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The initial oxidation of solid and liquid aluminium
Applied Surface Science 27 (1987) 393-400 North-Holland, Amsterdam
393
THE INITIAL OXIDATION OF SOLID AND LIQUID ALUMINIUM F. STUCKI *, M. ERBUDAK and G. KOSTORZ lnstitut p~r A ngewandte Physik, Eidgeni~ssische Technische Hochschule, CH-8093 Ziirich, Switzerland Received 6 July 1986; accepted for publication 10 September 1986
The initial stages of low-pressure oxidation of solid and liquid AI are compared using Auger electron spectroscopy and electron energy loss spectroscopy. In contrast to the solid state, no chemisorption could be detected on the liquid surface, and only after about 1000 L, formation of a surface oxide starts. After about 3000 L a marked decrease in the oxidition rate indicates the completion of a compact oxide on the liquid surface. Thereafter the oxidation is probably controlled by the diffusion of A1 ions through the solid oxide film.
1. Introduction
The oxidation of aluminium has been the subject of many detailed studies, for fundamental as well as for practical reasons. The results for polycrystalline and single crystal surfaces have recently been reviewed by Batra and Kleinman [1]. For single crystal surfaces, it seems reasonable to approximate the dynamics of the early stages of oxidation by a three-step model, as originally suggested by Bachrach et al. [2]. According to this model, oxygen is first chemisorbed on the initially clean surface, then incorporated below it, and finally A1203 is formed. For the liquid phase, only macroscopic data have been reported. Several authors [3,4] have determined the surface tension of pure and alloyed A1 and its change with oxygen exposure. For oxide-free A1, the surface tension has a maximum value of 1100 m J / m 2 at 973 K and drops very rapidly with increasing oxygen exposure until a saturation is reached at an oxide coverage of about one monolayer (ML). Auger electron spectroscopy (AES) experiments by Laty et al. [5] indicate that the ratio of the Auger signals from an oxidized and a clean liquid surface lies between 0.1 and 0.3. Goumiri and Joud [3] reported that in the liquid state no appreciable change in signal strength could be observed during the initial stages of oxidation. * Present address: Brown Boveri Research Center, CH-5405 Baden-D~ttwil, Switzerland.
0169-4332/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
394
F. Stucki et al. / Initial o vitiation of solid and liquid alu.unium
Here we report on spectroscopic measurements, AES and electron energy loss spectroscopy (EELS), performed under ultra-high vacuum (UHV) conditions on pure polycrystalline and liquid AI surfaces. The experiments show that for the liquid AI no chemisorption takes place. The oxygen is immediately incorporated into the liquid up to a certain amount of oxygen exposure, where the growth of a surface oxide starts. This behavior is essentially different from that of solid A1, where first chemisorption takes place and after one ML has been formed, the oxidation rate, under UHV conditions, rapidly decreases. Loss of the long range order upon melting will make some weak features of the electronic band structure of AI disappear, as pointed out by Hague [6]. Since AES and EELS, employed in this work, are local probes and therefore sensitive to atomic properties, no large changes are expected in the spectra upon this phase transition. Indeed they are very similar for solid and liquid AI, except for a shift of the plasmon energies to lower values due to the 7% volume expansion upon melting. We made use of this shift in order to distinguish collective oscillations from single particle excitations.
2. Experimental The experiments were carried out in an UHV apparatus equipped with a cylindrical mirror analyzer and a hybrid electron-ion gun [7]. The latter is designed to emit electrons and ions, both from the same source point, depending on the polarity of the accelerating voltage. Thus a mechanical adjustment of the sample is not needed for the different modes of operation. The vacuum was in the 10 s Pa range during all measurements. The AI sample was placed in a boron nitride crucible and directly heated by a resistor-type heating plate, made of beryllium oxide with a high temperature insulating glaze to protect the resistive heater element. The temperature was monitored by a W - 3 % R e / W 25%Re thermocouple plunged directly into the liquid. AI of extremely high purity was necessary (zone-molten A1 from the "Vereinigte Aluminium-Werke', Germany), in order to suppress segregation of trace impurities present in the bulk. This has always been encountered with commercially pure A1 (99.99%) standards during the earlier stages of this work. To remove the oxide film, the solid sample was ion sputtered (At ~, 2000 eV) at 500°C until no oxide signals (oxygen or oxidized AI) could be detected in AES. We monitored the cleaning procedure using the L~3VV A1 Auger transition at 67 eV (minimum position of the first derivative) and the interatomic L A 2,3~ v ° v ° Auger transition at 53 eV from oxidized AI [8]. Owing to the small kinetic energies involved in these transitions, the emitted electrons have an extremely small escape depth (about 0.2-0.5 nm). This leeds to a very sensitive method to test the oxygen content of the surface.
