General gas phase assisted high sintering active silver powder and preparation method thereof

Abstract

The application discloses a general gas-phase assisted high-sintering active silver powder and a preparation method thereof. The preparation method comprises the following steps: 1) ozone oxidation: after the end of the silver powder growth process, precipitation is carried out in a reaction kettle, mother liquor is discharged, and the silver powder is stirred, and ozone is introduced into the reaction kettle to oxidize the silver powder during the stirring process; 2) after the end of the ozone introduction, the silver powder is washed alternately with water and ethanol, and silver carboxylate is added during the ethanol washing, so that the silver carboxylate is coated on the surface of the oxidized silver powder; 3) then, the silver powder obtained in the step (2) is post-treated, and finally, the general gas-phase assisted high-sintering active silver powder is obtained. The application forms an in-situ silver oxide shell layer on the surface of the silver powder by means of ozone oxidation, and the sintering activity of the silver powder is successfully improved by means of coating with multiple silver carboxylates without changing other performances of the silver powder.

Description

Technical Field

[0001] This application relates to a general gas-phase assisted high-activity sintering silver powder and its preparation method, belonging to the field of metal powder material preparation. Background Technology

[0002] The importance of silver powder in the photovoltaic industry cannot be ignored. It plays a crucial role in the manufacturing process of solar cells, from the conductive grid to the reflective layer, barrier layer, and metal back electrode, all of which are essential for improving the efficiency, stability, and reliability of photovoltaic cells. In the manufacturing process of solar cells, silver powder is commonly used as a conductive grid material. The conductive grid's function is to draw current out of the photovoltaic cell and provide a low-resistance path to reduce resistance loss. Silver powder has excellent conductivity and current carrying capacity, which can effectively achieve high-efficiency conversion of solar cells. Silver powder can also be used to prepare the reflective layer of solar cells. The reflective layer can improve the light absorption efficiency of photovoltaic cells and reduce light loss. Due to the good light reflection properties of silver powder, it can effectively increase the utilization rate of sunlight by solar cells. In the operating environment of photovoltaic cells, factors such as humidity and oxidation may affect cell performance, leading to a decline in cell performance. Silver powder can be made into a barrier layer, covering the cell surface, to protect against the intrusion of moisture and oxides, improving the stability and lifespan of photovoltaic cells. Some types of photovoltaic cells, such as thin-film solar cells, require a metal back electrode to provide current harvesting functionality. Silver powder, due to its excellent conductivity and adhesion, is often used as the main material for metal back electrodes, enabling efficient current collection and transmission. The application of silver powder has promoted the development of the photovoltaic industry and driven the utilization of clean energy and sustainable development.

[0003] Raw silver powder readily reacts with oxygen in the air at room temperature to form an oxide layer, leading to a decrease in its surface conductivity. Untreated silver powder, lacking a protective layer, is more susceptible to oxidation, resulting in reduced conductivity and stability. Furthermore, untreated silver powder often exhibits large particle size and aggregation, making it difficult to disperse uniformly in the substrate material. This leads to inhomogeneity in the properties of the prepared material, affecting its conductivity, mechanical properties, and other characteristics. Additionally, its poor adhesion to other materials (such as the substrate material and adhesives) can result in insufficient bond strength, leading to peeling or detachment. During the oxidation process, the oxide layer formed by untreated silver powder may be uneven or inconsistent in thickness. This results in unstable performance, with variations between different batches or samples, affecting product consistency and reliability. Silver powder is easily corroded in high humidity or corrosive environments, especially without a protective surface layer. Untreated silver powder has poor corrosion resistance, potentially leading to decreased long-term stability and durability of the material. Untreated silver powder has drawbacks in terms of oxidation sensitivity, dispersibility, adhesion, oxide layer uniformity, and corrosion resistance. Appropriate surface treatment can improve these shortcomings, enhancing the performance and stability of the silver powder and making it more suitable for various applications.

