WO2010101418A2

WO2010101418A2 - Composition for conductive paste containing nanometer-thick metal microplates

Info

Publication number: WO2010101418A2
Application number: PCT/KR2010/001339
Authority: WO
Inventors: Yoon-Jin Kim; Chang-Mo Ko; Ho-Souk Cho
Original assignee: Ls Cable Ltd.
Priority date: 2009-03-04
Filing date: 2010-03-03
Publication date: 2010-09-10
Also published as: KR20100099970A; WO2010101418A3; KR101207363B1

Abstract

Disclosed is a composition for a conductive paste containing micrometer- size plate-shaped metal particles having a thickness of 200 nm or less. The composition comprises silver nano particles having an average particle size between 1 nm to 100 nm; and plate-shaped metal particles. Preferably, the micrometer-size plate-shaped metal particles have an average thickness of 200 nm or less. Preferably, the micrometer- size plate-shaped metal particles have horizontal and vertical sides of 1 to 20 µm size on average. The composition may further comprise metal nano particles such as copper, palladium and so on. And, the composition may further comprise carbon nanotubes, and the surface of the carbon nanotubes may be coated with metal nano particles. The use of such composition can reduce a sintering temperature to a low temperature of 150 °C or lower and decrease the thickness of a circuit wire for achieving the same resistance.

Description

COMPOSITION FOR CONDUCTIVE PASTE CONTAINING NANOMETER-THICK METAL MICROPLATES

The present invention relates to a conductive paste, and in particular, to a composition for a conductive paste containing nanometer-thick plate-shaped metal particles and silver nano particles.

<Cross-reference to related application>

This application claims priority to Korean Patent Application No. 10-2009-0018582 filed in Republic of Korea on March 4, 2009, the entire contents of which are incorporated herein by reference.

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. The 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.

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.

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.

A conductive ink suitable for forming a circuit wire should have a high conductivity corresponding to a low specific resistance of 1×10^-5Ω·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 so as to reach the level of pure metallic silver (d=10.5 g/cm³). The use of a large amount of silver produces very disadvantageous results in aspects of cost and storage stability.

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 to larger metal particles. As a result, it makes a continuous metallization easier.

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 increase the sintering temperature again, which was lowered once due to particle size reduction of silver.

As mentioned above, no prior art has suggested any promising solution to achieve a low content of silver, a low specific resistance of 1×10^-5Ω·cm or less and a low sintering temperature of 150 ^OC or lower.

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

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 particles, and be sintered at temperature of 150 ^OC or lower.

According to an aspect of the present invention, the composition for a conductive paste comprises 3 to 20 weight% of silver nano particles having an average particle size between 1 nm to 100 nm, and 40 to 70 weight% of metal microplates having an average thickness of 200 nm or less, wherein the metal microplates are referred to as metal particles having a micrometer-sized plane perpendicular to a thickness direction.

According to another aspect of the present invention, a composition for a conductive paste comprises 1 to 10 weight% of the silver nano particles, 40 to 60 weight% of the metal microplate and 0.01 to 2 weight% of carbon nanotubes having an average diameter between 2 to 40 nm.

According to yet another aspect of the present invention, the carbon nanotubes are surface-coated with metal nano particles.

In the composition for a conductive paste according to the present invention, the horizontal and vertical sides of the metal microplates preferably have 1 to 20 μm size on average.

In one embodiment of the present invention, the composition for a conductive paste may further comprise metal nano particles selected from the group consisting of copper, tin, gold, platinum and palladium.

In addition to the above components, the composition for a conductive paste according to the present invention may further comprise a solvent or both a solvent and an additive.

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

The conductive paste of the present invention can be sintered at temperature of 150 ^OC or lower, and consequently, can be printed on circuit boards of various materials. And, the conductive paste allows a low specific resistance of 10^-5Ω·cm or less and a thinner circuit wire than the prior art. In addition to these effects, 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.

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. 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 gold, and so on.

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

FIG. 2 is a view illustrating conductivity of a conductive paste containing solely silver nano particles and a conductive paste containing silver nano particles and carbon nanotubes (CNT).

FIG. 3 is a view illustrating silver-coated carbon nanotubes in which the surface of carbon nanotubes is coated with silver nano particles.

FIG. 4 is a view of a process for producing silver-coated carbon nanotubes from silver ions and carbon nanotubes according to an embodiment of the present invention.

FIG. 5 is a TEM (Transmission Electron Microscope) image of silver nano particles obtained according to a manufacturing example.

