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Combustion nitridation of titanium wires in liquid nitrogen
Materials Research Bulletin, Vol. 32, No. 10, pp. 1341-1347, 1997 Copyright 0 1997 Elsevier Science Ltd Printed in the USA. All rights reserved 0025540&97 $17.00 + .OO
PII SO0255408(97)00112-S
COMBUSTION NITRIDATION OF TITANIUM WIRES IN LIQUID NITROGEN
M. Shihuya*, J.F. Despres, and 0. Odawara Department of Electronic Chemistry, Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama 226, Japan (Communicated by M. Koizumi) (Received September 26, 1996; accepted December 16, 1996)
ABSTRACT Titanium nitridation in liquid nitrogen has been investigated by heating titanium wires electrically with various electric powers to make clear the reaction characteristics and the possibility of self-sustaining reaction occurrence. It has been possible to define the critical electric power necessary for an initiation of Ti wire combustion. The value was determined to be 1.58 W/mm2 for the wires 0.1-0.3 mm in diameter tested in the present work. The temperature of the combustion initiation was confirmed to be close to the melting point of Ti. The Ti wire heated at the critical electric power in liquid N, could be almost completely nitrided within one second. copyrighr6 1997 Elsevier Science L.td
KEYWORDS: A. metals, A. nitrides, C. electron microscopy, D. thermodynamic properties INTRODUCTION By using combustion synthesis technology [ 1,2] which is characterized with self-sustaining exothermic reaction, it is possible to obtain high-temperature materials such as carbides, borides, and nitrides in a short time. Among such materials, TiN is superior in hardness, chemical stability, corrosion resistance, and electric conductivity. Nitrogen sources used in TiN combustion synthesis have been gaseous N, [3-81, liquid N, [9], and solid sodium azide [lo]. The achievable conversion was less than 40% when Ti compact reacted with gaseous N, at atmospheric pressure. Therefore, a method of reaction in
*To whom correspondence should be addressed. 1341
1342
M. SHIBUYA et al.
Vol. 32, No. 10
FIG.1 Equipment for nitridation of Ti wire with liquid nitrogen. (a) Constant electric power supply. (b) Transient recorder. (c) Standard resistance (1 a). (d) Dewar vessel. (e) Ti wire. gaseous N, at high pressure (400 MPa) [3] was proposed for full conversion. However, full conversion could not be achieved even in such environmental applications, because the penetration rate of N, into Ti powder compact is reduced by increasing liquid Ti phase at the combustion zone followed with the increase of combustion velocity. The combustion velocity was decreased by increasing the packing density of powder compact [4-71, which improves the conversion rate. Utilizing NaN, as a solid source of nitrogen was also proposed for full conversion in which the environment pressure is sufficient at atmospheric pressure [lo]. Addition of diluent TiN was proposed in order to decrease the maximum temperature to prevent appearance of a liquid phase during the reaction process. In this way, fully converted TiN could be obtained under gaseous N, at more than 0.5 MPa [8]. However, the disadvantages of these methods include impurities in final products and the need for high-pressure equipment. In previous work [9] carried out on combustion synthesis of TiN in liquid N, under atmospheric pressure, highly converted TIN was obtained by controlling the reaction process without any special pressurized furnace. However, the reaction process was not made clear. We must understand the initiation phenomena of Ti combustion in liquid N, during the induction period, in order to obtain a good product by combustion synthesis of TiN in liquid N,. Ti wire was used under gaseous N, to determine initiation phenomena in previous work [6]. In the present work, the temperature effect for Ti wire combustion in liquid N, was investigated by electric power supply, to clarify the initiation phenomena of the TiN combustion synthesis in liquid N,. We have determined the temperature gradients, the minimum electric power (critical value: P,J which is necessary to initiate Ti wire combustion, and the morphology of products in the case of liquid N,. EXPERIMENTAL As the scheme of the apparatus in Figure 1 shows, Ti wire was placed between two electrodes and a dewar vessel was filled with liquid N2. The Ti wire was 0.1,0.2, or 0.3 mm in diameter
Vol. 32, No. 10
TITANIUM NITRIDE
1343
Time [msec] FIG. 2 Thermograms of Ti wire (4: 0.2 mm) at various constant electric powers in liquid nitrogen. P[W/mn?]: (a) 2.17, (b) 1.63, (c) 1.60, (d) 1.50, and (e) 1.08.
