Loveleen K. Brar,
Gourav Singla and
O. P. Pandey*
School of Physics and Materials Science, Thapar University, Patiala, India. E-mail: oppandey@thapar.edu
First published on 2nd November 2016
Cubic phase carbon-coated nano tantalum carbide (TaC) has been synthesized at 800 °C in a single step from tantalum oxide using the carbon and hydrogen produced in situ via decomposition of acetone in an autoclave. In the product phase(s) carbon exists: (a) inside the carbide, (b) on the surface of the carbide particles and (c) as free carbon (amorphous as well as graphitic). The effects of initial carbon concentration on the final carbon content inside as well as outside the TaC have been studied. The structural features of the final product have a complex dependency on the initial carbon concentration. The thermal behaviour of the final product clearly delineates the effects of internal and external carbon content. The soaking time studies show that the grain growth of TaC within the autoclave follows the simultaneous grain boundary migration and grain rotation model. The DSC/TG, XRD and microstructure analysis results along with thermal calculations have been used to predict the formation mechanism for the carbide particles. The reaction mechanism analysis brings forth the role of Mg in lowering the reaction temperature. In this process the carbon content of TaC, the size as well as the strain of the synthesized powders and the %free carbon content can be tailored as per the requirement for the given application.
Over last few years many methods have emerged for the synthesis of TaC nano-particles. Ta2O5 is the most common low cost precursor for the synthesis of TaC. Ta2O5 along with various carburization sources have been used for the synthesis of nano-TaC either in vacuum or inert atmosphere where the single phase TaC nano-particles are obtained only at temperatures greater than 1000 °C.4,15–22 Addition of catalysts such as Mg, Ni and/or halogenation agents such as NaF can reduce the reduction temperature.7,17,21
Carbon content in the cubic carbides like TaC dictates the mechanical properties as well as the surface stoichiometric properties which ultimately affect the catalytic response of the material.1,4 So the synthesized TaC powders need to be characterized for size, specific surface area as well as stoichiometry.23 The final characteristics of the synthesized product depend largely on the initial carbon content in the reaction mixture as well as the homogeneity of mixing.24 In most of the currently prevalent methods the reduction in reaction temperature and product size is achieved by multiple steps involving formation of nano – Ta2O5 followed by its mixing with carbon source and/or heating in reducing carbon rich atmosphere.8,14,25,26 Slight variation in any one of the steps can change the properties of the final product drastically.15,23,24
For particle size control as well as maintaining the purity of the synthesized transition metal carbide nano powder, the single step chemical reaction method in autoclave is well documented.27–29 In the present study we report the role of carbon in structural evolution during synthesis. In this work the carbon concentration in the reaction mixture and soaking time were varied to obtain nano TaCx powders of desired composition. The idea behind this work was to control x, the fraction of carbon in TaCx, for different industrial applications. Commercial grade Ta2O5 has been used with acetone as carbon source. The in situ hydrogen and carbon, produced during the decomposition of acetone, have been utilized for the reduction and carburization in the presence of reducing agent Mg at 800 °C to get TaC nanopowder. To the best of our knowledge single step synthesis of such fine grained (5–8 nm) nano TaC from Ta2O5 has not been reported so far at such a low temperature. Only reported work with lower synthesis temperature which we have come across has been by Ma et al. in which TaC has been synthesized from TaCl5 at 600 °C in 8 h with the average particle size is 40 nm.7
The evolution of the microstructure and properties of the synthesized powders with initial carbon concentrations and increasing soaking times have been studied. The initial carbon concentration emerges as a complex parameter for determining the final properties of the powders. It seems to control the concentration dependent diffusion of carbon into the particles which is actually the rate determining step. At the same time it also determines the amount of carbonaceous network outside the particles which in turn hinders the carbon diffusion and also strains the embedded particles. The carbon diffusion also plays an important role while determining the properties of the particles with increasing soaking time. One important result which emerged from the soaking time experiments is that the grain growth inside the autoclave follows the simultaneous grain boundary migration and grain rotation model.
