J Therm Anal Calorim DOI 10.1007/s10973-014-4136-1
Thermal stability and structural properties of Ta nanopowder synthesized via simultaneous reduction of Ta2O5 by hydrogen and carbon Loveleen K. Brar • Gourav Singla Navjot Kaur • O. P. Pandey
•
Received: 22 January 2014 / Accepted: 26 August 2014 Ó Akade´miai Kiado´, Budapest, Hungary 2014
Abstract Tantalum nanoparticles have been synthesized by the single-step chemical reaction route. Simultaneous reduction of Tantalum Pentoxide (Ta2O5) with the in situ produced hydrogen and carbon at 600 °C is a new approach for the production of Ta nanopowder. DH values obtained from thermodynamic calculations are used to predict the entire mechanism of reduction of bulk Ta2O5 into Ta nanoparticles. The results of X-ray diffraction studies show that the final product consists of predominately nano a-Ta with b-Ta as the minority phase. The lattice strain in the final product was calculated using Williamson–Hall formula. The effect of lattice strain on thermal stability of the samples was analyzed by differential scanning calorimetry and thermal gravimetry in the air atmosphere. The morphology and particle size distribution of Ta nanosized powders have been analyzed by scanning electron microscope and transmission electron microscope. The results show that average crystallite size of the product Ta nanopowder is about 2–7 nm. Keywords Nanoparticles Tantalum a-Ta b-Ta Chemical reaction method
Introduction Transition metals and their compounds exhibit high potential for industrial application due to their inherent metal-like properties as well as those like refractory materials [1]. Among them, Tantalum (Ta) belongs to the L. K. Brar G. Singla N. Kaur O. P. Pandey (&) School of Physics and Materials Science, Thapar University, Patiala, India e-mail:
[email protected]
group of high-performance metals that have gained acceptance due to their several unique properties. Ta has a high melting point, good ductility along with high strength, good antioxidation properties, and also exhibits excellent thermal resistance [2]. Because of these characteristics, Ta and its oxides have been studied extensively as important materials for electronic, chemical, and biological applications [3]. Currently, its demand is mainly in electrical and electronic industries where 55 % tantalum is used as highperformance capacitor. The electrolytic capacitors made with sintered Ta powder tend to miniaturize the electronics circuit using pure and fine Ta powder [3]. Other applications of Ta are as its carbide (TaC) which is used in cutting tools as well as in the catalyst manufacturing [4]. In order to fulfill the demand of Ta powder, many chemical methods have been adopted to synthesize it from Ta2O5, TaCl5, or K2TaF7 [2, 5]. In actual practice, it is difficult to synthesize pure Ta nanopowder. Traditionally, Ta metal powder has been produced by reduction of potassium heptofluorotantalate (K2TaF7) with sodium [6]. The K2TaF7 is produced by hydrofluoric acid dissolution and purification from tantalite [(Fe,Mn)Ta2O5] ore [7]. The metallic Ta powder produced with this method is of high quality, but the process itself is energy consuming, complicated, and also generates environmentally harmful fluorides as byproducts [8, 9]. Nanosized Ta powder can also be obtained by various methods like hydrogen arc plasma method [3], hydrogen reduction [10–12], metallo-thermic reduction in the presence of hydrogen [13], and calciothermic reduction [14] of Ta2O5 and TaCl5 powders of micron size. But all these reduction processes are not too well suited for commercial production of Ta nanopowders because of high sensitivity to contamination, high reaction temperature, and lack of control for particle morphology as well as purity [2, 3, 15, 16]. This has led to the exploration
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for new synthesis routes which can simplify the processing steps and/or reduce the reaction temperature. Any such technique which can yield nanosized Ta powder will be highly useful for industrial applications. So, the process of the reduction of Ta2O5 at low reaction temperature with high degree of purity of the obtained products is the subject of extensive research. To avoid contamination from reaction container as well as to control particle size and purity of synthesized transition metal nanopowder, a new chemical reaction method has been introduced [17, 18]. This route has been developed for the preparation of nanosized powders, and the final product characteristics mainly depend on chemical and thermodynamic parameters [19]. Present study deals with the production of Ta nanopowder by this simple chemical reaction route. So, in the present work, synthesis of Ta nanoparticles is reported using Ta2O5, acetone (C3H6O), and magnesium (Mg) as reducing agent where simultaneous reduction of original Ta2O5 with in situ produced carbon and hydrogen takes place.
