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A novel chemical solution technique for the preparation of nano size titanium powders from titanium dioxide
Advanced Powder Technol., Vol. 11, No. 4, pp. 415 – 422 (2000) © VSP and Society of Powder Technology, Japan 2000.
Original paper A novel chemical solution technique for the preparation of nano size titanium powders from titanium dioxide S. AMARCHAND, T. R. RAMA MOHAN and P. RAMAKRISHNAN Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Powai, Bombay 400 076, India Received 14 January 2000; accepted 8 May 2000 Abstract—Nano size titanium powder was prepared by a novel chemical solution synthesis route from titanium dioxide (TiO2 ). TiO2 was allowed to form a complex, titanium catecholate precursor, in the presence of ammonium sulfate and concentrated sulfuric acid. The complex was filtered, washed with cold isopropyl alcohol and dried. Titanium hydride was prepared by heating the titanium catecholate precursor at 800◦ C in a hydrogen atmosphere. The product was then dehydrogenated in 8 × 10−6 torr vacuum to get nano size titanium powders. The powder characteristics have been studied by X-ray diffraction, ICP, laser particle size analysis, and scanning and transmission electron microscopy. The latter indicated that the powders were 21.6 nm in size and selected area diffraction showed the powders to be crystalline.
1. INTRODUCTION
Due to its light weight, high specific strength, corrosion resistance and biocompatibility, titanium is an attractive material for aerospace and terrestrial systems, chemical processing industries, and biomedical applications [1– 4]. However, a major concern with titanium-based materials is their high cost compared to the competing materials. This had led to investigations on various potentially lower cost processes, including near net shape powder metallurgy techniques [5, 6]. The commercial production of titanium metal involves the chlorination of natural or synthetically produced rutile (TiO2 ) in the presence of carbon [7]. The resulting tetrachloride is then reduced to titanium sponge by the Kroll magnesium process [8] or the Hunder sodium process [9] and also by the reduction of TiO2 with calcium hydride [10]. Further, titanium powders that are produced from titanium sponge, hydride–dehydride processes and a variety of centrifugal atomization techniques are relatively coarse [11, 12]. If fine titanium powders can be produced, then the
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grain size in the sintered titanium can be small, thereby contributing to improved density and strength of the product [13, 14]. In the present investigation, nano size titanium powders of high purity have been synthesized from easily and economically available titanium dioxide, by chemical methods. Catechol (1,2-dihydroxy benzene) is a powerful bidentate chelating agent which has a particular affinity for metal ions that exhibit high oxidation states or high charge to metal ion radius ratios [15]. Hence, the enormous affinity of catechol for strong Lewis acids Ti(IV) as water soluble sulfate. Comparing with chlorination of TiO2 one has to deal with the intermediate highly volatile and corrosive TiCl4 , whereas this method does not involve these problems. The catecholate route used here gives titanium nano particles of size of the order of 9.37 nm. The fine titanium powders obtained are pure and there is no further purification is necessary. Titanium catecholate is synthesized from TiO2 and catechol in the presence of ammonium sulfate and concentrated sulfuric acid. The titanium catecholate obtained is dried and heated in a hydrogen atmosphere at 800◦ C for 3 h. The resulting titanium hydride powder obtained is reheated at temperatures upto 900◦ C for 1 h in a 6 × 10−6 torr vacuum in order to carry out dehydrogenation. The product is then rapidly cooled in vacuum from this temperature to obtain nano size titanium powder.
