Abstract:
A method for preparing core-shell and hollow silver particles is provided. In the method silver salts and glycine nitrate or starch are mixed with solvent to form precursor solution. The mole percentage of the silver salts over the silver salts plus glycine nitrate or starch is 5 to 50 mol %. The precursor solution is then atomized to form precursor droplets. The precursor droplets are heated by pyrolysis to form silver particles. The composition of the precursor solution can be adjusted to finely manipulate the structure of the silver particles.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to Taiwan Application Serial Number 102123840, filed Jul. 3, 2013, which is herein incorporated by reference. 
       BACKGROUND 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to a method of preparing silver particles. More particularly, the present invention relates to a method of preparing silver particles and core-shell silver particles, and the formed core-shell silver particles. 
         [0004]    2. Description of Related Art 
         [0005]    In the recent years, the core-shell and hollow silver particles have received much attention for their excellent natures and wide applicability such as conductors, electrical contacts and wound dressing. For example, the wound dressing of antibiosis and silver conductive adhesive are the major commercialized applications of the silver particles. However, the major processes of making silver particle, like reduction process and sol-gel process, are both conducted in batch and thus have less potential for mass production in view of business. 
         [0006]    At present, most of the applications use solid silver particle, which has some issues for further improvement. For example, the solid silver particle most likely is not totally consumed after the wound dressing of antibiosis expires, which is not economic and wasting. On the other hand, the solid silver particles may sediment after a period of time of idle due to large difference of evident density between the solid silver particles and the gel solution, which makes uneven distribution of the particles in gel solution and deteriorates the properties of the product. 
         [0007]    Concerning the silver particles with hollow structure, the major measures of preparing the hollow structure includes soft template, hard template, and Kirkendall effect. The soft template needs additional heating process step to remove polymer template and may involve carbon contamination problem. The hard template needs to use acid/base to remove the template, causing environmental pollution and need to deal with the post-treatment of acid/base. Kirkendall effect also has defects due to its complex procedures and high instrument cost. 
       SUMMARY 
       [0008]    Therefore, the present disclosure provides a method of preparing hollow and core-shell silver particle in a continuous process, which can provide silver particles in a mass production way for the industry and choose using glycine nitrate method as the hollow formation mechanism for the silver particles. The glycine nitrate can be removed easily during the heating process due to its low molecular weight, so as to improve the defects in prior art. The prepared core-shell and hollow silver particles can replace the solid silver particles using on the market now to improve the efficiency of silver particle. 
         [0009]    One aspect of the present disclosure is a method for preparing core-shell and hollow silver particles includes mixing a silver salt with a glycine nitrate or starch as a solute in a polar solvent to form a precursor solution, in which the mole percentage of the silver salt over the silver salts plus glycine nitrate or starch is 5-50 mol % and the silver salt plus glycine nitrate or starch are 0.01-10 wt % to the precursor solution, atomizing the precursor solution to form a plurality of precursor droplets, and heating the precursor droplets to pyrolyze the precursor droplets and to form core-shell silver particles and hollow silver particles. 
         [0010]    In various embodiments of the present disclosure, the silver salt is silver nitrate or silver acetate. 
         [0011]    In various embodiments of the present disclosure, the polar solvent is water. 
         [0012]    In various embodiments of the present disclosure, the solute weight percentage concentration of the precursor solution is 1 wt %. 
         [0013]    In various embodiments of the present disclosure, the mole percentage of the silver salts over the silver salts plus glycine nitrate or starch of the precursor solution is between 12.5-50 mol %, the formed silver particles are the core-shell silver particles. 
         [0014]    In various embodiments of the present disclosure, the mole percentage of the silver salts over the silver salts plus glycine nitrate or starch of the precursor solution is less than 12.5 mol %, the formed silver particles are the hollow silver particles. 
         [0015]    In various embodiments of the present disclosure, the step of heating the precursor droplets to pyrolyze the precursor further includes evaporating the polar solvent of the precursor droplets, precipitating the solute of the precursor droplets, and pyrolyzing the solute. 
         [0016]    Another aspect of the present disclosure provides a core-shell silver particle includes a silver core, a silver shell, encapsulating the silver core, and a hollow structure, between the silver core and the silver shell. 
         [0017]    In various embodiments of the present disclosure, the diameter of the core-shell silver particle is about 100-1,000 nanometers. 
