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Effect of surface macrodefects on the mechanism of oxygen interaction with the boundary layer of Fe2O3
Reactivity of Solids, 8 (1990) 41-50 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
41
Effect of surface macrodefects on the mechanism of oxygen interaction with the boundary layer of Fe,O, F. Adam ‘, B. Dupr6 ‘, C. Gleitzer ‘, J. Jauowski ‘, J. Nowotuy 37* and M. Sloma ’ ’ UniversitP de Nancy I, Laboratoire de Chimie du Solide, 54506 Vandoeuvre-Ies-Nancy (France) 2 Academy of Mining and Metallurgy, Institute of Metallurgy, 30-059 Krakow (Poland) ‘Australian Nuclear Science and Technology Organisation, Lucas Heights Research Laboratories, Menai, N.S. W. 2234 (Australia) (Received 10 May 1989; accepted 3 October 1989)
Abstract The interaction between gaseous oxygen and the surface of sintered polycrystalline Fe,O, was studied by using “in situ” work function measurements at 1140 K (867“ C). The treatment of the specimen (after preliminary sintering at 1673 K in air) involved polishing of the oxide surface with a diamond powder resulting in the formation of a high density of grooves of different orientations along the surface. Sorption of oxygen on the freshly polished surface resulted in the formation of a positive surface charge which has been considered in terms of the formation of subsurface microdipoles, induced either by specific active centres formed during the mechanical treatment, or by donors formed at the same centres. The centres were closely related to the presence of grooves. The charge decreased during the thermal treatment and finally changed in sign. It was also observed that the treatment led to disappearance of grooves except some with specific orientation. The time of the equilibration, whose rate is controlled by the surface diffusion, was about 300 h at 1140 K.
Introduction
Mechanical treatment applied to the surface of solids leads to the formation of defects and resulting changes in surface properties. In the case of ionic solids these changes may even involve phase transitions which are limited to the outer layer. Ohtani and Senna [l] have reported that milling of CdS results in the formation of metastable amorphous phases. Similar effects have been observed for yttria stabilized zirconia. Reed and Lejus [2] have reported a near-surface phase transition of the cubic phase of ZrO, into the tetragonal and the monoclinic structure as a result of grinding and polishing. It has been also observed that polishing of yttria doped zirconia has a strong effect on the mechanism of oxygen interaction with this material [3]. This effect has been considered in terms of changes in composi0168-7336/90/$03.50
0 1990 - Elsevier Science Publishers B.V.
42
tion of the overlayer structure. It seems, however, that both composition and structural changes should be taken into account. Surface defects as well as metastable phases formed on surfaces as a result of mechanical treatment exhibit a substantial excess of energy and resulting specific properties. These properties involve the formation of surface active sites for sorption of gases and consequent changes in several properties such as catalytical and electrical properties. High temperature annealing should lead to equilibration and should restore the surface properties of the material to those applying before the mechanical treatment. The purpose of the present work is to study the effect of polishing on the interaction of oxygen with the Fe,O, surface. The interaction of oxygen with Fe,O, at various stages of the annealing was studied by means of work function measurements.
Experimental Polycrystalline samples of Fe,O, were sintered at 1673 K in air for 10 h from a powder supplied by Merck. The average grain size was about 50 pm. The pellets were obtained in the form of plates 1 mm thick and 8 X 8 mm dimensions. The surface of the sintered sample was polished with 0.1 pm diamond powder.
HEMATITE
I
I
I
-15 log ~~2
-
5
-5 [p.
in 2
PaI
Fig. 1. The phase diagram of the Fe-O2 system illustrating the stability
of Fe,O,.
