Metal powder

Abstract

To provide a metal powder in which flocculation between metal particles is effectively suppressed.SOLUTION: A metal powder in this invention is a ferromagnetic metal powder, in which, when the metal powder is analyzed using an Auger electron spectroscopic analysis device to compare respective spectra about each of a case where the direction of the metal powder with respect to an electron detector is 0° and a case where the direction thereof is 90° in an energy region where an electronic energy value is 50 eV or less, a difference in kinetic energy values at the rise position of electron detection intensities in their spectra is less than 1.0 eV.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to metal powders. [Background technology]

[0002] Metal powders, such as nickel powder, are sometimes used as electrode materials for multilayer ceramic chip capacitors (MLCCs) used in electronic computers, including multi-function mobile phones, as well as materials for nickel-metal hydride batteries and lithium-ion batteries, due to their excellent heat dissipation and electrical properties.

[0003] Among these, multilayer ceramic chip capacitors have a structure in which dielectric layers and internal electrode layers are alternately laminated, with external electrodes provided on both ends. The dielectric layers are made primarily of a ceramic with a high dielectric constant, such as barium titanate. The internal electrode layers can be made of various metal or alloy powders. In particular, in recent years, development of multilayer ceramic chip capacitors using fine nickel powder for the internal electrode layers has been progressing.

[0004] The internal electrode layers of multilayer ceramic chip capacitors are formed by heating a metal powder paste placed between green sheets that will become the dielectric layers, removing organic components from the paste and sintering the metal powder. If the metal powder contains agglomerates, they can penetrate the dielectric layers and cause short circuits between the electrodes. For applications such as these, it is particularly important to minimize metal powder agglomerations.

[0005] In relation to this, Patent Document 1 states, "We have concluded that the polarity of the OH group of nickel hydroxide (e.g., Na(OH)2) contained in the oxide film of the metallic nickel powder causes the powder to aggregate and impair dispersibility," and further states, "When the oxygen content is 0.1 to 2.0 wt. % and the wave number in the infrared absorption spectrum is 3600 to 3700 cm -1 A metallic nickel powder that has no absorption peak at 1000 nm has been proposed.

[0006] Furthermore, Patent Document 2 is based on the finding that "in addition to hydroxides on the surface of metallic nickel powder, the presence of trace amounts of silicic acid causes nickel powder to aggregate and generate coarse particles," and describes a method for producing nickel powder with an average particle size of 10 nm to 1000 nm and a spectrophotometer with a wavelength of 1200 cm using a Fourier transform infrared spectrophotometer equipped with an MCT detector. -1 From 900cm -1 S / N ratio of the absorption spectrum signal (X) and 3700 cm -1 From 3600cm -1 The article discloses a metallic nickel powder characterized in that the S / N ratio (Y) of the absorption spectrum signal is Y≦−1.0X+23.0. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-045002 [Patent Document 2] International Publication No. 2013 / 151172 Summary of the Invention [Problem to be solved by the invention]

[0008] As described in Patent Documents 1 and 2, metal powders determined based on the presence of OH groups and silicic acid using infrared absorption spectroscopy have some degree of suppression of aggregation, but this is not sufficient from the viewpoint of further reducing minute aggregation.

[0009] Furthermore, elemental analysis using characteristic X-rays with an electron probe microanalyzer (EPMA) or scanning electron microscope (SEM) may not be able to effectively evaluate agglomeration if the constituent elements of agglomerated and non-agglomerated metal particles are the same. In addition, because the characteristic X-rays excited by the electron beam in these elemental analyses have no electric charge, even if disturbances in electric or magnetic fields exist, it is difficult to confirm the presence or absence of an electromagnetic field using characteristic X-rays that are not affected by these disturbances.

[0010] The present invention is intended to solve these problems, and its object is to provide a metal powder in which aggregation of metal particles is effectively suppressed. [Means for solving the problem]

[0011] As a result of extensive research, the inventors have newly discovered that certain metal powders are magnetic and that this magnetism affects the aggregation of the metal powders.

