JP6927687B2 - Nanodiamond dispersion liquid manufacturing method and nanodiamond dispersion liquid - Google Patents

Description

The present invention relates to a method for producing a nanodiamond dispersion and a nanodiamond dispersion.

In recent years, the development of fine-grained diamond materials called nanodiamonds has been promoted. As for nanodiamonds, so-called single-digit nanodiamonds having a particle size of 10 nm or less may be required depending on the application. Techniques for producing such nanodiamonds are described, for example, in Patent Documents 1 to 3 below.

Japanese Unexamined Patent Publication No. 2005-001983 JP-A-2010-126669 Japanese Patent Application Laid-Open No. 2016-5200035

Nanodiamonds with a primary particle size of 10 nm or less can exhibit high mechanical strength, high thermal conductivity, high refractive index, etc., as do bulk diamonds. Nanoparticles, which are fine particles, generally have a large proportion of surface atoms (coordinatively unsaturated), so that the sum of van der Waals forces that can act between the surface atoms of adjacent particles is large and agglutination occurs. It is easy to occur. In addition to this, in the case of nanodiamond particles, a phenomenon called agglutination can occur in which the Coulomb interaction between the crystal planes of adjacent crystal faces contributes and very strongly aggregates. Nanodiamond has such a unique property that superimposition interaction can occur between crystallites or primary particles, for example, to create a state in which nanodiamond primary particles are dispersed in a solvent, and so on. Maintaining a good condition often involves technical difficulties. Nanodiamonds, for example, in the products obtained by the detonation method, first take the form of cohesive particles (secondary particles) in which the primary particles are assembled due to the very strong interaction between the primary particles. However, there are technical difficulties in crushing the secondary particles into primary particles and maintaining the dispersed state of the primary particles in a desired solvent.

Wet crushing treatment such as bead milling is known as an effective crushing method from secondary particles to primary particles of nanodiamond. Then, in the nanodiamond-containing slurry to be subjected to the treatment and the nanodiamond aqueous dispersion obtained through the treatment, a large zeta potential is secured for the nanodiamond particles in the aqueous dispersion to stabilize the dispersion. In many cases, a pH adjuster is added to adjust the pH for the purpose of achieving the change. For example, in order to secure a large positive zeta potential for nanodiamond particles in an aqueous dispersion, an acid such as hydrochloric acid is used as a pH adjuster.

However, the addition of such a pH adjuster causes an increase in the electrolyte concentration of the obtained nanodiamond aqueous dispersion, which is not preferable. In the process of producing a composite material of nanodiamond and, for example, a resin, a step of mixing a predetermined organic material such as an organic solvent and a nanodiamond aqueous dispersion is often performed, and the electrolyte concentration of the nanodiamond aqueous dispersion is high. The higher the value, the lower the miscibility or dispersibility of the aqueous dispersion with respect to the organic material. The amount tends to decrease, which is not preferable for producing a composite material. Further, the addition of an acid as a pH adjuster causes an increase in the acid concentration of the obtained nanodiamond aqueous dispersion, which is not preferable. When using the nanodiamond aqueous dispersion, the higher the acid concentration of the nanodiamond aqueous dispersion, the more likely the nanodiamond aqueous dispersion is to cause corrosion of the metal material. When a nanodiamond aqueous dispersion is used to prepare a composite material of nanodiamond and resin, the higher the acid concentration of the nanodiamond aqueous dispersion, the more the nanodiamond aqueous dispersion causes deterioration and decomposition of the resin. Easy to make.

The present invention has been devised under the above circumstances, and is suitable for producing a nanodiamond dispersion liquid in which the zeta potential of the nanodiamond particles is positive and the electrolyte concentration is low. It is an object of the present invention to provide a method and to provide such a nanodiamond dispersion.

According to the first aspect of the present invention, a method for producing a nanodiamond dispersion liquid is provided. This production method includes an oxygen oxidation step for oxygen-oxidizing the nanodiamond and a hydrogenation step for hydrogenating the nanodiamond that has undergone the oxygen oxidation step. In the present invention, the oxygen oxidation treatment is a treatment for causing an oxygen oxidation reaction in a state where nanodiamonds are placed in an atmosphere in which oxygen is present. In the present invention, the hydrogenation treatment is a treatment for causing a hydrogenation reaction in a state where nanodiamonds are placed in an atmosphere in which hydrogen is present.

When non-diamond carbon such as graphite or amorphous carbon is present on the surface of nanodiamond, if an oxygen oxidation reaction occurs in the non-diamond carbon, carbon dioxide is generated and the non-diamond carbon is removed from the surface of nanodiamond. The Rukoto. Further, when an oxygen oxidation reaction occurs on the surface of the nanodiamond itself, the surface of the nanodiamond is oxygen-modified. Specifically, a functional group or a partial structure containing an oxygen atom will be generated on the surface of nanodiamond. Examples of such a functional group or a partial structure include a carbonyl group, a carboxy group, a lactone structure, an acid anhydride structure and the like. On the other hand, when a functional group such as a carboxyl group containing an oxygen atom or a partial structure is present on the surface of nanodiamond and a hydrogenation reaction occurs in the functional group, does the functional group form a hydrogen reducer? Alternatively, it will be removed from the nanodiamond surface. Wet crushing of nanodiamonds after the hydrogenation treatment is performed by performing the above-mentioned oxygen oxidation treatment for causing the oxygen oxidation reaction before the hydrogenation treatment for causing such a hydrogenation reaction. The present inventors have found that it is possible to obtain a nanodiamond dispersion having a positive zeta potential of nanodiamond particles without performing pH adjustment accompanied by addition of a pH adjuster before and after the step. For example, as shown in the examples below. In this manufacturing method, which does not require pH adjustment before and after the wet crushing step to obtain a nanodiamond dispersion having a positive zeta potential of the nanodiamond particles, the zeta potential of the nanodiamond particles is positive and the electrolyte concentration is high. Suitable for obtaining low nanodiamond dispersions.