F. Stucki et a L / Initial oxidation of solid and liquid aluminium
395
EELS also proved to be a very sensitive tool to estimate the oxide coverage, because the plasmon loss intensities change rapidly with oxygen present on the surface [9]. To obtain a better resolution in the EELS spectra, the negative second derivative of the signal was registered in order to remove the smooth but strong background of inelastically scattered electrons. Throughout this work, the current density of the primary electron beam was kept below 1 /~A/cm 2 to avoid electron stimulated desorption or distortion of the oxide. 3. Results AES: The spectrum of a pure polycrystalline A1 sample at room temperature (RT) shows a strong LVV AI Auger transition and no traces of oxygen or impurities within the detection limit of the method. Fig. 1 shows the peak-topeak values of the Auger transitions of pure A1 (LVV), the oxygen (KLL) and the LAIV°V ° interatomic transition between O and A1 in A1203 as a function of oxygen exposure. Initially, the signal due to pure A1 decreases very rapidly while the oxygen signal grows. The interatomic transition marking the formation of A1203 only appears at exposures as high as 20 L (1 L = 10 -6 Torrs). Since this latter transition is characteristic for a charge transfer between AI and O as observed in A1203, we conclude that for less than 20 L, oxygen is in a chemisorbed state. At about 100 L a marked decrease in the growth of the signals due to oxygen is interpreted as the completion of a first compact oxide layer [3,5]. The weak increase afterwards is attributed to the slow incorporation of oxygen underneath the first oxide layer. Therefore, these findings are in good agreement with the three-step model proposed by Bachrach et al. [2].
AI (solidi
T = 2Z.°C P02 =1×10q Torr
c
.4
"monolcly er"
Z AI 2 0 3 ,,-.z" iii
0
i
20L
100
i
i
1
200
300
400
Fig. 1. Variation of the AI (m), oxidized A1 (A) and oxygen (O) Auger peak-to-peak heights versus oxygen exposure on a clean polycrystalline A1 surface at room temperature.
F. Stuckt el al. / l n i t t a l o.~tdation ~/ solid and liquM aluminium
396
AI (liquid)
T : 7oo°c
c
,4
"monotayer
'
"
10
15
•,
"
~9
. s 5
20
25
30
35
z.o
L5
E X P O S U R E [103 L]
Fig. 2. O x y g e n exposure of a clean liquid AI surface.
Fig. 2 shows, analogously, the oxygen uptake characteristics for liquid AI at 700°C, The contraction of the abscissa indicates that for liquid A1 one needs much higher oxygen exposures to reach a saturation coverage. Within the first 1000 L no appreciable change has been detected in the Auger signals. After this initial phase, the A1 signal drops, and the oxygen signal as well as the signal due to oxidized AI grow simultaneously. Therefore we conclude that there is no chemisorbed phase for oxygen on the liquid A1 surface. The first detectable oxygen signals already indicate a bond like that in AI203. This behavior is essentially different from that encountered on solid A1. Around 3000 L, for all signals a drastic change in slope is observed, which we interpret, in analogy to the solid surface, as the completion of a first oxide layer. These findings are in good agreement with previous observations by Laty et al. [5] and Goumiri and Joud [3]. However, it is impossible to give an estimate about the amount of oxygen incorporated into the liquid, mainly because of our limited knowledge of the details of the interaction mechanism between oxygen and the liquid AI surface. EELS: Fig. 3 shows the EELS spectra of solid and liquid A1, excited with primary electrons with a kinetic energy of 250 eV. The abscissa gives directly the energy loss. One can distinguish four energy loss structures. The plasmon losses at h~0 b = 15.2 eV (bulk) and at h ~ = 10.3 eV (surface), both for the solid surface, shift to lower energies upon melting. This behavior is consistent with the 7% volume expansion from room temperature to the liquid state, since the energies of the plasmon oscillations are proportional to the square root of the electron density in the material [10]. The energy positions of the single-particle excitations, on the other hand, are insensitive to such a change in volume. Hence the additional structures
F. Stucki et al. / Initial oxidation of solid and fiquid aluminium
AL
397
Ep = 250 eV
% LLI Z
• . . uqupcl
co b ~.~
w s .~
1~2
1(23 6.6 RI~
1
% I
30 -
20 10 ENERGY LOSS [ eV ]
0
Fig. 3. EELS spectra from a solid and a liquid A1 surface. Surface and bulk plasmon losses are denoted by % and w b.