FIG. 6 is an SEM (Scanning Electron Microscope) image of silver microplates having an average thickness of 200 nm and horizontal and vertical sides of 2 to 7 μm size on average, obtained according to a manufacturing example, and in particular, FIG. 6(a) is a top view and FIG. 6(b) is a side view.

FIG. 7 is an SEM image of silver-coated multi-walled carbon nanotubes synthesized by an exemplary manufacturing method of the present invention.

FIG. 8 is an SEM image illustrating the surface of an electrode circuit wire manufactured according to example 1 of the present invention.

FIG. 9 is a graph illustrating of differential scanning calorimetry analysis results to find out a sintering temperature range of silver nano particles obtained according to a manufacturing example.

FIG. 10 is an SEM image of a paste obtained according to comparative example 3.

Hereinafter, the present invention will be described in detail. The present invention relates to a composition for a conductive paste containing plate-shaped silver particles having a nanometer thickness and a micrometer size, and silver nano particles. The use of the composition for a conductive paste according to the present invention ensures a high conductivity and a low sintering temperature with less silver nano particles than the prior art.

According to an aspect of the present invention, a composition for a conductive paste comprises 3 to 20 weight% of silver nano particles and 40 to 70 weight% of metal microplates.

The silver nano particles of the present invention may have various shapes including, without limitation, sphere, flake and so on. Preferably, the silver nano particles have an average particle size between 1 nm to 100 nm. If the average particle size of the silver 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 silver 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 microplates 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 silver nano particles having an average diameter more than 20 nm. Meanwhile, it is preferable to use silver nano particles having a particle size between 5 to 40 nm in terms of a synthesis yield, workability and formation of a conductive network.

The silver nano particles used in the conductive paste of the present invention may be used without coating or surface modification, or may be surface-coated with a protective colloid forming material or the like.

In the present invention, the composition for a conductive paste free of carbon nanotubes preferably contains 3 to 20 weight% of silver nano particles relative to the total weight of the composition. This content range ensures a high electrical conductivity with a lower content of silver nano particles than the prior art. 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 costs is insignificant.

In an embodiment of the present invention, in addition to the silver nano particles, one or more other metal nano particles may be included in the composition without sacrificing conductivity and low temperature sintering. At this time, available metals may include copper, tin, gold, platinum and palladium, exhibiting good conductivity without deteriorating low temperature sintering. Preferably, these metal nano particles have an average particle size between 1 to 100 nm, and are included at an amount between 0.5 to 5 weight% in the composition containing silver nano particles and metal microplates. The addition of the metal nano particles enables a cost reduction and improvement in welding at particle interfaces as well as maintenance of high conductivity and low temperature sintering.

In the specification, the metal microplates are metal particles having a flat plate shape and a thickness of 200 nm or less, wherein the metal microplates have a micrometer-sized plane approximately perpendicular to a thickness direction. The expression "a plane approximately perpendicular" is used to describe that the metal microplates have a shape of a plate, not exactly a shape of a regular hexahedron. 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 metal microplates help accomplish a high electrical conductivity with a minimum content of silver nano particles and support low temperature sintering, and improve storage stability and attain a cost reduction due to micrometer size. The use of micrometer-size plate-shaped particles leads to better sintering at low temperature than readily available micrometer-size metal particles of other shapes. 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 metal flakes do not support low temperature sintering. For example, metallic silver has a melting point of about 960 ^OC, while micrometer-size silver flakes have a sintering temperature of 750 ^OC or higher. For this reason, it is not too great with a sintering temperature reduction effect obtained by reduction of particle size.

However, if metal microplates of the present invention are used together with silver nano particles at a specific ratio as will be described in accompanying claims, it results in rapid sintering at 120 ^OC to the lowest, a dramatic reduction in a usage amount of silver nano particles, a high conductivity and a higher packing ratio than other particles due to a plate-shaped polygon. Accordingly, the conductive paste of the present invention using the metal 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, the conductive paste of the present invention has a reduction effect of manufacturing costs. Although it is not intended to be tied to a specific theory, it can be said that the plate-shaped metal microparticles have a small thickness of 200 nm or less, and thus, they perform better sintering at low temperature than other metal particles such as micrometer-size flake-shaped particles or the like.

In the present invention, the metal microplates may be made of silver, copper, tin, gold, platinum, palladium and aluminum, singularly or in combination. For example, the metal microplates may include mixtures of individual pure metal particles and composite metal microplates such as copper microparticles surface-coated with silver.

