw slurry, the need to heat the raw slurry with high-temperature water is eliminated or reduced, so the feed amount of the raw slurry is greatly improved over the conventional supercritical flow reaction. Can do. As a result, it becomes possible to apply a production process by a continuous flow type which is excellent in mass productivity, and the efficiency of the production process of barium titanate fine particles can be greatly improved.

(Embodiment 1)
The first embodiment shows an application example to a method for producing barium titanate fine particles using a continuous flow production process that easily generates a supercritical state. FIG. 2 is a schematic configuration diagram schematically showing a continuous flow manufacturing apparatus applied to the first embodiment. This continuous flow type manufacturing apparatus was produced by heating a slurry supplied via a valve 2 from a slurry stirrer 1 that stirs a slurry containing raw materials, a heater 13 that generates a high-temperature slurry, and heating by the heater 13. A slurry pump 4 for supplying the high-temperature slurry as a high-pressure high-temperature slurry to the reaction tube 3 constituting the reaction field, a heater 7 for heating pure water supplied from the tank 5 through the valve 6 to generate high-temperature water, A high-temperature water generated through heating by the heater 7 is mixed with a high-temperature high-pressure slurry and supplied to the reaction tube 3, a cooling tank 9 disposed on the discharge side of the reaction tube 3, A recovery tank 11 connected to the discharge side via a valve 10. In addition, the continuous flow type manufacturing apparatus also includes a heater 12 for heating for bringing the inside of the reaction tube 3 into a supercritical state (or subcritical state) around the reaction tube 3.

  In the example of the continuous flow manufacturing apparatus shown in FIG. 2, the fluid is allowed to flow from the lower side to the upper side, but two lines of high-temperature water and high-temperature and high-pressure slurry on the upstream side are placed at the top of the reaction tube 3. It may be installed so that the fluid flows from the upper side to the lower side. As the thickness of the reaction tube 3, a commercially available pipe having a diameter of 1/32 inch to several centimeters may be used.

  Here, when the raw slurry is not heated, the supercritical water is preheated to a temperature higher than the critical condition (for example, 200 ° C. to 300 ° C. higher than the critical temperature) in order to make the mixed fluid high temperature and high pressure. The supercritical water and the raw slurry should have a higher mixing ratio of the supercritical water, that is, more preheated supercritical water than the raw slurry. For example, in order to exceed the supercritical condition of water, the mixing ratio of supercritical water: raw material slurry needs to be about 4: 1 or more. Therefore, the density | concentration of the raw material in the fluid after mixing will become low. On the other hand, the slurry supplied to the reaction tube 3 by the heater 13 is also set to a high temperature and a high pressure, so that there is little or no need to heat the slurry with water (supercritical water). 1: 1 or more, that is, the amount of the raw slurry can be made larger than that of water. In addition, also when supplying the mixed solution of water and a solvent from the liquid feed pump 8 side, the mixing ratio of raw material slurry: mixed solution can be set to 1: 1 or more. That is, (volume flow rate of the whole slurry containing the raw material (metal salt raw material) (cc / min)) / (volume flow rate of water (or mixed solution) not containing the raw material) can be 1 or more. Thereby, the raw material density | concentration of a reaction part can be made high and productivity can be made high.

  Furthermore, in the example shown in FIG. 2, the heater 7 is used to heat the water, but the present invention is not limited to this, and the water is supplied to the reaction tube 3 at room temperature, that is, as room temperature water. It may be. Since the raw slurry can be heated to a high temperature and the proportion of water to be mixed can be reduced, even if the water supplied to the reaction tube 3 is room temperature water, the inside of the reaction tube 3 can be brought into a subcritical or supercritical state. .

  Next, a method for producing the barium titanate fine particles of the first embodiment will be described. FIG. 3 is a schematic process diagram showing a barium titanate production method using the continuous flow production apparatus as shown in FIG.

