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Solid–liquid synthesis of Ti3SiC2 particulate by fluctuation procedure
Scripta Materialia 49 (2003) 249–253 www.actamat-journals.com
Solid–liquid synthesis of Ti3SiC2 particulate by fluctuation procedure Y. Zhang *, Y.C. Zhou, Y.Y. Li Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China Received 19 November 2002; received in revised form 12 April 2003; accepted 14 April 2003
Abstract Reaction mechanisms in the fluctuation synthesis of Ti3 SiC2 are studied to show that the Ti3 SiC2 particulates precipitate directly out from a silicon-rich melt. 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Ti3 SiC2 ; Synthesis; Ceramic
1. Introduction Ti3 SiC2 has been the focus of many recent investigations because it combines the merit of both metals and ceramics [1,2]. Like metals, it is electrically and thermally conductive, not susceptible to thermal shock and easy to machine with conventional tools; like ceramics, it has a high modulus and good oxidation resistance. The good combination of these properties makes Ti3 SiC2 a candidate material for high-temperature application. The high modulus combined with good thermal and electrical conductivity makes Ti3 SiC2 a good reinforcement for metal matrix composites [3]. Further research and application of Ti3 SiC2 material are halted because it is difficult to syn* Corresponding author. Address: Ecomaterial Center, National Institute for Material Science, Sengen 1-2-1, Tsukuba, Ibaraki 3050047, Japan. Tel.: +81-298-592653; fax: +81-298592601. E-mail address: [email protected] (Y. Zhang).
thesize pure Ti3 SiC2 in large quantity. Although several synthesis methods have been developed [1,4–13], there are still many controversies on the synthesis of Ti3 SiC2 . One of questions is whether only solid reactions would take place under a traditional SiC tube furnace condition, i.e., a low heating rate and below 1300 C, for the synthesis of Ti3 SiC2 . In other words, can Ti3 SiC2 be synthesized easily using a traditional SiC tube furnace? Pampuch et al. [7] concluded that the synthesis of Ti3 SiC2 at a low heating rate is difficult since only solid-state reactions take place. Based on the results of Pampuch, high temperature and high pressure are used for the synthesis of Ti3 SiC2 [14]. We have successfully used a mixture of Ti, Si, and graphite powders as starting materials to produce Ti3 SiC2 material through a socalled fluctuation procedure, using a traditional SiC tube furnace [15]. As for the fluctuation synthesis of Ti3 SiC2 , we thought that a silicon rich melt had formed instead of a TiC melt since TiC is impossible to melt under such conditions because
1359-6462/03/$ - see front matter 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6462(03)00218-5
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Y. Zhang et al. / Scripta Materialia 49 (2003) 249–253
of its high melting point [7]. Li et al. [16] also used a mixture of Ti, Si, and graphite powders as starting materials to produce Ti3 SiC2 . Their results suggested that the formation of Ti3 SiC2 is related to the formation of liquid phases in the Ti–Si system. The presence of a liquid phase is attributed to the following facts. Firstly, the best synthesis temperature for Ti3 SiC2 is between 1300 and 1400 C, which correspond to two eutectic reactions: one between Ti and Ti5 Si3 and the other between TiSi2 and Si. Secondly, the homogeneity of the starting elemental powders is less important. Nevertheless, there is no evidence for the existing of silicon rich melt until recently. In this paper, the mechanism of synthesis of Ti3 SiC2 by the fluctuation procedure was investigated using an indirect approach including phase diagram analysis, and growth morphology observation.
perature was heated to 1300 C and kept for 10 min, and then reduced to 1200 C at 20 C/min and kept for 30 min. Then, the furnace temperature was increased to 1280 C and kept for 10 min. Finally, the furnace temperature was reduced to 1200 C at 20 C/min and kept for 50 min. Four samples were prepared by the above procedure but with different heating rates. The heating rates were 10 C/min for Sample 1 and 5 C/min for Sample 2 in the whole procedure, respectively. The heating rate of Sample 3 was 5 C/min up to 1200 C, 3 C/ min up to 1300 and 1280 C, respectively. Sample 4 was heated up to 1300 and 1280 C by the rate of 10 C/min, but kept for 20 min at the summit. The as-prepared samples were analyzed by X-ray diffraction (XRD). The growth morphology of asprepared particulate was observed using an S-360 scanning electron microscope (Cambridge Instruments, Ltd.).
