Solid state electrotransport of erbium

Journal of the Less-Common

Metals, 166 (1990)

173

173- 188

SOLID STATE ELECTROTRANSPORT

OF ERRIUM

D. FORT School of Metallurgy and Materials, University of Birmingham, (U.K.) B. J. BEAUDRY

and K. A. GSCHNEIDNER,

P.O. Box 363, Birmingham BIS 2TT

JR.

Ames Laboratory and Department of Materials Science and Engineering, IA 50011 (U.S.A.)

Iowa State University, Ames,

(Received May 23,199O)

Summary Solid state electrotransport has been used as the final stage in the purification of erbium, which is one of the highest vapour pressure rare earth metals to which this technique has been applied. An improvement in the overall purity from 99.91 at.% (99.988 wt.%) for the as-sublimed start metal to 99.97 at.% (99.996 wt.%) at the cathode end after electrotransport was achieved after processing for 2355 h. This is believed to be the highest purity ever measured for erbium.

1. Introduction As a group of metals, the rare earths have gained a reputation for being particularly difficult to refine to the high purity levels necessary for physical property research. While the separation of one rare earth from another was a major preoccupation in the early years of rare earth purification research, basic preparation techniques have now advanced to the stage whereby the main contaminants in the rare earth metals produced at the Ames Laboratory are normally the interstitial impurities (e.g. oxygen, hydrogen, nitrogen and carbon) rather than other metallic species [l]. Although solid state electrotransport (SSE) has been shown to be an effective technique for further refining many of the rare earth metals with respect to interstitial impurities [2, 31, the applicability of SSE is dependent upon the vapour pressure characteristics of the metal concerned. As SSE involves heating a metal at temperatures approaching its melting point for a considerable length of time (typically weeks or months), loss of metal by volatilization is a problem which is encountered with all but the lowest vapour pressure rare earths. With the majority of elements, volatilization problems necessitate reducing SSE processing temperatures and run lengths below the values ideally required for maximum purification and/or processing under an inert gas rather than vacuum. 0022-5088/90/$3.50

0 Elsevier Sequoia/Printed in The Netherlands

174

Dysprosium, holmium and erbium form a subgroup of heavy rare earth metals whose vapour pressures at their respective melting points are moderately high, being of the order of 0.1 mbar. All can be purified by sublimation, which, when performed under ultrahigh vacuum (UHV) conditions, can reduce the oxygen contents to a few hundred atomic ppm. While such levels are approaching the purities attained using SSE on lower vapour pressure rare earths, SSE has two significant advantages over sublimation as a purification method in that as-purified SSE samples are generally in a form suitable for direct cutting of specimens for further research (sublimates, even when collected on a hot surface, require consolidation by remelting before they acquire a usable form), and that actual single-crystal rods can be SSE-refined or crystal growth can be induced in an initially polycrystalline sample during refining [3, 41. Some SSE experiments on dysprosium and holmium have been published previously [3], although results were only assessed indirectly using resistance ratio measurements rather than by analysis. The results did suggest, however, that SSE could be used to purify these elements even though an inert gas environment had to be used to counter excessive volatilization along with reduced processing temperatures and times. To date, the only SSE work on erbium known to have been published [5] involved the use of comparatively low temperatures and short times so as to allow processing in vacuum (e.g. 70 h at 1050 “C). Analysis of results was by way of residual resistance ratio (RRR) measurements; in one run an initial -RRR value of 7.5 was raised to between 31 (anode end) and 46 (cathode end). As there was an overall increase in RRR throughout this and other samples, it could be postulated that much of the improvement was due to vacuum degassing of the comparatively low purity start metal, in particular the removal of hydrogen, rather than the effects of electrotransport. The aims of the present work were twofold: (i) to ascertain, by way of comprehensive analysis, the degree of further purification attainable using SSE for previously sublimed erbium metal; (ii) to determine whether processing under an inert gas, rather than the preferred UHV conditions, has a limiting effect upon the SSE purification of rare earth metals. 1.1. The SSE technique SSE simply involves passing a direct current along a rod of metal. The flow of electrons can induce the migration of both host atoms (self-electrotransport) and impurity atoms, and, as the current is increased, Joule heating of the metal by the electron flow significantly increases electromigration rates. In all of the rare earths so far studied, the electromigration rate of the typical interstitial impurities (and also some substitutional impurities) have proved to be far greater than the rate of self-electrotransport resulting in the transport of interstitial impurity atoms towards the anode ends of samples [6]. The redistribution during SSE of any particular species of impurity atom continues until a steady-state distribution is reached whereby the forward motion of atoms due to electrotransport is balanced by the opposing motion due to diffusion. Thus most impurities (hydrogen may be an exception) are not actually removed from a rare earth sample during SSE, but redistributed within it. An important feature of SSE purification is that the final