         [0018]    It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention can be more fully understood by reading the following detailed description of the embodiment, with reference made to the accompanying drawings as follows: 
           [0020]      FIG. 1  is a flowchart of a method preparing hollow and core-shell silver particles according to various embodiments of the present disclosure; 
           [0021]      FIG. 2  is a schematic diagram of an apparatus for preparing hollow and core-shell silver particles according to various embodiments of the present disclosure; 
           [0022]      FIGS. 3A-3B  are Thermogravimetric analysis graph of silver nitrate and glycine nitrate according to various embodiments of the present disclosure; 
           [0023]      FIGS. 4A-4D  are transmission electron microscopy (TEM) images of silver particles prepared in comparison 1 and experiments 1-3 according to various embodiments of the present disclosure; 
           [0024]      FIGS. 5A-5C  are TEM images of silver particles prepared in comparison 2 and experiments 4-5 according to various embodiments of the present disclosure; 
           [0025]      FIG. 6  is a TEM image of silver particles prepared in experiment 6 according to various embodiments of the present disclosure; and 
           [0026]      FIG. 7  is a sectional view of the core-shell silver particle according to various embodiments of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    Reference will now be made in detail to the present embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
         [0028]    One aspect of the present disclosure is a method for preparing core-shell and hollow silver particles includes the following steps. Referring to  FIG. 1 , step  100  is mixing a silver salt with a glycine nitrate or starch as a solute in a polar solvent to form a precursor solution, in which the mole percentage of the silver salt over the silver salts plus glycine nitrate or starch is 5-50 mol % and the silver salt plus glycine nitrate or starch are 0.01-10 wt % to the precursor solution. some embodiments of the present disclosure, the silver salt is silver nitrate or silver acetate. In various embodiments of the present disclosure, the polar solvent is water. In various embodiments of the present disclosure, the mole percentage of the silver salts over the silver salts plus glycine nitrate or starch of the precursor solution is between 12.5-50 mol %, the prepared silver particles have a core-shell structure. In various embodiments of the present disclosure, the mole percentage of the silver salts over the silver salts plus glycine nitrate or starch of the precursor solution is less than 12.5 mol %, the prepared silver particles have a hollow structure. In various embodiments of the present disclosure, the mole percentage of the silver salts over the silver salts plus starch of the precursor solution is about 50 mol %, the manufactured silver particles have a hollow structure. 
         [0029]    Step  110  is atomizing the precursor solution to form a plurality of precursor droplets. In various embodiments of the present disclosure, using an ultrasonic humidifier to atomize the precursor solution, where the ultrasonic frequency is 1.65 MHz when the polar solvent is water. In various embodiments of the present disclosure, the diameter of the precursor droplets is about 3-20 μm, the diameter of the precursor droplets has positive correlation with the precursor concentration. The preferable concentration range for atomizing the solution is the silver salt plus glycine nitrate or starch is 0.01-10 wt % to the precursor solution. 
         [0030]    Step  120  is heating the precursor droplets to pyrolyze the precursor droplets to form the core-shell silver particles and the hollow silver particles. Other parts of the precursor droplets are pyrolyzed to gas. In various embodiments of the present disclosure, the heating apparatus is a temperature controllable tube furnace, which can control the temperature in the tube to three sections: preheating section (200-400° C.), calcining section (500-800° C.), and cooling section (300-500° C.). In various embodiments of the present disclosure, the heating step includes evaporating the polar solvent of the precursor droplets, precipitating the solute of the precursor droplets, and pyrolyzing the solute. 
         [0031]      FIG. 2  is a schematic diagram of an apparatus for preparing hollow and core-shell silver particles according to various embodiments of the present disclosure. But the method in the present disclosure is not limit to prepare by the apparatus only. As illustrated in  FIG. 2 , a precursor solution  210  is added to an ultrasonic humidifier  220 , the precursor solution  210  is atomizing to form the precursor droplets. The precursor droplets first enter a quartz tube  230  than enter a tube furnace  240 . The precursor droplets are pyrolyzed to form silver particles and gas in the tube furnace  240 . In some embodiments of the present disclosure, using an electrostatic deposition collector  250  with a discharge equipment  260  to collect the silver particles. The gas formed after pryolysis in the tube furnace  240  is passing through cooling water  270  and a filter  280 , than exhaust by a pump  290 . 