0.0170° (2h). Crystallite size and strain for the synthesized powder were determined by X-ray diffraction profile technique using the Williamson–Hall equation:
Experimental
Results and discussion
Synthesis of Ta nanopowder
Morphology study of the reactants
For the synthesis of Ta nanopowder, Ta2O5 (99.99 %, Sigma Aldrich), Mg (99 %, LobaChemie), and anhydrous C3H6O (99.5 %, LobaChemie) in molar ratios of 1:7:12 were sealed in a stainless steel autoclave of 50 mL capacity. The entire assembly was placed inside the furnace. The temperature of the furnace was raised from room temperature to 600 °C slowly (5 °C min21), and autoclave was soaked at the extreme temperature (600 °C) for 15 h (15h sample) and 20 h (20h sample). The selection of temperature and time was done based on our earlier experiments for the reduction of WO3 to W [18]. The resultant product was collected and leached with diluted HCl (1:1) to remove magnesium oxide (MgO) and unreacted Mg. After leaching, the powder was washed several times with double-distilled water to remove any traces of unreacted acid. Finally, the powder was washed with acetone and dried in vacuum at 100 °C.
Figure 1a shows the SEM micrograph of Ta2O5 powder. The particles are spherical in shape having size variation in between 130 nm and 220 nm. Some of the particles are agglomerated. Figure 1b presents the SEM micrograph of Mg powder. The individual particles are large and elongated. However, the topographical features indicate that Mg flakes have been crushed to obtain the powder. The high-magnification micrograph of an individual particle shown in inset indicates that the surface is rough having flakey morphology which can provide high surface area for reaction.
Characterization of Ta nanopowder The X-ray diffraction (XRD) study of the product phase was done to identify crystalline phases. The XRD of the samples was performed using PANalytical Xpert Pro with ˚ ) obtained from the copper Cu Ka radiation (k = 1.54 A target using an inbuilt Ni filter. The X-ray powder diffraction data were collected for both samples at room temperature between 20° B 2h B 80° with a step size of
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bhkl Coshhkl ¼ ½0:89k=t þ ½4esinhhkl ;
ð1Þ
where b (2h) is the full width at half maximum (FWHM), k is the X-ray wavelength, t is the crystallite mean size, and e is strain. Simultaneous differential scanning calorimetry and thermogravimetry (DSC/TG) was done to determine the phase transitions and stability of the samples in air. The DSC/TG experiment was done with a heating rate of 5 °C min-1 from room temperature to 800 °C in air atmosphere by NETZSCH STA 449F3. The micro-structural features of Ta2O5, Mg powder, and synthesized Ta powders were analyzed with Jeol 840 SEM and also with field-emission scanning electron microscope (FE-SEM) (QUANTA 200 FEG) operating at 20 kV. Transmission electron microscope (TEM) study was done by Philips CM-10 operating at 200 kV.
X-ray diffraction analysis The XRD patterns of the as-synthesized product for 15h as well as 20h samples showed the presence of a-Ta (JCPDF card no: 089-4901), indicating that the reduction of Ta2O5 to metallic Ta has been completed during the reaction processes. b-Ta (JCPDF card no: 025-1280) is also present as minority phase. The observed XRD peaks (Fig. 2) for the 15h and 20h samples after acid leaching are broad and have shifted toward lower scattering angles as compared to standard reference for Ta. This clearly shows that product phase is small in size as well as strained. The shift in the XRD peak positions toward lower angles is observed even in case of formation of hydrides [12]. However, it is only possible in
Thermal stability and structural properties of Ta nanopowder Fig. 1 SEM micrographs of the reactants a Ta2O5 powder b Mg powder, inset shows the surface of an individual particle at high magnification
• α -Ta (JCPDS card no. 89-4901)
•
* β -Ta (JCPDS card no. 25-1280)
° TaO (JCPDS card no. 65-2903) *
•
•
20 hr •
•
•
15 hr
*
Intensity/a.u.