observed at 6.6 and 3.1 eV energy loss cannot be attributed to collective oscillations. The 6.6 eV energy loss peak is interpreted as an excitation out of the oxygen 2p level into an empty state just above the Fermi level of the metal [8,11]. This shows that trace amounts of oxygen must be present on the particular surface. The structure at 3.1 eV can be attributed to an interband transition in A1 [12], which changes slightly in shape upon phase transition, because of the small modifications in the electronic density of states. Fig. 4 shows the evolution of the loss features for solid AI during oxidation. The fast decrease of the intensity of the surface plasmon peak is remarkable and in contrast to the behavior of the bulk plasmon, indicating that only electrons at the surface are now involved in bonds to oxygen. As expected, the signal due to the oxygen 2p excitation increases with coverage, whereas the bulk A1 transition at 3.1 eV remains unchanged. Fig. 5 represents similar EELS spectra for liquid A1 with larger oxygen exposures. As discussed before, the growth rate of an oxide layer at the liquid surface is much lower than in the case of a solid surface. Both plasmon loss peaks decrease simultaneously with oxygen uptake, indicating that for the liquid, electrons on and below the surface are involved in bonding to oxygen and no longer contribute to plasmon oscillations as free conduction electrons would do. This observation is direct evidence for the immediate incorporation of oxygen into the liquid, before a compact surface oxide is formed. At about 30 000 L the plasmon features are not observable anymore. At this stage an
398
F. Stucki et ul. / Initial owdation of solid and liquid alummium
AI (solid)
E~: 250 eV p% : 54@?Torr COt,
CO,
90L ~ 36L
'L
ISL SL
Z
2L IL
t
0.5L ~ dean
~
r3
+0
i
h ~
]1~
4
20
10 ENERGY LOSS[eV]
0
Fig. 4. E E L S s p e c t r a of solid AI a f t e r d i f f e r e n t s t a g e s o f o x y g e n cxpo,~urc.
Al(liquid)
Ep= 250 eV Po2 5×106T°rr --
36000 L --
wb
~
COs
I
20020 . . . . . . . . . . . . . . . . . . .
J~l . . . . - 4 \
20000 • . . . . . . . . .
r
. j-
~-._- - =~<
I
I
I ,I" " V/' { ,"\ , , /
't ~'
% 1:3 Z {-4 1D I
5000L
/
3000 . . . . .
/ ]\,
20[30 L
I "~"
II-
-, I . ~*",,
/ /r-\
[~.,
/
xX[\\~}/
1000 L ~. . . . . . . . . . . 500 L __
--J
"
,
.!_ " 'I,\.'L,/ /~ l l l l
lJI
'
I \'~/ 6.6
3'0
2'0 --1'o ~ENERGY
LOSS[eV]--
Fig. 5. E v o l u t i o n o f the E E L S s p e c t r u m o f a liquid AI s u r f a c e w i t h o x y g e n e x p o s u r e
F. Stucki et al. / Initial oxidation of solid and liquid alurniniurn
399
oxide layer on the surface is formed with a thickness comparable to the mean escape depth of the emitted electrons.