43
The polished specimen was the subject of redox experiments during prolonged annealing at 1140 K, while the partial pressure of oxygen in the gas phase was continuously changed between two extreme values, 2 X lo* and 4.4 x lo4 Pa. The surface potential during successive oxidation and reduction runs was recorded. An Ar-0, gas mixture flowed through the reaction chamber, and partial pressure of oxygen was controlled by a zirconia oxygen gauge. The po, range in these studies remained within the stability of the Fe,O, phase (Fig. 1). Surface electrical effects accompanying redox processes were monitored by “in situ” work function measurements, using the High Temperature Kelvin Probe [4,5]. This method permits determinations of the contact potential difference (CPD) between the studied sample and the reference electrode (ref) :
Platinum was used as the interaction of oxygen with resulting parameters of Pt have been described before corresponds to the increase study.
reference. The electrical effects accompanying the Pt surface at elevated temperatures and the as a reference for work function measurements [6]. According to eq. 1 the increase in CPD of work function of the oxide specimen under
Annealing of Fe,O, and accompanying surface electrical effects Figure 2 illustrates the way the CPD changes oxidation (0) and reduction (R) when the sample K. The absolute CPD values as well as the CPD tal runs clearly exhibit variations over the 300 h
0
C
D
during alternating runs of had been annealed at 1140 changes during experimenof the experiment. Initially,
E Fe203
1140K (867°C)
-R
s
R
40
R
60
120 TIME
160
240
200
OF ANNEALING
260
320
[hl
Fig. 2. Changes in the CPD of the F%O,-Pt system during annealing at 1140 K. Changes in CPD during oxidation (0) and reduction (R) runs were performed in the range 2X10* and 4.4 X lo4 Pa, respectively.
44
sorption of oxygen resulted in a positive surface charge rather than the negative charge usually observed for the process of chemisorption of oxygen and its incorporation into an oxide lattice. The measured value of ACPD decreases with time of annealing and after 90 h ACPD changes its polarity. The surface state as this stage of the experiment does not produce any change in the CPD during either reduction or oxidation runs. However, after the 90 h point the electrical effects observed during redox processes have reverse polarity and exhibit further changes as time goes on. It is interesting that the absolute CPD value at low po, (2 x lo* Pa) does not change markedly with the time of annealing. Initially it remains at +O.ll V (within 100 h). After 140 h the CPD decreases by 0.15 V and stabilizes at the level of - 0.04 V. At the same time the changes in CPD vary between -0.2 V at the beginning of the experiment and stabilises at +0.25 after 300 h. Let us consider the observed effects in terms of reactions which may take place at the boundary of the metal oxide/oxygen phase boundary. Oxygen exhibits typical electron-acceptor properties. Accordingly, oxygen adsorption results in a negative surface charge. For a p-type semiconductor the following surface reaction can be written: l/20,
% O*-+
2h
(2)
where O*- is the doubly ionized form of oxygen localized at the surface and h’ denotes an electronic hole in the space charge layer. Incorporation of of cation oxygen O*- into the oxide lattice can lead to the formation vacancies. For Fe,O, this process can be expressed by the following equilibrium: 3/20,
% 2V,k” + 30, + 6h
(3)
Reactions (2) and (3) should lead to an increase in work function. The observed decrease of work function at the beginning of annealing (below 90 h) indicates that electrical effects resulting from reactions (2) and (3) do not dominate and another mechanism of oxygen sorption may be controlling the surface charge, which is apparently related to the nature of surface microdefects arising from polishing. The nature of these defects and their evolution during annealing was studied by scanning electron microscopy (SEM).
Electron microscopy studies Figure 3 presents annealing. In Fig. 3a enclosures. Once can orientations produced (Fig. 3b), corresponds
SEM pictures of the polished Fe,O, specimen before one can distinguish small spots corresponding to iron see a very high density of linear grooves of different by polishing. The width of the grooves, about 0.1 pm closely to the dimensions of the diamond powder
45
Fig. 4. The SEM picture of the Fe,O, surface (polished) after annealing at 1140 K for 300 h; enlargements:
(a) 1000, (b) 1800, (c) 5600, (d) 104.
