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Influence of Carbon on creep of NiO Polycrystals
J. Phys Chew Solids Vol. 47, No. 3, pp. 331-334, Printed in Great Britain.
0022-3697/86 $3.00 + .XJ 0 I986 Pergamon Press Ltd.
1986
TECHNICAL INFLUENCE
OF
CARBON
ON
NOTE
CREEP
OF
NiO
POLYCRYSTALS
M. JIM~NEZ-MELENDO,J.CASTAINGI,A.DOM~NGUEZ-RODR~GUEZ and R. MARQUEZ Dpto. de Optica, Facultad de Fisica. Aptdo. 1065, 41080 Sevilla, Spain. (Received 24 May 1985; accepted 26 September 1985)
Abstract-The use of CO/CO2 atmospheres is shown to alter the high temperature creep of NiO polycrystals. It is suggested that carbon dissolution in grain-boundaries affects the point defect structure.
INTRODU(7HON
former process and 88% for the latter, with grain-sizes in the 10 pm range. More details on the materials can be found in [ 121.Compression tests at constant load were performed on 5 x 3 x 3 mm3 specimens in a creep machine already used in previous works [9-l 11. The temperatures were between 1120 and 12OO”C,the engineering stresses between 10 MPa and 29 MPa and PO, was established by nowing air, argon/ oxygen and CO/CO2 mixtures. Po, values were controlled with a zirconia cell and tuned by injecting a small amount of CO in a CO2 flow in the case of CO/CO2 mixtures. Creep curves were analysed using the well established creep law [13]:
There has been some concern about the influence of carbon on the defect structure of nonstoichiometric oxides such as Co0 Il. 21. Recently. it has been shown that the use. of CO/ CO2 mix&s to control the oxygen chemical potential at high temperature, could influence atomic and electrical transport in MnO [3, 41; these results are interpreted as evidence of dissolution of carbon from the gas phase, although there is no proof based on chemical analysis, as in the case of MgO and alkaline earth oxides [5]. The above mentioned defect studies are baseci on the use of electrical conductivity, oxidation kinetics etc., which are sensitive to dominant point defects such as cation vacancies in Co0 or NiO. We have undertaken high temperature creep experiments which give access to the minority defects controlling the motion of the slowest species in compounds such as oxides [6, 71. Previous studies have shown that in Ni,_,O, the dominant defects being nickel vacancies VNi and Pki [8], the minority defects are oxygen vacancies VZ 19. 101.This was deduced from hiah temperature creep studies relating the plastic deformation rate i to the oxygen partial pressure PO, established by flowing argonoxygen mixtures through the mechanical testing machine. The extension of the results to lower temperature range required to reach Iow PO, which are classically obtained by mixing CO and CO* in variable ratios. Experiments on NiO single crystals gave results consistent with the proposed minority point defects viz V;; or Vb [6]. Deformation of fine grained polycrystals may occur via direct transport of matter (Nabarro-Herring and Coble creep). This type of creep mechanism has been activated in NiO and was found to be related to the same point defects V;; at PO, > lo-’ atm [I 1, 121. In this short note, we show that creep tests carried out in CO/COz atmospheres (PO2< 10m5atm) on polycrystals are in disagreement; this is probably due to carbon dissolution from the gas phase. 2. EXPERIMENTAL
t = AoW”&
exp(-AH/kT)
A, n, a, m and AH are constants; 0 = stress; d = grain size;
AH and m can be related to minority point defects via oxygen diffusion [6, 71.
3. RESULTS AND DISCUSSION Typical creep curves plotted as log g - e are shown in Figs. 1 and 2 for single and polycrystals. In order to have information about the creep mechanism and, therefore, on the defect structure, the parameters of the creep law are determined by changing one of the conditions (a, T or Pa) leaving the others constant. In the present note, we focus on the influence of PO,; that is, on m values. Determinations are shown in Figs. 1 and 2. After some strain 6, the specimen is unloaded and the gas mixture is changed to obtain a new Po2. The specimen is annealed at the temperature of the test to reach the corresponding new thermodynamic equilibrium (duration = 5 to 20 hours). The creep test is then resumed by setting the same load. The m values are deduced from the ratio of i extrapolated at c corresponding to the PO, change (Figs. 1 and 2). For single crystals the changes from argon (with about 10 ppm oxygen) to CO/COz were reversible (Fig. 1) giving m = -0.34 f 0.08 (four changes). This is not true for polycrystals (Fig. 2) where the first change from argon to CO/CO?, gave m = -0.19 + 0.03 (three experiments) followed by no influence of Pe (m = 0.00 2 0.03, 12 changes). This result suggests that carbon dissolves from the gas into the polycrystal and cannot be removed by annealing in argon. We have then performed one test where, after various CO/ CO*-Ar changes, the specimen was annealed 45 hours in air before introducing argon for 8 hours. A further change to CO/ CO* gave m = -0.10, suggesting that some carbon had been eliminated from the specimen.