175

purity attainable is directly related to the purity of the start metal. Thus the main value of SSE comes in further refining already high purity metal, rather than trying to “clean up” poor quality metal, a process better suited to alternative, less timeconsuming techniques. Because of the length of time required to attain an appreciable redistribution of impurity atoms and the generally reactive nature of rare earth metals, it is essential that SSE refining of these elements is performed in an “ultraclean” environment or contamination of the sample from its surroundings will outweigh any advantages resulting from electromigration. Normally this means refining in UHV conditions of, typically, lo-r0 mbar; if it is necessary to use an inert gas environment to suppress sample volatilization, this gas must approach these levels of “cleanliness” with respect to reactive gases. It is worth pointing out that excessive volatilization losses need to be avoided not just because the sample gets smaller, as this could be compensated for by starting with a larger sample, but because weight losses tend not to be uniform along the length of a sample but are concentrated at certain positions leading to local hot spots, necking and failure by melting through. Furthermore, the impurity concentrations will increase since some of the matrix metal is lost by volatilization, while impurities generally do not vapourize. More detailed descriptions of the theory and practice of electrotransport refining can be found elsewhere [2,3,7-91.

2. Experimental

procedures

and results

2.1. Determination of processing parameters

The processing parameters for a SSE run can be listed as the sample environment, sample temperature, processing time and sample dimensions. As indicated in Section 1, SSE purification of erbium under idealized conditions (i.e. 50-100 “C below the melting point in UHV) is not possible; under such conditions volatilization losses of about 10% of the sample every minute would be encountered. To process erbium in vacuum, the sample temperature would need to be reduced to around 950 “C before volatilization rates were reduced sufficiently to allow a run of even 200-300 h without overall weight losses exceeding 20%. However, such temperatures, about 67% of the melting temperature, would result in impurity mobilities which would probably be too low to maximize the effectiveness of electrotransport purification. Thus the case in favour of processing erbium under an inert gas rather than vacuum, so as to reduce volatilization and allow higher temperatures, was strong. The chosen argon pressure during processing of 0.7 bar (although this would have increased as the sample was heated) was largely determined by consideration of the capabilities of the available SSE equipment, which was constructed using standard UHY components. Pressures in excess of 1 bar were avoided for fear of damaging the system viewport and electrical feedthroughs. (Separate weight loss tests using a small erbium ingot heated in an r.f. coil indicated that weight losses of approximately 0.7% per day were likely given an argon pressure of 0.7 bar for a sample temperature of 1200 “C. These tests further indicated

176

that to reduce losses to, say, 0.1% per day a pressure of 20 bar would be required, a pressure far above that attainable in the SSE equipment with major modifications.) Given that the inert gas pressures attainable in a conventional SSE system will only reduce volatilization, not prevent it, an optimum temperature still had to be decided for SSE processing which would be high enough to give meaningful electrotransport rates without unacceptable volatilization losses. In a previous paper one of the present authors [3] suggested, from an analysis of several SSE runs, that a processing temperature which gave a sample vapour pressure of approximately 5 X 10 - 3 mbar would allow controllable SSE refining over a period of several hundred hours using 0.5 to 0.7 bar of argon, before volatilization losses became troublesome. This equates to a temperature of about 1200 “C for erbium, or 82% of the melting temperature. Although the agreement between theoretically predicted and actual results for electrotransport in rare earths is generally rather poor (the predicted extremely low interstitial impurity levels have never been achieved in practice for a number of possible reasons [2, 3, lo]), SSE theories provide the only available guide for estimating the length of run required to approach steady-state conditions. Unfortunately, no values for electrotransport mobilities and diffusivities of common impurities in erbium have been published, so it was not possible to calculate the theoretically ideal length for a SSE purification run. The most similar element for which relevant data exists is gadolinium, and in a previous paper [3] it was calculated (using a formula proposed by Peterson [2]) that the time required for oxygen, nitrogen and carbon impurities to approach within 1% of steady-state distributions would be 165, 470 and 940 h respectively given vacuum conditions, a sample length of 10 cm, a diameter of 0.64 cm and a processing temperature of 90% of the melting temperature. Assuming that the electrotransport mobilities and diffusivities of these impurities in erbium are similar to those in gadolinium, a period of at least 1000 h at 1200 “C was envisaged for erbium given the proportionately lower processing temperature. The sample dimensions also have a bearing upon SSE purification. Although the longer the sample the greater the degree of theoretically attainable impurity redistribution, this can only be achieved by having longer run times. Little advantage is gained by making long samples for high vapour pressure metals as the time required to approach the improved steady-state distributions cannot be reached because of volatilization losses; therefore, a moderate sample length of approximately 12 cm was chosen. A diameter of 7.1 mm was chosen primarily to give a sufficiently large sample to allow cutting of analysis samples. A larger diameter, while giving more material, would reduce the rate and potential degree of purification as the lower surface-to-volume ratio would result in proportionately less heat loss from the sample, thus reducing the electric field. Processing under an inert gas, rather than vacuum, does provide some theoretical benefits as higher currents are required to reach a given temperature owing to the cooling effects of the gas. The increase in electric field should reduce the time required to approach steady-state conditions, but any overall improvements in the scale of purification are dependent, of course, upon the absence of contamination from the environment as a result of using the gas.

