Nanoscale crystal mass made of precious metal and method for manufacturing the same

Abstract

This paper presents the specific structure of nanoscale crystalline aggregates made of precious metals, a type of material never before reported, and clarifies its manufacturing method. [Solution] The nanoscale crystal mass of a precious metal according to the present invention is a nanoscale crystal mass made of a polycrystalline body of a precious metal with a purity of 99% or more, wherein the median diameter of the crystal grains constituting the polycrystalline body is 0.1 μm or less. Precious metals are difficult to refine by strong cold processing and heat treatment. Therefore, the nanoscale crystal mass of a precious metal according to the present invention is manufactured by a powder sintering method using fine precious metal powder as a raw material. This manufacturing method includes a sintering step in which the precious metal powder is compacted into a powder and then sintered, and the sintering temperature at this time is set to 0.4 × Tm (°C) or less (Tm: melting point of the precious metal (°C)). Furthermore, in the sintering step of the present invention, after raising the temperature of the compacted powder to the sintering temperature, it is necessary to rapidly lower the temperature without raising the temperature or holding the temperature.

Description

Technical Field

[0001] The present invention relates to a nanoscale crystal mass composed of noble metals such as Pt, Au, Ag, and a method for producing the same.

Background Art

[0002] Many of the metal materials that have long been the base materials in various industrial fields are polycrystals. A polycrystalline metal is formed by binding a large number of crystal grains composed of metal crystals, and the crystal grains are partitioned by grain boundaries (hereinafter abbreviated as grain boundaries). In a polycrystalline metal, the crystal grains have a regular structure in which metal crystals of a predetermined structure are periodically arranged three-dimensionally. On the other hand, the grain boundary is an existence that interrupts the above-described periodic arrangement of the crystal grains. Despite the grain boundary being such an existence, it is assumed that the polycrystal maintains the shape as a bulk because the metal atoms in the vicinity of the grain boundary have a specific bonding mode while having a geometric arrangement.

[0003] In recent years, the action of the grain boundary, which is a special existence in polycrystalline metals as described above, has been attracting attention. And, as a new metal material that maximally utilizes the grain boundary, a research example of a bulk-shaped polycrystalline metal having nanoscale crystal grains has been reported. In the present specification, such a polycrystalline metal material composed of such fine crystal grains is referred to as a nanoscale crystal mass.

[0004] Ordinary polycrystalline metals are composed of crystal grains on the micron order (10 μm or more). In a nanoscale crystal mass, the crystal grain size is controlled to be finer to the nanometer order. Due to the increase in the number of particles due to the refinement of the crystal grain size, the volume ratio of the grain boundaries in the bulk rapidly increases. It is predicted that the above-described specificity of the grain boundary will exert a new effect on the mechanical properties and electrical and magnetic properties of the metal material by the rapid increase in its ratio. A nanoscale crystal mass is a polycrystalline metal considered with such an idea.

[0005] Currently, the most noteworthy advantage of creating nanoscale crystalline structures from metallic materials is the improvement in material strength. In metallic polycrystalline materials, the influence of grain boundaries on metal strength has long been known: the increase in grain boundaries due to refinement of crystal grain size inhibits dislocation movement and contributes to increased strength. It is thought that the rapidly increasing grain boundaries in nanoscale crystalline structures can act additively and synergistically on this well-known strengthening mechanism. For example, it has been reported that nanoscale crystalline structures can increase strength without adding other metallic or nonmetallic elements to the metal (alloying).

[0006] To date, examples of applications of nanoscale crystal aggregates have been studied, including ferrous and non-ferrous metal materials as structural materials and high-melting-point metal materials. As mentioned above, since nanoscale crystal aggregates are metallic materials composed of fine crystal grains, their manufacturing process can be said to be an extension of known crystal grain refinement techniques. As a crystal grain refinement technique for ferrous and non-ferrous metal materials, a method is known in which lattice defects such as dislocations are introduced to a high degree by severe cold working, followed by appropriate heat treatment. Furthermore, a method using severe strain working (ultra-severe working) has been proposed as a method for creating nanoscale crystal aggregates of ferrous and non-ferrous metal materials. Severe strain working is a process that applies extremely large plastic strain that cannot be introduced by normal plastic working. Various processing processes such as ECAE processing (Equal Channel Angular Extrusion) and ARB processing (Accumulative Roll Bonding) have been proposed as severe strain working, and a manufacturing process for nanoscale crystal aggregates combining these has been reported (Non-Patent Literature 1).

[0007] Furthermore, for high-melting-point metallic materials such as W alloys, examples of nanoscale crystal masses produced by sintering methods using fine metal powders (nanocrystalline powders) have been reported. Patent Document 1 describes a process for producing nanoscale crystal masses made of W alloy by sintering nanocrystalline powder of W alloy.