The method for producing nanodiamond dispersion liquid in the first aspect of the present invention preferably comprises a production step for producing nanodiamonds by a roaring method in an inert gas atmosphere, and the nanodiamonds obtained in the production step. A purification step for purifying the crude product is included prior to the oxygen oxidation step. The purification step preferably comprises a mixed acid treatment performed with a mixed acid containing sulfuric acid and nitric acid. According to the detonation method, it is possible to appropriately generate nanodiamonds having a particle size of primary particles of 10 nm or less. The detonation method in an inert gas atmosphere is suitable for producing nanodiamonds having a small particle size and a small amount of functional groups on the surface of the primary particles. As the inert gas, for example, at least one selected from nitrogen, argon, carbon dioxide, and helium can be used. The configuration in which the production method includes the above-mentioned purification step preferably including a mixed acid treatment before the oxygen oxidation step is suitable for increasing the degree of purification of nanodiamonds to be subjected to the oxygen oxidation step. The higher the degree of purification of nanodiamonds used in the oxygen oxidation step, that is, the less non-diamond carbons such as graphite and amorphous carbon are present on the surface of the nanodiamonds used in the oxygen oxidation step, the more nanodiamonds are produced. It is easy to realize a large positive zeta potential for nanodiamond particles in a diamond dispersion.

According to the second aspect of the present invention, a nanodiamond dispersion is provided. This nanodiamond dispersion liquid contains an aqueous dispersion medium and nanodiamond particles dispersed in the dispersion medium and having a positive zeta potential, and has an electric conductivity of 30 μS / cm · wt% or less. In the present invention, the aqueous dispersion medium is a dispersion medium containing 50% by mass or more of water as a dispersion medium component having the largest mass ratio. The nanodiamond particles in this dispersion include nanodiamond primary particles and / or nanodiamond secondary particles. The nanodiamond primary particles are nanodiamonds having a particle size of 10 nm or less. In the present invention, the zeta potential of the nanodiamond particles contained in the nanodiamond dispersion is a value measured at 25 ° C. for the nanodiamond particles in the nanodiamond dispersion having a nanodiamond concentration of 0.2% by mass. When it is necessary to dilute the stock solution of the nanodiamond dispersion for the preparation of the nanodiamond dispersion having a nanodiamond concentration of 0.2% by mass, ultrapure water shall be used as the diluent. The electrical conductivity of the solution can be measured according to the method described in JIS K 0130 (2008) "General Rules for Measuring Electrical Conductivity".

As described above, nanodiamond particles having a positive zeta potential are dispersed in this nanodiamond dispersion liquid. Such a nanodiamond dispersion can be used, for example, as a material for supplying nanodiamond particles having a positive zeta potential in producing a nanodiamond-containing composite material. The electrical conductivity, which is an index of the electrolyte concentration of the nanodiamond dispersion, is 30 μS / cm · wt% or less as described above. The lower the electrolyte concentration of the nanodiamond dispersion, the higher the dispersibility of the dispersion with respect to the organic material. However, the electrical conductivity of the nanodiamond dispersion is 30 μS / cm · wt% or less. The configuration is suitable for achieving sufficiently high dispersibility with respect to the organic material. The composition of the low electrolyte concentration in which the electrical conductivity of the nanodiamond dispersion is 30 μS / cm · wt% or less is, by extension, to the organic material phase in the solution obtained by mixing the aqueous dispersion and the organic material. It is suitable for realizing a high nanodiamond dispersion amount. In addition, the configuration of the low electrolyte concentration in which the electrical conductivity of the nanodiamond dispersion is 30 μS / cm · wt% or less is such that even if the nanodiamond dispersion contains an acid as an electrolyte, the nano is concerned. When using the aqueous diamond dispersion, it is suitable for suppressing corrosion of metal materials such as other materials and deterioration / decomposition of resin materials.

The zeta potential of the nanodiamond dispersion is preferably 30 mV or higher. Such a configuration is suitable for stably maintaining the dispersed state of the nanodiamond particles.

The pH of the nanodiamond dispersion is preferably in the range of 5 or more. Such a configuration is suitable for suppressing the above-mentioned corrosion, deterioration and decomposition of other materials when using the present nanodiamond aqueous dispersion.

It is a process drawing of the nanodiamond dispersion liquid manufacturing method which concerns on one Embodiment of this invention. It is an enlarged schematic diagram of the nanodiamond dispersion liquid which concerns on one Embodiment of this invention.

FIG. 1 is a process diagram of a method for producing a nanodiamond dispersion liquid according to an embodiment of the present invention. This method is a method for producing a dispersion liquid in which nanodiamond particles having a positive zeta potential are dispersed, and is a solution of a production step S1, a purification step S2, an oxygen oxidation step S3, and a hydrogenation step S4. It includes at least the crushing step S5.

