Title of Invention: COMPOSITION FOR CONDUCTIVE PASTE

CONTAINING NANOMETER-THICK METAL MICROPLATES

WITH SURFACE-MODIFYING METAL NANO PARTICLES

Technical Field

[1] The present invention relates to a conductive paste, and in particular, to a composition for a conductive paste containing nanometer-thick plate-shaped silver micro particles or silver-coated micro particles.

[2]

[3] <Cross-reference to related application>

[4] This application claims priority to Korean Patent Application No. 10-2009-0026577 filed in Republic of Korea on March 27, 2009, the entire contents of which are incorporated herein by reference. Background Art

[5] Information communication devices such as liquid crystal displays move toward miniaturization and high performance, and attempts have been steadily made to incorporate these devices on flexible material supports. A circuit wire of said devices is generally formed by forming a film by vapor deposition such as chemical vapor deposition (CVD), sputtering and so on, and etching out an unnecessary portion by photolithography and so on.

[6] However, the conventional method for forming a circuit wire has disadvantages caused by repetition of film formation and etching, for example a low usage efficiency of raw materials, generation of a large amount of waste, a long manufacturing time and a considerable facility cost. And, the conventional method encounters with many problems in forming a fine circuit wire required for miniaturization of said devices.

[7] To solve the problems, recently the related industries pay attention to an ink-jet printing, gravure printing and screen printing techniques that allow a low loss of raw materials, non-use of hazardous components such as lead or the like, and a simple process for forming a circuit wire. To form a circuit wire by these techniques, it needs development of a high-performance conductive paste or ink.

[8] A conductive ink suitable for forming a circuit wire should have a high conductivity corresponding to a low specific resistance of IxIO ⁵ Ω-cm or less. Conventionally, a conductive ink was suggested to include a large amount of silver particles, for example, 50 to 80% relative to the total weight of the ink, so as to accomplish a continuous metallization. To form a continuous conductive network with solely silver particles, 75 weight% or more of silver should be used to reach the level of pure metallic silver (d=10.5 g/cm ³ ). The use of a large amount of silver produces very disadvantageous results in aspects of cost and storage stability.

[9] And, when printing the conductive paste on a flexible circuit board, one of the important things is a sufficiently low sintering temperature because plastics or the like have a low glass transition temperature (T _g ). The smaller metal particles have higher surface energy, and accordingly, the sintering temperature tends to be even lower than an intrinsic melting point of a metal. Such correlation between a sintering temperature and a metal particle size is illustrated in FIG. 1. The graph of FIG. 1 shows the relationship between a silver particle size and a lowest sintering temperature for a continuous metallization. Generally, as a metal particle size becomes smaller, the surface energy increases exponentially. Thus, when sintering metal particles, surface diffusion occurs to smaller metal particles with less energy (at lower sintering temperature) than it does to larger metal particles. As a result, it makes a continuous metallization easier.

[10] However, if it increases the content of silver particles so as to ensure a high conductivity and it reduces the silver particle size to a nanometer level so as to lower the sintering temperature as the prior art did so, agglomeration of silver particles is accelerated. Thus, to obtain storage stability of a paste or ink, it is inevitable to use an additive such as a dispersant, a stabilizer and so on. However, these additives raise the sintering temperature again, which was lowered once due to particle size reduction of silver.

[11] As mentioned above, no prior art has suggested any promising solution to achieve a low content of silver, a low specific resistance of IxIO ⁵ Ω-cm or less and a low sintering temperature of 150 ⁰ C or lower. Disclosure of Invention Technical Problem

[12] It is an object of the present invention to develop a conductive paste which exhibits a high electrical conductivity by forming a continuous conductive network of metallization without the need for a high content of silver nano particles and which can be sintered at a low temperature. Solution to Problem

[13] The present invention provides a composition for a conductive paste that can accomplish a continuous metallization of silver without the need for a high content of silver nano particles, and be sintered below temperature between 150 to 300 ⁰ C.

[14] According to an aspect of the present invention, a composition for a conductive paste is provided, comprising metal nano particle-modified silver microplates or silver- coated microplates as a conductive material. The composition comprises 1 to 8 weight% of a binder, 4 to 35 weight% of a solvent, and 60 to 95 weight% of metal nano particle-modified silver microplates or metal nano particle-modified silver-coated microplates. At this time, the metal nano particle-modified silver microplates or silver- coated microplates are plate-shaped silver particles or silver-coated particles with a thickness between 50 to 200 nm on average, which have a micrometer- sized plane approximately perpendicular to a thickness direction, and of which surface is modified with metal nano particles having an average particle size between 1 to 500 nm. In an embodiment of the present invention, the silver-coated microplates may have a core of plate-shaped copper particles, a silver coating layer on the core, and metal nano particles modifying the surface of the silver coating layer. Preferably, the metal nano particles for surface modification may be selected from the group consisting of silver, tin, palladium, platinum and gold.

