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Heat and mass transfer phenomenon from an oxygen plasma to a semiconductor surface
Surface and Coatings Technology 142᎐144 Ž2001. 959᎐963
Heat and mass transfer phenomenon from an oxygen plasma to a semiconductor surface C. GuyonU , S. Cavadias, J. Amouroux Laboratoire de Genie ´ des Procedes ´ ´ Plasma et Traitement de Surface, Uni¨ ersite´ Pierre and Marie Curie, ENSCP-11, rue P. and M. Curie, 75231 Paris Cedex 05, France
Abstract Heat and mass transfer from a non-equilibrium low-pressure plasma of oxygen to an oxide semiconductor Žn- or p-type. target have been measured. The aim of this work is to establish a correlation between the electronic properties of the semiconductors Želectronic gap. and their reactivity Žactivation energy of the recombination reaction.. The energy transfer from a gas to a material is defined by the recombination and accommodation coefficients, ␥ and , respectively. The ␥ coefficient is measured in a pulsed radio frequency plasma reactor Ž13.56 MHz. in non-equilibrium conditions using an actinometric method. The test materials are n- and p-type semiconductors having band gap energies varying from 0.3 to 7.3 eV. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Plasma; Catalycity; Recombination; Surface; Oxygen; Semiconductors
1. Introduction The efficiency of a catalyst changes with the oxidation mechanisms involved in catalytic reactions. The electronic interactions between the adsorbed species and the semiconductor surface are important for the set-up of catalytic materials and the qualification of materials submitted to high thermal constraints w1x. Usually, experimental data for the atomic oxygen recombination do not take into account the ageing of the material w2᎐9x. In this study, we present the measurement of the atomic oxygen recombination on several metallic oxides surfaces. The mass transfer to a surface can be evaluated with the recombination coefficient ␥ Žgamma.. The recombination of oxygen atoms leads to the formation of an oxide layer on the surface, resulting in a modification of the nature of the surface and its properties. In order to point out the influence of electronic properties of the material on its catalytic activity U
Corresponding author.
Žactivation energy of the recombination reaction., the measurements were performed on n- and p-type semiconductors having different band gap energies.
2. Experimental set-up 2.1. Experimental de¨ ice The experimental device includes a plasma tubular reactor, the controlling and acquisition apparatus ŽFig. 1.. Tests are performed at 110 Pa pressure, with an air gas flow of 400 sccm, and an Argon gas flow of 25 sccm Žapproximately 5% of the total gas flow.. The plasma is created by a 13.56-MHz generator. Discharge pulses are short, reproducible and very stable. The emission spectroscopy signal was transmitted via fibre optics, analysed by a monochromator, and detected by an Optical Multichannel Analyser ŽO.M.A... The acquisition of spectra is synchronised with the pulsed discharge. Using a discharge time of 2000 ms, the total
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 1 2 5 - 2
C. Guyon et al. r Surface and Coatings Technology 142᎐144 (2001) 959᎐963
960
Fig. 1. Experimental set-up for the recombination coefficient measurement in pulsed discharge.
exposure time of the sample for each measurement is 60 s.
tion and can be calculated from the atomic oxygen concentration profile along the reactor w17x:
2.2. The actinometric method applied to recombination coefficient measurement
␥s
The recombination coefficients are calculated using the actinometry spectroscopic method w10x. Actinometry involves the use of optical emission intensity ratios to provide an estimate of ground state species concentrations. The intensity emitted from the species is divided by the emission intensity from an inert gas Žthe actinometer. which is added to the plasma in small quantities. In our case, the O atom concentration has been monitored with optical emission from O Ž844.6 nm. and Ar Ž811.5 nm.. The ratio IO rIAr obtained by actinometry is related to the concentration ratio by the following relation w11x; IO rIAr s k Ž O . r Ž Ar.