[0008] As described above, there are several reported examples of nanoscale crystal aggregates, but the variety of metals involved is still not very large. In particular, there are no studies on nanoscale crystal aggregates of precious metals such as Pt, Au, and Ag. These precious metals have few applications as structural materials, but they are widely used industrially as functional materials such as electrical and electronic materials. It is thought that the nanoscale crystallization of precious metals would be useful for improving the properties of these industrial materials.

[0009] Furthermore, due to their rarity, many precious metals possess asset and monetary value, and are frequently used in jewelry and ornaments. For items worn by people, such as jewelry and ornaments, surface scratches are a concern. Therefore, precious metals used in jewelry and ornaments require a certain degree of hardness to resist scratches. As mentioned above, nanoscale crystal masses are considered particularly effective in improving material strength, making them suitable for such applications.

[0010] Furthermore, considering the asset value of jewelry and other items, it is preferable that the constituent materials have a high precious metal content with a low amount of base metal added. The properties of nanoscale crystal masses, which can achieve high strength without alloying with other elements, are also useful for maintaining the purity of precious metals. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Japanese Patent Publication No. 2020-073731 [Non-patent literature]

[0012] [Non-Patent Document 1] "Structure and Mechanical Properties of Nanoscale Crystal Masses Fabricated by Ultra-Strong Processing," Naoya Kamikawa, Materia, Vol. 49 (No. 6), June 2010, published by The Japan Institute of Metals. [Overview of the project] [Problems that the invention aims to solve]

[0013] However, there are very few reports on the manufacturing process and effects, such as strength, of nanoscale crystal aggregates made of precious metals. Therefore, the present invention aims to disclose manufacturing examples of nanoscale crystal aggregates made of various precious metals such as Pt and to clarify their specific composition. In this study, the inventors conducted a detailed investigation into a method for manufacturing nanoscale crystal aggregates that takes into account the properties of precious metals, and to clarify an efficient manufacturing process for precious metal nanoscale crystal aggregates. [Means for solving the problem]

[0014] Regarding the manufacturing processes for the nanoscale crystal masses of the various metals mentioned above, the inventors' studies have shown that precious metals are difficult to refine by high-strain processing. This is because precious metals such as Pt, Au, and Ag tend to undergo self-annealing. Self-annealing is a phenomenon in which a metal that has been temporarily hardened by the introduction of lattice defects through cold working softens over time even without heat treatment after processing, and is also called age softening. Due to this self-annealing characteristic, even if high-density lattice defects are introduced into precious metals by high-strain processing, recrystallization occurs before heat treatment, making it impossible to refine the crystal grains. In this respect, precious metals differ from general ferrous and non-ferrous metal materials.

[0015] Since grain refinement cannot be expected through high-strain processing, the inventors decided to investigate the formation of nanoscale crystal aggregates of precious metals using powder metallurgy. Precious metals have high chemical stability, and sintered bodies can be formed even in the state of fine powder without surface oxidation. The inventors decided to form nanoscale crystal aggregates made of precious metals by molding and sintering nano-order fine powder in advance.

[0016] However, further investigation by the inventors revealed that applying the general sintering method in powder metallurgy to precious metal powder makes it difficult to obtain a sintered body that can be called a nanoscale crystal mass. As a result of diligent research, the inventors discovered sintering conditions for precious metal nanoscale crystal masses by powder metallurgy, particularly strict temperature control. They then clarified the structure of the nanoscale crystal mass obtained through the sintering process of precious metal powder by the inventors, leading to the present invention.

[0017] In other words, the present invention relates to a nanoscale crystal mass made of a polycrystalline material of a precious metal with a purity of 99% or more, wherein the median diameter of the crystal grains constituting the polycrystalline material is 0.1 μm or less. The following describes the structure of the precious metal nanoscale crystal mass according to the present invention and the method for manufacturing the precious metal nanoscale crystal mass based on powder metallurgy.

[0018] A. Structure of the noble metal nanoscale crystal mass according to the present invention As described above, the nanoscale crystal mass according to the present invention consists of a polycrystalline material of precious metals. Specifically, the precious metals are Au and Ag and the platinum group metals Pt, Ru, and Ir, and are either single metals or alloys of these metals. The nanoscale crystal mass according to the present invention is composed of precious metals with a purity of 99% or higher. Purity refers to the mass-based concentration of each of the aforementioned precious metals. The purpose of specifying purity in this invention is to clearly demonstrate that the present invention improves hardness and other properties without the addition of elements other than precious metals. Furthermore, by achieving a high purity of 99% or higher, the economic value of the precious metal is ensured. A precious metal purity of 99.9% or higher is preferable.