and 66 mm in length, with 99.8% of Ti metal. Electric power of 30-100 W was passed through th,e Ti wire by use of a constant electric power supply. The electrical parameters of the wire were measured using a four-probe method [l l] and recorded with a transient recorder. ‘The sampling rate was 6800 cps. The temperature gradient of the Ti wire was estimated from Ti wire resistivity by means of an electrothermographic method [ 12,131. X-ray powder diffraction patterns were measured by a stepped angle scanning method (scan speed 1.2”/min, scan step 0.02”, angle 30”-80”) with a RAD-C apparatus. A cross section of the heated wire was observed by optical microscopy after polishing and adequate etching. Analysis of the surface and cross section of the products was made using scanning electron microscopy (SEM).
RESULTS AND DISCUSSION In the case of combustion synthesis in liquid N,, the reaction was thought to be inhibited by increased escape of the heat generated. Liquid N, has a very high density (under atmospheric pressure), the density of liquid N, is 808 g/dm3 (77 K) and that for gaseous N, is 1.25 g/dm3 (273 K). The reaction between Ti and nitrogen gas is Ti + l/2 N, + TiN - AH. The heat generation, approximately 336 kJ/mol, is sufficient to evaporate liquid N,, the heat evaporation of which is approximately 5.6 kJ/mol. If the reaction can be induced, self-sustaining occurs. The Ti phase transition from OLtype (hexagonal close packed structure) to p type (body-centered cubic structure) occurs at 1155 K, and its melting point is 1943 K. Ti nitridation mechanism on solid-gas phase reaction is reported [14,15] in which the growth of TiN phase takes place after the penetration of N, into cx phase (homogeneous concentration of N, into (Yphase). Figure 2 shows the thermograms of Ti wire (0.2 mm in diameter) at various constant electric powers in liquid nitrogen. In Figure 2, curves a, b, and c show that the temperature rapidly increases after melting point of Ti, via phase transition point. The greater the electric power, the shorter is the keeping time at melting point. Curve d in Figure 2 shows the case of no-ignition, in which the wire temperature does not increase after melting point. In this case, penetration of N, into Ti is prevented because an
Vol. 32, No. 10
M. SHIBUYA et al.
1344
TABLE Effect
of Various
1
and Electrical
Diameters
Powers on Temperature
0.1
d (mm)
and Time 0.3
0.2 t(msec)
P(W/mm*)
t1
t2
t3
4
t2
t3
t1
t2
t3
1.84 (P > P,,) 1.58 (P = P,,) 1.34 (P c P,,)
70 95 625
95 135
10 40
85 200 780
120 280
5 240
30 125 780
80 230
10 70
t,(msec):
77-l
155 K, t, (msec): 77- 1943 K, t, (msec): sustaining time at 1943 K.
impermeable TiN film on the wire surface is generated for a long keeping time at melting point and this film does not break with increasing temperature [6]. Curve e in Figure 2 shows low-rate nitridation in which combustion does not occur at the low temperature. Therefore, it is obvious that the initiation of TiN combustion synthesis occurs at the melting point. The value of P,, required to ignite Ti wire 0.2 mm in diameter is equal to 1.60 W/mm’. In the same manner, the value of P,, for Ti wire 0.1 in diameter is 1.56 W/mm2 and that for Ti wire 0.3 in diameter is 1.58 W/mm2. In other words, the P,, values of the Ti wires 0.1-O-3 mm in diameter are approximately equal. Table 1 shows the effect of the various diameters and electrical powers on temperature and time: t, is the necessary time to reach phase transition (1155 K) from liquid nitrogen temperature (77 K) and b is the necessary time to reach Ti melting (1943 K) from 77 K. T, is the sustaining time at 1943 K. Sustaining time (ts) at melting point of Ti is very short over P,, On the other hand, it is apparent that the sustaining time (ts) is relatively long at P,, These results confirm that combustion synthesis in liquid N, as well as in gaseous N, is possible. The value of our P,, is coherent with P,, in the case of gaseous N, (Ti wire 0.1 mm in diameter), which is equal to 1.43 W/mm2 [6]. The difference can be explained by vaporization and heating N, (from 77 K to room temperature). Figure 3 shows X-ray diffraction pattern of a Ti wire (a: 0.3, P = P,,) heated in liquid nitrogen. We used several wires at P,, to detect phases. The products basically consisted of ol-Ti and TiN; @-Ti was not observed.
