Studies On The Preparation And Properties Of Silver Selenide Materials Of Large Positive Linear Magnetoresistance

Non-stoichiometric silver chalcogenides has aroused wide attention from researchers because of its outstanding linear magnetoresistance effect (LMR) in a wide temperature range (4 K-300K) and extra wide range of magnetic field (1 mT ~ 55T) ,and its LMR has not yet reached saturation in the super magnetic field up to 55 T. In this thesis, silver selenide material has been suceessfully synthesized by the effective hydrothermal method and room temperature-solid state method, the preparation condition and performance of the prepared material was also studied. Furthermore, we present a new physical model to explain the linear dependent magnetoresistance of silver chalcogenides.Firstly, we synthesized orthogonal phase silver selenide crystal with a variety of morphology by hydrothermal method. We study the effect of [OH~-] concentration, reaction temperature and time on the morphology of the final product, and the crystal structure and morphology of final product is also investigated. The results show that the reaction temperature and [OH~-] concentration is the key factors to determine the structure and morphology of the product. When temperature lower than 240℃, the hydrothermal reaction is not complete. The nucleate-speed of Ag2Se is slowe in low [OH~-] concentration, and the nucleus growth in various directions, the prepared product is dendrite-like crystal; with the increase of [OH~-] concentration, nucleation growth in orientation, and the product is rods-like crystal, when the [OH~-] concentration high, impurities began to appear in the product because the hydrothermal reaction is too fast.Secondly, we prepare silver selenium bulk by using room temperature– solid state method. The silver selenium nanopowder is prepared firstly by room temperature method, and then the nanopowder is pressed and sintered into bulk, and we analyse the structure and morphology of the prepared silver selenium powder and bulk. We investigate the effects of different sintering technology on the prepared silver selenium bulk, the study showed that 400℃is the best sintering temperature. And the heat treatment at 300℃of the nanopowder before sintered is helpful to reduce the hole in the bulk.Finally, against the novel magnetoresistance of silver chalcogenide films, we proposed a two-phase composite two-dimensional resistance network model according to experimental data. This new model not only description the novel LMR effect of silver chalcogenide films quantitatively, but also reveals the corresponding micro-mechanism in a certain scale. The point of this model is as follow: considered the film to be a number of silver metal particles embedded in the two-phase composite materials, and the internal structure of materials is discretized into quasi-random four-terminal resistor network, the specific forms quasi-random distribution is determined by the silver volume fraction f of composite materials. The model simulate the law of MR changes with the magnetic field H and temperature T of silver chalcogenides and the simulation are basically the same as the experimental results.