2. Experimental procedure 3. Results and discussion Ti (average particle size 40 lm), Si (average particle size 80 lm) and graphite powders were used as starting materials. The powders with nearstoichiometric composition were mixed in a polypropylene jar for 16 h and then put into an Al2 O3 crucible. Fig. 1 shows the morphology of powder mixture. The fluctuation procedures were conducted in a SiC tube furnace under a flowing Ar atmosphere as follows. Firstly, the furnace tem-
Fig. 2 shows XRD patterns of as-prepared samples. All the samples consisted mainly of Ti3 SiC2 , TiC, SiC and TiSi2 phases. The Ti3 SiC2 content of Sample 3 was higher than 85 wt.%, Ti3SiC2 TiC SiC TiSi2
? Unkown phase
4#
?
?
3#
2#
1#
30
Fig. 1. SEM micrograph of powder mixtures. The gray platelike particulate was graphite phase and the others were Ti and Si powders.
35
40
45
50
55
60
65
70
75
80
85
Fig. 2. XRD patterns of as-prepared Samples. The heating rates of Sample 1 and 2 were 10 and 5 C/min in the whole procedure, respectively. The heating rate of Sample 3 was 5 C/min up to 1200 C, 3 C/min up to 1300 and 1280 C, respectively. Sample 4 was heated up to 1300 and 1280 C by the rate of 10 C/min, but kept for 20 min at the summit.
Y. Zhang et al. / Scripta Materialia 49 (2003) 249–253
which was estimated by the Rietveld method [17,18]. The fluctuation method in this work was thus effective for the synthesis of Ti3 SiC2 powders. Compared with previous work [5], the reaction time was dramatically shortened. More importantly, the highest furnace temperature and heating rate for the synthesis of Ti3 SiC2 were reduced and could be fulfilled using a traditional SiC tube furnace. Fig. 2 also indicates that the purity of Ti3 SiC2 had obvious dependence on the high temperature duration and the heating rate. When the heating rate below 1200 C decreased from 10 C to 5 C/min, there was no obvious change in the relative amount of Ti3 SiC2 . The purity of Ti3 SiC2 increased with the decrease of heating rate during 1200–1300 C. In Fig. 2, the relative amount of Ti3 SiC2 in Sample 4 seemed to be higher than in Sample 1 or 2, but there were some unknown phases, suggesting that the reaction products would be more complicated with longer duration at high temperature.
251
Fig. 3(a) is SEM image of as-prepared Sample 3. Characters of Ti3 SiC2 lamellae were clearly seen in the figure. The growth morphologies of these lamellae were mainly many polygonal steps and hexagonal hillocks. Fig. 3(b) shows that the interaction-free polygonal growth steps would be hexagonal. The hexagonal hillocks were identified as spiral-like growth and two-dimensional nuclei under higher magnification, as shown in Fig. 3(c). Samples 1, 2 and 4 also had similar morphologies. The above morphology features revealed that the Ti3 SiC2 particulate prepared in this work had typical morphologies as same as those of the particles grown from melt. Fig. 3(c) also shows that there was a spiral pit-like morphology. The existence of spiral pits indicated that the formation and decomposition of Ti3 SiC2 particulate had occurred at the same time. The lamella particulate shape was the characteristic crystallite shape of Ti3 SiC2 [11]. The top faces are (0 0 2) and the side faces are (1 0 1) [11].
Fig. 3. (a) SEM micrograph of as-prepared Ti3 SiC2 particulate, (b) hexagonal growth steps at top of Ti3 SiC2 particulate and (c) spiral pits adjacent to the growth spirals and two-dimensional nuclei.
