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Ta alloy
Corrosion Science 45 (2003) 1779–1792 www.elsevier.com/locate/corsci
High resistivity magnesium-rich layers and current instability in anodizing a Mg/Ta alloy S. Mato a, G. Alcala a, P. Skeldon a,*, G.E. Thompson a, D. Masheder b, H. Habazaki d, K. Shimizu c a
Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 1QD, UK b AVX Ltd., Tantalum Division, Long Rd., Paignton, Devon TQ4 7ER, UK c University Chemical Laboratory, Keio University, 4-1-1 Hiyoshi, Yokohama 223, Japan Graduate School of Engineering, Hokkaido University, N13-W8, Kita-ku, Sapporo 060-8628, Japan
d
Received 4 September 2002; accepted 5 December 2002
Abstract Strikingly different morphologies of amorphous anodic films on a Mg/40 at.%Ta alloy are shown to result from single-stage and sequential anodizing procedures. The alloy, prepared by magnetron sputtering, was anodized galvanostatically in ammonium pentaborate (pH 8.3) and sodium silicate (pH 12.6) electrolytes at 293 K and studied by transmission electron microscopy, Rutherford backscattering spectroscopy, glow discharge optical emission spectroscopy and X-ray photoelectron spectroscopy. For one-step anodizing in the pentaborate electrolyte, a single-layered film, of approximate composition Ta2 O5 MgO, forms at a ratio of 1.8 nm V1 . In the silicate electrolyte, an outer, magnesium-rich layer, containing silicon species, also forms, with a ratio of 0.8 nm V1 . The outer layer can develop due to relatively fast migration of magnesium ions in the inner layer and the stabilization of the pH at the film surface, probably linked to generation of a silica gel that also limits loss of magnesium species to the electrolyte. Further thickening of the anodic film, in ammonium pentaborate electrolyte, produces fingers of low resistivity, inner oxide that penetrate the pre-existing, high resistivity outer layer, with a bi-modal distribution of finger sizes. When fingers reach the film surface, magnesium ions are ejected to the electrolyte. The absence of fingers in films grown in sodium silicate electrolyte is possibly due to prevention, by the silica gel, of their initiation. Ó 2003 Published by Elsevier Science Ltd.
*
Corresponding author. Tel.: +44-161-200-4872; fax: +44-161-200-4865. E-mail address: [email protected] (P. Skeldon).
0010-938X/03/$ - see front matter Ó 2003 Published by Elsevier Science Ltd. doi:10.1016/S0010-938X(02)00258-5
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1. Introduction Anodic films on tantalum are employed in the manufacture of capacitors and provide high levels of performance. The films, which are formed on sintered tantalum electrodes, are composed primarily of amorphous tantala [1,2]. The electrodes contain a number of impurities at low levels, particularly nitrogen, iron and magnesium, which generally appear to have minor influence on the growth of the film. However, such impurities may have local influences that affect leakage currents, the generation of flaws and the occurrence of field-induced crystallization. In the present work, attention is focused on a Mg/Ta alloy containing 40 at.%Ta in order to understand the possible effects on film growth of local high concentrations of magnesium in tantalum. Although tantalum alloy systems have been studied extensively for diverse applications [3], relatively little is known about Ta/Mg alloys and their anodizing behaviours. The resultant films also provide insight into formation of anodic films on magnesium, particularly since bi-layered films are produced at high pH in sodium silicate electrolyte, with an outer magnesium-rich layer, free of tantalum species, that can be sectioned readily by ultramicrotomy for examination by transmission electron microscopy (TEM).
2. Experimental procedure A Mg/40 at.%Ta alloy was deposited by magnetron sputtering, using an Atom Tech Ltd facility, on to electropolished and anodized aluminium substrates that provide smooth surfaces. The aluminium substrates were electropolished at 20 V for 180 s in a mixture of ethanol and perchloric acid (4:1 by vol.) at 278 K, rinsed in ethanol and in distilled water, and dried in a cool air stream. The electropolished substrates were then anodized to 150 V at 5 mA cm2 in 0.1 M ammonium pentaborate electrolyte at 293 K, rinsed in distilled water, and finally dried in a cool air stream. Following initial evacuation to 5 107 mbar, sputtering was carried out in 99.998% argon at 5 103 mbar. In order to obtain the desired alloy composition, appropriate currents were applied to the separate 99.9% magnesium and 99.99% tantalum targets. X-ray diffraction revealed a crystalline alloy; the pattern could not be identified in reference files. Specimens of the alloy were anodized at 5 mA cm2 as follows: (i) to 150 V in 0.1 M ammonium pentaborate electrolyte (pH 8.3), (ii) to 80 V, 150 V and 210 V in 0.1 M sodium silicate electrolyte (pH 12.6) and (iii) firstly to 80 V in 0.1 sodium silicate electrolyte and secondly to 200 V in 0.1 M ammonium pentaborate electrolyte. The temperature of the electrolyte was 293 K in all cases. The voltage–time response was recorded for each specimen. Anodized specimens were examined by TEM, using a JEOL 2000 FX II instrument, with preparation of cross-sections, of nominal thickness 10 nm, by ultramicrotomy. The compositions of film and alloy regions were determined by Rutherford backscattering spectroscopy (RBS) using 2.08 MeV Heþ ions with a current of 60 nA and beam diameter of 1 mm. The beam was incident normal to the
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surface of the specimen, with scattered ions detected at 165° to the direction of the incident beam. The data were interpreted using the RUMP program [4]. Spectra for anodized specimens were fitted using thicknesses of film regions from TEM. Composition profiles of a film formed in sodium silicate electrolyte were determined by glow discharge optical emission spectroscopy (GDOES), using a Jobin-Yvon 5000 instrument, with the analysis conditions given previously [5]. Anodized alloys were also examined by X-ray photoelectron spectroscopy (XPS) in a Shimadzu ESCA-850 electron spectrometer, using Mg Ka excitation. The binding energies of electrons were calibrated using a method described elsewhere [6]. The binding energies of Au 4f7=2 and 4f5=2 electrons of gold were taken as 84.07 and 87.74 eV, and those of Cu 2p3=2 and 2p1=2 electrons of copper as 932.53 and 952.35 eV respectively; the kinetic energy of Cu L3 M4;5 M4;5 Auger electrons was taken as 918.65 eV. The energy shifts, through charging of the specimen, were corrected using the C 1s peak for contaminant hydrocarbon at 285.0 eV.
3. Results 3.1. Voltage–time responses and transmission electron microscopy After an initial voltage surge of 1–2 V, due to the air-formed oxide on the alloy, the voltage–time response for anodizing in ammonium pentaborate electrolyte was linear with a slope of 1:2 0:1 V s1 (Fig. 1(a)). Transmission electron micrographs disclosed a uniform anodic film of amorphous appearance, with the thickness of 270 5 nm indicating a formation ratio of 1:80 0:03 nm V1 (Fig. 2(a)).
Fig. 1. Voltage–time responses of Mg/40 at.%Ta alloy anodized at 5 mA cm2 and 293 K (a) to 150 V in 0.1 M ammonium pentaborate electrolyte, (b) to 150 V in 0.1 M sodium silicate, and (c) to 80 V in 0.1 sodium silicate and then re-anodized to 200 V in 0.1 M ammonium pentaborate.