2. EXPERIMENTAL PROCEDURE
2.1. Preparation of titanium catecholate The titanium catecholate required for the preparation of titanium hydride is prepared on the basis of the technique reported by Davis et al. [16]. Commercially available or LR grade TiO2 of 99.5% purity is used for the synthesis of titanium catecholate. All other chemicals used are of high purity analytical reagent grade, unless otherwise specified. A Nicolet Impact-400 was used for the FT-IR spectra and Varian EM-360 L for NMR spectra, respectively. X-ray analysis was done using a Philips PW 1724 X-ray generator and PW-1710 diffractometer control. A Cambridge Instruments Stereoscan 90 was used for scanning electron microscopy (SEM) and Philips 200 for transmission electron microscopy (TEM). Particle size analysis was done by using a Galai laser particle size analyzer. TiO2 Ti(IV) + 3 1,2-C6 H4 (OH)2
H2 SO4 (NH4 )2 SO4
NH4 OH
Ti(IV)
[NH4 ]2 [Ti(1,2-C6 H4 O2 )3 ] · 2H2 O
(1) (2)
Titanium dioxide (13 mmol), ammonium sulfate (127 mmol) and concentrated sulfuric acid (43 ml) were loaded into a 100 ml round-bottomed flask equipped with an air condenser and heated in a sand bath using Bunsen burner until a clear, yellow
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solution was obtained. The addition of ammonium sulfate to elevate the boiling point of the sulfuric acid solvent was critical for the effective dissolution of the TiO2 reagent. With a sand bath temperature of about 500◦ C, dissolution takes 4–8 h. The solution was then allowed to cool to room temperature, at which point it was clear and colorless. The concentrated acid solution was then added carefully to 130 ml distilled water and the diluted titanyl sulfate solution then transferred to a 250 ml bypass dropping funnel. Separately, concentrated ammonium hydroxide solution (400 ml) was taken in a 1 l three-necked, round-bottomed flask equipped with the bypass dropping funnel, a nitrogen inlet and a glass stopper, and containing a magnetic stirrer bar. The ammonium hydroxide solution was deaerated by bubbling with nitrogen for 1 h. Catechol (39 mmol) was added and the contents were stirred under nitrogen until a clear solution results. The titanyl sulfate solution was then added from the bypass dropping funnel over a period of 5 min while the contents of the flask were stirred under nitrogen. During the addition, a red suspension was produced and this was stirred for an additional 4 h at room temperature of 30◦ C. The solid product was isolated and washed with cold isopropyl alcohol. The yield at this stage was found to be about 90%. 2.2. Preparation of titanium hydride Titanium hydride is prepared by heating the titanium catecholate in a hydrogen atmosphere. The furnace containing titanium catecholate was evacuated to 10−6 torr followed by flushing with hydrogen gas. The process was repeated thrice before heating the titanium catecholate in order to avoid the formation of unwanted products. The furnace was then heated to 800◦ C and hydrogen passed continuously for 3 h. The titanium hydride formed was cooled under an argon atmosphere and stored in the same atmosphere. 2.3. Preparation of nano size titanium powders Nano size titanium powders were prepared by the dehydrogenation of titanium hydride. The endothermic, reversible thermal decomposition reaction of titanium hydride is TiH2 + 145 kJ → Ti + H2
(3)
The dehydrogenation was carried out in a furnace which was evacuated to 10−6 torr at 900◦ C for 1 h. 3. RESULTS AND DISCUSSION
The precursor titanium catecholate prepared from LR grade TiO2 was characterized systematically. Proton NMR spectrum of the DMSO-d6 solution of the synthesized titanium catecholate exhibited resonances due to aromatic protons of the catecholate ligands.
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Figure 1. Typical FT-IR spectrum of titanium catecholate.
The IR spectrum exhibited broad features due to ammonium ion and the water of crystallisation of about 3700–2800 cm−1 and sharp bands due to catecholate ligands in the region below 1600 cm−1 . A typical FT-IR spectrum is shown in Fig. 1. The systematic synthesis of titanium hydride and its characterization is discussed in previous papers [17– 19]. Based on these results a rough composition of the hydrogenated precursor was computed which agreed well with the weight loss measurements. The particle size of the hydrogenated product at 8 × 10−6 torr as determined by the laser particle size analyzer indicated a number average particle size of 0.85 μm with about 90% of the particles below 1 μm. From the X-ray diffraction (XRD) of the hydrogenated product, it is confirmed that titanium catecholate can successfully be used for the preparation of titanium hydrides. A typical SEM image of titanium hydride is shown in Fig. 2 indicating the agglomerated particles. The dehydrogenation of the titanium hydride at 800◦ C and 8 × 10−6 torr vacuum resulted in 100% intensity titanium peak (Fig. 3). The number average particle size of the dehydrogenated product decreased to 0.71 μm with about 94% of particles below 1 μm size, indicating a decrease in particle size due to dehydrogenation. From our experience on samples dehydrogenated under various conditions, the background is due to the various types of Tix Hy non-stoichiometric phases which are nearly microcrystalline. Thus the peaks in Fig. 3 correspond to titanium, while
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Figure 2. SEM micrograph of agglomerated titanium hydride prepared by the hydrogenation of titanium catecholate.
