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Electrotransport of carbon in niobium and tantalum
JOURNAL
OF THE LESS-COMMON
Elsevier Sequoia
S.A., Lausanne
ELECTROTRANSPORT
F. A. SCHMIDT
AND
247
METALS
~ Printed
in The Netherlands
OF CARBON
IN NIOBIUM
AND TANTALUM*
0. N. CARLSON
Institute for Atomic Research and Department of Metallurgy, Iowa State University, Ames, Iowa 50010 (U.S.A.) (Received
September
lst, 1971)
SUMMARY
The electrotransport velocity of carbon in niobium and tantalum was measured at temperatures above 1900°C using 14C as a tracer. In both metals carbon was found to migrate in the opposite direction to the electron flow. The diffusion coefficients and effective charge were also calculated for this solute at the same temperatures. A least-squares treatment was made of the combined diffusion data of this study and that obtained at lower temperatures by other investigators. Values of Do and AH in the equation D = D, exp (- AHIRT), were determined for the extended temperature range and are as follows ; niobium D, = 0.0100 cm2/sec 130-2340” C 1 AH = 33.92 kcal/mol tantalum D, = 0.0067 cm’/sec 190-268O’C AH = 38.60 kcal/mol .
INTRODUCTION
Several reviews have been published in recent years’ - ’ on the migration of solute atoms in metals upon passage of a d.c. current through them. The authors and colleagues have shown that electrotransport is an excellent technique for preparing small amounts of refractory metals containing very low concentrations of interstitial solutes’-ii. The purpose of this investigation was to determine the migration direction and velocity of carbon in niobium and tantalum in order to complete our studies of solute migration in the group VA metals. Frantsevich and Koven’skiy” have reported that carbon migrates in the opposite direction to the flow of electrons (toward the cathode) in tantalum. In our earlier work on vanadium13, carbon, oxygen and nitrogen were also found to migrate toward the cathode. The electrotransport mobilities of these solutes were determined by observing the rate of movement of a discontinuity in the concentration profile and the diffusion coefficients were calculated by the Grube method. The same experimental method was used in the present investigation but the * Work was performed
in the Ames Laboratory
of the Atomic Energy Commission. J. Less-Common
Contribution
Metals,
NO. 3098.
26 (1972) 247-253
248
F. A. SCHMIDT, 0. N. CARLSON
electrotransport and diffusion of oxygen and nitrogen could not be determined in niobium and tantalum by this technique because of the high vapor pressures of their oxides and nitrides. EXPERIMENTAL METHOD
The experimental method used in this investigation was very similar to that described by Peterson et al. l4 in their work on thorium. The apparatus and procedures are described in detail in that paper and need not be repeated here. The niobium metal used in this study was prepared by the aluminothermic reduction of NbzOS and was subsequently purified by electron-beam melting”. It contained 20 ppm carbon, 50 ppm nitrogen, 60 ppm oxygen, 180 ppm silicon and less than 100 ppm total metallic impurities excluding tantalum. The tantalum concentration was approximately 300 ppm. The tantalum was obtained from the National Research Corporation in the form of sheet which had been rolled from an electron-beam melted ingot. It contained 20 ppm carbon, 90 ppm nitrogen, 40 ppm oxygen, 150 ppm niobium and less than 150 ppm total other metallic impurities. The specimens used in the electrotransport mobility measurements were rods, 6.6 cm long and 0.260 cm in diam. One third of the rod consisted of the pure material and the remainder had a higher concentration of carbon. A sharp concentration step existed initially at the butt-welded junction of these two sections and the displacement of the median concentration point gave a direct measure of the mean migration distance in a specific period of time. The segments of niobium and tantalum containing the higher concentration of carbon were prepared by adding carbon during arc-melting. Specimens containing approximately 200 ppm of natural carbon were prepared after which the specimens were electron-beam melted under a pressure of 2 x 10m6 Torr. In this manner the residual oxygen present in the samples was reacted with the carbon and removed as carbon monoxide. To these deoxidized specimens amorphous carbon that contained 8 mc of 14C per g was added during a second arc melting step. Samples prepared in this manner had a uniform concentration of i4C and thus did