[0012] Based on this knowledge, the metal powder of the present invention is Used as an electrode material or battery material A ferromagnetic metal powder, the particle size is 10 nm to 1000 nm, and when a filtration evaluation is performed in which a slurry obtained by adding the metal powder to pure water is subjected to suction filtration through a filter having an opening of 1 μm, and the metal powder remaining on the filter is dried at 120°C for 30 minutes in an inert gas atmosphere, the pass rate of the filter is 90% or more, The metal powder is analyzed using an Auger electron spectrometer, and when the spectra for when the orientation of the metal powder relative to the electron detector is 0° and 90° are compared in the energy region where the electron energy value is 50 eV or less, the difference in kinetic energy values ​​at the rising position of the electron detection intensity in those spectra is less than 1.0 eV.

[0014] The sulfur content of the metal powder is preferably 10,000 ppm by mass or less.

[0015] The metal powder may be, for example, nickel powder. [Effects of the Invention]

[0016] The metal powder of the present invention is one in which aggregation of metal particles is effectively suppressed. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is an SEM image of nickel powder A of an example. [Figure 2] 1 is an SEM image of nickel powder B of the example. [Figure 3] FIG. 10 is a schematic diagram showing a state in which the orientation of the sample relative to the electron detector (analyzer) is changed in the AES evaluation of the example. [Figure 4]1 is a graph showing a spectrum of the analysis results of AES evaluation of the copper plate of the example. [Figure 5] 5 is a graph showing an enlarged view of a part of FIG. 4. [Figure 6] 1 is a graph showing an enlarged spectrum of the analytical results of AES evaluation of nickel powder B of the example. [Figure 7] 1 is a graph showing an enlarged spectrum of the analytical results of AES evaluation of nickel powder B2 of the example. [Figure 8] 1 is a graph showing how to determine the rising position of the electron detection intensity in the spectrum of the analysis results of AES evaluation. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, an embodiment of the present invention will be described in detail. The metal powder according to one embodiment of the present invention is a ferromagnetic metal powder. When this metal powder is analyzed using an Auger electron spectrometer, the spectra obtained when the orientation of the metal powder relative to the electron detector of the Auger electron spectrometer is 0° and 90° are compared in an energy range where the electron energy value is 50 eV or less, and the difference in kinetic energy at the rising edge of the electron detection intensity between the spectra is less than 1.0 eV. Preferably, the spectra obtained when the orientation of the metal powder relative to the electron detector is 0° and 90° are similar in shape.

[0019] If the difference in kinetic energy values ​​at the rising edge of the electron detection intensity in each spectrum of a ferromagnetic metal powder is within the above range, it is considered to have almost no magnetism and almost no aggregation due to magnetism. Therefore, aggregation between metal particles in such metal powder is sufficiently suppressed.

[0020] (composition) The metal powder is a ferromagnetic material. Specifically, the metal powder is at least one selected from the group consisting of nickel powder, iron powder, cobalt powder, nickel powder, and gadolinium powder, all of which are elemental metals. In applications such as electrode materials for multilayer ceramic chip capacitors (MLCCs) and materials for nickel-metal hydride batteries and lithium-ion batteries, nickel powder may be evaluated.

[0021] The metal powder may contain sulfur to suppress activation during the production of the multilayer ceramic chip capacitor. The sulfur content in the metal powder is preferably 10,000 ppm by mass or less, more preferably 5,000 ppm by mass or less, more preferably 3,000 ppm by mass or less, and particularly preferably 2,000 ppm by mass or less. The lower limit of the sulfur content is not particularly limited, but may be the detection limit of a component analysis instrument or a value lower than the detection limit.

[0022] (Analysis by Auger electron spectroscopy) When the metal powder is analyzed using an Auger electron spectrometer, the difference in kinetic energy at the rising edge of the electron detection intensity between the spectra obtained when the metal powder is oriented at an angle of 0° to the electron detector and when the metal powder is oriented at an angle of 90° to the electron detector is less than 1.0 eV. Preferably, the spectra are similar in shape.

