Figure 1 - uploaded by George E. Thompson
Content may be subject to copyright.
XRD patterns of the Nb-50 atom % O films in the as-deposited condition and following annealing in vacuum at 923 K.
Source publication
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...
Contexts in source publication
Context 1
... previously, the niobium and Nb-20 atom % O films have a body-centered cubic bcc structure, with the latter having a reduced grain size. 12 The XRD pattern of the Nb-50 atom % O films reveals only the broad peak of an amorphous structure, while fol- lowing vacuum annealing films are transformed to crystalline NbO with a NaCl-type cubic structure Fig. ...
Context 2
... atom % O films, with the latter having a finer grain size of approximately 4 nm, whereas it is absent for the Nb-50 atom % O film in either amorphous or NaCl-type NbO structures. In contrast to anodizing, crystalline oxide develops readily on the crystalline NbO substrate, but not the amorphous substrate, during thermal oxidation in air at 723 K Fig. ...
Similar publications
Thermal treatment was performed on DC magnetron sputtered zirconium niobium (Zr0.7Nb0.3) films in the oxygen-enriched environment at different temperatures in the range 400 – 700ºC to transform from Zr0.7Nb0.3 to Zr0.7Nb0.3O2 films. The films oxidized at 700ºC were of tetragonal Zr0.7Nb0.3O2 with a crystallite size of 22 nm. There was a significant...
Citations
... Firstly, the system under study is multicomponent and, at least, contains three different chemical compounds: PA, which is amorphous [43,129,130], niobia V (nanocolumns embedded in the PA), and niobia IV (sublayer located on the glass surface and on which nanocolumns stand), which are also amorphous [117,131]. The state of niobia is not so unambiguous, since, for example, field crystallization of niobia V is possible, stimulated by the presence of mechanical stresses (the compressive stresses in the amorphous anodic niobia that can facilitate crystal growth), which are guaranteed to be present during the growth of niobia nanocolumns into the alumina pores [132]. It is also clear that there cannot be a sharp interface between niobia IV and V, and there certainly is a non-stoichiometric transition region that is a mixture of oxides and has a sufficiently large thickness. ...
Three types of niobia nanostructured films (so-called native, planarized, and column-like)
were formed on glass substrates by porous alumina assisted anodizing in a 0.2 M aqueous solution of oxalic acid in a potentiostatic mode at a 53 V and then reanodizing in an electrolyte containing 0.5 M boric acid and 0.05 M sodium tetraborate in a potentiodynamic mode by raising the voltage to 230 V, and chemical post-processing. Anodic behaviors, morphology, and optical properties of the films have been investigated. The interference pattern of native film served as the basis for calculating the effective refractive index which varies within 1.75–1.54 in the wavelength range 190–1100 nm. Refractive index spectral characteristics made it possible to distinguish a number of absorbance bands of the native film. Based on the analysis of literature data, the identified oxide absorbance bands were assigned. The effective refractive index of native film was also calculated using the effective-medium models, and was in the range of 1.63–1.68. The reflectance spectra of all films show peaks in short- and long-wave regions. The presence of these peaks is due to the periodically varying refractive index in the layers of films in two dimensions. FDTD simulation was carried out and the morphology of a potential 2-D photonic crystal with 92% (wavelength 462 nm) reflectance, based on
the third type of films, was proposed.
... Niobium is a candidate since its physical and chemical properties are similar to those of Ta and the anodizing behavior of these two metals is also similar to each other. It is, however, often claimed that the reliability of niobium capacitor is lower than that of Ta one, because of higher susceptibility of field crystallization of anodic niobium oxide [12][13][14][15][16][17][18], which increases dielectric loss of the capacitor, and large bias potential dependence of the capacitance originating from the n-type semiconductor character of the niobium oxide [19]. Alloying of niobium to form a nonequilibrium, uniform solid solution is effective in suppressing the field crystallization and avoiding the degradation of the dielectric properties [14,15,[20][21][22][23], although the non-equilibrium solid-solution alloys are not suitable for application to the current capacitor technology, which includes high-temperature sintering of the metal powders. ...