TiN 0 aTi
0
l
20 FIG.
3
XRD pattern of heated Ti wire (@ 0.3 mm, P: 1.58 W/mm2) in liquid nitrogen.
Vol. 32, No. 10
TITANIUM NITRIDE
FIG. 4 SEM photographs of the cross section and surface of heated Ti wire (@: 0.2 mm) in liquid N, at various electric powers: (a) 2.17 W/mm*, (b) 1.60 W/mm*, and (c) 1.08 W/mm*.
Figure 41shows the SEM photographs of the cross section and surface of heated Ti wire 0.2 mm in diameter in liquid N, at various electric powers. The electric powers represented in Figures .4(a), (b), and (c) are 2.17, 1.60 (PC), and 1.08 W/mm*, respectively. In Figure 4(a), the cross section of a wire after reaction over critical power exhibits three different morphologies. We observe acicular crystals in the center, a thin white film on the surface, and an amorphous8 shape without peculiar texture between these parts. The surface of the wire in Figure 4(a’) shows a white coating with some round outgrowths. Some exploded round shapes reveal that the surface of the wire is covered with bubbles. The cross section in Figure
1346
Vol. 32, No. 10
FIG. 5 SEM photograph of a cross section of the deformed edge of heated Ti wire (@: 0.2 mm, P: 1.60 W/mm’) in liquid N,.
4(b) shows very few acicular crystals, but the nontextured part seems bigger than that in the case of reaction over P,,. The cross section appears much more homogeneous; we observe some small holes near the center. On the surface of the wire in Figure 4(b’), the white film is homogeneous and no bubbles are observed. Figure 4(c) shows a cross section of a Ti wire with a low rate of nitridation. Some acicular crystals can be seen from the center until the edges of the wire, and small holes can be observed in the center. Figure 4(c’) shows the aspect of the surface. In this case, it looks like Ti metal. Each heated wire shows small holes in the center. This phenomenon is simply explained by shrinkage during cooling. On the other hand, the big cavities result from titanium vaporization [16]. Each wire shows the acicular phase that characterizes (Yphase of Ti metal obtained after heating over transition temperature (cx + p at 1155 K) and a cooling. The p phase is transformed in o-acicular phase at the same transition temperature. The cw-acicular phase gets anisotropic shape because the growth is leading by some preferential planes in p phase. The o-acicular phase is easily revealed by optical microscopy after polishing and adequate etching. The nontextured phase is only seen at the place very close to the ignition point. This is thought to be the result of fast quenching. Sustaining time at melting point and the conversion rate decrease with power increasing at over P,,. Heat generation is weaker at over P,, than at P,,. Because of these facts, we can interpret that more rapid quenching until inside, when the electric power is near P,,, leads to a large nontextured part. This, combined with the continuous decrease of transition temperature with cooling rate increasing [17] or the increase of transition temperature with presence of nitrogen [18], allows us to suppose that the nontextured phase is B phase or a-equiaxed phase (primary o). From some X-ray diffraction data, it seems that it must be primary OL,but this is still under discussion. Figure 5 shows a SEM photograph of the cross section of the deformed edge of a Ti wire. We recognize the nontextured part and a trace of a cavity is visible on the left. This picture confirms that the cross section of the wire is bent in the diameter direction. One of the reasons for this effect may be the well-known cracking of nitride film combined with vaporization of Ti [ 161. Another hypothesis is that it results from the increasing pressure of vaporized liquid N, around melting Ti wire during heating.
Vol. 32, No. 10
TITANIUM NITRIDE
1347
CONCLUSION In this work, it has been possible to define the PGr necessary for initiation of Ti wire combustion in liquid N,. The temperature at initiation of Ti wire combustion was confirmed to be close to the melting point of Ti in gaseous N,. The P,, does not depend on the diameter in the range 0.1-0.3 mm. The value of P,. is approximately 1.58 W/mm’, a little bit more than with. gas (1.43 W/mm2). In the case of an electric power greater than P,,, the heated Ti wire in lilquid N, clearly exhibited TiN film and o-acicular phase, based on X-ray diffraction data and SEM photographs of the cross section and surface. On the basis of these results, we can hope to obtain good conversion to some materials by combustion synthesis in liquid N2.