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Fig. 2. Transmission electron micrograph of Mg/40 at.%Ta alloy anodized at 5 mA cm2 and 293 K (a) to 150 V in 0.1 M ammonium pentaborate electrolyte, (b) to 150 V in 0.1 M sodium silicate electrolyte, (c) to 80 V in 0.1 M sodium silicate electrolyte and then re-anodized to 200 V in 0.1 M ammonium pentaborate electrolyte; (d) enlargement of the alloy/film interface of (c). The lines in the film of (a) near the top of the micrograph are knife marks from ultramicrotomy.
For anodizing in sodium silicate electrolyte, the voltage surge was followed by a linear voltage–time response, with a slope of 2:5 0:1 V s1 (Fig. 1(b)). Transmission electron micrographs revealed two layers in the anodic film formed to 150 V, both of uniform and amorphous appearance, but the outer layer, representing 40 4% of the film thickness, is lighter than the inner layer (Fig. 2(b)). The total thickness of 170 5 nm indicates a formation ratio of 1:13 0:03 nm V1 . The ratio of thicknesses of the inner and the outer layers and the formation ratio were also similar to the previous values for anodizing to 80 and 210 V. However, with increasing voltage, the interface between the layers developed increased undulations, associated with local variations in the thickness ratio of the inner and outer layers between 1.8 and 1.2, with peaks separated by about 60 nm. In order to determine whether dissolution of the film occurred in the sodium silicate electrolyte, a specimen was anodized to 50 V and the current was switched off with the specimen left immersed in the elec-
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trolyte for 300 s. On re-anodizing, a voltage surge of 50 V indicated no significant degradation of the film between the two stages of anodizing. For a specimen anodized sequentially, to 80 V in sodium silicate electrolyte and to 200 V in ammonium pentaborate electrolyte, the voltage at the start of the second anodizing rose to about 73 V, 9% less than the initial voltage, suggesting some dissolution of the first-formed film following the brief immersion in the second electrolyte before switching on the current (Fig. 1(c)). Thereafter, following a short stage where the voltage increased very slowly, the response was linear with a slope of 1:2 0:1 V s1 , similar to that for anodizing in ammonium pentaborate electrolyte. Extending the period of immersion in the second electrolyte to 300 s eliminated the initial region of low slope, with the voltage immediately increasing at 1.2 V s1 . Further, the initial voltage at the start of re-anodizing was reduced by 25% compared with the final voltage in the first stage of anodizing suggesting additional film dissolution. Transmission electron micrographs disclosed an anodic film of amorphous appearance with several distinct regions (Fig. 2(c)). The inner layer, representing 55% of the film thickness, was similar to that of the inner layer and the total thickness of films formed in sodium silicate and ammonium pentaborate electrolytes respectively. Above this inner layer, a lighter layer, resembling material developed at the outer region of films formed in sodium silicate electrolyte, was present with primary fingers of inner layer material, of width about 30 nm, penetrating its thickness. Above the lighter material, narrow, secondary fingers of light material, of width about 5 nm, are evident in a layer of relatively dark material. The outer 10 nm of the film consisted of the darker material, with thicker regions at locations of the main fingers. The total thickness of the film is 333 5 nm, indicating a formation ratio of 1:66 0:03 nm V1 , intermediate between values for anodizing in the individual electrolytes. Inspection of the alloy beneath the anodic films in the previous micrographs discloses a layer of about 2 nm thickness which is darker than the bulk alloy. The contrast results from enrichment of tantalum. The enriched layer is shown at increased magnification in Fig. 2(d). The underlying alloy is striated with light and dark bands, of thickness about 2 nm, which were caused by corrosion of the section in water during ultramicrotomy. 3.2. Rutherford backscattering spectroscopy The RBS spectra for the alloy, prior to anodizing, revealed yields from tantalum and magnesium, as well from the aluminium substrate, indicating a layer of Mg/40 at.%Ta alloy of thickness about 500 nm, using the weighted average of the atomic densities of the pure metals (Fig. 3(a)). The spectra following anodizing to 150 V in 0.1 M ammonium pentaborate electrolyte disclosed a single-layered film containing 4:2 1017 Ta atoms cm2 and 2:1 1017 Mg atoms cm2 , based on units of MgO and Ta2 O5 (Fig. 3(b)). The atomic concentration of magnesium in the film, considering cations only, is 33 at.%. The measured amounts of magnesium in this and in later films are possibly subject to an error of about 15% due to the uncertainty of the background level. The uncertainty in the tantalum content is about 5%. The outer regions of the film may contain boron species, derived from anions in the electrolyte;
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Fig. 3. Experimental and simulated RBS spectra for the Mg/40 at.%Ta alloy prior to anodizing (a) and following anodizing at 5 mA cm2 and 293 K (b) to 150 V in 0.1 M ammonium pentaborate electrolyte, (c) to 150 V in 0.1 M sodium silicate electrolyte, (d) as (c) showing details of the region corresponding to magnesium and silicon peaks, and (e) to 80 V in sodium silicate electrolyte and then to 200 V in 0.1 M ammonium pentaborate electrolyte. The leading edge for tantalum at the surface in (e) is shifted compared with that of (a), (b) and (c) due to a differing calibration of the spectrum.
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however, they would not be detected by RBS due to their low atomic mass. The charge of the cations in the film corresponded to 0.42 C cm2 , compared with 0.62 C cm2 from the current density and time of anodizing, indicating an efficiency of 66%. The reduced magnesium content of the film compared with that of the alloy indicates that a charge of about 0.20 C cm2 was associated with loss of Mg2þ ions to the electrolyte, and hence an efficiency of 76% that is consistent with the previous estimate given the uncertainties in the compositions. The RBS data for a specimen anodized to 150 V in 0.1 M sodium silicate electrolyte revealed a primarily two-layered film, consistent with results of TEM (Fig. 3(c) and (d)). The outer layer contained magnesium, oxygen and silicon species, with an absence of tantalum ions. The magnesium content of the outer layer was 2:4 1017 Mg atoms cm2 , with an atomic ratio of silicon to magnesium of about 0.25. The inner layer consisted of two sub-layers. An inner sub-layer contained 1:6 1017 Ta atoms cm2 and 8:1 1016 Mg atoms cm2 ; the overlying sub-layer contained 6:7 1016 Ta atoms cm2 , 2:5 1016 Mg atoms cm2 and also 3:5 1016 Si atoms cm2 . The average atomic ratio of magnesium to tantalum in the inner layer was 33 at.%Mg, similar to the composition of the previous film. Further, an enrichment of about 1:5 1016 Si atoms cm2 was present at the surface of the film. Accurate analysis of the silicon and magnesium contents of the inner parts of the film is hindered by overlap of silicon and magnesium signals and the background. The relative concentration of magnesium in the anodic film, 57 at.%, was similar to that in the alloy and is consistent with growth of the film with negligible loss of cation species to the electrolyte. The charge associated with the principal cations in the film is 0.29 C cm2 , compared with the charge of 0.31 C cm2 estimated from the current density and time of anodizing, consistent with a high efficiency of growth. Experimental and simulated spectra for a specimen anodized to 200 V in 0.1 M ammonium pentaborate electrolyte, and previously anodized to 80 V in 0.1 M sodium silicate electrolyte, disclosed an anodic film with several layers (Fig. 3(e)). The innermost layer contained about 33 at.%Mg. The following layers contained tantalum, magnesium, silicon and oxygen except for the outermost layer where silicon species were absent. Assignment of accurate compositions to the various layers is prevented by overlap of silicon, magnesium and background signals. A small peak indicated a silicon enrichment of about 5:0 1016 Si atoms cm2 remaining at the surface of the film from the first stage of anodizing. The estimated concentration of magnesium ions in the anodic film, 40%, is low compared with the alloy composition. The estimated charge of cations in the film was 0.43 C cm2 , compared with 0.76 C cm2 from the current density and time of anodizing, indicating an efficiency of 57%. 3.3. Glow discharge optical emission spectroscopy GDOES analysis of a film formed to 150 V in sodium silicate electrolyte confirmed the findings of RBS, with an outer magnesium-rich layer above an inner layer containing both magnesium and tantalum species (Fig. 4). The sputtering rate of the outer layer was slower than that of the inner layer; hence, the sputtering time ratio does not correspond with the thickness ratio from TEM. Silicon species are revealed
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Fig. 4. Results of GDOES analysis of Mg/40 at.%Ta alloy anodized to 150 V at 5 mA cm2 in 0.1 M sodium silicate electrolyte at 293 K.