Figure 3. Typical XRD analysis of pure titanium powder.
the background indicates the less crystalline non-stoichiometric Tix Hy . The ICP of the powder showed that the powder is pure with minimum impurities below 0.1 p.p.m.
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Figure 4. Typical TEM photograph of nano size titanium powder.
The TEM images of the powders indicated the particles to be 21.6 nm in size (range 9.37–38 nm). Particles of about 21.6 nm average particle size were found to be present along with agglomerated particles. Some of the agglomerates were as large as 500 nm. A typical TEM photograph is shown in Fig. 4. The selected area diffraction showed that the powders are microcrystalline (Fig. 5).
4. CONCLUSION
Titanium powder was prepared by a novel chemical solution technique from TiO2 through a titanium catecholate route. The results are summarized as follows: (1) Titanium catecholate has been successfully prepared from TiO2 through an intermediate sulfate route. (2) Hydrogenation of titanium catecholate at 800◦ C for 3 h yields titanium hydrides.
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Figure 5. Selected area diffraction of the microcrystalline titanium powder produced.
(3) Vacuum dehydrogenation (8 × 10−6 torr) of titanium hydrides at 900◦ C gave pure nano size titanium powders of particle size 21.6 nm with a microcrystalline nature. Acknowledgements The authors wish to thank the International Indo-Israeli collaboration program and the International Division of Department of Science and Technology of the Government of India for funding the project.
REFERENCES 1. K. H. Kramer, Titanium application — a critical review, in: Proc. 6th World Conf. on Titanium, P. Lacombe, R. Tricot and G. Beranger (Eds), vol. 1, p. 521. Societe Francise de Metallurgie, Cannes (1988).
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2. T. W. Farthing, Designing with titanium, in: Proc. 6th World Conf. on Titanium, P. Lacombe, R. Tricot and G. Beranger (Eds), vol. 1, p. 37. Societe Francise de Metallurgie, Cannes (1988). 3. F. H. Froes, L.-G. Li, P. R. Taylor, P. Chelman, F. Hehmann and C. M. Ward-Close, Mater. Trans. 37, 389 (1996). 4. A. Shenhar, I. Gotman, E. Y. Gutmanas and P. Ducheyne, Mater. Sci. Engng A268, 40– 46 (1999). 5. F. H. Froes and D. Eylon, Powder metallurgy of titanium alloys, Int. Mater. Rev. 35, 162 (1990). 6. S. Ankowitz, S. M. Abkowitz, P. F. Weihrauch and M. G. H. Wells, Low cost P/M manufacture of titanium alloy components for fatigue critical applications, in: P/M in Aerospace, Defense and Demanding Applications, E. H. Froes (Ed.). Metal Powder Industries Federation, Princeton, NJ (1993). 7. F. H. Hayes, H. B. Bomberger, R. H. Fores, L. Kanfman and H. M. Burte, Advances in titanium extraction metallurgy, J. Met. 136, 70 (1984). 8. W. J. Kroll, The production of ductile titanium, Trans. Electrochem. Soc. 78, 35 (1940). 9. M. A. Hunter, Metallic titanium, J. Amer. Chem. Soc. 32, 330 (1910). 