not lose the radioactive carbon as volatile 14C0 during the electrotransport experiments, which were carried out under pressures of 5 x lo-’ to 5 x lo-* Torr. The combined natural carbon and 14C concentration was 330 ppm for the doped niobium specimens and 300 ppm for the tantalum specimens. These concentrations were below the carbon solubility limit in both metals as indicated by the values cited in the literature16*17 and confirmed by’ microscopic examination of specimens quenched from 190°C. After each electrotransport run was completed the specimen was cut into samples 0.3 cm in length and the concentration of carbon traced by counting the /l activity of each end of the segment. An average of these values was used to obtain the level of activity and, hence, the relative carbon contents for that segment. RESULTS AND DISCUSSION
In previous work I4 we have shown how the displacement of the center of a diffusion couple from the original weld interface may be used to measure the electroJ. Less-Common Meruls. 26 (1972) 247-253
OF
ELECTROTRANSPORT
C
IN
Nb
AND
249
Ta
transport mobility of an interstitial solute in a metal. The net effect of the electric field is a displacement, AX, of the concentration profile by an amount AX= UEt, where U is the mobility, E the electric field, and t the time. The mobilities of carbon in niobium and tantalum were obtained by plotting the carbon concentration in each portion of a specimen against the distance of the midpoint of that portion from the cathode end. The displacement of the median composition relative to the weld interface was determined from the plot and the mobility calculated. The voltage drop per unit length, E, was calculated from the current density, assuming a cross-sectional area of the specimen of 0.054 cm’, and from the resistivity for the respective metal at temperature. A current density of 25W 3550 A per cm2 was used for the niobium specimens and 31254450 A per cm’ for the tantalum specimens. The movement of carbon in niobium under the influence of an electric field was measured at 1920°, 2125”, 2285” and 234O’C. The concentration profile for the
C-10
I
2
3
DISTANCE Fig.
1. CarbonI
I
activity
I
of niobium
I
I
50 -
4
5
6
(+I7
,CM
composite
sample
I I 254oOC 4 HOURS
after heating
to 2125°C for 8 h.
I
\
DISTANCE,
Fig. 2. CarbonI
activity
of composite
CM
sample
of tantalum
after heating
to 2540°C
for 4 h.
J. Less-Common Met&,
26 (1972) 247-253
F. A.
250
SCHMIDT,
0. N. CARLSON
movement at 2125°C is shown in Fig. 1 and is typical in shape to the profiles obtained at the other temperatures. The movement of carbon in tantalum is shown in Fig. 2 which is a profile of the concentration aher heating at 25 C for 4 h. The migration of carbon in tantalum was also determined at 22W’, 23700 and 26g@C. In both metals, carbon was found to migrate toward the cathode, that is, in the opposite direction to the electron flow. The length of time of the migration experiment was sufhcient to allow a measureable movement of carbon to occur but not so much that the high con~ntration plateau expired into the concentration increase at the negative end of the specimen. The diffusion coefficient, D, was calculated from the concentration profiles by the Grube method, taking the origin at the distance, AX, from the weld interface. The values were obtained by plotting the concentration on probability paper so that a best straight line could be drawn through the points. The effective valence, 2’: was calculate from the mobility and the diffusion c~~cients. The electrotranspo~ mobihtics, diffusion coefficients, and effective valences obtained are given in Table I. TABLE
1
EI.FCTROTRAUSPORT NIOBIUM AND TAN7
VbLOCITKFs, DITFUSIO~
COEFFICIENTS,
AI\‘D
FFIECTIVE
CHARGE.
Z*. FOR
CARRON
IN
AI-UN I”._
_.~._.
_~-..
z*
u
Temp.
AX
t
ec)
(cm)
(h)
[kk2hm-cm)
&/cm)
(10-b12/V-s)
1920 2125 2285 23443
1.2 0.7 0.6 0.5
18 8 5.5 3
87 95 101 103
0.22 0‘27 0.34 0.36
8.5 8.9 9.2 12.7
2.9 9.5 14.4 18.0
5.6 1.9 1.4 1.6
2200 2370 2540 2680
0.9 0.2 0.4 0.4
24 3 4 2
92 97 102 105
0.28 0.34 0.4 I 0.47
3.7 5.5
2.3 3.1 5.6 11.6
3.5 4.1 2.9 2.6
%m’/s)
Niobium
Tantalum
._-_-
“---~-
6.7 11.8
-I-~
--_^_lllll_- -
The electrotransport mobilities and diffusion coeflicients of carbon in niobium and tantalum were compared with the values we previously obtained for vanadium. This comparison was made at a value of 0.90 of the melting point, T,,and the results are shown in Table If. Values for U and D were calculated for carbon in tantalum from the data of
V 12.5
Nb 9.0 l___l--
J. ~ss-~~~rnrn~~
Ta 11.8 ~~.Metals.