In the production step S1, nanodiamonds are produced, for example, by a detonation method. Specifically, first, a molded explosive equipped with an electric detonator is installed inside a pressure-resistant container for detonation, and the container is in a state where a predetermined gas and the explosive used coexist in the container. To seal. The container is made of iron, for example, and the volume of the container is, for example, 0.5 to 40 m ³ . As the explosive, a mixture of trinitrotoluene (TNT) and cyclotrimethylene trinitroamine or hexogen (RDX) can be used. The mass ratio of TNT to RDX (TNT / RDX) is, for example, in the range of 40/60 to 60/40. The amount of explosive used is, for example, 0.05-2.0 kg. The gas sealed in the container together with the explosive used may have an atmospheric composition or may be an inert gas. From the viewpoint of producing nanodiamonds having a small amount of functional groups on the surface of the primary particles, the gas sealed in the container together with the explosive used is preferably an inert gas. That is, from the viewpoint of producing nanodiamonds having a small amount of functional groups on the surface of the primary particles, the detonation method for producing nanodiamonds is preferably performed in an inert gas atmosphere. As the inert gas, for example, at least one selected from nitrogen, argon, carbon dioxide, and helium can be used.

In the generation step S1, the electric detonator is then detonated and the explosive is detonated in the container. Detonation refers to an explosion that accompanies a chemical reaction in which the flame surface on which the reaction occurs moves at a high speed that exceeds the speed of sound. At the time of detonation, nanodiamonds are produced by the action of the pressure and energy of the shock wave generated by the explosion, using the carbon released by the explosive used as a partial incomplete combustion as a raw material. According to the detonation method, it is possible to appropriately generate nanodiamonds having a particle size of primary particles of 10 nm or less. Nanodiamond is a product obtained by the detonation method. First of all, between adjacent primary particles or crystallites, in addition to the action of van der Waals force, the Coulomb interaction between crystal planes contributes to make nanodiamond very strong. Assemble and form a cohesive body.

In the production step S1, the container and the inside thereof are then cooled by leaving it at room temperature for, for example, 24 hours. After this cooling, the nanodiamond crude products (including the nanodiamond adherents and soot produced as described above) adhering to the inner wall of the container are scraped off with a spatula, and the nanodiamond coarse products are scraped off. Collect the product. By the detonation method as described above, a crude product of nanodiamond particles can be obtained. Further, by performing the production step S1 as described above a required number of times, a desired amount of crude nanodiamond product can be obtained.

In the present embodiment, the purification step S2 includes an acid treatment in which a strong acid is allowed to act on the raw material nanodiamond crude product, for example, in an aqueous solvent. The nanodiamond crude product obtained by the detonation method tends to contain metal oxides, but this metal oxide is an oxide such as Fe, Co, Ni derived from the container used in the detonation method. be. For example, by allowing a predetermined strong acid to act in an aqueous solvent, the metal oxide can be dissolved and removed from the crude nanodiamond product (acid treatment). The strong acid used in this acid treatment is preferably a mineral acid, and examples thereof include hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid, and aqua regia. In the acid treatment, one kind of strong acid may be used, or two or more kinds of strong acids may be used. The concentration of the strong acid used in the acid treatment is, for example, 1 to 50% by mass. The acid treatment temperature is, for example, 70 to 150 ° C. The acid treatment time is, for example, 0.1 to 24 hours. Further, the acid treatment can be performed under reduced pressure, normal pressure, or pressure. After such acid treatment, the solid content (including the nanodiamond adherent) is washed with water, for example, by decantation. It is preferable to repeatedly wash the solid with water by decantation until the pH of the precipitate reaches, for example, 2-3. When the content of the metal oxide in the crude nanodiamond product obtained by the detonation method is small, the above acid treatment may be omitted.

In the purification step S2, in the present embodiment, a solution oxidation treatment for removing non-diamond carbon such as graphite and amorphous carbon from the crude nanodiamond product (nanodiamond adherent before completion of purification) using an oxidizing agent is performed. include. The nano-diamond crude product obtained by the detonation method contains non-diamond carbon such as graphite and amorphous carbon, but this non-diamond carbon causes the explosive used to partially incompletely burn. It is derived from the free carbon that did not form nanodiamond crystals. For example, after undergoing the above acid treatment, non-diamond carbon can be removed from the crude nanodiamond product by, for example, allowing a predetermined oxidizing agent to act in an aqueous solvent (solution oxidation treatment). Examples of the oxidizing agent used in this solution oxidation treatment include chromic acid, chromic anhydride, dichromic acid, permanganic acid, perchloric acid, and salts, nitric acids, and mixed acids (mixtures of sulfuric acid and nitric acid) thereof. Can be mentioned. In the solution oxidation treatment, one kind of oxidizing agent may be used, or two or more kinds of oxidizing agents may be used. The concentration of the oxidizing agent used in the solution oxidation treatment is, for example, 3 to 50% by mass. The amount of the oxidizing agent used in the solution oxidation treatment is, for example, 300 to 2000 parts by mass with respect to 100 parts by mass of the nanodiamond crude product subjected to the solution oxidation treatment. The solution oxidation treatment temperature is, for example, 50 to 250 ° C. The solution oxidation treatment time is, for example, 1 to 72 hours. The solution oxidation treatment can be performed under reduced pressure, normal pressure, or pressure. After such a solution oxidation treatment, the solid content (including the nanodiamond adherent) is washed with water, for example, by decantation. Since the supernatant liquid at the beginning of washing with water is colored, it is preferable to repeatedly wash the solid content with water by decantation until the supernatant liquid becomes visually transparent.

