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Surface segregation in binary alloy first wall candidate materials
831
Journal of Nuclear Materials 111 L 112 (1982) 831-838 North-Holland Publishing Company
SURFACE D.M.
SEGREGATION
GRUEN,
A.R.
IN BINARY ALLOY FIRST WALL CANDIDATE
KRAUSS,
M.H.
MENDELSOHN
and S. SUSMAN
Chemistry Division and f Solid State Science Division, Argonne National Laboratory,
MATERIALS
t
Argonne, Illinois 60439, USA
We have been studying the conditions necessary to produce a self-sustaining stable lithium monolayer on a metal substrate as a means of creating a low-Z film which sputters primarily as secondary ions. It is expected that because of the toroidal field, secondary ions originating at the first wall will be returned and contribute little to the plasma impurity influx [1,2]. Aluminum and copper have, because of their high thermal conductivity and low induced radioactivity, been proposed [3-51 as first wall candidatematerials. The mechanical properties of the pure metals are very poorly suited to structural applications and an alloy must be used to obtain adequate hardness and tensile strength. In the case of aluminum, mechanical properties suitable for aircraft manufacture are obtained by the addition of a few at% Li. In order to investigate alloys of a similar nature as candidate structural materials for fusion machines we have prepared samples of Li-doped aluminum using both a pyro-metallurgical and a vapor-diffusion technique. The sputtering properties and surface composition have been studied as a function of sample temperature and heating time, and ion beam mass. The erosion rate and secondary ion yield of both the sputtered Al and Li have been monitored by secondary ion mass spectroscopy and Auger analysis providing information on surface segregation, depth composition profiles, and diffusion rates. The surface composition and lithium depth profiles are compared with previously obtained computational results based on a regular solution model of segregation, while the partial sputtering yields of Al and Li are compared with results obtained with a modified version of the TRIM computer program.
1. Introduction One of the principal factors limiting the performance of magnetic fusion devices is the energy lost to line radiation from plasma impurities arising from the erosion of structural components. A significant and perhaps dominant mechanism for impurity release is that of sputtering. In current devices the erosion appears to be principally the result of self-sputtering, although it is to be expected that as the plasma parameters approach ignition, light ion sputtering will become dominant [6,7]. Since the energy lost to line radiation increases very rapidly with the atomic number of the impurity atoms, it is especially important to limit the concentration of high-Z atoms in the plasma. There are basically two approaches to this problem: One is to use high-Z materials but limit their influx to the plasma either by choosing materials with a low sputtering yield, or to use a device such as a divertor to prevent sputtered high-Z atoms from reaching the plasma. If however, the selfsputtering yield is greater than unity at the energy at which redeposited impurity atoms impinge on the device surfaces, there is a possibility of run-away selfsputtering even if the light ion sputtering yield is very low. 0022-3 115/82/0000-0000/$02.75
0 1982 North-Holland
An alternative approach is to use low-Z materials on all surfaces exposed to the plasma. The thermal and mechanical properties of most low-Z materials are such that it is necessary to use them in the form of a coating deposited on or bonded to a structural substrate. A number of problems associated with the interface then occur as a result of thermal shock and build-up of implanted hydrogen and helium. The lifetime of the coating is also limited by factors relating to redeposition and mechanical integrity of the coating. It is therefore desirable to keep the initial coating as thin as possible, provided that it limits substrate sputtering, and to provide a means of replenishing the low-Z material at locations where it is preferentially eroded. Several ideas have been suggested as a means of providing redeposition in a self-sustaining manner. Norem (81 has proposed the use of pellet ablation by the plasma as a means of recoating the surface, and has estimated that six to ten atomic monolayers are sufficient to protect the substrate. Rehn et al. [9] have suggested that radiation-induced segregation might be used to produce self-healing low-Z coatings. Krauss and Gruen [ 1, lo] have investigated the possible role of Gibbsian segregation in producing self-sustaining monolayer films of alkali metal in dilute alkali
832
D.M. Gmen et al. / Surface segregarion
metal-bearing alloys. Lithium was chosen as the alkali metal component for detailed investigation because of its low atomic number and because it forms a stable alloy with a number of metals. Aluminum and copper have been chosen as the host metals because they alloy readily with lithium and because they are expected to provide a strong driving force for Gibbsian segregation. Aluminum and copper have also been proposed as candidate first wall materials [3-51, although we prefer to regard these alloys more as a convenient means of studying fundamental processes. Aluminum was chosen for the present experiment in order to facilitate comparison with previous theoretical results. The model of Williams and Nason [ 1 l] was used to calculate the Gibbsian segregation profile of lithium in a dilute lithium-bearing aluminum alloy. The surface concentration of lithium was calculated to be nearly 94%, the concentration of the second layer was actually below that of the bulk, and the concentration had essentially reached the bulk value by the third atomic layer. Such a narrow profile may lead one to question whether the low-Z layer is thick enough to provide adequate shielding of the substrate material. A modified version of the TRIM computer code was used to calculate the depth of origin of the atoms sputtered from a
.lO
Is;*
,05
?‘I;; ,005
I .I
.
M/AI-L,: “;=z.w ev
A
LI/AI-LI,
H,=l.b7
eV
0 o
LI/AI-LI,
HS=2.17
eV
LI/AI-Ll.
Hs=2,67
0
I
I
I
.2
.5
1.0
I 2.0
E0 CkeV) Fig. 1, Results of TRIM calculation for the deuterium sputtering yield of aluminum from bulk aluminum, and aluminum and lithium from a sample consisting of one monolayer of lithium on an aluminum substrate.