TECHNIQUES
High purity NiO powder provided by Johnson Matthey was used to make polycrystals by a sintering process. The powder was cold pressed and received a preliminary sintering anneal at 1200°C in air during three days. In order to achieve a higher density, the material was either submitted to a large deformation (c - 100%) at 1200°C or submitted to a high temperature anneal (1500°C in air during three days). The resulting fractions of the theoretical density were 93% for the
t Permanent address: Laboratoire de Physique des Materiaux, CNRS Bellevue, 92 195 Meudon Cedex, France. 331
Technical Note
332
. .
.
.
:
I
Technical Note
333
9 ii
C
In
‘0 Y N
334
Technicai Note
The use of CO/COz mixtures brings quite a different behaviour for single and polycrystals (Figs. 1 and 2) at variance with the use of argon/oxygen [ 1I]. This can be understood by referring to the details of the rate controlling mechanism which is oxygen diffusion for both types of NiO. However, it has been shown that the deformation of single crystals is controlked by the climb of dislocations [lo] which occurs everywhere in the bulk of the specimen; it is then related to oxygen self-diffusion in the volume. It has recently been demonstrated that the creep rate for polycrystals is related to oxygen diffusion by grain boundaries [ 121 which act as short-circuits. These grain boundaries can be easily contaminated by carbon, which is stable in argon (low Pm) and dominates the oxygen transport, giving no dependence of i- with 13,. The presence of carbon has been studied in MgO which has many similarities with NiO. It has been shown that MgO could contain up to 1000 at. ppm of C with extremely pronounced segregation at room temperature in the 5-10 nm thick subsurface layer [5]. By heating, the segregation may be modified and carbon can be eliminated at high Po, . This kind ofbehaviour is likely to occur at grain boundaries;n NiO and we have observed its influence on point defect structure in an analogous way to Kofstad 13, 41 in MnO. In this case where defect clusters dominate, it has been suggested that carbon is incorporated in the clusters. This type of explanation cannot be extrapolated to NiO where it is believed that isolated VNi dominate [8]; in addition, our results describe minority defects in the oxygen sublattice. These defects are in very small concentrations; their behaviour can be changed by carbon amounts that cannot be detected by analytical techniques. These techniques are used on specimens at room temperature, and their results cannot be extrapolated to high temperature. In summary, the use of CO/CO, mixtures to control oxygen chemical activity at high temperature may be misleading,
especially when studying surface or interface dependent phenomena in oxide. REFERENCES 1. Bransky I. and Wimmer J. M., J. Phys. Chem. Solids 33, 801 (1972). 2. Kramer L. D. and Simkovich G., Oxid. Met. f&91 (1973). 3. Kofstad P., Oxid. Met. 19, 129 ( 1983). 4. Kofstad P., Solid St. Ionics 12, 101 (1984). 5. Freund F.. Kina B.. Knobel R. and Kathrein H.. Structure and Propertiesof MgO and A&O3Ceramics. In Advances in Ceramics (Edited by W. D. Kingery). The American Ceramic Societv, Ohio, 10, 119 ( 1984). A. and Monty C., 6. Castaing J., ~rn~ngu~z-~~~~ez Reformation of Ceramic ~ater~ais If (Edited bv R. E. Tressler and RI C. Bradt) Vol. 18, p. 14‘1.Plenum Press, New York ( 1984). I. Dominguez-Rodriguez A. and Castaing J., Rad. E&cts 75, 309 (1983). 8. Fahri R. and Petot-Ervas G., J. Phys. Chem. Solids 39, 1175 (1978). 9. tibrera-Caiio J. and Castaing J., J. Phys. Lett. 41, L-i 19 (1980). 10. Cabrem-Caiio J., Domingnez-Rodriguez A., Marquez R., Castaing J. and Phil&art J., Phil. Msg. A 46, 397 (1982). 11. Jiminez-Melendo M., Cabrera-Cafio J., DominguezRodriguez A. and Castaing J., J. Phys. Lett. 44, L-339 (1983). 12 Jimenez-Melendo M., Estudio de 10s mecanismos de la defo~a~~n p&s&a a alta temperatura de1 oxido de niquel. Tesis Doctoral, Universidad de Sevilla, Spain f 1984). 13. Bretheau T., Castaing J., Rabier J. and Veyssiere P., Adv. Phys. 28, 835 (1979).