10. V. S. Maxson, O. N. Senkov and F. H. Froes, Production and applications of low-cost titanium powder products, Int. J. Powder Metall. 34, 45– 53 (1998). 11. H. Shiraishi, E. Fukasawa, K. Fujisawa and H. Tajima, Titan 45 (1997). 12. S. J. Gerdemann, L. L. Oden and J. C. White, in: Proc. Conf. on Titanium Extraction and Processing, pp. 49– 50. Minerals, Metals and Materials Society/AIME, Indianapolis, IN/Warrandale, PA (1997). 13. R. M. German, Novel powder metallurgy technique for refractory metals and hard metals, in: Proc. 14th Int. Plansee Seminar ’97 Vol. 1: Metallic High Temperature Materials, p. 194. Triol, Austria (1997). 14. J. W. Sears, Process developments for titanium alloys, in: Synthesis/Processing of Lightweight Metallic Materials, F. H. Froes, C. Suryansarayana and C. M. Ward-Close (Eds), p. 331. TMS, Warrendale, PA (1995). 15. B. A. Borgais, S. R. Cooper, Y. B. Koh and K. N. Raymond, Synthetic, structural and physical studies of titanium complexes of catechol and 3,5-di-tert-butylcatechol, Inorg. Chem. 23, 1009– 1016 (1984). 16. J. A. Davies and S. Dutremez, J. Am. Ceram. Soc. 73, 1429 (1990). 17. S. Amarchand, T. R. Rama Mohan and P. Ramakrishnan, Synthesis of fine titanium powders via solution route, in: Proc. Int. Conf. on Reactive and Refractory Materials. Gordon & Breach, London (2000). 18. S. Amarchand, T. S. Murthy, T. R. Rama Mohan, P. Ramakrishnan and E. Y. Gutmanas, Some studies on titanium powders and composites, in: Proc. Meet. on Metal Matrix Composites, R. C. Agarwala, V. Agarwala, S. Kumar and S. Ray (Eds), p. 65. Institute Industry Interaction Meeting, University of Roorkee, Roorkee, India (1999). 19. S. Amarchand, T. R. Rama Mohan and P. Ramakrishnan, Synthesis and characterization of ultra fine titanium powders, in: Ultrafine Grained Materials: Proc. TMS Annual Meet, Nashville, TN, R. S. Mishra, S. L. Semiatin, C. Swyanarayana, N. N. Thodhani and T. C. Lowes (Eds), p. 33 (2000).
Original paper A novel chemical solution technique for the preparation of nano size titanium powders from titanium dioxide S. AMARCHAND, T. R. RAMA MOHAN and P. RAMAKRISHNAN Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Powai, Bombay 400 076, India Received 14 January 2000; accepted 8 May 2000 Abstract—Nano size titanium powder was prepared by a novel chemical solution synthesis route from titanium dioxide (TiO2 ). TiO2 was allowed to form a complex, titanium catecholate precursor, in the presence of ammonium sulfate and concentrated sulfuric acid. The complex was filtered, washed with cold isopropyl alcohol and dried. Titanium hydride was prepared by heating the titanium catecholate precursor at 800◦ C in a hydrogen atmosphere. The product was then dehydrogenated in 8 × 10−6 torr vacuum to get nano size titanium powders. The powder characteristics have been studied by X-ray diffraction, ICP, laser particle size analysis, and scanning and transmission electron microscopy. The latter indicated that the powders were 21.6 nm in size and selected area diffraction showed the powders to be crystalline.
