FIGURE 13 XRD patterns of the AlN ceramics (a) S1; (b) S3; (c) S5
As shown in Figure 12a, the AlN ceramic S1 is dense without any pores. The AlN grains are well-developed of a grain size about 3-5 µm. The bending fracture of the AlN ceramic mainly takes place intergranularly, but the cleavage fracture of couples of large AlN grains also occurs. Several bright grains in Figure 12a are situated at the AlN grain boundaries, the EDS testing result shows that they contain high Y, Al and O contents (Figure 12b). During sintering the AlN ceramics, the O impurity in the AlN powder reacts with Y2O3 to form an Al2O3-Y2O3melt, promoting the rapid densification of the AlN ceramics. The melt is then transformed into YAP (YAlO3), YAM (Y4Al2O9) and YAG (Y3Al5O12) in the cooling process of the AlN ceramics [34, 35]. As shown in Figure 13a, two oxide phases, YAP and YAM, are formed in the AlN ceramic S1, and the intensities of the diffraction peaks of them are rather weak, because the O content of the AlN powder P1 is the lowest. In the AlN ceramics S3 and S5, YAG is also detected by XRD, besides YAP and YAM. As the O content of the AlN powder increased from P3 to P5, the intensities of diffraction peaks of the three oxides increase, indicating more YAP, YAM and YAG in the AlN ceramics (Figure 13b, c). These oxides are distributed along the AlN grain boundaries to form a strong interfacial bond with the AlN grains, resulting in the significant increase of the bending strength of the AlN ceramics [36]. However, the thermal conductivities of these oxides are very low, e.g., AlN: 320 W/(m·K), YAG: 10.2 W/(m·K) [37], the thermal conductivity of the AlN ceramics is apparently decreased with increasing the amount of the oxides.
In summary, the comprehensive properties of the AlN ceramics are excellent, using the high-quality AlN powder synthesized in this work as raw materials, i.e., the thermal conductivity 176.3 W/(m·K), and the bending strength 421.3 MPa. They are comparable to those of the internationally-advanced SH-15 grade AlN ceramics (184 W/(m·K), 357 MPa, respectively) and SH-30 grade ones (174 W/(m·K), 511 MPa, respectively) developed by Tokuyama, Japan [38].
Conclusions
  1. The dark gray AlN powder particles were nearly spherical with a nonuniform microzone composition distribution. The existence of the Al2OC mesophase in the CRN-synthesized AlN powder was first identified by XRD, XPS, and TEM investigations. A small amount of the Al2OC mesophase and residual C and Al2O3 were detected in the AlN powder, resulting in high concentrations of O and C impurities in the AlN powder.
  2. During AlN powder synthesis in the CRN process, a core-shell structure composed of the Al2O3 core wrapped by the AlN shell was formed with the Al2OC mesophase at the AlN/Al2O3 interface. The Al2OC mesophase is an incomplete reduction product of the Al2O3 particles and has a highly similar crystal structure to AlN. It is also one of the major sources of C and O impurities of the AlN powder synthesized in the CRN process.
  3. Based on the thermodynamic calculations, the isothermal section of the AlN-Al2O3-Al2OC ternary phase diagram at 1973.15 K was constructed. When the PN2 was lower than 10-1.846 kPa, there was an ABC triangular region contained within, corresponding to the Al2OC single phase region. When thePN2 was as low as 10-5 kPa, the Al2OC mesophase was stable in thePCO range of 10-0.008-100.973 kPa. In contrast, the Al2OC mesophase was not stable and decomposed into AlN by increasing the PN2 and/or lowering thePCO in the synthetic furnace.
  4. The optimal process for AlN powder synthesis in the CRN process was determined as follows: calcination at 1700 °C for 12 h with a N2 flow rate of 400 L/min, while the other technical parameters were unchanged. Under these conditions, the AlN powder synthesized in batch quantities was light grey with high N content and low O and C contents. At these conditions, a high-quality AlN powder was stably produced.
5. The O content of the AlN powder has great influences on the thermal conductivity and strength of the AlN ceramics. The AlN ceramics made of the AlN powder of the O content as low as 0.84 wt% have a thermal conductivity of 176.3 W/(m·K) and a bending strength of 421.3 MPa.