A. Raw material preparation step First, a raw material preparation step is performed. In the raw material preparation step, the raw material is prepared, and the raw material pulverization, slurry preparation, and slurry dispersion are sequentially performed.

In the raw material pulverization treatment, the raw material particles are pulverized to a size of about several μm by any of ball mill, bead mill, and planetary pulverization in order to facilitate delivery from the slurry pump 4. In order to reduce the variation of the product, it is desirable to further grind to a size of about submicron. Here, in the first embodiment, since the commercially available TiO ₂ powder as a raw material is in the submicron, there is no need for pulverization. When barium titanyl oxalate is used as a raw material, the raw material particles are pulverized by the following treatment. As a pulverization method, a wet ball mill method using zirconia as a ball, ion-exchanged water as a solvent and ethanol as a polar solvent can be used. The grinding time is 24 hours, the volume ratio is raw material: ball: solvent = 1: 4: 8, the pot volume of the ball mill is 700 mL, the content volume ratio to the pot is 70% or more, and the raw material powder size before grinding is about 70 μm on average. The raw material powder size after pulverization was about 0.5 μm on average.

The preparation of barium titanyl oxalate slurry requires an alkaline atmosphere with a pH> 12 as a stable condition for barium titanate, so the pH is adjusted by adding an alkaline aqueous solution (electrolyte added, or alkaline water or ammonia water) To do. In Embodiment 1, NaOH is added. The specific amount of NaOH added depends on the amount of CO ₂ contained in the raw material, but as a guide, the NaOH: CO ₂ ratio is set to be greater than 2: 1. This is to prevent the formation of BaCO ₃ . In the case where the TiO ₂ and Ba _{(OH) 2} as a raw material, or not filled with an alkali agent, may be added a pH adjusting agent such as NaOH or TMAH.

  In the dispersion treatment of the slurry, it is dispersed for about 5 to 10 minutes by ultrasonic waves before the reaction. In the reaction process, the slurry is stirred by the slurry stirrer 1 and sent to the slurry pump 4 side. In addition, before mixing the reaction accelerator, the supercritical state of supercritical ethanol having a low surface tension is realized by setting the solvent on the raw slurry side to be above the critical condition of, for example, ethanol. Keep the raw powder before the reaction in good dispersion.

B. Reaction environment generation step The reaction environment generation step is a step of generating a reaction environment in a subcritical mixed solvent or supercritical mixed solvent state, and performs adjustment of mixed solvent critical conditions, and heating, pressurization, and mixing.

  As the adjustment process of the mixed solvent critical condition, the ratio of water and the solvent (ethanol) is adjusted. Here, the amount of water is about 5 wt% to 95 wt% by weight%. Thereby, the critical temperature of the mixed solvent is adjusted between 260 ° C. and 370 ° C., and the critical pressure is adjusted between 8 MPa and 22 MPa.

  In the heat treatment, the slurry is heated by the heater 13 to form a high temperature slurry. Further, pure water is heated by the heater 7 to be in a high temperature water state. In addition, the reaction tube 3 in which slurry containing high-temperature water and an organic solvent (such as ethanol) is mixed is held by the heater 12 above the adjusted critical temperature of the mixed solvent.

  In the pressurizing process, the slurry containing the raw material to be supplied and the organic solvent (ethanol or the like) is pressurized from the slurry agitator 1 by the slurry pump 4 to a pressure higher than the critical pressure of the organic solvent, and further, the heater 13 as described above. By heating, the raw material slurry is supplied to the reaction tube 3 side as a high-temperature and high-pressure slurry. Pure water is also pressurized by the liquid feed pump 8 and heated by the heater 7 to supply pure water to the reaction tube 3 side as high-temperature high-pressure water. The pure water may be room temperature and high pressure water without heating.