[0004] Common surface treatment methods for silver powder mainly include the following: Oxidation treatment: Silver powder is easily oxidized when exposed to air, forming a silver oxide film. Oxidation treatment can be achieved by placing the silver powder in an oxidizing agent solution or by heat treatment. Oxidation treatment can increase the stability and oxidation resistance of the silver powder surface. Surface coating: By depositing other metals such as nickel, zinc, and copper onto the surface of silver powder to form a coating, the conductivity, corrosion resistance, and adhesion of the silver powder can be improved. Coating methods include electroplating and electroless plating, and appropriate coating materials and process conditions can be selected according to specific needs. Surface modification: Surface modification refers to adding a covering material, such as organic molecules or polymers, to the surface of silver powder to increase the dispersibility, stability, and particle size control performance of the silver powder. This can be achieved through solvent methods, wet synthesis, etc. Spray coating: Silver powder can be directly coated onto the surface of a target object using spray coating technology. Spray coating can improve the bonding strength and adhesion between the silver powder and the substrate material. Common spray coating methods include spray spraying and electrophoretic spraying. Surface Repair: During the application of silver powder, factors such as wear and oxidation may cause a decline in surface quality. Surface repair can restore the surface smoothness and flatness of the silver powder through methods such as grinding and polishing.

[0005] However, each of the commonly used modification methods has its advantages and disadvantages. Oxidation treatment can form a thin oxide film on the surface of silver powder, improving its oxidation resistance and corrosion resistance. The oxide layer can also improve the dispersibility and adhesion properties of silver powder. Conventional heating oxidation is difficult to control precisely; if the oxide layer is uneven or inconsistent in thickness, it may lead to unstable performance of the silver powder. Silicon coating can improve the stability and oxidation resistance of silver powder, while reducing the contact between silver powder and the environmental medium, thus reducing the sensitivity of silver powder to humidity and oxygen. However, silicon coating increases the density and particle size of silver powder, which may affect its dispersibility and flowability. In addition, the silicon coating process requires specialized equipment and process control.

[0006] Treatment with surfactants is also a common method. Surfactants can form a protective organic molecular layer on the surface of silver powder, improving its dispersibility and stability. Surfactants can also regulate the interaction between silver powder and the substrate material, improving adhesion. However, some surfactants may reduce the conductivity of silver powder, especially when added in excessive amounts. Furthermore, surfactants may also have adverse effects on specific applications, such as interfering with the adhesion of adhesives.

[0007] Meanwhile, these methods are all post-processing methods, compared to in-situ integrated processing: In-situ processing allows the surface treatment of silver powder to be performed simultaneously with its preparation process. This means that surface treatment can be performed directly during the production or application of silver powder without removing the powder or performing additional processing steps. This saves time, labor, and costs. It also maintains the stability of particle shape and distribution: In-situ processing ensures the stability of the silver powder's particle shape and distribution. By treating the silver powder in-situ during preparation, particle aggregation or precipitation can be prevented, and consistency in particle size and dispersion can be maintained. In-situ processing can create a better bond between the silver powder surface and the substrate material. By performing surface treatment during preparation, the adhesion and consistency between the silver powder and the substrate can be improved, thereby improving the material's performance and stability. In production, in-situ processing can be closely coupled with other preparation steps, facilitating the optimization and control of process parameters. This helps ensure that surface treatment matches other production parameters, improving product performance and consistency. In-situ processing can adjust the properties and performance of silver powder, such as increasing its conductivity, oxidation resistance, or mechanical strength. By performing in-situ treatment of silver powder during the preparation process, customized surface modification can be achieved to meet specific application requirements. Summary of the Invention

[0008] To address at least one of the aforementioned problems, this application provides a general gas-phase assisted method for preparing highly active sintering silver powder. This method utilizes ozone oxidation to form an in-situ silver oxide shell on the surface of the silver powder, and then coats it with various silver carboxylic acids. This successfully enhances the sintering activity of the silver powder without altering its other properties.

[0009] The technical solution provided in this application is as follows:

[0010] According to one aspect of this application, a general method for preparing highly active sintered silver powder with vapor phase assistance is provided, comprising the following steps:

[0011] (1) Ozone oxidation

[0012] After the silver powder growth process is completed in the reactor, precipitation is carried out, the mother liquor is discharged, and the silver powder is stirred. During the stirring process, ozone is introduced into the reactor to oxidize the silver powder.

[0013] (2) After the ozone is introduced, the silver powder is washed with water and ethanol alternately. Silver carboxylate is added during the ethanol washing so that the silver carboxylate coats the surface of the oxidized silver powder.

