Patent Application
20220062985
This application claims the benefit of Korean Patent Application No. 10-2020-0110160, filed with the Korean Intellectual Property Office on Aug. 31, 2020, the disclosure of which is incorporated herein by reference in its entirety.
The present invention relates to porous silver powder and a preparation method thereof. More specifically, the present invention relates to porous silver powder that is easy to prepare, improves a sterilization effect because an specific surface area and a porosity are easily adjustable, improves electrical conductivity when molded as sintered body, contributes to reducing use of expensive silver when applied in various industrial fields, thus achieving price competitiveness, and is harmless to the human body because a particle size is adjustable to prevent the porous silver powder from being absorbed into the body; and a preparation method thereof.
Silver (Ag) powder has drawn much attention, for example, as a material of a solar cell, an adhesive material of a semiconductor chip package, a material of a touch panel pattern, a filler material of a vehicle product, medical and biomaterials, etc. in various industrial fields.
Particularly, there is a growing need for nano ink to which silver nano powder is applied to print a micro-electronic circuit. Nano ink has been applied for various purposes in the field of solar cells, sensors, touch screens, flexible display, etc., and is used in the form of conductive ink, dielectric ink, or the like.
However, ink containing silver (Ag) is still expensive and is limited in exhibiting desired performance. Accordingly, research has been conducted on using inexpensive copper instead of silver but an additional process is needed to maintain an inert gas environment so as to prevent oxidation of copper in a sintering process, thereby increasing manufacturing costs.
A silver (Ag)-tin (Sn)-based alloy including low-melting-point metals is applied to conductive paste or ink but electrical conductivity decreases significantly due to generation of an intermetallic compound of silver (Ag) and tin (Sn).
When a surface of silver nano powder combines with oxygen atoms in the air and bacterial cell membrane, the bacterial cell membrane is oxidized, thus achieving a sterilization effect. However, when the content of expensive silver is increased, manufacturing costs increase and thus research is being conducted to increase a surface area of silver powder per unit mass so as to effectively increase a sterilization effect but there are technical limits in a manufacturing process.
Generally, nano powder is formed to a very small size of less than 100 nm and thus may penetrate the skin, thus causing adverse health effects, and is not likely to be discharged from the body when absorbed into the body due to the very small size. In particular, when silver nano powder is applied in a medical or bio field, the silver nano powder may be harmful to the human body.
Therefore, porous silver powder, which is easy to prepare, improves a sterilization effect because an specific surface area and porosity are easy to adjust, increases electrical conductivity when molded as sintered body, contributes to reducing use of expensive silver when applied in various industrial fields, thus achieving price competitiveness, and is harmless to the human body because a particle size is adjustable to prevent the porous silver powder from being absorbed into the body, and a preparation method thereof are required.
The present invention is directed to providing porous silver powder that is easy to prepare, improves a sterilization effect because an specific surface area and a porosity are easily adjustable, improves electrical conductivity when molded as sintered body, contributes to reducing use of expensive silver when applied to various industrial fields, thereby securing price competitiveness, and is harmless to the human body because a particle size is adjustable to prevent the porous silver powder from being absorbed into the body; and a preparation method thereof.
To achieve the object, the present invention provides a porous silver powder, wherein pores are formed by etching alloy powder containing silver (Ag) and an alloying element other than silver (Ag) with an acid solution or a basic solution to remove at least part of the alloying element.
Also, the present invention provides the porous silver powder, wherein residual content of the alloying element is 0.5 to 3% by weight based on the total weight of the porous silver powder.
And, the present invention provides the porous silver powder, wherein the alloying element comprises at least one material selected from the group consisting of silicon (Si), bismuth (Bi), germanium (Ge), copper (Cu), iron (Fe), nickel (Ni), chromium (CR), and cobalt (Co).
Further, the present invention provides the porous silver powder, wherein the alloying element further comprises at least one transition metal element.