Ž1.
and it can be used as a tracer for atomic oxygen concentration. The validity of the actinometry method has been previously verified by titration of the atomic oxygen with nitric oxide w11x. The movement of oxygen atoms in the boundary layer near the sample is controlled by diffusion and described by the general diffusion equation. The recombination coefficient can be deduced from this equa-
y4Dⵜ Ž IO rIA . x c w yⵜŽ IO rIA . x L q Ž IO rIA . 0 x
Ž2.
where D is the binary coefficient Žm2 sy1 ., c is the atomic velocity Žm sy1 ., L is the width of the boundary layer Žm., ⵜŽ IO rIAr . x is the axial concentration gradient and Ž IO rIAr . 0 the steady state concentration.
3. Results and interpretation 3.1. Measurement of the recombination coefficient and the acti¨ ation energy from the recombination on p-type oxide semiconductors The measurements were performed on samples of metallic oxide powder deposited on porous quartz support. The gap energy of the material has been determined by optical reflection measurement w15x. The recombination coefficients of atomic oxygen on each metallic oxide sample have been measured in a temperature range of 300᎐473 K in order to follow the evolution of the mass transfer to the surface with increasing temperature ŽTable 1.. The electronic properties of the material seem to influence greatly the heterogeneous recombination of the oxygen atoms to the surface of the sample. The results for the p-type oxide semiconductor shows that
C. Guyon et al. r Surface and Coatings Technology 142᎐144 (2001) 959᎐963
961
Table 1 Determination of the recombination coefficients of atomic oxygen on p-type semiconductors, evolution with the surface temperature, and recombination activation energy of p-type semiconductors
CoO MnO PbO Sb2 O3
Egap ŽeV.
Recombination coefficient ␥ Ž"20%. 300 K
Recombination coefficient ␥ Ž"20%. 473 K
Ea ŽkJ moly1 . Ž"15%.
Ca Žatoms my2 . Ž"15%.
0.8 1.3 2.3 4.2
29 = 10y3 17 = 10y3 13 = 10y3 8.2= 10y3
34 = 10y3 25 = 10y3 18 = 10y3 21 = 10y3
4.1 5.9 6.4 9.9
2.3= 1020 2.8= 1020 3.0= 1020 6.3= 1020
the lower the gap energy the higher the recombination coefficients is. The recombination coefficient is linked to the surface temperature by the following relation w11x: ␥s
T0 E exp y a T RT
ž
/
Ž3.
Cah2 , Ea s activation energy of the sur2 m0 k face reaction ŽJ moly1 ., C a s number of active sights Žatoms my2 . and m O s mass of an oxygen atom Žkg.. The activation energy of the process and the number of active sites can be deduced by plotting lnŽ ␥T . vs. 1rT ŽFig. 2.. Results reported in Table 1 show that the activation energy depends clearly on the gap energy of the semiconductor. Indeed, the activation energy increases with increasing gap energy ŽFig. 3.. where T0 s
3.2. Measurement of the recombination coefficient and the acti¨ ation energy from the recombination on n-type oxide semiconductors
The samples used are high purity metallic oxide powders transformed in pellets of 25-mm diameter. The gap energy of the materials were determined by optical reflection measurements w15x. The recombination coefficients of atomic oxygen on each sample have been measured in a temperature range of 313᎐773 K ŽTable 2.. All sample recombination coefficients increase with surface temperature. However, in this case, there is no correlation between activation and gap energy as for the p-type semiconductors ŽTable 2..
3.3. Interpretation of results It is known that oxygen atoms are adsorbed as Oy ions on the surface, and the reactivity of the material can be influenced by the mobility of adsorbed oxygen
Fig. 2. Determination recombination activation energy of CoO: DŽO 2 . s 50 sccm miny1 ; DŽN2 . s 200 sccm miny1 ; DŽAr. s 15 sccm miny1 ; pressure s 110 Pa; power of the RFs 240 W; and discharge time of 2000 ms.
Fig. 3. Correlation between the recombination activation energy and the gap energy of p-type semiconductors surfaces.