used for polishing. Figures 4a and 4b are micrographs of samples after 300 h of annealing at 1140 K i.e. after the surface had been equilibrated. Individual grains evidently become clearly defined with clear-cut boundaries (Fig. 4a). One can also see that certain individual grains retain their grooves, characteristically of only one specific orientation while grooves of other orientations disappear or become very small (Fig. 4b). The remaining grooves are much more uniformly oriented that those seen before thermal treatment. Their orientation is an individual property of grains. Discussion The surface electrical effects monitored by work function measurements can be assigned to 5 ranges (Fig. 2). The range A corresponds to the beginning of the experiment when changes in contact potential difference (ACPD) during redox processes are at their largest ( - 0.2 V) and are of negative sign. At the same time the absolute value CPD at higher po, is at its highest. The range B corresponds to a decrease in ACPD during the redox experiments. After 90 h of annealing (range C) the surface state is such that neither reduction nor oxidation have any effect on CPD. Further annealing results in the increase of ACPD of an opposite sign (range D). Finally, after 300 h, all electrical effects stabilize. Then the measured values of the ACPD during redox processes assume a maximum value which remains stable in time (range E). The electron microscopy studies show how work function changes can be closely related to the history of surface grooves formed during polishing, specifically the disappearance of all grooves other than those having a specific orientation. One would expect that the way the surface energy changes as a result of the formation of grooves depends essentially on the crystallographic plane and the orientation of grooves. The grooves which remain stable during annealing are apparently oriented in a way which corresponds to the lowest energy state. All other grooves exhibiting higher energy are annealed out during the thermal treatment. Therefore, one would expect that disappearance of high energy grooves is closely related to the surface electrical phenomena accompanying redox processes involving the cycles of sorption/ desorption of oxygen. The positive surface potential formed through oxygen sorption on the freshly polished surface may be explained in two ways. (a) Dipoles may be formed as a result of oxygen bonding at specific surface centres corresponding to high energy grooves (Fig. 5). Similar dipoles can be produced by oxygen sorption in the subsurface layer of metals [7]. The dipoles disappear during the thermal treatment as the groves are annealed out and related surface centres are eliminated. Figure 6 illustrates the evolution of high energy grooves within various stages of the annealing.
+ 0 Fig. 5. Diagram of the formation of positive dipoles at the grooves as a result of oxygen sorption.
(b) Donor centres can be considered, such as iron interstitials (Fei) and oxygen vacancies (V,). However, it seems unlikely that sorption of oxygen would result in the formation of oxygen vacancies. On the other hand the formation of iron interstitials seems possible. The formation of similar defects at the surface upon adsorption of oxygen have already been observed for Co0 [S]. The following defect mechanisms have been considered for Fe,O, [9-111: .... 2Fe, + 2h’% 2Fe, + 2Vk: (4) or 2Fe,,
+ 30, + 6h’% 2Fef”‘+ 3/20,
(5)
Equilibrium (5) implies that the increase of oxygen partial pressure results in a decrease of Fe interstitials. Therefore, eq. 5 cannot be considered to explain the effect observed here. One may assume, however, that oxygen may stabilize these defects at the specific active centres which are formed during the mechanical treatment. The mechanism of oxygen interaction with the oxide surface can be considered in terms of the following relation between work function (+) and oxygen partial pressure [5]: 1 _=-n
(6)
where n is the parameter sensitive to the defect structure of the near-surface oxide layer. The parameter n in eq. 6 usually assumes positive values when
A
B
C
D
Fig. 6. Evolution of high energy grooves during thermal treatment.
49
1140K (867°C)
u_ 0
3 g100i? u 8 -150-
I,
I
I
100 200 TIME OF ANNEALING [hl
Fig. 7. The parameter
, 300
n in eq. 6 as a function
of time of annealing.
sorption or chemisorption of oxygen is taken into account [12]. As seen from Fig. 7 n is initially negative within regions A and B and drops to infinity at about 90 h. Its value becomes positive during incorporation of oxygen into the oxide lattice or by chemisorption. The negative value of n and its changes over time can be considered either in terms of dipoles formed within high energy grooves formed during polishing or donor-type point defects formed at special active centres within the grooves. Disappearance of this effect during the thermal treatment together with disappearance of the high energy grooves indicates that the grooves are responsible for the observed effect. After about 300 h the parameter n stabilizes at about 3 and this value may be considered characteristic of the defect structure of the Fe,O, surface layer in equilibrium. This defect structure will be the subject of a forthcoming paper.
