DC Plasma Synthesis of Aluminum Nitride Ceramic Powders
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DC Plasma Synthesis of Aluminum Nitride Ceramic Powders Z . P. Lu and E. Pfender Department of Mechanical Engineering University of Minnesota Minneapolis, Minnesota 55455, U. S. A. A novel Triple DC Torch Plasma Reactor (TTPR) has been developed for the plasma synthesis of fine ceramic powders. The reactor consists of three identical plasma torches. The plasma jets from these torches form a converging plasma volume into which the starting powder is fed. Thermodynamic equilibrium calculations have been performed for predicting the behavior of an aluminum-nitrogen system. Aluminum nitride has been synthesized by using the TTPR operating in the non-transferred mode. Product characterizations indicate that single hexagonal phase AIN ceramic powder has been obtained. INTRODUCTION AIN ceramics have a relatively high thermal, but very low electrical conductivity [1], and they have approximately the same thermal expansion coefficient as silicon [2]. Because of these properties this material is attracting increasing attention in the electronic packaging industry [3]. This man-made ceramic material has been synthesized by carbothermal reduction of alumina. Thermal plasma synthesis of AIN has been studied over the past decades because the unique features of thermal plasmas, such as high energy content and controlled atmosphere, are useful for producing a high purity product. A brief review of the history of thermal plasma synthesis of AIN has been given in a previous paper [4]. In the present work, AIN has been synthesized in a novel Triple Torch Plasma Reactor. The characterizations of the product agree with the predictions from the equilibrium thermodynamic considerations. Single hexagonal phase AIN is confirmed by X-ray powder diffraction and TEM micrographs. Both TEM and centrifugal sedimentation reveal that the powder produced has a submicron median size (number-wise). EQUILIBRIUM MODELING OF THE SYNTHESIS A computer program, SOLGASMIX-PV [5], was used for calculating the chemical equilibria of a nitrogen-aluminum system with various aluminum to nitrogen ratios. The results show that for the system with an aluminum to nitrogen mole ratio smaller than 0.5, the reaction 2 Al + N 2 -- 2 AIN is complete. One of the computational results is shown in Fig. 1. The results also show that AIN is the only solid phase product below 2700 K, i.e. complete conversion is possible. Because there is no liquid phase of AIN, gas phase AIN begins to sublimate at a temperature of 2700 K and a proper quenching temperature around 2500 K is suggested.
id0
R Mole Al + 0.5 Mole N2
P = 1 atm.
0u•
N2(G) 10
N(G). /O, -(G) or
102I+(G) "G13" 'A
AI(G 104 Q 105 100 2000 3000 4000 Temperature(K)
AIN 0 5000
6000
Fig. 1: Equilibrium Composition of an Al-N System
EXPERIMENTAL SETUP AND RESULTS Due to the short residence time in the plasma region, full evaporation of the solid particles, which is essential for vapor phase reactions requires small particle sizes (micron size). However, feeding of such small powder of raw material represents a sever
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