[0023] In analysis using an Auger electron spectrometer (Auger electron spectroscopy), a sample is irradiated with an electron beam, and the Auger electrons emitted as secondary electrons are detected and energy analysis is performed. More specifically, an electron beam of several eV to several keV is irradiated or scanned in an ultra-high vacuum. During this process, Auger electrons with energies specific to each element are emitted from the very surface of the sample. In Auger electron spectroscopy, an electron detector (an electrostatic hemispherical analyzer) can be used to target and detect electrons in a specific energy range with high energy resolution, enabling qualitative and quantitative analysis by accurately measuring the peak intensity of selected Auger electrons.

[0024] Unlike the characteristic X-rays detected by electron probe microanalyzers (EPMA) and scanning electron microscopes (SEM), Auger electrons have an electric charge, and therefore the spectrum of the analysis results changes significantly due to the influence of the Lorentz force caused by the magnetism of ferromagnetic metal powder samples. This makes it possible to determine whether the metal powder is magnetized and whether agglomeration occurs as a result.

[0025] In elemental analysis using an electron probe microanalyzer or scanning electron microscope, the detection target is characteristic X-rays, so the presence of magnetism in the sample does not cause any change in the spectrum. Furthermore, when methods using electron beams are generally applied to magnetized samples, the Lorentz force bends the irradiated electrons, making analysis difficult. In particular, when magnetism is concentrated in a localized area, even irradiated electrons accelerated by high voltage can be bent by the Lorentz force. This can make it difficult to focus when observing a magnetized sample, resulting in distorted images.

[0026] The inventors then focused on the fact that Auger electron spectroscopy can observe charges that cannot be observed with an electron probe microanalyzer or a scanning electron microscope. As a result, they discovered a new finding: when metal powders are magnetized to the extent that they do not cause image distortion with an electron probe microanalyzer or a scanning electron microscope, the magnetism causes the metal powders to aggregate. Based on this finding, it is now possible to evaluate whether aggregation is occurring based on the presence or absence of magnetism in metal powders, which can be determined by Auger electron spectroscopy.

[0027] Generally, magnetic force microscopy (MFM) is used to measure magnetic fields in microscopic regions. In a magnetic force microscope, magnetic attractive and repulsive forces are detected by the deflection and phase difference of a magnetic probe brought close to a sample. While its spatial resolution is comparable to that of an electron microscope, it cannot currently be applied to samples with irregularities, such as metal powders, due to constraints such as the flatness and size of the sample. In contrast, electron spectroscopy can be used to effectively determine the presence or absence of magnetism, even in metal powders with irregularities, because charged electrons emitted from magnetized metal powders are subjected to the Lorentz force, causing a change in the spectrum.

[0028] When analyzing metal powders using an Auger electron spectrometer, an Auger electron spectrometer equipped with a single-sided electron detector can be used, with an energy resolution of ΔE / E = 0.5% at 10 kV and 10 nA. There are no particular restrictions on the accelerating voltage or probe current, but if it is below 100 V, there is a risk of being affected by the magnetic field of the metal powder sample. It is preferable to use an electron beam of 1 kV or higher.

[0029] The analysis of metal powder using an Auger electron spectrometer is performed at least twice by changing the orientation of the metal powder relative to the electron detector. In the first analysis, the orientation of the metal powder relative to the electron detector is set to 0°. In the second analysis, the orientation of the metal powder is rotated from the orientation in the first analysis to 90° relative to the electron detector.

[0030] For non-magnetic metal powders, the secondary electrons emitted from the metal powder during analysis are not affected by any magnetism. Therefore, even electrons with low kinetic energies (0 eV to 50 eV) can be detected by the electron detector without their trajectories being deflected. Therefore, even if the intensity differs, the energy spectra obtained have similar or similar shapes. On the other hand, for magnetic metal powders, the secondary electrons emitted from the metal powder during analysis are subjected to the Lorentz force derived from the sample's magnetism (magnetic flux density). Therefore, changing the orientation of the metal powder between multiple analyses significantly deflects the trajectories of low-kinetic-energy electrons (0 to 50 eV). This causes changes in the amount of electrons captured by the electron detector. As a result, differences are observed in the spectral shape and the rising position of the main secondary electron peak between multiple analyses. This allows for a more accurate determination of the magnetic or non-magnetic properties of the metal powder. Based on this, it is possible to evaluate whether the metal powder is agglomerating.