0022-3697/86 $3.00 + .XJ 0 I986 Pergamon Press Ltd.
1986
TECHNICAL INFLUENCE
OF
CARBON
ON
NOTE
CREEP
OF
NiO
POLYCRYSTALS
M. JIM~NEZ-MELENDO,J.CASTAINGI,A.DOM~NGUEZ-RODR~GUEZ and R. MARQUEZ Dpto. de Optica, Facultad de Fisica. Aptdo. 1065, 41080 Sevilla, Spain. (Received 24 May 1985; accepted 26 September 1985)
Abstract-The use of CO/CO2 atmospheres is shown to alter the high temperature creep of NiO polycrystals. It is suggested that carbon dissolution in grain-boundaries affects the point defect structure.
INTRODU(7HON
former process and 88% for the latter, with grain-sizes in the 10 pm range. More details on the materials can be found in [ 121.Compression tests at constant load were performed on 5 x 3 x 3 mm3 specimens in a creep machine already used in previous works [9-l 11. The temperatures were between 1120 and 12OO”C,the engineering stresses between 10 MPa and 29 MPa and PO, was established by nowing air, argon/ oxygen and CO/CO2 mixtures. Po, values were controlled with a zirconia cell and tuned by injecting a small amount of CO in a CO2 flow in the case of CO/CO2 mixtures. Creep curves were analysed using the well established creep law [13]:
There has been some concern about the influence of carbon on the defect structure of nonstoichiometric oxides such as Co0 Il. 21. Recently. it has been shown that the use. of CO/ CO2 mix&s to control the oxygen chemical potential at high temperature, could influence atomic and electrical transport in MnO [3, 41; these results are interpreted as evidence of dissolution of carbon from the gas phase, although there is no proof based on chemical analysis, as in the case of MgO and alkaline earth oxides [5]. The above mentioned defect studies are baseci on the use of electrical conductivity, oxidation kinetics etc., which are sensitive to dominant point defects such as cation vacancies in Co0 or NiO. We have undertaken high temperature creep experiments which give access to the minority defects controlling the motion of the slowest species in compounds such as oxides [6, 71. Previous studies have shown that in Ni,_,O, the dominant defects being nickel vacancies VNi and Pki [8], the minority defects are oxygen vacancies VZ 19. 101.This was deduced from hiah temperature creep studies relating the plastic deformation rate i to the oxygen partial pressure PO, established by flowing argonoxygen mixtures through the mechanical testing machine. The extension of the results to lower temperature range required to reach Iow PO, which are classically obtained by mixing CO and CO* in variable ratios. Experiments on NiO single crystals gave results consistent with the proposed minority point defects viz V;; or Vb [6]. Deformation of fine grained polycrystals may occur via direct transport of matter (Nabarro-Herring and Coble creep). This type of creep mechanism has been activated in NiO and was found to be related to the same point defects V;; at PO, > lo-’ atm [I 1, 121. In this short note, we show that creep tests carried out in CO/COz atmospheres (PO2< 10m5atm) on polycrystals are in disagreement; this is probably due to carbon dissolution from the gas phase. 2. EXPERIMENTAL
t = AoW”&
exp(-AH/kT)
A, n, a, m and AH are constants; 0 = stress; d = grain size;
AH and m can be related to minority point defects via oxygen diffusion [6, 71.
3. RESULTS AND DISCUSSION Typical creep curves plotted as log g - e are shown in Figs. 1 and 2 for single and polycrystals. In order to have information about the creep mechanism and, therefore, on the defect structure, the parameters of the creep law are determined by changing one of the conditions (a, T or Pa) leaving the others constant. In the present note, we focus on the influence of PO,; that is, on m values. Determinations are shown in Figs. 1 and 2. After some strain 6, the specimen is unloaded and the gas mixture is changed to obtain a new Po2. The specimen is annealed at the temperature of the test to reach the corresponding new thermodynamic equilibrium (duration = 5 to 20 hours). The creep test is then resumed by setting the same load. The m values are deduced from the ratio of i extrapolated at c corresponding to the PO, change (Figs. 1 and 2). For single crystals the changes from argon (with about 10 ppm oxygen) to CO/COz were reversible (Fig. 1) giving m = -0.34 f 0.08 (four changes). This is not true for polycrystals (Fig. 2) where the first change from argon to CO/CO?, gave m = -0.19 + 0.03 (three experiments) followed by no influence of Pe (m = 0.00 2 0.03, 12 changes). This result suggests that carbon dissolves from the gas into the polycrystal and cannot be removed by annealing in argon. We have then performed one test where, after various CO/ CO*-Ar changes, the specimen was annealed 45 hours in air before introducing argon for 8 hours. A further change to CO/ CO* gave m = -0.10, suggesting that some carbon had been eliminated from the specimen.
