Electrical properties of niobium based oxides: Ceramics and single crystal fibers grown by the laser-heated pedestal growth (LHPG) technique

This thesis is dedicated to gain understanding of the dielectric behavior of Niobium-based oxide ceramics and single crystals. The work presents the results on the Nb₂O₅ system in three specific areas: (i) Unusual enhancement of dielectric constant of Nb₂O ₅ has been observed through the small substituents such as TiO ₂, SiO₂, and Al₂O₃. The ceramics of these solid solutions are prepared by the conventional mixed oxide method. Single crystal fibers are grown by the Laser-Heated Pedestal Growth (LHPG) technique, which is considered to be a powerful tool for rapid growth of high-melting temperature oxides and incongruent melting compositions. Optimized growth parameters are established to obtain the best quality of single crystal fibers. (ii) This work is the first report on the single crystal growth of the solid solution and the anisotropic dielectric behavior of the crystals. Interesting dielectric relaxation has been observed in these materials and analyzed by Arrhenius, Vogel-Fulcher, and Cole-Cole relationship. The direct-current (dc) electric field dependence of the dielectric constant has been studied. The results show the strong dielectric dispersion which exists in a large frequency range implies that the relaxation process involved is not of a simple Debye type. Nb₂O₅ system is known to have Magneli's phases of the polar group symmetry. The large dielectric constants, the anisotropic behavior and the field dependence of the dielectric properties could be associated with the presence of polar clusters or the similar groups of Magneli's phases in the solid solution compositions studied. The influence of the cluster size dispersion is one assumption. The data of do bias field dependence of the dielectric constant has been analyzed by the modified Devonshire relation including a cluster term giving the fitted parameters: cluster sizes distribution and their polar cluster polarization. (iii) The nanoscale distribution of Magneli's phases also influences the thermal expansion behavior of the samples. The measurements are made on the solid-solution Nb₂O ₅-based oxide samples in the ceramics and single crystal in the temperature range of −200 to 500°C by using the dilatometer technique. Both single crystals and ceramics show negative thermal expansion in this temperature range. Nevertheless, the thermal expansion coefficient of these solid solutions can be designed to achieve a desired overall coefficient by either annealing or processing history. (Abstract shortened by UMI.)...