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Techniques for the preparation and examination of partially reduced oxides
METALLOGRAPHY 17:367-379 (1985)
367
Techniques for the Preparation and Examination of Partially Reduced Oxides
S. P. MATTHEW, D. H. ST. JOHN, J. V. HARDY, ANDP. C. HAYES University of Queensland, St. Lucia, Queensland 4067, Australia
A technique has been developed that allows the controlled reduction of small oxide particles in high-velocity gas streams. The samples may be quenched after the desired reduction times, thus preserving the growth structures present at temperature. The paper also discusses a novel way of preparing delicate samples for examination in the scanning electron microscope and gives examples of the improved quality of the metallographic information attainable using these techniques.
Introduction The gaseous reduction of hematite (Fe203) results in the formation of a n u m b e r of intermediate oxides before the final formation of iron, i.e., Fe203 --->Fe304 --~ Fel - yO --* Fe. The reduction kinetics and m e c h a n i s m s of each of these transformations are directly dependent on the product microstructure f o r m e d at each stage of reduction. The two iron morphologies m o s t c o m m o n l y e n c o u n t e r e d on wustite ( F e l _ y O ) reduction are p o r o u s iron (sponge) and dense iron (Fig. 1). The dense iron product f o r m s a layer on the wustite surface preventing direct gas access to the oxide phase. I f this microstructure prevails further o x y g e n m a y be r e m o v e d f r o m the sample only by diffusion of oxygen species through the solid iron layer f r o m the i r o n - i r o n oxide interface to the g a s - i r o n surface. Since oxygen permeability through solid iron is very low, this latter reaction step is e x t r e m e l y slow. H o w e v e r , if a porous iron structure is f o r m e d the gas m a y diffuse through the pores in the structure to the oxide surface where direct reaction b e t w e e n gas and oxide m a y occur. The sponge iron structure is desirable in industrial applications as it leads to relatively fast reduction rates. Despite the fact that researchers h a v e been aware of these types of final p r o d u c t structure for a n u m b e r of decades their m e c h a n i s m s of for© Elsevier Science Publishing Co., Inc., 1985
52 Vanderbilt Ave., New York, NY 10017
0026-0800/85/$03.30
FiG 1. Conventional metallographic sections of the product morphologies obtained from the reduction of wustite at 1273 K. (a) porous iron; (b) dense iron. 368
Partially Reduced Oxides
369
mation have remained unclear. One of the major difficulties in finding out about these reactions has been the lack of suitable techniques for the preparation of sections of the extremely soft and delicate iron structures. Conventional polished metallographic sections of the growth interfaces between metal and oxide phases are unsatisfactory because of the uneven polishing rates of the hard oxide and the soft iron phases. In addition, smearing of the soft iron structures cannot be avoided during polishing; thus, the original growth structures are destroyed or distorted. The preparation of thin foils of the reaction interface by ion beam thinning has also proved unsuccessful. Orientating the specimen to obtain a section of the growth interface, including both iron and oxide, at an angle parallel to the growth direction is difficult. In addition the large pore sizes (0.1-2 ~m) relative to the foil thickness and the problems of uneven thinning contribute not only to the difficulties in attaining specimens but also to the uncertainty that these are indeed representative samples of the interface. The production of reliable metallographic evidence to indicate the growth mechanisms has also been hindered by the types of samples used by previous researchers. The use of large (e.g., 25 mm in diameter) porous oxide samples means that there is little control over the gas conditions at the reaction interface, and thus creates uncertainty as to the actual experimental conditions. In addition samples of this size cannot be successfully quenched, hence, growth morphology may alter either as the reaction gas is flushed from the system or as the specimen is cooled to room temperature. Other workers have used fine (1-10 p~m) diameter powders in their experiments, but there are obvious difficulties in obtaining information on the internal structural changes in these very small particles. It is apparent, therefore, that experimental techniques need to be developed that l) allow the chemical reactions and mass transport processes in the samples to be stopped at any stage of the reduction, and 2) provide clean distortion-free sections of the composite specimens.