In the present invention, it is proper that the plate of the metal microplates approximately perpendicular to a thickness direction has 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 of the metal microplates have 1 to 8 μm size on average. When the size of the metal microplates 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 metal microplates 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 metal microplates is more than 20 μm, it is not preferable because voids between particles increase and consequently a large amount of particles are needed, so that resolution of an electrode circuit wire is deteriorated.

Preferably, the metal microplates have a thickness of 200 nm or less, more preferably 50 nm or less. If the thickness of the metal microplates exceeds 200 nm, it is not preferable because a sintering temperature is raised, a packing ratio is lowered, and consequently thickness of a circuit wire is increased. On the contrary, if the thickness of the metal microplates is 50 nm or less, it is advantageous because a sintering temperature is lowered. In practice, it is not too significant with a minimum thickness limit of the metal microplates. Theoretically, the minimum thickness of the metal microplates corresponds to a thickness of a single metal atom. However, 200 nm or less thickness limit and 50 nm or less thickness limit are all proper in consideration of economical efficiency and easiness to manufacture and obtain the metal microplates.

In another aspect of the present invention, in addition to the silver nano particles and the metal microplates, carbon nanotubes (CNT) are further included in the composition for a conductive paste according to the present invention, and accordingly, it can enhance formation of a conductive network in the composition and enable a further reduction in the content of the silver nano particles.

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. Thus, the carbon nanotubes act to easily form a conductive network. This conductivity improvement effect of the carbon nanotubes is illustrated in FIG. 2. Accordingly, the use of carbon nanotubes enables a reduction in silver content required to attain the same level of conductivity.

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, the carbon nanotubes can enhance adhesion of the conductive paste to the surface of the circuit board.

The (non-treated) 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-modified with various functional groups. Preferably, the (non-treated) 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.

The use of such carbon nanotubes enables a dramatic reduction in the content of silver nano particles required to achieve a specific resistance of 1×10^-5Ω·cm or less. If the composition for a conductive paste according to the present invention containing the (non-treated) carbon nanotubes comprises 1 to 10 weight% of silver nano particles having an average particle size between 1 to 100 nm, 40 to 60 weight% of metal microplate, and 0.01 to 2 weight% of carbon nanotubes having an average diameter between 2 to 40 nm, it ensures low temperature sintering and high conductivity. If the content of the carbon nanotubes is less than 0.01 weight%, it is not preferable because a conductivity improvement effect is not obtained. If the content of the carbon nanotubes exceeds 2 weight%, it is not preferable because an additional conductivity improvement effect is insignificant and deterioration may occur to dispersion and the rheological properties of a resulting paste as will be mentioned below.

The carbon nanotubes without metal particles coating are advantageous to improve conductivity, 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, the carbon nanotubes are not uniformly dispersed in the structure of a conductive ink, but tend to concentrate on a specific part, making it difficult to effectively connect the silver particles. If the carbon nanotubes are not uniformly dispersed in the composition for a conductive paste, there is a difference in mechanical properties between a part where the carbon nanotubes exist and a part where the carbon nanotubes do not exist. As a result, a film having an uneven circuit wire or a low leveling is formed, so that the reliability of end-products is likely to go down.

To minimize these problems caused by use of pure carbon nanotubes and improve conductivity, a composition for a conductive paste according to yet another aspect of the present invention comprises silver nanotubes, metal microplates and metal-coated carbon nanotubes.

In the present invention, the metal-coated carbon nanotubes are carbon nanotubes surface-coated with metal nano particles. As an example of the metal-coated carbon nanotubes, silver-coated carbon nanotubes is illustrated in FIG. 3. As shown in FIG. 3, the silver-coated carbon nanotubes are produced by attaching silver atoms or silver ions onto the surface of carbon nanotubes. In the silver-coated carbon nanotubes, a high aspect ratio of carbon nanotubes is lowered and entanglement is prevented. Accordingly, a separate process for dispersing carbon nanotubes may be omitted. The silver-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 silver particles exist on the surface of carbon nanotubes, the surface area of a conductor increases. Furthermore, the silver-coated carbon nanotubes have a higher conductivity than carbon nanotubes without silver coating.

In the metal-coated carbon nanotubes of the present invention, a metal for coating carbon nanotubes may include nano particles of copper, tin, gold, platinum and palladium. These metal nano particles for coating may be in a reduced state or an ion state in a complex. If the metal nano particles for 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.