As such a solution oxidation treatment, a mixed acid treatment performed using a mixed acid is preferable from the viewpoint of increasing the degree of purification of nanodiamonds to be subjected to the oxygen oxidation step described later. The ratio (volume ratio) of concentrated sulfuric acid to concentrated nitric acid for preparing the mixed acid is preferably 1: 1 to 10: 1, more preferably 2: 1 to 9: 1, and more preferably 3: 1 to 8 :. It is 1. When the mixed acid treatment is adopted as the solution oxidation treatment, the mixed acid treatment temperature is preferably 80 to 200 ° C., more preferably 100 to 190 ° C., more preferably 120 to 180 ° C., and the mixed acid treatment time is preferably 1 to 1 to 1. It is 96 hours, more preferably 5 to 84 hours, more preferably 10 to 72 hours.

Even after undergoing the above acid treatment and solution oxidation treatment, detonation nanodiamonds are in the form of adherents (secondary particles) in which the primary particles interact very strongly and are assembled. Take. In order to promote the separation of the primary particles from the adherent, in the present embodiment, the nanodiamond may then be allowed to act on the nanodiamonds with a predetermined alkali and hydrogen peroxide in an aqueous solvent. Thereby, for example, when the metal oxide that could not be completely removed by the above-mentioned acid treatment remains in the nanodiamond, the metal oxide can be removed, and the nanodiamond primary from the nanodiamond adhering body can be removed. Separation of particles is promoted (alkaline overwater treatment). Examples of the alkali used in this treatment include sodium hydroxide, ammonia, potassium hydroxide and the like. In this treatment, the concentration of alkali is, for example, 0.1 to 10% by mass, the concentration of hydrogen peroxide is, for example, 1 to 15% by mass, the treatment temperature is, for example, 40 to 100 ° C., and the treatment time is, for example, 0. .5 to 5 hours. Further, this treatment can be performed under reduced pressure, normal pressure, or pressure. After the supernatant is removed from the nanodiamond-containing solution that has undergone this treatment by, for example, decantation, the residual fraction is subjected to a drying treatment to obtain a dry powder. Examples of the drying treatment method include spray drying performed using a spray drying device and evaporative drying performed using an evaporator.

In this method, the oxygen oxidation step S3 is then performed. In this step, the nanodiamond powder that has undergone the purification step S2 is heated in a gas atmosphere having a predetermined composition containing oxygen by using a gas atmosphere furnace. Specifically, nanodiamond powder is arranged in a gas atmosphere furnace, oxygen-containing gas is supplied or passed through the furnace, and the temperature inside the furnace is raised to a temperature condition set as a heating temperature. Oxygen oxidation treatment is carried out. The temperature condition of this oxygen oxidation treatment is preferably 250 to 500 ° C. The lower limit of the temperature condition of the oxygen oxidation treatment is more preferably 280 ° C, more preferably 320 ° C. The upper limit of the temperature condition of the oxygen oxidation treatment is more preferably 450 ° C., more preferably 400 ° C. Further, in the present embodiment, the oxygen-containing gas is a mixed gas containing an inert gas in addition to oxygen. Examples of the inert gas include nitrogen, argon, carbon dioxide, and helium. The oxygen content, that is, the oxygen concentration of the mixed gas is preferably 1 to 35% by volume, more preferably 1 to 10% by volume, and more preferably 2 to 5% by volume.

In this method, the hydrogenation step S4 is then performed. In this step, the nanodiamond powder that has undergone the oxygen oxidation step S3 is heated in a gas atmosphere having a predetermined composition containing hydrogen using a gas atmosphere furnace. Specifically, hydrogen-containing gas is supplied or passed through a gas atmosphere furnace in which nanodiamond powder is arranged, and the temperature inside the furnace is raised to the temperature condition set as the heating temperature to generate hydrogen. Chemical processing is carried out. The temperature condition of this hydrogenation treatment is preferably 400 to 800 ° C. The lower limit of the temperature condition of the hydrogenation treatment is more preferably 500 ° C., more preferably 550 ° C. The upper limit of the temperature condition of the hydrogenation treatment is more preferably 700 ° C., more preferably 650 ° C. Further, in the present embodiment, the hydrogen-containing gas is a mixed gas containing an inert gas in addition to hydrogen. Examples of the inert gas include nitrogen, argon, carbon dioxide, and helium. The hydrogen content, that is, the hydrogen concentration of the mixed gas is preferably 0.1 to 99.9% by volume, more preferably 0.5 to 50% by volume, and more preferably 1 to 10% by volume.

By going through a series of processes including the above-mentioned production step S1, purification step S2, oxygen oxidation step S3, and hydrogenation step S4, nanodiamonds that can be used for preparing a nanodiamond dispersion can be obtained.

