- Email: [email protected]
The co-reduction route to TiC nanocrystallites at low temperature
26 November 1999
Chemical Physics Letters 314 Ž1999. 37–39 www.elsevier.nlrlocatercplett
The co-reduction route to TiC nanocrystallites at low temperature Qingyi Lu, Junqing Hu, Kaibin Tang ) , Bin Deng, Yitai Qian, Guien Zhou, Xianming Liu Structure Research Laboratory and Department of Chemistry, UniÕersity of Science and Technology of China, Hefei, Anhui 230026, China Received 31 August 1999
Abstract TiC nanocrystallites have been synthesized through a co-reduction process by using TiCl 4 and CCl 4 as reactants and metal Na as co-reductant. The reaction was carried out in an autoclave at 4508C, which is much lower than those of traditional methods. X-ray diffraction pattern indicates that the obtained sample is cubic phase TiC. Transmission electron microscopy imaging reveals that it is produced as nanoparticles with a narrow size distribution Ž10–20 nm.. q 1999 Elsevier Science B.V. All rights reserved.
Titanium carbide, TiC, is well known for its high melting point Ž32608C., extreme hardness, low density Ž4.93 grcm3 ., and high resistance to corrosion and oxidation w1,2x. It has been applied widely in mechanical industry, chemistry, and microelectronics w3x. Nanostructural materials have received much attention for decades because they possess distinguished mechanical, physical and chemical properties w4,5x. However few reports about the synthesis of nanocrystalline TiC could be found. The industrial method for the production of crystalline TiC powders is carbon-thermal reduction of TiO 2 in a temperature range from 1900 to 27008C and associated with the following reaction w6x:
ied w7,8x. Some scientists proposed a reaction between TiCl 4 , H 2 and C in contact with an incandescent tungsten or carbon filament w9x. Due to the corrosive effects of the by-product HCl, special precautions must be taken w9x. Recently, some physical methods such as nontransferred-arc thermal-plasma w10x, mechanical alloying w11x, and spark erosion w12x technologies have been developed to obtain TiC ultrafine powders. In this Letter, we report a co-reduction method to synthesize TiC nanocrystallites at 4508C. The synthetic reaction was carried out in an autoclave and may be associated with the following reaction:
TiO 2 Ž s . q 3C Ž s . ™ TiC Ž s . q 2CO Ž g . .
TiCl 4 Ž l . q CCl 4 Ž l . q 8Na Ž s .
Ž 1.
The self-propagating high-temperature synthesis ŽSHS. from the elements has been extensively stud-
) Corresponding author. Fax: q86-0551-363-1760; e-mail: [email protected]
™ TiC Ž s . q 8NaCl Ž s . .
Ž 2.
In this process we take advantage of the high pressure in the autoclave coming from the vaporization of reagents and the high exothermal energy of the synthetic reaction to make the produced TiC crystal-
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 1 1 0 9 - 4
38
Q. Lu et al.r Chemical Physics Letters 314 (1999) 37–39
lize at 4508C, which is much lower than those of traditional methods. All manipulations were carried out in a dry glovebox with Ar flowing. In a typical procedure, 10 ml TiCl 4 Žexcess., 5 ml CCl 4 and 10 g metal Na were put into an autoclave of 50 ml capacity. After sealing the autoclave was maintained at 4508C for 8 h, and then cooled to room temperature in the furnace. The unreacted TiCl 4 was removed and the product was treated with dilute acid and distilled water, respectively, several times to remove NaCl and impurities. After drying in vacuum at 708C for 4 h, the final sample was obtained. The X-ray powder diffraction ŽXRD. pattern was recorded on a Japan Rigaku Dmax gA X-ray diffrac˚ .. In the tometer with Cu K a radiation Ž l s 1.5418 A XRD pattern ŽFig. 1., all the strong intensity peaks can be indexed as cubic TiC. After refinement, the ˚ is very close to the reported cell constant a s 4.34 A ˚ JCPDS card, No. value for TiC Ž a s 4.3274 A, 32-1383.. A transmission electron microscopy ŽTEM. image of the product was taken with a Hitachi H-800 transmission electron microscope, using an accelerating voltage of 200 kV. Fig. 2 shows that typical TiC crystallites are uniform nanoparticles with sizes ranging from 10 to 20 nm. A feature of this synthetic method is the high pressure in the autoclave. In our experiment, with the increase of the processing temperature, TiCl 4 and CCl 4 began to vaporize Žthe boiling point of TiCl 4 is 136.48C and that of CCl 4 is 76.88C. w13x. Estimated with the ideal gas law, the pressure in the autoclave under our experimental conditions is ; 15 MPa, which could promote the crystallization of the TiC produced at a relatively low temperature. Another feature of this synthetic reaction is that the synthetic
Fig. 1. The XRD pattern of the obtained TiC sample.