•
°
Ta2O5 (JCPDS card no. 25-0922)
20
25
30
35
40
45
50
55
60
65
70
75
80
2θ /°
Fig. 2 X-ray diffraction patterns of pure Ta2O5 and Ta nanoparticles after acid treatment for 15h and 20h samples
case of higher concentration of hydrogen [12]. The hydrogen in our method is being produced in situ by decomposition of acetone. Moreover, the presence of high concentration of carbon results in thicker carbon coating, which hinders the diffusion of hydrogen, as has been reported by other workers also [20]. The reaction mechanism for the current work (discussed later) predicts that before the final reduction step, the nanoparticles are coated by carbon layer (Fig. 4). The particle size and strain were calculated using Williamson–Hall Eq. (1) and are given in Table 1. The particle size of the starting Ta2O5 as determined from the XRD (Fig. 2) is 130 nm which is in good agreement with the results obtained from SEM. The calculated positive value of strain in the synthesized samples indicates that the
samples are in tensile-strained state. The induced tensile strain in the synthesized samples of Ta indicates lower stability of samples [21]. As given in Table 1, 15h sample has higher value of lattice strain as well as lower particle size as compared to 20h sample which clearly designates that 15h sample has higher surface energy as well as higher surface reactivity as compared to 20h sample. The presence of TaO phase in 15h sample (Fig. 2) is due to its higher surface energy and reactivity which results in its oxidation in air. This has been confirmed with DSC/TG analysis (discussed later). The prepared Ta powder from this method is fine and homogenous. The comparison of a and b phase peaks of Ta in XRD patterns for the two samples (Fig. 2) clearly shows that the transformation of b-Ta phase into a-Ta occurs with increasing the synthesis time. The reaction temperature of 600 °C is lower than that reported by Read and Altman [22]. It is essential to consider various reactions that occur inside the autoclave during heating. Mg being highly reactive substance absorbs oxygen from the air present in the autoclave atmosphere and forms MgO. MgO is one of the most active catalysts for reduction especially as compared to other oxides like Al2O3 [23]. So in the presence of this active catalyst, at temperatures above 200 °C, acetone decomposes into hydrogen and carbon with some intermediate products [23]. It is difficult to comment about the intermediate products formed within the closed environment of the autoclave at such low-temperature range. Since the acetone was added in excess so the carbon produced will be enough to completely encapsulate the Ta2O5 particles. Both the carbon (C) and the hydrogen (H2) produced as a result of decomposition of acetone can act as reducing agents for Ta2O5. The first step of reduction of Ta2O5 to Ta can occur through conversion to TaO2 via two reactions as expressed below: Ta2 O5 þ C ! 2TaO2 þ CO
ð2Þ
Ta2 O5 þ H2 ! 2TaO2 þ H2 O:
ð3Þ
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L. K. Brar et al. Table 1 Structural and thermal data of synthesized powders Sample name
Synthesis time/h
15h
15
20h
Particle size from W–H method/nm
20
360
-869.30
10
-3
354
-1027.57
8.24 9 10
ð4Þ