Experimental Techniques Materials Preparation The dense wustite samples used in the present investigation were made by oxidizing spectrographically pure metal strip in controlled gas mixtures at constant temperature. If metal strip is not available or if the preparation of dense oxide samples with controlled impurity levels is required the following procedure may be adopted. About 3.5 g of high purity iron powder is mixed with appropriate weights of oxide impurity. The mixtures
370
M a t t h e w et al.
are then pressed in a 12 mm diameter cylindrical steel die under a pressure of 400 MN m - 2 and sintered in a recrystallized alumina boat in a flowing high purity hydrogen gas stream at 1223 K for 4.5 days. The polycrystalline iron samples are removed cold from the furnace and repressed between mild steel plates at 1000 MN m -2 to remove residual porosity. Control of both oxygen partial pressure and reaction temperature is necessary if a reproducible oxide stoichiometry is to be obtained. The samples used in the present investigation were oxidized for two days, in a predried 50% CO, 50% CO2 gas mixture at 1523 K in a platinum crucible, to produce large wustite grains (I-3 mm in diameter x 0.5 mm thickness) and uniform cation distributions. In wustite the mobility of the cations is much greater than the oxygen ions. Consequently, as oxidation of the iron proceeds, the metal ions diffuse out through the wustite layer to form fresh oxide at the gas-oxide interface. This results in the formation of a void at the center, and when completely oxidized the sample may be carefully split into two using a scalpel. Small (0.5-1 mm 3) samples may be prepared by cleaving the oxide with a scalpel, or if close size control is required, the oxide may be cut using a fine diamond saw. Selected samples may be analyzed using an electron probe microanalyzer (EPMA) to check the chemical composition of the material and to ensure that homogeneity of the samples has been achieved.
REDUCTION APPARATUS A schematic diagram of the reaction furnace used for the reduction experiments is shown in Figure 2. The premixed gases are introduced into the top of the reaction tube and removed at the bottom. This enables the bottom of the furnace to be opened during subsequent quenching of the samples without affecting the gas mixture flowing over the sample in the hot zone. The sample is also introduced into the top of the furnace, in this case by way of a two-way tap, that has been sealed to prevent gas flow but at the same time allows space for the sample to sit. The tap is made of teflon on glass and may be used without lubricant, thus avoiding problems of contamination of the sample by grease. The sample drops under gravity into the hot zone of the reaction furnace where it is held by a "bell" device (Fig. 3). The bell is attached to an alumina rod which enables the position of the bell to be controlled. When the rod is pulled up then there is a tight fit between the bell and the surrounding tube. The bell is shaped so that wherever a sample falls on the surface it will gravitate to the same position in the furnace. The sample is positioned at the entrance to the
Partially Reduced Oxides
371 A
e"
;1 0
,,"~'"2 °
0
o~H
li-
B
I
) " - " H 2 ,H20
]
FIG. 2. Schematic diagram of the reduction furnace used for single crystal experiments; (a) Teflon tap; (b) Pyrex tube; (c) alumina rod (1.5 mm diameter) with O-ring seals; (d) alumina tube (8 mm OD); (e) alumina tube (25 mm OD) and alumina cement filling; (f) Pt/ Pt-Rh thermocouple; (g) Alumina stopper; (h) Furnace windings; (j) P.V.C. tubing; (k) Clamp; and (1) Plastic bottle containing thermal insulation and a beaker of liquid nitrogen.
only hole that allows gas flow through the furnace. As all the gas flows through this small orifice, the local gas velocity is high and thus the local mass transfer coefficient between the bulk gas and the sample surface is high. After the desired residence time the sample is removed from the furnace by lowering the bell. This allows the sample to drop through the gap to the bottom of the reaction chamber. A plastic flask containing liquid ni-
_-----2.
/ \1
j3.
I
1 4 .
1.
