JP2020029611A - Production method of copper nanoparticle - Google Patents

Abstract

To provide a production method of copper nanoparticles, by which impurities in copper nanoparticles can be efficiently removed while reducing the amount of water for cleaning, and copper nanoparticles having good dispersibility can be obtained while suppressing oxidation.SOLUTION: The production method of copper nanoparticles according to one aspect of the present invention includes: a step of preparing a dispersion of copper nanoparticles having an average particle diameter of 200 nm or less by reducing copper ions in an aqueous solution to which a reducing agent and a dispersant are added; a step of subjecting the dispersion of copper nanoparticles after the preparation step to centrifugal separation to separate the dispersion into a liquid phase and a solid phase containing the copper nanoparticles; a step of adding pure water to the solid phase containing the copper nanoparticles after the centrifugal separation step; and a step of cleaning the solid phase containing the copper nanoparticles after the step of adding pure water, by use of a high-speed rotary homogenizer equipped with a turbine and a stator.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing copper nanoparticles.

  A liquid phase reduction method for depositing metal nanoparticles in a solution is known. In this liquid phase reduction method, metal nanoparticles are precipitated in a solution by reducing metal ions with a reducing agent in a solution containing a complexing agent and a dispersing agent, for example. After removing impurities in the solution, the metal nanoparticles precipitated in the solution by the liquid phase reduction method are used as a metal nanoink by adjusting the concentration by adding a solvent such as pure water.

  As a method for removing impurities other than metal nanoparticles from the solution, for example, a method of centrifuging metal nanoparticles using a centrifuge has been proposed (see JP-A-2006-183092).

JP 2006-183092 A

  However, the production method described in the above publication has a problem in improving the dispersibility of the solid phase containing the copper nanoparticles after centrifugation. That is, in the liquid phase reduction method, copper ions are generally reduced in an aqueous solution to which a reducing agent and a dispersant are added in order to suppress aggregation of the metal nanoparticles. On the other hand, copper nanoparticles synthesized in an aqueous solution need to be repeatedly subjected to a step of adding pure water and washing after dehydration by centrifugation to remove impurities. As a result, depending on this manufacturing method, if a large amount of pure water is added during washing of the copper nanoparticles, the dispersant bound to the surface of the copper nanoparticles will be dissolved in the pure water and excessively removed, and after centrifugation, There is a possibility that the dispersibility of the solid phase containing the copper nanoparticles is reduced. Furthermore, since the copper nanoparticles are easily oxidized, it is necessary to suppress the oxidation of the copper nanoparticles during the cleaning step.

  The present invention has been made based on such circumstances, and can reduce the amount of water at the time of washing to efficiently remove impurities of the copper nanoparticles, and has excellent dispersibility while suppressing oxidation. It is an object of the present invention to provide a method for producing copper nanoparticles capable of obtaining the following.

  Solution to Problem The method for producing metal nanoparticles according to one embodiment of the present invention, which has been made to solve the above-described problems, includes copper nanoparticles having an average particle diameter of 200 nm or less by reducing copper ions in an aqueous solution to which a reducing agent and a dispersant are added. A step of preparing a dispersion of the copper nanoparticles, a step of centrifuging the dispersion of the copper nanoparticles after the step of preparing into a liquid phase and a solid phase containing the copper nanoparticles, and the step of centrifuging the copper nanoparticles A step of adding pure water to a solid phase containing the particles; and a step of washing the solid phase containing the copper nanoparticles with a high-speed rotating homogenizer having a turbine and a stator after the step of adding the pure water.

  ADVANTAGE OF THE INVENTION The manufacturing method of the copper nanoparticle of this invention can reduce the amount of water at the time of washing | cleaning, can remove the impurity of a copper nanoparticle efficiently, and can obtain the copper nanoparticle with favorable dispersibility while suppressing oxidation.

It is a flow figure showing the manufacturing method of the copper nanoparticles concerning one embodiment of the present invention.

[Description of Embodiment of the Present Invention]
First, embodiments of the present invention will be listed and described.