4. Conclusions A careful study of the initial oxidation of liquid A1 requires very pure samples and good vacuum conditions in order to get an oxide free surface to start with. AES is a suitable tool to study the initial oxidation of A1; the small escape depth of the Auger electrons leads to a high surface sensitivity, and together with the LVV interatomic Auger transition between O and A1 and the K L L oxygen transition it becomes possible to distinguish between chemisorption and A120 3 formation. The change in electron density upon melting leads to an energy shift of the plasmon features. This shift identifies collective and single electron excitations. Intensity variations of the plasmon oscillations in EELS show that for solid A1 only electrons from the surface region take part in bonding to oxygen. In the case of liquid A1, there exists an initial phase (about 1000 L) before the growth of a surface oxide is detectable. The simultaneous decrease of the plasmon intensities indicates that during this initial phase oxygen is incorporated. After a compact oxide layer on the liquid surface is formed, the oxidation rate decreases drastically, and further oxidation is controlled by the diffusion of A1 (not oxygen, which requires a much higher thermal activation energy) through the solid oxide layer. On the basis of our experience with commercial A1, we anticipate that alloying strongly influences the initial oxidation of liquid A1, if the alloying components segregate to the surface. This will probably influence the oxidation of liquid A1 and as well as the properties of the oxide layer on solidified samples.
Acknowledgements This work was supported by the Aluminium fond Neuhausen and by the Swiss National Science Foundation. Discussions with Professor E.B. Bas (now retired) and skillful technical assistance by A. Meier are highly appreciated.
References [1] I.P. Batra and L. Kleinman, J. Electron Spectrosc. Related Phenomena33 (1984) 175. [2] R.Z. Bachrach, S.A. Flodstr~Sm,R.S. Bauer, S.B.M. Hagstr~Smand D.J. Chadi, J. Vacuum Sci. Technol. 15 (1978) 488. [3] L. Goumiri and J.C. Joud, Acta Met. 30 (1982) 1397.
400 [41 [5] [6] [7] [8] [9] [10] [11]
F. Stucki et a L / Initial oxidatton of solid and liquid aluminium
A. Pamier, C. Garcia Cordovilla and E. Louis, Scripta Met. 18 (1984) 869. P. Laty. J.C. Joud and P. Desire, Surface Sci. 104 (1981) 117. C.F. Hague, Ph-vs. Rev. B25 (1982) 3529. E.B. Bas, E. Gisler and F. Stucki, J. Phys. E (Sci. Instr.) 17 (1984) 405. D.T. Quinto and W.D. Robertson, Surface Sci. 27 (1977) 645. C. Benndorf, G. Keller, H. Seidel and F. Thieme, Surface Sci. 67 (1977) 589. C.J. Powell, Phys. Rev. 175 (1968) 972. K.Y. Yu, J.N. Miller, P. Chye, W.E. Spicer, N.D. Lang and A.R. Williams. Phys. Rex. B14 (1976) 1446. [12] D. Massignon, F. Pellerin, J.M. Fontaine, C. Le Gressus and T. Ichinokawa, J. Appl. Phys. 51 (1980) 808.
393
THE INITIAL OXIDATION OF SOLID AND LIQUID ALUMINIUM F. STUCKI *, M. ERBUDAK and G. KOSTORZ lnstitut p~r A ngewandte Physik, Eidgeni~ssische Technische Hochschule, CH-8093 Ziirich, Switzerland Received 6 July 1986; accepted for publication 10 September 1986
The initial stages of low-pressure oxidation of solid and liquid AI are compared using Auger electron spectroscopy and electron energy loss spectroscopy. In contrast to the solid state, no chemisorption could be detected on the liquid surface, and only after about 1000 L, formation of a surface oxide starts. After about 3000 L a marked decrease in the oxidition rate indicates the completion of a compact oxide on the liquid surface. Thereafter the oxidation is probably controlled by the diffusion of A1 ions through the solid oxide film.
1. Introduction
The oxidation of aluminium has been the subject of many detailed studies, for fundamental as well as for practical reasons. The results for polycrystalline and single crystal surfaces have recently been reviewed by Batra and Kleinman [1]. For single crystal surfaces, it seems reasonable to approximate the dynamics of the early stages of oxidation by a three-step model, as originally suggested by Bachrach et al. [2]. According to this model, oxygen is first chemisorbed on the initially clean surface, then incorporated below it, and finally A1203 is formed. For the liquid phase, only macroscopic data have been reported. Several authors [3,4] have determined the surface tension of pure and alloyed A1 and its change with oxygen exposure. For oxide-free A1, the surface tension has a maximum value of 1100 m J / m 2 at 973 K and drops very rapidly with increasing oxygen exposure until a saturation is reached at an oxide coverage of about one monolayer (ML). Auger electron spectroscopy (AES) experiments by Laty et al. [5] indicate that the ratio of the Auger signals from an oxidized and a clean liquid surface lies between 0.1 and 0.3. Goumiri and Joud [3] reported that in the liquid state no appreciable change in signal strength could be observed during the initial stages of oxidation. * Present address: Brown Boveri Research Center, CH-5405 Baden-D~ttwil, Switzerland.