[0019] The nanoscale crystal mass according to the present invention is most preferably composed solely of the above-mentioned precious metals (100% purity), but the inclusion of unavoidable impurities is acceptable. Examples of unavoidable impurities that may be present in the present invention include Rh, Pd, Cu, Fe, Na, Cl, K, and Pb. Furthermore, Au, Ag, Pt, and Ru are also unavoidable impurities if their addition is not intended during the manufacturing of the nanoscale crystal mass. These unavoidable impurities are elements derived from the precious metal powder used as raw material, or elements derived from the manufacturing equipment in the manufacturing process. The total content of unavoidable impurities is preferably 0.1% by mass or less.

[0020] Furthermore, the noble metal nanoscale crystal mass according to the present invention has a median diameter of 0.1 μm or less. This is because making the nanoscale crystal mass a polycrystalline material with such fine crystal grains increases the volume fraction of the grain boundaries, thereby effectively exhibiting effects such as increased strength. It is more preferable that the median diameter of the crystal grains of the nanoscale crystal mass be 0.08 μm or less. In addition, it is preferable that the lower limit of the median diameter of the crystal grain be 1 nm (0.001 μm) or more. This is because it is difficult to manufacture noble metal powder with a diameter of less than 1 nm as the raw material. It is more preferable that the lower limit of the median diameter be 10 nm (0.01 μm) or more.

[0021] In this invention, the median diameter of a crystal grain is the crystal grain size at which the cumulative number of crystal grains in the cumulative grain size distribution based on the number of crystal grains of a nanoscale crystal mass reaches 50%, and is also referred to as the central grain size (D50). It is preferable to use the equivalent circle diameter for the crystal grain size at this time. As a method for measuring the median diameter of crystal grains in a nanoscale crystal mass, it is preferable to observe and image the material structure of an arbitrary cross-section, measure the grain size of multiple crystal grains (preferably 250 or more) from the observed images, and create a cumulative grain size distribution based on the number of crystal grains. The grain size value at which the cumulative number of crystal grains in the cumulative grain size distribution reaches 50% is defined as the median. For observing the material structure of a nanoscale crystal mass, it is preferable to use an electron microscope (SEM, TEM) or EBSD (electron backscatter diffraction analysis) using these in combination. Image analysis devices and software may be used as appropriate for measuring the crystal grain size.

[0022] The nanoscale crystallite according to the present invention requires the above-mentioned median diameter as a condition regarding crystal grains, and no other restrictions are imposed. However, for the nanoscale crystallite according to the present invention, the maximum particle size of the crystal grains is preferably 0.5 μm or less. Also, as the crystal orientation of the crystal grains, it is preferably randomly oriented. Specifically, it is preferable that the ratio of large-angle grain boundaries with an orientation difference of 15° or more at grain boundaries to the total grain boundaries is 80% or more.

[0023] The noble metal nanoscale crystallite according to the present invention is a sintered body formed as a polycrystal by sintering noble metal powder under the sintering conditions described later. Generally, in a sintered body, pores that existed between metal powders during the sintering process remain. Therefore, the actual density (measured density) of the noble metal nanoscale crystallite according to the present invention is lower than the true density (theoretical density). Regarding the relative density, which is the ratio of the actual density to the true density (the percentage (%) of the value obtained by dividing the measured density of the sintered body by the true density), it is preferably 80% or more for the nanoscale crystallite of the present invention. It is more preferable that this relative density is 90% or more.

[0024] And the noble metal nanoscale crystallite according to the present invention has high strength and hardness with respect to a general dissolving material due to the ultra-fine refinement of the crystal grain size and the resulting increase in the grain boundary volume ratio. Specifically, the Vickers hardness when the noble metal is Pt is 300 HV or more. Also, the Vickers hardness when the noble metal is Au or Ag is 100 HV or more. In addition, the noble metal nanoscale crystallite according to the present invention, in which the strength and strength are not adjusted by introducing processing strain, hardly undergoes a change in hardness due to self-annealing.

[0025] There are no restrictions on the dimensions of the noble metal nanoscale bulk crystals according to the present invention. However, since it is referred to as "bulk", it should be distinguished from the forms of thin films and fine particles. Defining the noble metal nanoscale bulk crystals according to the present invention in terms of dimensions, the minimum dimension when measuring between the ends of the nanoscale bulk crystals in any direction should be 100 μm or more. There are also no restrictions on the shape of the noble metal nanoscale bulk crystals. In particular, the noble metal nanoscale bulk crystals according to the present invention are produced by powder metallurgy. In powder metallurgy, since a sintered body with a shape close to the target can be easily produced, not only shapes such as plate-like and块状 formed only by straight lines, but also shapes such as ring-like including curves and even more complex hair-like shapes can be made.