V 10.2
Nb 11.8 --.
26 (1972) 247-253
Ta 11.6 .~
ELECTROTRANSPORTOF
C IN
Nb AND Ta
251
TABLE III DIFFUSION
PARAMETERS
OF CARBON
IN NIOBIUM
AND
TANTALUM
Metal
Investigators
D, (cm’/s)
AH(kcaf/mole)
Temp. (“C)
Niobium
Powers and Doyle” Son, Miyake and Sane*’ This work
0.004 0.033 0.026
33.02 31.9 37.8
13G280 93&1800 1920-2340
Tantalum
Powers and Doyle” Son, Miyake and Sane” Frantsevich and Koven’skiy” This work
0.0061 0.012 0.0028 0.038
38.5 1 40.3 24.6 48.2
19G360 1450-2200 6W1400 2200-2680
Frantsevich and Koven’skiyi2 at 0.90 T, (2670°C). The values obtained were U = 7.9 x lop4 cm’/volt-set and D=7.2 x lo-’ cm2/sec which are approximately six times larger than the values obtained in this investigation. From an Arrhenius plot of D us. l/T the activation energy for diffusion, AH, and the value for the pre-exponential factor, D,,for carbon in niobium and tantalum were determined. These values are shown in Table III along with the values reported by Powers and Doyle19 obtained by mechanical relaxation measurements. Also
0=00067ex&36,600/RT)
10-e-
_
I o-l0 7
10’6 -
I
10-a
5
IO
15
20
I
25
IO4 /TPK-‘1
104/TPK-‘)
Fig. 3. Least-squares
plot of diffusion
coefficient
us. l/T for carbon
in niobium
Fig. 4. Least-squares
plot of diffusion
coefficient
OS. l/T for carbon
in tantalum
J. Less-Common
metal. metal. Metals,
26 (1972) 247-253
252
F. A. SCHMIDT,
0. N. CARLSON
included in the Table are the data of Son, Miyake and Sano who have studied the diffusion of carbon in Group VA metals using a concentration gradient technique employing 14C. Three data points were generated from the data of Powers and Doyle and also that of Son et al. These points were within the temperature range in which their original experimental measurements were made. A least-squares treatment combining these data with those obtained in the present investigation was made for both metals and the standard deviations determined. Plots of D vs.l/T of these data points for carbon in niobium and tantalum over the extended temperature range are shown in Figs. 3 and 4, respectively. For carbon in niobium, the standard deviation for D, is +0.0013 cm2/s and for AH it is + 0.215 kcal/mole. The standard deviations of the diffusion parameters for carbon in tantalum are kO.007 cm’/s for D, and 0.180 kcal/mole for AH. These “standard deviations” are not the true standard deviations since the least-square treatment was made using log values, but they do represent a good measure of the deviations for the diffusion parameters. CONCLUSIONS
The electrotransport mobility of carbon in niobium and tantalum was found to be similar to that previously obtained for vanadium at the same fraction of the melting temperature, 0.90 T,. This indicates that electrotransport could be successfully used as a refining method for preparing small amounts of ultra-high-purity niobium or tantalum as the authors have already demonstrated for vanadium. Carbon was found to have a positive effective charge in niobium and tantalum, which is the same sign as carbon, nitrogen and oxygen exhibit in vanadium. The movements of oxygen and nitrogen were not measured in this investigation due to the high volatility of niobium and tantalum oxides and nitrides. In our previous electrotransport studies of interstitial solutes in metals, we have found that the direction of migration of carbon, nitrogen and oxygen is the same in a given metal. It is therefore reasonable to conclude that oxygen and nitrogen would also have a positive effective charge in niobium and tantalum. The diffusion coefficients of carbon in niobium and tantalum were determined and found to be in good agreement with values obtained at lower temperatures by other investigators. A least-squares treatment of the combined diffusion data was made and the diffusion parameters D, and AH were determined for the extended temperature range. ACKNOWLEDGEMENTS