[15] According to another aspect of the present invention, a composition for a conductive paste is provided, comprising 40 to 70 weight% of metal nano particle-modified silver microplates or silver-coated microplates, and 3 to 20 weight% of metal nano particles.

[16] According to yet another aspect of the present invention, a composition for a conductive paste is provided, comprising 40 to 60 weight% of metal nano particle- modified silver microplates or silver-coated microplates, 1 to 10 weight% of metal nano particles, and 0.01 to 2 weight% of carbon nanotubes.

[17] The metal nano particles of the content between 3 to 20 weight% or 1 to 10 weight% have an average particle size between 1 to 100 nm, and are at least one selected from the group consisting of silver, copper, tin, gold, platinum, palladium and aluminum. In an embodiment of the present invention, the carbon nanotubes may be surface-coated with metal nano particles.

[18] In an embodiment of the present invention, the composition for a conductive paste may further comprise 5 to 20 weight% of at least one conductivity improver selected from the group consisting of carbon nanotubes, silver nano particle-coated carbon nanotubes, flake-shaped copper particles and flake-shaped silver particles so as to improve conductivity.

[19] The present invention also provides a conductive circuit board having a circuit wire formed thereon using the conductive paste.

Advantageous Effects of Invention

[20] The conductive paste of the present invention can be sintered at temperature between

150 to 300 ⁰ C, and consequently, can be printed on circuit boards of various materials. And, the conductive paste has a low specific resistance below IxIO ⁵ Ω-cm. Furthermore, the conductive paste can dramatically reduce a usage amount of silver while achieving such level of conductivity and low temperature sintering, resulting in excellent economical efficiency. In addition to these effects, the conductive paste can prevent agglomeration of silver nano particles, and thus it is good to store.

[21] The conductive paste of the present invention also has excellent rheological properties, and thus, can be widely used to form a circuit wire by printing techniques, in particular, by screen printing. For example, the conductive paste of the present invention can be used to form circuit wires for a printed circuit board and a display device such as a liquid crystal display, a plasma display panel, an organic light- emitting diode and so on, to form an antenna for a radio-frequency identification (RFID) system, to produce an electrode and a reflective film for a solar cell, to form an electrode circuit wire for a semiconductor chip instead of a gold circuit wire, and so on. Brief Description of Drawings

[22] FIG. 1 is a graph illustrating the correlation between a silver particle size and a possible lowest sintering temperature.

[23] FIG. 2 is a view illustrating metal nano particle-modified silver microplates (A) and metal nano particle-modified silver-coated microplates (B) in cross section.

[24] FIG. 3 is a view illustrating silver-coated carbon nanotubes surface-coated with metal nano particles, particularly, silver nano particles.

[25] FIG. 4 is a view of a process for producing silver-coated carbon nanotubes from silver ions and carbon nanotubes.

[26] FIG. 5 is a SEM (Scanning Electron Microscope) image of silver nano particle- modified silver microplates obtained according to a manufacturing example of the present invention.

[27] FIG. 6 is an enlarged SEM image of silver nano particles for surface modification in the silver nano particle-modified silver microplates of FIG. 5.

[28] FIG. 7 is a graph illustrating differential scanning calorimetry analysis results to find out a sintering temperature range of silver nano particles obtained according to a manufacturing example of the present invention.

[29] FIG. 8 is a SEM image illustrating the surface of a circuit wire manufactured by printing a paste obtained according to example 2 of the present invention on a circuit board and sintering at 250 ⁰ C for 10 minutes.

[30] FIG. 9 is a photograph illustrating a chip bonding portion with a circuit wire for an

RFID antenna manufactured using a paste obtained according to example 3 of the present invention.

[31] FIG. 10 is a SEM image illustrating the surface and cross section of a circuit wire manufactured by printing and sintering a paste obtained according to comparative example 1. Best Mode for Carrying out the Invention

[32] Hereinafter, the present invention will be described in detail. The present invention relates to a composition for a conductive paste containing nanometer-thick plate- shaped silver particles or silver-coated particles surface-modified with metal nano particles wherein the plate has a micrometer size. In the present invention, the silver- coated particle means that the surface of plate-shaped copper particles is coated with silver to improve conductivity.

