Electrotransport of interstitial atoms in yttrium

JOURNAL

OF THE

LESS-COMMON

ELECTROTRANSPORT

0. N.

ChliLSON,

P. A.

METALS

OF INTERSTITIAL

SCHlMIDT

AND D. T.

Imtitztte ,fov Atomic Hesearch and Department (L’..S..4 .) (Received

I

ATOMS

IN YTTRIUM*

PETEKSON

ofMetallurgy. Iowa

State

Uuaioevsity,

Ames,

Iowa

July 3 Ist, 1965)

SliMMARY

The electrotransport velocities of carbon, oxygen and nitrogen in yttrium were determined in the temperature range of 1235 to 1460°C. Diffusion coefficients and effective charges were calculated for these elements for the same temperatures. Yttrium metal purified by the electrotransport method showed a decrease in oxygen content from 780 to 90 p.p.m. and an increase in the resistance ratio from 12 to 45.

INTROI)UCTION

The movement of solute atoms in solid metals when an elect:-ic field is applied has been of interest for over thirty years. Most of the work to date has been concerned with the migration of the substitutional elements with comparatively little data reported for the interstitials. The earliest work of this type was that of SEITH AND KUBASCHEWSKI~ in 1935 on the movement of carbon in iron. It was shown and later confirmed by DAYAL AND DARKENS that carbon migrates toward the cathode in iron. In 1940 DE BOER AND FASTS reported that oxygen moves toward the anode in zirconium and CLAISSE AND KOENIG4 observed that oxygen moves in the same direction in titanium. Nitrogen migrates toward the anode in iron5 and hydrogen toward the cathode in palladiume. It therefore appears that solute atoms in a given metal can migrate either with or against the electron flow and each case must be considered individually. A review of both the experimental and theoretical aspects of this phenomenon has been made by VERHOEVEN’. The purpose of this investigation was to determine the migration velocities of carbon, oxygen, nitrogen and hydrogen in yttrium and to obtain complementary data on approximate diffusion coefficients and effective charges. Such information is needed in order to furnish a basis for an understanding of electrotransport as well as in evaluating its feasibility and potential as a purification method. The measurements on carbon, oxygen and nitrogen were made in the temperature range from 1235 to 1460°C and on hyd-rogen from 775 to 950°C. Yttrium was selected for this study because of a growing interest in the preparation and physical properties of this *

Work

performed

in the Ames

Laboratory

of the U.S.A.E.C.

J. Less-Common

Metals,

IO (1966)

I-II

0. N. CARLSON, F. A. SCHMIDT, D. T. PETERSON

2 metal* and its resulting availability AND HUFFINE~ showed

that

in a high-purity form. Earlier work by WILLIAMS

oxygen,

nitrogen,

iron, manganese,

nickel,

boron,

titanium and cobalt all move toward the anode in yttrium with the oxygen content near the cathode being reduced by 80%. There are several methods by which the velocity of electrotransport of an interstitial solute can be measured. Usually the assumption is made that the temperature is uniform throughout the specimen. The lattice is considered to be rigid and to constitute a fixed reference frame. The mass transport of the interstitial solute can then be taken to be due to the sum of two independent fluxes: the electrotransport flux and the chemical-diffusion

flux. Differential

flux and solved for various initial and boundary

equations can be written for each conditions. The solution is very

simple if a volume element can be found into and out of which mass transport is due only to electrotransport

and within which the electrotransport

flux either initiates

or stops. In an ideal specimen with impermeable ends and no temperature gradients, each half of the specimen would constitute such a volume element until electrotransport had proceeded

to an extent such that a concentration gradient appeared at the

midpoint. The electrotransport flux would be equal to the time rate of increase or decrease of the quantity of interstitial solute in one half of the specimen divided by the cross-sectional

area at the midpoint.