177

2.2. Equipment Although it was anticipated that most of the SSE processing would be performed under an inert gas (argon), it was nevertheless considered essential that UHV-rated equipment was used since only this can provide sufficient leak-tightness combined with the ability to degas the system walls thoroughly by baking. The SSE equipment consisted of a water-cooled stainless steel chamber pumped by ion and sublimation pumps. All demountable joints were of the “conflat” type and used copper gaskets. A more detailed description of the equipment has been given previously [3,11]. The procedure adopted for processing under argon was to pump and bake the system (with the water cooling off) to UHV levels before admitting purified argon to 0.7 bar, the run then taking place in a static argon environment. This procedure was followed, rather than the alternative method which is to flush the system continually with purified gas, as it proved difficult to purify inert gases routinely to significantly better than one volume part per million total impurities. Such purity levels in a continuous gas flow could possibly be considered as being of “equivalent cleanliness” to a vacuum of the order of 1O- 3 mbar (although the exact analogy should take account of the flushing rate); such a vacuum level would be far too poor for successful SSE purification. When processing under static argon, however, gases which diffise through and degas from the system walls are not removed by flushing but gradually build up in the system during the run. (The amount of impurities admitted when initially filling the system with argon purified to 1 vol. ppm. are not significant, amounting to a total increase in sample contamination levels of only a few ppm. even if all the impurities in the argon react with, or are absorbed by, the sample.) No method could be devised to monitor continuously impurity levels in the argon during a run as this would involve measuring gas partial pressures of lower than 10e9 mbar in 0.7 bar of argon. It was relatively simple, however, to measure degassing rates in static vacuum by closing the gate valve to the ion pump and turning off the sublimation pump. While this does not equate exactly to the situation when the system contains an inert gas, as the gas may be thought to suppress degassing rates to some degree, a residual gas analysis of the system as a function of time when under static vacuum should at least indicate which impurity gases are likely to become present during a run under static argon as a result of system leaks and degassing. Table 1 shows the variation of total pressure, together with the partial pressures of the major gases, as the SSE system was held under static vacuum for 440 h. The initial vacuum level after pumping and baking was 5 X lo- lo mbar. By far the highest partial pressure of any gas evolved was that of argon, present after a series of trial SSE runs. Of the “active” gases present, the hydrogen partial pressure remained virtually unchanged while the oxygen, nitrogen and water vapour pressures increased only marginally. Methane progressively degassed into the system throughout the period and the final partial pressure for HCl is noteworthy. 2.3. The start metal The start metal for the SSE run was prepared at the Materials Preparation Center, Ames Laboratory, Iowa State University, using the methods described by

5 2x lo-‘” 8X 10-10 8x lo-‘” 1 x 10-s

9x 10-s 2x10-7

1 2 x 10-I” 7x10-1” 8x10-to 3x10-9

2x lo-” 6x10-s

2 x lo-‘”

5x10-‘”

2 hydrogen 1S methane 16 methane 18 water vapour 20 argon 28 nitrogen/CO 32 oxygen 36 HQ 40 argon

Total

Time(h)

0

a.m.u./gas

1 x10-s 3x 10-c 9x10-h

3x1o-y 9x 10-7 3x10-6

8 x lo-’

4x10-7

1 x10-9

2x 10-I” 7 x 10-y 8x lO-y 2x 10-y 5x10-7

I#

2 x 10-10 2x 10-y 2x10-y 5x10-‘fl 1x10-7

50

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20

3 x 10-s

3 x lo-‘” 2x10-a 3 x 10-a 6 x lo-’ 2 x 10-e 3 x lo-‘” 4x lo-‘” 4 x 10-s 8x10-”

440

Degassing rate of the SSE system in static vacuum over 440 h. The partial pressures (in mbar) of the main peaks for each of the major degassing products are shown. Missing values mean partial pressures of less than 1 x lo- ‘” mbar

