# Dry Printing of Ag-Ni Conductive Particles Using Toner-Type Printed Electronics
## 2 Materials and Methods  
Ag micro-powder particles with a nominal diameter of 2-3.5 \(\upmu\)m (Sigma-Aldrich Co. LLC, St. Louis, MO, USA) were used in this study. Ni nanopowder particles with a nominal diameter of \(\sim\)150 nm were used in the Ag-Ni composite toner (Sigma-Aldrich Co. LLC). Several commercially purchased micro- and nanoparticles are naturally aggregated; as a result, the actual diameter of these particles was generally larger than the reported nominal diameters. Therefore, before use, the naturally aggregated Ni particles were ball-milled to retain the original fine nanoparticles. Conversely, the Ag microparticles were used as purchased and were not milled because Ag is significantly soft; thus, milling would promote Ag aggregation. The Ag:Ni weight ratio in the Ag-Ni composite particles was 5:1. The volume ratio was Ag:Ni = 1:0.236. The role of the Ni particles is to prevent the natural aggregation of the conductive toner. Therefore, this ratio satisfies the required Ni composition and is not too sensitive to be used as a conductive toner. An excessive amount of Ni would lead to an increase in the electrical resistivity of the conductive toner after sintering. The Ag micro-powder and Ni nanopowder were gently mixed, obtaining Ag-Ni composite toner particles. Figure 1a,b demonstrate the repose angle estimation for Ag and Ag-Ni conductive particles. The higher repose angle for the Ag particles (72-85\({}^{\circ}\)) shown in Figure 1a compared to that of the Ag-Ni particles (54-75\({}^{\circ}\)) shown in Figure 1b indicates that the bare Ag particles have lower fluidity compared to that of the Ag-Ni particles, owing to the higher natural aggregation between bare Ag particles. Therefore, the Ni particles distributed on the surface of the Ag particles inactivated the Ag-Ni surface and prevented the aggregation of the conductive toner particles. As a result, the particle size distribution was apparently different for the bare Ag and Ag-Ni particles, as shown in Figure 1c,d. Figure 1c,d demonstrate Ag and Ag-Ni particle size distributions that were analyzed using an optical micrograph and image processing software (Image) [35; 36]), respectively. Figure 1c indicates that the Ag particle size distribution has a peak diameter of approximately 7 \(\upmu\)m. Although the nominal particle diameter reported by the manufacturer was 2-3.5 \(\upmu\)m, the Ag particles became larger, owing to the natural aggregation demonstrated. In contrast, the Ag-Ni composite particles presented different size distributions, and a lower-side tail and sub-peak appeared at approximately 3 \(\upmu\)m, which indicates that the addition of the Ni particles effectively prevented the natural aggregation of the Ag particles. Figure 2a presents an SEM image of a Ag-Ni conductive particle. Ag and Ni particles are difficult to distinguish based on their appearance. However, the energy-dispersive X-ray spectroscopy of this particle revealed the elemental distribution of the composite particles. Figures 2b,c demonstrate the mapping of the Ag K\(\alpha\) X-ray detected points and that of Ni-K\(\alpha\), respectively. The Ag K\(\alpha\) distribution reflects the entire particle shape (\(\sim\)5 \(\upmu\)m), as shown in Figure 2a. In contrast, the Ni K\(\alpha\) signal distribution indicates that the Ni particles are smaller (\(<\)1 \(\upmu\)m) and they stick to the surface of the larger Ag particles. If we carefully examine the SEM image after Ag, Ni, and the superimposed EDX image in Figure 2d, it is possible to distinguish Ni and Ag by the brightness in the SEM image. The dark gray particles are the Ni particles that cover the entire surface, while Ag is the bright region slightly observed among the Ni particles. The apparent difference in brightness in the SEM images is due to the secondary electron emission probability between Ag and Ni. As  
a result, it was confirmed that the surfaces of the Ag particles were covered with Ni particles.