Refining of tantalum by silicon deoxidation

Journal of Alloys and Compounds 265 (1998) 190–195

L

Refining of tantalum by silicon deoxidation a, a a a b a Alok Awasthi *, N. Krishnamurthy , Y.J. Bhatt , R. Venkataramani , Y. Ueda , S.P. Garg a

Materials Processing Division, Bhabha Atomic Research Centre, Mumbai-400 085, India b Department of Energy Science and Engineering, Kyoto University, Kyoto 606, Japan Received 20 May 1997; received in revised form 10 July 1997

Abstract The removal of residual oxygen from tantalum by pyrovacuum treatment using silicon deoxidation (as SiO(v) ) was studied. The possibility of eliminating oxygen from a Ta–Si–O solid solution by preferential evaporation of SiO (v) was indicated by calculations based on the experimental or estimated values for the activities of silicon and oxygen in the solid solution as well as for the standard free energies of formation of tantalum suboxide (TaO (v) ) and silicon monoxide (SiO(v) ). The experimental results obtained by treating Ta–Si–O alloys of known silicon and oxygen contents at temperatures ranging from 2073 to 2273 K under 1310 23 Pa pressure for 2 h indicated the occurrence of silicon deoxidation. In this temperature range, oxygen removal has occurred faster and to a lower residual level in Ta–O alloys containing silicon as compared to Ta–O alloys containing no silicon. When both silicon and oxygen concentrations have reached relatively lower levels, viz., ¯1 at % O and 0.3 at % Si, oxygen removal by SiO (v) vapourisation decreases with an increasing contribution of sacrificial deoxidation (as TaO(v) ).  1998 Elsevier Science S.A. Keywords: Tantalum; Deoxidation; Refining; SiO (v) ; TaO (v)

1. Introduction As reduced tantalum metal, obtained by sodium reduction of potassium tantalum fluoride [1] or by oxide reduction methods [2] may contain up to about 7.5 at % (|7000 ppm) of residual oxygen. During subsequent pyrovacuum treatments, the residual oxygen is removed from tantalum either by sacrificial deoxidation involving the vapourisation of the suboxide TaO (v) [3] or by carbon deoxidation involving the volatilization of carbon monoxide, CO [4]. Sacrificial deoxidation involves tantalum loss. For example, the decrease of oxygen content from 7.5 at % to 0.5 at % O by sacrificial deoxidation involves .7% loss of tantalum. This was carried out by treating the Ta–O alloy at 2320 K for 2 h under 7310 23 Pa. Under the same conditions, if tantalum mixed with carbon corresponding to a C / O mole ratio of 0.91 was treated, oxygen content decreased to about 0.1 at % with essentially no tantalum loss. The metal, however, contains approximately 0.15 at % residual carbon. This carbon could not be removed from tantalum by pyrovacuum methods except by introducing more oxygen to result in the evaporation of CO. However, introduction of oxygen in such Ta–C alloy is extremely difficult. Silicon appears to be an attractive *Corresponding author. 0925-8388 / 98 / $19.00  1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 97 )00433-7

alternative to carbon. Silicon deoxidation works essentially in the same way as carbon deoxidation with the possibility of distillation of the remaining residual silicon at the end of deoxidation as an added advantage. Silicon deoxidation of tantalum has been investigated in this work.

2. Theoretical considerations A complete assessment of the silicon deoxidation of tantalum involves consideration of thermodynamics of each of the major vapourisation processes expected from the Ta–Si–O alloy, viz., vapourisation of TaO (v) , vapourisation of SiO (v) and vapourisation of Si (v) and their relative extents. The reference states throughout this thermodynamic analysis have been taken as pure liquid / solid Si and Ta, pure vapour of Si (v) , SiO (v) and TaO (v) and pure O 2(g ) at the given temperature and at 1 atm (1.0133310 5 Pa) pressure.

2.1. Sacrificial deoxidation There can be only two phases in equilibrium during sacrificial deoxidation of tantalum (below the saturation

A. Awasthi et al. / Journal of Alloys and Compounds 265 (1998) 190 – 195

solubility limit of oxygen in tantalum). These are Ta–O solid solution (henceforth referred as phase ss) and a vapour phase containing TaO (v) (henceforth referred as phase v). The equilibrium reaction can be represented as [O] ss 1 [Ta] ss ⇔TaO (v) .

