CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 646–649
Communications Morphology Evolution of TiCx Grains During SHS in an Al-Ti-C System Shenbao Jin, Ping Shen, Binglin Zou, and Qichuan Jiang* Key Laboratory of Automobile Materials, Department of Materials Science and Engineering, Jilin UniVersity, No. 5988 Renmin Street, Changchun, 130025, P. R. China ReceiVed May 18, 2008; ReVised Manuscript ReceiVed December 4, 2008
ABSTRACT: The morphologies of the TiCx grains extracted from the self-propagating high-temperature synthesis (SHS) products in an Al-Ti-C system were examined by field-emission scanning electron microscope (FESEM) and their lattice parameters were calculated from the XRD results. It is believed that the combustion temperature plays an important role in determining their morphologies, which may evolve from octahedron to truncated octahedron and finally to sphere with the increase in the temperature. Self-propagating high-temperature synthesis (SHS), as a more time- and energy-efficient method, has been extensively applied to the preparation of refractory ceramics, of which titanium carbide, TiCx, has attracted much attention because of its high melting point, high modulus, high hardness, good chemical stability, etc. However, the fact that it is rather difficult to control the morphology of the resultant phases more or less limits its industrial applications. In recent years, many researches have been carried out on the control of the morphology of the SHS products,1-5 particularly for the typical TiCx phase. For instance, Cochepin et al.2,3 investigated the morphology (the grain size especially) of the TiCx grains in a Ti-C system and they suggested that the combustion temperature should play an important role in the TiCx crystal growth. However, detailed description of the morphology of the TiCx grains formed in the Al-Ti-C system, which is a classical reactive system and widely used to produce TiCx,6-9 is quite few. As the morphology of the TiCx grains may affect certain technology (such as sinterability) and final mechanical properties of a sintered TiCx-base material or abrasive properties of the TiCx powder, it is highly desirable to be able to understand and control it during the SHS reaction in the Al-Ti-C system. In the work done by Brinkman et al.,6 severe agglomeration of TiCx was observed in the samples with higher reactant C/Ti ratio, whereas Choi and Rhee9 reported that with the increase in the Al content, TiCx changed its morphology from a sinteredlike microstructure to spheral and monodispersed small particulates coated with a thin layer of Al. However, after removal of the Al film using a diluted HCl solution, which does not erode the TiCx grains,10 our experimental results, as shown in the present paper, indicated that many flat planes and growth steps emerged on the grain surfaces. On the basis of the careful examinations of the growth shapes and steps of the TiCx grains, we inferred the main factor that controls the morphologies of the TiCx grains and possible growth mechanism of TiCx in the SHS reaction in the Al-Ti-C system. * Corresponding author. Tel/Fax: 86 431 8509 4699. E-mail: jqc@ jlu.edu.cn.
Table 1. Compositions in the Al-Ti-C Reactant Compacts series I sample no. Al (wt %) C/Ti (molar ratio)
1 50 1.0
2 50 0.7
preheating sample
series II 3 30 0.6
4 30 0.7
5 30 0.8
6 30 1.0
7 30 1.2
8 30 0.6
The raw materials used in this study were commercial powders of Ti (>99.5% purity, ∼48 µm), C (>99.5% purity, ∼38 µm) and Al (>99% purity, ∼29 µm). The powders were proportioned with different C/Ti ratios and different Al contents (see Table 1) and then homogeneously mixed. The powder mixtures were then pressed into cylindrical compacts of ∼22 mm in diameter and ∼15 mm in height with green densities of ∼60 ( 2% of theoretical. The SHS experiments were conducted in a self-made vacuum vessel in an Ar atmosphere using an arc as ignition source. During the SHS process, the temperature in the position about 3 mm beneath the center of the compact top surface was measured by W5-Re26 thermocouples and the signals were recorded and processed by a data acquisition system using an acquisition speed of 50 ms/point. Two series of experiments were performed. In series I, the reactant C/Ti atomic ratios were assigned to 1.0 and 0.7, respectively, with 50 wt % Al. In series II, the reactant C/Ti atomic ratios varied from 0.6 to 1.2 with 30 wt % Al, as shown in Table 1. In addition, a sample with C/Ti ) 0.6 and 30 wt % Al was preheated to 600 °C and then ignited. The aim is to yield a higher combustion temperature and then to see the effect of the temperature on the TiCx morphology. The phase compositions in the reacted samples were identified by X-ray diffraction (XRD, Rigaku D/Max 2500PC, Japan) with Cu KR radiation using a scanning speed of 4°/min and an angle (2θ) step of 0.05°. The bulk samples were then dissolved in a 18 vol. % HCl-distilled water solution to remove the Al coating on the surfaces of the TiCx grains. In order to evaluate the lattice parameter of the synthesized TiCx grains, a much slower scanning was performed for the extracted TiCx powders around the (220)
10.1021/cg800527q CCC: $40.75 2009 American Chemical Society Published on Web 01/16/2009
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Figure 3. Variations in lattice parameter a0 and stoichiometry x of the synthesized TiCx phase with the reactant C/Ti atomic ratio in the samples of series I and II.