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Y. Zhang et al. / Scripta Materialia 49 (2003) 249–253
(0 0 2) face is the only closest-packed face or singular interface for Ti3 SiC2 . The spiral-like and two-dimensional nuclei-like morphologies reflected the symmetrical elements of (0 0 2) face. It is known that only under low supersaturation conditions, the macro-morphology follows the symmetry elements of the crystal structure and the micro-morphology follows the symmetrical elements of the face [19]. We could thus draw the conclusion that the Ti3 SiC2 crystallites prepared in this work had grown from melt under low supersaturation conditions. The synthesis conditions used here were not equilibrium, but the synthesis of Ti3 SiC2 could be discussed based on the Ti–Si– C phase diagram due to the slow heating rate. The phase diagram data for the Ti–Si–C system are limited, however, and no experimental information is available about its liquid projection until very recently. The latest work of Du et al. [20] provided the liquid projection and the reaction scheme for the ternary system. Based on the work, it is possible to investigate the reactions that occurred in the fluctuation synthesis of Ti3 SiC2 . In this work, the elemental powders were selected as starting material. Fig. 1 demonstrates that the mixing homogeneity was low. Therefore, at the beginning of the heating procedure, the Ti– Si–C system here should be broken down into three dual systems to be analyzed: Ti–Si, Ti–C, and Si–C systems. During the heating procedure, Ti, C, Si element would diffuse into each other to form solution or react with each other. At the Ti– C side, the exothermic reaction Ti + C ¼ TiC would happen above 870 C [7]. At the Ti–Si side, the lowest melting point was 1330 C because of the eutectic reaction: Liquid ¼ TiSi2 + Si [20]. The TiSi2 phase would be formed due to the solid diffusion during the heating. The melt consisted of about 85 at.% Si and 15 at.% Ti [20]. Due to the slow heating, the exothermic reaction of Ti + C ¼ TiC did not happen heavily but locally and continually, which would still make the temperature around the reaction region higher. Then, when the furnace temperature reached 1300 C, the eutectic reaction, Liquid ¼ TiSi2 + Si, would occur because of heat released from the exothermic reaction occurred nearby. SiC phase was formed at the Si–C side during the heating. It has been found
that the reactivity of SiC with a titanium compound depends on the amount of free Si in the ceramic [21]. Due to the presence of free silicon, the titanium atoms diffuse into SiC to form Ti3 SiC2 [22]. In the present fluctuation procedure, there was liquid silicon so that the reactivity of SiC would be high. As a result, the SiC phase transformed into Ti3 SiC2 during the fluctuation procedure. If the local temperature reached 1414 C, L $ Si + SiC would occur. In one word, a silicon rich melt containing dissolved Ti and C would form when the furnace temperature was kept at 1300 C because of the exothermic reaction and element bulk diffusion. Then the furnace was lowered to 1200 C, which had been known as a suitable temperature for the formation of Ti3 SiC2 powder [5]. Ti3 SiC2 was formed preferentially and rapidly because of higher element activity in melt. The formation of Ti3 SiC2 was such an exothermic reaction that it is considered SHS-like [14]. Thus, more silicon melt would form to compensate the assumed melt and the formation of Ti3 SiC2 continued. The rising of the temperature to 1280 C in the second step of temperature vs. time schedule was for the same aim. The essence of the fluctuation method was that the silicon rich melt formed at high furnace temperature was retained at low furnace temperature due to the exothermic reactions. The phenomenon that the purity of Ti3 SiC2 was higher with the slower rate of fluctuation procedure resulted from more melt being formed. The lesser amount of TiC with decreasing fluctuation rate supported the previous conclusion that the very stable transition monocarbides decompose in the presence of metametals; in general, a peritectic reaction TC + liquid ¼ T2 MC + X occurs [23]. TiC could combine with TiSi to form Ti3 SiC2 : TiSi + TiC fi Ti3 SiC2 [9,24]. However, we must acknowledge that much more work is needed to confirm the second and third reaction mechanisms.
4. Conclusion This work indirectly confirmed the growth of Ti3 SiC2 from silicon rich melt in the novel fluctuation procedure. The hexagonal step, spiral and
Y. Zhang et al. / Scripta Materialia 49 (2003) 249–253
two-dimensional morphologies showed that the Ti3 SiC2 lamella was the results of growth on (0 0 2) face from low supersaturation conditions. The above results indicated that Ti3 SiC2 material can be synthesized through liquid/solid reactions at low heating rate and below 1300 C, which can be fulfilled using a traditional SiC tube furnace. In addition, this work suggests that bulk Ti3 SiC2 material can be prepared through a fluctuation synthesis and simultaneous densification at relatively low temperature and low hot pressing pressure because of the presence of silicon-rich liquid.