variety of homogeneous and layered materials. Fig. I shows the results for a monolayer of lithium on an aluminum substrate. The relative sputtering yields obtained with TRIM have been found to be accurate to within 20-3056 although the absolute yields are considerably less accurate. The effective surface binding energy for a lithium monolayer on aluminum has not yet been determined, so the calculation was carried out for three different heats of sublimation. The figure shows the calculated sputtering yield of lithium and aluminum for 0.1-2 keV deuterons incident on a pure aluminum target, and an aluminum target with a lithium overlayer. The principal result is that, depending on the energy of the deuteron, the aluminum sputtering yield is reduced by a factor of 3-10 X by the overlayer. The lithium overlayer has about the same sputtering yield as does aluminum from a bare aluminum surface. Consequently the effect of the overlayer is essentially that of replacing aluminum erosion by the sputtering of lithium. Since the lithium has much lower atomic number than aluminum and because it is expected to sputter with a larger percentage of secondary ions, the coated surface is expected to produce less plasma contamination [ 1,2]. Experimentally. Krauss and Gruen [12] have found that for low energy (E Q 1 keV) light ion bombardment (Z < 2) almost all the sputtered atoms originate in the uppermost atomic layer. By comparing relative erosion rates of monolayer and multilayer films of K on MO, it was found that the monolayer sputtered more slowly than the multilayer film and that the multilayer film sputtered as neutral atoms whereas the monolayer sputtered predominantly as ions. The difference in sputtering yield for the monolayer versus the multilayer film was interpreted in terms of a variation in surface binding energy for the first and second potassium overlayers and a first layer binding energy was derived which was in reasonable agreement with the thermal desorption measurements of Langmuir and Kingdon [ 131. Subsequent TRIM calculations however, indicate that although the surface binding energy of the second layer is probably close to that of bulk potassium, the sputtering yield of a potassium bi-layer should be significantly higher than that of the bulk because the heavy substrate is more effective at directing particle motion away from the surface. In a Tokamak environment, radiation effects are likely to substantially alter the surface and near-surface composition [ 141 of a Li-bearing alloy. Recoil implantation will broaden the Li-enriched region and preferential sputtering may reduce the surface lithium concentration. Since the monolayer lifetime in a device such as FED is on the order of one second, the rate at which
D.M. Gwen
ei al. / Surfacesegregation
segregation occurs can be very important. Radiation-enhanced diffusion can substantially increase the segregation rate, especially since there will be a high defect production rate caused by the fusion neutrons which do not cause significant sputtering. Radiation-induced segregation may result in the enhancement of either component of the binary alloy. Characteristically, the enhanced region will extend very deeply into the bulk
833
placed into a 600°C furnace for - 120 h and cooled to room temperature for - 6 h. Wet chemical analysis yielded 0.32 at% Li, in reasonable agreement with the estimates based on electrochemical extrapolation.
3. Experimental
[151. 2. Sample preparation The preparation of homogeneous, solid solution samples of Li in Al was accomplished by an isothermal diffusion process. The activity of Li in Al metal was estimated from electrochemical data [16]. From the Nernst equation and the known vapor pressure of Li, it was estimated that at 6OO’C an equilibrium Li concentration of 1.3 at% would be obtained by diffusion from the vapor phase. The diffusion vessel is a heavy-walled 310 SS bomb with a pyrolitic BN liner. An annealed Ni gasket forms a pressure tight seal with the 90’ knife edge. The bomb components are vacuum baked at 1000°C and lop6 Torr. All manipulations proceeded in a glove box with a purified, recirculating Ar atmosphere. The assembly was
The experimental layout is shown in fig. 2. The sample is mounted on a heatable stage in an ultra-high vacuum system containing a double-pass cylindrical mirror electron energy analyzer with coaxial electron gun, a differentially pumped ion gun, a secondary ion mass and energy analyzer, and two Faraday cups. Cine Faraday cup (not shown) is rigidly fixed relative to the sample center and can be moved into the normal sample position where it is used to established beam alignment. The ion beam is incident at an angle of 11.25’ to the sample normal and the electron beam is incident at an angle of 56.25“. It is then possible to perform simultaneous Auger (AES) and secondary ion (SIMS) spectroscopy without moving the sample. Scan control and data acquisition are controlled by an LSI-11 microcomputer. The electron beam and ion positions can be adjusted manually, rastered, or placed under program control by the computer. In the present
DPCMA
Fig. 2. Experimental KS = ion scattering
setup: CMA = cylindrical mirror electron energy analyzer, spectrometer, SIMS = secondary ion mass spectrometer.
FC = Faraday
cup,
DIG = differential
ion gun,
834
D.M. Gruen et al.
/
Surface segregation
experiment, the ion beam was rastered over an area of 6.15 mm2 to minimize crater edge effects in depth-profiling and the electron beam was sequentially stepped over a rectangular array of points on a grid which encompassed most of the area rastered by the ion beam. The secondary ion and Auger scans were multiplexed so that the ‘Li and 27A1 secondary ion mass peaks were sampled sequentially. Concurrently, sequential scans were made of the Auger energy windows corresponding to the Li(KLL), Al(LMM), O(KLL), and Al(KLL) Auger lines. After an initial outgassing period, no oxygen was detected on the sample surface. The erosion of a monolayer film proceeds according to the expression dn/dt
= NdO,‘dt,
Because of the low energies corresponding to the Li(KLL) and Al(LMM) Auger transitions (43 and 67 eV respectively), the mean free path of the Auger electrons is less than 5 Angstroms, and it was therefore anticipated that Auger spectroscopy would be sensitive to changes in the composition of the uppermost atomic layer.
4. Results The as-prepared sample was placed in a scanning electron microscope to check for gross deposits, and grain boundary precipitation. The grain size was found to be rather large, 200-300 microns in diameter, probably as a result of annealing during the lithium diffusion process. The grain boundaries were well-defined and showed no evidence of grain boundary precipitation. There were two large areas near the sample edge which had the appearance of deposits. However, scanning Auger analysis showed no detectable lithium on the surface. The two areas were identified as aluminum oxide. The sample was then placed in the sputtering chamber and simultaneous Auger (AES) and secondary ion mass spectroscopy (SIMS) data were recorded during heating and cooling cycles. The sample was initially cleaned by heating, followed by a prolonged period of room temperature sputter-etching. During this time the
(1)
where n is the number of adatoms adsorbed on the surface per unit area, N is the total number of atoms on the surface per unit area, and 0 = n/N is the fractional coverage of the surface by adatoms. In terms of the monolayer sputtering yield S, and the ion beam current density i, eq. (1) is equjvalent to dn/dt = - S(j/q)O. Consequently, @( 1) = 0, e-sjf/Nq
(2)
where t is the elapsed time, and q is the electronic charge. By plotting experimental values of In 0 (0 in arbitrary units) vs sputtering time, it is therefore possible to determine the monolayer sputtering yield.
