Abstract:
The present invention relates to a method for manufacturing nickel nanoparticles and more particularly to a method including preparing a mixture solution by adding a reducing agent, a dispersing agent and a nickel salt to a polyol; stirring and heating the mixture solution; and producing nickel nanoparticles by reacting the mixture solution, so that it allows mass production of nickel nanoparticles having uniformity of size 30 to 50 nm and high dispersibility.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2007-0073598 filed on Jul. 23, 2007, with the Korea Intellectual Property Office, the contents of which are incorporated here by reference in their entirety. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention related to a method for manufacturing nickel nanoparticles and more particularly, to a method for manufacturing nickel nanoparticles having uniform size and high dispersibility. 
         [0004]    2. Description of the Related Art 
         [0005]    In response to demands for much smaller electronic components, the multi layer ceramic capacitor has been widely used as a capacitor having a miniaturized size and high capacity. Highly expensive materials such as Pd, Pt, etc. have been used for the internal electrode of the multi layer ceramic capacitor but they are recently replaced with nickel particles for a cost matter. Particularly, researches using nickel as a material of the internal electrode of the multi layer ceramic capacitor having high capacity has been greatly increased. 
         [0006]    Since a nickel electrode layer of the multi layer ceramic capacitor has a lower packing density than that of a nickel molding obtained by the powder metallurgy and much larger shrinkage than a dielectric substance during the sintering process, the defect rate is high with shorten-turn and broken phenomena. In order to avoid such defects nickel particles should not contain large size particles but have narrow and uniform particle distribution and good dispersibility without coagulation. 
         [0007]    Various methods for manufacturing nickel powders to be used as a material of the internal electrode of the multi layer ceramic capacitors have been introduced but any method until now has not satisfied to manufacture nickel powders having uniform size of less than 100 nm which is suitable for multi-layered and high capacity capacitors. 
         [0008]    Particularly, conventionally nickel chloride is carried for the gas phase reduction with hydrogen at a high temperature of about 1000° C. However, even if the surface of particles is smooth due to thermal history of the reaction at a high temperature, the particle distribution is wide and the size of particles is large, larger than 1.0 μm, since nucleation and growth are occurred at the same time. Thus, these particles are insufficient for thinning layers of the internal electrode. 
         [0009]    Further, a wet reduction for manufacturing nickel powders is introduced which reduces a solution of nickel chloride and nickel sulfate with a reducing agent of hydrazine and hydrazine hydrate in the presence of a strong alkali. Even if this method provides a narrower particle distribution compared to the gas phase reduction, the surface of particles is not smooth and thus it is not suitable for the internal electrode. 
         [0010]    Further, after adding ethylene glycol, a capping molecule and a reducing agent, a metal precursor and an alcohol compound are added and then acetone and ethylene glycol are added to produce metal nanoparticles. In this method, the reaction temperature is increased before the metal precursor is added. This method is not simple. 
         [0011]    In addition, various methods for manufacturing metal nanoparticles have been introduced and among these methods the wet reduction is relatively easy to control shape and size of particles and provides fine particles having a sub-micron size. However, since there are several reaction variables during the reaction process, the reaction can be ununiform and it is difficult to manufacture fine particles having uniformity of a size of 200 nm to 1 μm which is less than 100 nm. Further, it requires an additional reduction step, so that it is not suitable for mass production of uniform nickel nanoparticles. 
       SUMMARY 
       [0012]    An aspect of the present invention is to provide a method for manufacturing nickel nanoparticles which have uniform particle distribution and high dispersibility and allows mass production with a simple process. 
         [0013]    In order to resolve the aforementioned problems associated with the conventional methods, is a method for manufacturing nickel nanoparticles provided, the method including: preparing a mixture solution by adding a reducing agent, a dispersing agent and a nickel salt to a polyol; stirring and heating the mixture solution; and producing nickel nanoparticles by reacting the mixture solution. 
         [0014]    According to an embodiment of the present invention, the reducing agent may be at least one chosen from sodium hypophosphite (NaH 2 PO 2 ), hydrazine (N 2 H 4 ), hydrochloride, sodium borohydride (NaBH 4 ), and sodium hydroxymethylsulfoxylate (NaHSO 2 ◯CH 2 O◯2H 2 O). 
         [0015]    According to an embodiment of the present invention, the dispersing agent may be at least one chosen from a cationic surfactant, an anionic surfactant, an analogue of cellulose, a polymer, a copolymer and a terpolymer. 
         [0016]    According to an embodiment of the present invention, the dispersing agent may be at least one chosen from cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), sodium carboxymethyl cellulose (Na-CMC), polyvinylpyrrolidone (PVP), vinylpyrrolidone/vinylacetate (PVP/VA), and vinylcaprolactam/vinylpyrrolidone/propylmethacylamide. 