[0014] (3) Then the silver powder obtained in (2) is post-processed to finally obtain general gas phase assisted high sintering active silver powder.

[0015] Untreated silver powder was grown in a reactor. After the growth process, ozone was generated using an ozone generator and introduced into the reactor. The stirring rate was controlled according to the quality of the silver powder to ensure thorough mixing of the ozone and the silver powder, resulting in a uniform silver oxide shell on the surface of the powder. The thickness of the silver oxide was controlled by adjusting the ozone introduction time. Then, silver carboxylate was added during the ethanol washing process to coat the surface of the silver powder. Finally, high-activity sintered silver powder was obtained through post-processing steps such as washing, centrifugation, and drying.

[0016] Ozone gas-phase oxidation eliminates the need for high temperatures, reducing energy consumption and requiring minimal equipment modifications to the production process. Compared to heated oxidation, this room-temperature gas-phase oxidation reaction is slower, produces a more uniform oxide layer thickness, and is easier to control. Furthermore, unlike physical modification methods, the mixing of silver powders of different particle sizes does not alter the particle size and shape of the silver powder, allowing for universal application. This preparation method can be integrated into the production process, occurring simultaneously with the washing and drying of the silver powder. This significantly saves time, labor, and costs.

[0017] Optionally, in step (2), the ozone is introduced for 5-60 minutes, the ozone concentration is 80-120 ppm, and the stirring speed of the reactor during the oxidation of silver powder by ozone is 150-320 r / min.

[0018] Furthermore, in step (2), the ozone is introduced for 30-60 minutes, the ozone concentration is 90-110 ppm, and the stirring speed of the reactor during the oxidation of silver powder by ozone is 160-310 r / min.

[0019] Furthermore, in step (2), the ozone is introduced for 30-40 minutes, the ozone concentration is 100 ppm, and the stirring speed of the reactor during the oxidation of silver powder by ozone is 170-300 r / min.

[0020] In step (2), if the ozone oxidation time is too long, the oxidation degree on the surface of the silver powder will deepen, and after the formation of the silver oxide shell, the difficulty of further oxidation will increase, and the amount of ozone required will increase sharply. As the thickness of the silver oxide increases, an excessively thick silver oxide layer will reduce the conductivity of the silver oxide, which will have an adverse effect on the silver powder.

[0021] Optionally, in step (3), the mass of the silver carboxylate is 1‰-6‰ of the mass of the silver powder;

[0022] Furthermore, the mass of the silver carboxylate is 2‰-4‰ of the mass of the silver powder;

[0023] Furthermore, the mass of the silver carboxylate is 3‰ of the mass of the silver powder.

[0024] Optionally, the silver carboxylate includes one or more of silver stearate, silver palmitate, silver oleate, and silver neocaprate.

[0025] Optionally, in step (1), the capacity of the reactor is 30L, and the particle size of the silver powder before oxidation is 1-5μm;

[0026] Preferably, the particle size of the silver powder before oxidation is 1-2 μm.

[0027] Optionally, in step (1), the ozone oxidation temperature is room temperature, which does not require cooling or heating, thus reducing energy consumption.

[0028] Optionally, in step (3), after adding silver carboxylate during ethanol washing, the stirring speed is 150-300 r / min and the stirring time is 10-20 min.

[0029] Furthermore, in step (3), after adding silver carboxylate during ethanol washing, the stirring speed is 180-250 r / min and the stirring time is 12-18 min.

[0030] According to another aspect of this application, a general-purpose vapor-phase assisted high-sintering active silver powder is provided, which is obtained by any of the preparation methods described above.

[0031] Optionally, the weight loss on ignition of the general-purpose vapor-assisted high-sintering active silver powder is not less than 0.490 w%.

[0032] The beneficial effects of this application include, but are not limited to:

[0033] 1. According to the general gas-phase assisted method for preparing highly active sintered silver powder of this application, untreated silver powder is grown in a reactor. After the silver powder growth process is completed, ozone is generated using an ozone generator and introduced into the reactor. The stirring rate is controlled according to the quality of the silver powder to ensure thorough mixing of ozone and silver powder, resulting in a uniform silver oxide shell layer on the surface of the silver powder. The thickness of the silver oxide layer is controlled by adjusting the ozone introduction time. Then, silver carboxylate is added during the ethanol washing process to coat the surface of the silver powder. Finally, highly active sintered silver powder is obtained through post-processing steps such as washing, centrifugation, and drying.

