Ammonium fluoride-activated synthesis of cubic δ-TaN nanoparticles at low temperatures
© Lee et al.; licensee Springer. 2013
Received: 14 December 2012
Accepted: 5 February 2013
Published: 15 March 2013
Cubic delta-tantalum nitride (δ-TaN) nanoparticles were selectively prepared using a K2TaF7 + (5 + k) NaN3 + k NH4F reactive mixture (k being the number of moles of NH4F) via a combustion process under a nitrogen pressure of 2.0 MPa. The combustion temperature, when plotted as a function of the number of moles of NH4F used, was in the range of 850°C to 1,170°C. X-ray diffraction patterns revealed the formation of cubic δ-TaN nanoparticles at 850°C to 950°C when NH4F is used in an amount of 2.0 mol (or greater) in the combustion experiment. Phase pure cubic δ-TaN synthesized at k = 4 exhibited a specific surface area of 30.59 m2/g and grain size of 5 to 10 nm, as estimated from the transmission electron microscopy micrograph. The role of NH4F in the formation process of δ-TaN is discussed with regard to a hypothetical reaction mechanism.
KeywordsCubic δ-TaN Combustion synthesis Ammonium fluoride Nanoparticles
Among the various transition-metal nitrides, TaN is a material that has potential for application in microelectronic components such as capacitors, thin-film resistors, and barrier materials that prevent the diffusion of copper into silicon [1, 2]. In addition, TaN has been used in high-temperature ceramic pressure sensors because of its good piezoresistive properties . Also, it is an attractive histocompatible material that can be used in artificial heart valves . Among the various tantalum nitride phases, cubic delta-tantalum nitride (δ-TaN), with a NaCl-type structure (space group: Fm3m), exhibits excellent properties such as high hardness, stability at high temperature, and superconductivity .
In general, it is difficult to produce δ-TaN under ambient conditions since its formation requires high temperature and nitrogen pressure. According to the data reported in another study , δ-TaN is normally made at more than 1,600°C and 16 MPa of nitrogen pressure. Kieffer et al. synthesized cubic TaN by heating hexagonal TaN above 1,700°C at a N2 pressure of 6 atm . Matsumoto and Konuma were successful in producing cubic TaN by heating hexagonal TaN at a reduced pressure using a plasma jet . Mashimo et al. were able to transform hexagonal TaN into cubic TaN by both static compression and shock compression at high temperature . Cubic TaN in powder form was also synthesized by self-propagating high-temperature synthesis technique [10, 11]. In this process, the combustion of metallic tantalum from 350 to 400 MPa of nitrogen pressure resulted in micrometer size δ-TaN at a temperature above 2,000°C.
More recently, two approaches, solid-state metathesis reaction and nitridation-thermal decomposition [12–14], were adopted for the synthesis of nanosized particles of δ-TaN. O’Loughlin et al. used the metathesis reaction of TaCl5 with Li3N and 12 mol of NaN3 to produce δ-TaN . The authors concluded that significant nitrogen pressure created by the addition of NaN3 enabled cubic-phase TaN to form, along with hexagonal Ta2N. Solid-state metathesis reaction applied to the TaCl5-Na-NH4Cl mixture resulted in a bi-phase product at 650°C comprising both hexagonal and cubic phases of TaN . More recently, Liu et al. reported the synthesis of cubic δ-TaN through homogenous reduction of TaCl5 with sodium in liquid ammonia, with a subsequent annealing process at 1,200°C to 1,400°C under high vacuum . Nitridation-thermal decomposition, a two-step process for the synthesis of cubic δ-TaN, was also reported . In the first step, nanosized Ta2O5 was nitrided at 800°C for 8 h under an ammonia flow. The as-prepared product was then thermally decomposed at 1,000°C in nitrogen atmosphere, and cubic nanocrystalline δ-TaN was obtained.
In most cases, the products prepared by the above-mentioned methods were often mixtures containing other compounds such as TaN0.5 or other nonstoichiometric phases. Therefore, synthesis inefficiency of cubic δ-TaN nanoparticles by known approaches coupled with the multiphase composition of products makes this topic challenging and scientifically attractive.
In this paper, an attractive and rapid approach for synthesizing cubic δ-TaN nanoparticles is developed. This approach includes the combustion of K2TaF7 + (5 + k)NaN3 + k NH4F exothermic mixture under nitrogen atmosphere and water purification of final products to produce cubic δ-TaN. The approach described in this study is simple and cost-effective for the large-scale production of δ-TaN.
For sample preparation, the following chemicals were used: K2TaF7 (prepared at the Graduate School of Green Energy Technology, Chungnam National University, Korea), NaN3 powder (99.0% purity; particle size < 50 μm; Daejung Chemical and Metals Co., Ltd., Shiheung City, South Korea). Chemical-grade ammonium halides (NH4F and NH4Cl) were purchased from Samchun Pure Chemical Co., Ltd., Pyeongtaek City, South Korea. All salts were handled in a glove box in dry argon atmosphere (99.99%; Messer, Northumberland, UK).
