The mechanism of synthesis of titanium nitride by self-sustaining reactions
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I. INTRODUCTION
Because of their exceptionally high thermodynamic stabilities, many refractory carbides, nitrides, and borides can be prepared by self-sustaining reactions. This method of preparation, commonly referred to as self-propagating high-temperature synthesis (SHS) has been the focus of many investigations during recent years. Details of this method appear in two recent reviews,1'2 and recent research activities in this area are described elsewhere.3 In the case of refractory metal nitrides, selfsustaining reactions can be achieved between the metals and gaseous nitrogen even at relatively low gas pressure (e.g., 1 atm). However, despite the ease with which self-sustaining reactions can be initiated and propagated, the process of combustion synthesis of refractory metal nitrides is complex. In part, this is due to the nature of the phase equilibria between nitrogen and the refractory metals. As shown by the example of the Ti-N system,4 Fig. 1, extensive solid solubility between the refractory metal and nitrogen exists. Both the a (hep) and /3 (bec) phases of Ti form solid solution, with the former having a more extensive compositional range up to —22 at. % N. In addition, the nitride phase TiN exists over a relatively large region of nitrogen content (-28-50 at.% N). The complexity of the phase equilibria between refractory metals and nitrogen has focused attention on the mechanism of the self-sustaining reactions and the nature of the product(s) of combustion. In general, SHS reactions between gases and solids are not confined to the combustion wave front and in fact a major portion of the interaction takes place after the passage of the front, as has been observed in the case of the Ti-N2 reaction.5 Moreover, the nature of the reaction associated with the combustion front appears to depend on the specific refractory
metal. It has been reported that in the case of zirconium and vanadium, the reaction at the wave results in the formation of the appropriate solid solution with nitrogen.6"8 The implication here is that the dissolution of nitrogen in these metals is significantly exothermic to lead to self-propagating reactions.8 In contrast, the propagation of the wave in hafnium and tantalum is reported to result in the formation of HfN* and Ta2N, respectively.7'9 The fact that the passage of the wave represents a relatively small fraction of the total reaction between nitrogen and the refractory metal leads to the conclusion that the phases formed during the passage of the wave are not necessarily the final products of combustion. Experimental verifications for this conclusion have been provided.10'11
Atomic Percent Nitrogen 20
*
30
40
3290 20.9
L /
/
JO
O
2000
-
2350X // / 101 "4.98/ 7.0
^2020
V •/7[if
TiN
U.O
•
—1670°C
a.
7/
/
(ceTi) ""
BSZC
8
JV
-12.7% 1100°C
A
1 12.6/
Ti2N —
r \
-
15 16
51
n
l800°C 0
5
10
15
Weight Percent Nitrogen FIG. 1. The Ti-N phase diagram. 4
J. Mater. Res., Vol. 5, No. 10, Oct 1990
http://journals.cambridge.org
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