AI-Li 6
,
l7.DAT
,
G; El
9.0
nA/mm=
700
eV
3 >
5_
Ar+ *
:
AI-Li
0.32%
*
2 E
*
4-
*
: e-----300 F? : >
E
3_
4-215 I
2-35
*
I’ **
C
> Ly
*
*
w
* *
**
2 5
C-----,
c+
*
I I_
*
***
*
*
ii ,+*****
+ <
0;,*
* ’
’
* ’ 80
* j
a
’
ELAPSED
Fig. 3. Lithium
secondary
ion yield during
’ 85 TIME
a heating
*
*
’
’ 90
’
’
a
95
sequence
from room temperature
to 300°C.
835
D.M. Gruen et al. / Surface segregation
lithium secondary ion yield was monitored; etching was continued until the Li+ signal appeared to have reached a stable minimum value which was taken as indicative of the bulk lithium concentration. It should be noted that since diffusion of defects occur in aluminum at room temperature, this minimum lithium secondary ion signal represents at best, an upper limit for the bulk lithium concentration. As long as the lithium surface concentration is sufficiently low that there is a preponderance of Li-Al rather than Li-Li bonds, it is to be expected that the lithium secondary ion fraction will remain constant. Since lithium is less electronegative than aluminum, it is possible that there will be some tendency to produce a reduction in the secondary ion fraction of the aluminum sputtered from the alloy although the amount of reduction is not expected to be significantly greater than the lithium surface concentration. A typical heating sequence is shown in fig. 3. The Lit signal started to rise as soon as the sample heater
+ + -+
9.0
+ x
700 AI-LI
nA/mmZ e”
was turned on, reaching equilibrium about the same time the sample reached thermal equilibrium. At 215”C, the Li+ signal was double what it had been at room temperature. Upon further heating, the lithium secondary ion yield increased further, again reaching equilibrium at the same time the sample reached thermal equilibrium. At 300°C the Lif signal was six times greater than the starting value. The lithium surface concentration at 3OO’C should then be 1.9%, assuming that the lithium secondary ion fraction is constant in this concentration range, and that the initial secondary ion signal corresponded to the bulk lithium concentration of 0.32%. In accord with this expectation, the aluminum secondary ion yield was unchanged throughout the course of the run. Upon further heating, (fig. 4) the Li+ signal again increased, but continued to increase long after the sample had reached thermal equilibrium. The Li secondary ion yield eventually equilibrated at a value 22 times greater than the starting value, corresponding to an expected surface concentration of 7%. Concurrently, the aluminum secondary ion yield dropped to 45% of its initial value. The observed increase in the lithium secondary ion yield is in good agreement with the 7.5% lithium concentration obtained by Auger spectroscopy (AES) after
Ar+ 0.32%
\ .L++
Table
1
Experimental
Fig. 4. Lithium and aluminum secondary heating sequence from 300°C to 440°C.
ion yield during
a
and Predicted
Sputter
Yield Values a)
Beam/Target
Experimental yield
TRIM yield
700 eV He+/Al 700 eV He+/Li 700 eV He+/Al-Li room Li+: temp. Al+: 490°C Li+: Al+: 700 eV Ar+/AI 700 eV Ar+/Li 700 eV Ar+/Al-Li 520°C Li: Al:
0.21 b’
0.31 0.15
0.23 ”
0.23 ” 0.08 0.30 e’ 0.08 0.91 0.03
0.44
1.24d’
0.77 e) 0.19
‘) From the present work unless otherwise noted. b, J. Roth, J. Bohdansky and W. Ottenberger, ref. [18]. ‘) Sample was heated lo 490°C and then cooled before turning ion beam on. d, N. Laegreid and G.K. Wehner, ref. [19]. ‘) Eb=2.17 eV assumed for a lithium monolayer on an aluminum substrate.
836
D.M.
Gruen et al. / Surface segregation
the sample had been heated. However, after the initial heating period the lithium concentration value obtained by AES became nearly constant and did not exhibit the variation found for the lithium secondary ion signal. Examination in the scanning electron microscope after the conclusion of the experiment revealed a rectangular array of very small holes with regular spacing, approximately centered on the crater caused by the ion beam when the raster was turned off during beam alignment. The array constituted a 4 X 5 point grid, with the spacing in one direction just double that of the spacing in the other direction, corresponding to the computer-controlled positions of the electron beam incident on the sample at an angle of 56.25” to the normal. The Auger analysis therefore appears to have been monitoring the lithium concentration in the near-subsurface rather than the surface concentration. The sputtering yield calculated by TRIM for 700 eV Ar+ incident on bare aluminum is 0.91, and the value for the aluminum sputtering yield from aluminum covered by a monolayer of lithium is 0.19 (table 1). Assuming that the drop in the aluminum secondary ion yield shown in fig. 4 was solely the result of decreased aluminum sputtering associated with a lithium overlayer. we can estimate the required lithium coverage by means of the relation S,, =0.91(1
-O,,)
+ 0.19 o,,,
(3)
where O,, is the fraction of the surface which is covered by lithium. The observed reduction in the aluminum secondary ion yield then corresponds to OLi = 0.69. This value is clearly inconsistent with the previous estimate of the lithium surface concentration. It should be borne in mind however, that eq. (3) is based on the assumption that the aluminum secondary ion fraction is independent of lithium coverage. The validity of this assumption will be checked in future experiments by the use of ion scattering spectroscopy (ISS). In order to determine the depth profile of the Li-enriched surface region, the sample was heated under ion bombardment to 49O’C. The heater and ion beam were switched off once the Li+ signal had attained equilibrium. When the sample temperature had dropped below 2OO”C, the ion beam was switched back on and a depth profile of the lithium was made by very rapidly scanning the Li and Al secondary ion mass peaks. The resultant profile is shown in fig. 5 for 700 eV helium ion bombardment. Immediately after turning the ion beam back on, the lithium secondary ion yield was found to be nearly double what it had been prior to turning the beam off. The signal decayed rapidly to a level which was higher than the initial value and then decayed very
*
7u1r-, 23
33 ELAPSED
Fig. 5. Lithium from 49O’C.