We used the simulation software ‘Thermo’ to predict adiabatic combustion temperature (Tad) and concentrations of equilibrium phases in the combustion wave . Calculations of equilibrium characteristics were based on minimizing the thermodynamic potential of the system. The initial parameters (temperature and pressure) of the system were set as 25°C and 2.0 MPa, respectively.
The crystal structure and morphology of the TaN nanoparticles were characterized X-ray diffraction (XRD) with Cu Kα radiation (D5000, Siemens AG, Munich, Germany), field-emission scanning electron microscopy (FESEM; JSM 6330F, JEOL Ltd., Akishima, Tokyo, Japan), and transmission electron microscopy (TEM; JEM 2010, JEOL Ltd.). The specific surface area of the nanoparticles was determined from the linear portion of the Brunauer, Emmett, and Teller plot.
Results and discussion
Adiabatic combustion temperature and equilibrium phases
DSC-TGA curves and combustion parameters
Characteristics of combusted samples and powders
Content of nitrogen in TaN
As shown above, the forming of cubic TaN from the exothermic mixture of K2TaF7 + 5NaN3 composition does not occur despite a relatively high combustion temperature (1,170°C). Under conditions, however, the addition of ammonium fluoride to the reaction mixture had a favorable effect on the cubic-phase δ-TaN nanoparticle synthesis, despite large drops in the combustion temperature (850°C; k = 4). The replacement of NH4F with NH4Cl slightly lowered the combustion temperature to 850°C (k = 4). However, cubic-phase δ-TaN nanoparticles were obtained. Therefore, the addition of ammonium halides to the combustion reaction can provide low pressure and temperature route for the synthesis of the cubic TaN.
In this process, the total amount of NaN3 was set at 10.35 mol to produce 15.5 mol of N2, as seen in the reaction (Equation 2). The combustion temperature of the K2TaF7 + 5.175ZnF2 + 10.35 NaN3 mixture measured by thermocouples was 900°C. The reaction product after acid leaching was a black powder and was a component from hexagonal ε-TaN and Ta2N according to XRD analysis. No peaks matching to cubic TaN was found on the XRD patterns. The response of cubic TaN to ammonium halides raised the question about the mechanism of the process. At present, we do not have a clear explanation of the role that ammonium halide has during the synthesis process. However, a plausible hypothesis can be offered with respect to the underlying mechanism. We believe that the hydrogen that is released from ammonium halide may stimulate a process of hydration-dehydration of Ta in the intermediate stages of the combustion process and may lead to vacancies in the tantalum lattice without affecting its crystal structure. These free vacancies created by hydrogen atoms could be easily occupied by nitrogen atoms at higher combustion temperatures, thus leading to the formation of cubic δ-TaN.
Another possible explanation for the cubic phase may involve the formation of tantalum amido- or imido-fluorides (Ta(NH2)2F3.4NH3 or Ta(NH2)2F4.6NH3) in a manner similar to the previously reported formation of tantalum amido- or imido-chlorides (Ta(NH2)2Cl3.4NH3 or Ta(NH2)2Cl4.6NH3) [18, 19]. However a further, detailed investigation is needed to clarify the mechanism behind the formation of cubic tantalum nitride using ammonium halides.
Nanocrystalline cubic δ-TaN was prepared by a solid combustion synthesis method using the K2TaF7 + (5 + k)NaN3 + k NH4F reactive mixture. It was shown that without NH4F, the maximum temperature of K2TaF7 + 5NaN3 mixture is 1,170°C, and the combustion product is multiphase consisting of hexagonal TaN as well as TaN0.8 and Ta2N phases. However, the addition of NH4F to the reactive mixture stimulates the formation of cubic δ-TaN. Phase-pure cubic δ-TaN was obtained when NH4F in the amount of 4.0 mol (or greater) was used in the combustion experiments. The formation temperatures for cubic δ-TaN were as low as 850°C to 950°C. Cubic δ-TaN synthesized using 4.0 mol of NH4F exhibited a specific surface area of 30.59 m2/g and a grain size of 5 to 10 nm, as estimated from its TEM micrograph. The approach developed in this study is a simple and cost-efficient method for the large-scale production of δ-TaN.
YJL is under the Ph.D. course in Green Energy Technology in Chungnam National University. DYK is under the master course in Green Energy Technology in Chungnam National University. KKB and KSK are principal researchers in Korea Institute of Energy Research. KHL and JHL are professors at the Graduate School of the Department of Metallurgical Engineering of Chungnam National University. MHH is a professor at the Graduate School of Green Energy Technology of Chungnam National University.
This research was supported by KIER R&D program (Project number KIER B2-2144-03) under Korea Institute of Energy, Republic of Korea.
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