Conclusions It has been shown that mechanical treatments such as grinding and polishing lead to the formation of defects which exhibit specific properties. These defects have a strong influence on the mechanism of oxygen interaction with the oxide surface and can lead to the formation of either positive oxygen dipoles or donor centres at the active surface sites which appear at high energy grooves formed during surface polishing. The model involving dipoles seems to be more consistent with the results. The surface defects lead
50
to an unusual interaction of the Fe,O, surface with oxygen, involving the positive charge. The charge alters as grooves of different orientations disappear during thermal treatment. Some grooves remain stable, namely those with a specific orientation which correspond to the lowest surface energy and which are stable in experimental conditions. The equilibration time for the Fe,O, surface at 1140 K is about 300 h. It would be of interest to study the “in situ” processes of change of grooves at different and well defined crystallographic planes.
References 1 G. Ohtani and M. Set-ma, Mater. Sci. Monographs No. 10, Elsevier, Amsterdam, 1982, p. 668. 2 J.S. Reed and A.-M. Lejus, Mater. Res. Bull., 12, (1977) 949. 3 J. Nowotny, M. Sloma and W. Weppner, Solid State Ionics, 28-30 (1988) 1445. 4 L.S. Darken and R.W. Gurry, J. Am. Chem. Sot., 67 (1945) 1398. 5 J. Nowotny, M. Sloma and W. Weppner, Adv. Ceram., 23 (1987) 159. 6 J. Nowotny, M. Sloma, J. Phys. (France), 47 (1986) Cl-807. 7 B. Hayden, S. Hachicha and A.M. Bradshow, Le Vide, Les Couches Minces, Sppl., Mater. 14th Intern. Conf., 201 (1981) 1125. 8 J. Nowotny, M. Sloma, and W. Weppner, in J. Nowotny and W.W. Weppner (Eds.), Non-Stoichiometric Compounds, Kluwer Acad., 1989, p. 265. 9 P. Kofstad, Electrical Conductivity, Diffusion and Nonstoichiometry in Binary Metal Oxides, Wiley, 1972. 10 R. Chang and J.B. Wagner, Jr., J. Am. Ceram. Sot., 55 (1972) 211. 11 Hj. Matzke, in T.O. Sorensen (Ed.), Nonstoichiometric Oxides, Academic Press, New York, 1981, p. 220. 12 J. Nowotny and M. Sloma, in J. Nowotny and L.C. Dufour (Eds.), Surface and NearSurface Chemistry of Oxide Materials, Elsevier, 1988, p. 281.
41
Effect of surface macrodefects on the mechanism of oxygen interaction with the boundary layer of Fe,O, F. Adam ‘, B. Dupr6 ‘, C. Gleitzer ‘, J. Jauowski ‘, J. Nowotuy 37* and M. Sloma ’ ’ UniversitP de Nancy I, Laboratoire de Chimie du Solide, 54506 Vandoeuvre-Ies-Nancy (France) 2 Academy of Mining and Metallurgy, Institute of Metallurgy, 30-059 Krakow (Poland) ‘Australian Nuclear Science and Technology Organisation, Lucas Heights Research Laboratories, Menai, N.S. W. 2234 (Australia) (Received 10 May 1989; accepted 3 October 1989)
Abstract The interaction between gaseous oxygen and the surface of sintered polycrystalline Fe,O, was studied by using “in situ” work function measurements at 1140 K (867“ C). The treatment of the specimen (after preliminary sintering at 1673 K in air) involved polishing of the oxide surface with a diamond powder resulting in the formation of a high density of grooves of different orientations along the surface. Sorption of oxygen on the freshly polished surface resulted in the formation of a positive surface charge which has been considered in terms of the formation of subsurface microdipoles, induced either by specific active centres formed during the mechanical treatment, or by donors formed at the same centres. The centres were closely related to the presence of grooves. The charge decreased during the thermal treatment and finally changed in sign. It was also observed that the treatment led to disappearance of grooves except some with specific orientation. The time of the equilibration, whose rate is controlled by the surface diffusion, was about 300 h at 1140 K.
Introduction
Mechanical treatment applied to the surface of solids leads to the formation of defects and resulting changes in surface properties. In the case of ionic solids these changes may even involve phase transitions which are limited to the outer layer. Ohtani and Senna [l] have reported that milling of CdS results in the formation of metastable amorphous phases. Similar effects have been observed for yttria stabilized zirconia. Reed and Lejus [2] have reported a near-surface phase transition of the cubic phase of ZrO, into the tetragonal and the monoclinic structure as a result of grinding and polishing. It has been also observed that polishing of yttria doped zirconia has a strong effect on the mechanism of oxygen interaction with this material [3]. This effect has been considered in terms of changes in composi0168-7336/90/$03.50
0 1990 - Elsevier Science Publishers B.V.