secondary
43 TIME
53
i3
CSEC/l000)
ion profile during cooling sequence
slowly with depth. The narrow portion of the depth profile has been analyzed in accord with the procedure presented in the previous section for determining monolayer sputtering yields. The resulting value for the sputtering yield is 0.23, in excellent agreement with the value predicted by TRIM for a lithium monolayer on an aluminum substrate (table 1). The time required to reach the depth at which the signal decays slowly is approximately twice the e-folding time for the monolayer removal as defined in the previous section, for the experimentally determined sputtering yield. A similar result with a higher sputtering yield (table 1) was obtained for a profile obtained with the sample maintained at a temperature of 49O”C, indicating that thermal segregation brought additional lithium to the surface during the monolayer removal time, and that during sputtering, the equilibrium surface concentration of lithium is lower than that obtained by Gibbsian segregation alone. In order to investigate the enrichment of lithium in the sub-surface region, the sample was heated to 520°C thermally quenched with the ion beam off and then profiled with a 700 eV Ar+ beam (fig. 6a). In accord with previous measurements, the lithium secondary ion yield increased rapidly during the heating cylce and continued to increase at a slower rate once the sample reached thermal equilibrium. The ion beam and sample heater were turned off simultaneously, and the ion beam
D. M. Gwen et al. / Surface segregation
837
5. Conclusion
1
,+,+++&+ ++++ P
+++
+++ +i +
I
+T++
+*
CL
0 -100
200 ELAPSED
300 TIME
I----&+ 400 500
Fig. 6. (a) Lithium secondary ion room temperature profile. (b) Aluminum secondary ion room temperature profile.
depth depth
was turned back on when the sample temperature reached 1OO“C. The time resolution employed was not sufficient to observe the initial secondary ion yield, but the same phenomenon of a very fast decay followed by a much slower drop in the lithium secondary ion yield was observed. An approximate depth scale was obtained assuming a sputtering yield corresponding to that of pure aluminum, and the depth of the altered layer was found to exceed 3000 A. The aluminum secondary ion profile (fig. 6b) was complementary to the Li+ profile, although it exhibited a region about 140 A thick in which the Al+ yield changed rapidly with profiled depth. Related structure can be seen in fig. 6a at the point where this region ends.
At temperatures between 200 and 5OO“C lithium segregates to the surface and near-surface regions of a dilute aluminum alloy, forming two and possibly three distinct regions of altered composition. Gibbsian segregation produces a single monolayer of enriched lithium concentration in accord with the bond enthalpy model of segregation. Radiation-induced segregation produces a region of enhanced lithium concentration extending approximately 3000 Angstroms in from the surface, in accord with observations made on transition metal alloys [ 151. For argon ion bombardment a third region is observed extending approximately 100 Angtroms into the material. This distance is roughly commensurate with the damage layer and is tentatively identified as a region in which enhanced defect diffusion carries lithium to the surface. This region is not observed for helium bombardment, consistent with the helium-induced damage profile which extends over a very long distance and corresponds to a low defect production rate at all points along the profile. The initial segregation occurs very quickly, essentially as fast as the sample could be heated. Subsequent enhancement of the surface and near-surface lithium concentration requires an increased bulk diffusion rate which occurs as a result of radiation enhanced diffusion and radiation-induced segregation but requires an “induction period”. The equilibrium surface lithium concentration during irradiation was lower than the concentration determined solely by Gibbsian segregation, but the extent to which neutron irradiation in a Tokamak would increase the diffusion rate and consequent lithium surface concentration has not yet been determined. It was not possible to quantitatively determine either the absolute concentration or the secondary ion fraction of the sputtered lithium. Roughly speaking, the lithium surface concentration appears to lie between 7 and 70%, and the secondary ion fraction lies between 1 and 10 times that of the sputtered aluminum. The published value for the ion fraction of sputtered aluminum is 2% [17]. These values are derived on the assumption that the lithium and aluminum secondary ion fractions are independent of surface composition, and the range of uncertainty of the experimental values appears to indicate that the assumption is invalid. The surface properties of alloys as they apply to Tokamak first wall, limiter and divertor surfaces are rather complicated, but it appears to be possible to make use of some of these properties to reduce plasma contamination.
838
D.M.
Gruen et al. / Surface
Acknowledgement The authors would like to acknowledge the contribution of A.B. DeWald for developing the multilayer, multicomponent version of TRIM.
References [1] Alan R. Krauss and Dieter M. Gruen, Proc. Second. Top. Conf. Fusion Reactor Materials, Seattle, 198 1. [2] A.B. DeWald, J.N. Davidson, A.R. Krauss and D.M. Gruen, these proceedings. [3] O.K. Harling, G.P. Yu, J.E. Meyer and N.J. Grant, Proc. Second Top. Conf. Fusion Reactor Materials, Seattle, 1981. [4] Yu. M. Patov et al., Fizika i Khimia Obrabotki Materialov (Akad. Nauk., USSR, 1981) p. 53. [S] K. Miyaharea, these proceedings. [6] C.H. Muller III and K.H. Burrell, Phys. Rev. Letters 47 (1981) 330; also R.A. Zuhr, J.B. Roberto and S.P. Withrow, Proc. 2nd Top. Meeting Fusion Reactor Materials, Seattle, 1981. [7] R. Behrisch, Proc. Symp. on Atomic and Surface Physics, Maria Alm (Austria), 1980.
segregation
[8] J.H. Norem and B.A. Cramer. Proc. 2nd. Top. Meeting Fusion Reactor Reactor Materials, Seattle, 1981. [9] L.E. Rehn. P.R. Okamoto, D.I. Potter and H. Wiedersich, J. Nucl. Mater. 85/86 (1979) 1139. [lo] A.R. Krauss. D.M. Gruen and A.B. DeWald, Proc. 9th Symp. on Engineering Problems of Fusion Research, Chicago, 1981. [ 111 F.L. Williams and D. Nason, Surface Sci. 45 (1974) 377. [12] A.R. Krauss and D.M. Gruen. J. Nucl. Mater. 93/94 (1980) 686. [ 131 I. Langmuir and K.H. Kingdon. Proc. Roy. Sot. (London) A107 (1925) 61. [14] H. Wiedersich, to be published in: Surface Modification and Alloying, Eds. J.M. Poate and G. Foti. NATO Materials Science Series (Plenum, New York). [15] L.E. Rehn. S. Danyluk and H. Wiedersich. Phys. Rev. Letters 43 (1979) 1764. [16] C.J. Wen, W. Weppner. B.A. Boukamp and R.A. Huggins. Met. Trans. B, IlB (1980) 131. [ 171 A. Benninghoven, Surface Sci. 35 (1973) 457. (181 J. Roth, J. Bohdansky and W. Ottenberger. Max Planck Institut fur Plasmaphysik Report IPP 9/26, (1979). [19] N. Laegreid and G.K. Wehner, J. Appl. Phys. 32 (1961) 365.
