Reduction of titanium oxide in the presence of nickel by nonequilibrium hydrogen gas
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Shigeo Sato Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
Hiroaki Kakiuchi Department of Precision Science and Technology, Division of Precision Science & Technology and Applied Physics, Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan
Yasuhiro Awakura Department of Materials Science and Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan (Received 16 December 2008; accepted 22 April 2009)
We investigated the reduction of TiO2 in the presence of Ni by nonequilibrium hydrogen gas, including low-temperature hydrogen plasma at 800 C and supercooled monatomic hydrogen at 1000 C. TiO2 was reduced to Ti2O3, which is not in equilibrium phase, by low-temperature hydrogen plasma. The results of x-ray diffraction and energy dispersive x-ray analysis in experiments at 1000 C indicate that the thermodynamical reduction potential of supercooled monatomic hydrogen is almost the same as atmospheric hydrogen gas. However, the wide Ti3O5 layer formed only in the case of the reduction at 1000 C by supercooled monatomic hydrogen. With these experimental facts, we speculate that the reduction mechanism by nonequilibrium hydrogen consists of two steps; the releasing energy process and the relaxation process. We can explain the difference of reduction products by nonequilibrium hydrogen gas on the assumption of the rate of the relaxation process between 800 and 1000 C.
I. INTRODUCTION
Titanium alloys are attractive materials because they have various unique properties. Ni–Ti alloy shows shape memory and a superelasticity effect. The Nb–Ti alloy is a superconducting material. Some titanium alloys show excellent corrosion resistance and heat resistance.1,2 However, such materials are expensive. One of the reasons is that they are made from an alloy metal and pure titanium, which is an expensive material because of its high production cost. The major production process of titanium is the Kroll process, which consists of carbochlorination of TiO2, purification of TiCl4, magnesiothermic reduction of TiCl4 to metallic titanium, and vacuum distillation of MgCl2 and excess Mg from metallic titanium. This process has problems of process complication, low productivity of titanium, and high energy consumption.3,4 Therefore, direct reduction of titanium oxide to titanium alloy has an advantage of simplicity.3 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0286 J. Mater. Res., Vol. 24, No. 7, Jul 2009
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Reduction by hydrogen gas may be the simplest process because the only by-product is water vapor. In this study, the reduction of titanium oxide in the presence of nickel as the alloying element to titanium alloy by hydrogen gas was investigated. II. THERMODYNAMICS OF Ti–Ni–O TERNARY SYSTEM
Figure 1(a) shows the equilibrium chemical potential of oxygen, RT ln pO2 ; in equilibrium with some titanium oxides as a function o
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