The authors wish to thank D. T. Peterson and J. D. helpful guidance throughout this work and M. E. Thompson electrotransport experiments. We also wish to acknowledge Group of the Ames Laboratory for their services and Analytical for making the analytical measurements. J. Less-Common Metals, 26 (1972) 247-253
Verhoeven for their for performing the the Health Physics Chemistry Group I
ELECTROTRANSPORT OF
C
IN
Nb
AND
Ta
253
REFERENCES
1 The Physical Chem~str.v of Metalli& ~~~ut~onsand Intermeta~lic &ompounds, Vol. I, Paper 2C, H.M. Stationery Office, London, 1959. 2 J. D. VERHOEVEN, Met. Rer., 8 (1963) 311. 3 A. LOIXXN~;.in C. A. HAMPEI.(ed.), Enq,c/opedia of’ EIectrochemistr.v. Reinhold, New York, 1964, Q. 491. 4 H. B. HUNTINGTON, in C. A. HAMPEL(ed.), Encyclopedia of Electrochemistry, Reinhold, New York, 1964,496. 5 R. A. ORIANI AND 0. D. GONZALEZ,Trans. AIME, 239 (1967) 1041. 6 H. B. HUNTINGTON,Trans. AIME, 245 (1969) 2571. 7 D. T. PETERSON, Proc. Europhysics Conf. Atomic Transport in Solids and Liquids, June 15-19, 1970,
Marstrand Sweden, to be published. 8 0. N. CARLSON,F. A. S~H%UIXAND D. T. PETEKSON,J. Less-Common Met& 10 (1966) 1. 9 D. T. PEITERSON AND F. A. SCHMIDT,J. Less-Common Metals 18 (1969) 111. 10 F. A. SCHMIDT,0. N. CARLSONAND C. E. SWANSON,JR., Met. Trans., 1 (1970) 1371. 11 D. T. PETERSON ANDF. A. SCHMIDT,J. ~ss-~orn~lan Metals, 24 (1971) 223. 1.2 I. N. FRANTSEVICH ANDI. I. KOVEN‘SKIY,Dopouidi Akad. Nauk Ukr. RSR, No. II, I961 ; Eng. Trans. NASA TT F-263, Nov. 1964. 13 F. A. SCHMIDTAND J. C. WARNER,J. Less-Common Metals, 13 (1967) 493. 14 D. T. PETERSON, F. A. SCHMIDTAND J. D. VERHOEVEN, Trans. AIME, 236 (1966) 1311. 15 H. A. WILHELM,F. A. SCHMIDTAND T. G. ELLIS,J. Metals, 18 (1966) 1303. 16 R. P. ELLIOTT,Trans. Am. Sot. Metals, 53 (1961) 13. 17 H. R. OGDEN. F. F. S~HWDT ANDE. S. BARTLETT,Trans. AIME, 227 (1963) 1458. 18 Mctabr ~~~6n~~b~~~~k. Vol. 1. Am. Sot. Metals. Novelty. Ohio, 1961. pp. 1202 and 1223. 19 R. W. POWERS AND M. V. DOYLE,J. Appt. Phw.. 30 (1959) 514. 20 P. SON, M. MIYAKEAND T. SANO. Tech&. Rep. Osaka Unir.. 18 (1968) 317. J. Less-Common Metals, 26 (1972) 247-253
OF THE LESS-COMMON
Elsevier Sequoia
S.A., Lausanne
ELECTROTRANSPORT
F. A. SCHMIDT
AND
247
METALS
~ Printed
in The Netherlands
OF CARBON
IN NIOBIUM
AND TANTALUM*
0. N. CARLSON
Institute for Atomic Research and Department of Metallurgy, Iowa State University, Ames, Iowa 50010 (U.S.A.) (Received
September
lst, 1971)
SUMMARY
The electrotransport velocity of carbon in niobium and tantalum was measured at temperatures above 1900°C using 14C as a tracer. In both metals carbon was found to migrate in the opposite direction to the electron flow. The diffusion coefficients and effective charge were also calculated for this solute at the same temperatures. A least-squares treatment was made of the combined diffusion data of this study and that obtained at lower temperatures by other investigators. Values of Do and AH in the equation D = D, exp (- AHIRT), were determined for the extended temperature range and are as follows ; niobium D, = 0.0100 cm2/sec 130-2340” C 1 AH = 33.92 kcal/mol tantalum D, = 0.0067 cm’/sec 190-268O’C AH = 38.60 kcal/mol .