In the composition for a conductive paste according to the present invention comprising the metal-coated carbon nanotubes, the content of silver-coated carbon nanotubes is preferably 0.01 to 2 weight% relative to the total weight of the paste. When the content of the silver-coated carbon nanotubes is in this range, it is preferable because a conductive paste has a high electrical conductivity, a low sintering temperature and mechanical and rheological properties suitable for screen printing. If the content of the silver-coated carbon nanotubes is less than 0.01 weight%, it results in a poor electrical contact between silver particles, and resistance of a conductive paste becomes larger. If the content of the silver-coated 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.

In the silver-coated carbon nanotubes used in the present invention, a carbon nanotube portion has preferably an average length between 5 to 50 μm, exclusive of a silver-coated portion.

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 be other methods, for example,

1) surface-coating carbon nanotubes with a metal, such as silver or the like, by chemical reduction (NaBH₄ treatment and reduction in hydrogen atmosphere) or thermal reduction.

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

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.

Taking silver as a metal, the third method is illustrated in FIG. 4.

In addition to the metal conductive component including the silver nano particles, 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 ketones such as butyl carbitol acetate, butyl acetate and so on, alpha-terpineol, a glycol-based solvent and an alcohol-based solvent. 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%.

The composition for a conductive paste containing 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 specific resistance between 2×10^-6 to 10×10^-6Ω·cm and a low sintering temperature between 120 to 150 ^OC, and thus, achieves both a high conductivity and a low sintering temperature.

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 ink jet, spin coating, 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 conductive paste is sprayed in conformity with the scanned circuit pattern to form a film including conductive filler. The circuit board having the conductive paste printed thereon is sintered to remove a solvent and so on, and to fuse silver particles. If necessary, a multilayered circuit board may be fabricated through subsequent processes including stacking, thin-film forming, plating and so on.

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.

To compare a conductive paste of the present invention with a conventional paste, compositions for conductive pastes were prepared according to examples and comparative examples. The conductive pastes of examples according to the present invention utilized silver microplates as metal microplates, silver-coated carbon nanotubes as metal-coated carbon nanotubes and an acryl resin as a binder. As a solvent, a mixture of tetradecane, terpineol and butyl carbitol acetate was used, which act to disperse the silver nano particles, the silver microplates and the binder, respectively. The formulation for the pastes according to examples and comparative examples is shown in the following Table 1.

Table 1

	Examples					Comparative examples
	1	2	3	4	5	1	2	3
Silver nano particles(%)	14	7	7	7	7	63	-	14
Silver microplates(%)	50	50	50	50	50	-	-	-
Non-treated carbon nanotubes(%)	-	1	-	-	-	-	-	-
Silver-coated carbon nanotubes (%)	-	-	1	1	1	-	-	-
Flake-shaped silver microparticles(%)	-	-	-	-	-	-	78	50
Binder(%)	3	3.6	3.6	3.6	3.6	3	3	3
Solvent(%)	33	38.4	38.4	38.4	38.4	34	19	33

Sintering temperature(^OC)	130	130	130	120	110	130	130	130
Sintering time(min)	2

The pastes according to examples and comparative examples are manufactured as follows.

Manufacturing example of silver nano particles

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 trilethylamine 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), it was found that the silver nano particles were sintered in the range between 80 to 140 ^OC (See FIG. 9 and the below-mentioned experimental examples). A transmission electron microscope (TEM) image of the silver nano particles obtained in this manufacturing example is illustrated in FIG. 5.

Manufacturing example of silver microplates

M27-SA made by Tokusen in Japan (plate-shaped silver particles of which horizontal and vertical sides have 2 to 7 μm size) was used. M27-SA has a thickness of 200 nm or less and is plate-shaped silver microplates wherein the plate has horizontal and vertical sides of 2 to 7 μm size and a shape of a polygon including a triangle, a hexagon, an octagon and so on. A scanning electron microscope (SEM) image of the silver microplates is illustrated in FIG. 6.

Manufacturing example of silver-coated CNT

50 g/L multi-walled carbon nanotubes (diameter: 10 nm, length: tens of μm, liquid phase modification and high-temperature thermal treatment, made by LS Cable Ltd.) were dispersed in 10% sodium hydroxide aqueous solution. 50g silver nitrate was dissolved in 1L CNT-sodium hydroxide aqueous solution and 12g sodium borohydride (NaBH₄, Mw=40 g/mol) was added to reduce silver ions, so that the surface of the carbon nanotubes was coated with silver. The resultant product was filtered and thermally treated at 500 ^OC for one hour under a hydrogen atmosphere to obtain silver-coated carbon nanotubes, in which silver was attached more stably to the surface of the carbon nanotubes. An SEM image of the silver-coated carbon nanotubes is illustrated in FIG. 7.

