Pulse Electric Current Sintering Of AIN Transparent Ceramics

Because aluminum nitride (A1N) demonstrates high thermal conductivity, dielectric constant, thermal expansion match to silicon, good electrical insulation properties and other potential properties, it is expected to be a high-performance ceramic substrate and packaging material for high-power, high-speed microelectronics. However, as a covalent chemical compound with the wurtzite crystal structure, A1N is not easy to sinter. Densification of A1N is typically achieved by the addition of alkaline earth oxides, rare earth oxides or mixtures of oxides, fluorides or carbides. There are little reports about sintering of transparent A1N ceramics at home and abroad.Pulse electric current sintering (PECS), developed by Sodick, Ltd., of Japan, is a recently developed densification method which consists of a pulsed electrical discharge step when plasma is generated between powder particles, followed by resistance heating while a uniaxial pressure is applied. The following are expected merits of PECS: rapid heating and cooling; short sintering time; lowering sintering temperature. This process has been successfully used to sinter FGM, MMC, FRC and so on.The most important condition of sintering A1N is the purification of A1N powder, especially the content of Si, Fe and O purities. At the same time, the A1N powder should also present high fine and scatter properties.The samples were sintered at 1750 C, 1800 C and 1850 C. The measured relative density ranged between 93.1% and 99.2%. With the increase of sintering temperature, the relative density and transmittance of the samples was also increasing. The fracture SEM micrographs of A1N ceramics shown that grains became more uniform along with the increase of sintering temperature.The samples were sintered for 5min, 8min, 10min and 15min. The measured relative density ranged between 93.5% and 99.2%, With the prolong of soaking time d, density and transmittance increased and then decreased. Density became a little higher and a little grain growth was found with the prolong of soaking time.The samples were sintered with heat rate of 50 C/min, 80 C/rnin, 120 C/min, 150 C/min. The measured relative density ranged between 97.2% and 99.2%, showing that high density could achieve with any heat rate. The fracture SEMmicrographs of A1N ceramics shown that decreasing heat rate could help the growth of grain, but it hadn't much effect to the properties of the samples.The samples sintered in N2 and in vacuum were almost the same. But the transmittance of the sample sintered in N2 was better than in vacuum. So A1N transparent ceramics must sintered in N2.TEM observation displayed consolidated and uniform structure, which contributed to transmittance of A1N transparent ceramics. From TEM micrographs, dislocation, pores, impurity elements were also observed, which were adverse to transmittance of A1N transparent ceramics.Sintering additives could help decrease the sintering temperature and increase the density and transmittance of the A1N ceramics. CaF2 was a good additive, which could volatilize from the crystal lattice and purify the crystal lattice in the process of sintering.The samples were sintered with 2%, 3% and 4% CaF2. The result of experiment shown that with 3% CaF2, the sample had the highest transmittance, which was 59.6%.The study on the transparent mechanism and transmissivity factors of transparent ceramics indicates that the absorption coefficient, reflectivity and scattering coefficient all have effect on the transmittance of transparent ceramics. And the most important factor was scattering coefficient, especial those of pores, secondary grain boundary phase and impurities.