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Effect of crystal orientation on galvanoluminescence of aluminium
Elmrroctinlea lcez ,977. Val. 2_1, pp . 851 855. Pergenon Drew . Printed
w Green Britain
EFFECT OF CRYSTAL ORIENTATION ON GALVANOLUMINESCENCE OF ALUMINIUM S . TAJ1MA, K . SHttmzu, N . 6AaA and S. MATSUZAWA Laboratory of Electrochemistry and Inorganic Chemistry, Tokyo City University, Tokyo, 15R, Japan (Receipec I August 1976)
Abstract-When high purity aluminium sheets containing large single crystals were anodized galvanostatically in barrier-loaning organic acid solutions, such as tartaric, citric acids etc, the distinct difference in the intensity of galvanoluminescence appeared to correspond exactly with the variation of the rate of oxide growth on underlying crystal planes, The intensity of the luminescence was stronger with a crystal plane where the rate of the film growth was higher . Therefrom, the effect of underlying metal orientation on the luminescence was interpreted as a secondary phenomenon due to the difference in the thickness of the film growing on the different crystal planes .
INTROOCrT1ON
Luminescence during anodic oxidation of Al, Ta, Ti, Zr and some other metals is well known and termed °gahanolumincscence°[7] . One of the interesting problems in the study of galvanolurninescence relates to the information that can be gleaned from the luminescence in the growth process of anodic oxide films. Indeed, in aa previous paper[2], we have shown that the luminescence during anodic oxidation of atunxmium depends essentially on the features or the film formed, such as the existence of flaws, inclusion of anions etc . Annealed aluminium sheet of higher purity (99 .99`%, 99.999% Al) is an aggregate of randomly orientated single crystals large enough to allow the determination of their orientations by the back-reflection Laue method- Employing such specimens, we found that the intensity of the luminescence in organic electrolytes was affected by the underlying metal orientation . This paper describes the relationship between the intensity of the luminescence and grain orientation of the substrate metal, and elucidates how this effect is related to the process of the oxide growth . EXPERIMENTAI, 99.999% aluminium sheets 50 x 12 x 0 .5 mm, containing 0.003% Si . 0.002% Fe and 0.003% Cu as major
impurities, were annealed at 600'C for an hour in the air and cooled They were etched in 10°/a HF and electropolished in the bath consisting of 20 v/o of 60'y perchloric acid and 80 v.'o ethanol, rinsed with ethanol and dried . In order to secure uniform anodizing of a definite area and to avoid non-uniform anodizing at the waterline, the upper part of the specimen (20 x 12 x 0.5 mm) was preliminarily coated with insulating barrier oxide film galvanostatically formed in 0.05 M ammonium borate up to ca 440 V . Electrolytes employed were 0 .1 M aqueous solution of tartaric, succinic, citric and malic acids and further ammonium tartrate and ammonium citrate . Anodizing was carried out at 20rC with a constant current of 100mA for the effective specimen area of about 7 .5 cm 2 . The orientation of crystal grain was determined by the back-rfloction Lane method.
RESULTS Figure 1 shows that the annealed aluminium sheet is an aggregate of randomly orientated crystals large enough to allow the determination of their orientations by the back-reflection Laue method . Electropolishing of such a specimen in the ethanol-perchloric acid bath at -t0 1&-0'C for 5 min at 100 mA/cm 2 gives a surface without grain boundary grooves or
Fig . I . Aluminium sheet after rumeahng . 851
552
S. TAnMA,
K . Smwzu, N .