beneath the boundary between the outer and inner layers. Further, the outer layer contained enhanced levels of hydrogen species relative to other film regions, and hydrogen was also enriched near the alloy/film interface. 3.4. X-ray photoelectron spectroscopy The X-ray photoelectron spectra of specimens anodized (i) to 150 V in sodium silicate electrolyte and (ii) by sequential anodizing, firstly to 80 V in sodium silicate electrolyte and secondly to 200 V in ammonium pentaborate electrolyte, disclosed peaks of oxygen, silicon and magnesium. Results were similar generally for both specimens; results are presented for the specimen anodized sequentially. The Si 2p peak at 102.6 eV is consistent with presence of silica or silicate (Fig. 5(a)) [7]. The Mg 2p3=2 peak at 50.4 eV, corresponds to divalent magnesium in the form of MgO, Mg(OH)2 or magnesium silicate (Fig. 5(b)) [7–9]. The specimen sequentially anodized reveals also a Ta 4f peak, located at 26.5 eV, which partly overlaps the O 2s peak, corresponding to pentavalent tantalum (Fig. 5(c)) [7]. The O 1s peak was located at 532.2 eV, consistent with species suggested by the previous peaks. The Ta 4f peak was not observed following anodizing in the silicate electrolyte only. 4. Discussion 4.1. Anodic oxidation of alloys in ammonium pentaborate and sodium silicate electrolytes Relatively dilute, solid–solution, aluminium alloys can reveal enrichments of the alloying element in the alloy immediately beneath the anodic film as a consequence
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Fig. 5. Results of XPS analysis of Mg/40 at.%Ta alloy anodized to 80 V at 5 mA cm2 in 0.1 M sodium silicate electrolyte at 293 K and then re-anodized to 200 V in 0.1 M ammonium pentaborate electrolyte at 293 K: (a) silicon 2p peak; (b) magnesium 2p peak; (c) tantalum 4f peak.
of film growth [10]. Limited evidence for magnesium alloys suggest similar enrichments occur [11]. For aluminium alloys, the initial composition is dependent upon the relative values of the Gibbs free energy per equivalent (DG0 =n) for formation of the oxides of the constituent elements [12]. Elements forming oxides with higher values of DG0 =n than that for formation of Al2 O3 initially enrich in the alloy and the anodic film is free of their oxide species. Tantalum alloys appear to behave differently, with oxidation of the alloying elements commencing without the requirement for enrichment [13]. Consistent with previous indications for magnesium and the relative magnitudes of DG0 =n for MgO and Ta2 O5 [12], enrichment of tantalum, in a layer of thickness 1–2 nm, evident by atomic number contrast in TEM, occurred during anodizing of the present alloy. The enrichment is observable by TEM, but the layer is too thin for resolution by the RBS or GDOES analyses. Following enrichment, Ta5þ and Mg2þ ions are incorporated into the anodic film, as units of MgO and Ta2 O5 , in proportion to the composition of the alloy.
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The growth of amorphous anodic oxides usually involves contributions of cation and anion migration [14,15]. The transport numbers of cations and anions in anodic tantala are about 0.24 and 0.76 respectively [16]. There appear to be no published transport numbers for magnesium oxide. However, transport of Ta5þ , Mg2þ and O2 /OH ions is consistent with the evidence of layered films on the present alloy. Migration rates of cation species in anodic films often correlate with the energies of metal–oxygen, single-bonds [17]. With bond energies of about 166 and 347 kJ mol1 for Mg2þ –O and Ta5þ –O respectively, faster outward migration of Mg2þ ions relative to that of Ta5þ ions is anticipated. Neglecting other factors, an outer, magnesiumrich, layer should develop if the surface pH is >10 when the solubility of MgO/ Mg(OH)2 reduces [18]. At lower pH, extensive loss of Mg2þ ions to solution by either chemical dissolution or field-assisted ejection would be expected. The expected growth mechanisms are illustrated schematically in Fig. 6(a) and (b). In agreement with the previous discussion, the innermost layers of the present films are depleted in magnesium compared with the alloy, due to relative fast outward migration of Mg2þ ions through the inner layer, with a composition, by RBS, of approximately Ta2 O5 MgO. The formation ratio, 1.8 nm V1 , indicated by the single-layered film formed in ammonium pentaborate electrolyte, compares with 1.6 nm V1 for anodic tantala formed under similar conditions [19]. In sodium silicate electrolyte, an outer, magnesium-rich layer, containing silicon species, develops above the inner layer, and silicon species penetrate to the inner layer. The outer layer also contains an increased concentration of hydrogen relative to that in the inner layer, as evident from GDOES. Using the previous formation ratio for inner layer material, that for the outer layer is about 0.8 nm V1 . The formation ratio has been reported previously as 0.6 nm V1 for anodic films formed on magnesium at 5 mA cm2 in KOH solution [20]. Although more resistive to ionic current, penetration of the outer layer by fingers of the inner layer material does not occur for the present films. However, variable thicknesses of the inner and outer layers, particularly with increase of voltage, suggests some non-uniformity of current. 4.2. Role of silicon species during anodizing in sodium silicate electrolyte Silicon species are present at the surface and within the film formed in sodium silicate electrolyte. The surface species probably indicate a silica-based gel formed by reaction between silicate ions of the electrolyte and Hþ ions produced by anodizing [21]. The gel, assisted by stabilization of the pH at the surface of the film to about 10 [18], probably limits loss of magnesium species to the electrolyte. Silicate ions pass readily through the gel, allowing incorporation of silicon species into the film as 4þ either an anion species, such as SiO2 3 or HSiO3 ions, or a cation species, such as Si 4þ 1 ions in units of SiO2 . The relatively high Si –O bond energy, 465 kJ mol , would suggest that Si4þ ions, if present, are of low mobility. In anodic tantala, silicon species incorporated directly from the electrolyte are immobile [22]. The outer film layer grows at the film/electrolyte interface and is effectively destroyed at the inner layer/outer layer interface to supply oxygen species for growth of the inner layer material. Consequently, silicon species may pass to the inner layer/outer layer in-
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Fig. 6. Schematic diagrams of the mechanisms of anodic film growth on Mg/40 at.%Ta alloy in (a) ammonium pentaborate electrolyte, (b) sodium silicate electrolyte and (c) ammonium pentaborate electrolyte after previous anodizing in sodium silicate electrolyte. For simplicity incorporation of species derived from electrolyte anions has been neglected and anion migration assumed to involve O2 ions only.
terface, whether of relatively low mobility outward, immobile, or mobile inward. The fate of the silicon species transferred to the inner layer may be to enter the inner layer material as an inward migrating anion or as a low mobility or immobile cation species. Further, a non-uniform region may develop with silicon-rich regions interspersed with usual inner layer material. The concentration of silicon species in the film is relatively high, with an atomic ratio of silicon to magnesium of about 0.25. The silicon species are probably important in preventing degradation of the magnesium-rich outer layer during ultramicrotomy, which exposes the sections to water. Possibly of relevance, MgSiO3 is relatively insoluble in water. The presence of hydrogen species in the outer layer, which was not quantified, indicates incorporation of OH or HSiO 3 ions into the film or the presence of hydrated material. Contributions to the hydrogen concentration in the film may also arise from oxidation of any hydrogen impurity in the alloy.
