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Using a cobalt activator to synthesize titanium carbide (TiC) nanopowders
Int. Journal of Refractory Metals and Hard Materials 41 (2013) 363–365
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Using a cobalt activator to synthesize titanium carbide (TiC) nanopowders Hua Lin a,b,⁎, Bowan Tao b, Jie Xiong b, Qing Li a a b
School of Materials Science and Engineering, Southwest University, Chongqing 400715, PR China State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China
a r t i c l e
i n f o
Article history: Received 11 April 2013 Accepted 20 May 2013 Keywords: TiC Nanopowders Ceramics X-ray diffraction
a b s t r a c t Titanium carbide (TiC) nanopowders were successfully synthesized by in-situ reduction and carbonization using a precursor comprising elemental titanium, carbon, and cobalt. The samples prepared at different conditions were characterized by X-ray diffraction and transmission electron microscopy techniques. The thermolysis process of the precursor was investigated by themogravimetric analysis and differential thermal analysis. The experimental results show that the samples have average sizes ranging within 30–40 nm. In addition, the experimental temperature is 100–900 °C lower and the reaction time is 2–90 h shorter than those used in the conventional carbothermal reduction method. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Titanium carbide (TiC) is one of the most important metal carbides with excellent properties, such as high melting temperature, hardness, strength, wear, corrosion resistance, electrical conductivity, and thermal conductivity. Consequently, TiC is widely applied in the manufacture of cutting tools, grinding wheels, and aerospace materials [1–4]. TiC can substitute for tungsten carbide (WC), which is commonly used to produce cemented carbides because of its superior properties and low cost [5]. Recently, nanoscale or nanostructured TiC has received increased attention because the mechanical behavior of TiC-based tools can be improved by decreasing the TiC particle size [6]. Conventionally, TiC is synthesized by methods such as direct chemical reaction between element Ti and C [5,7], carbothermal reduction of titanium oxide using carbon (C) [8,9], mechanical alloying [10,11], thermal plasma processing [3], magnesium reduction of TiCl4 and CCl4 [4], gas phase reaction of TiCl4 and appropriate gaseous hydrocarbons [12], etc. Carbothermal reduction is the most common route to TiC production among the abovementioned methods because using TiO2 as a raw material can reduce the cost and be easily handled. However, carbothermal reduction method difficultly produces TiC nanoparticles because it requires high reaction temperatures (1700–2100 °C) and long holding times (10–24 h) and depends on the size of the TiO2 powders used [13,14]. Increasing the contact area between TiO2 and C can effectively synthesize TiC nanopowders because the resulting improvement in reaction efficiency and decrease in synthesis temperature prevent
⁎ Corresponding author at: School of Materials Science and Engineering, Southwest University, Chongqing 400715, PR China. Tel.: +86 23 68253204; fax: +86 23 68254373. E-mail address: [email protected] (H. Lin). 0263-4368/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2013.05.010
the grain growth of TiC. Koc [15] synthesized TiC through a precursor of C-coated TiO2. Yasuo [4] reported the preparation of TiC from a composite consisting of nanosized TiO2 and an organic polymer by carbothermal reduction. However, only microscale powders can be obtained and the reaction temperatures are >1300 °C. Meanwhile, an activator is known to decrease the activation energy of reactants and accelerate the solid state reaction. Thus, an activator can be used to shorten the reaction process and reduce the synthesis temperature to prevent grain growth. Cobalt (Co) is an ideal activator in the synthesis of metal carbides, such as WC [16]. However, reports on the synthesis of TiC nanopowders using Co-activator are limited. Accordingly, a low-temperature route to synthesizing TiC nanopowders was proposed in this work. Similar to our previous work [17], the solution-derived precursor method was used to prepare a precursor comprising elemental Ti, C, and Co. TiC nanopowders were then obtained by in-situ reduction and carbonization of the precursor at milder conditions. The reaction process, phase composition, microstructure, and chemical change of the precursor during thermal processing were also investigated. 