  The method for producing copper nanoparticles according to one embodiment of the present invention includes a step of preparing a dispersion of copper nanoparticles having an average particle diameter of 200 nm or less by reducing copper ions in an aqueous solution to which a reducing agent and a dispersant are added, A step of centrifuging the dispersion of the copper nanoparticles after the step of preparing into a liquid phase and a solid phase containing the copper nanoparticles, and pure water into the solid phase containing the copper nanoparticles after the step of centrifuging. A step of adding, and after the step of adding the pure water, a step of washing the solid phase containing the copper nanoparticles with a high-speed rotary homogenizer having a turbine and a stator.

  The method for producing copper nanoparticles includes a step of washing the solid phase containing the copper nanoparticles with a high-speed rotating homogenizer having a turbine and a stator after the step of adding the pure water. Excellent phase dispersibility in pure water. Therefore, the effect of removing impurities can be greatly improved with a small amount of pure water, and the number of times of cleaning can be greatly reduced. In addition, the surface of the copper nanoparticles is bound with the dispersant used during the reduction of copper ions in the step of preparing the dispersion of the copper nanoparticles, but the step of washing with a large amount of pure water. , The dispersant may be excessively removed from the surface of the copper nanoparticles. The method for producing the copper nanoparticles, by using a high-speed rotary homogenizer having a turbine and a stator in the cleaning step, the amount of water during cleaning can be reduced, so that the dispersant is excessively removed from the surface of the copper nanoparticles. Can be suppressed. Furthermore, the high-speed rotary homogenizer including the turbine and the stator can moderately suppress the stirring force, so that the progress of oxidation of the copper nanoparticles due to excessive stirring can be reduced. Therefore, the method for producing the copper nanoparticles, while reducing the amount of water during washing, can efficiently remove the impurities of the copper nanoparticles, while suppressing the oxidation of the copper nanoparticles, copper nanoparticles with good dispersibility. Obtainable.

  It is preferable that the number of revolutions of the high-speed rotation type homogenizer in the washing step is not less than 3000 rpm and not more than 25000 rpm. As described above, the rotation speed of the high-speed rotating homogenizer in the washing step is 3000 rpm or more and 25000 rpm or less, so that the effect of removing impurities of copper nanoparticles, the effect of suppressing oxidation of copper nanoparticles, and the dispersibility are reduced. Can be further improved.

  In the method for producing copper nanoparticles, the washing step is preferably performed under reduced pressure. As described above, by performing the above-described washing step under reduced pressure, dissolved oxygen in pure water that causes oxidation can be removed, so that oxidation of copper nanoparticles that are easily oxidized can be suppressed.

  In the method for producing copper nanoparticles, it is preferable to add the dispersant to the pure water in advance in the step of adding the pure water. In this way, by adding the dispersant to the pure water in the step of adding the pure water in advance, it is possible to suppress the dispersant from being excessively removed from the surface of the copper nanoparticles in the washing step.

  In the method for producing copper nanoparticles, it is preferable that in the step of adding the pure water, dissolved oxygen in the pure water is removed by performing an inert gas bubbling on the pure water in advance. In this way, by performing inert gas bubbling on pure water in advance, by removing dissolved oxygen in pure water that causes oxidation, oxidation of copper nanoparticles that are easily oxidized can be suppressed.

  In the present invention, the “average particle diameter” of the copper nanoparticles in the copper nanoparticle dispersion refers to a median diameter (D50) calculated from a volume-based cumulative distribution measured by a dynamic light scattering method.

[Details of Embodiment of the Present Invention]
<Method for producing copper nanoparticles>
Hereinafter, each embodiment of the method for producing copper nanoparticles according to the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a procedure of a method for producing copper nanoparticles according to one embodiment of the present invention. The method for producing copper nanoparticles includes a step of preparing a dispersion of copper nanoparticles having an average particle diameter of 200 nm or less by reducing copper ions in an aqueous solution to which a reducing agent and a dispersant are added (a copper nanoparticle dispersion liquid preparation step). : Step S1), a step of centrifuging the dispersion liquid of the copper nanoparticles after the preparation step into a liquid phase and a solid phase containing the copper nanoparticles (centrifugation step: Step S2), and the step of centrifugation The subsequent step of adding pure water to the solid phase containing the copper nanoparticles (pure water adding step: step S3), and the solid phase containing the copper nanoparticles after the step of adding the pure water is provided with a turbine and a stator. Washing with a high-speed rotating homogenizer (washing step: step S4). Further, the step of adding the pure water and the step of washing can be repeated a plurality of times as necessary. The copper nanoparticles obtained by the method for producing copper nanoparticles can be used to prepare a copper nanoink by adjusting the concentration with a solvent. This copper nanoink is used, for example, to form a sintered body layer of copper nanoparticles on a base film of a printed wiring board.