0169-4332/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
394
F. Stucki et al. / Initial o vitiation of solid and liquid alu.unium
Here we report on spectroscopic measurements, AES and electron energy loss spectroscopy (EELS), performed under ultra-high vacuum (UHV) conditions on pure polycrystalline and liquid AI surfaces. The experiments show that for the liquid AI no chemisorption takes place. The oxygen is immediately incorporated into the liquid up to a certain amount of oxygen exposure, where the growth of a surface oxide starts. This behavior is essentially different from that of solid A1, where first chemisorption takes place and after one ML has been formed, the oxidation rate, under UHV conditions, rapidly decreases. Loss of the long range order upon melting will make some weak features of the electronic band structure of AI disappear, as pointed out by Hague [6]. Since AES and EELS, employed in this work, are local probes and therefore sensitive to atomic properties, no large changes are expected in the spectra upon this phase transition. Indeed they are very similar for solid and liquid AI, except for a shift of the plasmon energies to lower values due to the 7% volume expansion upon melting. We made use of this shift in order to distinguish collective oscillations from single particle excitations.
2. Experimental The experiments were carried out in an UHV apparatus equipped with a cylindrical mirror analyzer and a hybrid electron-ion gun [7]. The latter is designed to emit electrons and ions, both from the same source point, depending on the polarity of the accelerating voltage. Thus a mechanical adjustment of the sample is not needed for the different modes of operation. The vacuum was in the 10 s Pa range during all measurements. The AI sample was placed in a boron nitride crucible and directly heated by a resistor-type heating plate, made of beryllium oxide with a high temperature insulating glaze to protect the resistive heater element. The temperature was monitored by a W - 3 % R e / W 25%Re thermocouple plunged directly into the liquid. AI of extremely high purity was necessary (zone-molten A1 from the "Vereinigte Aluminium-Werke', Germany), in order to suppress segregation of trace impurities present in the bulk. This has always been encountered with commercially pure A1 (99.99%) standards during the earlier stages of this work. To remove the oxide film, the solid sample was ion sputtered (At ~, 2000 eV) at 500°C until no oxide signals (oxygen or oxidized AI) could be detected in AES. We monitored the cleaning procedure using the L~3VV A1 Auger transition at 67 eV (minimum position of the first derivative) and the interatomic L A 2,3~ v ° v ° Auger transition at 53 eV from oxidized AI [8]. Owing to the small kinetic energies involved in these transitions, the emitted electrons have an extremely small escape depth (about 0.2-0.5 nm). This leeds to a very sensitive method to test the oxygen content of the surface.
F. Stucki et a L / Initial oxidation of solid and liquid aluminium
395
EELS also proved to be a very sensitive tool to estimate the oxide coverage, because the plasmon loss intensities change rapidly with oxygen present on the surface [9]. To obtain a better resolution in the EELS spectra, the negative second derivative of the signal was registered in order to remove the smooth but strong background of inelastically scattered electrons. Throughout this work, the current density of the primary electron beam was kept below 1 /~A/cm 2 to avoid electron stimulated desorption or distortion of the oxide. 3. Results AES: The spectrum of a pure polycrystalline A1 sample at room temperature (RT) shows a strong LVV AI Auger transition and no traces of oxygen or impurities within the detection limit of the method. Fig. 1 shows the peak-topeak values of the Auger transitions of pure A1 (LVV), the oxygen (KLL) and the LAIV°V ° interatomic transition between O and A1 in A1203 as a function of oxygen exposure. Initially, the signal due to pure A1 decreases very rapidly while the oxygen signal grows. The interatomic transition marking the formation of A1203 only appears at exposures as high as 20 L (1 L = 10 -6 Torrs). Since this latter transition is characteristic for a charge transfer between AI and O as observed in A1203, we conclude that for less than 20 L, oxygen is in a chemisorbed state. At about 100 L a marked decrease in the growth of the signals due to oxygen is interpreted as the completion of a first compact oxide layer [3,5]. The weak increase afterwards is attributed to the slow incorporation of oxygen underneath the first oxide layer. Therefore, these findings are in good agreement with the three-step model proposed by Bachrach et al. [2].
