| Summary: | 博士 === 國立成功大學 === 資源工程研究所 === 90 === In the nonconventional preparation of spinel ferrite powders, the pyrolytic decomposition of precursors often occurs through multi-stage transformation paths, which involve different metastable intermediate phases. The present investigation deals with the synthesis of nano-sized zinc ferrite powders using the tartrate precursor technique. The focus of this investigation is to study the development of the intermediate phase during the decomposition of precursor, as well as the formation mechanism of zinc ferrite. Accordingly, the nano-sized zinc ferrite powders can be obtained at low-temperature (< 500℃) in this study.
Characterizations of the various experimental products have been conducted as: (i) thermal behavior by DTA/TG, (ii) crystalline phase determined by XRD and TEM method, (iii) crystallite and particle sizes measured by Scherrer formula-XRD powder method, BET surface area diameters, and TEM, and (iv) magnetic properties and electronic structure of Zn conducted by SQUID and XPS techniques.
The experimental results are given as follows:
1.The thermal reaction sequence during the precursor decomposition to synthesize zinc ferrite powders as performed in this work can be noted as follows:
Step 1: Decomposition of the tartrate precursor
Zn1Fe2-tartrate precursor 2/3Fe3O4＋ZnO
Step 2: Oxidation of Fe3O4
2/3 Fe3O4 +1/6O2 γ-Fe2O3
Step 3: Formation of inverse spinel Zn-ferrite
γ-Fe2O3＋ZnO (Fe3+)A[Zn2+Fe3+]BO2-4
Step 4: Inverse-normal spinel transformation
(Fe3+)A[Zn2+Fe3+]BO2-4 (Zn)A[Fe2]BO2-4
2.The intermediate phase, Fe3O4, could not effectively react with ZnO to synthesize zinc ferrite at low temperature. The key step of preparation of ultrafine zinc ferrite powders by tartrate technique is the preheating process, in which the precursor is thermally treated between 300°and 400℃ for several hours to ensure complete conversion of the Fe3O4 into γ-Fe2O3. Therefore the ultrafine and mono-phase ZnFe2O4 can be obtained at low temperature by preventing the formation of α-Fe2O3 via preheating process.
3.It has been proven that the octahedral sites are preferentially exposed on the spinel surfaces. The γ-Fe2O3 is a cubic spinel, chemical formula of which is (Fe3+)[Fe3+5/3□1/3]O4, where □ stands for the vacancy of cation and distribution on the octahedral sites. Thus the γ-Fe2O3 can provide the cation vacancies on octahedral sites for Zn2+ ions diffusing into the octahedral-structured clusters, and then forming the inverse spinel zinc ferrite. The model of reaction mechanism between γ-Fe2O3 and ZnO is also built-up from the experimental results.
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