Details of samples synthesized under different conditions along with sample names are given in Tables 1 and 2.
Sample name | Carbon conc. | χ2 | Rwp | a (nm) | x | Dv (nm) | Ds (nm) | RMSS (x 10−3) at 5 nm |
---|---|---|---|---|---|---|---|---|
01C | 1 | 0.510 | 4.20 | 0.44508 | 0.96 | 6.6 | 4.06 | 1 |
02C | 2 | 0.540 | 3.96 | 0.44498 | 0.95 | 6.8 | 4.1 | 2.19 |
03C | 3 | 0.565 | 4.09 | 0.44497 | 0.95 | 6.3 | 3.8 | 3.14 |
05p5C | 5.5 | 0.577 | 4.43 | 0.44496 | 0.95 | 7.1 | 4.1 | 5.31 |
07p5C | 7.5 | 0.744 | 4.75 | 0.44551 | 0.98 | 6.9 | 4.0 | 3.21 |
15C | 15 | 0.945 | 5.51 | 0.44555 | 0.99 | 6.2 | 3.8 | 4.34 |
40C | 40 | 1.55 | 7.10 | 0.44532 | 0.98 | 7.3 | 4.2 | 4.21 |
60C | 60 | 1.31 | 6.40 | 0.44534 | 0.98 | 6.6 | 4.0 | 4.37 |
90C | 90 | 1.46 | 6.88 | 0.44534 | 0.98 | 7.5 | 4.6 | 4.31 |
Sample name | Soaking time (h) | χ2 | Rwp | a (nm) | x | Dv (nm) | Ds (nm) | RMSS (x 10−3) at 5 nm |
---|---|---|---|---|---|---|---|---|
01Q | 1 | 0.571 | 4.13 | 0.44445 | 0.92 | 6.5 | 4.1 | 6.22 |
01T | 1 | 0.575 | 4.12 | 0.44458 | 0.93 | 9.2 | 5.2 | 7.39 |
02T | 2 | 0.565 | 4.09 | 0.44497 | 0.95 | 6.3 | 3.8 | 3.14 |
07T | 7 | 0.534 | 4.03 | 0.44547 | 0.99 | 5.1 | 2.6 | 14.45 |
10T | 10 | 0.538 | 4.06 | 0.44485 | 0.95 | 7.4 | 4.4 | 1.34 |
15T | 15 | 0.542 | 4.26 | 0.44551 | 0.99 | 6.2 | 3.8 | 4.10 |
F(x) = ηL(x) + (1 − η)G(x) | (1) |
Based on the better results for our earlier work29 we use the Voigt double-line integral-breadth methods for the XRD line profile analysis. The double line analysis was carried out using Origin™ software.
(2) |
(3) |
(4) |
This method is called “Double-Voigt” method (D-V method). βL and βG2 are plotted w.r.t. s2 and the corresponding strain and size components βSL, βDL(hkl), βSG, βDG(hkl) are obtained. The analytical expressions for the volume weighted domain size, 〈Dv〉, mean square strain, 〈ε2〉 and size coefficients, As(L) are:30–32
(5) |
(6) |
As(L) = exp(−2LβSL − πL2βSG2) | (7) |
Eqn (6) is used to determine the m.s.s. strain distribution w.r.t. size (L) and direction. The data for one arbitrary size (5 nm) was used for comparison of strains (root mean square strain) between different samples. The 5 nm was chosen based on the particle sizes (surface and volume weighted) obtained for our samples and literature survey.32 The initial slope of the As(L) vs. L graph is used to determine the surface weighted domain size (〈Ds〉).30 Surface weighted domain size determination of the powder is important for the applications where the active surface area is important e.g. catalysis.33
DSC/TG (NETZSCH STA 449F3) was done at a heating rate of 5 °C min−1 in air atmosphere to determine the phase transitions and thermal stability of the materials. The micro-structural features of synthesized TaC powders were analyzed with field-emission scanning electron microscope (FE-SEM) (SIGMA Carl Zeiss) operating at 5 kV and transmission electron microscope (TEM) (JEOL 2100F) operating at 200 kV. The N2 sorption studies for surface analysis were conducted using a Tristar 3000 (Micromeritics) to determine the Brunauer–Emmett–Teller (BET) surface area, the pore size, and the pore volume.