[0031] In particular, Auger electron spectroscopy, when using high-energy resolution (energy resolution of 0.1% or less), has been found to produce spectra with sharp peaks, with different peak shapes and positions obtained through multiple analyses depending on the environment (e.g., chemical state) of the metal powder. The first and second spectra are compared in the energy range where the kinetic energy is 50 eV or less, typically 30 eV or less. While this depends on various conditions, spectral changes due to the presence or absence of magnetism in the metal powder are almost nonexistent in the relatively high kinetic energy range, but tend to be significant in the energy range below 50 eV. Furthermore, in the energy range below 50 eV, the spectrum contains a true secondary electron peak. Depending on the magnetism of the metal powder, the electron trajectory is bent, reducing the amount of electrons captured by the electron detector. This results in differences in the kinetic energy values ​​of the spectral shape and the onset position of the main secondary electron peak between the first and second spectra.

[0032] More specifically, the analysis is performed at least twice to obtain at least a first spectrum and a second spectrum. When the orientation of the metal powder relative to the electron detector in the analysis to obtain the first spectrum is set to 0°, it is preferable that the orientation of the metal powder relative to the electron detector be set to 90° in the analysis to obtain the second spectrum. By rotating the orientation of the metal powder by this angle when obtaining the first spectrum and the second spectrum, the magnetism of the metal powder can be determined with high accuracy.

[0033] To perform multiple analyses with different orientations of the metal powder, it is preferable to use an Auger electron spectrometer equipped with an electron detector and a sample stage that allows the orientation of the sample relative to the electron detector to be changed. It is preferable that the sample stage be capable of freely rotating and controlling the orientation of the sample through 360°. It is particularly preferable to use an Auger electron spectrometer equipped with a field emission electron gun, a high-precision eucentric sample stage that enables the analysis of insulators, and a floating-type ion gun. This enables versatile analysis of compositional and chemical information for metal and insulator samples.

[0034] By performing at least two analyses as described above, at least two spectra (first spectrum and second spectrum) are obtained as the results of each analysis. The spectra can be represented on a graph with the horizontal axis representing kinetic energy and the vertical axis representing electron detection intensity. In the metal powder of this embodiment, when the first spectrum and the second spectrum are compared in an energy range where the electron energy value is 50 eV or less, the difference between the kinetic energy value at the rising position of the electron detection intensity in the first spectrum and the kinetic energy value at the rising position of the electron detection intensity in the second spectrum is less than 1.0 eV. The magnitude of the difference in kinetic energy value between the spectra varies depending on the magnetism of the metal powder.

[0035] The rising position of the electron detection intensity in a spectrum such as the first spectrum or the second spectrum can be determined as shown in Figure 8. That is, in many cases, a spectrum composed of many measurement points has a shape in which the electron detection intensity increases and rises as the kinetic energy value increases from 0 eV toward the peak. In the rising position of such a spectrum, a rising line is drawn through three or more measurement points that are closest to a position at half (Pmax / 2) of the maximum peak intensity (Pmax) of the initial peak that first appears on the low kinetic energy side, and that are on the same line. The intersection of this rising line and the X-axis is the rising position of the spectrum.

[0036] If the difference in kinetic energy values ​​at the rising edge of the electron detection intensity between the first spectrum and the second spectrum is less than 1.0 eV, it can be determined that the metal powder is substantially not magnetized and that there is almost no aggregation. In particular, if the first spectrum and the second spectrum have similar shapes, it can be further determined that there is no aggregation. On the other hand, if the difference in kinetic energy values ​​at the rising edge is 1.0 eV or more, it can be determined that the metal powder is magnetized and that there is aggregation in the metal powder.

[0037] (particle size) The particle size of the metal powder is preferably 10 nm to 1000 nm, preferably 1 nm to 1000 nm, preferably 0.5 nm to 1000 nm, preferably 0.1 nm to 1000 nm. Such fine metal powder can be mixed with an organic solvent, a plasticizer, an organic binder, etc. to form a metal powder paste, which can be suitably used to form internal electrode layers of multilayer ceramic chip capacitors. However, since fine metal powders are prone to aggregation, it is effective to evaluate the presence or absence of aggregation using the evaluation method described here.

