A Common Regularity of Stoichiometry-Induced Morphology Evolution

A Common Regularity of Stoichiometry-Induced Morphology Evolution of Transition Metal Carbides, Nitrides, and Diborides during Self-Propagating ...
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A Common Regularity of Stoichiometry-Induced Morphology Evolution of Transition Metal Carbides, Nitrides, and Diborides during Self-Propagating High-Temperature Synthesis Shenbao Jin, Ping Shen, Dongshuai Zhou, and Qichuan Jiang* Key Laboratory of Automobile Materials, Ministry of Education, and Department of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, P.R. China ABSTRACT: The morphologies of the transition metal carbide (TMC) (ZrCx, NbCx, and TaCx), transition metal nitride (TMN) (TiNx), and transition metal diboride (TMD) (NbB2x and TaB2x) particles formed during the selfpropagating high-temperature synthesis (SHS) were investigated. The results indicate that the ceramics with wide stoichiometric ranges all show a stoichiometry-induced morphology evolution, i.e., octahedron → truncated-octahedron → spherelike → sphere, for TMCs and TMNs, and hexagonal prism → polyhedron → spherelike, for TMDs. For TMCs and TMNs, the increase in the stoichiometry leads to the increase in the growth rate in the ⟨111⟩ crystalline direction. Hence, their morphologies show an evolution process of gradual exposure of the {100} surfaces and shrinkage of the {111} surfaces. When the exposed {100} surfaces are roughed because of the extremely high combustion temperatures during the SHS and thus turn round, the growth shapes of the TMC and TMN crystals change to spherelike. On the other hand, when the TMCs and TMNs are stoichiometric or near stoichiometric, the critical transition temperature for thermodynamic roughening of the {100} surfaces could be very high. Then, the rounded {100} will restore to the flat surfaces, and the cubic and truncatedcubic TMCs and TMNs particles are formed. For TMDs, the morphology evolution could be caused by the decrease in the stability of the {0001} and {101̅0} surfaces at high stoichiometries. With the increase in the stoichiometry, these two surfaces are less-exposed gradually while the {110̅ 1} surfaces are exposed and expand. The growth shapes of TMDs change from regular hexagonal prism to polyhedron. With the rounding (roughening) transition of the {11̅01} surfaces at high temperatures, the TMDs particles become spherical. hardness,3,4 modulus,5 wettability with the molten metals,6,7 and critical transition temperature for superconductivity.8−13 Recently, a morphology evolution of one of the TMCs, TiCx, was reported by us as from octahedron to truncated-octahedron and further to spherelike during the SHS.14 This morphology evolution was explained later by relating it to the TiCx stoichiometry,15 i.e., the quite low stoichiometry of TiCx makes the {111} become the most stable surfaces, leading to the formation of the octahedral TiCx particles; however, with the increase in the TiCx stoichiometry, the {100} emerge on the TiCx surfaces and expand, and the TiCx particles change their shapes to the truncated-octahedron. Considering the importance of the shapes of the ceramic particles in their some applications, such as the reinforcement in composites16−19 and the refiners during casting,20,21 the above results are of great significance to understand the growth mechanism of the ceramics with wide stoichiometric ranges and then to realize the shape control.

1. INTRODUCTION Self-propagating high-temperature synthesis (SHS), as a more time- and energy-efficient method, has been extensively applied to the synthesis of various materials, including intermetallic compounds, composite materials, and especially, refractory ceramics such as transition metal (TM) carbides (TMCs), nitrides (TMNs), and diborides (TMDs).1 These refractory ceramics have attracted much interest from both scientific and technological perspectives due to their unique combinations of properties, such as high melting point, hardness, chemical stability, high thermal conductivity, low electrical resistance, and low work function, and these advantages made these materials particularly important in several engineering applications for structural materials, hard and corrosionresistant coatings, and reinforcing phases in the composites.2 As known, some of the TMCs, TMNs, and TMDs can vary widely in their stoichiometry, such as TiCx (0.47 < x < 0.98), NbCx (0.54 < x < 0.96), ZrCx (0.49 < x < 1.0), TiNx (0.44 < x < 1.0), ZrNx (0.68 < x < 0.99), NbB2x (1.78 < 2x < 3.16), and TaB2x (1.84 < 2x < 2.92). Because of the great influence of the stoichiometry on the atomic bonds in these ceramic crystals, the variation in the stoichiometry usually leads to a significant change to their physical properties, such as the melting point, © 2012 American Chemical Society

Received: December 4, 2011 Revised: March 13, 2012 Published: May 8, 2012 2814

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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. The phase compositions in the reacted samples were identified by X-ray diffraction (XRD, Rigaku D/Max 2500PC) with Cu Kα 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 ceramic particles. The morphologies of the extracted particles were then observed using a field emission scanning electron microscope (FESEM, JSM 6700F).