  In the mixing process, high-temperature high-pressure water and high-temperature high-pressure slurry are mixed, or room temperature high-pressure water and high-temperature high-pressure slurry are mixed and supplied to the reaction tube 3. In the mixed state, the reaction environment in the reaction tube 3 is preferably close to the critical condition of the mixed solvent, and is preferably in a supercritical state exceeding the critical temperature and critical pressure.

  In particular, the minimum temperature that can be synthesized into tetragonal barium titanate is greater than 215 ° C., and the minimum pressure is greater than 5 MPa. For this reason, in order to improve the squareness, it is desirable that the reaction environment in the reaction tube 3 is a temperature and pressure higher than the critical condition of the mixed solvent.

C. Powder production process In the powder production process, a slurry containing barium titanyl oxalate is allowed to stay in a reaction tube 3 produced as a reaction environment in a supercritical mixed solvent state (or subcritical mixed solvent state) for a predetermined time. In this step, barium titanate fine particles are produced, and the decomposition or dissolution step and the crystallization step are successively performed.

C-1. Decomposition or dissolution process In the decomposition or dissolution process, reactants (metal oxides, hydroxides, complex metal complexes, etc.) in the reaction environment of the supercritical mixed solvent state (or subcritical mixed solvent state) in the reaction tube 3 By supplying a slurry containing, the reactant components are decomposed or dissolved by supercritical solvolysis. More specifically, for example, if TiO ₂ + Ba (OH) ₂ is used as a raw material, Ba ⁺² , TiO ₂ hydrate, Ti (OH) ₄ hydrate, Ti (OH) ₃ ⁺¹ , Ti (OH) ₅ ⁻ It is dissolved as ions such as ¹ . Taking oxalate as an example, the complex molecular structure is completely destroyed by the decomposition force caused by the violent collision between the molecule and the subcritical mixed solvent or supercritical mixed solvent. For example, titanium oxide becomes ions such as Ti (OH) ₄ (aq) or Ti (OH) ₃ ⁺¹ and Ti (OH) ₅ ^−1, and the organic components of oxalate are CO, CO ₂ , H ₂ O. It is completely broken down to the simple molecular level. The decomposed components such as Ti and Ba are dissolved in the subcritical mixed solvent or the supercritical mixed solvent. As a result, the decomposed inorganic component achieves high saturation (supersaturation). That is, since powder TiO ₂ , Ba (OH) ₂ , and barium titanyl oxalate are used as raw materials, a high supersaturation degree can be easily achieved locally when decomposed into a subcritical mixed solvent or supercritical mixed solvent. It is suitable for nano-fabrication of the generated barium titanate fine particles. The time required for this decomposition or dissolution process varies depending on the temperature, pressure, and type of reactant.

C-2. Crystallization Step In the crystallization step, after barium titanyl oxalate is decomposed, the reaction environment in the subcritical mixed solvent or supercritical mixed solvent state is allowed to stay in the reaction tube 3 for a predetermined reaction time and decomposed. Barium titanate with high crystallinity in the order of nm is generated by forming and crystallizing barium titanate nuclei by synthesizing a supercritical mixed solvent that synthesizes ions containing Ba and Ti. .

  At this time, the time for staying in the reaction tube 3 constituting the reaction field (reaction time) is adjusted and controlled. In the treatment of the crystallization step, it is necessary to stay for at least several seconds under the conditions that simultaneously satisfy the temperature and pressure conditions for achieving the subcritical mixed solvent or supercritical mixed solvent state as described above. In order to obtain higher crystallinity, it is desirable to stably crystallize by staying for several seconds or more in a subcritical mixed solvent or supercritical mixed solvent state. The reaction time should be within tens of seconds so that the length of the reaction tube is not limited (that is, the length is not too long) and the particle size does not grow beyond the desired size. Is desirable. Thereby, barium titanate with high crystallinity is generated with an optimum size of about 50 nm to 150 nm. For barium titanate, the optimum size with the highest relative dielectric constant is about 50 nm to 150 nm, and the relative dielectric constant of the barium titanate powder tends to decrease if it is too small or too large. It is known that there is.

