Plasma Activated Sintering Of Aluminum Nitride Ceramics

Aluminum nitride?AlN?ceramic is an ideal packaging and heat dispersing material which demonstrates excellent performances.It is appropriate for large scale integrated circuits and high power devices.However,its application and development are limited by its characteristics of easily becoming oxidized and being difficult to sinter.To obtain high purity and high density AlN ceramics,plasma activated sintering?PAS?is used to densify the AlN powders and NH₄F reactant is used to deoxidize during sintering.First,aluminum nitride micro powders were sintered by the plasma activated sintering.The influences of process parameters on phase compositions,elements compositions and morphologies were systematically discussed.The plasma activated sintering process of aluminum nitride ceramic could be divided into three stages,namely,AlN powders expand with heat at low temperatures,the particles rearrange and the mass transfer from 900°C to 1400°C and the density of AlN increases furtherly at high temperatures.The decrease of heating rate,the prolongation of holding time,the using of high purity N₂ as sintering atmosphere,as well as the increase of axial pressure and sintering temperature are beneficial to the sintering of aluminum nitride.The most appropriate process parameter was determined as follows:1800°C-40MPa-5min-100°C/min.Relatively high density?>98%?AlN ceramics were obtained under the condition.Second,NH₄F was added during plasma activated sintering of AlN powders.The effects of NH₄F addition amount on phase compositions,microstructure of grains,O-N contents,and thermal properties were explored.According to the Thermodynamic calculation result,NH₃ which was released by NH₄F could react with the Al₂O₃ impurity in AlN powder and could inhibit the reaction between Al₂O₃ and AlN.The experiments showed that the addition of NH₄F effectively reduced the oxygen content of AlN ceramics,purified the grain boundaries and change shape of the grains from lamellar to hexagonal.The appropriate molar ratio of NH₄F/O was determined to exceed 0.8.AlN ceramics with enough NH₄F demonstrate dense structures,well-growth grains,clear grain boundaries and O-N contents near to theoretical values.NH₄F showed purification effects on the phases and the improvement effects on the microstructure of AlN micro powder and AlN nano powder.The addition of NH₄F not only improves the density of sintered body but also greatly benefits the growth and purification of AlN crystal as well as the thermal conductivity of AlN ceramics.