Nickel nanowires and their manufacturing method

Abstract

The present invention provides a nickel nanowire that does not break readily under stress. The present invention relates to a nickel nanowire having a face-centered cubic lattice structure and a crystallite size of 10 nm or less in the (111) lattice plane orientation.

Description

[Technical Field]

[0001] The present invention relates to nickel nanowires and a method for producing the same. [Background technology]

[0002] Nickel nanowires are ferromagnetic, so they can be used not only as conductive materials such as transparent conductive films and high-dielectric-constant materials, but also as magnetic materials such as radio wave absorbers. Nanowires are characterized by the anisotropy of their fiber shape (high aspect ratio), which allows them to exhibit percolation and magnetic anisotropy, thereby providing performance that cannot be achieved with particles (Patent Document 1).

[0003] For example, the nickel nanowires disclosed in Patent Document 1 are produced by reducing one type of nickel salt, and have a crystallite size of more than 10 nm and 15 nm or less in the direction of the (111) lattice plane. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2019 / 073833 Brochure Summary of the Invention [Problem to be solved by the invention]

[0005] The inventors of the present invention have found that conventional nickel nanowires have the problem of being easily broken by stress.

[0006] Specifically, to utilize the performance of nickel nanowires in industrial products, they need to be molded into various shapes such as sheets and housings. To mold nickel nanowires, they need to be mixed with other substances, but nickel nanowires, like metal nanowires such as silver nanowires, are vulnerable to stress. Therefore, when mixed with other substances by kneading or other methods and molded, the stress breaks them down into particles, causing anisotropy to be lost, preventing the nanowires from exhibiting the expected performance (for example, magnetic properties such as magnetic anisotropy).

[0007] The present invention aims to solve the above-mentioned problems and to provide nickel nanowires that are resistant to breakage due to stress. [Means for solving the problem]

[0008] The present inventors have found that the above object can be achieved by controlling the crystallite size within a specific range, and have arrived at the present invention.

[0009] That is, the gist of the present invention is as follows. <1> Nickel nanowires with a face-centered cubic lattice structure and a crystallite size of 10 nm or less in the direction of the (111) lattice plane. <2> The crystallite size in the direction of the (111) lattice plane is larger than the crystallite size in the direction of the (110) lattice plane. <1> The nickel nanowire according to claim 1. <3> The crystallite size in the direction of the (110) lattice plane is larger than the crystallite size in the direction of the (100) lattice plane. <2> The nickel nanowire according to claim 1. <4> The average diameter is 50 nm or more and less than 1 μm. <1> ~ <3> The nickel nanowire according to any one of the above. <5> The average length is 5 μm or more. <1> ~ <4> The nickel nanowire according to any one of the above. <6> The saturation magnetic susceptibility is 20 emu / g or more. <1> ~ <5> The nickel nanowire according to any one of the above. <7> The crystallite size in the direction of the (111) lattice plane is 1 to 8 nm. <1> ~ <6> The nickel nanowire according to any one of the above. <8> <1> ~ <7> 2. A dispersion liquid containing the nickel nanowires according to any one of claims 1 to 11. <9> <1> ~ <7> A molded body comprising the nickel nanowires according to any one of the preceding items. <10> In a reaction solution, two or more nickel salts are reduced while applying a magnetic field, <1> ~ <7> 1. A method for producing nickel nanowires, comprising the steps of: obtaining the nickel nanowires according to any one of the preceding items; <11> the two or more nickel salts include nickel chloride and nickel sulfate; the ratio of the nickel chloride to the total of the nickel chloride and the nickel sulfate is 70 to 98 mol %; <10> 10. The method for producing nickel nanowires according to claim 9. <12> the two or more nickel salts include nickel chloride and nickel acetate; the ratio of the nickel chloride to the total of the nickel chloride and the nickel acetate is 70 to 98 mol %; <10> 10. The method for producing nickel nanowires according to claim 9. <13> the two or more nickel salts include nickel acetate and nickel sulfate; the ratio of the nickel acetate to the total of the nickel acetate and the nickel sulfate is 70 to 98 mol %; <10> 10. The method for producing nickel nanowires according to claim 9. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide nickel nanowires that are resistant to fracture due to stress. The nickel nanowires of the present invention are resistant to fracture due to stress even when mixed with other substances by kneading or the like and then molded, and therefore can be suitably used for various applications. [Brief explanation of the drawings]