[0038] The particle size refers to the average diameter of the smallest encompassing circle enclosing each metal particle constituting the metal powder in an SEM image obtained by observing the metal powder with a scanning electron microscope (SEM). To determine particle size, image analysis software can be used on SEM images containing a relatively large number of metal particles, such as approximately 40,000. Image analysis software can be used as long as it complies with JIS Standard Z8827-1 and can evaluate and analyze the major axis, minor axis, aspect ratio, and other primary particle diameters. One example is Mac View from Mountech Co., Ltd. Given the large number of particles, software capable of automatic measurement is preferred. For spherical metal particles, the software recognizes the three points of a triangle within the particle and automatically recognizes the circle passing through those three points as a particle. Furthermore, in order to analyze irregular metal particles or metal particles that are difficult to distinguish using software capable of automatic measurement, software with a built-in manual analysis function that allows direct tracing along the periphery of the metal particle's outline is even more preferable.

[0039] (Manufacturing method) The metal powders described above can be produced by, for example, a gas phase method, a liquid phase method, etc. Among metal powders, in the production of nickel powder, it is particularly preferable to use a gas phase reduction method in which nickel chloride gas is brought into contact with a reducing gas, or a spray pyrolysis method in which a pyrolyzable nickel compound is sprayed and pyrolyzed, because it is easy to control the particle size and spherical nickel particles can be efficiently obtained.

[0040] In the gas-phase reduction method, a reduction step is carried out in which nickel chloride gas is reacted with a reducing gas such as hydrogen. Although pre-vaporized nickel chloride gas can be used as the nickel chloride gas, from the viewpoints of inhibiting oxidation or moisture absorption of nickel chloride and energy efficiency, it is preferable to further carry out a chlorination step in which solid nickel chloride is evaporated by heating to produce nickel chloride. Specifically, for example, nickel chloride gas can be continuously generated by contacting metallic nickel with chlorine gas in the chlorination step, and then this nickel chloride gas can be supplied to the reduction step and contacted with a reducing gas to continuously reduce the nickel chloride gas.

[0041] The solid raw material containing nickel as a single metal to be subjected to the chlorination step can be in the form of granules, lumps, plates, or the like with a particle size of approximately 5 mm to 20 mm, and the nickel purity is preferably 99.5 mass% or more. In the chlorination step, the solid raw material is contacted with chlorine gas while being heated. The temperature at this time can be set to 800°C or higher to sufficiently promote the reaction and 1453°C or lower, which is the melting point of nickel. In consideration of the reaction rate and the durability of the chlorination furnace, the temperature is preferably in the range of 900°C to 1100°C. Nickel chloride gas is thereby produced.

[0042] In the reduction step, the nickel chloride gas is brought into contact with a reducing gas such as hydrogen to cause a reaction. An inert gas such as nitrogen or argon may be mixed with the nickel chloride gas at 1 mol % to 30 mol %. Chlorine gas can also be supplied in addition to the nickel chloride gas during the reduction step. When an inert gas or chlorine gas is supplied during the reduction step, the partial pressure of the nickel chloride gas can be adjusted, making it possible to control the particle size of the nickel powder and suppress particle size variation. The temperature of the reduction reaction may be at least the temperature required for the reduction reaction. However, to produce a solid nickel powder that is easy to handle, the temperature can be set below the melting point of nickel, preferably between 900°C and 1100°C.

[0043] In the reduction process, nickel atoms are generated the moment nickel chloride gas and reducing gas come into contact, and ultrafine particles are generated and grow as the nickel atoms collide with each other. Nickel powder of a predetermined particle size can be obtained depending on the partial pressure, temperature, and other conditions of the nickel chloride gas in the reduction process. Since the amount of nickel chloride gas generated in the chlorination process depends on the amount of chlorine gas supplied, the amount of nickel chloride gas supplied to the reduction process can be adjusted by controlling the amount of chlorine gas supplied. This allows the particle size of the nickel powder to be effectively controlled.