(1)

Since, the standard states for oxygen and tantalum in the Ta–O solid solution are respectively pure solid Ta and pure O 2( g) at 1.0 atm pressure, the standard free energy change of reaction (1) is same as the standard free energy of formation of TaO (v) . The equilibrium constant for reaction (1) can, therefore, be written as o pTaO /(a Ta a O ) 5 exp(2D form G TaO /RT ) (v)

(2)

where p is the partial pressure, a is the activity and D form G o is the standard free energy of formation (of the elements and compound shown in subscript) at temperature T. The values of a Ta is close to 1 as the tantalum contents in the Ta–O alloys during this work are generally more than 95 at % Ta. Eq. (2), therefore, gives pTaO 5 a O exp(2D form G oTaO (v) /RT ) .

2153 K [6,7], the activity coefficient of oxygen in the solid solution at the solubility limit calculated on the basis of this equilibrium can be extrapolated to higher temperatures. That this extrapolation is valid is confirmed by the agreement between the oxygen activity coefficient values obtained using Eq. (8) and from the following equation obtained from the data reported by Fromm et al. [8] a O 5 XO exp(11.54 2 46060 /T )

(9)

which is based on experimental investigation of the Ta–O equilibrium between 1573 and 2273 K. However, Eq. (8) is used presently for maintaining consistency in the estimated values of pTaO , pSi and pSiO . Combination of Eqs. (4) and (8) gives pTaO 5 XO exp(32.15 2 68400 /T )Pa .

(10)

For example, over the saturated solid solution of oxygen in tantalum, 5.4 at % O at 2000 K [6,7], pTaO is equal to 7310 23 Pa.

2.2. Silicon deoxidation (3)

Substituting reported expression [6,7] for D form GTaO (v) as a function of temperature in Eq. (3), pTaO (in Pa) is obtained as pTaO 5 a O exp(21.82 2 22390 /T )Pa .

191

(4)

The removal of oxygen from Ta–Si–O alloy by silicon deoxidation can be represented as [O] ss 1 [Si] ss ⇔SiO (v)

(11)

for which o

At the solubility limit, oxygen dissolved in tantalum is in equilibrium with solid Ta 2 O 5 [6,7]. This equilibrium can be represented by [O] ss 1 2 / 5[Ta] ss ⇔1 / 5kTa 2 O 5 l

(5)

for which both a Ta and a Ta 2 O 5 are nearly equal to 1 and, therefore sat

o

a O 5 exp(1 / 5D form G Ta 2 O 5 /RT ) .

(6)

The superscript sat refers to the value at the saturation solubility limit. In the range of solid solubility, it has been reported [8] that oxygen in Ta–O solid solution obeys Henry’s law. The activity of oxygen may therefore be expressed by a O 5 a Osat XO /X sat O .

Substituting the reported values [5] for D form G oSiO (v) Eq. (12) gives pSiO 5 a Si a O exp(20.90 1 12790 /T ) .

(13)

At the solubility limit, silicon and oxygen in tantalum are in equilibrium with Ta 3 Si [6–8] and Ta 2 O 5 [6,7] respectively. Therefore, as described for Eq. (6), o a sat Si 5 exp(D form G Ta 3 Si /RT )

(14)

Assuming Henry’s law to be valid for the Ta–Si solid solution within the solid solubility range, as described for Eq. (7), sat a Si 5 (XSi /X Si ) exp(D form GTa o3 Si o /RT ) .

(15) o

(7)

where XO is the atom fraction of oxygen. Substituting the o reported values [6,7] for D form G Ta and the solid solu2O 5 bility of oxygen in tantalum, Eq. (7) gives a O 5 XO exp(10.33 2 46010 /T ) .

(12)

Substituting the reported values [10] for D form G Ta 3 Si and solubility of silicon in tantalum,

By substituting Eq. (6), o a O 5 (XO /X sat O ) exp(1 / 5D form G Ta 2 O 5 /RT )

pSiO 5 a Si a O exp(2D form G SiO (v) /RT ) .

a Si 5 XSi exp(1.09 2 4500 /T ) .

