LI Jilian ¹, ZHANG Jing ¹, QIN Zixuan ¹, BAI Xiaolong ², SUN Xudong ^{1, 3}

(1. School of Materials Science and Engineering, Northeastern University, Shenyang 110819, Liaoning, China; 2. Acro New Materials (Dalian) Co., Ltd., Dalian 116423, Liaoning, China; 3. Foshan Graduate School of Innovation,Northeastern University, Foshan 528311, Guangdong, China)

Extended Abstract:[Significance] Silicon nitride (Si₃N₄) has excellent mechanical, thermal and electrical properties, which is widely used in manufacturing and modern industries, such as electronics, machinery, information and so on. It has high strength, high hardness, excellent high temperature resistance, high thermal conductivity, excellent chemical stability, oxidation resistance and biocompatibility. In the backplane materials of smartphones, compared with traditional materials, such as plastic, aluminum-magnesium alloy and glass, silicon nitride has higher overall performance, which has important research value and application prospects. However, among ceramic materials, zirconia has been researched earlier and more widely used as mobile phone backplanes. Nevertheless, zirconia has higher density and dielectric constant, so that the overall weight of the phone is heavier and is not conducive to signal transmission. The toughness of Si₃N₄ is slightly inferior to zirconia, while the appearance of a single gray, which cannot meet consumer demand for cell phones, so it is important to prepare Si₃N₄ with different colors and excellent performance. Rare earth oxide and spinel oxide have been widely used as coloring agent, which have high-temperature resistance and can maintain color purity. At the same time, they can act as both colorant and sintering aid to promote the sintering of the Si₃N₄ ceramics, which is very helpful to achieve high density and mechanical properties. The coloring of Si₃N₄ is discussed. The rare earth oxide and spinel oxide colorants and their coloring principles are introduced, while the effect of colorants on the mechanical properties of Si₃N₄ is analyzed. More importantly, combined with the current progress in the preparation and research of high performance multicoloured Si₃N₄ ceramics, the development prospect of silicon nitride materials for application in mobile phone backplane is envisioned.[Progress] Firstly, rare earth oxides, which are widely used in silicon nitride ceramics, are introduced. The coloring principle of rare earth oxide is mainly attributed to the electron leaps of the coloring ions and the selective absorption and reflection of light. The rare earth oxides have high hardness, high-temperature resistance, excellent electrical and thermal conductivity. By adding different rare earth oxides, Si₃N₄ can be made to show different colors. For example, Nd₂O₃ makes Si₃N₄ blue-green (cyan), Eu₂O₃ yellow, Dy₂O₃ yellow-green, Er₂O₃ pink, Yb₂O₃ brown or green, etc. Meanwhile, by adjusting the types and quantities of rare earth oxides, the color of Si₃N₄ can be optimized. In addition, rare earth oxides can be used as sintering aid to improve the conversion rate from α-Si₃N₄ to β-Si₃N₄, promote liquid phase flow and effectively improve the mechanical strength of the Si3N4 ceramics, such as fracture toughness, flexural strength and hardness. Secondly, the spinel oxides are discussed. The coloring principle of spinel oxides is similar to that of the rare earth oxides. The mainstream colors of mobile phone backplanes are blue and black. The d-d electron leap generated by Co²⁺ and Ni²⁺ in tetrahedral coordination reflects blue light. The black colorant hue is caused by the electron leap of the coloring ions (Fe³⁺, Mn³⁺, Cr³⁺, etc.). When they are added to Si₃N₄ with right ratio, the ions can absorb all light in the visible range, giving rise to black color. In addition, the spinel oxides have high high-temperature stability and proper dielectric constant, thus reducing decomposition and volatilization of the colorant and making the color stable and uniform. It may also react with SiO₂ on the surface of Si₃N₄ and sintering aids to generate coloring compounds and improving the mechanical properties. However, the spinel oxides have disadvantages, such as strict synthesis conditions and increased process flow, which are currently less used in Si₃N₄ ceramics. So further research and development are necessary. Finally, the process of Si₃N₄ needs to be further optimized. Some solutions have been proposed, such as low-temperature sintering.

[Conclusions and Prospects] Silicon nitride has various advantages in the application of smartphone backplanes. By choosing suitable colorants, stable and uniform colors could be achieved, while the mechanical properties could be effectively enhanced simultaneously. Rare earth oxides and spinel oxides have been widely used as colorants to prepare high performance multicolored Si₃N₄ ceramics, which make the color of Si₃N₄ more diverse and less prone to decomposition and volatilization at high temperatures. At the same time, both of them can be used as sintering additives to promote the sintering of Si₃N₄ and improve the comprehensive properties. However, the types of colorants used at present are limited, resulting in single color, so it is necessary to develop new colorants. Moreover, there is still room for improvement in the fracture toughness of Si₃N₄. In addition, in-depth research should be carried out to achieve low-temperature sintering of Si₃N₄ ceramics. The low temperature liquid phase could be employed to promote the densification of Si₃N₄, with potential large-scale industrial applications.

Key words: silicon nitride; mobile phone backplane; colourants; colouring principle; fracture toughness