Micro-, submicron-, and nano-scale titanium dioxide particles were reduced by reduction with a metallic calcium reductant in calcium chloride molten salt at 1173 K, and the reduction mechanism of the oxides by the calcium reductant was explored. These oxide particles, metallic calcium as a reducing agent, and calcium chloride as a molten salt were placed in a titanium crucible and heated under an argon atmosphere. Titanium dioxide was reduced to metallic titanium through a calcium titanate and lower titanium oxide, and the materials were sintered together to form a micro-porous titanium structure in molten salt at high temperature. The reduction rate of titanium dioxide was observed to increase with decreasing particle size; accordingly, the residual oxygen content in the reduced titanium decreases. The obtained micro-porous titanium appeared dark gray in color because of its low surface reflection. Micro-porous metallic titanium with a low oxygen content (0.42 wt%) and a large surface area (1.794 m 2 g -1 ) can be successfully obtained by reduction under optimal conditions. Key words: Titanium, Calcium, Calciothermic reduction, Nano-particles, Electrolytic capacitor IntroductionMetallic calcium can easily reduce a large number of metal oxides directly to metals because calcium has a strong reducing ability. The reduction of metal oxides with a calcium reductant in calcium chloride molten salt at high temperature, well known as the calciothermic reaction, has long been widely investigated by many researchers. During oxide reduction, calcium chloride molten salt acts as a suitable solvent and has a high solubility for calcium and calcium oxide as a byproduct formed by reduction. [11] are formed via reduction of these oxides with a calcium reductant in calcium chloride molten salt. The residual oxygen in the reduced metal decreases continuously by deoxidation with a calcium reductant [4]. Pure metals, alloys, and intermetallic compounds with a lower residual oxygen content can be successfully fabricated by a one-step reduction technique. Reduction with a calcium reductant is a simpler technique for the production of these metals without complicated processes such as the Kroll process, although reduction is a batch-type production method.In recent years, a method for the successive reduction of metal oxides was developed by electrolysis in calcium chloride and calcium oxide mixture molten salts. Ono and Suzuki reported that calcium oxide in molten calcium chloride becomes the reductant source for the reduction of metal oxides during constant-voltage electrolysis [12]. For example, metallic calcium formed electrochemically at a cathode reacts with metal oxides, and reduced metals with a low residual oxygen content can be formed (OS (Ono and Suzuki) Kyoto process). Metallic titanium [13][14][15], niobium [16], nickel [17], and other alloys and intermetallic compounds [18,19] can be formed via the OS process. In addition, the electrochemical decomposition of carbon dioxide gas by an advanced OS process using ...

This paper reports unusual diffusion-controlled growth of TiO 2 mesoporous anodic films on titanium in hot phosphate/glycerol electrolytes. The formation behavior was investigated by cyclic voltammetry (CV) between 0 and 5 V vs. Pt at 433 K. The current density became almost constant above 1.5 V vs. Pt during the positive potential sweep, and was maintained even during the negative potential sweep. This is contrast to a drastic decrease in current density in changing the direction of potential sweep from the positive to negative in fluoride-containing ethylene glycol electrolyte. The constant current density between 1.5 and 5 V vs. Pt increased with an increase in the basicity of the hot phosphate electrolyte, suggesting that the rate-determining step of the film formation in the hot phosphate electrolyte was diffusion process of oxygen sources in the electrolyte, not the ion migration in the thin barrier layer under the high electric field. When CV measurements were conducted to higher potentials up to 20 V vs. Pt, anatase was developed above 7 V vs. Pt, leading to generate oxygen gas. The film morphology was also potential-dependent and the diffusion current was also influenced by the film morphology as well as oxygen gas generation.

Recently, we reported the formation of TiO2 mesoporous films by anodizing titanium in hot phosphate/glycerol electrolytes [1-3]. The resultant films showed interesting features, for example: (i) the pore size was as small as ~10 nm leading to a high surface area and did not change linearly with the formation voltage; and (ii) formation of the films at an 20 V generated the crystalline anatase phase without the requirement of post-annealing, whereas as prepared films anodized in the electrolytes containing fluorides were usually amorphous. However, the details of formation behavior of the crystalline TiO2mesoporous films have yet to be revealed. Therefore, in this study, we investigated the growth process of the films during the anodizing in hot alkaline glycerol electrolytes containing phosphate by cyclic voltammetry and changing electrolyte anion species. The specimens were 99.5% pure titanium plates, which were electropolished in 1.0 mol dm-3 NaCl/ethylene glycol solution. The electropolished specimens were anodized in stirred glycerol electrolytes containing selected amounts of phosphate such as K2HPO4 and K3PO4 and/or NOH, and 0.03 wt% H2O at 433 K. Anodizing by cyclic voltammograms (CVs) was performed using a three-electrode system with a platinum foil as a counter electrode and a platinum wire as a reference electrode. The potential sweep rate and stirring rate were 30 mV s-1and 330 rpm, respectively. Anodizing at constant voltage of 20 V was performed using a two-electrode system with the platinum foil as the counter electrode. Fig. 1 shows the CVs of titanium at the first cycle in the hot glycerol electrolyte containing 0.6 mol dm-3 K3PO4 + 0.2 mol dm-3 K2HPO4 (Fig. 1a), together with that in the ethylene glycol electrolyte containing fluoride (Fig. 1b). In the case of the anodizing in the electrolyte containing the phosphates, the current density became almost constant at potentials higher than 2 V vs Pt. Similar constant currents were obtained even during the negative sweep of the potential, whereas the current density decreased during the negative sweep in the electrolyte containing fluoride. This result suggested that the ionic transport in the barrier layer under the high electric field was not a rate-determining step during the anodizing in the hot phosphate/glycerol electrolyte. The Cottrell plot during anodizing in the phosphate/glycerol electrolyte at 3 V vs Pt reveals a linear correlation between the reciprocal of the current density and the square root of anodizing time. Thus, the anodizing process is diffusion-controlled. In addition, the limiting current became smaller with decreasing basicity of the electrolyte, implying some basic species, such as OH-, should be diffusing ones to grow the anodic TiO2films during anodizing in the hot phosphate/glycerol electrolyte. The crystallinity of the films anodizing the electrolyte containing phosphate or not was investigated by XRD. The results suggested that a small amount of the phosphate contained the anodic films may have important roles to crystallize during anodizing.  References [1] E. Tsuji, Y. Taguchi, Y. Aoki, T. Hashimoto, P. Skeldon, G.E. Thompson and H. Habazaki, Appl. Surf. Sci. 2014, 301, 500. [2] E. Tsuji, N. Hirata, Y. Aoki and H. Habazaki, Mater. Lett., 2013, 91,39. [3] Y. Taguchi, E. Tsuji, Y. Aoki and H. Habazaki, Appl. Surf. Sci. 2012, 258, 9810.  Figure caption Fig. 1 The cyclic voltammogram of titanium specimens during anodizing in (a) 0.6 mol dm-3 K3PO4 and 0.2 mol dm-3 K2HPO4 glycerol electrolytes containing 0.03 mass% water at 433 K and (b) 0.25 wt% NH4F /ethylene glycol electrolyte containing 1 vol% water at 293 K with stirring. The right axis shows the current density during anodizing in (a) and the left one shows that in (b), respectively. Figure 1