C. Guyon et al. r Surface and Coatings Technology 142᎐144 (2001) 959᎐963
962
Table 2 Determination of the recombination coefficients of atomic oxygen on n-type semiconductors, evolution with the surface temperature, and recombination activation energy of n-type semiconductors
WO3 BaTiO3 TiO2 CaTiO3 Al2 O3 SiCrSiO2
Egap ŽeV.
Recombination coefficient ␥ Ž"20%. 313 K
Recombination coefficient ␥ Ž"20%. 773 K
Ea ŽkJ moly1 . Ž"15%.
Ca Žatoms my2 . Ž"15%.
2.8 3.1 3.1 3.5 7.3 11
12 = 10y3 11 = 10y3 13 = 10y3 9.7= 10y3 14 = 10y3 4 = 10y3
78 = 10y3 24 = 10y3 28 = 10y3 61 = 10y3 31 = 10y3 41 = 10y3
4.3 5.2 5.4 5.8 10.3 7
1.1= 1020 1.6= 1020 2.5= 1020 2.0= 1020 8.1= 1020 1.1= 1020
atoms during the recombination process. The mobility can be considered as the result of the diffusion of oxygen ions on the oxide surface w18x. It had been shown w13,14x that the coverage of adsorbed oxygen molecules is higher for p-type oxides than for n-type oxides and the same behaviour for atomic oxygen. Indeed, the adsorption of an oxygen atom on a n type semiconductor is due to the creation of surface ionic species using the lattice electrons. As the coverage of the surface increases, only few electrons can move to the surface because of the electrostatic repulsion between the surface and the moving electrons. For these oxides the number of defects is a restricted factor. Moreover, the p-type oxides oxygen adsorption is the result of the creation of surface ionic species using electrons released from charges separation, corresponding to moving holes. The mobility of the adsorbed species is easier in this case, and the surface coverage is increasing until saturation. The p type conduction is the result of electrons transfer from the valence band to the conduction band allowing oxygen adsorption which create electronic holes in the valence band. The lattice oxygen atoms near the surface will also release some electrons which will allow an oxygen adsorption and holes creation. At low temperature, the holes are trapped on the lattice ions and some Oy ions are created. A large concentration of holes in the valence band can lead to the formation of covalent bonds as O᎐O. The transfer of electrons from the valence band to the surface, that means to the conduction band, is easier if the width of the forbidden zone Žgap energy. is small. In this case, the surface coverage of oxygen increases and the reactivity of the material is higher ŽTable 1.. A simulation of recombination on oxide semiconductors performed in our laboratory w12x showed that the activation energy of the reaction depends essentially on the energy gap for the p-type semiconductors and on the surface density of the active sites for the n-type semiconductors w16x.
4. Conclusion We have determined the role of the electronic properties of materials with respect to the heterogeneous recombination reactions of oxygen atoms on the surface by measurements of gamma coefficient on metallic oxide semiconductors. The p-type semiconductors are more efficient to recombine the oxygen atoms than the n-type semiconductors. On n-type oxides, which present an excess of electrons in the conduction band, the oxygen atoms are adsorbed on the surface as Oy ions and their mobility is reduced. On p-type semiconductors, there is no excess of electrons in the conduction band and, thus no electrostatic repulsion with the adsorbed species on the surface. For p-type oxides, it has been shown that the activation energy of recombination can be correlated to the gap energy of the material. Low gap energies lead to low activation energies of recombination resulting in important surface reactivity. A kinetic model confirms the experimental data. In conclusion, in order to characterise and predict the catalytic reactivity of the surface, it is essential to know the electronic conduction of the material. References w1x J. Warnatz, Proceedings of the Twenty-Fourth International Conference on Combustion, Pittsburgh, 1992, pp. 553᎐579. w2x F. Nguyen-Xuan, O. Mallard, S. Cavadias, J. Amouroux, A. Le Bozec and M. Rapuc, Proceedings of the S European Symposium on Aerothermo-dynamics for Space Vehicles, Noordwijk, The Netherlands, 1994, pp. 457᎐461. w3x G.A. Melin, R.J. Madix, Trans. Faraday Soc. 67 Ž1971. 198᎐211. w4x A.L. Myerson, J. Chem. Phys. 50 Ž1969. 1228᎐1234. w5x K. Nakada, Bull. Chem. Soc. Jpn. 32 Ž1959. 1072᎐1078. w6x J.C. Greaves, J.W. Linnett, Trans. Faraday Soc. 54 Ž1958. 1323᎐1330. w7x C.D. Scott, AIAA Paper 80᎐1477, Snowmass, CO, 1980. w8x E.V. Zoby, R.N. Gupta, A.L. Simmonds, AIAA Paper 84᎐0224, Reno, Nevada, 1984. w9x R.H. Krech, J. Spacecraft Rockets 30 Ž1993. 509᎐513. w10x D. Pagnon, J. Phys. D; Appl. Phys. 2B’ Ž1995. 1856᎐1868. w11x F. Nguyen-Xuan, Thesis of Pierre and Marie Curie University, 1997.
