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Oxidation of tungsten and tungsten carbide in dry and humid atmospheres
ht. J. of Refractory Metals & Hard Materials 14 (1996) 345-353 0 1996 Elsevier Science Limited Printed
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Oxidation of Tungsten and Tungsten Carbide in Dry and Humid Atmospheres Anna Warren, Anders Nylund & Ingemar Department (Received
of Engineering 12 October
Olefjord
Metals, Chalmers University of Technology,
1995; accepted
S-412 96 Giiteborg,
Sweden
14 May 1996)
Abstract:
Oxidation experiments were performed on pure tungsten and hotpressed tungsten carbide. The chemical state and thickness of the oxide products were determined by ESCA. The oxidation of W and WC in dry atmosphere was performed in oxygen at temperatures ranging from 20 to 500°C. The oxide formed is WO,. The thickness of the oxide layer increases slowly up to 2OO”C, after which the oxide growth is rapid. The oxidation behaviour of W and WC in humid atmospheres was studied at room temperature in air at relative humidities of 60 and 95%. It was found that the thickness of the oxide layer increases with increased humidity. No formation of hydroxide was observed. Exposing W to water for one week results in a thick layer of WO,, WO, and hydroxide. In the case of WC no oxide at all was visible after exposure to water. Furthermore, WC is resistant to further oxidation after exposure. 0 1996 Elsevier Science Limited
INTRODUCTION
Gulbransen and Wysong” investigated the oxidation of tungsten in oxygen in the temperature range 25-550°C and found that the oxidation rate was strongly dependent on the temperature with a marked increase in oxidation rate at temperatures above 300°C. The oxidation behaviour of tungsten is affected by humidity. Webb et ~1.~ observed an increase in oxidation rate when oxygen was saturated with H,O at room temperature. This tendency was also noted by Friedrich” who suggested that the oxide formed corresponds to the formula WO, - 2H,O. In another study concerning oxidation in humid air, involving ESCAanalysis, it was suggested that the oxidation product consisted of a mixed tungsten oxidehydroxide.” However, an alternative ESCAstudy’ only revealed the presence of WO, on the surface. The oxidation of tungsten in water has been investigated very sparsely. In an article by Johnson8 it was observed that exposure of a tungsten specimen to deionized water resulted in a weight loss of the specimen. An ESCA-study’ of W exposed to water suggested the presence of
The oxidation behaviour of W and WC during milling and storage is of interest in the powder processing of tungsten hard alloys and hard metals. Therefore, this study has concentrated on the oxidation of pure W and WC at low temperatures ( <5OO”C) in dry and humid atmospheres. In addition the influence of water which is sometimes used as a milling fluid has been investigated by studying the oxidation behaviour of W and WC in water. Two stable tungsten oxides, WO, and W03, exist. Of these, WO, is the most thermodynamically stable oxide at low temperatures and atmospheric pressures. WO, is mostly prepared by reduction of WO, or tungstates, since direct oxidation of tungsten often results in the formation of WO,.’ A variety of other oxides have been reported, for example WO,.,, and WO,.,,, ’ but these are usually formed at high temperatures and according to Kellett and Rogers2 the distinction between these is one of structural perfection and orientation rather than of composition. 345
346
A. Warren, A. Nylund, I. Olefiord
both WV’+ and Wlv + in the surface oxide, but no interpretation of the results was carried out. In the case of WC the carbon affects the oxidation behaviour. Webb et al.,’ for example, found that at least at higher temperatures (700-1000°C) the oxidation rate is higher for WC than for pure tungsten. The oxidation behaviour at lower temperatures has so far not been elucidated. A literature survey of the oxidation behaviour of W and WC at low temperatures has revealed that the information available is meagre and often contradictory, in particular in the case of humid atmospheres and water. The need for more accurate information is however increasing. The aim of this paper is to determine the thickness and composition of the oxides formed on tungsten and tungsten carbide exposed to the mentioned atmospheres. This is accomplished by using ESCA-analysis.
EXPERIMENTAL
The oxidation experiments were performed on bulk polycrystalline tungsten and tungsten carbide. The tungsten samples were cut out of a l-mm-thick, 99.95% pure, tungsten foil and were approximately 1 x 1 cm’. The tungsten carbide samples were cut from 3-mm-thick, hotpressed test bars with a porosity of 0.005 v/o and a Co-content of 0.75 w/o. However, no trace of cobalt was found on the surface during The carbon/tungsten atomic ESCA-analysis. ratio of the carbide was 1.01. All samples were mechanically polished with a 1 pm diamond susfollowed by ultrasonic cleaning in pension ethanol and 2-propanol. To obtain a metallically clean surface the oxide formed during polishing and cleaning was removed by Ar ion-etching in the ESCA-instrument prior to the oxidation experiments. The oxidation of tungsten in dry atmosphere was performed in oxygen (P,,.=O*2 atm) for 1 h at five different temperatures- 20, 100, 200, 400 and 500°C. The furnace used for these experiments is connected to the ESCA-instrument in order to avoid contact with air after the ion etching step. To gain more precise information on the various oxidation states of tungsten in the oxide layer a W sample with a thick WO,-layer (formed by oxidation at 500°C) was reduced at
800°C for 15 min, in a hydrogen/nitrogen (15/85) atmosphere. In order to study the oxidation behaviour of W in humid atmospheres at room temperature the specimens were exposed to air at relative humidities (RH) of 60% and 95% for one week. A relative humidity of 95% was obtained simply by placing the specimens in a closed container partly filled with water. The humidity of 60% was achieved by using a controlled humidity chamber. A more detailed description of the humidity chamber can be found in Ref. 10. The oxidation of tungsten in water was performed by placing the samples in deionized water at room temperature for one week. Before exposure, half of the samples were cleaned by Ar ion-etching in the ESCA-instrument. The ion-etched samples, though metallically clean after etching, reoxidize slightly when transported from the ESCA-instrument to the beaker containing water. Therefore, the remaining samples were ground directly in the water with a PlOOO, SIC grinding paper. This was done to obtain a metallic surface in direct contact with water. The above oxidation experiments performed on tungsten were repeated on the tungsten carbide samples with few differences. The oxidized samples were analysed by ESCA (PHI 5500) at a take-off angle (the angle between the axis of the spectrometer lens and the sample surface) of 45” using a monocremated Al Ka X-ray source (1486.6 eV). The reported binding energy values are all references to the Au 4f,,, line taken as 84-O eV and the Cu 2pz,, line taken as 932.7 eV. ’ ’ Utilizing the formalism described in Ref. 12 the thickness of a thin oxide layer, d, on top of a metal surface can be calculated according to:
where /1 is the attenuation length of the photoelectrons, 0 is the take-off angle, I is the measured intensity, S is the experimental sensitivity factor and D is the atomic density of the oxide and metal states, respectively. For WO, and the atomic densities are metallic tungsten DG=O*O31 mole/cm3 and D$3;“‘=0.105 mole/ cm-‘. The R value was estimated from the empirical formulas fitted by Seat and Dench,” and was found to be: iL’$=32 A. Calibrations
347
Oxidation of Wand WC in humid atmospheres
performed at this laboratory on pure W and WO, gave the values of SK (4f) and SF;et (4f) to be 1110 and 460, respectively. In the case of WC the atomic density, DE’ and sensitivity factor, S r’, were replaced by D$‘:“=O*OSO mole/ cm3 and S$:” (4f) =550, respectively.
