Abstract:
An electroless plating method for use in metal plating on a porous substrate is disclosed. It uses selective contact of the plating metal salt solution with a reducing solution on the activated surface on or inside the porous substrate. This electroless plating method in the setup is useful for unmanned, automatic operation resulting in almost 100% membrane (pure/composite) with substantially no pinholes or cracks.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention provides an electroless plating process, particularly, the present invention provides a continuous process for electroless plating using porous substrate, metal salt solution and reducing solution only to the extent that the membrane is formed; 
         [0002]    The present invention further provides a continuous process for electroless plating by using reducing agent separately. 
       BACKGROUND AND PRIOR ART OF THE INVENTION 
       [0003]    Plating is a surface covering in which a metal is deposited on a surface. Plating has been done for hundreds of years, but is critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improve solderability, to harden, to improve wearability, to reduce friction, to improve paint adhesion, to alter conductivity, for radiation shielding, and for other purposes. Plating also fins applications in the field of nanotechnology where thin-film deposition has plated objects as small as an atom. 
         [0004]    There are several plating methods, and many variations. In one method, a solid surface is covered with a metal sheet, and then heat and pressure are applied to fuse them (a version of this is Sheffield plate). Other plating techniques include vapor deposition under vacuum and sputter deposition. Recently, plating processes often refer to use of liquids. Metallizing refers to coating metal on non-metallic objects. 
         [0005]    Electroless plating, also known as chemical or auto-catalytic plating, is a non-galvanic type of plating method that involves several simultaneous reactions in an aqueous solution, which occur without the use of external electrical power. The reaction is accomplished when hydrogen is released by a reducing agent, usually sodium hypophosphite (Note: the hydrogen leaves as a hydride ion), and oxidized thus producing a negative charge on the surface of the part. The most common electroless plating method is electroless nickel plating, although silver, gold and copper layers can also be applied using electroless plating method, as in the technique of Angel gilding. 
         [0006]    References may be made to U.S. Pat. No. 6,761,929, which provides for a method for preparation of metal/porous substrate composite membranes by flowing a solution of metal to be plated and the use of the resulting membrane for the production of highly purified hydrogen gas. But the cited U.S. application and other prior arts suffer from several drawbacks including that the metal solution and reducing solution are added together in this process, therefore metal is separated before reaching the membrane/substrate which needs to be plated. The separated metal gets deposited in tubing, pipe lines etc resulting in enormous wastage. This deposited metal cannot be recovered, and can prohibitively add to the cost of plating in case of precious metals. Another disadvantage is that there is no assurance that all parts of substrate are plated. Also there is no assurance of complete filling, so that there are no pinholes, cracks. To overcome the drawbacks of prior art processes, the inventor has developed a novel process of electroless plating. 
       OBJECTIVE OF THE INVENTION 
       [0007]    The main object of the present invention is to provide an electroless plating process. 
         [0008]    Another object of the present invention is to provide a process of electroless plating that result in 100% membrane preparation. 
         [0009]    Still another object of the present invention is to provide an automatic process for electroless plating that results in 100% membrane preparation. 
         [0010]    Yet another object of the present invention is to provide an automatic electroless plating process that prevents any wastage of metal to be plated on the substrate. 
       SUMMARY OF THE INVENTION 
       [0011]    Accordingly, present invention provides an electroless plating process for 100% membrane preparation using a porous substrate comprising the steps of:
       i. providing cleaned, sensitized and activated porous substrate selected from the group consisting of glass, sintered metal and ceramics;   ii. contacting first surface of the substrate as provided in step (i) with plating solution;   iii. contacting second surface which is located opposite to the first surface of the substrate as provided in step (i) with reducing solution;   iv. applying pressure to one of the said plating and reducing solutions causing permeation of the pressurized solution to the other side of the substrate resulting contact between plating solution and reducing solution thereby causing deposition of the metal on the substrate surface;   v. rinsing the metal deposited substrate surface with water and heat treating the same at predetermined temperature to obtain the metal plated surface. In an embodiment of the present invention, said process is characterized by contacting one surface of the substrate with plating solution and contacting second surface of substrate with reducing solution.       
 
