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).