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4.3. Film growth by sequential anodizing When a two-layered film is further thickened in ammonium pentaborate electrolyte, primary fingers of tantalum-rich material, of 30 nm width, penetrate the magnesium-rich layer due to the differing ionic resistivities of the inner and outer layers [23]. Their absence in films formed only in sodium silicate electrolyte suggests that initiation of fingers is not associated with the compositions and structures of the silicon-rich sub-layer or magnesium-rich outer layer alone, since these factors are common in single and sequential anodizing. However, the lack of the buffering action from gel formation may allow local dissolution of the outer layer, with the consequent enhancement of the electric field leading to initiation of fingers. Finger development results in the expected reduced slope of the voltage–time response at the start of re-anodizing [23]. The region of reduced slope is absent if the specimen is immersed in the second electrolyte for 300 s prior to re-anodizing, suggesting chemical attack of the outer layer facilities finger penetration. Although current is channelled preferentially through the fingers, current flow re-distributes in response to local film thickening; the latter thickening is evident in transmission electron micrographs as shallow peaks at the film/electrolyte interface above the fingers. Redistribution of current to the original magnesium-rich layer appears to develop secondary, finer fingers, of 5 nm width. Further, the channelled current carries both Mg2þ and Ta5þ ions, the faster migration of the former resulting in secondary banding. The sites of initiation of secondary fingers are possibly related to effects of local thickness, structural and compositional variability in the original, magnesiumrich layer, and the underlying silicon-rich region of the inner layer. Some regions of the outer layer are effectively short-circuited from the ionic transport, and hence these act as an immobile marker revealing the formation of the film by both cation and anion migration. Magnesium ions from the inner layer move relatively rapidly through the primary fingers, with their loss to the electrolyte on reaching the surface of the film. The primary fingers develop a thin, tantalum-rich layer adjacent to the film surface. In contrast, magnesium ions in the secondary fingers move more slowly toward the film/electrolyte interface, with the overlying, lower resistivity, tantalumrich layer providing a stable ordering of film resistivities. The proposed mechanism of film growth is illustrated in Fig. 6(c).
4.4. Efficiencies of film growth From results of RBS analyses and previous considerations of the mechanisms of film growth, a reduced efficiency of film growth occurs for films formed in ammonium pentaborate electrolyte due primarily to loss of magnesium species to the electrolyte. The efficiency is enhanced in sodium silicate electrolyte, when loss of magnesium species is either reduced or eliminated. In contrast, sequential anodizing results in an intermediate efficiency, with significant losses of magnesium species occurring in the second stage of anodizing. Comparison with charges passed during anodizing suggests lower efficiencies than indicated by the film compositions from
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RBS. The differences, which do not affect the main conclusions of the study, may be due to side reactions associated with the use of the sputtering-deposited specimens.
5. Conclusions (1) Anodic oxidation of sputtering-deposited Mg/40 at.%Ta alloy results in a tantalum-rich alloy layer, about 2 nm thick, immediately beneath the anodic film, which is expected from consideration of the Gibbs free energies per equivalent for formation of tantalum and magnesium oxides. This suggests that the anodizing behaviour of magnesium alloys is similar to that of aluminium alloys with respect to enrichment of the alloying elements. (2) Anodic films grown on sputtering-deposited Mg/40 at.%Ta alloy have diverse morphologies depending upon the anodizing electrolyte. Anodizing in ammonium pentaborate electrolyte develops a uniform, amorphous anodic film, based on units of Ta2 O5 and MgO. Mg2þ ions migrate faster than Ta5þ ions through the film and are lost to the electrolyte on reaching the surface of the film, resulting in a reduced concentration of Mg2þ species compared with the alloy composition and a reduced efficiency of film growth. The formation ratio is about 1.8 nm V1 . (3) When the alloy is anodized in sodium silicate electrolyte, the resultant film is two-layered. The composition of the inner layer is similar to that of the previous film. The outer magnesium-rich layer forms due to the relatively fast outward migration of magnesium ions in the inner layer and the stability of the magnesiumrich film material at the highly alkaline pH of the electrolyte at the film surface, stabilized by formation of a protective silica gel. The outer layer of the anodic film also contains silicon species derived from the electrolyte anions, and hydrogen species derived from the electrolyte and possibly also from the alloy. The formation ratio for the magnesium-rich material of the outer layer is about 0.8 nm V1 . (4) When the alloy is re-anodized in ammonium pentaborate electrolyte, following previous anodizing in sodium silicate electrolyte, differences in ionic resistivities of the magnesium-rich and tantalum-rich layers, and the reduced pH at the film surface, assist development of fingers of the less resistive tantalum-rich material that penetrate the more resistive magnesium-rich material. Subsequent re-distribution of current leads to secondary fingers of finer dimensions. Acknowledgements The authors are grateful to the Engineering and Physical Sciences Research Council (UK) and AVX Ltd for support of this work. They also wish to thank Dr C. Ortega of the Groupe de Physique des Solides, Universites Paris 7 et 6, for assistance with RBS measurements (work partially funded by Centre Nationale de la Recherche Scientifique (GDR86)).
