Toru H. Okabe

Titanium powder production by preform reduction process (PRP)

Abstract To develop an effective process for titanium powder production, a new preform reduction process (PRP), based on the calciothermic reduction of preform containing titanium oxide (TiO 2 ), was investigated. The feed preform was fabricated from slurry, which was made by mixing TiO 2 powder, flux (e.g. CaCl 2 ) and binder. Various types of preforms in the form of plates, spheres, or tubes were prepared using a conventional technique, and the fabricated preform was sintered at 1073 K before reduction in order to remove the binder and water. The sintered solid preform containing TiO 2 was then placed in a stainless steel container, and reacted with calcium vapor at a constant temperature ranging between 1073 and 1273 K for 6 h. Titanium powder was recovered from the reduced preform by leaching it with acid. As a result, pure titanium powder with 99 mass% purity was obtained. This process was found to be suitable for producing a homogeneous fine powder when the composition of flux and the size of the preform are controlled.

Electrochemical deoxidation of titanium

Removal of oxygen in titanium using an electrochemical technique was examined at temperatures around 1223 K with the purpose of obtaining nearly oxygen-free titanium. Titanium and carbon electrodes, immersed in molten CaCl2, served as cathode and anode, respectively, with an external DC source. CaCl2 was employed to produce the deoxidant calcium and to facilitate the reaction by decreasing the activity of the by-product CaO. By applying about 3 V between the electrodes, the calcium potential in CaCl2 was increased at the titanium cathode surface and titanium samples of the cathode could be deoxidized by the electrolytically produced deoxidant calcium or by calcium of high activity in the CaCl2 flux. Resulting O2− species, mainly present as the deoxidation product CaO in the flux, reacted at the carbon anode to form CO (or CO2) gas which was removed from the system. Titanium wires containing 1400 mass ppm oxygen were deoxidized to less than 100 mass ppm, whereas the carbon concentration increased by about 50 mass ppm. In some cases, the oxygen concentration in titanium samples was lowered to a level less than 10 mass ppm that could be determined by conventional inert gas fusion analysis. The behavior of contaminants, such as carbon and nitrogen, is also discussed.

Preparation and characterization of extra-low-oxygen titanium

Abstract Removal of oxygen in titanium by reaction with chemically active calcium dissolved in CaCl2 was examined between 1273 and 1473 K with the purpose of obtaining extra low-oxygen titanium. CaCl2 was used as a flux to facilitate the reaction by decreasing the activity of the by-product CaO. Titanium wires and small pieces of titanium were deoxidized to 20–60 mass ppm oxygen by use of calcium-saturated CaCl2 at a temperature of 1273 K. Trace element analysis (e.g. glow discharge mass spectrometry), micro Vickers hardness measurements, and electrical resistivity measurements were carried out to characterize the deoxidized titanium. The deoxidation of electrolytically refined titanium wire produced titanium with a high residual resistivity ratio ( ga 298 /α 4.2 ⋍100 ). The “ideal resistivities”, or hypothetical resistivities of pure titanium, at 77 and 298 K were determined to be 40 and 440 nΩm respectively. The influence of oxygen on resistivity at 4.2 K was also measured by using titanium containing 30 and 500 mass ppm O, and was determined to be 88 nΩm (mol.% O)−1.

Production of niobium powder by electronically mediated reaction (EMR) using calcium as a reductant

Explored in this study is an electronically mediated reaction (EMR) route for the production of niobium powder using calcium as a reductant for niobium oxide (Nb2O5). Feed material, Nb2O5, and reductant calcium alloy containing aluminum and nickel were charged into electronically isolated locations in a molten salt (e.g. CaCl2) at 1173 K. The current flow through an external path between the feed and reductant locations was monitored. A current approximately 0.4 A was measured during the reaction in the external circuit connecting cathode and anode location. Niobium powder with low aluminum and nickel content was obtained although liquid Ca–Al–Ni alloy was used as the reductant. This clearly demonstrates that niobium metal powder can be produced by an electronically mediated reaction (EMR), without direct physical contact between feed (Nb2O5) and reductant (calcium). Mechanism of calciothermic reduction of Nb2O5 in the molten salt is discussed using an isothermal chemical potential diagram.

Electrochemical deoxidation of yttrium-oxygen solid solutions

Oxygen was removed from yttrium by an electrochemical method in which the metal is made the cathode in a cell consisting of a carbon anode and molten CaCl2 electrolyte. At 1223 K yttrium containing 5700 ppm oxygen was deoxidized down to less than 100 ppm. The method can be used to deoxidize other highly reactive metals. Furthermore, in principle it should be possible to remove other impurities besides oxygen.

Metallothermic reduction as an electronically mediated reaction

The commonly held view that metallothermic reduction is strictly a chemical reaction and that the process is rate limited by mass transfer has been found to be incomplete. In a study of the production of tantalum powder by the reaction of K 2 TaF 7 with sodium, it has been shown that there are two dominant kinetic pathways, both involving electron transfer. Furthermore, the overall rate of reaction is limited by electron transport between the reactants. This indicates that metallothermic reduction is an “electronically mediated reaction” (EMR). Experiments found that the location of the tantalum deposit and its morphology are governed by the reaction pathway.