Except TiCx, many TMCs, TMNs, and TMDs also have wide stoichiometric ranges. Moreover, the TMCs and TMNs, such as NbCx, ZrCx, TaCx, TiNx, and ZrNx, have the same crystal structure as TiCx, a simple rocksalt structure (Fm3̅m, 255). Hence, whether there is similar stoichiometry-induced morphology evolution regularity during the SHS for these ceramics becomes an interesting question. Among these ceramics, the morphologies of the ZrCx and TiNx particles synthesized through the SHS were investigated mostly.22−32 The results indicated that the ZrCx particles formed in the Fe− Zr−C systems with high combustion temperatures generally display spherical morphology,22,23 while in the Al−Zr−C system, with the increase in the content of the Al addition from 0 to 50 wt % and thus the decrease in the combustion temperature from 3100 to 1200 °C, the synthesized ZrCx particles change their morphology from sintered particles with rounded surfaces to the monodispersed octahedral particles with only flat {111} surfaces exposed.28 On the other hand, the TiNx particles formed in the Ti−N2 gas system (nitrogen pressure is 2 MPa) had a variety of shapes, including dendrite, octahedron, truncated-cube, and spherelike.32 The variety of the shapes of the TiNx particles was explained with the competition of the growth rates in the {100} and {111} crystalline directions. However, the dominating factor for this competition has not been decided. In fact, the mechanism for the morphology transition of the ZrCx and TiNx particles during the SHS is still unclear, and the influence of the stoichiometry on their growth shapes has not been considered. Except ZrCx and TiNx, the work on the morphologies of the NbCx, TaCx and NbB2x particles synthesized through the SHS are rather limited. Therefore, in this paper, we first investigated the growth shapes of the TMC (ZrCx, NbCx and TaCx), TMN (TiNx), and TMD (NbB2x and TaB2x) particles formed with different stoichiometries during the SHS and presented the morphology evolution processes for them. Then, the mechanism for this stoichiometry-induced morphology evolution was discussed.

3. RESULTS AND DISCUSSION 3.1. Morphology Evolution of the TMC, TMN, and TMD Particles during the SHS. 3.1.1. TMCs. Figure 1a shows

2. EXPERIMENTAL SECTION

Figure 1. (a) Maximum combustion temperatures of the Al−Nb−C, Al−Zr−C, and Al−Ta−C samples, as functions of the C/TM molar ratio in the reactant mixtures. (b) Maximum combustion temperatures of Al−Ti−C samples and the schematic illustration of the morphology evolution of the TiCx particles during SHS (image modified from ref 14). (c and d) FESEM images of the TiCx particles formed at C/Ti = 1.4 and 1.6 in Al−Ti−C system, respectively.

Particles of TMCs (ZrCx, NbCx and TaCx), TMNs (TiNx), and TMDs (NbB2x and TaB2x) were synthesized in the Al−TM−C, −BN (boron carbide), and −B systems in our experiments, respectively, and their different stoichiometries were gained through changing the C/ TM, BN/TM, and B/TM molar ratios in the reactants. The stoichiometries of the products, of course, do not equal the molar ratios in the reactants. Nevertheless, when the C, BN, or B content in the reactants increases, their concentrations in the Al−TM melts which formed first during the heating process will increase, and accordingly, the stoichiometries of the growing TMCx, TMNx, and TMBx particles are supposed to increase. On the other hand, the increase in the C/TM or BN/TM molar ratio usually leads to the increase in the combustion temperature, which accelerates the solution rates of C and BN into the Al−TM melts. Therefore, it is reasonable to believe that the stoichiometries of the growing TMCx, TMNx, and TMBx increase with the increase in the C/TM, BN/TM, and B/TM molar ratios in the reactants. The raw materials were commercial powders of Ti (>99.5% purity, ∼48 μm), Zr (>99.5% purity, ∼48 μm), Nb (>99.5% purity, ∼48 μm), Ta (>99.5% purity, ∼48 μm), carbon black (∼99%, in nanosize), and Al (>99% purity, ∼29 μm). The powders were proportioned with different C/TM, BN/TM, and B/TM molar ratios and 20 wt % Al 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, as determined from weight and geometric measurements. The SHS experiments were