[0026] B. Method for manufacturing noble metal nanoscale bulk crystals according to the present invention Next, the method for manufacturing noble metal nanoscale bulk crystals according to the present invention will be described. As described above, the nanoscale bulk crystals of the present invention are composed of a sintered body of noble metal powder. Therefore, in the present invention, nanoscale bulk crystals are manufactured by powder metallurgy. In powder metallurgy, it is basically to sinter and densify the metal powder by heating the formed metal powder. The sintering densification of the metal powder proceeds by the formation and growth of necks between particles, and the particle surface changes to the grain boundary. Originally, sintering is a process intended to cause densification by expanding (diffusing) the particle size in a state where the particle volume is constant. However, in the actual sintering process, the decrease in the particle surface due to the coalescence of particles accompanying the mass transfer between particles, that is, grain growth is also progressing. Grain growth is a process of grain boundary migration that proceeds in a state where the grain boundary area is constant. And the normal sintering process proceeds with both the processes of sintering (grain boundary diffusion) and grain growth (grain boundary migration) competing. Therefore, when manufacturing nanoscale bulk crystals by powder metallurgy, it is necessary to preferentially proceed with grain boundary diffusion while suppressing grain boundary migration to form a sintered body (polycrystalline body) with a fine particle size.

[0027] The inventors focused first on controlling the sintering temperature as a means of promoting grain boundary diffusion (sintering) while suppressing grain boundary migration during the sintering process. According to the inventors' research, in a polycrystalline metal at high temperatures, the activation enthalpy, which acts as a barrier to grain boundary migration, tends to decrease with increasing ambient temperature. Therefore, by setting the sintering temperature in a temperature range where the activation enthalpy of grain boundary migration is high, sintering can be preferentially promoted. As a result of the research, the inventors set the upper limit of this sintering temperature to 0.4 × Tm (°C) based on the melting point Tm (°C) of the precious metal to be manufactured.

[0028] Furthermore, the inventors argue that in order to suppress grain boundary migration while promoting grain boundary diffusion during the sintering process of precious metal powder, in addition to setting the sintering temperature as described above, attention should also be paid to temperature control after reaching the sintering temperature. Figure 1 shows a diagram illustrating the control of the sintering temperature in the sintering process in the present invention in comparison with a general sintering process.

[0029] In a typical sintering process, as shown in Figure 1(a), the material is heated to a set sintering temperature and then held at that temperature for several hours to allow sintering to proceed. In the sintering process of inorganic oxide powder (ceramic powder), as shown in Figure 1(b), after reaching the sintering temperature, the material may be held at a slightly lower temperature for several hours to more than 10 hours to allow sintering to proceed. According to the inventors' research, under such typical temperature control, grain growth proceeds even in the high temperature range of the activation enthalpy of grain boundary movement, making it difficult to maintain fine crystal grains.

[0030] The inventors have found that in order to form nanoscale crystal aggregates of precious metal powder by a sintering process, as shown in Figure 1(c), it is necessary to cool the powder down after reaching the sintering temperature (0.4 × Tm (°C) or lower) without raising the temperature or holding it at a temperature near the sintering temperature.

[0031] Based on the above findings, the present inventors have clarified a method for manufacturing nanoscale crystal aggregates made of precious metals. Specifically, the method for manufacturing nanoscale crystal aggregates according to the present invention includes a pressurization step of pressurizing powder made of a precious metal with a median diameter of primary particles of 0.1 μm or less to form a compact, and a sintering step of sintering the compact. The sintering step involves raising the temperature of the compact until it reaches the sintering temperature, and then lowering the temperature without raising the temperature or holding it after reaching the sintering temperature. The sintering temperature is 0.4 × Tm (°C) or less, where Tm (°C) is the melting point of the precious metal. The method for manufacturing nanoscale crystal aggregates of precious metals according to the present invention will be explained in detail, taking into account the two points to note regarding the sintering temperature mentioned above.

[0032] B-1. Raw material powder The precious metal powder used as a raw material for the nanoscale crystal mass according to the present invention is a powder made of a precious metal with a median diameter of primary particles of 0.1 μm or less. Primary particles are the smallest units of particles when the powder is divided to the maximum extent possible. Since the nanoscale crystal mass according to the present invention is composed of fine crystal grains with a median diameter of 0.1 μm or less, the precious metal powder used as a raw material is also required to be composed of minute primary particles. However, the primary particles of the precious metal powder do not need to be single crystals and may be polycrystalline. The median diameter of the primary particles of the raw material powder is more preferably 0.05 μm or less. The lower limit of the primary particle diameter is preferably 1 nm (0.001 μm) or more, and more preferably 10 nm (0.01 μm) or more. Furthermore, the purity of the precious metal powder used as a raw material is preferably 95.0% by mass or more, and more preferably 97.0% by mass or more.

