Phase transition and oxide dissolution processes in vacuum-annealed anodic Nb2O5/Nb systems

Oechsner, H.; Giber, J.; Füßer, Hans-Jürgen; Darlinski, A.

doi:10.1016/0040-6090(85)90266-4

Cited by 32 publications

(21 citation statements)

References 19 publications

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“…Nb can act as a NEG. This is consistent with AES depth profiling measurements [25] showing that the oxide of the Nb 2 O 5 /Nb system is dissolved into the Nb bulk when heated to 325 °C while at 150 °C no oxide dissolution occurs.In a previous study of TiZr and TiZrV NEG coatings [11] a correlation has been found between the NEG activation temperature and the temperature needed to decrease the SEY to values close to those of the clean metals.In the present case of the Nb thin film only 1 h heating at 160 °C is sufficient to reduce its maximum SEY to about 1.4. However, heating temperatures above 250 °C are needed to decrease the SEY of chemically cleaned bulk Nb to the same value [8].…”

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confidence: 79%

The secondary-electron yield of air-exposed metal surfaces

Hilleret

Scheuerlein

Taborelli

2003

Applied Physics A: Materials Science & Processing

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The secondary electron yield (SEY) variation of atomically clean metal surfaces due to air exposures and during subsequent heat treatments is described with the example of a sputter-deposited Nb thin film. Corresponding variations of the surface chemical composition have been monitored using AES and SSIMS. On the basis of these results and of previously obtained SEY results on metals and metal oxides the origin of the SEY variations is discussed.The SEY increase, which is generally observed during long lasting air exposures of clean metals, is mainly caused by the adsorption of an airborne carbonaceous contamination layer. The estimated value of about 3 for the maximum SEY of this layer is higher than that of all pure metals.Only in some cases the air-formed oxide can contribute to the air exposure induced SEY increase while many oxides have a lower SEY than their parent metals. From the experimental data it can also be excluded that the SEY increase during air exposures is mainly due to an increased secondary electron escape probability. Paper submitted to Applied Physics A Geneva, SwitzerlandJune 2002 -2- INTRODUCTIONThe secondary electron yield (SEY) of the internal surfaces of ultra high vacuum systems is of great importance for devices the operation of which can be perturbed by the occurrence of a resonant electron multiplication (multipacting) [1]. Examples for such devices are superconducting RF cavities for particle acceleration [2], and the beam vacuum system of CERN's next accelerator, the Large Hadron Collider (LHC) [3].Reliable SEY data for most atomically clean metals have been published already several decades ago [4]. However, despite the fact that for many applications the SEY of air exposed surfaces is more relevant than the SEY of atomically clean surfaces, SEY data of air exposed surfaces are rare and the reasons for the SEY variation during air exposures of atomically clean metals are still discussed controversially. There are mainly two reasons for this.Firstly, it is difficult to define precisely an air exposed surface because of the many parameters influencing the surface properties (e.g. initial sample state, air exposure time, humidity, concentration of contaminants in the ambient air, etc.). For initially clean copper, as an example, the maximum SEY value after air exposure can vary between 1.2 and above 2 for an air exposure lasting a few minutes and a few days, respectively [5].The second reason why SEY data of air exposed surfaces is often questionable is the fact that the SEY of such surfaces is much more sensitive to electron irradiation than the SEY of atomically clean surfaces [6]. Electron doses as low as 10 -6 C mm -2 remove efficiently a part of the surface species and, as a result, reduce the SEY [7]. Therefore, measurements on air exposed metal surfaces provide in some cases the SEY of an electron beam damaged surface, which has a much lower SEY than the as-received surface.Several SEY and surface analysis measurements of differently treated air exposed Nb surfaces h...

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confidence: 79%

The secondary-electron yield of air-exposed metal surfaces

Hilleret

Scheuerlein

Taborelli

2003

Applied Physics A: Materials Science & Processing

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“…In Fig. (c), we can find three peaks which are assigned to Nb 2 O 5 , NbO 2 and NbO, and they are located at ~208.6 eV, 206.2 eV and 203.6 eV . Results about the oxidation of Nb, either by exposure to air or in a controlled atmosphere of O 2 , the composition of surface layers and about the consequences of chemical treatments, are well established.…”

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confidence: 76%

The investigation of NbO₂ and Nb₂O₅ electronic structure by XPS, UPS and first principles methods

Zhang

Wang

et al. 2013

Surface & Interface Analysis

121

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In this paper, the electronic structures of NbO2 and Nb2O5 are theoretically and experimentally analyzed. The oxides in the samples are mainly consisted of NbO2 and NbO, whereas the outmost layer of the samples is NbO2. After exposure to air, the outermost layer on all niobium samples is Nb2O5. The photoelectrons from the first 2–4 Å contribute to the spectra, so the valence band structure of NbO2 and Nb2O5 can be confirmed from ultraviolet photoelectron spectroscopy (UPS). By comparing the UPS with density of state results, the electronic structure of NbO2 and Nb2O5 can be distinguished from each other, and then the electronic structure was deconvoluted into several electronic states. The agreement between experimental result and theory is, in the best case, satisfactory. Copyright © 2013 John Wiley & Sons, Ltd.