         [0017]    In another embodiment of the present invention, the plating solution used is selected from metal salt solutions that can produce metal/metal alloy ions and can be reduced by reducing ion like hydrogen to produce metal. 
         [0018]    In yet another embodiment of the present invention, the reducing solution used is selected from solution that can produce a reducing ion like hydrogen that can reduce the metal salt solution to metal. 
         [0019]    In yet another embodiment of the present invention, the reducing solution used is selected from hydrazine, sodium hypophosphite or formaldehyde. 
         [0020]    In yet another embodiment of the present invention, the step of drying the metal deposited substrate comprises heat treating the same at a temperature in the range of 70 to 80° C. followed by annealing at temperature in the range of 350 to 370° C. to obtain plated metal substrate surface. 
         [0021]    In yet another embodiment of the present invention, said metal membrane is without any air leakage. 
         [0022]    In yet another embodiment of the present invention, a system for performing electroless plating process for plating of a porous substrate with metal wherein said system comprises:
       a) a compartment divided into a first section and a second section by a substrate to be plated with metal; each of the said first and the second section comprises
           i. an inlet for supplying plating solution and reducing solution respectively;   ii. an outlet for draining reacted solution;   
           b) a first burette containing the plating solution connected with the inlet of the first section;   c) a second burette containing the reducing solution connected with an inlet of the second section;   d) a pressurizing means connected with the anyone of the said first or the second burette.       
 
         [0029]    In yet another embodiment of the present invention, the first section and the second section are securable with each other to define the said compartment. 
         [0030]    In yet another embodiment of the present invention, the substrate is disposed between the first section and the second section when the said sections are secured with other. 
         [0031]    In yet another embodiment of the present invention, the burettes are connected with the inlets using pipes and the burettes are provided with feed control valves. 
         [0032]    In yet another embodiment of the present invention, the pressurizing means is selected from a balloon, compressor, gas cylinder. 
         [0033]    In yet another embodiment of the present invention, further comprises a heating means heating for any one of the plating or reducing solution. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1  illustrates a system for performing an electroless plating process according to an embodiment of the present invention. 
           [0035]      FIG. 2  illustrate a system for performing an electroless plating process according to another embodiment of the present invention 
           [0036]      FIG. 3  depicts SEM image of substrate of example 2 showing the deposition of Pd on the substrate. 100% plating was visual &amp; also no unplated surface was observed in SEM. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    The present invention provides an electroless plating process wherein 100% plating can be achieved. The present invention provides an automatic process for cleaning, activation and electroless plating for almost 100% plating of substrate surface with substantially no pinholes/cracks of metal on porous substrate comprises:
       a. contacting one surface of the substrate with plating solution;   b. contacting the second surface of substrate with reducing solution;   c. applying pressure to one of the said plating and reducing solutions resulting in permeation of pressurized solution to the other side of the substrate resulting in contact between the two solution causing deposition of metal on one side of the substrate.       
 
         [0041]    The plating operation is carried out in three stages:
       Cleaning   Sensitization &amp; Activation   Electroless plating       
 
