Kinetics of In-situ formation of AlN in Al alloy melts by bubbling ammonia gas

Abstract

In-situ processing of AlN-Al alloy composites by the gas bubbling method was investigated using ammonia as the gaseous precursor in the temperature range of 1373 to 1523 K. The products were characterized using X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray microanalysis. It was found that in-situ formation of AlN reinforcing particles was feasible by bubbling ammonia through aluminum and aluminum alloy melts. The AlN particles formed in situ were small in size and enriched in the top part of the product formed in the crucible. In comparison with the purified nitrogen bubbling gas, using ammonia as the nitrogen precursor enhanced the formation of AlN. Hydrogen gas generated from the dissociation of ammonia is an effective oxygen getter in the system, which can offset the deleterious effect of oxygen impurities and enhance the formation of AlN. The rate of formation of AlN was controlled by the diffusion of nitrogen atoms in the liquid boundary layer. A kinetic model was developed to describe the rate of formation of AlN, which was in excellent agreement with the experimental results. The influences of various processing variables on the rate of formation of AlN were also investigated. The rate of formation of AlN changed little with the content of silicon in the matrix melt, increased slightly with increase in temperature and decrease in nozzle size, and increased significantly with the increasing gas flow rate.

In situ synthesis of cold-rollable aluminum–aluminum nitride composites via arc plasma-induced accelerated volume nitridation

“…34 h AlN in the gas bubbling method was calculated by the equation, h AlN 5 a D N 0.5 C N,i (a is constant) since the formation of AlN occurs mainly at the gas-melt interface and C N is much smaller than C Al in Al melt (surface nitridation). 6 Based on the same assumption, h AlN in the APAVN process was calculated to be about ten times higher than that in the gas bubbling method since T N was higher over 1000 K, and C N,i was about 5.6 times greater. However, it was insufficient to explain about 400 times higher specific weight gain normalized by t N in the APAVN process compared to that in the gas bubbling method.…”

“…Since the AlN formation rate (h AlN ) in the gas bubbling method was evaluated by considering concentration and diffusion of the dissolved N in Al melt, 6,8 the predominance of N diffusion in this study was investigated by determining concentration profiles. The diffusion of N in Al melt was estimated by Fick's second law as follows:…”

“…6 R is gas constant and T l is temperature of Al melt. With boundary conditions (at x 5 0, C N 5 C N,i and at x 5 ∞, C N 5 0), Eq.…”

“…7 Although P N of APAVN process (0.2 atm) was lower than that of the gas bubbling method (1.0 atm), APAVN process exhibited higher C N,i (9.22 Â 10 À4 mol/L) than the gas bubbling method (1.65 Â 10 À4 mol/L) 7 due to the difference in k in Eq. (6). In APAVN process, the k can be assumed as 1 since dissociated N by arc plasma is instantaneously dissolved into Al melt without overcoming activation energy (Step 2 in Fig.…”

“…1 However, AMCs manufactured by conventional ex situ processes are economically unfavorable due to expensive reinforcing materials and thermodynamic instability resulting from interfacial reaction between matrix and reinforcements. 2 Thus, in situ fabrication of AMCs with a ceramic second phase such as TiC, Al 2 O 3 , and SiC has been investigated for several decades and various processes like self-propagating high temperature synthesis, 3 directed melt oxidation, 4,5 or gas bubbling method 2,[6][7][8][9][10] have been developed as a feasible process depending on the formation of ceramic second phase. Recently, aluminum nitride (AlN) has been considered as an attractive second phase in AMCs due to excellent TC and low CTE.…”