42
tion of the overlayer structure. It seems, however, that both composition and structural changes should be taken into account. Surface defects as well as metastable phases formed on surfaces as a result of mechanical treatment exhibit a substantial excess of energy and resulting specific properties. These properties involve the formation of surface active sites for sorption of gases and consequent changes in several properties such as catalytical and electrical properties. High temperature annealing should lead to equilibration and should restore the surface properties of the material to those applying before the mechanical treatment. The purpose of the present work is to study the effect of polishing on the interaction of oxygen with the Fe,O, surface. The interaction of oxygen with Fe,O, at various stages of the annealing was studied by means of work function measurements.
Experimental Polycrystalline samples of Fe,O, were sintered at 1673 K in air for 10 h from a powder supplied by Merck. The average grain size was about 50 pm. The pellets were obtained in the form of plates 1 mm thick and 8 X 8 mm dimensions. The surface of the sintered sample was polished with 0.1 pm diamond powder.
HEMATITE
I
I
I
-15 log ~~2
-
5
-5 [p.
in 2
PaI
Fig. 1. The phase diagram of the Fe-O2 system illustrating the stability
of Fe,O,.
43
The polished specimen was the subject of redox experiments during prolonged annealing at 1140 K, while the partial pressure of oxygen in the gas phase was continuously changed between two extreme values, 2 X lo* and 4.4 x lo4 Pa. The surface potential during successive oxidation and reduction runs was recorded. An Ar-0, gas mixture flowed through the reaction chamber, and partial pressure of oxygen was controlled by a zirconia oxygen gauge. The po, range in these studies remained within the stability of the Fe,O, phase (Fig. 1). Surface electrical effects accompanying redox processes were monitored by “in situ” work function measurements, using the High Temperature Kelvin Probe [4,5]. This method permits determinations of the contact potential difference (CPD) between the studied sample and the reference electrode (ref) :
Platinum was used as the interaction of oxygen with resulting parameters of Pt have been described before corresponds to the increase study.
reference. The electrical effects accompanying the Pt surface at elevated temperatures and the as a reference for work function measurements [6]. According to eq. 1 the increase in CPD of work function of the oxide specimen under
Annealing of Fe,O, and accompanying surface electrical effects Figure 2 illustrates the way the CPD changes oxidation (0) and reduction (R) when the sample K. The absolute CPD values as well as the CPD tal runs clearly exhibit variations over the 300 h
0
C
D
during alternating runs of had been annealed at 1140 changes during experimenof the experiment. Initially,
E Fe203
1140K (867°C)
-R
s
R
40
R
60
120 TIME
160
240
200
OF ANNEALING
260
320
[hl
Fig. 2. Changes in the CPD of the F%O,-Pt system during annealing at 1140 K. Changes in CPD during oxidation (0) and reduction (R) runs were performed in the range 2X10* and 4.4 X lo4 Pa, respectively.
44
sorption of oxygen resulted in a positive surface charge rather than the negative charge usually observed for the process of chemisorption of oxygen and its incorporation into an oxide lattice. The measured value of ACPD decreases with time of annealing and after 90 h ACPD changes its polarity. The surface state as this stage of the experiment does not produce any change in the CPD during either reduction or oxidation runs. However, after the 90 h point the electrical effects observed during redox processes have reverse polarity and exhibit further changes as time goes on. It is interesting that the absolute CPD value at low po, (2 x lo* Pa) does not change markedly with the time of annealing. Initially it remains at +O.ll V (within 100 h). After 140 h the CPD decreases by 0.15 V and stabilizes at the level of - 0.04 V. At the same time the changes in CPD vary between -0.2 V at the beginning of the experiment and stabilises at +0.25 after 300 h. Let us consider the observed effects in terms of reactions which may take place at the boundary of the metal oxide/oxygen phase boundary. Oxygen exhibits typical electron-acceptor properties. Accordingly, oxygen adsorption results in a negative surface charge. For a p-type semiconductor the following surface reaction can be written: l/20,
% O*-+
2h
(2)
where O*- is the doubly ionized form of oxygen localized at the surface and h’ denotes an electronic hole in the space charge layer. Incorporation of of cation oxygen O*- into the oxide lattice can lead to the formation vacancies. For Fe,O, this process can be expressed by the following equilibrium: 3/20,
% 2V,k” + 30, + 6h
(3)
Reactions (2) and (3) should lead to an increase in work function. The observed decrease of work function at the beginning of annealing (below 90 h) indicates that electrical effects resulting from reactions (2) and (3) do not dominate and another mechanism of oxygen sorption may be controlling the surface charge, which is apparently related to the nature of surface microdefects arising from polishing. The nature of these defects and their evolution during annealing was studied by scanning electron microscopy (SEM).