         [0045]    In the process of the present invention, the substrate is cleaned with water, followed by sensitization and activation of the substrate surface. In the sensitization and activation step, the substrate is treated with SnCl 2  solution and then PdCl 2  solution. 
         [0046]    The porous substrate comprises a first surface and a second surface which is located opposite to the first surface. The first surface of the substrate is contacted with the plating solution containing metal salts and the second surface of the substrate is contacted with a reducing solution. Thereafter a pressure is applied on any one of the said plating and reducing solution so that solution from one side of the substrate permeates through the substrate and reaches the other side causing contact between the plating and reducing solution thereby resulting in deposition of metal on the said other side of the substrate. 
         [0047]    For example, when a pressure is applied at the plating solution on the side of the first surface of the substrate, the plating solution permeates through the porous substrate and reaches on to the second surface where it comes in contact with the reducing solution. The contact between the reducing solution and the plating solution which has permeated through the substrate at or near the second surface causes deposition of the metal on the second surface of the substrate. Alternatively, when a pressure is applied at the reducing solution, on the side of the second surface of the substrate, the reducing solution permeates through the porous substrate and reaches on to the first surface where it comes in contact with the plating solution. The contact between the plating solution and the reducing solution which has permeated through the substrate at or near the first surface causes deposition of the metal on the first surface of the substrate. 
         [0048]    As it can be clearly noticed, in the process of the present invention the metal is deposited at a surface which is located opposite to the side where pressure is applied. As can be clearly understood, the pressure applied is sufficient so as to result or facilitate permeation of the plating or reducing solution through the porous substrate. 
         [0049]    Electroless plating process of the present invention can be used for plating metal on a porous substrate selected from glass, sintered metal and such like preferably ceramic. 
         [0050]    In the process of the present invention, the plating solution can be selected from metal salt solutions that can produce metal/metal alloy ions and can be reduced by reducing ion like hydrogen to produce metal. Also, the reducing solution is selected from solution that can produce a reducing ion like hydrogen that can reduce the metal salt solutions to metal. In an aspect of the present invention, the reducing solution is selected from hydrazine, sodium hypophosphite or formaldehyde. 
         [0051]    After the metal is deposited on the first or the second surface of the substrate, the substrate is rinsed with water and the metal deposited first or second surface is heat treated to obtain the metal plated substrate surface. In an aspect of the present invention, the heat treatment of the metal deposited first or second surface includes drying the substrate at a temperature in the range of 70 to 80° C. followed by annealing at temperature in the range of 350 to 370° C. to obtain metal plated substrate surface. 
         [0052]    In the process of the invention, the porous substrate facilitates the transport of a solution (plating) on one side, when subjected to pressure. The solution (plating) passes to the other side through the porous substrate where it comes in contact with another solution (reducing) on the activated surface and immediately the reduced metal is deposited on the activated surface. The process continues till all the pores are blocked by the reduced metal. As soon as the whole surface area of the activated side of substrate is plated, plating stops automatically. Thereafter, further processing of plated membrane is done by drying, annealing if required, leak free testing and testing of permeability of H 2  (if the membrane is permeable to H 2  gas). The present invention also provides a system for performing an electroless plating process for plating of a porous substrate with metal.  FIG. 1  shows system for performing an electroless plating process according to an embodiment of the present invention. 
         [0053]    Referring to  FIG. 1 , the system for performing electroless plating process of the present invention comprises a compartment divided into a first section and a second section by a substrate to be plated with metal. As shown in  FIG. 1 , the system for performing electroless plating process of the present invention comprises a compartment ( 1 ) which is separated into an upper (or first) section ( 2 ) and a lower (or second) section ( 3 ) when the substrate ( 4 ) is fitted in the said compartment ( 1 ). For this purpose, the compartment can be dismantle in the lower and upper section for removing and fitting the substrate. In other words, the lower ( 3 ) and upper ( 2 ) sections when secured together forms the compartment ( 1 ). As shown in  FIG. 1 , each of the upper ( 2 ) and the lower ( 3 ) sections comprises an inlet ( 5 ,  6 ) for feed solution (i.e. plating or reducing solution). Also, and each of the upper ( 2 ) and the lower ( 3 ) sections comprises an outlet ( 8 ,  7 ) for draining out the reacted solution. The inlet of the upper section ( 2 ) is connected with a first burette ( 12 ) through pipe and provided with a valve for feed control. Similarly, inlet of the lower section ( 3 ) is connected with a second burette ( 12 ′) through pipe and provided with a valve for feed control. A pressurizing means ( 11 ) is connected with one of burettes ( 12 ,  12 ′) to provide a desired pressure to the feed solution. In embodiment of the present invention, the pressurizing means ( 11 ) is provided at the burette ( 12 ) connected with the inlet ( 5 ) of the upper section. The pressurizing means ( 11 ) can be a balloon or any other suitable pressurizing means or device such as compressor or gas cylinder, etc fixed on a three-way valve can be fitted to one of the burettes to provide desired pressure. Some electroless platings require temperature to be maintained in the range of 70° C. to 90° C. For this purpose, a magnetic stirrer heater ( 9 ) can be provided as shown in  FIG. 1 . The lower section of the compartment can be placed over the magnetic heater ( 9 ) and it is turned on for uniform heating. All parts (i.e. compartments ( 1 ), burettes ( 12 ,  12 ′), pipes etc) of the system can be made airtight for better functioning and preventing leakages using conventional sealing arrangements. 
         [0054]    The process described on the above paragraph and equipment can also be used for activation of the substrate. To carry out sensitizing-activation, SnCl 2  is subjected to pressure on one side of the cleaned substrate to react with PdCl 2  on the other (activating side) or vice versa. Activation will occur on the surface opposite to the pressurized solution. 
         [0055]    The following paragraphs describe performing the electroless plating process of the present invention by the system as shown in  FIG. 1 . 
         [0056]    All parts (i.e. compartments ( 1 ), burettes ( 12 ,  12 ′), pipes etc) of the system ( 1 ) are washed with de-ionized water. The bottom section ( 6 ) of the compartment ( 1 ) is placed over the magnetic heater ( 9 ) and the substrate is fixed in the compartment ( 1 ). The burettes ( 12 ,  12 ′) are connected to the two sections ( 2 ,  3 ) of the compartment ( 1 ). A balloon ( 11 ) is fixed on the burette ( 12 ) connected to upper section ( 2 ) of the compartment ( 1 ). The outlet ( 7 ) of the lower section ( 3 ) was fitted with a thermometer to measure the temperature of the magnetic heater ( 9 ). Sensitizing-activating solution, plating solution and reducing solutions are prepared. Activating solution of PdCl 2  should be preferably prepared overnight for it to stabilize, while SnCl 2  solution should be preferably prepared fresh. Activating solution was put in to burette ( 12 ) which is fixed with an air balloon ( 11 ), to apply desired pressure. The sensitizing solution is put in the other burette ( 12 ′) (i.e. the burette connected with the lower section). After activation of the substrate ( 4 ), the solutions were drained out. Burettes containing activating and sensitizing solutions were replaced by those containing plating solution and reducing solution respectively. In other words, burette containing the plating solution in connected with the inlet ( 5 ) of the upper section ( 2 ) and burette containing the reducing solution is connected with an inlet ( 6 ) of the lower section ( 3 ). The valve of the burette containing reducing solution is opened and the entire lower section was filled up. The magnetic heater is switched on for heating the reducing solution. As the temperature of the reducing solution reached the required range, the valve of the burette containing the plating solution was opened thereby filling the upper section with the plating solution and the desired pressure is maintained through the balloon. Due to the pressurization, the plating solution permeated through the porous substrate ( 4 ) and comes in contact with the reducing solution in the lower section causing deposition or plating of the metal or forming a membrane on the surface of the substrate. After plating, further processing of the synthesized membrane was carried out, such as drying and annealing. The prepared membrane was tested for leaks. Its hydrogen permeability was also tested, as exemplified herein. With reference to example 1, with no air passing through the metal coated substrate, the process of the invention resulting in almost 100% coating with substantially no pinholes or cracks is adequately proven. 
         [0057]      FIG. 2  illustrate a system for performing an electroless plating process according to another embodiment of the present invention, where
   A 1 =Heater &amp; Magnetic Stirrer   A 2 =Plating Solution   A 3 =Peristaltic Pump   A 4 =Plating Compartment   A 5 =Membrane Enclosure   A 6 =Reducing Compartment   A 7 =Membrane   A 8 =Pressurized Reducing Solution   A 9 =Balloon   
 
