Combustion synthesis of nanostructured tungsten and its morphological study
Abstract
A tungsten trioxide, zinc, and sodium chloride (WO3 + 3Zn + kNaCl) mixture was used as a precursor material for the manufacture of tungsten (W) nanopowders. Precursor material components were dry mixed and heated up to 450 °C under a constant argon gas flow to promote a combustion reaction. Combustion products were subjected to acid enrichment followed by isothermal heat treatment under hydrogen flow at temperatures ranging from 650 to 850 °C to reduce oxygen concentration. As-synthesized W nanopowders were characterized with respect to average particle size, particle size distribution, oxygen concentration, and phases. Under optimized reaction conditions, W particles with diameters less than 100 nm and an average interstitial oxygen concentration lower than 1.0 wt.% were obtained.
Graphical abstract
We demonstrate the high potential of the salt assisted combustion method for the synthesis of nanostructured tungsten from WO3 + 3Zn + kNaCl mixture. Under optimized reaction conditions W particles with diameters less than 100 nm were obtained. The procedure of nanoparticle production is experimentally quite facile and delivers reproducibly different sizes of nanostructured tungsten.
Introduction
In recent years, considerable researches have been performed to refine the grain size of pure tungsten for increasing the propensity for shear localization [1], [2], [3]. The effect of grain size on the yield strength of metals has long been recognized [4], e.g., the Hall–Petch relation. It also has been shown [5] that the strain rate sensitivity of BCC metals is proportional to the inverse of grain size. As a result, there has been tremendous recent interest in pure tungsten ultrafine powder (≤ 100 nm).
In the last few decades, several methods have been proposed and developed to produce W nanopowders: salt assisted combustion reaction (SACR) [6], plasma processing technique (PPT) [7], electrical explosion of wires [8], high energy ball-milling [9] and physical vapor deposition (PVD) [10]. Among those, SACR and PPT are considered as scalable processes capable of producing from 10 to 100 nm diameter tungsten nanopowders. In PPT, tungsten trioxide (WO3) is injected in hydrogen (H2) plasma where WO3 is vaporized and the oxide vapor phase reacts with H2 to induce a chemical reaction to form a W vapor phase. The W vapor phase is quenched rapidly and nano-size W particles are solidified. In contrast, the chemical reaction in SACR involves solid phases, and it has been reported [6] that SACR processes were used to synthesize W nanopowders using WO3, and a variety reducing agents, such as magnesium (Mg), sodium borohydride (NaBH4), sodium azide (NaN3), and sodium chloride (NaCl) as starting materials. W nanopowders with average particle sizes smaller than 100 nm were successfully synthesized.
The present work proposes a robust and scalable method to synthesize W nanopowders using inexpensive raw materials that are suitable for commercial production of large quantity and high quality W nanopowders. The combustion process of WO3 + 3Zn + kNaCl system (where, k is the mole fraction of NaCl) was investigated with respect to its robustness and scalability. Moderate chemical activity, low melting point, high density and relatively low cost of Zn make it highly attractive as a promising reducing agent in the synthesis of W nanopowder.
Section snippets
Experimental
WO3 powder (> 99.9% pure, and particle size < 45 μm, Grand Chemical and Material Co., Ltd., South Korea), Zn powder (99% pure and particle size < 10 μm, Daejung Chemicals and Metals Co., Ltd, South Korea) and NaCl powder (99.5% pure and particle size < 150 μm, Samchun Pure Chemicals Co., Ltd., South Korea) were used as starting materials for the synthesis of tungsten nanopowders. WO3 powder was first ground into fine powder (≤ 200 nm) and then thoroughly mixed with Zn and NaCl powders by
Results and discussion
In order for combustion to occur in a “weakly exothermic” WO3 + 3Zn + kNaCl system (250 cal/g at k = 0)) it is necessary to increase the total energy content of the system. The proper way for introducing an additional energy source is the preheating of the reaction mixture to a temperature at which the combustion reaction can be self-initiated and sustained. For this purpose the basic experiments were carried out into laboratory box furnace under an inert atmosphere with a constant argon flow of
Conclusion
The combustion wave parameters (Tc, Uc) of the mixture WO3 + 3Zn + kNaCl and the characteristics of the final products have been experimentally obtained over the NaCl concentration range of 0–6 mol. The combustion temperature associated with the WO3 + 3Zn mixture is experimentally measured to be 1200 °C. By adding NaCl, the heat released per unit volume decreased. The NaCl acts as a heat sink thermodynamically; therefore, both the combustion temperature (Tc) and wave velocity (Uc) decrease. Because
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