Processing, Microstructure And Thermal Properties Of AlN Ceramics And Ain Powder Prepared By Modified Carbothermal Reduction Method

Aluminum nitride (AlN) has attracted large interest recently as a suitable material for hybrid integrated circuit substrates because of its high thermal conductivity. However AlN ceramics with high thermal conductivity are mostly determined by the quality of AlN powder. High nitridation temperature is required to obtain high quality AlN powders in conventional carbothermal reduction reaction, hence it is essential to modify and improve the carbothermal reduction process for decreasing production cost.The paper consists of synthesis of AlN micron powder via carbothermal reduction method and AlN nanopowder prepared from low temperature precursors. The effects of reaction temperature, atmosphere and various aluminum sources on composition and microstructure of AlN micron powder were studied. The formation process, effect factor and mechanism of AlN nanopowder prepared from precursors were systematically discussed. Sintering characteristics of AlN powder, thermal conductivity of AlN and hot-pressed bulk AlN are also investigated.AlN micropowders were prepared by carbon thermal reduction method in this paper Thermal stability of condensed phase in Al₂O₃-C-N₂ system was analyzed by thermodynamics calculation. Based on the thermodynamics analysis, effects of starting raw materials such as γ -Al₂O₃, Al(OH)₃ and empholite on phase, particle size and microstructure were discussed. The aids and reaction temperature in carbon thermal reduction reaction have been explored, and the optimal contont of AlN seed and CaF₂ were ca. 5wt% and2.5³.5wt%, respectively. AlN powders synthesized by carbothermal reduction method were characterized by XRD, SEM, BET, IR spectrum and chemical analysis methods. The results show that activated carbon and Al(OH)₃ have made a contribution to nitridation reaction rate andnitrogen content. AIN powders were obtained with the content of 33.3wt% nitrogen, 1.18wt% oxygen and 0.12wt% residual carbon at 1550°C for 6 h, employing Al(0H)3 and activated carbon as starting material in graphite tube furnace.Precursors containing atomic level uniform mixing of alumina and carbon were obtained by low temperature combustion of aluminum nitrate-urea-sucrose (starch or glucose) solution. A combustion mechanism is proposed via HREM, EDS and IR spectrum: firstly a reaction intermedium of A1(NO3)3 x ? CO(NH2)2 * yH2O is formed in precursor solution, then in the process of combustion the intermedium transformed into amorphous alumina, and carbon could be formed from glucose/sucrose/starch in aqueous mixture by pyrolysis at high temperature or dehydration in the condition of strong nitric acid.Ultrafine AIN powders were synthesized from low-temperature combustion precursors, which were thermochemically converted to AIN powders by calcining in a flowing nitrogen stream. It was found that nitridation began at much lower calcination temperatures (1300°C) than in conventional carbothermal reduction. The crystallites of the as-synthesized AIN powders are very small, ranging from 60 to 100 nm, and are not agglomerated. AIN powder prepared at 1500°C for 3 h. Its size distribution is very narrow with specific surface area of 21.91-30.5m2/g.The synthesized powders were characterized by XRD, SEM, XPS, PL, TEM,HREM, NMR and elemental analysis. The lower calcination temperatures are attributed to the formation of high surface area Y -alumina during heating of the precursor powders to the synthesis temperature. In addition, the lower formation temperature of AIN in nitrogen gas is attributed to the direct nitridation of amorphous or Y -phase alumina without the formation of a -phase alumina, because solid solution of carbon into AIN crystal detain crystal transformation of Y -phase alumina.Sintering characteristics of AIN powder are systematically studied. For AIN samples after pressureless and free-additive sintering, high density (-98%) was obtained for nano-size AIN at 1700°C, and the micro-grade AIN was not densified at even 1800°C. For the AIN samples after pressureless and 3wt% Y2O3-doped sintering, nano-size AIN powder could be fully densified at low sintering temperature about 1700°C, which is lower than that of micro-grade AIN at 1800°C.The low-temperature sintering for nano-size A1N powder proceeded in two stages. The first stage were attributed to particles rearrange and interdiffusion between yttrium aluminates, then the second stage was due to liquid-phase sintering in Al-O-Y-N system.The powder was pressureless sintered at 1800 °C for 4 hours to neartheoretical density. The measured thermal conductivity of the sintered samples was 168 W/m ? K. For the nano-AIN samples undoped with aids, its measured thermal conductivity was 56 W/m ? K. However The measured thermal conductivity of the samples (micrograde A1N) doped with 5wt% Y2O3 was 97.6W/m ? K at the same sintering condition. This result differs from different microstructure of sintered samples. High thermal conductivity for A1N nanopowder compares favorably with the best values reported in the literature. However, on theoretical grounds, the thermal conductivity of pure A1N at room temperature is predicted to be 320 W/m ? K. Future work will attempt to achieve this theoretical goal.Bulk texture A1N with preferential orientation perpendicular to (002) plane parallel to c-axis and pressing direction was obtained. This bulk A1N was produced from nanosize A1N powders by hot pressing at 2000 °C for 2 h under 35 MPa. The texturing of nanosize bulk A1N are probable attributed to stick-like original A1N grains which act as self-seed of preferential orientation via dissolve and precipitate transport, and that the growth rate of nuclei oriented with self-seed stick-like grain parallel to hot pressing direction are fastest under high pressure.Preferential orientation with (002) plane for nanosize bulk A1N progressed as following: (1) random arrange of nanosize A1N grains during initial sintering; (2) nuclei on the initial A1N grains via dissolve-precipitate transport; (3) nuclei and grain grow under high pressure; (4) preferential orientation with (002) plane occurs since the growth rate self-seeded stick-like A1N grains dominate along the pressing direction. The nuclei growth rate of nanosize A1N oriented with (002) plane are faster because of high surface energy and small particle size.