References [1] Barsoum MW, El-Raghy T. J Am Ceram Soc 1996;79:1953. [2] Low M, Lee SK, Lawn BR. J Am Ceram Soc 1998;81:225. [3] Zhang Y, Sun ZM, Zhou YC. Mater Res Innovations 1999;3:80. [4] Lis J, Miyamoto Y, Pampuch R, Tanihata K. Mater Lett 1995;22:163. [5] Arunajatesan S, Carim A. J Am Ceram Soc 1995;78:667. [6] Racault C, Langlais F, Naslain R. J Mater Sci 1994;29:3384. [7] Pampuch R, Raczka M, Lis J. Int J Mater Prod Technol 1995;10:316.
253
[8] Pampuch R, Lis J, Stobierski L, Tymkiewicz M. J Eur Ceram Soc 1989;5:283. [9] Goto T, Hirai T. Mater Res Bull 1987;22:1195. [10] Orthner HR, Tomasi R, Botta WJ. Mater Sci Eng A 2002;336(1–2):202. [11] Riley DP, Kisi EH, Hansen TC, Hewat AW. J Am Ceram Soc 2002;85:2417. [12] Tang K, Wang CA, Wu LF, Guo XJ, Huang Y. Ceram Int 2002;28:761. [13] Sun ZM, Zhang ZF, Hashimoto H, Abe T. Mater Trans 2002;43:428. [14] Feng AG, Orling T, Munir ZA. J Mater Res 1999;14: 925. [15] Sun ZM, Zhou YC. Mater Res Innovations 1999;2:227. [16] Li JF, Sato F, Wantanbe R. J Mater Sci Lett 1999;18:1595. [17] Young RA. The Rietveld Method. Oxford: Oxford University Press; 1993. [18] Wiles DB, Young RA. J Appl Crystallogr 1981;14:149. [19] Sunagawa I. In: Crystal growth and characterization: Proceedings of the ISSCG2 Springschool, Japan, 1974. p. 347. [20] Du Y, Schuster JC, Seifert JH, Aldinger F. J Am Ceram Soc 2000;83:197. [21] Gorsse S, Le Petitcorps Y. Composites Part A 1998;29A:1221. [22] Vahlas C, Hall IW, Ni C-Y, Chevalier P-Y. Key Eng Mater 1997;127–131:359. [23] Jeischko W, Nowtotny H, Benesovsky F. J Less Common Metals 1964;7:133. [24] Klemm H, Tanihata K, Miyamoto Y. J Mater Sci 1993;28:1557.
Solid–liquid synthesis of Ti3SiC2 particulate by fluctuation procedure Y. Zhang *, Y.C. Zhou, Y.Y. Li Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China Received 19 November 2002; received in revised form 12 April 2003; accepted 14 April 2003
Abstract Reaction mechanisms in the fluctuation synthesis of Ti3 SiC2 are studied to show that the Ti3 SiC2 particulates precipitate directly out from a silicon-rich melt. 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Ti3 SiC2 ; Synthesis; Ceramic
1. Introduction Ti3 SiC2 has been the focus of many recent investigations because it combines the merit of both metals and ceramics [1,2]. Like metals, it is electrically and thermally conductive, not susceptible to thermal shock and easy to machine with conventional tools; like ceramics, it has a high modulus and good oxidation resistance. The good combination of these properties makes Ti3 SiC2 a candidate material for high-temperature application. The high modulus combined with good thermal and electrical conductivity makes Ti3 SiC2 a good reinforcement for metal matrix composites [3]. Further research and application of Ti3 SiC2 material are halted because it is difficult to syn* Corresponding author. Address: Ecomaterial Center, National Institute for Material Science, Sengen 1-2-1, Tsukuba, Ibaraki 3050047, Japan. Tel.: +81-298-592653; fax: +81-298592601. E-mail address: [email protected] (Y. Zhang).