[33] According to an aspect of the present invention, a composition for a conductive paste is provided, comprising metal nano particle-modified silver microplates as a conductive material. The composition comprises 60 to 95 weight% of metal nano particle-modified silver microplates or silver-coated microplates, 1 to 8 weight% of a binder, and 4 to 35 weight% of a solvent.

[34] In the present invention, the metal nano particle-modified silver microplates are plate-shaped silver micro particles surface-modified with metal nano particles, having a size of several micrometers and a thickness of 200 nm or less. The silver microplates are plate-shaped metal particles having a micrometer-sized plane approximately perpendicular to the direction of the thickness of 200 nm or less as mentioned above. The expression "a plane approximately perpendicular" is used to describe that the silver microplates have a shape of a plate, not exactly a shape of a regular hexahedron, but the micrometer- sized plane has a polygon shape including a triangle, a square, a rectangle, an octagon and so on. It will not be difficult for an ordinary person skilled in the art to understand the shape of the microplates from such expression. In the present invention, the silver microplates help to accomplish a high electrical conductivity with a minimum content of silver nano particles, support low temperature sintering, and improve storage stability and attain a cost reduction due to micrometer size. The use of micrometer- size plate-shaped metal particles leads to better sintering at low temperature than readily available micrometer-size metal particles of other shapes.

[35] The effects and features of the silver microplates of the present invention will be apparent more clearly by comparison with conventional silver micro particles. The conventional micrometer- size metal particles may be flake-shaped particles that can be easily obtained using an attrition mill. If the flake-shaped particles are used together with conductive metal nano particles, it results in reduced content of metal nano particles and a relatively good conductive network, thereby attaining cost reduction. Disadvantageously, however, the micrometer-size flake-shaped metal particles do not support low temperature sintering. For example, metallic silver has a melting point of about 960 ⁰ C, while the conventional micrometer- size flake-shaped silver particles have a sintering temperature between 550 to 900 ⁰ C. For this reason, a sintering tern- perature reduction effect by use of flake-shaped silver particles is almost little, and if any, it is not too great. Thus, it is not suitable for plastic or glass substrates.

[36] However, if the silver microplates of the present invention are used singularly or in combination with silver nano particles at a designated ratio in the appended claims, it allows rapid sintering around 150 ⁰ C, a remarkably reduced usage amount of silver nano particles, and a high conductivity.

[37] Although it is not intended to be tied to a specific theory, because the plate-shaped silver micro particles have a thickness of 200 nm or less, it could be explained that the plate-shaped silver micro particles enable better sintering at low temperature than other metal particles such as micrometer- size flake-shaped particles and so on. And, the plate-shaped silver micro particles have a higher packing ratio than other particles due to a plate-shaped polygon. Accordingly, the conductive paste of the present invention using such silver microplates ensures a sufficient electrical conductivity while forming a circuit wire of 1 to 2 μm thickness. If typical micrometer- size flake-shaped particles are used to form a circuit wire, the circuit wire has a thickness between 4 to 8 μm. In this sense, it will be obvious that the conductive paste of the present invention can reduce the manufacturing costs.

[38] In the metal nano particle-modifed silver microplates or silver-coated microplates of the present invention, a silver (silver-coated) microplate portion, exclusive of a nano particle portion, has preferably a thickness of 200 nm, more preferably 50 to 200 nm, for the purpose of a lower sintering temperature and a thinner circuit wire. If the thickness exceeds 200 nm, it may cost less to produce plate-shaped silver microplates themselves, but it is not preferable because a sintering temperature of a paste is raised, a packing ratio of the paste is lowered, and consequently thickness of a circuit wire is increased. On the contrary, if the thickness is below 50 nm, it is not preferable because it is difficult to uniformly produce silver micro particles and the silver micro particles may be destroyed unintentionally during agitation.