C. Guyon et al. r Surface and Coatings Technology 142᎐144 (2001) 959᎐963 w12x w13x w14x w15x w16x
P. Cauquot, Thesis of Pierre and Marie Curie University, 1999. P.B. Weisz, J. Chem. Phys. 20 Ž1952. 1483᎐1484. P.B. Weisz, J. Chem. Phys. 21 Ž1953. 1531᎐1538. J.I. Pankove, Opt. Process Semicond. 1975. C. Guyon, S. Cavadias, J. Amouroux, Thermal Plasma Process 6 Ž2000. in publication.
963
w17x M.F Golde, A.E Roche, F. Kaufman, J. Chem. Phys. 59 Ž8. Ž1973.. w18x P.J. Crane, P.G. Dickens, R.E. Thomas, Recombination of oxygen atoms on oxides surfaces’, Trans. Faraday Soc. 63 Ž1967. 693.
Heat and mass transfer phenomenon from an oxygen plasma to a semiconductor surface C. GuyonU , S. Cavadias, J. Amouroux Laboratoire de Genie ´ des Procedes ´ ´ Plasma et Traitement de Surface, Uni¨ ersite´ Pierre and Marie Curie, ENSCP-11, rue P. and M. Curie, 75231 Paris Cedex 05, France
Abstract Heat and mass transfer from a non-equilibrium low-pressure plasma of oxygen to an oxide semiconductor Žn- or p-type. target have been measured. The aim of this work is to establish a correlation between the electronic properties of the semiconductors Želectronic gap. and their reactivity Žactivation energy of the recombination reaction.. The energy transfer from a gas to a material is defined by the recombination and accommodation coefficients, ␥ and , respectively. The ␥ coefficient is measured in a pulsed radio frequency plasma reactor Ž13.56 MHz. in non-equilibrium conditions using an actinometric method. The test materials are n- and p-type semiconductors having band gap energies varying from 0.3 to 7.3 eV. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Plasma; Catalycity; Recombination; Surface; Oxygen; Semiconductors
1. Introduction The efficiency of a catalyst changes with the oxidation mechanisms involved in catalytic reactions. The electronic interactions between the adsorbed species and the semiconductor surface are important for the set-up of catalytic materials and the qualification of materials submitted to high thermal constraints w1x. Usually, experimental data for the atomic oxygen recombination do not take into account the ageing of the material w2᎐9x. In this study, we present the measurement of the atomic oxygen recombination on several metallic oxides surfaces. The mass transfer to a surface can be evaluated with the recombination coefficient ␥ Žgamma.. The recombination of oxygen atoms leads to the formation of an oxide layer on the surface, resulting in a modification of the nature of the surface and its properties. In order to point out the influence of electronic properties of the material on its catalytic activity U
Corresponding author.