I
i
2ram
t
Scale
j2. J
j3. J
't
j 4 . J
B_m
I I I
i
2 m
iica~
i
FIG. 3. The sample holding device in the reduction furnace. (a) front elevation; (b) side elevation. 1--A1203 stopper (made from concentric tubes); 2--A1203 rod; 3--A1203 tube; and 4--sample. 372
Partially Reduced Oxides
373
trogen is attached to the furnace end so that the sample drops directly into the liquid. The sample is quenched in two ways: 1) the vaporizing nitrogen effectively dilutes the partial pressure of the reducing gas, stopping the chemical reaction; and 2) the liquid nitrogen lowers the temperature of these small samples very quickly, preventing structural modifications in the sample during cooling. PREPARATION OF METALLOGRAPHIC SECTIONS The delicate partially reduced wustite specimens are handled at all times not with ordinary tweezers but magnetically using a pointed soft iron rod and a strong magnet thus avoiding mechanical damage. Following reduction the samples were vacuum impregnated with acetone in a vacuum chamber at approximately 25 kN m -2, i.e., the vapour pressure of acetone at room temperature. The samples were removed one at a time from the acetone and snap frozen on an austenitic stainless steel anvil submerged in liquid nitrogen (Fig. 4). Each specimen was then broken by the blow from a small hammer into 3-10 pieces. At this temperature, iron, acetone, as well as the oxide, all fail in a brittle manner, thus 4.
3.
2.
1.
/
! 1Omm !
Scale
FIG. 4. Section of the anvil used for the fracturing of partially reduced samples. 1 stainless steel anvil; 2--sample; 3--stainless steel container; 4--insulating fiber; and 5 - liquid nitrogen.
374
M a t t h e w et al.
Partially Reduced Oxides
375
N
N
e~
E
,,.t
e~
0 U
2
e~
2
376
M a t t h e w et al.
there is little plastic deformation of the structure. The purpose of the acetone is to provide additional support for the porous iron during fracture. The fractured samples are removed from the liquid nitrogen using the magnetized soft iron rod; this technique enabling even very small fragments to be collected. The fractured shards are allowed to fall directly onto a sample stub covered with double sided tape. Water condensation on these cold fractured samples was minimized by warming the stub with an incandescent light source. The acetone also evaporates during this period. The samples are then examined using an optical stereomicroscope to select both the specimens to be examined further and the fracture faces which best illustrate the features under examination. As a general rule best results are obtained using samples which are only about 5% reduced. These samples are easily broken without bulk distortion and provide plenty of exposed reaction interface. The initial stages of iron nucleation and growth can also be studied, but the likelihood of obtaining interfaces which have been fractured at a suitable position and orientation is obviously reduced in these circumstances and it may be necessary to prepare a number of samples before satisfactory results may be obtained. The samples are gold sputtered to improve the secondary electron emission in subsequent SEM examination. Results
Examples of the microstructures of partially reduced wustite samples, prepared using the techniques outlined above, are shown in Figures 57. It is immediately apparent that the quality of the micrographs from these specimens is far superior to those obtained from conventional metallographic sections. (Fig. 1). The high resolution and great depth of field of the SEM makes it the ideal instrument for examining these types of specimens. The improvement in image quality is not due solely to the superior optics of the SEM but also to the improved sharpness of the section. It can be seen that the local deformation during fracture of the samples is negligible. Thus the sections accurately reflect the structure of the material during reduction. Figure 5 shows the resultant morphology generated in pure H2 at 1273 K after a reaction time of 30 sec. The coarse tunnels form in the oxide before iron nucleation (1). When iron finally nucleates the surfaces of the now porous oxide are covered with a dense iron layer, a structure that is still visible on the coarse pores. Starting at the original specimen surface the dense iron layer breaks down and allows the initiation of the fine-
Partially Reduced Oxides
377
Fro. 7. SEM micrograph of fractured section of partially reduced wustite sample (pure CO, 1173 K, 40 sec).