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“…12 They have shown, that the annealing causes the O content of the oxide layer dissolve partially into the metal substrate. After the annealing, the composition of the remaining Ta 2 O 5 did not change, only an oxygen-depleted region was formed at the oxide/metal interface.…”

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Anodic Oxidation of Niobium Sheets and Porous Bodies

Kovács

Kiss

Stenzel³

et al. 2003

J. Electrochem. Soc.

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The anodic oxidation behavior of niobium has been investigated under nearly industrial conditions. High porosity sintered bodies as well as flat niobium sheets were anodized between 20-100 V in 1 wt % aqueous solution of o-phosphoric acid at 65°C, applying current densities between 0.01 and 0.067 mA/cm 2 . During formation at constant current, changes in the rate of voltage increase has been observed. Stopping oxidation at such changes in the V(t) curve, the electrical properties, surface composition and in-depth homogeneity of the oxide was analyzed. The formed oxide layers have also been investigated after a heat-treatment in vacuum and in air. Based on the measured composition profiles, the role of the solid suboxide phases ͓Nb͑O͒, NbO, NbO 2 ] in the oxide growth and in the annealing could be clarified.The Nb/Nb 2 O 5 system is a candidate to replace the widely used Ta/Ta 2 O 5 system in solid electrolyte capacitors in some applications. 1-5 Advantages of the Nb/Nb 2 O 5 system are the high dielectric permittivity 6 ( r Ϸ 42) of Nb 2 O 5 , the relatively low price of niobium, and the vastness of niobium reserves. 7 However, some properties of niobium, like its high oxygen absorbing capacity, 2,8 strong chemical affinity to oxygen, carbon, and nitrogen, 2 as well as the existence of its suboxides, NbO 2 , NbO, and Nb͑O͒ beside the desirable Nb 2 O 5 , makes this system hard to handle. [8][9][10][11]12 Nevertheless, the usage of Nb as anode material is feasible considering the following facts. ͑i͒ The stable working voltage of this system is significantly lower than that of the Ta/Ta 2 O 5 system, 6 because the r value of Nb 2 O 5 is higher. This lower electrical field in the oxide leads to the same ionic current. ͑ii͒ The maximum working temperature of the system is 125°C, which is lower than that of the Ta/Ta 2 O 5 system ͑typically 150°C͒. Therefore, despite the problems mentioned, the Nb/Nb 2 O 5 based capacitors are reliable and stable devices. 13 The anodization of niobium has been widely studied 6,14-18 at the basic research level. This process proved to be basically similar to the anodization of tantalum. The investigation of the effect of heattreatment is important, as it causes changes in the capacity ͑C͒ and leakage current of the capacitors. The thermal stability of the Ta/Ta 2 O 5 system has been widely studied. [19][20][21][22][23][24][25] The capacity of such systems changes after heat-treatment due to the incorporation of O from the oxide into the Ta metal. A change in the structure of the oxide layer was found by X-ray diffraction ͑XRD͒. 21 Smith and Tripp 26 have extended their studies onto the anodically oxidized Nb/Nb 2 O 5 system. The effect of annealing was much more pronounced than in the Ta/Ta 2 O 5 system. The microphysical properties and structure of various niobium oxides are described in detail. 27 Grundner and Halbritter 28 have investigated the heattreatment induced transformation of the chemical structure of various thin film oxides ͑grown both anodically and thermally͒. Like Hahn...

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Phase transition and oxide dissolution processes in vacuum-annealed anodic Nb2O5/Nb systems

Cited by 32 publications

References 19 publications

The secondary-electron yield of air-exposed metal surfaces

The secondary-electron yield of air-exposed metal surfaces

The investigation of NbO₂ and Nb₂O₅ electronic structure by XPS, UPS and first principles methods

Anodic Oxidation of Niobium Sheets and Porous Bodies

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Phase transition and oxide dissolution processes in vacuum-annealed anodic Nb2O5/Nb systems

Cited by 32 publications

References 19 publications

The secondary-electron yield of air-exposed metal surfaces

The secondary-electron yield of air-exposed metal surfaces

The investigation of NbO2 and Nb2O5 electronic structure by XPS, UPS and first principles methods

Anodic Oxidation of Niobium Sheets and Porous Bodies

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The investigation of NbO₂ and Nb₂O₅ electronic structure by XPS, UPS and first principles methods