thesize pure Ti3 SiC2 in large quantity. Although several synthesis methods have been developed [1,4–13], there are still many controversies on the synthesis of Ti3 SiC2 . One of questions is whether only solid reactions would take place under a traditional SiC tube furnace condition, i.e., a low heating rate and below 1300 C, for the synthesis of Ti3 SiC2 . In other words, can Ti3 SiC2 be synthesized easily using a traditional SiC tube furnace? Pampuch et al. [7] concluded that the synthesis of Ti3 SiC2 at a low heating rate is difficult since only solid-state reactions take place. Based on the results of Pampuch, high temperature and high pressure are used for the synthesis of Ti3 SiC2 [14]. We have successfully used a mixture of Ti, Si, and graphite powders as starting materials to produce Ti3 SiC2 material through a socalled fluctuation procedure, using a traditional SiC tube furnace [15]. As for the fluctuation synthesis of Ti3 SiC2 , we thought that a silicon rich melt had formed instead of a TiC melt since TiC is impossible to melt under such conditions because
1359-6462/03/$ - see front matter 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6462(03)00218-5
250
Y. Zhang et al. / Scripta Materialia 49 (2003) 249–253
of its high melting point [7]. Li et al. [16] also used a mixture of Ti, Si, and graphite powders as starting materials to produce Ti3 SiC2 . Their results suggested that the formation of Ti3 SiC2 is related to the formation of liquid phases in the Ti–Si system. The presence of a liquid phase is attributed to the following facts. Firstly, the best synthesis temperature for Ti3 SiC2 is between 1300 and 1400 C, which correspond to two eutectic reactions: one between Ti and Ti5 Si3 and the other between TiSi2 and Si. Secondly, the homogeneity of the starting elemental powders is less important. Nevertheless, there is no evidence for the existing of silicon rich melt until recently. In this paper, the mechanism of synthesis of Ti3 SiC2 by the fluctuation procedure was investigated using an indirect approach including phase diagram analysis, and growth morphology observation.
perature was heated to 1300 C and kept for 10 min, and then reduced to 1200 C at 20 C/min and kept for 30 min. Then, the furnace temperature was increased to 1280 C and kept for 10 min. Finally, the furnace temperature was reduced to 1200 C at 20 C/min and kept for 50 min. Four samples were prepared by the above procedure but with different heating rates. The heating rates were 10 C/min for Sample 1 and 5 C/min for Sample 2 in the whole procedure, respectively. The heating rate of Sample 3 was 5 C/min up to 1200 C, 3 C/ min up to 1300 and 1280 C, respectively. Sample 4 was heated up to 1300 and 1280 C by the rate of 10 C/min, but kept for 20 min at the summit. The as-prepared samples were analyzed by X-ray diffraction (XRD). The growth morphology of asprepared particulate was observed using an S-360 scanning electron microscope (Cambridge Instruments, Ltd.).
2. Experimental procedure 3. Results and discussion Ti (average particle size 40 lm), Si (average particle size 80 lm) and graphite powders were used as starting materials. The powders with nearstoichiometric composition were mixed in a polypropylene jar for 16 h and then put into an Al2 O3 crucible. Fig. 1 shows the morphology of powder mixture. The fluctuation procedures were conducted in a SiC tube furnace under a flowing Ar atmosphere as follows. Firstly, the furnace tem-
Fig. 2 shows XRD patterns of as-prepared samples. All the samples consisted mainly of Ti3 SiC2 , TiC, SiC and TiSi2 phases. The Ti3 SiC2 content of Sample 3 was higher than 85 wt.%, Ti3SiC2 TiC SiC TiSi2
? Unkown phase
4#
?
?
3#
2#
1#
30
Fig. 1. SEM micrograph of powder mixtures. The gray platelike particulate was graphite phase and the others were Ti and Si powders.
35
40
45
50
55
60
65
70
75
80
85
Fig. 2. XRD patterns of as-prepared Samples. The heating rates of Sample 1 and 2 were 10 and 5 C/min in the whole procedure, respectively. The heating rate of Sample 3 was 5 C/min up to 1200 C, 3 C/min up to 1300 and 1280 C, respectively. Sample 4 was heated up to 1300 and 1280 C by the rate of 10 C/min, but kept for 20 min at the summit.
Y. Zhang et al. / Scripta Materialia 49 (2003) 249–253
which was estimated by the Rietveld method [17,18]. The fluctuation method in this work was thus effective for the synthesis of Ti3 SiC2 powders. Compared with previous work [5], the reaction time was dramatically shortened. More importantly, the highest furnace temperature and heating rate for the synthesis of Ti3 SiC2 were reduced and could be fulfilled using a traditional SiC tube furnace. Fig. 2 also indicates that the purity of Ti3 SiC2 had obvious dependence on the high temperature duration and the heating rate. When the heating rate below 1200 C decreased from 10 C to 5 C/min, there was no obvious change in the relative amount of Ti3 SiC2 . The purity of Ti3 SiC2 increased with the decrease of heating rate during 1200–1300 C. In Fig. 2, the relative amount of Ti3 SiC2 in Sample 4 seemed to be higher than in Sample 1 or 2, but there were some unknown phases, suggesting that the reaction products would be more complicated with longer duration at high temperature.