(16)

Combining Eqs. (8) and (13) and 16, the partial pressure of SiO (v) can be expressed as pSiO 5 XO XSi exp(32.32 2 37720 /T )Pa .

(17)

(8)

Even though the equilibrium between oxygen saturated solid tantalum and tantalum pentoxide occurs only up to

An important assumption made during derivation of Eq. (17) is that the activity data for oxygen and silicon estimated for binary alloys are also valid for the ternary

A. Awasthi et al. / Journal of Alloys and Compounds 265 (1998) 190 – 195

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Ta–Si–O alloy. This assumption would be more valid for the dilute ternary alloys. If the reported [6,7,9] saturated solubility data for oxygen (5.4 at %) and silicon (1.0 at %) in tantalum are substituted in Eq. (17), the p sat SiO value is calculated 380 Pa at 2000 K. This is a significant pressure for silicon deoxidation to be practically useful.

be silicon deoxidation (evaporation of SiO (v) ). This is so because of large values for the vapour pressure ratios, pSiO /pTaO and pSiO /pSi even at low silicon and oxygen contents respectively.

2.3. Silicon and sacrificial deoxidation

3.1. Materials

The relative contribution of silicon and sacrificial deoxidation in oxygen removal from Ta–Si–O alloys may be assessed using the following expression obtained by combining Eqs. (10) and (17), pSiO ]] 5 XSi exp(0.17 1 30680 /T ) . (18) pTaO

Tantalum powder containing 7.38 at % O (produced by sodium reduction of potassium tantalum fluoride) obtained from Nuclear Fuel Complex, Hyderabad was used in the investigation. The silicon powder used (99.9% pure) was obtained from NewMet, Essex. Calculated amounts of tantalum powder and silicon were thoroughly mixed and pelletized at a pressure of 20310 6 Pa without any binder. The pellet diameter and height were 6 mm each.

This expression indicates even on simple inspection that pSiO is more than pTaO at all temperatures of practical interest even at very low silicon content. At 2200 K, for example, pSiO 6 ]] 5 1.35 3 10 XSi . pTaO

(19)

This indicates that silicon deoxidation should predominate over sacrificial deoxidation for silicon contents more than 10 23 at % Si in the Ta–Si–O alloys. The partial pressure of Si (v) over the Ta–Si–O alloy can be expressed as o pSi 5 a Si p Si

where p is the partial pressure of pure silicon. The vapour pressure of pure silicon between 1687 K (melting point of Si) and 2600 K can be expressed [11] as (21)

and, therefore, pSi 5 XSi exp(25.90 2 51410 / T) Pa.

(22)

Another relevant factor is the relative extent of silicon removal as SiO (v) and as Si (v) . This can be assessed from the following expression obtained by combining Eqs. (17) and (22), pSiO ]] 5 XO exp(6.42 1 13690 /T ) . pSi

(23)

Eq. (23) indicates that pSiO is also more than pSi at all temperatures and oxygen concentrations of practical interest. Eq. (23) gives, for example, at 2200 K, pSiO 5 ]] 5 3.1 3 10 XO . pSi

3.2. Deoxidation by vacuum sintering Pellets of mass about 3 g with silicon to oxygen mole ratios of 1, 0.95, 0.91, 0.83, 0.77 and 0.71 were placed in a tantalum crucible and heated in an electron bombardment furnace. The pellets were heated at 2073, 2173 and 2273 K at 10 23 Pa vacuum for 2 h. They were then analyzed for their residual silicon and oxygen contents. Few pellets containing 7.38 at % O but no silicon were also treated similarly and analyzed for their oxygen contents.

(20)

o Si

p oSi 5 exp(24.81 2 46910 /T ) Pa.