Fig. 1 XRD results of the Ta2O5 powder and the acid leached samples synthesized from it with different soaking temperatures. |
The final soaking temperature of 800 °C was thus chosen for the further studies: (a) effects of changing initial carbon concentration (1 to 90) on the final product (Fig. S1, (ESI†), Table 1) and (b) evolution of TaC with soaking time (1 h to 15 h) (Fig. S2, ESI,† Table 2).
Fig. 2(a) gives the Rietveld refinement plots for the 03C sample. The lattice parameter (a), Rwp and χ2 – values obtained from the Rietveld refinement are listed in the Tables 1 and 2. The lattice parameter of the synthesized samples from the Rietveld refinement has been used to determine x in TaCx:35
a (Å) = 4.3007 + 0.1563x | (8) |
Fig. 2 (a) Rietveld refinement plots for the 03C sample. Tick marks indicate allowed peak positions. (b) The result of the pseudo-Voigt curve fit routine for the (111) peak of 03C sample. |
Fig. 2(b) shows the results obtained from the fitting of (111) peak with the pseudo-Voigt function for the 03C sample. Fig. S3 in ESI† gives the D-V integral breadth method analysis graphs for the 03C sample. Similar graphs have been used to analyse all the samples. Fig. S4 in ESI† shows the isotropic nature of the strain and variation of Fourier transform coefficients as a function of column length for 03C sample. Tables 1 and 2 also give the details of the size and strain values (root mean square strain, RMSS) obtained for the D-V integral breadth methods.
Thus, the diffusion of carbon into the particle and hence the final carbon content in the TaC is a complex function of initial C concentration since on one side an increase in carbon concentration creates a concentration gradient which will enhance the process of diffusion but at the same time the extra carbon forms graphitic carbon network outside the particles which hinders the diffusion of carbon into the particle.
A closer look at the further time evolution of the size and strain data (Table 2) shows that the crystallites are now in turn becoming smaller in size and strained followed by the larger size and lower strain. We believe this to be a clear indication that once the carbon inside the TaC crystallites have achieved the lowest energy configuration the further heating results in grain growth which seems to follow the simultaneous grain rotation and grain boundary (GB) migration model.36–38 In this mechanism initially the grain boundaries grow/migrate resulting in decreased grain size and increased strain in the system. This is followed by the grain rotation across the lowest angle grain boundary resulting in a larger grain as the GB between them vanishes. At the time of the grain shrinkage the GB becomes vastly enhanced so the samples show higher average strain and smaller grain size. An important consequence of this type of grain growth for TaC is that the manifold increase in strain just before rotation also results in expulsion of some of the carbon from the crystallites. So for these samples the carbon content of the carbide phase also alternates with soaking time and grain growth.
In the present system the evolution of the TaC nanopowder with increasing soaking time at 800 °C can essentially be divided into two regimes: (i) the formation of low strain TaC followed by (ii) grain growth via simultaneous grain rotation and GB movement mechanism. Fig. 4 show the proposed grain evolution mechanisms for TaC.