AI (solidi
T = 2Z.°C P02 =1×10q Torr
c
.4
"monolcly er"
Z AI 2 0 3 ,,-.z" iii
0
i
20L
100
i
i
1
200
300
400
Fig. 1. Variation of the AI (m), oxidized A1 (A) and oxygen (O) Auger peak-to-peak heights versus oxygen exposure on a clean polycrystalline A1 surface at room temperature.
F. Stuckt el al. / l n i t t a l o.~tdation ~/ solid and liquM aluminium
396
AI (liquid)
T : 7oo°c
c
,4
"monotayer
'
"
10
15
•,
"
~9
. s 5
20
25
30
35
z.o
L5
E X P O S U R E [103 L]
Fig. 2. O x y g e n exposure of a clean liquid AI surface.
Fig. 2 shows, analogously, the oxygen uptake characteristics for liquid AI at 700°C, The contraction of the abscissa indicates that for liquid A1 one needs much higher oxygen exposures to reach a saturation coverage. Within the first 1000 L no appreciable change has been detected in the Auger signals. After this initial phase, the A1 signal drops, and the oxygen signal as well as the signal due to oxidized AI grow simultaneously. Therefore we conclude that there is no chemisorbed phase for oxygen on the liquid A1 surface. The first detectable oxygen signals already indicate a bond like that in AI203. This behavior is essentially different from that encountered on solid A1. Around 3000 L, for all signals a drastic change in slope is observed, which we interpret, in analogy to the solid surface, as the completion of a first oxide layer. These findings are in good agreement with previous observations by Laty et al. [5] and Goumiri and Joud [3]. However, it is impossible to give an estimate about the amount of oxygen incorporated into the liquid, mainly because of our limited knowledge of the details of the interaction mechanism between oxygen and the liquid AI surface. EELS: Fig. 3 shows the EELS spectra of solid and liquid A1, excited with primary electrons with a kinetic energy of 250 eV. The abscissa gives directly the energy loss. One can distinguish four energy loss structures. The plasmon losses at h~0 b = 15.2 eV (bulk) and at h ~ = 10.3 eV (surface), both for the solid surface, shift to lower energies upon melting. This behavior is consistent with the 7% volume expansion from room temperature to the liquid state, since the energies of the plasmon oscillations are proportional to the square root of the electron density in the material [10]. The energy positions of the single-particle excitations, on the other hand, are insensitive to such a change in volume. Hence the additional structures
F. Stucki et al. / Initial oxidation of solid and fiquid aluminium
AL
397
Ep = 250 eV
% LLI Z
• . . uqupcl
co b ~.~
w s .~
1~2
1(23 6.6 RI~
1
% I
30 -
20 10 ENERGY LOSS [ eV ]
0
Fig. 3. EELS spectra from a solid and a liquid A1 surface. Surface and bulk plasmon losses are denoted by % and w b.