And, the present invention provides the porous silver powder, wherein the basic solution comprises at least one selected from the group consisting of sodium hydroxide, potassium hydroxide, lithium hydroxide, an ammonia solution, zinc chloride, tetramethyl ammonium hydroxide (TMAH), and ethyltrimethyl ammonium hydroxide (ETMAH).
Also, the present invention provides the porous silver powder, wherein an average particle size D50 of the porous silver powder is in a range of 100 to 20,000 nm.
Further, the present invention provides the porous silver powder, wherein a porosity is in a range of 5 to 50% and a specific surface area (BET) is in a range of 4.0 to 8.0 m2/g.
And, the present invention provides the porous silver powder, wherein an atomic ratio between silver (Ag) and the alloying element contained in the alloy powder is 1:9 or 8:2.
Also, the present invention provides a preparation method of the porous silver powder, comprising: preparing a master alloy containing silver (Ag) and an alloying element other than silver (Ag); manufacturing an alloy casting by dissolving and rapidly solidifying the master alloy; grinding the alloy casting into alloy powder; and removing at least part of the alloying element by etching the alloy powder with an acid solution or a basic solution.
Further, the present invention provides the preparation method of claim 9, wherein the rapidly solidifying of the master alloy is carried out by melt spinning, wherein, in the melt spinning, an average cooling rate is 106 to 108° C./sec.
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The present invention is, however, not limited thereto and may be embodied in many different forms. Rather, the embodiments set forth herein are provided so that this disclosure will be thorough and complete, and fully convey the scope of the invention to those of ordinary skill in the art.
Porous silver powder according to the present invention may be formed in a porous structure by a chemical method. That is, the porous silver powder may be formed of alloy powder of a binary or higher system including silver (Ag) and an alloying element other than silver (Ag), and specifically, multiple pores may be formed on a surface or the inside of the alloy powder by removing at least part of the alloying element by etching the alloy powder with an acid solvent or a basic solvent, and preferably, a polar solvent such as the basic solvent.
Here, the etching of the alloy powder performed to form pores in the porous silver powder refers to forming pores in the alloy powder by dissolving the alloying element from the alloy powder with an acid solution or a basic solution and extracting the dissolved alloying element with a solvent.
Accordingly, in the porous silver powder according to the present invention, a porosity, i.e., a ratio of total volume of pores to total volume of the porous silver powder, may be easily adjusted by adjusting the content of the alloying element in the alloy powder excluding silver (Ag) during the preparation of the alloy powder or adjusting a cooling rate when a molten alloy is cooled to prepare the alloy powder.
That is, the porosity increases as the content of the alloying element contained in the alloy powder excluding silver (Ag), i.e., the content of the alloying element in the pores in the alloy powder from which the alloying element is extracted and removed using the solvent during the etching of the alloy powder, increases or increases as a cooling rate of the molten alloy decreases during the cooling of the molten alloy because the alloying element is removed during the etching of the alloy powder due to the growth of crystal grains due to the alloying element other than silver (Ag).
Specifically, the porous silver powder according to the present invention may have a porosity of 5 to 50% and thus particularly have a specific surface area (BET) of 4.0 to 8.0 m2/g and preferably 5.0 to 6.0 m2/g. Here, when the porosity is less than 5%, a surface area of the porous silver powder is insufficient and thus a sterilization effect, an antibacterial effect, an antifungal effect, an antiviral effect or the like may decrease, and a melting point of a metal may increase due to a reduction of the amount of surface energy due to the insufficient surface area of the porous silver powder and thus a sintering process should be performed under relatively high pressure and at relatively high temperature, e.g., a temperature higher than 300° C., when sintered body is formed of the porous silver powder, thus increasing manufacturing time and costs.
[34] When the porosity is greater than 50%, the shape of the porous silver powder is difficult to maintain and the sterilization effect, electrical conductivity, etc. may decrease due to a reduction of the content of silver (Ag).
Part of the alloying element may remain when the porous silver powder according to the present invention is etched, thus improving some physical properties of the porous silver powder, and the residual content of the alloying element may be 0.5 to 3% by weight based on the total weight of the porous silver powder.