Journal of Nuclear Materials 111 L 112 (1982) 831-838 North-Holland Publishing Company
SURFACE D.M.
SEGREGATION
GRUEN,
A.R.
IN BINARY ALLOY FIRST WALL CANDIDATE
KRAUSS,
M.H.
MENDELSOHN
and S. SUSMAN
Chemistry Division and f Solid State Science Division, Argonne National Laboratory,
MATERIALS
t
Argonne, Illinois 60439, USA
We have been studying the conditions necessary to produce a self-sustaining stable lithium monolayer on a metal substrate as a means of creating a low-Z film which sputters primarily as secondary ions. It is expected that because of the toroidal field, secondary ions originating at the first wall will be returned and contribute little to the plasma impurity influx [1,2]. Aluminum and copper have, because of their high thermal conductivity and low induced radioactivity, been proposed [3-51 as first wall candidatematerials. The mechanical properties of the pure metals are very poorly suited to structural applications and an alloy must be used to obtain adequate hardness and tensile strength. In the case of aluminum, mechanical properties suitable for aircraft manufacture are obtained by the addition of a few at% Li. In order to investigate alloys of a similar nature as candidate structural materials for fusion machines we have prepared samples of Li-doped aluminum using both a pyro-metallurgical and a vapor-diffusion technique. The sputtering properties and surface composition have been studied as a function of sample temperature and heating time, and ion beam mass. The erosion rate and secondary ion yield of both the sputtered Al and Li have been monitored by secondary ion mass spectroscopy and Auger analysis providing information on surface segregation, depth composition profiles, and diffusion rates. The surface composition and lithium depth profiles are compared with previously obtained computational results based on a regular solution model of segregation, while the partial sputtering yields of Al and Li are compared with results obtained with a modified version of the TRIM computer program.
1. Introduction One of the principal factors limiting the performance of magnetic fusion devices is the energy lost to line radiation from plasma impurities arising from the erosion of structural components. A significant and perhaps dominant mechanism for impurity release is that of sputtering. In current devices the erosion appears to be principally the result of self-sputtering, although it is to be expected that as the plasma parameters approach ignition, light ion sputtering will become dominant [6,7]. Since the energy lost to line radiation increases very rapidly with the atomic number of the impurity atoms, it is especially important to limit the concentration of high-Z atoms in the plasma. There are basically two approaches to this problem: One is to use high-Z materials but limit their influx to the plasma either by choosing materials with a low sputtering yield, or to use a device such as a divertor to prevent sputtered high-Z atoms from reaching the plasma. If however, the selfsputtering yield is greater than unity at the energy at which redeposited impurity atoms impinge on the device surfaces, there is a possibility of run-away selfsputtering even if the light ion sputtering yield is very low. 0022-3 115/82/0000-0000/$02.75
0 1982 North-Holland
An alternative approach is to use low-Z materials on all surfaces exposed to the plasma. The thermal and mechanical properties of most low-Z materials are such that it is necessary to use them in the form of a coating deposited on or bonded to a structural substrate. A number of problems associated with the interface then occur as a result of thermal shock and build-up of implanted hydrogen and helium. The lifetime of the coating is also limited by factors relating to redeposition and mechanical integrity of the coating. It is therefore desirable to keep the initial coating as thin as possible, provided that it limits substrate sputtering, and to provide a means of replenishing the low-Z material at locations where it is preferentially eroded. Several ideas have been suggested as a means of providing redeposition in a self-sustaining manner. Norem (81 has proposed the use of pellet ablation by the plasma as a means of recoating the surface, and has estimated that six to ten atomic monolayers are sufficient to protect the substrate. Rehn et al. [9] have suggested that radiation-induced segregation might be used to produce self-healing low-Z coatings. Krauss and Gruen [ 1, lo] have investigated the possible role of Gibbsian segregation in producing self-sustaining monolayer films of alkali metal in dilute alkali
832
D.M. Gmen et al. / Surface segregarion
metal-bearing alloys. Lithium was chosen as the alkali metal component for detailed investigation because of its low atomic number and because it forms a stable alloy with a number of metals. Aluminum and copper have been chosen as the host metals because they alloy readily with lithium and because they are expected to provide a strong driving force for Gibbsian segregation. Aluminum and copper have also been proposed as candidate first wall materials [3-51, although we prefer to regard these alloys more as a convenient means of studying fundamental processes. Aluminum was chosen for the present experiment in order to facilitate comparison with previous theoretical results. The model of Williams and Nason [ 1 l] was used to calculate the Gibbsian segregation profile of lithium in a dilute lithium-bearing aluminum alloy. The surface concentration of lithium was calculated to be nearly 94%, the concentration of the second layer was actually below that of the bulk, and the concentration had essentially reached the bulk value by the third atomic layer. Such a narrow profile may lead one to question whether the low-Z layer is thick enough to provide adequate shielding of the substrate material. A modified version of the TRIM computer code was used to calculate the depth of origin of the atoms sputtered from a
.lO
Is;*
,05
?‘I;; ,005
I .I
.
M/AI-L,: “;=z.w ev
A
LI/AI-LI,
H,=l.b7
eV
0 o
LI/AI-LI,
HS=2.17
eV
LI/AI-Ll.
Hs=2,67
0
I
I
I
.2
.5
1.0
I 2.0
E0 CkeV) Fig. 1, Results of TRIM calculation for the deuterium sputtering yield of aluminum from bulk aluminum, and aluminum and lithium from a sample consisting of one monolayer of lithium on an aluminum substrate.