1. INTRODUCTION
Due to its light weight, high specific strength, corrosion resistance and biocompatibility, titanium is an attractive material for aerospace and terrestrial systems, chemical processing industries, and biomedical applications [1– 4]. However, a major concern with titanium-based materials is their high cost compared to the competing materials. This had led to investigations on various potentially lower cost processes, including near net shape powder metallurgy techniques [5, 6]. The commercial production of titanium metal involves the chlorination of natural or synthetically produced rutile (TiO2 ) in the presence of carbon [7]. The resulting tetrachloride is then reduced to titanium sponge by the Kroll magnesium process [8] or the Hunder sodium process [9] and also by the reduction of TiO2 with calcium hydride [10]. Further, titanium powders that are produced from titanium sponge, hydride–dehydride processes and a variety of centrifugal atomization techniques are relatively coarse [11, 12]. If fine titanium powders can be produced, then the
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grain size in the sintered titanium can be small, thereby contributing to improved density and strength of the product [13, 14]. In the present investigation, nano size titanium powders of high purity have been synthesized from easily and economically available titanium dioxide, by chemical methods. Catechol (1,2-dihydroxy benzene) is a powerful bidentate chelating agent which has a particular affinity for metal ions that exhibit high oxidation states or high charge to metal ion radius ratios [15]. Hence, the enormous affinity of catechol for strong Lewis acids Ti(IV) as water soluble sulfate. Comparing with chlorination of TiO2 one has to deal with the intermediate highly volatile and corrosive TiCl4 , whereas this method does not involve these problems. The catecholate route used here gives titanium nano particles of size of the order of 9.37 nm. The fine titanium powders obtained are pure and there is no further purification is necessary. Titanium catecholate is synthesized from TiO2 and catechol in the presence of ammonium sulfate and concentrated sulfuric acid. The titanium catecholate obtained is dried and heated in a hydrogen atmosphere at 800◦ C for 3 h. The resulting titanium hydride powder obtained is reheated at temperatures upto 900◦ C for 1 h in a 6 × 10−6 torr vacuum in order to carry out dehydrogenation. The product is then rapidly cooled in vacuum from this temperature to obtain nano size titanium powder.
2. EXPERIMENTAL PROCEDURE
2.1. Preparation of titanium catecholate The titanium catecholate required for the preparation of titanium hydride is prepared on the basis of the technique reported by Davis et al. [16]. Commercially available or LR grade TiO2 of 99.5% purity is used for the synthesis of titanium catecholate. All other chemicals used are of high purity analytical reagent grade, unless otherwise specified. A Nicolet Impact-400 was used for the FT-IR spectra and Varian EM-360 L for NMR spectra, respectively. X-ray analysis was done using a Philips PW 1724 X-ray generator and PW-1710 diffractometer control. A Cambridge Instruments Stereoscan 90 was used for scanning electron microscopy (SEM) and Philips 200 for transmission electron microscopy (TEM). Particle size analysis was done by using a Galai laser particle size analyzer. TiO2 Ti(IV) + 3 1,2-C6 H4 (OH)2
H2 SO4 (NH4 )2 SO4
NH4 OH
Ti(IV)
[NH4 ]2 [Ti(1,2-C6 H4 O2 )3 ] · 2H2 O
(1) (2)
Titanium dioxide (13 mmol), ammonium sulfate (127 mmol) and concentrated sulfuric acid (43 ml) were loaded into a 100 ml round-bottomed flask equipped with an air condenser and heated in a sand bath using Bunsen burner until a clear, yellow
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solution was obtained. The addition of ammonium sulfate to elevate the boiling point of the sulfuric acid solvent was critical for the effective dissolution of the TiO2 reagent. With a sand bath temperature of about 500◦ C, dissolution takes 4–8 h. The solution was then allowed to cool to room temperature, at which point it was clear and colorless. The concentrated acid solution was then added carefully to 130 ml distilled water and the diluted titanyl sulfate solution then transferred to a 250 ml bypass dropping funnel. Separately, concentrated ammonium hydroxide solution (400 ml) was taken in a 1 l three-necked, round-bottomed flask equipped with the bypass dropping funnel, a nitrogen inlet and a glass stopper, and containing a magnetic stirrer bar. The ammonium hydroxide solution was deaerated by bubbling with nitrogen for 1 h. Catechol (39 mmol) was added and the contents were stirred under nitrogen until a clear solution results. The titanyl sulfate solution was then added from the bypass dropping funnel over a period of 5 min while the contents of the flask were stirred under nitrogen. During the addition, a red suspension was produced and this was stirred for an additional 4 h at room temperature of 30◦ C. The solid product was isolated and washed with cold isopropyl alcohol. The yield at this stage was found to be about 90%. 2.2. Preparation of titanium hydride Titanium hydride is prepared by heating the titanium catecholate in a hydrogen atmosphere. The furnace containing titanium catecholate was evacuated to 10−6 torr followed by flushing with hydrogen gas. The process was repeated thrice before heating the titanium catecholate in order to avoid the formation of unwanted products. The furnace was then heated to 800◦ C and hydrogen passed continuously for 3 h. The titanium hydride formed was cooled under an argon atmosphere and stored in the same atmosphere. 2.3. Preparation of nano size titanium powders Nano size titanium powders were prepared by the dehydrogenation of titanium hydride. The endothermic, reversible thermal decomposition reaction of titanium hydride is TiH2 + 145 kJ → Ti + H2
(3)
The dehydrogenation was carried out in a furnace which was evacuated to 10−6 torr at 900◦ C for 1 h. 3. RESULTS AND DISCUSSION
The precursor titanium catecholate prepared from LR grade TiO2 was characterized systematically. Proton NMR spectrum of the DMSO-d6 solution of the synthesized titanium catecholate exhibited resonances due to aromatic protons of the catecholate ligands.