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References [1] D.A. Vermilyea, J. Electrochem. Soc. 104 (1957) 485. [2] J.J. Randall, W.J. Bernard, R.R. Wilkinson, Electrochim. Acta 10 (1965) 183. [3] S.M. Cardonne, P. Kumar, C.A. Michaluk, H.D. Schwartz, Int. J. Refract. Met. Hard Mater. 13 (1995) 187. [4] L.R. Doolittle, Nucl. Instrum. Meth. B 9 (1985) 344. [5] K. Shimizu, H. Habazaki, P. Skeldon, G.E. Thompson, G.C. Wood, Surf. Interface Anal. 27 (1999) 998. [6] K. Asami, K. Hashimoto, Corros. Sci. 17 (1977) 559. [7] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronic, Minnesota, USA, 1995. [8] K. Asami, S. Ono, J. Electrochem. Soc. 147 (2000) 1408. [9] J.H. Nordlien, S. Ono, N. Masuko, K. Nisancioglu, Corros. Sci. 39 (1997) 1397. [10] X. Zhou, H. Habazaki, K. Shimizu, P. Skeldon, G.E. Thompson, G.C. Wood, Thin Solid Films 293 (1997) 327. [11] F.A. Bonilla, A. Berkani, P. Skeldon, G.E. Thompson, H. Habazaki, K. Shimizu, C. John, K. Stevens, Corros. Sci. 44 (2002) 1941. [12] H. Habazaki, K. Shimizu, P. Skeldon, G.E. Thompson, G.C. Wood, Trans. Inst. Met. Finish. 75 (1997) 18. [13] S. Mato, G.E. Thompson, P. Skeldon, K. Shimizu, H. Habazaki, D. Masheder, Corros. Sci. 43 (2001) 993. [14] J.A. Davies, B. Domeij, J. Electrochem. Soc. 110 (1963) 849. [15] F. Brown, W.D. Mackintosh, J. Electrochem. Soc. 120 (1973) 1096. [16] J.P.S. Pringle, J. Electrochem. Soc. 120 (1973) 398. [17] K. Shimizu, K. Kobayashi, J. Surf. Finish. Soc. 46 (1995) 402. [18] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association of Corrosion Engineers, Houston, 1974. [19] K. Shimizu, K. Kobayashi, P. Skeldon, G.E. Thompson, G.C. Wood, Philos. Mag. B 73 (1996) 461. [20] O. Khaselev, J. Yahalom, Corros. Sci. 40 (1998) 1149. [21] J.R. Morlidge, P. Skeldon, G.E. Thompson, H. Habazaki, K. Shimizu, G.C. Wood, Electrochim. Acta 44 (1999) 2423. [22] K. Shimizu, K. Kobayashi, G.E. Thompson, P. Skeldon, G.C. Wood, Philos. Mag. A 72 (1995) 1409. [23] J.P.S. Pringle, Electrochim. Acta 25 (1980) 1423.
High resistivity magnesium-rich layers and current instability in anodizing a Mg/Ta alloy S. Mato a, G. Alcala a, P. Skeldon a,*, G.E. Thompson a, D. Masheder b, H. Habazaki d, K. Shimizu c a
Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 1QD, UK b AVX Ltd., Tantalum Division, Long Rd., Paignton, Devon TQ4 7ER, UK c University Chemical Laboratory, Keio University, 4-1-1 Hiyoshi, Yokohama 223, Japan Graduate School of Engineering, Hokkaido University, N13-W8, Kita-ku, Sapporo 060-8628, Japan
d
Received 4 September 2002; accepted 5 December 2002
Abstract Strikingly different morphologies of amorphous anodic films on a Mg/40 at.%Ta alloy are shown to result from single-stage and sequential anodizing procedures. The alloy, prepared by magnetron sputtering, was anodized galvanostatically in ammonium pentaborate (pH 8.3) and sodium silicate (pH 12.6) electrolytes at 293 K and studied by transmission electron microscopy, Rutherford backscattering spectroscopy, glow discharge optical emission spectroscopy and X-ray photoelectron spectroscopy. For one-step anodizing in the pentaborate electrolyte, a single-layered film, of approximate composition Ta2 O5 MgO, forms at a ratio of 1.8 nm V1 . In the silicate electrolyte, an outer, magnesium-rich layer, containing silicon species, also forms, with a ratio of 0.8 nm V1 . The outer layer can develop due to relatively fast migration of magnesium ions in the inner layer and the stabilization of the pH at the film surface, probably linked to generation of a silica gel that also limits loss of magnesium species to the electrolyte. Further thickening of the anodic film, in ammonium pentaborate electrolyte, produces fingers of low resistivity, inner oxide that penetrate the pre-existing, high resistivity outer layer, with a bi-modal distribution of finger sizes. When fingers reach the film surface, magnesium ions are ejected to the electrolyte. The absence of fingers in films grown in sodium silicate electrolyte is possibly due to prevention, by the silica gel, of their initiation. Ó 2003 Published by Elsevier Science Ltd.
*
Corresponding author. Tel.: +44-161-200-4872; fax: +44-161-200-4865. E-mail address: [email protected] (P. Skeldon).
0010-938X/03/$ - see front matter Ó 2003 Published by Elsevier Science Ltd. doi:10.1016/S0010-938X(02)00258-5
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1. Introduction Anodic films on tantalum are employed in the manufacture of capacitors and provide high levels of performance. The films, which are formed on sintered tantalum electrodes, are composed primarily of amorphous tantala [1,2]. The electrodes contain a number of impurities at low levels, particularly nitrogen, iron and magnesium, which generally appear to have minor influence on the growth of the film. However, such impurities may have local influences that affect leakage currents, the generation of flaws and the occurrence of field-induced crystallization. In the present work, attention is focused on a Mg/Ta alloy containing 40 at.%Ta in order to understand the possible effects on film growth of local high concentrations of magnesium in tantalum. Although tantalum alloy systems have been studied extensively for diverse applications [3], relatively little is known about Ta/Mg alloys and their anodizing behaviours. The resultant films also provide insight into formation of anodic films on magnesium, particularly since bi-layered films are produced at high pH in sodium silicate electrolyte, with an outer magnesium-rich layer, free of tantalum species, that can be sectioned readily by ultramicrotomy for examination by transmission electron microscopy (TEM).
2. Experimental procedure A Mg/40 at.%Ta alloy was deposited by magnetron sputtering, using an Atom Tech Ltd facility, on to electropolished and anodized aluminium substrates that provide smooth surfaces. The aluminium substrates were electropolished at 20 V for 180 s in a mixture of ethanol and perchloric acid (4:1 by vol.) at 278 K, rinsed in ethanol and in distilled water, and dried in a cool air stream. The electropolished substrates were then anodized to 150 V at 5 mA cm2 in 0.1 M ammonium pentaborate electrolyte at 293 K, rinsed in distilled water, and finally dried in a cool air stream. Following initial evacuation to 5 107 mbar, sputtering was carried out in 99.998% argon at 5 103 mbar. In order to obtain the desired alloy composition, appropriate currents were applied to the separate 99.9% magnesium and 99.99% tantalum targets. X-ray diffraction revealed a crystalline alloy; the pattern could not be identified in reference files. Specimens of the alloy were anodized at 5 mA cm2 as follows: (i) to 150 V in 0.1 M ammonium pentaborate electrolyte (pH 8.3), (ii) to 80 V, 150 V and 210 V in 0.1 M sodium silicate electrolyte (pH 12.6) and (iii) firstly to 80 V in 0.1 sodium silicate electrolyte and secondly to 200 V in 0.1 M ammonium pentaborate electrolyte. The temperature of the electrolyte was 293 K in all cases. The voltage–time response was recorded for each specimen. Anodized specimens were examined by TEM, using a JEOL 2000 FX II instrument, with preparation of cross-sections, of nominal thickness 10 nm, by ultramicrotomy. The compositions of film and alloy regions were determined by Rutherford backscattering spectroscopy (RBS) using 2.08 MeV Heþ ions with a current of 60 nA and beam diameter of 1 mm. The beam was incident normal to the
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surface of the specimen, with scattered ions detected at 165° to the direction of the incident beam. The data were interpreted using the RUMP program [4]. Spectra for anodized specimens were fitted using thicknesses of film regions from TEM. Composition profiles of a film formed in sodium silicate electrolyte were determined by glow discharge optical emission spectroscopy (GDOES), using a Jobin-Yvon 5000 instrument, with the analysis conditions given previously [5]. Anodized alloys were also examined by X-ray photoelectron spectroscopy (XPS) in a Shimadzu ESCA-850 electron spectrometer, using Mg Ka excitation. The binding energies of electrons were calibrated using a method described elsewhere [6]. The binding energies of Au 4f7=2 and 4f5=2 electrons of gold were taken as 84.07 and 87.74 eV, and those of Cu 2p3=2 and 2p1=2 electrons of copper as 932.53 and 952.35 eV respectively; the kinetic energy of Cu L3 M4;5 M4;5 Auger electrons was taken as 918.65 eV. The energy shifts, through charging of the specimen, were corrected using the C 1s peak for contaminant hydrocarbon at 285.0 eV.