This method

of measuring the electro-

transport velocity by measuring the electrotransport flux is not valid unless there are no thermal gradients and there is no solute movement into and out of the end of the specimen. These conditions are almost impossible to achieve simultaneously solid state. The electrotransport movement

velocity

in the

can also be measured by observing the rate of

of an initial variation in the slope of the concentration

profile. In their

work on titanium, CLAISSE AND KOENIG~ introduced a Gaussian-shaped peak in the oxygen concentration

profile and measured the rate of movement

of the center of

mass of this peak. Similar measurements have been made on the electrotransport of carbon in iron by using a concentration peak of radioactive carbon. Diffusion coefficients can also be calculated from the change in the peak shape with time. An experimentally simpler variation in the initial uniform concentration to another. The measured by observing the distance which in an interval of time. A smaller number

concentration profile is a step from one velocity of electrotransport can then be the median concentration point has moved of determinations is necessary to establish

the concentration profile with a given accuracy for this case, which corresponds to a pair of semi-infinite bars, than for the concentration-peak method, which corresponds approximately to the thin-film diffusion source. The diffusion problem for the pair of semi-infinite bars is very simple and either the Grube method or the Matano interface method can be used to calculate the diffusion coefficient+‘. Either the concentration peak or step can be placed near the center of the specimen where the thermal gradient is smallest and where gain or loss of solute at the ends of the specimen does not affect the measurement. EXPERIMENTAL PROCEDURE A sketch of the apparatus used in this investigation is shown in Fig. I. The basic unit consisted of a sample chamber and a vacuum base-plate. The stainless J. Less-Common

Metals, IO (1966) I-II

ELECTROTRANSPORT

OF INTERSTITIAL

3

ATOMS IN YTTRIUM

steel chamber was 4 in. in diameter and was equipped with an observation window and electrodes fitted with “0”-ring seals and compression bushings. The yttrium specimen was threaded and screwed into tantalum metal adapters attached to the ends of the electrodes. The “U”-shaped design of the adapters, shown in Fig. 1, accommodated the thermal expansion of the sample. The cross-sectional area of the

0

YTTRIUM

SAMPLE

@

SIGHT

@

TANTALUM

ADAPTERS

8

VACUUM

@

INSULATDRS

@

“GETTER”

@

S STEEL

CATHODE

@

“0”

@

S STEEL

ANODE

@

CONNECTING

Fig. I. Apparatus

used

in study

of electrotransport

GLASS LINE WIRE

RING FLANGES

of interstitial

solute

atoms

in yttrium

metal.

adapters was adjusted so that they were heated by the current to nearly the same temperature as the specimen. This greatly reduced the temperature gradients toward the ends of the specimen. The base-plate was equipped with two electrical “feedthroughs” to which a small getter wire was attached. This wire was heated by a separate electrical circuit. A d.c. current served to heat the specimen by internal resistance as well as producing the electrotransport phenomenon. It was supplied by a saturable core stepdown transformer which provided a full-wave rectified current with a fifteen per cent ripple. The amperage was determined by means of a standard resistance and a potentiometer and the temperature was measured with an optical pyrometer sighted on the specimen surface. The observed temperatures were corrected for the emissivity of the sample by adding a factor obtained by measuring the apparent surface temperature and the corresponding true temperature of an yttrium bar at various power settings. The true temperature was determined from sightings on black-body J. Less-Common

Metals,

IO (1906) I--II

0. N. CARLSON,F. A. SCHMIDT,D. T. PETERSON

4

holes drilled at intervals along the specimen. A correction was also applied for absorption by the viewing window. The yttrium metal used throughout this investigation was prepared by the method described by CARLSON et al.11 and purified by electron-beam melting which removed the more volatile impurities such as hydrogen, fluorine and magnesium. This material was then swaged at room temperature into rods 0.254 cm in diam. The total of all metallic impurities in this metal was less than 600 p.p.m., the principal ones being iron, tantalum, nickel and silicon. An analysis of the fabricated rod for the nonmetallic elements is presented in Table I. TABLE I ANALYSIS OF

YTTRIUM

Element Concentvation (p.p.m.)