INTRODUCTION
Several reviews have been published in recent years’ - ’ on the migration of solute atoms in metals upon passage of a d.c. current through them. The authors and colleagues have shown that electrotransport is an excellent technique for preparing small amounts of refractory metals containing very low concentrations of interstitial solutes’-ii. The purpose of this investigation was to determine the migration direction and velocity of carbon in niobium and tantalum in order to complete our studies of solute migration in the group VA metals. Frantsevich and Koven’skiy” have reported that carbon migrates in the opposite direction to the flow of electrons (toward the cathode) in tantalum. In our earlier work on vanadium13, carbon, oxygen and nitrogen were also found to migrate toward the cathode. The electrotransport mobilities of these solutes were determined by observing the rate of movement of a discontinuity in the concentration profile and the diffusion coefficients were calculated by the Grube method. The same experimental method was used in the present investigation but the * Work was performed
in the Ames Laboratory
of the Atomic Energy Commission. J. Less-Common
Contribution
Metals,
NO. 3098.
26 (1972) 247-253
248
F. A. SCHMIDT, 0. N. CARLSON
electrotransport and diffusion of oxygen and nitrogen could not be determined in niobium and tantalum by this technique because of the high vapor pressures of their oxides and nitrides. EXPERIMENTAL METHOD
The experimental method used in this investigation was very similar to that described by Peterson et al. l4 in their work on thorium. The apparatus and procedures are described in detail in that paper and need not be repeated here. The niobium metal used in this study was prepared by the aluminothermic reduction of NbzOS and was subsequently purified by electron-beam melting”. It contained 20 ppm carbon, 50 ppm nitrogen, 60 ppm oxygen, 180 ppm silicon and less than 100 ppm total metallic impurities excluding tantalum. The tantalum concentration was approximately 300 ppm. The tantalum was obtained from the National Research Corporation in the form of sheet which had been rolled from an electron-beam melted ingot. It contained 20 ppm carbon, 90 ppm nitrogen, 40 ppm oxygen, 150 ppm niobium and less than 150 ppm total other metallic impurities. The specimens used in the electrotransport mobility measurements were rods, 6.6 cm long and 0.260 cm in diam. One third of the rod consisted of the pure material and the remainder had a higher concentration of carbon. A sharp concentration step existed initially at the butt-welded junction of these two sections and the displacement of the median concentration point gave a direct measure of the mean migration distance in a specific period of time. The segments of niobium and tantalum containing the higher concentration of carbon were prepared by adding carbon during arc-melting. Specimens containing approximately 200 ppm of natural carbon were prepared after which the specimens were electron-beam melted under a pressure of 2 x 10m6 Torr. In this manner the residual oxygen present in the samples was reacted with the carbon and removed as carbon monoxide. To these deoxidized specimens amorphous carbon that contained 8 mc of 14C per g was added during a second arc melting step. Samples prepared in this manner had a uniform concentration of i4C and thus did not lose the radioactive carbon as volatile 14C0 during the electrotransport experiments, which were carried out under pressures of 5 x lo-’ to 5 x lo-* Torr. The combined natural carbon and 14C concentration was 330 ppm for the doped niobium specimens and 300 ppm for the tantalum specimens. These concentrations were below the carbon solubility limit in both metals as indicated by the values cited in the literature16*17 and confirmed by’ microscopic examination of specimens quenched from 190°C. After each electrotransport run was completed the specimen was cut into samples 0.3 cm in length and the concentration of carbon traced by counting the /l activity of each end of the segment. An average of these values was used to obtain the level of activity and, hence, the relative carbon contents for that segment. RESULTS AND DISCUSSION
In previous work I4 we have shown how the displacement of the center of a diffusion couple from the original weld interface may be used to measure the electroJ. Less-Common Meruls. 26 (1972) 247-253
OF
ELECTROTRANSPORT
C
IN
Nb
AND
249
Ta
transport mobility of an interstitial solute in a metal. The net effect of the electric field is a displacement, AX, of the concentration profile by an amount AX= UEt, where U is the mobility, E the electric field, and t the time. The mobilities of carbon in niobium and tantalum were obtained by plotting the carbon concentration in each portion of a specimen against the distance of the midpoint of that portion from the cathode end. The displacement of the median composition relative to the weld interface was determined from the plot and the mobility calculated. The voltage drop per unit length, E, was calculated from the current density, assuming a cross-sectional area of the specimen of 0.054 cm’, and from the resistivity for the respective metal at temperature. A current density of 25W 3550 A per cm2 was used for the niobium specimens and 31254450 A per cm’ for the tantalum specimens. The movement of carbon in niobium under the influence of an electric field was measured at 1920°, 2125”, 2285” and 234O’C. The concentration profile for the
C-10
I
2
3
DISTANCE Fig.