2. Experimental All reagents used were analytically pure and purchased from Chendu Kelong Chemical Plant. Titanium tetrachloride (TiCl4), glucose (C6H12O6), and cobalt chloride (CoCl2) were used as the sources of Ti, C, and Co, respectively. In a typical procedure, C6H12O6 (1.82 g) and CoCl2 (0.03 g, 3 wt.%) were dissolved in 20 ml of absolute ethyl alcohol at 80 °C, and then the solution was cooled to room temperature. TiCl4 (2 ml) was slowly added dropwise to 30 ml of absolute ethyl alcohol with machine stirring at room temperature. A large amount of hydrochloric acid gas was exhausted, and a transparent yellowish solution was obtained. After complete dissolution, the two solutions were
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H. Lin et al. / Int. Journal of Refractory Metals and Hard Materials 41 (2013) 363–365
mixed together and evaporated to dryness at 60 °C. The precursor, which was a uniform mixture of elemental Ti, C, and Co, was then obtained. TiC nanopowders were prepared by thermal processing of the precursor in a vacuum at 1200 °C for 0.5 h. Phase identification was performed with an X-ray diffractometer (XD-3, Purkinje, Beijing, China) using Cu Kα radiation (λ = 0.15406 nm) at a scanning rate of 0.02° s−1 within the 2θ range of 30° to 80°. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were obtained with a Tecnai G2F20 S-TWIN transmission electron microscope at an accelerating voltage of 200 kV. Thermogravimetry (TG) and differential scanning calorimetry (DSC) analysis was performed on a NETZSCH STA 499C simultaneous thermal analyzer. Samples (approximately 12 mg) were heated in corundum crucibles with non-hermetic lids, with corundum serving as the standard. Heating was performed in a dynamic inert atmosphere (argon, 30 ml min−1) from room temperature to 1350 °C at a temperature rate increase of 10 °C min−1.
3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns of the precursor and as-prepared samples under different reaction conditions. Fig. 1a shows the diffraction peaks of the precursor obtained by evaporation to dryness. No apparent diffraction peaks exist, indicating that the precursor has an amorphous structure. Fig. 1b shows the XRD patterns of the sample obtained by heating the precursor at 600 °C for 0.5 h. All diffraction peaks are identified as rutile-structure TiO2 (JCPDS 87-0710). Fig. 1c shows the XRD patterns of the sample obtained by heating the precursor at 1000 °C for 0.5 h. The reflections correspond to the peaks of TiO2, Ti2O3, and TiO composites, indicating that the reduction process is already well under way at that temperature and a part of TiO2 is reduced to titanium suboxide (TiOx, x b 2). Fig. 1d shows the XRD patterns of the sample obtained by heating the precursor at 1100 °C for 0.5 h. The major products are TiC with a few Ti2O3 and TiO and no trace of TiO2. Figs. 1c and 1d show that the rate of the reduction–carbonization reaction increases with increased temperature. Fig. 1e shows the XRD patterns of the sample obtained by heating the precursor at 1200 °C for 0.5 h. All peaks are indexed to cubic-structure TiC with the calculated lattice parameter α = pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 0.4321 nm (using the equation α ¼ d h þ k þ l ), in agreement with the reported value (JCPDS 89–3828, α = 0.4317 nm). No peak of any other phase or impurity has been detected. Thus, the carbonization reaction is complete under the condition, and the optimum parameters for preparing TiC are obtained. The reaction temperature is
Fig. 1. XRD patterns of the precursor (a) and the samples prepared at: (b) 600 °C, 0.5 h; (c) 1000 °C, 0.5 h; (d) 1100 °C, 0.5 h; and (e) 1200 °C, 0.5 h.