Figure 1. XRD patterns for the reacted Al-Ti-C samples in series I and II.
peak using a scanning speed of 0.05°/min and an angle (2θ) step of 0.002°. The morphologies of the extracted TiCx grains were then observed using a field emission scanning electron microscope (FESEM, JSM 6700F, Japan). As shown in Figure 1, the reaction products in samples 3-7 (Series II) mainly consist of TiCx and Al. Because of the deficiency of C in the samples with the reactant C/Ti < 0.8, Al3Ti and Ti3AlC2 were also found. In contrast, a small amount of Al4C3 was found in the sample with C/Ti ) 1.2. In the samples of series I, the XRD results indicate that the reaction products are mainly TiCx, Al and Al3Ti. The increase in the Al reactant resulted in the decrease in the combustion temperature (see Figure 2i) and the increase in the quantity of the Al3Ti phase. Figure 2 shows the representative FESEM images of the extracted TiCx grains in various samples together with the variation in the maximum combustion temperature with the C/Ti ratio in the reactants. As indicated, the TiCx grains formed in sample 1 with the lowest combustion temperature (1393 °C) are in typical
octahedron shapes with sizes about 50-200 nm. With the increase in the combustion temperature, as shown in images b and c in Figure 2 for samples 2 and 3, respectively, the synthesized TiCx grains are still in the octahedron shapes but with larger sizes, which are about 0.1-0.6 and 0.5-1.2 µm, respectively. In sample 4, as the maximum combustion temperature reaches about 1900 °C, the TiCx grains are of truncated octahedron shapes with sizes about 0.8-2.5 µm [Figure 2 (d)]. In samples 5-7, the sizes of the TiCx grains are about 1-4, 0.8-3.5 and 0.8-2 µm, respectively (Figure 2e-g). As the TiCx nucleates in the vicinity of the C particles, more TiCx nuclei will form if the number of the C particles (depending on the reactant C/Ti ratio) increases. Hence, in contrast to that in sample 5, the decrease in the grain size in sample 7 with similar combustion temperature (see Figure 2i) is presumed to result from the more serious jam of the growing grains. The grain shapes are generally spherelike; however, the sphericity, which can be characterized by the proportion of the flat planes to the curved ones on their surfaces, is varied. In sample 6 (Figure 2f) with the highest combustion temperature, the area of the flat planes is relatively small and the TiCx grains exhibit the highest sphericity. Whereas, in samples 5 and 7, the area of the flat planes seems to increase considerably. It is worth mentioning that these fine planes displaying on the TiCx surface give us a hint to disclose the TiCx growth mechanism during
Figure 2. (a-h) FESEM images of the TiCx grains extracted from the reacted Al-Ti-C samples (1-8) and (i) variation in maximum combustion temperature, Tc, with the reactant C/Ti ratio (b, series I; 0, series II; 2, sample 8). Inset: a schematic of the typical temperature profile recorded in the SHS reaction.
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Figure 4. (a-f) FESEM images of the TiCx grains synthesized in the Al-Ti-C samples: (a) sample 1, (b-d) sample 4, (e) sample 5, and (f) sample 6. (g-k) Schematic illustration of the morphology evolution of the TiCx grain during SHS.
Figure 5. FESEM images of the jam-packed TiCx grains synthesized in (a) sample 5 and (b) sample 6.