RESULTS Figure 1 depicts the ESCA spectra recorded from ion-etched tungsten and tungsten oxidized at 23°C and 500°C in pure oxygen for 1 h. The signals are curve fitted into peaks representing different chemical states and bondings of the elements. The W-signal in Fig. l(a) only shows peaks representing pure metallic tungsten. The binding energy of the main Wm”’ peak (W4f,J is 31.2 +0-l eV. The additional peak at 37.1 eV arises from electrons excited from the W5p
0 1s
Cls
II
W4f
II
PURE W
02- GIDIZED
50072 ih
X5 I
I
535
530
-
290
285
BINDING ENERGY
I
I
40
35
I
31
(eV)
Fig. 1. ESCA-spectra recorded from: (a) pure W, (b) W oxidized in pure oxygen at 23°C for 1 h, (c) W oxidized in pure oxygen at 500°C for 1 h.
Table 1. A summary of the thickness of the formed on W and WC after various treatments Experiment
Oxygen, 23”C, 1 h Oxygen, lOo”C, 1 h Oxygen, 2OO”C, 1 h Oxygen, 4Oo”C, 1 h Oxygen, 5Oo”C, 1 h RH=60%, 1 week RH=95%, 1 week Water, 1 week
W Oxide thickness (A)
10 10 16 > 100 >lOO 20 > 100 > 100
oxide
WC Oxide thjckness (A) 6 8
12 > 100 24 34 X0
levelI and is not included in the quantification procedure. The W-signal recorded from tungsten oxidized at 23°C can be divided into a series of peaks denoted as W”” and W”‘. Therefore, it can be concluded that the metal surface is covered by the oxide WO,. The presence of a W”“‘- signal shows that the oxide layer is very thin. In this case the oxide thickness was estimated to be 10 A. A summary of the estimated oxide thicknesses for all the oxidation experiments can be found in Table 1. From the sample oxidized at 500°C the only detectable W-signal is W6+ at 35.6 eV which means that the oxide layer is thicker than 100 A. This can be decided from the fact that 95% of the signal intensity is derived from a distance approximately 3i under the sample surface.’ ’ The oxygen signal is split into two peaks representing oxygen bound in oxide (O*-) and hydrocarbon compounds O(C), respectively. The binding energy of the 0’ -peak is at 530.6 eV. The carbon signal originates from a thin contamination layer present on the sample surface. It is curve fitted into two peaks representing C-H and C-OH. The intensity ratio between the high energy peak denoted O(C) in the O-signal and the carbon-oxygen peak originating from the contamination layer is about 2.5, which is the expected ratio between the 01s and Cls signals arising from the contamination layer. ” On the ion-etched sample [see Fig. l(a)] no carbon or oxygen is present since these species have been displaced during etching. The spectrum recorded from a specimen exposed to a relative humidity of 60% is shown in Fig. 2(a). The only peaks evident from the W-signal are W”“’ and W”‘. From Table 1 it is seen that the thickness of the oxide product
A. Warren. A. Nylund, I. Oleford
348
after exposure is 20 A. The W-signal from a sample exposed to 95% RH for one week [see Fig. 2(b)] only exhibits the W’+-peaks indicating a more rapid oxide growth than for W exposed to 60% RH. In addition, all the sample surfaces were blotchy indicating an uneven oxide formation. Figure 3 illustrates the spectrum recorded from a specimen exposed to water for one week. After exposure the surface is covered with a thick, yellow-brown, porous layer. The curve fitting procedure has been accomplished
[: 60% REL. HUM. 1 WEEK
95% REL. HUM. 1WEEK
535
530
290
265
40
3(
35
BINDING ENERGY (eV 1 L
Fig. 2. ESCA-spectra recorded from tungsten: (a) after exposure to air at a RH of 60% for 1 week, (b) after exposure to air at a RH of 95% for 1 week.
535
530
290
according to the outline in the Appendix. As in the case of the partially reduced tungsten trioxide, W”’ and W”+-signals arising from WO, and WO, are observed. In addition to these signals peaks representing W”’ in hydroxide are observed. Consequent curve fitting of the oxygen signal also reveals the presence of hydroxide, OH-, at a 1.3 eV higher binding energy than O* in WO,. The spectra recorded from the W-samples that were ground in water to obtain a clean metallic surface were identical with the spectra recorded from the samples cleaned by ion-etching before water exposure. It was therefore concluded that the thin oxide layer that forms on the ion-etched samples before exposure to water is of no significance for the oxidation behaviour. Figure 4 depicts the ESCA spectra recorded from ion-etched WC and WC oxidized at 23°C and 500°C in pure oxygen for 1 h. Figure 4(a) shows the spectrum of pure WC. In this case the binding energy of W”“ is 31.6 +0-l eV compared to 31.2kO.l eV for metallic tungsten. The carbon signal can be curve fitted into two peaks representing carbon bound to W [car. in Fig. 4(a)] and free carbon [C in Fig. 4(a)].‘“,” The chemical shift between bound carbon and free C is l-2 eV. From a comparison between the oxidation behaviour of tungsten and WC in oxygen it can be seen that the oxide layer is slightly thinner for WC (see Table 1). It is also seen that an oxide layer results in a sharp decrease in intensity for the peaks representing carbon bound to W and free carbon on the surface. By comparing Figs 2(a), (b) with 5(a), (b) it can be concluded that the oxidation behaviour
285
40
35
BINDING ENERGY (eV) Fig. 3.
ESCA-spectra
recorded
from tungsten
after exposure to water for 1 week.
349
Oxidation of Wand WC in humid atmospheres
of WC in humid atmospheres is similar to that of W. However, the oxide layer formed on WC after exposure to RH=95% is essentially thinner than that formed on W under the same conditions. Figure 5(c) illustrates the spectrum recorded from a WC sample exposed to water for one week. The W-signal reveals that there is no oxide present on the WC surface after exposure. This discovery is also supported by the very weak 02--peak in the oxygen signal. This is a result which differs completely from that recorded for W (see Fig. 3).