BABA AND
S
MATSUZAWA
Fig 2. The effect of crystal onenLaliun on galvanolununcsuence : 0.1 M tartaric acid eulutiun, foruting voltage of about 160 V . level difference between grains ; longer polishing times are usually required to obtain an uniform and featureless surface . For all the electrolytes employed in our experiments, the different patterns in the intensity of luminescence, depending upon the underlying metal orientation, appeared at about 100V and became more distinct with the rise of the forming voltage . A typical aspect of the luminescence is shown in Fig . 2 . together with the orientation of the grains. The orientation of each grain is represented by a small circle in the stereographic triangle bound by the direction [001], LI I1] and [011] . It is clear that the luminous pattern exactly corresponds to the variation of crystal planes . Luminescence during anodizing of aluminium is a phenomenon associated with the growing film on the metal surface, and accordingly this result clearly indicates that there may be some differences among the films formed on the different crystal planes . The effect of underlying metal orientation on the oxide growth is well known . Indeed, Lacombe and Beaujardf31, for example, demonstrated this effect in a striking manner by a polarized micrograph study of anodically oxidized films on aluminium. Herenguel and LelongL4] have also reported that anodizing of 99.95% Al - 3% Mg alloy in dilute sulphuric acid solution (10 0, V/0) with current density as high as 270mA/col a gave non-uniform films, and that the thickness was the greatest in the [111] direction and least in the L0011 The orientation of each grain was determined by etch figure, and a stereographic contour chart of the relative rate of oxide growth was presented for the alloy. It should be noted here that, in these cases, the factor limiting the rate of oxide growth is the potential barrier at the metal-oxide interface for
metal ion injection into the oxide, and accordingly that the relative rate of oxide growth corresponds to the relative height of the potential harrier at the interface. On the basis of these earlier observations, an attempt was made to correlate the experimentally observed relationship between the intensity of luminescence and metal grain orientation with the relative rate of oxide growth shown in the stereographic contour chart . More than 20 samples were tested from this point of view using mainly tartaric acid solution, and the intensity of luminescence proved to be exactly corresponding to the rate of oxide growth of the underlying metal grains, ie, the luminescence was more intense with the crystal plane where the film growth was faster . Some of the results are shown in Fig. 3 . In the case of Fig. 3(a), for instance, all the grains of the specimen have the same zone axis [I10] and their orientations vary from the [001] direction to the [111] ; as the orientation of the grain approaches the [1111 direction, ie the direction giving the most rapid oxide growth, the intensity of luminescence increases correspondingly . An aluminium surface anodized up to a voltage near the final breakdown voltage of each system exhibited various interference colours, such as green, pink, purple and white, varying from grain to grain (Fig . 4a) . In the case of 0.1 M tartaric acid solution, for example, the voltage at which interference colours began to appear was about 220V for the final breakdown voltage of 260 V . Examination of such a surface by optical microscopy revealed the distinct difference in the roughness among the films grown on the different crystal planes, as shown in Fig . 4(b) . The surface of greater roughness proved to he corresponding to the grain where the rate of oxide growth was higher .
Effect of crystal orientation
luminescence intensity 2>1>4)5>3
3-1)2)4)5)6
853
5x4)3>2> 1=6
1>2)4>3)5
Fig. 3 . The variation of the intensity of luminescencc-with the orientation of underlying grain .
(a)
(b )
Fig . 4 . (a) Aluminium surface anodized in 0 .1 M tartaric acid solution up to 240 V ; (b) optical micrograph of three adjacent grain of the same sample exhibiting different interference colours .
85 4
S .. . Turn , K.