Fig. 2. The TEM image of the obtained TiC sample.
reaction is an exothermal reaction, according to the reaction enthalpy Ž D H s y613.4 kcalrmol. w13x. The formation of the product results in an instantaneously high temperature, which makes the crystalline TiC form. The excess TiCl 4 is important to the formation of nanocrystalline TiC, not only by increasing the pressure in the autoclave but it also acts as a diluent to absorb the reaction enthalpy and therefore maintain a relatively low overall reaction temperature. This results in the formation of TiC nanocrystallites instead of an agglomeration product. In our previous experiment, when carbon powders and metal titanium were substituted for CCl 4 and TiCl 4 , respectively, the synthetic reaction needed a high processing temperature Ž650–7008C. w14x. These experimental results are consistent with the suggestion above. Using these solid state reactants with high melting-temperature Žthe melting point of Ti is 16608C, and that of C powders is 36528C w13x. did not result in a high pressure in the autoclave, which cannot exert a favorable effect on the crystallization of the product. Moreover, using CCl 4 and TiCl 4 as the reactants results in a higher reaction enthalpy than using C or Ti as reactant, respectively, according to the reaction heats: D H1 s y613.4 kcalrmol Žusing TiCl 4 and CCl 4 as reactants.; D H2 s y412.6 kcalrmol Žusing Ti and CCl 4 as reactants., and D H3 s y245.0 kcalrmol Žusing TiCl 4 and C as
Q. Lu et al.r Chemical Physics Letters 314 (1999) 37–39
reactants. w13x, which is beneficial to the formation of crystalline TiC. In the co-reduction process, the processing temperature and time also play important roles to the formation of TiC nanocrystallites. Although the synthetic reaction is thermodynamically spontaneous and exothermal, crystalline TiC cannot form at a temperature lower than 3708C or in a time shorter than 3 h. Heating to a temperature of higher than 6008C will result in the increase of the size of the TiC produced. Varying the processing time between 3 and 15 h at 4508C did not affect the crystallinity and the particle size greatly. The optimum processing temperature and time for TiC nanocrystallites are 420–5008C and 6–8 h, respectively. Certainly, a deeper understanding of the reaction mechanism is needed, but we believe that this method could in principle produce other stable carbide nanocrystallites.
Acknowledgements This work is supported by Chinese National Natural Science Research Foundation.
39
References w1x R.J. Meyer, E.H.E. Pietsch, Gmelins Handbook of Anorganic Chemistry, vol. 41, 8th ed., Springer, Berlin, 1951, p. 357 Žin German.. w2x P. Pascal, Nouveau traite´ de chimie minerale, vol. 9, Masson, ´ Paris, 1963, p. 150 w3x High Temperature Chemistry of Inorganic Materials, The Chemical Society, London, 1977. w4x H. Brune, M. Giovannini, K. Bromann, K. Kern, Nature ŽLondon. 394 Ž1998. 451. w5x R.N. Barnett, U. Landman, Nature ŽLondon. 387 Ž1997. 788. w6x I.N. Mihailescu, N. Chitica, V.S. Teodorescu, M. Popescu, M.L. De Giorge, A. Luches, A. Perrone, C. BoulmerLeborgne, J. Hermann, B. Dubreuil, S. Urdea, A. Barborica, I. Iova, J. Appl. Phys. 75 Ž1994. 5286. w7x K.S. Vecchio, J.C. LaSalvia, M.A. Meyers, G.T. Gray, Metall. Trans. 23A Ž1992. 87. w8x Z.A. Munir, W. Lai, Combust. Sci. Technol. 88 Ž1993. 210. w9x K. Thorne, S.J. Ting, C.J. Chu, J.D. Mackenzie, T.D. Getman, M.F. Hawthorne, J. Mater. Sci. 27 Ž1992. 4406. w10x P.R. Tayler, S.A. Pirzada, T.D. McColm, Plasma Synthesis Processing of Materials, Proc. Symp., 1993, pp. 179–190. w11x L.L. Ye, M.X. Quan, Nanostruct. Mater. 5 Ž1995. 25. w12x M.S. Hsu, M.A. Meyers, A. Berkowitz, Scr. Metall. Mater. 32 Ž1995. 805. w13x J.A. Dean, Lange’s Handbook of Chemistry, 13th ed., McGraw-Hill, New York, 1985. w14x J. Hu, Q. Lu, K. Bin, B. Deng, Y. Qian, G. Zhou, X. Liu, J. Wu, unpublished.