DISCUSSION All the oxidation experiments conducted in dry oxygen on W and WC resulted in the formation of WO, regardless of the oxidation tempera-
ture. It was noted that the oxide formed at temperatures up to 200°C is relatively thin. At 400°C and over the oxide thickness increases markedly which is in agreement with earlier studies.’ The fact that the oxidation of WC results in a thinner oxide layer compared to W, at least at low temperatures, is not surprising since WC is covalently bonded and therefore a higher activation energy is required for reaction. In the case of oxidation of W in humid atmospheres the information from earlier studies is contradictory. It has, for example, been suggested that the oxidation product is a mixed tungsten oxide-hydroxide6 or alternatively a hydrated oxide corresponding to the formula WO, - 2H,O.’ No evidence of hydroxide formation is found in the ESCA-spectra recorded from the W-samples exposed to relative
0 1s
01s
!!
Cls
!!
Cls
II
W4f
W4f
II
95% ~RE~~iut4.1 WEEK I I Oq-6XlDlZED
23-C lh
WATER EXPOSED 1WEEK
!
535
530
290
285
40
35
3(
BINDING ENERGY (eV) Fig. 4. ESCA-spectra recorded from (a) pure WC, (b) WC oxidized in pure oxygen at 23°C for 1 h, (c) WC oxidized in pure oxygen at 500°C for 1 h.
535
530
290
285
LO
35
31
BINDING ENERGY (eV) Fig. 5. ESCA-spectra recorded from WC, (a) after exposure to air at a RH of 60% for 1 week, (b) after exposure to air at a RH of 95% for 1 week, (c) after exposure to water for 1 week.
350
A. Warren. A. Nylund, I. Okfiord
humidities of 60% and 95%. If hydroxide had been present this should have been seen as an OH -peak located at a higher binding energy than the O’--peak. This observation does, however, not completely exclude the formation of a thin hydroxide layer on the surface. An earlier study performed on Al” has shown that hydroxide can decompose under the UHV-conditions of the ESCA-spectrometer and form oxide. Contrary to the exposure to humid atmospheres hydroxide was observed when W was exposed to water. There are two plausible reasons for this behaviour; the greater amount of water present provides a larger source of OH and secondly the hydroxide particles that form may be so thick that they are not completely decomposed to oxide in the UHV-chamber. As a result of the limited amount of oxygen dissolved in the water WO, was also found on the surface. During the exposure to water the oxide will react further to form tungstate ions.‘” In neutral and alkaline solutions tungsten exists ion, but if the pH as the tetrahedral WOl is drops below 7“’ or if the O/W-relationship less than four,“’ polytungstates can form. In fact, the pH-value recorded from the water solutions in this study after storage of W was 2.6 which suggests the presence of polytungstates. To study the long term effect of exposing tungsten to water a W sample was stored in a beaker of water for three months which resulted in the precipitation of a yellow powder on the bottom of the beaker. This powder is probably a oxide hydrate with the formula tungstic WO,.2H,O”.” which, in principle, is identical with the tungstic acid H,WO; H,O.” The latter in its protolysed form corresponds to wo: -. The behaviour of WC in humid air is more or less identical with that of W, with the exception that the oxide layer formed is thinner. In water, however, WC reacts in a completely different way. There is no sign of any oxide formation on the WC samples stored in water. In fact, it was found that even the thin oxide layer formed on the sample during the transport from the ESCA-instrument to the beaker containing water had disappeared. To study this phenomena further, two WC samples were preoxidized at 200°C to obtain a WO,-layer on the surface. The samples were then exposed to water for 17 days after which
one sample was analysed directly after removal from the water and the second sample was exposed to air for 36 h prior to analysis. Neither of these samples showed any oxide layer on the surface. After analysis the first sample was placed in air for one week to determine whether exposure to the ultra high vacuum in the ESCA-system influences the oxidation behaviour, but after one week in air there was still no sign of oxide formation. It was therefore concluded that the WO,-layer present prior to exposure reacts with the water to form tungstates after which the surface is resistant to further oxidation. This was verified by an analysis of the tungsten content of the water. It was found that the tungsten content was 0.6 ppm which more or less corresponds to the amount of tungsten bound in the perforated WO,-layer. However, an analysis of the water from a beaker containing a W sample of the same size revealed that the tungsten content was 30 ppm. indicating that in this case the oxidation of tungsten and formation of tungstates is a continuous process. The reason for the passivation of the WCsurface is not yet fully understood. It can, however, be assumed that the crystalline diamond structure of WC is disturbed in the surface region resulting in W atoms of incomplete bonding. These will oxidize quite readily to form WO., and dissolve in the water in accordance with the reactions described above. When all the incompletely bonded W atoms have been oxidized and dissolved in this way the outermost layer of the crystal will only consist of carbon atoms. A possibility is that these atoms will form covalent bondings with the carbon atoms in the second layer resulting in a very stable surface structure which will be almost immune to further reactions. The above remarkable behaviour of WC is clearly of practical significance to the preparation and application of WC-based materials, e.g. in the PM-industry.
CONCLUSIONS The oxide formed on W and WC exposed in dry oxygen for 1 h from room temperature to 500°C is WO, regardless of the oxidation temperature. a The oxide layer formed on WC is slightly thinner than the oxide layer formed on W. l
Oxidation of Wand
0
0 0
0
351
WC in humid atmospheres
In air at room temperature the oxidation rate of W and WC increases with increasing humidity. No evidence of any hydroxide formation in humid atmospheres was found. When W is exposed to water the oxidation is considerable resulting in a thick layer in which WOZ, WO, and hydroxide are present. When WC is stored in water there is no sign of any oxide formation. In fact, storing preoxidized WC in water will result in the removal of the oxide layer formed prior to exposure. Furthermore, it has been shown that WC exposed to water is resistant to further oxidation.
ACKNOWLEDGEMENTS Elis Carlstriim at the Swedish Ceramic Institute is acknowledged for valuable discussions. The Swedish National Board for Technical and Industrial Development, NUTEK, is acknowledged for financial support.
12. Nylund, A. & Olefjord, I., Hydration of Al,O, and decomposition of Al(OH), in a vacuum as studied by ESCA. SUI$ Integace Anal., 21 (1994) 283-9. 13. Seah, M. P. & Dench, W. A., Quantitative electron spectroscopy of surfaces: a standard data base for electron inelastic mean free paths in solids. Su$ InterfaceAnal., 1(1979) 2-11. 14. Himpsel, F. J., Morar, J. F. & McFeely, F. R., et al., Core-level shifts and oxidation states of Ta and W: electron spectroscopy for chemical analysis applied to surfaces. Physical Review B, 30 (1984) 7236-41. 15. Olefjord, I., Brox, B. & Jelvestam, U., Surface composition of stainless steel during anodic dissolution and passivation studied by ESCA. J. Electrochem. Sot., 132 (1985) 2854-61.
16. Nakazawa, M. & Okamoto, H., Surface composition of prepared tungsten carbide and its catalytic activity. Appl. Su$
Sci., 24 (1985) 75-86.
