Transport numbers of metal and oxygen species in anodic tantala

Lu, Qiongqiong; Skeldon, P.; Thompson, G.E.; Masheder, D.; Habazaki, H.; Shimizu, K.

doi:10.1016/j.corsci.2004.03.021

Cited by 28 publications

(18 citation statements)

References 20 publications

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“…From the compositions of the anodic film (Table 1) and the alloy, t + is estimated to be 0.29, which is lower than that films formed in ammonium pentaborate electrolyte at ambient temperature (t + = 0.42). The reduced t + is consistent with the fact that the t + values for anodic tantala and niobia decrease with an increase in the electrolyte temperature [17,18]. The ejection of all of the outward migrating titanium ions to the electrolyte would reduce the efficiency of film growth to about 71%, which is close to the efficiency indicated from the film composition according to RBS and the charge passed during anodizing.…”

Section: Discussionsupporting

confidence: 68%

Formation of porous anodic films on Ti–Si alloys in hot phosphate-glycerol electrolyte

Habazaki

Oikawa

Fushimi

et al. 2007

Electrochimica Acta

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Porous anodic films, with pore size of ~10 nm, have been developed by anodizing of magnetron sputtered Ti-25 at% Si alloy at constant formation voltages in glycerol electrolyte containing dibasic potassium phosphate at 433 K. The films, of amorphous structure, contain titanium and silicon species, as units of TiO 2 and SiO 2 , throughout the film thicknesses, with negligible amounts of phosphorus species. The silicon is enriched in the film relative to the composition of the alloy, the level of enrichment suggesting that anion migration is increased in comparison with amorphous film growth at ambient temperature. In contrast to the behaviour of the alloy, essentially barrier films were formed on commercially pure titanium in the glycerol electrolyte, when a main anodic reaction was generation of oxygen, which was probably promoted by the development of anatase.

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Section: Discussionsupporting

confidence: 68%

Formation of porous anodic films on Ti–Si alloys in hot phosphate-glycerol electrolyte

Habazaki

Oikawa

Fushimi

et al. 2007

Electrochimica Acta

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“…In the present anodic films, an outer layer, essentially free from silicon species, is developed at the film/electrolyte interface by egress of titanium ions, while an inner layer containing silicon species is developed at the alloy/film interface by ingress of O 2-/OH -ions. The transport number of cations, t + , can be estimated from the following equation, The transport numbers of cations in growing anodic niobium oxide and tantalum oxide are dependent upon current density and temperature of the electrolyte; for anodic tantalum oxide, the value of t + increases with increasing current density and decreases with increasing temperature [22]. A similar trend has been reported for anodic niobium oxide [23].…”

Section: Transport Numbers Of Cations and Anionsmentioning

confidence: 62%

Influence of silicon on the growth of barrier-type anodic films on titanium

Tanvir

Fushimi

Shimizu

et al. 2007

Electrochimica Acta

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Amorphous anodic titania, stabilised by incorporation of silicon species, is shown to grow to high voltages on sputter-deposited, single-phase Ti-Si alloys during anodizing at a constant current density in ammonium pentaborate electrolyte. The films comprise two main layers, with silicon species confined to the inner layers. An amorphous-to-crystalline transition occurs at ~60 V on the Ti-6 at% Si alloy, while the transition is suppressed to voltages above 140 V on alloys with 12 and 26 at% silicon. The crystalline oxide, nucleated at a depth of ~40% of the film thickness, is associated with the presence of a precursor of crystalline oxide in the pre-existing air-formed oxide. The modified structure of the air-formed oxide due to increased incorporation of silicon species suppresses the amorphous-to-crystalline transition until the onset of dielectric breakdown. The transport numbers of cations and anions during growth of the anodic oxides are independent of the concentration of silicon species in the inner layer, despite the marked change in the field strength.