251
Fig. 3(a) is SEM image of as-prepared Sample 3. Characters of Ti3 SiC2 lamellae were clearly seen in the figure. The growth morphologies of these lamellae were mainly many polygonal steps and hexagonal hillocks. Fig. 3(b) shows that the interaction-free polygonal growth steps would be hexagonal. The hexagonal hillocks were identified as spiral-like growth and two-dimensional nuclei under higher magnification, as shown in Fig. 3(c). Samples 1, 2 and 4 also had similar morphologies. The above morphology features revealed that the Ti3 SiC2 particulate prepared in this work had typical morphologies as same as those of the particles grown from melt. Fig. 3(c) also shows that there was a spiral pit-like morphology. The existence of spiral pits indicated that the formation and decomposition of Ti3 SiC2 particulate had occurred at the same time. The lamella particulate shape was the characteristic crystallite shape of Ti3 SiC2 [11]. The top faces are (0 0 2) and the side faces are (1 0 1) [11].
Fig. 3. (a) SEM micrograph of as-prepared Ti3 SiC2 particulate, (b) hexagonal growth steps at top of Ti3 SiC2 particulate and (c) spiral pits adjacent to the growth spirals and two-dimensional nuclei.
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Y. Zhang et al. / Scripta Materialia 49 (2003) 249–253
(0 0 2) face is the only closest-packed face or singular interface for Ti3 SiC2 . The spiral-like and two-dimensional nuclei-like morphologies reflected the symmetrical elements of (0 0 2) face. It is known that only under low supersaturation conditions, the macro-morphology follows the symmetry elements of the crystal structure and the micro-morphology follows the symmetrical elements of the face [19]. We could thus draw the conclusion that the Ti3 SiC2 crystallites prepared in this work had grown from melt under low supersaturation conditions. The synthesis conditions used here were not equilibrium, but the synthesis of Ti3 SiC2 could be discussed based on the Ti–Si– C phase diagram due to the slow heating rate. The phase diagram data for the Ti–Si–C system are limited, however, and no experimental information is available about its liquid projection until very recently. The latest work of Du et al. [20] provided the liquid projection and the reaction scheme for the ternary system. Based on the work, it is possible to investigate the reactions that occurred in the fluctuation synthesis of Ti3 SiC2 . In this work, the elemental powders were selected as starting material. Fig. 1 demonstrates that the mixing homogeneity was low. Therefore, at the beginning of the heating procedure, the Ti– Si–C system here should be broken down into three dual systems to be analyzed: Ti–Si, Ti–C, and Si–C systems. During the heating procedure, Ti, C, Si element would diffuse into each other to form solution or react with each other. At the Ti– C side, the exothermic reaction Ti + C ¼ TiC would happen above 870 C [7]. At the Ti–Si side, the lowest melting point was 1330 C because of the eutectic reaction: Liquid ¼ TiSi2 + Si [20]. The TiSi2 phase would be formed due to the solid diffusion during the heating. The melt consisted of about 85 at.% Si and 15 at.% Ti [20]. Due to the slow heating, the exothermic reaction of Ti + C ¼ TiC did not happen heavily but locally and continually, which would still make the temperature around the reaction region higher. Then, when the furnace temperature reached 1300 C, the eutectic reaction, Liquid ¼ TiSi2 + Si, would occur because of heat released from the exothermic reaction occurred nearby. SiC phase was formed at the Si–C side during the heating. It has been found
that the reactivity of SiC with a titanium compound depends on the amount of free Si in the ceramic [21]. Due to the presence of free silicon, the titanium atoms diffuse into SiC to form Ti3 SiC2 [22]. In the present fluctuation procedure, there was liquid silicon so that the reactivity of SiC would be high. As a result, the SiC phase transformed into Ti3 SiC2 during the fluctuation procedure. If the local temperature reached 1414 C, L $ Si + SiC would occur. In one word, a silicon rich melt containing dissolved Ti and C would form when the furnace temperature was kept at 1300 C because of the exothermic reaction and element bulk diffusion. Then the furnace was lowered to 1200 C, which had been known as a suitable temperature for the formation of Ti3 SiC2 powder [5]. Ti3 SiC2 was formed preferentially and rapidly because of higher element activity in melt. The formation of Ti3 SiC2 was such an exothermic reaction that it is considered SHS-like [14]. Thus, more silicon melt would form to compensate the assumed melt and the formation of Ti3 SiC2 continued. The rising of the temperature to 1280 C in the second step of temperature vs. time schedule was for the same aim. The essence of the fluctuation method was that the silicon rich melt formed at high furnace temperature was retained at low furnace temperature due to the exothermic reactions. The phenomenon that the purity of Ti3 SiC2 was higher with the slower rate of fluctuation procedure resulted from more melt being formed. The lesser amount of TiC with decreasing fluctuation rate supported the previous conclusion that the very stable transition monocarbides decompose in the presence of metametals; in general, a peritectic reaction TC + liquid ¼ T2 MC + X occurs [23]. TiC could combine with TiSi to form Ti3 SiC2 : TiSi + TiC fi Ti3 SiC2 [9,24]. However, we must acknowledge that much more work is needed to confirm the second and third reaction mechanisms.