SHUmzu,
N . BABA ANn S. MArSUZAWA
Fig. 5. Luminescence near the final breakdown voltage : 0 .1 M tartaric acid solution, forming voltage of about 250 V . Roughening of the anodized surface is an indication of the film breakdown . The term "breakdown" used here is not in the electrochemical sense of a fall in film resistance but in the sense of the electric or dielectric rupture . Contrary to anodic oxide films on tantalum- those on aluminium exhibit no interference colours in the true sense owing to their low refractive index of 1 .66[5] . Since surface irregularities are another cause of interference colours, the above results lead clearly to the conclusion that the colours are due to the scattering of light at the surface irregularities caused by the film breakdown and vary according to the degree of roughness. Depending upon the nature, concentration, and temperature of the electrolytes, the film breakdown is accompanied by two distinct types of sparking : (1) random sparking- that is, isolated bright spots "sparks", appearing and disappearing in succession initially at the edge of the anode, and then also on the entire surface leaving trucks upon them ; and (2) tiny and uniformly distributed sparking which fpves to the naked eye uniform radiation, then alters the emission to yellowish (Fig . 5) and which gives an uniformly roughened surface (Fig. 4b). Under the anodizing conditions employed in our experiment, the latter type of sparking appeared. Sparking is another kind of luminous effect seen during galvanostatic anodizing of valve metals. DISCUSSION
It is now clear that the intensity of luminescence and, when the specimens were anodized up to a voltage near the final breakdown voltage of each system, the degree of surface roughness which corresponds to the amount of sparking, are closely related to the orientation, and hence the rate of oxide growth, of
the underlying metal grains . In obtaining a possible explanation for these results, the first point to be discussed is the distinct difference in the roughness of films grown on the grains of different orientation (Fig . 4b) . It may be reasonable to assume that the thickness attainable in each electrolyte is the same for all the grains regardless of their orientations, since electrolytic breakdown, which restricts the thickness attainable in barrier-forming electrolytes, has been shown by Yahalom annd Zahavi[6I to be controlled not by the "history" of the film, that is, the properties of oxide bulk or the metal-oxide contact . but only by the oxide-electrolyte interface . The fact that the greater roughness corresponds to the grain of higher rate of oxide growth, therefore, strongly implies that the thickness is greater on the grain of higher film growth rate, namely, that the higher the rate of film growth the closer is the thickness to a critical point, then the easier is the breakdown and thus the greater is the roughness. In the previous paper [2], we have shown that the thickness is one of the most influential factors for the luminescence intensity . Accordingly, the effect of the orientation of underlying metal grains on the luminescence can reasonably be interpreted to be a secondary phenomenon, due to the difference in the rate of oxide growth, and hence in its thickness . At present, the determination of the factor controlling the rate of film growth may be one of the important problems in the kinetic investigation . The rate is controlled either by the defect injection at the metal oxide, or at the oxide electrolyte interface, or by ion transport within the film . Recently, Dignam[7] based upon an extensive study of the data in the literature, has concluded that the rate is controlled most likely at the oxide- electrolyte interface with the rate controlling step of the injection of anion defects into the oxide. However, our experimental observations clearly indicate that, under the anodizing conditions employed in our experiment by which the luminescence intensity was controlled by the grain orientation of the substrate metal, the rate ought to be controlled at the metal-oxide interface, with the rate controlling step possibly being the metal ion injection into the oxide . The rate controlling step may be expected to vary depending upon anodizing conditions, such as the nature of the electrolyte, anodizing rd, thickness of growing film etc, and the phenomenon of galvanolumhtescence could be a powerful tool for the investigation of such aspects . Acknowledgement-Thanks are due to Prof. Dr . Y. Tanabe and Dr . T. Watanabe of the university for the useful advice in the determination of crystal orientation and also to Light Metal Educational Foundations (Keikinzoku Shogakukai, OsaksI for the financial support . REFERENCES
t . SS tkonopisov, Efecrrochtm . Acre 20, 783 (1975) . 2 . S . Tajima, K . Shimizu, N . Baba, and S. Matsuzawa, Electrochim. Acta 22, 845 11977) . 3 . P. Laeombe and L. Beanjard, "Etudes cur tea Aspects de Pititicule d'Oxydation Anodique", edition du Comite General d'Organization des Industrie Mechaniques, Paris, 1944. 4. 7 . Hcrcngucl and P. Ldong, C .r, hebd. Seanc. Aced. Set ., Paris 232, 2219 (1951) .