[0033] One method for producing precious metal powders with a median diameter of 0.1 μm or less is the wet reduction method (liquid phase method). The wet reduction method involves reducing a solution of a precious metal salt (base metal compound) to precipitate fine particles of the precious metal. For example, in the production of platinum powder, dinitrodiammineplatinum(II), chloroplatinic acid(II), diamminedinitroplatinum(II) nitric acid solution, hexachloroplatinum(IV) acid hexahydrate, hexaammineplatinum(IV) chloride solution, tetraammineplatinum(II) chloride, and tetraammineplatinum(II) hydroxylate solution are preferably used as platinum salts. Water is preferred as the solvent. The reduction treatment of the solution involves adding a reducing agent such as ethanol or other alcohol, formic acid, hydrazine, or sodium borohydride to the solution, and heating (reflux heating) as needed to precipitate metallic platinum and produce fine-particle platinum. Powders of other precious metals can be produced by the same method.

[0034] B-2. Manufacturing of compacted powder As a step prior to the sintering process, a compact is produced by pressurizing the base metal powder. This pressurizing step brings the precious metal powder into close proximity to accelerate the sintering process in the next step. Furthermore, this step also serves to obtain the dimensions and shape of the compact as a nanoscale crystalline mass. There are no restrictions on the dimensions and shape of the compact. Powder metallurgy makes it possible to create a near-net shape that approximates the desired shape.

[0035] The pressure used to form the compacted material is preferably 500 MPa or higher. As this pressure increases, the density of the compacted material increases, which can raise the relative density when it becomes a sintered body and also increase the strength of the nanoscale crystal mass. This pressure is more preferably 750 MPa or higher. Furthermore, the pressure used to form the compacted material is preferably 5000 MPa or lower, taking into account the upper limit pressure set in the pressurizing device and mold.

[0036] B-3. ​​Sintering Process B-3-1. Sintering temperature In the production of nanoscale crystal aggregates according to the present invention, setting and controlling the sintering temperature during the sintering process is crucial. As described above, the sintering temperature is set to Tm × 0.4 (°C) or less relative to the melting point Tm of the target precious metal. If this temperature is exceeded, grain growth due to grain boundary movement becomes dominant during the sintering process, making it difficult to produce the desired nanoscale crystal aggregate.

[0037] The sintering temperature can be set according to the type of precious metal being treated, based on the above conditions. For precious metals such as Ag and Au, which tend to self-sinter at relatively low temperatures, it is preferable to set the sintering temperature on the lower side of the above-mentioned upper limit. Specifically, it is preferable to set the sintering temperature for Ag and Au between Tm × 0.02 (°C) and Tm × 0.25 (°C). There is no particular advantage to increasing the sintering temperature excessively in order to promote sintering. On the other hand, since self-sintering does not easily occur in Pt, Ir, and Ru, it is preferable to set the sintering temperature near the upper limit mentioned above. Specifically, the sintering temperature for Pt and Ir is preferably between Tm × 0.35 (°C) and Tm × 0.4 (°C).

[0038] B-3-2. Control of Sintering Temperature In the sintering process, the compacted powder is heated and the temperature is increased until it reaches the sintering temperature described above. The heating rate at this time is preferably between 1°C / min and 20°C / min. If the heating rate is too slow, grain boundary migration will progress between the crystal grains, causing the crystal grains to coarseen and potentially leading to a decrease in hardness. On the other hand, if the heating rate is too high, strain may occur during the sintering process, potentially causing cracks. The atmosphere during heating and cooling in the sintering process can be a vacuum, air, or an inert gas. Even in an air atmosphere, within the sintering temperature range described above, precious metals are less likely to oxidize, so sintering is rarely hindered. Furthermore, there is no significant difference in the strength of sintered bodies processed in these atmospheres. However, assuming all other conditions are the same, sintered bodies heated in an inert gas atmosphere will have the highest strength, albeit by a small margin.

[0039] After the compacted powder reaches the sintering temperature, it is cooled down without further heating or temperature maintenance. Rapid temperature reduction after reaching the sintering temperature, without further heating or temperature maintenance, is essential temperature control for forming nanoscale crystal aggregates from precious metal powder. If heating is continued to increase the temperature or the temperature is maintained near the sintering temperature after reaching the sintering temperature, grain boundary migration will progress, resulting in coarser crystal grains. In the case of precious metal powder, by setting the sintering temperature within the range described above, sintering (grain boundary diffusion) continues even as the temperature decreases. This is thought to be because precious metal powder is resistant to oxidation, making it difficult for an oxide film that would hinder sintering to form on the surface of the precious metal particles even near the sintering temperature.