TABLE 1 2

179

Beaudry and Gschneidner, Jr. [l]. This involved conversion of the pure oxide to the fluoride which was then reduced using high purity calcium. The ensuing metal was vacuum melted in tantalum crucibles to remove excess calcium, residual fluoride and hydrogen, then it was sublimed at 1300 “C to remove dissolved tantalum and also to reduce interstitial impurity contents (oxygen, nitrogen and carbon impurities, which form stable compounds, tend to remain in the residue when the metal is sublimed at a slow rate). A full analysis is given in a later section. 2.4. Temperature measurement During the SSE processing, all sample temperature measurement was performed using a disappearing filament optical pyrometer calibrated to an emissivity of 0.37. For most previous SSE work [3], pyrometers were calibrated for emissivity against the reported melting point of the relevant metal by holding some of the metal just molten in a cold boat. With erbium, however, this procedure did not prove possible as vapour from the molten sample clouded the silica tube around the cold boat too quickly to allow an accurate measurement to be made. An emissivity value of 0.37 was chosen as being a value representative of heavy lanthanide metals. Given the uncertainty inherent in this assumed value, most of the temperatures quoted are believed to be accurate to within * 15 “C at best. The accuracy of temperature measurement was also affected by clouding of the viewport and surface roughness of the sample, as explained in the text. 2.5. Details of the SSE processing

The erbium start metal was cast into a crude rod in a cold boat using arc melting. This rod was mechanically swaged to a uniform diameter of 7.1 mm except for one end, intended to be the anode, which was left slightly thicker so that it would run cooler during SSE and be less likely to melt if impurity build-up became excessive. After swaging, the sample was thoroughly cleaned before loading at the cold boat for vacuum degassing and stress relieving at 850°C. The degassed rod was next cut to length, its ends were drilled and tapped and “I?’ shaped tantalum end rods of diameter 4.2 mm were screwed in. This arrangement, which has been explained in detail elsewhere [3], can be seen clearly in the photograph of the sample taken after SSE had been completed (Fig. 1). The sample was next loaded into the SSE equipment, being held by the tantalum end rods which were connected to the main electrodes in the system via soft copper sheet expansion links. In order that volatilized vapour would tend to rise away from the rest of the sample during processing under argon, it was loaded at an angle of approximately 40” to the vertical, Holding the sample horizontally was not attempted for fear of excessive grain boundary sliding. The SSE processing was performed in six stages, some in UHV and some under argon. Although the current passing through the sample and the voltage across its ends were monitored continuously during each stage of refining, the sample temperature could be measured only infrequently so as to prevent excessive clouding of the system viewport by volatilized erbium (when not in use the viewport was protected by a “Venetian blind” type of shutter). Figures 2 and 3, which accompany the descriptions of the separate stages of refining (described

Fig. 1. The erbium SSE sample, as removed after the final stage of processing, with the cathode end to the left. The extent of whiskering can be seen clearly. The curved rods screwed into the ends of the sample are the tantalum end rods used to connect the sample to the main electrodes within the SSE system.