[39] The metal nano particle-modified silver microplates of the present invention depict that the surface of the silver microplates is modified with metal nano particles. Preferably, the metal nano particles for surface modification have an average particle size between 1 to 500 nm. The metal nano particles may include, but not limited to, nano particles of metals capable of supporting a target electrical conductivity of the present invention, for example, silver, tin, palladium, platinum and gold. Since silver nano particles for surface modification can be melted at low temperature and establish a continuous connection of interfaces between micro particles, the metal nano particle- modified silver microplates have higher electrical conductivity than silver microplates without surface modification, in particular, conductivity in the direction of a Z-axis rather than conductivity in a plane, i.e., an X-Y surface. [40] A method for surface-modifying plate-shaped silver micro particles with metal nano particles is well known in the art, and its detailed description is omitted herein. To put it briefly, for example, metal nano particles, such as silver nano particles and so on, are deposited on silver microplates as host particles by sputtering while the silver mi- croplates are agitated in a vacuum bath. In this conventional manufacturing method, the sputtered metal nano particles generally form a partial alloy with silver of silver microplates or a silver coating layer of silver-coated microplates on the surface of the silver or silver coating layer. In the manufacture, the content of the metal nano particles is preferably 0.02 to 2 weight% of the plate-shaped silver micro particles. If the content of the metal nano particles is less than 0.02 weight%, it is not preferable because the metal nano particles cannot effectively make a connection of interfaces between the plate-shaped silver micro particles. If the content of the metal nano particles exceeds 2 weight%, it is not preferable because the silver nano particles agglomerate to increase a particle size, resulting in deteriorated sintering characteristics at low temperature.

[41] Meanwhile, the silver microplates of the present invention may be made of pure silver. Alternatively, the silver microplates may be made by coating particles of a high- conductivity and low-cost material such as copper with silver, which is hereinafter referred to as silver-coated microplates. In the context of the present specification, micrometer-size plate-shaped silver-coated particles, shortened as silver-coated particles may be named instead of silver-coated microplates. The silver-coated microplates may be produced with the same shape and size as silver microplates by coating the surface of micrometer-sized copper particles with silver, and replace silver microplates. The metal nano particle-modified silver-coated microplates may be obtained from the silver-coated microplates in the same manner as the silver microplates. FIG. 2 is a view illustrating comparison of metal nano particle-modified silver microplates and metal nano particle-modified silver coated microplates. FIG. 2(B) shows metal nano particle- modified silver-coated microplates produced by coating plate-shaped copper micro particles with silver to form a silver coating layer and modifying the silver coating layer with metal nano particles.

[42] In the present invention, the silver coating layer of the silver-coated microplates may have any thickness to ensure a target electrical conductivity of a paste and enable a reduction in manufacturing cost of pure silver microplates. For example, preferably plate-shaped copper micro particles or a copper core equivalent thereto may occupy 50 to 95% of the weight of silver-coated microplates without surface modification, and the silver coating layer may occupy 5 to 50%. Electrolysis may be used to form the silver coating layer. In this case, if silver is educed on the surface of copper micro particles by electrolysis, the resulting silver coating layer may have a thickness of 20 nm or more. This thickness range allows sufficient performance of a paste.

[43] In the present invention, a plate of silver microplates or silver-coated microplates approximately perpendicular to the thickness direction may have horizontal and vertical sides of about 1 to 20 μm size, wherein the horizontal and vertical sides form a plane. Preferably, the horizontal and vertical sides may have 2 to 7 μm size on average. When the size of the plate is in this range, it is preferable because only a small amount of a polymer binder is required to wet the particles, which is favorable to electrical conductivity, and dispersion is improved. If the size of the horizontal side or vertical side of the plate is less than 1 μm, it is not preferable because the tap density decreases and consequently a large amount of a binder is needed, thereby deteriorating the electrical conductivity. If the size of the horizontal side or vertical side of the plate exceeds 20 μm, it is not preferable because voids between particles increase and consequently a large amount of particles are needed and resolution of an electrode circuit wire is deteriorated.

[44] In the composition for a conductive paste, the content of the metal nano particle- modified silver microplates or silver-coated microplates may be preferably 60 to 95 weight% per the total weight of the composition. If the content is less than 60 weight%, it is not preferable because a desired level of electrical conductivity is not achieved. If the content exceeds 95 weight%, it is not preferable because viscosity of a paste excessively increases and printing performance decreases.

[45] According to another aspect of the present invention, a composition for a conductive paste is provided, comprising at least two components of metal nano particle-modified silver (or silver-coated) microplates and metal nano particles as a conductive material. The composition comprises 3 to 20 weight% of metal nano particles having an average particle size between 1 to 100 nm, and 40 to 70 weight% of metal nano particle- modified silver microplates or metal nano particle-modified silver-coated microplates. At this time, the metal nano particles having an average particle size between 1 to 100 nm may be nano particles of at least one metal selected from the group consisting of silver, copper, tin, gold, platinum, palladium and aluminum. The metal nano particle- modified silver microplates or silver-coated microplates used in this composition may be those of the previous embodiment.

[46] When compared with the composition for a conductive paste comprising solely the metal nano particle-modified silver microplates as a conductive material, this composition further comprising metal nano particles ensures a relatively lower specific resistance and can be sintered at low temperature more rapidly.

