The Choice of Substrate Material and Electrodeposition of High Purity Niobium Coatings

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The interaction of different materials with the NaCl-KCl-NaF(10 wt.%)-K2NbF7(8 wt.%)-Nb melt was investigated. As substrate materials the ceramic, beryllium and carbopyroceram of different shape (spheres, cylinders, parallelepipeds) were chosen. The time of the experiments was varied from 10 minutes to 12 hours. The spherical ceramic samples were coated with protective molybdenum film by magnetron sputtering. Porous area of these films was at an average 200 μm2. After the experiment (exposition time 10 min) the exfoliation of molybdenum film was observed due to interaction of the substrate with the melt and the porous area was increased in to 27 times. The XRD data showed niobium oxides formation such as NbO, Nb4O5, NbO2 on the ceramic surface. The same XRD data were obtained for the samples without protective films. The beryllium samples regardless of the shape and the presence of the protective films were dissolved in the niobium containing melt due to more negative electrode potential comparing with niobium one. The carbopyroceram samples were exposed in the melt during 3 and 12 hours. After 3 hours of exposition, no changes of XRD data and weight of samples were found, but after 12 hours, XRD data indicated the formation of niobium carbide (NbC) of insignificant thickness. On the base obtained results, the carbopyroceram was chosen as a perspective material for electrodeposition of high purity niobium coatings because it not corrodes in the niobium containing melt. The electrolyte mentioned above was used for niobium coatings electrodeposition on carbopyroceram spheres. Due a spherical shape of substrates, a special form of the cathode should be used for plating of samples by niobium in molten salts. In the present study construction of the cathode was developed. It consists of two parts: top part is fixed, bottom part is rotating by digital controlled stirrer. Construction of the cathode provides the electrical contact in random point of the sample. Due to such contact a uniform coatings were obtained. The optimal regimes (temperature, current density, time of electrolysis and rotation speed of the cathode) for electrodeposition of smooth uniform niobium coatings with the thickness up to 50 µm on carbopyroceram spheres were found. Purity of the coatings was determined by spectral quantitative analysis. The impurity content is given below (ppm): Mn < 2.0; Mg < 3.0; Si < 10; Fe = 2.3; Ni < 5.0; Pb < 5.0; Sn < 5.0; Ti < 10; Al < 5.0; Co < 10; Mo < 10; Ca < 10; Zr < 20; V< 3.0; Cu ≤ 10; Cr < 5.0.