[Copper nanoparticle dispersion liquid preparation step]
In the copper nanoparticle dispersion preparation step, a copper nanoparticle dispersion having an average particle diameter of 200 nm or less is prepared by reducing copper ions in an aqueous solution to which a reducing agent and a dispersant are added. For example, a water-soluble copper compound which forms copper ions forming copper nanoparticles in water, a dispersant and a complexing agent are dissolved, and a reducing agent is added to cause a reduction reaction of the copper ions for a certain period of time. The copper nanoparticles produced by the liquid phase reduction method can be fine particles having a uniform spherical or granular shape and an average particle diameter of 50 nm or less. Examples of the water-soluble copper compound from which the copper ions are formed include copper (II) nitrate trihydrate (Cu (NO ₃ ) ₂ .3H ₂ O) and copper (II) sulfate pentahydrate (CuSO ₄ .5H ₂ O) and the like.

  As the reducing agent, various reducing agents capable of reducing and precipitating copper ions in a liquid phase (aqueous solution) reaction system can be used. Examples of the reducing agent include sodium borohydride, sodium hypophosphite, hydrazine, ions of transition metals such as trivalent titanium ions and divalent cobalt ions, ascorbic acid, reducing sugars such as glucose and fructose, Examples include polyhydric alcohols such as ethylene glycol and glycerin. Among them, a trivalent titanium ion is preferable as the reducing agent. The liquid phase reduction method using trivalent titanium ions as a reducing agent is referred to as a titanium redox method. In the titanium redox method, copper ions are reduced by redox action when trivalent titanium ions are oxidized to tetravalent, and copper nanoparticles are precipitated. According to the titanium redox method, it is easy to form copper nanoparticles having a fine and uniform particle diameter.

  The dispersant preferably does not contain sulfur, phosphorus, boron, halogen and alkali from the viewpoint of preventing deterioration of peripheral members. As preferred dispersants, nitrogen-containing polymer dispersants such as polyethyleneimine and polyvinylpyrrolidone, polyacrylic acid, hydrocarbon-based polymer dispersants having a carboxy group in a molecule such as carboxymethylcellulose, poval (polyvinyl alcohol), Polymer dispersants having a polar group such as a styrene-maleic acid copolymer, an olefin-maleic acid copolymer, and a copolymer having a polyethyleneimine moiety and a polyethylene oxide moiety in one molecule can be exemplified.

  Examples of the complexing agent include sodium citrate, sodium tartrate, sodium acetate, gluconic acid, sodium thiosulfate, ammonia, ethylenediaminetetraacetic acid, and the like, and one or more of these can be used. Among them, sodium citrate is preferable as the complexing agent.

  To adjust the particle size of the copper nanoparticles, adjust the type and blending ratio of the copper compound, dispersant and reducing agent, and adjust the stirring speed, temperature, time, pH, etc., when the copper compound undergoes the reduction reaction. do it. The lower limit of the pH of the reaction system is preferably 7, and the upper limit of the pH of the reaction system is preferably 13. By setting the pH of the reaction system within the above range, copper nanoparticles having a fine particle diameter can be obtained. At this time, the pH of the reaction system can be easily adjusted to the above range by using the pH adjuster. As the pH adjuster, common acids or alkalis such as hydrochloric acid, sulfuric acid, nitric acid, sodium hydroxide, sodium carbonate, and ammonia can be used. Particularly, in order to prevent deterioration of peripheral members, an alkali metal, an alkaline earth, or the like is used. Nitric acid and sodium carbonate which do not contain impurities such as metals, halogens, sulfur, phosphorus and boron are preferred.

  The content ratio of the copper nanoparticles in the dispersion liquid of the copper nanoparticles is preferably, for example, 0.1% by mass or more and 5.0% by mass or less.