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“…The amorphous niobia formed in this stage is developed by simultaneous countermigrations of Nb 5+ ions outward and O 2− ions inward, 15 with a transport number of cations of about 0.25, 16 with the precise value depending upon the field strength and hence, current density and temperature. 17,18 For the present anodizing conditions, the precur- sors or developing fine crystal nuclei, presumed to be essentially immobile in the surrounding amorphous oxide, are located at a depth of ϳ25% of the anodic film thickness during the stage of galvanostatic anodizing. From these sites, relatively large crystals develop during the subsequent potentiostatic stage of anodizing.…”

Section: Discussionmentioning

confidence: 99%

Suppression of Field Crystallization of Anodic Niobia by Oxygen

Habazaki

Ogasawara

Konno

et al. 2006

J. Electrochem. Soc.

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Solid-solution Nb-O films containing up to 50 atom % oxygen, prepared by magnetron sputtering, were used to investigate the influence of the oxygen on field crystallization during anodizing at 100 V in 0.1 mol dm −3 ammonium pentaborate electrolyte at 333 K. The findings reveal that field crystallization is hindered dramatically by addition of 20 atom % oxygen to the substrate, while no crystallization occurs for a Nb-50 atom % O substrate. Prior thermal treatment accelerates field crystallization of niobium, but not the Nb-50 atom % O substrate. The thermal treatment is considered to promote generation of precursor sites for crystal nucleation. However, sufficient oxygen in the substrate may restrict precursor development and/or reduce the compressive stresses in the amorphous anodic niobia that can facilitate crystal growth. Niobium is a potential alternative material to tantalum for electrolytic capacitors due to its many attractive properties 1 and its relatively high natural abundance. However, niobium capacitors are more susceptible to field crystallization during both growth of the anodic oxide dielectric and packaging of capacitors. Field crystallization in niobium and tantalum leads to degradation of the initial amorphous oxide, involving cracking and peeling due to growth of crystalline oxide, 2 which increases the leakage current. The effects are enhanced at increased anodizing temperatures and forming voltages.2-4 Inclusions 5,6 and roughness, especially convex surfaces, of the substrate assist crystal nucleation. 7,8 Recent studies using flat, inclusion-free, magnetron-sputtered niobium suggested nucleation is associated with precursor sites in the original, air-formed film, while incorporation of foreign species from the electrolyte or from the substrate at precursor regions can hinder nucleation.9 Other investigations indicate an enhanced susceptibility to field crystallization due to oxygen impurity in the substrate. 5,10,6,11 Oxide precipitates, possibly at grain boundaries, may be involved, although the detailed processes are not well understood. Here, the influence of oxygen in the substrate is examined on field crystallization of the anodic niobia. ExperimentalNiobium and Nb-͑20 and 50͒ atom % O films were deposited by sputtering 99.9% niobium in either argon or a mixture of argon and oxygen at ϳ0.1 Pa using magnetron sputtering enhanced with a radio frequency plasma source to increase the plasma density. Three types of flat substrate were employed: glass plates, silicon wafers, and electropolished and anodized aluminum sheets. Subsequent thermal treatment of the deposited films, either in air at 523-623 K for 1.8 ks or in vacuum ͑ϳ10−5 Pa͒ at 923 K, was employed selectively. Structures of the deposits were determined by X-ray diffraction ͑XRD͒ ͑Rigaku RINT 2000͒ using Cu K␣ radiation, with patterns obtained in an ␣−2 ͑␣ = 1°͒ mode.The films were anodized at 50 A m −2 to 100 V with current decay in stirred 0.1 mol dm −3 ammonium pentaborate electrolyte at 333 K. A platinum sheet was used as a...

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Transport numbers of metal and oxygen species in anodic tantala

Cited by 28 publications

References 20 publications

Formation of porous anodic films on Ti–Si alloys in hot phosphate-glycerol electrolyte

Formation of porous anodic films on Ti–Si alloys in hot phosphate-glycerol electrolyte

Influence of silicon on the growth of barrier-type anodic films on titanium

Suppression of Field Crystallization of Anodic Niobia by Oxygen

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