Example 1: Paste containing silver nano particles and silver microplates

Silver nano particles, silver microplates and a binder described in the following A, B and C were prepared, respectively.

A) 20g tetradecane dispersant solution containing 70 weight% of spherical silver nano particles having an average particle size between 5 to 20 nm, obtained in the manufacturing example

B) 50 g silver microplates obtained according to the manufacturing example, dispersed in 20g butyl carbitol acetate solution

C) 10g acryl resin dissolved in terpineol solution as a binder (resin solid content: 30 weight%)

The above A, B and C 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 PET substrate at a size of 6×6 cm by screen printing using Sus 325 (350 meshes) to form an uniform film. The substrate was sintered at 130 ^OC for 2 minutes in a convection oven to manufacture a specimen. An SEM image of the surface of an electrode circuit wire according to example 1 is illustrated in FIG. 8.

Example 2: Paste containing silver nano particles, silver microplates and CNT

A specimen was manufactured in the same way as example 1 except that 12g a binder resin solution and 27 g butyl carbitol acetate solvent were used, and as non-coated carbon nanotubes,

D) 1g multi-walled carbon nanotubes having an average diameter of 10 nm and an average length of tens of μm

were added to the mixture of the above A, B and C components.

Example 3: Paste containing silver nano particles, silver microplates and silver-coated CNT

A specimen was manufactured in the same way as example 2 except that, instead of D) the non-coated carbon nanotubes,

E) 1g spherical silver-coated carbon nanotubes having an average size between 5 to 20 nm, obtained according to the manufacturing example,

were added to the mixture of the above A, B and C components.

Example 4: Paste containing silver nano particles, silver microplates and silver-coated CNT

A specimen was manufactured in the same way as example 3 except that sintering was made at 120 ^OC for 2 minutes.

Example 5: Paste containing silver nano particles, silver microplates and silver-coated CNT

A specimen was manufactured in the same way as example 3 except that sintering was made at 110 ^OC for 2 minutes.

Comparative example 1: Paste containing solely silver nano particles

A) 90g tetradecane dispersant solution containing 70 weight% of spherical silver nano particles having an average particle size between 5 to 20 nm, obtained according to the manufacturing example

B) 10g acryl resin dissolved in terpineol solution, as a binder (resin solid content: 30 weight%)

C) 34 g butyl carbitol acetate

The above A, B and C 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 PET substrate at a size of 6×6 cm by screen printing using Sus 325 (350 meshes) to form an uniform film. The substrate was sintered at 130 ^OC for 2 minutes in a convection oven to manufacture a specimen.

Comparative example 2: Paste containing solely flake-shaped silver microparticles

A specimen was manufactured in the same way as comparative example 1 except that the following A, B and C components were preliminarily mixed:

A) 78g flake-shaped silver microparticles having an average particle size between 2 to 7 μm

C) 19 g butyl carbitol acetate.

Comparative example 3: Paste containing silver nano particles and flake-shaped silver microparticles

A specimen was manufactured in the same way as comparative example 1 except that the following A, B, C and D components were preliminarily mixed:

B) 50g flake-shaped silver microparticles having an average particle size between 2 to 7 μm

C) 33 g butyl carbitol acetate

D) 10g acryl resin dissolved in terpineol solution, as a binder (resin solid content: 30 weight%).

An SEM image of an electrode circuit wire according to comparative example 3 is illustrated in FIG. 10.

Experimental example: Sintering temperature of silver nano particles

A differential scanning calorimetry (DSC) was performed to find out a possible sintering temperature of the silver nano particles obtained according to the manufacturing example. The melting point range of silver was measured at a temperature increase rate of 10 ^OC per minute. As shown in the graph of FIG. 9,it was found that sintering was completed between 80 to 140 ^OC.

The pastes according to examples and comparative examples were printed on PET substrates, 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.