         [0067]    As can be understood from the  FIG. 2 , the pressure is applied using balloon (A 9 ) to the reducing solution in the reducing compartment (A 9 ) so that the reducing solution permeates through the membrane (A 7 ) contained in the membrane enclosure (A 5 ) and comes in contact with the plating solution in plating compartment (A 4 ) and forms a metal deposition/plating on the surface of the membrane (A 7 ). In the plating compartment (A 4 ), the plating solution is being heated using the magnetic heater stirrer (A 1 ) and circulated with the help of Peristaltic Pump (A 3 ). 
         [0068]    The membrane formed inside the porous substrate (middle of the substrate) is called a composite membrane consisting of metal and substrate itself. It acts as a membrane. This is different from the pure metal membrane formed on the “surface” of the substrate. The basic differences are, the membrane formed on the surface is almost a pure metal membrane &amp; can be separated from the substrate &amp; may have additional advantages like its thickness is very small. But the membrane formed inside the substrate is a composite of metal &amp; the substrate itself. Although it may function as a membrane, its thickness may be huge and it may not be separable from the substrate. 
         [0069]    Both types of membranes can be prepared by the system and process of the present invention. 
       EXAMPLES The following examples are given by way of illustration and therefore should not construe to limit the scope of the present invention. 
     Example 1 (Copper Electroless Plating) 
       [0070]    Substrate used: Ceramic disk 
         [0071]    Copper plating bath composition: 
         [0000]    
       
         
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Components 
                 Amount (100 ml solution in water) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 CuSO 4   
                 6.225  
                 g/l CuSO 4 •5H 2 O 
               
               
                 Na 2 EDTA 
                 20.0988  
                 g/l 
               
               
                 NaOH 
                 20  
                 g/l 
               
               
                 Reducing solution: 
                   
                   
               
               
                 Formaldehyde (37%) 
                 14.039  
                 ml/l (100 ml solution in water) 
               
               
                   
               
             
          
         
       
     
         [0072]    Steps for Cu membrane preparation using electroless plating:
       1. Cleaning of substrate   2. Activation of substrate using FeCl 2  &amp; CuCl 2  solution.   3. Metal film deposition.       
 