         [0017]    According to an embodiment of the present invention, the nickel salt may be added in a concentration of 0.001 to 1M with respect to the mixture solution. 
         [0018]    According to an embodiment of the present invention, the reducing agent may be added in a mole ratio of 2 to 10 with respect to the nickel salt. 
         [0019]    According to an embodiment of the present invention, the dispersing agent may be added in a mole ratio of 1 to 20 with respect to the nickel salt. 
         [0020]    According to an embodiment of the present invention, the nickel salt may be at least one chosen from NiCl 2 , Ni(NO 3 ) 2 , NiSO 4  and (CH 3 COO) 2 Ni. 
         [0021]    According to an embodiment of the present invention, the polyol may be at least one chosen from ethylene glycol, diethylene glycol, triethylene glycol, and polyethylene glycol. 
         [0022]    According to an embodiment of the present invention, the mixture solution may be heated to 80 to 160° C. 
         [0023]    According to an embodiment of the present invention, the method may further include washing, isolating, and drying the produced nickel nanoparticles. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a SEM image of nickel nanoparticles produced in Example 1 of the present invention. 
           [0025]      FIG. 2  is a XRD graph of nickel nanoparticles produced in Example 1 of the present invention. 
           [0026]      FIG. 3  is a SEM image of nickel nanoparticles produced in Comparison Example 1. 
           [0027]      FIG. 4  is a XRD graph of nickel nanoparticles produced in Comparison Example 1 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    Hereinafter, the method for manufacturing nickel nanoparticles according to the invention will be described in more detail. 
         [0029]    Nickel nanoparticles are produced by preparing a mixture solution by adding a reducing agent, a dispersing agent, a nickel salt to a polyol, stirring and heating the mixture solution, producing nickel particles through a reducing reaction under control of a reaction temperature and a reaction time, and washing, isolating, and drying. 
         [0030]    The nickel salt may be a water soluble salt such as NiCl 2 , Ni(NO 3 ) 2 , NiSO 4 , and (CH 3 COO) 2 Ni and it may be used alone or a combination of at least two. The nickel salt may be NiCl 2 . The nickel salt may be added in a concentration of 0.001 to 1M. When the concentrating of the nickel salt is less than 0.001M, the efficiency is not preferable due to low concentration of nickel ions, while when it exceeds more than 1M, it causes overgrowth coagulation of particles. Here, the less amount of a nickel precursor is used the smaller the nickel nanoparticles are produced. 
         [0031]    The polyol such as ethylene glycol, diethylene glycol, triethylene glycol, and polyethylene glycol may be used alone or as a combination of at least two, preferably ethylene glycol alone. 
         [0032]    Ethylene glycol reduces the metal precursor along with the reducing agent by preventing remaining of unreacted compounds, so that it increases the yield of manufacturing. Further, ethylene glycol may not only be used as a solvent to dissolve the metal precursor but also removes unreacted PVP with addition of excess amount of acetone and completes the reaction. 
         [0033]    Examples of the reducing agent include dimethylformamide (DMF), glucose, ascorbic acid, tannic acid, tetrabutyl ammonium borohydride, sodium hypophosphite (NaH 2 PO 2 ), hydrazine (N 2 H 4 ), hydrochloride, sodium borohydride (NaBH 4 ), sodium hydroxymethylsulfoxylate (NaHSO 2 ◯CH 2 O◯2H 2 O), etc., preferably sodium hypophosphite (NaH 2 PO 2 ). 
         [0034]    The may be added in a mole ratio of 2 to 10 with respect to the nickel salt. When it is added less than 2 mole ratio, the nickel ions cannot be sufficiently reduced, while when it is added more than 10 mole ratio, excess amount of byproducts are produced and thus it is not economical since the reducing agent is used much more than the amount to reduce 100% of the nickel ions. 
         [0035]    The dispersing agent may be a cationic or anionic surfactant such as cetyltrimethylammonium bromide (CTAB) or sodium dodecyl sulfate (SDS), an analogue of cellulose such as sodium carboxymethyl cellulose (Na-CMC), a polymer such as polyvinylpyrrolidone (PVP), a copolymer such as vinylpyrrolidone/vinylacetate (PVP/VA), a terpolymer such as vinylcaprolactam/vinylpyrrolidone/propylmethacrylamide, etc., and it can be used alone or a combination of at least two, preferably PVP alone, and more preferably PVP having a molecular weight of 40,000 alone. 
         [0036]    Further, the dispersing agent may be used in a mole ratio of 1 to 20 with respect to the nickel salt. When it is less than 1 mole ratio, it can be difficult to control the shape and size of particles and thus cannot provide sufficient dispersibility of the produced particles, while when it is more than 20 mole ratio, the viscosity of the precursor solution is rapidly increased, it may be difficult to mix uniformly, the reaction cannot be performed uniformly, it produces excess amount of unreacted compounds or byproducts, and it requires large amount of a solvent to wash and isolate which is then uneconomical. 