378
M a t t h e w et al.
scale porous iron growth (2). This porous iron growth advances into the oxide at approximately 5 p~m/sec under these reaction conditions. The rapid sintering of the porous iron structure and the resulting change in the pore size can also be clearly seen, a phenomenon not identified in previous investigations. Increasing the H20 content results in fewer coarse pores before iron nucleation. However, porous growth is still initiated by the breakdown of an initial dense iron layer until the H2/H20 composition is reached where the breakdown no longer occurs. The presence of H20 results in slower reaction rates and a coarser porous iron structure, as illustrated in Fig. 6. This micrograph shows a sample reduced in 85% H2/15% H20 at 1273 K after a reaction time of 150 sec. Note that the iron actually covers the oxide surface at the growth front, and it appears that the mechanism of porous iron growth involves the repeated breakdown of this thin iron layer. A porous iron morphology may also be obtained on the reduction of wustite in CO/CO2 gas mixtures. Figure 7 shows the resultant morphology generated in pure CO at 1173 K after 40 sec. In this case the wustite surface has well-developed facets upon which a dense iron layer has nucleated and grown. As the iron layer spreads over the surface it breaks down, resulting in the initiation of porous growth. Figure 7b shows a more detailed section of the porous iron-wustite interface in an area where the iron is growing into the bulk oxide. In this sample, the porous iron product appears as a result of a eutectoid-like transformation, the two product phases being iron and gas. The continuous tunnels or pores connecting the bulk gas mixture and the reaction interface may be clearly seen,
Summary The development of new techniques of specimen preparation and examination of partially reduced solid metal oxides have lead to significant improvements in the quality of information that may be obtained from metallographic examination of the specimens. The structure of metaloxide interfaces can now be clearly characterized and quantitative information on the relationships between pore size and growth rate of the product phase may now be obtained. Preliminary investigations indicate that there are two types of growth mechanism that lead to porous iron morphologies. One mechanism resembles a eutectoid type of transformation with continuous growth of the iron product. The other mechanism is a discontinuous or stop-start process involving the repeated formation and subsequent breakdown of iron layers at the reaction interface.
Partially Reduced Oxides
379
References 1. D. H. St. John, S. P. Matthew, and P. C. Hayes, Establishment of product morphology during the initial stages of wustite reduction, Metall. Trans. B, 15B:709-717 (1984). 2. D. H. St. John, S. P. Matthew, and P. C. Hayes, The breakdown of dense iron layers on wustite in CO/CO2 and Hz/H20 systems, MetaU. Trans. B, 15B:701-708 (1984).
Received December 1984; accepted May 1985.
367
Techniques for the Preparation and Examination of Partially Reduced Oxides
S. P. MATTHEW, D. H. ST. JOHN, J. V. HARDY, ANDP. C. HAYES University of Queensland, St. Lucia, Queensland 4067, Australia
A technique has been developed that allows the controlled reduction of small oxide particles in high-velocity gas streams. The samples may be quenched after the desired reduction times, thus preserving the growth structures present at temperature. The paper also discusses a novel way of preparing delicate samples for examination in the scanning electron microscope and gives examples of the improved quality of the metallographic information attainable using these techniques.
Introduction The gaseous reduction of hematite (Fe203) results in the formation of a n u m b e r of intermediate oxides before the final formation of iron, i.e., Fe203 --->Fe304 --~ Fel - yO --* Fe. The reduction kinetics and m e c h a n i s m s of each of these transformations are directly dependent on the product microstructure f o r m e d at each stage of reduction. The two iron morphologies m o s t c o m m o n l y e n c o u n t e r e d on wustite ( F e l _ y O ) reduction are p o r o u s iron (sponge) and dense iron (Fig. 1). The dense iron product f o r m s a layer on the wustite surface preventing direct gas access to the oxide phase. I f this microstructure prevails further o x y g e n m a y be r e m o v e d f r o m the sample only by diffusion of oxygen species through the solid iron layer f r o m the i r o n - i r o n oxide interface to the g a s - i r o n surface. Since oxygen permeability through solid iron is very low, this latter reaction step is e x t r e m e l y slow. H o w e v e r , if a porous iron structure is f o r m e d the gas m a y diffuse through the pores in the structure to the oxide surface where direct reaction b e t w e e n gas and oxide m a y occur. The sponge iron structure is desirable in industrial applications as it leads to relatively fast reduction rates. Despite the fact that researchers h a v e been aware of these types of final p r o d u c t structure for a n u m b e r of decades their m e c h a n i s m s of for© Elsevier Science Publishing Co., Inc., 1985
