**1. Introduction**

Metals with high intrinsic conductivity, such as gold, silver, and copper, are mainly used as electrodes. Conductive powders using noble metals are easily synthesized into metals even in an oxidizing atmosphere, whereas transition metals are synthesized into metal in a reducing atmosphere [1].

Generally, the powder synthesis technology is divided into top-down and bottomup methods. The solid phase method can be classified as a top-down method, while the liquid phase method and gas phase method are regarded as bottom-up methods. In the solid phase method, mechanical energy is applied to the raw materials of the

desired composition in order to induce bonding and pulverization repeatedly between the powders, which reduce the particle size and synthesize powder while continuously contacting a new surface. Then, synthesis is completed at room temperature or a desired composition is obtained through post-treatment. The mechanical alloying method is a representative solid phase method. Compared to the bottom-up method, this method has the advantages of relatively easy process operation, low cost, and fewer by-products. However, it is limited in reducing the particle size, and homogeneity of the particles is not secured locally when synthesizing multi-component particles [2–6]. In particular, in pulverization of metals, it is difficult to atomize through crushing due to the ductility of the metal. Furthermore, since spheroidization through crushing is impossible, synthesizing conductive spherical powders through the solid phase method is not possible.

The liquid phase method ionizes a metal salt in a solvent to recover the synthesized particles using a reducing or precipitating agent. Since the raw materials can be mixed evenly at the molecular level, it is possible to control the composition uniformly, and the particle size and distribution can be controlled uniformly as well. Synthesis can be realized at a relatively low temperature, and thus, the method has the advantage of low process cost. However, this method generates substantial amount of chemical solvent byproducts, because particles are precipitated by chemical reaction. Moreover, it is difficult to improve the purity due to residual chemical substances in the particles. In addition, it has the disadvantage of low crystallinity, since it is synthesized at low a temperature [7–10].

The gas phase method is a process in which the raw material is divided into atomic or molecular units by vaporizing a metal salt or solution of metal salt, and then, particles are synthesized through nucleation, growth, and agglomeration during condensation. Examples of such a method include Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD). In the gas phase method, the vaporized molecules are nucleated again to grow into particles, and thus, fine particles can be synthesized. It is an advantageous process for producing high-purity particles with low residues of C, N, S, etc., because the synthesis occurs at a high temperature. However, there are some difficulties in synthesizing alloy powders, because non-uniformity may occur due to the difference in vapor pressure during the vaporizing stage. Additionally, the production cost is also high [11–16].

The spherical shape is preferred for conductive powders, because the density of the electrode must be high after formation. As the dispersion is suboptimal when significantly small particles are applied to the manufacturing paste, mixtures of particles with sub-micron sizes are mainly used. Therefore, synthesis has been performed mainly through the liquid phase process. However, in the case of the liquid phase process, since metal particles are synthesized through a reducing agent, residual carbon is present in the particles, which lowers the purity, thereby lowering the conductivity.

In order to overcome the disadvantages of the liquid phase method, a method for synthesizing metals at high temperature using a gas phase method is introduced, but in the case of PVD and CVD, the process for atomization of 0.2 μm or less. This is due to the nature of the process in which vaporized molecules are condensed and powdered. This characteristic is very advantageous, but there is a limit to synthesizing particles with a size of sub-micron to 1 μm.

The ultrasonic spray pyrolysis process is a process that simultaneously utilizes the advantages of the bottom-up, liquid phase, and gas phase methods. The desired raw material is placed in a solvent to prepare a solution, and fine droplets are generated using an ultrasonic generator, followed by heating in a high-temperature furnace.

#### *Conductive Powder Synthesis Technology for Improving Electrical Conductivity by One-Pot… DOI: http://dx.doi.org/10.5772/intechopen.108937*

It is a process of synthesizing particles in within seconds by passing through drying and thermal decomposition. This is similar to a process of synthesizing particles by generating fine-sized droplets in the form of aerosols, similar to vaporizing a solution using a homogeneous solution at the molecular level. Additionally, it is a process that can utilize both the effects of the gas phase and liquid phase methods, as it starts from the liquid phase to synthesize the powder using the sprayed droplets. Since the spray pyrolysis process performs synthesis using the initial raw material as a solution, it is also advantageous to produce multi-component particles that contain trace elements. Furthermore, this process facilitates synthesizing composite powders using various additives during solution preparation. In particular, the mist comprising a solution with a uniform composition passes through the heating furnace to form particles such that the composition of the entire solution is kept the same, and spherical particles are mainly synthesized in a droplet-like form.

The main requirements for conductive powders are spherical shape, low resistivity, high electrical conductivity, high purity, high crystallinity, low temperature sintering, oxidation resistance, etc. The spray pyrolysis process produces spherical shaped powders, since it maintains the shape of the droplets. Moreover, the pyrolysis process enables synthesizing high-purity and highly crystalline particles with a size of 1 μm or less.

When manufacturing an electrode using a conductive powder, a paste is prepared by mixing the conductive powder and glass frit as an inorganic binder with an organic binder, and printing is used to produce an electrode layer. It is essential to control the surface oxidation of the conductive powder in order to improve the density of the electrode, and the presence or absence of glass frit and particle characteristics are very important [17]. The mixing ratio of the conductive powder of the electrode and the glass frit is generally at least 8–20 times different. In particular, amorphous glass frit generally synthesizes powders through crushing, and realizing uniform dispersion becomes difficult due to the irregular size. Further, in the case of transition metal conductive powders, in addition to the problem of mixing glass frit, an oxide layer is formed on the surface powder after synthesis, since it is easily oxidized even at room temperature. This inhibits the densification of the electrode by decreasing sinterability when forming electrodes and leads to poor conductivity due to oxidation.

In this chapter, a method for synthesizing a composite electrode powder with a core-shell structure using a single-step phase-segregation mechanism using the ultrasonic spray pyrolysis process is described, and the corresponding characteristics are studied. The proposed method overcomes the limitations of the conductive paste process. Moreover, this chapter summarizes some representative studies on the subject.