         [0037]    The polyol mixture solution, in which the reducing agent, the dispersing agent and the nickel salt are dissolved, may be heated to 80 to 160° C. When the temperature is higher than 160° C., the reaction may proceed rapidly and thus the stability gets decreased and the produced particles are not uniform, while it is lower than 80° C., the reduction is not sufficiently performed. 
         [0038]    When the mixture solution is heated as described above, the reduction is performed at a temperature of 100 to 140° C. according the mole ratio of the nickel salt and the reducing agent. The reaction time is in a range of 1 minute to 1 hour. When the reaction time is within 1 minute, the reduction is not performed sufficiently and the yield thus becomes lowered. On the other hand, when the reaction time is more than 1 hour, it causes overgrowth of particles and ununiformity. 
         [0039]    When the reduction completes to produce nickel nanoparticles, the reaction is quickly cooled by using ice-water to prevent overgrowth of particles and the nickel nanoparticles produced are isolated by centrifuge, etc. The isolated nickel nanoparticles are washed with water and acetone to remove byproducts and any remained compound. The washed nickel nanoparticles are then dried in a vacuum oven at a temperature of 30 to 80° C. for 2 to 8 hours. 
         [0040]    While the present invention has been described with reference to particular embodiments, it is to be appreciated that various changes and modifications may be made by those skilled in the art without departing from the spirit and scope of the present invention, as defined by the appended claims and their equivalents. Throughout the description of the present invention, when describing a certain technology is determined to evade the point of the present invention, the pertinent detailed description will be omitted. 
       EXAMPLE 1 
     Preparation of Nickel Nanoparticles 
       [0041]    Nickel chloride 95.04 g (0.4M), sodium hypophosphite 106 g (1.2M), PVP 444 g (1.2M), ethylene glycol 500 ml were mixed in a beaker. The mixture solution was dissolved while stirring and the temperature was slowly increased up to 120° C. at a rate of 2° C./min. The reaction mixture was turned to black due to the reduction at a temperature of 120° C. and preceded for 30 min. The reaction mixture was then cooled down quickly by using ice-water and black nickel nanoparticles were recovered from the reaction mixture by centrifuge. The produced nickel nanoparticles were washed with acetone and distilled water 3 times and dried in a vacuum oven at a temperature of 50° C. for 3 hours to provide target nickel nanoparticles 12 g. 
         [0042]    A SEM image of the nickel nanoparticles produced in Example 1 is shown in  FIG. 1 . According to  FIG. 1 , it is noted that the nickel nanoparticles have a size of 30 to 50 nm and are uniform. 
         [0043]    A XRD graph of the nickel nanoparticles produced in Example 1 is shown in  FIG. 2 . According to  FIG. 2 , it is noted that pure nickel crystalline having face-centered cubic lattice structure (FCC structure) is formed without any impurity and oxidized compound. When pure nickel particles having FCC structure are formed, 3 distinctive peaks are appeared at 111, 200, and 220 corresponding to each FCC structure as shown in  FIG. 2 . 
       COMPARISON EXAMPLE 1 
       [0044]    In order to compare with the result of Example 1, a reaction in which a metal salt was added after increasing the reaction temperature was performed as follows. 
         [0045]    Sodium hypophosphite 106 g, PVP 444 g, ethylene glycol 400 g were mixed in a beaker and the mixture solution was dissolved while stirring and increasing the temperature up to 120° C. at a rate of 2° C./min. Nickel chloride 95.04 g was dissolved in ethylene glycol 150 g and the mixture solution was then heated to 120° C. The mixture solution of nickel chloride was added at once to the mixture solution of sodium hypophosphite, PVP, ethylene glycol and then thoroughly mixed using a stirrer while keeping the temperature at 120° C. The reaction mixture was turned slowly to black and further preceded for 60 minutes. The reaction mixture was then cooled down quickly by using ice-water and black nickel nanoparticles were recovered from the reaction mixture by centrifuge. The produced nickel nanoparticles were washed with acetone and distilled water 3 times and dried at a vacuum oven at a temperature of 50° C. for 3 hours to provide target nickel nanoparticles 8 g. 
         [0046]    A SEM image of the nickel nanoparticles produced in Comparison Example 1 is shown in  FIG. 3 . According to  FIG. 3 , it is noted that the nickel nanoparticles are ununiform with heavy coagulation. 
         [0047]    A XRD graph of the nickel nanoparticles produced in Comparison Example 1 is shown in  FIG. 4 . According to  FIG. 4 , it is noted that the nickel crystalline is not a face-centered cubic lattice structure (FCC structure). It is also noted that nickel crystalline is not formed smoothly according to the conventional method which adds a metal salt after the temperature is increased as shown in Comparison Example 1.