The surface weighted size for all the samples is smaller than the volume weighted size but follows the same trend as the volume weighted size for the time and carbon variation. This we believe is an indicator that for the temperature and time scales used in present setup the grains do not have anisotropic growth.33
Sample name | Initial stability (°C) | C removal peak (°C) | Oxidation peak (°C) | Mass gain (%) | Final mass (%) | Free carbon (%) | Initial mass loss (%) |
---|---|---|---|---|---|---|---|
02T | 255 | 305 | 434 | 5.9 | 91 | 20.4 | 5.3 |
10T | 238 | 482 | 582 | 1.9 | 89 | 22 | 4.7 |
01C | 212 | 330 | 507 | 7.6 | 102 | 10.7 | 2.7 |
02C | 185 | 313 | 462 | 7.5 | 102 | 10.7 | 2.2 |
03C | 255 | 305 | 434 | 5.9 | 91 | 20.4 | 5.3 |
20C | 195 | 342 | 572 | 4.3 | 73 | 35.9 | 2.2 |
60C | 211 | 325 | 556 | 1.06 | 46 | 59.7 | 2.3 |
If the sample consists of pure TaC, its oxidation will result in 14.5% increase in mass. In our samples this is never achieved. For our samples the maximum increase encountered is 2–8 mass% after the initial weight loss stops. The TG curves become stable for all the samples at ∼800 °C. The residual mass at these temperatures is used to calculate the free carbon content in the synthesized samples using the formula:39
(9) |
The smaller increase in mass for our samples as compared with the expected value is due to the presence of carbonaceous content (graphitic as well as amorphous) in the synthesized samples.9 As the sample is heated the mass increase due to oxidation is being offset by the decrease in mass due to oxidation of carbonaceous residue. This implies that if we have more free carbonaceous content in the samples the %mass gain will be smaller. The data in Table 3 shows as the initial carbon content is increased the %free carbon in the synthesized samples increases with a corresponding decrease in %mass gain in TG curves.
As we increase the initial C the net stability of the synthesized samples (the location of the C removal and oxidation peaks in DSC) decreases. This can be attributed to smaller size and increased strain in the particles (Table 2). As we increase the initial carbon content from 2 (02C) to 3 (03C), the free carbon content in the samples nearly doubles whereas the carbon within the TaC remains the same (Table 2). Thus this thermal data supports our earlier conjecture that extra initial C forms a thick carbonaceous network around the particles which prevents the movement of carbon into the particles.
The large initial mass loss for 03C sample corresponds with the smallest surface weighted size for this sample (Table 1). When the soaking time for the sample is increased (02T to 10T) the locations of the DSC peaks indicate that the particles are more stable against oxidation. This is supported by the XRD data which shows that the sample consists of low strain large sized particles. The decrease in the initial stability for 10T sample is due to the increased free carbon content. This conjecture is further supported by the thermal data for the 20C and 60C data. For both the cases the initial stability of the sample decreases as the free carbon content increases.
Thus we conclude that for a nano-carbide sample initially the increased free carbon content increases the initial stability of the powders but on further increase (>20%) it decreases. Comparing 01C and 60C samples clearly shows that very high free carbon content in the system is detrimental to the quality of the sample by reducing not only the amount of the useful material but also deceasing the stability.
Complete analysis of XRD and thermal data from Tables 1–3 indicates that the initial stability and hence the handling temperature of the samples depends on the free carbon content whereas the oxidation stability depends on the carbon content in the carbide phase as well as the size of the particles.
Fig. 6(b) shows the representative TEM images of the synthesized TaC nanoparticles for 01C sample. The presence of carbon coating on the surface of each individual particle is clearly visible. Also visible is the carbonaceous network in which the particles are embedded. The morphology of carbon coated nanoparticles varies from faceted to spherical and they have a tendency to aggregate. Fig. 6(c) gives the High Resolution-TEM (HRTEM) image of a single crystalline TaC nanoparticle and shows the (111) facets of the particle. The distance between the adjacent lattice fringes is the interplanar distance of cubic TaC (111), which is 0.26 nm (ICDD pattern – 01-077-0205). This confirms that the synthesized TaC powder has cubic crystalline structure.