[0011] [Figure 1] 1 shows a WAXD (wide-angle X-ray diffraction) diffraction pattern of the nickel nanowires produced in Example 1. [Figure 2] 1 shows a WAXD diffraction pattern of nickel nanowires produced in Comparative Example 2. [Figure 3] 1 shows a WAXD diffraction pattern of nickel nanowires produced in Comparative Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0012] [Nickel nanowires] The nickel nanowires of the present invention must have an fcc structure (i.e., a face-centered cubic lattice structure) as their crystal structure. The lattice structure (or crystal structure) can be analyzed by WAXD.

[0013] The nickel nanowires having an fcc structure means that they exhibit one or more (particularly three) main peaks specific to the so-called fcc crystal structure at a predetermined angle of incidence in X-ray diffraction under the following conditions: Examples of the main peaks specific to the fcc structure include the peak (111) at 2θ=44.4°, the peak (200) at 2θ=51.6 to 51.9°, and the peak (220) at 2θ=76.3°. Conditions: CuKα ray = 1.54Å, 50kV, 300mA, 2θ / θ method.

[0014] The nickel nanowires of the present invention do not necessarily have to have only an fcc structure as a crystal structure, but may also include other crystal structures (e.g., an hcp structure (i.e., a hexagonal close-packed structure)). For example, the nickel nanowires of the present invention may mainly have an fcc structure and may also include an hcp structure.

[0015] The hcp structure content (hcp / fcc) of the nickel nanowire of the present invention is usually 0.15 or less, and from the viewpoint of magnetic properties, it is preferably 0.1 or less, and more preferably 0. The hcp structure content (hcp / fcc) is the ratio of the hcp structure to the fcc structure in the nickel nanowire. Specifically, the hcp structure content is calculated as the ratio (hcp(010) / fcc(200)) of the integral value of the peak (010) at 2θ=37.2° in the hcp structure to the integral value of the peak (200) at 2θ=51.6 to 51.9° in the fcc structure in the diffraction pattern of WAXD (wide-angle X-ray diffraction measurement, CuKα radiation=1.54 Å, 50 kV, 300 mA, 2θ / θ method).

[0016] The smaller the crystallite size of nickel nanowires, the easier it is for sliding to occur at the crystallite grain boundaries, and therefore, stress applied to the nickel nanowires is more easily relieved at the crystallite grain boundaries. On the other hand, the larger the crystallite size, the less likely it is for sliding to occur at the crystallite grain boundaries, and therefore, the more difficult it is for stress applied to the nickel nanowires to be relieved at the crystallite grain boundaries. Therefore, to make nickel nanowires less susceptible to stress-induced fracture, it is sufficient to increase the number of locations where stress can be relieved by sliding at the crystallite grain boundaries. Therefore, a small crystallite size of nickel nanowires is preferable. Specifically, the crystallite size in the (111) lattice plane direction must be 10 nm or less. From the viewpoint of more sufficient stress relaxation and excellent magnetic properties, it is preferably 1 to 10 nm, more preferably 1 to 9 nm, even more preferably 1 to 8 nm, particularly preferably 5 nm or more and 8 nm or less, and most preferably more than 5 nm and 8 nm or less. If the crystallite size in the (111) lattice plane direction is too large, stress is not sufficiently relaxed, making the nanowires more susceptible to stress-induced fracture. Furthermore, magnetic properties may be degraded. Considering the process in which minute nickel nuclei that become crystal nuclei are generated from nickel ions and grow into nanowires, it is not realistic to expect the crystallite size to be smaller than the crystal nuclei (1 to 5 nm). To achieve a crystallite size of 10 nm or less in the direction of the (111) lattice plane, nickel nanowires can be obtained by reducing two or more nickel salts in a reaction solution while applying a magnetic field, adjusting the dissociation constant of each nickel salt, the size and coordination state of the counter ion, the amount of crystal nuclei, the nanowire generation time, and the timing of nanowire growth, as described below. The direction of the (111) lattice plane refers to the direction perpendicular to the (111) lattice plane.