Table 2

	Examples					Comparative examples
	1	2	3	4	5	1	2	3
Thickness of circuit wire(μm)	1.8	1.8	1.8	1.8	1.8	2.5	3.3	3.2
Specific resistance(Ω·cm)	7×10^-6	7×10^-6	5×10^-6	6×10^-6	6×10^-6	7×10^-6	800×10^-6	10×10^-6

The examples 1 to 5 have equal level of specific resistance, which are within the range of 10^-5Ω·cm suitable for an antenna of an RFID system. Among the comparative examples, such high conductivity is shown in the comparative example 1 containing solely silver nano particles at a high content (63%). However, the pastes of examples according to the present invention can accomplish such high level of conductivity with an even lower content of silver nano particles. Due to the use of metal microplates, the present invention (example 1) enables a reduction in the content of silver nano particles required for a desired specific resistance by one fifth of comparative example 1, and if carbon nanotubes are added (examples 2 to 4), the content of silver nano particles can be reduced by one tenth. This leads to cost reduction, and as shown in Table 2, it can diminish the thickness of a circuit wire for ensuring the same target specific resistance, thereby supporting miniaturization of a circuit wire.

Furthermore, examples 4 and 5 further reduced a sintering temperature necessary for a thin circuit wire and a high conductivity, and shortened a sintering time (less than 2 minutes), thereby improving productivity of a manufacturing process.

The advantageous effects resulted from the use of metal microplates according to the present invention are prominent when compared with comparative examples 2 and 3. The comparative example 2 contains a large amount of micrometer-size flake-shaped silver particles, but exhibits a very low conductivity. The comparative example 3 has equal level of sintering temperature and specific resistance as examples of the present invention, but involves an even thicker circuit wire than examples of the present invention so as to exhibit such level of specific resistance. For this reason, it takes much cost to form an electrode circuit wire using the paste of comparative example 3.

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.

Claims

A composition for a conductive paste, comprising:

3 to 20 weight% of silver nano particles having an average particle size between 1 nm to 100 nm; and

40 to 70 weight% of metal microplates having an average thickness of 200 nm or less and a micrometer-sized plane approximately perpendicular to a thickness direction.
A composition for a conductive paste, comprising:

1 to 10 weight% of silver nano particles having an average particle size between 1 nm to 100 nm;

40 to 60 weight% of metal microplate having an average thickness of 200 nm or less and a micrometer-sized plane approximately perpendicular to a thickness direction; and

0.01 to 2 weight% of carbon nanotubes having an average diameter between 2 to 40 nm.
A composition for a conductive paste, comprising:

1 to 10 weight% of silver nano particles having an average particle size between 1 nm to 100 nm;

40 to 60 weight% of the metal microplate having an average thickness of 200 nm or less and a micrometer-sized plane approximately perpendicular to a thickness direction; and

0.01 to 2 weight% of metal-coated carbon nanotubes in which the surface of carbon nanotubes is coated with metal nano particles.
The composition according to any one of claims 1 to 3, further comprising:

0.5 to 5 weight% of metal nano particles,

wherein the metal nano particles are at least one selected from the group consisting of copper, tin, gold, platinum and palladium, and

wherein the metal nano particles have an average particle size between 1 to 100 nm.
The composition according to any one of claims 1 to 3,

wherein the silver nano particles have an average particle size between 5 to 40 nm.
The composition according to any one of claims 1 to 3,

wherein the metal microplates are made of at least one selected from the group consisting of silver, copper, tin, gold, platinum, palladium and aluminum.
The composition according to any one of claims 1 to 3,

wherein the micrometer-sized plane perpendicular to the thickness direction has horizontal and vertical sides of 1 to 20 μm size on average.
The composition according to any one of claims 1 to 3, further comprising:

0.1 to 10 weight% of at least one additive 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.
The composition according to any one of claims 1 to 3, further comprising:

a solvent selected from the group consisting of ketones, alpha-terpineol, glycols and alcohols; and

a binder selected from the group consisting of acryls, vinyls, nitrocellulose, ethyl cellulose and modified resins thereof.
The composition according to any one of claims 1 to 3,

wherein the composition for a conductive paste is used to manufacture a conductive ink, and

wherein the conductive ink has a specific resistance between 2×10^-6 to 10×10^-6Ω·cm.
The composition according to any one of claims 1 to 3,

wherein the composition for a conductive paste is used to manufacture a conductive ink, and

wherein the conductive ink has a sintering temperature between 110 to 150 ^OC.
The composition according to claim 3,

wherein the metal-coated carbon nanotubes are carbon nanotubes with a metal which is at least one selected from the group consisting of silver, tin, platinum, gold, palladium and ion complexes thereof.
A conductive substrate having a circuit wire formed from the composition for a conductive paste according to any one of claims 1 to 3.