100–900 °C lower and the holding time is 2–90 h shorter than those reported using mechanical alloying and direct element reaction method [11,13,14]. This finding can be attributed to two factors: (1) given that the precursor is homogeneous at the molecular level, the pathways of element diffusion are short during the reduction–cabonization process [16], and (2) the co-activator can effectively reduce the activation energy of the reactants and accelerate the solid-state reaction. Fig. 2 shows a typical TEM image and SAED pattern of the sample obtained by heating the precursor at 1200 °C for 0.5 h. The powders show good dispersion and consist of uniformly sized spherical particles 30–40 nm in diameter. Moreover, the as-prepared powders are more uniform and smaller than the powders prepared by direct elemental reaction or mechanical alloying synthesis [10,18]. The SAED pattern (inset of Fig. 2) shows four clear ring patterns corresponding to the (111), (200), (220), and (311) reflections of cubic TiC, indicating that the phase composition is simple. The lattice parameter is measured as ~0.4319 nm from the crystal habit planes and is near the value obtained from XRD peaks (Fig. 1d). To determine whether the addition of Co is effective in accelerating the reaction, contrastive experiments were carried out for comparison. The XRD patterns of the samples prepared under different conditions are shown in Fig. 3. Fig. 3a is the diffraction pattern of the sample synthesized at 1200 °C for 0.5 h without Co addition. The majority of peaks correspond to those of TiC, and Ti2O3 is also detected. This finding indicates that the reduction–carbonization reaction is incomplete. Fig. 3b shows the diffraction pattern of the sample prepared by heating the precursor without Co addition at 1500 °C for 0.5 h. The main product is TiC, and a small quantity of TiCxOy is also detected. The result is same as the previous report [4]. Fig. 3c shows the diffraction pattern of the sample when the precursor was heated with 3% CoCl2 at 1200 °C for 0.5 h. All peaks can be indexed to the face-centered cubic phase of TiC (JCPDS 89-3828), and no diffraction peak of other species can be detected. This finding indicates that the sample comprises single-phase TiC. A comparison of Fig. 3b to c reveals that the synthesis temperature of the precursor with CoCl2 is 300 °C lower than that of the precursor without. Moreover, the synthesis temperature does not decrease with increased CoCl2 addition. When the amount of added CoCl2 exceeds 5%, a small amount of Co is detected with the main product TiC, as shown in Fig. 3d. Therefore, the method is a novel route to synthesizing nanocomposites of TiC-Co powders. Simultaneous TG–DSC measurements of the precursor were carried out to reveal the chemical changes of the precursor during thermal processing, and the results are shown in Fig. 4. The reaction during
Fig. 2. TEM and SAED patterns of the sample with 3% CoCl2 prepared at 1200 °C for 1 h.
H. Lin et al. / Int. Journal of Refractory Metals and Hard Materials 41 (2013) 363–365
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4. Conclusions We demonstrate a novel method of preparing TiC nanopowders by subjecting a precursor to thermal processing at 1200 °C for 0.5 h. The as-prepared powders are spherical particles 30–40 nm in diameter. Using inexpensive equipment, relatively low reaction temperature, and no protection atmosphere, the process is remarkably convenient and low cost. The TiC nanopowders produced by the method can also well substitute for WC in cement carbide industries. Acknowledgement This work is supported by Fundamental Research Funds for the Central Universities (No. XDJK2013B017) and Chongqing Key Natural Science Foundation (CSTC2012jjB50011). Fig. 3. XRD pattern of the samples obtained by thermo-processing the composite at different conditions: (a) at 1300 °C for 0.5 h, without addition; (b) at 1500 °C for 0.5 h, without addition; (c) at 1200 °C for 0.5 h, with 3% CoCl2; and (d) at 1200 °C for 0.5 h, with 5% CoCl2.
thermal processing is divided into three stages, namely, I, II, and III. At first stage I (0–300 °C), the weight of the precursor rapidly reduces, and a large endothermic peak occurs at 90–220 °C in the DSC curve because of the loss of adsorbed and included water of the precursor. At stage II (300–700 °C), the endothermic peak at approximately 400 °C is formed by the carbonization of glucose, and the reaction is shown in Eq. (1). In a previous research [19], the starting material CoCl2 is reduced to metal Co at stage II at about 600 °C. At the third stage III (700–1200 °C), a large and broad endothermic peak at 750–1190 °C is attributed to a series of reductions of TiO2 → TiOx (x b 2; e.g., Ti2O3 and TiO) → TiC and corresponds to a large weight loss in the TG curve. This process can be expressed as Eqs. (2) and (3). Beyond 1200 °C, the TG curve is a flat line, indicating completion of the entire reaction. This finding agrees with the XRD result in Fig. 1d. C6 H12 O6 →C þ H2 O
ð1Þ
TiO2 þ C→TiOx þ CO
ð2Þ
TiOx þ C→TiC þ CO
ð3Þ
Fig. 4. TG–DSC curves of thermoanalysis of the precursor running at 10 °C min−1 in Ar flow.
