| Because aluminum nitride (A1N) demonstrates high thermal -conductivity (theory thermal conductivity is 320W/(m.k)), dielectric constant (about 8.8), thermal expansion match to silicon(293-773K, 4.8 X 10-6K-1), 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. A1N will replace other substrate materials such as SiC, BeO, Al2O3, which are now used because of their high thermal conductivity. 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.Plasma spark sintering (SPS), 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 SPS: rapid heating and cooling; short sintering time; lowering sintering temperature. This process has been successfully used to sinter FGM, MMC, FRC and so on. In this paper, SPS was selected to reduce the need for additives and to rapidly and successfully sinter transparent A1N ceramics.Experiment condition: without any sintering aids, at a pressure of 25MPa, sintered at 1700-1850, with heat rate of 100-550/min for 4-20min, in vacuum of 6Pa or in nitrogen of 0.56 X 105Pa.The samples were sintered at 1700, 1730, 1800 and 1850. The measured relative density ranged between 95.9 and 99.3%, demonstrating that high density could achieve just at 1730. The fracture SEM micrographs of A1N ceramics shown that grains became more uniform along with the increase of sintering temperature.The samples were sintered with heat rate of 100/min, 150/min, 200 /min, 300/min, 550/min. The measured relative density ranged between 97.6 and 99.1%, and shown that high density could achieve with any heat rate,demonstrating that SPS technique could short consolidation process and could achieve quick sintering.The samples were sintered for 4min, lOmin, ISmin and 20min. The measured relative density ranged between 97.5 and 99.1%, demonstrating that high density could achieve just for 4min. Density became a little higher and a little grain growth was found with the prolong of soaking time.The samples were sintered in Na of 0.56 X 105Pa. The fracture SEM micrographs of A1N ceramics shown that grains grow better in nitrogen.The test results of X-ray, XPS and EPMA indicated that the ceramics prepared by this technique had high purity. Optical micrographs of A1N ceramics displayed uniform microstructure and fine grains. Together with high density measured by the Archimedes method and consolidated structure viewed by SEM, demonstrated that the sintered samples basically met with transparent ceramics sintering conditions.The samples were transparent observed by naked eyes after coarse-grinded, fine-grinded and polished to about O.5Omm-thickness.The samples were transparent in infrared wavelength(2.5-25 u m). The maximum transmittance was changed with heat rate, soaking time and sintering temperature. With the slow of heat rate, prolong of soaking time and enhanced sintered temperature, the maximum transmittance was increased.TEM observation displayed consolidated and uniform structure, which contributed to transmittance of A1N transparent ceramics. From TEM micrographs, dislocation, intergranular pores, pores in crystals and secondary grain boundary phase were also observed, which were adverse to transmittance of A1N transparent ceramics.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 tran...
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