[0017] The magnetic properties of nickel nanowires vary depending on the direction of the lattice plane. In nickel, the (111) lattice plane direction is the easiest direction of magnetization, followed by the (110) lattice plane direction, and then the (100) lattice plane direction. For use as a magnetic material, the larger the crystallite size in the (111) lattice plane direction, the more advantageous it is. Therefore, for nickel nanowires to be used as a magnetic material, it is preferable that the crystallite size in the (111) lattice plane direction is larger than the crystallite size in the (110) lattice plane direction, and even more preferable that the crystallite size in the (110) lattice plane direction is larger than the crystallite size in the (100) lattice plane direction.

[0018] The crystallite size in the (110) lattice plane direction of the nickel nanowire of the present invention is typically 8.0 nm or less (particularly 1.0 to 8.0 nm), and from the viewpoint of more sufficient stress relaxation and excellent magnetic properties, it is preferably 4.0 to 7.8 nm, more preferably 4.8 nm or more and 7.8 nm or less, and even more preferably 5.3 nm or more and 6.0 nm or less.

[0019] The crystallite size in the (100) lattice plane direction of the nickel nanowire of the present invention is typically 7.0 nm or less (particularly 0.8 to 7.0 nm), and from the viewpoint of more sufficient stress relaxation and excellent magnetic properties, it is preferably 2.0 to 6.4 nm, more preferably 3.0 nm or more and 5.4 nm or less, and even more preferably 3.0 nm or more and 4.5 nm or less.

[0020] In this specification, the crystallite size in each lattice plane direction is calculated from the peak of WAXD. In the case of fcc nickel, the reflections of the (100) and (110) lattice planes cannot be directly observed due to the extinction law, so the values ​​are calculated from the peaks of the (200) and (220) lattice planes, respectively.

[0021] Nanowires are generally fibrous materials with an average diameter on the nanoscale. The average diameter of the nickel nanowires of the present invention is necessarily larger than the crystallite size in the direction of each lattice plane. In the present invention, from the viewpoints of ease of handling, more sufficient stress relaxation, and excellent magnetic properties, the average diameter of the nickel nanowires is preferably 50 nm or more and less than 1 μm, more preferably 50 to 500 nm, even more preferably 70 to 200 nm, particularly preferably 90 to 200 nm, and most preferably 90 to 150 nm.

[0022] In this specification, the average diameter of nickel nanowires is the average value of nickel nanowire diameters at 100 arbitrary points in 10 fields of view observed under a transmission electron microscope (600,000 magnification).

[0023] The average length of the nickel nanowires is preferably 5 μm or more (particularly 5 to 50 μm), more preferably 10 to 30 μm, from the viewpoints of mixing with other substances by kneading or the like, handling properties, stress relaxation, and magnetic properties.

[0024] In this specification, the average length of nickel nanowires is the average value of the lengths of 200 randomly selected nickel nanowires measured under a scanning electron microscope (2000 to 6000 magnifications).

[0025] The aspect ratio (average length / average diameter) of the nickel nanowires of the present invention is typically 50 or more, and from the viewpoint of magnetic properties, preferably 100 or more, more preferably 150 or more, even more preferably 200 or more, and particularly preferably 250 or more. There is no particular upper limit to the aspect ratio, and the aspect ratio is typically 400 or less, particularly 300 or less.

[0026] The nickel nanowires of the present invention are ferromagnetic and have a saturation magnetic susceptibility of 20 emu / g or more. From the viewpoint of further improving magnetic properties, the saturation magnetic susceptibility of the nickel nanowires of the present invention is preferably 30 emu / g or more, more preferably 40 emu / g or more, and even more preferably 45 emu / g or more. There is no particular upper limit to the saturation magnetic susceptibility, and the saturation magnetic susceptibility is usually 60 emu / g or less, particularly 55 emu / g or less.

[0027] In this specification, the saturation magnetic susceptibility can be measured by a vibrating sample magnetometer (VSM) as described later. In particular, nickel nanowires with a hcp structure content of more than 0.1 (particularly more than 0.15) do not have sufficient magnetic properties and have a saturation magnetic susceptibility of less than 20 emu / g.

[0028] [Method of manufacturing nickel nanowires] The nickel nanowires of the present invention can be obtained by reducing two or more nickel salts in a reaction solution while applying a magnetic field, while adjusting the dissociation constant of the nickel salt, the size of the counterion, the amount of crystal nuclei, the nanowire generation time, and the timing of nanowire growth. Conventionally, no technology for controlling the crystallite size of nickel nanowires has been known. In the present invention, by using two or more nickel salts, nickel nanowires with relatively small crystallite sizes can be obtained compared to when a single nickel salt is used, and the crystallite size can be controlled over a wide range.