variety of homogeneous and layered materials. Fig. I shows the results for a monolayer of lithium on an aluminum substrate. The relative sputtering yields obtained with TRIM have been found to be accurate to within 20-3056 although the absolute yields are considerably less accurate. The effective surface binding energy for a lithium monolayer on aluminum has not yet been determined, so the calculation was carried out for three different heats of sublimation. The figure shows the calculated sputtering yield of lithium and aluminum for 0.1-2 keV deuterons incident on a pure aluminum target, and an aluminum target with a lithium overlayer. The principal result is that, depending on the energy of the deuteron, the aluminum sputtering yield is reduced by a factor of 3-10 X by the overlayer. The lithium overlayer has about the same sputtering yield as does aluminum from a bare aluminum surface. Consequently the effect of the overlayer is essentially that of replacing aluminum erosion by the sputtering of lithium. Since the lithium has much lower atomic number than aluminum and because it is expected to sputter with a larger percentage of secondary ions, the coated surface is expected to produce less plasma contamination [ 1,2]. Experimentally. Krauss and Gruen [12] have found that for low energy (E Q 1 keV) light ion bombardment (Z < 2) almost all the sputtered atoms originate in the uppermost atomic layer. By comparing relative erosion rates of monolayer and multilayer films of K on MO, it was found that the monolayer sputtered more slowly than the multilayer film and that the multilayer film sputtered as neutral atoms whereas the monolayer sputtered predominantly as ions. The difference in sputtering yield for the monolayer versus the multilayer film was interpreted in terms of a variation in surface binding energy for the first and second potassium overlayers and a first layer binding energy was derived which was in reasonable agreement with the thermal desorption measurements of Langmuir and Kingdon [ 131. Subsequent TRIM calculations however, indicate that although the surface binding energy of the second layer is probably close to that of bulk potassium, the sputtering yield of a potassium bi-layer should be significantly higher than that of the bulk because the heavy substrate is more effective at directing particle motion away from the surface. In a Tokamak environment, radiation effects are likely to substantially alter the surface and near-surface composition [ 141 of a Li-bearing alloy. Recoil implantation will broaden the Li-enriched region and preferential sputtering may reduce the surface lithium concentration. Since the monolayer lifetime in a device such as FED is on the order of one second, the rate at which
D.M. Gwen
ei al. / Surfacesegregation
segregation occurs can be very important. Radiation-enhanced diffusion can substantially increase the segregation rate, especially since there will be a high defect production rate caused by the fusion neutrons which do not cause significant sputtering. Radiation-induced segregation may result in the enhancement of either component of the binary alloy. Characteristically, the enhanced region will extend very deeply into the bulk
833
placed into a 600°C furnace for - 120 h and cooled to room temperature for - 6 h. Wet chemical analysis yielded 0.32 at% Li, in reasonable agreement with the estimates based on electrochemical extrapolation.
3. Experimental
[151. 2. Sample preparation The preparation of homogeneous, solid solution samples of Li in Al was accomplished by an isothermal diffusion process. The activity of Li in Al metal was estimated from electrochemical data [16]. From the Nernst equation and the known vapor pressure of Li, it was estimated that at 6OO’C an equilibrium Li concentration of 1.3 at% would be obtained by diffusion from the vapor phase. The diffusion vessel is a heavy-walled 310 SS bomb with a pyrolitic BN liner. An annealed Ni gasket forms a pressure tight seal with the 90’ knife edge. The bomb components are vacuum baked at 1000°C and lop6 Torr. All manipulations proceeded in a glove box with a purified, recirculating Ar atmosphere. The assembly was
The experimental layout is shown in fig. 2. The sample is mounted on a heatable stage in an ultra-high vacuum system containing a double-pass cylindrical mirror electron energy analyzer with coaxial electron gun, a differentially pumped ion gun, a secondary ion mass and energy analyzer, and two Faraday cups. Cine Faraday cup (not shown) is rigidly fixed relative to the sample center and can be moved into the normal sample position where it is used to established beam alignment. The ion beam is incident at an angle of 11.25’ to the sample normal and the electron beam is incident at an angle of 56.25“. It is then possible to perform simultaneous Auger (AES) and secondary ion (SIMS) spectroscopy without moving the sample. Scan control and data acquisition are controlled by an LSI-11 microcomputer. The electron beam and ion positions can be adjusted manually, rastered, or placed under program control by the computer. In the present
DPCMA
Fig. 2. Experimental KS = ion scattering
setup: CMA = cylindrical mirror electron energy analyzer, spectrometer, SIMS = secondary ion mass spectrometer.
FC = Faraday
cup,
DIG = differential
ion gun,
834
D.M. Gruen et al.
/
Surface segregation
experiment, the ion beam was rastered over an area of 6.15 mm2 to minimize crater edge effects in depth-profiling and the electron beam was sequentially stepped over a rectangular array of points on a grid which encompassed most of the area rastered by the ion beam. The secondary ion and Auger scans were multiplexed so that the ‘Li and 27A1 secondary ion mass peaks were sampled sequentially. Concurrently, sequential scans were made of the Auger energy windows corresponding to the Li(KLL), Al(LMM), O(KLL), and Al(KLL) Auger lines. After an initial outgassing period, no oxygen was detected on the sample surface. The erosion of a monolayer film proceeds according to the expression dn/dt
= NdO,‘dt,
Because of the low energies corresponding to the Li(KLL) and Al(LMM) Auger transitions (43 and 67 eV respectively), the mean free path of the Auger electrons is less than 5 Angstroms, and it was therefore anticipated that Auger spectroscopy would be sensitive to changes in the composition of the uppermost atomic layer.
4. Results The as-prepared sample was placed in a scanning electron microscope to check for gross deposits, and grain boundary precipitation. The grain size was found to be rather large, 200-300 microns in diameter, probably as a result of annealing during the lithium diffusion process. The grain boundaries were well-defined and showed no evidence of grain boundary precipitation. There were two large areas near the sample edge which had the appearance of deposits. However, scanning Auger analysis showed no detectable lithium on the surface. The two areas were identified as aluminum oxide. The sample was then placed in the sputtering chamber and simultaneous Auger (AES) and secondary ion mass spectroscopy (SIMS) data were recorded during heating and cooling cycles. The sample was initially cleaned by heating, followed by a prolonged period of room temperature sputter-etching. During this time the
(1)
where n is the number of adatoms adsorbed on the surface per unit area, N is the total number of atoms on the surface per unit area, and 0 = n/N is the fractional coverage of the surface by adatoms. In terms of the monolayer sputtering yield S, and the ion beam current density i, eq. (1) is equjvalent to dn/dt = - S(j/q)O. Consequently, @( 1) = 0, e-sjf/Nq
(2)
where t is the elapsed time, and q is the electronic charge. By plotting experimental values of In 0 (0 in arbitrary units) vs sputtering time, it is therefore possible to determine the monolayer sputtering yield.