A 250

b

im ~~-,~ 100

200

300

hours

600 hours

500

:g, 600

703

SE0

0 0

100 hours

200

A

~~~~

0

~~1

~~

IW

200 hours

300

0

100

200

300 hours

400

SW

0

100

200 hours

300

Fig. 2. Plots of the current passing through the sample as a function of time for the six stages of SSE processing. The eight numbered arrows indicate the times at which the temperature profiles shown in Fig. 3 were measured.

below), show plots of the current passing through the sample as a function of time with some actual temperature profiles. Stage 1. Sample degassing in UHX This stage was used to degas the sample and its t~t~urn end rods, p~cul~ly with respect to surface contests and dissolved hydrogen (unlike the other interstitial impurities, a substantial amount of the hydrogen dissolved in rare earth metals can be removed by heating the metals in vacuum). A maximum sample temperature of 920 “C was achieved. At such

181

Cathode VA+1

tz4

_---

_-

_/d

6 (191AArl

1 l193A.Ar)

Fig. 3. Schematic diagrams of the SSE sample for stages 1 to 3 (top) and stages 4 to 6 (bottom) showing the sections removed for analysis (hatched areas) and, vertically below, the temperature profiles (numbered 1 to 8) measured at the times indicated in Fig. 2. The currents passing through the sample and the environment (argon or vacuum) are given in parentheses after the temperature profile number. The cooling effect of whiskering is illustrated by a comparison of profiles 6 and 7, which were taken at nearly identical currents but separated by 320 h, during which time significant whisker growth had occurred. The temperatures indicated along profile 4 are probably slightly low as a result of clouding of the viewport by erbium vapour.

temperatures the vapour pressure of the erbium was high enough to coat the system walls partly, which had already been thoroughly degassed by baking during the initial pumpdown, with a thin film of erbium (estimated to be two orders of magnitude greater in surface area than the sample) which would have a gettering effect upon any active gas impurities contained in the inert gas during the subsequent processing under argon. This first stage lasted a total of 340 h. Stage 2. Processing under argon. Purified argon was admitted to the system to approximately 0.7 bar via a British Oxygen Company inert gas purifier specified to reduce the total of all gaseous impurities to below one volume part per million. This stage of electrotransport lasted a total of 770 h and a maximum sample temperature of 1210 “C was reached. After about 220 h the viewport had become visibly clouded, presumably because the shutter had failed to close properly, making subsequent temperature readings unreliably low. The temperature varia-

182

tion along the length of the sample was rather larger than ideal as the cathode end was running cooler than anticipated as a result of the tantalum end rods being made slightly too thick. Recondensation of erbium vapour onto the sample eventually led to the growth of dendritically shaped “whiskers” on parts of the sample surface. The first visual evidence of whiskering occurred after about 150 h and the proportion of the sample surface covered by clusters of whiskers gradually increased until most of the central (hotter) section of the sample was covered. This stage of the processing was eventually terminated when whiskering, which can significantly lower the sample temperature, was considered to have become excessive. Stage 3. Degassing in vacuum. Without removing the sample, the system was again pumped to UHV levels both to ensure that no leaks had developed during the argon processing and to allow sample degassing, particularly with respect to hydrogen. A maximum sample temperature of about 900 “C was reached in this stage which lasted for 175 h. Before continuing the SSE refining, the sample was removed and a 2 cm length was cut from its cathode end for analysis as indicated in Fig. 3. The whiskers were carefully removed from the remainder of the rod using a sharp blade; the sample diameter at its thinnest point in the centre was 6.6 mm after removal of the whiskers, a reduction of 7% from the original diameter. A new hole was drilled and tapped for the cathode end rod, which was replaced after it had been cut half-way through in several positions so that it would run hotter during SSE and so reduce the temperature gradient in the upper half of the sample. The final three stages of SSE processing mirrored the first three, so only a brief description will be given. Stage 4. Sample degassing in UHV. A maximum sample temperature of 950 “C was attained in a stage lasting 290 h. Stage 5. Processing under argon. Although some difficulties were experienced in measuring sample temperatures as the rough surfaces left after whisker removal gave higher indicated temperatures than neighbouring smooth areas, the temperature gradient along the sample appeared to be more uniform than in the previous argon stage as a result of the modifications made to the tantalum end rod. An initial maximum sample temperature of nearly 1200 “C was set, although this decreased with time owing to the cooling effects of whiskering. The changing temperature profiles, shown in Fig. 3, clearly illustrate the effects of whiskering which were far more extensive in the hotter (cathode end) half of the sample. The total stage length was 560 h. Stage 6. Degassing in UT-IV.The final degassing stage lasted 220 h and a maximum sample temperature in excess of 900 “C was reached. A photograph of the as-removed sample is shown in Fig. 1. Sections intended for analysis were cut from the cathode, centre and anode positions as indicated in Fig. 3. 2.6. Analyses Specimens were spark-cut from the analysis sections, followed by chemical cleaning, for vacuum fusion, mass spectrographic, carbon combustion and chemical analyses. The results, together with the start metal analysis and the cathode end