[47] The metal nano particles (not used to modify the surface of the microplates) may have various shapes including, without limitation, sphere, flake and so on. Preferably, the metal nano particles may have an average particle size between 1 to 100 nm. If the average particle size of the metal nano particles is less than 1 nm, it may result in a very low viscosity of a resulting paste, which makes it difficult to form a circuit wire of a predetermined thickness or above. If the average particle size of the metal nano particles exceeds 100 nm, it is not preferable because it is difficult to have an advantageous effect attained at a nanometer level such as low temperature sintering. However, it does not necessarily need silver nano particles having an average particle size of 20 nm or less so as to ensure a high conductivity and a low sintering temperature as the prior art did so. As will be described below, addition of metal mi- croplates and/or carbon nanotubes allows both a low sintering temperature and a high electrical conductivity. The larger nano particles become, the better effects appear in aspects of cost and storage stability. Thus, it is possible to use metal nano particles having an average diameter more than 20 nm. Meanwhile, it is preferable to use silver nano particles having an average particle size between 5 to 40 nm as the metal nano particles in terms of a synthesis yield, workability and formation of a conductive network. In certain embodiments of the present invention, mixed nano particles of silver nano particles and one or more other metal may be used as the metal nano particles without sacrificing conductivity and low temperature sintering.

[48] The metal nano particles used in the conductive paste of the present invention may be used without coating or surface modification, or may be surface-modified for hy- drophilic or hydrophobic property or surface-coated with a coating material such as a protective colloid forming material or the like. In particular, in the case of silver nano particles are used as the metal nano particles, if tertiary fatty acid is used as a dispersant, it effectively prevents agglomeration of silver nano particles.

[49] Preferably, the composition for a conductive paste free of carbon nanotubes, comprising only silver microplates and metal nano particles as a conductive material, may comprise 3 to 20 weight% of metal nano particles per the total weight of the composition. This content range ensures a high electrical conductivity with a lower content of silver nano particles than a conventional content. If the content of the silver nano particles is less than 3 weight%, it results in a poor electrical contact between silver particles and consequently a large resistance of a resulting paste. If the content of the silver nano particles exceeds 20 weight%, it is not preferable because costs rise but a conductivity improvement effect for the increased costs is insignificant.

[50] In yet another aspect of the present invention, a composition for a conductive paste is provided, comprising carbon nanotubes (CNT) together with the metal nano particles and the metal nano particle-modified silver microplates (or silver-coated microplates). The addition of the carbon nanotubes can enhance formation of a conductive network in the composition and enable a further reduction in the content of the silver nano particles. The composition comprises 3 to 20 weight% of metal nano particles having an average particle size between 1 to 100 nm, 40 to 60 weight% of metal nano particle- modified silver microplates (or silver-coated microplates), and 0.01 to 2 weight% of carbon nanotubes.

[51] In the present invention, the carbon nanotubes (CNT) are interposed between silver particles to establish an electrical connection between the silver particles, or are attached to the surface of the silver particles to substantially increase the surface area of the silver particles, thereby facilitating formation of a conductive network. Accordingly, the use of carbon nanotubes enables a reduction in silver content required to attain the same level of conductivity.

[52] Meanwhile, the carbon nanotubes advantageously improve adhesion between a circuit board material and a paste, and easily control the viscosity of the paste to a suitable level for printing. Typical carbon nanotubes have some extent of surface defects in a graphene sheet. Thus, a functional group, such as a carboxyl group and so on, juts out from the surface of the carbon nanotubes in the manufacture. Although it is not intended to be tied to a specific theory, because the carbon nanotubes have such a surface functional group, it can enhance adhesion of the conductive paste to the surface of the circuit board.

[53] The carbon nanotubes used in the composition for a conductive paste according to the present invention may include single- walled, double-walled and multi-walled carbon nanotubes, and may be surface-coated with various functional groups. Preferably, the carbon nanotubes used in the composition of the present invention has a diameter between 2 to 40 nm and a length between several micrometers to tens of micrometers.

[54] The carbon nanotubes used in the composition of the present invention may be metal-coated carbon nanotubes of which surface is coated with metal nano particles having an average particle size between 1 to 100 nm. Carbon nanotubes not coated with metal particles are a good conductivity improving material, but have a very high aspect ratio of 10,000 or more. Thus, it may cause entanglement in the composition for a conductive paste as if a skein of thread is entangled. If such entanglement occurs, it is difficult to form a continuous conductive network and the carbon nanotubes are not uniformly dispersed in a paste. As a result, an uneven film is formed or leveling is deteriorated after printing, so that the reliability of end-products is likely to go down.