[0040] Rapid cooling after reaching the sintering temperature means starting the cooling process within 30 minutes of reaching the temperature. As mentioned above, prolonged holding at or above the sintering temperature may lead to grain coarsening. Cooling is initiated by stopping the heating device, such as the sintering furnace. A cooling medium may be used for air cooling or other purposes during cooling, or the cooling process may be initiated by introducing a cooling medium. It is more preferable to start the cooling process within 20 minutes of reaching the sintering temperature, even more preferable within 10 minutes, and particularly preferable within 5 minutes. Cooling may even be initiated within 1 minute. The cooling rate at this time is preferably between 2°C / min and 100°C / min. Too high a cooling rate will damage the sintering furnace body, while too low a cooling rate will result in a long recovery time for the sintered body. These cooling rates can be achieved by furnace cooling or air cooling. It is preferable to continue the cooling process until the sintered body is at least 100°C below the sintering temperature. This is because if the temperature is lowered by 100°C or more below the sintering temperature, grain growth will not proceed easily, and sintering will be almost complete. It is preferable to continue cooling from the sintering temperature until the temperature of the sintered body falls below 50°C, and more preferably to room temperature.

[0041] Through the above steps, precious metal nanoscale crystal aggregates can be manufactured from precious metal powder. The manufactured precious metal nanoscale crystal aggregates can be processed as needed. Cold working is also possible at this stage. Furthermore, as already mentioned, powder metallurgy can produce sintered bodies in a near-net shape, so the manufactured precious metal nanoscale crystal aggregates can be used directly for various applications. [Effects of the Invention]

[0042] As described above, the present invention relates to nanoscale crystal masses composed of precious metals, a type of material for which no manufacturing examples have existed to date. The precious metal nanoscale crystal mass according to the present invention is a polycrystalline material composed of unprecedentedly fine crystal grains compared to precious metal materials manufactured using various conventional manufacturing processes. Due to the rapid increase in grain boundary volume, various property changes can be expected. In particular, there are high expectations for improvements in strength and hardness. [Brief explanation of the drawing]

[0043] [Figure 1] A diagram illustrating the temperature control (heating up to sintering temperature to cooling down) in the sintering process. [Figure 2] SEM images of Pt nanoscale crystal masses from Examples 1, 2, and 4, manufactured according to the first embodiment. [Figure 3] EBSD crystal orientation map of the Pt nanoscale crystal mass of Example 1 manufactured in the first embodiment. [Modes for carrying out the invention]

[0044] First Embodiment The embodiments of the present invention are described below. In this embodiment, Pt was selected as the precious metal, and nanoscale crystal masses were produced from Pt powder.

[0045] The raw material used was Pt powder (Pt Black) produced by a wet reduction method. The purity of this Pt powder was 97% by mass or higher, and the median diameter of the primary particles was 10 nm (0.01 μm). 1 g of this Pt powder was filled into a mold with an inner diameter of 10 mm, and then compressed to produce a compact. In this embodiment, a manual hydraulic press and a mechanical hydraulic press were used to produce Pt compacts by compressing at pressures of 750 MPa (Example 1), 1500 MPa (Example 2), 2250 MPa (Example 3), and 4500 MPa (Example 4). These compacts were then placed in an electric furnace and sintered.

[0046] The sintering process involved setting the sintering temperature to 640°C (Tm × 0.36) and heating in an inert gas atmosphere (Ar atmosphere) at a heating rate of 5°C / min. After confirming that the sintering temperature had been reached, the electric furnace was immediately stopped and allowed to cool to room temperature. The cooling rate at this time was approximately 5°C / min. After cooling to room temperature, the sintered body was removed to obtain a Pt nanoscale crystal mass.

[0047] The Pt nanoscale crystal mass produced in this embodiment was subjected to density measurement using the Archimedes method, and its relative density was calculated. Subsequently, the material structure was observed using SEM and EBSD, and the grain size was measured, the grain size distribution was created, and the median diameter was calculated.

[0048] Figure 2 shows SEM images of the Pt nanoscale crystal mass from Example 1, produced from a compacted powder with the minimum pressure (750 MPa) applied during compaction; the Pt nanoscale crystal mass from Example 2, produced with a medium pressure (1500 MPa); and the Pt nanoscale crystal mass from Example 4, produced with the maximum pressure (4500 MPa).

