The organic binder, dispersant, plasticizer and solvent were dissolved
in absolute ethanol. The powder mixture of AlN and 5 wt%
Y2O3 sintering additives were then added
into the solution and sealed in a corundum jar. The corundum balls of 10
mm in diameter were used as the milled medium. The mixture was milled
for 48 h at a speed of 50 rounds/min, till a uniform slurry was
obtained. After tape casting, stacking, and cold isostatic pressing at
20 MPa, the AlN green pieces were made. And then, the
adhesive evacuating of the AlN
green pieces was performed. The main process parameters were listed in
Table 2. Finally, the AlN green pieces were sintered in a graphite
furnace at 1800 ℃ for 4 h. The main sintering process parameters were
also listed in Table 2. After that, the AlN ceramics were cooled to 1300
℃ at a rate of 5 ℃/min, and then cooled to room temperature with the
furnace. The whole adhesive evacuating and sintering processes are
protected under a flowing N2 atmosphere at a rate of 200
L/min.
Characterization
The O, N and C contents of the as-calcined, as-decarburized, and
as-pulverized AlN powder were detected using an ON836 type
oxygen-nitrogen content analyzer and a CS844-MC type carbon content
analyzer, respectively. For each test, at least three samples were
employed to obtain the average value. In the as-decarburized products,
dark grey particles were chosen and dry ground in an agate mortar to
prepare the powder sample. A SU8020 type field emission scanning
electron microscopy (FE-SEM) equipped with an Oxford INCA type energy
dispersive spectrometer (EDS) was used to observe the powder morphology
and its micro-zone compositions. Due to the poor conductivity of the
powder sample, gold spraying was
necessary before SEM observation. The gold spraying parameters were set
as 35 mA for 90 s. Phases of the powder sample was analyzed using an
X’Pert PRO MPD type X-ray diffractometer (XRD) with a Cu target (Kα,
λ=0.154 nm), and the tube voltage and current were 40 kV and 40 mA,
respectively. The scanning rate was set as 0.02° per 20 s and the
scanning angle range (2θ ) was from 10° to 90°. An ESCALAB250xi
type X-ray photoelectron energy dispersive spectrometer (XPS) was
employed for qualitative and quantitative analysis of the elemental
valences of the powder sample. A JEM-2100F type field emission
high-resolution transmission electron microscopy (HRTEM) equipped with
an energy dispersive spectrometer (EDS) was used to observe and analyze
the morphology, composition distribution, and crystal structure of the
powder sample. Before the TEM analyses, the powder sample was first
ground for 10 min, ultrasonically dispersed in alcohol, dropped on a
carbon film, and finally glued on a copper net. By the way, for the
planar spacing measurement, three points were tested to calculate the
average value.
The densities of the sintered AlN ceramic specimens were determined by
the Archimedes method. The phases of the AlN specimens were identified
by XRD, using the same test parameters above. The thermal conductivities
of the AlN specimens of 10 mm×10 mm×2 mm in shape were measured at room
temperature, employing an LFA447 type laser thermal conductivity meter.
The bending strengths of the AlN specimens of 40 mm×3 mm×2 mm in shape
were measured in accordance with the China national standard GB/T
6569-2006 (ISO14704:2000) “Fine ceramics test method for flexural
strength of monolithic ceramics at room temperature”. The test was
conducted using an AG-X PLUS type microcomputer control electronic
universal testing machine with a testing span of 30 mm and a beam
displacement rate of 0.5 mm/min. The bending fracture morphologies and
microzone compositions of the AlN specimens were tested by SEM and EDS,
respectively.
Results and Discussion
Morphologies and compositions of the AlN powder
After calcination at 1700 °C for 12 h and then decarbonization, most of
the AlN products were light grey on the outside and at the fracture
surfaces. However, a small amount of unsatisfactory AlN products exist
with dark grey outer and fracture surfaces (Figure 1).