  Here, the reaction time for staying in order to control the crystallinity and particle size of barium titanate is adjusted by adjusting the tube diameter and length of the reaction tube 3 or adjusting the flow rate by adjusting the pumps 4 and 8 and the valves 2 and 6. Furthermore, it is controlled by adjusting the rapid cooling timing for stopping the reaction.

The stop of barium titanate (BaTiO ₃ ) particle growth is performed by rapid cooling using the cooling bath 9. The cooling rate is, for example, a rapid rate of about 30 ° C./second or more, and is cooled to room temperature. Furthermore, due to the excellent dispersion characteristics of the organic solvent in the supercritical mixed solvent, the generated barium titanate particles are quickly flowed into the supercritical mixed solvent and diffused to disperse the barium titanate particles. Thus, particle aggregation can be suppressed by reducing contact between particles immediately after generation. When importance is attached to prevention of particle aggregation after the crystallization step, the mixing ratio may be adjusted so that the ratio of the organic solvent to the mixed solvent is increased.

D. Recovery Step In the recovery step, the pressure is reduced to atmospheric pressure by the valve 10 and gas is released to perform gas-liquid separation, and the slurry is recovered in the recovery tank 11. The recovered slurry is washed with ion-exchanged water in order to remove the additive Na and the like. Further, the barium titanate powder and the liquid are separated by centrifugation at a high speed of at least several thousand rpm. Further, the pH adjustment is repeated until neutrality, and then dried at about 100 ° C. with a dryer to finally obtain barium titanate fine particles.

  In the present embodiment, the ethanol constituting the mixed solvent is mixed into the slurry together with the ion-exchanged water. However, the present invention is not limited to this. For example, ethanol is mixed with water in the high-temperature water supply path. It may be. In any case, it is sufficient that the mixing ratio of water and ethanol is known. According to Reference Document 1, if the temperature is about 350 ° C. or lower, ethanol itself is not decomposed by supercritical water. Moreover, since the hydrolysis rate of ethanol was slow even in the temperature of 400 to 500 degreeC by actual experiment, the mixed solvent of this invention was able to be produced | generated.

In the present embodiment, the liquid mixed with the slurry in the reaction tube (reaction region) 3 may be a reaction accelerator containing at least water for accelerating the synthesis of the powder. As described above, ethanol (organic solvent) Or a mixed solution with another solvent not related to the reaction. In the reaction tube 3 where the slurry and the reaction accelerator are mixed, it is necessary to be in a subcritical or supercritical state. Further, the slurry may be in a state of containing a liquid as long as it does not contain water. In the present embodiment, it is desirable that the raw material slurry contains less water of crystallization or alcohol contained in the raw material slurry, but Ba (OH) ₂ .2H ₂ O, Ba (OH) _2. Reactants with crystal water such as 8H ₂ O, commercially available alcohols of about 99% or more can also be used.

  In this way, the supplied slurry is supplied as a high-temperature and high-pressure slurry to the reaction tube 3 constituting the reaction field from the slurry stirrer 1 for stirring the slurry not containing water through the valve 2, the slurry pump 4, and the heater 13. By supplying the slurry to the reaction field at a high temperature in this way, it can be brought into a subcritical or supercritical state without heating the slurry with water to be mixed. This makes it possible to synthesize the powder under conditions where the ratio of the solvent or the concentration of the raw material is high, that is, under the condition where the ratio of water mixed with the slurry is small. In addition, basically, it is not necessary to heat the slurry with water, so that it is not necessary to increase the temperature of the water. In addition, it is not necessary to heat the slurry with a large amount of high-temperature water to bring it into a subcritical or supercritical state. Thereby, the temperature of the water supplied to the reaction tube can be lowered, and the piping for supplying the water can be prevented from being heated to a high temperature and causing corrosion or the like.