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Figure 1. Typical FT-IR spectrum of titanium catecholate.
The IR spectrum exhibited broad features due to ammonium ion and the water of crystallisation of about 3700–2800 cm−1 and sharp bands due to catecholate ligands in the region below 1600 cm−1 . A typical FT-IR spectrum is shown in Fig. 1. The systematic synthesis of titanium hydride and its characterization is discussed in previous papers [17– 19]. Based on these results a rough composition of the hydrogenated precursor was computed which agreed well with the weight loss measurements. The particle size of the hydrogenated product at 8 × 10−6 torr as determined by the laser particle size analyzer indicated a number average particle size of 0.85 μm with about 90% of the particles below 1 μm. From the X-ray diffraction (XRD) of the hydrogenated product, it is confirmed that titanium catecholate can successfully be used for the preparation of titanium hydrides. A typical SEM image of titanium hydride is shown in Fig. 2 indicating the agglomerated particles. The dehydrogenation of the titanium hydride at 800◦ C and 8 × 10−6 torr vacuum resulted in 100% intensity titanium peak (Fig. 3). The number average particle size of the dehydrogenated product decreased to 0.71 μm with about 94% of particles below 1 μm size, indicating a decrease in particle size due to dehydrogenation. From our experience on samples dehydrogenated under various conditions, the background is due to the various types of Tix Hy non-stoichiometric phases which are nearly microcrystalline. Thus the peaks in Fig. 3 correspond to titanium, while
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Figure 2. SEM micrograph of agglomerated titanium hydride prepared by the hydrogenation of titanium catecholate.
Figure 3. Typical XRD analysis of pure titanium powder.
the background indicates the less crystalline non-stoichiometric Tix Hy . The ICP of the powder showed that the powder is pure with minimum impurities below 0.1 p.p.m.
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Figure 4. Typical TEM photograph of nano size titanium powder.
The TEM images of the powders indicated the particles to be 21.6 nm in size (range 9.37–38 nm). Particles of about 21.6 nm average particle size were found to be present along with agglomerated particles. Some of the agglomerates were as large as 500 nm. A typical TEM photograph is shown in Fig. 4. The selected area diffraction showed that the powders are microcrystalline (Fig. 5).
4. CONCLUSION
Titanium powder was prepared by a novel chemical solution technique from TiO2 through a titanium catecholate route. The results are summarized as follows: (1) Titanium catecholate has been successfully prepared from TiO2 through an intermediate sulfate route. (2) Hydrogenation of titanium catecholate at 800◦ C for 3 h yields titanium hydrides.
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Figure 5. Selected area diffraction of the microcrystalline titanium powder produced.
(3) Vacuum dehydrogenation (8 × 10−6 torr) of titanium hydrides at 900◦ C gave pure nano size titanium powders of particle size 21.6 nm with a microcrystalline nature. Acknowledgements The authors wish to thank the International Indo-Israeli collaboration program and the International Division of Department of Science and Technology of the Government of India for funding the project.
REFERENCES 1. K. H. Kramer, Titanium application — a critical review, in: Proc. 6th World Conf. on Titanium, P. Lacombe, R. Tricot and G. Beranger (Eds), vol. 1, p. 521. Societe Francise de Metallurgie, Cannes (1988).