3. Results 3.1. Voltage–time responses and transmission electron microscopy After an initial voltage surge of 1–2 V, due to the air-formed oxide on the alloy, the voltage–time response for anodizing in ammonium pentaborate electrolyte was linear with a slope of 1:2 0:1 V s1 (Fig. 1(a)). Transmission electron micrographs disclosed a uniform anodic film of amorphous appearance, with the thickness of 270 5 nm indicating a formation ratio of 1:80 0:03 nm V1 (Fig. 2(a)).
Fig. 1. Voltage–time responses of Mg/40 at.%Ta alloy anodized at 5 mA cm2 and 293 K (a) to 150 V in 0.1 M ammonium pentaborate electrolyte, (b) to 150 V in 0.1 M sodium silicate, and (c) to 80 V in 0.1 sodium silicate and then re-anodized to 200 V in 0.1 M ammonium pentaborate.
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Fig. 2. Transmission electron micrograph of Mg/40 at.%Ta alloy anodized at 5 mA cm2 and 293 K (a) to 150 V in 0.1 M ammonium pentaborate electrolyte, (b) to 150 V in 0.1 M sodium silicate electrolyte, (c) to 80 V in 0.1 M sodium silicate electrolyte and then re-anodized to 200 V in 0.1 M ammonium pentaborate electrolyte; (d) enlargement of the alloy/film interface of (c). The lines in the film of (a) near the top of the micrograph are knife marks from ultramicrotomy.
For anodizing in sodium silicate electrolyte, the voltage surge was followed by a linear voltage–time response, with a slope of 2:5 0:1 V s1 (Fig. 1(b)). Transmission electron micrographs revealed two layers in the anodic film formed to 150 V, both of uniform and amorphous appearance, but the outer layer, representing 40 4% of the film thickness, is lighter than the inner layer (Fig. 2(b)). The total thickness of 170 5 nm indicates a formation ratio of 1:13 0:03 nm V1 . The ratio of thicknesses of the inner and the outer layers and the formation ratio were also similar to the previous values for anodizing to 80 and 210 V. However, with increasing voltage, the interface between the layers developed increased undulations, associated with local variations in the thickness ratio of the inner and outer layers between 1.8 and 1.2, with peaks separated by about 60 nm. In order to determine whether dissolution of the film occurred in the sodium silicate electrolyte, a specimen was anodized to 50 V and the current was switched off with the specimen left immersed in the elec-
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trolyte for 300 s. On re-anodizing, a voltage surge of 50 V indicated no significant degradation of the film between the two stages of anodizing. For a specimen anodized sequentially, to 80 V in sodium silicate electrolyte and to 200 V in ammonium pentaborate electrolyte, the voltage at the start of the second anodizing rose to about 73 V, 9% less than the initial voltage, suggesting some dissolution of the first-formed film following the brief immersion in the second electrolyte before switching on the current (Fig. 1(c)). Thereafter, following a short stage where the voltage increased very slowly, the response was linear with a slope of 1:2 0:1 V s1 , similar to that for anodizing in ammonium pentaborate electrolyte. Extending the period of immersion in the second electrolyte to 300 s eliminated the initial region of low slope, with the voltage immediately increasing at 1.2 V s1 . Further, the initial voltage at the start of re-anodizing was reduced by 25% compared with the final voltage in the first stage of anodizing suggesting additional film dissolution. Transmission electron micrographs disclosed an anodic film of amorphous appearance with several distinct regions (Fig. 2(c)). The inner layer, representing 55% of the film thickness, was similar to that of the inner layer and the total thickness of films formed in sodium silicate and ammonium pentaborate electrolytes respectively. Above this inner layer, a lighter layer, resembling material developed at the outer region of films formed in sodium silicate electrolyte, was present with primary fingers of inner layer material, of width about 30 nm, penetrating its thickness. Above the lighter material, narrow, secondary fingers of light material, of width about 5 nm, are evident in a layer of relatively dark material. The outer 10 nm of the film consisted of the darker material, with thicker regions at locations of the main fingers. The total thickness of the film is 333 5 nm, indicating a formation ratio of 1:66 0:03 nm V1 , intermediate between values for anodizing in the individual electrolytes. Inspection of the alloy beneath the anodic films in the previous micrographs discloses a layer of about 2 nm thickness which is darker than the bulk alloy. The contrast results from enrichment of tantalum. The enriched layer is shown at increased magnification in Fig. 2(d). The underlying alloy is striated with light and dark bands, of thickness about 2 nm, which were caused by corrosion of the section in water during ultramicrotomy. 3.2. Rutherford backscattering spectroscopy The RBS spectra for the alloy, prior to anodizing, revealed yields from tantalum and magnesium, as well from the aluminium substrate, indicating a layer of Mg/40 at.%Ta alloy of thickness about 500 nm, using the weighted average of the atomic densities of the pure metals (Fig. 3(a)). The spectra following anodizing to 150 V in 0.1 M ammonium pentaborate electrolyte disclosed a single-layered film containing 4:2 1017 Ta atoms cm2 and 2:1 1017 Mg atoms cm2 , based on units of MgO and Ta2 O5 (Fig. 3(b)). The atomic concentration of magnesium in the film, considering cations only, is 33 at.%. The measured amounts of magnesium in this and in later films are possibly subject to an error of about 15% due to the uncertainty of the background level. The uncertainty in the tantalum content is about 5%. The outer regions of the film may contain boron species, derived from anions in the electrolyte;
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Fig. 3. Experimental and simulated RBS spectra for the Mg/40 at.%Ta alloy prior to anodizing (a) and following anodizing at 5 mA cm2 and 293 K (b) to 150 V in 0.1 M ammonium pentaborate electrolyte, (c) to 150 V in 0.1 M sodium silicate electrolyte, (d) as (c) showing details of the region corresponding to magnesium and silicon peaks, and (e) to 80 V in sodium silicate electrolyte and then to 200 V in 0.1 M ammonium pentaborate electrolyte. The leading edge for tantalum at the surface in (e) is shifted compared with that of (a), (b) and (c) due to a differing calibration of the spectrum.