SHS, as to be discussed later. From Figure 2, it seems that the morphology of the TiCx grains formed under a low temperature condition is octahedron. As the combustion temperature increases, the anisotropy growth of TiCx decreases, leading to the spherical shapes. Moreover, similar combustion temperatures give the similar morphologies (the grain shape especially) (e.g., see Figures 2e-g for samples 5-7). These findings inform us that the combustion temperature might play an important role in influencing the morphology of TiCx during the SHS reaction in the Al-Ti-C system. To further confirm this speculation, the sample with 30 wt % Al and C/Ti ) 0.6 was first preheated to 600 °C in a furnace preplaced in the vacuum chamber and then immediately ignited by arc. The maximum combustion temperature, Tc, increased from approximately 1750 °C (without preheating) to 1950 °C. As indicated in Figure 2h, the morphology of TiCx formed in this sample is quite different from that in sample 3 with the same reactant composition. In fact, most of the TiCx grains in sample 8 show the shapes of the truncated octahedron similar to those in sample 4, however, some have the curved planes. The morphology of the TiCx grains in sample 8 is virtually between that in samples 4 and 5. This result further demonstrates that the combustion temperature should be an important factor in determing the morphology of the resultant TiCx during the SHS reaction in the Al-Ti-C system. On the other hand, it is well-known that TiCx has a wide stoichiometric composition (from 0.47 to 0.98).11 Although the crystal structure does not change with its stoichiometry,12 the lattice parameter does. It was postulated that the stoichiometry of TiCx might also affect the TiCx morphology since the C/Ti ratio in the reactants was quite different. The lattice parameter a0 of the synthesized TiCx phase was then calculated on the basis of the XRD
results using a slow scanning speed. The stoichiometry of TiCx was estimated from the available relationship between the TiCx lattice parameter and its stoichiometry.10 The calculated lattice parameter a0 and stoichiometry x as a function of the reactant C/Ti ratio are plotted in Figure 3. It can be seen that the stoichiometry, x, of TiCx generally increases with the increase in the reactant C/Ti ratio as well as with the increasing combustion temperature, however, there are several exceptional cases. For example, for sample 1 with the reactant C/Ti ) 1.0 and 50 wt % Al, the TiCx stoichiometry, x, is slightly lower than that for sample 6 with the same C/Ti ratio, and it is higher than that for sample 3 (both sample 6 and sample 3 contain 30 wt % Al). Whereas, the combustion temperature for sample 1 is considerably lower than that for the latter two. Also, the value of x for sample 2 (50 wt % Al) is appreciably higher than that for sample 4 (30 wt % Al) having the same reactant C/Ti ratio (C/Ti ) 0.7), whereas the combustion temperature for the former is substantially lower than that for the latter. In fact, the stoichiometry of the synthesized TiCx phase can be affected by many factors, such as the reactant C/Ti ratio, combustion temperature, formation of the Al-Ti intermediate phases, and dissolution of residual Ti or C in Al in the cooling stage. Yang et al.13 suggested that for C/Ti < 1.0, the formation of the Al-Ti intermediate phases in the SHS products consumed many Ti and thus led to the increase in the stoichiometry, x, of TiCx. In fact, with the decrease in the reactant C/Ti ratio or the increase in the Al content, the combustion temperature decreases and the amount of the Al3Ti phase increases (see Figures 1 and 2i), which could have opposite effects on the stoichiometry of the synthesized TiCx. Thus, the variation in the stoichiometry shown in Figure 3 could be understood if these influencing factors were taken into account. Furthermore, as indicated in Figure 3, a considerable difference exists in the lattice parameter and the TiCx stoichiometry in the samples of series I, but the shapes of the TiCx grains (see images a and b in Figure 2) are similar. Similar behavior and phenomenon were also observed in samples 5 and 7 (with C/Ti ) 0.8 and 1.2) in series II (see images e and g in Figure 2). On the other hand, the values a0 and x of TiCx do not differ significantly in the samples with C/Ti ) 0.7 and 1.0 in the reactants, but their morphologies vary considerably (see images d and f in Figure 2). Therefore, we speculate that there is no corresponding relationship between the TiCx stoichiometry and its morphology. Namely, the morphology is seemingly independent of the stoichiometry of TiCx. The preceding results suggest that it is the combustion temperature rather than the stoichiometry of the synthesized TiCx phase