References [1] Shin Y, Li XS, Wang C, Coleman JR, Exarhos GJ. Synthesis of hierarchical titanium carbide from titania-coated cellulose paper. Adv Mater 2004;16:1212–5. [2] Durlu N. Titanium carbide based composites for high temperature applications. J Eur Ceram Soc 1999;19:2415–9. [3] Tong L, Reddy RG. Synthesis of titanium carbide nano-powders by thermal plasma. Scr Mater 2005;52:1253–8. [4] Yasuo G, Kensaku F, Mikio K, Yutaka O, Masanobu N, Kensuke A, et al. Synthesis of titanium carbide from a composite of TiO2 nanoparticles/methyl cellulose by carbothermal reduction. Mater Res Bull 2001;36:2263–5. [5] Pierson HO. Handbook of refractory carbides and nitrides. Properties, Characteristics, Processing and Applications. Westwood, NJ: Noyes Publications; 1996 . [6] Arora A. Ceramics in nanotech revolution. Adv Eng Mater 2004;6:244–7. [7] Daniel BM, Lipsitt HA. Mechanical properties of fine-grained substoichiomebic titanium carbide. J Am Ceram Soc 1983;66:592–7. [8] Koc R, Folmer JS. Carbothermal synthesis of titanium carbide using ultrafine titania powders. J Mater Sci 1997;32:3101–11. [9] Woo YC, Kang HJ, Kim GJ. Formation of TiC particle during carbothermal reduction of TiO2. J Eur Ceram Soc 2007;27:719–22. [10] Rahaei MB, Rad RY, Kazemzadeh A, Ebadzadeh T. Mechanochemical synthesis of nano TiC powder by mechanical milling of titanium and graphite powders. Powder Technol 2012;217:369–76. [11] Ali M, Basu P. Mechanochemical synthesis of nano-structured TiC from TiO 2 powders. J Alloys Compd 2010;500:220–3. [12] Lee DW, Alexandrovskii SV, Kim BK. Novel synthesis of substoichiometric ultrafine titanium carbide. Mater Lett 2004;58:1471–4. [13] Huber P, Manova D, Mandl S, Rauschenbach B. Formation of TiN, TiC and TiCN by metal plasma immersion ion implantation and deposition. Surf Coat Technol 2003;174–5:1243–7. [14] Thorne K, Ting SJ, Chu CJ, Mackenzie JD, Getman TD, Hawthorne MF. Synthesis of TiC via polymeric titanates: the preparation of fibres and thin films. J Mater Sci 1992;27:4406–14. [15] Swift GA, Koc R. Formation studies of TiC from carbon coated TiO2. J Mater Sci 1999;34:3083–93. [16] Zawrah MF. Synthesis and characterization of WC–Co nanocomposites by novel chemical method. Ceram Int 2007;33:155–61. [17] Lin H, Tao BW, Li Q, Li YR. In situ synthesis of V8 C7 nanopowders from a new precursor. Int J Refract Met Hard Mater 2012;31:138–40. [18] Yuan Q, Zheng Y, Yu HJ. Synthesis of nanocrystalline Ti(C, N) powders by mechanical alloying and influence of alloying elements on the reaction. Int J Refract Met Hard Mater 2009;27:121–5. [19] Lin H, Tao BW, Li Q, Li YR. In situ synthesis of WC–Co nanocomposite powder via core–shell structure formation. Mater Res Bull 2012;47:3283–6.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Using a cobalt activator to synthesize titanium carbide (TiC) nanopowders Hua Lin a,b,⁎, Bowan Tao b, Jie Xiong b, Qing Li a a b
School of Materials Science and Engineering, Southwest University, Chongqing 400715, PR China State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China
a r t i c l e
i n f o
Article history: Received 11 April 2013 Accepted 20 May 2013 Keywords: TiC Nanopowders Ceramics X-ray diffraction
