Colored Passive Layers On Nb With A Wide Spectrum Of Well-defined Colors By Anodic Oxidation In Oxalic Acid

Niobium is a kind of high melting metal, with a melting point of2468℃. Niobium is very stable which results from the excellent corrosion resistance of niobium oxide. Niobium and its oxides are widely used in waveguide, catalysis, solar energy, and other electronic equipment. Nb3Sn and Nb3Al are used in producing superconductive solenoids. Because of its unique physical and chemical properties, high refractive index, band gap, excellent chemical stability and corrosion resistance, Nb3O5film is widely applied in many fields of modern technology, such as optical interference filters, electrochemical colorizing film and gas sensors etc. Niobium oxide has also been proposed as an ideal substitute for Ta2O5in solid electrolyte tantalum/tantalum pent oxide (Ta2O5) capacitors. Compared to Ta2O5, the higher dielectric constant of Nb2O5that make it is a logical substitute for Ta2O5, in addition the content of Nb2O5is more abundant in nature. Furthermore, Niobium, as well as many other metals, such as titanium, tantalum, aluminum and zirconium are well-known to develop colored passive layers at elevated temperatures in an atmosphere containing O2or oxidized by applying a direct current (DC) or alternating current (AC) in an aqueous electrolyte. The coloration originates from iridescence, i.e. constructive interference of light reflected at the outer air/oxide and the inner oxide/metal interfaces. Colorized niobium has attracted extensive research and attention as a promising substrate to achieve cost effective and stable colorful display. Niobium with colored passive (oxide) layers was selected as core metal for a special25Euro bi-metallic collected coin issued by the Austrian Mint. In addition, niobium with gorgeous and bright colors can also be used in art, such as costume jewelry, body piercing jewelry and wedding rings. So, in order to strengthen the application and development of Nb in the field of art and decoration, it is very necessary and meaningful that researching a set of effective and simple coloring technology.In this article, we studied the coloration of Nb by electrochemical anodic oxidation, oxalic acid as the electrolyte. Following conclusions are the results of these experiments:(1) during a very short time (10s), it is so easy to get a dozen of bright colors just by changing the voltage (15V-130V) in oxalic acid (0.05mol/L);(2) a serious of experiments have been performed to survey the coloration at different temperature and concentration, it is found that the colors are less florid at room temperature than high temperature (70℃or40℃), the higher temperature the faster reaction, and there is not obvious distinction resulting from the concentration;(3) with regard to one color, the required anodic oxidation time is shorter at high voltage than at low voltage;(4) if we set all of the colored samples made at different voltage and reaction time, it can be found that they almost contain a majority of the visible light colors;(5) finally, by means of researching the influence of time, temperature and electrolyte concentration on colors, we explore a reasonable and simple coloration process for Nb.The components, valence state and crystal structure of the passive layers were studied by X-ray Energy Disperse spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS) and X-ray Diffractometry (XRD), respectively. The results show that passive layer on colored Nb is amorphous or crystallographic Nb2O5, which depends on the applied DC voltage and reaction time. Field Emission Scanning Electron Microscope (FESEM) characterization of the colored passive layers demonstrates that the films are compact without cracks if the voltage is less than100V. Besides, the cohesion between the passive layers and the substrate Nb is measured by scratching experiment, and the hardness of the passive layers is evaluated by hardness testing. The results of Spectroscopic Ellipsometry (SE) thickness analysis show that the films with various colors have different thickness. At the same oxidation time (10s), the higher applied DC voltage the thicker passive layer, which results in a wide spectrum of colors.And the colors can be tuned by controlling the DC voltage or reaction time, because the thickness of the compact Nb2O5passive layers ascends with the increasing DC voltage and the oxidation time. The main reason of colored passive layers is the constructive interference behavior of light reflected at the outer air/oxide and the inner oxide/metal interfaces.With respect to different applied voltage, the relationship between color and anodic oxidation time exhibits some differences. If the applied DC voltage is low (15V-30V), the colors change slowly with the reaction time, just2or3kinds of colors can be received. If elevating the DC voltage to65V-100V, it can be found that the colors vary continuously and dozens of colors can be made just controlling the reaction time. However, once the voltage is elevated excessively, such as120V,125V and130V, it will keep at green after20s, even8s, and then transform slowly into greyish-green, ultimately. Combined with the results of XRD and SEM, it indicates that the appearance of green is the the beginning of the crystallization, and with the intensification of the crystallization, green fades, and turns into sage green, even gray, gradually.