ACKNOWLEDGMENT The authors are grateful to Mr. M. Ozaki in the Department of Chemistry at Tokai University, for his skillful assistance. REFERENCES 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
A.G. Metzhanov and I.P. Borovinskaya, Dokl. Chem. 2&I(2), 429 (1972). A.G. Merzhanov and I.P. Borovinskaya, Combust. Sci. Technol. 10, 195 (1975). I.P. Borovinskaya and V.E. Loryan, Poroshk. Metal. 11(191), 42 (1978). Z.A. Munir, High Temp.-High Press. 20, 19 (1988). M. Eslamloo-Grami and Z.A. Munir, J. Am. Ceram. Sot. 73, 1235 (1990). H. Kudo and 0. Odawara, Proc. 32nd Jpn. Congress of Muter. Res., 168 (1989). H. Kudo and 0. Odawara, J. Mater. Sci. 24, 4030 (1989). M. Eslamloo-Grami and Z.A. Munir, J. Am. Ceram. Sot. 73, 2222 (1990). 0. Odawara, T. Narita, and W. Hua, J. Chem. Sot. Jpn. 10, 1461 (1991). J.B. Holt and D.D. Kingman, Muter. Sci. Res. 17, 167 (1984). U. Seydel and W. Fucke, J. Phys. F: Metal Phys. 10, L203 (1980). L.T. Abramova, V.G. Abramov, and A.G. Metzhanov, Russ. J. Phys. Chem. 43(5), 647 (1969). A.G. Merzhanov, AIAA Paper No. 74-144, 1 (1974). S.G. Vadchenko and Y.M. Grigor’yev, Russ. Metal. 1, 156 (1979). M. Otto and T. Yoshida, Nippon Kagaku Gakkaishi 2, 195 (1979). A.M. Mellor and I. Glassman, Pyndynamics 3, 43 (1965). M.J. Bibby and J.C.T. Parr, J. Inst. Metals 92, 341 (1964). S.Z. IBokshtein, T.A. Emelyanova, S.T. Kishkin, and L.M. Musky, The Science, Technology and Application of Titanium, p. 43, Pergamon Press, Oxford (1970).
PII SO0255408(97)00112-S
COMBUSTION NITRIDATION OF TITANIUM WIRES IN LIQUID NITROGEN
M. Shihuya*, J.F. Despres, and 0. Odawara Department of Electronic Chemistry, Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama 226, Japan (Communicated by M. Koizumi) (Received September 26, 1996; accepted December 16, 1996)
ABSTRACT Titanium nitridation in liquid nitrogen has been investigated by heating titanium wires electrically with various electric powers to make clear the reaction characteristics and the possibility of self-sustaining reaction occurrence. It has been possible to define the critical electric power necessary for an initiation of Ti wire combustion. The value was determined to be 1.58 W/mm2 for the wires 0.1-0.3 mm in diameter tested in the present work. The temperature of the combustion initiation was confirmed to be close to the melting point of Ti. The Ti wire heated at the critical electric power in liquid N, could be almost completely nitrided within one second. copyrighr6 1997 Elsevier Science L.td
KEYWORDS: A. metals, A. nitrides, C. electron microscopy, D. thermodynamic properties INTRODUCTION By using combustion synthesis technology [ 1,2] which is characterized with self-sustaining exothermic reaction, it is possible to obtain high-temperature materials such as carbides, borides, and nitrides in a short time. Among such materials, TiN is superior in hardness, chemical stability, corrosion resistance, and electric conductivity. Nitrogen sources used in TiN combustion synthesis have been gaseous N, [3-81, liquid N, [9], and solid sodium azide [lo]. The achievable conversion was less than 40% when Ti compact reacted with gaseous N, at atmospheric pressure. Therefore, a method of reaction in