METAL

ROD

C 170

0 780

N 20

H

F

Cl

15

< 20

< 10

cow

DISTANCE

Fig. 2. Ideal concentration profile of composite samples before and after electrotransport experiments.

The specimens used in this study were rods of yttrium 6.6 cm long and 0.254 cm in diam. One third of the bar had a low concentration of the solute of interest and the remainder of the bar had the higher concentration. The initial concentrations in a specimen are represented by Ci and Cz in Fig. 2 while the ideal profile after an electrotransport experiment is indicated by the solid curve. The displacement, dx, of the median concentration gives a direct measure of the mean migration distance in a specific length of time. The diffusion coefficient can be calculated from the shape of the concentration curve between Cr and CZ. For the carbon, nitrogen and hydrogen measurements, electron-beam-melted yttrium was used for the low concentration, cathode section. Yttrium alloys containing 500 p.p.m. nitrogen and 800 p.p.m. carbon were prepared by introducing carbon or nitrogen during arc melting and these ingots were fabricated into rods for use as the high-concentration ends. The high-hydrogen specimen was prepared from the base metal which contained 120 p.p.m. hydrogen prior to beam melting. The yttrium rod containing 780 p.p.m. of oxygen was considered suitable for the highoxygen section, and the oxygen content of this material was decreased to 150 p.p.m. by electrotransport for use at the low-concentration end. Sections of yttrium rod of two different solute concentrations were cut to the desired lengths and threaded at one end. These were then electropolishedia and the J. Less-Common

Metals, IO (1966) I--II

ELECTROTRANSPORT

OF INTERSTITIAL

ATOMS IN YTTRIUM

5

two segments butt-welded together under an inert atmosphere. The welding conditions were carefully controlled to avoid contamination and to preserve the sharpness of the interface. A fiducial mark inscribed at the weld prior to an electrotransport experiment identified its original position. In a typical run, the specimen was positioned in the apparatus with the low-concentration section at the cathode end. The specimen chamber was evacuated, outgassed and filled with argon to a pressure of I p.s.i. gauge. The argon atmosphere was purified by heating the zirconium wire to 12oo’C for one hour. The specimen was heated under argon rather than vacuum to minimize the loss of yttrium by vaporization. At temperatures above 120o’C in vacuum, thinning of the specimen resulting from volatilization of yttrium caused the formation of hot spots in the rods where melting eventually occurred. No detectable change in the specimen diameter was observed when the runs were made under an argon atmosphere. A d.c. current sufficient to heat the yttrium specimen to the desired temperature was then passed through the bar for the specified length of time. After cooling, the specimen was cut into 0.6 cm long samples for chemical analysis. Oxygen, nitrogen and hydrogen were determined by vacuum fusion in a platinum bath and carbon was determined by combustion in oxygen. The use of hardness measurements to trace the movement of the concentration profile was investigated but proved to be unsatisfactory. This was due primarily to the dependence of the which resulted in large variations in the hardness hardness on crystal orientation13 along the polycrystalline rod. RESULTS AXI)

DISCITSSIOS

To obtain electrotransport velocities of the individual interstitial solutes in yttrium, the concentration of the solute in each portion of a specimen was plotted against the distance of the midpoint of that portion from the cathode end. The displacement, dx, of the median concentration point from the weld was determined from this graph. Figures 3-6 show typical concentration profiles for carbon at 1235°C

1000

1235 oc 16 HOURS

900 WELD

- 600 x p J

500 400

L--F_

1;

2

‘-4

3 DISTANCE,

Fig. 3. Carbon content of composite

7 Cm

sample after heating to 1235°C for 16 h. J. Less-Common

Metals,

IO

(1966) I-I

I

6

0. N. CARLSON, F. A. SCHMIDT, D. T. TEPERSON

nitrogen at 135o”C, oxygen at 1460°C and hydrogen at 835°C. The accuracy of these experiments would have been improved if shorter lengths of the bar could have been used for the analytical samples; however, a sample of about 0.15 g was required for satisfactory precision. A specimen of larger diameter would, of course, have permitted

1350 oc 4 HOURS 600 t 2

700

;

600

I

2

3 4 5 DISTANCE, cm

Fig. 4. Nitrogen content of composite

I

2000

-

6

7

sample after heating to 135o'c for 4 h.