17. Qin, D.-Y. & Gao, Z., XPS study of tungsten carbide. Chin. J. Chem., 4 (1990) 301-S. 18. Freier, R. K., Aqueous Solutions Data for Inorganic and Organic Compounds, Vol. 1. Walter de Gruyter, Berlin, 1976. 19. Griffith, W. P. & Lesniak, P. J. B., Raman studies on species in aqueous solutions. Part III. Vanadates, molybdates and tungstates. J. Chem. Sot. (A), (1969) 1066-71. 20. H%gg, G., Allmiin och Oorganisk Kemi. Almqvist & Wiksell, Stockholm, 1963. 21. Kepert, D. L., Isopolytungstates. ProgE Org. Chem., 4 (1962)
199-274.
22. Freedman,
M. L., The tungstic
acids. J. Am. Chem.
Sot., 81 (1959) 3834-9.
REFERENCES 1. Gmelin Handbuch der Anorganischen Chemie, B2, Springer, 1979. 2. Kellett, E. A. & Rogers, S. E., The structure of oxide layers on tungsten. .I. Electrochemical Sot., 110 (1963)502-4. 3. Gulbransen,
E. A. & Wysong, W. S., Thin oxide films on tungsten. AZME Trans., 175 (1948) 61 l-27. 4. Webb, W. W., Norton, J. T. & Wagner, C., Oxidation of tungsten. J. Electrochemical Sot., 103 (1956) 107-11. 5. Friedrich,
K., Oxidation of molybdenum, tungsten and rhenium powders at room temperature in air of different humidity. J. Less-Common Metals, 16 (1968)
147-56. 6. Barr, T. L., An ESCA study of the termination of the passivation of elemental metals. J. Phys. Chem., 82 (1978) 1801-10. 7. Helm, R. & Storp, S., Nachweis von Oxiden niedriger Wertigkcitsstufen auf Metallen mit ESCA. VbkuumTechnik., 25 (1976) 172-5. B. A., Corrosion of metals in deionized 8. Johnson,
water at 38°C (100°F). NASA-Tm-X-1791 (1969) l-7. J. T. & Wagner, C., Oxidation studies in metal-carbon systems. J. Electrochemical
23. Griinert, W., Shpiro, E. S., Feldhaus, R., t’t al., Reduction behavior and metethesis activity of WO,/ AIZO, catalysts: I. An XPS investigation of WO,/ Al,O, catalysts. J. Catal., 107 (1987) 522-34. 24. Wachs, I. E., Cherish, C. C. & Hardenbergh, J. H., Reduction of WO,/AI,O, and unsupported WO,: a comparative ESCA-study. Appl. Catal., 13 (1985) 335-46.
25. Haber, J., Stoch, J. & Ungier, L., Electron spectroscopic studies of the reduction of WO,. J. Solid State Chem., 19 (1976) 113-5. 26. Goodenough, J. B., Metallic oxides. Prog. Solid State Chem., 5 (i971).
27. Beatham, N. & Orchard, A. F., X-ray and UV photoelectron snectra of the oxides NbO,, MOO, and RuO,. J. Eiectron Spec. Rel. Phen., 16 (1979) 77-86. 28. Werfel, F. & Minni, E., Photoemission study of the electronic structure of MO and MO oxides. J. Phys. C: Solid State Phys., 16 (1983) 6091-100.
29. Brox, B. & Olefjord, I., ESCA studies of MoOz and MOO,. Su$ Integace Anal., 13 (1988) 3-6.
APPENDIX
9. Webb, W. W., Norton, Sot., 103 (1956)
10. Nylund,
112-17.
A. & Olefjord, exposed rapidly solidified alloy powders. Powd. Met., Il. Briggs, D. & Seah, M. P., sis by Auger
and
Wiley, Chichester,
I., Surface analysis of airaluminium and aluminium 36 (1993) 193-7. ed., Practical Surfhce Analy-
X-ray Photoelectron
1983.
Spectroscopy.
It has been reported in the literature23-2” that W oxides containing the cations W2+, W4+, W”+ and W”’ are found from ESCA measurements. However, from the chemical point of view only two pure simple oxides, WO, and WO, exist.’ Beside these two oxides a large
352
A. Warren. A. Nylund, I. Olejjord
number of mixed oxides, e.g. W,O, and W,O, have been reported. ’ The structure of the two oxides is described as follows; at room temperature WO, forms a structure with monoclinic symmetry.’ Each tungsten ion is coordinated into the centre of a WO, octahedra, sharing each oxygen ion with another tungsten ion. The structure of WO, as shown in Fig. Al is similar to the r-utile structure of TiO,.“‘” Each W atom is coordinated to six oxygen atoms. The octahedra are joined by sharing edges to form strings which are mutually connected into a three-dimensional structure by corner-sharing of the octahedra. Within the strings the W-W distances are alternatively short (2.5 A) or long (3.1 A). This results in the formation of pairs of W atoms and a consequent distortion of the WO, octahedra which is
t C
Fig. Al.
The tetragonal
structure
of WO,.
not present in the common undistorted rutile structure of TiO,. The structure of WO, is exactly the same as for Mo02? It is therefore expected that the interpretation of ESCA spectra can be performed in the same way, due to the fact that MOO, and WO, have the same type of energy level diagram.*’ The W4f spectra recorded from W oxides are complex. Figure A2 illustrates the spectrum recorded after partially reducing a W sample covered by a thick WO,-layer. The positions of the doublet representing the metallic state are 31.2 and 33.4 eV. The chemical shift for the W”+ -peaks recorded from pure WO, is 4.4 eV. A linear interpolation between the metallic and six valent states predicts that the chemical shift of W4+ is at 3.2 eV. As seen from the figure, small peaks exist at a shift of 3.2 eV. However, the strongest peak representing W4+ is shifted by only 1.6 eV. In the literature this peak has often been denoted by Wz+ .23-25 In this paper it is suggested that the small chemical shift of the main peak of W”’ is due to the structural effect of WO,. The same behaviour has been observed from the recording of ESCA spectra from MoO,.*~-~” The interaction between the metal ions gives a partly filled WW G bond at the Fermi level which imparts high conductivity to the oxide.2” It is proposed that the W-W-bonds which give this metallic character screen the core electrons and thereby lower the effective charge and the coulomb interaction. Thus, the final electron state is lowered and the main peak appears at a position corresponding to a lower apparent oxidation number. The low intensity peak is positioned at
W4f
BINDING ENERGY (eV) Fig. A2.
ESCA-spectra
recorded
after partially reducing a W sample covered nitrogen atmosphere at 800°C for 15 min.
by a thick WO,-layer
in a hydrogen/
Oxidation of Wand
WC in humid atmospheres
the binding energy which is expected if WO, had been the same type of insulator as WO,. It is suggested that the position of the low intensity peak is caused partly by molecular screen-
353
ing of the core level. The W ions situated at long distances from each other give no overlapping orbitals and thereby the screening of the core electrons is partly ionic.
