| In this thesis the single-phase LiMn2O4 spinels with various Li/Mn ratios (Li/Mn=0. 50,0.52,0.54) synthesized by Sol-Gel method were sintered at 400 to 750 in the air and characterized by X-ray diffraction (XRD),Differential Thermal Analysis and Thermogravimetric Analysis (DTA-TG) and Transmission Electron Microscopy (TEM) .The low-temperature resistance was also investigated by Physical Property Measure System (PPMS).The main results came as following:(1) Compared to the Solid-State Reaction method the materials were synthesized with high efficiency at low temperature by Sol-Gel method and the LiMn2O4 spinel resulted when sintered at 400 .(2) The oxygen-rich spinel material from sintering the precursor at low temperature changed into perfect spinel structure due to releasing the excess oxygen when raised the sintering temperature slowly ,which caused the lattice expanded and at the same time the system energy lowered and the bonding energy raised, and tansformed into the stoichiometric LiMn2O4 at 700 . When raised the sintering temperature further we got oxygen-poor LiMn2O4 spinel.(3) The samples stabilized more because of the lattice parameter reduction and the crystalline bonding increase with the ratio Li/Mn increase (Li/Mn=0.50,0.52,0.54).(4) The Mn electron orbits of stoichiometric LiMn2O4 became degenerated and the electrons settled in disordered states at 320K~280K and therefore the resistance of the material increased slowly with the temperature decrease. Below 280k the degenerated states were relieved and the electrons turned to the ordered states from the disordered , leading to the resistance increase suddenly and the transition from the cubic to thetetragonal structure.(5) The increase of Mn+4/Mn+3 in octahedron with the addition of Lithium tampered the disordered states , and the electrons around 280K tended ordered so that the resistance showed no significant changes. |
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