... It is, however, often claimed that the reliability of niobium capacitor is lower than that of Ta one, because of higher susceptibility of field crystallization of anodic niobium oxide [12][13][14][15][16][17][18], which increases dielectric loss of the capacitor, and large bias potential dependence of the capacitance originating from the n-type semiconductor character of the niobium oxide [19]. Alloying of niobium to form a nonequilibrium, uniform solid solution is effective in suppressing the field crystallization and avoiding the degradation of the dielectric properties [14,15,[20][21][22][23], although the non-equilibrium solid-solution alloys are not suitable for application to the current capacitor technology, which includes high-temperature sintering of the metal powders. ...
Heat treatment of Zr-24 at% Ti alloy with barrier-type dielectric anodic oxide films was conducted at 473 K in air to examine the thermal stability of the dielectric oxide films for possible electrolytic capacitor application. The anodic oxide film was formed by anodizing of the alloy at 50 V for 30 min in 0.1 mol dm−3 ammonium pentaborate electrolyte. The anodic oxide film of 125 nm thickness was crystalline, containing both monoclinic and tetragonal ZrO2 phase. It was found that marked thickening of the oxide film with generation of cracks occurred during heat treatment at 473 K. Thus, the dielectric loss was largely increased along with the capacitance increase. In contrast, the anodic oxide film formed on the oxygen-incorporated alloy remained uniform, and no significant increase in dielectric loss was observed even after the heat treatment. The capacitance of the anodic film became as high as 4.8 mF m−2, which was nearly twice that on Ta. The high capacitance was associated with the preferential formation of tetragonal ZrO2 phase in the anodic oxide film on the oxygen-incorporated alloy. Findings indicated that the oxygen-incorporated Zr-Ti alloy is a promising novel material for capacitor application.
... The best volume-capacitance ratio is presently obtained by electrolytic capacitors based on Nb 2 O 5 and Ta 2 O 5 [16]. Owing to the larger permittivity of the Nb 2 O 5 and more natural abundance of Nb, the niobium oxide is considered as substitute of Ta 2 O 5 in solid electrolyte tantalum/Ta 2 O 5 capacitor [17,18]. The porous structure on tantalum and niobium is also widely used in optical and electronic devices [19,20]. ...
The anodic oxide films were prepared on the niobium and tantalum in aqueous electrolyte mixtures containing 1 M CH3COOH + 1 M H3PO4 or 1 M CH3COOH + 1 vol.% HF or 1 M CH3COOH + 1 M H3PO4 + 1 vol.% HF at 30 V for 30 min. The barrier films were obtained on both niobium and tantalum surfaces in all electrolyte mixtures except niobium oxide film formed in 1 M CH3COOH + 1 vol.% HF which is porous in nature. The anodic oxide films were characterized by FESEM. Also, electrochemical impedance spectroscopy at open-circuit potential on Nb and Ta was applied and obtained data were analyzed by fitting with four different equivalent circuits.
... Niobium oxide has also been proposed as an ideal substitute for Ta 2 O 5 in solid electrolyte tantalum/ tantalum pentoxide (Ta 2 O 5 ) capacitors, where the oxide dielectric layer is formed from a porous metal powder compacted by anodic oxidation. Due to the larger permittivity of Nb 2 O 5 compared to Ta 2 O 5 it is a logical substitute since it has a similar anodization behavior and in addition offers the advantage of greater natural abundance and hence, lower raw material price [25][26][27][28][29][30][31]. ...