Sample name | BET surface area (m2 g−1) | Average pore diameter (nm) | Pore volume (cm3 g−1) |
---|---|---|---|
01C | 54.81 | 8.26 | 0.097 |
02C | 56.4 | 10.52 | 0.083 |
03C | 72.97 | 6.78 | 0.112 |
01Q | 78.53 | 8.14 | 0.139 |
02T | 72.97 | 6.78 | 0.112 |
10T | 61.07 | 8.31 | 0.122 |
Fig. S6 in ESI† shows the N2 adsorption–desorption isotherms for the samples with increasing soaking times. The adsorption isotherms exhibit the characteristics of a type-II isotherm according to IUPAC classification.3,40 An empirical classification of the hysteresis loops which gives information about the texture of the adsorbent has also been given by IUPAC and the isotherms show H-4 hysteresis characteristics.40,41 This implies that the powders are forming a complex structure with both micropores and mesopores. This behaviour is because of the agglomeration of particles in the synthesized product.
2Ta2O5 + C → 4TaO2 + CO2 | (10) |
Ta2O5 + C → 2TaO2 + CO | (11) |
Ta2O5 + H2 → 2TaO2 + H2O | (12) |
Ta2O5 + Mg → 2TaO2 + MgO | (13) |
The ΔG values for the above reactions at the standard pressure at different temperatures can be calculated by:
ΔG = ΔG0 + ΔH − TΔS | (14) |
(15) |
(16) |
Cp = Δa + ΔbT + ΔcT−2 | (17) |
The variation of ΔG with temperature for the Ta2O5 reduction with C, H2 and Mg is shown in Fig. 7(a). The curves show that although all the reactions are non-spontaneous (ΔG > 0) the reduction becomes more favorable as the temperature increases. The reaction involving Mg is more favourable at all temperatures. The reduction reactions involving Mg and C are a solid-solid reaction whereas that for H2 is solid–gas reaction.44
The H2 being a small molecule has a large diffusion coefficient. So once formed (especially in excess) it is able to diffuse into the Ta2O5 particles whereas C coats the outside of the Ta2O5 particles. Even at lower temperatures (<650 °C) the reduction due to hydrogen (which is exothermic) is possible due to the size of the initial powder (130–220 nm) which allows for large contact and solid–gas nature of the reaction.45 This reduction is extremely small in amount and only occurs for the diffused hydrogen because of low partial pressure of H2O inside.44,45 The exothermic reduction reaction leads to formation of steam which fragments Ta2O5 powders. This conjecture is supported by the broadened XRD peaks at lower temperatures and no signature of TaO2. The small final size of the synthesized powders in our work is attributed to this initial reduction of size for Ta2O5.34
The fragmented Ta2O5 gets coated immediately with carbon present in the autoclave. This carbon coating prevents the fragmented oxide particles from coalescing. The carbon coating also helps to increase the reaction rate for reduction and carburization by ensuring the close proximity of the reactants. For the fragmented Ta2O5 the reduction process proceeds at a faster rate due to reduced size and higher surface area of fragmented particles.46 The decrease in the transformation temperature seen in our work is a combined effect of the reduced size of the Ta2O5 as well as addition of Mg which is energetically more favourable reductant. This is supported by existence of critical initial stoichiometry of Mg needed for complete transformation into TaC at 800 °C. We believe once the reaction starts the reduction proceeds via C and H2 reduction pathways as well as Mg. This will be expected since the C and H2 are in more intimate contact with the initial powders whereas Mg is in the form of solid powder.34 Also all the reactions are exothermic (data not shown) so once the reaction starts the local temperature will increase making all reaction pathways possible. The detailed morphology of the Mg powder may also have an effect on its critical stoichiometry and needs to be studied further. As the temperature increases the reduction reactions become more favourable increasing the reaction rate.
For the further reduction of TaO2 the following reactions are possible:
TaO2 + C → TaO + CO | (18) |
TaO2 + H2 → TaO + H2O | (19) |
TaO2 + Mg → TaO + MgO | (20) |
TaO2 + 2C → Ta + 2CO | (21) |
TaO2 + 2H2 → Ta + 2H2O | (22) |
TaO2 + 2Mg → Ta + 2MgO | (23) |
Fig. 7(b) clearly shows that reduction of TaO2 to Ta in single step is a spontaneous process with reduction reactions involving C and H2 being comparable but that for Mg being more probable from the energy point of view at the temperature of interest (≤700 °C). Since the TaO2 particles are small in size, completely, encapsulated in carbon and reaction is spontaneous in nature so this reaction proceeds at a very fast rate and the Ta particles formed will be strained. This is supported by the XRD data which clearly shows the formation of pure cubic phase highly strained Ta nanoparticles at 700 °C.
