Aluminothermic Reduction of Ta2o5 Ignited by Plasma

Aluminothermic Reduction of Ta2O5 Ignited by Plasma

R.A. Brito1, F.F.P. Meeiros1, C. Alves Jr.1, A.G.P. da Silva2, C.P. Souza3, F.A. Costa4

1UFRN – DFTE – Laboratório de Processamento de Materiais por Plasma, Campus Universitário, 59072-570, Natal, RN, Brasil [email protected] 2 Laboratório de Materiais Avançados – CCT – UENF, Campos dos Goytacazes, RJ, Brasil 3UFRN – DEQ, Laboratório de Termodinâmica e Reatores, Natal, RN, Brasil 4UFRN – DFTE, Laboratório de Materiais Cerâmicos e Metais Especiais, Natal, RN, Brasil

Keywords: Aluminothermic Reduction, Plasma, Ignition, Tantalum.

Abstract: In this work, the production of metallic tantalum by reduction of tantalum pentoxide by aluminum in a plasma rector is investigated. A hydrogen plasma is utilized as heat source to cause the ignition of the strong exothermal reaction between the aluminum powder and the tantalum oxide intimately mixed. The yield of the reaction and its development in the mixed powder bed are investigated by means of SEM and XRD.

Introduction

Tantalum is a very interesting metal for industry due to some properties such as high melting point, ductility, high corrosion resistance, attractive electric and thermal conductivities [1]. Its main industrial application is the production of capacitors due to the excellent dielectric properties of its oxide [2,3], contributing to the miniaturization of the electronic circuitry and its operation in hazardous environments [4]. Tantalum is also used as carbide (TaC) in cutting tools as additive to control grain size [5,6] and in catalysis [7]. Ta powder is usually produced by the reduction of its oxide by reducers such as C, Si, Ca, Mg and Al [2,3,8,9]. Among these alternatives the reduction by Al, called aluminothermic reduction, has been industrially used to produce Nb, Ta and their alloys due to the easy of Al and Al2O3 removal after the reaction [9-11]. In the conventional aluminothermic reduction, the heat to ignite reaction is supplied by an electric resistance [12]. Since it starts, the heat generated by the reaction is enough to propagate it through the powder bed. The reaction products are metallic tantalum and Al2O3. Usually to ensure the complete reduction of the tantalum oxide, an excess Al is added to the powder mixture. This excess is found in the reacted material in solid solution or forming intermediated phases with tantalum. Recently, Nb was produced by the aluminothermic route using plasma as the heat source for ignition [13]. In the present work the same procedure is used to reduce tantalum oxide.

Experimental

Tantalum oxide powder (99.9%) and aluminum powder (9.8%) were the reactants. H2 (99.5%) was used to produce the plasma. Figs. 1A and 1B show particles of tantalum oxide and aluminum, respectively. Ta2O5 particles tend to agglomerate. The mean particle size for this powder determined by laser scattering was 0.8µm and for the Al powder was 5.2µm.

Figure 1: Agglomerate of Ta2O5 particles (A) and Al particles (B).

Aluminum and tantalum oxide powders were mixed manually and using the high energy mill model micro mill Fritsch Pulverisette 7. Four powder mixtures were prepared. The first was manually mixed. The others were milled for 2, 6 and 10 hours. The amount of Ta2O5 was kept in 10g for all samples and the amount of Al 2.14g. This corresponds to an overstoichiometric Al content of 5% taking Eq. 1 as reference.

3Ta2O5 +10Al → 6Ta + 5Al2O3 Eq.1 Details of the powder mixtures are given in Table 1.

Table 1: Mixtures Ta2O5 and Al. Sample Mass of Al – Mass of Ta2O5 Milling Time (h) (g) A-1 Manual A-2 2 A-3 2.14 – 10.00 6 A-4 10

A schematic of the plasma reactor is shown in Fig. 2. It consists in a glass cylinder closed in the extremities by two stainless steel flanges and two electrodes. The cathode, in the bottom, works as the sample holder. It is a hollow cylinder. All reductions were performed under the same conditions: Temperature – 800°C, Hydrogen flow – 16cm3/min., initial pressure – 300Pa, final pressure – 1000Pa and heating rate of approximately 20°C/min. The power source has maximum output voltage and DC current of respectively 1500V and 1.5A. Voltage, current, pressure and cathode temperature are measured and controlled in the panel. The sample is placed in the sample holder. The reactor is closed and pumped until the pressure reaches 300Pa. The electrodes are polarized and hydrogen is introduced in the reactor chamber. The gas is ionized and the temperature is raised up to 800°C. The heating rate is controlled by appropriately adjusting the voltage. After reaching the final temperature, the power is turned off. The reaction starts at an instant during heating. After cooling the samples are characterized by SEM and XRD.