52 Vanderbilt Ave., New York, NY 10017
0026-0800/85/$03.30
FiG 1. Conventional metallographic sections of the product morphologies obtained from the reduction of wustite at 1273 K. (a) porous iron; (b) dense iron. 368
Partially Reduced Oxides
369
mation have remained unclear. One of the major difficulties in finding out about these reactions has been the lack of suitable techniques for the preparation of sections of the extremely soft and delicate iron structures. Conventional polished metallographic sections of the growth interfaces between metal and oxide phases are unsatisfactory because of the uneven polishing rates of the hard oxide and the soft iron phases. In addition, smearing of the soft iron structures cannot be avoided during polishing; thus, the original growth structures are destroyed or distorted. The preparation of thin foils of the reaction interface by ion beam thinning has also proved unsuccessful. Orientating the specimen to obtain a section of the growth interface, including both iron and oxide, at an angle parallel to the growth direction is difficult. In addition the large pore sizes (0.1-2 ~m) relative to the foil thickness and the problems of uneven thinning contribute not only to the difficulties in attaining specimens but also to the uncertainty that these are indeed representative samples of the interface. The production of reliable metallographic evidence to indicate the growth mechanisms has also been hindered by the types of samples used by previous researchers. The use of large (e.g., 25 mm in diameter) porous oxide samples means that there is little control over the gas conditions at the reaction interface, and thus creates uncertainty as to the actual experimental conditions. In addition samples of this size cannot be successfully quenched, hence, growth morphology may alter either as the reaction gas is flushed from the system or as the specimen is cooled to room temperature. Other workers have used fine (1-10 p~m) diameter powders in their experiments, but there are obvious difficulties in obtaining information on the internal structural changes in these very small particles. It is apparent, therefore, that experimental techniques need to be developed that l) allow the chemical reactions and mass transport processes in the samples to be stopped at any stage of the reduction, and 2) provide clean distortion-free sections of the composite specimens.
Experimental Techniques Materials Preparation The dense wustite samples used in the present investigation were made by oxidizing spectrographically pure metal strip in controlled gas mixtures at constant temperature. If metal strip is not available or if the preparation of dense oxide samples with controlled impurity levels is required the following procedure may be adopted. About 3.5 g of high purity iron powder is mixed with appropriate weights of oxide impurity. The mixtures
370
M a t t h e w et al.
are then pressed in a 12 mm diameter cylindrical steel die under a pressure of 400 MN m - 2 and sintered in a recrystallized alumina boat in a flowing high purity hydrogen gas stream at 1223 K for 4.5 days. The polycrystalline iron samples are removed cold from the furnace and repressed between mild steel plates at 1000 MN m -2 to remove residual porosity. Control of both oxygen partial pressure and reaction temperature is necessary if a reproducible oxide stoichiometry is to be obtained. The samples used in the present investigation were oxidized for two days, in a predried 50% CO, 50% CO2 gas mixture at 1523 K in a platinum crucible, to produce large wustite grains (I-3 mm in diameter x 0.5 mm thickness) and uniform cation distributions. In wustite the mobility of the cations is much greater than the oxygen ions. Consequently, as oxidation of the iron proceeds, the metal ions diffuse out through the wustite layer to form fresh oxide at the gas-oxide interface. This results in the formation of a void at the center, and when completely oxidized the sample may be carefully split into two using a scalpel. Small (0.5-1 mm 3) samples may be prepared by cleaving the oxide with a scalpel, or if close size control is required, the oxide may be cut using a fine diamond saw. Selected samples may be analyzed using an electron probe microanalyzer (EPMA) to check the chemical composition of the material and to ensure that homogeneity of the samples has been achieved.