a b s t r a c t Titanium carbide (TiC) nanopowders were successfully synthesized by in-situ reduction and carbonization using a precursor comprising elemental titanium, carbon, and cobalt. The samples prepared at different conditions were characterized by X-ray diffraction and transmission electron microscopy techniques. The thermolysis process of the precursor was investigated by themogravimetric analysis and differential thermal analysis. The experimental results show that the samples have average sizes ranging within 30–40 nm. In addition, the experimental temperature is 100–900 °C lower and the reaction time is 2–90 h shorter than those used in the conventional carbothermal reduction method. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Titanium carbide (TiC) is one of the most important metal carbides with excellent properties, such as high melting temperature, hardness, strength, wear, corrosion resistance, electrical conductivity, and thermal conductivity. Consequently, TiC is widely applied in the manufacture of cutting tools, grinding wheels, and aerospace materials [1–4]. TiC can substitute for tungsten carbide (WC), which is commonly used to produce cemented carbides because of its superior properties and low cost [5]. Recently, nanoscale or nanostructured TiC has received increased attention because the mechanical behavior of TiC-based tools can be improved by decreasing the TiC particle size [6]. Conventionally, TiC is synthesized by methods such as direct chemical reaction between element Ti and C [5,7], carbothermal reduction of titanium oxide using carbon (C) [8,9], mechanical alloying [10,11], thermal plasma processing [3], magnesium reduction of TiCl4 and CCl4 [4], gas phase reaction of TiCl4 and appropriate gaseous hydrocarbons [12], etc. Carbothermal reduction is the most common route to TiC production among the abovementioned methods because using TiO2 as a raw material can reduce the cost and be easily handled. However, carbothermal reduction method difficultly produces TiC nanoparticles because it requires high reaction temperatures (1700–2100 °C) and long holding times (10–24 h) and depends on the size of the TiO2 powders used [13,14]. Increasing the contact area between TiO2 and C can effectively synthesize TiC nanopowders because the resulting improvement in reaction efficiency and decrease in synthesis temperature prevent
⁎ Corresponding author at: School of Materials Science and Engineering, Southwest University, Chongqing 400715, PR China. Tel.: +86 23 68253204; fax: +86 23 68254373. E-mail address: [email protected] (H. Lin). 0263-4368/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2013.05.010
the grain growth of TiC. Koc [15] synthesized TiC through a precursor of C-coated TiO2. Yasuo [4] reported the preparation of TiC from a composite consisting of nanosized TiO2 and an organic polymer by carbothermal reduction. However, only microscale powders can be obtained and the reaction temperatures are >1300 °C. Meanwhile, an activator is known to decrease the activation energy of reactants and accelerate the solid state reaction. Thus, an activator can be used to shorten the reaction process and reduce the synthesis temperature to prevent grain growth. Cobalt (Co) is an ideal activator in the synthesis of metal carbides, such as WC [16]. However, reports on the synthesis of TiC nanopowders using Co-activator are limited. Accordingly, a low-temperature route to synthesizing TiC nanopowders was proposed in this work. Similar to our previous work [17], the solution-derived precursor method was used to prepare a precursor comprising elemental Ti, C, and Co. TiC nanopowders were then obtained by in-situ reduction and carbonization of the precursor at milder conditions. The reaction process, phase composition, microstructure, and chemical change of the precursor during thermal processing were also investigated. 