**4. Air stable coated copper powder**

Ag powder has high utility, but the price of precious metal raw materials is high. Thus, the demand for metals to replace Ag is continuously increasing [25, 26]. Copper, which is relatively inexpensive and has low resistivity, can be used as an alternative, but low atmospheric stability is a major problem. In addition, as the size decreases, the specific surface area increases, and thus, the oxidation problem becomes more pronounced.

In order to solve this problem, efforts have been made to improve atmospheric stability by coating surfactants, conductive polymers, silver, titanium, silica, etc. [27–35]. However, such attempts have led to lowering the conductivity. Further, the method of coating the copper surface with metal requires an additional coating process after synthesizing the copper powder, which increases the production cost in industrial applications. In addition, these methods present a difficulty in realizing uniform coating.

In this study, a technique for synthesizing coated copper composite particles using the spray pyrolysis process was studied in order to solve problems such as low atmospheric stability, complicated process, and uniform coating. The Cu composite particles developed in this study were prepared into a mixed solution by simultaneously adding copper nitrate trihydrate and silver nitrate salt for producing Ag-coated Cu powder when preparing the precursor solution to be input to the droplet generating device. And hydrogen mixed gas was used as carrier gas for reduction and synthesizing silver-coated copper powder [36]. The melting points of Ag and Cu are 962 and 1084°C, respectively. The results of this study showed that when the synthesis was conducted at 900°C, Cu remained in the core, and Ag was uniformly pushed to the surface. This is because Ag with a lower melting point has relatively better mobility. **Figure 6** shows the result of evaluating the air stability of these uncoated particles, after being left in the atmosphere for 1 month. The uncoated bare Cu powder developed substantial surface irregularities due to oxidation, and its HRTEM elemental profile showed that the oxygen peak on the surface was relatively higher than that on the inside. Further, as shown in **Figure 7**, the surface analysis of the coated particles after 1 month showed smooth characteristics similar to the initial stage, and the HRTEM analysis showed lattice spacing (Ag on the surface and Cu on the core). The elemental profile also confirmed atmospheric stability, showing relatively high Ag on the surface and no major change in oxygen peak. **Figure 8a** shows the sheet resistances of the electrode according to the Ag content of the Ag-coated Cu electrode layer. All samples were sintered at 700°C for 10 min under N2 atmosphere. In **Figure 8a**, the sheet resistance value showed the lowest value of 2 mΩ square<sup>−</sup><sup>1</sup> when Ag was 15 wt.%, which showed the characteristic that the porosity of the electrode was low, as shown in **Figure 8c**. When Ag was as high as 50 wt.%, large pores were exhibited due to the high mobility of Ag, and the sheet resistance value increased.

Glass-coated Cu particles have been proposed as another structure for synthesizing particles with improved atmospheric stability and electrode properties [37]. In the reported study, one-pot synthesis was performed using the spray pyrolysis process in the same manner as the previously shown process. To prepare glass-coated copper particles, The precursor solution was obtained by combining copper nitrate trihydrate

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

#### **Figure 7.**

*(a) SEM, (b) TEM, (c) HRTEM images of 20 wt.% silver coated copper, and (d) element profile across the particle diameter direction. The sample was exposed to air for 1 month [36].*

(Cu(NO3)2∙3H2O), barium carbonate (BaCO3), tetraethyl orthosilicate (SiC8H20O4), and boric acid (H3BO3). The content of the glass precursors in the Cu@BBS particles was varied such that the final weight portion of the glass comprised 0.5–10 wt% of the Cu.

In the Cu electrode, the glass material also serves as an inorganic binder, helping the electrode sinter to increase the density of the electrode, and as an auxiliary agent to improve the electrode conductivity. It simultaneously acts as an inorganic binder and passivation to derive two positive effects in the case of Cu, enabling the synthesis of Cu particles without an additional process, which provides several advantages for mass production.

**Figure 9** shows the synthesis process of Cu@BBS particles synthesized in the core-shell structure by the one-pot spray pyrolysis process. It was analyzed that CuOx is changed to Cu during dry pyrolysis in a reducing atmosphere, and the materials constituting BBS that are not easily reduced are formed into BBS glass. Then, Cu is pushed to the center, and BBS glass is pushed to the surface due to the difference in melting point.

As shown in **Figure 10**, the surface of the synthesized particles was uniformly coated, and the coating layer was amorphous. In addition, the resistance values were compared by forming an electrode using the powder before and after coating, and the Cu and Cu@BBS particles exposure to air for 1 month, followed by heat treatment. As a result, the resistance values of bare Cu and Cu@BBS particles were lowered to 5.1 and

**Figure 8.**

*(a) Sheet resistance of conducting films obtained from silver-coated copper particles with various loadings of silver, sintered at 700°C for 10 min in N2 atmosphere. SEM photographs of surface section of the prepared conductive films obtained from (b) bare copper, (c) 15 wt.%, and (d) 50 wt.% silver-coated particles [36].*

#### **Figure 9.**

*(a) Experimental setup for ultrasonic spray pyrolysis used in the current investigation. (b) Formation of BBS glass-coated copper particles by phase segregation [37].*

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

#### **Figure 10.**

*(a) Transmission electron microscopy (TEM) images of Cu@BBS particles (b) high-resolution TEM image magnified from the red box in (a) showing the amorphous coating layer and (c) resistivities of Cu conductive films based on bare Cu particles and Cu@BBS particles (measured after sintering at 800°C for 10 min under N2 atmosphere) [37].*

2.01 μΩ∙cm, respectively, and those for the electrodes formed after 1 month of exposure were 18.3 and 2.26 μΩ∙cm, respectively. As a result, the resistivity of bare Cu particles and Cu@BBS particles was lowered to 5.1 and 2.01 μΩ∙cm, respectively, and after 1 month of exposure, these values were 18.3 and 2.26 μΩ∙cm, respectively. Thus, the resistivity of the electrode comprising bare Cu increased by approximately three times or more than that of the existing electrode, while the Cu@BBS particle showed similar resistivity and presented improved atmospheric stability and electrode characteristics.