3. Experimental details

4. Results and discussion The silicon and oxygen contents 1 of the tantalum refined using silicon deoxidation are given in Table 1, and the residual oxygen contents of the metal deoxidized by the sacrificial mode are also given in the last column in Table 1. The decrease in the oxygen contents of tantalum deoxidized at 2073, 2173 and 2273 K under 10 23 Pa vacuum for 2 h is plotted in Fig. 1. The data in Table 1 indicate that silicon is effective in removing oxygen from tantalum. On treating at 2073 K for 2 h under 10 23 Pa vacuum, the oxygen content decreased from 7.38 at % to only 4.97 at % when no silicon was present in the pellet and the only way oxygen could have gone away was by evaporation of TaO (v) . Under the same conditions of treatment, when the pellet contained silicon (e.g., corresponding to Si / O mole ratio of 1), the oxygen

1

(24)

It is, therefore, to be anticipated from the thermodynamic considerations of the Ta–Si–O system that at high temperatures the main mode of oxygen removal will

The analysis values for the ternary Ta–Si–O alloys (in the text, Table 1 and Figures) are given as, at%Si 5 [1 /(Ta 1 Si)] 3 100 and at% O 5 [O /(Ta 1 O)] 3 100 and not as, at%Si 5 [Si /(Ta 1 Si 1 O)] 3 100 and at% O 5 [O /(Ta 1 Si 1 O)] 3 100 The present procedure is adopted so that silicon and oxygen contents in the ternary alloys are independent of Si / O ratio.

A. Awasthi et al. / Journal of Alloys and Compounds 265 (1998) 190 – 195

193

Table 1 Analysis values for the silicon deoxidation of Tantalum at different Si / O ratios Si / O

Analysis (at %)b of deoxidized metal for the following Si / O mole ratios

Deoxidation parameter

1.00 Si

O

Initial charge 2073 K, 0.001 Pa, 2 h 2173 K, 0.001 Pa, 2 h 2273 K, 0.001 Pa, 2 h

7.38 0.369 0.013 0.013

7.38 0.864 0.674 0.506

0.95 Si 7.01 a

0.113 0.001

O

0.91 Si

O

0.83 Si

7.38 1.89 1.12 0.259

6.72 0.113 0.013 0.013

7.38 1.23 0.931 0.361

O

0.77 Si

O

0.71 Si

O

6.13

7.38

5.68

7.38

5.24

7.38

a

a

a

a

a

a

a

a

a

a

a

a

0.372

a

0.282

0.013

0.271

0.039

0.0 Si

O

0 0 0 0

7.38 4.97 3.93 2.00

a

Analysis not available. b In the Ta–Si–O alloys, analysis is given as at %. Si5[Si /(Ta1Si)]3100; at % O5[O /(Ta1O)]3100.

Fig. 1. Extent of oxygen removal from tantalum by sacrificial and silicon deoxidation. Dotted line only indicates the starting oxygen content at room temperature and the composition after pyrovacuum treatment at 2073 K.

content decreased from 7.38 at % to less than 1 at %. Such decrease in oxygen content can be attributed to the evaporation of SiO (v) which predominates over the evaporation of both TaO (v) and Si (v) . All of the oxygen and approximately 95% of the silicon removal occurring during the treatment can be attributed to silicon deoxidation. The marked influence of silicon in oxygen removal is very clearly brought out by the data presented in the Fig. 1. Once the oxygen content and also the silicon content decrease to lower values, the situation begins to change. The role of silicon so far was however anticipated from the large values for the SiO (v) and TaO (v) vapour pressure ratios. These values at the deoxidation temperatures are given in Table 2. According to the Table, from a Ta–Si–O Table 2 Relative extents of silicon deoxidation and sacrificial deoxidation in Ta–Si–O alloys Temperature, K pSiO /pTaO pSiO /pSi