Chemical Physics Letters 314 Ž1999. 37–39 www.elsevier.nlrlocatercplett
The co-reduction route to TiC nanocrystallites at low temperature Qingyi Lu, Junqing Hu, Kaibin Tang ) , Bin Deng, Yitai Qian, Guien Zhou, Xianming Liu Structure Research Laboratory and Department of Chemistry, UniÕersity of Science and Technology of China, Hefei, Anhui 230026, China Received 31 August 1999
Abstract TiC nanocrystallites have been synthesized through a co-reduction process by using TiCl 4 and CCl 4 as reactants and metal Na as co-reductant. The reaction was carried out in an autoclave at 4508C, which is much lower than those of traditional methods. X-ray diffraction pattern indicates that the obtained sample is cubic phase TiC. Transmission electron microscopy imaging reveals that it is produced as nanoparticles with a narrow size distribution Ž10–20 nm.. q 1999 Elsevier Science B.V. All rights reserved.
Titanium carbide, TiC, is well known for its high melting point Ž32608C., extreme hardness, low density Ž4.93 grcm3 ., and high resistance to corrosion and oxidation w1,2x. It has been applied widely in mechanical industry, chemistry, and microelectronics w3x. Nanostructural materials have received much attention for decades because they possess distinguished mechanical, physical and chemical properties w4,5x. However few reports about the synthesis of nanocrystalline TiC could be found. The industrial method for the production of crystalline TiC powders is carbon-thermal reduction of TiO 2 in a temperature range from 1900 to 27008C and associated with the following reaction w6x:
ied w7,8x. Some scientists proposed a reaction between TiCl 4 , H 2 and C in contact with an incandescent tungsten or carbon filament w9x. Due to the corrosive effects of the by-product HCl, special precautions must be taken w9x. Recently, some physical methods such as nontransferred-arc thermal-plasma w10x, mechanical alloying w11x, and spark erosion w12x technologies have been developed to obtain TiC ultrafine powders. In this Letter, we report a co-reduction method to synthesize TiC nanocrystallites at 4508C. The synthetic reaction was carried out in an autoclave and may be associated with the following reaction:
TiO 2 Ž s . q 3C Ž s . ™ TiC Ž s . q 2CO Ž g . .
TiCl 4 Ž l . q CCl 4 Ž l . q 8Na Ž s .
Ž 1.
The self-propagating high-temperature synthesis ŽSHS. from the elements has been extensively stud-
) Corresponding author. Fax: q86-0551-363-1760; e-mail: [email protected]
™ TiC Ž s . q 8NaCl Ž s . .
Ž 2.
In this process we take advantage of the high pressure in the autoclave coming from the vaporization of reagents and the high exothermal energy of the synthetic reaction to make the produced TiC crystal-
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 1 1 0 9 - 4
38
Q. Lu et al.r Chemical Physics Letters 314 (1999) 37–39
lize at 4508C, which is much lower than those of traditional methods. All manipulations were carried out in a dry glovebox with Ar flowing. In a typical procedure, 10 ml TiCl 4 Žexcess., 5 ml CCl 4 and 10 g metal Na were put into an autoclave of 50 ml capacity. After sealing the autoclave was maintained at 4508C for 8 h, and then cooled to room temperature in the furnace. The unreacted TiCl 4 was removed and the product was treated with dilute acid and distilled water, respectively, several times to remove NaCl and impurities. After drying in vacuum at 708C for 4 h, the final sample was obtained. The X-ray powder diffraction ŽXRD. pattern was recorded on a Japan Rigaku Dmax gA X-ray diffrac˚ .. In the tometer with Cu K a radiation Ž l s 1.5418 A XRD pattern ŽFig. 1., all the strong intensity peaks can be indexed as cubic TiC. After refinement, the ˚ is very close to the reported cell constant a s 4.34 A ˚ JCPDS card, No. value for TiC Ž a s 4.3274 A, 32-1383.. A transmission electron microscopy ŽTEM. image of the product was taken with a Hitachi H-800 transmission electron microscope, using an accelerating voltage of 200 kV. Fig. 2 shows that typical TiC crystallites are uniform nanoparticles with sizes ranging from 10 to 20 nm. A feature of this synthetic method is the high pressure in the autoclave. In our experiment, with the increase of the processing temperature, TiCl 4 and CCl 4 began to vaporize Žthe boiling point of TiCl 4 is 136.48C and that of CCl 4 is 76.88C. w13x. Estimated with the ideal gas law, the pressure in the autoclave under our experimental conditions is ; 15 MPa, which could promote the crystallization of the TiC produced at a relatively low temperature. Another feature of this synthetic reaction is that the synthetic
Fig. 1. The XRD pattern of the obtained TiC sample.