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however, they would not be detected by RBS due to their low atomic mass. The charge of the cations in the film corresponded to 0.42 C cm2 , compared with 0.62 C cm2 from the current density and time of anodizing, indicating an efficiency of 66%. The reduced magnesium content of the film compared with that of the alloy indicates that a charge of about 0.20 C cm2 was associated with loss of Mg2þ ions to the electrolyte, and hence an efficiency of 76% that is consistent with the previous estimate given the uncertainties in the compositions. The RBS data for a specimen anodized to 150 V in 0.1 M sodium silicate electrolyte revealed a primarily two-layered film, consistent with results of TEM (Fig. 3(c) and (d)). The outer layer contained magnesium, oxygen and silicon species, with an absence of tantalum ions. The magnesium content of the outer layer was 2:4 1017 Mg atoms cm2 , with an atomic ratio of silicon to magnesium of about 0.25. The inner layer consisted of two sub-layers. An inner sub-layer contained 1:6 1017 Ta atoms cm2 and 8:1 1016 Mg atoms cm2 ; the overlying sub-layer contained 6:7 1016 Ta atoms cm2 , 2:5 1016 Mg atoms cm2 and also 3:5 1016 Si atoms cm2 . The average atomic ratio of magnesium to tantalum in the inner layer was 33 at.%Mg, similar to the composition of the previous film. Further, an enrichment of about 1:5 1016 Si atoms cm2 was present at the surface of the film. Accurate analysis of the silicon and magnesium contents of the inner parts of the film is hindered by overlap of silicon and magnesium signals and the background. The relative concentration of magnesium in the anodic film, 57 at.%, was similar to that in the alloy and is consistent with growth of the film with negligible loss of cation species to the electrolyte. The charge associated with the principal cations in the film is 0.29 C cm2 , compared with the charge of 0.31 C cm2 estimated from the current density and time of anodizing, consistent with a high efficiency of growth. Experimental and simulated spectra for a specimen anodized to 200 V in 0.1 M ammonium pentaborate electrolyte, and previously anodized to 80 V in 0.1 M sodium silicate electrolyte, disclosed an anodic film with several layers (Fig. 3(e)). The innermost layer contained about 33 at.%Mg. The following layers contained tantalum, magnesium, silicon and oxygen except for the outermost layer where silicon species were absent. Assignment of accurate compositions to the various layers is prevented by overlap of silicon, magnesium and background signals. A small peak indicated a silicon enrichment of about 5:0 1016 Si atoms cm2 remaining at the surface of the film from the first stage of anodizing. The estimated concentration of magnesium ions in the anodic film, 40%, is low compared with the alloy composition. The estimated charge of cations in the film was 0.43 C cm2 , compared with 0.76 C cm2 from the current density and time of anodizing, indicating an efficiency of 57%. 3.3. Glow discharge optical emission spectroscopy GDOES analysis of a film formed to 150 V in sodium silicate electrolyte confirmed the findings of RBS, with an outer magnesium-rich layer above an inner layer containing both magnesium and tantalum species (Fig. 4). The sputtering rate of the outer layer was slower than that of the inner layer; hence, the sputtering time ratio does not correspond with the thickness ratio from TEM. Silicon species are revealed
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Fig. 4. Results of GDOES analysis of Mg/40 at.%Ta alloy anodized to 150 V at 5 mA cm2 in 0.1 M sodium silicate electrolyte at 293 K.
beneath the boundary between the outer and inner layers. Further, the outer layer contained enhanced levels of hydrogen species relative to other film regions, and hydrogen was also enriched near the alloy/film interface. 3.4. X-ray photoelectron spectroscopy The X-ray photoelectron spectra of specimens anodized (i) to 150 V in sodium silicate electrolyte and (ii) by sequential anodizing, firstly to 80 V in sodium silicate electrolyte and secondly to 200 V in ammonium pentaborate electrolyte, disclosed peaks of oxygen, silicon and magnesium. Results were similar generally for both specimens; results are presented for the specimen anodized sequentially. The Si 2p peak at 102.6 eV is consistent with presence of silica or silicate (Fig. 5(a)) [7]. The Mg 2p3=2 peak at 50.4 eV, corresponds to divalent magnesium in the form of MgO, Mg(OH)2 or magnesium silicate (Fig. 5(b)) [7–9]. The specimen sequentially anodized reveals also a Ta 4f peak, located at 26.5 eV, which partly overlaps the O 2s peak, corresponding to pentavalent tantalum (Fig. 5(c)) [7]. The O 1s peak was located at 532.2 eV, consistent with species suggested by the previous peaks. The Ta 4f peak was not observed following anodizing in the silicate electrolyte only. 4. Discussion 4.1. Anodic oxidation of alloys in ammonium pentaborate and sodium silicate electrolytes Relatively dilute, solid–solution, aluminium alloys can reveal enrichments of the alloying element in the alloy immediately beneath the anodic film as a consequence
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Fig. 5. Results of XPS analysis of Mg/40 at.%Ta alloy anodized to 80 V at 5 mA cm2 in 0.1 M sodium silicate electrolyte at 293 K and then re-anodized to 200 V in 0.1 M ammonium pentaborate electrolyte at 293 K: (a) silicon 2p peak; (b) magnesium 2p peak; (c) tantalum 4f peak.
of film growth [10]. Limited evidence for magnesium alloys suggest similar enrichments occur [11]. For aluminium alloys, the initial composition is dependent upon the relative values of the Gibbs free energy per equivalent (DG0 =n) for formation of the oxides of the constituent elements [12]. Elements forming oxides with higher values of DG0 =n than that for formation of Al2 O3 initially enrich in the alloy and the anodic film is free of their oxide species. Tantalum alloys appear to behave differently, with oxidation of the alloying elements commencing without the requirement for enrichment [13]. Consistent with previous indications for magnesium and the relative magnitudes of DG0 =n for MgO and Ta2 O5 [12], enrichment of tantalum, in a layer of thickness 1–2 nm, evident by atomic number contrast in TEM, occurred during anodizing of the present alloy. The enrichment is observable by TEM, but the layer is too thin for resolution by the RBS or GDOES analyses. Following enrichment, Ta5þ and Mg2þ ions are incorporated into the anodic film, as units of MgO and Ta2 O5 , in proportion to the composition of the alloy.
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The growth of amorphous anodic oxides usually involves contributions of cation and anion migration [14,15]. The transport numbers of cations and anions in anodic tantala are about 0.24 and 0.76 respectively [16]. There appear to be no published transport numbers for magnesium oxide. However, transport of Ta5þ , Mg2þ and O2 /OH ions is consistent with the evidence of layered films on the present alloy. Migration rates of cation species in anodic films often correlate with the energies of metal–oxygen, single-bonds [17]. With bond energies of about 166 and 347 kJ mol1 for Mg2þ –O and Ta5þ –O respectively, faster outward migration of Mg2þ ions relative to that of Ta5þ ions is anticipated. Neglecting other factors, an outer, magnesiumrich, layer should develop if the surface pH is >10 when the solubility of MgO/ Mg(OH)2 reduces [18]. At lower pH, extensive loss of Mg2þ ions to solution by either chemical dissolution or field-assisted ejection would be expected. The expected growth mechanisms are illustrated schematically in Fig. 6(a) and (b). In agreement with the previous discussion, the innermost layers of the present films are depleted in magnesium compared with the alloy, due to relative fast outward migration of Mg2þ ions through the inner layer, with a composition, by RBS, of approximately Ta2 O5 MgO. The formation ratio, 1.8 nm V1 , indicated by the single-layered film formed in ammonium pentaborate electrolyte, compares with 1.6 nm V1 for anodic tantala formed under similar conditions [19]. In sodium silicate electrolyte, an outer, magnesium-rich layer, containing silicon species, develops above the inner layer, and silicon species penetrate to the inner layer. The outer layer also contains an increased concentration of hydrogen relative to that in the inner layer, as evident from GDOES. Using the previous formation ratio for inner layer material, that for the outer layer is about 0.8 nm V1 . The formation ratio has been reported previously as 0.6 nm V1 for anodic films formed on magnesium at 5 mA cm2 in KOH solution [20]. Although more resistive to ionic current, penetration of the outer layer by fingers of the inner layer material does not occur for the present films. However, variable thicknesses of the inner and outer layers, particularly with increase of voltage, suggests some non-uniformity of current. 4.2. Role of silicon species during anodizing in sodium silicate electrolyte Silicon species are present at the surface and within the film formed in sodium silicate electrolyte. The surface species probably indicate a silica-based gel formed by reaction between silicate ions of the electrolyte and Hþ ions produced by anodizing [21]. The gel, assisted by stabilization of the pH at the surface of the film to about 10 [18], probably limits loss of magnesium species to the electrolyte. Silicate ions pass readily through the gel, allowing incorporation of silicon species into the film as 4þ either an anion species, such as SiO2 3 or HSiO3 ions, or a cation species, such as Si 4þ 1 ions in units of SiO2 . The relatively high Si –O bond energy, 465 kJ mol , would suggest that Si4þ ions, if present, are of low mobility. In anodic tantala, silicon species incorporated directly from the electrolyte are immobile [22]. The outer film layer grows at the film/electrolyte interface and is effectively destroyed at the inner layer/outer layer interface to supply oxygen species for growth of the inner layer material. Consequently, silicon species may pass to the inner layer/outer layer in-
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Fig. 6. Schematic diagrams of the mechanisms of anodic film growth on Mg/40 at.%Ta alloy in (a) ammonium pentaborate electrolyte, (b) sodium silicate electrolyte and (c) ammonium pentaborate electrolyte after previous anodizing in sodium silicate electrolyte. For simplicity incorporation of species derived from electrolyte anions has been neglected and anion migration assumed to involve O2 ions only.