  In addition, even if the slurry is heated to a subcritical or supercritical state, the reaction rate (the rate at which the raw material is decomposed and the particles are generated) is slow in the state that does not contain water. Generation) does not proceed. As a result, the powder synthesis does not proceed basically even if heated first, and is mixed with water in the reaction tube 3 and is synthesized when it becomes a subcritical or supercritical state to produce a powder. Is done.

(Embodiment 2)
The second embodiment is an electronic component manufacturing method for manufacturing an electronic component that includes an electronic material manufactured using barium titanate fine particles, which is a powder manufactured by the manufacturing method of the first embodiment, as a constituent element. An application example is shown. In the second embodiment, an example of a multilayer ceramic capacitor is shown as an electronic component.

  FIG. 4 is a cross-sectional view illustrating a configuration example of the multilayer ceramic capacitor 30. The multilayer ceramic capacitor 30 includes a capacitor element body 40 having a configuration in which dielectric layers 31 and internal electrode layers 32 are alternately stacked. At both ends of the capacitor element body 40, a pair of external electrodes 33 that are respectively connected to the internal electrode layers 32 arranged alternately in the capacitor element body 40 are formed.

  The internal electrode layers 32 are laminated so that each end face is alternately exposed on the surface of the two opposite ends of the capacitor element body 40. The conductive material contained in the internal electrode layer 32 is not particularly limited, and a noble metal or a base metal (for example, Ni or Ni alloy) can be used. The pair of external electrodes 33 are connected to the exposed end surfaces of the alternately arranged internal electrode layers 32 to constitute a capacitor circuit. The conductive material contained in the external electrode 33 is not particularly limited, but inexpensive Ni, Cu, and alloys thereof can be used.

  The dielectric layer 31 is a thin film layer of a dielectric material made from barium titanate fine particles produced by the production method of the first embodiment.

  The multilayer ceramic capacitor 30 having such a structure is manufactured by producing a green chip by a normal printing method or a sheet method using a paste, firing the green chip, and printing or transferring the external electrode 33 and firing. Is done.

  When the printing method is used, the dielectric layer paste and the internal electrode layer paste are printed and laminated on a substrate such as PET, cut into a predetermined shape, and then peeled from the substrate to obtain a green chip. When the sheet method is used, a dielectric layer paste is used to form a green sheet, the internal electrode layer paste is printed thereon, and these are stacked to form a green chip. Further, before firing, the green chip is subjected to binder removal processing.

  The atmosphere at the time of firing the green chip may be appropriately determined according to the type of the conductive material in the internal electrode layer paste, but when a base metal such as Ni or Ni alloy is used as the conductive material, a reducing firing atmosphere Is preferred. The holding temperature during firing is preferably 1000 ° C to 1400 ° C.

  The capacitor element body 40 obtained as described above is subjected to end surface polishing, and an external electrode paste is applied and baked to form the external electrode 33. Then, a coating layer by plating or the like is formed on the surface of the external electrode 33 as necessary.

  A multilayer ceramic capacitor 30 including a thin film layer of a dielectric material made of barium titanate fine particles as a dielectric layer 31 and thus manufactured is mounted on a printed circuit board or the like by soldering or the like, and various electronic devices, etc. Used for.

  Although the second embodiment has been described as an application example to a method of manufacturing an electronic component that is a capacitor using the barium titanate fine particles manufactured by the manufacturing method of the first embodiment as a dielectric material, the present invention is not limited thereto. The present invention can be similarly applied to an electronic component that uses barium titanate fine particles as an electronic material such as a piezoelectric material or a semiconductor, such as a PTC element.