[55] The use of the metal-coated carbon nanotubes can minimize these problems caused by use of pure carbon nanotubes and improve conductivity. Advantageously, the metal-coated carbon nanotubes also lower a high aspect ratio of non-coated carbon nanotubes and prevent entanglement of nanotubes. Accordingly, a separate process for dispersing carbon nanotubes may be omitted. The metal-coated carbon nanotubes have a proper dispersion and consequently the improved rheological properties such as repetitive printing ability, leveling, storage stability and so on. And, because metal nano particles exist on the surface of carbon nanotubes, the surface area of a conductor increases. Thus, the metal-coated carbon nanotubes have a higher conductivity than non-coated carbon nanotubes. Such metal-coated carbon nanotubes are shown in FIG. 3.

[56] In the metal-coated carbon nanotubes of the present invention, a metal for surface coating of carbon nanotubes may include nano particles of silver, tin, platinum, gold and palladium. These metal nano particles for surface coating may be in a reduced state or an ion state in a complex. If the metal nano particles for surface coating exist as ions in a complex, a dispersant or a solvent may serve as a reducing agent during sintering or a separate reducing agent may be added to a paste. After sintering, the ions may be reduced to a metal having an oxidation number of zero (0) and may function as a circuit wire.

[57] The use of such carbon nanotubes enables a dramatic reduction in the content of silver nano particles required to achieve a specific resistance below IxIO ⁵ Ω-cm. In the composition for a conductive paste according to the present invention comprising (non-coated or metal-coated) carbon nanotubes, the content of the carbon nanotubes may be preferably 0.01 to 2 weight% of carbon nanotubes per the total weight of the paste. This content range ensures high conductivity, low sintering temperature, and suitable mechanical and rheological properties for screen printing. If the content of the carbon nanotubes is less than 0.01 weight%, it results in poor electrical contact of silver particles and consequently a high resistance of the paste. If the content of the carbon nanotubes exceeds 2 weight%, it results in cost rise and dispersion reduction, leading to an insignificant conductivity improvement effect. In this case, the sintering temperature is increased since a large amount of a polymer binder should be added.

[58] The metal-coated carbon nanotubes used in the conductive paste of the present invention may be obtained by various methods. For example, there is a method for coating, with silver, carbon nanotubes produced by thermal chemical vapor deposition, laser ablation, arc discharge and so on. The method for coating carbon nanotubes with a metal such as silver is well known in the art, and its description is made in brief. And, there may exemplary methods, for example,

[59] 1) coating the surface of carbon nanotubes with a metal, such as silver or the like, by chemical reduction (NaBH ₄ treatment or reduction in hydrogen atmosphere) or thermal reduction.

[60] 2) adding carbon nanotubes while reducing a metal ion complex into metal particles.

[61] 3) forming a metal ion complex intermediate on the surface of carbon nanotubes by attaching a functional group to the surface of the carbon nanotubes, and reducing the resulting product. [62] As the third method, a process for attaching silver nano particles to carbon nanotubes is illustrated in FIG. 4.

[63] The conductive paste of the present invention may further comprise micrometer- size flake-shaped copper particles and/or silver particles as additional conductive materials. Preferably, the content of these components may be 5 to 20 weight% per the total weight of the composition for a conductive paste.

[64] In addition to the above conductive materials, the conductive paste of the present invention may further comprise a binder and a solvent. Selectively, the conductive paste of the present invention also may further comprise an additive. For example, the binder may be nitrocellulose, an acryl-based resin, a vinyl-based resin, ethyl cellulose and modified resins thereof. The solvent and the additive may be properly selected from all typical solvents and additives depending on the desired end-use properties with reference to the prior art by an ordinary person skilled in the art, and their description is omitted herein. For example, the solvent may be a nonpolar hydrocarbon solvent such as tetradecan, an ester-based solvent such as butyl carbitol acetate, butyl acetate and so on, a ketone-based solvent, a glycol-based solvent, and an alcohol-based solvent such as alpha-terpineol, butyl alcohol and so on. Preferably, the additive may be at least one selected from the group consisting of a stabilizer, a dispersant, a reducing agent, a surfactant, a wetting agent, a thixotropic agent, a leveling agent, an antifoaming agent, a coupling agent, a surface tension adjusting agent and a thickener, and the content of the additive is preferably 0.1 to 10 weight%.

[65] The composition for a conductive paste comprising the silver nano particles and the metal microplates according to the present invention may be used to manufacture a conductive ink. A method for manufacturing the conductive ink is well known in the art, and its description is omitted herein. The conductive ink according to the present invention has a low specific resistance of 10 ⁶ Ω-cm level and a low sintering temperature between 150 to 250 ⁰ C, and thus, achieves both a high conductivity and sintering at low temperature.

