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Study On Hydrothermal & Solvothermal Synthesis Of Metastable Phases Of Functional Materials

Posted on:2004-06-15Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y C ZhangFull Text:PDF
GTID:1101360092992031Subject:Materials science
Abstract/Summary:PDF Full Text Request
The present thesis is aimed at exploring novel low-temperature hydrothermal & solvothermal methods to synthesize metastable phases of functional materials, and studing the formation mechanism of the metastable phases under the hydrothermal & solvothermal conditions. The main works achieved are summed up as following:1. Hydrothermal synthesis of metastable transitional metal sulfides For the first time, we succeeded in the synthesis of phase-pure metastable γ-MnS nanocrystallites by a simple hydrothermal method at 60~130oC. The effects of the reaction temperature and during time, the reactants and their molar ratio, the kinds of additives and their concentrations on the formed phase(s) of MnS during the hydrothermal process were also investigated, the results indicated that they are all influencing factors on the phase(s) of the final products. Based on the knowledge that the addition of glycol and glycerol favor the synthesis of pure metastable γ-MnS in the hydrothermal process, we used them as the solvents and also obtained pure metastable γ-MnS by a solvothermal method at 130oC. Using a similar hydrothermal method, we also succeeded in the synthesis of metastable β-CdS at 120~150oC. When investigating the effects of additives on the formed phase(s) of CdS in the hydrothermal process, we found a novel phenomenon, that is, Cl- can catalyze the phase transition from metastable β-CdS to stable α-CdS. Take the hydrothermal synthesis of metastable γ-MnS nanocrystallites as an example, the formation mechanism of the metastable phases under the hydrothermal and solvothermal conditions was discussed. Based on the calculation and experimental results, we draw a conclusion that kinetics played a predominant role in the formation of metastable phases during the hydrothermal and solvothermal processes. 2. Hydrothermal synthesis of Li4Mn5O12 nanocrystallitesFor the first time, we make an attempt at the synthesis of metastable Li4Mn5O12 by a low-temperature hydrothermal method. We proposed a precursor-hydrothermaltreatment route: firstly, the precursor (Li-birnessite) was prepared from LiOH, H2O2 and Mn(NO3)2 through a room temperature redox precipitation process; then the as-prepared precursor was subjected to hydrothermal treatment in LiOH aqueous solutions to obtain spinel-type lithium manganese oxides. Through this soft solution process, nearly stoichiometric Li4Mn5O12 nanocrystallites with the average valence of Mn ion being 3.96 could be obtained at 110oC. To our knowledge, the reaction temperature used in this case is lower than those in any other methods reported on the synthesis of spinel-type lithium manganese oxides up to now. However, in the precursor-hydrothermal treatment route mentioned above, the precursor (Li-birnessite) has to be firstly prepared from commercial LiOH, H2O2 and Mn(NO3)2 through a room temperature redox precipitation process, during which a great deal of time and excessive LiOH were wasted. So, we improved on that method: instead of using Li-birnessite as the precursor, we directly conducted the hydrothermal treatment on the sol-like solution from a room temperature redox reaction of LiOH, H2O2 and Mn(NO3)2. By so doing, spinel-type lithium manganese oxide nanocrystallites with the composition of Li1+xMn2-xO4 (0.09≤x≤0.28) were btained at 110oC for 8 h. Compared with the precursor-hydrothermal treatment method, the reaction time used in the latter route was greatly reduced. Finally, the role of basic principles of crystal chemistry in the formation of lithium-rich spinel Li1+xMn2-xO4 (0
Keywords/Search Tags:Functional material, Metastable phase, Hydrothermal, Solvothermal
PDF Full Text Request
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