the maximum combustion temperatures of the Al−Nb−C, Al− Zr−C, and Al−Ta−C samples, as functions of the C/TM molar ratio in the reactant mixtures. As indicated, with the increase in the C/Nb ratio in the Al−Nb−C system, the combustion temperature first increases monotonously from about 1645 °C (C/Nb = 0.6) to 1750 °C (C/Nb = 1.0), and then decreases slightly to 1730 °C at C/Nb = 1.2. Similar variations can be found for the combustion temperatures in the Al−Zr−C system, and the sample with C/Zr = 1.0 has the highest combustion temperature, about 1500 °C. For Al−Ta−C system, the heat released by the formation reaction of TaCx is so small that the combustion reaction cannot be selfsustained if the heat source is removed except in the sample with C/Ta = 1.0 (although the combustion temperature of this sample is very low, only about 1200 °C). Hence, the TaCx particles formed in other Al−Ta−C samples were prepared with continuous heating. In contrast to the combustion temperatures of the Al−Ti−C samples shown in Figure 1b (1730−2050 °C), those of the Al−Nb−C, Al−Zr−C, and Al− Ta−C samples are relatively lower. The decrease in the combustion temperature will of course limit the dissolution and diffusion of carbon during the SHS, resulting in the deficiency 2815

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of [C] in the Al−TM melts. As a consequence, the intermediate products of intermetallic compounds Al3Nb, Al3Zr, and Al3Ta are formed in these samples, as shown in Figure 2. Nevertheless, the main products are still the ceramics of NbCx, ZrCx, and TaCx.

particles are stoichiometry-dependent, but not combustiontemperature-dependent, which can be further proved here, as when more carbon was incorporated in the Al−Ti−C reactants (C/Ti = 1.4 and 1.6), the combustion temperature decreases, while the TiCx particles are still spherical (Figure 1c,d). Then, the serious deficiency of [C] in the Al−Nb, Al−Zr, and Al−Ta melts caused by the relatively low combustion temperatures, as mentioned before, will limit the augmentation of the stoichiometry of the growing ceramic particles during the SHS and thus limit their morphology evolution. Figure 3 shows the morphologies of the NbCx, ZrCx, and TaCx particles formed in the Al−Nb−C, Al−Zr−C, and Al− Ta−C samples with C/TM = 0.6 and 1.2. As indicated, when C/TM = 0.6, the NbCx, ZrCx, and TaCx particles are in the shapes of an octahedron (Figure 3a,d,g), while when C/TM = 1.2, their shapes change to a truncated-octahedron (Figures 3b,e,h). In the samples with C/TM = 0.8 and 1.0, the octahedral and truncated-octahedral growth shapes were found simultaneity for the synthesized ceramic particles. The shapes of the NbCx, ZrCx, and TaCx particles with increasing C/TM from 0.6 to 1.2 display the same evolution process as that of the TiCx particles with increasing C/Ti from 0.6 to 0.7. Obviously, the morphology evolution process of the NbCx, ZrCx, and TaCx particles are restrained, just as we inferred, due to the relatively lower combustion temperatures and the stoichiometry. In order to increase the combustion temperature and the stoichiometry, we ignited the Al−Nb−C, Al−Zr−C, and Al− Ta−C samples (C/TM = 1.2) with a thermite package (Al and Fe2O3 with molar ratio of 1:2), for which the adiabatic temperature is about 3500 °C.33 However, because the combustion rate of the thermite is far higher than the rates of the formation reactions of the NbCx, ZrCx, and TaCx, the time for heat transfer from the thermite to the samples is very limited. Nevertheless, it was found that the shapes of the NbCx, ZrCx, and TaCx particles formed in the position near the surfaces of the samples had changed to spherelike, with the

Figure 2. XRD patterns for the reacted Al−Nb−C, Al−Zr−C, and Al−Ta−C samples.