Even after being purified through the above series of processes, detonation nanodiamonds are cohesive bodies (secondary particles) in which primary particles interact very strongly and are assembled. Take the form. In this method, the crushing step S5 is then performed in order to separate the primary particles from the adherent. Specifically, first, nanodiamonds that have undergone the hydrogenation step S4 are suspended in pure water to prepare a slurry containing the nanodiamonds. In preparing the slurry, a centrifugation treatment may be performed to remove a relatively large aggregate from the nanodiamond suspension, or the nanodiamond suspension may be subjected to ultrasonic treatment. Then, the slurry is subjected to a wet crushing process. The crushing treatment can be performed using, for example, a high shear mixer, a high shear mixer, a homomixer, a ball mill, a bead mill, a high-pressure homogenizer, an ultrasonic homogenizer, or a colloid mill. A crushing treatment may be carried out by combining these. From the viewpoint of efficiency, it is preferable to use a bead mill.

The bead mill, which is a crusher or a disperser, includes, for example, a cylindrical mill container, a rotor pin, a centrifuge mechanism, a raw material tank, and a pump. The rotor pin has a common axis with the mill container and is configured to be rotatable at high speed inside the mill container. The centrifuge mechanism is located at the top of the mill vessel. In the bead milling by the bead mill in the crushing step S5, a predetermined amount of beads are filled in the mill container and the rotor pin is stirring the beads, and the raw material tank is moved to the lower part of the mill container as a raw material by the action of a pump. The above slurry (including nanodiamond adhering material) is charged. The slurry reaches the top of the mill vessel through the beads being stirred at high speed in the mill vessel. In this process, the nanodiamond adherents contained in the slurry are subjected to the action of crushing or dispersing by contact with the violently moving beads. As a result, the crushing of nanodiamond adhering bodies (secondary particles) into primary particles proceeds. The slurry and beads that have reached the upper centrifuge mechanism in the mill container are centrifuged using the difference in specific gravity by the operating centrifugation mechanism, the beads stay in the mill container, and the slurry is sent to the centrifugation mechanism. It is discharged to the outside of the mill container via a hollow line slidably connected. The discharged slurry is returned to the raw material tank, and then put into the mill container again by the action of the pump (circulation operation). In such bead milling, the crushing medium used is, for example, zirconia beads, the diameter of which is, for example, 15-500 μm. The amount of beads (apparent volume) filled in the mill container is, for example, 50 to 80% of the volume of the mill container. The peripheral speed of the rotor pin is, for example, 8 to 12 m / min. The amount of slurry to be circulated is, for example, 200 to 600 ml, and the flow rate of the slurry is, for example, 5 to 15 L / hour. The processing time (circulation operation time) is, for example, 30 to 300 minutes. In the present embodiment, a batch type bead mill may be used instead of the continuous type bead mill as described above.

By going through such a crushing step S5, a nanodiamond dispersion liquid containing the primary particles of nanodiamond dispersed as colloidal particles can be obtained.

For the slurry that has undergone the crushing step S5, a classification operation for removing coarse particles may be performed. For example, a classification device can be used to remove coarse particles from the slurry by a classification operation utilizing centrifugation. As a result, a black transparent nanodiamond dispersion liquid in which the primary particles of nanodiamond are dispersed as colloidal particles can be obtained.

As described above, a nanodiamond dispersion liquid in which the primary particles of nanodiamond are dispersed as colloidal particles can be produced.

In the oxygen oxidation step S3 described above, when non-diamond carbon such as graphite or amorphous carbon is present on the surface of nanodiamond, if an oxygen oxidation reaction occurs in the non-diamond carbon, carbon dioxide is generated and the non-diamond carbon is generated. Diamond carbon is removed from the nanodiamond surface. Further, in the above-mentioned oxygen oxidation step S3, when an oxygen oxidation reaction occurs on the surface of the nanodiamond itself, the surface of the nanodiamond is oxygen-modified. Specifically, a functional group or a partial structure containing an oxygen atom will be generated on the surface of nanodiamond. Examples of such a functional group or a partial structure include a carbonyl group, a carboxy group, a lactone structure, an acid anhydride structure and the like. Then, in the above-mentioned hydrogenation step S4, when a functional group such as a carboxyl group containing an oxygen atom or a partial structure is present on the surface of the nanodiamond and a hydrogenation reaction occurs in the functional group or the like, the functional group is concerned. The groups and the like form a hydrogen reducer or are removed from the surface of the nanodiamond. The nanodiamonds are subjected to the above-mentioned oxygen oxidation treatment for causing an oxygen oxidation reaction prior to the hydrogenation treatment for causing such a hydrogenation reaction, whereby wet crushing after the hydrogenation treatment is performed. The present inventors have found that it is possible to obtain a nanodiamond dispersion having a positive zeta potential of nanodiamond particles without performing pH adjustment accompanied by addition of a pH adjuster before and after step S5. For example, as shown in the examples below. In this production method, which does not require pH adjustment before and after the wet crushing step S5 to obtain a nanodiamond dispersion having a positive zeta potential of the nanodiamond particles, the zeta potential of the nanodiamond particles is positive and the electrolyte concentration. It is suitable for obtaining a nanodiamond dispersion having a low concentration.