I

I

I

I

2

3 4 5 DISTANCE, cm

6

I

‘460 OC 2 HOURS

1600 1600 -

600400 -

I

Fig. 5. Oxygen content of composite

7

sample after heating to 1460°C for 2 h.

a greater number of samples from a given length of rod but the current density and migration velocity would have been lower and the displacement of the median concentration smaller so there would be no gain in the accuracy of the experiment. The length of time selected for each electrotransport experiment was enough to allow a measurable movement to occur but not so much that the high-concentration plateau disappeared into the concentration increase at the end of the specimen. This conJ. Less-Common Metals, IO (1966) I--II

ELECTROTRANSPORT

OF INTERSTITIAL

ATOMS

IN YTTRIUM

7

centration increase at the anode end of the specimen confirmed the direction of migration. In the case of hydrogen, the transport by electrotransport was small compared to that by diffusion and the hydrogen increase in the anode section was vital in establishing the direction of migration. Carbon increases were not observed in the anode section apparently due to the rapid migration of carbon into the tantalum adapters

835 -.Z 4 HOURS

WELD INTERFACE I”’

I I

I 2

___ I

Fig. 0. Hydrogen

content

I

I 5

I I 3 4 DISTANCE, of composite

^-

6

7

Cm

sample after heating to 835T

for 4 h.

All four interstitial solutes were found to migrate toward the anode, in the same direction as the electron flow. The electrotransport velocities, in centimeters per hour per volt per centimeter, at various temperatures are given in Table II together with the data from which they were calculated. The voltage drop per centimeter was calculated from the current density assuming a cross-sectional area of the specimen of 0.050 cm2 and the specific resistivity

THE

ELECTROTRANSPORT

Carbon Oxygen

1235 1350 1qho ‘235 1350 1460

Nitrogen Hydrogen

X23.5 1350 1460 775 835 950

VELOCITIES

OF

CARBON,

0.8 0.6

o.a 1.2

0.8 0.7 0.7 0.4 0.5 o-3 0.2 0.2

OYYGEN,

16

NITROGEN

AND

01.2

0.20

65.4 72.9

0.56

IO

59.8

0.3’

4 2

66.2 72.0

O-73 I.14

16

59.5

o.ra

4 2

65.4 71.0

0.37 0.83

IO.5

39.x

0.24

4 2

41.4 45.0

0.37 0.63

4 2

1.30

HYDROGEN

IN

YTTRIUM

0. N. CARLSON, F. A. SCHMIDT, D. T. PETERSON

8

g

0.80

E s

0.40

g Y d

0.20

1

1250

I 1350 TEMPERATURE,

I

1450 OC

Fig. 7. Temperature us. migration velocities for carbon, oxygen and nitrogen in yttrium.

H

0.40

si !s $ IB d

0.20

750

950

850 TEMPERATURE.

*C

Fig. 8. Temperature vs. migration velocity for hydrogen in yttrium.

data of HARERMANN AND DAANE 14. These values were 204, 210 and 216 $&cm at 1235, 1350 and 1460°C respectively and 160, 165 and 177 @-cm at 775, 835 and 950°C respectively. The transport velocities are plotted as a function of temperature in Figs. 7 and 8 and show an increase with temperature due to the increased mobility of the interstitial solutes at higher temperatures. The diffusion coefficients listed in Table III were calculated from the concentration profiles by the Grube method. This solution of Fick’s law assumes that the J. Less-Common