Uf ect of crystal orientation
855
7 . M . 1 . Dignasn . Mechanism of Ionic Trmrsporl Through 5 . S. Tajima, "Anodic oxidation of aluminum", in Advances in Corrosion Science and Technology, (Edited by Fon- Oxide Films, The Anodic Behavior of Metals and Semi. Plenum Press, New York, conductors Series, Vol . 1, p. 215 . Marvel Dekker, New tana aid Staehle), p . 287 1970 . York 1972 . 6. 1 Yahalom and J . Zahavi, Elecrrochim. Acra 16 . 603 (1971) .
w Green Britain
EFFECT OF CRYSTAL ORIENTATION ON GALVANOLUMINESCENCE OF ALUMINIUM S . TAJ1MA, K . SHttmzu, N . 6AaA and S. MATSUZAWA Laboratory of Electrochemistry and Inorganic Chemistry, Tokyo City University, Tokyo, 15R, Japan (Receipec I August 1976)
Abstract-When high purity aluminium sheets containing large single crystals were anodized galvanostatically in barrier-loaning organic acid solutions, such as tartaric, citric acids etc, the distinct difference in the intensity of galvanoluminescence appeared to correspond exactly with the variation of the rate of oxide growth on underlying crystal planes, The intensity of the luminescence was stronger with a crystal plane where the rate of the film growth was higher . Therefrom, the effect of underlying metal orientation on the luminescence was interpreted as a secondary phenomenon due to the difference in the thickness of the film growing on the different crystal planes .
INTROOCrT1ON
Luminescence during anodic oxidation of Al, Ta, Ti, Zr and some other metals is well known and termed °gahanolumincscence°[7] . One of the interesting problems in the study of galvanolurninescence relates to the information that can be gleaned from the luminescence in the growth process of anodic oxide films. Indeed, in aa previous paper[2], we have shown that the luminescence during anodic oxidation of atunxmium depends essentially on the features or the film formed, such as the existence of flaws, inclusion of anions etc . Annealed aluminium sheet of higher purity (99 .99`%, 99.999% Al) is an aggregate of randomly orientated single crystals large enough to allow the determination of their orientations by the back-reflection Laue method- Employing such specimens, we found that the intensity of the luminescence in organic electrolytes was affected by the underlying metal orientation . This paper describes the relationship between the intensity of the luminescence and grain orientation of the substrate metal, and elucidates how this effect is related to the process of the oxide growth . EXPERIMENTAI, 99.999% aluminium sheets 50 x 12 x 0 .5 mm, containing 0.003% Si . 0.002% Fe and 0.003% Cu as major
impurities, were annealed at 600'C for an hour in the air and cooled They were etched in 10°/a HF and electropolished in the bath consisting of 20 v/o of 60'y perchloric acid and 80 v.'o ethanol, rinsed with ethanol and dried . In order to secure uniform anodizing of a definite area and to avoid non-uniform anodizing at the waterline, the upper part of the specimen (20 x 12 x 0.5 mm) was preliminarily coated with insulating barrier oxide film galvanostatically formed in 0.05 M ammonium borate up to ca 440 V . Electrolytes employed were 0 .1 M aqueous solution of tartaric, succinic, citric and malic acids and further ammonium tartrate and ammonium citrate . Anodizing was carried out at 20rC with a constant current of 100mA for the effective specimen area of about 7 .5 cm 2 . The orientation of crystal grain was determined by the back-rfloction Lane method.
RESULTS Figure 1 shows that the annealed aluminium sheet is an aggregate of randomly orientated crystals large enough to allow the determination of their orientations by the back-reflection Laue method . Electropolishing of such a specimen in the ethanol-perchloric acid bath at -t0 1&-0'C for 5 min at 100 mA/cm 2 gives a surface without grain boundary grooves or
Fig . I . Aluminium sheet after rumeahng . 851
552
S. TAnMA,
K . Smwzu, N .