[66] According to still another aspect, the present invention provides a conductive circuit board having a circuit wire formed using the conductive paste. An example of a method for fabricating a conductive circuit board is described in brief as follows. A circuit wire is formed by printing the conductive paste on a circuit board made of metal, glass, plastic and so on, by gravure printing, flexo printing, offset printing, screen printing and so on. At this time, the circuit wire is formed on a base film positioned on the surface of the circuit board. The base film on the circuit board may have a circuit pattern scanned thereon in advance by photolithography or screen printing. The paste is sprayed in conformity with the scanned circuit pattern to form a film including conductive filler. Subsequently, the circuit board having the paste printed thereon is sintered to remove a solvent and so on, and to fuse silver particles. Subsequently, if necessary, a multilayered circuit board may be fabricated through subsequent processes including stacking, thin-film forming, plating and so on. Mode for the Invention

[67] Hereinafter, the present invention will be described in detail through examples and manufacturing examples. The description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention, so it should be understood that other equivalents and modifications could be made thereto without departing from the spirit and scope of the invention.

[68] For the purpose of performance comparison between conductive pastes of the present invention with conventional pastes, compositions for pastes were prepared according to examples and comparative examples. The pastes of examples according to the present invention utilized, as metal nano particle-modified silver microplates and metal nano particles, respectively, silver nano particle-modified silver microplates and silver nano particles according to a manufacturing example as described below. And, the pastes of examples according to the present invention utilized an acrylic resin as a binder. The formulation for the pastes according to examples and comparative examples is shown in the following Table 1.

[69] Table 1 [Table 1] [Table ]

[70] * Described briefly are a manufacturing example of silver nano particle-modified silver microplates and methods for preparing compositions for pastes according to examples and comparative examples. [71] Manufacturing example of silver nano particle-modified silver microplates

[72] Silver microplates, as host particles, having an average thickness of 0.2 μm and a size

(horizontal and vertical sides) of 4 μm were put into a vacuum sputterer system having an impeller mounted at the bottom. While the silver microplates were agitated, sputtered silver nano particles were deposited on the silver microplates to produce silver nano particle-modified silver microplates. In the obtained silver nano particle- modified silver microplates, silver nano particles had a size between 10 to 300 nm, and a ratio of silver nano particles for surface modification to non-modified silver microplates was 100:1 to 100:2.

[73] SEM (Scanning Electron Microscope) images of the silver nano particle-modified silver microplates are shown in FIGs. 5 and 6. FIG. 5 is an SEM image of silver nano particle-modified silver microplates produced by modifying, with silver nano particles, the surface of silver particles having a polygon including a triangle, a pentagon and a hexagon. FIG. 6 is an enlarged SEM image of silver nano particles for surface modification, showing the size of the nano particles.

[74] Manufacturing example of silver nano particles

[75] Silver nano particles were manufactured by LS Cable, Ltd. as follows: Silver nitrate was reacted with tertiary fatty acid to form a silver complex. The silver complex was dissolved in a toluene solvent, and triethylamine as a reducing agent was added thereto. The resulting silver nano particles had a spherical shape and a particle size between 5 to 20 nm. According to UV- visible spectroscopy, a characteristic peak of the silver nano particles was observed at 427 nm. According to differential scanning calorimetry (DSC) at a temperature increase rate of 10 ⁰ C /min, it was found that the silver nano particles were sintered in the range between 80 to 140 ⁰ C (See FIG. 7).

[76] Example 1: Paste containing silver nano particle-modified silver microplates

[77] Silver nano particle-modified silver microplates, a binder and a solvent were prepared as described in the following A and B:

[78] A) a suspension solution in which 70 g of silver nano particle-modified silver microplates obtained in the manufacturing example are dispersed in 20 g of butyl carbitol acetate solvent

[79] B) as a binder, 10 g of a terpineol solution having 3g of acrylic resin dissolved therein (resin solid content: 30 weight%)

[80] The above A and B components were preliminarily mixed and agitated using a 3-roll mill till the components are uniformly dispersed. Next, a resultant paste was uniformly printed on a glass substrate at a size of 6x6 cm by screen printing using Sus 325 (350 meshes) to form an uniform film. The substrate was sintered at 150 ⁰ C for 60 minutes in a convection oven to manufacture a specimen. [81] Example 2: Paste containing silver nano particle-modified silver microplates

[82] A specimen was manufactured in the same way as example 1 except that sintering was made at 250 ⁰ C for 10 minutes. FIG. 8 is a SEM image illustrating the surface of a circuit wire formed by printing the paste obtained according to example 2 and sintering at 250 ⁰ C for 30 minutes. FIG. 8 shows continuous metallization is accomplished such that it is difficult to distinct individual silver microplates and individual silver nano particles.