observed at 6.6 and 3.1 eV energy loss cannot be attributed to collective oscillations. The 6.6 eV energy loss peak is interpreted as an excitation out of the oxygen 2p level into an empty state just above the Fermi level of the metal [8,11]. This shows that trace amounts of oxygen must be present on the particular surface. The structure at 3.1 eV can be attributed to an interband transition in A1 [12], which changes slightly in shape upon phase transition, because of the small modifications in the electronic density of states. Fig. 4 shows the evolution of the loss features for solid AI during oxidation. The fast decrease of the intensity of the surface plasmon peak is remarkable and in contrast to the behavior of the bulk plasmon, indicating that only electrons at the surface are now involved in bonds to oxygen. As expected, the signal due to the oxygen 2p excitation increases with coverage, whereas the bulk A1 transition at 3.1 eV remains unchanged. Fig. 5 represents similar EELS spectra for liquid A1 with larger oxygen exposures. As discussed before, the growth rate of an oxide layer at the liquid surface is much lower than in the case of a solid surface. Both plasmon loss peaks decrease simultaneously with oxygen uptake, indicating that for the liquid, electrons on and below the surface are involved in bonding to oxygen and no longer contribute to plasmon oscillations as free conduction electrons would do. This observation is direct evidence for the immediate incorporation of oxygen into the liquid, before a compact surface oxide is formed. At about 30 000 L the plasmon features are not observable anymore. At this stage an
398
F. Stucki et ul. / Initial owdation of solid and liquid alummium
AI (solid)
E~: 250 eV p% : 54@?Torr COt,
CO,
90L ~ 36L
'L
ISL SL
Z
2L IL
t
0.5L ~ dean
~
r3
+0
i
h ~
]1~
4
20
10 ENERGY LOSS[eV]
0
Fig. 4. E E L S s p e c t r a of solid AI a f t e r d i f f e r e n t s t a g e s o f o x y g e n cxpo,~urc.
Al(liquid)
Ep= 250 eV Po2 5×106T°rr --
36000 L --
wb
~
COs
I
20020 . . . . . . . . . . . . . . . . . . .
J~l . . . . - 4 \
20000 • . . . . . . . . .
r
. j-
~-._- - =~<
I
I
I ,I" " V/' { ,"\ , , /
't ~'
% 1:3 Z {-4 1D I
5000L
/
3000 . . . . .
/ ]\,
20[30 L
I "~"
II-
-, I . ~*",,
/ /r-\
[~.,
/
xX[\\~}/
1000 L ~. . . . . . . . . . . 500 L __
--J
"
,
.!_ " 'I,\.'L,/ /~ l l l l
lJI
'
I \'~/ 6.6
3'0
2'0 --1'o ~ENERGY
LOSS[eV]--
Fig. 5. E v o l u t i o n o f the E E L S s p e c t r u m o f a liquid AI s u r f a c e w i t h o x y g e n e x p o s u r e
F. Stucki et al. / Initial oxidation of solid and liquid alurniniurn
399
oxide layer on the surface is formed with a thickness comparable to the mean escape depth of the emitted electrons.
4. Conclusions A careful study of the initial oxidation of liquid A1 requires very pure samples and good vacuum conditions in order to get an oxide free surface to start with. AES is a suitable tool to study the initial oxidation of A1; the small escape depth of the Auger electrons leads to a high surface sensitivity, and together with the LVV interatomic Auger transition between O and A1 and the K L L oxygen transition it becomes possible to distinguish between chemisorption and A120 3 formation. The change in electron density upon melting leads to an energy shift of the plasmon features. This shift identifies collective and single electron excitations. Intensity variations of the plasmon oscillations in EELS show that for solid A1 only electrons from the surface region take part in bonding to oxygen. In the case of liquid A1, there exists an initial phase (about 1000 L) before the growth of a surface oxide is detectable. The simultaneous decrease of the plasmon intensities indicates that during this initial phase oxygen is incorporated. After a compact oxide layer on the liquid surface is formed, the oxidation rate decreases drastically, and further oxidation is controlled by the diffusion of A1 (not oxygen, which requires a much higher thermal activation energy) through the solid oxide layer. On the basis of our experience with commercial A1, we anticipate that alloying strongly influences the initial oxidation of liquid A1, if the alloying components segregate to the surface. This will probably influence the oxidation of liquid A1 and as well as the properties of the oxide layer on solidified samples.
Acknowledgements This work was supported by the Aluminium fond Neuhausen and by the Swiss National Science Foundation. Discussions with Professor E.B. Bas (now retired) and skillful technical assistance by A. Meier are highly appreciated.
References [1] I.P. Batra and L. Kleinman, J. Electron Spectrosc. Related Phenomena33 (1984) 175. [2] R.Z. Bachrach, S.A. Flodstr~Sm,R.S. Bauer, S.B.M. Hagstr~Smand D.J. Chadi, J. Vacuum Sci. Technol. 15 (1978) 488. [3] L. Goumiri and J.C. Joud, Acta Met. 30 (1982) 1397.
400 [41 [5] [6] [7] [8] [9] [10] [11]
F. Stucki et a L / Initial oxidatton of solid and liquid aluminium
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