Žactivation energy of the recombination reaction., the measurements were performed on n- and p-type semiconductors having different band gap energies.
2. Experimental set-up 2.1. Experimental de¨ ice The experimental device includes a plasma tubular reactor, the controlling and acquisition apparatus ŽFig. 1.. Tests are performed at 110 Pa pressure, with an air gas flow of 400 sccm, and an Argon gas flow of 25 sccm Žapproximately 5% of the total gas flow.. The plasma is created by a 13.56-MHz generator. Discharge pulses are short, reproducible and very stable. The emission spectroscopy signal was transmitted via fibre optics, analysed by a monochromator, and detected by an Optical Multichannel Analyser ŽO.M.A... The acquisition of spectra is synchronised with the pulsed discharge. Using a discharge time of 2000 ms, the total
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 1 2 5 - 2
C. Guyon et al. r Surface and Coatings Technology 142᎐144 (2001) 959᎐963
960
Fig. 1. Experimental set-up for the recombination coefficient measurement in pulsed discharge.
exposure time of the sample for each measurement is 60 s.
tion and can be calculated from the atomic oxygen concentration profile along the reactor w17x:
2.2. The actinometric method applied to recombination coefficient measurement
␥s
The recombination coefficients are calculated using the actinometry spectroscopic method w10x. Actinometry involves the use of optical emission intensity ratios to provide an estimate of ground state species concentrations. The intensity emitted from the species is divided by the emission intensity from an inert gas Žthe actinometer. which is added to the plasma in small quantities. In our case, the O atom concentration has been monitored with optical emission from O Ž844.6 nm. and Ar Ž811.5 nm.. The ratio IO rIAr obtained by actinometry is related to the concentration ratio by the following relation w11x; IO rIAr s k Ž O . r Ž Ar.
Ž1.
and it can be used as a tracer for atomic oxygen concentration. The validity of the actinometry method has been previously verified by titration of the atomic oxygen with nitric oxide w11x. The movement of oxygen atoms in the boundary layer near the sample is controlled by diffusion and described by the general diffusion equation. The recombination coefficient can be deduced from this equa-
y4Dⵜ Ž IO rIA . x c w yⵜŽ IO rIA . x L q Ž IO rIA . 0 x
Ž2.
where D is the binary coefficient Žm2 sy1 ., c is the atomic velocity Žm sy1 ., L is the width of the boundary layer Žm., ⵜŽ IO rIAr . x is the axial concentration gradient and Ž IO rIAr . 0 the steady state concentration.
3. Results and interpretation 3.1. Measurement of the recombination coefficient and the acti¨ ation energy from the recombination on p-type oxide semiconductors The measurements were performed on samples of metallic oxide powder deposited on porous quartz support. The gap energy of the material has been determined by optical reflection measurement w15x. The recombination coefficients of atomic oxygen on each metallic oxide sample have been measured in a temperature range of 300᎐473 K in order to follow the evolution of the mass transfer to the surface with increasing temperature ŽTable 1.. The electronic properties of the material seem to influence greatly the heterogeneous recombination of the oxygen atoms to the surface of the sample. The results for the p-type oxide semiconductor shows that
C. Guyon et al. r Surface and Coatings Technology 142᎐144 (2001) 959᎐963
961
Table 1 Determination of the recombination coefficients of atomic oxygen on p-type semiconductors, evolution with the surface temperature, and recombination activation energy of p-type semiconductors
CoO MnO PbO Sb2 O3
Egap ŽeV.
Recombination coefficient ␥ Ž"20%. 300 K
Recombination coefficient ␥ Ž"20%. 473 K
Ea ŽkJ moly1 . Ž"15%.
Ca Žatoms my2 . Ž"15%.
