TECHNICAL FIELD OF THE INVENTION

Present invention uses a facile and cost effective methods to synthesize highly conductive silver nanoparticle based paste (Silver nanopaste ) which can be used for industrial production.
BACKGROUND OF THE INVENTION
Nanostructure silver paste is highly conducting adhesive material which is used for different applications such as solar panel, bio-sensors, textile industry and 3D printing for making the electronic contacts, antimicrobial spray and conducting thin films. The nano-dimensional size of the silver particles provides the high surface to volume ratio and other ingredients used to synthesize silver paste are also highly conductive, which enhance the conductivity of synthesized silver paste and makes it very significant and cost effective in compare to micro-particle based silver paste.
Synthesis of nano-silver paste is very important and process involved, plays an important role in its quality and efficiency as well cost of production. Previously reported techniques use high temperature curing material as binder, which includes extra step and enhances cost of the production. Also use of non-conducting binder, mixture reduce and bulk material reduces the conductivity of the final product. Previous methods also used carcinogenic materials such as PbO3, which are harmful for human health and environment.
For example, US 2015/0243812 A1 discloses a method for synthesis of silver nano-paste however the method used for nanoparticle synthesis was not adequate in the terms of particle size distribution. The silver powder used in paste consist the particles ranges from nano to micro-dimensions with average diameter between 10nm to 1000nm. The additional binder materials an adhesive medium comprises more than 50 % w/w of the paste which alter the electronic properties of the material and compromise the conductivity of the paste.
The literature “Silver nanopaste: Synthesis, reinforcements and application (International Journal of Heat and Mass Transfer; Volume 127, Part A, December 2018) compares the present technologies to develop cost effective silver nano-paste for electronic and biomedical application.

SUMMARY
The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the protective scope of the claims.
The present application provides a cost effective method for synthesis of the highly conductive nanoparticles based silver paste using new composition of precursor materials with a polymer based binder, which have ability to cure at room temperature. The conductivity and viscosity of the silver nanoparticle based paste can be tuned by the changing the concentration of silver metal and binding agent.
The present inventors have tested and screened various combinations and found that only PMMA as binding material is suitable. The binder materials (PMMA) used to make the slurry have room temperature curing point which eliminate the extra step of heating to get the desire viscosity. Also the particle size can be control easily without using any reducing and capping agents. Most of the silver paste developed through other processes available for use are made up of silver particle with micro-dimensional particle, which restrict their electrical properties. The nanoparticle based silver paste show high conductivity in compare to available silver pastes. The nano size of the particle provides high surface to volume ratio that affect the physical and chemical performance additionally can be use in less quantity to get the same results.
Based on the above reasons, the method for producing nano silver in the present application has a high application value.
Optionally, synthesis of silver nanoparticle based paste can be altered by changing the silver concentration, ratio of mixture, viscosity of the binder material. These factor can alter the physical and chemical properties of the paste. The nano-dimension precursor material were synthesized using ball milling which is a physical method to synthesize homogenous particle size distribution. This step can be substituted by chemical reaction route but include the addition of reducing and capping agents and lacks homogenous particle size distribution.
Compared with the related art, the present application has the advantages as follows.
(1) In the present application, nanoparticle based silver paste is produced that show high conductivity in compare to available silver pastes.
(2) In the present application, nano size of the particle is used that provides high surface to volume ratio that affect the physical and chemical performance additionally can be used in less quantity to get the same results.
(3) In the present application, the methods used to synthesize these nano-particles are accurate and cost effective. The particle size can be controlled easily without using any reducing and capping agents.
(4) In the present application, binder materials (PMMA) or Ethyl cellulose used to make the slurry have room temperature curing point which eliminate the extra step of heating to get the desire viscosity.
(5) In the present application, physical and chemical properties of the synthesize nanoparticle based silver paste can be easily tuned by composition of the precursor materials and binding agent.
After reading and understanding the detailed description, other aspects can be understood.
DETAILED DESCRIPTION OF THE EMBODIMENTS
In order to facilitate understanding of the present application, the present application lists two embodiments as follows. Those skilled in the art should understand that the embodiments are intended merely to help understand the present application and should not be considered as a specific limitation to the present application.
Embodiment 1: 80% silver metal powder and 8% mixture of glass frits, aluminum oxide powder, boric acid powder and silicon dioxide powder are ball milled individually for 12 hours at 300 rpm under ambient condition. Slurry of desired viscosity of respective mixture is obtained after ball milling. This slurry was filtered with the help of micron size filter paper to get a uniform agglomeration/pinholes free precipitate. The precipitates of both mixtures were green heated at 100 ?C in a vacuum oven to remove the excessive solvent and moisture content. Further, the obtained powders were mixed together and grinded with the help of mortar pestle for nanostructured synthesis. To bind the mixture, a conductive polymeric binder 12% PMMA [poly (methyl methacrylate)] crystal was used. To prepare this first PMMA [poly (methyl methacrylate)] crystals were dissolved in acetone to prepare a transparent homogeneous solution for easy synthesis or pasting. PMMA solution was added in the respective mixture drop wise with continuously stirring the mixture at room temperature manually with the help of cleaned glass rod after that have use the triple roll mill machine for homogenous mixture of the paste. Finally, a cost effective nanostructured silver paste with desired viscosity was obtained.
Embodiment 2: 80% silver metal powder and 18% mixture of glass frits, aluminum oxide powder, boric acid powder and silicon dioxide powder are ball milled individually for 12 hours at 300 rpm under ambient condition. Slurry of desired viscosity of respective mixture is obtained after ball milling. This slurry was filtered with the help of micron size filter paper to get a uniform agglomeration/pinholes free precipitate. The precipitates of both mixtures were green heated at 100 ?C in a vacuum oven to remove the excessive solvent and moisture content. Further, the obtained powders were mixed together and grinded with the help of mortar pestle for nanostructured synthesis. To bind the mixture, binder Ethyl Cellulose is used . Binder is prepared by prepared by dissolving 2% of cellulose in 60% toluene and 40% ethanol solution and leaving overnight for swelling. Ethyl Cellulose solution was added in the respective mixture drop wise with continuously stirring the mixture at room temperature manually with the help of cleaned glass rod after that have use the triple roll mill machine for homogenous mixture of the paste. Finally, a cost effective nanostructured silver paste with desired viscosity was obtained.
EXPERIMENT AND RESULT(S)
The nano-dimensionality and purity of the synthesized nano-powders were investigated by different optical and physical characterization techniques. To confirm the phase formation and purity to synthesize nanoparticles, X-Ray Diffraction (XRD) measurement has been performed and shown in figure 1. The XRD peak centered at diffraction angle at 2?= 38.140, 44.320, 64.500, 77.450 are assigned to the (111), (200), (220) and (311) reflections of the face centred cubic (FCC) structure of metallic silver respectively. The diffraction pattern is in well agreement with the literature report JCPDS File No. 04-0783. X-Ray Fluorescence analysis also complemented the XRD data. XRF reports shows the 97.667 % of silver metal with negligible presence of other metal ions.
TEM measurement has been done to get the particle size and morphology of the synthesized particles. TEM images were taken at different resolutions to get the best insight about the shape and size. Figure 2 (a) shows the homogenous particle size distribution of AgNPs at the resolution at 100 nm with the average diameter of 40 nm. Most of the particles were spherical in size. SEAD pattern also confirm the crystalline structure of the AgNPs and have showed doted bright rings corresponding to diffraction planes. EDX graph also confirm the purity of the material and showed the maximum intense peak for Ag, followed by the carbon and oxygen.
Applicant declares that in the present application, the above embodiments are used to describe the process flow of the present application, but the present application is not limited to the above-mentioned process flow. That is, it does not mean that the present application must rely on the above-mentioned specific process flow to be implemented.