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2. T. W. Farthing, Designing with titanium, in: Proc. 6th World Conf. on Titanium, P. Lacombe, R. Tricot and G. Beranger (Eds), vol. 1, p. 37. Societe Francise de Metallurgie, Cannes (1988). 3. F. H. Froes, L.-G. Li, P. R. Taylor, P. Chelman, F. Hehmann and C. M. Ward-Close, Mater. Trans. 37, 389 (1996). 4. A. Shenhar, I. Gotman, E. Y. Gutmanas and P. Ducheyne, Mater. Sci. Engng A268, 40– 46 (1999). 5. F. H. Froes and D. Eylon, Powder metallurgy of titanium alloys, Int. Mater. Rev. 35, 162 (1990). 6. S. Ankowitz, S. M. Abkowitz, P. F. Weihrauch and M. G. H. Wells, Low cost P/M manufacture of titanium alloy components for fatigue critical applications, in: P/M in Aerospace, Defense and Demanding Applications, E. H. Froes (Ed.). Metal Powder Industries Federation, Princeton, NJ (1993). 7. F. H. Hayes, H. B. Bomberger, R. H. Fores, L. Kanfman and H. M. Burte, Advances in titanium extraction metallurgy, J. Met. 136, 70 (1984). 8. W. J. Kroll, The production of ductile titanium, Trans. Electrochem. Soc. 78, 35 (1940). 9. M. A. Hunter, Metallic titanium, J. Amer. Chem. Soc. 32, 330 (1910). 10. V. S. Maxson, O. N. Senkov and F. H. Froes, Production and applications of low-cost titanium powder products, Int. J. Powder Metall. 34, 45– 53 (1998). 11. H. Shiraishi, E. Fukasawa, K. Fujisawa and H. Tajima, Titan 45 (1997). 12. S. J. Gerdemann, L. L. Oden and J. C. White, in: Proc. Conf. on Titanium Extraction and Processing, pp. 49– 50. Minerals, Metals and Materials Society/AIME, Indianapolis, IN/Warrandale, PA (1997). 13. R. M. German, Novel powder metallurgy technique for refractory metals and hard metals, in: Proc. 14th Int. Plansee Seminar ’97 Vol. 1: Metallic High Temperature Materials, p. 194. Triol, Austria (1997). 14. J. W. Sears, Process developments for titanium alloys, in: Synthesis/Processing of Lightweight Metallic Materials, F. H. Froes, C. Suryansarayana and C. M. Ward-Close (Eds), p. 331. TMS, Warrendale, PA (1995). 15. B. A. Borgais, S. R. Cooper, Y. B. Koh and K. N. Raymond, Synthetic, structural and physical studies of titanium complexes of catechol and 3,5-di-tert-butylcatechol, Inorg. Chem. 23, 1009– 1016 (1984). 16. J. A. Davies and S. Dutremez, J. Am. Ceram. Soc. 73, 1429 (1990). 17. S. Amarchand, T. R. Rama Mohan and P. Ramakrishnan, Synthesis of fine titanium powders via solution route, in: Proc. Int. Conf. on Reactive and Refractory Materials. Gordon & Breach, London (2000). 18. S. Amarchand, T. S. Murthy, T. R. Rama Mohan, P. Ramakrishnan and E. Y. Gutmanas, Some studies on titanium powders and composites, in: Proc. Meet. on Metal Matrix Composites, R. C. Agarwala, V. Agarwala, S. Kumar and S. Ray (Eds), p. 65. Institute Industry Interaction Meeting, University of Roorkee, Roorkee, India (1999). 19. S. Amarchand, T. R. Rama Mohan and P. Ramakrishnan, Synthesis and characterization of ultra fine titanium powders, in: Ultrafine Grained Materials: Proc. TMS Annual Meet, Nashville, TN, R. S. Mishra, S. L. Semiatin, C. Swyanarayana, N. N. Thodhani and T. C. Lowes (Eds), p. 33 (2000).