         [0032]      FIGS. 3A-3B  are Thermogravimetric analysis graph of silver nitrate and glycine nitrate according to various embodiments of the present disclosure. Referring to  FIG. 3A , the graph shows that silver nitrate has two stages of pyrolysis. The first stage is the temperature rise from room temperature to 442° C., in which the specimen losing 5% weight, estimating that the silver nitrate decomposed to silver nitrite in this stage. The second stage is the temperature rise from 442° C. to 700° C., in which the weight of the specimen is lost from 95% to 67%, estimating that the silver nitrite is pyrolyzed to silver only. Referring to  FIG. 3B , the graph shows that glycine nitrate has two stages of pyrolysis. The first stage is the temperature rise from room temperature to 260° C., in which the specimen losing 51% weight. The second stage is the temperature rise over 650° C., the glycine nitrate is totally pyrolyzed, no weight left. Therefore to decide the pyrolysis temperature is at least 700° C. 
         [0033]    Following are some embodiments to further elaborate the method of the present disclosure, but only for explanation, should not be limited to the description of the embodiments contained herein. The scope of protection for present disclosure depends on the following claims. 
       Embodiments 
       [0034]    A. Preparing the silver particles with the precursor solution prepared from different ratio of silver nitrate and glycine nitrate. 
         [0035]    Comparison 1: No Glycine Nitrate in the Precursor Solution 
         [0036]    Preparing 1 wt % silver nitrate in water as the precursor solution, the experiment apparatus is as illustrated in  FIG. 2 . The precursor solution  210  is added to an ultrasonic humidifier  220 , the precursor solution  210  is atomizing to form the precursor droplets. The precursor droplets first enter a quartz tube  230  than enter a tube furnace  240 , passing through the preheating section (200-400° C.) calcining section (500-800° C.), and cooling section (300-500° C.) The precursor droplets are pyrolyzed to form silver particles and gas in the tube furnace  240 . Using an electrostatic deposition collector  250  with a discharge equipment  260  to collect the silver particles. The gas formed after pryolysis in the tube furnace  240  is passing through cooling water  270  and a filter  280 , than exhaust by a pump  290 . The silver particles formed by this method have solid structure. 
         [0037]    Referring to  FIG. 4A ,  FIG. 4A  is a TEM image of silver particles prepared in comparison 1 according to various embodiments of the present disclosure. The dark parts in the image represent to solid silver particles. The particle diameter is about 360 to 1120 nanometer. 
         [0038]    Experiment 1: the mole percentage of the silver nitrate over the silver nitrate plus glycine nitrate is 25 mol % in the precursor solution 
         [0039]    Preparing 1 wt % precursor solution, in which the mole percentage of the silver nitrate over the silver nitrate plus glycine nitrate is 25 mol %. The experiment process is the same as comparison 1. 
         [0040]    Referring to  FIG. 4B ,  FIG. 4B  is a TEM image of silver particles prepared in experiment 1 according to various embodiments of the present disclosure. The core-shell silver particles are shown in the image, including the core and shell structure as the dark portion and the hollow structure as the brighter portion between the core and shell in the image. The particle diameter is about 118 to 216 nanometers, and the porosity is 22.6%. 
         [0041]    Experiment 2: the mole percentage of the silver nitrate over the silver nitrate plus glycine nitrate is 14.2 mol % in the precursor solution 
         [0042]    Preparing 1 wt % precursor solution, in which the mole percentage of the silver nitrate over the silver nitrate plus glycine nitrate is 14.2 mol %. The experiment process is the same as comparison 1. 
         [0043]    Referring to  FIG. 4C ,  FIG. 4C  is a TEM image of silver particles prepared in experiment 2 according to various embodiments of the present disclosure. The core-shell silver particles are shown in the image, including the core and shell structure as the dark portion and the hollow structure as the brighter portion between the core and shell in the image. The particle diameter is about 127 to 195 nanometers, and the porosity is 27.2%. 
         [0044]    Experiment 3: the mole percentage of the silver nitrate over the silver nitrate plus glycine nitrate is 12.5 mol % in the precursor solution 
         [0045]    Preparing 1 wt % of precursor solution, in which the mole percentage of the silver nitrate over the silver nitrate plus glycine nitrate is 12.5 mol %. The experiment process is the same as comparison 1. 
         [0046]    Referring to  FIG. 4D ,  FIG. 4D  is a TEM image of silver particles manufactured in experiment 3 according to various embodiments of the present disclosure. The hollow silver particles are shown in the image, including the shell structure as the dark portion and the hollow structure as the brighter part inside the shell in the image. The particle diameter is about 143 to 235 nanometers, and the porosity is 36.5%. 
