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
- 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.
- 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.
- 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.
- 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.