FIG. 2 is an enlarged schematic view of the ND dispersion liquid 10 which is the nanodiamond dispersion liquid according to the embodiment of the present invention. The ND dispersion liquid 10 contains ND particles 11 and an aqueous dispersion medium 12, and has an electrical conductivity of 30 μS / cm · wt% or less. The ND dispersion liquid 10 can be used, for example, as a nanodiamond supply material when producing a composite material containing nanodiamonds.

The ND particles 11 contained in the ND dispersion liquid 10 are primary particles or secondary particles of nanodiamonds, and are separated from each other in the aqueous dispersion medium 12 and dispersed as colloidal particles. The particle size D50 (median diameter) of the ND particles 11 is, for example, 60 nm or less, preferably 30 nm or less, more preferably 28 nm or less, more preferably 25 nm or less, more preferably 22 nm or less, and more preferably 20 nm or less. The particle size D50 (median diameter) of the nanodiamond primary particles forming the ND particles 11 is, for example, 10 nm or less, preferably 8 nm or less, and more preferably 6 nm or less. For example, when the ND dispersion liquid 10 is used as a material for adding or supplying nanodiamonds to a transparent resin or the like when forming a nanodiamond-containing transparent member, the smaller the particle size D50 of the ND particles 11, the smaller the particle size D50 of the transparent member. It tends to be preferable for achieving high transparency. On the other hand, the lower limit of the particle size D50 of the ND particles 11 is, for example, 1 nm. The particle size D50 of the particles in the dispersion can be measured by a so-called dynamic light scattering method.

The zeta potential of the ND particles 11 contained in the ND dispersion liquid 10 is positive (that is, takes a positive value), and is preferably 30 mV or more from the viewpoint of stably maintaining the dispersed state of the ND particles 11. , More preferably 35 mV or more, more preferably 40 mV or more. In the present invention, the zeta potential of the nanodiamond particles contained in the nanodiamond dispersion is a value measured at 25 ° C. for the nanodiamond particles in the nanodiamond dispersion having a nanodiamond concentration of 0.2% by mass. When it is necessary to dilute the stock solution of the nanodiamond dispersion for the preparation of the nanodiamond dispersion having a nanodiamond concentration of 0.2% by mass, ultrapure water shall be used as the diluent.

The aqueous dispersion medium 12 contained in the ND dispersion liquid 10 is a medium for appropriately dispersing the ND particles 11 in the ND dispersion liquid 10. The aqueous dispersion medium 12 is a dispersion medium containing 50% by mass or more of water as a dispersion medium component having the largest mass ratio, and is preferably water. Examples of the dispersion medium component of the aqueous dispersion medium 12 other than water include methanol, ethanol, ethylene glycol, dimethyl sulfoxide, and N-methylpyrrolidone.

As described above, the electrical conductivity of the ND dispersion liquid 10 is 30 μS / cm · wt% or less, preferably 28 μS / cm · wt% or less, more preferably 26 μS / cm · wt% or less, and more preferably 25 μS /. It is cm · wt% or less, more preferably 23 μS / cm · wt% or less, more preferably 20 μS / cm · wt% or less. The electrical conductivity of a solution, which serves as an index of the electrolyte concentration of the solution, can be measured in accordance with the method described in JIS K 0130 (2008) "General Rules for Measuring Electrical Conductivity".

The pH of the ND dispersion is preferably in the range of 5 or more, more preferably 6 to 10, and more preferably 7 to 9.

The ND dispersion liquid 10 having the above configuration can be produced, for example, by the nanodiamond dispersion liquid production method described above with reference to FIG.

As described above, the ND particles 11 having a positive zeta potential are dispersed in the ND dispersion liquid 10. Such an ND dispersion liquid 10 can be used as a material for supplying ND particles 11 having a positive zeta potential in producing a nanodiamond-containing composite material. The electrical conductivity, which is an index of the electrolyte concentration of the ND dispersion liquid 10, is 30 μS / cm · wt% or less, preferably 28 μS / cm · wt% or less, and more preferably 26 μS / cm · wt% as described above. % Or less, more preferably 25 μS / cm · wt% or less, more preferably 23 μS / cm · wt% or less, more preferably 20 μS / cm · wt% or less. The lower the electrolyte concentration of the ND dispersion liquid 10, the higher the dispersibility of the dispersion liquid with respect to the organic material. It is suitable for achieving sufficiently high dispersibility. The configuration of the low electrolyte concentration in which the ND dispersion liquid 10 exhibits such low electrical conductivity is, by extension, a high nano-diamond dispersion in the organic material phase in a solution or the like obtained by mixing the aqueous dispersion liquid and the organic material. It is suitable for achieving the quantity. In addition, the low electrolyte concentration configuration in which the ND dispersion 10 exhibits such low electrical conductivity is such that the nanodiamond aqueous dispersion is used even when the ND dispersion 10 contains an acid as an electrolyte. Therefore, it is suitable for suppressing corrosion of metal materials such as other materials and deterioration / decomposition of resin materials.

Further, as described above, the pH of the ND dispersion liquid 10 is preferably in the range of 5 or more, more preferably 6 to 10, and more preferably 7 to 9. Such a configuration is suitable for suppressing the above-mentioned corrosion, deterioration and decomposition of other materials when using the ND dispersion liquid 10.

[Nanodiamond dispersion]
A nanodiamond dispersion was produced through the following production steps, purification steps, oxygen oxidation steps, hydrogenation steps, and crushing steps.