         [0047]    B. Preparing the silver particles with the precursor solution prepared from different ratio of silver acetate and glycine nitrate. 
         [0048]    Comparison 2: No Glycine Nitrate in the Precursor Solution 
         [0049]    The experiment process is the same as comparison 1, but to change the precursor solution including 1 wt % silver nitrate to 1 wt % silver acetate. 
         [0050]    Referring to  FIG. 5A , it is a TEM image of silver particles prepared in comparison 2 according to various embodiments of the present disclosure. The image shows that the silver particles prepared in comparison 2 have solid is structures. 
         [0051]    Experiment 4: the mole percentage of the silver acetate over the silver acetate plus glycine nitrate is 25 mol % in the precursor solution 
         [0052]    The experiment process is the same as comparison 1, but to change the precursor solution to 1 wt % silver acetate and glycine nitrate mixture solution, in which the mole percentage of the silver acetate over the silver acetate plus glycine nitrate is 25 mol %. 
         [0053]    Referring to  FIG. 5B ,  FIG. 5B  is a TEM image of silver particles prepared in experiment 4 according to various embodiments of the present disclosure. The core-shell silver particles are shown in the image, including the core and shell structure as the dark portion and the hollow structure as the brighter portion between the core and shell in the image. The porosity is 36.5%. 
         [0054]    Experiment 5: the mole percentage of the silver acetate over the silver acetate plus glycine nitrate is 16.7 mol % in the precursor solution 
         [0055]    The experiment process is the same as comparison 1, but to change the precursor solution to 1 wt % silver acetate and glycine nitrate mixture solution, in which the mole percentage of the silver acetate over the silver acetate plus glycine nitrate is 16.7 mol %. 
         [0056]    Referring to  FIG. 5C ,  FIG. 5C  is a TEM image of silver particles prepared in experiment 5 according to various embodiments of the present disclosure. The core-shell silver particles are shown in the image, including the core and shell structure as the dark portion and the hollow structure as the brighter portion between the core and shell in the image. The porosity is 33.81%. 
         [0057]    C. Preparing the silver particles with the precursor solution prepared from silver nitrate and starch. 
         [0058]    Experiment 6: the mole percentage of the silver nitrate over the silver nitrate plus starch is 50 mol % in the solution 
         [0059]    The experiment process is the same as comparison 1, but to change the precursor solution to 1 wt % silver nitrate and starch mixture solution, in which the mole percentage of the silver nitrate over the sliver nitrate plus starch is 50 mol %. 
         [0060]    Referring to  FIG. 6 ,  FIG. 6  is a TEM image of silver particles prepared in experiment 6 according to various embodiments of the present disclosure. The hollow silver particles are shown in the image, including the shell structure as the dark portion and the hollow structure as the brighter portion inside the shell in the image. 
         [0061]    Referring to  FIG. 7 ,  FIG. 7  is a sectional view of the core-shell silver particle according to various embodiments of the present disclosure. The core-shell silver particle includes a silver core, a silver shell, which encapsulating the silver core, and a hollow structure, which is between the silver core and the silver shell. In various embodiments of the present disclosure, the diameter of the core-shell silver particle is about 100-1000 nanometer. 
         [0062]    The present disclosure provides the method of continuously preparing the core-shell and hollow silver particles. The method not only is a continuous process that can apply in industry mass production, but also can control porosity of silver particles that can apply in the commercialized silver particle products to save the amount of silver using and the cost. Also the porosity of the silver particles can be controlled depend on the using time of the products to maximize the utility. 
         [0063]    For example, the wound dressing of antibiosis sometimes needs to be thrown away due to reach their expiration date, but there may still have some non-reacted silver inside the silver particles. So the porosity controllable silver particle provided in the present disclosure can adjust the silver amount in the silver particle base on the expiration date to minimize the silver waste. 
         [0064]    In silver conductive adhesive, the solid silver particles may sediment after a period of time of idle due to large difference of evident density between the solid silver particles and the gel solution, which makes uneven distribution of the particles in gel solution and deteriorates the properties of the product. Hollow silver particles, because of having air inside the particles, can evidently decrease the density of the silver particles to minimize the density difference between the solid silver particles and the gel solution to decrease the sedimentation. 
         [0065]    Although the present invention has been described in considerable detail with reference to certain embodiments thereof, other embodiments are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein. 
         [0000]    It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the present disclosure cover modifications and variations of this disclosure provided they fall within the scope of the following claims.