*To whom correspondence should be addressed. 1341
1342
M. SHIBUYA et al.
Vol. 32, No. 10
FIG.1 Equipment for nitridation of Ti wire with liquid nitrogen. (a) Constant electric power supply. (b) Transient recorder. (c) Standard resistance (1 a). (d) Dewar vessel. (e) Ti wire. gaseous N, at high pressure (400 MPa) [3] was proposed for full conversion. However, full conversion could not be achieved even in such environmental applications, because the penetration rate of N, into Ti powder compact is reduced by increasing liquid Ti phase at the combustion zone followed with the increase of combustion velocity. The combustion velocity was decreased by increasing the packing density of powder compact [4-71, which improves the conversion rate. Utilizing NaN, as a solid source of nitrogen was also proposed for full conversion in which the environment pressure is sufficient at atmospheric pressure [lo]. Addition of diluent TiN was proposed in order to decrease the maximum temperature to prevent appearance of a liquid phase during the reaction process. In this way, fully converted TiN could be obtained under gaseous N, at more than 0.5 MPa [8]. However, the disadvantages of these methods include impurities in final products and the need for high-pressure equipment. In previous work [9] carried out on combustion synthesis of TiN in liquid N, under atmospheric pressure, highly converted TIN was obtained by controlling the reaction process without any special pressurized furnace. However, the reaction process was not made clear. We must understand the initiation phenomena of Ti combustion in liquid N, during the induction period, in order to obtain a good product by combustion synthesis of TiN in liquid N,. Ti wire was used under gaseous N, to determine initiation phenomena in previous work [6]. In the present work, the temperature effect for Ti wire combustion in liquid N, was investigated by electric power supply, to clarify the initiation phenomena of the TiN combustion synthesis in liquid N,. We have determined the temperature gradients, the minimum electric power (critical value: P,J which is necessary to initiate Ti wire combustion, and the morphology of products in the case of liquid N,. EXPERIMENTAL As the scheme of the apparatus in Figure 1 shows, Ti wire was placed between two electrodes and a dewar vessel was filled with liquid N2. The Ti wire was 0.1,0.2, or 0.3 mm in diameter
Vol. 32, No. 10
TITANIUM NITRIDE
1343
Time [msec] FIG. 2 Thermograms of Ti wire (4: 0.2 mm) at various constant electric powers in liquid nitrogen. P[W/mn?]: (a) 2.17, (b) 1.63, (c) 1.60, (d) 1.50, and (e) 1.08.
and 66 mm in length, with 99.8% of Ti metal. Electric power of 30-100 W was passed through th,e Ti wire by use of a constant electric power supply. The electrical parameters of the wire were measured using a four-probe method [l l] and recorded with a transient recorder. ‘The sampling rate was 6800 cps. The temperature gradient of the Ti wire was estimated from Ti wire resistivity by means of an electrothermographic method [ 12,131. X-ray powder diffraction patterns were measured by a stepped angle scanning method (scan speed 1.2”/min, scan step 0.02”, angle 30”-80”) with a RAD-C apparatus. A cross section of the heated wire was observed by optical microscopy after polishing and adequate etching. Analysis of the surface and cross section of the products was made using scanning electron microscopy (SEM).
RESULTS AND DISCUSSION In the case of combustion synthesis in liquid N,, the reaction was thought to be inhibited by increased escape of the heat generated. Liquid N, has a very high density (under atmospheric pressure), the density of liquid N, is 808 g/dm3 (77 K) and that for gaseous N, is 1.25 g/dm3 (273 K). The reaction between Ti and nitrogen gas is Ti + l/2 N, + TiN - AH. The heat generation, approximately 336 kJ/mol, is sufficient to evaporate liquid N,, the heat evaporation of which is approximately 5.6 kJ/mol. If the reaction can be induced, self-sustaining occurs. The Ti phase transition from OLtype (hexagonal close packed structure) to p type (body-centered cubic structure) occurs at 1155 K, and its melting point is 1943 K. Ti nitridation mechanism on solid-gas phase reaction is reported [14,15] in which the growth of TiN phase takes place after the penetration of N, into cx phase (homogeneous concentration of N, into (Yphase). Figure 2 shows the thermograms of Ti wire (0.2 mm in diameter) at various constant electric powers in liquid nitrogen. In Figure 2, curves a, b, and c show that the temperature rapidly increases after melting point of Ti, via phase transition point. The greater the electric power, the shorter is the keeping time at melting point. Curve d in Figure 2 shows the case of no-ignition, in which the wire temperature does not increase after melting point. In this case, penetration of N, into Ti is prevented because an