183 TABLE 2 Results of the analysis of the erbium start metal and after three and six stages of SSE processing. Concentrations in ppm. atomic with ppm. weight in parentheses for oxygen, hydrogen, nitrogen, carbon and fluorine. Only elements with concentrations greater than 1 ppm.at are shown Element

Start metal

After three stages

After six stages Cathode

Centre

Anode

705( 67) 1503(9) 96(8) 195( 14) 26(3) 2.0 3 4.1 0.6 0.4 1.5 co.1 co.4 3.5 2.6 7.2 1.5 2564 2499 99.74 99.988

Cathode 0

H N C F Ho Fe Ni Ca cu Ta MO Mn Pb Cl Na K

638(61) BLDa 48(4) 97(7) 79(9) 8.8 7 2 1.8 3.6 6 6 11 3.8 4 co.1 < 0.2

200( 19) 167( 1) 24(2) 153(11) 26(3) 3.5 0.8 co.1 0.6 5.4 2.5 co.1 0.4 0.9 6.5 3.5 1.0

168( 16) BLDa BLDa ill(8) 44(5) 3.7 G.2 0.5 1.6 1.3 co.1 co.1 2.0 0.2 3.8 1.8

305( 29) BLDa 12(l) 306(22) 141( 14) 2.8 1.4 0.5 0.9 0.4 1.0 co.1 co.1 2.3 0.2 5.5 2.5

TotaP’ O+H+N+C at.% wt.%

921 783 99.91 99.988

595 544 99.94 99.995

338 279 99.97 99.996

790 623 99.92 99.991

1

“BLD means below limit of detection (less than 1 ppm.wt. for H, N). bPositively identified elements only.

analysis after the first three stages of SSE, are given in Table 2. The vacuum fusion results (for oxygen, hydrogen and nitrogen), carbon combustion and chemical analyses (for fluorine) were actually measured to an accuracy of + 1 ppm. weight which translates to a far higher numerical error when converted to ppm. atomic, particularly for hydrogen. Both ppm. atomic and ppm. weight values are given in Table 2 for these elements so that the magnitude of the error can be assessed readily. 3. Discussion As indicated in Table 2, the SSE purification of erbium resulted in a 63% reduction in overall impurity levels from 921 ppm. atomic in the start metal to 338 ppm. atomic in the cathode section after all six stages of processing. This equates to

184

a final purity at the cathode end of 99.97 at.% or 99.996 wt.%, compared with the start level of 99.91 at.% or 99.988 wt.%. The centre section of the refined sample showed a marginal improvement over the start metal, while the anode section was considerably less pure as would be expected given the anticipated electromigration of interstitial impurities in the cathode-to-anode direction. The cathode end analysis is believed to be the lowest total impurity concentration ever reported for erbium. The changes in overall impurity levels generally reflect the electromigration behaviour of oxygen which was the main contaminant of the start metal at 640 ppm. atomic. This figure had been reduced to 200 ppm. atomic at the cathode end after three stages of processing, and to 168 ppm. atomic after six stages. The oxygen contents at the cathode, centre and anode positions of the final sample show the progressive increase typical of electrotransport purified materials. For the cathode end analyses, a reduction of nearly 70% in oxygen impurity levels took place during the first three stages of processing, while the reduction between the third and sixth stage was about 20-25% (an exact figure cannot be calculated as the cathode end of the SSE rod was cropped after three stages to provide an analysis sample). By analogy with the theoretically predicted results for gadolinium, referred to in Section 2.1, it may have been expected that little further change in the oxygen profile would have occurred in the final three stages of processing as steady-state conditions were expected to be approached within a few hundred hours. That a further 20%-25% reduction in oxygen content took place in this period indicates either that the assumption that erbium would behave similarly to gadolinium was invalid, or that the general theoretical basis for SSE in rare earth metals is unreliable. It is possible that the tantalum end rod also acts as a sink for the oxygen (depending upon how well it bonds to the sample), which could effectively lengthen the time required to approach steady-state conditions. A comparison of the final analyses with that of the start metal gives no indication of any oxygen contamination having taken place during the processing, which is consistent with the degassing study described earlier (see Table 1). As with oxygen, the nitrogen levels after SSE show a progressive increase along the length of the sample, again following the expected electromigration direction of cathode to anode. The nitrogen content at the cathode was below the limit of detection (1 ppm. weight) after six stages of processing compared with a start metal value of 48 ppm. atomic (4 ppm. weight). Overall, however, the nitrogen contents were too low, and the percentage errors too high as a consequence, for any further conclusions to be drawn. Although no hydrogen could be detected in the start metal, a hydrogen level of 1500 ppm. atomic (9 ppm. weight) was measured at the anode end of the sample at the completion of SSE, while the hydrogen content at the other positions was below the detection limits. This would appear to indicate that some hydrogen contamination occurred during SSE processing and/or sample preparation, the hydrogen then electromigrating towards the anode. The fact that the hydrogen at the anode was not removed by the vacuum degassing stages could be explained if the time allowed was insufficient.

185