TECHNIQUES
High purity NiO powder provided by Johnson Matthey was used to make polycrystals by a sintering process. The powder was cold pressed and received a preliminary sintering anneal at 1200°C in air during three days. In order to achieve a higher density, the material was either submitted to a large deformation (c - 100%) at 1200°C or submitted to a high temperature anneal (1500°C in air during three days). The resulting fractions of the theoretical density were 93% for the
t Permanent address: Laboratoire de Physique des Materiaux, CNRS Bellevue, 92 195 Meudon Cedex, France. 331
Technical Note
332
. .
.
.
:
I
Technical Note
333
9 ii
C
In
‘0 Y N
334
Technicai Note
The use of CO/COz mixtures brings quite a different behaviour for single and polycrystals (Figs. 1 and 2) at variance with the use of argon/oxygen [ 1I]. This can be understood by referring to the details of the rate controlling mechanism which is oxygen diffusion for both types of NiO. However, it has been shown that the deformation of single crystals is controlked by the climb of dislocations [lo] which occurs everywhere in the bulk of the specimen; it is then related to oxygen self-diffusion in the volume. It has recently been demonstrated that the creep rate for polycrystals is related to oxygen diffusion by grain boundaries [ 121 which act as short-circuits. These grain boundaries can be easily contaminated by carbon, which is stable in argon (low Pm) and dominates the oxygen transport, giving no dependence of i- with 13,. The presence of carbon has been studied in MgO which has many similarities with NiO. It has been shown that MgO could contain up to 1000 at. ppm of C with extremely pronounced segregation at room temperature in the 5-10 nm thick subsurface layer [5]. By heating, the segregation may be modified and carbon can be eliminated at high Po, . This kind ofbehaviour is likely to occur at grain boundaries;n NiO and we have observed its influence on point defect structure in an analogous way to Kofstad 13, 41 in MnO. In this case where defect clusters dominate, it has been suggested that carbon is incorporated in the clusters. This type of explanation cannot be extrapolated to NiO where it is believed that isolated VNi dominate [8]; in addition, our results describe minority defects in the oxygen sublattice. These defects are in very small concentrations; their behaviour can be changed by carbon amounts that cannot be detected by analytical techniques. These techniques are used on specimens at room temperature, and their results cannot be extrapolated to high temperature. In summary, the use of CO/CO, mixtures to control oxygen chemical activity at high temperature may be misleading,
especially when studying surface or interface dependent phenomena in oxide. REFERENCES 1. Bransky I. and Wimmer J. M., J. Phys. Chem. Solids 33, 801 (1972). 2. Kramer L. D. and Simkovich G., Oxid. Met. f&91 (1973). 3. Kofstad P., Oxid. Met. 19, 129 ( 1983). 4. Kofstad P., Solid St. Ionics 12, 101 (1984). 5. Freund F.. Kina B.. Knobel R. and Kathrein H.. Structure and Propertiesof MgO and A&O3Ceramics. In Advances in Ceramics (Edited by W. D. Kingery). The American Ceramic Societv, Ohio, 10, 119 ( 1984). A. and Monty C., 6. Castaing J., ~rn~ngu~z-~~~~ez Reformation of Ceramic ~ater~ais If (Edited bv R. E. Tressler and RI C. Bradt) Vol. 18, p. 14‘1.Plenum Press, New York ( 1984). I. Dominguez-Rodriguez A. and Castaing J., Rad. E&cts 75, 309 (1983). 8. Fahri R. and Petot-Ervas G., J. Phys. Chem. Solids 39, 1175 (1978). 9. tibrera-Caiio J. and Castaing J., J. Phys. Lett. 41, L-i 19 (1980). 10. Cabrem-Caiio J., Domingnez-Rodriguez A., Marquez R., Castaing J. and Phil&art J., Phil. Msg. A 46, 397 (1982). 11. Jiminez-Melendo M., Cabrera-Cafio J., DominguezRodriguez A. and Castaing J., J. Phys. Lett. 44, L-339 (1983). 12 Jimenez-Melendo M., Estudio de 10s mecanismos de la defo~a~~n p&s&a a alta temperatura de1 oxido de niquel. Tesis Doctoral, Universidad de Sevilla, Spain f 1984). 13. Bretheau T., Castaing J., Rabier J. and Veyssiere P., Adv. Phys. 28, 835 (1979).