We Claim:

1. A Nano Silver Paste composition comprising:
a) a nano silver metal powder mixture
b) a binder; and
c) a base medium

2. The composition as claimed in claim 1 wherein,
a nano silver metal powder comprising about 60-80% w/w silver metal powder

3. The composition as claimed in claim 1 wherein,
The binder is a conductive polymeric binder comprising about 6-12% w/w PMMA [poly (methyl methacrylate)] crystal or Ethyl cellulose comprising 2% of cellulose dissolved in 60% toluene and 40% ethanol.

4. The composition as claimed in claim 1 wherein,
The base medium is 8% mixture of glass frits, aluminum oxide powder, boric acid powder and silicon oxide powder.

5. A process for preparing a mixture of comprising about 60-80% w/w silver metal powder and mixture of glass frits, aluminum oxide powder, boric acid powder and silicon oxide powder comprising:

a) ball milling individual components for 12 hours at 300 rpm under ambient condition.
b) Filtering this slurry with the help of micron size filter paper to get a uniform agglomeration/pinholes free precipitate.
c) Heating the precipitates of both mixtures at 100 ?C in a vacuum oven to remove the excessive solvent and moisture content.
d) Mixing the obtained powders together and grinding with the help of mortar pestle for nanostructured synthesis.

6. A process for preparing a conductive polymeric binder as in Claim 3 comprising
a) Adding PMMA [poly (methyl methacrylate)] crystal in acetone
b) Dissolving the components to prepare a transparent homogeneous solution

7. A process for preparing a nanostructured silver paste of Claim 1 comprising
a) Adding PMMA solution or Ethyl cellulose in the respective mixture drop wise
b) continuously stirring the mixture at room temperature manually with the help of cleaned glass rod
c) mixing the mixture using the triple roll mill machine to obtain homogenous mixture of the paste.