[0044] The nickel powder obtained in the reduction step can be cooled. To prevent the formation of secondary particles due to aggregation of primary particles in the nickel powder produced in the reduction step and obtain nickel powder of the desired particle size, it is desirable to rapidly cool the gas flow from around 1000°C after the reduction reaction to about 400 to 800°C by blowing in an inert gas such as nitrogen. The nickel powder is then separated and recovered using, for example, a bag filter.

[0045] In the spray pyrolysis method, a thermally decomposable nickel compound is used as a raw material, and a solution of the raw material is sprayed to form fine droplets and heated at a high temperature to thermally decompose the nickel compound and produce nickel powder. Specifically, the raw material may contain at least one selected from the group consisting of nitrates, sulfates, oxynitrates, oxysulfates, chlorides, ammonium complexes, phosphates, carboxylates, and alkoxy compounds. When spraying the raw material to form droplets, water, alcohol, acetone, ether, etc. may be used as a solvent. The spraying method may be performed using ultrasound or a double-jet nozzle. The temperature to which the droplets are heated is preferably above the thermal decomposition temperature of the specified nickel compound used as the raw material and near the melting point of the metal.

[0046] In the liquid phase method, an aqueous nickel solution containing nickel sulfate, nickel chloride, or a nickel complex is contacted with an alkali metal hydroxide such as sodium hydroxide by addition or the like to produce nickel hydroxide. The nickel hydroxide is then reduced with a reducing agent such as hydrazine to obtain nickel powder. The nickel powder thus produced is optionally crushed to obtain uniform particles.

[0047] The nickel powder obtained by the various methods described above is preferably treated by suspending it in a carbonated aqueous solution under specific conditions in which the pH and temperature are controlled. Treatment with a carbonated aqueous solution not only sufficiently removes impurities such as chlorine adhering to the surface of the nickel particles, but also removes hydroxides such as nickel hydroxide present on the surface of the nickel particles and fine particles formed away from the surface due to friction between nickel particles, etc., making it possible to form a uniform nickel oxide coating on the surface of the nickel particles. Washing with a carbonated aqueous solution is not limited to this, and treatment may also be performed by blowing carbon dioxide gas or adding a carbonated aqueous solution to the aqueous slurry after washing with pure water.

[0048] Furthermore, the amount of Ni(OH)2 on the surface of the nickel particles can be controlled by subjecting the nickel powder to heat treatment in an environment where the oxygen partial pressure is controlled. Specifically, for example, heat treatment is performed in an atmosphere where the oxygen partial pressure is controlled while stirring using a fluidized bed stirrer or the like. The heat treatment temperature and time are determined depending on the particle size of the nickel powder, the thickness of the oxide film, and other factors, but the heat treatment temperature is, for example, 200 to 400°C, preferably 200 to 300°C, and more preferably 200 to 250°C. The heat treatment time may be 1 minute to 10 hours.

[0049] The nickel powder thus obtained is dispersed again in a solvent such as water, if necessary. Then, coarse particles and interlocked particles are removed by filtration through a filter. Nickel powder has good dispersibility, so coarse particles and interlocked particles can be removed efficiently. Known methods can be used for filtration. For example, filters made of organic polymers (nylon, polypropylene, tetrafluoroethylene resin, cellulose, melamine, phenolic resin, acrylic, etc.), metals, or inorganic compounds can be used. To increase the efficiency of filtration, other classifications, such as centrifugal classification (hydrocyclone), may be performed before passing through the filter.

[0050] In the above-described method for producing a metal powder such as nickel powder, it is preferable to perform demagnetization (erasure) at some stage after the metal powder is obtained, for example, before or after filtration. Demagnetization reduces the magnetism of the ferromagnetic metal powder, thereby eliminating aggregation caused by the magnetism. There are no particular limitations on the demagnetization method, but examples include heating the metal powder to a temperature above the Curie temperature or using a commercially available demagnetizer.

[0051] The Curie temperature varies depending on the material, with iron being approximately 770°C, cobalt being approximately 1131°C, and nickel being approximately 358°C. When demagnetizing by heating, the heating temperature needs to be at least the Curie temperature, and there is no problem if it is at least 10°C higher than the Curie temperature. The time for heating the metal powder to the Curie temperature or higher can be 30 minutes or more.