PII:
ELSEVIER
in Great
Britain.
All rights reserved 0263-4368/96/$15.00
SO263-4368(96)00027-3
Oxidation of Tungsten and Tungsten Carbide in Dry and Humid Atmospheres Anna Warren, Anders Nylund & Ingemar Department (Received
of Engineering 12 October
Olefjord
Metals, Chalmers University of Technology,
1995; accepted
S-412 96 Giiteborg,
Sweden
14 May 1996)
Abstract:
Oxidation experiments were performed on pure tungsten and hotpressed tungsten carbide. The chemical state and thickness of the oxide products were determined by ESCA. The oxidation of W and WC in dry atmosphere was performed in oxygen at temperatures ranging from 20 to 500°C. The oxide formed is WO,. The thickness of the oxide layer increases slowly up to 2OO”C, after which the oxide growth is rapid. The oxidation behaviour of W and WC in humid atmospheres was studied at room temperature in air at relative humidities of 60 and 95%. It was found that the thickness of the oxide layer increases with increased humidity. No formation of hydroxide was observed. Exposing W to water for one week results in a thick layer of WO,, WO, and hydroxide. In the case of WC no oxide at all was visible after exposure to water. Furthermore, WC is resistant to further oxidation after exposure. 0 1996 Elsevier Science Limited
INTRODUCTION
Gulbransen and Wysong” investigated the oxidation of tungsten in oxygen in the temperature range 25-550°C and found that the oxidation rate was strongly dependent on the temperature with a marked increase in oxidation rate at temperatures above 300°C. The oxidation behaviour of tungsten is affected by humidity. Webb et ~1.~ observed an increase in oxidation rate when oxygen was saturated with H,O at room temperature. This tendency was also noted by Friedrich” who suggested that the oxide formed corresponds to the formula WO, - 2H,O. In another study concerning oxidation in humid air, involving ESCAanalysis, it was suggested that the oxidation product consisted of a mixed tungsten oxidehydroxide.” However, an alternative ESCAstudy’ only revealed the presence of WO, on the surface. The oxidation of tungsten in water has been investigated very sparsely. In an article by Johnson8 it was observed that exposure of a tungsten specimen to deionized water resulted in a weight loss of the specimen. An ESCA-study’ of W exposed to water suggested the presence of
The oxidation behaviour of W and WC during milling and storage is of interest in the powder processing of tungsten hard alloys and hard metals. Therefore, this study has concentrated on the oxidation of pure W and WC at low temperatures ( <5OO”C) in dry and humid atmospheres. In addition the influence of water which is sometimes used as a milling fluid has been investigated by studying the oxidation behaviour of W and WC in water. Two stable tungsten oxides, WO, and W03, exist. Of these, WO, is the most thermodynamically stable oxide at low temperatures and atmospheric pressures. WO, is mostly prepared by reduction of WO, or tungstates, since direct oxidation of tungsten often results in the formation of WO,.’ A variety of other oxides have been reported, for example WO,.,, and WO,.,,, ’ but these are usually formed at high temperatures and according to Kellett and Rogers2 the distinction between these is one of structural perfection and orientation rather than of composition. 345
346
A. Warren, A. Nylund, I. Olefiord
both WV’+ and Wlv + in the surface oxide, but no interpretation of the results was carried out. In the case of WC the carbon affects the oxidation behaviour. Webb et al.,’ for example, found that at least at higher temperatures (700-1000°C) the oxidation rate is higher for WC than for pure tungsten. The oxidation behaviour at lower temperatures has so far not been elucidated. A literature survey of the oxidation behaviour of W and WC at low temperatures has revealed that the information available is meagre and often contradictory, in particular in the case of humid atmospheres and water. The need for more accurate information is however increasing. The aim of this paper is to determine the thickness and composition of the oxides formed on tungsten and tungsten carbide exposed to the mentioned atmospheres. This is accomplished by using ESCA-analysis.
EXPERIMENTAL
The oxidation experiments were performed on bulk polycrystalline tungsten and tungsten carbide. The tungsten samples were cut out of a l-mm-thick, 99.95% pure, tungsten foil and were approximately 1 x 1 cm’. The tungsten carbide samples were cut from 3-mm-thick, hotpressed test bars with a porosity of 0.005 v/o and a Co-content of 0.75 w/o. However, no trace of cobalt was found on the surface during The carbon/tungsten atomic ESCA-analysis. ratio of the carbide was 1.01. All samples were mechanically polished with a 1 pm diamond susfollowed by ultrasonic cleaning in pension ethanol and 2-propanol. To obtain a metallically clean surface the oxide formed during polishing and cleaning was removed by Ar ion-etching in the ESCA-instrument prior to the oxidation experiments. The oxidation of tungsten in dry atmosphere was performed in oxygen (P,,.=O*2 atm) for 1 h at five different temperatures- 20, 100, 200, 400 and 500°C. The furnace used for these experiments is connected to the ESCA-instrument in order to avoid contact with air after the ion etching step. To gain more precise information on the various oxidation states of tungsten in the oxide layer a W sample with a thick WO,-layer (formed by oxidation at 500°C) was reduced at
800°C for 15 min, in a hydrogen/nitrogen (15/85) atmosphere. In order to study the oxidation behaviour of W in humid atmospheres at room temperature the specimens were exposed to air at relative humidities (RH) of 60% and 95% for one week. A relative humidity of 95% was obtained simply by placing the specimens in a closed container partly filled with water. The humidity of 60% was achieved by using a controlled humidity chamber. A more detailed description of the humidity chamber can be found in Ref. 10. The oxidation of tungsten in water was performed by placing the samples in deionized water at room temperature for one week. Before exposure, half of the samples were cleaned by Ar ion-etching in the ESCA-instrument. The ion-etched samples, though metallically clean after etching, reoxidize slightly when transported from the ESCA-instrument to the beaker containing water. Therefore, the remaining samples were ground directly in the water with a PlOOO, SIC grinding paper. This was done to obtain a metallic surface in direct contact with water. The above oxidation experiments performed on tungsten were repeated on the tungsten carbide samples with few differences. The oxidized samples were analysed by ESCA (PHI 5500) at a take-off angle (the angle between the axis of the spectrometer lens and the sample surface) of 45” using a monocremated Al Ka X-ray source (1486.6 eV). The reported binding energy values are all references to the Au 4f,,, line taken as 84-O eV and the Cu 2pz,, line taken as 932.7 eV. ’ ’ Utilizing the formalism described in Ref. 12 the thickness of a thin oxide layer, d, on top of a metal surface can be calculated according to:
where /1 is the attenuation length of the photoelectrons, 0 is the take-off angle, I is the measured intensity, S is the experimental sensitivity factor and D is the atomic density of the oxide and metal states, respectively. For WO, and the atomic densities are metallic tungsten DG=O*O31 mole/cm3 and D$3;“‘=0.105 mole/ cm-‘. The R value was estimated from the empirical formulas fitted by Seat and Dench,” and was found to be: iL’$=32 A. Calibrations
347
Oxidation of Wand WC in humid atmospheres
performed at this laboratory on pure W and WO, gave the values of SK (4f) and SF;et (4f) to be 1110 and 460, respectively. In the case of WC the atomic density, DE’ and sensitivity factor, S r’, were replaced by D$‘:“=O*OSO mole/ cm3 and S$:” (4f) =550, respectively.