2073

2173 6

3.17310 XSi 0.45310 6 XO

2273 6

1.61310 XSi 0.33310 6 XO

0.86310 6 XSi 0.25310 6 XO

alloy silicon deoxidation should be the predominant mode of oxygen removal even when the alloy contains very low amount of silicon. In practice however the proportion of total oxygen removed from the alloy as SiO (v) decreases markedly by the time the oxygen content has decreased to around 1 at %. The reason is kinetics. Even though silicon deoxidation is highly favoured in Ta–Si–O alloys by thermodynamic considerations, sacrificial deoxidation is favoured by kinetic considerations. The kinetic factor appear to assume greater importance at low levels of residual silicon and oxygen contents. In the Ta–Si–O alloy, the probability of an oxygen atom contacting the abundantly available tantalum atom to form TaO and evaporate as TaO (v) is much greater than the probability of oxygen contacting the scarce silicon atom to form SiO and evaporate as SiO (v) . Clearly, sacrificial deoxidation is more likely to occur than silicon deoxidation at low levels of silicon and oxygen even though the calculated are still large. As a result of these two opposing factors, while silicon deoxidation occurs predominantly at higher silicon and oxygen contents, sacrificial deoxidation also sets in at low silicon and oxygen contents, but at silicon levels much higher than those expected on the basis of Eq. (18). The vapour pressure of TaO (v) being much lower than that of SiO(v) , the removal of oxygen slows down when sacrificial deoxidation sets in at lower oxygen and silicon contents. The course of deoxidation in Ta–Si–O alloys is summarized in Figs. 2 and 3. The plots given in the figures give the direction in which the composition of the pellets (silicon and oxygen contents) move along for various Si / O ratios. Exclusive silicon deoxidation will cause the composition to move along the line with slope of unity. If sacrificial deoxidation also occurs, the slope will be more than unity. The extent of silicon deoxidation is given by the reciprocal of the slope. Fig. 2 refers to the overall composition change between the starting material and the pellets treated up to 2273 K. The slope is ¯1 when the Si / O ratio in the starting material was 0.91 to 1.0 and the slope is 1.14 and 1.22 respectively when the Si / O ratio was 0.83 and 0.71. In the two later cases, overall 88 and 82 percent of total oxygen removal occurs by silicon deoxidation. An intercept value of more than zero in Fig. 2 for charge with Si / O ratio equal to 1.0 is indicative of certain evaporation of silicon as Si (v) .

194

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The extent of silicon deoxidation compared to sacrificial deoxidation will come down as the concentrations of silicon and oxygen decrease to below certain values. This is indicated in Fig. 3 where residual silicon and oxygen contents of the samples treated at 2073 K and above are plotted. The data indicates strong dependence of the extent of silicon deoxidation on residual silicon content for tantalum containing comparatively low residual oxygen. For oxygen content of ¯1.0 at %, the extent of silicon deoxidation is maximum when silicon content is more than 0.37 at % Si (Fig. 3a). However, it decreased to ¯10% as silicon content comes down to ¯0.1 at % Si (Fig. 3b, c). Sacrificial deoxidation becomes the significant mode of oxygen removal at low silicon levels.

Fig. 2. Composition changes in the Ta–Si–O alloys on deoxidation – between undeoxidized pellets and the pellets treated at 2273 K under 10 23 Pa for 2 h.

5. Conclusions Silicon is a useful deoxidizing agent for tantalum. It can quickly bring down the oxygen content of tantalum from as much as 7.38 at % O to about 1 at % O at a relatively low temperature of 2073 K at 10 23 Pa vacuum, on a treatment lasting 2 h. Under similar pyrovacuum treatment, the oxygen levels decreased from 7.38 at % O to only 4.97 at % O when no silicon was added to the charge. Silicon deoxidation has been studied with initial charge containing Si / O ratio of 0.77 to 1.0. On treatment at higher temperatures 2173 and 2273 K, under 10 23 Pa vacuum, the oxygen content in the silicon deoxidized alloy decreased further to less than 0.5 at % and simultaneously silicon also goes away resulting in the residual concentration of less than 0.02 at % Si. During this stage, the deoxidation is not exclusively by SiO (v) vapourisation but is also by simultaneous TaO (v) vapourisation due to kinetic reasons. It is not necessary to resort to a small Si / O ratio (,0.91) in the initial charge in silicon deoxidation, since removal of residual silicon is not a problem here unlike the residual carbon in carbon deoxidation because of relatively high vapour pressure of silicon.

Acknowledgements Authors are thankful to Professor H.W. Franzen, Ames Laboratory, Iowa State University for his valuable suggestions in presentation of theoretical discussions in this paper.

References Fig. 3. Composition changes in the Ta–Si–O alloys on deoxidation for three different Si / O ratios in the initial charge (a) 1.0, (b) 0.95 and (c) 0.91. Starting composition refers to alloy after deoxidation at 2073 K under 10 23 Pa for 2 h. Final composition refers to alloy after deoxidation at 2273 K under 10 23 Pa for 2 h.

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