[0034] Ozone gas-phase oxidation eliminates the need for high temperatures, reducing energy consumption and requiring minimal equipment modifications to the production process. Compared to heated oxidation, this room-temperature gas-phase oxidation reaction is slower, produces a more uniform oxide layer thickness, and is easier to control. Furthermore, unlike physical modification methods, the mixing of silver powders of different particle sizes does not alter the particle size and shape of the silver powder, allowing for universal application. This preparation method can be integrated into the production process, occurring simultaneously with the washing and drying of the silver powder. This significantly saves time, labor, and costs.

[0035] 2. The general gas-phase assisted high-activity silver powder preparation method of this application can be carried out at room temperature, reducing energy consumption; compared with physical methods, gas-phase reaction makes the surface and thickness uniform and does not change the morphology.

[0036] 3. The general gas-phase assisted high-sintering active silver powder preparation method according to this application uses simple equipment and processes, mild reaction conditions, short production cycle, easy operation, good repeatability, energy saving and environmental protection, and is suitable for industrial scale-up and industrial application. Attached Figure Description

[0037] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments of this application and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings:

[0038] Figure 1 SEM image of the highly sintered active silver powder 1# prepared in Example 1 of this application;

[0039] Figure 2 SEM image of the highly sintered active silver powder 2# prepared in Example 2 of this application;

[0040] Figure 3 SEM image of the highly sintered active silver powder 3# prepared in Example 3 of this application;

[0041] Figure 4 SEM image of comparative silver powder 1'# prepared for Comparative Example 1 of this application;

[0042] Figure 5 SEM image of comparative silver powder 2'# prepared in Comparative Example 2 of this application;

[0043] Figure 6 This is a SEM image of the comparative silver powder 3'# prepared for Comparative Example 3 of this application. Detailed Implementation

[0044] The present invention will be described in detail below with reference to specific embodiments. The following embodiments are only for the purpose of enabling those skilled in the art to understand the technical solutions of the present invention, implement or use the present invention, and are not intended to limit the scope of protection of the present invention.

[0045] Unless otherwise specified, the materials and equipment used in the embodiments of this application are all commercially purchased or commonly used in the art. The methods in the embodiments, unless otherwise specified, are conventional methods in the art.

[0046] In this application, silver stearate, silver palmitate, silver oleate, and silver neocaprate are all of analytical grade concentration. In this application, the morphology of silver powder was detected by Inspect S50 scanning electron microscope, the specific surface area of ​​silver powder was tested by QDS-30 fully automatic nitrogen adsorption surface area analyzer, the weight loss on ignition of silver powder was tested by YX1207 high temperature furnace, and the tap density was tested by BT-302 tap density analyzer.

[0047] It should be noted that the silver powder manufacturing process without surface treatment in this application (i.e., step (1) silver powder growth process) is prior art. Please refer to patent CN112475311A.

[0048] Example 1

[0049] Preparation of high-sintering active silver powder #1:

[0050] 1) Pre-treatment of silver powder

[0051] After the silver powder growth process is completed in a 30L glass reactor, the mixture settles, the mother liquor is extracted, and the mixture is rinsed with water once. After the solution is discharged, the next step is carried out.

[0052] 2) Ozone oxidation

[0053] Ozone is introduced during the stirring of silver powder. The ozone is introduced at the bottom of the reactor at a concentration of 100 ppm. The ozone introduction time is controlled to be 30 min. During the ozone introduction, the silver powder is stirred, and a uniform reaction is carried out on the surface of the silver powder, resulting in a uniform silver oxide coating on the surface of the silver powder.

[0054] 3) Surface silver carboxylate coating

[0055] After the ozone was introduced, the mixture was washed alternately with water and ethanol. During the ethanol washing, 6 grams of silver stearate were added. The silver stearate accounted for about 3‰ of the mass of the silver powder. The mixture was stirred at 200 r / min for 15 min.

[0056] 4) The reactants obtained in step 3) are subjected to post-processing such as solid-liquid separation, washing, and drying to obtain silver powder 1# with high sintering activity.