2. Experimental All reagents used were analytically pure and purchased from Chendu Kelong Chemical Plant. Titanium tetrachloride (TiCl4), glucose (C6H12O6), and cobalt chloride (CoCl2) were used as the sources of Ti, C, and Co, respectively. In a typical procedure, C6H12O6 (1.82 g) and CoCl2 (0.03 g, 3 wt.%) were dissolved in 20 ml of absolute ethyl alcohol at 80 °C, and then the solution was cooled to room temperature. TiCl4 (2 ml) was slowly added dropwise to 30 ml of absolute ethyl alcohol with machine stirring at room temperature. A large amount of hydrochloric acid gas was exhausted, and a transparent yellowish solution was obtained. After complete dissolution, the two solutions were
364
H. Lin et al. / Int. Journal of Refractory Metals and Hard Materials 41 (2013) 363–365
mixed together and evaporated to dryness at 60 °C. The precursor, which was a uniform mixture of elemental Ti, C, and Co, was then obtained. TiC nanopowders were prepared by thermal processing of the precursor in a vacuum at 1200 °C for 0.5 h. Phase identification was performed with an X-ray diffractometer (XD-3, Purkinje, Beijing, China) using Cu Kα radiation (λ = 0.15406 nm) at a scanning rate of 0.02° s−1 within the 2θ range of 30° to 80°. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were obtained with a Tecnai G2F20 S-TWIN transmission electron microscope at an accelerating voltage of 200 kV. Thermogravimetry (TG) and differential scanning calorimetry (DSC) analysis was performed on a NETZSCH STA 499C simultaneous thermal analyzer. Samples (approximately 12 mg) were heated in corundum crucibles with non-hermetic lids, with corundum serving as the standard. Heating was performed in a dynamic inert atmosphere (argon, 30 ml min−1) from room temperature to 1350 °C at a temperature rate increase of 10 °C min−1.
3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns of the precursor and as-prepared samples under different reaction conditions. Fig. 1a shows the diffraction peaks of the precursor obtained by evaporation to dryness. No apparent diffraction peaks exist, indicating that the precursor has an amorphous structure. Fig. 1b shows the XRD patterns of the sample obtained by heating the precursor at 600 °C for 0.5 h. All diffraction peaks are identified as rutile-structure TiO2 (JCPDS 87-0710). Fig. 1c shows the XRD patterns of the sample obtained by heating the precursor at 1000 °C for 0.5 h. The reflections correspond to the peaks of TiO2, Ti2O3, and TiO composites, indicating that the reduction process is already well under way at that temperature and a part of TiO2 is reduced to titanium suboxide (TiOx, x b 2). Fig. 1d shows the XRD patterns of the sample obtained by heating the precursor at 1100 °C for 0.5 h. The major products are TiC with a few Ti2O3 and TiO and no trace of TiO2. Figs. 1c and 1d show that the rate of the reduction–carbonization reaction increases with increased temperature. Fig. 1e shows the XRD patterns of the sample obtained by heating the precursor at 1200 °C for 0.5 h. All peaks are indexed to cubic-structure TiC with the calculated lattice parameter α = pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 0.4321 nm (using the equation α ¼ d h þ k þ l ), in agreement with the reported value (JCPDS 89–3828, α = 0.4317 nm). No peak of any other phase or impurity has been detected. Thus, the carbonization reaction is complete under the condition, and the optimum parameters for preparing TiC are obtained. The reaction temperature is
Fig. 1. XRD patterns of the precursor (a) and the samples prepared at: (b) 600 °C, 0.5 h; (c) 1000 °C, 0.5 h; (d) 1100 °C, 0.5 h; and (e) 1200 °C, 0.5 h.