In the generation process, first, a molded explosive equipped with an electric detonator was installed inside a pressure-resistant container for detonation, and the container was sealed. The container is made of iron and the volume of the container is 15 m ³ . As the explosive, 0.50 kg of a mixture of trinitrotoluene (TNT) and cyclotrimethylene trinitroamine or hexogen (RDX) was used. The mass ratio of TNT to RDX (TNT / RDX) in the explosive is 50/50. Next, the electric detonator was detonated and the explosive was detonated in the container. Next, the temperature of the container and its inside was lowered by leaving it at room temperature for 24 hours. After this cooling, the crude nanodiamond products adhering to the inner wall of the container (including the adherents of nanodiamond particles and soot generated by the above detonation method) are scraped off with a spatula, and the nanodiamonds are removed. The crude product was recovered.

Next, the crude nanodiamond product obtained by performing the above-mentioned production step a plurality of times was subjected to acid treatment in the purification step. Specifically, the slurry obtained by adding 6 L of 10% by mass hydrochloric acid to 200 g of the crude nanodiamond product was heat-treated for 1 hour under reflux under normal pressure conditions. The heating temperature in this acid treatment is 85 to 100 ° C. Next, after cooling, the solid content (including nanodiamond adherents and soot) was washed with water by decantation. The solid content was repeatedly washed with water by decantation until the pH of the precipitate was from the low pH side to 2.

Next, a solution oxidation treatment or a mixed acid treatment in the purification step was performed. Specifically, 6 L of a 98 mass% sulfuric acid aqueous solution and 1 L of a 69 mass% nitric acid aqueous solution were added to a precipitate (including a nanodiamond adherent) obtained through decantation after acid treatment to prepare a slurry. After that, this slurry was heat-treated for 48 hours under reflux under normal pressure conditions. The heating temperature in this oxidation treatment is 140 to 160 ° C. Next, after cooling, the solid content (including the nanodiamond adherent) was washed with water by decantation. The supernatant liquid at the beginning of washing with water was colored, and the solid content was repeatedly washed with water by decantation until the supernatant liquid became visually transparent.

Next, 1 L of a 10 mass% sodium hydroxide aqueous solution and 1 L of a 30 mass% hydrogen peroxide aqueous solution were added to the precipitate (including the nanodiamond adhering body) obtained through decantation after the solution oxidation treatment. Then, the slurry was heat-treated for 1 hour under reflux under normal pressure conditions (alkaline superwater treatment). The heating temperature in this process is 50-105 ° C. Then, after cooling, the supernatant was removed by decantation. Then, the residual fraction was subjected to a drying treatment to obtain a dry powder. As a method of drying treatment, evaporative drying performed using an evaporator was adopted.

Next, an oxygen oxidation step was performed using a gas atmosphere furnace (trade name "gas atmosphere tube furnace KTF045N1", manufactured by Koyo Thermo System Co., Ltd.). Specifically, 4.5 g of the nanodiamond powder obtained as described above was allowed to stand in the core tube of the gas atmosphere furnace, and nitrogen gas was continuously passed through the core tube at a flow rate of 1 L / min for 30 minutes. After that, the flowing gas was switched from nitrogen to a mixed gas of oxygen and nitrogen, and the mixed gas was continuously passed through the core tube at a flow rate of 1 L / min. The oxygen concentration in the mixed gas is 4% by volume. After switching to the mixed gas, the temperature inside the furnace was raised to the set heating temperature of 350 ° C. The heating rate was set to 10 ° C./min up to 330 ° C., which is 20 ° C. lower than the set heating temperature, and then 1 ° C./min from 330 ° C. to the set heating temperature of 350 ° C. Then, while maintaining the temperature condition in the furnace at 350 ° C., the nanodiamond powder in the furnace was subjected to oxygen oxidation treatment. The processing time was 3 hours. As described above, nanodiamond powder that has undergone the oxygen oxidation step or the oxygen oxidation treatment was obtained.

Next, the hydrogenation step was carried out by continuing to use the above-mentioned gas atmosphere furnace. Specifically, nitrogen gas is continuously passed through a gas atmosphere furnace in which nanodiamond powder that has undergone an oxygen oxidation step is arranged at a flow rate of 1 L / min for 30 minutes, and then the flowing gas is transferred from nitrogen. The gas was switched to a mixed gas of hydrogen and nitrogen, and the mixed gas was continuously passed through the core tube at a flow rate of 1 L / min. The hydrogen concentration in the mixed gas is 2% by volume. After switching to the mixed gas, the temperature inside the furnace was raised to the set heating temperature of 600 ° C. The heating rate was 10 ° C./min. Then, while maintaining the temperature condition in the furnace at 600 ° C., the nanodiamond powder in the furnace was subjected to hydrogen oxidation treatment. The processing time was 5 hours. As described above, nanodiamond powder that has undergone a hydrogen oxidation step or a hydrogen oxidation treatment was obtained.