Another explanation is as follows. The leakage of methane into the system, as indicated by the static vacuum degassing experiment, could account for the high hydrogen concentration at the anode end, and also the increase in the carbon concentration during SSE processing (see next paragraph). Presumably when a methane molecule strikes the erbium rod it will decompose into hydrogen and carbon, but since some of the hydrogen (approximately one-fourth) will be trapped by the carbon atom, its partial pressure is lowered sufficiently such that it does not vapourize as readily as the untrapped hydrogen, especially in the colder regions near the anode end. Also since in the SSE purification process hydrogen is being driven continua~y towards the anode region, the trapped hydrogen does not have sufficient time to evaporate and a steady-state condition is established. The analysis results for carbon do not show the expected progressive increase from cathode to anode, although the cathode value is lowest at 111 ppm. atomic. Even this figure, however, is slightly higher than the start metal concentration (97 ppm. atomic), clearly indicating that some carbon contamination of the sample had occurred. As was the case with hydrogen, the two most likely sources of this contamination were during the preparation of the SSE rod, where hydrocarbon oils were used as a lubricant during swaging, and during electrotransport under argon, where methane was expected to be one of the main gases evolved from the system walls. The former explanation appears to fit with the fact that the carbon content at the cathode was lower after six stages than it was after three, for if carbon conta~nation was occurring during the SSE processing under argon, an increase might have been expected, However, the high final carbon value of 306 ppm. atomic for the centre section, compared with 195 for the anode, could be explained (providing that the figures are reliable) by assuming that any contamination during the SSE stages under argon would have been greatest in the central parts of the sample which were most affected by whiskering and therefore had the largest surface area. Possibly a combination of both factors would best account for the contamination encountered. Other explanations for the carbon profile could be advanced, however, which are not connected with preferential contamination. The profile could be a result of the temperature gradients within the sample when SSE processing, either because of thermotransport (unfortunately neither the direction nor the likely rate of thermotr~sport of carbon in erbium are known), or because the anode end was too cool to allow a si~~icant rate of carbon electrotransport, meaning that carbon moving in the cathode to anode direction tended to pile up in the centre upon approaching the cooler anode section. As with carbon, the levels of fluorine show no consistent trend along the length of the sample after SSE and similar explanations based upon the thermal gradient within the sample can be advanced. However, a more likely reason for this behaviour is connected with the probability that fluorine was distributed inhomogeneously in the start metal. The problem of fluorine contamination in erbium arises because the vapour pressures of ErF, and erbium are comparable, although the former is slightly higher. Thus in the initial purification of the metal erbium and the fluoride tend to co-sublime, although the sublimate collected first will be somewhat higher in fluorine than that collected later. This fluorine gradient

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is probably then arc-melted into the metal ingots (i.e. the start metal for the SSE) which are prepared from the sublimate. Of the metallic impurities, no consistent trend was generally evident, owing, at least in part, to their relatively low concentrations, although some movement of nickel and possibly lead in the electron flow direction (towards the anode) was observed, while copper appeared to migrate towards the cathode. The levels of manganese and molybdenum in the start metal analysis appear to be anomalously high compared with those after SSE. It is possible that the original analysis for these impurities in the starting metal was in error. The consistent and low values of manganese and molybdenum in the SSE-purified rod indicate this possibility. Overall, SSE has been demonstrated to be of significant value as a final step in the preparation of high purity erbium. The very low oxygen levels (i.e. below 50 ppm. atomic) achieved by using combined zone refining/double stage SSE in previous purification studies on gadolinium [ 121 have not been fully matched, but the present results are comparable with those reached using SSE alone on several other lower vapour pressure rare earths. (Because of its high vapour pressure, erbium is not readily amenable to zone refining.) The oxygen content at the cathode end of the erbium sample shows a considerable improvement upon that achieved by sublimation during the production of the start metal. On the question of possible contamination of the sample as a result of SSE processing under an inert gas rather than UHV, no increase in the overall oxygen or nitrogen contents was evident, and the hydrogen contamination, although present, did not prove to be a significant problem as this impurity was concentrated at the low purity anode end. From the above discussions it is clear that the most problematical impurities in erbium were fluorine and carbon, notwithstanding the fact that oxygen was actually present in greater concentrations. Fluorine contamination has not been encountered to any noticeable degree in the previous SSE work on the lower vapour pressure lanthanide metals (because the vapour pressure of the fluoride is much higher than the metal making its removal by distillation more effective), but it is probably a hurdle which will have to be overcome if the higher vapour pressure metals are to be refined to ultrahigh purities. The fluorine contents of as-prepared erbium start metals are being investigated currently for inhomogeneous distributions, the collection of such data being necessary before any solution can be suggested. The exact form that fluorine takes in the metal (i.e. dissolved, as ErF, or as minute pockets of gas) also needs to be clarified. The difficulties with carbon arose both because of an increase in the overall level as a result of contamination and also because the SSE refining was not particularly effective at redistributing carbon. A primary objective of any future SSE purification work on high vapour pressure rare earths under an inert gas must be to reduce, or to eliminate, such carbon contamination. It is unfortunate that the requirement for uniformly shaped samples is paramount when working with high vapour pressure metals (because hot spots at narrow parts of such samples will rapidly thin further as a result of volatilization leading to failure by melting) as this invariably involves some form of mechanical shaping since it is difficult to cast a sufficiently uniform rod. Rather than the swaging used in the present work, how-