REDUCTION APPARATUS A schematic diagram of the reaction furnace used for the reduction experiments is shown in Figure 2. The premixed gases are introduced into the top of the reaction tube and removed at the bottom. This enables the bottom of the furnace to be opened during subsequent quenching of the samples without affecting the gas mixture flowing over the sample in the hot zone. The sample is also introduced into the top of the furnace, in this case by way of a two-way tap, that has been sealed to prevent gas flow but at the same time allows space for the sample to sit. The tap is made of teflon on glass and may be used without lubricant, thus avoiding problems of contamination of the sample by grease. The sample drops under gravity into the hot zone of the reaction furnace where it is held by a "bell" device (Fig. 3). The bell is attached to an alumina rod which enables the position of the bell to be controlled. When the rod is pulled up then there is a tight fit between the bell and the surrounding tube. The bell is shaped so that wherever a sample falls on the surface it will gravitate to the same position in the furnace. The sample is positioned at the entrance to the
Partially Reduced Oxides
371 A
e"
;1 0
,,"~'"2 °
0
o~H
li-
B
I
) " - " H 2 ,H20
]
FIG. 2. Schematic diagram of the reduction furnace used for single crystal experiments; (a) Teflon tap; (b) Pyrex tube; (c) alumina rod (1.5 mm diameter) with O-ring seals; (d) alumina tube (8 mm OD); (e) alumina tube (25 mm OD) and alumina cement filling; (f) Pt/ Pt-Rh thermocouple; (g) Alumina stopper; (h) Furnace windings; (j) P.V.C. tubing; (k) Clamp; and (1) Plastic bottle containing thermal insulation and a beaker of liquid nitrogen.
only hole that allows gas flow through the furnace. As all the gas flows through this small orifice, the local gas velocity is high and thus the local mass transfer coefficient between the bulk gas and the sample surface is high. After the desired residence time the sample is removed from the furnace by lowering the bell. This allows the sample to drop through the gap to the bottom of the reaction chamber. A plastic flask containing liquid ni-
_-----2.
/ \1
j3.
I
1 4 .
1.
I
i
2ram
t
Scale
j2. J
j3. J
't
j 4 . J
B_m
I I I
i
2 m
iica~
i
FIG. 3. The sample holding device in the reduction furnace. (a) front elevation; (b) side elevation. 1--A1203 stopper (made from concentric tubes); 2--A1203 rod; 3--A1203 tube; and 4--sample. 372
Partially Reduced Oxides
373
trogen is attached to the furnace end so that the sample drops directly into the liquid. The sample is quenched in two ways: 1) the vaporizing nitrogen effectively dilutes the partial pressure of the reducing gas, stopping the chemical reaction; and 2) the liquid nitrogen lowers the temperature of these small samples very quickly, preventing structural modifications in the sample during cooling. PREPARATION OF METALLOGRAPHIC SECTIONS The delicate partially reduced wustite specimens are handled at all times not with ordinary tweezers but magnetically using a pointed soft iron rod and a strong magnet thus avoiding mechanical damage. Following reduction the samples were vacuum impregnated with acetone in a vacuum chamber at approximately 25 kN m -2, i.e., the vapour pressure of acetone at room temperature. The samples were removed one at a time from the acetone and snap frozen on an austenitic stainless steel anvil submerged in liquid nitrogen (Fig. 4). Each specimen was then broken by the blow from a small hammer into 3-10 pieces. At this temperature, iron, acetone, as well as the oxide, all fail in a brittle manner, thus 4.
3.
2.
1.
/
! 1Omm !
Scale
FIG. 4. Section of the anvil used for the fracturing of partially reduced samples. 1 stainless steel anvil; 2--sample; 3--stainless steel container; 4--insulating fiber; and 5 - liquid nitrogen.
374
M a t t h e w et al.
Partially Reduced Oxides
375
N
N
e~
E
,,.t
e~
0 U
2
e~
2
376
M a t t h e w et al.