  The lower limit of the average particle diameter of the copper nanoparticles is preferably 1 nm, more preferably 10 nm. On the other hand, the upper limit of the average particle diameter of the copper nanoparticles is 200 nm, preferably 150 nm. When the average particle diameter of the copper nanoparticles is less than the lower limit, for example, when the copper nanoparticles are used in the copper nanoink, the dispersibility and stability of the copper nanoparticles are reduced, so that the base of the printed wiring board substrate is reduced. There is a possibility that uniform lamination on the surface of the film may not be easy. On the other hand, when the average particle diameter of the copper nanoparticles exceeds the upper limit, for example, when copper nanoparticles are used for the copper nanoink, the voids in the sintered body layer of the dried coating film of the formed copper nanoink increase, Sufficient conductivity may not be obtained.

[Centrifugation step]
In the centrifugation step, the dispersion liquid of the copper nanoparticles after the preparation step is centrifuged into a liquid phase and a solid phase containing the copper nanoparticles. The centrifugation can be performed by a centrifuge.

  The lower limit of the centrifugal acceleration in the centrifugation step is preferably 10,000 G, more preferably 20,000 G. If the centrifugal acceleration is less than the lower limit, the copper nanoparticles may not be sufficiently centrifuged. The upper limit of the centrifugal acceleration is not particularly limited, but may be, for example, 120,000 G. If the centrifugal acceleration exceeds the upper limit, the concentration of the solid phase containing the copper nanoparticles after centrifugation becomes too high, and the solid phase containing the copper nanoparticles may adhere to a container or the like, and the yield may be reduced. .

[Pure water addition step]
In the pure water addition step, pure water is added to the solid phase containing the copper nanoparticles after the centrifugation step. Specifically, after taking out the solid phase containing the copper nanoparticles after centrifugation into a container, the mixture is stirred with a glass rod while adding pure water.

  In the pure water adding step, it is preferable to add the dispersant to the pure water in advance. The lower limit of the amount of the dispersant added is preferably 0.1% by mass, more preferably 1.0% by mass, based on the solid phase. If the amount of the dispersant added is less than the lower limit, there is a possibility that the effect of suppressing the excessive removal of the dispersant from the surface of the copper nanoparticles in the washing step may not be sufficiently obtained. The upper limit of the amount of the dispersant added is preferably 10.0% by mass with respect to the solid phase, and more preferably 5.0% by mass. When the amount of the dispersant exceeds the upper limit, the dispersant may easily remain after the particles are washed, and may be a residue in the sintering step.

  In the pure water addition step, it is preferable to remove dissolved oxygen in the pure water by performing an inert gas bubbling on the pure water in advance. By removing the dissolved oxygen in the pure water by performing inert gas bubbling on the pure water in advance, it is possible to suppress the oxidation of the copper nanoparticles that are easily oxidized.

  The inert gas is a gas that does not corrode the copper nanoparticles and has a low oxygen concentration, specifically a gas having a volume content of oxygen of 10% or less, preferably 1% or less. Examples of the inert gas include a nitrogen gas and an argon gas. Among them, inexpensive nitrogen gas is preferable.

  The bubbling amount of the inert gas with respect to the pure water in the above inert gas bubbling is preferably 100 ml / L or more and 500 ml / L or less. When the bubbling amount of the inert gas is within the above range, dissolved oxygen in pure water can be effectively removed.

  Further, the final dissolved oxygen content in pure water is preferably 1.0 mg / L or less. By setting the final dissolved oxygen amount in pure water within the above range, the oxidation of the copper nanoparticles that are easily oxidized can be effectively suppressed.

[Washing process]
In the washing step, the dispersion of the copper nanoparticles is stirred by the homogenizer having the turbine and the stator after the pure water addition step. In this step, the amount of water at the time of washing can be reduced by stirring the dispersion liquid of the copper nanoparticles with the high-speed rotating homogenizer including the turbine and the stator, so that the dispersant is excessively removed from the surface of the copper nanoparticles. Can be suppressed. Furthermore, the high-speed rotation type homogenizer including the turbine and the stator can moderately suppress the stirring force, so that the progress of oxidation of the copper nanoparticles due to excessive stirring can be reduced.