1. CarbonI
I
activity
I
of niobium
I
I
50 -
4
5
6
(+I7
,CM
composite
sample
I I 254oOC 4 HOURS
after heating
to 2125°C for 8 h.
I
\
DISTANCE,
Fig. 2. CarbonI
activity
of composite
CM
sample
of tantalum
after heating
to 2540°C
for 4 h.
J. Less-Common Met&,
26 (1972) 247-253
F. A.
250
SCHMIDT,
0. N. CARLSON
movement at 2125°C is shown in Fig. 1 and is typical in shape to the profiles obtained at the other temperatures. The movement of carbon in tantalum is shown in Fig. 2 which is a profile of the concentration aher heating at 25 C for 4 h. The migration of carbon in tantalum was also determined at 22W’, 23700 and 26g@C. In both metals, carbon was found to migrate toward the cathode, that is, in the opposite direction to the electron flow. The length of time of the migration experiment was sufhcient to allow a measureable movement of carbon to occur but not so much that the high con~ntration plateau expired into the concentration increase at the negative end of the specimen. The diffusion coefficient, D, was calculated from the concentration profiles by the Grube method, taking the origin at the distance, AX, from the weld interface. The values were obtained by plotting the concentration on probability paper so that a best straight line could be drawn through the points. The effective valence, 2’: was calculate from the mobility and the diffusion c~~cients. The electrotranspo~ mobihtics, diffusion coefficients, and effective valences obtained are given in Table I. TABLE
1
EI.FCTROTRAUSPORT NIOBIUM AND TAN7
VbLOCITKFs, DITFUSIO~
COEFFICIENTS,
AI\‘D
FFIECTIVE
CHARGE.
Z*. FOR
CARRON
IN
AI-UN I”._
_.~._.
_~-..
z*
u
Temp.
AX
t
ec)
(cm)
(h)
[kk2hm-cm)
&/cm)
(10-b12/V-s)
1920 2125 2285 23443
1.2 0.7 0.6 0.5
18 8 5.5 3
87 95 101 103
0.22 0‘27 0.34 0.36
8.5 8.9 9.2 12.7
2.9 9.5 14.4 18.0
5.6 1.9 1.4 1.6
2200 2370 2540 2680
0.9 0.2 0.4 0.4
24 3 4 2
92 97 102 105
0.28 0.34 0.4 I 0.47
3.7 5.5
2.3 3.1 5.6 11.6
3.5 4.1 2.9 2.6
%m’/s)
Niobium
Tantalum
._-_-
“---~-
6.7 11.8
-I-~
--_^_lllll_- -
The electrotransport mobilities and diffusion coeflicients of carbon in niobium and tantalum were compared with the values we previously obtained for vanadium. This comparison was made at a value of 0.90 of the melting point, T,,and the results are shown in Table If. Values for U and D were calculated for carbon in tantalum from the data of
V 12.5
Nb 9.0 l___l--
J. ~ss-~~~rnrn~~
Ta 11.8 ~~.Metals.
V 10.2
Nb 11.8 --.