there is little plastic deformation of the structure. The purpose of the acetone is to provide additional support for the porous iron during fracture. The fractured samples are removed from the liquid nitrogen using the magnetized soft iron rod; this technique enabling even very small fragments to be collected. The fractured shards are allowed to fall directly onto a sample stub covered with double sided tape. Water condensation on these cold fractured samples was minimized by warming the stub with an incandescent light source. The acetone also evaporates during this period. The samples are then examined using an optical stereomicroscope to select both the specimens to be examined further and the fracture faces which best illustrate the features under examination. As a general rule best results are obtained using samples which are only about 5% reduced. These samples are easily broken without bulk distortion and provide plenty of exposed reaction interface. The initial stages of iron nucleation and growth can also be studied, but the likelihood of obtaining interfaces which have been fractured at a suitable position and orientation is obviously reduced in these circumstances and it may be necessary to prepare a number of samples before satisfactory results may be obtained. The samples are gold sputtered to improve the secondary electron emission in subsequent SEM examination. Results
Examples of the microstructures of partially reduced wustite samples, prepared using the techniques outlined above, are shown in Figures 57. It is immediately apparent that the quality of the micrographs from these specimens is far superior to those obtained from conventional metallographic sections. (Fig. 1). The high resolution and great depth of field of the SEM makes it the ideal instrument for examining these types of specimens. The improvement in image quality is not due solely to the superior optics of the SEM but also to the improved sharpness of the section. It can be seen that the local deformation during fracture of the samples is negligible. Thus the sections accurately reflect the structure of the material during reduction. Figure 5 shows the resultant morphology generated in pure H2 at 1273 K after a reaction time of 30 sec. The coarse tunnels form in the oxide before iron nucleation (1). When iron finally nucleates the surfaces of the now porous oxide are covered with a dense iron layer, a structure that is still visible on the coarse pores. Starting at the original specimen surface the dense iron layer breaks down and allows the initiation of the fine-
Partially Reduced Oxides
377
Fro. 7. SEM micrograph of fractured section of partially reduced wustite sample (pure CO, 1173 K, 40 sec).
378
M a t t h e w et al.
scale porous iron growth (2). This porous iron growth advances into the oxide at approximately 5 p~m/sec under these reaction conditions. The rapid sintering of the porous iron structure and the resulting change in the pore size can also be clearly seen, a phenomenon not identified in previous investigations. Increasing the H20 content results in fewer coarse pores before iron nucleation. However, porous growth is still initiated by the breakdown of an initial dense iron layer until the H2/H20 composition is reached where the breakdown no longer occurs. The presence of H20 results in slower reaction rates and a coarser porous iron structure, as illustrated in Fig. 6. This micrograph shows a sample reduced in 85% H2/15% H20 at 1273 K after a reaction time of 150 sec. Note that the iron actually covers the oxide surface at the growth front, and it appears that the mechanism of porous iron growth involves the repeated breakdown of this thin iron layer. A porous iron morphology may also be obtained on the reduction of wustite in CO/CO2 gas mixtures. Figure 7 shows the resultant morphology generated in pure CO at 1173 K after 40 sec. In this case the wustite surface has well-developed facets upon which a dense iron layer has nucleated and grown. As the iron layer spreads over the surface it breaks down, resulting in the initiation of porous growth. Figure 7b shows a more detailed section of the porous iron-wustite interface in an area where the iron is growing into the bulk oxide. In this sample, the porous iron product appears as a result of a eutectoid-like transformation, the two product phases being iron and gas. The continuous tunnels or pores connecting the bulk gas mixture and the reaction interface may be clearly seen,
Summary The development of new techniques of specimen preparation and examination of partially reduced solid metal oxides have lead to significant improvements in the quality of information that may be obtained from metallographic examination of the specimens. The structure of metaloxide interfaces can now be clearly characterized and quantitative information on the relationships between pore size and growth rate of the product phase may now be obtained. Preliminary investigations indicate that there are two types of growth mechanism that lead to porous iron morphologies. One mechanism resembles a eutectoid type of transformation with continuous growth of the iron product. The other mechanism is a discontinuous or stop-start process involving the repeated formation and subsequent breakdown of iron layers at the reaction interface.
Partially Reduced Oxides
379
References 1. D. H. St. John, S. P. Matthew, and P. C. Hayes, Establishment of product morphology during the initial stages of wustite reduction, Metall. Trans. B, 15B:709-717 (1984). 2. D. H. St. John, S. P. Matthew, and P. C. Hayes, The breakdown of dense iron layers on wustite in CO/CO2 and Hz/H20 systems, MetaU. Trans. B, 15B:701-708 (1984).
Received December 1984; accepted May 1985.