Introduction
Due to its high thermal conductivity (up to 320 W/(m·K) in theory), low thermal expansion coefficient matched with semiconductor materials such as Si and GaAs (4.2 × 10-6 /K, Si and GaAs: 3-4 × 10-6 /K, 25-200 °C), low dielectric constant (1 MHz, 8.0) and dielectric loss, as well as good electrical insulation properties, aluminum nitride (AlN) has become one of the most ideal ceramic substrates and electronic packaging materials [1-3]. High-property AlN ceramic substrates cannot be fabricated without high-quality AlN powder raw materials of high N concentration and low O and C concentrations. Currently, the carbothermal reduction nitridation process (CRN) is the mainstream technology used around the world [4-6] for synthesizing AlN powder in batch quantities. Other techniques such as direct nitridation, self-propagating high temperature synthesis and sol-gel method have been also widely used [7-10].
The CRN process in AlN powder synthesis is carried out in terms of a gas-solid reaction, including two sub-processes of the reduction reaction of Al2O3 by C and the reaction of the reduction products with N2 to form AlN [11]. In detail, as the reaction temperature increases above 1500 °C, the reaction between the carbon black and the Al2O3 powder takes place to produce vapors of Al(g) and/or low-valent oxides of Al, such as Al2O(g). The reduction products of Al(g) and/or Al2O(g) have high reactivity and react rapidly with N2 to synthesize AlN [12]. The concentrations of the C and O impurities are the main factors affecting the quality of the synthesized AlN powder. Generally, the C impurity content in the AlN powder comes from residual carbon black [13], while the O impurity content is derived from the residual Al2O3 and O atoms dissolved in the AlN crystal lattice in the CRN process [14]. The O atoms dissolved in the AlN crystal lattice occupy the N atom positions, which induce Al vacancies and generates a strong phonon scattering effect. The thermal conductivity of the AlN ceramics is therefore sharply reduced [15]. Almost no other sources of C and O impurities have been reported in the CRN process.
Regarding the solid-state reaction process, if the thermodynamic and kinetic driving forces of the reaction are inadequate, one or more mesophases will be formed. Using the reduction reaction of Al2O3 with C as an example, Al4C3 is first formed, and then it reacts with Al2O3 to form Al2OC and/or Al4O4C [16]. AlN powder is synthesized by the CRN process employing C and Al2O3 as the main raw materials. It is unknown which of the following compounds, Al2OC, Al4O4C, or Al4C3, acts as the mesophase in the CRN process, so far. If these mesophases form in the CRN process and exist in the AlN powder, the O and C concentrations of the AlN powder are highly increased and the N concentration of the AlN powder is decreased, accordingly. The quality of the AlN powder is therefore highly degraded. It is of great value to study the formation and characteristics of the mesophase in the AlN powder synthesized by the CRN process both theoretically and industrially.
In the current work, high-purity and ultrafine Al2O3 powder and nanoscale carbon black are used as raw materials to prepare AlN powder in batch quantities via the CRN process. Taking the AlN powder of low N content and high C and O contents as samples, its composition, morphology, crystal structure, and phases were investigated to identify the mesophase in the AlN powder. Furthermore, the formation mechanism and condition of the mesophase was determined via thermodynamic calculations. Accordingly, an improved process plan was proposed to eliminate the mesophase in the AlN powder in the CRN process. Finally, the AlN powders of different C and O contents synthesized in this work were used as raw materials to prepare bulk AlN ceramics via a commonly-used process of tape casting and subsequently pressureless sintering [17, 18]. The thermal conductivities and bending strengths of the AlN ceramics were mainly concerned to verify the significant improvement of the AlN ceramic performance by reducing the C and O contents through eliminating the mesophase in the AlN powder. This study is believed to provide important background for the production of high-quality AlN powder in the CRN process, both theoretically and experimentally.
Material and methods
Synthesis of the AlN powder
TABLE 1 Characteristics of ultrafine Al2O3 and nanoscale carbon black powders