Specifically, the remaining alloying element improves dislocation density of the porous silver powder and thus overall mechanical properties, such as strength and durability, of the porous silver powder may increase, thereby maintaining the shape of the porous silver powder for a long time. When the content of the remaining alloying element is less than 0.5% by weight, it is difficult to expect an improvement in additional mechanical properties due to the alloying element, and when the content of the remaining alloying element is greater than 3% by weight, electrical conductivity of the porous silver powder decreases and a melting point increases, thus increasing costs of a sintering process.
An atomic ratio (at %) between silver (Ag) and the alloying element contained in the alloy powder may be 1:9 or 8:2, and preferably, 2:8 or 6:4.
If it is assumed that a total atomic weight of the alloy powder is 10, when an atomic weight of silver (Ag) is less than 1, the content of the alloy element is greater than that of silver (Ag) and thus the content of the alloying element to be removed by etching increases, thereby making it difficult to maintain the shape of the powder after etching, and when an atomic weight of silver (Ag) is greater than 8, the content of the alloying element to be removed by etching decreases and a porosity of pores formed in the porous silver powder decreases, thereby decreasing a sterilization effect of the porous silver powder and decreasing electrical conductivity of sintered body and the like formed of the porous silver powder.
The alloying element contained in the alloy powder may be an element that does not form an intermetallic compound when combined and reacted with silver (Ag). Specifically, the alloying element may include at least one material selected from the group consisting of silicon (Si), bismuth (Bi), germanium (Ge), copper (Cu), iron (Fe), nickel (Ni), chromium (Cr) and cobalt (Co), and preferably, silicon (Si), bismuth (Bi), germanium (Ge) or the like having high brittleness may be selectively used for ease of grinding.
The alloying element may further include at least one transition metal element, and the transition metal element may be selected from among elements of period 4 to 7 and elements of Group 3 to 12 listed in the periodic table of elements, and preferably, scandium (Sc), titanium (Ti), chromium (Cr) or the like may be used. The transition metal element may additionally improve the sterilization effect, electrical conductivity, etc. of silver (Ag).
The acid solvent serving as the polar solvent may include at least one among a phosphoric acid (H3PO4) solution, a sulfuric acid (H2SO4) solution, an acetic acid (CH3COOH) solution, and a hydrofluoric acid (HF) solution. The basic solvent serving as the polar solvent may include at least one among a sodium hydroxide (NaOH) solution, a potassium hydroxide (KOH) solution, a lithium hydroxide (LiOH) solution, a sodium cyanide (NaCN) solution, an ammonium hydroxide (NH4OH) solution, a tetra methyl ammonium hydroxide (TMAH)solution, an ethyltrimetyl ammonium hydroxide (ETMAH) solution, an ammonia (NH3) aqueous solution, and a zinc chloride (ZNCL) solution, and preferably, sodium hydroxide or potassium hydroxide with low toxicity and low persistence may be selected and used. The acid solvent or the basic solvent may be diluted with distilled water for efficient solvent extraction.
An average particle size D50 of the porous silver powder according to the present invention may be set to 100 to 20,000 nm, and preferably, 300 to 3,000 nm, so that the porous silver powder may be applicable to an electronic ink material.
Here, the average particle size D50 refers to a particle size defined with respect to 50% of a distribution of particle sizes (diameters) of powder, based on the volume of the porous silver powder, and may be measured by, for example, a laser diffraction method.
Generally, silver powder is formed to a particle size of less than 100 nm and thus is difficult to be discharged and promotes platelet aggregation and causes cardiovascular disease due to the small particle size when absorbed into skin cells, thereby causing a bad effect on the human body. Therefore, it is difficult to apply the silver powder to the bio industry and the medical field.
However, an average particle size of the porous silver powder according to the present invention is adjustable to 100 nm or more, and preferably, 1,000 nm or more, and thus, the porous silver powder may be prevented from being absorbed into the skin and may be easily discharged out of the body even when absorbed into the body through the respiratory organ, thereby securing safety to the body.