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ever, it would probably be preferable to use a dry (i.e. u~ub~cated) shaping or cutting technique as this should eliminate any possibility of contamination from hydrocarbon oils. The other identified source of possible carbon contamination, i.e. degassing from the system walls into the argon, may be more difficult to overcome. To a large degree, the gas species which are evolved are a product of the environment to which the system is routinely exposed. Possibly systems which are intended for SSE in inert gas work should only be exposed to inert gases. This would mean that sample loading and unloading, system maintenance and (dry abrasive) cleaning would have to be performed using an inert gas filled glove box or glove bag; while this would be un~eidy it would be possible. Other changes which may prove beneficial would be to employ a higher bake-out temperature for a longer time, more efficient system wall cooling and, possibly, the use of a liquid nitrogen cooled shield around the sample. Besides the carbon contamination difficulties and presence of fluorine, the other problem encountered while SSE refining erbium over and above those encountered when processing lower vapour pressure rare earths under vacuum was the whiskering phenomenon. Whiskering limited the stage lengths when processing under argon and also meant that the temperature distribution along the sample could not be kept at an ideal, or measurable, level. One effect of this was to make difficult the scientific assessment of the effects of different processing variables; in severe cases the extent of possible thermotransport of impu~ties between the cooler (whiskered) and hotter regions of a sample may have to be evaluated. It is difficult, however, to conceive of a method of eliminating whiskering which would not create more difficulties than the original problem, although some reduction of whiskering as a result of a moderate increase in argon pressure should be relatively straightforward. The use of 0.7 bar argon pressure was probably excessively cautious and an increase to 1.5 bar should be possible without significant modification to the equipment. Weight loss tests have indicated that this approximate doubling of pressure would be anticipated to reduce volatilization by 3 5%, with a presumed equivalent reduction in whiskering.

4. Conclusions ( 1) SSE processing of previously sublimed erbium has given a worthwhile improvement in purity, illustrating the value of this purification method when applied to the higher vapour pressure rare earth metals. (2) The technique of SSE processing under an inert gas was successful as regards redistributing oxygen, nitrogen and hydrogen impurities, giving results comparable with those obtained with lower vapour pressure rare earths under vacuum, but the procedures used were not successful with carbon because of contamination. (3) In erbium, oxygen, hydrogen, nitrogen and probably carbon impu~ties migrate in the cathode to anode direction during SSE.

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(4) The presence of fluorine at relatively high levels in the start metal, and its apparent inhomogeneous distribution after SSE, need to be investigated further. (5) The growth of dendritically shaped whiskers on the SSE sample, due to the recondensation of erbium vapour, severely affected the temperature profile along the sample and limited the time for which it could be processed under argon.

Acknowledgments Financial support for the work carried out at the University of Birmingham came from the U.K. Science and Engineering Research Council, while that at the Ames Laboratory, Iowa State University, came from the U.S. Department of Energy, Director of Energy Research, Office of Basic Energy Science under Contract No. W-7405ENG-82.

References 1 B. J. Beaudry and K. A. Gschneidner, Jr., in K. A. Gschneidner, Jr. and L. Eyring (eds.), Handbook on the Physics and Chemistry of Rare Earths, Vol. 1, North-Holland, Amsterdam, 1978, p. 173. 2 D. T. Peterson, in R. E. Hummel and H. B. Huntington (eds.), Electra- and Thermo-transport in Metals and Alloys, American Institute of Mining and Petroleum Engineers, New York, 1977, p. 54. 3 D. Fort, J. Less-Common Met., I34 (1987) 45. 4 R. G. Jordan, D. W. Jones and M. G. Hall, J. Crysf. Growth, 24-25 (1974) 568. 5 K. Maezawa, Y. Saito and S. Wakabayashi, Jpn. J. Appl. Phys., 24 (1985) 28. 6 F. A. Schmidt and 0. N. Carlson, Met. Trans. A, 7( 1976) 127. 7 D. T. Peterson, in A. Lodding and T. Lagerwall (eds.), Atomic Transport in Solids and Liquids, Verlag der Zeitschrift fur Naturforschung, Tubingen, 1971, p. 104. 8 0. N. Carlson and F. A. Schmidt, J. Less-Common Met., 53 (1977) 73. 9 R. G. Jordan, Contemp. Phys., 15 (1974) 375. 10 0. N. Carlson, F. A. Schmidt and D. T. Peterson, J. Less-Common Met., 39( 1975) 277. 11 R. G. Jordan and D. W. Jones, J. Less-Common Met., 31(1973) 125. 12 D. Fort, B. J. Beaudry and K. A. Gschneidner, Jr., J. Less-Common Met., I34 (1987) 27.