where Da, Db, and Dc are the thermodynamic data of reactants and products at the standard state. The change of DH with the temperature of the Ta2O5 for reaction with C as well as H2 is shown in Fig. 3a. At temperatures higher than 350 °C, the DH values for both the reactions are lower than zero. This indicates that at these temperatures, both the reactions (2 and 3) are highly exothermic in nature. The curve for reduction due to carbon has slightly higher enthalpy than that for reduction due to hydrogen. This clearly indicates that the reaction with carbon is more favorable. But, due to quite small energy differences at the temperature of interest (600 °C), both the reactions actually occur simultaneously. The carbon which is encapsulating the Ta2O5 particles will start to get adsorbed on to the surface of the Ta2O5 particles and start the reduction process. The encapsulating carbon with more energetically favorable reduction hinders the absorption of hydrogen on to the surface. The hydrogen, which is also present in excess within the autoclave chamber, being a small molecule has a large diffusion coefficient. So it is able to diffuse into the Ta2O5 particles. The reduction due to carbon takes place at the surface of Ta2O5 particles, while the reduction due to hydrogen predominantly takes place inside the particle. The released oxygen inside the particles reacts with hydrogen to form steam. This generated steam and growth of TaO2 within the Ta2O5 particles stresses the Ta2O5 and as the reaction progresses, it leads to fragmentation of Ta2O5 powders. For the fragmented Ta2O5, the reduction process proceeds at a faster rate due to reduced size and higher surface area of fragmented particles [24]. The decrease in the transformation temperature in our work may be attributed to this reduced size of particles during the first reduction reaction. Such kind of phase transformation at lower temperature has also been reported by many authors during synthesis of their own nanocrystalline system [18, 25]. On the basis of calculated DH values, it is concluded that sequence for reduction of Ta oxide may be Ta2O5, TaO2, TaO, and Ta. The possible steps for further reduction are
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DH Oxidation/J g-1
8.93 9 10-3
1 DHT DHr ¼ r CP dT ¼ DaT þ DbT DcT 1 ; 2 T0 2
Oxidation peak temperature/°C
9
The DH values for the above reactions at the standard pressure at different temperatures can be calculated by T
Strain
TaO2 þ C ! TaO þ CO
ð5Þ
TaO2 þ H2 ! TaO þ H2 O
ð6Þ
TaO þ C ! Ta þ CO
ð7Þ
TaO þ H2 ! Ta þ H2 O:
ð8Þ
Figure 3b and c clearly indicates that the reduction of tantalum oxide(s) to Ta proceeds mainly via carbon reaction. The reacting carbon is completely encapsulating the fractured particles so the rate of reaction is expected to be high. The calculated positive value of strain for both the Ta samples clearly indicates the higher oxygen exchange rate from tantalum oxide to encapsulating carbon. The entire mechanism of transformation and formation of Ta nanoparticles is illustrated in Fig. 4. Reduction of Ta from Ta2O5 particles by carbon is energetically more favored (Fig. 3), but it is the simultaneous presence of hydrogen which is reducing the size of the particles in the first step and, hence, results in lower reaction temperature and faster reaction rate in the present work. As reported in our earlier publication [18], the formation of intermediate Magneli oxide phase (TaOx) enhances the reduction rate of oxides, but this reduction is a multistep process. However, the absence of any intermediate oxide in X-ray pattern (Fig. 2) indicates that these oxides may not be stable at synthesis temperature and directly reduced to pure Ta nanopowder. Thermal analysis The thermal stability and oxidation resistance of the obtained powder were analyzed by DSC/TG curve. TG curve can provide information in two forms: mass loss as a function of either temperature or time which is a direct indication of mass loss and the derivative of mass loss (DTG) as a function of temperature or time which helps pinpoint any shoulders present [26, 27]. Figure 5 represents the DSC-TG-DTG curves of the sample. There is an obvious exothermic process in the range of 200–500 °C, which can be ascribed to the oxidation of Ta nanoparticles to form Ta2O5 because TG curve shows an increase in mass at the corresponding temperature. From TG curve, it is clear that the powders were stable up to approximately 200 °C, and then the mass started to increase due to oxidation of Ta and formation of Ta2O5 until the mass gain