RESULTS Figure 1 depicts the ESCA spectra recorded from ion-etched tungsten and tungsten oxidized at 23°C and 500°C in pure oxygen for 1 h. The signals are curve fitted into peaks representing different chemical states and bondings of the elements. The W-signal in Fig. l(a) only shows peaks representing pure metallic tungsten. The binding energy of the main Wm”’ peak (W4f,J is 31.2 +0-l eV. The additional peak at 37.1 eV arises from electrons excited from the W5p
0 1s
Cls
II
W4f
II
PURE W
02- GIDIZED
50072 ih
X5 I
I
535
530
-
290
285
BINDING ENERGY
I
I
40
35
I
31
(eV)
Fig. 1. ESCA-spectra recorded from: (a) pure W, (b) W oxidized in pure oxygen at 23°C for 1 h, (c) W oxidized in pure oxygen at 500°C for 1 h.
Table 1. A summary of the thickness of the formed on W and WC after various treatments Experiment
Oxygen, 23”C, 1 h Oxygen, lOo”C, 1 h Oxygen, 2OO”C, 1 h Oxygen, 4Oo”C, 1 h Oxygen, 5Oo”C, 1 h RH=60%, 1 week RH=95%, 1 week Water, 1 week
W Oxide thickness (A)
10 10 16 > 100 >lOO 20 > 100 > 100
oxide
WC Oxide thjckness (A) 6 8
12 > 100 24 34 X0
levelI and is not included in the quantification procedure. The W-signal recorded from tungsten oxidized at 23°C can be divided into a series of peaks denoted as W”” and W”‘. Therefore, it can be concluded that the metal surface is covered by the oxide WO,. The presence of a W”“‘- signal shows that the oxide layer is very thin. In this case the oxide thickness was estimated to be 10 A. A summary of the estimated oxide thicknesses for all the oxidation experiments can be found in Table 1. From the sample oxidized at 500°C the only detectable W-signal is W6+ at 35.6 eV which means that the oxide layer is thicker than 100 A. This can be decided from the fact that 95% of the signal intensity is derived from a distance approximately 3i under the sample surface.’ ’ The oxygen signal is split into two peaks representing oxygen bound in oxide (O*-) and hydrocarbon compounds O(C), respectively. The binding energy of the 0’ -peak is at 530.6 eV. The carbon signal originates from a thin contamination layer present on the sample surface. It is curve fitted into two peaks representing C-H and C-OH. The intensity ratio between the high energy peak denoted O(C) in the O-signal and the carbon-oxygen peak originating from the contamination layer is about 2.5, which is the expected ratio between the 01s and Cls signals arising from the contamination layer. ” On the ion-etched sample [see Fig. l(a)] no carbon or oxygen is present since these species have been displaced during etching. The spectrum recorded from a specimen exposed to a relative humidity of 60% is shown in Fig. 2(a). The only peaks evident from the W-signal are W”“’ and W”‘. From Table 1 it is seen that the thickness of the oxide product
A. Warren. A. Nylund, I. Oleford
348
after exposure is 20 A. The W-signal from a sample exposed to 95% RH for one week [see Fig. 2(b)] only exhibits the W’+-peaks indicating a more rapid oxide growth than for W exposed to 60% RH. In addition, all the sample surfaces were blotchy indicating an uneven oxide formation. Figure 3 illustrates the spectrum recorded from a specimen exposed to water for one week. After exposure the surface is covered with a thick, yellow-brown, porous layer. The curve fitting procedure has been accomplished
[: 60% REL. HUM. 1 WEEK
95% REL. HUM. 1WEEK
535
530
290
265
40
3(
35
BINDING ENERGY (eV 1 L
Fig. 2. ESCA-spectra recorded from tungsten: (a) after exposure to air at a RH of 60% for 1 week, (b) after exposure to air at a RH of 95% for 1 week.
535
530
290
according to the outline in the Appendix. As in the case of the partially reduced tungsten trioxide, W”’ and W”+-signals arising from WO, and WO, are observed. In addition to these signals peaks representing W”’ in hydroxide are observed. Consequent curve fitting of the oxygen signal also reveals the presence of hydroxide, OH-, at a 1.3 eV higher binding energy than O* in WO,. The spectra recorded from the W-samples that were ground in water to obtain a clean metallic surface were identical with the spectra recorded from the samples cleaned by ion-etching before water exposure. It was therefore concluded that the thin oxide layer that forms on the ion-etched samples before exposure to water is of no significance for the oxidation behaviour. Figure 4 depicts the ESCA spectra recorded from ion-etched WC and WC oxidized at 23°C and 500°C in pure oxygen for 1 h. Figure 4(a) shows the spectrum of pure WC. In this case the binding energy of W”“ is 31.6 +0-l eV compared to 31.2kO.l eV for metallic tungsten. The carbon signal can be curve fitted into two peaks representing carbon bound to W [car. in Fig. 4(a)] and free carbon [C in Fig. 4(a)].‘“,” The chemical shift between bound carbon and free C is l-2 eV. From a comparison between the oxidation behaviour of tungsten and WC in oxygen it can be seen that the oxide layer is slightly thinner for WC (see Table 1). It is also seen that an oxide layer results in a sharp decrease in intensity for the peaks representing carbon bound to W and free carbon on the surface. By comparing Figs 2(a), (b) with 5(a), (b) it can be concluded that the oxidation behaviour
285
40
35
BINDING ENERGY (eV) Fig. 3.
ESCA-spectra
recorded
from tungsten
after exposure to water for 1 week.
349
Oxidation of Wand WC in humid atmospheres
of WC in humid atmospheres is similar to that of W. However, the oxide layer formed on WC after exposure to RH=95% is essentially thinner than that formed on W under the same conditions. Figure 5(c) illustrates the spectrum recorded from a WC sample exposed to water for one week. The W-signal reveals that there is no oxide present on the WC surface after exposure. This discovery is also supported by the very weak 02--peak in the oxygen signal. This is a result which differs completely from that recorded for W (see Fig. 3).