100–900 °C lower and the holding time is 2–90 h shorter than those reported using mechanical alloying and direct element reaction method [11,13,14]. This finding can be attributed to two factors: (1) given that the precursor is homogeneous at the molecular level, the pathways of element diffusion are short during the reduction–cabonization process [16], and (2) the co-activator can effectively reduce the activation energy of the reactants and accelerate the solid-state reaction. Fig. 2 shows a typical TEM image and SAED pattern of the sample obtained by heating the precursor at 1200 °C for 0.5 h. The powders show good dispersion and consist of uniformly sized spherical particles 30–40 nm in diameter. Moreover, the as-prepared powders are more uniform and smaller than the powders prepared by direct elemental reaction or mechanical alloying synthesis [10,18]. The SAED pattern (inset of Fig. 2) shows four clear ring patterns corresponding to the (111), (200), (220), and (311) reflections of cubic TiC, indicating that the phase composition is simple. The lattice parameter is measured as ~0.4319 nm from the crystal habit planes and is near the value obtained from XRD peaks (Fig. 1d). To determine whether the addition of Co is effective in accelerating the reaction, contrastive experiments were carried out for comparison. The XRD patterns of the samples prepared under different conditions are shown in Fig. 3. Fig. 3a is the diffraction pattern of the sample synthesized at 1200 °C for 0.5 h without Co addition. The majority of peaks correspond to those of TiC, and Ti2O3 is also detected. This finding indicates that the reduction–carbonization reaction is incomplete. Fig. 3b shows the diffraction pattern of the sample prepared by heating the precursor without Co addition at 1500 °C for 0.5 h. The main product is TiC, and a small quantity of TiCxOy is also detected. The result is same as the previous report [4]. Fig. 3c shows the diffraction pattern of the sample when the precursor was heated with 3% CoCl2 at 1200 °C for 0.5 h. All peaks can be indexed to the face-centered cubic phase of TiC (JCPDS 89-3828), and no diffraction peak of other species can be detected. This finding indicates that the sample comprises single-phase TiC. A comparison of Fig. 3b to c reveals that the synthesis temperature of the precursor with CoCl2 is 300 °C lower than that of the precursor without. Moreover, the synthesis temperature does not decrease with increased CoCl2 addition. When the amount of added CoCl2 exceeds 5%, a small amount of Co is detected with the main product TiC, as shown in Fig. 3d. Therefore, the method is a novel route to synthesizing nanocomposites of TiC-Co powders. Simultaneous TG–DSC measurements of the precursor were carried out to reveal the chemical changes of the precursor during thermal processing, and the results are shown in Fig. 4. The reaction during
Fig. 2. TEM and SAED patterns of the sample with 3% CoCl2 prepared at 1200 °C for 1 h.
H. Lin et al. / Int. Journal of Refractory Metals and Hard Materials 41 (2013) 363–365
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4. Conclusions We demonstrate a novel method of preparing TiC nanopowders by subjecting a precursor to thermal processing at 1200 °C for 0.5 h. The as-prepared powders are spherical particles 30–40 nm in diameter. Using inexpensive equipment, relatively low reaction temperature, and no protection atmosphere, the process is remarkably convenient and low cost. The TiC nanopowders produced by the method can also well substitute for WC in cement carbide industries. Acknowledgement This work is supported by Fundamental Research Funds for the Central Universities (No. XDJK2013B017) and Chongqing Key Natural Science Foundation (CSTC2012jjB50011). Fig. 3. XRD pattern of the samples obtained by thermo-processing the composite at different conditions: (a) at 1300 °C for 0.5 h, without addition; (b) at 1500 °C for 0.5 h, without addition; (c) at 1200 °C for 0.5 h, with 3% CoCl2; and (d) at 1200 °C for 0.5 h, with 5% CoCl2.
thermal processing is divided into three stages, namely, I, II, and III. At first stage I (0–300 °C), the weight of the precursor rapidly reduces, and a large endothermic peak occurs at 90–220 °C in the DSC curve because of the loss of adsorbed and included water of the precursor. At stage II (300–700 °C), the endothermic peak at approximately 400 °C is formed by the carbonization of glucose, and the reaction is shown in Eq. (1). In a previous research [19], the starting material CoCl2 is reduced to metal Co at stage II at about 600 °C. At the third stage III (700–1200 °C), a large and broad endothermic peak at 750–1190 °C is attributed to a series of reductions of TiO2 → TiOx (x b 2; e.g., Ti2O3 and TiO) → TiC and corresponds to a large weight loss in the TG curve. This process can be expressed as Eqs. (2) and (3). Beyond 1200 °C, the TG curve is a flat line, indicating completion of the entire reaction. This finding agrees with the XRD result in Fig. 1d. C6 H12 O6 →C þ H2 O
ð1Þ
TiO2 þ C→TiOx þ CO
ð2Þ
TiOx þ C→TiC þ CO
ð3Þ
Fig. 4. TG–DSC curves of thermoanalysis of the precursor running at 10 °C min−1 in Ar flow.
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