| Since the first discovery of superconductivity in1911by Dutch scientist Onnes, superconducting materials have gained widespread attentions from the researchers for their special properties and complex physical mechanisms. Till now, there have been a number of superconducting researches won the Nobel Prize for physics, which proved the importance of superconducting science. It is known that when the temperature is under the critical transition temperature, Tc, superconducting materials have three unique features:zero resistance, perfect diamagnetism and Josephson effects, based on which the superconducting materials have shown great potentials in many application fields.In2008, the first iron-based superconducting materials was reported based on LaOFeAs system, which was different from conventional superconductors, because their transition temperatures Tc can break through the Macmillan limit of39K predicted by BCS theory, which is based on the phonon-electron coupling mehanism. It offered a new opportunity on the investigation of the long-standing mystery of high-temperature superconductivity mechanisms, and it will also play a very important role in many related fields in condensed matter physics. Meanwhile, the Fe base superconducting materials have excellent current-carrying performances and mechanical properties, therefore they have shown a great potential in the field of practical application.Based on the different structures, the iron-based superconductors can be distinguished into four systems:"1111" system,"122" the system,"111" system and "11" system. FeSe-based superconductor has the typical lattice structure of "11" system, and the simplest structure of Fe-based superconducting materials currently found. Based on its advantage that the simple preparation method and low cost on the raw materials, it has become one of the hottest new iron-based superconducting materials. Researchers have developed many techniques for the preparation of FeSe-based superconducting bulks, wires and films for the basic research and applications. In this thesis, we choose FeSe superconducting material as the research object. FeSe-based superconducting bulks have been prepared by solid state sintering process and high energy ball mill (HEBM) aided solid state sintering process. The phase evolution mechinasms involved in different process have been systematically compared and discussed. New techniques for the low dimensional FeSe materials synthesis, including the preparation of FeSe nanoparticles with low temperature chemical process and the deposition of FeSe films with electrochemical deposition technique also have been developed. The main conclusions obtained are as follows:First, this thesis have studied the phase evolution mechanism involved during the solid state sintering process, and the influence of many important processing parameters on the evolution of Fe-Se phase system have been analyzed. It showed that during the heating process, it underwent three steps, including the melting of Se, the reaction of Se melt with Fe into hexagonal8-FeSe, and the transition from8-FeSe to tetragonal phase P-FeSe. While, during the cooling process, there also happens a phase transition from8-FeSe to P-FeSe due to the decrease of solubility of Se in8-FeSe, which further increased the tetragonal phase content in the final bulks. The distribution between Fe and Se is the crucial process during sintering, therefore the optimization of sintering parameters should consider the distribution process. It can be concluded that the higher Fe content in the precursor powders is beneficial to the formation of tetragonal phase β-FeSe. The increase of initial bulk density can promote the diffusion between Fe and Se, therefore increase the final tetragonal phase content. Meanwhile, the smaller cooling rate can also improve β-FeSe content. There highest tetragonal phase content we obtained is88.4%, with the Fe/Se ratio of1.30and sintered at900℃for12h.In order to further promote the distribution process during sintering, high-energy ball milling has been adoped for the pretreatment of precursor powders. The change of phase evolution process can be obviously noticed during sintering. During the HEBM process, both the particles sizes of Fe and Se decreased and amorphous of Se appeared after the ball milling for4h. The formation of Fe-Se solid solution can be detected due to the change of Fe diffraction patterns. After the high-energy ball mill for6h, only small content of amorphous Se and Fe-Se solid solution with the smallest average particle size can be detected. With the further extension of high-energy ball milling time, Fe and Se were alloyed and compounds with different Fe/Se ratio, such as Fe3Se4, FeSe2etc. can be found. Therefore the ball milling time of6h has been considered as the optimal parameter. Due to the formation of solid solution, the diffusion distance between Fe and Se atiom has been effectively decrease, which lead to the direct synthesis of P-FeSe phase. The tetragonal phase content in the sintered bulks has been greatly increased to97.0%with the initial Fe/Se ratio of1.15.Based on above, low temperature chemical method has been adoped for β-FeSe nano-powders, and the electrochemical doposition has been designed for FeSe film.β-FeSe nano-powders can be directly obtained with low temperature chemical method. It has also been proved that both the ions of Se2-and SeO32-obtained with the reaction of Se and NaOH can participate in to the formation of FeSe. Therefore the optimal Fe2+:Se ratio is1:1. Meanwhile, with the increase of pH value, the particles size of FeSe particles decreased gradually. While pH value increased to12, the particles size has been decreased from～100nm to～10nm.The appatus for the electrochemical doposition have been designed. After the deposition, both Fe and Se elements can be detected on the thin film by XPS. However the ratio of Fe/Se is not1:1, and the phase obtained is Fe2O(SeO3)2. Therefore further optimization should be made in order to have the right composition deposited on the substrate and to avoid the oxidation of films. |
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