  The lower limit of the rotation speed of the high-speed rotating homogenizer in the washing step is preferably 3000 rpm, and more preferably 5000 rpm. If the rotation speed of the high-speed rotation type homogenizer is less than the lower limit, the effect of removing impurities may not be sufficient. On the other hand, the upper limit of the rotation speed is preferably 25,000 rpmPa, and more preferably 20,000 rpmPa. When the rotation speed exceeds the upper limit, oxidation of the nanoparticles may proceed due to excessive stirring.

  The washing step is preferably performed under reduced pressure. By performing the above-mentioned washing step under reduced pressure, dissolved oxygen in pure water that causes oxidation can be removed, so that oxidation of copper nanoparticles that are easily oxidized can be suppressed.

  The reduced pressure condition is preferably 100,000 Pa or less, more preferably 10,000 Pa or less.

  Copper nanoparticles produced by the method for producing copper nanoparticles, for example, after concentration by centrifugation again after the washing step described above, by adjusting the concentration of copper nanoparticles in the dispersion of copper nanoparticles, copper nano-ink Can be manufactured.

  According to the method for producing copper nanoparticles, the amount of water at the time of washing can be reduced, impurities in the copper nanoparticles can be efficiently removed, and copper nanoparticles having good dispersibility while suppressing oxidation can be obtained.

[Other Embodiments]
The embodiments disclosed this time are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is not limited to the configuration of the above-described embodiment, but is indicated by the appended claims, and is intended to include all modifications within the scope and meaning equivalent to the appended claims. You.

  In the above-described embodiment, it has been described that the inert gas bubbling is performed in advance on the pure water in the step of adding the pure water, but the dissolution in the pure water is performed by performing the inert gas bubbling on the pure water during the cleaning process. Oxygen may be removed.

  Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

<Copper nanoparticles>
[No. 1]
(1) Copper nanoparticle dispersion liquid preparation step First, 160 g of 0.1% of a 20% titanium trichloride solution as a reducing agent, 100 g of sodium carbonate as a pH adjuster, and sodium citrate as a complexing agent were added to a reaction tank. 180 g of polyvinylpyrrolidone (molecular weight: 30,000) as a dispersing agent were dissolved in 1.8 L of pure water, and the aqueous solution was kept at 40 ° C. To this aqueous solution, 40 g of a 50% aqueous solution of copper nitrate kept at the same temperature was added in 2 seconds while stirring a 0.04 M aqueous solution to precipitate 5 g of copper nanoparticles to prepare a dispersion of copper nanoparticles.

(2) Centrifugation Step The dispersion of copper nanoparticles prepared as described above was centrifuged at a centrifugal acceleration of 20000 G for 10 minutes and concentrated.

(3) Pure Water Addition Step Next, 300 g of pure water was added to the solid phase containing 5 g of the copper nanoparticles.

(4) Cleaning Step Next, the dispersion liquid of the copper nanoparticles after the addition of the pure water was subjected to a rotation at 8000 rpm using a high-speed rotary homogenizer “Lab Lution” having a turbine and a stator manufactured by Primix Co., Ltd. at 5 rpm. The copper nanoparticles were washed by stirring for minutes.

(5) Re-centrifugation step Next, the dispersion liquid of the copper nanoparticles after the washing step was centrifuged again at a centrifugal acceleration of 20000 G for 10 minutes, and concentrated.

(Manufacture of copper nano ink)
After adding 20 g of pure water to the copper nanoparticles concentrated after the washing step, the concentration was adjusted to 25% by mass to produce a copper nanoink.

[No. 2-No. 6]
The conditions of No. A copper nanoink was manufactured in the same procedure as in Example 1.

(impurities)
Copper nanoparticles no. 1 to No. The presence or absence of impurities in the copper nano-ink obtained in No. 6 was measured using the ion chromatography system “ICS-2100” manufactured by Thermo Fisher Scientific Co., Ltd. and the ICP emission spectrometer “iCAP6300” manufactured by the company, and evaluated according to the following criteria. did.
A: no impurities were detected B: impurities were slightly detected C: impurities were detected in large quantities

(Dispersibility)
After the washing step, centrifugation was performed, and the size of the copper nanoparticle aggregate in a state where pure water was added to the solid phase containing the copper nanoparticles was measured. The dispersibility of the copper nanoparticles was evaluated according to the following criteria using a laser diffraction particle size distribution analyzer, SALD-2300 manufactured by Shimadzu Corporation, capable of measuring to the order of microns.
A: No copper nanoparticle aggregate peak is present, and the dispersibility is excellent. B: Copper nanoparticle aggregate peak is less than 0.5 μm and the dispersibility is good. C: Copper nanoparticle aggregate peak. Is 0.5 μm or more, and dispersibility is poor.