4. Conclusion This work indirectly confirmed the growth of Ti3 SiC2 from silicon rich melt in the novel fluctuation procedure. The hexagonal step, spiral and
Y. Zhang et al. / Scripta Materialia 49 (2003) 249–253
two-dimensional morphologies showed that the Ti3 SiC2 lamella was the results of growth on (0 0 2) face from low supersaturation conditions. The above results indicated that Ti3 SiC2 material can be synthesized through liquid/solid reactions at low heating rate and below 1300 C, which can be fulfilled using a traditional SiC tube furnace. In addition, this work suggests that bulk Ti3 SiC2 material can be prepared through a fluctuation synthesis and simultaneous densification at relatively low temperature and low hot pressing pressure because of the presence of silicon-rich liquid.
References [1] Barsoum MW, El-Raghy T. J Am Ceram Soc 1996;79:1953. [2] Low M, Lee SK, Lawn BR. J Am Ceram Soc 1998;81:225. [3] Zhang Y, Sun ZM, Zhou YC. Mater Res Innovations 1999;3:80. [4] Lis J, Miyamoto Y, Pampuch R, Tanihata K. Mater Lett 1995;22:163. [5] Arunajatesan S, Carim A. J Am Ceram Soc 1995;78:667. [6] Racault C, Langlais F, Naslain R. J Mater Sci 1994;29:3384. [7] Pampuch R, Raczka M, Lis J. Int J Mater Prod Technol 1995;10:316.
253
[8] Pampuch R, Lis J, Stobierski L, Tymkiewicz M. J Eur Ceram Soc 1989;5:283. [9] Goto T, Hirai T. Mater Res Bull 1987;22:1195. [10] Orthner HR, Tomasi R, Botta WJ. Mater Sci Eng A 2002;336(1–2):202. [11] Riley DP, Kisi EH, Hansen TC, Hewat AW. J Am Ceram Soc 2002;85:2417. [12] Tang K, Wang CA, Wu LF, Guo XJ, Huang Y. Ceram Int 2002;28:761. [13] Sun ZM, Zhang ZF, Hashimoto H, Abe T. Mater Trans 2002;43:428. [14] Feng AG, Orling T, Munir ZA. J Mater Res 1999;14: 925. [15] Sun ZM, Zhou YC. Mater Res Innovations 1999;2:227. [16] Li JF, Sato F, Wantanbe R. J Mater Sci Lett 1999;18:1595. [17] Young RA. The Rietveld Method. Oxford: Oxford University Press; 1993. [18] Wiles DB, Young RA. J Appl Crystallogr 1981;14:149. [19] Sunagawa I. In: Crystal growth and characterization: Proceedings of the ISSCG2 Springschool, Japan, 1974. p. 347. [20] Du Y, Schuster JC, Seifert JH, Aldinger F. J Am Ceram Soc 2000;83:197. [21] Gorsse S, Le Petitcorps Y. Composites Part A 1998;29A:1221. [22] Vahlas C, Hall IW, Ni C-Y, Chevalier P-Y. Key Eng Mater 1997;127–131:359. [23] Jeischko W, Nowtotny H, Benesovsky F. J Less Common Metals 1964;7:133. [24] Klemm H, Tanihata K, Miyamoto Y. J Mater Sci 1993;28:1557.