DISCUSSION All the oxidation experiments conducted in dry oxygen on W and WC resulted in the formation of WO, regardless of the oxidation tempera-
ture. It was noted that the oxide formed at temperatures up to 200°C is relatively thin. At 400°C and over the oxide thickness increases markedly which is in agreement with earlier studies.’ The fact that the oxidation of WC results in a thinner oxide layer compared to W, at least at low temperatures, is not surprising since WC is covalently bonded and therefore a higher activation energy is required for reaction. In the case of oxidation of W in humid atmospheres the information from earlier studies is contradictory. It has, for example, been suggested that the oxidation product is a mixed tungsten oxide-hydroxide6 or alternatively a hydrated oxide corresponding to the formula WO, - 2H,O.’ No evidence of hydroxide formation is found in the ESCA-spectra recorded from the W-samples exposed to relative
0 1s
01s
!!
Cls
!!
Cls
II
W4f
W4f
II
95% ~RE~~iut4.1 WEEK I I Oq-6XlDlZED
23-C lh
WATER EXPOSED 1WEEK
!
535
530
290
285
40
35
3(
BINDING ENERGY (eV) Fig. 4. ESCA-spectra recorded from (a) pure WC, (b) WC oxidized in pure oxygen at 23°C for 1 h, (c) WC oxidized in pure oxygen at 500°C for 1 h.
535
530
290
285
LO
35
31
BINDING ENERGY (eV) Fig. 5. ESCA-spectra recorded from WC, (a) after exposure to air at a RH of 60% for 1 week, (b) after exposure to air at a RH of 95% for 1 week, (c) after exposure to water for 1 week.
350
A. Warren. A. Nylund, I. Okfiord
humidities of 60% and 95%. If hydroxide had been present this should have been seen as an OH -peak located at a higher binding energy than the O’--peak. This observation does, however, not completely exclude the formation of a thin hydroxide layer on the surface. An earlier study performed on Al” has shown that hydroxide can decompose under the UHV-conditions of the ESCA-spectrometer and form oxide. Contrary to the exposure to humid atmospheres hydroxide was observed when W was exposed to water. There are two plausible reasons for this behaviour; the greater amount of water present provides a larger source of OH and secondly the hydroxide particles that form may be so thick that they are not completely decomposed to oxide in the UHV-chamber. As a result of the limited amount of oxygen dissolved in the water WO, was also found on the surface. During the exposure to water the oxide will react further to form tungstate ions.‘” In neutral and alkaline solutions tungsten exists ion, but if the pH as the tetrahedral WOl is drops below 7“’ or if the O/W-relationship less than four,“’ polytungstates can form. In fact, the pH-value recorded from the water solutions in this study after storage of W was 2.6 which suggests the presence of polytungstates. To study the long term effect of exposing tungsten to water a W sample was stored in a beaker of water for three months which resulted in the precipitation of a yellow powder on the bottom of the beaker. This powder is probably a oxide hydrate with the formula tungstic WO,.2H,O”.” which, in principle, is identical with the tungstic acid H,WO; H,O.” The latter in its protolysed form corresponds to wo: -. The behaviour of WC in humid air is more or less identical with that of W, with the exception that the oxide layer formed is thinner. In water, however, WC reacts in a completely different way. There is no sign of any oxide formation on the WC samples stored in water. In fact, it was found that even the thin oxide layer formed on the sample during the transport from the ESCA-instrument to the beaker containing water had disappeared. To study this phenomena further, two WC samples were preoxidized at 200°C to obtain a WO,-layer on the surface. The samples were then exposed to water for 17 days after which
one sample was analysed directly after removal from the water and the second sample was exposed to air for 36 h prior to analysis. Neither of these samples showed any oxide layer on the surface. After analysis the first sample was placed in air for one week to determine whether exposure to the ultra high vacuum in the ESCA-system influences the oxidation behaviour, but after one week in air there was still no sign of oxide formation. It was therefore concluded that the WO,-layer present prior to exposure reacts with the water to form tungstates after which the surface is resistant to further oxidation. This was verified by an analysis of the tungsten content of the water. It was found that the tungsten content was 0.6 ppm which more or less corresponds to the amount of tungsten bound in the perforated WO,-layer. However, an analysis of the water from a beaker containing a W sample of the same size revealed that the tungsten content was 30 ppm. indicating that in this case the oxidation of tungsten and formation of tungstates is a continuous process. The reason for the passivation of the WCsurface is not yet fully understood. It can, however, be assumed that the crystalline diamond structure of WC is disturbed in the surface region resulting in W atoms of incomplete bonding. These will oxidize quite readily to form WO., and dissolve in the water in accordance with the reactions described above. When all the incompletely bonded W atoms have been oxidized and dissolved in this way the outermost layer of the crystal will only consist of carbon atoms. A possibility is that these atoms will form covalent bondings with the carbon atoms in the second layer resulting in a very stable surface structure which will be almost immune to further reactions. The above remarkable behaviour of WC is clearly of practical significance to the preparation and application of WC-based materials, e.g. in the PM-industry.
CONCLUSIONS The oxide formed on W and WC exposed in dry oxygen for 1 h from room temperature to 500°C is WO, regardless of the oxidation temperature. a The oxide layer formed on WC is slightly thinner than the oxide layer formed on W. l
Oxidation of Wand
0
0 0
0
351
WC in humid atmospheres
In air at room temperature the oxidation rate of W and WC increases with increasing humidity. No evidence of any hydroxide formation in humid atmospheres was found. When W is exposed to water the oxidation is considerable resulting in a thick layer in which WOZ, WO, and hydroxide are present. When WC is stored in water there is no sign of any oxide formation. In fact, storing preoxidized WC in water will result in the removal of the oxide layer formed prior to exposure. Furthermore, it has been shown that WC exposed to water is resistant to further oxidation.
ACKNOWLEDGEMENTS Elis Carlstriim at the Swedish Ceramic Institute is acknowledged for valuable discussions. The Swedish National Board for Technical and Industrial Development, NUTEK, is acknowledged for financial support.
12. Nylund, A. & Olefjord, I., Hydration of Al,O, and decomposition of Al(OH), in a vacuum as studied by ESCA. SUI$ Integace Anal., 21 (1994) 283-9. 13. Seah, M. P. & Dench, W. A., Quantitative electron spectroscopy of surfaces: a standard data base for electron inelastic mean free paths in solids. Su$ InterfaceAnal., 1(1979) 2-11. 14. Himpsel, F. J., Morar, J. F. & McFeely, F. R., et al., Core-level shifts and oxidation states of Ta and W: electron spectroscopy for chemical analysis applied to surfaces. Physical Review B, 30 (1984) 7236-41. 15. Olefjord, I., Brox, B. & Jelvestam, U., Surface composition of stainless steel during anodic dissolution and passivation studied by ESCA. J. Electrochem. Sot., 132 (1985) 2854-61.
16. Nakazawa, M. & Okamoto, H., Surface composition of prepared tungsten carbide and its catalytic activity. Appl. Su$
Sci., 24 (1985) 75-86.