The final possible carburization reactions are:
Ta + C → TaC | (24) |
Ta + C → Ta2C | (25) |
Ta2C + C → 2TaC | (26) |
Reactions (25) and (26) are competing reactions. Thermodynamic calculations show that the formation of Ta2C is more favourable (Fig. 7(c)). So the Ta nanoparticles are carburized initially to Ta2C and finally to TaC. Both these reaction steps will be very fast as both the reactions are spontaneous. This is supported by the XRD data where Ta2C and TaC are formed simultaneously for the 750 °C sample. But the complete conversion to TaC takes place only at 800 °C. This indicates that the diffusion of carbon inside the particles is the reaction rate hindrance step. Increase in temperature increases the diffusion and results in complete final phase formation in 1 h.
So the complete reaction pathway for the synthesis of TaC from Ta2O5 is:
The formation mechanism is depicted in Fig. 8.
Fig. 8 The mechanism for transformation of Ta2O5 into TaC nanoparticles. The arrows within the stressed oxide particle represent the beginning of cracks. |
Studying the evolution of TaC powders with increasing soaking time revealed that initially the powders are strained and require about 2 hours of soaking at 800 °C to achieve the low strain state with carbon diffusion being the rate limiting step. Soaking for higher times leads to increase in crystallite size which follows the simultaneous grain boundary migration and grain rotation model. We believe that the grain rotation effects are visible in our samples even at the comparatively low temperatures (for TaC) like 800 °C due to the small size of the initial powders (since the grain rotation rate varies inversely as 1/r−5 with the grain size) and the high pressure environment of the autoclave.38,47
A detailed study of increasing the initial C concentration and soaking times brings forward complexity of the system dependence on these parameters. The initial C stoichiometry of 1 results in the formation of TaCx with x = 0.96. As the C in the initial mixture is increased a strong carbon network (consisting of amorphous and graphitic components) is formed outside the particle which hinders the flow of carbon into the carbide. Quench cooled sample for 1 h results in TaCx with x = 0.92. As the soaking time is increased the carbon diffusion inside the carbide increases along with grain size via the simultaneous grain boundary movement and grain rotation model. So the variation of initial C as well as soaking time can be used for tailoring the C concentration in carbide. The best TaCx with x = 0.99 is achieved either for initial C ratio of 15 in 2 h or for C stoichiometry of 3 in 7 hours. With former having higher carbonaceous content on the outside but smaller strain. Depending on the requirement of the system/application the desired sample can be prepared. The overall stability of the powders against oxidation is determined by the particle size as well as the carbon content inside the TaC. The surface area and pore size/volume is determined by the particle size.
Thermodynamic calculations, thermal analysis and XRD analysis results have been used to predict the mechanism for the formation of the carbon coated nano-TaC particles from Ta2O5. It emerges from the analysis that the fine size in our synthesized powders is due to initial production of H2 in the autoclave and, the lowering of the reaction temperature is due to the combined effect of the initial size reduction as well as addition of Mg. In this process the carbon content of TaC, size as well as the strain of the synthesized powders and the %free carbon content can be tailored as per the requirement for the given application. The microstructure evolution studies carried out here if extended can also help to predict the stabilization of dense nano-structures as expected from rapid densification techniques such as spark plasma sintering.38,47
Footnote |
† Electronic supplementary information (ESI) available: Supplementary material (XRD data for the carbon and time series, double-Voigt analysis graphs and BET graphs) is available in the online version. See DOI: 10.1039/c6ra24484j |
This journal is © The Royal Society of Chemistry 2016 |