International Journal of Refractory Metals and Hard Materials
Effect of reaction time on phase composition and microstructure of chromium carbide nanopowders
Introduction
The transition metal carbides have very high temperature strength [1], [2], high mechanical hardness [3], [4], [5] and high melting points [6]. Besides, they also have good electrical and thermal conductivities [7]. Therefore, they have wide uses in metallurgy, electronics, catalysts, etc. [8], [9], [10]. Particularly, chromium carbides (Cr3C2) demonstrate excellent strength, hardness, anti-erosion and corrosion properties and permanent non-magnetizability [11]. Therefore, chromium carbides have been widely used in shaft bearings, seals, high-temperature furnaces, etc. [12].
Commonly, carbide powders are synthesized by carbon thermal reduction of micron-sized oxides and carbon. This method has a number of weaknesses such as a higher reaction temperature (> 1400 °C), a longer reaction time (> 4 h) and a higher cost. Furthermore, the prepared carbide powders exhibit grains in the μm-range, which cannot satisfy the demands of carbide powders for modern industry. Nowadays, various methods for synthesizing carbide powders have been researched, e.g. by direct element reaction [13], mechanical alloying [14], [15], temperature programmed reaction [16], [17] and by gas-reduction [18], [19]. However, industrial applications of the methods are still limited due to the agglomeration problems [13], wide size distributions [13], low yields [13], [14], complex monitoring [15], [16], [17] and high costs [18], [19].
In this study, chromium carbide (Cr3C2) nanopowders were synthesized by the carbon thermal reduction method using nanometer Cr2O3 and carbon black. Because those nanometer materials exhibit a higher specific surface area and activity, the reaction temperature and reaction time can be reduced and shortened, respectively. This not only helps to save energy and causes lower costs, but one can also prepare chromium carbide (Cr3C2) nanopowders with excellent properties. Besides, this method is characterized by a simple production process, so it will be favorable for industrial production. As we know, it was seldom reported that chromium carbide nanopowders were synthesized by the method using nanometer Cr2O3 and nanometer carbon black as raw materials. In this investigation, the effect of reaction time on phase composition and microstructure of chromium carbide nanopowders was researched.
Section snippets
Experimental
Nanometer Cr2O3 (average particle size < 60 nm) and nanometer carbon black (average particle size < 50 nm) were used as raw materials. 72 wt.% Cr2O3 and 28 wt.% C were put into a ball milling pot according to the reaction equation between Cr2O3 and C (3Cr2O3 + 13C = 2Cr3C2 + 9CO↑), and the milling medium was absolute ethyl alcohol. Cemented carbide balls were used for milling. After being milled from 2 to 12 h, the mixture was dried in a vacuum drying oven under the conditions of 90 to 150 °C and 0.5 to 6 h. Cr3
Results and discussion
To determine the physical phenomena occurring during carbon thermal reduction, simultaneous TG–DSC measurements were carried out for the mixture and the results are given in Fig. 1. As shown in Fig. 1, the ultimate weight loss revealed by TG curve is 15.7 wt.%. The change of weight is relatively smooth (9.4 wt.%) below 1100 °C, but it is abrupt between 1100 °C and 1200 °C (6.3 wt.%). A broad exothermic peak occurs at 600 to 900 °C in the DSC curve, which is ascribed to the oxidation-reduction reaction
Conclusions
In this paper, chromium carbide (Cr3C2) nanopowders have been synthesized by carbon thermal reduction method, and the raw materials are nanometer Cr2O3 and nanometer carbon black. The results show that powders with the single phase of Cr3C2 can be synthesized at 1100 °C for 1 h, and an average crystallite size of 26 nm. The powders show good dispersion and are mainly composed of spherical or nearly spherical particles with a mean diameter of about 30 nm. When the reaction time is 0.5 h, the product
Acknowledgments
The research was supported by the Key Scientific and Technological Project of Henan Province (132102210163), the Development of Science and Technology Plan Projects of Zhengzhou City (20110305), the Plan for Scientific Innovation Talent of Henan University of Technology (11CXRC15) and the Doctor Foundation of Henan University of Technology (2009BS014), China.
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