[0057] Example 2

[0058] Preparation of high-sintering active silver powder #2:

[0059] 1) Pre-treatment of silver powder

[0060] After the silver powder growth process is completed in a 30L glass reactor, the mixture settles, the mother liquor is extracted, and the mixture is rinsed with water. After the solution is discharged, the next step is carried out.

[0061] 2) Ozone oxidation

[0062] Ozone is introduced during the stirring of silver powder. The ozone is introduced at the bottom of the reactor at a concentration of 100 ppm. The ozone introduction time is controlled to be 30 min. During the ozone introduction, the silver powder is stirred, and a uniform reaction is carried out on the surface of the silver powder, resulting in a uniform silver oxide coating on the surface of the silver powder.

[0063] 3) Surface silver carboxylate coating

[0064] After the ozone was introduced, the mixture was washed alternately with water and ethanol. During the ethanol washing, 6 grams of silver palmitate were added. The silver palmitate accounted for about 3‰ of the mass of the silver powder. The mixture was stirred at 200 r / min for 15 min.

[0065] 4) The reactants obtained in step 3) are subjected to post-processing such as solid-liquid separation, washing, and drying to obtain silver powder 2# with high sintering activity.

[0066] Example 3

[0067] Preparation of high-sintering active silver powder #3:

[0068] 1) Pre-treatment of silver powder

[0069] After the silver powder growth process is completed in a 30L glass reactor, the mixture settles, the mother liquor is extracted, and the mixture is rinsed with water once. After the solution is discharged, the next step is carried out.

[0070] 2) Ozone oxidation

[0071] Ozone is introduced during the stirring of silver powder. The ozone is introduced at the bottom of the reactor at a concentration of 100 ppm. The ozone introduction time is controlled to be 30 min. During the ozone introduction, the silver powder is stirred, and a uniform reaction is carried out on the surface of the silver powder, resulting in a uniform silver oxide coating on the surface of the silver powder.

[0072] 3) Surface silver carboxylate coating

[0073] After the ozone was introduced, the mixture was washed alternately with water and ethanol. During the ethanol washing, 6 grams of silver oleate were added. The silver oleate accounted for about 3‰ of the mass of the silver powder. The mixture was stirred at 200 r / min for 15 min.

[0074] 4) The reactants obtained in step 3) are subjected to post-processing such as solid-liquid separation, washing, and drying to obtain silver powder 3# with high sintering activity.

[0075] It should be noted that the ozone oxidation steps in Examples 1-3 were all carried out at room temperature. The theoretical yield of the untreated silver powder precursor was 2 kg, and the morphology was 1-2 μm spherical silver powder. The amount of deionized water used in the cleaning after the mother liquor was drained in step 1) was the same as the volume of the mother liquor.

[0076] Comparative Example 1

[0077] Preparation of silver powder 1'#

[0078] The difference between the preparation of silver powder 1' and Example 1 is that step 2) ozone oxidation and step 3) surface silver carboxylate coating were not performed.

[0079] Comparative Example 2

[0080] Preparation of silver powder 2'#

[0081] The difference between the preparation of silver powder 1' and Example 1 is that step 3) of surface silver carboxylic acid coating was not performed.

[0082] Comparative Example 3

[0083] Preparation of silver powder 3'#

[0084] The difference between the preparation of silver powder 1' and Example 1 is that step 2) ozone oxidation was not performed.

[0085] Example 4

[0086] The test results of specific surface area, tap density and weight loss on ignition of high sintering active silver powder 1#-3# and control silver powder 1'#-3'# are shown in Table 1 below:

[0087] Table 1

[0088]

[0089] The above performance tests show that the high-sintering active silver powder 1#-3#, which has undergone ozone oxidation and surface silver carboxylate coating, has a larger specific surface area and higher weight loss on ignition. The surface area and weight loss on ignition of 1'#-3'#, which has not undergone ozone oxidation and / or surface silver carboxylate coating, are not as good as those of the high-sintering active silver powder 1#-3#.

[0090] from Figures 1-6 From the SEM images, there is almost no difference in morphology between the SEM images of Examples 1-3 and Comparative Examples 1-3. One of the silver carboxylate coating treatments is to coat the surface with a small amount of silver carboxylate film, and the other is to change the surface composition of the silver powder. Neither of these treatments will change the morphology of the silver powder.