[0029] Examples of nickel salts include nickel chloride, nickel sulfate, nickel nitrate, and nickel acetate. The salts may be hydrated or anhydrous. Among these nickel salts, two or more (particularly two) types of nickel salts with different dissociation constants and / or counter ions are used. Preferred combinations of nickel salts are those with a large difference in dissociation constants and / or a large difference in the size of counter ions. Examples of such combinations include a combination of nickel chloride and nickel sulfate (hereinafter sometimes referred to as Combination A), a combination of nickel chloride and nickel acetate (hereinafter sometimes referred to as Combination B), and a combination of nickel acetate and nickel sulfate (hereinafter sometimes referred to as Combination C). Among these, Combination A of nickel chloride and nickel sulfate is more preferred from the viewpoints of reducing crystallite size (particularly the crystallite size in the (111) lattice plane direction) and improving magnetic properties.

[0030] The preferred total concentration of nickel salts in the reaction solution varies greatly depending on the type of nickel salt used, the type of solvent, and the reaction temperature. If the total concentration is too high, nanowires cannot be formed, while if the concentration is too low, production efficiency tends to decrease. From the viewpoint of reducing crystallite size (particularly the crystallite size in the (111) lattice plane direction) and improving magnetic properties, the total concentration of nickel salts in the reaction solution is preferably 0.01 to 1 mmol / g, more preferably 0.015 to 0.25 mmol / g, and even more preferably 0.015 to 0.030 mmol / g.

[0031] The preferred concentration ratio of each nickel salt in the reaction solution varies greatly depending on the dissociation constant of the nickel salt used, etc. For each combination of nickel salts, the following concentration ratios are preferred from the viewpoints of reducing the crystallite size (particularly the crystallite size in the direction of the (111) lattice plane) and improving the magnetic properties.

[0032] (1A) When the nickel salt used is a combination A of nickel chloride and nickel sulfate, the ratio of nickel chloride to the total of nickel chloride and nickel sulfate is preferably 70 to 98 mol%, more preferably 70 to 95 mol%, and even more preferably 85 to 95 mol%. If the ratio is 50 mol% or more but less than 70 mol%, nanowires may not be formed and may become particulate. If the ratio is less than 50 mol%, the crystallite size in the direction of the (111) lattice plane of the nanowires increases.

[0033] (1B) When the nickel salt used is a combination B of nickel chloride and nickel acetate, the ratio of nickel chloride to the total of nickel chloride and nickel acetate is preferably 70 to 98 mol%, more preferably 70 to 95 mol%, and even more preferably 85 to 95 mol%. If the ratio is 50 mol% or more but less than 70 mol%, nanowires may not be formed and may become particulate. If the ratio is less than 50 mol%, the crystallite size in the direction of the (111) lattice plane in the nanowires increases.

[0034] (1C) When the nickel salt used is a combination C of nickel acetate and nickel sulfate, the ratio of nickel acetate to the total of nickel acetate and nickel sulfate is preferably 70 to 98 mol%, more preferably 70 to 95 mol%, and even more preferably 85 to 95 mol%. If the ratio is 50 mol% or more but less than 70 mol%, nanowires may not be formed and may become particulate. If the ratio is less than 50 mol%, the crystallite size in the direction of the (111) lattice plane in the nanowires increases.

[0035] The solvent used for the reaction solution is not particularly limited, but highly polar solvents such as water, alcohol, and NMP, and glycol-based solvents such as ethylene glycol and propylene glycol, which have a high boiling point and polarity, are preferred because they easily dissolve nickel salts.

[0036] The reducing agent used to reduce the nickel salt is not particularly limited, but hydrazine monohydrate (hydrazine) is preferred from the perspective of reducing crystallite size (especially in the (111) lattice plane direction) and improving magnetic properties. Phosphorus-based and borane-based reducing agents, such as hypophosphorous acid and dimethylamine borane, which are commonly used as reducing agents for electroless nickel plating, cause phosphorus and boron to become impurities in the metal, reducing the crystallinity of the metal itself. This makes them undesirable, as they may prevent nanowire formation or may degrade the magnetic properties of the resulting nickel nanowires. Organic reducing agents, such as glycol and ascorbic acid, are also undesirable because they require high temperatures of 200°C or higher, which makes the magnetic field and solvent conditions (temperature, boiling, etc.) used in the reaction unstable.