Vol. 32, No. 10
M. SHIBUYA et al.
1344
TABLE Effect
of Various
1
and Electrical
Diameters
Powers on Temperature
0.1
d (mm)
and Time 0.3
0.2 t(msec)
P(W/mm*)
t1
t2
t3
4
t2
t3
t1
t2
t3
1.84 (P > P,,) 1.58 (P = P,,) 1.34 (P c P,,)
70 95 625
95 135
10 40
85 200 780
120 280
5 240
30 125 780
80 230
10 70
t,(msec):
77-l
155 K, t, (msec): 77- 1943 K, t, (msec): sustaining time at 1943 K.
impermeable TiN film on the wire surface is generated for a long keeping time at melting point and this film does not break with increasing temperature [6]. Curve e in Figure 2 shows low-rate nitridation in which combustion does not occur at the low temperature. Therefore, it is obvious that the initiation of TiN combustion synthesis occurs at the melting point. The value of P,, required to ignite Ti wire 0.2 mm in diameter is equal to 1.60 W/mm’. In the same manner, the value of P,, for Ti wire 0.1 in diameter is 1.56 W/mm2 and that for Ti wire 0.3 in diameter is 1.58 W/mm2. In other words, the P,, values of the Ti wires 0.1-O-3 mm in diameter are approximately equal. Table 1 shows the effect of the various diameters and electrical powers on temperature and time: t, is the necessary time to reach phase transition (1155 K) from liquid nitrogen temperature (77 K) and b is the necessary time to reach Ti melting (1943 K) from 77 K. T, is the sustaining time at 1943 K. Sustaining time (ts) at melting point of Ti is very short over P,, On the other hand, it is apparent that the sustaining time (ts) is relatively long at P,, These results confirm that combustion synthesis in liquid N, as well as in gaseous N, is possible. The value of our P,, is coherent with P,, in the case of gaseous N, (Ti wire 0.1 mm in diameter), which is equal to 1.43 W/mm2 [6]. The difference can be explained by vaporization and heating N, (from 77 K to room temperature). Figure 3 shows X-ray diffraction pattern of a Ti wire (a: 0.3, P = P,,) heated in liquid nitrogen. We used several wires at P,, to detect phases. The products basically consisted of ol-Ti and TiN; @-Ti was not observed.
TiN 0 aTi
0
l
20 FIG.
3
XRD pattern of heated Ti wire (@ 0.3 mm, P: 1.58 W/mm2) in liquid nitrogen.
Vol. 32, No. 10
TITANIUM NITRIDE
FIG. 4 SEM photographs of the cross section and surface of heated Ti wire (@: 0.2 mm) in liquid N, at various electric powers: (a) 2.17 W/mm*, (b) 1.60 W/mm*, and (c) 1.08 W/mm*.
Figure 41shows the SEM photographs of the cross section and surface of heated Ti wire 0.2 mm in diameter in liquid N, at various electric powers. The electric powers represented in Figures .4(a), (b), and (c) are 2.17, 1.60 (PC), and 1.08 W/mm*, respectively. In Figure 4(a), the cross section of a wire after reaction over critical power exhibits three different morphologies. We observe acicular crystals in the center, a thin white film on the surface, and an amorphous8 shape without peculiar texture between these parts. The surface of the wire in Figure 4(a’) shows a white coating with some round outgrowths. Some exploded round shapes reveal that the surface of the wire is covered with bubbles. The cross section in Figure
1346
Vol. 32, No. 10
FIG. 5 SEM photograph of a cross section of the deformed edge of heated Ti wire (@: 0.2 mm, P: 1.60 W/mm’) in liquid N,.