[0052] A demagnetizer reduces or removes the magnetism of an object using a magnetic field generated by an alternating current or the like. When using a demagnetizer, metal powder can be passed through the demagnetizer one or more times. When passing the metal powder through the demagnetizer, the metal powder may be placed in a bag. There are no particular restrictions on the material of the bag, but a bag made of resin such as polyethylene or polypropylene is preferred.

[0053] The metal powders described above are effectively prevented from agglomerating with each other and are particularly suitable for use as electrode materials for multilayer ceramic chip capacitors. This is because such metal powders are free of even minute agglomerates, which can prevent short circuits between electrodes caused by agglomeration. [Example]

[0054] Next, a metal powder according to the present invention was produced as a prototype and its effects were confirmed, which will be described below. However, the description here is for illustrative purposes only and is not intended to be limiting.

[0055] As a standard, a non-magnetic copper plate was prepared, along with two types of nickel powder A and B, which are presumed to retain some magnetic properties. The following observations and evaluations were carried out on these nickel powders A and B.

[0056] (SEM observation) To prepare microscopic observation samples for each of nickel powders A and B, tap water was filtered through a 0.3 μm filter (Organo Corporation) and activated carbon (Organo Corporation). The filtered water was then passed through an Organo Corporation cartridge water purifier (Model G-10C) and a 0.1 μm filter (Organo Corporation), followed by a 0.1 μm filter (Organo Corporation). A slurry with a concentration of approximately 50% by mass was obtained. One scoop of this nickel powder slurry was applied to the top edge of a Matsunami Glass Industry Co., Ltd. white-green polished glass slide (76 mm x 26 mm, 1.0–1.2 mm thick) with a microspatula. After the nickel powder slurry was applied to the slide, it was spread to a thickness of approximately 5 cm with a uniform speed and force using a 30 μm 4-sided 360 film applicator (Erichsen). Finally, the coating slurry was air-dried until the water was completely dry, yielding a microscopic observation sample.

[0057] The above microscopic observation samples were observed using a JEOL Ltd. scanning electron microscope (SEM) JSM-6060 at an accelerating voltage of 20 kV and a magnification of 500x. SEM images of nickel powders A and B are shown in Figures 1 and 2, respectively. Table 1 also shows the presence or absence of aggregation as determined by electron microscope observation.

[0058] (filtration evaluation) For each of nickel powders A and B, 100 g of nickel powder was added to 1,900 g of pure water to prepare a 5% by mass nickel powder slurry. This was then subjected to suction filtration using a filter with 1 μm mesh size. The nickel powder remaining on the filter was then dried at 120°C for 30 minutes under an inert gas atmosphere, its weight was measured, and the pass-through rate ((100 g - weight of metallic nickel powder on the filter in g) / 100 g) was calculated to evaluate the aggregation of the metal powder. The results are shown in Table 1. A pass-through rate of 90% or more is indicated by "○," and a pass-through rate of less than 90% is indicated by "×."

[0059] [Table 1]

[0060] (AES evaluation) For the AES evaluation, a standard copper plate measuring approximately 1 cm square and 2 mm thick was fixed with double-sided tape to prevent it from moving. For each of the nickel powders A and B, a diamond cutter was used to make cuts in the substrate, and the nickel powder was then imprinted into the cuts. The excess nickel powder was then blown away with nitrogen gas to prepare the AES evaluation sample.

[0061] Auger electron spectroscopy was performed on the above-mentioned AES evaluation sample using a single-sided detection AES (JEOL Auger electron spectrometer JAMP-9510F) with the sample facing directly toward the electron detector (analyzer) as shown in Figure 3, and with the sample rotated 90° from that position. The accelerating voltage was 10 kV, the probe current was 10 nA, and the energy resolution was ΔE / E = 0.5%, and the analysis was performed in 1 eV steps over the energy range of 0 to 2000 eV. The dwell time was 20 ms, and the number of integrations was 10.