BABA AND
S
MATSUZAWA
Fig 2. The effect of crystal onenLaliun on galvanolununcsuence : 0.1 M tartaric acid eulutiun, foruting voltage of about 160 V . level difference between grains ; longer polishing times are usually required to obtain an uniform and featureless surface . For all the electrolytes employed in our experiments, the different patterns in the intensity of luminescence, depending upon the underlying metal orientation, appeared at about 100V and became more distinct with the rise of the forming voltage . A typical aspect of the luminescence is shown in Fig . 2 . together with the orientation of the grains. The orientation of each grain is represented by a small circle in the stereographic triangle bound by the direction [001], LI I1] and [011] . It is clear that the luminous pattern exactly corresponds to the variation of crystal planes . Luminescence during anodizing of aluminium is a phenomenon associated with the growing film on the metal surface, and accordingly this result clearly indicates that there may be some differences among the films formed on the different crystal planes . The effect of underlying metal orientation on the oxide growth is well known . Indeed, Lacombe and Beaujardf31, for example, demonstrated this effect in a striking manner by a polarized micrograph study of anodically oxidized films on aluminium. Herenguel and LelongL4] have also reported that anodizing of 99.95% Al - 3% Mg alloy in dilute sulphuric acid solution (10 0, V/0) with current density as high as 270mA/col a gave non-uniform films, and that the thickness was the greatest in the [111] direction and least in the L0011 The orientation of each grain was determined by etch figure, and a stereographic contour chart of the relative rate of oxide growth was presented for the alloy. It should be noted here that, in these cases, the factor limiting the rate of oxide growth is the potential barrier at the metal-oxide interface for
metal ion injection into the oxide, and accordingly that the relative rate of oxide growth corresponds to the relative height of the potential harrier at the interface. On the basis of these earlier observations, an attempt was made to correlate the experimentally observed relationship between the intensity of luminescence and metal grain orientation with the relative rate of oxide growth shown in the stereographic contour chart . More than 20 samples were tested from this point of view using mainly tartaric acid solution, and the intensity of luminescence proved to be exactly corresponding to the rate of oxide growth of the underlying metal grains, ie, the luminescence was more intense with the crystal plane where the film growth was faster . Some of the results are shown in Fig. 3 . In the case of Fig. 3(a), for instance, all the grains of the specimen have the same zone axis [I10] and their orientations vary from the [001] direction to the [111] ; as the orientation of the grain approaches the [1111 direction, ie the direction giving the most rapid oxide growth, the intensity of luminescence increases correspondingly . An aluminium surface anodized up to a voltage near the final breakdown voltage of each system exhibited various interference colours, such as green, pink, purple and white, varying from grain to grain (Fig . 4a) . In the case of 0.1 M tartaric acid solution, for example, the voltage at which interference colours began to appear was about 220V for the final breakdown voltage of 260 V . Examination of such a surface by optical microscopy revealed the distinct difference in the roughness among the films grown on the different crystal planes, as shown in Fig . 4(b) . The surface of greater roughness proved to he corresponding to the grain where the rate of oxide growth was higher .
Effect of crystal orientation
luminescence intensity 2>1>4)5>3
3-1)2)4)5)6
853
5x4)3>2> 1=6
1>2)4>3)5
Fig. 3 . The variation of the intensity of luminescencc-with the orientation of underlying grain .
(a)
(b )
Fig . 4 . (a) Aluminium surface anodized in 0 .1 M tartaric acid solution up to 240 V ; (b) optical micrograph of three adjacent grain of the same sample exhibiting different interference colours .
85 4
S .. . Turn , K.