Next, a crushing step was performed. Specifically, first, 0.9 g of nanodiamond powder and 29.1 ml of pure water that had undergone the above-mentioned hydrogenation step were added to a 50 ml sample bottle and mixed to obtain about 30 ml of slurry. Next, the slurry was subjected to a centrifugal separation treatment (centrifugal force of 20000 × g for 10 minutes) and a subsequent ultrasonic treatment. In the ultrasonic treatment, an ultrasonic irradiator (trade name "ultrasonic cleaner AS-3", manufactured by AS ONE) was used to irradiate the slurry with ultrasonic waves for 2 hours. .. After that, bead milling was performed using a bead milling device (trade name "parallel four-cylinder sand grinder LSG-4U-2L type", manufactured by IMEX Co., Ltd.). Specifically, 30 ml of the slurry after ultrasonic irradiation and zirconia beads having a diameter of 30 μm are put into a 100 ml mill container Vessel (manufactured by IMEX Co., Ltd.) and sealed, and the device is driven to perform bead milling. bottom. In this bead milling, the input amount of zirconia beads is, for example, 33% with respect to the volume of the mill container, the rotation speed of the mill container is 2570 rpm, and the milling time is 2 hours.

Next, the slurry or suspension that had undergone the crushing step as described above was subjected to a centrifugation treatment using a centrifugation device (classification operation). The centrifugal force in this centrifugation treatment was 20000 × g, and the centrifugation time was 10 minutes. Next, 10 ml of the supernatant of the nanodiamond-containing solution that had undergone the centrifugation treatment was collected. In this way, a nanodiamond dispersion liquid in which nanodiamonds are dispersed in pure water was obtained. The solid content concentration or the nanodiamond concentration of this nanodiamond dispersion was 2.1% by mass, and the pH was 8.07.

<Grain size D50>
With respect to the nanodiamond dispersion obtained as described above, the particle size distribution of nanodiamonds was measured by a dynamic light scattering method. Specifically, the particle size distribution of nanodiamonds was measured by a dynamic light scattering method (non-contact backscattering method) using an apparatus manufactured by Malvern (trade name “Zetasizer Nano ZS”). The nanodiamond dispersion liquid subjected to the measurement has a solid content concentration or a nanodiamond concentration of 2.1% by mass, and has undergone ultrasonic irradiation with an ultrasonic cleaner. As a result of the measurement, the particle size D50 (median diameter) of the nanodiamond was 5.05 nm, and the particle size D90 of the nanodiamond was 7.54 nm.

<Zeta potential>
The zeta potential of the nanodiamond particles contained in the nanodiamond dispersion obtained as described above was measured by laser Doppler electrophoresis using an instrument manufactured by Malvern (trade name "Zetasizer Nano ZS"). It was measured. The nanodiamond dispersion liquid subjected to the measurement was diluted with ultrapure water so that the solid content concentration or the nanodiamond concentration was 0.2% by mass, and then ultrasonically irradiated with an ultrasonic cleaner. The zeta potential measurement temperature is 25 ° C. As a result of this measurement, the zeta potential of the nanodiamond dispersion was 41.8 mV.

<Measurement of electrical conductivity>
The nanodiamond dispersion obtained as described above is electrically conducted at a measurement temperature of 25 ° C. using an electric conductivity measuring device (trade name "TWIN-COND B-771", manufactured by HORIBA, Ltd.). The degree was measured. The nanodiamond dispersion liquid subjected to the measurement has a solid content concentration or a nanodiamond concentration of 2.1% by mass, and has undergone ultrasonic irradiation with an ultrasonic cleaner. As a result of this measurement, the electrical conductivity of the nanodiamond dispersion was 25.2 μS / cm · wt%.

[evaluation]
In the nanodiamond dispersion obtained by the above-mentioned nanodiamond production method, the zeta potential of the nanodiamond particles is 41.8 mV, which is a relatively large positive value, and the electrolyte concentration is 25.2 μS / cm · wt%. It is low and the pH is 8.07, which is in the alkaline range. On the other hand, when the electrical conductivity of "Nanoamand", which is a zeta potential positive nanodiamond aqueous dispersion manufactured by Nanocarbon Research Institute Co., Ltd., was measured, it was as high as 66.6 μS / cm · wt%.

10 ND dispersion (nanodiamond dispersion)
11 ND particles (nanodiamond particles)
12 Aqueous dispersion medium S1 Generation step S2 Purification step S3 Oxygen oxidation step S4 Hydrogenation step S5 Crushing step

Claims (5)

An oxygen oxidation step at a temperature condition of 250 to 400 ° C. for oxygen oxidation treatment of nanodiamonds,
Including a hydrogenation step at a temperature condition of 500 to 650 ° C. for hydrogenating the nanodiamonds that have undergone the oxygen oxidation step.
Dispersed in a water-based dispersion medium comprises a nanodiamond particles having a zeta potential 30mV or more, electric conductivity obtain nanodiamond dispersion is below 30μS / cm · wt%, nanodiamond dispersion manufacturing process. The production process for producing nanodiamonds by the detonation method in an inert gas atmosphere,
The method for producing a nanodiamond dispersion liquid according to claim 1, further comprising a purification step for purifying the crude nanodiamond product obtained in the production step before the oxygen oxidation step. The method for producing a nanodiamond dispersion liquid according to claim 2, wherein the purification step includes a mixed acid treatment performed using a mixed acid. Aqueous dispersion medium and
It contains nanodiamond particles dispersed in the aqueous dispersion medium and having a zeta potential of 30 mV or more.
A nanodiamond dispersion having an electrical conductivity of 30 μS / cm · wt% or less. The nanodiamond dispersion according to claim 4 , wherein the pH is in the range of 5 or more.

Images