[0062] The spectra obtained by AES analysis of a non-magnetic copper plate when the sample orientation was changed between 0° and 90° are shown in Figures 4 and 5. As shown in Figures 4 and 5, the spectra of the copper plate show that the rising position of the electron detection intensity remains almost the same even when the sample orientation is changed between 0° and 90°.

[0063] For nickel powder B, which was determined to have no agglomeration in the SEM observations described above, the spectra of the analysis results when the sample orientation was changed by AES between 0° and 90° are shown in Figure 6. As can be seen from Figure 6, for nickel powder B, a characteristic peak in the electron intensity was confirmed in the energy region where the kinetic energy value was 50 eV or less, whether the sample orientation was 0° or 90°.

[0064] More specifically, as shown in Figure 6, the kinetic energy value at the onset of the spectrum when the sample orientation was changed to 90° differed by 3.8 eV from the kinetic energy value at the onset of the spectrum when the sample orientation was 0°. Furthermore, the spectra were not similar in shape. Considering the results of the filtration evaluation described above in Table 1, it is surmised that nickel powder B is significantly affected by minute agglomerations due to magnetism that cannot be confirmed by SEM observation.

[0065] (Confirmation of magnetic influence) The above nickel powder B was demagnetized (demagnetized) by the following two methods to obtain nickel powder B1 and nickel powder B2, which were then subjected to the same AES evaluation and filtration evaluation.

[0066] Two demagnetization methods were used: a method using a demagnetizer, as described below, and a method using heat.

[0067] In the method using a demagnetizer, the SW was used as a substrate and cut into the notches with a diamond cutter. Next, nickel powder (nickel powder B) was imprinted into the notches and placed in a plastic bag. Finally, a demagnetizer, HC-33 (maximum magnetic flux density 68 mT) manufactured by Hozan Corporation, was used to gradually move the demagnetizer away from the plastic bag containing the nickel powder, and then held over the plastic bag containing the nickel powder to obtain demagnetized nickel powder (nickel powder B1).

[0068] For the heating method, a SW was used as the substrate and incisions were made with a diamond cutter. Next, nickel powder (nickel powder B) was imprinted into the incisions. The stainless steel tray containing the nickel powder was then heated to 400°C (a temperature above the Curie temperature) in a dryer. After reaching 400°C, heating was continued for 30 minutes, maintaining that temperature. After heating was completed, the temperature was lowered to room temperature, and finally the stainless steel tray containing the nickel powder was removed from the dryer to obtain nickel powder that had been demagnetized by heating (nickel powder B2). Note that the Curie temperature of nickel is approximately 358°C, and once heated above the Curie temperature, it will not return to its magnetic state even when returned to room temperature.

[0069] The spectra obtained by AES analysis of nickel powder B2, which had been demagnetized by heating, are shown in Figure 7. The difference in kinetic energy at the rising edge of each spectrum was 0.0 eV, meaning there was no change, and the spectra were similar in shape.

[0070] The same filtration evaluation as above was carried out for each of the nickel powders B1 and B2, and the pass rate for both nickel powders B1 and B2 was 95% or more, resulting in a rating of "◎", confirming that magnetic micro-agglomeration was further suppressed.

[0071] From the above, it has been found that according to the present invention, a metal powder can be obtained in which aggregation of metal particles is effectively suppressed.

Claims

1. A ferromagnetic metal powder used as an electrode material or a battery material, comprising: The particle size is 10 nm to 1000 nm, When a filtration evaluation was performed in which a slurry obtained by adding the metal powder to pure water was subjected to suction filtration through a filter having an opening of 1 μm, and the metal powder remaining on the filter was dried at 120° C. for 30 minutes under an inert gas atmosphere, the pass rate of the filter was 90% or more, The metal powder was analyzed using an Auger electron spectrometer, and the spectra obtained when the orientation of the metal powder relative to the electron detector was 0° and 90° were compared in an energy region where the electron energy value was 50 eV or less. The difference in kinetic energy values ​​at the rising positions of the electron detection intensities in the spectra of the metal powders is less than 1.0 eV.

2. 2. The metal powder according to claim 1, having a sulfur content of 10,000 ppm by mass or less.

3. 3. The metal powder according to claim 1, wherein the metal powder is nickel powder.

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