Thermal stability and structural properties of Ta nanopowder
(a)
1: Ta2O5 + C = 2TaO2 + CO –5
2: Ta2O5 + H2 = 2TaO2 + H2O
ΔH/kJ mol–1
–10 –15 –20 –25
2
–30 1
–35 400
500
600
700
800
Temperature/°C
(b)
0
ΔH/kJ mol–1
–2 2
–4
–6 1: TaO2 + C = TaO + CO –8
2: TaO2 + H2 = TaO + H2O
1
–10 400
500
600
700
800
Temperature/°C
(c)
1
2
ΔH/kJ mol–1
0
By stoichiometry, complete oxidation of pure Ta to Ta2O5 yields a mass gain of 22 %. The gap between the theoretical and the observed mass gains may be due to the presence of oxygen content [12] or the presence of excess carbonaceous residue in the final samples. This carbonaceous residue will oxidize when heated in air and result in decrease of sample mass. So, during the heating of the samples, there are two simultaneous processes that are taking place: (i) oxidation of Ta (250 °C–450 °C) which gives rise to gain in mass and (ii) oxidation of carbonaceous residue (250 °C–750 °C) which results in decrease in mass. This results in limiting the overall mass gain to a much smaller value (*5 %) as compared to the expected value (22 %). The DH values represent the amount of heat required for oxidation and were determined from the total area enclosed by the thermal analysis peak using software package provided with the thermal analyzer, which are given in Table 1 [28, 29]. A comparison of these values indicates that 20h sample requires more amount of heat as compared to 15h sample for oxidation. This lower value of DH for 15h sample may be due to higher surface reactivity as a result of the higher surface energy due to higher lattice strain [21]. This result corroborates the strain values obtained from Williamson–Hall equation. From the analysis of the DSC curves, it is concluded that for 15h sample the oxidation exothermic peak is at 360 °C, whereas for the 20h sample it is at 354 °C. This confirms the existence of the TaO protective layer on the surface of 15h synthesized particles which is protecting the particle from being oxidized and making it more thermally stable in spite of its more strained and, hence, reacting nature. After 440 °C, the TG curve is close to a straight line showing that all the Ta have been oxidized, leaving only the stable Ta2O5.
–1
Microstructure analysis of synthesized Ta powders –2 –3 1: TaO + C = Ta + CO
1
2: TaO + H2 = Ta + H2O
–4 –5 400
500
600
700
800
Temperature/°C Fig. 3 Change of DH with temperature for reduction of a Ta2O5, b TaO2, and c TaO when reacting with carbon and hydrogen
leveled off at 450 °C. As we further increase the temperature, there is a slight loss in mass after 650 °C and this loss is getting leveled off at 800 °C. The powders attain the final mass gain of approximately 5 % at 800 °C.
Figure 6 shows the representative FE-SEM images of the final product synthesized for 20h sample. It is clear from the images that particles have a tendency to agglomerate and the size distribution is inhomogeneous. The analysis of the morphology of particles in FE-SEM images shows that Ta powder varied in forms from lamellar to spherical shape which may depend upon synthesis time as well as temperature. In the present experiment as we have explained earlier in the reaction mechanism, the starting Ta2O5 particles are fractured by hydrogen in the first step, and then these particles are further reduced into Ta by carbon. The smaller size reduced Ta nanoparticles are unstable and start joining with each other. Thus in due course of time leading to the formation of lamellar structures. However, the bigger size nanoparticles retain their spherical shape [30].