0.8 1.3 2.3 4.2
29 = 10y3 17 = 10y3 13 = 10y3 8.2= 10y3
34 = 10y3 25 = 10y3 18 = 10y3 21 = 10y3
4.1 5.9 6.4 9.9
2.3= 1020 2.8= 1020 3.0= 1020 6.3= 1020
the lower the gap energy the higher the recombination coefficients is. The recombination coefficient is linked to the surface temperature by the following relation w11x: ␥s
T0 E exp y a T RT
ž
/
Ž3.
Cah2 , Ea s activation energy of the sur2 m0 k face reaction ŽJ moly1 ., C a s number of active sights Žatoms my2 . and m O s mass of an oxygen atom Žkg.. The activation energy of the process and the number of active sites can be deduced by plotting lnŽ ␥T . vs. 1rT ŽFig. 2.. Results reported in Table 1 show that the activation energy depends clearly on the gap energy of the semiconductor. Indeed, the activation energy increases with increasing gap energy ŽFig. 3.. where T0 s
3.2. Measurement of the recombination coefficient and the acti¨ ation energy from the recombination on n-type oxide semiconductors
The samples used are high purity metallic oxide powders transformed in pellets of 25-mm diameter. The gap energy of the materials were determined by optical reflection measurements w15x. The recombination coefficients of atomic oxygen on each sample have been measured in a temperature range of 313᎐773 K ŽTable 2.. All sample recombination coefficients increase with surface temperature. However, in this case, there is no correlation between activation and gap energy as for the p-type semiconductors ŽTable 2..
3.3. Interpretation of results It is known that oxygen atoms are adsorbed as Oy ions on the surface, and the reactivity of the material can be influenced by the mobility of adsorbed oxygen
Fig. 2. Determination recombination activation energy of CoO: DŽO 2 . s 50 sccm miny1 ; DŽN2 . s 200 sccm miny1 ; DŽAr. s 15 sccm miny1 ; pressure s 110 Pa; power of the RFs 240 W; and discharge time of 2000 ms.
Fig. 3. Correlation between the recombination activation energy and the gap energy of p-type semiconductors surfaces.
C. Guyon et al. r Surface and Coatings Technology 142᎐144 (2001) 959᎐963
962
Table 2 Determination of the recombination coefficients of atomic oxygen on n-type semiconductors, evolution with the surface temperature, and recombination activation energy of n-type semiconductors
WO3 BaTiO3 TiO2 CaTiO3 Al2 O3 SiCrSiO2
Egap ŽeV.
Recombination coefficient ␥ Ž"20%. 313 K
Recombination coefficient ␥ Ž"20%. 773 K
Ea ŽkJ moly1 . Ž"15%.
Ca Žatoms my2 . Ž"15%.
2.8 3.1 3.1 3.5 7.3 11
12 = 10y3 11 = 10y3 13 = 10y3 9.7= 10y3 14 = 10y3 4 = 10y3
78 = 10y3 24 = 10y3 28 = 10y3 61 = 10y3 31 = 10y3 41 = 10y3
4.3 5.2 5.4 5.8 10.3 7
1.1= 1020 1.6= 1020 2.5= 1020 2.0= 1020 8.1= 1020 1.1= 1020
atoms during the recombination process. The mobility can be considered as the result of the diffusion of oxygen ions on the oxide surface w18x. It had been shown w13,14x that the coverage of adsorbed oxygen molecules is higher for p-type oxides than for n-type oxides and the same behaviour for atomic oxygen. Indeed, the adsorption of an oxygen atom on a n type semiconductor is due to the creation of surface ionic species using the lattice electrons. As the coverage of the surface increases, only few electrons can move to the surface because of the electrostatic repulsion between the surface and the moving electrons. For these oxides the number of defects is a restricted factor. Moreover, the p-type oxides oxygen adsorption is the result of the creation of surface ionic species using electrons released from charges separation, corresponding to moving holes. The mobility of the adsorbed species is easier in this case, and the surface coverage is increasing until saturation. The p type conduction is the result of electrons transfer from the valence band to the conduction band allowing oxygen adsorption which create electronic holes in the valence band. The lattice oxygen atoms near the surface will also release some electrons which will allow an oxygen adsorption and holes creation. At low temperature, the holes are trapped on the lattice ions and some Oy ions are created. A large concentration of holes in the valence band can lead to the formation of covalent bonds as O᎐O. The transfer of electrons from the valence band to the surface, that means to the conduction band, is easier if the width of the forbidden zone Žgap energy. is small. In this case, the surface coverage of oxygen increases and the reactivity of the material is higher ŽTable 1.. A simulation of recombination on oxide semiconductors performed in our laboratory w12x showed that the activation energy of the reaction depends essentially on the energy gap for the p-type semiconductors and on the surface density of the active sites for the n-type semiconductors w16x.