(Average particle size)
Copper nanoparticles no. 1 to No. The average particle size (D50) of the copper nanoparticles contained in the copper nanoink obtained in 6 was obtained from the volume-based cumulative distribution measured by dynamic light scattering using "NanoTrac Wave" manufactured by Microtrac Bell. Calculated.

(Particle size distribution)
Using “NanoTrac Wave” manufactured by Microtrac Bell Co., Ltd., copper nanoparticle No. 1 to No. The particle size distribution of the copper nanoparticles contained in the copper nanoink obtained in 6 was measured. Table 1 shows the minimum and maximum values of the particle size of the copper nanoparticles.

(Degree of oxidation)
The degree of oxidation was measured by an X-ray diffraction method for a film obtained by applying a dispersion liquid of copper nanoparticles on a glass substrate and drying.

  Copper nanoparticles no. 1 to No. Table 2 shows the evaluation results of the dispersibility, impurities, average particle size, particle size distribution, and oxidation degree of copper nanoparticles in Example 6.

  As shown in Table 2, the cleaning process was performed using a high-speed rotary homogenizer having a turbine and a stator. 1 to No. No. 4 was able to obtain excellent copper nanoparticles in all of the impurity removing effect, dispersibility, and oxidation inhibiting property despite the small number of washings. In particular, the copper nanoparticles No. to which the dispersant was added in the pure water addition step. 1 and No. 1 In No. 2, the particle size and the particle size distribution were small. In addition, by setting the rotation speed of the high-speed rotation type homogenizer to 8000 rpm or more, a high impurity removing effect was obtained even when the number of times of washing was one. Furthermore, the copper nanoparticles No. From the result of No. 3, it can be seen that by performing the washing step using a high-speed rotation type homogenizer, the dispersibility and the average particle diameter are good even without adding a dispersant.

  On the other hand, a copper nanoparticle No. which was subjected to a washing step using a mixer instead of a high-speed rotating homogenizer. 5-No. Sample No. 6 was inferior in the effect of removing impurities and dispersibility, and the impurities remained even when the number of times of cleaning was increased. In addition, the copper nanoparticles No. From the result of No. 6, it is understood that the degree of oxidation of the copper nanoparticles is increased by increasing the number of times of washing.

  From the above results, the method for producing copper nanoparticles can reduce the amount of water during washing to efficiently remove impurities of copper nanoparticles, and obtain copper nanoparticles having good dispersibility while suppressing oxidation. It was shown that it could be.

  INDUSTRIAL APPLICABILITY The present invention can be widely applied to the formation of a sintered layer of a dried coating film of copper nanoink, and can be suitably used particularly for the production of electronic components such as printed wiring boards.

S1 Copper nanoparticle dispersion liquid preparation step S2 Centrifugal separation step S3 Pure water addition step S4 Washing step

Claims (5)

A step of preparing a dispersion of copper nanoparticles having an average particle diameter of 200 nm or less by reduction of copper ions in an aqueous solution to which a reducing agent and a dispersant have been added,
A step of centrifuging the dispersion of the copper nanoparticles after the preparing step into a liquid phase and a solid phase containing the copper nanoparticles,
A step of adding pure water to the solid phase containing the copper nanoparticles after the step of centrifugation,
Washing the solid phase containing the copper nanoparticles with a high-speed rotary homogenizer having a turbine and a stator after the step of adding the pure water.   The method for producing copper nanoparticles according to claim 1, wherein the number of revolutions of the high-speed rotation type homogenizer in the washing step is 3000 rpm or more and 25000 rpm or less.   The method for producing copper nanoparticles according to claim 1, wherein the washing step is performed under reduced pressure.   The method for producing copper nanoparticles according to claim 1, wherein the dispersant is added to the pure water in advance in the step of adding the pure water.   The copper nanoparticles according to any one of claims 1 to 4, wherein in the step of adding the pure water, dissolved oxygen in the pure water is removed by performing an inert gas bubbling on the pure water in advance. Manufacturing method.

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