  Moreover, it is preferable that the manufacturing apparatus performs gold plating on a region where raw materials are reacted to synthesize particles, that is, a surface in contact with the raw materials. For example, in the case of a continuous flow manufacturing apparatus, it is preferable to perform gold plating at least on the reaction tube. As described above, by applying gold plating to the flow path of the particles and raw materials, impurities mixed in the particles can be reduced. Further, it is more preferable to apply gold plating to a region through which raw materials and particles pass, piping, and the like. Note that the treatment performed on the region where the raw materials are reacted to synthesize the particles, and further on the region where the raw materials and the particles pass, is not limited to gold plating, as long as the reactivity can be further reduced. You may make it comprise with platinum and gold | metal | money. Moreover, when synthesizing barium titanate as in the present embodiment, the surface in contact with the raw material may be made of titanium. By forming the surface in contact with the raw material with titanium which is the same element as barium titanate, impurities can be prevented from being mixed into the generated particles.

  In addition, in the said embodiment, although demonstrated as a case where it produces | generates by synthesizing | combining barium titanate, the method of this invention is the synthesis | combining method of various metal oxide nanoparticles other than barium titanate, for example, ferrite, It can be used as a synthesis method of a piezoelectric material, a positive electrode material of a lithium secondary battery, a phosphor, and the like.

Further, by using a conventional supercritical hydrothermal synthesis method, fine particles such as a dielectric, ferrite, a positive electrode material of a lithium secondary battery, and a phosphor can be synthesized. In the case of synthesizing these various fine particles, by applying the method of the present invention in the same manner, nanoparticles having the above-mentioned better characteristics can be produced. For example, examples of the synthesis of dielectric nanoparticles include SrTiO ₃ , (Ba, Sr) TiO ₃ , (Ba, Ca) TiO ₃ , (Ca _0.25 Cu _0.75 ) TiO _{3 in} addition to BaTiO _3. . It can also be used for synthesis of LiCoO ₂ or LiFePO ₄ as a positive electrode material of a lithium secondary battery, and synthesis of BaO · 6F ₂ O ₃ or the like as a synthesis of ferrite nanoparticles.

(Measurement Example 1)
As a measurement example 1, using a continuous flow manufacturing apparatus as shown in FIG. For example, a mixed liquid of 20 MPa / 480 ° C. high-temperature high-pressure water and ethanol (ethanol about 80%) at a flow rate of 20 cc / min and adjusted raw material slurry (barium titanyl oxalate, NaOH, ethanol) 4 cc / min Mixed. The temperature of the mixed fluid introduced into the reaction tube 3 was about 280 ° C. Thus, in order to make the temperature after mixing 250-300 ° C. when using a slurry at room temperature and setting the ethanol ratio after mixing to about 4 mol% to 74 mol% at a wide pressure of 20 MPa to 35 MPa, The mixing ratio of room temperature slurry: high temperature aqueous solution had to be about 1: 5-8.

Example 1
Next, the case where the raw slurry is heated using a continuous flow manufacturing apparatus as shown in FIG. 2 and then mixed with a reaction accelerator containing water to synthesize barium titanate will be described. The diameter of the piping was 1/8 inch. In Example 1, first, the adjusted room temperature raw material slurry (a charge amount of methanol, TiO ₂ and Ba (OH) ₂ of 2 g / L) was increased to 20 MPa, heated to about 400 ° C., and 20 cc / min. The flow rate was supplied to the reaction tube 3 side. Further, a room temperature NaOH (4 g / L) aqueous solution was joined to the slurry flowing toward the reaction tube 3 at a flow rate of about 4 cc / min. That is, mixing was performed under a mixing condition of raw material slurry: aqueous solution = about 5: 1. The temperature after mixing of the mixed fluid introduced into the reaction tube 3 was 295 ° C. Then, the mixed fluid in the reaction tube 3 became a value higher than the critical condition of the mixed solvent in which the temperature and pressure were estimated, and a reaction environment by the supercritical mixed solvent was generated in the reaction tube 3. The transit time for the mixed solvent containing the slurry to pass through the reaction tube 3 was about 3 seconds. And the mixed fluid (product produced | generated by the reaction tube 3) which passed the reaction tube 3 was cooled to room temperature by the cooling tank 9 for several seconds.