4(b) shows very few acicular crystals, but the nontextured part seems bigger than that in the case of reaction over P,,. The cross section appears much more homogeneous; we observe some small holes near the center. On the surface of the wire in Figure 4(b’), the white film is homogeneous and no bubbles are observed. Figure 4(c) shows a cross section of a Ti wire with a low rate of nitridation. Some acicular crystals can be seen from the center until the edges of the wire, and small holes can be observed in the center. Figure 4(c’) shows the aspect of the surface. In this case, it looks like Ti metal. Each heated wire shows small holes in the center. This phenomenon is simply explained by shrinkage during cooling. On the other hand, the big cavities result from titanium vaporization [16]. Each wire shows the acicular phase that characterizes (Yphase of Ti metal obtained after heating over transition temperature (cx + p at 1155 K) and a cooling. The p phase is transformed in o-acicular phase at the same transition temperature. The cw-acicular phase gets anisotropic shape because the growth is leading by some preferential planes in p phase. The o-acicular phase is easily revealed by optical microscopy after polishing and adequate etching. The nontextured phase is only seen at the place very close to the ignition point. This is thought to be the result of fast quenching. Sustaining time at melting point and the conversion rate decrease with power increasing at over P,,. Heat generation is weaker at over P,, than at P,,. Because of these facts, we can interpret that more rapid quenching until inside, when the electric power is near P,,, leads to a large nontextured part. This, combined with the continuous decrease of transition temperature with cooling rate increasing [17] or the increase of transition temperature with presence of nitrogen [18], allows us to suppose that the nontextured phase is B phase or a-equiaxed phase (primary o). From some X-ray diffraction data, it seems that it must be primary OL,but this is still under discussion. Figure 5 shows a SEM photograph of the cross section of the deformed edge of a Ti wire. We recognize the nontextured part and a trace of a cavity is visible on the left. This picture confirms that the cross section of the wire is bent in the diameter direction. One of the reasons for this effect may be the well-known cracking of nitride film combined with vaporization of Ti [ 161. Another hypothesis is that it results from the increasing pressure of vaporized liquid N, around melting Ti wire during heating.
Vol. 32, No. 10
TITANIUM NITRIDE
1347
CONCLUSION In this work, it has been possible to define the PGr necessary for initiation of Ti wire combustion in liquid N,. The temperature at initiation of Ti wire combustion was confirmed to be close to the melting point of Ti in gaseous N,. The P,, does not depend on the diameter in the range 0.1-0.3 mm. The value of P,. is approximately 1.58 W/mm’, a little bit more than with. gas (1.43 W/mm2). In the case of an electric power greater than P,,, the heated Ti wire in lilquid N, clearly exhibited TiN film and o-acicular phase, based on X-ray diffraction data and SEM photographs of the cross section and surface. On the basis of these results, we can hope to obtain good conversion to some materials by combustion synthesis in liquid N2.
ACKNOWLEDGMENT The authors are grateful to Mr. M. Ozaki in the Department of Chemistry at Tokai University, for his skillful assistance. REFERENCES 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
A.G. Metzhanov and I.P. Borovinskaya, Dokl. Chem. 2&I(2), 429 (1972). A.G. Merzhanov and I.P. Borovinskaya, Combust. Sci. Technol. 10, 195 (1975). I.P. Borovinskaya and V.E. Loryan, Poroshk. Metal. 11(191), 42 (1978). Z.A. Munir, High Temp.-High Press. 20, 19 (1988). M. Eslamloo-Grami and Z.A. Munir, J. Am. Ceram. Sot. 73, 1235 (1990). H. Kudo and 0. Odawara, Proc. 32nd Jpn. Congress of Muter. Res., 168 (1989). H. Kudo and 0. Odawara, J. Mater. Sci. 24, 4030 (1989). M. Eslamloo-Grami and Z.A. Munir, J. Am. Ceram. Sot. 73, 2222 (1990). 0. Odawara, T. Narita, and W. Hua, J. Chem. Sot. Jpn. 10, 1461 (1991). J.B. Holt and D.D. Kingman, Muter. Sci. Res. 17, 167 (1984). U. Seydel and W. Fucke, J. Phys. F: Metal Phys. 10, L203 (1980). L.T. Abramova, V.G. Abramov, and A.G. Metzhanov, Russ. J. Phys. Chem. 43(5), 647 (1969). A.G. Merzhanov, AIAA Paper No. 74-144, 1 (1974). S.G. Vadchenko and Y.M. Grigor’yev, Russ. Metal. 1, 156 (1979). M. Otto and T. Yoshida, Nippon Kagaku Gakkaishi 2, 195 (1979). A.M. Mellor and I. Glassman, Pyndynamics 3, 43 (1965). M.J. Bibby and J.C.T. Parr, J. Inst. Metals 92, 341 (1964). S.Z. IBokshtein, T.A. Emelyanova, S.T. Kishkin, and L.M. Musky, The Science, Technology and Application of Titanium, p. 43, Pergamon Press, Oxford (1970).