[83] Example 3: Paste containing silver nano particles, silver microplates and silver- modified CNT

[84] Silver nano particles, silver microplates and a binder were prepared as described in the following A, B and C:

[85] A) 20 g of a tetradecan dispersant solution containing 35 weight% of spherical silver nano particles obtained in the manufacturing example

[86] B) a suspension solution in which 60 g of silver nano particle-modified silver microplates obtained in the manufacturing example are dispersed in 10 g of butyl carbitol acetate solvent

[87] C) as a binder, 10 g of a terpineol solution having an acrylic resin dissolved therein

(resin solid content: 30 weight%)

[88] The above A, B and C components were preliminarily mixed and agitated till the components are uniformly dispersed. Next, a resultant paste was uniformly printed on a glass substrate at a size of 6x6 cm by screen printing using Sus 325 (350 meshes) to form an uniform film. The substrate was sintered at 150 ⁰ C for 2 minutes in a convection oven to manufacture a specimen. FIG. 8 is a SEM image illustrating the surface of a circuit wire formed using the paste obtained according to example 1. FIG. 9 is a photograph illustrating a chip bonding portion with a circuit wire for an RFID antenna manufactured using the paste obtained according to example 3.

[89] Comparative example 1: Paste containing solely silver nano particles

[90] A specimen was manufactured in the same way as example 1 except that sintering was made at 150 ⁰ C for 10 minutes. FIG. 10 is an SEM image illustrating the surface and cross section of a circuit wire formed by printing and sintering the paste obtained according to example 1 at 150 ⁰ C for 10 minutes. In FIG. 10, it is found that continuous metallization is not accomplished under the sintering conditions of comparative example 1 such that it is possible to distinct individual silver microplates and individual silver nano particles.

[91] Comparative example 2: Paste containing solely flake-shaped silver micro particles

[92] A specimen was manufactured in the same way as example 2 except that the following components were preliminarily mixed: [93] A) 78 g of flake-shaped silver micro particles having an average particle size between 2 to 7 μm [94] B) as a binder, 1Og of acrylic resin dissolved in a terpineol solution (resin solid content: 30 weight%)

[95] C) 12 g of butyl carbitol acetate [96] The pastes according to examples and comparative examples were printed on PET circuit boards, and the resulting circuit wires were measured in thickness and a specific resistance. The specific resistance was measured using a 4-probe tester (LORESTA-GP of Mitsubishi Chemical in Japan) according to the ASTM D 991 specifications. First, a sheet resistance was metered, and a thickness of the printed film was measured. The sheet resistance was multiplied by the film thickness to obtain a specific resistance. The measurement results are shown in the following Table 2.

[97] Table 2 [Table 2] [Table ]

[98] The examples 1 to 3 have equal level of specific resistance, which are within the range of 10 ^~5 Ω-cm suitable for an RFID antenna. The examples 1 and 2 containing solely silver microplates as a conductive material have no silver nano particles, but they exhibit a high conductivity as shown in Table 2. Moreover, the example 1 supports sintering at low temperature of 150 ⁰ C. The effect of the silver nano particle- modified silver microplates according to the present invention becomes prominent when compared with comparative example 2. The comparative example 2 contains a larger amount of flake-shaped silver particles having the same micrometer size and was sintered at higher temperature than example 1. However, it is found that the comparative example 2 has specific resistance 56 times higher than example 1. Therefore, the metal nano particle-modified silver microplates such as silver nano particle- modified silver microplates according to the present invention enables a conductive paste to give better performance than a conventional paste containing silver particles of the same content and size.

[99] Meanwhile, it is found through example 3 that adding metal nano particles to the metal nano particle-modified silver microplates allows an even lower specific re- sistance and more rapid sintering at low temperature.

[100] The comparative example 1 having the same formula as example 1 achieves a relatively low specific resistance. It is found that a desired specific resistance of a circuit wire can be attained by controlling the sintering conditions including sintering time and sintering temperature through comparison of the comparative example 1 and the examples 1 and 2. The circuit wire of comparative example 1 is twice or more as thick as that of example 1, rather has much larger specific resistance. In this sense, it can be seen that optimization of sintering conditions is an important factor directly related to economical efficiency.

[101] Hereinabove, the examples of the present invention were described. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.