[0037] When hydrazine monohydrate is used as the reducing agent, the molar amount of hydrazine monohydrate is preferably 1.1 to 2.0 times, and more preferably 1.2 to 1.8 times, the total amount of nickel salts. If the molar amount of hydrazine monohydrate is less than 1.1 times the total amount of nickel salts, unreacted nickel salts remain, resulting in poor efficiency. On the other hand, if the molar amount exceeds 2.0 times, the reaction becomes too active, causing foaming of the reaction solution and inhibiting nanowire formation.

[0038] When reducing nickel salts with hydrazine monohydrate, the reaction temperature and the pH of the solution are important. If the reaction temperature is too high, the reaction system becomes unstable due to foaming caused by the gas generated, while if the reaction temperature is too low, the reduction reaction itself tends not to occur. The reaction temperature is preferably below the boiling point of hydrazine at atmospheric pressure (114°C). From the viewpoint of adjusting the reaction temperature and the amount of gas generated and convective diffusion, a temperature of 80 to 100°C, particularly 80 to 95°C, is preferred. When the reaction temperature is 80 to 100°C, particularly 80 to 95°C, it is preferable to make the solution alkaline. To make the solution alkaline, it is preferable to use a hydroxide salt such as sodium hydroxide. However, depending on the concentration of the hydroxide salt, precipitation of insoluble nickel hydroxide may occur. In this case, precipitation can be suppressed by using a combination of sodium hydroxide and ammonia. When sodium hydroxide is used, the concentration of sodium hydroxide in the reaction solution is preferably 0.020 to 1 mmol / g (particularly 0.025 to 1 mmol / g), and more preferably 0.020 to 0.5 mmol / g (particularly 0.025 to 0.5 mmol / g). Ammonia converts the nickel hydroxide precipitate into an ammine complex and redissolves it. The amount of ammonia added is not particularly limited, but an excess amount relative to the nickel hydroxide is required for redissolution. However, excessive ammonia destabilizes the reaction system due to endothermic heat of vaporization. Therefore, a range of 3 to 30 mol per mol of sodium hydroxide is usually preferred. From the viewpoints of reducing crystallite size (particularly crystallite size in the (111) lattice plane direction) and improving magnetic properties, a range of 10 to 30 mol is more preferred, and a range of 10 to 20 mol is even more preferred. Ammonia is preferably added in the form of aqueous ammonia from the viewpoint of procurement management, etc. The amount of ammonia added per mol of sodium hydroxide in the reaction solution may be within the above range.

[0039] A complexing agent such as citrate may be added to the reaction solution. Addition of the complexing agent increases the amount of crystal nuclei, which tends to reduce the crystallite size of the nickel nanowires. From the viewpoint of reducing the crystallite size (particularly the crystallite size in the direction of the (111) lattice plane) and improving the magnetic properties, the concentration of the complexing agent is preferably 1 to 20 mol%, more preferably 5 to 15 mol%, and even more preferably 5 to 10 mol%, relative to the total moles of nickel salt. If the concentration of the complexing agent is too high, the reduction reaction may not occur easily, which may reduce the production efficiency.

[0040] The reaction is carried out in a magnetic field. The central magnetic field is preferably 10 to 200 mT, and more preferably 80 to 180 mT. If a magnetic field is not applied during the reaction, nickel nanowires cannot be produced.

[0041] The reduction time for the reduction reaction is not particularly limited as long as nickel nanowires are produced, but it is usually one hour or less, and preferably about 10 to 40 minutes. Crystal nuclei that affect the crystallite size form within a few minutes, and the crystallites grow into nanowires in about 10 minutes. It is presumed that the size of the crystal nuclei is determined by several factors, such as the structure and concentration of the raw material salt.

[0042] After the reduction reaction, nickel nanowires can be obtained by purifying and recovering the nickel nanowires by centrifugation, filtration, magnetic adsorption, etc. After the reaction, ammonia may be added before recovering the nickel nanowires. This dissolves the precipitate of nickel hydroxide that is produced as a by-product, making it easier to remove impurities.