Electron microscopy studies Figure 3 presents annealing. In Fig. 3a enclosures. Once can orientations produced (Fig. 3b), corresponds
SEM pictures of the polished Fe,O, specimen before one can distinguish small spots corresponding to iron see a very high density of linear grooves of different by polishing. The width of the grooves, about 0.1 pm closely to the dimensions of the diamond powder
45
Fig. 4. The SEM picture of the Fe,O, surface (polished) after annealing at 1140 K for 300 h; enlargements:
(a) 1000, (b) 1800, (c) 5600, (d) 104.
used for polishing. Figures 4a and 4b are micrographs of samples after 300 h of annealing at 1140 K i.e. after the surface had been equilibrated. Individual grains evidently become clearly defined with clear-cut boundaries (Fig. 4a). One can also see that certain individual grains retain their grooves, characteristically of only one specific orientation while grooves of other orientations disappear or become very small (Fig. 4b). The remaining grooves are much more uniformly oriented that those seen before thermal treatment. Their orientation is an individual property of grains. Discussion The surface electrical effects monitored by work function measurements can be assigned to 5 ranges (Fig. 2). The range A corresponds to the beginning of the experiment when changes in contact potential difference (ACPD) during redox processes are at their largest ( - 0.2 V) and are of negative sign. At the same time the absolute value CPD at higher po, is at its highest. The range B corresponds to a decrease in ACPD during the redox experiments. After 90 h of annealing (range C) the surface state is such that neither reduction nor oxidation have any effect on CPD. Further annealing results in the increase of ACPD of an opposite sign (range D). Finally, after 300 h, all electrical effects stabilize. Then the measured values of the ACPD during redox processes assume a maximum value which remains stable in time (range E). The electron microscopy studies show how work function changes can be closely related to the history of surface grooves formed during polishing, specifically the disappearance of all grooves other than those having a specific orientation. One would expect that the way the surface energy changes as a result of the formation of grooves depends essentially on the crystallographic plane and the orientation of grooves. The grooves which remain stable during annealing are apparently oriented in a way which corresponds to the lowest energy state. All other grooves exhibiting higher energy are annealed out during the thermal treatment. Therefore, one would expect that disappearance of high energy grooves is closely related to the surface electrical phenomena accompanying redox processes involving the cycles of sorption/ desorption of oxygen. The positive surface potential formed through oxygen sorption on the freshly polished surface may be explained in two ways. (a) Dipoles may be formed as a result of oxygen bonding at specific surface centres corresponding to high energy grooves (Fig. 5). Similar dipoles can be produced by oxygen sorption in the subsurface layer of metals [7]. The dipoles disappear during the thermal treatment as the groves are annealed out and related surface centres are eliminated. Figure 6 illustrates the evolution of high energy grooves within various stages of the annealing.
+ 0 Fig. 5. Diagram of the formation of positive dipoles at the grooves as a result of oxygen sorption.