Metals,

IO (1966) I-I I

ELECTROTRANSPORT

OF INTERSTITIAL

ATOMS IN YTTRIUM

9

diffusion coefficient is independent of concentration. The concentration data were plotted on probability paper so that a best straight line could be drawn through the points. Due to the limited number of concentration points, the diffusion coefficients are probably only accurate to f 25%. The effective valence, Z*, of all four interstitial solutes was calculated from the electrotransport velocity and the diffusion coefficient (see Table III). This effective valence cannot be considered as representing the TABLE

III

THE ELECTROTRANSPORTVELOCITIES, DIFFUSION COEFFICIENTS AND EFFECTIVE CHARGE, CARBON,

OYYGEN,

NITROGEN

AND

Temperature

HYDROGEN

u

PC)

Carbon

Oxygen

Nitrogen

Hydrogen

I235 1350 I460

0.20

1235 1350 1460

0.31 0.73 1.14 0.18

1235 1350 1460 775 835 950

0.56 1.30

0.37 0.83 0.24 o-37 0.63

z*,

FOR

IN YTTRIUM

.Z*

D (cmz/sec)

1.40 x 10-5 5.00 0.97

x 10-S x 10-s

1.10

x

10-5

2.60 x 10-5 0.3’ x 10-S 0.53 x 10-S 3.80 x 10-j 0.7 x 10-s 3.1 x 10-s 7.2 x 10~~

PO.5 -1.1 -1.2 _ 2.0 _

1.8

---2.1 _

2.8

-0.9 -0.9

PO.32 -0.26

ionic charge of the solute and is included only to allow comparison with similar values which have been reported for other systems. The effective valences are all negative and tend to increase in the order of hydrogen, carbon, oxygen and nitrogen. The lower values for hydrogen and carbon indicate that these solutes would not be as easily removed from yttrium by electrotransport as oxygen and nitrogen. A longer specimen of yttrium was treated by electrotransport for a length of time sufficient to approach the steady-state solute distribution as a preliminary indication of the potential of this method in the purification of yttrium. At the steady state, the interstitial solutes have been transported toward the anode until the electrotransport fluxes are balanced by the diffusion fluxes due to the concentration gradients. In this experiment a bar 0.254 cm in diam. and 12 cm long was treated at 1175°C for 90 h under a vacuum of 2 x 10-7 torr. This experiment was done under vacuum because of the contamination encountered upon heating of a similar specimen for extended periods under an argon atmosphere, therefore necessitating the use of a lower temperature to avoid vaporization of the yttrium. The velocities of the interstitials were thus reduced due to the lower temperature and also the decreased current density which resulted from the lower heat-transfer rate from the specimen. The resistance ratio of the original material was 12 and its composition that of the metal shown in Table I with the exception of the carbon content which was about IOO p.p.m. After a 90 h electrotransport run, the resistance ratios were measured for z-cm intervals along the rod. The results of this are shown in Table IV. The anode and cathode ends (sections I and 6) were cut off for analysis and theremainder of the rod was treated for an additional IOO hat I 175°C. The resistance ratios were again measured and J. Less-Common

Metals.

IO (1966)

I-II

IO

0. N. CARLSON, F. A. SCHMIDT, D. T. PETERSON

corresponding

segments

of the rod analyzed

for carbon,

results of which are shown in the right hand column The extent

oxygen,

of Table

TABLE

for carbon,

oxygen,

and nitrogen

in yttrium

velocities

estimated

and diffusion

by extrapolation

IV

RESISTANCE

RATI~ANDCHEMICALANALYSES(~.~.~.)~FPURIFIEDYTTRI~M

Section

90 h at

After

CO

N

I

37 33 22

4

18 I5 9

190 h at 1175°C c

0

N

-

-

-

-

45 4’5

IZO 115

go 125

8 8

34 25

85 I30 -

170 725 -

14 21 -

R3011°K

&OK

2 3

ROD

After

1175°C

&lO°K

to

the

to which the purest half of the bar is purified has been considered

by VERHOEVEN~~. His results indicate that, with the transport coefficients

and nitrogen,

IV.