[0049] Figure 2 shows that the Pt nanoscale crystal mass produced in this embodiment is a sintered body of Pt fine powder and contains voids due to residual pores inside. Comparing Examples 1, 2, and 4, it can be seen that the Pt nanoscale crystal mass in Example 4, which had the highest pressure applied to the compacted powder, has fewer voids. Figure 3 is the EBSD crystal orientation map of the Pt nanoscale crystal mass of Example 1. From Figure 3, it can be confirmed that the Pt nanoscale crystal mass of Example 1 is composed of crystal grains with relatively uniform particle sizes, and there are no crystals with exceptionally large particle sizes. This trend was similar in the other examples. In this embodiment, the particle size of more than 250 crystal grains within the observation area was measured based on this crystal orientation map, a cumulative particle size distribution was created, and the median diameter (D50) was calculated. The measurement results of the median diameter of the crystal grains in the Pt nanoscale crystal mass of each example are shown in Table 1, which will be described later.

[0050] The hardness of the Pt nanoscale crystal masses from each example was measured. The hardness was measured using a micro-Vickers hardness tester (Mitutoyo, product name HM-103) under a load of 0.3 kN. In addition to the Pt nanoscale crystal masses from each example, the hardness of a pure Pt mass that was melt-cast (Comparative Example 1) and a commercially available Pt material used for jewelry (Conventional Example 1) were also measured. The Pt material designated as Conventional Example 1 is a material obtained by cold-rolling melt-cast Pt with a purity of 99.9% by mass or higher at a processing rate of 90%, and is known as a relatively high-hardness Pt material. Table 1 summarizes the grain size (median diameter, maximum grain size), relative density, and hardness of the crystal grains of the Pt nanoscale crystal masses from each example, as well as the purity of the Pt measured by GD-MS.

[0051] [Table 1]

[0052] Table 1 shows that the Pt nanoscale crystal masses of Examples 1 to 4 are all high-purity (99.95% or higher) polycrystalline Pt with a median grain diameter of 0.1 μm or less. Furthermore, the maximum grain size of all of them was 0.5 μm or less. The relative density of the Pt nanoscale crystal masses of Examples 1 to 4 was 80% or higher (approximately 90% or higher).

[0053] Referring to the hardness measurement results for each example, it was confirmed that all Pt nanoscale crystal masses exhibited high hardness of 300 HV or higher. The comparative example, the molten casting, had the lowest hardness at 52 HV, indicating that the Pt nanoscale crystal masses exhibited extremely high hardness compared to the molten casting. Furthermore, even the conventional Pt material in Example 1, which has relatively high strength, had a hardness of 210 HV, confirming that the nanoscale crystal masses exhibited a remarkable increase in hardness.

[0054] Furthermore, as the pressure applied during the manufacturing process to produce the compacted powder increases, the hardness of the Pt nanoscale crystal mass increases, reaching a maximum hardness of over 500 HV (Example 4). However, even the Pt nanoscale crystal mass in Example 1, which uses the lowest pressure, exhibits a hardness exceeding 300 HV. In addition, the relative density of the Pt nanoscale crystal mass increases with increasing pressure applied to the compacted powder. Moreover, the Pt nanoscale crystal masses in each example are of high purity (99.95% or higher) and have good quality as a precious metal. While the high-purity Pt material in Conventional Example 1 is used for jewelry applications, the Pt nanoscale crystal masses in this embodiment can be considered a substitute for Conventional Example 1 as a Pt material that achieves high hardness while maintaining high purity.

[0055] Second Embodiment In this embodiment, Pt fine powder was compacted into a compacted powder in the same manner as in the first embodiment, and this compacted powder was sintered at different sintering temperatures to produce Pt nanoscale crystal aggregates.

[0056] Using the same Pt ​​powder as in the first embodiment, a compacted Pt powder was prepared using the same process and a pressurized pressure of 750 MPa. Then, the compacted powder was sintered at sintering temperatures of 490°C (0.28 × Tm), 530°C (0.30 × Tm), 580°C (0.33 × Tm), 685°C (0.39 × Tm), and 730°C (0.42 × Tm) to produce Pt nanoscale crystal masses.

[0057] Then, the median diameter and Vickers hardness of the crystal grains were measured for the Pt nanoscale crystal masses produced at each sintering temperature, in the same manner as in the first embodiment. The results are shown in Table 2. Table 2 also includes the measured values ​​for Example 1 of the first embodiment (sintering temperature 640°C (Tm × 0.36)) with the same applied pressure.

[0058] [Table 2]

[0059] Table 2 shows that the sintered body (No. 1) with a sintering temperature of 490°C (0.28 × Tm) had the lowest hardness at less than 300 HV (294 HV). This is thought to be due to insufficient sintering because the sintering temperature was too low. Furthermore, the median diameter of the crystal grains tends to increase with increasing sintering temperature. In the sintered body (No. 6) with a sintering temperature of 740°C (0.41 × Tm), the median diameter of the crystal grains exceeded 0.1 μm, and the hardness value was also less than 300 HV (294 HV). It can be said that in order to produce nanoscale crystal masses with a median diameter of 0.1 μm or less, the sintering temperature needs to be lower than approximately 0.40 × Tm (706°C).