[0043] [Dispersions, paints, pastes and molded products] The nickel nanowires of the present invention can be prepared into a dispersion by dispersing them in a medium such as water, an organic solvent, or a mixed solvent thereof, and / or a curable resin. As the organic solvent, any organic solvent conventionally used as a medium for nanowire dispersions can be used, such as acetone, isobutyl alcohol, isopropyl alcohol, isopentyl alcohol, ethanol, ethyl ether, ethylene glycol, ethylene glycol monoethyl ether, ethylene glycol monoethyl ether acetate, ethylene glycol mono-n-butyl ether, ethylene glycol monomethyl ether, dichlorobenzene, xylene, cresol, chlorobenzene, isobutyl acetate, isopropyl acetate, isopentyl acetate, ethyl acetate, n-butylethylene glycol acetate, n-propyl acetate, n-pentyl acetate, methyl acetate, cyclohexanol, cyclohexanone, N,N-dimethylformamide, tetrahydrofuran, 1,1,1-trichloroethane, toluene, n-hexane, propylene glycol, 1-butanol, 2-butanol, methanol, methyl ethyl ketone, methylcyclohexanol, methylcyclohexanone, methyl-n-butyl ketone, etc. As the curable resin, acrylic resin, epoxy resin, silicone resin, phenolic resin, etc. can be used.

[0044] The content of the nickel nanowires in the dispersion is not particularly limited, and may be, for example, 0.01 to 50 parts by mass, particularly 0.1 to 10 parts by mass, relative to 100 parts by mass of the medium.

[0045] The dispersion containing the nickel nanowires of the present invention can be used as a coating material, adhesive, or molding material by mixing it with a binder resin or a curing agent that cures a curable resin. Other additives such as a leveling agent, a wetting agent, an antifoaming agent, and an inorganic filler for the purpose of thermal conductivity can also be added to the dispersion.

[0046] Examples of binder resins and curable resins include acrylic resins, urethane resins, epoxy resins, silicone resins, phenolic resins, etc. Examples of curing agents include aldehydes, amines, isocyanates, imidazoles, carboxylic acids, acid anhydrides, hydrazides, and formaldehyde-based compounds.

[0047] The dispersion, paint, and paste containing the nickel nanowires of the present invention can be used for coating, etc., as has been conventionally done. The coating film is a conductor or a high dielectric constant material, and is suitable for electrical wiring, electrode materials, radio wave shielding materials, antenna substrates, radio wave absorbing materials, etc.

[0048] Because the nickel nanowires of the present invention are less susceptible to fracture under stress than conventional ones, they can also be mixed with other substances (e.g., polymers), melted, kneaded, and molded into a molded article. Examples of other substances include polymers (particularly thermoplastic polymers) similar to the binder resins described above. The method for mixing, melting, and kneading with other substances is not particularly limited, and examples include methods using a mixer, a screw extruder, or the like for mixing, melting, and kneading. Furthermore, the molding method is also not particularly limited, and examples include press molding and injection molding.

[0049] In particular, a sheet-like molded product obtained by mixing, melting, kneading, and molding the nickel nanowires of the present invention with a binder resin is useful as an electrical wiring, electrode material, radio wave shielding material, antenna substrate, radio wave absorbing material, etc. [Example]

[0050] The present invention will be described below with reference to examples, but the present invention is not limited to these examples. The physical properties of the nickel nanowires were measured by the following methods.

[0051] (1) Average diameter The obtained nanowires were dispersed in ethanol, thinly coated on a grid with a support film, and dried. The obtained sample was photographed using a transmission electron microscope at 600,000 magnification. The diameters of the nickel nanowires were measured at 100 random points within 10 fields of view, and the average value was calculated.

[0052] (2) Average length As in (1), the sample was coated and dried on a sample stage and photographed at 2000 to 6000 magnifications using a scanning electron microscope. The lengths of 200 randomly selected nickel nanowires were measured and the average value was calculated.

[0053] (3) Crystal structure The obtained nickel nanowires were packed into a glass sample plate and subjected to WAXD (wide-angle X-ray diffraction). The crystal structure was identified from the diffraction pattern. The measurement conditions were CuKα radiation, 50 kV, 300 mA, and 2θ / θ method.