terface, whether of relatively low mobility outward, immobile, or mobile inward. The fate of the silicon species transferred to the inner layer may be to enter the inner layer material as an inward migrating anion or as a low mobility or immobile cation species. Further, a non-uniform region may develop with silicon-rich regions interspersed with usual inner layer material. The concentration of silicon species in the film is relatively high, with an atomic ratio of silicon to magnesium of about 0.25. The silicon species are probably important in preventing degradation of the magnesium-rich outer layer during ultramicrotomy, which exposes the sections to water. Possibly of relevance, MgSiO3 is relatively insoluble in water. The presence of hydrogen species in the outer layer, which was not quantified, indicates incorporation of OH or HSiO 3 ions into the film or the presence of hydrated material. Contributions to the hydrogen concentration in the film may also arise from oxidation of any hydrogen impurity in the alloy.
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4.3. Film growth by sequential anodizing When a two-layered film is further thickened in ammonium pentaborate electrolyte, primary fingers of tantalum-rich material, of 30 nm width, penetrate the magnesium-rich layer due to the differing ionic resistivities of the inner and outer layers [23]. Their absence in films formed only in sodium silicate electrolyte suggests that initiation of fingers is not associated with the compositions and structures of the silicon-rich sub-layer or magnesium-rich outer layer alone, since these factors are common in single and sequential anodizing. However, the lack of the buffering action from gel formation may allow local dissolution of the outer layer, with the consequent enhancement of the electric field leading to initiation of fingers. Finger development results in the expected reduced slope of the voltage–time response at the start of re-anodizing [23]. The region of reduced slope is absent if the specimen is immersed in the second electrolyte for 300 s prior to re-anodizing, suggesting chemical attack of the outer layer facilities finger penetration. Although current is channelled preferentially through the fingers, current flow re-distributes in response to local film thickening; the latter thickening is evident in transmission electron micrographs as shallow peaks at the film/electrolyte interface above the fingers. Redistribution of current to the original magnesium-rich layer appears to develop secondary, finer fingers, of 5 nm width. Further, the channelled current carries both Mg2þ and Ta5þ ions, the faster migration of the former resulting in secondary banding. The sites of initiation of secondary fingers are possibly related to effects of local thickness, structural and compositional variability in the original, magnesiumrich layer, and the underlying silicon-rich region of the inner layer. Some regions of the outer layer are effectively short-circuited from the ionic transport, and hence these act as an immobile marker revealing the formation of the film by both cation and anion migration. Magnesium ions from the inner layer move relatively rapidly through the primary fingers, with their loss to the electrolyte on reaching the surface of the film. The primary fingers develop a thin, tantalum-rich layer adjacent to the film surface. In contrast, magnesium ions in the secondary fingers move more slowly toward the film/electrolyte interface, with the overlying, lower resistivity, tantalumrich layer providing a stable ordering of film resistivities. The proposed mechanism of film growth is illustrated in Fig. 6(c).
4.4. Efficiencies of film growth From results of RBS analyses and previous considerations of the mechanisms of film growth, a reduced efficiency of film growth occurs for films formed in ammonium pentaborate electrolyte due primarily to loss of magnesium species to the electrolyte. The efficiency is enhanced in sodium silicate electrolyte, when loss of magnesium species is either reduced or eliminated. In contrast, sequential anodizing results in an intermediate efficiency, with significant losses of magnesium species occurring in the second stage of anodizing. Comparison with charges passed during anodizing suggests lower efficiencies than indicated by the film compositions from
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RBS. The differences, which do not affect the main conclusions of the study, may be due to side reactions associated with the use of the sputtering-deposited specimens.
5. Conclusions (1) Anodic oxidation of sputtering-deposited Mg/40 at.%Ta alloy results in a tantalum-rich alloy layer, about 2 nm thick, immediately beneath the anodic film, which is expected from consideration of the Gibbs free energies per equivalent for formation of tantalum and magnesium oxides. This suggests that the anodizing behaviour of magnesium alloys is similar to that of aluminium alloys with respect to enrichment of the alloying elements. (2) Anodic films grown on sputtering-deposited Mg/40 at.%Ta alloy have diverse morphologies depending upon the anodizing electrolyte. Anodizing in ammonium pentaborate electrolyte develops a uniform, amorphous anodic film, based on units of Ta2 O5 and MgO. Mg2þ ions migrate faster than Ta5þ ions through the film and are lost to the electrolyte on reaching the surface of the film, resulting in a reduced concentration of Mg2þ species compared with the alloy composition and a reduced efficiency of film growth. The formation ratio is about 1.8 nm V1 . (3) When the alloy is anodized in sodium silicate electrolyte, the resultant film is two-layered. The composition of the inner layer is similar to that of the previous film. The outer magnesium-rich layer forms due to the relatively fast outward migration of magnesium ions in the inner layer and the stability of the magnesiumrich film material at the highly alkaline pH of the electrolyte at the film surface, stabilized by formation of a protective silica gel. The outer layer of the anodic film also contains silicon species derived from the electrolyte anions, and hydrogen species derived from the electrolyte and possibly also from the alloy. The formation ratio for the magnesium-rich material of the outer layer is about 0.8 nm V1 . (4) When the alloy is re-anodized in ammonium pentaborate electrolyte, following previous anodizing in sodium silicate electrolyte, differences in ionic resistivities of the magnesium-rich and tantalum-rich layers, and the reduced pH at the film surface, assist development of fingers of the less resistive tantalum-rich material that penetrate the more resistive magnesium-rich material. Subsequent re-distribution of current leads to secondary fingers of finer dimensions. Acknowledgements The authors are grateful to the Engineering and Physical Sciences Research Council (UK) and AVX Ltd for support of this work. They also wish to thank Dr C. Ortega of the Groupe de Physique des Solides, Universites Paris 7 et 6, for assistance with RBS measurements (work partially funded by Centre Nationale de la Recherche Scientifique (GDR86)).
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