         [0076]    Step 1: Cleaning of Support/Substrate
       a) Place the support in the Ultrasonicator to remove the dirt particle for 30 min with detergent, acetic acid, hot water, and 2-propanol each.   b) Dry the support in furnace for constant temperature of 200° C.       
 
         [0079]    Step 2: Activation of Support/Substrate (One Side Only)
       a) Immerse the support side in FeCl 2  solution for 5 min.   b) Then immerse the same support side in CuCl 2  solution for 5 min.   c) Then immerse the support in 0.1 N HCl solution for 2 min.   d) Then immersed the support in DI water for 3 min.   e) Repeat steps c to f for 8 to 10 times.       
 
         [0085]    Observations 
         [0086]    The support becomes reddish brown in colour. 
         [0087]    Step: 3 Copper Electroless Plating
       a) Plating solution and reducing solution were prepared as per Table 1 separately.   b) The prepared plating solution was made up to 100 ml using DI (deionized water) water.   c) The substrate was placed in the apparatus with the plating side on the plating solution. Both solutions were placed in respective chambers. A small continuous pressure was applied on the formaldehyde solution side.   d) The plating solution was maintained at a constant temperature of 65° C.   e) After plating, the support was rinsed in the DI water several times &amp; dried at 80° C. in air.   f) Finally the membrane was annealed in N2 at 350° C. to prepare composite membrane.   g) The membrane was tested for leaks in air (4 bar) at room temperature (27° C.) and no air leakage was found.       
 
         [0095]    This confirmed that the substrate was coated almost 100% with the metal. 
       Example 2 (Palladium Electroless plating) 
       [0096]    Components used for Pd electroless plating are reported in table given below. Electroless plating solutions are widely reported in literature. In the present invention, same plating solution is used but use the reducing agent from this solution for plating has done separately. 
         [0097]    Substrate used: Sintered SS plate (Sintered stainless steel plate) 
         [0098]    Plating solution composition: 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Component 
                 Composition 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Pd(NH 3 ) 4 Cl 2   
                 4  
                 g/l 
               
               
                   
                 EDTA 2 Na 
                 67.2 
                 g/l 
               
               
                   
                 NH 3 _H 2 O (28%) 
                 350  
                 ml/l 
               
               
                   
                 Reducing Solution: 
                   
                   
               
               
                   
                 N 2 H 4 H 2 O (0.5 mol/l) 
                 10 
                 ml/l 
               
             
          
           
               
                   
                 pH 
                 11.2 
               
               
                   
                 Temperature 
                 60° C. 
               
               
                   
                   
               
             
          
         
       
     
         [0099]    Table 2 was used for Pd membrane preparation (similar to the copper membrane). Procedure was same. The electroless plating solution was replaced by the Pd electroless plating solution of table 2. 
       Example 3 (Paladium-silver Electroless Plating) 
       [0100]    Components used for Pd electroless plating are reported in table given below. 
         [0101]    Substrate used: Ceramic disk 
         [0102]    Plating solution composition: 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Concentrations 
                 Pd-bath 
                 Ag-bath 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 PdCl 2   (g/l) 
                 05 
                 0.5 
               
               
                   
                 AgNO 3  (g/l) 
                 — 
                 5 
               
               
                   
                 Na 2 EDTA•2H 2 O (g/l) 
                 70 
                 35 
               
               
                   
                 NH 4 OH (28%) (ml/l) 
                 500 
                 500 
               
               
                   
                 Reducing Solution: 
                   
                   
               
               
                   
                 N 2 H 4 •H 2 O (ml/l) 
                 10 
                 5 
               
               
                   
                 pH value 
                 10 
                 10 
               
               
                   
                 Temperature (° C.) 
                 60 
                 50 
               
               
                   
                   
               
             
          
         
       
     
         [0103]    Table 3 was used for Pd—Ag membrane preparation (similar to the copper membrane). Procedure was same. The electroless plating solution was replaced by the Pd—Ag electroless plating solution of table 3. 
       ADVANTAGES OF THE INVENTION 
       [0000]    
       
         
           
             1. The process provides for 100% plating 
             2. The process can be conducted unmanned in automatic mode 
             3. The process prevents of the wastage of metals, especially of precious metals.