[0060] Third Embodiment In this embodiment, Au nanoscale crystal masses (Example 5) and Ag nanoscale crystal masses (Example 6) were produced as precious metal nanoscale crystal masses.

[0061] Example 5 (Production of Au nanoscale crystal masses) The raw material used was Au powder produced by a wet reduction method. The purity of this Au powder was 97% by mass or higher, and the median diameter of the primary particles was 200 nm (0.2 μm). 1 g of this Au powder was filled into the same mold as in the first embodiment, and a compact was produced by pressing it under pressure at 750 MPa using a manual hydraulic press. The compact was then sintered in an electric furnace.

[0062] In this embodiment, the sintering temperature for producing the Au nanoscale crystal mass was set to 380°C (Tm × 0.35). The sintering process involved heating at a heating rate of 5°C / min, and immediately after confirming that the sintering temperature had been reached, the electric furnace was stopped and the furnace was cooled to room temperature. The cooling rate at this time was approximately 5°C / min. After cooling to room temperature, the sintered body was removed to obtain the Au nanoscale crystal mass.

[0063] Example 6 (Production of Ag nanoscale crystal masses) The raw material used was Ag powder produced by a wet reduction method. The purity of this Ag powder was 97.8% by mass or higher, and the median diameter of the primary particles was 200 nm (0.2 μm). 1 g of this Ag powder was packed into the same mold as in the first embodiment, and a compact was produced by pressing it under pressure at 750 MPa using a manual hydraulic press. The compact was then sintered in an electric furnace. The sintering temperature for producing the Ag nanoscale crystal mass in this embodiment was set to 95°C (Tm × 0.1). Since Ag powder is self-sintering, the sintering temperature was set to a low temperature. In the sintering process, the temperature was increased and heated at a rate of 5°C / min, and immediately after confirming that the sintering temperature had been reached, the electric furnace was stopped and the mixture was cooled to room temperature. The cooling rate at this time was approximately 5°C / min. After cooling to room temperature, the sintered body was removed to obtain an Ag nanoscale crystal mass.

[0064] The manufactured Au nanoscale crystal mass (Example 5) and Ag nanoscale crystal mass (Example 6) were subjected to relative density measurement, median diameter measurement (EBSD), and purity (grade) measurement in the same manner as in the first embodiment, followed by Vickers hardness measurement. The load used for Vickers hardness measurement was 0.3 kN for both Au and Ag. These measurement results are shown in Table 3. For comparison between the examples, the hardness of the melted and cast materials was also measured.

[0065] [Table 3]

[0066] Table 3 confirms that the nanoscale crystal masses of the noble metals in Examples 5 and 6 exhibited more than twice the hardness of the molten castings (Au, Ag). [Industrial applicability]

[0067] As described above, the present invention reveals nanoscale crystalline masses made of precious metals, whose manufacture and properties have not been reported until now. The precious metal nanoscale crystalline mass according to the present invention can show a clear increase in hardness compared to conventional precious metal materials, based on the effect of rapidly increasing grain boundary volume due to the ultra-fine grain size. In addition to applications in jewelry and ornaments, the precious metal nanoscale crystalline mass according to the present invention is also expected to be applied to contact materials for various motors and switches where both conductivity and hardness are required.

Claims

1. A nanoscale crystal mass consisting of a polycrystalline material of a precious metal with a purity of 99% or higher, A nanoscale crystal mass in which the median diameter of the crystal grains constituting the polycrystalline material is 0.1 μm or less.

2. The nanoscale crystalline mass according to claim 1, which is a sintered body with a relative density of 80% or more.

3. The nanoscale crystal mass according to claim 1 or claim 2, wherein the precious metal is Pt and the Vickers hardness is 300 HV or higher.

4. The nanoscale crystal mass according to claim 1 or claim 2, wherein the precious metal is Au or Ag, and the Vickers hardness is 100 HV or more.

5. A method for producing nanoscale crystal aggregates according to claim 1 or claim 2, The process includes a pressurization step of pressurizing a powder made of a noble metal with a median diameter of primary particles of 0.1 μm or less to form a compact, and a sintering step of sintering the compact. The sintering step involves raising the temperature of the compacted powder until it reaches the sintering temperature, and then lowering the temperature without further heating or temperature maintenance after reaching the sintering temperature. A method for producing nanoscale crystal masses, wherein the sintering temperature is 0.4 × Tm (°C) or less, when the melting point of the noble metal is Tm (°C).

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