... In spite of the large number of applications and interesting properties only a relatively small number of reports are available on the preparation and properties of niobium oxide films. Recent papers describe the preparation by thermal evaporation [32], anodization [26,28,30], sputtering [19,29,[33][34][35][36][37][38][39][40][41], sol-gel processes [42][43][44][45], pulsed laser ablation [1,46], spray deposition [22,47], atomic layer deposition [23] and chemical vapor deposition [48]. ...
Amorphous niobium oxide thin films were deposited by unbalanced reactive magnetron sputtering under different conditions of pressure (2 to 4 Pa) and oxygen percentage (9, 17, and 23%). The films were characterized to obtain the relationships between the deposition parameters and the most relevant physical properties (structural, optical, mechanical, surface morphology and optical). The composition was determined by Rutherford backscattering spectroscopy and energy dispersive X-ray spectroscopy. From the results of X-ray diffraction and the composition, we can conclude that for all of the deposition pressures and flow ratios used, the films were stoichiometric Nb 2 O 5 and amorphous. Similarly, the mechanical and optical properties did not show significant variation between the different deposition conditions; all the films were transparent with a bandgap of about 3.4 eV and a hardness around 5 GPa. Concerning the electrochemical properties, the response of the films to DC polarization in a 0.89% NaCl solution was significantly different. The parameters used to compare the electrochemical response were the polarization resistance and the corrosion resistance obtained from the Tafel analysis.
... For the as-deposited Nb, the current turns to increase approximately at 1.0 ks, being associated with the development of crystalline oxide in the initially formed amorphous anodic niobium oxide film. In our previous study, the nucleation of crystalline oxide was confirmed at the current minimum, while no crystalline oxide was present during constant current anodizing to 100 V for the as-deposited niobium [10]. The development of crystalline oxide is accelerated by prior thermal treatment of niobium in air; the current minimum appears at shorter anodizing time for the thermally treated specimen. ...
The present work demonstrates effective inhibition of field crystallization of amorphous anodic niobium oxide by incorporation of silicon species from substrate. The field crystallization, detrimental for capacitor application of niobium, occurs during anodizing of magnetron sputtered niobium at 100 V in 0.1 mol dm-3 ammonium pentaborate electrolyte at 333 K, while amorphous structure of the anodic oxide is totally retained during anodizing of magnetron sputtered Nb-12 at% Si. Even after prior thermal treatment in air, which accelerates field crystallization of anodic oxide on niobium, no crystallization occurs on the Nb-12 at% Si. Through examination of the crystallization behaviours of anodic films formed on a thin Nb-12 at% Si layer superimposed on a niobium layer as well as on a thin niobium layer superimposed on an Nb-12 at% Si layer, it has been confirmed that air-formed oxide or thermal oxide becomes a nucleation site for crystallization. Modification of the air-formed or thermal oxide by incorporation of silicon species inhibits the nucleation of crystalline oxide. The modification, however, does not influence the growth of crystalline oxide. The growth is suppressed by continuous incorporation of silicon species into anodic film from the substrate during anodizing.
... [2][3][4][5] It has been recently demonstrated that incorporation of foreign species from metal substrate avoids the degradation of the film properties and even improves the dielectric properties. Examples are niobium supersaturated with silicon, 4) nitrogen 6,7) and oxygen 8,9) and titanium supersaturated with molybdenum, 10) silicon, 11,12) tungsten 13,14) and zirconium. 15) Such non-equilibrium alloy thin films were prepared by magnetron sputtering. ...
Niobium films with isolated columnar morphology have been prepared by oblique angle magnetron sputtering for capacitor application. Anodizing of the deposited mobium to form dielectric mobium oxide reduces the surface roughness, since the gaps between the neighboring columns are filled with the oxide due to large Pilling-Bedworth ratio for Nb/Nb2O5. To increase the gaps between neighboring columns, the influences of the angle of mobium flux to substrate and substrate surface roughness on the columnar morphology of the deposited films have been investigated using scanning electron microscopy and the electrochemical measurements. The deposition on the textured rough substrate surface and at higher angle of the mobium flux from normal to the substrate surface fabricates the mobium films with higher surface roughness.