[0054] Specifically, a face-centered cubic lattice structure (fcc) was identified in the diffraction pattern based on the presence of a peak at 2θ = 44.4°, a peak at 2θ = 51.6 to 51.9°, and a peak at 2θ = 76.3° (see, for example, Figures 1 and 2). Figures 1 and 2 show WAXD (wide-angle X-ray diffraction) diffraction patterns of the nickel nanowires produced in Example 1 and Comparative Example 2, respectively. On the other hand, the presence of peaks at 2θ = 37.2°, 2θ = 43.2°, and 2θ = 62.8° identified a hexagonal close-packed (hcp) structure (see, for example, FIG. 3). FIG. 3 shows the WAXD (wide-angle X-ray diffraction) diffraction pattern of the nickel nanowires produced in Comparative Example 4. When peaks corresponding to both of the above crystal structures were present, it was determined that both of the crystal structures were formed. The ratio of the hexagonal close-packed (hcp) structure to the face-centered cubic (fcc) structure was calculated from the diffraction pattern. Specifically, the ratio of the integral value of the peak (010) at 2θ = 37.2° in the hexagonal close-packed structure to the integral value of the peak (200) at 2θ = 51.6 to 51.9° in the face-centered cubic structure (hcp(010) / fcc(200)) was calculated.

[0055] (4) Crystallite size From the diffraction pattern obtained by WAXD, multiple peaks were separated using JADE software, and the corrected half-width β (rad) of the peaks corresponding to (111), (220), and (200) was calculated using equation (1). The crystallite size for each lattice direction was calculated using Scherrer's equation (2). Specifically, the corrected half-width β was calculated using equation (1) with a deconvolution constant of 1.3 and an instrument constant of 0.1. The crystallite size was calculated using equation (2) with constants K of 0.9, λ of 1.5406 (the wavelength of CuKα1 X-rays used), β as the corrected half-width, and θ as the diffraction angle. The measurement conditions for WAXD were as follows: CuKα radiation = 1.54Å, 50kV, 300mA, 2θ / θ method.

number

[0056] (5) Magnetic properties The obtained nanowires were packed into a sample holder, and the saturation magnetic susceptibility (emu / g) was measured using a VSM (vibrating sample magnetometer). ◎◎: 45 emu / g or more (best); ◎: 40mu / g% or more and less than 45mu / g (excellent); ○: 30 mu / g or more and less than 40 mu / g (good); △: 20 mu / g or more and less than 30 mu / g (pass); ×: Less than 20 mu / g (failed).

[0057] (6) Ease of cutting under stress 1 g of the obtained nickel nanowires was dispersed in 50 g of ethylene glycol and treated with 42 kHz ultrasound for 2 minutes, and the ease of breaking (fracturing) of the nanowires under the applied stress was evaluated. The retention rate was calculated from the average length of the nickel nanowires after treatment and evaluated according to the following criteria. In the present invention, a grade of "△" or better is considered acceptable, and a grade of "◎" is preferable. ◎: 90% or more (excellent); ○: 70% or more but less than 90% (good); △: 50% or more but less than 70% (pass); ×: Less than 50% (fail).

[0058] Example 1 3.61 g (15.2 mmol) of nickel chloride hexahydrate, 0.442 g (1.68 mmol) of nickel sulfate hexahydrate, and 0.375 g (1.27 mmol) of trisodium citrate dihydrate were added to ethylene glycol to make a total of 500 g. The solution was heated to 90°C and dissolved. In a separate container, 1.00 g (25.0 mmol) of sodium hydroxide was added to ethylene glycol to make a total volume of 499 g. The solution was heated to 90° C. to completely dissolve the solution, and then 1.00 g (20.0 mmol) of hydrazine monohydrate was added. The above two solutions were mixed, placed in a magnetic circuit capable of applying a magnetic field of 150 mT to the center, and a reduction reaction was carried out for 15 minutes while maintaining the temperature at 90 to 95°C. After the reaction, 25 g of 28% aqueous ammonia (amount of ammonia: 7 g (=411.8 mmol)) was added, and the nickel nanowires were collected by filtration.

[0059] Examples 2 to 6 and Comparative Examples 1 to 4 The same operation as in Example 1 was carried out except that the type and amount of nickel salt used were changed to those shown in Table 1, and nickel nanowires were collected.