SHUmzu,
N . BABA ANn S. MArSUZAWA
Fig. 5. Luminescence near the final breakdown voltage : 0 .1 M tartaric acid solution, forming voltage of about 250 V . Roughening of the anodized surface is an indication of the film breakdown . The term "breakdown" used here is not in the electrochemical sense of a fall in film resistance but in the sense of the electric or dielectric rupture . Contrary to anodic oxide films on tantalum- those on aluminium exhibit no interference colours in the true sense owing to their low refractive index of 1 .66[5] . Since surface irregularities are another cause of interference colours, the above results lead clearly to the conclusion that the colours are due to the scattering of light at the surface irregularities caused by the film breakdown and vary according to the degree of roughness. Depending upon the nature, concentration, and temperature of the electrolytes, the film breakdown is accompanied by two distinct types of sparking : (1) random sparking- that is, isolated bright spots "sparks", appearing and disappearing in succession initially at the edge of the anode, and then also on the entire surface leaving trucks upon them ; and (2) tiny and uniformly distributed sparking which fpves to the naked eye uniform radiation, then alters the emission to yellowish (Fig . 5) and which gives an uniformly roughened surface (Fig. 4b). Under the anodizing conditions employed in our experiment, the latter type of sparking appeared. Sparking is another kind of luminous effect seen during galvanostatic anodizing of valve metals. DISCUSSION
It is now clear that the intensity of luminescence and, when the specimens were anodized up to a voltage near the final breakdown voltage of each system, the degree of surface roughness which corresponds to the amount of sparking, are closely related to the orientation, and hence the rate of oxide growth, of
the underlying metal grains . In obtaining a possible explanation for these results, the first point to be discussed is the distinct difference in the roughness of films grown on the grains of different orientation (Fig . 4b) . It may be reasonable to assume that the thickness attainable in each electrolyte is the same for all the grains regardless of their orientations, since electrolytic breakdown, which restricts the thickness attainable in barrier-forming electrolytes, has been shown by Yahalom annd Zahavi[6I to be controlled not by the "history" of the film, that is, the properties of oxide bulk or the metal-oxide contact . but only by the oxide-electrolyte interface . The fact that the greater roughness corresponds to the grain of higher rate of oxide growth, therefore, strongly implies that the thickness is greater on the grain of higher film growth rate, namely, that the higher the rate of film growth the closer is the thickness to a critical point, then the easier is the breakdown and thus the greater is the roughness. In the previous paper [2], we have shown that the thickness is one of the most influential factors for the luminescence intensity . Accordingly, the effect of the orientation of underlying metal grains on the luminescence can reasonably be interpreted to be a secondary phenomenon, due to the difference in the rate of oxide growth, and hence in its thickness . At present, the determination of the factor controlling the rate of film growth may be one of the important problems in the kinetic investigation . The rate is controlled either by the defect injection at the metal oxide, or at the oxide electrolyte interface, or by ion transport within the film . Recently, Dignam[7] based upon an extensive study of the data in the literature, has concluded that the rate is controlled most likely at the oxide- electrolyte interface with the rate controlling step of the injection of anion defects into the oxide. However, our experimental observations clearly indicate that, under the anodizing conditions employed in our experiment by which the luminescence intensity was controlled by the grain orientation of the substrate metal, the rate ought to be controlled at the metal-oxide interface, with the rate controlling step possibly being the metal ion injection into the oxide . The rate controlling step may be expected to vary depending upon anodizing conditions, such as the nature of the electrolyte, anodizing rd, thickness of growing film etc, and the phenomenon of galvanolumhtescence could be a powerful tool for the investigation of such aspects . Acknowledgement-Thanks are due to Prof. Dr . Y. Tanabe and Dr . T. Watanabe of the university for the useful advice in the determination of crystal orientation and also to Light Metal Educational Foundations (Keikinzoku Shogakukai, OsaksI for the financial support . REFERENCES
t . SS tkonopisov, Efecrrochtm . Acre 20, 783 (1975) . 2 . S . Tajima, K . Shimizu, N . Baba, and S. Matsuzawa, Electrochim. Acta 22, 845 11977) . 3 . P. Laeombe and L. Beanjard, "Etudes cur tea Aspects de Pititicule d'Oxydation Anodique", edition du Comite General d'Organization des Industrie Mechaniques, Paris, 1944. 4. 7 . Hcrcngucl and P. Ldong, C .r, hebd. Seanc. Aced. Set ., Paris 232, 2219 (1951) .
Uf ect of crystal orientation
855
7 . M . 1 . Dignasn . Mechanism of Ionic Trmrsporl Through 5 . S. Tajima, "Anodic oxidation of aluminum", in Advances in Corrosion Science and Technology, (Edited by Fon- Oxide Films, The Anodic Behavior of Metals and Semi. Plenum Press, New York, conductors Series, Vol . 1, p. 215 . Marvel Dekker, New tana aid Staehle), p . 287 1970 . York 1972 . 6. 1 Yahalom and J . Zahavi, Elecrrochim. Acra 16 . 603 (1971) .