R.l°K 90

230

-85

1475

8

9

1175”C, the cathode half of a ro-cm bar should contain less than one per cent of the

original

concentration

of these impurities

state in 25 days for carbon purification,

it is necessary

surroundings adapters.

and oxygen

reach the steady

state,

content

in agreement

with the predicted

was also reasonably

reason for this is not known adapters

the specimen

and oxygen

from

contents

in nitrogen

value although

The reduction

close to the expected

changed

of the cathode were indicat-

concentration

was

contents

in oxygen from 780 to about however,

the carbon

in this experiment.

of carbon into the specimen

It has been observed

into the tantalum

this

by the

the tantalum

the nitrogen

result;

by electrotransport

but the migration

is suspected.

moves out of the yttrium

To achieve

of the specimen

was much less than that required to

by VERHOEVEN. The reduction

was not significantly

tantalum

reach the steady

of the original values and in nitrogen to 50%

were too low for accurate determinations. IOO p.p.m.

into

decreases in the carbon

ed by the calculations substantially

of impurities

the time of this experiment

half of the specimen to 4%

effectively

that there be no contamination

and no transfer

Although

and should

and 80 days for nitrogen.

The

from the

that at the anode end-carbon

connector

and the converse effect at the

cathode end would be expected. Although electrotransport content

this

experiment

to about

as a higher temperature, purification

done

method,

under

conditions

it was possible

which

handicapped

to reduce

the

of its original value and to increase the resistance

10%

factor of four. If electrotransport

were applied under more favorable

higher current density

should be achieved.

most effective

was

as a purification

way of purifying

This method

conditions

and no contamination,

appears

small quantities

oxygen

ratio by a such

much better

at the present time to be the

of yttrium

metal.

ACKNOWLEDGEMENTS The authors wish to thank Dr. J. D. VERHOEVEN for his consultation M. E. THOMPSON for assisting in the experimental J. Less-Common Metals,

IO (1966) I--II

and Mr.

work. Special acknowledgement

is

ELECTROTRANSPORT

OF INTERSTITIAL

ATOMS

If

IN YTTRICI1f

also made to the Analytical and Spectrographic Sections of the Ames Laboratory, and especially to Mr. C. C. HILL, for their work in performing the analyses on the yttrium metal. IIElXIIENCES I 2 3 4 5 6 7 8 9 10 II 12 13 14 I5

W. SEITH AND 0. KUBASCHEWSKI, 2. Electrockem., 4r(Ig35) 551. 0. DAYAL AND L. S. DARKEN, Trans. AZME, 188 (1950) 1156. J. H. DE BOER AND J. D. FAST, Rec. Trau. Chim., 59 (1940) 161. F. CLAISSE AND H. P. KOENIG, Acta Met., 4 (1956) 650. W. SEITH AND TH. DAUR, Z. Electrochem., 44 (1938) 256. C. WAGNER AND G. HELLER,~. Phys. Chem, (B), 46 (1940) 242. J. D. VERHOEVEN, Met. Rev., 8 (1963) 3 I I. F. H. SPEDDINC AND A. H. DAANE, The Rare Earths, Wiley, New York and London, 1961 J. M. WILLIAMS AND C. L. HUFFINE, AT&. Sci. Eng., 9 (1961) 500, P. G. SHEWMON, Diffusion in Solids, McGraw-Hill, New York and London, 1963. 0. N. CARLSON, J. A: HAEFLING, F. A. SCHMIDT AND F. H. SPEDDING, J. Electiochem. Sot., (1960) 540. I>. T. PETERSON AND E. N. HOPKINS, U. S. A.E.C. Rept. IS-1036, 1964. 0. N. CARLSON, D. W. BARE, E. D. GIBSON AND F. .4. SCHMIDT, Symposium ASTM Spec. Tech. Publ. No. 272, 1960, p. 144. C. E. HABERMANN AND A. H. DAANE, J. Less-Common Metals, 5 (1963) 134. J, D. VERHOEVEN, personal communication. J. Less-Common

107

on Lvewer Metals,

Metals,

IO (1966)

I--II