An average pore size of the porous silver powder according to the present invention may be 10 to 1,000 nm, and preferably, 1 to 500 nm, and it was confirmed that an average particle size of pores of the porous silver powder formed when etched was 100 nm or less. Here, the average particle size of the pores of the porous silver powder may be the same or substantially the same as an average particle size of the alloying element removed by etching.
The preparation method of porous silver powder according to the present invention includes preparing a master alloy silver (Ag) and an alloying element other than silver (Ag) (S110), manufacturing an alloy casting by dissolving and rapidly solidifying the master alloy (S120), grinding the alloy casting into alloy powder (S130), and removing at least part of the alloying element by etching the alloy powder with an acid solvent or a basic solvent (S140).
In the preparing of the master alloy (S110), a master alloy may be prepared by mixing and dissolving silver (Ag) and the alloying element. In the mixing of silver (Ag) and the alloying element, silver (Ag) and the alloying element may be weighed using an electronic scale such that an atomic ratio therebetween is 1:9 or 8:2 and thereafter may be dissolved to prepare a master alloy with a uniform composition.
Silicon (Si) may be selected as the alloying element to be mixed with silver (Ag) in the preparing of the master alloy (S110). A melting point of silver (Ag) is about 961° C. and a melting point of silicon (Si) is about 1,414° C., and thus, the difference between the melting points thereof is about 450° C.
Accordingly, when silver (Ag) and silicon (Si) are put and dissolved together in the same container, silver (Ag) with a relatively low melting point is easily dissolved but much energy is needed to completely dissolve silicon (Si) with a relatively high melting point. Therefore, as silver (Ag) that is first dissolved is volatilized or sublimated, an atomic ratio between silver (Ag) and silicon (Si) that constitute the alloying elements changes, thus making it difficult to prepare a master alloy with a uniform composition.
Therefore, in the preparing of the master alloy (S110), the master alloy may be cast using casting equipment such as a vacuum arc furnace or a vacuum induction melting (VIM) furnace, after an inert atmosphere is formed by injecting an inert gas such as argon (Ar) in a vacuum state to evenly dissolve the alloying element without being oxidized. Alternatively, in order to save energy, a molten alloy may be applied to subsequent operation (S120) without being solidified in the form of a master alloy.
Thereafter, in the manufacturing of the alloy casting (S120), the master alloy prepared in the preparing of the master alloy (S110) is dissolved and rapid solidified to manufacture an alloy casting. Here, in the rapid solidification, an average cooling rate is high and thus crystal growth of a certain alloying element may be suppressed as much as possible due to rapid cooling of the molten metal, and thus, the rapid solidification is effective for preparation of porous silver powder of a nano composite structure.
In the manufacturing of the alloy casting (S120), the rapid solidification may be performed by melting spinning at an average cooling rate of 106 to 108° C./sec, and pores of porous silver powder prepared by rapid solidification may be formed in nano sizes, thereby improving a specific surface area may be improved and easily controlling a particle size of resultant powder.
Unlike general metal casting methods, in the melt spinning, a rate of cooling from a liquid state to a solid state is very high, i.e., 106 to 108° C./sec, and in this case, production of a solid nucleus is promoted during the melt spinning, thereby quickly completing a phase change to the solid state.
The alloy casting manufactured by melt spinning may be in the form of a ribbon having an ultra-thin thickness and a certain width, and a microstructure thereof may be a nanostructure or an amorphous structure.
In the grinding of the alloy casting into alloy powder (S130), the alloy casting rapidly solidified by melt spinning in the manufacturing of the alloy casting (S120) is mechanically ground into alloy powder.
In the grinding of the alloy casting into alloy powder (S130), the alloy casting is first ground using a paint shaker, a pin crush or the like to prepare alloy powder having an average particle size of 1 to 50 μm.
The alloy powder prepared by first grinding the alloy casting may be second ground into more fine powder by a dry or wet method. Here, in the dry method, the alloy powder is ground using an air classifier mill or the like by feeding the alloy powder to a high-speed gas flow to be collided against a blade or the like, and in the wet method, the alloy powder is ground using a bead mill or the like by putting water or an organic solvent and 0.1 to 0.5φ ceramic balls into a container and grinding the alloy powder using a shear force generated by starting a rotating part.