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L. K. Brar et al. Fig. 4 The mechanism for transformation of Ta2O5 into Ta nanoparticles. The arrows within the stressed oxide particle represent the beginning of cracks
Carbon coated tantalum nanoparticles
Carbon coated fragmented oxide
Carbon coated reduced oxide
(a) 106
0.25
TG
1.2
0.20
1.0
DTG
0.8 DSC
0.6 0.4
DTG/% min–1
104
Mass/%
1.4
0.30
360 °C
Exo up
1.6
DSC/mW mg–1
Fig. 5 DSC/TG/DTG curves of Ta nanopowder synthesized for a 15h and b 20h (DTG curve in the above graph is the differential thermogravimetric data)
Stressed oxide particle
Diffused hydrogen Carbon encapsulated oxide particle
Ta2O5 particle
102
100
0.2
0.15 0.10 0.05 0.00 –0.05
0.0
98
–0.10
–0.2 100
200
300
400
500
600
700
800
Temperature/°C
106
104
1.0
Mass/%
DSC/mW mg–1
0.2
TG
1.2
DTG
0.8 0.6 0.4
DSC
102
100
0.2
0.1
0.0
–0.1
0.0
98
–0.2
–0.2 100
200
300
400
500
Temperature/°C
123
DTG/% min–1
1.4
0.3
354 °C
Exo up
(b)1.6
600
700
800
Thermal stability and structural properties of Ta nanopowder Fig. 6 SEM micrographs of Ta nanopowder for 20h sample
Fig. 7 TEM micrograph of Ta nanopowder for 20h sample
Fig. 9 HRTEM micrograph of Ta nanopowder for 20h sample
1.5
The log-normal probability distribution (Fig. 8) is calculated by adopting the following equations: ( ) ðlogd loglg Þ2 1 f ðdÞ ¼ pffiffiffiffiffiffi exp ð9Þ 2pdlogrg 2ðlogrg Þ2 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Rðlog di log lg Þ2 ð10Þ logrg ¼ Rni
1.0
loglg ¼
3.5 3.0
f (d )
2.5 2.0
0.5 0.0 2
3
4
5
6
7
Particle size/nm
Fig. 8 Particle size distribution for 20h sample (from TEM)
Figure 7 shows the representative TEM image of the synthesized Ta nanoparticles for 20h sample. The morphologies of nanoparticles are approximately spherical. Some nanoparticles are aggregated. It is observed from TEM images that the particle size distribution is narrow. The particle size measurement was done on more than 50 particles from different areas/scans and follows the well-documented log-normal distribution [31].
Rlogdi ; Rni
ð11Þ
where f(d) denotes the log-normal distribution, d is the particle diameter, ni is the number of particles with diameter di, log lg is the mean diameter, and rg is the geometrical standard deviation, respectively. The difference in the particle sizes obtained by XRD and TEM can be due to agglomeration of the particles. The average size of the prepared Ta nanoparticles is nearly in the range of 2–7 nm for the reaction time of 20 h. Figure 9 gives the highresolution TEM (HRTEM) image of single crystalline Ta nanoparticles and shows the (330) facets of the particles. The distance between the adjacent lattice fringes is interplanar distance of a-Ta (330), which is 0.77 nm (JCPDF card no: 089-4901). This suggests that the synthesized Ta powder has cubic crystalline structure.
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L. K. Brar et al.
Conclusions In summary, (1)
(2)
(3)
(4)
(5)
Ta nanosized powder has been synthesized successfully in a single step by the chemical reaction method using Ta2O5 as the precursor. From thermodynamic point of view, the initial reduction process occurs by simultaneous reduction of Ta2O5 by carbon as well as hydrogen. Carburization reaction occurs at the surface of the Ta2O5 particles, and hydrogen diffuses into the particles. The reduction due to hydrogen results in production of steam within the particles which breaks the particles resulting in an increase in reduction reaction rate, decrease in the size of the final product as well as lowering of the reaction temperature significantly. Ta nanoparticles synthesized with 15 h soaking possess high surface activity which results in an oxide layer on top. As the soaking time is increased, the stress is decreasing. The microstructure of powder was studied and analyzed by SEM and TEM. These particles are of nanosized having varied morphology with average particle size in the range of 2–7 nm as determined from TEM.
Acknowledgements One of the author (O. P. Pandey) is thankful to Department of Science and Technology (DST), New Delhi, India for which proposal has been submitted. The authors are also grateful to Central Research facilities (IIT Ropar) for providing XRD, IIT Roorkee for providing FE-SEM, SAI Labs, Thapar University for providing SEM and AIIMS, Delhi for providing TEM.
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