AI-Li 6
,
l7.DAT
,
G; El
9.0
nA/mm=
700
eV
3 >
5_
Ar+ *
:
AI-Li
0.32%
*
2 E
*
4-
*
: e-----300 F? : >
E
3_
4-215 I
2-35
*
I’ **
C
> Ly
*
*
w
* *
**
2 5
C-----,
c+
*
I I_
*
***
*
*
ii ,+*****
+ <
0;,*
* ’
’
* ’ 80
* j
a
’
ELAPSED
Fig. 3. Lithium
secondary
ion yield during
’ 85 TIME
a heating
*
*
’
’ 90
’
’
a
95
sequence
from room temperature
to 300°C.
835
D.M. Gruen et al. / Surface segregation
lithium secondary ion yield was monitored; etching was continued until the Li+ signal appeared to have reached a stable minimum value which was taken as indicative of the bulk lithium concentration. It should be noted that since diffusion of defects occur in aluminum at room temperature, this minimum lithium secondary ion signal represents at best, an upper limit for the bulk lithium concentration. As long as the lithium surface concentration is sufficiently low that there is a preponderance of Li-Al rather than Li-Li bonds, it is to be expected that the lithium secondary ion fraction will remain constant. Since lithium is less electronegative than aluminum, it is possible that there will be some tendency to produce a reduction in the secondary ion fraction of the aluminum sputtered from the alloy although the amount of reduction is not expected to be significantly greater than the lithium surface concentration. A typical heating sequence is shown in fig. 3. The Lit signal started to rise as soon as the sample heater
+ + -+
9.0
+ x
700 AI-LI
nA/mmZ e”
was turned on, reaching equilibrium about the same time the sample reached thermal equilibrium. At 215”C, the Li+ signal was double what it had been at room temperature. Upon further heating, the lithium secondary ion yield increased further, again reaching equilibrium at the same time the sample reached thermal equilibrium. At 300°C the Lif signal was six times greater than the starting value. The lithium surface concentration at 3OO’C should then be 1.9%, assuming that the lithium secondary ion fraction is constant in this concentration range, and that the initial secondary ion signal corresponded to the bulk lithium concentration of 0.32%. In accord with this expectation, the aluminum secondary ion yield was unchanged throughout the course of the run. Upon further heating, (fig. 4) the Li+ signal again increased, but continued to increase long after the sample had reached thermal equilibrium. The Li secondary ion yield eventually equilibrated at a value 22 times greater than the starting value, corresponding to an expected surface concentration of 7%. Concurrently, the aluminum secondary ion yield dropped to 45% of its initial value. The observed increase in the lithium secondary ion yield is in good agreement with the 7.5% lithium concentration obtained by Auger spectroscopy (AES) after
Ar+ 0.32%
\ .L++
Table
1
Experimental
Fig. 4. Lithium and aluminum secondary heating sequence from 300°C to 440°C.
ion yield during
a
and Predicted
Sputter
Yield Values a)
Beam/Target
Experimental yield
TRIM yield
700 eV He+/Al 700 eV He+/Li 700 eV He+/Al-Li room Li+: temp. Al+: 490°C Li+: Al+: 700 eV Ar+/AI 700 eV Ar+/Li 700 eV Ar+/Al-Li 520°C Li: Al:
0.21 b’
0.31 0.15
0.23 ”
0.23 ” 0.08 0.30 e’ 0.08 0.91 0.03
0.44
1.24d’
0.77 e) 0.19
‘) From the present work unless otherwise noted. b, J. Roth, J. Bohdansky and W. Ottenberger, ref. [18]. ‘) Sample was heated lo 490°C and then cooled before turning ion beam on. d, N. Laegreid and G.K. Wehner, ref. [19]. ‘) Eb=2.17 eV assumed for a lithium monolayer on an aluminum substrate.
836
D.M.
Gruen et al. / Surface segregation
the sample had been heated. However, after the initial heating period the lithium concentration value obtained by AES became nearly constant and did not exhibit the variation found for the lithium secondary ion signal. Examination in the scanning electron microscope after the conclusion of the experiment revealed a rectangular array of very small holes with regular spacing, approximately centered on the crater caused by the ion beam when the raster was turned off during beam alignment. The array constituted a 4 X 5 point grid, with the spacing in one direction just double that of the spacing in the other direction, corresponding to the computer-controlled positions of the electron beam incident on the sample at an angle of 56.25” to the normal. The Auger analysis therefore appears to have been monitoring the lithium concentration in the near-subsurface rather than the surface concentration. The sputtering yield calculated by TRIM for 700 eV Ar+ incident on bare aluminum is 0.91, and the value for the aluminum sputtering yield from aluminum covered by a monolayer of lithium is 0.19 (table 1). Assuming that the drop in the aluminum secondary ion yield shown in fig. 4 was solely the result of decreased aluminum sputtering associated with a lithium overlayer. we can estimate the required lithium coverage by means of the relation S,, =0.91(1
-O,,)
+ 0.19 o,,,
(3)
where O,, is the fraction of the surface which is covered by lithium. The observed reduction in the aluminum secondary ion yield then corresponds to OLi = 0.69. This value is clearly inconsistent with the previous estimate of the lithium surface concentration. It should be borne in mind however, that eq. (3) is based on the assumption that the aluminum secondary ion fraction is independent of lithium coverage. The validity of this assumption will be checked in future experiments by the use of ion scattering spectroscopy (ISS). In order to determine the depth profile of the Li-enriched surface region, the sample was heated under ion bombardment to 49O’C. The heater and ion beam were switched off once the Li+ signal had attained equilibrium. When the sample temperature had dropped below 2OO”C, the ion beam was switched back on and a depth profile of the lithium was made by very rapidly scanning the Li and Al secondary ion mass peaks. The resultant profile is shown in fig. 5 for 700 eV helium ion bombardment. Immediately after turning the ion beam back on, the lithium secondary ion yield was found to be nearly double what it had been prior to turning the beam off. The signal decayed rapidly to a level which was higher than the initial value and then decayed very
*
7u1r-, 23
33 ELAPSED
Fig. 5. Lithium from 49O’C.