26 (1972) 247-253
Ta 11.6 .~
ELECTROTRANSPORTOF
C IN
Nb AND Ta
251
TABLE III DIFFUSION
PARAMETERS
OF CARBON
IN NIOBIUM
AND
TANTALUM
Metal
Investigators
D, (cm’/s)
AH(kcaf/mole)
Temp. (“C)
Niobium
Powers and Doyle” Son, Miyake and Sane*’ This work
0.004 0.033 0.026
33.02 31.9 37.8
13G280 93&1800 1920-2340
Tantalum
Powers and Doyle” Son, Miyake and Sane” Frantsevich and Koven’skiy” This work
0.0061 0.012 0.0028 0.038
38.5 1 40.3 24.6 48.2
19G360 1450-2200 6W1400 2200-2680
Frantsevich and Koven’skiyi2 at 0.90 T, (2670°C). The values obtained were U = 7.9 x lop4 cm’/volt-set and D=7.2 x lo-’ cm2/sec which are approximately six times larger than the values obtained in this investigation. From an Arrhenius plot of D us. l/T the activation energy for diffusion, AH, and the value for the pre-exponential factor, D,,for carbon in niobium and tantalum were determined. These values are shown in Table III along with the values reported by Powers and Doyle19 obtained by mechanical relaxation measurements. Also
0=00067ex&36,600/RT)
10-e-
_
I o-l0 7
10’6 -
I
10-a
5
IO
15
20
I
25
IO4 /TPK-‘1
104/TPK-‘)
Fig. 3. Least-squares
plot of diffusion
coefficient
us. l/T for carbon
in niobium
Fig. 4. Least-squares
plot of diffusion
coefficient
OS. l/T for carbon
in tantalum
J. Less-Common
metal. metal. Metals,
26 (1972) 247-253
252
F. A. SCHMIDT,
0. N. CARLSON
included in the Table are the data of Son, Miyake and Sano who have studied the diffusion of carbon in Group VA metals using a concentration gradient technique employing 14C. Three data points were generated from the data of Powers and Doyle and also that of Son et al. These points were within the temperature range in which their original experimental measurements were made. A least-squares treatment combining these data with those obtained in the present investigation was made for both metals and the standard deviations determined. Plots of D vs.l/T of these data points for carbon in niobium and tantalum over the extended temperature range are shown in Figs. 3 and 4, respectively. For carbon in niobium, the standard deviation for D, is +0.0013 cm2/s and for AH it is + 0.215 kcal/mole. The standard deviations of the diffusion parameters for carbon in tantalum are kO.007 cm’/s for D, and 0.180 kcal/mole for AH. These “standard deviations” are not the true standard deviations since the least-square treatment was made using log values, but they do represent a good measure of the deviations for the diffusion parameters. CONCLUSIONS
The electrotransport mobility of carbon in niobium and tantalum was found to be similar to that previously obtained for vanadium at the same fraction of the melting temperature, 0.90 T,. This indicates that electrotransport could be successfully used as a refining method for preparing small amounts of ultra-high-purity niobium or tantalum as the authors have already demonstrated for vanadium. Carbon was found to have a positive effective charge in niobium and tantalum, which is the same sign as carbon, nitrogen and oxygen exhibit in vanadium. The movements of oxygen and nitrogen were not measured in this investigation due to the high volatility of niobium and tantalum oxides and nitrides. In our previous electrotransport studies of interstitial solutes in metals, we have found that the direction of migration of carbon, nitrogen and oxygen is the same in a given metal. It is therefore reasonable to conclude that oxygen and nitrogen would also have a positive effective charge in niobium and tantalum. The diffusion coefficients of carbon in niobium and tantalum were determined and found to be in good agreement with values obtained at lower temperatures by other investigators. A least-squares treatment of the combined diffusion data was made and the diffusion parameters D, and AH were determined for the extended temperature range. ACKNOWLEDGEMENTS
The authors wish to thank D. T. Peterson and J. D. helpful guidance throughout this work and M. E. Thompson electrotransport experiments. We also wish to acknowledge Group of the Ames Laboratory for their services and Analytical for making the analytical measurements. J. Less-Common Metals, 26 (1972) 247-253
Verhoeven for their for performing the the Health Physics Chemistry Group I
ELECTROTRANSPORT OF
C
IN
Nb
AND
Ta
253
REFERENCES
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