(b) Donor centres can be considered, such as iron interstitials (Fei) and oxygen vacancies (V,). However, it seems unlikely that sorption of oxygen would result in the formation of oxygen vacancies. On the other hand the formation of iron interstitials seems possible. The formation of similar defects at the surface upon adsorption of oxygen have already been observed for Co0 [S]. The following defect mechanisms have been considered for Fe,O, [9-111: .... 2Fe, + 2h’% 2Fe, + 2Vk: (4) or 2Fe,,
+ 30, + 6h’% 2Fef”‘+ 3/20,
(5)
Equilibrium (5) implies that the increase of oxygen partial pressure results in a decrease of Fe interstitials. Therefore, eq. 5 cannot be considered to explain the effect observed here. One may assume, however, that oxygen may stabilize these defects at the specific active centres which are formed during the mechanical treatment. The mechanism of oxygen interaction with the oxide surface can be considered in terms of the following relation between work function (+) and oxygen partial pressure [5]: 1 _=-n
(6)
where n is the parameter sensitive to the defect structure of the near-surface oxide layer. The parameter n in eq. 6 usually assumes positive values when
A
B
C
D
Fig. 6. Evolution of high energy grooves during thermal treatment.
49
1140K (867°C)
u_ 0
3 g100i? u 8 -150-
I,
I
I
100 200 TIME OF ANNEALING [hl
Fig. 7. The parameter
, 300
n in eq. 6 as a function
of time of annealing.
sorption or chemisorption of oxygen is taken into account [12]. As seen from Fig. 7 n is initially negative within regions A and B and drops to infinity at about 90 h. Its value becomes positive during incorporation of oxygen into the oxide lattice or by chemisorption. The negative value of n and its changes over time can be considered either in terms of dipoles formed within high energy grooves formed during polishing or donor-type point defects formed at special active centres within the grooves. Disappearance of this effect during the thermal treatment together with disappearance of the high energy grooves indicates that the grooves are responsible for the observed effect. After about 300 h the parameter n stabilizes at about 3 and this value may be considered characteristic of the defect structure of the Fe,O, surface layer in equilibrium. This defect structure will be the subject of a forthcoming paper.
Conclusions It has been shown that mechanical treatments such as grinding and polishing lead to the formation of defects which exhibit specific properties. These defects have a strong influence on the mechanism of oxygen interaction with the oxide surface and can lead to the formation of either positive oxygen dipoles or donor centres at the active surface sites which appear at high energy grooves formed during surface polishing. The model involving dipoles seems to be more consistent with the results. The surface defects lead
50
to an unusual interaction of the Fe,O, surface with oxygen, involving the positive charge. The charge alters as grooves of different orientations disappear during thermal treatment. Some grooves remain stable, namely those with a specific orientation which correspond to the lowest surface energy and which are stable in experimental conditions. The equilibration time for the Fe,O, surface at 1140 K is about 300 h. It would be of interest to study the “in situ” processes of change of grooves at different and well defined crystallographic planes.
References 1 G. Ohtani and M. Set-ma, Mater. Sci. Monographs No. 10, Elsevier, Amsterdam, 1982, p. 668. 2 J.S. Reed and A.-M. Lejus, Mater. Res. Bull., 12, (1977) 949. 3 J. Nowotny, M. Sloma and W. Weppner, Solid State Ionics, 28-30 (1988) 1445. 4 L.S. Darken and R.W. Gurry, J. Am. Chem. Sot., 67 (1945) 1398. 5 J. Nowotny, M. Sloma and W. Weppner, Adv. Ceram., 23 (1987) 159. 6 J. Nowotny, M. Sloma, J. Phys. (France), 47 (1986) Cl-807. 7 B. Hayden, S. Hachicha and A.M. Bradshow, Le Vide, Les Couches Minces, Sppl., Mater. 14th Intern. Conf., 201 (1981) 1125. 8 J. Nowotny, M. Sloma, and W. Weppner, in J. Nowotny and W.W. Weppner (Eds.), Non-Stoichiometric Compounds, Kluwer Acad., 1989, p. 265. 9 P. Kofstad, Electrical Conductivity, Diffusion and Nonstoichiometry in Binary Metal Oxides, Wiley, 1972. 10 R. Chang and J.B. Wagner, Jr., J. Am. Ceram. Sot., 55 (1972) 211. 11 Hj. Matzke, in T.O. Sorensen (Ed.), Nonstoichiometric Oxides, Academic Press, New York, 1981, p. 220. 12 J. Nowotny and M. Sloma, in J. Nowotny and L.C. Dufour (Eds.), Surface and NearSurface Chemistry of Oxide Materials, Elsevier, 1988, p. 281.