Electrochemical deoxidation of RE–O (RE=Gd, Tb, Dy, Er) solid solutions

The removal of oxygen from rare-earth metals (RE, RE=Gd, Tb, Dy, Er) by an electrochemical deoxidation method was investigated. A titanium basket containing the rare-earth metal sample, submerged in molten CaCl2 electrolyte, formed the cathode of an electrolysis cell. A high-purity graphite anode was used. The calcium metal produced at the cathode effectively deoxidized the rare-earth metal. Carbon monoxide and dioxide were generated at the graphite anode. Rare-earth metals containing more than 2000 mass ppm oxygen were deoxidized to 10–50 mass ppm level by electrolysis at 1189 K for 36 ks (10 h). Cyclic voltammetry was used to characterize the molten salt at different stages of the process. The effectiveness of the process is discussed with the aid of a chemical potential diagram for RE–O solid solutions. The new electrochemical technique is compared with the conventional deoxidation methods reported in the literature. The possibility of nitrogen removal from the rare-earth metals by the electrochemical method is outlined.

Deoxidation of titanium aluminide by Ca-Al alloy under controlled aluminum activity

Removal of oxygen in titanium aluminide (TiAl) by chemically active calcium-aluminum (Ca-AI) alloy was carried out around 1373 K with the purpose of obtaining extra-low-oxygen TiAl. The deoxidation experiments were preceded by an investigation of the phase equilibria of the system Ti-Al-Ca at 1273 and 1373 K. The compositions of the Ca-AI alloy deoxidant, which equilibrates with TiAl, and the experimental conditions suitable for the deoxidation were of particular interest. In experiments in which Ti-Al samples were submerged in liquid Ca-AI alloys at 1373 K, the surfaces of the samples severely deteriorated and became nodular. When TiAl powders were mixed with CaO and the deoxidant was supplied in vapor form, powders which initially contained 510, 1100, and 4200 ppm O were deoxidized to about 160, 490, and 670 ppm O after deoxidation at 1373 K in 86.4 ks (1 day). Among many conditions tested, the use of TiAl powders mixed with CaCl2 was most effective for deoxidation at 1373 K. CaCl2 was used as a flux to facilitate the deoxidation by decreasing the activity of the deoxidation product CaO. In the case that TiAl powders mixed with CaCl2 and reacted with Ca-AI vapor at 1373 K for 86.4 ks, the powders initially containing 510, 1100, and 4200 mass ppm O were deoxidized to a level of 62, 140, and 190 mass ppm O, respectively. No significant change in morphology of the particle after deoxidation was observed. The titanium and nitrogen concentrations in the powders remained constant, whereas calcium, which was present only in trace amounts initially, increased up to 160 mass ppm after the deoxidation treatment.

Thermodynamic properties of oxygen in RE-O (RE = Gd, Tb, Dy, Er) solid solutions

The oxygen potentials of four rare-earth metal – oxygen (RE–O: RE=Gd, Dy, Tb, Er) solid solutions have been measured by equilibration with yttrium – oxygen (Y–O) and titanium – oxygen (Ti–O) solid solutions. Rare-earth metal, yttrium and titanium samples were immersed in calcium-saturated CaCl2 melt at temperatures between 1093 and 1233 K. Homogeneous oxygen potential was established in the metallic samples through the fused salt, which contains some dissolved CaO. The metallic samples were analyzed for oxygen after quenching. The oxygen potentials of RE–O solid solutions were determined using either Y–O or Ti–O solid solution as the reference. This method enabled reliable measurement of extremely low oxygen potentials at high temperature (circa pO2=10−48 atm at 1173 K). It was found that the oxygen affinity of the metals decreases in the order: Y>Er>Dy>Tb>Gd>Ti. Values for the standard Gibbs energy of solution of oxygen in RE metals obtained in this study, permit assessment of the extent of deoxidation that can be achieved with various purification techniques. It may be possible to achieve an oxygen level of 10 mass ppm using an electrochemical deoxidation method.

Evaluation of Ti-Cr-Cu alloys for dental applications

This study examined the characteristics of as-cast Ti-Cr(7–19%)-Cu(3–7%) (all percentages in this article are mass%) alloys to evaluate their suitability for dental applications; studies on the alloy structures and mechanical properties, grindability, and corrosion behavior were included in the investigation. The alloys were centrifugally cast and bench-cooled in investment molds. The x-ray diffractometry of the as-cast alloys bench-cooled in the molds indicated the following phases: α+β+ω in the 7% Cr and 7% Cr+3% Cu; β+ω in the 13%Cr; and β in the 13%Cr+3% Cu through the 19%Cr+3% Cu alloys. The strengths of the binary β Ti-Cr and ternary β Ti-Cr-Cu alloys with 13 and 19% Cr were approximately two times higher than those of CP Ti. The alloy ductility was dependent on the chemical composition and thus, the microstructure. The 7% Cr alloys were extremely brittle and hard due to the ω phase, but the ductility was restored in the 13 and 19% Cr alloys. The hardness (HV) of the cast 13 and 19% Cr alloys was approximately 300–350 compared with a value of 200 for CP Ti. The grindability of the cast alloys was examined using a rotating SiC wheel at speeds (circumferential) of 500 and 1250 m/min. At the higher speed, the grindability of the 13 and 19% Cr alloys increased with the Cu content. The grindability of the 13% Cr alloy with 7% Cu was similar to that of CP Ti. Evaluation of the corrosion behavior in an artificial saliva revealed that the alloys are like many other titanium alloys within the normal intraoral oxidation potential. The wear resistance testing of these alloys also showed favorable results.