**3. Results and Discussion**

## *3.1. Comparison of the Light Absorption and Thermal Decomposition Properties of the Copper Compounds*

We first obtained diffuse reflectance spectra of the copper compounds and found that diffuse reflectance increased in the order CuO > Cu3N > Cu2O > Cu (Figure 1). We then examined the thermal decomposition of Cu3N, CuO, and Cu2O under an air or N2 atmosphere (Figure 2). Under the N2 atmosphere, the weight of Cu3N was decreased at 100 ◦C, and at 250 and 500 ◦C the weight losses were −5.4% and −5.1%, respectively. In contrast, the weights of CuO and Cu2O did not change until 500 ◦C, at which point the weights had increased by 0.1 and 1.1%, respectively. Under the air atmosphere, the weight of copper nitride decreased until 188 ◦C, after which, it increased via oxidation; at 500 ◦C, the weight change of copper nitride was −0.7%. The weight of Cu2O remained constant until 333 ◦C at which point it began to increase as a result of oxidation; the weight was 7.2% at 500 ◦C. The weight of CuO remained constant, and the change was only 0.3% at 500 ◦C. Thus, Cu3N decomposed at a lower temperature than CuO and Cu2O, which is consistent with previous reports that CuO and Cu2O melt at 1193 and 1229 ◦C [30]. The light absorption and thermal decomposition properties of the

materials are summarized in Table 1. Among the copper compounds examined, copper nitride had the second highest light absorbency and lowest decomposition temperature; therefore, we concluded that copper nitride was the most suitable material for producing a wiring ink that is sintered by means of IPL irradiation.

**Figure 1.** Ultraviolet–visible diffuse reflection spectra of copper nitride (Cu3N), copper(I) oxide (Cu2O), copper(II) oxide (CuO), and copper (Cu) particles.

**Figure 2.** Thermogravimetric-differential thermal analysis curves of copper nitride (Cu3N), copper(I) oxide (Cu2O), and copper(II) oxide (CuO) in an air or N2 atmosphere.