In the present study, porous Nb–Si alloy films with isolated nano-column morphology have been successfully developed by oblique angle magnetron sputtering on to aluminum substrate with concave cell structure. The deposited films are amorphous with the 15at% silicon supersaturated into niobium. The porous Nb-15at% Si films, as well as niobium films with similar morphology, are anodized at several voltages up to 50V in 0.1moldm−3 ammonium pentaborate electrolyte. Due to the presence of sufficient gaps between neighboring columns, the gaps are not filled with anodic oxide, despite the large Pilling–Bedworth ratio (for instance, 2.6 for Nb/Nb2O5) and hence, a linear correlation between the reciprocal of capacitance and formation voltage is obtained for the Nb-15at% Si. From the comparison with the anodic films formed on porous niobium films, it has been found that silicon addition improves the thermal stability of anodic niobium oxide; the change in capacitance and increase in leakage current become small for the Nb–Si. The findings indicate the potential of oblique angle deposition to tailor porous non-equilibrium niobium alloy films for high performance niobium-base capacitor.
The influence of thermal aging, at intermediate temperature (1 h at 250 degrees C) and in different environments, on the electronic and solid-state properties of stabilized 160 nm thick amorphous anodic niobia, grown on magnetron-sputtered niobium metal, has been studied. A detailed physicochemical characterization of the a-Nb(2)O(5)/0.5 M H(2)SO(4) electrolyte junction has been carried out by means of photocurrent and electrochemical impedance spectroscopy as well as by differential admittance (DA) measurements. A change in the optical bandgap (3.45 eV) of niobia film has been observed after aging (3.30 eV) at 250 degrees C in air for 1 h. A cathodic shift (0.15-0.2 V) in the flatband potential of the junction has been observed. The frequency dependence of DA data agrees with expectations of the theory of amorphous semiconductor Schottky barrier. The fitting of both components of DA allowed to get information on the distribution of the electronic density of states as a function of energy and distance from the metal oxide interface. The DA measurements evidenced for vacuum-treated niobia film an insulating to semiconductor transition. These findings can help to explain the large changes in the measured values of capacitance, after aging, and the larger leakage current observed in niobia electrolytic capacitors.
The present work demonstrates the formation of porous niobium films with separated columnar structures by oblique angle magnetron sputtering for capacitor application. The mobium films deposited on textured aluminium substrates, which had concave cell structures with the cell sizes ranging from 125 nm to 550 nm, consist of isolated columns of niobium with wider gaps between columns developing on the substrates with larger cell sizes. The surface areas of the deposited films. evaluated by the capacitance of the anodic films formed at several voltages, increased with an increase in the cell size of substrate. The surface area decreases with an increase in the formation potential of anodic films from 2 V to 10 V vs Ag/AgCl, because the gaps are filled with anodic oxide as a consequence of the large Pilling-Bedworth ratio of 2.6 for the Nb/Nb(2)O(5) system. The reduction of the surface area is suppressed when the substrate with larger cell size is used, due to the formation of niobium columns with wider gaps, which are not filled with anodic oxide. The high surface area even at higher formation voltages of the anodic films is a requisite for capacitor application.
Anodic oxide films with nanocrystalline tetragonal ZrO(2) precipitated in an amorphous oxide matrix were formed on Zr-Si and Zr-Al alloys and had significantly enhanced capacitance in comparison with those formed on zirconium metal. The capacitance enhancement was associated with the formation of a high-temperature stable tetragonal ZrO(2) phase with high relative permittivity as well as increased ionic resistivity, which reduces the thickness of anodic oxide films at a certain formation voltage. However, there is a general empirical trend that single-phase materials with higher permittivity have lower ionic resistivity. This study presents a novel material design based on a nanocrystalline-amorphous composite anodic oxide film for capacitor applications.