As reported in our pervious paper, when the C/Ti molar ratio in the Al−Ti−C reactants increases from 0.6 to 1.2, the TiCx particles will change their growth shapes from octahedron → truncated-octahedron → spherelike → sphere (Figure 1b),14 and this was suggested to be caused by the increase in the TiCx stoichiometry.15 In other words, the growth shapes of the TiCx

Figure 3. Morphologies of the ZrCx, NbCx, and TaCx particles formed in the Al−Nb−C, Al−Zr−C, and Al−Ta−C samples: (a, d, and g) C/TM = 0.6, (b, e, and h) C/TM = 1.2, and (c, f, and i) C/TM = 1.2 with a thermite package. 2816

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Figure 4. (a) Schematic illustration of decreasing concentration gradient of nitrogen from the surface of the Ti melt to its interior. (b−g) Morphologies of the TiNx particles formed in the Ti−N2 gas system. (Reprinted from ref 32. Copyright 2006 American Chemical Society.) Morphologies of the TiNx particles formed in the Al−Ti−BN system: (h) BN/Ti = 0.6, (i) BN/Ti = 1.0, (j) BN/Ti = 1.8, (k) BN/Ti = 2.4.

rounded {100} and flat {111} being exposed simultaneity (Figures 3c,f,i). This result indicates that the TMCs of TiCx, NbCx, ZrCx and TaCx can have the same stoichiometryinduced morphology evolution during the SHS: octahedron → truncated-octahedron → spherelike → sphere. 3.1.2. TMNs. Most of the TMNs such as TiNx and ZrNx also possess wide stoichiometric ranges and NaCl crystal structure. Among these TMNs, TiNx has the closest atomic bonding characters and crystal parameters with TiCx.34,35 Moreover, the dendritic, octahedral, spherical, truncated-cubic, and cubic TiNx particles were found in the Ti−N2 gas system during the SHS, as shown in Figure 4b−g.32 Considering the similarities of the TiNx and TiCx in the intrinsic atomic bonding characters and the crystal structure, it is reasonable to speculate that the variety of the growth shape of the TiNx particles is also induced by the different stoichiometries. In fact, because of the high combustion temperature of the Ti−N2 gas system (about 2000 °C), Ti will melt during the combustion process. In this case, the TiNx nucleates and grows through the dissolving of the N2 molecules or N atoms from the N2 atmosphere into the Ti liquid phase. 32 Then, there must be a decreasing concentration gradient of nitrogen from the surface of the Ti melt to its interior (Figure 4a), which could induce the formation of the TiNx particles with different stoichiometries and thus the different growth shapes. As can be seen clearly from Figure 4b−g, the sizes of the differently shaped TiNx particles are also different, which supports the speculation that they are formed in the different zones of the Ti melt. The cubic TiNx particles with the largest sizes are supposed to be formed in the zone with the highest nitrogen concentration (point 5 in Figure 4a), whereas the dendritic TiNx particles with the finest sizes are formed in the zone with the lowest nitrogen concentration (point 1 in Figure 4a). As another consequence of the varying nitrogen concentration, the differently shaped

TiNx particles should have different stoichiometries. The cubic TiNx particles should be highly stoichiometric, while the dendritic TiNx particles should be quite substoichiometric. To further prove that the morphology of the TiNx particles is stoichiometry-dependent, we synthesized the TiNx particles in the Al−Ti−BN system, as it is easier to control the nitrogen content in the reactants with using the solid nitrogen source.36 The stoichiometry of the growing TiNx should increase with the increase in the reactant BN/Ti molar ratio. Figure 4h−k show the morphologies of the synthesized TiNx particles. It can be seen that the TiNx particles change their shapes from octahedron to truncated-octahedron to spherelike and finally to sphere with the increase in the BN/Ti ratio from 0.6 to 2.4. On the basis of this result, it is reasonable to believe that the cubic structured TMCs and TMNs display a similar stoichiometryinduced morphology evolution regularity during the SHS as from octahedron to sphere. The detailed mechanism for this morphology evolution and the formation of the cubic growth shapes will be discussed in section 2.1. 3.1.3. TMDs. Different from the cubic structured TMCs and TMNs, the TMDs such as TiB2, ZrB2, and NbB2x crystallize in the simple hexagonal structure (P6/mmm, 191), in which the graphite-like B layers and close-packed TM layers stack alternately and thus gain higher structural anisotropy. Because of the strong bonding between B atoms in the B layer and the interlayer interaction, the {0001} and {1010̅ } surfaces are quite stable.36,37 Therefore, the crystal shapes of the AlB2-type TMDs tend to be hexagonal plates or prisms.38−42 In fact, almost all the TMDs are stoichiometric compounds or only present a small homogeneity range (