Fig. 2. The TEM image of the obtained TiC sample.
reaction is an exothermal reaction, according to the reaction enthalpy Ž D H s y613.4 kcalrmol. w13x. The formation of the product results in an instantaneously high temperature, which makes the crystalline TiC form. The excess TiCl 4 is important to the formation of nanocrystalline TiC, not only by increasing the pressure in the autoclave but it also acts as a diluent to absorb the reaction enthalpy and therefore maintain a relatively low overall reaction temperature. This results in the formation of TiC nanocrystallites instead of an agglomeration product. In our previous experiment, when carbon powders and metal titanium were substituted for CCl 4 and TiCl 4 , respectively, the synthetic reaction needed a high processing temperature Ž650–7008C. w14x. These experimental results are consistent with the suggestion above. Using these solid state reactants with high melting-temperature Žthe melting point of Ti is 16608C, and that of C powders is 36528C w13x. did not result in a high pressure in the autoclave, which cannot exert a favorable effect on the crystallization of the product. Moreover, using CCl 4 and TiCl 4 as the reactants results in a higher reaction enthalpy than using C or Ti as reactant, respectively, according to the reaction heats: D H1 s y613.4 kcalrmol Žusing TiCl 4 and CCl 4 as reactants.; D H2 s y412.6 kcalrmol Žusing Ti and CCl 4 as reactants., and D H3 s y245.0 kcalrmol Žusing TiCl 4 and C as
Q. Lu et al.r Chemical Physics Letters 314 (1999) 37–39
reactants. w13x, which is beneficial to the formation of crystalline TiC. In the co-reduction process, the processing temperature and time also play important roles to the formation of TiC nanocrystallites. Although the synthetic reaction is thermodynamically spontaneous and exothermal, crystalline TiC cannot form at a temperature lower than 3708C or in a time shorter than 3 h. Heating to a temperature of higher than 6008C will result in the increase of the size of the TiC produced. Varying the processing time between 3 and 15 h at 4508C did not affect the crystallinity and the particle size greatly. The optimum processing temperature and time for TiC nanocrystallites are 420–5008C and 6–8 h, respectively. Certainly, a deeper understanding of the reaction mechanism is needed, but we believe that this method could in principle produce other stable carbide nanocrystallites.
Acknowledgements This work is supported by Chinese National Natural Science Research Foundation.
39
References w1x R.J. Meyer, E.H.E. Pietsch, Gmelins Handbook of Anorganic Chemistry, vol. 41, 8th ed., Springer, Berlin, 1951, p. 357 Žin German.. w2x P. Pascal, Nouveau traite´ de chimie minerale, vol. 9, Masson, ´ Paris, 1963, p. 150 w3x High Temperature Chemistry of Inorganic Materials, The Chemical Society, London, 1977. w4x H. Brune, M. Giovannini, K. Bromann, K. Kern, Nature ŽLondon. 394 Ž1998. 451. w5x R.N. Barnett, U. Landman, Nature ŽLondon. 387 Ž1997. 788. w6x I.N. Mihailescu, N. Chitica, V.S. Teodorescu, M. Popescu, M.L. De Giorge, A. Luches, A. Perrone, C. BoulmerLeborgne, J. Hermann, B. Dubreuil, S. Urdea, A. Barborica, I. Iova, J. Appl. Phys. 75 Ž1994. 5286. w7x K.S. Vecchio, J.C. LaSalvia, M.A. Meyers, G.T. Gray, Metall. Trans. 23A Ž1992. 87. w8x Z.A. Munir, W. Lai, Combust. Sci. Technol. 88 Ž1993. 210. w9x K. Thorne, S.J. Ting, C.J. Chu, J.D. Mackenzie, T.D. Getman, M.F. Hawthorne, J. Mater. Sci. 27 Ž1992. 4406. w10x P.R. Tayler, S.A. Pirzada, T.D. McColm, Plasma Synthesis Processing of Materials, Proc. Symp., 1993, pp. 179–190. w11x L.L. Ye, M.X. Quan, Nanostruct. Mater. 5 Ž1995. 25. w12x M.S. Hsu, M.A. Meyers, A. Berkowitz, Scr. Metall. Mater. 32 Ž1995. 805. w13x J.A. Dean, Lange’s Handbook of Chemistry, 13th ed., McGraw-Hill, New York, 1985. w14x J. Hu, Q. Lu, K. Bin, B. Deng, Y. Qian, G. Zhou, X. Liu, J. Wu, unpublished.