secondary
43 TIME
53
i3
CSEC/l000)
ion profile during cooling sequence
slowly with depth. The narrow portion of the depth profile has been analyzed in accord with the procedure presented in the previous section for determining monolayer sputtering yields. The resulting value for the sputtering yield is 0.23, in excellent agreement with the value predicted by TRIM for a lithium monolayer on an aluminum substrate (table 1). The time required to reach the depth at which the signal decays slowly is approximately twice the e-folding time for the monolayer removal as defined in the previous section, for the experimentally determined sputtering yield. A similar result with a higher sputtering yield (table 1) was obtained for a profile obtained with the sample maintained at a temperature of 49O”C, indicating that thermal segregation brought additional lithium to the surface during the monolayer removal time, and that during sputtering, the equilibrium surface concentration of lithium is lower than that obtained by Gibbsian segregation alone. In order to investigate the enrichment of lithium in the sub-surface region, the sample was heated to 520°C thermally quenched with the ion beam off and then profiled with a 700 eV Ar+ beam (fig. 6a). In accord with previous measurements, the lithium secondary ion yield increased rapidly during the heating cylce and continued to increase at a slower rate once the sample reached thermal equilibrium. The ion beam and sample heater were turned off simultaneously, and the ion beam
D. M. Gwen et al. / Surface segregation
837
5. Conclusion
1
,+,+++&+ ++++ P
+++
+++ +i +
I
+T++
+*
CL
0 -100
200 ELAPSED
300 TIME
I----&+ 400 500
Fig. 6. (a) Lithium secondary ion room temperature profile. (b) Aluminum secondary ion room temperature profile.
depth depth
was turned back on when the sample temperature reached 1OO“C. The time resolution employed was not sufficient to observe the initial secondary ion yield, but the same phenomenon of a very fast decay followed by a much slower drop in the lithium secondary ion yield was observed. An approximate depth scale was obtained assuming a sputtering yield corresponding to that of pure aluminum, and the depth of the altered layer was found to exceed 3000 A. The aluminum secondary ion profile (fig. 6b) was complementary to the Li+ profile, although it exhibited a region about 140 A thick in which the Al+ yield changed rapidly with profiled depth. Related structure can be seen in fig. 6a at the point where this region ends.
At temperatures between 200 and 5OO“C lithium segregates to the surface and near-surface regions of a dilute aluminum alloy, forming two and possibly three distinct regions of altered composition. Gibbsian segregation produces a single monolayer of enriched lithium concentration in accord with the bond enthalpy model of segregation. Radiation-induced segregation produces a region of enhanced lithium concentration extending approximately 3000 Angstroms in from the surface, in accord with observations made on transition metal alloys [ 151. For argon ion bombardment a third region is observed extending approximately 100 Angtroms into the material. This distance is roughly commensurate with the damage layer and is tentatively identified as a region in which enhanced defect diffusion carries lithium to the surface. This region is not observed for helium bombardment, consistent with the helium-induced damage profile which extends over a very long distance and corresponds to a low defect production rate at all points along the profile. The initial segregation occurs very quickly, essentially as fast as the sample could be heated. Subsequent enhancement of the surface and near-surface lithium concentration requires an increased bulk diffusion rate which occurs as a result of radiation enhanced diffusion and radiation-induced segregation but requires an “induction period”. The equilibrium surface lithium concentration during irradiation was lower than the concentration determined solely by Gibbsian segregation, but the extent to which neutron irradiation in a Tokamak would increase the diffusion rate and consequent lithium surface concentration has not yet been determined. It was not possible to quantitatively determine either the absolute concentration or the secondary ion fraction of the sputtered lithium. Roughly speaking, the lithium surface concentration appears to lie between 7 and 70%, and the secondary ion fraction lies between 1 and 10 times that of the sputtered aluminum. The published value for the ion fraction of sputtered aluminum is 2% [17]. These values are derived on the assumption that the lithium and aluminum secondary ion fractions are independent of surface composition, and the range of uncertainty of the experimental values appears to indicate that the assumption is invalid. The surface properties of alloys as they apply to Tokamak first wall, limiter and divertor surfaces are rather complicated, but it appears to be possible to make use of some of these properties to reduce plasma contamination.
838
D.M.
Gruen et al. / Surface
Acknowledgement The authors would like to acknowledge the contribution of A.B. DeWald for developing the multilayer, multicomponent version of TRIM.
References [1] Alan R. Krauss and Dieter M. Gruen, Proc. Second. Top. Conf. Fusion Reactor Materials, Seattle, 198 1. [2] A.B. DeWald, J.N. Davidson, A.R. Krauss and D.M. Gruen, these proceedings. [3] O.K. Harling, G.P. Yu, J.E. Meyer and N.J. Grant, Proc. Second Top. Conf. Fusion Reactor Materials, Seattle, 1981. [4] Yu. M. Patov et al., Fizika i Khimia Obrabotki Materialov (Akad. Nauk., USSR, 1981) p. 53. [S] K. Miyaharea, these proceedings. [6] C.H. Muller III and K.H. Burrell, Phys. Rev. Letters 47 (1981) 330; also R.A. Zuhr, J.B. Roberto and S.P. Withrow, Proc. 2nd Top. Meeting Fusion Reactor Materials, Seattle, 1981. [7] R. Behrisch, Proc. Symp. on Atomic and Surface Physics, Maria Alm (Austria), 1980.
segregation
[8] J.H. Norem and B.A. Cramer. Proc. 2nd. Top. Meeting Fusion Reactor Reactor Materials, Seattle, 1981. [9] L.E. Rehn. P.R. Okamoto, D.I. Potter and H. Wiedersich, J. Nucl. Mater. 85/86 (1979) 1139. [lo] A.R. Krauss. D.M. Gruen and A.B. DeWald, Proc. 9th Symp. on Engineering Problems of Fusion Research, Chicago, 1981. [ 111 F.L. Williams and D. Nason, Surface Sci. 45 (1974) 377. [12] A.R. Krauss and D.M. Gruen. J. Nucl. Mater. 93/94 (1980) 686. [ 131 I. Langmuir and K.H. Kingdon. Proc. Roy. Sot. (London) A107 (1925) 61. [14] H. Wiedersich, to be published in: Surface Modification and Alloying, Eds. J.M. Poate and G. Foti. NATO Materials Science Series (Plenum, New York). [15] L.E. Rehn. S. Danyluk and H. Wiedersich. Phys. Rev. Letters 43 (1979) 1764. [16] C.J. Wen, W. Weppner. B.A. Boukamp and R.A. Huggins. Met. Trans. B, IlB (1980) 131. [ 171 A. Benninghoven, Surface Sci. 35 (1973) 457. (181 J. Roth, J. Bohdansky and W. Ottenberger. Max Planck Institut fur Plasmaphysik Report IPP 9/26, (1979). [19] N. Laegreid and G.K. Wehner, J. Appl. Phys. 32 (1961) 365.