Next, in the removing of the alloying element (S140), the alloying element is partially removed by etching the alloy powder, which is finally ground in the grinding of the alloy casting into alloy powder (S130), with an acid or basic solvent. The etching of the alloy powder may be performed for about thirty minutes to five hours but embodiments are not limited thereto.
In the removing of the alloying element (S140), pores may be formed in a region of the alloy powder from which the alloying element is removed, thereby obtaining the porous silver powder of the present invention, and as described above, an average particle size D50 of the finally obtained porous silver powder may be in a range of 100 to 20,000 nm and be adjustable according to a field to which the porous silver powder is applied, a porosity thereof may be in a range of 5 to 50%, and a specific surface area (BET) thereof may be in a range of 4.0 to 8.0 m2/g.
The porous silver powder and the preparation method thereof will be described with respect to examples below.
1. Preparation Example of Porous Silver Powder
silver (Ag) and silicon (Si) were mixed at an atomic ratio of 0.67:1 and dissolved at about 1500° C. to prepare a master alloy. The master alloy was dissolved by melt spinning and rapidly solidified to manufacture an alloy casting. Next, the alloy casting was mechanically ground using a paint shaker to prepare alloy powder with an average particle size D50 of 10 μm.
100 ml of distilled water and 25 g of NaOH were mixed and then heated at 60° C. The alloy powder was added to the heated basic solution and silicon (Si) was removed to prepare porous silver powder.
Porous silver powder was prepared under the same condition as in Example 1 except that silver (Ag) and silicon (Si) were mixed at an atomic ratio of 0.43:1.
Porous silver powder was prepared under the same condition as in Example 1 except that silver (Ag) and silicon (Si) were mixed at an atomic ratio of 0.25:1.
2. Evaluation of Etching Results
Referring to
It is expected from the results of
Specifically, generally, 60% or more of silver nano powder is contained in a conductive paste, an adhesive or an ink to achieve electromagnetic wave shielding performance, whereas it is expected that high electromagnetic wave shielding performance of 60 db can be achieved with only 35% of porous silver powder according to Examples 1 to 3 of the present invention, thereby reducing manufacturing costs.
In addition, when a sterilization test was conducted using the porous silver powder according to Examples 1 to 3 of the present invention for about one hour, 99.9% or more of both colon bacillus and staphylococcus aureus were removed and thus germicidal, antibacterial, antifungal, and antiviral effects of the porous silver powder were verified.
3. Evaluation of Mechanical Characteristics of Porous Silver Powder
The residual content of alloying elements in porous silver powder was measured by wet spectroscopy (ICP), and as shown in
As shown in Table 1 above, a change of the volume of the porous silver powder before/after the compression decreased significantly when the residual content of the alloying elements was 0.5 wt % or more and thus the mechanical properties of the porous silver powder were improved, whereas a change of the volume of the porous silver powder before/after the compression increased when the residual content of the alloying elements was 0.4 wt % or less and thus the porous silver powder clumped and an average particle size, porosity, a specific surface area, etc. were not maintained when compressed.
While the present invention has been described above with respect to exemplary embodiments thereof, it would be understood by those of ordinary skilled in the art that various changes and modifications may be made without departing from the technical conception and scope of the present invention defined in the following claims. Thus, it is clear that all modifications are included in the technical scope of the present invention as long as they include the components as claimed in the claims of the present invention.
In porous silver powder according to the present invention, an alloy can be easily prepared using certain alloying elements, a specific surface area and a porosity can be easily adjusted to improve a sterilization effect, electrical conductivity can be improved when molded as sintered body at a low temperature and under low pressure, use of silver can be reduced when the porous silver powder is applied in various industrial fields, thus securing prices competitiveness, and the porous silver powder can be prevented from being absorbed into the body by adjusting a particle size and thus is harmless to the human body.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2020-0110160 | Aug 2020 | KR | national |