[0060] Example 7 3.61 g (15.2 mmol) of nickel chloride hexahydrate, 0.442 g (1.68 mmol) of nickel sulfate hexahydrate, and 0.375 g (1.27 mmol) of trisodium citrate dihydrate were added to ethylene glycol to make a total of 500 g. The solution was heated to 90°C and dissolved. In a separate container, 1.00 g (25.0 mmol) of sodium hydroxide was added to ethylene glycol to make a total volume of 499 g. This solution was heated to 90°C to completely dissolve it, and then 25 g of 28% aqueous ammonia (ammonia amount: 7 g (= 411.8 mmol)) and 1.00 g (20.0 mmol) of hydrazine monohydrate were added in that order. The above two solutions were mixed, placed in a magnetic circuit capable of applying a magnetic field of 150 mT to the center, and a reduction reaction was carried out for 15 minutes while maintaining the temperature at 90 to 95°C. After the reaction, the nickel nanowires were collected by filtration.

[0061] Comparative Example 5 An attempt was made to obtain nickel nanowires by performing the same operation as in Example 1, except that the amounts of nickel chloride hexahydrate and nickel sulfate hexahydrate used were changed to the amounts shown in Table 1. However, because the concentrations of nickel chloride hexahydrate and nickel sulfate hexahydrate were equimolar, nanowires could not be obtained.

[0062] [Table 1]

[0063] The nickel nanowires of Examples 1 to 7 had an fcc crystal structure and a crystallite size of 10 nm or less in the (111) lattice plane direction, so that even when stress was applied to the nanowires, the average length was maintained, and the maintenance rate was 50% or more. In particular, the nickel nanowires of Examples 1 and 7 had a crystallite size in the (111) lattice plane direction of 1 to 8 nm (particularly greater than 5 nm and less than 8 nm), which was in the more preferable range, so that even when stress was applied to the nanowires, the average length was largely maintained, with the maintenance rate being 90% or more.

[0064] The nickel nanowires of Comparative Examples 1 to 4 were produced using one type of nickel salt, and therefore the crystallite size in the (111) lattice plane direction exceeded 10 nm. Therefore, when stress was applied to the nanowires, the average length became shorter and the retention rate was less than 50%. [Industrial Applicability]

[0065] The nickel nanowires of the present invention are conductors or high dielectric constant materials, and are suitable for use in electrical wiring, electrode materials, radio wave shielding materials, antenna substrates, radio wave absorbing materials, and the like.

Claims

1. Nickel nanowires having a face-centered cubic lattice structure, a crystallite size in the direction of a (111) lattice plane of 1 to 10 nm, an average diameter of 50 nm or more but less than 1 μm, and an average length of 5 μm or more, The crystallite size in the direction of the (111) lattice plane is larger than the crystallite size in the direction of the (110) lattice plane, The nickel nanowires have a crystallite size in the direction of the (110) lattice plane that is larger than the crystallite size in the direction of the (100) lattice plane.

2. The nickel nanowire according to claim 1, having a saturation magnetic susceptibility of 20 emu / g or more.

3. The nickel nanowire according to claim 1 or 2, wherein the crystallite size in the direction of the (111) lattice plane is 1 to 8 nm.

4. A dispersion containing the nickel nanowires according to any one of claims 1 to 3.

5. A molded body comprising the nickel nanowires according to any one of claims 1 to 3.

6. A method for producing nickel nanowires, comprising reducing two or more nickel salts in a reaction solution while applying a magnetic field to obtain the nickel nanowires according to any one of claims 1 to 3.

7. the two or more nickel salts include nickel chloride and nickel sulfate; 7. The method for producing nickel nanowires according to claim 6, wherein a ratio of the nickel chloride to the total of the nickel chloride and the nickel sulfate is 70 to 98 mol%.

8. the two or more nickel salts include nickel chloride and nickel acetate; 7. The method for producing nickel nanowires according to claim 6, wherein a ratio of the nickel chloride to the total of the nickel chloride and the nickel acetate salt is 70 to 98 mol %.

9. the two or more nickel salts include nickel acetate and nickel sulfate; 7. The method for producing nickel nanowires according to claim 6, wherein a ratio of the nickel acetate to the total of the nickel acetate and the nickel sulfate is 70 to 98 mol%.

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