Communications that plays the dominating role in determing its morphology. If so, the TiCx crystal growth mechanism during SHS can be inferred by comparing the morphologies of TiCx formed in the samples with different combustion temperatures. Following this idea, we carefully examined the morphologies of TiCx developed in the samples with elevated combustion temperatures, and suggested a mechanism, as illustrated in Figure 4. At relatively low combustion temperatures, a mass of the octahedral TiCx grains with fine sizes begin to form [Figure 4 (a)]. The octahedron might be the nucleation unit as well as the growth unit of TiCx, as proposed by Jin et al.14 If the octahedron could grow freely in a liquid environment under an equilibrium condition, the final ideal morphology of TiCx would be octahedron.15 As known, TiCx is a typically faceted crystal with a NaCl-type structure, in which, {111} planes share the highest surface atomic density and the lowest surface energy. According to the Bravais empirical law,16 the octahedral TiCx grains developed at low combustion temperatures should be enclosed by eight {111} planes. The flat surfaces of the octahedral TiCx grains indicate that, at this stage, the {111} planes are smooth, on which it is difficult to form the nuclei required for the subsequent growth. Therefore, the growth rate of these planes is the lowest. With the increase in the combustion temperature, in addition to the increase in size, the octahedral TiCx grains change their morphology to truncated octahedron, as shown in Figure 4h. According to ref 17, the shape of an fcc crystal is mainly determined by the ratio of the growth rate in the 〈100〉 direction to that in the 〈111〉 direction. When the ratio is 1.73, octahedron bounded by the eight {111} planes will form; when the ratio is 1.15, the truncated octahedron shape similar to that shown in Figure 4h will form. Therefore, we speculate that the growth rate of the TiCx crystal along the 〈111〉 direction could be accelerated with the increase in the combustion temperature during SHS. On the other hand, as shown in images c and d in Figure 4, some TiCx growth steps with hexagonal geometry emerged on the {111} planes. The presence of these growth steps indicate that the two-dimensional nucleation growth of the {111} planes is favorable. This is possibly attributed to the higher reaction rate and atomic diffusion rate when the combustion temperature increases, leading to a higher density of defects (such as impurities and dislocations) in the TiCx crystal structure and larger concentrations of the C and Ti atoms at the solid-liquid interface. The former implies that there will be more steps created by crystal defects for the growth unit to deposit on the {111} planes, and the latter means that there will be a greater driving force to form TiCx. Both of them accelerate the growth of TiCx along the 〈111〉 direction. When the combustion temperature further increases, the growth steps change their shapes from hexagon to a close-to-orbicular geometry and form multilayer stack to construct a round surface, as shown in Figure 4e. By such a way, the TiCx grains with the spherical morphology could be finally developed. However, the practical crystal growth is very complicated, especially for the ceramics such as TiCx under the conditions far from equilibrium. It is possible that in addition to combustion temperature, other factors such as interaction time for the TiCx precursors in the liquid environment might also play an important role. Nevertheless, such parameters are not completely independent. For example, a higher combustion temperature brings rapider dissolution and then diffusion rates of C in the Al-Ti melt to form TiCx as well as a longer duration for the TiCx precursors to grow
Crystal Growth & Design, Vol. 9, No. 2, 2009 649 in the liquid. In this communication article, we ascribed the primary factor that influences the TiCx morphology to the maximum combustion temperature because it is most noticeable and easy to understand. More profound investigations on the TiCx crystal growth, however, are still in progress. In addition, as we have mentioned before, most TiCx grains observed in this study are not in perfectly spherical shapes but with fine planes more or less on their surfaces. It can be explained by the fact that the multilayer stacking growth of the {111} planes could be inhibited by the lack of free space around them. As shown in Figure 5a, the jam of the TiCx grains in some regions is quite obvious, making them still in the octahedron or truncated octahedron shapes. Whereas, those situated in the regions with sufficient free space have already evolved into the close-to-sphere shapes. On the other hand, in sample 6 with the highest combustion temperature (2042 °C), there are a few TiCx grains with visible octahedron appearance (Figure 5b) in the crowded locations, which also supports our conclusion that the TiCx grains may evolve from an octahedron shape to a sphere one during the SHS reaction. In summary, the combustion temperature plays an important role in determining the morphology of the TiCx grains developed from the SHS reaction in the Al-Ti-C system. The typical sphere shape of the TiCx grains may develop from the initial octahedron and then truncated octahedron with the increase in the combustion temperature, as long as the free space and the concentration conditions for the crystal growth are satisfied.
Acknowledgment. This work is supported by the NNSFC (50531030) and project 985-Automotive Engineering of Jilin University.
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