Figure 2: Schematic of the plasma reactor.

Results and Discussion

Fig. 3 shows the diffraction patterns of the Ta2O5 – Al prepared by manual mixture and milling. The diffraction peaks correspond only to those of tantalum oxide and aluminum. It can be noted that the intensity of the peaks decrease, and the widths decrease as the milling time increases. The cause of this is the intensity of milling.

z Tantalum oxide 4000 ‹Aluminum z 2000 z z z z z z ‹ ‹ zz z z‹ z ‹ A-4 0 4000

2000 A-3 0

Intensity 4000 2000 A-2 0 4000

2000 A-1

0 20 30 40 50 60 70 80 2θ Figure 3: XRD patterns of the mixtures A-1, A-2, A-3 and A-4. Only peaks of tantalum oxide and Al are present. The peaks become wider for longer milling times.

Fig. 3 shows the XRD patterns of all mixtures after reduction. In all cases, the reaction products (Ta and Al2O3) are present. But the reactants are still in the samples as individual phases. This indicates that the reaction did not come to completion. The amount of tantalum oxide in each sample after reduction varied as pointed out by the relative intensity of the respective diffraction peaks. For the milled samples, the amount of tantalum oxide in the reduced samples decreased as the milling time increases. However the hand mixed sample presents comparable amount of the oxide as the sample milled for 2 hours. The positive effect of milling on the reduction level is explained by the better dispersion of the reactants produced by longer milling. Aluminum is a very soft material. During the first moments of milling it is expected that the aluminum particles be severely deformed and agglomerated by cold welding. At this stage the quality of the dispersion can even become poorer. This can explain the results obtained in the hand mixed and 2 hours milled samples.

z __ Ta 3000 z 3000 __ Ta O A-3 A-4 † 2 5 __ Al O z ‹ 2 3 2000 2000  __ Al

1000 1000 z z z z ‹ ‹ ‹ † †‹ ‹† ‹ ‹ †† ‹ ‹ ‹ ‹ ‹ 0 0 400 1000 A-1 A-2 z Intensity

z

200 500 z † ‹ † z z z ‹ † † ‹ † † ‹ † † ‹ ‹ † † ‹ ‹ ‹  ‹ † †‹ ‹†‹ ‹ † †† ‹ † † † 0 0 20 30 40 50 60 70 80 20 30 40 50 60 70 80 2θ

Figure 4: XRD patterns of the reduced samples. A-1 hand mixed, A-2 milled for 2 h. A-3 milled for 6 h. A-4 milled for 10 h.

Micrographs of the mixture A-3 before and after reduction are exhibited in Fig. 4. In the milled powder the largest particles are around 20µm (Figs. 5A and 5B). The particles are composite. Al and tantalum oxide constitute each particle. Thus each particle is a system that can react. These particles are formed during milling. The soft aluminum is deformed and agglomerated with the harder tantalum oxide. As milling proceeds, more tantalum oxide is incorporated to the aluminum particles, producing better dispersion, and the aluminum particles hardens up to the fracture. The balance between cold welding and fracture plus the milling time determine the final size of the particles. Each particle of the reduced material is a mixture of Ta, Al2O3 and non-reacted materials. The particles are bulky and larger than before the reaction (Figs. 5C and 5D). In the conventional aluminothermic procedure, the whole material (reactants and reduction products) is fused mainly due to the highly exothermal reaction, but due to the low thermal losses of the reactor as well. After cooling the material is the form of a dense block. In the present case, the final material is in form of powder. This indicates that partial fusion occurred. This fusion promoted the enlargement of initial composite particles by agglomeration. Complete fusion did not occur due to high thermal loss of the particles to the surrounding atmosphere. The existence of unreacted Al and Ta2O5 after cooling is result of both short reaction time and bad dispersion. In conventional aluminothermic reduction, reaction occurs in both solid and liquid phases. When the whole material is liquid the dispersion improves. But in the present case the melting is partial. This means that each particle agglomerate is an individual reducing system. If in a certain agglomerate there is more Ta2O5 than the stoichiometry then there will be residual Ta2O5 in that particle. On the inverse case, the whole Ta2O5 present in the particle will be reduced. The Al excess is in solid solution, not as a phase. This explains the occurrence of Ta2O5 after reduction as a separate phase.