4. Conclusion We have determined the role of the electronic properties of materials with respect to the heterogeneous recombination reactions of oxygen atoms on the surface by measurements of gamma coefficient on metallic oxide semiconductors. The p-type semiconductors are more efficient to recombine the oxygen atoms than the n-type semiconductors. On n-type oxides, which present an excess of electrons in the conduction band, the oxygen atoms are adsorbed on the surface as Oy ions and their mobility is reduced. On p-type semiconductors, there is no excess of electrons in the conduction band and, thus no electrostatic repulsion with the adsorbed species on the surface. For p-type oxides, it has been shown that the activation energy of recombination can be correlated to the gap energy of the material. Low gap energies lead to low activation energies of recombination resulting in important surface reactivity. A kinetic model confirms the experimental data. In conclusion, in order to characterise and predict the catalytic reactivity of the surface, it is essential to know the electronic conduction of the material. References w1x J. Warnatz, Proceedings of the Twenty-Fourth International Conference on Combustion, Pittsburgh, 1992, pp. 553᎐579. w2x F. Nguyen-Xuan, O. Mallard, S. Cavadias, J. Amouroux, A. Le Bozec and M. Rapuc, Proceedings of the S European Symposium on Aerothermo-dynamics for Space Vehicles, Noordwijk, The Netherlands, 1994, pp. 457᎐461. w3x G.A. Melin, R.J. Madix, Trans. Faraday Soc. 67 Ž1971. 198᎐211. w4x A.L. Myerson, J. Chem. Phys. 50 Ž1969. 1228᎐1234. w5x K. Nakada, Bull. Chem. Soc. Jpn. 32 Ž1959. 1072᎐1078. w6x J.C. Greaves, J.W. Linnett, Trans. Faraday Soc. 54 Ž1958. 1323᎐1330. w7x C.D. Scott, AIAA Paper 80᎐1477, Snowmass, CO, 1980. w8x E.V. Zoby, R.N. Gupta, A.L. Simmonds, AIAA Paper 84᎐0224, Reno, Nevada, 1984. w9x R.H. Krech, J. Spacecraft Rockets 30 Ž1993. 509᎐513. w10x D. Pagnon, J. Phys. D; Appl. Phys. 2B’ Ž1995. 1856᎐1868. w11x F. Nguyen-Xuan, Thesis of Pierre and Marie Curie University, 1997.
C. Guyon et al. r Surface and Coatings Technology 142᎐144 (2001) 959᎐963 w12x w13x w14x w15x w16x
P. Cauquot, Thesis of Pierre and Marie Curie University, 1999. P.B. Weisz, J. Chem. Phys. 20 Ž1952. 1483᎐1484. P.B. Weisz, J. Chem. Phys. 21 Ž1953. 1531᎐1538. J.I. Pankove, Opt. Process Semicond. 1975. C. Guyon, S. Cavadias, J. Amouroux, Thermal Plasma Process 6 Ž2000. in publication.
963
w17x M.F Golde, A.E Roche, F. Kaufman, J. Chem. Phys. 59 Ž8. Ž1973.. w18x P.J. Crane, P.G. Dickens, R.E. Thomas, Recombination of oxygen atoms on oxides surfaces’, Trans. Faraday Soc. 63 Ž1967. 693.