[0091] To obtain silver powder with high surface activity, this application improves the silver powder surface treatment process and develops an ozone-assisted gas-phase surface treatment method to prepare silver powder with high sintering activity. The gas-phase oxidation used in this application does not require high temperatures, reducing energy consumption and having extremely low equipment requirements, without necessitating extensive modifications to the production process. This room-temperature gas-phase oxidation reaction is slower than heated oxidation, resulting in a more uniform oxide layer thickness that is easier to control. Furthermore, unlike physical modification methods, the mixing of silver powders of different particle sizes does not alter the particle size and shape of the silver powder, allowing for universal application without impact. Following ozone oxidation treatment, silver carboxylate is added for surface treatment, coating the silver oxide shell with an organic silver layer, which significantly improves the sintering activity of the silver powder. The preparation method of the high-sintering-activity silver powder in this application can be added to the production process, carried out simultaneously during the washing and drying of the silver powder, thus greatly saving time, labor, and costs.

[0092] The above description is merely an embodiment of this application, and the scope of protection of this application is not limited to these specific embodiments, but is determined by the claims of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the technical concept and principles of this application should be included within the scope of protection of this application.

Claims

1. A general method for preparing highly active sintered silver powder with gas phase assistance, characterized in that, Includes the following steps: (1) Ozone oxidation After the silver powder growth process is completed in the reactor, precipitation is carried out, the mother liquor is discharged, and the silver powder is stirred. During the stirring process, ozone is introduced into the reactor to oxidize the silver powder. (2) After the ozone is introduced, the silver powder is washed with water and ethanol alternately. Silver carboxylate is added during the ethanol washing so that the silver carboxylate coats the surface of the oxidized silver powder. The mass of the silver carboxylate is 1‰-6‰ of the mass of the silver powder. (3) Then the silver powder obtained in (2) is post-processed to finally obtain general gas phase assisted high sintering active silver powder.

2. The general gas-phase assisted method for preparing highly active sintered silver powder according to claim 1, characterized in that, In step (1), the ozone is introduced for 5-60 minutes, the ozone concentration is 80-120 ppm, and the stirring speed of the reactor during the oxidation of silver powder by ozone is 150-320 r / min.

3. The general gas-phase assisted method for preparing highly active sintered silver powder according to claim 2, characterized in that, In step (1), the ozone is introduced for 30-60 minutes, the ozone concentration is 90-110 ppm, and the stirring speed of the reactor during the oxidation of silver powder by ozone is 160-310 r / min.

4. The general gas-phase assisted method for preparing highly active sintered silver powder according to claim 1, characterized in that, In step (2), the mass of the silver carboxylate is 2‰-4‰ of the mass of the silver powder.

5. The general gas-phase assisted method for preparing highly active sintered silver powder according to claim 4, characterized in that, In step (2), the mass of the silver carboxylate is 3‰ of the mass of the silver powder.

6. The general gas-phase assisted method for preparing highly active sintered silver powder according to claim 1, characterized in that, The silver carboxylate includes one or more of silver stearate, silver palmitate, silver oleate, and silver neodecanoate.

7. The general gas-phase assisted method for preparing highly active sintered silver powder according to claim 1, characterized in that, In step (1), the capacity of the reactor is 30L, and the particle size of the silver powder before oxidation is 1-5μm.

8. The general gas-phase assisted method for preparing highly active sintered silver powder according to claim 7, characterized in that, In step (1), the particle size of the silver powder before oxidation is 1-2 μm.

9. The general vapor-phase assisted method for preparing highly active sintered silver powder according to claim 1, characterized in that, In step (1), the ozone oxidation temperature is room temperature.

10. The general vapor-phase assisted method for preparing highly active sintered silver powder according to claim 1, characterized in that, In step (2), after adding silver carboxylate during ethanol washing, the stirring speed is 150-300 r / min and the stirring time is 10-20 min.

11. A general-purpose vapor-phase assisted high-sintering active silver powder, characterized in that, Obtained by the preparation method described in any one of claims 1-10.

12. The universal vapor-phase assisted high-sintering active silver powder according to claim 11, characterized in that, The weight loss on ignition of the general-purpose vapor-assisted high-sintering active silver powder is not less than 0.490%.

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