A B

C C

Figure 5: SEM micrographs of the mixture A-3 after milling for 6 hours (A and B) and after reduction (C and D).

Conclusions

The aluminothermic reduction of composite particles of Ta2O5 and Al ignited by plasma is able to produce metallic tantalum in form of powder. The reduction occurs in each composite particle. The reduction level depends on the dispersion of the reactants. Even after 10 hours of milling the dispersion was not enough to complete the reduction. The reduced particles are also composite constituted of Ta with aluminum in solid solution, Al2O3 and Ta2O5 not reacted. The complete reaction could be reached with the total fusion of the reacting material but powder would not be produced. Another possibility is to improve the dispersion of the reactants. This would require finer starting powders and/or longer milling times.

Acknowledgements

The authors wish to acknowledge CAPES and CNPq for the financial support.

References

[1] M.L.A.Andrade; L.M.S. Cunha; M.C. Silva, Tantalum: The importance of the Brazilian Production. Boletim Mineração e Metalurgia. Partnership between BNDES, FINAME and BNDESPAR. Nº04, August/2002 (in Portuguese). [2] Il Park; T.H. Okabe; Y. Waseda. Tantalum powder production by magnesothermic reduction of TaCl5 through an electronically mediated reaction (EMR). Journal of Alloys and compounds 280. 1998. 265-272. [3] M. Baba; Y. Ono; R.O. Suzuki. Tantalum and niobium powder preparation from their oxides by calciothermic reduction in the molten CaCl. Journal of Physics and Chemistry of solids 66 (2005) 466-470. [4] T. Balaji, R. Govindaiah, M.K. Sharma, Y. Purushotham, Arbind Kumar, T.L. Prakash. Sintering and electrical properties of tantalum anodes for capacitor applications. Materials Letters 56 (2002) 560–563. [5] L. López-de-la-Torre; B. Winklerb; J. Schreuerb; K. Knorrc; M. Avalos-Borjad. Elastic properties of tantalum carbide (TaC). Solid State Communications 134 (2005) 245–250. [6] S.K. Kim; B.C. Cha. Deposition of tantalum nitride thin films by D.C. magnetron sputtering. Thin Solid Films 475 (2005) 202– 207. [7] C.M. Visinescu; R. Sanjines; F. Lévy; V. Marcuc; V.I. Pârvulescu. Tantalum doped titania photocatalysts: Preparation by dc reactive sputtering and catalytic behavior. Journal of Photochemistry and Photobiology A: Chemistry 174 (2005) 106–112. [8] A Awasthi; Y.J. Bhatt; N. Krishnamurthy; Y. Ueda; S.P. Garg. The reduction of niobium and tantalum pentoxides by silicon in vacuum. Journal of Alloys na Compounds 315. 2001. 187-192. [9] H.R.Z. Sandin; D.G. Pinatti; C.A. Baldan; R.A. Conte; C.R. Dainesi; C.A. Nunes. Processing of Niobium and Its Alloys Through Aluminothermic Reduction and Refine in in Electron Bean Furnace. In: Annual Symposium of ACIESP on Science and Technology of Niobium. São Paulo, FINEP/ACIESP, 1995. p.1-9. (in portuguese). [10] J.R. Darnell; L.F. Yntema. The element columbium and its compounds. In:Technology of columbium (Niobium) – Symposium on Columbium (Niobium) of the electrothermics and metallurgy division of the electrochemical society, Washington D.C., May 15-16, 1958, p. 1-9. [11] F. Habashi. Principles of extractive metallurgy. New York, Gordon and Breach, V.3, 1986, p.19. [12] U. U. Gomes. Study on sintering anodic oxidation and development of a new kind of Nb- Ta electrolytic capacitor, doctor thesis, UNICAMP, Campinas/SP, 1987. (in Portuguese). [13] M.W.D. Mendes, Production of Nb powder from the aluminothermic reduction of its oxide ignited by plasma. Master thesis, UFRN, Natal/RN, 2005. (in Portuguese).