  Since the mixing ratio of the slurry and water was about 5: 1, it was possible to synthesize barium titanate under conditions of a mixing ratio (ratio of slurry and water) that was about 10 times higher than that in Measurement Example 1. This makes it possible to synthesize barium titanate with the same high crystallinity and an average size of several tens of nanometers even at a high mixing ratio. It can be seen that it can be improved by about one digit.

(Example 2)
As another example, as in Example 1, the continuous flow production apparatus as shown in FIG. 2 was used to measure the slurry: high temperature water mixing ratio at 6: 1. Specifically, the prepared raw material slurry (barium titanyl oxalate charge amount 2 g / L, NaOH addition amount 1.5 g / L, methanol) was heated to 315 ° C., and water and 6 at room temperature at a flow rate of 18 cc / min. The mixture was mixed at a ratio of 1 and introduced into the reaction tube 3. And the reaction environment by a supercritical mixed solvent was produced | generated in the reaction tube 3 by making temperature and pressure into 275 degreeC and 15 Mpa higher than the critical condition of the estimated mixed solvent. The transit time for the mixed solvent containing the slurry to pass through the reaction tube 3 was about 3 seconds. And the product was cooled to room temperature by the cooling tank 9 in several seconds.

As a result of the generation under the above conditions, the average particle size d50 was about 30 nm, FWHM (111) was 0.35, and the specific surface area SSA was 42.4 m ² / g. Thus, the synthesized particles were able to be produced by suitably synthesizing the particles even when the mixing ratio was 6: 1. This makes it possible to synthesize barium titanate with the same high crystallinity and an average size of several tens of nanometers even at a high mixing ratio, increase mass productivity, and reduce the ratio of the amount of raw material to the water supplied to the reaction tube. It can be seen that it can be improved by about one digit.

  As described above, the method for synthesizing powders and the method for producing electronic components according to the present invention are useful for synthesizing metal oxide powders, and are particularly suitable for synthesizing barium titanate powders.

DESCRIPTION OF SYMBOLS 1 Slurry stirrer 2, 10 Valve 3 Reaction tube 4 Slurry pump 5 Tank 6 Valve 7, 12, 13 Heater 8 Liquid feed pump 9 Cooling tank 11 Recovery tank 30 Multilayer ceramic capacitor 31 Dielectric layer 32 Internal electrode layer 33 External electrode 40 Capacitor Element body

Claims (6)

A first step of pressurizing and heating a raw material and a raw material slurry mixed with a solvent that becomes a supercritical state at a lower pressure and lower temperature than in the case of water alone;
A second step of separately supplying a pressurized and heated raw material slurry and a reaction accelerator containing water to the reaction path, and generating a subcritical or supercritical reaction environment by pressurizing the reaction path; ,
A third step of generating fine powder particles by allowing the raw slurry to stay in the reaction field for a predetermined time using the generated reaction environment in the subcritical or supercritical state as a reaction field;
And a fourth step of cooling the generated fine powder particles and stopping the growth of the fine powder particles.   2. The powder according to claim 1, wherein the second step supplies the pressurized and heated raw material slurry to the reaction path at a volume ratio higher than that of water contained in the reaction accelerator. Synthesis method. And further, heating the reaction accelerator.
The method for synthesizing a powder according to claim 1 or 2, wherein the second step supplies the heated reaction accelerator to the reaction path.   The said 2nd process makes the said raw material slurry into a supercritical state before the said raw material slurry and the said reaction accelerator are mixed, The any one of Claim 1 to 3 characterized by the above-mentioned. Powder synthesis method.   The method for synthesizing powder according to any one of claims 1 to 4, wherein the solvent includes an organic solvent that can be continuously dissolved in water in a supercritical state.   An electronic component comprising an electronic material manufactured by using the fine powder particles produced by the powder synthesis method according to claim 1 as a constituent element. Manufacturing method.

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