17. Qin, D.-Y. & Gao, Z., XPS study of tungsten carbide. Chin. J. Chem., 4 (1990) 301-S. 18. Freier, R. K., Aqueous Solutions Data for Inorganic and Organic Compounds, Vol. 1. Walter de Gruyter, Berlin, 1976. 19. Griffith, W. P. & Lesniak, P. J. B., Raman studies on species in aqueous solutions. Part III. Vanadates, molybdates and tungstates. J. Chem. Sot. (A), (1969) 1066-71. 20. H%gg, G., Allmiin och Oorganisk Kemi. Almqvist & Wiksell, Stockholm, 1963. 21. Kepert, D. L., Isopolytungstates. ProgE Org. Chem., 4 (1962)
199-274.
22. Freedman,
M. L., The tungstic
acids. J. Am. Chem.
Sot., 81 (1959) 3834-9.
REFERENCES 1. Gmelin Handbuch der Anorganischen Chemie, B2, Springer, 1979. 2. Kellett, E. A. & Rogers, S. E., The structure of oxide layers on tungsten. .I. Electrochemical Sot., 110 (1963)502-4. 3. Gulbransen,
E. A. & Wysong, W. S., Thin oxide films on tungsten. AZME Trans., 175 (1948) 61 l-27. 4. Webb, W. W., Norton, J. T. & Wagner, C., Oxidation of tungsten. J. Electrochemical Sot., 103 (1956) 107-11. 5. Friedrich,
K., Oxidation of molybdenum, tungsten and rhenium powders at room temperature in air of different humidity. J. Less-Common Metals, 16 (1968)
147-56. 6. Barr, T. L., An ESCA study of the termination of the passivation of elemental metals. J. Phys. Chem., 82 (1978) 1801-10. 7. Helm, R. & Storp, S., Nachweis von Oxiden niedriger Wertigkcitsstufen auf Metallen mit ESCA. VbkuumTechnik., 25 (1976) 172-5. B. A., Corrosion of metals in deionized 8. Johnson,
water at 38°C (100°F). NASA-Tm-X-1791 (1969) l-7. J. T. & Wagner, C., Oxidation studies in metal-carbon systems. J. Electrochemical
23. Griinert, W., Shpiro, E. S., Feldhaus, R., t’t al., Reduction behavior and metethesis activity of WO,/ AIZO, catalysts: I. An XPS investigation of WO,/ Al,O, catalysts. J. Catal., 107 (1987) 522-34. 24. Wachs, I. E., Cherish, C. C. & Hardenbergh, J. H., Reduction of WO,/AI,O, and unsupported WO,: a comparative ESCA-study. Appl. Catal., 13 (1985) 335-46.
25. Haber, J., Stoch, J. & Ungier, L., Electron spectroscopic studies of the reduction of WO,. J. Solid State Chem., 19 (1976) 113-5. 26. Goodenough, J. B., Metallic oxides. Prog. Solid State Chem., 5 (i971).
27. Beatham, N. & Orchard, A. F., X-ray and UV photoelectron snectra of the oxides NbO,, MOO, and RuO,. J. Eiectron Spec. Rel. Phen., 16 (1979) 77-86. 28. Werfel, F. & Minni, E., Photoemission study of the electronic structure of MO and MO oxides. J. Phys. C: Solid State Phys., 16 (1983) 6091-100.
29. Brox, B. & Olefjord, I., ESCA studies of MoOz and MOO,. Su$ Integace Anal., 13 (1988) 3-6.
APPENDIX
9. Webb, W. W., Norton, Sot., 103 (1956)
10. Nylund,
112-17.
A. & Olefjord, exposed rapidly solidified alloy powders. Powd. Met., Il. Briggs, D. & Seah, M. P., sis by Auger
and
Wiley, Chichester,
I., Surface analysis of airaluminium and aluminium 36 (1993) 193-7. ed., Practical Surfhce Analy-
X-ray Photoelectron
1983.
Spectroscopy.
It has been reported in the literature23-2” that W oxides containing the cations W2+, W4+, W”+ and W”’ are found from ESCA measurements. However, from the chemical point of view only two pure simple oxides, WO, and WO, exist.’ Beside these two oxides a large
352
A. Warren. A. Nylund, I. Olejjord
number of mixed oxides, e.g. W,O, and W,O, have been reported. ’ The structure of the two oxides is described as follows; at room temperature WO, forms a structure with monoclinic symmetry.’ Each tungsten ion is coordinated into the centre of a WO, octahedra, sharing each oxygen ion with another tungsten ion. The structure of WO, as shown in Fig. Al is similar to the r-utile structure of TiO,.“‘” Each W atom is coordinated to six oxygen atoms. The octahedra are joined by sharing edges to form strings which are mutually connected into a three-dimensional structure by corner-sharing of the octahedra. Within the strings the W-W distances are alternatively short (2.5 A) or long (3.1 A). This results in the formation of pairs of W atoms and a consequent distortion of the WO, octahedra which is
t C
Fig. Al.
The tetragonal
structure
of WO,.
not present in the common undistorted rutile structure of TiO,. The structure of WO, is exactly the same as for Mo02? It is therefore expected that the interpretation of ESCA spectra can be performed in the same way, due to the fact that MOO, and WO, have the same type of energy level diagram.*’ The W4f spectra recorded from W oxides are complex. Figure A2 illustrates the spectrum recorded after partially reducing a W sample covered by a thick WO,-layer. The positions of the doublet representing the metallic state are 31.2 and 33.4 eV. The chemical shift for the W”+ -peaks recorded from pure WO, is 4.4 eV. A linear interpolation between the metallic and six valent states predicts that the chemical shift of W4+ is at 3.2 eV. As seen from the figure, small peaks exist at a shift of 3.2 eV. However, the strongest peak representing W4+ is shifted by only 1.6 eV. In the literature this peak has often been denoted by Wz+ .23-25 In this paper it is suggested that the small chemical shift of the main peak of W”’ is due to the structural effect of WO,. The same behaviour has been observed from the recording of ESCA spectra from MoO,.*~-~” The interaction between the metal ions gives a partly filled WW G bond at the Fermi level which imparts high conductivity to the oxide.2” It is proposed that the W-W-bonds which give this metallic character screen the core electrons and thereby lower the effective charge and the coulomb interaction. Thus, the final electron state is lowered and the main peak appears at a position corresponding to a lower apparent oxidation number. The low intensity peak is positioned at
W4f
BINDING ENERGY (eV) Fig. A2.
ESCA-spectra
recorded
after partially reducing a W sample covered nitrogen atmosphere at 800°C for 15 min.
by a thick WO,-layer
in a hydrogen/
Oxidation of Wand
WC in humid atmospheres
the binding energy which is expected if WO, had been the same type of insulator as WO,. It is suggested that the position of the low intensity peak is caused partly by molecular screen-
353
ing of the core level. The W ions situated at long distances from each other give no overlapping orbitals and thereby the screening of the core electrons is partly ionic.